Symposium F.EN04—Beyond Lithium-Ion Batteries—Materials, Architectures and Techniques (Includes Symposium S.EN07—Next-Generation Electrical Energy Storage—Beyond Intercalation-Type Lithium Ion, and Symposium S.NM08—2D Atomic and Molecular Sheets—Electronic and Photonic Properties and Device Applications)

Next generation of energy storage devices may largely benefit from fast and solid Li+ ceramic electrolyte conductors to allow for safe and efficient batteries and fast data calculation. For those applications, the ability of Li-oxides to be processed as thin film structures and with high control over Lithiation and phases at low temperature is of essence to control conductivity. In particular, Li-garnet structures are attractive due to their fast transfer properties and safe operation over a wide electrochemical stability range for batteries. In the first part we focus on low temperature processing of Li-garnet thin films and nanostructures. Examining the structure-Li transport in solid state and asking the provocative question: How do Li-amorphous to crystalline structures conduct? How to alter processing of Li-oxide films to compensate for the challenge that most of Lithium phases lead to sluggish transport and hard phase stabilization? Bringing down battery solid electrolyte processing temperature and decreasing form factor opens new opportunities in battery design and new doors to give more functions for similar material classes for neuromorphic computing. Through the talk, we focus on new processing opportunities to Lithiate thin film structures in crystalline state and to assure cubic and fast conduction on the example of Li-garnet for thin film form using some active Lithiation strategies at film deposition1-3. For this we will review the field of thin film processing and introduce to new processing routes based on vacuum and wet-chemical techniques to control various strategies of Lithiation and phase evolution. Excitingly, one has the opportunity for Li-garnet solid state conductors to not only synthesize crystalline structures, but also amorphous films in various frozen-in states4. Insights on degree of amorphous to crystalline Li-garnet thin films are presented based on model experiments, incl. JMAK and TTT-diagram analysis of Li-garnet structure types. Dependent on either a vacuum or wet-chemical based method chosen the phase evolution differs when synthesizing the glass states, with significant impact on condensation and Li-transport5-6. We find that the different amorphous structures that Li-garnets are no Zachariasen glasses as they violate several of Zachariasen rules by text book: These amorphous Li-garnets differ therefore significantly from LIPON but have a wider room to arrange the higher coordinated polyhedral, network former and builders. In amorphous garnets there is room for manipulation in densification and Li-transport and one may potentially profit from high stability window to design batteries with no grain boundaries at the Li electrolyte interface. Collectively, the insights on solid state energy storage provide evidence for the functionalities that those Li-materials can have in the future. The challenges are deeply connected to low temperature processing and design of structure-transport properties, however, solving those allows us to dream a little more and give materials more functions. 1. A Low Ride on Processing Temperature for a Fast Li Conduction in Garnet Solid State Battery Films R Pfenninger, M. Struzik, I Garbayo, E Stilp , JLM Rupp, Nature Energy, 4, 475-4832019, 2019 2. Lithium-Containing Thin Films JLM Rupp, R Pfenninger, M. Struzik, A Nenning, I Garbayo, US 62/718,838 (2018) 3. Methods of Fabricating Thin Films Comprising Lithium-Containing Materials JLM Rupp, R Pfenninger, M. Struzik, A Nenning, I Garbayo, US 62/718,842 (2018) 4. Glass-Type Polyamorphism in Li-Garnet Thin Film Solid State Battery Conductors I Garbayo, M Struzik, WJ Bowman, R Pfenninger, JLM Rupp, Advanced Energy Materials, 1702265 (2018) 5. Solid-state Electrolyte and Method of Manufacture Thereof. ZD Hood, Y Zhu, L Miara, JLM Rupp. US 62/713,366 (2018) 6. Solution-Processed Solid-state Electrolyte and Method of Manufacture Thereof. Y Zhu, ZD Hood, L Miara, JLM Rupp US 62/713,428 (2018)

All-solid-state lithium (Li) metal batteries (ASSLMBs) attract increasing attention due to the use of high-capacity Li metal anode and solid electrolytes (SEs) with high safety. Recently, significant research achievements have been made in developing Li-ion conductors. In particular, sulphide SEs generally offer higher ionic conductivity (10?4?10?3 S cm?1 at room temperature) and lower grain boundary resistance. However, the practical applications of sulfide-based ASSLMBs are significantly hampered due to the interfacial challenges originating from (1) poor solid-solid contact between SE and electrode; (2) significant reaction at SE/Li interface. To solve the these issues and enhance the electrochemical performance of sulfide-based ASSLMBs, here, we employed a polymer interlayer between Li6PS5Cl SE and electrodes. It was found that the contact resistance was remarkably eliminated and the instability of SE against Li metal was resolved owing to the engineered polymer interlayer. In addition, long-term cycling stability was achieved due to the chemical compatibility between SE and this polymer interlayer. This method provides an effective strategy to address the interfacial challenge between sulfide SEs and Li metal, enabling the successful utilization of Li metal and sulfide SEs in ASSLMBs towards next-generation safe and high-energy density energy storage systems.

Recently, the beneficial combination of two different lithium storage mechanisms in a single compound, referred to as conversion and alloying materials (CAMs), has been noticed to complement the shortcomings of the conversion and alloying mechanisms [1,2]. This could lead to the excellent synergistic effects due to their atomically homogeneous and finer distribution than the physically mixed nanocomposite between two materials. However, the combination between alloying and insertion reaction or conversion and insertion reaction materials has been rarely investigated even it is expected to show superior cycle retention with an intermediate specific capacity than that of CAMs or other anode materials. Unlike the known CAMs, the realization of dual reaction with insertion reaction in a single compound requires a consideration of the crystal structure relation between two different materials, not simple comprising elements in the compound. Since the insertion reaction anodes have sufficient spaces to accommodate the reversible lithium ion insertion/extraction in their crystal structure, the dual reaction material should be designed with the similar crystal structure, which could be the solid solution compounds exhibiting a high solubility limit with an insertion-reaction type structure. The lack of consideration for the structural relation prevents the realization of dual reaction anode with an insertion reaction mechanism in a single compound. In this study, we suggest a new concept of highly stable alloying/insertion dual reaction anode of Mn1-xVxP solid solution by combining alloying reaction type MnP and insertion reaction type VP based on their structural similarity. The crystal structures of MnP and VP are basically similar, but their electrochemical performance as anodes for lithium-ion batteries (LIBs) is entirely different [3,4]. In this regard, the combination of MnP and VP into a single compound could form a fascinating solid solution anode with synergistic effects on electrochemical performance through alloying/insertion dual reaction mechanism derived its structural similarity. The series of Mn1-xVxP compounds (x = 0, 0.25, 0.5, 0.75, and 1.0) are firstly synthesized with a facile high energy mechanical milling and their electrochemical properties as an anode for LIBs were systematically studied and compared with those of MnP/VP mixture composite, particularly focusing on the verification of simultaneous alloying/insertion dual anode reaction in the solid solution compounds. The Mn1-xVxP (x = 0.25) solid solution electrode showed the excellent high-rate cyclability delivering the reversible capacity of 310 mAh g-1 after 400 cycles with a negligible capacity fading at 2 A g-1 by the synergistic effects of two reaction mechanisms. [1] D. Bresser et al., Energy Environ. Sic., 9, 3348-3367 (2016). [2] F. Wu et al., Nanoscale, 7, 17211-17230 (2015). [3] L. Li et al., Electrochim. Acta, 95, 230 (2013). [4] C.-M. Park et al., Chem. Mater., 21, 5566-5568 (2009).

A number of Li battery processes, including growth of Li dendrites, growth of SEI, and the effect of pressure on the internal resistance in solid state batteries, have been understood using a square root scaling, in line with simple models such as diffusion-limited metal/semiconductor oxidation. This model performs sufficiently well for some empirical predictions of capacity fade via SEI growth in lithium-ion batteries, for example. However, the t0.5 model is not theoretically well-justified to describe SEI growth in many cases. Furthermore, this model is not necessarily the best-fitting empirical model for published literature data. In this work, we illustrate these limitations and suggest an alternative approach for accurate estimates and predictions of SEI growth: evaluating and comparing the performance of multiple models. We also provide other examples where a square root scaling has been improperly assumed.

Implementing alkali metal anodes and replacing flammable liquid electrolytes with solids promises to deliver higher energy densities and greater safety than conventional Li-ion batteries. However, challenges at both the metal-electrolyte and cathode-electrolyte interfaces must be addressed to realise this potential. It is imperative to maintain good interfacial contact between the solid-state components to achieve low interfacial resistances. Alkali metal dendrite growth, even at modest rates, when paired with dense, well-sintered solid electrolytes, leads to short circuits, and ultimately cell failure. We have used a number of techniques to investigate the effect of cycling conditions on the formation of voids and dendrites and how the latter is intimately connected to the former. These results and their implications for the operation of a failure free all-solid-state battery will be discussed.

Recent experiments have shown that applying a stack pressure can strongly alter both the electrical and mechanical behavior of Li metal-solid electrolyte (SE) systems. To understand quantitatively the relationship between pressure, Li microstructure, and the Li-SE interface resistance, we have developed a 3D time-dependent contact model for describing the Li-SE interface evolution under a stack pressure. Our simulation simultaneously considers the surface roughness of the Li and the SE, Li elastoplasticity, Li creep, and the Li metal plating/stripping process. Our spatial scale runs from tens of nm to sub-mm, and our time scale runs from 50 ms to 50 hours. We describe quantitatively the competition between charge/discharge and creep, and how that competition controls the interfacial resistance. We demonstrate how to predict whether plating/stripping is stable or unstable under given conditions. And we describe a simple technique that can allow determination of the absolute area-specific resistance of the Li-SE interface. Simultaneous fitting to very recent experiments from two different research groups requires an effective yield strength of the Li used in those experiments of 16 ñ 2 MPa, substantially higher than traditional values.

There is tremendous interest in making the next super battery, but state-of-the-art Li-ion technology works well and has inertia in several commercial markets. Supplanting Li-ion will be difficult. Recent material breakthroughs in Li and Na metal solid-state electrolytes could enable a new class of non-combustible solid-state batteries (SSB) delivering twice the energy density (1,200 Wh/L) compared to Li-ion. However, technological and manufacturing challenges remain. The discussion will consist of recent milestones and attempts to bridge knowledge gaps to include: The physical and mechano-electrochemical phenomena that affect the stability and kinetics of the Li and Na metal-solid electrolyte interface Thin film processing and Li integration with LLZO Plating and stripping dynamics of Li and Na metal Despite the challenges, SSB technology is rapidly progressing. Multi-disciplinary research in the fields of materials science, solid-state electrochemistry, and solid-state mechanics will play an important role in determining if SSB will make the lab-to-market transition.

Mechanical constriction theory and simulation technique were recently developed by our group to quantitatively describe the expansion of voltage stability window and modification of instability decomposition for solid state batteries based on sulfide solid electrolyte.[1,2,3] In solid state batteries, lithium dendrites form when the applied current density is higher than a critical value. In this talk, an advanced mechanical constriction technique is applied on a solid-state battery constructed with Li10GeP2S12 (LGPS) as the electrolyte and lithium metal as the anode. The decomposition pathway of LGPS at the anode interface is modified by this mechanical constriction and the growth of lithium dendrite is inhibited, leading to excellent rate and cycling performances. A combination of electrochemical battery tests, SEM, XPS and XRD characterizations, and DFT simulations were used. 1. Advanced sulfide solid electrolyte by core-shell structural design, Nature Communications, (9), 4037 (2018) 2. Strain-stabilized ceramic-sulfide electrolytes, Small, 15, 1901470 (2019) 3. The effects of mechanical constriction on the operation of sulfide based solid-state batteries, Journal of Materials Chemistry A: 7, 23604 - 23627 (2019)

Li metal is considered as one of the most promising anode materials for high energy batteries, owing to its high theoretical specific capacity and low electrochemical potential. However, notorious issues with the formation of Li dendrites and insufficient coulombic efficiency (CE) during the plating/stripping process prevent its practical applications in rechargeable batteries. These limitations are primarily attributed to fragile and inhomogeneous solid electrolyte interphases (SEIs), which cause continuous consumption of fresh Li and electrolyte during cycling, leading to short battery life. Of late, many efforts have been taken to enhance CE and suppress Li dendrite growth, including design of three-dimensional current collectors, artificial SEI construction on the surface of Li, employment of solid-state electrolytes, separator modification and optimization of electrolyte compositions. In this talk, we will show some recent works in developing novel electrolyte additives and polymer coatings to improve the CE and cycling stability of Li-metal anodes. We demonstrate reasonably stable cell cycling performance under practical conditions such as lean electrolytes and low N/P ratios. The roles of these additives and polymer coatings were also explored to elucidate their function in tuning Li-metal morphology and SEI composition.

By utilizing a combination of ab initio random structure searching, genetic algorithms, and prototyping, we have identified a new phase of copper phosphides, Fm-3m Cu2P, which is a potential candidate for a high capacity conversion anode [1]. Within the copper phosphide phase diagram, Cu3P is currently the highest capacity material with a capacity of 363 mAh/g, however Cu2P has a theoretical capacity of 508 mAh/g, which is higher than both Cu3P and the commonly used graphite anodes. Although many conversion anodes exhibit volume expansions above 150%, Cu2P has a limited volume expansion of 99%, similar to Cu3P which has been previously used as an anode for Li-ion and Na-ion batteries. We calculate the dynamical stability of Cu2P using the vibrational effects from 0 K phonon modes, which suggest that Cu2P is stable up to temperatures of 600 K. We predict that Cu2P, if synthesized experimentally, would therefore be a successful conversion anode for future battery applications, and outperform other conversion anodes within the copper phosphide phase diagram.

[1] Harper et al., Chem. Mater. 2020, 32, 15, 6629–6639

The importance of electrochemical energy storage systems for large-scale application has been growing increasingly over the last decade. Due to high cost, limited source and several technical barriers of the lithium ion system, sodium ion batteries are considered as one of promising candidates for large-scale energy storage systems considering their cost-effective, earth abundant and recyclable characteristics.[1] As cathode materials for sodium ion batteries, sodium transition metal (TM) oxide with layered structures (NaxTMO2, 0.5 < x < 1.0) demonstrated high energy density similar to cathode for lithium ion batteries.[2] However, the capacity retention of the cell with layered structure cathode is not optimized especially when it is charged and discharged fully to obtain higher capacity. The major reason for this is a phase transition induced by gliding of transition metal slabs, which causes irreversible structural changes gradually during cycling.[3] To address this phase transition problem, we doped electrochemically inactive components in the transition metal sites with precise ratio control. The introduction of the inactive components (by substituting with existing transition metal(s)) consumed some of reversible capacity, but it is able to provide improved capacity retention due to their enhanced structural stability. In addition, we systematically studied the reversible capacity and corresponding phase transition behavior of layered type oxide materials with/without dopant(s) under various voltage window ranges to figure out the role of electrochemically inactive dopants. By tracing the structural changes using in-situ XRD, we determined the optimized ratio of electrochemically inactive components to minimize and/or detain the phase transition. The results can provide the insights of the phase transition mechanism of layered type cathode materials and the strategies to suppress them by stabilizing the structure, thus enabling rational design of new cathode materials for the development of next generation advanced Na-ion batteries. [1] M. D. Slater, D. Kim, E. Lee, C. S. Johnson, Adv. Funct. Mater. 23 (2013) 947-958. [2] C. Delmas, C. Fouassier, P. Hagenuller, Physica B+C 99 (1980) 81-85. [3] E. Talaie, V. Duffort, H. L. Smith, B. Fultz, L. F. Nazar, Energy Environ. Sci., 5 (2015) 2512.

Amorphous carbon is a promising anode materials for Na-ion batteries since it has been reported to exhibit high Na storage capacity (180~420 mAh g-1) as compared to graphite (~35 mAh g-1) and other potential candidates. Previous studies suggested that Na ions can be easily intercalated into amorphous carbon primarily because of its large interlayer distance as well as the presence of vacancy defects. However, the fundamental mechanisms remain unclear yet to date. In this work, we employed first-principles density functional theory calculations to investigate the atomistic mechanism of Na intercalation into amorphous carbon. Furthermore, we recalibrated the dispersion coefficients for C and Na in the DFT-D2 method to reproduce the d-spacing and the Na/Li storage capacity of bulk graphite measured in the experiments. This refined potential was then used to describe the van der Waals interactions between Na and amorphous carbon in this study. Our calculations first revealed that the interlayer spacing is not the main factor that hinders the intercalation of Na into the carbon-based electrodes. When the equilibrium d-spacing of graphite increased from 3.35 to 4.40 Å by weakening the C-C van der Waals interaction, the Na capacity was only slightly raised up from 35 to 70 mAh g-1, which is still much lower than the Na capacity of amorphous carbon. We next investigated the C vacancy effect on the Na capacity of graphene monolayer. Our results showed that the achievable Na capacity limit is only half of the achievable Li capacity limit on both monovacancy and divacancy defects in graphene, indicating a much weaker binding strength of Na to graphene as compared to Li. Moreover, our calculations showed that the Na storage capacity can be effectively enhanced by increasing the degree of structural disorders and non-hexagonal rings in the graphene sheets, which can even turn the neighboring hexagonal rings into the active sites for Na intercalation. Our results suggest that the presence of large amounts of non-hexagonal rings in the bond network is the main driving force that triggers the intercalation of Na into amorphous carbon, which can be more effective than creating vacancies on the surface of the carbon electrodes. It was also found that the amorphous bond network can become much softer as the number of non-hexagonal rings increases. The increased softness of the amorphous bond network can be beneficial in reducing the strain energy induced by Na intercalation, thereby contributing to the Na storage capacity of the amorphous carbon electrode. Accordingly, our study can provide a new insight toward a mechanistic understanding of Na intercalation into the amorphous carbon electrode.

There is a significant interest in understanding the factors that influence multivalent ion intercalation kinetics into oxide host materials. Recently, the addition of water into non-aqueous Mg2+ electrolytes was proposed as a strategy to decrease interfacial charge transfer resistance and increase solid state Mg2+ diffusion into different transition metal-based intercalation hosts. To investigate the role of this adscititious water, we added known amounts of H2O to a non-aqueous magnesium ion electrolyte (Mg(ClO4)2 in acetonitrile) and performed cyclic voltammetry with a tungsten oxide (WO3) electrode. Similar to other reports, we find increased kinetic reversibility as the electrolyte water content increases. The WO3 electrode undergoes reversible electrochromism indicative of an intercalation process. We investigated the nature of the water in the non-aqueous electrolyte using solution 1H NMR and molecular dynamics simulations. To investigate the nature of the intercalated species (and in particular, the potential for proton intercalation) and the energy storage mechanism, we performed ex situ X-ray diffraction, solid state 1H NMR, and inductively coupled plasma-optical emission spectroscopy. Our combined results suggest that adscititious water in non-aqueous electrolytes may increase the surface, and not solid-state, diffusivity and thus lead to enhanced energy storage kinetics.

The current landscape of energy storage systems is dominated by lithium-ion batteries because of their high energy densities, and continuous improvement in performance through the last few decades for use in a number of applications. They certainly have been a boon for rapid societal development; however, they also have had major disadvantages like high cost, severe toxicity, high chances of flammability and ethical concerns about the use of cobalt. Aqueous batteries containing manganese dioxide (MnO2) and zinc (Zn) have the theoretical capacities to deliver high energy densities comparable to some variations of lithium-ion batteries, have low cost and toxicity, and high material abundance to be used as an alternative battery compared to the current status quo. However, MnO2 and Zn have been highly irreversible and accessing close to their theoretical capacities has been very challenging. The current status quo in aqueous batteries has been to intercalate Zn and H-ions into layered structures to deliver modest capacities, which also has unfortunately resulted in limited energy densities. These layered structures, although novel, face limitations like their layered counterparts in lithium (Li)-ion batteries, where the capacity is limited to the host's intercalation capacity. Low voltage of ~1.1-1.4V is another Achilles heel of aqueous Zn-anode batteries, where it is simply not comparable to the high voltage properties of Li-ion. MnO2|Zn batteries can compete with Li-ion because of its safe and abundant raw materials, nonflammable electrolyte and theoretical energy density, however, sufficient advances are required for it to be considered a true challenger. In this talk, we will present a new strategy to enable a new generation of energy dense aqueous- based batteries, where we exploit the conversion reactions of MnO2 and Zn electrodes to extract significantly higher capacity compared to intercalation systems. Accessing the conversion reactions allows us to achieve theoretical capacities of 617 mAh/g (~30 mAh/cm2) from MnO2 and 810 mAh/g (~30 mAh/cm2) from Zn anodes, respectively. The high areal capacities help to attain unprecedented energy densities of 210 Wh/L, which is the highest of all aqueous-based batteries. We will also present our work on identifying new Mn-based conversion compounds that give higher capacities and demonstrate its application in the case of small-scale automobile. We will also present our breakthrough work on breaking the 2V barrier in aqueous Zn batteries, where we have demonstrated for the first time in the field of energy storage 2.45V and 2.8V MnO2|Zn aqueous batteries capable of accessing the theoretical capacity, which can truly challenge Li-ion's dominance. Finally, we will briefly present our commercialization experience, and how market forces and application have guided the design and research of these batteries.

Rapidly growing demands for electrical energy storage have motivated active research into new safe, low-cost technologies, such as sodium-based batteries. Solid-state sodium-ion conductors are central elements of emerging molten sodium, solid-state, and even sodium-ion batteries designed for effective long-term applications. Traditional sodium-ion conductors, such as ?-alumina or the sodium super ion conductor, NaSICON, however, can be expensive or difficult to process, they often suffer from limited ionic conductivity at desirable low to intermediate temperatures, and they can be mechanically fragile, limiting their long-term durability. A simple, low-cost alternative ion conducting candidate proposed by researchers at Sandia National Laboratories aims to utilize layered sodium-montmorillonite (MMT) clay and MMT/polymer composites. In this presentation, the microstructural, mechanical and electrochemical properties of the MMT-based solid electrolytes are investigated and correlated. Characterization methods such as x-ray diffraction, atomic force microscopy, and scanning electron microscopy together with measurement techniques such as nanoindentation and bulk and local electrochemical impedance spectroscopy (EIS) inform important structure-property relationships in materials with room temperature ionic conductivities rivaling conventional ceramic electrolytes. Utilizing these techniques, we have discovered that MMT processing can impact the layered microstructure, chemical integrity, and subsequently not only the ionic conductivity, but critically the elastic modulus and hardness of prepared bulk MMT ion conductors. Recognizing the importance of mechanical integrity for high performance, reliable solid-state ion conductors, these studies reveal that many of the processing parameters that degrade ionic conductivity also diminish the mechanical properties in these materials. Understanding these relationships will help guide new studies to engineer the development of robust, highly conductive MMT-based separators.

Nickel-rich layered oxide cathodes are promising candidates to achieve high energy density lithium ion batteries (LIB) because of lower content of expensive and toxic cobalt. However, transition metal dissolution, increased structural disorder, material amorphization upon cycling and parasitic reactions between organic electrolyte and nickel-rich composition lead to both capacity and voltage fading upon cycling. Extensive work has been reported on NMC532, NMC622 and NMC811 compositions, which are already commercialized. Recently, the battery research community is increasing nickel content in NMC based layered oxide cathodes to increase gravimetric capacity. Nickel-rich cathodes with nickel content above 80% aren't fully understood and their synthesis at larger scales requires optimization. This presentation reports cobalt-free, nickel-rich LiNi0.9Mn0.1O2 cathode material synthesized at a moderate level (500 g), using co-precipitation method in a 1 liter Taylor Vortex Reactor. The physical and chemical properties of the cathode material were measured with scanning electron microscopy (SEM), x-ray diffraction (XRD), and x-ray absorption spectroscopy (XAS). In-operando XAS was measured at Ni and Mn K-edges to examine oxidation state and local geometry of transition metal atoms as a function of state of charge. Additionally, ex-situ XAS and synchrotron XRD before and after long term cycling were utilized to understand the degradation mechanism and provide insight for designing cathode material with better performance.

This work explores a new class of material for Li-ion cathodes, based on disordered rock salt structures, in particular the composition Li1.2Mn0.6Nb0.2O2 and the oxyfluoride versions Li1.2Mn0.6Nb0.2O2-xFx. The classical approach of layered oxides, as their name indicates, consist in a superposition of separated layers of transition metals (TM) and Li in an oxygen network. On the contrary disordered rocksalt materials still compose of an oxygen network but in which all cations share their positions. This structure of material was ignored for a long time, the reason being that it generally did not seem to allow for Li ions conduction. However this structure very easily accommodate with Li-rich compositions. Based on previous studies carried out by the Ceder's group[1], it was demonstrated that in the case of Li-rich materials Li ions conduction was possible. Therefore, this class of materials was the perfect choice to attain higher energy densities via the increase of Li content. Considering that in Li-rich materials the redox capabilities of the TM are smaller than the Li content, we usually observe oxygen redox activity for these materials. Depending on the composition of the material, this contribution can go from very to hardly reversible[2]. Knowing that the oxygen contribution is inevitable, two approaches are valid to improve cycling performances: stabilise the oxygen or increase the redox part of the TM. To achieve both we introduce fluoride in the oxygen network. This work aims to deepen the understanding of the material structure and electrochemical behaviour via an experimental approach. We synthesised various samples of disordered rock salt materials using the solid-state approach. The oxide material is metastable and the synthesis requires special apparatus. The oxyfluoride is even more constrained with the use of high temperatures and corrosive precursors such as LiF and (TM)Fx. Materials synthesised were then characterised using advanced microscopy and spectroscopy technics, especially, a particular structure with short range order (SRO) was observed by TEM (transmission electron microscope) despite the long range order observed using XRD (X-ray diffraction). These structural observations are explained and are put in perspective with the electrochemical performances. In regards to oxygen stability, which is one of the main structural issues in Li-rich materials, we investigate the benefits of a fluoride doping. The cycling performances indicate an influence of surface reactivity and conductivity. To respond to such limitations new coating strategies were set in place. A carbon coating has led to improved electrochemical performances, which can come from either surface protection or enhanced electronic conductivity (or combination). [1] Urban, Lee, et Ceder, « The Configurational Space of Rocksalt-Type Oxides for High-Capacity Lithium Battery Electrodes ». [2] Yabuuchi, « Material Design Concept of Lithium-Excess Electrode Materials with Rocksalt-Related Structures for Rechargeable Non-Aqueous Batteries ».

We report a new approach for nanosilicon-graphene hybrids with uniquely stable solid electrolyte interphase. Expanded graphite is gently exfoliated creating "defect-free" graphene that is non-catalytic towards electrolyte decomposition, simultaneously introducing high mass loading (48wt.%) Si nanoparticles. Silane surface treatment creates epoxy chemical tethers, mechanically binding nano-Si to CMC binder through epoxy ring-opening reaction while stabilizing the Si surface chemistry. Epoxy-tethered Silicon pristine-Graphene hybrid "E-Si-pG" exhibits state-of-the-art performance in full battery opposing commercial mass loading (12 mg cm-2) LiCoO2 (LCO) cathode. At 0.4 C, with areal capacity of 1.62 mAh cm-2 and energy of 437 Wh kg-1, achieving 1.32 mAh cm-2, 340.4 Wh kg-1 at 1C. After 150 cycles, it retains 1.25 mAh cm-2, 306.5 Wh kg-1. Sputter-down XPS demonstrates survival of surface C-Si-O-Si groups in E-Si-pG after repeated cycling. The discovered synergy between support defects, chemical-mechanical stabilization of Si surfaces, and SEI-related failure may become key LIB anode design rule.

From as far back as 1971 researchers have been intensively working on developing strategies for integrating silicon into commercial lithium-ion batteries with the goal of developing more energy-dense energy storage devices. Relatively quickly, the research community determined that silicon-carbon nanocomposites were a promising avenue to address this technical challenge, allowing for long cycle-life electrodes as well as overcoming the swelling experienced by silicon.1 However, the majority of proposed synthesis methods of these complex and intricate nanostructures have proven to be too expensive, slow, or unable to scale to meet potential demand. Within this contribution we present a cost-effective solution suitable for large-volume production through a chemical vapor deposition -CVD- process based on a cheap acetylene precursor and largely available off-the-shelf silicon nanoparticles -SNPs. Key advantages of this method are the precise and decoupled control of both the carbon layer thickness and its graphitization degree. This is achieved by separating the CVD process into two steps: first the SNPs are placed into a furnace and exposed to pure acetylene at 650oC permitting thermal cracking of the precursor and causing a uniform deposition of amorphous carbon on the particles, with shell thickness depending on the amount of time spent in CVD; next the reactor is drained of acetylene, refilled with argon at low pressure, and brought up to 1000oC to instigate graphitization. Notably, this approach achieves the described result of a silicon-core coated with a uniform graphitic shell without any protective oxide shell or the formation of silicon carbide phases, both of which would negatively affect the battery performance. By taking advantage of the flexibility of the described approach, we were also able to delve into the effects on electrochemical performance and charging mechanism resulting from the interplay between a silicon-core and carbon-shell of varying graphitization degree. Through the use of core-shell particles with identical silicon cores and carbon layer thicknesses, but by varying the degree of shell graphitization (i.e. amorphous carbon and highly graphitic carbon), this work highlights how this simple property of the carbon-coating bears a profound influence on the material's electrochemical characteristics. In-situ TEM lithiation experiments revealed insights into the surprisingly poorly understood interaction between silicon and carbon in lithium-ion batteries, showing that a highly graphitized carbon layer significantly promotes the lithium ion diffusion dynamics into the silicon core with respect to an amorphous coating. As a result, lithium-ion battery anodes produced with graphitic-carbon coated SNPs exhibit markedly enhanced coulombic efficiencies and capacity retention upon cycling with respect to amorphous-carbon coated SNPs.2 Finally, the optimized composite produced with our CVD method can operate both as a pure high-charge-density anode material as well as a "drop-in" additive for graphite-dominant anodes. The pure composite boasts 1800mAh g-1 in half-cell testing and a first cycle coulombic efficiency -FCCE- of 87%, while replacing 10% in weight of a standard graphite anode increases capacity from 330mAh g-1 to 600mAh g-1 with FCCE of 90% and capacity retention of 81% over 100 cycles. [1] Zuo, X.; Zhu, J.; Muller-Buschbaum, P.; Cheng, Y.-J. Silicon based lithium-ion battery anodes: A chronicle perspective review. Nano Energy. 2017, 31, 113?143. [2] Nava, G.; Schwan, J.; Boebinger, M.; McDowell M.; Mangolini, L. Silicon-Core-Carbon-Shell Nanoparticles for Lithium Ion Batteries: Rational Comparison between Amorphous and Graphitic Carbon Coatings. Nano Letters. 2019, 19 (10), 7236-7245.

While the modern Li-ion battery is a scientific and engineering marvel, having enabled substantial technological innovation spanning from mobile electronics to commercially viable electric vehicles, the intercalation chemistry of the Li-ion battery has a fundamental upper limit on its achievable energy density owing to the fact that the same structure must remain stable in the fully lithiated and delithiated states. While further advances within the Li-ion chemistry are possible, significantly higher theoretical energy densities are achievable with conversion chemistries, such as Li-O2 and Li-S batteries1, where the structure and/or phase of the electrode materials change with its state of charge. While the importance of electrolyte electrochemical/chemical stability and ionic conductivity has been known since the very first batteries, owing to the partial solubility of reaction intermediates in conversion chemistries (such as Li+-O2- and Li2Sx), electrolyte can play an even more critical role in determining their reaction pathway and kinetics. In this talk, the various mechanisms through which electrolyte has been found to interact with the Li-O2 battery chemistry are explored. This includes recent results on the influence of solvent, counter anion and lithium ion concentration on the thermodynamics and kinetics of oxygen reduction2 as well as solvent-water-halide interactions and their impact on the performance of lithium halide redox mediators3-5. This work highlights that a complex combination of solvent, salt and additives can alter the thermodynamics and kinetics of redox reactions, the understanding of which can be extended beyond the Li-O2 battery reactions studied here to any small molecule redox reaction. References 1. P. G. Bruce, S. A. Freunberger, L. J. Hardwick, and J.-M. Tarascon: Li-O2 and Li-S batteries with high energy storage. Nature Materials 11(1), 19 (2011). 2. G. Leverick, R. Tatara, S. Feng, E. Crabb, A. France-Lanord, M. Tulodziecki, J. Lopez, R. Stephens, J. Grossman, and Y. Shao-Horn: Solvent- and anion-dependent Li+-O2- coupling strength and implications on the thermodynamics and kinetics of Li-O2 batteries. Submitted (2019). 3. M. Tulodziecki, G. M. Leverick, C. V. Amanchukwu, Y. Katayama, D. G. Kwabi, F. Barde, P. T. Hammond, and Y. Shao-Horn: The role of iodide in the formation of lithium hydroxide in lithium-oxygen batteries. Energy Environ. Sci. 10(8), 1828 (2017). 4. G. Leverick, M. Tulodziecki, R. Tatara, F. Barde, and Y. Shao-Horn: Solvent-Dependent Oxidizing Power of LiI Redox Couples for Li-O2 Batteries. Joule 3, 1 (2019). 5. G. Leverick and Y. Shao-Horn: In preparation (2019).

Na-ion batteries present a promising alternative to Li-ion batteries due to the low cost and abundancy of sodium and the existence of well-studied Na-superionic conductors. The design of new Na-ion battery electrode materials, however, is severely limited by the larger size of Na⁺ compared to Li⁺. One approach to designing new electrode materials for Na-ion solid-state batteries is to modify solid-state electrolytes. That is, because solid-state electrolyte materials are known for their high levels of ionic conductivity and minimal levels of electronic conductivity, they are not even considered as candidates for electrode materials which require both ionic and electronic conduction. However, if the chemistry of fast ion conductors can be suitably modified to produce electronic conduction and their defect structures are maintained so that their high ion conductivities are preserved, then these materials represent a unique opportunity to creating a new class of battery electrode materials. An electrolyte-centered material design method takes advantage of not only the inherently fast ionic conduction within the material but also of a more continuous chemical potential across the electrode-electrolyte interface for solid-state batteries, leading to increased interfacial stability. The research presented in this paper demonstrates that partial substitution of aluminum by iron in the superionic conductor Na β''-Al₂O₃ (stabilized as Na_1.67Li_0.33Al₁₁O₁₇) results in a mixed ionic-electronic conductor. A series of Fe-substituted compositions exhibit reasonable capacities as a negative electrode for Na-ion batteries. Fe-substitution for up to 80% of the aluminum sites is possible through a combination of sol-gel chemistry and high-temperature heat treatment, resulting in Fe²⁺/Fe³⁺ in both the octahedral (O) and tetrahedral (T) sites of the spinel block. Structural refinement and Mössbauer spectroscopy have helped to elucidate the nature of Fe-substitution within the structure and its limitations. Successful octahedral site doping with Fe-ions appears to be the key to appreciable electronic conduction due to the shorter bond distances between O-O sites compared to T-T and O-T of the spinel block. The electrochemical properties of ~50% Fe-substituted Na β''-Al₂O₃ (Na_1+x(Al,Fe)₁₁O₁₇) have been investigated in non-aqueous electrolytes, demonstrating 115 mAh g-1 at C/5 and 72 mAh g^-1 at 20C (0.2-2.5 V vs Na/Na⁺) through Fe²⁺/Fe³⁺ redox, while maintaining the β'' structure. Electrochemical analysis indicates a capacitive-like charge storage mechanism and excellent capacity retention at high rates, despite the micron-sized particles.

Due to the high specific capacity of 3860 mAh g-1 and low electrochemical potential of -3.04 V vs standard hydrogen electrode, Li metal as anode is one of most attractive approaches in high energy-density lithium ion batteries (LiBs). Nevertheless, mossy/dendritic Li growth induces numerous safety issues and the sustained SEI formation causes Li depletion, which greatly limits further applications. We present the lamination of Ti3C2Tx MXene film on 30 microns Li metal foil using a rolling process, which significantly improves the battery cycling performance in both symmetric and full cells. The excellent Li+/e- transport behavior of the 2D MXene provides the LiCx-Ti intercalated surface, interlayer spacing and inter-block spacing for Li metal storage. The improved electrical conductivity also reduces un-uniform distribution of current density, leading to a homogeneous Li plating/stripping along with a suppressed mossy/dendritic Li growth and reduced Li and electrolyte depletion from continuous SEI formation. Moreover, the coexisted organic-rich and inorganic-rich SEI of both cycled MXene-Li and Li foil electrodes were investigated via XPS depth profiling spectroscopy. A full cell battery paired with commercial LiNi0.8Co0.15Al0.05O2 (NCA) cathode with lean electrolyte condition and an N/P ratio as low as 1.5 was also studied. The 2D MXene coated Li metal anode presents a great potential in high energy-density batteries for portable devices and electric vehicles.

There are multiple types of electrochemical storage mechanisms in energy storage materials. The most widely adopted is the insertion mechanism where on reduction, a positive ion inserts into the structure of the host material and is removed from the structural lattice upon oxidation. A second type of mechanism is conversion where a chemical reaction leads to a new material or phase on reduction with reformation of the original material on oxidation. Insertion materials contain space in the lattice to accommodate Li+ or other positive ions and can be depicted as nLi+ + ne? + MxOy? LinMxOy. Functionally, these materials exhibit reversible or quasi-reversible electrochemistry with the transfer of limited numbers of electrons/lithium ions per metal center resulting in moderate lattice expansion but also providing moderate capacity. Conversion materials differ in that they often involve multielectron transfers per active center represented here for a metal oxide type material as MxOy + 2yLi+ + 2ye?? yLi2O + xM resulting in significantly higher theoretical capacity. However, the challenge with many conversion reactions is the degradation of delivered capacity with continued electrochemical oxidation and reduction. The mechanisms of conversion reactions will be examined including those of densely packed ferrites with spinel or inverse spinel structures where the theoretical capacity exceeds 900 mAh/g. In-situ and operando methods that demonstrate the governing roles of crystallite size, aggregation, and interfacial reactivity on rate capability and capacity retention will be featured.

Transition metal fluorides display the combination of high capacity and high electrode potential that are essential to boosting the energy density of lithium ion batteries. However, their application as cathode materials has been hindered by an incomplete understanding of their electrochemical capabilities and limitations. Herein, we present a system of single crystalline, monodisperse iron (II) fluoride nanorods and detail how their morphology and uniformity make them ideal for mechanistic study. High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships, and oxidation state changes that redefine the conversion reaction with unprecedented spatial resolution. We first introduce the presence of surface specific reactions and examine how they critically influence phase evolution, diffusion kinetics and cell failure. Next, we examine how the reversibility of the conversion reaction is governed by topotactic cation diffusion through an invariant lattice of fluoride anions and the nucleation of metallic particles on semi-coherent interfaces. Finally, we demonstrate how this new understanding informs a more effective application of metal fluorides and establishes a framework for improving reaction hysteresis, kinetics and reversibility. To ensure a pertinent result, ex-situ data was extracted from coin cells that deliver near theoretical capacity (570 mAh g-1) and extraordinary cycling stability (>90 % capacity retention after 100 cycles). The role of the electrolyte in enabling this performance as well as the collection of meaningful HRTEM data is briefly discussed.

Over the past few decades lithium ion batteries have become utmost technological importance for energy storage devices for stationary applications and electric vehicles. Currently, Graphite is the widely used anode materials for lithium ion batteries due to its low intercalation voltage and high stability. But due to its low specific capacity research is focussed to find alternative anode materials for lithium ion batteries. Among the transition metal oxides, ?-Fe2O3 as anode material is extensively studied owing to its high theoretical capacity (1006 mAh. g-1), environment-friendly, good catalytic properties and stability. But capacity fading is found to be inhibitory to be used as the anode materials for lithium ion battery. In order to reduce the capacity fading, different porous carbon supports have been reported in the literature. In this work, we have synthesised ?-Fe2O3 coated with thermally etched carbon nanotubes (?-Fe2O3/TECNTs) using solid state pyrolysis method. In the presence of nanotube structure, high rate capability is observed for ?-Fe2O3/TECNT anode material due to the fact that lithium insertion/de-insertion in carbon matrix at lower voltage is governed by the intercalation mechanism while for ?-Fe2O3, lithium interaction is mainly observed due to the conversion reaction. The rate capability studies in the range 0.05-20 A/g shows 100% retention of capacity even after extensive cycling. Also, good cyclic stability and 100 % coulombic efficiency are achieved in the presence of carbon matrix. Further to analyse the role of carbon nanotubes, amount of carbon matrix is varied which also corroborates the crucial role of carbon in enhancing the electrochemical performance of lithium ion battery.

The use of deterministic design is a growing area of interest to achieve both high energy and power density for energy storage electrodes. Traditionally manufactured energy storage electrodes consist of a multi-component slurry containing an electrochemically active material as well as conductive carbon and polymer binder additives cast onto a flat conductive substrate. Inhomogeneity in the dispersion of the slurry components limits areal power and energy densities due to tortuous and poorly controlled electron and ion transport pathways. In this work, a nanostructured, high capacity transition metal oxide (MoO3) is electrodeposited onto a conductive carbon nanotube (CNT) foam scaffold to attain high mass loading energy storage electrodes with fast Li+ intercalation kinetics. CNT foams provide surface area for the electrodeposition of MoO3 while maintaining sufficient electrical conduction throughout electrodes. Ample porosity between the CNTs provides good ion transport pathways. The MoO3 is a model Li+ intercalation oxide with a high gravimetric capacity that can be easily electrodeposited from aqueous solutions. This presentation will describe how control over the properties of the CNT foam and MoO3 electrodeposition are used to tailor the energy storage performance of the electrode architecture, along with characterization to understand the how the structure of the electrode affects its energy storage behavior. Overall, this research seeks to understand the relationships between electrode architecture and energy storage behavior to achieve simultaneous high power and high energy density.

Solid-state batteries are attracting world-wide interest for their potential to enhance safety by eliminating the need for flammable liquid electrolytes and to increase the energy density by facilitating the use of Li metal anodes and high-capacity cathodes. When incorporated into real devices, the solid electrolyte interacts with Li anodes and cathodes to form a nanometer-thick amorphous/disordered interfacial layer known as the solid electrolyte interphase (SEI), which determines the resistivity and stability of the interfaces between the electrode and electrolyte. The inherent complexity and the dynamic evolution of the SEI cannot be handled directly with first-principles methods, such as density functional theory, because of the computational hurdle associated with the large system size (e.g., more than one thousand atoms) and time scale. In order to gain insights into the structural properties and the interfacial reactions at the SEI, an integrated approach that combines high-performance computing and machine learning tools with state-of-the-art synchrotron X-ray absorption/electron energy-loss spectroscopy, is developed to tackle the complex electrochemical interfaces. The approach and some recent results from the structural modeling and spectroscopy studies of the SEI at the LiTMO2 (TM=Ni, Co, Mn) cathode/ Li7La3Zr2O12 solid electrolyte interfaces will be presented.

Graphite-based Al-ion battery has been attracted large attention due to its extremely fast charge/discharge rate and excellent cycle stability. However, the intercalation mechanism of AlCl4 into graphite is still vague. The concern comes with the large size of the AlCl4 molecule while the experimentally observed gallery height for graphite cathode is not large enough (~ 5.7 Å). In the past theoretical studies, the computational models mostly represent that the stable structure of intercalated AlCl4 is the tetragonal form which resulted in a large expansion of the intercalation gallery to 8~9 Å. In those studies, researchers commonly considered thermodynamically stable structures that allow free expansions of electrode structures. In this study, we suggest that a strain effect in a realistic electrode structure governs the stable form of intercalating AlCl4. We have performed first-principles calculations to show a strain-structure relation of intercalated AlCl4 in graphite and found that a planar-shape of AlCl4 molecule is more probable in realistic conditions. Using the computational results, we made model energy for AlCl4 intercalation at given structural conditions such as volume expansion rate and stage number of graphite electrode. The model energy shows good agreements with the experimental observations on the structure-engineered graphite electrodes. In addition, to examine new candidate materials for Al-ion cathode, we also conduct calculations on MoS2 and MoS2/graphene hetero-structure for AlCl4 intercalation. As a result, we believe our study provides a deeper understanding of the charging/discharging mechanism and a modeling rule to find more enhanced electrode materials in Al-ion batteries.

Lithium sulfur batteries (LSBs) are attracting attention as a next generation energy storage device because of their high energy density, low cost, and environmental friendliness surpassing that of lithium ion batteries (LIBs). LIBs are considered insufficient for many applications such as grid energy storage and electric vehicles due to their high cost and low energy density. However, LSBs still have some problems that need to be overcome before they can be used commercially. The major problem that limits their capacity is the dissolution of long chain lithium polysulfide into the electrolyte which leads to a loss of active material and capacity decay during cycling. Here, we have developed a cost-effective method for the lithium polysulfide battery to achieve high performance by employing a nanostructured carbon black/ carbon cloth electrode that exhibits high electrical conductivity, good mechanical properties and high surface area. These benefits enable a high sulfur loading mass, capture the lithium polysulfide intermediate reaction products and avoid structural changes. As a result, this simple and novel design delivered a high specific discharge capacity exceeding 1350 mAhg-1 at 0.2 C and good capacity retention, providing a promising path towards practical lithium sulfur batteries.

The lithium sulfur battery presents unique polysulfide chemistry to exhibit high theoretical capacity of 2,600 Wh/kg. However, the life cycle of Li-S battery is greatly affected due to unwanted shuttling of soluble lithium polysulfides (LiPSs) such as Li2S8 and Li2S6, that corrodes the Li anode by forming an inactive sulfide layer. Previously, pristine 2D materials with polar surface were used as a catalyst in sulfur cathodes to overcome this issue by acting as polysulfide anchor and prevent shuttling effect. However, the concentration and weight loading of catalyst was high and lack the practical applicability in Li-S battery. To overcome this issue, we adopted novel strategy of synthesizing 2D alloys with very low concentration loading on carbon nanotube paper as a sulfur host cathode to efficiently catalyze the polysulfide reactions. The 2D alloy with mixed 2H-1T phase exhibited synergetic properties and showed high specific capacity of 1,228 mAh/g at 0.1 C and improved the cyclic stability to 400 cycles, as compared to 100 cycles for pristine cathode. The reduced shuttle effect and increased life cycle was due to accelerated transformation of the dissolved polysulfides to the insoluble LiPSs and back to sulfur. Our study shows novel approach with binder-free ultra-light weight catalytic cathodes for practical application of Li-S battery.

We propose a low cost and simple method to prepare three-dimensional (3D) carbon structures by Chemical Vapor Deposition (CVD) and characterized their structural and electrochemical properties. The 3D pore system were obtained using different zeolites (NaY, SBA-15, and ZMS-5) as solid template structures to assist in the synthesis of 3D carbon network structures. The carbon source consisted of radicals formed from the thermal decomposition of acetone, in a relatively facile and fast chemical deposition process. The 3D carbon network structures were characterized with X-ray diffraction, Raman spectroscopy, spectroscopy, surface area measurement, scanning electron microscopy, and transmission electron microscopy. The carbon networks deposited in the pores consist of reduced graphene oxide and multiwalled carbon nanotubes; their size distribution correlates with the size of the pores of zeolites. The electrical conductivity and dispersed morphology of the 3Ds carbone network structures -based conductive electrode were examined by atomic force microscope (AFM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Using the standard method we assembled the Lithium Ion Battery in the Ar-filled glove box, we used LiPF6 with EC:DMC::1:2 ratio as the electrolyte, lithium foil as the counter electrode and the 3D carbon network material. The specific capacity and cycle-life stability of the 3Ds carbon network-based lithium ion battery was investigated by cyclic voltammetry analysis.

MXene is a promising candidate for energy storage devices owing to its versatile characteristics. However, as 2D structured material, layer restacking besides its surface oxidation is a serious issue. This re-stacking compromises significantly surface active sites, compelling the material to exhibit sluggish electro-kinetics behavior, and thus deterring the overall capacitive performance. Here, a tremella-like 3D architecture of carbon-coated Ti3C2Tx MXene (T-MXene@C) is fabricated as an efficient anode material for lithium- (LIBs) and sodium-ion batteries (SIBs), respectively. The nanohybrid is synthesized via self-polymerization of dopamine over the surface of pristine Ti3C2Tx MXene nanosheets succeeded by carbonization. The self-polymerization of dopamine in this case, facilitates the transformation of MXene sheets into 3D architectures where its post-carbonization subsequently forms a thin carbon layer over the structure, preserving the surface facets from oxidation and structural aggregation. The incursion of carbon layers within MXene not only contributes in provision of additional active area, but also enhances the structural durability of the material during charge/discharge process. Therefore, the T-MXene@C nanohybrid exhibits ultrahigh capacities, superior rate performance, and stable cyclability as anodes in both LIBs and SIBs. Moreover, the discussed strategy could be applied as general route to fabricate diverse MXene-based 3D structures with potential applications in many other areas.

Graphene Oxide was synthesized using a Modified Hummer's Method. It is fairly known that SnO2 is used as anode material for lithium-ion batteries applications due to their high specific capacity. Hence, we use a non-conventional synthesis route to obtain a final composite, G/GO/SnO2. G/GO, with its good surface-to-volume ratio, serves as a support and matrix for SnO2 nanoparticles that buffers the huge volume changes the nanoparticles are used alone as anode for lithium ion batteries and then enhances its electrochemical properties. For the structural characterization of the synthesized material, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and Raman Spectroscopy were performed; showing a uniform distribution of the nanomaterial in the carbon matrix. Electrochemical analysis, such as charge/discharge profiles show that the composite can be used as anode in lithium ion batteries. In fact, at different current densities- 25, 75, 100, and 200 mA/g- the material deliver in the first cycle very high specific capacity-1808, 1253, 903, 886 mAhg-1 respectively. After repeated cycling the composite is still offering reversible capacities over 400 mAhg-1 for each current density, showing that G/GO is providing contact areas which results in more energy density at the interface electrode/electrolyte and that the SnO2 provides additional ions to the EDL and the electrochemical process.

Ab initio modelling can provide valuable insights into the electronic and structure behaviour of battery materials, including the prediction of sodium, potassium and magnesium cathodes.12 Nevertheless, it is crucial that the accuracy of such ab initio calculations be established. The most common method to theoretically the strong correlation in the 3d valence electrons of transition metals is to use the addition of 'U' parameters to standard Density Functional Theory (DFT), enabling high throughput studies of Na-based materials;3 however these parameters must be tuned to experimental measurements, and are highly sensitive to changes in oxidation state and composition - crucial properties during the intercalation of AMO2 (A=Li, Na, K) layered cathodes.4,5 Recent high-level quasiparticle self-consistent GW (QSGW) calculations performed on the layered LiMO2 (M=Co, Ni, Mn) cathodes suggest that the charge transfer transition in LiMO2 phases is underestimated, even when the inaccuracy of standard DFT at describing band gaps is accounted for, but it is well predicted by hybrid Density Functional Theory (DFT), which mixes exact exchange from Hartree-Fock theory.6 In this study, we examine how electronic correlation is described in layered sodium cathode materials with various levels of theory. We calculate the electronic structures of NaCoO2, NaNiO2 (in direct comparison to their Li counterparts) and ?-NaFeO2 - ternaries typically alloyed to form stable and efficient sodium-based cathodes7 -- using DFT+U, hybrid DFT (HSE06) and QSGW to establish a thorough comparison of the electronic properties at each level, and to establish that tuning is necessary to reproduce the valence electronic structure of such materials. We further demonstrate the practical impact of this theoretical description by calculating and comparing voltage intercalation profiles of these systems as a function of Na content, at both the DFT+U and hybrid DFT levels via a cluster expansion approach, in order to establish the most reliable comparison to experiment. We establish the necessity to use accurate, transferable methods to describe such systems, with an aim to inform ab initio studies on the future generation of first row transition metal-based cathodes. (1) Zhang, W.; Liu, Y.; Guo, Z. Sci. Adv. 2019, 5 (5), eaav7412. (2) Gao, Y.; Wang, Z.; Lu, G. J. Mater. Chem. A 2019, 7 (6), 2619. (3) Zhang, X.; Zhang, Z.; Yao, S.; Chen, A.; Zhao, X.; Zhou, Z. npj Comput. Mater. 2018, 4 (1), 13. (4) Urban, A.; Seo, D.-H.; Ceder, G. npj Comput. Mater. 2016, 2 (October 2015), 16002. (5) Zhou, F.; Cococcioni, M.; Marianetti, C. A.; Morgan, D.; Ceder, G. Phys. Rev. B - Condens. Matter Mater. Phys. 2004, 70 (23), 235121. (6) Savory, C. N.; Morgan, B.J., Walsh, A.; Scanlon, D. O. (in preparation) 2019 (7) Thorne, J. S.; Zheng, L.; Lee, C. L. D.; Dunlap, R. A.; Obrovac, M. N. ACS Appl. Mater. Interfaces 2018, 10 (26), 22013.

Solid state batteries have advantages such as safeness and high energy density. However, their conductivity at room temperature is low which limits their further commercialization. Gold nanorods can promote the Li ion transport by decreasing the crystallization of solid state electrolytes, and locally heat the electrolytes based on their photothermal conversion capability, providing a more efficient way to improve the performance of solid state batteries.

Spinel ferrites have emerged as an attracted class of materials that can find application in various fields, ranging from information storage, biomedical application to energy-related areas. As the anode in LIBs, spinel CoFe2O4 attracts much attention because of its high theoretical specific capacity (916 mAh g-1). However, some issues impede its practical application such as drastic capacity fading and poor rate capability arising from an inherent low electronic conductivity, severe aggregation, and lithiation-induced volume expansion in the cycling process. Increasing surface-to-volume ratio could provide relatively short transport distance for lithium-ion diffusion and alleviate the volume expansion, which is one of the possible strategies to solve the above-mentioned problem. In this frame, an aminolytic method is described to synthesize ultra-small (<10 nm) CoFe2O4 nanoparticles with good dispersion at a relatively low temperature without further annealing being required. Here, the 9.1 nm CoFe2O4 nanoparticles synthesized by aminolytic method delivered a reversible specific capacity of 1238 mAh g-1 after 100 cycles at a current density of 0.1C, a much better performance compared with CoFe2O4 synthesized by traditional methods in previous reports. By adjusting the reaction temperature, 3.4 nm CoFe2O4 nanoparticles could also be synthesized, delivering a slightly enhanced specific capacity (1343 mAh g-1 after 100 cycles at 0.1C). For comparison, the CoFe2O4 nanoparticles were also prepared by coprecipitation method and its electrochemical performance was studied in detail. It can be concluded that spinel CoFe2O4 synthesized by aminolytic method is a promising anode for LIBs based on its superior lithium storage performance. Further, the aminolytic method can also be used to prepare other spinel materials in different fields. The systematically studied electrochemical performance of CoFe2O4 nanoparticles is beneficial in the design of future anode materials for LIBs.

Sulfur is a promising active material because of a high theoretical capacity (1675 mA h gS?1) and low cost. Nevertheless, practical applications of S electrode have been hindered by low S fraction in a cell. Excess Li in negative electrode, auxiliary materials such as heavy current collectors of metals, and large amount of electrolyte (Electrolyte/Sulfur ratio (E/S) > 7) have been used in previous works [1], which lowers energy density of the Li-S cells based on total mass/volume (including electrodes, separator and electrolyte). We previously demonstrated the high gravimetric and volumetric energy densities based on an electrode using self-supporting sponge-like paper of few wall carbon nanotubes (FWCNTs) as current collector. The unique properties of FWCNTs with high conductivity of ~100 S cm?1 and high surface area of ~500 m2 g?1 enhanced the sulfur utilization; however, E/S ratio was still high [2]. In this work, we report the Li2Sx-CNT (4?x?8) positive electrode that works effectively even with lean electrolyte. Self-supporting paper of FWCNTs was fabricated by dispersion and filtration. Then, three types of Li2Sx (x = 4, 6, 8) in 1,2-dimethoxyethane (DME) solution were drop-casted on the papers and dried for 1 h to obtain the Li2Sx-CNT electrodes. As a reference, the S-CNT electrode was also prepared by the S vapor deposition method [2]. The Li2Sx-CNT and S-CNT (areal density of 2 mgS cm?2 and 1 mgCNT cm?2) electrodes were evaluated by assembling 2032-type coin cell with Li foil (50 µm thick) as a negative electrode, a polyethylene separator, and an electrolyte of 1 M LiTFSI + 0.2 M LiNO3 in 1:1 (v/v) DOL/DME. Although the S-CNT exhibited very small capacity, the Li2S6-CNT exhibited 1471 mA h gS?1 with lean electrolyte of E/S ratio of 3.2 µL mgS?1. Furthermore, the Li2S6-CNT achieved 526 W h kgcell?1 based on the cell including positive and negative electrodes, separator, and electrolyte at second discharge. The value is encouragingly high compared to the high-performance Li-ion battery of ~300 W h kgcell?1 [1]. [1] M. Hagen et al., Adv. Energy Mater., 5, 1401986 (2015). [2] K. Hori, et al., J. Phys. Chem. C, 123, 3951 (2019).

Li metal is a promising negative electrode because of its high theoretical capacity (3860 mAh gLi?1) and low potential (?3.04 V vs SHE). However, it has a serious safety problem of Li dendrite which penetrates a separator and causes short circuit [1]. Surface modification of Li foils by texture or additional layer have shown effective in suppressing the dendrite growth [2] [3], but many of such methods make the electrode too thick (several hundred µm), and excessive Li were used (5 times or more). In this work, we propose thin and light-weight electrode to achieve high gravimetric, volumetric and areal capacities. Self-supporting paper of few-wall carbon nanotubes (FWCNTs; areal mass of 0.3 mg cm?2 and thickness of ~15 µm), having high conductivity of 40 S cm?1 and large specific surface area of ~500 m2 g?1, was used as non-metal 3D current collector. Cu nanoparticles (25 nm; 0.1-0.2 mg cm?2) were held in this 3D matrix as seeds for stable Li plating/stripping. The Cu-CNT film was prepared by dispersion and filtration [4]. 2032 coin cells of symmetrical configurations (Li-CNT//Li-CNT and Li-Cu-CNT//Li-Cu-CNT) were prepared by performing electrochemical pre-cycling of the Li foil (50 µm)/CNT//CNT and Li foil (50 µm)/Cu-CNT//Cu-CNT stacks with polypropylene separator (25 µm). Charge-discharge test was performed with an areal capacity of 4 mA h cm?2 and current density of 0.4 mA cm?2. The former cell showed unstable plating/stripping behavior in several cycles due to micro-short circuits while the latter worked stably over 20 cycles with small excess (1.5-2 times of the areal capacity) in Li. The Cu nanoparticles working as the nucleation sites and mesopores in the FWCNT paper working as growth sites realized the stable operation of the cells. [1] L. Wang, et al., Energy Storage Mater. 14, 22 (2018). [2] Rodrigo V. Salvatierra, et al., Adv. Mater. 30, 50, (2018). [3] Y. Zhao, et al., Nano energy. 43, 368, (2018). [4] K. Hori, et al., J. Phys. Chem. C 123, 3951 (2019).

The development of high-performance batteries is fundamental to meeting global energy demands, while also storing energy affordably and safely. An area of intense research in the battery field has been the replacement of the liquid or gel electrolyte with a solid polymer material, namely a solid polymer electrolyte (SPE). SPEs can allow the use of metal anodes due to its suppression of dendrite formation, which could thereby double the energy density, besides further addressing concerns over the volatility and flammability of liquid electrolytes. Crosslinked polymer electrolytes are quite attractive owing to their ability to enhance conductivity, while also retaining mechanical stability.1 Studies on SPEs not only focus on different polymer systems, but also on improving the interaction of existing and new polymer networks to ions other than lithium-ion to achieve even less expensive and more efficient rechargeable batteries. Other univalent ions, in particular sodium cations, can be highlighted due to its wide availability and low cost comparing to lithium. Multivalent cations, such as Ca2+ and Al3+, are also featured in SPE studies for their high energy density and mineral abundance.2 In those studies, poly(ethylene oxide) (PEO) has been extensively employed as the electrolyte's backbone. However, polytetrahydrofuran (PTHF) has been reported as having a looser coordination with Li+ than PEO, due to PTHF's fewer oxygen heteroatoms in its chain.3 As a result, lower activation energy for ion motion is observed, leading to higher conductivity and transference numbers. In this work, we describe the synthesis of crosslinked SPEs for lithium, sodium, aluminum, and calcium conduction using a PTHF backbone. The SPEs were crosslinked through the photo-copolymerization of PTHF and 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate. The crosslinked samples were evaluated for their ionic conductivity, thermal stability, amorphousness, and mechanical properties. Ionic conductivity of the order of 10-5 - 10-4 S/cm at room temperature was achieved, which is superior to some previously reported Ca, Na, and Al dry SPEs.4,5 Thermogravimetric analysis of the electrolytes indicated that, within the operating temperature of a battery, all electrolytes were stable and showed minimal weight loss. Differential scanning calorimetry confirmed the amorphous feature of the polymer. The electrolytes also showed excellent mechanical stiffness and flexibility. Further electrochemical studies will be presented and discussed. This work represents a new route for cleaner and safer energy storage devices with the potential for a broad variety of applications. References 1. Placke, T., Kloepsch, R., Dühnen, S. & Winter, M. Lithium ion, lithium metal, and alternative rechargeable battery technologies: the odyssey for high energy density. J. Solid State Electrochem. 21, 1939-1964 (2017). 2. Xu, C. et al. Secondary batteries with multivalent ions for energy storage. Nat. Publ. Gr. 1-8 (2015). doi:10.1038/srep14120 3. Mackanic, D. G. et al. Crosslinked Poly ( tetrahydrofuran ) as a Loosely Coordinating Polymer Electrolyte. 1800703, 1-11 (2018). 4. Wang, J., Genier, F. S., Li, H., Biria, S. & Hosein, I. D. A Solid Polymer Electrolyte from Cross-Linked Polytetrahydrofuran for Calcium Ion Conduction. ACS Appl. Polym. Mater. 1, 1837-1844 (2019). 5. Yao, T., Genier, F. S., Biria, S. & Hosein, I. D. A solid polymer electrolyte for aluminum ion conduction. Results Phys. 10, 529-531 (2018).

Nuclear batteries, or betavoltaic cells (BV), convert the energy of beta radiation directly into electricity via a semiconductor stack. The decades-long lifetime and high-energy density open a broad range of applications that are not able to use chemical batteries. Such low-power electronic applications (nW-mW) are the internet-of-things, internal medical devices, and unattended equipment in space and other hostile environments. We are developing simulation methods to model radiation sources and degradation of the contact interface under beta irradiation. Methods were developed to adequately simulate radiation sources for BVs with Monte Carlo N-Particle (MCNP) and CASINO. Methods were also developed to adequately simulate electron generation and device performance with ATLAS. The simulations have been used to design the accelerated aging experiments of tritium beta-induced degradation at a GaN contact.

Ceramic electrolytes exhibiting high sodium-ion conductivity are a key component in the development of low temperature molten sodium batteries. Traditionally, molten sodium batteries utilize Na+-ion conducting solid electrolytes such as ?-Al2O3 or NaSICON. These ceramics require high operating temperatures of ~300°C, not only for improved Na+-ion conductivity, but also to facilitate wetting of molten sodium on the solid electrolyte. Lowering the operating temperature to near 100°C drastically reduces interfacial wetting of the molten sodium to the ceramic electrolyte, increasing interfacial resistance in the cell and ultimately producing poor battery performance. Here, we describe the use of engineered coatings to enhance wetting of molten sodium on NaSICON ceramic electrolyte and improve charge transfer across this key interface. We observe that these coatings dramatically lower cell-limiting interfacial resistance, and thereby increase battery cycling current densities and overall battery performance at low operating temperatures. Continued optimization of such engineered, interfacial coatings promises continued advances of low temperature sodium batteries. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.

High entropy materials are the "multi-metallic cocktails" in metallurgy, which are having five or more than five metals, all in equal proportions. Due to the high configurational entropy arising from such conditions, these types of materials form a simple single-phase structure. The high entropy alloys are high strength, corrosion resistant materials which find application in several industries. However, the exploration on the other class of high entropy oxides have caught the attention of the researchers only recently. High entropy oxide nanoparticles with multifunctional properties can have potential applications in several technologies. In this work, a high entropy oxide nanoparticle (HEO) in a simple bottom up process is synthesized and the possibility of application of such materials as cathode electrocatalysts in Li-O2 batteries is explored. Reversible formation and decomposition of Li2O2 during the respective discharge and charge operations is the main challenge in the development of a secondary Li-O2 battery. Several approaches to address this issue include development of an efficient bifunctional catalyst material in aprotic media and liquid phase redox mediators. In the present work, we have prepared HEO containing Co, Cr, Ni, Al, Fe by a simple sol-gel process. The oxide crystallizes in spinel (Fd3m) phase. The synthesized material is characterized extensively to understand the structure and morphology. Li-O2 cells are fabricated with synthesized HEO coated on a gas diffusion layer as cathode, a redox mediated electrolyte (1M LiTFSI and 0.1M LiI in tetraethylene glycol dimethyl ether) and lithium metal as anode. The detailed investigations result in a capacity of 1000 mAh g-1 for a constant current of 250 mA g-1 in the capacity limited cyclic stability test for 50 cycles.

Joachim Maier Max Planck Institute for Solid State Research, Physical Chemistry of Solids, Stuttgart, 70569, Germany, e-mail: [email protected] One part of the contribution considers the various storage mechanisms (insertion, phase change and interfacial storage) together with thermodynamic and kinetic constraints [1-3]. Special emphasis is laid on the difference between Lithium and Sodium based battery systems [2]. The helpful role of nano-structuring and confinement is stressed. A second part of the contribution refers to the equally important role of the circuity [4]. A detailed modelling of the storage kinetics provides useful guidelines of how to optimize the electrode morphology [5]. In all cases practical examples and experimental results are discussed. References [1] J. Maier, Angewandte Chemie 52 (2013) 4998 [2] Chen et al., Nature 536 (2016) 159 [3] Chen et al., Nature Energy 3 (2018) 102 [4] Zhu et al., Science 358 (2017) eaao2808(1--8) [5] Usiskin et al., Physical Chemistry Chemical Physics 20 (2018) 16449

Lithium-ion secondary batteries (LIBs) are one of the most popular electrochemical systems, especially in home-electronics and electric vehicles due to their relatively high energy density, high operating voltage, and low self-discharge. Further advancements of Lithium-ion batteries are imperative in order to improve their capacities and life cycles. Silicon anode materials are regarded as one of the most promising candidates for Lithium-ion batteries based on their substantial specific capacity in comparison with conventional graphite anodes. However, a severe drawback is the large volume changes of anodes during the charge (up to ?300% volume expansion during lithiation) and discharge (delithiation) cycles. These changes cause high mechanical stress, breakdown of the conductive layers of composite electrodes, and undesirable side reactions within batteries. In addition, the solid electrolyte interphase (SEI) formed on the silicon anodes surfaces are unstable and cannot efficiently protect the electrodes from reacting with electrolytes. The importance of SEI on battery performance motivates us to study the interphase interaction in batteries. In this work, the formation of SEI components such as C2H4, CO2, and CO gases, and organic and inorganic products are predicted by using Reactive molecular dynamics (ReaxFF) simulations. For this investigation, we analyzed the effect of different types of Si anodes (pristine Si and SiOx), compositions of various carbonate electrolytes and additives on SEI formation. Additionally, the degree of lithiation and temperature effects on gas evolution are investigated. Overall, these simulation results indicate that ReaxFF will be a very useful tool for the development of novel electrolytes or additives in Lithium-ion batteries.

Electron microscopy and spectroscopy diagnosis, both in-situ and ex-situ, appears to be one of the essential methods for gaining insights as how an electrode material evolves and eventually fails, therefore feeding back for designing and creating new materials with enhanced battery performances. In this presentation, I will focus on recent progress on ex-situ, in-situ and operando S/TEM studies for probing into the structural and chemical evolution of electrode for lithium ion batteries, especially layer structured transition meal oxide cathode and Li metal anode. I will highlight several recent key observations, which appear to be well documented, while essentially are poorly understood, and therefore act as the bottleneck for the advances of cathode and anode for better batteries. It would be expected that this presentation can stimulate new ideas as how to attack these bottlenecks to advance the cathode based on layer structured materials and Li anode.

Si-based materials have recently received great interest for their much higher theoretical Li capacity than graphite. However, their practical use is still largely hampered by many of their inherent problems, particularly for the massive volume expansion and the anisotropic lithiaion behavior that result in the pulverization of the Si electrode upon lithiation. To solve this problem, it would require a detailed understanding of the fundamental mechanism of the Si-based anode during the lithiation processes. Regarding this aspect, molecular dynamic (MD) simulation is a powerful tool to explore the structural evolution and dynamic properties of the Si-based anode upon lithiation. However, molecular dynamic simulation requires a reliable force-field model to accurately describe the atomic interactions between the constituent elements. In this study, we have developed a new ReaxFF parameter set for the Li-Si alloy system based on first-principles calculations. This new potential parameter set has been examined through a series of validations, which has proved to accurately predict the fundamental properties of Li, Si, and LixSi alloys, including the structure parameters, elastic moduli, and formation energies, etc. We then employed this newly developed model in conjunction with MD simulations to investigate the lithiation kinetics and the associated underlying mechanisms for the Si-based anode. Our MD simulations first show that this newly developed ReaxFF model can accurately describe the anisotropic lithiation behavior of the Si-based anode as observed in the experiments. The distinct properties in each facets were further analyzed to find out the mechanism of the anisotropic lithiation. More specifically, the Li insertion energy barrier is found to be the controlling factor in the initial stage lithiation. The high surface Si density of Si(111) facet leads to its high Li insertion energy barrier, which makes it "almost immobile" as observed in our MD simulations, consistent with the experimental results. Meanwhile, the local Li/Si ratio and the stress concentration generated at the Si/LixSi interface (phase boundary layer), which affects the bond-breaking and Li insertion rate, was found to play a critical role in the steady-state lithiation process. These structural characteristics lead to the distinct lithiation rates and anisotropic morphologies on different crystallographic orientations of Si. Finally, our results show that the anisotropic lithiation behavior of the Si anode is primarily determined by the lithiation kinetics on different facets of Si rather than the thermodynamic factors as suggested by early first-principles calculations.

Clathrates are comprised of a cage framework of covalently bonded Group IV elements in which guest atoms reside in the center of each cage. Clathrates are typically known for their superconducting or thermoelectric applications, while their electrochemical properties are relatively unexplored. Our group has been investigating the electrochemistry of Si and Ge clathrates for Li-ion battery anodes and have found that in some cases the Li-alloying pathways and reactions are distinctly different than those seen in the diamond Si/Ge case. For example, the Ba8Al16Si30 Type I clathrate does not undergo a Li-alloying reaction1 while the Ba8Ge43 clathrate does2. In the case of Ba8Ge43, the reaction pathway appears to follow an amorphous phase transformation. This is different than diamond cubic Ge which undergoes several phase transformations to crystalline phases3. In the case of the Type II Si clathrate (NaxSi136), the alloying reaction depends on the Na content. When the cages are full of Na (Na24Si136, the alloying reaction looks similar to diamond cubic Si4, where if the cages are empty (Si136) the voltage profile is significantly more sloped5, more similar to the alloying of amorphous Si. In this work, we employ synchrotron x-ray pair-distribution function analysis (PDF), X-ray diffraction, and in-situ heating experiments to elucidate the structural differences between Ba8Ge43 and Si136 and their diamond cubic analogues at similar states of charge. The PDF method allows us to probe the local structure of the amorphous phases that are present in the lithiated samples. In situ heating experiments enable us to observe structural changes to amorphous phases upon moderate heating. The insight learned from the structural characterizations are then used to explain the differences in the electrochemistry between the Si and Ge clathrates and their diamond cubic analogues. The results show that at various states of lithiation of Ba8Ge43, Ba8Ge43 is converted to a an X-ray amorphous phase while diamond cubic Ge undergoes crystalline phase transformations similar to previous work3. Interesting, when heating the fully lithiated Ba8Ge43 phase and monitoring with X-ray diffraction, crystalline peaks corresponding Li15Ge4 appear at 420 K. Upon further heating to 480 K, the reflections corresponding to Ba4Li2Ge6 and other binary Ge-Li phases appear. These results suggest that the amorphous phases that are part of the final lithiation state have local structures similar to that of crystalline phases which can than be accessed upon moderate heating (480 K). These results would aid in giving insight to the fundamentals of how the initial structure of group IV materials affect their Li-alloying properties and structures. (1) Zhao, R.; Bobev, S.; Krishna, L.; Yang, T.; Weller, J. M.; Jing, H.; Chan, C. K. Anodes for Lithium-Ion Batteries Based on Type I Silicon Clathrate Ba 8 Al 16 Si 30 - Role of Processing on Surface Properties and Electrochemical Behavior. ACS Appl. Mater. Interfaces 2017, 9, 41246-41257. (2) Dopilka, A.; Zhao, R.; Weller, J. M.; Bobev, S.; Peng, X.; Chan, C. K. Experimental and Computational Study of the Lithiation of Ba8AlyGe46- y Based Type I Germanium Clathrates. ACS Appl. Mater. Interfaces 2018, 10, 37981-37993. (3) Jung, H.; Allan, P. K.; Hu, Y. Y.; Borkiewicz, O. J.; Wang, X. L.; Han, W. Q.; Du, L. S.; Pickard, C. J.; Chupas, P. J.; Chapman, K. W.; et al. Elucidation of the Local and Long-Range Structural Changes That Occur in Germanium Anodes in Lithium-Ion Batteries. Chem. Mater. 2015, 27, 1031-1041. (4) Wagner, N. A.; Raghavan, R.; Zhao, R.; Wei, Q.; Peng, X.; Chan, C. K. Electrochemical Cycling of Sodium-Filled Silicon Clathrate. ChemElectroChem 2014, 1, 347-353. (5) Langer, T.; Dupke, S.; Trill, H.; Passerini, S.; Eckert, H.; Pöttgen, R.; Winter, M. Electrochemical Lithiation of Silicon Clathrate-II. J. Electrochem. Soc. 2012, 159159, 1318-1322.

Group IV clathrates have cage structures that host guest atoms and exhibit a wide range of interesting properties including superconductivity, hydrogen storage, and thermoelectricity. Recently, there have been interest in the possibility of clathrates and other group IV frameworks as anodes for Li-ion and Na-ion batteries. The type and presence of guest atoms have a direct influence on the properties that are observed. For Li-ion battery applications, we found with DFT (density functional theory) calculations that the presence of Ba inside the clathrate cages in Ba8Ge43 impedes Li mobility by raising the migration barrier for diffusion. If Ba was absent, the barrier was much lower suggesting the need for guest atoms to be absent for diffusion to be feasible1. In this work, the favorable Li and Na positions are determined in the type I clathrate frameworks (Si46, Ge46, and Sn46) and then the nudged elastic band (NEB) method is used to evaluate the Li and Na mobility. The results show that it is energetically favorable for a Li atom to occupy the center position inside the small Tt20 cages while preferring the off-center positions in the larger Tt24 cages. The lowest Li migration barriers are found to be 0.35, 0.13 and 0.37 eV for Si46, Ge46, and Sn46, respectively, with the dominant diffusion pathway along channels of Tt24 cages connected by hexagonal faces. Li accessibility to the Si20 cage in Si46 appears to be restricted in the dilute regime due to a high energy barrier (2.0 eV) except for the case in which Li atoms are present in adjacent cages; this lowers the migration barrier to 0.77 eV via a mechanism where a Si?Si bond is temporarily broken. In contrast, Na atoms show preference for the cage centers and display higher migration barriers than Li. Overall, the Tt24 channel sizes in the guest-free, type I clathrates are ideal for fast Li diffusion, while Na is too large to migrate effectively between cages, except in the case of hexagonal migration of Na in Sn46 (0.43 eV). The energy landscape for Li inside the type I clathrates is uniquely different than that in diamond cubic structures, leading to significantly lower energy barriers for Li migration. These results suggest that open frameworks of group IV elements may enable facile Li migration and, in some cases, facile Na migration. By the gained understanding the structural requirements for fast ion diffusion in group IV open frameworks, new materials can be identified that have potential use as Li-ion or Na-ion anodes. (1) Dopilka, A.; Zhao, R.; Weller, J. M.; Bobev, S.; Peng, X.; Chan, C. K. Experimental and Computational Study of the Lithiation of Ba8AlyGe46- y Based Type I Germanium Clathrates. ACS Appl. Mater. Interfaces 2018, 10, 37981-37993.

Lithium-Sulfur (Li-S) batteries exhibit a very high theoretical specific capacity of 1675 mA h g-1 and high energy density of 2650 W h Kg-1, which are 2~3 times higher than the current Li ion batteries and thus are considered next-generation batteries. The major challenges of current Li-S batteries are cathode degradation related to the "shuttle" effect and Li dendrite formation on anode surface, leading to the low Coulombic efficiency, poor cyclic stability, and serious safety problems. We have used new nanomaterials to solve these issues and improved substantially the performance of Li-S batteries. The shuttle effect has been reduced by adding a BN nanosheet-graphene interlayer on cathode surface to trap lithium polysulfide ions (Fan 2017). Coating the separator with a thin layer of negatively-charged BN nanosheets can prevent polysulfide migration through the separator effectively, further suppressing the shuttle effect (Fan 2019); Dendrite formation is successfully avoided by constructing a 3D Li anode using a 3D host of MnO2-decorated graphene foam (Yu 2018). These research effort have substantially improved the battery performance with a long cycle life of over 1000 cycles, a high Coulombic efficiency of 98.9%, and high rate capacities (7C) both in symmetric-cell and full-cell cells (Tao 2018). REFERENCES Fan, Y. Chen, Y. et al "Functionalized Boron Nitride Nanosheets/Graphene Interlayer for Fast and Long Life Lithium-Sulfur Batteries", Advanced Energy Materials, 2017, 7,1602380 Fan, Y, Chen, Y. et al "Repelling Polysulfide Ions by Boron Nitride Nanosheets Coated Separators in Lithium-Sulfur Batteries", ACS Applied Energy Materials, 2019, DOI: 10.1021/acsaem.8b02205 Yu, B. Chen, Y. et al "Nanoflake Arrays of Lithiophilic Metal Oxides for the Ultra Stable Anodes of Lithium-Sulfur Batteries", Advaned Functional Materials, 2018, 1803023. Tao, T. Chen, Y et al "Anode Improvement in Rechargeable Lithium-Sulfur Batteries", Advanced Materials, 2017, 29, 1700542.

The use of ether species has shown great promises for equimolar (super-concentrated) ionic liquid for novel organic electrolyte applications and as model systems for solid-state polymer electrolyte [1-5]. However, the moderately-sized combinatorial space for designing supramolecular chemistry for ether-ion interactions is on the order of 10^3 ~ 10^4 and has remained largely unexplored. We applied graph neural network-based potentials to accelerate the exploration of novel species for equimolar ionic liquid using a self-consistent strategy that accelerates the sampling and training of transferable neural networks[6-10]. The proposed method combines deep neural network training and active sampling of both chemical and configurational space in an end-to-end fashion to 1) bypass expensive ab initio MD sampling and 2) leverage the transferable configurational information across species. This workflow has discovered novel molecules for potential novel organic electrolyte applications for several different ions including Lithium and Sodium and shows great promises in theory-guided engineering for supramolecular chemistry. [1]Seki, S., Serizawa, N., Takei, K., Dokko, K. & Watanabe, M. Charge/discharge performances of glyme-lithium salt equimolar complex electrolyte for lithium secondary batteries. J. Power Sources 243, 323-327 (2013). [2]Ueno, K. et al. Glyme-lithium salt equimolar molten mixtures: Concentrated solutions or solvate ionic liquids? J. Phys. Chem. B 116, 11323-11331 (2012). [3]Ueno, K. et al. Li+ solvation in glyme-Li salt solvate ionic liquids. Phys. Chem. Chem. Phys. 17, 8248-8257 (2015). [4]Westman, K. et al. Diglyme Based Electrolytes for Sodium-Ion Batteries. ACS Appl. Energy Mater. 1, 2671-2680 (2018). [5]Zhang, L., Han, J., Wang, H., Car, R. & E, W. Deep Potential Molecular Dynamics: a scalable model with the accuracy of quantum mechanics. (2017). [6]Schütt, K. T., Sauceda, H. E., Kindermans, P.-J., Tkatchenko, A. & Müller, K.-R. SchNet-A deep learning architecture for molecules and materials. J. Chem. Phys. 148, 241722 (2018). [7]Behler, J. & Parrinello, M. Generalized neural-network representation of high-dimensional potential-energy surfaces. Phys. Rev. Lett. 98, 1-4 (2007). [8]Yao, K., Herr, J. E., Toth, D. W., Mckintyre, R. & Parkhill, J. The TensorMol-0.1 model chemistry: a neural network augmented with long-range physics. Chem. Sci. 9, 2261-2269 (2018). [9]Smith, J. S., Isayev, O. & Roitberg, A. E. ANI-1: an extensible neural network potential with DFT accuracy at force field computational cost. Chem. Sci. 8, 3192-3203 (2017). [10]Smith, J. S., Nebgen, B., Lubbers, N., Isayev, O. & Roitberg, A. E. Less is more: Sampling chemical space with active learning. J. Chem. Phys. 148, 241733 (2018).48, 241733 (2018).

Graphene is a 2-dimensional carbon layer. Because of its potential properties such as chemical inertness and atomically thin nature, graphene has been considered as a thin diffusion layer in metal interconnect structures. Furthermore, as the feature size of device is decreased to sub-nm size, the contact resistance between metal and Si in source/drain has become one of the critical issues. We demonstrated that reduced contact resistivity of 1.47 nohm-cm2 via insertion of graphene between metal and Si which approaches the theoretical limit of 1.3 nohm-cm2.[1] In order to adapt graphene to source and drain, direct and selective graphene growth on semiconductor substrates is required. In our previous report, we have already suggested that the directly grown nanocrystalline graphene (nc-G) at 560 oC can act as a liner reducing the resistivity of W as well as a diffusion barrier in a W interconnect structure by realizing W/nc-G/TiN.[2] However, the selective graphene growth on the non-catalytic semiconductor-insulator substrates is barely reported. In this study, we addressed the origin of selective growth of graphene on a semiconductor over the dielectric materials. Based on Ab initio simulations, we found that the adsorption energy of CH4 on SiO2 is much higher than that on Si and Ge. The growth differences of graphene on Ge and SiO2 were also experimentally confirmed. While a continuous graphene layer was formed on the Ge surface at the initial growth stage that became thicker at increasing growth times, an incubation time was required for the nucleation of graphene on SiO2. The interface analysis between nc-G and SiO2 indicates that initially incorporated carbon onto SiO2 cannot contribute to the nc-G growth because of its reaction with SiO2 producing CO. Finally, we selectively grew 3 nm-thick nc-G layer on a SiO2-patterned Ge substrate.[3] In addition, as the feature size of semiconductor devices decreases, hard mask films with high etch selectivity are required to achieve fine pattern transfer during lithography. We are going to address the potential and properties of nc-G layer as a future hardmask film.[4] [1] M.-H. Lee et al.(SAIT) "Two-Dimensional Materials Inserted at the Metal/Semiconductor Interface: Attractive Candidates for Semiconductor Device Contacts", Nano Letters 18, 4878 (2018) [2] C.S. Lee et al.(SAIT) "Fabrication of Metal/Graphene Hybrid Interconnects by Direct Graphene Growth and Their Integration Properties", Advanced Electronic Materials 4, 170064 (2018). [3] K.W. Shin et al.(SAIT) "Study of Selective Graphene Growth on Non-Catalytic Hetero-Substrates", 2D Materials 7, 011002 (2020). [4] S. W Kim et al.(SAIT) (In preparation).

Graphene and other 2D materials possess desirable mechanical and functional properties for incorporation into or onto novel colloidal particles, potentially granting them unique electronic and optical functions. However, this application has not yet been realized because conventional top-down lithography scales poorly for the production of colloidal solutions. Herein, we describe an "autoperforation" technique providing a means of spontaneous assembly for colloidal microparticles comprised of 2D molecular surfaces at scale. Such particles demonstrate remarkable chemical, mechanical and thermal stability. They can function as aerosolizable memristor arrays capable of storing digital information, as well as dispersible and recoverable probes for large-scale collection of chemical information in water and soil. Liu, A. T.;* Liu, P.;* Kozawa, D.; Dong, J.; Yang, J. F.; Koman, V. B.; Saccone, M.; Wang, S.; Son, Y.; Wong, M. H.; Strano, M. S.-Nature Materials (2018) 17, 1005-1012.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are some of the more promising material candidates for future sensors requiring detection of molecular species at extremely low concentrations. This is primarily due to the multifunctional properties including a high surface to volume ratio and strict control of surface reaction sites. In this talk, two different scalable synthesis approaches towards large area TMD films using magnetron sputtering are presented. Firstly, direct sputtering of MoS2 and WSe2 thin onto various substrate and subsequent laser crystallization was shown to enable local control of defect density and crystal structure. Additionally, wafer-scale crystallization of stretchable 2D photodetectors with the use of a broadband pulsed lamp source demonstrate the feasibility of large scale transformation as a means to create unique device constructs. The second large area technique to be discussed is the sputtering of metal precursor films and sulfurization/selenization using various gas phase reaction sources. Heterostructures and superlattice constructs using stacked metal layers with alternating materials were also demonstrated using this technique. Finally, the large area TMD films and heterostructures were evaluated in both electrical and optical molecular sensors constructs for future low analyte detection devices.

The atomically thin two-dimensional materials have emerged as a new type of thin film semiconductors that are promising for flexible electronics, thanks to their unique electronic and mechanical properties. As many important components of flexible electronics (for example, transistors, sensors and memory devices based on two-dimensional materials) have been developed, an efficient, flexible and always-on energy-harvesting solution is highly desirable yet still missing. In this work, we present a fully flexible Wi-Fi band rectenna based on atomically thin MoS2 semiconducting-metallic phase heterojunctions. Through a self-aligned phase engineering technology, we successfully improved the cutoff frequency of MoS2-based rectifiers to 10 gigahertz, which is high enough to cover the increasingly ubiquitous electromagnetic radiation from Wi-Fi and cellular systems. By integrating the MoS2 rectifier with a flexible Wi-Fi band antenna, we realized wireless energy harvesting of electromagnetic radiation at zero external bias. In addition, the phase-engineered MoS2 rectifier can also function as a flexible mixer, realizing frequency conversion beyond 10 gigahertz. This work provides a flexible energy-harvesting building block that can be integrated with various flexible electronic systems.

Van der Waals semiconductor materials and heterostructures possessing room temperature strongly bound exciton states exhibit emergent physical phenomena and are of a great promise for optoelectronic applications. Likewise, their self-passivated surfaces enable facile integration for novel heterostructures for device applications. In the first part of the talk, we will present novel light-matter interactions that are observed at the hybrid interface of two-dimensional (2D) semiconductors and bulk gold (Au) surface. We will demonstrate that nanostructured multilayer transition metal dichalcogenides (TMDCs) by themselves provide an ideal cavity-less platform for exciton-photonics. Hence, we show that by patterning the TMDCs into nanoresonators, strong dispersion and avoided crossing of excitons and photon-polaritons with interaction potentials exceeding 410 meV may be controlled with great precision. We further observe that inherently strong TMDC exciton absorption resonances may be completely suppressed due to excitation of hybrid photon states and their interference. In the second part of this talk we will demonstrate, hybrid 2D/3D semiconductor heterojunctions using MoS2 as the prototypical 2D semiconductor laid upon Si and GaN as the heavily doped 3D semiconductor layers. At the interface of these junctions we demonstrate that Fermi-level is tunable in the transition metal dichalcogenides (TMDCs) to achieve highly tunable rectification ratios in the diodes across seven orders of magnitude i.e from 0.1 to 106. Concurrently, we have also demonstrated that these triodes can effectively serve the purpose of a switching device with on/off ratios exceeding 107. Our results suggest that the 2D/3D semiconductor heterojunction system is highly tunable, near-ideal and effective for electronic applications despite the lack the perfect passivation at the interface. Given the hybrid nature of the device it can serve the function in both switching and rectifying applications. With a wealth of 2D semiconductors now isolated and combined with the availability of controlled complementary doping in 3D semiconductors, our work opens up new opportunities in high performance and multi-functional, hybrid heterostructure devices.

Ultra-clean van der Waals interfaces can be achieved between soft indium metal and monolayer 2D transition metal dichalcogenide semiconductors. Such interfaces lead to low contact resistance and n-type field effect transistors with high mobilities- in excess of 100 cm2-V-1-s-1. It has been, however, challenging to make similarly clean interfaces between refractory metals with high work functions to achieve efficient hole injection. Here, I will present our efforts on realizing p-type contacts using high work function metals and alloys. We show that it is possible to deposit a thin layer of indium and then a high work function metal on top of it to form an alloy by annealing at 200oC. This method preserves the ultra-clean interface between the monolayer semiconductor and alloy while increasing the work function so that p-type devices can be realized. We also demonstrate clean interfaces using metals such as Au and Pt via direct deposition. These interfaces reveal low contact resistance and also high mobility p-type devices.

Synthesis of van der Waals (vdW) metal-organic framework (MOF) nanosheets with controlled crystallinity and interlayer coupling strength are one of the bottlenecks in 2D materials that have limited its successful transition to large-scale applications. Especially for MOFs involving transition metals possessed versatile oxidation states have been highly rated in electrocatalytic applications. Here, the synthesis of vdW MnBDC with huge crystal size through a novel bi-phase strategy is demonstrated. The results show Mn2+ and BDC- species diffusing along vertical directions in described bi-phase system slows down the reaction speed and enhances the crystallinity of MnBDC significantly. Moreover, the existence of capping agent - pyridine promotes the nanosheets that are stacking in 2D landscape and controls over the crystallinity further by competing with ligands carboxylic acid. The attained MnBDC crystals exhibit classic vdW nature, submillimeter of lateral size and great crystallinity. The high quality of vdW MnBDC nanosheets enables us to perform thickness-dependent kelvin probe force microscopy and optical absorption to discover the electron transition and interlayer hybridization in 2D MOF nanosheets for the first time. Further, we conducted a series of electrocatalytic characterizations to investigate the catalytic water splitting efficiency of MnBDC. Theoretical calculation and experimental results highlighted the contributions of 2D crystal structure and multiple oxidation states of MnBDC crystals.

The choice of growth substrate often has a profound influence on the electronic [1] and optical [2] behavior of atomically thin 2D crystals (2DCs). In order to exploit this influence, we have developed a synthetic technique for creating ordered, porous metal films with various suspended 2DCs, which we dub Oriented Porous mEtallic Networks (OPEN) [3]. These OPEN films exhibit characteristic optical phenomena, including enhanced photoluminescent (PL) intensity in suspended regions and remote quantum emission. Aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) enables us to probe the various resonances of the suspended 2D crystals, the metal/2DC hybrid, and the interfacial region between these two distinct areas. Similar to recent work that uses gold films for generation of large-area 2DCs [4], OPEN films are synthesized by annealing metal films capped by 2DCs. The annealing causes lattice registry between the metal layer and the 2DCs. We show a single MoS2 monolayer can affect the structural order of a comparatively thick (13-25nm) gold thin film. Electron back scatter diffraction and aberration-corrected STEM high-angle annular dark-field (HAADF) imaging of annealed MoS2/Au layers reveals that the gold films become highly textured and in close crystallographic registry with each other. With further annealing, the gold films can locally dewet beneath the MoS2 layer to form the OPEN film structure with suspended regions of monolayer 2DC above the pore sites. The unique optical behavior of these hybrid systems include a 1000x increase in PL intensity in suspended 2DCs vs 2DCs on the metal film, and single-photon emitters manifests in remote quantum emission, with evidence for surface plasmon-polariton (SPP) propagation of > 10 µm. Further, low-loss EELS measurements at 60 kV, reveal that the OPEN films have resonances at ~ 2 eV, which are associated with the so-called A and B excitons in 2DCs, while resonances at several other energies occur in both the interfacial region and the "bulk" MoS2/Au regions. We will discuss the origin of these resonances, and how both the metal overlayer and disordered carbon contamination affect them. References: [1] W Bao et al., Appl Phys Lett 102 (2013), p. 042104. [2] M Buscema et al., Nano Research 7 (2014), p. 561. [3] J. Fonseca, submitted (2019) [4] MA Islam et al., Nano Lett 17 (2017), p. 6157.

We introduce polycaprolactone (PCL) dry transfer which is a powerful material for picking up non- adhesive van der Waals materials such as NiPS3, NbSe2, Fe3GeTe2, ZnPS3 and bismuth strontium calcium copper oxide (BSCCO) and etc. The heterostructures of Van der Waals were composed of layered atomically thin 2D materials that provide access to new properties important to the quantum information sciences. However, one of the main constraints is the stacking techniques. (PPC) has been known as the highest quality in dry transfer because PPC left less residue on vdW heterostructure through high temperature vacuum annealing but yield of the transfer was low. Here, we present the new polymer, polycaprolactone (PCL) for stacking vdW heterostructure with Rare layered 2D materials by dry transfer at low temperature. These 2D materials were easily detached from hexagonal boron nitride (hBN) assisted with PPC during dry transfer process. We succeeded to pick up many rare superconducting or magnetic 2D materials which were hard to be picked up by conventional dry transfer method from the oxide or silicon nitride substrates.

Low dimensional, van der Waals (vdW) materials, from graphene to 2D magnets such as Fe₃GeTe₂ and CrI₃, show exotic behaviors at the atomic layer limit, including superconductivity, long range magnetism and topological edge states. Their “stackability” further creates new opportunities to explore quantum phenomena previously studied only in 3D crystals. Here we demonstrate that magneto-Raman spectroscopy is a unique measurement capability that is exceptionally well suited to study these exotic 2D materials.
Specifically, in the MPX₃ (M=Fe, Mn; X =S, Se) compounds, inter-layer antiferromagnetic ordering has been suggested to survive in the monolayer limit through the measurement of spin-phonon coupling. Using temperature-, polarization-, and wavelength-dependent Raman scattering, we studied the Neel-type antiferromagnet MnPSe₃ through its ordering temperature, and as a function of applied magnetic field. Surprisingly, the previously assigned one-magnon scattering peak showed no change in frequency with an increasing in-plane magnetic field. Instead, the Raman data revealed a more surprising story, including changes in resonant Raman scattering due to magnetic ordering as well as the hybridization of phonons with a 2-magnon scattering continuum. These results, which are supported using first-principles calculations, reveal the complex phase space of the interactions of phonons and magnons with regards to temperature, laser excitation wavelength, and magnetic field.

Electronic devices based on 2D and quasi-2D materials promise several performance advantages. However, the physical properties and small length scales that enhance low dimensional electronic devices can also make them more susceptible to parasitic effects. Contact resistance and power dissipation [1] are noteable challenges to making practical 2D devices. This study measures thermionic heating and cooling at heterojunctions of 2D nanoribbon semiconductor channel and metal contacts in both β-Ga2O3 FET on insulator [2] and graphene FET on insulator. High resolution thermoreflectance microscope images show temperature difference of 60 K between gold drain and source contacts separated by a 15 micron long, two micron wide, 70 nm thick β-Ga2O3 nanoribbon channel deposited on insulator substrate. [2] Current density was 9×104 A/cm2, power was 3 mW. Thermionic effects at the metal-2D semiconductor contact interface were consistent with current flux, reversing heating and cooling at respective drain-source contacts when applied current polarity was reversed. Heating in the β-Ga2O3 channel increased quadratically with current, consistent with Joule's law. However, temperature dependence on current at the gold-β-Ga2O3 contacts divereged significantly from a simple model combining Joule and Peltier contributions, suggesting thermionic effects at the metal-2D semiconductor interface dominate over any thermoelectric contribution from the β-Ga2O3 channel. [3] Thermionic parasitics at the contacts may be exacerbated by use of low dimensional β-Ga2O3 as channel. The high temperature gradient along the quasi-2D channel could impact transport properties, and device performance and reliability. Thermoreflectance images of a monolayer graphene nanoribbon FET modulated by backgate substrate revealed similar location, magnitude, and polarity dependent thermionic heating and cooling at the metal-graphene source and drain contacts. Thermionic heating and cooling, especially the large spatial temperature gradient between source and drain, would be difficult to measure using purely electrical methods. Future study will look at both geometric dependence and reliability considerations related to parasitic thermionic heating and cooling in 2D material electronic devices. [1] Zhou, H., Maize, K., Noh, J., Shakouri, A. & Ye, P. D. Thermodynamic Studies of β-Ga 2 O 3 Nanomembrane Field-Effect Transistors on a Sapphire Substrate. ACS Omega 2, 7723-7729 (2017). [2] Zhou, H., Maize, K., Qiu, G., Shakouri, A. & Ye, P. D. ?-Ga 2 O 3 on insulator field-effect transistors with drain currents exceeding 1.5 A/mm and their self-heating effect. Applied Physics Letters 111, 092102 (2017). [3] Zeng, T. & Chen, G. Interplay between thermoelectric and thermionic effects in heterostructures. Journal of Applied Physics 92, 3152-3161 (2002).

There has been tremendous interest in recent years to study two-dimensional (2D) atomic layers which form building blocks of many bulk layered materials. This talk will focus on the use of 2D atomic layer building blocks to create various types of heterostructures and composite architectures that takes advantage of the varied properties of individual layers. The concept of nanoscale engineering and the goal of creating new artificially stacked van der Waals solids and 3D constructs will be discussed through a number of examples including graphene and other 2D layer compositions. The talk will explore the emerging landscape of multi-component 2D layers such as doped, intercalated and alloy systems. The anticipated multifunctional properties of these structures will be discussed.

The charge density wave (CDW) phase is a quantum state consisting of a periodic modulation of the electronic charge density accompanied by a periodic distortion of the atomic lattice in quasi-1D or quasi-2D metallic crystals. Several layered transition metal dichalcogenides (TMDs) exhibit unusually high transition temperatures to different CDW symmetry-reducing phases opening possibility for practical device applications. One of the most promising materials, 1T-TaS₂, has the CDW transition between the nearly-commensurate (NC-CDW) and the incommensurate (IC-CDW) phases at 350 K. The transition from the IC-CDW phase to the normal metal phase is observed at even higher temperature. In this invited talk, I will review our recent experimental results on controlling the CDW phase transitions with an applied electric field and discuss device applications of quasi-2D CDW materials. We have demonstrated the room-temperature voltage controlled oscillators and logic circuits, which operate on the basis of the NC-to-IC CDW transition in 1T-TaS₂ channels, triggered by the applied voltage [1-2]. We found that the 1T-TaS₂ CDW devices reveal exceptional hardness against X-ray and proton radiations [3-4]. We explained this property of the CDW devices by the high carrier concentration in all their phase states, two-terminal design, and the thin-film channel geometry. The low-frequency electronic noise spectroscopy has been used as an effective tool for monitoring the CDW phase transitions, particularly the switching from the IC-CDW phase to the normal metal phase in 1T-TaS2 [5]. The noise spectral density exhibits sharp increases at the phase transition points, which correspond to the step-like changes in resistivity. The noise spectroscopy was instrumental in revealing the “hidden phase transitions” in vertical 1T-TaS2 devices [6]. Preliminary data on the “narrow-band noise” in quasi-2D CDW devices will also be presented.

This work was supported, in part, by the National Science Foundation (NSF) through the DMREF: Collaborative Research (UC Riverside – Stanford): Data Driven Discovery of Synthesis Pathways and Distinguishing Electronic Phenomena of van der Waals Bonded Solids, and by the University of California – National Laboratory Collaborative Research and Training Program LFR-17-477237, and by the Semiconductor Research Corporation (SRC) contract 2018-NM-2796: One-Dimensional van-der-Waals Metals: Ultimately Downscaled Interconnects with Exceptional Current-Carrying Capacity and Reliability.

[1] G. Liu, et al., A charge-density-wave oscillator based on an integrated tantalum disulfide–boron nitride–graphene device operating at room temperature, Nature Nanotechnology, 11, 845 (2016).
[2] A. G. Khitun, et al., Transistor-less logic circuits implemented with 2-D charge density wave devices, IEEE Electron Device Letters, 39, 1449 (2018).
[3] G. Liu, et al., Total-ionizing-dose effects on threshold switching in 1T-TaS₂ charge density wave devices, IEEE Electron Device Letters, 38, 1724 (2017).
[4] A. K. Geremew, et al., Proton-irradiation-immune electronics implemented with two-dimensional charge-density-wave devices, Nanoscale, 11, 8380 (2019).
[5] A. K. Geremew, et al., Bias-voltage driven switching of the charge-density-wave and normal metallic phases in 1T-TaS2 thin-film devices, ACS Nano, 13, 7231 (2019).
[6] R. Salgado, et al., Low-frequency noise spectroscopy of charge- density-wave phase transitions in vertical quasi-2D 1T-TaS2 devices, Appl. Phys. Express, 12, 037001 (2019).

The deterministic creation of single-photon emitters in a solid-state system is an important step towards scalable photonic quantum processors [1]. Two-dimensional materials transferred onto nanostructures are promising candidate thanks to their ability to host quantum emitters with high probability at specifically fabricated sites [2,3]. However, the emission energies of quantum emitters, a crucial factor for generating indistinguishable photons and entangled states, are broadly distributed, and the precise relationship between this strain and the resultant emission energy remains elusive. Here, we study the localized emitters hosted in monolayer WSe2 transferred onto micron-sized structures, and the corresponding free-exciton energy shift induced by local strain. The localized emission energy show positive correlation with the strain-modulated free-exciton energy together with minimum offset value ~ 40 meV. Our results show local strain plays a crucial role not only in the formation but also in the emission energy of the localized emitters, elucidates the origin of single-photon emission in two-dimensional semiconductors. [1] I. Aharonovich, D. Englund, M. Toth, Nature Photonics 2016, 10, 631. [2] A. Branny, S. Kumar, R. Proux, B. D. Gerardot, Nature Communications 2017, 8, 15053. [3] C. Palacios-Berraquero, D. M. Kara, A. R.-P. Montblanch, M. Barbone, P. Latawiec, D. Yoon, A. K. Ott, M. Loncar, A. C. Ferrari, M. Atatüre, Nat. Commun. 2017, 8, 15093.

2D materials have a number of intriguing value proposition that could be harnessed for compact, tunable, high-performance optoelectronic devices when heterogeneously integrated in photonic circuits. Here I review our latest work including; (1) tunable TMD-based microring resonator with engineered critical-coupling condition, (2) a broadband graphene plasmon-slot photodetector (R=0.7A/W), (3) a 200mV bandgap-shifted strain-engineered absorption-enhanced MoTe2 photodetector (R=0.5A/W, low-dark-current <10nA@-1V), (4) a record-high responsivity (R=1.36A/W) slot-plasmon exciton-modulated MoTe2 photodetector, (5) a MoS2 electro-absorption modulator all enabled by our recently developed method of cross-contamination-free yet deterministic dry transfer 2D material 'printer' mimicking a 3D printer for enabling rapid prototyping. These devices are based on heterogeneous integration of 2D materials into Silicon and SiN photonics, with the latter used for on-exciton modulation or exciton absorption.

Following the developments of two-dimensional (2D) graphene with distinct properties, tremendous progress has been achieved for the broad applications of 2D materials in nanoscale electronics and optoelectronics. The layered metal chalcogenides (LMC) contribute to a dominant group of the layered 2D structure with various properties and applications. Pursuing the precise structural identification of functional two-dimensional (2D) layered metal chalcogenides (LMCs) are key factors dominating the origins of the unique electronic and ferroelectric properties. Presently, indium selenide (In2Se3) has become one of the most studied LMC materials due to the flexible phases with broad applications in phase-change memory, lithium batteries, optoelectronic and even photovoltaic devices. In2Se3 is particularly attractive in the atomically thin film research due to the flexible phase modulations and unique electronic behaviors. However, the complicated phase change of In2Se3 and their high sensitivity towards the intrinsic defects still requires the advanced technology to identify the origins of the inhomogeneous charge distribution induced in-plane and out-of-plane ferroelectricity. In this work, we have presented comprehensive theoretical research to reveal the simulated scanning tunneling microscope (STM) images as a toolbox for the experimental results to distinguish the structural features. By detailed analysis of the simulated STM images and phonon behaviors under different defects, this work supplies the insightful structure clues of the nanoscale charge distribution fluctuations, explaining the unique electronic behaviors in mono-layered ?- In2Se3. We also confirm the high possibility of surface defects in the few-layered ?-In2Se3 rather than sub-layered defects. Moreover, the corresponding electron-phonon behaviors of ?-In2Se3 with major intrinsic defects provide pivotal references to explain the unique in-plane and out-of-plane electronic and ferroelectric properties in different applications. The modulations of the electronic properties by different intrinsic defects regarding the charge couplings and bandgap have been revealed, which is of significance in the precise identification and modulation of the ultra-thin LMC based ferroelectric materials, shedding new light for the synthesis of other relevant functional materials for broad applications.

Two-dimensional materials with tunable band gap in the range of 0.3-1.5 eV are highly desirable for electronic and optoelectronic applications. Palladium diselenide (PdSe₂), a less explored group-10 transition metal dichalcogenide, is one such material of particular interest that demonstrates a gradual transition from a semiconductor (monolayer) to semimetal (bulk) [1, 2]. In this work, we report our recent results on laser-induced phase transformation of a few layer PdSe₂ films [3]. We demonstrate that controlled laser irradiation can be used to directly ablate PdSe2 thin films using high power or trigger the local transformation of PdSe2 into a metallic phase PdSe_2-x using lower laser power. Scanning transmission electron microscopy is used to reveal the laser-induced Se-deficient phases of PdSe₂ material. The process sensitivity to the laser power allows patterning flexibility for resist-free device fabrication. The laser-patterned devices demonstrate that a laser-induced metallic phase PdSe_2-x is stable with increased conductivity by a factor of about 20 compared to PdSe2. [1] L. H. Zeng et al. Adv. Funct. Mater. 2019, 1806878, 1-9. [2] A. D.Oyedele et al. J. Am. Chem. Soc. 2017, 139, 14090?14097. [3] V. Shautsova et al. ACS Nano 2019, 13, 12, 14162-14171

The discovery of two-dimensional (2D) magnets in van der Waals (vdW) crystals provide a new platform for studying fundamental 2D magnetism and spintronic devices. In this talk, I will first present 2D itinerant ferromagnetism in atomically thin Fe3GeTe2 (FGT). FGT is a ferromagnetic metal in the bulk form, with a strong out-of-plane anisotropy and high Curie temperature of 220-230 K. Our layer-number-dependent studies reveal a crossover from 3D to 2D Ising ferromagnetism for thicknesses less than 4 nm (five layers), accompanied by a fast drop of the Curie temperature from 207 K to 130 K in the monolayer. For FGT flakes thicker than ~15 nm, a distinct magnetic behavior emerges in an intermediate temperature range, which we show is due to the formation of labyrinthine domain patterns. In the second part of my talk, I will present the pressure control of the magnetic states in bi- and tri-layer chromium triiodide (CrI3), which exhibits layered antiferromagnetic ground states at ambient pressure. In bilayer CrI3, pressure can change the stacking arrangement which induces a transition from layered antiferromagnetic to ferromagnetic phases. In trilayer CrI3, we found that pressure can create coexisting domains of three phases, one ferromagnetic and two distinct antiferromagnetic. Our work provides ample opportunities to engineer spintronic devices based on vdW magnets and heterostructures.

Tellurium is one of the special elemental materials which have 1D helical atomic structures and formed by van der Waals force between helical atomic chains. 2D tellurium films can be synthesized in a liquid solution. [1,2] In this talk, we will review the fundamental studies of this new 2D material at atomic scale in terms of its electrical, optical, thermal and mechanical properties. The helical atomic structure offers strong anisotropic properties of this 1D van der Waals material. The band-structure of this 2D material also offers some excellent material properties such as a high value of thermoelectric figure-of-merit ZT [3] and high carrier mobilities for both electrons and holes. Magneto-transport properties, such as SdH oscillations and QHE in 2D tellurium, were reported before. [4] More interestingly, topological non-trivial properties as Weyl node at conduction band edge are also unveiled in magneto-transport under 45 Tesla at tens of mK temperatures, showing Berry phase and radial spin texture being significantly different from Rashba or Dresselhaus spin-orbit coupling materials [5,6] The work is in close collaborations with Prof. Wenzhuo Wu at Purdue University. References: [1] Wang, Y. et al., Nature Electronics 2018, 1, 228. [2] Du, Y. et al., Nano Letters 2017, 17, 3965. [3] Qiu, G. et al., Nano Letters 2019, 19, 1955. [4] Qiu, G. et al., Nano Letters 2018, 18, 5760. [5] Qiu, G. et al.,arXiv. 1908.11495. [6] Niu, C. et al., arXiv. 1909.06659.

Recent developments in the device architecture and operation of state-of-the-art superconducting qubits has allowed the technology to advance to practical applications in quantum computing. However, the central circuit element in superconducting qubits, the Josephson junction, has mainly focused on using the amorphous AlOx tunnel barrier. New materials for fabricating Josephson junctions are expected to introduce novel functionalities and new circuit elements to superconducting qubits. Two-dimensional (2D) materials are one such promising candidate. The van der Waals layered structures of 2D materials can enable the precise design of the tunnel barrier at the sub-nanometer scale via layer-by-layer stacking of atomically thin monolayers, wherein each layer can be a different 2D material. This ability to generate heterostructure barriers would allow, for example, the design of the tunnel barrier's band structure using 2D materials with different band gaps and band offsets, or the fabrication of novel quantum circuit components such as ?-junctions using 2D magnets. In order to realize the implementation of 2D materials in these systems, it is essential to have a fabrication method that not only maintains oxide-free interfaces between the easily-oxidized superconductors and the 2D material tunnel barriers, but is also scalable for technological applications. Here, we demonstrate Josephson junctions and superconducting qubits that employ 2D materials as the tunnel barrier [1]. We batch-fabricate and tune the critical Josephson current of the device arrays over orders of magnitudes via layer-by-layer stacking of N layers of MoS2. In addition, we observe a transition of the junction J-V characteristics when N is increased. Based on these junctions, we demonstrate, for the first time, the engineering and operation of MoS2 superconducting qubits using a bulk superconducting microwave resonator. Our work here allows Josephson junctions to access the diverse physical properties of 2D materials, offering a platform to study the effects of these properties in superconducting qubit circuits under different geometries, and engineer novel quantum circuit elements for quantum computing. [1] K. -H. Lee, S. Chakram, S. E. Kim, F. Mujid, A. Ray, H. Gao, C. Park, Y. Zhong, D. A. Muller, D. I. Schuster, J. Park, "Two-Dimensional Material Tunnel Barrier for Josephson Junctions and Superconducting Qubits," Nano Letters, https://doi.org/10.1021/acs.nanolett.9b03886 (2019).

Ferroelectric 2D materials have emerged as a promising platform for nonvolatile memories and nonlinear optoelectrical devices working at the ultimate atomic thickness. However, most of the experimental investigations have been limited in the 2D ferroelectric with out-of-plane polarization, which causes the depolarization field at the metal/ferroelectric interface with decreasing the thickness. Although monolayer SnS has been theoretically expected to be a purely in-plane multiferroic semiconductor with ferroelectricity and ferroelasticity,[1] experimental demonstration is challenging because it is difficult to obtain a high-quality monolayer SnS.[2,3] Bao et al. have recently reported a ferroelectric device of bulk SnS (~15 nm), utilizing an external electric field to break the centrosymmetry.[4] As the few layer SnS has been investigated only by probe measurements owing to its small size of several tens of nanometers, the ferroelectric characteristic of few-to-monolayer SnS is still unidentified. Here, we report an in-plane ferroelectric device of a micrometer-size monolayer SnS grown via physical vapor deposition, where the growth conditions are precisely controlled to balance the adsorption and desorption of SnS. The Raman spectrum for monolayer SnS indicates high crystalline quality and strong anisotropy. As a prerequisite for ferroelectricity, second harmonic generation (SHG) spectroscopy reveals a non-centrosymmetry in monolayer SnS. Room temperature ferroelectric switching in monolayer is successfully demonstrated by the ferroelectric evaluation system. SnS has been commonly regarded to exhibit the odd/even effect, where the centrosymmetry breaks only in the odd-number layers to exhibit ferroelectricity. Remarkably, however, both the SHG signal and ferroelectric switching are observed in both odd and even-number layers below a critical thickness (~15 layers), thus overcoming the odd/even effect. This suggests an unusual stacking sequence lacking centrosymmetry due to a strong interaction between SnS and mica substrate. Given that SnS is the semiconductor with multiferroicity, pyroelectricity, and piezoelectricity, this work will open up possibilities of providing novel multifunctionalities in van der Waals heterostructure devices. [1] M. Wu and X.C. Zeng, Nano Lett. 16, 3236 (2016). [2] N. Higashitarumizu et al., MRS Adv. 3, 2809 (2018). [3] N. Higashitarumizu et al., Nanoscale 10, 22474 (2018). [4] Y. Bao et al., Nano Lett. 19, 5109 (2019).

Electronic devices based on atomic layered two-dimensionalmaterials and related heterostructures have recently attracted greatresearch attentiondue to their unique electronic properties and the feasibility of hybrid integration, which provide the unprecedented opportunities for various van der Waals heterojunctions. We have studied the performance improvement based on black phosphorus and molybdenum disulfide from the carrier velocity and operating frequency. High frequency transistor and circuits operating at gigahertz range based on molybdenum disulfide are demonstrated with record high maximum oscillation frequency. High hole velocity of up to 1.5 x 107cm/s has been demonstrated for short channel BP transistors and transport approaching ballistic limit are predicted for ultimately scaled channel length. Moreover, bandgap engineering using lateral heterojunctions has been carried out with multifunctionality for reconfigurable logic operations, showing great promise for future electronics. Ultrathin indium tin oxide has been also systematically investigated for high performance electronics with high on off rate and frequency response.

The most common approach for establishing electrical contacts to 2D semiconductors, such as MoS2, is by fabricating metal contacts on top of the inert crystal surface. From an electron transport perspective, these top-contact interfaces can be awkward, with a van der Waals gap or some difficult-to-control form of interfacial bonding. Pure edge contacts (where the interface between the metal and 2D semiconductor occurs completely along the open bonds of the 2D crystal edge) provide a more natural bonding structure and potentially better carrier transport behavior; however, edge contacts have proven more difficult to realize. In this talk, recent success with establishing clean, pure edge contacts between metals and 2D semiconductors will be discussed, including an approach involving an in situ ion beam within a thin-film deposition system. Evaluation of these edge contacts compared to other approaches will be provided, along with demonstration of how edge-contact scaling affects the performance of 2D transistors compared to top-contact scaling. While there remains much to be learned about these interesting metal-2D edge interfaces, results thus far show significant promise for yielding a more scalable and potentially reproducible contact for 2D-based devices.

With the ever-increasing demand for new memory technologies, tremendous efforts have been made to develop non-volatile, high-density, fast-operation and flexible memory devices. To continue scaling and reducing cell size, our future towards high-density and flexible memory would be made up of cells with 2D materials. On the other hand, as the devices continue to scale down, the time and energy spent transporting data across �Von-Neumann bottleneck� between memory and processor has become an unavoidable issue, which is believed to be resolved by non-Von Neumann computing system as in human brain. In the last few years, the scaling of dense non-volatile memory crossbar arrays, which has few-nm critical dimensions, has been recognized as one promising way to build the neuromorphic computing system that can mimic the massive parallelism and low-power operation in the human brain. In our work, we reported the observation of non-volatile resistive switching (NVRS) effect in various single-layer atomic sheets, including transition metal dichalcogenides (TMDs) and hexagonal boron nitride (h-BN) atomic sheets which can be labeled as �atomristor� - memristor effect in atomically thin nanomaterials. The atomristors in vertical metal-insulator-metal (MIM) device structure are fabricated using CVD-grown or high-quality exfoliated TMDs and h-BN with various process realization techniques. The memory devices show low switching voltage (down to < 1V), high on/off current ratio (up to ~ 6 orders of magnitude), fast switching speed (< 15ns) and good endurance and retention. What�s more, the intrinsic low resistance values, approaching ~5?, opens up a new application for low-power non-volatile electronic RF switches. With the utilization of MoS2 and h-BN atomristors, the RF switches can be realized with low insertion loss (< 0.2dB) and high isolation (> 15dB) up to 110 GHz. Importantly, the operating frequencies cover the RF, 5G and mm-wave bands, which is suitable for diverse communication and connectivity front-end systems. In this work, the application of the atomristors in neuromorphic computing system will be presented and discussed. The energy consumption is expected to be minimized due to the fast switching speed. Moreover, as a result of the large on/off current ratio, more resistance states are committed to mimic the gradual analog-like change in neuromorphic computing. The discovery of atomristors indicates a potential universal phenomenon of non-volatile resistance switching in 2D monolayers, inspiring the in-depth study of the intriguing mechanisms. In addition, with the desired characteristics presented, the atomristors will stand out as a promising candidate in the applications of flexible electronics, non-volatile memory arrays, low-power RF switches and neuromorphic synapse.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are promising for next-generation electronics due to their layered structure and electrical properties. In particular, they can maintain good carrier mobilities compared to silicon at sub-nanometer thickness [1], making them attractive for digital or flexible electronics applications. Often overlooked is the fact that monolayer (1L) TMDs also have relatively large electronic band gaps and large effective masses [2], potentially enabling transistors with ultra-low off-state currents (Ioff). These could enable significant improvements in standby power consumption, or in dynamic random access memory (DRAM) where retention and performance are dependent on minimizing leakage currents. Ioff in exfoliated multilayer MoS2 [3], and more recently in chemical vapor deposition (CVD) grown monolayer MoSe2 and WSe2 [4] have been reported. However, WS2, which possesses the largest band gap and thus the lowest expected Ioff, has not been investigated. Here we measure, for the first time, record-low Ioff in CVD-grown monolayer WS2 transistors. We first grow such layers directly on amorphous SiO2/Si substrates by CVD utilizing a perlyene-3,4,9,10 tetracarboxylic acid tetrapotassium salt (PTAS) seeding molecule alongside solid sulfur and WO3 precursors [5]. Material quality was characterized using Raman spectroscopy and photoluminescence (PL), confirming monolayer uniformity. We note that the PL (optical) band gap of monolayer WS2 is ~1.9 eV but its electronic band gap (on SiO2) is ~2.4 eV [6], due to the large exciton binding energy in 2D semiconductors. Back-gated field-effect transistors (FETs) were fabricated using electron-beam lithography with channel lengths down to L ? 100 nm. Au contacts were used and devices were capped with ~15 nm Al2O3 by atomic layer deposition (ALD) to improve stability and hysteresis [7]. In order to avoid the noise floor of our instrumentation when measuring the true off-state leakage, ultra-wide FETs were fabricated with an interdigitated structure resulting in widths up to W = 2275 ?m. These ultra-wide FETs enabled our measurement of Ioff down to ~10 aA/?m at drain bias of VDS = 0.1 V in a device with channel length L = 250 nm. At higher drain bias, we measured Ioff as low as 15 aA/?m in a FET with L = 750 nm at VDS = 1 V, i.e. an average lateral field > 1 V/?m. In both devices, on/off current ratios were >1010, demonstrating viability for memory and power gating applications which require ultra-low leakage transistors. In comparison, silicon transistors have off-state leakage more than three orders of magnitude higher [8]. In summary, we demonstrated the first measurements of "true" off-state leakage current in monolayer WS2 transistors, reaching ~10 aA/?m with on/off current ratio ~1010. These represent the lowest currents measured to date in any monolayer semiconductor, particularly at lateral fields > 1 V/?m, and suggest significant advantages over silicon in ultra-low leakage applications such DRAM and power gating. [1] C. English et al., Nano Lett. 16, 3824 (2016) [2] J. Kang et al., IEDM Tech. Dig., 32.2.1 (2017) [3] C. Kshirsagar et al., ACS Nano, 10, 8457 (2016) [4] C. Bailey et al., MRS-EMC, 125 (2019) [5] K. McCreary et al., Sci. Rep., 6, 19159 (2016) [6] H. Hill et al., Nano Lett. 16, 4831 (2016) [7] Y. Illarionov et al., IEEE-EDL, 38, 1763 (2017) [8] J. Song et al., ISDRS (2009)

Despite the great progress of ferroelectric gated field-effect transistors (Fe-FETs) based on graphene and other 2D materials, a device model that accurately describes the hysteretic transfer characteristics and provides guidelines on performance enhancement of the Fe-FET is still lacking. Here, we present an experimentally validated analytical model that couples charge displacement of the ferroelectric layer with the charge transport in the graphene layer. The model describes hysteretic transfer characteristics of the FeFETs with good accuracy and predicts that the on/off ratio of the graphene Fe-FET is determined by Dirac bias and the charge carrier mobility. We show unsuitability of an ideal graphene layer for memory application and outline the conditions to achieve the best memory performance in graphene Fe-FETs. Using the insight gained from the model, we theoretically and experimentally demonstrate ternary and quaternary graphene memory elements. The model is generic and can be equally well used for Fe-FETs based on other 2D materials.

Two-dimensional transition metal dichalcogenides (TMDs) are fertile ground for fundamental material science and emergent applications in high-performance electronics. The mechanical flexibility of 2D materials allows their incorporation in variable form factor designs such as smart wearables and foldable electronics, where their electronic and optical properties outperform conventional flexible materials such as organic polymers. In the current work, we present high-performance ALD grown MoS2 based FET devices transferred on polyimide substrates. The growth method employs ALD to grow an MoO3 layer on a 6 inch wafer which is subsequently annealed in an H2S atmosphere to convert to MoS2. The ALD allows for excellent uniformity and because the layer acts as a template the process is scalable to larger sizes. The resulting MoS2 film is 2-3 layers thick. The film was transferred on the patterned target substrate using polystyrene assisted transfer of MoS2 layer. More than 90% device yield, excellent current ON to OFF ratio of 106 and mobility values up to 32 cm2 V-1 s-1 have been measured from micron scale devices with L/W in the range of two. We will present results on the yield and homogeneity on large area memory networks aiming for low power neuromorphic circuits.

The natural crystal anisotropy of van der Waals materials allows them to support a range of nanophotonic phenomena not observable in other natural materials. One such example is anisotropic phonon polaritons (PhPs), which form due to polar optic phonons in materials such as hexagonal boron nitride. Such hyperbolic polaritons have been shown to enable sub-diffraction guiding of light waves, sensing of ultra-thin analytes, and hyperlensing for super resolution imaging in the mid infrared (IR). Recently, the biaxial van der Waals semiconductor α-phase molybdenum trioxide (α-MoO₃) has received significant attention due to its ability to support PhPs which are anisotropic along all three crystal axes. These offer an unprecedented platform for controlling the flow of energy at the nanoscale, in particular for manipulating the polarization state of light. However, to fully exploit the extraordinary IR response of this material, an accurate dielectric function is required.

In this work, we report an accurate IR dielectric function of α-MoO₃ by modelling far-field, polarized IR reflectance spectra acquired on a single thick flake of this material. Unique to our work, the far-field model is refined using the experimental dispersion and damping of PhPs revealed by using scattering-type scanning near-field optical microscopy (s-SNOM) on thin flakes of α-MoO3. Through these correlative efforts, exceptional quantitative agreement is attained to both far- and near-field properties for multiple flakes, thus providing strong verification of the accuracy of our model. This also provides a novel approach to extracting dielectric functions of different 2D materials, usually too small or inhomogeneous for establishing accurate models only from standard far-field methods. Finally, by employing density functional theory (DFT), we provide insights into the various vibrational states dictating our dielectric function model and the intriguing optical properties of α-MoO₃.

Two-dimensional materials have been identified as interesting candidates for various applications in nano-electronics, and the characterization of electrical breakdown limits is a crucial step in device development. However, methods for repeatable measurements have been scarce in two-dimensional materials, where breakdown studies have been limited to destructive methods. This lack of repeatability restricts our ability to fully account for variability in local electronic properties induced by the fabrication process and surface contaminants.

To tackle this, we implement a two-step deep-learning model to predict the breakdown mechanism and breakdown voltage of monolayer MoS₂ devices with varying channel lengths and resistances using current measured in the low-voltage regime as inputs. A deep neural network (DNN) first classifies between Joule and avalanche breakdown mechanisms with partial current traces from 0–20V. Following this, a convolutional long short-term memory network (CLSTM) predicts breakdown voltages of these classified devices based on user-defined partial current traces.

We test our model with feedback-controlled electrical measurements to achieve non-destructive electrical measurements that limit device damage and increase the amount of data collected. We show that the DNN classifier achieves an accuracy of 74% while the CLSTM model has a 14% error when requiring only 80% of the current trace as inputs. Our results indicate that information encoded in the current behavior far from the breakdown point can be used for predictions, thus enabling non-destructive and fast material characterization for 2D material device development.

Atomically thin two-dimensional (2D) materials such as graphene, MoS2, WS2 show excellent material properties and therefore studied extensively for a variety of applications including electronics, optoelectronics, electrocatalysis, energy storage, etc. More recently, there has been a growing interest in 2D heterostructure devices as they offer unique opportunities to design and study possibilities for controlling and manipulating the carrier confinement and transport of charge within these device structures. Tungsten disulfide (WS2) is an excellent material for optoelectronic applications such as photodetectors as monolayer WS2 has a direct bandgap of ~ 2.05 eV. Also, recently emerged as MXenes has shown great promise in energy storage and electromagnetic shielding applications due to excellent metallic conductivity. The MXenes can show either metallic or semiconducting behavior depending on the functional groups attached to the surface during the etching process. In particular, their tunable work function property can be exploited to control the properties of the heterostructure of MXenes with other layered 2D materials. Here, we report the characterization results of one such heterostructure formed by combining MXenes with WS2. MXene is synthesized by etching the Al layer from the parent MAX phase, and WS2 is synthesized on SiO2/Si substrates using the CVD process. The electrical properties of these as-synthesized materials can be modified using different approaches such as chemical doping etc. We present results from different morphological, spectroscopic and electrical characterization techniques that are used to study the transport characteristics of these MXene-WS2 heterostructure devices.

The dominating mechanism of photodetection in a single-layer graphene (SLG) based device depends on the design of the detector [1,2,3], its chosen operation [1,3], and the band diagram in the photo-active region [2,4]. Here we demonstrate, over a ?0.5eV doping range, that the photo-thermoelectric effect [5] has a long-range (exceeding the cooling length in SLG) contribution, even in uniformly doped SLG, in the absence of a Seebeck coefficient gradient, whenever non-uniform absorption creates an electronic temperature gradient. This is achieved by integrating SLG with on-chip Si waveguides to generate an heterogeneous absorption profile over tens ?m and placing electrical probes along the propagation direction of light. We show the presence of a photovoltage Vph along the waveguide. The doping-dependence of Vph suggests signal generation by hot electrons via the Seebeck effect. This has general implications for the design and optimization of optoelectronic devices based on SLG, in particular for integrated photonics, where planar integration of layered materials leads to non-uniform absorption. [1] F. H. L. Koppens et al., Nat. Nanotechnol., 9, 780, (2014). [2] T. Echtermeyer et al., Nano Lett., 7, 3733, (2014) [3] M. Romagnoli et al., Nat. Rev. Mater., 3, 392, (2018). [4] V. Shautsova et al., Nat. Comm., 9, 5190, (2018). [5] N. Gabor et al., Science, 334, 648, (2011).

Transition metal dichalcogenide (TMD) semiconductors are layered van der Walls materials that exhibit exceptional optoelectronic properties in monolayer form. Their atomically thin nature and reduced long-range dielectric screening make them ideal systems in which to study many-body electronic states. Notably, excitonic many-body phenomena govern the light-matter interactions in TMD semiconductors and open up new possibilities for novel optoelectronic devices, including new classes of quantum light sources. Thus, understanding the rich suite of many-body phenomena in these systems is motivated by potential applications in on-chip optoelectronics and by the insights that can be gained into the fundamental interactions that govern TMD material properties. Here, the dynamics of several higher-order excitons including trions, biexcitons, and charged biexcitons in monolayer-WSe2 are probed using temperature-, energy-, and power-dependent time-resolved optical spectroscopy. The properties of these multiexciton states build on the unique many-body physics of the single-excitons in TMDs.These studies reveal a complex interplay between the multiexcitons and single-excitons that depends on both the density and excitation energy of the initial exciton population. In addition, the presence of defect-bound excitons is found to drastically alter the formation of multiexcitons. This competition between exciton trapping and multiexciton formation highlights the need for high-quality materials to enhance multiexciton physics. By exploring this parameter space of excitation energy, excitation density, and defect density, complex many-body interactions are revealed that suggest means to optimize multiexciton yields and thus capitalize on the emergent characteristics of multiexciton emission and exciton-multiexciton interactions that are unique to monolayer-WSe2. These observations open new doors to a number of applications such as entangle photon pair production and lasing, highlighting the importance of understanding the formation and relaxation dynamics of the rich manifold of excitons in order to leverage 2D semiconductors for advanced technologies.

The emergence of two dimensional (2D) materials opened up many potential avenues for novel device applications such as nanoelectronics, topological insulators, field effect transistors, microwave and terahertz photonics and many more. To date there are over 1,000 theoretically predicted 2D materials. Of those theoretical materials, only 55 have been experimentally synthesized due to the difficulty in finding a suitable substrate which simultaneously stabilizes the 2D phase over of bulk phase while preserving the novel materials properties resulting from quantum confinement. One avenue to aid in the development of these novel 2D materials is to explore and predict suitable substrates which will stabilize the 2D growth using computational methods. In this work a high-throughput approach to density functional theory is used to explore the viability of various common substrates to stabilize and grow these novel theoretically predicted 2D materials. This work analyzes three key factors which determine the stability of synthesizing 2D materials via substrate assisted methods such as chemical vapor deposition. The results will provide guidance to future experimental efforts to synthesize these 2D materials. Using various 2D materials databases and van der Waals corrected density functional theory we investigate the suitability of 12 substrates to stabilize 2D growth. First, the formation energy of the 2D material is computed relative to the most stable bulk phase. A 2D material with a high formation energy implies the material will likely only be synthesized via substrate assisted methods thus making it a good candidate material for this study. Computational synthesis can be broken down into a two-step process: the formation of the 2D flake from the bulk phase, the formation energy, and adsorption of the 2D material on a symmetry-matched and low lattice-matched substrate surface, the binding energy. When the adsorption energy, the difference between the formation from the binding energy, is less than zero the adsorption is spontaneous, exothermic, and stable. For materials which meet this criteria, the density of states is computed to characterize the electronic properties of these materials for device applications. 1. We gratefully acknowledge ASU's HPC staff for support and assistance with computing resources along with the Extreme Science and Engineering Discovery Environment (XSEDE), for support by National Science Foundation grant number ACI-1548562, through award number TG-DMR-150006. We gratefully acknowledge the National Science Foundation grant DMR-1308085 and Arizona State University for funding.

Black phosphorus (BP) has gained intense interests because of its tunable direct bandgap, leading to emission with photon energies from ~1.7eV for monolayer to ~0.35eV for bulk [1]. This makes few-layer BPs one of the very few 2D infrared materials, especially in communication wavelengths. Besides, BP has a strong anisotropy compared with other 2D materials due to its puckered honeycomb lattice. It leads to many interesting phenomena such as strong coupling between phonon and carriers [2]. Thus BP is an appealing 2D platform for investigating novel physics and device applications. However, few-layer BP degrades quickly in the ambient environment due to the interaction with oxygen under light excitation. Such instability causes great difficulty in investigating properties of few-layer BP and in exploring the practical applications. Although BP protection methods are reported with verified good electronic properties, there is no report on stabilized BP with enhanced photoluminescence (PL) under ambient environment. Here we report an approach to realize BP with greatly enhanced PL (by as much as 160 times) that can be stable under ambient environment for months. We adopt a three-step method to prepare BP covered by boron nitride (BN) from both sides. First, thin flakes of BP and BN are exfoliated onto PDMS layers, followed by successive transfers of the bottom BN and then BP onto SiO2 (270nm)/ Si substrate. Second, oxygen plasma is used to thin down BP to desired thickness or numbers of layers. The oxygen plasma also leaves an oxide layer on top of the thinned-down BP at the end of the process. Third, we transfer the top BN onto the oxide covered BP which is then followed by thermal annealing. This process leads to oxygen doped few-layer BP with BN binding tightly from both sides to protect BP from the environment, highly improving the stability of BP. Raman spectroscopy indicates the sandwiched BP is defective. The breathing mode shows broadening and redshift. Ag1 and Ag2 modes both split into two subpeaks. These are the signs of introduction of oxygen defects in BP. PL of oxygen doped BP shows two-order of magnitude enhancement compared with freshly etched BP, which can be explained by bound exciton luminescence. Few-layer BP produced this way shows remarkable stability. The PL line-shape and intensity of sandwiched BP are barely changed when kept in the ambient environment under room temperature. A 3-layer BP sample prepared by such method, 5 months after preparation, PL intensity is still ~80% of that measured as prepared. At an elevated temperature of 90 °C, oxygen can escape from BP, which will result in redshift of PL peak and decreased PL intensity. 7 months after preparation, the 3-layer BP sample still shows strong PL, at ~70% of the original PL intensity right before TEM measurement. TEM results show close contact between BP and BN and three BP layers can be clearly identified from side view. Quite low oxygen concentration in BP layers is consistent with the assumption that oxygen escapes from BP with time. A 4-layer BP with comparable intensity to that of the 3-layer sample made by this method shows the possible general validity of this approach. In summary, our method can produce highly stable BP with excellent PL intensity induced by oxygen defects, allowing the study of rich physics in defective BP. Furthermore, it can pave the way for research on BP-based photonic devices such as lasers, modulators, etc. [1] Li, Likai, et al. "Direct observation of the layer-dependent electronic structure in phosphorene." Nature nanotechnology 12.1 (2017): 21. [2] Pelant, Ivan, and Jan Valenta. Luminescence spectroscopy of semiconductors. Oxford University Press, 2012.

Dynamics of water is one kind of clean energy source which readily available in our surrounding environment. Recently, the direct conversion of liquid motion energy into electricity by two-dimensional (2D) materials such as graphene has gained much interest [1-3]. However, the reported output voltage values generated by liquid motion on the graphene surface are limited to millivolts order. With these output voltage values, the graphene nanogenerators might be difficult to drive conventional silicon-based electronics. Thus, an alternative to semimetallic graphene as an active material is needed to boost the output voltage. Here we report a single-layer MoS2 nanogenerator to generate electricity from liquid movement on its surface. The centimeter-scale single-layer MoS2 sheets were prepared by using chemical vapor deposition on sapphire substrate (3 × 1 cm) with MoO3 and sulfur powder as precursors. Then, the MoS2 was transferred onto polyethylene naphthalate (PEN) substrate. The device was finalized by applying silver epoxy as electrical contact on both ends of MoS2 and a silicone film as electrode protection from liquid. To generate electricity, aqueous 1 M NaCl were dropped on the MoS2 surface on a stage inclined at 45°. Due to the high hydrophobicity of MoS2, the droplets easily slid on the MoS2 surface, generating an open-circuit voltage of ~6 V. This output voltage value is significantly higher than graphene nanogenerators previously reported [1-3]. The short-circuit current of ~5 nA was generated from liquid movement on the MoS2 surface. The power produced by droplet movement is about 1.7 nW at load resistance of 200 M?. We also show the high scalability of our MoS2 nanogenerator by arranging them in series and parallel connections. The open-circuit voltage and short-circuit current were multiplied about three times by connecting three identical MoS2 nanogenerators in series and parallel connection, respectively. In conclusion, we showed the large-area single-layer MoS2 nanogenerator, which is capable of generating output voltage more than 5 V from liquid motion. Seeing the exceptional mechanical and electrical property of single-layer MoS2, the realization of self-powering untethered wearable device based on 2D materials will soon become fulfilled. Reference: [1] J. Yin et al., Nat. Nanotechnol. 9, 378 (2014) [2] D. Park et al., Nano Energy 54, 66 (2018) [3] S. S. Kwak et al., ACS Nano 10, 7297 (2016)

Recently, there has been active interest in developing greenhouse gas sensors for understanding the influence of these gases in controlling the atmosphere and the weather (1,2). We wish to report here, a greenhouse gas sensor that has been developed using 2D graphene quantum dots (3) platform on alumina substrate containing platinum interdigitated electrodes on one side and heating resistors on the other side of the platform with an electroactive conducting polymer, as the 2D structure provides a large surface area for the active material making it more sensitive for detection and measurements. The sensor operates in two stages; in the first stage the response of the interfering gases is measured and in the second stage a thermal pulse is applied corresponding to the desorption energies of the interfering gases. The temperature on the active material was measured using an infrared thermometer. The electroactive conducting polymer was electrochemically synthesized at a graphene quantum dots electrode by a potential step electrolysis at 1.00 V vs SCE. The experiments conducted with graphene quantum dots dispersed in the electrolytic medium, shows the monomer well coupled with graphene prior to the polymerization. The polymer coupled graphene shows D and G bands arising from graphene in Raman imaging spectroscopy. The mechanistic pathways has been unraveled by Raman imaging, Fourier transform infra-red spectroscopy, UV-VIS absorption spectroscopy, and scanning electron microscope. The results suggest that 2D platform for the active material for sensing enhances the performance of the sensor with a temperature-controlled selectivity. 1. D. Shukman, Greenland's ice faces melting 'death sentence', https://www.bbc.com/news/science-environment-49483580 2. K.S.V. Santhanam and N. Ahamed, Chemical Engineering (MDPI), 2,38 (2018) 3. K.S.V. Santhanam, S. Kandlikar, M. Valentina and Y. Yang, US patent No. 9840782, December 12, 2017

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as a promising transistor channel material because of their inherent atomic thinness, which can allow further scaling of channel length. In order to miniaturize transistors in future technology nodes, not only the channel length, but also the contact length (Lc) needs to be scaled as Lc constitutes a significant portion of the device's integrated pitch/density. Researchers have suggested the use of edge contacts to enable maximum scalability; in this configuration, the channel is contacted via its side edges along the same lateral plane as the contacts. Since the injection area is practically independent of the actual Lc, the performance of edge contacts can be Lc-independent [1], thus allowing much better scalability compared to traditional top contacts. In addition, edge contacts are hypothesized to form covalent bonds with the TMD channel, which should translate to better carrier injection. Published work on edge-contacted TMD transistors has predominantly focused on MoS2 as the channel material. Since chemically dissimilar TMDs interact distinctly with different metal contacts, further research on other promising TMDs, such as MoTe2 and WSe2 is warranted. In this work, we created clean edge contacts to TMD channels utilizing our previously established method [1] using an in-situ Ar+ ion beam in an integrated electron beam evaporation chamber. We discovered that the etching behavior of the TMDs varied, and thus the realization of clean edge contacts required custom fabrication processes. The role of contact metal is elucidated for each TMD, along with analysis of top- versus edge-contacted devices on the same TMD flake or domain. These results provide key insights on how metal selection and channel thickness influence the device performance for various edge-contacted TMD transistors. References [1] Cheng, Zhihui, et al. "Immunity to Contact Scaling in MoS2 Transistors Using in Situ Edge Contacts." Nano letters 19.8 (2019): 5077-5085.

While graphitic intercalant compounds (GICs) have been thoroughly studied for application in battery technologies, use of these materials for transparent conductive electrodes (TCEs) has remained fairly unexplored. Although individual layers of 2D graphene promise high intrinsic mobility and transparency, its inherent properties do not rival those of current industry-standard materials. However, with lithium-ion intercalated multi-layer graphene grown via chemical vapor deposition (CVD), we produce large-scale LiC6 films whose electrical and optical properties are expected to surpass TCE requirements. With the developed sample design, we present preliminary data suggesting a path toward scalable and stable films for TCE application.

This presentation will describe the impacts of steric and electronic interactions of zwitterionic polymers in contact with two-dimensional (2D) materials. Modulation of the work function of 2D materials facilitates charge injection and improves device performance for several applications such as solar cells, electronic displays, and organic light emitting diodes. The work function of 2D materials has been tailored by gas absorption, mechanical strain, and chemical doping methods. Polymer coatings have also been applied to tune the work function of 2D materials via non-covalent adsorption, offering possibilities for designing novel hybrid organic-inorganic functional materials. However, little is known about the precise role of functional group orientation on work function modulation. Here, we select graphene as a model 2D system and systematically examine work function modulation arising from coatings of functional poly(sulfobetaine methacrylate) (PSBMA). This approach maintains the desirable structural and electronic properties of graphene while exclusively altering its surface potential via the polymer coating. Work function modulation was observed by ultraviolet photoelectron spectroscopy and Kelvin probe force microscopy. First-principles density functional theory calculations provided insights into the preferred configurations of zwitterionic SBMA pendent groups physically adsorbed on graphene sheet, resultant dipole moment, and possible range of work function shifts that are achievable. Overall, this method of work function engineering permits scalable patterning of polymer films and precise spatial control over carrier doping for fabrication of monolayer graphene-based devices.

Graphene, one of the most popular two-dimensional (2-D) materials, has been studied with high interests owing to its outstanding properties such as high thermal and electrical conductivity, easy surface decoration, and transparency with flexibility. These remarkable advantages led to the studies of graphene in varied applications including gas sensors. However, for various applications, graphene-based gas sensors have to solve various problems such as power consumption by external heaters, insufficient selectivity, and method of mass production. In this work, we report ethanol gas detection of nickel nanoparticles decorated on graphene layers which are self-activated by applied bias voltage from 1 to 5 V. The graphene is prepared by chemical vapor deposition (CVD) method and we designed a narrow graphene channel to generates heat by using a joule heating effect while working as a sensing area. Since graphene is a 2-D material, both active sensing area and electrodes composed of graphene could be easily defined by facile lithography processes. Nickel nanoparticles are deposited by the e-beam evaporator. The gas sensing properties to various gases such as NO2, H2, C2H5OH, and NH3 have been tested at room temperature while regulating the input bias voltages. Our results show superior ethanol gas detection of nickel nanoparticles decorated graphene sensors at room temperature. These results broaden the potential of graphene-based gas sensors for real applications.

Graphenic semiconductors (GSCs) are an emerging class of energy materials based on the sp2-hybridized graphene motif that either by themselves or in composites, have demonstrated excellent performance for the electrochemical reduction of CO2, the photoelectrochemical splitting of water to generate H2, and the photocatalytic reduction of CO2. GSCs have a range of astonishing properties such as high catalytic activity, strong visible light absorption, exceptional chemical stability, thermal stability until at least 350 deg. C, and facile compositional and electrical doping. GSCs are typically synthesized by low cost solvothermal and annealing processes that lend themselves to scale-up and mass production. The most prominent member of this family of compounds is g-C3N4, which has a relatively wide bandgap of 2.6 eV. Herein, we report on the synthesis, characterization and solar energy applications of a number of new GSCs [1-5]. C3N5 is a novel modified carbon nitride framework with a remarkable 3:5 C:N stoichiometry and an electronic bandgap of 1.76 eV which was formed by thermal deammoniation of a precursor consisting of melem hydrazine [1]. The two s-heptazine units in the polymeric C3N5 are bridged together with an azo linkage, which constitutes an unconventional bonding arrangement, different from g-C3N4 where three heptazine units are linked together with tertiary nitrogen. C3N5 exhibits extended conjugation due to overlap of azo nitrogens and increased electron density [1]. C3N5 was found to be an excellent electron transport layer for perovskite solar cells, and a good photocatalyst for the photoreduction of 4-nitrobenzenethiol into 4,4?-dimercaptoazobenzene. Even lower bandgap analogs of C3N5 were synthesized and characterized. We also formed highly luminescent quantum dots of g-C3N4 doped with P, F or Cl. Intimate heterojunctions of doped g-C3N4 quantum dots with BiOI and TiO2 were demonstrated to be excellent photoanodes for visible light-driven water splitting, achieving photocurrent densities as high 2.5 mA cm-2 under AM1.5G simulated sunlight [2-3]. REFERENCES 1. P Kumar, E Vahidzadeh, UK Thakur, P Kar, KM Alam, A Goswami, N Mahdi, K Cui, GM Bernard, VK Michaelis and K Shankar, "C3N5: A Low Bandgap Semiconductor Containing an Azo-Linked Carbon Nitride Framework for Photocatalytic, Photovoltaic and Adsorbent Applications", Journal of the American Chemical Society, 2019, 141, 13, 5415-5436. 2. P Kumar et al. "Noble Metal Free, Visible Light Driven Photocatalysis Using TiO2 Nanotube Arrays Sensitized by P-doped C3N4 Quantum Dots", Advanced Optical Materials (in press), 2019. 3. KM Alam, P. Kumar, P Kar, A Goswami, UK Thakur, S Zeng, E Vahidzadeh, K Cui and K Shankar, "Heterojunctions of halogen-doped carbon nitride nanosheets and BiOI for sunlight-driven water-splitting", Nanotechnology, 2019, doi: 10.1088/1361-6528/ab4e2c. 4. C Ott et al. "Flexible and Ultrasoft Inorganic 1D Semiconductor and Heterostructure Systems Based on SnIP", Advanced Functional Materials, 2019, 29, 1900233. 5. KM Alam, P Kumar, AP Manuel, E Vahidzadeh, A Goswami, S Zeng, W Wu, N Mahdi, K Cui, AE Kobryn, S Gusarov, Y Song and K Shankar, "CVD grown nitrogen doped graphene is an exceptional visible-light driven photocatalyst for surface catalytic reactions", 2D Materials, 2019, 7, 015002.

A significant improvement in the power conversion efficiency (PCE) and the environmental stability of n-Graphene/p-Si solar cells is indicated through effective n-doping of graphene, using low work function oxide capping layers. AlOx, deposited through atomic layer deposition, is particularly effective for such doping and in addition serves as an antireflection coating and a cell encapsulating layer. It is shown that the related charge transfer doping and interfacial engineering was crucial to achieve a record PCE of 12.5%. The work indicates a path forward, through work function engineering, for further efficiency gains in Gr-based solar cells

X-rays have been used for decades in medical diagnostic imaging, but the presence of metal scattering interference centers can limit their effectiveness. Longer wavelength sources may mitigate this effect, but risk poor imaging from low x-ray fluence. To produce a sufficient density of low-power x-rays to allow imaging of targets with metal inclusions, the x-ray current source would require a proportionate increase in output. Graphene nanoribbons crafted by CVD[1] or arc discharge[2] have been shown to exhibit electron emission under bias, with theoretical outputs exceeding thermionic or field emission sources of similar dimensionality and applied bias conditions. Epitaxial graphene, able to be synthesized on the wafer-scale and compatible with current fabrication techniques, can be readily processed into ribbons with superior mechanical and electrical capabilities to its CVD or arc discharge counterparts. With even low applied bias (3 - 5 V) graphene nanoribbons (0.5 - 1 um long, 10-30 nm wide) suspended in high vacuum exhibited phonon-assisted electron emission (PAEE)[1][2], resulting in emission currents being produced at internal electric fields and external pulling voltages both nearly 10^2 less than comparable thermionic or field emission sources. Emissions exhibited repeatable directionality out of the plane of the ribbons, offering a means to mechanically control the orientation of the output current; ribbons crafted from epitaxial graphene, with deposited electrodes to provide the bias, would emit out of the plane of the substrate, enabling even macro-scale directional control. Fabrication of epitaxial graphene microribbons is readily accomplished via a combination of electron beam lithography and plasma etching, while relevant metal depositions are completed with electron beam evaporation. Patterned-photoresist masks allow for precise plasma etching to isolate the ribbons and electrodes in order to craft emission arrays of arbitrary numbers, dimensions, and positions, facilitating arrangements for rapid testing of device parameters and capabilities. Metals deposited on plasma-etched surfaces allow for external ball- and wedge-bonding of gold wire, resulting in test setup and device controls being significantly cheaper, faster, easier, and more robust than those presented in referenced works. Emission currents can be measured via a straightforward setup with the devices (cathodes) and a phosphor screen (anode) positioned in a vacuum chamber with feed-through contacts for control and measurement. Results of captured currents from various dimensions, arrangements, and applied biases of the graphene microribbon emitters and arrays are provided in both absolute terms, as well as normalized to more conventional dimensions and energizing voltages of comparable thermionic and field emission electron sources. By crafting microribbon emitters in various combinations of dimensions and arrangements, and isolating control circuits from each other, it should even be possible to craft a single physical device that could alternately function as multiple different sources, either independently or in concert as need dictates. Electron sources crafted in this fashion are expected to exhibit greater output, durability, flexibility, and cost savings compared to other electron sources of comparable dimensionality and power requirements. Presented here is a treatment of the preparation, fabrication, and testing of these epitaxial graphene microribbon electron emission arrays intended as sources for low-power x-ray imaging applications. 1. Wei, X. L.; Golberg, D.; Chen, Q.; Bando, Y.; Peng, L. M.; Phonon-Assisted Electron Emission from Individual Carbon Nanotubes. Nano Lett. 2011, 11, 734-739. 2. Wei, X.; Bando, Y.; Golberg, D.; Electron Emission from Individual Graphene Nanoribbons Driven by Internal Electric Field. ACS Nano, 2012, 6, 705.

The in-plane acoustic phonon scattering in graphene is solved by considering fully inelastic acoustic phonon scatterings in two-dimensional (2D) Dirac materials for large range of temperature (T ) and chemical potential (?). Rigorous analytical solutions and symmetry properties of Fermionic and Bosonic functions are obtained. We illustrate how doping alters the temperature dependence of acoustic phonon scattering rates. It is shown that the quasi-elastic and ansatz equations previously derived for acoustic phonon scatterings in graphene are limiting cases of the inelastic-scattering equations derived here. For heavily-doped graphene, we found that the high-T behavior of resistivity is better described by ? (T, ?) ~T [1 - ?a ?2/3( kB T )2] rather than a linear T behavior, and in the low T regime we found ?-1 ~ (kBT )4 but with a different prefactor (i.e. ~ 3 times smaller) in comparison with the existing quasi-elastic expressions. Furthermore, we found a simple analytic "semi-inelastic" expression of the form ?-1 ~ ( kBT )4 /(1 + cT 3) which matches nearly perfectly with the full inelastic results for any temperature up to 500 K and ? up to 1 eV. Our simple analytic results agree well with previous first-principles studies and available experimental data. Our analyses pave a way for investigating scatterings between electrons and other fundamental excitations with linear dispersion relation in 2D Dirac material-based heterostructures such as bogolon-mediated electron scattering in graphene-based hybrid Bose-Fermi systems.

Semiconducting transition metal dichalcogenides (TMDs) represent a novel and sustainable material class for ultrathin optoelectronic devices. Although various approaches towards light emitting devices have been reported, e.g. based on exfoliated or chemical vapor deposited TMD monolayers, they all suffer from limited scalability required for practical applications. Recently, a first scalable approach has been suggested combining WS2 monolayers grown in a commercial horizontal multi-wafer AIXTRON metal organic chemical vapor deposition (MOCVD) reactor on sapphire (0001) substrates [1] with a vertical p-n device concept [2]. With this, large-scale light emitting devices based on TMDs as active material, organic injection layers on the anode side and an inorganic supporting layer on the cathode side have been demonstrated.[3] One unique property of this ultra-thin semiconductor family is their flexiblility. Taking advantage of this, we realized for the first time a large-scale WS2 monolayer based light emitting device on a flexible substrate. Our devices are fabricated in a vertical p-n architecture on ITO covered PEN foil, with PEDOT:PSS / poly-TPD hole injection layers on the anode side and a ZnO nanoparticle layer as an electron supporting layer on the cathode side. By applying an external voltage, those devices show diode-like behavior in their I-V characteristic and pronounced red electroluminescence purely originating from the WS2 monolayer. Light emission starts at a turn-on voltage below 3 V from an emissive area of 6 mm2, which is just limited by the size of the cathode contact. By bending the device, an energy shift of the electroluminescence with a gauge factor of 30 meV/% is observed, caused by the strain-induced shift of the bandgap. These novel large-scale and flexible LEDs with a TMD emissive monolayer and an emission energy depending on the bending radius opens a new field of potential applications for flexible optoelectronics. References: [1] A. Grundmann et al., MRS Adv. 4, 593 (2019) [2] D. Andrzejewski et al., Nanoscale 11, 8372 (2019) [3] D. Andrzejewski et al., ACS Photonics 6, 1832 (2019)

Organic field effect transistors (OFETs) are the major building blocks of the next generation flexible electronics. From logic gates to health monitoring sensors, different flexible electronics have been developed based on the OFETs. In this talk, I will focus on one of the most promising OFET fabrication methods which has excellent compatibility with large area deposition, the meniscus guided coating (MGC). Four elementary processing parameters (i) shearing speed (v), (ii) solute concentration, (iii) deposition temperature (T) and, (iv) solvent boiling point (Tb) are utilized to analyze crystal growth behavior in the meniscus-guided coating. By carefully varying and studying these four key factors, we can confirm that v is the thickness regulation factor while c is proportional to crystal growth rate. The MGC crystal growth rate is also correlated to latent heat (L) of solvents and deposition temperature in an Arrhenius form. The latent heat of solvents is proportional to Tb. I will also provide a generalized formula to estimate the effects of these fabrication parameters which can serve as the crystal growth guidelines for the MGC approach. It is also an important cornerstone towards scaling up the OFETs for the sophisticated organic circuits or mass production. Furthermore, I will also discuss how to use the ultra-low speed version of MGC to develop 2D monolayer organic crystal and long range in-plane ordering. By using this method, we can obtain 2,9-didecyldinaphtho[2,3-b:2?,3?-f]thieno[3,2-b]thiophene (C10-DNTT) monolayer with single-crystalline domain size ranging from millimeters to centimeters. Intrinsic field-effect mobility around 12 cm2V-1s-1 is achieved. More importantly, the ultrathin thickness minimizes the charge injection barrier for the top-contact electrodes, allowing an ohmic hole injection and a low contact resistance (RcW) of 40±15 ohm-cm, one of the lowest values reported for the n-shape organic semiconductors. The detailed analysis of this ohmic metal/organic contact will be also be discussed.

The integration of single layer graphene (SLG) with on-chip photonic circuitry may offer improved performance and new functionalities for active components in telecom modules [1]. Graphene based photodetectors (GPDs) promise high-speed [1,2], broad-band operation [3,4] and the direct generation of a photovoltage [1,2,4,5], potentially removing the need of using noise-prone trans-impedance amplifiers (TIAs) [1]. Here we present a micrometer scale, on-chip integrated, plasmonic enhanced photo-thermoelectric [6] GPD for telecom wavelengths operating at zero dark current [1,5]. The GPD is designed and optimized to directly generate a photovoltage and has an external responsivity ? 12.2 V/W with a 3dB bandwidth ? 42 GHz. We utilize Au split-gates with a ? 100 nm gap to electrostatically create a p-n-junction and simultaneously guide a surface plasmon polariton gap-mode. This increases light-graphene interaction and optical absorption and results in an increased electronic temperature and steeper temperature gradient across the channel. Implemented with scalable CVD SLG, our work paves the way to compact, on-chip integrated, power-efficient GPDs for receivers in tele/datacom. [1] M. Romagnoli et al., Nat. Rev. Mater., 3, 392, (2018). [2] S. Schuler et al., Nano Lett.,16, 7107, (2016) [3] F. Bonaccorso et al., Nat. Photonics, 4, 611, (2010). [4] F. H. L. Koppens et al., Nat. Nanotechnol., 9, 780, (2014). [5] J. E. Muench et al., arxiv: 1905.04639, (2019) [6] N. Gabor et al., Science, 334, 648, (2011).

Perovskite Chalcogenides are a new class of semiconductors, which have tunable band gap in the visible to infrared part of the electromagnetic spectrum. Besides this band gap tunability, they offer a unique opportunity to realize large density of states semiconductors with high carrier mobility. In this talk, I will discuss some of the advances made both in my research group and in the research community on the theory, synthesis of these materials and understanding their optoelectronic properties. The basic building block of the perovskite structure is the octahedral cage and the nature of its connectivity gives rise to both the perovskite and related structures with varying dimensionality. I will discuss how low dimensional connectivity can influence areas spanning optoelectronics to thermal transport. First, I will show unusual band gap evolution in two dimensional Ruddlesden Popper perovskites; Second, I will discuss giant optical anisotropy in one dimensional perovskite-like phases, which also show ultralow thermal conductivity. These examples show unique opportunities to explore novel dimensional controlled physical properties in bulk materials. Finally, I will provide a general outlook for future studies on these exciting new class of materials. References: Nature Photonics, 12, 392-396 (2018). Advanced Materials 29, 1604733 (2017). Chemistry of Materials, 30 (15), 4897-4901 (2018). Chemistry of Materials, 30 (15), 4882-4886 (2018).

The insulator/2D channel interface property is highly detrimental for the performance of 2D field effect transistors. Although severe interface degradation has been reported so far, a common understanding of the origin of the interface states has not been obtained yet. Here, for the first time, we present the full energy spectra of interface states density for both n- and p- MoS2 FETs, as shown below, based on the comprehensive and systematic studies, i.e., wide thickness range from monolayer to bulk and various gate stack structures including 2D heterostructure with h-BN as well as typical high-k top gate structure (more than 100 devices). For n-MoS2, the lowest interface states density (Dit = 5×1011 cm-2eV-1) was achieved for the heterostructure FET with h-BN, while the interface states density remained high, ~1013 cm-2eV-1, even for the heterostructure FET for p-MoS2. The systematic study elucidates that the strain induced externally through the substrate surface roughness and high-k deposition process is the origin for the interface degradation at the conduction band side, while sulfur-vacancy-induced defect-states dominate the interface degradation at the valance band side. The present understanding on the interface properties provides the key to further improving the performance of 2D FETs. Finally, based on the recent understanding, the perspective on 2D electronics will be discussed.

Herein, we report on a facile one-pot synthesis of polyoxometalates encapsulated zeolite imidazolate framework cages (PMo₁₀V₂@ZIF-67). The morphological and structural properties of the as-synthesized materials were investigated via FESEM, EDS, XRD and FTIR techniques. Moreover, N₂ adsorption/desorption isotherms, and XPS measurements were carried out to elucidate the textural properties and composition of the fabricated materials. Upon their use as supercapacitor electrodes, the PMo₁₀V₂@ZIF-67 electrode exhibited a maximum specific capacitance of 765 F/g at a scan rate of 5 mV/s. Furthermore, a supercapacitor device was assembled using activated carbon as the negative electrode and PMo₁₀V₂@ZIF-67 as the positive electrode, delivering a specific power of 702 W/kg and corresponding specific energy of 20.9 Wh/kg at a charging current density of 1 A/g. In addition, the device shows excellent long term stability and high Columbic efficiency over 5000 charging and discharging cycles at charging and discharging current density of 5 A/g.

The intensive implementation of Li-ion batteries in many markets makes it increasingly urgent to address the recycling of strategic materials from spent batteries. Batteries typically contain toxic chemicals and cannot be disposed of at will. In this study, Li-Ni-Mn-Co hydroxides are successfully recycled from spent Li-ion batteries electrodeposited on nickel foam and fully characterized using different techniques such as field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDXS), inductively coupled plasma (ICP), and X-ray photoelectron spectroscopy (XPS) techniques. The recycled nanostructured films are tested in a three-electrode electrochemical system to investigate their capacitance behavior. The recycled electrodes show a high capacitance of 951 F.g^-1 (Specific capacity of 523.5 C. g^-1) at 1 A g^-1. Moreover, recycled materials are used as positive electrodes to construct asymmetric supercapacitor devices. The device shows a Coulombic efficiency of 100%, capacitance retention reaching ∼90% with excellent cycling stability after 10,000 cycles as well as reasonable power and energy densities.

The application of metal oxides has been increasing for energy storage device due to excellent electrochemical properties, multifunctional energy application, environmentally friendly property and lower price. Due to rapid growth of energy use, there is very high demand for new ways to store energy. It is challenged to develop efficient way to store energy without polluting environment, which is possible by using transition metal oxide. We have prepared Fe_xCo_3-xO₄ (x=0.0, 0.2,0.4,0.6,0.8,1) successfully via hydrothermal method from which we can prepared electrode to make supercapacitor device. The structural properties were investigated by X-ray diffraction (XRD) and found phase pure Fe_xCo_3-xO₄. The morphology of the series obtained from scanning electron microscopy (SEM) varies from thinner nano-plates to thicker nano-plates as the concentration of Fe increase from x=0.0 to x=1.0. Furthermore, Fe_xCo_3-xO₄ were characterized by X-ray Photoelectron spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR) and Quantachrome Surface area analyzer. The maximum BET surface area obtained from Quantachrome Instrument for x=1 is 87.5 m2/g. The XPS gives elemental composition of Fe_xCo_3-xO₄ as in desired amount. The higher specific capacitance of 268 F/g at a current density of 1 A/g, 856 F/g at 2 mV/s scan rate, the energy density of 16 Wh/kg and power density of 170 W/kg in 3M KOH electrolyte was observed for x =1 sample. An increased retention capacity ∼122 % measured at 5 A/g current density and Coulombic efficiency of 100%. All these results indicate Fe dope Co₃O₄ composites electrode shows promising electrocatalytic activity results in the high intrinsic electronic conductivity, can largely improve the interfacial electroactive sites, increase charge transfer rates and help to stabilize the structure of compound.

We report on the correlation of the microstructural evolution and electrochemical properties of an ex-situ N-doping KOH activated carbon derived from Arachis Hypogea Shell (NAC-AHS) and compared to the non-doped (AC-AHS). Interconnected cavities with irregular-shaped structure are reported by SEM analysis for AC-AHS. After nitrogen doping, the morphology of the NAC-AHS materials exhibited a less porous structure as compared to AC-AHS. The surface specific area (SSA) of NAC-AHS was found to be 1442 m² g^-1 lower than the value of AC-AHS (1557 m² g^-1), the pore volumes were also found to follow the same trend since the reported values are as follow 0.76 cm³ g^-1 and 0.69 cm³ g^-1, respectively for AC-AHS and NAC-AHS. The AC-AHS and NAC-AHS exhibited in a half cell a specific capacitance (C_s) values of 167 F g^-1 and 216 F g^-1, respectively at 1 A g^-1, corresponding to an improvement of 30% after ex-situ N-doping, which agrees with the higher current response recorded in the CV profiles of the samples. A symmetric device fabricated from the NAC-AHS materials displayed a Cs of 251.2 F g^-1 at 1 A g^-1 in a wide operating voltage of 2.0 V in 2.5 M KNO₃ aqueous electrolyte. In terms of stability, it achieved 83.2% as capacity retention up to 20,000 cycles, which was, improved through the floating/ aging measurement at 5 A g^-1over a long period of time (∼7+ days). A specific energy of 35 Wh kg^-1 with a corresponding specific power of 1 kW kg^-1 at 1 A g^-1 was delivered with the device still retaining up to 22 Wh kg^-1 and a 20 kW kg^-1 specific power even at 20 A g^-1.

Activated coconut charcoal is unique in meso and micro pore distribution, BET surface area exceeding 2000 m²g^-1 and pore volume ~ 1.3 cm³g^-1. It is the bio-derived carbon material of choice for supercapacitor electrodes, cryogenic gas absorption in vacuum technology and water purification. Coconut shell carbon is also the hardest biochar - hardness 3-4 in the Mohs scale. For supercapacitors, batteries and several other applications, high electrical conductivity is an advantageous parameter. Owing to large surface to volume ratio of the material, the carrier trapping sites on the surface play a crucial role in electrical transport. The DC electrical conductivity is found be greatly influenced by mode of activation, surface treatments and impurities. The most studies on electrical conductivity of activated carbons were based on measurements conducted using compacted pellets. The higher hardness of coconut shell charcoal enable use of sizable samples made from uncrushed flakes to carry out chemical treatments and conduct measurements under varying conditions. The results of measurements and interpretations will be presented, emphasizing supercapacitor applications of coconut shell charcoal.

Electrochemical energy storage (EES) units are vital for mobile electronic devices that can provide high energy and power densities. Current studies mostly aim at improving the performance of the electrode active materials. Nevertheless, inert components such as electrolyte, separator and current collector occupy 70-80 wt.% of the EES devices and reducing their weight/volume can significantly improve the performance of these devices. Dielectric materials are used extensively in EES devices as separators such as ceramics in conventional capacitors. In supercapacitors, polypropylene membranes and glassy fiber films with typical thicknesses larger than 25 μm are the common separator materials. Two dimensional (2D) dielectric materials with flat and smooth surfaces at a much smaller thickness can in fact be used as efficient separators.
In this work, 2D hexagonal boron nitride (h-BN) thin films were utilized as efficient separator layers in asymmetric MXene supercapacitors. In these devices, titanium carbide (Ti₃C₂) and manganese dioxide (MnO₂) were used as the negative and positive electrodes, respectively. Electrochemical tests such as cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy were conducted to determine specific capacity, rate capability and cycling stability of the asymmetric supercapacitor devices. Fabricated supercapacitor devices showed promising electrochemical performance in a PVA/H₂SO₄ gel electrolyte compared to the control device with conventional Celgard separator. The reduced thickness of the h-BN separator decreased the volume of an entire supercapacitor device and significantly improved the overall capacitance.

Bioinspired ion transport membranes have been widely investigated for energy storage applications. High theoretical specific energy density (2600Wh/kg) and high specific capacity (1675mA/g) along with natural abundance and low toxicity of sulfur have been attracting significant attention for development of an alternative battery system to replace traditional lithium ion batteries which suffer from safety and capacity/energy density limitations in various applications. However, challenges such as polysulfide dissolution and shuttling prevent mass commercialization of metal sulfur batteries. Inspired from biological ion transport mechanisms, we show a practical yet comprehensive approach for development of highperformance metal sulfur batteries. Aramid nanofiber (ANF) based composite ion transport membranes not
only prevent dendrite formation but also confine polysulfides on the cathode side. ANF composite battery separators provide diverse and opposing properties including high mechanical properties, high ionic conductivity and high thermal/chemical stability. Highly selective ion sieving properties of these biomimetic separators provide safe and high-performance batteries. Fabrication of such biocompatible, affordable,
flexible and high energy density battery is quite crucial in powering next-generation electronics including but not limited to portable, wearable and implantable biomedical devices.

Rechargeable Mg batteries present a “beyond lithium-ion” technology with high theoretical energy density and a comparatively small susceptibility for dendrite formation^1,2. However, rechargeable Mg batteries suffer from slow migration and irreversible trapping of Mg ions in most cathode materials.

The MXene family is a group of electronically conductive 2D materials which have been investigated as electrode material for both Mg and Li batteries. Here, DFT is used to assess the voltage, specific capacity and migration barriers for Li and Mg ions in MXenes with compositions V₂CO_2, V₂CF₂, V₂C(OH)₂, Ti₃C₂O₂, Ti₃C₂F₂ and Ti₃C₂(OH)₂ as reported in our recent work³. Bader charge analysis and ionization energies are used to understand the differences in performance. Guidelines for MXene electrode design and views on how to best overcome the fundamental challenges with divalent battery chemistries are presented.


References
1. M. Matsui, Journal of Power Sources 196 (2011), 7048-7055
2. P. Canepa, G. S. Gautam, D. C. Hannah, R. Malik, M. Liu, K. G. Gallagher, K. A. Persson, G. Ceder, Chemical Reviews 117 (2017) 4287-4341
3. H. Kaland, J. Hadler-Jacobsen, F. H. Fagerli, N. P. Wagner, Z. Wang, S. M. Selbach, F. Vullum-Bruer, K. Wiik, S. K. Schnell, Sustainable Energy Fuels, 2020, 4, 2956-2966

Dendrite formation on Li metal anodes leads to reduced Coulombic efficiency, poor cycling stability and short-circuiting, and prevents commercialization of high energy density rechargeable Li metal batteries. The migration energy barrier for transport along the metal surface can describe the tendency of the surface to form dendrites. Density functional theory calculations of the migration energy barrier show inherent differences in transport properties between Li and Mg¹, which may explain the higher propensity of Li to form dendrites compared to Mg². We use density functional theory to show that differences in the atomic and electronic structure of the Li and Mg surface lead to different migration energy barriers³. The atomic and electronic surface structure of Li and Mg are in turn affected by the bulk crystal structure, the atomic and electronic structure of the substrate, and surface impurities. We investigate how and why these factors impact the migration energy barrier to shed light on the mechanisms of dendrite formation on the metal anodes. Thus, we may guide the development of dendrite-free metal anodes for battery applications.

References:
1. M. Jäckle, A. Groß, J. Chem. Phys. 141 (2014) 174710
2. M. Matsui, K. Takahasi, K. Sakamoto, A. Hirano, Y. Takeda, O. Yamamoto, N. Imanishi, Dalton trans. 43 (2014) 1019
3. I. T. Roee, S. M. Selbach, S. K. Schnell, J. Phys. Chem. Lett. 11 (2020) 2891-2895

Rechargeable Mg batteries have the potential to take advantage of Mg metal anodes, as they are less prone to dendritic growth compared to Li metal [1]. This leads to the possibility of having high capacity batteries based on Mg-ions, with competitive energy density and materials cost compared to the state-of-the art Li-ion batteries [2]. However, this requires a cathode material that allows for reversible reactions with significant amounts of Mg-ions at a sufficiently high operating potential. Since the first prototype of a rechargeable Mg battery was reported in 2000 [3], only a limited number of cathode materials have been demonstrated to work reversibly, and the energy density has not been sufficient to compete with Li-ion battery technology [2]. In this project, we have investigated the possibility of using two-dimensional MXenes as possible cathode materials for rechargeable Mg batteries, based on their previously reported ion intercalating properties [4,5]. Here, we present the synthesis and surface modification of two MXenes, Ti₃C₂T_x and V₂CT_x (T = F, O or OH), in order to optimize the possibility for Mg-intercalation by increasing the O terminations [6]. Various methods, such as hydrolysation and vacuum annealing, were used to shift the surface terminations towards more oxygen content, where EDX and XPS were used to quantify the different termination groups. The surface modified MXenes demonstrate more promising electrochemical performance compared to the pristine MXene, which is assigned to the improved properties of the O-terminated surfaces.


References:
[1] M. Masaki, Journal of Power Sources 196 (2011), 7048-7055
[2] P. Canepa et al., Chemical Reviews 117 (2017) 4287-4341
[3] D. Aurbach et al., Nature 407 (2000) 724-727
[4] Naguib et al., J. Am. Chem. Soc. 135 (2013) 15966-15969
[5] C. Eames, M. S. Islam, J. Am. Chem. Soc. 136 (2015) 16270-16276
[6] H. Kaland, J. Hadler-Jacobsen, F. H. Fagerli, et al., Sustainable Energy Fuels, 4 (2020) 2956-2966

Secondary batteries are crucial to contemporary life, powering everything from personal electronics to electric vehicles. The growth of these technologies has led to an unprecedented demand for rechargeable batteries, a market currently dominated by Li-ion batteries. Despite its high performance, Li-ion is an unideal candidate for meeting this growing demand due to the scarcity of its component elements, resulting in higher costs and an inflexible supply chain. Ca-ion batteries would avoid these issues while providing higher volumetric energy density and the potential for improved safety through suppressed dendrite growth. However, their development is limited by a lack of suitable electrolytes. Many electrolytes decompose to form an impermeable layer at the anode, preventing plating and stripping and compromising the cyclability of the battery. Our work aims to elucidate the reactions occurring at the electrolyte-anode interface, which will allow for more informed Ca-ion battery electrolyte selection.

As a starting point for this analysis, we use ab initio molecular dynamics (AIMD) to simulate several anode-electrolyte systems. Motivated by recent successes in reversible Ca-ion plating and stripping, tetrahydrofuran (THF) is selected as one of the solvents. We contrast THF with ethylene carbonate (EC), a successful Li-ion solvent and popular choice for Ca-ion battery experiments. In addition to pure solvent, we also investigate 1.2 M solutions of four different salts (Ca(BH₄)₂; Ca(BF₄)₂; Ca(BCl₄)₂; and Ca(ClO₄)₂) in each solvent for a total of ten AIMD trajectories. Over a 15 ps trajectory at 450 K in the presence of a Ca anode, pure THF was found to be non-reactive, while pure EC decomposed soon upon contact with the anode. Preliminary results have shown that similar behavior persists upon addition of salt molecules. We supplement these trajectories with density functional theory (DFT) calculations of electronic structure and reaction barriers as well as charge analysis over time. Lastly, we include a final simulation to test the reactivity of THF while gradually adding electrons to the Ca-THF system, reflecting the changes in anode charge during battery cycling. Our multifaceted characterization will both illuminate past experiments’ results as well as guide new research, shedding light on a major obstacle in the development of Ca-ion batteries.

The lithium-metal anode is the main factor constraining the cyclability of energy-dense lithium-sulfur batteries. The irreversible plating and stripping of lithium in the liquid ether-based electrolyte engenders poor cycling efficiencies and severe parasitic side reactions with the electrolyte. Under conditions of limited lithium inventory and limited electrolyte supply, which are necessary for practically relevant cell designs, this leads to rapid capacity fade and premature cell failure. Commercially viable Li-S batteries with high energy density and long cycle life can only be realized by designing strategies towards stabilizing lithium deposition and reducing electrolyte decomposition.

The generation of soluble polysulfide intermediates that shuttle between the sulfur cathode and the lithium-metal anode renders the chemistry of Li-S batteries fundamentally unique compared to other systems. They transform the chemistry of the lithium-electrolyte interface by reducing on the lithium surface and forming lithium sulfide (Li₂S) and lithium disulfide (Li₂S₂) as components of the SEI layer. These species have been observed to have a unique stabilizing effect on lithium deposition, which is intrinsic to Li-S batteries. This suggests that tuning polysulfide species with other elements or functional groups could be a potential pathway towards further stabilization of lithium deposition in this system.

We discovered that elemental tellurium (Te⁰) reacts with ethereal solutions of polysulfides to form mixed polytellurosulfide species that are also soluble in the ether-based solvent. On the reducing surface of lithium metal, these polytellurosulfide species decomposed to form red-colored lithium thiotellurate (Li₂TeS₃). As a lithium SEI component, Li₂TeS₃ is anticipated to enable a homogenous lithium-ion flux on the lithium surface, thereby leading to uniform plating of lithium. This is due to the lower diffusion barrier for lithium ions through Li₂TeS₃ when compared with Li₂S, as the electron density around sulfur is significantly softened when bonded with tellurium.

When Te⁰ was added to sulfur/Li₂S cathodes in a 1:10 molar ratio, polytellurosulfide species were generated in-situ during cell operation by the reaction of polysulfides with Te⁰ additive. These species crossed over to the lithium anode and formed Li₂TeS₃ as a thin interfacial layer on the lithium surface. The favorable properties of this tellurized, sulfide-rich SEI allows lithium to be deposited without undesirable high-surface area mossy/dendritic growths. With a limited lithium inventory (N/P ratio = 1) in an anode-free full cell, which combines a fully-lithiated Li₂S cathode with a bare Ni foil current collector on the anode side, the enhancement in lithium cycling efficiency with the addition of Te⁰ drastically reduced capacity fade per cycle by a factor of 7.

It was also found using time-of-flight secondary ion mass spectroscopy (ToF-SIMS) that electrolyte decomposition on the lithium surface was significantly mitigated with the addition of tellurium to the cathode. This enabled the limited electrolyte supply to be maintained for a longer number of cycles. Under lean-electrolyte conditions (E/S ratio = 4.5 µl mg^-1) in large-area pouch cells employing a high-loading sulfur cathode (5.2 mg cm^-2), tellurium-stabilized lithium deposition allowed the cell to be stably operated for nearly 100 cycles. This was a nearly four-fold improvement over the control cell with no additive.

A unique and reliable strategy for stabilizing lithium deposition and reducing electrolyte decomposition in Li-S batteries is demonstrated. This is achieved by a simple and scalable addition of elemental tellurium to the cathode. The stabilizing layer is formed in-situ during cell operation and no expensive or complicated pre-treatments of the lithium surface are required. Importantly, a pathway for commercially viable Li-S batteries with high energy density and long cycle life is realized.

Lithium-sulfur batteries hold promising potential for next-generation high-energy-density energy storage. One of their major technical problems is the loss of sulfur active material and its significant volume change during the charge-discharge process, resulting in rapid capacity fading. Here, we introduce sulfur-inserted polymer-anchored edge exfoliated graphite as a new class of positive electrode, which can accommodate the conflicting requirement of physically restrain sulfur dissolution while maintaining structural flexibility to cope with the volume expansion.
The introduction of sulfur between the flexible polymer-anchored graphene layer is achieved by a simple chemical reaction at ambient temperature. The obtained sulfur-carbon composite demonstrates superior sulfur efficiency and cyclability compare to mesoporous carbon-based counterparts. The strong interfacial attraction between sulfur and highly-conductive graphene sheet at the confined interlayer space enables (1) rapid charge transfer and (2) effective inhibition of the polysulfide dissolution, resulting in improved redox reaction reversibility and sulfur efficiency. Furthermore, the structural flexibility of layered structure, derived from polymer-anchor, guarantees the stable cycling by accommodating the significant volume expansion of sulfur active materials.
Therefore, our work provides a simple, proof-of-concept strategy for improving the overall performance of carbon-based positive electrode for lithium-sulfur batteries.

Introduciton
Lithium-Sulfur (Li-S) batteries are attracting attention as high energy density rechargeable battery. Elemental sulfur (S₈) have high theoretical capacity (1,672 mAhg^-1) as positive electrode active material. However, S₈ electrode is degraded by redox shuttle effect of lithium-polysulfide (Li₂S_n), which are generated during charge-discharge reactions. To solve this problem, many electrolyte designs were proposed. As one of many designs, sulfolane (SL) based high Li salt concentration electrolyte solution was reported as low solubility electrolyte of Li₂S_n [1]. The aim of this concept is chemically protection of Li₂S_n dissolution into electrolyte solution. Therefore, we proposed new polymer electrolyte, which protects chemically and physically dissolution and diffusion of Li₂S_n. In this study, we prepared SL based solid gel polymer electrolytes (GPEs), and thermal and ionic conductive properties of the samples were evaluated. Furthermore, Li-S batteries containing GPEs were prepared and evaluated by charge-discharge measurements.

Experimental
Sulfolane (SL), LiN(SO₂CF₃)₂ (LiTFSA), and polyether-based P(EO/PO) macromonomer were used as electrolyte materials for GPEs. First, electrolyte solution samples were prepared by mixing LiTFSA and SL at each molar ratio of SL : LiTFSA = x : 1 (x=1, 1.5 and 2, respectively). Next, prepared electrolyte solution and P(EO/PO) macromonomer were mixed at each weight ratio of electrolyte solution : polymer = y : (10-y) (y=7, 8 and 9, respectively), and added DMPA as photo initiator. After that, these materials were radical polymerized by UV irradiation. In this research, GPEs are expressed as (x),yEl+(10-y)Po. Thermal property and ionic conductivity of prepared GPEs were investigated by TG-DTA, DSC and AC impedance measurements.

Results & Discussion
First, in TG-DTA, TG curves of all GPEs were exhibited 3 step weight trends of decreasing. First weight decreasing observed the range of 380 K to 490 K, which might due to evaporation or decomposition of SL. From first weight decreasing, thermal stability was improved with Li salt concentration and electrolyte solution amount. Therefore, a part of Li⁺ should be changed coordination site from SL to P(EO/PO).
Next, from impedance measurements, obtained ionic conductivity (σ) showed non-Arrhenius type temperature dependence with all GPEs. All GPEs exhibited high σ compared with genuine solid polymer electrolyte (mixture of P(EO/PO) and LiTFSA) within a range of 244 K to 353 K. (2),9El+1Po sample observed relatively high σ (4.0×10^-4 Scm^-1) at 303 K. σ increased with decrease of Li salt concentration (inverse of x). Also, σ increased with electrolyte solution composition (y) at low temperature region. Therefore, ionic transport mechanism and thermal property should be affected by Li salt concentration and electrolyte solution amount into GPE.
In presentation, charge-discharge properties of Li-S batteries containing GPE will be also discussed.


Reference
[1] A. Nakanishi et al., J. Phys. Chem. C, 123, 14229-14238 (2019).

Electrically rechargeable zinc–air may achieve higher rechargeable specific energy than Li-ion batteries, but today’s air cathodes limit zinc–air performance. To overcome this challenge, researchers have investigated hundreds of air-cathode variations. Unfortunately, the efficacy of these variations remains ambiguous due to non-standardized cycling protocols. We show using a meta-analysis of 100 zinc–air cells that reported areal-energy values span five orders of magnitude. Based on an analytical model we present here, researchers should cycle zinc–air cells at 35 mWh cm_geo^–2 to compete with lithium-ion and 240 mWh cm_geo^–2 to compete with lithium–air. Once the community cycles zinc–air cells at relevant areal-energy values and understands failure mechanisms, lab-scale results will translate to practical advancements.

Presently, nitrogen-doped carbon nanotube (NCNT) has been widely considered as one of the promising carbon materials replacing the platinum group metal catalysts in energy conversion and storage technologies. It has been essentially concentrated on rational design of NTCNT to overcome the generalized limitations of hydrophobic surface and base-growth model for the enhancement of catalytic performance in renewable energy devices such as zinc-air battery (ZAB). Here, the we demonstrated a concept of an apically dominant mechanism controlled NCNT, encapsulating CoNi nanoparticles (CoNi@NCNT) as highly efficient catalysts for ZAB. Density functional theory calculations reveals that the outstanding catalytic performance of CoNi@NCNT can be ascribed to the synergetic contributions from the apical active sites nearby CoNi and NCNT. Remarkably, the ZAB using CoNi@NCNT as air electrode exhibits an excellent battery performance with a power density of 127 mW cm^-2 and energy density of 845 Wh kg_zn^-1.

Lithium-air batteries are an active area of research because of their potential to have a much higher energy density than traditional lithium-ion batteries. However, they are not yet commercially viable due to poor efficiency, high charging voltages, and low cycle lifetimes. Many of these issues could be addressed with a deeper fundamental understanding of the atomistic behavior of these batteries. One tool to model such atomic scale behavior is ab initio molecular dynamics (AIMD) simulations. However, AIMD simulations are limited to timescales of tens of picoseconds due to their high computational cost. As a result, equilibration and sampling methodologies can have a significant effect on the behavior of AIMD simulations. We thus compared two equilibration methods for AIMD simulations of systems of common solvents and salts found in lithium air batteries: (1) using an AIMD temperature ramp and (2) using a classical MD simulation followed by a short AIMD simulation all at the target simulation temperature of 300 K. We also compared two different classical all-atom force fields (PCFF+ and OPLS) and performed multiple simulations for each system. In this presentation, I will discuss how the differences between our simulation results and experimental results for properties such as coordination number illustrate the importance of both equilibration method and independent sampling for extracting experimentally relevant quantities from AIMD simulations.

Rechargeable alkaline Zn–MnO₂ batteries are a promising candidate for grid-level energy storage owing to their high theoretical energy density rivaling Li-ion systems (~400 Wh/L), safe aqueous electrolyte, established supply chain, and projected costs as low as $50/kWh at scale.[1] Unfortunately, due to inherent issues at both electrodes, the goal of a Zn–MnO₂ battery with long lifetime (5000+ cycles, equating to 10-15 years of service life) at appreciable energy densities (200+ Wh/L) has not yet been achieved. Over repeated cycling at high capacity utilization, Zn anodes suffer from irreversible redistribution of active material and passivation, while MnO₂ cathodes suffer from irreversible breakdown of the crystal structure and formation of inactive phases.

To mitigate anode degradation, we recently showed that pre-saturating the electrolyte with zincate [Zn(OH)₄]^2-, which is the soluble discharge product of the anode, improved cycle life of Zn–Ni cells by 100-200% at 14-35% Zn utilization.[2] However, this strategy alone cannot be used with MnO₂ cathodes because zincate reacts with Mn(III) species to irreversibly form ZnMn₂O₄, which is insulating and electrochemically inactive. Conventional cellophane and polyolefin separators used in alkaline batteries have no ability to block zincate from migrating to the cathode, whereas ceramic conductors such as NaSICON, which we previously studied in limited-depth-of-discharge Zn–MnO₂ batteries,[3] are impervious to zincate but are thick, brittle, and less conductive overall, limiting their utility.

To address this issue, this presentation will discuss various aspects of our efforts to develop flexible polymeric separators that selectively impede zincate transport without sacrificing overall conductivity. We have synthesized freestanding membranes of polymers incorporating charged functional groups, as well as thin films of ion-selective polymers on commercial separator materials. Using a rapid anodic stripping voltammetry assay,[4] our separator materials show a strong selectivity for hydroxide over zincate transport, while maintaining comparable ionic conductivity to commercial battery separators. Most importantly, our selective separators impart significant performance improvements to both Zn–Ni cells and Zn–MnO₂ cells with Bi/Cu-stabilized cathodes[5] that can be cycled at or near the full two-electron capacity of MnO₂ (617 mAh g^-1). Notably, Zn–Ni cells show a 460+% and 115+% increase in cycle life at substantial Zn utilizations of 20% and 50% respectively. In Zn–MnO₂ cells, our separators not only slow down zincate poisoning of MnO₂ but also hinder growth of zinc through the separator pores, which can lead to shorting.

This work was supported by the U.S. Department of Energy, Office of Electricity, and the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

[1] S. Banerjee, “Alkaline Zn-MnO₂ Battery Development: A University-Private-Public Partnership”. U.S. Department of Energy Office of Electricity Peer Review, 2018. https://eesat.sandia.gov/wp-content/uploads/2018/10/1_SanjoyBanerjee_Presentation.pdf
[2] M. B. Lim, T. N. Lambert, E. I. Ruiz, J. Electrochem. Soc. 2020, 167, 060508.
[3] J. Duay, M. Kelly, T. N. Lambert, J. Power Sources 2018, 395, 430.
[4] J. Duay, T. N. Lambert, R. Aidun, Electroanalysis 2017, 29, 2261.
[5] G. G. Yadav, J. W. Gallaway, D. E. Turney, M. Nyce, J. Huang, X. Wei, S. Banerjee, Nat. Commun. 2017, 8, 14424.

Introduction
Lithium-sulfur (Li-S) batteries are attracted attention as new secondly battery system. The sulfur positive electrode of this system have theoretical capacity of 1672 mAhg^-1, which shows approximately 10 times higher than conventional Li-ion battery positive electrode material (LiCoO₂). A problem of the Li-S batteries is dissolution of sulfur component of positive electrode, which reached as state of reaction intermediate (Li₂S_m: m=2~8) into electrolyte during charge / discharge processes. The dissolution and diffusion of Li₂S_m into electrolyte solution lead significant degradation of cycle characteristics and irreversible side reaction of Li-S batteries. Currently, as functional electrolyte solution that suppresses dissolution of Li₂S_m, “Solvate Ionic Liquid (SIL)” has been proposed [1]. We propose further functional SIL electrolyte for high performance Li-S battery. In order to exhaustive suppress to dissolve/diffuse of Li₂S_m into electrolyte, and we investigated the increase of salt composition in the SIL electrolyte and the solidification by introducing polymer matrix.

Experimental
In this study, all procedures of sample preparation were conducted in an argon-filled glovebox. At first, as highly concentrated SILs, [Li_a(G4)₁]TFSA_a (a = 1 and 1.25) (G4 : CH₃-O-(C₂H₄O)₄-CH₃, LiTFSA: LiN(SO₂CF₃)₂ = 1 : a) were prepared [2]. Gel electrolyte were prepared by mixing of SILs (a = 1, 1.25), polyether-based P(EO/PO) macromonomer and photo initiator. Composition ratio of gel electrolyte were SILs : polymer = 7 : 3, 8 : 2, 9 : 1 (wt%), respectively. After stirring, samples casted to glass substrate and polymerized by UV irradiation to obtain thin-films, which was transparent and self-standing gel electrolyte films. Prepared electrolyte films ware evaluated by differential scanning calorimetry (DSC) and AC impedance measurements.

Result and discussion
Thermal properties of prepared SILs and gel electrolytes were measured by DSC. For the results, all electrolytes exhibited glass transition temperatures (T_gs). T_g of SIL electrolytes was shifted high temperature with Li salt concentration in electrolyte. Gel electrolyte showed lower T_g than that neat polymer electrolyte (without SIL). From the results, it was considered that mobility of PEO chain was increased by SIL electrolyte solutions. With increasing SIL amount into the gel electrolyte, T_g appeared low temperature side, and therefore, the gel electrolyte with high SIL composition should indicate high ionic conductivity.
Prepared SILs and gel electrolyte was measured dependence temperature of ionic conductivity (s) by AC impedance method. Owing to increase of viscosity with Li salt concentration, s value of high Li salt composition SIL electrolyte (a=1.25) was lower than equimolar SIL (a=1). At 303 K, s of [Li₁(G4)₁]TFSA₁ : polymer = 8 : 2 electrolyte was 1.5×10⁻³ Scm⁻¹, which was almost same s value with SIL electrolyte solution. Generally, the introduction of polymer matrix in electrolyte solution should suppress the diffusion of Li⁺. However, our result exhibited another result.
Gel electrolyte of [Li₁(G4)₁]TFSA : polymer = 8 : 2 was applied for charge/discharge test in Li-S cell. The Li-S battery using the equimolar SIL of gel electrolyte was confirmed first discharge capacity of 1,100 mAhg^-1. However, this system showed unstable voltage profiles during charge process, and this charge capacity exhibited larger than that discharging capacity. From the unstable and large voltage in this charging process, Li₂S_m dissolution into gel electrolyte and side reactions were estimated. This result correlates with the report of the co-migration of glyme molecule and breaking of Li complex cation with introduction of PEO matrix by NMR spectroscopy [3].

Reference
[1] K. Dokko et. al., J. Electrochem. Soc., 160, A1304 (2013).
[2] K. Takahashi et.al., RSC Adv., 9, 24922 (2019).
[3] R. Kido, et.al., Electrochem. Acta, 170, 5 (2015).

The solid-electrolyte interphase (SEI) formed at the negative electrode in Li-ion batteries (LIB) is a passivation layer that prevents recurrent electrolyte decomposition reactions at the electrode-electrolyte interface.¹ The consumption of the charge carrier inventory due to these irreversible reactions are the origin of capacity fading and increasing internal resistances and ultimately reduce the cycle life of a cell.
For more than three decades the SEI layer of LIB has been characterized and optimized in order to arrive at the outstanding cycle life of high energy density, state-of-the-art LIB. A major contribution to its success is the graphite negative electrode that offers high reversibility, electrode potentials close to that of Li metal at a small volume expansion of the active material particles.² Post-Li systems might also benefit from the stability of graphite intercalation compounds and are therefore an interesting material choice for Na- (in ether-electrolytes)³ and K-batteries⁴ as well. However, their performance has not yet reached that of a corresponding Li-cell.⁵

Li- and K-systems are highly comparable because, despite the alkali metal ion, the same electrode and electrolyte formulations can be employed. A comparison between the graphite-electrolyte interface formed in a Li- and K-system, respectively, can thus allow valuable insights on how to work towards a more benign SEI layer formation in K-batteries. For this study, the surface compositions of graphite electrodes cycled in Li- or K-half cells was investigated post-mortem by in-house X-ray photoelectron spectroscopy (XPS). Our results show a comparatively high degree of crosstalk originating from the K-metal counter electrode and a different distributions of the characteristic decomposition products in presence of K- instead of Li-ions. The presentation will further discuss strategies to mitigate charge losses related to SEI layer formation on graphite electrodes as negative electrodes in K-ion batteries.

References
(1) Verma, P.; Maire, P.; Novák, P. A Review of the Features and Analyses of the Solid Electrolyte Interphase in Li-Ion Batteries. Electrochim. Acta 2010, 55 (22), 6332–6341.
(2) Berg, E. J.; Villevieille, C.; Streich, D.; Trabesinger, S.; Novák, P. Rechargeable Batteries: Grasping for the Limits of Chemistry. J. Electrochem. Soc. 2015, 162 (14), A2468–A2475.
(3) Jache, B.; Adelhelm, P. Use of Graphite as a Highly Reversible Electrode with Superior Cycle Life for Sodium-Ion Batteries by Making Use of Co-Intercalation Phenomena. Angew. Chem. Int. Ed. 2014, 53 (38), 10169–10173.
(4) Komaba, S.; Hasegawa, T.; Dahbi, M.; Kubota, K. Potassium Intercalation into Graphite to Realize High-Voltage/High-Power Potassium-Ion Batteries and Potassium-Ion Capacitors. Electrochem. Commun. 2015, 60, 172–175.
(5) Carboni, M.; Naylor, A. J.; Valvo, M.; Younesi, R. Unlocking High Capacities of Graphite Anodes for Potassium-Ion Batteries. RSC Adv. 2019, 9 (36), 21070–21074.

Ionogels are pseudo-solid state electrolytes in which an ionic liquid electrolyte confined in a mesoporous inorganic matrix leads to a material that possesses the electrochemical, thermal and chemical properties of the ionic liquid despite being a macroscopic solid. Because of their unique architecture, ionogels maintain a nanoscale fluidic state and thus mitigate the interfacial resistances which commonly arise at solid-solid interfaces. The use of sol-gel synthesis enables the materials to be prepared as liquids and to penetrate porous electrodes before becoming solid, an especially beneficial property for solid-state batteries. The incorporation of Li-ion conducting ionogel electrolytes into Li-ion batteries has been reported previously and generally show behavior comparable to that of the ionic liquid electrolyte.

In this work, we report on the development of Na-ion conducting ionogels, a topic which has received only limited investigation to date. We use sol-gel synthesis to form a siloxane-modified silica scaffold that traps an ionic liquid electrolyte comprised of 0.5M NaFSI in 1-Butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide [PYR14][TFSI]. The low vapor pressure of the ionic liquid ensures that a dense gel is formed. The interconnected porosity allows for good ionic conductivity (0.7 mS cm^–1), although it is somewhat less than the corresponding ionic liquid electrolyte. The activation energy for conduction of 0.3eV is, as expected, higher than that of Li-ion conducting ionogels. The material possesses a 5V electrochemical window and is thermally stable up to 150°C. Using the positive Na-ion electrode material, Na₃V₂(PO₄)₃, the charge transfer properties of the ionogel/NVP interface were characterized and found to exhibit low interfacial impedance. This led us to demonstrate the operation of a Na-ion solid state battery which contained a NaTi₂(PO₄)₃ negative electrode in conjunction with the NVP. The results shown here underscore the versatility of ionogels as a thermally stable electrolyte with designed ion transport properties for use in future solid-state electrochemical devices

Rechargeable aluminum metal batteries have seen a resurgence of interest as an emerging energy storage technology. Aluminum (Al) is low cost, inherently safe, and the most abundant metal in the earth’s crust. Despite these opportunities, technological development of rechargeable aluminum batteries has been hindered due to lack of cathode materials that permit reversible transport of aluminum ions, mainly due to high Coulombic charge on aluminum cations rendering solid-state diffusion of aluminum ions difficult into crystalline materials.
The chevrel phases Mo₆X₈ (X= S, Se) are among the few materials known to electrochemically intercalate multitude of multivalent ions, including trivalent aluminum cations. Thus, understanding the ion intercalation and charge transfer mechanism within this unique structure may provide insights into the design of new intercalation electrodes for rechargeable aluminum-ion batteries. Here, electrochemical and multi-nuclear solid-state nuclear magnetic resonance (NMR) measurements of aluminum-ion intercalation into the chevrel Mo₆S₈ are correlated and quantitatively analyzed, revealing new insights into the ion intercalation mechanism. Solid-state ²⁷Al NMR measurements reveal aluminum ions in two different environments associated with distinct cavities in the chevrel framework. Quantitative analyses of their relative populations establish that they reversibly (de)intercalate into the cavities simultaneously, as opposed to sequentially, upon discharge (charge).
In addition, for Mo₆Se₈ electrodes, we perform ex-situ ⁷⁷Se and ⁹⁵Mo Hahn-echo NMR measurements along with single-pulse ²⁷Al NMR at charged and discharged states to probe the crystalline frameworks to experimentally determine the charge transfer mechanism during the intercalation process. Replacing S with Se in the chevrel phase Mo₆X₈ (X=S, Se) enables use of ⁷⁷Se and ⁹⁵Mo NMR which reveal changes in the chemical shift of only the ⁷⁷Se NMR signal suggesting that the electrons prefer going to the Se atoms rather than the Mo atoms, unlike traditional transition metal oxide cathodes where the electrons are stored within the d-orbitals of the transition metal. Thus, for the first time, we demonstrate reversible electrochemical anionic redox as a charge storage mechanism for rechargeable batteries, which preserves the crystalline framework structure and does not involve the breaking and forming of chemical bonds.
Overall, the results yield quantitative molecular-level understanding of aluminum ion intercalation and charge transfer mechanism in a model crystalline transition metal electrode and establish solid-state ²⁷Al NMR as a powerful characterization tool for rechargeable aluminum-ion batteries.

Driven by the potential applications of ionic liquids (ILs) in many emerging electrochemical technologies, recent research efforts have been directed at understanding the complex ion ordering in these systems, to uncover novel energy storage mechanisms at IL/electrode interfaces. Here, we discover that surface-active ionic liquids (SAILs), which contain amphiphilic structures inducing self-assembly, exhibit enhanced charge storage performance at electrified surfaces. Unlike conventional non-amphiphilic ILs (NAILs), for which ion distribution is dominated by Coulombic interactions, SAILs exhibit significant and competing van der Waals interactions owing to the nonpolar surfactant tails, leading to unusual interfacial ion distributions. We reveal that at an intermediate degree of electrode polarization SAILs display optimal performance, because the low-charge-density alkyl tails are effectively excluded from the electrode surfaces, whereas the formation of nonpolar domains along the surface suppresses undesired overscreening effects. This work represents a crucial step towards understanding the unique interfacial behavior and electrochemical properties of amphiphilic liquid systems showing long-range ordering, and offers insights into the design principles for high-energy-density electrolytes based on spontaneous self-assembly behavior. (Reference: Mao et al., Nature Materials 2019, 18, 1350–1357.)

Different from commercial liquid electrolyte batteries, where multiple interfaces can react under no mechanical constriction, the close contact between electrolyte particles and between electrolyte and electrode particles enforces a local mechanical constriction inside these particles as well as on their interfaces. Our work shows how such mechanical constriction effect can modulate battery performances at the material particle, interface coating and device levels. Design principles for advanced solid state batteries based on various types of solid electrolytes and advanced electrodes will be discussed, including sulfide, halide, oxide electrolytes, high voltage cathodes and the lithium metal anode.

All-state-state lithium batteries (ASSLBs) have gained worldwide attention because of intrinsic safety and increased energy density. Compared with other types of solid-state electrolytes including oxide-based, polymer-based and sulfide-based electrolytes, recently-developed halide-based solid-state electrolytes (SSEs) have garnered considerable attention for all-solid-state lithium batteries (ASSLBs) due to the high ionic conductivity, high oxidation voltage and good stability toward oxide cathode materials [1]. However, there are still many challenges in halide-based solid-state electrolytes for ASSLBs including controllable and mass-production synthesis, achieving high humidity tolerance and demonstrate high-performance of ASSLBs; in particular, increased understanding of mechanisms during synthesis and tuning their properties of the electrolytes as well as interface with electrode materials[1].

In this talk, (i) I will demonstrate synthesis strategy [2-3], in particular, new and salable water-mediated synthesis method [2]. (ii) I will report a systematic study on the correlations among structural evolution, Li⁺ migration properties, and humidity stability resulting of the halide-based electrolytes, along with in-situ characterization for understanding of the mechanisms [4], (iii) Full cell battery performance will be optimized [5], (iv) humidity ability [6] and (v) other applications such as Li-O2 batteries [7]. In the end, energy densities of ASSLBs using different solid-state electrolytes in ASSLBs will be discussed.


References:
1. X. Li, J. Liang, X. Yang, K. Adair, C. Wang, F. Zhao, X. Sun. Progress and Perspectives of Halide-based Lithium Conductors for All-Solid-State Batteries. Energy Environ. Sci., 13, 1429-1461 (2020).
2. X. Li, J. Liang, X. Sun, et al., H₂O-Mediated Synthesis of Superionic Halide Solid Electrolyte. Angewandte Chemie International Edition, 58, 16427-16432 (2019).
3. X. Li, J. Liang, X. Sun, et al.,. Air-Stable Li3InCl6 Electrolyte with High Voltage Compatibility for All-Solid-State Batteries. Energy Environ. Sci., 12, 2665 - 267 (2019).
4. X. Li, J. Liang, X. Sun, et al.,. Origin of Superionic Halide Solid Electrolytes with High Humidity Tolerance, J. Am. Chem. Soc. 142, 7012-7022 (2020).
5. C. Wang, X. Li, J. Liang, X. Sun, et al., Eliminating Interfacial Resistance in All-Inorganic Batteries by In-situ Interfacial Growth of Halide-based Electrolyte. Nano Energy, 2020, in press.
6. X. Li, J. Liang, X. Sun, et al., Origin of Superionic Li₃Y_1-xIn_xCl₆ Halide Solid Electrolytes with High Humidity Tolerance, Nano Letters, in press, 2020.
7. Zhao, J. X. Sun, et al., Halide-based solid-state electrolyte as an interfacial modifier for high performance solid-state Li-O2 batteries. Nano Energy, 2020, in press

Sodium-ion and potassium-ion batteries have attracted attention in the search for cost-effective batteries with a minimum sacrifice on the performance. Similar to Li-ion batteries, performance of these batteries depends on mechanical integrity of the cathodes and interfacial stability associated with solid-electrolyte interface formation. However, larger alkali metal-ions are expected to cause greater volumetric changes and lattice distortions in the cathode structure. Even small changes in volume upon alkali metal-ion intercalation can cause particle fracturing in brittle cathode materials. Also, diffusion of these larger alkali metal ions is slower in the electrode matrix due to their larger ionic size, compared to the Li ions. Therefore, it is expected that Na-ion and K-ion cathodes experience more mechanical instabilities at faster scan rates due to diffusion-limited ion transport. Understanding the interplay between alkali-ion chemistry, structural stability and electrochemical performance of cathode materials is crucial to develop novel cathode structures, which are suitable for Na-ion and K-ion batteries.

In this study, we investigated electrochemical strain generation in composite iron phosphate (FePO₄) cathode during Li⁺, Na⁺ and K⁺ ion intercalation. Strains were monitored using in situ, optical, full-field digital image correlation technique. A host FePO₄ framework was initially formed by the electrochemical delithiation of the LiFePO₄ composite electrode in order to establish the identical electrodes in terms of active material loading in composite electrode, particle size, binder, and porosity of the composite electrode. FePO₄ composite electrodes were cycled at 1C, C/10 and C/25 in against Na and Li counter electrode, and K ions were only intercalated into FePO₄ at C/25 rate.

Striking differences in the physical response of FePO₄ were observed during the intercalation of three different alkali-metal ions. First of all, Li intercalation caused strain hysteresis at higher scan rates. Linear relationship between strains and capacity was observed during Na intercalation at any scan rate. However, the electrodes experience additional 0.05 and 0.15% more composite strains at C/10 and 1C rates compared to the electrode cycled at C/25 rate when the state of discharge was same during Na intercalation. An analytical model was developed to predict Na concentration gradients and misfit strain generation in the NFP particles. Second, only about 30 mAh/g discharge capacity was achieved at fourth cycle during the intercalation of potassium into FePO₄, whereas at the same scan rate, Li and Na intercalation delivered about 130 and 140 mAh/g, respectively. Despite of the size difference between Na and K ions, at the same discharge capacity, similar volumetric expansion was observed upon Na and K intercalation. Third, strain derivatives with respect to voltage were calculated using the finite difference method. Local minima in strain derivatives closely matched the current peaks during Li, Na and K insertion into the FePO₄ structure. Phase transformation during Li and Na intercalation, measured by ex-situ XRD, corresponds well with the current peaks observed during the cycling. Intercalation of K into FePO₄ was reported to cause amorphization in the crystalline FePO₄. Surprisingly, strain derivatives during K ion intercalation demonstrated distinct features, which aligned well with the electrochemical responses. Therefore, digital image correlation can be used to monitor electrochemical strains in other amorphous electrodes too. In conclusion, our results provide new insights about how Li, Na and K alkali ions influence the phase transitions and mechanical response of the iron phosphate cathodes.

Sulfide-based lithium (Li)-ion conductors represent one of the most popular solid electrolytes (SEs) for solid-state Li metal batteries (SSLMBs) with high safety. However, the practical application of sulfide-based SEs is significantly limited by their chemical instability in air and electrochemical instability with electrode materials (metallic Li anode and oxide cathodes). To address these difficulties, here, we design and demonstrate a novel sulfide-based composite electrolyte (SCE) through the combination of inorganic sulfide Li argyrodite (Li₇PS₆) with a polymer matrix. The developed SCE achieves excellent air-stability, impressive flexibility, and great chemical/electrochemical stability. Meanwhile, the presence of sulfide fillers facilitates the Li-ion transport in SCE, leading to a superior ionic conductivity at room temperature. Using the SCE enables with enhanced stability in Li symmetric cells and excellent suppression of Li dendrites. In addition, LiFePO₄ (LFP)/Li cells can deliver impressive specific capacity and excellent stability over long-term cycles. These features indicate this sulfide-based SCE as a promising candidate, and whose design will contribute to the practical development of SSLMBs.

Metal-organic frameworks (MOFs) are considered a novel type of crystalline porous structures. These structures are composed of inorganic vertices and organic linkers coupled through the coordination bonds. MOFs have distinct purposes such as high specific surface area, controllable pore size and good thermal stability that makes them good candidates to be used as porous templates.
In this work, the designed synthesis of NiCo₂O₄(Co₃O₄) double shell nanocages (DSNCs)from zeolitic imidazolate framework-67 is studied. The DSNCs are later embedded with graphene to enhance their conductivity and electrochemical performance. The strategy includes the synthesis of zeolite imidazolate framework-67(ZIF-67)/Ni-Co layer doubled hydroxide(LDH) precursor and subsequent transformation to NiCo₂O₄(Co₃O₄) DSNC by thermal annealing. The as-prepared DSNC structure is wrapped with graphene structure through a simple electrostatic self-assembly. Here, we focused on tuning the microstructure of NiCo₂O₄(Co₃O₄) DSNCs by controlling the crystal size of ZIF-67 and Ni-Co LDH formation while etching the initial MOF template.
Herein, we report NiCo₂O₄(Co₃O₄)@rGO composite structure with a capacity of 969 F.g^-1 at current density of 1 A.g^-1 showing a remarkable stability of showing 88% of initial capacitance after 10000 cycles of charge-discharge with a high mass loading of electrodes ( mg.cm^-2).
This work shows a universal process for controlled synthesis of different complex spinel structures that can be used for various applications.

Understanding and tailoring electrode-electrolyte interfaces (EEIs) is critical for enhancing the rate capability, cycling stability, and thermal safety of lithium ion batteries (LIBs). Although artificial coatings have been explored to tailor EEIs, no clear mechanisms have been identified that explain how these coatings engineer the EEIs of LIB electrodes. For the first time, we study Li⁺ kinetics in different polymer-based artificial coatings and at EEIs using a combination of experimental techniques (electrochemical impedance, neutron reflectometry and depth profiling) and density functional theory (DFT) calculations. Small binding energy with Li⁺ and the presence of sufficient binding sites for Li⁺ allow poly(3,4-ethylenedioxythiophene) (PEDOT) based artificial coatings to enable fast charging of LiCoO₂. Operando synchrotron X-ray diffraction experiments suggest that the superior Li⁺ transport property of the PEDOT coating further improves current homogeneity in the LiCoO₂ electrode during cycling, and, therefore, determines the electrochemical performance of the electrode. The LiCoO₂ 4.5 V cycle life tested at C/2 is increased by over 1700 % after applying the PEDOT coating. In comparison, other polymer coatings have negligible or negative effects on LiCoO₂ performance. Additionally, X-ray photoelectron spectroscopy and DFT calculations demonstrate that S, O present in PEDOT form chemical bonds with Co in LiCoO₂. These chemical bonds reduce Co dissolution and inhibit electrolyte decomposition, improving the cycling stability of LiCoO₂. These insights provide us practical design rules for polymer selection that will help shorten the research time and reduce the trial-and-error effort involved in tailoring EEIs for developing advanced batteries.

Hybrid supercapacitors, which exhibit the beneficial characteristics of both their battery (high energy density) and supercapacitor (high power density and durability) constituents, are in demand to meet current and future technological and infrastructural needs. While substantial efforts have already been made to modify the anode, commonly Li₄Ti₅O₁₂ (LTO), to increase rate performance, lesser attention has been placed on the activated carbon (AC) cathode, which has been reported as the energy density limiting electrode and is so prime for optimization. Using galvanostatic charge-discharge cycling and high-resolution imaging techniques, it was determined that systematically adding LiMn₂O₄ (LMO), a common battery cathode material, and Super P, a common conductive element, to the AC supercapacitor cathode changes the structure of the electrode and increases the energy density of a LTO//AC Li-ion capacitor without significantly impacting rate performance. Future work will help determine the energy storage kinetics of the composite cathode and how modifications to the LTO anode in tandem with the cathode may increase electrochemical capacity without affecting the capacitor’s typical high-power density and long-term cyclability.

Lithium (Li) dendrite growth poses serious challenges for the development of Li metal batteries. Replacing liquid electrolyte with solid composite electrolyte embedded with nanofiller additives can potentially suppress the Li dendrite growth. However the underlying mechanism is still not fully understood, and most theoretical works focus on pure liquid electrolyte and ignore the mechanical strain effects. Here we developed a phase-field model to simulate the Li dendrite growth by incorporating the microstructure of the solid composite electrolyte, and considering the mechanical effects of the electrolyte. Using aluminum oxide nanofiber embedded poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) as an example, we discovered two key factors, the elastic modulus and the electrolyte nanochannel width that govern the Li dendrite growth. The difference of the Young’s modulus between the Li metal and solid electrolyte acts as the additional mechanical driving force, which partially offsets the electrochemical driving force to either promote or inhibit the dendrite growth. We also discovered that the introduction of the 1D nanofiber arrays could confine the Li ion transport along vertical direction, reduce the concentration gradient across the metal/electrolyte interface, and inhibit the Li dendrite growth. Finally the dependence of overall Li ion conductivity on the nanofiller is discussed. Our work provides deep understanding and designing strategy for the solid composite electrolyte for improved Li anode stability and Li ion conductivity.

Solid electrolyte is the key component for all solid state batteries. Extensive research attentions are being attracted to the design and development of high performance solid electrolytes. An ideal solid electrolyte should simultaneously satisfy properties in multiple categories, including ionic and electronic conductivities, chemical stabilities against air and moisture, good mechanical properties, and good electrochemical stability and compatibility with both cathode and anode, etc. Cost and feasibility for scale production and battery fabrication are also important considerations.
Here we report our recent work on design and development of high performance novel solid electrolytes, including oxide, sulfide and halide compounds, and the comparison of their performances in batteries. These new materials are designed based on either computational prediction or empirical hypothesis. LiTaSiO5 series oxide materials show good ionic conductivity and outstanding stability. The synthesis and processing of this material will be discussed. Novel sulfide and halide solid electrolytes will also be reported. They all show very high room temperature ionic conductivities and good performance in the cycling test of all solid state batteries. High resolution synchrotron X-ray and neutron scattering techniques, solid state NMR and electron microscopy were complementarily used to reveal the structure-property relationships in these compounds. The impacts of synthesis condition, chemical composition and processing to the ionic conductivity and battery performances will be discussed.

The upcoming Na-ion battery system has advantages over the Li-ion battery system in terms of the more widespread availability and lower cost of Na-precursors. Among the potential cathode material classes, O3-type layered Na_xT_MO₂ (T_M => transition metal ion; x~1) is important due to the inherently high Na-storage capacity. However, the O3-type Na_xT_MO₂ suffers from instabilities caused due to multiple structural changes during Na-extraction/insertion, T_M-dissolution into electrolyte and, more importantly, inherent sensitivity to moisture. The moisture sensitivity necessitates the usage of binders that dissolve only in toxic non-aqueous solvents like N-Methyl-2-pyrrolidone (NMP) during electrode preparation. Against this backdrop, a carefully designed composition has been developed here to address the aforementioned problems, in particular, the air/water-instability. Among other things, Ti-ion substitution for Mn-ion (partially/completely) has led to excellent air/water-stability; resulting in no phase/structural change (from the original O3 structure) even after 40 days of exposure to air and 12 h of stirring in water. By contrast, under similar conditions, O’3 and P3 phases progressively evolve for the predominantly Mn-ion containing counterparts. The excellent stability of the optimized O3-type NaT_MO₂ enables the usage of environment/health friendly and economical ‘aqueous-binder’ (viz., Na-alginate) and water (as solvent) for electrode preparation. Furthermore, Ti-substitution also prevented the presence of Mn-ion in its +3 oxidation state; thus, addressing the Mn-dissolution problem and concomitantly supressing the increase in impedance, as well as voltage hysteresis, during electrochemical cycling. Overall, the ‘aqueous-processed’ cathode exhibits excellent long-term cyclic stability, with a capacity retention of ~56% after 750 cycles at C/5, including just ~4% capacity fade over the last 250 cycles investigated. Hence, this is a significant step towards the development of alkali metal-ion batteries having electrochemically stable electrodes being prepared in a health/environment friendly and cost-effective manner (by enabling the usage of water-based, as opposed to NMP-based, binder). A part of this work has been published as; Kumar et al., J. Mater. Chem. A (2020; DOI: 10.1039/D0TA05169A).
Keywords: Na-ion battery; layered transition metal oxide cathode; air/water-stability; aqueous processing; electrochemical behaviour

In contrast to the Li-ion battery system, graphite does not work as anode material for the upcoming Na-ion battery system. Even though hard carbon has been widely explored as anode material, it possesses safety issues since the Na-insertion potential is very low and close to that of Na-plating. In this context, the present work reports a bi-phase Na-titanate based composite anode material, which is safe, electrochemically stable, possesses very high ‘rate-capability’ and long-term cyclic stability, even at very high current densities. ‘Bi-phase NTO’, having ~66% Na₂Ti₃O₇ and ~34% Na₂Ti₆O₁₃ shows contributions from both the constituent phases, which is confirmed via operando synchrotron X-ray diffraction studies. ‘Bi-phase NTO’ is electrochemically more stable than Na₂Ti₃O₇ and its operating potential is well above the Na-plating potential. Furthermore, the performance of ‘Bi-phase NTO’ has been further improved by the addition of functionalized multi-walled carbon nanotubes (MWCNTs), which bestows the same with reversible Na-storage capacity of ~162 mAh g^-1 at C/5 and outstanding cyclic stability. The very stable ‘charge-averaged’ discharge/charge potentials and only minor increment in impedance over multiple electrochemical cycles support the excellent stability of ‘bi-phase NTO’/MWCNTs composites. Of special note is that the composite electrode exhibits 1^st cycle reversible capacity of >140 mAh g^-1 even at 50C, with capacity retentions of ~83% and ~58%, after 100 and 2000 cycles, respectively, including negligible capacity fade from the 300^th cycle onwards. This indicates the feasibility for long-term cycling at very high current densities. Overall, this will allow the development of Na-ion cells possessing outstanding rate-capability and cyclic stability; thus addressing some of the major concerns associated with the same. A part of this work has been published as Pradeep et al., Electrochim. Acta (2020).

Keywords: Bi-phase Na-titanate; operando synchrotron X-ray diffraction; high rate performance; long-term cyclic stability; Na-ion battery
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* Corresponding author’s e-mail-id: [email protected]; phone (o): +91 22 25767612

As an alternative to Li-based technologies to develop stationary energy storage systems, Na-ion batteries have attracted substantial interest. Unlike the Li-ion, layered sodium transition metal oxide (NaTMO₂) cathodes can diversify their redox centers from expensive Co to cheap Mn and Fe to achieve high energy density, offering a wide variety of chemistry selection to design low-cost, high-performance batteries. These cathode materials often undergo discontinuous, yet reversible, phase transitions upon Na intercalation due to complex interatomic interactions, which result in multiple plateau-like features in their voltage profiles. However, large desodiation of Fe-containing Na-ion cathodes often leads to an unclarified, irreversible phase transition at high voltage, hindering the full use of Fe redox.

In this talk, we will discuss how transition metal selection affects the thermodynamic stability of desodiated structures of NaTMO₂. Specifically, our recent findings on a quaternary transition metal system, NaTi_0.25Fe_0.25Co_0.25Ni_0.25O₂, will be underlined. We will present structural evidence and characterization of the highly-desodiated, high-voltage phase that exhibits peculiar oxygen stacking. This phase consists of alternating octahedral and prismatic Na layers, namely OP2 stacking, in which formation is afforded by distortion-tolerant Ti and Jahn-Teller-active Fe. Moreover, we will demonstrate that this new phase participates in redox reaction reversibly, fundamentally distinct from inactive high-voltage phases in many Na-ion cathodes. Finally, a strategy to stabilize Fe-containing NaTMO₂ materials to create layered cathodes with high reversible Na capacity will be briefly discussed. Our results demonstrate that we can manipulate the redox reaction and associated structure change by transition metal mixing.

Layered metal vanadate, such as sodium vanadate (Na_1+xV₃O₈ or NVO), has recently attracted immense interest as the promising aqueous Zn ion battery cathode material owing to its metal ion pillars that facilitate Zn ion diffusion within the structure. Here, reaction mechanisms of hydrated Na₂V₆O₁₆*2H₂O slabs and non-hydrated Na_1.25V₃O₈ nanorods were investigated in detail by employing transmission electron microscopy. We observed the full discharge of the thin (30-50 nm) Na₂V₆O₁₆*2H₂O system with successful Zn ion insertion into the bulk while detecting Zn ion insertion only at the surface for thick (120-170 nm) Na_1.25V₃O₈, demonstrating that both the hydration and the nanostructure thickness determine the reaction mechanism of NVO. Interestingly, electrochemical cycling of Zn/NVO cells induced the irreversible formation of the novel by-product phase, Na-inserted Zinc pyrovanadate (Zn₃Na_x(OH₂)V₂O₇ or ZNVO), which is electrochemically active. Its presence could eventually lead to a bifurcated discharge/charge process, where Zn redox reactions happen in both NVO and ZNVO contributing to the overall cell capacity. This microscopy work offers important insights into the effects of morphology and hydration on reaction mechanisms of NVO, which hold some implications on its electrochemistry for aqueous Zn ion batteries.

Sodium-ion batteries (NIBs) are the nearest of the "Beyond Li-ion Batteries" systems to commercialization. Attractive features include the large abundance of sodium precursor materials, which reduce supply security worries and lower cost, and the ability to use Co-free cathodes and aluminum current collectors on the anode side. Further improvements require the identification of suitable replacements for the hard carbon anodes currently used, which limit energy density and present safety issues. We have recently been working with several types of sodium titanium oxides with stepped layered structures, which undergo reductive intercalation of sodium at extremely low potentials. These include "sodium nonatitanate", which can deliver up to 200 mAh/g at an average potential of about 0.3V vs. Na⁺/Na, and lepidocrocite-structured titanates, whose redox properties vary depending on exact chemistry and structure. Both types of materials show excellent reversibility in sodium half-cells, making them appealing candidates for use as anode materials for NIBs. In this talk, we will discuss their structure, chemistry, and electrode optimization.

Na_xCoO₂ is a layered cathode material which shares many attributes with widely used LiCoO₂. Several geopolitical and economic factors make lithium a potentially unreliable raw material for rechargeable batteries therefore an earth abundant alternative is eagerly sought after. Sodium shares many chemical similarities with lithium however does underperform in the areas of specific energy and charge density¹; as such understanding the electronic structure and electrochemical behaviour is an essential steppingstone to optimising said material and rationally designing new layered sodium ion cathode materials. A combination of structural dynamism, magnetic complexity and inter layer dispersion make Na_xCoO₂ a challenging material to model; thus far its electrochemical voltage profile has not been convincingly reproduced by theoretical methods.² The motivation of this study is to ascertain the correct level of theory necessary to robustly reproduce experimental structures and voltage profiles of Na_xCoO₂ , and thus guide the accurate simulation of other layered Na-ion cathodes.

This study simulates octahedral and prismatic structures of Na_xCoO2 within the stoichiometric range of 0 ≤ x ≤ 1 using the cluster expansion approach as implemented in the CASM code^3-5 as well as high throughput ab-initio calculation management within the atomate program.⁶ The hierarchical survey of methodology uses both generalised-gradient and hybrid functionals to examine the role of electron exchange. Our hierarchical survey examines the voltage profile of Na_xCoO₂ calculated with 6 different methodologies; these variously comprise hybrid DFT and GGA+U methods to correct for correlated d electrons, and include or exclude the effects of Van der Waals interactions to explicitly account for how each of these affects the voltage profile and thus establish the optimal ab-initio methodology to accurately reproduce such profiles in the future. This survey will inform predictions of future Na-ion cathodes, while detailed electronic structure analysis will develop our understanding of the redox processes present in LiCoO₂. DFT is also used to inform electronic structure analysis, specifically the composition of the valance band, with a view to ascertain the species participating in the redox process during cycling. Collectively, this approach will inform computational analyses into similar layered sodium open shell transition metal oxides for cathode applications to take the guess work out of voltage predictions.

References
1. Sawicki, M. & Shaw, L. L., Advances and challenges of sodium ion batteries as post lithium ion batteries, RSC Adv., The Royal Society of Chemistry, 2015, 5, 53129-53154
2. Kaufman, J. L. & Van der Ven, A., Na_xCoO₂ phase stability and hierarchical orderings in the O3/P3 structure family, Phys. Rev. Materials, American Physical Society, 2019, 3, 015402
3. J. C. Thomas, A. Van der Ven, Finite-temperature properties of strongly anharmonic and mechanically unstable crystal phases from first principles, Physical Review B, 88, 214111 (2013).
4. B. Puchala, A. Van der Ven, Thermodynamics of the Zr-O system from first-principles calculations, Physical Review B, 88, 094108 (2013).
5. A. Van der Ven, J. C. Thomas, Q. Xu, J. Bhattacharya “Linking the electronic structure of solids to their thermodynamic and kinetic properties”, Mathematics and Computers in Simulation, 80(7) 1393-1410 (2010).
6. Mathew, K., Montoya, J. H., Faghaninia, A., Dwarakanath, S., Aykol, M., Tang, H., Chu, I., Smidt, T., Bocklund, B., Horton, M., Dagdelen, J., Wood, B., Liu, Z.-K., Neaton, J., Ong, S. P., Persson, K., Jain, A., Atomate: A high-level interface to generate, execute, and analyse computational materials science workflows. Comput. Mater. Sci. 139, 140–152 (2017)

The low cost and abundance of sodium have driven the growing interest in Na-ion batteries (NIBs) as a possible alternative for Li-ion batteries. However, NIBs are still in a developmental research phase and the exploration of proper electrode materials is necessary ^[1]. Among the candidate electrode materials, Na₂Ti₃O₇ has emerged as a promising electrode material because of its low cost, nontoxicity ^[2,3], and high theoretical capacity of 177 mAhg^{-1 [4]}. Nevertheless, Na₂Ti₃O₇ has not been used as main electrodes owing to its low electric/ionic conductivity and poor structural stability, which causes significant capacity fading upon cycling ^[5]. Efforts to improve the cycling stability of sodium titanates have discovered Na₂Ti₆O₁₃ which has properties contradictory to Na₂Ti₃O₇; Na₂Ti₆O₁₃ exhibits high ionic conductivity and structural stability but suffers from low Na storage capacity ^[6]. In this regard, proper hybridization of Na₂Ti₃O₇ and Na₂Ti₆O₁₃ could break the limitations of each structure and offer a composite electrode for high performance NIBs ^[7].
Hydrogenation treatment, which can transform Na₂Ti₃O₇ into Na₂Ti₆O₁₃ ^[8,9], has recently emerged as a facile way to synthesize hybrid Na₂Ti₃O₇/Na₂Ti₆O₁₃ electrode ^[7]. From this perspective, a comprehensive understanding of the phase stability of Na₂Ti₃O₇ during hydrogenation treatment can enable further performance enhancements of sodium titanate electrodes. In this study, using hybrid HSE06 DFT functional implemented in VASP code, we have analyzed the phase stability of Na₂Ti₃O₇ during hydrogenation treatment and its effect on battery performance. The ternary phase diagram of Na-Ti-O system and subsequent Gibbs free energy calculations reveal that, as a result of the thermal energy applied during hydrogenation treatment, Na₂Ti₃O₇ can thermodynamically decompose into six different phases of Na₂O, Na₄TiO₄, Na₈Ti₅O₁₄, Na₄Ti₅O₁₂, Na₂Ti₆O₁₃, and TiO₂. The calculated band structures and their alignments show that the formation of Na₂Ti₆O₁₃ can reduce the band gap of initial Na₂Ti₃O₇ by up to 0.19 eV, which enhances electrochemical performances of the electrode. In addition to these results, the formation of defects (O vacancy and -OH group) associated with hydrogenation treatment and their effect on electrochemical performance will be also discussed. The results provided here can help designing high performance sodium titanate electrodes for future NIBs.

(1) Chemical reviews 117.21 (2017): 13123-13186.
(2) Advanced Functional Materials 26.21 (2016): 3703-3710.
(3) Nano Energy 18 (2015): 20-27.
(4) Journal of Materials Chemistry A 3.44 (2015): 22280-22286.
(5) Advanced Energy Materials 6.11 (2016): 1502568.
(6) Small 12.22 (2016): 2991-2997.
(7) Advanced Science 5.9 (2018): 1800519.
(8) The Journal of Physical Chemistry B 110.9 (2006): 4030-4038.
(9) Journal of Raman Spectroscopy 41.10 (2010): 1331-1337.

Layered transition-metal (TM) oxides have played a central role as cathodes for Na-ion and Li-ion rechargeable batteries. Alleviating the deleterious structural transitions that occur in these materials during electrochemical cycling is still a critical challenge which requires a fundamental understanding of the mechanisms taking place at the atomistic level. In this work we investigated how Te⁶⁺ doping into the A_xNiO₂ (A=Na and Li) family of materials leads to changes in the local structure and electrochemical performance. The Na-Ni-Te-O system provides an ideal case study into the link between local structure and electrochemical performance of Na-cathode materials. The inclusion of Te⁶⁺ results in the suppression of TM layer shearing during desodiation after an initial transformation from an O’3 to a P’3 structure. Using a combination of X-ray diffraction and solid-state paramagnetic NMR it can be shown that the inclusion of Na-ions into the TM layer suppresses long range Na-ordering in the P’3 structure which facilitates favorable electrochemical properties. Paramagnetic NMR measurements show that while the long-range Na ordering is suppressed in the Na-Ni-Te-O system, there is significant local ordering of the Na sites in the TM layer, with a strong preference for the formation of nearest neighbor Na-Na motifs that are distinct from analogous Li compounds.

Complimentary DFT calculations are used to understand energetics of local structural orderings and to assign the complex paramagnetic NMR spectra. The DFT calculations are coupled with an advanced basin-hopping methodology to probe the intricate relationship between structural transitions and Na-vacancy ordering as a function of sodium content in the O’3 and P’3 structures. The combined experimental and computational approach developed in this work is further extended to understand the changes in the local structure during desodiation of the isostructural Na-Ni-Fe-Sb-O family of materials.

In the analogous Li-Ni-Te-O family, two layered polymorphs and one disordered rock salt phase can be synthesized. In contrast to the Na-Ni-Te-O system, Ni²⁺ migration occurs in layered Li polymorphs which leads to rapid capacity fading. The disordered rock salt compound displayed the highest reversible capacity out of the three structures, which could be rationalized based on the presence of fast Li-transport on the partially ordered Li/Ni sublattice. The conclusions from this work have wider implications for the design of new Na-ion and Li-ion cathode materials.

[1] Grundish et al. Chem. Mater. 31 (2019)

Recently, new energy storage materials for K-ion batteries (KIBs), which emerged as a low cost alterative battery system, have been developed by using Na⁺/K⁺ ion exchange. For example, P2-type K_0.5CoO₂ was prepared from P2-Na_0.84CoO₂ via electrochemical Na⁺/K⁺ ion exchange by Baskar et al.¹ P3-type K_xCrO₂ and K_xCr_0.9Ru_0.1O₂, both cathodes which display good cycle life, have been derived from their Na analogues through ion exchange.^2-4 There are many other examples.^5-7 In fact, ion exchange reaction has been widely used to produce metastable inorganic materials, unlike those prepared by direct chemical reactions at high temperature. New materials design for Li-ion batteries has frequently used Li⁺/Na⁺ ion-exchange reactions to attain structural features of Na layered oxides not directly achievable via conventional synthesis methods for Li compounds. For example, Delmas et al. developed O2-type LiCoO₂ from P2-type Na_0.7CoO₂, even though O3-type LiCoO₂ is the commonly observed structure.⁸ Similarly, layered LiMnO₂ was first synthesized from α-NaMnO₂ via Li⁺/Na⁺ ion exchange by Armstrong et al. and Delmas et al.,^{9, 10} whereas an orthorhombic structure (space group Pmmn) is the stable form of LiMnO₂ under typical synthesis conditions.¹¹ Nevertheless, the microscopic theory of solid-state ion-exchange is not well developed. The microscopic mechanism for Na⁺/K⁺ ion-exchange has been even less investigated than that of Li⁺/Na⁺.
In this work, we investigate how ion-exchange occurs in compounds, using electrochemical Na⁺/K⁺ ion exchange in Na₃Ni₂SbO₆ as a model system. Using in-situ XRD we find a surprising and remarkable sequences of phase to occur as K is inserted into a compound with Na ions remaining. We establish that the origin of this remarkable behavior upon Na/K ion exchange is the strong repulsion between the two ions. We discover that K ion insertion into the desodiated Na_xNi₂SbO₆ pushes the remaining Na ions away and redistributes them, thereby separating Na-rich and K-rich phases in a particle. Interestingly, this phenomenon creates a complex three-phase equilibrium during most of the de-sodiation and potassiation of Na_xK_yNi₂SbO₆: either one K-rich and two Na-rich phases or two K-rich and one Na-rich phase coexist, thereby perfectly satisfying the Gibbs phase rule. Our computational study further demonstrates that the phase separation originates from the large lattice mismatch along the c-axis between the two phases (K-rich and Na-rich phases). Our observations and interpretations demonstrate that “ion-exchange” systems may be more complex to interpret than previously understood. Our analysis should be applicable to other ion-exchange systems where the exchanged ions are not well miscible in the host structure.

References
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[8] C, Delmas et al. Mater. Res. Bull. 1982, 17, 117.
[9] A. R. Armstrong et al. Nature 1996, 381, 499.
[10] F. Capitaine et al. Solid State Ionc. 1996, 89, 197.
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To resolve energy issues, the demand of large-scaled capacity storage batteries are increasing. Therefore, the realization of lithium-sulfur (Li-S) battery, which is one candidate of the next-generation high-performance innovative battery systems, is strongly desired. In general, lithium metal is used as the negative electrode for Li-S batteries. On the other hand, elemental sulfur (S₈) is also used as the positive electrode, and it has high theoretical reversible capacity of 1,672 mAhg^-1. However, effect of Li metal negative electrode is unclear due to their chemical properties with charge and discharge process. In this study, we applied Li-doped negative electrode for suppressing the Li-dendrite formation. Therefore, we obtained Li₄Ti₅O₁₂ (LTO) electrode as stable negative electrode, which is strain-free intercalation material, and high reversible characteristics of battery system are expected.
To applying LTO for Li-S batteries, electrochemical doping of Li into LTO is required owing to their deficiency of carrier ion into LTO. Therefore, at first [Li| electrolyte |LTO] cells were assembled by using disassemble cell, which can be easily taken out cell materials without short circuit into inert atmosphere (e.g., glove box). After takeoff of Li-doped LTO (Li-LTO), lithium (Li-doped) -sulfur battery was fabricated by using SPAN as sulfur positive electrode ([Li-LTO|1M-LiFSI EC/DEC=3/7|SPAN] cell). (SPAN: developed non-degraded S positive electrode). Charge and discharge performance of prepared cells were evaluated. Experimental conditions of prepared cells are weight of active material: 0.602 mgcm^-2, current density: 328 mAcm^-2, range of voltage: -0.5~1.5 V, temperature: 303K, respectively.
Although operating voltage and capacity are relatively low, charge and discharge over 2000 cycles were observed in the charge and discharge profiles. Therefore, this cell has suitable constitution in battery materials. Within 200 cycles, high coulombic efficiencies (about 99.9%) were always observed in the cycle number dependences of coulombic efficiency. Therefore, it suggests the possibility of realizing a non-degraded battery, which has high reversible characteristics by using both Li-LTO negative electrode and SPAN positive electrode.

ies is a mature and successful energy-storage technology. The continuous development of lithium-ion battery cathodes has pushed the cathode performance approaching their theoretical values, which shows a challenge for the future development. A high-capacity sulfur cathode in the lithium-sulfur battery enables a complete two-electron conversion reaction per sulfur atom and therefore provides an order of magnitude higher theoretical capacity (1,672 mA h g^-1) than the currently used oxide cathodes. Such a significant progress in the electrochemical characteristics let sulfur cathode getting a high attention. In addition, sulfur is inexpensive and environmentally benign. However, as a new energy-storage technology, lithium-sulfur battery cathodes suffer from unique electrochemical challenges. The insulating nature of sulfur limits the high and reversible electrochemical utilization of the active material. The irreversible migration of polysulfides within the cells impact the cycle life and electrochemical stability of lithium-sulfur cells. These challenges further restrict the lithium-sulfur battery cathode to attain a sufficient sulfur loading and content of above 4.0 mg cm^-2 and 65 wt.%, respectively, in a cell with a reasonable low electrolyte-to-sulfur ratio of less than 10.0 uL mg^-1.

In our presentation, we present innovations on electrode design enabling lithium-sulfur batteries to operate excellently with a high amount of sulfur (i.e., 14.4 mg cm^-2 and 70 wt.%) and a low electrolyte-to-sulfur ratio of 4.0 uL mg^-1. The cathode substrate is prepared by carbonized electrospinning nanofibers having 20 wt.%, 40 wt.%, and 100wt.% polyacrylonitrile mixing with Nafion as the sacrificial polymer. With a high content of the sacrificial Nafion, the carbonized nanofibers generate a high specific surface area of up to 640 m² g^-1 with 150 m² g^-1 contributed from micropores. Although a high surface area and micropores of the electrode substrate benefit the polysulfide retention, most of the electrolyte added in the cell is initially absorbed by this highly porous cathode substrate. This explains why a high amount of electrolyte that would cause a low energy density is needed in the lithium-sulfur research. This also expounds why our high-loading cathodes with a low amount of electrolyte either dry out in a short cycle life nor cannot get sufficient charges transferred via the electrolyte, resulting in a poor cell performance. On the other hand, the carbonized electrospinning nanofiber with no sacrificial polymer possesses a relatively low specific surface area of 130 m² g^-1 and no micropores. Benefiting from a unique electrode-design criterion, the limited surface area ameliorates the fast consumption of electrolyte, while still blocking the fast polysulfide diffusion. Therefore, the cells with the 100CNF substrate output a stable discharge capacity and Coulombic efficiency of above 650 mA h g^-1 and 98%, respectively, for 100 cycles. Such excellent electrochemical efficiency and stability demonstrate a high capacity retention of 98% with a high areal capacity and energy density of 10 mA h cm^-2 and 20 mW h cm^-2, respectively, in a cell with a low electrolyte-to-sulfur ratio of only 4 uL mg^-1. In conclusion, the electrochemical enhancements and engineering designs of the cathode substrates with limited nanopores make them advanced cathode designs for the development of high-loading/content sulfur cathodes in high-energy-density lithium-sulfur batteries.

Recently, large-scaled rechargeable batteries are expected as load-leveling systems for in/output natural energy, such as solar / wind power. Thus, lithium-sulfur(Li-S) batteries are attracted attention because of sulfur-based positive electrodes have high theoretical reversible capacity (1,672 mAh/g(S₈)), low-cost of active material and so on. On the other hand, Li-S batteries still have some issues for practical use. For instance, dissolution of intermediate products (Li₂S_n) into electrolyte and deposition of Li-dendrite during charge process are serious problems for Li-S batteries. Then, we applied sulfolane (SL) based electrolyte because of their prevent properties for disolution of Li₂S_n, and prepared Li-doped graphite (Li_xC₆) negative electrodes by electrochemical process to prevent deposition of Li-dendrite. However, Li_xC₆ electrodes should be handled under inert atmosphere. Therefore, we used to disassemble-type cell, which can easily pick up Li_xC₆ electrodes. In addition, increasing for loading amount of elemental S₈ positive electrodes are required for actual high-energy-density systems. Therefore, we fabricated 3D-S₈ electrode prepared by using 3D-Al current collector, which can easily obtain electron conductive pathway. In this study, we fabricated Li-S battery using 3D-S₈ positive electrode and Li_xC₆ negative electrode (litihium-ion-sulfur battery), and evaluated by constant current charge-discharge cycle test.
S₈ and ketjen black (KB) were mixed in mortar, and mixed compound was heated at 438K into sealed bottle for compositing S₈ and KB. Slurry was prepared by the mixed compounds and binders (CMC+SBR) into H₂O. 3D-S₈ electrodes were fabricated with putting on the slurry to 3D-Al cermet current collector. Prepared 3D-S₈ electrodes were dried, punched out f13 mm and pressed at 0.3t. First, [Li | electrolyte | C₆] cells were prepared using disassemble-type cell, and examined with constant current charge-discharge tests. Li_xC₆ (x<1) negative electrodes were picked out by disassemble-type cell in the inert Ar-filled glove box, and washed with DMC. [Li_xC₆ | SL-based electrolyte | 3D-S₈] cells were fabricated with 2032-type coin cell, and evaluated by constant current charge-discharge cycle test.

[Li_xC₆ | SL based electrolyte | 3D-S₈] cell showed good charge-discharge behavior and high discharge capacity (ca. 1,000 mAhg^-1) at 1st cycle. Similar capacity and low over potential were observed comparison with normal Li-S batteries, because of low resistance derived from Li_xC₆ electrode than Li-metal electrode. In addition, reaction resistances of second plateau of discharge process and beginning charge process declined gradually as cycle passes. Those phenomena are showed frequently normal Li-S battery using Li-metal negative electrode, and regard as constitution of ionic conductive pathway on positive electrode. Furthermore, capacity fluctuations of 1^st plateaus were quite lower than 2^nd plateaus during discharge process at any cycles. Also, capacity degradation of 1^st plateau during discharge process for Li-S battery using SL based electrolytes was reported. From the above, we considered that low capacity fluctuation was due to the balance between capacity increasing due to constitution of conductive pathway and capacity degradation. Moreover, this cell showed high cycle performance. Quite high coulombic efficiencies (ca. 98.5 %) were confirmed except for 1st cycle. Also, capacity retention at 50 cycles was high (ca. 87.8 %). Therefore, high performance lithium-ion-sulfur battery was achieved. In addition, we will report actual energy density of litihum-ion-sulfur battery for practical application.

For the application in consumer electronics, magnesium metal can be used as anode material in a rechargeable magnesium batteries (RMB) which are proposed as one of the next generation batteries beyond lithium-ion batteries given their high capacity (3833 mAh*L^-1 and 2205 mAh*g^-1). More advantages beside the dendrite-free stripping/plating of Mg with high reversibility (CE close to 100 % depending on the electrolyte) are high abundance, high standard reduction potential vs. SHE (-2.37 V) and safety. While significant advantages have been achieved in RMB electrolytes, especially for Mg-S battery architectures, Mg-based cathode materials are still challenging based on their low energy density and sluggish kinetics. MnS and ZnS as cathode materials follow a conversion-type storage mechanism to form MgS. The weaker coulombic attraction between the guest Mg²⁺ and the host sulfur lattice enhances ion mobility, which is expected to enhance Mg²⁺diffusion.
In this work, nanoparticles of MnS and ZnS were synthesized via hydrothermal synthesis and characterized by XRD, XPS, and SEM. These materials exhibited good theoretical specific densities (MnS: 569 Wh*g^-1 and ZnS: 705 Wh*g^-1) and high energy densities (MnS: 1770 Wh*L^-1 and ZnS: 2270 Wh*L^-1) in a RMB setup with a metallic Mg anode. Cyclic voltammetry and galvanostatic measurements were performed in conjunction with a non-nucleophilic Mg(HMDS)₂*2 AlCl₃ electrolyte and a salt electrolyte containing MgCl₂*AlCl₃/THF (MACC) to investigate the electrochemical performance.
To the best of our knowledge we present for the first time a MnS and a ZnS cathode material in a rechargeable magnesium battery with metallic Mg anode.

<Introduction>
In general, sodium-sulfur (Na-S) battery operates at high temperature (ca. 573 K) using β-Al₂O₃ ceramic as the membrane of Na-conductive electrolyte separated sodium negative electrode and sulfur positive electrode. During discharge in the Na-S battery, Na⁺migrates to the S electrode through the ceramic electrolyte and products sodium polysulfide. This system by operating high temperature increases the ionic conductivity of beta-alumina and ensures to form the interface between electrode and electrolyte. However, there is a concern about the fire risk attributable to an active reaction between molten sodium and molten sulfur in case of a rupture of the b-alumina ceramic membrane. To avoid these risks, we propose low-temperature operated Na-S battery using thermally stable organic gel electrolyte with polyether-based host polymer and sulfolane (SL)-based electrolyte solution. Herein, thermal stability and ionic conductivity of proposed Na conductive gel polymer electrolyte and charge-discharge property of Na-S cells were evaluated.


<Experimental>
Liquid electrolytes were prepared by mixing SL and NaTFSA using shading bottle in Ar-filled glove box. The molar ratio was changed to xSL:1NaTFSA (x=2,3,4,5 and 6, respectively). These liquid electrolytes and polyether-based macromonomer (P(EO)/(PO), polymer) were mixed with changing weight ratio as followed formula; y liquid electrolyte: (10-y)polymer (y=7 and 8). After added DMPA as photo-initiator, solid gel polymer electrolytes were fabricated by photo-polymerization by UV irradiation. Prepared samples were expressed as (xSL)yEl+(10-y)Po. Thermophysical properties of fabricated gel polymer electrolytes were evaluated by thermogravimetric analysis (TG) and differential scanning calorimeter analysis (DSC). In addition, ionic conductivities were measured by electrochemical impedance method. The reversibility of Na-S batteries was evaluated by charge and discharge measurements at 333 K.

<Results and discussion>
The prepared gel electrolytes were transparent film and showed high flexibility. Therefore, the P(EO/PO) polymer matrix might maintain electrolyte solvent as the framework of SL-based liquid electrolyte. Gel electrolyte of each composition of SL and polymer showed three-steps decreasing thermogravimetry by TG measurement and it was assumed thermally decomposed in the order SL, polymer, and NaTFSA. Moreover, the gel electrolytes showed the possibility of applying to room temperature operated Na-S battery because thermogravimetry decreasing wasn't observed by less than 373K. Ionic conductivity of the gel electrolytes showed a tendency of increasing with SL composition owing to decreasing viscosity of liquid electrolytes. Na-S cells using the gel electrolyte exhibited sufficient charge and discharge operations at relatively low temperature (333 K) and exhibited a high capacity of approximately 350 mAh g^-1.

Rechargeable Al metal batteries have recently garnered significant interest due to its low cost, earth abundance, inherent safety, and high volumetric and gravimetric capacities due to the trivalent nature of Al³⁺ ions. Commercialization of rechargeable aluminum battery systems has been limited, however, due to the small number of (i) electrolytes that enable reversible electrodeposition of Al metal and (ii) positive electrode materials that are compatible with the electrolytes while providing high capacity and cycle life. Sulfur (S) and selenium (Se) are promising positive electrode materials for rechargeable battery systems as they provide very high specific capacities. Although, sulfur is a very low-cost material and can provide a very high specific capacity. Battery systems with sulfur electrodes has been hindered due to its low electrical conductivity, high volume expansion during galvanostatic cycling, and deleterious polysulfide shuttle reactions during cycling. Selenium, a chalcogen like sulfur is a heavier element and consequently exhibits lower specific capacity, it possesses significantly higher electronic conductivity compared to sulfur that would be beneficial in mitigating key challenges that faces sulfur electrodes, such as low active material utilization and high charge transfer overpotentials during galvanostatic cycling. To date, there are few reports of Al-S, and Al-Se systems. There is no clear consensus on the electrochemical reaction mechanism during the charge/discharge process, while capacity fade plagues both systems during galvanostatic cycling.
Here, aluminum-sulfur (Al-S) and aluminum-selenium (Al-Se) cells were investigated with a chloroaluminate ionic liquid electrolyte, AlCl₃:[EMIm][Cl] (molar ratio of 1.5:1), whose electrochemical reaction mechanisms and ensuing reaction products were analyzed via a combination of bulk (XRD, NMR) and surface (XPS) analyzation techniques. Electrochemical experiments for Al-S batteries showed high initial capacities close to 1200 mAh g^-1. For Al-S batteries a higher capacity was observed upon charge, compared to discharge, pointing towards either an unwanted side reaction or polysulfide-shuttle-like behavior. This behavior occurred predominately at low current densities and affected the reversibility of the system. For the Al-Se batteries, the two different reaction mechanisms that have been previously reported were found to be dependent on (i) the applied current density and (ii) the crystallinity of the active material, as shown by XRD. We demonstrate that, with appropriate cycling conditions, both reaction mechanisms can occur during a single charge/discharge step, thus significantly increasing the capacity and the electrochemical window of the Al-Se cell. A significant capacity fade was also observed over a course of multiple cycles showing similar behavior compared to Al-S cells.
To understand the reaction mechanisms of Al-S and Al-Se batteries at a molecular level, solid-state magic-angle-spinning and liquid-state ²⁷Al NMR and ⁷⁷Se NMR measurements (on Al-Se systems) were acquired on the electrodes and electrolyte at different states-of-charge, respectively. In particular, ²⁷Al NMR measurements reveal aluminum moieties in different coordination environments and establish quantitatively their relative populations. Solid-state 2D ²⁷Al{²⁷Al} multiple-quantum MAS NMR measurements also reveal information about the local electronic environments and extents of disorder of the solid discharge products. XPS was also performed on cycled chalcogen electrodes to understand surface compositions. This work establishes the potential of conversion-type aluminum-chalcogen batteries as high-energy-density energy storage systems and highlights the importance that molecular-level analytical tools like NMR spectroscopy can have in clarifying electrochemical mechanisms in emerging electrochemical systems.

A major breakthrough in battery materials is required to meet the ever-increasing proliferation of portable electronic devices, electric vehicles and their variants, as well as the need for incorporating renewable energy resources into the main energy supply [1]. In this context, lithium-sulfur (Li-S) batteries attract attention owing to their very high energy density (2,600 Wh kg^-1) and specific capacity (1,675 mAh g^-1) and significantly lower weight and cost, compared to lithium-ion batteries (LIBs) [2]. Fully packaged, it is expected that future Li-S batteries can operate at close to 500 Wh kg^-1, which is more than twice the energy density of LIBs (200 Wh kg^-1) [3]. The problem of realizing the expected high energy density is defined by several issues including the dissolution of Li-Polysulfide (PS) species into the electrolyte, insulating properties of sulfur and Li-PS species, and volume change at the cathode [4]. Overcoming these challenges requires a fundamental understanding of the interplay between events scaling over wide spatial and temporal scales, and accurate prediction of electrode and electrolyte properties to obtain design metrics for new improved materials. In this talk, I will present how we are utilizing a multi-scale-data-driven approach to gain mechanistic insight into Li-S battery by combining density functional theory (DFT) with molecular dynamics (MD) simulations. I will describe our group’s newly developed computational workflow and analysis codes for generating data with high-throughput DFT and MD simulations within the framework of the Materials Project infrastructure [5]. I will discuss usage of these tools to mitigate dissolution of LiPS species by altering the atomistic interactions between electrode and electrolyte components through functionalizing the cathode material. This approach guides and accelerates our rational selection of functional groups that exhibit strong affinity with both the cathode material and LiPS moieties from a bigger set of available candidates through fully automated DFT calculations. We use the selected candidates in detailed MD studies to understand the effect of various electrolyte variables (components, PS chain length, salt concentration) on structural and dynamical properties at the functionalized interface [6, 7]. The approach allows for creating a database of well-characterized materials to be used in machine learning-based methods as well as for testing computationally identified structures in experiments. This work provides crucial information to alleviate the dissolution of PS species during cycling, which is the main reason for rapid capacity decay in Li-S batteries.

1. Larcher, D. and J.-M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage.Nature chemistry, 2015. 7(1): p. 19.
2. Manthiram, A., X. Yu, and S. Wang, Lithium battery chemistries enabled by solid-state electrolytes. Nature Reviews Materials, 2017. 2(4): p. 1-16.
3. Fang, R., et al., More reliable lithium-sulfur batteries: status, solutions and prospects. Advanced materials, 2017. 29(48): p. 1606823.
4. Manthiram, A., Y. Fu, and Y.-S. Su, Challenges and prospects of lithium–sulfur batteries. Accounts of chemical research, 2013. 46(5): p. 1125-1134.
5. Jain, A., et al., Commentary: The Materials Project: A materials genome approach to accelerating materials innovation. Apl Materials, 2013. 1(1): p. 011002.
6. Rajput, N.N., et al., Elucidating the solvation structure and dynamics of lithium polysulfides resulting from competitive salt and solvent interactions. Chemistry of Materials, 2017. 29(8): p. 3375-3379.
7. Andersen, A., et al., Structure and dynamics of polysulfide clusters in a nonaqueous solvent mixture of 1, 3-dioxolane and 1, 2-dimethoxyethane. Chemistry of Materials, 2019. 31(7): p. 2308-2319.

Recent advances in rechargeable Zn/MnO₂ alkaline batteries have shown promise for scalable energy storage systems which provide a safe, low-cost alternative to current Li-ion technologies and have a demonstrated lifetime of thousands of cycles.¹ This cathode technology is based on a 2-electron Mn redox process where a layered birnessite-type phase has been shown to form after the first cycle with excellent reversibility between the discharge product, Mn(OH)₂ when Bi₂O₃ and Cu constituents were added to the electrode architecture. For this system to reach costs < $100/kWh, the complete reduction to Mn²⁺ must be reversible with high mass loading (>20 mAh/cm²).² We investigate the reversible reaction between birnessite and Mn(OH)₂ with and without Bi₂O₃ and Cu additives using multimodal structural characterization techniques during active battery cycling to provide a fundamental understanding of the structural properties critical to reversibility. Diffraction results provide evidence of Bi³⁺ residing in the interlayer of birnessite, which prevents irreversible Mn₃O₄ formation by limiting Mn³⁺ diffusion within the crystal lattice. Also, upon charge no MnOOH intermediate phases are observed. Instead, X-ray absorption and Raman spectroscopy indicate a disordered, non-crystalline birnessite-type phase consisting of mostly neutral H₂O within the interlayer. Birnessite phases will reform without the constituents present, but Mn₃O₄ formation severely polarizes the potential they are formed at, leading to capacity fade. Also, we discuss the reversible Bi₂O₃ and Cu conversion and their contribution to the observed capacity. We expect the results will provide crucial insight into the development of aqueous, rechargeable battery systems utilizing MnO₂.

References
1. Yadav, G. G.; Gallaway, J. W.; Turney, D. E.; Nyce, M.; Huang, J.; Wei, X.; Banerjee, S., Nature Communications 2017, 8, 14424.
2. Gallaway, J. W.; Hertzberg, B. J.; Zhong, Z.; Croft, M.; Turney, D. E.; Yadav, G. G.; Steingart, D. A.; Erdonmez, C. K.; Banerjee, S., J. Power Sources 2016, 321, 135-142.

Iron(III) fluoride (FeF₃) is considered a potential cathode for sodium-ion batteries (SIBs) due to its high capacity and low cost. However, particle pulverization upon cycling generally results in poor cycling performance. Here, we introduce a free-standing nanoconfined FeF₃ cathode and explore the effect of different salts and solvents on the improvement of its cycling performance. 1M sodium-difluoro(oxalato)borate (NaDFOB) in ternary electrolytes solvents shows the best performance compared with others in this case. The achieved high performance can be attributed to the synergic protection provided by the nanoconfined FeF₃ electrode and the NaDFOB electrolyte. Post-mortem analysis and quantum mechanics show that DFOB anions facilitated the formation of a thin cathode electrolyte interphase (CEI) on the surface of FeF₃-carbon nanofibers (CNFs) via oligomerization.

Choice of the cathode materials underpins the overall performance ceiling of battery systems. Historically, the identification of new cathode materials had led to significant advances in the battery field. To achieve truly revolutionary advances, we can employ theoretical methods that explore cathode stoichiometries beyond those seen in traditional Li-ion batteries. Database-based structure prediction methods have been developed to automate the generation of unknown materials for a given chemical composition alone. However, they can suffer from significant bias originating from the databases themselves. In the current work, we apply ab initio random structure searching (AIRSS), an unbiased structure prediction method, to search for cathode materials in the under-explored regions of the chemical space. Our tests performed over a few known systems, such as LiFePO₄ and LiMnPO₄, show that the method can rapidly rediscover the known phases and predict many previously unknown polymorphs. We will present a few newly identified promising cathode systems. The use of crystal structure prediction helps us construct fundamental structure-property relationships in cathodes, and thus will also help the field to explore and find new cathode paradigms in the future.

Air stable precursors for alkali metal sulfides provide an unpredicted molecular approach to sulfur-based materials for battery applications. Theses molecular precursors facilitate a more flexible preparation of the desired material via electrospinning, based on the appropriate solubility of molecular precursors in different protic solvents like alcohol or water and DMSO compared to conventional spinning systems. We describe the design and synthesis of molecular precursors of general formula (MSEt)₂NMe (M = Li, Na, K) which were characterized by NMR analysis. Provision of suitable spinning solution enable the incorporation in 3D fiber mats forming networks of 1D electrospun polymer/(MSEt)₂NMe fiber hybrids. The starting material, an ethanolic PVP/(MSEt)₂NMe (M = Li, Na) spinning solution was stable under air and thus enabled simple handling and an innovative preparational approach of the desired materials, compared to literature known systems. These molecular precursors allow integration of M₂S (M = Li, Na) at the cathode side of the battery, which obviate the need of additionally M₂S materials at the anode side. Air stable 3D fiber mats of homogeneous 1D electrospun PVP/(LiSEt)₂NMe fibers were isolated at ambient conditions and characterized by electron microscopy. Calcination of the prepared fibers at 700°C led to homogeneous crystalline carbon based Li₂S fibers which were confirmed by XRPD, SEM and TEM analysis. These fiber mats were tested in a battery half-cell, which performed good cycle stability and efficiency comparable to literature known systems.

Inspired from their high theoretical capacity (1675 mAh g^-1) and high energy density (2600 Wh kg^-1) lithium-sulfur (Li-S) batteries are one of the most attractive next generation energy storage solutions to meet the emerging market requirements. Nevertheless, some challenges like the poor electrical conductivity, volume expansion during the charge-discharge process, as well as the shuttling effect of soluble polysulfides retard the commercial success of Li-S batteries. Considering these challenges for the sulfur cathode, Li₂S is an alternative
cathode material for Li–S batteries, offering several advantages like avoiding structural damages and increase the safety of the battery cell.
In this work, Li₂S/C nanofiber composites were synthesized via an electrospinning process to embed the active material in a defined, conductive 1D structure and additionally minimize the diffusion of polysulfides using metal oxides as trapping agents.
The high flexibility and mechanical stability of the fiber structure is expected to buffer the volume changes during electrochemical cycling and generates a freestanding electrode design making the use of a current collector and the preparation of a battery slurry obsolete.
Furthermore, changes in the setup of the electrospinning process allowed for the design of core/shell nanofibers, in which the active material Li₂S was completely surrounded by a carbon shell rather than just being randomly distributed in the carbon matrix. This coaxial fiber architecture facilitated handling of the materials under atmospheric conditions, slowed down the degradation of the active material by traces of water, and lowered the shuttle effect of polysulfides.

For many electrochemical applications in the energy sector such as batteries, water splitting, or supercapacitors, active oxygen functional groups (OFGs) at the electrode surface are considered essential. This has sparked a lot of scientific as well as industrial activities for instance in the domain of vanadium redox flow batteries, which focus on the optimization of the carbon electrodes by various oxidation treatments. The introduced OFGs are considered to catalyze the vanadium redox reactions effectively. However, these studies often disregard that the harsh attack of the surface also leads to an increase of graphitic edge sites which may also contribute to the observed catalytic activity. In a previous study, we could demonstrate that the relative amount of OFGs is less important for the catalysis than the electrochemical stability of structural defects. Further, the initial surface composition and defect structure was altered significantly in the electrochemical cell, which complicates a prediction of the electrode performance based on OFG composition. Therefore, a more systematic approach is taken to analyze the influence of OFGs and defects on the electrocatalytic activity. To separate the effect of OFGs and defects we took the reverse approach by thermally stripping oxygen groups under inert atmosphere of two different kinds of graphitic electrodes; a pristine one with a low amount of defects and oxygen species, and a thermally activated one with a high amount, respectively. By varying the stripping temperature and saturating resulting dangling bonds with hydrogen, specific OFGs were removed prior to electrochemical cycling experiments without changing surface area or defect density of the electrode. Subsequently, the electronic and defect structure of the samples was analyzed by X-Ray photoelectron and Raman spectroscopy. Cyclic voltammetry and impedance spectroscopy were used to study the electrochemical activity. Our results suggest that the initial amount or kind of OFG has no beneficial effect on the catalysis of the vanadium redox reactions. The pristine graphite felt exhibited even higher half-cell activity after the thermal treatment, which can be attributed to an increase of graphitic edge sites by the removal of OFGs. For the activated felt no remarkable change in activity was observed after stripping, most likely due to its already initially quite high amount of defects. However, both stripped samples showed an increase of the charge-transfer resistance during cycling by the formation of oxygen species. In a second step, the evolution of OFGs on the electrode was investigated by XPS after polarization experiments at the negative and positive half-cell as well as storage in the electrolyte without polarization. The results of our study disprove the universal opinion about the role of carbonaceous OFGs for the catalysis of the vanadium redox reactions. It is much rather the defect density that is responsible for the activity of the electrode.

As more and more electricity is generated from renewable sources such as wind or solar, there is a growing demand for improved energy storage devices to mitigate the intermittent nature of these sources [1]. Of particular interest are organic redox flow batteries (RFBs) due to the organic active material’s facility to undergo multiple electron transfers, its ability to be diversely sourced, and its tunability via functionalization [2]. The selection of optimal materials of such devices presents a challenge due to the sheer number of possible combinations of class of organic molecules, types and locations of functional group additions, solvents, and additives [2]. In the current work, our focus is on the selection of organic redox active materials in aqueous solvent, specifically the effect of type and position of functional groups attached to a base organic molecule on properties critical to battery performance. In particular, the effect of change in solvation structure around redox active molecules as a function of functional group and operating conditions (concentration, temperature) on the electrochemical and chemical stability, solubility, and transport properties is explored [3,4]. Such fundamental understanding allows establishing design rules to develop novel redox active molecules for flow batteries. Our approach involves using coupled high throughput ab initio and molecular dynamics simulations to estimate crucial properties of the electrolyte over a wide spatial and temporal scale and explore a large chemical and parameter space. We then develop computational databases of properties on which surrogate machine learning models can be trained to gain key insights into the effect of functional groups of the redox active species and the properties of the electrolyte, which is pivotal in the design of novel electrolytes for aqueous organic RFBs.

References:
[1] Després, J.; Mima, S.; Kitous, A.; Criqui, P.; Hadjsaid, N.; Noirot, I. Storage as a Flexibility Option in Power Systems with High Shares of Variable Renewable Energy Sources: A POLES-Based Analysis. Energy Econ. 2017, 64, 638–650.

[2] Leung, P.; Shah, A. A.; Sanz, L.; Flox, C.; Morante, J. R.; Xu, Q.; Mohamed, M. R.; Ponce de León, C.; Walsh, F. C. Recent Developments in Organic Redox Flow Batteries: A Critical Review. J. Power Sources 2017, 360, 243–283.

[3] Rajput, N. N.; Qu, X.; Sa, N.; Burrell, A. K.; Persson, K. A. The Coupling between Stability and Ion Pair Formation in Magnesium Electrolytes from First-Principles Quantum Mechanics and Classical Molecular Dynamics. J. Am. Chem. Soc. 2015, 137 (9), 3411–3420.

[4] Han, S. D.; Rajput, N. N.; Qu, X.; Pan, B.; He, M.; Ferrandon, M. S.; Liao, C.; Persson, K. A.; Burrell, A. K. Origin of Electrochemical, Structural, and Transport Properties in Nonaqueous Zinc Electrolytes. ACS Appl. Mater. Interfaces 2016, 8 (5), 3021–3031.

Bioinspired ion transport membranes have been widely investigated for energy storage applications. High theoretical specific energy density (2600Wh/kg) and high specific capacity (1675mA/g) along with natural abundance and low toxicity of sulfur have been attracting significant attention for development of an alternative battery system to replace traditional lithium ion batteries which suffer from safety and capacity/energy density limitations in various applications. However, challenges such as polysulfide dissolution and shuttling prevent mass commercialization of metal sulfur batteries. Inspired from biological ion transport mechanisms, we show a practical yet comprehensive approach for development of highperformance metal sulfur batteries. Aramid nanofiber (ANF) based composite ion transport membranes not
only prevent dendrite formation but also confine polysulfides on the cathode side. ANF composite battery separators provide diverse and opposing properties including high mechanical properties, high ionic conductivity and high thermal/chemical stability. Highly selective ion sieving properties of these biomimetic separators provide safe and high-performance batteries. Fabrication of such biocompatible, affordable,
flexible and high energy density battery is quite crucial in powering next-generation electronics including but not limited to portable, wearable and implantable biomedical devices.

New battery architectures have enabled large improvements in battery energy and power density by improving ion and electron transport through multiple solid and liquid materials as well as maximizing the volume fraction of energy storage materials. The standard battery architecture for lithium ion battery electrodes is made of small particles (to reduce solid transport distances) tightly packed together (to increase energy density and reduce interfacial resistances) with a small fraction of fillers that increase conductivity, adhesion between particles, and processability. Although effective, this slurry cast particle architecture has important limitations on energy and power. Thick electrodes increase energy density and reduce fabrication costs, however, the associated long transport distances of Li ions and electrons in thick slurry cast electrodes greatly hinder the power output. Delamination issues also limit the thickness of slurry cast electrode below ~100 μm. As a result, most electrode loadings are lower than 20 mg/cm², which correspond to 2−3 mAh/cm². Meanwhile, electrode porosity, carbon additives, and binder account for about one-third of the inactive volume in the electrode. These limitations in slurry cast particle electrodes cause significant trade-offs between energy, power, and cost for lithium and lithium-ion batteries.

Here, we demonstrate lithium cobalt oxide (LCO) based electrode architectures delivering 1516 Wh/L (over 3X higher capacity than typical slurry electrodes) electrode energy density at a loading of 52 mg/cm² (3X higher than typical slurry electrodes) while keeping a practical current density of 1.46 mA/cm², which correspond to 580 W/L power density. To achieve this performance, we electrochemically deposited hundreds of micrometer thick and up to 98% dense LCO directly onto current collectors from molten salts. Since no carbon and polymer binder was required, and the electrodes were almost completely dense, our monolithic LCO cathodes delivered close to theoretical electrode energy densities. Traditional approaches suggest that cathodes need to be ~1-10 μm thick to achieve good power density, but despite our electrode being over 100 μm thick, we extracted 63% of the electrode capacity at 1C rates (7.3 mA/cm²) because the electrode grains were aligned so that the fastest transport directions were normal to the current collector. With a working potential of 2.5-4.7 V, our 113 μm thick (52 mg/cm²) monolithic LCO electrode delivered 1516 Wh/L and 580 W/L at a current density of 1.46 mA/cm². The electrode delivered 639 Wh/L at an ultra-high 14.6 mA/cm² current density (5025 W/L), which is about 15% more energy than conventional electrodes discharged at low current. We realized 60% of the above energy density over the working potential range of 3-4.3 V. In addition to improved performance, these electrodes allowed for new manufacturing methods. We demonstrated this by fabricating lithium metal primary microbatteries with 400 Wh/kg and 900 Wh/L energy densities. These microbatteries had 4x larger energy density than previous size-comparable batteries.

The application of metal oxides has been increasing for energy storage device due to excellent electrochemical properties, multifunctional energy application, environmentally friendly property and lower price. Due to rapid growth of energy use, there is very high demand for new ways to store energy. It is challenged to develop efficient way to store energy without polluting environment, which is possible by using transition metal oxide. We have prepared Fe_xCo_3-xO₄ (x=0.0, 0.2,0.4,0.6,0.8,1) successfully via hydrothermal method from which we can prepared electrode to make supercapacitor device. The structural properties were investigated by X-ray diffraction (XRD) and found phase pure Fe_xCo_3-xO₄. The morphology of the series obtained from scanning electron microscopy (SEM) varies from thinner nano-plates to thicker nano-plates as the concentration of Fe increase from x=0.0 to x=1.0. Furthermore, Fe_xCo_3-xO₄ were characterized by X-ray Photoelectron spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR) and Quantachrome Surface area analyzer. The maximum BET surface area obtained from Quantachrome Instrument for x=1 is 87.5 m2/g. The XPS gives elemental composition of Fe_xCo_3-xO₄ as in desired amount. The higher specific capacitance of 268 F/g at a current density of 1 A/g, 856 F/g at 2 mV/s scan rate, the energy density of 16 Wh/kg and power density of 170 W/kg in 3M KOH electrolyte was observed for x =1 sample. An increased retention capacity ∼122 % measured at 5 A/g current density and Coulombic efficiency of 100%. All these results indicate Fe dope Co₃O₄ composites electrode shows promising electrocatalytic activity results in the high intrinsic electronic conductivity, can largely improve the interfacial electroactive sites, increase charge transfer rates and help to stabilize the structure of compound.

Li-O₂ batteries with high capacity have been studied for several decades because of their superior energy density compared to Li-ion batteries. The cathode composed of carbon materials and an liquid electrolyte could play a critical role for reducing the cathode weight. However, due to the limitation of carbon based cathode, the Li-O₂ cells can’t achieve both a high energy density and long cycles. It is well known that discharge product, Li₂O₂, formed during a discharge reaction with lithium ions and oxygen molecules decomposes the carbon based materials to produce Li₂CO₃.
In this study, we propose a porous inorganic electrode without an organic liquid electrolyte. The LiNi₂O₄ inorganic electrode is stable in saturated aqueous LiOH solution for 1 week. Porous LiNi₂O₄ electrode was prepared by slurry-coating by doctor-blade with sacrificial Poly(styrene-divinyl benzene) (PS-DVB) microspheres having 3 mm diameter. After thermal treatment, PS-DVB microspheres are clearly removed and introduce the pore architecture which facilitates fast oxygen transport and increases the discharge-charge reaction surface area to achieve high current density during cycles. It also acts as a storage of the discharge products, LiOH monohydrate. The reversible reaction in cathode is observed by SEM and FT-IR. With this strategy, we achieved 55 cycles at high current density (0.6 mA/cm²) and high capacity (3 mAh/cm²)

A key step to assure the practical application of Li–air batteries involves replacing O₂ with air. The presence of CO₂ in the air (>400 ppm), however, critically alters the underlying O₂ chemistry of the Li–air battery.[1] CO₂ added to Li–O₂ cells in a tetraglyme-electrolyte was shown to swiftly shift the discharge reaction mechanism toward the formation of soluble CO₄²⁻ and C₂O₆²⁻ products and solid-state Li₂CO₃. Surprisingly, CO₂ was shown to favorably act as an O₂⁻ scavenger during discharge, with the resulting soluble products being also correlated with enhanced full capacity and cell cyclability. Compared with the conventional Li–O₂ however, the introduced CO₂ resulted in higher recharge potentials (~4.5 V) required for the decomposition of Li₂CO₃, rendering impracticable the commercialization of these technologies.[2] In this study, we assessed the promise of incorporating a Br₃^–/Br₂-based redox mediator to suppress the overcharge potential.[3] Hindered overcharges (~0.5 V) driven by electrochemically-generated Br₂ were found to be a function of the LiBr concentration and the insulating Li₂CO₃ structure. Coupled electrochemical and spectroscopic analyses additionally discerned involved redox shuttling steps at progressive states-of-charge and, for the first time, the prominent presence of Br₂—Br₃⁻ loosely-bound anionic complexes in the electrolyte during the recharge step. As opposed to discrete polar Br₂ irreversibly precipitated on the CNT surface, these soluble Br₂—Br₃⁻ species were proposed to operate as redox mediators with advantageous facile diffusion. These findings foment the evaluation of the appropriateness of redox mediators beyond an overcharge decrease, and shed light for the development of Li–air devices.

References
[1] K. Takechi, T. Shiga, T. Asaoka, Chem. Commun. 2011, 47(12), 3463.
[2] Y. Qiao, J. Yi, S. Guo, Y. Sun, S. Wu, X. Liu, S. Yang, P. Hec, H. Zhou, Energy Environ. Sci. 2018, 11(5), 1211.
[3] F. Marques Mota, J.-H. Kang, Y. Jung, J. W. Park, M. Na, D. H. Kim, H. R. Byon, Adv. Energy Mater. 2020, 10(9), 1903486.

In contrast to silicon, germanium shows an electrical conductivity of ~1.0 S m^-1 that is 100 times higher than silicon, and a fast lithium ion diffusivity of 10^-12 -10^-8 cm² s^-1 that is 400 times faster than silicon, leading to a promising potential in rapid charge/discharge batteries. Similar as silicon, the large volume expansion upon lithiation/delithiation leads to capacity decay and thus poor cycling performance. The scalable production of complex nano-structures also pose a challenge. Recently MXene, a family of 2D transition-metal carbides, are attracting great interest in lithium-ion batteries due to their unique electrical, mechanical and electrochemical properties particularly with the high electrical conductivity. Here we present a novel 2D hybrid composite that is composed of 2D layered MXene as host covered with a thin layer of amorphous GeO_x, which showed an improved rate capability as anode electrode in lithium ion batteries (ACS Nano 2020, 14, 3678–3686). The battery with hybrid composite as anode maintains 90% of the initial capacity with a coulombic efficiency of 99.4 % after charging/discharging 500 cycles at 0.5 C. A high capacity retention under 10.0 C and 20.0 C and a wide temperature between -40 and 80 C was achieved. A full-cell battery paired with NMC as cathode also exhibited a high capacity.

We report on the correlation of the microstructural evolution and electrochemical properties of an ex-situ N-doping KOH activated carbon derived from Arachis Hypogea Shell (NAC-AHS) and compared to the non-doped (AC-AHS). Interconnected cavities with irregular-shaped structure are reported by SEM analysis for AC-AHS. After nitrogen doping, the morphology of the NAC-AHS materials exhibited a less porous structure as compared to AC-AHS. The surface specific area (SSA) of NAC-AHS was found to be 1442 m² g^-1 lower than the value of AC-AHS (1557 m² g^-1), the pore volumes were also found to follow the same trend since the reported values are as follow 0.76 cm³ g^-1 and 0.69 cm³ g^-1, respectively for AC-AHS and NAC-AHS. The AC-AHS and NAC-AHS exhibited in a half cell a specific capacitance (C_s) values of 167 F g^-1 and 216 F g^-1, respectively at 1 A g^-1, corresponding to an improvement of 30% after ex-situ N-doping, which agrees with the higher current response recorded in the CV profiles of the samples. A symmetric device fabricated from the NAC-AHS materials displayed a Cs of 251.2 F g^-1 at 1 A g^-1 in a wide operating voltage of 2.0 V in 2.5 M KNO₃ aqueous electrolyte. In terms of stability, it achieved 83.2% as capacity retention up to 20,000 cycles, which was, improved through the floating/ aging measurement at 5 A g^-1over a long period of time (∼7+ days). A specific energy of 35 Wh kg^-1 with a corresponding specific power of 1 kW kg^-1 at 1 A g^-1 was delivered with the device still retaining up to 22 Wh kg^-1 and a 20 kW kg^-1 specific power even at 20 A g^-1.

Two-dimensional (2D) materials are deemed as more efficient players regarding ionic intercalation compared to their bulk counterparts. This property makes 2D materials attractive for applications such as energy storage. Within this ever-ascending group, 2D transition metal carbides, carbonitrides and nitrides (MXenes) are studied since 2011 due to their versatile chemistry and consequent surface terminations, which provide hydrophilic surfaces, unlike graphene, for instance. Here we study three MXene species, namely Mo₂TiC₂, Mo₂Ti₂C₃, and Ti₃C₂ as anode materials for lithium (LIB), sodium (SIB), and potassium-ion batteries (KIB). In terms of electrochemical results, our findings indicate reasonable cycling stability under galvanostatic charge-discharge for all alkali metal ion batteries studied. Interestingly, Ti₃C₂ potassium-ion presented the highest first cycle capacity drop; however, it also yielded the highest specific first cycle charge capacity of 305.6 mAh g^-1. Detailed electrochemical impedance spectroscopy (EIS) studies are also presented for better understanding of the charge storage mechanisms taking place in the MXenes studied.

Potassium-ion batteries have currently been in the spotlight as an alternative to lithium- and sodium-ion batteries due to abundant availability, low reduction potential, and high theoretical energy density. Although some chemistry correlation with other monovalent-ion batteries may exist, research on potassium-ion battery chemistry is still in its infancy. Hence, a relevant research aspect of potassium-ion rechargeable batteries is the development of electrode materials that may efficiently intercalate the larger size K⁺ ions in its layered structure; thus, providing reasonable capacity and reversibility. With a peculiar electronic structure, here we report the semimetal tungsten ditelluride (WTe₂) as anode material for potassium-ion batteries. This layered material presents reasonable cycling stability and specific capacity within 0 to 3 V potential range. Results show that WTe₂ may store up to 3.3 K⁺ per formula unit, at least 4 times of WS₂ KIB electrode. In sum, this work unravels the potential of WTe₂ as anode for potassium-ion batteries with respect to its physical and electrochemical properties.

The need for high-performance microscale energy storage units for rapidly growing microelectronic devices have continued to attract much attention. Miniaturized batteries and electrochemical capacitors (ECs) are attractive candidates for on-chip energy storage applications. Typically, ECs can give advantageous characteristics such as higher power density, faster charge/discharge properties, and ultralong cyclability than batteries. However, the limited energy density of ECs represents a significant drawback. One strategy to overcome this issue is based on the combination of the bulk and surface storage mechanisms by a battery-type and a capacitor-type electrode. Lithium-ion capacitor (LIC) is one such systems, consisting of a high energy battery-type electrode and high power capacitor-type electrode. On-chip LICs could potentially be favorable for integration along with other components in microelectronic devices to provide a good combination of high energy and power densities within a limited footprint area. Carbon microelectromechanical systems (C-MEMS) technique is one of the most interesting 3D microelectrode fabrication approaches which generally involves photolithographic patterning and pyrolyzing photoresists in an inert atmosphere to obtain glassy carbon structures. C-MEMS process is a facile route of obtaining a wide variety of ordered 3D carbon microelectrode structures with high resolution. Given the prospects of providing enhanced areal capacity, this study demonstrates a novel first-generation 3D on-chip LIC based on C-MEMS-derived interdigital carbon microelectrodes. In this work, the total developed electrode geometric surface area of each side of the interdigital microelectrode was ~0.07 cm². An aqueous on-chip LIC was fabricated using carbon microelectromechanical systems (C-MEMS) technique. Oxygen plasma-treated 3D carbon microelectrode arrays served as the capacitor-type electrode in the LIC, while LiFePO₄ integrated on the 3D carbon microelectrode arrays was used as the battery-type cathode. Among the devices, the 5 min plasma-treated sample gives the highest discharge capacity. The discharge capacities calculated from the GCD curves for 0, 1, 5, and 10 min plasma treatment durations were about 0.001, 0.041, 0.297 and 0.138, μAh cm^-2, respectively, based on device footprint area. The rate performance results also shows that the 5 min plasma-treated device gave the highest discharge capacities at different current densities. The discharge capacities after the tenth cycle for the 5 min plasma-treated sample at current densities of 0.24, 0.48, and 0.60 mA cm^-2 were about 0.63, 0.18, and 0.13 μAh cm^-2, respectively, based on device footprint area. The decreased discharge capacities of the device upon further increasing the oxygen plasma treatment duration from 5 to 10 min may be attributed to the reduced effective surface area of the interdigital microelectrodes. Electrochemical performance evaluation shows that the aqueous on-chip LIC can deliver an areal density up to ~5.03 μWh cm^-2, which is ~5 times higher than the oxygen plasma-treated symmetric carbon microelectrode-based electrochemical capacitor. Our work is an initial full cell study on the application of the C-MEMS technique for the development of on-chip LIC in aqueous electrolytes. The C-MEMS approach demonstrated in this study exhibits promising potential for the facile development of on-chip LICs with 3D microelectrodes. The on-chip LIC could potentially be favorable as a high-performance compact and integrable energy storage system for practical application in microelectronic devices. The developed electrode preparation and its material and electrochemical characterizations will be discussed in detail at the conference.

Spark Plasma Sintering (SPS) has proven to be an effective, rapid, and energy-efficient tool in the sintering of a variety of materials, such as ultrahigh temperature ceramics, composites, thermoelectric and magnetic compounds. Its main difference from the traditional sintering techniques (hot pressing, pressureless sintering, microwave sintering, etc) consists in the use of the direct pulsed current along with a uniaxial pressure to obtain well-compacted materials. Due to the fast heating/cooling rates and internally generated heat (resistive heating), the obtained compounds experience little to no particle growth while shortened sintering times ensure the absence of the unwanted side-reactions. Recently, SPS has been demonstrated to be used not only for the sintering of materials but also for their synthesis making it an attractive technique in the fabrication of thick high energy density electrodes for Li- or Na-ion battery application as well as in the synthesis of battery’s active material compounds or the fabrication of all-solid-state batteries ^[1-5].
High energy density electrodes could be obtained by altering the electrodes’ architectures. In pursuit of increasing the energy density, some methods suggest fabrication of (ultra)thick electrodes while others concentrate on the production of electrodes with a minimum amount of inactive materials (binder-free, conductive additive-free, integrated current collectors, etc). Owing to a rapid and highly efficient sintering process, SPS offers a luxury of obtaining simultaneously binder-free and (ultra)thick electrodes which suffer no structural degradation. In 2018 Elango et. al reported a successful Spark Plasma Sintering of the LiFePO₄ and Li₄Ti₅O₁₂ electrodes with controlled porosity by hard templating method (NaCl leaching) ^[1]. Obtained by SPS and templating approach ultrathick electrodes (1 mm thick) showed remarkable electrochemical performance at C/20 against lithium metal delivering areal capacities 4 times higher than those of the conventional tape-casted electrodes. This study coming from our group demonstrated the proof-of-concept in the fabrication of Li-ion battery electrodes with an ultrathick design by means of SPS.
Such a fabrication technique is expected to be easily transferrable to other battery types like Na-ion batteries. However, commonly used Na-ion battery materials are not as widely produced as their Li counterparts and still require long and tedious synthesis procedures. In our work, we have recently demonstrated that SPS could be used to synthesize a common Na-ion battery cathode material – Na₃V₂(PO₄)₂F₃ – in under only 40 min^[2] which resembles a phase-pure compound with small particle size showing slightly enhanced electrochemical performance when compared to the solid-state synthesized NVPF. SPS-synthesized NVPF requires no further treatment and could be used directly as an active material component in the production of our thick binder-free electrodes.
In this presentation, apart from the Spark Plasma Sintering-assisted synthesis of active materials for Na-ion battery application, a new generation of thick binder-free electrodes will be presented. Correlation between the electrode architecture (porosity, pore size/shape distribution, tortuosity, etc), synthesis/sintering parameters, nature of precursors, and the electrochemical performance will be discussed.

[1] R. Elango et al., Advanced Energy Materials, Vol 8 Issue 15 (2018)
[2] A. Nadeina et al., Energy Technol., 8: 1901304 (2020)
[3] F. Lalère, et al, Journal of Power Sources, 247, 975-980 (2014)
[4] G. Delaizir, et al., Adv. Funct. Mater., 22: 2140-2147 (2012)
[5] A. Aboulaich, et al., Adv. Energy Mater., 1: 179-183 (2011)

Sn possesses three times higher capacity in comparison to graphite anode (372 mAhg^-1) which makes it a promising candidate for enhanced performance Li ion batteries. Contrary to Si, Sn is compliant and ductile in nature and thus is expected to readily relax the Li diffusion-induced stresses. The low melting point of Sn additionally allows for stress relaxations from time-dependent or creep deformations even at room temperature. In this study, intrinsic mechanical properties of lithiated Sn at various stages in the lithiation were evaluated using nanoindentation experiments using a special setup. In order to avoid oxidation of the highly reactive lithiated Sn, nanoindentation was performed on specimens submerged in a mineral oil bath. After careful calibration, hardness and modulus of different phases of lithiated Sn were evaluated. With increasing in the lithium content, both the hardness and modulus of the lithiated tin decreased, as expected, where the hardness and modulus are 28.6 GPa and 0.37 GPa, respectively, for fully lithiated Sn (Li₂₂Sn₅) and 58.0 GPa and 0.76 GPa, respectively, for unlithiated Sn. This is the first report on the mechanical properties of lithiated Sn, which is expected to be of importance in theoretical analysis of the diffusion induced stresses in Sn anodes.

Water electrolysis is an ideal method for producing hydrogen, which is expected as a next-generation energy carrier. However, the sluggish reaction kinetics of the oxygen evolution reaction (OER) limits the overall energy efficiency.^1,2
We here present a highly-efficient water electrolyzer, consist of the metal-deposited fibrous electrode supported on a nonwoven polarized sheet, synthesized via simple and scalable electrospinning and electroless deposition methods. We selected PVdF-HFP with tunable polarity as catalyst support, to promote the diffusion of the ions in the electrolyte and optimize the reaction energetics. Cu was used as an electrocatalyst, owing to its low cost and excellent electrical conductivity.
Various spectro(electro)chemical methods were performed, including infrared spectroscopy, X-ray photoelectron spectroscopy, and linear sweep voltammetry, to clarify the effect of the geometric structure of Cu and polarity of the catalyst support on the OER activity. It was found that the fibrous structure of Cu catalyst increases the active surface area as well as optimizes the mass transport and diffusion, which contributes to the improved OER activity. Furthermore, the high dipolar moment and large dielectric constant for a polarized PVdF-HFP can assist OER by its unique catalyst-support and reactant-support interaction, which can optimize the OH adsorption energetics and diffusion of the reactant (e.g., OH^-), respectively. We successfully build a prototype water electrolyzer with a nonwoven fabric-fibrous electrode assembly, which achieved the stable water electrolysis for more than 24 h at 2.5 V.
This work opens up a new avenue to fabricate a flexible and durable water electrolyzer in a scalable way, which expands the applicability of the water electrolyzer and provides a new strategy to simplify the fabrication procedure of electrocatalysts for various energy conversion/storage reactions.

REFERENCES
1 Seh, W. Z. et al., Science. 2017, 355, eaad4998.
2 Fabbri, E. et al., Catal. Sci. Technol. 2014, 4, 3800–3821.

Organic electrode materials hold great potential due to their low cost, minimal environmental footprint, and chemical diversity. However, their electrochemical performances could not reach the level of currently used electrode materials due to the relatively low redox potential, low electrical conductivity, and high solubility in organic solvents. Those drawbacks have not been simultaneously overcome with the single moiety-based tuning such as molecular tailoring and hybridization with conductive scaffolds. Herein, we report the novel design paradigm of organic charge-transfer complexes (OCTCs), an association of two or more types of organic molecules in which a fraction of electronic charge is transferred between the molecular entities, as a new category of organic electrode material candidates.[1, 2] The OCTCs exhibit superior electrochemical performances in terms of rate capability, energy density, and cyclability, compared to the single-moiety-based organic electrode materials discovered so far. The improved performances are attributed to the high electronic conductivity and structural stability. Well-ordered stacking of the molecular layers in OCTCs creates a charge-transport path through which electrons can move freely, and the strong intermolecular interaction such as hydrogen bonding and π-π interaction secure structural stability.
We first introduce the OCTC through a combination of phenazine (PNZ) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) via a room-temperature synthesis.[1] OCTC exhibited the enhancement in the electrical conductivity for 5 order magnitudes and reduction in the dissolution for 2-5 times, resulting in the high power and cycle performances that far outperform those of each single-moiety counterpart in Li-ion battery. In order to optimize the electrochemical performances in terms of energy- and power-density, two further steps were progressed in the molecular- and battery-level. First, dibenzo-1,4-dioxin (DD) is adopted due to the high redox potential of 4.1 V vs. Li/Li⁺, and DD-TCNQ OCTC showed the increased practical energy density of 560 Wh kg^-1.[1] On the other hand, an aqueous electrolyte is applied to further enhance the rate performance of OCTC. Zinc aqueous batteries could achieve the ultrahigh level of active contents (80%) and loading density (10 mg cm^-2) with fully utilizing the specific capacity (~250 mAh g^-1) from both PNZ and TCNQ.[2] Surprisingly, it is observed that the redox mechanisms are different according to the electrolyte types, and we revealed that the formation of the TCNQ-electrolyte complex affects the redox reaction path. These findings demonstrate the general applicability of the OCTCs and open up an uncharted pathway toward the development of high-performance organic electrode materials via the exploration of various combinations of donor-acceptor monomers with different stoichiometry.
[1] S. Lee et al., Energy Storage Mater. 20, 462 (2019) [2] S. Lee et al., In preparation to submit.

Advances in the basic science and engineering principles of electrochemical energy storage is imperative for significant progress in portable electronic devices. In this regard, metal batteries based on a reactive metal (like Li, Na) as anode have attracted remarkable attention due to their promise of improving the gravimetric energy density by at least 3-folds, compared to the current Li-ion battery that uses graphitic anode. However, a persistent challenge with metal batteries is their propensity to fail by short-circuiting due to dendrite growth, and by runaway of cell resistance because of internal side reactions. In this talk, I will discuss my research that utilizes multiscale transport modeling and experiments to fundamentally understand and thereby develop rational designs for polymer electrolytes and electrochemical interfaces to overcome these challenges. Specifically, we utilized linear stability analysis of metal electrodeposition and showed that the length-scale on which transport occurs near the electrodes can be as important as electrolyte modulus in stabilizing metal deposition. To evaluate this concept, we designed cross-linked polymer electrolytes with tunable nanostructure and quantified corresponding dendritic growth. Direct visualization of electrodeposition using optical microscopy showed remarkable agreement with the theoretical predictions.

References:
Choudhury, S., Mangal, R., Agrawal, A. & Archer, L. A. A highly reversible room-temperature lithium metal battery based on crosslinked hairy nanoparticles. Nat. Commun. 6, 10101 (2015).
Tikekar, M. D., Choudhury, S., Tu, Z. & Archer, L. A. Design principles for electrolytes and interfaces for stable lithium-metal batteries. Nat. Energy 1, 16114 (2016).
Tikekar, M. D., Archer, L. A. & Koch, D. L. Stabilizing electrodeposition in elastic solid electrolytes containing immobilized anions. Sci. Adv. 2, (2016).
Choudhury, S. et al. Confining electrodeposition of metals in structured electrolytes. Proc. Natl. Acad. Sci. 201803385, (2018).

Li-ion batteries with polymer-electrolytes still have several disadvantages related to costs, safety, and materials’ availability. It would be desirable to develop alternative storage devices comprising solid-state electrolytes and sodium ions, which may replace current Li-ion batteries for certain applications. The Na2Ge-GeSe2 chalcogenide glass-ceramic was proposed as a promising material to be used as a solid electrolyte for Na-ion batteries. The complex chemistry and structural heterogeneity of this material are hardly described by empirical potentials, calling for first-principles modeling. However, first-principles simulations involve high computational costs and cannot scale to the size and time scales necessary to characterize the functional behavior of this material, thus requiring a multiscale approach.
Initially, we show that parameter-free ab initio molecular dynamics (AIMD) simulations reproduce the experimental properties of Ge-Se melts at different stoichiometry, and predict the structural variations induced by the inclusion of Na. In this study, we overcome the size and time scale limitations of AIMD by constructing suitable machine-learning potentials - based on the Chebyshev Polynomials paradigm (ChIMES) - with which we can simulate quenching from the melt and Na diffusion. This study will allow us to finally characterize amorphous Na-intercalated GeSe2 and predict its ionic conductivity at various conditions.

Achieving a truly stable and reversible Li metal anode would enable a step-change in today’s battery energy densities if key hurdles of reactivity, insufficiently high Coulombic Efficiency, parasitic electrolyte consumption and loss of active Li can be addressed. These challenges arise invariably from reactivity at the solid electrolyte interphase (SEI) on Li. After decades of study, the native SEI, which is highly chemically nonuniform, fragile, and nanoscopically thin (< 100 nm), remains notoriously difficult to experimentally probe. To identify strategies that may rationally address these challenges by interface or electrolyte design, improved understanding of the chemistry, structure, and function of the SEI are still needed.

In this talk, I present our recent studies on dynamic processes occurring in SEI-forming reactions and also within a nominally-established SEI. First, I describe our studies on single SEI components, which we have conducted to isolate and simplify the multi-phase SEI for experimental study. We have developed synthetic approaches based on Li reactions with oxide- and fluoride-donating reactants that permit formation of a compositionally uniform SEI consisting entirely of lithium fluoride (LiF) or lithium oxide (Li₂O). The stability of LiF within the Li SEI is first examined. We find that, counter to some assertions in literature, LiF does not fully protect Li from electrolyte reactions under even mild cycling conditions; its breakdown invites infiltration and repair from the electrolyte, causing the SEI to evolve over time to reflect the electrolyte chemistry more than the initial interphase. We compare these results to those obtained with fluorinated electrolyte, high-concentration LiFSI in FEC, and discuss our conclusions regarding the origin of improved cycling and high Coulombic Efficiency in fluorinated electrolyte systems. Next, I will discuss recent findings on transport properties of Li₂O and LiF at the interface of Li. Using impedance measurements coupled to a transport model of the interface, we have extracted conductivity, diffusion coefficients, and carrier concentrations of these phases at the chemical potential of Li for the first time. Our results indicate significant differences between Li₂O and LiF that help to identify less-resistive phases that may support more facile ion transport. Finally, I will present our emerging understanding of the chemical stability of these individual phases, Li₂O and LiF, in contact with carbonate electrolytes. Using X-ray absorption near-edge spectroscopy, we have probed the reactivity of ionic SEI phases upon exposure to electrolyte, and find evidence of varying interfacial stability as a function of SEI and electrolyte composition due to reactions at the SEI-electrolyte interface. These results help illuminate the origins of commonly-observed changes in SEI impedance over hours or days in certain electrolytes, which have lacked chemical specificity to date, and identify motifs for improved stability. The integration of this knowledge to guide design of improved SEI chemistries will be discussed.

The wide range of operations performed by implantable bioelectronic devices mandates reliable, stable and biocompatible sources of power. Commercially available batteries, even those used in implantable devices, are predominantly based on toxic and volatile compounds that require bulky, rigid encapsulations to protect the body from exposure. Here, we introduce a battery that functions on the basis of the work-function difference between conducting polymers and inner metals, materials that are currently utilized clinically within implantable devices. To provide mechanical flexibility and high charge capacity, we developed physical processes that create dense particulate films from bulk material. Particles can slide freely within these films, ensuring flexibility that persists even when volume of material is substantially increased. We investigated how metal particle size affected anode conductivity, Young modulus, and battery discharge time. We further investigated how electrolyte membrane thickness affected metal dissolution, device capacitance, and battery cycling ability in digestive fluids. We evaluated the battery’s performance and its potential application to medical devices by developing an ingestible “electronic pill” that allows wireless pH sensing and drug delivery within the gastrointestinal tract. Overall, the mechanical flexibility and biocompatibility of material used in such batteries should allow them to become a critical component of implantable bioelectronic devices.

In this work, x-ray tomography data of silicon anodes is used to reveal the distribution of poly(acrylic acid)-based binder and silicon. This data reveals that processing conditions directly mediate the electrode (in)homogeneity. X-ray nanotomography shows poor binder and silicon intermixing resulting in segregation of silicon and binder where the binder is in the middle of the electrode. Electrode homogeneity could be improved by adding a dispersant to the slurry, which, in turn produced better cycling performance. This data may shed light on some of the reasons behind the large variability observed in the literature with respect to silicon electrode performance and proposes a way to increase electrode homogeneity.


Acknowledgement: This work was supported by the US Department of Energy’s Office of Energy Efficiency and Renewable Energy through the Vehicle Technology Office (Brian Cunningham Program Manager).

For long-duration standalone power needs such as unmanned vehicles, space applications, and implantable/portable medical devices, batteries need to have longer operation times and thus higher energy densities. Although today’s state-of-the-art Li primary battery, Li−SOCl₂, has high specific energy (~500 Wh/kg), it is highly toxic if vented or leaked. Therefore, research into safer primary batteries that can achieve high energy density remains essential. Our previous work into Li-perfluorinated gas batteries, such as Li-SF₆ and Li-NF₃ batteries, demonstrated that perfluorinated compounds that are nominally chemically inert (thus have high safety) can be electrochemically activated through multi-electron transfer reduction reactions, resulting in a high theoretical energy density (e.g. 3922 Wh/kg for SF₆). However, the intrinsically low solubility of perfluorinated gases in typical battery electrolytes (< 5 mM) makes it necessary to have a gas reservoir present in the cell, resulting in difficulties to further optimize volumetric capacity. To overcome these issues induced by the gas phase, but still retain a multiple electron transfer reaction, we have subsequently studied new battery chemistries using liquid fluorinated reactants: an iodo-fluorocarbon series (C₆F₁₃I, C₄F₉I, and C₃F₇I), or ‘Li-CFI’. This battery system utilizes lithium metal as the anode, and liquid CFI as both the electrolyte and active cathodic component. The Li−CFI batteries exhibit attractive electrochemical performance, with voltages up to 2.9 V and an areal capacity of > 5 mAh/cm² (at 0.3 mA/cm² on un-optimized low surface area gas diffusion layer (GDL) cathodes). The liquid nature of CFI also contributes to its outstanding rate capability: an areal capacity of ~3 mAh/cm² is achievable at a current density as high as 1 mA/cm² (on GDL). In addition to the cell configuration and performance, the discharge reaction mechanism, effect of molecular structure on the electrochemistry, and electrode/electrolyte balance strategy will be discussed. We will point to current challenges for the Li−CFI system, including relatively sluggish reduction kinetics after the first electron transfer and overpotential induced by electrode passivation. Strategies to address these issues in ongoing work will also be provided. In addition to the cathode reaction, the operation of the Li anode in these novel fluorinated electrolytes will be discussed, including the potential of using CFI to construct a stable solid electrolyte interface (SEI) on Li metal surface and improve the Li stability and cycle performance for secondary batteries.

Increasing the energy density of intercalation positive electrodes in Li- an Na-ion batteries necessitate high talent redox couples. Oxygen redox has emerged as a versatile and promising high-talent redox couple. However, the reversibility of this redox couple is low compared to conventional lower-valent transition metal redox couples. In this talk, I will present understandings of and design rules for high-talent redox in intercalation compounds.

Electrolytes comprising a Li-salt (or other alkali metal salts) dissolved in ionic liquids (ILs) are often seen as attractive candidates to replace flammable organic solvents, mainly due to their superior safety. However, relatively poor transport properties hinder their applicability. Due to high salt concentrations and long-range Coulombic interactions, these systems possess high degrees of inter-species ionic correlation, thus posing a crucial and interesting challenge to understanding their transport properties. We investigate the transport properties of IL-based electrolytes through molecular dynamics computer simulations, and report two major findings. First, by adopting rigorous concentrated multicomponent solution theory, we uncover anomalously low and even negative Li transference numbers. This is due to the strong ionic interactions resulting in significant correlations and deviations from ideal solution behavior. We show that a negative transference number is to be expected in a wide range of chemistries, suggesting a universal behavior for this class of materials. Additionally, we study the anion-cation clusters and show that lithium-containing clusters carry an overall negative charge in a remarkably wide range of compositions and concentrations. Second, we leverage the microscopic understanding developed in the previous point to suggest and test modifications to increase the cation transference number. The results have significant implications for the adoption of IL-based electrolytes as they provide a recipe for optimizing transport properties in next-generation highly concentrated and correlated electrolytes.

Earth abundant electrolytes are a vital paradigm in the realisation of truly sustainable solid-state rechargeable batteries. These materials must maintain a tight balancing act between chemical stability, ion mobility and earth abundance. Recent work has shown promising Li ion mobility in phosphidoaluminates owing to the favourable alignment of AlP₄ tetrahedra_.¹ The final characterisation of this as well as the exact Li migration mechanism is not fully understood. The sodium analogue (Na₃AlP₂) shares significant structural features which may make it a possible candidate for sodium ion electrolytes.²
In this study, we have performed ab initio Density Functional Theory calculations on M₃AlP₂ (M = Li, Na) to predict and critically assess their ion transport properties. The methodology makes use of an array of python-based tools to generate structures (pymatgen)³, defective supercells (bsym)⁴ and create workflows (atomate)⁵ to perform high throughput calculations. Ab initio molecular dynamics (AIMD) and nudged elastic band (NEB) approaches are employed to examine the ion mobility and diffusion coefficients of M₃AlP₂ with a view to assessing its suitability as an electrolyte. The thermodynamic stability is window is calculated with electronic alignment relative to cathodes. Dynamic stability of these electrolytes is investigated with phonon analysis within the harmonic approximation.
While quantitative analysis of AIMD informs the extent of diffusion, qualitative directional analysis gives insights into the preferred route of diffusion as well as help propose specific mechanisms for ion diffusion. Through multiple AIMD runs we can extract temperature dependent behaviour and compare with microscopic activation barriers from NEB. These results allow us to elucidate the favourable mechanism in the phosphidoaluminates and thus draw conclusions on promising future developments for solid state electrolytes in both Li-ion and Na-ion batteries.



1. T. M. F. Restle, C. Sedlmeier, H. Kirchhain, W. Klein, G. Raudaschl-Sieber, V. L. Deringer, L. van Wüllen, H. A. Gasteiger, T. F. Fässler, Angew. Chem. Int. Ed. 2020 , 59 , 5665.
2. Somer, M., Carrillo-Cabrera, W., Peters, E.-M., Peters, K., & von Schnering, H. G. (1995). Crystal structure of trisodium catena-di-μ-phosphidoaluminate, Na₃AlP₂, Zeitschrift für Kristallographie - Crystalline Materials, 210(10), 777-777.
3. Shyue Ping Ong, William Davidson Richards, Anubhav Jain, Geoffroy Hautier, Michael Kocher, Shreyas Cholia, Dan Gunter, Vincent L. Chevrier, Kristin A. Persson, Gerbrand Ceder, Python Materials Genomics (pymatgen): A robust, open-source python library for materials analysis, Computational Materials Science, Volume 68, 2013, Pages 314-319.
4. Bsym: Morgan, (2017), bsym: A basic symmetry module, Journal of Open Source Software, 2(16), 370.
5. Mathew, K., Montoya, J. H., Faghaninia, A., Dwarakanath, S., Aykol, M.,Tang, H., Chu, I., Smidt, T., Bocklund, B., Horton, M., Dagdelen, J., Wood, B., Liu, Z.-K., Neaton, J., Ong, S. P., Persson, K., Jain, A., Atomate: A high-level interface to generate, execute, and analyse computational materials science workflows. Comput. Mater. Sci. 139, 140–152 (2017).

Herein, we report on a facile one-pot synthesis of polyoxometalates encapsulated zeolite imidazolate framework cages (PMo₁₀V₂@ZIF-67). The morphological and structural properties of the as-synthesized materials were investigated via FESEM, EDS, XRD and FTIR techniques. Moreover, N₂ adsorption/desorption isotherms, and XPS measurements were carried out to elucidate the textural properties and composition of the fabricated materials. Upon their use as supercapacitor electrodes, the PMo₁₀V₂@ZIF-67 electrode exhibited a maximum specific capacitance of 765 F/g at a scan rate of 5 mV/s. Furthermore, a supercapacitor device was assembled using activated carbon as the negative electrode and PMo₁₀V₂@ZIF-67 as the positive electrode, delivering a specific power of 702 W/kg and corresponding specific energy of 20.9 Wh/kg at a charging current density of 1 A/g. In addition, the device shows excellent long term stability and high Columbic efficiency over 5000 charging and discharging cycles at charging and discharging current density of 5 A/g.

The intensive implementation of Li-ion batteries in many markets makes it increasingly urgent to address the recycling of strategic materials from spent batteries. Batteries typically contain toxic chemicals and cannot be disposed of at will. In this study, Li-Ni-Mn-Co hydroxides are successfully recycled from spent Li-ion batteries electrodeposited on nickel foam and fully characterized using different techniques such as field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDXS), inductively coupled plasma (ICP), and X-ray photoelectron spectroscopy (XPS) techniques. The recycled nanostructured films are tested in a three-electrode electrochemical system to investigate their capacitance behavior. The recycled electrodes show a high capacitance of 951 F.g^-1 (Specific capacity of 523.5 C. g^-1) at 1 A g^-1. Moreover, recycled materials are used as positive electrodes to construct asymmetric supercapacitor devices. The device shows a Coulombic efficiency of 100%, capacitance retention reaching ∼90% with excellent cycling stability after 10,000 cycles as well as reasonable power and energy densities.

The increase in demands of devices requiring batteries, which not only encompasses electronic devices, but also electric cars, is pushing towards the development of new materials and solutions for electrodes. Hard carbons obtained by pyrolysis of resorcinol-formaldehyde xerogels have gained in interest because of their promising properties as alternative anode materials in sodium ion batteries [1]. They are easily prepared, are non-toxic and have low production costs because a simple sol-gel process can be used to prepare them. The meso- and macroporosity and overall architecture of the xerogels can be controlled by tuning the composition of the sol mixture, and the concentration of the catalyst (pH) [2-3]. Aging and drying conditions also have a substantial effect on the final surface area and pore volume of these gels. Conversely, the microporosity is created during the pyrolysis step 800 °C for 1h under inert atmosphere, which leads to a fully carbonized porous xerogels [4]. The impact of the porosity and the pyrolysis step on the performance of the carbonized RF xerogels as electrodes have been extensively investigated.
Current synthetic methods do not permit to impart anisotropy to the macrostructure of the final material, and neither allow for a control of pores direction. By adapting a procedure developed by our group, in this work we show that the macroporous structure of carbon gels can be rendered naturally anisotropic by adding superparamagnetic colloids to the sol-gel precursor solution, and by applying a uniform magnetic field during the sol-gel transition. By ensuring that the superparamagnetic colloids are incorporated into the gel phase, the magnetic colloids will self-assemble into chain-like structures in the direction of the magnetic field and become templates for the growing gel phase [5]. The lower tortuosity brought by the structural anisotropy has known advantages [6]. As anode material, the structural control gives an optimized directional flow of the ions without interfering with the advantages brought by the well-engineered internal meso and microporous gel structure.

[1] Velez, V. et al., Carbon N. Y., 2019, 147, 214–226.
[2] Pekala, R. W. CA 94550, 1990, 171, 285–291.
[3] Al-Muhtaseb, S. A. & Ritter, J. A., Advanced Materials.,2003, 15, 101–114.
[4] Hasegawa, G. et al., J. Power Sources, 2016, 318, 41–48.
[5] Furlan, M. & Lattuada, M., Langmuir: the ACS Journal of surfaces and colloids, 2012, 28, 12655–12662.
[6] Billaud, J., Bouville, F., Magrini, T., Villevieille, C. & Studart, A. R., Nature Energy, 2016,1, 8-14.

Understanding how complex oxides conduct electronic charge has relevance to a wide range of energy materials. For instance, ternary spinel oxides are actively researched for applications in electronics, sensors, catalysis, data storage, and energy storage due to their useful magnetic, catalytic, and electronic properties. It is known that the most prominent mechanism of charge transport in oxides is through hopping that follows the polaron models. But the polaron hopping models are too crude to account for the configurational differences that can occur in complex oxides, like ternary transition metal spinels, where the two different cation types can exhibit large degrees of disorder between the two cation lattice sites, possess a variety of oxidation states, and have different energy barriers. The current model, the small polaron nearest neighbor hopping model, was proposed in 1961 and has multiple limitations. Understanding the mechanisms of charge transport in ternary and higher order spinels would enable design principles for enhancing their electronic and electrochemical properties for energy applications. Here, we develop a new method to characterize the donor/acceptor pairs that form the conduction pathway for polarons: advanced synchrotron x-ray emission spectroscopy (XES) is able to accurately determine both site location and oxidation state of the cations to a higher accuracy than other common methods such as XAS and XPS. We use XES to investigate colloidal nanoparticles of the ternary Co_xMn_3-xO₄ spinel system and determine the relationship between electronic properties and cation site occupation, based on the octahedral site, donor-acceptor formalization introduced in the polaron hopping model. We find that the stochiometric volcano trend in electronic conductivity correlates well with the concentration of Mn⁴⁺/Mn³⁺ hopping pairs, while the concentration of Mn³⁺/Mn²⁺ pairs is unchanged with stoichiometry. We also find that Co does not directly contribute to conductivity, however, the Co does create configurational disorder that leads to the generation of hopping pairs for Mn at octahedral sites. From these results we conclude that Mn⁴⁺/Mn³⁺ pairs are the dominant active species and the origin of the stoichiometry-dependent behavior for polaron charge transport. This work provides a starting point for understanding and optimizing charge transport for higher order spinels to improve energy electrodes.

Combining a recently developed variant of transition state theory (TST) for ionic transport in amorphous structures with molecular dynamics (MD) simulations facilitates unprecedented insights into the local and medium-range structure of glasses. Realistic structural models of network glasses are generated using MD simulations based on a reactive force field, which allows for the accommodation of dynamically variable coordination numbers for a given network former and dynamic charge transfer upon formation or breakage of covalent bonds. The TST model, which relies on mean-field statistical measures, allows us to identify the size of the spatial region affected by the activated ion hopping process. In combination with MD simulations, we can identify the specific molecular configurations that participate in the jump process and onto which the thermal energy for activation must be focused. Thus, MD simulations allow us to observe the behavior of all atoms involved in the ionic transport process, and identify the underlying mechanisms in the context of glass structure, its chemical character, and its bonding topology. Cation jumps at room temperature are a rare event, but in the supercooled liquid near Tg each cation jumps once every 1-10 ns. Since, structural relaxation comes to a halt upon cooling at Tg, we can expect meaningful results for solid state ionic conduction when studying cation mobility in this temperature range. This allows us to observe a number of jump events that is statistically significant but typically too few to rely on standard measures such as the mean squared displacements to evaluate cation mobility. We explore a potentially novel approach for estimating cation mobility that is based on the analysis of deviatory modes of motion using the construct of the velocity autocorrelation function, which can be described as an oscillatory function whose amplitude decays with time. The integral of the velocity correlation function yields the diffusion coefficient. The existence of this non-zero diffusion coefficient is a consequence of damping or decay in the velocity correlation function. Here we establish a working connection between cation mobility and the damping coefficients as a means to rapidly screen materials structures for their potential to be good ion conductors. This knowledge is used towards the development of a materials design criteria for solid-state electrolytes. (Acknowledgment: NSF DMR-1610742.)

While batteries are an essential part of modern technology, diagnostic techniques for commercial batteries are still in high demand. Magnetic Resonance Imaging (MRI) is a powerful technique with a variety of contrast options and measurable parameters, but its application to batteries in conductive enclosures poses a major challenge. Recently, the inside-out (ioMRI) technique enabled the recovery of essential information by mapping the field caused by the (changing) susceptibility of the battery associated with state-of-charge (SOC) or defects [1]. Current distributions have been measured with this technique as well [2] and the technique has been demonstrated with the simple acquisition of free induction decays (FID) paired with statistical analysis as well. A common technique in battery diagnostics is electrical impedance spectroscopy (EIS), which lacks spatial resolution in the case of commercial batteries. A novel technique has been developed, which provides the ability to measure the oscillating fields caused by alternating currents in a battery at different frequencies [3].

Although the inside-out technique provides important parameters, direct measurements of e.g. the lithium within the battery are often desirable. Due to the inability of RF radiation to penetrate through the metal casing, other ways to access the interior noninvasively are sought for. A method to incorporate the aluminum casing of a pouch cell into a resonant circuit, essentially making it a low quality rf coil was developed recently [4]. Employing this method, signals from three different characteristic environments can be observed: Metallic lithium, intercalated lithium and lithium in the electrolyte. This approach can enable deeper insights into processes within commercial batteries.


[1] Illot et al., Nat. Comm. 9 (1) (2018) 1-7; [2] Mohammadi et al., J. Magn. Res. 209 (2019) 206601; [3] Benders et al., J. Magn. Res. 319 (2020) 106811; [4] Benders et al., Sci. Rep. 10(2020) 13781.

Current lithium-ion batteries (LIBs) based on a lithium transition metal oxide cathode (LiCoO₂, LiMn₂O₄, etc) and a graphite anode (372 mAh g^-1) are approaching their theoretical energy density (~ 300 Wh kg^-1) and thus are unable to satisfy the increasing demand for high-energy-density power sources for electric vehicles, electronic devices, and other promising markets. A promising and feasible approach to boost energy density > 400 Wh kg^-1 is to turn to high-voltage rechargeable LMBs based on a Li metal anode (LMA) and a commercially-available high-Ni NMC (LiNi_xM_1-xO₂, M=Mn, Co and x ≥ 0.6) cathode materials. However, it still remains a great challenge to for developing electrolytes that are compatible with both the Li metal anode (LMA) and the high-Ni NMC cathodes.
In order to overcome the challenge, a group of new sulfonamide-based electrolytes beyond traditional carbonate and ether-based ones are rationally designed and developed. They successfully enable a highly reversible LMA, exhibiting an excellent Coulombic efficiency with rapid ramp-up to >99% within several cycles. Furthermore, due to the excellent compatibility with high-Ni NMC cathodes, the sulfonamide-based electrolyte successfully reach a stable cycle and high energy efficiency of commercial NMC811 at an ultra-high cutoff voltage of 4.7 V vs. Li⁺/Li, which is first ever reported in the literature. Benefitting from these unique performances, a full-cell energy density exceeding 400 Wh kg⁻¹ has been achieved under practical conditions with industrial-level high-loading cathode, extremely low negative to positive (N/P) ratio, and lean electrolyte which demonstrates huge technological advances for practical LMBs.

Water-in-salt electrolytes have enabled the development of novel high-voltage aqueous lithium-ion batteries due to their wide electrochemical stability window. In this study, we explore why analogous sodium electrolytes have struggled to reach the same level of electrochemical stability.^[1-3] We compare solution structure and electrochemical stability for eleven sodium salts, selected among the major classes of salts proposed for highly concentrated electrolytes. We relate the water environment established for each anion to its position in the Hofmeister series and find a surprisingly strong correlation between the chaotropicity of the anion and the resulting electrochemical stability of the electrolyte. We also discuss the importance of the solid-electrolyte interphase for stable battery performance and how it relates to the anion. We further show that the search for suitable sodium salts is complicated by the fact that higher salt concentrations are needed than for their lithium equivalents.^[4] Reaching such a high concentration of 25 to 30 mol/kg with one or a combination of multiple sodium salts that have the desired properties remains a major challenge. We conclude that alternative approaches such as mixed water/organic solvent or dual-cation electrolytes should be pursued to enable the development of high-voltage aqueous sodium-ion batteries. We demonstrate the potential of the mixed-solvent approach by showing that the water solubility of NaTFSI can be increased from 8 to 30 mol/kg in the presence of ionic liquids. Such a ternary electrolyte, based on highly chaotropic anions, enables stable cycling of a 2 V-class sodium-ion battery based on the NaTi₂(PO₄)₃/Na₂Mn[Fe(CN)₆] electrode couple for 300 cycles at 1C with a Coulombic efficiency of >99.5%.^[5]

References:
^[1] Kühnel, R.-S.; Reber, D.; Battaglia, C., A High-Voltage Aqueous Electrolyte for Sodium-Ion Batteries. ACS Energy Lett. 2017, 2, 2005-2006.
^[2] Reber, D.; Kühnel, R.-S.; Battaglia, C., Suppressing Crystallization of Water-in-Salt Electrolytes by Asymmetric Anions Enables Low-Temperature Operation of High-Voltage Aqueous Batteries. ACS Materials Letters 2019, 1, 44-51.
^[3] Reber, D.; Takenaka, N.; Kühnel, R.-S., Yamada, A.; Battaglia, C., Impact of Anion Asymmetry on Local Structure and Supercooling Behavior of Water-in-Salt Electrolytes. J. Phys. Chem. Lett. 2020, 11, 4720
^[4] Reber, D.; Kühnel, R.-S.; Battaglia, C., Stability of aqueous electrolytes based on LiFSI and NaFSI. Electrochim. Acta 2019, 321, 134644
^[5] Reber, D.; Becker, M.; Kühnel, R.-S.; Battaglia, C., Anion Chaotropicity as Indicator for Water-in-Salt Electrolyte Stability. submitted

Development of Li metal anodes is critical to the realization of next generation high specific energy batteries. Commercialization of Li metal anodes, however, is plagued by its poor cyclability. This is because Li metal inevitably reacts with electrolytes, forming mechanically vulnerable solid-electrolyte interphase (SEI). With the significant volume fluctuation during Li metal cycling, the SEI easily cracks, leading to dendritic growth and continuous capacity loss. Electrode structural design is one of the most promising approaches to address these issues. Various "host" frameworks have been developed to accommodate Li metal and ameliorate the volume fluctuation during cycling, enabling much improved SEI stability and Coulombic efficiency (CE). However, further optimizations of these "host" frameworks are still in demand. First, achieving completely zero volume change is still challenging. Second, most of current carbon-based structures possess decent electronic conductivity, promoting undesirable surface Li plating under high currents. Here, we report recently developed "host" frameworks that successfully address these issues, enabling further advancements in the Li metal anode cycling performance.
1. Wrinkled graphene cages (WGCs)
We report WGCs as a "host" framework for Li metal anodes. Different from previously reported amorphous carbon spheres, WGCs show highly improved mechanical stability, Li-ion conductivity and robust SEI for continuous Li metal protection. With embedded gold nucleation seeds, Li metal preferentially deposits inside the cages with minimal exposure to the electrolyte. As a result, CEs of 98.0% in a carbonate electrolyte, 99.1% in a high concentration electrolyte are realized. A Li-WGC||LFP full cell can run for ~350 cycles with minimal capacity decay.
2. Silicon (Si) nanoparticles embedded reduced graphene oxide (rGO)
A rGO "host" framework for Li metal anodes is further optimized by embedding Si nanoparticles between the graphene layers. These Si nanoparticles possess two critical functionalities. First, they serve as Li nucleation seeds to promote Li deposition within the framework even without pre-stored Li. Second, the Li_xSi alloy particles serve as supporting "pillars" between the graphene layers, enabling a negligible thickness shrinkage after full stripping of metallic Li. As a result, we achieve 99.4% Coulombic efficiency over ~600 cycles in Cu||Li half cells. Meanwhile, a Li-SirGO||NMC532 full cell runs for ~380 cycles with negligible capacity decay.

Next generation of energy storage devices may largely benefit from fast and solid Li+ ceramic electrolyte conductors to allow for safe and efficient batteries and fast data calculation. For those applications, the ability of Li-oxides to be processed as thin film structures and with high control over Lithiation and phases at low temperature is of essence to control conductivity. In particular, Li-garnet structures are attractive due to their fast transfer properties and safe operation over a wide electrochemical stability range for batteries. In the first part we focus on low temperature processing of Li-garnet thin films and nanostructures. Examining the structure-Li transport in solid state and asking the provocative question: How do Li-amorphous to crystalline structures conduct? How to alter processing of Li-oxide films to compensate for the challenge that most of Lithium phases lead to sluggish transport and hard phase stabilization? Bringing down battery solid electrolyte processing temperature and decreasing form factor opens new opportunities in battery design and new doors to give more functions for similar material classes for neuromorphic computing. Through the talk, we focus on new processing opportunities to Lithiate thin film structures in crystalline state and to assure cubic and fast conduction on the example of Li-garnet for thin film form using some active Lithiation strategies at film deposition1-3. For this we will review the field of thin film processing and introduce to new processing routes based on vacuum and wet-chemical techniques to control various strategies of Lithiation and phase evolution. Excitingly, one has the opportunity for Li-garnet solid state conductors to not only synthesize crystalline structures, but also amorphous films in various frozen-in states4. Insights on degree of amorphous to crystalline Li-garnet thin films are presented based on model experiments, incl. JMAK and TTT-diagram analysis of Li-garnet structure types. Dependent on either a vacuum or wet-chemical based method chosen the phase evolution differs when synthesizing the glass states, with significant impact on condensation and Li-transport5-6. We find that the different amorphous structures that Li-garnets are no Zachariasen glasses as they violate several of Zachariasen rules by text book: These amorphous Li-garnets differ therefore significantly from LIPON but have a wider room to arrange the higher coordinated polyhedral, network former and builders. In amorphous garnets there is room for manipulation in densification and Li-transport and one may potentially profit from high stability window to design batteries with no grain boundaries at the Li electrolyte interface. Collectively, the insights on solid state energy storage provide evidence for the functionalities that those Li-materials can have in the future. The challenges are deeply connected to low temperature processing and design of structure-transport properties, however, solving those allows us to dream a little more and give materials more functions. 1. A Low Ride on Processing Temperature for a Fast Li Conduction in Garnet Solid State Battery Films R Pfenninger, M. Struzik, I Garbayo, E Stilp , JLM Rupp, Nature Energy, 4, 475-4832019, 2019 2. Lithium-Containing Thin Films JLM Rupp, R Pfenninger, M. Struzik, A Nenning, I Garbayo, US 62/718,838 (2018) 3. Methods of Fabricating Thin Films Comprising Lithium-Containing Materials JLM Rupp, R Pfenninger, M. Struzik, A Nenning, I Garbayo, US 62/718,842 (2018) 4. Glass-Type Polyamorphism in Li-Garnet Thin Film Solid State Battery Conductors I Garbayo, M Struzik, WJ Bowman, R Pfenninger, JLM Rupp, Advanced Energy Materials, 1702265 (2018) 5. Solid-state Electrolyte and Method of Manufacture Thereof. ZD Hood, Y Zhu, L Miara, JLM Rupp. US 62/713,366 (2018) 6. Solution-Processed Solid-state Electrolyte and Method of Manufacture Thereof. Y Zhu, ZD Hood, L Miara, JLM Rupp US 62/713,428 (2018)

Ionogels are pseudo-solid state electrolytes in which an ionic liquid electrolyte confined in a mesoporous inorganic matrix leads to a material that possesses the electrochemical, thermal and chemical properties of the ionic liquid despite being a macroscopic solid. Because of their unique architecture, ionogels maintain a nanoscale fluidic state and thus mitigate the interfacial resistances which commonly arise at solid-solid interfaces. The use of sol-gel synthesis enables the materials to be prepared as liquids and to penetrate porous electrodes before becoming solid, an especially beneficial property for solid-state batteries. The incorporation of Li-ion conducting ionogel electrolytes into Li-ion batteries has been reported previously and generally show behavior comparable to that of the ionic liquid electrolyte.

In this work, we report on the development of Na-ion conducting ionogels, a topic which has received only limited investigation to date. We use sol-gel synthesis to form a siloxane-modified silica scaffold that traps an ionic liquid electrolyte comprised of 0.5M NaFSI in 1-Butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide [PYR14][TFSI]. The low vapor pressure of the ionic liquid ensures that a dense gel is formed. The interconnected porosity allows for good ionic conductivity (0.7 mS cm^–1), although it is somewhat less than the corresponding ionic liquid electrolyte. The activation energy for conduction of 0.3eV is, as expected, higher than that of Li-ion conducting ionogels. The material possesses a 5V electrochemical window and is thermally stable up to 150°C. Using the positive Na-ion electrode material, Na₃V₂(PO₄)₃, the charge transfer properties of the ionogel/NVP interface were characterized and found to exhibit low interfacial impedance. This led us to demonstrate the operation of a Na-ion solid state battery which contained a NaTi₂(PO₄)₃ negative electrode in conjunction with the NVP. The results shown here underscore the versatility of ionogels as a thermally stable electrolyte with designed ion transport properties for use in future solid-state electrochemical devices

There is tremendous interest in making the next super battery, but state-of-the-art Li-ion technology works well and has inertia in several commercial markets. Supplanting Li-ion will be difficult. Recent material breakthroughs in Li and Na metal solid-state electrolytes could enable a new class of non-combustible solid-state batteries (SSB) delivering twice the energy density (1,200 Wh/L) compared to Li-ion. However, technological and manufacturing challenges remain. The discussion will consist of recent milestones and attempts to bridge knowledge gaps to include: The physical and mechano-electrochemical phenomena that affect the stability and kinetics of the Li and Na metal-solid electrolyte interface Thin film processing and Li integration with LLZO Plating and stripping dynamics of Li and Na metal Despite the challenges, SSB technology is rapidly progressing. Multi-disciplinary research in the fields of materials science, solid-state electrochemistry, and solid-state mechanics will play an important role in determining if SSB will make the lab-to-market transition.

All-state-state lithium batteries (ASSLBs) have gained worldwide attention because of intrinsic safety and increased energy density. Compared with other types of solid-state electrolytes including oxide-based, polymer-based and sulfide-based electrolytes, recently-developed halide-based solid-state electrolytes (SSEs) have garnered considerable attention for all-solid-state lithium batteries (ASSLBs) due to the high ionic conductivity, high oxidation voltage and good stability toward oxide cathode materials [1]. However, there are still many challenges in halide-based solid-state electrolytes for ASSLBs including controllable and mass-production synthesis, achieving high humidity tolerance and demonstrate high-performance of ASSLBs; in particular, increased understanding of mechanisms during synthesis and tuning their properties of the electrolytes as well as interface with electrode materials[1].

In this talk, (i) I will demonstrate synthesis strategy [2-3], in particular, new and salable water-mediated synthesis method [2]. (ii) I will report a systematic study on the correlations among structural evolution, Li⁺ migration properties, and humidity stability resulting of the halide-based electrolytes, along with in-situ characterization for understanding of the mechanisms [4], (iii) Full cell battery performance will be optimized [5], (iv) humidity ability [6] and (v) other applications such as Li-O2 batteries [7]. In the end, energy densities of ASSLBs using different solid-state electrolytes in ASSLBs will be discussed.


References:
1. X. Li, J. Liang, X. Yang, K. Adair, C. Wang, F. Zhao, X. Sun. Progress and Perspectives of Halide-based Lithium Conductors for All-Solid-State Batteries. Energy Environ. Sci., 13, 1429-1461 (2020).
2. X. Li, J. Liang, X. Sun, et al., H₂O-Mediated Synthesis of Superionic Halide Solid Electrolyte. Angewandte Chemie International Edition, 58, 16427-16432 (2019).
3. X. Li, J. Liang, X. Sun, et al.,. Air-Stable Li3InCl6 Electrolyte with High Voltage Compatibility for All-Solid-State Batteries. Energy Environ. Sci., 12, 2665 - 267 (2019).
4. X. Li, J. Liang, X. Sun, et al.,. Origin of Superionic Halide Solid Electrolytes with High Humidity Tolerance, J. Am. Chem. Soc. 142, 7012-7022 (2020).
5. C. Wang, X. Li, J. Liang, X. Sun, et al., Eliminating Interfacial Resistance in All-Inorganic Batteries by In-situ Interfacial Growth of Halide-based Electrolyte. Nano Energy, 2020, in press.
6. X. Li, J. Liang, X. Sun, et al., Origin of Superionic Li₃Y_1-xIn_xCl₆ Halide Solid Electrolytes with High Humidity Tolerance, Nano Letters, in press, 2020.
7. Zhao, J. X. Sun, et al., Halide-based solid-state electrolyte as an interfacial modifier for high performance solid-state Li-O2 batteries. Nano Energy, 2020, in press

Implementing alkali metal anodes and replacing flammable liquid electrolytes with solids promises to deliver higher energy densities and greater safety than conventional Li-ion batteries. However, challenges at both the metal-electrolyte and cathode-electrolyte interfaces must be addressed to realise this potential. It is imperative to maintain good interfacial contact between the solid-state components to achieve low interfacial resistances. Alkali metal dendrite growth, even at modest rates, when paired with dense, well-sintered solid electrolytes, leads to short circuits, and ultimately cell failure. We have used a number of techniques to investigate the effect of cycling conditions on the formation of voids and dendrites and how the latter is intimately connected to the former. These results and their implications for the operation of a failure free all-solid-state battery will be discussed.

Li metal is considered as one of the most promising anode materials for high energy batteries, owing to its high theoretical specific capacity and low electrochemical potential. However, notorious issues with the formation of Li dendrites and insufficient coulombic efficiency (CE) during the plating/stripping process prevent its practical applications in rechargeable batteries. These limitations are primarily attributed to fragile and inhomogeneous solid electrolyte interphases (SEIs), which cause continuous consumption of fresh Li and electrolyte during cycling, leading to short battery life. Of late, many efforts have been taken to enhance CE and suppress Li dendrite growth, including design of three-dimensional current collectors, artificial SEI construction on the surface of Li, employment of solid-state electrolytes, separator modification and optimization of electrolyte compositions. In this talk, we will show some recent works in developing novel electrolyte additives and polymer coatings to improve the CE and cycling stability of Li-metal anodes. We demonstrate reasonably stable cell cycling performance under practical conditions such as lean electrolytes and low N/P ratios. The roles of these additives and polymer coatings were also explored to elucidate their function in tuning Li-metal morphology and SEI composition.

We introduce polycaprolactone (PCL) dry transfer which is a powerful material for picking up non- adhesive van der Waals materials such as NiPS3, NbSe2, Fe3GeTe2, ZnPS3 and bismuth strontium calcium copper oxide (BSCCO) and etc. The heterostructures of Van der Waals were composed of layered atomically thin 2D materials that provide access to new properties important to the quantum information sciences. However, one of the main constraints is the stacking techniques. (PPC) has been known as the highest quality in dry transfer because PPC left less residue on vdW heterostructure through high temperature vacuum annealing but yield of the transfer was low. Here, we present the new polymer, polycaprolactone (PCL) for stacking vdW heterostructure with Rare layered 2D materials by dry transfer at low temperature. These 2D materials were easily detached from hexagonal boron nitride (hBN) assisted with PPC during dry transfer process. We succeeded to pick up many rare superconducting or magnetic 2D materials which were hard to be picked up by conventional dry transfer method from the oxide or silicon nitride substrates.

Abstract

Low dimensional, van der Waals (vdW) materials, from graphene to 2D magnets such as Fe₃GeTe₂ and CrI₃, show exotic behaviors at the atomic layer limit, including superconductivity, long range magnetism and topological edge states. Their “stackability” further creates new opportunities to explore quantum phenomena previously studied only in 3D crystals. Here we demonstrate that magneto-Raman spectroscopy is a unique measurement capability that is exceptionally well suited to study these exotic 2D materials.
Specifically, in the MPX₃ (M=Fe, Mn; X =S, Se) compounds, inter-layer antiferromagnetic ordering has been suggested to survive in the monolayer limit through the measurement of spin-phonon coupling. Using temperature-, polarization-, and wavelength-dependent Raman scattering, we studied the Neel-type antiferromagnet MnPSe₃ through its ordering temperature, and as a function of applied magnetic field. Surprisingly, the previously assigned one-magnon scattering peak showed no change in frequency with an increasing in-plane magnetic field. Instead, the Raman data revealed a more surprising story, including changes in resonant Raman scattering due to magnetic ordering as well as the hybridization of phonons with a 2-magnon scattering continuum. These results, which are supported using first-principles calculations, reveal the complex phase space of the interactions of phonons and magnons with regards to temperature, laser excitation wavelength, and magnetic field.

Tellurium is one of the special elemental materials which have 1D helical atomic structures and formed by van der Waals force between helical atomic chains. 2D tellurium films can be synthesized in a liquid solution. [1,2] In this talk, we will review the fundamental studies of this new 2D material at atomic scale in terms of its electrical, optical, thermal and mechanical properties. The helical atomic structure offers strong anisotropic properties of this 1D van der Waals material. The band-structure of this 2D material also offers some excellent material properties such as a high value of thermoelectric figure-of-merit ZT [3] and high carrier mobilities for both electrons and holes. Magneto-transport properties, such as SdH oscillations and QHE in 2D tellurium, were reported before. [4] More interestingly, topological non-trivial properties as Weyl node at conduction band edge are also unveiled in magneto-transport under 45 Tesla at tens of mK temperatures, showing Berry phase and radial spin texture being significantly different from Rashba or Dresselhaus spin-orbit coupling materials [5,6] The work is in close collaborations with Prof. Wenzhuo Wu at Purdue University. References: [1] Wang, Y. et al., Nature Electronics 2018, 1, 228. [2] Du, Y. et al., Nano Letters 2017, 17, 3965. [3] Qiu, G. et al., Nano Letters 2019, 19, 1955. [4] Qiu, G. et al., Nano Letters 2018, 18, 5760. [5] Qiu, G. et al.,arXiv. 1908.11495. [6] Niu, C. et al., arXiv. 1909.06659.

The charge density wave (CDW) phase is a quantum state consisting of a periodic modulation of the electronic charge density accompanied by a periodic distortion of the atomic lattice in quasi-1D or quasi-2D metallic crystals. Several layered transition metal dichalcogenides (TMDs) exhibit unusually high transition temperatures to different CDW symmetry-reducing phases opening possibility for practical device applications. One of the most promising materials, 1T-TaS₂, has the CDW transition between the nearly-commensurate (NC-CDW) and the incommensurate (IC-CDW) phases at 350 K. The transition from the IC-CDW phase to the normal metal phase is observed at even higher temperature. In this invited talk, I will review our recent experimental results on controlling the CDW phase transitions with an applied electric field and discuss device applications of quasi-2D CDW materials. We have demonstrated the room-temperature voltage controlled oscillators and logic circuits, which operate on the basis of the NC-to-IC CDW transition in 1T-TaS₂ channels, triggered by the applied voltage [1-2]. We found that the 1T-TaS₂ CDW devices reveal exceptional hardness against X-ray and proton radiations [3-4]. We explained this property of the CDW devices by the high carrier concentration in all their phase states, two-terminal design, and the thin-film channel geometry. The low-frequency electronic noise spectroscopy has been used as an effective tool for monitoring the CDW phase transitions, particularly the switching from the IC-CDW phase to the normal metal phase in 1T-TaS2 [5]. The noise spectral density exhibits sharp increases at the phase transition points, which correspond to the step-like changes in resistivity. The noise spectroscopy was instrumental in revealing the “hidden phase transitions” in vertical 1T-TaS2 devices [6]. Preliminary data on the “narrow-band noise” in quasi-2D CDW devices will also be presented.

This work was supported, in part, by the National Science Foundation (NSF) through the DMREF: Collaborative Research (UC Riverside – Stanford): Data Driven Discovery of Synthesis Pathways and Distinguishing Electronic Phenomena of van der Waals Bonded Solids, and by the University of California – National Laboratory Collaborative Research and Training Program LFR-17-477237, and by the Semiconductor Research Corporation (SRC) contract 2018-NM-2796: One-Dimensional van-der-Waals Metals: Ultimately Downscaled Interconnects with Exceptional Current-Carrying Capacity and Reliability.

[1] G. Liu, et al., A charge-density-wave oscillator based on an integrated tantalum disulfide–boron nitride–graphene device operating at room temperature, Nature Nanotechnology, 11, 845 (2016).
[2] A. G. Khitun, et al., Transistor-less logic circuits implemented with 2-D charge density wave devices, IEEE Electron Device Letters, 39, 1449 (2018).
[3] G. Liu, et al., Total-ionizing-dose effects on threshold switching in 1T-TaS₂ charge density wave devices, IEEE Electron Device Letters, 38, 1724 (2017).
[4] A. K. Geremew, et al., Proton-irradiation-immune electronics implemented with two-dimensional charge-density-wave devices, Nanoscale, 11, 8380 (2019).
[5] A. K. Geremew, et al., Bias-voltage driven switching of the charge-density-wave and normal metallic phases in 1T-TaS2 thin-film devices, ACS Nano, 13, 7231 (2019).
[6] R. Salgado, et al., Low-frequency noise spectroscopy of charge- density-wave phase transitions in vertical quasi-2D 1T-TaS2 devices, Appl. Phys. Express, 12, 037001 (2019).

Single photon emitters (SPEs), or quantum emitters, are key components in a wide range of nascent quantum-based technologies. A solid state host offers many advantages for realization of a functional system, but creation and placement of SPEs are difficult to control. We describe here a novel paradigm for encoding strain into 2D materials to create and deterministically place SPEs in arbitrary locations with nanometer-scale precision [1]. We demonstrate the direct writing of SPEs in 2D semiconductors based on a materials platform consisting of a WSe₂ monolayer on a deformable substrate using an atomic force microscope nano-indentation process. This quantum calligraphy allows deterministic placement and real time design of arbitrary patterns of SPEs for facile coupling with photonic waveguides, cavities and plasmonic structures.

The weak interlayer bonding in van der Waals heterostructures (vdWh) enables one to rotate the layers at arbitrary azimuthal angles. For transition metal dichalcogenide vdWh, twist angle has been treated solely through the use of rigid-lattice moiré patterns. No atomic reconstruction has been observed to date, although reconstruction can be expected to have a significant impact on all measured properties, and its existence will fundamentally change our understanding of such systems. Here we demonstrate via conductive AFM and TEM that vdWh of MoSe₂/WSe₂ and MoS₂/WS₂ undergo significant atomic level reconstruction at twist angles ≤ 1° leading to discrete commensurate domains divided by narrow domain walls [2], rather than a smoothly varying rigid-lattice moiré pattern as has been assumed in prior work. We show that this occurs because the energy gained from adopting low energy vertical stacking configurations is larger than the accompanying strain energy [3]. Such reconstruction impacts both the local conductivity and the optical properties.

[1] M.R. Rosenberger et al, ACS Nano 13, 904 (2019). DOI: 10.1021/acsnano.8b08730
[2] M.R. Rosenberger et al, ACS Nano 14, 4550 (2020). https://dx.doi.org/10.1021/acsnano.0c00088
[3] M. Phillips and C.S. Hellberg, ArXiv 190902495 Cond-Mat (2019)

*Work done in collaboration with Matthew R. Rosenberger, Hsun-Jen Chuang, Madeleine Phillips, Kathleen M. McCreary, and C. Stephen Hellberg of the Naval Research Laboratory, Washington DC, and Chandriker Kavir Dass and Joshua R. Hendrickson of the Air Force Research Laboratory, Wright-Patterson AFB, OH.
† This work was supported by AFOSR and core programs at NRL.