Symposium ET06 : Advanced Materials and Chemistries for High-Energy and Safe Rechargeable Batteries

Over the last decade the energy density of lithium-based batteries has gradually increased, but commercially available cells have now topped out at around 200-250 Wh/kg at the cell level. I will describe several materials-centered approaches that will allow in excess of 300-350 Wh/kg and 700 Wh/l. First the carbon-based anode must be replaced by a higher capacity material, preferably lithium metal itself; we have found that tin-based anodes can achieve 50-100% greater capacity than carbons with coulombic efficiencies over 99.5%. Second on the cathode side, I will describe two options: two-electron systems using lithium, and high nickel NMCAs, both of which have theoretical energy densities of around 1 kWh/kg and have the capability of attaining 1 kWh/liter. The safety aspects will also be covered. This work is supported by US DOE, BES-EFRC and EERE-VTO-BMR.

Li-ion batteries are expected to play a vital role in stabilizing the electrical grid as solar and wind generation capacity becomes increasingly integrated into the electric infrastructure. In this work, different commercial Li-ion batteries based on LiNi_0.8Co_0.15Al_0.05O₂ (NCA), LiNi_xMn_yCo_zO₂ (NMC) and LiFePO₄ (LFP) cathode chemistries have been tested under the grid duty cycle protocols recently developed for frequency regulation (FR) and peak shaving (PS) with and without being subjected to electric vehicle (EV) drive cycles. The lifecycle comparison derived from capacity, round trip efficiency (RTE), resistance, charge/discharge energy and total utilized energy of the battery chemistries will be presented. Furthermore, degradation mechanisms of different battery chemistries will be discussed. The results can be used as a guideline for selection, deployment, operation and cost analyses of Li-ion batteries used for different applications.

Overcharge testing has long been used as a standard abuse test evaluation for lithium ion batteries, which have notably poor tolerance for overcharge and overvoltage conditions. While this has historically been strictly an abuse test, large potential gradients created by high rate charge and discharge operations in electric vehicle and stationary storage applications may lead to areas of localized overcharge or overpotential on the electrodes. This work looks at high capacity (10 AH) prismatic pouch cells, applying overcharge from 105 – 200% total State of Charge (SOC), up to and including energetic thermal runaway of cells. The mechanisms of overcharge failure are investigated using electrochemical techniques including EIS and differential capacity measurements to evaluate the degradation and failure mechanisms that occur during the overcharge condition. This is supported with materials evaluations to further evaluate the impact of overcharge on the constituent materials.
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.

The skyrocketing price of cobalt pushes battery manufacturers to look back into low cost manganese containing positive electrodes. However, manganese dissolution has been considered as a critical problem for the majority of manganese containing positive electrodes. Although many efforts have been devoted to stabilizing the crystal structure and exploring new electrolyte additives, less progress has been reported so far. In this work, we have developed a novel multifunctional separator, targeting the root cause of manganese dissolution. A lithium-ion exchanged zeolite has been coated on polymer separator as the ceramic coating, which provides multi-functions to mitigate the issues arose from sequential scenario associated with manganese dissolution, including: 1. Trapping trace water: the high surface area in zeolite traps the trace water in the electrolyte, mitigating the hydrolysis of lithium salt and HF generation. 2. HF scavenger: in case of the HF already existing in the electrolyte, Al₂O₃ in the zeolite can preferentially react with HF as the scavenger due to the high surface area, therefore protecting the oxides in positive electrodes 3. Trapping Mn ions: in the worst scenario of Mn dissolved into electrolyte, the Li ion in zeolite will have the ion exchange with Mn ions in the electrolyte and trap Mn ions in the separator to avoid its damage to the SEI layer on anode side. In addition, lithium-ion exchanged zeolite separator can improve the wettability and thermal stability of the plain separator on which zeolite is coated. Based on this technology, we have demonstrated that Lithium-ion exchanged zeolite separator leads to the enhanced cycle performance (high capacity and Coulombic efficiency) of graphite/ (LiNi_0.5Mn_0.3Co_0.2O₂+LiMn₂O₄) full cells at both room temperature and elevated temperature, comparing with the plain separator and commercial alumina coated separator. The coin cell with Li-zeolite coated separator exhibits an average Coulombic efficiency of 99.89% and achieves a capacity retention rate of 78.3% after 500 cycles at 25 °C. We find out that a lower amount of manganese is present on the cycled graphite electrode when Li-zeolite coated separator is used, suggesting less side reactions resulted from Mn.

Solid electrolytes with low area specific resistance must be developed to enable high energy density lithium metal batteries. In addition to high ionic conductivities and large electrochemical stability windows, solid electrolytes also require robust mechanical properties to allow for large-scale production and successful integration into conventional lithium battery cell designs. To achieve these features, a novel solid electrolyte separator design was developed, in which a 50 - 100 nm fully dense solid electrolyte layer was coated onto microporous Celgard separators. The supporting Celgard made the solid electrolyte more robust and flexible, which enabled integration into coin cells. Due to its thinness, the resistance of the solid electrolyte layer was 5 - 10 Ω-cm². The solid electrolyte also showed low interfacial resistance with liquid electrolyte. The total resistance of solid electrolyte-Celgard membrane was determined to be 40 Ω-cm² in alkyl carbonate electrolytes, which is much lower than Garnet and Ohara solid electrolytes. The solid electrolyte membrane also was also shown to inhibit the crossover between anode and cathode in Li-S cells. The Li-S cell enabled by the new solid electrolyte membranes showed high coulombic efficiency and stable cycling performance. The advent of the new robust solid electrolyte paves the way for the commercialization of high energy density lithium metal batteries.

This work was supported by the ARPA-E IONICS program, U.S. Department of Energy, award DE-AR0000775.

With the increased penetration of electrified vehicles in the automotive market, requirements pertaining to battery durability are becoming increasingly stringent. To meet the requirements of the automotive industry, lithium-ion batteries must exhibit extensive life before reaching a terminal state of capacity loss and impedance growth. For battery designers and manufacturers, it is therefore of paramount importance to understand and predict long-term battery cell degradation based on a necessarily limited set of accelerated degradation tests.

Long-term calendar aging, defined as the temperature-induced cell degradation in the absence of current, is especially difficult to predict at relevant battery operating temperatures (25^oC – 45^oC). Unlike cycling tests, which can be rapidly conducted to the end-of-life by removing rest periods between cycles, calendar tests cannot be directly accelerated. To this end, a variety of empirical and physics-based models have been developed to predict the long-term storage behavior of battery packs based on a set of accelerated storage tests conducted at high temperatures. However, the validity of these calendar predictions has not, to the best of our knowledge, been extensively studied against actual long-term storage data surpassing the 4 year mark.

In this presentation, we present a set of long-term storage experiments performed over the course of 4-to-6 years on LiFePO₄/graphite cells, at various states of charge and temperatures. Analysis of the storage data sheds light on the long-term degradation mechanism in the cell, and demonstrates a transition between a reaction-controlled to a diffusion-controlled growth of the anodic solid electrolyte interphase (SEI). The dependence of state of charge and temperature on the degradation rate is clarified, and the predictive performance of empirical calendar life models is assessed. This work provides a mechanistic analysis of the nature of long-term degradation mechanisms in Li-ion batteries and paves the way towards an improvement of the predictive ability of empirical calendar life models. The conclusions of this study can also serve to understand long-term calendar degradation in higher-voltage NMC/graphite batteries, in which both the anode and the cathode may experience calendar degradation at high states-of-charge.

High energy density rechargeable batteries are critical for the widespread adoption of EVs, however their high energy density leads to an inherent safety risk if an internal short circuit (ISC) forms and releases all the energy in the battery in seconds. When an ISC occurs – from dendrite formation, cell deformation/damage, or a manufacturing defect – the entire battery capacity rapidly discharges. This release of energy leads to extremely high temperatures near the short that can induce thermal runaway, cell rupture or venting, and fire. While the use of shutdown separators can help mitigate ISCs in smaller cells, they are often ineffective in the larger, high-energy batteries used in EVs and grid storage applications. Rather than relying on thermally-induced shutdown that may fail to shut down regions of the battery far from the ISC, it is preferable to directly detect and stop the internal current that flows during an ISC. In this presentation, I will introduce our work on adaptive current-collectors, which allow for direct control over the local current that flows between the current-collector and the active material by simple printed electronic circuits. I will show how the electrical properties of printed electronics can be tuned to meet the demanding requirements of adaptive current-collectors, including low resistance during normal operating currents and high resistance under abuse conditions. Our simulations indicate that adaptive current-collectors can reduce the current flowing through ISCs by more than 90%, converting this catastrophic failure mode into a graceful one. In addition to enabling safe high-energy rechargeable batteries, the development of adaptive current-collectors will give battery users finer control over current flow at the sub-cell level, which is expected to improve battery reliability and rate capability.

Lithium metal constitutes an attractive anode material mainly due to its high theoretical specific capacity of 3860 mAh g−1, ten times higher than graphite (372 mAh g−1). The use of lithium metal in rechargeable batteries with typical liquid organic solvent based electrolytes suffers so far from severe safety problems associated with the formation of high surface area metallic lithium (HSAL) upon repeated charge/discharge.[1] Solid polymer electrolytes (SPEs) designed to be compatible with lithium metal are able to mechanically suppress HSAL formation and are considered as viable alternative. Solvent-free SPEs exhibit advantages in terms of mechanical stability, operational safety and simplicity of cell design.[2] However, application of polymer electrolytes to all-solid-state lithium ion batteries (ASS-LIBs) and all-solid-state lithium ion batteries (ASS-LMBs), requires improvements in respect to lithium ion conductivity, especially at ambient temperature.
Although high ionic conductivities can be achieved by high chain mobility linked to low molecular weight polymers, they are mostly too soft and therefore cause deterioration in mechanical stability of the SPE. In order to use low molecular weight polymers for fast lithium ion transport with sufficient mechanical strength at the same time, one strategy is related to utilization of a hyperbranched co-polymer where one segment represents a stable, hard backbone while the second segment is derived from a soft polymer with high ionic conductivities.[3] With this in line, a new generation of Li+-conducting SPEs obtained from supramolecular self-assembly of PEO, cyclodextrin (CD) and lithium salt was designed and thoroughly investigated for application in lithium metal batteries (LMBs) and LIBs. When mixing an aqueous solution of PEO together with an aqueous solution of CD, a precipitate forms where the CD is threaded onto a PEO chain.[4] The channel-type structure formed by self-assembly of PEO and CD can be used as the backbone structure whereas the hydroxyl groups of CD rings can be modified. Here, we use the ability of CD being the initiator for ring-opening polymerization of cyclic carbonates. This strategy enables synthesis of grafted polycarbonate side chains with low molecular weight. The obtained inclusion complexes show impressive ionic conductivity up to 1 mS cm-1 at 60 °C, together with high oxidative stability and allow for application in LFP/Li cells at 40 °C for more than 200 charge/discharge cycles. Post mortem XPS and SEM studies confirm that the polymer/LiTFSI penetrates the cathode upon cycling, facilitating improved contacts. This new system provides a platform for further modifications of the polymer side-chains.

Lithium metal combines the lowest reduction potential in the electrochemical reactivity series with a high theoretical specific capacity, and using metallic Li as anode would therefore significantly improve the energy density of the Li-battery. There exist, however, some challenges in the application of the Li metal electrode, such as safety risks and low coulombic efficiency [1]. In recent years, there has been a growing interest to find more stable electrolytes when in contact with the reactive Li electrode in Li-metal batteries. It has in this context been found that solid polymer electrolytes (SPEs), formed by doping a polymer with a lithium salt, are promising candidates, which can provide both high mechanical stability and better battery safety [2,3]. The major disadvantage of SPEs – their low inherent ion conductivity – can be resolved by a somewhat higher operational temperature.
We have, in a number of studies [4-6], modelled the Li metal/electrolyte interface using different simulation techniques. In this current study, we apply computational materials modelling to investigate the interface between the ion-conductive polymeric systems and Li metal surfaces by first principle calculations. To this end, Density Functional Theory (DFT) have been used to study several potential SPE host polymers such as poly(trimethylene carbonate) (PTMC), poly(vinyl alcohol) (PVA) and polycaprolactone (PCL), in order to get insights into their electronic structures and their stability when in contact with the Li metal surface. Using this knowledge, conclusions are drawn on which ion-conductive polymers are stable at the Li-metal surface, and which can adhere well to it.

References:

[1] X. B. Cheng, R. Zhang, C.-Z. Zhao, F. Wei, J.-G. Zhang, Q. Zhang, Adv. Sci. 3 (2015) 1500213.
[2] J. Kalhoff, G. G. Eshetu, D. Bresser, S. Passerini, ChemSusChem 8 (2015), 2154.
[3] J. Mindemark, M. J. Lacey, T. Bowden, Daniel Brandell, Prog. Polym. Sci (2018), doi.org/10.1016/j.progpolymsci.2017.12.004
[4] M. Ebadi, D. Brandell, C.M. Araujo, J. Chem. Phys. 145 (2016) 204701.
[5] M. Ebadi, L.T. Costa, C.M. Araujo, D. Brandell, Electrochim. Acta. 234 (2017) 43.
[6] M. Ebadi, M. J. Lacey, D. Brandell, C. M. Araujo, J. Phys. Chem. C, 121 (2017) 23324.

More than forty years after the first report of a rechargeable lithium battery, electrochemical cells that utilize metallic lithium anodes are again under active study for their potential to provide more energy dense storage in batteries. Electrolytes based on small-molecule ethers and their polymeric counterparts are known to form stable interfaces with alkali metal electrodes and for this reason are among the most promising choices for rechargeable lithium batteries. Uncontrolled anionic polymerization of the electrolyte at the low anode potentials and oxidative degradation at the working potentials of the most interesting cathode chemistries have led to a quite concession in the field that solid-state or flexible batteries based on polymer electrolytes can only be achieved in cells based on low- or moderate-voltage cathodes. In this work, we show that cationic chain transfer agents in an ether electrolyte provide a fundamental strategy for limiting polymer growth at the anode, enabling long term (at least 2000) cycles of high-efficiency operation of asymmetric lithium cells. Building on these ideas, we also report that cathode electrolyte interphases composed of anionic polymers and the superstructures they form spontaneously at high electrode potentials provide as fundamental a strategy for extending the high voltage stability of ether-based electrolytes to potentials well above conventionally accepted limits. Through computational chemistry, we discuss the mechanistic processes responsible for the extended high voltage stability and on this basis report Li||NCM cells based on a simple diglyme electrolyte that offer unprecedented stability in extended galvanostatic cycling studies.

All-solid-state Li-ion battery is a promising next-generation energy storage technology, providing intrinsic safety and higher energy density. Currently, high interfacial resistance and interfacial degradation at the solid electrolyte-electrode interfaces are the critical issues limiting the cycling and rate performance of all-solid-state battery. Fundamental understanding about the interfaces is yet lacking due to the difficulties of direct experimental characterizations. In this presentation, I will show how we use first principles computation to bring new understanding about these buried interfaces. Using our developed computation approach based on large materials database, we calculated the true electrochemical stability window of solid electrolytes and predicted interphase decomposition products, which are verified by in-situ experiments at solid electrolyte-electrode interfaces. I will discuss the critical role of decomposition interphase layers at electrolyte-electrode interface and their effects on the battery performance. From these insights, we are able to classify different interface types for different solid-electrolyte and cathode pairs and to estimate their impacts on battery performance. Moreover, specific interfacial engineering strategies are proposed to address potential issues at these interfaces in all-solid-state Li-ion batteries. I will present the predicted novel chemistry and strategies to stabilize lithium metal anode, which is greatly impeded by the lack of knowledge about lithium-stable materials chemistry. With first-principles calculations based on large materials database, we found that most oxides, sulfides, and halides, which were commonly studied as protection materials, are reduced by lithium metal due to the reduction of metal cations. New materials chemistry that are stable against Li metal are predicted, as promising candidates for lithium metal anode protection to achieve long-term stability. This series of computational studies provides novel insights and general design principles for interface engineering in all-solid-state Li-ion batteries.

The electrodeposition of lithium metal is a core process in next-generation, high energy density energy storage devices. However, the high reactivity of the lithium metal causes short cycling lifetimes, and there are safety issues due to the growth of dendrites. Recently, a number of approaches have been pursued to stabilize the lithium metal interface, including soft polymeric coatings that have shown the ability to enable high-rate and high-capacity lithium metal cycling, but a clear understanding of how to design and modify these coatings has not yet been established. In this work, we studied the effects of several polymers with systematically varied chemical and mechanical properties as coatings on the lithium metal anode. By examining the early stages of lithium metal deposition we determine that while global morphology depends on the coating quality and mechanics, the local morphology of the lithium particles is strongly influenced by the chemistry of the polymer coating. We have identified polymer dielectric constant and surface energy as two key descriptors of the lithium deposit size. Low surface energy polymers were found to promote larger deposits with smaller surface areas. This is consistent with a reduction of the coatings interaction with the lithium surface and thus an increase in the interfacial energy. On the other hand, high dielectric constant polymers were found to increase the exchange current and gave larger lithium deposits, suggesting improved Li⁺ ion solvation in the coating and decreased nucleation rate. We also note that the thickness of the polymer coating should be optimized for each individual polymer, and that polymer reactivity is an important parameter to be considered as it was found to strongly influence the Coulombic efficiency. Overall, this work offers new fundamental insights into lithium electrodeposition processes and provides direction for the design of new polymer coatings to better stabilize the lithium metal anode.

Rechargeable battery technology based on the lithium (Li) metal anode is plagued by the unstable solid-electrolyte interphase (SEI), which grows upon cycling and is associated with dendritic/mossy Li deposition. A key challenge in improving SEI stability lies in regulating its chemical composition and nanostructure. Here we report a new approach that enables the design of SEI layers with tunable structure and stable properties. This involves the use of a reactive polymeric composite, which can generate a stable SEI layer in situ by reacting with Li to occupy surface sites and then electrochemically decomposing to form nanoscale SEI components. Cryo-TEM shows that the resulting SEI layer is composed of organic polymeric Li salts, nanoparticles of inorganic Li salts, and two-dimensional nanosheet components. This conformal nanocomposite SEI layer exhibits excellent passivation, homogeneity, ionic conductivity, and mechanical strength and stabilizes the interface for dendrite-free Li deposition in a conventional carbonate electrolyte. 950-cycle life was achieved in a full cell paired with a LiNi_0.5Co_0.2Mn_0.3O₂ cathode. Moreover, under lean electrolyte conditions, the full cells also show significantly extended cycle lives, owing to the excellent stability of the polymeric nanocomposite SEI.

Despite the development of new technologies, such as lithium-sulfur or sodium-ion cells, lithium-ion devices remain the most used batteries: they are found in a majority of electronic devices and the demand for electric vehicles keeps growing. Their performance and characteristics vary according to the chosen electrode but their main advantages are their high energy density and their low self-discharge. However, lithium-ion batteries suffer safety risks which are mainly due to the use of liquid electrolytes. These electrolytes are based on a lithium salt dissolved in a mix of organic solvents and therefore are highly flammable. A safer alternative is to replace this liquid electrolyte by a solid one. Ceramic electrolytes are a first possibility but they are often complex to synthesize and too rigid. Thus, solid polymer electrolytes are good candidates regarding their flexibility but they have a rather low conductivity and poor mechanical properties.
This work focuses on the conception of a solid hybrid electrolyte. The organic part is composed of a mix of polyethylene oxide (PEO), known for its lithium conductivity, and polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP), known for its mechanical properties. The inorganic part is a silica network formed in situ via a sol gel reaction. This network is functionalized with an immobilized ionic liquid in order to increase the conductivity of the electrolyte. These components are dissolved in N,N-dimethylformamide altogether with a lithium salt, LiTFSI, and a first set of solid electrolyte is accomplished by solvent casting. Conductivity and lithium transference number are measured in temperature while the structure and the transport at different scales are analyzed by NMR, SAXS-WAXS, and Differential Scanning Calorimetry (DSC). Electrochemical performances at different rates are assessed in full cell using LiFePO₄ as the cathode and metallic lithium as the anode. Then, the importance of microstructure is investigated by achieving a second set of solid electrolyte. A skeleton of PVDF-co-HFP and functionalized silica is electrospun, producing a mat of fibers of 100nm in diameter. The space between fibers is then filled by concentrated LiTFSI or PEO/LiTFSI salts. The structure as well as their performances will be discussed using the same techniques.

Electrolytes have become key players in the design of modern high-energy solutions as they can be leveraged to enhance various important aspects of the device such as recharge time and efficiency, anode/cathode stability, and safety. However, conductivity and general transport properties of the cation/anion pair(s) dissolved in the electrolyte often pose a technological limit to the viability of the battery, and progress in fundamental insights into the origin of transport limitations are challenging yet extremely valuable. In our group, molecular modelling techniques are adopted in order to shine light on transport properties and correlation effects in electrolyte system.

Poly(ethylene) oxide (PEO) -based solid polymer electrolytes have a long history of research due to the easy processability and good transport properties, yet new observations that challenge our conventional understanding are still reported, especially at high salt concentrations relevant for technological applications. Our molecular dynamics simulation study of such regimes for one of the most prominent materials, PEO-Li-TFSI, reveals the central role of the anion in coordinating and hindering Li ion movements. In particular we observed significant competition between the anion and the polymer backbone to coordinate lithium atoms and surprising formation of asymmetric cation-anion clusters. In particular, the latter novel observation resonates well with recent experimental findings, where negatively-charged Li-anion clusters were speculated to exist to justify the surprising negative lithium transference number observed in this system for high salt concentrations.

Ionic liquids (ILs) -based electrolytes are attractive candidates as they generally exhibit better diffusion properties then polymer-based electrolytes, and several recent studies focus on assessing the performance of different mixtures. Given the highly-correlated nature of these systems, understanding the role of species correlation is non-trivial yet crucial for a rational design of future solutions. As a case study we investigated the promising NaFSI-C3C1PyrrFSI system at different Na concentrations and focus on highlighting the correlations in this system as well as how they effect the transference number for high NaFSI concentrations.

Understanding the surface reactivity between electrode and electrolyte is critical for cycle life of Li-ion batteries. The composition, properties and mechanisms behind electrolyte/electrolyte interface (EEI) on positive electrode are still unidentified for most lithium ion battery positive electrode materials [1,2]. Especially at high potentials, EEI layer on positive electrode becomes more critical since it approaches electrolyte instability limit for oxidation. The EEI layers on carbon-free, binder-free and thin film LiCoO₂ electrodes were investigated by using X-ray photoelectron spectroscopy (XPS). The growth of oxygenated and carbonated species was observed together with salt decomposition starting at 4.1 V_Li [3]. By DFT calculations, we correlated the EEI composition to the thermodynamic tendency of the EC solvent for dissociative adsorption on the Li_xCoO₂ surface, which can have a role in the salt decomposition on oxide surfaces [4]. The salt decomposition products had been also observed by solution ¹⁹F-NMR measurements. Finally, we demonstrated that the addition of diphenyl carbonate to the electrolyte has a strong impact on EEI layer and salt decomposition on LiCoO₂ surface. With this study, we also highlight the strength of using the carbon-free, binder-free electrodes to get fundamental insights in the reactivity of the positive electrode with the electrolyte.

[1] Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104 (10), 4303–4418.
[2] Gauthier, M.; Carney, T. J.; Grimaud, A.; Giordano, L.; Pour, N.; Chang, H.-H.; Fenning, D. P.; Lux, S. F.; Paschos, O.; Bauer, C. Electrode–electrolyte Interface in Li-Ion Batteries: Current Understanding and New Insights. J. Phys. Chem. Lett. 2015, 6 (22), 4653–4672.
[3] Gauthier, M.; Karayaylali, P.; Giordano, L.; Feng, S.; Lux, S. F.; Maglia, F.; Lamp, P.; Shao-Horn, Y. Probing Surface Chemistry Changes Using LiCoO2-Only Electrodes in Li-Ion Batteries. J. Electrochem. Soc. 2018, 165 (7), A1377–A1387.
[4] Giordano, L.; Karayaylali, P.; Yu, Y.; Katayama, Y.; Maglia, F.; Lux, S.; Shao-Horn, Y. Chemical Reactivity Descriptor for the Oxide-Electrolyte Interface in Li-Ion Batteries. J. Phys. Chem. Lett. 2017, 8 (16), 3881–3887.

Recently, aqueous energy storage systems (AESSs), such as Na-ion battery & capacitor, have demonstrated their uniqueness compared with their non-aqueous counterparts due to the excellent safety performance in nature. Furthermore, the advantage of their low cost derived from the high abundance of sodium and the simplified assemble process in ambient endows AESSs the possibility of application in large-scale power grid. However, restricted by the narrow electrochemical window (~1.23 V) of water, the common used electrode materials in non-aqueous batteries/capacitors such as transition metal oxides/sulfides/selenides are directly excluded from the scope due to the high voltage platform, other than limited kinds of materials like NASICON-type NaTi₂(PO₄)₃ with appropriate voltage platform. Nevertheless, conventional NaTi₂(PO₄)₃ materials with irregular morphology and large size prepared by solid-state reaction still hinder the application of AESSs. Herein, a newly structured porous single-crystal NaTi₂(PO₄)₃ with uniform sizes was fabricated via a well-designed novel liquid transformation of ultrathin TiO₂ nanosheets, followed by coating a conductive carbon sheath. To best of our knowledge, this is the first report of the porous single-crystal structure of NaTi₂(PO₄)₃ materials. Examined in a three-electrode cell, this NaTi₂(PO₄)₃ electrode demonstrates an outstanding rate capability of 80-102 mA h g^-1 at varied current densities of 0.5-3 A g^-1 due to the synergistic effect between porous nanostructure and outstanding stability originated from single-crystal structure. The high-quality NaTi₂(PO₄)₃ was also assembled with N-doped carbon to fabricate an aqueous Na-ion battery with robust flexibility. This work paves the way for designing advanced NASICON based materials for aqueous energy storage systems.

Ni-rich cathode materials for lithium-ion batteries, commonly in the forms of LiNi_1-x-yCo_xMn_yO₂ (NCM) and LiNi_1-x-yCo_xAl_yO₂ (NCA), are of great interests in commercial applications due to their high reversible capacity and relative low cost compared to LiCoO₂. Due to the well-known Ni/Li cation mixing problem, such a multicomponent (high-entropy) compound cannot be synthesized at elevated temperatures, the more so the higher Ni concentration. This temperature doctrine suppresses the inter-diffusion kinetics between transitional metal ions (Ni, Co, Mn, Al etc.) during material synthesis, hindering a homogeneous distribution and electrochemical properties, and limits the achievable particle size, impairing electrode packing and volumetric energy density. This is part of the reason why solid state synthesis of NCM/NCA materials was not successful. The conundrum is tentatively solved by introducing a specialized co-precipitation technique and self-agglomerated secondary particles (~10 micron) for NCM/NCA systems. Yet the secondary particles would still crack along grain boundaries during electrode pressing and electrochemical cycling. Therefore, micron-size single-crystalline NCM/NCA with good electrochemical properties is a great treasure. In our recent work, we revisited the solid state synthesis of such single-crystalline Ni-rich cathodes and obtained competing capacities with the state-of-the-art co-precipitation technique, thus re-opening the doors to process Ni-rich cathodes in the same synthesizing route as classical LiCoO₂ and genius control of doping and coating at the primary-particle level.

Reactive superoxide triggers side reactions during discharge of lithium-oxygen batteries (LOBs) and therefore affects seriously harmful effects on LOB performances. In living organisms exposed to oxygen, superoxide dismutase (SOD) manages superoxide in a way of converting the reactive species to less reactive oxygen and peroxide. Herein we adopted a functionalized fullerene molecule (MA-C₆₀ where MA = maleic acid) that mimicked dismutation or disproportionation function of SOD. Superoxide-triggered side reactions during discharge were significantly reduced by MA-C₆₀ so that desired oxygen evolution reaction was dominantly encouraged during charge with less CO₂ evolution. Toroidal lithium peroxide particles were generated, which indicated that solution mechanism for peroxide formation was favored. Resultantly, MA-C₆₀-containing LOB cells exhibited tremendously improved capacities especially at high rates.

K-ion battery (KIB) is a new-type energy storage device that possesses potential advantages of low-cost and abundant resource of potassium. To develop advanced electrode materials and electrolytes for accommodating the relatively large size and high activity of potassium is of great interests. In order to address the fast capacity decay caused by severe volume expansion of Sb anode, a novel segment-like antimony nanorod encapsulated in hollow carbon nanotube electrode material (so called Sb@N-C) was prepared by hydrothermal synthesis, polymerization coating and followed by an in-situ pyrolysis and reduction process. The potassium storage performance and mechanism of Sb@N-C in rechargeable KIB were also investigated.
Beneficial from the virtue of abundant nitrogen doping in hollow carbon nanotube, one-dimensional nanotube structure and hollow structure advantages, Sb@N-C exhibited excellent potassium storage properties: based on potassium hexafluorophosphate (KPF₆) electrolyte, the reversible capacity could be maintained 318.6 mA h g^-1 at a current density of 0.5 A g^-1, whereas the cycle stability and rate performance were unsatisfactory. Electrolyte optimization strategy was further used to boost its potassium storage performance, and the optimization mechanism was also disclosed. In the potassium bis(fluorosulfonyl)imide (KFSI) electrolyte, Sb@N-C displayed a reversible capacity of up to 453.4 mA h g^-1 at a current density of 0.5 A g^-1. The rate performance (a reversible capacity of 316.9 mAh g^-1 could be achieved at a current density of 2 A g^-1). Additionally, Sb@N-C demonstrated excellent long-cycle stability at ultra-high current of 5 A g^-1 over 600 cycles, its reversible capacity could be retained at 234.7 mA h g^-1.The results confirm that Sb@N-C nanocomposite is an advanced and superior promising electrode material for KIB. Besides, electrolyte chemistry optimization is an effective strategy for greatly improving electrochemical performance.

Reference:
1. Advanced Energy Materials, 2018, DOI: 10.1002/aenm.201703237.
2. Advanced Energy Materials, 2018, DOI: 10.1002/aenm.201703288.
3. Nano Letters, 2016, 17(1): 544-550.

Multivalent battery chemistries are seen as a promising alternative to energy storage systems based on lithium-ions. Among those, aluminum-based systems have the potential to offer higher energy densities at a lower cost. Aluminum is the most abundant metal in the Earth's crust and the extraction/processing of its ores is one of the most cost-effective for transition metals.¹ Added to this, its ions are trivalent and bear a small ionic radii, which allows for extremely high theoretical gravimetric and volumetric capacities, 2980 mAh g^-1 and 8040 mAh cm^-3, respectively. In fact, the latter figure represents the highest value attained by metal-ion batteries.²

Research in the field of aluminum batteries has focused heavily on electrodes made of carbonaceous materials. Accordingly, it is believed that the high structural quality and low defect density of graphitic carbons are crucial to obtain superior performance and cycling stability in these batteries.^{3, 4} Still, and despite all effort, the capacities reported for these systems remain stubbornly low, particularly when compared to the >300 mAh g^-1 attained by commercial lithium-ion batteries.

We wish to communicate an Al-chloride battery where reduced graphene oxide powder, dried under supercritical conditions, is used as the active cathode material. This system boasts a gravimetric capacity of 171 mAh g^-1 at 100 mA g^-1 and remarkable stability over a wide range of current densities. These properties are thought to be the consequence of the cathode's tailored porosity.⁵ On one hand, its microporosity assists in breaking down the Coulombic ordering of the electrolyte; on the other, the mesoporosity (originated from the drying conditions) facilitates the movement of the large chloroaluminate ions within the active material.

REFERENCES
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2. G. A. Elia, K. Marquardt, K. Hoeppner, S. Fantini, R. Lin, E. Knipping, W. Peters, J.-F. Drillet, S. Passerini and R. Hahn, Advanced Materials, 2016, 28, 7564-7579.
3. H. Chen, F. Guo, Y. Liu, T. Huang, B. Zheng, N. Ananth, Z. Xu, W. Gao and C. Gao, Advanced Materials, 2017, 29, 1605958s.
4. D.-Y. Wang, C.-Y. Wei, M.-C. Lin, C.-J. Pan, H.-L. Chou, H.-A. Chen, M. Gong, Y. Wu, C. Yuan, M. Angell, Y.-J. Hsieh, Y.-H. Chen, C.-Y. Wen, C.-W. Chen, B.-J. Hwang, C.-C. Chen and H. Dai, Nature Communications, 2017, 8, 14283.
5. A. Alazmi, O. El Tall, S. Rasul, M. N. Hedhili, S. P. Patole and P. M. F. J. Costa, Nanoscale, 2016, 8, 17782-17787.

Lithium-sulfur batteries have been extensively considered as a promising alternative for lithium–ion batteries (LIBs) owing to their high theoretical energy density (2500 Wh kg^-1) based on the reaction of lithium with sulfur to form Li₂S. In addition, S is inexpensive, abundant and nontoxic. However, the practical application of lithium–sulfur batteries is hindered by the low conductivity and huge volume expansion of S during the charge-discharge process. Recently, selenium, a congener of sulfur, has also been used as a cathode material with a theoretical capacity of about 675 mA hg^-1. Compared with sulfur, selenium has a better conductivity and it is found that the polyselenides are insoluble in carbonate based electrolytes, indicating that the shuttle effect could be suppressed effectively. Combing the merits of sulfur and selenium, selenium–sulfur solid solution (like SeS₂) incorporated with carbon materials display an excellent performance when using as the cathode materials lithium–sulfur/selenium batteries. Herein, we designed a MOF derived N–doped microporous carbon nanofibers, which was encapsulated with SeS₂. The resultant cathode materials possesses a free standing structure, which could avoid the utilization of non–active additives, thus ensuring a higher energy density. As a result, the Li–S batteries with C–SeS₂ composites deliver a high specific capacity (950 mA hg^-1 @ 0.5C) and outstanding rate performance. The nanofibers could buffer the volume expansion of the S/Se species as well as increase the contact area between the cathode and electrolyte to decrease the ion transmission distance. Furthermore, the in–situ Raman technology was applied to observe the changes during charge/discharge process.

Silicon (Si) is a high energy, low cost anode material for lithium-ion batteries. With a theoretical specific capacity that is almost tenfold higher than graphite, Si has been the focus of extensive research recently for its potential use in electric vehicles and other devices. Unfortunately, Si anode technology has been held back by its lack of cyclability, largely attributed to the structural disintegration of Si electrodes associated with large volume changes during electrochemical cycling. This work aims to address this problem by stabilizing the electrode structure with a robust polymeric coating, allowing for reversible electrochemical reactions. Moreover, spatial molecular layer deposition (MLD) has been developed and applied here to realize the distinct all-organic polymer film chemistry deposited. The aromatic polyamide film chosen for this work has the desired mechanical properties of strength and elasticity that help stabilize Si anodes upon cycling. A very thin film polyamide coating on a Si anode enabled a stable and reversible capacity of 2186 mAh g^-1 over 100 electrochemical cycles. Using a spatial MLD configuration further enables fast growth rates on the order of minutes, thereby permitting ease of future scale-up. This spatially deposited, all-organic thin film coating is a promising material that may enable high energy, low cost lithium-ion batteries.

<span style="font-family:times new roman,serif; font-size:12pt; line-height:115%; margin:0px"><font color="#000000">The upsurge in the market for electric vehicles (EVs) as well as high-power portable electronics demands batteries with higher energy and higher power densities with longer cycle life. Among the available secondary batteries, lithium-sulfur (Li-S) batteries have become quite attractive for next-generation rechargeable batteries; they are known for the high-energy density (~2600 Wh kg^-1) which is five times higher than that of the commercial lithium-ion batteries. Lithium (Li) metal has been considered an ideal anode material for the next-generation high-capacity batteries. However, the practical use of Li metal for Li-S batteries is inhibited due to the parasitic growth of Li dendrites and high reactivity of Li with electrolyte and other active species of polysulfides. Here, we introduce an atomic layer two-dimensional MoS₂ as a passivation layer for Li metal anode. With the Li-intercalated atomic layer of MoS₂ formed, stable Li electrodeposition is realized with the nucleation sites for dendrite growth inhibited. The deposition/dissolution process of a symmetric cell for the MoS₂ coated Li metal operates at a current density of 10 mA cm^-2 with low voltage hysteresis; it shows three-fold improvement in cycle-life than that of the bare Li metal. Using Li-S full cell configuration, the MoS₂ coated Li anodes assembled with 3D carbon nanotube-sulfur cathodes provide superior electrochemical performance demonstrating specific energy density of over 652 Wh kg^-1 and capacity retention (~84%) for up to 1200 cycles with a nominal Coulombic efficiency of ~98%. These exceptional results open a new pathway towards the realization of high energy density and safe Li-metal based batteries.</font></span>

Demand with higher energy density LIB is increasing in many areas such as automobile and group IV elements, such as Si, Ge and Sn, have been received much interest for this purpose. However, the use of these elements is limited due to the large volumetric change during long term charge-discharge cycle which results in undesirable rapid capacity fading, low initial coulombic efficiency and poor rate performance.
Among various approaches to avoid this limitation, chemically modified graphene and SnO₂ has been hybridized to accommodate volume change and improve the capacity and cycling stability of the electrode material. Hybridization of SnO₂ with carbonaceous materials has been used to circumvent this limitation [1-3]. Reddy et al studied hybridization of SnO₂ with long chain alkylamine rafted graphene oxide and specific capacities decreased more with increasing alkylamine chain length [4].
Compared with traditional polymer binders, the self-healing chemistry is designed to enable spontaneous repair of the mechanical damage in the electrode and enhance the lifetime of the anode materials.
In the present study it is attempted to synthesize SnO₂ nanopillared carbon structures using dodecylamine grafted graphene oxide as templates. Self-healing polyurethane is synthesized with disulfide and it is compared with commercial PVDF as a binder.
Structural and morphological characterizations of self-healing polyurethane and electrode were done using FT-IR, XRD, SEM and TEM. Electrochemical studies, such as charge-discharge, cyclic voltametry abd impedence, were carried by fabricated 2032 type coin cells using RGO-SnO₂ as electrode.
Self-healing characteristics of prepared polyurethane is confirmed from restored tensile property and SEM photographs of cutted samples. Electrochemical measurements revealed that the SnO₂ pillared carbon based anode materials with self-healing polyurethane binder showed improved cycling performances with excellent reversible capacity relative to the electrode prepared by poly(vinylidene difluoride).
References
1. C.C. Chang, S.J. Liu, J.J. Wu, C.H. Yang, J. Phys. Chem. C111 (2007) 16423-16427.
2. Y. Fu, R. Ma, Y. Shu, Z. Cao, X. Ma, Mater. Lett. 63 (2009) 1946-1948.
3. X. Zhu, Y. Zhu, S. Murali, M.D. Stoller, R.S. Ruoff, J. Power Sources 196 (2011) 6473-6477.
4. M. Jeevan Kumar Reddy, Sung Hun Ryu, A. M. Shanmugharaj, Nanoscale 8 (2016) 471-482.

Since the introduction of lithium-ion batteries (LIBs) to market in 1991, their performance has improved significantly, which has been achievable through development in materials technologies. However, further breakthroughs are still needed to ameliorate cycle-life, safety and energy density of LIBs. This requires new electrode materials and a detailed understanding of the electrochemical mechanisms during cycling. Transition metal phosphates are interesting candidates as cathode materials for LIBs [1]. In this work, we report the electrochemical performance of FPHH/C and FPHH/CNT composites where FPHH represents Fe_1.19(PO₄)(OH)_0.57(H₂O)_0.43 while carbon black and carbon nanotubes (CNT) were used as precursors in the one-pot hydrothermal synthesis, respectively. We show that the addition of conducting carbon black into the solution has a strong influence on reducing the particle size and tailoring their morphology, but does not interfere with the formation of the FPHH phase. Thanks to its favorable microstructural characteristics, the FPHH-10 wt% C and FPHH-20 wt% C materials exhibited good performance [2]. The CNT also improve the performance of FPHH such as capacity retention (500 cycles at 1 C).
The mechanisms of lithiation-delithiation were investigated by combining operando X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy. FPHH undergoes a monophasic reaction based on Fe³⁺/Fe²⁺ redox process. However, the variations of the lattice parameters and ⁵⁷Fe quadrupole splitting indicate a more complex mechanism than a random occupation of the vacant sites within FPHH. This can be related to the peculiar structure of FPHH formed by chains of face sharing (Fe_{0.6 0.4})O₆ octahedra connected by PO₄ tetrahedra and by channels for Li diffusion along [100] and [010] directions. The existence of Fe vacancies provide interconnections between the one-dimensional channels, improving lithium diffusion within FPHH. This mechanism, combined with the addition carbon black or nanotubes in the solution prior to hydrothermal treatment as a simple and effective way to reduce particle size and improve electronic conductivity, provides good cycle life and rate capability for FPHH.

Acknowledgements
A. Mahmoud is grateful to University of Liege and FRS-FNRS for the grants and thanks to the Walloon region for a Beware Fellowship Academia 2015-1, RESIBAT n° 1510399. Part of this work was supported by the Walloon Region under the “PE PlanMarshall2.vert” program (BATWAL – 1318146).

References
1. C. Karegeya, A. Mahmoud, F. Hatert, B. Vertruyen, R. Cloots, P.E. Lippens, F. Boschini, Journal of Power Sources 388 (2018) 57-64.
2. C. Karegeya, A. Mahmoud, R. Cloots, B. Vertruyen, F. Boschini, Electrochim. Acta 250 (2017) 49-58.

Li-ion batteries (LIBs) took over the lead in rechargeable battery technologies since their introduction in the early 1990s. However, the price and abundance of lithium element are still the issues and alternative cell chemistries based on abundant elements might become important especially when stationary energy storage reaches its market breakthrough. Therefore, alternatives to “lithium-ion technology” are being examined. Sodium-ion batteries (SIBs) are recently being revisited as attractive alternative. The hope to realize more cost-effective batteries rises due to the large abundance of sodium. Although SIBs often have lower energy densities and cell voltages compared to their lithium analogues, the lower polarization of the sodium-ion might enable cells with peculiar advantages over conventional lithium-ion technology^[1].

Due to its low cost, safety and good cycling performance, graphite is currently the preferred choice as an anode material for LIBs. Graphite is also favored in SIBs but storage of sodium-ions is only possible by formation of ternary intercalation compounds ^[2]. Instead of the naked ion, the solvated ion is intercalated in between graphene layers. The co-intercalation of the ether based solvent molecule causes an enormous volume change in graphite lattice and so in the electrode. In order to observe this change, in situ electrochemical dilatometry (ECD) can be used to measure changes in the electrode thickness during charging and discharging. Phase transitions during de/insertion of ions, irreversible reactions such as solid electrolyte interphase (SEI) formation or structural changes such as delamination of graphite can be followed on-line. Overall, the studied electrode reaction behaves very different from conventional intercalation reactions of graphite. Finally, and maybe most intriguingly, the reaction is possibly the first case of an SEI-free graphite anode material ^[3]. Moreover, formation of gasses is examined during cycling by online electrochemical mass spectrometry (OEMS).

Moreover, was also present a systematic study on temperature effects related to the intercalation of solvated sodium ions into graphite. For this, a series of glymes (mono- to pentaglyme) and several crown ethers are used.

[1] P. K. Nayak, L. Yang, W. Brehm, P. Adelhelm, Angewandte Chemie International Edition 2017.
[2] B. Jache, P. Adelhelm, Angewandte Chemie International Edition 2014, 53, 10169-10173.
[3] M. Goktas, C. Bolli, E. J. Berg, P. Novák, K. Pollok, F. Langenhorst, M. v. Roeder, O. Lenchuk, D. Mollenhauer, P. Adelhelm, Advanced Energy Materials 2018, 1702724.

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. However, challenges such as polysulfide dissolution and shuttling prevent mass commercialization of sulfur cathode batteries. Here we show a practical yet comprehensive approach for development of high performance metal sulfur batteries. Aramid nanofiber (ANF) based composite asymmetric separator [1] not only prevent dendrite formation[2] 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. These separators therefore provide a safe and long cycle life as well as high performance metal sulfur batteries. Fabrication of such safe, 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.
References:
[1] M. Yang et al., “Aramid nanofiber-reinforced transparent nanocomposites,” J. Compos. Mater., vol. 49, no. 15, pp. 1873–1879, 2015.
[2] S.-O. Tung, S. Ho, M. Yang, R. Zhang, and N. A. Kotov, “A dendrite-suppressing composite ion conductor from aramid nanofibres.,” Nat. Commun., vol. 6, no. 2015, p. 6152, 2015.

Lithium-rich layer oxide, Li_1.2Ni_0.16Mn_0.56Co_0.08O₂ (NMC), is a potential cathode candidate for high-energy density batteries. Issues such as cycling stability, rate performance, and cost are yet to be overcome before successful commercialization of the material. Here, we report on the synthesis of Cr-doped lithium-rich phases Li_1.2Ni_0.16Mn_0.56Co_0.08-xCr_xO₂ (where x=0.0, 0.01 & 0.02) (NMC-Cr) by the sol-gel technique. Cr is homogeneously distributed in the crystal structure evidence by XRD, XPS and elemental mapping measurements. The Cr-doped materials exhibit much better cycling stability with 100% capacity retention versus 44% for the undoped sample after 50 cycles. The Cr-doped samples show excellent electrochemical performance at higher C-rate in comparison with the undoped NMC. The latter shows rapid capacity fading from 220 to 50mAhg^-1 at the 0.1 to 1C rates, respectively. Moreover, the Cr-containing materials do not show significant signs of voltage fading during cycling owing to the stabilization of the crystal lattice by chromium. The electrochemical impedance spectroscopy measurements also indicate the stable cell resistance on cycling for the Cr-doped phases compared to the undoped phase.

As Li-ion batteries are long advanced for its electrochemical properties in application to both electric vehicles and devices, many ongoing researches are focusing on the improvement of energy density, capacity, cycling stability and rate performance. These can divide into three main parts in Li-ion batteries; cathode, anode, and electrolyte. One of the solution for this enhancement is to find novel cathode materials. This work focuses on the synthesis and characterization of Li,Mn-rich cathode, LiCo_0.1625Ni_0.1625Mn_0.675O₂, for Li-ion batteries. The layered Mn-rich transition metal (Mn, Co, Ni) hydroxide precursor was synthesized via facile co-precipitation method in a continuous stirred tank reactor (CSTR). Under proper control of pH, temperature, time, concentration of the reactants, and feeding rates in the reactor, uniform spherical particles were obtained. The hydroxide precursor was then undergone two-step heat treatment process to achieve Li,Mn-rich cathode powder. Morphology and structure were examined using SEM and XRD, respectively, showing plate-like primary particles which intercalated into spherical secondary particle shape (average size ~ 17 μm). Chemical stoichiometry was confirmed by ICP-OES technique and electrochemical performance was studied to examine the reliability of this cathode for large scale production and its future commercial use in Li-ion batteries. Drawback of Li,Mn-rich, such as capacity fading and voltage decay, were discussed with an effort to minimize these issues.

The VO₂(M/R) system undergoes a structural change from monoclinic [VO₂(M)] to rutile [VO₂(R)] phase at a temperature that is easily accessible and corresponds to an electrical conductivity increase two orders of magnitude. The ability to exploit electrical conductivity makes the system attractive for study as a lithium ion battery electrode considering uniform electron access can be a limiting factor in producing electrodes that deliver high capacities. In the work presented, several forms of characterization were employed to gain insight on the relationship of structure and electrochemical function. Synchrotron based x-ray powder diffraction (XPD) data was used to monitor the structural changes as a function of temperature. Electrochemical impedance spectroscopy was utilized to track impedance as a function of temperature. Further, the VO₂ system was then tested in two electrode cells to determine the impact of the structural transition on functional electrochemistry. The results from the compliment of experiments provides a foundation for investigating charge transport properties in polymorphic materials and sets a precedent for understanding the impact of phase changes on electrochemistry in a complex energy storage system.

A critical challenge for electrical energy storage is to achieve more useful work (w) and minimize the generation of waste heat (q). Batteries have often been approached at the macro level, where bulk parameters are identified and manipulated, with optimization as an ultimate goal. However, such a strategy may not provide insight toward the complexities of electric energy storage, especially when addressing multiple length scales in application and demands on devices. Beginning from a fundamental approach of identifying and reducing sources of localized resistance facilitates the understanding of the inherent heterogeneity of ion and electron flux both at multiple interfaces and length scales.At a fundamental level, it is necessary to identify and reduce sources of localized resistance and to understand the inherent heterogeneity of ion and electron flux at numerous interfaces found at several scale lengths within a battery. Benefits from experimentation and characterization over multiple length scales will be highlighted in this presentation.

The suppression of lithium dendrite is critical to the application of lithium metal batteries. Many approaches have been applied previously to demonstrate the improved cycling performance with lithium mental anode, which include the 3D conductive framework, lithium metal surface protection and the formation of special SEI protection layer, etc. In this talk a combined simulation and experiment approach is used to understand the new principles behind these approaches. Innovative synthesis procedures, electrochemical battery tests and morphology and spectroscopy characterizations are used to demonstrate the advanced dendrite proof capabilities. Specifically, new approaches to construct the general 3D conductive framework from nonconductive materials and to apply the surface protection layers are demonstrated. Moreover, combined thermodynamic modeling, DFT simulation and new modeling approach is used to understand the dendrite growth thermodynamics and kinetics down to the atomic scale. We further propose some new design principles behind these technological approaches based on our experiment and theory.

Enabling stable and safe reversible Li metal anode is essential to achieve a specific energy density of 500 Wh/kg from a cell level in next-generation Li batteries. The low Coulombic efficiency and dendrite growth issues significantly hinder the commercialization of Li metal anode. It is well accepted that after electrochemical cycling, formation of "inactive" Li, consisting of Li ions that form SEI (Li⁺) and SEI wrapped Li metal (Li⁰), is a direct reason for capacity loss. Differentiating and quantifying Li⁺and Li⁰ after cycling is one of the most critical yet challenging problems that impedes the thorough understanding of the failure mechanism of Li metal anode. A new chemical analytic method has been introduced in this work and provides a solution enabling the quantitative measurement of Li⁰ content in cycled Li metal cells at microgram (µg) level for the first time. Combining with Cryo-FIB-3D reconstruction, Cryo-TEM and XPS, a correlation among mass content, microstructure, SEI nanostructure and chemcial composition has been established to investigate the properties of "inactive" Li generated in different electrolytes.

Lithium metal is known as the “Holy Grail” of anode materials, as it has the highest theoretical capacity, lowest density, and most negative electrochemical potential of known anode materials for rechargeable batteries. Unfortunately, dendrites of lithium form during repeated cycling, posing a safety hazard and deterring commercialization of lithium metal batteries. Previous studies, each with different methods of sample preparation and testing methods, show that Lithium’s yield strength may vary by more than 2 orders of magnitude (from ~1 MPa to 100-300 MPa). However, comprehensive knowledge of the mechanical behavior of Li remains a key obstacle to understanding how to engineer anode-separator interfaces that can mitigate or suppress dendrites. Through a combination of in-glovebox tensile testing and nanoindentation in as-received lithium ribbon, we probe the mechanical properties of Li metal at different length scales.

Formation and growth of dendrites on Li metal surfaces is a well-known problem and is one of the main reasons that prevent the successful operation of Li-metal batteries. Numerous mitigation strategies are continuously being developed such as electrode coatings, artificial solid electrolyte interphase (SEI) films, electrolyte design for specific SEI compositions, and use of solid electrolytes among others. In some of these processes, mitigation is induced by chemical or mechanical (or both) modifications of the environment surrounding the sites where Li ions are plated. Here we use first principles calculations first to investigate why dendritic nucleation occurs and what properties should be tuned to mitigate such phenomena. We then analyze the microscopic mechanisms behind some of the proposed strategies and determine which ones have the best probabilities of a long-term solution to this problem.

Li metal anode has been deemed as the ultimate choice of rechargeable Li batteries with high energy density due to highest capacity and lowest electrochemical potential. However several challenges, such as severe dendrite formation, poor Coulombic efficiency, and drastic volume expansion, impeded its application in practice. Herein, we utilize the lignin, one of the most abundant but underutilized biomaterial on earth, to fabricate a layered and holey carbon. This novel structure owns enlarged interlayered gap in micrometer scale and holes in each layer, which can decrease the area current density, accelerate the ion transference and stabilize the Li metal to some extent. The Li anode with novel host exhibits high Coulombic efficiency (~97% over 350 cycles), large areal capacity (20 mA h cm^-2), and long life-span in cycling (>500 h, 250 cycles) at a high current density of 4 mA cm^-2. In the full cell with LiFePO₄ cathode, a high capacity of 90 mA h g^-1 was achieved and kept stable for 1800 cycles with a high capacity retention (>92%) under a high current density of 10 C (corresponding to 4.5 mA cm^-2).

Next-generation energy storage devices, such as Li-ion batteries (LIBs) and Li-sulfur batteries (LiS), demand high energy, power and better safety. Conventional graphite anode in Li-ion batteries falls short of fulfilling all these necessities. Carbon nanostructural materials have gained the spotlight as promising active materials for energy storage; they exhibit unique physico-chemical properties such as large surface area, short Li⁺ ion diffusion length, and high electrical conductivity, in addition to their long-term stability. Carbon-nanostructured materials have issues with low areal and volumetric densities for the practical applications in electric vehicles, portable electronics, and power grid systems, which demand higher energy and power densities. One approach to overcoming these issues is to design and apply a three-dimensional (3D) electrode accommodating a larger loading amount of active materials (e.g., sulfur) while facilitating Li+ ion intercalation. Furthermore, 3D nanocarbon frameworks can impart a conducting pathway and structural buffer to high-capacity non-carbon nanomaterials, which results in enhanced Li⁺ ion storage capacity. Recent advance of two-dimensional (2D) materials enables us to design/fabricate atomic layer deposition on electrode materials for high-efficiency active electrode materials: atomic layered 2D MoS₂ - coated Li metal demonstrates a stable Li electrodeposition with the suppression of nucleation sites for dendrite growth. The MoS₂ coated Li anodes assembled with 3D carbon nanotube-sulfur cathodes provide superior electrochemical performance in Li-S batteries ever reported to date. The superior performance of 3D carbon nanotubes and 2D materials coated Li-metal in energy storages will be presented along with their mechanistic analysis.
References:
1. High performance rechargeable Li-S batteries using binder-free large sulfur-loaded three- dimensional carbon nanotubes, M Patel, E Cha, C Kang, B Gwalani, W Choi, Carbon http://dx.doi.org/10.1016/j (2017)
2. Recent development of 2D materials and their applications, Wonbong Choi, Nitin Choudhary, Juhong Park, Deji Akinwande, Younghee Lee, Materials Today, 116-130, 20, (2017).
3. 2D MoS₂ as an efficient protective layer for lithium metal anodes in high performance Li-S batteries, Cha, E., Patel, M.D., Park, J., Hwang, J., Prasad, V., Cho, K., and Choi, W., Nature Nanotechnology, 13, pages337–344 (2018).

Engineering the surface chemistry of lithium ion battery materials is necessary for the development of safe, stable, high energy-density cells. The application of ultrathin film coatings is a widely explored strategy to design surface chemistries and mediate electrochemical reactions. The preponderance of work in this field has investigated inorganic thin film coatings, very often metal oxide materials. This talk will describe work in our group to engineer organic surface chemistries using polymeric thin films. We have developed two approaches to apply polymer thin films on battery electrodes. The first approach is surface-initiated atom transfer radical polymerization. Polymerization initiators are tethered to the surface of a model thin film SI anode and then used to synthesize poly(methyl methacrylate) (PMMA) brushes of tunable thicknesses from approximately 20nm to several hundred nanometers. It was shown that the presence of 75nm brushes reduces the first cycle irreversibility on thin film Si anodes to 23.7%. The irreversibility in untreated Si is 37.6%. Post-mortem FTIR-ATR confirmed that less ethylene carbonate is reduced on the PMMA-coated Si in the first cycle, and electrochemical impedance spectroscopy showed that the PMMA brushes inhibit the growth of a resistive surface layer during extended cycling. In the second approach, initiated chemical vapor deposition (iCVD) is used to apply ultrathin conformal polymer layers on conventional lithium ion battery electrodes prepared by slurry casting. The conformality and coverage of the polymer coatings was confirmed by scanning electron microscopy and x-ray photoelectron spectroscopy. For coatings on lithium ion battery anodes, polymer compositions with high crosslinking densities were developed to exclude liquid electrolyte from the electrochemical interface. Three distinct polymer chemistries have been applied by iCVD: crosslinked poly(lithium methacrylate) (PLiMA), poly(ethylene glycol diacrylate) (PEGDA), and poly(1,3,5-trimethylcyclotrisiloxane) (PV3D3).With these crosslinked films, there is a trade-off between the reduction in irreversible side reactions and an increased area specific resistance. We have also demonstrated that capacity retention of full cells (NMC vs. graphite in 1M LiP6 in EC:DMC:DEC) at 55°C is improved by coating both cathode and anode with these ultrathin films. Our efforts to understand how polymer film chemistry and morphology influences the electrochemical reactions in lithium ion cells will be discussed.

The demand for large-scale renewable energy generation and electric mobility is rising the need for high capacity and safe energy storage systems. Utilizing metal anodes are gaining momentum, owing to their very high energy densities compared to conventional intercalation-based electrodes. Nevertheless, considering the hostless nature of the metal anodes and their interfacial instability, the practical utilization of such systems has been restricted. Inhomogeneous metal electrodeposition (dendrites) and unwanted byproduct formation during cycling limit the cycle life and safety of the metal anode-based batteries. Therefore, research community has focused on designing innovative approaches to regulate the deposition behavior of metal anodes. In this context, one of the most reliable methods is the use of ultra-thin and ultra-stable materials on the metal anode to both prevent the side reactions at the interface and also suppress the formation of dendritic deposits. Among the proposed solutions, 2D materials are promising candidates, owing to their ultrahigh mechanical strength, superflexibility and chemical stability. However, it is not truly clarified that how these approaches affect the nucleation and growth modes of the metal during the electrodeposition.
In this work, we have studied the nucleation and growth mechanism of lithium (Li) and zinc (Zn), as examples of metal anodes, in the presence of high-quality graphene (Gr) layer. Interestingly, addition of an ultra-thin layer of carbon is able to significantly regulate the morphology and electrochemical performance of these metal anodes. Utilizing electrochemical potential tests and scanning electron microscopy (SEM) the nucleation mechanism was explored. Accordingly, Li forms homogeneous spherical nucleation electrodeposits all over the Gr-coated electrode surface, being different from randomly whisker like deposition in case of bare electrode. Utilizing transmission and scanning electron microscopy we detected that upon further electrodeposition, in contrast to the expected highly dendritic Li deposition, Li spheres can develop into a uniform and compact structure composed of vertically aligned Li nanorods. Moreover, despite the randomly oriented inhomogeneous deposition of multi-crystalline Zn on the bare substrate, a planar Zn deposition, composed of single crystalline flat flakes, was observed in case of Gr-coated sample. Therefore, we can conclude that high quality graphene not only provides a homogenous metal-ions nucleation, but also regulates the morphology and growth orientation of the final deposition products, significantly. Further experimental and computational efforts are being carried out to provide comprehensive explanations for our observations is this research. Overall, we believe that such systematic studies can pave the way in the evolution of such surface engineering approaches into the industrial scale applications of rechargeable metal batteries.

A stable and high efficiency metal anode (such as Li, Na, Zn, Mg) is critical for all rechargeable metal batteries, including the batteries with various cathodes such as ion intercalation compounds, conversion materials, sulfur, and oxygen. An ideal metal anode not only needs to have a very high Coulombic efficiency, but also need to have a low concentration, high conductivity and low cost. Recently, we have developed a series of “localized high-concentration electrolytes (LHCE)” by diluting high-concentration electrolytes with electrochemically “inert” solvents or poorly solvating diluents. Unlike the high concentration electrolytes reported before, the electrolyte reported in this work exhibits low concentration, low cost, low viscosity, improved conductivity, and good wettability to separator and electrodes. With selected Li salt, solvent, and the diluent, we demonstrated a fire-retardant LHCE that enables stable, dendrite-free cycling of LMAs with high coulombic efficiency of up to 99.2%. Moreover, this electrolyte exhibits excellent anodic stability even up to 5.0 V and greatly enhances the cycling performance of LMBs. A Li|| LiNi_0.6Mn_0.2Co_0.2O₂ battery using this electrolyte can retain > 97% capacity after 600 cycles at 1C rate (ca. 1.6 mA cm^-2), corresponding to a negligible capacity decay of < 0.005% per cycle. Similar concept of “localized high-concentration electrolytes (LHCE)” have also been used to stabilize Na metal anode for more than 40,000 cycles. In addition, this concept was also used to expand the electrochemical windows of water based electrolytes in a salt concentration much less than those reported before. Therefore, this new approach opened a window for further development of novel electrolyte for practical high-energy metal batteries.

With the fast-growing demands for high energy storage, lithium (Li)-ion batteries (LIBs) can no longer satisfy the application needs due to their relatively low energy densities. Li metal batteries (LMBs) are regarded as one of the most promising next-generation energy storage systems. The key to enable long-term cycling stability of high-voltage LMBs is the development of functional electrolytes that are stable against both Li anodes and high-voltage (>4 V vs. Li/Li⁺) cathodes. Due to their limited oxidative stability (< 4 V