Symposium ES03 : Electrochemical Energy Materials Under Extreme Conditions

The intrinsic instability of carbon largely limits its use for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as a bifunctional catalyst in reversible fuel cells or water electrolyzers. Herein, we discovered that Mn doping has a promotional role in stabilizing nanocarbon catalysts for the ORR/OER in alkaline media. Stable nanocarbon composites are derived from an inexpensive carbon/nitrogen precursor (i.e., dicyandiamide) and quaternary FeCoNiMn alloy via a template-free carbonization process. In addition to FeCoNiMn metal alloys/oxides, the carbon composites comprise substantial carbon tube forests growing on a thick and dense graphitic substrate. The dense carbon substrate with high degree of graphitization results from Mn doping, while active nitrogen-doped carbon tubes stem from FeCoNi. Catalyst structures and performance are greatly dependent on the doping content of Mn. Various accelerated stress tests (AST) and life tests verify the encouraging ORR/OER stability of the nanocarbon composite catalyst with optimal Mn doping. Extensive characterization before and after ASTs elucidates the mechanism of stability enhancement resulting from Mn doping, which is attributed to (i) hybrid carbon nanostructures with enhanced resistance to oxidation and (ii) the in situ formation of the β-MnO₂ and FeCoNi-based oxides capable of preventing carbon corrosion and promoting activity. Note that the improvement in stability due to Mn doping is accompanied by a slight activity loss due to a decrease in surface area. This work provides a strategy to stabilize carbon catalysts by appropriately integrating transition metals and engineering carbon structures.

Polymer electrolyte fuel cells (PEMFCs) have been considered a promising mobile energy conversion system to replace internal combustion engines due to high energy conversion efficiency, moderate operating temperature, short fuel charging time, and zero emission. Accordingly, PEMFC is one of the promising candidates for vehicle electrification as exampled by fuel cell electric vehicle. Even though PEMFC technology has arrived at a commercialization stage, the durability of fuel cell stack needs to be further improved to guarantee reliable operations under harsh conditions. Various degradation modes critical to the durability have been identified; the hydroxyl radical formed by crossover H₂ or O₂ in the presence of Pt catalyst deteriorates membranes and ionomers in the catalyst layers; the stress generated during dynamic change in hydration level of the cell leads to a mechanical defect at the membrane/catalyst layer interface or membrane. These chemical and mechanical degradation issues, which eventually result in a cell failure, still remain unsolved. During the past few years, we have addressed the durability problems of PEMFC by engineering the membrane/catalyst layer interface. We witnessed that the interfacial durability between membrane and catalyst layer can be improved by introducing an interfacial bonding layer such as Lego-block structured and ball-socket joint structured interfaces, enhancing the humidity cycling stability. Recently, we found new possibilities that interface material and structure design can also improve the mechanical and chemical durability of membranes. In this talk, we will present our results on how the fuel cell durability under harsh conditions can be improved by advanced interfacial engineering.

The polymer electrolyte membrane fuel cell (PEMFC), which is an energy technology for directly transforming chemical energy into electrical energy by oxidizing hydrogen at the anode and reducing oxygen at the cathode, is a promising way to resolve the energy crisis and environmental pollution. However, realizing their implementation as a practical and highly efficient energy conversion system remains a big challenge because the chemical instability of metal bipolar plates in the harsh acidic (pH ~3-5) and humid operating environment inside PEMFCs leads to decreased performance and durability. To improve the chemical durability of metal plates, various protective layers such as TiC, CrC, and TiN/CrN₂ have been applied to their surfaces.^1,2 Even though the corrosion resistance of metal bipolar plates coated with protective layers has been enhanced, their contact resistances remain above the target value (technical targets for 2020 set by the US Department of Energy (DOE): interfacial contact resistance < ~10 mΩ cm² and corrosion current density < ~1 µA cm^-2).³ Besides, another obstacle to commercial utilization of metal bipolar plates is the difficulties in precise machining and designing of flow channels used as a distributor of reactant gases on the metal plate.
In this work, we present a novel method for coating highly crystalline multilayer graphene (Gr) as a superficial protective layer (thickness of ~12 nm) onto a 6 × 6 cm² Ni foam in short growth times (t ≤5 min) via the facile and rapid thermal annealing (RTA) of poly(methylmethacrylate) as a solid-state C source. The synthesized graphene layers have a low defect density and completely cover the three-dimensional (3D)-structured surface of the Ni foam, dramatically improving its corrosion resistance, interfacial contact resistance (ICR), and hydrophobicity. After stability tests in the operating environment of a PEMFC, the optimized Gr-coated Ni foam preserved its exceptionally low corrosion current density of 2.5 µA cm^-2 with an ICR of 9.3 mΩ cm² at 10.1 kgf cm^-2. A H₂/air PEMFC fabricated using the Gr-coated Ni foam as bipolar plates showed an exceptionally enhanced maximum power density of ~967 mW cm^-2, which is the best among those of the reported metal foam bipolar plates. Moreover, all the characteristic values meet the 2020 DOE technical target values for transportation applications of PEMFCs. This study demonstrates that the 3D Gr-coated Ni foam prepared using our proposed new coating method exhibits superior characteristics as an inhibitor for the highly efficient performance of a PEMFC with durability. Our facile coating approach can pave the way to further enhance energy conversion systems through interface engineering.

Reference
1. M. G. Wu, C. D. Lu, D. P. Tan, T. Hong, G. H. Chen and D. H. Wen, Thin Solid Films, 2016, 616, 507–514.
2. M. Omrani, M. Habibi, R. Amrollahi and A. Khosravi, Int. J. Hydrogen Energy, 2012, 37, 14676–14686.
3. 2020 DOE technical targets for polymer electrolyte membrane fuel cell components, http://energy.gov/eere/fuecells/, 2013.

Developing cost effective Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts are crucial for the wide application of many electrochemical energy conversion and storage devices. Due to the low cost and high activity, transition meal oxides have attracted great attention as the alternative electro-catalysts to replace currently used noble metal catalysts. Anion defects, such as oxygen vacancies and interstitials, recently were found to significantly change the electronic structure of the oxides, selectively enhance the stability of specific intermediates, and lead to unconventional chemical activities of reactants on the catalyst surface. Therefore, anion defect engineering presents an effective way to enhance the activity of transition metal oxides. In this work, we introduce controllable oxygen vacancies into perovskite cobaltite (La,Sr)CoO₃ and (Pr,Ba,Sr)₂(Co,Fe)O₅ by several approaches. The presence of oxygen vacancies was found to significantly impact the electro-activity of the materials. Combining experimental approaches and first principle calculation, the impact of the anion defects were investigated systematically.

Different primary and rechargeable battery technologies have been used in NASA missions, based on the requirements of energy and power, and more importantly those driven by the environments anticipated in the missions. In addition to the usual needs of high specific energy, high energy density, high power density and long calendar life, planetary surface missions, including landers, rovers and probes to outer planets, i.e., Mars and beyond, require power sources operating at ultra-low temperatures ranging from -40 to -100 ^oC, depending on the thermal management provided in the spacecraft. On the other hand, atmospheric aerial missions and surface missions on inner planets, e.g., Venus, require power technologies that can survive high temperatures up to 465 ^oC. Another unique requirement for the power sources in NASA missions, especially missions to Jupiter and its moons, is tolerance to high intensity radiation environment of ~10 Mrad. Further, for the missions to Ocean Worlds that are more likely to have extant life, e.g., Europa, Titan and Enceladus, the power sources need to be biologically sterile, which is currently accomplished only by irradiation to about 10 MRad.
In support of these various missions, we developed, and are still developing in some cases, different primary and rechargeable battery technologies for extreme environments. The primary battery technologies have been developed include lithium-thionyl chloride cells for operations at -80C, similar lithium oxyhalide chemistries for lower operating temperatures of -100 ^oC, and lithium-carbon fluoride batteries with high specific energy of >700 Wh/kg and good radiation tolerance. The rechargeable battery technologies that have been advanced include low temperature lithium-ion chemistries operating at low temperatures of -40 to -70 ^oC and high specific energy candidates offering high specific energy of 220 Wh/kg or higher combined with tolerance to ~20 Mrad of radiation. Finally, we have also focused, to less extent, on high temperature sodium rechargeable molten salt batteries with metal chloride cathode operational at 20-350 ^oC and lithium molten salt primary batteries that can survive and operate at 465 ^oC. In this paper, we will briefly describe these technological advances on energy storage technologies for extreme environments in conjunction with the mission descriptions.

Conventional bio-based plastics are usually low performance plastics in terms of thermo-mechanical properties. Here we have successfully synthesized bio-based polybenzimidazole consist of high thermo-mechanical properties comparable with engineering plastics, owing to strong π-π stacking interaction among aromatics and imidazole rings and H-bonding between N-H and N of imidazole ring. Imidazole proton can be easily modified by various substituents. Here we report N-boronation of the PBI via lithiation to be ionically conductive (scheme 1). The PBI was modified by triethylborane substitution to imidazole proton to create boronated PBI (B-PBI) with Li counter ion. Ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMImTFSI) was added to the B-PBI as a plasticizing agent with different concentration to prepare composite solid polymer electrolytes (SPEs). Impedance analysis of various composition SPEs show ionic conductivity with 1.4x10^-2 Scm^-1 at maximum whereas Li-ion transference number was obtained (t_Li⁺=0.623) at maximum. Ionic conductivity, Li-ion transference number, and carrier ion number increased with increasing amount of BMIMTFSI, to show maxima at a certain concentration. Further, charge-discharge measurement was performed by fabricating anodic half-cell composed of the highest li-ion conducting composite SPE as electrolyte, Si-electrode as anode, and Li-metal as cathode. The resulting value showed a good reversible capacity at discharge rates ranging 0.1 – 2.0 C and stable solid electrolyte interface formation takes place.

Protons are the relevant electric charge carriers in the sustainable hydrogen economy [1]. Because of humidity (H2O) in ambient environment and the crystalline imperfections (defects) in materials, water molecules like to enter some materials and settle as hydrates or hydrides structures. Oxygen and protons become part of the structure. Upon thermal excitation, hydroxyl bonds may become hydrogen bonds which eventually "melt ". The protons may then liberate and become electric charge carriers, which lend them a particular new function in solid electrolytes as proton conductors. We have in the last couple of years observed and investigated the biography and lifestyle of such protons from localization to de-localization. The proton is an elusive player and not always easy to make out. With a combination of neutron and synchrotron based scattering and spectroscopy methods, along with electroanalytical techniques, we have increased our understanding of the proton dynamics and its structural origin, which is important for super-protonic conductivity. We have investigated the oxygen vacancy filling of engineered oxygen deficient proton conductors by water molecules with impedance spectroscopy and ambient pressure XPS [2], which enabled us to sketch a detailed picture of the correlation of molecular and electronic structure changes, with concomitant onset of proton conductivity at higher temperatures. We thus could design experiments, where the proton-phonon coupling was quantitatively investigated with high pressure and high temperature impedance spectroscopy combined with quasi-elastic neutron scattering [3,4]. Supported by pressure dependent XRD and Raman scattering [5,6] we correlated the proton jumping parameters with the temperature and found that the proton jump times follow a polaron relation [Braun 2017] [7,8,9].
[1] Q. Chen Q, A. Braun, Protons and the hydrogen economy. MRS Energy & Sustainability 2017, 4.
[2] Q. Chen et al., Chem. Mater. 25 (23), 4690 (2013).
[3] Q. Chen et al., Solid State Ionics 252, 2 (2013).
[4] Q. Chen et al., High Pressure Research 32(4), 471 (2012).
[5] Q. Chen et al., J. Phys. Chem. C 115 (48), 24021 (2011).
[6] Q. Chen et al., J. Eur. Ceram. Soc. 31 (14), 2657 (2011).
[7] Q. Chen et al., Appl. Phys. Lett. 97, 041902 (2010)
[8] A. Braun et al., Appl. Phys. Lett., 95, 224103 (2009).
[9] A. Braun, Q. Chen, Experimental neutron scattering evidence for proton polaron in hydrated metal oxide proton conductors. Nature Communications 2017, 8:15830.

Lithium-sulfur (Li-S) batteries offer a theoretical energy density of ~2600 Wh/kg (compared to ~387 Wh/kg for Li-ion technology) and therefore offer great potential as a next generation energy storage device. However there are two major barriers to realization of high performance Li-S batteries: (1) poor cycle stability caused by dissolution of intermediate lithium polysulfides from the S cathode into the electrolyte and (2) nucleation and growth of dendritic structures on the Li metal anode, which can electrically short the battery. In this talk, I will discuss some possible solutions to these problems. Specifically, I will show that two-dimensional (2D) sheets of black phosphorous (i.e. phosphorene) are highly effective as a lithium polysulfide trapping agent. I will further show that the Li dendrite problem can be addressed by using self (Joule) heating to accelerate surface diffusion processes to heal (smoothen) the dendrites in situ. Such advances show potential in enabling the successfully deployment of Li-S batteries with breakthrough improvements in performance as compared to the incumbent Li-ion technology.

Related Publications:

(1) L. Li, L. Chen, S. Mukherjee, J. Gao, H. Sun, Z. Liu, X. Ma, T. Gupta, C. V. Singh, W. Ren, H.-M. Cheng, N. Koratkar, “Phosphorene as a Polysulfide Immobilizer and Catalyst in High-Performance Lithium-Sulfur Batteries”, Advanced Materials 29, 1602734 (2017).

(2) Lu Li, Swastik Basu, Yiping Wang, Zhizhong Chen, Prateek Hundekar, Baiwei Wang, Jian Shi, Yunfeng Shi, Shankar Narayanan and Nikhil Koratkar, "Self-Heating Induced Healing of Lithium Dendrites", Science 359, 1513-1516 (2018).

Lithium-sulfur batteries (LSBs), have been considered as promising power source for future electric vehicles (EVs) due to their high energy and power densities. Thus, many efforts have been made on new electrode materials that can bring the realization of these devices. Despite offering many fascinating advantages over conventional lithium-ion batteries (LIBs), such as low cost of sulfur, the safer operating voltage and in particular non-toxic nature, LSBs are facing numerous challenges, which are currently hindering LSBs from being a serious competitor to LIBs on commercial scale. Among the mainstream challenges, the polysulfide shuttle effect, insulating nature of sulfur, volume expansion, and self-discharge are very crucial factors which need to be resolved urgently to improve the electrochemical performance of LSBs. Moreover, structural disintegration, limited access to redox sites and loss of electrical contact have long been identified as common reasons for capacity loss and poor cyclic life of these materials. Thus, rational design can inhibit the side reaction by surface protection, make all redox sites accessible by increasing the intrinsic conductivity of the active materials, maintain a continues network for ionic and electronic flow and keep the structural integrity, resulting improved performance and excellent capacity retention with long cyclic life.
We have developed rationally designed carbon nanoarchitectures as the conductive matrix and 3D scaffold (such as hollow cubes, porous spheres, and nanoshells) decorated with different hybrid nanostructures (e.g., metal oxides & sulfides), as the host materials for high sulfur loading. The synthesized nanostructured hybrid materials exhibited extraordinary performances as the cathode for LSBs with long cyclic stability and excellent rate capability. For example, holey Fe-N co-doped graphene (HFeNG) synthesized through a green, scalable, and one-step calcination process revealed excellent PS adsorption capabilities due to Fe-N₂ moiety on the edges of holey graphene. HFeNG@S exhibited high rate capabilities of 810 mAh/g at 5 C and a stable cycling with a capacity decay of 0.083% per cycle at 0.5 C. In another study, novel N-doped graphene-NiS₂ (NiS₂@NG) core-shells showed excellent cycling stability by suppressing PS shuttle through physical encapsulation by polar N-doped graphene (NG) nanoshells, and chemical bonding along with catalyzing the conversion of PS species by NiS₂. A high sulfur loading up to 90% was achieved for the novel hollow NG nanoshells, as well as NiS₂@NG core-shells with a very stable cycling life. Our findings shows that PS shuttle effect can be suppressed in the rationally designed cathode material. The highly conducting carbon matrix boost the internal conductivity of the cathode and physically encapsulate the PS species. On the other hand, metal nanostructures provide chemical bonding and catalyze PS conversion reactions. These strategies to combine the different properties enhancing factors in one cathode material with engineered structures will bring the realization of these devices in the broad market.

Lithium-ion batteries have been widely used for portable electronics by virtue of its high energy density, and even higher energy density has been required for a power source of electric vehicle or energy storage system. Scale-up of lithium rechargeable batteries, however, inevitably encounters safety problems, because these high-capacity batteries consist of a bunch of cells and unexpected malfunction of a single cell can trigger chain explosion. Therefore, it is important not only to improve the energy density of the battery but also to control the inherent risks. Here, we study structural degradation in the lithium cobalt oxide (LiCoO₂) particles caused by overcharging which is one of the major issues for battery safety. LiCoO₂ is a most commonly used cathode material for a lithium-ion battery and its electrochemical intercalation mechanism has been well studied on the normal charge range of the formula Li_xCoO₂ where 0.5<x<1. In contrast, little research was conducted on overcharging in the range of x<0.5. To assure that the entire particle is overcharged, the cell, composed of LiCoO₂ cathode and lithium metal anode, has been continuously charged until the voltage reached 6 V. We have examined the overcharged particle exploiting an aberration-corrected transmission electron microscope. Interestingly, submicron-cracks running with a zigzag shape accompanied by nano-pores are found in the middle area of the particle where a small amount of lithium remains. In other words, passivated lithium-ion is existing in spite of overcharge process of which state of charge is seven or eight times greater. Furthermore, we observe diverse cobalt oxide phases in form of Co₂O₃, Co₃O₄ near cracks; these phases yield lattice shrinkage up to 10 % compared to its original structure, which suggests that crack has generated by great stress induced from uneven lattice shrinkage. The detailed analysis of the cracks will be discussed, focusing on its impact on safety.

The rate performance associated with the battery's power capabilities have always been an important factor in designing energy storage materials. Recently, with the gradual increasing of electric vehicles, the request of rapid charging has increased as much as the capacity of materials, accelerating the development of high rate performance materials.
We introduce high-pressure-assisted deposition method for the nanoporous structure that enable high capacity at the high current rates. The high porous nanostructure can be deposited due to diffusion-limited-aggregation phenomenon caused by the reduction of the mean free path of gas molecules under high pressure. Based on this method, we could fabricate an anode for a lithium ion battery by directly depositing a nanoporous Sn structure on a Cu current collector.
Since the deposited electrode did not contain a binder or a conductive additive, it was possible to realize a high energy density in addition to inherent high material density of Sn. Furthermore, the high porosity between nanoparticles contributed to the enhancement of the rate performance by increasing the diffusion rate of lithium ions and lowering the charge transfer resistance. Electrochemical test results showed a high rate performance at 20C, which could be up to 90% of the specific capacity at 0.1C. In addition, this structure can contribute to the improvement of cyclability by alleviating the electrode degradation associated with the large volume change during the lithium alloying and de-alloying.

To bridge the gap between existing lithium ion batteries and future high energy density lithium ion batteries, device manufacturers have moved towards fast charging. Fast charging methods have been widely adopted for many applications, especially in electric vehicles and is therefore important to understand the long-term effects of this type of charging. Fast charging is commonly used for charging EV’s; people misunderstand and misuse fast charging and this has introduced safety and capacity issues for the battery. A fast charging technique is proposed by using the internal resistance of the battery and adjusting the charging current based on it. The internal resistance was also measured over the cycling life of the battery and the charging algorithm will be adjusted accordingly, therefore making the charging algorithm adaptive. This type of charging should help reduce charging time for commercial lithium ion batteries and also help improve the cycling life of the battery.

Widespread acceptance for electric vehicles is deterred by (i) range anxiety and (ii) recharging time. To be competitive with conventional fuels, battery recharging time must be less than 10 min, which amounts to a very high charging rate (~6C). At such high rates (i.e., extreme fast charge, XFC), a host of anode centric issues arise, namely suboptimal charging capacity, self-heating, and irreversible plating losses. Fundamentally these observations are a result of sluggish kinetic and transport modes at the pore scale. Here in we analyze such complex physicochemical interactions for different commercial graphite electrodes (microstructures are obtained through tomography). Even though the porosity of these structures is spatially quite uniform, the pore and solid networks exhibit considerable variations, both within the sample and across electrodes. Based on these examinations, we outline the guidelines for improving the XFC response via appropriate scaling of structural features.

Structurally ordered oxides exhibit a broad range of structural, compositional, and functional properties, which can be further tuned by means of judicious elemental doping, strain and defect engineering. As such, they have found widespread application in energy storage and conversion devices, particularly for use as electrocatalysts, cathodes, and solid state ionics. However, as-designed materials can undergo dramatic changes under their operating and/or extreme conditions, which often leads to performance degradation and device failure.

This talk will highlight our most recent effort aiming to modify complex oxides through heteroepitaxy to achieve tunable functional properties. Combining in situ and environmental transmission electron microscopy (TEM), ¹⁸O₂ labeled time-of-flight secondary ion mass spectrometry (ToF-SIMS), and ab initio simulations, we elucidate the structural and chemical evolution pathways in selected materials systems, and reveal how such changes impact their functional properties. The first part of my talk focuses on Brownmillerite (BM)-structured SrCoO_2.5, SrFeO_2.5 (BM-SFO), and rhombohedral-structured SrCrO_2.8 (R-SCrO), which are perovskite (ABO₃)-associated structures that contain ordered oxygen vacancy channels. We show that at relatively low temperatures, a topotactic phase transition between BM-SFO (R-SCrO) and perovskite SrFeO₃ (SrCrO₃) can be promoted, delayed, or prohibited based on the interfacial strain conditions, highlighting the importance of interface engineering in designing robust and efficient ion conducting materials. In another example, I will present the epitaxial growth and in situ TEM studies of LiCoO₂ with or without overlayers to understand the Li transport processes and device failure mechanisms. In both cases, the high spatial and temporal resolution offered by advanced electron microscopy allow us to visualize the reaction onset, kinetics, intermediates, and final products, which are critical for the rational design of functional materials.

Performance of electrochemical energy systems is governed by ion transport through solid/liquid or solid/gas interfaces. Major breakthroughs are then intrinsically linked to a detailed understanding of how molecules, atoms, and electrons behave at these interfaces during operation. Recent progress in the development of ex situ and in situ characterization surface techniques using synchrotron radiation sources has enabled major scientific advancements toward this understanding. In this work, we studied interfacial processes in energy materials such as Li-ion systems or proton conductor solid oxide fuel cells (H⁺-SOFC) by using Synchrotron Infrared Nanospectroscopy (SINS) and Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) at the Advanced Light Source.
To unravel the mechanism behind an effective Solid electrolyte interphases (SEI) layer on Si anode, we used the unparalleled spatial resolution and surface sensitivity of the near-field IR probe (ca. 20 nm) and chemical selectivity of SINS. We leverage the influence of LiBOB additive to shed lights on SEI’s crucial components and structure at a nanoscale level, which corresponds to the size of the SEI building blocks. Through these studies, we find that the formation of a robust nanostructure SEI made of a li-ion conductive oligomers backbone at the silicon surface is a key factor.
We also examined proton and oxygen transport at the interface between a standard H⁺-SOFC perovskite electrolyte (BaCe_xZr_0.9-xY_0.1O_2.95 (x = 0.9 ; 0.2 ; 0)) and the fuel by using AP-XPS. We explored doping effects on surface reaction kinetics, at early stages, in situ, in fuel cell environments (100 mtorr of H₂O, H₂, O₂ or CO₂) at operating temperatures (>400°C). We demonstrated that the exceptional performance of BaCe_0.2Zr_0.7Y_0.1O_2.95 was a result of the optimization of the surface reaction kinetics, with a high number of fast proton accompanied by a limited hindrance of active sites by adsorb SO₄/OH^-.
Acknowledgement
This work was supported by the Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, Office of Naval Research, Nanostructures for Electrical Energy Storage (NEES) and the Advanced Light Source funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences.

It is increasingly recognized that extremely fast charging and discharging, along with long cyclability, will require energy storage materials that are able to respond with minimum mechanical deformation while at the same time storing significant amounts of energy. Our research shows that interlayer structural water in crystalline transition metal oxides appears to play an important role in mitigating the mechanical deformation during electrochemical ion intercalation in a class of model transition metal oxides, layered tungsten oxide hydrates. We investigated the electro-chemo-mechanics of proton intercalation into crystalline WO₃ and crystalline, layered WO₃●2H₂O using operando atomic force microscopy (AFM) dilatometry. The nanoscale deflections of the AFM tip were measured for both WO₃ and WO₃●2H₂O as a function of potential and cyclic voltammetry sweep rate for charge/discharge times on the order of minutes and seconds, pushing the extremes of fast charging and discharging of electrochemical energy storage materials. In addition, we correlated the AFM results with neutron scattering, solid state nuclear magnetic resonance (NMR), and electrochemical quartz crystal microbalance studies to provide a detailed overview of the role of interlayer structural water during electrochemical ion intercalation in this class of materials and how it can enable extremely fast charging and discharging.

Electrochemical gas-involving reactions, including gas-evolution reaction and gas-consumption reaction, are essential parts in current energy conversion processes and industries. Although the exploration of the highly active catalysts has been very mature, less attention was paid on the gas management during the gas-involving reactions. Inspired from bio-inspired materials, scientists find that bio-mimicked electrodes with superwetting property will influence the gas transportation process during the electrochemical reactions. Our group fortunately found that the interface behavior of electrode could be tuned by surface architecture construction, for example, transferring from aerophobic to superaerophobic by engineering a series of superwetting micro-/nanostructured electrodes, eg. MoS₂. Cu nanoarray and Pt pine-like films ^[1-4]; transferring from aerophibic to superaerophibic by poly(tetrafluoroethylene) (PTFE) modifying, eg. CoNCNT@CFP. ^[5] For gas-evolution reaction, constructing nanostructured superaerophobic electrodes is effective to improve the performance by enlarging the bubble contact angle and reducing the bubble adhesion force with the surface of the electrode, thus insuring smooth leaving of the gas products. ^[6-7] As to the gas consumption reactions, the superaerophibic electrodes are able to improve the performance by providing an unblocked gas diffusion pathway and a smooth electron transport. Therefore, construction of superwetting surface (auperserophobic for gas evolution reaction and superaerophibic for gas consumption reaction) can boost the performances of the electrodes by managing the surface bubbles.
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Exploitation of fossil energy has increased the atmospheric CO₂ concentration, which leads to severe climatic problems. Clean fuels such as hydrogen and oxygen produced from water splitting provide alternative solutions for green energy conversion and production. However, since the efficiency of the water-splitting reaction is largely limited by the high overpotential required by the oxygen evolution reaction (OER) and the relatively slow kinetics of the OER, high-performance and stable electrocatalysts are required. Particularly the OER is much limited in the acidic condition that very few electrocatalysts can survive. Considering the cost, efficiency, and stability of catalysts in extreme conditions, we used core-shell alloys, namely Pd core and Ir shell or Pd@Ir, to show much-improved OER activity (order of magnitude higher than IrO₂) in acidic condition with good stability. To understand the enhanced electrochemical performance, we have combined in situ X-ray diffraction and X-ray absorption spectroscopy to study nanocatalysts in reaction. We found that the Pd@Ir with concave surface undergoes atomic structural and oxidation state changes during the process, but shows higher activity than the flat one that shows no change with moderate activity. Our in situ measurements can be correlated with their stability and activity and provides the insights better design catalysts for applications in extreme electrochemical conditions.

Lithium ion batteries are usually defined by their bulk characteristics, but their actual response is often limited by reactions at their interfaces. One classic example of this disconnect can be found in the broad class of conversion reactions. For instance, the conversion of binary metal oxides into Li₂O and metal nanoparticles are known to have specific capacities far beyond comparable intercalation materials, but take place at potentials far below predicted values from their reaction enthalpies. This overpotential has been loosely associated with the formation of interfaces between the nanoscale metal and lithia products, but has been difficult to characterize due to the complex, three-dimensional nature of the phase separation. To better quantify phase change at the interface, we have studied the structure of ultrathin (< 2 nm thick) thin films of NiO grown on a Ni current collector using operando x-ray reflectivity. Votammetry on these nanoelectrodes show additional redox peaks near 2 V, in addition to lower voltage peaks associated with the bulk reduction of NiO. Reflectivity shows that the higher voltage peaks are due to an interfacial conversion reaction confined to the buried Ni/NiO interface that is catalyzed by the presence of a lithium space charge layer. Using density functional theory, we developed a nucleation theory picture that helps explain the overpotentials for conversion. In particular, DFT shows that the observed lithium space-charge layer significantly reduces the interfacial energy between the conversion products, effectively eliminating the reaction’s overpotential. These results provide a new window into the role of interfaces in conversion reactions and could provide a new template for studying lithium in model grain boundaries, such as those found in solid state batteries.

Graphene is a material with unique electronic and mechanical properties. It is also a model material for carbon electrodes used for ion intercalation reactions in ion batteries. Inherent to electrodes made with few layers of graphene (FLG), is that their ultra-thin bulk can also be understood as an interface. Therefore, FLG electrodes allow the inspection of interfacial aspects of ion intercalation with true surface sensitivity. Elucidating the mechanisms and kinetics of ion intercalation reactions beyond Li⁺ is key to the development of a diverse array of chemistries for next-generation energy storage. In this context, nanostructured continuous-film FLG electrodes[1] can contribute to understanding fundamental interfacial limitations that have prevented the development of efficient intercalation chemistries, for instance of other alkali ions such as K⁺, Na⁺, and even of multivalent cations such as Mg²⁺. Because these type of electrodes do not require the use of binders that affect the electrochemical response, and owing to their ultra-thin bulk, they create the opportunity to look at electrochemical processes with unprecedented fidelity.
In my talk, I will discuss the use of FLG electrodes for overcoming interfacial limitations preventing the fast and efficient intercalation of K⁺.[2] FLG electrodes pre-treated with an SEI formed in Li⁺ containing electrolyte displayed remarkably clear voltammetric features of ion intercalation for K⁺. These experiments allowed us to confirm the quantitative formation of KC₈, the observation of staging peaks indicating a similar mechanism for K⁺ and Li⁺ intercalation, and outstanding performance, with quantitative Coulombic efficiency even at C rates higher than 300. Pushing the limits of what these electrodes can do, we have explored voltammetric scan rates up to few V/s, representing unprecedented charge/discharge rates that will allow us to test the kinetics of ion intercalation. Through these studies, we aim at establishing new kinetic and thermodynamic relationships that allow us to have a better grasp of electrochemical intercalation for emerging ion-based batteries and insertion supercapacitors.
[1] Hui, J.; Burgess, M.; Zhang, J.; Rodríguez-López, J. Layer Number Dependence of Li⁺ Intercalation on Few-Layer Graphene and Electrochemical Imaging of its Solid-Electrolyte Interphase Evolution. ACS Nano 2016, 10, 4248-4257.
[2] Hui, J.; Schorr, N.B.; Qu, Z.; Pakhira, S.; Mendoza-Cortes, J.L.; Rodríguez-López, J. Achieving Fast and Efficient K⁺ Intercalation on Ultrathin Graphene Electrodes Modified by a Li⁺ Based Solid-Electrolyte Interphase. J. Am. Chem. Soc. 2018, 140, 13599-13603.

Since the risk of catch fire using non-aqueous electrolyte solution, aqueous solution-based rechargeable lithium batteries (ARLB) have been highlighted. However, the conventional positive electrodes of lithium transition-metal oxide such as LiCoO₂ (LCO) and LiNi_1/3Mn_1/3Co_1/3O₂ (NMC) have suffered from poor cyclability in aqueous medium. Representatively, the layered two-dimensional structure of LCO shows notably poor stability, possibly due to the surface degradation from water ^[1] and proton ^[2]. The understanding of interfacial reaction of LCO in the aqueous electrolyte solution is still superficial however.

Here we present degradation phenomena of LCO electrode in aqueous medium using various X-ray measurement techniques, and suggest the solution to avoid such an irreversible electrochemical reaction. The aqueous solution was prepared with 0.5 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and pH was controlled to ~6.8 and 10. In both cases, there was no evidence for the formation of cathode-electrolyte interphase (CEI) on LCO in contrast to the one with non-aqueous electrolyte solution. The direct contact of aqueous electrolyte solution to LCO surface results in the short-range disorder of LCO structure such as the distortion of octahedral CoO₆, and irreversible Li⁺ desertion during 10 cycles. To improve electrochemical reversibility and structural stability of LCO, we prepared the organic protection layer that opened the Li⁺ mass transport route while inhibiting H₂O contact from hydrophobic surface. As a result, the capacity retention was improved to ~85% during 30 cycles at pH ~ 6.8. Furthermore, we developed the way to protect LCO surface by anion engineering and in the absence of protection layer, which give insight into the inner Helmholtz plane (IHP) structure and its effect for LCO degradation in aqueous medium.

References:
[1] A. Ramanujapuram, D. Gordon, A. Magasinski, B. Ward, N. Nitta, C. Hang, G. Yushin, Energy, Environ. Sci., 9 (2016) 1841-1848.
[2] Y. G. Wang, J. Y. Lou, W. Wu, C. X. Wang, Y. Y. Xia, J. Electrochem. Soc., 154(3) (2007) A228-A234.

The development of extreme fast charging (XFC) protocols (<10 minute charge time) for lithium-ion batteries is critical for widespread adaptation of electric vehicles and other devices with time-sensitive applications. However, a limited understanding of degradation modes during XFC and the large manufacturing variability of commercial lithium-ion batteries are major challenges to the development of high-performing XFC protocols. In this work, we perform both “top down” optimization and “bottom up” characterization of extreme fast charging across multiple length scales. First, we employ Bayesian optimal experimental design to develop a six-step XFC protocol for commercial 18650 lithium-ion batteries, achieving 80% state of charge in ten minutes. Next, we rationalize their high performance via cell-level simulations and electrode-level physical characterization. Our results reveal that loss of active material from the graphitic negative electrode is a significant source of degradation, preceding more conventional fast charging degradation modes such as lithium plating. We also identify a critical charge rate beyond which the loss of active material, and thus the overall cell degradation, accelerates. Finally, we study the rate of SEI growth during fast charging via electrochemical methods, identifying a strong dependence on the applied C rate. This work provides novel insight into optimizing and characterizing extreme battery fast charging for time-sensitive applications and suggests avenues to improve both charging times and lifetimes during aggressive battery operation.

Here, we report in-depth study on basic principles of electrochromic devices (ECD) such as their structural design as well as operational method. Electrochemically deposited prussian blue (PB) and tungsten trioxide (WO₃) thin films were employed as a working and counter electrode, respectively, and sandwich-typed ECDs were fabricated using aqueous electrolyte. Electrochemical devices relying on interfacial charge-transfer and ion transport, normally suffer from a decline of its operational performance which is caused by irreversible redox reaction, ion accumulation or overvoltage at the solution/electrode interface, and it severely curtails the operational lifetime of the devices. After careful investigation of the redox properties of PB and WO₃ films and their complementary ECDs, we found that by balancing the charge during on/off switching (regarding bleaching and coloration), the ECD could exhibit improved long-term stability with broad optical modulation and decent coloration efficiency (△T < 1.7 % over 2000 cycles) under mild operational voltage. We expect that the results provide a general overview of the operational conditions and structural design in ECD researches.

This presentation will focus on the molecular scale understanding of key factors influencing ion transport, electrochemical stability and decomposition in a wide range of aqueous and non-aqueous electrolytes as a function of salt concentration. No one simulation technique is capable of efficiently capturing all transport and electrochemical properties at interfaces. Therefore, we will utilize a combination of a) density functional theory (DFT) studies of the solvent reactions on cathode surfaces and at the solid electrolyte interphase (SEI) covering lithium metal; b) representative quantum chemistry (QC) calculations performed on the representative small model electrolyte clusters to estimate oxidation and reduction; c) molecular dynamics (MD) using APPLE&P polarizable force field to examine bulk and interfacial properties of electrolytes and electrochemical interfaces. Combined together, information from these modeling studies expands on the previous work¹ and suggests numerous strategies for stabilizing the electrolyte – electrode interfaces for a number of aggressive high energy density cathodes combined coupled with graphite and metal anodes.
References
Borodin, O.; Ren, X.; Vatamanu, J.; von Wald Cresce, A.; Knap, J.; Xu, K., Modeling Insight into Battery Electrolyte Electrochemical Stability and Interfacial Structure. Acc. Chem. Res. 2017, 50, 2886-2894.

Enabling fast-charging capabilities in lithium-ion batteries (LIBs) is critical to the continued development of modern applications such as electric vehicles (EVs), unmanned robots, and grid-leveling applications. However, the current state-of-the-art in commercially available LIBs renders fast charging difficult, especially if energy density, and a long battery lifetime are also desired. While many researchers are currently looking for novel materials to alleviate this issue, this research takes a different approach: can we use currently existing materials in new ways, as to enable fast-charging capabilities, while retaining energy density, and a stable cycling life. As such we are able to demonstrate optimization of this triad through a combination of external temeratures, cycling protocols, and battery geometry. Using our novel cycling protocol, as well as in-operando temperature controls, we are able to demonstrate how to avoid traditional battery degradation mechanisms, such as a loss of Li-inventory, cathode dissolution, and electrolyte degradation, while optimizing the aforementioned triad. This provides valuable insight on how to expand the application suite of existing LIB technologies, as well as a guide into the types of materials and electrolytes that should be developed for next generation LIBs.

The general public reception of electric vehicles (EV) depends, among other factors, on meeting their expectations of similar comfort compared to conventional vehicles. Replicate the convenience of filling a tank of fuel in minutes is a very challenging task when one has to recharge a Li-ion battery. The primary goal of a fast charging protocol must be avoiding Li plating in the anode and maintaining temperature under control. Hence, a deeper understanding of the thermodynamics and kinetics of the anode charging process is crucial to improve the models and subsequently the design of battery cells. In the present work, an atomistic level analysis of Li intercalation and diffusion in graphite have been performed under fast charging conditions. In this presentation, we will show and discuss the effect of fast charging conditions on the intercalation and diffusion mechanism of Li in graphite.

The authors gratefully acknowledge support from the U. S. Department of Energy (DOE), Vehicle Technologies Office. Computer time allocations at the Argonne's Laboratory Computing Resource Center is gratefully acknowledged. Argonne National Laboratory is operated for DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357.

Redox flow batteries have several outstanding technological advantages over traditional static batteries (e.g. Li ion and Lead acids) for large scale energy storage including decoupled energy and power, safe energy storage, high current and power performance, and scalability. Recently, Aqueous organic redox flow batteries (AORFBs) have emerged as an attractive alternative RFB technology because redox active organic molecule materials are synthetically tunable, sustainable, and potentially low cost. The presentation will highlight our research efforts in developing viologen anolyte materials for both anion and cation exchange AORFBs with a variety of catholyte materials including ferrocene, TEMPO, ferrocyanide, and halides. The viologen AORFBs demonstrated outstanding battery performance including a power density up to 120 mW/cm², energy efficiency up to 72% at 60 mA/cm², and capacity retention up to 99.999% per cycle up to 1000 cycles, representing the state of the art organic RFBs. Particularly, the presentation emphasizes that fundamental understandings of redox active electrolytes at molecular level are crucial to develop new generations of redox flow batteries for large scale and dispatchable renewable energy storage.

Thin film materials, processes, and structures provide powerful leverage in meeting the wide spectrum of demands posed by energy storage in extreme conditions. In some applications safety and reliability (stability under cycling or at rest) are paramount, as in implanted biomedical devices or for defense/security applications. Here electrode protection layers or thin interphase layers represent an increasingly important opportunity, created by intentional introduction of material by vapor phase or electrochemical reaction. We have had substantial success in suppressing capacity degradation using thin coatings, where mechanical as well as chemical properties play a crucial role. Some important applications require simultaneous high energy and high power, in a variety of temporal profiles. Thin film processing enables construction of decidedly 3D components with high aspect ratios, a favorable pathway to meeting energy-power demands. At the same time, this route enables solid state batteries (SSBs) to be created in various form factors to allow integration into multifunctional systems with sensing, actuation, computation and communication capabilities for opportunities in applications from biomedicine to electronics to security.

Double-layer capacitors are well suited for both long life and wide temperature operation, based on the storage of charge at a solid/liquid interface. Since intercalation processes are not involved, the life limiting mechanisms related to electrode phase changes are of a lesser concern relative to batteries. For the same reason, the kinetics of the charging/discharging process are more favorable. Typical commercial devices are rated for operation between -40°C and +65°C, with temperature limits set by the nature of the electrolyte solvent. This talk will provide an update on efforts to expand beyond these temperature limits in symmetric devices through selection of suitable electrode/electrolyte materials. In addition, the potential for wider temperature operation in asymmetric cells featuring an intercalation electrode will be discussed.

Supercapacitor is regarded as the ideal power source in wearable electronic device, the electrode materials of which are always research focus due to it is a key component in the supercapacitor. Among numerous electrode materials, graphene has attracted significant attention for its outstanding electrochemical performance and mechanical properties, which endows graphene films with great potential of applications in future flexible electronics. Therefore, effective preparing methods of graphene films were researched and reported extensively in recent years. Herein, we fabricate a novel graphene oxide (GO) film with excellent mechanical properties via foam film method. Its thickness can be simply regulated by changing the concentration of the surfactant. After chemical reduction, the reduced GO (rGO) films exhibit impressive electrical conductivity of ~172 S cm^-1. The supercapacitors based on the fabicated rGO films exhibit satisfied capacitive performance of ~56 mF cm^-2 at 0.2 mA cm^-2 with 6 M KOH solution. Furthermore, the flexible all-solid-state supercapacitors (FSSCs) based on the rGO films also show great volumetric capacitance of ~2810 mF cm^-3 at 12 mA cm^-3 (~1607 mF cm^-3 at 613 mA cm^-3) with polyvinyl alcohol-KOH gel electrolyte, which indicates great rate performance of solid-state devices. Besides, the supercapacitor also show great cycling stability and flexibility: after 10000 cycles and continuously bent to 180° for 300 times, the volumetric capacitance of the FSSC remains at 81.4% and 90.4% of its initial capacitance value, respectively. All the results demonstrate the free-standing rGO films prepared via foam film method in this study could be considered as promising electrode materials for high performance flexible supercapacitors.

The strong need for the development of inexpensive, flexible, light-weight and environmentally friendly energy storage devices has resulted in large interest in new cellulose-based electrode materials that can be used in batteries and supercapacitors [1-3]. In this presentation it will be shown that flexible nanocellulose and polypyrrole composites, manufactured by chemical polymerization of e.g. pyrrole on a nanocellulose substrate, can be used as electrodes in charge storage devices containing either water or organic solvent based electrolytes. The aqueous flexible paper-based devices exhibit high charge storage capacities (e.g. 9 Wh/kg) as well as excellent power capabilities (e.g. 3.5 kW/kg) due to the large surface area (up to 250 m²/g) of the nanocellulose and the thin (i.e. 50 nm) layer of polypyrrole present on the nanocellulose fibers. The straightforward (paper-making) composite synthesis approach and the electrochemical properties of the resulting composites will be discussed. It will also be shown that high active mass paper electrodes [4-8] with mass loadings of up to 20 mg/cm² can be employed at high current densities without significant loss of electrochemical performance as a result of the advantageous structure of the electrodes. Devices with unprecedented areal and volumetric cell capacitances (e.g. 5.7 F/cm² and 240 F/cm³) that can cycle for thousands of cycles in aqueous electrolytes can likewise be realized. As the cellulose composites also can be used in lithium-ion batteries [9,10], functional (e.g. redox-active) separators [11] for lithium based batteries and in the realization of all-cellulose energy storage devices [12], the present materials provide new exciting possibilities for the development of green and foldable devices for a range of new applications, many of which are incompatible with conventional batteries and supercapacitors.

1) G. Nyström, A. Razaq, M. Stromme, L. Nyholm, A. Mihranyan, Nano Letters, 9 (2009) 3635.
2) L. Nyholm, G. Nyström, A. Mihranyan, M. Stromme, Adv. Mater., 23 (2011) 3751.
3) Z. Wang, P. Tammela, M. Stromme, L. Nyholm, Adv. Energy Mater, 7 (2017) 1700130.
4) Z. Wang, P. Tammela, P. Zhang, M. Stromme, L. Nyholm, J. Mater. Chem. A, 2 (2014) 7711.
5) Z. Wang, P. Tammela, P. Zhang, M. Stromme, L. Nyholm, J. Mater. Chem. A, 2 (2014) 16761.
6) Z. Wang, P. Tammela, P. Zhang, J. Huo, F. Ericson, M. Stromme, L. Nyholm, Nanoscale, 6 (2014) 13068.
7) Z. Wang, P. Tammela, M. Stromme, L. Nyholm, Nanoscale, 7 (2015) 3418.
8) Z. Wang, D. O. Carlsson, P. Tammela, K. Hua, P. Zhang, L. Nyholm, M. Stromme, ACS Nano, 9 (2015) 7563.
9) Z. Wang, C. Xu, P. Tammela, P. Zhang, K. Edstrom, T. Gustafsson, M. Stromme, L. Nyholm, Energy Techn., 3 (2015) 563.
10) Z. Wang, C. Xu, P. Tammela, J. Huo, M. Stromme, K. Edstrom, T. Gustafsson, L. Nyholm, J. Mater. Chem. A, 3 (2015) 14109.
11) Z. Wang, R. Pan, C. Ruan, K. Edstrom, M. Stromme, L. Nyholm, Adv. Sci. (2017) 201700663.
12) Z. Wang, R. Pan, R. Sun, K. Edstrom, M. Stromme, L. Nyholm, ACS Appl. Energy Mater., 1 (2018) 4341.

Implantable biomedical applications represent demanding operating environments for electrochemical energy storage materials and systems providing a unique set of challenges for successful design and implementation of such devices. While the capacity and power requirements can vary significantly depending on the specific application, several common challenges become evident. Volumetric rather than gravimetric density is the key consideration. The power source must operate effectively under elevated operating temperature. Reliable function under varied use conditions is essential. Long lifetimes as well as predictive state of discharge indicators are essential.
Advanced interrogation methods, particularly in-situ and operando approaches, can provide important mechanistic information to elucidate the behavior of the energy storage material. However, it is challenging yet important to design experiments which interrogate the system under application relevant conditions. Progress in investigation of energy storage materials for biomedical applications will be highlighted in this presentation, providing inspiration for design of new electrochemical energy storage solutions for a broad array of demanding applications.

The ever-growing demands for stretchable electronics in applications of electronic textiles, electronic skins, and wearable health monitors, require concerted stretchability in energy storage devices. As one of the main energy supplies for stretchable and wearable electronics, stretchable supercapacitors are drawing increasing attention in recent years owing to their superior power density and long calendar life under deformation. Although some progress has been made on stretchable supercapacitors, traditional stretchable supercapacitors fabricated by predesigning structured electrodes for device assembling still lack the device-level editability and programmability. To adapt to wearable electronics with arbitrary configurations, it is highly desirable to develop editable supercapacitors that can be directly transferred into customizable shapes and stretchable structure according to application scenarios. Herein, we proposed an editable strategy to fabricate stretchable supercapacitors for regulation of the strain on the electrodes. The editable supercapacitors with customizable structures can regulate the peak strain of the electrodes below the fracture of the electrode materials, which enables a wider range of electrode materials for stretchable supercapacitors. The mechanically reinforced flexible electrode materials for the editable supercapacitors further guarantee the editable process and improve the mechanical stability of customizable devices. Editable supercapacitors for customizable shapes and stretchability using electrodes based on mechanically strengthened ultralong MnO₂ nanowire composites are developed. A supercapacitor edited with honeycomb-like structure shows a specific capacitance of 227.2 mF cm⁻² and can be stretched up to 500% without degradation of electrochemical performance. Taking advantages of these superior properties, the editable supercapacitors are integrated with a strain sensor, and the system exhibits a stable sensing performance even under arm swing. Besides, based on flexible porous polypyrrole composite electrodes, the supercapacitor arrays rearranged into an expandable honeycomb structure can be further used to fabricate three-dimensional (3D) stretchable supercapacitors with customizable 3D shapes and enhanced areal energy storage performance. Being highly stretchable, easily programmable, as well as connectable in series and parallel, the editable supercapacitor with customizable stretchability and shapes is promising to produce stylish energy storage devices to power various portable, stretchable, and wearable devices. Our editable strategy provides a new design platform for the electrode materials in the customizable and stretchable electrochemical energy storage devices. Based on it, many other new methods and stretchable electronics could be further developed.

The demands of stretchable electrochemical energy storage devices that can operate under extreme environments are growing in various aspects of the society requirements. Here, we report on fiber based highly stretchable supercapacitors comprising of nine supercoiled fiber electrodes and quasi-solid-state electrolyte. Each supercoil fibers were fabricated by inserting a giant twist to supercoil (coiling of a coiled fiber) a spandex fiber that is helically wrapped by forest-drawn carbon nanotube sheets. Plying these supercoil fibers enables lower internal resistance, thereby resulting in improved rate-capability (54% retention of maximum capacitance at 1000mV/s scan rate) as well as high areal capacitance (4.23 mF/cm²). Moreover, the nine plied, supercoiled fiber based supercapacitors exhibited ultra-high stretchability in tensile direction (ε = 600%), while conserving 93% of initial capacitance during the reversible stretching. The new structured supercapacitors are posed for energy storage system with special electrochemical and mechanical properties that can work in extreme environments

The increasing power and energy demand for wearable electronics greatly stimulates the development of wearable energy storage devices with excellent flexibility, robust mechanical property and outstanding energy density. To power the wearable devices in a safe and cost-effective way, fabric-based flexible SCs have attracted intensive attention in both academical and industrial fields due to its significantly mechanical property and inherently safety. However, one major limiting factor regarding the practical application of fabric-based flexible SCs is insufficient areal energy density. The average areal energy density of wearable SCs is only in the range of 0.01 ~ 0.1 mWh cm^-2, which requires hundreds to thousands of square centimeters of SCs to powdering commonly used wearable devices, such as LED garments, wearable medical care devices and flexible displays. Therefore, considering the limited surface area of a human body (~ 2 m²), the tremendous challenge for powering future wearable electronics lies on rational fabric-based flexible SCs design for robust areal energy density with prominent wear-ability properties, such as flexible, tailorable, waterproof and fire-retardant.
In this study, we report a new electrode preparation method to fabricate thick and high mass-loading hybrid electrode with superior conductivity and excellent ion diffusion property though successively filtrating fresh mixture 1D core-shell pseudo-type materials and 2D rHGO nanosheets on carbon cloth to form a 3D soft hybrid scaffold. By using the afore-proposed electrode fabrication method, we herein demonstrate the preparation of soft 3D scaffold electrodes with controllable mass loading, increasing from 5 mg cm^-2 to more than 30 mg cm^-2. After cathode and anode pairing and simple vacuum encapsulation, we assemble the 3D soft hybrid scaffold-like electrodes into all-solid-state flexible SCs with ultrahigh mass loading of 68 mg cm^-2 and thickness of 1.05 mm, which delivers a remarkable areal energy density of 1.05 mWh cm^-2 at a power density of 17.07mW cm^-2, along with high electrochemical life (91% retention after 3000 charge/discharge cycles). More importantly, the hybrid 3D scaffolds SC shows outstanding wear-ability properties: excellent mechanical flexibility (only 2.5% capacitive decay after 4,000 bending tests), water-proof property (negligible capacitive decay after immersing in 20 cm deep water for 48 hours), fire-retardant ability (non-combustion-supporting even under butane flame thrower for 34 s) and tailor-abilities, which are critical for wearable applications.

Li-ion batteries are the energy storage sources of choice for rechargeable electronics applications due to their high energy density and long cycle life. Despite these benefits, Li-ion batteries suffer from safety concerns associated with the flammable organic solvents that are needed to maintain the high voltage window in the electrolyte. Water emerges as a natural, inherently safe alternative solvent. Its low electrochemical stability window of ~1.23 V limits the available energy density and cycle life of aqueous Li-ion batteries. Recent innovations in aqueous Li-ion batteries have introduced a new class of aqueous electrolytes termed as “water-in-salt” (WiS). In WiS electrolytes, a highly concentrated salt (>21 m) such as LiTFSI is typically used in water. At such high concentrations, water is bound and its electrochemical activity for gas generation is suppressed, extending the electrochemical stability to > 3.0 V.

In this work, significant advancements in WiS electrolytes are discussed, specifically through the incorporation of the WiS in a polymer matrix. When the liquid WiS is combined with a polymer, the need for a separator is eliminated and new form factors with minimal packaging can be realized. Using a high molecular weight polyvinyl alcohol (PVA) based Gel Polymer Electrolyte (GPE) containing water-in-bisalt (WiBS), a flexible symmetric cell with LiVPO₄F anode/cathode was built. This cell exhibited long cycle life (4000 cycles) at a 20C rate and continued to operate even after sustaining mechanical damage, such as cutting and exposure to air, immersion in salt water, and subjection to ballistic impact. The GPE properties can be dramatically enhanced when the high molecular weight polymers are replaced by UV cured acrylates. Using a combination of various acrylate monomers, cross-linkers, and photoinitators, aqueous UV cured GPEs can be fabricated in fully aerobic/ambient conditions in less than 10 minutes with excellent control over thickness and mechanical properties. In addition to simplifying manufacturing, this approach significantly improves electrochemical performance as the UV cured GPEs exhibit an expanded electrochemical stability window (5.0 V) while maintaining ionic conductivity > 1 mS/cm and enabled the use of Li₄Ti₅O₁₂ (LTO) as an anode. To further increase cycle stability and coulombic efficiency, the UV cured GPE can be combined with a dry polymer electrode passivation. This passivation layer, which can be cast or UV cured, further suppresses hydrogen evolution on the anode and expands the stability of LTO-based cells to > 200 cycles at 2C rate. This UV cured passivation concept can be combined with previously demonstrated passivation strategies on graphite to extend the cyclic stability of “4.0 V aqueous Li-ion batteries” to >100 cycles. Finally, in addition to its use in the GPE and passivation layers, acrylate chemistry can also be used to replace the PVDF binder that is typically used in Li-ion batteries. Using a novel cross-linking approach, acrylate binders can be incorporated in battery electrodes with loadings as high as 90%, exhibiting comparable performance to those made with PVDF.

As a new generation of supercapacitor, the Li-ion capacitor (LIC) is an advanced energy storage device which consists of an electric double-layer capacitor (EDLC) cathode and a pre-lithiated anode [1], between which the ions shuttle during charge and discharge processes. Because of using pre-lithiated and low surface anode materials, the LIC can be charged to a maximum voltage as high as 4.0 V, which is much higher than of EDLCs. The LIC cell not only retains all the advantages of EDLC such as high specific power >10 kW/kg and long cycle life >100,000 cycles, but also exhibits a higher specific energy of 15-25 Wh/kg and a higher maximum cell voltage than that of the EDLC [2].
In addition to high specific power and long cycle life, we have also developed new electrolytes for LICs and compared performance of the LICs at various temperatures from 70 to -60 °C. The effects of charging temperature and negative electrode material on the low temperature performance of LICs are also studied [3].
Because of electrodes’ potentials of LIC are comparable to that of Li-ion batteries (LIBs), it allows the LIC and LIB to be assembled in one package as a LIB/LIC hybrid energy storage cell. In this talk, we will also demonstrate a new hybrid energy storage source which combines LIC and LIB. The fundamental difference between the proposed hybrid energy storage source and the previous hybrid device (LIC) is that the new hybrid energy storage technology integrates two separate energy storage devices into one by synergistically combining battery and capacitor materials together to form positive (or cathode) and/or negative (or anode) composite electrodes, and it is rightly named as the “Internal Hybrid” energy source. This new hybrid energy storage device will not require any electric circuits for charge balancing and control. The overall internal hybrid system will be simple, light, compact, and cost effective when compared with the conventional (external) hybrid energy storage source.

W.J. Cao and J.P. Zheng, Li-ion capacitors with carbon cathode and hard carbon/stabilized lithium metal powder anode electrodes, Journal of Power Sources, 2012, 213: 180-185.
W.J. Cao, J.F. Luo, J. Yan, X.J. Chen, W. Brandt, M. Warfield, D. Lewis, S.R. Yturriaga, D.G. Moye and J.P. Zheng, High Performance Li-Ion Capacitor Laminate Cells Based on Hard Carbon/Lithium Stripes Negative Electrodes, Journal of the Electrochemical Society, 2017, 164(2), A93-A98.
3. A. Cappetto, W.J. Cao, J.F. Luo, M. Hagen, D. Adams, A. Shellikeri, K. Xu, and J.P. Zheng, Performance of Wide Temperature Range Electrolytes for Li-Ion Capacitor Pouch Cells, J. Power Sources, 2017, 359, 205-214.

Lithium-ion capacitors (LICs) and Hybrid LICs (H-LICs) are hybrid battery-capacitor energy storage devices that have been proposed as suitable alternative high-power power source technologies for operation in extreme environments with prolonged cycle-life performance. The LICs can accommodate both the non-faradaic (i.e., ion adsorption/desorption) and the faradaic (i.e., Li-ion insertion/extraction) processes of electrochemical double-layer capacitors (EDLCs) and lithium-ion batteries (LIBs) at the same time, respectively. The advantages of the hybrid LIC device includes higher power density and longer cycle life than a LIB and higher energy density than an EDLC. Recently, hybrid composite cathodes that combine capacitor and battery materials have shown promise for enhanced energy/power performance and extended life-cycle. The LICs were assembled as three-layered pouch cells in an asymmetric configuration employing Faradaic pre-lithiated hard carbon anodes and non-Faradaic ion adsorption-desorption activated carbon (AC) cathodes for LICs and lithium iron phosphate (LiFePO₄-LFP)/AC composite cathodes for H-LICs. The room temperature rate performance was evaluated after the initial LIC and H-LIC cell formation as a function of the electrolyte additives. The capacity retention was measured after charging at high temperature conditions, while the design factor explored was electrolyte additive formulation, with a focus on their stability.

Flexible and stretchable batteries are attractive for wide applications, such as health care and flexible electronics. It is challenging to realize high flexibility/stretchability and high energy density simultaneously. In this work, we propose feasible and scalable strategies to fabricate flexible lithium-ion batteries, which can concurrently have high energy density and high mechanical flexibility, including bendability, foldability, twistability and stretchability. Inspired by the structure of animal spine, the vertebrae-like hard segments with multiple layers of conventional anode/separator/cathode stack to offer energy storage are interconnected by the soft components as marrow with the monolayer stack to provide flexibility. Its energy density 242 Wh/L can be over 85% of a standard prismatic cell with the same components.
The bio-inspired design allows the battery to sustain stable electrochemical performance even upon continuous dynamic mechanical formation including twisting and bending. Additionally, a wrinkling structure replacing the flat interconnector in between two hard segments is introduced to realize stretchability up to 50%. To further improve energy density and mechanical stability, we minimize the width of soft components by asymmetrically folding one long strip of electrode stack, where complex fabrication process of wounding and cutting is not needed. Thanks to this improved design, the battery exhibit excellent mechanical and electrochemical stability upon up to 180 degree folding. Our facile and scalable designs of flexible lithium ion battery potentially play an important role in wearable electronics.

References:
1. Qian, G et al., Bio-inspired, spine-like flexible rechargeable lithium-ion batteries with high energy density, Advanced Materials, 30(12), 1704947 (2018).
2. Liao, X et al., High-Energy-Density Foldable Battery Enabled by Zigzag-like Design, Advanced Energy Materials. Online.

Li-ion batteries (LIB) are useful energy storage devices for portable electronics application, electric vehicles, and utility grids. However, there are substantial needs of higher energy density and lower price materials than what the LIB systems can provide. A rechargeable battery utilizing intercalation of divalent ions such as Mg, Zn and Ca could be one of the viable strategies to overcome the capacity limit of LIB and to produce lower price batteries. However, only a few materials have been reported for the electrode materials that can intercalate the divalent ions reversibly.
Recently, calcium-ion batteries (CIBs) have received attention as one of the post LIBs. The abundance of calcium resources can make CIBs cost effective. Calcium has a standard reduction potential of 0.17 V above lithium (E° = -2.869 V vs. SHE for Ca/Ca²⁺) that makes CIBs capable of a higher energy density compared to other multivalent ions such as magnesium, aluminum, and zinc. In addition, calcium ion has a lower charge density than other multivalent ions, which may lead to relatively faster diffusion in a host material. Recently, Palacin et al. reported the feasibility of calcium plating and stripping using conventional organic electrolytes at elevated temperatures.¹ To use calcium metal as a negative electrode in a cell, suitable cathode materials that store calcium ions are required. However, only a few cathode materials are reported so far to show electrochemical activity in calcium-containing electrolytes, such as V₂O₅, CaCo₂O₄, WO₃, and Prussian-blue analogues.
In this presentation, some exploratory experimental results of electrochemical intercalation chemistry of divalent calcium ions will be presented with several host materials in aqueous electrolytes as well as non-aqueous electrolytes.

1. A. Ponrouch, C. Frontera, F. Barde, and M. R. Palacin, Nat. Mater., 2016, 15, 169-172.

Sodium ion batteries (SIBs) are an ideal alternative to lithium ion batteries for grid-level energy storage systems due to the low cost and abundance of Na precursors. ^[1,2] The elements, such as SnO₂, Sb₂S₃ and SnS₂, undergoing the conversion and alloying reactions with Na hold great promise for high energy density SIBs. ^[3,4] However, their large volume changes and low electrical conductivities lead to poor electrochemical performance and sluggish sodiation kinetics. In this work, we report the Na storage performance of a 3D topological insulator (TI), Sb₂Te₃ phase, ^[5] and its composites with carbon nanotubes (CNTs) synthesized through a facile, low cost and scalable high-energy ball-milling (HEBM) approach. The composite electrodes exhibit an excellent reversible gravimetric and volumetric capacities of 422 mA h g^-1 and 1232 mA h cm^-3, respectively, at a current density of 100 mA g^-1 with ~ 97.5 % capacity retention after 300 cycles. The roles of HEBM to introduce the Sb–C, Te–C, Sb–O–C and Te–O–C chemical bonds between Sb, Te and functional groups on CNTs are elucidated by combined XPS characterization and the first-principles calculations. Ex-situ TEM analysis reveals the formation of Na₂Te and Na₃Sb phases after the first sodiation, which are recombined into Sb₂Te₃ after the first desodiation, supporting the reversible phase transitions in the composite electrodes. For practical application, the electrochemical performance of a sodium ion full cell (SIFC) consisting of an Sb₂Te₃/CNT anode and a Na₃V₂(PO₄)₂F₃ cathode is evaluated. The SIFC delivers a remarkable energy density of ~ 229 Wh kg^-1 at 0.5 C and excellent cyclic stability of more than 71 % and 66 % capacity retention at 5 C and 10 C, respectively, after 200 cycles. Even after fully charged at 40 C in 90 seconds, the SIFC presents an ultrahigh power density of 5384 W kg^-1. These findings may shed new insight into exploring the unique class of TI quantum materials as anodes for high performance SIFCs.

References
[1] J. Cui, S. Yao, and J. K. Kim, Energy Storage Mater., 7, 64–114, 2017.
[2] M. Ihsan-Ul-Haq, H. Huang, J. Cui, S. Yao, J. Wu, W. G. Chong, B. Huang, and J.-K. Kim, J. Mater. Chem. A, 6, 20184–20194, 2018.
[3] S. Yao, J. Cui, J. Huang, J.-Q. Huang, W. G. Chong, L. Qin, Y.-W. Mai, and J.-K. Kim, Adv. Energy Mater., 8, 1702267, 2018.
[4] J. Cui, S. Yao, Z. Lu, J.-Q. Huang, W.G. Chong, F. Ciucci, and J.K. Kim, Adv. Energy Mater., 8, 1702488, 2018.
[5] H. J. Zhang, C. X. Liu, X. L. Qi, X. Dai, Z. Fang, and S. C. Zhang, Nat. Phys., 5, 438–442, 2009.