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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP There is a pressing need to develop new methodologies to elucidate the complexity of the formed interphases. Labeling approaches can be applied to track chemical species throughout the reaction pathway and thereby to elucidate their role in the development of the final interphase product. For example, direct observation of solid-phase electrolyte products resulting from decomposition of acetonitrile on activated carbon has been investigated by 13C, 1H, and 15N solid-state nuclear magnetic resonance. Such approaches can be used to understand true interphase development. Realizing Simple but Representative SEIs: An important barrier to complete molecular-level understanding of interfacial transport and chemistry in energy storage systems arises from the difficulty in validation of theoretical predictions using well-characterized experiments performed on relevant model systems. An analogous impasse to progress in understanding surface reactivity of molecular agents was removed with the introduction of ultra-high vacuum surface science, which provided a fundamental experimental framework able to validate theoretical predictions for quantities such as heats of adsorption, rates of reactions, and mechanisms of surface reactions.37-41 This approach is, in principle, applicable to condensed matter and at electrified interfaces, but requires well-characterized systems of sufficient complexity. The systems must not only be designed to advance fundamental understanding of chemical physics at interfaces, but must ultimately be robust enough to couple with intrusive experiments able to mimic conditions used in theoretical work for accurately describing interfacial complexity. A recent example of such an experiment was achieved through the synthesis of well-defined planar electrodes for detailed desolvation studies of sodium ions, which showed a six-fold variation in desolvation energy depending on the anion donor number.42 Expanding these types of studies while identifying the interface structure and introducing additional functionality from binders or porosity will develop a foundation to support theoretical model development and the prediction of optimized artificial SEI layers needed to focus experimental developments. How to Understand and Build Ideal Electrochemical Interfaces: An interphase is an enabling aspect of the entire Li-ion state-of-the-art battery and also offers a pathway to next generation materials. In many cases an effective interphase, whether it is synthesized by in situ or ex situ chemistry, should be electronically insulating, have high ionic conductivities, exhibit self-passivating growth, and be stable against dissolution.43 As a result, electrolyte components operating outside their potential window of stability are stabilized. Artificial interphases can be created by coating electrodes with very thin (1-50 nm, typically 1-3 nm), insulating polymer44.45 or ceramic coatings,9,10,46-49 which mimic the effects of spontaneously formed interphases and may be deposited by techniques ranging from solution casting to atomic layer deposition. There is currently little understanding of the chemistry or mechanisms by which these coatings work, whether they are robust enough to protect the interface during extended cycling of metals at high current density, and how they fail. As a consequence, few insights are possible into optimal designs for coatings with specific functionality, mechanics, and lifetimes. New characterization tools are required to enable progress. When coupled with detailed mechanistic studies of electrolyte decomposition pathways, supported with theoretical and computational analyses of adsorption and reaction mechanisms, this research would fundamentally alter the current Edisonian trial-and-error approach to coating development. The free-energy landscape for migrating charges has its origins at the atomic- to nano-scale for charge transfer and at meso- to macro-length scales for charge transport. In a typical device architecture, these interfaces are buried and, therefore, difficult to study. To highlight the key roles played by interfaces, consider ion-transport selectivity in a membrane for a flow battery, which occurs at a membrane-electrolyte interface; in this case, eliminating crossover is key to preventing internal shuttling and deleterious cross-annihilation of constituent active materials that are dissolved in the electrolyte.50-54 Similarly, consider electron transfer across metal anode-electrolyte or cathode-electrolyte interfaces, which degrades the electrolyte and ultimately yields new interphases;55-58 if left uncontrolled, their continued growth increases cell impedance, and active materials are increasingly underutilized in the cell for a given operating voltage range.59,60 While still nascent, our emerging knowledge of these interphases and the interfacial processes generating them points to an emerging materials challenge: new materials, characterization tools, and theoretical frameworks are needed to enable interphases to be designed rationally for function, whether that function is chemical, physical, 46 PRIORITY RESEARCH DIRECTION – 3PDF Image | Next Generation Electrical Energy Storage
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