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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP separator inserted between electrodes in EES systems to prevent direct electrical contact also introduces additional interfaces, which present both challenges and opportunities in modulating function. Figure 2.3.1. Schematic of a multicomponent electrode interfacing with both the electrolyte (top) and current collector (bottom), presenting a multiplicity of internal interfaces, each with the possibility of evolving an interphase during the energy storage function. From Ref. 1. The hypothesis that frames this Priority Research Direction is that explicit design of interfaces and interphases for function will enable better energy storage devices, whether these devices use conventional liquid or solid electrolytes. From detailed knowledge of the composition and properties of electrochemical interfaces and interphases in both exceptional and poorly functioning cells, we foresee an emergence in expertise in design and synthesis of optimal interfaces/interphases for a desired energy storage device/context. Several questions then emerge: What can we learn from model systems or from aspects of working cells to guide development of interphases that allow explicit control of reactivity and electrochemical function? Which interfacial phenomena are key to achieving a desired energy storage function? How do we build adaptive interphases to respond to cues in closed systems to achieve high efficiency and extended lifetime? Furthermore, how would one learn about and curate knowledge of useful, ideally exceptional examples of functional interphases from research on best-in-class type electrochemical systems? How would one transform that knowledge into rational design principles for tailoring interfacial composition, morphology, stability, strength, etc., required for achieving electrochemical function? These questions define Priority Research Directions for future research on next generation electrical energy storage solutions. The goal is to enable new approaches that provide mechanistic insight into how interfacial function is determined and, from this, to rationally design and realize interphases as integral components that substantially improve the performance and robustness of electrochemical cells. 2.3.1 SCIENTIFIC CHALLENGES During the initial lithiation cycles of a Li-ion battery, an SEI forms on the anode surface due to the electrochemical instability of the electrolyte to lithiated anodes. The SEI allows Li-ion conduction but is electrically insulating, inhibiting further reduction of the electrolyte. SEI formation is one of the most important and fundamental reactions in Li-ion batteries and is critical to achieving reversible cycling performance. The SEI in these batteries has been under investigation for over 30 years, and a general understanding of the primary components has emerged from characterization studies.2-5 However, while the SEI is now understood to be a mixture of known insoluble lithium salts and Li-conductive organic polymers, very little is known about how the morphology and nanoscale structure of the materials influence properties and function. Even less is known about how basic variables such as current density, temperature, salt concentration, or mechanical properties of an electrolyte influence SEI structure and morphology. How the SEI structure and physical properties change in response to volume change at metallic electrodes (e.g., Si, Sn, Li, and Na) and why these changes compromise electrochemical stability of some liquids, but not others, are additional open questions. Widening the Electrochemical Stability Window of Liquid Electrolytes: Recent findings on concentrated liquid electrolytes show that compared to conventional electrolytes (salt concentration in the range of 1-1.5 M), interfaces formed in both aqueous5 and non-aqueous electrolytes6,7 display a host of unusual properties, including measurably expanded voltage stability windows. The appearance of such electrolytes has significantly changed what one can expect from a typical liquid-electrolyte electrochemical cell and is considered a breakthrough in the field. In one study, the electrochemical stability window of conventional aqueous electrolytes was shown to be expanded from less than 1.50 V to more than 3.0 V simply by increasing lithium salt concentration to > 20 M. This increase was found to coincide with an entirely new Li+-solvation sheath structure and unusual interphase electrochemistry (Figure 2.3.2a).5 Enclosed in the expanded stability window are most cathode materials that would have been otherwise impossible for aqueous cell applications (Figure 2.3.2b), 38 PRIORITY RESEARCH DIRECTION – 3PDF Image | Next Generation Electrical Energy Storage
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