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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP Towards Rational Design of Interphases and Interfaces: Better understanding of interphase formation in energy storage, and the critical components required to obtain interfaces with desired reactivity, can enable their rational design. An effective interphase, whether be it synthesized via in situ or ex situ chemistry, displays the following properties: it is electronically insulating, has high ionic conductivity, displays self-passivating growth, and is stable towards dissolution.68 There have already been many demonstrations of “artificial interphases” through coating electrodes with very thin (1-50 nm, but typically 1-3 nm) insulative films that mimic the effect of an in situ derived interphase. These coatings (e.g., Al2O3, B2O3, SiO2, ZrO2, and polymers)69-72 are deposited by various techniques ranging from solution casting to atomic layer deposition and have been shown to be highly effective for stabilizing the cycling of cathodes in Li-ion batteries at very high potentials, where anodic decomposition of electrolyte is a dominant failure mode. These coatings can also be effective in protecting reactive metal anodes where electrolyte consumption by parasitic reactions with the metal is an important mode of failure. That being stated, an effective ex situ derived interphase need not be electronically insulating,73 especially when the electrode surface is catalytic and triggers the electrolyte decomposition. In this case, replacing the catalytic surface of the electrode with a non-catalytic electronically conductive surface will effectively eliminate the degradation-related surface decomposition. The importance of this approach is that the electronic charge transfer to the external circuit is maintained. Further, recent studies have also shown that surface “layers” need not be distinct chemical or physical phases, but surface regions that are rich in extrinsically introduced ions or that enable strong composition gradients,74-76 leading to improved interfacial properties and electrolyte stability. 3.2.2 SCIENTIFIC CHALLENGES AND OPPORTUNITIES Advanced Analytical Characterization Techniques for Probing Interfaces and Interphases: A key challenge is unravelling the complexity of coupled processes that control the function of heterogeneous electrochemical interfaces. New approaches are required that use combinations of tools for complementary insights. At one end of the spectrum, opportunities exist for studies that take advantage of well-controlled model systems in which the functional aspects of the electrochemical cell are preserved to enable detailed understanding of how interfaces of well-defined chemistry contribute to overall function. At another extreme, opportunities exist for approaches that enable operando interrogation of specific features of a heterogeneous interface to unravel structure-function relationships in both explicit and buried interfaces. Each of these approaches brings obvious drawbacks; however, strategies for overcoming them are not unfamiliar in the chemical physics community. For example, well-controlled model systems can provide a wealth of valuable information about particular interfacial components or particular configurations of multiple components, but there is a gap in applying that knowledge to much more heterogeneous realistic electrodes if these model systems are designed to be so ideal that the electrochemical context is lost. There are also questions related to the best ways to corroborate the results obtained from multiple studies, particularly for specialized in situ setups. For example, how “universal” are the results? In this regard, round-robin studies of the same model systems by multiple teams of researchers must be encouraged. Operando investigations overcome the most serious drawbacks of model-system studies, but require more intensive, longer term commitment to instrument development and large-scale collaborative efforts among tool and subject matter experts. Additionally, there is poor understanding of the impact of interphase heterogeneities within the cell as it relates to spatiotemporal aspects of charge transfer associated with a cell’s accessible energy density and lifetime. Tools are needed to elucidate how ions transport at phase boundaries and how processes such as desolvation and non-specific surface binding influence the energetics of interfacial transport across length and time scales. There is, likewise, no concrete understanding of how the electric field in a liquid in contact with a solid electronic conductor is altered by separating the liquid and solid with a leaky dielectric, insulator, or semiconductor of nano-sized thickness that may permit some ingress of the liquid. Operando investigations and time-dependent studies are likewise needed to fully understand the potential of interfaces to evolve over time. To provide the required insights, such measurements should be performed under relevant conditions to enable coupling to ex situ observations and connections to bulk electrochemical observables to be determined. Beyond these specific needs, there is a critical overarching need for new methods suitable for isolating the role that electrochemical interfaces play in device degradation and for enabling accelerated analyses for predicting device failure and lifetime. 100 PANEL 2 REPORTPDF Image | Next Generation Electrical Energy Storage
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