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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP composed of multiple components. Numerous interfaces exist even when considering only the active materials (particle-particle, particle-binder, particle-conducting additive, particle-electrolyte, etc.). The specific form of the interphase will dictate the exact free-energy landscape for migrating charges. These landscapes, which have origins at the atomic level, are collectively responsible for the cell’s energy efficiency, rate capability, and stability. Further, the so-called “inactive” conducting additives and binders may also affect interfacial processes and the nature of the SEI that is formed on electrodes.85-87 Fundamental, mechanistic understanding is required to elucidate how individual components of the SEI contribute to the functional properties of this interphase. Individual components of the SEI (e.g., LiF, Li2CO3, and alkyl carbonates) have been studied in isolation, but their synergistic effects have only begun to be addressed. How multiple components and processes interact together is still not well understood. There is also the possibility that the behavior of individual components may not be “additive”, and that the interphase properties are a product of the heterogeneity, which cannot be captured through studying individual components in isolation. Hence, while a need exists for fundamental studies on model systems of reduced complexity to understand chemistry-structure-property-function relationships for individual components, a complementary need exists for detailed studies of realistic systems in which coupled contributions to overall SEI function are preserved. Addressing the challenge requires that electrolyte formulations be modified to vary the structure of the SEI in a systematic fashion. These components must be fully experimentally characterized, with the use of multiple tools to extract information regarding coupled and synergistic relationships. Such measurements would enable the development of a fundamental understanding of the structure and function of the SEI on lithium-ion battery anodes and enable the use of reactive metals, such as Si, Sn, Li, Na, and Al, as anodes for high capacity storage. New methodologies to elucidate the complexity of fully formed interphases are also needed, particularly if interphase formation occurs through the contribution of a number of components. There is especially a need for methods that take advantage of specific labeling, such as radio-isotopes,88-90 to track chemical species throughout the reaction pathway, thus allowing realization of their role in the development of the final interphase product. An example of isotope tagging88 is the direct observation of solid-phase electrolyte products resulting from decomposition of acetonitrile solvent on the mesoporous activated carbon positive electrode of a non- aqueous asymmetric hybrid supercapacitor by 13C, 1H, and 15N solid-state nuclear magnetic resonance. At least two of the observed nuclear magnetic resonance signals could be assigned to acetamide and a lithium-based carbon derivative, suggesting that the Li salt interacts with the acetonitrile solvent, leading to accumulation of decomposition products on the electrode particle surface. Such approaches can be used to understand true interphase development.89 A parallel computational investigation (e.g., the ionic-conducting properties of the known components as a function of different nano-structures) is also required to understand the structure- function relationship of the SEI. Towards Rational Design of Interphases and Interfaces: Metal anodes represent the most efficient use of mass and volume as the penalty of a host material (e.g., graphite, Si, or Sn) is eliminated, but the absence of a host scaffold results in the formation of a physically unsupported SEI.91 Attempts to create artificial SEIs through thin film growth on Si (Figure 3.2.7) and lithium anodes using coating techniques, including atomic and molecular layer deposition, have improved lithium anode stability with cycling.71,72,87, 92-95 Ultimate success for these or new approaches must solve both the coulombic efficiency and dimensional control (dendrite) problem. Management of the local volume change within a cell that employs a metal anode must also be addressed. Discharge of the anode produces local volume loss and threatens the ability to maintain a coherent, low-impedance interface with the artificial SEI membranes and films integrated with the anode. One solution to this problem is to design for volume change through an appropriately sized host scaffold (e.g., using porous structures), where the scaffold mass and volume are minimal relative to the metal. There are vast opportunities in materials synthesis/processing and electrode fabrication to direct interfacial reactions beyond the use of surface coatings to act as artificial SEIs. Such architectures could be designed to more efficiently use advances in materials synthesis (e.g., highly faceted nanostructures) and fabrication (e.g., 3D printing and self-assembly) to control interfacial processes and introduce adaptable interphases. All-solid-state thin film batteries have been fabricated with well-defined and stable interfaces. Recent work on the 5-volt Li/ LiPON/LiMn1.5Ni0.5O4 micro-battery revealed that it can cycle over 10,000 cycles with greater than 90% capacity retention with the formation of the right interfacial structure96 (see Figure 2.3.4 in Chapter 2). 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