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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP dense composite cathodes and alkali metal anodes. The electrolytes must be thin yet also mechanically robust to accommodate various stresses from integration with other cell components, assembly and packaging into a full battery module, and cyclic dilation during cycling. Conventional wisdom suggests that the alkali metal/electrolyte interface should be free of inhomogeneity, e.g., defects that can lead to non-uniform current densities and formation of lithium dendrites.55,56 A fundamental question for fabrication is whether a planar interface is a general requirement for the lithium-electrolyte interface. An alternative approach is to fabricate high surface area or 3D structures to reduce current densities at the interface.57 Also, it is unclear how inhomogeneity at the interface governs electrochemical phenomena. It is critically important to understand how much inhomogeneity can be tolerated in solid-state cells without appreciably compromising cell performance, a question that is fundamental to mesoscale science.58 At the cathode, on the other hand, the solid electrolyte must be mated to a solid-state composite (porous) cathode. While compliant sulfide and polymer electrolytes have been successfully integrated with porous cathodes, this integration is much more difficult to achieve with stiff ceramic oxide materials. Future research should seek to balance the development of feasible fabrication approaches with studies of how these approaches shape the physical and chemical nature of the electrode-electrolyte interface. More generally, the domain of solid-state storage presents qualitatively different structural permutations whose consequences in design and synthesis are profound at the mesoscale, posing a host of scientific questions. Thin film synthesis, already common in commercial solid-state cells, provides relatively smooth, predictable surfaces and films, but with greater limitation on material complexity. Thick film synthesis, typically formed from powders by sintering, accesses complex combinations of elements and stoichiometries but produces less controlled microstructure (particle shape and size) and varying tortuosity of transport paths. One can envision mixtures of the two as well, exemplified by thin film atomic layer deposition of an alumina layer to enable wetting of garnet electrolyte pores for successful in-filling of Li anode material.59 Given these considerations, what architecture and synthesis strategy can realize high performance and stability in the resulting storage structures? These outstanding questions emphasize the need for rational design principles to identify solid-state architectures for efficient charge capacity utilization, long-term cycleability, and mechanical robustness in electrochemical cells. Developing these design principles requires relevant model systems that allow study of correlated physical, chemical, and electrochemical phenomena during cell operation. Lithium thin-film batteries are exemplary systems and provide inspiration for these model systems.60 As progress in solid-state electrolytes continues and they are integrated into practical solid-state electrochemical cells, relevant model structures for these novel materials and devices must be developed. For example, a model structure could be a single crystal of a polycrystalline ceramic or a bi-crystal consisting of a crystalline electrode and crystalline electrolyte. Ideally, these structures should allow the interrogation of how material properties and structural features (such as crystal orientation, aliovalent substitutions, impurity phases, and interfacial layers) govern the correlated phenomena. Electrochemical system modeling coupled to these experimental observations is also needed. In particular, modeling of solid-state mechanics is especially important to understand the evolution of material and interfacial stresses associated with cycling and to discover how the cell architectures can be designed to accommodate reversible volume changes with each charge/discharge cycle. As the computational capabilities progress for model systems, an important challenge will be extending the coupled physics to cell architectures of real, practical solid-state devices. Operando characterization of solid-state cells will be useful in validating these computational models. Interaction of Chemistry and Mechanics of Solid-Solid Interfaces: While there have been recent advances in solid ion conductors exhibiting conductivities comparable to liquid electrolytes, how to best translate these materials into viable solid-state batteries is not currently known. What are the design rules? How are solid- solid interfaces formed? How does charge transfer occur at interfaces? What mechanical stresses arise during fabrication and during cycling? Essentially, there are two interfaces of interest, distinguished by the electrode type—the alkali metal electrode and the ceramic electrode. Several key scientific challenges related to these two interfaces must be addressed to mature solid-state batteries. First, despite the impact that metallic anodes could have on energy storage technology performance, very little is known about the chemical, electrochemical, and mechanical stability of the alkali-metal/solid electrolyte interface. 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