All-solid-state batteries, which use a solid electrolyte, are a promising technology for improving the safety of currently commercialized batteries based on liquid electrolytes. However, to enable all-solid-state batteries with high energy densities, we need to integrate solid electrolytes with high-voltage and high-capacity cathodes. The interface between solid electrolytes and high-energy cathodes is often thermodynamically unstable, which can lead to reactions and the formation of decomposition products which cause high interfacial resistance. One solution to improve resistance and poor contact at the interface is the application of a coating layer, which can act as a physical barrier between the solid electrolyte and the cathode and prevent decomposition.

I performed first-principles computation and thermodynamic analyses to study the thermodynamic stability and Li-ion transport in coating layers for solid-solid interfaces. I used a high-throughput systematic analysis of phase diagrams based on a materials database to study the decomposition energy and products of reactions between coating layer chemistries and layered and high-voltage cathodes. My thermodynamic stability analysis revealed that the strong reactivity of lithiated and delithiated cathodes greatly limits the possible choice of materials that are stable with the cathode under high-voltage cycling. The computation results reaffirmed previously demonstrated coating chemistries and identified several new chemistries for high energy cathodes. In particular, I found that lithium quaternary phosphates and lithium ternary fluorides were two promising materials classes, with good stability with high-voltage cathodes and sufficient lithium content to enable Li-ion transport.

I next studied the interface stability between solid electrolytes and common cathodes. The lithium garnet solid electrolytes are promising among known solid electrolytes because of their high temperature stability, good stability in air and moisture, and wide electrochemical window, but have limited stability against a variety of cathodes. To guide the development of coating layers for the garnet-cathode interface, I analyzed the stability of garnet with families of lithium ternary oxide (Li-M-O) coating chemistries and revealed factors governing the stability of materials with LLZO garnet and high-energy NMC cathodes. In addition to classifying known coating layers, I provide detailed guiding tables for coating layer selection and identify and discuss several new promising coating layer materials for stabilizing the interface between garnet and high-capacity cathodes.

The crystal structure of a coating material plays a major role in transport properties such as Li-ion diffusivity and conductivity, which are required in the coating layer to achieve low interfacial resistance and good battery performance. Alumina is widely used as a coating layer in batteries and other applications, and has decent stability against a wide range of solid electrolytes and cathodes. I used first-principles molecular dynamics simulations and nudged-elastic band calculations to study Li-ion transport and migration barriers in several crystalline polymorphs and amorphous alumina. I found structural features in the Al framework, specifically the Li-Al distance variation, determined migration barriers in both crystalline and amorphous structures. Based on this structure-property relationship, I investigated how Li content, defects and off-stoichiometry changed the Li-ion transport within selected polymorphs, and suggest lowering the Al-ion content as a strategy to achieve stable and Li-diffusive alumina coatings.

With this work, I provide an understanding of trends in stability between coating layers, cathodes, and the garnet solid electrolyte, new promising coating layer materials and families, and rational guidance for coating layer design and interfacial engineering for energy dense all-solid-state batteries. My thesis provides guiding principles for selecting materials with long-term cycling stability and good Li diffusivity as coatings for energy dense Li-ion batteries.