Insight into the Interfacial Evolutions between Halide and Sulfide Solid Electrolytes

Halide solid-state electrolytes (HSEs) exhibit electrochemical instability with the Li metal anode, which necessitates the adaptation of an interlayer, mostly sulfide solid-state electrolytes (SSEs). So far, the HSE/SSE compatibility has been overlooked, and their interfacial reactions remain undiscovered. Here, we comprehensively unveil HSE/SSE interfacial evolutions using Li3InCl6 (LIC) and Li6PS5Cl (LPSC) under two typical operating scenarios: static aging and electrochemical aging. We not only visualize the LIC/LPSC interfacial evolutions but also quantify their interfacial reaction determinants, products, and corresponding mechanisms. Our findings reveal that LIC/LPSC interfaces are unstable under all circumstances. Under static aging, the interfacial reactions are diffusion-controlled, and the main reaction products (In2S3 and InP) lead to successive diffusion and deteriorated structure. Under electrochemical aging, the interfacial reactions are both diffusion-controlled and charge-transfer-controlled, and the increased reaction product InCl3 leads to suppressed diffusion and a preserved structure.