Electrode/Electrolyte Interphase Formation by Lithium Iodide in a Li₂S-Based Positive Electrode for All-Solid-State Batteries

Because lithium sulfide (Li2S) as an active material is both ionically and electronically insulating, it is typically combined with sulfide solid electrolytes (SSEs) and conductive carbon in all-solid-state batteries to form a composite positive electrode. However, the decomposition of the SSEs at the interface causes capacity degradation. Among the many reported Li2S-based composite positive electrodes, sulfide active materials incorporating LiI specifically improve the battery performance by overcoming the decomposition of SSEs. In this study, the interface between the Li2S–LiI active material and SSE was analyzed, and the effects of LiI on the formation of this interphase were elucidated. Structural analyses revealed the formation of an amorphous interphase comprising lithium, phosphorus, sulfur, and iodine on the SSE surface during the initial charging process. This interphase contributed to the charge–discharge capacity and protected the pure SSE inside it from excessive redox decomposition, which led to an improvement in the battery performance. These findings will facilitate the design of all-solid-state batteries with sulfide-based active materials.