Understanding Electrochemical Reaction Mechanisms of Sulfur in All‐Solid‐State Batteries through Operando and Theoretical Studies **

Due to its outstanding safety and high energy density, all-solid-state lithium-sulfur batteries (ASLSBs) are considered as a potential future energy storage technology. The electrochemical reaction pathway in ASLSBs with inorganic solid-state electrolytes is different from Li-S batteries with liquid electrolytes, but the mechanism remains unclear. By combining operando Raman spectroscopy and ex situ X-ray absorption spectroscopy, we investigated the reaction mechanism of sulfur (S8 ) in ASLSBs. Our results revealed that no Li2 S8, Li2 S6, and Li2 S4 were formed, yet Li2 S2 was detected. Furthermore, first-principles structural calculations were employed to disclose the formation energy of solid state Li2 Sn (1≤n≤8), in which Li2 S2 was a metastable phase, consistent with experimental observations. Meanwhile, partial S8 and Li2 S2 remained at the full lithiation stage, suggesting incomplete reaction due to sluggish reaction kinetics in ASLSBs.