Understanding Mechanisms of All-solid-state Lithium-Sulfur Batteries through Operando Raman and Ex-situ XAS Studying

All-solid-state lithium-sulfur batteries (ASLSBs) have been considered a promising next-generation energy storage technology due to their remarkable safety and high energy density. In ASLSBs, the electrochemical pathways are intrinsically different from conventional Li-S batteries using liquid electrolytes. However, the mechanism still lacks clear identification and deep understanding. Herein, for the first time, we investigated the chemistries and explored the electrochemical reaction mechanism and kinetic in ASLSBs through coupling operando Raman spectroscopy and ex-situ X-ray absorption spectroscopy. We proved no long-chain lithium polysulfides (Li2Sn, 4≤n≤8) were formed during the redox reactions, but a short-chain polysulfide (Li2S2) intermediate phase formation was identified in the conversion between active material S8 and reduction product Li2S. The existence of intermediate phase Li2S2 results in low sulfur utilization and poor battery performance. In comparison to liquid cells, ASLSBs exhibit sluggish reaction kinetics due to the higher conversion barrier and slower charger transfer in solid-solid reactions. This study revealed the generation of Li2S2 intermediates in ASLSBs, inspiring future research to further improve the performance of ASLSBs through completing the conversions and promoting reaction kinetics in ASLSBs.