Real Time Raman Spectroscopy Reveals Hydrolysis Pathways in Sulfide Solid Electrolyte Li ₆ PS ₅ Cl Upon Moisture Exposure

ABSTRACT The sulfide solid electrolyte (SE) Li 6 PS 5 Cl is a promising material owing to its high lithium ionic conductivity. However, it suffers from pronounced instability in humid environments, which significantly hinders its practical application in all‐solid‐state batteries. Although many studies have examined the degradation behavior of Li 6 PS 5 Cl, its underlying hydrolysis mechanism remains poorly understood, especially from a mechanistic and time‐resolved perspective. In this work, we develop a real‐time Raman spectroscopy platform integrated with a quantitatively controlled humidity delivery system to analyze the hydrolysis process with high temporal resolution. Using this platform, we captured the real‐time structural and chemical evolution of Li 6 PS 5 Cl during exposure to moisture. Our observations indicate the presence of decomposition products and a distinct multistep progression of chemical‐state transitions, suggesting intermediate reaction pathways not previously captured in static analyses. This detailed analysis of the hydrolysis mechanism offers a fundamental understanding of how moisture reacts with sulfide solid electrolytes. Furthermore, the ability to monitor dynamic degradation behavior provides new insights for improving material stability through approaches such as targeted elemental substitution and microstructural engineering, ultimately contributing to the development of robust batteries.