Revealing the Structural Origins of Ionic Conductivity Degradation in Li ₆ PS ₅ Cl: A Detailed Spectroscopic Study of Moisture-Induced Chemical and Crystallographic Transformations

Sulfide solid electrolytes, while promising for all-solid-state batteries (ASSBs) due to their high ionic conductivity and favorable mechanical properties, are highly sensitive to moisture. In this work, we have elucidated the correlation between electrochemical performance degradation and a chemical structural change induced by exposure to humidified air with specific dew points. Moisture exposure leads to substantial losses in ionic conductivity accompanied by the formation of degradation products, including LiOH and LiCl, as well as structural changes of the PS43− tetrahedra. A trace amount of H2O has clearly reacted and changed the chemical structures of the sulfide solid electrolyte. On the other hand, although H2O molecules are incorporated into the sulfide solid electrolyte in close proximity to Li and P nuclei, the complete decomposition of the original framework has not proceeded. It was verified that subsequent heat treatment partially recovers the ionic conductivity by removing incorporated H2O; however, full recovery is not achieved because of the irreversible structural changes, such as LiCl formation, sulfur deficiency, and oxygen incorporation into PS43− tetrahedra. These results highlight the critical importance of moisture control during the ASSB manufacturing process—especially in dry room environments where severe control of dew point is quite difficult—and emphasize the need for developing the sulfide solid electrolytes with enhanced moisture resistance.