Investigation of the Crystal‐Structure‐Dependent Moisture Stability of the Sulfide Solid Electrolyte Li ₄ SnS ₄

The practical realization of all‐solid‐state batteries is a key technology for achieving future environmental sustainability. In this context, sulfide solid electrolyte Li 4 SnS 4 has attracted attention because of its superior moisture stability compared to that of other sulfide materials. Li 4 SnS 4 crystallizes in two crystal structures, hexagonal and orthorhombic; however, the relationship between the structure and moisture stability remains unclear. In this study, both crystal structures of Li 4 SnS 4 were synthesized and systematically examined under controlled moisture exposure by monitoring H 2 S evolution and evaluating electrochemical performance. The electrolytes were exposed to Ar gas at a dew point of 0°C for 1 h, during which H 2 S release was continuously monitored. Post‐exposure X‐ray diffraction and electrochemical impedance spectroscopy revealed that the orthorhombic phase produced significantly more H 2 S than the hexagonal phase. By contrast, the hexagonal phase displayed a higher degree of lithium ionic conductivity degradation. Both phases formed thermodynamically stable hydrate Li 4 SnS 4 ·4H 2 O after moisture exposure, with the hexagonal phase yielding a larger fraction. These results indicate that hydrate formation suppresses H 2 S generation but accelerates conductivity degradation. Overall, this study establishes the correlation among crystal structure, hydrate formation, and hydrolytic decomposition and proposes a revised definition of “moisture stability” for sulfide solid electrolytes.