Influence of Traces of Moisture on a Sulfide Solid Electrolyte Li4SnS4

The sulfide solid electrolyte Li4SnS4 has gained attention owing to its high moisture durability. In this study, we quantitatively investigated the changes in the electrochemical properties and chemical/physical states of Li4SnS4 resulted from moisture exposure using the XRD, Raman spectroscopy, and high-frequency electrochemical impedance spectroscopy (HF-EIS). Li4SnS4 was subjected to Ar gas flow at a dew point ranging from −20 °C to 0 °C for 1 h, and sulfide hydrolysis generated only a minute amount of H2S. The XRD patterns and Raman spectra revealed the formation of Li4SnS4·4H2O with increasing dew point. The HF-EIS analysis, which was conducted to clarify the spatial distribution of the hydrate within the particle, revealed a significant decrease in the ionic conductivity of Li4SnS4; this result can be attributed to the increased grain-boundary (SE/SE particle contact) resistance due to the formation of Li4SnS4·4H2O at the particle surface, despite the generation of a minute amount of H2S. By combining these multifaceted analytical methods, we demonstrated that the thermodynamically stable surface hydrate Li4SnS4·4H2O reduced the lithium-ion conductivity without H2S generation owing to the hydrolysis of sulfide. Thus, we chemically, spatially, and quantitatively verified the mechanism underlying the observed decrease in the ionic conductivity.