Influence of Traces of Moisture on a Sulfide Solid Electrolyte Li<sub>4</sub>SnS<sub>4</sub>

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.