Effects of Li Defects on Moisture Stability and Li-Ion Transport Properties on Li₁₀SiP₂S₁₂ Solid Electrolyte—A First-Principles Study

All-solid-state batteries (ASSBs) are expected to replace liquid electrolyte-based lithium-ion batteries (LIBs) as the next-generation rechargeable battery technology to meet the increasing demand for advanced energy storage systems because of their high energy density and safety. Solid electrolytes (SEs) with high Li-ion conductivity are required for ASSBs. Over the years, Li₁₀SiP₂S₁₂(LSPS) has attracted attention as a promising new Li-ion conductor, as it has the high Li-ion conductivity (2.3 mS/cm). However, the poor chemical stability of sulfide-based electrolytes in the presence of moisture causes the hydrolysis reaction on the LSPS surface, resulting in the formation of toxic H₂S gas. This disadvantage necessitates handling of the LSPS in an inert gas environment, limiting large-scale commercialization. Therefore, improving moisture stability without compromising ionic conductivity of LSPS-based SEs is a critical issue in the development of ASSBs. In this study, we used density functional theory (DFT) calculations to understand the reaction mechanisms of the hydrolysis reaction on the perfect and defective LSPS (101) surface. Furthermore, we investigated the effect of Li vacancy on the Li-ion conductivity of LSPS using ab initio molecular dynamics (AIMD) simulations and statistical thermodynamic methods. The low energy barrier of the hydrolysis reaction on the defective LSPS surface indicated that Li vacancy would decrease the moisture stability of the LSPS surface. On the other hand, the presence of Li vacancy increased Li inter-cage migration, which increased Li ionic conductivity in the LSPS. Our findings show that the Li vacancy has a significant impact on the moisture stability and Li-ion conductivity of LSPS.