First-Principles Study on Selenium-doped Li₁₀GeP₂S₁₂ Solid Electrolyte—Effects of Doping on Moisture Stability and Li-Ion Transport Properties

All-solid-state lithium-ion batteries (ASSBs) are currently at the forefront of the search for next-generation energy storage technologies. ASSBs use solid-state electrolytes (SSEs) rather than the liquid electrolyte, allowing for greater safety, higher energy density, and higher output power. Among the different SSEs, the ceramic-sulfide-based solid electrolyte, Li₁₀GeP₂S₁₂(LGPS) has attracted considerable attention because of its comparable ionic conductivity of 12 mS/cm with current organic liquid electrolytes. However, one of the most significant challenges in LGPS is the hydrolysis reactions at the interface, which are intrinsically unstable in ambient humidity, resulting in the release of toxic gas H₂S and, as a result, a significant decrease in ionic conductivity. Hence, improving moisture stability without degrading the ionic conductivity of LGPS-based SSEs is one of the key issues in developing efficient ASSBs. Herein, we report the reactivity of the LGPS surface towards H₂O, subsequent hydrolysis reactions for the H₂S formation, and effective methods to suppress the H₂S evolution on the LGPS surface using density functional theory (DFT) calculations. The pristine LGPS surface is highly unstable towards H₂O at room temperature, where H₂O dissociation occurs spontaneously. However, the H₂O adsorption and its activation are weakened upon the Se substitution on the LGPS surface. By doping the Se atom on LGPS, the H₂S evolution can be suppressed and significantly improve the chemical stability of the LGPS surface. In addition, we predict the Li⁺ ion transport properties of both pristine and Se-doped LGPS compounds using ab initio molecular dynamics (AIMD) simulations. The Se-doped LGPS material has a similar bandgap to pristine LGPS; it can have similar decomposition products and low electronic conductivity. The predicted ionic conductivity of Se-doped LGPS is found to be slightly lower than the pristine. These findings suggest that replacing sulfur atoms in LGPS with Se atoms could be an effective strategy for improving moisture stability.