Improved Air Stability of Li Argyrodites Through PS ₄ ³⁻ Rotation Suppression by Al and Se Co‐Substitution for All‐Solid‐State Batteries

Abstract Sulfide‐based solid electrolytes, particularly Li argyrodites, hold significant promise for practical all‐solid‐state batteries (ASSBs); however, their poor stability under humid conditions presents a critical challenge. Despite numerous efforts to address this issue, a comprehensive mechanistic understanding of moisture‐induced degradation remains limited. Herein, we introduce an Al and Se co‐substituted argyrodite, Li _{6‐3 x} Al _x PS _{5‐1.5 x} Se _{1.5 x} Cl, which enhances both the Li ⁺ conductivity and air stability. The optimized composition ( x = 0.05) exhibits an improved Li ⁺ conductivity of 4.91 mS cm ⁻¹ at 30 °C and a 22% conductivity reduction after dry‐air exposure (dew point: −40 °C for 5 h), compared with 3.71 mS cm ⁻¹ and a 42% decrease for the unsubstituted sample. Reduced surface degradation is validated by comprehensive experimental analyses. Complementary calculations indicate less favorable H ₂ O adsorption and further reveal that Al and Se co‐substitution inhibits the rotation of P[S ₂ SeO] ³⁻ and P[S ₂ O ₂ ] ^3−; tetrahedra via preferential surface‐oriented Se ²⁻ and Al─O interactions, which otherwise promote H ₂ O‐induced degradation, thereby minimizing moisture interactions. Finally, the improved electrochemical performance of the co‐substituted argyrodite is validated by its enhanced capacity retention following air exposure in NCM|Li ₆ PS ₅ Cl|(Li‐In) cells. This study highlights rotational dynamics as an overlooked mechanism underlying moisture‐induced degradation, and demonstrates that targeted co‐substitution is a viable strategy for advancing practical ASSBs.