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

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_xPS_{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 H₂O adsorption and further reveal that Al and Se co-substitution inhibits the rotation of P[S₂SeO]^3- and P[S₂O₂]^3-; tetrahedra via preferential surface-oriented Se^2- and Al─O interactions, which otherwise promote H₂O-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|Li₆PS₅Cl|(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.