Atomic insights into the oxidative degradation mechanisms of sulfide solid electrolytes

Electrochemical degradation of solid electrolytes is a major roadblock in the development of solid-state batteries. Combining X-ray absorption spectroscopy characterization, first-principles simulations, and machine learning, here we report the atomic-scale oxidative degradation mechanisms of sulfide electrolytes using Li3PS4 (LPS) as a model system. The degradation begins with a decrease of Li neighbor affinity to S atoms, followed by the formation of S-S bonds as the PS4 tetrahedron deforms. After the first cycle, the PS4 motifs become strongly distorted, and PS3 motifs start to form. The distortion of PS4 and the formation of S-S bonds are correlated with an increased interfacial impedance. We identify the spectral fingerprints of the local structural evolution and use them as a proxy for the electrochemical stability of phosphorus sulfide electrolytes, as demonstrated in argyrodite Li6PS5Cl. This study provides guidance for controlling macroscopic reactions through microstructural engineering and can advance the rational design of sulfide electrolytes.