Fluoride‐rich Sulfide Solid Electrolyte With Ultrahigh Air Stability for All‐Solid‐State Batteries

Argyrodites are among the most promising sulfide-based solid electrolytes (SEs) due to their high ionic conductivity and ductility. However, the poor atmospheric stability of sulfide-based SEs caused by the side reaction with moisture, which generates toxic H₂S gas and degrades its high ionic conductivity and low electronic conductivity, has limited the commercialization of ASSBs. Herein, the preparation of sulfide-based SEs consisting of a Li₆PS₅Cl (LPSCl) core and fluoride-rich LPSCl shell are described using a facial fluorine treatment following an annealing process to improve atmospheric stability. Most importantly, with the benefit of a fluoride-rich LPSCl shell, the prepared SE exhibits well-kept low electronic conductivity even after exposure to an atmosphere with 20% relative humidity at 25 °C, resulting in an improvement in electrochemical properties without short-circuiting. These results indicate that the fluoride-rich LPSCl shell effectively suppresses side reactions with moisture and mitigates the extent of irreversible side reactions. These experimental results and associated first-principles density functional theory (DFT) models profoundly understand the enhanced air stability of F-LPSCl. Full cells employing an air-exposed F-LPSCl exhibit an enhanced initial discharge capacity of 168.5 mAh g^-1 at 0.05 C, cycling stability at 0.3 C over 500 cycles, and rate capability.