The crucial role of oxygen substitution in argyrodite solid electrolytes from the bulk to the surface under atmospheric conditions

Abstract

Argyrodite-type sulfide-based solid electrolytes (SEs) have drawn immense interest because of their high ionic conductivity. In contrast, the poor structural and electrochemical stabilities of argyrodite-type SEs have recently emerged as a major issue. Based on the combined method of first principles calculations and electrochemical experiments, here, we present Li₆PO_xS_5−xBr_0.5Cl_0.5 (x = 0, 0.5) SEs and the oxygen (O) substitution mechanism from the surface to the bulk to improve the stability of sulfide-based Li argyrodite SEs, whose surfaces are vulnerable to air exposure. Using first principles calculations, we analyzed the mechanism of the introduction of O substituted for sulfur (S) depending on the different S sites and confirmed the improvement in the stability by calculating the atomic and electronic structures according to the O content for the bulk and the surface models. In particular, we confirmed that variations in the electronic structures of S on the surface and the changed electrochemical environment could facilitate the side reactions, which were suppressed by O introduction. In addition, these designed models were synthesized to verify the calculation results. Electrochemical experiments revealed the structural decomposition and O penetration depending on the depth from the surface to the bulk are suppressed by O introduction. Moreover, introducing O into Li₆PO_0.5S_4.5Br_0.5Cl_0.5 maintained the ionic conductivity sufficiently, and the conductivity retention after air exposure for a day was notably enhanced compared with that of the pristine Li₆PS₅Br_0.5Cl_0.5. These improved structural and electrochemical stabilities achieved by introducing O can enhance the electrochemical performances. This study provides a rational strategy for developing promising oxysulfide-based SEs for all-solid-state batteries.