Sulfide solid electrolytes (SEs) with high Li-ion conductivities (σion) and soft mechanical properties have limited applications in wet casting processes for commercial all-solid-state batteries (ASSBs) because of their inherent atmospheric and chemical instabilities. In this study, we fabricated sulfide SEs with a novel core-shell structure via environmental mechanical alloying, while providing sufficient control of the partial pressure of oxygen. This powder possesses notable atmospheric stability and chemical resistance because it is covered with a stable oxysulfide nanolayer that prevents deterioration of the bulk region. The core-shell SEs showed a σion of more than 2.50 mS cm-1 after air exposure (for 30 min) and reaction with slurry chemicals (mixing and drying for 31 min), which was approximately 82.8% of the initial σion. The ASSB cell fabricated through wet casting provided an initial discharge capacity of 125.6 mAh g-1. The core-shell SEs thus exhibited improved powder stability and reliability in the presence of chemicals used in various wet casting processes for commercial ASSBs.