Waxing Bare High-Voltage Cathode Surfaces to Enable Sulfide Solid-State Batteries

Ni-rich cathode materials (i.e., LiNi_xCo_yMn_1–x–yO₂, x ≥ 0.8) coupled with sulfide solid catholytes such as argyrodite (Li₆PS₅Cl) are promising materials for high-performance solid-state batteries. However, the chemical reactivity and electrochemical decomposition of argyrodite above 2.5 V cause fast capacity fading in the operating window of the cell, necessitating the use of coatings to suppress those reactions. Here, we report a simple organic-coating concept that generates an electronically insulating cathode–electrolyte interphase which stabilizes the argyrodite against oxidative degradation. Utilizing Li₆PS₅Cl as the solid electrolyte, the nanometric layer of “waxy soap” (fatty acid salt) coated on the cathode active material (CAM) enables a capacity retention of 90% after 200 cycles (2.8–4.3 V vs Li⁺/Li), compared to 38% retention for the bare CAM, confirming both its interfacial stability and permeability to Li ions. This emphasizes the suitability and potential of this low-cost, easily processable, flexible, and conformal coating to suppress interfacial degradation in sulfide-based composite cathodes.