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

Ni-rich cathode materials (i.e., LiNixCoyMn1–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.