Surface Coating Enabling Sulfide Solid Electrolytes with Excellent Air Stability and Lithium Compatibility

All-solid-state lithium metal batteries (ASSLMBs) featuring sulfide solid electrolytes (SEs) are recognized as the most promising next-generation energy storage technology because of their exceptional safety and much-improved energy density. However, lithium dendrite growth in sulfide SEs and their poor air stability have posed significant obstacles to the advancement of sulfide-based ASSLMBs. Here, a thin layer (approximately 5 nm) of g-C₃N₄ is coated on the surface of a sulfide SE (Li₆PS₅Cl), which not only lowers the electronic conductivity of Li₆PS₅Cl but also achieves remarkable interface stability by facilitating the in situ formation of ion-conductive Li₃N at the Li/Li₆PS₅Cl interface. Additionally, the g-C₃N₄ coating on the surface can substantially reduce the formation of H₂S when Li₆PS₅Cl is exposed to humid air. As a result, Li–Li symmetrical cells using g-C₃N₄-coated Li₆PS₅Cl stably cycle for 1000 h with a current density of 0.2 mA cm⁻². ASSLMBs paired with LiNbO₃-coated LiNi_0.6Mn_0.2Co_0.2O₂ exhibit a capacity of 132.8 mAh g⁻¹ at 0.1 C and a high-capacity retention of 99.1% after 200 cycles. Furthermore, g-C₃N₄-coated Li₆PS₅Cl effectively mitigates the self-discharge behavior observed in ASSLMBs. This surface-coating approach for sulfide solid electrolytes opens the door to the practical implementation of sulfide-based ASSLMBs.