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

Abstract

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 LiNi0.6Mn0.2Co0.2O2 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.