Molecular Surface Engineering of Sulfide Electrolytes with Enhanced Humidity Tolerance for Robust Lithium Metal All‐Solid‐State Batteries

Solid‐state electrolytes (SSEs) enable next‐generation batteries due to their intrinsic safety and compatibility with lithium (Li) metal anodes. However, many SSEs, particularly sulfide‐based systems, suffer from limited electrochemical stability and high moisture sensitivity. Here, the molecular surface engineering of Li argyrodite SSE, Li 6 PS 5 Cl 0.5 Br 0.5 (LPSClBr), is reported using octadecyl phosphonic acid (OPA) and its lithiated form (Li‐OPA) in a single‐step coating strategy to stabilize both anode and cathode interfaces. The Li‐OPA‐coated electrolyte maintains high ionic conductivity (>2.5 mS cm −1 ) and retains >92% of its initial conductivity after 24 h dry room exposure (dew point −50 °C). At 2 wt.% loading, Li‐OPA‐coated LPSClBr achieves a critical current density of 2.4 mA cm −1 and supports stable Li plating/stripping for over 400 h at 1.0 mAh cm −2 . In NCM811 cathode‐based all‐solid‐state cells, it delivers 160 mAh g −1 at 0.3 C with >99.7% Coulombic efficiency and 85% capacity retention after 100 cycles. In anode‐free cell configurations, Li‐OPA‐modified electrolytes enhance interfacial stability and cycling performance. These results demonstrate Li‐OPA as a scalable, high‐performance interfacial modifier for sulfide‐based solid‐state batteries.