Molecular Surface Engineering of Sulfide Electrolytes with Enhanced Humidity Tolerance for Robust Lithium Metal...

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, Li₆PS₅Cl_0.5Br_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.