Modification of Composite Cathode for Sulfide-Based All-Solid-State Batteries

All-solid-state batteries (ASSBs) based on sulfide electrolytes hold great promise for next-generation energy storage, yet their performance is critically constrained by unstable cathode-electrolyte interfaces. Here, we report a dual-modification strategy utilizing ionic liquids (ILs) in combination with lithium salts to simultaneously improve interfacial wettability, ionic transport, and electrochemical stability in NCM811 composite cathodes. Three ILs (EMIMTFSI, Pyr₁₄FSI, and PP₁₃FSI) and three lithium salts (LiTFSI, LiDFOB, and LiBOB) were systematically evaluated and screened. While neat ILs improved initial capacities by reducing solid-solid contact resistance, they also triggered parasitic reactions with sulfides, resulting in capacity fading. Among the lithium salts, LiBOB was identified as the most chemically compatible additive, forming thin and uniform hybrid interphases enriched with B-O species. This interphase effectively suppressed high-voltage side reactions and reduced electrode polarization. Strikingly, the synergistic combination of PP₁₃FSI and 1 wt% LiBOB transformed discontinuous point contacts into continuous ionic pathways, yielding a discharge capacity of 165.9 mAh g^-1 and maintaining excellent stability over 100 cycles at 0.1C. This work highlights a rational IL-Li salt pairing strategy that not only overcomes intrinsic limitations of sulfide-based composite cathodes but also provides a generalizable route to interfacial design in ASSBs. By integrating molecular-level ion transport regulation with interphase stabilization, our approach offers practical guidance toward realizing high-energy-density, long-cycle-life solid-state batteries.