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.