Trace ionic liquid enables homogeneous electrochemical reactions for stable interfaces and durable cycling in...

Sulfide-based quasi-solid-state batteries (QSSBs) are severely limited by heterogeneous electrochemical reactions and interfacial chemo-mechanical instability within composite cathodes, which lead to contact degradation, sluggish reaction kinetics, and rapid capacity decay during cycling. Herein, a trace amount of an imidazolium-based ionic liquid (IL) additive, composed of 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][TFSI]) (denoted as MI14), is introduced as a multifunctional interfacial modifier to regulate both cathode microstructure and interfacial chemical evolution. Owing to its superior wetting capability, the trace IL redistributes along interparticle boundaries during composite cathode fabrication, thereby mitigating localized strain accumulation and mechanical deformation. Consequently, interparticle contact continuity is improved, promoting a more uniform ionic-flux distribution within the electrode architecture. Meanwhile, localized electrochemical decomposition of the IL induces the in situ formation of a robust LiF-rich cathode-electrolyte interphase (CEI), which suppresses parasitic interfacial reactions and stabilizes interfacial Li⁺ transport. Multiscale characterizations combined with finite-element simulations reveal that the synergistic structural and chemical regulation sustains continuous ionic percolation pathways and preserves interfacial integrity throughout prolonged cycling. Consequently, Li-In|Li₆PS₅Cl|LiNi_0.8Co_0.1Mn_0.1O₂ cells achieve 80% capacity retention after 1750 cycles at 1C and deliver a high discharge capacity of 92 mAh g^-1 at 5C under 25 MPa. This work demonstrates an effective trace-additive strategy for mitigating coupled chemo-mechanical degradation in sulfide-based QSSBs.