Sulfide-based solid electrolytes (SEs) are promising enablers of next-generation solid-state batteries (SSBs), yet their practical implementation is limited by interfacial instability with anode and cathode active materials. Current interfacial mitigation strategies rely primarily on inorganic coatings, which are often unable to accommodate chemo-mechanical strain. Here, we introduce a new dynamic ion gel nanocoating that stabilizes both cathode and anode interfaces in sulfide-based SSBs. It is a type of ion-conductive polyelectrolyte complex (PEC), which is formed via complex coacervation between oppositely charged polyelectrolytes. Controlled mixing of a polycation bearing ammonium groups with TFSI- counteranions and a polyanion featuring pendant TFSI- groups with Li+ countercations yields a homogeneous, solution-processable dynamic ion gel accompanied by LiTFSI release. This enables uniform, thin (≈1-3 nm), and scalable PEC nanocoatings on active material particles via spray drying. Dynamic ionic cross-linking endows the PEC with enhanced viscoelasticity and improved ionic conductivity relative to the parent polycation, allowing it to adapt to solid-solid interfaces while maintaining efficient ion transport. SSBs employing PEC-coated silicon anodes and PEC-coated LiNiO2 cathodes exhibit improved cycling stability through suppression of interfacial side reactions. Thus, the PEC nanocoating acts as a scalable stabilizer of solid-solid interfaces in SSBs.
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