Unlocking the Interfacial Activity of Insoluble Lithium Salts for Stable Lithium Batteries

ABSTRACT Employing functional lithium salts in electrolytes is an effective strategy to suppress anode instabilities and sluggish charge‐transfer kinetics in lithium batteries. However, high solubility requirements intrinsically exclude a broad class of promising, high‐lattice‐energy inorganic salts for carbonate electrolytes. Here, we establish an immobilized functional platform (IFP) by deploying insoluble lithium salts with strong Lewis‐acidic Li + sites directly onto an aramid‐scaffolded separator. This design bypasses conventional dissolution–diffusion limitations, yielding a thin, ionically conductive, LiF‐rich solid electrolyte interphase (SEI) driven by regulated interfacial Li + solvation and enhanced anion‐derived decomposition. Systematic screening and 6 Li/ 7 Li isotope tracing identify Li 2 SO 4 (LS) as the optimal IFP candidate (IFP‐LS) owing to its moderate Lewis acidity, superior solvent affinity, and industrial viability. Remarkably, LS provides a finite sacrificial lithium contribution during continuous SEI evolution, mitigating irreversible lithium consumption. Consequently, IFP‐LS separators quadruple the lifespan of Li||LiNi 0.9 Co 0.05 Mn 0.05 O 2 cells at 0.7 C compared to bare separators, and deliver 88.4% capacity retention over 2400 cycles in Li||LiFePO 4 cells at 5 C. Scalability is further validated through 100 m roll‐to‐roll manufacturing across diverse Ah‐level formats, adding only 0.163 USD m −2 . This work overcomes the lithium salt solubility barrier and unlocks a vast library of potent materials for next‐generation high‐energy‐density batteries.