Salt‐Free Solid Polymer Electrolytes Enabling Inorganic‐Rich Solid‐Electrolyte Interphase for Stable and Cost‐Effective Li‐Metal Batteries

Abstract Constructing robust solid‐electrolyte interphase (SEI) on electrodes is crucial for achieving stable lithium metal batteries in liquid electrolytes. However, intrinsic issues associated with liquid electrolytes remain unavoidable, such as the continuous corrosion of SEI and the high costs involved. Herein, a novel lithium salt‐free solid‐state polymer electrolyte (SPE) is introduced enabled by in situ polymerization of 1,3‐dioxolane and 1,3,5‐trioxane with the addition of ionic liquid to eliminate the drawbacks of liquid electrolyte. The homogeneous interaction between the polymer and ionic liquid constructs a synergistic Li⁺ conduction pathway, promoting the extraction of Li⁺ out of the cathode/anode and the smooth transport throughout the polymer network to endow a higher Li⁺ transference number (0.63) and a comparable ionic conductivity (1.21 mS cm⁻¹) to the conventional liquid electrolyte (0.45 and 5.51 mS cm⁻¹). The absence of lithium salt prevents the oxidative decomposition of lithium salts in electrolytes to generate hazardous and corrosive acidic species. More intriguingly, an anion‐dominated solvation configuration can be realized by the incorporation of ionic liquid with the polymer matrix, inducing the formation of an inorganic‐rich and anti‐corrosive SEI on the electrodes. The resulting salt‐free SPE enables superior cycling stability in lithium||LiFePO₄ battery with a high capacity retention of over 92% after 780 cycles.