Lewis Acid–Base Synergistically Enhancing Practical Composite Electrolyte for Fluoride‐ion Batteries at Room Temperature

Abstract Fluoride‐ion batteries (FIBs) represent a potential “next‐generation” electrochemical storage device, offering high energy density. However, the practical implementation of FIBs at room temperature is impeded by the limitations of currently available ceramic electrolytes. Here, a composite NH ₄ HF ₂ @PEO@β‐PbSnF ₄ electrolyte with both high conductivity of 10 ⁻⁴ S cm ⁻¹ and wide electrochemical stability window (4.59 V vs Pb/PbF ₂ ) at room temperature is fabricated. Field emission transmission electron microscope (FETEM) demonstrates the presence of a space charge region, which enhances the conductivity. Furthermore, ¹⁹ F NMR and density functional theory (DFT) calculations elucidate that the interaction between Sn ²⁺ (Lewis acid) and HF ₂ ⁻ (Lewis base) induces significant modifications to the electronic structure, which critically contribute to the enhanced electrochemical stability window of the composite electrolyte. Integrating this promising electrolyte with high‐voltage CuF ₂ cathodes and Pb/PbF ₂ anodes, a reversible coin cell with a discharge capacity of 143 mAh g ⁻¹ up to 50 cycles is demonstrated. The rational design of such composite electrolytes offers a pathway toward the practical application of FIBs at room temperature.