Adjusting Chemical Hardness–Softness Balance of Electrolyte to Enable High‐Voltage Reversible Fluoride Ion Batteries

Abstract Fluoride ion batteries (FIBs), as a promising next‐generation high‐energy‐density storage technology, have attracted significant attention. However, developing an ideal fluoride‐ion electrolyte that suppresses the β‐H abstraction (caused by strong Lewis‐basicity F − ) and electrolyte decomposition remains challenging. To address this bottleneck, we design an electrolyte system based on commercial tetrabutylammonium fluoride (TBAF) salt and 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMImBF 4 ) ionic liquid solvent through anion–cation coordination engineering and hard–soft‐acid–base (HSAB) balance modulation, unveiling its multiscale mechanisms for mitigating interfacial parasitic reaction and enhancing metal anode stability. Experimental and theoretical analyses reveal that the soft‐acid BMIm⁺ participates in the solvation structure of hard‐base fluoride ions, effectively blocking the β‐H elimination pathway and expanding the electrochemical window to 4.5 V. The ionic conductivity of this ionic liquid based electrolyte reaches 5.0 × 10 −3 S cm −1 at 60 °C even after in situ polymerization. The Cu 2 O cathode coupling insertion and conversion reactions can alleviate the volume deformation and capacity decay of Cu 2 O||Li–LiF high‐voltage FIBs, with a high resting voltage (2.91 V) and a high initial capacity of 589.9 mAh g −1 . The Cu 2 O||Pb–PbF 2 FIBs maintain a high reversible capacity of 243.6 mAh g −1 even after 800 cycles under 200 mA g −1 . The work establishes a novel electrolyte design paradigm for high‐voltage reversible FIBs.