Imidazolium-Based\nIonic Liquid Electrolytes for Fluoride\nIon Batteries

The fluoride-ion battery (FIB) is a post-lithium anionic\nbattery\nthat utilizes the fluoride-ion shuttle, achieving high theoretical\nenergy densities of up to 1393 Wh L<sup>–1</sup> without relying\non critical minerals. However, developing liquid electrolytes for\nFIBs has proven arduous due to the low solubility of fluoride salts\nand the chemical reactivity of the fluoride ion. By introducing a\nchemically stable electrolyte based on 1,3-dimethylimidazolium [MMIm]\nbis(trifluoromethanesulfonyl)imide [TFSI] and tetramethylammonium\nfluoride (TMAF), we achieve an electrochemical stability window (ESW)\nof 4.65 V, ionic conductivity of 9.53 mS cm<sup><i>–</i>1</sup>, and a solubility of 0.67 m. The origin of this high solubility\nand the solvation structure were investigated using NMR spectroscopy\nand neutron total scattering, showing a fluoride solvation driven\nby strong electrostatic interactions and weak hydrogen bonding without\ncovalent H–F character. This indicates the chemical stability\nof 1,3-dimethylimidazolium toward the fluoride ion and its potential\nas an electrolyte for high-voltage FIBs.