Imidazolium-Based Ionic Liquid Electrolytes for Fluoride Ion Batteries

Abstract

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