Design and synthesis of asymmetric anhydrous quaternary ammonium fluoride electrolytes for fluoride ion batteries†

Fluoride ion batteries using quaternary ammonium fluoride-based liquid electrolytes have recently demonstrated promising performance. While liquid electrolytes are desirable for their high ionic conductivity at room temperature, finding solvent-fluoride salt systems with both high solubility and stability has been challenging. In this study, we synthesize novel asymmetric anhydrous quaternary ammonium fluoride salts with varying structural bulkiness to reveal how structural variations influence chemical properties such as solubility, ionic conductivity, and stability. The new fluoride salt electrolytes were successfully employed in a half-cell with BiF₃ electrodes, achieving 80% of the 302 mA h g⁻¹ theoretical capacity. We find that increasing the size of the cation led to higher conductivities for fluoride salts in bis(2,2,2-trifluoroethyl) ether solution. However, we also observe a loss of thermostability with increasing cation size. Optimizing this trade-off between ionic conductivity and thermostability will likely be critical for the future development of quaternary ammonium fluoride electrolytes.