Propylene Carbonate-Based Liquid Electrolyte with Enhanced Chemical Stability for Fluoride Ion Batteries

Fluoride ion batteries (FIBs) using liquid electrolytes containing quaternary ammonium fluoride salts have recently shown promising room-temperature performance. However, the most commonly used aprotic solvent, bis(2,2,2-trifluoroethyl) ether (BTFE), has a low boiling point (63 °C), making it unsuitable for practical FIB applications. In this study, we substituted BTFE with propylene carbonate (PC), which has a boiling point of 242 °C. We found that the anhydrous quaternary ammonium fluoride, N, N -dimethyl- N, N -dineopentylammonium fluoride (Np 2 F), was soluble in propylene carbonate at concentrations up to 1.6 M. At 0.9 M, this Np 2 F/PC electrolyte had an ionic conductivity of 4.7 mS/cm, accompanied by a high fluoride transport number of 0.69 and an exceptionally high electrochemical stability window of 4.2 V. Interestingly, this wide stability was achieved despite the dominant charge carrier being found to be bifluoride, HF 2 − . The Np 2 F/PC electrolyte supported reversible fluorination/defluorination in a test cell with BiF 3 electrodes at 0.1 C.