Electrolytes for Room-Temperature Rechargeable Fluoride Shuttle Batteries

An electrolyte capable of operating at room temperature has been developed for fluoride shuttle batteries, which are regarded as a promising type of high energy density batteries. This electrolyte uses a reasonably priced and widely available potassium salt called potassium bifluoride (KHF2) as the source of fluoride ions, propylene carbonate, which has a high dielectric constant, as the solvent, and the 18-crown-6 ether, which is capable of dissolving potassium salts, as an additive. The ionic conductivity of this electrolyte reaches 1.72 mS cm–1, which is sufficient to enable the operation of fluoride shuttle batteries. The charge/discharge test results using the bismuth fluoride (BiF3) electrode showed a discharge capacity of 263 mAh g–1 and a charge capacity of 184 mAh g–1. By the measurement of nuclear magnetic resonance (NMR), the state of the fluoride ions in the electrolyte is determined to be [(FH)F]−. It is considered that hydrogen fluoride (HF) generated by the equilibration reactions of [(FH)F]− dissolves metal fluorides that act as the active material of the electrodes, creating a mechanism that enhances the redox reaction of the metal fluorides.