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 has a high dielectric constant, as the solvent, and the 18-crown-6 ether, which K 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.