Improvement of cycling performance in bismuth fluoride electrodes by controlling electrolyte composition in fluoride shuttle batteries

We have developed a fluoride shuttle battery (FSB) which is a promising candidate for the next-generation high-energy-density secondary batteries. Using the bis [2-(2-methoxyethoxy) ethyl] ether (tetraglyme: G4) solvent containing 0.45 mol dm −3 cesium fluoride (CsF) and 0.5 mol dm −3 fluorobis (2,4,6-trimethylphenyl) borane (FBTMPhB) as an electrolyte for FSB, we have successfully conducted the discharge (BiF 3 + 3e − → Bi + 3F − ) and charge (Bi + 3F − → BiF 3 + 3e − ) reactions for a BiF 3 electrode; however, the discharge and charge capacities significantly decreased during cycling. Atomic absorption spectrometry results indicated that, in addition to the formation of BiF 3 , dissolution of Bi (Bi → Bi 3+ + 3e − ) occurred during the charge process. The dissolution of Bi indicated that the active material was lost from the electrode, which decreased the capacity during cycling. An increased CsF/FBTMPhB ratio in the electrolyte was found to suppress the dissolution of Bi during the charge process and, therefore, improve the cycling performance. Graphical Abstract