Influence of Electrolyte Composition on the Electrochemical Reaction Mechanism of Bismuth Fluoride Electrode in Fluoride Shuttle Battery

Fluoride shuttle battery (FSB) is a promising next-generation battery candidate. In the FSB, metal fluoride and organic solvent containing supporting electrolyte salt and anion acceptor were used as active material and electrolyte. In this study, using bis[2-(2-methoxyethoxy)ethyl] ether (tetraglyme: G4) containing cesium fluoride (CsF; 0.45 mol dm–3 or saturated) and triphenylboroxine (TPhBX; 0.50 mol dm–3) as electrolyte (CsF(0.45)–TPhBX(0.50)–G4 and CsF(sat.)–TPhBX(0.50)–G4), the electrochemical performance of bismuth fluoride (BiF3) was assessed. Although the discharge and charge reactions of BiF3 electrode proceeded in both electrolytes, the cycling performance of BiF3 electrode in CsF(0.45)–TPhBX(0.50)–G4 was poorer than that in CsF(sat.)–TPhBX(0.50)–G4. The cause of differences in the electrochemical properties was investigated using atomic absorption spectrometry (AAS), X-ray photoelectron spectroscopy (XPS), and cross-sectional scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDX). The AAS results indicate that the poor cycling performance with CsF(0.45)–TPhBX(0.50)–G4 was due to the dissolution of active material during charging. The XPS and cross-sectional SEM/EDX results indicate that the formation state of Bi, and the progress of electrolyte decomposition during discharging were affected by the CsF/TPhBX ratio in the electrolyte.