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.