Defluorination/fluorination mechanism of Bi₀.₈Ba₀.₂F₂.₈ as a fluoride shuttle battery positive electrode

Fluoride shuttle batteries (FSBs), which utilize F– ion migration in electrochemical reactions, have recently advanced in academic research as next-generation rechargeable batteries. Bismuth trifluoride (BiF₃) and its relatives are expected to be promising positive electrode materials for FSBs because of their high theoretical capacity. Herein, the defluorination/fluorination reaction of a BaF₂-doped BiF₃, Bi₀.₈Ba₀.₂F₂.₈, positive electrode was investigated using synchrotron-radiation X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy. The Bi₀.₈Ba₀.₂F₂.₈ electrode showed a higher reversible capacity in the first cycle and improved capacity retention compared to the BiF₃ electrode. The pristine Bi₀.₈Ba₀.₂F₂.₈ showed a tysonite-type structure, and metallic Bi and BaF₂ nanoparticles were observed in the fully defluorinated state. Moreover, we found that the (re-)fluorinated material consisted of BiF₃ and BaF₂ nanoparticles, indicating that bismuth is the only redox-active element, and that the tysonite structure is not recovered after the initial discharging. This suggests that the cycle performance of the Bi₀.₈Ba₀.₂F₂.₈ electrode may be improved due to the suppression of the coarsening of BiF₃ nanoparticles by the adhesion of BaF₂ nanoparticles formed after initial defluorination