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

•Bi0.8Ba0.2F2.8 is prepared as a new fluoride shuttle battery electrode material.•Bi0.8Ba0.2F2.8 decomposes into metallic Bi and BaF2 by defluorination.•Conversion reaction between Bi and BiF3 occurs in the subsequent cycles.•Cycle performance may be improved by suppression of coarsening of BiF3 particles. 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.