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

Abstract

• Bi 0.8 Ba 0.2 F 2.8 is prepared as a new fluoride shuttle battery electrode material. • Bi 0.8 Ba 0.2 F 2.8 decomposes into metallic Bi and BaF 2 by defluorination. • Conversion reaction between Bi and BiF 3 occurs in the subsequent cycles. • Cycle performance may be improved by suppression of coarsening of BiF 3 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 (BiF 3 ) 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 BaF 2 -doped BiF 3 , Bi 0.8 Ba 0.2 F 2.8 , positive electrode was investigated using synchrotron-radiation X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy. The Bi 0.8 Ba 0.2 F 2.8 electrode showed a higher reversible capacity in the first cycle and improved capacity retention compared to the BiF 3 electrode. The pristine Bi 0.8 Ba 0.2 F 2.8 showed a tysonite-type structure, and metallic Bi and BaF 2 nanoparticles were observed in the fully defluorinated state. Moreover, we found that the (re-)fluorinated material consisted of BiF 3 and BaF 2 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 Bi 0.8 Ba 0.2 F 2.8 electrode may be improved due to the suppression of the coarsening of BiF 3 nanoparticles by the adhesion of BaF 2 nanoparticles formed after initial defluorination.