Evolution of fluoride shuttle battery reactions and three-dimensional morphology changes of BiF3 microparticles in an ethylene carbonate-based liquid electrolyte

Abstract

Fluoride shuttle batteries (FSBs) use defluorination of metal fluorides and fluorination of metals, and they are considered as candidates of next-generation batteries with high energy densities. During FSB reactions of orthorhombic and cubic BiF₃ (o-BiF₃ and c-BiF₃, respectively) in an ethylene carbonate-based liquid electrolyte, in situ Raman mapping and in situ laser scanning confocal microscopy (LSCM) for three-dimensional analysis were conducted almost simultaneously. As the potential of o-BiF₃vs. Pb (E_WE) was decreased to 0.4 V, desorption of F⁻ started at the protrusions of o-BiF₃ particles. After defluorination, E_WE was increased to 0.6 V, and c-BiF₃ appeared at protrusions of the surfaces of Bi. However, at the surfaces where o-BiF₃ partially remained, o-BiF₃ grew rather than c-BiF₃. The apparent volumes of particles increased during defluorination and fluorination. The results are important for determining reaction mechanisms, and the results indicate the possibility of the use of ethylene carbonate-based liquid electrolytes.