Evolution of Fluoride Shuttle Battery Reactions of BiF₃ Microparticles in a CsF/LiBOB/Tetraglyme Electrolyte: Dependence on Structure, Size, and Shape

Fluoride shuttle batteries (FSBs) are possible candidates of next-generation batteries with high energy densities that operate by defluorination of metal fluorides and fluorination of metals. Understanding and control of FSB reactions are important for the development of FSBs with high performance. In the present work, evolution of FSB reactions of orthorhombic and cubic BiF3 (o-BiF3 and c-BiF3, respectively) microparticles partly embedded in a gold plating film (BiF3/gold) in a CsF/LiBOB (lithium bis(oxalato)borate)/tetraglyme electrolyte was studied by in situ Raman microscopy. Three-dimensional morphology of embedded BiF3 microparticles was also studied by laser scanning confocal microscopy (LSCM) to elucidate the detailed shape dependence of defluorination. Evolution of FSB reactions was found to be different from that in an ionic-liquid electrolyte reported previously. When the voltage of BiF3/gold vs a Pb counter electrode was decreased from OCV (about 0.5 V), direct defluorination of o-BiF3 and c-BiF3 started at 0.25 and 0.05 V, respectively, which are lower than those in the ionic-liquid electrolyte. The direct defluorination mechanism was always predominant at all CsF concentrations studied from saturation to zero. The sizes of most of the BiF3 microparticles that were defluorinated were less than 3 μm. However, partial defluorination of large (>10 μm) BiF3 (insulator) microparticles often occurred at protruded positions far from the contours (i.e., far from the gold current collector). The results suggest that defluorination occurred at positions with small radii of curvatures, probably due to stronger electric fields. On the other hand, defluorination of o-BiF3 particles with sizes of less than 3 μm started from their contours and proceeded toward their central positions. The observed phenomena have implications for controlling the rate-determining steps in FSB reactions that depend on electrolytes and the sizes and shapes of particles and for designing electrodes and electrolytes for superior FSBs.