Reactivity and Mechanisms in Fluoride Shuttle Battery Reactions: Difference between Orthorhombic and Cubic BiF₃ Single Microparticles

BiF3 is a strong candidate for a cathode material in fluoride shuttle batteries (FSBs), which utilize defluorination of metal fluorides and fluorination of metals. Much interest has been shown in FSBs as next-generation batteries with high energy densities. Elucidation and control of reactivity and mechanisms in FSB reactions are crucial for the development of FSBs with high performances. In the present work, structural transformation, reactivities, and mechanisms in FSB reactions of BiF3 microparticles partly embedded in a gold plating film (BiF3/gold, mainly orthorhombic BiF3 (o-BiF3) containing small amounts of cubic BiF3 (c-BiF3)) were studied by in-situ Raman microscopy. Reactivities and mechanisms in defluorination were found to be different for o-BiF3 and c-BiF3. In an ionic-liquid-based electrolyte, gradual transformation of o-BiF3 into cubic c-BiF3 occurred. When the voltage of BiF3/gold (cathode) vs a Pb counter electrode (anode) was decreased from OCV (0.7 V) to 0.05 V step by step, direct defluorination of the surfaces of only o-BiF3 started from their contours at 0.45 V and then extended to their center parts and was mostly completed at 0.2 V. Then defluorination of c-BiF3 started at a voltage below 0.2 V by both direct defluorination and dissolution–deposition mechanisms. In the former mechanism, the nucleus of Bi first appeared near the edge of c-BiF3 microparticles, and the nucleus grew for defluorination to proceed over the whole surface. The rate of direct defluorination of c-BiF3 was much slower than that of o-BiF3. Defluorination of c-BiF3 by the dissolution–deposition mechanism was dominant and fast when the excitation beam was strong, probably due to a thermal effect. Such an observation is important for the development of electrodes and electrolytes with proper solubility to better utilize reactions by the two mechanisms to realize FSBs with high performance.