Defluorination/Fluorination Reaction of BiF_3 Nanoparticles Prepared Via Sol-Gel Method for Fluoride Shuttle Battery

摘要

Introduction Rechargeable batteries with high energy density have been required for recent applications such as hybrid electric vehicles and large-scale energy storage. Rechargeable batteries based on fluoride shuttle (FSBs) have attracted much attention as a candidate above, owing to their high theoretical energy density by selecting appropriate metal/metal fluoride couples as active materials of negative- and positive-electrode[l-3]. One of the important things of the battery is adoption of the good conductive media for fluoride species, that is, an electrolyte. Another is reversible fluorination/defluorination of active materials at the interface with electrolyte. Previously, we have demonstrated archetypal reversible FSB using a liquid electrolyte containing organic fluoride at room temperature. However, this battery needed to improve the utilization and cyclability of its active material. Since the common metal fluoride shows low electronic- and ionic-conductivity, the active materials should be minified for FSBs. In this study, metal fluoride nanoparticle, e.g. BiF_3, was prepared via sol-gel method using a trifluoroacetate precursor for improvement of the charge/discharge reactions in FSB. Bismuth trifluoride having 302 mAh/g in theoretical capacity is considered as a candidate for positive active material[l-6]. Then, we studied reversible defluorination/fluorination reaction of thus prepared BiF_3 nanoparticles in a liquid electrolyte of FSB at room temperature, and valence change of bismuth species in the consequent electrode was evaluated using synchrotron ex-situ XANES measurement.