Fluorination/Defluorination Behavior of Y₂C in Fluoride-Ion Battery Anodes

Despite the high theoretical energy density of fluoride-ion batteries (FIBs), their practical applications are hindered by the large volume changes associated with the redox reactions (typically metal <-> metal fluoride interconversions) of most of the corresponding anode materials. Consequently, FIB anode materials that react at low potentials with small expansion and shrinkage are desired. Inspired by the low theoretical volume change (8%) of the Y2C <-> Y2CF2 interconversion, we herein evaluated Y2C as an FIB anode material and determined its initial discharge and charge capacities as 565 and 432 mAh g(-1), respectively. The first fluorination was characterized by a capacity plateau equivalent to a two-electron reaction at -2 V vs Pb/PbF2. The first and second halves of this region corresponded to the Y2C -> Y2CF2 intercalation reaction and Y2CF2 lattice expansion, respectively, whereas further fluorination led to a YF3-like structure. Y2CF2 formed at the end of the first plateau was reversibly defluorinated to Y2C upon charging. The reversible change in the shape of the C K-edge electron energy loss spectrum during charge-discharge indicated the contribution of carbon to the redox reaction. Thus, this paper presents, for the first time, an account of the reversible electrochemical intercalation of fluoride ions in FIB anode materials, paving the way for FIB commercialization.