Development of Fluoride-Ion Primary Batteries: the Electrochemical Defluorination of CF X.

The lithium-carbon monofluoride (Li-CFx) couple has the highest specific energy of any practical battery chemistry. However, the large polarization associated with the CFx electrode (>1.5 V loss) limits it from achieving its full discharge energy, motivating the search for new CFx reaction mechanisms with reduced overpotential. Here, using a liquid fluoride (F)-ion conducting electrolyte at room temperature, we demonstrate for the first time the electrochemical defluorination of CFx cathodes, where metal fluorides form at a metal anode instead of the CFx cathode. F-ion primary cells were developed by pairing CFx cathodes with either lead (Pb) or tin (Sn) metal anodes, which achieved specific capacities of over 700 mAh g(-1) and over 400 mAh g(-1), respectively. Solid-state F-19 and Sn-119{F-19} nuclear magnetic resonance (NMR), X-ray diffraction (XRD), Raman, inductively coupled plasma (ICP), and X-ray fluorescence (XRF) measurements establish that upon discharge, the CFx cathode defluorinates while Pb forms PbF2 and Sn forms both SnF4 and SnF2. Technological development of F-ion metal-CFx cells based on this concept represents a promising avenue for realizing primary batteries with high specific energy.