Development of Fluoride-Ion Primary Batteries: The Electrochemical Defluorination of CFx

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 19F and 119Sn{19F} 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.