Development of Fluoride-Ion Primary Batteries: The Electrochemical Defluorination of CF x .

The lithium-carbon monofluoride (Li-CF x ) couple has the highest specific energy of any practical battery chemistry. However, the large polarization associated with the CF x electrode (>1.5 V loss) limits it from achieving its full discharge energy, motivating the search for new CF x 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 CF x cathodes, where metal fluorides form at a metal anode instead of the CF x cathode. F-ion primary cells were developed by pairing CF x 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 19 F and 119 Sn{ 19 F} nuclear magnetic resonance (NMR), X-ray diffraction (XRD), Raman, inductively coupled plasma (ICP), and X-ray fluorescence (XRF) measurements establish that upon discharge, the CF x cathode defluorinates while Pb forms PbF 2 and Sn forms both SnF 4 and SnF 2 . Technological development of F-ion metal-CF x cells based on this concept represents a promising avenue for realizing primary batteries with high specific energy.