Development of Fluoride-Ion Primary Batteries: The Electrochemical Defluorination of CF_<i>x</i>

The lithium-carbon monofluoride (Li-CF _<i>x</i> ) couple has the highest specific energy of any practical battery chemistry. However, the large polarization associated with the CF _<i>x</i> electrode (>1.5 V loss) limits it from achieving its full discharge energy, motivating the search for new CF _<i>x</i> 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 _<i>x</i> cathodes, where metal fluorides form at a metal anode instead of the CF _<i>x</i> cathode. F-ion primary cells were developed by pairing CF _<i>x</i> 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 CF _<i>x</i> cathode defluorinates while Pb forms PbF₂ and Sn forms both SnF₄ and SnF₂. Technological development of F-ion metal-CF _<i>x</i> cells based on this concept represents a promising avenue for realizing primary batteries with high specific energy.