Development of\nFluoride-Ion Primary Batteries: The\nElectrochemical Defluorination of CF<sub><i>x</i></sub>

The lithium–carbon\nmonofluoride (Li-CF<sub><i>x</i></sub>) couple has the highest\nspecific energy of any practical\nbattery\nchemistry. However, the large polarization associated with the CF<sub><i>x</i></sub> electrode (>1.5 V loss) limits it from\nachieving\nits full discharge energy, motivating the search for new CF<sub><i>x</i></sub> reaction mechanisms with reduced overpotential.\nHere, using a liquid fluoride (F)-ion conducting electrolyte at room\ntemperature, we demonstrate for the first time the electrochemical\ndefluorination of CF<sub><i>x</i></sub> cathodes, where\nmetal fluorides form at a metal anode instead of the CF<sub><i>x</i></sub> cathode. F-ion primary cells were developed by pairing\nCF<sub><i>x</i></sub> cathodes with either lead (Pb) or\ntin (Sn) metal anodes, which achieved specific capacities of over\n700 mAh g<sup>–1</sup> and over 400 mAh g<sup>–1</sup>, respectively. Solid-state <sup>19</sup>F and <sup>119</sup>Sn{<sup>19</sup>F} nuclear magnetic resonance (NMR), X-ray diffraction (XRD),\nRaman, inductively coupled plasma (ICP), and X-ray fluorescence (XRF)\nmeasurements establish that upon discharge, the CF<sub><i>x</i></sub> cathode defluorinates while Pb forms PbF<sub>2</sub> and Sn\nforms both SnF<sub>4</sub> and SnF<sub>2</sub>. Technological development\nof F-ion metal-CF<sub><i>x</i></sub> cells based on this\nconcept represents a promising avenue for realizing primary batteries\nwith high specific energy.