Redox Reaction Mechanism of Graphite Fluoride (CF)n in Fluoride-Ion Batteries

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In theory, fluoride-ion batteries (FIBs) should possess a high energy density; however, only Cu/CuF2 has exhibited a capacity greater than 500 mA h g–1. Graphite fluoride, (CF)n, has a high theoretical capacity of 864 mA h g–1. Reversible defluorination and fluorination reactions of (CF)n have been observed in lithium- and sodium-ion batteries. In these alkaline-ion batteries, nearby alkali-metal ions enable the breakage of strong C–F bonds, whereas in FIBs, the C–F bonds must be broken electrochemically. We prepared (CF)n as the positive electrode for a liquid-electrolyte FIB and analyzed it via combustion ion chromatography, 19F nuclear magnetic resonance (NMR), and various X-ray analyses. The discharge and charge capacities of the first cycle were 731 and 262 mA h g–1, respectively. The discharging process defluorinated (CF)n and converted it into graphite-like carbon (GLC), indicating that the C–F bonds were electrochemically broken. After a discharge of approximately 650 mA h g–1, 19F NMR measurements revealed the presence of (C2F)n, and (C4F)n phases with hyperconjugated C–F bonds, suggesting that during defluorination, stable (C2F)n and (C4F)n phases were formed from (CF)n. During charging, primarily, the edges of GLC and acetylene black (carbon additive) domains were fluorinated to form >CF2 groups.