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

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.