Fast Solid-State Defluorination/Fluorination of FeF_x (x = 3–0) as Fluoride-Ion Battery Cathode

Fluoride-ion batteries, which use fluoride ions as charge carriers, are potential candidates for surpassing conventional lithium-ion batteries. Iron fluoride (FeF_x, x = 3-0) is a cathode material with a high theoretical capacity of 712 mAh g^-1. However, its electrochemical properties and reaction mechanism remain largely unexplored. In this study, a FeF_x thin film was fabricated to investigate its discharge/charge capability and defluorination/fluorination mechanism as a cathode for all-solid-state fluoride-ion batteries. At room temperature, the FeF_x cathode exhibited a reversible capacity that is 88-72% of the theoretical value at current rates of 0.1-1C. In addition, the cathode demonstrated excellent cyclability without any overvoltage increase or capacity degradation. The FeF_x cathode could be discharged/charged even at a very low temperature of -30 °C, suggesting that the activation energy required for defluorination/fluorination is low. Operando X-ray absorption spectroscopy quantitatively demonstrated that the reversible discharge/charge of the FeF_x cathode at room temperature was due to defluorination/fluorination between FeF₃, FeF₂, and Fe. The scanning electron microscopy and X-ray photoelectron spectroscopy results revealed that the electrode structure comprising uniformly distributed FeF₃ fine grains approximately 10 nm in size was maintained throughout the discharge/charge cycle, suggesting that this nanometer-scale structure is related to the excellent cyclability of the FeF_x cathode. Electrochemical analysis revealed that the defluorination/fluorination reaction of FeF₂/Fe was rate-limited by the charge transfer process, indicating the kinetic advantage of the solid-state FeF_x reaction. This study provides significant improvements in the practical properties of the fluoride-ion electrode and insights into solid-state defluorination/fluorination.