Lithium fluoride/iron difluoride composite prepared by a fluorolytic sol-gel method: Its electrochemical behavior and charge-discharge mechanism as a cathode material for lithium secondary batteries

Abstract

Owing to their high theoretical capacity, metal fluorides have attracted significant interest as materials for fabricating the cathode of lithium secondary batteries. In the present study, a nanocomposite of LiF and FeF₂ is prepared by a fluorolytic sol–gel method in an ethanol solution, for use as the cathode material of a lithium secondary battery. The produced LiF/FeF₂ composite is characterized by broad X-ray diffraction peaks attributed to the nanosized (∼10 nm) LiF and FeF₂ crystals, a large Brunauer–Emmett–Teller surface area of 119 m²g⁻¹, and adsorption–desorption hysteresis, attributed to the presence of mesopores. The results of charge–discharge tests indicates an initial discharge capacity of 225 mAh (g-LiF/FeF₂)⁻¹ through reversal conversion at a current rate of 10 mA (g-LiF/FeF₂)⁻¹. Based on a combination of galvanostatic intermittent titration, X-ray absorption, and X-ray diffraction investigations, a new reaction mechanism is developed, namely, the conversion of the local environment of an Fe atom from a rutile-type FeF₂ structure to a rhenium trioxide-type FeF₃ structure during charging, with the subsequent discharge resulting in the insertion of Li⁺ into the rhenium trioxide-type FeF₃ structure, followed by the conversion reaction to LiF and FeF₂.