A new synthetic method of titanium oxyfluoride and its application as an anode material for rechargeable lithium batteries

This paper introduces a novel, efficient method for the synthesis of TiOF2, and outlines the mechanisms of TiOF2 on discharge (reduction) and charge (oxidation). Anatase TiO2 is reacted with hydrofluoric acid, HF, to produce TiOF2. The synthesized TiOF2 is crystallized into a cubic perovskite structure with a Pm3m space group, as confirmed by Rietveld refinement of X-ray diffraction data. The particles show peculiar cubic-shaped secondary particle morphology. Electrochemical investigation indicated that the TiOF2 delivers a high charge capacity 526 mAh g−1. X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and transmission electron microscopy are employed to elucidate the reaction process. During lithiation, TiOF2 adopts Li+ ions into the cubic perovskite structure to form LiTiOF2, and the phase separates to LiF and TiO by further lithiation. The formed TiO is oxidized to TiO2 during charging although those oxides are in an amorphous state. The electrochemical reaction seems to be fairly stable, retaining 94.5% of its capacity during 100 cycles, starting from the second cycle. Also, the electrode is suitable for operation at high rates, namely, 400 mAh g−1 at 200 mA g−1, 345 mAh g−1 at 1.6 A g−1, and 288 mAh g−1 at 3.2 A g−1.