Fluorination of anatase TiO2 towards titanium oxyfluoride TiOF2: a novel synthesis approach and proof of the Li-insertion mechanism

Abstract

The reactivity of pure molecular fluorine F₂ allows the creation of new materials with unique electrochemical properties. We demonstrate that titanium oxyfluoride TiOF₂ can be obtained under molecular fluorine from anatase titanium oxide TiO₂, while the fluorination of rutile TiO₂ leads only to pure fluoride form TiF₄. Contrary to most fluorides, TiOF₂ is air-stable and hydrolyses poorly under humid conditions. Such a stability makes it possible for TiOF₂ to be studied as an electrode material in Li-ion secondary battery systems. It shows the capacity as high as 220 mA h g⁻¹ and good cyclability at high current rates at an average potential of 2.3 V vs. Li⁺/Li. At such a potential, only Li⁺ insertion occurs, as proven by in operando XRD/electrochemistry experiments.