Abstract
Reactivity of pure molecular fluorine F 2 allows the creation of new materials with unique electrochemical properties. We demonstrate that titanium oxyfluoride TiOF 2 can be obtained under molecular fluorine from anatase titanium oxide TiO 2 , while the fluorination of rutile TiO 2 leads only to pure fluoride form TiF 4 . Contrary to most fluorides, TiOF 2 is air-stable and hydrolyses poorly in humid conditions. Such stability makes it possible for TiOF 2 to be studied as an electrode material in Li-ion secondary batteries systems. It shows capacities as high as 220 mAh 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.
Highlights
The reaction of molecular fluorine F2 with rutile titanium dioxide TiO2 at 25 °C should lead to an equimolar mixture of TiF4 and O2: TiO2(s) + 2 F2(g) = TiF4(s) + O2(g) From room temperature, the reaction is supposedly quantitative, with solid TiF4 formed
In order to investigate the reactivity of titanium dioxide, anatase and rutile TiO2 powders have been fluorinated with a mixture of molecular fluorine and nitrogen gases at 250 °C for 18 hours in dedicated fluorination equipment
This relatively high capacity retention at C may stem from the presence of O2- ion in TiOF2, which makes it a semiconductor with a band-gap of approximately 2.5 eV,[8] whereas metal fluorides are commonly referred to as insulator with band-gaps larger than 4 eV that cannot be cycled at such high density current without nanocarbon grafting.[9,10,11] An impressive capacity of 200 mAh g-1, close to the theoretical one, can even be obtained when the half-cell is cycled at C/100 (Figure 4b)
Summary
On the contrary, when anatase is fluorinated in similar conditions (Figure 1), the pressure profile obtained is clearly dissimilar, suggesting a different reactivity with pure fluorine between both polymorphs, contrary to what was expected.
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