Metal oxyfluorides TiOF₂ and NbO₂F as anodes for Li-ion batteries

Abstract

Lithium insertion and extraction in to/from the oxyfluorides TiOF₂ and NbO₂F is investigated by galvanostatic cycling, cyclic voltammetry and impedance spectroscopy in cells using Li-metal as a counter electrode at ambient temperature. The host compounds are prepared by low-temperature reaction and characterized by powder X-ray diffraction (XRD), Rietveld refinement and Brunauer, Emmett and Teller (BET) surface area. Crystal structure destruction occurs during the first-discharge reaction with Li at voltages below 0.8-0.9 V for Li_xTiOF₂ as shown by ex situ XRD and at ≤1.4 V for Li_xNbO₂F to form amorphous composites, 'Li_xTi/NbO_y-LiF'. Galvanostatic discharge-charge cycling of 'Li_xTiO_y' in the range 0.005-3.0 V at a current density of 65 mA g⁻¹ gives a capacity of 400 (±5) mAh g⁻¹ during 5-100 cycles with no noticeable capacity fading. This value corresponds to 1.52 mol of recycleable Li/Ti. The coulombic efficiency (η) is > 98%. Results on 'Li_xNbO_y' show good reversibility of the electrode and a η >98% is achieved only after 10 cycles (range 0.005-3.0 V and at 30 mA g⁻¹) and a capacity of 180 (±5) mAh g⁻¹ (0.97 mol of Li/Nb) was stable up to 40 cycles. In both 'Li_xTiO_y' and 'Li_xNbO_y', the average discharge and charge voltages are 1.2-1.4 and 1.7-1.8 V, respectively. The impedance spectral data measured during the first cycle and after selected numbers of cycles are fitted to an equivalent circuit and the roles played by the relevant parameters as a function of cycle number are discussed.