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

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 0.8-0.9V for LixTiOF2 as shown by ex situ XRD and at < =1.4V for LixNbO2F to form amorphous composites, 'LixTi/NbOy-LiF'. Galvanostatic discharge-charge cycling of 'LixTiOy' in the range 0.005-3.0V at a current density of 65mAg-1 gives a capacity of 400 (+/-5)mAhg-1 during 5-100 cycles with no noticeable capacity fading. This value corresponds to 1.52mol of recycleable Li/Ti. The coulombic efficiency (eta) is > 98%. Results on 'LixNbOy'show good reversibility of the electrode and a eta > 98% is achieved only after 10 cycles (range 0.005-3.0V and at 30mAg-1) and a capacity of 180 (+/-5)mAhg-1 (0.97mol of Li/Nb) was stable up to 40 cycles. In both 'LixTiOy' and 'LixNbOy', the average discharge and charge voltages are 1.2-1.4 and 1.7-1.8V, 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.