(N,F)-Co-doped TiO2: synthesis, anatase-rutile conversion and Li-cycling properties

Nitrogen and fluorine co-doped Ti-oxide, TiO1.9N0.05F0.15 (TiO2(N,F)), with the anatase structure is prepared by the pyro-ammonolysis of TiF3. For the first time it is shown that TiO2(N,F) and anatase-TiO2 are converted to nanosize-rutile structure by high energy ball milling (HEB). The polymorphs are characterised by X-ray diffraction, Rietveld refinement, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and Raman spectra. The Li storage and cycling properties are examined by galvanostatic cycling and cyclic voltammetry in the voltage range 1-2.8 V vs. Li at 30 mA g(-1). The performance of TiO2(N,F) is much better than pure anatase-TiO2 and showed a reversible capacity of 95 (+/- 3) mA h g(-1) stable up to 25 cycles with a coulombic efficiency of similar to 98%. Nano-phase rutile TiO2(N,F) showed an initial reversible capacity of 210 mA h g(-1) which slowly degraded to 165 (+/- 3) mA h g(-1) after 50 cycles and stabilised between the 50th and 60th cycle whereas the nano-phase rutile-TiO2 (prepared by HEB of anatase-TiO2) exhibited a reversible capacity of 130 (+/- 3) mA h g(-1) which is stable in the range, 10-60 cycles. The crystal structure of anatase TiO2(N,F) is not destroyed upon Li-cycling and is confirmed by ex situ XRD and HR-TEM.