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

Abstract

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 ∼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 situXRD and HR-TEM.