Hollandite-type TiO2: a new negative electrode material for sodium-ion batteries

The electrochemical properties of TiO2 with the hollandite structure (TiO2(H)) as a negative electrode material for sodium-ion batteries are reported. TiO2(H) was obtained from hollandite K0.21TiO2 by an oxidation–ion extraction process. Na/TiO2(H) cells exhibit a large first discharge capacity of 280 mA h g−1 down to 0.2 V. After the first discharge the Na/TiO2(H) cells develop a reversible charge–discharge capacity of 85 mA h g−1 at C/8 rate in the 2.5–0.2 V voltage range; this corresponds to the reversible insertion of 0.25 Na per TiO2(H) formula unit. Chronoamperometry and potentiostatic intermittent titration techniques were used to further characterize the electrochemical reaction mechanism. Structural changes in the TiO2(H) electrode upon sodium insertion and extraction have been studied by ex situ XRD and high resolution in situ synchrotron diffraction techniques, for which appropriately modified coin-type cells were used. It is seen that sodium insertion into TiO2(H) is commenced with a single-phase solid solution followed by a structural transition from tetragonal I4/m to monoclinic I2/m symmetry, in which the skeleton framework is retained. The reversible transition includes few structural changes with a small volume change of only 1.1%. Fourier difference maps deduced from SXRD patterns revealed the location of Na ions in 4i sites in the tunnel space. The coordination arrangement around Na ions is distorted capped trigonal prisms formed by seven oxygen atoms. Although still far from the theoretical capacity (335 mA h g−1), the cycling properties at a low insertion potential together with the host framework stability indicate the feasibility of TiO2 with the hollandite structure as a negative electrode material for Na-ion batteries.