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 K_0.21TiO₂ 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.

Li₂Ti₆O₁₃ and H₂Ti₆O₁₃ were easily synthesized from Na₂Ti₆O₁₃ by successive Na⁺–Li⁺–H⁺ ion exchange. The crystal structures of Na₂Ti₆O₁₃, Li₂Ti₆O₁₃ and H₂Ti₆O₁₃ were investigated using neutron powder diffraction. Monovalent A⁺ cations (Na, Li and H) have been located using difference Fourier analysis. Although monoclinic lattice parameters (space group C2/m) of the three titanates remain almost unchanged with retention of the basic [Ti₆O₁₃ ²⁻] network, monovalent Na, Li and H cations occupy different sites in the tunnel space. By comparing the structural details concerning the A⁺ oxygen coordination, i.e. NaO₈ square prismatic coordination, LiO₄ square planar coordination and covalently bond H atoms, with results from ²³Na, ⁷Li and ¹H NMR spectroscopy we were able to obtain a more detailed insight into the respective local distortions and anharmonic motions. We were able to show that the site that the A⁺ cation occupies in the hexatitanate channel structure strongly influences the lithium insertion properties of these compounds and therefore their usefulness as electrode materials for energy storage.

Substitution of La³⁺ by Sr²⁺ in the double perovskite La₂CoTiO₆ yields materials of the La_2−xSr_xCoTiO₆ series showing a significant amount of trivalent cobalt ions when prepared at ambient atmosphere. The as-prepared compounds can be reduced in severe conditions retaining the perovskite structure while inducing the formation of a large amount of oxygen vacancies. The limit of aliovalent substitution in this series was found to extend up to x = 1. For substitution of La³⁺ up to 15% cobalt and titanium are ordered, though the order is progressively lost as x increases; for x ≥ 0.30 no ordering is observed as evidenced by magnetic measurements. The ability of these materials to present either cobalt ions in a mixed oxidation state or large amounts of anion vacancies depending on the atmosphere makes them interesting to be further investigated regarding their electrical and electrochemical properties, and hence, their usefulness in some electrochemical devices.

VO ₂ F with a ReO ₃ -type structure has been synthesized at high pressures. It reversibly inserts up to 1 Li ⁺ per vanadium above 2.15 V delivering a high specific capacity (250 mA h g ⁻¹ ).