Thin-film and bulk investigations of LiCoBO3 as a Li-ion battery cathode.

The compound LiCoBO3 is an appealing candidate for next-generation Li-ion batteries based on its high theoretical specific capacity of 215 mAh/g and high expected discharge voltage (more than 4 V vs Li(+)/Li). However, this level of performance has not yet been realized in experimental cells, even with nanosized particles. Reactive magnetron sputtering was therefore used to prepare thin films of LiCoBO3, allowing the influence of the particle thickness on the electrochemical performance to be explicitly tested. Even when ultrathin films (∼15 nm) were prepared, there was a negligible electrochemical response from LiCoBO3. Impedance spectroscopy measurements suggest that the conductivity of LiCoBO3 is many orders of magnitude worse than that of LiFeBO3 and may severely limit the performance. The unusual blue color of LiCoBO3 was investigated by spectroscopic techniques, which allowed the determination of a charge-transfer optical gap of 4.2 eV and the attribution of the visible light absorption peak at 2.2 eV to spin-allowed d → d transitions (assigned as overlapping (4)A2' to (4)A2″ and (4)E″ final states based on ligand-field modeling).

Substitutional mechanism of Ni into the wide-band-gap semiconductor InTaO4 and its implications for water splitting activity in the wolframite structure type.

The mechanism of Ni substitution into the oxide semiconductor InTaO(4) has been studied through a combination of structural and spectroscopic techniques, providing insights into its previously reported photoactivity. Magnetic susceptibility and X-ray absorption near-edge spectroscopy (XANES) measurements demonstrate that nickel is divalent within the host lattice. The combined refinement of synchrotron X-ray and neutron powder diffraction data indicates that the product of Ni doping has the stoichiometry of (In(1-x)Ni(2x/3)Ta(x/3))TaO(4) with a solubility limit of x ≈ 0.18, corresponding to 12% Ni on the In site. Single-phase samples were only obtained at synthesis temperatures of 1150 °C or higher due to the sluggish reaction mechanism that is hypothesized to result from small free energy differences between (In(1-x)Ni(2x/3)Ta(x/3))TaO(4) compounds with different x values. Undoped InTaO(4) is shown to have an indirect band gap of 3.96 eV, with direct optical transitions becoming allowed at photon energies in excess of 5.1 eV. Very small band-gap reductions (less than 0.2 eV) result from Ni doping, and the origin of the yellow color of (In(1-x)Ni(2x/3)Ta(x/3))TaO(4) compounds instead results from a weak (3)A(2g) → (3)T(1g) internal d → d transition not associated with the conduction or valence band that is common to oxide compounds with Ni(2+) in an octahedral environment.