Takeshi Kimijima

Hydrothermal synthesis of size- and shape-controlled CaTiO3 fine particles and their photocatalytic activity

Calcium titanate fine particles controlled precisely in size and morphology were synthesized by an optimized hydrothermal method. Their photocatalytic activity was evaluated as the effect of the particle morphology, that is, the exposed surface. The size and morphology were successfully controlled with aging temperature and alternative use of Ti-sources, such as a TiO2 dispersion and titanium–triethanolamine complex. The photocatalytic activity of the CaTiO3 fine particles was measured by the evolution amount of H2 from a methanolic aqueous solution and the decomposition of acetic acid in water. As a result, rod-like particles showed the highest photocatalytic activity for H2 evolution. On the other hand, cubic-shaped particles exhibited a higher activity for the decomposition of acetic acid. Judging from electron diffraction images, both the cubic and rod-like particles were bound by {110} and {001} facets, but the rate of the {110} faces of the rod-like particles was much more than that of the cubic ones. The proper selection of the particle shape, outer surface is one of the key factors for the photocatalytic activity.

Molybdate flux growth of idiomorphic Li(Ni1/3Co1/3Mn1/3)O2 single crystals and characterization of their capabilities as cathode materials for lithium-ion batteries

A Li2MoO4 flux enabled size-tunable idiomorphic Li(Ni1/3Co1/3Mn1/3)O2 (NCM) crystal growth. The crystal size was controlled from 0.26 to 4.4 μm simply by changing the experimental conditions (i.e., solute concentration and reaction temperature). The obtained crystals individually dispersed in both water and N-methylpyrrolidone without unexpected agglomeration; however, Li deficiency and a thin amorphous-like layer formed on the NCM crystal surface during flux removal with water. We found that post-heat-treatment with LiOH significantly improved the discharge capacity of NCM. Recovery of the surface quality of the NCM crystals was primarily responsible for the capacity improvement. The electrodes achieved a high capacity of 81 mA h g−1 under a high current density of 2000 mA g−1, while the electrodes prepared from commercially available NCM showed a maximum of 65 mA h g−1. The rate and cycle performance of the flux-grown NCM crystals was highly dependent on the crystal size. Smaller crystals showed excellent discharge capacities at high C rates, whereas larger crystals showed better cycle performances.

Low-temperature growth of idiomorphic cubic-phase Li7La3Zr2O12 crystals using LiOH flux

Micrometer-sized Li7La3Zr2O12 crystals with well-developed facets were grown from a LiOH flux at 700 °C. Supersaturation-controlled crystallization driven by the cooling of a homogeneous hot solution with a Li/Zr ratio of 70 achieved one-step formation of cubic-phase Li7La3Zr2O12 that potentially exhibits high lithium-ion conductivity. Excess LiOH flux decreased the reaction temperature dramatically to 700 °C, lower than that of solid-state-reaction processes. The flux growth of the Li7La3Zr2O12 crystals in response to varying reaction conditions was studied systematically and indicated that the initial Li/Zr ratio and the holding temperature significantly affected the crystal phase, shape, and size. We further demonstrated Li7La3Zr2O12 crystal growth at 500 °C, beginning with La2Zr2O7 powders that dissolve readily in hot LiOH. This will be examined in greater depth in the near future.

Thin and Dense Solid-solid Heterojunction Formation Promoted by Crystal Growth in Flux on a Substrate

In this work, we demonstrate the direct growth of cubic Li5La3Nb2O12 crystal layer on the LiCoO2 substrate through the conversion of ultra-thin Nb substrate in molten LiOH flux. The initial thickness of the Nb layer determines that of the crystal layer. SEM and TEM observations reveal that the surface is densely covered with well-defined polyhedral crystals. Each crystal is connected to neighboring ones through the formation of tilted grain boundaries with Σ3 (2–1–1) = (1–21) symmetry which show small degradation in lithium ion conductivity comparing to that of bulk. Furthermore, the sub-phase formation at the interface is naturally mitigated during the growth since the formation of Nb2O5 thin film limits the whole reaction kinetics. Using the newly developed stacking approach for stacking solid electrolyte layer on the electrode layer, the grown crystal layer could be an ideal ceramic separator with a dense thin-interface for all-solid-state batteries.