Kazuyoshi Uematsu

Enhancement of discharge capacity of Li3V2(PO4)3 by stabilizing the orthorhombic phase at room temperature

Abstract Li 3 V 2 (PO 4 ) 3 and solid solutions of Li 3−2 x (V 1− x Zr x ) 2 (PO 4 ) 3 were prepared by a solid state reaction. A high temperature orthorhombic phase of Li 3 V 2 (PO 4 ) 3 with a β-Fe 2 (SO 4 ) 3 -type was successfully stabilized at room temperature by substituting Zr for V with substitution ratios beyond x =0.05. The pure material of Li 3 V 2 (PO 4 ) 3 exhibited a cathode performance with two well defined regions of plateau at around 3.7 and 4.1 V vs. Li/Li + upon charging and 3.6 and 4.0 V vs. Li/Li + upon discharging, respectively, suggesting two types of phases produced upon the charge/discharge process. On the other hand, the cathode performance of the orthorhombic stabilized materials showed almost the same charge/discharge voltages as those of the pure material, but, with two plateaus slightly sloping, showed a considerably improved charge/discharge cycle performance compared to that of the pure material. Such improvement on the charge/discharge cycle performance is suggested to come from the disordered lithium ion arrangement in the orthorhombic phase.

New Silicate Phosphors for a White LED

We focus on the development of new silicate phosphors for a white LED. In the europium doped silicate system, four LED phosphor candidates-Li 2 SrSiO 4 :Eu 2+ , Ba 9 Sc 2 Si 6 O 24 :Eu 2+ , Ca 3 Si 2 O 7 :Eu 2+ and Ba 2 MgSi 2 O 7 :Eu 2+ were found. Luminescent properties under near UV and visible excitation were investigated for the new Eu 2+ doped LED silicate phosphors. These new phosphors have a relatively strong absorption band in a long wavelength region.

Preparation of iron phosphate cathode material of Li3fe2(PO4)3 by hydrothermal reaction and thermal decomposition processes

Abstract Well-crystallized Li 3 Fe 2 (PO 4 ) 3 with an orthorhombic β-Fe 2 (SO 4 ) 3 -type structure was able to be synthesized by using two synthetic routes, where hydrothermal reaction and successive heat treatment with relatively low temperatures were employed. The samples prepared were found to show orthorhombic symmetry, contrary to the fact that a conventional solid state synthesis heating at temperatures higher than 1000 °C usually gives monoclinic symmetry. The sample showed two plateau potentials of 2.8 and 2.7 V vs. Li/Li + upon discharge, and the total discharge capacity of the sample exhibited more than 80 mA h g −1 under 0.5 mA cm −2 .

Bulk and grain boundary ionic conduction in lithium rare earth-silicates LiLnSiO4 (Ln=La,Nd,Sm,Eu,Gd,Dy)

Abstract The lithium rare earth silicates of nominal composition “LiLnSiO 4 ” (Ln = La, Nd, Sm, Eu, Gd, Dy) have been synthesized. The lithium ion conductivity of their sintered samples was found to consist of two contributions; one is the grain boundary composed possibly of lithium silicate amorphous phases and the other the bulk phase with an apatite structure. Although the conductivity of the bulk phase was smaller than that of the grain boundary, it was apparently increased by several orders of magnitude than that of stoichiometric apatites of Li x Ln 10 − x Si 6 O 24 O 3 − x with x = 2 and x = 3. From the structure refinement of the bulk phase, this enhancement in the conductivity of the apatite phase in LiLnSiO 4 was reasonably interpreted by the model where lithium ions and vacancies were both partially introduced to the 4 f site of the apatite structure.

Stabilization of superionic conduction phase in Li3Sc2(PO4)3

Abstract Lithium superion conductors, Li3+2x(Sc1−xMgx)2(PO4)3, Li3−2x(Sc1−xMx)2(PO4)3 (M=Ti, Zr, Sn, Hf) and Li3−4x(Sc1−xMx)2(PO4)3 (M=Nb, Ta) were prepared by a solid-state reaction. TG–DTA analysis indicated no phase transition in Li3+2x(Sc1−xMgx)2(PO4)3 and Li3−2x(Sc1−xMx)2(PO4)3 (M=Ti, Zr, Sn, Hf) with x higher than 0.05, and in Li3−4x(Sc1− xMx)2(PO4)3 (M=Nb, Ta) with x higher than 0.025. The room temperature ionic conductivity of Li3Sc2(PO4)3 has been increased by three orders of magnitude with the highest conductivity observed in Li3−2x(Sc1−xTix)2(PO4)3 with x=0.20 and in Li3−2x(Sc1− xZrx)2(PO4)3 with x=0.10. It was ascribed to the stabilization of the high temperature superionic conduction phase and the introduction of vacancies on the Li+ sites by substituting Ti4+ or Zr4+ for Sc3+.

Structure refinement of lithium ion conductors Li3Sc2(PO4)3 and Li3−2x(Sc1−xMx)2(PO4)3 (M=Ti, Zr) with x=0.10 by neutron diffraction

Abstract The crystal structure of lithium ion conductors, Li3Sc2(PO4)3 and Li3−2x(Sc1−xMx)2(PO4)3 (M=Ti, Zr) with x=0.10, were refined by Rietveld analysis for neutron diffraction patterns in order to confirm the site location and site occupancy for Li ions. The Rietveld refinement confirmed that a high disorder was introduced to three kinds of Li sites in the high temperature superionic conduction phase of Li3Sc2(PO4)3. The high temperature phase was stabilized at room temperature by substituting Ti4+ or Zr4+ for Sc3+ sites. The stabilized materials were also found to have similar high disorder over the Li sites.

Characterization of Potassium Niobate Produced by Self-Assembled Nanosheet from Aqueous Solution.

Submicron particles of potassium niobate KNbO3 were prepared by a colloid chemistry method. A rock-salt-type KF block in a layered perovskite, K2NbO3F, with a K2NiF4 type structure was selectively dissolved in water to produce perovskite nanosheets at room temperature. The self-assembly between perovskite nanosheets in a colloidal solution may enable shape control of potassium niobate powder. The microstructure, phase transition and mechanism of the solution process were investigated by transmission electron microscopy, X-ray diffraction and determination of ultraviolet-visible absorption spectra.

Hydrothermal synthesis of meso/macroporous BiVO4 hierarchical particles and their photocatalytic degradation properties under visible light irradiation

An ordered hierarchical meso/macroporous monoclinic bismuth vanadate (BiVO4) particle was fabricated for the first time by a simple two-step melamine template hydrothermal method followed by calcination. The physiochemical parameters of as-prepared porous materials were characterized by means of X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, Raman, Barrett–Emmett–Teller, and UV–vis techniques. The nitrogen adsorption–desorption measurement and pore size distribution curve suggest that meso/macropores exist in these hierarchical microarchitectures. Further, it is found that melamine plays a significant role in the formation of porous BiVO4 particles, and when a known amount of melamine was added, the surface area and pore size of such porous BiVO4 particles were increased. The photocatalytic activities of the as-prepared hierarchical BiVO4 samples were measured for the photodegradation of Congo red aqueous dye solution under visible light irradiation. Surprisingly, the porous BiVO4 particles showed outstanding photocatalytic activities than polycrystalline BiVO4 sample. The possible enhancement of such catalytic performance has also been further discussed.

Site engineering concept of Ce 3+ -activated novel orange-red emission oxide phosphors

Abstract: Novel Sr6(Y1-xCex)2Al4O15 (0.005 ≤ x ≤ 0.20) phosphors were synthesized in a single phase form by a conventional solid-state reaction method. These phosphors can be achieved the emission color tunable from blue to orange-red by controlling the Ce3+ doping site in the Sr6Y2Al4O15 lattice and exhibit orange-red emission centered on 600 nm by blue light irradiation as the Ce3+ concentration was increased. To the best of our knowledge, this is the first report of Ce3+ doping oxide phosphor exhibiting orange-red emission centered on 600 nm under blue light excitation.

Room temperature synthesis and characterization of perovskite compounds

A new soft chemical method has been developed to remove the rock-salt type layers in the Ruddlesden–Popper-type layered perovskite at room temperature. The rock-salt type (KF) blocks in the layered perovskite, K2NbO3F, were selectively dissolved into water to give three-dimensional perovskite, KNbO3, at room temperature. In the alkali metal aqueous solution, the ion-exchange reaction of parent layered compound with coexisting ions in the alkaline solution results in the new perovskites containing the other metal ions for the A-site. The development of this synthetic route could lead to interesting new metastable perovskite materials at low temperature.