Masatsugu Oishi

Three-Dimensional Nanoelectrode by Metal Nanowire Nonwoven Clothes

Metal nanowire nonwoven cloth (MNNC) is a metal sheet that has resulted from intertwined metal nanowires 100 nm in diameter with several dozen micrometers of length. Thus, it is a new metallic material having both a flexibility of the metal sheet and a large specific surface area of the nanowires. As an application that utilizes these properties, we propose a high-cyclability electrode for Li storage batteries, in which an active material is deposited or coated on MNNC. The proposed electrode can work without any binders, conductive additives, and current collectors, which might largely improve a practical gravimetric energy density. Huge electrode surfaces provide efficient ion/electron transports, and sufficient interspaces between the respective nanowires accommodate large volume expansions of the active material. To demonstrate these advantages, we have fabricated a NiO-covered nickel nanowire nonwoven cloth (NNNC) by electroless deposition under a magnetic field and annealing in air. The adequately annealed NNNC was shown to be an excellent conversion-type electrode that exhibits a quite high cyclability, 500 mAh/g at 1 C after 300 cycles, compared to that of a composite electrode consisting of NiO nanoparticles. Thus, the present design concept will contribute to a game-changing technology in future lithium ion battery (LIB) electrodes.

An X-ray absorption spectroscopic study on mixed conductive La0.6Sr0.4Co0.8Fe0.2O3−δ cathodes. I. Electrical conductivity and electronic structure

The electrical conduction mechanism of mixed conductive perovskite oxides, La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ), for cathode materials of solid oxide fuel cells has been investigated from electronic structural changes during oxygen vacancy formation. La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ) was annealed under various oxygen partial pressures p(O(2))s at 1073 K and quenched. Iodometric titration indicated that the oxygen nonstoichiometry of La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ) depended on the annealing p(O(2)), with more oxygen vacancies introduced at lower than at higher p(O(2))s. X-Ray absorption spectroscopic measurements were performed at the O K-, Co L-, Fe L-, Co K-, and Fe K-edges. The valence states of the Co and Fe ions were investigated by the X-ray absorption near edge structure (XANES) at the Co and Fe L(III)-edges. While the Fe average valence was almost constant, the valence of the Co ions decreased with oxygen vacancy introduction. The O K-edge XANES spectra indicated that electrons were injected into the Co 3d/O 2p hybridization state with oxygen vacancy introduction. Both absorption edges at the Co and Fe K-edge XANES shifted towards lower energies with oxygen vacancy introduction. The shift at the Co K-edge resulted from the decrease in the Co average valence and that at the Fe K-edge appeared to be caused by changes in the coordination environment around the Fe ions. The total conductivity of La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ) decreased with decreasing p(O(2)), due to a decreasing hole concentration.

A new aspect of Chevrel compounds as positive electrodes for magnesium batteries

Chevrel compounds are regarded as potential positive-electrode materials for magnesium rechargeable batteries, but their redox potential is only about 1.2 V vs. Mg/Mg2+. In this work, we show logically and experimentally that the redox potential of Chevrel compounds can be as high as about 2–3 V vs. Mg/Mg2+. A crucial basis for this is that Cu cations can be extracted from CuxMo6S8 at around 1.2–1.6 V vs. Mg/Mg2+ in the conventional electrolyte (Grignard-reagent/tetrahydrofuran) while the anodic dissolution of Cu metal can occur above about 1.7 V vs. Mg/Mg2+ in the same electrolyte, which means that the chemical potential of Cu in Chevrel compounds is higher than that in pure Cu metal. This thermodynamic conflict inevitably compels us to consider a certain interaction with the solvent, rather than simple deintercalation from the compound, which is discussed throughout the paper. With the use of large-molecule solvents or ionic liquids, we have observed an intriguing relaxation phenomenon, where the cations move to find more stable sites, which directly indicates that the Chevrel compounds have several sites for cations.

Elastically constrained phase-separation dynamics competing with the charge process in the LiFePO4/FePO4 system

By using phase-field computer simulations, we have investigated the effects of the coherent strain due to the phase separation in the olivine-type LiFePO4. In this system, the coherent elastic-strain energy due to the lattice mismatch between LiFePO4 and FePO4 phases accompanied by insertion and extraction of Li ions is considered to play a crucial role in the phase separation kinetics during the charge/discharge process. The present phase-field micromechanics simulations reveal several significant features of the LiFePO4/FePO4 system accompanying the coherent strain, such as the retardation of the phase separation, the charge rate dependence, the thermodynamic stability of coherent interfaces between dual phases, etc. Nucleation of the new phase is found to be fundamentally unlikely in terms of the elastic strain energy, except in the vicinity of the surface of the particles, and thus the phase separation would be dominated by the spinodal decomposition process. When the nucleus is present precedently, however, the phase separation can proceed in the mixture mode of the domino cascade and spinodal decomposition processes.

In situ two-dimensional imaging quick-scanning XAFS with pixel array detector

Two-dimensional imaging quick-scanning XAFS measurements were performed using a pixel array detector.

Local structural analysis for oxide ion transport in La0.6Sr0.4FeO3−δ cathodes

The relationship between the local structural changes and the oxide ion diffusion in La0.6Sr0.4FeO3−δ was investigated. The oxygen vacancy concentration in La0.6Sr0.4FeO3−δ was varied by annealing under various oxygen partial pressures. Local structural changes of La0.6Sr0.4FeO3−δ with the introduction of oxygen vacancies were studied by the extended X-ray absorption fine structure (EXAFS) analysis. Oxygen vacancies are preferentially introduced near the La ions and local distortion around the oxygen vacancies is induced. The oxygen vacancy diffusion coefficient, Dv was calculated by means of electrical conductivity measurement. Dv decreased with increasing local distortion around the oxygen vacancy. Activation energies for Dv strongly depended on the bottle-neck size calculated from the result of EXAFS.

Revisit to diffraction anomalous fine structure

The diffraction anomalous fine structure method has been revisited by applying this measurement technique to polycrystalline samples and using an analytical method with the logarithmic dispersion relation.

Direct observation of layered-to-spinel phase transformation in Li2MnO3 and the spinel structure stabilised after the activation process

Li2MnO3 is an important parent component in lithium- and manganese-rich layered oxides (LMRs), which are one of the promising positive electrode materials for next-generation lithium ion rechargeable batteries. Here, we report the layered-to-spinel phase transformation in Li2MnO3 during the initial charging process to characterise its unique delithiation behaviour, which gives an insight into the relationship between the structure, superior capacities and degradation of LMR electrodes. The atomic-scale observation using scanning transmission electron microscopy (STEM) techniques suggests that the structural transformation occurs in a biphasic manner within a single particle. The formed phase has a Li-defect spinel structure, indicating that the delithiation leads to Mn migration from the transition-metal layer to the Li layer, accompanied by some oxygen release. This layered-to-spinel phase transformation is an essential bulk process in the initial activation of Li2MnO3. During the lithiation in the 1st discharge, the Mn remigration occurs and the layered structure is again formed with significant disordering. During the multiple cycles, the defect spinel structure is stabilised and becomes more oxygen-deficient with a lower Mn valency. As a consequence, the amount of inserted Li decreases, which corresponds to the capacity and voltage fading observed in Li2MnO3 and LMRs.

In situ two-dimensional micro-imaging XAFS with CCD detector

In situ two-dimensional (2D) micro-imaging X-ray absorption fine structure (XAFS) measurements were performed in transmission mode using a charge coupled device (CCD) detector, phosphor screen, and magnifying lens. This method makes it possible to display a 2D image with a spatial resolution of around 2 μm at each energy point in a XAFS spectrum. The method was applied to in situ transmission micro-imaging XAFS measurement with a quick scanning technique.

High temperature degradation mechanism of a red phosphor, CaAlSiN3:Eu for solid-state lighting

Thermal properties of a red phosphor CaAlSiN3:Eu (CASN) at elevated temperatures were evaluated. A heat treatment at 800 °C degraded the photoluminescence property of CASN and caused irreversible changes in both the excitation and emission intensities. The heat treatment in air simultaneously decreased the N elements and increased the O elements. Consequently, the Eu2+ luminescence center was oxidized and CASN lost its photoluminescence property. Although the crystal structure of CASN host was stable even after the heat treatments, the local structure change around the Eu2+ ions is the origin of the thermal degradation of CASN. We found that the heat treatment in N2 atmosphere suppresses the thermal degradation. This is due to the suppression of N evolutions and the incorporation of O elements, which sustains the optically active Eu2+ state.