Showing papers by "Ikuya Yamada published in 2019"

Structural and Electrochemical Analyses on the Transformation of CaFe2O4-Type LiMn2O4 from Spinel-Type LiMn2O4

Abstract: Lithium manganese oxides have received much attention as positive electrode materials for lithium-ion batteries. In this study, a post-spinel material, CaFe2O4-type LiMn2O4 (CF-LMO), was synthesize...

High-pressure synthesis of highly oxidized Ba0.5Sr0.5Co0.8Fe0.2O3−δ cubic perovskite

Abstract: A cubic perovskite oxide Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) has been termed as a representative energy-conversion material. This compound includes intrinsic oxygen deficiency denoted as δ. We obtained a highly oxidized cubic perovskite BSCF by using high-pressure and moderate-temperature (8 GPa and 500 °C) conditions with NaClO3 as the oxidizing agent, although the hexagonal phase was obtained when treated at conventional high-pressure and high-temperature (8 GPa and 1000 °C) conditions with KClO4 as the oxidizing agent. The highly oxidized BSCF sample contained a small amount of oxygen deficiency (δ = 0.12) compared to the BSCF samples (δ = 0.38) synthesized in typical conditions, thus proposing an alternative route to obtain the metastable phase of highly oxidized perovskite. The electric and magnetic measurements revealed that the metallic phase was predominant for the highly oxidized sample (δ = 0.12) in contrast to the non-metallic properties for the conventional oxygen-deficient sample (δ = 0.38). The catalytic activity for oxygen evolution reaction (OER) was evaluated within a wide range of oxygen content from heavily oxygen-deficient (δ = 0.51) to conventional (δ = 0.38) to highly oxidized (δ = 0.12) samples. Both the highly oxidized and heavily oxygen-deficient samples were inferior to the conventional one in OER catalytic activity, indicating that an adequate amount of oxygen deficiency is favorable for the activation of OER by BSCF.

Synthesis of Rhombohedral LiCo0.64Mn0.36O2 Using a High-Pressure Method

Abstract: Lithium transition metal (M) oxides with a rhombohedral structure, r-LiMO2, have attracted a great deal of attention as a positive electrode material for lithium-ion batteries. Despite intensive studies thus far, Mn-rich r-LiMO2 compounds have remained unattainable, due to a cooperative Jahn–Teller distortion of Mn3+ ions in the MnO6 octahedra. We employed a high-pressure method for synthesizing r-LiCoxMn1–xO2 (r-LCMO) with x = 0.5 and examined its electrochemical properties in a nonaqueous lithium cell. The high-pressure method successfully suppressed the Jahn–Teller distortion of Mn3+ ions, and the r-LCMO phase was observed in a wide temperature–pressure region when using a LiOH·H2O precursor. The rechargeable capacity of the sample synthesized at 600 °C and 12 GPa reached 126 mAh g–1, although the r-LCMO phase was contaminated with electrochemically inactive rock-salt LCMO and hexagonal LCMO phases. Compositional and structural analyses clarified that the actual Co/Mn ratio of the r-LCMO phase was 64/3...

Structure and thermoelectric transport analysis of defect-containing CuGaTe2 prepared by room-temperature high-pressure treatment

Abstract: The effect of room-temperature high-pressure (RTHP) treatment on the transport properties, crystal structure, and electronic structure of CuGaTe2 with chalcopyrite structure was investigated. Dense bulk samples were obtained without the application of heat. In addition, the thermal conductivity of the RTHP-treated CuGaTe2 was greatly reduced compared with that of its hot-pressed counterpart. Theoretical analysis based on the Debye–Callaway model indicated that this reduction in the thermal conductivity originated from point defects, stacking faults, and increased grain boundaries. These defects also decreased the carrier mobility and increased the carrier concentration, resulting in reductions of the electrical conductivity and Seebeck coefficient. Crystal structure analysis revealed that the RTHP-treated CuGaTe2 contained microstrains, which partially arose from a series of disorder at the Cu/Ga cation site with various degrees and geometries. From the calculated formation energies, we predicted that such disorder can be induced as a metastable structure by the RTHP treatment applied in this study. Electronic structure calculations clarified that this disorder at the Cu/Ga cation site affected the orbital hybridization. Specifically, the shape of the band structure near the Fermi level was modified, and the bandgap was considered to decrease compared with that of pristine CuGaTe2; these findings partially explain the experimentally observed transport properties of the RTHP-treated CuGaTe2.

Pressure-induced intersite charge transfer in SrCu 3 Fe 4 O 12

Abstract: The electronic properties of the perovskite oxide SrCu3Fe4O12 subjected to pressure in a diamond anvil cell were investigated by 57Fe Mossbauer spectroscopy and electrical resistance measurements. ...

Electrochemical properties of chromium oxyfluoride CrO2−xFx with 0 ≤ x ≤ 0.3

Abstract: To overcome the limitations of graphite as a negative electrode material for lithium-ion batteries (LIBs), transition metal oxyfluorides are under active development. In this study, chromium oxyfluorides CrO2−xFx with 0 ≤ x ≤ 0.3 were synthesized under a high-pressure/high-temperature (HP/HT) environment, and their electrochemical properties were examined in a nonaqueous lithium cell. The HP/HT-treated CrO2 maintained a rutile structure and exhibited a rechargeable capacity (Qrecha) of over 400 mA h g−1 at 298 K. The replacement of O2− ions with F− ions in CrO2 was confirmed by linear changes in the tetragonal lattice parameters, weaker ferromagnetic interactions between Cr4+ ions, and elemental mappings of F− ions. The Qrecha values of the x > 0 samples at 298 K decreased to 150–300 mA h g−1 because of low electric conductivity in CrO2−xFx. However, the Qrecha values at 318 K increased to 600–700 mA h g−1, and the cycle performance over 30 cycles was better than that of the HP/HT-treated CrO2 sample with no F− substitution. Hence, CrO2−xFx was found to be a promising negative electrode material for LIBs, although its cycle stability should be further improved.