Papers by Author: Shinya Suzuki

Abstract: Electrochemical properties of defect-introduces graphenes for lithium ion batteries were investigated. Graphene sheets (GSs) were prepared from graphite through treating with oxidizing agent followed by rapid thermal exfoliation. Defect concentration was controlled by selecting the number of times of oxidation of graphite. GSs electrodes derived from 1, 2 and 3 times-oxidized graphite oxides exhibited a high charge capacity of 1250, 1790 and 2310 mAh g¹, respectively, at the 20th cycle at a current density of 100 mA g¹. The enhanced capacity is assumed to be due to additional lithium storage sites such as defects and edges.

Abstract: The electrochemical capacitors (ECs) have attracted a great attention as a rechargeable strage device with a high power density and high safety. An increase in specific capacity is demanded to use ECs in various applications. MnO₂ are expected as electrodes of ECs because of their large oxidation state change (Mn⁴⁺ Mn²⁺), low cost and environmental compatibility. When all manganese ions in the MnO₂ are completely reduced to Mn²⁺ from Mn⁴⁺ over a potential window of 1.1 V, the theoretical capacity is estimated to about 2000 Fg^-1. However, the reported capacity of MnO₂ electrodes are 100250 Fg^-1 [1-2] for powders and around 700 Fg^-1 [3] for thin films. Tunnel structured MnO₂ are expected to show high capacities by utilizing high ionic mobility in the tunnel and high surface area of tunnel walls. Fig. 1 shows crystal structures of (a) Pyrolusite (Tunnel size: 1×1), (b) Hollandite (Tunnel size: 2×2) and (c) OMS-5 (Tunnel size: 2×4) and (d) MnO₆ unit. In the present study, the relationship between the tunnel size and the specific capacity was investigated for those tunnel structured MnO₂. In addition, Hollandite /carbon composites were synthesized to improve the electrode properties of Hollandite.

Abstract: Electrochemical properties of restacked nanosheets (RS-LDH) of nickel-aluminum layered double hydroxide were investigated for the first time. RS-LDH was prepared by delaminating nickel-aluminum layered double hydroxide powder (powder LDH) into nanosheets and restacking them. RS-LDH and powder LDH showed same discharge capacity of 230 mAhg¹ at a current density of 500 mAg¹, but RS-LDH showed a larger capacity than powder LDH at a large current density of 1000 mAg¹. This increase in capacity was assumed to be attributed to the increased specific surface area and ion conductivity.

Abstract: Lithium intercalation properties of lithium tetratitanate obtained by nanosheets process (NS-LT4) was examined and compared with those of conventional lithium tetratitanate. NS-LT4 was prepared by restacking of tetratitanate nanosheets with LiOH aqueous solution. NS-LT4 exhibited a reversible capacity of approximately 140 mAh g^-1, which corresponds to approximately two Li insertions per formula unit. Two Li insertions per formula unit mean that half of the Ti atoms were reduced from a tetravalent state to a trivalent state. The quasi open-circuit voltage of NS-LT4 was comparable with that of conventional lithium tetratitanate, and the voltage change of NS-LT4 as the change in lithium composition was greater than that of conventional lithium tetratitanate. This potential behavior would be caused by the unique stacking structure with stacking fault and random rotation in nanosheet-plane generated during the restacking of nanosheets.

Abstract: The lithium ion conducting properties of lithium- and MⅣ (M = Al, In, Y)-doped zirconium pyrophosphates synthesized via solid state reaction were investigated. The ionic conductivity of the compounds increased with increasing Li content. The activation energy of LixMxZr1-xP2O7 decreased as the lattice parameter increased owing to the enlargement of the size of bottleneck between the cavities. Li0.55(Li0.15Y0.1Zr0.75)P2O7 with a high Li content and a large lattice parameter exhibited a conductivity of 1.7×10-3 S cm-1 at 350 °C, which is sufficient for its application as a solid electrolyte for sensors.

Abstract: Lithium-ion conducting properties were investigated for a layered perovskite oxide Li2SrTa2O7 (LST) and defect-controlled LST, synthesized via solid state reactions. The ionic conductivities of A-site solid solutions Li2[Sr-(La2/3□1/3)-(La1/2Li1/2)]Ta2O7 (□ denotes vacancy.) suggested that lithium ions migrate in the Li-layer. The conductivity of Li-deficient (Li2-z□z)(LazSr1-z)Ta2O7 increased dramatically from 4.2 × 10-6 S cm-1 (z = 0, LST) to 1.6 × 10-3 S cm-1 (z = 0.2) at 400°C with increasing Li-vacancy concentration. This result obviously indicates that the conductivity of LST originate from the Li migration through vacancies in the Li-layer.

Abstract: The electrode properties of Ba0.5Sr0.5CoyFe1-yO3-δ {BSCF(5/5/10y/10(1-y))} on a Ce0.8Sm0.2O1.9 electrolyte were investigated. BSCF powders were synthesized by a solid-state reaction. The electronic conductivities of BSCFs increased with increasing Co content and BSCF(5/5/8/2) showed the highest conductivity of 38 S cm-1 at 600oC. The electrode resistance was measured on symmetric cells, BSCF/Ce0.8Sm0.2O1.9/BSCF, over the temperature range from 600oC to 800oC. The electrode resistances decreased with increasing Co content and BSCF(5/5/8/2) showed the lowest electrode resistance in the whole temperature range, having an electrode resistance of 0.62
cm2 at 600oC.

Abstract: Electrochemical properties of LiFePO4/carbon composites with various carbon materials were investigated to achieve high-rate charge and discharge properties. LiFePO4/carbon composites were synthesized by a pyrolysis of a LiFePO4 precursor solution added with porous graphite or particulate carbon powders. The LiFePO4/porous-graphite composite had a microstructure in which LiFePO4 particles existed on carbon surface and within pores. The LiFePO4/particulate-carbon composite had a microstructure in which each carbon particle was covered with LiFePO4 fine particles. The LiFePO4/porous-graphite and the LiFePO4/particulate-carbon composite electrodes showed high discharge capacities of 69 and 30 mAh g-1 at a high current density of 4000 mA g-1. The high electronic conductivity in the LiFePO4/porous-graphite composite contributed to achieving the large discharge capacity at the high current density.

Abstract: Effects of lattice defects on cathode properties of LiMn2O4 synthesized at low temperatures were investigated. LiMn2O4 powders were synthesized by a sol-gel method. The specific capacities of LiMn2O4 decreased from 134 to 81 mAh g-1 with decreasing heating temperature from 750 to 200°C. X-ray absorption spectroscopy showed that a large amount of lattice defects such as cation vacancies existed and cation mixing occurred in LiMn2O4 calcined at low temperatures. It was found that the low specific capacities of LiMn2O4 calcined at low temperatures were attributed to these lattice defects.

Abstract: Thin films of titanate were prepared by electrophoretic deposition (EPD) of a colloidal suspension of nanosheets, and their lithium intercalation properties were examined. Thickness of the obtained film increased approximately in proportion to the increase in deposition time and concentration of the colloidal suspension used for EPD bath. EPD method was revealed to be a convenient method for layer lamination of nanosheets. The reversible capacity for the obtained film was approximately 170 mA h g-1, and it was in common with anatase-type TiO2 or conventional titanate. Lithium diffusion coefficient along the thickness direction was estimated to be 6 × 10-14 cm2 sec-1.