Hiroyoshi Nakamura

Saga University, Сага Япония, Saga, Japan

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Publications (31)86.47 Total impact

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    ABSTRACT: A novel ion intercalation cell was fabricated using a MCMB cathode and a TiO2(B) anode which delivers 142 Wh Kg-1 .0 V. During the charging process, the ceiling potential of cathode reaches to 5.4 V while the bottom potential of anode drops down to 1.4 V Vs Li/Li+ . The average working voltage of this device is 3.1 V which is significantly higher than that of other hybrid capacitors. The better rate performance of the fabricated cell mainly attribute to the structural features of TiO2(B) electrode due to fast lithium insertion/de-insertion processes through the open channel parallel to the b-axis of TiO2(B). The ion intercalation mechanism is discussed using 4-electrode cell and ex-situ XRD data. XRD data revealed that peak position of MCMB does not fully come back to its original position after first charge/discharge process of the cell indicating structural changes of the graphite cathode. The fabricated cell is inherently safer than lithium ion batteries especially at low temperate and at high charge discharge rates.
    International journal of electrochemical science 09/2014; 9:6975. · 3.73 Impact Factor
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    ABSTRACT: The performance of KS-6/TiO2 full cell has been studied at different cut-off voltages, charge/discharge rates and cathode to anode mass ratios. The charge/discharge mechanism of the full cell is elucidated by ex-situ XRD data and analyzing the voltage profiles of each electrode by means of 4-electrode cell. Capacity retention of the cell between the 3rd and 50th cycles is 95.5 % though the ceiling potential of KS-6 exceeds 5.1 V vs. Li/Li+. Under such conditions this device delivers 100 Wh.kg-1 based on the total weight of the cathode and anode materials. Thus the energy density of this device is higher than AC/TiO2 type capacitors due to the higher working voltage and capacity of the KS-6 cathode. The KS-6:TiO2 weight ratio and charging potential have a crucial influence on the overall performance of this device. The KS-6/TiO2 weight ratio could be increased up to 2 without a significant capacity fade of the cell in the voltage window 1.5-3.5 V. These results demonstrate that the electrolyte could be used as the sole source of Li+ allowing development of novel type of energy storage devices without lithium rich cathodes as the lithium source
    International journal of electrochemical science 01/2013; 2014(9):195. · 3.73 Impact Factor
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    ABSTRACT: Graphite samples with different amount of graphitization, rhombohedral phase, particle shapes and surface area are electrochemically investigated at 25 oC and -5 oC. It was found that the degree of graphitization and the amount of rhombohedral phase in graphite play an important role for the lithium intercalation/deintercalation and amount of lithium deposition on the graphite samples. Due to low conductivity of electrolyte and slow diffusion of lithium ions into the graphite interlayers, the charge/discharge capacity of graphite sample was found to be significantly low at -5 oC. In addition, lithium deposition occurs on graphite at -5 oC. The graphite samples with high graphitization can easily affect by ambient temperatures, which shows a reduced capacity at low temperatures. The graphite with high rhombohedral phase shows high lithium deposition on the graphite surface, because lithium ion does not diffuse into the interlayer of graphites and accumulates at the edge plane of graphite. Increasing pathway for lithium ion intercalation into the graphite could help accelerating lithium intercalation and decreasing amounts of lithium deposition on the graphite.
    Journal of Power Sources 01/2012; 199:293. · 5.26 Impact Factor
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    Electrochemistry Communications 10/2011; 13:1116. · 4.29 Impact Factor
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    ABSTRACT: A cell employing a graphite cathode and a molybdenum (VI) oxide (MoO3) anode is investigated as a possible energy storage device. Graphite cathode allows raising the voltage well above the cathode materials of LIBs without causing safety issues. The bottom potential of this anode is 2.0V vs. Li/Li+, which is well above the lithium plating potential. Pulse polarization experiment reveals that no lithium deposition occurs, which further enhances the safety of the graphite/MoO3 full cell. Charge/discharge mechanism of this system results from intercalation and de-intercalation of the PF6− in the cathode (KS-6) and Li+ in the anode (MoO3). This mechanism is supported by in situ X-ray diffraction data of the graphite/MoO3 cell recorded at various states of charge.
    Journal of Power Sources 05/2011; 196(18):7886-7890. · 5.26 Impact Factor
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    ABSTRACT: The performance of the graphite (KS-6)/MoO3 full cell has been studied at different cut-off voltages, rates, temperatures, cathode to anode mass ratios. It was found that cathode to anode weight ratio and charging potential are the most influential parameters which determine the cycle life of this device. Under optimized condition (graphite:MoO3 weight ratio 1 and operation voltage window 1.5-3.3 V), the capacity retention of the cell between the 5th and 500th cycles was found to be 91%. At 0.3 C and 10 C rates this device delivers 88.9 mAh.g-1 and 35.5 mAhg-1, respectively. High and low temperature performance of this device is superior to that of the conventional EDLCs. The charge/discharge mechanism of the full cell was elucidated by ex-situ XRD data and analyzing the voltage profiles of each electrode by 4-electrode cell. Partial amorphization of the MoO3 anode was confirmed by XRD and SEM data. These results indicate that the electrolyte could be used as the sole source of lithium ions to develop a novel type of energy storage devices which do not contain traditional lithium rich cathode materials.
    Journal of Power Sources 01/2011; · 5.26 Impact Factor
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    Journal of Power Sources 01/2011; 196:9820. · 5.26 Impact Factor
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    ABSTRACT: A graphite/TiO2 full cell has been developed as a new safety energy storage system using a highly safety process. The crystal structures of the anatase TiO2 electrode have been investigated with respect to the performance of the electrodes. Due to the large anion intercalation into the graphite positive electrode, the possible charging potential can be raised to around 5.3V against the Li/Li+ electrode, which is a higher charging voltage than lithium-ion batteries (maximum voltage is around 4.3V vs. Li/Li+). In situ XRD measurements have been carried out on both the cathode and anode electrodes of the graphite/TiO2 cell during the charge process to elucidate the intercalation mechanism.
    Electrochimica Acta - ELECTROCHIM ACTA. 01/2010; 55(24):7305-7309.
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    ABSTRACT: To meet the high current load requirement from the high energy density realized by metal oxide and high power density graphite, we propose a novel hybrid supercapacitor consisting of Nb2O5 and KS6 graphite in 1.0 M LiPF6-EC:DEC (1:2). This new system exhibits a sloping voltage profile from 2.7 to 3.5 V during charging and presents a high operating voltage plateau between 1.5 and 3.5 V during discharging. The cell was tested at a current density of 100 mA/g with a cut-off voltage between 3.0 and 1.0 V. This novel energy storage system delivers the highest initial discharge capacity of 55 mAh/g and exhibits a good cycle performance.
    Bulletin of the Korean Chemical Society 02/2009; 30(4):817-820. · 0.84 Impact Factor
  • Gumjae Park, Hiroyoshi Nakamura, Yunsung Lee, Masaki Yoshio
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    ABSTRACT: 1,3-Propane sultone (PS) additive for graphite electrodes was studied for propylene carbonate (PC) and ethylene carbonate (EC)-based electrolytes in lithium batteries. Decomposition of solvents with graphite electrodes could be remarkably suppressed by addition of the PS additive in the PC-based electrolyte, leading to improvement of electrochemical performances of the cells. The 1,3-propane sultone additive showed very interesting properties for the graphite electrode. It is predicted to give a solid electrolyte interphase (SEI) on the surface of the graphite prior to solvent decomposition and bring about effects that not only suppress lithium deposition on the graphite electrode surface, but also accelerate lithium intercalation, leading to formation of LiC6 onto graphite electrode.
    Journal of Power Sources 01/2009; 189(1):602-606. · 5.26 Impact Factor
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    ABSTRACT: Graphite/TiO2 electrochemical capacitor has been developed as an electric energy storage system with high capacity as well as high energy density. The crystal structure of anatase and rutile structure of TiO2 have investigated with respect to the performance of capacitors. in situ XRD measurements have been also carried out on the both cathode and anode during the charge of capacitor
    ECS Transactions 12/2008; 16(1).
  • Hiroyoshi Nakamura, Masaki Yoshio, Yunsung Lee, Koji Takano
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    ABSTRACT: A novel energy storage system was developed using graphite and Nb2O5 for cathode and anode materials, respectively. This system exhibited a sloping voltage profile from 2.7 to 3.5 V during charging and presented a high operating voltage plateau between 1.5 and 3.5 V during discharging process. The adsorption of anions to the edge plane regions in graphite happened in the voltage region between 0.0 and 2.7 V, but anion intercalation into graphite occurred between 2.9 and 3.5 V.
    ECS Transactions 12/2008; 16(1).
  • Bohua Deng, Hiroyoshi Nakamura, Masaki Yoshio
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    ABSTRACT: The nominal LiMn2O4 and Li-doped spinels with different oxygen stoichiometry were prepared and investigated for capacity fading upon cycling at elevated temperatures. The discharge plateau at 3.2V originating from oxygen defects in manganese spinels is observed to grow very quickly to nearly a maximum scale in initial 15 cycles at 60°C. Meanwhile, the majority of capacity fading is lost. Therefore, the quick capacity fading in the initial stage is associated with the increase of oxygen deficiencies or oxygen loss upon cycling. It is proposed that the oxygen loss is originated from the decomposition of instable spinel phases that containing little Li cations on the 8a sites ([□1]8a[Mn2−x]16d[O4−δ□δ]32e, etc.), which are formed upon charging to the upper voltage limit. This phenomenon is much severe for nominal LiMn2O4 spinels with oxygen deficiencies. After partial substitution of Mn with Li, part of the Li cations on the 8a sites will be retained upon charging to the upper voltage limit. Thereafter, the cycling performance can be improved for the stabilized spinel phases formed upon charging.
    Journal of Power Sources 01/2008; 180(2):864-868. · 5.26 Impact Factor
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    ABSTRACT: Layered LiNi0.5Mn0.5O2 was prepared by the carbonate co-precipitation method at 800, 900, 950, and 1000°C for 15 h in air and characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), ICP, cyclic voltammetry and charge-discharge cycling. The hexagonal lattice parameter obtained for the compound prepared at 900C is: a = 2.879 Å and c = 14.265 Å. XPS studies indicated that the oxidation states of nickel and manganese are +2 and +4, respectively. The initial discharge capacity of 200 mAh g-1 was obtained in the voltage range of 2.8-4.5 V at a specific current density of 0.2 mA cm-2 at 30C. The sample, when charged to 2.8-4.6 V gives initial discharge capacity of 206 mAh g-1 in the first cycle and still delivered 205 mAh g-1 after 30 cycles at 30C. The sample synthesized by the carbonate co-precipitation was obtained as a homogeneous mixture of Ni and Mn consisting of spherical particles.
    ITE LETTERS on Batteries, New Technologies & Medicine. 06/2007;
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    ABSTRACT: Several series of Li1±xMn2−yO4±δ and Li1.05AlyMn1.95−yO4±δ samples with different oxygen defect degree have been synthesized by controlling synthesis temperature and procedures. The cycle performance of spinel as cathode in lithium batteries has been correlated with oxygen deficiency. The structure change of spinel during charge has also been investigated with respect with oxygen deficiency.
    Journal of Power Sources 04/2007; 166(2):485-491. · 5.26 Impact Factor
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    ABSTRACT: Li1+xAlyMn2−x−yO4±δ spinel cathode materials for lithium-ion batteries have been prepared by two methods, a specific two-step and the conventional one-step solid-state calcination methods. Compared with the conventional method, the new two-step method can guarantee the oxygen stoichiometry in spinel samples as well as reduced surface area. These characters lead to the improvement in cycling performance of spinel cathode even at elevated temperature. Moreover, the increase in doping amount of Al into Mn-spinel contributes to smearing the oxygen deficiency at high calcination temperature (1000 °C). The oxygen stoichiometric spinel samples exhibited greatly improved cycling performance. Further, Mn dissolution from spinel cathodes into the electrolyte was sufficiently suppressed even at elevated temperature of 60 °C. This beneficial influence would be reflected more remarkably in the cycles of lithium-ion full cells (spinel/C).
    Electrochimica Acta 04/2007; 52(14):4708–4714. · 4.09 Impact Factor
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    ABSTRACT: Graphitic carbon instead of activated carbon has been employed as the positive electrode material in the activated carbon (AC)/carbon capacitors using organic electrolytes. The advantageous electrochemical performance of the AC/graphite capacitors has been investigated as compared with the AC/AC capacitors. The charge storage mechanism of anions on the graphite positive electrodes in the AC/graphite capacitors has been studied by in situ XRD measurements.
    Journal of Power Sources 01/2007; 169(2):375-380. · 5.26 Impact Factor
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    ABSTRACT: Improvement in Electrochemical Performance of an Orthorhombic LiMnO2-yXy at High Temperature
    ITE LETTERS on Batteries, New Technologies & Medicine. 11/2006;
  • Bohua Deng, Hiroyoshi Nakamura, Masaki Yoshio
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    ABSTRACT: The rate capacity especially the high rate discharge performance is another important aspect for the application of Mn-based spinel cathodes for EV/HEV power sources besides the cycling performance that is now intensively investigated. In this paper, spinel materials differing in chemical composition and thermal processing history were investigated by discharging at constant current rates from C/10 to 4 C at ambient temperature. It was found that the high-rate discharge capability of Mn-based spinels is very excellent if prepared at temperatures below 850 °C, no matter cation doping or not. In contrast, spinels synthesized over 950 °C showed much poorer high rate performance, and some kinds of impurities were proposed to be responsible for the deteriorated behavior. Annealing at lower temperatures was found to be useful for the significant improvement of the high rate discharge capability of Mn spinels.
    Journal of Power Sources 02/2005; 141(1):116-121. · 5.26 Impact Factor
  • Bohua Deng, Hiroyoshi Nakamura, Masaki Yoshio
    Electrochemical and Solid State Letters - ELECTROCHEM SOLID STATE LETT. 01/2005; 8(3).