Publications (11)0 Total impact
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Irina D Gocheva,
Yuichi Kamimura,
Takayuki Doi,
Shigeto Okada,
Juna'ichi Yamaki,
Tetsuaki Nishida,
雄一 上村,
貴之 土井,
重人 岡田,
準一 山木,
哲明 西田,
ユウイチ カミムラ,
タカユキ ドイ,
シゲト オカダ, ジュンイチ ヤマキ,
テツアキ ニシダ
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ABSTRACT: We have synthesized a Cryolite type iron hexafluoroferrates by mild chemistry complex formation reaction. For the first time their electrochemical properties as positive electrode materials in rechargeable batteries were studied. The alpha-Li3FeF6 delivered a 116 mAh/g reversible capacity at an average discharge voltage of 3.0 V maintaining good capacity retention of about 30 cycles at a 0.2 mA/cm2 rate. The preliminary results are reported.
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ABSTRACT: The decomposition of LiPF$_6$ electrolytes with water was studied by measuring the water content after storage. The solvents used are ethylene carbonate (EC) + diethyl carbonate (DEC), EC + dimethyl carbonate (DMC), propylene carbonate (PC) + DEC and PC + DMC. The experimental results were in good agreement with $-d[{\rm H}_2{\rm O}]/dt = k [{\rm H}_2{\rm O}]^2 [{\rm LiPF}_6]$. We obtained rate constant (k) from this empirical equation and rate constant (k) increased in the order of EC+DMC < EC+DEC < PC+DMC < PC+DEC. This order is inverse proportion to order of dielectric constant of these solvents. Non-ionized LiPF$_6$ dissociates to PF$_5$ and LiF in organic solvents and PF$_5$ reacts with water. High dielectric constant solvent increases the ionization of LiPF$_6$. Therefore, the reaction with water is suppressed in high dielectric constant solvent.
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Bui Thi Hang,
Minato Egashira,
Izumi Watanabe,
Satoshi Hata,
Shigeto Okada,
Jun-Ichi Yamaki,
Seong-Ho Yoon,
港 江頭,
和泉 渡辺,
聰 波多,
重人 岡田,
準一 山木,
ミナト エガシラ,
イズミ ワタナベ,
サトシ ハタ,
シゲト オカダ, ジュンイチ ヤマキ
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ABSTRACT: In order to find the adequate material for Fe/air battery anode，two kinds of materials，Fe_2O_3-nano-loaded carbon and Fe_2O_3-nano mixed carbon material using various carbon materials, were prepared．The Fe_2O_3-nano-loaded carbon material was prepared by loading Fe_2O_3 on carbon using a chemical method．Fe(NO3)3 was impregnated on carbon with different weight ratios of iron to carbon in an aqueous solution，and the mixture was dried and then calcined for 1 h at 400℃ in flowing Ar．For the Fe_2O_3-nano mixed carbon material, Fe_2O_3-nano powder was mixed with carbon by mechanical method. The effect of various carbons on the physical and electrochemical properties of both Fe_2O_3-nano-loaded carbon and Fe_2O_3-nano mixed carbon electrodes was investigated and compared using X-ray diffraction(XRD), transmission electron microscopy(TEM), scanning electron microscopy(SEM) along with X-ray energy-dispersive spectroscopy(EDS), cyclic voltammetry(CV) and galvanostatic cycling performance．Transmission electron microscopy(TEM) coupled with X-ray diffraction measurements revealed that Fe_2O_3-nanoparticles were distributed on the carbon surface for Fe_2O_3-nano-loaded carbon．This distribution is similar to that for Fe_2O_3-nano mixed carbon electrodes observed by SEM and EDS．The CV results indicated that the redox currents of the Fe_2O_3-nano-loaded carbon are larger than that of correspondence Fe_2O_3-nano mixed carbon．Such properties are expected to provide the larger capacity for Fe_2O_3-nano-loaded carbon electrode than Fe_2O_3-nano mixed carbon electrode.
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ABSTRACT: The current-potential relationships for the electrochemical oxidation of organic solvent electrolytes on microelectrodes are confirmed for various lithium salts, namely ${\rm LiClO}_4$, ${\rm LiBF}_4$, ${\rm LiCF}_3{\rm SO}_3$ and ${\rm LiPF}_6$. The oxidation is expected to occur by means of a two stage process; the chemical degradation of the solvent followed by the electrochemical generation of anion radical species, and the direct electrochemical oxidation of the solvent. The dependence of the oxidation current on temperature and the lithium salt concentration confirms this two step reaction feature. In other words, a reaction based on electrochemically generated anion radicals seems to occur below 4.8V vs. Li/Li$^+$ and direct oxidation of the solvent occurs above 4.8V. Above 4.8V, we cannot disregard the distribution of the diffusion-limiting current of the anion radical into the whole oxidation current.
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Shigeto Okada,
Shoichiro Sawa,
Minato Egashira,
Jun-Ichi Yamaki,
重人 岡田,
港 江頭,
準一 山木,
シゲト オカダ,
ショウイチロウ サワ,
ミナト エガシラ, ジュンイチ ヤマキ
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ABSTRACT: Four types of orthorhombic olivine LiMPO_4 (M:Co, Fe, Ni, Mn) were investigated as high voltage cathode active materials for lithium secondary batteries. LiCoPO_4 exhibited the highest 4.8V discharge plateau of 100mAh/g vs. Li/Li^+ after initial charging to 5.1V and its energy density was comparable to that of layeredrocksalt LiCoO_2 (120mAh/g×4V=480mWh/g). In contrast, the open circuit voltage profile of LiFePO_4 was 160mAh/g on the 3V plateau. TG-DSC measurements showed that the thermal stability of charged Li_<1-x>CoPO_4 is better than that of fully-charged Li_<1-x>CoO_2. The chemical state of Co in LiCoPO_4 was also analyzed using HRXRF.
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Yasushi Uebou,
Toshiyasu Kiyabu,
Shigeto Okada,
Jun-Ichi Yamaki,
泰史 上坊,
敏康 木藪,
重人 岡田,
準一 山木,
ヤスシ ウエボウ,
トシヤス キヤブ,
シゲト オカダ, ジュンイチ ヤマキ
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ABSTRACT: Insertion of sodium into phosphates Na_3M_2 (PO_4)_3 (M=Fe, V) were investigated electrochemically to determine the usefulness as the possible cathodes for sodium batteries. The discharge curves show that sodium ion can be accommodated into these hosts. Specific charges of up to 45mAhg^<-1> and 140mAhg^<-1> were obtained for Na/Na_3Fe_2 (PO_4)_3 and Na/Na_3V_2(PO_4)_3 cells with liquid organic electrolytes at room temperature, respectively. Intercalation processes under constant current densities of 0.1mAcm^<-2> were reversible within the range of composition 3.0<x<3.4 for Na_xFe_2 (PO_4)_3 and 1.2<x<3.8 for Na_xV_2 (PO_4)_3.
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Yasushi Uebou,
Shigeto Okada,
Minato Egashira,
Jun-Ichi Yamaki,
泰史 上坊,
重人 岡田,
港 江頭,
準一 山木,
ヤスシ ウエボウ,
シゲト オカダ,
ミナト エガシラ, ジュンイチ ヤマキ
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ABSTRACT: We synthesized the two pyrophosphates LiVP_2O_7 and TiP_2O_7 by a solid state reaction, and the cathode properties of lithium secondary batteries were investigated. The structures were characterized by powder X-ray Rietveld refinement. Lithium cells using LiVP_2O_7 and TiP_2O_7 cathodes showed that approximately 0.5 Li per unit could be reversibly deintercalated from LiVP_2O_7 at 4.1V vs. Li/Li^+, 0.8 Li per unit could be reversibly intercalated into TiP_2O_7 at 2.6V vs. Li/Li^+. The redox potentials 4.1V of LiVP_2O_7 and 2.6V of TiP_2O_7 were higher than that of transition metal oxides such as VO_2 and TiO_2.
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ABSTRACT: Lithium rechargeable cells with lithium metal anodes are widely considered to have the highest energy density among comparable cells. However the cycle life and thermal stability of these cells must be improved The poor performance of lithium metal cells is mainly explained by lithium dendrite growth. With a view to overcoming this problem, the lithium deposition mechanism was considered. It is suggested that lithium dendrites may be called whiskers. Because their tip morphology remains unchanged during their growth, they grow from the base in the same way as whiskers of tin from thin films under stress. The deposited lithium under the protective film will be stressed by non-uniform deposition. To release the stress, the protective film will be broken. To support my assumption, the possible morphology of the lithium was calculated with the boundary condition that pressure induced by the surface tension is the same throughout the lithium surface. The calculation indicated three types of shape depending on the value of the surface tension and internal pressure. It was possible that the surface tension may act to maintainthe size of the hole in the film during whisker extrusion based on assuming that the whisker has higher creep strength than bulk lithium.
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ABSTRACT: For a wider application in batteries and electrochemical capacitors, an improvement of the electrochemical stability of imidazolium cation - based room - temperature molten salt is required. We found that a certain type of combination of anions can change the anodic stability and other properties of molten salt systems. The 1:1 mixture of 1-ethyl-3-methyl imidazolium (EMI) tetrafluoroborate and EMI bis(trifluoromethylsulphonyl)imide (TFSI) showed a similar anodic stability to that of EMITFSI, the most anodically stable single anion molten salt. This effect may originate from the difference of association of both cation and neighborhood anions. A molten salt which has BF^-_4 and TFSI^- anions has a high stability toward an anodic oxidation and provides the highest energy density double-layer capacitor in this study by enlarging the cell voltage.
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哲也 川村,
港 江頭,
高明 園田,
重人 岡田,
準一 山木,
Tetsuya Kawamura,
Minato Egashira,
Takaaki Sonoda,
Shigeto Okada,
Jun-Ichi Yamaki,
テツヤ カワムラ,
ミナト エガシラ,
タカアキ ソノダ,
シゲト オカダ, ジュンイチ ヤマキ
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ABSTRACT: It is known that LiPF_6 reacts with water. In this study LiC1, LiF, LiBr and LiI were added to the LiPF_6 electrolyte with about 5000ppm water to inhibit the decomposition of LiPF_6. When 0.1M LiC1 was added, the LiPF_6 electrolyte with water did not decompose for 50 hours. From DSC measurement, it was found that the amount of an exothermic heat at 265℃ did not change at all for 48 hours when LiC1 was added. It suggests that LiPF_6 salt was not decomposed.
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重人 岡田,
智雄 高田,
港 江頭,
準一 山木,
Shigeto Okada,
Tomoo Takada,
Minato Egashira,
Jun-Ichi Yamaki,
シゲト オカダ,
トモオ タカダ,
ミナト エガシラ, ジュンイチ ヤマキ
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ABSTRACT: We carried out redox-couple and counter-cation exchange in NASICON-type M_2(XO_4)_3 (M: Fe and Al; X: S, Mo and W) to control the cathode performance. The Fe^<3+>/Fe^<2+> redox potential in NASICON matrix is changed by the local crystalline field at the iron resulting from polarization of oxygen towards the counter cation in the polyanionic complex. It is confirmed by ^<57>Fe Mossbauer resonance, Fe-2p XPS, Mo K-XAFS and electrochemical measurements. This inductive effect in NASICON matrix is a significant material control technique for cathode design. We also found a promising anode candidate by ion exchange of Fe by Al in Fe_2(MoO_4)_3. The reversible capacity of Al_2(MoO_4)_3 reached almost 200mAh/g between 3.5V and 1.2V.
