Hiromasa Ikuta

High ionic conductivity in lithium lanthanum titanate

It has been discovered that the polycrystalline lithium lanthanum titanate Li0.34(1)La0.51(1)TiO2.94(2) shows high ionic conductivity more than 2 × 10−5 S cm−1 (D.C. method) at room temperature, which is compared with that of Li3.5V0.5Ge0.5O4. This compound has cubic perovskite structure whose cell parameter is 3.8710(2) A. By a.c. impedance analysis, the equivalent circuit of the sample could be divided into two parts; bulk crystal and grain boundary. The ionic conductivity of the bulk part is as high as 1 × 10−3 S cm−1 at room temperature. Such a high conductivity is considered to be attributed to the presence of a lot of equivalent sites for lithium ion to occupy and freely move in this perovskite. In addition, this compound is easy to react with lithium metal and the electronic conductivity has become much higher than before being in contact with Li. It can be explained that titanium ion was reduced by ii insertion into a vacant site and then an electron carrier was introduced.

The Spinel Phases LiM y Mn2 − y O 4 (M = Co, Cr, Ni) as the Cathode for Rechargeable Lithium Batteries

To improve the cycle performance of spinel LiMn 2 O 4 as the cathode of 4 V class lithium secondary batteries, the quaternary spinel phases LiM y Mn 2-y O 4 (M = Co, Cr, Ni ; y = 1/12, 1/9, 1/6, 1/3) were prepared at 750°C in air. X-ray diffraction and electrochemical studies were carried out using the Li/Li x M y Mn 2-y O 4 cells. These cathodes were more tolerant to repeated lithium extraction and insertion than a standard LiMn 2 O 4 spinel electrode in spite of a small reduction in the initial capacity. Considering both cycle life and capacity density, LiCo 1/6 Mn 11/6 O 4 showed good cycle performance with an energy density of 370 Wh kg at the 300th cycle. The improvement in cycling performance is attributed to the stabilization in the spinel structure by the doped metal cations.

The spinel phases LiM{sub y}Mn{sub 2{minus}y}O{sub 4} (M = Co, Cr, Ni) as the cathode for rechargeable lithium batteries

To improve the cycle performance of spinel LiMn 2 O 4 as the cathode of 4 V class lithium secondary batteries, the quaternary spinel phases LiM y Mn 2-y O 4 (M = Co, Cr, Ni ; y = 1/12, 1/9, 1/6, 1/3) were prepared at 750°C in air. X-ray diffraction and electrochemical studies were carried out using the Li/Li x M y Mn 2-y O 4 cells. These cathodes were more tolerant to repeated lithium extraction and insertion than a standard LiMn 2 O 4 spinel electrode in spite of a small reduction in the initial capacity. Considering both cycle life and capacity density, LiCo 1/6 Mn 11/6 O 4 showed good cycle performance with an energy density of 370 Wh kg at the 300th cycle. The improvement in cycling performance is attributed to the stabilization in the spinel structure by the doped metal cations.

The spinel phases LiAlyMn2−yO4 (y=0, 1/12, 1/9, 1/6, 1/3) and Li(Al,M)1/6Mn11/6O4 (M=Cr, Co) as the cathode for rechargeable lithium batteries

In order to improve the cycle performance of LiMn2O4, the spinel phases LiAlyMn2−yO4 (y=0, 1/12, 1/9, 1/6, 1/3) and Li(Al,M)1/6Mn11/6O4 (M=Cr, Co) were prepared at 650°C for 24 h in air. By substituting, the cycle performance was improved at the expense of an initial capacity. For LiAl1/12Cr1/12Mn11/6O4, the capacity of the cell maintained 90% of the initial capacity at the 200th cycle. By substituting, the chemical diffusion coefficient of lithium ion in the cathodes increased by one order of magnitude comparing with that of parent LiMn2O4. The phase transition due to the Jahn–Teller distortion caused by Mn3+ was suppressed in these partially substituted spinels.

Synthesis and characterization of MnV2O6 as a high capacity anode material for a lithium secondary battery

Abstract Crystalline MnV 2 O 6 has been synthesized by a polymer gellation method and investigated for its physical and electrochemical properties as an anode material for Li secondary battery. The physical characterization was carried out by thermal analysis (TG/DTA), FT–IR and SEM. Structural analysis by powder XRD and spectroscopic analysis by XANES showed that the synthesized compound is MnV 2 O 6 with brannerite structure. The Li insertion of MnV 2 O 6 anode during the first charge showed a large capacity of about 1400 mAh/g, accompanied by irreversible structural transformation into amorphous material. Despite its structural transformation to amorphous during the first lithiation, subsequent cycles showed a capacity of about 800 mAh/g. This paper presents the advantage of this material over existing anode material and discusses the mechanism underlying the electrode process.

Amorphous MoS2 as the cathode of lithium secondary batteries

Amorphous MoS2(a-MoS2) was synthesized by the thermal decomposition of (NH4)2MoS4 in a hydrogen gas flow at temperatures from 150 to 300 °C. The a-MoS2 synthesized at 150 and 200 °C were almost amorphous. However, those prepared at 250 and 300 °C showed very small and broad peaks in powder X-ray diffraction patterns. Discharge/charge cycling measurements, revealed that the sample prepared at 150 °C showed the highest cycle capacity (100 Ah/kg) at the 100th cycle. The chemical diffusion coefficients of lithium were also highest for a-MoS2 prepared at 150 °C, and the D values fell in the range of −10

Reaction mechanisms of MnMoO4 for high capacity anode material of Li secondary battery

Crystalline MnMoO4 was synthesized using a conventional solid reaction method and investigated for its physical and electrochemical properties as an anode material for Li secondary battery. The reversible amount of Li insertion/removal of MnMoO4 anode during the first cycle was about 800 mA h/g, accompanied by irreversible structural transformation into amorphous material. The amorphization during the first Li insertion was investigated by structural analysis using XRD of electrode. The charge compensation during Li insertion/removal was examined by measurement of X-ray Absorption Near Edge Structure (XANES) spectroscopy. Despite its irreversible structural transformation to amorphous during the first lithiation, subsequent cycles showed a reasonable cyclability. This paper presents the electrochemical properties of MnMoO4 and discusses the mechanism underlying the Li insertion/removal process.

Structural phase transition of the spinel-type oxide LiMn2O4

Abstract The crystal structure of the low-temperature (LT) form of spinel-type LiMn2O4 has been studied by neutron and X-ray powder diffraction as well as electron diffraction. Reflections of the LT phase in its neutron and X-ray diffraction patterns were indexed on the basis of not a tetragonal unit cell but roughly an orthorhombic one (space group Fddd) with lattice parameters a=8.2797(2), b=8.2444(3) and c=8.1981(2) A. However, the presence of extra reflections which cannot be indexed with this unit cell shows that the actual lattice parameters are different from the above ones. Superlattice reflections, which were missing in powder diffraction patterns, with tripled periodicity were observed in electron diffraction patterns of the LT phase.

Polymer electrolyte plasticized with PEG-borate ester having high ionic conductivity and thermal stability

Abstract We have focused on the PEG-borate ester as a new type of plasticizer for solid polymer electrolyte composed of poly(ethyleneglycol) methacrylate (PEGMA) and lithium bis-trifluoromethanesulfonimide (LiTFSI). The PEG-borate ester shows good thermal stability and high flash point. Ionic conductivity of the polymer electrolyte increases with increasing amount of the PEG-borate ester and exhibits values greater than 10−4 S cm−1 at 30 °C and 10−3 S cm−1 at 60 °C. Furthermore, PEG-borate ester has three EO chains whose lengths are variable, and various ionic conductivities are expected to depend on EO chain length. As a result, polymer electrolyte containing the PEG-borate ester whose EO chain length is n=3 shows highest ionic conductivity. Furthermore, polymer electrolytes containing PEG-borate esters show excellent thermal and electrochemical stability. The electrolytes are thermally stable up to 300 °C and electrochemically up to 4.5 V vs. Li+/Li.

Single phase region of cation substituted spinel LiMyMn2-yO4-δ (M = Cr, Co and Ni) and cathode property for lithium secondary battery

Abstract Non-stoichiometric spinel oxides LiM y Mn 2− y O 4−δ ( y =0, 1/6 and 1/3; M=Cr, Co and Ni) (space group Fd3m) were prepared by controlling oxygen partial pressure at 1023 K. The single phase region of each LiM y Mn 2− y O 4−δ was larger than that of parent LiMn 2 O 4−δ (0≤δ y increased in LiM y Mn 2− y O 4 , the more the δ extended at 1023 K (e.g. 0≤δ 1/6 Mn 11/6 O 4−δ and 0≤δ 1/3 Mn 5/3 O 4−δ ). The defect model for LiM y Mn 2− y O 4−δ was estimated from the data of the cell volume and the measured density. These defect structures were assumed to form metal excess structure (Li) 8 a [Li δ/(4−δ) M δ y /(4−δ) Mn δ(2− y )/(4−δ) ] 16 c [M y Mn 2− y ] 16 d [O 4 ] 32 e which has been the same as that of the parent LiMn 2 O 4−δ published previously. As the cathode for the lithium secondary battery, the initial capacity of LiCo 1/6 Mn 11/6 O 4−δ with δ=0.040 had a value of 127 Ah kg −1 . It was about 11% larger than that of the spinel with δ=0. The spinel with the defect showed a good cycle performance.