Masahisa Fujimoto

Advanced Structures in Electrodeposited Tin Base Negative Electrodes for Lithium Secondary Batteries

Tin anodes deposited electrochemically on a copper foil current collector are studied to develop a next-generation lithium-ion battery with higher energy density. Better cycle performance through ten initial cycles under full charge and discharge conditions was attained by annealing tin electrodeposited on a rough surface copper foil. The annealing process was found to change the main active material from Sn to Cu 6 Sn 5 with some minor compounds. Furthermore, a microcolumnar structure of the active material portion was found to he self-organized in accordance with the surface profile of the foil during the first charge-discharge cycle. Advantages of these structural features are discussed in terms of the initial charge and discharge performance. including specific capacity and coulombic efficiency measured by using a three-electrode cell.

Characteristics of a lithium secondary battery using chemically-synthesized conductive polymers

Several conductive polymers (polyacetylene, polypyrrole, polythiophene, polyiminodibenzyl, polycarbazole, polyfuran, polyphenothiazine) have been synthesized by a chemical polymerization method and examined for their suitability as positive electrode material for lithium secondary batteries. A test cell, using polypyrrole as a positive electrode, showed good charge/discharge characteristics. Oxidizing agents for the synthesis of polypyrrole have been investigated for further improvement, and polypyrrole prepared by a new method of synthesis using a Cu(BF4)2/nitrile system exhibited excellent performance.

Effect of Electrode Parameters on LiFePO4 Cathodes

LiFePO 4 electrodes with thicknesses from 15 to 120 μm were coated on Al current collectors. The electrochemical characteristics of these electrodes depend strongly on film thickness, with the largest rate capability for the thinnest film-a 15-μm electrode can be discharged at a current rate of 25 C and still give a capacity of 70 mAh/g. This shows great promise for high-power applications such as hybrid electrical vehicles. Increasing the amount of carbon in the electrode, decreasing the packing density, or using an electrolyte with lower viscosity and higher ionic conductivity improved the rate performance. This suggests that the thickness effect is caused by a larger electrode resistance and a slower Li-ion conduction through the electrolyte for thicker films. Electrode thickness in turn affects the energy density of a battery, because the percentage of inactive materials increases with decreasing film thickness. An energy density prospect for a 18650-type battery with these LiFePO 4 electrodes gives a maximum capacity of 1050 mAh at 1-C rate for a 60-μm electrode. This corresponds to a volumetric and gravimetric energy density of 214 Wh/L and 96.5 Wh/kg, respectively. The effective Li diffusivity in the active material is estimated to be of the order of 10 -13 cm 2 /s.

Characteristics of polypyrrole chemically synthesized by various oxidizing reagents

Polypyrrole was chemically synthesized using various oxidizing reagents and examined as a positive electrode material. Physical properties, morphologies and electrochemical characteristics of polypyrrole were greatly influenced by the oxidizing reagent used for polymerization. In general, polypyrrole with a smaller particle size and a larger specific surface area showed better discharge performance than that with a larger particle size and a smaller specific surface area. Polypyrrole was also synthesized on various conducting and nonconducting substrates using Fe(C104)3 as an oxidizing reagent. By using PP nonwoven fabric as a substrate material, a high discharge capacity of 72 mA h g−1 was obtained. Polypyrrole synthesized on only one side of PP nonwoven fabric was able to be used as an electrode.

Influence of solvent species on the charge-discharge characteristics of a natural graphite electrode

Abstract The charge–discharge characteristics of a natural graphite electrode are examined in a mixed solvent composed of ethylene carbonate (EC) and propylene carbonate (PC). The characteristics are influenced largely by the solvent species. Natural graphite electrode displays good charge–discharge characteristics in an electrolyte containing EC with a high volume fraction. In an electrolyte containing PC, however, the electrode cannot be charged and the solvent is decomposed. X-ray photoelectron spectroscopy is used to obtain information about the surface of natural graphite. A thin LiF layer, the decomposition product of lithium hexafluorophosphate (LiPF6), is formed on the surface of the natural graphite charged to 0.5 V (vs. Li/Li+) in an electrolyte containing a high volume fraction of EC. On the other hand, LiF and a carbonate compound are formed in the bulk and on the surface of natural graphite when the volume fraction of PC is high. These results suggest that the thin LiF layer, which is produced at a potential higher than 0.5 V (vs. Li/Li+) on the surface of natural graphite, enables the lithium ions to intercalate into the natural graphite without further decomposition of the electrolyte.

Electrochemical behaviour of carbon electrodes in some electrolyte solutions

Abstract The electrochemical properties of coke and natural graphite in some electrolyte solutions containing diethylcarbonate (DEC) are studied. It is found that natural graphite exhibits am excellent performance, such as high discharge capacity (370 mAh −1 g), when a mixed solvent composed of ethylene carbonate (EC) and DEC is used. The charge/discharge characteristics of the coke electrode are mot influenced by the species of the electrolyte solution, but those of the natural graphite electrode are very much influenced by the species of the electrolyte solution. It is confirmed that there are three patterns in the behaviour of the graphite electrode in the electrolyte solutions tested in this investigation. In the first pattern, natural graphite can be charged to C 6 Li and them discharged. In the second pattern, the charging and discharging of the natural graphite electrode is impossible and destruction of the natural graphite crystal structure is observed. In the third pattern, lithium is intercalated into the graphite layer but the de-intercalation of lithium does not take place.

Electrochemical characteristics of polyaniline synthesized by various methods

Abstract The influence of polymerization conditions on the electrochemical properties and morphology of polyaniline are studied. It is found that the charge/discharge performance of chemically synthesized polyaniline is greatly influenced by the oxidizing reagents used for the polymerization. Cu(BF 4 ) 2 and Fe(ClO 4 ) 3 are particularly suitable reagents. Protons in the reaction media exert a strong influence on the morphology and electrochemical properties of polyaniline in both chemical and electrochemical polymerization processes. A novel process for chemically synthesizing polyaniline is developed to obtain polyaniline with superior electrochemical properties. In this process, polyaniline/ HBF 4 salt is oxidized with Cu(BF 4 ) 2 /acetonitrile and produces fibrous polyaniline with good doping/undoping reversibility.

Ion conductive material for secondary battery

A secondary battery is comprised of a positive electrode having a material intercalating alkali earth metal ions reversibly, a negative electrode having a carbon composition intercalating those ions reversibly and an electrolyte having an organic solvent and a solute. The solute includes an alkali earth metal salt. The organic solvent is selected from a group comprising ethylene carbonate (EC), dimethyl carbonate (DMC) and vinylene carbonate (VC). The carbon composition is selected from a group comprising coke, refined coke with 99% or more purity, organic compound produced by calcined cellulose, graphite and glassy carbon.

A study on electrolytes for manganese dioxide-lithium cells

Abstract The physical properties of organic electrolyte used in manganese dioxide-lithium cells play a major role in determining various cell characteristics. The influence on various cell characteristics of electrolytes has been investigated with flat cells. LiCF 3 SO 3 is the suitable solute in terms of low-temperature, storage and overdischarge characteristics. Mixture of ethylene carbonate (EC), 1,2-butylene carbonate (BC) and 1,2-dimethoxyethane (DME) is the suitable solvent in terms of high-rate discharge and storage characteristics.