Masaru Yao

Indigo carmine: An organic crystal as a positive-electrode material for rechargeable sodium batteries

Using sodium, instead of lithium, in rechargeable batteries is a way to circumvent the lithiums resource problem. The challenge is to find an electrode material that can reversibly undergo redox reactions in a sodium-electrolyte at the desired electrochemical potential. We proved that indigo carmine (IC, 5,5′-indigodisulfonic acid sodium salt) can work as a positive-electrode material in not only a lithium-, but also a sodium-electrolyte. The discharge capacity of the IC-electrode was ~100 mAh g−1 with a good cycle stability in either the Na or Li electrolyte, in which the average voltage was 1.8 V vs. Na+/Na and 2.2 V vs. Li+/Li, respectively. Two Na ions per IC are stored in the electrode during the discharge, testifying to the two-electron redox reaction. An X-ray diffraction analysis revealed a layer structure for the IC powder and the DFT calculation suggested the formation of a band-like structure in the crystal.

Organic Positive-Electrode Materials Based on Dialkoxybenzoquinone Derivatives for Use in Rechargeable Lithium Batteries

The performance of 2,5-dialkoxy-1,4-benzoquinone derivatives as positive-electrode active materials for use in rechargeable lithium batteries was investigated. These derivatives showed stepwise two-electron redox behaviors in cyclic voltammetry, and also exhibited high initial discharge capacities of more than 200 mAh g in charge/discharge tests. In particular, a positive-electrode that used a methoxy derivative showed the largest discharge capacity (close to 300 mAh g) among the dialkoxy derivatives tested. These discharge curves imply benzoquinone-based twoelectron redox behavior. Theoretical quantum calculations based on the density functional theory (DFT) were also performed to discuss the electrochemical properties of these materials, and supported the experimental results.

Electrocatalytic oxidation of alcohols by a carbon-supported Rh porphyrin.

A Rh porphyrin on carbon black was shown to catalyze the electro-oxidation of several aliphatic alcohols (ethanol, 1-propanol, and 2-propanol) and benzyl alcohols. The overpotentials for alcohol oxidation were very low. The reaction mechanism and substrate specificity are discussed.

A pentakis-fused tetrathiafulvalene system extended by cyclohexene-1,4-diylidenes: a new positive electrode material for rechargeable batteries utilizing ten electron redox

Pentakis-fused TTF derivatives extended with two cyclohexene-1,4-diylidenes (2a and 2b) have been successfully synthesized. Cyclic voltammetry of the tetrakis(2-ethylhexylthio) derivative (2b) has revealed that it exhibits five stages of the redox process to form 2b10+. A coin-type cell composed of a positive electrode incorporating the tetrakis(methylthio) derivative (2a) exhibits a discharge capacity of 196 mA h g−1, which corresponds to participation of a ten electron redox. It shows a high performance of energy density of 700 mW h g−1 and an output power density of 69 W g−1 with a relatively stable cycle-life performance (72% of the initial discharge capacity after 30 cycles).

Nickel Substrate Having Three-Dimensional Micronetwork Structure for High-Power Nickel/Metal-Hydride Battery

A low-cost substrate for the positive electrode of nickel/metal-hydride battery was developed by using a nonwoven cloth with a three-dimensional micronetwork structure, in which the amount of plated nickel was less than half compared with the conventional foam-type substrate. The substrate was designed to have a finer structure than the foam-type substrate. The cylindrical sealed cell using the substrate exhibited superior high-rate discharging capability to the cell using the foam-type substrate, despite the reduction of plated nickel. The characteristic microporous structure of the substrate presumably contributes to the improvement of high-rate discharge performance.

Molecular ion battery: a rechargeable system without using any elemental ions as a charge carrier

Is it possible to exceed the lithium redox potential in electrochemical systems? It seems impossible to exceed the lithium potential because the redox potential of the elemental lithium is the lowest among all the elements, which contributes to the high voltage characteristics of the widely used lithium ion battery. However, it should be possible when we use a molecule-based ion which is not reduced even at the lithium potential in principle. Here we propose a new model system using a molecular electrolyte salt with polymer-based active materials in order to verify whether a molecular ion species serves as a charge carrier. Although the potential of the negative-electrode is not yet lower than that of lithium at present, this study reveals that a molecular ion can work as a charge carrier in a battery and the system is certainly a molecular ion-based “rocking chair” type battery.

High-capacity electric double layer capacitor using three-dimensional porous current collector

A three-dimensional porous current collector made of a nickel-chromium alloy was developed based on foamed polyurethane. The foam-type substrate exhibited a high tolerance at high voltages in nonaqueous electrochemical systems. The electric double layer capacitor (EDLC) using the foam-type substrate exhibited stable charge/discharge behavior and showed a superior high-rate discharge capability as compared to an EDLC using the conventional aluminum foil. Furthermore, impedance analysis revealed an improved current collecting ability for the foam-type substrate. To improve the performance of EDLCs, a methodology that uses a three-dimensional current collector was described.

Influence of Nickel Foam Pore Structure on the High-Rate Capability of Nickel/Metal-Hydride Batteries

The influence of nickel foam pore structure on the high-rate capability of nickel/metal-hydride (Ni/MH) batteries was studied to develop a substrate suitable for high-power use. Three types of nickel foam substrates with different pore structures were used for preparing the sealed cells. Among the three prepared sealed cells, the cell using the substrate with a smallest pore diameter and largest surface area exhibited the best high-rate discharging performance. The electrochemical impedance measurement also showed a close relationship between the charge-transfer resistance and the pore structure of the substrate. The contact area between the current collector and the active material is considered to significantly affect the contact resistance and the charge-transfer resistance. The pore-size reduction and the enlargement of the surface area of the substrate were revealed to be effective in improving the high-rate performance of Ni/MH batteries.