Tetsuya Ozaki

Structural Analysis by Synchrotron XRD and XAFS for Manganese-Substituted α - and β -Type Nickel Hydroxide Electrode

The detailed structural change in the charge-discharge process for the 10 and 20 mol % manganese-substituted nickel hydroxide was investigated by using high-energy synchrotron X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). On the charge-discharge process, the 10 and 20 mol % manganese-substituted nickel hydroxide showed the β-Ni(OH) 2 /γ-NiOOH and the α-Ni(OH) 2 /γ-NiOOH phase transformation, respectively. The manganese ions were inserted on the only nickel sites for the 10 mol % manganese-substituted nickel hydroxide, and on both nickel sites and 18h sites for the 20 mol % manganese-substituted nickel hydroxide. The α-Ni(OH) 2 structure could be stabilized by the presence of the manganese ions on the 18h sites. The structural refinement for the manganese-substituted nickel hydroxides has been done successfully on the basis of two phase models of the ideal phases and the fault ones. As compared to ideal phases, the fault phases were characterized by the shift of the nickel atoms, and showed a larger amount of the intercalated potassium ions and H 2 O (OH - ) molecules in the interlayer. The occupancy sites for the potassium ions and H 2 O molecules were contained for the refinements, bringing about a better agreement between the observed and calculated patterns.

A Nickel Electrode with Ni-Coated 3D Steel Sheet for Hybrid Electric Vehicle Applications

A Ni-coated 3D steel sheet, at a quarter of the cost of conventional Ni foam, was designed as a substrate for the positive electrode of a nickel/metal hydride battery. A new thin positive electrode was developed using this substrate, and the electrochemical properties were investigated. The thin positive electrode using the Ni-coated 3D sheet and polystyrene acrylic acid ester (PSA) as a binder maintained a discharge capacity of more than 80% of initial capacity even after 1000 cycles in cycle test under 25°C, which was comparable to the Ni foam electrode. However, the electrode showed capacity fade after 180 cycles in cycle test under 45°C, because of the decomposition of PSA. The results of Fourier transform IR analysis indicated that styrene-maleic acid resin (SMA) had high stability in alkaline solution at high temperatures and was more suitable as a binder than PSA. A new battery for hybrid electric vehicles (HEVs) was developed using the thin positive electrode with the Ni-coated 3D sheet and SMA. The power density of the new battery achieved 1055 W/kg, compared with 480 W/kg for the conventional one. In the HEV mode cycle test, both the capacity and the power output of the cell remained at more than 80% of the initial value even after 25,000 cycles.

Non-Foam Nickel Electrode with Quasi-Three-Dimensional Substrate for Ni-MH Battery

New paste-type positive electrodes for nickel/metal-hydride (Ni/MH) batteries were prepared using low-cost quasi-three-dimensional (Q-3D) substrate replacing conventional Ni foam, and their electrochemical properties werestudied. Q-3D substrate showed better packing efficiency of active material than punched metal. By using highly conductive CoOOH-coated Ni(OH) 2 as an active material, an unsealed cell with Q-3D substrate electrode showed relatively low resistance and 98% of utilization efficiency at 0.2 C discharging rate. A sealed cell with this positive electrode was slightly inferior to that with conventional Ni foam electrode in high rate capability and durability, but was able to discharge at 10 C rate and attained 510 cycles of cycle life. Swelling of the positive electrode and distribution of electrolyte in cells were investigated by means of X-ray computed tomography and atomic absorption spectrometry after cycle testing, and it was found that the cycle degradation was due to increase in cell resistance associated with transfer of electrolyte from the separator to the positive electrode.

Structural Analysis Using Synchrotron XRD and XAFS for Cobalt Oxyhydroxides Heat-Treated under Sodium Hydroxide Solution for Nickel Hydroxide Electrode

The structural analysis for the cobalt oxyhydroxide has been done by using high-energy synchrotron X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) analysis. The relationship between the structure and the electrical conductivity for cobalt oxyhydroxide was investigated. The structural refinement for cobalt oxyhydroxide heat-treated in the temperature range of 80-160°C has been done successfully on the basis of two phase models (L and S phase) with large and small c lattice constants. With increasing treatment temperature, the phase abundance for the L phases was increased, whereas the one for the S phases was decreased. By heat-treatment above 100°C, the cobalt ions for the cobalt oxyhydroxide were oxidized to the higher oxidation state over 3. The electrical resistivity was extremely decreased by treatment temperatures above 100°C. The increase in the electrical conductivity for the cobalt oxyhydroxide could be explained by the increase in cobalt oxidation state.

Non-Foam-Type Nickel Electrodes Using Various Binders for Ni-MH Batteries

New paste-type positive electrodes for nickel/metal-hydride (Ni/MH) batteries were prepared using low-cost punched metal substrate and binders with high adhesive strength. Their charge-discharge properties were evaluated using cylindrical sealed batteries. Electrochemical stability of the binders was investigated by means of cyclic voltammetry. For sealed batteries using epoxy resin, styrene butadiene rubber, and poly(vinyl acetate) as positive electrode binders, cell voltage continued to increase at the end stage of charging, and leakage was observed after charging test at 1 C rate. In cyclic voltammetry of these binders in alkaline solution, the anodic peak current was much larger than the cathodic peak current, suggesting their electrochemical oxidation. As a mechanism of leakage, consumption of oxygen by oxidation of binder and consequent formation of hydrogen at the negative electrode was proposed. The battery using polystyrene acrylic acid ester (PSA) as a binder showed no leakage during charging and the highest charge efficiency of more than 80% at 1 C rate. The cycle life of this battery was longer than 350 cycles. It was demonstrated that PSA is a suitable binder for electrodes with two-dimensional substrate.