H. Miyamura

Electrochemical impedance and deterioration behavior of metal hydride electrodes

Abstract Electrochemical impedance spectroscopy (EIS) was applied to metal hydride electrodes. Cole-Cole plots for the electrodes consisted of two obvious semicircles and a slope related to Warburg impedance. The semicircle in the high-frequency region was mainly related to the resistance and capacitance between the current collector and the pellet of alloy powder. The semicircle in the low-frequency region, which exhibited appreciable dependence on hydrogen content, was attributed to electrode reactions on the alloy particles and double-layer capacitance on the alloy particles. Resistance and capacitance between alloy particles in the electrodes also need to be taken into account. Deterioration processes of metal hydride electrodes using a mischmetal-based alloy, MmNi 3.5 Co 0.7 Al 0.8 , were also studied employing EIS. Deterioration of a metal hydride electrode using copper-coated alloy powder was dominated by a decrease in reactivity of the alloy surface. In contrast, an increase in the contact resistances and a decrease in the amount of electrochemically utilizable alloy particles were significant in the deterioration of electrodes using uncoated alloy powder. Deterioration of the electrodes was avoided to some extent by elevating the hot-press temperature during electrode preparation.

Some factors affecting the cycle lives of LaNi5-based alloy electrodes of hydrogen batteries

The electrode characteristics, the hydrogen equilibrium properties, the crystallographic and mechanical properties of simple ternary alloys of the type LaNi5−xMx(M ≡ Ni, Mn, Cu, Cr, AlandCo) were examined. The effect of substituting elements to improve the cycle life increased in the order: M ≡ Mn, Ni, Cu, Cr, AlandCo. The lower the capacity, the smaller the volume expansion ratio, the slower the pulverizing rate and the lower the Vickers hardness were, the longer the cycle life became. The alloy LaNi2.5Co2.5, which showed the highest durability as a negative electrode, satisfied all these requirements.

Rechargeable hydrogen batteries using rare-earth-based hydrogen storage alloys

Abstract Electrochemical properties of low cost MmNi 5 -based-hydrogen storage alloys (Mm ≡ mischmetal) were extensively examined. The alloy MmNi 3.5 Co 0.7 Al 0.8 showed a very long cycle life with reasonable discharge capacity (250 mA h g −1 ) and rate capability. An ingot with a very high endurance and low lattice strain was obtained under controlled conditions, including high rate cooling in the casting process for obtaining a columnar structure, no heat treatment and prevention of stoichiometric deviation. A columnar structure was formed so that the c -axis of the hexagonal structure was oriented parallel to the cooling plane. This alloy was significantly distinguished in crystal growth from a manganese-containing alloy (MmNl 3.5 Co 0.5 Al 0.3 Mn 0.4 ) which had an equiaxial structure with considerable lattice strain, which needed conventional heat treatment. Deviation from stoichiometric composition to the nickel-rich side caused a significant decrease in capacity and cycle life owing to the precipitation of AlNi 3 at grain boundaries. The decay in capacity of the MH electrode using MmNi 3.6 Co 0.7 Al 0.8 was only 10% after 2000 cycles. The cylindrical sealed cell also showed a very long cycle life (a capacity decay of 6% after 2000 cycles). The high capacity sealed cell had a 1.5–2 times higher energy density (210 W h dm −3 ; 65 W h kg −1 ) with a longer cycle life and better rate capability than the high capacity Ni-Cd battery.

The influence of small amounts of added elements on various anode performance characteristics for LaNi2.5Co2.5-based alloys

The addition of small amounts of elements such as silicon, aluminium and titanium to LaNi2.5Co2.5 greatly influenced anode performance characteristics such as usable temperature range, capacity and its decay rate during repeated cycles, rate capability, low temperature dischargeability and self-discharge rate. The capacity decay was suppressed by the addition of silicon, but the rate capability decreased and the self-discharge rate increased. The alloy containing titanium exhibited a much longer cycle life, but much lower storage capacity and worse low temperature dischargeability. The addition of aluminium was very useful for improving the usable temperature range, cycle life and charge retention, and it did not cause too great a decrease in capacity and/or increase in overpotentials.

Rare-earth-based alloy electrodes for a nickel-metal hydride battery

Abstract Extensive work has been carried out on utilizing MmNi 5 -based alloys (Mm, mischmetal) as a low cost negative battery electrode. The replacement of nickel by cobalt was effective in improving cycle lifetime but caused a decrease in capacity and high rate capability. The replacement of lanthanum by large amounts of cerium of neodymium gave the alloy a satisfactory cycle lifetime with small amounts of cobalt present and without impairing the high rate capability. The alloy MmNi 3.5 Co 0.7 Al 0.8 was selected consistent with the above requirements. Induction-melting tests showed that the surface region with a columnar structure had a longer cycle lifetime than the inner region with an equiaxed structure. Annealing treatment also caused a decrese in the cycle lifetime. It was considered that alloys with smaller crystal grains had a longer cycle lifetime because the protective surface layer on the grain would remain effective after pulverization. The deviation from stoichiometric composition to the nickel-rich side or the Mm-rich side also caused a decrease in cycle lifetime, accompanied by the precipitation of nickel or Mm on the grain boundaries. It was concluded that the surface layer of the crystal grain played a very important role in preventing capacity decay.

Metal hydride electrodes based on solid solution type alloy TiV3Nix (0 ≦ x ≦ 0.75)

Abstract Microstructures, pressure-composition isotherms, and charge-discharge characteristics of TiV 2 Ni x (0 ≦ x ≦ 0.75) alloys were investigated. The alloys for x ≧ 0.25 composed of a vanadium rich (βTi, V) main phase and a TiNi based b.c.c. secondary phase. The secondary phase formed a three dimensional network, enhancing electrode kinetics of the alloys. The maximum discharge capacity was 420 A h kg −1 for x = 0.56.

Hydrogen storage alloys rapidly solidified by the melt-spinning method and their characteristics as metal hydride electrodes

Abstract Rapidly solidified LaNi5-based hydrogen storage alloys were prepared by a melt-spinning method. The prepared melt-spun alloy ribbon had very fine crystal grain of below 10 μm. The hydrogen absorption behavior and electrode properties of the alloys were greatly improved. Heat treatment at 400 °C which did not cause enlargement of the grain further improved these properties.

Nickel-metal hydride battery using microencapsulated alloys

Abstract Overcharging properties of nickel-metal hydride prismatic single cells (300 mA h) were examined in high pressure cells. The rise in pressure during overcharging increased with increasing charging rate and decreased with increasing content of copper or nickel in the surface coating on the alloy, i.e. in the following order: a mixed alloy containing 20 wt.% Cu or Ni; an alloy coated with 10 wt.% Cu or Ni; and an alloy coated with 20 wt.% Cu or Ni. The kind of coated metal used had no influence on the overcharging behavior. The gas in the cell using a 20wt.%Cu-coated alloy was mostly oxygen, while the cell using a copper-mixed alloy contained a large amount of hydrogen in addition to oxygen. The rapid pressure rise on overcharging was ascribed to insufficient charging of the metal hydride electrode. A sealed cell using a microencapsulated alloy showed a higher resistance to overcharging at higher charging rates.

Nickel-metal hydride battery for electric vehicles

When a nickel-powder-mixed alloy electrode using MmNi3.5Co0.7Al0.8 (Mm = mischmetal) was held in a complete discharge state at 40 °C, a severe capacity decrease due to passivation was observed. This capacity lowering was prevented by a nickel (copper) coating or by a cobalt powder mixing. The alloy MmNi3.8Co0.5Mn0.4Al0.3 did not cause passivation even for the nickel-mixed alloy. Prismatic nickel-metal hydride batteries (30–60 A h) were constructed using the alloy electrodes and evaluated as an electric vehicle battery.

Characterization of metal hydride electrodes by means of electrochemical impedance spectroscopy

Abstract Resistive components of various metal hydride electrodes were evaluated by means of electrochemical impedance spectroscopy. Examples of applications of the technique are reported: characterization of (1) the difference in performance between a misch-metal-based alloy electrode and a titanium-based alloy electrode and (2) the influence of the storage period of misch-metal-based alloy powder on the alloy surface activity.