Alkaline storage batteries using a hydrogen storage alloy are widely employed as power sources for various cordless appliances or electronic equipment and the like. Among them, nickel-metal hydride storage batteries are harmless to the environment, and have high energy density. For this reason, nickel-metal hydride storage batteries are considered promising as power sources for electric tools or electric vehicles which necessitate charging and discharging at a large current, and also as back-up power sources, for which a long-term reliability is required.
Generally, an MmNi5 alloy having a CaCu5 type crystal structure, in which Ni is partly replaced with metal such as Co, Mn or Al, is employed as a hydrogen storage alloy for use in the nickel-metal hydride storage battery. Here, Mm refers to a mixture of rare earth elements.
The hydrogen storage alloy is produced, for example, by the processes such as: casting in which molten alloy is flowed into a mold; roll-quenching in which molten alloy is flowed between twin rolls, followed by quenching; centrifugal spraying in which molten alloy is injected onto a high-speed rotating disc; and atomization in which molten alloy is sprayed into an inert gas.
In order to use an alloy produced by casting or roll-quenching for the electrodes of a nickel-metal hydride storage battery, it is necessary to crush the alloy into a particle size of not more than 100 μm, either by a mechanical process or through a step of absorbing/desorbing hydrogen gas. In contrast, atomization produces a fine spherical alloy powder, which does not require any crushing step.
An alloy powder having a rough surface produced by crushing is likely to be pulverized during charge/discharge cycles, whereas such pulverization is suppressed for a spherical alloy powder because of the uniform alloy structure. A spherical alloy powder has a smaller specific surface area than a crushed alloy powder, so that it is also superior in corrosion resistance. As described in Japanese Unexamined Patent Publication No. Hei 3-116655, a spherical alloy powder can be filled into electrodes at a higher density than a crushed alloy powder.
For the above reasons, a spherical alloy powder is mainly used. A spherical alloy powder, however, has a small specific surface area. Moreover, the contacts between the particles of the alloy powder and between the alloy powder and the conductive core material of the electrode are point contacts. Accordingly, an electrode using the spherical alloy powder has a disadvantage that its current collection performance is inferior to that of one using a crushed alloy powder. FIG. 1 shows a sectional schematic view of a conventional electrode using a spherical alloy powder. From FIG. 1, it can be seen that the contact between a conductive core material 1 and a spherical alloy powder 2 of the electrode is given by point contact.
In view of the above, Japanese Unexamined Patent Publication No. Hei 11-97002 proposes the combined use of a spherical alloy powder and a crushed alloy powder. Additionally, Japanese Unexamined Patent Publication No. Hei 11-283618 suggests an electrode having a two-layer structure consisting of one layer comprising a crushed alloy powder and another layer comprising a spherical alloy powder. In other words, the crushed alloy powder and the spherical alloy powder are used in combination in order to improve the current collection performance of the electrode.
However, a crushed alloy powder is more susceptible to pulverization during charge/discharge cycles than a spherical alloy powder. Accordingly, the current collection performance of electrodes containing a crushed alloy powder is decreased gradually. Moreover, the pulverization increases the surface area of the alloy powder in contact with an electrolyte, thereby causing the constituent elements of the alloy to be dissolved and leading to unnecessary consumption of the electrolyte. This results in problems such as the increase in internal resistance and deterioration in discharge characteristics or cycle characteristics of the battery.
The present invention solves the above-discussed problems and an object of the present invention is to provide an alkaline storage battery excellent in both discharge characteristics and cycle characteristics, and a hydrogen storage alloy electrode used therefor.