1. Filed of the Invention
The present invention relates to a method for producing hydrogen storage alloy particles which can absorb and desorb hydrogen in a reversible manner and to a sealed-type nickel-metal hydride storage battery using the same alloy particles.
2. Description of the Related Art
Recently, nickel-metal hydride storage batteries utilizing a negative electrode comprising a metal hydride, i.e., a hydrogen storage alloy, capable of absorbing and desorbing hydrogen in a reversible manner at a low pressure have been developed and have been attracting attention as clean rechargeable batteries having a long cycle life and a high energy density and being substantially free from environmental concern.
The long cycle life and the high energy density of the batteries are based on the least probability of forming and subsequent deformation of dendrites theoretically, which might be a cause for short-circuiting. The least environmental concern is based on the constituent of the batteries containing no harmful substance such as cadmium (Cd).
The hydrogen storage alloy can be classified into two general groups, an AB.sub.5 type consisting mainly of rare earth elements and nickel (Ni), and an AB.sub.2 type consisting mainly of zirconium (Zr) and manganese (Mn).
Heretofore, an alloy of the above-mentioned AB.sub.5 type has been employed as the material for negative electrode Of the rechargeable batteries. The battery characteristics such as discharge capacity, inner pressure, preserving property and cycle life are well-balanced in the AB.sub.5 type alloy.
Typical manufacturing process of the negative electrode comprising the AB.sub.5 type alloy can be exemplified as follows. First, each of the predetermined amounts of the constituent metals of the hydrogen storage alloy is placed in a melting furnace such as high frequency melting furnace and molten completely therein. The melt is then poured into a water-cooled casting die to produce an ingot of the hydrogen storage alloy. After the ingot is subjected to an annealing treatment under vacuum or an argon atmosphere, it is mechanically pulverized by a jaw crusher or a jet mill into a powder having an average particle diameter of about 20-30 .mu.m (casting and pulverizing process).
Next, in order to enhance the electrode activity of the surface of the powder, the powder is subjected to a so-called alkali treatment whereby it is soaked in an aqueous solution of potassium hydroxide at a high temperature of 60.degree.-90.degree. C. for about 20 minutes to about 4 hours, and washed with water to obtain the hydrogen storage alloy powder for the negative electrode.
A paste for the negative electrode is prepared by mixing the powder thus obtained with a viscosity-enhancing agent such as carboxymethyl cellulose or polyvinyl alcohol, a rubber binder and water, as well as an electrically conductive agent such as carbon if required. A conductive core acting as a current collector of the negative electrode such as a punched or perforated metal sheet is coated with this paste, and the coated sheet is dried and pressed. In this manner, the negative electrode wherein bonding forces among the constituents of the electrode are further strengthened is produced.
In the above-mentioned manufacturing process of the negative electrode, fine powders having a smaller particle size than 10 .mu.m are however inevitably produced in a considerable amount during the pulverizing step of the alloy. The fine powders can hardly participate in a function of the negative electrode. The surfaces of the fine powders are very active and are covered with firm oxide films, and thus can not absorb or desorb the hydrogen. For this reason, the fine powders bring a decrease in the energy density and another decrease in the high-rate discharge characteristics of the battery configured with the hydrogen storage alloy powder containing such fine powders.
Further, the removal of the fine powders requires a large hike in the manufacturing cost. Since the alloy of the AB.sub.2 type is harder than the AB.sub.5 type alloy, it requires a longer time period in the pulverizing step than that of the AB.sub.5 type alloy, leading to a higher manufacturing cost.
Undesirable subsequent pulverization of the hydrogen storage alloy powder during the charging and discharging process can be pointed out as a cause for deteriorating the cycle life in the test. The hydrogen storage alloy powders obtained by the mechanical pulverization each have a polygonal shape with planes of cleavage containing acute angles, and thus have a disadvantage that they are liable to be further pulverized from the sites with acute angles by a volumetric expansion and contraction during the charging and discharging process.
In order to overcome this disadvantage, a pulverization by means of inert gas atomizing or centrifugal spraying has since been devised and proposed in, for instance, U.S. Pat. No. 5,219,678. The hydrogen storage alloy powders produced by the inert gas atomizing process or the centrifugal spraying process are themselves having substantially a spherical shape or a shape approximating to a sphere because the process contains no mechanical pulverizing step.
However, the inert gas atomizing process or the centrifugal spraying process is only capable of producing particles with an average particle diameter of about 50-60 .mu.m which is too large to obtain an optimum result, and thus requires a separate subsequent pulverizing step. Therefore, the process brings a decrease in the cycle life characteristics and a hike in the manufacturing cost. Further, the inert gas atomizing process requires inert gas such as expensive argon gas in large quantities, and thus any great reduction in the manufacturing cost cannot be expected.