Recently hydrogen absorbing alloys used as the negative electrode in alkaline battery have attracted much attention. The alloy absorbs and desorbs hydrogen reversibly, and the absorbed hydrogen is used as an active material. An effectively rechargeable battery using a hydrogen storage compound should have a large amount of capacity, high enough hydrogen diffusion rate to cause a small reaction resistance (overpotential) for high-rate charge/discharge, and low compositional change rate during repeated electrode reaction to prolong its cycle life.
The hydrogen absorbing alloys used in an alkaline battery were conventionally classified into an AB.sub.2 type and an AB.sub.5 type. The former had larger capacity but was more expensive. From the view of commercialization, the AB.sub.5 type would be more suitable. LaNi.sub.5 was chosen initially but the cycle life was too short.
To improve the cycle life, many compositions were proposed, such as MmNiCoMiAl system disclosed in JP63-175,339 and JP63-264,869 (1988). It was found that the partial replacement of nickel with Co and Al, as well as the substitution of the lanthanum content with mischmetal (a mixture of rare earth elements such as La, Ce, Pr, and Nd) were very useful in prolonging the cycle life.
Some other elements were added, too. In JP5-284,619 (1993), Zr was added to form an oxide film to prevent the other active elements from being oxidized. In U.S. Pat. No. 5,242,656 (1993), alkali metal was added to relieve the alkali metal ions M.sup.+ continuously in charge/discharge process of the alkaline battery, increasing the concentration of MOH within the battery, having the effect of protecting the cathodes and the anodes validly, and resulting in longer cycle life.
The substitution of Al, Zr, etc. was effective in improving the cycle life of the alloy. However, they increased the reaction resistance of the alloy, whereby the overpotential was increased and deteriorated the high-rate charge/discharge characteristics. H. S. Lim et al. reported in the 12th Battery Conference on Applications and Advances (1997) that the cycle life of LaNi.sub.4.8 In.sub.0.2 was shorter than that of LaNi.sub.5. In U.S. Pat. No. 4,925,748 (1990), In, Ga, etc. were added to raise the overvoltage in the hydrogen gas generation reaction so as to prevent the hydrogen generation in the process of high-rate charge. The atomic ratios of In and Ga were within the range of from 0.02 to 0.1. Nonetheless, the cycle life was not perfect.
On the other hand, to protect steel structures in sea water from being corroded, sacrificial anodes were used in cathode protection systems. Pure Aluminum supports a thin protective oxide film on the surface with an operational potential in sea water as nearly -0.8V (vs. standard hydrogen electrode), as reported in Material Protection 7 (1968) by B. M. Ponchel, which makes it useless as a pure metal in sacrificial anode protection system. However, the addition of very little In, Sn, Ga, Bi, Zn, Cd, Hg, etc. into aluminum alloys can depassivate the oxide film on the aluminum surface. By restraining or preventing the continuous formation of protective oxide film, those additives keep the activity of the aluminum surface with more electronegativity and higher exchange current density for sacrificial anode use. Among them, Al--In, Al--Zn, Al--Sn, Al--Zn--In, Al--Zn--Sn, Al--Zn--Ga, etc. are the most used alloys in sacrificial anodes.
On the contrary, Al added in the widely used alloy MmNi.sub.5-(a+b+c) CO.sub.a Mn.sub.b Al.sub.c, developed by Matsushita Co., Japan, can prevent the above hydrogen absorbing material from corroding by forming a dense oxide film, but the working current of the alloy is sacrificed.
That is to say, there is still difficulty in preparing hydrogen absorbing alloys that are satisfactory in all performances of discharge capacity, cycle life characteristics, and reactivity. By means of proper addition of the other elements, it is possible to enhance exchange current without significant deterioration of cycle life.