In recent years, a nickel hydride storage battery has attracted public attention because of its high energy density and low pollution as compared with a conventional nickel-cadmium storage battery. Further, many studies and developments have been made on it for use as power sources for portable equipment and electric motor etc. In the nickel hydride storage battery, a hydrogen absorbing electrode using a hydrogen absorbing alloy, which can absorb and release hydrogen in a reversible manner, is used for a negative electrode and a nickel electrode utilizing a nickel hydroxide as an active material is used for a positive electrode.
This nickel hydride storage battery is used for a sealed-type storage battery. In this case, its negative electrode capacity is made larger than its positive electrode capacity, so that oxygen gas produced from the positive electrode at time of over-charging is consumed by the negative electrode. Thereby, a sealed system is formed.
However, when charge/discharge cycles are repeated, a gas absorbing performance and a charge efficiency at the negative electrode are reduced due to oxidation etc. of the hydrogen absorbing alloy. For this reason, there have been such problems as a rise in battery inside pressure, an increase in internal resistance due to loss of electrolyte, and a lowering of battery performance.
It is well known that a surface of the hydrogen absorbing electrode deteriorated by oxidation etc. is covered with a lot of needle-like products and these needle-like products are composed of hydroxides of rare earth elements etc. These needle-like products are produced in such a manner that rare earth element forming principal component element of the hydrogen absorbing alloy is eluted and deposited. The products grow with an increase in charge/discharge cycle number, reduce a conductivity and lessen an utilization of negative electrode capacity.
As a means for solving such a phenomenon, a method is employed now wherein corrosion resistance is improved by decreasing a quantity of La in the hydrogen absorbing alloy. This is because La is strongest in its basicity among the rare earth element forming the component element of hydrogen absorbing alloy. However, this method has included such problems as a small effect of the improvement in corrosion resistance and a decrease in discharge capacity.
On the other hand, alkaline storage batteries such as a nickel hydride storage battery, a nickel-zinc storage battery and a nickel-cadmium storage battery are frequently used under high temperature conditions because they are installed particularly in various equipments. Therefore, it is required to improve an utilization of active material under high temperature. However, a charge efficiency of the nickel electrode lowers when its temperature rises, so that the utilization of active material decreases, the electrolyte is exhausted by gas generated and a battery life is shortened.
In order to improve the utilization of active material at high temperature, such methods are generally used as [1] a method for changing a component of electrolyte, for example, a method wherein lithium hydroxide aqueous solution is added to potassium hydroxide aqueous solution, and [2] a method wherein a quantity of Co existing as a solid solution in crystal of nickel hydroxide is increased. However, the method [1] includes such problems as a reduction in the utilization of active material at low temperature and a lowering of high-rate discharge performance. And, when the quantity of Co is increased excessively, the method [2] includes such troubles as a lowering of discharge voltage and an increase in manufacturing cost.
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