Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1997-03-07
1998-11-24
Phipps, Margery
Chemistry: electrical and wave energy
Apparatus
Electrolytic
429 29, 429501, 420900, 148530, 148556, 164479, 164480, 164463, 75255, H01M 402
Patent
active
058401660
DESCRIPTION:
BRIEF SUMMARY
FIELD OF ART
The present invention relates to a rare earth metal-nickel hydrogen storage alloy which achieves high capacity and long battery life when it is used for a hydrogen storage vessel, a heat pump, or as an anode material for a nickel-hydrogen rechargeable battery, a process for producing the same, and an anode for a nickel-hydrogen rechargeable battery.
An anode for a nickel-hydrogen rechargeable battery which is currently produced in a large amount is mainly produced with an AB.sub.5 type alloy which has a light rare earth elements such as La, Ce, Pr, Nd, or a mixture of these elements (Mm (misch metal)) in A-site, and Ni, Co, Mn, and/or Al in B-site. This alloy has properties of a larger hydrogen storage capacity than other alloys, and a usable hydrogen absorption-desorption pressure of 1 to 5 atmosphere at ordinary temperature.
The conventional rare earth metal-nickel alloy of AB.sub.5 type structure, however, exhibits low initial activity in absorbing hydrogen, so that several cycles to a dozen of cycles of hydrogen absorption and desorption are required at the initial stage for achieving 100% hydrogen storage capacity. Further, this alloy has drawbacks of expanding and contracting due to the absorption and desorption of hydrogen, thereby generating cracks and being decrepitated to deteriorate the properties of the battery.
Recently, electrodes for achieving still larger battery capacity are demanded, and an alloy has been developed having a composition wherein the ratio of transition metals containing nickel as a main component to rare earth metals is decreased in order to increase the capacity of the battery. For example, Japanese Laid-open Patent Application No. 6-145851 discloses a hydrogen storage alloy produced by rapidly cooling and solidifying an alloy melt mainly composed of La and Ni with the atomic ratio of Ni to La being not more than 4.9, in which alloy the crystal grains of the alloy are finely crystallized, i.e. the length of the alloy crystals along the short axis is not more than 10 .mu.m. It is also disclosed that the battery capacity and the battery life of a nickel-hydrogen rechargeable battery can be improved with this hydrogen storage alloy.
However, though this hydrogen storage alloy can achieve the effects of finely crystallized crystal grains, the battery capacity cannot be increased when the atomic ratio of Ni in the alloy is not higher than 4.5, in other words, when the composition is greatly shifted from LaNi.sub.5 to rare earth rich. It is believed that this is because in the composition rich in La, which has particularly strong affinity for hydrogen among other rare earth elements, hydrogen is trapped to generate hydrides (a hydrogen-induced amorphous phase) upon absorption of hydrogen, thereby decreasing the substantial capacity which contributes to the hydrogen absorption-desorption. The above patent application also discloses that a portion of La can be substituted by rare earth elements other than La, but specific examples of the elements and effects resulting therefrom are not described.
As discussed above, the rare earth metal-nickel hydrogen storage alloy which has been used as an anode material for a nickel-hydrogen rechargeable battery is demanded to have higher capacity and longer life.
For example, in order to prolong the life, there is proposed a method of increasing the content of Co or the like, or a method of subjecting the alloy itself to a heat treatment to clear elemental segregation and to relieve strain generated in casting. However, either of the methods results in decreased battery capacity. On the other hand, when the content of Mn is increased to enhance the capacity, the long life is sacrificed. Therefore, an alloy which achieves high initial activity and long life at the same time, and further high battery capacity when it is used as an anode for a nickel-hydrogen rechargeable battery, is not known.
As stated above, with the conventional nickel-hydrogen rechargeable battery of LaNi.sub.5 type structure, composition has mainly been discus
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Phipps Margery
Santoku Metal Industry Co. Ltd.
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