Alloys or metallic compositions – Nickel base – Rare earth containing
Reexamination Certificate
1999-03-01
2001-07-17
King, Roy (Department: 1742)
Alloys or metallic compositions
Nickel base
Rare earth containing
C148S555000, C420S900000, C420S580000, C429S218200, C429S223000, C429S224000
Reexamination Certificate
active
06261517
ABSTRACT:
FIELD OF ART
The present invention relates to a rare earth metal-nickel hydrogen storage alloy, a method for producing the same, and an anode for a nickel-hydrogen rechargeable battery. In particular, the present invention relates to an anode for a nickel-hydrogen rechargeable battery having high electrode capacity, long service life, and excellent high rate charge-discharge performance at an ordinary temperature as well as excellent high rate discharge performance at lower temperatures (collectively referred to as “high rate charge-discharge performance” hereinbelow), a rare earth metal-nickel hydrogen storage alloy that can be used as a starting material for such anode, and a method for producing the same.
BACKGROUND OF THE INVENTION
Hydrogen storage alloys predominantly used these days in preparation of anodes for nickel-hydrogen rechargeable batteries are AB
5
type alloys (CaCu
5
type structure) that have light rare earth elements such as La, Ce, Pr, Nd, or a mixture of these elements (misch metal) in A-site, and Ni, Co, Mn, and/or Al in B-site. This kind of alloys has a larger hydrogen storage capacity than other alloys, and hydrogen absorption-desorption pressure (equilibrium pressure) of 1 to 5 atmospheres at an ordinary temperature, which make the alloys usable. These alloys are generally composed of 0.4 to 0.8 of Co, 0.3 to 0.5 of Mn, 0.2 to 0.4 of Al, and less than 3.9 of Ni per one rare earth metals in atomic ratio, for the sake of adjustability of equilibrium pressure and corrosion resistance against the electrolyte in the battery.
The nickel-hydrogen rechargeable batteries have recently made rapid prevalence in the field of laptop computers, cellular phones, and portable audio equipment. Electric vehicles equipped with such batteries will be put to practical use before long. As a result of diversification of battery usage, there have developed demands not only for high electrode capacity and long service life (cycle life) of the batteries, but also for high rate charge-discharge performance, i.e. heavy current charge-discharge performance or high rate discharge performance at lower temperatures, as important characteristics, to keep up with higher output of equipment or use in cold district.
For the purpose of improving the electrode capacity, JP-A-6-145851, for example, proposes to reduce the Ni content with respect to rare earth metal content, that is, an alloy of rare earth-rich composition. In this measure, however, the higher content of rare earth metals causes the corrosion resistance against electrolyte to decrease, resulting in disadvantage for battery life.
JP-A-7-97648, for example, proposes another measure for improving the electrode capacity, wherein a portion of the alloy is substituted by Mn, and the alloy melt is rapidly cooled and solidified to form a columnar crystal structure of a particular size. This method gives the alloy a fine crystal structure by the rapid cooling and reduced segregation of Mn, thereby improving the electrode capacity and cycle life to a certain degree. However, excess amount of Mn causes corrosion at the segregation to lower the cycle life, and no remarkable improvement is achieved in the high rate charge-discharge performance compared to that of the conventional alloy.
In an attempt to improve the high rate charge-discharge performance, there is proposed to plate the surface of a hydrogen storage alloy with nickel for utilizing the catalytic effect of nickel. However, repeated charging and discharging of the battery decrepitates the alloy to form fresh surfaces, thereby diminishing the effect of nickel plating.
It is conventionally believed to be essential for improving the corrosion resistance against the electrolyte and for improving the battery life, to add Co in an amount of not less than 4% by weight, usually about 10% by weight. However, addition of Co adversely affects the activity of the hydrogen storage alloy (easiness to desorb hydrogen), and adds greatly to the cost of the alloy. Therefore, substitution for Co addition is demanded.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a rare earth metal-nickel hydrogen storage alloy which has both high electrode capacity and excellent cycle life, and which also has excellent high rate charge-discharge performance compared to conventional alloys, and a method for producing such alloy, and an anode for a nickel-hydrogen rechargeable battery produced with such alloy.
It is another object of the present invention to provide a rare earth metal-nickel hydrogen storage alloy which combines high electrode capacity, long cycle life, and high rate charge-discharge performance at low cost by remarkably reduced content of expensive cobalt, a method for producing such alloy, and an anode for a nickel-hydrogen rechargeable battery produced with such alloy.
The present inventors have made intensive studies on the effect of each alloy element and of production conditions such as cooling and heat treatment conditions, on the electrode capacity, cycle life, and high rate charge-discharge performance of R (rare earth metals)-Ni—Mn—Co—Al-containing hydrogen storage alloys. As a result, it has been revealed that an increased amount of Ni favorably affects the corrosion resistance and high rate charge-discharge performance under certain alloy production conditions. However, in order to compensate the lowered corrosion resistance due to Co reduction simply by increased amount of Ni, Ni content 1.5 times the Co content was required, which was found to result in decreased electrode capacity. In this case, the alloy was composed of two separate phases of an AB
5
type matrix (CaCu
5
type structure) and a Ni-rich phase of cubic system. It is believed that low hydrogen storage capacity of this cubic system leads to decrease in the hydrogen storage capacity and electrode capacity of the overall alloy.
Thus, the inventors have searched for measures for suppressing formation of the distinct Ni-rich phase to dissolve as much Ni as possible into AB
5
type matrix. As a result, they have revealed that addition of more than the conventional standard amount of Mn together with Ni, and control of alloy melt cooling conditions during alloy production could make more Ni and Mn dissolved in the solid solution, leading to improvement in the high rate charge-discharge performance. The inventors have also found that addition of a small amount of one or more elements selected from the group consisting of Fe, Cu, Zn, V, and Nb to the above-mentioned composition contributes to further increase in the amount of Ni and Mn dissolved in the solid solution. Specifically, the inventors prepared a R—Ni—Mn—Co—Al-containing alloy melt with the Mn content of not less than 0.45 and less than 1.5 per one R in atomic ratio, and solidified the alloy melt using a roll casting device having a roll with a surface roughness of 7 to 100 &mgr;m in ten-point mean roughness (Rz) into a 0.02 to 0.5 mm thick solidified alloy. As a result, the total content of Ni, Mn, and Co in B-site could be increased to the range of 5.2 to 7.5 per one R in atomic ratio beyond the conventional recognition, and thus the Ni content could also be increased to the range of 3.9 to 6, exceeding the conventional range. Therefore, the high rate charge-discharge performance was improved. Incidentally, the atomic ratio of the elements in B-site of the conventionally used AB
5
type alloy is less than 5.2.
However, aiming at reduction of the alloy cost by reducing Co content, which is one of the objects of the present invention, when the Co content in the above composition with higher Mn content was reduced to less than 0.3 in atomic ratio, a battery with an anode made of such alloy exhibited somewhat shorter cycle life compared to a conventional battery. The reasons for the shorter cycle life are believed to be the tendency of the more than the conventional amount of Mn in the solid solution matrix to dissolve in the alkali electrolyte in the battery, and relatively low solubility of Mn in the electrolyte, which cause precipi
Irikawa Masafumi
Kaneko Akihito
Tanibuchi Yuji
Coy Nicole
Darby & Darby
King Roy
Santoku Metal Industry Co. Ltd.
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