Metal treatment – Process of modifying or maintaining internal physical... – Treating loose metal powder – particle or flake
Reexamination Certificate
1999-03-10
2001-05-22
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Treating loose metal powder, particle or flake
C420S900000
Reexamination Certificate
active
06235130
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a hydrogen absorbing alloy powder and electrodes formed of the same. More particularly, it relates to a hydrogen absorbing alloy powder which can suitably be used to form the negative electrodes of nickel-metal hydride storage batteries, and to electrodes formed of the same. Still more particularly, it relates to a hydrogen absorbing alloy powder for use in nickel-metal hydride storage batteries having a high capacity, excellent initial characteristics and a long life.
2. Description of the Related Art
Since the discovery of hydrogen absorbing alloys capable of absorbing and releasing hydrogen, they not only have been used as hydrogen storage means, but also have been more and more widely applied to heat pumps, batteries and the like. In particular, alkaline storage batteries using a hydrogen absorbing alloy powder for the negative electrode have already been put to practical use, and various attempts have successively been made to improve the capacity and life of the batteries in view of the hydrogen absorbing alloy used therefor.
That is, with respect to the initially investigated LaNi
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alloy having a CaCu
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type crystal structure, an improvement in capacity and life has been achieved by replacing a part of La with Ce, Pr, Nd and other rare earth elements to form misch metal (Mm), or by replacing a part of Ni with a metallic element such as Al, Co or Mn.
However, when such hydrogen absorbing alloys are used to form electrodes for batteries, the capacity and life of the resulting batteries can be improved, but their initial characteristics are reduced.
Generally, the capacity tends to become higher as the La content in Mm is increased. When the La content is 100%, a maximum capacity is obtained, but the cycle life becomes extremely poor. Moreover, when a part of Ni is replaced with Co, Mn, Al or the like in order to achieve a long life and prevent passivation, the equilibrium pressure is lowered to cause a reduction in low-temperature characteristics and high rate discharge characteristics. Accordingly, in order to bring about an improvement in life, it has conventionally been proposed to use Mm in which a part of La is replaced with the other light rare earth element (Ce, Pr or Nd), decrease the Ce content of Mm (Japanese Patent Provisional Publication No. 62-223971/'87), or, on the contrary, add Ce thereto positively (Japanese Patent Provisional Publication No. 6-96766/'94). However, it has been impossible to obtain a battery having a high capacity, a long life and excellent characteristics solely by resorting to an improvement in alloy composition.
Initial characteristics are generally expressed in terms of the number of charging-discharging cycles repeated until a maximum capacity is reached, and they are considered to be higher as the number of cycles is smaller.
Usually, initial characteristics are evaluated by the capacity in the first cycle. However, a battery having low initial characteristics has the disadvantage that, when it is fabricated into a sealed type one, the balance between the positive and negative electrodes is lost to cause a reduction in the capacity and life of the battery.
In order to overcome the above-described disadvantages, it has been conventional practice to treat a hydrogen absorbing alloy with an aqueous alkaline solution or inorganic acid. However, the alkali treatment has been disadvantageous in that treating conditions such as a high concentration and a high temperature are required and it is difficult to wash the treated hydrogen absorbing alloy with water. In addition, the hydrogen absorbing alloy treated by the alkaline solution or by inorganic acid undesirably undergoes a compositional change because the alloy is subject not only to surface oxidation during treatment but also to oxidation during drying and storage. Hence, the difficulty in handleability and safety makes a procedure for forming negative electrodes for batteries more complicated.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a negative electrode for batteries which is formed of a hydrogen absorbing alloy, exhibits excellent initial characteristics, and is suitable for the fabrication of a sealed type battery having a high capacity and a long life, as well as a hydrogen absorbing alloy used therefor.
In order to overcome the above-described disadvantages, the present inventors made intensive investigations on hydrogen absorbing alloy powders for use in negative electrodes and processes for producing them. As a result, it was found that a hydrogen absorbing alloy powder having excellent storability and handleability can be easily produced by treating a hydrogen absorbing alloy powder with a solution containing a condensed phosphoric acid and/or phytic acid, and that, when this hydrogen absorbing alloy powder is used to form the negative electrodes of nickel-metal hydride storage batteries, the initial characteristics of the resulting batteries can be improved without reducing their capacity or life. This technique has already been proposed (Japanese Patent Provisional Publication No. 10-176201/'98). However, there still remains a need for further improvement in initial capacity and discharge capacity.
Accordingly, an object of the present invention is to provide a process for the production of a hydrogen absorbing alloy powder having excellent storability and handleability, as well as a negative electrode for use in nickel-metal hydride storage batteries which has excellent initial characteristics and a long life.
The present inventors made further investigations on the above-described treatment with a condensed phosphoric acid, and have now found that a further improvement in initial characteristics can be achieved by treating a hydrogen absorbing alloy powder with an acid solution prior to the treatment with a condensed phosphoric acid. The present invention has been completed on the basis of this finding.
Thus, the present invention relates to a process for the production of a hydrogen absorbing alloy powder which includes the steps of treating a hydrogen absorbing alloy powder with an acid solution, and subsequently treating the hydrogen absorbing alloy powder with a solution containing a phosphoric compound, especially a condensed phosphoric acid having 2 to 20 phosphorus atoms per molecule and/or phytic acid, and to an electrode formed of the hydrogen absorbing alloy powder produced by this process.
It has been found that, when a hydrogen absorbing alloy in which most of the rare earth elements present therein comprise Ce and La is treated with an acid solution, washed with water, and treated with a phosphorus compound, the initial characteristics of the resulting batteries can be improved more without reducing their capacity or life. The present invention has been completed on the basis of this finding.
More specifically, the present inventors made investigations with attention paid to the fact that the Ce content in Mm is an important point for the improvement of life and other characteristics, and have now found that, when a certain proportion of La present in a hydrogen absorbing alloy is replaced with Ce, and the hydrogen absorbing alloy powder is treated with an acid solution and then with a phosphorus compound to achieve a high initial capacity and a long life, negative electrodes for use in nickel-metal hydride storage batteries which have a high capacity and a long life can be formed by using the resulting hydrogen absorbing alloy. Thus, the present invention has been completed.
The above objects of the present invention are accomplished by a hydrogen absorbing alloy powder of the AB
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type in which A is exothermic metal and B is endothermic metal, the hydrogen absorbing alloy powder being obtained by providing a hydrogen absorbing alloy in which A preferably comprises a mixture of rare earth elements containing 20 to 50% by weight of Ce and 20 to 50% by weight of La, treating the hydrogen absorbing alloy with an acid s
Ishii Masatoshi
Kuribayashi Yukihiro
Shima Satoshi
Sugahara Hiroto
Coleman Henry D.
Sapone William J.
Shin-Etsu Chemical Co. , Ltd.
Sudol R. Neil
Wyszomierski George
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