Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-07-27
2001-10-09
Kalaput, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S218100, C420S900000
Reexamination Certificate
active
06300010
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to hydrogen absorbing alloy powders for use in the negative electrodes of alkaline rechargeable (secondary) batteries and processes for producing the same.
2. Description of the Related Art
For purposes associated with performance improvements including, for example, a reduction in the size and weight of electronic information equipment, a reduction in the size and an increase in the power output of electric tools, and a reduction in the size and weight of electric cars, it is desired to reduce the size and weight of rechargeable batteries used therefor and improve the performance thereof. As a result, it has become essential to improve the performance characteristics of rechargeable batteries, such as initial activity, a high rate discharge property and life characteristics.
However, in rechargeable batteries used for these purposes and, in particular, increasingly widely used nickel-metal hydride alkaline rechargeable batteries, their initial activity and a high rate discharge property and their life characteristics run counter to each other owing to the properties of the hydrogen absorbing alloy used as the negative electrode active material constituting the batteries. In order to make them consistent with each other, various attempts have been made from the viewpoint of alloy composition and alloy preparation, but no satisfactory result has been obtained.
As a means for improving the initial characteristics and a high rate discharge property of nickel-metal hydride alkaline rechargeable batteries, it is conceivable to reduce the hydrogen absorbing alloy powder constituting the negative electrode. To this end, various investigations on the particle size or particle diameter of the hydrogen absorbing alloy powder are made, for example, in Japanese Patent Provisional Publication Nos. 2-306541, 3-116655 and 4-292860. Although it is described therein to reduce the particle size or particle diameter, these techniques are less than practicable from the viewpoint of battery characteristics, production process, and handling of hydrogen absorbing alloys.
As to the addition of an electrically conductive material to the negative electrode of a nickel-metal hydride alkaline rechargeable battery, it is common practice to add the electrically conductive material, together with a binder and a thickener, during the preparation of a paste. Processes in which an electrically conductive material is mixed with an alloy powder are proposed in Japanese Patent Provisional Publication Nos. 6-44965 and 8-213005. However, since the alloy powder has a large particle size and is ground in a solvent system, these techniques are less than practicably from the viewpoint of battery characteristics and production process.
SUMMARY OF THE INVENTION
In view of the above-described problems, the present inventors made intensive investigations with a view to developing a hydrogen absorbing alloy powder for use in the negative electrodes of alkaline rechargeable batteries which has excellent initial characteristics and a high rate discharge property, exhibits satisfactory life characteristics, and is hence very beneficial from a practical point of view.
As a result, the present inventors have now found that the above-described problems can be solved by reducing the particle size of the alloy powder to increase the reaction area, reducing the oxygen content as compared with the surface area, and causing the oxygen to be present in the form of hydroxyl groups.
In order to accomplish the above object, the present invention provides a hydrogen absorbing alloy powder for use in the negative electrodes of alkaline rechargeable batteries which is characterized in that it has an average particle diameter of 5 to 20 &mgr;m and an oxygen content of 2,000 to 6,000 ppm, and in that the oxygen on the surfaces of hydrogen absorbing alloy particles is present in the form of hydroxyl groups.
Thus, the particle size of the powder is so small as to increase the reaction area, the oxygen content is low as compared with the surface area, and the oxygen is present in the form of hydroxyl groups. Consequently, the charging and discharging reactions proceed rapidly and the corrosion of the surface can be suppressed, so that the alloy powder has excellent initial characteristics and a high rate discharge property and can exhibit satisfactory life characteristics.
In another aspect, the present invention provides a process for the production of a hydrogen absorbing alloy powder for use in the negative electrodes of alkaline rechargeable batteries which comprises the steps of grinding a hydrogen absorbing alloy in an inert gas and, prior to exposure to ambient air, treating the resulting hydrogen absorbing alloy powder with an inert gas containing a very small amount of moisture.
In another embodiment, the process for the production of a hydrogen absorbing alloy powder for use in the negative electrodes of alkaline rechargeable batteries in accordance with the present invention may comprise the step of grinding a hydrogen absorbing alloy in an atmosphere of an inert gas containing a very small amount of moisture.
Furthermore, according to the present invention, the hydrogen absorbing alloy powder is mixed with an electrically conductive material during its grinding in an atmosphere of an inert gas. Consequently, the electrically conductive material adheres to the surfaces of hydrogen absorbing alloy particles owing to the surface energy of the surfaces newly formed by grinding, so that the electrically conductive material is uniformly dispersed without cohering together.
Thus, the present invention can provide a hydrogen absorbing alloy powder for use in the negative electrodes of alkaline rechargeable batteries which has excellent initial characteristics and a high rate discharge property, exhibits satisfactory life characteristics, and is hence very beneficial from a practical point of view.
REFERENCES:
patent: 5864072 (1999-01-01), Kobayashi et al.
patent: 6063524 (2000-05-01), Kasashima et al.
patent: 02306541 A (1990-12-01), None
patent: 03116655 A (1991-05-01), None
patent: 04292860 A (1992-10-01), None
patent: 06044965 A (1994-02-01), None
patent: 08213005 A (1996-08-01), None
Hamaya Noriaki
Nakano Hajime
Shima Satoshi
Kalaput Stephen
Myers Bigel & Sibley & Sajovec
Shin-Etsu Chemical Co, Inc.
Wills M.
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