Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-05-17
2001-07-10
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
Reexamination Certificate
active
06258482
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hydrogen storage alloy electrode, particularly suitable for use as a negative electrode of alkaline storage cells or batteries, and to a method for fabrication of the hydrogen storage alloy electrode.
2. Related Art
A nickel-hydrogen storage cell or battery is regarded as a prime candidate for the alkaline battery of the next generation, because of its high capacity, more than twice of that of a nickel-cadmium battery, and its environmentally friendly nature.
Such alkaline batteries typically use a hydrogen storage alloy electrode. There generally exists two types of hydrogen storage alloy electrodes; a sintered hydrogen storage alloy electrode prepared by sintering hydrogen storage alloy powder, and a pasted hydrogen storage alloy electrode prepared by coating or loading a paste containing hydrogen storage alloy powder particles on or in an electrically conductive core. However, these two types of hydrogen storage alloy electrodes suffer from a common deficiency. As the electrode undergoes a change in its volume during the charge-discharge cycles, the hydrogen storage alloy powder incorporated therein is often subjected to subdivision to result in the occurrence of falling-off or shedding of the subdivided powder particles. This has led to the reduced initial discharge capacities as well as the shortened charge-discharge cycle lives of alkaline batteries using those hydrogen storage alloy electrodes.
In order to overcome the above-described deficiency, a method has been proposed, for example in Japanese Patent Laying-Open No. 60-70665 (1985), which utilizes hydrogen storage alloy powder of smaller particle sizes. The use of the size-reduced hydrogen storage alloy powder results in the reduced occurrence of subdivision, so that the shedding of powder particles is effectively suppressed. Fabrication of an electrode from the size-reduced hydrogen storage alloy powder, in accordance with the proposed method, allows for a large electrode surface area. An alkaline storage battery using such an electrode is thus expected to produce a high level of discharge capacity.
However, the size-reduced hydrogen storage alloy powder is readily oxidized in the air to produce electrically inert surface films, such as oxide films. Accordingly, an alkaline storage battery incorporating such a hydrogen storage alloy powder in its negative electrode fails to enjoy the expected improvements in cycle life and discharge characteristics. In addition, the failure to suppress the generation of hydrogen gas during charge leads to an unfavorable pressure build-up in the battery.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a hydrogen storage alloy electrode which utilizes fine hydrogen storage alloy particles excellent in charge-discharge cycling characteristics, which exhibits excellent initial discharge characteristics and which can effectively suppress the build-up of an internal pressure of a battery during charge. Another object of the present invention is to provide a method for fabrication of such a hydrogen storage alloy electrode.
A hydrogen storage alloy electrode of the present invention is comprised of a hydrogen storage alloy powder which characteristically includes agglomerates of hydrogen storage alloy particles joined together through a metallic layer.
In accordance with the present invention, the hydrogen storage alloy powder includes the agglomerates of hydrogen storage alloy particles joined together through a metallic layer. In such agglomerates, the metallic layer covers surface portions of the hydrogen storage alloy particles to reduce exposed surface areas thereof which will be subjected to oxidation. Thus, the surface oxidation of hydrogen storage alloy is suppressed.
The coverage of alloy particles with the metallic layer lessens the surface energy of fine alloy particles, initially maintained at a high level, so that the alloy surfaces are allowed to shift to a more stable state. This also results in suppressing the surface oxidation of the hydrogen storage alloy.
Furthermore, since the hydrogen storage alloy particles are joined together by the metallic layer, the contact resistance therebetween is effectively reduced. Therefore, the use of the hydrogen storage alloy electrode of the present invention results not only in the increased initial discharge capacity of a cell or battery, but also in the improved gas-absorbing performance that effectively suppresses the build-up of internal pressure of the cell or battery during charge.
In accordance with a first aspect of the present invention, the agglomerates included in the hydrogen storage alloy powder are characterized as being comprised of hydrogen storage alloy particles of sizes not exceeding 25 &mgr;m.
The “hydrogen storage alloy particles of sizes not exceeding 25 &mgr;m”, as used herein, refer to those alloy particles which, when subjected to sieving with a 500-mesh screen, pass through the screen openings and have a maximum particle size of not exceeding 25 &mgr;m.
In the first aspect, the hydrogen storage alloy powder may be comprised of a combination of the aforementioned agglomerates of hydrogen storage alloy particles of sizes not exceeding 25 &mgr;m and the unjoined hydrogen storage alloy particles of sizes exceeding 25 &mgr;m but not exceeding 100 &mgr;m. The “hydrogen storage alloy particles of sizes exceeding 25 &mgr;m but not exceeding 100 &mgr;m”, as used herein, refer to those alloy particles which, when subjected to sieving, pass through a 200-mesh screen but remain on a 500-mesh screen. In such a combination, finer alloy particles, i.e., the hydrogen storage alloy particles of sizes not exceeding 25 &mgr;m are joined together by the metallic layer to take the form of agglomerates, so that the surface oxidation thereof in the air can be suppressed.
In a second aspect of the present invention, the aforementioned agglomerates are characterized as being comprised of hydrogen storage alloy particles of sizes not exceeding 100 &mgr;m. In this second aspect, those alloy particles of sizes exceeding 25 &mgr;m are also joined together by the metallic layer to take the form of agglomerates. This eliminates the necessity of sieving to classify the alloy particles of sizes not exceeding 25 &mgr;m. Accordingly, a procedure of obtaining the agglomerates is simplified.
Like the hydrogen storage alloy particles of sizes not exceeding 25 &mgr;m, the hydrogen storage alloy particles of sizes not exceeding 100 &mgr;m, if rendered into the form of the above-defined agglomeration, are less subjected to surface oxidation. The contact resistance between the alloy particles can also be reduced. This is considered due to the tendency of the fine hydrogen storage alloy particles to cohere to each other or to adsorb the larger-size alloy particles for shift to a more stable state.
The hydrogen storage alloy particles for use in the present invention can be made in an electric-arc melting furnace under argon atmosphere, for example. They can also be made by other techniques such as gas atomization and roll quenching.
In the present invention, the metallic layer which joins the alloy particles can be formed, for example, from at least one metal selected from the group consisting of scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), molybdenum (Mo), palladium (Pd), silver (Ag), indium (In), tin (Sn) and antimony (Sb).
The metallic layer can be produced in the form of a deposit of any of the listed metals on surfaces of the hydrogen storage alloy particles, for example, by immersing the alloy particles in an acid solution into which a metallic compound is dissolved containing at least one metal selected from the group consisting of scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), molybdenum (Mo), palladium (Pd), silver (Ag), indium (In), tin (Sn) and antim
Fujitani Shin
Higashiyama Nobuyuki
Imoto Teruhiko
Kato Kikuko
Kimoto Mamoru
Fasse W. F.
Fasse W. G.
Sanyo Electric Co,. Ltd.
Weiner Laura
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