Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-07-11
2002-11-05
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S217000
Reexamination Certificate
active
06475671
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a hydrogen absorbing alloy electrode containing hydrogen absorbing alloy powder and a binding agent and a nickel-metal hydride battery using the hydrogen absorbing alloy electrode as its negative electrode, and is particularly characterized in that the binding agent to be used in the hydrogen absorbing alloy electrode is modified so as to attain, in the nickel-metal hydride battery using the hydrogen absorbing alloy electrode as its negative electrode, an increased discharge voltage in a case where the battery is discharged at a high current as well as improved charge/discharge cycle performance.
2. Description of the Related Art
A nickel-metal hydride battery has been conventionally known as one of alkaline storage batteries. Such a nickel-metal hydride battery has been generally employed as its negative electrode a hydrogen absorbing alloy electrode using a hydrogen absorbing alloy.
In fabricating such a hydrogen absorbing alloy electrode, the method conventionally generally utilized is the one so adapted as to prepare a paste by adding a binding agent to hydrogen absorbing alloy powder and then apply the paste to a current collector composed of a punching metal or the like.
However, when the paste comprising the hydrogen absorbing alloy powder containing the binding agent added thereto is applied to the current collector as described above, contact between each hydrogen absorbing alloy powder is insufficient and further, contact between the hydrogen absorbing alloy powder and the current collector is degraded, resulting in increased resistance in a hydrogen absorbing alloy electrode. Accordingly, in a nickel-metal hydride battery employing such a hydrogen absorbing alloy electrode as its negative electrode, a discharge voltage in a case where the battery is discharged at a high current is decreased and charge/discharge cycle performance is degraded.
Therefore, in recent years, Japanese Patent Laid-Open No. Heil0(1998)-241693 has proposed to use, as a binding agent in a hydrogen absorbing alloy electrode, a copolymer of aromatic vinyl units, conjugated diene units, (meta)acrylic ester units, and functional group-containing compound units, in which the content of the (meta)acrylic ester units is in the range of 10 to 40% by weight of the whole copolymer. This binding agent serves to enhance binding between hydrogen absorbing alloy powder and a current collector in the hydrogen absorbing alloy electrode and hence, a nickel-metal hydride battery employing the hydrogen absorbing alloy electrode as its negative electrode is improved in charge/discharge cycle performance and the like.
Unfortunately however, a hydrogen absorbing alloy electrode using as a binding agent the copolymer as disclosed in the above-mentioned gazette suffers insufficient water retentivity. As a result, in a nickel-metal hydride battery employing such a hydrogen absorbing alloy electrode, there still remain problems that a discharge voltage in a case where the battery is discharged at a high current can not be sufficiently increased and that charge/discharge cycle performance is still inferior.
SUMMARY OF THE INVENTION
An object of the present invention is to enhance, in a hydrogen absorbing alloy electrode containing hydrogen absorbing alloy powder and a binding agent, the binding between each hydrogen absorbing alloy powder and between the hydrogen absorbing alloy powder and a current collector as well as water retentivity of the hydrogen absorbing alloy electrode.
Another object of the present invention is to attain, in a nickel-metal hydride battery employing the above-mentioned hydrogen absorbing alloy electrode as its negative electrode, an increased battery voltage when the battery is discharged at a high current as well as improved charge/discharge cycle performance, and to prevent a rise in internal pressure of the battery when the battery is overcharged.
A hydrogen absorbing alloy electrode according to the present invention is a hydrogen absorbing alloy electrode containing hydrogen absorbing alloy powder and a binding agent, wherein said binding agent is a copolymer of aromatic vinyl and at least one of acrylic ester and methacrylic acid ester and the total content of acrylic ester units and methacrylic acid ester units in the copolymer is in the range of 43 to 90% by weight.
The total content of acrylic ester units and methacrylic acid ester units in the above-mentioned copolymer is set in the range of 43 to 90% by weight because when the total content is less than 43% by weight, flexibility of the copolymer is degraded. If the binding agent comprising the copolymer with such degraded flexibility was used to apply hydrogen absorbing alloy powder to a current collector, it is considered that the hydrogen absorbing alloy powder is liable to fall off from the current collector and hence, binding between each hydrogen absorbing alloy powder and between the hydrogen absorbing alloy powder and the current collector is degraded, whereby resistance in a hydrogen absorbing alloy electrode is increased and an alkaline electrolyte can not be well dispersed in the hydrogen absorbing alloy electrode. On the other hand, when the total content of acrylic ester units and methacrylic acid ester units is more than 90% by weight, the copolymer is easily dissolved in an alkaline electrolyte. It is considered that when the copolymer is dissolved in the alkaline electrolyte, binding between each hydrogen absorbing alloy powder and between the hydrogen absorbing alloy powder and the current collector is degraded, whereby resistance in the hydrogen absorbing alloy electrode is increased and oxygen gas generated during overcharge can not be sufficiently absorbed in the hydrogen absorbing alloy powder.
As in the hydrogen absorbing alloy electrode according to the present invention, when the copolymer of aromatic vinyl and at least one of acrylic ester and methacrylic acid ester, in which the total content of acrylic ester units and methacrylic acid ester units is in the range of 43 to 90% by weight of the whole copolymer is used as the binding agent, sufficient contact between hydrogen absorbing alloy powder and between the hydrogen absorbing alloy powder and the current collector can be attained, so that resistance in the hydrogen absorbing alloy electrode is reduced. Further, water retentivity of the hydrogen absorbing alloy electrode is improved by the acrylic ester and/or methacrylic acid ester contained in the copolymer, and oxygen gas generated during overcharge can be sufficiently absorbed in the hydrogen absorbing alloy powder.
In a nickel-metal hydride battery employing the hydrogen absorbing alloy electrode as its negative electrode, a discharge voltage in a case where the battery is discharged at a high current is increased, charge/discharge cycle performance is improved, and internal pressure is prevented from rising when the battery is overcharged.
In the above-mentioned nickel-metal hydride battery, in order to increase the discharge voltage in a case where the battery is discharged at a high current as well as to improve the charge/discharge cycle performance, the total content of acrylic ester units and methacrylic acid ester units in the above-mentioned copolymer is preferably set in the range of 50 to 90% by weight, and more preferably 56 to 70% by weight.
Further, in the above-mentioned nickel-metal hydride battery, in order to prevent a rise in internal pressure during overcharge, the total content of acrylic ester units and methacrylic acid ester units in the above-mentioned copolymer is preferably set in the range of 46 to 70% by weight, and more preferably 46 to 56% by weight.
In obtaining the above-mentioned copolymer, examples of the above-mentioned acrylic ester and methacrylic acid ester include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, hexyl acrylate, hexyl methacrylate, 2-ethylhexyl acry
Maeda Reizo
Matsuura Yoshinori
Nohma Toshiyuki
Shinyama Katsuhiko
Tanaka Tadayoshi
Armstrong Westerman & Hattori, LLP
Chaney Carol
Sanyo Electric Co,. Ltd.
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