Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-02-01
2001-09-11
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S217000, C429S233000, C429S245000
Reexamination Certificate
active
06287725
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a sintered hydrogen-absorbing alloy electrode used as a negative electrode in an alkaline storage battery.
BACKGROUND ART
In accordance with recent spread of cordless equipment, there are rapidly increasing demands for secondary batteries. In particular, a nickel-metal hydride storage battery, which has a higher energy density than a nickel-cadmium storage battery and a lead storage battery, is regarded as a noticeable power source for an electric vehicle and the like.
A nickel-metal hydride storage battery uses a hydrogen-absorbing alloy electrode as a negative electrode and a nickel electrode as a positive electrode, and is designed to have a positive electrode capacity smaller than a negative electrode capacity. Thus, an oxygen gas generated from the positive electrode in over-charge is absorbed by the negative electrode so that the pressure within the battery can be prevented from increasing. Accordingly, the pressure within the battery can be prevented from increasing even when the battery is over-charged as far as the oxygen gas is rapidly absorbed by the negative electrode.
As the hydrogen-absorbing alloy electrode, any of various electrodes including a pasted electrode, a foamed metal electrode and a sintered electrode can be used. A pasted electrode is fabricated by shaping a punching metal coated with a hydrogen-absorbing alloy and a binder. A foamed metal electrode is fabricated by shaping a spongy conductive substrate coated with a hydrogen-absorbing alloy and a binder. A sintered electrode is fabricated by sintering a conductive substrate, such as a punching metal, coated with a hydrogen-absorbing alloy and a binder. The pasted electrode and the foamed metal electrode, which are not sintered, are poor at oxygen absorbing power because the binder prevents the hydrogen-absorbing alloy from absorbing oxygen.
In contrast, in the sintered electrode, almost all the binder is lost through decomposition during the sintering, and hence, the binder never prevents the hydrogen-absorbing alloy from absorbing oxygen. However, when an oxygen-containing binder (a binder including an oxygen atom in its molecule) or a binder aqueous solution (a water soluble binder dissolved in water) is used as a binder, which includes oxygen which oxidizes the alloy, the hydrogen-absorbing alloy is unavoidably oxidized during the sintering. Accordingly, development of a sintered hydrogen-absorbing alloy electrode with large oxygen absorbing power is significant for obtaining a highly reliable battery in which increase of the pressure is minimal.
The present invention was devised under these circumstances, and an object is providing a sintered hydrogen-absorbing alloy electrode exhibiting large oxygen absorbing power and a highly reliable nickel-metal hydride storage battery using the same.
DISCLOSURE OF INVENTION
The method of fabricating a sintered hydrogen-absorbing alloy electrode (the present method) comprises the steps of:
coating a conductive substrate with a paste in which a rare earth-nickel hydrogen-absorbing alloy having a CaCu
5
structure, an oxygen-containing binder or a water-soluble binder, and a carbon material are dispersedly mixed; and
sintering the conductive substrate coated with the paste under vacuum or in an atmosphere of a non-oxidizing gas.
Any of known hydrogen-absorbing alloys for alkaline storage batteries can be used as the hydrogen-absorbing alloy, and a spherical alloy obtained by an atomizing method is preferably used in order to attain a uniform density distribution of alloy particles in a sintered substance. Apart from the hydrogen-absorbing alloy obtained by the atomizing method, a granulated alloy obtained by grinding a hydrogen-absorbing alloy prepared by cast molding; a hydrogen-absorbing alloy prepared by a roll quenching method in which a melted hydrogen-absorbing alloy is dropped onto a rotating roll; and a hydrogen-absorbing alloy obtained by grinding the hydrogen-absorbing alloy prepared by the roll quenching method can be used. Examples of kinds of the hydrogen-absorbing alloy are those conventionally used for alkaline storage batteries, such as a rare earth-nickel hydrogen-absorbing alloy having a CaCu
5
structure, typically LaNi
5
; a hydrogen-absorbing alloy having a Laves structure; and a hydrogen-absorbing alloy having a BCC structure. Among these known hydrogen-absorbing alloys, a rare earth-nickel hydrogen-absorbing alloy represented by MmNi
a
Co
b
Al
c
Mn
d
(wherein 2.0≦a≦5.0, 0.1≦b≦3.0, 0.1≦c≦2.0, 0.1≦d≦2.0, 4.5≦a+b+c+d ≦5.5, and Mm indicates a mixture of rare earth elements) is preferred. The particle size of the hydrogen-absorbing alloy is not herein specified, but when the particle size is too large, formation of the electrode is difficult because the hydrogen-absorbing alloy cannot be uniformly rolled and coated over the conductive substrate. On the contrary, when the particle size is too small, the proportion of oxide is increased because the oxide of the hydrogen-absorbing alloy is formed on the particle surface. Therefore, the hydrogen-absorbing alloy preferably has a particle size of approximately 10 through 200 &mgr;m.
The binder is an oxygen-containing binder or water-soluble binder. In a conventional sintered electrode, the alloy is oxidized in the sintering when an oxygen-containing binder or a binder aqueous solution is used. In this invention, however, since the paste includes the carbon material, the alloy is remarkably suppressed from degrading through the oxidation during the sintering owing to a reducing function of the carbon material. Accordingly, when an oxygen-containing binder or a water-soluble binder is used as the binder, the effect of the invention is remarkably exhibited. Examples of the oxygen-containing binder are polyamide, polycarbonate, poly(methyl methacrylate), polyurethane, a phenol resin, silicone and poly(acrylic acid), and examples of the water-soluble binder are poly(ethylene oxide), poly(vinyl alcohol), poly(vinylpyrrolidone), hydroxypropyl cellulose, carboxymethyl cellulose and methyl cellulose.
Examples of the carbon material are graphite (natural graphite and artificial graphite), coke (oil coke and coal pitch coke), non-graphitizable carbon (such as glassy carbon), amorphous carbon (such as carbon black, acetylene black and Ketchen black). In using a carbon material in which volumetric shrinkage thereof or a side reaction derived from an impurity included therein can be caused during the sintering, the carbon material is preferably previously heated at a temperature higher than a sintering temperature so as to prevent the volumetric shrinkage or the side reaction, thereby suppressing deformation of the electrode as far as possible. Through this heat treatment, the carbon material is crystallized so as not to largely shrink during the sintering and the impurity included therein is removed. Examples of a carbon material that is difficult to shrink during the sintering are natural graphite, and a carbon material with a lattice spacing (d
002
) between lattice planes (002) of 0.345 nm or less obtained by heating graphitizable carbon at 1300° C. or more, and preferably 2000° C. or more.
Examples of the conductive substrate are a punching metal, a foamed metal, a lath board, a metallic fiber nonwoven fabric, and a corrugated board made from a corrugated substrate with irregularities formed thereon. When the conductive substrate is a punching metal or a lath board functioning as a two-dimensional current collector, the thickness is preferably approximately 0.01 through 0.1 mm, and when it is a foamed metal, a metallic fiber nonwoven fabric or a corrugated board functioning as a three-dimensional current collector, the thickness is preferably 0.5 through 2 mm.
The paste preferably includes the carbon material in a ratio of 0.1 through 20 wt % based on the total amount of the hydrogen-absorbing alloy, the oxygen-containing binder or a water-soluble binder and the carbon material. When the amount of the carbon mat
Maeda Reizo
Nakamura Hiroshi
Nishio Koji
Nogami Mitsuzo
Suzuki Syuuichi
Kubovcik & Kubovcik
Sanyo Electric Co,. Ltd.
Weiner Laura
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