Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2000-03-29
2001-08-07
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S232000, C419S030000, C419S019000, C419S020000
Reexamination Certificate
active
06270547
ABSTRACT:
TECHNICAL FIELD
The present invention relates to hydrogen absorbing alloy electrodes for use as negative electrodes of alkaline secondary batteries such as nickel-metal hydride batteries and a process for fabricating the electrode, and more particularly to improvements in the surface characteristics of hydrogen absorbing alloys.
BACKGROUND ART
While nickel-cadmium batteries and lead acid batteries are already in wide use as secondary batteries, it is desired especially in recent years to develop secondary batteries which have a high energy density and are yet clean with advances in compact information devices such as cellular phones and notebook computers. Accordingly attention has been directed to closed-type nickel-metal hydride batteries which are free from harmful substances such as cadmium and lead and wherein an electrode of hydrogen absorbing alloy is used as the negative electrode.
Nickel-metal hydride batteries comprise a negative electrode of hydrogen absorbing alloy, a positive electrode of nickel, an alkaline electrolyte, a separator, etc. The hydrogen absorbing alloy electrode serving as the negative electrode is prepared by adding a binder to a hydrogen absorbing alloy powder obtained by pulverizing a hydrogen absorbing alloy ingot, and forming the mixture to a shape of an electrode. The hydrogen absorbing alloys developed for use in the negative electrode include those of the Misch metal type comprising a Misch metal (hereinafter referred to as “Mm”) which is a mixture of rare-earth elements and Laves phase hydrogen absorbing alloys.
With alkaline secondary batteries, such as nickel-metal hydride batteries, wherein a negative electrode of hydrogen absorbing alloy is used, a gas-phase reaction and an electrochemical reaction proceed at the same time on the surface of the hydrogen absorbing alloy by virtue of the contact of the alloy surface with the alkaline electrolyte. More specifically, in the relationship between the hydrogen pressure and the temperature, hydrogen is absorbed by the hydrogen absorbing alloy, or the hydrogen absorbing alloy desorbs hydrogen (gas-phase reaction). In the relationship between the voltage and the current, on the other hand, application of voltage (charging) causes absorption of hydrogen by the hydrogen absorption alloy of the hydrogen produced by the electrolysis of water, and delivery of current (discharge) causes oxidation of hydrogen to water (electrochemical reaction).
To promote such gas-phase reaction and electrochemical reaction on the hydrogen absorbing alloy surface, a powder of electric conductor is added to the electrode, or the hydrogen absorbing alloy surface is chemically treated, whereas if the attempt to improve the hydrogen absorbing alloy is made singly, various characteristics of the hydrogen absorbing alloy are merely improved partially, and appearance of a new limiting factor hampers great improvements in the characteristics of the alkaline secondary battery.
An object of the present invention is to provide a hydrogen absorbing alloy electrode which is given outstanding battery characteristics by simultaneously ameliorating problems encountered with hydrogen absorbing alloys in the electric conductivity, gas affinity, etc. which are associated with the gas-phase reaction and electrochemical reaction.
DISCLOSURE OF THE INVENTION
A hydrogen absorbing alloy electrode embodying the present invention consists mainly of a mixture of a first hydrogen absorbing alloy powder A formed on surfaces of particles thereof with a surface layer
22
containing metallic copper or a copper oxide and a second hydrogen absorbing alloy powder B formed on surfaces of particles thereof with a surface layer
24
containing metallic cobalt or a cobalt oxide.
Copper is an element having a extremely high electric conductivity. The surface layer
22
formed on the surfaces of hydrogen absorbing alloy particles
21
and containing metallic copper or a copper oxide promotes the migration of electrons due to the acceptance of hydrogen atoms by the surface of the hydrogen absorbing alloy, contributing to improvements in high-rate discharge characteristics and suppressing the formation of gas due to a side reaction. Cobalt has high affinity for gases and has catalytic activity to promote dissociation of hydrogen molecules and combination thereof with oxygen atoms on the surface of the hydrogen absorbing alloy. Accordingly, the surface layer
24
formed on the surfaces of hydrogen absorbing alloy particles
23
and containing metallic cobalt or a cobalt oxide promotes absorption of hydrogen by the hydrogen absorbing alloy and a water forming reaction due to the recombination of the gaseous oxygen produced on the surface of the positive electrode and the hydrogen atoms produced on the surface of the negative electrode. This consequently suppresses the rise in the battery internal pressure during the charge-discharge reactions and leads to improved cycle characteristics.
Accordingly, the alkaline secondary batteries incorporating the hydrogen absorbing alloy electrode of the invention exhibit unprecedented excellent battery characteristics which are attributable to the presence of the two kinds of hydrogen absorbing alloys having surface layers of two kinds of metals which are different in effect.
Moreover, the surface layers of the hydrogen absorbing alloy powders protect the exposed active sites on the surface of the alloy from oxidation, thereby contributing also to improvements in cycle characteristics.
Stated specifically, the first hydrogen absorbing alloy powder A is smaller than the second hydrogen absorbing alloy powder B in particle size for use in the mixture. Further the amount (by weight) of the first hydrogen absorbing alloy powder A is not greater than the amount (by weight) of the second hydrogen absorbing alloy powder B in the mixture to be used.
The first hydrogen absorbing alloy powder A is itself capable of absorbing and desorbing hydrogen, therefore present as an active substance and serves also as a conductive material because the copper atoms present in the surface layer have satisfactory electric conductivity. Accordingly, when the particles of the first hydrogen absorbing alloy powder A form a bridge between the particles of the second hydrogen absorbing alloy powder B or between the particles of the second hydrogen absorbing alloy powder B and a current collector, electrons can be transmitted efficiently. Further when the first hydrogen absorbing alloy powder A is smaller than the second hydrogen absorbing alloy powder B in particle size, the first powder A is present in the interstices between the particles of the second powder B, giving an increased fractional solids content to the electrode alloy, so that the alkaline secondary battery incorporating such a hydrogen absorbing alloy electrode exhibits satisfactory battery characteristics.
To obtain satisfactory battery characteristics by using the hydrogen absorbing alloy electrode in alkaline secondary batteries, it is desired that the amount of the second hydrogen absorbing alloy powder B be greater than the amount of the first hydrogen absorbing alloy powder A in the mixture as will be apparent from the experimental result to be given later.
A process for preparing the hydrogen absorbing alloy electrode embodying the present invention has a step of preparing a first hydrogen absorbing alloy powder A, a step of preparing a second hydrogen absorbing alloy powder B, and a step of preparing an electrode by mixing together the two kinds of hydrogen absorbing alloy powders obtained by these steps and forming the mixture into a predetermined shape. In the step of preparing a first hydrogen absorbing alloy powder A, the alloy powder is subjected to a surface treatment with an acid solution containing a copper chloride (CuCl
2
) and adjusted to a pH value of 0.7 to 2.0, and in the step of preparing a second hydrogen absorbing alloy powder B, the alloy powder is subjected to a surface treatment with an acid solution containing a cobalt chloride (CoCl
2
) or cobalt hydroxide (
Fujitani Shin
Higashiyama Nobuyuki
Hirota Yohei
Imoto Teruhiko
Kato Kikuko
Arent Fox Kintner & Plotkin & Kahn, PLLC
Coy Nicole
King Roy
Sanyo Electric Co,. Ltd.
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