Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-07-07
2001-01-09
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S218100, C429S206000, C029S623500
Reexamination Certificate
active
06171728
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nickel electrode for an alkaline storage cell such as a nickel-hydrogen storage cell, a nickel-cadmium storage cell, a nickel-zinc storage cell and the like and a manufacturing method for the same. More particularly, the present invention relates to a nickel electrode made of a nickel hydroxide active material containing a conductive agent and a manufacturing method for the same.
2. Description of the Prior Art
In recent years, demands for a high performance storage cell have increased for use in rapid popularization of portable electronic and communication equipments. In the field of alkaline storage cells, there have been proposed various improvements for enhancing the utilization factor of a nickel hydroxide active material in a positive electrode of the alkaline storage cell. For example, there have been proposed a method for adding a cobalt compound or metal nickel powder to a nickel hydroxide active material as a conductive agent and a method for precipitating a cobalt compound or metal nickel powder on the surface of a nickel hydroxide active material.
Although the conductivity of a cobalt compound having an oxidation value of 2 is low, the cobalt compound becomes a higher order state when the oxidized at during initial charge and discharge of the cell. In addition, cobalt hydroxide is oxidized by charging of the cell and precipitated on the surface of the nickel hydroxide active material as oxy-cobalt hydroxide. When the cell is discharged, the oxy-cobalt hydroxide is partly reduced and dissolved in electrolyte. With such reaction of the cobalt hydroxide, a conductive network is uniformly formed on the surface of the nickel hydroxide active material. This is useful to reduce the electrical potential of an isolated portion and to enhance the utilization factor of the active material.
In the method of precipitating a cobalt compound on the surface of the nickel hydroxide active material, 1 discharge reaction of the active material occurs as represented by the following formula (1).
NiOOH+H
2
O+e
—
→Ni(OH)
2
+OH
—
(1)
The discharge reaction of the formula (1) is effected on the surfaces of nickel hydroxide particles in spherical form under the presence of a sufficient amount of H
2
O and OH
—
. For this reason, if the amount of H
2
O and OH
—
is insufficient, the cobalt compound ineffective in the reaction of formula (1) is uniformly precipitated on the surfaces of the nickel hydroxide particles. This decreases the utilization factor of the active material.
On the other hand, if the conductive layer is irregularly formed on the nickel hydroxide particles, the utilization factor of the active material is enhanced by the reaction of formula (1), but it becomes difficult to uniformly form irregular conductive layers on the nickel hydroxide active material.
SUMMARY OF THE INVENTION
It is, therefore, a primary object of the present invention to provide a nickel electrode for an alkaline storage cell superior in utilization factor of an active material and to provide a method capable of manufacturing the nickel electrode in a simple manner.
According to the present invention, the object is accomplished by providing a nickel electrode for an alkaline storage cell, which comprises a first active material essentially composed of nickel hydroxide particles covered with a conductive substance lower in dissolubility to aqueous alkaline solution, and a second active material essentially composed of nickel hydroxide particles without forming the conductive substrate, wherein the nickel electrode is formed by a mixture of the first and second active materials.
In a practical embodiment of the present invention, it is preferable that the conductive substance of the first active material is metal nickel or a cobalt compound of more than +2 in oxidation value and is 1 to 10 wt % with respect to the entirety of the first and second active materials, and it is also preferable that the second active material is 10 to 40 wt % with respect to the first active material.
According to an aspect of the present invention, there is provided a manufacturing method for a nickel electrode for an alkaline storage cell, which comprises the steps of precipitating cobalt hydroxide on the surface of a first nickel active material essentially composed of nickel hydroxide, subjecting the first nickel active material to heat treatment under the presence of an alkaline solution and oxygen so that the cobalt hydroxide precipitated on the first nickel active material is disordered in its crystal structure and is formed in a higher order cobalt compound containing alkaline cation, mixing the first nickel active material subjected to the alkali heat treatment with a second nickel hydroxide active material essentially composed of nickel hydroxide without forming any conductive substance lower in dissolubility to the alkaline solution.
In the manufacturing method of the nickel electrode, it is preferable that the second nickel hydroxide active material is 10 to 40 wt % with respect, to the first nickel hydroxide active material subjected to the alkali heat treatment, and it is also preferable that an amount of cobalt in the higher order cobalt compound is 1 to 10 wt % with respect to the entirety of the first and second nickel hydroxide active materials.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Preparation of Active material slurry
Embodiment
While an amount of aqueous solution mixed with nickel sulfate, zinc sulfate and cobalt sulfate at a ratio by weight of 100:3:1 was stirred, an amount of aqueous solution of sodium hydroxide and ammonia was gradually added so that pH of the reaction solution was maintained at 13-14 to precipitate nickel hydroxide particles. Thereafter, an amount of aqueous solution of cobalt sulfate of specific gravity 1.30 and 25 wt % sodium hydroxide was added to the solution containing the precipitated nickel hydroxide particles so that, the pH of the reaction solution was maintained at 9-10 to precipitate cobalt hydroxide around crystal cores in the form of the precipitated nickel hydroxide particles. The precipitated particles were collected, washed by water and dried to produce a nickel hydroxide active material of the nickel-zinc-cobalt type in the form of nickel hydroxide particles covered with the cobalt hydroxide. When the cobalt hydroxide was formed on the surface of the nickel hydroxide active material, 7 wt % cobalt hydroxide relative to the entirety of the nickel hydroxide active material was produced.
In the nickel hydroxide active material of the nickel-zinc-cobalt type, the surfaces of the nickel hydroxide particles were covered with the cobalt hydroxide precipitated thereon. Although a small amount of dissolved nickel compound was impregnated in the nickel hydroxide particles prior to addition of alkaline solution composed of the aqueous solution of cobalt sulfate and aqueous solution of sodium hydroxide, the dissolved nickel compound was precipitated in the nickel hydroxide particles integrally with the cobalt hydroxide by addition of the alkaline solution. As a result, the cobalt hydroxide was formed on the surfaces of the nickel hydroxide particles as a thick layer and formed in the interior of nickel hydroxide particles as a thin layer.
The nickel hydroxide particles covered with the cobalt hydroxide were sprayed with alkaline solution (35 wt % sodium hydroxide) under heat stream in an oxygen atmosphere. In this instance, the heat temperature of the nickel hydroxide particles was maintained at about 60° C. and raised up to 90° C. after an amount of alkaline solution (35 wt % sodium hydroxide) five times the amount of cobalt was sprayed on the nickel hydroxide particles. During the alkali heat treatment, the cobalt hydroxide formed on the nickel hydroxide particles was disordered in its crystal structure, while oxidation of the nickel hydroxide was strongly effected to produce a higher order cobalt compound of more than +2 in average oxidation va
Baba Yoshitaka
Tadokoro Motoo
Yano Takayuki
Kalafut Stephen
Martin Angela J.
Nixon & Vanderhye
Sanyo Electric Co,. Ltd.
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