Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-08-27
2001-11-27
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C420S900000
Reexamination Certificate
active
06322925
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a metal hydride alkaline storage cell and a manufacturing method thereof, and more particularly to a metal hydride alkaline storage cell having an improved hydrogen-absorbing alloy surface and a manufacturing method thereof.
(2) Description of the Prior Art
The cell performance of a metal hydride alkaline storage cell employing hydrogen-absorbing alloy as a negative electrode active material largely depends on the degree of activation of the hydrogen-absorbing alloy therein. For this reason, a hydrogen-absorbing alloy used in this type of storage cells is pulverized into fine powder to enlarge a reaction area involved in the electrochemical reaction, to intensify the filling density in the electrode substrate, and thereby to enhance the energy density.
However, since hydrogen-absorbing alloy is a very active substance, it is corroded as a charge-discharge cycle is repeated, which results in a short cycle life.
In light of this problem, there has been suggested a hydrogen-absorbing alloy electrode wherein a metal (cobalt, nickel, and the like) oxide or hydroxide layer is formed on the surface of hydrogen-absorbing alloy powder.
Nevertheless, this type of hydrogen-absorbing alloy electrode still has such shortcomings that, when oxygen gas is generated in overcharging, the contact of the oxygen gas with the hydrogen-absorbing alloy is hindered by the metal oxide layer etc. on the surface of the hydrogen-absorbing alloy powder, which results in deterioration of absorbing performance of the oxygen gas and, consequently, high-rate charge characteristic.
In view of the above problems, Japanese Unexamined Patent Application No. 08-333603 discloses a cell wherein a coating film dotted with cobalt or the like is formed directly on the surface of the hydrogen-absorbing alloy powder without intermediary of an oxide.
In the cell according to the above-mentioned composition, however, there is no oxide formed on the surface of the powder, and the alloy surface therefore becomes flat and smooth with a small reaction area. Consequently, although cobalt or the like is dotted on the alloy surface, its effect as a catalyst cannot be sufficiently utilized.
The manufacturing method of the hydrogen-absorbing alloy with the above-mentioned composition comprises the steps of removing an oxide film on the hydrogen-absorbing alloy powder, then dotting the surface with cobalt or the like by dipping the powder into an aqueous solution containing metallic ions of cobalt or the like, and then washing the powder with water. In the washing process, however, cobalt or the like coated on the surface of the hydrogen-absorbing alloy is likely to be exfoliated and/or oxidized, and therefore cannot exhibit sufficient effect as a catalyst. Moreover, the manufacturing method of the above-mentioned invention requires such complex steps as removing an oxide film on the surface of the hydrogen absorbing alloy powder, washing, and the like, and consequently incurs high manufacturing cost.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a metal hydride alkaline storage cell having a remarkably improved internal pressure characteristic (high-rate charge characteristic) by sufficiently utilizing the catalytic function of a metal or metal compound on the surface of a hydrogen-absorbing alloy.
It is another object of the present invention to provide a low-cost manufacturing method of a metal hydride alkaline storage cell without complex steps.
One of the aforementioned objects is achieved by a metal hydride alkaline storage cell comprising a positive electrode, a separator impregnated with an electrolyte, and a negative electrode comprising hydrogen-absorbing alloy powder, wherein the hydrogen-absorbing alloy powder has a layer of hydrogen-absorbing alloy oxide formed on the surface thereof, and a catalytic metal or metal compound is dotted on the oxide layer in a granular state by adding a substance selected from the group consisting of a metal fluoride, a metal chloride, a metal iodide, and a metal sulfide, and the proportion of the substance to be added is restricted within the range of from 0.1 to 2.5 wt. % based on the weight of the hydrogen-absorbing alloy powder.
The reason why the aforementioned object is achieved is as follows:
When a layer of hydrogen-absorbing alloy oxide is formed on the surface of hydrogen-absorbing alloy powder, the oxide layer has roughness and thereby results in a large reaction area on the alloy surface. Consequently, when a catalytic metal or metal compound id dotted on the alloy surface, the catalytic action of the metal or metal compound can be fully utilized, and thereby the internal pressure characteristic (high-rate charge characteristic) of the cell is improved.
It is to be noted that the proportion of the metal compound, such as a metal fluoride, to the hydrogen-absorbing alloy powder should be restricted within 0.1 to 2.5 wt. % in the above composition. This is due to the following reasons. First, if the proportion of the metal compound is less than 0.1 wt. %, the catalytic action of the catalytic metal is not sufficiently utilized. On the other hand, if the proportion exceeds 2.5 wt. %, various characteristics of the cell are deteriorated because the amount of the hydrogen-absorbing alloy is reduced in relation to that of the metal compound, and moreover, the absorption of the oxygen gas generated in overcharging is hindered since the surface of the hydrogen-absorbing alloy powder is entirely covered with a reduced metal or metal compound (i.e., a metal or metal compound is not dotted on the surface).
The metal fluoride in the above composition may be at least one metal fluoride selected from the group consisting of a cobalt fluoride, a nickel fluoride, an aluminum fluoride, and a copper fluoride.
When a cobalt fluoride and the like is employed, the deterioration of cell characteristics due to excessive oxidation of a hydrogen-absorbing alloy can be prevented because fluorides are capable of preventing a hydrogen-absorbing alloy from being oxidized.
The metal fluoride in the above composition may be CoF
2
and/or NiF
2
.
When CoF
2
or NiF
2
is employed, the inner pressure characteristic of the cell can be further improved because cobalt and nickel are excellent in catalytic function.
The metal chloride in the above composition may be a cobalt chloride and/or a nickel chloride.
The metal iodide in the above composition may be a cobalt iodide and/or a nickel iodide.
The metal sulfide in the above composition may be a cobalt sulfide and/or a nickel sulfide.
The hydrogen-absorbing alloy powder in the above composition may be selected from the group consisting of rare-earth element based hydrogen-absorbing alloy powder, Zr—Ni based hydrogen-absorbing alloy powder, Ti—Fe based hydrogen-absorbing alloy powder, Zr—Mn based hydrogen-absorbing alloy powder, Ti—Mn based hydrogen-absorbing alloy powder, and Mg—Ni based hydrogen-absorbing alloy powder.
In addition, the hydrogen-absorbing alloy powder in the above composition can comprise hydrogen-absorbing alloy that has a CaCu
5
type crystal structure and is expressed by the general formula, MmNi
a
Co
b
Al
c
Mn
d
(a>0, b>0,c>0, d≧0, and 4.4≦a+b+c+d≦5.4).
Another object of the present invention is achieved by a manufacturing method of a metal hydride alkaline storage cell comprising the following steps, which are:
the first step of producing a negative electrode by applying a paste on to a substrate, wherein the paste contains hydrogen-absorbing alloy powder and a metal compound which is soluble in the electrolyte and selected from the group consisting of a metal fluoride, a metal chloride, a metal iodide, and a metal sulfide, and the proportion of the aforementioned metal compound to the hydrogen-absorbing alloy powder is from 0 1 to 2.5 wt. %; and
the second step of placing the above negative electrode and a positive electrode into a cell can with disposing a separator therebetween, and then pourin
Maeda Reizo
Matsuura Yoshinori
Nishio Koji
Nogami Mitsuzo
Shinyama Katsuhiko
Armstrong Westerman Hattori McLeland & Naughton LLP
Chaney Carol
Sanyo Electric Co,. Ltd.
Tsang Susy
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