A hydrogen-absorbing alloy electrode for use in an alkaline storage cell, comprising a hydrogen-absorbing alloy for reversibly absorbing and desorbing hydrogen; and a metal oxide or metal hydroxide existing in the state of a metal in a range of electric potential where said hydrogen-absorbing alloy electrochemically absorbs and desorbs hydrogen in an alkaline electrolyte.
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4. A hydrogen-absorbing alloy electrode for use in an alkaline storage cell, the electrode comprising:
a hydrogen-absorbing alloy for reversibly absorbing and desorbing hydrogen; and a metal hydroxide existing in the state of a metal in a range of electric potential where said hydrogen-absorbing alloy electrochemically absorbs and desorbs hydrogen in an alkaline electrolyte; wherein said metal hydroxide is at least one selected from the group consisting of Cu(OH)2 , Ti(OH)3 and Au(OH)3.
1. A hydrogen-absorbing alloy electrode for use in an alkaline storage cell, the electrode comprising:
a hydrogen-absorbing alloy for reversibly absorbing and desorbing hydrogen; and a metal oxide existing in the state of a metal in a range of electric potential where said hydrogen-absorbing alloy electrochemically absorbs and desorbs hydrogen in an alkaline electrolyte; wherein said metal oxide is at least one selected from the group consisting of CuO, Cu2 O, Ag2 O, Ag2 O2, PbO, Ti2 O3, AuO2 and Au2 O3.
10. A manufacturing method of a hydrogen-absorbing alloy electrode for use in an alkaline storage cell, comprising the steps of:
producing hydrogen-absorbing alloy powders; kneading the above powders, a metal hydroxide existing in the state of a metal in a range of electric potential where the hydrogen-absorbing alloy electrochemically absorbs and desorbs hydrogen in an alkaline electrolyte, and a binder into a paste, wherein the metal hydroxide is at least one selected from the group consisting of Cu(OH)2 , Ti(OH)3 and Au(OH)3 ; and coating the paste on a conductive substrate.
7. A manufacturing method of a hydrogen-absorbing alloy electrode for use in an alkaline storage cell, comprising the steps of:
producing hydrogen-absorbing alloy powders; kneading the above powders, a metal oxide existing in the state of a metal in a range of electric potential where the hydrogen-absorbing alloy electrochemically absorbs and desorbs hydrogen in an alkaline electrolyte, and a binder into a paste, wherein the metal oxide is at least one selected from the group consisting of CuO, Cu2 O, Ag2 O, Ag2 O2, PbO, Ti2 O3, AuO2 and Au2 O3 ; coating the paste on a conductive substrate.
2. A hydrogen-absorbing alloy electrode of
3. A hydrogen-absorbing alloy electrode of
5. A hydrogen-absorbing alloy electrode of
6. A hydrogen-absorbing alloy electrode of
8. A manufacturing method of
9. A manufacturing method of
11. The method of
12. The method of
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(1) Field of the Invention
The present invention relates to a hydrogen-absorbing alloy electrode for use in an alkaline storage cell and its manufacturing method.
(2) Description of the Prior Art
In recent years, a metal-hydrogen alkaline storage cell employing a hydrogen-absorbing alloy for its negative electrode has attracted attention as a replacement for a nickel-cadmium storage cell. If a hydrogen-absorbing alloy of the right type having the right composition is employed, the metal-hydrogen alkaline storage cell remarkably enhances cell life and energy density. It means that developing an improved hydrogen-absorbing alloy electrode is essential for this type of storage cell.
Conventionally, a hydrogen-absorbing alloy, especially the one having a small grain size, is easy to oxidize on its surface during its pulverization, electrode production, or cell assembly. This oxidation increases contact resistance among the grains of the hydrogen-absorbing alloy, and thus lowers conductivity of the electrode for which the alloy is employed. As a result, charge-discharge efficiency, especially the one during a rapid charge discharge, is decreased. Furthermore, since the rapid charge-discharge generates hydrogen gas in the negative electrode through a side reaction, hydrogen gas partial pressure in the cell is raised. This brings about electrolyte leakage or dryout, thereby deteriorating the cell performance.
The following methods have been proposed in order to solve the above problems: a) adding a metal conductor to the hydrogen-absorbing alloy electrode as disclosed in Japanese Patent Publication Kokai No. 53-103543; and b) coating the hydrogen-absorbing alloy grains with a thin film of copper as disclosed in Japanese Patent Publication Kokai No. 50-111546.
However, these methods still involve the following problems.
a) The metal conductor, which is added in the state of a metal, not in the state of a compound, comprises large grains. Accordingly, it is extremely hard to uniformly distribute it in the electrode and its effects as a conductor are not fully obtained.
b) An additional process of coating the alloy grains with a thin film of copper complicates the procedure, which raises manufacturing cost.
Accordingly, a primary object of this invention is to offer a hydrogen-absorbing alloy electrode for use in an alkaline storage cell and its manufacturing method, the electrode having improved conductivity.
Another object of this invention is to offer a hydrogen-absorbing alloy electrode for use in an alkaline storage cell and its manufacturing method, the electrode having a conductor uniformly distributed therein for lowering contact resistance among grains of its hydrogen-absorbing alloy.
Still another object of this invention is to offer a hydrogen-absorbing alloy electrode for use in an alkaline storage cell and its manufacturing method, the electrode improving a cycle characteristic of its alkaline storage cell during rapid charge-discharge.
Still another object of this invention is to offer a manufacturing method of a hydrogen-absorbing alloy electrode for use in an alkaline storage cell for attaining the above objects without raising manufacturing cost.
The above objects are fulfilled by a hydrogen-absorbing alloy electrode for use in an alkaline storage cell, the electrode comprising a hydrogen-absorbing alloy for reversibly absorbing and desorbing hydrogen; and a metal oxide or hydroxide existing in the state of a metal in a range of electric potential where the hydrogen-absorbing alloy electrochemically absorbs and desorbs hydrogen in an alkaline electrolyte. The above electrode will be referred to as the electrode [A] hereinafter.
The electrode [A] is manufactured by a manufacturing method of a hydrogen-absorbing alloy electrode for use in an alkaline storage cell, comprising the steps of producing hydrogen-absorbing alloy powders; kneading the above powders, a metal oxide or hydroxide existing in the state of a metal in a range of electric potential where the hydrogen-absorbing alloy electro-chemically absorbs and desorbs hydrogen in an alkaline electrolyte, and a binder into a paste; and coating the paste on a conductive substrate.
The metal oxide may be at least one selected from the group consisting of CuO, Cu2 O, Ag2 O, Ag2 O2, PbO, VII VI according to the present invention
Cells B1 through B7 B6 were produced by the same method as Cell A except that Cu2 O, Cu(OH)2, Ag2 O, AuO2, Au(OH)3, and PbO, and Ti2 O3 were respectively used instead of CuO.
Cells B1 through B7 B6 , A and X were subjected to a cycle test to check the decreased weight of each cell after the 50th cycle. The results are shown in Table 1. The experiment conditions were the same as in Embodiment I.
| TABLE 1 |
| ______________________________________ |
| Metal oxide or |
| Cell hydroxide added |
| Decreased weight % |
| ______________________________________ |
| A CuO 0.07 |
| B1 CuO 0.08 |
| B2 Cu(OH)2 0.05 |
| B3 Ag2 O 0.09 |
| B4 AuO2 0.11 |
| B5 Au(OH)3 0.10 |
| B6 PbO 0.21 |
| B7 |
| Ti2 O3 |
| 0.19 |
| X None 0.49 |
| ______________________________________ |
As well as Cell A, Cells B1 through B7 B6 , each of which decreased smaller weight than Cell X, were excellent in cycle characteristic.
Moreover, Cells B1 through B5 (employing an oxide or hydroxide of Cu, Ag or Au, which belong to the Ib group) decreased smaller weight than Cell B6 and B7 (employing other another metal oxides oxide). This is attributed to that the excellent conductivity of Cu, Ag and Au generated by reduction in the hydrogen-absorbing alloy electrode highly enhances the conductivity of the above electrode.
The grain size of the metal oxide or hydroxide employed in the present invention is desirably as small as possible, considering its distribution in the cell. It is confirmed through experiments, though, that good enough effects are obtained if the average grain size is 50μm or less.
If the above metal oxide or hydroxide is added excessively, its reduction requires a huge amount of electricity in the hydrogen-absorbing alloy electrode during the charge-discharge cycles for electrochemical formation or the like. When this occurs, the negative electrode is not charged enough, which damages the balance in the charging amount between the positive and negative electrodes. This deteriorates the cycle characteristic of the cell. Experiments have confirmed that it is desirable to set the amount of the metal oxide or hydroxide in the following way: the amount of electricity required to reduce the metal oxide or hydroxide is obtained first, and the amount of the above metal oxide or hydroxide is set so that the above electricity amount is 15% or less of the capacity of the completed hydrogen-absorbing alloy electrode.
Employable as the hydrogen-absorbing alloy instead of MmNi3.2 CoAl0.2 Mn0.6 are a rare earth alloy such as LaNi3 Co2, Ti-Ni alloy, Ti-Mn alloy, Ti-Fe alloy, Mg-Ni alloy, Ti-Zr alloy or Zr-Mn alloy.
In the above embodiments, the hydrogen-absorbing alloy powders were obtained by pulverizing the hydrogen-absorbing alloy ingot. Needless to say, however, the hydrogen-absorbing alloy powders may be obtained by the atomizing method, the low pressure gas evaporation method, the active hydrogen - molten metal reaction method, the chloride reaction method, the pyrolyzing method or the volatile condensation method.
Although the present invention has been fully described by way of embodiments with references to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.
Tadokoro, Motoo, Furukawa, Nobuhiro, Nogami, Mitsuzo, Kameoka, Seiji
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