Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-08-11
2003-07-15
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S223000, C252S182100, C420S900000
Reexamination Certificate
active
06593031
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nickel metal-hydride cell. In particular, the present invention relates to a nickel metal-hydride cell having improved storage properties at a high temperature.
2. Prior Art
With the progress of the miniaturization of portable electronic equipment, safer secondary cells having larger capacities are desired. Thus, various investigations have been made to further increase the capacities of nickel metal-hydride cells comprising a hydrogen absorbing alloy as a negative electrode active material. As a hydrogen absorbing alloy used as the negative electrode active material, Laves alloys (AB
2
type alloys) comprising Zr, Ni, V and Mn, Mg—Ni alloys (A
2
B type alloys) comprising Mg and Ni, and also rare earth metal alloys (AB
5
type alloys) comprising a rare earth element and Ni are well known. Among them, misch metal (Mm) alloys (MmNi
5
) comprising a misch metal as a rare earth element is widely used as an electrode material since it can be easily activated and has a high ability to absorb and desorb hydrogen.
It is suitable for the hydrogen absorbing alloys to have a hydrogen equilibrium pressure of 20 to 500 kPa at room temperature judging from the charge-discharge efficiency, in addition to the high hydrogen absorbing capacity. However, the hydrogen equilibrium pressure of the MmNi
5
type alloys is as high as 1 MPa at room temperature, and therefore it should be lowered. Furthermore, the MmNi
5
type alloys have problems that the alloys are finely pulverized and suffer from the change of the composition and therefore the hydrogen absorbing ability and the reactivity tend to decrease, when they are repeatedly charged and discharged in an alkaline electrolytic solution.
Thus, MmNi
5
type alloys in which a part of Ni is replaced with other metal such as Mn attract attention, since they have a low hydrogen equilibrium dissociation pressure and good hydrogen absorbing abilities, and the life of the alloys can be extended in the charge-discharge reaction. Such misch metal alloys are disclosed in JP-B-5-15774, JP-B-5-86029, JP-A-1-162741, etc., and are characterized in that they have the substantially stoichiometric composition containing a relatively large amount of cobalt to prevent the pulverization and to improve the corrosion resistance against the electrolytic solution. In addition to such alloys having the stoichiometric composition, those having a non-stoichiometric composition containing the increased amount of elements on the B site (Ni site) are proposed.
With the miniaturization of the portable electronic equipment which uses a cell as a primary powder source, the occasions to carry and use the cells increase, and the cells are used in a wider variety of circumstances than ever. Thus, it is required for cells to exert the steady performances without being influenced by the change of environment, in particular, the change of temperature. For example, when a cell is used in a lap top type personal computer or a cellular phone, it is required to exhibit the same performances at low or high temperature as those at room temperature. Therefore, the requirements for the cells become severer. Accordingly, in the case of cells used in such electronic equipment, the further improvement of temperature characteristics is being studied.
However, a nickel metal-hydride cell, which comprises the above misch metal type alloy as a hydrogen absorbing alloy used as a negative electrode active material and a paste type nickel electrode as a non-sintered type positive electrode, suffers from the severe decrease of the cell voltage, when it is maintained at a high temperature atmosphere of 80° C. In particular, a multi-element type misch metal alloy, in which a part of Ni elements of MmNi
5
are replaced with at least Mn, suffers from the significant decrease of the voltage under the high temperature atmosphere.
Since the decrease of the voltage during the storage in a high temperature atmosphere will have large influences on the cell capacity after storage, it is very important to improve the high temperature storage properties of the nickel metal-hydride cells.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a nickel metal-hydride cell comprising a rare earth metal based hydrogen absorbing alloy as a negative electrode active material and a paste type nickel electrode as a non-sintered type positive electrode, which has improved storage properties at high temperature, that is, which suffers from less decrease of the voltage during the storage in a high temperature atmosphere and has a large recovery rate of a capacity after the storage in a high temperature atmosphere.
To achieve the above object, the present inventors studied the decrease of the voltage of a nickel metal-hydride cell after the storage in a high temperature atmosphere, when the cell comprises a non-sintered type positive electrode consisting of a paste type nickel electrode to which cobalt or a cobalt compound is added as a conducting aid, and a negative electrode comprising a MmNi
5
type hydrogen absorbing alloy containing Mn. Thus, it was found that, as shown as Curve
1
b
in
FIG. 2
(the results obtained in Comparative Example 1), the voltage decreased by two steps. As a result of the investigation into the causes of the voltage decrease, it was found that the first step decrease of the voltage down to about 0.9 V was due to the reduction of the positive electrode with hydrogen which was desorbed from the hydrogen absorbing alloy as the temperature rose, while the second step decrease of the voltage down to 0 V was due to the micro-short circuits formed in the cell.
Furthermore, it was found that the first step decrease of the voltage was caused as follows:
As the ambient temperature rises during the storage, the equilibrium dissociation pressure of the hydrogen absorbing alloy increases, and thus hydrogen, which is absorbed in the negative electrode as a discharge reserve and may not be essentially desorbed, is desorbed in the cell in the form of a hydrogen gas and then reduces the positive electrode. As a result, the storage properties at high temperature deteriorate. The reason for this phenomenon may be thought as follows:
The positive electrodes of the nickel metal-hydride cells include a sintered type positive electrode and a non-sintered positive electrode which is a paste type nickel electrode. The non-sintered type positive electrode may be produced by dispersing nickel hydroxide, a binder, a thickening agent, etc. in water or a solvent to form a paste, and filling the paste in a conductive porous material which functions as a collector. In this case, a distance between the active material and a substrate increases and thus the utilization ratio of the active material tends to decrease. To increase the utilization ratio to achieve the high capacity, a cobalt conducting aid such as metal cobalt, cobalt monoxide, cobalt hydroxide, etc. is added to the positive electrode. Such cobalt conducting aids form cobalt oxide, for example, CoOOH which has a higher valency than the divalent in the course of charging, and thus a conductive network of cobalt oxide oxide which electrically connects the particles of nickel hydroxide is formed.
In the nickel metal-hydride cell, the generation of gasses from the negative electrode in the final period of charging or discharging is suppressed by making the capacity of the negative electrode larger than that of the positive electrode so that the oxygen gas generated from the positive electrode can be absorbed with the negative electrode. Thus, the tight sealing of the cell is achieved. That is, even after the positive electrode is fully charged in the course of charging, or completely discharged, the negative electrode still has a non-charged or non-discharged part, which allows the positive electrode to preferentially generate oxygen gas. Therefore, the generation of the gasses from the negative electrode is suppressed. The excessive capacity of the negative electr
Fukunaga Hiroshi
Isogai Masato
Nagai Ryo
Onishi Masuhiro
Alejandro Raymond
Hitachi Maxell Ltd.
Kalafut Stephen
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