Nickel-hydrogen secondary cell

Details Nickel-hydrogen secondary cell Nickel-hydrogen secondary cell

2000-02-08

2002-08-27

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S217000, C429S218200, C420S900000

Reexamination Certificate

active

06440607

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a nickel-hydrogen secondary battery, and more particularly, to a nickel-hydrogen secondary battery which has low internal resistance, which is capable of large current discharge and has improved cycle life, compared with conventional nickel-hydrogen secondary batteries, which can suppress increase in the internal pressure thereof even at the time of quick charge, and which also can supply large current not only at ordinary temperatures but also at low temperatures.
BACKGROUND ART
Various secondary batteries are currently used because of their recharging capability and portability as the power supply for a diversity of applications such as electrically powered tools, electric power-assisted bicycles, and electric vehicles which have recently been developed.
Secondary batteries to be used in these applications are required to have large-current discharge capability, and conventionally nickel-cadmium secondary battery is often used in such applications for the reason stated below.
Namely, nickel-cadmium secondary battery has low internal resistance and provides increased discharge current (large discharge rate) per hour rate, and its battery characteristics are less liable to be deteriorated even if the battery is overcharged or over-discharged.
As the power supply for small-sized electronic devices such as notebook computers or portable telephones, on the other hand, nickel-hydrogen secondary battery, rather than the above nickel-cadmium secondary battery, is widely used for the reason stated below.
Namely, nickel-hydrogen secondary battery, despite its relatively high internal resistance and relatively small discharge rate, has a discharge capacity 1.5 to 2 times as large as that of a nickel-cadmium secondary battery of the same size, and therefore, an electronic device which is driven by a very small current can be driven for a long time even with a small-sized nickel-hydrogen secondary battery.
Nickel-hydrogen secondary battery can be classified into cylindrical type and rectangular parallelepiped type according to shape. The cylindrical type will be outlined below.
The production of positive and negative electrodes will be explained first.
To produce a positive electrode, paste of positive electrode mixture is first prepared by kneading together powder of nickel compound as an active material, such as nickel hydroxide, a binder such as PTFE, a cobalt compound as an electrically conductive material, for example, cobalt oxide or cobalt hydroxide, and water.
A predetermined amount of the paste is then filled, for example, in a spongy, porous metal sheet with a three-dimensional network structure or in an alkali-proof metal sheet (collector sheet) with a porous structure such as a metal fiber mat, dried, and subjected to pressure molding and cutting as needed, to obtain a positive electrode sheet having a predetermined thickness and a predetermined planar shape. In the positive electrode obtained in this manner, therefore, the dried positive electrode mixture is filled in internal voids of the collector sheet and is also supported on the surfaces of same. A small piece of nickel, for example, is attached as a tab terminal to an upper edge portion of the positive electrode.
To produce a negative electrode, on the other hand, paste of negative electrode mixture is prepared first which contains powder of hydrogen absorbing alloy as a major component and which is admixed with a thickener such as carboxymethyl cellulose and an electrically conductive material such as carbon powder.
A predetermined amount of the paste is then applied, for example, to a punched nickel sheet (collector sheet) having a predetermined rate of hole area, dried and subjected to rolling and cutting, to obtain a negative electrode sheet having a predetermined thickness and a predetermined planar shape. Thus, in the negative electrode obtained in this manner, the dried negative electrode mixture is filled in the holes of the collector sheet and is also supported on the surfaces of same.
Like the positive electrode, a tab terminal may be attached as needed to an edge portion of the negative electrode.
Subsequently, using the positive and negative electrodes obtained as described above, an electrode group is formed.
First, as shown in
FIG. 1
, a separator 3 having liquid retentivity and electrical insulation property, for example, nonwoven polyolefin cloth, is sandwiched between a negative electrode
1
, in which the negative electrode mixture is supported on the collector sheet (punched nickel sheet), and a positive electrode
2
, in which the positive electrode mixture is supported on the collector sheet (foamed nickel sheet) and a tab terminal
2
c
is attached to one edge portion thereof, to form a sheet laminate.
With a core placed on the positive electrode
2
of the sheet laminate, the laminate is rolled up in a manner such that the negative electrode
1
is located outside, thereby forming a spiral electrode group having a predetermined outer diameter.
Thus, as shown in
FIG. 2
, the resulting electrode group A has a sectional structure in which the negative electrode
1
and the positive electrode
2
are alternately arranged with the separator interposed therebetween. The core is thereafter removed, so that a hole
4
is left in the center of the electrode group, and the tab terminal
2
c
protrudes from an upper end of the of the electrode group.
The electrode group is then inserted into a battery case with a predetermined inner diameter, and after a predetermined alkali electrolyte is poured into the battery case, an upper opening of the battery case is closed with a sealing plate provided with a positive electrode terminal. Since, in this case, the negative electrode
1
of the electrode group comes into contact with the inner wall of the battery case, the battery case serves as a negative electrode terminal. When the electrode group is inserted into the battery case, the tab terminal
2
c
of the positive electrode
2
is connected to the sealing plate.
In the case of producing a rectangular parallelepiped type secondary battery, an electrode group having a predetermined thickness is formed by stacking negative and positive electrode sheets alternately with a separator interposed therebetween. Accordingly, also in this case, the electrode group has a laminate structure as viewed in section.
While nickel-cadmium secondary battery mentioned above is capable of supplying large current, recently there is a tendency to avoid using nickel-cadmium secondary battery as the power supply for electrically powered tools etc., because cadmium used as its electrode can be harmful to the environment. Attempts have therefore been made to replace nickel-cadmium secondary battery with nickel-hydrogen secondary battery, which is pollution-free and which has higher capacity than nickel-cadmium secondary battery where the batteries are of identical size.
Nickel-hydrogen secondary batteries conventionally available on the market, however, can provide a capacity equivalent to their nominal capacity only when they are discharged at a rate 1 to 3 times the 1-hour rate. Further, this performance is attained only at ordinary temperatures and is not available in a low-temperature environment of, for example, −10° C. or less. For example, if large current discharge is effected at a rate exceeding 5 times the 1-hour rate, the operating voltage greatly drops, and also if nickel-hydrogen secondary battery is used at low temperatures, the operating voltage drastically drops to such an extent that the battery is of no practical use.
Accordingly, although conventional nickel-hydrogen secondary batteries are useful as the power supply for small-sized electronic devices which can be driven by a very small current, they cannot be practically used as a large-current power supply, especially as the power supply for electrically powered tools or electric vehicles that are often used at low temperatures.
Also, in nickel-hydrogen secondary batteries in general, as the charge-discharge

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