Metal working – Battery-grid making
Reexamination Certificate
2002-11-04
2004-08-10
Tsang-Foster, Susy (Department: 1745)
Metal working
Battery-grid making
C429S236000, C205S160000, C205S169000, C442S052000, C442S379000
Reexamination Certificate
active
06772489
ABSTRACT:
The present application is based on Japanese application 2000-043017, filed on Feb. 21, 2000, and Japanese application 2001-002728, filed on Jan. 10, 2001 which are both hereby incorporated by reference in their entireties.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a collector for alkaline secondary batteries including a plated nonwoven fabric, to a method for making the same, and to an alkaline secondary battery using the same.
2. Description of the Background
Alkaline secondary batteries, which are highly reliable and are suitable for a reduction in weight, are widely used as power sources for various devices and apparatuses from portable devices to industrial large facilities. In most alkaline secondary batteries, nickel electrodes are used as positive electrodes. A nickel electrode has a structure including a collector for collecting electricity and a positive-electrode active material inducing a cell reaction supported on the collector. As collectors in this case, a sintered nickel plate formed by sintering nickel powder and a punched nickel plate have been widely used. The cell capacity is determined by the volume of the active material loaded in pores in such a nickel plate, and the volume of the loaded active material depends on the porosity of the nickel plate. Thus, it is preferable that the porosity of the nickel plate be as large as possible.
However, in sintered nickel plates and punched nickel plates, the porosity is as low as 75% to 80%. Moreover, the nickel content in a nitrate solution is low. Thus, the loading cycle for impregnation and neutralization must be repeated several times in order to load a predetermined amount of active material. Since the penetration of the nitrate solution into the interior of the nickel plate is impaired as the loading cycle is repeated, high density loading of the active material is barely achieved. Recently, a collector with a three-dimensional network structure has been used in order to enhance loading density of the active material into the collector to meet the requirements for higher capacity of batteries, since this structure has large porosity and thus can has high loading density for the active material.
The collector having the three-dimensional network structure is generally fabricated as follows. A porous network structure, such as a polyurethane foam sheet or an organic nonwoven fabric is plated with nickel by a known process, and is fired in a reducing atmosphere to pyrolyze the polyurethane sheet or the fabric so that the plated nickel network skeleton remains. In the resulting collector, a portion for an external terminal is flattened, the pores are filled with an active material paste, and a small nickel piece as an external terminal is spot-welded to the flattened position. Since the resulting collector has large pores and the porosity is as large as 90 to 98%, pasted nickel hydroxide can be directly loaded into the pores with high loading density. This collector contributes to an increase in capacity of alkaline secondary batteries.
However, this three-dimensional network structure does not have strength required for the collector and is too rigid. Thus, producing an electrode using this collector and assembling the electrode into a battery cause the following problems. When an active material paste with high viscosity is loaded into the collector, the active material paste is injected from the surface into the internal pores of the collector under a predetermined pressure. After the loaded active material paste is dried, the collector is rolled to increase the density and to optimize the electrode thickness, and is cut into pieces with a predetermined size. When the pressure applied to the paste is increased to improve the loading density of the paste, the nickel network skeleton of the collector may buckle or chip. Thus, the pressure on the active material paste must be reduced to avoid such buckling or chipping. However, desirable loading density of the paste is not achieved under a low pressure.
Since the nickel itself constituting the network skeleton is rigid, the network skeleton will leave cracks and projections such as scuffing on the outer periphery of the electrode using this collector, with chipping of the network skeleton in many cases, during winding the collector with a separator in the assembly of a cylindrical storage battery. These projections increase the electrical resistance of the electrode and impair the function of the collector and charge/discharge characteristics of the battery. In a prismatic storage battery using this collector, the collector swells due to a change in volume of the active material during charge/discharge cycles in some cases. Hence, separation may occur between the collector and the active material, or in the active material, resulting in deterioration of charge/discharge characteristics due to deterioration of the collector itself.
In addition, this collector with three-dimensional network structure is produced by many complicated steps with low productivity and relatively high cost. Moreover, the metal, i.e., nickel, which is only the constituent of the collector, precludes a decrease in thickness or weight of the collector. Accordingly, this metallic collector does not sufficiently meet the requirements for a decrease in weight and size.
In order to overcome this problem, Japanese Unexamined Patent Application Publication No. 8-329956 discloses a collector having a three-dimensional network structure. In this collector, a polyurethane foam sheet or a polyolefin nonwoven fabric is plated with nickel so as to impart conductivity to only the surface of the sheet or nonwoven fabric without pyrolyzing the sheet or nonwoven fabric. This collector can be produced by simpler steps, is flexible, and has relatively high strength, in comparison with the above-mentioned pyrolyzed collector with a three-dimensional network structure. No crack or projection causing scuffing forms during winding an electrode using this substrate together with a separator to assemble a cylindrical or prismatic battery. This collector exhibits improved charge/discharge characteristics and can meet the requirement for a reduction in weight and size.
However, in this collector, adhesion is insufficient between the polyurethane foam sheet or polyolefin nonwoven fabric and the plated nickel. When this collector is used as a nickel electrode of a nickel-hydrogen battery, the collector does not have a satisfactory function in a combination with a nickel hydroxide active material. Thus, it is difficult to assemble high-capacity batteries.
Japanese Unexamined Patent Application Publication No. 5-290838 discloses a method for making a nonwoven fabric electrode in which the nonwoven fabric is corona-treated prior to nickel plating. The corona-treated nonwoven fabric exhibits higher bonding strength to the plated layer compared to untreated nonwoven fabrics.
However, in this method, the bonding strength between the base material and the plated layer is still insufficient in practice. When this collector is used as a nickel electrode in a nickel-hydrogen battery, the plated layer undergoes a change in quality or partial scaling during assembling a battery and repeated charge/discharge cycles of the battery. The resulting battery shows a short charge/discharge cycle life at high temperatures, resulting in an abrupt decrease in capacity.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a collector for an alkaline secondary battery exhibiting improved adhesiveness to plated nickel and a method for making the same.
It is another object of the present invention to provide an alkaline secondary battery which can be easily assembled and exhibits a high discharge rate and improved charge/discharge cycle characteristics.
According to a first aspect of the present invention, a collector for an alkaline secondary battery comprises a nonwoven fabric hydrophilized by sulfonation, gaseous fluorine treatment, or vinyl monomer grafting, a
Imai Hiroyuki
Satou Kazuya
Takase Toshiaki
Yoshida Kaori
Mitsubishi Materials Corporation
Tsang-Foster Susy
LandOfFree
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