Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-04-03
2002-04-30
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S209000, C429S223000, C429S236000, C029S002000
Reexamination Certificate
active
06379845
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrically conductive porous body obtained by giving electrical conductivity onto the surface of the framework of a plastic porous body having a continuous-pore structure, to a metallic porous body produced by using the conductive porous body as an intermediate material, which metallic porous body is especially suitable for a plate used in batteries such as alkaline secondary batteries; and to a battery plate produced by using the metallic porous body.
2. Description of the Background Art
Alkaline secondary batteries have been widely used as a power source for various devices because they are highly reliable and can be reduced in size and weight. There are a variety of sizes from small types for portable devices to large types for industrial or large-scale equipment.
There are many types of alkaline secondary batteries have in terms of the combination of the positive and negative electrodes. While most cases, a nickel electrode is used as the positive electrode, various negative electrodes are used, such as, a cadmium electrode, a zinc electrode, an iron electrode, a hydrogen electrode, and so on. Of these, a cadmium electrode is the most common. Yet, a hydrogen electrode having a hydrogen-absorbing alloy as the active material has been the focus of attention with regard to capacity increase and pollution decrease.
Among the foregoing electrodes, the so-called “pocket type” nickel electrode was conventionally used. However, a new type of nickel electrode has been in popular use in recent years. The new type is produced by filling large quantities of particles of an active material for the positive electrode, such as nickel hydroxide, into the pores of a porous current-collecting plate made of a conductive material such as nickel. This type enables a battery to be hermetically sealed and can further improve the battery properties compared to the pocket type. Also, a cadmium electrode and a hydrogen electrode are produced by filling large quantities of active materials into the pores of a porous plate for the negative electrode, such as cadmium or hydrogen-absorbing alloy.
Formerly a sintered body of a nickel powder was used as a porous plate to be filled with an active material. However, a new type of metallic porous body has been increasingly used as the porous plate in recent years. The new type is produced by using a plastic porous body (a polyurethane foam, for example) that has a continuous-pore structure with high porosity as the core, because this type allows the filling of greater quantities of active materials compared to the sintered body, and is suitable for increasing the battery capacity.
Such a metallic porous body is usually produced by the following method:
First, conductivity is given to the plastic core by either of the following methods:
(1) The surface of the framework of the plastic core is treated with a catalyst such as palladium chloride. Next, conductivity is given onto the treated surface by electroless plating such as electroless nickel plating.
(2) A mixed-binder solution containing conductive carbon particles such as graphite is applied to the surface of the framework of the plastic core. Next, the solution is dried to complete the process of giving conductivity to the plastic core.
Second, a continuous metal-plated layer (a nickel-plated layer, for example) is formed on the surface of the conductive framework of the plastic core by electroplating with a metal (nickel, for example) with the conductive core (the conductive porous body) serving as the cathode. Finally, if necessary, the core is removed by heat treatment.
The electroless plating described in the method (1) above, however, is costly because it uses palladium, which is a noble metal. Moreover, if the palladium is admitted into the electroplating liquid, a treating liquid for the succeeding process, a rapid reducing reaction of nickel ions represented as “nickel-dust formation” occurs, consuming nearly all the nickel in the plating liquid, and which prevents further use of the liquid as a plating liquid. The term “nickel-dust formation” is used in the present invention to mean a phenomenon that nickel deposits onto palladium particles, being suspended in an electroplating liquid.
It is difficult to handle the conductive porous body produced by the method (1) above. The conductive porous body tends to increase its electrical resistance significantly when undergoing deformation during the following processes:
(a) In a continuous production system where a metallic porous body is produced continuously from the core through the conductive porous body, when the conductive porous body is supplied to the electroplating process, a process following the electroless plating, the conductive porous body is drawn longitudinally by bending or tension.
(b) When the required quantities of conductive porous bodies are grouped as a batch to be supplied to the subsequent electroplating process, the conductive porous body is wound into the form of a roll or hoop.
(c) After the required quantities of the cores wound into the form of a roll or hoop as a batch are treated in the process for obtaining conductivity, when the conductive porous body thus produced is supplied to the subsequent continuous-electroplating process, the conductive porous body is unwound from the form of a roll or hoop.
If the conductive porous body increases its electrical resistance significantly, this increase reduces the growth rate of the metal-plated layer, such as a nickel-plated layer, in the electroplating process. This rate reduction may reduce the productivity or production efficiency of the metallic porous body.
The conductive layer formed on the surface of the framework of a core by electroless plating is an extremely thin, continuous metallic film. Electroless nickel plating, for instance, produces a metallic film as thin as 0.1 &mgr;m or so. The metallic film cracks or folds easily when the conductive porous body undergoes deformation caused by the bending, drawing winding, or unwinding as described above. The cracks or folds reduce the conductivity of the conductive porous body, increasing the electrical resistance significantly as mentioned above. The result is the reduction in the growth rate of the metal-plated layer, such as a nickel-plated layer, in the electroplating process.
In order to reduce the tension applied to, the conductive porous body, for instance, a decrease in the feeding speed and an increase in the radius of curvature in bending and winding have been discussed. These solutions, however, pose new problems such as the reduction in the productivity or production efficiency of the metallic porous body and the necessity for a greater area of space for the production equipment or the facilities for material handling or storing.
Electroless nickel plating, the most popular electroless plating described in the method (1) above, has used sodium hypophosphite (NaH
2
PO
2
·H
2
O) or sodium boron hydride (NaBH
4
) as a reducing agent to deposit nickel ions in the plating liquid as a metal. Consequently, the conductive layer formed on the surface of the framework of the core is inevitably made of a nickel-phosphorus alloy or a nickel-boron alloy each of which contains several percentages of phosphorus or boron derived from the reducing agent as an impurity.
In this case, when the core is removed by heat treatment after the metal-plated layer, such as a nickel-plated layer, is formed on the conductive layer by electroplating, the foregoing impurity such as phosphorus diffuses into the metal-plated layer, increasing the electrical resistance of the metallic porous body produced. As a result, a battery having a plate made with such a metallic porous body may suffer a reduction in charging and discharging efficiency or may undergo deterioration in charging and discharging properties caused by the dissolution of the phosphorus into the electrolyte after prolonged and repeated charging and discharging.
Similarly, the conductive po
Inazawa Shinji
Kariya Ayao
Majima Masatoshi
Chaney Carol
Sumitomo Electric Industries Ltd.
Yuan Dah-Wei
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