Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-12-01
2002-12-31
Maples, John S. (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S059000, C429S218200, C429S223000
Reexamination Certificate
active
06500582
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of producing an electrode for a battery and the electrode produced by the method. More particularly, the present invention relates to a method of forming a metallic porous foil from metal powder, filling a void of the metallic porous foil with a powder of an active substance, and fixing it to a surface of a sheet of the metallic porous foil. The method is preferably used to produce a negative electrode of a nickel hydrogen battery. In addition, the method is preferably used to produce a positive electrode of the nickel hydrogen battery, and further to produce electrodes of various kinds of batteries such as a nickel cadmium battery, a lithium primary battery, a lithium secondary battery, an alkaline dry cell, a fuel cell, and a battery for a vehicle.
2. Description of the Related Art
Conventionally, a negative electrode of the nickel hydrogen battery is produced as follows: Hydrogen-storing alloy powders are kneaded with binder (binding agent), carbon (electrically conductive material), and the like to obtain a pasty active substance. Then, the pasty active substance is filled into three-dimensional pores of a three-dimensional metallic porous plate such as a foamed sheet-shaped metallic porous plate and a nonwoven fabric-like metallic porous plate used as a base material of an electrode; or the pasty active substance is applied to a metallic porous plate produced by forming pores on a metallic plate such as a punching metal, a lath, and the like. Finally, the metallic porous plate is passed between a pair of pressure rollers one to four times to pressurize the plate after the pasty active substance is dried to produce the electrode.
However when the metallic porus plate having the three-dimensional pores is pressurized after the pasty active substance is applied thereto, a skeleton surrounding the three-dimensional pores is destroyed by the powder of the active substance. Therefore, an electrode thus formed is not flexible and is hard.
More specifically, the thickness of the skeleton surrounding the pores of the foamed metallic porous plate and the nonwoven cloth-like metallic porous plate is as small as 30-50 &mgr;m, and the hydrogen-storing alloy powders used as the active substance of the nickel hydrogen battery are hard. Thus, the hydrogen-storing alloy powders may destroy the skeleton of the pores of the foamed metallic porous plate and the nonwoven cloth-like metallic porous plate.
On the other hand, the metallic porous plate produced by forming pores on the metallic plate such as the punching metal, the lath, and the like has a high degree of strength. Thus, the metallic porous plate is not destroyed by the hydrogen-storing alloy powders. The pores formed on the metallic plate are not three-dimensional. Therefore, to fix the hydrogen-storing alloy powders to the metallic porous plate, it is necessary to apply the pasty hydrogen-storing alloy powders thereto and pressurize the metallic porous plate repeatedly at a high degree after the pasty hydrogen-storing alloy powders are dried. However, when the metallic porous plate is pressurized repeatedly at a high degree, an electrode thus produced is very hard.
In the case where the negative electrode having base material, in which the hydrogen-storing alloy powders have been filled, is used for a cylindrical battery, the negative electrode is spirally wound together with a positive electrode via a separator to accommodate both electrodes in a battery can.
However, because the negative electrode produced as described above is hard, the electrode is cracked when it is wound. Normally, the electrode is accommodated in a battery can, with a crack left thereon. The occurrence of the crack causes an alloy powder layer of the active substance to drop from an electricity collecting material. As a result, flow of electric current is not favorable in the electrode and the electric resistance becomes high, which deteriorates the characteristic of the battery. Therefore, in a conventional method, preventive measures are taken by forming fine cracks intentionally in the electrode to prevent large cracks from being generated when it is wound. However, the alloy powder layer drops from the cracks. Thus, the preventive measures cannot solve the above-described problem that causes the characteristics of the battery to deteriorate.
Further, according to the conventional method, a pasty binder is kneaded with powder of hydrogen-storing alloy powder, and a mixture thereof is applied to a metallic porous plate. According to this method, the entire surface of alloy powders is likely to be coated with the binder. In this case, the alloy powders do not contact each other, and the binder disturbs the flow of electric current. Especially, electricity collection performance at a thickness of the electrode decreases, which deteriorates a characteristic of the battery.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-described situation. Thus, it is a first object of the present invention to provide an electrode for a battery which does not become hard when pressurization is repeated to securely fill an active substance such as hydrogen-storing alloy powder into a base material of an electrode and fix it thereto and which does not crack when the electrode is wound spirally.
It is a second object of the present invention to improve electricity collection performance by contacting powders of the active substance with each other directly.
In order to achieve the object, there is provided a method of producing an electrode for a battery comprising the continuous steps of:
forming a metallic porous foil consisting of metal powders in which adjacent powders are contacted and bonded with each other and gaps between non-contact powders form fine voids;
applying powders of an active substance not containing a binder to a surface of the metallic porous foil at a required position while the metallic porous foil is being conveyed continuously;
filling the powders of the active substance into the fine voids of the metallic porous foil and fixing the powders on the surface of the metallic porous foil under pressure by passing the metallic porous foil between a pair of rollers immediately after the powders of the active substance is applied to the metallic porous foil or while the powders of the active substance is being applied to the metallic porous foil;
forming a binder coating layer on surfaces of the powders of the active substance positioned at the surface of the metallic porous foil by introducing the metallic porous foil into a tank accommodating a liquid binder;
drying the binder coating layer by introducing the metallic porous foil into a drying oven;
and setting the metallic porous foil to a required thickness by passing the metallic porous foil sequentially between a plurality of a pair of pressure rollers arranged along a conveying path.
For example, the powders of the active substance not containing a binder are applied to both surfaces of the metallic porous foil by introducing the metallic porous foil into a hopper accommodating the powders of the active substance. The metallic porous foil, which has the powders of the active substance applied to it, is passed under pressure between a pair of rollers disposed at an exit position of the hopper so that the powders of the active substance are filled into the fine voids of the metallic porous foil and fixed to both surfaces of the metallic porous foil. Thereafter, the metallic porous foil is introduced into a tank accommodating a liquid binder so that binder coating layers are formed on the surfaces of the powders of the active substance, which have been applied to the both surfaces of the metallic porous foil.
According to the present invention, as described above, the metallic porous foil consisting of metal powders and having fine voids consisting of gaps between non-contact portions of the metal powders is used as the base material of the electrode. Because the metallic por
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