Process for producing a porous pasted electrode

Abstract: In a process for the production of an electrode for cell, a three-dimensional porous metal substrate is filled with an active material filler containing an active material. Then, heat and ultrasonic vibration is applied in order to denature or deform said active material filler to the substrate at a predetermined point where a collector leading portion is formed to remove the active material filler.

Description: The present invention relates to a process for preparing an electrode of the type having an electrode parent body comprising a three-dimensional porous metal substrate composed of a main body and a collector tab having a narrower width than the main body. One of electrode structures for an alkali storage battery is one having a three-dimensional porous metal substrate such as foamed nickel filled in an active material. An electrode having such a structure has a low strength and electrical conductivity in its substrate as compared with a sintered electrode having a core metal plate. Therefore, this type of an electrode is produced by a process consisting of filling a substrate entirely with an active material, removing the active material from a part of the substrate by applying an ultrasonic vibration or the like, welding a collector tab to the part of the substrate, and then connecting the substrate to an electrode terminal via the collector tab. Figs. 3A and 3B are sectional views illustrating an example of the structure comprising the foregoing electrode connected to an electrode terminal. Fig. 3A is a sectional view obtained by cutting the electrode in parallel to the surface of the electrode. Fig. 3B is a sectional view obtained by cutting the electrode perpendicularly to the surface of the electrode. A collector tab 7 welded to a metal terminal plate 4 having a rising portion provided inside the cell. The metal terminal plate 4 having a rising portion provided inside the cell is connected to a positive electrode terminal 1. A collector tab 7 is connected to the metal terminal plate 4. In this arrangement, an electrode 5 is connected to the metal terminal plate 4 via the collector tab 7. In Figs. 3A and 3B, the shaded portion indicates the portion of the electrode 5 where an active material has been removed. Reference numerals 2 and 3 represent an insulating packing and a cell case, respectively. A cell needs to satisfy requirements that it has a capacity as high as possible and is provided at a low price. The foregoing process involves the removal of an active material from a part of the substrate, and then, a collector tab is connected thereto. Accordingly, the process cannot provide effective utilization of a region from an active material has been removed, preventing the enhancement of capacity. The additional process of attaching a collector tab adds to the production cost. Further, this type of an electrode can be produced at a relatively easy process. This type of an electrode can provide good electricity-collecting properties even if an active material having a low electrical conductivity is used. This type of an electrode is also advantageous in that it can provide a cell having an enhanced capacity. However, a collector leading portion for taking in and out current can hardly be provided on the electrode prior to the filling of an active material. Therefore, a process normally employed including filling the entire substrate with an active material, removing the active material from the substrate at a predetermined area, and then connecting a collector member to the area to form a collector leading portion. If a collector lead is connected by this process, the active material or a resin used for retaining the active material must be completely removed from the surface of the substrate. As a preferred process for removing an active material, ultrasonic vibration is applied to the substrate at a predetermined point. In order to produce a cell having a capacity density as large as possible, the substrate should be filled with an active material as much as possible. In order to meet this demand, various countermeasures have been worked out. For example, the distribution of particle diameters of active material is adjusted. The amount of binder resin is reduced. Three-dimensional porous metal substrate skeleton is finely divided. The weight of three-dimensional porous metal substrate is reduced. The three-dimensional porous metal substrate filled with an active material is reduced. The pressing pressure in an electrode containing the foregoing three-dimensional porous metal substrate filled with an active material is increased. These countermeasures have successfully made it possible to gradually increase the cell capacity. On the other hand, however, as the packing density of active material filler in the substrate increases, the increase in the percent defective during the production process tends to occur. The reason for this phenomenon was investigated. As a result, it was found that as the packing density increases, the active material can be retained more rigidly in the substrate. Thus, the active material can be hardly removed by physical methods such as ultrasonic vibration. The active material cannot be completely removed from the predetermined surface of the substrate. The collector member connected to the substrate has a deteriorated connection strength. As a result, a defective such as falling of collector member can occur at the subsequent process of cell assembly. In order to inhibit the occurrence of such a defective, some countermeasures have been studied. Namely, the intensity of ultrasonic vibration is increased during the removal of active material, and the energy for spot welding or ultrasonic welding for connection of collector member is increased. However, the approach involving the increase of the intensity of ultrasonic vibration during the removal of active material causes the following problems. For example, the three-dimensional porous metal substrate is much damaged. The strength of the substrate at which the collector member is connected is deteriorated. As a result, the cutting of the substrate around the connection point causes the collector member to fall off. Thus, this approach cannot give a good solution to the problems. Further, the approach involving the increase of energy for connection of collector member is the following disadvantageous. Namely, since the strength of the substrate or the like is limited, the intensity of energy to be applied cannot be raised beyond some value. Further, if the intensity of energy is raised beyond some value, energy is wasted. Thus, this approach cannot give a good solution to the foregoing problems. Moreover, an approach including removing a binder resin alone with a solvent to reduce the retaining force of an active material, and then removing the active material has been studied. However, since a high capacity electrode containing a very large amount of a binder to have an enhanced packing density of active material and the majority of the retaining force of the active material greatly depends on the pressure developed by pressing the substrate, this approach involving the treatment of binder can exert little effect. From US-A-5,578,397, there is known a process for manufacturing an electrode having a main body and a collector tab which are integrally formed. By first compressing a metal foam support in the height direction and thereafter compressing said support in the thickness direction to form a collector tab. From JP-A-63040254, there is known a method for manufacturing a battery electrode wherein a collecting tab is welded to an electrode substrate made of three-dimensional porous metal plate by ultrasonic vibration. Said tab is welded by an ultrasonic horn. The part, to which the tab is to be welded, is formed by removing active material filled into the substrate by ultrasonic vibration. It is an object of the present invention to provide a process for the production of an electrode for cell capable of efficiently and completely removing an active material filler from a three-dimensional porous metal substrate filled with the active material filler at a predetermined point. The above object is achieved by the subject matter of claim 1 according to which a three-dimensional porous metal substrate composed of a main body and a collector tab is filled with an active material filler and thereafter heat and ultrasonic vibration is applied to the collector tab of said electrode parent body to denature or deform and to remove the active material filler from said collector tab. Preferred embodiments and further improvements of the inventive process are defined in depending subclaims 2-18. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Figs. 1A and 1B are diagrams illustrating the structure for connection of positive electrode in a nickel-hydrogen cell having an electrode welded to a metal terminal plate having a rising portion provided inside the cell; Figs. 2A and 2B are diagrams illustrating the structure for connection of positive electrode in a nickel-hydrogen cell having an electrode connected to a metal foil intermediate lead; Figs. 3A and 3B are sectional views illustrating an example of the structure having an electrode connected to an electrode terminal; Figs. 4A and 4B are diagrams illustrating the process of heating by a metallic trowel; Figs. 5A and 5B are diagrams illustrating the process of applying ultrasonic wave to remove the active material filler; Fig. 6 is a characteristic curve illustrating the relationship between the density of current flowing through an ear portion and the discharge capacity; Figs. 7A and 7B are diagrams illustrating the process of heating by a metallic trowel; Figs. 8A and 8B are diagrams illustrating the process of applying ultrasonic wave to remove an active material filler; Figs. 9A and 9B are schematic diagrams illustrating how the collector tab is attached; Fig. 10 is a diagram illustrating the process of allowing a heat fluid to come in contact with the substrate only at a predetermined point; Fig. 11 is a diagram illustrating the process of applying ultrasonic wave to remove the active material filler; Fig. 12 is a schematic diagram illustrating how a collector tab is attached; Fig. 13 is a graph illustrating the relationship between the heating time and the tensile strength; Fig. 14 is a schematic plan view of electrode illustrating an example of electrode structure having a collector tab attached thereto; and Fig. 15 is a schematic plan view of electrode illustrating an example of electrode structure having the same shape as that having a collector tab attached thereto. DETAILED DESCRIPTION OF THE INVENTION Detailed description of the present invention will be described as follows referring to the accompanying drawings. First Embodiment The first embodiment of the present invention provides an electrode structure having a three-dimensional porous metal substrate filled with an active material filler, which is composed of a main body portion filled with the active material filler and a collector tab where the three-dimensional porous metal substrate is exposed, the collector tab having a width narrower than the main body portion. In the first aspect of this embodiment, a cell has an electrode and an electrode terminal. The electrode contains a three-dimensional porous metal substrate filled with an active material filler. The electrode is composed of a main body portion filled with the active material filler and a collector tab where the three-dimensional porous metal substrate is exposed. The collector tab has a width narrower than the main body portion. The electrode terminal has a metal terminal plate having a rising portion inside the cell, and the collector tab is connected to the metal terminal plate. In the second aspect of this embodiment, a cell has an electrode and an electrode terminal. The electrode contains a three-dimensional porous metal substrate filled with an active material filler. The electrode is composed of a main body portion filled with the active material filler and a collector tab where the three-dimensional porous metal substrate is exposed. The electrode has a width narrower than the main body portion. The electrode terminal has a metal terminal plate provided inside the cell. One end of a metal foil intermediate lead having a thickness of not more than 0.08 mm is connected to the metal terminal plate and the other end is connected to the collector tab of the electrode. The three-dimensional porous metal substrate to be used in this embodiment is a substrate made of a metal skeleton having a number of three-dimensionally arranged pores which are large enough to be filled with an active material filler. Examples of the three-dimensional porous metal substrate include sintered metallic fiber, metallic felt such as nickel felt, and foamed metal. The kind of the substrate to be used herein may be properly selected depending on the kind of the electrode. If a nickel hydroxide electrode for nickel-hydrogen cell is produced, a substrate having a nickel skeleton is desirable. In particular, a plated three-dimensional porous nickel substrate having a metal skeleton formed by plating such as one produced by plating a porous foamed product is more desirable. The plated three-dimensional porous nickel substrate, if used, preferably has a nickel weight of from 400 to 600 g/m 2 . The active material filler with which the foregoing three-dimensional porous metal substrate is filled is mainly composed of an active material, optionally having a binder such as resin, an electrically-conductive material and other additives added thereto. For example, the active material filler with which a nickel hydroxide electrode for a nickel-hydrogen cell is filled is mainly composed of a powdered nickel hydroxide (Ni(OH) 2 ) as an active material having an electrically-conductive material such as graphite and metallic nickel, a thickening agent such as CMC and MC or a binder resin such as PFD and PTFE added thereto, optionally having an additive such as nickel oxyhydroxide (NiOOH) powder and cobalt hydroxide powder added thereto. The filling of the active material filler may be carried out by subjecting the substrate to coating, impregnation or spraying with a paste obtained by adding or the like to the foregoing active material filler. In practice, however, the substrate thus treated is preferably dried and then pressed to obtain a higher electrode density. The ear portion of the electrode of this embodiment acts as a collector tab. In order to maintain its collecting properties and strength of connection to the electrode terminal, the collector tab of the electrode is preferably pressed to have an enhanced density of metal skeleton and a reduced thickness as compared with the main body portion of this embodiment. However, if pressing is too strong, the strength of the border of the collector tab with the main body portion is weak. Thus, pressing needs to be effected properly. Pressing is preferably effected such that the difference in thickness between the main body portion and the collector tab is not too large. The density of the collector tab thus pressed, if a plated three-dimensional porous nickel substrate is used, is preferably from 0.83 to 5 g/cm 3 from the standpoint of lowest allowable value for sufficient electronic conductivity and upper value which can be easily obtained by pressing. As mentioned above, the electrode of this embodiment has a collector tab formed integrally therewith to provide a portion corresponding to the collector tab. Accordingly, a cell provided with this electrode requires no procedure of attaching a collector tab during its production. Accordingly, it is possible to simplify the production process and reduce the production cost. Further, there is no necessity of removing an active material from the electrode at a region for attachment of a collector tab. Accordingly, the main body portion can be made the efficient use of as an active electrode portion. Accordingly, it is possible to provide an increased capacity and an enhanced percent of utilization of active material. The electrode of this embodiment is preferably used such that the density of current flowing through the collector tab is not more than 800 A per net unit sectional area of the metal constituting the collector tab (i.e., not more than 800 A/cm 2 ). The use of the electrode of this embodiment within this range makes it possible to secure sufficient discharge capacity. Further, if a plated three-dimensional porous nickel substrate is used, it is preferred that the nickel weight be from 400 to 600 g/m 2 , the density of current flowing through the collector tab be not more than 800 A per net unit sectional area of the metal constituting the collector tab (i.e., not more than 800 A/cm 2 ) and the density of the collector tab be from 0.83 to 5 g/cm 3 . The formation of the collector tab as an exposed area of the three-dimensional porous metal substrate can be hardly accomplished by a process which includes preparing a three-dimensional porous metal substrate composed of a main body portion and a collector tab, and then filling the three-dimensional porous metal substrate with an active material filler at the main body portion alone except the collector tab. Therefore, a process is preferably used which includes filling a three-dimensional porous metal substrate formed of a main body portion and a collector tab having a width narrower than the main body portion entirely with an active material filler to prepare an electrode parent body, and then removing the active material filler from the electrode parent body at the collector tab. In this process, the removal of the active material filler needs to be effected such that the three-dimensional porous metal substrate is exposed to an extent such that sufficient electrical and mechanical connection such as welding can be effected. More preferably, the active material filler is completely removed. Further, it suffices so far as the substrate is exposed at least at the collector tab. The substrate may be unfilled with the active material filler at the portion other than the collector tab. The substrate may be exposed at the portion other than the collector tab. The removal of the active material filler from the three-dimensional porous metal substrate entirely filled with the active material filler at the collector tab is preferably carried out by a process which includes applying a heat and an ultrasonic vibration causing the denaturation or deformation of the active material filler to the ear portion of the electrode parent body to remove the active material filler therefrom. The electrode thus produced exhibits an excellent mechanical strength at the collector tab to advantage. This process is desirable when the void of the main body portion filled with the active material filler is not more than 30%, more desirable when the void of the main body portion filled with the active material filler is not more than 28%. The foregoing process which includes preparing an electrode parent body having a three-dimensional porous metal substrate composed of a main body portion and a collector tab having a width narrower than the main body portion filled with an active material filler, and then applying a heat and an ultrasonic vibration causing the denaturation or deformation of the active material filler to the collector tab of the electrode parent body to remove the active material filler from the collector tab is intended to apply a heat to the collector tab. Therefore, a thermal deformation or the like is given to cause the change in the contact condition of the active material or the like to reduce the retaining force of the active material. In other words, a heat is applied to the three-dimensional porous metal substrate or the active material filler. The three-dimensional porous metal substrate is thermally expanded or at least one component of the active material filler is denatured or deformed to reduce the retaining force of the active material filler with respect to the three-dimensional porous metal substrate. As a result, it can be removed from the three-dimensional porous metal substrate by ultrasonic vibration. More effectively, the foregoing process is intended to denature or deform at least one component of the active material filler. The three-dimensional porous metal substrate is thermally expanded so that a shearing force between the three-dimensional porous metal substrate and the active material filler in contact therewith is caused and the active material filler is peeled off the surface of the three-dimensional porous metal substrate. The entire active material filler retained in the substrate is shaken to produce a new gap that reduces the retaining force of the active material filler. Accordingly, in order to enhance this effect, it is preferred that the temperature of the substrate is raised higher. Further, in order to add a shrinking effect, the cycle of temperature rise and cooling is preferably short. However, the temperature rise is preferably effected up to a proper value depending on the kind of the material so far as the strength and electrical conductivity of the substrate are not deteriorated. The denaturation of at least one component of the active material filler is intended to cause a change resulting in the reduction of the binding power of one component with another or with the substrate. Hence, the retaining force of the active material filler is dropped. Further, the denaturation of at least one component of the active material filler causes a change in the filling condition of the active material filler or causes the production of a new gap or shift causing the active material filler to be peeled off the substrate to reduce the retaining force of the active material filler. This deformation is produced not only by thermal expansion and thermal shrinkage but also by denaturing similarly to the substrate. In particular, an active material filler having a high proportion of active material is expected to see an important role of the denaturation and deformation of active material. In this case, the thermal oxidation or reduction reaction of the active material is important. Thus, the temperature or atmosphere such as oxygen atmosphere and hydrogen atmosphere is adjusted depending on the kind of the active material. Referring to the order of applying heat and ultrasonic vibration, it is preferred that ultrasonic vibration is not applied prior to heat to enhance the efficiency for the purpose of applying heat. Thus, ultrasonic vibration is preferably applied at the same time with or after the application of heat. If ultrasonic vibration is applied at the same time with the application of heat, the step of removing the active material filler can be effected in a short period of time to advantage. On the contrary, if ultrasonic vibration is applied after the application of heat, a cooling effect can be exerted by adjusting the time required until the application of ultrasonic vibration or the atmospheric temperature. Accordingly, it is possible to remove the active material filler at a high energy efficiency and it is easy to adjust the heat application conditions. There are various heat application methods such as irradiating the substrate with infrared rays or laser beam, and spraying flame of a burner onto the substrate. The temperature of the heat thus applied is preferably so that the active material filler can be denatured or deformed. If a nickel hydroxide electrode containing nickel hydroxide as a main active material is produced, the temperature of the heat thus applied is preferably not lower than 100°C at which the water of crystallization can be removed, more preferably not lower than 220°C at which nickel hydroxide becomes nickel oxide. Alternatively, if the active material filler contains some resin, the temperature of the heat thus applied is even more preferably not lower than 250°C. This can apply when the active material filler contains a thickening agent such as CMC and MC or a binder resin such as PFD and PTFE. Further, the upper limit of the temperature of the heat thus applied is preferably not higher than 650°C if the substrate is a three-dimensional porous metal substrate made of metallic nickel. In particular, if the substrate is a three-dimensional porous metal substrate made of a skeleton formed by plating such as foamed nickel, this temperature is desirable. This is because if the heating time is prolonged at a temperature higher than this temperature, it causes the denaturation of the substrate besides the denaturation and deformation of the active material filler, resulting in the drop of strength and electrical conductivity. Further, if nickel hydroxide is used, heating may be effected in the atmosphere. In this case, the facilities to be used can be simplified to advantage. The cell according to the first aspect of this embodiment can be produced by connecting a metal terminal plate to the electrode of the embodiment at the collector tab thereof. The connection of the metal terminal plate to the electrode may be carried out by using an electrically-conductive solder or resin. In practice, however, spot welding, ultrasonic welding or laser welding may be used. In order to effect this welding rigidly without impairing the electrical conductivity, it is preferred that the surface of the collector tab is as clean as possible and is free of electrical conductivity-impairing film such as oxide layer. This is because the accomplishment of such a surface condition makes it possible to realize a rigid connection having a good electrical conductivity and reduce the energy required for connection. Accordingly, as mentioned with reference to the electrode of the embodiment, it is more desirable that an electrode is used produced by a process which includes preparing an electrode parent body made of a three-dimensional porous metal substrate composed of a main body portion and a collector tab having a width narrower than the main body portion filled with an active material filler, and then applying a heat and an ultrasonic vibration causing the denaturation or deformation of the active material filler to the collector tab of the electrode parent body to remove the active material filler from the collector tab. In this case, it is preferred that the three-dimensional porous metal substrate is a plated three-dimensional porous nickel substrate and the active material is nickel hydroxide. The cell of this embodiment is preferably arranged such that the density of current flowing through the collector tab be not more than 800 A per net unit sectional area of the metal constituting the collector tab (i.e., not more than 800 A/cm 2 ). Further, if an electrode employing a plated three-dimensional porous nickel substrate is used, it is preferred that the nickel weight be from 400 to 600 g/m 2 , the density of current flowing through the collector tab be not more than 800 A per net unit sectional area of the metal constituting the collector tab (i.e., not more than 800 A/cm 2 ) and the density of the collector tab be from 0.83 to 5 g/cm 3 . In the cell according to the second aspect of this embodiment, the foregoing electrode of the embodiment is connected to the metal terminal plate via a metal foil intermediate lead. Since the metal foil intermediate lead is enclosed warped in the cell, the collector tab of the electrode is subject to stress such as bending and compression from the metal foil intermediate lead. Therefore, if this stress is too large, the of the electrode is deformed, producing defective cells. In order to reduce this stress sufficiently, a metal foil intermediate lead made of nickel having a thickness of not more than 0.08 mm is used. The preferred form of the electrode used is the same as that of the cell according to the first aspect of this embodiment. Thus, the use of the metal foil intermediate lead having a thickness of not more than 0.08 mm makes it possible to produce cells in a good yield. In particular, if an electrode made of a plated three-dimensional porous nickel substrate is used, it is preferred that the nickel weight be from 400 to 600 g/m 2 and the thickness of the metal foil intermediate lead be not more than 0.08 mm, more preferably not more than 0.05 mm. The cell of this embodiment is preferably arranged such that the density of current flowing through the collector tab be not more than 800 A per net unit sectional area of the metal constituting the collector tab (i.e., not more than 800 A/cm 2 ). Further, if an electrode made of a plated three-dimensional porous nickel substrate is used, it is preferred that the nickel weight be from 400 to 600 g/m 2 , the density of current flowing through the collector tab be not more than 800 A per net unit sectional area of the metal constituting the collector tab (i.e., not more than 800 A/cm 2 ) and the density of the collector tab be from 0.83 to 5 g/cm 3 . Example 1 The electrode of this embodiment will be further described in the following examples. 90 parts by weight of a particulate nickel hydroxide active material having an average particle diameter of 10 µm and 10 parts by weight of a particulate cobalt hydroxide having an average particle diameter of 5 µm were dispersed in a 0.5 wt-% aqueous solution of carboxymethyl cellulose (CMC) to prepare a paste. The paste thus prepared was applied to a foamed nickel having a porosity of 95% and a weight of 500 g/m 2 (trade name: Cermet, produced by Sumitomo Electric Industries, Ltd.) so that the foamed nickel was filled with the paste. The foamed nickel was dried, pressed to have an enhanced packing density, and then stamped a rectangle having a collector tab out therefrom to prepare an electrode parent body. The size of the electrode parent body except the ear portion was 15 mm x 60 mm. The size of the collector tab was 3 mm (root) x 5 mm. The thickness of the unpressed foamed nickel was 1.6 mm. The thickness of the pressed foamed nickel was 0.7 mm. The porosity of the pressed foamed nickel was 26%. Subsequently, a heat causing the denaturation or deformation of the active material filler was applied to the collector tab of the electrode parent body from a metallic trowel. To the collector tab, ultrasonic vibration was applied to remove the active material filler therefrom. Figs. 4A and 4B are diagrams illustrating the process of heating by a metallic trowel according to this embodiment. Figs. 5A and 5B are diagrams illustrating the process of applying ultrasonic wave to remove the active material filler of the present example. Figs. 4A and 5A are a sectional view of the electrode, respectively. Figs. 4B and 5B are a plan view of the electrode, respectively. The present example will be further described in connection with these drawings. As shown in Figs. 4A, two metallic trowels 31, 32 were prepared. The metallic trowels 31, 32 were allowed to come in contact with the collector tab 6 which acts as a collector tab in such a manner that the collector tab 6 is vertically sandwiched therebetween. The surface of the metallic trowels 31, 32 which come in contact with the collector tab 6 is planar and its shape is rectangle. The temperature of the forward end of the metallic trowels 31, 32 had been kept to 480°C. Under these conditions, the metallic trowels 31, 32 were allowed to come in contact with the collector tab 6 in the air for 3 seconds. After the heat treatment, the color of the collector tab was changed from green to blackish. Thereafter, the ear portion 6 was allowed to cool to room temperature. As shown in Fig. 5A, a Ti horn 41 was then pressed against the collector tab 6 while applying an ultrasonic wave thereto. In this manner, the active material filler was completely removed from the collector tab 6. As a result, a very clean surface of foamed nickel skeleton appeared. The thickness and density of the collector tab 6 from which the active material filler had been removed were 0.6 mm and 0.83 g/cm 3 , respectively. The collector tab 6 thus treated exhibited a tensile strength of not less than 0.8 kgf and a peel strength of not less than 0.5 kgf. In this embodiment, since an ultrasonic wave is applied to the collector tab, the filler can be released also from the three sides. Thus, the filler can be efficiently removed as compared with the process in which an ultrasonic wave is applied to a part of the electrode main body portion. The cell of this embodiment will be described with reference to an example of cell including the foregoing electrode as a positive electrode. Figs. 1A and 1B are diagrams illustrating the structure for connection of positive electrode in a nickel-hydrogen cell having the foregoing electrode welded to a metal terminal plate having a rising portion provided inside the cell. Fig. 1A is a sectional view obtained by cutting the electrode in parallel to the surface of the electrode. Fig. 1B is a sectional view obtained by cutting the electrode perpendicularly to the surface of the electrode. As shown in Figs. 1A and 1B, to a positive electrode 1 is connected a metal terminal plate 4 having a rising portion provided inside the cell. A collector tab 6 which acts as a collector tab is connected to the metal terminal plate 4 by ultrasonic welding. In this arrangement, the electrode 5 is connected to the metal terminal plate 4 via the collector tab 6. The shade indicates the collector tab 6 of the electrode 5 from which an active material has been removed, the reference numeral 2 represents an insulating packing, and the reference numeral 3 represents a cell case. In Figs. 1A and 1B, no negative electrodes, separators, etc. are shown. However, the cell of the present example is made of three sheets of electrodes 5 each enclosed as a positive electrode in a bag-shaped separator (nonwoven cloth made of polyolefin) and four sheets of negative electrodes, one positioned between the electrodes 5 and one positioned on each side of the electrodes 5, filled with an aqueous solution of potassium hydroxide. The negative electrode used had been produced by dispersing 99 parts by weight of hydrogen-occluded alloy particles and 1 part by weight of carbon particles in a 1 wt-% aqueous solution of carboxymethyl cellulose to prepare a paste, applying the paste to a Ni-plated punched metal of iron, drying the coated metal, pressing the coated metal to a predetermined thickness, and then stamping almost the same size out of the material. In the process for the production of the cell, no defectives occurred due to cutting of the collector tab. If foamed nickel is used, the tensile strength of the collector tab may be not less than 0.2 kgf, preferably not less than 0.5 kgf, to keep the cell according to the first aspect of the embodiment out of strength problems. It was also found that if foamed nickel is used, the thickness of the collector tab may fall within the range of from 0.10 mm to 0.6 mm to inhibit problems on the strength of connection to the metal terminal plate. Fig. 6 is a characteristic curve illustrating the relationship between the density of current flowing through the collector tab 6 and the discharge capacity. The current density is defined as the quantity of current per net unit sectional area of metal constituting the collector tab. The current density can be calculated by the following equation: (Current density) = (Current flowing through collector tab )/(width of collector tab x net thickness) (Net thickness) = (Weight per area of 1 cm 2 )/(density of metal) In the present example, the width and net thickness of the collector tab are 3 mm and 0.05 (weight)/8.9 (density of nickel), respectively. These values are used to calculate the current density. For the evaluation of the cell characteristics, the foregoing cell was used. In some detail, the discharge capacity of the cell was measured at a temperature of 23°C with different values of current taken out of the cell. The ordinate of the graph indicates the discharge capacity. The abscissa of the graph indicates the value obtained by dividing the foregoing current by 3 as the current flowing through the collector tab (collector tab in the case of a comparative cell) calculated by the foregoing equation. In the graph, the square indicates the characteristics of a comparative cell produced by using the collector tab without any collector tab. The black dot indicates the characteristics of the present example. The comparative cell was quite the same as the cell of the present example except that a collector tab was provided. As the collector tab, there was used a nickel foil having a thickness of 30 µm and a portion protruding the electrode having a width of 3 mm and a length of 5 mm. As shown in the graph, the cell of the present example exhibits a discharge capacity which can compare favorably with the comparative cell within the range where the density of current flowing through the collector tab is not more than 800 A/cm 2 . The cell of this embodiment exhibits a great capacity as compared with the comparative cell because there is no necessity of removing the active material from the region (4 mm x 4.5 mm) at which a collector tab is attached. In the graph, a correction (capacity/0.98) for attachment of collector tab is applied to the discharge capacity of the comparative cell to give better understanding of the relationship between the current density and the discharge capacity. Figs. 2A and 2B are diagrams illustrating the structure for connection of positive electrode in a nickel-hydrogen cell made of the foregoing electrode connected to a metal foil intermediate lead. Fig. 2A is a sectional view obtained by cutting the electrode in parallel to the surface of the electrode. Fig. 2B is a sectional view obtained by cutting the electrode perpendicularly to the surface of the electrode. As shown in Figs. 2A and 2B, a metal terminal plate 4 inside the cell is connected to a positive electrode 1. One end of a metal foil intermediate lead 8 is connected to the metal terminal plate 4. The other end of the metal foil intermediate lead 8 is connected to the collector tab 6 of the foregoing electrode 5 by ultrasonic welding. The other structures are the same as that of the cell of the previous example as shown in Figs. 1A and 1B. As the metal foil intermediate lead 8, there was used a nickel foil having a width of 3 mm. When a nickel foil having a thickness of 0.1 mm was used, the collector tab 6 underwent deformation that produced some defective cells. When a nickel foil having a thickness of 0.05 mm was used, no deformation or defectives occurred. Further, when a nickel foil having a thickness of 0.08 mm was used, no defectives occurred but the intermediate lead was too hard to give good handleability. The thickness of the nickel foil deviated about 0.01 mm from the desired value. For example, a group of nickel foils having a nominal thickness of 0.08 mm included some samples having a thickness of 0.09 mm. The use of the electrode of this embodiment makes it possible to make the effective use of electrode area and eliminate the step of attaching the electrode to terminal. Further, the use of the electrode of this embodiment makes it possible to produce a cell having a greater capacity and less number of required production steps than the conventional cell. Second Embodiment According to the first aspect of second embodiment according to this embodiment, a process for the production of an electrode for cell is performed in the following manner. First, a three-dimensional porous metal substrate is filled with an active material filler containing an active material. Thereafter, a heat for denaturing or deforming mainly the active material filler and an ultrasonic vibration for removing mainly the active material filler is applied to the substrate to remove the active material filler. Then, a collector member is electrically connected to the substrate at the predetermined point where the active material filler has been removed. In other words, the concept of the first aspect of the second embodiment is to apply a heat to the three-dimensional porous metal substrate at a predetermined point, thereby giving a thermal deformation or the like causing the change in the contact condition of the active material or the like to reduce the retaining force of the active material. In other words, heat is applied to the three-dimensional porous metal substrate or the active material filler so that the three-dimensional porous metal substrate is thermally expanded. Alternatively, at least one component of the active material filler is denatured or deformed to reduce the retaining force of the active material filler with respect to the three-dimensional porous metal substrate. As a result, it can be removed from the three-dimensional porous metal substrate by ultrasonic vibration. More effectively, the foregoing process is intended to denature or deform at least one component of the active material filler. The three-dimensional porous metal substrate is thermally expanded to cause a shearing force between the three-dimensional porous metal substrate and the active material filler in contact therewith so that the active material filler is peeled off the surface of the three-dimensional porous metal substrate. The entire active material filler retained in the substrate is shaken to produce a new gap that reduces the retaining force of the active material filler. Accordingly, in order to enhance this effect, it is preferred that the temperature of the substrate be raised higher. Further, in order to add a shrinking effect, the cycle of temperature rise and cooling is preferably short. However, the temperature rise is preferably effected up to a proper value depending on the kind of the material so far as the strength and electrical conductivity of the substrate are not deteriorated. An active material filler is mainly composed of an active material (optionally an active material alone) having a binder such as resin, an electrically conductive material and other additives. At least one component of the active material filler is denatured to intend to cause a change resulting in the reduction of the binding power of one component with another or with the substrate. Hence, the retaining force of the active material filler is dropped. Further, at least one component of the active material filler is denatured to cause a change in the filling condition of the active material filler or causes the production of a new gap or shift causing the active material filler to be peeled off the substrate to reduce the retaining force of the active material filler. This deformation is produced not only by thermal expansion and thermal shrinkage but also by denaturing similarly to the substrate. In particular, an active material filler having a high proportion of active material is expected to see an important role of the denaturation and deformation of active material. In this case, the thermal oxidation or reduction reaction of the active material is important. Thus, the temperature or atmosphere such as oxygen atmosphere and hydrogen atmosphere is adjusted depending on the kind of the active material. Referring to the order of applying heat and ultrasonic vibration, it is preferred that ultrasonic vibration is not applied prior to heat to enhance the efficiency for the purpose of applying heat. Thus, ultrasonic vibration is preferably applied at the same time with or after the application of heat. If ultrasonic vibration is applied at the same time with the application of heat, the step of removing the active material filler can be effected in a short period of time to advantage. On the contrary, if ultrasonic vibration is applied after the application of heat, a cooling effect can be exerted by adjusting the time required until the application of ultrasonic vibration or the atmospheric temperature. Accordingly, it is possible to remove the active material filler at a high energy efficiency and it is easy to adjust the heat application conditions. In a process for the production of an electrode for cell according to the second aspect of this embodiment, heat is applied to cause the denaturation or deformation of mainly the active material filler to the substrate at a predetermined point from a solid heat heating member being in contact therewith. In other words, the process off the second aspect of this embodiment is characterized by a heating process. The use of a solid heating member makes it easy to determine the dimension of the heating site. Therefore, it is possible to prevent excess sites from being heated. Further, the temperature of the solid heating member can be easily controlled to a predetermined value. Consequently, it is easy to control the temperature of the heat applied to the electrode and it is possible to enhance the reliability of heat treatment. The second embodiment will be further described with reference to an example of nickel hydroxide electrode for nickel-hydrogen cell, etc. As the three-dimensional porous metal substrate there may be used a sintered metal fiber or foamed metal. If a nickel hydroxide electrode is produced by the process according to this embodiment, a plated three-dimensional porous nickel substrate such as foamed nickel produced by Sumitomo Electric Industries, Ltd. can be preferably used to make the best use of this embodiment. The active material filler is mainly composed of powdered nickel hydroxide (Ni(OH) 2 ) having an electrically conducting agent such as graphite, metallic nickel and cobalt compound (e.g., cobalt hydroxide, metallic cobalt) added thereto by various methods with a thickening agent such as CMC and MC or a binder resin such as PFD and PTFE properly added thereto, optionally having additives such as nickel oxyhydroxide (NiOOH) added thereto. Water or the like is added to the active material filler to make a paste which is then applied to the three-dimensional porous metal substrate so that the substrate is filled with the paste. The substrate is dried, and then pressed to have an enhanced packing density. Thus, an electrode is produced. For example, 100 parts by weight of powdered nickel hydroxide (Ni(OH) 2 ), 15 parts by weight of powdered nickel oxyhydroxide (NiOOH) and a 0.4 wt-% aqueous solution of carboxymethyl cellulose (CMC) are mixed to prepare a paste. The paste is applied to a foamed nickel plate which is then dried and pressed. Subsequently, a collector member such as collector tab made of nickel foil is electrically connected to the foregoing electrode. The electrical connection of the collector member to the electrode may be carried out by using an electrically-conductive solder or resin. In practice, however, spot welding, ultrasonic welding or laser welding may be used. In order to effect this welding rigidly without impairing the electrical conductivity, it is necessary that the active material filler is removed from the connection site to allow the metallic substrate to be exposed at the connection site. Further, it is preferred that the surface of the connection site is as clean as possible and is free of electrical conductivity-impairing film such as oxide layer. This is because the accomplishment of such a surface condition can realize a rigid connection having a good electrical conductivity and reduce the energy required for connection. In this embodiment, a solid heating member contacts with the substrate at a point where the collector member is connected so that the substrate is heated to remove the active material filler. Heat can be applied by irradiating the substrate with infrared rays or laser beam or spraying flame of a burner onto the substrate. However, the foregoing process involving the contact of a solid heating member is preferred. The region of the substrate to which heat is applied is preferably slightly greater than the connecting site of the collector member taking into account the margin for connection of the collector member. Thus, the area of the surface of the solid heating member in contact with the substrate is preferably greater than that of the connecting site of the collector member. The shape of the surface of the solid heating member in contact with the substrate is preferably similar to that of the connecting site of the collector member for the purpose of minimizing the extra area. Further, the shape of the collector member is preferably rectangle from the standpoint of production efficiency. Accordingly, the surface of the solid heating member in contact with the substrate, too, is preferably rectangular. The temperature of the heat thus applied is preferably such that the active material filler can be denatured or deformed. If a nickel hydroxide electrode, which contains nickel hydroxide as a main active material, is produced, the temperature of the heat thus applied is preferably not lower than 100°C, at which the water of crystallization can be removed, more preferably not lower than 220°C, at which nickel hydroxide becomes nickel oxide. Alternatively, if the active material filler contains some resin, the temperature of the heat thus applied is even more preferably not lower than 250°C. This can apply when the active material filler contains a thickening agent such as CMC and MC or a binder resin such as PFD and PTFE. Further, the upper limit of the temperature of the heat thus applied is preferably not higher than 650°C if the substrate is a three-dimensional porous metal substrate made of metallic nickel. In particular, if the substrate is a three-dimensional porous metal substrate having a skeleton formed by plating such as foamed nickel, this temperature is desirable. This is because if the heating time is prolonged at a temperature higher than this temperature, it causes the denaturation of the substrate besides the denaturation and deformation of the active material filler, resulting in the drop of strength and electrical conductivity. Further, if nickel hydroxide is used, heating may be effected in the atmosphere. In this case, the facilities to be used can be simplified to advantage. In a process, two of the solid heating member is allowed to come in contact with the substrate at a predetermined point so that the substrate is vertically sandwiched therebetween. This process makes it possible to apply heat to the active material filler in short time. Accordingly, it is possible to minimize the effect of heat on other sites requiring no heating. Further, this process can apply heat uniformly in the vertical direction. Therefore, it is possible to effect uniform heat treatment of the active material filler and to make sure that the active material filler can be removed completely. Further, after applying heat, ultrasonic vibration is applied to remove mainly the active material filler to the three-dimensional porous metal substrate on one vertical side. The active material filler is sucked therefrom on the other side via a predetermined time interval. These process is desirable because it can add a cooling effect, enhance the percent removal of the active material filler and prevent the scattering of the active material filler. Subsequently, ultrasonic vibration is applied to the connecting site thus heated to remove the active material filler the retaining force of which has been reduced. The region of the substrate to which ultrasonic vibration is applied is preferably slightly greater than the region of the substrate to which heat has been applied to cover the region of the substrate to which heat has been applied to leave none of the active material thermally denatured behind. Thus, if a solid ultrasonic wave application device such as metallic horn is pressed against the substrate at a predetermined point to apply ultrasonic wave thereto, the surface of the solid ultrasonic wave application device in contact with the substrate is preferably greater than that of the solid heating member in contact with the substrate. The shape of the surface of the solid ultrasonic wave application device in contact with the substrate is preferably similar to that of the surface of the solid heating member in contact with the substrate for the purpose of reducing the extra area. Further, the shape of the collector member is preferably square in view of its production efficiency. Accordingly, the surface of the solid heating member in contact with the substrate is preferably square. Further, the surface of the solid ultrasonic wave application device in contact with the substrate, too, is preferably square. Moreover, the application of ultrasonic vibration is preferably effected while pressing the three-dimensional porous metal substrate to remove the active material filler efficiently. Further, the solid heating member is preferably allowed to come in contact with the substrate to an extent so that heat can be transferred to the substrate. Further, the solid heating member also acting as an ultrasonic wave application member is desirable so that it can perform the application of heat and ultrasonic wave at the same time. Accordingly, it is possible to reduce the required number of steps. In this case, heat and ultrasonic wave are applied simultaneously with the solid heating member being in contact with one vertical side of the three-dimensional porous metal substrate while the active material filler is being sucked off at the other side. These process is more preferable because it can enhance the percent removal of the active material thus released and prevent the scattering of the active material. The foregoing treatment makes it possible to remove the active material filler efficiently or more completely and makes sure that the connecting member can be connected efficiently. Further, if a nickel hydroxide using a foamed plated three-dimensional porous nickel substrate is produced, the present embodiment can exert a remarkable effect on a structure having a porosity of not more than 30%, more remarkably not more than 28%, after pressing. Thus, the use of the process of the present embodiment makes it possible to remarkably reduce the percent defective. This effect becomes particularly remarkable when the porosity is not more than 28%. Example 2 The present embodiment will be further described in the following examples. 90 parts by weight of a particulate nickel hydroxide active material having an average particle diameter of 8 µm and 10 parts by weight of a particulate cobalt hydroxide having an average particle diameter of 6 µm were dispersed in a 0.4 wt-% aqueous solution of carboxymethyl cellulose to prepare a paste. The paste thus prepared was applied to a foamed nickel having a porosity of 95% (trade name: Cermet, produced by Sumitomo Electric Industries, Ltd.) so that the foamed nickel was filled with the paste. The foamed nickel was dried, and then pressed to almost half the original thickness to produce a nickel hydroxide electrode. The porosity of the electrode thus produced was 27%. A collector tab made of nickel foil was then ultrasonically welded to the electrode by applying this embodiment. Figs. 7A and 7B are diagrams illustrating the process of heating by a metallic trowel as a solid heating member according to this embodiment. Figs. 8A and 8B are diagrams illustrating the process of applying ultrasonic wave to remove an active material filler according to this embodiment. Figs. 3A and 3B are schematic diagrams illustrating how the collector tab is attached according to this embodiment. Figs. 1A, 2A and 3A each are a sectional view. Figs. 1B, 2B and 3B each are a plan view of electrode. The present example will be further described in connection with these drawings. In the present example, two metallic trowels 131, 132 were prepared. The metallic trowels 131, 132 were then allowed to come in contact with an electrode 101 at a predetermined point shown by the shade so that the electrode 101 (i.e., substrate) was vertically sandwiched therebetween. The surface of the metallic trowels 131, 132 which come in contact with the electrode 101 is planar and their shape is rectangle. The temperature of the forward end of the metallic trowels 131, 132 had been kept to 480°C. Under these conditions, the metallic trowels 131, 132 were allowed to come in contact with the electrode 101 in the air for 3 seconds. After the heat treatment, the color of the electrode thus treated was changed from green to blackish at almost the same area having almost the same shape as the metallic trowel. Thereafter, the electrode 101 was allowed to cool to room temperature. As shown in Fig. 8A, a Ti horn 104 was then pressed against the electrode 101 while applying an ultrasonic wave thereto. In this manner, the active material filler was removed from the electrode 101 while the electrode 101 was being compressed. During this procedure, the active material filler which had been released was sucked off through a vacuum suction port 105. The surface of the Ti horn 104 in contact with the electrode 101 is planar and almost square. In Figs. 2A and 2B, the shade indicates the region of the electrode with which the Ti horn 104 is in contact, and the network indicates the region thus heated. In this manner, the region shown by the shade had a slight amount of the active material filler left therein while the region shown by the network had got a very clean foamed nickel skeleton fully free of active material filler. To the network region thus free of active material filler was then ultrasonically welded a collector tab 102. For comparison, the active material filler was removed only by the application of ultrasonic wave without the foregoing heat treatment. A collector tab was then connected to the electrode. Thereafter, the subsequent various conditions were optimized. The resulting comparative product was then measured for connecting strength of collector tab. The measurements were then compared with that of the product of the present example. As a result, the product of the present example showed a connecting strength three times that of the conventional product. The electrode obtained according to the present example was then assembled into a cell. As a result, the percent defective due to malconnection of collector tab was almost halved as compared with the conventional process. Further, the process of the present example made it possible to produce an electrode having a higher capacity and density than ever. Further, the foregoing experiment procedure was followed except that the heating time was changed from 3 seconds to 30 seconds. As a result, almost the same connecting strength was obtained. Referring to the temperature of the metallic trowel, the highest connecting strength was obtained at a temperature range of from 400°C to 500°C, provided that the heating time is fixed. In accordance with the present embodiment, a collector member can be rigidly connected to an electrode made of a three-dimensional porous metal substrate filled with an active material filler containing an active material with a minimized connecting energy without reducing the strength of the substrate. Further, the energy required for the application of ultrasonic wave during the removal of the active material can be reduced from the conventional value. Moreover, the occurrence of a defective due to falling of collector member at the subsequent process of cell assembly can be reduced. Further, heat treatment can be effected without damaging the substrate. The working process is safe. Accurate dimensional control can be conducted for heat treatment. Further, if the present embodiment is applied to the production of an electrode made of metallic nickel as a three-dimensional porous metal substrate filled with nickel hydroxide as a main active material, the effect of this embodiment can be exerted drastically because nickel hydroxide is subject to denaturation or deformation by oxidation. Thus, the substrate can little be affected by heat to advantage. Further, as compared with the conventional removal process using ultrasonic wave alone, the substrate can be provided with a very clean surface after removal of the active material, although the reason is unknown. Third Embodiment In a process for the production of an electrode for cell according to the first aspect of this embodiment, a three-dimensional porous metal substrate is filled with an active material filler containing an active material, a heat is applied for denaturing or deforming mainly the active material filler to the substrate at a predetermined point from a heat fluid being in contact therewith, an ultrasonic vibration is applied for removing mainly the active material filler to the substrate to remove the active material filler, and then a collector member is electrically connected to the substrate at the predetermined point where the active material filler has been removed. The main concept of this embodiment is to apply a heat to the three-dimensional porous metal substrate at a predetermined point so as to give a thermal deformation or the like to cause the change in the contact condition of the active material or the like to reduce the retaining force of the active material. Namely, a heat is applied to the three-dimensional porous metal substrate or the active material filler so that the three-dimensional porous metal substrate is thermally expanded. Alternatively, at least one component of the active material filler is denatured or deformed to reduce the retaining force of the active material filler with respect to the three-dimensional porous metal substrate, so that it can be removed from the three-dimensional porous metal substrate by ultrasonic vibration. More effectively, the foregoing process is intended to denature or deform at least one component of the active material filler. With respect to the thermal expansion of the three-dimensional porous metal substrate in this embodiment, it is similar to that of the second embodiment. Concerning to an active material filler in this embodiment, it is similar to that of the second embodiment. Regarding to the order of applying heat and ultrasonic vibration, it is similar to that of the second embodiment. In a process for the production of an electrode for cell according to the second aspect of this embodiment, heat is applied to cause the denaturation or deformation of mainly the active material filler to the substrate at a predetermined point from a heat fluid being in contact therewith. In other words, the process for the production of an electrode for cell according to the second aspect of this embodiment is characterized by a heating process. The use of a heat fluid makes it easy to apply a high temperature heat to the substrate and cope with a larger area to be treated in a larger electrode. This embodiment will be further described with reference to an example of nickel hydroxide electrode for nickel-hydrogen cell, etc. The three-dimensional porous metal substrate used in this embodiment is similar to that of the second embodiment. The active material filler and a producing method thereof are similar to those of the second embodiment. A collector member connected to the foregoing electrode is similar to that of the second embodiment. In this embodiment, a heat is applied to the substrate at a point where the collector member is connected to remove the active material filler. The region of the substrate to which heat is applied is preferably slightly greater than the connecting site of the collector member taking into account the margin for connection of the collector member. The application of heat can be accomplished by irradiating the substrate with infrared rays or laser beam or spraying flame of a burner onto the substrate. Preferably, a heat fluid such as burner flame is used in this embodiment. Further, if as the three-dimensional porous metal substrate there is used metallic nickel and as the main active material there is used nickel hydroxide, the heat fluid is preferably a flame of combustible gas. More preferably, the time during which the flame of combustible gas is allowed to come in contact with the substrate is not more than 15 seconds. In this case, even if the substrate used has a softening point of not higher than 700°C and the flame used has a temperature of about 1,300°C, the substrate can neither lose its strength nor have unnecessary oxide layer formed thereon. The temperature of the heat thus applied is similar to that of the second embodiment. However, in particular, the use of a flame of combustible gas makes it possible to minimize the effect on the substrate and give a heat of high temperature. Further, if the main active material is nickel hydroxide, heating may be effected in the atmosphere. In this case, the facilities to be used can be simplified to advantage. Subsequently, ultrasonic vibration is applied to the connecting site thus heated to remove the active material filler the retaining force of which has been reduced. The region of the substrate to which ultrasonic vibration is applied is preferably slightly greater than the region of the substrate to which heat has been applied to cover the region of the substrate to which heat has been applied to leave none of the active material thermally denatured behind. Further, the foregoing heat treatment is preferably followed by the application of ultrasonic vibration via a predetermined time interval. Further, suction is preferably effected at the same time to enhance the percent removal of the active material thus released and prevent the scattering of the active material. In this case, it is preferred that ultrasonic vibration is applied to the three-dimensional porous metal substrate on one vertical side thereof while the active material filler is sucked off from the other side. The foregoing treatment makes it possible to remove the active material filler efficiently or more completely and makes sure that the connecting member can be connected efficiently. Further, if a nickel hydroxide having a foamed plated three-dimensional porous nickel substrate is produced, this embodiment can exert a remarkable effect on a structure having a porosity of not more than 30%, more remarkably not more than 28%, after pressing. Thus, the use of the process of the present embodiment makes it possible to remarkably reduce the percent defective. This effect becomes particularly remarkable when the porosity is not more than 28%. Example 3 The present embodiment will be further described in the following examples. A nickel hydroxide electrode is produced in the same manner as Example 2. The porosity of the electrode thus produced was 27%. A collector tab made of nickel foil was then ultrasonically welded to the electrode by applying the present embodiment. Fig. 10 is a diagram illustrating the process of allowing a heat fluid to come in contact with the substrate only at a predetermined point. Fig. 11 is a diagram illustrating the process of applying ultrasonic wave to remove the active material filler. Fig. 12 is a schematic diagram illustrating how a collector tab is attached according to this embodiment. The present example will be further described in connection with these drawings. As shown in Fig. 10, in order to allow a heat fluid to come in contact with the substrate only at a predetermined point, an iron square pillar rod 203 having a size of 3 mm x 3 mm, which is greater than 30 times the thickness of an electrode 201, was placed on the electrode 201. The forward end of a flame of propane gas was allowed to come in contact with the portion shown by the shade at one point while being moved for 5 seconds so that the portion shown by the shade was entirely heated. This heat treatment caused the color of the treated portion to be changed from green to blackish at the area almost corresponding to the shape of the portion shown by the shade. The temperature of the forward end of the flame was 1,300°C. This treatment was effected in the atmosphere. Thus, in the production process of this embodiment, it is preferred that a metallic rod is placed on one vertical side of the three-dimensional porous metal substrate along the border of the region where the active material filler is denatured or deformed by the contact of the heat fluid with the region where the active material filler is not denatured or deformed by the contact of the heat fluid. In this manner, the active material cannot be damaged or dropped at extra points. Thereafter, the electrode 201 was allowed to cool to room temperature. As shown in Fig. 11, a Ti horn 204 was then pressed against the electrode 201 while applying an ultrasonic wave thereto. In this manner, the active material filler was removed from the electrode 201 while the electrode 201 was being compressed. During this procedure, the active material filler which had been released was sucked off. The surface of the Ti horn 204 in contact with the electrode 201 is planar and rectangular. It is longer than the short side of the region shown by the shade in Fig. 10 but shorter than the long side of the region shown by the shade. The Ti horn 204 was repeatedly pressed against the substrate plural times while being moved longitudinally to remove the active material filler from the entire region. In this manner, the periphery of the region shown by the shade had a slight amount of the active material filler left therein while the region shown by the shade had got a very clean foamed nickel skeleton fully free of active material filler. To the network region thus free of active material filler was then ultrasonically welded a collector tab 202. For comparison, the active material filler was removed only by the application of ultrasonic wave without the foregoing heat treatment. A collector tab was then connected to the electrode. Thereafter, the subsequent various conditions were optimized. The resulting comparative product was then measured for connecting strength of collector tab. The measurements were then compared with that of the product of the present example. As a result, the product of the present example showed a connecting strength three times that of the conventional product. The electrode obtained according to the present example was then assembled into a cell. As a result, the percent defective due to malconnection of collector tab was almost halved as compared with the conventional process. Further, the process of the present example made it possible to produce an electrode having a higher capacity and density than ever. Fig. 13 is a graph illustrating the relationship between the heating time and the tensile strength showing the results of an examination of the tensile connecting strength of collector tab with different heating times for the same testing point in the present example. The results show that the time during which the flame of combustible gas comes in contact with the substrate is preferably not more than 15 seconds, more preferably not more than 10 seconds. It can be confirmed in these results that the active material filler can be completely removed in 3 seconds or longer. However, the active material filler cannot be completely removed in 1 second. Thus, it was found that the time during which the flame of combustible gas comes in contact with the substrate is preferably not less than 3 seconds, more preferably from not less than 5 seconds to not more than 10 seconds. In accordance with this embodiment, a collector member can be rigidly connected to an electrode made of a three-dimensional porous metal substrate filled with an active material filler containing an active material with a minimized connecting energy without reducing the strength of the substrate. Further, the energy required for the application of ultrasonic wave during the removal of the active material can be reduced from the conventional value. Moreover, the occurrence of a defective due to falling of collector member at the subsequent process of cell assembly can be reduced. Further, if this embodiment is applied to the production of an electrode made of metallic nickel as a three-dimensional porous metal substrate filled with nickel hydroxide as a main active material, the effect of this embodiment can be exerted drastically because nickel hydroxide is subject to denaturation or deformation by oxidation. Thus, the substrate can little be affected by heat to advantage. Further, as compared with the comparative removal process using ultrasonic wave alone, the substrate can be provided with a very clean surface after removal of the active material, although the reason is unknown. Fourth Embodiment In this embodiment, a process for the production of an electrode for cell, a three-dimensional porous metal substrate is filled with an active material filler containing an active material. Then, a heat and an ultrasonic vibration is applied to cause the denaturation or deformation of the active material filler to the substrate at a predetermined point where a collector leading portion is formed to remove the active material filler. The main concept of this embodiment is to apply a heat to the three-dimensional porous metal substrate at a predetermined point so as to give a thermal deformation or the like causing the change in the contact condition of the active material or the like to reduce the retaining force of the active material. In other words, a heat is applied to the three-dimensional porous metal substrate or the active material filler so that the three-dimensional porous metal substrate is thermally expanded. Alternatively, at least one component of the active material filler is denatured or deformed to reduce the retaining force of the active material filler with respect to the three-dimensional porous metal substrate, so that it can be removed from the three-dimensional porous metal substrate by ultrasonic vibration. More effectively, the foregoing process is intended to denature or deform at least one component of the active material filler. Regarding to the thermal expansion of the three-dimensional porous metal substrate according to this embodiment, it is similar to chat of the second embodiment. An active material filler used in this embodiment is similar to that of the second embodiment. Concerning to the order of applying heat and ultrasonic vibration, it is similar co that of the second embodiment. This embodiment will be further described with reference to an example of nickel hydroxide electrode for nickel-hydrogen cell, etc. The three-dimensional porous metal substrate used in this embodiment is similar to that of the second embodiment. The active material filler used in this embodiment is similar to that of the second embodiment. A collector member such as collector tab made of nickel foil electrically connected to the foregoing electrode is also similar to that of the second embodiment. Fig. 14 is a schematic plan view of electrode illustrating an example of electrode structure having a collector tab attached thereto. In Fig. 1, the reference numeral 301 indicates an electrode produced by the foregoing process including coating, drying and pressing. The reference numeral 2 indicates a collector tab. The shade indicates a point on the substrate which has been freed of active material filler. A process for the removal of the active material filler will be described hereinafter. In this embodiment, a heat is applied to the substrate at a point where the collector member is connected to remove the active material filler. The region of the substrate to which heat is applied is preferably slightly greater than the connecting site of the collector member taking into account the margin for connection of the collector member. The application of heat can be accomplished by irradiating the substrate with infrared rays or laser beam or spraying flame of a burner onto the substrate. The temperature of the heat thus applied is preferably such that the active material filler can be denatured or deformed. If a nickel hydroxide electrode, which contains nickel hydroxide as a main active material, is produced, the temperature of the heat thus applied is preferably not lower than 100°C, at which the water of crystallization can be removed, more preferably not lower than 220°C, at which nickel hydroxide becomes nickel oxide. Alternatively, if the active material filler contains some resin, the temperature of the heat thus applied is even more preferably not lower than 250°C. This can apply when the active material filler contains a thickening agent such as CMC and MC or a binder resin such as PFD and PTFE. Further, the upper limit of the temperature of the heat thus applied is preferably not higher than 650°C if the substrate is a three-dimensional porous metal substrate made of metallic nickel. In particular, if the substrate is a three-dimensional porous metal substrate made of a skeleton formed by plating such as foamed nickel, this temperature is desirable. This is because if the heating time is prolonged at a temperature higher than this temperature, it causes the denaturation of the substrate besides the denaturation and deformation of the active material filler, resulting in the drop of strength and electrical conductivity. Further, if nickel hydroxide is used, heating may be effected in the atmosphere. In this case, the facilities to be used can be simplified to advantage. Subsequently, ultrasonic vibration is applied to the connecting site thus heated to remove the active material filler the retaining force of which has been reduced. The region of the substrate to which ultrasonic vibration is applied is preferably slightly greater than the region of the substrate to which heat has been applied to cover the region of the substrate to which heat has been applied to leave none of the active material thermally denatured behind. Moreover, the application of ultrasonic vibration is preferably effected while pressing the three-dimensional porous metal substrate to remove the active material filler efficiently. Further, suction is preferably effected at the same time to enhance the percent removal of the active material thus released and prevent the scattering of the active material. The foregoing treatment makes it possible to remove the active material filler efficiently or more completely and makes sure that the connecting member can be connected efficiently. A process for the production of an electrode for cell has been described which includes filling a three-dimensional porous metal substrate with an active material filler containing an active material, applying a heat and an ultrasonic vibration causing the denaturation or deformation of the active material filler to the substrate at a predetermined point where a collector leading portion is formed to remove the active material filler so that the three-dimensional porous metal substrate is partially exposed, and then electrically connecting a collector member to the substrate at the predetermined point to effect collection of electricity from the exposed point. However, the use of this embodiment makes it possible to form a collector leading portion without connecting a collector member and hence produce an electrode for cell similar to that described above. This process will be further described hereinafter. Fig. 15 is a schematic plan view of electrode illustrating an example of electrode structure having the same shape as that having a collector tab attached thereto. In Fig. 2, the reference numeral 301 indicates an electrode produced by the foregoing process which includes coating, drying and pressing as mentioned above. The shade indicates a point on the substrate which has been freed of active material filler. This portion acts as the foregoing collector tab. A process for the production of this electrode will be described hereinafter. Firstly, coating, drying and pressing were effected in the same manner as mentioned above to produce an electrode having a rectangular collector tab (region shown by the shade) corresponding to collector tab portion as shown in Fig. 15. An electrode having such a shape can be produced by stamping out of an electrode parent body having a larger size obtained by coating, drying and pressing. Subsequently, the active material filler is removed from the collector tab in the same manner as mentioned above. In this manner, an electrode for cell having the same function as that having the collector tab connected thereto can be produced without connecting a collector member. The past study shows that the foregoing process for the production of an electrode having a collector leading portion which includes preparing a three-dimensional porous metal substrate filled with an active material filler, and then removing the active material filler from the substrate at a point where a collector leading portion is formed cannot be used for the actual production process. This is because the resulting electrode leaves something to be desired in strength for example. However, it has been found that the use of the process of this embodiment involving the application of a heat and ultrasonic vibration causing the denaturation or deformation of the active material filler instead of the conventional process of removing the active material filler makes it possible to put the foregoing process into use in the actual production. This is because the production process of this embodiment causes remarkably little damage on the three-dimensional porous metal substrate during the removal of the active material filler therefrom and thus does not impair the strength of the metallic substrate. This is also because the surface of the metallic substrate thus freed of active material filler is very clean similarly to that of the collector member separately provided. Further, if a nickel hydroxide using a foamed plated three-dimensional porous nickel substrate is produced, this embodiment can exert a remarkable effect on a structure having a porosity of not more than 30%, more remarkably not more than 28%, after pressing. In other words, even when the porosity is greater than 30%, the percent occurrence of defectives such as falling of connecting member due to the reduction of connecting strength of connecting member can be reduced to that of this embodiment by properly adjusting the process conditions even if the conventional process using ultrasonic wave alone is used. However, as the packing density increases to reduce the porosity to the foregoing value, the conventional process cannot cope with the foregoing difficulties. Thus, the use of the process of this embodiment makes it possible to remarkably reduce the percent defective. This effect becomes particularly remarkable when the porosity is not more than 28%. In accordance with this embodiment, a collector member can be rigidly connected to an electrode having a three-dimensional porous metal substrate filled with an active material filler containing an active material with a minimized connecting energy without reducing the strength of the substrate. Further, the energy required for the application or ultrasonic wave during the removal of the active material can be reduced from the conventional value. Moreover, the occurrence of a defective due to falling of collector member at the subsequent process of cell assembly can be reduced. Further, a collector leading portion can be formed without separately attaching a collector member. Further, if this embodiment is applied to the production of an electrode containing metallic nickel as a three-dimensional porous metal substrate filled with nickel hydroxide as a main active material, the effect of this embodiment can be exerted drastically because nickel hydroxide is subject to denaturation or deformation by oxidation. Thus, the substrate can little be affected by heat to advantage. Further, as compared with the conventional removal process using ultrasonic wave alone, the substrate can be provided with a very clean surface after removal of the active material, although the reason is unknown.

Claims: Process for preparing an electrode (5) comprising the steps of: preparing an electrode parent body comprising a three-dimensional porous metal substrate composed of a main body and a collector tab (6) having a narrower width than the main body, characterized in that the substrate is filled with an active material filler; and heat and ultrasonic vibration is applied to the collector tab (6) of the electrode parent body to denature or deform and to remove the active material filler from the collector tab (6). Process according to claim 1, wherein heat is applied at the collector tab (6) by a solid heating member being in contact therewith; thereafter the heating ultrasonic vibration is applied to said substrate to remove mainly said active material filler; and a collector member is electrically connected to said substrate at the collector tab (6) where said active material filler has been removed. Process according to claim 2, wherein two of said solid heating member are prepared and said solid heating members are allowed to come in contact with said substrate at the collector tab (6) so that said substrate is vertically sandwiched between two said solid heating members. Process according to claim 2, wherein after said heating step, ultrasonic vibration is applied for removing mainly said active material filler to said three-dimensional porous metal substrate on one vertical side and said active material filler is sucked therefrom on the other side via a predetermined time interval. Process according to claim 2, wherein said solid heating member also acts as an ultrasonic wave application member to effect the application of heat and ultrasonic wave at the same time. Process according to claim 5, wherein said solid heating member is allowed to come in contact with said three-dimensional porous metal substrate on one vertical side thereof to effect the application of heat and ultrasonic wave at the same time; and wherein said active material filler is simultaneously sucked off said three-dimensional porous metal substrate on the other side. Process according to claim 2, wherein said three-dimensional porous metal substrate comprises metallic nickel; said main active material comprises nickel hydroxide; and a temperature of said solid heating member is from not lower than 100°C to not higher than 650°C. Process according to claim 7, wherein the temperature of said solid heating member is from not lower than 250°C to not higher than 650°C. Process according to claim 1, wherein heat is applied at the collector tab (6) by a heat fluid being in contact therewith; thereafter the heating ultrasonic vibration is applied to said substrate to remove mainly said active material filler; and a collector member is electrically connected to said substrate at the collector tab (6) where said active material filler has been removed. Process according to claim 9, wherein said three-dimensional porous metal substrate comprises metallic nickel; said main active material comprises nickel hydroxide; and said heat fluid is a flame of combustible gas. Process according to claim 10, wherein the time during which the flame is allowed to come in contact with said substrate is not more than 15 seconds. Process according to claim 9, wherein after said heating step, ultrasonic vibration is applied for removing mainly said active material filler to said three-dimensional porous metal substrate on one vertical side and the suction of said active material filler therefrom on the other side via a predetermined time interval. Process according to any one of claims 9-12, wherein a metallic rod is placed on one vertical side of said three-dimensional porous metal substrate along the border of the region where said active material filler is denatured or deformed by the contact of said heat fluid with the region where said active material filler is not denatured or deformed by the contact of said heat fluid. Process according to claim 1, wherein heat and ultrasonic vibration are simultaneously applied. Process according to claim 14, wherein said three-dimensional porous metal substrate comprises metallic nickel and said main active material comprises nickel hydroxide. Process according to claim 15, wherein a temperature of said substrate at said predetermined point to be heated is from not lower than 100°C to not higher than 650°C. Process according to claim 16, wherein the temperature of said substrate at said predetermined point to be heated is from not lower than 250°C to not higher than 650°C. Process according to any one of claims 14 to 17, wherein an ultrasonic wave is applied to said three-dimensional porous metal substrate on one vertical side thereof while said active material filler is sucked off said three-dimensional porous metal substrate on the other side.

Inventors: HIRAMURA YASUAKI, IDA YASUYUKI, ITO YASUAKI, JINUSHI SHINICHI, KOMAI SABURO, MATSUBARA TAKEHITO, MIYAZAKI KAZUYA, MURATA TOSHIO, TAKEMURA HARUHIKO, HIRAMURA, YASUAKI, IDA, YASUYUKI, ITO, YASUAKI, JINUSHI, SHINICHI, KOMAI, SABURO, MATSUBARA, TAKEHITO, MIYAZAKI, KAZUYA, MURATA, TOSHIO, TAKEMURA, HARUHIKO, Hiramura, Yasuaki c/o GS-MELCOTEC Co., Ltd., Ida, Yasuyuki c/o GS-MELCOTEC Co., Ltd., Ito, Yasuaki c/o GS-MELCOTEC Co., Ltd., Jinushi, Shinichi Japan Storage Battery Co., Ltd., Komai, Saburo c/o GS-MELCOTEC Co., Ltd., Matsubara, Takehito Japan Storage Battery Co. Ltd., Miyazaki, Kazuya c/o GS-MELCOTEC Co., Ltd., Murata, Toshio c/o Japan Storage Battery Co., Ltd., Takemura, Haruhiko c/o GS-MELCOTEC Co., Ltd.
Applicants: GS MELCOTEC CO LTD, JAPAN STORAGE BATTERY CO LTD, GS-MELCOTEC CO., LTD., JAPAN STORAGE BATTERY COMPANY LIMITED
Application year: 1998
Main classification: H01M4
IPC codes: H01M4, H01M2