Porous metallic sheet used as an electrode substrate of a battery and method of manufacturing the porous metallic sheet

Abstract: Metallic powders are extruded from a spinning nozzle to form metallic fibers each having a diameter of 1.0µm - 100µm. Then, the resultant metallic fibers are formed into a sheet having a porous structure such as a nonwoven sheet, or the like. Thereafter, the sheet is sintered. An active substance is applied to pores of a resultant porous metallic sheet to be used as an electrode substrate of a battery. The metallic fibers are three-dimensionally intertwined with each other by using fluid at a high pressure and a high speed and then, surfaces of the metallic fibers intertwined with each other is fused under pressure at a temperature lower than the melting point of the metal to directly connect intersections of the intertwined metallic fibers so as to form a porous metallic sheet.

Description: Field of the Invention The present invention relates to a porous metallic sheet to be used as a substrate of a battery electrode, and an electrode plate of the battery. More particularly, the present invention relates to a porous metallic sheet to be used as the substrate of positive and negative plates of a nickel hydrogen battery, a nickel cadmium battery, a primary lithium battery, a secondary lithium battery, and the like; and an electrode plate formed by applying an active substance to the porous metallic sheet. Description of Related Art Heretofore, as the substrate of the electrode plate comprising positive and negative electrode plates of the nickel hydrogen battery and the nickel cadmium battery, principally, a nickel-plated punched iron metal on which pores have been formed by a press is used. The electrode plate is formed by applying an active substance to the punched metal. A cylindrical battery accommodates the belt-shaped positive and negative electrode plates coiled via a separator, while a rectangular solid battery accommodates the positive and negative electrode plates laminated one on the other via the separator. As the substrate of the electrode plate compris-ing the positive and negative electrode plates of the primary lithium battery, a metallic plate (SUS, Ti) formed into a lath net is mainly used. An active substance is applied to the lath net to form the electrode plate. In the case of a secondary lithium battery, the positive plate is formed by applying the active substance in a required thickness to both surfaces of a metallic core made of an aluminum foil, whereas the negative plate is formed by applying the active substance in a required thickness to both surfaces of a metallic core made of a copper foil. In recent years, a foamed sheet made of resin, a nonwoven cloth made of resin, and a mesh sheet made of resin are chemically plated to allow them to be conductive and then, electrically plated, and then, resin removal and wintering operations are performed to form a porous metallic sheet to use it as the substrate of the electrode plate of the nickel hydrogen battery, the nickel cadmium battery, and the primary lithium battery. The punched metal used as the substrate of the electrode plate of the nickel hydrogen battery and the like has the following disadvantages: (1) Portions to be formed into pores are cut off when the punched metal is punched by a press. For example, when the percentage of pores is 50%, half of the material is lost. As such, the production cost is high. (2) It costs high to operate a press to be used to form pores. (3) Because the pores are two-dimensional, the percentage of pores is 50% at most. Thus, there is a limitation in the amount of an active substance to be applied to the pores. (4) In order to increase the capacity of a battery, it is preferable to use a thin substrate having a high percentage of the pores so as to apply a large amount of active substance to the pores. But there is a limitation in the percentage of pores for the reason described above. Further, in order to reduce the thickness of the substrate from 60 - 80µm to less than 60µm, material cost is high and the punched metal is plated at a low efficiency. That is, processing cost is high. In addition, if the substrate is thin, it is easily deformed or a burr is easily formed thereon in forming the pores by the press. In the case of the metallic plate processed into the lath net used as the substrate of the electrode of the primary lithium battery, the metallic plate is deformed and warped due to stress locally concentrated in processing the metallic plate into the lath net. That is, the metallic plate becomes unflattered. The lath net deformed and warped is corrected by a leveler without changing the original size in order to produce the substrate at a low cost. Then, the active substance is applied to the lath net. Then, the lath net is cut to a plurality of pieces having the size which meets the battery standards. At this time, the deformation generated when the metallic plate has been processed into the lath net is regenerated and burrs are likely to occur. As a result, when the lath net is coiled via a separator, there is a possibility that the burr and the deformation cause leakage. It is preferable for the electrode of the primary lithium battery to have a high percentage of pores, so long as the lath net has a preferable strength. But structurally, the lath net should not have the percentage of pores not more than 63%. There is another problem that the higher the percentage of pores is, the higher material cost is. The porous metallic sheet formed by plating a base porous sheet made of resin, successively burning out and sintering the plated material sheet used as the substrate of the electrode plate developed to replace the punched metal and the lath net has a high percentage of pores and allows a great amount of active substance to be applied to pores thereof. But it is necessary to chemically and electrically plate the porous sheet made of resin and thus perform a complicated process. Hence, the plated porous metallic sheet is produced with an unfavorable productivity. Further, chemicals such as plating liquid is used and high power is consumed. Therefore, producing cost is high. In addition, the management of treatment liquid and countermeasure against pollution are required. Furthermore, after the surfaces of organic fibers are chemically plated with a conductive substance and the conductive substance is electrically plated in a thickness of 25µm - 50µm. As a result, the outer diameter of each fiber (mixture fiber) consisting of an organic fiber and plated metal becomes large. When the organic fiber is burnt out continuously the plated metal is sintered, the portion of the fiber occupied with the organic fiber becomes hollow. As a result, a resultant metallic fiber is annular and has a large diameter. That is, the resultant porous metallic sheet has a structure in which pores are surrounded with a framework made of metallic sleeves having cavities. In order to use the porous metallic sheet as an electrode plate, the active sub-stance is applied to pores of the plated porous metallic sheet. But it cannot be applied to the cavities of the sleeves. Thus, the cavities are reactive portions in the electrode plate. Further, because the outer diameter of each of the sleeves is large, the plated porous metallic sheet has a small volume of the pores surrounded with the metallic cylinders. Therefore, a large amount of the active substance cannot be applied to the plated porous metallic sheet. When the sheet structure composed of the organic fibers, for example, a nonwoven sheet composed of the organic fibers is electrically plated after it is allowed to be conductive, metal is deposited thickly on the surface of the nonwoven sheet while it is not deposited much in the interior (center portion in thickness direction of sheet), i.e., the deposition amount of the metal in the interior is about half as much as that in the surface. That is, it is difficult to make the amount of a metallic framework on the surface equal to that on the entire sheet. In order to use the porous metallic sheet as the substrate of the electrode of a battery, as described above, the active substance is applied to pores and thereafter, it is pressurised to adjust the thickness thereof to the standard one. It is easy to apply the active substance to the porous metallic sheet having a large thickness and a high percentage of pores. According to the conventional art, it is not easy to produce a thick porous metallic sheet formed by coating the surface of the nonwovenn sheet made of organic fibers with a conductive substance and electrical plating. That is, supposing that in the conventional nonwoven sheet comprising the organic fibers, quantity of fiber is 40 - 50g/m 2 ; quantity of resin binder is 20g/m2; the total weight is 60 - 70g/m 2 ; and the percentage of pores is 95%, the maximum thickness thereof is 2.5mm - 3.5mm. After the nonwoven sheet having the above thickness is subjected to electric conduction treatment, electric plating, resin burning out, and sintering, it is difficult to produce a porous metallic sheet having a thickness 1.6mm. Further, the conventional nonwoven sheet is produced by using short organic fibers. The short organic fibers are knitted into the nonwoven sheet by a spinning carding machine and resin binder is used to connect the short organic fibers to each other. In the nonwoven sheet composed of the organic fibers thus produced, resin binder (R) is collected at the intersections of organic fibers (f), thus forming lumps at the intersections, as shown in Fig. 28. When the nonwoven sheet is subjected to electric conduction treatment and electrical plating, the diameters of fibers are large at the intersections. Thus, when the organic fibers are subjected to resin burning out and wintering operations, the resin lumps become hollow at the intersections. Thus, as described above, the active substance cannot be applied to the hollow portions, and thus the hollow portions are unconductive and further, the diameters of the fibers are great at the intersections. Accordingly, the nonwoven sheet has a small volume of pores. Thus, a large amount of active substance cannot be applied to the porous metallic sheet. The higher the percentage of pores is, the greater the active substance can be applied to the porous metallic sheet to be used as the electrode plate, and thus, the electrode plate is allowed to have a long life. The smaller the area of each pore is, the greater the area of contact between metal and the active substance is. That is, the higher the percentage of pores is and the smaller the area of each pore is, the more favorable the performance of the electrode plate is. According to the conventional art, in order to reduce the areas of pores in producing the conventional porous metallic sheet by plating the surface of the organic fibers, it is necessary to make the dense meshes of the sheet comprising the organic fibers small. But the surface of each organic fiber is plated and the organic fibers are burnt out. As a result, the resultant porous metallic sheet has a small percentage of pores and many cavities formed as a result of the removal of the organic fibers. Aluminum foils and copper foils are hitherto used as the substrate of the electrode plate of the secondary lithium battery. The active substance is applied to both surfaces of each solid metallic foil in the same thickness while the metal foil is being drawn. But because the metallic foil has a small strength, the producing line cannot be operated at a high speed. Further, it is not easy to apply the active substance to upper and lower surfaces of the metallic foil in the same thickness. Actually, the thickness of the active substance applied to the upper surface of the foil is often different from that applied to the lower surface thereof. As a result, the active substance is not partly conductive in charge and discharge times and hence the active substance cannot be utilised efficiently in a battery case. The substrate of the electrode plate which satisfy all the following requirements has not been provided. (a) The substrate has a high conductivity: The internal resistance of a battery is set to a small value so as to perform electricity-collection action smoothly. (b) The substrate has a high percentage of pores: A large amount of active substance can be applied to the substrate so as to increase the capacity of the battery. Even though a large amount of active substance can be applied to the substrate, electricity-collection action cannot be performed smoothly when the area of contact between metal and the active substance is small. Thus, it is necessary for the substrate to have a high percentage of pores and a large area of contact between the metal and the active substance. (c) The thickness of the substrate is small and the substrate has a great tensile force: If the substrate is thin, the battery can be allowed to have a high performance by accommodating a large amount of substrate in the battery case. (d) The substrate is provided with a electricity-collection lead and produced in a required configuration at a low cost. Patent Abstracts of Japan, vol. 009, no. 015 (E-291), 22.01.85 + JPA-59-163 754, 14.09.84, discloses a porous sintered body made by sintering 4-50 µm nickel fibers for an alkaline storage battery. The sintered body is prepared by making a paste of an organic binder and Ni powder, extruding the paste from a nozzle for making fiber, heating the obtained fiber to remove the organic binder and sintering the heated fiber in a reducing atmosphere at around 1.000°C. DE-A-27 20 278 discloses a porous web of metallic fibers (stainless steel) which are sintered at the intersection points. The porous web may comprise superimposed layers of regular and/or irregular networks of stainless steel fibers having a polygonal cross section. US-A-3 835 514 discloses a laminated, flexible metal fiber electrode plaque made by providing a first layer of substantially parallel metallic fibers, placing a second layer of substantially parallel metallic fibers, placing a third layer of substantially parallel metallic fibers, metallurgically diffusion bonding the contact points of the fibers between the layers together, to provide annealed bonded plaques, coining the annealed bonded plaques to provide a plaque having a body portion of between 70 and 97 percent porosity, and then metallurgically bonding the contact points. US-A-3 895 960 discloses a flexible electrode plaque having 75 to 95 percent porosity made from at least one sheet of diffusion bonded iron fibers. WO 95/35 117A corresponding to EP 95 923 878.3, which forms state of the art in the sense of Article 54(3)EPC discloses a battery plate comprising a plurality of metallic fibers of a single or plural diameters randomly oriented and sintered to provide a conductive battery plate with a multiplicity of pores. The metallic fibers are formed by cladding and drawings a plurality of metallic wires to provide a fiber tow. The fiber tow is severed and the cladding is removed to form metallic fibers. The metallic fibers are arranged into a web and sintered to form a battery plate. None of the cited prior art documents describes or suggests a porous metallic sheet having the fatures of the present invention. SUMMARY OF THE INVENTION The present invention has been made in view of the above-described conventional problems, and it is an object of the present invention to provide a porous metallic sheet to be used as an electrode substrate of a battery having the above-described requirements and an electrode plate formed by applying an active substance to the porous metallic sheet. It is another object of the present invention to reduce manufacturing cost by eliminating conduction treatment such as chemical plating and the management of treating solution and by saving the rate of electric power consumption. In order to achieve the above-described objects, according to a first invention, there is provided a porous metallic sheet, to be used as an electrode substrate of a battery, having a porous fibrous structure or a three-dimensional net-shaped structure in which a framework surrounding pores of the porous fibrous structure or those of the three-dimensional net-shaped structure is formed of metallic fibers made of metallic powders, wherein the porous fibrous structure or the three-dimensional net-shaped structure has fine pores and a large number of through-holes formed therein. The porous fibrous structure consists of a nonwoven sheet, a woven sheet, a knitted sheet, a felt sheet, a screen-shaped sheet, an expanded sheet, a net-shaped sheet. The three-dimensional net-shaped structure consists of a foamed sheet, a spongelike sheet, a honey-comb-shaped sheet. As described above, metallic fibers made of metallic powders are formed into a sheet having the porous fibrous structure consisting of a nonwoven sheet, a woven sheet, a knitted sheet, and the like or into the three-dimensional net-shaped structure. The method of forming the metallic fibers into these structures eliminate plating process. Heretofore, the porous metallic sheet is formed by plating organic fibers and then burning out the organic fibers. Thus, cavities to which the active substance cannot be applied are generated in a resulting sheet. According to the present invention, because a framework surrounding pores consists solid metallic fibers, cavities to which the active substance cannot be applied are not generated in the porous metallic sheet. That is, a large amount of active sub-stance can be applied to the porous metallic sheet. Further, the conventional metallic fibers are hollow, whereas the metallic fibers according to the present invention are solid. Thus, the metallic fibers according to the present invention can be allowed to have small diameters, thus increasing the percentage of pores. Accordingly, a great amount of active substance can be applied to the porous metallic sheet. That is, supposing that the same spinning nozzle is used, according to the conventional method, the surfaces of the organic fibers extruded from the spinning nozzle are plated. Thus, the outer diameters of organic fibers become large. On the contrary, according to the present invention, fibers extruded from the spinning nozzle are metallic fibers. Therefore, it is unnecessary to plate the surface of the metallic fiber. When the porous sheet made of the metallic fibers is subjected to burning out and sintering operations, the diameters of the metallic fibers becomes small. At a high percentage of pores, a large amount of active substance can be applied to the porous metallic sheet per area. The performance of the battery cannot be improved even though large area pore is formed to apply a large amount of active substance thereto. That is, the performance of the battery can be improved when the gap between metallic fibers (framework) which flows electricity to the active substance is small and the area of contact between the metallic fibers and the active substance is great. Because the percentage of pores in the porous metallic sheet of the present invention is the same as that of the conventional sheet, the porous metallic sheet of the present invention can be composed of a larger number of metallic fibers than the conventional sheet . Therefore, in the metallic sheet of the present invention, the gap between the metallic fibers is allowed to be small and in addition, the area of contact between the metallic fibers and the active substance is great. This structure increases the electric conduction performance of the active substance, thus improving the performance of the electrode plate. Further, according to the present invention, because metallic fibers are woven or knitted and intertwined with each other to form the sheet having the porous fibrous structure, metallic frameworks are present uniformly on the surface and the interior of the sheet. On the other hand, according to the conventional art, when a porous metallic sheet is formed by electrically plating the organic fibers, the amount of metal which deposits on the organic fibers located in the interior of the sheet is about half as small as that of metal which deposits on those located on the surface of the sheet. The porous metallic sheet of the present invention does not have the disadvantage. The diameters of the metallic powders are in the range from 0.1µm to 5µm. As the metallic fiber made of the metallic powders, continuous or long metallic fibers having diameters 1.0µm - 100µm are preferable. The smaller the diameter of the fiber is, the higher the percentage of pores is and further, the area of one pore can be reduced without decreasing the percentage of pores in the sheet. The porous metallic sheet made of long or continuous metallic fibers have a tensile force 1.5 - 3 times as great as that of the porous metallic sheet produced by the conventional method, namely, made of electrically plating the organic fibers and then burning out and sintering them. Accordingly, because the porous metallic sheet has a high tensile force, it can be drawn at a high speed while it is being fed for applying the active substance. As necessary, the surface of the metallic fibers may be electrically plated in the present invention. The plating increases the strength of the porous metallic sheet and its tensile force, thus allowing the active substance to be applied to the metallic fibers at a higher speed. The metallic powders consist of metals, alloys, oxides of metals, or oxides of alloys. The metallic powders also consist of one kind of the above metallic powders or a mixture of the above metals, alloys, oxides of metals, and oxides of alloys. As the metallic powders, Ni, Cu. Al, Ag, Fe, Zn, Sn, In, Ti, Pb, V, Cr, Co, oxides of these metals, alloys of these metals, and mixtures of these metals are preferably used. In this manner, metallic fibers consisting of a mixture of the metallic powders can be formed. Preferably, the porous metallic sheet, to be used as the electrode substrate of a battery, formed of the metallic fibers made of metallic powders has percentage of pores at more than 90%, and its thickness is set to 5µm - 5000µm. In particular, the thickness of the porous metallic sheet in the present invention can be formed more thickly than the conventional one. That is, the maximum thickness of the conventional nonwoven sheet comprising the plating organic fibers is 2.5mm - 3.5mm as described previously, when the quantity of fiber is 40 - 50g/m 2 ; the quantity of resin binder is 20g//m 2 ; the total weight is 60 - 70g/m 2 ; and the percentage of pores is 95%. After the organic sheet having the above thickness is subjected to electric conduction treatment, electric plating, resin removal, and sintering, a thickness of a resulting porous metallic sheet reduces to 1.6mm. It is difficult to apply the active substance to the porous metallic sheet when it has such a small thickness. On the other hand, because the porous metallic sheet made of metallic fibers in the present invention has a high rigidity and elasticity, it can be allowed to have a thickness as great as 3.3mm - 5.0mm. After the porous metallic sheet is burnt to remove resin binder and sintered the metal, the weight thereof is decreased in a small extent. After the porous metallic sheet is passed through a pair of skin pass rolls to make the thickness thereof uniform, the porous metallic sheet has a thickness more than 1.6mm. Accordingly, the active substance can be applied to the porous metallic sheet in the present inven-tion more easily than the conventional one. After the application of the active substance to the porous metallic sheet, the porous metallic sheet is pressurised to obtain the predetermined thickness, namely, 1/2 to 1/3 of the thickness of an original one. Preferably, a plurality of porous metallic sheets are laminated one on the other. Further, the porous metallic sheet in the present invention may be laminated on a porous metallic sheet formed by plating a porous sheet made of organic or inor-ganic substance proposed and/or a conductive metallic foil having holes formed therein, so as to form a porous metallic sheet to be used as an electrode substrate of a battery. The organic substance consists of porous sheet such as a foamed sheet, a mesh sheet or a nonwoven sheet made of resin, the porous sheet is plated and then, burnt to remove resin and then sintered. According to a second embodiment the invention refers to a porous metallic sheet, to be used as an electrode substrate of a battery, having a porous fibrous structure or a three-dimensional net-shaped structure in which a framework surrounding pores of the porous fibrous structure or those of the three-dimensional net-shaped structure which is formed of metallic fibers made of metallic powders, wherein the porous metallic sheet made of metallic fibers is overlaid with one or two selected from the group consisting of a porous metallic sheet formed of a plated organic substance or an inorganic substance and a conductive metallic foil having a plurality of holes formed therein. Further, the porous metallic sheet in accordance with the first and second inventions is made of fine metallic fibers and thus flexible. In particular, unlike the conventional punched metal sheet or lath net sheet, the flexibility prevents an external force from being locally applied to the porous metallic sheet in a process of cutting it to a predetermined size after the active substance is applied thereto. Thus, the porous metallic sheet is hardly deformed and further, burr is not generated, which prevents leak from being generated when the porous metallic sheet serving as an electrode plate is accommodated in a battery. It is possible to overlay on the porous metallic sheet according to the first and second invention with porous metallic sheet formed by a punched conductive metallic foil, an plated organic porous sheet or an inorganic porous sheet, so as to use the resultant porous metallic sheet as the electrode substrate of the battery. The present invention provides an electrode plate of a battery formed by applying the active substance to the porous metallic sheet in accordance with the first and second inventions. The electrode plate is used as the electrode plate of a nickel hydrogen battery, a nickel cadmium battery, a primary lithium battery or a secondary lithium battery. More specifically, the electrode plate of the nickel hydrogen battery and the nickel cadmium battery is formed by applying the active substance to the porous metallic sheet made of nickel fibers. Cavities to which the active substance cannot be applied are not present in the electrode plate of the battery according to the present invention. Thus, a large amount of active substance can be applied and further, the area of contact between the applied active substance and the metallic fibers (framework) is great, which improves the performance of the battery. A method of manufacturing the porous metallic sheet of the first and second invention, to be used as an electrode substrate of a battery comprises the steps of forming a mixture by kneading metallic powders and resin binder; spinning metallic fibers by extruding the mixture from a spinning nozzle; and forming the spun metallic fibers into a porous fibrous structure or a three-dimen-sional net-shaped structure. Preferably, the porous fibrous structure or the three-dimensional net-shaped structure are burnt to remove the resin binder and then sintered. Resin binder is required to have a required viscosity in spinning process, and thus the following substances having the required viscosity are preferably used: polypropylene, polyester, polyethylene, polyacrylonitrile, polyvinyl polymer, polyimide, nylon polymer, polyurethane, cellulose, and polymer formed of organic fiber. Favorably, the mixing ratio between the metallic powder and the resin binder is 70% - 97% and 30% - 3%, and more favorably, 85% - 97% and 15% - 3%. Because the metallic powder is mixed with the resin binder at percentages as high as 70% - 97%, it is not always necessary to plate a resulting sheet in a later process, but the resulting sheet may be plated as necessary. In order to easily spin and draw the mixture of the resin binder and the metallic powder contained at a high percentage to form fine fibers, the mixture is extruded from the center of a spinning nozzle to form a core, while the resin is extruded from the periphery of the center of the spinning nozzle to form a sheath. In this manner, the mixture is extruded from the spinning nozzle as a compound metallic fiber consisting of the core and the sheath surrounding the core. The mixture of the resin binder and the metallic powder is extruded from a plurality of discharge portions to form a plurality of cores, and the resin is extruded to the spaces between the cores. In this manner, the mixture is extruded from the spinning nozzle as a compound metallic fiber consisting of a resin portion and a plurality of scattered cores surrounded with the resin portion. Further, the mixture of the resin binder and the metallic powder is extruded from the spinning nozzle, with the mixture being in contact with resin having a melting point different from the resin contained in the mixture to form a compound metallic fiber. The resultant fibers are curled when they are cooled. This method allows the spun fibers to be curled and a thick sheet to be composed of the curled metallic fibers. In the above-described methods, metallic powders and the resin binder are mixed with each other, but fibers may be formed of only metallic powders by extruding them from the spinning nozzle. The resultant metallic fibers are formed into the porous fibrous structure or the three-dimensional net-shaped structure so as to use them as the electrode substrate of the battery. In this case, pressurizing force to be applied to the metallic powder is set at about 30 - 70 times, preferably about 50 times as great as that to be applied to the mixture of the metallic powders and the resin binder. Because the resin binder is not used in this method, it is not necessary to burn out for removing the resin binder and hence they are only sintered. Preferably, the porous fibrous structure or the three-dimensional net-shaped structure is burnt out at 800°C - 1200° for about two minutes to remove the resin binder and then sintered at 1000°C - 1300° for about 2 - 10 minutes. Continuous metallic fibers extruded from the spinning nozzle are curved continuously into a coiled configuration by movement of blowing air. As described above, air is blown to the continuous metallic fibers extruded from the nozzle or air is sucked to bend the metallic fibers extruded from the nozzle into a continuously curved coil spring-configuration. Because the metallic fibers are curved, they are rigid and elastic and thus a sheet having a great thickness can be formed in correspondence to the curved amount. It is possible to heat surfaces of a plurality of the resulting porous metallic sheets laminated one on the other at a temperature lower than the melting point thereof so as to join them to each other. Further, in producing the porous metallic sheet by using fluid, the fluid is injected to the web placed on the upper surface of the finished porous metallic sheet at a high pressure and a high speed so as to increase the thickness thereof. Further, it is possible to inject fluid at a high pressure and a high speed to the web placed on the upper surface of the porous metallic sheet produced by other methods so as to increase the thickness thereof. The resulting porous metallic sheet laminated with a plurality of sheets are preferably pressed by punch to form through-holes. According to the method, of the present invention, for manufacturing the porous metallic sheet to be used as the electrode substrate of the battery, conduction treatment process and plating process required by the conventional method in forming organic firers into the porous metallic sheet can be eliminated. Thus, the problem of waste solution which occurs in plating the porous metallic sheet can be solved. Further, the method of the present invention reduces the rate of electric power consumption, thus greatly reducing the cost. If it is preferable to increase the strength of the porous fibrous structure sheet or the three-dimensional net-shaped structure produced sheet by the above-described methods, it is possible to electrically plate the sheet. Preferably, after they are electrically plated, sintering operations are carried out. Even though electrical plating is used to increase the strength of the porous metallic sheet, the amount of metal to be applied to the porous metallic sheet of the present invention is smaller than that to be applied to the conventional porous metallic sheet. Hence, the method of the present invention reduces the rate of electric power consumption, thus greatly reducing the cost. The present invention is also intended to provide a method for producing a battery electrode formed by applying the active substance to the finished porous metallic sheet. Because the porous metallic sheet produced by the method of the present invention is formed of metallic fibers, it has a high tensile strength. Therefore, the producing line speed can be increased when the active substance is applied thereto while it is being drawn continuously. Hence, the porous metallic sheet can be manufactured with a high productivity. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flowchart showing a manufacturing method for a porous metallic sheet in accordance with a first embodiment of the present invention; Fig. 2 is a schematic view showing a manufacturing apparatus for carrying out the manufacturing method; Fig. 3 is a partial plan view showing a net conveyor of the manufacturing apparatus; Fig. 4 is a schematic view showing a porous metallic sheet according to the first embodiment manufactured by the manufacturing method; Fig. 5A is a sectional view showing a metallic fiber used according to the present invention; Fig. 5B is a sectional view showing a conventional metallic fiber; Figs. 6A and 6C are plan views showing pores formed by metallic fibers used in accordance with the present invention; Fig. 6B is a plan view showing a pore formed by conventional metallic filers; Fig. 7A is a plan view showing connection portions of metallic fibers in accordance with the present invention before resin is burnt out; Fig. 7B is a plan view showing the connection portions of the metallic fibers in accordance with the present invention after the resin is burnt out; Figs. 8A through 8D are sectional views showing the body of a spinning nozzle and continuous fibers, in accordance with the present invention, formed by the spinning nozzle; Fig. 9 is a flowchart showing another manufacturing method; Fig. 10 is a flowchart showing still another manufacturing method; Fig. 11 is a flowchart showing a further manufacturing method; Fig. 12 is a schematic view showing another porous metallic sheet in accordance with a second embodiment of the present invention; Figs. 13A, 13B, and 13C are schematic sectional views each showing a porous metallic sheet in accordance with a further embodiment of the present invention; Fig. 14 is a flowchart showing a further manufacturing method; Fig. 15A, 15B, and 15C are views showing the relationship between the configuration of a convex of a supporter and a formed through-hole in accordance with the first embodiment of the present invention; and Fig. 16 is a plan view showing a conventional problem. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention are described below with reference to drawings. Initially, a first method for manufacturing a porous metallic sheet to be used as a substrate of a battery electrode, in accordance with a first invention, composed of metal fibers made out of metallic powder. In the first embodiment, in accordance with the flowchart of Fig. 1, metal fibers are made out of metallic nickel powder by using a manufacturing apparatus shown in Fig. 2, and the metallic fibers are formed into a nonwoven porous metallic sheet 40, to be used as a substrate of a battery electrode, shown in Fig. 4. Using fine nickel powder of 0.1µm-5µm in diameter as fine metallic powder and powder of nylon 6 as resin binder, 85 % of the fine nickel powder and 15% of nylon powder are fed to a kneader 4 from storing containers I and 2, respectively after the quantity of the fine nickel powder and that of the nylon powder are measured by quantity-measuring feeders 3A and 3B, respectively so as to knead them at step #1. A mixture (M) comprising the fine nickel powder and the nylon powder kneaded by the kneader 4 is extruded from a nozzle 4a provided at the leading end of the kneader 4 to form the mixture (M) into fibers about 3mm in diameter. The fiber-shaped mixture (M) is fed inside a cooler 5 by a conveyor 5a to cool it therein. A pump 12 of the cooler 5 circulates cold water stored in a water tank 11 so as to cool the fiber-shaped (M) with the cold water. While fiber-shaped mixture (M) taken out from the cooler 5 is being fed by a roller 6, it is cut to form pellets (P) of about 3mm in length by a cutter 7 at step #2. The pellets (P) are fed through a drier 8 to dry them and then, accommodated in a container 9. The pellets (P) are supplied to a fusion furnace 10. The pellets (P) are then heated therein at 260°C -280°C to fuse the resin binder at step #3. At this time, the fine metallic powder does not melt. Then, the pellets (P) are fed to an extruder 14 after the quantity thereof is measured by a feeder 13. Then, the mixture of melted resin binder and unmelted metallic powder is fed from the extruder 14 into a spinning nozzle 16 via a filter 15 while a gear pump (not shown) of the extruder 14 is applying pressure thereto. Large particles and aggregated masses are removed from the mixture by passing it through the filter 15. A heater 18 circulating heating medium is provided inside a heat retaining wall 17. The heater 18 surrounds a nozzle body 20. The mixture is supplied to the nozzle body 20 via the filter 15 so as to extrude it successively from the nozzle body 20 as fibers. In this manner, the fibers are formed at step #4. A cooling cylinder 21 is provided below the spinning nozzle 16. An air blowout port 22 is provided at a position proximate to the nozzle body 20 such that the air blowout port 22 is positioned at an upper portion of the cooling cylinder 21 so as to horizontally blow air to the continuous fibers being extruded from the nozzle body 20. In this manner, the fibers are drawn and cooled. Air sucked by a blower 24 through a filter 23 is fed to the air blowout port 22 through a cooler 25 in which cooling water circulates, a filter 26, and a heater 27 in which vapor circulates so as to adjust the temperature of air to a required one, and then blown to the inside of the cooling cylinder 21. The air blown to the cooling cylinder 21 is exhausted from an air exhaust port 28 located at the lower end of the cooling cylinder 21. As shown in Fig. 8A, the nozzle body 20 has a large number of discharge ports 51 through which the fibers (about 40 metallic fibers in the embodiment) are extruded successively in parallel with each other. It is possible to form hundreds to thousands of discharge ports in the nozzle body 20 to extrude hundreds to thousands of continuous fibers at a time. The diameter of each discharge port 51 of the nozzle body 20 is set to 100µm - 50µm. When the diameter of each discharge port 51 is set to 100µm, the diameter of each fiber extruded from the discharge port 51 is about 60µm and then reduced to 30µm owing to the air blowing-caused drawing thereof. When the diameter of each dis-charge port 51 is set to 50µm, the diameter of each fiber extruded therefrom is about 30µm and then reduced to 7µm owing to the air blowing-caused drawing thereof. When each of the fibers which have moved downward with air being blown thereto to draw and cool them in the cooling cylinder 21 is continuously bent into a coil spring configuration by a pair of crimpers 30 of suction drum type positioned below the cooling cylinder 21. The crimpers 30 suck air and blow it toward the fibers from the left and right sides thereof, thus bending the fibers moving downward. The crimpers 30 do not contact the fibers when it bends them. The fibers formed in the coil spring configuration by the crimpers 30 are supplied to a net conveyor 33 located horizontally below the crimpers 30 and placed thereon. As shown in Fig. 3, a large number of continuous fibers bent in the coil spring configuration are arranged in parallel with each other on the net conveyor 33. The net conveyor 33 is vibrated in the widthwise direction (W). As a result, the fibers arranged in parallel with each other on the net conveyor 33 are vibrated widthwise; intertwined with each other; and bonded to each other at random by means of the resin binder which has mixed to metallic fibers. In this manner, a sheet having a nonwoven porous structure is manufactured at step #5. As shown in Fig. 3, a means for vibrating the fibers in the widthwise direction (W) comprises a net conveyor-feeding apparatus comprising a driving motor, a pair of pulleys to be driven by the motor, and an endless conveyor 33 spanned between the pulleys. Rods 36 of cylinder 35 reciprocate to vibrate the entire net conveyor 33 in the widthwise direction (W). Needless to say, other appropriate means may be adopted instead of the above-described vibration means. Because each fiber of the sheet having a nonwoven porous structure has the coil spring configuration, the sheet having a nonwoven porous structure has a comparatively large thickness. The percentage of pores of the sheet having a nonwoven porous structure can be adjusted by controlling the feeding speed of the net conveyor 33. That is, with the increase in the feeding speed the density of the fibers becomes coarse and thus, the percentage of pores becomes high, whereas with the decrease in the feeding speed, the density of the fibers becomes small and thus, the percentage of pores becomes low. The sheet having a nonwoven porous structure composed of the fibers is heated at 800°C - 1200° for about two minutes to burn out the resin binder from the sheet having a nonwoven porous structure at step #6. The sheet having a nonwoven porous structure is heated at 1000°C- 1300° for about 2 - 10 minutes to sinter metallic powder at step #7. In this manner, the porous metallic sheet 40 having a construction shown in Fig. 4 is produced to use it as the electrode substrate of a battery. The percentage of pores of the porous metallic sheet 40 is set to 94% - 98%. The area of one pore is set to 0.005mm 2 - 0.942mm 2 . The thickness of the porous metallic sheet 40 is set to 0.5mm - 5mm. The porous metallic sheet 40 of the present invention to be manufactured by the method according to the first embodiment has advantages described below. That is, as a result of the removal of the resin binder and sintering of metallic powder, the diameter of each metallic fiber becomes smaller by 5 - 40% because the resin binder and metal oxide are burnt out. That is, as shown in Fig. 5A, each of metallic fibers (F) obtained in the first embodiment is solid and the diameter thereof is small. As shown in Fig. 5B, supposing that organic fibers are extruded from the dis-charge port 51, the diameter of a resulting conventional metallic fiber F' is greater than that of the metallic fiber (F). This is because according to the conventional art, the organic fibers are metal-plated. Further, because the organic fibers are removed from the conventional metallic fiber F', a cavity (C) is generated therein. Thus, an active substance cannot be applied to the cavity (C) . More specifically, in the case of the metallic fibers (F) of the present invention shown in Fig. 5A, the diameter of each of the metallic fibers (F) is 20µm in average, whereas in the case of the conventional metallic fiber F' shown in Fig. 5B, the diameter thereof is 30µm in average. Thus, the volume of each of the metallic fibers (F) is smaller by 55.6% than that of the conventional metallic fiber F'. Comparing the porous metallic sheet, in accordance with the present invention, formed of the metallic fibers (F) having a smaller diameter with the conventional porous metallic sheet formed by electrically plating organic fibers, a pore of the porous metallic sheet in accordance with the present invention shown by oblique lines of Fig. 6A has a greater area than that of the conventional porous metallic sheet shown by oblique lines of Fig. 6B. Let it be supposed that the fiber density of the present invention is equal to that of the conventional one. When the percentage of pores is 93% in the conventional porous metallic sheet, the percentage of pores is 96.9% in the porous metallic sheet of the present invention. When the percentage of pores is 96% in the conventional porous metallic sheet, it is 98.2% in the porous metallic sheet of the present invention. That is, the percentage of pores of the porous metallic sheet of the present invention is greater than that of the conventional porous metallic sheet. Thus, the entire area of pores of the present invention to which an active substance can be applied is equal to that of the conventional porous metallic sheet, the porous metallic sheet has the metallic fibers (F) 2.25 times as great as the conventional porous metallic sheet per area. That is, as shown in Fig. 6C, the metallic fibers (F) can be arranged at a higher density than the conventional metallic fibers F' per area. In this case, the gap between metallic fibers (F) which sandwich the active substance is smaller than that between the conventional metallic fibers F', and the area of contact between the active substance and the metal fibers (F) is greater than that between the conventional metallic fibers F'. Thus, the porous metallic sheet according to the present invention has a higher conductivity than the conventional one, thus allowing a battery to have a more favorable characteristic than the conventional one. When the weight of metal to be used per area to form the porous metallic sheet of the present invention is equal to that to be used per area to form the conventional porous metallic sheet, the former is allowed to have more metallic fibers per area than the latter by 1.75 times as large as that of the latter, without decreasing the per-centage of pores. Thus, the porous metallic sheet according to the present invention has a higher conductivity than the conventional one, thus allowing a battery to have a more favorable characteristic than the conventional one. Supposing that the same number of metallic fibers is used per centiare to form the porous metallic sheet of the present invention and the conventional one, 420g is required per area in the case of the latter, whereas 240g is required in the case of the former. In this case, there is no problem on the strength of the porous metallic sheet of the present invention even though the weight of metal is smaller per area than that used in the conventional porous metallic sheet because the metallic fibers of the present invention are solid and continuous, and thus they have a higher strength than the conventional ones. A conventional problem of a resin lump formed at connection points of the metallic fibers (F) as shown in Fig. 16 is described. Even though resin is collected at the connection point of the metallic fibers (F) shown in Fig. 7A, it is burnt out during a resin binder removal process and a wintering process. Consequently, as shown in Fig. 7B, the resin lump disappears from the connection point. Accordingly, the diameter of the metallic fibers (F) can be prevented from becoming partly larger and thus the percentage of pores can be prevented from being reduced. Because the metallic fibers (F) are solid and continuous, they have a strength 1.5 - 3 times higher than the conventional metallic fibers formed by electrically plating organic fibers. Accordingly, in applying an active substance into pores, the porous metallic sheet 40 can be drawn at a great force. Thus, the active substance can be applied to the pores at a high speed. Further, because the sheet having a nonwoven porous structure is formed by intertwining the solid metallic fibers with each other, the metallic fibers (F) is allowed to be present uniformly on the peripheral surface and the inner peripheral thereof, whereas in the case of the conventional metallic porous sheet formed by electrically plating organic fibers, a large amount of metal is deposited on the peripheral surface of the sheet and a small amount of metal is deposited in the inner of the sheet. Therefore, the porous metallic sheet of the present invention is capable of improving the performance of a battery. Further, because the continuous metallic fibers (F) are bent in the coil spring configuration and intertwined with each other, the thickness of the porous metal-lic sheet can be set to as large as 3.3mm-5mm. Thus, the active sub-stance can be applied to the porous metallic sheet 40 consisting of the metallic fibers (F) with high efficiency. In the first embodiment, the discharge ports 51 for extruding the mixture (M) comprising the resin binder and metallic powder therefrom are arranged in parallel with each other in the nozzle body 20 of the spinning nozzle 16. When the mixture (M) contains a large amount of metallic powder and thus when it is not easy to extrude the mixture (M) from the discharge ports 51, it is preferable to use the nozzle body 20 shown in Figs. 8B through 8D to form the mixture (M) into compound metallic fibers. That is, the nozzle body 20 shown in Fig. 8B has the discharge port 51 for discharging the mixture (M) at the center thereof and a discharge port 52 for discharging only resin in the periphery of the discharge port 51. Thus, the mixture (M) extruded from the center of the nozzle body 20 forms a core 60 while the resin (R) extruded from the periphery of the center of the nozzle body 20 forms a sheath 61. That is, the mixture (M) is extruded from the nozzle body 20 as a compound metallic fiber consisting of the core 60 and the sheath 61 surrounding the core 60. The sheath 61 of the compound metallic fiber thus formed serves as adhesive agent in forming the mixture (M) into the sheet having a nonwoven porous structure by vibrating the net conveyor 33 widthwise. Because the sheath 61 is burnt out during a resin binder-removal and a sintering process, the diameter of the metallic fibers can be prevented from becoming great. The nozzle body 20 shown in Fig. 8C has a large number of discharge ports 51, for discharging the mixture (M), at predetermined intervals inside the discharge port 52 having a large diameter. The mixture (M) extruded from the discharge ports 51 forms the cores 60, and the resin (R) is extruded between the cores 60. In this manner, the mixture (M) is extruded from the nozzle body 20 as a compound metallic fiber consisting of a resin portion 62 and a plurality of scattered cores 60 surrounded with the resin portion 62. In the multi-core compound metallic fiber, the cores 60 are connected to each other during the resin binder-removal and sintering processes. As a result, each metallic fiber has a great surface area. When the resin portion 62 made of polyester is dissolved in an alkaline solution to remove polyester, the metallic fiber can be composed of small diameter-metallic fibers separated from each other. The nozzle body 20 shown in Fig. 8D has the discharge port 51, for discharging the mixture (M), at one side of thereof and a resin discharge port 53 at the other side thereof. Resin different from the resin binder contained in the mixture (M) is discharged from the resin discharge port 53. The mixture (M) is extruded from the discharge ports 51 and 53, with the mixture (M) being in contact with the resin different from the resin binder contained in the mixture (M) to form a compound metallic fiber consisting of the core 60 of the mixture (M) connected with a resin portion 64 in a bimetallic state. The contraction coefficients of the core 60 and the resin portion 64 are different from each other. Therefore, when they are cooled in the cooling cylinder 21, the compound metallic fiber is curled. That is, the compound metallic fiber can be curved without using the crimpers 30 of suction drum type In the first embodiment, metallic powder and powdered resin binder are kneaded into filaments, and then, the filaments are cut to pellets, and then, the pellets are heated to fuse the powdered resin binder. Instead, it is possible to mix the metallic powder with the fused powdered resin binder, knead the mixture of the metallic powder and the fused resin binder, and extrude the mixture from the spinning nozzle. In the first embodiment, after the sheet having a nonwoven porous structure is formed, it is burnt out to remove the resin binder and the metallic powder is sintered. But it is possible to omit the burning out process at step #6 and the sintering process at step #7 as necessary, as shown in Fig. 9. If the metallic powder is mixed at high percentages, for example, 95\ - 97\ with the resin binder, it is unnecessary to burn out the sheet having a nonwoven porous structure to remove the resin binder. Further, as shown in Fig. 10, after the sheet having a nonwoven porous structure is formed at step #5 of the first embodiment, the sheet having a nonwoven porous structure may be electrically plated at step #6 and then, the resin binder may be burnt out at step #7, and then, metallic powder may be sintered at step #8. That is, if it is necessary to increase the weight of metal per area in forming the porous metallic sheet so as to allow metallic fibers to have a great strength, the sheet having a nonwoven porous structure may be plated. In this case, because metallic fibers of the sheet having a nonwoven porous structure are solid, a much smaller amount of metal is used to plate it compared with the conventional electrical plating method. Thus, a much smaller of electric power is consumed in plating the sheet having a nonwoven porous structure. It is possible to omit the burning out process and the sintering process as necessary, as shown in Fig. 11. The kind of metallic powder to be mixed with the powdered resin binder is not limited to one, but a plurality of kinds of metallic powders may be mixed with the resin binder so as to form compound metallic fibers. In the first and second embodiments, in order to form the sheet having a nonwoven porous structure, the method of bending the continuous metallic fibers formed by using the spinning nozzle and then transporting them to the net conveyor. But other methods can be adopted when a porous fiber sheet or a three-dimensional net-shaped sheet having a porous structure is formed of metallic fibers. For example, the continuous metallic fibers (F) may be woven in a mesh configuration to form a mesh sheet as shown in Fig. 12. It is also possible to cut continuous metallic fibers extruded from the spinning nozzle and put them into water so as to form short metallic fibers of 2mm - 60mm and then use dry nonwoven sheet-producing method to form the sheet having a nonwoven porous structure. That is, after drying the short metallic fibers, they are raveled or accumulated at random by applying air thereto so as to form the sheet having a nonwoven porous structure. It is also possible to use wet nonwoven sheet-producing method of dispersing the short metallic fibers in water or water containing adhesive agent and collecting them with a net so as to form the sheet having a nonwoven porous structure. It is also possible to use melt-blow nonwoven sheet-producing method of applying heated gas to the metallic fibers so as to draw and cut them to short fibers and then directly accumulating the short fibers on the conveyor so as to form the sheet having a nonwoven porous structure. It is also possible to use span bonding method of applying air to the metallic fibers so as to draw them, accumulating them on the conveyor directly to form a web consisting of continuous metallic fibers, and bonding fibers of the web to each other with adhesive agent. In the further embodiment, a plurality of porous metallic sheet 40 made of metallic powder is laminated one on the other to form a porous metallic sheet 100 to be used as a substrate of a battery electrode as shown in Fig. 13A. The porous metallic sheet 40 has a porous fibrous structure having a nonwoven sheet, a woven sheet, knitted sheet, a felt sheet, a screen-shaped sheet, an expanded sheet, a net-shaped sheet; and a three-dimensional net-shaped structure having a foamed sheet, a spongelike sheet, a honeycomb-shaped sheet. In addition, as shown in Fig. 13B, it is possible to overlay a conductive metallic foil 101 having a plurality of pores 101a formed thereon on the porous metallic sheet 40 to form a porous metallic sheet 100'. Further, as shown in Fig. 13C, it is possible to form a porous metallic sheet 100" by laminating the porous metallic sheet 40 and a porous metallic sheet 102 formed by plating foamed, mesh or nonwoven sheets made of resin and then burning out resin binder and sintering metallic powder. As shown in Fig. 14, an active substance is applied to the porous metallic sheets 40 of the first and second embodiment, 100, 100' or 100" of the further embodiment so as to form an electrode plate of a battery. As shown in Fig. 14, at step #8, the active substance is applied to the porous metallic sheet formed in the process of steps #1 through #7 of Fig. 1 showing the first embodiment. Needless to say, the active substance may be applied to the porous metallic sheet after performing the operation of the final process shown in Figs. 9, 10, and 11 showing the modifications of the first embodiment, after forming the porous metallic sheet by overlaying the porous metallic sheets 100, 100' or 100" on the porous metallic sheet 40. The active substance is applied to the nonwoven porous metallic sheet consisting of nickel fibers composed of nickel powder while the porous metallic sheet is being continuously drawn, so that an electrode of a nickel hydrogen battery is formed. Three experiments of the fourth embodiment are described below. In a first experiment, paste of an active sub-stance was applied to a nonwoven porous metallic sheet made of nickel. The paste was obtained by kneading a mixture of 100 parts by weight of nickel hydroxide powder, 10 parts by weight of cobalt powder, 0.2 parts by weight of methyl cellulose used as adhesive agent, and 20 parts by weight of water. After the paste is dried, it is molded under pressure to form a positive electrode plate of a nickel hydrogen battery having a thickness of 0.5mm. In a second experiment, paste of an active substance was applied to a sheet having a three-dimensional net-shaped structure made of nickel. The paste was obtained by kneading a mixture of 90 parts by weight of nickel hydroxide, 10 parts by weight of cobalt oxide, 0.4 parts by weight of carboxyl methyl cellulose, and 43 parts by weight of water. After the paste is dried, it is rolled by a roller press to form a positive electrode plate of a nickel cadmium battery having a thickness of 0.6mm. In a third experiment, paste of an active sub-stance was applied to the three-dimensional net-shaped sheet made of nickel. The paste was obtained by kneading a mixture of 90 parts by weight of cadmium oxide, 10 parts by weight of nickel powder, 2.8 parts by weight of polyethylene powder, and 2.5 parts by weight of polytetrafluoroethylene, and organic solvent. After the paste is dried, it is molded under pressure form a negative electrode plate of a nickel cadmium battery having a thickness of 0.45mm. In the porous metallic sheet to be used as a substrate of a battery electrode according to the first invention, the diameter of metallic powder is as small as 0.1-5µm. Thus, the metallic powder mixed with fused resin binder can be extruded from the spinning nozzle to form the mixture into fibers. Further, because the diameters of the fibers thus formed are as small as 1.0µm-100µm, the porous metallic sheet formed of the fibers has an improved percentage of pores and further, the area of one pore of the porous metallic sheet of the present invention is smaller area than that of one pore of the conventional porous metallic sheet, supposing that the percentage of pores of the former is the same as that of the latter. Thus, the former has a higher conductivity than the latter. Further, because the metallic fibers are continuous or long, the porous metallic sheet made of the fibers is allowed to have a high tension and thus the active substance can be applied to the porous metallic sheet at a high speed. It is possible to use various kinds of metals to form the porous metallic sheet in order to allow it to have a feature of each of the metals used. Further, a mixture of various kinds of metallic powders may be used to form fibers to allow the porous metallic sheet to have various characteristics which cannot be provided by the porous metallic sheet consisting of fibers made of only one kind of metal. The convex formed on the supporter is pyramidal as shown in Figs. 15B and 15C. When the columnar water current 200 is injected under a high pressure to the web (B) placed on the upper surface of the convex 208 formed on the supporter 207, the web (B) on the convex 208 flows downward from the upper end of the convex 208 and forced to drop to the upper surface of the supporter 207 in the periphery of the convex 208 along the peripheral surface of the convex 208, thus forming the annular web (B) in the periphery of the lower end of the convex 208. Thus, through-holes 218 corresponding to the sectional configuration of the lower end of the convex 208 is formed on the sheet (S). When the convexes 208 are conic and formed at regular intervals, as shown in Fig. 15A, the circular through-holes 218 are formed at regular intervals on the sheet (S). When the convexes 208 are pyramidal as shown in Figs. 15B and 15C, the rhombic or square through-hole 218 are formed on the porous metallic sheet 220. Thus, the lath-shaped through-holes 218 or the net-shaped through-holes 218 are formed on the porous metallic sheet 220. The three-dimensional nonwoven porous metallic sheet 220 having the circular through-holes 218, the lath-shaped through-holes 218 or the net-shaped through-holes 218 has not only the through-holes 218 formed therein at regular intervals, but also a great number of fine pores at the portion except for the through-holes 218. As apparent from the foregoing description, the porous metallic sheet to be used as the electrode substrate of a battery in accordance with the present invention has the following effects. (1) The thickness of the porous metallic sheet and the percentage of pores thereof can be controlled easily. That is, in making the thickness of the conventional punched metal small, for example as small as less than 60gm, the material cost such as an iron plate becomes high and workability becomes inferior. On the other hand, the porous metallic sheet in accordance with the present invention is formed of fine metallic fibers, having 1 -100µm in diameter, laminated one on the other. Thus, the thickness of the porous metallic sheet and the percentage of pores can be adjusted to 10µm - 500µm and 20 - 90%, respectively, by changing the amount of metallic fibers and the diameters thereof. In particular, the thickness of the porous metallic sheet can be reduced to less than 60µm although it is difficult for the conventional method to do so. As such, the weight of metal per area can be reduced because a thin porous metallic sheet can be formed and the porous metallic sheet can be allowed to have a high percentage of pores. Thus, according to the present invention, the thinner the porous metallic sheet is, the less expensive the material cost becomes. (2) Further, because the diameters of the metallic fibers composing the porous metallic sheet can be allowed to be small, a thin porous metallic sheet can be produced. Moreover, because the porous metallic sheet is allowed to have a high percentage of pores, a large amount of active substance can be applied thereto. That is, the porous metallic sheet has a high conductivity and increases the capacity of a battery, because a large amount of active substance can be applied thereto. In addition, as an electrode substrate is thin and, it is flexible, the substrate disperses an external force applied thereto when the porous metallic sheet is cut to a plurality of pieces having a predetermined size after the active substance is applied thereto. Thus deformation or burr hardly occurs in the substrate. When the substrate is coiled to be accommo-dated in a spiral type battery, it can be coiled without generating leak or crack. (3) Because a sheet having a porous structure is made of solid metallic fibers, metal surrounding pores is uniformly distributed on the surface thereof and in the interior thereof. According to the conventional method, the surface of organic fibers is plated and then burnt out. Therefore, cavities to which the active substance cannot be applied are present in the metallic fibers. On the other hand, because cavities are not present in the metallic fibers of the present invention, on the porous metallic sheet, there is no portion to which the active substance cannot be applied. Further, because the diameter of the metallic fiber of the present invention is smaller than that of the conventional metallic fiber having a cavity therein, the percentage of pores can be increased and thus the application amount of the active substance can be increased. In addition, supposing that the same amount of metal is used per (m2) both in the porous metallic sheet of the present invention and that the conventional one and the percentage of pores of the former is equal to that of the latter, the number of the metallic fibers to be used in the former is greater than that of the metallic fibers of the latter, while the area of one pore of the porous metallic sheet of the former is smaller than that of one pore of the porous metallic sheet of the latter. Thus, the porous metallic sheet of the present invention increases the conductivity of the substrate, thus improving the performance of the battery. (4) The method of manufacturing the porous metallic sheet to be used as the electrode substrate of the battery according to the present invention eliminates conductivity imparting process and plating process which are required conventionally. Thus the method eliminates the problem of a waste solution which is generated when plating used to manufacture the porous metallic sheet. That is, the manufacturing method is pollution-preventive. In addition, the manufacturing method saves electric power, thus reducing manufacturing cost greatly. (5) Even though electrical plating is used to increase the strength of the porous metallic sheet, the amount of metal to be applied to the porous metallic sheet of the present invention is smaller than that to be applied to the conventional porous metallic sheet. Hence, the method of the present invention reduces the rate of electric power consumption.

Claims: A porous metallic sheet, to be used as an electrode substrate of a battery, having a porous fibrous structure or a three-dimensional net-shaped structure in which a framework surrounding pores of the porous fibrous structure or those of the three-dimensional net-shaped structure is formed of metallic fibers made of metallic powders, wherein the porous fibrous structure or the three-dimensional net-shaped structure has fine pores and a large number of through-holes formed therein. A porous metallic sheet, to be used as an electrode substrate of a battery, having a porous fibrous structure or a three-dimensional net-shaped structure in which a framework surrounding pores of the porous fibrous structure or those of the three-dimensional net-shaped structure is formed of metallic fibers made of metallic powders, wherein the porous metallic sheet made of metallic fibers is overlaid with one or two selected from the group consisting of a porous metallic sheet formed of a plated organic substance or an inorganic substance and a conductive metallic foil having a plurality of holes formed therein. The porous metallic sheet according to claim 1 or 2, wherein the metallic fibers consist of short fibers each having a length in a range from 1mm to 60mm. The porous metallic sheet according to any one of claims 1 through 3, wherein the porous fibrous structure consists of a nonwoven sheet, a woven sheet, a knitted sheet, a felt sheet, a screen-shaped sheet, an expanded sheet, a net-shaped sheet; and the three-dimensional net-shaped structure consists of a foamed sheet, a spongelike sheet, a honeycomb-shaped sheet. The porous metallic sheet according to claim 1, wherein the through-holes are circular, rectangular or rhombic, thus allowing the porous metallic sheet to be a punched-like sheet, a net-shaped sheet or a lath-shaped sheet. The porous metallic sheet according to claim 1 or 5, wherein a belt-shaped portion to be used as a lead of an electrode plate is formed in a region in which the through-holes are not formed. The porous metallic sheet according to any one of claims 1 through 6, wherein surfaces of the metallic fibers are electrically plated. The porous metallic sheet according to claim 1, wherein a plurality of the metallic porous sheets having the same configuration or a plurality of the metallic porous sheets having different configuration is laminated one on the other. An electrode plate of a battery formed by applying an active substance to the porous metallic sheet according to any one of claims 1 through 8. The battery electrode plate according to claim 9, wherein the battery electrode is used as an electrode plate of a nickel hydrogen battery, a nickel cadmium battery, a primary lithium battery or a secondary lithium battery.

Inventors: SUGIKAWA HIROFUMI, SUGIKAWA, HIROFUMI
Applicants: KATAYAMA TOKUSHU KOGYO KK, KATAYAMA SPECIAL INDUSTRIES, LTD.
Application year: 1995
Main classification: H01M4
IPC codes: H01M4, B22F5, B22F3, B01D39, B22F1