Electrode for alkaline storage battery and method of manufacturing the same

Abstract: An electrode for an alkaline storage battery which can suppress reduction of the battery capacity in employment under high-temperature environment is provided. The electrode for an alkaline storage battery is formed by charging a collector consisting of a porous body of a nickel-boron alloy containing 0.001 to 3 percent by weight of boron with an active material. The porous body of the nickel-boron alloy can be formed by depositing nickel on a surface of a substrate consisting of a porous polymer by electroless plating with a boron compound serving as a reductant.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an electrode for an alkaline storage battery and a method of manufacturing the same, and more particularly, it relates to an electrode which is useful for providing an alkaline storage battery having a long life and a method of manufacturing the same. Description of the Background Art In recent years, high-capacity alkaline storage batteries such as a nickel-cadmium battery and a nickelhydrogen battery are employed as power sources for various electronic devices, particularly portable devices. In such a high-capacity alkaline storage battery, a cathode is generally prepared by charging a foam metal of nickel with an active material such as nickel hydroxide, for example, in order to improve the capacity. A foam metal is generally prepared by a technique disclosed in JP 53-128544, for example. According to this method, a porous body which is provided with conductivity by coating a porous insulator with carbon or the like is brought into close contact with a rotating feeder roll and dipped in a plating path, to be plated. The plated porous body is thereafter employed as a cathode and electroplating is performed from both surfaces, thereby forming a porous body consisting of a metal such as nickel, for example. When an electrode which is formed by charging a collector consisting of nickel with an active material is applied to an alkaline storage battery, however, the capacity of the battery is reduced if charge/discharge cycles are repeated under a high temperature, although the cause therefor is not clear. SUMMARY OF THE INVENTION An object of the present invention is to provide an electrode for an alkaline storage battery, which can suppress reduction of the battery capacity also when the same is used under environment of a relatively high temperature. The electrode for an alkaline storage battery according to the present invention comprises a collector consisting of a porous body having three dimensional continuous pores in its interior of a nickel-boron alloy containing 0.001 to 3 percent by weight of boron, and an active material which is charged in the collector. The collector consisting essentially of a porous body can be prepared in any mode so far as the same has three-dimensional continuous pores in its interior, such as a spongy or felty body, for example. The porous body serving as the collector can be formed by depositing a conductive layer on a substrate consisting of a porous polymer by electroless plating. While nickel is deposited in the electroless plating, a boron compound is employed as a reductant at this time. Therefore, the method of manufacturing an electrode for an alkaline storage battery according to the present invention comprises the steps of claim 6. The porous polymer for forming the substrate can be prepared from urethane foam, or polymer nonwoven fabric consisting of polyethylene or polypropylene, for example. The boron compound which is employed as the reductant for the electroless plating is prepared from boron hydride (borane), dimethylaminoborane or the like. The substrate is dipped in a plating solution containing nickel and the boron compound reductant, to be subjected to electroless plating. After the electroless plating, electronickeling may be carried out in order to adjust the target weight and boron concentration. The porous body consisting essentially of a nickel-boron alloy which is formed on the substrate is heat treated under a reducing atmosphere of hydrogen or the like, to obtain a nickel-boron alloy porous body for serving as a collector. The porous body for serving as a collector is charged with an active material, in order to form an electrode. The active material is prepared from a mixture which is mainly composed of nickel hydroxide, for example. The mixture may contain other components such as 3 to 15 percent by weight of cobalt, 1 to 5 percent by weight of cobalt hydroxide, 1 to 5 percent by weight of zinc oxide and the like, for example. In addition, a binder which is prepared by adding polyvinyl alcohol or carboxymethyl cellulose to water may also be employed. The electrode according to the present invention is applied to an alkaline storage battery such as a nickelcadmium battery or a nickel-hydrogen battery. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates performances of alkaline storage batteries employing electrodes according to the present invention and prior art, with the axis of ordinates showing standardized battery capacities in charge/discharge cycles shown on the axis of abscissas; and Fig. 2 illustrates performances of seven types of alkaline storage batteries comprising electrodes employing seven types of nickel-boron alloy porous bodies having different boron concentrations respectively. DESCRIPTION OF THE PREFERRED EMBODIMENTS An alkaline storage battery which is formed by an electrode according to the present invention is less deteriorated in capacity as compared with that employing a nickel porous body as an electrode particularly with respect to charge/discharge cycles under a high temperature as understood from Examples described later, although the cause for this is not clear. According to the present invention, a nickel-boron alloy has a boron concentration of 0.001 to 3 percent by weight, more preferably 0.01 to 1 percent by weight. In this range, the inventive electrode can further suppress deterioration of the capacity of the battery in charge/discharge cycles under a high temperature as compared with a conventional electrode. If the boron concentration is out of the aforementioned range, the effect of suppressing deterioration of the capacity is not much remarkable. In the range of 0.01 to 1 percent by weight of the boron concentration, on the other hand, the effect of suppressing deterioration of the capacity is more remarkable. Further, a nickel alloy containing 0.001 to 3 percent by weight of boron has low electric resistance, and is excellent in collectability. Examples of the present invention are now described, while it is noted that the present invention is not restricted to these Examples. Example 1 A urethane foam body of 1.4 mm in thickness, 200 mm in width and 300 mm in length having 50 continuous pores per inch was pretreated with a colloidal catalyst prepared from palladium chloride and tin chloride, and subjected to electroless plating with a nickeling solution containing boron hydride as a reductant. Then, the foam body having a nickel-boron alloy deposited on its surface was subjected to electronickeling, and thereafter heat treated under a hydrogen jet at 1000°C, thereby obtaining a spongy nickel-boron alloy porous body of 580 g/m 2 in density. This porous body contained 0.03 percent by weight of boron. The porous body obtained in this manner was charged with an active material consisting of 88 percent by weight of nickel hydroxide, 7 percent by weight of metallic cobalt, 2 percent by weight of cobalt hydroxide and 3 percent by weight of zinc oxide, to obtain a cathode for an alkaline storage battery. Comparative Example 1 A urethane foam body identical to that employed in Example 1 was coated with carbon powder and thereafter brought into contact with a feeder roll to be subjected to electronickeling, as described in JP 53-128544. Then, the urethane foam body depositing nickel was heat treated under a hydrogen jet at 1000°C, thereby obtaining a spongy pure nickel porous body of 580 g/m 2 in density. The porous body obtained in this manner was charged with the same active material as that for Example 1, to obtain a cathode. Other spongy pure nickel porous bodies prepared along comparative example 1 were charged with an active material which was mainly composed of a hydrogen absorbing alloy consisting of MmNi 3.8 Mn 0.4 Al 0.3 Co 0.5 (Mm: misch metal), to prepare anodes. These anodes were combined with the cathodes obtained in Example 1 and comparative example 1 respectively, to prepare nickel-hydrogen batteries of 22.5 mm in diameter and 49.2 mm in height with separators consisting of sulfonated polypropylene nonwoven fabric and an electrolyte which was aqueous potassium hydroxide having specific gravity of 1.2. Charge/discharge cycle tests at 1C rate were carried out at 45°C on the two types of batteries employing the respective cathodes, to examine capacity changes. Fig. 1 shows the results. Referring to Fig. 1, the capacities are standardized with reference to initial capacities of 1.0. It is clearly understood from Fig. 1 that the battery comprising the electrode employing the nickel-boron alloy porous body is less deteriorated in capacity. Example 2 A nickel-boron alloy porous body was prepared absolutely in a similar manner to Example 1, except that the amount of deposition in electroless plating was changed. The plating was so carried out that the porous body exhibited a boron concentration of 0.001 percent by weight. Thereafter a cathode was prepared similarly to Example 1. Example 3 A nickel-boron alloy porous body was prepared in a similar manner to Example 1, except the amount of deposition in electroless plating. The plating was so carried out that the porous body exhibited a boron concentration of 0.01 percent by weight. Thereafter a cathode was prepared similarly to Example 1. Example 4 A nickel-boron alloy porous body was prepared in a similar manner to Example 1, except the amount of deposition in electroless plating. The plating was so carried out that the porous body exhibited a boron concentration of 1 percent by weight. Thereafter a cathode was prepared similarly to Example 1. Example 5 A nickel-boron alloy porous body was prepared in a similar manner to Example 1, except the amount of deposition in electroless plating. The plating was so carried out that the porous body exhibited a boron concentration of 3 percent by weight. The porous body was employed to prepare a cathode similarly to Example 1. Comparative Example 2 Plating was carried out in a similar manner to Example 1 to attain a boron concentration of 0.0001 %, thereby forming a nickel-boron alloy porous body. A cathode was prepared from this porous body in a similar manner to Example 1. Comparative Example 3 Plating was carried out in a similar manner to Example 1 to attain a boron concentration of 5 %, thereby forming a nickel-boron alloy porous body. A cathode was prepared from this porous body in a similar manner to Example 1. The cathodes of Examples 1 to 5 employing the nickel-boron alloy porous bodies having boron concentrations of 0.03, 0.001, 0.01, 1 and 3 percent by weight respectively and those of comparative examples 2 and 3 employing the nickel-boron alloy porous bodies having boron concentrations of 0.0001 and 5 percent by weight respectively were employed to prepare nickel-hydrogen batteries, similarly to the above. Charge/discharge cycle tests at 1C rate were carried out at 45°C as to the seven types batteries obtained in the aforementioned manner, to examine capacity changes. Fig. 2 shows battery capacities after 600 charge/discharge cycles, which were standardized with reference to initial capacities of 1.0. Referring to Fig. 2, the batteries employing the nickel-boron alloy porous bodies having boron concentrations of 0.001 to 3 percent by weight exhibit standardized capacities exceeding that of comparative example 1. In the boron concentration range of 0.1 to 1 percent by weight, capacity deterioration is further remarkably suppressed as compared with comparative example 1. Further, polyester nonwoven fabric members of 20 µm in fiber diameter and 1.5 mm in thickness were employed in place of the urethane foam bodies to prepare nickelhydrogen batteries similarly to Examples 1 to 5 and comparative example 1. These batteries were evaluated similarly to the above, to obtain results which were similar to those shown in Figs. 1 and 2. Example 6 A nickel-boron alloy porous body was prepared in a similar manner to Example 1, except that dimethylamino borane was used as the reductant in place of boron hydride. The plating was so carried out that the porous body exhibited a boron concentration of 0.03 percent by weight. A cathode was prepared from the obtained porous body in a similar manner to Example 1 and then a nickelhydrogen battery was prepared similarly to the above. One charge/discharge cycle (1C) at 45°C was repeated 600 times to examine the capacity change of the obtained battery. After the 600 cycles, the final capacity of the battery was about 96% of the initial capacity which was similar to Example 1. As hereinabove described, the inventive electrode for an alkaline storage battery employing a porous body of a nickel-boron alloy can suppress capacity deterioration of the battery particularly in charge/discharge cycles in employment under a high temperature and increase the life of the battery. Further, an electrode for an alkaline storage battery having the aforementioned performance can readily be manufactured by carrying out electroless nickeling with a boron compound serving as a reductant in accordance with the present invention. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation.

Claims: An electrode for an alkaline storage battery, comprising: a collector consisting of a porous body having three-dimensional continuous pores in its interior of a nickel-boron alloy containing 0.001 to 3 percent by weight of boron; and an active material being charged in said collector. The electrode for an alkaline storage battery in accordance with claim 1, wherein said nickel-boron alloy contains 0.01 to 1 percent by weight of boron. The electrode for an alkaline storage battery in accordance with claim 1 or 2, wherein said active material is mainly composed of nickel hydroxide. Use of an electrode for an alkaline storage battery in accordance with any of claims 1 to 3, being employed as a cathode of a nickel-hydrogen battery. Use of an electrode for an alkaline storage battery in accordance with any of claims 1 to 3, being employed as a cathode of a nickel-cadmium battery. A method of manufacturing an electrode for an alkaline storage battery, comprising the steps of: a) depositing nickel on a surface of a substrate consisting of a porous polymer by electroless plating employing a boron compound as a reductant; b) optionally performing electronickeling after said electroless plating; c) subjecting such plated porous body to a heat treatment under reducing atmosphere, thereby forming a porous body of a nickel-boron alloy containing 0.001 to 3 percent by weight of boron on said substrate; d) charging said porous body of said nickel-boron alloy with an active material. The method of manufacturing an electrode for an alkaline storage battery in accordance with claim 6, wherein said boron compound includes boron hydride. The method of manufacturing an electrode for an alkaline storage battery in accordance with claim 6, wherein said boron compound includes dimethylaminoborane. The method of manufacturing an electrode for an alkaline storage battery in accordance with claim 6 or 7, wherein said porous polymer is polymer nonwoven fabric. The method of manufacturing an electrode for an alkaline storage battery in accordance with claim 6 or 7, wherein said porous polymer is a urethane foam. The method of manufacturing an electrode for an alkaline storage battery in accordance with any of claims 6 to 10, wherein said step of forming said porous body further comprises a step of performing electronickeling after said electroless plating.

Inventors: KARIYA AYAO, NISHI TETSUYA, SAKATOMO TAKESHI, TERAO TORU, UEMIYA TAKAFUMI, KARIYA, AYAO, NISHI, TETSUYA, SAKAMOTO, TAKESHI, TERAO, TORU, UEMIYA, TAKAFUMI, KARIYA, AYAO, C/O ITAMI WORKS, NISHI, TETSUYA, C/O ITAMI WORKS, SAKATOMO, TAKESHI, C/O OSAKA WORKS, TERAO, TORU, C/O ITAMI WORKS, UEMIYA, TAKAFUMI, C/O OSAKA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1995
Main classification: H01M4
IPC codes: H01M4, C23C18, H01M10