JPH0119619B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a cadmium negative electrode, and aims to improve battery characteristics such as improving the utilization rate of negative electrode active material, improving cycle life, and improving gas absorption ability during charging. There are three main methods for producing cadmium negative electrodes: The first is the sintering method, in which nickel powder is sintered in a high-temperature reducing atmosphere to create a porous substrate, and the pores of this substrate are filled with an active material, which has excellent high-rate discharge characteristics. However, it has the disadvantage that it is expensive because it requires the production of a nickel sintered body and the active material filling process is complicated. The second type is the press type, which is an electrode made by press-molding active material powder together with conductive powder, etc., and has the advantage of being inexpensive to manufacture, but has poor high rate discharge characteristics and cycle life characteristics. There is a drawback. The third method is called a paste type, in which a paste is created by kneading powder mainly composed of active materials with a binder solution, and the paste is applied to a porous core metal, which allows high rate discharge. It has the advantage that it has almost the same characteristics as the sintered type and can be manufactured at a lower cost than the sintered type. In this paste type, the general paste composition is cadmium oxide, a conductive material such as nickel powder or carbon powder, a reinforcing agent consisting of short synthetic resin fibers such as polyvinyl chloride resin fiber, and a binder such as polyethylene or polyvinyl alcohol. It consists of a binder solution made by dissolving a binder in a solvent. Among these compositions, the short synthetic resin fibers as a reinforcing agent exist between the active material layers coated on both sides of the core metal through the pores of the porous core metal, so it is particularly difficult for the active material to peel off from the core metal. In addition to preventing this, it is also effective in binding the active materials to each other. However, when increasing its quantity in order to further enhance its effect as a reinforcing agent, the disadvantage is that the electrical resistance of the electrode increases due to its lack of electrical conductivity, and the utilization rate of the active material is poor. Therefore, if thin metal fibers such as nickel fibers or iron fibers are used as an alternative, the active material utilization rate will improve, but metal fibers are difficult to plastically deform even when electrodes are pressed, so they protrude into needle-like shapes on the electrode surface, causing the battery There is a drawback that when configured as shown in FIG. The present invention relates to an improvement of the reinforcing agent in the conventional paste-type electrode manufacturing method, and solves the above problems by using cadmium-plated short synthetic resin fibers. explain. A mixed powder was prepared by adding 10 parts by weight of carbonyl nickel powder as a conductive material to 100 parts by weight of cadmium oxide, and then 0.5 parts by weight of polyvinyl chloride resin short fibers with a cadmium-plated surface and 5 deniers and a fiber length of 3 mm. In addition, 30c.c. of 1 part by weight of polyvinyl alcohol was dissolved in the mixture and further mixed.
Add ethylene glycol solution and knead the whole to make a paste. This paste is applied to both sides of a core made of nickel-plated perforated steel plate with a thickness of 0.8 mm, then dried to dissipate the ethylene glycol, and then pressed to form an unformed material with a porosity of approximately 40%. Use it as a pole plate. Next, this unformed electrode plate is reduced to 50% of its theoretical capacity in a 20% caustic soda aqueous solution, then washed with water, dried, and pressed to form a completed electrode plate. This completed electrode plate is cut into a predetermined size to form a unit electrode plate. Using this unit electrode plate, combine it with a sintered positive electrode and a separator made of polyamide resin nonwoven fabric,
A sealed nickel-cadmium storage battery with a capacity of 500mAh was fabricated. High rate discharge characteristics, cycle life characteristics, and gas absorption capacity characteristics of this battery A and battery B that uses a cadmium negative electrode in which conventional short synthetic resin fibers without cadmium plating are mixed into the paste as a reinforcing agent. I looked into it. In addition, battery A,
The only difference in B was the reinforcing agent of the negative electrode, and the other conditions were completely the same. FIG. 1 shows high rate discharge characteristics, and shows the discharge capacity when a fully charged battery is discharged at various discharge rates.
As is clear from the figure, it can be seen that the product A of the present invention is superior when the discharge is 5C (2.5A) or higher. Figure 2 shows the cycle life characteristics, charging at 1/2C (250mA) for 3 hours and discharging at 1C at room temperature.
(500mA). Product A of the present invention shows no capacity deterioration even after 1000 cycles or more, whereas conventional product B tends to deteriorate from around 1000 cycles. Figure 3 shows the gas absorption capacity characteristics. The sealed nickel-cadmium storage battery in this example is kept sealed by absorbing (oxidizing) oxygen gas generated from the positive electrode during overcharging with the cadmium in the negative electrode. This leads to time savings. Therefore, we investigated the absorption capacity especially at low temperatures where gas absorption reactions are slow. Figure 3 is 0
This is the value of the internal pressure of the battery when charging at various charging rates at °C. It can be seen that product A of the present invention has lower internal battery pressure and improved gas absorption ability than conventional product B. These improvements in battery characteristics are achieved by reducing the cadmium oxide near the cadmium-plated portion during chemical conversion treatment, and the cadmium-plated fibers communicate with the active materials on both sides of the core through the holes in the core. This is due to the formation of a good conductive network. In particular, the amount of metallic cadmium on the electrode surface has increased significantly compared to conventional electrodes, and this is thought to contribute to improvements in discharge characteristics and gas absorption. As described above, the product of the present invention has improved battery characteristics compared to the conventional product, but when the utilization rate of the cadmium negative electrode was investigated, the results were as follows. That is, the active material utilization rate (%) at each discharge rate was measured using negative electrodes A' and B' similar to those used in batteries A and B above in a caustic potassium aqueous solution with a concentration of 30% and a nickel plate as the other electrode. I got the results as shown in the table below.

[Table] As shown in the above table, the negative electrode of the present invention A' has an improved active material utilization rate, and thus the various battery characteristics described above are improved. Methods for plating synthetic resin short fibers include electroless plating and electrolytic plating, but the method used in this example first performs electroless nickel plating on the fiber surface, and then It is electrolytically cadmium plated. This plating resulted in cadmium plating to a thickness of approximately 0.5μ. It is conceivable to simply use nickel plating without this cadmium plating, but
Compared to cadmium plating, nickel plating alone is insufficient because the hydrogen overvoltage is small and the charging efficiency deteriorates when charging at a high charging rate at low temperatures. Therefore, the surface layer of the fibers is preferably plated with cadmium, and there are no restrictions on the method of plating. In the above example, nickel powder was added as a conductive material to the paste composition, but it was confirmed that the inclusion of cadmium-plated short fibers was more effective than the quantitative effect of this nickel powder. It was done. After conducting various studies on the amount of short fibers coated with cadmium, we found that cadmium oxide
Effects are seen at 0.1 parts by weight or more per 100 parts by weight.
A good upper limit is 2 parts by weight; if the amount is larger than this, the amount of fibers will be too large and dispersion in the paste will be poor. Further, if the amount is less than 0.1 part by weight, the amount of cadmium plated is too small to obtain the desired effect. Therefore, 0.1 to 2 parts by weight per 100 parts by weight of cadmium oxide is suitable. In addition, although the fiber diameter and length were studied, it was found that the effect of the above-mentioned weight ratio was also effective. Further, in the examples, the chemical conversion treatment is described as partial reduction of cadmium oxide, but the chemical conversion treatment may be reduction, reduction plus oxidation, such as 100% reduction or partial oxidation after 100% reduction.
There are no restrictions on this chemical conversion treatment method. As described above, the present invention improves battery performance by changing the reinforcing agent in the negative electrode base, and is of great value in batteries using cadmium negative electrodes, especially batteries using sealed cadmium negative electrodes.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the discharge rate characteristics of a sealed storage battery using a cadmium negative electrode in an example of the present invention;
FIG. 2 shows the cycle life, and FIG. 3 shows the gas absorption capacity characteristics. A...Product of the present invention, B...Conventional product.

Claims (1)

[Claims] 1. Cadmium-plated short synthetic resin fibers are added and dispersed in a paste-like material made by adding a binder to cadmium oxide, and the paste is applied to a porous core, and then A method for producing a cadmium negative electrode characterized by chemical conversion treatment. 2. The method for producing a cadmium negative electrode according to claim 1, wherein the cadmium-plated short synthetic resin fibers are added to the paste in an amount of 0.1 to 2 parts by weight per 100 parts by weight of cadmium oxide.