JPH0217910B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a cadmium negative electrode, and aims to improve battery performance such as high rate discharge characteristics, cycle life characteristics, and gas absorption characteristics. There are three types of manufacturing methods for cadmium negative electrodes: sintering method, press method, and paste method. The sintered type is made by sintering nickel powder and filling the pores of a porous sintered substrate with active material, which has excellent high-rate discharge characteristics, but is expensive due to the complicated manufacturing process. It has the following drawback. The press type mixes the active material powder with a conductive agent and then press-forms it into a predetermined shape, and has the advantage of being inexpensive to manufacture, but has the disadvantage of poor high rate discharge characteristics and cycle life characteristics. The paste type has attracted attention in recent years as it has high rate discharge characteristics close to that of the sintered type and can be produced at low cost.The paste type is made by adding a binder solution to a mixed powder material consisting mainly of active material powder and kneading it to form a paste. This paste is applied to both sides of the porous metal core. The conventional composition of this paste is to mix cadmium oxide, nickel powder as a conductive agent, and short synthetic resin fibers as a reinforcing agent, and add a binder solution to this mixture. . The short fibers as a reinforcing agent are connected to both sides of the core metal through the holes in the core metal, and are present between the active material powders, so they can prevent the active material from peeling off from the core metal, and can be used mechanically as an electrode. It is effective in increasing strength. On the other hand, efforts have been made in recent years to improve the active material utilization rate of this paste type electrode. Conventionally, the active material utilization rate of paste-type cadmium negative electrodes is around 60 to 70%.
In order to improve this utilization rate, it has already been proposed to plate the unformed electrode plate with cadmium after applying the paste. Although an improvement in the utilization rate due to this cadmium plating was certainly observed, no improvement in cycle life characteristics or gas absorption characteristics was confirmed, and issues remained. The present invention relates to an improvement of the conventional method of manufacturing paste-type electrodes, and is a composition improvement in a method of plating cadmium on an unformed electrode plate after applying a paste and drying it.
This is intended to improve cycle life characteristics and gas absorption characteristics. This will be explained below using examples. 100 parts by weight of cadmium oxide as carbon powder,
For example, add 2 parts by weight of graphite powder to make a mixed powder,
To this is added 0.5 part by weight of synthetic resin short fibers made of a copolymer of vinyl chloride resin and acrylic acid resin. Next, add 1 portion of polyvinyl alcohol as a binder.
Add 30 c.c. of dissolved ethylene glycol solution and knead to make a paste. This paste was applied to both sides of a nickel-plated perforated steel plate with a thickness of 0.8 mm, then dried to dissipate the ethylene glycol, and the porosity was adjusted to 40-65% by pressure to form an unformed Use it as a pole plate. Next, this unformed electrode plate is electrolytically reduced in a cadmium sulfate solution using cadmium as a counter electrode to electrodeposit metal cadmium. The amount of electrodeposited cadmium is 20 to 55% of the amount of electricity of cadmium oxide in the unformed electrode plate. After this electrolytic reduction, it is washed with water, dried, and further pressed if necessary to obtain a completed electrode plate. This completed electrode plate is cut into a predetermined size to form a unit electrode plate A. For comparison, two types of conventional electrodes were fabricated.
Conventional product B has a different conductive agent from the above example,
Nickel powder was used as a conductive agent, and the rest was the same as in the example. Conventional product c uses carbon powder as a conductive agent and electrolytically reduces a portion of cadmium oxide to cadmium in an unformed electrode plate in an alkaline electrolyte. The active material utilization rates of these two conventional products and product A of the present invention were compared and measured. In this case, the total amount of cadmium in the electrodes A, B, and C was the same, and the amount of metallic cadmium among the total cadmium at the time of the completed electrode plate was also adjusted to the same amount for measurement. The utilization rate was calculated from the discharge capacity discharged at each discharge rate after 100% charging in a caustic potassium aqueous solution with a concentration of 30%. The results are shown in the table below.

[Table] As is clear from the above table, invention product A has an active material utilization rate comparable to that of conventional product B. Next, using the unit electrode plates described above, a sealed nickel-cadmium storage battery with a nominal capacity of 500 mAh was manufactured by combining a sintered positive electrode and a separator made of a nonwoven fabric made of polyamide resin. Both batteries used the same negative electrode as in the measurement of the active material utilization rate, and the present invention was of type A, and the conventional products were of two types, B and C. Using this battery, high rate discharge characteristics, cycle life characteristics,
Tests were conducted on gas absorption properties. FIG. 1 shows high rate discharge characteristics, and product A of the present invention is equivalent to or slightly superior to conventional product B, and clearly different from product C. Figure 2 shows the cycle life characteristics. The life test conditions were charging at 1/2C (250mA) for 3 hours at room temperature, and discharging at 1C (500mA) to 1.0V. As a result, inventive product A shows almost no capacity decrease until 1500 cycles, whereas conventional products B and C tend to gradually decrease in capacity, clearly inventive product A.
It can be seen that the cycle life characteristics are good. Third
The figure shows a comparison of gas absorption performance. In this sealed nickel-cadmium storage battery, oxygen gas is generated from the positive electrode during overcharging, and the sealing is achieved by absorbing this oxygen gas with the cadmium of the negative electrode.The greater the gas absorption capacity, the more
It is possible to increase the charging rate and enable rapid charging. Therefore, we investigated the properties at low temperatures, where gas absorption performance is generally severe. In other words, charging is at 0℃
The battery was charged at various charging rates of 0.25, 0.5, 0.75, and 1C, and the pressure inside the battery was measured at each charging rate. As is clear from FIG. 3, product A of the present invention has a lower pressure value than conventional products B and C, indicating that it has a greater gas absorption capacity. As described above, the product of the present invention is superior to conventional products in high rate discharge characteristics, cycle life characteristics, and gas absorption performance, and the following may be the reason for this. In other words, the product of the present invention uses carbon powder as a conductive agent in the negative electrode paste, and cadmium is electrodeposited on the unformed electrode plate after the paste has been applied and dried. Moreover, it is also deposited on the surface of short synthetic resin fibers added as reinforcing agents. That is, carbon powder is scattered on the surface of the short fibers, and cadmium is precipitated using the carbon powder as nuclei, so that a relatively large amount of electrodeposited cadmium is present on the fiber surface. This is clearly different from the case where nickel powder is used as a conductive agent as in conventional product B. In the case of nickel powder, the presence of nickel powder is hardly seen on the surface of the short fibers, Therefore, no precipitation of cadmium on the fiber surface is observed. Next, the composition ratio of carbon powder and short synthetic resin fibers was investigated. As a result, a paste was prepared based on the composition of 100 parts by weight of cadmium oxide, 0.3 to 6 parts by weight of carbon powder, and 0.1 to 2 parts by weight of short synthetic resin fibers. When cadmium was electrodeposited, the effects of the present invention as described above were obtained. It has been found that it is effective if the amount of cadmium electrodeposited is at least 20% of the amount of cadmium oxide. Furthermore, if the amount of carbon powder is less than 0.3 parts by weight, less adhesion to the surface of the short fibers results in very little cadmium being precipitated and the effect is poor. On the other hand, if the amount is more than 6 parts by weight, the packing density of cadmium oxide decreases, resulting in an electrode with a small amount of active material, which is not preferable. Furthermore, if the amount of short fibers is less than 0.1 part by weight, the amount of fibers will be small, and the absolute amount of electrodeposited cadmium on the fibers will decrease, reducing the effect of the present invention. On the other hand, if the amount of short fibers exceeds 2 parts by weight, the entanglement between the short fibers becomes strong and it becomes difficult to disperse them uniformly in the paste, making it difficult to form a uniform electrode. As mentioned above, the effect of the present invention is that a relatively large amount of cadmium is electrodeposited on the surface of the carbon powder adhered to the short fibers which are blended in a preferable amount and uniformly dispersed in the paste. A conductive network of fibers is formed across the front and back surfaces of the electrode through the holes, and the presence of electrodeposited cadmium on these fibers improves battery performance such as high rate discharge characteristics, cycle life characteristics, and gas absorption performance. An improved negative electrode can be obtained.

[Brief explanation of drawings]

FIG. 1 shows the discharge rate characteristics of the cadmium negative electrode according to the present invention, FIG. 2 shows the cycle life characteristics, and FIG. 3 shows the gas absorption characteristics. A is a product of the present invention, B and C are conventional products.

Claims (1)

[Claims] 1 Carbon powder for 100 parts by weight of cadmium oxide
A binder solution is added to a mixed powder containing 0.3 to 6 parts by weight and 0.1 to 2 parts by weight of short synthetic resin fibers to form a paste, and the paste is applied to both sides of the porous core and dried. , a method for producing a cadmium negative electrode characterized by electrodepositing cadmium by electrolysis.