JP2989001B2 - Method for producing cadmium negative electrode for alkaline storage battery - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a paste-type cadmium negative electrode used for an alkaline storage battery.

(Prior art) In recent years, in the alkaline storage battery, the paste-type cadmium negative electrode has advantages such as a simpler manufacturing process, a lower cost, and a higher energy density than a sintered-type cadmium negative electrode. More and more are being used.

Conventionally, a method of manufacturing a paste-type cadmium negative electrode generally involves kneading cadmium oxide with a solvent together with a binder in order to improve mechanical strength, and applying the obtained paste to a conductive support. Met. In this method, the more the binder (polyvinyl alcohol) is added to the organic solvent (for example, ethylene glycol), the higher the mechanical strength is. On the other hand, the more polyvinyl alcohol is added, the longer it must be stirred for dispersing it. Must
In addition, since kneading is performed at a high temperature, polyvinyl alcohol is in a state of being dissolved in ethylene glycol, so that wetting of the ethylene glycol to the cadmium oxide powder is prevented, and solidification of the cadmium oxide powder occurs, making it difficult to obtain a stable paste. there were.

(Problems to be Solved by the Invention) In order to solve such a problem, polyvinyl alcohol is heated and dissolved in a solvent in advance, then allowed to cool, gelled, added to cadmium oxide, and kneaded to obtain a paste. Methods for shortening the kneading time have been proposed. But,
The stability of the paste is not yet sufficient.

Further, it has been proposed to add a thickener for improving the stability of the paste. For example, as described in JP-B-57-39024, polyvinyl alcohol and carboxymethylcellulose as a thickener are mixed, heated and dissolved in ethylene glycol, and then allowed to cool to obtain a binder solution. A method of obtaining a paste by kneading in addition to cadmium oxide has also been proposed.In this method, the binder solution is stirred and dissolved at 120 to 135 ° C., so that the reaction of polyvinyl alcohol is promoted, but There is a problem that methylcellulose breaks down the network structure between molecules under the influence of temperature and agitation and loses its effect as a thickener, and it is not yet enough to stably apply it to a conductive support. .

The present invention has been made in view of the above problems, and an object of the present invention is to produce a cadmium negative electrode for an alkaline storage battery that has a small amount of active material and a high mechanical strength with a simple manufacturing process. It is to provide a method that can be performed.

(Means for Solving the Problems) The present invention relates to a binder solution obtained by heating and dissolving polyvinyl alcohol in a solvent while stirring the mixture, and cooling the mixture to form a binder solution. The resulting paste-like active material is obtained by heating and dissolving while stirring at a low speed that does not break the network structure of the molecule, and then kneading the thickener solution, which has been cooled, with the active material powder mainly composed of cadmium oxide. And a method for producing a cadmium negative electrode for an alkaline storage battery, wherein the cadmium negative electrode is dried after being coated on a conductive support.

In the above production method, a preferable embodiment is that the thickener is carboxymethyl cellulose, the degree of saponification of polyvinyl alcohol is 85 mol% or more, and the drying temperature is room temperature to 150.
Temperatures in the range of ° C.

(Operation) In the present invention, since the binder solution and the thickener solution are separately prepared in advance, it is not necessary to sufficiently stir the thickener together with the binder at a high temperature. The properties of the paste can be sufficiently maintained, and the binder can improve the electrode strength. In addition, when the drying temperature at the time of coating is set to be equal to or higher than room temperature and equal to or lower than 150 ° C., cracks on the electrode surface during the reduced rate drying period are reduced, and the electrode strength after coating and drying is improved.

Hereinafter, the present invention will be described in detail with reference to examples.

 The following three solutions were made.

Solution 1 ... Polyvinyl alcohol was added to ethylene glycol, dissolved by heating at 120 ° C., and then cooled.

Solution 2 ... Carboxymethylcellulose was added to ethylene glycol, heated and dissolved at 120 ° C., and then cooled.

Solution 3 ... Polyvinyl alcohol and carboxymethyl cellulose were added to ethylene glycol, heated and dissolved at 120 ° C, and then cooled.

However, the solution 2 was stirred at a low speed so as not to break the network structure between the molecules of carboxymethyl cellulose.

These solutions are then used alone or in combination to form cadmium oxide, carbonyl nickel and vinyl chloride-
A paste was prepared in addition to the active material powder composed of acrylic copolymer short fibers. A paste prepared by adding both Solution 1 and Solution 2 to paste A (Example of the present invention), a paste formed by adding only Solution 1 to paste B (Comparative Example), and a paste formed by adding Solution 3 to paste C (Comparative Example), and paste D obtained by adding polyvinyl alcohol in a liquid state to a paste. The following table shows the stirring time until these pastes have the optimum viscosity for coating.

In paste D, a powder was formed as it was from the beginning, and it took a long time to dissolve.

FIG. 1 shows the results of measuring the change in viscosity while leaving these pastes. From this result, it can be seen that the viscosity change of paste B is remarkable. Also, the change of the paste C is larger than that of the paste A of the present invention. This is considered to be due to the fact that the paste C is sufficiently stirred to completely dissolve the polyvinyl alcohol at the time of producing the solution 3, so that the network structure between the molecules of carboxymethyl cellulose is broken.

Next, after the paste A according to the present invention was applied to a conductive support, the paste A was dried at a different drying temperature, and the falling rate at the time of forming the group was measured. The result is shown in FIG. From this result, it can be seen that as the drying temperature increases, the active material falls off more. This is because the higher the temperature, the more the active material cracks during the rate-reducing drying period after the surface layer of the applied paste is dried, and unlike the case of polyvinyl alcohol alone, the effect of carboxymethyl cellulose is affected by the temperature. Therefore, it is considered that the mechanical strength decreases. From the above, it is understood that the drying temperature is suitably from room temperature or higher to 150 ° C. or lower.

Next, pastes of the present invention were prepared using polyvinyl alcohols having different degrees of saponification, and the viscosity change upon standing was measured. Paste E with polyvinyl alcohol having a saponification degree of 71 to 75 mol%, paste F with 78.5 to 81.5 mol%, paste G with 86.5 to 89 mol%, paste 9
This is called paste H by 8 to 99 mol%. The result is shown in FIG. From this result, it can be seen that the viscosity of the pastes E and F sharply increases, and further, the pastes E and F are completely hardened. In addition, these two pastes do not become a gelled state even when polyvinyl alcohol is heated and allowed to cool during the production of the adhesive solution. Since pastes G and H are stable, it can be seen that a saponification degree of 85 mol or more is appropriate.

Further, in the above example, carboxymethylcellulose was used as the thickener, but similar results were obtained when methylcellulose or sodium polyacrylate was used instead.

(Effects of the Invention) As described above, according to the present invention, the mechanical strength, which is a drawback of the conventional paste-type cadmium negative electrode, can be greatly improved, and the defective rate in group formation can be reduced.
In addition, simplification of the kneading step and stabilization of the paste can be achieved, and as a result, there are effects such as an improvement in productivity and a decrease in the defective rate in the coating step.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the time of paste standing and the change in viscosity in Examples and Comparative Examples of the present invention, and FIG. 2 is a drying temperature when the paste according to the present invention is applied to a conductive support and then dried. FIG. 3 is a graph showing the relationship between the standing time and the change in viscosity when pastes were prepared using polyvinyl alcohols having different degrees of saponification in the present invention.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Seiji Ishizuka 3-4-1-10 Minamishinagawa, Shinagawa-ku, Tokyo Toshiba Battery Corporation (58) Field surveyed (Int. Cl. ⁶ , DB name) H01M 4 / 26 H01M 4/62

Claims (3)

(57) [Claims] 1. A binder solution obtained by heating and dissolving polyvinyl alcohol in a solvent while stirring, and allowing the polyvinyl alcohol to cool, and a gelling agent in the solvent and a thickening agent which does not break the network structure of the thickening agent molecules. The mixture is heated and dissolved while stirring at a low speed, and the thickener solution, which is left to cool, is kneaded with an active material powder mainly composed of cadmium oxide, and the obtained paste active material is applied to a conductive support. And then drying the cadmium negative electrode for an alkaline storage battery. 2. The saponification degree of polyvinyl alcohol is 85 mol.
%. The method for producing a cadmium negative electrode for an alkaline storage battery according to claim 1, wherein 3. The method for producing a cadmium negative electrode for an alkaline storage battery according to claim 1, wherein the drying temperature is from room temperature to 150 ° C.