JPH0963572A - Nonsintered cadmium negative electrode for alkaline storage battery and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonsintered cadmium negative electrode plate for an alkaline storage battery with high active material utilization factor, high oxygen gas absorbing performance, and high charging acceptance performance at low temperature by devising a binder with high binding capability and high decomposition resistance to an alkaline electrolyte. SOLUTION: A nonsintered cadmium negative electrode for an alkaline storage battery has a powdery cadmium active material and ethylene vinyl acetate copolymer for binding the active material. In the nonsintered cadmium electrode for the alkaline storage battery, the ethylene vinyl acetate copolymer in which the ratio of ethylene to vinyl acetate is 15-25wt.% is used as a binder.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-sintered cadmium negative electrode for alkaline storage batteries.

[0002]

2. Description of the Related Art A non-sintered cadmium negative electrode plate for an alkaline storage battery is prepared by kneading an active material powder with a binder to form a paste, applying the paste to a conductive core such as punching metal, and then drying. It is made by. Therefore, in the non-sintered cadmium negative electrode plate manufactured by such a manufacturing method, the binder plays an important role.

By the way, conventionally, as a binder for a non-sintered negative electrode plate, a hydrophobic polymer such as polyethylene or polytetrafluoroethylene, a hydrophilic polymer such as methyl cellulose or polyvinyl alcohol, carboxymethyl cellulose or hydroxyethyl cellulose is used. Alkali-soluble hydrophilic polymers such as sodium salts are used. Each of these polymers has advantages and disadvantages. For example, hydrophobic polymers such as polytetrafluoroethylene are
Although it has high chemical resistance (chemical stability), it has the drawback of weak binding strength.

A hydrophilic polymer such as polyvinyl alcohol which does not dissolve in an alkaline electrolyte has a strong binding force, but is inferior in chemical resistance to a hydrophobic polymer. Therefore, it is decomposed by an alkaline electrolytic solution, and the binding force gradually decreases, and there are drawbacks such as generation of carbonates and a decrease in the electrolytic solution concentration, and a film is formed on the surface of the active material to bring the active material into contact with the electrolytic solution. There is also a drawback that it inhibits.

On the other hand, an alkali-soluble hydrophilic polymer such as sodium salt of hydroxyethyl cellulose does not interfere with the permeation of the electrolytic solution by forming a film on the surface of the substance because the binder is eluted into the electrolytic solution. However, there are drawbacks such that the binding force is lost in a relatively short period of time, and like the above, it is easily decomposed into an alkaline electrolyte, and carbonate is generated by the decomposition to lower the concentration of the electrolyte.

From the above, Japanese Patent Laid-Open No. 61-240
Japanese Patent No. 579 proposes a method in which a water-soluble organic polymer dissolved in an alkaline electrolyte and an aqueous latex of a thermoplastic organic synthetic resin are used in combination. According to this method,
As a result of the two kinds of binders complementing each other, the mechanical strength of the electrode plate is increased, and the permeation inhibition of the electrolytic solution and the generation of carbonate radicals can be suppressed, and the active material utilization rate, oxygen gas absorption performance and low temperature It is said that it is possible to obtain a non-sintered negative electrode plate having excellent charge acceptance performance.

[0007]

However, merely using the water-based latex and the water-soluble organic polymer in combination cannot sufficiently suppress the deterioration of the battery performance due to the decomposition of these binders. Therefore, the present invention has devised a binder which is excellent in binding property and is not easily decomposed in an alkaline electrolyte, and therefore has an active material utilization rate, oxygen gas absorption performance and charge acceptance performance at low temperature for alkaline storage batteries. It is an object to provide a non-sintered cadmium negative electrode and a method for producing such a negative electrode.

[0008]

In order to achieve the above object, the present invention has the following arrangement. The invention according to claim 1 provides a non-sintered cadmium negative electrode for an alkaline storage battery, which comprises a powdery cadmium active material and an ethylene / vinyl acetate copolymer binding the active material.
The vinyl acetate copolymer is characterized in that the ratio of ethylene to vinyl acetate is regulated to 15 to 25% by weight.

According to a second aspect of the present invention, in the production of a non-sintered cadmium negative electrode for an alkaline storage battery, the ratio of ethylene to vinyl acetate is 15 to 2 as the active material binder.
It is characterized by using an emulsion of an ethylene / vinyl acetate copolymer defined at 5% by weight.

[0010]

[Embodiment]

[Production of electrode plate] An aqueous emulsion of five kinds of ethylene / vinyl acetate copolymers having different ratios of ethylene to vinyl acetate (hereinafter referred to as E / A ratio) was used as a binder to prepare an ethylene / vinyl acetate copolymer. Five types of non-sintered cadmium negative electrode plates having different E / A ratios were produced. The production of the negative electrode plate is as follows.

85% by weight of cadmium oxide powder and 15% by weight of metallic cadmium powder are mixed to obtain a cadmium active material powder. Based on this active material powder, a water-based ethylene / ethylene acetate / vinyl acetate copolymer containing 5% by weight of solid content. 2% by weight of polymer emulsion and an appropriate amount of water are added and kneaded to form an active material paste. Next, this paste was applied to punching metal (thickness 100 μm, opening ratio 45%) and dried to obtain a non-sintered cadmium negative electrode plate.

In the above, the ethylene / vinyl acetate copolymer emulsion has an E / A ratio of 10% by weight,
Those of 15% by weight, 20% by weight, 25% by weight and 30% by weight were used. Negative electrode plates produced using these emulsions are referred to as negative electrode plates A, B, C, D, and E, respectively, from the one having a smaller E / A ratio.

[Preparation of Storage Battery] The above negative electrode plate and a known sintered nickel positive electrode plate having a smaller capacity than this negative electrode plate were wound around a nylon nonwoven fabric and inserted into a battery can without an electrode body. Then, a 6N potassium hydroxide aqueous solution was poured into this battery can as an electrolyte, and the opening of the battery can was closed to form a sealed cylindrical nickel-cadmium storage battery (JIS, KR-SC size) with a nominal capacity of 1300 mAh. It was made.

[Measurement / Evaluation of Binding Strength and Battery Performance]
Using the negative electrode plate and the storage battery produced above, the influence of the difference in the E / A ratio of the ethylene / vinyl acetate copolymer (binder) on the binding strength and the performance of the storage battery was investigated. The contents of the test and the results thereof will be described below.

(Binding Strength) Each of the negative electrode plates was wound with a nylon nonwoven fabric, and the presence or absence of the active material particles falling off and the occurrence of cracks in the negative electrode plate were visually observed during the winding. As a result, it was confirmed that the active material particles did not fall off or crack in any of the negative electrode plates A to E. From this, when an emulsion of ethylene / vinyl acetate copolymer having an E / A ratio of 10 to 30% by weight is used as a binder, the active material particles form a conductive core with sufficient bonding strength. It was confirmed that the binder was held and at least at the time of assembling the battery, no trouble caused by insufficient bond strength occurred.

(Measurement of Carbonate Content) For each of the above storage batteries,
After charging for 16 hours at 130 mAh and discharging at 1300 mAh until the battery voltage reaches 0.8 V, after repeating 100 times at room temperature (18 to 27 ° C.),
The electrolytic solution was taken out from each storage battery, and the amount of carbonate radical in the electrolytic solution was measured by a titration method using hydrochloric acid. The results are shown in FIG. 1 as a relationship between the E / A ratio of the ethylene / vinyl acetate copolymer and the amount of carbonate. In FIG. 1, the amount of carbonate radicals (vertical axis) of each storage battery is the storage battery E with an E / A ratio of 30% by weight.
The amount of carbonate radicals in Table 1 is used as a reference (100) and is represented by an index value.

From FIG. 1, the amount of carbonate in the electrolytic solution is E / A.
It was recognized that the ratio decreased as the ratio increased. Further, the amount of carbonate radicals in the electrolytic solution remarkably increases when the E / A ratio is less than 15% by weight, but it is from 30% by weight to 30% by weight.
No significant variation was observed between the weight percentages. From this, it is understood that the E / A ratio of the ethylene / vinyl acetate copolymer should be regulated to 15% by weight or more in order to suppress the generation of carbonate radicals.

The results shown in FIG. 1 are considered to mean that the chemical stability (against alkalinity) of the ethylene / vinyl acetate copolymer increases as the E / A ratio increases.

(Measurement of Gas Pressure in Battery) Each storage battery was charged at 1300 mAh for 2 hours (room temperature) and the gas pressure in the battery was measured. The results are shown in the relationship between the E / A ratio of ethylene / vinyl acetate copolymer and the gas pressure in the battery.
Shown in The gas pressure in the battery (vertical axis) in FIG. 2 is represented by an index value with the battery gas pressure in the storage battery E (E / A ratio; 30% by weight) as the reference (100), as in FIG.

In FIG. 2, contrary to the result of the above-mentioned amount of carbonates, it was recognized that the gas pressure in the battery tends to remarkably increase when the E / A ratio exceeds 20% by weight. However, the gas pressure in the battery when the E / A ratio was 25% by weight was considerably smaller than when the E / A ratio was 30% by weight.
From this, in order to suppress gas generation during battery charging to a low level, it is more preferable that the E / A ratio of the ethylene / vinyl acetate copolymer is 20% by weight or less. But 2
Since there is no great difference in the gas pressure in the battery between 0% by weight or less and 25% by weight, it can be understood that the gas generation amount can be sufficiently suppressed by setting the E / A ratio to 25% by weight or less.

The ethylene-vinyl acetate copolymer E
When the A / A ratio exceeds 20% by weight, the reason why the gas pressure inside the battery increases is that the binding property of the ethylene / vinyl acetate copolymer increases as the E / A ratio increases and the active material particles are wrapped. It is considered that since the active material and the electrolytic solution are not smoothly contacted with each other because of strong binding, the oxygen gas consumption reaction of the negative electrode is hindered.

Combining the results of FIG. 1 and FIG. 2 above, in order to suppress the generation of carbonate radicals and the generation of gas,
It can be concluded that the E / A ratio of the ethylene / vinyl acetate copolymer should be regulated to 15% by weight to 25% by weight.

[0023]

As is apparent from the above, the ethylene / vinyl acetate copolymer having the E / A ratio of 15% by weight to 25% by weight is excellent in binding strength and alkali resistance, and has active material particles. The degree to which contact with the wrapping electrolyte is hindered is small. Therefore, the electrochemical reaction of the active material is not hindered, and the active material can be bound and held to the conductive core for a long period of time. Therefore, the cadmium negative electrode for alkaline storage batteries of the present invention using such an ethylene / vinyl acetate copolymer as a binder is excellent in active material utilization rate, oxygen gas absorption performance and charge acceptance performance at low temperature. .

On the other hand, since the emulsion containing the ethylene / vinyl acetate copolymer as a main component as described above is a low-viscosity liquid, a good activity in which the emulsion (binder) and the cadmium active material powder are uniformly mixed is obtained. Easy to prepare material slurry. Further, the active material slurry can be easily applied to the conductive core, and the active material powder can be strongly bound to the core by being easily solidified by the normal temperature electrode plate drying operation performed after the application.

Therefore, when such an ethylene / vinyl acetate copolymer emulsion is used as a binder, compared with the case where a thermoplastic synthetic resin of a type that is heated at a high temperature to solidify and bind is used, Uniform binding strength is obtained over the entire electrode plate. Further, since the decomposition of the binder can be suppressed, it is possible to prevent the deterioration of the battery performance due to the decomposition of the binder. Therefore, according to the method for producing a cadmium negative electrode for an alkaline storage battery of the present invention, it is possible to provide an effect of providing a cadmium negative electrode for an alkaline storage battery at a low cost with less deterioration in battery performance due to the binder.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the E / A ratio of an ethylene / vinyl acetate copolymer and the amount of carbonate radical formation.

FIG. 2 is a graph showing the relationship between the E / A ratio of an ethylene / vinyl acetate copolymer and the gas pressure in the battery.

Claims (2)

[Claims] 1. A non-sintered cadmium negative electrode for an alkaline storage battery, which comprises a powdery cadmium active material and an ethylene / vinyl acetate copolymer binding the active material, wherein the ethylene / vinyl acetate copolymer is A non-sintered cadmium negative electrode for an alkaline storage battery, wherein the ratio of ethylene to vinyl acetate is 15 to 25% by weight. 2. The ratio of ethylene to vinyl acetate is 1.
A method for producing a non-sintered cadmium negative electrode for an alkaline storage battery, which comprises using an emulsion of an ethylene / vinyl acetate copolymer in an amount of 5 to 25% by weight as an active material binder.