JPH0517658B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a sintered cadmium electrode used in alkaline storage batteries.

(B) Prior art Generally, as shown in Japanese Patent Application Laid-open No. 56-82573, the manufacturing method of electrodes used in alkaline storage batteries involves adding a binder to active material powder and kneading it into a paste. A non-sintering manufacturing method in which paste is applied to a conductive substrate and dried, and JP-A-51-
As shown in Japanese Patent No. 18834, a porous nickel sintered substrate is impregnated with a salt solution of an active material such as cadmium nitrate, and then an active material impregnation process of alkali treatment, water washing, and drying is performed several times to obtain the desired amount. There is a sintering method for filling active materials into a substrate.

The former electrode shown in JP-A-56-82573 uses polyvinyl alcohol as a binder and is heat-treated at a temperature of 120 to 220°C to prevent it from dissolving. Since electrodes use a binder to fix the active material, the binder exists between the active material particles, and this binder reduces the conductivity between the active material particles and between the active material and the conductive substrate. However, it cannot be said that sufficient electrode characteristics can be obtained.

In contrast, in the latter type of sintered electrode, the active material is in direct contact with the substrate without using a binder, etc.
There is no decrease in conductivity as in the non-sintered electrodes mentioned above, and furthermore, because the conductive matrix of the porous nickel sintered substrate exists within the electrode, the conductivity within the electrode is high, and the non-sintered It has superior electrode characteristics compared to formula electrodes.

However, in this highly conductive sintered electrode, when a cadmium active material such as cadmium hydroxide or cadmium oxide is used as the active material,
As charge/discharge cycles are repeated, the active material becomes inactive and metal cadmium, which cannot be discharged, accumulates, resulting in a decrease in electrode plate capacity. This is unique to highly conductive sintered electrodes,
A non-sintered electrode with low conductivity reaches the end of its cycle life due to other factors, such as shedding of the active material, before the aforementioned phenomenon of accumulation of undischarged metal cadmium occurs. In addition, the more methylcellulose is added to this type of non-sintered electrode to suppress shedding, the more the conductivity of the electrode decreases.
Electrode characteristics deteriorate.

(c) Problems to be solved by the invention The present invention suppresses the accumulation of metal cadmium that cannot be discharged in a sintered cadmium electrode caused by repeated charge/discharge cycles, and suppresses a decrease in electrode capacity. The present invention aims to provide a sintered cadmium electrode for alkaline storage batteries with excellent cycle characteristics.

(d) Means for solving the problems The cadmium electrode for alkaline storage batteries of the present invention includes:
The gist is that a polysaccharide or a derivative thereof is added to the inside of a nickel sintered substrate filled with a cadmium active material.

(E) Effect As mentioned above, the plate capacity of a sintered cadmium cathode decreases as charge and discharge cycles are repeated due to fine hydroxylation of discharge products on the surface of the metal cadmium, which is the active material during discharge. A film of cadmium particles begins to form, and this film of fine cadmium hydroxide particles prevents contact between the metal cadmium present inside and the electrolyte, leaving undischarged metal cadmium. It is thought that this is because of this. Here, by adding a polysaccharide or a derivative thereof to the inside of a nickel sintered substrate filled with a cadmium active material, and more specifically, by covering the surface of the active material with a polysaccharide or a derivative thereof, This suppresses the number of cadmium hydroxide nuclei that precipitate on metal cadmium during discharge, and prevents the precipitated cadmium hydroxide particles from becoming finer. Therefore, it is possible to suppress the generation and accumulation of metal cadmium that cannot be discharged due to the surface of metal cadmium being covered with many fine cadmium hydroxide particles and contact between metal cadmium and electrolyte being prevented.

(F) Example: A porous sintered nickel substrate is impregnated with an aqueous cadmium nitrate solution, and the active material impregnation process of alkaline treatment, water washing, and drying is repeated six times to fill the substrate with a desired amount of cadmium hydroxide to form an electrode plate. After producing the electrode plate, the electrode plate is subjected to chemical conversion treatment by charging and discharging in an aqueous alkaline solution, washed with water and dried.Then, the electrode plate after the chemical conversion treatment is immersed in a 1% aqueous solution of methylcellulose, and then the electrode plate is formed by brushing. The methylcellulose on the surface is removed to obtain electrode A of the present invention.

For comparison, comparative electrode B was prepared under the same conditions as in the above example except that immersion in the methylcellulose aqueous solution and brushing were not performed, and comparative electrode C was prepared under the same conditions as in the above example except that brushing was not performed.

The cycle characteristics of these electrodes A, B, and C were compared and studied. The results are shown in FIG.
In addition, the cycle conditions are 160% charging at 1/3C current,
A current of 2C discharges to -1.0V against the nickel plate counter electrode. In addition, the capacity is shown with the first cycle being 100%. Based on this result, sintered cadmium electrode A with the addition of methylcellulose,
C has better cycle characteristics than electrode B, which is not added. This is because, as shown in FIG. 2, the amount of metal cadmium that cannot be discharged is generated and accumulated as the number of cycles progresses, and the activity of the electrode is maintained at a high level. Note that the amount of non-dischargeable metal cadmium is shown here, assuming that the amount of non-dischargeable metal cadmium generated when charging and discharging is performed once is 100%.

Next, each of these electrodes A, B, and C is combined with a sintered nickel electrode plate obtained by a chemical impregnation method,
Fabricate SC size batteries a, b, and c and store them at 0°C.
Continuous charging was performed with a current of 0.2C. At this time, the internal pressure of the battery was measured, and the results were obtained as shown in FIG. From this, it is understood that the oxygen gas absorption performance of the battery a of the present invention in which the methylcellulose on the electrode surface was removed by brushing after immersion in the methylcellulose aqueous solution was comparable to that of the battery B to which methylcellulose was not added. On the other hand, in Battery C in which methylcellulose was simply added, the excellent oxygen gas absorption performance, which is a major feature of the sintered cadmium electrode, was not exhibited. The oxygen gas absorption reaction in this type of sintered cadmium electrode can be expressed as follows.

O ₂ +2H ₂ O+4e ^- →4OH ^- ... 4OH ^- +2Cd→2Cd(OH) ₂ +4e ^- ... Among these, the reaction mainly occurs on the electrode surface,
The process proceeds on a nickel sintered substrate that can be contacted with electrolyte and oxygen gas. Therefore, the battery C having the electrode C in which the nickel sintered substrate on the electrode surface is covered by adding methylcellulose has low oxygen gas absorption performance. Therefore, a battery a having an electrode in which methyl cellulose is added inside a nickel sintered substrate filled with a cadmium active material like electrode A of the present invention can maintain high oxygen gas absorption performance. As a result of the above studies, it was discovered that the generation and accumulation of metal cadmium that cannot be discharged is small, and this led to the completion of the present invention.

Although methylcellulose was used as the polysaccharide or its derivative in the examples, it is not limited thereto, and starch, pectin, carboxymethylcellulose, hydroxypropylcellulose, etc. may also be used.

In addition, as an example of adding polysaccharides or derivatives thereof, immersing the electrode plate in an aqueous solution has been disclosed, but it is also possible to spray them on or apply them with a roller. In order to improve oxygen gas absorption performance,
Although the material adhering to the electrode surface was removed by brushing, it may also be removed by washing with water, wiping, or the like.

(G) Effects of the Invention According to the present invention, the oxygen gas absorption performance of the sintered cadmium electrode is maintained at a high level without deterioration, and the generation and accumulation of metallic cadmium that cannot be discharged as the number of charge/discharge cycles progresses. The deactivation of the active material, which is the main cause, can be suppressed, and the synergistic effect of these can improve the cycle characteristics of alkaline storage batteries using such an electrode, and its industrial value is extremely large.

[Brief explanation of drawings]

FIG. 1 is a comparison diagram of the cycle characteristics of the electrodes, FIG. 2 is a diagram showing the relationship between the number of charge/discharge cycles and the ratio of the amount of undischargeable metal cadmium, and FIG. 3 is a diagram of changes in battery internal pressure during continuous charging. A... Electrode of the present invention, B, C... Comparison electrode.

Claims (1)

[Claims] 1. A sintered cadmium electrode for an alkaline storage battery, characterized in that a polysaccharide or a derivative thereof is added to the inside of a nickel sintered substrate filled with a cadmium active material.