JPH063730B2 - Paste type cadmium negative electrode - Google Patents

Description

TECHNICAL FIELD The present invention relates to a paste type cadmium negative electrode for alkaline storage batteries.

Configuration of conventional example and its problems Paste type cadmium negative electrode for alkaline storage battery is generally mainly cadmium oxide or cadmium hydroxide,
Conductive powders such as carbonyl nickel and graphite, binders such as polyvinyl alcohol and carboxymethyl cellulose, and solvents such as water and ethylene glycol are added and kneaded to form a paste. It is manufactured by applying it to a conductive core material, drying it, and then performing chemical conversion in an alkaline solution.

The purpose of the chemical conversion step is to convert a part or all of the cadmium compound in a discharged state such as cadmium oxide and cadmium hydroxide used as the active material into metal cadmium in a charged state to provide a precharged portion in the negative electrode. Especially.
When there is no pre-charged part in the negative electrode, the utilization factor of the negative electrode is lower than that of the positive electrode, so discharge becomes dominant and the high rate discharge characteristic of the battery deteriorates. Deterioration becomes remarkable. For this reason, formation is carried out. In this conversion process, the negative electrode capacity of 2
Since 0 to 10% of charging is performed, a large amount of power is required. Further, in the chemical conversion step, it is not easy to uniformly distribute the charged metal cadmium in the electrode, and variations in characteristics are likely to occur. This tendency is remarkable in the paste type electrode.

In order to solve such a problem, Japanese Patent Publication No. 57-379
89, Japanese Patent Application Laid-Open No. 57-5265, a method has been proposed in which an active metal cadmium powder is added at the time of mixing an active material in an amount corresponding to a precharge amount, thereby eliminating the need for a chemical conversion step. However, in the electrode having such a configuration,
Metal cadmium powder intervenes between cadmium oxide powder or cadmium hydroxide powder with low conductivity, and electrical contact between metal cadmium is small, so it contributes to charge and discharge more than metal cadmium in matrix form generated by chemical conversion. The proportion is small. In other words, the utilization rate of the added metal cadmium is low. Further, metal cadmium generated during charging is concentrated near the conductive core material and is unlikely to grow to the surface of the electrode plate. In a battery having a sealed structure such as a sealed nickel-cadmium storage battery, the oxygen gas generated from the positive electrode during overcharge is absorbed by the negative electrode metal cadmium.
In order to improve the absorption of oxygen, it is desirable that cadmium metal be present on the surface of the electrode plate. However, the above-mentioned type of electrode in which the metal cadmium powder is added does not make such a desirable result.

Further, the metal cadmium added as a precharge amount needs to be electrochemically active. However, when active cadmium metal is used, there is a problem that, due to its activity, it reacts with oxygen, water vapor, etc. in the air to form an oxide film and inactivates. To solve such a problem, it is proposed to use a spherical metal cadmium having a relatively low activity and a large particle size, which is formed by intentionally forming and stabilizing an oxide film (Japanese Patent Publication No. 57-37986). ,
In order to prevent inactivation, there is a method of using metal cadmium with a phosphate (Japanese Patent Laid-Open No. 57-5265) and the like, but these methods also leave a problem in terms of a decrease in the activity of metal cadmium. There was a place.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and to effectively use metal cadmium powder added as a precharge amount to provide a paste-type cadmium negative electrode that does not require a chemical conversion step.

Composition of the invention The cadmium negative electrode of the present invention is a surface layer of an electrode plate in which an active mixture mainly composed of cadmium oxide powder and metal cadmium powder is held on a conductive core material, by means of electroplating or chemical plating, nickel. It is provided with a plating layer.

The principle of the present invention will be described below.

As described above, the metal cadmium powder added as a portion corresponding to the amount of preliminary charge for the purpose of eliminating the chemical conversion step is scattered between cadmium oxide and cadmium hydroxide having low conductivity, and mutual electrical contact occurs. Since the amount is small, the contribution to charge / discharge is small and the utilization rate is low. Further, unlike the metal cadmium obtained in the chemical conversion process, since a conductive matrix is not formed, the metal cadmium generated by charging is concentrated near the core material of the electrode plate and can efficiently absorb the oxygen gas generated from the positive electrode. It is not so distributed on the surface of the electrode plate. However, when a nickel plating layer that has good adhesion to the active material is provided on the surface layer of the negative electrode by electroplating or chemical plating, the metal cadmium generated during charging is the core material that holds the active material and the nickel plating on the surface of the electrode plate. A matrix is formed through metal cadmium powder that grows from both layers and is interspersed within the active material layer. In this way, the utilization rate of the metal cadmium powder that has increased electrical contact is improved, and since a large amount of metal cadmium is distributed in the electrode plate surface layer, the oxygen gas absorption capacity is improved during overcharge.

Further, it is preferable to use metal cadmium having a small particle size and a large surface area and high activity, but such metal cadmium is inactivated during the operation as described above. However, if the electrode plate is formed using metal cadmium having a high activity in the initial stage and then electroplating or chemical plating is performed, the surface of the metal cadmium can be changed by the negative polarization during electroplating or the reducing agent in the chemical plating bath. The inactive oxide film formed on the surface is reduced, and the metal cadmium returns to its original highly active state. As described above, the reactivated metal cadmium is present in the cadmium oxide, so that it is not easily deactivated in the subsequent steps such as washing with water and drying, and is incorporated into the battery in an active state. be able to.

Heretofore, it has been proposed in JP-A-53-86442 to apply a metal powder to the surface of a paste type negative electrode having a chemical conversion step for the purpose of reinforcing the strength of the electrode plate. In the present invention,
It improves the efficiency of metal cadmium as a precharge added for the purpose of eliminating the chemical conversion step, and the metal coating layer does not need to be as strong as in the case of electrode plate reinforcement.

Description of Examples 70 parts by weight of cadmium oxide powder having an average particle size of about 1 μm,
30 parts by weight of metal cadmium powder is mixed, and an ethylene glycol solution of polyvinyl alcohol is added to this and kneaded to form a paste. This paste is 0.1mm thick
Was applied to the nickel-plated perforated iron plate and dried at about 140 ° C. for 30 minutes to obtain an electrode plate having a thickness of about 0.5 mm. This electrode plate was adjusted to pH 3-5 and liquid temperature 50 ° C.
A negative electrode was electrolyzed at a current density of 100 mA / cm ² in a mol / water solution, and the surface layer of the electrode plate was plated with nickel.

After the plating was completed, the electrode plate was washed with water, dried and cut into a predetermined size.

The metal cadmium powder used is an amorphous fine particle produced by an electrochemical treatment and has a large specific surface area. Moreover, the average particle diameter and the specific surface area were changed by changing the processing method. The amount of electroplating was adjusted by changing the energization time.

The above negative electrode is combined with a sintered nickel positive electrode to obtain 120
A sealed battery corresponding to 0 mAh was prototyped and tested for battery characteristics. The tests were a discharge rate characteristic test for evaluating the utilization rate of the metal cadmium powder and a battery internal pressure test for evaluating the oxygen gas absorption capacity during overcharge. The discharge rate characteristics were evaluated by the ratio of the discharge capacity when discharged at a current equivalent to 5c and the discharge capacity when discharged at equivalent to 0.2c at 20 ° C. The internal pressure was evaluated by the peak value of the internal pressure of the battery when charged at 20 ° C. with a current equivalent to 1 c.

FIG. 1 shows the relationship between the particle size of metal cadmium powder and the discharge capacity ratio. In the figure, a represents a battery using the above negative electrode, and b represents a battery using the same negative electrode as that described above except that nickel plating is not performed.

As described above, in a, the utilization rate of metal cadmium as the precharge amount is improved, so that the characteristics are significantly improved as compared with b. Regarding the metal cadmium powder used, the smaller the particle size, the more active, the higher the utilization rate and the better the discharge rate characteristics, but if it is too small, it will be buried in the space formed between the main active materials, cadmium oxide. Therefore, it is not incorporated in the conductive matrix formed by nickel plating, and conversely the utilization rate decreases. Therefore, the appropriate particle size is 0.5 to 2.5.
It is about μm.

FIG. 2 shows the relationship between the specific surface area of metal cadmium and the discharge capacity ratio. The specific surface area was determined by the BET method.
As the specific surface area increases, the activity of metallic cadmium increases and the discharge rate characteristic improves, but this tendency becomes saturated at some point. When the electrode plate containing metal cadmium is plated, the metal cadmium surface is naturally covered with the plating layer.
The phenomenon that the above discharge rate becomes saturated at a certain point is that as the surface area of metal cadmium increases, the proportion of metal cadmium covered with the plating layer increases, and the metal cadmium in this part can contribute to the charge / discharge reaction. It is thought that this is due to the disappearance. In addition, cadmium metal having a large specific surface area has too high an activity, and the risk of ignition during work increases. From the above, an appropriate specific surface area is about 0.5 to ⁵ cm ² / g.

FIG. 3 shows the relationship between the nickel plating amount on the negative electrode plate and the peak of the battery internal pressure. As described above, when a nickel plating layer is provided on the surface of the electrode plate, a large amount of metal cadmium generated during battery charging becomes distributed in the surface layer of the electrode plate, which improves the oxygen gas absorption capacity and reduces the internal pressure of the battery. descend. This tendency becomes remarkable when the plating amount is about 2 mg / cm ₂ per unit area of the electrode plate. However, if the amount of plating is too large, the surface layer of the electrode plate will be completely covered, and the movement of oxygen gas and the electrolytic solution will be hindered. From the above, the appropriate range of the plating amount is about ² to 20 mg / cm ² .

Although electroplating is performed in the above example, the same effect can be obtained by chemical plating.

Effects of the Invention As described above, according to the present invention, it is possible to obtain a high-performance paste-type cadmium negative electrode which does not require a chemical conversion step and has an excellent oxygen gas absorbing ability without decreasing the activity of the metal cadmium powder. it can.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the particle size of metal cadmium added to the negative electrode and the discharge capacity ratio of the battery, and FIG. 2 is a diagram showing the relationship between the specific surface area of the metal cadmium powder and the discharge capacity ratio of the battery. FIG. 3 is a diagram showing the relationship between the nickel plating amount on the negative electrode and the peak of the battery internal pressure.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Motohide Masui 1006 Kadoma, Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Minoru Yamaga 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (3)

[Claims] 1. A paste-type cadmium negative electrode in which a continuous nickel plating layer is formed on an electrode plate surface layer in which a conductive core material holds an active material mixture mainly containing cadmium oxide powder and metal cadmium powder. 2. The average particle size of the metal cadmium powder is 0.
The paste type cadmium negative electrode according to claim 1, which has a specific surface area of 5 to 2.5 μm and a specific surface area of 0.5 to ⁵ m ² / g. 3. The nickel plating layer has a weight of 2 to 20 mg.
The paste-type cadmium negative electrode according to claim 1, wherein the negative electrode is Cs / cm ² .