JPH069136B2 - Alkaline zinc storage battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to an alkaline zinc storage battery using zinc oxide or metallic zinc as a cathode active material, such as a nickel-zinc storage battery or a silver-zinc storage battery. .

<Prior Art> Zinc used as a cathode active material of a zinc electrode in an alkaline zinc storage battery as described above has advantages such as high energy density per unit weight and low cost, but zinc is not The zincate ion Zn (OH) ₄ ²⁻ produced by elution in the alkaline electrolyte grows as a dendritic or spongy crystal (hereinafter referred to as “dendrite”) on the surface of the zinc electrode during charging and grows. When the discharge is repeated, the electrodeposited zinc penetrates the separator and comes into contact with the counter electrode to cause an internal short circuit, which disadvantageously shortens the cycle life of the battery.

In order to eliminate the above-mentioned inconvenience caused by electrodeposited zinc and improve the cycle life of the zinc electrode and the battery, for example, as disclosed in JP-B-48-27099, the cathode active material is hydrated. A method of adding a hydroxide of an alkaline earth metal such as magnesium or barium hydroxide is known. By adding such a hydroxide, zincate ions eluted in the electrolytic solution during discharge immediately react with these hydroxides and are fixed in the cathode, so the amount of free zincate ions in the electrolytic solution is increased. As a result, the growth of zinc dendrites during charging can be prevented.

However, in such a method, in order to effectively prevent the growth of zinc dendrites, a large amount of alkaline earth metal hydroxide must be added to the cathode active material. Has the drawback of inviting.

Therefore, as disclosed in, for example, Japanese Patent Publication No. 54-9696, a metal such as cadmium, lead, indium, tin, etc. having a redox potential higher than that of zinc or an oxide of these metals is oxidized. A method of adding calcium to a cathode active material has been proposed. According to this method
The utilization of the zinc electrode is improved by the addition of the above metals or metal oxides, and the growth of the dendrite-like electrodeposited zinc is suppressed by the action of calcium hydroxide, so that the battery cycle life is improved without causing a decrease in capacity. The capacity of the zinc electrode can be maintained over a long cycle.

<Problems to be Solved by the Invention> However, according to the method of adding a metal or the like and calcium hydroxide as described above, although the battery cycle life is improved without causing a capacity decrease, a cycle of high rate discharge In the above, there is a drawback that a sufficient cycle life cannot be obtained because the active material peels off due to the coarsening of the cathode active material particles and the densification of the zinc electrode surface.

<Means for Solving Problems> The alkaline zinc storage battery of the present invention comprises a cathode active material containing at least one of zinc oxide and metallic zinc as a main component,
The gist of the present invention is to add strontium hydroxide and at least one additive selected from oxides or hydroxides of metals having a redox potential higher than that of zinc.

As a metal having a redox potential more noble than that of zinc as described above,
Metals such as cadmium, lead, indium, and tin are listed, and similar effects can be obtained by adding them as oxides or hydroxides.

The amount of strontium hydroxide added is based on the total amount of the cathode active material and all additives (at least one additive selected from oxides or hydroxides of the above metals, strontium hydroxide). A range of 1 to 10% by weight is preferable, and if it is less than 1% by weight, the desired effect is small.
If it exceeds 5% by weight, the amount uninvolved in the reaction increases and the capacity decreases.

<Operation> Strontium hydroxide is a substance that does not participate in the battery reaction of the alkaline zinc storage battery and at the same time is slightly soluble in water. It is considered that this strontium hydroxide suppresses the growth of zinc dendrites and exerts a capacity maintaining effect in a cycle of high rate discharge for the following reason. That is, it is considered that the portion where the cathode active material particles in the zinc electrode are likely to be coarsened in the cycle of high rate discharge is a portion where current concentration occurs, that is, a portion where the discharge reaction rate is high. The discharge reaction at the zinc electrode is represented by a reaction formula of Zn + 2OH ⁻ − → Zn (OH) ₂ + 2e. Therefore, at the portion where the discharge reaction is concentrated, the supply of hydroxide ions cannot keep up, and the pH of the electrolyte is locally reduced. Will drop. In such a place where the pH is lowered, the parallelism represented by the following equation is tilted to the right and a trace amount of strontium hydroxide is dissolved.

Sr (OH) ₂ Sr ²⁺ + 2OH ^{− When} the discharge progresses and the discharge products increase, the concentration of the current is relieved, so that the pH returns to the original level, and at the same time, the above parallel is left. The strontium ions that have been dissolved in the electrolyte due to the precipitation are deposited as strontium hydroxide and are taken into the active material.

By such a mechanism, it is considered that strontium hydroxide, which is a non-reaction-related substance, is taken in at the place where the current of the zinc electrode is concentrated, and the action of this strontium hydroxide suppresses the coarsening of the cathode active material particles. .
In addition, strontium hydroxide does not easily react with zinc and solidify like calcium hydroxide to solidify the surface of the zinc electrode.

In addition, at least one additive selected from oxides or hydroxides of metals having a redox potential higher than that of zinc improves the utilization rate of the zinc electrode and maintains the capacity of the zinc electrode over a long cycle. Work.

<Examples> As shown in Table 1, a mixture of metallic zinc as a cathode active material and AE obtained by adding an additive such as indium hydroxide to zinc oxide, and polytetrafluoroethylene as a binder, respectively. In addition, after adding water and kneading, it was rolled into a sheet by a roller. These sheets are 5 cm long and 4 cm wide
Pressed on both sides of the current collector plate, and then dried to prepare five types of zinc electrodes A to E.

Each of the 5 types of zinc electrodes A to E made as described above is 4
Three types of known nickel electrodes of the same size as these are used in combination with a separator made of a microporous film and a nylon nonwoven fabric to provide five types of nickel-zinc storage batteries (nominal capacity of about 2
Ah) AE was constructed.

For these five types of nickel-zinc storage batteries A to E, 50
A series of charge and discharge cycles of charging at 0 mA for 5 hours and discharging at 4 A until the battery voltage reached 1.3 V was repeated until the battery capacity dropped to 1.5 Ah, and the number of cycles at that time was taken as the cycle life. The results are shown in Table 2. The number of cycles shown in Table 2 is the remaining 8 cycle life excluding the one with the lowest cycle number when 10 nickel-zinc storage batteries A to E were made and each was subjected to a charge / discharge cycle test. It is an average value.

From the above table, it can be seen that the cycle life of the zinc electrode containing strontium hydroxide is remarkably excellent, and that strontium hydroxide contributes to the improvement of cycle characteristics in the cycle of high rate discharge.

Thus, it is presumed that the fact that only strontium hydroxide contributes to the improvement of the characteristics of the high rate discharge cycle is closely related to the solubility product. Therefore, magnesium hydroxide, calcium hydroxide, strontium hydroxide used in the experiment,
The solubility product (Ksp) of barium hydroxide is shown in Table 3.

Table 3 Solubility Product (Ksp) Magnesium hydroxide 1.8 × 10 ^-11 Calcium hydroxide 5.5 × 10 ^-6 Strontium hydroxide 9 × 10 ^-4 Barium hydroxide (Easily soluble) These four hydroxides Among them, magnesium hydroxide has a very large solubility product, so that as a particle not involved in the reaction, the particle shape at the time of electrode preparation is kept as it is, and magnesium hydroxide does not elute into the electrolyte solution to supplement the hydroxide ion. No, it does not contribute to the improvement of cycle characteristics in high rate discharge. In addition, the solubility product of calcium hydroxide is not so large that it dissolves in the electrolytic solution and acts to replenish the hydroxide ions, but it reacts with zincate ions which are discharge products, and the produced calcium zincate is Since the zinc electrode surface is densified due to solidification, peeling of the cathode active material from the current collector easily occurs, and conversely the cycle characteristics deteriorate. Furthermore, since barium hydroxide is easily soluble, it has the effect of being dissolved in the electrolytic solution and replenishing hydroxide ions, but the detailed reason is unknown, but there is a difference in improving cycle characteristics in high-rate discharge. Does not contribute. For the above reasons, it is considered that only strontium hydroxide contributes to the improvement of cycle characteristics in high rate discharge.

Next, in order to investigate the preferable addition amount of strontium hydroxide, the addition amount of strontium hydroxide to the total amount of the cathode active material composed of zinc oxide and metallic zinc, indium hydroxide, and strontium hydroxide was set to 0.
5, 1, 5, 10, 15, and 20% by weight of zinc electrodes were combined with known nickel electrodes to form nickel-zinc storage batteries having a nominal capacity of about 2 Ah, and these batteries were subjected to the same conditions as above. Then, a series of charge / discharge cycle tests were conducted to determine the cycle life. The results are shown in the attached drawings, and it can be judged that the effective addition amount of strontium hydroxide is in the range of 1 to 10% by weight.
If it is less than 10%, the effect of improving the cycle life is small, and if it exceeds 10% by weight, the capacity of the zinc electrode is significantly decreased due to an increase in the amount of strontium hydroxide not involved in the reaction.

<Effects of the Invention> As described above, according to the alkaline zinc storage battery of the present invention, coarsening of the cathode active material particles in the cycle of high rate discharge is effectively prevented, and therefore even for high rate discharge. The effect that a sufficient cycle life can be obtained is exhibited.

[Brief description of drawings]

The drawing is a graph showing the relationship between the amount of strontium hydroxide added and the cycle life of the battery.

Claims (2)

[Claims] 1. A cathode active material containing zinc oxide or zinc metal as a main component, and at least one additive selected from oxides or hydroxides of metals having a redox potential higher than that of zinc. And an strontium hydroxide are added, the alkaline zinc storage battery characterized by the above-mentioned. 2. The alkaline zinc storage battery according to claim 1, wherein the addition amount of strontium hydroxide is 1 to 10% by weight with respect to the total amount of the cathode active material, the additive and strontium hydroxide. .