JPH0136673B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to the improvement of air-zinc batteries, particularly small air-zinc batteries that have been attracting attention for use in hearing aids in recent years.
The purpose is to improve zinc batteries by regulating the electrolyte composition within a certain range.
The object of the present invention is to provide a battery that has excellent overdischarge leakage resistance, discharge characteristics, and short activation time. Traditionally, small air-zinc batteries have been required to have a large discharge capacity per volume, leakage resistance, especially less leakage during overdischarge, excellent storage stability, and a high open circuit voltage when the battery is activated. It has been done. However, until now, electrical conductivity, which is one of the basic requirements for electrolytes, has been emphasized, and a potassium hydroxide (KOH) concentration of approximately 30% by weight, which has the highest electrical conductivity, has been used. It's here. As a result of intensive research into the composition of this electrolyte, the inventors discovered that air is present in areas unrelated to conductivity.
We were able to find an electrolyte composition that exhibits excellent properties for zinc batteries. In other words, in the ternary composition diagram of Figure 3, which shows potassium hydroxide (KOH), zinc oxide (ZnO), and water (H ₂ O) in weight percent, the line connecting points A, B, C, and D By using the electrolyte composition within the enclosed area, it is possible to provide an air-zinc battery that satisfies the above requirements. Hereinafter, the present invention will be explained in detail with reference to Examples. Various solutions consisting of three components, KOH, ZnO, and H ₂ O, were prepared as electrolytes. The specific adjustment method is
Create an aqueous solution of KOH and H ₂ O with a certain concentration,
After adjusting the specific gravity to various concentrations, ZnO was dissolved and readjusted to each composition. Table 1 shows the concentration (weight %) of each component of the prepared electrolytic solution, and FIG. 1 shows the overall condition of each electrolytic solution.

【table】

[Table] As is clear from Figure 1, ZnO is difficult to dissolve in areas where KOH is low in concentration. Therefore, the insoluble portion of zinc oxide is removed. An air-zinc battery with a size of R44 (diameter 11.6 mm, height 5.4 mm) was created using these electrolytes of each composition. The configuration of the tested battery is explained in Figure 2. The positive electrode 1 contains 50 parts by weight of activated carbon,
Catalyst layer 2 consisting of 80 parts by weight of manganese dioxide, 20 parts by weight of acetylene black and 25 parts by weight of polytetrafluoroethylene (PTFE) heat-treated at 550°C.
and a nickel net 3, the thickness of which is 0.45 mm. The negative electrode 4 is mainly made of zinc chloride powder, and is composed of a weight with a theoretical zinc reaction capacity of 440 mAh, 120 μ of the various electrolytes described above, and a small amount of thickener. The separator 5 is composed of a porous polypropylene membrane 6 and a liquid-containing material 7. In addition, 8 is a porous PTFE membrane, 9 is a positive electrode case, and 10 is a porous membrane of PTFE.
1 is a negative electrode sealing plate, 11 is an insulating gasket, 12 is a supporting paper, 13 is an air hole for oxygen intake provided in the positive electrode case, and 14 is peelable to close the air hole 13 during storage and open the hole when activated. It is a sealing paper made of a material that is difficult for gas and moisture to pass through. The discharge performance of 51 types of batteries with various electrolyte compositions shown in Table 1 was investigated using a load of 180Ω, continuous discharge in an environment of 20°C, and the capacitance obtained up to a final voltage of 0.9V. . Also, over discharge leakage
Continuous discharge is performed with a load of 180Ω, and 30
After a few days, the presence or absence of leakage was checked by checking for discoloration of the cresol red. Furthermore, in order to determine the activation time of the battery, the sealing paper was removed and the open circuit voltage was examined after 1 minute. The respective results are shown in Table 2. In addition, in order to clarify the concentration range of each component of the electrolyte, we prepared a partially enlarged view of the three-component composition diagram in Figure 1, and inserted the characteristic values of each battery investigated into this to show the good characteristics. A ternary composition diagram incorporating the regions was created as Figure 3.

【table】

【table】

[Table] As is clear from Table 2 and Figure 3, discharge performance is controlled by the concentration of KOH, especially the concentration of hydroxyl group ions. In Figure 3, the area where a discharge capacity performance of approximately 90% or more of the theoretical zinc capacity, that is, 400mAh or more is obtained, is indicated by dots. In a concentration range where the KOH concentration is approximately 33% by weight or more, a discharge capacity of 400mAh or more can be secured. On the other hand, over-discharge leakage is caused by KOH
Leakage is noticeable in the concentration range of 42.5% by weight or higher. Although these relationships are not clear, the following mechanism is presumed. In other words, it is estimated that _the following two discharge reactions of zinc occur in this electrolyte composition region: Zn+2OH→Zn(OH) ₂ +2e ...(1) Zn+4OH→ZnO ^2- 2 +2H ₂ O+2e ...(2) be done. In particular, when the KOH concentration is low, the reaction (1) proceeds predominantly, and when the concentration is high, the reaction (2) predominately proceeds. That is, in a region where the KOH concentration is low, the water consumption reaction progresses and H ₂ O in the electrolyte decreases, so the amount of electrolyte that contributes to discharge decreases, making it difficult to obtain a sufficient discharge capacity. However, since no free electrolyte is generated, leakage from the air holes becomes difficult during overdischarge. On the other hand, in the region where the KOH concentration is high, the electrolyte is not consumed, so discharge is possible until the final stage, and the discharge capacity increases. However, since water is not easily consumed, the synergistic effect with the volume expansion of the reactants during discharge prevents overdischarge. If this is done, leakage from the air holes is likely to occur. On the other hand, the open circuit voltage after the air hole is opened after removing the sealing paper is controlled by the ZnO concentration, and when the ZnO concentration is low, it shows 1.40 V or more as shown in the shaded area C. Conversely, when the ZnO concentration becomes high, the open circuit voltage increases. descend. The reason for this is extremely simple: the unipolar potential of zinc roughly corresponds to the ZnO concentration, and the potential of zinc becomes more noble in the low ZnO concentration region. Generally, ZnO is often added to alkaline sealed batteries. This is to suppress the generation of hydrogen gas from zinc during storage, and it is common to add 4% by weight or more of zinc oxide. In air-zinc batteries, the air holes for oxygen intake are sealed with sealing paper, and some hydrogen gas will pass through, so there is no need to increase the ZnO concentration that much.In fact, it is necessary to increase the zinc oxide concentration. Reducing the open circuit voltage will shorten the activation time required to make the battery usable after removing the sealing paper, which will be effective when using the battery. If this activation time is within 1 minute, the ZnO concentration will be approximately at its maximum concentration of 3.5% by weight. The lower limit concentration was set at 0.2% by weight because adding a small amount of ZnO significantly reduces hydrogen gas generation from zinc. Comprehensive consideration of these discharge capacity performance of 400mAh or more, elimination of over-discharge leakage, maintenance of open circuit voltage of 1.40V or more, and shortening of battery activation time, points in the three-component composition diagram shown in Figure 3. A,
A battery using an electrolytic solution having a composition within the area surrounded by the line connecting B, C, and D exhibits extremely excellent overall performance. In addition, the higher the concentration of KOH, the better the storage stability will be.
Because the vapor pressure decreases and the product no longer dries, it is improved compared to solutions with a KOH concentration of about 30% by weight, which have been mainly used in the past.

[Brief explanation of drawings]

Figure 1 is a three-component composition diagram of a KOH-ZnO-H ₂ O electrolyte, and Figure 2 is an air-
FIG. 3 is a half-cut side view showing the structure of a zinc battery, and is a three-component composition diagram in which characteristic values of batteries using electrolytes of various compositions are inserted. 1...Positive electrode, 4...Negative electrode, 5...Separator,
9... Positive electrode case, 13... Air hole for oxygen intake.

Claims (1)

[Claims] 1 A battery that uses oxygen as a positive electrode active material, zinc as a negative electrode active material, and a three-component solution consisting of potassium hydroxide, zinc oxide, and water as an electrolytic solution, and has an oxygen intake hole in the battery case, Point A (33.25% by weight of potassium hydroxide, 0.2% by weight of zinc oxide, 66.55% of water) in the three-component composition diagram in Figure 3, which shows potassium hydroxide, zinc oxide, and water as liquids in weight%.
(wt%), point B (33.25 wt%, 3.5 wt%, 63.25 wt%), point C (38.60 wt%, 3.5 wt%, 57.90 wt%), point D (39.92 wt%, 0.2 wt%, 59.88 wt%) ) with the composition within the area bounded by the line connecting the