JPS58188067A - Zinc air cell - Google Patents

Abstract

PURPOSE:To provide a zinc air cell having good leakage resistance to excess discharge, good discharge performance, and short activation time by spacifying an electrolyte composition in a desired region. CONSTITUTION:An electrolyte composition existing inside a region surrounded by lines connecting A, B, C, and D in three- component system diagram indicated by weight percentage of potassium hydroxide (KOH), zinc oxide (ZnO), and water (H2O). A positive electrode 2 consists of a catalytic layer 2 and a nickel net 3 and has a thickness of 0.45mm.. The catalytic layer 2 is consisted of 50pts.wt. active carbon, 80pts.wt. manganese dioxide which is treated at 550 deg.C, 20pts.wt. acetylene black, and 25pts.wt. polytetrafluoroethylene (PTFK). A negative electrode 4 is consisted of amalgamated zinc powder having the weight corresponding to a theoretical capacity of 440mAh and 120mul of various kinds of electrolytes and small amount of thickening agent. A separator 5 is consisted of polypropylene porous film 6 and absorvent material 7.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in air-zinc batteries, and in particular to improvements in small-sized air-zinc batteries, which have been attracting attention for use in hearing aids in recent years. The purpose of this invention is to provide a battery that is easy to operate and has excellent overdischarge and leakage resistance, discharge characteristics, and short activation time.

Conventionally, small air-zinc batteries have a large discharge capacity per volume, are resistant to leakage, especially less leakage during overdischarge, have good shelf life, and have low open circuit voltage when the battery is activated. There is a growing demand for higher standards.

However, in the current situation 1, emphasis is placed on electrical conductivity, which is one of the basic conditions for electrolytes, and potassium hydroxide (KO), which has a maximum electrical conductivity of approximately 30% by weight, is emphasized.
H) Concentration has been used.

As a result of intensive research into this electrolyte composition, the present inventors were able to discover an electrolyte composition that exhibits all excellent characteristics for air-zinc batteries, regardless of conductivity.

That is, potassium hydroxide (KOH), zinc oxide (Zn
O) and water (H2O) i are each shown in weight% in the ternary composition diagram in Figure 3, using the electrolyte composition within the area surrounded by the lines connecting dots M, B, C, and D'i. This makes it 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 H2O, were prepared as electrolytes. The specific adjustment method was to prepare an aqueous solution of KOH and H2O with a constant concentration, adjust the specific gravity to various concentrations, and then dissolve ZnOf and readjust to each composition.

The concentration (weight %) of each component of the prepared electrolytic solution is shown in Figure 1, and the overall condition of each electrolytic solution is shown in Figure 1.

As is clear from Figure 1 in the margin below (in the region of low KOH concentration, Zn
O is difficult to dissolve. Therefore, the insoluble portion of zinc oxide was removed. Using electrolytes of each composition, the size is R44.
(Diameter: 11.6 m+n, height: 6.4 mm) A complete air-zinc battery was fabricated. The configuration of the tested battery is explained in FIG. 2. The positive electrode 1 was made of 50 parts by weight of activated carbon, 8 parts of manganese dioxide heat-treated with 550', 20 parts by weight of acetylene black, and polytetrafluoroethylene (PTFE).
) 25 parts by weight of the catalyst layer 2 and the nickel mesh 3, the thickness of which was 0.45 mm. Negative electrode 4 is mainly made of frozen zinc powder, and has a theoretical zinc reaction capacity of 44
It consists of a weight of 0mAh, 120μ of the various electrolytes mentioned above, and a small amount of thickener. The seven craters 6 are composed of a polypropylene porous membrane 6 and a liquid-containing material 7. In addition, 8 is a porous membrane of PTFIE, 9 is a positive electrode case, 1
o is a negative electrode sealing plate, 11 is an insulating gasket meter, 12 is a support paper, 13 is an air hole provided in the positive electrode case for inserting oxygen, and 14 is an air hole that closes the air hole 13 when activated and opens when activated. It is a sealing paper that can be peeled off due to the holes, and is made of a material that is difficult for gas and moisture to pass through.

Using the various electrolyte compositions shown in Table 1, that is, the discharge performance of 47 types of batteries, using a total load of 180 Ω, the electric capacity obtained up to a final voltage of 0.9 V during continuous discharge in an environment of 20°C was investigated. Ta. In addition, over-discharge leakage was investigated by conducting a continuous series test under a load of 180Ω and checking for leakage or discoloration of all cresol red 30 days after the start of discharge. Furthermore, in order to check the activation time of the battery, after removing all the sealing paper,
The open circuit voltage after 1 minute was checked. All the results are shown in Table 2. In addition, in order to clarify the concentration range of each component of the electrolyte, we have prepared a partial enlarged view of the three-component composition diagram in Figure 1, and have inserted all the characteristic values of each battery investigated to determine good characteristics. A three-component composition diagram incorporating the indicated regions was created as Figure 3.

Table 2 As is clear from Table 2 and Figure 3, the discharge performance is KO
It is determined by the concentration of H, especially the concentration of hydroxyl ions.
The region where discharge capacity performance of mAh or more can be obtained is indicated by a dotted circle. In a concentration range where the KOH concentration is approximately 35% by weight or more, a discharge capacity of 400 mAh or more can be secured. On the other hand, overdischarge leakage is noticeable in the KOH concentration range of 42.6% by weight or more, as shown by the horizontal parallel lines in the area where there is no leakage. Although these relationships are not clear, the following mechanism is presumed. That is, the discharge reaction of zinc is Zn +2(:IH-+ Zn(OH) 2 + 2e
−=−−−−−−・(11Z n−1−40H−?
Zn02z + 2H'20+215-- (21) It is presumed that the two reactions occur within this electrolyte composition region. In particular, when the KOH concentration is low, reaction (1) proceeds dominantly, and the concentration increases. In the dark region, reaction (2) proceeds dominantly.

That is, in a region where the KOH concentration is low, the water consumption reaction progresses and H2O 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 free electrolyte is not produced, it becomes difficult for the electrolyte to leak from the air holes during overdischarge. On the other hand, in the region where the KOH concentration is high, the electrolyte is not consumed and discharge is possible until the final stage, increasing the discharge capacity, but water is less consumed and the
It is thought that overdischarge, combined with the volumetric expansion of reactants during discharge, causes liquid leakage from the air holes.

On the other hand, the open circuit voltage after opening the air hole excluding the sealing paper @9 is:
It is controlled by the ZnO concentration, and when the ZnO concentration is low, the voltage exceeds 1.40 V as shown by the hatched area, and conversely, when it becomes high, the open circuit voltage decreases. The reason for this is extremely simple;
This is because the unipolar potential of zinc is approximately proportional to the ZnO concentration, and the potential of zinc becomes base in a 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 use less than 2% zinc oxide.

In an air-zinc battery, the air holes for introducing oxygen v are sealed with sealing paper, and some hydrogen gas passes through, so there is no need to increase the ZnO concentration that much. Reducing the concentration is effective in terms of open circuit voltage, ie, the time required for activation to become usable after removing the sealing paper, which is effective when using the battery.

If the activation time is within 1 minute, the ZnO concentration will be 3. f5 weight % is almost the maximum limit. The lower limit concentration was set at 0.2% by weight because the addition of a small amount of ZnO significantly reduces hydrogen gas generation from zinc.

Comprehensively examining the discharge capacity, the presence or absence of overdischarge leakage, the quality of the open-circuit voltage, and the length of the battery activation time, it is found that in the three-component composition diagram of Figure 3, point person, B.

A battery using an electrolytic solution having a composition within the area surrounded by the line connecting C and D exhibits extremely excellent overall performance.

In addition, the higher the concentration of KOH, the higher the storage stability1, the higher the vapor pressure;
Since the concentration of KO'H decreases and does not dry, this is improved compared to solutions with a KO'H concentration of approximately 30% by weight, which have been mainly used in the past.

[Brief explanation of the drawing]

Figure 1 is a three-component composition diagram of a KOH-ZnO-H20 electrolyte, Figure 2 is a half-cut side view showing the configuration of an air-zinc battery in an embodiment of the present invention, and Figure 3 is a diagram showing electrolytes of each composition. It is a three-component composition diagram in which characteristic values of the battery used are inserted. 1...Positive electrode, 4...Negative electrode, 6...
... Senokureta, 9... Positive electrode case, 13.
...Air hole for oxygen intake. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 〆nO (weight%) Figure 2

Claims (1)

[Claims] A battery that uses oxygen as a positive electrode active material, zinc as a negative electrode active material, and a solution consisting of three components of potassium hydroxide, zinc oxide, and water as an electrolytic solution, and has an oxygen intake hole in the battery case, wherein the electrolytic solution is In the three-component composition diagram of Figure 3, which shows potassium hydroxide, zinc oxide, and water in weight percent, the composition sound within the area surrounded by the line connecting dots B, C, and D is air. zinc battery.