JPH0756802B2 - Manufacturing method of hydrogen storage electrode - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a hydrogen storage electrode using, as a negative electrode material, a hydrogen storage alloy that reversibly stores and releases hydrogen in an electrolytic solution, and in particular, a sealed battery. Regarding the electrodes.

2. Description of the Related Art A hydrogen storage alloy can be used as a negative electrode material for a secondary battery by electrochemically storing and releasing hydrogen. Of these, by selecting an alloy capable of absorbing and desorbing hydrogen near room temperature and having a large absorption amount, a hydrogen absorption electrode having a large discharge electricity amount becomes possible. Therefore, for example, by combining with a nickel oxide positive electrode, an alkaline storage battery having a large energy density can be expected. From such a background, a high-capacity storage battery using a hydrogen storage electrode has been attracting attention. In this type of electrode, the corrosion resistance of the hydrogen storage alloy, the decrease in discharge capacity due to atomization due to charge and discharge, and the ability to absorb oxygen generated from the positive electrode during overcharge in the closed battery system combined with the nickel oxide positive electrode The negative effect of increasing the internal pressure of the battery due to the decrease has hindered its practical use.

On the other hand, as an electrode material of this kind, an alloy represented by the general formula LnM ₅ (Ln is a mixture of rare earth metals, M is Ni, Cu, Co, Mn, Fe, A
Two or more metals selected from l, etc.) have been proposed. Among these metals, Mn plays an important role because it can lower the equilibrium pressure at the time of hydrogen absorption and desorption with a small amount.
However, on the other hand, Mn has a higher vapor pressure than other metals, and is evaporated near the surface of the alloy by evaporating during melting of the alloy,
Difficult to form a uniform alloy. Therefore, the adverse effect of the above-mentioned electrode is promoted.

Problems to be Solved by the Invention Since the problems as described above are left in the conventional electrode configurations as described above, there is a problem that the cycle life is shortened, and the electrolyte leaks in the sealed battery due to an increase in the battery internal pressure. was there.

In order to solve such problems, the present invention aims to improve the stability of oxygen gas absorption characteristics and the cycle life by homogenizing a hydrogen storage alloy that is a negative electrode material and improving the corrosion resistance of the alloy powder surface. It is intended.

Means for Solving the Problems In order to solve this problem, the present invention provides an arc melting method,
By heat-treating an alloy represented by the general formula LnM ₅ (Ln is a mixture of rare earth elements) obtained by high-frequency electrolysis in a vacuum, the alloy composition can be homogenized, and this alloy can be used as an electrode. After crushing into a fine powder, it is immersed in a high temperature alkaline aqueous solution to form an oxide film that is evenly distributed on the powder surface, improving corrosion resistance, stability against oxygen, and corrosion of the alloy inside the powder. It is the one that restrains the progress.

Operation With this structure, a homogeneous intermetallic compound is formed in the alloy. Due to charging and discharging, some of the metals in the alloy are adversely affected by melting, for example, the hydrogen storage capacity is reduced and the discharge electricity is reduced. Phenomena can be prevented. further,
By forming a thin oxide film uniformly distributed on the powder surface, corrosion resistance can be improved, and battery life can be improved.

EXAMPLES Hereinafter, the present invention will be described in detail with reference to Examples.

(Example 1) Using a misch metal (Mm) containing lanthanum (La), nickel (Ni), cobalt (Co), and manganese (Mn) having a purity of 99.5% or more and a rare earth element content of 98.5% or more, an alloy is formed. Each metal was weighed so that the composition was La _0.3 Mm _0.7 Ni _3.5 Co _1.2 Mn _0.3 , and an alloy was prepared using an arc melting furnace. This alloy is vacuum-heat treated at 900 ℃, 950 ℃, 1050 ℃, 110 ℃.
Keep the degree of vacuum below 10 ^-2 Torr at 0 ℃ and 1150 ℃.
Heat treatment was performed for an hour. After cooling, crush these alloys
It was made into powder of 400 mesh or less. Then, each of the 5 kinds of powders was kept for 5 hours with stirring in an aqueous solution of caustic potash of 30 ° C. at 60 ° C. for 5 hours, and then washed with water to remove caustic potash and dried.

100 g of the dried powder was mixed with 25 g of a 1.5% by weight aqueous solution of polyvinyl alcohol to prepare a muddy paste. These pastes were uniformly filled into a foamed nickel porous body (size 260 × 38 mm, thickness 0.9 mm) having a porosity of 94 to 96% and dried. After that, pressure pressing was performed to obtain a negative electrode.

Next, as a nickel oxide positive electrode, a foaming nickel electrode obtained by a known method (theoretical charging electricity amount: 2920 to 3030 mAh)
Polyamide non-woven fabric is used for the separator, and 30 wt% of potassium hydroxide aqueous solution containing 30 g of lithium hydroxide is used as the electrolytic solution. In combination with the negative electrode, a sealed size of C2 with a nominal capacity of 2.8 Ah. A nickel-metal hydride storage battery was constructed.

Under the condition that these batteries are charged at 0.1C for 15 hours at a constant temperature of 20 ° C for 15 hours in the first cycle, and at 7.5C for 0.2 hours after the second cycle, all discharges are performed at a current of 0.2C and a cutoff voltage of 0.9V. The battery was continuously discharged until the cycle life of the battery was examined. At the same time, a hole with a diameter of 1.5 mm was opened at the bottom of the battery, a pressure sensor was attached, and the pressure change inside the battery was measured. The result is first
Shown in the table. For comparison, the battery characteristics of some alloys are shown in the negative electrode using the alloy powder in which the step of immersing the alloy in the caustic potash solution is omitted.

In Table 1, paying attention to the influence of the heat treatment temperature, it was found that the optimum range was 950 to 1100 ° C, although the discharge capacity and the battery internal pressure differed depending on the presence or absence of the alkali treatment step. The reason why the optimum conditions were obtained in this range is that, on the high temperature side, unalloyed simple metal, especially metal with high vapor pressure, preferentially evaporates, causing a change in alloy composition, resulting in an AB ₅ type alloy. It is considered that the deterioration of the negative electrode characteristics increased due to the collapse of the system and the change in hydrogen storage pressure.

On the low temperature side, it is considered that the effect of the heat treatment did not appear because the temperature range where homogenization of the alloy obtained by the ordinary heat treatment effect was possible has not been reached.

On the other hand, when the alkali treatment was performed, the effect of the alkali treatment was confirmed when compared under the same heat treatment conditions. Therefore, in order to form a stable oxide film layer on the surface by subjecting the alloy powder to alkali treatment and water washing and drying in advance, in a sealed battery system, the oxygen gas absorption capacity may slightly decrease at the initial stage, but the charge and discharge cycle The stability was confirmed by repeating the above procedure, and it was effective to prevent the rise of the internal pressure of the battery. Therefore, it is considered that the oxidation of the alloy was limited only to the vicinity of the surface, did not proceed to the inside, and the capacity reduction of the negative electrode did not appear.

From the above results, the heat treatment condition and the alkali treatment of the alloy powder are effective manufacturing methods for the hydrogen storage electrode showing stable electrode characteristics for a long period of time.

(Example 2) Using an alloy having a heat treatment temperature of 1050 ° C, 100 g of alloy powder after pulverization
On the other hand, an aqueous solution of 1.5% by weight of polyvinyl alcohol was mixed at a ratio of 25 g to prepare a muddy paste. These pastes were uniformly filled into a foamed nickel porous body (having the same dimensions as in Example 1) having a porosity of 94 to 96% and dried. After that, pressure pressing was performed, and the plate was immersed and held in an aqueous solution of caustic potash or caustic soda at the temperature and concentration shown in Table 5 for 5 hours. Then, after washing with water and drying, the sealed nickel-metal hydride storage batteries K to T of size A (size C) were prepared under the battery constitution conditions shown in Example 1, and the same charging and discharging conditions as in Example 1 were used. The test results are shown in Table 2.

From this result, the following was found. First, regarding the temperature of the alkaline aqueous solution, the effect was recognized even at a low temperature, but it can be said that it is good that the temperature is not less than 45 ° C. Good results are obtained even on the high temperature side, but it is considered that 90 ° C or lower is appropriate from the viewpoints of risk and concentration control. Also, regarding the alkali concentration, even at low concentrations Although the effect was recognized, it was found that 20% by weight or more was more effective. In order to compare these results, the battery of E shown in Table 1 does not undergo alkali treatment at all, and its effects can be compared.

Further, alloy system containing no Mn, on the contrary, a system containing a large amount, for example, as a result of examining the effect of the present invention on an alloy of La _0.3 Mm _0.7 Ni _2.8 Co _1.2 Mn, in the former case, since Mn is not contained, The effect of lowering the equilibrium hydrogen pressure of the hydrogen storage alloy was small, and the storage capacity was decreased and the internal pressure of the battery was increased. The latter causes the flatness of the hydrogen equilibrium pressure to deteriorate, and makes it difficult to form a homogenous alloy, resulting in deterioration of the characteristics at the hydrogen storage electrode, and the amount of Mn is 0.2 to 0.8 in M of LnM _5. The degree seems optimal.

Therefore, the present invention is considered to be effective in alloys containing Mn. Further, as described above, Mn can be said to be an effective metal for changing the characteristics of the hydrogen storage alloy even in a small amount, so the present invention is an effective method for providing a hydrogen storage electrode.

As described above, according to the present invention, particularly in a hydrogen storage alloy containing Mn, a stable oxide film is formed on the surface of a homogeneous alloy by a combination of selection of effective heat treatment conditions and an alkali treatment step. As a result, a hydrogen storage electrode in which a decrease in discharge capacity and a decrease in oxygen gas absorption capacity due to charging and discharging are suppressed can be obtained, and it is possible to provide a battery having a long life and a high capacity.

Claims (4)

[Claims] 1. An alloy represented by the general formula LnM ₅ (Ln is a mixture of rare earth elements), wherein M in the formula contains at least Mn is 950 ° C. to 1 ° C.
After heat treatment in a vacuum atmosphere at 100 ° C, a step of crushing this alloy, a step of immersing the crushed powder in a high-temperature alkaline aqueous solution, and then, washing and drying the alloy powder with a binder into a porous metal body. A method for producing a hydrogen storage electrode, comprising a step of forming an electrode by filling or coating both surfaces of a metal porous body. 2. An electrode is formed by filling the alloy powder obtained by the steps of heat treatment and pulverization together with a binder into a porous metal body or coating both surfaces of the porous metal body to form an electrode, and then forming a high temperature alkali. Immerse in aqueous solution, wash with water, dry,
The method for producing a hydrogen storage electrode according to claim 1, wherein pressure is applied. 3. The method for producing a hydrogen storage electrode according to claim 1 or 2, wherein the temperature of the aqueous alkaline solution is 45 to 90 ° C. 4. The hydrogen according to claim 1, wherein the alkaline aqueous solution is an aqueous solution of caustic potash or caustic soda, and the concentration of each is 20% by weight or more. Manufacturing method of storage electrode.