JPH0794177A - Manufacture of hydrogen storage alloy electrode and storage battery hydrogen storage alloy - Google Patents

Abstract

PURPOSE:To provide a hydrogen storage alloy electrode having high cycle characteristics by producing a hydrogen storage alloy having own pertinent mechanical strength without any process of special machining, and forming the electrode using the alloy. CONSTITUTION:Regarding a hydrogen storage alloy electrode using misch metal- nickel hydrogen storage alloy, this alloy is formed so as to have the dendrite cell size of the crystal grain thereof equal to or less than 15mum. Also, a method for manufacturing the misch metal-nickel hydrogen storage alloy for a storage battery is to melt an element constituting the alloy to prepare molten metal and to quickly cool the molten metal at a cooling rate equal to or above 1000 deg.C/sec.

Description

Detailed Description of the Invention

[0001]

The present invention relates to the reversible storage of hydrogen.
The present invention relates to a hydrogen storage alloy electrode containing a hydrogen storage alloy to be released and a method for manufacturing a hydrogen storage alloy for a storage battery.

[0002]

2. Description of the Related Art Conventionally used storage batteries include alkaline storage batteries such as nickel-cadmium storage batteries and lead storage batteries. Capable metal-hydrogen alkaline storage batteries are being used. In this metal-hydrogen alkaline storage battery, an electrode made of a hydrogen storage alloy that reversibly stores and releases hydrogen, which is a negative electrode active material, at normal pressure is used as a negative electrode, and a metal oxide such as nickel hydroxide is used as a positive electrode active material. It is a battery configured by using an electrode as a positive electrode.

By the way, for the negative electrode of this type of alkaline storage battery, an ingot of alloy is produced by a method of pouring a molten liquid of a predetermined composition into a mold, and the alloy ingot is crushed and adjusted to a proper particle size to obtain a hydrogen storage alloy powder. It is used. However, since the hydrogen storage alloy has a property of expanding and contracting due to storage and release of hydrogen, the hydrogen storage alloy in the negative electrode is cracked due to expansion and contraction stress associated with charge and discharge cycles, and gradually becomes fine powder. Further, since pulverization of the alloy exponentially increases the specific surface area of the alloy as a whole, the hydrogen storage alloy in the negative electrode is much more susceptible to the chemical action of the alkaline electrolyte solution than at the beginning. Therefore, the corrosion of the negative electrode is accelerated due to the progress of charge and discharge cycles, the hydrogen storage capacity of the alloy is reduced, and the hydroxide formed on the alloy surface reduces the mutual conductivity of the alloys. Cause deterioration. That is, the pulverization phenomenon of the alloy consequently deteriorates the corrosion resistance of the alloy and deteriorates the battery cycle characteristics.

For these reasons, a conventional electrode using the hydrogen storage alloy as it is cannot obtain a sufficient cycle life. Therefore, recently, the alloy was nickel-treated,
It has been proposed that the nickel thin film formed on the surface reinforces the alloy strength to prevent alloy cracking due to expansion and contraction. By subjecting the alloy surface to nickel processing in this way, the mechanical strength of the alloy particles is improved, and alloy cracking is suppressed. Therefore, the cycle life is improved to some extent in the electrode formed by using this processed alloy.

[0005]

However, when the surface-treated hydrogen storage alloy as described above is produced,
Since a new step for forming the nickel thin film has to be provided, the cost becomes higher accordingly. Further, since the nickel thin film forming process is not always easy and easy, variations occur in the degree of thin film formation, resulting in variations in alloy strength. Further, even an electrode using a nickel-treated alloy has a drawback that the cycle life cannot be sufficiently improved. Then, the inventors of the present invention paid attention to the corrosion resistance and reviewed the conventional hydrogen storage alloy from the viewpoint of physical properties. As a result, they have found that the conventional hydrogen storage alloy itself does not have sufficient mechanical strength, and found that the hydrogen storage alloy having certain physical properties has improved mechanical strength.

Based on the above findings, the present invention creates a hydrogen storage alloy having suitable mechanical strength by itself without adding special processing to the hydrogen storage alloy, and using this hydrogen storage alloy, hydrogen It is intended to provide a hydrogen storage alloy electrode having excellent cycle characteristics by forming a storage alloy electrode. Further, the present invention intends to provide a method for producing a hydrogen storage alloy which itself has suitable mechanical strength.

[0007]

The above object can be achieved by the following constitution. The invention configuration of claim 1 is a hydrogen storage alloy electrode using a hydrogen storage alloy of a misch metal-nickel system, wherein the hydrogen storage alloy has a dendrite cell size value of 15 μm or less of its crystal grains. It is characterized by being

According to the second aspect of the present invention, there is provided a misch metal-
In the method for producing a nickel-based hydrogen storage alloy for a storage battery, the first step of melting the elements constituting the hydrogen storage alloy to prepare a molten metal, and the molten metal at 1000 ° C./se
and a second step of quenching at a cooling rate of c or more.

[0009]

According to the invention of claim 1, a hydrogen storage alloy having a crystal grain dendrite cell size of 15 μm or less is used as a main constituent material of the hydrogen storage alloy electrode (negative electrode). Since the hydrogen storage alloy of the size does not have an extremely brittle portion in the metal structure, the mechanical strength of the alloy itself is high. Therefore, the hydrogen storage alloy having such physical properties can sufficiently withstand the expansion and contraction stress associated with the storage and release of hydrogen, and is less likely to be pulverized due to alloy cracking. That is, in the hydrogen storage alloy electrode formed by using the hydrogen storage alloy having such physical properties, the phenomenon that the alloy in the electrode is finely pulverized as the charge / discharge cycle progresses and the corrosion resistance is gradually lowered is unlikely to occur. Therefore, as a result, the cycle characteristics of the electrode are improved.

According to the second aspect of the invention, the melt of the misch metal-nickel alloy component is heated to 1000 ° C. /
Since the hydrogen absorbing alloy ingot is rapidly cooled and solidified at a cooling rate of sec or more to produce a hydrogen storage alloy ingot, it is possible to suppress segregation of a specific alloy composition at grain boundaries in the metal structure. Therefore, it is possible to obtain an ingot having a metallographic structure in which each composition is dispersed and solidified more uniformly and has less brittle portions.

The dendrite cell size means the primary branch grown from the nucleus in the dendritic metal structure that appears when the alloy melt is solidified, and the primary branch (main axis) is It is the thickness of the secondary branch grown in the orthogonal direction. In general, it is considered that in the alloy, the larger the dendrite cell size, the more nonuniform the alloy structure.

[0012]

【Example】

Example 1 (Preparation of Hydrogen Storage Alloy Ingot) Mm (Misch metal; mixture of rare earth elements), Ni, Co, Al, and Mn alloy components in an element ratio of 1: 3.2: 1: 0. 0.2: Weighed 0.6 and mixed, and then melted in a high frequency melting furnace in an inert gas atmosphere. The melt was cooled at a cooling rate of 1.2 × 10 ⁴ ° C./sec by a roll method to obtain a composition formula MmNi.
_3.2 to obtain an alloy ingot represented by CoAl _{0. 2} Mn _0.6.
The dendrite cell size (DCS) of this alloy ingot was measured by the method shown below,
It was 6 μm.

Next, the alloy ingot is mechanically crushed,
An alloy powder having an average particle size of about 50 microns was produced. Then, 1.2 g of carbonyl nickel as a conductive agent and PTFE (polytetrafluoroethylene) powder as a binder were added to 1 g of this alloy powder and mixed to prepare an alloy paste. The alloy paste was wrapped with a nickel mesh and compressed. Then, a hydrogen storage alloy electrode having a thickness of 2 mm and a diameter of 20 mm was produced.

The hydrogen storage alloy electrode thus prepared is
Hereinafter, the electrode A _{1 of the} present invention is used. [Example 2] The alloy melt was treated at 1.1 x 10 ³ ° C / sec.
A hydrogen storage alloy electrode was produced in the same manner as in Example 1 except that the cooling rate was 1. However, the DCS of the alloy ingots cooled and cast under the cooling rate condition was 8.5 μm.

This hydrogen storage alloy electrode is hereinafter referred to as electrode A _{2 of the} present invention. [Example 3] A hydrogen storage alloy electrode was prepared in the same manner as in Example 1 except that the alloy melt was poured into a mold by a normal cooling method and cooled at a cooling rate of 0.9 x 10 ² ° C / sec. Was produced. The DCS of the alloy ingots cooled and cast under the cooling rate condition was 14.5 μm.

This hydrogen storage alloy electrode is hereinafter referred to as electrode A _{3 of the} present invention. [Comparative Example 1] A hydrogen storage alloy electrode was produced in the same manner as in Example 1, except that the ordinary cooling method of pouring the melt into the mold was used and cooling was performed at a cooling rate of 10 ° C / sec. The DCS of this alloy ingot was 23 μm. The hydrogen storage alloy electrode thus manufactured was used as a comparative electrode B ₁ .

(Method of measuring DCS) In the Mm-Ni type hydrogen storage alloy ingot, the secondary branch of the solidification structure does not sufficiently develop,
As shown in FIG. 3, the cells are solidified into cells. Therefore, a line was drawn perpendicularly to the crystal growth direction, the number of cell boundaries intersecting this line was counted, the average distance between cell boundaries was determined, and this was defined as the dendrite cell size (DCS).

[Experiment] Each of the electrodes prepared above was used as a negative electrode.
Make the test cells shown below and
Cycle characteristics test is performed to evaluate the degree of cycle deterioration of each electrode
I paid. (Test cell) Fig. 4 shows a schematic diagram of the test cell.
The outline of the test cell will be described with reference to FIG. 1 is the negative electrode
And the hydrogen storage alloy electrode formed into a cylindrical shape in this 1.
Pole (A ₁~ A₃And B₁) Is installed. 2 is a known person
With a known sintered nickel electrode (positive electrode) made by the
is there. The sintered nickel electrode 2 has an upper surface 5 of the closed container 3.
Is held by the positive electrode lead 4 connected to
The electrode 1 is perpendicular to the center of the cylinder of the sintered nickel electrode 2.
Negative electrode connected to the upper surface 5 of the closed container 3 so as to be positioned
It is held by the lead 6. Furthermore, the positive electrode lead 4 and
And each end of the negative electrode lead 6 penetrates the upper surface 5 of the closed container 3.
Exposed to the outside, and the positive electrode terminal 4a and the negative electrode terminal, respectively.
6a is connected. And, in the closed container 3,
30% by weight potassium hydroxide aqueous solution is added as potassium electrolyte
In this solution, the hydrogen storage alloy electrode 1 and
The sintered nickel electrode 2 is immersed. 7 is pressure
A total of 8 is a relief pipe, and 9 is a gas relief valve.

The test cell having such a structure will be described below in accordance with the type (A _{1 to} A ₃ ) of the hydrogen storage alloy electrode used as the negative electrode.
And B ₁ ) corresponding to XA ₁ , XA ₂ and XA, respectively.
₃ and XB ₁ . (Cycle characteristic test) After charging for 8 hours at 50 mA per 1 g of alloy, discharging at 50 mA per 1 g of alloy until the cell voltage becomes 1.0 V is defined as one cycle, and this series of operations is repeated 100 times. The discharge capacity C ₁₀₀ at the 100th cycle was determined. Then, to the maximum discharge capacity Cmax of the cell, the discharge capacity ratio of C _{10 0} at the 100th cycle (cell volume ratio V ₁₀₀₎ were calculated according to the following equation. Cell capacity ratio V ₁₀₀ = [C ₁₀₀ / Cmax] × 100 (Experimental result) Table 1 shows the cell capacity ratio V ₁₀₀ of each cell together with the dendrite cell size of the hydrogen storage alloy. Further, FIG. 1 shows the relationship between the cell capacity ratio V ₁₀₀ and DCS,
FIG. 2 shows the relationship between DCS and the cooling rate of the alloy melt.
The horizontal axis of FIG. 1 is a logarithmic scale.

[0020]

[Table 1] As is clear from Table 1, all the cells (XA _{1 to} XA ₃ ) using the electrode of the present invention are XB using the reference electrode.
Compared with ₁ , the cell capacity ratio V ₁₀₀ was significantly high.

Here, the only difference between the cells is the difference in the DCS of the hydrogen storage alloy that is the negative electrode constituent material. Therefore, it can be concluded that the difference in cycle characteristics of the cells represented by the cell capacity ratio V ₁₀₀ is caused by the difference in DCS, and the difference in cycle characteristics is the difference in corrosion resistance of hydrogen storage alloys. It can be considered that the result is caused by.

On the other hand, from FIG. 1 showing the relationship between the cell capacity ratio V ₁₀₀ and DCS, it can be seen that the cycle characteristics improve as the DCS decreases, and the graph line has a refraction point near 15 μm. It can be seen that the improvement tendency of the cycle characteristics becomes large in DCS of 15 μm or less. On the other hand, from FIG. 2 showing the relationship between DCS and cooling rate, DCS
Is found to decrease as the cooling rate of the alloy melt increases, and since the graph line has a refraction point near 1.6 × 10 ² ° C / sec, 1.6 × 10 ² ° C / s
It can be seen that the DCS can be effectively reduced at a cooling rate of ec or higher. However, considering the effect of improving the cycle characteristics described above, it is preferable that the cooling rate is 1 × 10 ³ ° C. or more. This is because the DCS is 15 μm or less at the cooling rate or higher. The cooling rate may be 1 × 10 ³ ° C or higher, and the upper limit is not particularly specified, but in reality, the cooling rate is 1 × 10 ⁵ ° C.
The above is not realistic. Therefore, 1 × 10 ³ ° C is considered to be the upper limit.

The reason why the cycle characteristics are improved when the DCS becomes small is considered to be as follows. That is, the hydrogen storage alloy repeats storage and release of hydrogen according to the charge and discharge cycle, but the alloy expands and contracts with the storage and release of hydrogen. In this case, if the alloy structure is non-uniform, a fragile portion exists in the alloy structure, and strain is likely to occur inside the alloy during expansion and contraction.

That is, the more inhomogeneous the alloy structure is, the more easily the alloy cracks due to the expansion and contraction accompanying the absorption and desorption of hydrogen. The alloy cracks increase the specific surface area of the alloy and make it more susceptible to the chemical action of the alkaline electrolyte, so that the alloy in the negative electrode gradually corrodes and deteriorates the cycle characteristics. However, in the negative electrode using a hydrogen storage alloy having a DCS of 15 μm or less, the nonuniformity of the alloy structure is small, so that the strain generated by expansion and contraction can be sufficiently resisted, and thus alloy cracking is less likely to occur. Therefore, deterioration of the negative electrode due to corrosion is suppressed, and as a result, cycle characteristics are improved.

[0025]

As described above, according to the hydrogen storage alloy electrode of the present invention, the corrosion resistance of the hydrogen storage alloy which is the active material does not deteriorate due to repeated charging and discharging, so that excellent cycle characteristics are obtained. be able to. Further, according to the method for producing a hydrogen storage alloy of the present invention, the hydrogen storage alloy having excellent corrosion resistance can be easily obtained without changing the alloy composition as it is conventionally and without adding special processing for adding mechanical strength. Obtainable. Therefore, by constructing a hydrogen storage alloy electrode using this hydrogen storage alloy produced by the method of the present invention, an electrode having excellent cycle characteristics can be easily and economically obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a discharge capacity ratio (V ₁₀₀ ) and a dendrite cell size at the 100th cycle of an experimental cell.

FIG. 2 is a diagram showing a relationship between a dendrite cell size and a cooling rate of an alloy melt.

FIG. 3 is an explanatory diagram of a dendrite cell size.

FIG. 4 is a schematic diagram of an experimental cell.

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[Procedure amendment]

[Submission date] November 2, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0011

[Correction method] Change

[Correction content]

The dendrite cell size means the primary branch grown from the nucleus in the dendritic metal structure that appears when the alloy melt is solidified, and the primary branch (main axis) is It is the thickness of the secondary branch grown in the orthogonal direction. In general, it is considered that the larger the dendrite cell size of the alloy, the more uneven the metal structure.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0022

[Correction method] Change

[Correction content]

On the other hand, from FIG. 1 showing the relationship between the cell capacity ratio V ₁₀₀ and DCS, it can be seen that the cycle characteristics improve as the DCS decreases, and the graph line has a refraction point near 15 μm. It can be seen that the improvement tendency of the cycle characteristics becomes large in DCS of 15 μm or less. On the other hand, from FIG. 2 showing the relationship between DCS and cooling rate, DCS
Is found to decrease as the cooling rate of the alloy melt increases, and since the graph line has a refraction point near 1.6 × 10 ² ° C / sec, 1.6 × 10 ² ° C / s
It can be seen that the DCS can be effectively reduced at a cooling rate of ec or higher. However, considering the effect of improving the cycle characteristics described above, the cooling rate is 1 × 10 ³ ° C./se
It is preferably c or more. This is because the DCS is 15 μm or less at the cooling rate or higher.
The cooling rate may be 1 × 10 ³ ° C./sec or more, and the upper limit is not particularly specified, but it is not realistic to set the cooling rate to 1 × 10 ⁵ ° C./sec or more. . Therefore, 1 × 10 ⁵ ° C./sec is considered to be the upper limit.

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] 0023

[Correction method] Change

[Correction content]

The reason why the cycle characteristics are improved when the DCS becomes small is considered to be as follows. That is, the hydrogen storage alloy repeats storage and release of hydrogen according to the charge and discharge cycle, but the alloy expands and contracts with the storage and release of hydrogen. In this case, if the metallographic structure is not uniform, the metallographic structure has a fragile portion, and distortion is likely to occur inside the alloy during expansion and contraction.

[Procedure amendment 4]

[Document name to be amended] Statement

[Name of item to be corrected] 0024

[Correction method] Change

[Correction content]

That is, the more inhomogeneous the metallographic structure is, the more easily the alloy cracks due to the expansion and contraction accompanying the absorption and desorption of hydrogen. The alloy cracks increase the specific surface area of the alloy and make it more susceptible to the chemical action of the alkaline electrolyte, so that the alloy in the negative electrode gradually corrodes and deteriorates the cycle characteristics. However, in a negative electrode using a hydrogen storage alloy having a DCS of 15 μm or less, the nonuniformity of the metal structure is small, so that the strain generated by expansion and contraction can be sufficiently resisted, and thus alloy cracking is unlikely to occur. Therefore, deterioration of the negative electrode due to corrosion is suppressed, and as a result, cycle characteristics are improved.

Claims (2)

[Claims] 1. A hydrogen storage alloy electrode using a misch metal-nickel-based hydrogen storage alloy, wherein the hydrogen storage alloy is configured so that the dendrite cell size of its crystal grains is 15 μm or less. A hydrogen storage alloy electrode characterized by being present. 2. A method for producing a hydrogen storage alloy of a Misch metal-nickel system for a storage battery, the first step of melting the elements constituting the hydrogen storage alloy to prepare a molten metal, and the molten metal of 1000 ° C./sec or more. And a second step of rapidly cooling at a cooling rate of 1. A method for producing a hydrogen storage alloy.