JPH0883615A - Processing method for hydrogen storage alloy powder - Google Patents

Abstract

PURPOSE: To provide hydrogen storage alloy powder for the negative electrode of an alkaline secondary battery effectively eluting and removing a segregation constituent when an alloy is ingotted, suppressing the rise of the battery internal voltage at the time of overcharging, and increasing the capacity recovering factor after the voltage drop. CONSTITUTION: The powder 2 of a hydrogen storage alloy is cathode-polarized in a thermal alkaline solution 3 to manufacture this alloy powder. When the Co component is contained in the hydrogen storage alloy in particular, cathode polarization is conducted at the potential lower than the Hg/HgO single electrode potential by -0.8V to manufacture the alloy powder.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating a hydrogen storage alloy powder, and more particularly, to produce a hydrogen storage alloy powder useful as a main material of a hydrogen storage alloy electrode incorporated as a negative electrode of a nickel-hydrogen secondary battery. On how to do.

[0002]

2. Description of the Related Art Recently, nickel-hydrogen secondary batteries have been attracting attention as high-capacity alkaline secondary batteries. This nickel-hydrogen secondary battery operates with hydrogen as a negative electrode active material, and a negative electrode in which a hydrogen storage alloy capable of reversibly storing and releasing hydrogen is carried on a current collector,
A power generating element is formed by stacking a positive electrode, which is also a positive electrode active material nickel hydroxide supported on a current collector, through a separator having electrical insulation and liquid permeability, and forming the power generating element. After being placed in an alkaline electrolyte such as potassium hydroxide, the entire structure is hermetically sealed.

The negative electrode used in this battery is usually prepared by mixing a hydrogen-absorbing alloy powder, a conductive material powder, and a binder powder in a predetermined ratio, and pouring the mixed powder into an aqueous thickener solution and stirring it to form a slurry. It is manufactured by preparing the slurry, coating the slurry on a current collector, drying and rolling it into a desired shape.
Conventionally, as a hydrogen storage alloy for a negative electrode, LaNi is mainly used.
_Although alloys such as ₅ have been used, there is a problem in that a battery incorporating a negative electrode made of this alloy has a short charge / discharge cycle life of about 30 cycles and is not practical.

Therefore, recently, a part of Ni of LaNi ₅ is replaced with Co, Al, Mn or the like to form a multi-component alloy,
In addition, from the viewpoint of economic efficiency, La is replaced with La, Ce,
Hydrogen storage alloys substituted with misch metal (Mm), which is a mixture of rare earth elements such as r and Nd, have begun to be widely used. Such an alloy is usually manufactured by mixing predetermined amounts of the respective component elements so as to be compatible with the target alloy composition, and then smelting in an arc melting furnace, for example.

By the way, the potential at which the hydrogen storage alloy forming the negative electrode of the nickel-hydrogen secondary battery causes a charging reaction is a value close to the electrolysis potential of water in the alkaline electrolyte.
At the time of overcharging, a large amount of hydrogen gas is generated by electrolysis of water, which causes an increase in battery internal pressure. Normally, the nickel-hydrogen secondary battery is designed so that when the internal pressure of the battery exceeds 20 atm, the safety valve provided in the battery is activated to discharge the internal gas. When the condition is repeated, the amount of electrolyte is reduced each time and the battery life becomes short.

In order to suppress the rise in the internal pressure of the battery during overcharge, the hydrogen storage alloy forming the negative electrode is made to contain a Co component as described above. Further, when the hydrogen storage alloy used for the negative electrode is not completely alloyed during its melting and the constituent elements are segregated, when the negative electrode manufactured by the hydrogen storage alloy is incorporated into a battery, the segregation The ingredients dissolve in the alkaline electrolyte. Then, the dissolved ions of the segregated component may be oxidized, for example, at the time of discharge in the process of repeating the charge / discharge cycle, or may be deposited in a metallic state on the separator or the like due to the solubility. When such a situation occurs, the electrical insulation of the separator is broken, a short circuit phenomenon occurs in the battery, and the discharge capacity of the battery decreases rapidly.

The above-mentioned phenomenon due to the presence of the segregation component is remarkably exhibited at high temperature. It is considered that this is because the dissolution rate and the amount of the segregated component dissolved in the alkaline electrolyte increase at high temperatures. To solve such a problem, in JP-A-61-176063, a segregated component is eluted by crushing a melted hydrogen storage alloy and then washing the powder with an alkaline solution at 40 to 80 ° C. A method of removal is disclosed.

However, when this method is applied to the powder of the hydrogen storage alloy containing the Co component, for example,
Most of the Co component that suppresses the increase in battery internal pressure during overcharging is also eluted and removed, which may result in an increase in battery internal pressure. In addition, in the case of nickel-hydrogen secondary battery, if the battery is left in a discharged state and left at high temperature, it exhibits a peculiar behavior that the battery voltage continues to drop to the open voltage even after the battery capacity becomes zero. . Then, there is a problem that the battery capacity does not return to the original state even if the battery whose battery voltage has dropped to the open voltage is overcharged again.

Even when a negative electrode composed of a hydrogen storage alloy treated by the method disclosed in the above-mentioned Japanese Patent Laid-Open No. 61-176063 is used, the nickel-hydrogen secondary battery incorporating the negative electrode is not affected by the above high-temperature discharge. There is a problem in that the effect of suppressing the decrease in battery voltage is insufficient and the capacity recovery rate is not so high.

[0010]

DISCLOSURE OF THE INVENTION The present invention is disclosed in Japanese Patent Laid-Open No. Sho 61-61.
By improving the method disclosed in Japanese Patent Publication No. 176063, the above-mentioned problems in a hydrogen storage alloy having segregation components are solved, and when used as a constituent material of a negative electrode, in the battery, the internal pressure of the battery during overcharge is reduced. It is an object of the present invention to provide a method for treating a hydrogen storage alloy powder, which can suppress an increase in temperature, suppress a decrease in battery voltage when left to discharge at high temperature, and achieve an improvement in capacity recovery rate after voltage decrease. In particular, it is intended to provide an effective treatment method when applied to a hydrogen storage alloy powder containing a Co component.

[0011]

In order to achieve the above-mentioned object, in the present invention, the hydrogen storage alloy powder is treated by cathodic polarization in a hot alkaline solution. There is provided a method, in particular, a treatment method in which the hydrogen storage alloy contains a Co component, and the cathodic polarization is performed at a base potential lower than -0.8 V with respect to a Hg / HgO single electrode potential.

The processing method of the present invention will be described below in detail with reference to the drawings. In FIG. 1, a hydrogen storage alloy powder 2 to be treated is stored in a treatment tank 1 together with an alkaline solution 3. Then, an electrode 4 such as a foamed nickel plate is embedded in the hydrogen storage alloy powder 2, and an electrode 5 such as a nickel plate is embedded in the alkaline solution 3 so as not to come into contact with the alloy powder 2. And the electrodes 4 and 5 are connected to the negative and positive poles of the power source 6, respectively, and the electrode 4 is set as the cathode and the electrode 5 is set as the anode.

Then, the treatment tank 1 is heated by a heating means 7 such as a burner or an electric heater to turn the alkaline solution 3 into a hot alkaline solution having a predetermined temperature, and the electrodes 4 and 5 are energized.
Here, the alkaline solution 3 is not particularly limited, and examples thereof include potassium hydroxide aqueous solution and sodium hydroxide aqueous solution. In particular, it is preferable to use the same type and concentration as the alkaline electrolyte used in the battery to be assembled.

The temperature of the alkaline solution 3 is usually 30 to 1
It is preferably set to 00 ° C. This is because when the temperature is lower than 30 ° C., the elution of the segregated components does not proceed promptly and the processing time becomes long. Further, if the temperature is higher than 100 ° C., the evaporation of the alkaline solution will proceed rapidly. In addition, the reaction progresses rapidly and its control becomes difficult.

By maintaining a predetermined potential between the electrodes 4 and 5 and conducting electricity, the alloy powder 2 becomes a cathode as a whole and its cathode polarization proceeds. As a result, alloy powder 2
The segregation component contained in (1) is forced to be ionized in addition to the elution action by the hot alkaline solution, so that it is surely and quickly eluted from the alloy powder 2. At this time, the voltage applied to the electrodes 4 and 5 is set in relation to the polarization potential of the segregated component to be eluted, but it is set to a potential nobler than the polarization potential of the segregated component to promote the elution of the segregated component. It is preferable that

In the present invention, Co is used as the hydrogen storage alloy.
When treating a material containing a component, in order to prevent the elution of the Co component, which has an inhibitory effect on the rise in the internal pressure of the battery, the polarization potential of Co, that is, -0.8 V with respect to the Hg / HgO unipolar potential is lower than It is preferable to treat at a different potential. This is because a useful Co component will be eluted if treated with a potential nobler than -0.8V.

However, if the potential is made lower than -0.95 V, hydrogen gas will be generated in the processing system and the elution of other segregated components will be disturbed. In the treatment of the alloy powder containing the components, it is preferable to advance the cathode polarization within the range of -0.8 to -0.95 V (value for Hg / HgO single electrode potential).

[0018]

Examples of the invention

In Examples 1-9, Comparative Examples 1 to 10 arc melting method, composition: MmNi _3.4 Co _1.0 Mn _0.4
After producing a hydrogen storage alloy represented by Al _0.3 , this was pulverized into a powder under 150 mesh (Tyler sieve).

Then, as shown in FIG. 1, this powder was inserted into a mixed alkaline aqueous solution of potassium hydroxide and lithium hydroxide (specific gravity: 1.37), and Hg / Hg / Cathodic polarization was performed at -0.85 V with respect to the HgO single electrode potential. During this process, the state of the alkaline aqueous solution was observed. A hydrogen storage alloy electrode is manufactured using the obtained treated powder, and the same mixed alkaline aqueous solution as that described above is used as the alkaline electrolyte, and the rated AA size is 1100.
A mAh nickel-hydrogen secondary battery was assembled.

Each of these batteries was subjected to 450% overcharge at 1 C at a temperature of 20 ° C., and the internal pressure of the battery at this time was measured. The results are shown in Table 1. Further, each battery was placed in a discharged state and left in a constant temperature bath at a temperature of 80 ° C., and the time until the battery voltage dropped to 1.0 V was measured. The results are shown in Table 1. Furthermore, after continuing to discharge each battery and lowering the open voltage to 0.6V, the batteries are overcharged under the above conditions and the battery capacity is measured.
The capacity recovery rate was calculated based on the value. The results are shown in Table 1.

For comparison, a battery similar to that of the example was assembled using the powder immediately after crushing (comparative example 1), and its characteristics were examined in the same manner as the example. Further, in Examples 1 to 9, without performing cathodic polarization, at the temperature shown, the time shown,
For alloy powder that has just been treated with a hot alkaline solution,
The same battery as the example was assembled and the battery characteristics were examined. At this time, the state of the alkaline aqueous solution was observed. The results are shown in Table 1.

[0022]

[Table 1]

The following is clear from Table 1. (1) Alloy powders of the present invention that were subjected to cathodic polarization treatment in a hot alkaline solution (Examples 1 to 9) and alloy powders of comparative examples that were not subjected to cathodic polarization and were only subjected to hot alkaline treatment (Comparative Examples 2 to 2). 10) means that in comparison with the alloy powder immediately after crushing (Comparative Example 1), suppression of battery internal pressure during overcharge,
It is effective in suppressing the drop in battery voltage and improving the capacity recovery rate when it is left to discharge at a temperature of 80 ° C.

(2) Alloy powders of Examples 1 to 9 and Comparative Example 2
Comparing the alloy powders of No. 10 with each other, the alloy powders of the examples subjected to the cathodic polarization treatment have an extremely effective effect. This is because in Comparative Examples 2 to 10, the color of the alkaline solution turned blue and the elution of the Co component was observed, but in the case of the present invention, the color of the alkaline solution did not change and the elution of the Co component was observed. This is because the cathode polarization treatment prevents the elution of the Co component, which proves the usefulness of the cathode polarization treatment.

(3) Alloy powders of Examples 1 to 9 and Comparative Example 2
With all of the 10 alloy powders, when the treatment time exceeds 2 hours, the effect exhibited is almost saturated. Therefore, considering economic problems, the processing time is 2
It may be about 3 hours.

[0026]

As is apparent from the above description, when the treatment method of the present invention is applied to the powder of hydrogen storage alloy, the segregation component in the alloy can be eluted and removed very effectively. When a hydrogen storage alloy electrode is manufactured using the obtained treated powder, and an alkaline secondary battery is assembled by incorporating the electrode,
The increase in the battery internal pressure during overcharging of the battery is suppressed, the decrease in the battery voltage when left to discharge at high temperature is suppressed, and the capacity recovery rate when the battery is overcharged after being discharged is also improved. To do.

Particularly, in the case of the method of claim 2, the object to be treated is Co which exhibits an effect of suppressing an increase in the internal pressure of the battery.
In the case of the hydrogen storage alloy powder containing the components, the cathodic polarization is carried out at a potential lower than −0.8 V with respect to the Hg / HgO single electrode potential, so that it is eluted and removed by the treatment only with the hot alkaline solution. This is effective because the elution of the Co component can be selectively suppressed and the elution and removal of other segregated components can be promoted.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a processing system when carrying out the method of the present invention.

[Explanation of Codes] 1 treatment tank 2 hydrogen storage alloy powder 3 hot alkaline solution 4 cathode plate 5 anode plate 6 power supply 7 heating means

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[Procedure amendment]

[Submission date] August 10, 1995

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 1

[Correction method] Change

[Correction content]

FIG.

Claims (2)

[Claims] 1. A method for treating a hydrogen storage alloy powder, which comprises subjecting the hydrogen storage alloy powder to cathodic polarization in a hot alkaline solution. 2. The hydrogen storage alloy contains a Co component,
The cathode polarization is -0.8 with respect to the Hg / HgO single electrode potential.
The method for treating a hydrogen storage alloy powder according to claim 1, which is carried out at a potential lower than V.