JP2000182612A - Manufacture of hydrogen storage alloy powder and alkaline secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of hydrogen storage alloy powder exhibiting higher initial activity, higher capacity, higher utilization factor and longer service life than the alloy powder made by a conventional method. SOLUTION: After a molten alloy is cooled at a speed not to cause component segregation in the alloy or a speed so that alloy is formed with polyphase dispersed as uniformly as possible even if the polyphase is easily deposited under an inert atmosphere to form the alloy, the alloy is heat-treated under a reduced pressure or in an inert atmosphere such as argon gas. Hydrogen is held by the alloy at a low temperature in a closed vessel. After alloy powder is provided by grinding the alloy by releasing the hydrogen from the alloy at high temperature or by a mechanical grinding method, the alloy powder is classified to a desired grading distribution under the inert atmosphere. An objective alloy powder can be provided by heat-treating the alloy powder individually or with fine particles of rare earth oxide or rare earth hydroxide addingly mixed into the alloy under reduced pressure or inert atmosphere such as argon gas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen storage alloy and an alkaline secondary battery using the same as a negative electrode active material, and more particularly, to an initial activity, capacity and utilization when used as a negative electrode material of an alkaline secondary battery. It relates to an alloy powder having a high rate and a long life.

[0002]

2. Description of the Related Art The secondary batteries used in electronic information devices have been reduced in size and weight for the purpose of reducing the size and weight of electronic information equipment, reducing the weight and increasing the output of electric power tools, and increasing the performance of electric vehicles. Higher performance is required, and as a result, performance improvement mainly with high initial activity, high capacity, high utilization, and long life is essential. However, in the secondary batteries used therefor, particularly in the alkaline secondary batteries which have come to be widely used, the initial activity, capacity, and utilization factor depend on the characteristics of the hydrogen storage alloy which is the negative electrode active material of the constituent members. And long life are in conflict with each other, and various attempts have been made on alloy composition, alloy production methods, and the like in order to make these compatible, but no satisfactory one has been obtained.

For example, in improving the alloy composition,
Case of requiring a high capacity and high initial activity, hydrogen easily absorbing element, for example in such AB ₅ -type hydrogen absorbing alloy but generally that considered in the direction of increasing the amount of rare earth elements is carried out, easily absorb hydrogen Since the elements are easily corroded by the alkaline aqueous solution used in the battery electrolyte, the life of the alloy is shortened, resulting in a short-life battery. Also,
Conversely, when extending the life, a method is adopted to reduce the corrosion of the alloy by the alkaline electrolyte by reducing the elements that easily absorb hydrogen, but this results in a reduction in the hydrogen storage capacity of the alloy. Battery capacity will be reduced. Therefore, it is difficult to satisfy all the requirements in the examination of the composition at the present stage, and it is necessary to specialize the performance and satisfy only the specific requirements. In addition, due to the manufacturing method, when the composition extremely deviates from the stoichiometric ratio, or when an element that is difficult to alloy is added, composition segregation occurs in the alloy, so that the originally intended alloy is obtained. And there is also a problem that the composition is restricted by the alloyable composition.

[0004] Therefore, in order to reduce the improvement limits and restrictions on these compositions, improvements have been made in the alloy manufacturing method. In order to reduce the composition segregation inside the alloy as much as possible and to extend the life, there is a need to mechanically pulverize the alloy. Methods for improving the initial activity and other properties by modifying the surface of the obtained alloy powder are generally studied, and although showing reasonable results, it is still not enough Was out. This is because heat treatment of the alloy ingot is generally performed to reduce the composition segregation of the alloy, but this can reduce the macroscopic segregation, but the particle diameter is several μm to 50 μm. In general, the particle diameter used for battery electrodes is in the range of several μm to 200 μm because the segregation of fine regions cannot be sufficiently improved. In this case, the battery life was not improved, and it was not a sufficient solution.

For this reason, Japanese Patent Publication No. 6-93358 and Japanese Patent Publication
As disclosed in JP-63007, a method of rapidly cooling the molten alloy and a surface treatment of the alloy powder with an alkaline aqueous solution are also being studied as methods for reducing the microsegregation and extending the life, but they are one of the alloy components. Because of the elution of the part, the volume per alloy volume and weight per unit weight is reduced, and there is a side that goes against the improvement of the utilization rate of the alloy. In addition, in these surface treatments, a large amount of high-concentration wastewater containing various metal ions is generated. Therefore, processing equipment and cost for such wastewater treatment are required, and particularly in recent years, cost reduction is desired. In practice, it is very problematic.

[0006]

Therefore, an alloy powder exhibiting a higher initial activity, a higher capacity, a higher utilization factor and a longer life than the alloy powder produced by the conventional method can be obtained without being restricted by the alloy composition as much as possible. The present invention provides a method for producing a hydrogen storage alloy powder which can be obtained and hardly generates waste such as surface treatment waste liquid during production of the alloy powder, and is extremely useful in practical use.

[0007]

According to the present invention, an alloy is formed in a state in which these components are dispersed as uniformly as possible even when a speed that does not cause compositional segregation or a multiphase is likely to precipitate in an alloy under an inert atmosphere. After cooling the molten alloy at such a speed to form an alloy, heat-treat the alloy under reduced pressure or an inert atmosphere such as argon gas, and then store hydrogen in the alloy at a low temperature in a closed container, Further, alloy powder is obtained by a method of pulverizing the alloy by releasing hydrogen from the alloy at a high temperature or a mechanical pulverizing method, and then classified into a desired particle size distribution under an inert atmosphere, under reduced pressure or argon gas. Under an inert atmosphere such as that described above, heat treatment may be performed by adding and mixing alloy powder alone or, in some cases, fine particles of rare earth oxide or rare earth hydroxide. Obtain an alloy powder.

That is, by quenching the molten alloy, alloy segregation is reduced microscopically, and an alloy having fine crystal grains is formed, or an alloy composition whose composition ratio is extremely shifted from the stoichiometric ratio is obtained. And the formation of an alloy in which the polyphases generated when elements that are difficult to alloy are added are dispersed as evenly as possible, followed by heat treatment to release the strain that occurs when the multiphases appear inside the alloy, thereby forming a crystal lattice. After further reducing the compositional segregation that has been forcedly precipitated, the alloy is pulverized along the crystal grain interface by absorbing and releasing hydrogen, and the individual crystal grain interface, in other words, the alloy particle surface The surface of the alloy, in which hydrogen atoms are easily diffused, can be separated without eluting alloy components by performing dry surface modification by heat treatment of the alloy. By characteristics are processed into a homogeneous alloy particles, high initial activity, high capacity, high utilization, long life can be realized.

Further, rare earth oxides or rare earth hydroxides are added to and mixed with the alloy powder, and the rare earth oxides or the rare earth hydroxides are dispersed on the surface of the alloy particles. Since the substance acts to suppress the elution of the alloy particle component with respect to the alkaline electrolyte, an even longer life can be realized.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The method for each step of the present invention will be described. The alloy composition is not particularly limited, and AB _n (n
Represents a number of 1 to 6. A hydrogen storage alloy having a composition having a crystal structure of (a) can be used. Each element is introduced so as to have an alloy composition suitable for the purpose, and the raw material is melted by a known melting method such as high-frequency melting or arc melting. Thereafter, the alloy melt may be rapidly cooled to produce an alloy.

With regard to the rapid cooling of the molten alloy, the molten alloy can be rapidly solidified by an atomizing method, a strip casting method or the like in an atmosphere of an inert gas such as argon. A casting method, in particular, a cooling method using a rotating single roll is preferable. The cooling conditions vary depending on the alloy composition, but while the molten alloy exists on the cooling roll, a single phase is formed, or even when forming a multi-phase, cooling can be performed until these multi-phases are uniformly precipitated. The melt temperature, melt supply amount, roll temperature, roll peripheral speed, and atmospheric pressure may be determined.

The heat treatment before the pulverization may be carried out at 800 to 1100 ° C. for 4 to 20 hours in the same manner as in the heat treatment of the alloy ingot. Since it is necessary to match them and the shape is such that the effect of the heat treatment easily appears, 4 to 10 at 800 to 1000 ° C. in an inert gas atmosphere such as argon.
It is also possible to change to conditions with a mild time.

In the pulverization of the alloy, the particle diameter is 0.5 to 200 μm.
In order to meet the purpose, timely classification is performed so as to be in the range of about m, for example, a mechanical pulverization method such as a ball mill or a pin mill under an inert gas atmosphere such as argon or nitrogen, or a pulverization method by absorbing and releasing hydrogen. The alloy may be pulverized by the method described above. In the pulverization method using hydrogen, the hydrogen storage alloy causes a volume change in the process of storing and releasing hydrogen, and the alloy is pulverized by utilizing a change in strain generated with the hydrogen storage alloy. Efficient pulverization can be performed by increasing the rate of change in strain, so conditions that take advantage of the general properties of hydrogen-absorbing alloys that generally absorb more hydrogen at low temperatures and release more hydrogen at high temperatures It is desirable to perform the grinding underneath. Because these conditions vary greatly depending on the composition of the alloy, it can not be said sweepingly, for example, in the AB ₅ type hydrogen storage alloy used in the alkaline secondary battery, to occlude hydrogen at about 0 to 30 ° C. in particular from about 10 ° C. , 40-70 ° C., especially about 60 ° C., it is possible to perform efficient pulverization from the viewpoint of cost on a mass production scale.

[0014] Classification of the alloy powder can be performed in the air, but from the viewpoints of oxidation of the alloy particles during classification and deterioration of various characteristics and prevention of fire, and under an inert atmosphere, the alloy powder is classified. It is preferred to do so.

Further, in the heat treatment of the pulverized alloy powder,
Unlike the conventional case of heat-treating an alloy lump, since the heat conductivity of the alloy powder is lower than that of the alloy lump, heat treatment spots are likely to occur. Therefore, a method for preventing this is necessary. For example, a rotary kiln that stores alloy powder in a container and heats it while rotating it, or a method in which alloy powder is spread thinly in a pad-shaped container and stored and sent to a processing furnace in sequence to heat it, A method capable of uniformly or batchwise heat-treating under reduced pressure or an inert atmosphere such as argon gas may be used. The processing conditions may be selected temperature and time characteristics in accordance with the composition of the alloy is highest, for example AB ₅ type hydrogen storage alloy, 100 to 1000 ° C. (preferably 25
(0 to 950 ° C.) for about 0.5 to 4 hours.

In addition, when a rare earth oxide or a rare earth hydroxide is added to and mixed with the alloy powder, the alloy should be dispersed so that the rare earth oxide or the rare earth hydroxide can be scattered as uniformly as possible on the surface of the alloy powder. The particle size ratio and mixing ratio between the powder and the rare earth oxide or the rare earth hydroxide are adjusted, and this is also mixed under an inert atmosphere from the viewpoint of preventing the deterioration of the alloy particle characteristics and preventing fire as in the case of the heat treatment. The device used for mixing is not particularly limited as long as it has a function of mixing under an inert atmosphere. After the rare earth oxide or rare earth hydroxide powder is added and mixed as described above, the above heat treatment is performed.

The rare earth used for the rare earth oxide or the rare earth hydroxide is not particularly limited as long as the oxide and hydroxide cause the above-mentioned protective action on the alloy surface, but La, Ce, Pr, Nd, Gd, Dy, Er, T
m, Yb, Lu, Y, etc., among which Gd, D
Rare earth oxides or hydroxides such as y, Er and Yb are preferred. The amount of the rare earth oxide or the rare earth hydroxide or the rare earth hydroxide is 0.1 to
10% by weight, preferably 0.3 to 2% by weight, may be used alone or in combination.

In the present invention, the thus treated alloy powder is pasted using a binder such as polyvinyl alcohol, carboxymethylcellulose, methylcellulose, PTFE, or polyethylene oxide to form a three-dimensional conductive support such as a nickel foam or a nickel fiber. A negative electrode can be easily obtained by filling and coating a two-dimensional conductive support such as a body or a punched metal. The binder is preferably used in an amount of 0.1 to 20 parts by weight with respect to the alloy. The negative electrode thus obtained is housed in a closed container together with a nickel positive electrode, an alkaline electrolyte, a separator such as PP, and the like, and can be manufactured as a battery.

[0019]

EXAMPLES Example 1 The alloy composition was R _1.0 Ni _4.4 Co _0.2 Mn _0.3 Al in atomic ratio.
_The alloy material was subjected to high-frequency melting in an argon atmosphere so as to obtain _0.3 (R is composed of 80% by weight of La and 20% by weight of Ce). The alloy was formed by supplying it at about 150 g / sec on a water-cooled copper roll rotating at sec., cooled to room temperature and taken out (formation of an alloy ribbon), and then heat-treated at 900 ° C. for 6 hours in argon gas. And then put it in a closed container and once depressurize the container to remove air, moisture, etc.
After occlusion of hydrogen at 0 ° C. and a pressure of 0.9 MPa, the alloy was pulverized by releasing hydrogen up to a pressure of 200 Pa at a temperature of 60 ° C., and then classified in a nitrogen gas so as to have an average particle diameter of 23 μm. This at 400 ° C in argon gas
Heat treatment was performed for 1 hour to obtain the desired alloy powder.

For 0.5 g of the obtained alloy powder, 1.5 g
g of nickel fine powder (Inco # 255) was added and mixed well, and then pressure-molded on a nickel mesh (# 100) to which a current collecting lead was welded to form a φ10 mm pellet electrode, and a polypropylene separator, An open battery was constructed by combining with a sintered nickel positive electrode, an 8N-KOH electrolyte, and a Hg / HgO reference electrode. Then, at a temperature of 25 ° C., charging was performed at 50 mA / g for 3.8 hours, charging was suspended for 30 minutes, and then the battery was discharged at 50 mA / g to a potential of −0.7 V with respect to a reference electrode. Approximately 25 cycles of the test were performed, and the discharge capacity of the alloy was calculated from the average of the discharge capacities obtained for the top three discharge times of the longer discharge time. evaluated.

Further, 1 g of the alloy powder obtained by the above method is used.
And 0.25 g of a 3% by weight aqueous PVA solution were added and mixed, and a nickel lead for current collection was applied to a spot-welded fiber nickel substrate, dried under reduced pressure, and the substrate was sandwiched with fiber nickel. A paste electrode (negative electrode) was prepared by using a press to maintain the pressure at about 560 kgf / cm ² for 1 minute. The prepared electrode (negative electrode) is wrapped with a polypropylene separator, sandwiched on both sides by a sintered nickel positive electrode, placed in a polypropylene container together with a Hg / HgO reference electrode, and poured with 8N-KOH electrolyte to open the electrode. The battery was assembled. The open-type battery thus prepared was charged at a discharge capacity of 1 C and a temperature of 2C.
At 5 ° C, charge was performed at 0.3 C for 4 hours, discharge pause for 30 minutes, and then discharged at 0.3 C to a potential of -0.7 V with respect to the reference electrode. And the life as an electrode was evaluated (FIG. 1).

Examples 2 to 4 An alloy ribbon was prepared and heat-treated in the same manner as in Example 1. After the alloy was placed in a closed container, the inside of the container was once depressurized to remove air, moisture, etc. ° C, pressure 0.9
After absorbing hydrogen at MPa, the alloy was pulverized by releasing hydrogen at a temperature of 60 ° C. and a pressure of 200 Pa. Then, after classification in nitrogen gas so that the average particle diameter becomes 23 μm, the obtained alloy powder is mixed with the rare earth oxide and hydroxide powder in the combination and weight% (weight% with respect to the alloy) as shown in Table 1. Was added and mixed in a nitrogen gas for 10 minutes using a W cone-type blender, and further heat-treated at 400 ° C. for 1 hour in an argon gas to obtain a target alloy powder. Next, a battery was prepared in the same manner as in Example 1, and the capacity, initial activity, and life were measured.

Example 5 An alloy ribbon was prepared and heat-treated in the same manner as in Example 1, and the alloy was pulverized using a pin mill under nitrogen gas. Then, the average particle size was adjusted to 23 μm in nitrogen gas. This was classified and subjected to a heat treatment at 400 ° C. for 1 hour in an argon gas to obtain a target alloy powder. Then, Example 1
A battery was prepared in the same manner as described above, and the capacity, initial activity, and life were measured.

Examples 6 to 8 Alloy powders were prepared in the same manner as in Example 3, and then classified in a nitrogen gas so that the average particle diameter became 23 μm. Rare earth oxide and hydroxide powders were added to the obtained alloy powder in combinations and weight% (weight% based on the alloy) as shown in Table 1 and mixed in a nitrogen gas for 10 minutes using a cone-type blender. After that, this is
Heat treatment was performed at 1 ° C. for 1 hour to obtain a target alloy powder.
Next, a battery was prepared in the same manner as in Example 1, and the capacity, initial activity, and life were measured.

COMPARATIVE EXAMPLE 1 The alloy composition is represented by an atomic ratio of R _1.0 Ni _4.4 Co _0.2 Mn _0.3 Al
_The alloy raw material was subjected to high frequency melting in an argon atmosphere so as to obtain _0.3 (R is composed of 80% by weight of La and 20% by weight of Ce). After pouring into a mold to form an alloy, cooling to room temperature and taking out, the alloy lump is placed in a closed container, and the inside of the container is once depressurized to remove air, moisture, etc., and then 1050 ° C. in an argon gas atmosphere. Temperature, 10 as it is
After heating and holding for a time, the alloy was pulverized using a pin mill under nitrogen gas using a pin mill under nitrogen gas, and coarse powder was removed using a 32-mesh sieve in nitrogen gas to obtain an alloy powder. This alloy powder was immersed in 2 liters of an 8N-KOH aqueous solution per 1 kg of the alloy powder, stirred at 80 ° C. for 60 minutes, filtered under pressure, and then the pH of the washing solution was adjusted using pure water.
Wash the alloy powder until it is 7-8. The washed alloy powder was filtered under pressure, dried under reduced pressure, and classified in a nitrogen gas so as to have an average particle size of 23 μm to obtain a target comparative alloy powder. Next, a battery was prepared in the same manner as in Example 1, and the capacity, initial activity, and life were measured.

Table 1 shows the discharge capacity at the pellet electrode and the number of cycles until the discharge capacity is reached, alone and in combination of the alloy powder and the rare earth oxide and hydroxide added to the alloy powder. FIG. 1 shows the initial characteristics / utilization rate and life of the paste electrode.

[Table 1]

Referring to Table 1 which summarizes the results of the above Examples and Comparative Examples and FIG. 1, when comparing the discharge capacities at the pellet electrodes in Table 1, each Example shows a higher capacity than the Comparative Example. This shows that the utilization rate is improved, the number of cycles required to reach the maximum discharge capacity is shorter than in the comparative example, and the initial activity is high. Further, comparing the lifetimes of the paste electrodes in FIG. 1, it can be seen that all of the examples have a long lifetime. FIG.
In Example 2 to make the graph easier to see,
In Example 4, Example 3 is described, and in Examples 6 to 8, Example 7 is described as a representative example.

[0028]

According to the present invention, an alloy is formed by a quenching method, heat-treated, and then pulverized, and then heat-treated under reduced pressure or an inert atmosphere in a state in which alloy powder alone or a rare earth oxide or a rare earth hydroxide is added and mixed. By doing so, when used as a negative electrode material of an alkaline secondary battery, an alloy powder having a high initial activity, a high capacity, a high utilization factor, and a long life can be obtained. The effects obtained by this method can be adapted to various alloy compositions, and so have been developed but have not been put to practical use.For example, Laves, whose theoretical capacity is high but the desired capacity was not actually obtained, was obtained. It is considered that application to a composition or a composition extremely deviating from the stoichiometric ratio greatly contributes to improvement of the characteristics of the secondary battery.

[Brief description of the drawings]

FIG. 1 is a view showing initial characteristics / utilization rate and life of a paste electrode.

Claims (4)

[Claims] 1. A step of quenching a molten hydrogen storage alloy in an inert atmosphere to form an alloy, a first heat treatment step of heat-treating the obtained alloy under reduced pressure or an inert atmosphere, and A method for producing a hydrogen storage alloy powder, wherein an alloy powder is obtained by a pulverizing step of obtaining a powder by pulverizing, and a second heat treatment step of heat-treating the pulverized alloy powder under reduced pressure or an inert atmosphere. 2. The method according to claim 1, wherein the pulverizing step comprises:
The method for producing a hydrogen storage alloy powder according to claim 1, wherein the method is a step of obtaining a powder by crushing by absorbing and releasing hydrogen. 3. The second heat treatment step is a step of subjecting the alloy powder obtained by the pulverization to a heat treatment under reduced pressure or an inert atmosphere in a state where a rare earth oxide or a rare earth hydroxide is added thereto and mixed. Item 3. The method for producing a hydrogen storage alloy powder according to Item 1 or 2. 4. An alkaline secondary battery using the alloy powder obtained according to claim 1 as a negative electrode active material.