JPH0831414A - Hydrogen storage alloy for alkaline storage battery and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain the hydrogen storage alloy for alkaline storage battery, which can solve the contrary themes of the improvement of hydrogen absorbing ability and the improvement of corrosion resistance, and to obtain the manufacture of this hydrogen storage alloy. CONSTITUTION:Composition formula of the hydrogen storage alloy for alkaline storage battery is expressed by MmNiaCobMncAld (Mm means the mixture of rare earth group elements, and a, b, c, d respectively mean the number satisfying 2.5<=a<=3.8, 0.2<=b<=1.0, 0.2<=c<=0.8, 0.1<=d<=0.5, and 3.8<=a+b+c+d<=4.85), and dendrite cell size thereof is 8mum or less. Mm (misch metal) includes La at 20% or more and 50% or less by weight to the total weight of the rare earth group elements.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a hydrogen storage alloy,
More specifically, it relates to an improvement of a hydrogen storage alloy used as an electrode active material of a metal / hydride alkaline storage battery, and a method for producing such a hydrogen storage alloy.

[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. In recent years, these storage batteries can be replaced by lighter weight and higher capacity. Metal-hydride alkaline storage batteries are becoming popular.
In this metal-hydride 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. The electrode is configured as a positive electrode.

By the way, as a hydrogen storage alloy used in this type of battery, a rare earth-based (MmNi-based) hydrogen storage alloy having a CaCu ₅ type hexagonal structure represented by LaNi ₅ is used. Alloys are Mm and Ni
When the stoichiometric ratio with the system element is 1: 5, a CaCu ₅ type hexagonal structure with a three-dimensional ordered array can be formed, and hydrogen can be occluded and released in this structure. There was a problem that the storage capacity was not sufficient. Therefore, various attempts have heretofore been made to improve the hydrogen storage / release characteristics of this type of alloy. Against this background, Japanese Patent Laid-Open No. 2-277737 discloses a technique of increasing the hydrogen storage capacity of an MmNi-based alloy by making _x of MmNi _x smaller than 5 (having a non-stoichiometric composition). Proposed.

This technique is intended to further increase the hydrogen storage capacity of the alloy by causing a partial disorder (strain) in the crystal structure of the alloy. The discharge equilibrium pressure can be made sufficiently low, and therefore the discharge capacity can be increased when batteries are constructed using such alloys.

[0005]

However, although the above technique can improve the discharge capacity of the battery,
There has been a problem that the discharge capacity and the cycle characteristics, which are important performance factors of the secondary battery, deteriorate. The reason is as follows. The composition formula is A ₁ where Mm of the MmNi-based hydrogen storage alloy is A, and other elements (such as Ni) are B.
When expressed as B _x , if a hydrogen storage alloy with x smaller than 5 is formed according to the above technique, CaCu ₅ having a strain in the crystal structure
A hydrogen storage alloy having a hexagonal crystal structure is obtained, and the strained hydrogen storage alloy has a space that is convenient for storing and releasing hydrogen, and is therefore suitable as a hydrogen storage alloy for alkaline storage batteries. However, Ca with such distortion
Since the Cu ₅ type hexagonal structure lacks stability, if excess Mm (mixture of rare earth elements) is present in the molten metal in excess of the stoichiometric ratio in the solidification process of the alloy, excess Mm is generated from the parent phase. Excludes Mm, more stable CaC with less distortion
The phenomenon of attempting to adopt the u ₅ type hexagonal crystal structure occurs. Here, making x smaller than 5 is equivalent to M for Ni-based elements.
Since m is excessively present, a hydrogen storage alloy having an inhomogeneous alloy structure in which the specific component is segregated is inevitable, and the segregation of the specific component causes deterioration of cycle characteristics.

To further explain this, when x is smaller than 5, excess Mm is repelled in the alloy solidification process, resulting in a mother phase having a composition close to the stoichiometric ratio (1: 5) and a rare earth rich element. A second phase is possible. This rare earth-rich second phase does not contribute to the storage and release of hydrogen and makes the alloy fragile. Such a hydrogen storage alloy easily cracks due to alloy cracking due to expansion and contraction of the alloy accompanying storage and release of hydrogen. Since the refinement of the alloy causes an increase in the specific surface area,
The surface of the alloy is corroded by the alkaline electrolyte, and the hydrogen storage capacity of the alloy gradually decreases.

That is, the above-mentioned technique (a hydrogen storage alloy having a non-stoichiometric composition) can improve the hydrogen storage / release capacity by causing alloy strain, but deteriorates the corrosion resistance to the alkaline electrolyte. Therefore, there is a problem that cycle characteristics are deteriorated. The present invention has been made for the purpose of solving such a problem, and it is possible to solve the contradictory problems of improving the hydrogen storage capacity and the corrosion resistance, and a hydrogen storage alloy for alkaline storage batteries, and the like. An object of the present invention is to provide a method for producing a simple hydrogen storage alloy.

[0008]

In order to solve the above problems, the present invention is constructed as follows. The invention of claim 1 has a composition formula of MmNi _a Co _b Mn _c Al _d (however,
Mm is a mixture of rare earth elements, and _a , _b , _c , _d are 2.5 ≦ _a ≦ 3.8, 0.2 ≦ _b ≦ 1.0, respectively.
0.2 ≤ _c ≤ 0.8, 0.1 ≤ _d ≤ 0.5,
It represents a number of 3.8 ≦ _a + _b + _c + _d ≦ 4.85. ) And the dendrite cell size is 8μ
It is characterized in that it is a hydrogen storage alloy for alkaline storage batteries having a m or less.

According to a second aspect of the present invention, in the hydrogen storage alloy for alkaline storage batteries according to the first aspect, the Mm, which is a mixture of rare earth elements, has a La content of 20% by weight or more based on the total weight of the rare earth elements. It is characterized by being less than or equal to wt%. According to a third aspect of the present invention, at least a first step of melting each element composing the hydrogen storage alloy, and a second step of cooling the melt melted in the first step at a cooling rate of 1000 ° C./sec or more. The method for producing a hydrogen storage alloy for an alkaline storage battery, which comprises the step of.

[0010]

According to the invention of claim 1, the element as a chemical characteristic
Elemental composition is MmNi_aCo_bMn _cAl_d(However, 2.5
≤_a ≤ 3.8, 0.2 ≤_b ≦ 1.0, 0.2 ≦_c≤
0.8, 0.1 ≦_d ≦ 0.5, a + b + c + d 3.
8 ≦_a +_b +_c+_d ≦ 4.85), and
The dendrite cell size as a physical property is 8 μm or less
The hydrogen storage alloy for alkaline storage batteries is

In such a hydrogen storage alloy, the components constituting the chemical composition suitably act to synergistically act to enhance the discharge characteristics and the cycle characteristics. Furthermore,
Physical properties such as a dendrite cell size of 8 μm or less are given to those having such a chemical composition,
The discharge capacity and the cycle characteristics are further improved, and especially the cycle characteristics are remarkably improved.

Here, the dendrite cell size is 8 μm.
Although the reason why the discharge capacity and the cycle characteristics are significantly improved is not clear in the following, it can be considered as follows. Generally, Mm and N of MmNi-based hydrogen storage alloy
The stoichiometric ratio of the i-type element is considered to be 1: 5, and at this ratio, CaCu ₅ having no defect in elemental arrangement.
Form a hexagonal crystal structure. However, in the alloy according to the present invention, the ratio is 1: [3.8 to 4.85], and in the alloy having such a composition having a non-stoichiometric ratio, as described above, excess alloy is generated in the alloy formation process. Since a phenomenon occurs in which Mm is excluded and an alloy having a composition close to the stoichiometric ratio (1: 5) is produced, a hydrogen storage alloy having an alloy structure in which specific components are segregated is apt to be formed. Here, when the alloy ingot is produced, if the cooling rate is set to 1000 ° C./sec or more and the molten alloy is rapidly cooled, the movement of each composition in the molten alloy is suppressed, so that segregation of excess Mm is prevented. It Therefore, it is possible to form a CaCu ₅ type hexagonal structure having a favorable strain in terms of hydrogen absorption and desorption, and to obtain a hydrogen storage alloy with less segregation of Mm. It is considered that the hydrogen storage alloy having physical properties has a dendrite cell size of 8 μm or less. That is, a hydrogen storage alloy having a dendrite cell size of 8 μm or less and the above chemical composition has a convenient degree of strain, and the alloy structure is more uniform and strong, so that the discharge capacity is A hydrogen storage alloy for alkaline storage batteries, which is excellent and has excellent cycle characteristics, can be obtained.

According to a second aspect of the invention, in the hydrogen storage alloy for alkaline storage batteries according to the first aspect, Mm (mixture of rare earth elements) is contained at least in an amount of La of 20 to 50% by weight. In the hydrogen storage alloy using Mm, although the reason is not clear, the discharge capacity is excellent and the corrosion resistance is further improved. According to the third aspect of the invention, the molten alloy composition is cooled at a cooling rate of 1000 ° C./sec. However, when cooled at such a cooling rate, a hydrogen storage alloy having a dendrite cell size of 8 μm or less is obtained. You can

[0014]

The contents of the present invention will be described in detail below based on experiments. [Experiment 1] In Experiment 1, the dendrite cell size was measured for the alloy ingots produced by changing the cooling rate variously, and the relationship between the cooling rate of the molten alloy and the dendrite cell size was investigated. A test cell was prepared using this alloy ingot, and the effect of the dendrite cell size on the cell capacity and cycle characteristics was investigated. In the following, first, a method for producing a hydrogen storage alloy ingot, a method for measuring a dendrite cell size, a method for producing a test cell and a method for measuring cell characteristics will be described, and then the experimental results will be described.

(Preparation of hydrogen storage alloy) La 30%, Ce
Mm (Misch metal) which is a mixture of 45%, Pr7% and Nd18% (weight%), Ni, Co and Mn
And Al in the element ratio of 1: 3.2: 0.6: 0.4:
The mixture is mixed at a ratio of 0.3, and the mixture is melted in a high frequency melting furnace in an inert gas atmosphere to obtain a molten alloy. This molten metal was subjected to 10000 ° C./sec (Example A ₁ ) and 3000 ° C.
/ Sec (Example _{A 2), 1000 ℃ / sec} ( Example _{A 3), 100 ℃ / sec} ( Comparative Example B _1), 10 ℃ / s
ec (Comparative Example B ₂ ) was cooled at each cooling rate, and the compositional formula was represented by MmNi _3.2 Co _0.6 Mn _0.4 Al _0.3.
The same alloy ingots A _{1 to} A ₃ and B _{1 to} B ₂ (Table 1) were produced.

The alloy ingot is produced at 10 ° C./sec and 1
The alloy ingot of 00 ° C./sec was cast by the usual method of pouring the molten alloy into the mold, and the alloy ingot of cooling rates other than these was performed by the roll method. (Measurement of dendrite cell size) The dendrite cell size of each of the five alloy ingots produced above was measured as follows. That is, in the Mm-Ni-based hydrogen storage alloy ingot, the secondary branch of the solidification structure does not sufficiently develop, and the solidified state becomes a cell as shown in FIG.
Therefore, a line is drawn perpendicularly to the crystal growth direction, the number of boundaries of dendrite cells intersecting this line is counted, the average distance between dendrite cell boundaries is calculated, and this value is taken as the dendrite cell size (DCS). According to the method.

(Preparation of Test Cell) The various alloy ingots prepared above are mechanically crushed to obtain an average particle size of 50.
Various alloy powders having a diameter of .mu.m were produced, and 1.2 g of carbonyl nickel as a conductive agent and PTFE (polytetrafluoroethylene) powder as a binder of 0.
2 g was added and kneaded to prepare an alloy paste. Furthermore,
By wrapping this alloy paste in a nickel mesh and press-working, five types of hydrogen storage alloy electrodes differing only in dendrite cell size were prepared, and further using this electrode, a nickel electrode having a discharge capacity sufficiently larger than this electrode was formed. Using a 30 wt% KOH aqueous solution, FIG.
The test cells A _{1 to} A ₃ and B _{1 to} B _{2 shown} in FIG.

The structure of the test cell thus manufactured will be described with reference to FIG. In the figure, reference numeral 1 denotes a negative electrode, and the hydrogen storage alloy electrode formed in a cylindrical shape is installed on the negative electrode 1. Reference numeral 2 is a known sintered nickel electrode (positive electrode) manufactured by a known method. This sintered nickel electrode 2
It is held by a positive electrode lead 4 connected to the upper surface 5 of the hermetically sealed container 3, and the electrode 1 is placed on the upper surface 5 of the hermetically sealed container 3 such that the electrode 1 is vertically positioned in the substantially center of the cylinder of the sintered nickel electrode 2. It is held by the connected negative electrode lead 6. Further, each end of the positive electrode lead 4 and the negative electrode lead 6 penetrates the upper surface 5 of the closed container 3 and is exposed to the outside, and is connected to the positive electrode terminal 4a and the negative electrode terminal 6a, respectively. Then, a 30 wt% potassium hydroxide aqueous solution as an alkaline electrolyte is put in the closed container 3, and the hydrogen storage alloy electrode 1 and the sintered nickel electrode 2 are immersed in this solution. In addition, 7 is a pressure gauge, 8 is a relief pipe, and 9 is a gas relief valve.

(Measurement of Cell Characteristics) With respect to each of the test cells produced as described above, after charging at 50 mA per gram of alloy for 8 hours, discharging was performed at 50 mA until the cell voltage reached 1.0 V 100 times. Repeatedly, the percentage of the discharge capacity at the 100th cycle with respect to the maximum capacity C _max of the cell was obtained, and this percentage R ₁₀₀ % was used as the cycle characteristic. Cycle characteristic value R ₁₀₀ % = [C ₁₀₀ / Cmax] × 100 (Result of Experiment 1) Cooling rate (° C./sec) at the time of ingot production of various alloys, dendrite cell size (μm),
Table 1 shows a list of cycle characteristic values R ₁₀₀ %. Further, the relationship between the cooling rate and the dendrite cell size is shown in FIG.
The relationship between the dendrite cell size and the cycle characteristic value R ₁₀₀ % is shown in FIG. The horizontal axis of FIG. 1 is a logarithmic scale.

It can be seen from FIG. 1 that the dendrite cell size decreases hyperbolically in relation to the logarithmic value of the cooling rate when the cooling rate in the production of the alloy ingot is increased, and the cooling rate is 1000 ° C./sec. It can be seen that when it exceeds the limit, the decreasing tendency of the dendrite cell size slows down.
On the other hand, FIG. 2 shows that the graph line showing the relationship between the cycle characteristic value R ₁₀₀ % and the dendrite cell size has a refraction point at the dendrite cell size of 8 μm. From these results, it was clarified that the cycle life can be efficiently increased by using the hydrogen storage alloy having the dendrite cell size of 8 μm or less.

[0021]

[Table 1]

[Experiment 2] In Experiment 2, MmNi _a Co _b
A: b: c: d = 3.2: 0 in Mn _c Al _d .
6: 0.4: 0.3 and changing the value of a + b + c + d = x, alloy ingots of seven compositions were cooled at a cooling rate of 3000 ° C.
/ Sec (roll method), and in the same manner as in Experiment 1 described above, seven types of test cells were produced using each of these alloy ingots. With respect to these test cells, the effect of the difference in x on the discharge capacity at the first cycle was examined.

The experimental results are shown in FIG. The following is clear from FIG. When x is smaller than 5, the discharge capacity is remarkably increased and the high discharge capacity is maintained in the range of 4.85 to 3.8. On the other hand, when x becomes 3.8 or less, the discharge capacity sharply decreases again. From this, it is preferable that x satisfies 3.8 ≦ x ≦ 4.85.

Here, it is considered that the reason why the cell discharge capacity increases when x is 3.8 ≦ x ≦ 4.85 is as follows. That is, when x is set to 3.8 ≦ x ≦ 4.85, the Ni-based element is appropriately deficient with respect to Mm, so that the crystal structure is distorted and a space structure convenient for hydrogen storage is formed. To be done. On the other hand, when x is set to 5, which is a stoichiometric ratio, a suitable spatial structure like the former cannot be formed, and the hydrogen storage / release capacity is poor. Also,
It is considered that when x is 3.8 or less, the balance between Mm and the Ni-based element is greatly inclined to the excess of Mm, so that the proportion of the rare earth-rich phase that cannot contribute to hydrogen storage / release is relatively large.

[Experiment 3] In Experiment 3, MmNi _a Co _b
A + b + c + d = 4.5 (constant) in Mn _c Al _d
And b: c: d = 0.6: 0.4: 0.3
(Invariant), only a was changed, and the effect of the amount of nickel on the discharge capacity was examined. Incidentally, a method for producing an alloy ingot,
The test cell manufacturing method and the like are the same as in Experiment 2.

The experimental results are shown in FIG. The following is clear from FIG. When the amount of nickel (a) exceeds 3.8, or when the amount of nickel (a) is less than 2.5, the discharge capacity sharply decreases, and the amount of nickel (a) becomes 2. When it is in the range of 5 to 3.8, a high discharge capacity can be obtained. From these results, the amount of nickel is
2.5 ≦ a ≦ 3.8 is preferable.

Here, the reason why the discharge capacity sharply decreased when the nickel amount was less than 2.5 is considered to be because the electrode catalytic ability decreased, and the discharge capacity sharply decreased when the nickel amount was 3.8 or more. It is considered that the hydrogen equilibrium pressure increased and the hydrogen storage amount decreased. [Experiment 4] In Experiment 4, MmNi _a Co _b Mn _c Al _d
A + b + c + d = 4.5 (constant) and a: c: d = 3.2: 0.4: 0.3 (invariant), only b is changed, and the discharge capacity of the cobalt amount (b) is changed. And the effect of R ₁₀₀ % (cell capacity ratio after 100 cycles) was examined. The method for producing the alloy ingot and the method for producing the test cell are the same as in Experiment 2.

Experimental results are shown in FIGS. It can be seen from FIG. 5 that the discharge capacity sharply decreases when the cobalt amount (b) exceeds 1.0, and R ₁₀₀ % sharply decreases when the cobalt amount (b) is less than 0.2. I understand that From these results, the cobalt amount b is 0.2 ≦ b ≦
It became clear that 1.0 is preferable.

Here, when the cobalt amount (b) exceeds 1.0, it is considered that the discharge capacity sharply decreases because the plateau property of the PCT characteristic deteriorates and the hydrogen storage amount decreases. Further, it is considered that the reason why the R ₁₀₀ % sharply decreased when the amount of cobalt (b) was less than 0.2 is that the alloy activity decreased due to the corrosion of the alloy surface. The reason for this is that if the amount of cobalt compounded is reduced, the expansion of the alloy during hydrogen storage increases, and the expansion / contraction stress associated with charging / discharging increases. Therefore, in a hydrogen storage alloy containing a small amount of cobalt, alloy cracking occurs as the charging / discharging cycle progresses.
As a result, the specific surface area of the alloy increases, the corrosion resistance to the alkaline electrolyte decreases, and the alloy gradually corrodes and becomes inactive.

[Experiment 5] In Experiment 5, MmNi _a Co _b
A + b + c + d = 4.5 (constant) in Mn _c Al _d
And a: b: d = 3.2: 0.6: 0.3
(Invariant), changing only c, manganese amount (c)
The effect of discharge on discharge capacity was investigated. The method for producing the alloy ingot and the method for producing the test cell are the same as in Experiment 2.

The experimental results are shown in FIG. From FIG. 7, it is understood that when the manganese amount (c) is less than 0.2 or exceeds 0.8, the discharge capacity decreases, but the high discharge capacity is maintained at 0.2 to 0.8. From this,
The manganese amount c is preferably 0.2 ≦ c ≦ 0.8. Here, the reason why the discharge capacity decreased when the manganese amount (c) was less than 0.2 is considered to be because the equilibrium pressure of hydrogen absorption / desorption of the alloy increased when the manganese amount (c) was less than 0.2. ) Exceeds 0.8, the discharge capacity also decreases,
Contrary to the above, it is considered that this is because the equilibrium pressure for hydrogen storage / release is too low, which makes it rather difficult to release hydrogen.

[Experiment 6] In Experiment 6, MmNi _a Co _b
A + b + c + d = 4.5 (constant) in Mn _c Al _d
And a: b: c = 3.2: 0.6: 0.4
(Invariant), only d was changed, and the influence of aluminum (d) on the discharge capacity and R ₁₀₀ % (cell capacity ratio after 100 cycles) was examined. The method for producing the alloy ingot and the method for producing the test cell are the same as in Experiment 2.

Experimental results are shown in FIGS. From FIG.
When aluminum (d) is 0.5 or more, the discharge capacity is
It turns out that it decreases sharply. Moreover, from FIG.
When the frame (d) is less than 0.1, R ₁₀₀% Drops sharply
I understand that From this, the aluminum amount d is
0.1 ≦ c ≦ 0.5 is preferable.

Here, when the amount of aluminum (d) is 0.5 or more, the amount of discharge sharply decreases because the hydrogen storage capacity of the alloy decreases due to the increase of aluminum. If less than 0.1,
It is considered that the reason why the R ₁₀₀ % sharply decreases is that the corrosion resistance of the alloy greatly decreases at the boundary of 0.1. [Experiment 7] In Experiment 7, MmNi _3.2 Co _0.6 Mn _0.4
In the alloy represented by the composition formula of Al _0.3 , the La content in the misch metal (Mm) is changed from 10% to 60% so that the amount of lanthanum in the misch metal is the discharge capacity and R
_The effect on ₁₀₀ % (cell capacity ratio after 100 cycles) was investigated. The method for producing the alloy ingot and the method for producing the test cell are the same as in Experiment 2.

Experimental results are shown in FIGS. Figure 10
It can be seen from FIG. 11 that the discharge capacity sharply decreases when the lanthanum amount is less than 20%.
It can be seen that when it exceeds 0%, R ₁₀₀ % sharply decreases. From this, the lanthanum content in the misch metal is preferably 20% or more and 50% or less.

The reason why the discharge capacity sharply decreased when the lanthanum content was less than 20% is considered to be because the hydrogen equilibrium pressure increased and the hydrogen storage capacity of the alloy decreased, and the lanthanum content exceeded 50%. Then, it is considered that the reason why the R ₁₀₀ % decreased sharply was that the corrosion resistance of the alloy decreased.

[0037]

As described above, according to the present invention, in the hydrogen storage alloy for alkaline storage batteries, both the improvement of the discharge capacity and the improvement of the cycle characteristics, which have been technically contradictory to each other in the past, can be improved. It is possible to obtain a remarkable effect.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a cooling rate (° C./sec) of a molten hydrogen-absorbing alloy and a dendrite cell size (μm) of crystal grains.

FIG. 2 shows the dendrite cell size (μm) of the hydrogen storage alloy electrode and R ₁₀₀ % (100
It is a graph which shows the relationship of the cell capacity ratio after a cycle.

FIG. 3 a in MmNi _a Co _b Mn _c Al _d :
The ratio of b: c: d is unchanged (3.2: 0.6: 0.4:
3 is a graph showing the relationship between x and the cell discharge capacity when a + b + c + d = x is changed.

FIG. 4 a + b in MmNi _a Co _b Mn _c Al _d
Keep + c + d constant (4.5) and b: c: d
6 is a graph showing the relationship between a (nickel amount) and cell discharge capacity when only a is changed, where = 0.6: 0.4: 0.3 (invariant).

FIG. 5 a + b in MmNi _a Co _b Mn _c Al _d
Keep + c + d constant (4.5) and b: c: d
6 is a graph showing the relationship between b (cobalt amount) and the cell discharge capacity when only b is changed with 0.6 = 0.4: 0.3 (invariant).

FIG. 6 a + b in MmNi _a Co _b Mn _c Al _d
While keeping + c + d constant (4.5), a: c: d
= 3.2: 0.4: 0.3 (invariant), b (cobalt amount) and R ₁₀₀ % (1
It is a graph showing the relationship with the cell capacity ratio after 00 cycles).

FIG. 7 a + b in MmNi _a Co _b Mn _c Al _d
While keeping + c + d constant (4.5), a: b: d
= 3.2: 0.6: 0.3 (invariant), it is a graph showing the relationship between c (amount of manganese) and cell discharge capacity when only c is changed.

FIG. 8 a + b in MmNi _a Co _b Mn _c Al _d
+ C + d is fixed (4.5), and a: b: c
2 is a graph showing the relationship between d (aluminum amount) and the cell discharge capacity when only d is changed, where 3.2: 0.6: 0.4 (invariant).

FIG. 9 a + b in MmNi _a Co _b Mn _c Al _d
+ C + d is fixed (4.5), and a: b: c
= 3.2: 0.6: 0.4 (invariant), d (aluminum amount) and R ₁₀₀ % when only d is changed
It is a graph which shows the relationship with (cell capacity ratio after 100 cycles).

FIG. 10 shows MmNi _3.2 Co _0.6 Mn _0.4 Al _0.3
It is a graph which shows the relationship between lanthanum content (%) and cell discharge capacity when changing the lanthanum content in Mm.

FIG. 11 shows MmNi _3.2 Co _0.6 Mn _0.4 Al _0.3
In the case of changing the lanthanum content in mm, it is a graph showing the relationship between the lanthanum content (%) and R _100% (cell capacity ratio after 100 cycles).

FIG. 12 is an explanatory diagram of a dendrite cell size.

FIG. 13 is a schematic diagram showing the structure of an experimental cell.

Claims (3)

[Claims] 1. The composition formula is MmNi _a Co _b Mn _c Al _d.
(However, Mm is the a mixture of rare earth elements, _{a, b, c, d} are each 2.5 ≦ _a ≦ 3.8,
0.2 ≦ _b ≦ 1.0, 0.2 ≦ _c ≦ 0.8,
0.1 ≤ _d ≤ 0.5, 3.8 ≤ _a + _b + _c +
_d represents a number of 4.85. ), And a hydrogen storage alloy for alkaline storage batteries having a dendrite cell size of 8 μm or less. 2. The Mm, which is a mixture of rare earth elements, is
The hydrogen storage alloy for alkaline storage batteries according to claim 1, wherein the content of La is 20% by weight or more and 50% by weight or less with respect to the total weight of the rare earth elements. 3. A first step of melting at least each element constituting the hydrogen storage alloy, and a second step of cooling the melt melted in the first step at a cooling rate of 1000 ° C./sec or more. A method for producing a hydrogen storage alloy for an alkaline storage battery, comprising: