JP4462909B2 - Hydrogen storage alloy - Google Patents

Description

  The present invention relates to a hydrogen storage alloy, and more particularly to a hydrogen storage alloy that obtains excellent pulverization characteristics by suppressing a rise in battery capacity and battery internal pressure.

In recent years, nickel-hydrogen storage batteries (secondary batteries) using a hydrogen storage alloy as a negative electrode have attracted attention as high-capacity alkaline storage batteries that can replace nickel-cadmium storage batteries. As this hydrogen storage alloy, a five-element hydrogen storage alloy of Mm (Misch metal), which is a rare earth-based mixture, and Ni, Al, Mn, and Co, is currently widely used.

This Mm-Ni-Mn-Al-Co alloy can form a negative electrode with a relatively inexpensive material compared to that of La-based, has a long cycle life, and has a small increase in internal pressure due to the generated gas during overcharge. Since a storage battery can be obtained, it is widely used as an electrode material.

The currently used Mm-Ni-Mn-Al-Co alloy suppresses the pulverization of the alloy and extends the cycle life. Generally, in order to suppress the pulverization, about 10 % by mass is required. It is known that Co (at an atomic ratio of 0.6 to 1.0) is required. In order to obtain excellent hydrogen storage characteristics and corrosion resistance, it is necessary to contain a certain amount of Co.

However, if the Co content is increased in order to improve the pulverization characteristics, the hydrogen storage amount is lowered, and there is a problem in terms of raw material costs. In particular, when the content of rare earth elements is increased in order to increase the battery capacity, corrosion of the alloy in the electrolytic solution is promoted, the internal pressure of the battery is increased, and the pulverization characteristics are also decreased.

In order to eliminate the drawbacks of the prior art, for example, Patent Document 1 discloses that the high capacity and cycle life characteristics are improved by defining the Mn content in the hydrogen storage alloy. Although Mn has a high vapor pressure, it tends to segregate during casting and has the disadvantage of causing variations in characteristics. ing. In order to increase the capacity, it is necessary to reduce the amount of the hydrogen storage alloy, which is a negative electrode active material, in order to increase the space volume in the battery.

JP 11-111283 A

An object of the present invention is to provide a hydrogen storage alloy that is excellent in pulverization characteristics and hydrogen storage characteristics, has high capacity in battery characteristics, and can suppress an increase in battery internal pressure.

As a result of various studies conducted by the present inventors, the AB ₅ type alloy composition has a specific stoichiometric composition (B site rich), and the La content and the Ni, Mn, Al, and Co contents in the rare earth element It was found that the above-mentioned purpose can be achieved by optimizing the above. That is, the lifetime can be improved by increasing ABx, but the capacity and homogeneity are reduced. In the present invention, it has been found that an alloy having high capacity and good life characteristics can be obtained by optimizing the crystal lattice length by changing ABx and optimizing the crystal lattice length by the heat treatment temperature.

The present invention has been made on the basis of the above knowledge, and has a general formula MmNi _a Mn _b Al _c Co _d (where Mm is a misch metal, 3.60 ≦ a ≦ 3.90, 0.30 ≦ b ≦ 0. 5, 0.20 ≦ c ≦ 0.4, 0.60 ≦ d ≦ 0.80, 5.10 ≦ a + b + c + d ≦ 5.30) and an AB ₅ type hydrogen storage alloy having a CaCu ₅ type crystal structure The present invention provides a hydrogen storage alloy characterized in that the c-axis lattice length is 4.055 mm or more.

Further, the present invention provides the hydrogen storage alloy as described above, wherein in the above general formula, a + b + c + d is 5.10 or more and less than 5.20, and the c-axis lattice length is 4.055 mm or more. It is to provide.

Further, the present invention provides the above hydrogen storage alloy according to the above general formula, wherein a + b + c + d is 5.20 or more and 5.30 or less and the c-axis lattice length is 4.058 mm or more. It is to provide.

The present invention also provides the hydrogen storage alloy described above, wherein the content of La in the hydrogen storage alloy is 20 % by mass or more and less than 30 % by mass .

The present invention also provides the hydrogen storage alloy as described above, wherein the hydrogen storage alloy is produced at a heat treatment temperature of 1000 ° C. or higher and lower than 1100 ° C. and a heat treatment time of 3 to 6 hours.

The present invention also provides a nickel-hydrogen secondary battery using the hydrogen storage alloy described above.

The hydrogen storage alloy of the present invention is excellent in pulverization characteristics and hydrogen storage characteristics, has high capacity in battery characteristics, and can suppress an increase in battery internal pressure.

The hydrogen storage alloy of the present invention have the general formula _{MmNi a Mn b Al c Co d} ( wherein, Mm is the mischmetal, 3.60 ≦ a ≦ 3.90,0.30 ≦ b ≦ 0.5,0.20 ≦ c ≦ 0.4, 0.60 ≦ d ≦ 0.80, 5.10 ≦ a + b + c + d ≦ 5.30), and an AB ₅ type hydrogen storage alloy having a CaCu ₅ type crystal structure.

Here, Mm is a misch metal which is a rare earth-based mixture of La, Ce, Pr, Nd, Sm and the like. This hydrogen storage alloy is an AB ₅ type hydrogen storage alloy having a CaCu ₅ type crystal structure and has a B site rich non-stoichiometric composition of AB _5.10 to _5.30 .

In the present invention, the La content in the hydrogen storage alloy is 20 % by mass <La <30 % by mass , preferably 22 % by mass <La <28 % by mass . If it is in the range of 20 % by mass <La <30 % by mass , there is no effect of reducing the hydrogen storage amount, and there is little effect of reducing the life characteristics due to elution of La and the like.

In the hydrogen storage alloy, the composition ratio (atomic ratio) of Ni _a Mn _b Al _c Co _d has the following relationship. That is, the proportion of Ni is 3.60 ≦ a ≦ 3.90, the proportion of Mn is 0.30 ≦ b ≦ 0.50, the proportion of Al is 0.20 ≦ c ≦ 0.40, and the proportion of Co is 0.60 ≦ d ≦ 0.80, And a + b + c + d is in the range of 5.10 ≦ a + b + c + d ≦ 5.30.

As described above, the Ni ratio a should be adjusted within the range of 3.60 ≦ a ≦ 3.90, preferably 3.65 ≦ a ≦ 3.85, and more preferably 3.70 ≦ a ≦ 3.80. If it is in the range of 3.60 ≦ a ≦ 3.90, there is no adverse effect on the pulverization characteristics.

The ratio (b) of Mn is adjusted within the range of 0.30 ≦ b ≦ 0.50, preferably 0.35 ≦ b ≦ 0.50, and more preferably 0.40 ≦ b ≦ 0.50. If it is in the range of 0.30 ≦ b ≦ 0.50, there is no adverse effect on the reduction of hydrogen storage amount and pulverization characteristics.

The proportion (c) of Al is adjusted within the range of 0.20 ≦ c ≦ 0.40, preferably 0.25 ≦ c ≦ 0.40, and more preferably 0.25 ≦ c ≦ 0.35. If it is within the range of 0.20 ≦ c ≦ 0.40, the plateau pressure, which is the hydrogen storage alloy discharge pressure, becomes high, and the influence of deteriorating the energy efficiency of charge / discharge is small, and the influence of reducing the hydrogen storage amount is small.

The Co ratio d should be adjusted within the range of 0.60 ≦ d ≦ 0.80, preferably 0.65 ≦ d ≦ 0.80, and more preferably 0.65 ≦ d ≦ 0.75. If it is in the range of 0.60 ≦ d ≦ 0.80, the hydrogen storage characteristics and the pulverization characteristics are not adversely affected, and the benefits of cost reduction can be enjoyed.

The composition ratio a + b + c + d of Ni, Mn, Al and Co is adjusted within the range of 5.10 ≦ a + b + c + d ≦ 5.30, preferably 5.15 ≦ a + b + c + d ≦ 5.25. Is good. If it is in the range of 5.10 ≦ a + b + c + d ≦ 5.30, the battery life characteristics, pulverization characteristics, hydrogen storage characteristics, and output characteristics will not be adversely affected.

The hydrogen storage alloy of the present invention has a c-axis lattice length of 4.055 mm or more. When the c-axis lattice length is less than 4.055 mm, the pulverization characteristics are inferior and the battery life characteristics are impaired.

Next, the manufacturing method of the hydrogen storage alloy of this invention is demonstrated. First, the hydrogen storage alloy raw material is weighed and mixed so as to have the alloy composition as shown above, and the hydrogen storage alloy raw material is melted to form a molten metal using, for example, an induction heating high-frequency heating melting furnace. This is poured into a mold, for example, a water-cooled mold, and a hydrogen storage alloy is cast at 1350 to 1550 ° C. Moreover, the casting temperature in this case is 1200-1450 degreeC. The casting temperature here is the temperature of the molten metal in the crucible at the start of casting, and the casting temperature is the mold pouring port temperature (temperature before casting). The present invention does not depend on the casting method.

Next, the obtained hydrogen storage alloy is heat-treated in an inert gas atmosphere, for example, argon gas. The heat treatment conditions are 1000 to 1080 ° C. and 3 to 6 hours. The reason why such a heat treatment is performed is that fine grain boundary segregation mainly composed of Mn is usually observed in the structure of the cast alloy, but it is homogenized by heating.

In this way, a hydrogen storage alloy is obtained. This hydrogen storage alloy is suitably used as a negative electrode for a long-life alkaline storage battery by performing coarse pulverization and fine pulverization. Such an alkaline storage battery is excellent in pulverization characteristics and hydrogen storage characteristics.

Hereinafter, the present invention will be specifically described based on examples and the like.

[Examples 1-6, Comparative Examples 1-3] Each hydrogen storage alloy raw material is weighed and mixed so as to have the alloy composition shown in Table 1, and the mixture is put in a crucible and fixed in a high-frequency melting furnace, After evacuating to 10 ⁻⁴ to 10 ⁻⁵ Torr, the mixture was heated and dissolved in an argon gas atmosphere, poured into a water-cooled copper mold, and cast at 1350 ° C. (casting temperature 1250 ° C.) to obtain an alloy. . Further, this alloy was heat-treated at 1040 ° C. for 3 hours in an argon atmosphere to obtain a hydrogen storage alloy.

[Characteristic Evaluation] With respect to the hydrogen storage alloys obtained in Examples 1 to 6 and Comparative Examples 1 to 3, the lattice length, PCT capacity, and pulverization residual rate were measured by the methods described below. The results are shown in Table 2.

[Examples 7 to 11, Comparative Examples 4 to 6] Each hydrogen storage alloy raw material was weighed and mixed so as to have the alloy composition shown in Example 3 above, and the mixture was put in a crucible and put into a high-frequency melting furnace. After fixing and ^evacuating to 10 ⁻⁴ to 10 ⁻⁵ Torr, the mixture is heated and dissolved in an argon gas atmosphere, poured into a water-cooled copper mold, cast at 1350 ° C. (casting temperature 1250 ° C.), and alloyed. Got. Furthermore, this alloy was heat-treated in an argon atmosphere at different treatment temperatures and treatment times to obtain a hydrogen storage alloy. The lattice length, PCT capacity, and pulverization residual rate were measured by the methods described below. The results are shown in Table 3.

<Lattice length> The obtained alloy was pulverized and the particle size was adjusted to -20 µm, and then measured by a powder X-ray diffraction method using CuKα rays. In the measurement of the lattice constant, silicon was used as an internal standard substance, and a peak between diffraction angles 20 ° ≦ Θ ≦ 80 ° was used, and the lattice constant was refined by the least square method.

<PCT capacity> Apart from X-ray diffraction, the alloy before X-ray diffraction is pulverized, the particle size is adjusted to a particle size width of + 300 / -500 μm, and activation treatment is performed in a hydrogen pressure state at 250 ° C. After repeating occlusion / desorption of 2 times, the hydrogen occlusion characteristic (PCT curve) was measured at 45 ° C. The following numerical values were used as indicators.
Hydrogen storage amount (H / M 0.5); H / M at a pressure of 0.5 MPa
(2) Plateau pressure (P0.5); equilibrium pressure at H / M = 0.5 MPa

<Pulverization residual ratio> Apart from X-ray diffraction, the alloy before X-ray diffraction is pulverized, the particle size is adjusted to a particle size width of + 20 / -53 μm, and activation treatment is performed in a hydrogen pressure state at 250 ° C. Then, hydrogen was occluded / desorbed twice to prepare a sample. At 45 ° C., the sample was subjected to a cycle test by repeating hydrogen occlusion / desorption for 10, 100, and 300 times in a range of hydrogen pressure of 0 to 1.7 MPa. A value obtained by dividing the average alloy particle size after the cycle test by the average alloy particle size before the cycle test was used. The particle size was measured using a laser diffraction particle size distribution meter.

As is clear from the results in Table 2, Examples 1 to 6 are particularly excellent in long-term (300 cycles) pulverization residual rate characteristics and hydrogen storage characteristics as compared with Comparative Examples 1 to 3. Further, the results in Table 3 show that the heat treatment temperature of the alloy is excellent in long-term (100, 300 cycles) pulverization characteristics at 1000 to 1080 ° C.

Further, as can be seen from Examples 1 and 2, when a + b + c + d is 5.10 or more and less than 5.20 and the c-axis lattice length is 4.055 mm or more, the pulverization residual rate characteristic and the hydrogen storage characteristic are good. It is.

As can be seen from Examples 3 to 5, when a + b + c + d is 5.20 or more and 5.30 or less and the c-axis lattice length is 4.058 mm or more, the pulverization residual rate characteristic and the hydrogen storage characteristic are It is good.

The life can be improved by increasing a + b + c + d, but the capacity and homogeneity are reduced. In contrast, in the present invention, a hydrogen storage alloy with high capacity and good life characteristics can be obtained by optimizing the crystal lattice length by changing a + b + c + d and optimizing the crystal lattice length by adjusting the heat treatment conditions. It is done.

Claims (6)

General formula MmNi _a Mn _b Al _c Co _d (where Mm is Misch metal, 3.60 ≦ a ≦ 3.90, 0.30 ≦ b ≦ 0.5, 0.20 ≦ c ≦ 0.4, 0.60 ≦ d ≦ 0.80, 5.10 ≦ a + b + c a + d ≦ 5.30) AB ₅ type hydrogen storage alloy having a CaCu ₅ type crystal structure represented by the content of La in the hydrogen absorbing alloy is 20 mass% or more and less than 30 wt%, and A hydrogen storage alloy having a c-axis lattice length of 4.055 mm or more ( excluding the AB ₅ type hydrogen storage alloy represented by the general formula MmNi _3.90 Mn _0.40 Al _0.30 Co _0.65 ) .   2. The hydrogen storage alloy according to claim 1, wherein a + b + c + d is not less than 5.10 and less than 5.20, and the c-axis lattice length is not less than 4.055 mm. .   2. The hydrogen storage alloy according to claim 1, wherein a + b + c + d is 5.20 or more and 5.30 or less and the c-axis lattice length is 4.058 mm or more. . After adjusting the particle size of the hydrogen storage alloy to a particle size width of over 20 μm and under 53 μm , activation treatment was performed under hydrogen pressure at 250 ° C., and hydrogen storage / desorption was repeated twice. In a cycle test in which hydrogen occlusion / desorption is repeated 300 times at a hydrogen pressure in the range of 0 to 1.7 MPa at a temperature of 0 ° C., the average particle size of the alloy after the cycle test is divided by the average particle size of the alloy before the cycle test. The hydrogen storage alloy according to any one of claims 1 to 3 , wherein the residual ratio is 82.3 to 94.5%. The hydrogen storage alloy according to any one of claims 1 to 4 , wherein the heat storage temperature of the hydrogen storage alloy is 1000 ° C or higher and lower than 1100 ° C, and the heat treatment time is 3 to 6 hours. Nickel-hydrogen secondary battery using the hydrogen storage alloy according to claim 1.