JP4511298B2 - Nickel metal hydride storage battery - Google Patents

Description

  The present invention relates to a nickel metal hydride storage battery.

  This type of nickel metal hydride storage battery is disclosed in, for example, Patent Document 1 and Patent Document 2. The nickel electrode applied to the battery of Patent Document 1 includes nickel hydroxide powder as a positive electrode active material and a cobalt compound as a conductive agent. These nickel hydroxide powder and cobalt compound are oxidized in an aqueous sodium hydroxide solution, and the average valences of nickel and cobalt are both greater than 2. Such a higher-order cobalt compound is considered to form a strong conductive network between the nickel hydroxide powders, and to improve the overdischarge characteristics and the high rate discharge characteristics of the battery.

On the other hand, the hydrogen storage alloy electrode applied to the battery of Patent Document 2 includes a hydrogen storage alloy powder capable of storing and releasing hydrogen as a negative electrode active material. Although this hydrogen storage alloy powder contains Mn element in its composition, it is considered that elution of Mn element into the alkaline electrolyte is suppressed by surface treatment.
JP2003-173771 Japanese Patent Laid-Open No. 11-185797

  In the case of the nickel metal hydride storage battery using the nickel electrode and the hydrogen storage alloy electrode of Patent Documents 1 and 2 described above as the positive electrode and the negative electrode, respectively, although Mn elution from the hydrogen storage alloy powder is prevented at the beginning of use, it goes through a charge / discharge cycle. In the meantime, cracks occur in the particles of the hydrogen storage alloy powder. When the powder particles are cracked in this way, the active surface generated by the cracking is exposed to the alkaline electrolyte, and the Mn element is eluted from the active surface. The eluted Mn element is deposited on the surface of the nickel hydroxide powder of the positive electrode, causing a decrease in conductivity in the positive electrode, that is, a decrease in charge / discharge cycle characteristics of the battery.

  The present invention has been made based on the above-mentioned circumstances, and an object thereof is to provide a nickel-metal hydride storage battery that has good discharge characteristics and overdischarge characteristics and can prevent deterioration of charge / discharge cycle characteristics. It is in.

The present inventor has conducted various studies to achieve the above object, the AB ₅ type hydrogen absorbing alloy powder composition is limited to a predetermined range, mixing a rare earth -Mg-Ni-based hydrogen storage alloy powder When the mixed powder obtained in this way was applied to the negative electrode, it was found that the elution of Mn element into the alkaline electrolyte from the AB ₅ type hydrogen storage alloy powder was prevented, and the present invention was conceived. That is, according to the present invention, a positive electrode containing as a main component nickel hydroxide powder having at least a part of the surface coated with a cobalt compound having an average valence of cobalt greater than divalent, and the following general formula (I And a negative electrode containing a second hydrogen storage alloy powder having a composition represented by the following general formula (II), and having the general formula (I) )
_{La p Mm 1-p Ni a} Co b Mn c T1 d
(In the formula, Mm represents at least one element selected from the group consisting of Ce, Pr, and Nd, T1 represents Al, and p, b, and d are 0.6 ≦ p, 0.5, respectively. ≦ b, 0.1 ≦ c ≦ 0.3, and a, b, c, d satisfy the relationship represented by 5.0 <a + b + c + d ≦ 5.5), the above general formula ( II)
Ln _1-x Mg _x (Ni _1-y T2 _y ) _z
(In the formula, Ln is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ca, Sr, Sc, Y, Ti, Zr, and Hf. T2 represents at least one element selected from the group consisting of Co and Al , and x, y, and z are 0 <x <1, 0 ≦ y, respectively. ≦ 0.5 and 2.5 ≦ z ≦ 4.5.
A nickel-metal hydride storage battery characterized by the above is provided (claim 1).

In the above-described positive electrode of the battery of the present invention, at least a part of the surface of the nickel hydroxide powder is coated with a cobalt compound having an average valence of cobalt larger than divalent, and cobalt particles between the nickel hydroxide powder particles are coated with cobalt. A strong conductive network made of a compound is formed.
On the other hand, the negative electrode of the battery of the present invention described above includes not only the first hydrogen storage alloy powder containing Mn element but also the second hydrogen storage alloy powder not containing Mn element. The total amount of elements has been reduced. Therefore, according to the battery of the present invention, the elution amount of Mn element into the alkaline electrolyte when the charge / discharge cycle is performed is reduced.

  In the case of the first hydrogen storage alloy powder, the molar ratio of La occupying the A site is set to be high in view of its crystal structure (0.6 ≦ p), and the elements occupying the A site (La, Mm) The molar ratio of the element (Co, Mn, T1) occupying the B site with respect to is set high (5.0 <a + b + c + d), and the molar ratio of Co and Mn is limited (0.5 ≦ b, 0), respectively. .1 ≦ c ≦ 0.3). Therefore, the first hydrogen storage alloy powder has a hydrogen equilibrium pressure suitable for a nickel metal hydride storage battery. On the other hand, even after a charge / discharge cycle, particle cracking of the first hydrogen storage alloy powder is suppressed. The active surface generated by the particle cracking is not exposed to the alkaline electrolyte, and thus the elution of Mn element from the active surface is suppressed. Moreover, according to the battery of the present invention, the molar ratio of the Mn element is originally limited to less than 0.3, and the Mn elution amount is further reduced because the Mn content is small.

The AB ₅ type first hydrogen storage alloy powder, when it does not contain Mn element, greatly increases its hydrogen equilibrium pressure and becomes inappropriate for nickel metal hydride storage batteries. Even if the 2nd hydrogen storage alloy powder does not contain Mn element, it has a hydrogen equilibrium pressure suitable for a nickel metal hydride storage battery. However, the second hydrogen storage alloy powder is poor in alkali resistance, and when used alone, not only the operating voltage is significantly reduced when it is subjected to a charge / discharge cycle, but also the life characteristics of the battery are deteriorated. Used with powder.

As a preferred embodiment, the nickel hydroxide powder contains nickel having an average valence of more than 2 (claim 2). In this case, the chemical strength between the nickel hydroxide powder particles and the coating layer of the cobalt compound covering the surface thereof increases, and a strong conductive network can exist stably.
As a preferred embodiment, the first hydrogen storage alloy powder exhibits a hydrogen equilibrium pressure in the range of 0.08 to 0.20 MPa when the hydrogen storage amount H / M is 0.5 at a temperature of 40 ° C. Item 3). In this case, the discharge performance of the battery is ensured, and in the case of a battery having a safety valve, the alkaline electrolyte is prevented from leaking out of the battery due to an increase in internal pressure.

  In a preferred aspect, the negative electrode further includes carbon powder, and a mass ratio of the carbon powder to a total mass of the first and second hydrogen storage alloy powders is in a range of 0.1 to 1.0% (invoice). Item 4). In this case, the presence of the carbon powder improves the liquid retention of the alkaline electrolyte in the negative electrode, and the oxidation of the first and second hydrogen storage alloy powders during the charge / discharge cycle is suppressed.

  The nickel metal hydride storage battery according to the first to fourth aspects of the present invention has a strong conductive network formed on the positive electrode, and has good discharge characteristics and overdischarge characteristics. On the other hand, since the battery of the invention suppresses the elution of Mn element from the negative electrode into the alkaline electrolyte even after the charge / discharge cycle, it also suppresses the precipitation of the eluted Mn element on the surface of the nickel hydroxide powder of the positive electrode Is done. As a result, since the conductive network of the positive electrode is maintained, deterioration of charge / discharge cycle characteristics is suppressed.

According to the nickel metal hydride storage battery of claim 2, since the conductive network becomes stronger, the discharge characteristics, the supersaturated discharge characteristics, and the charge / discharge cycle characteristics are further improved.
According to the nickel metal hydride storage battery of the third aspect, the discharge performance of the negative electrode is ensured and the decrease of the alkaline electrolyte is prevented, so that the discharge performance and the charge / discharge cycle characteristics are further improved.
According to the nickel-metal hydride storage battery of claim 4, since the oxidation of the first and second hydrogen storage alloy powders is suppressed, the charge / discharge cycle characteristics are further improved.

FIG. 1 shows a nickel metal hydride storage battery according to an embodiment of the present invention.
This battery is, for example, a cylindrical battery, and includes a bottomed cylindrical outer can 10 having an open upper end, and the outer can 10 functions as a negative electrode terminal whose bottom wall has conductivity. In the opening of the outer can 10, a stepped disk-shaped lid plate 14 having conductivity is disposed via a ring-shaped insulating packing 12, and the lid plate 14 and the insulating packing 12 are arranged at the opening edge of the outer can 10. The outer can 10 is fixed to the opening edge by caulking.

  The cover plate 14 has a gas vent hole 16 in the center, and a disc-shaped valve element 18 is disposed on the outer surface of the cover plate 14 so as to close the gas vent hole 16. The seat 20 is overlapped. Further, a flanged cylindrical positive terminal 22 is fixed on the outer surface of the cover plate 14 so as to surround the valve body 18 and the spring seat 20, and the spring seat 20 is interposed between the spring seat 20 and the positive terminal 22. A coil spring 24 that presses the valve body 18 against the cover plate 14 is arranged. Accordingly, at the normal time, the outer can 10 is airtightly closed by the cover plate 14 via the insulating packing 12 and the valve body 18. On the other hand, when gas is generated in the outer can 10 and its internal pressure increases, the coil spring 24 is compressed and gas is released from the outer can 10 through the gas vent hole 16. That is, the cover plate 14, the valve body 18, the spring seat 20, the positive terminal 22 and the coil spring 24 form a safety valve. Although not shown, the outer surface of the battery extends from the periphery of the cover plate 14 to the periphery of the bottom wall of the outer can 10 and is covered with an outer tube.

  In the outer can 10, a substantially cylindrical electrode group 26 is accommodated together with an alkaline electrolyte (not shown). More specifically, the electrode group 26 is formed by winding a belt-like positive electrode 28 and a negative electrode 30 with a separator 32 interposed therebetween, and the positive electrode 28 and the negative electrode 30 are overlapped with each other with the separator 32 interposed therebetween. . The positive electrode 28 and the lid plate 14 of the electrode group 26 are electrically connected via a positive electrode lead 34, while the disc-shaped current collector plate 36 is connected between the negative electrode 30 and the inner bottom wall of the outer can 10. It is electrically connected via.

Hereinafter, the positive electrode 28 and the negative electrode 30 will be described in detail.
<Positive electrode>
The positive electrode 28 includes a conductive positive electrode substrate and a positive electrode mixture held on the positive electrode substrate. As the positive electrode substrate, for example, a plate-like nickel material having a three-dimensional porous structure can be used. The positive electrode mixture includes nickel hydroxide powder as a positive electrode active material, a conductive agent made of a Co compound, and a binder that adheres the nickel hydroxide powder and the conductive agent to the positive electrode substrate. More specifically, as schematically shown in the circle of FIG. 1, the nickel hydroxide powder was partially or entirely covered with a coating layer 42 made of a Co compound as a conductive agent. In the state, it is bound by the binder 44. Both nickel of the nickel hydroxide powder and Co of the Co compound of the coating layer 42 have an average valence greater than two. In addition, the nickel hydroxide particle powder may be dissolved in zinc or cobalt.

In the following, nickel hydroxide having a nickel valence greater than 2 is referred to as high-order nickel hydroxide, cobalt compound having a cobalt valence greater than 2 is referred to as a higher-order cobalt compound, and higher-order cobalt compound. The coated high-order nickel hydroxide powder is referred to as high-order Co-coated nickel hydroxide powder.
The positive electrode 28 described above is produced, for example, as follows.

First, the nickel hydroxide powder and the cobalt compound powder are heated and mixed in an atmosphere containing oxygen while simultaneously adding an alkaline aqueous solution, and then dried to obtain a high-order Co-coated nickel hydroxide powder. Thereafter, a slurry made of high-order Co-coated nickel hydroxide powder, a binder and water is prepared, and this positive electrode substrate is filled with this slurry. Then, the positive electrode substrate filled with the slurry is rolled and cut after drying, and the positive electrode 28 is produced.
<Negative electrode>
The negative electrode 30 includes a conductive negative electrode substrate and a negative electrode mixture held on the negative electrode substrate. As the negative electrode substrate, for example, a punching metal can be used. The negative electrode mixture is composed of a first hydrogen storage alloy powder, a second hydrogen storage alloy powder, and a binder, each of which can store and release hydrogen as a negative electrode active material. That is, as schematically shown in the circle of FIG. 1, the first hydrogen storage alloy powder particles 46 and the second hydrogen storage alloy powder particles 48 are bound together by the binder 50. Yes.

The first hydrogen storage alloy powder is composed of a hydrogen storage alloy having a crystal structure of AB ₅ type (CaCu ₅ type) (hereinafter referred to as AB ₅ type alloy), and its composition is represented by the general formula (I):
_{La p Mm 1-p Ni a} Co b Mn c T1 d
(In the formula, Mm represents at least one element selected from the group consisting of Ce, Pr, and Nd, T1 represents Al, and p, b, and d are 0.6 ≦ p, 0.5, respectively. ≦ b, 0.1 ≦ c ≦ 0.3, and a, b, c, d satisfy the relationship represented by 5.0 <a + b + c + d ≦ 5.5.)
Indicated by

The second hydrogen-absorbing alloy powder, the hydrogen storage alloy of the super lattice structure of the crystal structure is AB ₅ type and AB ₂ type (hereinafter referred to as the superlattice alloy) consists, the composition of the general formula (II):
Ln _1-x Mg _x (Ni _1-y T2 _y ) _z
(In the formula, Ln is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ca, Sr, Sc, Y, Ti, Zr, and Hf. T2 represents at least one element selected from the group consisting of Co and Al , and x, y, and z are 0 <x <1, 0 ≦ y, respectively. ≦ 0.5 and 2.5 ≦ z ≦ 4.5.
Indicated by Due to this composition, the superlattice alloy is also referred to as a rare earth-Mg—Ni-based hydrogen storage alloy.

About the ratio of the 1st and 2nd hydrogen storage alloy powder mentioned above in a negative electrode mixture, when the total mass of 1st hydrogen storage alloy powder is set to 1 for the reason mentioned above, 2nd hydrogen storage alloy powder The total mass of is preferably in the range of 0.1-1. The negative electrode mixture preferably further contains a predetermined amount of carbon powder.
The negative electrode 30 is prepared by adjusting a slurry made of the first and second hydrogen storage alloy powders, a binder, water and, if necessary, carbon powder, and drying the negative electrode substrate coated with the slurry. It is produced by rolling and cutting.

Further, the second hydrogen storage alloy powder is produced as follows.
First, metal raw materials are weighed and mixed so as to have the composition shown in the general formula (II), and this mixture is melted in, for example, a high-frequency melting furnace to form an ingot. The obtained ingot is subjected to heat treatment in an inert gas atmosphere at a temperature of 900 to 1200 ° C. for 5 to 24 hours, so that the crystal structure of the ingot becomes a superlattice structure of AB ₅ type structure and AB ₂ type structure. . Thereafter, the ingot is pulverized and classified to a desired particle size by sieving to produce a second hydrogen storage alloy powder.

In the above-described positive electrode 28 of the nickel-metal hydride storage battery, at least a part of the surface of each particle 40 of the nickel hydroxide powder is coated with a high-order cobalt compound, and the nickel hydroxide powder particles 40 are made of a strong cobalt compound. A conductive network is formed.
On the other hand, the negative electrode 30 of this battery contains not only the first hydrogen storage alloy powder containing Mn element but also the second hydrogen storage alloy powder not containing Mn element. The total amount has been reduced. Therefore, from this negative electrode 30, the elution amount of the Mn element to the alkaline electrolyte when the charge / discharge cycle is performed is reduced.

  Further, the first hydrogen storage alloy powder has a high molar ratio of La occupying the A site (0.6 ≦ p) in view of its crystal structure, and the B site for the elements (La, Mm) occupying the A site. Is set high (5.0 <a + b + c + d), and the molar ratio of Co and Mn is limited (0.5 ≦ b, 0.1 ≦), respectively. c ≦ 0.3), cracking of the powder particles 46 during the charge / discharge cycle is suppressed while having a hydrogen equilibrium pressure suitable for a nickel metal hydride storage battery. That is, in the particles 46 of the first hydrogen storage alloy powder, the active surface caused by the cracking is exposed to the alkaline electrolyte, and the elution of Mn element from the active surface is suppressed. Therefore, in this battery, the molar ratio of the Mn element is originally limited to less than 0.3, the Mn content may be small, and the Mn elution amount is further reduced.

The AB ₅ type first hydrogen storage alloy powder, when not containing Mn element, greatly increases its hydrogen equilibrium pressure and becomes unsuitable for nickel metal hydride storage batteries, whereas the second hydrogen storage alloy powder The occlusion alloy powder has a hydrogen equilibrium pressure suitable for a nickel metal hydride storage battery even if it does not contain Mn element. However, the second hydrogen storage alloy powder has poor alkali resistance, and when used alone, not only will the operating voltage decrease significantly after a charge / discharge cycle, but also the life characteristics of the battery will decrease. Used with storage alloy powder.

  As is clear from the above description, the above-described nickel-metal hydride storage battery has a strong conductive network formed on the positive electrode 28 and has good discharge characteristics and overdischarge characteristics. On the other hand, in this battery, since the elution of Mn element from the negative electrode 30 into the alkaline electrolyte is suppressed even after the charge / discharge cycle, the eluted Mn element is deposited on the surface of the nickel hydroxide powder of the positive electrode 28. Is also suppressed. As a result, since the conductive network of the positive electrode 28 is maintained, deterioration of charge / discharge cycle characteristics in the battery is suppressed.

  The present invention is not limited to the above-described embodiment, and various modifications are possible. For example, the nickel hydroxide powder may have an average valence of nickel of two. However, when the average valence of nickel is larger than 2, the chemical strength between each particle 40 of the nickel hydroxide powder and the coating layer 42 of the cobalt compound covering the surface increases, and the conductive network Becomes stronger, and the discharge characteristics, the supersaturated discharge characteristics, and the charge / discharge cycle characteristics are further improved.

  In one embodiment, the battery has a cylindrical shape, but may have a rectangular shape, and the mechanical structure of the battery is not particularly limited.

Example 1
1. Preparation of positive electrode While stirring a mixed aqueous solution of nickel sulfate aqueous solution, zinc sulfate aqueous solution, and cobalt sulfate aqueous solution at a predetermined concentration, sodium hydroxide aqueous solution is gradually added thereto, and the deposited precipitate is washed and dehydrated and then dried. Thus, a nickel hydroxide powder having a zinc solid solution amount of 3% by mass and a cobalt solid solution amount of 1% by mass was obtained.

This nickel hydroxide powder plus cobalt hydroxide powder is heated in an oxygen-containing atmosphere and mixed while adding an alkaline aqueous solution, followed by drying to obtain a high-order Co-coated nickel hydroxide powder. It was. Thereafter, a 40 mass% HPC (hydroxypropylcellulose) dispersion solution was added to the high-order Co-coated nickel hydroxide powder and stirred, and the resulting slurry was filled into a nickel material having a porous structure. Then, the nickel material filled with the slurry was rolled and cut after drying, and a positive electrode 28 for an SC size nickel-metal hydride storage battery was produced.
2. Production of Negative Electrode Metal raw materials were weighed and mixed so as to have the composition shown in Table 1, and this mixture was melted in a high-frequency melting furnace to obtain an ingot. This ingot was heated for 10 hours in an argon atmosphere at a temperature of 1000 ° C., and the crystal structure of the ingot was changed to the AB ₅ type. Thereafter, the ingot was pulverized and sieved to produce a first hydrogen storage alloy powder.

On the other hand, metal raw materials were weighed and mixed so that the composition was (La _0.25 Ce _0.05 Pr _0.35 Nd _0.35 ) _0.7 Mg _0.3 Ni _2.5 Co _0.5 Al _0.2, and this mixture was melted in a high-frequency melting furnace to obtain an ingot. . This ingot was heated for 10 hours in an argon atmosphere at a temperature of 1000 ° C., and the crystal structure of the ingot was changed to an AB ₅ type and AB ₂ type superlattice structure. Thereafter, the ingot was pulverized and sieved to produce a second hydrogen storage alloy powder.

The first and second hydrogen storage alloy powders thus obtained were weighed and mixed so that the total mass of the first hydrogen storage alloy powder: the total mass of the second hydrogen storage alloy powder = 9: 1. . In the obtained mixed powder, sodium polyacrylate 0.4%, carboxymethylcellulose 0.1%, polytetrafluoroethylene dispersion (dispersion medium: water, solid content 60% by mass) with respect to the total mass of the mixed powder. ) 2.5% and carbon powder 0.5% were added and kneaded to obtain a slurry. After applying this slurry to both sides of the punching metal at a constant thickness, the punching metal was rolled and cut after drying the slurry, and the negative electrode 30 for the SC-size nickel metal hydride storage battery was produced.
3. Assembling the Nickel Metal Hydride Battery The positive electrode 28 and the negative electrode 30 produced as described above are wound through a separator 32 made of a polyolefin-based non-woven fabric mainly composed of polypropylene and polyethylene, and the obtained electrode group 26 is packaged in an outer can. 10 and was subjected to a predetermined attachment process. Thereafter, a 7N alkaline aqueous solution containing potassium hydroxide as a main component is injected into the outer can 10 as an electrolytic solution, and then the opening of the outer can 10 is sealed with a cover plate 14 or the like, so that an SC size nickel-metal hydride storage battery is obtained. (Nominal capacity 3.0 Ah) was assembled.
Examples 2-9 and Comparative Examples 1-4
The nickel hydrogen storage batteries of Examples 2 to 9 and Comparative Examples 1 to 4 are assembled in the same manner as in Example 1 described above except that the first hydrogen storage alloy powder has the composition shown in Table 1. It was.

Examples 10-13
In the negative electrode, the carbon powder was added in such a manner that the ratio of the total mass of the carbon powder to the total mass of the first and second hydrogen storage alloy powders was the value shown in Table 2, respectively. The nickel metal hydride storage batteries of Examples 10 to 13 were assembled in the same manner as in the case.
4). Hydrogen storage alloy and battery evaluation test (1) For each of the first hydrogen storage alloy powders of Examples 1 to 13 and Comparative Examples 1 to 4, the hydrogen storage amount (H / M) in an atmosphere at a temperature of 40 ° C. The equilibrium pressure of hydrogen was measured when N was 0.5, and the results are shown in Tables 1 and 2. In addition, the equilibrium pressure of hydrogen was measured based on JIS H7201 (1991) “Measurement Method of Pressure-Composition Isotherm (PTC Line) of Hydrogen Storage Alloy”. The measurement temperature was 40 ° C., which is an average value of the actual use temperature exhibited by the battery in a general use environment.
(2) Charging / discharging cycle characteristics For each of the nickel-metal hydride storage batteries of Examples 1 to 13 and Comparative Examples 1 to 4, charging for 1.2 hours at a current value of 3A and a final voltage of 1.0V at a current value of 15A Charging / discharging with one discharge cycle was repeated, and the number of cycles until the discharge capacity reached 80% in the first cycle was counted. These results are shown in Tables 1 and 2 as indexes with the result of Comparative Example 1 as 100.
(3) Overdischarge characteristics Each of the nickel-metal hydride storage batteries of Examples 1 to 13 and Comparative Examples 1 to 4 is connected to the positive and negative electrodes through an electric resistance of 0.5Ω in an atmosphere at a temperature of 60 ° C. Left for 2 weeks. After that, each battery was repeatedly charged and discharged with one cycle of charging for 1.2 hours at a current value of 3A and discharging to a final voltage of 1.0V at a current value of 15A. It was measured. These results are shown in Tables 1 and 2 as indexes with the result of Comparative Example 1 as 100.

5). Test results From Tables 1 and 2, the following is clear.
(1) Examples 1 to 3 having a La molar ratio of 0.6 or more have better charge / discharge cycle characteristics and overdischarge characteristics than Comparative Example 1 having the same ratio of 0.5.
(2) In Example 3 in which the molar ratio of Co is 0.5 or more, charge / discharge cycle characteristics and overdischarge characteristics are superior to Comparative Example 2 in which the ratio is 0.4.
(3) Examples 6 and 7 having a molar ratio of Mn of 0.1 or more and 0.3 or less are superior in charge / discharge cycle characteristics and overdischarge characteristics than Comparative Example 4 having the same ratio of 0.4.

(4) Examples 5 and 6 in which the hydrogen equilibrium pressure is in the range of 0.20 MPa or less have better charge / discharge cycle characteristics than Example 9 in which the same pressure is 0.26. This is because in Example 9, because the equilibrium pressure was increased, the safety valve frequently operated due to an increase in internal pressure during the charge / discharge cycle, resulting in the ejection of alkaline electrolyte from the battery, that is, reduction of alkaline electrolyte. As a result, it is considered that the internal resistance of the battery has increased. Accordingly, it is preferable to use a hydrogen storage alloy powder having a hydrogen equilibrium pressure of 0.20 MPa or less as the first hydrogen storage alloy powder when the hydrogen storage amount H / M is 0.5 at a temperature of 40 ° C. In order to ensure discharge performance, when the hydrogen storage amount H / M is 0.5 at a temperature of 40 ° C., the hydrogen storage alloy powder having a hydrogen equilibrium pressure of 0.08 MPa or more is used as the first hydrogen storage alloy powder. It is preferable to use as.

(5) Examples 5, 11 and 12 in which the mass ratio of the carbon powder to the total mass of the first and second hydrogen storage alloy powders is in the range of 0.1 to 1.0% Charge / discharge cycle characteristics are superior to Example 10 and Example 13 having the same ratio of 2.0%. This is because, in Examples 5, 11 and 12, the liquid retention of the alkaline electrolyte in the negative electrode is moderately improved by the presence of the carbon powder, and the first and second hydrogen storage alloy powders after the charge / discharge cycle are obtained. This is probably because oxidation was suppressed. On the other hand, in Example 13, since the mass ratio of the carbon powder is as large as 2.0%, it is considered that the liquid retention is too high and the charge / discharge cycle characteristics are deteriorated. That is, in Example 13, the oxygen gas absorbability is lowered due to excessively high liquid retention in the negative electrode, and the safety valve frequently operates due to an increase in the internal pressure of the battery during the charge / discharge cycle. It is thought that the increase in internal resistance was brought about. Thus, the mass ratio of the carbon powder is preferably in the range of 0.1 to 1.0%.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially cutaway perspective view showing a nickel-metal hydride storage battery according to an embodiment of the present invention, and the inside of the circle is a cross-sectional view schematically showing a positive electrode mixture or a negative electrode mixture.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Exterior can 28 Positive electrode 30 Negative electrode 32 Separator 40 Nickel hydroxide powder particle 42 Coating layer 46 First hydrogen storage alloy powder particle 48 Second hydrogen storage alloy powder particle

Claims (4)

A positive electrode comprising, as a main component, nickel hydroxide powder having at least a part of the surface coated with a cobalt compound having an average valence of cobalt greater than divalent;
A first hydrogen storage alloy powder having a composition represented by the following general formula (I), and a negative electrode comprising a second hydrogen storage alloy powder having a composition represented by the following general formula (II): ,
The general formula (I) is
_{La p Mm 1-p Ni a} Co b Mn c T1 d
(In the formula, Mm represents at least one element selected from the group consisting of Ce, Pr, and Nd, T1 represents Al, and p, b, and d are 0.6 ≦ p, 0.5, respectively. ≦ b, 0.1 ≦ c ≦ 0.3, and a, b, c, d satisfy the relationship represented by 5.0 <a + b + c + d ≦ 5.5).
The general formula (II) is
Ln _1-x Mg _x (Ni _1-y T2 _y ) _z
(In the formula, Ln is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ca, Sr, Sc, Y, Ti, Zr, and Hf. T2 represents at least one element selected from the group consisting of Co and Al , and x, y, and z are 0 <x <1, 0 ≦ y, respectively. ≦ 0.5 and 2.5 ≦ z ≦ 4.5.
A nickel metal hydride storage battery characterized by the following.   The nickel hydride storage battery according to claim 1, wherein the nickel hydroxide powder contains nickel having an average valence of more than two.   The first hydrogen storage alloy powder has a hydrogen equilibrium pressure in the range of 0.08 to 0.20 MPa when the hydrogen storage amount H / M is 0.5 at a temperature of 40 ° C. The nickel metal hydride storage battery according to claim 1 or 2. The negative electrode further includes carbon powder,
The mass ratio of the carbon powder to the total mass of the first and second hydrogen storage alloy powders is in the range of 0.1 to 1.0%. Nickel metal hydride storage battery.

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