Hydrogen storage alloy, negative electrode containing this hydrogen storage alloy, and nickel-hydrogen secondary battery containing this negative electrode

Abstract

To provide a hydrogen storage alloy, a negative electrode containing the hydrogen storage alloy, and a nickel-metal hydride secondary battery containing the negative electrode, which can improve the low temperature discharge characteristics without deteriorating the cycle life characteristics of the nickel hydrogen secondary battery.SOLUTION: A nickel-metal hydride secondary battery 2 includes an exterior can 10, and an electrode group 22 housed in the exterior can 10 together with an alkaline electrolyte, the electrode group 22 includes a positive electrode 24 and a negative electrode 26 superimposed via a separator 28, and the negative electrode 26 is a hydrogen storage alloy for a nickel-metal hydride secondary battery and has a single composition, and includes a hydrogen storage alloy composed of a plurality of crystal phases.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a hydrogen storage alloy, a negative electrode containing this hydrogen storage alloy, and a nickel-metal hydride secondary battery containing this negative electrode. [Background technology]

[0002] Nickel-metal hydride secondary batteries are known as a type of alkaline secondary battery. Nickel-metal hydride secondary batteries have a higher capacity than nickel-cadmium secondary batteries and are also environmentally safe. Because of this, their applications are expanding, with their use in a variety of devices, including various portable devices and hybrid electric vehicles. As their applications expand, there is a demand for even higher performance nickel-metal hydride secondary batteries.

[0003] One of the performance requirements for nickel-metal hydride secondary batteries is their low-temperature discharge characteristics, which refer to the degree to which they can discharge in a low-temperature environment. A battery with excellent low-temperature discharge characteristics is one that can discharge a large amount of electricity for a long period of time even in a low-temperature environment.

[0004] Much research has been conducted into improving the low-temperature discharge characteristics of nickel-metal hydride secondary batteries, and one example is the nickel-metal hydride secondary battery shown in Patent Document 1. The nickel-metal hydride secondary battery in Patent Document 1 improves its low-temperature discharge characteristics by modifying the surface of the hydrogen storage alloy particles. Other approaches to improving the low-temperature discharge characteristics of nickel-metal hydride secondary batteries include improving the composition of the hydrogen storage alloy and reducing the particle size of the hydrogen storage alloy. These approaches increase the surface activity of the hydrogen storage alloy, thereby improving the reactivity of the negative electrode, enabling the battery to exhibit good discharge characteristics even in low-temperature environments. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-030702 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the above-mentioned methods for improving low-temperature discharge characteristics essentially increase the surface activity of the hydrogen storage alloy, which tends to result in excessive reaction between the hydrogen storage alloy and the alkaline electrolyte. This facilitates corrosion of the hydrogen storage alloy, resulting in early deterioration of the hydrogen storage alloy. Deterioration of the hydrogen storage alloy makes it difficult to absorb and release hydrogen, inhibiting the battery reaction and shortening the battery's cycle life. Furthermore, as the reaction between the hydrogen storage alloy and the alkaline electrolyte progresses, the alkaline electrolyte is consumed and reduced. Furthermore, the reduction in the alkaline electrolyte causes the separator to dry out, increasing the battery's internal resistance and making discharge more difficult. As a result, the battery's cycle life shortens.

[0007] As described above, if the activity of the surface of the hydrogen storage alloy is increased in order to improve the low-temperature discharge characteristics, the low-temperature discharge characteristics can be improved, but there is a problem in that the cycle life characteristics are reduced.

[0008] Nickel-metal hydride secondary batteries are also required to have a long life so that they can withstand as many repeated uses as possible. Therefore, there is a need to develop nickel-metal hydride secondary batteries that combine excellent low-temperature discharge characteristics with excellent cycle life characteristics.

[0009] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a hydrogen storage alloy that can improve the low-temperature discharge characteristics without deteriorating the cycle life characteristics of a nickel-metal hydride secondary battery, a negative electrode containing this hydrogen storage alloy, and a nickel-metal hydride secondary battery containing this negative electrode. [Means for solving the problem]

[0010] According to the present invention, there is provided a hydrogen storage alloy for a nickel-metal hydride secondary battery, which has a single composition and is composed of a plurality of crystalline phases. [Effects of the Invention]

[0011] The hydrogen storage alloy of the present invention is a hydrogen storage alloy for nickel-metal hydride secondary batteries, which has a single composition and is composed of multiple crystalline phases. Nickel-metal hydride secondary batteries incorporating a negative electrode containing the hydrogen storage alloy of the present invention can achieve both improved low-temperature discharge characteristics and improved cycle life characteristics. Therefore, the present invention can provide a hydrogen storage alloy that contributes to achieving both improved low-temperature discharge characteristics and improved cycle life characteristics of nickel-metal hydride secondary batteries. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a partially cutaway perspective view of a nickel-metal hydride secondary battery according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0013] An embodiment will be described below using an AA-size cylindrical nickel-metal hydride secondary battery (hereinafter referred to as battery) 2 as shown in FIG.

[0014] As shown in FIG. 1 , the battery 2 includes a cylindrical outer can 10 with a bottom and an open top. The outer can 10 is electrically conductive, and its bottom wall 35 functions as a negative electrode terminal. A sealing body 11 is fixed to the opening of the outer can 10. This sealing body 11 includes a lid plate 14 and a positive electrode terminal 20, and seals the outer can 10 while providing the positive electrode terminal 20. The lid plate 14 is a disc-shaped member that is electrically conductive. The lid plate 14 and a ring-shaped insulating gasket 12 that surrounds the lid plate 14 are disposed within the opening of the outer can 10. The insulating gasket 12 is fixed to the opening edge 37 of the outer can 10 by crimping the opening edge 37 of the outer can 10. In other words, the lid plate 14 and the insulating gasket 12 cooperate to hermetically close the opening of the outer can 10.

[0015] The cover plate 14 has a central through-hole 16 in the center, and a rubber valve body 18 that closes the central through-hole 16 is disposed on the outer surface of the cover plate 14. Furthermore, a metallic positive electrode terminal 20 that is cylindrical with a flange and covers the valve body 18 is electrically connected to the outer surface of the cover plate 14. This positive electrode terminal 20 presses the valve body 18 toward the cover plate 14. The positive electrode terminal 20 is provided with a gas vent hole (not shown).

[0016] Normally, the central through-hole 16 is airtightly closed by the valve body 18. On the other hand, if gas is generated inside the outer can 10 and the internal pressure increases, the valve body 18 is compressed by the internal pressure and opens the central through-hole 16, resulting in the gas being released from inside the outer can 10 to the outside through the central through-hole 16 and a gas vent hole (not shown) in the positive terminal 20. In other words, the central through-hole 16, the valve body 18, and the positive terminal 20 form a safety valve for the battery 2.

[0017] The outer can 10 accommodates an electrode group 22. The electrode group 22 includes a strip-shaped positive electrode 24, a strip-shaped negative electrode 26, and a strip-shaped separator 28. Specifically, the positive electrode 24 and the strip-shaped negative electrode 26 are spirally wound with the separator 28 sandwiched therebetween. That is, the positive electrode 24 and the strip-shaped negative electrode 26 are stacked on top of each other with the separator 28 interposed therebetween. The outermost periphery of the electrode group 22 is formed by a part (outermost periphery) of the negative electrode 26, and is in contact with the inner circumferential wall of the outer can 10. That is, the negative electrode 26 and the outer can 10 are electrically connected to each other.

[0018] A positive electrode lead 30 is disposed within the exterior can 10 between one end of the electrode group 22 and the cover plate 14. More specifically, one end of the positive electrode lead 30 is connected to the positive electrode 24, and the other end is connected to the cover plate 14. Therefore, the positive electrode terminal 20 and the positive electrode 24 are electrically connected to each other via the positive electrode lead 30 and the cover plate 14. A circular upper insulating member 32 is disposed between the cover plate 14 and the electrode group 22, and the positive electrode lead 30 extends through a slit 39 provided in the upper insulating member 32. A circular lower insulating member 34 is also disposed between the electrode group 22 and the bottom of the exterior can 10.

[0019] Furthermore, a predetermined amount of alkaline electrolyte (not shown) is poured into the exterior can 10. This alkaline electrolyte is impregnated into the electrode group 22 and promotes charge-discharge reactions between the positive electrode 24 and the negative electrode 26. As this alkaline electrolyte, it is preferable to use an alkaline aqueous solution containing KOH, NaOH, LiOH, or the like as a solute.

[0020] Examples of materials that can be used for the separator 28 include polyamide fiber nonwoven fabrics to which hydrophilic functional groups have been added, and polyolefin fiber nonwoven fabrics, such as polyethylene and polypropylene, to which hydrophilic functional groups have been added. Specifically, it is preferable to use nonwoven fabrics primarily composed of polyolefin fibers that have been sulfonated to provide sulfonic groups. Here, the sulfonic groups are provided by treating the nonwoven fabric with an acid containing sulfonic groups, such as sulfuric acid or fuming sulfuric acid. Batteries using separators containing such fibers with sulfonic groups exhibit excellent self-discharge characteristics.

[0021] The positive electrode 24 includes a conductive positive electrode substrate having a porous structure and a positive electrode mixture held in the pores of the positive electrode substrate.

[0022] As the positive electrode substrate, for example, a nickel-plated metal body in the form of a mesh, sponge, or fiber, or foamed nickel can be used.

[0023] The positive electrode mixture contains a positive electrode active material, a conductive material, a positive electrode additive, and a binder. The binder functions to bind the positive electrode active material, the conductive material, and the positive electrode additive together and to bind the positive electrode mixture to the positive electrode substrate. Examples of binders that can be used here include carboxymethyl cellulose, methyl cellulose, PTFE (polytetrafluoroethylene) dispersion, and HPC (hydroxypropyl cellulose) dispersion.

[0024] The positive electrode active material particles are nickel hydroxide particles or high-order nickel hydroxide particles, and it is preferable that at least one of zinc, magnesium, and cobalt is solid-dissolved in these nickel hydroxide particles.

[0025] The conductive material may be, for example, one or more selected from a cobalt compound and cobalt (Co). Examples of the cobalt compound include cobalt oxide (CoO) and cobalt hydroxide (Co(OH)2). This conductive material is added to the positive electrode mixture as needed, and may be added in the form of a powder or a coating layer covering the surface of the positive electrode active material.

[0026] The positive electrode additive is added to improve the characteristics of the positive electrode, and for example, yttrium oxide, zinc oxide, etc. can be used.

[0027] The positive electrode 24 can be produced, for example, as follows. First, a conductive material, a positive electrode additive, water, and a binder are added to a positive electrode active material powder, which is an aggregate of positive electrode active material particles, and the mixture is kneaded to prepare a positive electrode mixture slurry. The resulting positive electrode mixture slurry is filled into, for example, a foamed nickel and dried. After drying, the foamed nickel filled with nickel hydroxide particles and the like is rolled and then cut. This results in a positive electrode 24 carrying the positive electrode mixture.

[0028] Next, the negative electrode 26 will be described. The negative electrode 26 has a strip-shaped conductive negative electrode substrate (core), and a negative electrode mixture is held on this negative electrode substrate.

[0029] The negative electrode substrate is a sheet-like metal material with distributed through-holes, and may be, for example, a punched metal sheet or a sintered substrate made by molding and sintering metal powder. The negative electrode mixture is not only filled into the through-holes of the negative electrode substrate, but is also held in layers on both sides of the negative electrode substrate.

[0030] The negative electrode mixture contains particles of a hydrogen storage alloy capable of absorbing and releasing hydrogen as a negative electrode active material, a conductive material, and a binder. The binder functions to bind the hydrogen storage alloy particles, the negative electrode additive, and the conductive material to each other, and also to bind the negative electrode mixture to the negative electrode substrate. Here, a hydrophilic or hydrophobic polymer can be used as the binder, and carbon black or graphite can be used as the conductive material. Furthermore, a negative electrode additive can be added as needed.

[0031] Here, hydrogen storage alloys are generally a combination of metal elements with high affinity for hydrogen (hereinafter referred to as element A) and metal elements with low affinity for hydrogen (hereinafter referred to as element B), and are classified by the ratio of element A to element B (AB2, AB3, AB5, etc.). The crystalline phase also changes mainly depending on the ratio of these elements. In other words, hydrogen storage alloys with a single composition generally have a single crystalline phase. These crystalline phases differ in their pulverization (deterioration) behavior when hydrogen is repeatedly absorbed and released. Generally, crystalline phases with a high ratio of element B to element A, such as CaCu5-type, are less likely to pulverize when hydrogen is repeatedly absorbed and released, while crystalline phases with a low ratio of element B to element A, such as Ce2Ni7-type, tend to be more susceptible to the aforementioned pulverization.

[0032] When a hydrogen storage alloy is used in a nickel-metal hydride secondary battery, the pulverization behavior of the crystalline phase of the hydrogen storage alloy has conflicting effects on the battery's characteristics of low-temperature discharge and cycle life. Specifically, a crystalline phase that is prone to pulverization generates many highly active new surfaces, which improves the low-temperature discharge of the battery when incorporated into the battery, but reacts excessively with the alkaline electrolyte, reducing the cycle life. On the other hand, a crystalline phase that is less prone to pulverization reduces the reaction with the alkaline electrolyte to some extent when incorporated into the battery, extending the cycle life of the battery, but reduces the low-temperature discharge performance due to the low surface activity of the hydrogen storage alloy.

[0033] Since conventional hydrogen storage alloys have a single crystal phase for a single composition, in nickel-metal hydride secondary batteries, in order to extend the cycle life characteristics, the low-temperature discharge characteristics had to be sacrificed to some extent, and in order to extend the low-temperature discharge characteristics, the cycle life characteristics had to be sacrificed to some extent.

[0034] As a result of intensive research to achieve both low-temperature discharge characteristics and cycle life characteristics of nickel-metal hydride secondary batteries, the inventors of the present application found that by changing the ratio of constituent elements, manufacturing conditions (casting method, heat treatment conditions), etc., a hydrogen storage alloy in which a plurality of different crystal phases are combined can be obtained even with a single composition, and obtained a hydrogen storage alloy having a single composition and composed of a plurality of crystal phases. Since the hydrogen storage alloy according to the present application has a plurality of crystal phases, a crystal phase that contributes to improving the low-temperature discharge characteristics and a crystal phase that contributes to improving the cycle life characteristics are mixed. That is, a crystal phase with high surface activity that contributes to the battery reaction and a crystal phase that has resistance to the corrosion reaction by the alkaline electrolyte are mixed. By these crystal phases being mixed in a balanced manner, a nickel-metal hydride secondary battery employing the hydrogen storage alloy can increase the low-temperature discharge performance while maintaining the cycle life characteristics.

[0035] In the hydrogen storage alloy according to the present application, regarding the plurality of crystal phases contained therein, when measuring an X-ray diffraction pattern using Cu-Kα rays as an X-ray source, the intensity of the diffraction peak at 2θ = 30.3° is defined as I1, the intensity of the diffraction peak at 2θ = 32.8° is defined as I2, and the intensity of the diffraction peak at 2θ = 31.5° is defined as I3, the intensity ratio A represented by I1 / I2 is in the range of 0.02 ≦ A < 0.14, and the intensity ratio B represented by I3 / I2 is in the range of 0.02 < B < 0.31. Here, the diffraction peak at 2θ = 30.3° belongs to the CaCu5-type crystal phase, and the diffraction peak at 2θ = 31.5° is Ce5Co 19The diffraction peak at 2θ=32.8° is attributed to the CeNi7-type crystalline phase, and the diffraction peak at 2θ=32.8° is attributed to the CeNi7-type crystalline phase. If the intensity ratio of these is within the above-mentioned range, a crystalline phase with high surface activity that contributes to the battery reaction and a crystalline phase that is resistant to the corrosion reaction caused by the alkaline electrolyte are mixed in a balanced manner, and in a nickel-metal hydride secondary battery incorporating such a hydrogen storage alloy, it is possible to improve the low-temperature discharge characteristics without deteriorating the cycle life characteristics.

[0036] The hydrogen storage alloy according to the present invention may be, for example, a rare earth-Mg-Ni based hydrogen storage alloy containing rare earth elements, Mg, and Ni. Specifically, it is preferable to use a hydrogen storage alloy having a composition represented by the following general formula (I):

[0037] Ln 1-x Mg x Ni y-z Al z (I) In the general formula, Ln represents at least one element selected from Zr and rare earth elements, and the subscripts x, y, and z satisfy x≦0.30, 3.3≦y≦3.6, and z≦0.25, respectively. Here, the rare earth elements specifically refer to La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y.

[0038] As described above, if x, representing the Mg element ratio, is outside the range of 0.30 or less, y, representing the Ni element ratio, is between 3.3 and 3.6, and z, representing the Al element ratio, is outside the range of 0.25 or less, the resulting hydrogen storage alloy will be unable to store a large amount of hydrogen or will have a high equilibrium pressure during hydrogen absorption / desorption, making it unsuitable for use as a negative electrode for a nickel-metal hydride secondary battery. Therefore, it is preferable to set the subscripts x, y, and z within the above-mentioned ranges. Furthermore, it is preferable to use only La as the rare earth element. La is a relatively inexpensive rare earth element, contributing to reduced production costs for hydrogen storage alloys.

[0039] The above-mentioned hydrogen storage alloy particles can be obtained, for example, as follows. First, metal raw materials are weighed and mixed to obtain a predetermined composition. This mixture is melted, for example, in an induction melting furnace, and then cooled to form an ingot. The resulting ingot is heat-treated in an inert gas atmosphere at 900 to 1200°C for 5 to 24 hours. The ingot is then cooled to room temperature and mechanically crushed in an inert gas atmosphere. The ingot is then sieved to obtain hydrogen storage alloy particles of the desired particle size.

[0040] Next, the negative electrode 26 can be produced, for example, as follows. First, a negative electrode mixture paste is prepared by kneading a hydrogen storage alloy powder, which is an aggregate of hydrogen storage alloy particles, a conductive material, a binder, and water. The resulting negative electrode mixture paste is applied to a negative electrode substrate and dried. After drying, the negative electrode substrate with the hydrogen storage alloy particles and other particles attached thereto is rolled and cut. This results in a negative electrode 26.

[0041] The positive electrode 24 and negative electrode 26 obtained as described above are spirally wound with a separator 28 interposed therebetween, thereby forming the electrode group 22.

[0042] The electrode group 22 thus obtained is housed in an outer can 10. Subsequently, a predetermined amount of alkaline electrolyte is poured into the outer can 10. Thereafter, the outer can 10 housing the electrode group 22 and alkaline electrolyte is sealed with a lid plate 14 equipped with a positive electrode terminal 20, thereby obtaining a battery 2. The obtained battery 2 is subjected to an initial activation process and is ready for use.

[0043] [Example] 1. Battery manufacturing Example 1 (1) Preparation of hydrogen storage alloy and negative electrode

[0044] First, La, Mg, Ni, and Al were weighed and a mixture was prepared with a molar ratio of 0.763:0.237:3.30:0.10. The resulting mixture was melted in an induction melting furnace, and the molten metal was poured into a mold and cooled to room temperature (25°C) to form an ingot of hydrogen storage alloy. A sample taken from this ingot was subjected to composition analysis by inductively coupled plasma spectroscopy (ICP). The results showed that the composition of the hydrogen storage alloy was La, 0.763 Mg 0.237 Ni 3.30 Al 0.10 It was.

[0045] The resulting ingot was then loaded into a container, the interior of the container was replaced with argon, and the container was then sealed. The container was then placed in a heat treatment furnace and held at 1000°C for 10 hours, subjecting the ingot to heat treatment in an argon gas atmosphere. After this heat treatment, the hydrogen storage alloy ingot was cooled to room temperature and mechanically pulverized in an argon gas atmosphere to obtain a hydrogen storage alloy powder, which was an aggregate of hydrogen storage alloy particles. The particle size of the resulting hydrogen storage alloy powder was measured using a laser diffraction / scattering particle size distribution analyzer, and the volume average particle size (MV) of the hydrogen storage alloy particles was found to be 65 μm.

[0046] Furthermore, X-ray diffraction measurements (XRD measurements) were carried out on the obtained hydrogen storage alloy powder. A parallel beam X-ray diffractometer manufactured by Rigaku Corporation was used for the measurements. The measurement specifications were: X-ray source: Cu-Kα, tube voltage: 50 kV, tube current: 300 mA, scan speed: 1° / min, and sample rotation speed: 60 rpm. From the profile of the measurement results, the peak intensity I1 of the diffraction peak at 2θ=30.3° attributed to the CaCu5 type crystal phase, the peak intensity I2 of the diffraction peak at 2θ=32.8° attributed to the Ce2Ni7 type crystal phase, and the peak intensity I3 of the diffraction peak at 2θ=32.8° attributed to the Ce5Co 19 The peak intensity I3 of the diffraction peak at 2θ=31.5°, which is attributed to the crystalline phase, was measured. The intensity ratio A represented by I1 / I2 and the intensity ratio B represented by I3 / I2 were calculated. The calculation results are shown in Table 1.

[0047] Next, 0.4 parts by weight of sodium polyacrylate, 0.1 parts by weight of carboxymethyl cellulose, 1.0 part by weight of a 50% solids dispersion of styrene butadiene rubber (SBR), 0.5 parts by weight of carbon black (hollow carbon black whose primary particles have a hollow shell-like structure, specifically, Ketjenblack (registered trademark) manufactured by Lion Specialty Chemicals Co., Ltd.), and 30 parts by weight of water were added to 100 parts by weight of the hydrogen storage alloy powder obtained as described above, and the mixture was kneaded to prepare a paste of a negative electrode mixture.

[0048] The negative electrode mixture paste was applied evenly and to a uniform thickness to both sides of a perforated iron plate serving as a negative electrode substrate. The perforated iron plate had a thickness of 60 μm and its surface was nickel-plated.

[0049] After the paste was dried, the perforated plate with the hydrogen storage alloy powder attached was further rolled to increase the amount of alloy per volume, and then cut to obtain an AA size negative electrode 26.

[0050] (2) Preparation of the positive electrode active material and positive electrode Nickel sulfate, zinc sulfate, magnesium sulfate, and cobalt sulfate were weighed to give a nickel-to-nickel ratio of 3.0 wt% zinc, 0.4 wt% magnesium, and 1.0 wt% cobalt. These were then added to a 1N aqueous sodium hydroxide solution containing ammonium ions to prepare a mixed aqueous solution. While stirring the resulting mixed aqueous solution, 10N aqueous sodium hydroxide was gradually added to the mixed aqueous solution to allow the reaction to proceed. The pH during the reaction was stabilized at 13-14, producing nickel hydroxide particles composed primarily of nickel hydroxide with zinc, magnesium, and cobalt dissolved therein. The resulting nickel hydroxide particles were washed three times with 10 times the amount of pure water, then dehydrated and dried to obtain a positive electrode active material powder consisting of an aggregate of nickel hydroxide particles. The particle size of the resulting positive electrode active material powder was measured using a laser diffraction / scattering particle size distribution analyzer, and the volume average particle size (MV) of the positive electrode active material particles was found to be 10 μm.

[0051] Next, 100 parts by weight of the positive electrode active material powder was mixed with 10 parts by weight of cobalt hydroxide, 0.5 parts by weight of yttrium oxide, 40 parts by weight of HPC (hydroxypropyl cellulose), 0.3 parts by weight of zinc oxide, and 30 parts by weight of water to prepare a positive electrode mixture slurry, which was then filled into a sheet-shaped foamed nickel as a positive electrode substrate. After the positive electrode mixture slurry was dried, the foamed nickel filled with the positive electrode mixture was rolled and then cut into a predetermined shape to obtain an AA-size positive electrode 24.

[0052] (3) Assembly of nickel-metal hydride secondary batteries The obtained positive electrode 24 and negative electrode 26 were spirally wound with a separator 28 sandwiched therebetween to produce an electrode group 22. The separator 28 used in producing the electrode group 22 here was formed of a nonwoven fabric made of sulfonated polypropylene fibers, and had a thickness of 0.1 mm (basis weight 53 g / m 2 ) was.

[0053] On the other hand, an aqueous solution containing KOH, NaOH, and LiOH was prepared as an alkaline electrolyte, where the ratio of KOH:NaOH:LiOH was 5.0:1.5:1.0.

[0054] Next, the electrode group 22 was placed in a cylindrical outer can 10 with a bottom, and a predetermined amount of the prepared alkaline electrolyte was poured into it. After that, the opening of the outer can 10 was sealed with a sealing member 11, and an AA-size nickel-metal hydride secondary battery 2 with a nominal capacity of 2300 mAh was assembled. Here, the nominal capacity was the discharge capacity of the battery when it was charged at 0.23 A for 16 hours in an environment at a temperature of 25°C, and then discharged at 0.46 A until the battery voltage reached 1.0 V.

[0055] (4) Initial activation process Battery 2 was subjected to an initial activation treatment by charging it at 0.23 A for 16 hours in an environment at a temperature of 25°C, and then discharging it at 0.46 A until the battery voltage reached 1.0 V. This charge-discharge cycle was repeated five times to prepare Battery 2 for use.

[0056] (Examples 2 and 3, Comparative Examples 1 and 2) Nickel-metal hydride secondary batteries were assembled in the same manner as in Example 1, except that the heat treatment temperature of the hydrogen storage alloy was changed to give the values ​​of intensity ratio A and intensity ratio B shown in Table 1.

[0057] 2. Evaluation of nickel-metal hydride secondary batteries (1) Low-temperature discharge characteristics The batteries fabricated in Examples 1 to 3 and Comparative Examples 1 and 2 were charged at a charging current of 2.3 A in an environment of 25°C until the battery voltage reached its maximum value and then dropped by 10 mV, after which the batteries were rested for 1 hour. After the 1-hour rest, the batteries were discharged at a discharging current of 2.3 A in an environment of 25°C until the battery voltage reached 1.0 V. The discharge capacity at this time was measured, and this discharge capacity was defined as the initial capacity of the battery.

[0058] After measuring the initial capacity, a charging current of 2.3 A was applied to the batteries of Examples 1 to 3 and Comparative Examples 1 and 2 in an environment of 25°C, and the batteries were charged until the battery voltage reached its maximum value and then dropped by 10 mV, and then the batteries were left to rest for 3 hours in an environment of -10°C.

[0059] Next, after resting for 3 hours, the battery was discharged in an environment of −10° C. at a discharge current of 2.3 A until the battery voltage reached 1.0 V, and the discharge capacity at this time was determined. This discharge capacity was defined as the capacity under a low-temperature environment.

[0060] Next, the ratio of the capacity in a low-temperature environment of -10°C to the capacity (initial capacity) in a room temperature environment of 25°C was calculated using the following formula (II). This ratio was defined as the low-temperature discharge capacity ratio. Note that the larger the value of this low-temperature discharge capacity ratio, the smaller the degree of decrease in discharge capacity at low temperatures. Low-temperature discharge capacity ratio = capacity under low-temperature environment / initial capacity x 100 (II)

[0061] Here, the value of the low-temperature discharge capacity ratio of Comparative Example 1 was set to 100, and the ratio of the low-temperature discharge capacity ratio of each battery was calculated. The results are shown in Table 1 as low-temperature discharge characteristic ratios.

[0062] The larger the value of the low-temperature discharge characteristic ratio, the more excellent the low-temperature discharge characteristic.

[0063] (2) Cycle life characteristics The batteries fabricated in Examples 1 to 3 and Comparative Examples 1 and 2 were charged at a charging current of 2.3 A in a 25°C environment until the battery voltage reached its maximum and then dropped by 10 mV, after which the battery was rested for one hour. After the one-hour rest, the batteries were discharged at a discharging current of 2.3 A in a 25°C environment until the battery voltage reached 1.0 V, after which the battery was rested for one hour. This charge-discharge cycle was repeated, and the number of cycles was counted. The discharge capacity in each cycle was measured. The number of cycles at which the discharge capacity in each cycle fell below 60% of the discharge capacity in the first cycle was defined as the cycle life.

[0064] Here, the cycle life value of Comparative Example 1 was set to 100, and the ratio of the cycle life value of each battery was calculated. The results are shown in Table 1 as cycle life characteristic ratios.

[0065] The larger the value of this cycle life characteristic ratio, the more excellent the cycle life characteristic.

[0066] [Table 1]

[0067] (3) Discussion From the results in Table 1, it was confirmed that Examples 1, 2, and 3 and Comparative Example 2 all had superior low-temperature discharge characteristics and longer discharge times in a low-temperature environment than Comparative Example 1. It was also confirmed that Examples 1, 2, and 3 had cycle life characteristics equal to or better than Comparative Example 1. On the other hand, it was confirmed that Comparative Example 2 had inferior cycle life characteristics compared to Comparative Example 1.

[0068] From these results, a hydrogen storage alloy having a single composition and composed of a plurality of crystal phases, such as those in Examples 1, 2, and 3, when measuring an X-ray diffraction pattern with Cu-Kα rays as the X-ray source, if the intensity of the diffraction peak at 2θ = 30.3° is designated as I1, the intensity of the diffraction peak at 2θ = 32.8° is designated as I2, and the intensity of the diffraction peak at 2θ = 31.5° is designated as I3, the intensity ratio A represented by I1 / I2 is in the range of 0.02 ≦ A < 0.14, and the intensity ratio B represented by I3 / I2 is in the range of 0.02 < B < 0.31. A nickel-hydrogen secondary battery equipped with a negative electrode containing such a hydrogen storage alloy can improve the low-temperature discharge characteristics without degrading the cycle life characteristics compared to a nickel-hydrogen secondary battery equipped with a negative electrode containing a hydrogen storage alloy outside the above-described ranges of the intensity ratio A and the intensity ratio B, such as in Comparative Example 1 and Comparative Example 2.

[0069] That is, it can be said that setting the above-described ranges of the intensity ratio A and the intensity ratio B to 0.02 ≦ A < 0.14 and 0.02 < B < 0.31 is effective for improving the low-temperature discharge characteristics without degrading the cycle life characteristics. In particular, as in Comparative Example 2, when the intensity ratio A and the intensity ratio B exceed the above-described ranges, the cycle life characteristics deteriorate, and it becomes difficult to achieve both the low-temperature discharge characteristics and the cycle life characteristics.

[0070] From the above, by setting the above-described ranges of the intensity ratio A and the intensity ratio B to 0.02 ≦ A < 0.14 and 0.02 < B < 0.31, it is possible to improve the contradictory relationship between the low-temperature discharge characteristics and the cycle life characteristics, which was a problem in conventional nickel-hydrogen secondary batteries, and obtain a nickel-hydrogen secondary battery with high low-temperature discharge characteristics without degrading the cycle life characteristics of the nickel-hydrogen secondary battery.

Explanation of Reference Numerals

[0071] 2 Nickel-hydrogen secondary battery 22 Electrode group 24 Positive electrode 26 Negative electrode 28 Separator

Claims

1. A hydrogen storage alloy for a nickel-metal hydride secondary battery, which has a single composition and is composed of a plurality of crystalline phases, When X-ray diffraction patterns of the plurality of crystalline phases are measured using Cu-Kα radiation as an X-ray source, the intensity of the diffraction peak at 2θ=30.3° is defined as I1, the intensity of the diffraction peak at 2θ=32.8° is defined as I2, and the intensity of the diffraction peak at 2θ=31.5° is defined as I3, an intensity ratio A represented by I1 / I2 is in the range of 0.02≦A<0.14, and an intensity ratio B represented by I3 / I2 is in the range of 0.02<B<0.31, The composition is a hydrogen storage alloy represented by the general formula: Ln1-xMgxNiy-zAlz (wherein Ln represents at least one element selected from Zr and rare earth elements, and the subscripts x, y, and z satisfy x≦0.30, 3.3≦y≦3.6, and z≦0.25, respectively).

2. 2. The hydrogen storage alloy according to claim 1, wherein said Ln is La.

3. A negative electrode for a nickel-metal hydride secondary battery, comprising the hydrogen storage alloy according to claim 1 or 2.

4. a container; and an electrode group accommodated in the container together with an alkaline electrolyte; the electrode group includes a positive electrode and a negative electrode stacked on top of each other with a separator interposed therebetween, A nickel-metal hydride secondary battery, wherein the negative electrode is the negative electrode according to claim 3 .

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