Hydrogen storage alloy, negative electrode comprising said hydrogen storage alloy, and nickel hydrogen secondary battery comprising said negative electrode

Abstract

A nickel hydrogen secondary battery 2 comprises an outer can 10 and an electrode group 22 housed in the outer can 10 together with an alkaline electrolyte. The electrode group 22 includes a positive electrode 24 and a negative electrode 26 overlapped via a separator 28; the negative electrode 26 is a hydrogen storage alloy represented by the general formula Ln1-aMgaNib-cAlc (where Ln represents one or more elements selected from rare earth elements, Y, and Zr, and the subscripts a, b, and c satisfy the respective relationships a ≤ 0.30, 3.4 ≤ b ≤ 3.7, and c ≤ 0.25). For the Ln described above, at least La among the rare earth elements is selected, and the La accounts for 89.0 mol% or more of the Ln.

Description

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

[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.

[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, so they are used in a variety of fields, and are attracting particular attention in the automotive field.

[0003] Nickel-metal hydride secondary batteries used in the automotive field are required to have high capacity, long cycle life, and excellent capacity recovery rate after storage in a high-temperature environment. Much research has been conducted to obtain such nickel-metal hydride secondary batteries, and for example, the nickel-metal hydride secondary battery disclosed in Patent Document 1 has been researched and developed. The nickel-metal hydride secondary battery disclosed in Patent Document 1 has high capacity and excellent cycle life characteristics and high-temperature storage characteristics.

[0004] Japanese Patent Application Laid-Open No. 2001-307721

[0005] Nickel-metal hydride secondary batteries used in the automotive field are used in a wide range of environmental temperatures. For example, they may be used in room temperature environments of 18°C ​​to 28°C, in high temperature environments exceeding 28°C, or in low temperature environments below 18°C. Specifically, the interior of an automobile may be in a high temperature environment exceeding 50°C under the blazing sun of midsummer, or conversely, in a low temperature environment of below freezing in midwinter.

[0006] In nickel-metal hydride secondary batteries stored in such high-temperature environments, corrosion of the hydrogen storage alloy progresses due to the alkaline electrolyte (high-concentration alkaline aqueous solution) in the battery. As corrosion progresses, a passivation film forms on the surface of the hydrogen storage alloy. The formation of this passivation film inhibits the discharge reaction, resulting in a deterioration in discharge characteristics. In particular, discharge characteristics in low-temperature environments (hereinafter also referred to as low-temperature discharge characteristics) are significantly reduced. In other words, nickel-metal hydride secondary batteries have a problem in that their low-temperature discharge characteristics are significantly reduced after storage in high-temperature environments.

[0007] Although the nickel-metal hydride secondary battery disclosed in the above-mentioned Patent Document 1 has improved high-temperature storage characteristics, no consideration has been given to the problems that may arise when the battery is used in a low-temperature environment after being stored in a high-temperature environment.

[0008] Nickel-metal hydride secondary batteries are required to be able to stably supply power over as wide a temperature range as possible, and therefore there is a need to develop nickel-metal hydride secondary batteries that combine excellent high-temperature storage characteristics with excellent low-temperature discharge 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 contributes to a nickel-metal hydride secondary battery exhibiting excellent discharge characteristics even when placed in a low-temperature environment after being stored in a high-temperature environment, a negative electrode containing this hydrogen storage alloy, and a nickel-metal hydride secondary battery containing this negative electrode.

[0010] According to the present invention, the general formula: Ln １－ａ Mg ａ Ni ｂ－ｃ Al ｃ (wherein Ln represents one or more elements selected from rare earth elements, Y, and Zr, and the subscripts a, b, and c satisfy the relationships a≦0.30, 3.4≦b≦3.7, and c≦0.25, respectively), wherein Ln is at least La selected from the rare earth elements, and La accounts for 89.0 mol % or more of Ln.

[0011] The hydrogen storage alloy of the present invention has the general formula: Ln １－ａ Mgａ Ni ｂ－ｃ Al ｃ (where Ln represents one or more elements selected from rare earth elements, Y, and Zr, and the subscripts a, b, and c respectively satisfy the relationships a≦0.30, 3.4≦b≦3.7, and c≦0.25), and Ln is at least La selected from the rare earth elements, and La accounts for 89.0 mol % or more of Ln. With this configuration, the hydrogen storage alloy of the present invention is resistant to corrosion by alkaline electrolytes even when placed in a high-temperature environment, and the formation of a passivation film can be reduced. Therefore, a nickel-metal hydride secondary battery incorporating a negative electrode containing the hydrogen storage alloy of the present invention exhibits excellent discharge characteristics even when placed in a low-temperature environment after being stored in a high-temperature environment. In other words, the present invention can provide a hydrogen storage alloy that contributes to the nickel-metal hydride secondary battery exhibiting excellent discharge characteristics even when placed in a low-temperature environment after being stored in a high-temperature environment, a negative electrode containing this hydrogen storage alloy, and a nickel-metal hydride secondary battery including this negative electrode.

[0012] 1 is a partially cutaway perspective view of a nickel-metal hydride secondary battery according to an embodiment of the present 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. 1 as an example.

[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 conductive, disc-shaped member. The lid plate 14 and a ring-shaped insulating gasket 12 surrounding 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] Under normal conditions, 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 exterior 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 portion (outermost periphery) of the negative electrode 26, and is in contact with the inner circumferential wall of the exterior can 10. That is, the negative electrode 26 and the exterior 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 resistance.

[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 the binder 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] As the conductive material, for example, one or more selected from a cobalt compound and cobalt (Co) can be used. The cobalt compound can be cobalt oxide (CoO), cobalt hydroxide (Co(OH)), or the like. ２ The conductive material is added to the positive electrode mixture as needed, and may be added in the form of a powder or in the form of a coating layer that covers the surface of the positive electrode active material and is included in the positive electrode mixture.

[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 manufactured, 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 as described above, and the mixture is kneaded to prepare a positive electrode mixture slurry. The obtained positive electrode mixture slurry is filled into, for example, foamed nickel, and then 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, a description will be given of the negative electrode 26. The negative electrode 26 has a strip-shaped conductive negative electrode core, and a negative electrode mixture is held in this negative electrode core.

[0029] The negative electrode core is a sheet-like metal material having through holes distributed therein, and may be, for example, a punched metal sheet. The negative electrode mixture is not only filled into the through holes of the negative electrode core, but also held in layers on both sides of the negative electrode core.

[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 core. 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] The hydrogen storage alloy used in this embodiment is a rare earth-Mg—Ni-based hydrogen storage alloy containing rare earth elements, Mg, and Ni. Specifically, the rare earth-Mg—Ni-based hydrogen storage alloy has a composition represented by the following general formula (I):

[0032] Ln １－ａ Mg ａ Ni ｂ－ｃ Al ｃ ... (I) In general formula (I), Ln represents one or more elements selected from rare earth elements, Y, and Zr, and the subscripts a, b, and c satisfy the relationships a≦0.30, 3.4≦b≦3.7, and c≦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.

[0033] As described above, if a, which represents the Mg element ratio, is out of the range of 0.30 or less, b, which represents the Ni element ratio, is out of the range of 3.4 to 3.7, and c, which represents the Al element ratio, is out of the range of 0.25 or less, the amount of hydrogen that can be stored in the obtained hydrogen storage alloy will be small, and the equilibrium pressure during hydrogen absorption and desorption of the hydrogen storage alloy will be high, making it unsuitable for use as a negative electrode for a nickel-metal hydride secondary battery. Therefore, the subscripts a, b, and c are set within the above ranges.

[0034] Furthermore, in the above-mentioned Ln, at least La is selected from the rare earth elements. The content of this La is set to 89.0 mol% or more of Ln. In this way, by including 89.0 mol% or more of La in Ln, corrosion of the surface of the hydrogen storage alloy is suppressed, and the formation of a passivation film is suppressed. Therefore, a nickel-metal hydride secondary battery containing the hydrogen storage alloy of this embodiment in the negative electrode has the effect of suppressing deterioration of low-temperature discharge characteristics after storage in a high-temperature environment.

[0035] Furthermore, in the hydrogen storage alloy according to this embodiment, the hydrogen storage capacity, expressed as H / M, where H is the ratio of the total atomic number M of the elements constituting the hydrogen storage alloy to the number of hydrogen atoms H in the hydrogen storage alloy, is represented by the pressure-composition-isothermal characteristics of the hydrogen storage alloy, which show the relationship between the equilibrium hydrogen pressure during the hydrogen absorption process (Pa) and the equilibrium hydrogen pressure during the hydrogen release process (Pd), at a temperature condition of 80°C. It is preferable to use a hydrogen storage alloy whose hydrogen storage pressure is 0.07 MPa or higher and whose hydrogen release pressure is 0.04 MPa or higher when the above-mentioned H / M ratio reaches 0.4. The above-mentioned effects are more pronounced when the hydrogen storage alloy has a hydrogen storage pressure of 0.07 MPa or higher and a hydrogen release pressure of 0.04 MPa or higher when the above-mentioned H / M ratio reaches 0.4.

[0036] If the hydrogen absorption pressure is less than 0.07 MPa and the hydrogen desorption pressure is less than 0.04 MPa when H / M reaches 0.4, it becomes difficult to fully obtain the above-mentioned effects. On the other hand, if the hydrogen absorption pressure and hydrogen desorption pressure exceed 0.9 MPa when H / M reaches 0.4, the internal pressure of the sealed nickel-metal hydride secondary battery becomes too high during charging, which can lead to problems such as alkaline electrolyte leakage. Therefore, it is preferable that the hydrogen absorption pressure (Pa) and hydrogen desorption pressure (Pd) when H / M reaches 0.4 be within the ranges of 0.07 MPa < Pa < 0.9 MPa and 0.04 MPa < Pa < 0.9 MPa, respectively.

[0037] 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 obtained ingot is subjected to a heat treatment 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, followed by sieving to obtain hydrogen storage alloy particles of the desired particle size.

[0038] Next, the negative electrode 26 can be manufactured, 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 obtained negative electrode mixture paste is applied to a negative electrode core and dried. After drying, the negative electrode core to which the hydrogen storage alloy particles and the like are attached is rolled and cut. This results in the negative electrode 26.

[0039] The positive electrode 24 and the negative electrode 26 obtained as described above are spirally wound with a separator 28 interposed therebetween, thereby forming the electrode group 22 .

[0040] 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.

[0041] [Examples] 1. Battery Production (Example 1) (1) Preparation of hydrogen storage alloy and negative electrode

[0042] First, La, Mg, Ni, and Al were weighed to prepare a mixture having 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 a hydrogen storage alloy ingot.

[0043] The resulting ingot was then loaded into a container, the atmosphere in 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 950°C for 10 hours, whereby the ingot was subjected 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 hydrogen storage alloy particles had a mean particle size by volume (MV) of 25 μm.

[0044] A sample taken from the obtained hydrogen storage alloy powder was subjected to composition analysis by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (hereinafter also referred to as ICP analysis). As a result, the composition of the hydrogen storage alloy was found to be La ０．７６３ Mg ０．２３７ Ni ３．３０ Al ０．１０ The mol% of each element constituting the hydrogen storage alloy was also determined. The results are shown in Table 1.

[0045] Furthermore, the ratio (mol %) of La in Ln in the general formula (I) showing the composition of the hydrogen storage alloy was determined. The results are shown in Table 2.

[0046] Furthermore, samples taken from the obtained hydrogen storage alloy powder were evaluated for PCT (pressure-composition-isothermal) characteristics. Specifically, the evaluation was carried out as follows.

[0047] The hydrogen storage alloy powder was set in a PCT characteristic evaluation device. The hydrogen storage alloy constituting the hydrogen storage alloy powder was hydrogenated at a temperature of 80°C until the maximum hydrogen pressure reached 1 MPa, and then dehydrogenated to activate the hydrogen storage alloy. The activated hydrogen storage alloy was hydrogenated again at a temperature of 80°C until the maximum hydrogen pressure reached 1 MPa. The equilibrium hydrogen pressure during hydrogen absorption was measured. The equilibrium hydrogen pressure at which H / M = 0.4 was determined as the hydrogen absorption pressure (Pa). Next, the hydrogenated hydrogen storage alloy was dehydrogenated at a temperature of 80°C, and the equilibrium hydrogen pressure during hydrogen release was measured. The equilibrium hydrogen pressure at which H / M = 0.4 was determined as the hydrogen release pressure (Pd). The obtained hydrogen absorption pressure (Pa) and hydrogen release pressure (Pd) are shown in Table 2.

[0048] Next, 100 parts by weight of the hydrogen storage alloy powder obtained as described above was mixed with 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.), 0.5 parts by weight of yttrium oxide, and 30 parts by weight of water, and the mixture was kneaded to prepare a paste of a negative electrode mixture.

[0049] 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 was nickel-plated on its surface.

[0050] 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 the negative electrode 26 for AA size.

[0051] (2) Preparation of Positive Electrode Active Material and Positive Electrode Nickel sulfate, zinc sulfate, magnesium sulfate, and cobalt sulfate were weighed out so that the zinc, magnesium, and cobalt contents were 3.0 wt % relative to nickel, 0.4 wt %, and 1.0 wt % relative to nickel, respectively. These were then added to a 1N aqueous solution of sodium hydroxide containing ammonium ions to prepare a mixed aqueous solution. While stirring the resulting mixed aqueous solution, 10N aqueous solution of sodium hydroxide was gradually added to the mixed aqueous solution to allow a reaction to occur. The pH during the reaction was stabilized at 13-14, producing nickel hydroxide particles composed primarily of nickel hydroxide and containing zinc, magnesium, and cobalt as a solid solution. The resulting nickel hydroxide particles were washed three times with 10 times the volume of pure water, and then subjected to a dehydration and drying process to obtain a positive electrode active material powder, which was 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 mean particle size by volume (MV) of the positive electrode active material particles was found to be 10 μm.

[0052] 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 a hydroxypropyl cellulose (HPC) dispersion, 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 serving 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 a positive electrode 24 for AA size.

[0053] (3) Assembly of a Nickel-Metal Hydride Secondary Battery The obtained positive electrode 24 and negative electrode 26 were spirally wound with a separator 28 sandwiched therebetween to prepare an electrode group 22. The separator 28 used in preparing the electrode group 22 here was formed of a sulfonated polypropylene fiber nonwoven fabric, and had a thickness of 0.1 mm (basis weight 53 g / m ２ ) was.

[0054] On the other hand, an aqueous solution containing KOH and LiOH was prepared as an alkaline electrolyte, where the ratio of KOH:LiOH was 7.0N:1.0N.

[0055] 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. The opening of the outer can 10 was then sealed with a sealing member 11, and an AA-size nickel-metal hydride secondary battery 2 with a nominal capacity of 1000 mAh was assembled. Here, the nominal capacity was defined as the discharge capacity of the battery when it was charged at 0.1 A for 16 hours in an environment at a temperature of 25° C., and then discharged at 0.2 A until the battery voltage reached 1.0 V.

[0056] (4) Initial activation treatment: Battery 2 was charged at 0.1 A for 16 hours in an environment at a temperature of 25° C., and then discharged at 0.2 A until the battery voltage reached 1.0 V. This charge-discharge cycle was repeated five times to perform initial activation treatment. In this way, Battery 2 was brought into a usable state.

[0057] Example 2 A nickel-metal hydride secondary battery was assembled in the same manner as in Example 1, except that a hydrogen storage alloy was fabricated by weighing La, Sm, Zr, Mg, Ni, and Al and preparing a mixture of these elements in a molar ratio of 0.675:0.067:0.008:0.25:3.47:0.10.

[0058] Here, the hydrogen storage alloy of Example 2 was subjected to ICP analysis, and the composition was found to be La ０．６７５ Sm ０．０６７ Zr ０．００８ Mg ０．２５ Ni ３．４７ Al ０．１０ The mol % of each element constituting the hydrogen storage alloy of Example 2 was also determined. The results are shown in Table 1.

[0059] Furthermore, for the hydrogen storage alloy of Example 2, the ratio (mol %) of La in Ln, the hydrogen absorption pressure (Pa), and the hydrogen desorption pressure (Pd) are shown in Table 2.

[0060] Example 3 A nickel-metal hydride secondary battery was assembled in the same manner as in Example 1, except that a hydrogen storage alloy was fabricated by weighing La, Sm, Zr, Mg, Ni, and Al and preparing a mixture of these elements in a molar ratio of 0.72:0.072:0.008:0.20:3.47:0.10.

[0061] Here, the hydrogen storage alloy of Example 3 was subjected to ICP analysis, and the composition was found to be La ０．７２ Sm ０．０７２ Zr ０．００８ Mg ０．２０ Ni ３．４７ Al ０．１０ The mol % of each element constituting the hydrogen storage alloy of Example 3 was also determined. The results are shown in Table 1.

[0062] Furthermore, for the hydrogen storage alloy of Example 3, the ratio (mol %) of La in Ln, the hydrogen storage pressure (Pa), and the hydrogen desorption pressure (Pd) are shown in Table 2.

[0063] Example 4 A nickel-metal hydride secondary battery was assembled in the same manner as in Example 1, except that a hydrogen storage alloy was fabricated by weighing La, Sm, Zr, Mg, Ni, and Al and preparing a mixture of these elements in a molar ratio of 0.693:0.069:0.008:0.23:3.47:0.10.

[0064] Here, the hydrogen storage alloy of Example 4 was subjected to ICP analysis, and the composition was found to be La ０．６９３ Sm ０．０６９ Zr ０．００８ Mg ０．２３ Ni ３．４７ Al ０．１０ The mol % of each element constituting the hydrogen storage alloy of Example 4 was also determined. The results are shown in Table 1.

[0065] Furthermore, for the hydrogen storage alloy of Example 4, the ratio (mol %) of La in Ln, the hydrogen storage pressure (Pa), and the hydrogen desorption pressure (Pd) are shown in Table 2.

[0066] Example 5 A nickel-metal hydride secondary battery was assembled in the same manner as in Example 1, except that a hydrogen storage alloy was produced by weighing La, Sm, Zr, Mg, Ni, and Al and preparing a mixture of these elements in a molar ratio of 0.693:0.069:0.008:0.23:3.60:0.10.

[0067] Here, the hydrogen storage alloy of Example 5 was subjected to ICP analysis, and the composition was found to be La ０．６９３ Sm ０．０６９ Zr ０．００８ Mg ０．２３ Ni ３．６０ Al ０．１０ The mol % of each element constituting the hydrogen storage alloy of Example 5 was also determined. The results are shown in Table 1.

[0068] Furthermore, for the hydrogen storage alloy of Example 5, the ratio (mol %) of La in Ln, the hydrogen absorption pressure (Pa), and the hydrogen desorption pressure (Pd) are shown in Table 2.

[0069] Comparative Example 1 A nickel-metal hydride secondary battery was assembled in the same manner as in Example 1, except that a hydrogen storage alloy was fabricated by weighing Nd, Zr, Mg, Ni, and Al and preparing a mixture containing these in a molar ratio of 0.88:0.01:0.110:3.23:0.17.

[0070] Here, the hydrogen storage alloy of Comparative Example 1 was subjected to ICP analysis, and the composition was found to be Nd ０．８８ Zr ０．０１ Mg ０．１１０ Ni ３．２３ Al ０．１７ The mol % of each element constituting the hydrogen storage alloy of Comparative Example 1 was also determined. The results are shown in Table 1.

[0071] Furthermore, for the hydrogen storage alloy of Comparative Example 1, the ratio (mol %) of La in Ln, the hydrogen storage pressure (Pa), and the hydrogen desorption pressure (Pd) are shown in Table 2.

[0072] Comparative Example 2 A nickel-metal hydride secondary battery was assembled in the same manner as in Example 1, except that a hydrogen storage alloy was fabricated by weighing La, Sm, Zr, Mg, Ni, and Al and preparing a mixture containing these elements in a molar ratio of 0.269:0.619:0.009:0.103:3.23:0.17.

[0073] Here, the hydrogen storage alloy of Comparative Example 2 was subjected to ICP analysis, and the composition was found to be La ０．２６９ Sm ０．６１９ Zr ０．００９ Mg ０．１０３ Ni ３．２３ Al ０．１７ The mol % of each element constituting the hydrogen storage alloy of Comparative Example 2 was also determined. The results are shown in Table 1.

[0074] Furthermore, for the hydrogen storage alloy of Comparative Example 2, the ratio (mol %) of La in Ln, the hydrogen storage pressure (Pa), and the hydrogen desorption pressure (Pd) are shown in Table 2.

[0075] 2. Evaluation of Nickel-Metal Hydride Secondary Batteries (1) Low-Temperature Discharge Characteristics The batteries fabricated in Examples 1 to 5 and Comparative Examples 1 and 2 were charged in a 25°C environment with a charging current of 1.0 A until the battery voltage reached its maximum and then dropped by 5 mV. Each battery was then placed in a -30°C environment and allowed to rest for 3 hours. After the 3-hour rest, the battery was discharged under a constant wattage of 1.0 W in the same -30°C environment until the battery voltage reached 0.6 V. The discharge capacity at this time was determined, and this discharge capacity was designated as the pre-storage capacity A [mAh] of the battery. This pre-storage capacity A is shown in Table 2.

[0076] Here, discharging under constant wattage conditions means discharging while adjusting the current value during discharging so that the battery power [W] value obtained by the following formula (II) is always a constant value: Battery power [W] = Actual battery voltage value [V] × Actual current value [A] (II)

[0077] Next, after determining the pre-storage capacity A, each battery was again charged in a 25°C environment with a charging current of 1.0 A until the battery voltage reached its maximum value and then dropped by 5 mV. Each battery was then placed in a 60°C environment and stored for a total of 180 days. During this storage period, a supplemental charge was performed every 30 days of storage. The supplemental charge conditions were as follows: after 30 days of storage, the battery was left in a 25°C environment for 3 hours, and then charged to a fully charged state at a current equivalent to 0.1 C. After supplemental charge, the battery was again placed in a 60°C environment and stored for the remaining days.

[0078] Next, the battery that had been stored at high temperature for a total of 180 days was placed in a -30°C environment and rested for 3 hours. After the 3-hour rest, the battery was discharged under a constant wattage condition of 1.0 W in the same -30°C environment until the battery voltage reached 0.6 V. The discharge capacity at this time was determined, and this discharge capacity was designated as the post-storage capacity B [mAh] of the battery.

[0079] Next, the ratio of the low-temperature discharge capacity before and after high-temperature storage was calculated using the following formula (III). This ratio was defined as the low-temperature discharge maintenance rate C [%] after high-temperature storage and is shown in Table 2. The larger the value of this low-temperature discharge maintenance rate C, the closer the discharge capacity in a low-temperature environment after storage in a high-temperature environment is to the discharge capacity in a low-temperature environment before storage in a high-temperature environment. In other words, the larger the value of the low-temperature discharge maintenance rate C, the more suppressed the deterioration of the discharge characteristics after storage in a high-temperature environment. Low-temperature discharge maintenance rate C [%] after high-temperature storage = capacity B after storage / capacity A before storage × 100 (III)

[0080]

[0081]

[0082] (2) Discussion From the results in Table 2, it was confirmed that Examples 1 to 5 had higher low-temperature discharge maintenance ratios C after high-temperature storage than Comparative Examples 1 and 2, and that the deterioration of discharge characteristics after storage in a high-temperature environment was suppressed.

[0083] From these results, it is possible to determine whether the compounds of the general formula: Ln １－ａ Mg ａ Ni ｂ－ｃ Al ｃ (where Ln represents one or more elements selected from rare earth elements, Y, and Zr, and the subscripts a, b, and c respectively satisfy the relationships a≦0.30, 3.4≦b≦3.7, and c≦0.25), in which Ln in the above general formula is at least La among the rare earth elements, and this La accounts for 89.0 mol % or more of Ln. It can be seen that a nickel-metal hydride secondary battery equipped with a negative electrode containing a hydrogen storage alloy of this type is able to suppress deterioration in discharge characteristics after storage in a high-temperature environment, compared to nickel-metal hydride secondary batteries of Comparative Example 1 and Comparative Example 2 equipped with a negative electrode containing a hydrogen storage alloy that does not contain La or in which La accounts for less than 89.0 mol % of Ln.

[0084] This hydrogen storage alloy, which is represented by the general formula above and in which La accounts for 89.0 mol % or more of Ln, is thought to be resistant to corrosion and to inhibit the formation of a passivation film even when in contact with the alkaline electrolyte in a nickel-metal hydride secondary battery under high-temperature conditions. Therefore, a nickel-metal hydride secondary battery equipped with a negative electrode containing this hydrogen storage alloy is thought to be able to sufficiently initiate a battery reaction and inhibit the deterioration of low-temperature discharge characteristics even when stored in a high-temperature environment and then placed in a low-temperature environment.

[0085] Furthermore, from the results in Table 2, it can be seen that Examples 2 to 5, in which the hydrogen absorption pressure Pa was 0.07 MPa or higher and the hydrogen desorption pressure Pd was 0.04 MPa or higher, had higher low-temperature discharge retention ratios C after high-temperature storage than Example 1, in which the hydrogen absorption pressure Pa was less than 0.07 MPa and the hydrogen desorption pressure Pd was less than 0.04 MPa, and thus demonstrated a greater effect of suppressing the deterioration of low-temperature discharge characteristics. From this, it can be said that a nickel-metal hydride secondary battery having a negative electrode containing a hydrogen storage alloy in which La accounts for 89.0 mol % or more of Ln and further in which the hydrogen absorption pressure Pa is 0.07 MPa or higher and the hydrogen desorption pressure Pd is 0.04 MPa or higher, is more effective in suppressing the deterioration of discharge characteristics after storage in a high-temperature environment.

[0086] 2 Nickel-metal hydride secondary battery 22 Electrode group 24 Positive electrode 26 Negative electrode 28 Separator

Claims

1. General formula: Ln １－ａ Mg ａ Ni ｂ－ｃ A ｃ (wherein Ln represents one or more elements selected from rare earth elements, Y and Zr, and the subscripts a, b and c respectively satisfy the relationships a≦0.30, 3.4≦b≦3.7 and c≦0.25), wherein Ln is at least La selected from among the rare earth elements, and La accounts for 89.0 mol % or more of Ln.

2. The hydrogen storage alloy according to claim 1, wherein in the pressure-composition-isothermal characteristics of the hydrogen storage alloy, which shows the relationship between the hydrogen storage capacity represented by H / M, which is the ratio of the total atomic number M of the elements constituting the hydrogen storage alloy to the number of hydrogen atoms H in the hydrogen storage alloy, and the hydrogen absorption pressure which is the equilibrium hydrogen pressure in the hydrogen absorption process and the hydrogen release pressure which is the equilibrium hydrogen pressure in the hydrogen release process, at a temperature condition of 80°C, the hydrogen absorption pressure is 0.07 MPa or more and the hydrogen release pressure is 0.04 MPa or more when H / M=0.

4.

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

4. A nickel-metal hydride secondary battery comprising: a container; and an electrode group housed in the container together with an alkaline electrolyte, the electrode group including a positive electrode and a negative electrode stacked together with a separator interposed therebetween, the negative electrode being the negative electrode defined in claim 3.

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