Negative electrode for alkaline secondary battery, and alkaline secondary battery comprising this negative electrode

Abstract

[Problem] The present invention provides: a negative electrode for an alkaline secondary battery with which it is possible to achieve both improvement of low-temperature discharge characteristics and suppression of the occurrence of an internal short circuit; and an alkaline secondary battery which comprises the negative electrode. [Solution] A battery 2 is provided with an electrode group 22 that is composed of a positive electrode 24 and a negative electrode 26 that are overlapped with each other with a separator 28 being interposed therebetween. The negative electrode 26 includes a negative electrode core body 40 and a negative electrode mixture 42 that is press-bonded to the negative electrode core body 40. The negative electrode mixture 42 contains a hydrogen storage alloy powder which is an aggregate of hydrogen storage alloy particles. The hydrogen storage alloy particles include first hydrogen storage alloy particles and second hydrogen storage alloy particles. The volume average particle diameter of the second hydrogen storage alloy particles is not less than three times the volume average particle diameter of the first hydrogen storage alloy particles, and the ratio of the first hydrogen storage alloy particles to the total of the first hydrogen storage alloy particles and the second hydrogen storage alloy particles is 80 wt% to 95 wt% inclusive.

Description

Negative electrode for alkaline secondary battery and alkaline secondary battery including said negative electrode

[0001] The present invention relates to a negative electrode for an alkaline secondary battery and to an alkaline secondary battery including this negative electrode.

[0002] Nickel-metal hydride batteries are a type of alkaline secondary battery that uses an alkaline aqueous solution as the electrolyte. Nickel-metal hydride batteries are being used in a variety of applications, including as replacement batteries for alkaline dry batteries and as backup power sources.

[0003] The negative electrode in a nickel-metal hydride secondary battery includes a negative electrode core and a negative electrode mixture supported on the negative electrode core, and the negative electrode mixture contains a hydrogen storage alloy.

[0004] For the negative electrode substrate, a thin metal plate is used because of its advantages of being inexpensive and having excellent mechanical strength. For example, a punched metal plate, which is a smooth metal plate with many openings, each about several millimeters in diameter, formed therein, is used.

[0005] The negative electrode is formed, for example, by applying a slurry of a negative electrode mixture prepared by kneading a hydrogen storage alloy, water, a binder, etc., to the punched metal, and then drying the resulting mixture.

[0006] However, conventional negative electrodes formed as described above have relatively low adhesion between the negative electrode mixture and the negative electrode core, which can cause the negative electrode mixture to partially peel off from the negative electrode core. When peeling of the negative electrode mixture occurs, pieces of the peeled negative electrode mixture may bridge between the negative electrode and positive electrode components within the nickel-metal hydride secondary battery, causing an internal short circuit. To prevent this problem, as shown in Patent Document 1, for example, hydrogen storage alloy particles constituting the hydrogen storage alloy powder are embedded in the negative electrode core to increase the adhesion between the negative electrode core and the negative electrode mixture.

[0007] Japanese Patent Application Publication No. 04-328251

[0008] The applications of nickel-metal hydride secondary batteries are expanding, and in recent years, their use in vehicles has been increasing. When nickel-metal hydride secondary batteries are installed in vehicles used in cold regions, they may be used in low-temperature environments below 0°C. In low-temperature environments, the battery reaction does not proceed easily, resulting in a significant deterioration in low-temperature discharge characteristics and a decrease in output. For vehicle applications, hydrogen storage alloy particles with a small particle size, for example, 15 μm or less, are used to avoid a decrease in output in low-temperature environments. The smaller the particle size, the greater the reaction area, allowing the battery reaction to proceed sufficiently even in low-temperature environments, thereby achieving high output.

[0009] On the other hand, when the particle size of the hydrogen storage alloy becomes smaller, the hydrogen storage alloy particles are less likely to penetrate into the negative electrode substrate, and the negative electrode mixture is more likely to peel off, which makes the above-mentioned internal short circuit more likely to occur.

[0010] One way to prevent peeling of the negative electrode mixture due to the reduction in particle size of the hydrogen storage alloy is to increase the amount of binder, but increasing the amount of binder has adverse effects on battery quality, such as a decrease in the cycle characteristics of the battery and a decrease in battery capacity due to a relative decrease in the amount of hydrogen storage alloy, so increasing the amount of binder is not the best solution.

[0011] For these reasons, it is difficult to simultaneously improve low-temperature discharge characteristics and suppress the occurrence of internal short circuits while keeping the amount of binder low.

[0012] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a negative electrode for an alkaline secondary battery that can achieve both improved low-temperature discharge characteristics and suppression of the occurrence of internal short circuits, and an alkaline secondary battery including this negative electrode.

[0013] According to the present invention, there is provided a negative electrode for an alkaline secondary battery, comprising a negative electrode core and a negative electrode mixture compressed onto the negative electrode core, the negative electrode mixture containing a hydrogen storage alloy powder which is an aggregate of hydrogen storage alloy particles, the hydrogen storage alloy particles including first hydrogen storage alloy particles and second hydrogen storage alloy particles, the second hydrogen storage alloy particles having a volume average particle size three times or more the volume average particle size of the first hydrogen storage alloy particles, and the first hydrogen storage alloy particles accounting for 80 wt % to 95 wt % of the total of the first hydrogen storage alloy particles and the second hydrogen storage alloy particles. The first hydrogen storage alloy particles and the second hydrogen storage alloy particles may have the same composition.

[0014] With the above configuration, the first hydrogen storage alloy particles with a small volume average particle size contribute to expanding the reaction area involved in the battery reaction, thereby improving the low-temperature discharge characteristics of alkaline secondary batteries. Meanwhile, when the negative electrode mixture is pressed onto the negative electrode core, the second hydrogen storage alloy particles with a large volume average particle size penetrate into the negative electrode core, preventing the negative electrode mixture from peeling off, thereby suppressing the occurrence of internal short circuits. Furthermore, the amounts of the first and second hydrogen storage alloy particles are well balanced, providing a sufficient anchoring effect while maintaining good low-temperature discharge characteristics. Furthermore, to prevent preferential and excessive deterioration of the alloy with poor corrosion resistance, it is preferable that the first hydrogen storage alloy particles and the second hydrogen storage alloy have the same composition.

[0015] According to the present invention, it is possible to provide a negative electrode for an alkaline secondary battery that can achieve both improved low-temperature discharge characteristics and suppressed occurrence of internal short circuits, and an alkaline secondary battery including this negative electrode.

[0016] 1 is a partially cutaway perspective view of a nickel-metal hydride secondary battery according to an embodiment of the present invention;

[0017] An embodiment will be described below using an AA size cylindrical nickel-metal hydride secondary battery (hereinafter also simply referred to as battery) 2 as shown in FIG. 1 as an example.

[0018] As shown in FIG. 1 , the battery 2 includes an outer can 10, which is a cylindrical container 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 (plastic deformation processing) 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.

[0019] 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 cylindrical metal positive electrode terminal 20 with a flange is electrically connected to the outer surface of the cover plate 14 so as to cover the valve body 18. The 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).

[0020] 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.

[0021] The exterior can 10 accommodates an electrode group 22. This 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.

[0022] 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.

[0023] 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 a charge / discharge reaction between the positive electrode 24 and the negative electrode 26.

[0024] 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.

[0025] 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.

[0026] As the positive electrode substrate, for example, a nickel-plated mesh-like, sponge-like or fibrous metal body, or foamed nickel can be used.

[0027] 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 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.

[0028] 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.

[0029] 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.

[0030] The positive electrode additive is added to improve the characteristics of the positive electrode, and for example, yttrium oxide, zinc oxide, niobium oxide, etc. can be used.

[0031] 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, a nickel foam and dried. After drying, the nickel foam filled with nickel hydroxide particles and the like is rolled and then cut. This results in a non-sintered positive electrode 24 carrying the positive electrode mixture. In particular, a non-sintered positive electrode using a positive electrode mixture containing nickel hydroxide particles with cobalt dissolved therein improves battery output and increases the charge / discharge capacity per unit volume of the positive electrode, contributing to the miniaturization and high output of batteries.

[0032] Next, a description will be given of the negative electrode 26. The negative electrode 26 has a strip-shaped conductive negative electrode core 40, and this negative electrode core 40 holds a negative electrode mixture 42.

[0033] The negative electrode core 40 is a sheet-like metal material. A flat sheet without holes can be used as this sheet-like metal material. However, it is more preferable to use a punched metal 46 having a plurality of through holes 44 that penetrate from the front surface to the back surface. When the punched metal 46 is used for the negative electrode core 40, the through holes 44 are filled with the negative electrode mixture 42, which is advantageous for improving the bonding between the negative electrode core 40 and the negative electrode mixture 42. Here, the negative electrode mixture 42 is not only filled into the through holes 44 of the punched metal 46, but is also applied in layers on the flat front and back surfaces of the punched metal 46 other than the through holes 44. The negative electrode mixture 42 applied to the flat portions on the front and back surfaces of the negative electrode core 40 is pressed against the negative electrode core 40 by applying pressure, resulting in a pressure-bonded state.

[0034] The negative electrode mixture 42 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 one another, while also binding the negative electrode mixture 42 to the negative electrode core 40. The binder may be a hydrophilic or hydrophobic polymer, carboxymethyl cellulose, or the like, and the conductive material may be carbon black or graphite. Furthermore, a negative electrode additive may be added as needed. Examples of the negative electrode additive include styrene butadiene rubber and sodium polyacrylate.

[0035] Here, the hydrogen storage alloy is not particularly limited, and any hydrogen storage alloy that is generally used in nickel-metal hydride secondary batteries can be used.

[0036] 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. Preferably, the heat treatment is performed in an argon gas atmosphere at a temperature of 900°C or higher and 1000°C or lower for 10 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.

[0037] In this embodiment, first and second hydrogen storage alloy particles having different particle sizes are prepared by adjusting the particle size. The composition of the first and second hydrogen storage alloy particles may be the same. It is easy to achieve the same composition by producing the first and second hydrogen storage alloy particles from the same ingot. When adjusting the particle size, the volume average particle size (MV) of the second hydrogen storage alloy particles is set to be at least three times the volume average particle size (MV) of the first hydrogen storage alloy particles. Furthermore, the volume average particle size (MV) of the second hydrogen storage alloy particles is preferably no more than eight times the volume average particle size (MV) of the first hydrogen storage alloy particles. Here, the particle size is defined as the volume average particle size (MV) measured using a laser diffraction particle size analyzer (manufactured by Malvern Panalytical).

[0038] The volume average particle size of the first hydrogen storage alloy particles is preferably 10 μm or more and 20 μm or less in order to maintain a high level of battery reaction in a low-temperature environment of 0° C. or less. If the volume average particle size is less than 10 μm, the reactivity of the hydrogen storage alloy may be too high, which may result in a decrease in the corrosion resistance of the hydrogen storage alloy. On the other hand, if the volume average particle size exceeds 20 μm, the reactive area of ​​the hydrogen storage alloy may be reduced, which may result in a decrease in reactivity in a low-temperature environment and a corresponding decrease in low-temperature discharge characteristics. Therefore, it is preferable that the volume average particle size of the first hydrogen storage alloy particles be a value within the above range. The volume average particle size of the first hydrogen storage alloy particles is preferably 15 μm or more and 20 μm or less, and more preferably 10 μm or more and 15 μm or less.

[0039] The volume average particle size of the second hydrogen storage alloy particles is preferably equal to or greater than the thickness of the negative electrode core 40, which will be described later. If the volume average particle size of the second hydrogen storage alloy particles is less than the thickness of the negative electrode core 40, the degree to which the second hydrogen storage alloy particles are embedded in the negative electrode core 40 will be reduced, and a sufficient anchoring effect may not be obtained. Furthermore, the volume average particle size of the second hydrogen storage alloy particles is preferably equal to or greater than 40 μm and equal to or less than 80 μm, and is preferably equal to or less than twice the thickness of the negative electrode core 40.

[0040] The first hydrogen storage alloy particles are contained in the negative electrode mixture at a ratio (ratio of first hydrogen storage alloy particles) of 80 wt% to 95 wt% relative to the total of the first and second hydrogen storage alloy particles. If the ratio is less than 80 wt%, the first hydrogen storage alloy particles are relatively scarce, resulting in reduced reactivity in low-temperature environments and ultimately in reduced low-temperature discharge characteristics of the battery. On the other hand, if the ratio exceeds 95 wt%, the second hydrogen storage alloy particles are relatively scarce, resulting in reduced penetration of the second hydrogen storage alloy particles into the negative electrode substrate. As a result, a sufficient anchoring effect cannot be obtained, which makes the negative electrode mixture more susceptible to peeling and induces internal short circuits in the battery. Therefore, the ratio of the first hydrogen storage alloy particles to the total of the first and second hydrogen storage alloy particles is set to a value within the above range. From the viewpoint of low-temperature discharge characteristics in a low-temperature environment, the ratio of the first hydrogen storage alloy particles is preferably 85 wt % or more and 95 wt % or less, and more preferably 90 wt % or more and 95 wt % or less.

[0041] 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 substrate 40 and dried. After drying, the negative electrode substrate 40 to which the negative electrode mixture 42 containing hydrogen storage alloy particles is attached is rolled and cut. This results in the negative electrode 26.

[0042] The positive electrode 24 and the negative electrode 26 obtained as described above are wound into a spiral shape with a separator 28 interposed therebetween, thereby forming the electrode group 22 .

[0043] The electrode group 22 obtained as described above is housed in an outer can 10 serving as a container. 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 brought into a usable state.

[0044] The negative electrode mixture of this embodiment contains first and second hydrogen storage alloy particles and forms a single layer. In other words, the first hydrogen storage alloy particles with a small particle size and the second hydrogen storage alloy particles with a large particle size are mixed in the negative electrode mixture forming a single layer. The pressure applied by the roll rolling described above causes the second hydrogen storage alloy particles with a large particle size to bite into the flat portions of the negative electrode substrate 40, thereby exerting an anchoring effect. This improves the bonding between the negative electrode substrate 40 and the negative electrode mixture 42 and prevents the negative electrode mixture 42 from peeling off from the negative electrode substrate 40. As a result, it is possible to prevent the occurrence of an internal short circuit due to pieces of the negative electrode mixture 42 peeling off from the negative electrode substrate 40. Furthermore, the first hydrogen storage alloy particles with a small particle size contribute to an expansion of the reaction area during the battery reaction, thereby contributing to excellent discharge characteristics even in low-temperature environments.

[0045] Therefore, the obtained battery 2 is an excellent battery that can achieve both improved low-temperature discharge characteristics and suppressed occurrence of internal short circuits.

[0046] Here, in the negative electrode of this embodiment, the binding strength between the negative electrode core and the negative electrode mixture layer is improved due to the anchoring effect of the second hydrogen storage alloy as described above, so peeling of the negative electrode mixture is suppressed even when wound. In conventional cylindrical batteries formed by winding an electrode group, peeling of the negative electrode mixture 42 is likely to occur at the start of winding the electrode group. However, the negative electrode of this embodiment has significantly improved binding strength at the start of winding the electrode group compared to conventional batteries, making it particularly suitable for use in cylindrical batteries formed by winding an electrode group.

[0047] [Examples] 1. Battery Production (Example 1)

[0048] (1) Preparation of hydrogen storage alloy powder and negative electrode First, La, Mg, Ni, and Al were prepared and mixed in a predetermined ratio. The resulting mixture was melted in a high-frequency induction melting furnace, and the molten metal was poured into a mold and cooled to room temperature to form an ingot of hydrogen storage alloy. A sample taken from this ingot was placed in an optical emission spectrometer and subjected to composition analysis by inductively coupled plasma atomic emission spectroscopy (ICP-AES). As a result, the composition of the hydrogen storage alloy was determined to be La, Mg, Ni, and Al. ０．７６３ Mg ０．２３７ Ni ３．３０ Al ０．１０ It was.

[0049] The resulting ingot was then filled into a container, the atmosphere inside 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, 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 resulting hydrogen storage alloy powder was then sieved to obtain two types of hydrogen storage alloy particles with different volume average particle sizes. Specifically, a first hydrogen storage alloy powder, which was an aggregate of first hydrogen storage alloy particles having a first volume average particle size, and a second hydrogen storage alloy powder, which was an aggregate of second hydrogen storage alloy particles having a second volume average particle size, were obtained. The particle size of the obtained hydrogen storage alloy powder was measured using a laser diffraction particle size distribution analyzer manufactured by Malvern Panalytical. The results showed that the mean volume diameter (MV) of the first hydrogen storage alloy particles was 15 μm, and the mean volume diameter (MV) of the second hydrogen storage alloy particles was 45 μm.

[0050] Then, the first hydrogen storage alloy particles and the second hydrogen storage alloy particles were mixed so that the ratio of the first hydrogen storage alloy particles to the total of the first hydrogen storage alloy particles and the second hydrogen storage alloy particles (ratio of the first hydrogen storage alloy particles) was 95 wt %, thereby obtaining a mixed powder of hydrogen storage alloys.

[0051] To 100 parts by weight of the mixed powder of hydrogen storage alloy obtained as described above, 0.3 parts by weight of carboxymethyl cellulose, 0.5 parts by weight of hollow carbon black (Ketjenblack (registered trademark) manufactured by Lion Specialty Chemicals Corporation), and 30 parts by weight of water were added and kneaded to prepare a paste of a negative electrode mixture.

[0052] Next, the negative electrode mixture paste was applied evenly and to a uniform thickness to both sides of a punched metal 46 serving as a negative electrode core 40. The punched metal 46 used was a nickel-plated cold-rolled steel sheet (SPCC steel sheet) strip with a plurality of through holes 44 arranged in a houndstooth pattern from the front to the back. The punched metal 46 had a thickness of 45 μm and an aperture ratio of 43.2%. The diameter of the through holes 44 was 1 mm.

[0053] The paste was dried to produce an intermediate negative electrode product. The intermediate negative electrode product 26, which held the negative electrode mixture 42 containing the hydrogen storage alloy powder, was then rolled to increase the amount of alloy per volume, and the second hydrogen storage alloy particles were pressed into the flat surface of the punched metal. The resulting product was then cut to a predetermined size to obtain the negative electrode 26 carrying a predetermined amount of hydrogen storage alloy.

[0054] (2) Preparation of Positive Electrode Nickel sulfate, zinc sulfate, magnesium sulfate, and cobalt sulfate were weighed out so that the zinc, magnesium, and cobalt contents were 3% by weight relative to nickel, 0.4% by weight, and 1% by weight, respectively, and these were 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, a 10N aqueous solution of sodium hydroxide was gradually added to the mixed aqueous solution to cause a reaction. The pH during this reaction was stabilized at 13 to 14, producing nickel hydroxide particles mainly composed of nickel hydroxide with zinc, magnesium, and cobalt dissolved therein.

[0055] The obtained nickel hydroxide particles were washed three times with 10 times the amount of pure water, followed by dehydration and drying. In this way, nickel hydroxide powder (positive electrode active material powder) was obtained, which was an aggregate of nickel hydroxide particles. The particle size of the obtained nickel hydroxide particles was measured using a scanning electron microscope, and it was confirmed that they were spherical with an average particle size of 10 μm.

[0056] Next, 100 parts by weight of the positive electrode active material powder, which was an aggregate of nickel hydroxide particles prepared as described above, was mixed with 1.0 part by weight of cobalt hydroxide powder, and further mixed with 0.3 parts by weight of yttrium oxide, 1.0 part by weight of zinc oxide, 0.6 parts by weight of niobium oxide, 1.0 part by weight of HPC dispersion liquid, and 30 parts by weight of water to prepare a positive electrode mixture slurry. This positive electrode mixture slurry was then filled into a sheet-shaped foamed nickel as a positive electrode substrate. The filled positive electrode mixture slurry was then dried to produce a positive electrode intermediate product. The obtained positive electrode intermediate product was rolled and then cut to a predetermined size to obtain a positive electrode 24 carrying a predetermined amount of positive electrode active material.

[0057] (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 made of a sulfonated polypropylene fiber nonwoven fabric, and had a thickness of 0.1 mm (basis weight 53 g / m ２ ) was.

[0058] On the other hand, an alkaline electrolyte solution was prepared, which was an aqueous solution containing KOH and LiOH in a ratio of KOH:LiOH=7.0:1.0.

[0059] Next, the electrode group 22 was housed in a cylindrical outer can 10 with a bottom, and a predetermined amount of the prepared alkaline electrolyte was poured into the can 10. Thereafter, the opening of the can 10 was 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.

[0060] (4) Initial Activation Treatment The obtained nickel-metal hydride secondary battery 2 was charged at a current of 0.1 C for 16 hours in an environment at a temperature of 25° C., and then discharged at a current of 0.2 C until the battery voltage reached 0.5 V. This initial activation treatment was repeated twice. In this way, the nickel-metal hydride secondary battery 2 was ready for use.

[0061] (Example 2) A nickel-metal hydride secondary battery was fabricated in the same manner as in Example 1, except that the ratio of the first hydrogen storage alloy particles to the total of the first hydrogen storage alloy particles and the second hydrogen storage alloy particles (ratio of the first hydrogen storage alloy particles) was 90 wt %.

[0062] (Example 3) A nickel-metal hydride secondary battery was fabricated in the same manner as in Example 1, except that the second hydrogen storage alloy powder used was made of second hydrogen storage alloy particles having a volume average particle size of 60 μm.

[0063] (Example 4) A nickel-metal hydride secondary battery was fabricated in the same manner as in Example 1, except that the ratio of the first hydrogen storage alloy particles to the total of the first hydrogen storage alloy particles and the second hydrogen storage alloy particles (ratio of the first hydrogen storage alloy particles) was 80 wt %.

[0064] Comparative Example 1 A nickel-metal hydride secondary battery was fabricated in the same manner as in Example 1, except that the second hydrogen storage alloy powder was not used and only the first hydrogen storage alloy powder was used.

[0065] (Comparative Example 2) A nickel-metal hydride secondary battery was fabricated in the same manner as in Example 1, except that the second hydrogen storage alloy powder used was made of second hydrogen storage alloy particles having a volume average particle size of 20 μm.

[0066] (Comparative Example 3) A nickel-metal hydride secondary battery was fabricated in the same manner as in Example 1, except that the second hydrogen storage alloy powder used was made of second hydrogen storage alloy particles having a volume average particle size of 30 μm.

[0067] (Comparative Example 4) A nickel-metal hydride secondary battery was fabricated in the same manner as in Example 1, except that the second hydrogen storage alloy powder used was made of second hydrogen storage alloy particles having a volume average particle size of 40 μm.

[0068] (Comparative Example 5) A nickel-metal hydride secondary battery was fabricated in the same manner as in Example 1, except that the ratio of the first hydrogen storage alloy particles to the total of the first hydrogen storage alloy particles and the second hydrogen storage alloy particles (ratio of the first hydrogen storage alloy particles) was 65 wt %.

[0069] (Comparative Example 6) A nickel-metal hydride secondary battery was fabricated in the same manner as in Example 1, except that the ratio of the first hydrogen storage alloy particles to the total of the first hydrogen storage alloy particles and the second hydrogen storage alloy particles (ratio of the first hydrogen storage alloy particles) was 50 wt %.

[0070] 2. Battery Evaluation (1) Low-Temperature Discharge Characteristics Test The batteries of Examples 1 to 4 and Comparative Examples 1 to 6 were charged in a 25°C environment with a charging current of 1000 mA until the battery voltage reached its maximum value and then dropped by 5 mV, a so-called -ΔV control, and then placed in a -30°C environment and allowed to rest for 3 hours. Next, after the 3-hour rest, the batteries were discharged in a -30°C environment with the discharge output controlled to 3 W until the battery voltage reached 0.8 V. The discharge time until the battery voltage reached 0.8 V was measured. The discharge time measurement results are shown in Table 1 as low-temperature discharge characteristics. A longer discharge time indicates better low-temperature discharge characteristics.

[0071] (2) Inspection of peeling of negative electrode mixture The batteries of Examples 1 to 4 and Comparative Examples 1 to 6 were inspected using an X-ray CT inspection device to check for peeling of the negative electrode mixture at the start of winding the electrode group. 100 batteries of each type were inspected, and the number of batteries in which peeling of the negative electrode mixture occurred was counted. The proportion of batteries in which peeling occurred was calculated as a percentage, and the results are shown in Table 1 as the rate of peeling of the negative electrode mixture. A higher rate of peeling of the negative electrode mixture indicates a higher possibility of an internal short circuit.

[0072]

[0073] 3. Discussion The battery of Comparative Example 1, which corresponds to a conventional example, has excellent low-temperature discharge characteristics, but the rate of peeling of the negative electrode mixture is high, and there is a high possibility of an internal short circuit occurring.

[0074] In addition, the batteries of Comparative Examples 2 to 4, in which the volume average particle size of the second hydrogen storage alloy particles is less than three times the volume average particle size of the first hydrogen storage alloy particles, also have a high rate of peeling of the negative electrode mixture and a high possibility of internal short circuits occurring.

[0075] On the other hand, the batteries of Examples 1 to 4 and Comparative Examples 5 to 6 had a 0% incidence of anode mixture peeling and an extremely low possibility of internal short circuit occurrence. This is thought to be because the volume average particle size of the second hydrogen storage alloy particles was set to at least three times the volume average particle size of the first hydrogen storage alloy particles, thereby obtaining a sufficient anchor effect and effectively suppressing peeling of the anode mixture.

[0076] However, the batteries of Comparative Examples 5 and 6 had inferior low-temperature discharge characteristics compared to the batteries of Examples 1 to 4. This is thought to be because the proportion of the first hydrogen-absorbing alloy particles in the total hydrogen-absorbing alloy particles was less than 80 wt %, resulting in a relatively small amount of the first hydrogen-absorbing alloy particles, and as a result, their reactivity in low-temperature environments was reduced.

[0077] In contrast, in the batteries of Examples 1 to 4, the ratio of first hydrogen storage alloy particles to the total hydrogen storage alloy particles is 80 wt % or more, so there is a relatively large amount of first hydrogen storage alloy particles, and the low-temperature discharge characteristics are also excellent, and it is thought that this has enabled the batteries to achieve both improved low-temperature discharge characteristics and the suppression of internal short circuits.

[0078] From the above, it can be said that by making the volume average particle size of the second hydrogen storage alloy particles at least three times the volume average particle size of the first hydrogen storage alloy particles and adjusting the ratio of the first hydrogen storage alloy particles to the total hydrogen storage alloy particles, as in the batteries of Examples 1 to 4, it is possible to provide an excellent battery that achieves both improved low-temperature discharge characteristics and suppression of internal short circuits.

[0079] The present invention is not limited to the nickel-metal hydride secondary batteries described in the above embodiments and examples, and various modifications are possible. The battery to which the present invention is applicable may be any alkaline secondary battery containing a hydrogen storage alloy in the negative electrode, and in addition to nickel-metal hydride secondary batteries, examples include hydrogen-air secondary batteries.

[0080] 2 Nickel-metal hydride secondary battery 22 Electrode group 24 Positive electrode 26 Negative electrode 28 Separator 40 Negative electrode substrate 42 Negative electrode mixture 44 Through-hole 46 Punched metal

Claims

1. A negative electrode for an alkaline secondary battery, comprising a negative electrode core and a negative electrode mixture pressed onto the negative electrode core, wherein the negative electrode mixture contains hydrogen storage alloy powder which is an aggregate of hydrogen storage alloy particles, wherein the hydrogen storage alloy particles include first hydrogen storage alloy particles and second hydrogen storage alloy particles, wherein the volume average particle size of the second hydrogen storage alloy particles is at least three times the volume average particle size of the first hydrogen storage alloy particles, and wherein the proportion of the first hydrogen storage alloy particles to the total of the first hydrogen storage alloy particles and the second hydrogen storage alloy particles is 80 wt% or more and 95 wt% or less.

2. The negative electrode for an alkaline secondary battery according to claim 1, wherein the hydrogen storage alloys constituting the first hydrogen storage alloy particles and the second hydrogen storage alloy particles are of the same type.

3. The negative electrode for an alkaline secondary battery according to claim 1, wherein the first hydrogen storage alloy particles and the second hydrogen storage alloy particles have the same composition.

4. The negative electrode for an alkaline secondary battery according to any one of claims 1 to 3, wherein the volume average particle size of the second hydrogen storage alloy particles is equal to or greater than the thickness of the negative electrode substrate.

5. An alkaline secondary battery comprising: an electrode group consisting of a positive electrode and a negative electrode stacked together with a separator interposed therebetween; and a container accommodating the electrode group together with an alkaline electrolyte, wherein the positive electrode includes a positive electrode substrate and a positive electrode mixture held by the positive electrode substrate, and the negative electrode is the negative electrode for an alkaline secondary battery according to claim 1.

6. The alkaline secondary battery according to claim 5, wherein the positive electrode mixture contains nickel hydroxide as a positive electrode active material.

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