Nickel-metal hydride secondary battery

Abstract

To provide a nickel hydrogen secondary battery, capable of achieving both high output and an improvement in charge acceptance at high temperatures.SOLUTION: A nickel hydrogen secondary battery 2 includes an electrode group 22 comprising a positive electrode 24 and a negative electrode 26 overlapped through a separator 28. The negative electrode 26 includes a hydrogen storage alloy. The hydrogen storage alloy has a composition represented by general formula: Ln1-x-yMgxZryNiz-a-bAlaCrb (in the formula, Ln represents a rare earth element, x, y, z, a and b satisfy the conditions of 0.1≤x≤0.2, 0≤y≤0.02, 3.5≤z≤3.8, 0.08≤a≤0.2, and 0≤b≤0.05, respectively, and at least one of Zr and Cr is contained.). In a pressure-composition-isotherm diagram at 80°C, a hydrogen equilibrium pressure at hydrogen storage capacity H / M=0.5 is 0.1 MPa or more and 0.13 MPa or less, and a ratio N / P of capacity N of the negative electrode to capacity P of the positive electrode is 2.7 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a nickel-metal hydride secondary battery. [Background technology]

[0002] Nickel-metal hydride secondary batteries are now being used in a variety of applications, such as as replacement batteries for alkaline batteries, backup power sources, power sources for electric vehicles, etc. In particular, nickel-metal hydride secondary batteries are becoming increasingly important for in-vehicle use in electric vehicles.

[0003] Nickel-metal hydride secondary batteries for automotive applications are required to have excellent durability and charge / discharge characteristics under high-temperature conditions. However, when charging under high-temperature conditions, the charging reaction at the positive electrode competes with the electrolysis reaction of water, resulting in a decrease in charge acceptance.

[0004] As a means for improving charge acceptance under such high-temperature conditions, for example, yttrium is added to the positive electrode to lower the potential of the charge reaction at the positive electrode (see Patent Document 1), and sodium ions are added to the alkaline electrolyte to increase the water decomposition potential. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-130249 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the above-mentioned measures for improving charge acceptance under high-temperature conditions may result in a decrease in the output of nickel-metal hydride secondary batteries. In particular, in low-temperature environments, the battery reaction is less likely to proceed, and output is already reduced. Therefore, attempts to improve charge acceptance under high-temperature conditions result in a significant decrease in discharge characteristics in low-temperature environments. Therefore, in applications requiring high output over a wide temperature range, such as in-vehicle applications, there is a limit to how much charge acceptance can be improved.

[0007] In other words, it is difficult to achieve both improved charge acceptance at high temperatures and high output. Therefore, there is a demand for the development of a nickel-metal hydride secondary battery that has excellent charge acceptance at high temperatures and high output.

[0008] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a nickel-metal hydride secondary battery that can achieve both improved charge acceptance at high temperatures and high output. [Means for solving the problem]

[0009] According to the present invention, a battery includes an electrode group consisting of a positive electrode and a negative electrode stacked with a separator interposed therebetween, and a container accommodating the electrode group together with an alkaline electrolyte, wherein the positive electrode contains nickel hydroxide, and the negative electrode contains a hydrogen storage alloy represented by the general formula: Ln 1-x-y Mg x Zr y Ni z-a-b Al a Cr b (wherein Ln represents a rare earth element, and x, y, z, a, and b respectively satisfy the conditions of 0.1≦x≦0.2, 0≦y≦0.02, 3.5≦z≦3.8, 0.08≦a≦0.2, and 0≦b≦0.05, and the battery contains at least one of Zr and Cr), and in a pressure-composition-isotherm diagram at 80°C, the hydrogen equilibrium pressure at which the hydrogen storage capacity H / M=0.5 is 0.1 MPa or more and 0.13 MPa or less, and the ratio N / P of the capacity N of the negative electrode to the capacity P of the positive electrode is 2.7 or more. [Effects of the Invention]

[0010] The nickel-metal hydride secondary battery of the present invention comprises an electrode group consisting of a positive electrode and a negative electrode stacked with a separator interposed therebetween, and a container accommodating the electrode group together with an alkaline electrolyte, wherein the positive electrode contains nickel hydroxide, and the negative electrode contains a hydrogen storage alloy represented by the general formula: Ln 1-x-y Mg x Zr y Ni z-a-b Al a Cr b (wherein Ln represents a rare earth element, and x, y, z, a, and b respectively satisfy the conditions of 0.1≦x≦0.2, 0≦y≦0.02, 3.5≦z≦3.8, 0.08≦a≦0.2, and 0≦b≦0.05, and contain at least one of Zr and Cr), and in a pressure-composition-isotherm diagram at 80°C, the hydrogen equilibrium pressure at a hydrogen storage capacity H / M=0.5 is 0.1 MPa or more and 0.13 MPa or less, and the ratio N / P of the capacity N of the negative electrode to the capacity P of the positive electrode is 2.7 or more. With this configuration, the nickel-metal hydride secondary battery can have improved charge acceptance at high temperatures and can also exhibit high power. Therefore, according to the present invention, it is possible to provide a nickel-metal hydride secondary battery that can achieve both improved charge acceptance at high temperatures and high power. [Brief explanation of the drawings]

[0011] [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

[0012] An embodiment will be described below using an AAA-sized cylindrical nickel-metal hydride secondary battery (hereinafter referred to as battery) 2 as shown in FIG. 1 as an example.

[0013] As shown in FIG. 1 , the battery 2 includes a cylindrical outer can (container) 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, and 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.

[0014] 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).

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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 as described above, and the mixture is kneaded to prepare a positive electrode mixture slurry. The resulting positive electrode mixture slurry is then 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.

[0027] 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.

[0028] 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.

[0029] 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, while also binding the negative electrode mixture to the negative electrode substrate. Hydrophilic or hydrophobic polymers 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. Examples of the negative electrode additive include styrene butadiene rubber and sodium polyacrylate.

[0030] Here, the hydrogen storage alloy used has a composition represented by the following general formula (I). Ln 1-x-y Mg x Zr y Niz-a-b Al a Cr b (I)

[0031] In general formula (I), Ln represents a rare earth element, specifically at least one element selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y. The subscripts x, y, z, a, and b satisfy the following conditions: 0.1≦x≦0.2, 0≦y≦0.02, 3.5≦z≦3.8, 0.08≦a≦0.2, and 0≦b≦0.05, respectively. Furthermore, it is essential that at least one of Zr and Cr is contained. The presence of at least one of Zr and Cr in the composition of the hydrogen storage alloy allows these elements to migrate from the negative electrode to the positive electrode, increasing the oxygen evolution potential of the positive electrode and improving charge acceptance in high-temperature environments. However, if Zr and Cr are added in excess, the effect on the hydrogen release pressure of the hydrogen storage alloy becomes significant, making it difficult to apply the hydrogen storage alloy to a negative electrode. Therefore, y, which defines the amount of Zr, and b, which defines the amount of Cr, are added within the ranges of 0≦y≦0.02 and 0≦b≦0.05. However, as mentioned above, since it is necessary to contain either Zr or Cr, it is necessary to avoid the values ​​of y and b both being 0.

[0032] Here, a hydrogen storage alloy is a combination of a metal element with a high affinity for hydrogen (hereinafter referred to as element A) and a metal element with a low affinity for hydrogen (hereinafter referred to as element B). If the value of the subscript z, which indicates the stoichiometric ratio between element A and element B, is less than 3.5, the battery's output characteristics will decrease, and if the value of z exceeds 3.8, pulverization will occur easily during charging and discharging, significantly reducing the battery's lifespan. Therefore, the value of z is set to 3.5≦z≦3.8 as described above.

[0033] Furthermore, in the hydrogen storage alloy of this embodiment, the hydrogen equilibrium pressure at a hydrogen storage capacity (H / M) of 0.5 in a pressure-composition-isotherm diagram (PCT diagram) at 80°C is 0.1 MPa or more and 0.13 MPa or less. If the hydrogen equilibrium pressure is less than 0.1 MPa, the supply of hydrogen becomes insufficient, the cell reaction does not proceed smoothly, and the output characteristics of the cell may be reduced. On the other hand, if the hydrogen equilibrium pressure exceeds 0.13 MPa, the charging efficiency may be reduced.

[0034] The above-mentioned hydrogen storage alloy particles can be obtained, for example, as follows. First, metal raw materials are weighed and mixed to achieve 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. Preferably, the ingot is heat-treated 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 and sieved to obtain hydrogen storage alloy particles of the desired particle size.

[0035] 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.

[0036] 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.

[0037] Here, assuming that the capacity [mAh] of the positive electrode 24 is P and the capacity [mAh] of the negative electrode 26 is N, the ratio of the capacity N to the capacity P, N / P, is set to 2.7 or greater. If the N / P value is less than 2.7, there is a concern that corrosion of the hydrogen storage alloy in the negative electrode will shorten the battery's lifespan, and that insufficient surface area between the positive and negative electrodes will result in insufficient battery output. Furthermore, if the N / P value is less than 2.7, the amount of Zr and Cr migrating from the negative electrode will be reduced, and the effect of improving the charging temperature characteristics achieved by including at least one of Zr and Cr in the hydrogen storage alloy in the negative electrode may not be achieved. For this reason, the N / P value is set to 2.7 or greater.

[0038] Furthermore, it has been confirmed that an N / P value of up to 3.1 is effective in achieving both improved charge acceptance at high temperatures and higher output.

[0039] Here, the positive electrode capacity P [mAh] can be calculated by measuring the unit capacity of the positive electrode active material in a single-electrode evaluation and multiplying it by the amount of the positive electrode active material. The unit capacity of the positive electrode active material can be calculated as follows. First, a predetermined amount of the positive electrode active material is mixed with Ni powder and molded into a tablet to create a reference positive electrode. The resulting reference positive electrode and a predetermined negative electrode as a counter electrode are placed in an alkaline electrolyte and charged and discharged. The discharge capacity at this time is then determined, and the unit capacity of the positive electrode active material is calculated from the obtained discharge capacity.

[0040] On the other hand, the negative electrode capacity N [mAh] can be calculated by measuring the unit capacity of the hydrogen storage alloy in a single-electrode evaluation and multiplying it by the amount of hydrogen storage alloy. The unit capacity of the hydrogen storage alloy can be calculated as follows. First, a predetermined amount of the hydrogen storage alloy is mixed with Ni powder and molded into a tablet to create a reference negative electrode. The resulting reference negative electrode and a predetermined positive electrode as a counter electrode are placed in an alkaline electrolyte and charged and discharged. The discharge capacity at this time is then determined, and the unit capacity of the hydrogen storage alloy is calculated from the obtained discharge capacity.

[0041] The positive electrode capacity P can be adjusted by the amount of positive electrode active material supported on the positive electrode, and the negative electrode capacity N can be adjusted by the amount of hydrogen storage alloy supported on the negative electrode. The N / P value can be adjusted by adjusting the amounts of the positive electrode active material and hydrogen storage alloy supported. For example, the amounts of the positive electrode active material and hydrogen storage alloy supported can be adjusted by cutting the intermediate positive and negative electrode products to predetermined dimensions. This allows the amounts of the positive electrode active material and hydrogen storage alloy supported in the resulting positive and negative electrodes to be adjusted.

[0042] The electrode group 22 obtained as described above 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 cover 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] The electrolyte contained in the alkaline electrolyte is preferably not sodium hydroxide. This is because using sodium hydroxide as the electrolyte improves charge acceptance but reduces discharge performance. In this embodiment, the electrolyte contained in the alkaline electrolyte is preferably at least one of potassium hydroxide and lithium hydroxide.

[0044] In addition, in the above-mentioned negative electrode 26, it is preferable to partially coat the surface of the negative electrode with perfluoroalkoxyalkane (PFA). PFA has water repellency, which makes it possible to adjust the contact area between the alkaline electrolyte and the negative electrode surface, thereby improving the gas absorption capacity of the negative electrode. This makes it possible to suppress an increase in the internal pressure of the battery. The amount of PFA coated is 0.6 mg / cm. 2 More than 2.6mg / cm 2 It is preferable that the amount of PFA applied is 0.6 mg / cm or less. 2 If the amount of PFA applied is less than 2.6 mg / cm, the effect of suppressing the increase in the internal pressure of the battery is small. 2If the amount of PFA applied exceeds this range, the negative electrode surface will be excessively coated with PFA, resulting in a decrease in the reactive area of ​​the negative electrode, which will result in a lack of gas absorption capacity and a decrease in the output characteristics of the battery. Therefore, it is preferable that the amount of PFA applied be within the above range.

[0045] [Example] 1. Battery manufacturing Example 1

[0046] (1) Preparation of hydrogen storage alloy powder and anode First, La, Sm, Mg, Ni, Al, and Zr were prepared, and a mixture containing these elements in a predetermined ratio was prepared. 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 its composition was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). As a result, the composition of the hydrogen storage alloy was determined to be La, 0.54 Sm 0.36 Mg 0.10 Ni 3.71 Al 0.09 Zr 0.01 It was.

[0047] The resulting ingot was then loaded into a heat treatment vessel, the interior of the vessel was replaced with argon, and the vessel was then sealed. The heat treatment vessel 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 35 μm.

[0048] A sample for measuring hydrogen equilibrium pressure was taken from the obtained hydrogen storage alloy powder, and the pressure-composition-isotherm (PCT line) of the sample was determined under hydrogen pressure at 80°C by the Sieverts method in accordance with Japanese Industrial Standards (JIS H7201), and the effective hydrogen storage capacity (H / M) and the hydrogen equilibrium pressure at H / M = 0.5 were determined. The results are shown in Table 1.

[0049] To 100 parts by weight of the hydrogen storage alloy powder obtained as described above, 0.4 parts by weight of sodium polyacrylate, 0.1 parts by weight of carboxymethyl cellulose, 2.0 parts by weight of a 50% solids dispersion of styrene butadiene rubber (SBR), 0.5 parts by weight of hollow carbon black (Ketjenblack (registered trademark) manufactured by Lion Specialty Chemicals Co., Ltd.), and 30 parts by weight of water were added and kneaded to prepare a paste of a negative electrode mixture.

[0050] Next, 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 core. The perforated iron plate had a thickness of 60 μm and was nickel-plated on its surface.

[0051] The paste was dried to produce an intermediate negative electrode product, which then was rolled to increase the amount of alloy per volume and cut to a predetermined size to obtain an intermediate negative electrode 26 carrying a predetermined amount of hydrogen storage alloy.

[0052] Furthermore, in this example, PFA was partially applied to the surface of the obtained negative electrode 26. At this time, the amount of PFA applied was 1.5 mg / cm 2 In this way, a negative electrode 26 partially coated with PFA was obtained.

[0053] In the battery fabricated in this example, the negative electrode capacity of the negative electrode is adjusted together with the positive electrode capacity of the positive electrode, which will be described later, so that N / P, which is the ratio of the negative electrode capacity N [mAh] of the negative electrode to the positive electrode capacity P [mAh] of the positive electrode, is 3.1. The negative electrode capacity can be calculated by measuring the unit capacity of the hydrogen storage alloy in a single-electrode evaluation and multiplying it by the amount of hydrogen storage alloy held in the negative electrode. The unit capacity of this hydrogen storage alloy can be determined as follows.

[0054] For example, a measurement sample taken from the hydrogen storage alloy powder obtained as described above and nickel powder are prepared. 0.25 g of the hydrogen storage alloy powder as the measurement sample is mixed with 0.75 g of nickel powder to prepare a mixed powder, which is then molded to produce a circular tablet-shaped electrode with a diameter of 10 mm.

[0055] Next, 100 mL of an 8 mol / L KOH aqueous solution was poured into a cylindrical resin container, and a tablet electrode and a mercury oxide reference electrode were placed in the center of the container within the KOH solution. Furthermore, a nickel hydroxide counter electrode with a capacity sufficiently large relative to the negative electrode (tablet electrode) was placed along the inner circumferential wall of the container. A charge-discharge test was conducted on this battery, in which the battery was charged at 0.5 It for 200 minutes and then discharged at 0.5 It until the negative electrode potential reached -0.3 V relative to the mercury oxide reference electrode. The electrochemical alloy capacity (unit capacity of the hydrogen storage alloy) was determined. Note that in the charge-discharge test using the tablet electrode described above, the negative electrode capacity calculated assuming an alloy capacity of 300 mAh / g was 1 It.

[0056] Once the unit capacity of the hydrogen storage alloy is determined, it is possible to determine how much hydrogen storage alloy should be supported on the negative electrode to obtain the desired negative electrode capacity. One method for adjusting the amount of hydrogen storage alloy supported is, for example, to adjust the longitudinal length of the intermediate negative electrode product when cutting it. For an intermediate negative electrode product in which the negative electrode mixture is applied at a constant thickness, the longitudinal length of the intermediate product is proportional to the amount of hydrogen storage alloy supported thereon. Therefore, the amount of hydrogen storage alloy can be adjusted by adjusting the length of the intermediate negative electrode product.

[0057] (2) Preparation of the positive electrode Nickel sulfate, zinc sulfate, magnesium sulfate, and cobalt sulfate were weighed out so that the ratio of zinc to nickel was 3% by weight, magnesium 0.4% by weight, and cobalt 1% by weight, 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, 10N aqueous solution of sodium hydroxide was gradually added to the mixed aqueous solution to cause a reaction, and the pH during the reaction was stabilized at 13 to 14, producing nickel hydroxide particles mainly composed of nickel hydroxide with zinc, magnesium, and cobalt dissolved therein.

[0058] The obtained nickel hydroxide particles were washed three times with 10 times the amount of pure water, and then dehydrated and dried. In this way, nickel hydroxide powder (positive electrode active material powder) was obtained, which was an aggregate of nickel hydroxide particles. The obtained nickel hydroxide particles were spherical with an average particle size of 10 μm.

[0059] 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.

[0060] Here, the positive electrode capacity of the positive electrode 24 can be calculated by measuring the unit capacity of the positive electrode active material in a single-electrode evaluation and multiplying it by the amount of positive electrode active material retained in the positive electrode. This unit capacity of the positive electrode active material can be determined as follows.

[0061] For example, a measurement sample taken from the positive electrode active material powder obtained as described above and nickel powder were prepared. 0.25 g of the measurement sample positive electrode active material powder and 0.75 g of nickel powder were mixed to prepare a mixed powder, which was then molded to prepare a circular tablet-shaped electrode with a diameter of 10 mm.

[0062] Next, 100 mL of an 8 mol / L KOH aqueous solution was poured into a cylindrical resin container, and a tablet electrode and a mercury oxide reference electrode were placed in the center of the container within the KOH solution. Furthermore, a hydrogen storage alloy counter electrode with a capacity sufficiently large relative to the positive electrode (tablet electrode) was placed along the inner circumferential wall of the container. This battery was subjected to a charge / discharge test, in which the battery was charged at 0.5 It for 200 minutes and discharged at 0.5 It until the positive electrode potential reached −0.3 V relative to the mercury oxide reference electrode. The electrochemical alloy capacity (unit capacity of the positive electrode active material) was determined. Note that in the charge / discharge test using the tablet electrode, the positive electrode capacity was calculated as 1 It, assuming a positive electrode active material capacity of 300 mAh / g.

[0063] Once the unit capacity of the positive electrode active material is determined, it is possible to determine how much positive electrode active material should be loaded onto the positive electrode to obtain a desired positive electrode capacity. One method for adjusting the amount of positive electrode active material loaded is, for example, adjusting the longitudinal length of a positive electrode intermediate product when cutting the intermediate product. In a positive electrode intermediate product in which a fixed amount of positive electrode mixture is filled, the longitudinal length of the intermediate product is proportional to the amount of positive electrode active material loaded therein. Therefore, by adjusting the length of the positive electrode intermediate product, the amount of active material can be adjusted. Furthermore, when adjusting the ratio between the positive electrode capacity of the positive electrode and the negative electrode capacity of the negative electrode, the N / P value can be easily adjusted to a desired value by adjusting the length of the negative electrode and the length of the positive electrode.

[0064] (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 made of a nonwoven fabric made of sulfonated polypropylene fibers, and had a thickness of 0.1 mm (basis weight 53 g / m 2 ) was.

[0065] 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.

[0066] 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 in. Thereafter, the opening of the outer can 10 was sealed with a sealing member 11, and an AAA-size nickel-metal hydride secondary battery 2 with a nominal capacity of 220 mAh was assembled.

[0067] The assembled nickel-metal hydride secondary battery 2 was subjected to an initial activation process by repeating five charge-discharge cycles, each cycle consisting of charging for 16 hours at a charging current of 0.022 A in an environment at a temperature of 25°C, followed by discharging at a discharging current of 0.044 A until the battery voltage reached 1.0 V.

[0068] Example 2 La, Sm, Mg, Ni, Al, and Cr were prepared, and the composition of the hydrogen storage alloy was La 0.54 Sm 0.36 Mg 0.10 Ni 3.70 Al 0.09 Cr 0.01 A nickel-metal hydride secondary battery was fabricated in the same manner as in Example 1, except that:

[0069] (Comparative Example 1) La, Sm, Mg, Ni, and Al were prepared, and the composition of the hydrogen storage alloy was La 0.54 Sm 0.36 Mg 0.10 Ni 3.71 Al 0.09A nickel-metal hydride secondary battery was fabricated in the same manner as in Example 1, except that:

[0070] 2. Battery evaluation (1) High-temperature charging test The batteries of Examples 1 and 2 and Comparative Example 1 were rested for 3 hours in a 25°C environment, and then charged for 16 hours with a charging current of 0.022 A. Then, the batteries were rested for 3 hours. Then, after the 3-hour rest, the batteries were discharged in a 25°C environment with a discharge current of 0.22 A until the battery voltage reached 0.6 V. The discharge capacity at this time was measured. The result was taken as the discharge capacity during charging at 25°C.

[0071] The discharged battery was then rested for 3 hours in a 55°C environment. After resting for 3 hours in a 55°C environment, the battery was charged for 16 hours at the same temperature with a charging current of 0.022 A. The battery was then rested for 3 hours and cooled to 25°C. The battery cooled to 25°C was discharged at a discharge current of 0.22 A in a 25°C environment until the battery voltage reached 0.6 V. The discharge capacity at this time was measured. This result was designated as the discharge capacity during charging at 55°C.

[0072] Next, the ratio of the discharge capacity when charging at 25°C to the discharge capacity when charging at 55°C was calculated using the following formula (II). This ratio was taken as the charge acceptance ratio and is shown in Table 2. A higher value of this charge acceptance ratio indicates better charge acceptance in a high-temperature environment. Charge acceptance ratio = (discharge capacity when charged at 55°C / discharge capacity when charged at 25°C) × 100 (II)

[0073] (2) Low-temperature discharge test The batteries of Examples 1 and 2 and Comparative Example 1 were charged for 16 hours in a 25°C environment with a charging current of 0.022 A, and then left to rest for 3 hours in a -30°C environment.

[0074] Next, after resting for 3 hours, the battery was discharged in an environment of -30°C at a discharge current of 0.11 A until the battery voltage reached 0.6 V. The discharge capacity at this time was determined. This discharge capacity was defined as the discharge capacity during discharge at -30°C.

[0075] The discharged battery was then rested for 3 hours in a 25°C environment. After resting for 3 hours in a 25°C environment, the battery was charged for 16 hours at the same temperature with a charging current of 0.022 A. After resting for 3 hours, the battery was discharged at a 25°C environment with a discharging current of 0.11 A until the battery voltage reached 0.6 V. The discharge capacity at this time was measured. This result was used as the discharge capacity at 25°C.

[0076] Next, the ratio of the discharge capacity when discharged at 25°C to the discharge capacity when discharged at -30°C was calculated using the following formula (III). This ratio was taken as the low-temperature discharge ratio and is shown in Table 2. A higher low-temperature discharge ratio indicates better discharge characteristics in a low-temperature environment, and can be said to be a high-output battery. Low temperature discharge ratio = (discharge capacity at -30℃ discharge / discharge capacity at 25℃ discharge)×100...(III)

[0077] [Table 1]

[0078] [Table 2]

[0079] 3. Discussion Examples 1 and 2 have higher charge acceptance ratios than Comparative Example 1. Furthermore, Examples 1 and 2 exhibit low-temperature discharge ratios equivalent to that of Comparative Example 1. In other words, Examples 1 and 2 achieve improved charge acceptance at high temperatures without sacrificing output characteristics.

[0080] Previous methods for improving charge acceptance have been limited to examining improvements to the positive electrode and electrolyte, with no examples of examining other elements. Therefore, the present inventors focused on the negative electrode and attempted to improve the negative electrode in addition to improving the positive electrode and electrolyte. Unlike conventional technology, the present invention achieves both improved charge acceptance at high temperatures and high output by combining the addition of specific elements (Cr and Zr) to the negative electrode, setting the ratio of the negative electrode capacity to the positive electrode capacity within a specific range, and setting the hydrogen equilibrium pressure of the hydrogen storage alloy within a specific range.

[0081] From the above, it can be said that the present invention can provide a nickel-metal hydride secondary battery that can achieve both improved charge acceptance at high temperatures and high output. [Explanation of symbols]

[0082] 2 Nickel-metal hydride secondary battery 22 electrode groups 24 Positive electrode 26 negative electrode 28 Separator

Claims

1. an electrode group consisting of a positive electrode and a negative electrode stacked with a separator interposed therebetween; a container that contains the electrode group together with an alkaline electrolyte, the positive electrode contains nickel hydroxide, the negative electrode contains a hydrogen storage alloy, The hydrogen storage alloy has the general formula: Ln １－ｘ－ｙ Mg ｘ Zr ｙ Ni ｚ－ａ－ｂ Al ａ Cr ｂ (wherein Ln represents a rare earth element, and x, y, z, a, and b respectively satisfy the conditions of 0.1≦x≦0.2, 0≦y≦0.02, 3.5≦z≦3.8, 0.08≦a≦0.2, and 0≦b≦0.05, and the material contains at least one of Zr and Cr), and in a pressure-composition-isotherm diagram at 80°C, the hydrogen equilibrium pressure at a hydrogen storage capacity H / M=0.5 is 0.1 MPa or more and 0.13 MPa or less, A nickel-metal hydride secondary battery, wherein the ratio N / P of the capacity N of the negative electrode to the capacity P of the positive electrode is 2.7 or more.

2. 2. The nickel-metal hydride secondary battery according to claim 1, wherein the positive electrode is a non-sintered positive electrode.

3. 3. The nickel-metal hydride secondary battery according to claim 1, wherein the nickel hydroxide contained in the positive electrode contains cobalt as a solid solution.

4. a surface of the negative electrode coated with a perfluoroalkoxyalkane; The amount of the perfluoroalkoxyalkane applied was 0.6 mg / cm ２ Above, 2.6mg / cm ２ 4. The nickel-metal hydride secondary battery according to claim 1, wherein:

5. 5. The nickel-metal hydride secondary battery according to claim 1, wherein the electrolyte contained in the alkaline electrolyte does not include sodium hydroxide.

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