Negative electrode for alkaline secondary battery, alkaline secondary battery, and method for producing negative electrode for alkaline secondary battery

Abstract

The negative electrode for an alkaline secondary battery has a negative electrode current collector and a negative electrode mixture supported on the negative electrode current collector. The negative electrode mixture contains a hydrogen storage alloy in which the mass content of Fe is 200 ppm to 900 ppm, and carbon black in which the mass content of Fe is 1000 ppm to 2800 ppm.

Description

Technical Field

[0001] The present invention relates to a negative electrode for an alkaline secondary battery, an alkaline secondary battery, and a method for manufacturing a negative electrode for an alkaline secondary battery. Background Art

[0002] Compared with nickel-cadmium secondary batteries, nickel-metal hydride secondary batteries have high capacity and excellent environmental safety, and for these reasons, they are used in a variety of applications such as various electronic devices, electrical equipment, and hybrid electric vehicles. In recent years, with the further expansion of the applications of nickel-metal hydride secondary batteries, further high performance is required, specifically, an increase in discharge capacity and cycle characteristics.

[0003] The negative electrode of a nickel-metal hydride secondary battery generally has a negative electrode current collector and a negative electrode mixture containing a hydrogen storage alloy, a conductive assistant, and a binder, which is held on the negative electrode current collector. Here, it has been investigated to improve the conductivity of the negative electrode mixture by using carbon black having a large surface area as the conductive assistant, and thereby improve the discharge capacity or cycle characteristics.

[0004] For example, Patent Document 1 discloses a negative electrode obtained by coating a negative electrode mixture slurry containing a hydrogen storage alloy, Ketjen black, and a sulfonic acid group-containing polymer on a negative electrode current collector. Patent Document 2 discloses an electrode containing composite particles in which carbon microparticles are supported on active material particles.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-319401

[0008] Patent Document 2: Japanese Patent Application Laid-Open No. 10-241677 Summary of the Invention

[0009] Problems to be Solved by the Invention

[0010] However, even when using the negative electrodes shown in Patent Documents 1 or 2, it is not possible to sufficiently improve the discharge capacity or cycle characteristics of the battery.

[0011] The present invention has been completed based on the above circumstances, and an object thereof is to provide a negative electrode for an alkaline secondary battery, an alkaline secondary battery using the negative electrode for an alkaline secondary battery, and a method for manufacturing a negative electrode for an alkaline secondary battery, which can sufficiently improve the discharge capacity and cycle characteristics.

[0012] Means for Solving the Problems

[0013] The present invention relates to the following negative electrode for an alkaline secondary battery, alkaline secondary battery, and method for manufacturing a negative electrode for an alkaline secondary battery.

[0014] [1]A negative electrode for an alkaline secondary battery, comprising: a negative electrode current collector; and a negative electrode mixture carried on the negative electrode current collector, the negative electrode mixture comprising: a hydrogen storage alloy with an Fe mass content of 200 ppm - 900 ppm; and carbon black with an Fe mass content of 1000 ppm - 2800 ppm.

[0015] [2]The negative electrode for an alkaline secondary battery according to [1], wherein the content of the carbon black in the negative electrode mixture is 0.5% by mass - 1% by mass relative to the hydrogen storage alloy.

[0016] [3]An alkaline secondary battery, comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte, the negative electrode having a negative electrode current collector and a negative electrode mixture carried on the negative electrode current collector, the negative electrode mixture comprising a hydrogen storage alloy with an Fe mass content of 200 ppm - 900 ppm and carbon black with an Fe mass content of 1000 ppm - 2800 ppm.

[0017] [4]The alkaline secondary battery according to [3], wherein the content of the carbon black in the negative electrode mixture is 0.5% by mass - 1% by mass relative to the hydrogen storage alloy.

[0018] [5]The alkaline secondary battery according to [3] or [4], wherein the positive electrode has a positive electrode current collector and a positive electrode mixture carried on the positive electrode current collector, the positive electrode mixture comprising nickel hydroxide.

[0019] [6]A method for manufacturing a negative electrode for an alkaline secondary battery, comprising: a step of preparing a negative electrode mixture slurry containing a hydrogen storage alloy with an Fe mass content of 200 ppm - 900 ppm, carbon black with an Fe mass content of 1000 ppm - 2800 ppm, and a binder; and a step of coating the negative electrode mixture slurry on a negative electrode current collector and drying the negative electrode mixture slurry.

[0020] Advantages of the Invention

[0021] According to the present invention, it is possible to provide a negative electrode for an alkaline secondary battery, an alkaline secondary battery using the negative electrode for an alkaline secondary battery, and a method for manufacturing a negative electrode for an alkaline secondary battery, which can sufficiently improve the discharge capacity and cycle characteristics. Description of the Drawings

[0022] Figure 1 is a perspective view showing a partial cross-section of a nickel-metal hydride secondary battery according to an embodiment of the present invention. Detailed Embodiments

[0023] As described above, it is difficult to sufficiently improve the discharge capacity and cycle characteristics by using conventional carbon black or the like.

[0024] In this regard, the present inventors have found that carbon black containing an appropriate amount of Fe can sufficiently improve the discharge capacity and cycle characteristics of a battery. The present inventors have also found that this effect is particularly significantly obtained when combined with a hydrogen storage alloy containing an appropriate amount of Fe.

[0025] Although the principle is not clear, it is presumed as follows.

[0026] Carbon black with a high Fe content contains more Fe, which is a metal with high conductivity, and thus has higher conductivity than conventional carbon black with a low Fe content. Therefore, it is easy to reduce the internal resistance of the battery, and the discharge capacity and cycle characteristics can be sufficiently improved.

[0027] On the other hand, since the ionization tendency of Fe is relatively high, it is easy to dissolve from the carbon black into the electrolyte. In this regard, by combining with a hydrogen storage alloy containing an appropriate amount of Fe, the dissolution of Fe from the hydrogen storage alloy is prioritized, so the dissolution of Fe from the carbon black is suppressed. Thereby, the effect of improving conductivity brought by Fe in the carbon black is more easily exerted, and the discharge capacity or cycle characteristics can be further improved.

[0028] That is, the negative electrode for an alkaline secondary battery of the present invention contains a hydrogen storage alloy containing an appropriate amount of Fe and carbon black containing an appropriate amount of Fe as a negative electrode binder. Hereinafter, the battery using the negative electrode for an alkaline secondary battery of the present invention will be described in detail.

[0029] 1. Nickel-metal hydride secondary battery

[0030] Hereinafter, as an example of the alkaline secondary battery according to the present embodiment, a nickel-metal hydride secondary battery will be described.

[0031] Figure 1 FIG. 23 is a perspective view showing a partial cutaway of a nickel-metal hydride secondary battery 10 according to an embodiment of the present invention. In this figure, a part of the wound body 16 is omitted from the illustration.

[0032] As Figure 1 shown, the nickel-metal hydride secondary battery 10 according to the present embodiment is, for example, a cylindrical battery of FA size, and includes an outer can 12, a sealing body 14, a wound body 16 (electrode group), an electrolyte (not shown), an upper insulating member 18, and a lower insulating member 20.

[0033] The outer can 12 is a container for accommodating the wound body 16. In the present embodiment, it is a bottomed cylindrical container with an open upper end. The outer can 12 has conductivity, and its bottom wall 12A functions as a negative terminal.

[0034] The material constituting the outer can 12 may be any material that has conductivity and is corrosion-resistant to the electrolyte or the electrochemical reaction inside the battery, and usually includes metal materials such as iron and steel.

[0035] The sealing body 14 is fixed to the opening of the outer can 12 with an insulating gasket 22 interposed therebetween, and while sealing the outer can 12, a positive terminal is provided. The sealing body 14 includes a cover plate 24, a valve body 26, and a positive terminal 28.

[0036] The cover plate 24 is a conductive circular plate-shaped member having a through-hole 24A at the center. The insulating gasket 22 has an annular shape surrounding the cover plate 24 and is interposed between the outer can 12 and the sealing body 14. The insulating gasket 22 is fixed to the opening edge 12B of the outer can 12 by caulking the opening edge 12B of the outer can 12. Thus, the cover plate 24 and the insulating gasket 22 cooperate with each other to hermetically seal the opening of the outer can 12.

[0037] The valve body 26 is a rubber member disposed on the outer surface of the cover plate 24 so as to block the through-hole 24A.

[0038] The positive terminal 28 is a metal member having a flanged cylindrical shape and is electrically connected to the outer surface of the cover plate 24 so as to cover the valve body 26. The positive terminal 28 presses the valve body 26 toward the cover plate 24. The positive terminal 28 has an exhaust hole (not shown).

[0039] Furthermore, during normal operation, the through-hole 24A is hermetically closed by the valve body 26. On the other hand, when gas is generated inside the outer can 12 and its internal pressure increases, the valve body 26 is compressed by the internal pressure and the through-hole 24A is opened. As a result, the gas is discharged from inside the outer can 12 to the outside through the through-hole 24A and the exhaust hole (not shown) of the positive terminal 28. That is, the through-hole 24A, the valve body 26, and the positive terminal 28 form a safety valve for the battery.

[0040] The wound body 16 includes a positive electrode 30, a negative electrode 32, and a separator 34. That is, the wound body 16 is an electrode group formed by winding the positive electrode 30 and the negative electrode 32 with the separator 34 interposed therebetween in a laminated state. Specifically, the wound body 16 is wound in such a manner that a structure in which the separator 34, the positive electrode 30, the separator 34, and the negative electrode 32 are laminated has the negative electrode 32 on the outside.

[0041] The negative electrode 32 is disposed on the outermost peripheral surface of the wound body 16, and the negative electrode 32 is in contact with the inner wall surface of the outer can 12. That is, the negative electrode 32 is electrically connected to the outer can 12 serving as a negative terminal.

[0042] On the other hand, a positive electrode lead 36 is connected to the positive electrode 30 of the wound body 16. One end of the positive electrode lead 36 is connected to the positive electrode 30, and the other end is connected to the cover plate 24. Thus, the positive electrode 30 and the positive terminal 28 are electrically connected to each other via the positive electrode lead 36 and the cover plate 24.

[0043] The upper insulating member 18 is disposed between the wound body 16 and the cover plate 24. Thus, the negative electrode 32 of the wound body 16 does not contact the sealing body 14. In addition, the upper insulating member 18 has a slit 18A, and the positive electrode lead 36 passes through the slit 18A.

[0044] The lower insulating member 20 is disposed between the wound body 16 and the bottom of the outer can 12. Thus, the positive electrode 30 of the wound body 16 does not contact the inner wall surface of the outer can 12.

[0045] The electrolytic solution (not shown) is an alkaline electrolytic solution and is sealed in the outer can 12. The wound body 16 is impregnated with the alkaline electrolytic solution, and the alkaline electrolytic solution is mainly retained in the separator 34.

[0046] The alkaline electrolytic solution is preferably an aqueous solution containing at least one of KOH, NaOH, and LiOH as a solute. The solute concentration of the alkaline electrolytic solution is not particularly limited, and can be set to 7.0N - 10.0N, for example.

[0047] Next, each component constituting the wound body 16 will be described.

[0048] (Positive electrode)

[0049] The positive electrode 30 includes a positive electrode current collector and a positive electrode mixture.

[0050] The positive electrode current collector can be, for example, a metal foil, a mesh, a sponge, a fibrous or felt-like metal porous body, a metal punched plate, or a steel wire mesh. The metal material constituting the positive electrode current collector only needs to be a metal material that is stable even at the reaction potential of the positive electrode. For example, it is nickel, stainless steel, and preferably nickel. That is, the positive electrode current collector can be nickel foam, a nickel-made or nickel-plated mesh, sponge, or fibrous metal body.

[0051] The positive electrode mixture is retained on the positive electrode current collector and includes a positive electrode active material. As the positive electrode active material, nickel hydroxide can be cited. The form of nickel hydroxide is, for example, powder form. In addition, the nickel hydroxide particles may be solid-solved with at least one of cobalt (Co), zinc (Zn), and magnesium (Mg). In addition, the nickel hydroxide particles may be nickel hydroxide particles containing a cobalt compound with a higher valence of 3 or more.

[0052] The positive electrode mixture may further include a conductive assistant, a positive electrode additive, a binder, etc. as needed.

[0053] As the conductive assistant, it includes cobalt compounds such as cobalt oxide or cobalt hydroxide, and cobalt.

[0054] The positive electrode additive can be added for the purpose of suppressing the expansion of the positive electrode active material during charging or improving the control of the oxygen evolution potential during charging. Examples of the positive electrode additive include yttrium oxide or zinc oxide.

[0055] The binder functions as follows: it binds the positive electrode active material and the positive electrode additive to each other, and at the same time binds the positive electrode active material and the positive electrode additive to the positive electrode current collector. As the binder, for example, hydrophilic or hydrophobic polymers can be cited, such as hydroxypropyl cellulose (HPC) or carboxymethyl cellulose (CMC), sodium polyacrylate, fluorine-based polymers (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), etc.).

[0056] (Negative electrode)

[0057] The negative electrode 32 includes a negative electrode current collector and a negative electrode mixture agent.

[0058] The shape of the negative electrode current collector can be the same as above, and it is preferably a sheet-like metal substrate having through holes such as a metal punched plate, a steel wire mesh, or a sintered body of metal powder. The metal material constituting the negative electrode current collector may be any metal material that is stable even at the reaction potential of the negative electrode, and nickel or iron is preferred.

[0059] The negative electrode mixture agent is held on the negative electrode current collector, and it contains a hydrogen storage alloy containing a specified amount of Fe as the negative electrode active material and carbon black containing a specified amount of Fe as the conductive assistant.

[0060] The crystal structure of the above hydrogen storage alloy is not particularly limited, and it can be any one of AB5 type, AB2 type, A2B7 type, A2B type, AB type, and others.

[0061] The composition of the above hydrogen storage alloy is not particularly limited, and for example, it preferably has a composition represented by the general formula (1).

[0062] Ln 1-x Mg x Ni y-a Al a ···(1)

[0063] In the general formula (1),

[0064] Ln represents at least one element selected from rare earth elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y), Ti, and Zr. Among them, Y, Ti, and Zr are preferred.

[0065] x, y, and a are integers satisfying 0 < x < 0.30, 2.80 ≤ y ≤ 3.90, and 0.10 ≤ a ≤ 0.25, respectively.

[0066] A part of Ni may also be replaced by other elements (for example, at least one selected from V, Nb, Ta, Cr, Mo, Mn, Fe, Co, Ga, Zn, Sn, In, Cu, Si, P, and B).

[0067] In general formula (1), Ln and Mg are component A, and Ni and Al are component B. Further, the hydrogen storage alloy represented by general formula (1) forms a crystal structure in which AB2-type units and AB5-type units are stacked, that is, a so-called superlattice structure having an A2B7-type structure or an A5B 19 type structure. The hydrogen storage alloy having such a superlattice structure has both the advantages of stable storage and release of hydrogen characteristic of an AB5-type alloy and the advantage of a large hydrogen storage capacity characteristic of an AB2-type alloy. Therefore, the hydrogen storage alloy having the composition represented by general formula (1) has excellent hydrogen storage capacity and can increase the capacity of the battery.

[0068] As described above, the hydrogen storage alloy contains a specified amount of Fe. The Fe contained in the hydrogen storage alloy may be from the molten salt electrolysis product of rare earth metals as raw materials or impurities in the manufacturing process. In the present invention, by using a hydrogen storage alloy containing a specified amount of Fe, the effect of improving conductivity brought by carbon black containing Fe can be easily exerted.

[0069] That is, the mass content of Fe in the above hydrogen storage alloy is 200 ppm - 900 ppm. If the mass content of Fe in the above hydrogen storage alloy is 200 ppm or more, Fe will moderately dissolve from the hydrogen storage alloy into the electrolyte, so that the dissolution of Fe from the above carbon black into the electrolyte can be suppressed. As a result, the effect of improving the conductivity of the negative electrode mixture is easily exerted, and the discharge capacity or cycle characteristics are easily improved. If the mass content of Fe in the hydrogen storage alloy is 900 ppm or less, the Fe dissolved from the hydrogen storage alloy can be suppressed from moving to the positive electrode and deteriorating the charge and discharge characteristics of the positive electrode. Therefore, in particular, the deterioration of the cycle characteristics can be suppressed. From the same viewpoint, the mass content of Fe in the above hydrogen storage alloy is more preferably 230 ppm - 750 ppm, and further preferably 230 ppm - 500 ppm.

[0070] The content of Fe in the above hydrogen storage alloy can be measured by the following method.

[0071] 1) First, according to JIS K 0116; 2014, the hydrogen storage alloy particles are pretreated by acid decomposition method to be dissolved and made into a solution.

[0072] 2) Use an ICP emission analyzer PS3520 UVDDII manufactured by Hitachi, Ltd. to measure the content of Fe ions / atoms in the obtained solution.

[0073] The content of the above hydrogen storage alloy only needs to be within the range where a specified discharge capacity can be obtained, and can be, for example, 94.0 mass% - 99.4 mass% with respect to the negative electrode mixture.

[0074] The conductive additive has the effect of improving the conductivity of the negative electrode mixture. The conductive additive contains carbon black containing a specified amount of Fe. Carbon black containing a specified amount of Fe has higher conductivity than conventional carbon black with a lower Fe content. Therefore, it is easy to reduce the internal resistance of the battery and easy to improve the discharge capacity or cycle characteristics of the battery. It should be noted that the Fe contained in the carbon black can also be metal impurities from raw materials or manufacturing processes, as described above.

[0075] The mass content of Fe in the above carbon black is preferably 1000 ppm - 2800 ppm relative to the carbon black. If the mass content of Fe in the carbon black is 1000 ppm or more, the conductivity of the negative electrode mixture can be sufficiently improved, and thus the discharge capacity or cycle characteristics of the battery can be sufficiently improved. If the mass content of Fe in the carbon black is 2800 ppm or less, during the preparation of the electrode, cross-linking reactions of rubbery polymers, etc. used as binders are less likely to occur, and the processability is less likely to be impaired. From the same perspective, the mass content of Fe in the above carbon black is more preferably 1000 ppm - 2600 ppm relative to the carbon black, and further preferably 2000 ppm - 2600 ppm.

[0076] The content of Fe in the carbon black can be measured by the same method as the Fe content in the hydrogen storage alloy.

[0077] The specific surface area of the above carbon black is not particularly limited, and is preferably 500 m 2 / g or more, more preferably 500 m 2 / g - 1500 m 2 / g. If the specific surface area is 500 m 2 / g or more, the carbon black can form a structure well. The specific surface area of the carbon black can be measured by the BET method using nitrogen adsorption according to ASTM-D6556.

[0078] The content of the above carbon black is preferably 0.5 mass% - 1 mass% relative to the above hydrogen storage alloy. If the content of the above carbon black is 0.5 mass% or more relative to the above hydrogen storage alloy, the conductivity is more likely to be further improved, and the charge-discharge characteristics or cycle characteristics are more likely to be further improved. If the content of the above carbon black is 1 mass% or less relative to the above hydrogen storage alloy, the content of the negative electrode active material can be easily ensured, and thus the discharge capacity is less likely to be impaired. From the same perspective, the content of the above carbon black is more preferably 0.5 mass% - 0.7 mass% relative to the above hydrogen storage alloy.

[0079] The negative electrode mixture can further contain a binder, a thickener, and a negative electrode auxiliary agent as needed.

[0080] The binder functions as follows: it binds the hydrogen storage alloy particles and the conductive additive to each other, and at the same time binds the hydrogen storage alloy particles and the conductive additive to the negative electrode current collector. Examples of the binder include rubber-like polymers such as styrene-butadiene rubber (SBR), and fluorine-based polymers (such as polytetrafluoroethylene (PTFE)). The content of the binder, for example, relative to the above-mentioned hydrogen storage alloy, is preferably 0.1% by mass to 5% by mass, more preferably 0.5% by mass to 1% by mass.

[0081] The thickener can impart viscosity to the negative electrode mixture paste described later, making it easier to form the negative electrode. Examples of the thickener include carboxymethyl cellulose (CMC) or sodium polyacrylate.

[0082] (Separator)

[0083] As described above, the separator 34 is disposed between the positive electrode 30 and the negative electrode 32 (refer to Figure 1 ).

[0084] As the separator 34, for example, there can be mentioned: a separator in which a hydrophilic functional group is imparted to a non-woven fabric made of polyamide fiber, a separator in which a hydrophilic functional group is imparted to a non-woven fabric made of polyolefin fibers such as polyethylene and polypropylene, a polyolefin-based microporous membrane such as polyethylene or polypropylene, etc. The separator 34 can be used alone or in combination of two or more.

[0085] (Function)

[0086] The negative electrode 32 of the nickel-metal hydride secondary battery 10 according to the above-described embodiment contains the following components as a negative electrode mixture: a hydrogen storage alloy containing a specified amount of Fe and carbon black containing a specified amount of Fe. The carbon black containing a specified amount of Fe can easily improve the conductivity of the negative electrode mixture. In addition, by the coexistence of the hydrogen storage alloy containing a specified amount of Fe, the elution of Fe from the carbon black can be reduced. Thus, the effect of improving the conductivity of the negative electrode mixture brought about by the carbon black can be more easily exerted, and the discharge capacity and cycle characteristics of the battery can be sufficiently improved.

[0087] 2. Manufacturing method of nickel-metal hydride secondary battery

[0088] The above-mentioned nickel-metal hydride secondary battery can be manufactured by any method. For example, the above-mentioned nickel-metal hydride secondary battery can be manufactured through the following steps: 1) a step of preparing electrodes (positive electrode and negative electrode); and 2) a step of obtaining a nickel-metal hydride secondary battery using the prepared electrodes.

[0089] Regarding the step of 1)

[0090] In this step, the positive electrode and the negative electrode are prepared.

[0091] The positive electrode 30 can be prepared, for example, through the following steps.

[0092] First, mix the positive electrode active material, conductive assistant, positive electrode additive, binder, and water or solvent, and knead them to obtain a positive electrode mixture paste. Then, coat the obtained positive electrode mixture paste on the positive electrode current collector, dry it, press it, and cut it into a specified size to obtain the positive electrode 30.

[0093] Similarly, the negative electrode 32 can be prepared, for example, through the following steps.

[0094] First, mix the above-mentioned negative electrode active material containing the hydrogen storage alloy, the above-mentioned conductive assistant containing carbon black, the binder, and an appropriate amount of water or solvent as needed to obtain a negative electrode mixture paste. After coating the obtained negative electrode mixture paste on the surface of the negative electrode current collector and drying it, press it as needed and cut it into a specified size to obtain the negative electrode 32.

[0095] Regarding the process of 2)

[0096] In this process, use the prepared electrodes to fabricate a nickel-metal hydride secondary battery.

[0097] Specifically, prepare a wound body in which the prepared positive electrode 30 and negative electrode 32 are wound in a state of being stacked with the separator 34 in between. Weld a positive electrode lead 36 to one end in the length direction of the positive electrode 30. Then, for example, stack the separator 34, the positive electrode 30, the separator 34, and the negative electrode 32 in sequence, and then wind them along the length direction with the negative electrode 32 on the outside to obtain the wound body 16.

[0098] Accommodate the obtained wound body 16 in the outer can 12, inject the electrolyte, and then seal the opening of the outer can 12 with the sealing body 14.

[0099] After standing for a certain period of time, perform an activation treatment by charging under specified conditions. The activation conditions can be adjusted according to the properties of the electrode active materials (positive electrode active material and negative electrode active material). Thus, the nickel-metal hydride secondary battery 10 can be obtained.

[0100] 3. Variation

[0101] It should be noted that in the above-described embodiment, an example of a cylindrical nickel-metal hydride secondary battery is shown, but it is not limited thereto, and it can also be a square nickel-metal hydride secondary battery.

[0102] In addition, in the above-described embodiments, a negative electrode containing a hydrogen storage alloy containing Fe and carbon black containing Fe is used for the negative electrode of a nickel-metal hydride secondary battery, but it is not limited thereto. For example, it can also be used for the negative electrode of a hydrogen / air secondary battery. In a hydrogen / air secondary battery, as the positive electrode, an air electrode mixture in which an air electrode catalyst (for example, a redox catalyst such as a pyrochlore-type composite oxide), nickel powder as a conductive aid, and a binder are supported on a positive electrode current collector such as nickel foam can be used.

[0103] Examples

[0104] Hereinafter, the present invention will be described with reference to examples. The interpretation of the scope of the present invention is not limited by the examples.

[0105] 1. Preparation of Materials

[0106] 1-1. Negative Electrode Active Material

[0107] Prepare the hydrogen storage alloys 1-5 shown in Table 1 below.

[0108] Table 1

[0109]

[0110] (Fe content)

[0111] The Fe content in the hydrogen storage alloy was measured by the following method.

[0112] According to JIS K 0116; 2014, the hydrogen storage alloy particles were pretreated by acid decomposition to dissolve them and made into a solution. The content of Fe ions and atoms in the obtained solution was measured using an ICP emission analyzer PS3520 UVDDII manufactured by Hitachi, Ltd.

[0113] 1-2. Conductive Aid

[0114] Prepare the carbon blacks 1-5 shown in Table 2 below.

[0115] Table 2

[0116]

[0117] The physical properties of the carbon black were measured by the following method.

[0118] (Specific surface area)

[0119] According to ASTM-D6556, the specific surface area was measured by nitrogen adsorption using the BET method.

[0120] (Fe content)

[0121] It is measured by the same method as the Fe content in the hydrogen storage alloy.

[0122] 2. Preparation of the battery

[0123] <Example 1>

[0124] (1) Preparation of the negative electrode

[0125] To 100 parts by mass of the powder of hydrogen storage alloy 1 (Fe mass content is 230 ppm), 0.4 part by mass of sodium polyacrylate, 0.1 part by mass of carboxymethyl cellulose (CMC), 1.0 part by mass of a dispersion of 50 mass% of the solid component of styrene-butadiene rubber (SBR), 0.5 part by mass of carbon black 1 (Fe mass content is 1080 ppm), and 30 parts by mass of water are added and kneaded to prepare a negative electrode mixture paste. The paste is evenly coated on both sides of an iron punched plate with nickel plating on the surface as the negative electrode current collector and then dried. After roll pressing it, it is cut into a specified size to prepare the negative electrode.

[0126] (2) Preparation of the positive electrode

[0127] 88 mass% of nickel hydroxide containing 3 mass% of zinc and 1 mass% of cobalt relative to metallic nickel as the positive electrode active material, 10 mass% of cobalt hydroxide as the conductive assistant, 0.5 mass% of yttrium oxide and 0.5 mass% of zinc oxide as the positive electrode additives, and 1 mass% of hydroxypropyl cellulose (HPC) as the binder are mixed to prepare a positive electrode mixture paste. The positive electrode mixture paste is filled into nickel foam as the positive electrode current collector and dried. After roll pressing it, it is cut into a specified size to prepare the positive electrode.

[0128] (3) Preparation of the electrolyte

[0129] KOH, NaOH, and LiOH are mixed in a mass ratio of 5.0:1.5:1.0, and further diluted with water to prepare an 8N alkaline aqueous solution to make the electrolyte.

[0130] (4) Preparation of the battery

[0131] The positive electrode and negative electrode prepared above are wound together with a separator (polyolefin-based microporous membrane). Specifically, a structure laminated in the order of separator / positive electrode / separator / negative electrode is wound. It is put into a cylindrical outer can, and a specified amount of the above electrolyte is injected to prepare a nickel-metal hydride secondary battery with a nominal capacity of 2300 mAh.

[0132] Then, the obtained battery is charged at 0.23 A for 16 hours and then discharged at 0.46 A until the battery voltage becomes 1.0 V, and this cycle is repeated 5 times for activation.

[0133] <Examples 2 - 7, Comparative Examples 1 - 5 and 7>

[0134] Except for changing at least one of the type of hydrogen storage alloy, the type of carbon black, and its content as shown in Table 3, the negative electrode was prepared and the battery was prepared in the same manner as in Example 1.

[0135] <Comparative Example 6>

[0136] Except for changing carbon black 1 to carbon black 5, an attempt was made to prepare the negative electrode in the same manner as in Example 1. However, the viscosity of the negative electrode mixture paste increased, and the paste could not be uniformly coated, so the negative electrode could not be prepared.

[0137] <Evaluation>

[0138] (1) Initial charge-discharge characteristics

[0139] The obtained battery was charged at 2.3 A in an environment of 25°C. When the battery voltage dropped by 10 mV from the maximum value, charging was stopped, and then paused for 1 hour. Then, it was discharged at 6.9 A until the battery voltage reached 1.0 V. Further, the discharge capacity ratio was calculated with the discharge capacity of Comparative Example 1 as 100%.

[0140] (2) Cycle characteristics

[0141] The obtained battery was charged at 1.0 C in an environment of 25°C. When the battery voltage dropped by 10 mV from the maximum value, charging was stopped, and then paused for 1 hour. Then, it was discharged at 1.0 C until the battery voltage reached 1.0 V, and then paused for 1 hour. The above was regarded as one cycle, and the charge-discharge test was repeated.

[0142] Further, when the discharge capacity reached 60% of that in the first cycle, it was regarded as the cycle life, and the cycle life ratio with the cycle life of Comparative Example 1 as 100% was obtained.

[0143] (3) Plate manufacturability

[0144] When the viscosity of the negative electrode mixture paste increased during the preparation of the negative electrode and it could not be uniformly coated, it was marked as ×, and when it could be uniformly coated, it was marked as ○.

[0145] The evaluation results of Examples 1-7 and Comparative Examples 1-7 are shown in Table 3.

[0146] Table 3

[0147]

[0148] As shown in Table 3, it can be seen that for the batteries of Examples 1-7 using negative electrodes containing a hydrogen storage alloy containing a specified amount of Fe and carbon black containing a specified amount of Fe, both the discharge capacity ratio and the cycle life ratio exceeded 100%, and they were more excellent than the batteries of Comparative Example 1.

[0149] In contrast, it is known that when the content of Fe in carbon black is low (when Ketjen black is used), both the discharge capacity ratio and the cycle life ratio are low (see Comparative Examples 1-3). It is speculated that this is because the effect of improving conductivity cannot be obtained due to too little Fe content. On the other hand, it is known that when the content of Fe in carbon black is too high, the coatability of the negative electrode mixture slurry deteriorates and it is impossible to prepare a plate electrode (see Comparative Example 6). It is considered that when the amount of Fe in carbon black is too large, it thickens due to the crosslinking reaction with the binder, resulting in poor coatability.

[0150] In addition, it is known that even when using carbon black containing a specified amount of Fe, if the mass content of Fe in the hydrogen storage alloy is as low as 100 ppm, the discharge capacity ratio or the cycle life ratio is still low (see Comparative Example 7). It is implied that in order to exhibit the conductivity of Fe in carbon black, it is preferable to contain a certain amount of Fe in the hydrogen storage alloy. That is, it is speculated that by containing a certain amount of Fe in the hydrogen storage alloy, the dissolution of Fe from the hydrogen storage alloy is prioritized and the dissolution of Fe from the carbon black is suppressed, thereby improving conductivity.

[0151] On the other hand, it is known that when the mass content of Fe in the hydrogen storage alloy is as high as 1000 ppm, especially the cycle life ratio does not improve (see Comparative Examples 3-5). It is speculated that this is because Fe dissolved from the hydrogen storage alloy into the electrolyte moves to the positive electrode, deteriorating the charge and discharge characteristics of the positive electrode.

[0152] This application claims priority based on Japanese Patent Application No. 2022-154917 filed on September 28, 2022. All of the content described in the specification of this application is incorporated herein by reference.

[0153] Industrial Applicability

[0154] According to the negative electrode for an alkaline secondary battery of the present invention, the discharge capacity and cycle characteristics of the battery can be sufficiently improved. Therefore, the negative electrode for an alkaline secondary battery can be suitably used, for example, as the negative electrode of a nickel-metal hydride secondary battery or a hydrogen / air secondary battery.

[0155] Explanation of Reference Numerals

[0156] 10: Nickel-metal hydride secondary battery;

[0157] 12: Outer can;

[0158] 12A: Bottom wall;

[0159] 12B: Opening edge;

[0160] 14: Sealing body;

[0161] 16: Wound body (electrode group);

[0162] 18: Upper insulating member;

[0163] 20: Lower insulating component;

[0164] 22: Insulating gasket;

[0165] 24: Cover plate;

[0166] 26: Valve body;

[0167] 28: Positive terminal;

[0168] 30: Positive electrode;

[0169] 32: Negative electrode;

[0170] 34: Diaphragm;

[0171] 36: Positive electrode lead;

[0172] 38: Negative electrode current collector.

Claims

1. A negative electrode for an alkaline secondary battery, comprising: a negative electrode current collector; and a negative electrode mixture carried on the negative electrode current collector, wherein the negative electrode mixture comprises: a hydrogen storage alloy with an Fe mass content of 200 ppm - 900 ppm; and carbon black with an Fe mass content of 1000 ppm - 2800 ppm.

2. The negative electrode for an alkaline secondary battery according to claim 1, wherein the content of the carbon black in the negative electrode mixture is 0.5 mass% - 1 mass% relative to the hydrogen storage alloy.

3. An alkaline secondary battery, comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture carried on the negative electrode current collector, and the negative electrode mixture comprises a hydrogen storage alloy with an Fe mass content of 200 ppm - 900 ppm and carbon black with an Fe mass content of 1000 ppm - 2800 ppm.

4. The alkaline secondary battery according to claim 3, wherein the content of the carbon black in the negative electrode mixture is 0.5 mass% - 1 mass% relative to the hydrogen storage alloy.

5. The alkaline secondary battery according to claim 3 or 4, wherein the positive electrode has a positive electrode current collector and a positive electrode mixture carried on the positive electrode current collector, and the positive electrode mixture comprises nickel hydroxide.

6. A method for manufacturing a negative electrode for an alkaline secondary battery, comprising: a step of preparing a negative electrode mixture slurry containing a hydrogen storage alloy with an Fe mass content of 200 ppm - 900 ppm, carbon black with an Fe mass content of 1000 ppm - 2800 ppm, and a binder; and a step of coating the negative electrode mixture slurry on the negative electrode current collector and drying the negative electrode mixture slurry.

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