Alkaline storage battery

Abstract

Provided is an alkaline storage battery having excellent reliability. An alkaline storage battery according to the present disclosure includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an alkaline electrolytic solution. The negative electrode includes a hydrogen absorbing alloy capable of electrochemically occluding and releasing hydrogen. The separator includes carbon fibers having hydrophobicity on the surface layer on the side that faces the negative electrode.

Description

alkaline storage battery

[0001] The present disclosure relates to alkaline storage batteries.

[0002] An alkaline storage battery includes, for example, an electrode group including a positive electrode, a negative electrode, and a separator disposed therebetween, an alkaline electrolyte, and a metal battery case (external can) that houses these. Among alkaline storage batteries, nickel-metal hydride storage batteries primarily use nickel oxides, including nickel oxyhydroxide and nickel hydroxide, as the positive electrode active material, and a hydrogen storage alloy as the negative electrode active material. Attempts have been made to improve components such as the electrodes and separator in order to improve the performance of alkaline storage batteries.

[0003] Patent Document 1 proposes a hydrogen storage alloy electrode for alkaline storage batteries having hydrophilized carbon powder on the electrode surface.

[0004] Patent Document 2 proposes a separator for alkaline storage batteries coated with a water repellent agent, the water repellent agent being present from one surface to the interior of the separator.

[0005] JP-A-8-315810 JP-A-5-121061

[0006] Although the reliability of the alkaline storage batteries has been improved to some extent, there is a demand for alkaline storage batteries with even higher reliability so that they can be used in more severe environments.

[0007] One aspect of the present disclosure relates to an alkaline storage battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an alkaline electrolyte, wherein the negative electrode includes a hydrogen storage alloy capable of electrochemically absorbing and desorbing hydrogen, and the separator includes a hydrophobic carbon fiber on a surface layer facing the negative electrode.

[0008] It is possible to provide an alkaline storage battery with excellent reliability.

[0009] FIG. 1 is a partial cross-sectional view of a cylindrical alkaline storage battery according to one embodiment of the present invention.

[0010] In recent years, there has been a demand for high reliability in alkaline storage batteries. In particular, there is a strong demand for high output and long-term durability. Meanwhile, oxygen gas is generated at the positive electrode of alkaline storage batteries during overcharge, which easily increases the battery's internal pressure. To achieve long-term durability, it is desirable to suppress this increase in battery internal pressure. Furthermore, even though alkaline storage batteries have high output at room temperature (e.g., temperatures between 20°C and 35°C), their output tends to decrease at low temperatures (e.g., −10°C). Therefore, there is a demand for alkaline storage batteries that can provide high output even at low temperatures.

[0011] In view of the above, (1) an alkaline storage battery according to one aspect of the present disclosure includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an alkaline electrolyte. The negative electrode includes a hydrogen storage alloy capable of electrochemically absorbing and desorbing hydrogen. The separator includes a hydrophobic carbon fiber on a surface layer facing the negative electrode.

[0012] By placing hydrophobic carbon fibers on the negative electrode side of the separator, it is possible to achieve high output even at low temperatures. This is thought to be because the electrical conductivity of the carbon fibers reduces the resistance between the negative electrode and the separator compared to separators that do not contain hydrophobic carbon fibers, and improves current collection performance.

[0013] However, when carbon fibers are applied to the surface layer of the negative electrode, they tend to fall off during the process of assembling the electrode assembly. On the other hand, carbon fibers contained in the surface layer of the separator are less likely to fall off, and the amount of carbon fibers that fall off can be reduced.

[0014] In addition, the hydrophobicity of carbon fiber increases gas permeability between the negative electrode and the separator, allowing oxygen gas generated at the positive electrode during overcharge to smoothly move to the negative electrode and be absorbed by the negative electrode, thereby suppressing the increase in internal battery pressure during overcharge.

[0015] As described above, the present disclosure provides an alkaline storage battery with excellent reliability, which can maintain high output (particularly high output at low temperatures) for a long period of time, ensure high capacity even after repeated charge and discharge, and achieve excellent cycle characteristics (cycle life).

[0016] Conventionally, water repellents have been used to improve gas permeability, and conductive materials such as carbon materials have been used to provide conductivity. In contrast, in the present disclosure, by disposing hydrophobic carbon fibers on the surface layer of the separator facing the negative electrode, it is possible to ensure both high gas permeability and conductivity. This also reduces the number of steps and materials required in the production of batteries.

[0017] (2) In the above (1), the separator has a density of 0.25 g / m per projected area in the thickness direction of the separator. ２ 2.50g / m or more ２ The carbon fibers may be contained at the following areal density: In this case, the internal pressure of the battery during overcharge can be further reduced, higher output can be secured, and better cycle characteristics can be obtained.

[0018] (3) In the above (1) or (2), the carbon fiber may contain carbon nanotubes (CNTs). CNTs are highly hydrophobic, and therefore can further reduce the internal pressure of the battery during overcharge.

[0019] (4) In any one of (1) to (3) above, the separator may be made of a nonwoven fabric containing the carbon fibers. The carbon fibers are entangled with the fibers of the nonwoven fabric, which reduces the amount of carbon fibers that fall off during separator production or electrode assembly assembly. This makes it easier for the carbon fibers to exert their effects in the surface layer of the separator.

[0020] The alkaline storage battery of the present disclosure will be described in more detail below, including the above (1) to (4). At least one selected from the components described below can be arbitrarily combined with at least one of the above (1) to (4) of the alkaline storage battery of the present disclosure, as long as such combination is technically possible.

[0021] [Alkaline Storage Battery] The alkaline storage battery of the present disclosure includes a positive electrode, a negative electrode, a separator interposed between the electrodes, and an alkaline electrolyte. The negative electrode includes a hydrogen storage alloy capable of electrochemically absorbing and desorbing hydrogen.

[0022] (Separator) In the alkaline storage battery of the present disclosure, the separator contains hydrophobic carbon fibers in the surface layer facing the negative electrode. This ensures high conductivity between the negative electrode and the surface layer, resulting in high output. For example, high output can be ensured even in low-temperature environments such as -10°C. Furthermore, compared to a negative electrode containing the above-mentioned carbon fibers in the surface layer, when the surface layer of the separator contains the above-mentioned carbon fibers, the amount of carbon fiber shedding is more likely to be reduced. Therefore, high capacity can be easily maintained even after repeated charge and discharge, and high cycle characteristics can be easily obtained. Furthermore, the hydrophobicity of the carbon fibers allows oxygen gas generated at the positive electrode during overcharge to smoothly permeate the separator and be absorbed by the negative electrode. This can suppress an increase in internal battery pressure.

[0023] The separator may be a porous membrane, a fibrous nonwoven fabric, a woven fabric, or the like, or may have a laminate structure of two or more of these. It is preferable that at least the surface layer of the separator facing the negative electrode contains fibers. By including fibers in at least the surface layer, carbon fibers are easily entangled with these fibers. Therefore, the amount of carbon fibers falling off from the separator during the separator manufacturing process and the electrode assembly formation process can be reduced. This allows the carbon fibers to be stably maintained in the surface layer of the separator for a long period of time, maintaining high conductivity. The separator may, for example, include a nonwoven fabric containing the above-mentioned carbon fibers. In this case, at least the surface layer of the separator is a nonwoven fabric, and the surface layer of this nonwoven fabric contains carbon fibers.

[0024] The separator may have a surface layer of a nonwoven fabric containing hydrophobic carbon fibers. Such a separator may have a base layer (or main layer) having a nonwoven fiber structure and a composite layer containing hydrophobic carbon fibers formed on the surface of the base layer. The composite layer may contain hydrophobic carbon fibers and other fibers (e.g., fibers made of a resin material). The composite layer may also have a nonwoven fabric structure continuous with the nonwoven fabric structure of the base layer. In these cases, the carbon fibers are more likely to entangle with other fibers or fibers in the nonwoven fabric structure, which is advantageous in reducing the amount of carbon fiber shedding. Hydrophobic carbon fibers function as both a water-repellent and conductive material. Therefore, the composite layer imparts appropriate water-repellent and conductive properties to the surface layer of the separator.

[0025] The separator includes, for example, a resin material. The separator may include a porous film including the resin material, or a nonwoven or woven fabric of fibers including the resin material. Examples of the resin material include polyolefin resins (polyethylene, polypropylene, ethylene-propylene copolymer, etc.), polyamide resins (aromatic polyamide, etc.), vinyl resins (polyvinyl acetate or its saponification product, etc.), acrylic resins, cellulose or its derivatives (cellulose ether, cellulose ester, etc.). The separator may include one of these resin materials or a combination of two or more. Since separators for alkaline storage batteries are required to have appropriate hydrophilicity and appropriate hydrophobicity, the separator preferably includes a polyolefin resin.

[0026] The average fiber diameter of the fibers constituting the nonwoven fabric may be 0.8 μm or more and 20 μm or less, or 1 μm or more and 15 μm or less. The nonwoven fabric may contain multiple types of fibers with different average fiber diameters to increase strength, etc. The nonwoven fabric may contain, for example, first fibers with an average fiber diameter of 5 μm or more and 20 μm or less and second fibers with an average fiber diameter of 0.8 μm or more and less than 5 μm (e.g., 0.8 to 4 μm).

[0027] The basis weight of the nonwoven fabric constituting the separator is 35 g / m without including the carbon fiber. ２ 75g / m or more ２or less, and ２ 60g / m or more ２ It may be the following:

[0028] To ensure the wettability of the separator with the alkaline electrolyte, the separator may be hydrophilized. For example, a porous membrane, a nonwoven fabric, or a woven fabric may be hydrophilized and used as the separator. For example, in the case of a separator containing a polyolefin resin, a hydrophilized nonwoven fabric is used as the separator.

[0029] The hydrophilization treatment may be performed using known treatments such as sulfuric acid treatment, fluorine treatment, plasma treatment, etc. The hydrophilization treatment provides sulfonic acid groups (—SO ) to at least a portion of the separator (for example, fibers of a nonwoven fabric). ３ Hydrophilic functional groups such as -H, -OH, and -COOH are introduced. This can improve wettability with alkaline electrolyte. In particular, sulfuric acid-treated separators (such as separators having sulfonic groups) have high alkaline electrolyte retention, which makes the battery's internal pressure more likely to increase. In the present disclosure, even when a sulfuric acid-treated separator is used, the surface layer facing the negative electrode contains hydrophobic carbon fibers, so the increase in battery internal pressure during overcharge can be suppressed.

[0030] The sulfonation degree of the separator (or the nonwoven fabric constituting the separator) is 1.9 × 10 －３ 3.6 x 10 －３ It may be 2×10 or less, －３ 3x10 or more －３ The degree of sulfonation can be expressed as the ratio of the number of sulfur atoms to the number of carbon atoms contained in the separator (or nonwoven fabric).

[0031] Hydrophobic carbon fibers contained in the surface layer of the separator facing the negative electrode include hydrophobic and conductive carbon fibers. Examples of such carbon fibers include carbon nanotubes (CNTs) and carbon nanofibers (CNFs). The carbon fibers preferably contain CNTs. Due to their special structure, CNTs have a large aspect ratio and specific surface area, resulting in highly hydrophobic surfaces. The surface layer of the separator facing the negative electrode may contain CNTs and other carbon fibers. The proportion of CNTs in the total carbon fibers contained in the surface layer may be 60% by mass or more, or even 85% by mass or more. The proportion of CNTs in the total carbon fibers is 100% by mass or less. The carbon fibers contained in the surface layer may be composed entirely of CNTs.

[0032] The CNT may be a single-wall carbon nanotube (SWCNT), a multi-wall carbon nanotube (MWCNT), or a mixture of SWCNT and MWCNT. MWCNT is a general term for carbon nanotubes with two or more walls. Examples of MWCNT include double-wall carbon nanotubes (DWCNT), triple-wall CNTs, and CNTs with four or more walls.

[0033] CNF is a type of MWCNT and is produced as vapor grown carbon fiber (VGCF).

[0034] The outer diameter (diameter) of the CNT may be 0.4 nm or more and 200 nm or less, or may be 1 nm or more and 50 nm or less.

[0035] The length of the CNTs may be 0.5 μm or more and 25 μm or less. When N number of CNTs (e.g., 100≦N) are arbitrarily selected, 50% or more (0.5N), or even 80% (0.8N) or more of the CNTs may have a length of 1 μm or more and 25 μm or less.

[0036] In the separator, the surface density of the hydrophobic carbon fiber is 0.25 g / m per projected area in the thickness direction of the separator. ２ 3.00g / m or more ２ or less, and ２ 2.50g / m or more２ When the surface density is in this range, the internal pressure of the battery during overcharge can be further reduced, and higher output characteristics can be obtained. In addition, relatively high cycle characteristics can be easily obtained. The surface density is calculated by multiplying the mass (g) of the hydrophobic carbon fiber in the surface layer of the separator facing the negative electrode by the projected area (m ２ ) is the value obtained by dividing the areal density of the hydrophobic carbon fibers in the separator before assembly of the electrode group by the mass of the hydrophobic carbon fibers. In the separator before assembly of the electrode group, it is preferable that the areal density of the hydrophobic carbon fibers is within the above range. However, if a separator before assembly of the electrode group is not available, the areal density of the hydrophobic carbon fibers determined for the separator removed from the battery may be within the above range. The areal density of the carbon fibers may be determined by measuring the mass of the hydrophobic carbon fibers contained in a small piece of the separator having a predetermined small projected area, for example, by a method such as thermal analysis using a differential scanning calorimeter or thermogravimetry using a thermogravimetric measuring device.

[0037] It is preferable that the separator does not contain hydrophobic carbon fibers in the surface layer facing the positive electrode, or that the surface density of the carbon fibers is lower than the surface density of the carbon fibers in the surface layer facing the negative electrode, thereby achieving better cycle characteristics.

[0038] The surface density of the carbon fibers in the surface layer of the separator facing the positive electrode is calculated by multiplying the mass (g) of the carbon fibers contained in the surface layer facing the positive electrode by the projected area (m) in the thickness direction of the separator. ２ ) and calculate it by dividing by

[0039] The separator may be formed, for example, by transfer printing. For example, a layer containing hydrophobic carbon fibers is formed on the surface of a substrate, and a layer serving as the base of the separator (such as a porous membrane, nonwoven fabric, woven fabric, or a laminate thereof) is placed on the surface of this layer. Pressurization is applied in the thickness direction, and the substrate is peeled off to obtain a separator having a surface layer containing the carbon fibers. The carbon fiber layer is formed, for example, by applying a dispersion containing the carbon fibers (and other fibers, if necessary) to the surface of the substrate and drying it. The substrate may be a resin or metal substrate sheet.

[0040] The thickness of the separator may be 50 μm or more and 300 μm or less, or may be 100 μm or more and 250 μm or less.

[0041] (Negative Electrode) The negative electrode includes, for example, a negative electrode mixture layer. The negative electrode may include a core material and the negative electrode mixture layer attached to the core material.

[0042] Examples of the negative electrode core material include known core materials. Examples of the negative electrode core material include a conductive substrate. The substrate may be in the form of a sheet. The substrate may be a porous substrate having a plurality of through holes, or may be a substrate without through holes. Examples of the porous substrate include punched metal, sintered metal powder, expanded metal, and metal net (such as nickel net).

[0043] Examples of materials constituting the core include iron or iron alloys (such as stainless steel), nickel or its alloys, etc. The core may be plated as needed.

[0044] The negative electrode mixture layer is formed, for example, on at least the surface of the core material. The negative electrode mixture layer may be formed on one surface of the sheet-shaped core material, or may be formed on both surfaces. When the core material is porous, the negative electrode mixture layer may be formed by filling the pores of the core material with the negative electrode mixture. The negative electrode mixture layer can be formed by molding the negative electrode mixture or by attaching it to the core material.

[0045] The negative electrode mixture contains a hydrogen storage alloy powder capable of electrochemically absorbing and releasing hydrogen as a negative electrode active material, and may further contain a dispersion medium. Furthermore, the negative electrode mixture may contain known components used in negative electrode mixtures, such as a binder, a conductive agent, a thickener, etc., as needed. Specifically, the negative electrode mixture layer can be formed, for example, by applying the negative electrode mixture to a core material, removing the dispersion medium by drying, and then applying pressure in the thickness direction.

[0046] The hydrogen storage alloy constituting the hydrogen storage alloy powder is, for example, a material that can absorb hydrogen electrochemically generated in an alkaline electrolyte during charging and can easily release the absorbed hydrogen during discharging. Hydrogen storage alloys known in the field of nickel-metal hydride batteries may also be used.

[0047] Among these, hydrogen storage alloys containing Ni and Mg are preferred, and in addition to these elements, the alloy may further contain at least one element selected from the group consisting of elements of periods 4 and 6 of groups 2 to 6 of the periodic table (including lanthanoid elements), elements of periods 4 of groups 7 to 9 and 11 to 12 of the periodic table, and elements of periods 3 to 5 of groups 13 and 14 of the periodic table.

[0048] The hydrogen storage alloy preferably contains the elements Ln, Mg, Ni, and Al. In addition to these elements, the alloy may further contain the element M (at least one element selected from the group consisting of V, Nb, Ta, Cr, Mo, Mn, Fe, Co, Ga, Zn, Sn, In, Cu, Si, P, and B). The element Ln is at least one element selected from the group consisting of Group 3 and Group 4 elements of the periodic table. The element Ln is preferably at least one element selected from the group consisting of Y, lanthanoid elements, Zr, and Ti (particularly, at least one element selected from the group consisting of La, Pr, Nd, Sm, and Zr). The element Ln may contain Y and an element other than Y.

[0049] In a hydrogen storage alloy containing the elements Ln, Mg, Ni, and Al, the molar ratio x of Mg to the total of the elements Ln and Mg is, for example, 0.01≦x≦0.5, preferably 0.1≦x≦0.4, and more preferably 0.25≦x≦0.35. The molar ratio y of Ni to the total of the elements Ln and Mg is, for example, 1.6≦y≦4, preferably 2≦y≦4, and more preferably 2.5≦y≦3.5. The molar ratio α of Al to the total of the elements Ln and Mg is, for example, 0.01≦α≦0.3, preferably 0.01≦α<0.06, and more preferably 0.03≦α≦0.055 (e.g., 0.03≦α≦0.05).

[0050] When the hydrogen storage alloy contains the element M, the molar ratio z of the element M to the total of the elements Ln and Mg is 0.01≦z≦0.8, preferably 0.1≦z≦0.75.

[0051] The element M preferably contains at least Co. The molar ratio z1 of Co to the total of the elements Ln and Mg is, for example, 0.25≦z1≦0.75, preferably 0.25≦z1≦0.7 or 0.25≦z1≦0.6. The element M may further contain Sn in addition to Co.

[0052] The hydrogen storage alloy is, for example, AB ２ Type, AB ３ type (i.e., CeNi ３ type), AB ５ Mold (LaNi ５ , MmNi ５ (Mm indicates mischmetal) etc.), A ２ B ７ type (i.e., Ce ２ Ni ７ It may have a crystal structure such as AB ３ Type A ２ B ７ In the hydrogen storage alloy of this type, among the above elements, Mg and La are present in the A site, and Ni, Co and Al are present in the B site. ３ Type A ２ B ７ It is preferable to use a hydrogen storage alloy of this type.

[0053] As the dispersion medium, known media such as water, organic media, and mixtures thereof can be used. Examples of organic media include alkanols such as ethanol and isopropanol; aliphatic ketones such as acetone; aliphatic nitriles such as acetonitrile; ethers such as diethyl ether and tetrahydrofuran; and N-methyl-2-pyrrolidone. Although it depends on the types of other components contained in the negative electrode mixture, such as the binder, it is preferable that the dispersion medium contains at least water.

[0054] Examples of binders include resin materials (thermoplastic resins, thermosetting resins, etc.), for example, rubber-like materials such as styrene-butadiene copolymer rubber (SBR); polyolefin resins such as polyethylene and polypropylene; fluororesins such as PTFE, tetrafluoroethylene copolymer, polychlorotrifluoroethylene, chlorotrifluoroethylene copolymer (for example, copolymers with olefins such as ethylene), polyvinylidene fluoride, and vinylidene fluoride copolymer; acrylic resins such as ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, and ethylene-methyl acrylate copolymer, and Na ion crosslinkers thereof. These binders can be used alone or in combination of two or more.

[0055] The amount of the binder may be 0.01 to 5 parts by mass, or may be 0.05 to 2 parts by mass, per 100 parts by mass of the hydrogen storage alloy powder.

[0056] As the conductive agent, various electron conductive materials can be used. Examples of the conductive agent include graphite (natural graphite (e.g., flake graphite), artificial graphite, expanded graphite, etc.), carbon black (e.g., acetylene black, ketjen black, etc.), conductive fiber (e.g., carbon fiber, metal fiber, etc.), metal powder (e.g., copper powder), and organic conductive material (e.g., polyphenylene derivative). The conductive agent may be used alone or in combination of two or more.

[0057] The amount of the conductive agent may be 0.01 to 5 parts by mass, or may be 0.05 to 2 parts by mass, relative to 100 parts by mass of the hydrogen storage alloy powder.

[0058] The conductive agent may be added to the negative electrode mixture and mixed with other components. Alternatively, the surface of the hydrogen storage alloy powder may be coated with the conductive agent in advance. The conductive agent can be coated by a known method, such as sprinkling the conductive agent on the surface of the hydrogen storage alloy powder, applying a dispersion containing the conductive agent and drying it, or mechanically coating the surface by a mechanochemical method. These coating methods may also be combined.

[0059] The thickener imparts viscosity to the negative electrode mixture (slurry or paste-like negative electrode mixture). The thickener can be appropriately selected depending on the type of dispersion medium. Examples of thickeners include cellulose derivatives such as carboxymethyl cellulose (CMC) and its modified forms (including salts such as Na salts), methyl cellulose, etc.; acrylic resins having acrylic acid units or methacrylic acid units, such as polyacrylic acid and polymethacrylic acid, or salts thereof; saponified polymers having vinyl acetate units, such as polyvinyl alcohol; and polyalkylene oxides, such as polyethylene oxide. These thickeners can be used alone or in combination of two or more.

[0060] When the dispersion medium contains water, it is preferable to use, among the above-mentioned thickeners, a component having a hydrophilic group such as a carboxyl group (or a salt thereof), a hydroxyl group, or a polyoxyethylene unit.

[0061] The amount of the thickener may be 0.01 to 5 parts by mass, or may be 0.05 to 1 part by mass, per 100 parts by mass of the hydrogen storage alloy powder.

[0062] The negative electrode mixture layer may further contain known additives such as an oxidation inhibitor, for example, yttrium oxide or ytterbium oxide.

[0063] The amount of the additive may be 0.01 to 5 parts by mass, or may be 0.05 to 1 part by mass, per 100 parts by mass of the hydrogen storage alloy powder.

[0064] In the present disclosure, the surface layer of the separator facing the negative electrode contains hydrophobic carbon fibers, so that this surface layer (more specifically, the carbon fibers) faces the surface of the negative electrode. This ensures high gas absorption in the negative electrode while minimizing the amount of carbon fiber shedding during the manufacturing process. This allows the negative electrode to efficiently absorb hydrogen gas and oxygen gas generated within the battery. This, in turn, minimizes the increase in internal battery pressure during overcharge, resulting in a highly reliable alkaline storage battery.

[0065] (Positive Electrode) As the positive electrode, for example, a known positive electrode for an alkaline storage battery (such as a known positive electrode for a nickel-metal hydride storage battery) is used.

[0066] The positive electrode may include a core material and an active material or an active material layer attached to the core material. The positive electrode may be a positive electrode formed by sintering an active material powder, or a non-sintered positive electrode.

[0067] The positive electrode can be formed, for example, by applying a positive electrode mixture (slurry or paste-like positive electrode mixture) containing at least a positive electrode active material to a core material. More specifically, the positive electrode can be formed by applying the positive electrode mixture to the core material, drying to remove the dispersion medium, and rolling.

[0068] The positive electrode core material can be a known core material. Examples of the positive electrode core material include a porous substrate (nickel foam, sintered nickel plate, etc.) formed of nickel or a nickel alloy. When a porous substrate is used as the positive electrode core material, the positive electrode mixture is filled into the pores of the positive electrode core material.

[0069] The positive electrode active material may be, for example, a nickel compound (specifically, nickel oxide) such as nickel hydroxide or nickel oxyhydroxide. The positive electrode active material may be used alone or in combination of two or more.

[0070] The positive electrode mixture in a slurry or paste form usually contains a dispersion medium, which may be selected from the dispersion mediums exemplified for the negative electrode mixture.

[0071] If necessary, known components used in positive electrodes (conductive agents, binders, etc.) may be added to the positive electrode mixture.

[0072] Examples of the binder include hydrophilic or hydrophobic polymers, and may be selected from the binders and thickeners exemplified for the negative electrode mixture. One binder may be used alone, or two or more binders may be used in combination. The amount of binder may be 0.1 to 15 parts by mass, or 0.5 to 10 parts by mass, per 100 parts by mass of the positive electrode active material.

[0073] The conductive agent may be selected from the conductive agents exemplified for the negative electrode mixture, or a conductive cobalt oxide such as cobalt hydroxide or γ-type cobalt oxyhydroxide may be used. The conductive agent may be used alone or in combination of two or more. The amount of the conductive agent may be 0.1 to 10 parts by mass, or 0.5 to 5 parts by mass, per 100 parts by mass of the positive electrode active material.

[0074] The positive electrode mixture may contain known additives, for example, metal compounds (oxides, hydroxides, etc.) such as zinc oxide, zinc hydroxide, and cadmium compounds (cadmium oxide, etc.).

[0075] (Electrode Group) In a nickel-metal hydride storage battery, the negative electrode, the positive electrode, and the separator may form an electrode group with the separator disposed between the positive electrode and the negative electrode. The electrode group may be an electrode group in which the negative electrode, the positive electrode, and the separator disposed therebetween are wound in a spiral shape, or an electrode group in which the negative electrode, the positive electrode, and the separator disposed therebetween are stacked in a zigzag shape or the like.

[0076] (Alkaline Electrolyte) As the alkaline electrolyte, for example, an aqueous solution containing an alkaline solute is used. Examples of the solute include alkali metal hydroxides such as lithium hydroxide, potassium hydroxide, and sodium hydroxide. The solute may be used alone or in combination of two or more.

[0077] The concentration of the alkaline solute in the alkaline electrolyte may be 3 to 10 mol / L, or 5 to 9 mol / L.The specific gravity of the alkaline electrolyte may be 1.03 to 1.55, or 1.11 to 1.32.

[0078] FIG. 1 is a partial cross-sectional view of a cylindrical alkaline storage battery according to one embodiment of the present invention.

[0079] The alkaline storage battery includes a cylindrical battery case (external can) 2 having a bottom and an opening at the top, an electrode group and an alkaline electrolyte (not shown) housed in the battery case 2, and a sealing plate 7 that seals the opening of the battery case 2.

[0080] The electrode group includes a strip-shaped (long sheet-like) positive electrode 1 having an active material-filled portion and an unfilled portion 13, a negative electrode 3 containing a hydrogen storage alloy, and a separator 4 disposed therebetween, which are superimposed and spirally wound. At the end face of this spirally wound electrode group, the unfilled portion 13 of the positive electrode 1 is welded to a flat positive electrode current collector 5. This electrode group is inserted into a battery case 2, which also serves as the negative electrode terminal. Meanwhile, the negative electrode 3 uses a plate-shaped core material, and the unfilled portion of the negative electrode active material is welded to a negative electrode current collector 9. The positive electrode current collector 5 is electrically connected to a sealing plate 7, which also serves as the positive electrode terminal, via a lead 8, and the negative electrode current collector 9 is electrically connected to the battery case 2. An electrolyte is then injected, and the sealing plate 7 and the battery case 2 are crimped together to form an alkaline storage battery of the present invention.

[0081] In the present disclosure, the surface layer of the separator 4 facing the negative electrode 3 contains hydrophobic carbon fiber. The conductivity of the carbon fiber ensures high conductivity between the negative electrode 3 and the separator 4, resulting in high output of the alkaline storage battery. By including carbon fiber in the separator 4 rather than the negative electrode 3, shedding of the carbon fiber during the manufacturing of the negative electrode 3 is suppressed, making it easier to maintain the high quality of the negative electrode 3. The hydrophobicity of the carbon fiber improves gas permeability between the negative electrode 3 and the separator 4. This allows oxygen gas generated at the positive electrode 1 during overcharge to be smoothly absorbed by the negative electrode 3, thereby suppressing an increase in the internal pressure of the battery. This results in a highly reliable alkaline storage battery.

[0082] [Examples] Hereinafter, the present invention will be specifically described based on examples and comparative examples, but the alkaline storage battery of the present disclosure is not limited to only the following examples.

[0083] Examples 1 to 5 and Comparative Examples 1 to 9 An SC-type cylindrical nickel-metal hydride storage battery with a capacity of 3000 mAh was fabricated according to the following procedure.

[0084] (1) Preparation of Negative Electrode: La, Ce, Ni, Al, Mn, and Co were melted in a mass ratio of 20.6:12.0:49.7:1.80:5.40:10.5 in an induction melting furnace, and an ingot was prepared from the melt. The resulting ingot was heated in an argon atmosphere at 1000°C for 10 hours to obtain an ingot of a superlattice structure alloy. The heated ingot was crushed into coarse particles. The resulting coarse particles were further mechanically crushed in an inert gas atmosphere and classified to produce a hydrogen storage alloy powder with an average particle size of approximately 30 μm.

[0085] To 100 parts by mass of the obtained hydrogen storage alloy powder, 0.7 parts by mass of SBR as a binder, 0.15 parts by mass of CMC as a thickener, 0.3 parts by mass of Ketjen black as a conductive agent, and 0.7 parts by mass of yttrium oxide as an oxidation inhibitor were added, and an appropriate amount of water was further added and kneaded to prepare a negative electrode slurry. The SBR was used in the form of a dispersion containing 48% by mass of SBR and 52% by mass of ion-exchanged water.

[0086] The obtained negative electrode slurry was applied to both sides of a nickel-plated iron punched metal (thickness 60 μm, hole diameter 1 mm, porosity 40%) as a negative electrode core material. The coating of the negative electrode slurry was dried at 95 ° C. for 10 minutes, and then the coating film and the core material were pressed with a roller to form a negative electrode mixture layer so that the total thickness of the core material and the negative electrode mixture layer was 0.3 mm. The obtained product was cut to a width of 35 mm and a length of 310 mm. In this way, a strip-shaped negative electrode (negative electrode A) was produced.

[0087] A strip-shaped negative electrode B was also prepared by the following procedure.

[0088] First, an ultrasonic homogenizer (amplitude 80 μm, frequency 20 kHz) was used to mix the auxiliary agent, CMC, and water in a mass ratio of 1:0.4:23.6 to prepare a dispersion. The dispersion was applied to the surface of a PET film as a substrate film and dried to form an auxiliary agent layer. The amount of the dispersion applied was adjusted so that the surface density of the auxiliary agent layer (surface layer) on one main surface of the negative electrode was the value shown in Table 1.

[0089] In Comparative Example 6, an auxiliary agent layer was placed on one main surface of a strip-shaped negative electrode A obtained in the same manner as above, and pressure was applied in the thickness direction. The PET film was then peeled off, transferring the auxiliary agent layer to the negative electrode A. In this manner, a negative electrode B having a surface layer containing an auxiliary agent on one main surface side was produced. In Comparative Examples 7 and 8, a dispersion containing carbon black or PTFE was applied to one main surface of the negative electrode A and dried to produce a negative electrode C or a negative electrode D having a surface layer containing an auxiliary agent on one main surface side. The amount of dispersion applied was adjusted so that the areal density of the auxiliary agent layer (surface layer) on one main surface of the negative electrode was the value shown in Table 1.

[0090] (2) Preparation of Separator A dispersion was prepared by mixing the auxiliary agent, CMC, and water in a mass ratio of 1:0.4:23.6 using an ultrasonic homogenizer (amplitude 80 μm, frequency 20 kHz). The dispersion was applied to the surface of a PET film as a substrate film and dried to form an auxiliary agent layer. The amount of dispersion applied was adjusted so that the surface density of the surface layer (auxiliary agent layer) in the completed separator would be the value shown in Table 1.

[0091] A PP nonwoven fabric (thickness: 150 μm, basis weight: 64 g / m) that has been hydrophilized by sulfonation treatment is applied to the surface of the auxiliary layer. ２), and pressure was applied in the thickness direction, and the PET film was peeled off, thereby transferring the auxiliary agent layer to the nonwoven fabric. In this way, a separator (separator A) having a surface layer containing an auxiliary agent on one of the main surfaces was produced. Two laminates each having an auxiliary agent layer formed on the surface of a PET film were prepared, and the above-mentioned PP nonwoven fabric was sandwiched between the two laminates so that the auxiliary agent layers were in contact. Pressure was applied in the thickness direction, and the auxiliary agent layers were transferred to both sides of the nonwoven fabric, and the PET film was peeled off. In this way, a separator (separator B) having surface layers containing an auxiliary agent on both main surfaces was produced. The above-mentioned PP nonwoven fabric was prepared as separator C.

[0092] In the PP nonwoven fabric used, the fibers constituting the nonwoven fabric included fibers with an average fiber diameter of 10 μm and ultrafine fibers with a fiber diameter of 1 to 3 μm, and the content of ultrafine fibers was 20 mass % or less of the total fibers.

[0093] The auxiliary agents used in the preparation of the negative electrode or separator are as follows.

[0094] CNT: multi-walled CNT (average fiber length 5 μm, average fiber diameter 12 nm) PTFE: polytetrafluoroethylene particles (average particle size D50 1 μm) GF: artificial graphite (average particle size D50 7 μm) CB: ketjen black (primary particle size 40 nm) (3) Preparation of positive electrode A non-sintered nickel positive electrode was prepared by the following procedure.

[0095] First, nickel hydroxide powder containing 2.5% by mass of zinc and 1.0% by mass of cobalt as coprecipitated components was added to an aqueous cobalt sulfate solution. While stirring the resulting mixture, a sodium hydroxide aqueous solution (sodium hydroxide concentration: 1 mol / L) was gradually added dropwise to adjust the pH to 11, and stirring was then continued for a predetermined period of time. The precipitate was filtered out from the resulting mixture. The filtered precipitate was washed with water and vacuum dried to obtain a powder in which the surfaces of nickel hydroxide particles were coated with 5% by mass of cobalt hydroxide.

[0096] To 1 part by mass of the powder obtained above, 10 parts by mass of an aqueous sodium hydroxide solution (sodium hydroxide concentration: 48% by mass) was added. The resulting mixture was heated at 85°C for 8 hours while stirring, and then washed with water and dried at 65°C. This heat treatment caused a portion of the cobalt hydroxide in the cobalt hydroxide-containing layer on the surface of the nickel hydroxide particles to be highly ordered and converted to cobalt oxyhydroxide, and sodium was also introduced. Composite particles were obtained in which a coating layer containing cobalt oxyhydroxide and 1% by mass of sodium was formed on the surface of the nickel hydroxide particles.

[0097] A positive electrode slurry was prepared by adding 25 parts by mass of an aqueous solution containing CMC (CMC concentration: 0.2% by mass) as a binder to 100 parts by mass of a mixed powder of 100% by mass of the obtained composite particles and 2% by mass of zinc oxide and mixing them.

[0098] The obtained positive electrode slurry was mixed with nickel foam (areal density (weight per unit area) about 360 g / m) as a positive electrode core material. 2 The mixture was filled into holes (approximately 1.2 mm thick) and dried. The dried product was rolled to a thickness of 0.5 mm and then cut into a strip-shaped positive electrode having a width of 35 mm and a length of 270 mm.

[0099] (4) Fabrication of Nickel-Metal Hydride Storage Battery The negative electrode obtained in (1) above and the positive electrode obtained in (3) above were offset so that their unfilled portions protruded upward or downward, and the separator obtained in (2) above was arranged and spirally wound to fabricate an electrode group. In fabricating the electrode group, in the Example and Comparative Examples 3 to 5, separator A was arranged so that the surface layer containing the auxiliary agent contacted the negative electrode. In Comparative Example 2, separator A was arranged so that the surface layer containing the auxiliary agent contacted the positive electrode. In Comparative Example 9, separator B was used, and in Comparative Examples 1 and 6 to 8, separator C was used. In Comparative Examples 6, 7, and 8, negative electrode B, negative electrode C, and negative electrode D were arranged so that the surface layer containing the auxiliary agent contacted the separator, respectively. In the Example, Comparative Examples 1 to 5, and Comparative Example 9, negative electrode A was used.

[0100] Next, flat, disk-shaped current collectors were electrically resistance-welded to the unfilled portions of the positive and negative electrodes from above. The resulting electrode group was inserted into a bottomed, cylindrical metal battery case (outer diameter 23 mm) of an SC type. The negative current collector was resistance-welded to the inner bottom of the battery case, and the positive current collector and sealing plate were electrically connected using a lead.

[0101] Next, an alkaline electrolyte was poured into the battery case, and the opening of the battery case was covered with a sealing plate equipped with a safety valve and sealed by crimping with an insulating packing. The battery case was then pressed in the height direction to crimp the groove formed on the opening side of the battery case so that the total height of the battery was 42.5 mm. The alkaline electrolyte used was an aqueous solution containing potassium hydroxide at a concentration of 6.6 mol / L and sodium hydroxide at a concentration of 0.5 mol / L.

[0102] (5) Evaluation In the examples and comparative examples, the areal density of the auxiliary agent layer on one main surface of the negative electrode or separator, and the amount of auxiliary agent shed in the process up to the production of the electrode plate assembly were determined by the following procedure (a). The alkaline storage batteries (nickel-metal hydride storage batteries) obtained in the examples and comparative examples were also evaluated in the following procedures (b) to (d). Prior to the evaluations (b) to (d), the nickel-metal hydride storage batteries were activated as follows.

[0103] (Activation of nickel-metal hydride storage battery) The fabricated nickel-metal hydride storage battery was charged at room temperature (25° C.) with a charging current of 0.1 It (300 mA) for 16 hours, then rested for 1 hour, and then discharged at a discharging current of 0.2 It (600 mA) until the final voltage reached 1.0 V, after which the battery was rested for 1 hour. This charge-discharge cycle was repeated 5 times at room temperature (25° C.) to activate the nickel-metal hydride storage battery.

[0104] (a) Amount of assistant agent removed from surface layer (assistant layer) A separator provided with an assistant agent layer or a negative electrode provided with an assistant agent layer was vibrated in a vibration tester at a frequency of 5 times / second and an amplitude of 2 mm for 1 minute to determine the amount (g) of assistant agent that actually removed. This amount was calculated based on the projected area (m2) of the separator or negative electrode subjected to the vibration test in the thickness direction. ２ ) to obtain the amount of auxiliary agent that falls off (g / m ２) was sought.

[0105] (b) Battery Internal Pressure Characteristics A hole with a diameter of 1.0 mm was drilled in the bottom of the battery case of the nickel-metal hydride storage battery, and a pressure sensor was attached. The nickel-metal hydride storage battery in this state was charged at 25°C at 1.0 It (3000 mA) to 120% of the positive electrode capacity. The internal pressure of the battery during charging was measured with the pressure sensor. Similar measurements were performed on five nickel-metal hydride storage batteries for each example, and the average value of the internal battery pressure was calculated.

[0106] (c) Battery Output Characteristics (Low-Temperature Discharge Rate Characteristics) The nickel-metal hydride storage battery was charged at 25° C. at 0.1 It (300 mA) for 15 hours, and then discharged at 0.2 It (600 mA) until the battery voltage reached 1.0 V. The discharge capacity at this time was determined as the initial capacity.

[0107] After measuring the initial capacity, the nickel-metal hydride storage battery was charged at 25°C for 15 hours at 0.1 It (300 mA), then left to stand at -10°C for 3 hours and discharged at 3 It (9000 mA) until the battery voltage reached 0.8 V, and the discharge capacity at this time was calculated. The percentage (%) of this discharge capacity relative to the initial capacity (100%) was calculated, and the output characteristics of the battery were evaluated based on this percentage (large current discharge rate). A higher percentage indicates better output characteristics.

[0108] The discharge capacity was measured for five nickel-metal hydride batteries for each example, and the average value was calculated.

[0109] (d) Battery Cycle Characteristics The nickel-metal hydride storage battery was charged at 1.0 It (3000 mA) for 1.2 hours at 25°C, and then discharged at 1.0 It (3000 mA) until the battery voltage reached 1.0 V. This charge and discharge constituted one cycle, and the number of cycles at which the discharge capacity fell below 60% of the discharge capacity at the first cycle was determined. Measurements were performed on five nickel-metal hydride storage batteries for each example, and the average number of cycles was calculated. The cycle characteristics of each battery were expressed as a relative value when the number of cycles for Comparative Example 1 was set to 100.

[0110] The evaluation results of the Examples and Comparative Examples are shown in Table 1. In Table 1, A1 to A5 are Examples 1 to 5, and B1 to B9 are Comparative Examples 1 to 9.

[0111]

[0112] As shown in Table 1, when the separator contains hydrophobic carbon fibers in the surface layer facing the positive electrode, the output characteristics and internal charging pressure are almost the same as when the separator does not contain such carbon fibers (comparison between B1 and B2). In contrast, when the separator contains the above-mentioned carbon fibers in the surface layer facing the negative electrode, the internal charging pressure can be significantly reduced, and the output characteristics are greatly improved, resulting in high reliability (A1 to A5), compared to B1 and B2.

[0113] When the surface layer of the negative electrode contains hydrophobic carbon fibers, a certain level of high output characteristics can be obtained, but the effect of reducing the internal pressure during charging is low (comparison of A1 to A5 with B6).In addition, the amount of carbon fibers that fall off is also large (comparison of A1 to A5 with B6).

[0114] Furthermore, when the surface layer of the separator facing the negative electrode contains carbon black or graphite, the internal charging pressure is higher and the output characteristics are lower than when using hydrophobic carbon fiber (comparison of A1 to A5 with B3 and B4). Furthermore, when the surface layer of the separator facing the negative electrode contains a highly water-repellent resin such as a fluororesin, high gas permeability is ensured, so the internal charging pressure is reduced, but the output characteristics are lower (comparison of A1 to A5 with B5).

[0115] High cycle performance can be achieved when the separator or negative electrode surface layer contains a highly water-repellent resin such as a fluororesin (B5 and B7). However, when the separator surface layer facing the positive electrode contains carbon fiber, or when the separator surface layer facing the negative electrode or the negative electrode surface layer contains other carbon materials, it is difficult to achieve high cycle performance (B2 to B4, B6, B8). In contrast, high cycle performance can be achieved when the separator surface layer facing the negative electrode contains carbon fiber (A1 to A5). From the perspective of achieving high cycle performance, it is preferable that the separator surface layer facing the positive electrode contain a small amount of carbon fiber, and carbon fiber may not be present at all (compare A1 to A5 with B9).

[0116] The alkaline storage battery of the present disclosure can suppress an increase in the internal pressure of the battery, even when the battery has a high capacity or when multiple batteries are used. Therefore, leakage of alkaline electrolyte due to an increase in the internal pressure of the battery can be effectively suppressed. In addition, high output can be obtained. Thus, because alkaline storage batteries have high reliability, they are suitable for use as a replacement for dry batteries and as a power source for various devices.

[0117] REFERENCE SIGNS LIST 1 positive electrode 2 battery case 3 negative electrode 4 separator 5 positive electrode current collector 7 sealing plate 8 lead 9 negative electrode current collector 13 unfilled portion

Claims

1. An alkaline storage battery comprising: a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an alkaline electrolyte, wherein the negative electrode comprises a hydrogen storage alloy capable of electrochemically absorbing and releasing hydrogen, and the separator comprises hydrophobic carbon fiber on a surface layer facing the negative electrode.

2. The separator has a density of 0.25 g / m per projected area in the thickness direction of the separator. ２ 2.50g / m or more ２ 2. The alkaline storage battery of claim 1 comprising the carbon fibers at an areal density of:

3. The alkaline storage battery according to claim 1 or 2, wherein the carbon fibers include carbon nanotubes.

4. The alkaline storage battery according to claim 1 or 2, wherein the separator is made of a nonwoven fabric containing the carbon fiber.

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