Alkaline storage battery

Abstract

The present disclosure suppresses deterioration in discharge capacity and discharge characteristics of an alkaline storage battery after trickle charging. An alkaline storage battery (10) comprises a positive electrode (1), a negative electrode (2), a separator (3), and an alkaline electrolyte. The positive electrode (1), the negative electrode (2), and the separator (3) constitute a wound body. The positive electrode (1) includes a positive electrode current collector and a positive electrode mixture supported by the positive electrode current collector. The positive electrode mixture includes Fe and a nickel compound serving as a positive electrode active material. In the wound body, the Fe content Cinm in the positive electrode mixture in an innermost circumferential portion of the positive electrode is smaller than the Fe content Coutm in the positive electrode mixture in an outermost circumferential portion of the positive electrode.

Description

Alkaline storage battery

[0001] The present disclosure relates to an alkaline storage battery.

[0002] Alkaline storage batteries such as nickel-metal hydride storage batteries are used in various applications. Various proposals have conventionally been made for alkaline storage batteries.

[0003] Patent Document 1 proposes "a nickel hydroxide active material powder used for a battery, wherein at least one selected from the group consisting of cadmium, calcium, zinc, magnesium, iron, cobalt, and manganese is contained in the nickel hydroxide active material powder in an amount of 1 to 7 wt% when producing a positive electrode, and the powder is a mixture of spherical or spherical-like particles and non-spherical nickel hydroxide active material powder".

[0004] Patent Document 2 proposes "a positive electrode active material for an alkaline storage battery, characterized by comprising active material particles containing a positive electrode active material, and an iron compound attached to the surfaces of the active material particles".

[0005] Japanese Unexamined Patent Publication No. Hei 5-21064, Japanese Unexamined Patent Publication No. 2004-303599

[0006] As a positive electrode active material for alkaline storage batteries, nickel compounds such as nickel hydroxide are used. When γ-oxy nickel hydroxide is generated in the positive electrode active material, the positive electrode swells, the amount of electrolyte contained in the positive electrode increases, and the amount of electrolyte retained by the separator decreases, thereby degrading discharge characteristics. In contrast, including Fe element in the positive electrode can suppress the generation of γ-oxy nickel hydroxide during charging.

[0007] However, when Fe element is included in the positive electrode, the discharge capacity may significantly decrease or the discharge characteristics may significantly degrade after trickle charging. Such a phenomenon becomes more pronounced as the concentration of Fe element contained in the positive electrode increases.

[0008] One aspect of this disclosure relates to an alkaline storage battery comprising a positive electrode, a negative electrode, a separator, and an alkaline electrolyte, wherein the positive electrode, the negative electrode, and the separator constitute a winding body, the positive electrode comprises a positive electrode current collector and a positive electrode mixture supported by the positive electrode current collector, the positive electrode mixture comprises a nickel compound which is a positive electrode active material and the element Fe, and in the winding body, the content of the element Fe in the positive electrode mixture at the innermost circumference of the positive electrode, Cinm, is smaller than the content of the element Fe in the positive electrode mixture at the outermost circumference of the positive electrode, Coutm.

[0009] Another aspect of the present disclosure relates to an alkaline storage battery comprising a positive electrode, a negative electrode, a separator, and an alkaline electrolyte, wherein the positive electrode, the negative electrode, and the separator constitute a winding, the positive electrode comprises a positive electrode current collector and a positive electrode mixture supported by the positive electrode current collector, the positive electrode mixture comprises a nickel compound which is a positive electrode active material and the element Fe, and in the winding, the content of the element Fe in the positive electrode mixture Cin on the inner circumference of the positive electrode is smaller than the content of the element Fe in the positive electrode mixture Cout on the outer circumference of the positive electrode.

[0010] According to this disclosure, it is possible to suppress the decrease in discharge capacity and the decrease in discharge characteristics after trickle charging in an alkaline storage battery.

[0011] This is a schematic exploded perspective view showing an alkaline storage battery according to an embodiment of the present disclosure.

[0012] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and other materials may be applied as long as they allow the invention of this disclosure to be carried out. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "numerical value A or greater and numerical value B or less". In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either of the given lower limits and either of the given upper limits can be arbitrarily combined, as long as the lower limit does not exceed the upper limit.

[0013] (Alkaline Battery) The alkaline battery according to this embodiment may be referred to as "alkaline battery (A)" below. Alkaline battery (A) includes a positive electrode, a negative electrode, a separator, and an alkaline electrolyte. The positive electrode, negative electrode, and separator constitute a wound body. That is, the positive electrode and the negative electrode are each in the shape of a long strip, and the positive electrode and the negative electrode are wound together with the separator in between.

[0014] The positive electrode comprises a positive electrode current collector and a positive electrode mixture supported by the positive electrode current collector. The positive electrode mixture contains a nickel compound, which is the positive electrode active material, and the element Fe.

[0015] The wound material and alkaline electrolyte are housed, for example, in a bottomed cylindrical battery case. The opening of the battery case is sealed with a sealing body. When the battery case is bottomed cylindrical, a cylindrical alkaline storage battery (A) is obtained.

[0016] The proportion of the positive electrode active material (nickel compound) in the positive electrode mixture may be, for example, 80% by mass or more, or 90% by mass or more.

[0017] The alkaline battery (A) satisfies the following condition (X).

[0018] Condition (X): In a wound body, the Fe element content Cinm in the positive electrode mixture at the innermost circumference of the positive electrode is smaller than the Fe element content Coutm in the positive electrode mixture at the outermost circumference of the positive electrode.

[0019] Here, the innermost part of the positive electrode refers to the portion of the positive electrode that is located at the innermost part of the winding. In other words, the innermost part of the positive electrode is the portion that rotates 360° outward from the end where the winding begins. The outermost part of the positive electrode refers to the portion of the positive electrode that is located at the outermost part of the winding. In other words, the outermost part of the positive electrode is the portion that rotates 360° inward from the end where the winding ends.

[0020] According to alkaline storage battery (A), the action of the element Fe is thought to suppress the formation of γ-oxyhydroxide nickel and the expansion of the positive electrode during charging, thus ensuring excellent discharge characteristics even after trickle charging.

[0021] On the other hand, the element Fe reduces the oxygen overpotential of the positive electrode active material, which is thought to make it easier for oxygen gas to be generated at the positive electrode during trickle charging. When a large amount of oxygen gas is generated at the positive electrode, the pressure of the gas is thought to cause the positive electrode active material to lift away from the positive electrode current collector, reducing the conductive path between the positive electrode current collector and the positive electrode active material. Therefore, when the element Fe is added to the positive electrode, the discharge capacity gradually decreases and the discharge characteristics deteriorate.

[0022] In contrast, in alkaline storage batteries (A), the Fe element content Cinm in the positive electrode mixture at the innermost circumference of the positive electrode is smaller than the Fe element content Coutm in the positive electrode mixture at the outermost circumference of the positive electrode. Therefore, it is thought that the amount of oxygen gas generated is small at the innermost circumference of the positive electrode, and large at the outermost circumference of the positive electrode. The innermost circumference of the positive electrode is in nearest contact with the hollow of the winding body and is less susceptible to pressure from the surroundings, so it is thought that the positive electrode active material is most likely to float away from the positive electrode current collector. However, in such areas where the positive electrode active material is likely to float away, the amount of oxygen gas generated is small, so it is thought that the floating of the positive electrode active material from the positive electrode current collector is suppressed. On the other hand, the outermost circumference of the positive electrode is in nearest contact with the inner surface of the battery case and is more susceptible to pressure from the surroundings, so it is thought that the positive electrode active material is least likely to float away from the positive electrode current collector. In areas where the positive electrode active material is less likely to float up, it is thought that the floating of the positive electrode active material from the positive electrode current collector will be suppressed, even if a large amount of oxygen gas is generated.

[0023] In other words, when the Fe element content Cinm in the positive electrode mixture at the innermost circumference of the positive electrode is smaller than the Fe element content Coutm in the positive electrode mixture at the outermost circumference of the positive electrode, it is considered that the Fe element effectively suppresses the formation of γ-oxyhydroxide nickel and the expansion of the positive electrode, while also efficiently suppressing the decrease in the holding power of the positive electrode active material by the positive electrode current collector.

[0024] As described above, this disclosure is believed to ensure excellent discharge characteristics and effectively suppress the decrease in discharge capacity and discharge characteristics after trickle charging due to the generation of oxygen gas.

[0025] The relationship Cinm < Outm is sufficient, but Cinm is preferably 0% to 50% of Outm. Within this range, the smaller Cinm is relative to Outm, the more oxygen gas generation at the innermost part of the positive electrode is suppressed, thus suppressing the decrease in discharge capacity after trickle charging. On the other hand, the larger Cinm is relative to Outm, the more the formation of γ-nickel oxyhydroxide at the innermost part of the positive electrode is suppressed, thus suppressing the decrease in discharge characteristics. Considering the balance between discharge capacity and discharge characteristics after trickle charging, Cinm may be 10% to 40% or 20% to 40% of Outm.

[0026] The Fe element content (Cotm) in the positive electrode mixture at the outermost periphery of the positive electrode may be, for example, within the range of 0.005% by mass to 0.1% by mass. Within this range, excellent discharge characteristics can be ensured, and the decrease in discharge capacity and the decrease in discharge characteristics after trickle charging can be suppressed in a balanced manner.

[0027] The alkaline battery (A) may satisfy the following condition (Y) instead of condition (X).

[0028] Condition (Y): In a wound body, the Fe element content Cin in the positive electrode mixture on the inner circumference of the positive electrode is smaller than the Fe element content Cout in the positive electrode mixture on the outer circumference of the positive electrode.

[0029] When condition (X) is met, the Fe content in parts of the positive electrode other than the innermost and outermost parts is usually an intermediate value between content Cinm and content Coutm. In that case, condition (Y) is also met. When condition (Y) is met, excellent discharge characteristics can be ensured, as in the case when condition (X) is met, and the decrease in discharge capacity and discharge characteristics after trickle charging due to the generation of oxygen gas can be effectively suppressed.

[0030] Here, the inner circumference of the positive electrode refers to the inner part when a long positive electrode is divided into two equal parts with the center in the longitudinal direction as the boundary. The outer circumference of the positive electrode refers to the outer part when a long positive electrode is divided into two equal parts with the center in the longitudinal direction as the boundary.

[0031] The relationship Cin content < Cout content is sufficient, but it is preferable that Cin content is, for example, 0% to 50% of Cout content. Considering the balance between discharge capacity and discharge characteristics after trickle charging, Cin content may be 10% to 40% or 20% to 40% of Cout content.

[0032] The Fe element content (Cout) in the positive electrode mixture at the outer peripheral side of the positive electrode may be, for example, within the range of 0.0045% by mass to 0.09% by mass. Within this range, excellent discharge characteristics can be ensured, and a good balance can be maintained between the decrease in discharge capacity and the decrease in discharge characteristics after trickle charging.

[0033] The Fe element content in the cathode mixture of each part of the cathode can be measured by inductively coupled plasma (ICP) emission spectroscopy.

[0034] First, the alkaline battery is disassembled, the positive electrode is removed, washed with deionized water, and dried. Then, the positive electrode is divided into three parts: the innermost part, the outermost part, and the remaining part (the middle part). Typically, the innermost part is the smallest and the middle part is the largest. Alternatively, the positive electrode can be divided into two equal parts: the inner side and the outer side. The inner and outer sides are the same size.

[0035] The innermost and outermost parts are each divided into three equal parts in the width direction of the positive electrode, and a sample of the predetermined size is cut from the part of each section as close to the center of gravity as possible. Alternatively, the inner and outer sides are each divided into three equal parts in the length direction of the positive electrode, and then further divided into three equal parts in the width direction of the positive electrode, and a sample of the predetermined size is cut from the part of each section as close to the center of gravity as possible. The size of the sample is, for example, about 1 cm. ２ That's fine.

[0036] Three samples, arranged in the width direction of the positive electrode plate at the innermost and outermost circumferences, are immersed in acetic acid in separate containers, and ultrasonic waves are applied to remove the positive electrode mixture. Alternatively, three x three samples, each at the inner and outer circumferences, are immersed in acetic acid in separate containers, and ultrasonic waves are applied to remove the positive electrode mixture. After drying the removed positive electrode mixture, it is heated in nitric acid to dissolve it. After the resulting solution is allowed to cool, the insoluble components are filtered out to obtain the sample solution. The sample solution is analyzed using an ICP emission spectrometer as specified in JIS K0116. The amount of Fe element in each part can be determined from the emission intensity of Fe element in the sample solution of each part. The content percentages Cinm and Coutm are the average values ​​of the content percentages obtained from three samples. The content percentages Cin and Coutm are the average values ​​of the content percentages obtained from nine samples.

[0037] (Method for manufacturing the positive electrode) There are no limitations on the method for manufacturing the positive electrode; it may be manufactured in the same manner as known methods, except that the positive electrode contains the element Fe. In one example of a manufacturing method, first, a positive electrode paste containing nickel compound particles (particles of the positive electrode active material) is prepared.

[0038] The positive electrode paste typically contains a dispersion medium and, if necessary, other components (such as conductive materials, binders, and thickeners). The dispersion medium can be water, an organic medium, or a mixture of two or more liquid mediums selected from these.

[0039] Examples of conductive materials and binders will be described later. Examples of thickeners include carboxymethylcellulose and its modified forms (including salts such as Na salts and ammonium salts), cellulose derivatives such as methylcellulose; saponified polymers having vinyl acetate units such as polyvinyl alcohol; and polyalkylene oxides such as polyethylene oxide. These thickeners can be used individually or in combination of two or more. The amount of thickener may be 5 parts by mass or less, or in the range of 0.01 to 3 parts by mass, per 100 parts by mass of positive electrode active material.

[0040] Next, after applying or filling the positive electrode paste onto a positive electrode current collector, the paste is dried to form a positive electrode mixture (positive electrode mixture layer) supported by the positive electrode current collector. At this time, the positive electrode mixture layer may be compressed (or rolled) as necessary.

[0041] Fe element is added to the positive electrode obtained through the above steps such that condition (X) or condition (Y) is satisfied. The method is not particularly limited. For example, an Fe element-containing liquid containing Fe element may be prepared, and the Fe element-containing liquid may be sprayed onto the surface of the positive electrode such that condition (X) or condition (Y) is satisfied. The Fe element-containing liquid may be, for example, a dispersion liquid obtained by dispersing iron powder or iron compound powder in a dispersion medium such as water or alcohol, or a solution obtained by dissolving an iron compound in a solvent such as water or alcohol.

[0042] When iron powder or iron compound powder is used, a smaller particle diameter of the iron powder or iron compound powder is more preferable. The particle diameter D50 (median diameter) at 50% cumulative volume in the volume-based particle size distribution determined by laser diffraction scattering method is preferably 5 μm or less, and the particle diameter D90 at 90% cumulative volume is preferably 10 μm or less. Note that D50 means that 50% of the particles constituting the powder as a population have a particle diameter smaller than this diameter, and D90 means that 90% of the particles constituting the powder as a population have a particle diameter smaller than this diameter.

[0043] When an iron compound is used, examples of the iron compound that can be used include iron oxide, iron hydroxide, iron sulfide, iron chloride, and organometallic compounds such as ferrocene.

[0044] When iron or an iron alloy is used as the material of the battery case, instead of adding Fe element to the positive electrode before it is accommodated in the battery case, an iron component supply source may be disposed between the wound body inside the battery case and the battery case, and the iron component may be supplied from this supply source into the wound body via the electrolyte. As an example of the supply source, Fe element may be eluted from the material of the battery case. For example, after accommodating the wound body and an alkaline electrolyte in the battery case to constitute an alkaline storage battery, the constituted battery may be subjected to a predetermined treatment. Examples of the predetermined treatment include allowing the battery to stand under predetermined temperature conditions and time schedule, or charging the battery under predetermined charging conditions. In this case, since Fe element diffuses from the outermost periphery toward the innermost periphery of the wound body, a configuration satisfying both conditions (X) and (Y) is completed after the battery is constituted.

[0045] (Manufacturing method of alkaline storage battery (A)) The alkaline storage battery (A) may be produced by winding a long positive electrode and a long negative electrode via a long separator to form a wound body, then accommodating the wound body and an alkaline electrolyte in a bottomed cylindrical battery case, and sealing the opening of the battery case with a sealing member. However, as the long positive electrode, a positive electrode satisfying condition (X) or (Y) is used.

[0046] Alternatively, except for using a positive electrode that does not satisfy condition (X) or (Y), after constituting the alkaline storage battery in the same manner as described above, a predetermined treatment for eluting Fe element from the material of the battery case may be performed. In this case, it is essential to use a battery case made of iron or an iron alloy.

[0047] Hereinafter, examples of the configuration and constituent elements of the alkaline storage battery (A) will be further described. However, the configuration and constituent elements of the alkaline storage battery (A) are not limited to the examples shown below. Known constituent elements may be applied to constituent elements other than those characteristic of the present disclosure.

[0048] (Positive electrode) The positive electrode comprises a positive electrode current collector and a positive electrode mixture (positive electrode mixture layer) supported by the positive electrode current collector. The positive electrode may be either a paste-type positive electrode or a sintered-type positive electrode.

[0049] There are no particular limitations on the positive electrode current collector, and known positive electrode current collectors may be used. Examples of positive electrode current collectors include porous current collectors made of metal (such as nickel or nickel alloys). Specifically, examples of positive electrode current collectors include nickel foam and sintered nickel sheets.

[0050] The positive electrode mixture contains particles of a nickel compound (positive electrode active material) and may optionally contain other components (conductive materials, binders, etc.). The nickel compound particles may be those of a known nickel compound used in alkaline batteries (e.g., nickel hydroxide). Some of the nickel hydroxide in the positive electrode mixture may be converted to nickel oxyhydroxide.

[0051] Nickel compound particles may contain trace components other than nickel hydroxide and nickel oxyhydroxide. The surface of the nickel compound particles may be coated with other compounds. Examples of compounds used for such coatings include metal hydroxides. Specifically, examples of compounds used for coatings include cobalt hydroxide, γ-cobalt oxyhydroxide, and β-cobalt oxyhydroxide.

[0052] The conductive material is not particularly limited, and known conductive materials may be used. Examples of conductive materials include graphite such as natural graphite (such as flaky graphite), artificial graphite, and expanded graphite; carbon black such as acetylene black and Ketjen black; conductive fibers such as carbon fibers and metal fibers; metal particles such as nickel powder and cobalt powder; and organic conductive materials such as polyphenylene derivatives. These conductive materials may be used individually or in combination of two or more. Conductive cobalt compounds (such as cobalt hydroxide and γ-type cobalt oxyhydroxide) may also be used as the conductive material.

[0053] The amount of conductive material may be in the range of 0.01 to 20 parts by mass (for example, in the range of 0.1 to 10 parts by mass) per 100 parts by mass of active material.

[0054] The conductive material may be added to the positive electrode paste and mixed with other components before use. Alternatively, the surface of the active material particles may be pre-coated with the conductive material. There are no limitations on the method of coating with the conductive material, and known methods may be used. For example, coating may be performed by sprinkling the conductive material onto the surface of the active material particles. Alternatively, coating may be performed by applying a dispersion containing the conductive material to the surface of the active material particles and drying it. Alternatively, coating may be performed by a mechanochemical method or the like.

[0055] There are no particular limitations on the binder, and known binders used in alkaline storage batteries may be used. Examples of binders include rubber-like materials such as styrene-butadiene copolymer rubber; polyolefin resins such as polyethylene and polypropylene; fluororesins such as polyvinylidene fluoride; acrylic resins such as ethylene-acrylic acid copolymer and ethylene-methyl acrylate copolymer and their Na ion crosslinked products. These binders can be used individually or in combination of two or more. The amount of binder may be 7 parts by mass or less per 100 parts by mass of positive electrode active material, and may be in the range of 0.01 to 5 parts by mass.

[0056] (Negative electrode) There are no particular limitations on the negative electrode; it can be selected according to the type of alkaline battery (A). A known negative electrode used as the negative electrode for alkaline batteries may be used.

[0057] The negative electrode may include a negative electrode current collector and a negative electrode mixture layer supported by the negative electrode current collector. The negative electrode current collector is not limited, and known negative electrode current collectors may be used. Examples of negative electrode current collectors include sheets of porous or non-porous metal (such as stainless steel, nickel, or nickel alloys).

[0058] A negative electrode can be formed by attaching a negative electrode mixture containing a negative electrode active material to a negative electrode current collector. The negative electrode mixture is usually used in the form of a paste containing a dispersion medium. In one example of a negative electrode manufacturing method, first, a negative electrode mixture paste containing a negative electrode active material is prepared. Next, the negative electrode paste is applied to or filled into a negative electrode current collector, and then dried and rolled. In this way, a negative electrode can be manufactured that includes a negative electrode current collector and a negative electrode mixture layer supported by the negative electrode current collector.

[0059] Examples of alkaline batteries (A) include nickel-metal hydride batteries and nickel-cadmium batteries. When alkaline battery (A) is a nickel-metal hydride battery, a hydrogen storage alloy capable of electrochemically absorbing and releasing hydrogen can be used as the negative electrode active material. There are no particular limitations on the hydrogen storage alloy, and known hydrogen storage alloys may be used. When alkaline battery (A) is a nickel-cadmium battery, a cadmium compound (such as cadmium hydroxide) can be used as the negative electrode active material.

[0060] The negative electrode mixture may, if necessary, contain components other than the negative electrode active material (such as conductive agents, binders, and thickeners). The dispersion medium, conductive material, binder, and thickener may be the same as those exemplified for the positive electrode. The amounts of conductive material, binder, and thickener per 100 parts by mass of negative electrode active material may be within the range exemplified for amounts per 100 parts by mass of positive electrode active material.

[0061] The outermost periphery of the negative electrode may be positioned outside the outermost periphery of the positive electrode. In this case, the mass of the negative electrode active material (e.g., hydrogen storage alloy) per unit area at the outermost periphery of the negative electrode may be smaller than the mass of the negative electrode active material per unit area at the other parts.

[0062] Note that the mass of the negative electrode active material (e.g., hydrogen storage alloy) per unit area at the negative electrode refers to the mass of the negative electrode active material present on both sides when negative electrode composite layers are present on both sides of the negative electrode current collector. For example, consider a negative electrode current collector that is 1 cm long and 1 cm wide, with A grams of negative electrode active material on one side and B grams of negative electrode active material on the other side. In that case, the mass of the negative electrode per unit area at that point is... ２ The mass of the negative electrode active material per unit is (A + B) grams.

[0063] (Alkaline Electrolyte) As the alkaline electrolyte, an aqueous solution containing an alkaline solute can be used. Examples of solutes include alkali metal hydroxides, specifically lithium hydroxide, potassium hydroxide, sodium hydroxide, etc. One solute may be used alone, or two or more may be used in combination.

[0064] The concentration of the solute (specifically, alkali metal hydroxide) contained in the alkaline electrolyte may be in the range of 2.5 to 13 mol / L (for example, 3 to 12 mol / L). The specific gravity of the alkaline electrolyte may be in the range of 1.1 to 1.6 (for example, 1.2 to 1.5).

[0065] The alkaline electrolyte preferably contains sodium hydroxide. The alkaline electrolyte may also contain sodium hydroxide and other alkali metal hydroxides (lithium hydroxide and / or potassium hydroxide). The alkaline electrolyte may contain only sodium hydroxide as the solute.

[0066] The concentration of sodium hydroxide in the alkaline electrolyte may be higher than the concentration of potassium hydroxide. In other words, the concentration of sodium ions in the alkaline electrolyte may be higher than the concentration of potassium ions. Examples of electrolytes having this configuration include alkaline electrolytes in which both sodium hydroxide and potassium hydroxide are dissolved, and alkaline electrolytes in which sodium hydroxide is dissolved but potassium hydroxide is not. That is, examples of electrolytes having this configuration include electrolytes in which the concentration of potassium hydroxide is zero.

[0067] When an alkaline battery (A) is charged in a high-temperature environment, it is preferable to use sodium hydroxide as the solute. Therefore, when an alkaline battery (A) is charged in a high-temperature environment, it is preferable to use an alkaline electrolyte in which the concentration of sodium hydroxide is higher than the concentration of potassium hydroxide.

[0068] The concentration of sodium hydroxide in the alkaline electrolyte may be in the range of 2.5 to 11.5 mol / L, preferably in the range of 3.5 to 10.5 mol / L (for example, in the range of 4 to 10 mol / L). When the concentration of sodium hydroxide is in this range (especially high concentration), it is possible to increase the charging efficiency even when charging at high temperatures. Furthermore, when the concentration of sodium hydroxide is in the above range (especially high concentration), it is possible to suppress a decrease in the average discharge voltage while maintaining high charging efficiency, and as a result, it is possible to increase the cycle life.

[0069] (Separator) There are no particular limitations on the separator, and known separators used in alkaline storage batteries may be used. Examples of separator forms include microporous membranes, nonwoven fabrics, and woven fabrics. The separator can be formed from an insulating material. Examples of separator materials include polyethylene, polyolefin resins such as polypropylene; fluororesins; and polyamide resins.

[0070] Separators made of highly hydrophobic materials such as polyolefin resins may have hydrophilic groups introduced through hydrophilization treatment. Examples of hydrophilization treatments include corona discharge treatment, plasma treatment, sulfonation treatment, and fluorine gas treatment. Among these, fluorine gas treated polyolefin separators are preferred.

[0071] Fluorine-treated separators have high oxygen gas permeability and are considered suitable for suppressing the lifting of positive electrode active material from the positive electrode current collector. Fluorine gas treatment is a method of generating hydrophilic functional groups on the surface of a separator using fluorine gas. Fluorine-treated separators are obtained by exposing the separator to fluorine gas. Specifically, fluorine gas diluted with nitrogen gas or argon gas is mixed with at least one gas selected from oxygen gas, carbon dioxide gas, sulfur dioxide gas, etc., and the resulting mixed gas is brought into contact with the web of the separator, thereby generating carboxyl groups, carbonyl groups, hydroxyl groups, etc. on the surface of the separator.

[0072] Furthermore, in the alkaline storage battery (A), the separator constituting the winding body may include a strip-shaped first separator and a strip-shaped second separator. In the winding body, the first separator may be arranged in the region on the inner circumference side with respect to the strip-shaped positive electrode, and the second separator may be arranged in the region on the outer circumference side with respect to the strip-shaped positive electrode. In other words, in the winding body, the components may be arranged in the order of first separator, positive electrode, second separator, negative electrode, first separator, etc., from radially outward. The electrolyte absorption rate of the second separator may be higher than that of the first separator. In the winding body, the positive electrode is prone to larger cracks on the outer circumference side, and due to these cracks, the electrolyte tends to be unevenly distributed in the positive electrode, with the distribution shifting from the inner circumference side to the outer circumference side. It is believed that by making the electrolyte absorption rate of the second separator higher than that of the first separator, the uneven distribution of the electrolyte in the positive electrode can be suppressed. A separator that has been sulfonated may be used as the first separator, and a separator that has been treated with hydrophilic treatment such as fluorine gas treatment may be used as the second separator.

[0073] Here, the following method can be used to confirm the liquid absorption rate of each separator. First, each separator is removed from the prepared alkaline battery and washed with deionized water. The washed separators are dried, and three test pieces of a predetermined size are cut from each dried separator. Each cut test piece is placed in a test chamber at a temperature of 20°C and a relative humidity of 65% in accordance with JIS Z8703 to reach a moisture equilibrium state. These test pieces are arranged on a horizontal rod and secured with pins at a constant height above a water tank containing a potassium hydroxide solution with a specific gravity of 1.30 (at 20°C). The horizontal rod, with the lower ends of each test piece at the same height, is lowered into the water tank and then positioned vertically so that the lower end of each test piece is submerged in the solution by 5 mm. The height to which the potassium hydroxide solution rises due to capillary action in each test piece is measured 30 minutes after the start of immersion.

[0074] Furthermore, in a wound body, it is not necessary for the first separator to be located at the innermost circumference and the second separator at the outermost circumference. It is sufficient for the first separator to be located on the inner circumference side of the positive electrode and the second separator to be located on the outer circumference side of the positive electrode in at least a portion of the wound body.

[0075] (Other) There are no particular limitations on components other than those mentioned above (outer casing, leads, etc.), and known components used in alkaline storage batteries may be used. When the alkaline storage battery (A) is a cylindrical battery, one example of an outer casing includes, as described above, a bottomed cylindrical battery case, and a sealing body and gasket that seal the battery case.

[0076] Hereinafter, an example of this embodiment will be specifically described with reference to the drawings. The components of the example embodiment described below can be the components described above. Furthermore, the components of the example embodiment described below can be modified based on the above description. In addition, the matters described below may be applied to the above embodiment. Furthermore, in the example embodiment described below, components that are not essential to the alkaline storage battery according to this disclosure may be omitted.

[0077] (Embodiment 1) Figure 1 shows an alkaline storage battery 10 of Embodiment 1. Figure 1 is a schematic exploded perspective view showing the structure of the alkaline storage battery 10. The alkaline storage battery 10 includes a battery case 4, an electrode group housed in the battery case 4, and an alkaline electrolyte (not shown). The battery case 4 is a bottomed cylindrical case. The electrode group is formed by winding a negative electrode 1, a positive electrode 2, and a separator 3 such that a separator 3 is placed between the negative electrode 1 and the positive electrode 2. The opening of the battery case 4 is sealed by a sealing body 7 and an insulating gasket 8. The sealing body 7 includes a positive electrode terminal 5 and a safety valve 6. The positive electrode 2 and the sealing body 7 are electrically connected via a positive electrode lead 9. The battery case 4 is electrically connected to the negative electrode 1 and functions as a negative electrode terminal.

[0078] The positive electrode 2 includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The alkaline storage battery 10 has the above-described configuration.

[0079] [Note] The above description of embodiments discloses the following technologies.

[0080] (Technical 1) An alkaline storage battery comprising a positive electrode, a negative electrode, a separator, and an alkaline electrolyte, wherein the positive electrode, the negative electrode, and the separator constitute a wound body, the positive electrode comprises a positive electrode current collector and a positive electrode mixture supported by the positive electrode current collector, the positive electrode mixture comprises a nickel compound which is a positive electrode active material and the element Fe, and in the wound body, the content of the element Fe in the positive electrode mixture at the innermost circumference of the positive electrode, Cinm, is smaller than the content of the element Fe in the positive electrode mixture at the outermost circumference of the positive electrode, Coutm.

[0081] (Technology 2) The alkaline storage battery according to Technology 1, wherein the content Cinm is 0% to 50% of the content Cotton.

[0082] (Technical 3) The alkaline storage battery according to Technical 1 or 2, wherein the separator includes a first separator and a second separator, and in the wound body, the first separator is arranged in a region on the inner circumference side of the positive electrode, and the second separator is arranged in a region on the outer circumference side of the positive electrode, and the liquid absorption rate of the second separator is greater than the liquid absorption rate of the first separator.

[0083] (Technical 4) An alkaline storage battery comprising a positive electrode, a negative electrode, a separator, and an alkaline electrolyte, wherein the positive electrode, the negative electrode, and the separator constitute a wound body, the positive electrode comprises a positive electrode current collector and a positive electrode mixture supported by the positive electrode current collector, the positive electrode mixture comprises a nickel compound which is a positive electrode active material and the element Fe, and in the wound body, the content of the element Fe in the positive electrode mixture in the inner circumference of the positive electrode, Cin, is smaller than the content of the element Fe in the positive electrode mixture in the outer circumference of the positive electrode, Cout.

[0084] The present disclosure will be described in more detail below with reference to examples. In the following examples, several nickel-metal hydride batteries were fabricated and evaluated.

[0085] (Experimental Example 1) In Experimental Example 1, several types of nickel-metal hydride batteries were fabricated using sulfonated polypropylene nonwoven fabric separators. Specifically, batteries with a structure similar to that shown in Figure 1 were fabricated using the following procedure.

[0086] (1) Preparation of the positive electrode A positive electrode paste was prepared by mixing nickel hydroxide particles coated with cobalt oxyhydroxide (100 parts by mass as nickel hydroxide), 0.1 parts by mass of methylcellulose (thickener), and water. Next, the positive electrode paste was filled into a sheet of foamed nickel porous material (positive electrode current collector) and dried. The resulting sheet was compressed in the thickness direction and then cut to a predetermined size to produce the positive electrode.

[0087] An ethanol dispersion of iron powder (D50 = 5 μm) was sprayed onto both the outermost and innermost circumferences of the obtained positive electrode, along its entire length, such that the amount of sprayed decreased from the outermost to the innermost circumference, so that the content of Outm and Cinm satisfied the relationship shown in Table 1. The content of Outm was adjusted to 0.1% by mass.

[0088] (2) Preparation of the negative electrode 100 parts by mass of LaNiCoMnAl-based hydrogen storage alloy, 0.2 parts by mass of carboxymethylcellulose (thickener), 0.2 parts by mass of Ketjenblack (conductive material), and 0.5 parts by mass of styrene-butadiene rubber (binding agent) were mixed. Water was added to the resulting mixture and mixed further to prepare the negative electrode paste.

[0089] A negative electrode paste was applied to both sides of the negative electrode current collector to form a coating. Nickel-plated iron perforated metal was used as the negative electrode current collector. After drying the resulting coating, a negative electrode mixture layer was formed by pressing it together with the negative electrode current collector. Next, the negative electrode was obtained by cutting the sheet consisting of the negative electrode current collector and the negative electrode mixture layer to a predetermined size.

[0090] (3) Fabrication of alkaline storage battery A winding body was fabricated by winding the fabricated positive electrode and negative electrode with a separator. A sulfonated polypropylene nonwoven fabric was used as the separator.

[0091] Next, the coiled body and alkaline electrolyte were placed in the battery case. The alkaline electrolyte used had an alkali metal hydroxide concentration of 5.5 mol / L. The alkali metal hydroxide consisted of a mixture of KOH, NaOH, and LiOH in a molar ratio of KOH:NaOH:LiOH = 2:7:1. The alkaline electrolyte was applied to the apparent area of ​​the separator, approximately 1 cm². ２ The liquid was poured into the battery case so that each portion contained 20 mg. A nickel-plated iron can was used as the battery case.

[0092] Next, the opening of the battery case was sealed with a gasket and a sealing body. At this time, the foamed nickel porous body (positive electrode current collector) and the sealing body (positive electrode terminal) were electrically connected via a connecting member. Also, the negative electrode and the battery case (negative electrode terminal) were electrically connected via a connecting member. In this way, several types of alkaline storage batteries with different positive electrodes were manufactured. For the sealing body, an iron material with a nickel-plated surface was used.

[0093] As described above, several AA-sized alkaline storage batteries (nickel-metal hydride storage batteries) with a theoretical capacity of 1200 mAh were fabricated. The fabricated batteries were activated by performing one charge and one discharge cycle, and then the following evaluations were performed.

[0094] (4) Evaluation (Initial Capacity) The initial discharge capacity was confirmed by performing the following charge and discharge on the batteries manufactured and activated according to the above procedure. Note that It is the current value expressed as theoretical capacity / 1h, and 1It = 1200mA.

[0095] (a) Charge at 20°C with a current of 0.1 It for 16 hours.

[0096] (b) At 20°C, discharge the battery to a cutoff voltage of 1.0V with a current of 0.2It.

[0097] (Standard capacity after trickle charging) After the batteries manufactured and activated according to the above procedure and whose initial discharge capacity was confirmed were fully charged under the conditions of (a) above, trickle charging was performed under the conditions of (c) below.

[0098] (c) Charge at 5°C with a current of 0.05 It for 3 months.

[0099] Subsequently, the battery was discharged under the conditions described in (b) above, and the standard capacity after trickle charging was determined. If the standard capacity after trickle charging is 70% or more of the initial discharge capacity, the battery can be judged to be a good product.

[0100] (Discharge characteristics after trickle charging) The batteries prepared and activated according to the above procedure were charged under the conditions in (a) above, trickle charged under the conditions in (c) above, and then discharged under the conditions in (d) below.

[0101] (d) At 20°C, discharge the battery to a cutoff voltage of 1.0V with a current of 1.0It.

[0102] If the discharge duration of an alkaline battery after trickle charging is 30 minutes or more, 10 minutes or more but less than 30 minutes, or less than 10 minutes, it can be judged as a good product, a good product, or a defective product, respectively.

[0103] The evaluation results are shown in Table 1. Batteries A1 to A12 are the batteries of the embodiment according to this disclosure, and batteries Z1 to Z3 are comparative example batteries.

[0104] The content (Cotm) of batteries A1 and Z2 is 0.1% by mass and 0.1% by mass, respectively, and the content (Cinm) of battery Z3 is 0.1% by mass.

[0105]

[0106] The standard capacity after trickle charging decreased as the ratio of the content Cinm to the content Coutm increased. When Coutm = Cinm, the standard capacity after trickle charging was less than 70% of the initial capacity. This trend is thought to strongly reflect the degree to which the positive electrode active material lifts away from the positive electrode current collector due to oxygen gas generation.

[0107] The discharge characteristics after trickle charging improved as the ratio of Cinm content to Cotton content increased, within the range of 50% or less. This is because the formation of γ-hydroxyhydroxide nickel during charging is suppressed. In the range where the ratio of Cinm content to Cotton content exceeds 50%, the discharge characteristics after trickle charging deteriorated as the ratio increased. This is because the effect of oxygen gas generation causing the positive electrode active material to lift away from the positive electrode current collector gradually increases. In other words, when the Cinm content is between 0% and 50% of the Cotton content, an alkaline storage battery with a better balance of characteristics is obtained, although the standard capacity decreases slightly.

[0108] (Experimental Example 2) In Experimental Example 2, a sulfonated polypropylene nonwoven fabric separator was used for the first separator located in the region on the inner circumference side of the positive electrode, and a fluorine gas-treated polypropylene nonwoven fabric separator was used for the second separator located in the region on the outer circumference side of the positive electrode. Multiple types of nickel-metal hydride batteries were fabricated and evaluated in the same manner as in Experimental Example 1.

[0109] The evaluation results are shown in Table 2. Batteries A13 to A24 are the batteries of the embodiment according to this disclosure, and batteries Z4 to Z6 are comparative example batteries.

[0110]

[0111] From the results in Table 2, it can be seen that using the first and second separators described above improves the overall standard capacity and discharge characteristics after trickle charging.

[0112] This disclosure can be used in alkaline storage batteries.

[0113] 1: Negative electrode 2: Positive electrode 3: Separator 4: Battery case 7: Sealing body 8: Insulating gasket 9: Positive electrode lead 10: Alkaline battery

Claims

1. An alkaline storage battery comprising a positive electrode, a negative electrode, a separator, and an alkaline electrolyte, wherein the positive electrode, the negative electrode, and the separator constitute a wound body, the positive electrode comprises a positive electrode current collector and a positive electrode mixture supported by the positive electrode current collector, the positive electrode mixture comprises a nickel compound which is a positive electrode active material and the element Fe, and in the wound body, the content of the element Fe in the positive electrode mixture at the innermost circumference of the positive electrode, Cinm, is smaller than the content of the element Fe in the positive electrode mixture at the outermost circumference of the positive electrode, Coutm.

2. The alkaline storage battery according to claim 1, wherein the content Cinm is 0% to 50% of the content Coutm.

3. The alkaline storage battery according to claim 1, wherein the separator includes a first separator and a second separator, and in the wound body, the first separator is arranged in a region on the inner circumference side of the positive electrode, and the second separator is arranged in a region on the outer circumference side of the positive electrode, and the liquid absorption rate of the second separator is greater than the liquid absorption rate of the first separator.

4. An alkaline storage battery comprising a positive electrode, a negative electrode, a separator, and an alkaline electrolyte, wherein the positive electrode, the negative electrode, and the separator constitute a wound body, the positive electrode comprises a positive electrode current collector and a positive electrode mixture supported by the positive electrode current collector, the positive electrode mixture comprises a nickel compound which is a positive electrode active material and the element Fe, and in the wound body, the content of the element Fe in the positive electrode mixture in the inner circumference of the positive electrode (Cin) is smaller than the content of the element Fe in the positive electrode mixture in the outer circumference of the positive electrode (Cout).

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