Alkaline secondary battery

Abstract

Provided is an alkaline secondary battery which contains zinc metal, a zinc alloy or a zinc compound as a negative electrode active material and is excellent in battery characteristics such as charge / discharge cycle characteristics and load characteristics.　An alkaline secondary battery according to the present invention has: a negative electrode having a negative electrode active material layer containing zinc metal, a zinc alloy or a zinc compound as a negative electrode active material; a positive electrode; and a separator, wherein the separator has a porous layer that contains insulating inorganic fine particles.

Description

Alkaline rechargeable battery

[0001] This invention relates to an alkaline secondary battery containing zinc metal, zinc alloy, or zinc compound as the negative electrode active material, and exhibiting excellent battery characteristics such as charge-discharge cycle characteristics and load characteristics.

[0002] Alkaline batteries, which have a negative electrode containing zinc metal, zinc alloy, or zinc compound as the negative electrode active material and an alkaline electrolyte, are widely used as primary batteries, but their application to secondary batteries is also being considered. However, in such alkaline secondary batteries, repeated charging and discharging can cause zinc dendrites to form at the negative electrode and grow to the positive electrode, impairing the battery's performance.

[0003] On the other hand, technologies to suppress the occurrence of the aforementioned problems are also being considered. For example, Patent Document 1 proposes an alkaline secondary battery in which an anion-conducting film is placed between the negative electrode and the separator, the film having a polymer as a matrix and in which particles of at least one metal compound selected from the group consisting of metal oxides, hydroxides, carbonates, sulfates, phosphates, borates, and silicates are dispersed in the matrix. In the alkaline secondary battery described in Patent Document 1, the anion-conducting film suppresses the generation and growth of zinc dendrites even after repeated charging and discharging, thereby suppressing the occurrence of internal short circuits and ensuring good charge-discharge cycle characteristics.

[0004] Furthermore, Patent Document 2 discloses a zinc battery in which a porous film containing a metal oxide having an isoelectric point of 5 to 11 is placed between the positive and negative electrodes. Patent Document 2 states that the metal oxide becomes negatively charged in the electrolyte and electrostatically repels the tetrahydroxyzincate ions generated at the negative electrode, thereby trapping the tetrahydroxyzincate ions inside or on the surface of the negative electrode and suppressing dendrite growth, thus improving the lifespan characteristics of the zinc battery.

[0005] International Publication No. 2017 / 047628, Japanese Patent Publication No. 2019-216057

[0006] However, even with these conventional technologies, there is still room for further improvement in battery characteristics.

[0007] The present invention has been made in view of the above circumstances, and its purpose is to provide an alkaline secondary battery that contains zinc metal, zinc alloy, or zinc compound as the negative electrode active material, and has excellent battery characteristics such as charge-discharge cycle characteristics and load characteristics.

[0008] The alkaline secondary battery of the present invention has a negative electrode, a positive electrode, and a separator, the negative electrode having a negative electrode active material layer containing zinc metal, a zinc alloy, or a zinc compound as the negative electrode active material, and the separator is characterized by having a porous layer containing insulating inorganic fine particles.

[0009] According to the present invention, it is possible to provide an alkaline secondary battery containing zinc metal, zinc alloy, or zinc compound as the negative electrode active material, which exhibits excellent battery characteristics such as charge-discharge cycle characteristics and load characteristics.

[0010] This is a schematic cross-sectional view showing an example of the alkaline secondary battery of the present invention. This is a graph showing the voltage change during the first charging cycle when evaluating the charge-discharge cycle characteristics of the button-type alkaline secondary batteries of the examples and comparative examples. This is a graph showing the voltage change during the first discharging cycle when evaluating the charge-discharge cycle characteristics of the button-type alkaline secondary batteries of the examples and comparative examples.

[0011] The alkaline secondary battery of the present invention (hereinafter sometimes simply referred to as "battery") has a negative electrode, a positive electrode, and a separator, the negative electrode having a negative electrode active material layer containing zinc metal, zinc alloy, or zinc compound (hereinafter, zinc metal, zinc alloy, and zinc compound may be collectively referred to as "zinc, etc.") as the negative electrode active material, and the separator comprises a porous layer containing insulating inorganic fine particles.

[0012] In alkaline secondary batteries that use zinc or similar materials as the negative electrode active material, as mentioned above, repeated charging and discharging can cause zinc dendrites to grow at the negative electrode. These dendrites can penetrate the separator and reach the positive electrode, potentially impairing the battery's performance.

[0013] However, in the battery of the present invention, the porous layer containing insulating inorganic fine particles in the separator prevents zinc dendrites from penetrating the separator and reaching the positive electrode. As a result, the alkaline secondary battery of the present invention can effectively suppress the degradation of performance associated with repeated charging and discharging.

[0014] Figure 1 shows a schematic cross-sectional view illustrating an example of the alkaline secondary battery of the present invention. In the alkaline secondary battery 1 shown in Figure 1, a sealing can 3 containing a negative electrode 5 is fitted into the opening of an outer can 2 containing a positive electrode 4 and separators 61 and 62 via an L-shaped, annular gasket 7. The opening end of the outer can 2 is tightened inward, causing the gasket 7 to contact the sealing can 3, thereby sealing the opening of the outer can 2 and creating a sealed structure inside the battery. In other words, in the alkaline secondary battery 1 shown in Figure 1, the power generation elements including the positive electrode 4 and negative electrode 5 are loaded into the space (sealed space) inside the battery container consisting of the outer can 2, the sealing can 3, and the gasket 7, and an alkaline electrolyte (not shown) is also contained within. Furthermore, in the alkaline secondary battery 1 shown in Figure 1, the peripheral edge of the positive electrode 4 is positioned between the inner bottom surface of the outer can 2 and the bottom surface of the gasket 7 (hereinafter, this structure will be referred to as the "bottom structure"). In the battery shown in Figure 1, the outer casing 2 also serves as the positive terminal and the sealing casing 3 also serves as the negative terminal. However, in the battery of the present invention, the outer casing can also serve as the negative terminal and the sealing casing can also serve as the positive terminal.

[0015] The alkaline secondary battery 1 shown in Figure 1 has two separators 61 and 62, one of which (for example, separator 61) has a porous layer containing insulating inorganic fine particles. For example, the porous layer is a separator consisting of a laminated film formed on a microporous film made of resin. Although separator 61 is composed of a laminated film with multiple structures, each layer constituting separator 61 is not shown in Figure 1 to avoid making the drawing too complex. Similarly, separator 62 is represented as a single layer, but as will be described later, it is also possible to use a multilayer structure.

[0016] The details of the battery of the present invention will be described below.

[0017] (Separator) The separator of the battery has a porous layer containing insulating inorganic fine particles (hereinafter referred to as "porous layer (I)").

[0018] In the present invention, a porous layer (I) composed solely of a composition containing insulating inorganic fine particles and a binder described later can be used as a separator. However, it is preferable to integrate the composition with a resin substrate to form the separator, as this makes it easier to handle the porous layer (I) as a self-supporting film during battery assembly. Therefore, the following embodiments will describe such a configuration.

[0019] Furthermore, when using a porous layer (I) composed solely of the aforementioned composition as a separator, it may be used in a manner integrated with the electrodes, for example, by forming the porous layer (I) on the positive or negative electrode by coating or other means.

[0020] As the resin substrate, a microporous film or nonwoven fabric can be used. For example, by applying a coating liquid (slurry) obtained by dispersing a composition constituting the porous layer (I) (insulating inorganic fine particles and a binder as needed) in a solvent such as water onto a microporous film and drying it, a laminated film is obtained in which a porous layer (I) composed of the composition is formed on the microporous film, and the porous layer (I) can be integrated with the resin substrate. Alternatively, by impregnating the coating liquid into the pores of a substrate such as a nonwoven fabric and drying it, a porous layer (I) consisting of a composite film in which the composition is held within the pores of the substrate such as a nonwoven fabric can be obtained, and a porous layer (I) in which the resin substrate is part of the composition can be produced.

[0021] In the aforementioned laminated film, the microporous film plays a role in isolating the positive electrode and the negative electrode, as well as serving as a substrate for holding the porous layer (I) containing insulating inorganic fine particles. In the aforementioned composite film, the nonwoven fabric plays a similar role to that of the microporous film.

[0022] Microporous films can be made from resins such as polyolefin, polyester, polyimide, polyamide, and polyurethane, but it is preferable to use a microporous film made of polyolefin because it can be used in conjunction with the separator of lithium-ion secondary batteries. Nonwoven fabrics made from the same resin as microporous films can also be used.

[0023] Examples of polyolefins include polyethylene (PE), such as low-density polyethylene, high-density polyethylene, and ultra-high molecular weight polyethylene; and polypropylene (PP). Only one of these may be used, or two or more may be used in combination. For example, a microporous film using two or more polyolefins may be a three-layer microporous film in which a PP layer is laminated on a PP layer with a PE layer in between.

[0024] As the microporous film constituting the laminated film, for example, an ion-permeable microporous film having many pores formed by known solvent extraction methods or dry or wet stretching methods (microporous films commonly used as battery separators) can be used. Similarly, nonwoven fabrics known to be used as separators in alkaline batteries and alkaline secondary batteries can also be used.

[0025] The insulating inorganic fine particles to be included in the porous layer (I) of the laminated film or the composite film are preferably fine particles that are stable in an alkaline electrolyte, such as boehmite, alumina, silica, and titanium oxide, and one or more of these can be used.

[0026] There are no particular restrictions on the shape of the insulating inorganic fine particles; various shapes such as approximately spherical (including perfectly spherical), approximately ellipsoidal (including ellipsoidal), irregular shapes, and plate-shaped particles can be used. However, it is preferable to use plate-shaped particles because they are easier to orient and have a better effect in suppressing zinc dendrites from penetrating the separator.

[0027] The average particle size of the insulating inorganic fine particles is preferably 0.1 μm or larger, more preferably 0.3 μm or larger, and particularly preferably 0.5 μm or larger, because if it is too small, the ion permeability decreases. Furthermore, if the insulating inorganic fine particles are too large, the surface irregularities of the porous layer (I) become large, which may cause the charge-discharge reaction on the electrode to become uneven. Therefore, the average particle size is preferably 5 μm or smaller, more preferably 2 μm or smaller, and particularly preferably 1 μm or smaller.

[0028] In this specification, the particle size of insulating inorganic fine particles and other particles (silver oxide particles, graphite particles, carbon black particles, insulating inorganic particles contained in the positive electrode, and zinc particles related to the negative electrode) is measured using a laser scattering particle size analyzer (e.g., Horiba LA-920) by dispersing these particles in a non-dissolving medium, and the average particle diameter is the 50% diameter value in the volume-based integrated fraction when determining the integrated volume from the smallest particle size (D ５０ This means...

[0029] The porous layer (I) preferably contains a binder for bonding insulating inorganic fine particles together or for bonding insulating inorganic fine particles to a substrate such as a microporous film or nonwoven fabric. The binder is not particularly limited, but it is preferable to use a resin with high alkali resistance, and it is preferable to use ethylene-vinyl acetate copolymer (EVA, having 20 to 35 mol% of structural units derived from vinyl acetate), ethylene-acrylic acid copolymer such as ethylene-ethyl acrylate copolymer, fluororesin, styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), hydroxyethylcellulose (HEC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), acrylic resin, polyurethane, epoxy resin, etc., and one or more of these can be used.

[0030] The content of the insulating inorganic fine particles in the porous layer (I) (however, when the porous layer (I) is a composite film in which the above-described composition and a base material such as a nonwoven fabric are integrated, the insulating inorganic fine particles in the above-described composition (the composition containing insulating inorganic fine particles, a binder, etc.) constituting the composite film) is preferably 70% by mass or more, more preferably 90% by mass or more, and preferably 99% by mass or less, more preferably 97% by mass or less in the total mass of the components constituting the porous layer (I) (in the total solid content excluding the pore part). The balance may be, for example, the above-described binder).

[0031] The thickness of the entire laminated film constituting the separator is preferably 5 μm or more, more preferably 10 μm or more, and preferably 30 μm or less, more preferably 20 μm or less.

[0032] Further, from the viewpoint of favorably suppressing the penetration of zinc dendrites, the thickness of the porous layer (I) is preferably 1 μm or more, more preferably 2 μm or more, particularly preferably 3 μm or more. From the viewpoint of suppressing an increase in the thickness of the porous layer (I) and increasing the energy density of the battery, it is preferably 15 μm or less, more preferably 10 μm or less, particularly preferably 8 μm or less.

[0033] Further, the porosity of the entire laminated film constituting the separator is preferably 30 to 60%. The porosity of the porous layer (I) is preferably 20 to 60%.

[0034] The battery of the present invention uses an aqueous alkaline electrolyte. However, the porous layer (I) and the resin base material may be poor in hydrophilicity. In order to enhance the wettability of the alkaline electrolyte, it is preferable to perform a hydrophilic treatment on at least one of the porous layer (I) (the composition containing insulating inorganic fine particles) and the resin base material, or on the entire laminated film or composite film including the porous layer (I). Examples of the hydrophilic treatment include a treatment of applying a liquid (such as an aqueous solution) containing various known surfactants (such as nonionic surfactants) to the target and drying it; ultraviolet treatment; corona discharge treatment; plasma discharge treatment; chemical treatment with a chemical solution such as sulfonation treatment with sulfuric acid; and various known hydrophilic treatments such as these.

[0035] As the separator of the battery, the porous layer (I) composed only of a composition containing insulating inorganic fine particles, etc., or the laminated film or the composite film may be used alone, but other separators can also be used in combination. For example, when silver oxide is used as the active material of the positive electrode, it is preferable to use a laminate of a graft film composed of a graft polymer having a main chain of polyolefin and a side chain derived from (meth)acrylic acid or its derivative bonded to the main chain and a cellophane film (hereinafter sometimes referred to as "graft film / cellophane film laminate") together with the separator.

[0036] In addition, the graft polymer constituting the graft film in the laminate only needs to have the above form, and it does not necessarily have to be produced by a method of graft-polymerizing (meth)acrylic acid or its derivative to polyolefin.

[0037] The (meth)acrylic acid or its derivative constituting the graft polymer is represented by the following general formula (1). In the following general formula (1), R １ is H or CH ３ and R ２ is H or a hydrophilic substituent such as NH ４ , Na, K, Rb, Cs.

[0038]

[0039] The aforementioned graft films and cellophane films are characterized by the fact that the polymers constituting these films themselves have the function of absorbing electrolytes and allowing ions to pass through.

[0040] The graft polymer constituting the graft film preferably has a graft rate of 160% or more, as defined by the following formula (2). Since there is a correlation between the graft rate of the graft polymer and the electrical resistance of the graft film, using a graft polymer with a graft rate of the above value allows the electrical resistance of the graft film to be 20 to 120 mΩ·in ２ The value can be controlled to a suitable level. The electrical resistance of the graft film is obtained by the AC voltage drop method (1 kHz). The ambient temperature is 20 to 25°C, the film is immersed in a 40% KOH (specific gravity: 1,400 ± 0.005) aqueous solution at 25 ± 1°C, and after 5 to 15 hours, it is removed and the electrical resistance is measured.

[0041] Graft rate (%) = 100 × (A - B) / B (2)

[0042] In formula (2) above, A: mass of the graft polymer (g), and B: mass of the polyolefin that forms the main chain in the graft polymer (g). Note that in formula (2), "B (mass of the polyolefin that forms the main chain in the graft polymer)" can be determined by, for example, measuring the mass of the polyolefin that forms the main chain used in the graft polymerization when the graft polymer is formed by graft polymerization of (meth)acrylic acid or its derivatives onto the polyolefin that forms the main chain. Furthermore, the grafting rate in the graft polymer may exceed 100% because the monomers used in the graft polymerization [(meth)acrylic acid or its derivatives] polymerize with each other, resulting in long-chain graft molecules (side chains). The upper limit of the grafting rate of the graft polymer defined in formula (2) is preferably 400%. Note that "(meth)acrylic acid" refers collectively to acrylic acid and methacrylic acid.

[0043] In the case of a separator composed of a laminate of a graft film and a cellophane film, the total thickness of the graft film and the cellophane film is preferably 30 μm or more, more preferably 40 μm or more, and preferably 70 μm or less, and more preferably 60 μm or less.

[0044] Furthermore, in the case of a separator composed of a laminate of graft film and cellophane film, the thickness of the graft film is preferably 15 μm or more, more preferably 25 μm or more, and preferably 30 μm or less.

[0045] Examples of laminates of graft film and cellophane film used to constitute a separator include those commercially available from GS Yuasa Membrane Corporation under the names "YG9132," "YG9122," "YG2122," and "YG2152."

[0046] When using a separator with a porous layer (I) along with another separator such as a graft film / cellophane film laminate, it is preferable to place the former on the negative electrode side and the latter on the positive electrode side to better suppress internal short circuits caused by zinc dendrite penetration. When the separator with a porous layer (I) is a laminated film having a porous layer (I) and a microporous film, in the alkaline secondary battery 1 shown in Figure 1, separator 61 is composed of a laminated film having a porous layer (I) and a microporous film, and separator 62 is composed of a graft film / cellophane film laminate.

[0047] Furthermore, regarding the laminated film itself, the arrangement of the porous layer (I) and the microporous film when the porous layer (I) is formed on only one side of the microporous film is not particularly limited.

[0048] (Negative electrode) For the negative electrode of the battery, a negative electrode active material layer containing negative electrode active material can be used as is, or the negative electrode active material layer can be formed on one or both sides of the current collector.

[0049] The negative electrode active material layer may contain one or more of the following as the negative electrode active material: zinc metal (elemental zinc), zinc alloy, and zinc compounds (such as zinc oxide). The negative electrode active material layer may be composed solely of the negative electrode active material, or it may be formed by combining it with other constituent materials. It can also be molded into a molded body by pressure molding.

[0050] Examples of alloying components in zinc alloys include indium, bismuth, and aluminum, and one or more of these elements may be included. The content of each alloying component in the zinc alloy is preferably, for example, indium: 0.005 to 0.1 mass%, bismuth: 0.002 to 0.5 mass%, and aluminum: 0.0001 to 0.15 mass%.

[0051] As for the particle size of the negative electrode active material particles, for example, it is preferable that the proportion of particles with a particle size of 75 μm or less in the total powder is 30% by mass or less, and that the average particle diameter is preferably in the range of 100 to 200 μm. The particle size of the zinc particles referred to here is a value obtained by the same measurement method as the average particle diameter measurement method for insulating inorganic fine particles described above.

[0052] The negative electrode may contain, for example, a gelling agent (such as sodium polyacrylate or carboxymethylcellulose) in addition to the zinc particles as needed, and a negative electrode composition (gel-like negative electrode) may be used, which is formed by adding an alkaline electrolyte to this. The amount of gelling agent in the negative electrode is preferably, for example, 0.5 to 1.5% by mass.

[0053] In the negative electrode mixture, the zinc content included as the negative electrode active material is preferably 85 to 99% by mass, in the form of elemental zinc, zinc alloy, or zinc compound.

[0054] The negative electrode mixture may contain a conductive additive. Examples of conductive additives include carbon materials such as carbon black (furnace black, channel black, acetylene black, thermal black, etc.) and graphite (natural graphite (flaky graphite, etc.), artificial graphite), and one or more of these may be used. The content of the conductive additive in the negative electrode mixture is preferably 0.01 to 5% by mass.

[0055] The negative electrode mixture may contain a binder. Examples of binders include fluororesins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE); styrene-butadiene rubber (SBR); fluororesins are preferred, and PTFE is more preferred. The binder content in the negative electrode mixture is preferably 0.1 to 2% by mass.

[0056] In the case of a negative electrode consisting only of a negative electrode mixture, for example, the mixture can be prepared by mixing it with a negative electrode active material and, if necessary, a conductive additive, a binder, and an alkaline electrolyte (the same alkaline electrolyte used in batteries can be used). If necessary, the negative electrode mixture may be pressure-molded into a predetermined shape before use.

[0057] Furthermore, in the case of a negative electrode having a molded negative electrode mixture (negative electrode mixture layer) and a current collector, for example, a negative electrode mixture-containing composition (slurry, paste, etc.) can be prepared by dispersing the negative electrode active material and conductive additive in water or an organic solvent such as N-methyl-2-pyrrolidone (NMP), applying this to the current collector and drying it, and then, if necessary, performing a pressing process such as calendering.

[0058] However, the negative electrode is not limited to those manufactured by the methods described above, but may be manufactured by other methods.

[0059] The thickness of the negative electrode active material layer is preferably 0.15 to 4 mm. On the other hand, in the case of a configuration having a negative electrode active material layer and a current collector, the thickness of the negative electrode active material layer (thickness per side of the current collector) is preferably 30 to 300 μm.

[0060] When using a current collector for the negative electrode, examples of such current collectors include those made of nickel; stainless steels such as SUS316, SUS430, and SUS444; copper and copper alloys. Examples of their forms include plain woven wire mesh, expanded metal, lath, punched metal, metal foam, foil (sheet), etc. The thickness of the current collector is preferably, for example, 0.05 to 0.2 mm. It is also desirable to apply a paste-like conductive material such as carbon paste or silver paste on the surface of such a current collector.

[0061] (Positive electrode) For the positive electrode of the battery, a molded body formed by molding a positive electrode mixture containing a positive electrode active material, etc., or a structure having a layer (positive electrode mixture layer) made of a positive electrode mixture on one or both sides of a current collector can be used.

[0062] As the positive electrode active material, silver oxides (such as silver(I) oxide, silver(II) oxide, silver-nickel composite oxide, etc.); manganese oxides (MnO ２ O ３ , Mn ３ O ４ , MnOOH, MnO ２ , ZnMn ２ O ４ , LiMn ２ O ４ and other oxides or complex oxides containing Mn, etc.); nickel oxides (such as nickel hydroxide, nickel oxyhydroxide, etc.); etc. can be mentioned, and one or more of these can be used. Among these, since a relatively stable and high discharge voltage can be ensured from the initial stage to the final stage of discharge, it is preferable to use silver oxide.

[0063] The silver oxide is not particularly limited with respect to its particle size, but the average particle diameter is preferably 10 μm or less, and more preferably 2 μm or less. Especially when the battery is a secondary battery, when silver oxide of such a size is used, the utilization rate during charging is improved, and a large charge capacity can be obtained even if the charge termination voltage is relatively low, so the charge-discharge cycle characteristics of the battery can be further enhanced. Also, for example, it is possible to suppress the swelling of the battery that may occur by increasing the charge termination voltage.

[0064] However, since silver oxide with very small particle sizes is difficult to manufacture and handle afterward, the average particle size of the silver oxide is preferably 0.01 μm or larger, and more preferably 0.03 μm or larger.

[0065] Examples of conductive additives for the positive electrode mixture include carbon materials such as carbon black and graphite. It is more preferable to use both carbon black and graphite as conductive additives.

[0066] By using carbon black, a good conductive network can be easily formed in the molded body of the positive electrode mixture. Compared to using graphite alone, for example, there are more contact points with the silver oxide particles, which are the positive electrode active material. This effectively reduces the electrical resistance within the molded body of the positive electrode mixture, thereby improving the reaction efficiency of the positive electrode active material during charging.

[0067] On the other hand, when using only carbon black, depending on the thickness of the molded body of the positive electrode mixture, it may be necessary to use a binder to improve its moldability. However, when graphite is also used, the moldability of the molded body of the positive electrode mixture is improved. For example, even when the molded body of the positive electrode mixture is thin, such as 0.4 mm or less, more preferably 0.3 mm or less, its moldability is good, making it easier to prevent manufacturing defects without using a binder.

[0068] For the graphite in the positive electrode mixture, one or more of the types previously exemplified as possible in the negative electrode mixture may be used.

[0069] As mentioned above, graphite has the function of improving the moldability of the positive electrode mixture molded body. From the viewpoint of exhibiting this function more effectively, the graphite preferably has an average particle diameter of 1 μm or more, more preferably 2 μm or more, and from the viewpoint of improving conductivity, it is preferably 7 μm or less, and more preferably 5 μm or less.

[0070] For the carbon black in the positive electrode mixture, one or more of the types previously exemplified as potentially being included in the negative electrode mixture may be used.

[0071] Furthermore, when using silver oxide as the positive electrode active material, it is preferable to further include insulating inorganic particles in the positive electrode mixture, thereby further improving the charge-discharge cycle characteristics of the battery. In addition, when using insulating inorganic particles, further including carbon black particles and graphite particles in the positive electrode mixture can further improve the charge-discharge cycle characteristics of the battery.

[0072] Examples of insulating inorganic particles related to the positive electrode mixture include particles such as oxides of at least one element selected from Si, Zr, Ti, Al, Mg, and Ca. Specific examples of the oxides include Al ２ O ３ , TiO ２ SiO ２ , ZrO ２ , MgO, CaO, AlOOH, Al(OH) ３ Examples include particles that do not dissolve in alkaline electrolytes or are sparingly soluble, which are preferably used. These insulating inorganic particles may be used individually or in combination of two or more types.

[0073] If the particle size of insulating inorganic particles is too large, there is a risk that the effect of improving the charge-discharge cycle characteristics of the battery will be reduced. Therefore, from the viewpoint of further improving the charge-discharge cycle characteristics of the battery, the average particle size of the insulating inorganic particles is preferably 0.5 μm or less, and more preferably 0.3 μm or less.

[0074] Furthermore, if the particle size of the insulating inorganic particles is too small, there is a risk that the effect of improving the battery's charging efficiency (initial capacity) will be reduced. Therefore, from the viewpoint of further improving the battery's charging efficiency, the average particle size of the insulating inorganic particles is preferably 0.01 μm or larger, and more preferably 0.05 μm or larger.

[0075] Regarding the composition of the positive electrode mixture, in order to ensure sufficient volume, when silver oxide is used as the positive electrode active material, its content is preferably 60% by mass or more, more preferably 80% by mass or more, and particularly preferably 90% by mass or more, based on 100% by mass of the total solid content constituting the positive electrode mixture.

[0076] Furthermore, the content of the conductive additive in the positive electrode mixture is preferably 0.2% by mass or more, preferably 0.5% by mass or more, and particularly preferably 1% by mass or more, from the viewpoint of conductivity. On the other hand, to prevent capacity reduction and gas generation during charging, it is preferably 8% by mass or less, more preferably 7% by mass or less, even more preferably 5% by mass or less, and particularly preferably 3% by mass or less.

[0077] Furthermore, when carbon black and graphite are included in the positive electrode mixture, the graphite content is preferably 1% by mass or more, and more preferably 2% by mass or more, from the viewpoint of ensuring a good improvement in the battery's charging efficiency and charge-discharge cycle characteristics through the combined use of carbon black and graphite. In addition, when carbon black and graphite are included in the positive electrode mixture, the graphite content is preferably 7% by mass or less, and more preferably 4% by mass or less, from the viewpoint of preventing a decrease in battery capacity due to, for example, too little positive electrode active material in the positive electrode mixture.

[0078] Furthermore, when carbon black and graphite are included in the positive electrode mixture, the carbon black content is preferably 0.1% by mass or more, and more preferably 0.5% by mass or more, from the viewpoint of ensuring a good improvement in the battery's charging efficiency and charge-discharge cycle characteristics through the combined use of carbon black and graphite. However, if the amount of carbon black particles in the positive electrode mixture is too high, there is a risk that the positive electrode will swell significantly, for example, when the battery is stored at high temperatures. Therefore, from the viewpoint of suppressing the swelling of the positive electrode during battery storage (especially storage at high temperatures of around 60°C) and improving the battery's storage characteristics, the carbon black content when carbon black and graphite are included in the positive electrode mixture is preferably 1.5% by mass or less, and more preferably 1% by mass or less.

[0079] Furthermore, when insulating inorganic particles are included in the positive electrode mixture, the content is preferably 0.1% by mass or more, and more preferably 3% by mass or more, from the viewpoint of ensuring a good effect from their use (particularly the effect of improving the charge-discharge cycle characteristics of the battery). However, if the amount of insulating inorganic particles in the positive electrode mixture is too large, the amount of positive electrode active material filled will decrease, leading to a decrease in battery capacity. In addition, depending on the type of insulating inorganic particles, the discharge capacity may suddenly decrease as the charge-discharge cycle progresses. Therefore, the content of insulating inorganic particles in the positive electrode mixture is preferably 7% by mass or less, and more preferably 5% by mass or less.

[0080] As described above, the positive electrode mixture can be formed without using a binder, but a binder may be used when it is necessary to increase strength (for example, when graphite is not used as a conductive additive). For the binder in the positive electrode mixture, one or more of the binders previously exemplified for inclusion in the negative electrode mixture can be used. When a binder is used, the binder content in the positive electrode mixture is preferably 0.1 to 20% by mass.

[0081] In the case of a positive electrode consisting only of a molded positive electrode mixture, it can be manufactured by, for example, mixing a positive electrode active material, a conductive additive, and, if necessary, an alkaline electrolyte (the same alkaline electrolyte used in batteries can be used), and then press-molding the prepared positive electrode mixture into a predetermined shape.

[0082] Furthermore, in the case of a positive electrode having a molded positive electrode mixture (positive electrode mixture layer) and a current collector, for example, it can be manufactured by dispersing a positive electrode active material and a conductive additive in water or an organic solvent such as N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture-containing composition (slurry, paste, etc.), applying this to a current collector and drying it, and then performing a press treatment such as calendering as necessary.

[0083] However, the positive electrode is not limited to those manufactured by the methods described above, but may be manufactured by other methods.

[0084] When only a molded body of the positive electrode mixture is used as the positive electrode, its thickness is preferably 0.15 to 4 mm. On the other hand, in the case of a positive electrode having a positive electrode mixture layer and a current collector, the thickness of the positive electrode mixture layer (thickness per side of the current collector) is preferably 30 to 300 μm.

[0085] When a current collector is used for the positive electrode, examples of materials for the current collector include nickel; stainless steel such as SUS316, SUS430, and SUS444; and examples of its form include plain weave wire mesh, expanded metal, lath mesh, perforated metal, metal foam, and foil (plate). The thickness of the current collector is preferably, for example, 0.05 to 0.2 mm. It is also desirable to apply a paste-like conductive material such as carbon paste or silver paste to the surface of such a current collector.

[0086] (Alkaline Electrolyte) An alkaline aqueous solution is used as the alkaline electrolyte for the battery. Suitable electrolyte salts to be included in the alkaline electrolyte include alkali metal hydroxides (such as sodium hydroxide, potassium hydroxide, and lithium hydroxide), with potassium hydroxide being particularly preferred. The concentration of the alkaline electrolyte is, for example, in the case of an aqueous solution of potassium hydroxide, preferably 20% by mass or more, and more preferably 28% by mass or more. On the other hand, to increase ionic conductivity, the concentration of potassium hydroxide is preferably 40% by mass or less, and more preferably 35% by mass or less. By adjusting the concentration of the aqueous solution of potassium hydroxide to these values, a battery with superior load characteristics can be constructed.

[0087] In addition to the components described above, various known additives may be added to the alkaline electrolyte as needed, provided that they do not impair the effects of the present invention. For example, when using pure zinc or a zinc alloy for the negative electrode of the battery, zinc oxide may be added to prevent corrosion (oxidation) of these materials. As mentioned above, zinc oxide can also be added to the negative electrode.

[0088] Furthermore, one or more compounds selected from the group consisting of manganese compounds, tin compounds, and indium compounds may be dissolved in the alkaline electrolyte.

[0089] In alkaline secondary batteries having a positive electrode containing silver oxide as the positive electrode active material, silver is generated from the silver oxide in the positive electrode during discharge. However, when the battery is charged, silver oxide crystals form around the silver, effectively reducing the reaction area of ​​the positive electrode active material and inhibiting subsequent battery reactions. However, when these compounds are dissolved in the alkaline electrolyte, ions derived from these compounds (manganese ions, tin ions, indium ions) adsorb onto the positive electrode, suppressing the growth of silver oxide crystals and refining the resulting silver oxide crystals. Therefore, the problem of silver oxide crystals forming during battery charging inhibiting battery reactions is suppressed, making it possible to improve the charge-discharge cycle characteristics of the secondary battery, for example.

[0090] Examples of manganese compounds that can be dissolved in an alkaline electrolyte include manganese chloride, manganese acetate, manganese sulfide, manganese sulfate, and manganese hydroxide. Examples of tin compounds that can be dissolved in an alkaline electrolyte include tin chloride, tin acetate, tin sulfide, tin bromide, tin oxide, tin hydroxide, and tin sulfate. Examples of indium compounds that can be dissolved in an alkaline electrolyte include indium hydroxide, indium oxide, indium sulfate, indium sulfide, indium nitrate, indium bromide, and indium chloride.

[0091] The concentrations of indium compounds, manganese compounds, and tin compounds in the alkaline electrolyte (the concentration of only one of these compounds if only one is dissolved, and the total concentration if two or more are dissolved) are preferably 50 ppm or more, more preferably 500 ppm or more, and more preferably 10,000 ppm or less, and more preferably 5,000 ppm or less, on a mass basis, from the viewpoint of ensuring the above-mentioned effects more effectively.

[0092] Furthermore, it is preferable to include polyalkylene glycols or calcium compounds in at least one of the negative electrode, alkaline electrolyte, and separator. In this case, the growth of zinc dendrites on the negative electrode can be suppressed by the action of the polyalkylene glycols or calcium compounds, thereby improving the charge-discharge cycle characteristics and storage characteristics of the battery.

[0093] Furthermore, it is preferable to include tellurium or a compound thereof (such as tellurium dioxide) in at least one of the components within the battery, such as the positive electrode, negative electrode, and separator, or in the alkaline electrolyte, thereby improving the battery's charge-discharge cycle characteristics and load characteristics.

[0094] (Outer casing) For the battery outer casing, for example, a battery container consisting of an outer can, a sealing can, and a gasket as shown in Figure 1; a sheet-like outer casing made of resin film or metal-resin laminate film; a battery container having a metal outer can with a bottomed cylindrical shape (cylindrical or rectangular) and a sealing structure that seals its opening; etc. can be used.

[0095] In the case of a battery container consisting of an outer casing, a sealing casing, and a gasket, the outer casing can be made of materials such as nickel-plated iron or stainless steel.

[0096] In the case of a battery container consisting of an outer can, a sealing can, and a gasket, the sealing can can be made of, for example, iron with nickel plating or stainless steel. When the negative electrode active material, such as pure zinc or a zinc alloy, is in direct contact with the inner surface of the sealing can, it is preferable to form a metal layer made of copper or a copper alloy such as brass on the surface of the sealing can that is in contact with the negative electrode, and it is even more preferable to form a layer of tin on the surface of the metal layer. The reason for forming a metal layer made of copper or a copper alloy on the surface of the sealing can that is in contact with the negative electrode is to suppress the formation of local galvanic cells with pure zinc or a zinc alloy and prevent corrosion of these cells, but the corrosion prevention effect can be further enhanced by forming a layer of tin on the surface of the metal layer.

[0097] In the case of a battery container consisting of an outer casing, a sealing casing, and a gasket, the gasket can be made of materials such as nylon or polypropylene.

[0098] Furthermore, in the case of batteries having a battery container composed of an outer casing, a sealing casing, and a gasket, or batteries having a sheet-like outer casing, the plan view shape may be circular, or it may be a polygon such as a square or rectangle. In the case of a polygon, its corners may be curved.

[0099] The present invention will be described in detail below based on examples. However, the following examples are not intended to limit the present invention.

[0100] Example 1 (Preparation of positive electrode) Silver(I) oxide (Ag) with an average particle size of 1.4 μm and containing 3.7% (by mass) of Bi relative to the total amount of silver. ２ O) and carbon black (BET specific surface area of ​​68 m²) ２ A mixture was prepared by mixing acetylene black (with an average primary particle size of 35 nm) at a mass ratio of 98:2 with the aforementioned mixture. Furthermore, the mixture and graphite particles (BET specific surface area: 20 m²) were mixed. ２ ( / g, average particle size: 3.7 μm) and TiO ２ A positive electrode mixture was prepared by mixing particles (average particle size: 250 nm) in a mass ratio of 95.2:3.8:1.

[0101] This positive electrode mixture: 300 mg is filled into the mold, with a filling density of 5.7 g / cm³. ３ The positive electrode mixture molded body was then fabricated by pressure molding it into a disc shape with a diameter of 10.7 mm and a height of 0.6 mm.

[0102] (Fabrication of the negative electrode) For the negative electrode active material, mercury-free zinc alloy particles commonly used in alkaline primary batteries were used, containing In: 500 ppm, Bi: 400 ppm, and Al: 10 ppm as additive elements. The particle size of the zinc alloy particles determined by the method described above was the average particle diameter (D ５０ The particle size was 120 μm, and the proportion of particles with a particle size of 75 μm or less was 25% by mass or less.

[0103] The zinc alloy particles and ZnO were mixed in a ratio of 97:3 (by mass) to obtain a composition for constructing the negative electrode (negative electrode composition). 78 mg of this composition was weighed out and used to prepare the negative electrode.

[0104] (Alkaline Electrolyte) For the alkaline electrolyte, a mixed solution was used, which was prepared by dissolving potassium hydroxide at a concentration of 35% by mass, dissolving zinc oxide at a concentration of 3% by mass in an aqueous solution, and further dissolving lithium hydroxide at a concentration of 1% by mass, polyethylene glycol at 1% by mass, and tellurium dioxide at a concentration of 9.2% by mass. The tellurium content in the electrolyte was 7.4% by mass.

[0105] (Preparation of Laminated Film) 100 parts by mass of commercially available plate-shaped boehmite particles (including secondary aggregates) were mixed with 100 parts by mass of ion-exchanged water and 10 parts by mass of a dispersant (aqueous polycarboxylate ammonium salt, solid content concentration 40% by mass). The mixture was then crushed in a ball mill at 40 revolutions / minute for 10 hours to prepare a dispersion. A portion of the dispersion after treatment was vacuum-dried at 120°C and observed with a scanning electron microscope (SEM). The boehmite was found to be almost plate-shaped. The average particle size of the boehmite after treatment was 1 μm.

[0106] To 500 g of the dispersion, 0.5 g of xanthan gum was added as a thickening agent, and 17 g of acrylic resin binder dispersion (modified polybutyl acrylate, solid content 45% by mass) was added as a binder. The mixture was then stirred with a stirrer for 3 hours to prepare a uniform slurry [slurry for forming porous layer (I), solid content ratio 50% by mass].

[0107] Corona discharge (discharge rate 40 W・min / m) was applied to a microporous PE film for lithium-ion secondary batteries (thickness 12 μm, porosity 40%, average pore size 0.08 μm, melting point of PE 135°C). ２ After hydrophilization treatment with ), a slurry for forming a porous layer (I) was applied to one side using a microgravure coater and dried to form a porous layer (I) with a thickness of 3 μm, obtaining a laminated film with a total thickness of 15 μm [a laminated film having a porous layer (I) and a microporous film]. The porosity of the porous layer (I) in this laminated film was 50%.

[0108] The aforementioned laminated film was immersed in water to which a fluorine-based surfactant was added, then removed and dried to make the entire film hydrophilic, and used in the assembly of the battery shown below.

[0109] (Battery Assembly) The battery separator used was a laminated film having a porous layer (I) and a microporous film, and a graft film / cellophane film laminate. The graft film / cellophane film laminate used was a laminate (YG2122, manufactured by GS Yuasa Membrane Co., Ltd.) consisting of a graft film (thickness: 30 μm) made of a graft copolymer having a structure in which acrylic acid is graft copolymerized onto a polyethylene main chain, and a cellophane film (thickness: 20 μm). These separators were cut into circles with a diameter of 11.3 mm for use.

[0110] An electrolyte absorbent made of vinylon-rayon nonwoven fabric, with a diameter of 11.1 mm and made of steel plate with a gold-plated interior, was placed on the inner bottom surface of the outer can. The positive electrode (positive electrode mixture molded body) was then placed on top of the absorbent, and 9 μL of alkaline electrolyte was dropped onto it. A button-type alkaline secondary battery with a diameter of 11.5 mm and a thickness of 3.0 mm was assembled by fitting together an outer can and a sealing can made of copper-stainless steel (SUS304)-nickel clad plate, which houses the negative electrode (negative electrode composition) and is fitted with an annular gasket made of nylon 66, between the positive and negative electrodes, sandwiching them in the order of a separator made of a laminated film having a porous layer (I) and a microporous film, followed by a separator made of a graft film / cellophane film laminate, and then sealing by crimping, with the electrolyte absorber being the same structure as shown in Figure 1. Note that in the separator made of a laminated film having a porous layer (I) and a microporous film, the porous layer (I) was positioned facing the negative electrode.

[0111] Comparative Example 1 A button-type alkaline secondary battery was assembled in the same manner as in Example 1, except that only a graft film / cellophane film laminate was used as the separator.

[0112] Comparative Example 2: 5 g of an aqueous dispersion of polytetrafluoroethylene (solid content: 60% by mass), 2.5 g of an aqueous solution of sodium polyacrylate (concentration: 2% by mass), and 2.5 g of hydrotalcite particles (average particle size: 0.4 μm) were kneaded together, rolled to produce a film with a thickness of 100 μm, and then punched out into a circular shape with a diameter of 11.3 mm to produce an anion-conducting film.

[0113] A button-type alkaline secondary battery was assembled in the same manner as in Example 1, except that the anion-conducting film was used to assemble the battery instead of a separator consisting of a laminated film having a porous layer (I) and a microporous film.

[0114] For the button-type alkaline secondary batteries of Example 1 and Comparative Examples 1 and 2, a constant current discharge at a current of 1.0 mA (discharge termination: 1 V) was performed, followed by constant current charging at a current of 3.5 mA down to 1.8 V, and then constant voltage charging at 1.8 V (charging terminated when the current value dropped to 0.35 mA). This constant current-constant voltage charging cycle was repeated 20 times, with each cycle consisting of constant current discharge at a current of 1.0 mA (discharge termination: 1 V).

[0115] Table 1 shows the discharge capacity at the 4th cycle and the capacity retention rate (the value obtained by dividing the discharge capacity at the 20th cycle by the discharge capacity at the 4th cycle) for each battery, expressed as a percentage.

[0116]

[0117] As shown in Table 1, the battery of Example 1, which uses only a graft film / cellophane film laminate as a separator, was able to achieve a higher capacity retention rate than Comparative Example 1, which does not have a porous layer containing insulating inorganic fine particles as a separator. Furthermore, despite having a thinner overall separator thickness than the battery of Comparative Example 2, which uses a conventional anion-conducting film as a separator instead of the porous layer, it was possible to achieve a battery with excellent charge-discharge cycle characteristics, similar to the battery of Comparative Example 2.

[0118] Furthermore, Figure 2 shows the voltage changes of each battery during the first charging cycle, and Figure 3 shows the voltage changes of each battery during the first discharging cycle.

[0119] As shown in Figure 2, the charging voltage of the battery in Example 1 in the constant current charging region is lower than that of the battery in Comparative Example 2, which uses a conventional anion-conducting film as a separator. Thus, the battery in Example 1 was able to reduce polarization compared to the battery in Comparative Example 2. For this reason, the battery in Example 1 can be charged more efficiently than the battery in Comparative Example 2.

[0120] Furthermore, as shown in Figure 3, the discharge voltage of the battery in Example 1 was higher than that of the battery in Comparative Example 2, which used a conventional anion-conducting film as a separator, demonstrating that it was possible to produce a battery with excellent load characteristics.

[0121] Example 2: As the positive electrode, a positive electrode mixture molded body was used, which was obtained by pressure molding 140 mg of positive electrode mixture into a disc shape with a diameter of 9.0 mm and a height of 0.38 mm. As the negative electrode, 32 mg of negative electrode composition was used, the diameter of the electrolyte absorber was set to 9.3 mm, and the amount of alkaline electrolyte to be dropped was changed to 15 μL. A button-type alkaline secondary battery with a diameter of 9.5 mm and a thickness of 2.0 mm was assembled in the same manner as in Example 1.

[0122] Example 3 A button-type alkaline secondary battery was assembled in the same manner as in Example 2, except that the microporous film of the separator, which is made of a laminated film having a porous layer (I) and a microporous film, was placed facing the negative electrode.

[0123] Comparative Example 3 A button-type alkaline secondary battery was assembled in the same manner as in Example 2, except that only a graft film / cellophane film laminate was used as the separator.

[0124] Comparative Example 4 A button-type alkaline secondary battery was assembled in the same manner as in Example 2, except that the same anion-conducting film as that prepared in Comparative Example 2 was punched out into a circle with a diameter of 9.3 mm, and the separator, which consisted of a laminated film having a porous layer (I) and a microporous film, was replaced with the anion-conducting film used to assemble the battery.

[0125] For the button-type alkaline secondary batteries of Examples 2 and 3 and Comparative Examples 3 and 4, constant current discharge at a current of 1.0 mA (discharge termination: 1 V) was performed, followed by constant current charging at a current of 3.5 mA down to 1.8 V, and constant voltage charging at 1.8 V (charging terminated when the current value dropped to 0.35 mA). Constant current discharge at a current of 1.0 mA (discharge termination: 1 V) was also performed. For four batteries of each type, the open-circuit voltage (OCV) of the battery at the end of charging and the AC impedance at 1 kHz at the end of discharge were measured, and the average values ​​were calculated. The results are shown in Table 2.

[0126]

[0127] Furthermore, for each of the two batteries, the discharge capacity during the initial discharge (first discharge capacity) was measured, and then the charge-discharge cycle was repeated four times under the same charge-discharge conditions. The discharge capacity during the final discharge (fifth cycle discharge capacity) was measured, and the average value was calculated for each. The results are shown in Table 3.

[0128]

[0129] As shown in Tables 2 and 3, the batteries of Examples 2 and 3 had lower AC impedance and reduced internal resistance than the battery of Comparative Example 4, which used a conventional anion-conducting film as a separator instead of a porous layer containing insulating inorganic fine particles.

[0130] Furthermore, the batteries of Examples 2 and 3 were able to achieve a larger initial discharge capacity and higher charge-discharge efficiency than the battery of Comparative Example 4 and the battery of Comparative Example 3, which uses only a graft film / cellophane film laminate as a separator and does not have a porous layer containing insulating inorganic fine particles as a separator.

[0131] Furthermore, the batteries of Examples 2 and 3 have a larger discharge capacity on the fifth cycle than the battery of Comparative Example 3, and exhibit improved charge-discharge cycle characteristics. By incorporating a porous layer containing insulating inorganic fine particles as a separator, the overall thickness of the separator was reduced compared to the battery of Comparative Example 2, while simultaneously achieving batteries with excellent charge-discharge cycle characteristics similar to those of Comparative Example 2.

[0132] In the battery of the present invention, the overall thickness of the separator can be made thinner than in conventional batteries that use an anion-conducting film as a separator. Therefore, the amount of active material can be increased by the amount of the reduction in separator thickness, making it possible to increase the capacity of the battery.

[0133] The present invention can also be implemented in forms other than those described herein, without departing from its spirit. The embodiments disclosed herein are examples, and the present invention is not limited to these embodiments. The scope of the present invention shall be interpreted in accordance with the claims attached, which take precedence over the description herein, and all modifications within the scope equivalent to the claims are included in the claims.

[0134] The alkaline secondary battery of the present invention can be applied to the same uses as known alkaline secondary batteries.

[0135] 1. Alkaline rechargeable battery 2. Outer casing 3. Sealed casing 4. Positive electrode 5. Negative electrode 61. Separator (Laminated film having a porous layer (I) and a microporous film) 62. Separator (Graft film / cellophane film laminate) 7. Gasket

Claims

1. An alkaline secondary battery having a negative electrode, a positive electrode, and a separator, the negative electrode having a negative electrode active material layer containing zinc metal, a zinc alloy, or a zinc compound as the negative electrode active material, characterized in that the separator comprises a porous layer containing insulating inorganic fine particles.

2. The alkaline secondary battery according to claim 1, wherein the porous layer has plate-shaped particles as the insulating inorganic fine particles.

3. The alkaline secondary battery according to claim 1 or 2, wherein the average particle size of the insulating inorganic fine particles is 5 μm or less.

4. The alkaline secondary battery according to claim 3, wherein the average particle size of the insulating inorganic fine particles is 0.1 μm or more.

5. The alkaline secondary battery according to claim 1 or 2, wherein the thickness of the porous layer containing the insulating inorganic fine particles is 15 μm or less.

6. The alkaline secondary battery according to claim 5, wherein the thickness of the porous layer is 1 μm or more.

7. The alkaline secondary battery according to claim 1 or 2, wherein the porous layer is integrated with a resin substrate to form a separator.

8. The alkaline secondary battery according to claim 7, wherein the resin substrate is a microporous film, and the separator comprises a laminated film having the porous layer on the microporous film.

9. The alkaline secondary battery according to claim 1 or 2, wherein the positive electrode contains silver oxide as a positive electrode active material.

10. The alkaline secondary battery according to claim 1 or 2, further comprising a laminate of a graft film composed of a graft polymer having a polyolefin main chain and side chains derived from (meth)acrylic acid or a derivative thereof bound to the main chain, and a cellophane film, as the separator.

11. The alkaline secondary battery according to claim 1 or 2, wherein the separator is hydrophilic.

12. The alkaline secondary battery according to claim 1 or 2, wherein the content of insulating inorganic fine particles in the porous layer containing the insulating inorganic fine particles is 70% by mass or more.

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