Negative electrode and sodium ion battery

Abstract

To provide a negative electrode that can improve the cycle characteristics of a sodium ion battery, and a sodium ion battery using the same.SOLUTION: The present invention provides a negative electrode including a current collector and a negative electrode composite layer, the negative electrode composite layer includes an active material capable of inserting and desorbing sodium ions, and a binder containing a first polymer, the first polymer includes acidic group-containing monomer units in a proportion of 1 weight% to 25 weight%. The present invention also provides a sodium ion battery using the negative electrode.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a negative electrode and a sodium ion battery using the same. [Background technology]

[0002] Among the batteries currently in practical use, lithium-ion batteries have a high energy density and are widely used, especially in small electronic devices. In addition to small applications, lithium-ion batteries are also expected to be used in automobiles.

[0003] On the other hand, because lithium-ion batteries use lithium, a rare metal element, there are concerns about a lithium supply shortage and associated cost increases as the market expands. Against this background, research into sodium-ion batteries, which use sodium, a resource that is abundant, has been progressing in recent years (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2017 / 119171 Summary of the Invention [Problem to be solved by the invention]

[0005] It is known that a solid electrolyte interphase (SEI) film is usually formed on the surface of the negative electrode of a sodium-ion battery as the electrolyte decomposes, and this SEI film inhibits further decomposition of the electrolyte. However, in sodium-ion batteries, the SEI film tends to dissolve easily, and sufficient cycle characteristics (durability) may not be obtained.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a negative electrode that can improve the cycle characteristics of a sodium ion battery, and a sodium ion battery using the same. [Means for solving the problem]

[0007] The present inventors have conducted extensive research to achieve the above object and have found that the cycle characteristics of a sodium ion battery can be improved by using a polymer containing a predetermined amount of acidic group-containing monomer units as a binder used in a negative electrode, and have thus completed the present invention.

[0008] The present invention includes the following. <1> A negative electrode comprising a current collector and a negative electrode mixture layer, the negative electrode mixture layer containing a negative electrode active material capable of inserting and desorbing sodium ions, and a binder containing a first polymer, the first polymer containing acidic group-containing monomer units in a proportion of 1% by weight or more and 25% by weight or less. <2> The first polymer further contains a (meth)acrylic acid ester monomer unit. <1> The negative electrode according to claim 1. <3> the first polymer further comprises an aromatic vinyl monomer unit; <1> or <2> The negative electrode according to claim 1. <4> the first polymer further comprises an aliphatic conjugated diene monomer unit; <1> ~ <3> The negative electrode according to any one of the preceding claims. <5> The glass transition temperature of the first polymer is −40° C. or higher and 50° C. or lower. <1> ~ <4> The negative electrode according to any one of the preceding claims. <6> The content of the binder in the negative electrode mixture layer is less than 5% by weight. <1> ~ <5> The negative electrode according to any one of the preceding claims. <7> A sodium ion battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is <1> ~ <6> A sodium ion battery, wherein the negative electrode is the negative electrode according to any one of claims 1 to 4. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a negative electrode that can improve the cycle characteristics of a sodium ion battery, and a sodium ion battery using the same. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will be described in detail below with reference to embodiments and examples. However, the present invention is not limited to the embodiments and examples shown below, and can be implemented with any modifications within the scope of the claims of the present invention and their equivalents. The components of the embodiments shown below can be combined as appropriate.

[0011] In the following description, in a polymer produced by copolymerizing multiple types of monomers, the ratio of a monomer unit formed by polymerizing a certain monomer in the polymer usually coincides with the ratio (feed ratio) of the certain monomer to all the monomers used in the polymerization of the polymer, unless otherwise specified.

[0012] In the following description, the structure of a molecule or a part thereof, such as a monomer unit, is not limited by its production method. For example, an aromatic vinyl monomer unit is a unit having a structure formed by polymerization of an aromatic vinyl monomer, but the aromatic vinyl monomer unit also includes units formed by other formation methods that have the same structure as the structure formed by polymerization of an aromatic vinyl monomer.

[0013] In the present invention, the "content ratio (wt%)" of each monomer unit (each repeating unit) contained in a polymer represents the weight ratio of each monomer unit when all monomer units (all repeating units) contained in the polymer are taken as 100 wt%, unless otherwise specified. The content ratio (wt%) of each monomer unit contained in a polymer is 1 H-NMR and 13 It can be measured using nuclear magnetic resonance (NMR) techniques such as C-NMR.

[0014] <1. Overview of the negative electrode> A negative electrode according to one embodiment of the present invention includes a current collector and a negative electrode mixture layer, the negative electrode mixture layer including a negative electrode active material capable of inserting and desorbing sodium ions and a binder containing a first polymer, wherein the first polymer includes a predetermined proportion of acidic group-containing monomer units.

[0015] The negative electrode according to this embodiment can be generally used as a negative electrode of a sodium ion battery.

[0016] According to this embodiment, the binder contains the first polymer containing a predetermined amount of acidic group-containing monomer units, so that the negative electrode can have good cycle characteristics of a sodium ion battery.

[0017] Furthermore, according to this embodiment, the binder usually contains a first polymer containing a predetermined amount of acidic group-containing monomer units, which can reduce the rate of increase in internal resistance of the negative electrode.

[0018] Furthermore, according to this embodiment, the binder usually contains a first polymer containing a predetermined amount of acidic group-containing monomer units, which can improve the adhesion of the negative electrode mixture layer to the current collector.

[0019] Although the mechanism by which the binder containing the first polymer provides the above-mentioned effect is unclear, the present inventors speculate that it may be as follows: However, the technical scope of the present invention is not limited to the mechanism described below.

[0020] The negative electrode composite layer is typically formed using a slurry composition containing at least a negative electrode active material, a binder, and a dispersion medium. Conventional binders, such as polyacrylic acid, tend to aggregate in such a slurry composition. When a negative electrode composite layer is formed, the binder adheres locally to the surface of the negative electrode active material. Therefore, when a conventional binder is used, the surface of the negative electrode active material is largely exposed in the negative electrode composite layer. When a negative electrode having such a negative electrode composite layer is used in a sodium-ion battery, the SEI coating formed on the surface of the negative electrode active material dissolves in the exposed areas.

[0021] Furthermore, if the binder adheres locally in the negative electrode mixture layer, the binder will be unevenly distributed, which may result in portions of the negative electrode mixture layer that are low in binder. Such portions with low binder tend to have low mechanical strength. Therefore, if the negative electrode active material expands and contracts during charging and discharging, causing stress in the negative electrode mixture layer, fractures may occur originating from the portions with low binder, which may lead to the severance of the conductive paths in the negative electrode mixture layer.

[0022] The present inventors speculate that, in the past, sufficient cycle characteristics could not be obtained because the SEI film was easily dissolved and the conductive path was easily disconnected during charging and discharging.

[0023] In contrast, in this embodiment, the first polymer used as the binder in the negative electrode mixture layer contains a predetermined amount of acidic group-containing monomer units, thereby increasing the dispersibility of the first polymer in the slurry composition and allowing the binder to be uniformly attached to the surface of the negative electrode active material. Typically, the first polymer is dispersed in particulate form in the slurry and is contained in the negative electrode mixture layer while maintaining this dispersed state. Therefore, the negative electrode mixture layer contains the first polymer in the form of uniformly dispersed particles. Therefore, when a binder containing the first polymer is used, the surface of the negative electrode active material is less exposed in the negative electrode mixture layer. When a negative electrode including such a negative electrode mixture layer is used in a sodium-ion battery, the SEI coating formed on the surface of the negative electrode active material can be protected by the binder, thereby suppressing dissolution of the SEI coating.

[0024] Furthermore, when the first polymer is uniformly dispersed in the negative electrode mixture layer, the formation of areas with little binder can be suppressed, which in turn suppresses the occurrence of breakdown starting points in the negative electrode mixture layer and thus prevents the disconnection of the conductive path during charge and discharge. Therefore, the present inventors speculate that this embodiment can suppress dissolution of the SEI film and disconnection of the conductive paths, thereby improving cycle characteristics.

[0025] Incidentally, it is known that an SEI coating is also formed on the surface of the negative electrode of a lithium ion battery by decomposition of the electrolyte. However, the SEI coating of a lithium ion battery is a coating primarily composed of organic components. In contrast, the SEI coating formed on the surface of the negative electrode of a sodium ion battery is a coating primarily composed of inorganic components. Therefore, the SEI coating of a sodium ion battery tends to dissolve more easily in the electrolyte and have lower durability than the SEI coating of a lithium ion battery. The decrease in cycle characteristics due to dissolution of the SEI coating on the negative electrode is a problem specific to sodium ion batteries, and the inventors speculate that the negative electrode according to this embodiment can solve this specific problem.

[0026] Furthermore, in this embodiment, since dissolution of the SEI coating can be suppressed, decomposition of the electrolyte solution accompanying charge and discharge can be suppressed, and gas generation due to the decomposition can be suppressed, thereby suppressing an increase in resistance due to the gas. Furthermore, as described above, in this embodiment, disconnection of the conductive path due to destruction of the negative electrode composite layer can be suppressed. Therefore, the rate of increase in internal resistance due to charge and discharge can be reduced.

[0027] Furthermore, in this embodiment, as described above, it is possible to prevent the occurrence of portions with little binder in the negative electrode mixture layer. Therefore, it is possible to prevent the occurrence of starting points for fracture due to stress. Therefore, it is possible to prevent delamination due to fracture of the negative electrode mixture layer, thereby improving the adhesion of the negative electrode mixture layer to the current collector.

[0028] <2. Negative electrode composite material layer> The negative electrode mixture layer contains at least a negative electrode active material and a binder.

[0029] <2.1. Binder> The binder contains at least a first polymer.

[0030] <2.1.1. First polymer> In the negative electrode mixture layer, the first polymer has the function of binding the components in the negative electrode mixture layer together or binding the components to the current collector.

[0031] (1) Properties of the first polymer Generally, the first polymer can be entirely or partially, preferably entirely, a particulate polymer, that is, a material that exists in particulate form in the positive electrode mixture layer.

[0032] The particulate polymer can be made into polymer particles that can be dispersed in an aqueous medium such as water while maintaining a particulate shape. The particulate polymer usually has an insoluble content of 90% by weight or more when 0.5 g of the particulate polymer is dissolved in 100 g of water at 25°C. In this specification, the "particulate" shape refers to a shape having an aspect ratio of 1 or more and less than 10 as measured by a scanning electron microscope.

[0033] The first polymer may be a particle with a single phase structure formed from a single polymer, or may be a particle with a heterogeneous phase structure formed by physically or chemically bonding two or more different polymers. Specific examples of heterogeneous phase structures include a core-shell structure in which the center (core) and outer shell (shell) of spherical particles are formed from different polymers; and a side-by-side structure in which two or more polymers are arranged side by side.

[0034] The volume average particle diameter (D 50 ) can be appropriately adjusted to a size that allows it to be uniformly present on the surface of the negative electrode active material. The volume average particle diameter is usually 50 nm or more, preferably 70 nm or more, more preferably 100 nm or more, and usually 10 μm or less, preferably 5 μm or less, more preferably 1 μm or less. When the volume average particle diameter of the particulate polymer is equal to or greater than the lower limit, particles of the negative electrode active material can be well bound together, and when the volume average particle diameter is equal to or less than the upper limit, the negative electrode resistance due to the inclusion of the particulate polymer can be reduced.

[0035] The volume average particle diameter (D 50) means the particle size at which the cumulative volume calculated from the small diameter side is 50% in the particle size distribution (volume basis) measured by a laser diffraction method. The particle size of the particulate polymer can be measured by a wet method using a particle size distribution measuring device (e.g., Microtrac MT3300EX II; manufactured by Microtrac Bell Co., Ltd.).

[0036] The glass transition temperature of the first polymer is usually -40°C or higher, preferably -35°C or higher, more preferably -30°C or higher, and usually 50°C or lower, preferably 40°C or lower, more preferably 30°C or lower. When the glass transition temperature of the first polymer is in the above range, a good binding function can be exhibited. The glass transition temperature can be measured using a differential scanning calorimeter in accordance with JIS K7121 at a measurement temperature of -100°C to 180°C and a heating rate of 5°C / min.

[0037] (2) Monomer unit The first polymer typically contains a predetermined content of acidic group-containing monomer units. Preferably, the first polymer further contains at least one of (meth)acrylic acid ester monomer units, aromatic vinyl monomer units, and aliphatic conjugated diene monomer units in addition to the acidic group-containing monomer units. The first polymer may optionally contain monomer units (hereinafter sometimes referred to as "optional repeating units") other than the above-described (meth)acrylic acid ester monomer units, aromatic vinyl monomer units, and aliphatic conjugated diene monomer units. Details of the monomer units that the first polymer may contain are described below.

[0038] (Acidic group-containing monomer unit) The acidic group-containing monomer unit refers to a unit having a structure formed by polymerizing an acidic group-containing monomer. Examples of the acidic group-containing monomer that can form the acidic group-containing monomer unit include a carboxylic acid group-containing monomer, a sulfonic acid group-containing monomer, and a phosphoric acid group-containing monomer. The acidic group contained in the acidic group-containing monomer unit may form a salt with an alkali metal, ammonia, or the like.

[0039] Examples of carboxylic acid group-containing monomers capable of forming carboxylic acid group-containing monomer units include monocarboxylic acids and their derivatives, dicarboxylic acids and their acid anhydrides and their derivatives, etc. These monomers are also called ethylenically unsaturated carboxylic acid monomers.

[0040] Examples of monocarboxylic acids include acrylic acid, methacrylic acid, crotonic acid, etc. Examples of monocarboxylic acid derivatives include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E-methoxyacrylic acid, etc.

[0041] Examples of dicarboxylic acids include maleic acid, fumaric acid, and itaconic acid. Examples of dicarboxylic acid derivatives include methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, and fluoromaleic acid, as well as maleic acid monoesters such as nonyl maleate, decyl maleate, dodecyl maleate, octadecyl maleate, and fluoroalkyl maleate. Examples of dicarboxylic acid anhydrides include maleic anhydride, acrylic anhydride, methylmaleic anhydride, and dimethylmaleic anhydride. Furthermore, acid anhydrides that generate a carboxylic acid group upon hydrolysis can also be used as carboxylic acid group-containing monomers.

[0042] Examples of sulfonic acid group-containing monomers that can form sulfonic acid group-containing monomer units include vinyl sulfonic acid, methyl vinyl sulfonic acid, (meth)allyl sulfonic acid, styrene sulfonic acid, (meth)acrylic acid-2-ethyl sulfonate, 2-acrylamido-2-methylpropanesulfonic acid, and 3-allyloxy-2-hydroxypropanesulfonic acid. The term "(meth)allyl" encompasses "allyl", "methallyl", and combinations thereof.

[0043] Examples of the phosphate group-containing monomer capable of forming the phosphate group-containing monomer unit include 2-(meth)acryloyloxyethyl phosphate, methyl-2-(meth)acryloyloxyethyl phosphate, and ethyl-(meth)acryloyloxyethyl phosphate. The term "(meth)acryloyl" includes "acryloyl," "methacryloyl," and combinations thereof.

[0044] The above-mentioned acidic group-containing monomers may be used alone or in combination of two or more. The acidic group-containing monomer capable of forming the acidic group-containing monomer unit is preferably a carboxylic acid group-containing monomer, more preferably a monocarboxylic acid, further preferably itaconic acid, methacrylic acid, or acrylic acid, and particularly preferably itaconic acid or methacrylic acid. This is because the cycle characteristics of a sodium ion battery using the negative electrode can be effectively improved.

[0045] The content of the acidic group-containing monomer unit in the first polymer is usually 1% by weight or more, preferably 2% by weight or more, more preferably 4% by weight or more, and usually 25% by weight or less, preferably 22% by weight or less, more preferably 21% by weight or less, when the total of all monomer units (repeating units) contained in the first polymer is taken as 100% by weight. This is because the content of the acidic group-containing monomer unit in the above range can effectively improve the cycle characteristics of a sodium ion battery using a negative electrode.

[0046] ((Meth)acrylic acid ester monomer) The (meth)acrylic acid ester monomer unit represents a unit having a structure formed by polymerizing a (meth)acrylic acid ester monomer. The term "(meth)acrylic" includes "acrylic", "methacrylic", and combinations thereof. For example, (meth)acrylic acid means acrylic acid, methacrylic acid, or a mixture thereof.

[0047] Examples of (meth)acrylic acid ester monomers that can form (meth)acrylic acid ester monomer units include alkyl acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, isobutyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, and stearyl acrylate; and alkyl acrylates such as 2-methoxyethyl acrylate and 2-ethoxyethyl acrylate. Alkoxy esters include alkyl methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, isobutyl methacrylate, pentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, and stearyl methacrylate; and alkoxy methacrylates such as 2-methoxyethyl methacrylate and 2-ethoxyethyl methacrylate. Among these, alkyl (meth)acrylates are preferred, with methyl acrylate, n-butyl acrylate, isobutyl acrylate, and 2-ethylhexyl acrylate being more preferred, and n-butyl acrylate and 2-ethylhexyl acrylate being even more preferred. This is because they can improve the cycle characteristics of the negative electrode mixture layer and can improve the adhesion of the negative electrode mixture layer to the current collector. The (meth)acrylic acid ester monomer units may be used alone or in combination of two or more.

[0048] In the present invention, hydroxyl group-containing (meth)acrylic acid ester monomer units are not included in the "(meth)acrylic acid ester monomer units" but are included in the "hydroxyl group-containing monomer units" described below.

[0049] There is no limitation on the content of the (meth)acrylic acid ester monomer units in the first polymer, but it can usually be appropriately selected within the range of 1% by weight or more and 90% by weight or less.

[0050] (aromatic vinyl monomer unit) The aromatic vinyl monomer unit refers to a unit having a structure formed by polymerizing an aromatic vinyl monomer. Examples of aromatic vinyl monomers that can form the aromatic vinyl monomer unit include styrene, α-methylstyrene, pt-butylstyrene, butoxystyrene, vinylnaphthalene, vinyltoluene, and chlorostyrene. Among these, styrene is preferred. These can be used alone or in combination of two or more.

[0051] There is no limitation on the content of the aromatic vinyl monomer units in the first polymer, but it can usually be adjusted within the range of 1% by weight to 70% by weight.

[0052] (Aliphatic conjugated diene monomer unit) The aliphatic conjugated diene monomer unit refers to a unit having a structure formed by polymerizing an aliphatic conjugated diene monomer. Examples of conjugated diene monomers that can form the aliphatic conjugated diene monomer unit include 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, 1,3-pentadiene, 2-chloro-1,3-butadiene (chloroprene), and piperylene. Among these, 1,3-butadiene is preferred. These can be used alone or in combination of two or more.

[0053] There is no limitation on the content of the aliphatic conjugated diene monomer units in the first polymer, but it can usually be adjusted within the range of 1% by weight or more and 40% by weight or less.

[0054] (any repeating unit) The first polymer according to this embodiment may optionally contain any repeating unit other than the above-mentioned acidic group-containing monomer unit, (meth)acrylic acid ester monomer unit, aromatic vinyl monomer unit, and aliphatic conjugated diene monomer unit. Examples of the optional repeating unit include a hydroxyl group-containing monomer unit, an unsaturated carboxylic acid amide monomer unit, a nitrile group-containing monomer unit, a crosslinkable monomer unit, and an alkylene structural unit. The particulate polymer may contain one type of optional repeating unit, or two or more types. The first polymer preferably contains a hydroxyl group-containing monomer unit as the optional monomer unit.

[0055] The hydroxyl group-containing monomer unit refers to a structural unit having a structure formed by polymerizing a hydroxyl group-containing monomer. Examples of the hydroxyl group-containing monomer that can form the hydroxyl group-containing monomer unit include a hydroxyl group-containing (meth)acrylic acid ester monomer and a hydroxyl group-containing (meth)acrylamide monomer.

[0056] Examples of hydroxyl group-containing (meth)acrylic acid ester monomers include 2-hydroxymethyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, 2-hydroxymethyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, and 2-hydroxybutyl methacrylate.

[0057] Examples of the hydroxyl group-containing (meth)acrylamide monomer include N-hydroxymethylacrylamide, N-hydroxyethylacrylamide, N-hydroxypropylacrylamide, N-hydroxymethylmethacrylamide, N-hydroxyethylmethacrylamide, and N-hydroxypropylmethacrylamide.

[0058] Of the hydroxyl group-containing monomer units mentioned above, hydroxyl group-containing (meth)acrylic acid ester monomers are preferred, and 2-hydroxyethyl acrylate is more preferred.

[0059] Examples of unsaturated carboxylic acid amide monomers that can form unsaturated carboxylic acid amide monomer units include acrylamide, methacrylamide, N-thyrol acrylamide, N-methylol methacrylamide, N,N-dimethylacrylamide, and hydroxyethyl acrylamide.

[0060] The nitrile group-containing monomer unit refers to a structural unit having a structure formed by polymerizing a nitrile group-containing monomer. Examples of the nitrile group-containing monomer that can form the nitrile group-containing monomer unit include α,β-unsaturated nitrile monomers such as acrylonitrile and methacrylonitrile. Among these, acrylonitrile is preferred.

[0061] The crosslinkable monomer unit refers to a structural unit having a structure formed by polymerizing a crosslinkable monomer. The crosslinkable monomer refers to a monomer that can form a crosslinked structure when polymerized. Examples of the crosslinkable monomer include a monomer having two or more reactive groups per molecule. Specific examples of the crosslinkable monomer include a monofunctional crosslinkable monomer having a thermally crosslinkable crosslinkable group and one olefinic double bond per molecule; and a polyfunctional crosslinkable monomer having two or more olefinic double bonds per molecule.

[0062] Examples of the thermally crosslinkable crosslinkable group possessed by the crosslinkable monomer include an epoxy group, an N-methylolamide group, an oxetanyl group, an oxazoline group, and combinations thereof. Among these, an epoxy group is preferred. Examples of monofunctional crosslinkable monomers having an epoxy group as a thermally crosslinkable crosslinkable group and an olefinic double bond include unsaturated glycidyl ethers such as vinyl glycidyl ether, allyl glycidyl ether, butenyl glycidyl ether, and o-allylphenyl glycidyl ether; diene or polyene monoepoxides such as butadiene monoepoxide, chloroprene monoepoxide, 4,5-epoxy-2-pentene, 3,4-epoxy-1-vinylcyclohexene, and 1,2-epoxy-5,9-cyclododecadiene; 3,4-epoxy phenyl glycidyl ether; and 3,4-epoxy phenyl glycidyl ether. alkenyl epoxides such as 1,2-epoxy-1-butene, 1,2-epoxy-5-hexene, and 1,2-epoxy-9-decene; and glycidyl esters of unsaturated carboxylic acids such as glycidyl acrylate, glycidyl methacrylate, glycidyl crotonate, glycidyl 4-heptenoate, glycidyl sorbate, glycidyl linoleate, glycidyl 4-methyl-3-pentenoate, glycidyl ester of 3-cyclohexenecarboxylic acid, and glycidyl ester of 4-methyl-3-cyclohexenecarboxylic acid. Among these, unsaturated glycidyl ethers and glycidyl esters of unsaturated carboxylic acids are preferred, allyl glycidyl ether and glycidyl methacrylate are more preferred, and glycidyl methacrylate is even more preferred.

[0063] Examples of polyfunctional crosslinkable monomers having two or more olefinic double bonds include allyl (meth)acrylate, ethylene di(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, dipropylene glycol diallyl ether, polyglycol diallyl ether, triethylene glycol divinyl ether, hydroquinone diallyl ether, tetraallyloxyethane, trimethylolpropane diallyl ether, allyl or vinyl ethers of polyfunctional alcohols other than those mentioned above, triallylamine, methylenebisacrylamide, and divinylbenzene. Among these, ethylene di(meth)acrylate, ethylene glycol di(meth)acrylate, and divinylbenzene are preferred, ethylene glycol di(meth)acrylate is more preferred, and ethylene glycol dimethacrylate is even more preferred.

[0064] The particulate polymer may contain one type of crosslinkable monomer unit alone or two or more types in combination.

[0065] The alkylene structural unit has the general formula: -C n H 2n - [where n is an integer of 2 or more]. Examples of the alkylene structural unit include structural units obtained by hydrogenating an aliphatic conjugated diene monomer unit derived from the above-mentioned aliphatic conjugated diene monomer.

[0066] The content of any repeating unit in the first polymer is not limited, but can usually be adjusted appropriately within the range of 0% by weight to 30% by weight.

[0067] (3) Preferred Examples of the First Polymer The first polymer contains a predetermined amount of acidic group-containing monomer units, and preferably further contains at least one of (meth)acrylic acid ester monomer units, aromatic vinyl monomer units, and aliphatic conjugated diene monomers. More preferred examples of the first polymer include polymer (I), polymer (II), and polymer (III), which will be described below.

[0068] (Polymer (I)) The polymer (I) is a polymer containing an acidic group-containing monomer unit, a (meth)acrylic acid ester monomer unit, and an aromatic vinyl monomer unit. The polymer (I) can usually be in the form of particles having a single-phase structure.

[0069] When the first polymer is polymer (I), it preferably contains styrene units as aromatic vinyl monomer units, and more preferably contains acidic group-containing monomer units, (meth)acrylic acid ester monomer units, and styrene units in a predetermined proportion.

[0070] The content of the acidic group-containing monomer unit in the polymer (I) is usually within the same range as the content of the acidic group-containing monomer unit in the first polymer described above.

[0071] The content of (meth)acrylic acid ester monomer units in polymer (I) is usually 50% by weight or more, preferably 60% by weight or more, more preferably 65% ​​by weight or more, and preferably 83% by weight or less, more preferably 80% by weight or less, and even more preferably 78% by weight or less. The content of (meth)acrylic acid ester monomer units in the above range can improve the flexibility of polymer (I) and can improve the adhesion of polymer (I) to the surface of the negative electrode active material. Furthermore, polymer (I) can exhibit good binding properties, which can improve the adhesion of the negative electrode mixture layer to the current collector.

[0072] The content of styrene units in polymer (I) is usually 7% by weight or more, more preferably 10% by weight or more, and even more preferably 14% by weight or more, and usually 33% by weight or less, more preferably 30% by weight or less, and even more preferably 26% by weight or less. The content of styrene units in the above range can improve the flexibility of polymer (I) and can improve the adhesion of polymer (I) to the surface of the negative electrode active material.

[0073] The polymer (I) contains at least the above-mentioned acidic group-containing monomer units, (meth)acrylic acid ester monomer units, and styrene units, and may optionally contain the above-mentioned aliphatic conjugated diene monomer units and any repeating units. The polymer (I) preferably contains hydroxyl group-containing monomer units. When the polymer (I) contains hydroxyl group-containing monomer units, the content of the hydroxyl group-containing monomer units in the polymer (I) is preferably 0.1% by weight or more, more preferably 0.5% by weight or more, and is preferably 10% by weight or less, more preferably 5% by weight or less, and even more preferably 2.5% by weight or less.

[0074] (Polymer (II)) The polymer (II) is a polymer containing an acidic group-containing monomer unit and a (meth)acrylic acid ester monomer unit.

[0075] The polymer (II) can usually have a structure having particles containing the polymer (A) containing (meth)acrylic acid ester monomer units inside a surface layer composed of the polymer (B) containing acidic group-containing monomer units.

[0076] The particles containing polymer (A) located inside the surface layer portion are not particularly limited, but include (i) particles composed of polymer (A) (particles with a single-phase structure), or (ii) core-shell particles having a core portion and a shell portion, one of the core portion and the shell portion being formed using polymer A. When the particles containing polymer (A) are of type (i), polymer (II) can also be considered to have a core-shell particle structure having a core portion containing polymer (A) and a shell portion containing polymer (B).

[0077] When the particles containing the polymer (A) are core-shell particles described in (ii) above, the core portion is preferably a core-shell particle containing the polymer (A), and more preferably a core-shell particle having a core portion containing the polymer (A) and a shell portion containing the polymer (C) containing an aromatic vinyl monomer unit.

[0078] When the particles containing polymer (A) are core-shell particles, the proportion of the core portion in polymer (II) is preferably 50% by weight or more and 95% by weight or less. Furthermore, the proportion of the shell portion in polymer (II) is preferably 1% by weight or more and 40% by weight or less. The proportion of the surface layer portion in polymer (II) is preferably 0.5% by weight or more and 30% by weight or less. By ensuring that the proportions of the core portion, shell portion, and surface layer portion in polymer (II) are within the above ranges, the binding properties of polymer (II) and its dispersibility when prepared as a slurry composition can be improved.

[0079] The polymer (A) contains (meth)acrylic acid ester monomer units, and may optionally contain polymers other than (meth)acrylic acid ester monomer units.

[0080] The content of (meth)acrylic acid ester monomer units in polymer (A) is usually 80% by weight or more and 100% by weight or less. The content of (meth)acrylic acid ester monomer units in polymer (II) is usually 50% by weight or more, preferably 55% by weight or more and more preferably 70% by weight or more, and usually 90% by weight or less, preferably 80% by weight or less, more preferably 77% by weight or less.

[0081] The polymer (A) may contain the above-described acidic group-containing monomer unit and any repeating unit as an optional monomer unit. Among them, the polymer (A) preferably contains at least one of an unsaturated carboxylic acid amide monomer unit, a nitrile group-containing monomer unit, and a crosslinkable monomer unit. This is because the cycle characteristics of the sodium ion battery can be improved.

[0082] The content of the optional monomer unit in polymer (A) is usually 0.1% by weight or more, preferably 0.5% by weight or more, more preferably 1% by weight or more, and usually 10% by weight or less, preferably 9% by weight or less, more preferably 8% by weight or less. When polymer (A) contains two or more types of optional monomer units, the total content of all optional monomer units usually falls within the above range. Furthermore, when polymer (A) contains unsaturated carboxylic acid amide monomer units, nitrile group-containing monomer units, or crosslinkable monomer units, the content in polymer (A) can be appropriately adjusted within the above range.

[0083] The polymer (B) contains an acidic functional group-containing monomer unit. The polymer (B) may optionally contain a monomer unit other than the acidic functional group-containing monomer unit. When the polymer (A) contains an acidic group-containing monomer unit, the acidic group-containing monomer unit contained in the polymer (B) may be the same as or different from the acidic group-containing monomer unit contained in the polymer (A).

[0084] The content of the acidic group-containing monomer units in polymer (B) is usually 80% by weight or more and 100% by weight or less. The content of the acidic group-containing monomer units in polymer (II) is the same as the range of the content of the acidic group-containing monomer units in the first polymer described above.

[0085] The polymer (C) contains an aromatic vinyl monomer unit. The polymer (C) may optionally further contain a monomer unit other than the aromatic vinyl monomer unit. When the polymer (A) contains an aromatic vinyl monomer unit, the aromatic vinyl monomer unit contained in the polymer (C) may be the same as or different from the aromatic vinyl monomer unit contained in the polymer (A).

[0086] Here, the content of aromatic vinyl monomer units in polymer (C) can be usually 80% by weight or more and 100% by weight or less. The content of aromatic vinyl monomer units in polymer (II) is usually 1% by weight or more, preferably 5% by weight or more, more preferably 10% by weight or more, and usually 30% by weight or less, preferably 25% by weight or less, more preferably 20% by weight or less. The proportion of aromatic vinyl monomer units in the monomer units in polymer (C) can be 80% by weight or more and 100% by weight or less. When the content is within the above range, the tackiness of the composition can be improved.

[0087] (Polymer (III)) The polymer (III) is a polymer containing an acidic group-containing monomer unit, an aromatic vinyl monomer unit, and an aliphatic conjugated diene monomer unit. The polymer (III) is usually in the form of particles having a single-phase structure.

[0088] The content of the acidic group-containing monomer unit in the polymer (III) is within the same range as the content of the acidic group-containing monomer unit in the first polymer described above.

[0089] The content of aromatic vinyl monomer units in polymer (III) is usually 1% by weight or more, more preferably 20% by weight or more, even more preferably 30% by weight or more, and 75% by weight or less, more preferably 70% by weight or less, even more preferably 65% ​​by weight or less.

[0090] The content of aliphatic conjugated diene monomer units in polymer (III) is usually 1% by weight or more, preferably 10% by weight or more, more preferably 15% by weight or more, and usually 35% by weight or less, more preferably 30% by weight or less, and even more preferably 25% by weight or less.

[0091] In the polymer (III), the aromatic vinyl monomer units and the aliphatic conjugated diene monomer units may form a copolymer. In this case, the copolymer preferably contains 1,3-butadiene units as the aliphatic conjugated diene monomer units and styrene units as the aromatic vinyl monomer units (i.e., it is a styrene-butadiene copolymer).

[0092] The polymer (III) may contain the (meth)acrylic acid ester monomer unit and any repeating unit as optional monomers. The content of the optional monomer unit in the polymer (III) is usually 0.1% by weight or more, preferably 0.5% by weight or more, more preferably 1% by weight or more, and usually 10% by weight or less, preferably 9% by weight or less, more preferably 8% by weight or less. When the polymer (III) contains two or more types of optional monomer units, the total content of all optional monomer units is usually within the above range.

[0093] (4) Method for preparing the first polymer The method for producing the particulate polymer is not particularly limited, and any method such as solution polymerization, suspension polymerization, or emulsion polymerization may be used. Among them, emulsion polymerization and suspension polymerization are preferred because they can be polymerized in water and can be used as a material for a slurry for a porous membrane as is. In addition, when producing the particulate polymer, it is preferred to add a dispersant to the reaction system.

[0094] Furthermore, when the first polymer is a polymer having a core-shell particle structure, it can be prepared by using a monomer for the core polymer and a monomer for the shell polymer, and polymerizing them stepwise while changing the ratio of these monomers over time. Specifically, the particulate polymer can be prepared by a continuous multi-stage emulsion polymerization method or multi-stage suspension polymerization method in which a polymer in a previous stage is successively coated with a polymer in a later stage. When the first polymer has a core portion, a shell portion, and a surface layer portion, it can also be prepared by polymerizing them stepwise while changing the ratio of the monomers constituting each portion.

[0095] (5) Content of the first polymer in the binder Although there is no limitation on the content of the first polymer in the binder, since a higher content of the first polymer can enhance binding properties, the content is usually 90% by weight or more, preferably 95% by weight or more, and more preferably 98% by weight or more. The content of the first polymer in the binder may be 100% by weight.

[0096] 2.1.2. Optional components of binder The binder may contain any component, examples of which include polymerization additives used in the polymerization of the polymer, such as surfactants, emulsifiers, dispersion stabilizers, polymerization initiators, and chain transfer agents.

[0097] 2.1.3. Binder content in negative electrode mixture layer In the negative electrode according to this embodiment, the content of the binder in the negative electrode mixture layer is not limited, but is usually 5% by weight or less, preferably 4% by weight or less, and more preferably 3% by weight or less, and is usually greater than 0% by weight, preferably 0.5% by weight or more, and more preferably 1% by weight or more. By keeping the content of the binder at or below the upper limit, a negative electrode with low resistance can be obtained.

[0098] <2.2. Negative electrode active material> The negative electrode active material may be any active material capable of inserting and extracting sodium ions, and known negative electrode active materials for sodium ion batteries can be used. Specifically, hard carbon can be used as the negative electrode active material. Hard carbon refers to carbon that is difficult to graphitize. For example, it is a carbon material that is difficult to form graphite even when heated in an inert atmosphere. Usually, this hard carbon has a large interlayer spacing of the (002) plane of 0.37 nm or more. The negative electrode active material is usually in the form of particles.

[0099] In addition, as the carbon material other than the above-mentioned hard carbon for the negative electrode active material, artificial graphite, natural graphite, graphene sheet, non-graphitizable amorphous carbon, and graphitizable amorphous carbon can be used.

[0100] In addition, as the negative electrode active material, for example, metal oxides such as titanium-containing oxides and tin oxides can be used. Examples of titanium-containing oxides include Na2Ti3O7, Na₂Ti₆O 13 , NaTiO₂, Na₄Ti₅O 12 , Li₄Ti₅O 12 , and TiO₂. Furthermore, the negative electrode active material can be a compound represented by the general formula A: Na P M 1 Q TiO R (where 0 < P < 0.5, 0 < Q < 0.5, and 1 ≤ R ≤ 2, and M 1 contains an alkali metal element other than Na). The M 1 contains an alkali metal element other than Na, and preferably contains lithium (Li), potassium (K), and a combination of lithium and potassium. In the compound represented by the general formula A, a part of the M 1 may be substituted by a rare earth element. In addition, as the tin-containing oxide, for example, SnO can be used.

[0101] Furthermore, the negative electrode active material can use sulfides such as WS₂.

[0102] In this embodiment, among the above, it is preferable that the negative electrode active material is hard carbon.

[0103] The content of the negative electrode active material in the negative electrode mixture layer is usually 80% by weight or more, particularly 85% by weight or more, preferably 90% by weight or more, and 99% by weight or less, particularly 98.5% by weight or less, preferably 98% by weight or less. This is because the content of the negative electrode active material in the above range can improve the cycle characteristics of the sodium ion battery.

[0104] 2.3. Optional Components of Negative Electrode Mixture Layer The negative electrode mixture layer may optionally contain a water-soluble polymer and a conductive material.

[0105] <2.3.1.Water-soluble polymer> The water-soluble polymer, together with the binder, functions to bind the components in the negative electrode mixture layer together or to bind the components to the current collector. The water-soluble polymer can usually be present in a non-particulate form in the negative electrode mixture layer.

[0106] The term "water-soluble" for a polymer means that when 0.5 g of the polymer is dissolved in 100 g of water at 25°C, the insoluble content is less than 1.0 wt %.

[0107] Examples of water-soluble polymers include cellulose-based polymers such as cellulose compounds such as carboxymethyl cellulose (CMC), carboxyethyl cellulose, ethyl cellulose, hydroxymethyl cellulose, hydroxypropyl cellulose, and carboxyethyl methyl cellulose, and salts thereof (ammonium salts, alkali metal salts, etc.); starches such as oxidized starch and starch phosphate; casein; various modified starches; polyethylene oxide, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, polysulfonic acid, polycarboxylic acid, polyacrylic acid; (meth)acrylic acid copolymers, acrylic acid-acrylamide copolymers, and other (meth)acrylamide-based polymers, and salts thereof (ammonium salts, alkali metal salts, etc.). These may be used alone or in combination of two or more. Among these, the water-soluble polymer is preferably a cellulose-based polymer, a (meth)acrylic acid copolymer, a (meth)acrylamide-based polymer, or a combination thereof.

[0108] Furthermore, the water-soluble polymer more preferably has a carboxyl group, which can improve the electrical characteristics of the secondary battery.

[0109] The content of the water-soluble polymer in the negative electrode mixture layer is preferably 0.01 wt% or more, more preferably 0.02 wt% or more, even more preferably 0.03 wt% or more, and preferably 1.0 wt% or less, more preferably 0.5 wt% or less, and even more preferably 0.3 wt% or less. This is because a negative electrode with low resistance can be obtained by having the content of the water-soluble polymer in this range. This is also because the dispersibility of a slurry composition containing the material of the negative electrode mixture layer and a dispersion medium can be improved.

[0110] <2.3.2. Conductive materials> The conductive material is used to promote electrical contact between the negative electrode active materials. Examples of conductive materials include conductive carbon materials such as carbon black (e.g., acetylene black, Ketjen Black (registered trademark), furnace black, etc.), graphite, carbon fiber, and carbon flakes; as well as fibers and foils of various metals. Among these, the conductive material preferably contains carbon fiber, more preferably contains ultrashort carbon fibers such as carbon nanotubes and vapor-grown carbon fibers, even more preferably contains carbon nanotubes, and particularly preferably contains single-walled carbon nanotubes. These materials may be used alone or in combination of two or more.

[0111] The content of the conductive material in the negative electrode mixture layer is preferably 0.01 wt % or more, and more preferably 0.05 wt % or more. When the content of the conductive material is equal to or more than the lower limit, electrical contact between the negative electrode active materials can be further promoted.

[0112] <2.4. Other matters related to the negative electrode mixture layer> There is no limitation on the thickness of the negative electrode mixture layer, and it can be appropriately selected depending on the application of the sodium ion battery, etc.

[0113] <3. Current Collector> The current collector material is preferably a material that is electrically conductive and electrochemically durable. Specific examples of materials that can be used for the current collector include metals, carbon, and conductive polymers, with metals being preferred. Examples of metals include copper, aluminum, platinum, nickel, tantalum, titanium, stainless steel, and alloys thereof. Among these, copper, aluminum, and aluminum alloys are preferred in terms of conductivity and voltage resistance. When high voltage resistance is required, high-purity aluminum as disclosed in JP-A-2001-176757 is preferably used. Of these, copper is preferred as the current collector material. These materials may be used alone or in combination of two or more.

[0114] The current collector generally has a film or sheet shape. The thickness of the current collector may be appropriately selected depending on the intended use, and is preferably 1 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more, and is preferably 200 μm or less, more preferably 100 μm or less, and even more preferably 50 μm or less.

[0115] <4. Optional negative electrode configuration> The negative electrode according to this embodiment only needs to include a current collector and a negative electrode composite layer, and may include any other components as needed. An example of such a component is a conductive adhesive layer provided between the current collector and the negative electrode composite layer. The conductive adhesive layer in the negative electrode can improve adhesion between the current collector and the negative electrode composite layer.

[0116] The conductive adhesive layer typically contains a conductive material and a binder. The conductive material is not limited, and a material known as a conductive material used in sodium ion batteries can be used. The conductive material can be appropriately selected from, for example, the materials described above as conductive materials used in the negative electrode composite layer, but a carbon material is preferred. The binder is also not limited, and a material known as a binder used in sodium ion batteries can be used, but from the viewpoint of improving adhesion with the negative electrode composite layer, it is preferable to use the same binder as the binder contained in the negative electrode composite layer described above. A conductive adhesive layer containing carbon is also called a carbon undercoat.

[0117] In the conductive adhesive layer, the amount of binder per 100 parts by weight of conductive material can usually be adjusted within the range of 0.1 parts by weight to 50 parts by weight. When the amount of binder is equal to or greater than the lower limit, the adhesion between the current collector and the negative electrode mixture layer can be improved, and when the amount of binder is equal to or less than the upper limit, the resistance of the negative electrode can be reduced.

[0118] The conductive adhesive layer may optionally contain carboxymethyl cellulose and a surfactant.

[0119] The conductive adhesive layer can be obtained, for example, by applying a conductive adhesive composition to a current collector, which is a mixture of a conductive material, a binder, and optionally carboxymethyl cellulose and a surfactant, in a solvent, and then drying the mixture. The thickness of the conductive adhesive layer can be adjusted appropriately depending on the intended use of the sodium ion battery. The conductive adhesive layer can also be, for example, the conductive adhesive layer described in JP 2010-108971 A.

[0120] <5. Negative electrode manufacturing method> A method for producing the negative electrode according to this embodiment can generally include a slurry composition preparation step of preparing a slurry composition containing a negative electrode active material, a binder, and a dispersion medium, a coating step of applying the slurry composition to the surface of a current collector to form a coating film, and a drying step of drying the coating film to obtain a negative electrode composite layer.

[0121] <5.1. Slurry composition preparation step> The slurry composition preparation step is a step of preparing a slurry composition containing a negative electrode active material, a binder, and a dispersion medium. The slurry composition preparation step results in a slurry composition. This slurry composition typically contains a solid component containing a negative electrode active material capable of inserting and desorbing sodium ions and a binder containing a first polymer, and a dispersion medium, where the first polymer contains acidic group-containing monomer units in a proportion of 1% by weight or more and 25% by weight or less. The slurry composition may also optionally contain at least one of the water-soluble polymer and the conductive material described above as a solid component other than the above-described solid components.

[0122] The solid components used in the slurry composition are as described above. Generally, in the slurry composition, the negative electrode active material and the first polymer are dispersed in the form of particles. The dispersion medium used in the slurry composition is usually water or a mixture of water and a water-soluble organic solvent, and water is preferred.

[0123] The slurry composition can usually be obtained by mixing the above-mentioned solid components with a dispersion medium and dispersing the solid components in the dispersion medium. There is no limitation on the method for mixing the solid components and the dispersion medium, and they can be mixed by a known method. Specific mixing methods include methods using mixers such as a ball mill, a sand mill, a bead mill, a pigment disperser, a crusher, an ultrasonic disperser, a homogenizer, a planetary mixer, and a Filmix.

[0124] The solid content of the slurry composition can be set to a concentration that allows each component to be uniformly dispersed, for example, 30% by weight or more and 90% by weight or less.

[0125] <5.2. Coating process> The coating step is a step of applying the slurry composition to the surface of the current collector to form a coating film.

[0126] The method for applying the slurry composition to the surface of the current collector is not particularly limited, and any known method can be used, such as a doctor blade method, a dipping method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, or a brush coating method.

[0127] The amount of the negative electrode mixture layer per unit area is sometimes referred to as the “basis weight.” The amount of the slurry composition applied in the application step can usually be adjusted appropriately depending on the basis weight of the negative electrode mixture layer. There is no limit to the weight of the negative electrode mixture layer, but in one example, it is usually 1 mg / cm 2 or more, preferably 2 mg / cm 2 More preferably, 5 mg / cm 2 or more, usually 100 mg / cm 2 Less than 50 mg / cm 2 Less than 30 mg / cm, more preferably 30 mg / cm 2 The following is the result.

[0128] <5.3. Drying process> The drying step is a step of drying the coating film obtained in the coating step to obtain a negative electrode mixture layer.

[0129] The method for drying the coating film is not particularly limited and any known method can be used, such as drying with warm air, hot air or low humidity air, vacuum drying, or drying by irradiation with infrared rays or electron beams.

[0130] After the drying step, the electrode mixture layer may be subjected to pressure treatment using a mold press, a roll press, etc. Pressure treatment can improve the peel strength of the electrode.

[0131] 6. Sodium-ion batteries A sodium ion battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte. The sodium ion battery includes the above-described negative electrode. The sodium ion battery may be a primary battery or a secondary battery, but is typically a secondary battery.

[0132] According to the sodium ion battery according to this embodiment, since it has the negative electrode described above, it can be a sodium ion battery with good cycle characteristics.

[0133] <6.1. Negative electrode> The sodium ion battery according to this embodiment includes the negative electrode according to the present invention.

[0134] <6.2. Positive electrode> The positive electrode generally includes a current collector and a positive electrode composite material layer. The material of the current collector used for the positive electrode can be appropriately selected from the materials described as the materials of the current collector of the negative electrode. Among them, aluminum and aluminum alloys are preferable, and aluminum is more preferable.

[0135] The positive electrode composite material layer generally contains at least a positive electrode active material. The positive electrode active material is not limited as long as it can be used in a sodium ion battery, and examples thereof include a layered active material, a spinel-type active material, an oxoacid salt active material, etc. Specific examples of the positive electrode active material include NaFeO2, NaNiO2, NaCoO2, NaCrO2, NaMnO2, NaVO2, Na(Ni X Mn 1-X )O2(0 < X < 1), Na(Fe X Mn 1-X )O2(0 < X < 1), NaVPO4F, Na2FePO4F, Na3V2(PO4)3, etc. Further, metallic sodium can be used as the positive electrode active material.

[0136] In addition, as the positive electrode active material, for example, a Prussian blue analog represented by the general formula B: A x M 2 c [M 3 (CN)6] y ·zH2O (in the general formula B, A is at least one of Li, Na, K, Ca, Mg, Zn, and Al, M 2 and M 3 are at least one of Fe, Co, Ni, Cu, Zn, Ti, V, Cr, and Mn, 1 < x ≦ 2, 0 < y ≦ 1, 0 < c ≦ 1, 0 < z ≦ 16) can be used.

[0137] The positive electrode mixture layer contains at least a positive electrode active material and may contain a conductive additive and a binder as necessary. Examples of binders that may be contained in the positive electrode mixture layer include those described as binders that may be contained in the negative electrode mixture layer, as well as known binders such as fluorine-based binders such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE), rubber-based binders such as styrene butadiene rubber (SBR), olefin-based binders such as polypropylene (PP) and polyethylene (PE), and cellulose-based binders such as carboxymethyl cellulose (CMC). The conductive additive may be the same material as the conductive additive that may be contained in the negative electrode mixture layer. The content of the binder in the positive electrode mixture layer and the content of the conductive additive in the positive electrode mixture layer may be the same as the content of the binder in the negative electrode mixture layer and the content of the conductive additive in the negative electrode mixture layer described above.

[0138] <6.3. Electrolyte> The electrolyte typically contains an electrolyte and a solvent. Examples of electrolytes that can be used in sodium-ion batteries include inorganic sodium salts such as NaPF, NaBF, NaClO, and NaAsF; and organic sodium salts such as NaCF, NaN(CF, SO), NaN(CF, SO), NaN(F, SO), NaN(F, SO), and NaC(CF, SO).

[0139] The solvent used in the electrolytic solution is usually a non-aqueous solvent, which may be any non-aqueous solvent capable of dissolving the electrolytic chamber, and examples thereof include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, and fluoroethylene carbonate, chain carbonates such as diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate, cyclic esters such as γ-butyrolactone and γ-valerolactone, chain esters such as methyl acetate and methyl propionate, cyclic ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, and dioxane, chain ethers such as dimethoxyethane and diethyl ether, and sulfone compounds and sulfoxide compounds such as dimethyl sulfoxide and sulfolane.

[0140] There is no limitation on the concentration of the electrolyte in the electrolytic solution, but it can usually be adjusted in the range of 0.5 mol / L or more and 3 mol / L or less.

[0141] <6.4. Separator> As the separator, for example, those described in JP 2012-204303 A can be used. Among these, a microporous film made of a polyolefin resin (polyethylene, polypropylene, polybutene, polyvinyl chloride) is preferred because it allows the thickness of the entire separator to be thin, thereby increasing the ratio of electrode active material in the lithium ion secondary battery and increasing the capacity per volume.

[0142] 7. Sodium-ion battery manufacturing method The sodium ion battery according to this embodiment can be manufactured by, for example, stacking a positive electrode and a negative electrode with a separator interposed therebetween, rolling or folding the stack as necessary according to the battery shape, placing the stack in a battery container, injecting an electrolyte into the battery container, and sealing the container. To prevent internal pressure buildup and overcharging / discharging of the sodium ion battery, a fuse, an overcurrent protection element such as a PTC element, an expanded metal, a lead plate, or the like may be provided as necessary. The shape of the secondary battery may be, for example, a coin type, a button type, a sheet type, a cylindrical type, a rectangular type, a flat type, or the like. [Example]

[0143] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the examples shown below, and can be implemented with any modifications within the scope of the claims of the present invention and their equivalents.

[0144] In the following description, the units "%" and "parts" that represent amounts are by weight unless otherwise specified. Furthermore, the operations described below were carried out in air at room temperature and normal pressure (23°C, 1 atmosphere) unless otherwise specified.

[0145] [Evaluation method] <Volume average particle size of binder> The volume-average particle diameter of the binder prepared in the examples was measured by laser diffraction. Specifically, an aqueous dispersion containing the prepared binder (adjusted to a solids concentration of 0.1 wt%) was used as a sample. The particle size distribution (volume basis) measured using a laser diffraction particle size distribution analyzer (manufactured by Beckman Coulter, Inc., "LS-13320") was used. The particle diameter D50 at which the cumulative volume calculated from the smallest diameter side reached 50% was defined as the volume-average particle diameter.

[0146] <Adhesion> The negative electrode was cut into a rectangle measuring 1.0 cm wide x 10 cm long to prepare a test specimen. Cellophane tape (as specified in JIS Z1522) was attached to the surface of the negative electrode composite layer of this test specimen, and the stress was measured when the cellophane tape was peeled off from one end of the specimen in a 90° direction at a rate of 50 mm / min. The measurement was performed three times, and the average value was calculated as the peel strength (N / m) and evaluated according to the following criteria. A higher peel strength indicates better adhesion of the negative electrode composite layer and stronger adhesion to the current collector. A: Peel strength is 6N / m or more B: Peel strength is 5N / m or more and less than 6N / m C: Peel strength is 4N / m or more and less than 5N / m D: Peel strength is less than 4N / m

[0147] <Battery evaluation> (Initialization) The batteries (2032-type coin cells) obtained in the examples and comparative examples were initialized in a thermostatic chamber maintained at 25°C ± 1.0°C. Specifically, CC-CV charging (cell voltage 0.010 V) was performed at a constant current of 20 mA / g (based on the weight of the negative electrode active material). After the cell voltage reached 0.010 V, charging was stopped when the current value decayed to 2 mA / g. Next, CC discharging was performed to 2.00 V at a constant current of 20 mA / g. This charge / discharge at 20 mA / g was repeated three times to initialize the batteries.

[0148] (Cycle characteristics) To evaluate the cycle characteristics of the battery, charge and discharge were repeated 50 times at a current value of 20 mA / g and a cell voltage in the range of 0.010 V to 2.00 V. The discharge capacity of the first cycle was defined as X1, and the discharge capacity of the 50th cycle as X2. The discharge capacities X1 and X2 were used to calculate the capacity retention rate, expressed as ΔC = (X2 / X1) × 100 (%), and evaluated according to the following criteria. The larger the value of this capacity retention rate ΔC, the better the cycle characteristics of the fabricated battery. A: Capacity retention rate is 90% or more B: Capacity retention rate is 85% or more but less than 90% C: Capacity retention rate is less than 85%

[0149] (Internal resistance increase rate) The internal resistance of the battery was determined under the same charging conditions as described above. The discharge capacity Y1 at a CC discharge current of 20 mA / g was calculated, and the discharge capacity Y2 at a CC discharge current of 100 mA / g was calculated. The internal resistance was measured before and after the cycle performance evaluation described above. The internal resistance increase rate, ΔR=(R2 / R1)×100(%), was calculated using the initial internal resistance R1 and the internal resistance R2 after 50 cycles, and was evaluated according to the following criteria. The smaller the capacity retention rate ΔR, the better the cycle performance of the battery. A: Internal resistance increase rate is less than 10% B: Internal resistance increase rate is 10% or more but less than 20% C: Internal resistance increase rate is 20% or more

[0150] [Example 1] <Production of Particulate Polymer (First Polymer)> A 1 L flask (reaction vessel) equipped with a stirrer and a septum was charged with 90 parts of ion-exchanged water and 0.1 parts of sodium lauryl sulfate as an emulsifier, the gas phase was replaced with nitrogen gas, and the temperature was raised to 70°C. After that, 0.3 parts of potassium persulfate (KPS) as a polymerization initiator was dissolved in 10.0 parts of ion-exchanged water and added. Meanwhile, in a separate container (emulsion container), 80 parts of ion-exchanged water, 0.5 parts of sodium lauryl sulfate as an emulsifier, 2.0 parts of itaconic acid as an acidic group-containing monomer, 75 parts of 2-ethylhexyl acrylate as a (meth)acrylic acid ester monomer, 22 parts of styrene as an aromatic vinyl monomer, and 1.0 part of 2-hydroxyethyl acrylate as a hydroxyl group-containing monomer were mixed to obtain a monomer composition. This monomer composition was continuously added to the septum-equipped 1 L flask over a period of 3 hours to allow polymerization. The reaction was carried out at 70°C during the addition. After completion of the addition, the mixture was further stirred at 80°C for 2 hours to obtain a binder composition. A 5% aqueous solution of sodium hydroxide was added to the aqueous dispersion containing the polymer thus obtained, and the pH was adjusted to 8, to obtain an aqueous dispersion of a particulate polymer.

[0151] <Formation of the negative electrode> A slurry composition for a negative electrode was prepared by stirring 97 parts of hard carbon (volume average particle diameter: 8 μm) as a negative electrode active material, 2 parts of a particulate polymer as a binder, 1 part of carboxymethyl cellulose as a thickener, and an appropriate amount of water in a planetary mixer. This negative electrode slurry composition was coated on a 15 μm thick copper foil current collector using a comma coater so that the weight after drying was 5 mg / cm 2 The copper foil was then coated and dried so that the density of the negative electrode composite layer became 1.0 g / cm. This drying was performed by conveying the copper foil at a speed of 0.5 m / min through an oven at 60°C for 2 minutes. Thereafter, the copper foil was heat-treated at 120°C for 2 minutes to obtain a negative electrode blank. The negative electrode blank was then rolled using a roll press to obtain a negative electrode composite layer having a density of 1.0 g / cm. 3 A negative electrode of 1000 .mu.m was obtained.

[0152] <Making a sodium-ion battery> A 2032-type coin cell was fabricated using the obtained negative electrode, a separator (manufactured by Celgard, product name "Celgard 2500"), a metallic sodium counter electrode pressed onto an aluminum current collector, and an electrolyte (solvent: ethylene carbonate / diethyl carbonate = 50 / 50 (volume ratio), electrolyte: NaPF6 with a concentration of 1M).

[0153] [Example 2] A particulate polymer was produced, and a negative electrode and a battery were fabricated in the same manner as in Example 1, except that the composition of the particulate polymer was changed to 2.0 parts of itaconic acid as an acidic group-containing monomer, 80 parts of butyl acrylate as a (meth)acrylic acid ester monomer, 17 parts of styrene as an aromatic vinyl monomer, and 1.0 part of 2-hydroxyethyl acrylate as a hydroxyl group-containing monomer.

[0154] [Example 3] A negative electrode and a sodium ion battery were produced in the same manner as in Example 1, except that a particulate polymer was produced by the following production method.

[0155] First, in the first-stage polymerization, a reactor equipped with a stirrer was charged with 76.5 parts of butyl acrylate as a (meth)acrylic acid ester monomer, 1.0 parts of methacrylic acid as an ethylenically unsaturated carboxylic acid monomer, 1.5 parts of N-methylolacrylamide as an unsaturated carboxylic acid amide monomer, 1.5 parts of allyl glycidyl ether as an unsaturated carboxylic acid epoxy monomer, 1.5 parts of acrylonitrile as a nitrile group-containing monomer, 0.3 parts of sodium dodecylbenzenesulfonate as an emulsifier, 0.5 parts of ammonium persulfate as a polymerization initiator, and 300 parts of ion-exchanged water. After thorough stirring, the mixture was heated to 70 ° C and the reaction was allowed to proceed for 4 hours. Next, in the second-stage polymerization, 15.0 parts of styrene as an aromatic vinyl monomer was added to the polymerization system within 1 hour of addition. After completion of the addition, the mixture was heated to 80 ° C and the reaction was allowed to proceed for 2 hours. Then, in the third-stage polymerization, 3.0 parts of methacrylic acid was added as an acidic functional group-containing monomer (ethylenically unsaturated carboxylic acid monomer), and the reaction was continued for another 4 hours. The aqueous dispersion containing the resulting polymerization product was cooled to 30°C or below. As a result, an aqueous dispersion (binder composition for negative electrodes) was obtained, which contained water and a particulate binder having a structure in which the surface layer of core-shell particles having a core portion containing a polymer formed in the first-stage polymerization and a shell portion containing a polymer formed in the second-stage polymerization contained a polymer formed in the third-stage polymerization.

[0156] [Example 4] The binder composition containing the particulate binder was prepared in the same manner as in Example 3, except that the amount of butyl acrylate used as the (meth)acrylic acid ester monomer in the first polymerization stage was changed from 76.5 parts to 59.5 parts, and the amount of methacrylic acid used as the ethylenically unsaturated carboxylic acid monomer in the third polymerization stage was changed from 3.0 parts to 20.0 parts.

[0157] [Example 5] A negative electrode and a sodium ion battery were produced in the same manner as in Example 1, except that a particulate polymer was produced by the following production method.

[0158] A 5 MPa pressure vessel equipped with a stirrer was charged with 62 parts of styrene, 33 parts of 1,3-butadiene, 4 parts of itaconic acid, 1 part of 2-hydroxyethyl acrylate, 0.3 parts of t-dodecyl mercaptan as a molecular weight modifier, 5 parts of sodium dodecylbenzenesulfonate as an emulsifier, 150 parts of ion-exchanged water, and 1 part of potassium persulfate as a polymerization initiator. After thorough stirring, the mixture was heated to 55°C to initiate polymerization. The reaction was stopped by cooling when the monomer consumption reached 95.0%. A 5% aqueous solution of sodium hydroxide was added to the resulting aqueous dispersion containing the polymer to adjust the pH to 8. Unreacted monomer was then removed by heated vacuum distillation. The mixture was then cooled to a temperature of 30°C or below to obtain an aqueous dispersion containing a particulate polymer (anode binder composition).

[0159] [Comparative Example 1] The procedure of Example 1 was repeated except that the particulate polymer was not used as the binder when forming the negative electrode, and 2 parts of the water-soluble polymer sodium polyacrylate was used instead. Sodium polyacrylate can also function as a thickener together with carboxymethyl cellulose.

[0160] Table 1 shows the compositions of the negative electrodes of Examples 1 to 5 and Comparative Example 1, and Table 2 shows the evaluation results. The abbreviations in Table 1 have the following meanings. "IA": Itaconic acid "MAA": methacrylic acid "2-EHA": 2-ethylhexyl acrylate "BA": Butyl acrylate "ST": Styrene "BD": 1,3-butadiene "β-HEA": 2-hydroxyethyl acrylate "NMA": N-methylolacrylamide "AGE": Allyl glycidyl ether "AN": acrylonitrile "HC": Hard carbon "CMC": Carboxymethyl cellulose "AA": sodium polyacrylate In Table 1, "AA (100%) & CMC" for Comparative Example 1 indicates that the content of sodium polyacrylate in the binder of the negative electrode was 100% by weight, and further, carboxymethyl cellulose similar to that in Example 1 was used as the water-soluble polymer.

[0161] [Table 1]

[0162] [Table 2]

[0163] As in Examples 1 to 5, it was confirmed that a negative electrode using a particulate polymer containing a predetermined amount of acidic group-containing monomer units as a binder can improve the cycle characteristics of a sodium ion battery. It was also confirmed that this negative electrode can reduce the rate of increase in internal resistance and improve the adhesion (peel strength) between the current collector and the negative electrode composite layer. On the other hand, as shown in Comparative Example 1, a negative electrode using sodium polyacrylate as a binder could not sufficiently improve the cycle characteristics, rate of increase in internal resistance, or adhesion (peel strength) between the current collector and the negative electrode composite layer of a sodium ion battery.

Claims

1. a current collector and a negative electrode mixture layer, a negative electrode, wherein the negative electrode mixture layer includes a negative electrode active material capable of inserting and desorbing sodium ions and a binder containing a first polymer, The negative electrode, wherein the first polymer contains an acidic group-containing monomer unit in a proportion of 1% by weight or more and 25% by weight or less.

2. The negative electrode of claim 1 , wherein the first polymer further comprises a (meth)acrylic acid ester monomer unit.

3. The negative electrode of claim 1 , wherein the first polymer further comprises an aromatic vinyl monomer unit.

4. 10. The negative electrode of claim 1, wherein the first polymer further comprises an aliphatic conjugated diene monomer unit.

5. 2. The negative electrode according to claim 1, wherein the glass transition temperature of the first polymer is −40° C. or higher and 50° C. or lower.

6. 2. The negative electrode according to claim 1, wherein the content of the binder in the negative electrode mixture layer is less than 5% by weight.

7. A sodium ion battery comprising a positive electrode, a negative electrode, and an electrolyte, A sodium ion battery, wherein the negative electrode is the negative electrode according to any one of claims 1 to 6.

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