Binder for nonaqueous secondary battery electrode and slurry for nonaqueous secondary battery electrode

Abstract

The present application provides an adhesive for a non-aqueous secondary battery electrode, which can greatly improve the peeling strength of an electrode active material layer to a current collector while suppressing the occurrence of cracks in the electrode active material layer formed on the current collector. An adhesive for a non-aqueous secondary battery electrode, which contains a copolymer (P) having a structural unit (a) derived from a monomer (A) represented by formula (1), a structural unit (b) derived from at least one monomer (B) selected from the group consisting of (meth)acrylic acid and a salt thereof, and a structural unit (c) derived from a monomer (C) of an olefinically unsaturated carboxylic acid ester compound which is an aromatic alcohol.

Description

TECHNICAL FIELD

[0001] The present application relates to a binder for a nonaqueous secondary battery electrode and a slurry for a nonaqueous secondary battery electrode.

[0002] This application claims priority based on Japanese Patent Application No. 2020-098514 filed on June 5, 2020, the contents of which are incorporated herein. BACKGROUND

[0003] Secondary batteries using a nonaqueous electrolyte (nonaqueous secondary batteries) are more superior to secondary batteries using an aqueous electrolyte in terms of high voltage, miniaturization, and light weight. Therefore, nonaqueous secondary batteries are widely used as power sources for notebook computers, mobile phones, power tools, and electronic / communication machines. In addition, in recent years, nonaqueous secondary batteries are also used in electric vehicles or hybrid vehicles from the viewpoint of environmental vehicles, but high output, high capacity, long life, and the like have been strongly required. As a representative example of a nonaqueous secondary battery, a lithium ion secondary battery can be cited.

[0004] A nonaqueous secondary battery has a positive electrode having a metal oxide or the like as an active material, a negative electrode having a carbon material such as graphite as an active material, and a nonaqueous electrolyte solvent having a carbonate or a flame-retardant ionic liquid as a center. The nonaqueous secondary battery is a secondary battery that performs charge and discharge of the battery by moving ions between the positive electrode and the negative electrode. Specifically, the positive electrode is obtained by coating a slurry composed of a metal oxide and a binder on the surface of a positive electrode current collector such as an aluminum foil, drying it, and then cutting it to an appropriate size. The negative electrode is obtained by coating a slurry composed of a carbon material and a binder on the surface of a negative electrode current collector such as a copper foil, drying it, and then cutting it to an appropriate size. The binder has a function of bonding the active materials to each other and the active material to the current collector in the positive electrode and the negative electrode, and preventing the active material from peeling off from the current collector.

[0005] As a binder, a polyvinylidene fluoride (PVDF)-based binder using an organic solvent-based N-methyl-2-pyrrolidone (NMP) as a solvent is widely known. However, this binder has low adhesion between the active materials and between the active material and the current collector, and a large amount of the binder is necessarily used in actual use. Therefore, there is a disadvantage that the capacity of the nonaqueous secondary battery is reduced. In addition, since an expensive organic solvent NMP is used in the binder, it is difficult to suppress the manufacturing cost.

[0006] As a method for solving these problems, development of a water dispersion-based binder has been conducted. As a water dispersion-based binder, for example, a styrene-butadiene rubber (SBR)-based water dispersion using carboxymethyl cellulose (CMC) as a tackifier is known.

[0007] In Patent Literature 1, an adhesive composition for an adherent containing a sodium acrylate-N-vinylacetamide copolymer is disclosed. Further, in Patent Literature 2, a composition for a hydrogel-containing body containing a sodium acrylate-N-vinylacetamide (55 / 45 (molar ratio)) copolymer is disclosed.

[0008] In Patent Literature 3, an adhesive for a non-aqueous battery electrode containing a sodium acrylate-N-vinylacetamide copolymer (copolymerization ratio: N-vinylacetamide / sodium acrylate = 10 / 90 mass ratio) is disclosed.

[0009] In Patent Literature 4, a copolymer for an adhesive for a non-aqueous battery electrode containing a structural unit derived from N-vinylacetamide, a structural unit derived from sodium (meth)acrylate, and a structural unit derived from methoxypolyethylene glycol methacrylate is described.

[0010] Prior Art Documents

[0011] [Patent Literature]

[0012] [Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 2005-336166

[0013] [Patent Literature 2] Japanese Patent Application Laid-Open (JP-A) No. 2006-321792

[0014] [Patent Literature 3] International Publication No. 2017 / 150200

[0015] [Patent Literature 4] International Publication No. 2020 / 017442 SUMMARY

[0016] However, the SBR-based adhesive described in Patent Literature 1 necessarily uses a tackifier, carboxymethyl cellulose, and the slurry production step is complicated. Further, the adhesion of the active material to each other and the adhesion of the active material to the current collector are not sufficient in the adhesive, and when an electrode is produced with a small amount of the adhesive, there is a problem that a part of the active material is peeled off in the step of cutting the current collector.

[0017] The sodium acrylate-N-vinylacetamide copolymer disclosed in Patent Literatures 1 and 2 contains a large amount of a component derived from N-vinylacetamide. When such a polymer is mixed with a negative electrode active material and water to be used as a slurry for an electrode, agglomerates are easily generated in the slurry.

[0018] The adhesive for a non-aqueous battery electrode disclosed in Patent Literature 3 has a problem that cracks are easily generated for an electrode having a thick film thickness, that is, a large weight per unit area, as shown in Comparative Example 1 described later.

[0019] The adhesive for a non-aqueous battery electrode described in Patent Literature 4 has room for improvement in the peeling strength of the electrode active material layer from the current collector, as shown in Comparative Example 2 described later.

[0020] Therefore, the object of the present invention is to provide an adhesive for non-aqueous secondary battery electrodes and a slurry for non-aqueous secondary battery electrodes that can significantly improve the peel strength of the electrode active material layer to the current collector while suppressing the occurrence of cracking of the electrode active material layer formed on the current collector.

[0021] Furthermore, the purpose of this invention is to provide a non-aqueous secondary battery electrode with less cracking and high peel strength of the electrode active material layer to the current collector.

[0022] Furthermore, the object of the present invention is to provide a non-aqueous secondary battery with an electrode having fewer cracks and a high peel strength between the electrode active material layer and the current collector.

[0023] [Methods used to solve problems]

[0024] In order to solve the above problems, the present invention is as described below [1] to

[14] .

[0025] [1] An adhesive for a non-aqueous secondary battery electrode, comprising a copolymer (P) having a structural unit (a) derived from a monomer (A) represented by formula (1), a structural unit (b) derived from at least one monomer (B) selected from (meth)acrylic acid and its salts, and a structural unit (c) derived from a monomer (C) of an olefinic unsaturated carboxylic acid ester compound as an aromatic alcohol, characterized in that,

[0026] The content of each structural unit in the aforementioned copolymer (P) is shown below;

[0027] The content of structural unit (a) is more than 0.5% by mass and less than 20.0% by mass.

[0028] The content of structural unit (b) is 50.0% by mass or more and 98.0% by mass or less.

[0029] The content of structural unit (c) is between 0.3% and 28.0% by mass.

[0030] The combined content of structural unit (a), structural unit (b) and structural unit (c) is 85% by mass or more;

[0031]

[0032] (where R is in the formula) 1 R 2 Each is independently an alkyl group having 1 to 5 hydrogen atoms or carbon atoms.

[0033] [2] The binder for a nonaqueous secondary battery electrode according to [1], which contains a structural unit (d) derived from a monomer (D) represented by formula (2) and has a content of the structural unit (d) of 0.3 mass% or more and 18.0 mass% or less.

[0034]

[0035] (In the formula, R 3 , R 4 , R 6 each independently is a hydrogen atom or an alkyl group having 1 to 5 carbon atoms; R 5 is an alkyl group having 1 to 6 carbon atoms and is more carbon atoms than R 4 ; n is an integer of 1 or more, m is an integer of 0 or more, and n + m ≥ 20).

[0036] [3] The binder for a nonaqueous secondary battery electrode according to [2], wherein, in the aforementioned formula (2), n + m ≤ 500.

[0037] [4] The binder for a nonaqueous secondary battery electrode according to [2] or [3], wherein, in the aforementioned formula (2), n + m ≥ 30.

[0038] [5] The binder for a nonaqueous secondary battery electrode according to any one of [1] to [4], wherein the aforementioned monomer (A) is an N-vinylformamide or an N-vinylacetamide.

[0039] [6] The binder for a nonaqueous secondary battery electrode according to any one of [1] to [5], wherein the aforementioned monomer (B) is a (meth)acrylic acid salt.

[0040] [7] The binder for a nonaqueous secondary battery electrode according to any one of [1] to [6], wherein the aforementioned monomer (C) is a (meth)acrylate ester containing an aromatic alcohol.

[0041] [8] The binder for a nonaqueous secondary battery electrode according to any one of [1] to [7], wherein the weight average molecular weight of the aforementioned copolymer (P) is 1,000,000 or more and 10,000,000 or less.

[0042] [9] The binder for a nonaqueous secondary battery electrode according to any one of [1] to [8], wherein the content of the structural unit (b) derived from the aforementioned monomer (B) in the aforementioned copolymer (P) is 60.0 mass% or more and 90.0 mass% or less.

[0043]

[10] A binder composition for a nonaqueous secondary battery electrode, which contains the binder for a nonaqueous secondary battery electrode according to any one of [1] to [9] and an aqueous medium.

[0044]

[11] The binder composition for a nonaqueous secondary battery electrode according to

[10] , wherein the nonaqueous secondary battery is a lithium ion secondary battery.

[0045]

[12] A slurry for a nonaqueous secondary battery electrode, comprising

[0046] the binder for a nonaqueous secondary battery electrode according to any one of [1] to [9],

[0047] an electrode active material, and

[0048] an aqueous medium.

[0049]

[13] A nonaqueous secondary battery electrode having

[0050] a current collector, and

[0051] an electrode active material layer containing the binder for a nonaqueous secondary battery electrode according to any one of [1] to [9] and an electrode active material formed on the surface of the current collector.

[0052]

[14] A lithium ion secondary battery comprising the electrode according to

[13] .

[0053] Effects of the Invention

[0054] According to the present application, it is possible to provide a binder for a nonaqueous secondary battery electrode and a slurry for a nonaqueous secondary battery electrode, which can greatly improve the peeling strength of an electrode active material layer from a current collector while suppressing the generation of cracks in the electrode active material layer formed on the current collector.

[0055] Further, according to the present application, it is possible to provide a nonaqueous secondary battery electrode having few cracks and high peeling strength of an electrode active material layer from a current collector.

[0056] Further, according to the present application, it is possible to provide a nonaqueous secondary battery having an electrode with few cracks and high peeling strength of an electrode active material layer from a current collector. DETAILED DESCRIPTION

[0057] Hereinafter, an embodiment of the present application will be described in detail. In the present embodiment, the battery is a secondary battery in which ion movement occurs between a positive electrode and a negative electrode during charge and discharge. The positive electrode comprises a positive electrode active material, and the negative electrode comprises a negative electrode active material. These electrode active materials are materials that allow ions to be intercalated and deintercalated. As a preferred example of such a secondary battery, a lithium ion secondary battery can be given.

[0058] "(Meth)acrylic acid" refers to either or both of methacrylic acid and acrylic acid. "(Meth)acrylic monomer" refers to either or both of a methacrylic monomer and an acrylic monomer. "(Meth)acrylate" refers to either or both of a methacrylate and an acrylate.

[0059] "Weight average molecular weight" is a value calculated using gel permeation chromatography (GPC) in terms of pullulan.

[0060] <1. Binder for electrode of non-aqueous secondary battery>

[0061] The binder for electrode of non-aqueous secondary battery (or non-aqueous secondary battery electrode binder. Hereinafter, sometimes referred to as "electrode binder") of the present embodiment contains the copolymer (P) described below. The electrode binder can also contain other components, for example, a polymer other than the copolymer (P), a surfactant, and the like.

[0062] Here, the electrode binder contains components that do not volatilize and remain in the step with heating in the manufacturing step of the battery described later. Specifically, the components constituting the electrode binder are the components remaining after 1 g of a mixture containing the electrode binder is weighed in an aluminum dish of 5 cm in diameter, dried at 110°C for 5 hours in a desiccator with air circulation at atmospheric pressure.

[0063] The content ratio of the copolymer (P) in the electrode binder is preferably 80% by mass or more, more preferably 90% by mass or more, further preferably 95% by mass or more, and still further preferably 98% by mass or more. This is because the copolymer (P) contributes greatly to the effects of the present application as the object.

[0064] The copolymer (P) contains a structural unit (a) derived from the monomer (A) represented by the formula (1) described later, a structural unit (b) derived from at least one monomer (B) selected from the group consisting of (meth)acrylic acid and a salt thereof, and a structural unit (c) derived from the monomer (C) which is an olefinically unsaturated carboxylic acid ester compound as an aromatic alcohol. The copolymer (P) can also contain a structural unit (d) derived from the monomer (D) represented by the formula (2) described later. The copolymer (P) can also contain a structural unit (e) derived from other monomer (E) that can be copolymerized with the monomer (A), the monomer (B), and the monomer (C), and does not correspond to any one of the monomer (A), the monomer (B), the monomer (C), and the monomer (D).

[0065] The weight average molecular weight of the copolymer (P) is preferably 1 million or more, more preferably 1.5 million or more, and further preferably 2 million or more. The weight average molecular weight of the copolymer (P) is preferably 10 million or less, more preferably 7.5 million or less, and further preferably 5 million or less.

[0066] <1-1. Monomer (A) >

[0067] The monomer (A) is a compound represented by the following formula (1). The monomer (A) can also contain a plurality of compounds represented by the formula (1).

[0068]

[0069] (In the formula (1), R 1 , R 2 each independently is a hydrogen atom or an alkyl group having 1 to 5 carbon atoms.

[0070] In the formula (1), R 1 , R 2 each independently is preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, R 1 , R 2 each independently is more preferably a hydrogen atom or a methyl group.

[0071] More preferable specific examples of the combination of R 1 , R 2 are R 1 : H, R 2 : H (i.e., the monomer (A) is an N-vinyl formamide), or R 1 : H, R 2 : CH3 (i.e., the monomer (A) is an N-vinyl acetamide).

[0072] <1-2. Monomer (B) >

[0073] The monomer (B) contains at least one selected from the group consisting of (meth)acrylic acid and a salt thereof. The (meth)acrylic acid salt preferably contains a salt of (meth)acrylic acid with a monovalent cation; more preferably contains at least one selected from the group consisting of sodium (meth)acrylate, potassium (meth)acrylate, and ammonium (meth)acrylate. Of these, it is particularly more preferable to contain at least one of sodium (meth)acrylate and ammonium (meth)acrylate; and most preferable to contain sodium (meth)acrylate. The (meth)acrylic acid salt is obtained by neutralizing (meth)acrylic acid with a hydroxide, ammonia water, or the like, and it is preferable to use sodium hydroxide from the viewpoint of ease of obtaining.

[0074] In order to adjust the pH, the monomer (B) preferably contains 60% by mass or more of the (meth)acrylic acid salt, more preferably contains 80% by mass or more, and further preferably contains 95% by mass or more.

[0075] Here, when (meth)acrylic acid is used as monomer (B), and a neutralizing agent is used for neutralization after polymerization, the structural units derived from (meth)acrylic acid are considered to form a salt to the extent that the equivalent amount (valence of the cation × number of moles of the cation, the same applies below) of the cation contained in the neutralizing agent is present. If the equivalent amount of the cation contained in the neutralizing agent is greater than the number of moles of (meth)acrylic acid used in polymerization, it is considered that all of the (meth)acrylic acid has formed a salt. On the other hand, if the equivalent amount of the cation contained in the neutralizing agent is less than the number of moles of (meth)acrylic acid used in polymerization, it is considered that all of the cation has formed a salt with (meth)acrylic acid. Furthermore, when the cation contained in the neutralizing agent has a valence of 2 or higher, it is considered that one cation is bonded with a number of structural units derived from (meth)acrylic acid of the same valence.

[0076] <1-3. Monomer (C)>

[0077] The monomer (C) is an olefinic unsaturated carboxylic acid ester compound of an aromatic alcohol. The monomer (C) may contain only one compound or two or more compounds. The monomer (C) preferably contains an aromatic alcohol (meth)acrylate, and more preferably is composed of an aromatic alcohol (meth)acrylate.

[0078] Examples of (meth)acrylates of aromatic alcohols include benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, phenoxyethylene glycol (meth)acrylate, phenoxydiethylene glycol (meth)acrylate, phenoxy polyethylene glycol (meth)acrylate, phenoxy polyethylene glycol (meth)acrylate, ethoxylated o-phenylphenol (meth)acrylate, and 2-hydroxy-3-phenoxypropyl (meth)acrylate. The monomer (C) is more preferably benzyl (meth)acrylate or phenoxyethyl (meth)acrylate among these compounds.

[0079] <1-4. Monomer (D)>

[0080] Monomer (D) is a compound represented by the following formula (2). Monomer (D) may also include multiple compounds represented by formula (2).

[0081]

[0082] (where R is in the formula) 3 R 4 R 6 Each is independently an alkyl group having 1 to 5 hydrogen atoms or carbon atoms. R 5 It is an alkyl group with 1 to 6 carbon atoms, and is more than R 4 There are more carbon atoms. n is an integer greater than or equal to 1, m is an integer greater than or equal to 0, and n + m ≥ 20).

[0083] In formula (2), R 3 , R 4 , R 6 , R 3 , R 4 , R 6 , R 6 , R 4 , R 5 , R 3 , R 4 , R 5 , R 6 , n, and m are each independently selected from the group consisting of a hydrogen atom, a methyl group, and an ethyl group.

[0084] In formula (2), n is an integer of 1 or more, and m is an integer of 0 or more, and n + m > 20. This is because when the copolymer (P) is used as a binder for an electrode active material to produce an electrode, the flexibility of the electrode is improved, and the generation of cracks is suppressed. From this viewpoint, it is preferable that n + m > 30, and more preferably n + m > 40. In addition, it is preferable that n + m < 500, and more preferably n + m < 200, and further preferably n + m < 150. This is because the adhesion of the binder becomes higher.

[0085] Further, in formula (2), the number of structural units containing R 4 is n, and the number of structural units containing R 5 is m, but the arrangement of these structural units is not limited. That is, when m > 1, in formula (2), each of the structural units can have a block structure in which all or a part of the structural units are continuous, can have a periodic regular arrangement structure in which two structural units are alternately arranged, or can have a structure in which two structural units are randomly arranged. The preferable form of the copolymer of formula (2) is a periodic regular arrangement structure or a randomly arranged structure. This is because such a structure suppresses the deviation in the distribution of each structural unit in the molecular chain of formula (2). The more preferable form of the copolymer of formula (2) is a randomly arranged structure. This is because the copolymer can be polymerized by a radical polymerization initiator without using a special catalyst, and the manufacturing cost can be reduced.

[0086] In formula (2), R 3 , R 4 , R 5 , R 6 , n, and m are each independently selected from the group consisting of a hydrogen atom, a methyl group, and an ethyl group.

[0087] Table 1

[0088]

[0089] More preferable in formula (2) is m = 0. As examples of the monomer (D) in which m = 0, monomer (meth)acrylates of polyethylene glycol can be cited, and more specifically, methoxypolyethylene glycol (meth)acrylate (e.g., monomers d1, d2 of Table 1), and the like can be cited. As an example of the methoxypolyethylene glycol methacrylate, VISIOMER (registered trademark) MPEG2005MA W manufactured by EVONIK INDUSTRIES can be cited. In this product, R 3 = CH3, R 4 = H, R 6 = CH3, n = 45, m = 0. As another example of the methoxypolyethylene glycol methacrylate, VISIOMER (registered trademark) MPEG5005MA W manufactured by EVONIK INDUSTRIES can be cited. In this product, R 3 = CH3, R 4 = H, R 6 = CH3, n = 113, m = 0.

[0090] As another example of the monomer (D) in which m = 0, monomer (meth)acrylates of polypropylene glycol can be cited, and more specifically, methoxypolypropylene glycol (meth)acrylate (e.g., monomers d3, d4 of Table 1), and the like can be cited.

[0091] <1-5. Other monomer (E)>

[0092] As the other monomer (E) which does not correspond to any of the monomer (A), the monomer (B), the monomer (C), and the monomer (D), there is no particular limitation, and it is preferable to consist of a hydrophilic ethylenically unsaturated compound, but a hydrophobic ethylenically unsaturated compound can also be contained.

[0093] Hydrophilic ethylenically unsaturated monomers, for example, compounds having at least one polymerizable ethylenically unsaturated bond and having a polar group such as a carboxyl group, a hydroxyl group, an amide bond, a cyano group, etc. can be exemplified. Among these compounds, (meth)acrylic acid and salts thereof are excluded as monomer (B). As ethylenically unsaturated monomers having a carboxyl group, for example, itaconic acid, β-carboxyethyl acrylate, maleic acid, fumaric acid, crotonic acid, half esters of unsaturated dicarboxylic acids, etc. can be exemplified. As ethylenically unsaturated monomers having a hydroxyl group, for example, (meth)acrylic acid-2-hydroxyethyl ester, (meth)acrylic acid-2-hydroxypropyl ester, acrylic acid 4-hydroxybutyl ester, 1,4-cyclohexanedimethanol monacrylate, vinyl alcohol, etc. can be exemplified. The aforementioned vinyl alcohol can also include that obtained by using an ester such as vinyl acetate as a monomer to be polymerized and then subjected to a treatment such as saponification. As ethylenically unsaturated monomers having an amide bond, for example, (meth)acrylamide, N-alkyl(meth)acrylamide, N,N-dialkyl(meth)acrylamide, N-hydroxyalkyl(meth)acrylamide having 1 to 3 carbon atoms in the alkyl group, diacetone(meth)acrylamide, dimethylaminoalkyl(meth)acrylamide having 1 to 5 carbon atoms in the alkyl group other than the dimethylamino moiety, (meth)acrylamide-2-methylpropane sulfonic acid, etc. can be exemplified. As ethylenically unsaturated monomers having a cyano group, for example, (meth)acrylonitrile, etc. can be exemplified.

[0094] <1-6. Content of Structural Units in Copolymer (P)

[0095] The content of each structural unit in the copolymer (P) will be described below. Here, when (meth)acrylic acid is used as a monomer (B) and neutralized with a neutralizing agent after polymerization, the structural unit derived from (meth)acrylic acid is considered to form a salt of the equivalent amount of the cation contained in the neutralizing agent (valence of the cation x the number of moles of the cation, and the same applies hereinafter). Details are described in the item of <1-2. Monomer (B)>.

[0096] The content of the structural unit (a) is 0.5% by mass or more, preferably 1.0% by mass or more, more preferably 3.0% by mass or more, and further preferably 7.0% by mass or more. This is because the dispersibility of the electrode active material, the conductive aid, etc. at the time of electrode slurry production described later is excellent, and an electrode slurry having good coatability can be produced. The content of the structural unit (a) is 20.0% by mass or less, preferably 15.0% by mass or less, and more preferably 12.5% by mass or less. This is because the generation of cracks in the electrode described later is suppressed, and the productivity of the electrode is improved.

[0097] The content ratio of the structural unit (b) ((total amount of (meth)acrylic acid and (meth)acrylic acid salt) is 50.0% by mass or more, preferably 60.0% by mass or more, and more preferably 70.0% by mass or more. This is because the electrode active material layer having high peeling strength to the current collector can be obtained. The content ratio of the structural unit (b) ((total amount of (meth)acrylic acid and (meth)acrylic acid salt) is 98.0% by mass or less, preferably 94.5% by mass or less, more preferably 93.0% by mass or less, and further preferably 90.0% by mass or less. This is because the dispersibility of the solid components such as the electrode active material and the conductive aid at the time of manufacturing the electrode slurry described later is further improved.

[0098] The content ratio of the structural unit (c) is 0.3% by mass or more, preferably 0.5% by mass or more, more preferably 3.0% by mass or more, and further preferably 6.0% by mass or more. This is because the cracking of the electrode described later is suppressed, and the productivity of the electrode is improved. The content ratio of the structural unit (c) is 28.0% by mass or less, preferably 23.0% by mass or less, more preferably 18.0% by mass or less, and further preferably 15.0% by mass or less. This is because the peeling strength of the electrode active material layer is improved and the expansion of the electrode active material layer is suppressed in the electrode described later. In addition, this is because the cycle characteristics (discharge capacity maintenance rate) are improved in the nonaqueous secondary battery described later.

[0099] The content ratio of the structural unit (d) is preferably 0.3% by mass or more, more preferably 0.9% by mass or more, and further preferably 3.0% by mass or more. The content ratio of the structural unit (d) is preferably 18.0% by mass or less, more preferably 12.0% by mass or less, and further preferably 7.0% by mass or less. This is because the cracking of the electrode described later is suppressed, the peeling strength of the electrode active material layer is improved, and the expansion of the electrode active material layer is suppressed. In addition, this is because the cycle characteristics (discharge capacity maintenance rate) are improved in the nonaqueous secondary battery described later.

[0100] The total content ratio of the structural units (a), (b), and (c) is 85% by mass or more, more preferably 90% by mass or more, and further preferably 93% by mass or more. This is because the contribution of the structural units (a), (b), and (c) to the target effects of the present application is improved.

[0101] <1-7. Method for producing copolymer (P)>

[0102] The synthesis of the copolymer (P) is preferably performed by radical polymerization in an aqueous medium. As the polymerization method, for example, a method in which all the monomers used for polymerization are added at once to perform polymerization, a method in which the monomers used for polymerization are continuously supplied while polymerization is performed, or the like can be applied. The content ratio of each monomer in all the monomers used for the synthesis of the copolymer (P) is the content ratio of the structural unit corresponding to the monomer in the copolymer (P). For example, the content ratio of the monomer (A) in all the monomers used for the synthesis of the copolymer (P) is the content ratio of the structural unit (a) in the copolymer (P) to be synthesized. However, when (meth)acrylic acid is used as the monomer (B) and neutralization is performed using a neutralizing agent after polymerization, the structural unit derived from (meth)acrylic acid is considered to form a salt to the extent of the equivalent number of the cation contained in the neutralizing agent (the valence number of the cation x the number of moles of the cation, and the same applies hereinafter). When the equivalent number of the cation contained in the neutralizing agent is more than the number of moles of (meth)acrylic acid used for polymerization, it is considered that (meth)acrylic acid is entirely formed into a salt. On the other hand, when the equivalent number of the cation contained in the neutralizing agent is less than the number of moles of (meth)acrylic acid used for polymerization, it is considered that the cation is entirely formed into a salt with (meth)acrylic acid (that is, (meth)acrylic acid is formed into a salt to the extent of the amount of the cation contained in the neutralizing agent). The radical polymerization is preferably performed at a temperature of 30 to 90°C. Furthermore, specific examples of the polymerization method of the copolymer (P) are described in detail in the Examples described later.

[0103] As the radical polymerization initiator, for example, ammonium persulfate, potassium persulfate, hydrogen peroxide, t-butyl hydroperoxide, an azo compound, or the like can be exemplified, but the present application is not limited thereto. As the azo compound, for example, 2,2'-azobis(2-methylpropionamidine) 2-hydrochloride can be exemplified. When polymerization is performed in water, a water-soluble polymerization initiator is preferably used. Furthermore, a redox polymerization can be performed by using a radical polymerization initiator and a reducing agent in combination as needed at the time of polymerization. As the reducing agent, sodium bisulfite, rongalite, ascorbic acid, or the like can be exemplified.

[0104] As the aqueous medium, water is preferably used, but a hydrophilic solvent can be added to water as the aqueous medium as long as the polymerization stability of the obtained binder copolymer is not impaired. As the hydrophilic solvent to be added to water, methanol, ethanol, N-methylpyrrolidone, or the like can be exemplified.

[0105] <2. Binder composition for nonaqueous secondary battery electrode>

[0106] The binder composition for nonaqueous secondary battery electrode (or nonaqueous secondary battery electrode binder composition. Hereinafter, it is also sometimes referred to as "electrode binder composition") of the present embodiment contains an electrode binder and an aqueous medium as a medium. Furthermore, the electrode binder composition can contain other components such as a pH adjustor, a surfactant, or the like as needed.

[0107] The aqueous medium contained in the electrode binder composition contains water. The aqueous medium contained in the electrode binder composition can also contain a hydrophilic solvent. Examples of the hydrophilic solvent include methanol, ethanol, and N-methylpyrrolidone. The content ratio of water in the aqueous medium contained in the electrode binder composition is preferably 80% by mass or more, more preferably 90% by mass or more, and further preferably 95% by mass or more.

[0108] The medium contained in the electrode binder composition can be the same as the aqueous medium used in the synthesis of the copolymer (P), or can be a medium obtained by further adding water or the like to the aqueous medium. In addition, in the electrode binder composition of the present embodiment, the electrode binder can be dissolved or dispersed in the medium.

[0109] The content ratio of the electrode binder in the electrode binder composition is preferably 30% by mass or less, and more preferably 20% by mass or less. This is because the viscosity of the electrode binder composition is inhibited from increasing, and the electrode active material or the like is efficiently dispersed when the electrode slurry is produced by mixing the electrode active material or the like with the electrode binder composition as described later.

[0110] The content ratio of the electrode binder in the electrode binder composition is preferably 3.0% by mass or more, more preferably 5.0% by mass or more, and further preferably 8.0% by mass or more. By inhibiting the amount of volatile components, the electrode slurry and the electrode can be produced from less electrode binder composition.

[0111] The pH of the electrode binder composition is preferably 4.0 or more, more preferably 5.0 or more, and further preferably 6.0 or more. This is because the electrode active material or the like is efficiently dispersed when the electrode slurry is produced by mixing the electrode active material or the like with the electrode binder composition as described later. The pH of the electrode binder composition is preferably 10 or less, more preferably 9.0 or less, and further preferably 8.0 or less. This is because the electrode active material or the like is efficiently dispersed when the electrode slurry is produced by mixing the electrode active material or the like with the electrode binder composition as described later. Here, the pH is a value measured at a liquid temperature of 23°C by a pH meter.

[0112] <3. Non-aqueous secondary battery electrode slurry>

[0113] In the slurry for nonaqueous secondary battery electrode (or nonaqueous secondary battery electrode slurry. Hereinafter, also referred to as "electrode slurry") of the present embodiment, the electrode binder and the electrode active material are dissolved or dispersed in the aqueous medium. The electrode slurry of the present embodiment can also contain, as necessary, a conductive aid, a tackifier, and the like, but in order to simplify the electrode slurry production step, it is preferable not to contain a tackifier. In order to prepare the electrode slurry, there is no particular limitation as long as each material is uniformly dissolved or dispersed. The method of preparing the electrode slurry is not particularly limited, and for example, a method of mixing the necessary components using a mixing device such as a stirring type, a rotating type, or a shaking type can be exemplified.

[0114] The nonvolatile component concentration of the electrode slurry is preferably 30% by mass or more, and more preferably 40% by mass or more. This is because a larger electrode active material layer is formed with a smaller amount of electrode slurry. The nonvolatile component concentration of the electrode slurry is preferably 70% by mass or less, and more preferably 60% by mass or less. The nonvolatile component concentration can be adjusted by the amount of the aqueous medium.

[0115] Here, the nonvolatile component concentration, unless otherwise specified, refers to the proportion of the mass of the components remaining after 1 g of the mixture is weighed into an aluminum dish having a diameter of 5 cm, dried at 130°C in a desiccator while air is circulated at atmospheric pressure for 1 hour, to the mass before drying.

[0116] <3-1. Content Rate of Electrode Binder>

[0117] The content of the electrode binder in the electrode slurry is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and further preferably 2.0% by mass or more, with respect to the total mass of the electrode active material (described later), the conductive aid (described later), and the electrode binder. This is because the adhesion between the electrode active materials and between the electrode active material and the current collector can be ensured by the electrode binder. The content of the electrode binder in the electrode slurry is preferably 7.0% by mass or less, more preferably 5.0% by mass or less, and further preferably 4.0% by mass or less, with respect to the total mass of the electrode active material, the conductive aid, and the electrode binder. This is because the charge and discharge capacity of the electrode active material layer formed from the electrode slurry can be increased, and the internal resistance when used as a battery can be reduced.

[0118] <3-2. Electrode Active Material>

[0119] The non-aqueous secondary battery is not particularly limited, and in the case of a lithium ion secondary battery, examples of the negative electrode active material include conductive polymers, carbon materials, lithium titanate, silicon, silicon compounds, and the like. Examples of the conductive polymer include polyacetylene, polypyrrole, and the like. Examples of the carbon material include cokes of petroleum coke, pitch coke, and the like; carbon blacks of carbides of organic compounds, carbon fibers, acetylene black, and the like; graphites of artificial graphite, natural graphite, and the like. Examples of the silicon compound include SiO x (0.1≤x≤2.0) and the like.

[0120] Further, the electrode active material can also use a composite material containing Si and graphite (Si / graphite) and the like. Among these active materials, from the viewpoint of the energy density per unit volume being large, it is particularly preferable to use a carbon material, lithium titanate, silicon, a silicon compound, and the like. Further, if the carbon material is a coke, a carbide of an organic compound, graphite, and the like, a silicon-containing material such as SiO x (0.1≤x≤2.0), Si, Si / graphite, and the like, the effect of improving the adhesion due to the electrode binder of the present embodiment is remarkable. For example, as a specific example of artificial graphite, SCMG (registered trademark) -XRs (manufactured by Showa Denko K.K.) can be given. Furthermore, as the negative electrode active material, two or more of the materials listed here can also be compounded.

[0121] Examples of the positive electrode active material of the lithium ion secondary battery include lithium cobaltate (LiCoO2); lithium complex oxides containing nickel; spinel-type lithium manganate (LiMn2O4); olivine-type lithium iron phosphate; oxygen group element compounds of TiS2, MnO2, MoO3, V2O5, and the like. The positive electrode active material can contain any of these compounds alone or a plurality of them. Further, an oxide of another alkali metal can also be used. The lithium complex oxide containing nickel can include lithium complex oxides of the Ni-Co-Mn system, lithium complex oxides of the Ni-Mn-Al system, lithium complex oxides of the Ni-Co-Al system, and the like. As specific examples of the positive electrode active material, LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2 or LiNi 3 / 5 Mn 1 / 5 Co 1 / 5 and the like can be given.

[0122] <3-3. Conductive aids>

[0123] The electrode slurry can also contain carbon black, vapor-phase carbon fibers, and the like as a conductive aid. As a specific example of the vapor-phase carbon fiber, VGCF (registered trademark) -H (manufactured by Showa Denko K.K.) can be given.

[0124] <3-4. Aqueous medium>

[0125] The aqueous medium of the electrode slurry contains water. As the aqueous medium of the electrode slurry, a hydrophilic solvent can also be contained. As the hydrophilic solvent, methanol, ethanol, N-methylpyrrolidone, and the like can be exemplified. The content ratio of water in the aqueous medium is preferably 80% by mass or more, more preferably 90% by mass or more, and further preferably 95% by mass or more. The aqueous medium of the electrode slurry can also be the same as the aqueous medium contained in the electrode binder composition.

[0126] <4. Electrode>

[0127] The electrode of the present embodiment has a current collector and an electrode active material layer formed on the surface of the current collector. The electrode active material layer contains an electrode active material and the electrode binder of the present embodiment. The shape of the electrode can be, for example, a laminate or a wound body, but is not particularly limited. The current collector is not particularly limited, and is preferably a thin sheet-shaped metal having a thickness of 0.001 to 0.5 mm. As the metal, iron, copper, aluminum, nickel, stainless steel, and the like can be exemplified. When the nonaqueous secondary battery is a lithium ion secondary battery, the material of the current collector for the positive electrode is preferably aluminum, and the material of the current collector for the negative electrode is preferably copper.

[0128] The electrode of the present embodiment can be produced, for example, by applying the electrode slurry to the current collector and drying, but is not limited to this method.

[0129] The method of applying the electrode slurry to the current collector can be exemplified by, for example, a reverse roll method, a direct roll method, a doctor blade method, a knife method, an extrusion method, a curtain method, a gravure method, a bar coater method, a dip method, a squeeze method, and the like. Among these, the doctor blade method, the knife method, or the extrusion method is preferable; and more preferably, the electrode slurry is applied using a doctor blade. This is because this is appropriate for the respective properties such as the viscosity of the electrode slurry and the drying property, and a coated film having a good surface state can be obtained.

[0130] The electrode slurry can be applied to only one surface of the current collector, or can be applied to both surfaces. When the electrode slurry is applied to both surfaces of the current collector, one surface can be applied at a time, or both surfaces can be simultaneously applied. In addition, the electrode slurry can be continuously applied to the surface of the current collector, or can be intermittently applied. The application amount and the application range of the electrode slurry can be appropriately decided depending on the size of the battery and the like. The weight per unit area of the electrode active material layer after drying is preferably 4 to 20 mg / cm 2 , more preferably 6 to 16 mg / cm 2 .

[0131] The electrode sheet can be obtained by drying the electrode slurry applied to the current collector. The drying method is not particularly limited, and for example, hot air, vacuum, (far) infrared rays, electron beams, microwaves, and low-temperature air can be used alone or in combination. The drying temperature is preferably 40°C or higher and 180°C or lower, and the drying time is preferably 1 minute or longer and 30 minutes or shorter.

[0132] The electrode sheet can be used directly as an electrode, or can be cut to have an appropriate size or shape. The method of cutting the electrode sheet is not particularly limited, and for example, a slitter, a laser, a wire cutting machine, a cutter, a Thomson knife, or the like can be used.

[0133] The electrode sheet can be pressed before or after being cut, as needed. This firmly bonds the electrode active material by the electrode, and makes it possible to further densify the nonaqueous battery due to the thinning of the electrode. The method of pressing can use a general method, and a die pressing method or a roll pressing method is particularly preferred. The pressing pressure is not particularly limited, and is preferably in a range where doping / dedoping of lithium ions or the like to / from the electrode active material does not occur. 2 .

[0134] <5. Battery>

[0135] As a preferred example of the battery of the present embodiment, a lithium ion secondary battery will be described, but the configuration of the battery is not limited to the configuration described below. The lithium ion secondary battery of the example described here has a positive electrode, a negative electrode, an electrolyte, and parts such as a separator, which are accommodated in an exterior body. At least one of the positive electrode and the negative electrode contains the electrode binder of the present embodiment.

[0136] <5-1. Electrolyte>

[0137] As the electrolyte, a nonaqueous liquid having ion conductivity is used. The electrolyte can include a solution in which an electrolyte is dissolved in an organic solvent, an ionic liquid, and the like, and the former is preferred because a battery having a low manufacturing cost and a low internal resistance can be obtained.

[0138] The electrolyte can use an alkali metal salt, and can be appropriately selected depending on the type of the electrode active material and the like. The electrolyte can include, for example, LiClO4, LiBF6, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiB 10 Cl 10 , LiAlCl4, LiCl, LiBr, LiB(C2H5)4, CF3SO3Li, CH3SO3Li, LiCF3SO3, LiC4F9SO3, Li(CF3SO2)2N, lithium aliphatic carboxylate, and the like. In addition, other alkali metal salts can be used as the electrolyte.

[0139] The organic solvent as the dissolving electrolyte is not particularly limited, and examples thereof include carbonate compounds such as ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC); nitrile compounds such as acetonitrile; and carboxylic acid esters such as ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. These organic solvents can be used alone or in combination of two or more.

[0140] <5-2. Outer Container>

[0141] The outer container can be appropriately made of metal or aluminum laminate, etc. The shape of the battery can be any of a coin type, a button type, a sheet type, a cylindrical type, a square type, and a flat type, etc.

[0142] [Examples]

[0143] Hereinafter, examples and comparative examples of the negative electrode binder, the negative electrode slurry, the negative electrode, and the lithium-ion secondary battery will be shown to describe the present application in more detail. Further, the present application is not limited to these examples.

[0144] <1. Production of Negative Electrode Binder (Copolymer (P))>

[0145] The composition of the monomers used in Examples 1 to 7 and Comparative Examples 1 to 8 is shown in Table 2. The production method of the negative electrode binder in Examples 1 to 7 and Comparative Examples 1 to 8 is the same except for the composition of the monomers. Details of the monomers and reagents are described below. When the monomers are used in the form of a solution, the amount of the monomers in the table indicates the amount of the monomers themselves excluding the solvent.

[0146] Monomer (A-1): N-vinylacetamide (NVA) (manufactured by Showa Denko K.K.)

[0147] Monomer (B-1): Sodium acrylate (AaNa) (aqueous solution of 28.5 mass%)

[0148] Monomer (C-1): Benzyl acrylate

[0149] Monomer (C-2): Phenoxyethyl acrylate

[0150] Monomer (D-1): Methoxypolyethylene glycol methacrylate (manufactured by EVONIK INDUSTRIES; VISIOMER (registered trademark) MPEG2005MA W) (50.0 mass% aqueous solution of R 3 = CH3, R 4 = H, R 6 = CH3, n = 45, m = 0, m + n = 45)

[0151] Monomer (E-1): Styrene

[0152] Polymerization initiator: 2,2'-azobis(2-methylpropionamidine) 2 hydrochloride (manufactured by Wako Pure Chemical Industries, Ltd.; V-50) and ammonium persulfate (manufactured by Wako Pure Chemical Industries, Ltd.)

[0153] In a separable flask equipped with a cooling tube, a thermometer, a stirrer, and a dropping funnel, 100 parts by mass of the monomers of the composition shown in Table 2, 0.2 parts by mass of 2,2'-azobis(2-methylpropionamidine) 2 hydrochloride, 0.05 parts by mass of ammonium persulfate, and 693 parts by mass of water were added at 30°C. This was warmed to 80°C, and polymerization was performed for 4 hours.

[0154] Then, water was added so that the content ratio of the negative electrode binder (copolymer (P)) would be 10.0% by mass (the amount of water added was adjusted taking into account the water contained in the monomer (B-1)), and negative electrode binder compositions Q1 to Q7 and CQ1 to CQ8 were prepared. In the following description, there are cases in which "each of the copolymers P1 to P7 and CP1 to CP8" is referred to as "copolymer (P)", and "each of the negative electrode binder compositions Q1 to Q7 and CQ1 to CQ8" is referred to as "negative electrode binder composition (Q)".

[0155] <2. Various measurements on the negative electrode binder composition>

[0156] The following measurements were performed on the copolymer (P) and the negative electrode binder composition (Q). The measurement results are shown in Table 2.

[0157] <2-1. Weight average molecular weight of the copolymer (P)>

[0158] The weight average molecular weight of the copolymer (P) was measured using gel permeation chromatography (GPC) under the following conditions.

[0159] GPC device: GPC-101 (manufactured by Showa Denko K.K.)

[0160] Solvent: 0.1M aqueous NaNO3

[0161] Sample column: Shodex Column Ohpak SB-806HQ (8.0 mm I.D. x 300 mm) x 2

[0162] Reference column: Shodex Column Ohpak SB-800RL (8.0 mm I.D. x 300 mm) x 2

[0163] Column temperature: 40°C

[0164] Sample concentration: 0.1% by mass

[0165] Detector: RI-71S (manufactured by Shimadzu Corporation)

[0166] Pump: DU-H2000 (manufactured by Shimadzu Corporation)

[0167] Pressure: 1.3 MPa

[0168] Flow rate: 1 ml / min

[0169] Molecular weight standards: Prunamose (P-5, P-10, P-20, P-50, P-100, P-200, P-400, P-800, P-1300, P-2500 (manufactured by Showa Denko Co., Ltd.))

[0170] <2-2. pH of the electrode binder composition (Q)>

[0171] The pH of the electrode binder composition (Q) was measured using a pH meter (manufactured by Toa DKK) at a liquid temperature of 23°C.

[0172] <3. Preparation of negative electrode slurry>

[0173] 76.8 parts by weight of SCMG (registered trademark)-XRs (manufactured by Showa Denko Co., Ltd.) as graphite, 19.2 parts by weight of silicon monoxide (SiO) (manufactured by Sigma-Aldrich), 1 part by weight of VGCF (registered trademark)-H (manufactured by Showa Denko Co., Ltd.), 30 parts by weight of binder composition (Q) (containing 3 parts by weight of copolymer (P) and 27 parts by weight of water), and 20 parts by weight of water were mixed. Mixing was performed using a stirring mixer (rotational-revolutionary mixer) at 2000 rpm for 4 minutes. 53 parts by weight of water were further added to the resulting mixture, and it was further mixed using the same mixing device at 2000 rpm for 4 minutes to prepare the negative electrode slurry.

[0174] <4. Appearance Evaluation of Negative Electrode Slurry>

[0175] Visually inspect the appearance of the negative electrode slurry prepared during the battery manufacturing process, and measure the size of the aggregates using a micrometer. Mark × if there are aggregates with a maximum size of 1 mm or more in 10 g of negative electrode slurry, and mark ○ if none are present.

[0176] <5. Fabrication of the negative electrode and battery>

[0177] <5-1. Making the Negative Electrode>

[0178] The prepared negative electrode slurry was applied to one side of a 10μm thick copper foil (current collector), and the dried slurry was weighed at a density of 8 mg / cm² per unit area. 2using a doctor blade in a manner such that the weight per unit area after drying becomes 22.5 mg / cm 2 was further dried at 100°C for 5 minutes. The negative electrode sheet formed with the negative electrode active material layer was pressed using a mold at a press pressure of 1 t / cm

[0179] <5-2. Production of a positive electrode>

[0180] LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2 90 parts by mass, acetylene black 5 parts by mass, and polyvinylidene fluoride 5 parts by mass were mixed, and then N-methylpyrrolidone 100 parts by mass was mixed to prepare a positive electrode slurry (the proportion of LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2 was 0.90).

[0181] The prepared positive electrode slurry was coated on one side of an aluminum foil (current collector) having a thickness of 20 μm using a doctor blade in a manner such that the weight per unit area after drying becomes 22.5 mg / cm 2 (22.5 x 10 -3 g / cm 2 ). The aluminum foil coated with the positive electrode slurry was dried at 120°C for 5 minutes, and then pressed by rolling to prepare a positive electrode sheet formed with a positive electrode active material layer having a thickness of 100 μm. The obtained positive electrode sheet was cut into 20 mm x 20 mm (2.0 cm x 2.0 cm), and a conductive handle was attached to prepare a positive electrode.

[0182] The theoretical capacity of the prepared positive electrode was calculated based on the weight per unit area after drying of the positive electrode slurry (22.5 x 10 -3 g / cm 2 ) x the coating area of the positive electrode slurry (2.0 cm x 2.0 cm) x the capacity of LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2 as a positive electrode active material (160 mAh / g) x the proportion of LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2 in the solid content (0.90), and the calculated value was 13 mAh.

[0183] <5-3. Preparation of an electrolyte>

[0184] A mixed solvent was prepared by mixing ethylene carbonate (EC), methyl ethyl carbonate (EMC) and fluoroethylene carbonate (FEC) at a volume ratio of 30:60:10. LiPF6was dissolved in the mixed solvent to a concentration of 1.0 mol / L, and vinyl carbonate (VC) was dissolved in the mixed solvent to a concentration of 1.0 mass%. An electrolyte was prepared.

[0185] <5-4. Assembly of the battery>

[0186] A positive electrode and a negative electrode were disposed with their respective active material layers facing each other through a separator made of a polyolefin porous film, and housed in an aluminum laminated outer body (battery pack). An electrolyte was injected into the outer body, which was sealed with a vacuum heat sealer to obtain a laminated battery.

[0187]

[0188] <6. Evaluation of the negative electrode and the battery>

[0189] The negative electrodes and the batteries of each of the examples and the comparative examples were evaluated. The evaluation method is described below, and the evaluation results are shown in Table 2.

[0190] <6-1. Number of wrinkles of the negative electrode>

[0191] The appearance of the surface of the negative electrode sheet was visually confirmed, and the number of wrinkles in the rectangular range of 5 cm x 20 cm was counted.

[0192] <6-2. Peeling strength of the negative electrode active material layer>

[0193] A sample for evaluation of the peeling strength was prepared by adhering the negative electrode active material layer formed on the negative electrode sheet to a SUS plate using double-sided tape (NITTO TAPE (registered trademark) No 5, manufactured by Nitto Electric Industrial Co., Ltd.) at 23°C. Using the sample, the negative electrode active material layer was peeled from the negative electrode sheet at a peeling width of 25 mm and a peeling speed of 100 mm / min at 180°, and the value obtained by dividing the peeling force by the peeling width of 25 mm was used as the peeling strength.

[0194] <6-3. Initial efficiency of the battery>

[0195] The initial efficiency of the battery was measured at 25°C by the following procedure. First, charge to 4.2 V (CC charge) at a current of 0.2 C, then charge to a current of 0.05 C (CV charge) at a voltage of 4.2 V. After 30 minutes, discharge to 2.75 V (CC discharge) at a current of 0.2 C. The series of CC charge, CV charge, and CC discharge was taken as one cycle, and this was repeated for 5 cycles. The sum of the time integral of the current during CC charge and CV charge of the nth cycle was taken as the charge capacity (mAh) of the nth cycle, and the time integral of the current during CC discharge of the nth cycle was taken as the discharge capacity (mAh) of the nth cycle. The average of the discharge capacities of the 4th and 5th cycles was taken as the initial discharge capacity, and the initial efficiency was calculated from the following calculation formula [1]. The positive electrode theoretical capacity was the value obtained in the description of the preparation of the positive electrode.

[0196] Initial efficiency (%) = {initial discharge capacity / 13 mAh (positive electrode theoretical capacity)} x 100 [1]

[0197] <6-4. Negative electrode expansion>

[0198] The electrode expansion of the negative electrode was measured by the following procedure. First, the thickness of the negative electrode was measured at 5 points using a micrometer (Mitutoyo Co. MDH-25MB) before assembly of the battery, and the average was taken as the initial thickness (μm). The battery was then assembled, and after measuring the initial efficiency as described above, CC charge and CV charge were performed to fully charge the battery. The battery was then disassembled, and the negative electrode was removed and measured for thickness at 5 points without drying, and the average was taken as the disassembly thickness (μm). The negative electrode expansion was calculated from the following calculation formula [2].

[0199] Negative electrode expansion (%) = {1 - (disassembly thickness / initial thickness)} x 100 [2]

[0200] <6-5. Discharge capacity maintenance rate of battery (100 cycles)

[0201] The measurement of the discharge capacity maintenance rate of the battery (charge-discharge cycle test of the battery) was performed under the condition of 25°C by the following steps. First, charging was performed at a current of 1C until the voltage became 4.2V (CC charging), and then charging was performed at a voltage of 4.2V until the current became 0.05C (CV charging). After 30 minutes of standing, discharging was performed at a current of 1C until the voltage became 2.75V (CC discharging). One cycle was defined as a series of CC charging, CV charging, and CC discharging. The sum of the time integral values of the current in the CC charging and CV charging of the nth cycle was defined as the charge capacity (mAh) of the nth cycle, and the time integral value of the current in the CC discharging of the nth cycle was defined as the discharge capacity (mAh) of the nth cycle. The discharge capacity maintenance rate of the battery of the nth cycle was the ratio (%) of the discharge capacity of the nth cycle to the discharge capacity of the 1st cycle. In the present example and the comparative examples, the discharge capacity maintenance rate of the 100th cycle was evaluated.

[0202] <7. Evaluation Results>

[0203] As shown in Table 2, the negative electrode binder and the negative electrode slurry produced in Examples 1 to 7 can greatly increase the peeling strength of the electrode active material layer from the current collector while suppressing the generation of cracks in the electrode active material layer formed on the current collector. It was found that the negative electrode binder produced in accordance with Examples 1 to 7 can suppress the generation of agglomerates in the negative electrode slurry.

[0204] It was found that the negative electrode produced in Examples 1 to 7 has few cracks and high peeling strength of the electrode active material layer from the current collector. It was found that the negative electrode produced in Examples 1 to 7 has small expansion of the negative electrode accompanying the use of the battery when used in a lithium ion secondary battery.

[0205] The lithium ion secondary battery produced in Examples 1 to 7 has a negative electrode with few cracks and high peeling strength of the electrode active material layer from the current collector. The lithium ion secondary battery produced in Examples 1 to 7 has high initial efficiency and discharge capacity maintenance rate, and can suppress the expansion of the negative electrode.

[0206] In Comparative Example 1, the monomer (C) was not used in the synthesis of the copolymer (P). The negative electrode produced in Comparative Example 1 was found to have cracks.

[0207] In Comparative Example 2, the monomer (C) was not used and the monomer (D) was used in the synthesis of the copolymer (P). The negative electrode produced in Comparative Example 2 was not found to have cracks, but had low peeling strength of the negative electrode active material layer. The negative electrode produced in Comparative Example 2 had large expansion of the negative electrode accompanying the use of the battery when used in a lithium ion secondary battery. The lithium ion secondary battery produced in Comparative Example 2 had low initial efficiency and discharge capacity maintenance rate.

[0208] In Comparative Example 3, the proportion of monomers (A), (B) and (C) in the total monomers used in the synthesis of the copolymer (P) was small. In Comparative Example 5, the proportion of monomer (A) used in the synthesis of the copolymer (P) was large and the proportion of monomer (B) was small. The electrode slurry prepared in Comparative Example 3 and Comparative Example 5 contained agglomerates. The negative electrode prepared in Comparative Example 3 and Comparative Example 5 exhibited cracking and the peeling strength of the negative electrode active material layer was low. The negative electrode prepared in Comparative Example 3 and Comparative Example 5 swelled greatly when used in a lithium ion secondary battery. The lithium ion secondary battery prepared in Comparative Example 3 and Comparative Example 5 exhibited low initial efficiency and low discharge capacity maintenance rate.

[0209] In Comparative Example 4, monomer (A) was not used in the synthesis of the copolymer (P). The negative electrode slurry prepared in Comparative Example 4 contained agglomerates. The negative electrode slurry prepared in Comparative Example 4 could not be coated flat on the current collector and could not be used to prepare a negative electrode and a battery that could be evaluated.

[0210] In Comparative Example 6 and Comparative Example 7, the proportion of monomer (C) used in the synthesis of the copolymer (P) was large. In Comparative Example 8, styrene was used instead of monomer (C) in the synthesis of the copolymer (P). The negative electrode slurries prepared in these Comparative Examples contained agglomerates. The negative electrodes prepared in these Comparative Examples exhibited low peeling strength of the negative electrode active material layer and also swelled greatly when used in a battery. The lithium ion secondary batteries prepared in these Comparative Examples exhibited low initial efficiency and low discharge capacity maintenance rate.

Claims

1. An adhesive for non-aqueous secondary battery electrodes, characterized in that, A copolymer P comprising only structural unit a derived from monomer A (represented by formula (1), structural unit b derived from monomer B, and structural unit c derived from monomer C, wherein monomer B is at least one selected from (meth)acrylic acid and its salts, and monomer C is benzyl (meth)acrylate. The content of each structural unit in the copolymer P is shown below: The content of structural unit a is above 0.5% by mass and below 20.0% by mass. The content of structural unit b is above 50.0% by mass and below 98.0% by mass. The content of structural unit c is above 0.3% by mass and below 28.0% by mass. The combined content of structural unit a, structural unit b, and structural unit c is 85% by mass or more. In the formula, R 1 R 2 Each is an alkyl group, which is independently composed of 1 or more hydrogen atoms and 5 or fewer carbon atoms.

2. The adhesive for non-aqueous secondary battery electrodes as described in claim 1, wherein monomer A is N-vinylformamide or N-vinylacetamide.

3. The adhesive for non-aqueous secondary battery electrodes as described in claim 1, wherein monomer B is (meth)acrylate.

4. The adhesive for non-aqueous secondary battery electrodes as described in claim 1, wherein the copolymer P has a weight-average molecular weight of 1 million or more and 10 million or less.

5. The adhesive for non-aqueous secondary battery electrodes as claimed in claim 1, wherein the content of structural unit b derived from monomer B in copolymer P is 60.0% by mass or more and 90.0% by mass or less.

6. An adhesive composition for a non-aqueous secondary battery electrode, comprising the adhesive for a non-aqueous secondary battery electrode according to any one of claims 1 to 5 and an aqueous medium.

7. The adhesive composition for a non-aqueous secondary battery electrode as described in claim 6, wherein the non-aqueous secondary battery is a lithium-ion secondary battery.

8. A slurry for a non-aqueous secondary battery electrode, comprising a binder for a non-aqueous secondary battery electrode, an electrode active material, and an aqueous medium as described in any one of claims 1 to 5.

9. A non-aqueous secondary battery electrode having a current collector and an electrode active material layer formed on the surface of the current collector, the electrode active material layer containing an adhesive and an electrode active material for a non-aqueous secondary battery electrode as described in any one of claims 1 to 5.

10. A lithium-ion secondary battery comprising the electrode as described in claim 9.

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