Binder for secondary battery electrode and use thereof

Abstract

Provided is a binder for a secondary battery electrode, which is capable of improving the cycle characteristics of a secondary battery. Also provided are a composition for a secondary battery electrode mixture layer containing the binder, a secondary battery electrode obtained using the composition, and a secondary battery. A binder for a secondary battery electrode, the binder comprising: a carboxyl group-containing non-crosslinked polymer comprising a structural unit derived from an ethylenically unsaturated carboxylic acid monomer, a structural unit derived from an amide group-containing ethylenically unsaturated monomer, and a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer, or a salt thereof, the carboxyl group-containing non-crosslinked polymer comprising a structural unit derived from an ethylenically unsaturated carboxylic acid monomer, a structural unit derived from an amide group-containing ethylenically unsaturated monomer, and a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer; the total amount of the amide group-containing ethylenically unsaturated monomer is 5.0 mass% or less relative to the total amount of the carboxyl group-containing non-crosslinked polymer and the salt thereof.

Description

TECHNICAL FIELD

[0001] The present application relates to a binder for secondary battery electrodes and use thereof. BACKGROUND

[0002] As secondary batteries, various power storage devices such as nickel-hydrogen secondary batteries, lithium-ion secondary batteries, and electric double layer capacitors have been put into practical use. The electrodes used in these secondary batteries are produced by coating a composition for forming an electrode mixture layer containing an active material and a binder or the like on a current collector and drying or the like. For example, in a lithium-ion secondary battery, as a binder used in a composition for a negative electrode mixture layer, an aqueous binder containing a styrene butadiene rubber (SBR) latex and a carboxymethyl cellulose (CMC) is used. On the other hand, as a binder used in a composition for a positive electrode mixture layer, an organic solvent-based binder such as an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) is widely used.

[0003] In recent years, with the expansion of the use of various secondary batteries, there is a tendency for the requirements for improvement in energy density, reliability, and durability to be strengthened. For example, in order to improve the capacity of a lithium-ion secondary battery, the specifications for using a silicon-based active material as an active material for a negative electrode have gradually increased. However, it is known that a silicon-based active material has a large volume change at the time of charge and discharge, and peeling or flaking of an electrode mixture layer occurs with repeated use, as a result of which there are problems in that the capacity of the battery decreases and the cycle characteristics (durability) deteriorate. In order to suppress such adverse situations, it is generally effective to improve the adhesiveness of the binder, and research related to improving the adhesiveness of the binder has been conducted for the purpose of improving the durability.

[0004] Among them, as a binder that has good cycle characteristics and has an effect of improving the durability of a negative electrode mixture layer obtained by using a silicon-based active material, it has been reported that an acrylic polymer is effective.

[0005] Patent Document 1 discloses a binder containing a cross-linked acrylic polymer obtained by cross-linking a polyacrylic acid with a specific cross-linking agent, and discloses that even in the case of using an active material containing silicon (hereinafter also referred to as "silicon-based active material"), the electrode structure is not destroyed and good cycle characteristics are exhibited.

[0006] In addition, Patent Document 2 discloses a binder containing a non-cross-linked acrylic polymer formed from an acrylic acid or a salt of an acrylic acid derivative and an acrylonitrile or an acrylonitrile derivative, and discloses that since it is possible to follow the expansion and contraction of a silicon-based active material, it is possible to improve the cycle characteristics.

[0007] PRIOR ART DOCUMENTS

[0008] PATENT DOCUMENTS

[0009] Patent Literature 1: International Publication No. 2014 / 065407

[0010] Patent Literature 2: Japanese Patent Application Publication No. 2015-115109 SUMMARY

[0011] PROBLEMS TO BE SOLVED BY THE INVENTION

[0012] However, the secondary battery electrode binder disclosed in Patent Literatures 1 and 2, while being able to impart good adhesiveness, in the case where the ratio of the silicon-based active material is increased in order to improve the performance of the secondary battery, sometimes the cycle characteristics become insufficient, which becomes a problem.

[0013] The present application was completed in view of such circumstances, and aims to provide a secondary battery electrode binder capable of improving the cycle characteristics of a secondary battery. In addition, a secondary battery electrode binder layer composition containing the above binder, a secondary battery electrode obtained using the composition, and a secondary battery are also provided.

[0014] SOLUTION TO THE PROBLEM

[0015] The present inventors and others conducted intensive studies in order to solve the above problem, and as a result, found that, in a secondary battery electrode binder containing a carboxyl group-containing non-crosslinking polymer or a salt thereof, by causing the polymer to have structural units derived from specific three kinds of monomers, and setting the content of an amide group-containing ethylenically unsaturated monomer to be a specific value or less, the cycle characteristics of a secondary battery can be improved, thereby completing the present application.

[0016] The present application is as described below.

[0017] [1] A secondary battery electrode binder comprising: a carboxyl group-containing non-crosslinking polymer or a salt thereof,

[0018] The aforementioned carboxyl group-containing non-crosslinking polymer contains: a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as “monomer (a)”), a structural unit derived from an amide group-containing ethylenically unsaturated monomer (hereinafter also referred to as “monomer (b)”), and a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter also referred to as “monomer (c)”), and

[0019] The total amount of the aforementioned monomer (b) is 5.0 mass% or less with respect to the total amount of the aforementioned carboxyl group-containing non-crosslinking polymer and salt thereof.

[0020] [2] The secondary battery electrode binder according to [1], wherein the total amount of the aforementioned monomer (a) and salt thereof, converted to the unneutralized form, is 5.0 mass% or less with respect to the total amount of the aforementioned carboxyl group-containing non-crosslinking polymer and salt thereof.

[0021] [3] The binder for a secondary battery electrode according to [1] or [2], wherein the aforementioned carboxyl group-containing non-crosslinking polymer contains 40 mass% or more and 98 mass% or less of the structural unit derived from the monomer (a), 1 mass% or more and 50 mass% or less of the structural unit derived from the monomer (b), and 1 mass% or more and 50 mass% or less of the structural unit derived from the monomer (c), with respect to the total structural units thereof.

[0022] [4] The binder for a secondary battery electrode according to any one of [1] to [3], wherein the number average molecular weight of the aforementioned carboxyl group-containing non-crosslinking polymer or salt thereof is 4,000 or more and 200,000 or less.

[0023] [5] The binder for a secondary battery electrode according to any one of [1] to [4], wherein the salt of the aforementioned carboxyl group-containing non-crosslinking polymer is a salt in which 40 mole% or more of the carboxyl groups possessed by the non-crosslinking polymer are neutralized.

[0024] [6] A composition for a secondary battery electrode mixture layer, comprising: the binder for a secondary battery electrode according to any one of [1] to [5], an active material, and water.

[0025] [7] The composition for a secondary battery electrode mixture layer according to [6], further comprising carboxymethyl cellulose (CMC).

[0026] [8] A secondary battery electrode provided with a mixture layer formed of the composition for a secondary battery electrode mixture layer according to [6] or [7] on the surface of a current collector.

[0027] [9] A secondary battery provided with the secondary battery electrode according to [8].

[0028] Effects of the Invention

[0029] The binder for a secondary battery electrode according to the present invention makes it possible to obtain a secondary battery having excellent cycle characteristics. DETAILED DESCRIPTION

[0030] The secondary battery electrode binder of the present invention (hereinafter also referred to as "the present binder") contains a carboxyl group-containing non-crosslinked polymer (hereinafter also referred to as "the present non-crosslinked polymer") having a structural unit derived from monomer (a), a structural unit derived from monomer (b), and a structural unit derived from monomer (c), or a salt thereof, and by mixing with an active material and water, a secondary battery electrode mixture layer composition (hereinafter also referred to as "the present composition") can be prepared. From the aspect of exerting the effect of the present invention, the above composition is preferably an electrode slurry in a state of being able to be coated on a current collector, but can also be prepared in a wet powder state so as to be able to cope with press processing on the surface of a current collector. By forming a mixture layer formed of the above composition on the surface of a current collector such as a copper foil or an aluminum foil, a secondary battery electrode of the present invention can be obtained.

[0031] Here, the present binder is preferred in the case of being used for a secondary battery electrode mixture layer composition containing a silicon-based active material described later as an active material, from the aspect of the effect of the present invention being particularly great.

[0032] Hereinafter, the present non-crosslinked polymer or a salt thereof, a method for producing the same, a secondary battery electrode mixture layer composition obtained using the present binder, a secondary battery electrode, and a secondary battery will be described in detail, respectively.

[0033] Note that in the present specification, "(meth)acrylic" means acrylic and / or methacrylic, "(meth)acrylate" means acrylate and / or methacrylate. In addition, "(meth)acryloyl" means acryloyl and / or methacryloyl.

[0034] In the numerical ranges described in stages in the present specification, the upper limit value or the lower limit value described in one numerical range can be replaced with the upper limit value or the lower limit value of another numerical range described in stages, and the upper limit value or the lower limit value of the numerical range can be replaced with the value shown in the examples.

[0035] With regard to the carboxyl group-containing non-crosslinked polymer or a salt thereof of the present invention, the non-crosslinked polymer has a structural unit derived from monomer (a), a structural unit derived from monomer (b), and a structural unit derived from monomer (c), and by polymerizing a monomer component containing monomer (a), monomer (b), and monomer (c), it can be introduced into the present non-crosslinked polymer.

[0036] <Structural unit derived from an ethylenically unsaturated carboxylic acid monomer>

[0037] The present non-crosslinked polymer has structural units derived from an ethylenically unsaturated carboxylic acid monomer (monomer (a)) (hereinafter also referred to as "(a) component"). The present non-crosslinked polymer has improved adhesion to a current collector when having a carboxyl group by having the structural units, and has excellent desolvation effect of lithium ions and ionic conductivity, and thus an electrode having a small resistance and excellent high-rate characteristics can be obtained, and dispersion stability of an active material or the like in the present composition can be improved.

[0038] The above (a) component can be introduced into the present non-crosslinked polymer, for example, by polymerizing the monomer (a). In addition, it can be obtained by (co)polymerizing a (meth)acrylate monomer and then hydrolyzing. Alternatively, it can be obtained by polymerizing a (meth)acrylamide, a (meth)acrylonitrile, or the like, and then treating with a strong base. Alternatively, it can be obtained by reacting an acid anhydride with a polymer having a hydroxyl group.

[0039] As the monomer (a), for example, the following can be listed: (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; (meth)acrylamide alkyl carboxylic acids such as (meth)acrylamide hexanoic acid and (meth)acrylamide dodecanoic acid; ethylenically unsaturated monomers containing a carboxyl group such as succinic acid monohydroxyethyl (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, β-carboxyethyl (meth)acrylate, or (partial) base neutralization products thereof, and one of them can be used alone or two or more of them can be used in combination. Among the above, from the viewpoint of being able to obtain a polymer having a long primary chain length due to a fast polymerization rate and good adhesion of the binder, a compound having an acryloyl group as a polymerizable functional group is preferred, and acrylic acid is particularly preferred. In the case where acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer having a high carboxyl group content can be obtained.

[0040] The content of the (a) component in the present non-crosslinked polymer can be 40% by mass or more and 98% by mass or less with respect to the total structural units of the present non-crosslinked polymer. By containing the (a) component in this range, excellent adhesion to a current collector can be easily ensured, and the cycle characteristics of a secondary battery can be improved. In the case where the lower limit is 40% by mass or more, the dispersion stability of the present composition becomes good, and a higher adhesion can be obtained, and thus it is preferred that it be 50% by mass or more, it be 60% by mass or more, it be 70% by mass or more, or it be 80% by mass or more. In addition, the upper limit is, for example, 97% by mass or less, for example, 96% by mass or less, for example, 95% by mass or less, for example, 93% by mass or less, for example, 91% by mass or less, or for example, 90% by mass or less.

[0041] The total amount of the monomer (a) and the salt thereof, converted into the unneutralized form (i.e., the form in which the entire olefinically unsaturated carboxylic acid monomer present is converted into a carboxylic acid), relative to the total amount of the present non-crosslinked polymer and the salt thereof (hereinafter also referred to as "the content of the remaining carboxylic acid monomer (salt)") may be, for example, 5.0% by mass or less.

[0042] The content of the remaining carboxylic acid monomer (salt) is, for example, further 4.0% by mass or less, further 3.0% by mass or less, further 2.0% by mass or less, further 1.5% by mass or less, further 1.0% by mass or less, further 0.5% by mass or less. If the content of the remaining carboxylic acid monomer (salt) is 5.0% by mass or less, high adhesiveness can be obtained.

[0043] The lower limit of the content of the remaining carboxylic acid monomer (salt) is not particularly limited and may be, for example, 0.05% by mass or more. It is, for example, further 0.1% by mass or more, further 0.2% by mass or more, further 0.3% by mass or more, further 0.4% by mass or more. This is because, from the viewpoint of cleaning cost and the like, the content of the remaining carboxylic acid monomer (salt) is not advantageous when it is less than 0.05% by mass, for example.

[0044] Note that the content of the remaining carboxylic acid monomer (salt) is measured by gas chromatography (GC) described in the examples.

[0045] <Structural unit derived from the amide group-containing olefinically unsaturated monomer>

[0046] The present non-crosslinked polymer has a structural unit derived from an amide group-containing olefinically unsaturated monomer (monomer (b)) (hereinafter also referred to as "(b) component"). The present non-crosslinked polymer has excellent cycle characteristics of a secondary battery by having this structural unit.

[0047] The above (b) component can be introduced into the present non-crosslinked polymer by polymerizing a monomer containing the monomer (b).

[0048] As the monomer (b), (meth)acrylamide, a monomer represented by the following formula (1), and a (meth)acrylamide derivative (which is a monomer other than the monomer represented by the following formula (1)) can be exemplified. Among them, (meth)acrylamide is preferred from the aspect that the cycle characteristics of a secondary battery are more excellent.

[0049] CH2=C(R 5 )CONR 6 R 7 (1)

[0050] [In the formula, R 5 represents a hydrogen atom or a methyl group, R 6 represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and R7 represents a hydrogen atom or a monovalent organic group.

[0051] The monomer represented by the above formula (1) is a (meth) acrylamide derivative having a hydroxyl group or a hydroxyl alkyl group having 1 to 8 carbon atoms. In formula (1), R 7 represents a hydrogen atom or a monovalent organic group. As the above monovalent organic group, there is no particular limitation, and examples thereof include an alkyl group which can have a linear, branched or cyclic structure, and an aryl group and an alkoxyalkyl group, and the like, and it is preferable that the carbon atom number thereof be 1 to 8. Furthermore, R 7 It can also be a hydroxyl group or a hydroxyl alkyl group having 1 to 8 carbon atoms.

[0052] As the monomer represented by the above formula (1), for example, there can be mentioned: a hydroxyl (meth) acrylamide; a (meth) acrylamide derivative having a hydroxyl alkyl group having 1 to 8 carbon atoms such as N-hydroxyethyl (meth) acrylamide, N-(2-hydroxypropyl) (meth) acrylamide, N-hydroxybutyl (meth) acrylamide, N-hydroxyhexyl (meth) acrylamide and N-hydroxyoctyl (meth) acrylamide, N-methylhydroxyethyl (meth) acrylamide and N-ethylhydroxyethyl (meth) acrylamide; and an N,N-dihydroxyalkyl (meth) acrylamide such as N,N-dihydroxyethyl (meth) acrylamide. One of these can be used alone, or two or more of them can be used in combination.

[0053] Among the monomers represented by the above formula (1), from the aspect of excellent cycle characteristics of the secondary battery, a (meth) acrylamide derivative having a hydroxyl alkyl group having 1 to 8 carbon atoms is further preferable, and N-hydroxyethyl (meth) acrylamide, N-(2-hydroxypropyl) (meth) acrylamide, N-hydroxybutyl (meth) acrylamide are still further preferable.

[0054] As the (meth) acrylamide derivative, for example, there can be mentioned: an N-alkyl (meth) acrylamide compound such as N-isopropyl (meth) acrylamide, N-tert-butyl (meth) acrylamide; an N-alkoxyalkyl (meth) acrylamide compound such as N-n-butoxymethyl (meth) acrylamide, N-isobutoxymethyl (meth) acrylamide; an N,N-dialkyl (meth) acrylamide compound such as N,N-dimethyl (meth) acrylamide, N,N-diethyl (meth) acrylamide. One of these can be used alone, or two or more of them can be used in combination.

[0055] Regarding the content of component (b) in this non-crosslinked polymer, it can be 1% by mass or more and 50% by mass or less relative to all structural units of this non-crosslinked polymer. By containing component (b) within this range, excellent adhesion to the current collector can be easily ensured, and the cycle characteristics of the secondary battery can be improved. When the lower limit is 1% by mass or more, the dispersion stability of this composition becomes good, and higher adhesion can be obtained. Therefore, it is preferable that it is 2% by mass or more, 3% by mass or more, or 5% by mass or more. In addition, the upper limit is, for example, 49% by mass or less, 45% by mass or less, and for example, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less.

[0056] From the perspective of improving the cycle characteristics of the secondary battery, the total amount of monomer (b) relative to the total amount of the non-crosslinked polymer and its salt (hereinafter also referred to as "the content of residual monomer (b)") is 5.0% by mass or less, preferably 4.0% by mass or less, more preferably 3.0% by mass or less, even more preferably 2.0% by mass or less, even more preferably 1.5% by mass or less, even more preferably 1.0% by mass or less, and particularly preferably 0.5% by mass or less.

[0057] There is no specific lower limit for the content of the remaining monomer (b), for example, it can be set to 0.05% by mass or more. Other examples include 0.1% by mass or more, 0.2% by mass or more, 0.3% by mass or more, and 0.4% by mass or more.

[0058] It should be noted that the content of the remaining monomer (b) was determined by gas chromatography (GC) as described in the examples.

[0059] <Structural units derived from nitrile-containing olefinic unsaturated monomers>

[0060] This non-crosslinked polymer has a structural unit derived from an olefinic unsaturated monomer containing a nitrile group (monomer (c)) (hereinafter also referred to as "(c) component"). This non-crosslinked polymer exhibits excellent cycle characteristics in secondary batteries due to the presence of this structural unit.

[0061] The above-mentioned component (c) can be introduced into the non-crosslinked polymer by polymerizing a monomer containing monomer (c).

[0062] Examples of monomers (c) include: (meth)acrylonitrile; (meth)acrylic acid cyanomethyl ester, (meth)acrylic acid cyanoethyl ester, and other (meth)acrylic acid cyanoalkyl ester compounds; cyano-containing unsaturated aromatic compounds such as 4-cyanostylene, 4-cyano-α-methylstyrene; and vinylidene cyanide, etc. One of these can be used alone, or two or more can be used in combination. Among the above, acrylonitrile is preferred due to its high nitrile content and superior cycle characteristics in secondary batteries.

[0063] Regarding the content of component (c) in this non-crosslinked polymer, it can be 1% by mass or more and 50% by mass or less relative to all structural units of this non-crosslinked polymer. By containing component (c) within this range, excellent adhesion to the current collector can be easily ensured, and the cycle characteristics of the secondary battery can be improved. When the lower limit is 1% by mass or more, the dispersion stability of this composition becomes good, and higher adhesion can be obtained. Therefore, it is preferable that it is 3% by mass or more, 5% by mass or more, 10% by mass or more, 15% by mass or more, 20% by mass or more, or 25% by mass or more. In addition, the upper limit is, for example, 45% by mass or less, 40% by mass or less, and for example, 35% by mass or less, and for example, 30% by mass or less.

[0064] The total amount of monomer (c) relative to the total amount of the non-crosslinked polymer and its salts (hereinafter also referred to as "the content of residual monomer (c)") can, for example, be set to less than 5.0% by mass.

[0065] The content of the remaining monomer (c) is, for example, 4.0% by mass or less, for example, 3.0% by mass or less, for example, 2.0% by mass or less, for example, 1.5% by mass or less, for example, 1.0% by mass or less, for example, 0.5% by mass or less. If the content of the remaining monomer (c) is 5.0% by mass or less, high adhesion between the active substances can be obtained.

[0066] There is no specific lower limit for the content of the remaining monomer (c), for example, it can be set to 0.05% by mass or more. Other examples include 0.1% by mass or more, 0.2% by mass or more, 0.3% by mass or more, and 0.4% by mass or more.

[0067] It should be noted that the content of the remaining monomer (c) was determined by gas chromatography (GC) as described in the examples.

[0068] <Other structural units>

[0069] In addition to components (a), (b), and (c), this non-crosslinked polymer may also contain structural units derived from other olefinic unsaturated monomers that can copolymerize with them (hereinafter also referred to as "component (d)"). Examples of components (d) include, for instance, structural units derived from hydroxyl-containing olefinic unsaturated monomers (monomers represented by formula (2) below), olefinic unsaturated monomer compounds having anionic groups other than carboxyl groups, such as sulfonic acid groups and phosphate groups, or nonionic olefinic unsaturated monomers (excluding monomers classified as nitrogen-containing olefinic unsaturated monomers). These structural units can be introduced by copolymerizing monomers containing hydroxyl-containing olefinic unsaturated monomers, olefinic unsaturated monomer compounds having anionic groups other than carboxyl groups, such as sulfonic acid groups and phosphate groups, or nonionic olefinic unsaturated monomers.

[0070] CH2=C(R) 1 COOR 2 (2)

[0071] [In the formula, R] 1 R represents a hydrogen atom or a methyl group. 2 Represents a monovalent organic group with 1 to 8 carbon atoms and a hydroxyl group, (R 3 O) m H or R 4 O[CO(CH2)5O] n H. It should be noted that R 3 R represents an alkylene group having 2 to 4 carbon atoms. 4 This indicates an alkylene group with 1 to 8 carbon atoms, where m represents an integer from 2 to 15, and n represents an integer from 1 to 15.

[0072] The proportion of component (d) relative to all structural units of the non-crosslinked polymer can be set to 0% by mass or more and 50% by mass or less. The proportion of component (d) can be 0.1% by mass or more and 45% by mass or less, 0.5% by mass or more and 40% by mass or less, 1.0% by mass or more and 30% by mass or less, 5.0% by mass or more and 20% by mass or less, or 3% by mass or more and 10% by mass or less. Furthermore, when component (d) contains 0.1% by mass or more relative to all structural units of the non-crosslinked polymer, the affinity for the electrolyte is improved; therefore, an improvement in lithium-ion conductivity can also be expected.

[0073] As component (d), among the above, from the perspective of the excellent adhesive properties of the adhesive containing the non-crosslinked polymer or its salt, a hydroxyl-containing olefinic unsaturated monomer is preferred.

[0074] The monomer shown in formula (2) above is a (meth)acrylate compound having hydroxyl groups. In R 2In the case of a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, the number of hydroxyl groups can be only one or more. There are no particular limitations on the aforementioned monovalent organic group; examples include alkyl groups having straight-chain, branched, or cyclic structures, as well as aryl and alkoxyalkyl groups. Furthermore, in R... 2 For (R) 3 O) m H or R 4 O[CO(CH2)5O] n In the case of H, R 3 or R 4 The alkylene groups represented can be either straight-chain or branched.

[0075] Examples of monomers represented by formula (2) above include: hydroxyalkyl methacrylates having 1 to 8 carbon atoms, such as 2-hydroxyethyl methacrylate, 3-hydroxypropyl methacrylate, 4-hydroxybutyl methacrylate, hydroxyhexyl methacrylate, and hydroxyoctyl methacrylate; polyalkylene glycol mono(methacrylate), such as polyethylene glycol mono(methacrylate), polypropylene glycol mono(methacrylate), polybutylene glycol mono(methacrylate), and polyethylene glycol-polypropylene glycol mono(methacrylate); dihydroxyalkyl methacrylates such as glycerol mono(methacrylate); caprolactone-modified hydroxymethacrylate (manufactured by Daicel, trade names "PLACCEL FM1", "PLACCEL FM5", etc.), caprolactone-modified hydroxyacrylate (manufactured by Daicel, trade names "PLACCEL FA1", "PLACCEL FA10L", etc.), etc. The monomers shown in formula (2) above can be used alone or in combination of two or more.

[0076] Furthermore, from the viewpoint of obtaining an electrode with good bending resistance, structural units derived from nonionic olefinic unsaturated monomers are preferred.

[0077] Examples of nonionic olefinic unsaturated monomers include olefinic unsaturated monomers containing alicyclic structures.

[0078] Examples of olefinic unsaturated monomers containing alicyclic structures include: cyclopentyl methacrylate, cyclohexyl methacrylate, methylcyclohexyl methacrylate, tert-butylcyclohexyl methacrylate, cyclodecyl methacrylate, and cyclododecyl methacrylate, which can have aliphatic substituents; isobornyl methacrylate, adamantyl methacrylate, cyclopentenyl methacrylate, dicyclopentenoxyethyl methacrylate, dicyclopentyl methacrylate, and cyclohexanediethanol mono(meth)acrylate and cyclodecanediethanol mono(meth)acrylate, etc., which are cycloalkyl polyol mono(meth)acrylates, etc., which can be used alone or in combination of two or more.

[0079] From the perspective of excellent adhesive properties, the non-crosslinked polymer preferably contains structural units derived from monomers shown in formula (2) above, olefinic unsaturated monomers with alicyclic structures, etc. Among them, as component (c), from the perspective of excellent effect in improving the adhesive properties of the adhesive, structural units derived from monomers shown in formula (2) above are more preferred.

[0080] Among the monomers shown in formula (2) above, hydroxyalkyl esters of (meth)acrylate having 1 to 8 carbon atoms are further preferred, and 2-hydroxyethyl ester, 3-hydroxypropyl ester and 4-hydroxybutyl ester of (meth)acrylate are even more preferred.

[0081] Furthermore, as component (d), when a structural unit derived from a hydrophobic olefinic unsaturated monomer with a solubility of less than 1 g / 100 ml in water is incorporated, it can exhibit strong interaction with the electrode material and provide good adhesion to the active material. This results in a robust and well-integrated electrode mixture layer. Therefore, as the aforementioned "hydrophobic olefinic unsaturated monomer with a solubility of less than 1 g / 100 ml in water," an olefinic unsaturated monomer containing an alicyclic structure is particularly preferred.

[0082] In addition, other nonionic olefinic unsaturated monomers, such as (meth)acrylates, can be used. Examples of (meth)acrylates include: methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and other alkyl (meth)acrylate compounds.

[0083] Aromatic (meth)acrylate compounds such as phenyl methacrylate, phenyl methyl methacrylate, phenyl ethyl methacrylate, and phenoxy ethyl methacrylate;

[0084] 2-Methoxyethyl ester (meth)acrylate, 2-ethoxyethyl ester (meth)acrylate, and other alkoxyalkyl ester compounds (meth)acrylates can be used alone or in combination of two or more.

[0085] From the viewpoint of adhesion to active materials and cycling characteristics, aromatic (meth)acrylate compounds are preferred. From the viewpoint of further improving lithium-ion conductivity and high-rate characteristics, compounds with ether bonds, such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate alkoxyalkyl esters, are preferred, and 2-methoxyethyl (meth)acrylate is more preferred.

[0086] Among nonionic olefinic unsaturated monomers, compounds having acryloyl groups are preferred from the perspective of obtaining polymers with long primary chains due to fast polymerization speed and good adhesive strength of binders. Furthermore, as a nonionic olefinic unsaturated monomer, compounds with a glass transition temperature (Tg) of 0°C or lower are preferred from the perspective of good flexural strength of the resulting electrode.

[0087] This non-crosslinked polymer salt is a salt obtained by neutralizing some or all of the carboxyl groups contained in the polymer. The type of salt is not particularly limited, but examples include: alkali metal salts such as lithium, sodium, and potassium salts; alkaline earth metal salts such as magnesium, calcium, and barium salts; other metal salts such as aluminum salts; ammonium salts and organic amine salts, etc. Among these, alkali metal salts and alkaline earth metal salts are preferred from the perspective of minimizing adverse effects on battery characteristics, and alkali metal salts are more preferred. Lithium salts are particularly preferred from the perspective of excellent cycle characteristics and excellent low-temperature battery characteristics of secondary batteries.

[0088] Characteristics of the present non-crosslinkable polymer salt

[0089] This non-crosslinked polymer is preferably used in the form of a salt obtained by neutralizing acid groups such as carboxyl groups from olefinic unsaturated carboxylic acid monomers in this composition, with a neutralization degree of 40 mol% or more. A neutralization degree of 40 mol% or more is preferred from the viewpoint of easily obtaining dispersion stabilization effects.

[0090] From the perspective of achieving excellent charge / discharge capacity retention over longer periods of use than before, the aforementioned degree of neutralization is more preferably 50 mol% or more, further preferably 70 mol% or more, even more preferably 75 mol% or more, even more preferably 80 mol% or more, and particularly preferably 85 mol% or more. The upper limit of the degree of neutralization is 100 mol%, but it can be 98 mol% or 95 mol%. In this specification, the aforementioned degree of neutralization can be calculated based on the amount of monomers having acidic groups such as carboxyl groups and the amount of neutralizing agent used for neutralization. It should be noted that, regarding the degree of neutralization, the non-crosslinked polymer salt can be dissolved in a mixed solvent of ethanol / water = 1 / 1 (wt ratio), and the determination can be made by the ratio of the amount of carboxyl salt titrated with acid to the amount of acidic groups such as carboxyl groups titrated with alkali.

[0091] Molecular weight of the present non-crosslinkable polymer or salt thereof

[0092] The number-average molecular weight (Mn) of the non-crosslinked polymer or its salt was determined by aqueous gel permeation chromatography (GPC) as described in the examples, and expressed as a sodium polyacrylate equivalent. From the viewpoint of improving the dispersibility of the active material and achieving excellent charge / discharge capacity retention, Mn is preferably 4,000 or more and 200,000 or less, and can be adjusted by monomer concentration and initiator dosage.

[0093] Mn is more preferably 5,000 or more and 150,000 or less, even more preferably 7,500 or more and 100,000 or less, and even more preferably 10,000 or more and 50,000 or less.

[0094] The weight-average molecular weight (Mw) of the non-crosslinked polymer or its salts was determined according to the aqueous GPC described in the examples and in the form of sodium polyacrylate conversion. From the viewpoint of improving adhesion to active materials and / or current collectors and achieving excellent charge / discharge capacity retention, Mw is preferably 500,000 or more and 4,000,000 or less, and can be adjusted by monomer concentration and initiator dosage.

[0095] Mw is more preferably 600,000 or more and 3,500,000 or less, even more preferably 700,000 or more and 3,000,000 or less, even more preferably 800,000 or more and 2,500,000 or less, and even more preferably 900,000 or more and 2,000,000 or less.

[0096] From the perspectives of improving adhesion to active materials and / or current collectors, improving the dispersibility of active materials, and achieving excellent charge / discharge capacity retention, the molecular weight distribution (Mw / Mn) of the non-crosslinked polymer or its salt is preferably 5 or more and 300 or less, and can be adjusted by monomer concentration and polymerization temperature.

[0097] More preferably, the Mw / Mn ratio is 10 or more and 300 or less; further preferably, 20 or more and 290 or less; even more preferably, 30 or more and 280 or less; still more preferably, 40 or more and 270 or less; and particularly preferably, 50 or more and 260 or less. It can be inferred that by increasing the Mw / Mn ratio, both the high molecular weight component, which improves adhesion to the active material and / or current collector, and the low molecular weight component, which improves dispersibility of the active material, can coexist and perform their respective functions.

[0098] 2. Method for producing the present non-crosslinkable polymer or salt thereof

[0099] This non-crosslinked polymer can be obtained by polymerizing a monomeric component comprising monomer (a), monomer (b), and monomer (c).

[0100] Furthermore, the polymerization method can include solvent-free bulk polymerization, solvent-based solution polymerization, aqueous emulsion polymerization, microemulsion polymerization, or suspension polymerization. Among these, solution polymerization is preferred from the perspective of enabling uniform dissolution of the non-crosslinked polymer or its salts and facilitating easier dispersion when added to the electrode mixture layer composition in a slurry state.

[0101] As a specific polymerization solvent, water is preferred from the perspective of being able to uniformly dissolve monomers (a), (b), and (c) for polymerization.

[0102] Besides water, other water-soluble solvents include methanol, tert-butanol, acetone, methyl ethyl ketone, acetonitrile, and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane, and n-heptane. One of these solvents can be used alone or in combination of two or more. Alternatively, they can be used as a mixed solvent of water. In this invention, a water-soluble solvent refers to a solvent with a solubility in water greater than 10 g / 100 ml at 20°C.

[0103] Furthermore, to ensure the neutralization reaction proceeds stably and rapidly during the neutralization process, it is preferable to pre-add a small amount of a highly polar solvent to the polymerization solvent. Water and methanol are preferred examples of such highly polar solvents. The amount of highly polar solvent used is preferably 0.05 to 20.0% by mass based on the total mass of the medium, more preferably 0.1 to 10.0% by mass, further preferably 0.1 to 5.0% by mass, and even more preferably 0.1 to 1.0% by mass. If the proportion of the highly polar solvent is 0.05% by mass or more, the effect on the neutralization reaction is confirmed; if it is 20.0% by mass or less, no adverse effect on the polymerization reaction is observed. In addition, in the polymerization of highly hydrophilic olefinic unsaturated carboxylic acid monomers such as acrylic acid, the addition of a highly polar solvent increases the polymerization rate, making it easier to obtain polymers with long primary chains. Among highly polar solvents, water, in particular, has a significant effect on increasing the polymerization rate and is therefore preferred.

[0104] The polymerization initiator can be any known polymerization initiator such as azo compounds, organic peroxides, or inorganic peroxides, without particular limitation. The operating conditions can be adjusted to achieve an appropriate amount of free radical generation through known methods such as thermal initiation, redox initiation with a reducing agent, or UV initiation. To obtain a non-crosslinked polymer with a long primary chain, it is preferable to set the conditions to minimize the amount of free radical generation within the limits of the manufacturing time.

[0105] Here, as the polymerization initiator in aqueous solution polymerization, a water-soluble polymerization initiator is preferred. Examples include compounds having hydrophilic groups (e.g., carboxyl groups) and / or their salts or hydrates. Among these, 4,4'-azobis(4-cyanopentanoic acid), 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(imidazolin-2-yl)propane] disulfate dihydrate, 2,2'-azobis(2-methylpropanediamine) dihydrochloride, and 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropanediamine] hydrate are preferred.

[0106] When the total amount of monomer components used is set to 100 parts by mass, the preferred amount of polymerization initiator is, for example, 0.001 to 3 parts by mass, or, for example, 0.005 to 2.5 parts by mass, or, for example, 0.01 to 2 parts by mass. If the amount of polymerization initiator is 0.001 parts by mass or more, the polymerization reaction can proceed stably; if it is 1.5 parts by mass or less, polymers with long primary chains are easily obtained.

[0107] The polymerization temperature also depends on the type and concentration of the monomer used, and is preferably 0~100℃, more preferably 20~80℃. The polymerization temperature can be constant or it can vary during the polymerization reaction. In addition, the polymerization time is preferably 1 minute to 20 hours, more preferably 1 hour to 10 hours.

[0108] Here, the non-crosslinked polymer may contain structural units derived from monomer (a) at a rate of 40% to 98% by mass relative to all its structural units, structural units derived from monomer (b) at a rate of 1% to 50% by mass, and structural units derived from monomer (c) at a rate of 1% to 50% by mass, wherein the preferred range of the content of these structural units is as described above. Furthermore, the types of monomer (a), monomer (b), and monomer (c) are as described above.

[0109] 3. Composition for secondary battery electrode mixture layer

[0110] The composition for the secondary battery electrode adhesive layer of the present invention comprises the binder, active substance and water.

[0111] The amount of the binder in this composition is preferably 0.5 parts by mass or more and 7.0 parts by mass or less relative to 100 parts by mass of the total active material. Other examples include 0.8 parts by mass or more and 3.0 parts by mass or less, 1.0 parts by mass or more and 2.5 parts by mass or less, and 1.2 parts by mass or more and 1.5 parts by mass or less. If the amount of binder is 0.5 parts by mass or more, sufficient adhesion can be obtained. Furthermore, the dispersion stability of the active material, etc., can be ensured, and a uniform adhesive layer can be formed. If the amount of binder is 1.5 parts by mass or less, the composition will not become highly viscous, ensuring coatability for the current collector. As a result, an adhesive layer with a uniform and smooth surface can be formed.

[0112] Among the aforementioned active materials, lithium salts of transition metal oxides can be used as positive electrode active materials, such as layered rock salt type and spinel type lithium-containing metal oxides. Specific compounds that can be listed as layered rock salt type positive electrode active materials include: lithium cobalt oxide, lithium nickel oxide, and NCM{Li(Ni)}, which is known as a ternary system. x Co y Mn z ), x+y+z=1} and NCA{Li(Ni 1-a-b Co a Al bIn addition, lithium manganese oxide and other materials can be used as spinel-type positive electrode active materials. Besides oxides, phosphates, silicates, and sulfur can also be used. For example, olivine-type lithium iron phosphate can be used as a phosphate. As a positive electrode active material, one of the above-mentioned materials can be used alone, or two or more can be combined in the form of a mixture or compound.

[0113] It should be noted that when a positive electrode active material containing a layered rock salt-type lithium metal oxide is dispersed in water, lithium ions on the surface of the active material exchange with hydrogen ions in the water, resulting in an alkaline dispersion. Therefore, common positive electrode current collector materials such as aluminum foil (Al) may be corroded. In such cases, it is preferable to use an unneutralized or partially neutralized non-crosslinked polymer as a binder to neutralize the alkaline components dissolved from the active material. Furthermore, regarding the amount of the unneutralized or partially neutralized non-crosslinked polymer used, it is preferable to use it in such a way that the amount of unneutralized carboxyl groups in the non-crosslinked polymer is equivalent to or greater than the amount of alkali dissolved from the active material.

[0114] Since the conductivity of the positive electrode active material is generally low, conductive additives other than carbon nanotubes can be added. Examples of such conductive additives include carbon black, carbon fiber, graphite powder, and other carbon-based materials. Among these, carbon black and carbon fiber are preferred for easily obtaining excellent conductivity. Ketjen black and acetylene black are preferred as carbon black. One of the above-mentioned conductive additives can be used alone, or two or more can be used in combination. From the viewpoint of balancing conductivity and energy density, the amount of conductive additive other than carbon nanotubes relative to 100 parts by mass of the total active material can be, for example, 0.2 to 20 parts by mass, or, for example, 0.2 to 10 parts by mass. Furthermore, the positive electrode active material can be a substance obtained by surface coating with a conductive carbon-based material.

[0115] On the other hand, examples of negative electrode active materials include carbon-based materials, lithium metal, lithium alloys, and metal oxides, and one or more of these can be used. Among these, active materials formed from carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based active materials") are preferred, and graphite such as natural graphite and artificial graphite, as well as hard carbon, are more preferred. Furthermore, in the case of graphite, from the perspective of battery performance, spherical graphite is suitable, with a preferred particle size range, for example, 1 to 20 μm, and another example, 5 to 15 μm.

[0116] In addition, to improve energy density, metals or metal oxides capable of lithium absorption and storage, such as silicon and / or tin, can be used as negative electrode active materials. Silicon has a higher capacity than graphite, and active materials formed from silicon-based materials such as silicon, silicon alloys, and silicon oxides (SiO) can be used (hereinafter also referred to as "silicon-based active materials"). From the viewpoint of improving the capacity of the secondary battery, the amount of silicon-based active material used relative to the total amount of active material is 5.0% by mass or more, and for example, it can be set to 10.0% by mass or more, or for example, 20.0% by mass or more.

[0117] Carbon-based active materials inherently possess good electrical conductivity, thus eliminating the need for conductive additives other than carbon nanotubes. When conductive additives are added for purposes such as further reducing resistance, from an energy density perspective, their amount relative to the total amount of active material should be, for example, 10 parts by mass or less, or, for example, 5 parts by mass or less.

[0118] When this composition is in slurry form, the amount of active material relative to the total amount of the composition is, for example, in the range of 10 to 75% by mass, and more specifically, in the range of 30 to 65% by mass. If the amount of active material is 10% by mass or more, the migration of binders, etc., can be suppressed, and it is also advantageous in terms of the drying cost of the medium. On the other hand, if it is 75% by mass or less, the flowability and coatability of the composition can be ensured, and a uniform adhesive layer can be formed.

[0119] This composition uses water as a medium. Furthermore, to adjust the properties and drying characteristics of this composition, it can be prepared as a mixed solvent with water-soluble organic solvents such as lower alcohols like methanol and ethanol, carbonates like ethylene carbonate, ketones like acetone, tetrahydrofuran, and N-methyl-2-pyrrolidone. The proportion of water in the mixed medium is, for example, 50% by mass or more, and also, for example, 70% by mass or more.

[0120] When the composition is prepared into a coatable slurry, from the viewpoint of the coatability of the slurry and the energy cost and productivity required for drying, the content of the water-containing medium in the whole composition can be set to, for example, in the range of 25 to 60% by mass, or, for example, 35 to 60% by mass.

[0121] This composition can be further combined with other binder components such as styrene-butadiene rubber (SBR) latex, carboxymethyl cellulose (CMC), acrylic latex, and polyvinylidene fluoride latex. When other binder components are used, their amount relative to 100 parts by mass of the total active material can be, for example, 0.1 to 5 parts by mass or less, or, for example, 0.1 to 2 parts by mass or less, or, for example, 0.1 to 1 part by mass or less. If the amount of other binder components exceeds 5 parts by mass, the resistivity may increase, and the high-rate characteristics may become insufficient. Of the above, considering the excellent balance between adhesion and flexural strength, SBR latex and CMC are preferred, and a combination of SBR latex and CMC is more preferred.

[0122] The aforementioned SBR-based latex refers to an aqueous dispersion of a copolymer having structural units derived from aromatic vinyl monomers such as styrene and structural units derived from aliphatic conjugated diene monomers such as 1,3-butadiene. As aromatic vinyl monomers, in addition to styrene, examples include α-methylstyrene, vinyltoluene, and divinylbenzene; one or more of these may be used. From the viewpoint of adhesion, the structural units derived from the aforementioned aromatic vinyl monomers in the copolymer may, for example, be in the range of 20-70% by mass, or, for example, in the range of 30-60% by mass.

[0123] As examples of the aforementioned aliphatic conjugated diene monomers, in addition to 1,3-butadiene, other examples include 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-butadiene, and one or more of these may be used. From the viewpoint of good adhesion of the binder and good flexibility of the resulting electrode, the structural units derived from the aforementioned aliphatic conjugated diene monomers in the copolymer may, for example, be set in the range of 30 to 70% by mass, or, for example, in the range of 40 to 60% by mass.

[0124] In styrene / butadiene latexes, in addition to the monomers mentioned above, other monomers such as (meth)acrylonitrile, (meth)acrylonitrile, (meth)acrylic acid, itaconic acid, maleic acid, (meth)acrylate ...

[0125] The structural units derived from the other monomers in the copolymer can be set in the range of 0 to 30% by mass, or in the range of 0 to 20% by mass.

[0126] The aforementioned CMC refers to the carboxymethyl substituted derivatives and their salts of nonionic cellulose-based semi-synthetic polymers. Examples of such nonionic cellulose-based semi-synthetic polymers include: alkyl celluloses such as methylcellulose, methylethylcellulose, ethylcellulose, and microcrystalline cellulose; and hydroxyalkyl celluloses such as hydroxyethylcellulose, hydroxybutylmethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, hydroxypropylmethylcellulose stearyl ether, carboxymethyl hydroxyethylcellulose, alkyl hydroxyethylcellulose, and nonylphenol polyether hydroxyethylcellulose.

[0127] The composition for the electrode layer of the secondary battery of the present invention uses the above-mentioned binder, active material, and water as essential components, and is obtained by mixing the components using known methods. There are no particular limitations on the mixing method of each component; known methods can be used. Preferably, the active material, conductive additive, and binder powder components are dry-mixed, then mixed with a dispersion medium such as water, and dispersed and kneaded. When the composition is obtained in a slurry state, it is preferable to prepare a slurry without poor dispersion and / or aggregation. As a mixing means, known mixers such as planetary mixers, thin-film rotary mixers, and rotation-revolutionary mixers can be used. From the perspective of obtaining a good dispersion state in a short time, a thin-film rotary mixer is preferred. Furthermore, when using a thin-film rotary mixer, it is preferable to pre-dispersettle with a mixer such as a disperser. The pH of the slurry is not particularly limited as long as the effects of the present invention are achieved; it is preferably less than 12.5. For example, when CMC is mixed, from the perspective of minimizing concerns about its hydrolysis, it is more preferably less than 11.5, and even more preferably less than 10.5. Furthermore, the viscosity of the slurry is not particularly limited as long as the effects of the present invention are achieved. For example, a type B viscosity at 20 rpm (25°C) can be set in the range of 100~30,000 mPa·s, or 500~20,000 mPa·s, or 1,000~10,000 mPa·s. If the viscosity of the slurry is within the above range, good coatability can be ensured.

[0128] 4. Secondary battery electrode

[0129] The secondary battery electrode of the present invention is formed by having an agent layer made of the secondary battery electrode agent layer composition of the present invention on the surface of a current collector such as copper or aluminum. The agent layer is formed by coating the surface of the current collector with the present composition and then drying to remove media such as water. The method of coating the present composition is not particularly limited, and known methods such as doctor blade coating, dip coating, roller coating, comma coating, curtain coating, gravure coating, and extrusion can be used. In addition, the above-mentioned drying can be carried out by known methods such as hot air blowing, depressurization, (far)infrared radiation, and microwave irradiation.

[0130] Typically, the dried binder layer is compressed using methods such as metal molding and rolling. Compression brings the active material and binder together more tightly, improving the strength of the binder layer and its adhesion to the current collector. Compression can adjust the thickness of the binder layer to, for example, about 30-80% of its original thickness, resulting in a thickness of approximately 4-200 μm.

[0131] 5. Secondary battery

[0132] By incorporating a separator and an electrolyte in the electrode of the secondary battery according to the present invention, a secondary battery can be manufactured. The electrolyte can be liquid or gel-like.

[0133] A separator is positioned between the positive and negative electrodes of the battery to prevent short circuits caused by electrode contact and / or to retain the electrolyte to ensure ionic conductivity. The separator is preferably a thin-film insulating microporous membrane with good ion permeability and mechanical strength. Specific raw materials can include polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene (PTFE).

[0134] Electrolytes can be well-known electrolytes that are commonly used depending on the type of active material. In lithium-ion secondary batteries, specific solvents include cyclic carbonates with high dielectric constants and high electrolyte solubility, such as propylene carbonate and ethylene carbonate, and chain carbonates with low viscosity, such as methyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. These can be used alone or in mixed solvents. Electrolytes are used by dissolving lithium salts such as LiPF6, LiSbF6, LiBF4, LiClO4, and LiAlO4 in these solvents. In nickel-metal hydride secondary batteries, an aqueous solution of potassium hydroxide can be used as the electrolyte. Secondary batteries are obtained by forming positive and negative electrode plates separated by separators into a spiral or stacked structure and housing them in a casing.

[0135] Example

[0136] The present invention will now be described in detail based on embodiments. It should be noted that the present invention is not limited to these embodiments. Unless otherwise specified, "parts" and "%" refer to parts by mass and percentage by mass, respectively.

[0137] In the following examples, the evaluation of carboxyl-containing non-crosslinked polymers or their salts is carried out by the following methods.

[0138] (Determination of solid component concentration)

[0139] Take about 1.0 g of the sample into a weighing bottle whose weight has been previously measured [weight of the weighing bottle = B (g)], weigh it accurately together with the weighing bottle [W0 (g)], place the sample together with the weighing bottle in a windless dryer, dry it at 155 °C for 45 minutes, and measure the weight at this time together with the weighing bottle [W1 (g)]. Calculate the solid component concentration using the following formula.

[0140] Solid component concentration (mass %) = (W1 - B) / (W0 - B) × 100

[0141] (Determination of the contents of monomer (a) and its salt, monomer (b), and monomer (c))

[0142] First, weigh 4.8 g of an aqueous solution of a non-crosslinked polymer (salt) containing carboxyl groups and 40.2 g of ion-exchanged water into a 50 mL screw tube, mix well, and thus prepare a 2 mass % aqueous solution. Weigh 15 g of this solution into another 50 mL screw tube, further add 15 g of methanol, and mix well, thus preparing a 1 mass % solution.

[0143] Next, centrifuge the above 1 mass % solution (4000 rpm × 10 minutes), add 4 g of the supernatant after centrifugation, 1 g of a 2 mass % methanol solution of propylene glycol monomethyl ether, and 1 g of ion-exchange resin to a 9 mL screw tube, and mix with a stirring rotor for 1 hour. Filter the supernatant with a 0.45 μm filter, and use the filtered liquid as a sample for gas chromatography (GC) measurement.

[0144] It should be noted that through the above ion-exchange operation, monomer (a) and its salt contained in the non-crosslinked polymer salt containing carboxyl groups can be extracted. Therefore, after forming the form of monomer (a) (the form of unneutralized acid), GC measurement is carried out.

[0145] Based on the obtained measurement results, calculate the contents (%) of monomer (a) and its salt, monomer (b), and monomer (c) contained in the non-crosslinked polymer containing carboxyl groups or its salt.

[0146] <GC measurement conditions>

[0147] Apparatus: Agilent 7820A-1 (manufactured by Agilent Technologies)

[0148] Column: HP-INNOWAX 60 m × 0.32 mm, df = 0.5 μm

[0149] Column temperature: 40 °C (held for 7 minutes) → 260 °C (held for 1 minute)

[0150] Temperature increase rate: 10 °C / minute

[0151] The content of structural units derived from each monomer in non-crosslinked polymer salts containing carboxyl groups was determined using nuclear magnetic resonance (NMR).

[0152] The carboxyl-containing non-crosslinked polymer salts obtained in the manufacturing examples and comparative manufacturing examples were developed on an aluminum cup and vacuum dried at 70°C for 2 hours to obtain a dried solid. The dried solid was then micronized using a mortar and pestle and filled into a zirconia sleeve to prepare a sample for testing. Solid-state NMR was performed under the conditions described below to determine the content (mass %) of structural units derived from each monomer in the carboxyl-containing non-crosslinked polymer salt.

[0153] <Solid-state NMR Measurement Conditions>

[0154] Device: JNM-ECA400 (manufactured by JEOL)

[0155] Test tube: Zirconia sleeve

[0156] Sample rotation speed: 15kHz

[0157] Iononite determination: 13C

[0158] Chemical shift reference: The adamantane high magnetic field signal is set to 29.5 ppm (external standard).

[0159] Pulse sequence: CPMAS method

[0160] (Determination of molecular weight)

[0161] The molecular weight of carboxyl-containing non-crosslinked polymer salts was determined by gel permeation chromatography (GPC).

[0162] 0.1 g (0.02 g based on the solid content of the polymer salt) of each of the carboxyl-containing non-crosslinked polymer salts obtained in the manufacturing examples and comparative manufacturing examples were diluted with 40 g of 0.1 M sodium nitrate aqueous solution to prepare test samples. For the above test samples, aqueous GPC analysis was performed under the conditions described below to obtain the number-average molecular weight (Mn) and weight-average molecular weight (Mw) based on sodium polyacrylate. Furthermore, the molecular weight distribution (Mw / Mn) was calculated from the obtained values.

[0163] <GPC Measurement Conditions for Aquatic Systems>

[0164] Pillars: 2 x TSKgel GMPW from Tosoh Corporation

[0165] Solvent: 0.1M sodium nitrate aqueous solution

[0166] Temperature: 40℃

[0167] Detector: RI

[0168] Flow rate: 0.5 mL / min

[0169] Manufacturing of Non-Crosslinked Polymer Salts

[0170] (Manufacturing Example 1: Manufacturing of carboxyl-containing non-crosslinked polymer salt R-1)

[0171] The polymerization process utilizes a reactor equipped with stirring blades, a thermometer, a reflux condenser, and a nitrogen inlet pipe.

[0172] Under a nitrogen atmosphere, 450.0 parts of ion-exchanged water, 53.0 parts of acrylic acid (hereinafter also referred to as "AA"), 27.0 parts of acrylamide (hereinafter also referred to as "AAm"), and 20.0 parts of acrylonitrile (hereinafter also referred to as "AN") were added to a reactor and heated to 60°C. 1.20 parts of 2,2'-azobis(2-methylpropanediamine) dihydrochloride (manufactured by Fujifilm and Koko Pure Chemical Industries, Ltd., trade name "V-50", hereinafter also referred to as "V-50") as an initiator were added to the solution, and the reaction was allowed to proceed for 6 hours. Then, after reacting at 80°C for 2 hours, the polymerization reaction solution was cooled. After the internal temperature dropped to 45°C, 18.5 parts of lithium hydroxide monohydrate (hereinafter also referred to as "LiOH·H2O") powder were added, and the temperature was maintained at 45°C. The mixture was stirred for 30 minutes, thereby obtaining an aqueous solution of a carboxyl-containing non-crosslinked polymer salt R-1.

[0173] The solids concentration of the above aqueous solution was 18.9% by mass. The contents of AA and its salts in R-1, determined by GC, were 0.1% by mass, 0.1% by mass of AAM, and 0.1% by mass of AN. Furthermore, the Mn of R-1, determined by GPC, was 5,000, Mw was 1,270,000, and the Mw / Mn ratio was 254.

[0174] (Manufacturing Examples 2-15: Manufacture of carboxyl-containing non-crosslinked polymer salts R-2-R-15)

[0175] The amounts of each raw material are shown in Table 1. Otherwise, the same operation as in Manufacturing Example 1 is performed to obtain an aqueous solution of non-crosslinked polymer salts R-2 to R-15 containing carboxyl groups.

[0176] The concentrations of solid components in the above aqueous solutions, the contents of AA and its salts in R-2 to R-15 as determined by GC, the contents of AAM and AN, and the contents of Mn, Mw and Mw / Mn in R-2 to R-15 as determined by GPC are shown in Table 1.

[0177] (Comparative manufacturing examples 1-6: Manufacturing of carboxyl-containing non-crosslinked polymer salts R-16-R-21)

[0178] The amounts of each raw material are shown in Table 1. The same operation as in Manufacturing Example 1 was performed to obtain an aqueous solution of non-crosslinked polymer salts R-16 to R-21 containing carboxyl groups.

[0179] The concentrations of solid components in the above aqueous solutions, the contents of AA and its salts, AAM, and AN in R-16~R-21 as determined by GC, and the Mn, Mw, and Mw / Mn in R-16~R-21 as determined by GPC are shown in Table 1.

[0180] [Table 1]

[0181]

[0182] The details of the compounds used in Table 1 are shown below.

[0183] AA: Acrylic acid

[0184] • AAM: Acrylamide

[0185] AN: Acrylonitrile

[0186] V-50: 2,2'-Azobis(2-methylpropanediamine) dihydrochloride (manufactured by Fujifilm and Kazuko Pure Chemical Industries, Ltd.)

[0187] ·LiOH·H2O: Lithium hydroxide monohydrate

[0188] Example 1

[0189] (Preparation of the composition for the electrode mixture layer)

[0190] As active materials, artificial graphite (manufactured by Showa Denko under the trade name "SCMG-CF") and SiO (5μm manufactured by Osaka Titanium Technology Co., Ltd.) are used. As binders, an aqueous solution of carboxyl-containing non-crosslinked polymer salt R-1, a mixture of styrene / butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC) are used.

[0191] In a planetary mixer (PRIMIX HIVIS MIX 2P-03 model), water was added as a diluent with a solid component concentration of 55% by mass in the composition for electrode paste layer, and the mixture was mixed for 1 hour and 30 minutes to prepare an electrode paste layer composition (electrode paste) in slurry form.

[0192] (Making of the negative electrode plate)

[0193] Next, using a variable applicator, the aforementioned electrode paste was applied to the current collector (copper foil, thickness: 16.5 μm), and dried in a ventilated dryer at 80°C for 15 minutes to form an adhesive layer. Subsequently, the adhesive layer thickness was set to 50 ± 5 μm, and the adhesive density to be 1.60 ± 0.10 g / cm³. 3 After being rolled, the plates are punched into 3cm squares for battery evaluation, thus obtaining the negative electrode plate.

[0194] (Making of the positive electrode plate)

[0195] In an N-methylpyrrolidone (NMP) solvent, LiNi is added as the positive electrode active material. 0.5 Co 0.2 Mn 0.3 A composition for the positive electrode binder layer is prepared by mixing 100 parts of O2 (NCM), 2 parts of acetylene black, and 4 parts of polyvinylidene fluoride (PVDF) as a binder for the positive electrode.

[0196] Next, using a variable applicator, the above-mentioned positive electrode compound layer composition was applied to the current collector (aluminum foil, thickness: 20 μm) and dried to form the compound layer. Subsequently, the compound layer thickness was set to 125 μm ± 1 μm, and the compound density to 3.0 ± 0.10 g / cm³. 3 After being rolled, the plate is punched into 3cm squares for battery evaluation, thus obtaining the positive electrode plate.

[0197] (Preparation of electrolyte)

[0198] A non-aqueous electrolyte was prepared by adding 1 wt% vinylene carbonate (VC) and 2 wt% fluoroethylene carbonate (FEC) to a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC:DMC = 3:7 by volume) to dissolve 1.2 mol / L of LiPF6.

[0199] (Making of a secondary battery)

[0200] Regarding the battery's construction, lead terminals were installed at the positive and negative terminals. Electrodes, separated by a separator (made of polyethylene: 16μm thick, 47% porosity), were placed inside an object using an aluminum laminate as the battery's outer casing. Liquid was injected and the casing was sealed to create the experimental battery. It should be noted that the designed capacity of this prototype battery is 50mAh. The battery's design capacity was based on a charging termination voltage up to 4.2V.

[0201] < Evaluation of cycle characteristics >

[0202] Furthermore, under conditions of 2.5 to 4.2V and repeated charge-discharge cycles at a charge-discharge rate of 0.5C at 25°C, the capacity C after 100 cycles was measured. 100 .

[0203] Here, the cyclic characteristic (ΔC) is obtained using the following formula.

[0204] ΔC=C 100 / C0×100(%)

[0205] The ΔC calculated using the above formula is 87.5%, and the cycle characteristics based on the following benchmarks are rated as "A".

[0206] It should be noted that the higher the value of ΔC, the better the cycling characteristics.

[0207] (Criteria for determining cyclic characteristics)

[0208] A: The charge / discharge capacity retention rate is above 86.0%.

[0209] B: Charge / discharge capacity retention is above 83.0% and below 86.0%.

[0210] C: Charge / discharge capacity retention is above 81.0% and below 83.0%.

[0211] D: Charge / discharge capacity retention rate is less than 81.0%.

[0212] Examples 2 to 17 and Comparative Examples 1 to 6

[0213] The formulation is as described in Table 2. Except for this, the same procedures as in Example 1 were performed to prepare the electrode slurry, and the cycle characteristics of the battery with the negative electrode plate obtained using the electrode slurry were evaluated. The results are shown in Table 2.

[0214] [Table 2]

[0215]

[0216] The details of the compounds used in Table 2 are shown below.

[0217] SBR: Styrene-butadiene rubber

[0218] CMC: Sodium carboxymethyl cellulose

[0219] Evaluation Results

[0220] As can be seen from the results of Examples 1-17, the secondary battery using the carboxyl-containing non-crosslinked polymer salt of the present invention exhibits excellent cycle characteristics. It can be inferred that this is because by giving the carboxyl-containing non-crosslinked polymer structural units derived from amide-containing olefinic unsaturated monomers (monomer (b)) and nitrile-containing olefinic unsaturated monomers (monomer (c)), the adhesion is improved, and furthermore, by making the content of monomer (b) in the carboxyl-containing non-crosslinked polymer salt 5.0% by mass or less, the cycle characteristics are improved.

[0221] In particular, considering the content of structural units derived from monomer (c), compared to the case where the content is 10.0% by mass (Example 5), the secondary battery exhibits superior cycle characteristics in a wider range of 15.0 to 29.0% by mass (Examples 14 to 16). This can be presumably because the higher the proportion of nitrile groups in this non-crosslinked polymer, the better the adhesion.

[0222] Furthermore, when considering Mn, the cycle characteristics of the secondary battery are better when Mn is below 200,000 (Example 3-6) compared to when Mn exceeds 200,000 (Example 7).

[0223] Furthermore, considering the Mw / Mn ratio, compared to the range of 5.1 to 17.1 (Examples 5 to 7), the secondary battery exhibits superior cycle characteristics in the wider range of 58.9 to 254 (Examples 3 and 4). This can be presumably because increasing the Mw / Mn ratio allows the high molecular weight component, which enhances adhesion to the active material and / or current collector, and the low molecular weight component, which improves dispersibility of the active material, to coexist and perform their respective functions.

[0224] Furthermore, considering the presence or absence of CMC in the electrode slurry, a comparison of the case containing CMC (Example 8) and the case without CMC (Example 5) reveals superior cycle characteristics of the secondary battery. This can be presumed to be because the addition of CMC improves the dispersibility of the electrode slurry.

[0225] In contrast, when the non-crosslinked polymer salt containing carboxyl groups does not have structural units derived from monomer (b) or from monomer (c) (Comparative Examples 1 and 2), and when the content of monomer (b) exceeds 5.0% by mass (Comparative Examples 3 to 6), the cycling characteristics deteriorate.

[0226] Industrial availability

[0227] Secondary batteries containing the binder for secondary battery electrodes of the present invention exhibit good durability (cycle characteristics). Therefore, it is anticipated that secondary batteries with electrodes obtained using the above-described binder will ensure good integrity and exhibit good durability (cycle characteristics) even with repeated charge and discharge cycles, which is expected to contribute to the high-capacity development of secondary batteries for vehicles and the like.

[0228] The binder for secondary battery electrodes of the present invention is particularly suitable for use in non-aqueous electrolyte secondary battery electrodes, especially for high-energy-density non-aqueous electrolyte lithium-ion secondary batteries.

Claims

1. A binder for a secondary battery electrode, comprising: a carboxyl group-containing non-crosslinked polymer or a salt thereof, the carboxyl group-containing non-crosslinked polymer comprises: a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter referred to as "monomer (a)"), a structural unit derived from an amide group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (b)"), and a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (c)"), the total amount of the monomer (b) is 5.0 mass% or less relative to the total amount of the carboxyl group-containing non-crosslinked polymer and the salt thereof.

2. The binder for secondary battery electrodes according to claim 1, wherein, the total amount of the monomer (a) and the salt thereof, converted into an unneutralized form, is 5.0 mass% or less relative to the total amount of the carboxyl group-containing non-crosslinked polymer and the salt thereof.

3. The binder for secondary battery electrodes according to claim 1 or 2, wherein, the carboxyl group-containing non-crosslinked polymer comprises 40 mass% or more and 98 mass% or less of the structural unit derived from the monomer (a), 1 mass% or more and 50 mass% or less of the structural unit derived from the monomer (b), and 1 mass% or more and 50 mass% or less of the structural unit derived from the monomer (c), relative to the total structural units thereof.

4. The binder for secondary battery electrodes according to claim 1 or 2, wherein, the number average molecular weight of the carboxyl group-containing non-crosslinked polymer or the salt thereof is 4,000 or more and 200,000 or less.

5. The binder for secondary battery electrodes according to claim 1 or 2, wherein, the salt of the carboxyl group-containing non-crosslinked polymer is a salt in which 40 mol% or more of the carboxyl groups possessed by the non-crosslinked polymer are neutralized.

6. A composition for a secondary battery electrode mixture layer, comprising: the binder for a secondary battery electrode according to claim 1 or 2, an active material, and water.

7. The composition for a secondary battery electrode mixture layer according to claim 6, further comprising carboxymethyl cellulose (CMC).

8. A secondary battery electrode provided with a mixture layer formed from the composition for a secondary battery electrode mixture layer according to claim 6 on the surface of a current collector.

9. A secondary battery provided with the secondary battery electrode according to claim 8.

Images