Binder composition for secondary battery electrode, composition for forming secondary battery negative electrode, negative electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

To provide a binder composition that can improve the electrical characteristics of a lithium ion secondary battery when used as a binder together with an electrode active material, a conductive aid, etc. and in an electrode (electrode mixture layer) of a lithium ion secondary battery or the like.SOLUTION: A binder composition for a secondary battery electrode containing a copolymer obtained by polymerizing a monomer composition containing a (meth)acrylic acid ester, the binder composition satisfying all of the following conditions: condition a, condition b-1, condition b-2, condition c-1, condition c-2, condition d-1, and condition d-2. Condition a, condition b-1, condition b-2, condition c-1, condition c-2, condition d-1, and condition d-2 are as defined in the specification.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a binder composition for secondary battery electrodes, a composition for forming a secondary battery negative electrode, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery. [Background technology]

[0002] In recent years, energy storage devices such as lithium-ion secondary batteries and capacitors, which are small, lightweight, and high-output, have become increasingly sophisticated, and as a result, they are increasingly being used not only in small electrical appliances but also in large products such as automobiles. The improved performance of lithium-ion secondary batteries has been achieved primarily through improvements in battery components such as electrolytes, positive electrodes, negative electrodes, and separators. However, even the negative electrode, for example, contains constituent materials such as a negative electrode active material, binder, conductive additive, and current collector, and further evolution of these constituent materials is thought to be extremely important for improving the performance of energy storage devices such as lithium-ion secondary batteries.

[0003] Patent Document 1 discloses an invention relating to a dispersion liquid of a conductive material for electrodes, which contains a polymer in which the content of an aromatic vinyl monomer and an ethylenically unsaturated nitrile monomer is not more than a specific amount, and reports that the dispersion liquid has excellent dispersibility of the conductive material and excellent adhesion between the electrode mixture layer and the current collector. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 065471 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of one embodiment of the present invention is to provide a binder composition for secondary battery electrodes, which, when blended as a binder together with an electrode active material and the like and used in an electrode (electrode mixture layer) of a lithium ion secondary battery or the like, can improve the electrical characteristics of the lithium ion secondary battery. Another object of the present invention is to provide a composition for forming a secondary battery negative electrode, which contains the binder composition for a secondary battery electrode and a negative electrode active material; a lithium ion secondary battery negative electrode including a negative electrode mixture layer formed using the composition for forming a secondary battery negative electrode; and a lithium ion secondary battery including a positive electrode, the lithium ion secondary battery negative electrode, a non-aqueous electrolyte, and a separator. [Means for solving the problem]

[0006] As a result of extensive research aimed at solving the above-mentioned problems, the present inventors have found that the electrical characteristics of lithium ion secondary batteries can be improved by using a copolymer that satisfies specific conditions and is obtained by polymerizing a monomer composition containing a (meth)acrylic acid ester in an electrode of the battery or the like, and have thus completed the present invention. That is, one aspect of the present invention is as follows.

[0007] <1> A binder composition for a secondary battery electrode, comprising a copolymer obtained by polymerizing a monomer composition containing a (meth)acrylic acid ester, A binder composition for a secondary battery electrode, which satisfies all of the following conditions: condition a, condition b-1, condition b-2, condition c-1, condition c-2, condition d-1, and condition d-2. Condition a: The copolymer forms a particle shape, and the volume average particle diameter of the particles measured by dynamic light scattering is 50 nm to 500 nm. Condition b-1: The monomer composition has a glass transition temperature (Tg HP ) contains (meth)acrylic acid esters whose temperature is -15°C or lower. Condition b-2: The glass transition temperature (Tg HP The total charged mass of the (meth)acrylic acid esters having a temperature of -15°C or lower is 39% by mass to 78% by mass when the total charged mass of all the monomers is taken as 100% by mass. Condition c-1: The monomer composition contains (meth)acrylonitrile. Condition c-2: The total charged mass of the (meth)acrylonitrile in the monomer composition is 21% by mass to 60% by mass when the total charged mass of all the monomers is taken as 100% by mass. Condition d-1: The monomer composition contains a crosslinkable monomer. Condition d-2: The total charged mass of the crosslinkable monomers in the monomer composition is 0.1% by mass to 15% by mass when the total charged mass of all the monomers is taken as 100% by mass. <2> the crosslinkable monomer contains at least one functional group selected from the group consisting of a methylol group, an epoxy group, a hydrolyzable silyl group, an allyl group, and a vinyl group; <1> The binder composition for a secondary battery electrode according to claim 1. <3> Furthermore, the following conditions e-1 and e-2 are met: <1> or <2> The binder composition for a secondary battery electrode according to claim 1. Condition e-1: The monomer composition has a Hansen solubility parameter (HSP value) of 17.0 MPa. 1 / 2 ~20.0MPa 1 / 2 Contains (meth)acrylic acid esters of the above. Condition e-2: The Hansen solubility parameter (HSP value) of the monomer composition is 17.0 MPa. 1 / 2 ~20.0MPa 1 / 2 The total charged mass of the (meth)acrylic acid ester is 39% by mass to 70% by mass when the total charged mass of all the monomers is taken as 100% by mass. <4> The glass transition temperature (Tg HP ) is a compound represented by the following formula (I): <1> ~ <3> 10. The binder composition for a secondary battery electrode according to claim 9, wherein the binder composition is a cellulose acylate.

[0008] [ka]

[0009] (In formula (I), R 1represents a hydrocarbon group having 4 to 20 carbon atoms which may contain at least one group selected from the group consisting of an oxa group (-O-) and a carbonyl group (>C=O) as a substituent, and R represents a hydrogen atom or a methyl group (-CH3). <5> <1> ~ <4> 10. A composition for forming a secondary battery negative electrode, comprising the binder composition for a secondary battery electrode according to any one of 1 to 9 and a negative electrode active material. <6> the negative electrode active material contains at least one selected from the group consisting of carbon particles, silicon particles, silicon oxide particles, and silicon carbide particles; <5> The composition for forming a secondary battery negative electrode according to claim 1. <7> <5> or <6> 2. A negative electrode for a lithium ion secondary battery, comprising a negative electrode mixture layer formed using the composition for forming a secondary battery negative electrode according to claim 1. <8> A lithium ion secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, The negative electrode is <7> A lithium ion secondary battery, which is the negative electrode for a lithium ion secondary battery according to claim 1. [Effects of the Invention]

[0010] According to one aspect of the present invention, the electrical characteristics of a lithium ion secondary battery, in particular the capacity retention rate after a charge-discharge cycle test at high temperature, can be improved. According to another aspect of the present invention, there can be provided a composition for forming a secondary battery negative electrode, which contains the binder composition for a secondary battery electrode and a negative electrode active material; a lithium ion secondary battery negative electrode including a negative electrode mixture layer formed using the composition for forming a secondary battery negative electrode; and a lithium ion secondary battery including a positive electrode, the lithium ion secondary battery negative electrode, a non-aqueous electrolyte, and a separator. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic cross-sectional view showing a stacked lithium ion secondary battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] In explaining the present invention, specific examples will be given, but the present invention is not limited to the following content and can be implemented with appropriate modifications without departing from the spirit of the present invention.

[0013] In the present disclosure, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. In the present disclosure, in the numerical ranges described in stages, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In addition, in the numerical ranges described in the present disclosure, the upper or lower limit value of the numerical range may be replaced with the value shown in the examples. In the present disclosure, when a plurality of substances corresponding to each component are present in the composition, the amount of each component in the composition means the total amount of the plurality of substances present in the composition, unless otherwise specified. In the present disclosure, combinations of preferred aspects are more preferred aspects. In the present disclosure, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0014] <Binder composition for secondary battery electrodes> A binder composition for secondary battery electrodes (hereinafter sometimes abbreviated as "binder composition"), which is one aspect of the present invention, is a binder composition containing a "copolymer" (hereinafter sometimes abbreviated as "copolymer") obtained by polymerizing a "monomer composition" (hereinafter sometimes abbreviated as "monomer composition") containing a "(meth)acrylic acid ester," and satisfies all of the following conditions a, b-1, b-2, c-1, c-2, d-1, and d-2: Condition a: The copolymer forms a particle shape, and the volume average particle diameter of the particles measured by dynamic light scattering is 50 nm to 500 nm. Condition b-1: The monomer composition has a glass transition temperature (Tg HP) contains (meth)acrylic acid esters whose temperature is -15°C or lower. Condition b-2: The glass transition temperature (Tg HP The total charged mass of the (meth)acrylic acid esters having a temperature of -15°C or lower is 39% by mass to 78% by mass when the total charged mass of all the monomers is taken as 100% by mass. Condition c-1: The monomer composition contains (meth)acrylonitrile. Condition c-2: The total charged mass of the (meth)acrylonitrile in the monomer composition is 21% by mass to 60% by mass when the total charged mass of all the monomers is taken as 100% by mass. Condition d-1: The monomer composition contains a crosslinkable monomer. Condition d-2: The total charged mass of the crosslinkable monomers in the monomer composition is 0.1% by mass to 15% by mass when the total charged mass of all the monomers is taken as 100% by mass.

[0015] The present inventors have conducted extensive research in search of a material useful as a binder for electrodes (electrode mixture layers) of lithium ion secondary batteries and the like, and have found that a copolymer obtained by polymerizing a monomer composition containing a (meth)acrylic acid ester and satisfying all of the above-mentioned conditions a, b-1, b-2, c-1, c-2, d-1, and d-2 can improve the electrical characteristics of lithium ion secondary batteries, particularly the capacity retention rate after a charge-discharge cycle test at high temperatures, when used as a binder.

[0016] Although the mechanism by which the capacity retention rate is improved when the copolymer of the present disclosure is used as a binder has not been specifically elucidated, it is believed that the introduction of (meth)acrylonitrile and a crosslinkable monomer into particles formed by the copolymer (hereinafter sometimes referred to as "copolymer particles") improves the breaking strength and suppresses the expansion and contraction of the active material due to charge and discharge. This is thought to suppress the separation of the active material due to charge and discharge cycles at high temperatures in the battery, improve the capacity retention rate, reduce DC resistance, and improve the cycle characteristics of the battery.

[0017] Furthermore, since the copolymer of the present disclosure has a relatively small volume average particle size and the monomer composition of the present disclosure contains a monomer having a low glass transition temperature (-15°C or lower) as a homopolymer, it is believed that the copolymer adheres thinly and uniformly to the surface of the electrode active material. + It is presumed that when ) is inserted into and removed from the electrode active material, the obstacles at the interface between the electrode active material and the electrolyte become smaller, thereby reducing the DC resistance.

[0018] The term "binder" refers to a compound that exhibits the function of binding particles of an electrode active material or the like together and / or the function of fixing an electrode mixture layer on a current collector, and the term "binder composition" refers to a composition that contains at least the compound. Furthermore, the term "(meth)acrylic acid ester" refers to an acrylic acid ester and / or a methacrylic acid ester. Also, "(meth)acrylonitrile" means acrylonitrile and / or methacrylonitrile. Furthermore, the term "crosslinkable monomer" refers to a monomer that is incorporated into copolymer molecules and acts to link the copolymer molecules together. Specifically, the "crosslinkable monomer" is a monomer that contains a reactive functional group. The type of functional group contained in the "crosslinkable monomer" is not limited, but is usually a functional group that reacts with the same groups to link them together, or a functional group that reacts with another group in the copolymer to link them together. Furthermore, the term "monomer composition" refers to a composition containing all of the monomers required for the polymerization reaction, and in the present invention, the composition contains at least a (meth)acrylic acid ester, (meth)acrylonitrile, and a crosslinkable monomer as monomers.

[0019] In addition, the glass transition temperature (Tg HP "(Meth)acrylic acid ester having a glass transition temperature (Tg) of -15°C or lower" means a (meth)acrylic acid ester having a polymer having a glass transition temperature (Tg) of -15°C or lower when polymerized alone, and the glass transition temperature (Tg) of the polymer as a homopolymer is not included. HP ) is referred to herein as "Tg HPFor monomers for which the numerical values ​​are specifically described, the described temperature is referred to as "Tg HP For monomers for which the numerical values ​​are not specifically stated, the actual values ​​determined by differential scanning calorimetry (DSC, heating rate: 1°C / min) shall be used.

[0020] Hereinafter, "copolymer," "monomer composition," "(meth)acrylic acid ester," and "Tg HP The terms "(meth)acrylic acid ester having a temperature of -15°C or less," "(meth)acrylonitrile," and "crosslinkable monomer" will be explained in detail.

[0021] The copolymer is obtained by polymerizing a monomer composition containing a (meth)acrylic acid ester, and the (meth)acrylic acid ester has a Tg HP The (meth)acrylic acid ester having a Tg≦−15° C. is included (satisfying condition b-1), but the specific type of (meth)acrylic acid ester having a Tg≦−15° C. is not particularly limited and can be appropriately selected depending on the purpose. Specific examples include compounds represented by the following formula (I):

[0022] [ka]

[0023] (In formula (I), R 1 represents a hydrocarbon group having 4 to 20 carbon atoms which may contain at least one group selected from the group consisting of an oxa group (-O-) and a carbonyl group (>C=O) as a substituent, and R represents a hydrogen atom or a methyl group (-CH3).

[0024] The "hydrocarbon group" in formula (I) is not limited to an aliphatic hydrocarbon group having a straight-chain structure, but may be a group having at least one structure selected from the group consisting of a branched structure, a cyclic structure, and a carbon-carbon unsaturated bond structure (a carbon-carbon double bond structure and a carbon-carbon triple bond structure), and the number of these structures is not limited. Therefore, (acyclic) aliphatic hydrocarbon groups, monocyclic aliphatic hydrocarbon groups, polycyclic aliphatic hydrocarbon groups, monocyclic aromatic hydrocarbon groups, and polycyclic aromatic hydrocarbon groups are all included in the "hydrocarbon group." Furthermore, naturally, alkyl groups, alkenyl groups, alkynyl groups, aryl groups, etc. are all included in the "hydrocarbon group." Furthermore, the phrase "may contain at least one group selected from the group consisting of an oxa group (-O-) and a carbonyl group (>C=O) as a substituent" means that some methylene groups (-CH2-) in the hydrocarbon group may be substituted with an oxa group (-O-) or a carbonyl group (>C=O), and the number of substituted methylene groups is not particularly limited. The glass transition temperature (Tg HP ) is R and R 1 First, methacrylates where R is a methyl group have a higher Tg than acrylates where R is a hydrogen atom. HP In addition, R 1 The larger the number of carbon atoms in HP tends to become smaller, but R 1 When the number of carbon atoms in exceeds a certain number, R 1 The properties of Tg HP tends to become larger.

[0025] Tg HP (Meth)acrylic acid esters with a Tg of ≦-15°C include ethyl acrylate (ethyl acrylate, HP : -22℃), n-butyl acrylate (n-butyl acrylate, Tg HP : -56℃), isobutyl acrylate (isobutyl acrylate, Tg HP : -25℃), 2-ethylhexyl acrylate (2-ethylhexyl acrylate, TgHP : -70℃), 2-ethylhexyl methacrylate (2-ethylhexyl methacrylate, Tg HP :-20℃), isomyristyl acrylate (isomyristyl acrylate, Tg HP These compounds are commercially available and can be used as monomers. HP The (meth)acrylic acid ester having a temperature of ≦−15° C. is not limited to one type, and two or more types may be used for copolymerization.

[0026] [ka]

[0027] The monomer composition has a Hansen solubility parameter (HSP value) of 17.0 MPa. 1 / 2 ~20.0MPa 1 / 2 It is preferable that the (meth)acrylic acid ester of the formula (satisfying condition e-1) is contained. The monomer composition is determined by the glass transition temperature (Tg HP ) is -15°C or less, and the Hansen solubility parameter (HSP value) is 17.0MPa 1 / 2 ~20.0MPa 1 / 2 It is more preferable that the (meth)acrylic acid ester is The Hansen solubility parameter (HSP value) can be calculated as the square root of the dispersion component δd, the polar component δp, and the hydrogen bonding component δh, and can be calculated using the software HSPiP (Hansen Solubility Parameters in Practice) version 5.3.06.

[0028] Hansen solubility parameter (HSP value) is 17.0MPa 1 / 2 ~20.0MPa 1 / 2 The (meth)acrylic acid ester is ethyl acrylate (ethyl acrylate, HSP value: 18.9 MPa). 1 / 2), n-butyl acrylate (n-butyl acrylate, HSP value: 18.6 MPa 1 / 2 ), acrylate isobutyl (isobutyl acrylate, HSP value: 18.1 MPa 1 / 2 ), lauryl methacrylate (lauryl methacrylate, HSP value: 17.4 MPa 1 / 2 ), 2-ethylhexyl acrylate (2-ethylhexyl acrylate, HSP value: 18.5 MPa 1 / 2 ), 2-ethylhexyl methacrylate (2-ethylhexyl methacrylate, HSP value: 18.3 MPa 1 / 2 ), isomyristyl acrylate (isomyristyl acrylate, HSP value: 17.0 MPa 1 / 2 ) etc.

[0029] The Hansen solubility parameter (HSP value) of the (meth)acrylic acid ester is preferably 17.3 MPa. 1 / 2 More preferably, 17.8 MPa 1 / 2 More preferably, 18.0 MPa 1 / 2 or more, preferably 19.5 MPa 1 / 2 Less than or equal to 19.0 MPa, preferably 1 / 2 or less, more preferably 18.8 MPa 1 / 2 When the Hansen solubility parameter of the (meth)acrylic acid ester is within the above range, it becomes easier for the (meth)acrylic acid ester to exhibit optimal swelling properties in an electrolyte solution.

[0030] Tg in the monomer composition HP ≦-15℃ (meth)acrylic acid ester total charge mass (Tg HP When two or more types of (meth)acrylic acid esters are copolymerized, the total mass of all charged monomers (when the total mass of all monomers is copolymerized) is 39% to 78% by mass (satisfies condition b-2), but is preferably 45% by mass or more, more preferably 50% by mass or more, and is preferably 75% by mass or less, more preferably 70% by mass or less, and even more preferably 60% by mass or less. HP When the total charged mass of the (meth)acrylic acid ester at temperatures ≦−15° C. is within the above range, the adhesion to the substrate can be improved.

[0031] HSP value of the monomer composition: 17.0 MPa 1 / 2 ~20.0MPa 1 / 2 Total charge mass of (meth)acrylic acid ester (HSP value 17.0 MPa 1 / 2 ~20.0MPa 1 / 2 When two or more types of (meth)acrylic acid esters are copolymerized, the total mass of all charged monomers is preferably 39% to 70% by mass (satisfying condition e-2), more preferably 45% by mass or more, even more preferably 50% by mass or more, more preferably 65% ​​by mass or less, even more preferably 60% by mass or less, and particularly preferably 55% by mass or less, when the total mass of all monomers is taken as 100% by mass (satisfying condition e-2). HSP value: 17.0 MPa 1 / 2 ~20.0MPa 1 / 2 When the total charged mass of the (meth)acrylic acid ester is within the above range, it becomes easier for the (meth)acrylic acid ester to exhibit optimal swelling properties in the electrolyte solution.

[0032] The copolymer is obtained by polymerizing a monomer composition containing a (meth)acrylic acid ester, and the monomer composition contains (meth)acrylonitrile (satisfying condition c-1), but the monomer composition may also be a composition containing acrylonitrile, methacrylonitrile, or both acrylonitrile and methacrylonitrile.

[0033] The total mass of (meth)acrylonitrile in the monomer composition is 21% by mass to 60% by mass (satisfies condition c-2) when the total mass of all monomers is 100% by mass, but is preferably 25% by mass or more, preferably 55% by mass or less, and more preferably 50% by mass or less. When the total mass of (meth)acrylonitrile is within the above range, the capacity retention rate of the battery is easily improved.

[0034] The copolymer is obtained by polymerizing a monomer composition containing a (meth)acrylic acid ester, and the monomer composition contains a crosslinkable monomer (satisfying condition d-1), but the specific type of the monomer composition is not particularly limited and can be appropriately selected depending on the purpose. The functional group contained in the crosslinkable monomer may be at least one selected from the group consisting of a methylol group, an epoxy group, a hydrolyzable silyl group, an allyl group, and a vinyl group. Examples of the crosslinkable monomer containing a methylol group include N-methylolacrylamide, N-methoxymethyl(meth)acrylamide, and N-butoxymethyl(meth)acrylamide. Examples of the crosslinkable monomer containing an epoxy group include glycidyl (meth)acrylate and (3,4-epoxycyclohexyl)-methyl (meth)acrylate. Examples of crosslinkable monomers containing a hydrolyzable silyl group include γ-methacryloyloxypropyltrimethoxysilane, γ-acryloyloxypropyltrimethoxysilane, vinyltriethoxysilane, vinyltrimethoxysilane, 2-styrylethyltrimethoxysilane, and vinyltris(methoxyethoxy)silane. Examples of crosslinkable monomers containing an allyl group or a vinyl group include allyl (meth)acrylate, (poly)ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, divinylbenzene, and trimethylolpropane tri(meth)acrylate.

[0035] The total mass of the crosslinkable monomers in the monomer composition is 0.1% by mass to 15% by mass (satisfies condition d-2) when the total mass of all the monomers is taken as 100% by mass, but is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and preferably 7% by mass or less. When the total mass of the crosslinkable monomers is within the above range, it becomes easier to suppress expansion and contraction of the active material during charge and discharge.

[0036] The copolymer is obtained by polymerizing a monomer composition containing a (meth)acrylic acid ester, but the monomer composition may contain a monomer other than a (meth)acrylic acid ester. Examples of the monomer other than a (meth)acrylic acid ester include unsaturated carboxylic acids and salts thereof, unsaturated sulfonic acids and salts thereof, unsaturated amides, etc., in addition to the above-mentioned (meth)acrylonitrile and crosslinkable monomers. The "unsaturated carboxylic acids and salts thereof," "unsaturated sulfonic acids and salts thereof," "unsaturated amides," etc. will be described in detail below.

[0037] Examples of unsaturated carboxylic acids and salts thereof include monocarboxylic acids (including monocarboxylic acid salts) such as acrylic acid, methacrylic acid, and crotonic acid, and dicarboxylic acids (including dicarboxylic acid salts) such as maleic acid, fumaric acid, and itaconic acid. Note that examples of counter ions of unsaturated carboxylic acid salts include lithium ions, potassium ions, sodium ions, and ammonium ions.

[0038] The total amount of the unsaturated carboxylic acid and its salt in the monomer composition is usually 1% by mass to 10% by mass, preferably 1.5% by mass or more, and preferably 7% by mass or less, when the total amount of the monomers charged is 100% by mass.

[0039] Examples of unsaturated sulfonic acids and salts thereof include 2-methyl-2-propene-1-sulfonic acid, 2-sulfoethyl methacrylate, t-butylacrylamidosulfonic acid, styrenesulfonic acid, etc. Examples of counter ions of unsaturated sulfonic acid salts include lithium ions, potassium ions, sodium ions, and ammonium ions.

[0040] The total charged mass of the unsaturated sulfonic acid in the monomer composition is usually 1% by mass to 10% by mass, preferably 1.5% by mass or more, and preferably 7% by mass or less, when the total charged mass of all the monomers is 100% by mass.

[0041] Examples of unsaturated amides include acrylamide, methacrylamide, N-isopropylacrylamide, N,N-dimethylacrylamide, N,N-dimethylmethacrylamide, N,N-diethylacrylamide, N,N-diethylmethacrylamide, N-methylolmethacrylamide, N-methylolacrylamide, diacetoneacrylamide, and maleic acid amide.

[0042] The total charged mass of the unsaturated amide in the monomer composition is usually 1% by mass to 10% by mass, preferably 1.5% by mass or more, and preferably 7% by mass or less, when the total charged mass of all the monomers is 100% by mass.

[0043] Other monomers that may be contained in the monomer composition include dienes such as styrene, vinyl chloride, vinyl ether, vinyl ketone, vinylamide, chloroprene, ethylene, propylene, isoprene, butadiene, vinylpyrrolidone, divinylbenzene, and vinyl esters, and silane coupling agents such as trimethoxyvinylsilane.

[0044] The total charged mass of other monomers in the monomer composition is usually 0.05% by mass to 10% by mass, but is preferably 0.1% by mass or more, and preferably 8% by mass or less, more preferably 9% by mass or less, and even more preferably 7% by mass or less, when the total charged mass of all monomers is 100% by mass.

[0045] As the polymerization method for producing the copolymer, any known polymerization method such as emulsion polymerization, solution polymerization, block (bulk) polymerization, or suspension polymerization can be appropriately adopted, but emulsion polymerization (aqueous solution polymerization) carried out in a solvent containing water as the main component is particularly preferred. "Emulsion polymerization" and the like will be explained in detail below.

[0046] The polymerization procedure and conditions in the emulsion polymerization method (aqueous solution polymerization method) are not particularly limited, and known procedures and conditions can be appropriately adopted. Examples of such a procedure include a method in which a dispersion (emulsion) of a monomer composition emulsified with a surfactant is added dropwise to an aqueous solution (dispersion) in which a polymerization initiator is dissolved or dispersed and kept at a polymerization temperature, to initiate polymerization, and the pH of the resulting dispersion is adjusted. The polymerization temperature is usually 30 to 95°C, preferably 50°C or higher, and preferably 85°C or lower.

[0047] In emulsion polymerization, a polymerization initiator, a surfactant, and a pH adjuster (neutralizer) are typically used, but other additives may be added to the solvent before, during, or after polymerization. Examples of additives include organic solvents, chelating agents, pigments, antistatic agents, antioxidants, preservatives, UV absorbers, light stabilizers, fluorescent brighteners, colorants, penetrants, and release agents. Ethylenediaminetetraacetic acid (EDTA) and its salts can be used as appropriate because they act as pH adjusters and chelating agents and can improve polymerization stability during polymerization.

[0048] The polymerization initiator is preferably a water-soluble initiator, and specific examples thereof include persulfates such as potassium persulfate and ammonium persulfate, as well as hydrogen peroxide, ammonium persulfate, azobiscyanovaleric acid, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-phenylamidino)propane] dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidino]propane} dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidino]propane} dihydrochloride, 2,2'-azobis[2-(N-benzylamidino)propane] dihydrochloride, 2,2'-azobis[2-(N-allylamidino)propane] dihydrochloride, 2,2'-azobis{2-[N-(2-hydroxyethyl) azo compounds such as 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide}, 2,2'-azobis[2-methyl-N-[2-hydroxyethyl]propionamide], and 2,2'-azobis(isobutylamide) dihydrate; and organic peroxides such as cumene hydroperoxide, t-butyl hydroperoxide, benzoyl peroxide, t-butyl peroxy-2-ethylhexanoate, t-butyl peroxybenzoate, and lauroyl peroxide.

[0049] The total amount of the polymerization initiator charged in the emulsion polymerization method is usually 0.1 to 5 parts by mass when the total amount of the monomers charged is 100 parts by mass.

[0050] Examples of surfactants include anionic surfactants such as dodecylbenzenesulfonate, lauryl sulfate, alkyldiphenyletherdisulfonate, alkylnaphthalenesulfonate, dialkylsulfosuccinate, stearate, oleate, dioctyl sulfosuccinate, polyoxyethylene alkyl ether sulfate, polyoxyethylene alkyl ether sulfate, polyoxyethylene alkylphenyl ether sulfate, dialkyl sulfosuccinate, and t-octylphenoxyethoxypolyethoxyethyl sulfate; Nonionic surfactants such as polyoxyethylene lauryl ether, polyoxyethylene stearyl ether, polyoxyethylene styrenated phenyl ether, polyoxyethylene distyrenated phenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene oleylphenyl ether, polyoxyethylene nonylphenyl ether, oxyethylene-oxypropylene block copolymer, t-octylphenoxyethyl polyethoxyethanol, and nonylphenoxyethyl polyethoxyethanol are examples thereof.

[0051] The total charged mass of the surfactants in the emulsion polymerization method is usually 0 to 100 parts by mass, preferably 3 parts by mass or more, and preferably 80 parts by mass or less, when the total charged mass of all the monomers is 100 parts by mass.

[0052] Examples of pH adjusters include ammonia, organic amines, potassium hydroxide, sodium hydroxide, lithium hydroxide, etc. The pH adjustment using a pH adjuster is usually carried out after polymerization using aqueous ammonia to adjust the pH to a range of 7 to 11.

[0053] Examples of the organic solvent include alkyl alcohols having 1 to 4 carbon atoms, such as n-butanol, isobutanol, 2-butanol, propanol, and isopropanol, and N-methyl-2-pyrrolidone.

[0054] In the emulsion polymerization method, the total charged mass of the organic solvents is usually 0 to 40 parts by mass, and preferably 30 parts by mass or less, when the charged mass of water is 100 parts by mass.

[0055] The glass transition temperature (Tg) of the copolymer means a theoretical value calculated from the following Fox's formula, excluding the crosslinkable monomer. Fox formula: 100 / Tg = (W1 / Tg HP1 )+(W2 / Tg HP2 )+···+(W n / Tg HPn ) Tg: Glass transition temperature of the copolymer (K) W1: Content of monomer 1 (charge mass, mass%) Tg HP1 : Glass transition temperature (K) of the homopolymer of monomer 1 W2: Content of monomer 2 (charge mass, mass%) Tg HP2 : Glass transition temperature (K) of the homopolymer of monomer 1 W n : Content of monomer n (charge mass, mass%) Tg HPn : Glass transition temperature (K) of homopolymer of monomer n

[0056] From the viewpoint of substrate adhesion, the glass transition temperature (Tg) of the copolymer excluding the crosslinkable monomer is preferably from -47°C to 10°C, more preferably from -20°C to 10°C, and even more preferably from 0°C to 10°C.

[0057] The Hansen solubility parameter (HSP value) of the copolymer is usually 17.5 MPa. 1 / 2 ~24.0MPa 1 / 2 However, preferably 17.8 MPa 1 / 2 More preferably, 17.9 MPa 1 / 2 or more, preferably 23.8 MPa 1 / 2 Less than or equal to 23.5 MPa, preferably 1 / 2 When the Hansen solubility parameter of the copolymer is within the above range, it becomes easy to form a thin and uniform binder layer at the interface between the electrode active material and the electrolyte. The weight average molecular weight (Mw) and number average molecular weight (Mn) of the copolymer are not particularly limited, but are usually ultra-high molecular weight.

[0058] The copolymer forms a particle shape, and the volume average particle diameter of the particles measured by dynamic light scattering is 50 nm to 500 nm (satisfies condition a). Specific shapes of the copolymer particles include spherical and flat shapes. The shape of the copolymer particles can be confirmed by observation with a scanning electron microscope (SEM).

[0059] The volume average particle diameter of the copolymer particles is 50 nm to 500 nm, preferably 70 nm or more, more preferably 100 nm or more, and preferably 400 nm or less, more preferably 350 nm or less, even more preferably 300 nm or less, and particularly preferably 250 nm or less. When the volume average particle diameter of the copolymer particles is within this range, a thin and uniform binder layer is easily formed at the interface between the electrode active material and the electrolyte. The volume average particle diameter of the copolymer particles is measured by dynamic light scattering, and for example, the binder composition can be diluted 500 times with ion-exchanged water and then measured using an FPAR-1000 or nanoSAQLA manufactured by Otsuka Electronics Co., Ltd.

[0060] The binder composition may contain a compound other than the copolymer, for example, a solvent. Examples of the solvent include water, acetonitrile, N-methylpyrrolidone, acetylpyridine, cyclopentanone, dimethylformamide, dimethyl sulfoxide, methylformamide, methyl ethyl ketone, furfural, and ethylenediamine. The solvent may be a mixed solvent of the above-mentioned solvents.

[0061] <Composition for forming secondary battery negative electrode> Another embodiment of the present invention, a composition for forming a secondary battery negative electrode (hereinafter sometimes abbreviated as "negative electrode composition"), contains the aforementioned binder composition for a secondary battery electrode and a "negative electrode active material." A typical negative electrode can be manufactured by dispersing a negative electrode active material, a binder, and optionally a conductive additive and a thickener in a solvent to form a slurry, applying the slurry to a current collector, drying, and compressing it to form a negative electrode composite layer (also referred to as a "negative electrode active material layer") on the current collector. Therefore, the "negative electrode composition" refers to a composition for forming this negative electrode composite layer. The "negative electrode active material" and the like will be described in detail below.

[0062] (Negative electrode active material) The negative electrode active material or compound can be classified into (1) carbon and carbon compounds capable of doping / dedoping lithium ions, (2) metals and alloys capable of alloying with lithium, and (3) oxides, nitrides, and carbides capable of doping / dedoping lithium ions. When the negative electrode active material is carbon, a particulate (powder) carbon or compound is typically used. Specific examples of the negative electrode active material include at least one selected from the group consisting of carbon particles, silicon particles, silicon oxide particles, and silicon carbide particles. Examples of carbon particles include graphite particles (natural graphite and artificial graphite), carbon black particles, activated carbon particles, and amorphous carbon particles. The negative electrode active material used is not limited to one type, and two or more types may be mixed together.

[0063] The negative electrode active material preferably contains carbon particles and at least one selected from the group consisting of silicon particles, silicon oxide particles, and silicon carbide particles. It is particularly preferable to contain graphite (natural graphite, artificial graphite) particles and silicon oxide particles, or graphite (natural graphite, artificial graphite) particles and silicon carbide particles. When the negative electrode active material contains silicon particles, silicon oxide particles, or silicon carbide particles, a high energy density can be achieved. However, when doped with lithium ions, the volume tends to expand significantly, resulting in a low capacity retention rate during charge-discharge cycles. The binder composition of the present disclosure is believed to have the effect of alleviating this problem, and is therefore suitable for use in such negative electrode active materials.

[0064] When the element or compound serving as the negative electrode active material is in the form of particles (powder), specific shapes include fibrous, spherical, potato-like, and flake-like shapes.

[0065] When the negative electrode active material contains particles of simple carbon, the median diameter D50 of the simple carbon is usually 1 μm to 30 μm, preferably 10 μm or more, more preferably 15 μm or more, and preferably 25 μm or less, more preferably 20 μm or less.

[0066] When the negative electrode active material contains carbon particles, the BET specific surface area of ​​the carbon particles is usually 1.0 m 2 / g~5.0m 2 / g, preferably 2.0m 2 / g or more, more preferably 3.0m 2 / g or more, preferably 4.5m 2 / g or less, more preferably 4.0m 2 / g or less.

[0067] Silicon oxide is SiO x where x is a variable, i.e., the content of oxygen atoms in silicon oxide is not particularly limited, but x is usually 0≦x<2, preferably 0.2 or more, more preferably 0.4 or more, even more preferably 0.6 or more, and preferably 1.8 or less, more preferably 1.6 or less, even more preferably 1.4 or less.

[0068] When the negative electrode active material contains silicon particles, silicon oxide particles, or silicon carbide particles, the median diameter D50 of the silicon particles, silicon oxide particles, or silicon carbide particles is usually 0.5 μm to 20 μm, preferably 1.0 μm or more, more preferably 3.0 μm or more, and preferably 15 μm or less, more preferably 10 μm or less.

[0069] When the negative electrode active material contains silicon particles, silicon oxide particles, or silicon carbide particles, the BET specific surface area of ​​the silicon particles, silicon oxide particles, or silicon carbide particles is usually 1.0 m 2 / g~5.0m 2 / g, preferably 1.5m 2 / g or more, more preferably 2.0m 2 / g or more, preferably 4.5m 2 / g or less, more preferably 4.0m 2 / g or less.

[0070] When the negative electrode active material contains carbon particles and at least one selected from the group consisting of silicon particles, silicon oxide particles, and silicon carbide particles, the total charged mass of the silicon particles, silicon oxide particles, and silicon carbide particles in the negative electrode active material is usually 1% by mass to 20% by mass, but is preferably 3% by mass or more, more preferably 5% by mass or more, and is preferably 18% by mass or less, more preferably 15% by mass or less, when the total charged mass of the entire negative electrode active material is taken as 100% by mass.

[0071] When the negative electrode active material contains carbon particles and at least one selected from the group consisting of silicon particles, silicon oxide particles, and silicon carbide particles, the total mass of the carbon particles in the negative electrode active material is usually 70% by mass to 99% by mass, but is preferably 80% by mass or more, preferably 95% by mass or less, and more preferably 90% by mass or less, when the total mass of the negative electrode active material is taken as 100% by mass. When the total mass of the silicon particles and the like is within this range, it becomes easier to ensure a balance between the energy density and capacity retention rate of the lithium ion secondary battery.

[0072] The total content of the negative electrode active material in the negative electrode mixture layer is usually 70% by mass to 99.5% by mass, preferably 75% by mass or more, and preferably 99% by mass or less, when the entire negative electrode mixture layer is taken as 100% by mass.

[0073] The total content of the copolymer of the binder composition in the negative electrode mixture layer is usually 0.1% by mass to 5% by mass, but is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and is preferably 3% by mass or less, more preferably 2% by mass or less, when the entire negative electrode mixture layer is taken as 100% by mass.

[0074] The negative electrode-forming composition is characterized by containing the above-described binder composition and negative electrode active material, and preferably further contains a conductive additive such as carbon black (e.g., acetylene black), carbon nanotubes, amorphous whiskers, and graphite.

[0075] The total content of the conductive additive in the negative electrode mixture layer is usually 0.01% by mass to 3% by mass, but is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and is preferably 2% by mass or less, more preferably 1% by mass or less, when the entire negative electrode mixture layer is taken as 100% by mass.

[0076] The negative electrode-forming composition is characterized by containing the aforementioned binder composition and negative electrode active material, but preferably further contains a thickener. The inclusion of a thickener makes it easier to adjust the viscosity of the negative electrode-forming composition, improving productivity. Examples of thickeners for negative electrodes include cellulose derivatives such as carboxymethyl cellulose (CMC), carboxyethyl cellulose, and hydroxyethyl cellulose; polyoxyethylene and its modified products; polyvinyl alcohol and its modified products; and polysaccharides.

[0077] The total content of the thickener in the negative electrode mixture layer is usually 0.1% by mass to 5% by mass, but is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and is preferably 3% by mass or less, more preferably 2% by mass or less, when the entire negative electrode mixture layer is taken as 100% by mass.

[0078] The negative electrode-forming composition may contain a solvent. Examples of the solvent include water, acetonitrile, N-methylpyrrolidone, acetylpyridine, cyclopentanone, dimethylformamide, dimethyl sulfoxide, methylformamide, methyl ethyl ketone, furfural, and ethylenediamine. The solvent may be a mixed solvent of the above-mentioned solvents.

[0079] <Anode for lithium-ion secondary batteries> Another embodiment of the present invention relates to a lithium-ion secondary battery anode, which includes a cathode composite layer formed using the aforementioned anode-forming composition. As described above, the anode can be manufactured by applying, for example, a slurry of the anode-forming composition to a current collector, drying the composition, and compressing the anode composite layer on the current collector. The application, drying, and compression conditions for the anode-forming composition are not particularly limited, and known conditions can be appropriately adopted. Examples of materials for the current collector of the negative electrode include copper, nickel, stainless steel, and nickel-plated steel.

[0080] <Lithium-ion secondary battery> A lithium ion secondary battery according to another embodiment of the present invention includes a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, and the negative electrode is the aforementioned negative electrode for a lithium ion secondary battery. The "positive electrode," "non-aqueous electrolyte," "separator," etc. will be described in detail below.

[0081] (positive electrode) Typically, a positive electrode can be manufactured by dispersing a positive electrode active material and a binder, and optionally a conductive additive and a thickener, in a solvent to form a slurry, applying this slurry to a current collector, drying, and compressing it to form a positive electrode composite layer (also referred to as a "positive electrode active material layer") on the current collector.

[0082] The positive electrode active material is transition metal oxides or sulfides such as MoS2, TiS2, MnO2, V2O5; LiCoO2, LiMnO2, LiMn2O4, LiNiO2, LiNi X Co (1-X) O2(0 <X<1)、LiNi x Co y Mn z O2 (x, y, and z are each independently greater than 0 and less than 1.00, and the sum of x, y, and z is 0.99 to 1.00) (so-called "NCM"; for example, LiNi 0.33 Co 0.33 Mn 0.33 O2, LiNi 0.5 Co 0.3 Mn 0.2 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2) and other composite oxides of transition metals; Li t Ni 1-x-y Co x Al y O2 (t is 0.95 or more and 1.15 or less, x is 0 or more and 0.3 or less, y is 0.1 or more and 0.2 or less, and the sum of x and y is less than 0.5) (so-called "NCA"; e.g., LiNi 0.8 Co 0.15 Al 0.05 O2), etc., composite oxides consisting of lithium, transition metals, and typical metals; Conductive polymer materials such as polyaniline, polythiophene, polypyrrole, polyacetylene, polyacene, dimercaptothiadiazole, and polyaniline composites; Lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium manganese iron phosphate (LiMn x Fe 1-xLithium metal phosphates such as LiFePO4 (where 0 < x < 1), lithium cobalt phosphate (LiCoPO4), lithium nickel phosphate (LiNiPO4); and the like.

[0083] Examples of the binder for the positive electrode include polyvinylidene fluoride and the like. Examples of the conductive assistant for the positive electrode include carbon black (e.g., acetylene black), amorphous whiskers, graphite, and the like. Examples of the thickener for the positive electrode include carboxymethyl cellulose and the like. In addition, examples of the solvent for the slurry for forming the positive electrode include organic solvents such as N-methylpyrrolidone.

[0084] When the total content of the positive electrode active material in the positive electrode composite layer is based on 100% by mass of the total content of the positive electrode composite layer, it is usually 70% to 97% by mass, preferably 75% by mass or more, and preferably 95% by mass or less.

[0085] Examples of the material for the current collector of the positive electrode include aluminum, aluminum alloy, stainless steel, nickel, titanium, tantalum, carbon cloth, carbon paper, and the like.

[0086] (Non-aqueous electrolyte) Generally, the non-aqueous electrolyte contains a non-aqueous solvent, an electrolyte, and an additive. Examples of the non-aqueous solvent include cyclic carbonates, fluorine-containing cyclic carbonates, chain carbonates, fluorine-containing chain carbonates, aliphatic carboxylic acid esters, fluorine-containing aliphatic carboxylic acid esters, γ-lactones, fluorine-containing γ-lactones, cyclic ethers, fluorine-containing cyclic ethers, chain ethers, fluorine-containing chain ethers, nitriles, amides, lactams, nitromethane, nitroethane, sulfolane, trimethyl phosphate, dimethyl sulfoxide, dimethyl sulfoxide phosphate, and the like. Examples of the cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and the like. Examples of fluorine-containing cyclic carbonates include fluoroethylene carbonate (FEC). Examples of chain carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), and dipropyl carbonate (DPC). Examples of the aliphatic carboxylic acid esters include methyl formate, methyl acetate, methyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethylbutyrate, ethyl formate, ethyl acetate, ethyl propionate, ethyl butyrate, ethyl isobutyrate, and ethyl trimethylbutyrate.

[0087] The content of the non-aqueous solvent is preferably 60% by mass or more, more preferably 70% by mass or more, and preferably 99% by mass or less, preferably 97% by mass or less, and even more preferably 90% by mass or less, when the entire non-aqueous electrolyte solution is taken as 100% by mass.

[0088] Examples of the electrolyte include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium hexafluorotantalate (LiTaF6), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (Li(CF3SO2)2N), and lithium bis(pentafluoroethanesulfonyl)imide (Li(C2F5SO2)2N).

[0089] When the non-aqueous electrolyte solution contains an electrolyte, the concentration of the electrolyte in the non-aqueous electrolyte solution is preferably 0.1 mol / L or more and 3 mol / L or less, more preferably 0.2 mol / L or more and 2 mol / L or less, and even more preferably 0.5 mol / L or more and 2 mol / L or less.

[0090] Examples of additives include fluoroethylene carbonate (FEC), vinylene carbonate (VC), lithium difluorophosphate (LiPO2F2), lithium fluorosulfonate (LiSO2F), and lithium bis(oxalato)borate (LiBOB).

[0091] When the non-aqueous electrolyte solution contains an additive, the content is usually 0.01 mass % or more and 5.0 mass % or less, preferably 0.05 mass % or more, more preferably 0.1 mass % or more, even more preferably 0.3 mass % or more, and preferably 4.0 mass % or less, more preferably 3.0 mass % or less, even more preferably 2.0 mass % or less, and particularly preferably 1.5 mass % or less.

[0092] The separator may be a porous resin plate, and its material may be a resin, a nonwoven fabric containing the resin, etc. Examples of the resin include polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polyester, cellulose, polyamide, etc. The separator is preferably a porous resin sheet having a single layer or multilayer structure.

[0093] The thickness of the separator is preferably 5 μm or more and 30 μm or less.

[0094] Examples of the case (exterior body) include a case including a laminate film, and a case consisting of a battery can and a battery can lid.

[0095] 1 is a schematic cross-sectional view showing a laminated lithium-ion secondary battery according to one embodiment of the present invention. Specifically, in the lithium-ion secondary battery 1, a battery element 10 is enclosed inside an exterior body 30. The exterior body 30 is formed of a laminate film. A positive electrode lead 21 and a negative electrode lead 22 are attached to the battery element 10. The positive electrode lead 21 and the negative electrode lead 22 are led out in opposite directions, from the inside to the outside of the exterior body 30.

[0096] As shown in FIG. 1 , battery element 10 is formed by laminating positive electrode 11, separator 13, and negative electrode 12. Positive electrode 11 has positive electrode composite layer 11B formed on both main surfaces of positive electrode current collector 11A. Negative electrode 12 has negative electrode composite layer 12B formed on both main surfaces of negative electrode current collector 12A. Positive electrode composite layer 11B formed on one main surface of positive electrode current collector 11A of positive electrode 11 and negative electrode composite layer 12B formed on one main surface of negative electrode current collector 12A of negative electrode 12 adjacent to positive electrode 11 face each other with separator 13 interposed therebetween.

[0097] A non-aqueous electrolyte solution is injected into the interior of the exterior housing 30 of the lithium ion secondary battery 1. The non-aqueous electrolyte solution permeates the positive electrode composite layer 11B, the separator 13, and the negative electrode composite layer 12B. In the lithium ion secondary battery 1, one single cell layer 14 is formed by the adjacent positive electrode composite layer 11B, the separator 13, and the negative electrode composite layer 12B. Note that the positive electrode and the negative electrode may each have a composite layer formed on one side of each current collector.

[0098] Although the lithium ion secondary battery 1 in Fig. 1 is a stacked type lithium ion secondary battery, it may also be a wound type lithium ion secondary battery. A wound type lithium ion secondary battery is formed by stacking a positive electrode, a separator, a negative electrode, and a separator in this order and winding them into layers. Wound type lithium ion secondary batteries include cylindrical lithium ion secondary batteries and prismatic lithium ion secondary batteries. [Example]

[0099] The present invention will be explained in more detail below by way of examples, but modifications can be made as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

[0100] [Example 1] <Preparation of binder composition and measurement of physical properties> (Preparation of Binder Composition) 769.1 g of ion-exchanged water and 4.2 g of Dow Fax2A1 manufactured by The Dow Chemical Company were charged into a reaction vessel equipped with a stirrer, reflux condenser, dropping device, and thermometer, and the temperature was raised to 75°C while stirring and replacing the atmosphere with nitrogen. The internal temperature was maintained at 75°C, and 4.8 g of potassium persulfate was added as a polymerization initiator. After dissolution, 399 g of ion-exchanged water, 4.2 g of Dow Fax2A1, and a crosslinkable monomer, N-methylolacrylamide (HSP value: 31.3 MPa), were added. 1 / 2 ) 19.0 g (2.0 mass % based on the total mass of all monomers), acrylonitrile (Tg HP : 110℃, HSP value: 27.9MPa 1 / 2 ) 427.5g (45.0% by mass based on the total mass of all monomers), n-butyl acrylate (Tg HP : -56℃, HSP value: 18.9MPa 1 / 2 ) 427.5g (45.0% by mass based on the total mass of the monomers), methacrylic acid (Tg HP : 186℃, HSP value: 23.8MPa 1 / 2 ) 27g (3.0% by mass based on the total mass of all monomers), 2-hydroxyethyl methacrylate (Tg HP : 55℃, HSP value: 24.1MPa 1 / 2 5 g (5.0% by mass based on the total charged mass of all monomers) of ethanol was added under stirring to prepare an emulsion, which was then continuously added dropwise to the reaction solution over a period of 5 hours. After the dropwise addition was completed, the emulsion was aged for 3 hours. The resulting aqueous emulsion was cooled to room temperature, and then ion-exchanged water and an aqueous ammonium solution were added to adjust the solid content to 45% by mass, and the pH was adjusted to 8.0, thereby obtaining a binder composition. The theoretical value of the glass transition temperature (Tg) of the copolymer excluding the crosslinkable monomer contained in the binder composition was calculated using the Fox equation and was found to be 9.7°C. On the other hand, the Hansen solubility parameter (HSP value) of the copolymer was 22.6 MPa. 1 / 2 It was.

[0101] (Observation of copolymers using a scanning electron microscope (SEM)) A part of the obtained binder composition was processed into a sample that could be photographed with a scanning electron microscope (SEM), and the copolymer was observed, and it was confirmed that the copolymer formed a particle shape.

[0102] (Measurement and calculation of volume average particle size of copolymer particles) The obtained binder composition (acrylic emulsion) was diluted 500 times with ion-exchanged water and stirred to form a uniform dispersion, which was used as a measurement sample. The volume average particle diameter of the copolymer particles was measured by dynamic light scattering using a concentrated particle size analyzer FPAR-1000 manufactured by Otsuka Electronics Co., Ltd., and was found to be 150 nm.

[0103] (Measurement of Breaking Strength of Dry Coating Film of Binder Composition) The obtained binder composition was weighed into a support container so that the film thickness after drying would be 250 μm to 300 μm, and a coating film was formed on the support container. This coating film was dried in an environment of 23°C and 55% RH for 24 hours, and then further dried at 80°C for 24 hours. The dried coating film was further subjected to humidity control at 23°C and 55% RH for 24 hours or more, and used to measure the breaking strength (unit: MPa). The dried coating film was prepared into a size of 10 mm x 50 mm, and the breaking strength of the dried coating film was measured using a universal testing machine, Model 201X, manufactured by Intesco Co., Ltd. The test was carried out at a tensile speed of 100 mm / min, in an environment of 23°C and 55% RH, with a chuck distance of 20 mm. The copolymer can be said to be excellent if it has a breaking strength of 3.2 MPa or more.

[0104] <Preparation and evaluation of lithium-ion secondary batteries> (Preparation of non-aqueous electrolyte) As a non-aqueous solvent, ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 30:35:35 to obtain a mixed solvent. LiPF6 as an electrolyte was dissolved in the obtained mixed solvent so that the electrolyte concentration in the finally prepared non-aqueous electrolyte solution was 1.0 mol / L. Vinylene carbonate (VC) was added to the obtained solution so that the content relative to the total mass of the non-aqueous electrolyte was 1.0 mass %, and further fluoroethylene carbonate (FEC) was added so that the content relative to the total mass of the non-aqueous electrolyte was 2.0 mass %, thereby obtaining a non-aqueous electrolyte.

[0105] (Preparation of positive electrode) The positive electrode active material is lithium nickel cobalt manganese oxide (Ni 0.5 Co 0.2 Mn 0.3 A paste-like positive electrode composite slurry was prepared by kneading 92 parts by mass of O2 (hereinafter also referred to as "NCM"), acetylene black (Super P (Imerys), 2 parts by mass) and acetylene black (KS6 (Imerys), 2 parts by mass) as a conductive additive, and polyvinylidene fluoride (4 parts by mass) as a binder, using N-methylpyrrolidine as a solvent to give a solids ratio of 60%. Next, this positive electrode composite slurry was applied to a 20 μm thick strip-shaped positive electrode current collector made of aluminum foil, dried, and then compressed with a roll press to obtain a sheet-like positive electrode consisting of the positive electrode current collector and a positive electrode composite layer. The coating density of the positive electrode composite layer at this time was 19 mg / cm. 2 and the packing density was 3.0 g / mL.

[0106] (Preparation of negative electrode) As the negative electrode active material, natural graphite (85.95 parts by mass, D50 = 20 μm, BET specific surface area = 3.0 m 2 / g) and silicon oxide SiO x (x=1.0)(9.55 parts by mass, D50=4.8μm, BET specific surface area=2.7m 2 / g), carboxymethyl cellulose (1.5 parts by mass, #2200 manufactured by Daicel Corporation) as a thickener, carbon nanotubes (0.02 parts by mass) and carbon black (0.95 parts by mass, Super P (Imerys)) as conductive additives, and a copolymer of the binder composition (2 parts by mass) as a binder were kneaded with an aqueous solvent to a solids ratio of 44%, to prepare a paste-like negative electrode mixture slurry. Next, this negative electrode composite slurry was applied to a 10 μm thick strip of copper foil negative electrode current collector, dried, and then compressed with a roll press to obtain a sheet-like negative electrode consisting of the negative electrode current collector and a negative electrode composite layer. The coating density of the negative electrode composite layer at this time was 6.5 mg / cm. 2 and the packing density was 1.5 g / mL.

[0107] <Preparing the separator> A microporous polyethylene film with a thickness of 20 μm was prepared as a separator.

[0108] (Fabrication of lithium-ion secondary batteries) The sheet-like negative electrode was punched into a rectangular shape of 42 mm in length and 31 mm in width, the sheet-like positive electrode was punched into a rectangular shape of 40 mm in length and 29 mm in width, and the separator was punched into a rectangular shape of 45 mm in length and 35 mm in width, to obtain a rectangular negative electrode, a rectangular positive electrode, and a rectangular separator. The obtained rectangular negative electrode, rectangular separator, and rectangular positive electrode were stacked in this order in a bag-shaped aluminum laminate, and then 250 μL of nonaqueous electrolyte was poured into the aluminum laminate to impregnate the rectangular separator, rectangular positive electrode, and rectangular negative electrode. The battery was then sealed by closing the opening of the bag with a heat sealer. In this way, a laminated lithium ion secondary battery was obtained.

[0109] <Aging> The lithium-ion secondary battery was charged in a thermostatic chamber at 25°C at a charge rate of 0.1 C using CC-CV (Constant Current - Constant Voltage) to 4.2 V, and then discharged at a discharge rate of 0.1 C to 2.5 V. After repeating this procedure twice, the same procedure was repeated with the charge rate set to 0.5 C and the discharge rate set to 0.5 C.

[0110] <High-temperature charge-discharge cycle test> Next, the lithium-ion secondary battery whose initial discharge capacity (0.5C) had been measured was subjected to CC-CV charging at a charge rate of 0.5C at 45°C to 4.2V, followed by CC-CV discharging at a discharge rate of 0.5C at 45°C to 2.5V. This cycle was repeated 49 times, and then a cycle with a charge rate and discharge rate of 0.1C was performed (i.e., a total of 50 cycles were performed, hereinafter sometimes referred to as a "high-temperature charge-discharge cycle test"). The charge-discharge cycles up to this point are considered one "high-temperature charge-discharge cycle test."

[0111] <Capacity retention rate after high-temperature charge-discharge cycle test> For the lithium ion secondary batteries after six high-temperature charge-discharge cycle tests, the capacity retention rate after the high-temperature charge-discharge cycle test was calculated using the following formula. The results are shown in Table 1. A capacity retention rate of 84.0% or higher after the high-temperature charge-discharge cycle test is considered excellent.

[0112] Capacity retention rate after high-temperature charge-discharge cycle test (%) = [(discharge capacity at 0.1 C in the final high-temperature charge-discharge cycle test) / (initial discharge capacity at 0.5 C)] × 100

[0113] <Measurement of DC resistance> After six high-temperature charge-discharge cycle tests, the lithium-ion secondary battery was removed and charged to 4.2 V at a charge rate of 0.5 C using CC-CV (Constant Current - Constant Voltage) in a thermostatic chamber at 25°C, and then left for 15 minutes. The voltage was measured after leaving the battery for 15 minutes. The battery was then discharged for 10 seconds at a discharge rate of 1 C. The voltage was measured again after 10 seconds of discharge. The DC resistance was calculated using the following formula. The results are shown in Table 1. A DC resistance of 2.60 Ω or less after the high-temperature charge-discharge cycle test is considered excellent.

[0114] DC resistance (Ω) = [(Voltage after 15 minutes of rest (V)) - (Voltage after 10 seconds of discharge (V))] / (Current value at 1 C (A))

[0115] [Table 1]

[0116] Details of the monomers listed in Table 1 are as follows: AN: acrylonitrile [(meth)acrylonitrile of the present disclosure] BA: n-butyl acrylate (Tg HP ≦-15℃ and HSP value 17.0MPa 1 / 2 ~20.0MPa 1 / 2 (Meth)acrylic acid ester] MAC: methacrylic acid [unsaturated sulfonic acid of the present disclosure] HEMA: 2 hydroxyethyl methacrylate [crosslinkable monomer of the present disclosure] N-MAM: N-methylolacrylamide [crosslinkable monomer of the present disclosure] [Industrial Applicability]

[0117] The binder composition of one embodiment of the present invention can be blended as a binder with an electrode active material, a conductive additive, and the like, and used in electrodes (electrode mixture layers) of power storage devices such as lithium ion secondary batteries and capacitors. [Explanation of symbols]

[0118] 1. Lithium-ion secondary battery 10 Battery element 11 Positive electrode 11A positive electrode current collector 11B Positive electrode composite layer 12 Negative electrode 12A negative electrode current collector 12B Negative electrode composite layer 13 Separator 14 Cell layer 21 Positive lead 22 Negative lead 30 Exterior body

Claims

1. A binder composition for a secondary battery electrode, comprising a copolymer obtained by polymerizing a monomer composition containing a (meth)acrylic acid ester, A binder composition for a secondary battery electrode, which satisfies all of the following conditions: condition a, condition b-1, condition b-2, condition c-1, condition c-2, condition d-1, and condition d-2: Condition a: The copolymer forms particles, and the volume average particle size of the particles measured by dynamic light scattering is 50 nm to 500 nm. Condition b-1: The monomer composition has a glass transition temperature (Tg ＨＰ ) contains a (meth)acrylic acid ester having a temperature of −15° C. or lower. Condition b-2: The glass transition temperature (Tg ＨＰ The total charged mass of the (meth)acrylic acid esters having a temperature of −15° C. or lower is 39% by mass to 78% by mass when the total charged mass of all the monomers is 100% by mass. Condition c-1: The monomer composition contains (meth)acrylonitrile. Condition c-2: The total charged mass of the (meth)acrylonitrile in the monomer composition is 21% by mass to 60% by mass when the total charged mass of all the monomers is 100% by mass. Condition d-1: The monomer composition contains a crosslinkable monomer. Condition d-2: The total charged mass of the crosslinkable monomers in the monomer composition is 0.1% by mass to 15% by mass when the total charged mass of all the monomers is 100% by mass.

2. 2. The binder composition for a secondary battery electrode according to claim 1, wherein the crosslinkable monomer contains at least one functional group selected from the group consisting of a methylol group, an epoxy group, a hydrolyzable silyl group, an allyl group, and a vinyl group.

3. The binder composition for a secondary battery electrode according to claim 1, further satisfying the following conditions e-1 and e-2: Condition e-1: The monomer composition has a Hansen solubility parameter (HSP value) of 17.0 MPa １／２ ~20.0 MPa １／２ (Meth)acrylic acid esters of the formula: Condition e-2: The Hansen solubility parameter (HSP value) of the monomer composition is 17.0 MPa １／２ ~20.0 MPa １／２ The total charged mass of the (meth)acrylic acid esters is 39% by mass to 70% by mass when the total charged mass of all the monomers is 100% by mass.

4. The glass transition temperature (Tg ＨＰ 2. The binder composition for a secondary battery electrode according to claim 1, wherein the (meth)acrylic acid ester having a temperature of −15° C. or less is a compound represented by the following formula (I): 【Chemical 1】 (In formula (I), R １ represents a hydrocarbon group having 4 to 20 carbon atoms which may contain at least one group selected from the group consisting of an oxa group (—O—) and a carbonyl group (>C═O) as a substituent, and R represents a hydrogen atom or a methyl group (—CH ３ ) represents.

5. A composition for forming a secondary battery negative electrode, comprising the binder composition for a secondary battery electrode according to any one of claims 1 to 4 and a negative electrode active material.

6. The composition for forming a secondary battery negative electrode according to claim 5 , wherein the negative electrode active material comprises at least one selected from the group consisting of carbon particles, silicon particles, silicon oxide particles, and silicon carbide particles.

7. A negative electrode for a lithium ion secondary battery, comprising a negative electrode mixture layer formed using the composition for forming a secondary battery negative electrode according to claim 5 .

8. A lithium ion secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, A lithium ion secondary battery, wherein the negative electrode is the negative electrode for a lithium ion secondary battery according to claim 7.

Images