Binder composition for secondary battery, slurry composition for secondary battery, functional layer for secondary battery, separator for secondary battery, electrode for secondary battery, and secondary battery

Abstract

The present invention provides a secondary binder composition which enables the achievement of a secondary battery that has good cycle characteristics, while exhibiting excellent adhesion between a base material and a layer that is obtained using the binder composition. A binder composition for secondary batteries, said binder composition containing a polymer and a solvent, wherein: the polymer is in the form of core-shell particles, each of which has a core part formed of a polymer (a1) and a shell part formed of a polymer (a2); the polymer (a2) contains a predetermined amount of a monofunctional unsaturated carboxylic acid ester monomer unit, a predetermined amount of one or more monomer units selected from among monofunctional unsaturated carboxylic acid monomer units and monofunctional unsaturated sulfonic acid monomer units, a predetermined amount of a monomer unit having two or more vinyl functional groups, and a predetermined amount of an aromatic vinyl monomer unit; and the amount of the one or more monomer units selected from among monofunctional unsaturated carboxylic acid monomer units and monofunctional unsaturated sulfonic acid monomer units relative to one mole of the monomer unit having two or more vinyl functional groups is from 0.1 mole to 15,000 moles.

Description

[Technical Field]

[0001] The present invention relates to a binder composition for a secondary battery, a slurry composition for a secondary battery, a functional layer for a secondary battery, a separator for a secondary battery, an electrode for a secondary battery, and a secondary battery. [Background technology]

[0002] Non-aqueous secondary batteries, including lithium-ion secondary batteries, are small, lightweight, have high energy density, and can be repeatedly charged and discharged, making them suitable for a wide range of applications. Secondary batteries generally include battery components such as electrodes (positive and negative electrodes) and a separator that separates the positive and negative electrodes to prevent short circuits between them.

[0003] Generally, a separator for a secondary battery is provided on a separator substrate with an adhesive layer containing a binder or a porous membrane layer containing a binder and non-conductive particles as functional particles. Also, an electrode for a secondary battery is provided on a current collector with an electrode mixture layer containing a binder and electrode active material particles as functional particles, or an electrode substrate having an electrode mixture layer on a current collector, and further provided with the above-mentioned adhesive layer or porous membrane layer.

[0004] In order to achieve further performance improvements of secondary batteries, attempts have been made to improve binder compositions containing binders, and a technique for improving adhesion by using a binder made of core-shell particles has been proposed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2016 / 039067 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-201444 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-513554 [Patent Document 4] Japanese Patent Application Laid-Open No. 2013-073921 [Patent Document 5] Japanese Patent Application Publication No. 6-302797 [Patent Document 6] International Publication No. 2017 / 110901 [Patent Document 7] International Publication No. 2017 / 188055 [Patent Document 8] International Publication No. 2018 / 043200 [Patent Document 9] Japanese Patent Application Publication No. 11-025989 [Patent Document 10] Japanese Patent Application Laid-Open No. 2013-182765 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the conventional techniques described above have a problem in that, in the production of each layer, a phenomenon called migration, in which the binder made of core-shell particles migrates to the surface of the layer, occurs easily. When migration occurs, the binder is unevenly distributed near the surface, while the amount of binder near the interface with the substrate decreases, which reduces the adhesion between the substrate and the layer and can degrade the battery performance of the secondary battery, particularly the cycle characteristics.

[0007] Regarding repeated charge-discharge cycles, a problem specific to lithium-ion secondary batteries is known to be the deposition of lithium metal on the negative electrode during charge-discharge cycles. The deposition of lithium metal on the negative electrode during charge-discharge cycles can cause short circuits, and therefore, it is required to be strictly suppressed in order to improve safety and reliability.

[0008] An object of the present invention is to provide a binder composition containing core-shell particles, which can provide excellent adhesion between a layer obtained using the binder composition and a substrate in the production of separators, electrodes, etc. for secondary batteries, thereby resulting in secondary batteries with good cycle characteristics. [Means for solving the problem]

[0009] The present inventors have aimed to solve the above problems and have conducted extensive research focusing on suppressing the migration of binders, and have completed the present invention.

[0010] That is, an object of the present invention is to advantageously solve the above-mentioned problems, and provides a binder composition for a secondary battery, comprising a polymer and a solvent, the polymer is a core-shell particle having a core portion made of polymer (a1) and a shell portion made of polymer (a2), the polymer (a2) contains 50.0% by mass or more and 89.9% by mass or less of monofunctional unsaturated carboxylic acid ester monomer units, 10.0% by mass or more and 49.9% by mass or less of one or more monomer units selected from monofunctional unsaturated carboxylic acid monomer units and monofunctional unsaturated sulfonic acid monomer units, 0.1% by mass or more and 10.0% by mass or less of monomer units having two or more vinyl functional groups, and 5.0% by mass or less of aromatic vinyl monomer units, The amount of one or more monomer units selected from the monofunctional unsaturated carboxylic acid monomer unit and the monofunctional unsaturated sulfonic acid monomer unit is 0.1 mole or more and 15,000 moles or less per mole of the monomer unit having two or more vinyl functional groups. A binder composition for a secondary battery.

[0011] Here, the term "core-shell type particle" refers to a particle having a multi-layer structure (core-shell structure) in which the surface of the core is covered with the shell. The phrase "containing a monomer unit" means that "a polymer obtained using that monomer contains a repeating unit derived from the monomer." "Unsaturated carboxylic acid ester monomer" refers to an unsaturated carboxylic acid ester containing an ethylenically unsaturated bond. The term "unsaturated carboxylic acid monomer" refers to an unsaturated carboxylic acid containing an ethylenically unsaturated bond, including an anhydride thereof. The term "monofunctional unsaturated carboxylic acid ester monomer" refers to an unsaturated carboxylic acid ester containing one ethylenically unsaturated bond. The term "monofunctional unsaturated carboxylic acid monomer" refers to an unsaturated carboxylic acid containing one ethylenically unsaturated bond, and includes an anhydride thereof. The term "monofunctional unsaturated sulfonic acid monomer" refers to an unsaturated sulfonic acid containing one ethylenically unsaturated bond, and includes an anhydride thereof.

[0012] In a secondary battery using the binder composition of the present invention, the mechanism by which excellent adhesion is obtained between the layer obtained using the binder composition and the substrate is presumed to be as follows.

[0013] When one or more monomer units selected from monofunctional unsaturated carboxylic acid monomer units and monofunctional unsaturated sulfonic acid monomer units are introduced into the shell portion of a core-shell particle, these units are soluble in the water typically contained in binder compositions and become charged in the composition. Electrical repulsion causes the molecular chains constituting the shell portion to expand, solubilizing the shell portion. The simultaneous presence of monofunctional unsaturated carboxylic acid ester monomer units in the shell portion enhances the solubility of the shell portion due to the polarity of the ester bonds in the monomer units. The solubilized shell portion, resulting from the expansion of the molecular chains, is thought to suppress migration of the core-shell particle, which serves as a binder, during the production of layers (e.g., electrode composite layers, porous membrane layers, etc.) in secondary batteries. This suppression of migration is thought to result in excellent adhesion between the layer obtained using the binder composition and the substrate.

[0014] On the other hand, if the shell portion is simply provided with a monofunctional unsaturated carboxylic acid ester monomer unit and one or more monomer units selected from a monofunctional unsaturated carboxylic acid monomer unit and a monofunctional unsaturated sulfonic acid monomer unit, the amount of the core-shell particles (binder) that are insoluble in the electrolyte may decrease when a secondary battery is assembled and an electrolyte is injected, which may deteriorate the cycle characteristics of the secondary battery. In the present invention, it is presumed that the decrease in the amount of insoluble in the electrolyte is suppressed by introducing a predetermined amount of monomer units having two or more vinyl functional groups.

[0015] In this case, it is presumed that by setting the ratio of the monomer unit having two or more vinyl functional groups to one or more monomer units selected from the monofunctional unsaturated carboxylic acid monomer unit and the monofunctional unsaturated sulfonic acid monomer unit within a predetermined range, it is possible to suppress both migration due to the molecular chain of the shell portion spreading into the aqueous phase and a decrease in the amount of insoluble material in the electrolyte.

[0016] It is presumed that, due to the above mechanism, in a secondary battery using the binder composition of the present invention, excellent adhesion is obtained between the layer obtained using the binder composition and the substrate, and deterioration of adhesion is suppressed even after repeated charge-discharge cycles, and that the secondary battery using the binder composition of the present invention exhibits good cycle characteristics.

[0017] Furthermore, by using the binder composition of the present invention in the production of a lithium ion secondary battery, it is possible to obtain a lithium ion secondary battery in which, in addition to the above-mentioned advantages, precipitation of lithium metal is significantly suppressed. It is presumed that the suppression of precipitation of lithium metal is due to the fact that the core-shell particles serving as the binder have excellent electrolyte retention properties and that uneven distribution is prevented by suppressing migration.

[0018] The present invention relates to the binder composition for secondary batteries, wherein the polymer (a1) has a tetrahydrofuran (THF)-insoluble content of 40% to 97%. By using the polymer (a1) having a THF-insoluble content within the above range as the core part, the core-shell particles serving as the binder are endowed with an appropriate elastic modulus, thereby achieving good film-forming properties between the particles and sufficiently improving adhesion to the substrate. The THF insoluble content can be measured by the method described in the Examples of this specification.

[0019] The present invention relates to the binder composition for secondary batteries, wherein the ratio of the mass of the polymer (a1) to the mass of the polymer (a2) is 95 / 5 or less and 5 / 95 or more. In the core-shell particles in which the ratio of the mass of the polymer (a1) to the mass of the polymer (a2) is within the above range, the core portion is sufficiently covered with the shell portion, and migration can be sufficiently suppressed.

[0020] The present invention relates to the binder composition for secondary batteries, in which the radius of gyration of the core-shell particles is 10 nm or more and 1000 nm or less. Core-shell particles having a radius of gyration within the above range have a shell with sufficient molecular chain extension, which can sufficiently suppress migration. Furthermore, the number of particles with excellent electrolyte retention properties is relatively sufficient, which suppresses uneven distribution of the electrolyte. This is particularly advantageous when used in lithium-ion secondary batteries, as it can sufficiently prevent lithium metal precipitation. Here, the radius of gyration can be measured by the method described in the examples of this specification.

[0021] Furthermore, the present invention relates to a slurry composition for a secondary battery having the above-mentioned binder composition for a secondary battery. The present invention relates to the above-mentioned slurry composition for a secondary battery containing non-conductive particles, and also to the above-mentioned slurry composition for a secondary battery containing an electrode active material.

[0022] The present invention also relates to a secondary battery separator having a secondary battery functional layer formed on a separator substrate using the above-mentioned slurry composition for secondary batteries containing non-conductive particles, and a secondary battery electrode having a secondary battery functional layer formed on a current collector using the above-mentioned slurry composition for secondary batteries containing electrode active material particles.

[0023] Furthermore, the present invention relates to a secondary battery including the functional layer for a secondary battery, a secondary battery including the separator for a secondary battery, and a secondary battery including the electrode for a secondary battery. [Effects of the Invention]

[0024] According to the binder composition for secondary batteries of the present invention, excellent adhesion can be obtained between a layer obtained using the binder composition and a substrate in the production of separators, electrodes, etc. of secondary batteries, and a secondary battery having good cycle characteristics can be obtained. When the binder composition for secondary batteries of the present invention is used, deterioration of adhesion is suppressed even after repeated charge-discharge cycles. In addition to the above advantages, the lithium ion secondary battery using the binder composition of the present invention significantly suppresses the deposition of lithium metal. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, embodiments of the present invention will be described in detail. The binder composition for secondary batteries of the present invention is used in the production of secondary batteries (e.g., non-aqueous secondary batteries such as lithium ion secondary batteries), and can be used, for example, to prepare a slurry composition for secondary batteries of the present invention. The slurry composition for a secondary battery of the present invention can be used to form any functional layer (e.g., an electrode mixture layer, a porous membrane layer, an adhesive layer) that has a function of donating and receiving electrons, or a function of reinforcement or adhesion in a secondary battery (e.g., a non-aqueous secondary battery such as a lithium ion secondary battery). The functional layer for a secondary battery of the present invention is formed from the slurry composition for a secondary battery of the present invention. The secondary battery separator and the secondary battery electrode of the present invention are provided with the secondary battery functional layer of the present invention. Furthermore, the secondary battery of the present invention (for example, a non-aqueous secondary battery such as a lithium ion secondary battery) comprises at least one of the functional layer for secondary batteries, the separator for secondary batteries, and the electrode for secondary batteries of the present invention.

[0026] <Binder composition for secondary batteries> The binder composition for a secondary battery of the present invention contains a polymer and a solvent.

[0027] [Polymer] The binder composition for a secondary battery of the present invention contains a polymer, which is a core-shell particle having a core portion made of polymer (a1) and a shell portion made of polymer (a2).

[0028] (Polymer (a1)) The core portion made of the polymer (a1) is not particularly limited, and any polymer known as a binder can be used.

[0029] -THF insoluble content of polymer (a1)- The polymer (a1) preferably has a THF insoluble content of 40% or more and 97% or less, more preferably 45% or more, even more preferably 50% or more, and more preferably 95% or less, even more preferably 93% or less.

[0030] The THF-insoluble content can be adjusted by the type and amount of the monomer used in preparing the polymer (a1), etc. Alternatively, an aqueous dispersion of a polymer having the same composition as the polymer (a1) may be prepared, and the THF-insoluble content of the polymer (a1) may be measured and used as the THF-insoluble content.

[0031] -Composition of polymer (a1)- The polymer (a1) may be a polymer of any composition, and examples thereof include a polymer containing unsaturated carboxylic acid ester monomer units and unsaturated carboxylic acid monomer units.

[0032] The unsaturated carboxylic acid ester capable of forming the unsaturated carboxylic acid ester monomer unit is not particularly limited, and examples thereof include octyl acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, and 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, and stearyl acrylate; and alkyl acrylates (e.g., alkyl esters having 1 to 20 carbon atoms, preferably having 1 to 10 carbon atoms). the following alkyl esters); octyl methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, pentyl methacrylate, hexyl methacrylate, heptyl methacrylate, and 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, and stearyl methacrylate (for example, alkyl esters having from 1 to 20 carbon atoms, preferably alkyl esters having from 1 to 10 carbon atoms). Among these, from the viewpoints of polymerization stability and core-shell structure formation, methyl methacrylate, ethyl methacrylate, and n-butyl acrylate are preferred, and a combination of methyl methacrylate and n-butyl acrylate is more preferred.

[0033] The unsaturated carboxylic acid ester monomer unit may be used alone or in combination of two or more kinds in any ratio.

[0034] The unsaturated carboxylic acid capable of forming the unsaturated carboxylic acid monomer is not particularly limited, and examples thereof include acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid, etc. The unsaturated dicarboxylic acid may be an anhydride. Among these, from the viewpoints of polymerization stability and core-shell structure formation, acrylic acid, methacrylic acid, and itaconic acid are more preferred, and acrylic acid and methacrylic acid are even more preferred.

[0035] When the polymer (a1) is taken as 100.0 mass%, the unsaturated carboxylic acid ester monomer unit is preferably 20.0 mass% or more, more preferably 30.0 mass% or more, and also preferably 99.9 mass% or less, more preferably 90.0 mass% or less. If it is equal to or greater than the lower limit, the shell portion can sufficiently cover the core portion and be fixed to the core portion, thereby suppressing foaming of the binder composition or slurry composition. If it is equal to or less than the upper limit, the amount of insolubility in the electrolyte is sufficient, resulting in good cycle characteristics.

[0036] The content of the unsaturated carboxylic acid monomer unit is preferably 0.1% by mass or more, more preferably 1.0% by mass or more, and also preferably 5.0% by mass or less, more preferably 4.5% by mass or less. If it is equal to or greater than the lower limit, the core-shell particles are sufficiently stabilized, and good adhesion to the substrate can be achieved. If it is equal to or less than the upper limit, the shell portion can sufficiently cover the core portion, and foaming of the binder composition or slurry composition can be suppressed.

[0037] Polymers containing unsaturated carboxylic acid ester monomer units and unsaturated carboxylic acid monomer units may contain other monomer units. Examples of monomers that can form the other monomer units include aliphatic conjugated dienes such as 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), and 2,3-dimethyl-1,3-butadiene; and aromatic vinyls such as styrene, styrene sulfonic acid and its salts, α-methylstyrene, butoxystyrene, and vinylnaphthalene. The content of the other monomer units may be 10.0% by mass or less.

[0038] To reduce the amount of insolubility in the electrolyte solution, other monomer units, such as aliphatic conjugated diene monomer units, may be included. For example, polymer (a1) may be composed of unsaturated carboxylic acid ester monomer units, unsaturated carboxylic acid monomer units, and aliphatic conjugated diene monomer units. In this case, the aliphatic conjugated diene monomer may be 5.0% by mass or more, preferably 10.0% by mass or more, and 50.0% by mass or less, preferably 45.0% by mass or less. The amounts of unsaturated carboxylic acid ester monomer units and unsaturated carboxylic acid monomer units are subject to the above-mentioned examples and preferred ranges.

[0039] In addition to the above polymers, examples of polymer (a1) that can be used include aliphatic conjugated diene polymers such as polybutadiene and polyisoprene, aromatic vinyl-aliphatic conjugated diene copolymers such as styrene-butadiene polymer (SBR) and styrene-butadiene-styrene polymer (SBS), vinyl cyanide-conjugated diene copolymers such as acrylonitrile-butadiene polymer (NBR), hydrogenated SBR, hydrogenated NBR, etc. Also usable are polymers described in Japanese Patent No. 6,520,720.

[0040] (Shell portion made of polymer (a2)) The shell portion is composed of polymer (a2), which contains 50.0 to 89.9% by mass of monofunctional unsaturated carboxylic acid ester monomer units, 10.0 to 49.9% by mass of one or more monomer units selected from monofunctional unsaturated carboxylic acid monomer units and monofunctional unsaturated sulfonic acid monomer units, 0.1 to 10.0% by mass of monomer units having two or more vinyl functional groups, and 5.0% by mass or less of aromatic vinyl monomer units. The content of each monomer unit is based on 100.0% by mass of polymer (a2). In the polymer (a2), the amount of one or more monomer units selected from the group consisting of monofunctional unsaturated carboxylic acid monomer units and monofunctional unsaturated sulfonic acid monomer units is 0.1 moles or more and 15,000 moles or less per mole of the monomer units having two or more vinyl functional groups.

[0041] -Amount of each monomer unit- If the monofunctional unsaturated carboxylic acid ester monomer unit is less than 50.0% by mass, the shell portion does not sufficiently cover the core portion, resulting in insufficient fixation to the core portion, and the binder composition or slurry composition becomes prone to foaming. On the other hand, if the monofunctional unsaturated carboxylic acid ester monomer is more than 89.9% by mass, the shell portion becomes less soluble in water, the molecular chain of the shell portion does not expand, the migration suppression ability of the shell portion decreases, and it becomes difficult to improve adhesion.

[0042] If the content of one or more monomer units selected from monofunctional unsaturated carboxylic acid monomer units and monofunctional unsaturated sulfonic acid monomer units (hereinafter also referred to as "monofunctional unsaturated carboxylic acid and / or sulfonic acid monomer units") is less than 10.0% by mass, the shell portion becomes less soluble in water, the molecular chain of the shell portion does not expand, the migration suppression ability decreases, and it becomes difficult to improve adhesion. On the other hand, if the content of monofunctional unsaturated carboxylic acid and / or sulfonic acid monomer units exceeds 49.9% by mass, the shell portion does not sufficiently cover the core portion, the fixation to the core portion is insufficient, and the binder composition or slurry composition becomes prone to foaming. The content of monofunctional unsaturated carboxylic acid and / or sulfonic acid monomer units is preferably 13.0% by mass or more, more preferably 16.0% by mass or more, and is preferably 49.0% by mass or less, more preferably 48.0% by mass or less.

[0043] If the content of the monomer units having two or more vinyl functional groups is less than 0.1% by mass, the solubility of the core-shell particles serving as the binder in the electrolyte solution increases, weakening their function as a binder and resulting in reduced cycle characteristics and peel strength after cycle testing. Furthermore, the electrolyte retention capacity decreases, making it difficult to sufficiently suppress lithium metal precipitation when used in a lithium-ion secondary battery. On the other hand, if the content of the monomer units having two or more vinyl functional groups exceeds 10.0% by mass, the crosslinked structure in the shell portion increases, making the shell portion less soluble in water. This prevents the molecular chains of the shell portion from expanding, reducing migration suppression and making it difficult to improve adhesion. The content of the monomer units having two or more vinyl functional groups is preferably 0.2% by mass or more, more preferably 0.5% by mass or more, and also preferably 8.0% by mass or less, more preferably 3.0% by mass or less.

[0044] If the aromatic vinyl monomer unit content exceeds 5.0% by mass, the shell portion does not sufficiently cover the core portion, and the binder composition or slurry composition tends to foam. The aromatic vinyl monomer unit content is preferably 4.0% by mass or less, more preferably 2.0% by mass or less, and may be 0.0% by mass.

[0045] The polymer (a2) may contain other monomer units within a range that does not impair the effects of the present invention, but is preferably composed of 50.0% by mass or more and 89.9% by mass or less of monofunctional unsaturated carboxylic acid ester monomer units, 10.0% by mass or more and 49.9% by mass or less of monofunctional unsaturated carboxylic acid and / or sulfonic acid monomer units, 0.1% by mass or more and 10.0% by mass or less of monomer units having two or more vinyl functional groups, and 5.0% by mass or less (or may be 0.0% by mass) of aromatic vinyl monomer units.

[0046] If the ratio of monofunctional unsaturated carboxylic acid and / or sulfonic acid monomer units to one mole of monomer units having two or more vinyl functional groups is less than 0.1 moles, the molecular chain of the shell portion does not expand, the migration suppression ability is reduced, and it becomes difficult to improve adhesion. Furthermore, if the ratio of monofunctional unsaturated carboxylic acid and / or sulfonic acid monomer units to one mole of monomer units having two or more vinyl functional groups is more than 15,000 moles, the solubility of the core-shell particles serving as the binder in the electrolyte increases, weakening their function as binders and reducing cycle characteristics and peel strength after cycle testing. The ratio of monofunctional unsaturated carboxylic acid and / or sulfonic acid monomer units to one mole of monomer units having two or more vinyl functional groups is preferably 0.15 moles or more, more preferably 0.25 moles or more, and preferably 14,000 moles or less, more preferably 13,500 moles or less.

[0047] -Monofunctional unsaturated carboxylic acid ester monomer unit- The monofunctional unsaturated carboxylic acid ester capable of forming the monofunctional unsaturated carboxylic acid ester monomer unit may be a mono- or polycarboxylic acid ester, and from the viewpoint of forming a core-shell structure, it is preferably a monocarboxylic acid ester, more preferably an acrylic acid ester, a methacrylic acid ester, etc. Examples of the ester include alkyl esters, and from the viewpoint of polymerization stability and forming a core-shell structure, it may be an alkyl ester having from 1 to 20 carbon atoms, and preferably an alkyl ester having from 1 to 10 carbon atoms.

[0048] Specific examples include octyl acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, and 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, and stearyl acrylate; octyl methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, pentyl methacrylate, hexyl methacrylate, heptyl methacrylate, and 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, and stearyl methacrylate. Among these, methyl methacrylate, ethyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, etc. are preferred from the viewpoint of forming a core-shell structure.

[0049] The monofunctional unsaturated carboxylic acid ester monomer unit may be used alone or in combination of two or more kinds in any ratio.

[0050] -Monofunctional unsaturated carboxylic acid and / or sulfonic acid monomer unit- The monofunctional unsaturated carboxylic acid capable of forming the monofunctional unsaturated carboxylic acid monomer unit can be a mono- or polycarboxylic acid, and from the viewpoint of the molecular chain extent of the shell portion, is preferably a monocarboxylic acid, more preferably acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, or itaconic acid, and even more preferably acrylic acid, methacrylic acid, or itaconic acid.

[0051] Examples of the monofunctional unsaturated sulfonic acid capable of forming the monofunctional unsaturated sulfonic acid monomer include vinyl sulfonic acid, p-styrene sulfonic acid, and (meth)allyl sulfonic acid, with p-styrene sulfonic acid being preferred.

[0052] The monofunctional unsaturated carboxylic acid and / or sulfonic acid monomer units may be in the form of a salt, such as an alkali metal salt (such as a sodium salt or a potassium salt), an alkaline earth metal salt (such as a magnesium salt), or an ammonium salt.

[0053] The monofunctional unsaturated carboxylic acid and / or sulfonic acid monomer unit may be one of these alone or a combination of two or more of these in any ratio.

[0054] -Monomer unit having two or more vinyl functional groups- Regarding the polymer (a2), the "monomer unit having two or more vinyl functional groups" does not include anything equivalent to the "aromatic vinyl monomer unit." The number of vinyl functional groups in the monomer unit having two or more vinyl functional groups may be two or more, but from the viewpoint of the solubility of the shell portion in water, it is preferably two or more and ten or less, more preferably two or more and six or less.

[0055] Examples of monomers having two or more vinyl functional groups that can form monomer units having two or more vinyl functional groups include aliphatic dienes such as 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), and 2,3-dimethyl-1,3-butadiene; vinyl ethers or allyl ethers of unsaturated carboxylic acids such as allyl acrylate, allyl methacrylate, vinyl acrylate, and vinyl methacrylate; and acrylic acid esters or methacrylic acid esters of polyhydric alcohols such as ethylene glycol dimethacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, tetraethylene glycol diacrylate, ethoxylated isocyanuric acid triacrylate, ethoxylated glycerin triacrylate, ditrimethylolpropane tetraacrylate, ethoxylated pentaerythritol tetraacrylate, dipentaerythritol polyacrylate, and dipentaerythritol hexaacrylate. Among these, ethylene glycol dimethacrylate, 1,3-butadiene, ethoxylated pentaerythritol tetraacrylate, etc. are preferred from the viewpoint of the solubility of the shell portion in water.

[0056] The monomer unit having two or more vinyl functional groups may be used alone or in any combination of two or more kinds in any ratio.

[0057] -Aromatic vinyl monomer unit- Regarding the polymer (a2), the term "aromatic vinyl monomer unit" does not include units corresponding to either "monofunctional unsaturated carboxylic acid monomer unit" or "monofunctional unsaturated sulfonic acid monomer unit."

[0058] When an aromatic vinyl monomer unit is present, examples of the monomer capable of forming the aromatic vinyl monomer include styrene, α-methylstyrene, butoxystyrene, vinylnaphthalene, and the like.

[0059] The aromatic vinyl monomer units may be used alone or in combination of two or more kinds in any ratio.

[0060] -Molecular weight of polymer (a2)- The weight-average molecular weight of polymer (a2) is preferably 10,000 or more, more preferably 30,000 or more, and preferably 10,000,000 or less, more preferably 4,500,000 or less. Within the above ranges, migration due to the shell portion is sufficiently suppressed, good adhesion is obtained, and the core-shell particles serving as the binder are sufficiently dispersed in the binder composition or slurry composition, resulting in good cycle characteristics.

[0061] The weight average molecular weight can be measured by the measurement method described in the Examples of this specification.

[0062] (core-shell particles) The core-shell particles have a core portion made of polymer (a1) and a shell portion made of polymer (a2).

[0063] - Mass ratio of polymer (a1) to polymer (a2) - In the core-shell particles, the ratio of the mass of polymer (a1) to the mass of polymer (a2) ((mass of a1) / (mass of a2)) is preferably 95 / 5 or less, more preferably 90 / 10 or less, even more preferably 87 / 13 or less, and is preferably 5 / 95 or more, more preferably 10 / 90 or more, and more preferably 13 / 87 or more.

[0064] - Radius of gyration of core-shell particles - The radius of gyration of the core-shell particles is preferably 10 nm or more, more preferably 30 nm or more, and even more preferably 50 nm or more, and is preferably 1000 nm or less, more preferably 800 nm or less, and even more preferably 600 nm or less.

[0065] -Method of manufacturing core-shell particles- The method for producing the core-shell type particles is not particularly limited, but emulsion polymerization, solution polymerization, bulk polymerization, suspension polymerization, etc. are preferred because they can be easily obtained.

[0066] The emulsion polymerization method includes a method in which a monomer for forming polymer (a1) is radically polymerized in an aqueous solvent in the presence of an emulsifier and a polymerization initiator to obtain polymer (a1), and then a monomer for forming polymer (a2) is added and polymerized. In order to cover the outer surface of the core with the shell, it is preferable to supply the monomer for forming the shell polymer to the polymerization system in multiple batches or continuously.

[0067] The emulsifier is not particularly limited, and examples thereof include anionic surfactants such as sodium dodecylbenzenesulfonate and sodium dodecyl sulfate (sodium lauryl sulfate); and cationic surfactants such as octadecylamine acetate. The emulsifier may be used alone or in combination of two or more kinds in any ratio.

[0068] The polymerization initiator is not particularly limited, and examples thereof include peroxides such as t-butylperoxy-2-ethylhexanoate, potassium persulfate, ammonium persulfate, and cumene peroxide; and azo compounds such as 2,2'-azobis(2-methyl-N-(2-hydroxyethyl)-propionamide) and 2,2'-azobis(2-amidinopropane) hydrochloride. The polymerization initiator may be used alone or in combination of two or more kinds in any ratio.

[0069] A chain transfer agent can be used in the reaction. The chain transfer agent is not particularly limited, but is preferably an alkyl mercaptan, and more preferably t-dodecyl mercaptan.

[0070] The aqueous solvent can be water, and may contain other hydrophilic solvents as long as they can dissolve or disperse the monomers used in the production of core-shell particles. Examples of hydrophilic solvents include alcohols such as methanol, ethanol, n-propanol, and isopropanol; ketones such as acetone and methyl ethyl ketone; polyalkylene glycols such as ethylene glycol, diethylene glycol, and propylene glycol; alkyl ethers of polyalkylene glycols; and lactams such as N-methyl-2-pyrrolidone.

[0071] In the case of core-shell particles, the shell portion is charged and the molecular chains are spread by the repulsive force thereof, so that it is preferable to adjust the pH using a basic compound after polymerization. When an aqueous solvent is used, the pH of the reaction solution after polymerization is usually about 0.2 to 6, and adjusting the pH with a basic compound is usually an operation equivalent to neutralization.

[0072] From the viewpoint of slurry stability, the pH is preferably adjusted to 4 or more and 9 or less. Examples of basic compounds include organic amines such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, 2-aminoethanol, and 2-dimethylaminoethanol; inorganic basic compounds such as ammonia (water), sodium hydroxide, potassium hydroxide, and lithium hydroxide; and quaternary ammonium hydroxides such as tetramethylammonium hydroxide, tetra-n-butylammonium hydroxide, and trimethylbenzylammonium hydroxide. Among these, inorganic basic compounds such as sodium hydroxide and potassium hydroxide are preferred. The basic compounds may be used alone or in combination of two or more in any ratio.

[0073] The core-shell particles are preferably produced so that the ratio of the viscosity after adjustment to the viscosity before pH adjustment of the binder composition (viscosity after adjustment / viscosity before adjustment) is 1.5 or more, more preferably 1.6 or more, and although there is no particular upper limit, it is usually 500 or less. Within the above range, the shell portion has a sufficient spread of molecular chains, migration is sufficiently suppressed, the core-shell particles as a binder are sufficiently dispersed in the binder composition or slurry composition, and good cycle characteristics can be obtained.

[0074] [solvent] The binder composition for a secondary battery of the present invention contains a solvent. The solvent can be an aqueous solvent, such as water or a mixed solvent of water and a hydrophilic organic solvent, with water being preferred. As the aqueous solvent, the same description (including examples and preferred examples) as used in the production of core-shell particles can be applied. The aqueous solvent used in the production of core-shell particles can be used as is.

[0075] [Production of binder composition, etc.] The binder composition for secondary batteries of the present invention can be produced by dispersing the core-shell particles obtained by emulsion polymerization in an aqueous solvent, either directly or, if necessary, by adjusting the pH, removing unreacted monomers, removing the solvent, etc. The method for removing the unreacted monomers and removing the solvent is not particularly limited, and examples thereof include heating, distillation under reduced pressure, and a combination thereof.

[0076] The binder composition has an electrolyte insoluble content of preferably 50% or more, more preferably 55% or more. The upper limit is 100%, and the electrolyte insoluble content may be 100%. Within the above range, the core-shell particles maintain the core-shell structure, sufficient adhesion is obtained, and good cycle characteristics can be maintained. The amount of insoluble material in the electrolyte can be measured by the method described in the Examples of this specification.

[0077] The binder composition may contain binders other than core-shell particles, but advantageously, the amount of core-shell particles is 5 parts by mass or more, preferably 20 parts by mass or more, per 100 parts by mass of binder. Core-shell particles alone may also be used as the binder. Examples of binders other than core-shell particles include aliphatic conjugated diene polymers such as polybutadiene and polyisoprene, aromatic vinyl-aliphatic conjugated diene copolymers such as styrene-butadiene polymers (SBR) and styrene-butadiene-styrene polymers (SBS), vinyl cyanide-conjugated diene copolymers such as acrylonitrile-butadiene polymers (NBR), hydrogenated SBR, hydrogenated NBR, etc.

[0078] The binder composition may further contain components such as a thickener, a wetting agent, a conductive aid, a reinforcing material, a leveling agent, an electrolyte additive, an antifoaming agent, etc. These components may be used alone or in any combination of two or more in any ratio, provided that the effects of the present invention are not impaired.

[0079] The solid content concentration of the binder composition is not particularly limited, and can be, for example, from 5% by mass to 70% by mass.

[0080] <Slurry composition for secondary batteries> The slurry composition for a secondary battery of the present invention is a composition used to form an optional functional layer in a secondary battery (e.g., a non-aqueous secondary battery such as a lithium-ion secondary battery), and contains the binder composition for a secondary battery of the present invention and can further contain functional particles and other components. Because it contains the binder composition for a secondary battery of the present invention, a functional layer with excellent adhesion can be obtained, for example, by applying the slurry composition for a secondary battery of the present invention to a substrate and drying it.

[0081] [Binder composition] The amount of the binder composition for a secondary battery of the present invention in the slurry composition is not particularly limited, and can be set depending on the functional layer. When the slurry composition is used to prepare an electrode mixture layer, the binder composition can be used in an amount such that the binder (core-shell particles and any binder) is 0.5 parts by mass or more and 15 parts by mass or less in terms of solid content per 100 parts by mass of electrode active material particles. When the slurry composition is used to prepare a porous membrane layer, the binder composition can be used in an amount such that the binder (core-shell particles and any binder) is 0.5 parts by mass or more and 30 parts by mass or less in terms of solid content per 100 parts by mass of non-conductive particles.

[0082] [Functional particles] The slurry composition may contain functional particles depending on the desired functional layer. When the functional layer is an electrode mixture layer, examples of the functional particles include electrode active material particles, and when the functional layer is a porous membrane layer, examples of the functional particles include non-conductive particles.

[0083] (electrode active material particles) The electrode active material particles are not particularly limited, and examples thereof include particles made of known electrode active materials used in secondary batteries. For example, for the electrode mixture layer of a lithium ion secondary battery, examples thereof include particles made of the following electrode active materials:

[0084] -Cathode active material- The cathode active material blended in the cathode composite material layer of the lithium-ion secondary battery is not particularly limited, and examples include compounds containing transition metals, such as transition metal oxides, transition metal sulfides, composite metal oxides of lithium and transition metals, and the like. Here, examples of the transition metal include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, and the like. Specifically, lithium-containing cobalt oxide (LiCoO2), lithium manganate (LiMn2O4), lithium-containing nickel oxide (LiNiO2), lithium-containing composite oxide of Co-Ni-Mn, lithium-containing composite oxide of Ni-Mn-Al, lithium-containing composite oxide of Ni-Co-Al, olivine-type lithium iron phosphate (LiFePO4), olivine-type lithium manganese phosphate (LiMnPO4), Li , 0.5 , 1.5 , , 0.56 ,

[0085] , , , , Mn 2-x O4 (0 < X < 2) spinel compound with excess lithium, Li[Ni 0.17 Li 0.2 Co 0.07 Mn 0.56 O2, LiNi 0.5 Mn 1.5 O4, and the like. The cathode active material may be used alone or in combination of two or more in any ratio.

[0085] -Anode active material- The anode active material blended in the anode composite material layer of the lithium-ion secondary battery is not particularly limited, and examples include carbon-based anode active materials, metal-based anode active materials, anode active materials combining these, and the like. Here, the carbon-based anode active material refers to an active material having a carbon main skeleton into which lithium can be inserted (also referred to as "doped"), such as carbonaceous materials such as coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers, pyrolytic gas-phase grown carbon fibers, phenolic resin fired bodies, polyacrylonitrile-based carbon fibers, quasi-isotropic carbon, furfuryl alcohol resin fired bodies (PFA), hard carbon; graphite materials such as natural graphite and artificial graphite. The metal-based negative electrode active material is an active material containing a metal, and typically refers to an active material that contains an element capable of inserting lithium in its structure and has a theoretical electrical capacity per unit mass of 500 mAh / g or more when lithium is inserted, such as lithium metal, elemental metals that can form lithium alloys (e.g., Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, Ti, etc.), and oxides, sulfides, nitrides, silicides, carbides, phosphides, etc. of these. Further examples include oxides such as lithium titanate. The negative electrode active material may be a single material or a combination of two or more materials in any ratio.

[0086] (non-conductive particles) The non-conductive particles are not particularly limited, and examples thereof include known non-conductive particles used in secondary batteries. The non-conductive particles may be inorganic or organic fine particles, but inorganic fine particles are usually used. Among them, materials that are stable in the use environment of the secondary battery and are electrochemically stable are preferred. From this viewpoint, preferred examples of non-conductive particles include oxide particles such as aluminum oxide (alumina), hydrated aluminum oxide (boehmite), silicon oxide, magnesium oxide (magnesia), calcium oxide, titanium oxide (titania), BaTiO3, ZrO, and alumina-silica composite oxide; nitride particles such as aluminum nitride and boron nitride; covalently bonded crystalline particles such as silicon and diamond; sparingly soluble ionic crystalline particles such as barium sulfate, calcium fluoride, and barium fluoride; and clay fine particles such as talc and montmorillonite. These particles may be subjected to element substitution, surface treatment, solid solution formation, etc., as necessary. The non-conductive particles may be of a single kind or a combination of two or more kinds in any ratio.

[0087] [Other ingredients] The slurry composition may contain other components as long as the effects of the present invention are not impaired. As for other components, the descriptions (including examples and preferred examples) for the binder composition are applicable. The other components may be used alone or in combination of two or more kinds in any ratio.

[0088] [Production of slurry composition, etc.] The method for producing the slurry composition is not particularly limited. For example, when the slurry composition is used to prepare an electrode mixture layer, the binder composition, electrode active material particles, and other components used as needed can be mixed in the presence of a solvent to produce the slurry composition. When the slurry composition is used to prepare a porous membrane layer, the binder composition, non-conductive particles, and other components used as needed can be mixed in the presence of a solvent to produce the slurry composition. When the slurry composition is used to prepare an adhesive layer, the binder composition can be used as is or diluted with a solvent to form a slurry composition, or the binder composition and other components used as needed can be mixed in the presence of a solvent to produce a slurry composition. The solvent used in producing the slurry composition includes the solvent in the binder composition. The solvent is not particularly limited, and examples thereof include water, alcohol, ketone, etc. The mixing method is not particularly limited, and can be performed using a known stirrer, disperser, etc.

[0089] <Functional layer for secondary batteries> The functional layer for a secondary battery of the present invention is a layer that performs functions such as transferring electrons, or reinforcing or adhering, within a secondary battery (e.g., a non-aqueous secondary battery such as a lithium ion secondary battery), and examples thereof include an electrode mixture layer that transfers electrons via an electrochemical reaction, a porous membrane layer that improves heat resistance and strength, and an adhesive layer that improves adhesion. The functional layer of the present invention is formed from the slurry composition for a secondary battery of the present invention, and can be formed, for example, by applying the slurry composition to, for example, a substrate or the like and drying it. Since the slurry composition for a secondary battery of the present invention contains the binder composition for a secondary battery of the present invention, excellent adhesion can be obtained between the functional layer formed from the slurry composition and the substrate.

[0090] [Base material] The substrate to which the slurry composition is applied is not particularly limited. When forming an electrode mixture layer, a current collector may be used as the substrate, and the slurry composition may be applied onto the current collector and dried to form a layer. When forming a porous membrane layer or an adhesive layer, a separator substrate or an electrode substrate may be used as the substrate, and the slurry composition may be applied onto the substrate and dried to form the layer. The slurry composition can be applied to the surface of a release substrate and dried to form a functional layer, and the release substrate can be peeled off, and the functional layer peeled off from the release substrate can be used as a free-standing film in the production of a secondary battery.

[0091] (current collector) The current collector is not particularly limited, and any electrically conductive and electrochemically durable material can be used, such as a current collector made of iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, or platinum. Copper foil is particularly preferred as a current collector for the negative electrode. Aluminum foil is particularly preferred as a current collector for the positive electrode. The current collector may be made of a single material or a combination of two or more materials in any ratio.

[0092] (separator substrate) The separator substrate is not particularly limited, and examples thereof include known separator substrates such as organic separator substrates. The organic separator substrate is a porous member made of an organic material, and examples thereof include a microporous membrane or nonwoven fabric containing a polyolefin resin such as polyethylene or polypropylene, or an aromatic polyamide resin. Microporous membranes and nonwoven fabrics made of polyethylene are preferred because of their excellent strength.

[0093] (electrode base material) The electrode substrates (positive electrode substrate and negative electrode substrate) are not particularly limited, and examples thereof include electrode substrates in which an electrode mixture layer containing electrode active material particles and a binder is formed on the above-mentioned current collector. The electrode active material particles and binder contained in the electrode mixture layer are not particularly limited, and binders other than the electrode active material particles described in the slurry composition and the core-shell type particles described in the binder composition can be used. The functional layer of the present invention may be used as the electrode mixture layer in the electrode substrate.

[0094] [Formation of functional layer] The method for forming a functional layer on a substrate such as a current collector, separator substrate, or electrode substrate is not particularly limited, and examples thereof include the following. 1) A method in which the slurry composition of the present invention is applied to the surface of a substrate (in the case of an electrode substrate, the surface on the electrode mixture layer side) and then dried; 2) a method of immersing a substrate in the slurry composition of the present invention and then drying the same; and 3) A method in which the slurry composition of the present invention is applied to a release substrate, dried to produce a functional layer, and the resulting functional layer is transferred to the surface of the substrate. Among these, the method 1) is preferred because it is easy to control the thickness of the functional layer. Specifically, the method 1) includes a step of applying a slurry composition onto a substrate (application step) and a step of drying the slurry composition applied onto the substrate to form a functional layer (drying step).

[0095] (coating process) The method for applying the slurry composition onto the substrate is not particularly limited, and examples thereof include a doctor blade method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method.

[0096] (drying process) The method for drying the slurry composition on the substrate is not particularly limited, and known methods can be used, such as drying with warm air, hot air, or low-humidity air, vacuum drying, and drying by irradiation with infrared rays or electron beams.

[0097] When preparing an electrode mixture layer as a functional layer, it is preferable to apply pressure to the electrode mixture layer using a mold press, a roll press, or the like after the drying step.

[0098] <Secondary battery> The secondary battery of the present invention (e.g., a nonaqueous secondary battery such as a lithium-ion secondary battery) comprises a positive electrode, a negative electrode, a separator, and an electrolyte, and at least one of the positive electrode, negative electrode, and separator comprises the secondary battery functional layer of the present invention. The secondary battery of the present invention can also be produced using the secondary battery separator of the present invention and / or the secondary battery electrode of the present invention for one or both of the positive electrode and the negative electrode. The positive electrode, negative electrode, and separator that do not comprise the secondary battery functional layer of the present invention are not particularly limited, and known positive electrodes, negative electrodes, and separators can be used.

[0099] [Electrolyte] The electrolyte is not particularly limited, and examples thereof include organic electrolytes in which a supporting electrolyte is dissolved in an organic solvent. Examples of supporting electrolytes used in lithium-ion secondary batteries include lithium salts, such as LiPF6, LiAsF6, LiBF4, LiSbF6, LiAlCl4, LiClO4, CF3SO3Li, CF4F9SO3Li, CF3COOLi, (CF3CO)2NLi, (CF3SO2)2NLi, and (C2F5SO2)NLi. Among these, LiPF6, LiClO4, and CF3SO3Li are preferred because they are easily soluble in solvents and exhibit a high degree of dissociation. The electrolyte may be a single electrolyte or a combination of two or more electrolytes in any ratio. Typically, the use of a supporting electrolyte with a higher degree of dissociation tends to result in higher lithium ion conductivity, so the lithium ion conductivity can be adjusted by the type of supporting electrolyte.

[0100] The organic solvent used in the electrolyte is not particularly limited as long as it can dissolve the supporting electrolyte. Examples of organic solvents used in lithium-ion secondary batteries include carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), ethyl methyl carbonate (EMC), and vinylene carbonate (VC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; and sulfur-containing compounds such as sulfolane and dimethyl sulfoxide. A mixture of these solvents may also be used. Among these, carbonates are preferred due to their high dielectric constant and wide stable potential range. Generally, the lower the viscosity of the solvent used, the higher the lithium ion conductivity, so the lithium ion conductivity can be adjusted by the type of solvent. The concentration of the electrolyte in the electrolytic solution can be adjusted as appropriate. Known additives may be added to the electrolytic solution.

[0101] [Secondary battery manufacturing, etc.] The secondary battery of the present invention can be produced, for example, by stacking a positive electrode and a negative electrode with a separator interposed therebetween, rolling or folding the stack as necessary, placing the stack in a battery container, injecting an electrolyte into the battery container, and sealing the container. In this case, at least one of the positive electrode, negative electrode, and separator is provided with the functional layer for a secondary battery of the present invention. If necessary, the battery container may contain an expanded metal, an overcurrent prevention element such as a fuse or a PTC element, a lead plate, etc. to prevent pressure buildup inside the battery and overcharging / discharging. The shape of the battery is not particularly limited, and examples include coin type, button type, sheet type, cylindrical type, square type, and flat type. [Example]

[0102] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" representing amounts are based on mass unless otherwise specified. In a polymer produced by copolymerizing multiple types of monomers, the proportion of a repeating unit (monomer unit) formed by polymerizing a certain monomer in the polymer usually coincides with the ratio (feed ratio) of that certain monomer to all the monomers used in the polymerization of the polymer, unless otherwise specified.

[0103] In the examples and comparative examples, the following measurements and evaluations (1) to (11) were carried out. (1) Weight average molecular weight of the shell (2) Amount of binder composition insoluble in electrolyte (3) Viscosity ratio of binder composition before and after neutralization (4) THF-insoluble content of core polymer (5) Radius of gyration of core-shell particles (6) Foaming of the slurry composition (7) Electrode peel strength (8) Peel strength of separator coating layer (9) Cycle characteristics of secondary batteries (10) Peel test after cycle test (11) Lithium metal deposition test

[0104] (1) Weight average molecular weight of the shell A solution (pH 8.0) of the polymer containing only the shell portion was prepared, and its weight-average molecular weight was measured by gel permeation chromatography (GPC) to determine the weight-average molecular weight of the shell portion. The method for preparing the solution of the polymer containing only the shell portion is as described below. The measurement was performed as follows. First, a polymer consisting of the shell portion alone was added to approximately 5 mL of eluent so that the solids concentration was approximately 0.5 g / L. A stirring bar was added at room temperature (25°C), and the mixture was stirred at 300 rpm using a magnetic stirrer for 10 minutes to dissolve. After visually confirming dissolution, the mixture was filtered through a 0.45 μm filter to prepare a measurement sample. Then, a calibration curve was created using a standard substance, and the weight-average molecular weight was calculated as a value converted from the standard substance. The measurement conditions are as follows. <Measurement conditions> Column: Showa Denko Co., Ltd., product name "Shodex OHpak (SB-G, SB-807HQ, SB-806MHQ)" Eluent: 0.1M Tris buffer (0.1M potassium chloride added) Flow rate: 0.5mL / min Sample concentration: 0.05g / L (solid concentration) Injection volume: 200μL Column temperature: 40℃ Detector: Differential refractive index detector RI (manufactured by Tosoh Corporation, product name "RI-8020") Standard material: Monodisperse pullulan (Showa Denko)

[0105] (2) Amount of binder composition insoluble in electrolyte The binder composition was applied to a Teflon (registered trademark) substrate and dried for 24 hours in an environment of 50% relative humidity and 23 to 25°C temperature to form a film with a thickness of 0.5±0.3 mm. The formed film was dried for 10 hours in a vacuum dryer at 60°C, then cut into 2 mm square pieces, and the obtained film pieces were weighed (mass W1).

[0106] Next, the obtained film piece was added to 10 mL of a 1.0 M LiPF6 solution (solvent: a mixed solvent of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 3 / 7 (volume ratio), additive: vinylene carbonate 2 vol% (solvent ratio)) as an electrolyte in an environment at 60°C. After confirming that the entire film piece was immersed in the electrolyte, it was stored in an environment at 60°C for 72 hours. The film was then removed from the container using tweezers and placed in another container, to which 10 mL of diethyl carbonate was added. After confirming that the entire film was immersed in the liquid, the film was left to stand at 25°C for 2 hours for cleaning. The film piece was then removed from the container using tweezers, placed in another container, and 10 mL of ethanol was added. After confirming that the entire film piece was immersed in the liquid, it was left to stand at 25°C for 2 hours for cleaning. The film piece was removed, placed on a Kimwipe, and dried in a draft for 2 hours. The film piece was then placed in a vacuum dryer, dried at 25°C under a gauge pressure of -0.09 MPa or less, and then weighed (mass W2).

[0107] The amount of insoluble matter in the electrolyte was calculated from the mass W1 and the mass W2 using the following formula. Electrolyte insoluble amount (%)=W2 / W1×100

[0108] (3) Viscosity ratio of binder composition before and after neutralization The viscosity was measured using a Brookfield viscometer under the conditions of a temperature of 25° C., a spindle rotation speed of 60 rpm, and a spindle rotation time of 60 seconds. The unneutralized solution after polymerization was diluted with ion-exchanged water to a solids concentration of 10%, and the viscosity was measured (viscosity η1). Next, after polymerization, the solution was stirred and neutralized to pH 8.0 by adding an alkaline aqueous solution. The solid content was adjusted to 10%, and the viscosity after neutralization was measured (viscosity η2). The viscosity ratio η3 of the binder composition before and after neutralization was calculated from the viscosity η1 and viscosity η2 using the following formula. η3=η2 / η1 The higher the value of the viscosity ratio η3, the more the migration of the binder composition is suppressed.

[0109] (4) THF-insoluble content of core polymer The core polymer solution was applied to the substrate and dried for 24 hours in an environment of 50% humidity and 22 to 25°C temperature to form a film with a thickness of approximately 0.3 mm. The formed film was cut into 2 to 3 mm square pieces, and the obtained film pieces were precisely weighed (mass w0). This film piece was immersed in 75 mL of tetrahydrofuran (THF) for 24 hours at a temperature of 25°C. The film piece was then removed from the THF, dried in vacuum at a temperature of 105°C for 3 hours, and precisely weighed to obtain the mass of the insoluble matter (mass w1).

[0110] The THF insoluble content was calculated from the mass w0 and mass w1 using the following formula. THF insoluble amount (mass%)=(w1 / w0)×100

[0111] (5) Radius of gyration of core-shell particles The radius of gyration was determined by creating a Zimm plot using static light scattering in a field-flow fractionation (FFF) apparatus connected to a multi-angle light scattering (MALS) detector. The FFF apparatus allows a sample solution to pass through a channel with a size of 100 μm to 500 μm, and a field is applied during the passage of the solution. 50 μL of the measurement sample containing the core-shell particles was injected into the FFF device connected to a MALS detector, and static light scattering measurements were performed at a flow rate of 1.0 mL / min. The measurement conditions are as follows. <Measurement conditions> MALS detector: Postnova, product name "PN3621 MALS" FFF device: Postnova, product name "AF2000" RI detector: Postnova, product name "PN3150 RI" Channel: Polyethersulfone membrane 10kDa Developing solution: phosphate buffer (1 mM) Measurement sample: 100 μL of core-shell particles diluted to 1% with ion-exchanged water was diluted with 900 μL of pH 7.4 phosphate buffer (1 mM) to adjust the solid content to 0.1%.

[0112] (6) Foaming of the slurry composition 100 g of the slurry composition for secondary batteries was placed in a container with an inner diameter of 6 cm and mixed at 2000 rpm for 5 minutes at 25°C using a disperser equipped with a 3 cm diameter sawtooth disk turbine blade. The composition was then placed in a pressure-compatible case, and the internal pressure of the case was adjusted to 0.1 MPa with nitrogen gas and held for 3 minutes. After removal, the number of bubbles with a diameter of 0.1 mm or more present on the liquid surface of the composition was counted using a 20x magnifying glass. A: Less than 2 B: 2 or more but less than 6 C: 6 or more but less than 10 D: 10 or more A lower number of bubbles indicates a less foaming composition.

[0113] (7) Electrode peel strength A rectangular test piece measuring 100 mm in length and 10 mm in width was cut from an electrode (negative or positive electrode). The test piece was fixed to a test table with the electrode mixture layer surface facing up. Cellophane tape (specified in JIS Z1522) was applied to the surface of the electrode mixture layer of the fixed test piece, and the stress was measured when the cellophane tape was peeled off from one end of the test piece in a 180° direction at a rate of 50 mm / min. The same measurement was performed five times, and the average value was taken as the peel strength and evaluated according to the following criteria. A: Peel strength is 10N / m or more B: Peel strength is 8N / m or more and less than 10N / m C: Peel strength is 4N / m or more and less than 8N / m D: Peel strength is less than 4N / m The higher this value, the better the adhesion to the substrate.

[0114] (8) Peel strength of separator coating layer A rectangular test piece measuring 100 mm long x 10 mm wide was cut from the separator coated with the functional layer. Cellophane tape (specified in JIS Z1522) was fixed to the test table in advance. The obtained test piece was attached to the cellophane tape with the separator's functional layer facing down, and one end of the separator was then pulled vertically at a pulling rate of 100 mm / min to measure the stress when peeled off. The measurement was performed three times, and the average value was taken as the peel strength of the separator coating layer and evaluated according to the following criteria. A: Peel strength is 100N / m or more B: Peel strength is 80N / m or more and less than 100N / m C: Peel strength is 60N / m or more and less than 80N / m D: Peel strength is less than 60N / m The higher this value, the better the adhesion to the substrate.

[0115] (9) Cycle characteristics of secondary batteries After injecting the electrolyte into the laminate cell type lithium ion secondary battery, the battery was left standing for 24 hours in an environment at a temperature of 25°C, and then the battery was charged to a cell voltage of 4.35 V by a constant current method of 0.1 C, and then discharged to a cell voltage of 3.0 V, and the initial capacity C0 was measured. Furthermore, in an environment at a temperature of 45°C, the battery was charged to a cell voltage of 4.35 V using a constant current method of 1.0 C, and then repeatedly discharged in a charge mode and a constant current method until the cell voltage reached 3.0 V, and the capacity C2 after 100 cycles was measured. The capacity retention rate C3 was calculated from the capacity C0 and the capacity C2 using the following formula. C3(%) = (C2 / C0) x 100 A: Capacity retention rate C3 is 95% or more B: Capacity retention rate C3 is 90% or more but less than 95% C: Capacity retention rate C3 is 80% or more but less than 90% D: Capacity retention rate C3 is less than 80% The larger this value, the better the cycle characteristics.

[0116] (10) Peel test after cycle test After the above (9) secondary battery cycle characteristic test, the cell was disassembled in a glove box, and the electrode (negative electrode or positive electrode) was recovered. The recovered electrode was immersed in a solvent (a mixed solvent of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 30 / 70 (volume ratio)) for 10 minutes so that the entire electrode composite layer was in contact with the solvent, and then immersed in ethyl methyl carbonate for 10 minutes in the same manner. After immersion, the electrode was removed and dried for 2 hours in a draft in a dry room.

[0117] A rectangular test piece measuring 100 mm in length and 10 mm in width was cut from the obtained electrode (negative electrode or positive electrode). The test piece was fixed to a test table with the electrode mixture layer surface facing up. Cellophane tape (specified in JIS Z1522) was attached to the surface of the electrode mixture layer of the fixed test piece, and the stress was measured when the cellophane tape was peeled from one end of the test piece in a 180° direction at a rate of 50 mm / min. The same measurement was performed five times, and the average value was taken as the peel strength and evaluated according to the following criteria. Electrode (positive or negative) peel strength standard A: Peel strength is 4N / m or more B: Peel strength is 3N / m or more and less than 4N / m C: Peel strength is 2N / m or more and less than 3N / m D: Peel strength is less than 2N / m The separator was also recovered from the cell after the cycle characteristic test, and a test piece was prepared in the same manner, and stress was measured in the same manner as in (8) Peel strength test of separator coating layer. Separator peel strength standard A: Peel strength is 40N / m or more B: Peel strength is 15N / m or more and less than 40N / m C: Peel strength is 5N / m or more and less than 15N / m D: Peel strength is less than 5N / m Higher values ​​indicate better adhesion to the substrate.

[0118] (11) Lithium metal deposition test After injecting the electrolyte into the laminated cell type lithium ion secondary battery, the battery was left standing for 24 hours in an environment at a temperature of 25°C, and then the battery was charged to a cell voltage of 4.35 V using a constant current method of 0.1 C, and then discharged to a cell voltage of 2.75 V. Next, in an environment at -10°C, the battery was charged at a constant current of 1.5C up to a cell voltage of 4.35V (CCCV charging with a 0.05C cut), and then discharged at the same rate (CC discharge) down to a cell voltage of 2.75V. This charge-discharge cycle was repeated five times. Finally, the battery was charged at a constant current of 0.2C up to a cell voltage of 4.35V in an environment at 25°C. After the above-mentioned operation, the lithium-ion secondary battery cell was disassembled to remove the negative electrode, and the percentage of the area where lithium metal was deposited (Li metal deposition area) relative to the entire negative electrode surface was calculated by image processing. The Li metal deposition area was determined from the color of the electrode surface. Specifically, on the negative electrode facing the positive electrode, the gray to black area was determined to be the Li metal deposition area, and the gold area was determined to be the non-deposition area. A: The proportion of the Li metal deposition area is less than 3% of the entire negative electrode surface B: The proportion of the Li metal deposition area is 3% or more but less than 10% of the entire negative electrode surface C: The proportion of the Li metal deposition area is 10% or more but less than 30% of the entire negative electrode surface D: The proportion of the Li metal deposition area is 30% or more of the entire negative electrode surface The smaller this ratio, the more the deposition of lithium metal in the negative electrode of the lithium ion secondary battery is suppressed.

[0119] Example 1 The binder composition, negative electrode, separator, and secondary battery of Example 1 were prepared as follows.

[0120] [Preparation of core polymer] A 5 MPa pressure vessel A equipped with a stirrer was charged with 33 parts of methyl methacrylate, 33 parts of n-butyl acrylate (butyl acrylate in Table 1), 31 parts of 1,3-butadiene (butadiene in Table 1), 3 parts of methacrylic acid, 2 parts of sodium lauryl sulfate as an emulsifier, 400 parts of ion-exchanged water, 0.5 parts of t-dodecyl mercaptan as a chain transfer agent, and 1.0 part of ammonium persulfate as a polymerization initiator. After thorough stirring, the mixture was heated to 55°C to initiate polymerization and reacted for 4 hours. The temperature was then raised to 70°C and the reaction continued for an additional 4 hours to obtain a core polymer. This core polymer solution was used to measure the THF-insoluble content.

[0121] [Preparation of mixture for preparing shell polymer] A 5 MPa pressure vessel B equipped with a stirrer was charged with 0.35 parts sodium lauryl sulfate as an emulsifier and 67 parts ion-exchanged water. Then, 15 parts of ethyl acrylate and 14.6 parts of n-butyl acrylate (butyl acrylate in Table 1) as monofunctional unsaturated carboxylic acid ester monomers, 12.9 parts of methacrylic acid as a monofunctional unsaturated carboxylic acid monomer, 0.43 parts of ethylene glycol dimethacrylate as a monomer having two or more vinyl functional groups, and 0.1 parts of t-dodecyl mercaptan as a chain transfer agent were added, and the mixture was stirred at 25°C for 30 minutes to obtain a mixture for preparing the shell polymer. The shell polymer solution for measuring the weight-average molecular weight of the shell portion was prepared by adding 0.5 parts of ammonium persulfate as a polymerization initiator to the mixture without the core portion, polymerizing at 75°C for 3 hours, and then adjusting the pH to 8 by adding 5% aqueous sodium hydroxide.

[0122] [Preparation of binder composition containing core-shell particles] After the reaction of the core polymer, the 5 MPa pressure vessel A equipped with a stirrer was heated to a temperature of 75° C. The mixture for preparing the shell polymer obtained above was added to the pressure vessel A over 3 hours, and at the same time, the addition of 0.5 parts of ammonium persulfate as a polymerization initiator to the pressure vessel A was started to initiate polymerization of the shell polymer.

[0123] Three hours after the start of the shell polymerization, the addition of the entire mixture was completed. The mixture was then heated to 85°C and reacted for four hours, then cooled to terminate the reaction. The polymerization conversion rate at the time of termination was 97%. This reaction product solution was used as the unneutralized solution after polymerization in measuring the viscosity ratio of the binder composition before and after neutralization.

[0124] A 5% aqueous solution of sodium hydroxide was added to the reaction product solution to adjust the pH from 2.5 to 8.0. The viscosity after neutralization in measuring the viscosity ratio of the binder composition before and after neutralization was measured using a solution of this reaction product adjusted to a solids concentration of 10%. Thereafter, unreacted monomers were removed by heating and vacuum distillation, and the mixture was then cooled to obtain a binder composition.

[0125] The obtained binder composition contained core-shell type particles, and the monomer compositions of the core and shell parts were as follows, which were substantially equal to the monomer unit composition in the polymer. Of the 100% core monomers, the content is 33% methyl methacrylate, 33% n-butyl acrylate, 31% 1,3-butadiene, and 3% methacrylic acid. Of the 100% shell monomer, the content is 35% ethyl acrylate, 34% n-butyl acrylate, 30% methacrylic acid, and 1% ethylene glycol dimethacrylate. The ratio of the mass of the core portion to the mass of the shell portion (mass of the core portion / mass of the shell portion) is 70 / 30.

[0126] [Preparation of Slurry Composition for Secondary Battery] The planetary mixer is fitted with an electrode active material with a specific surface area of ​​3.5 m 2 98 parts of 1 / g artificial graphite (volume average particle diameter: 25 μm) and 1.0 part of sodium carboxymethylcellulose (degree of etherification: 1.0, aqueous solution viscosity at 1.0% solids: 2500 mPa·s) were added, adjusted to a solids concentration of 58% with ion-exchanged water, and mixed at 45 rpm for 60 minutes at room temperature (25°C). Next, the mixture was diluted with ion-exchanged water in two stages to achieve solids concentrations of 53% and 50% (mixed for 15 minutes at 45 rpm for each stage). Furthermore, 1.0 part of the binder composition was added, and mixed at 45 rpm for 15 minutes. The resulting mixture was degassed at 10 rpm for 3 minutes under a reduced pressure of -0.09 MPa or less to obtain a slurry composition for secondary batteries. The resulting slurry composition had excellent fluidity. This slurry composition for secondary batteries was used in the foaming test of the slurry composition.

[0127] [Negative electrode (prepared using the slurry composition for secondary batteries of the present invention)] The slurry composition for secondary batteries was applied to a copper foil (current collector) with a thickness of 18 μm using a comma coater so that the film thickness after drying was 105 μm and the coating amount was 10 mg / cm. 2The copper foil coated with this composition was transported at a speed of 0.5 m / min through an oven at 75°C for 2 minutes and then through an oven at 120°C for 2 minutes to dry the composition on the copper foil, thereby obtaining a negative electrode blank. This negative electrode blank was rolled using a roll press to obtain a negative electrode having a negative electrode composite layer thickness of 80 μm.

[0128] [Secondary battery production] 295 parts of LiCoO having a spinel structure as a positive electrode active material, 3 parts of PVDF (polyvinylidene fluoride) as a binder for the positive electrode composite layer in terms of solid content, 2 parts of acetylene black as a conductive material, and 20 parts of N-methylpyrrolidone as a solvent were added to a planetary mixer and mixed. The resulting composition was applied to a 20 μm thick aluminum foil (current collector) using a comma coater so that the film thickness after drying would be approximately 100 μm. The aluminum foil coated with this composition was transported at a speed of 0.5 m / min through an oven at 60°C for 2 minutes, and then through an oven at 120°C for 2 minutes to dry the composition on the aluminum foil and obtain a positive electrode blank. This positive electrode blank was rolled using a roll press to obtain a positive electrode with a positive electrode composite layer thickness of 70 μm. The resulting positive electrode was used to fabricate a secondary battery.

[0129] A single-layer polypropylene separator (width 65 mm, length 500 mm, thickness 25 μm; manufactured by a dry method; porosity 55%) was cut into a 5 cm×5 cm square and used to fabricate a secondary battery.

[0130] An aluminum packaging material was prepared as the battery exterior. The positive electrode was cut into a 4 cm x 4 cm square and placed so that the surface on the current collector side was in contact with the aluminum packaging material exterior. Next, the square separator was placed on the surface of the positive electrode composite layer of the positive electrode. Furthermore, the negative electrode was cut into a 4.2 cm x 4.2 cm square and placed on the separator so that the surface on the negative electrode composite layer side faced the separator. Next, a 1.0 M LiPF solution (solvent: a mixed solvent of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 30 / 70 (volume ratio), containing 2 vol% vinylene carbonate (solvent ratio) as an additive) was filled as the electrolyte. Furthermore, the aluminum packaging material exterior was closed by heat sealing at 150 °C to seal the opening of the aluminum packaging material, and a laminate cell-type lithium-ion secondary battery was produced.

[0131] <Examples 2 to 11, 26, Comparative Examples 1 to 8> A binder composition, a slurry composition for a secondary battery, a negative electrode, and a secondary battery were prepared in the same manner as in Example 1, except that the composition ratio and type of the shell monomers were changed as shown in Table 1 when preparing the shell portion. Various evaluations were then performed in the same manner as in Example 1. The ethoxylated pentaerythritol tetraacrylate used in Example 9 and Comparative Example 8 was ATM35E, a product manufactured by Shin-Nippon Chemical Industry Co., Ltd.

[0132] <Examples 12 and 13> A binder composition, a slurry composition for a secondary battery, a negative electrode, and a secondary battery were prepared in the same manner as in Example 1, except that the amount of t-dodecyl mercaptan, a chain transfer agent used in preparing the core portion (0.5 parts), was changed to 0.9 parts in Example 10 and to 0.05 parts in Example 11. Then, various evaluations were performed in the same manner as in Example 1.

[0133] <Examples 14 to 17, Comparative Examples 9 and 10> A binder composition, a slurry composition for a secondary battery, a negative electrode, and a secondary battery were produced in the same manner as in Example 1, except that the amounts of the core portion and the shell portion charged were adjusted and the mass ratio of the core portion to the mass of the shell portion (mass of the core portion / mass of the shell portion) was changed as shown in Table 1. Then, various evaluations were performed in the same manner as in Example 1.

[0134] <Examples 18 to 20> A slurry composition for a secondary battery, a negative electrode, and a secondary battery were produced in the same manner as in Example 1, except that an SBR binder was blended with the binder composition of Example 1 in a mass ratio as shown in Table 1 (the solid content of the binder in the binder composition is the same as in Example 1). Then, various evaluations were performed in the same manner as in Example 1.

[0135] The SBR binder used was prepared as follows. 3.15 parts of styrene, 1.66 parts of 1,3-butadiene, 0.19 parts of itaconic acid, 0.2 parts of sodium lauryl sulfate as an emulsifier, 20 parts of ion-exchanged water, and 0.03 parts of potassium persulfate as a polymerization initiator were placed in a 5 MPa pressure vessel A equipped with a stirrer, and after thorough stirring, the mixture was heated to a temperature of 60°C to initiate polymerization, and the mixture was allowed to react for 6 hours to obtain seed particles. After the above reaction, the temperature was raised to 75°C, and a mixture of 58.85 parts of styrene as an aromatic-containing monomer, 33.34 parts of 1,3-butadiene as an aliphatic conjugated diene monomer, 0.81 parts of itaconic acid as other monomers, 1 part of methyl methacrylate as a (meth)acrylic acid ester monomer which is other monomer, 0.25 parts of t-dodecyl mercaptan as a chain transfer agent, and 0.35 parts of sodium lauryl sulfate as an emulsifier was added from another container B to pressure vessel A, and at the same time, addition of 1 part of potassium persulfate as a polymerization initiator to pressure vessel A was started to initiate the second-stage polymerization. Furthermore, 4 hours after the start of the second-stage polymerization (after 70% of the entire monomer composition had been added), 1 portion of 2-hydroxyethyl acrylate as another monomer was added to the pressure vessel A over 1.5 hours. That is, the overall monomer composition used was 62 parts of styrene, 35 parts of 1,3-butadiene, 1 part of itaconic acid, 1 part of 2-hydroxyethyl acrylate, and 1 part of methyl methacrylate as another monomer. Five and a half hours after the start of the second-stage polymerization, the addition of the entire mixture containing these monomer compositions was completed, and the mixture was then further heated to a temperature of 85° C. and reacted for 6 hours. When the polymerization conversion rate reached 97%, the mixture was cooled to stop the reaction. A 5% aqueous solution of sodium hydroxide was added to the mixture containing the polymer to adjust the pH to 8. Unreacted monomers were then removed by heated, reduced-pressure distillation. The mixture was then cooled to yield an SBR binder.

[0136] <Example 21> A slurry for a secondary battery, a negative electrode, and a secondary battery were produced in the same manner as in Example 1, except that an SBS binder was blended with the binder composition of Example 1 in a mass ratio as shown in Table 1 (the solid content of the binder in the binder composition was the same as in Example 1). Then, various evaluations were performed in the same manner as in Example 1.

[0137] The SBS binder used was prepared as follows. [Preparation of cyclohexane solution of block polymer] A pressure-resistant reactor was charged with 233.3 kg of cyclohexane, 54.2 mmol of N,N,N',N'-tetramethylethylenediamine (hereinafter referred to as "TMEDA"), and 30.0 kg of styrene as an aromatic vinyl monomer. While stirring these at a temperature of 40°C, 1806.5 mmol of n-butyllithium as a polymerization initiator was added, and polymerization was carried out for 1 hour while the temperature was raised to 50°C. The polymerization conversion rate of styrene was 100%. Subsequently, 70.0 kg of 1,3-butadiene as an aliphatic conjugated diene monomer was continuously added to the pressure-resistant reactor over 1 hour while controlling the temperature to maintain it at 50 to 60°C. After the addition of 1,3-butadiene was completed, the polymerization reaction was continued for another 1 hour. The polymerization conversion of 1,3-butadiene was 100%. Next, 722.6 mmol of dichlorodimethylsilane as a coupling agent was added to the pressure reactor, and a coupling reaction was carried out for 2 hours to form a styrene-butadiene coupled block copolymer. Thereafter, in order to deactivate the active terminals, 3612.9 mmol of methanol was added to the reaction solution and mixed well. Next, 0.05 parts of 4-[[4,6-bis(octylthio)-1,3,5-triazin-2-yl]amino]-2,6-di-tert-butylphenol (H1) as a hindered phenol-based antioxidant and 0.09 parts of 3,9-bis(octadecyloxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane (P1) as a phosphite-based antioxidant were added to 100 parts of this reaction liquid (containing 30.0 parts of the polymer component), and they were mixed. The resulting mixed solution was gradually added dropwise to warm water at a temperature of 85 to 95°C to volatilize the solvent, thereby obtaining a precipitate. The precipitate was then pulverized and dried with hot air at a temperature of 85°C, thereby recovering a dried product containing the block polymer. The recovered dried product was then dissolved in cyclohexane to prepare a block polymer solution with a block polymer concentration of 5.0%.

[0138] [Phase inversion emulsification] Sodium alkylbenzene sulfonate was dissolved in ion-exchange water to prepare a 0.15% aqueous solution. 1,400 g of the resulting aqueous solution and 1,000 g of the block polymer solution obtained above were placed in a tank and premixed by stirring. The premix was then transferred from the tank to a high-pressure emulsifying and dispersing machine "LAB1000" (manufactured by SPXFLOW) using a metering pump, and circulated (number of passes: 5) to obtain an emulsion by phase inversion of the premix. Next, cyclohexane in the obtained emulsion was distilled off under reduced pressure using a rotary evaporator, and the distilled emulsion was then centrifuged at 7000 rpm for 10 minutes using a centrifuge (Hitachi Koki Co., Ltd., product name "Himac CR21N"), and the upper layer was removed for concentration. Finally, the upper layer was filtered through a 100-mesh wire screen to obtain an aqueous dispersion containing particulate block polymer (block polymer latex).

[0139] [Graft polymerization and crosslinking] The obtained block polymer latex was diluted with distilled water to a ratio of 850 parts water to 100 parts (solids equivalent) of the particulate block polymer. This diluted block polymer latex was placed in a nitrogen-substituted polymerization reaction vessel equipped with a stirrer and heated to 30°C with stirring. In addition, a diluted methacrylic acid solution was prepared in a separate vessel by mixing 4 parts of methacrylic acid as an acidic group-containing monomer with 15 parts of distilled water. This diluted methacrylic acid solution was added over 30 minutes to the polymerization reaction vessel heated to 30°C, thereby adding 4 parts of methacrylic acid to 100 parts of the block polymer. In addition, a solution (g) containing 7 parts distilled water and 0.01 parts ferrous sulfate (manufactured by Chubu Cherest Co., Ltd., trade name "Frost Fe") as a reducing agent was prepared in a separate container. The resulting solution was added to a polymerization reaction vessel, followed by 0.5 parts of 1,1,3,3-tetramethylbutyl hydroperoxide (manufactured by NOF Corporation, trade name "Perocta H") as an oxidizing agent. The reaction was carried out at 30°C for 1 hour, followed by a further 2 hours at 70°C to obtain a reaction product. The polymerization conversion rate was 99%. This reaction product was used as an SBS binder.

[0140] <Example 22> A binder composition, a slurry composition for a secondary battery, a negative electrode, and a secondary battery were produced in the same manner as in Example 1, except that the thickener used in preparing the slurry composition for a secondary battery was changed from sodium carboxymethyl cellulose to a lithium salt of an acrylic acid / acrylamide / hydroxyethylacrylamide copolymer. Various evaluations were then carried out in the same manner as in Example 1. The lithium salt of an acrylic acid / acrylamide / hydroxyethylacrylamide copolymer used was prepared as follows.

[0141] [Preparation of Lithium Salt of Acrylic Acid / Acrylamide / Hydroxyethylacrylamide Copolymer] 789 parts of ion-exchanged water was placed in a 1 L glass flask and heated to 40°C. The atmosphere in the flask was replaced with nitrogen gas at a flow rate of 100 mL / min. Next, 45 parts of acrylamide, 25 parts of acrylic acid, and 30 parts of hydroxyethyl acrylamide were mixed and poured into the flask. Then, 8.9 parts of a 2.5% aqueous solution of potassium persulfate as a polymerization initiator was added to the flask using a syringe. 15 minutes after the addition of the polymerization initiator, 22.2 parts of a 2.0% aqueous solution of tetramethylethylenediamine as a polymerization accelerator was added using a syringe to initiate the polymerization reaction. Four hours after the addition of the polymerization initiator, 4.4 parts of a 2.5% aqueous solution of potassium persulfate as a polymerization initiator was added to the flask, and 11.1 parts of a 2.0% aqueous solution of tetramethylethylenediamine as a polymerization accelerator was added. The temperature was raised to 60°C and maintained at this temperature to allow the polymerization reaction to proceed. Three hours after the addition of the polymerization initiator, the flask was opened to the air to terminate the polymerization reaction. An 8% aqueous solution of lithium hydroxide was added to the resulting polymer, and the mixture was stirred at 80°C for 6 hours while adjusting the pH to 8.0, yielding a lithium salt of a hydroxyethylacrylamide-acrylic acid-acrylamide copolymer as a thickener.

[0142] Example 23 A binder composition, a slurry composition for a secondary battery, a negative electrode, and a secondary battery were produced in the same manner as in Example 1, except that the SBR binder used in Example 18 was used as the core polymer solution and the composition ratio and type of the shell monomer were changed as shown in Table 1. Then, various evaluations were performed in the same manner as in Example 1.

[0143] Example 24 A binder composition, a slurry composition for a secondary battery, a negative electrode, and a secondary battery were prepared in the same manner as in Example 1, except that the reaction product in the previous stage of [graft polymerization and crosslinking] of the SBS binder used in Example 21 was used as a solution of the core polymer, and the composition ratio and type of 100 parts of the shell monomer were changed as shown in Table 1. Then, various evaluations were performed in the same manner as in Example 1.

[0144] Example 25 The active material used in preparing the slurry composition for secondary batteries was changed from graphite to graphite-silicon active material (mass of graphite:mass of silicon-based active material SiOx=70:30), and the thickener was changed from sodium carboxymethylcellulose to lithium salt of acrylic acid / acrylamide / hydroxyethylacrylamide copolymer (same as used in Example 22). Except for this, a binder composition, a slurry composition for secondary batteries, a negative electrode, and a secondary battery were produced in the same manner as in Example 1. Then, various evaluations were performed in the same manner as in Example 1.

[0145] Example 27 Using the binder composition prepared in Example 1, a separator including a slurry composition for a porous membrane layer and a porous membrane layer was prepared as follows. A secondary battery was prepared in the same manner as in Example 1 except that a separator including this porous membrane was used. Then, various evaluations were performed in the same manner as in Example 1.

[0146] [Preparation of Slurry Composition for Porous Membrane Layer] A mixture was obtained by mixing 100 parts of alumina (volume average particle diameter: 0.5 μm) as non-conductive particles, 1.0 part of ammonium polycarboxylate (manufactured by Toa Gosei, product name "Aron A-6114") as a dispersant, and water. The amount of water was adjusted so that the solids concentration was 50%. The mixture was treated using a media-less dispersing device to disperse the alumina and obtain a dispersion. To the resulting dispersion, 2.0 parts of sodium carboxymethylcellulose (etherification degree: 1.0, aqueous solution viscosity at solids concentration of 1.0%: 500 mPa s) was added and mixed. The added sodium salt of carboxymethylcellulose dissolved in the mixed liquid. Next, to this mixed solution, 5.0 parts (solid content equivalent) of the binder composition of Example 1 and 0.2 parts of the aliphatic polyether type nonionic surfactant as wetting agent are added, and then water is added so that the solid content concentration becomes 40%, thereby obtaining the slurry composition for porous membrane layer.

[0147] [Fabrication of separator with porous membrane layer] A single-layer polypropylene separator substrate (width 250 mm, length 1000 m, thickness 12 μm) manufactured by a wet method was prepared. Then, the redispersed slurry composition for the porous membrane layer was applied to both surfaces of the separator substrate using a gravure coater (application speed: 20 m / min) so that the thickness after drying was 2.5 μm. Next, the separator substrate coated with the slurry composition for the porous membrane layer was dried in a drying oven at a temperature of 50 ° C. and wound up to produce a separator having a porous membrane layer on both sides of the separator substrate. This separator was cut into a 5 cm × 5 cm square and used to produce a secondary battery. The test piece for measuring the peel strength of the separator coating layer is a separator having a porous membrane layer on one side prepared in the same manner as above, except that the slurry composition for the porous membrane layer is applied to one surface of the separator substrate.

[0148] Example 28 A positive electrode slurry composition was prepared as follows using the binder composition prepared in Example 1. The negative electrode slurry composition was prepared in the same manner as the slurry composition for secondary batteries described in Example 1, but the SBR binder described in Examples 18 to 20 was used as the binder composition. A secondary battery was prepared in the same manner as in Example 1 except that these positive electrode slurry compositions and negative electrode slurry compositions were used. Various evaluations were then performed in the same manner as in Example 1.

[0149] [Preparation of Positive Electrode Slurry Composition] A powder was prepared by dry-mixing 100 parts of carbon-coated lithium iron phosphate and 10 parts of acetylene black (carbon conductive material) in a sealed container. To this powder, 100 parts of a 2% aqueous solution of sodium carboxymethylcellulose (etherification degree: 1.0, aqueous solution viscosity at 1.0% solids concentration: 500 mPa·s) was added as a water-soluble thickener, and the mixture was thoroughly mixed with a planetary mixer to prepare a premix paste. The resulting premix paste was dispersed in a bead mill using 1 mm diameter zirconia beads, and then 3 parts (solids equivalent) of the binder composition of Example 1 was added and thoroughly mixed to obtain a positive electrode slurry composition.

[0150] [Preparation of positive electrode (using positive electrode slurry composition)] The obtained positive electrode slurry composition was applied to a pure aluminum foil current collector using a film applicator to a thickness of 120 μm, and then thoroughly dried in a hot air dryer for 10 minutes in a 50°C atmosphere. The dry weight was 10 mg / cm. 2 It was.

[0151] <Comparative Example 11> The amount of the shell portion charged was as shown in Table 1, and a binder composition, a slurry composition for a secondary battery, a negative electrode, and a secondary battery were prepared in the same manner as in Example 1, except that the amount of t-dodecyl mercaptan used as a chain transfer agent during preparation of the shell portion (0.1 parts in Example 1) was changed to 3.5 parts. Various evaluations were then performed in the same manner as in Example 1. Various evaluations were then performed in the same manner as in Example 1.

[0152] <Comparative Example 12> The amount of shell portion charged was as shown in Table 1, and the amount of t-dodecyl mercaptan used as a chain transfer agent during preparation of the shell portion (0.1 parts in Example 1) was changed to 0 parts, and the amount of ammonium persulfate used as a polymerization initiator during polymerization of the shell portion (0.5 parts in Example 1) was changed to 0.1 parts. Except for this, a binder composition, a slurry composition for a secondary battery, a negative electrode, and a secondary battery were prepared in the same manner as in Example 1. Various evaluations were then performed in the same manner as in Example 1.

[0153] [Table 1] TIFF0007750229000002.tif255153TIFF0007750229000003.tif255155TIFF0007750229000004.tif255156TIFF00077502290 00005.tif255148TIFF0007750229000006.tif255156TIFF0007750229000007.tif255149TIFF0007750229000008.tif255154 [Industrial Applicability]

[0154] According to the present invention, a binder composition for a secondary battery is provided together with a slurry composition for a secondary battery, a functional layer for a secondary battery, a separator for a secondary battery, an electrode for a secondary battery, and a secondary battery. According to the present invention, in the production of a separator or electrode for a secondary battery, excellent adhesion is obtained between a layer obtained using the binder composition and a substrate, resulting in a secondary battery with good cycle characteristics. Even after repeated charge-discharge cycles, deterioration of adhesion is suppressed, and in lithium-ion secondary batteries, precipitation of lithium metal is significantly suppressed.

Claims

1. A binder composition for a secondary battery comprising a polymer and a solvent, the polymer is a core-shell particle having a core portion made of polymer (a1) and a shell portion made of polymer (a2), the polymer (a1) is a copolymer composed of unsaturated carboxylic acid ester monomer units, unsaturated carboxylic acid monomer units, and aliphatic conjugated diene monomer units, an aliphatic conjugated diene polymer, or an aromatic vinyl-aliphatic conjugated diene copolymer, the polymer (a2) contains from 50.0% by mass to 89.9% by mass of monofunctional unsaturated carboxylic acid ester monomer units, from 10.0% by mass to 49.9% by mass of one or more monomer units selected from monofunctional unsaturated carboxylic acid monomer units and monofunctional unsaturated sulfonic acid monomer units, from 0.1% by mass to 10.0% by mass of monomer units having two or more vinyl functional groups, and from 5.0% by mass to 5.0% by mass of aromatic vinyl monomer units, the amount of one or more monomer units selected from the group consisting of monofunctional unsaturated carboxylic acid monomer units and monofunctional unsaturated sulfonic acid monomer units is 0.1 moles or more and 15,000 moles or less per mole of the monomer units having two or more vinyl functional groups, The solvent is water Binder composition for secondary batteries.

2. The binder composition for a secondary battery according to claim 1, wherein the polymer (a1) has a tetrahydrofuran insoluble content of 40% or more and 97% or less.

3. 3. The binder composition for a secondary battery according to claim 1, wherein a ratio of the mass of the polymer (a1) to the mass of the polymer (a2) is 95 / 5 or less and 5 / 95 or more.

4. 4. The binder composition for a secondary battery according to claim 1, wherein the core-shell particles have a radius of gyration of 10 nm or more and 1000 nm or less.

5. A slurry composition for a secondary battery, comprising the binder composition for a secondary battery according to any one of claims 1 to 4.

6. The slurry composition for a secondary battery according to claim 5 , further comprising non-conductive particles.

7. A functional layer for a secondary battery formed using the slurry composition for a secondary battery according to claim 5 or 6.

8. A secondary battery separator comprising a separator substrate and a functional layer for a secondary battery formed using the slurry composition for a secondary battery according to claim 6 .

9. A secondary battery comprising the functional layer for secondary batteries according to claim 7.

10. A secondary battery comprising the separator for a secondary battery according to claim 8.

11. A slurry composition for a secondary battery, comprising a binder composition for a secondary battery, the binder composition comprising a polymer and a solvent, the polymer is a core-shell particle having a core portion made of polymer (a1) and a shell portion made of polymer (a2), the polymer (a2) contains from 50.0% by mass to 89.9% by mass of monofunctional unsaturated carboxylic acid ester monomer units, from 10.0% by mass to 49.9% by mass of one or more monomer units selected from monofunctional unsaturated carboxylic acid monomer units and monofunctional unsaturated sulfonic acid monomer units, from 0.1% by mass to 10.0% by mass of monomer units having two or more vinyl functional groups, and from 5.0% by mass to 5.0% by mass of aromatic vinyl monomer units, the amount of one or more monomer units selected from the group consisting of monofunctional unsaturated carboxylic acid monomer units and monofunctional unsaturated sulfonic acid monomer units is 0.1 moles or more and 15,000 moles or less per mole of the monomer units having two or more vinyl functional groups, the solvent is water, The slurry composition for a secondary battery further comprises electrode active material particles.

12. An electrode for a secondary battery comprising a functional layer for a secondary battery formed on a current collector using the slurry composition for a secondary battery according to claim 11.

13. A secondary battery comprising the secondary battery electrode described in claim 12.

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