Binder and binder composition for lithium ion secondary battery positive electrode, lithium ion secondary battery positive electrode, and lithium ion secondary battery

Abstract

This lithium ion secondary battery positive electrode comprises a current collector and an active material layer containing an active material and a nonaqueous binder, wherein the nonaqueous binder contains a conjugated diene copolymer that satisfies the following requirements (a) to (d).　Requirement (a): the content of aromatic vinyl monomer unit is at least 6 mass% and not more than 80 mass% with respect to the total amount of the conjugated diene copolymer.　Requirement (b): the amount of 1,2-vinyl bond with respect to the conjugated diene monomer unit in the conjugated diene copolymer is at least 10 mol% and not more than 60 mol%.　Requirement (c): the ratio between 1,4-cis bonds and 1,4-trans bonds in the conjugated diene copolymer is 30 : 70 to 50 : 50.　Requirement (d): the weight-average molecular weight is at least 100,000 and not more than 2,000,000.

Description

Binder and binder composition for positive electrode of lithium ion secondary battery, positive electrode for lithium ion secondary battery, and lithium ion secondary battery

[0001] The present invention relates to a binder and binder composition for a positive electrode of a lithium ion secondary battery, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

[0002] Conventionally, a method for producing electrodes for use in electrochemical devices such as lithium-ion secondary batteries involves applying a liquid composition containing an electrode active material and a binder, thickener, etc. to the surface of a current collector and drying the composition to form an electrode layer on the current collector. Polyvinylidene fluoride (PVdF) is known as a binder that can form an electrode layer that has high adhesion to the metal constituting the positive electrode current collector and is also highly flexible. The binder functions to improve adhesion between the electrode layer containing the active material and the current collector or separator.

[0003] However, in recent years, alternative materials for PVdF have been investigated in view of the harmful effects of organic fluorine compounds (PFAS) on the environment and the human body, rising prices, and supply stability.

[0004] For example, Patent Document 1 discloses a positive electrode for a lithium secondary battery that uses styrene-butadiene rubber as a binder.

[0005] International Publication No. 2023 / 008953

[0006] As candidates for alternative materials for positive electrode binders, styrene-butadiene copolymers obtained by emulsion polymerization, nitrile rubber, polyimide, and polyacrylic acid have been investigated. For batteries with relatively low voltages, such as lithium iron phosphate batteries, emulsion-polymerized styrene-butadiene copolymers such as those described in Patent Document 1 are also effective materials. However, there is room for further improvement in the adhesion to the current collector and crack resistance required for positive electrodes of lithium-ion secondary batteries using a positive electrode binder, as well as the cycle characteristics of secondary batteries using such positive electrodes.

[0007] Therefore, the present inventors have conducted extensive research to solve the problems of the conventional techniques described above, and have come up with an object to provide a positive electrode for a secondary battery that has excellent adhesion to a current collector and suppresses cracking, and a secondary battery that has excellent cycle characteristics, by using a conjugated diene-based copolymer that meets specific requirements as a positive electrode binder.

[0008] <1> A positive electrode for a lithium ion secondary battery, comprising: a current collector; and an active material layer containing an active material and a non-aqueous binder, wherein the non-aqueous binder contains a conjugated diene copolymer satisfying the following requirements (a) to (d): Requirement (a): The content of aromatic vinyl monomer units is 6% by mass or more and 80% by mass or less relative to the total amount of the conjugated diene copolymer; Requirement (b): The amount of 1,2-vinyl bonds relative to the conjugated diene monomer units in the conjugated diene copolymer is 10 mol% or more and 60 mol% or less; Requirement (c): The ratio of 1,4-cis bonds to 1,4-trans bonds in the conjugated diene copolymer is 30:70 to 50:50; and Requirement (d): The weight-average molecular weight is 100,000 or more and 2,000,000 or less. <2> The positive electrode for a lithium ion secondary battery according to <1>, wherein the conjugated diene copolymer has a Mooney viscosity of 30 to 200 measured at 100°C. <3> The positive electrode for a lithium ion secondary battery according to <1> or <2>, wherein the content of the aromatic vinyl monomer units is 30 to 70% by mass, relative to the total amount of the conjugated diene copolymer. <4> The positive electrode for a lithium ion secondary battery according to any one of <1> to <3>, wherein the content of aromatic vinyl monomer blocks is 5 to 40% by mass, relative to the total amount of the conjugated diene copolymer. <5> The positive electrode for a lithium ion secondary battery according to any one of <1> to <4>, wherein a blocking rate of aromatic vinyl monomer blocks in the conjugated diene copolymer is 15 to 85%. <6> The positive electrode for a lithium ion secondary battery according to any one of <1> to <5>, wherein the active material includes lithium iron phosphate. <7> The positive electrode for a lithium ion secondary battery according to any one of <1> to <6>, wherein the content of each of zinc, aluminum, copper, and iron is 50 ppm or less relative to the total amount of the conjugated diene-based copolymer. <8> The positive electrode for a lithium ion secondary battery according to any one of <1> to <7>, wherein the total content of zinc, aluminum, copper, and iron is 50 ppm or less relative to the total amount of the conjugated diene-based copolymer.<9> The positive electrode for a lithium ion secondary battery according to any one of <1> to <8>, wherein the conjugated diene copolymer has a total hydrogenation rate of 10% to 99%. <10> The positive electrode for a lithium ion secondary battery according to any one of <1> to <9>, wherein the conjugated diene copolymer has a 1,2-hydrogenation rate of 80% or more. <11> The positive electrode for a lithium ion secondary battery according to any one of <1> to <10>, wherein the non-aqueous binder further contains polyvinylidene fluoride. <12> The positive electrode for a lithium ion secondary battery according to any one of <1> to <11>, wherein the active material layer further contains a conductive additive, and the conductive additive contains carbon black. <13> The positive electrode for a lithium ion secondary battery according to any one of <1> to <12>, wherein the current collector contains aluminum. <14> A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to any one of <1> to <13> and an electrolyte. <15> The lithium ion secondary battery according to <14>, having an operating voltage of 4.0 V or less.

[0009] According to the present invention, by using a conjugated diene copolymer having specific requirements as a positive electrode binder, it is possible to provide a positive electrode for a secondary battery that has excellent adhesion to a current collector and is suppressed from cracking, and a secondary battery that has excellent battery cycle characteristics.

[0010] Hereinafter, a mode for carrying out the present invention (hereinafter referred to as "the present embodiment") will be described in detail. Note that the following present embodiment is an example for explaining the present invention, and the present invention is not limited to the mode shown below. The present invention can be carried out in various modifications within the scope of its gist.

[0011] <Positive Electrode for Lithium-Ion Secondary Battery> The positive electrode for a lithium-ion secondary battery of this embodiment includes a current collector and an active material layer containing an active material and a non-aqueous binder, wherein the non-aqueous binder contains a conjugated diene copolymer satisfying the following requirements (a) to (d): Requirement (a): The content of structural units derived from aromatic vinyl monomers is 6% by mass or more and 80% by mass or less with respect to the total amount of the conjugated diene copolymer; Requirement (b): The amount of 1,2-vinyl bonds in the conjugated diene copolymer is 10 mol% or more and 60 mol% or less; Requirement (c): The ratio of the amount of 1,4-cis bonds to the amount of 1,4-trans bonds in the conjugated diene copolymer is 30:70 to 50:50; Requirement (d): The weight-average molecular weight is 100,000 or more and 2,000,000 or less.

[0012] [Current Collector] The current collector used in the positive electrode for a lithium-ion secondary battery of this embodiment can be one that has high conductivity without inducing chemical changes in the battery. Examples of current collector materials include stainless steel, aluminum, nickel, titanium, and calcined carbon. In the case of aluminum or stainless steel, surface treatment with carbon, nickel, titanium, silver, or the like can also be used. The current collector may also be in the form of a film, sheet, foil, net, porous material, foam, nonwoven fabric, or the like. Furthermore, the adhesive strength of the active material can be enhanced by providing fine irregularities on the surface to increase the surface area. Furthermore, the average thickness of the current collector can be appropriately set to 3 to 500 μm, taking into account the conductivity and total thickness of the positive electrode to be manufactured. Aluminum is preferred as the current collector material from the standpoints of price, weight, and supply stability.

[0013] [Active Material Layer] The active material layer of this embodiment contains an active material and a non-aqueous binder. The active material layer may further contain a conductive additive.

[0014] The active material layer of this embodiment is formed on one or both surfaces of the current collector, and is prepared by drying a positive electrode binder composition containing an active material and a non-aqueous binder.

[0015] [Active Material] As the active material used in the positive electrode for the lithium ion secondary battery of this embodiment, any active material commonly used for a positive electrode can be used. The active material for the positive electrode is not limited to the following, but inorganic particles are preferred. For example, LiMnO ２ , LiMn ２ O ４ , and Li ２ Mn ２ O ４ Lithium manganese oxides, such as LiCoO ２ Lithium cobalt oxides, such as LiNiO ２ Lithium nickel oxides such as NMC532 and NMC811; lithium iron oxides such as LiFeO; LiFePO ４ or lithium iron phosphate, a lithium composite oxide combining these, can be used.

[0016] The lower limit of the content of the active material is not particularly limited, but from the viewpoint of battery capacity, it is preferably 80 parts by mass or more, more preferably 85 parts by mass or more, even more preferably 88 parts by mass or more, and particularly preferably 90 parts by mass or more, relative to 100 parts by mass of the active material layer. On the other hand, from the viewpoint of optimizing the mixing ratio with additives such as a conductive additive described below, the upper limit of the content of the active material is preferably 99 parts by mass or less, more preferably 98 parts by mass or less, and even more preferably 97 parts by mass or less.

[0017] [Conductive Aid] In the positive electrode of the lithium secondary battery of this embodiment, the active material layer may further contain a conductive aid. The conductive aid can be used to improve the performance of the positive electrode, such as electrical conductivity, and can be one or more selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, and carbon fiber. For example, the conductive aid may include carbon black.

[0018] From the viewpoint of improving electrical contact between the positive electrode active materials, the content of the conductive additive may be 1 to 10 parts by mass, preferably 1 to 8 parts by mass, and more preferably 1 to 6 parts by mass, per 100 parts by mass of the active material layer.

[0019] [Non-aqueous Binder] The positive electrode of the lithium secondary battery of this embodiment contains a non-aqueous binder to bind the active material and the current collector and hold the active material on the current collector. The non-aqueous binder contains a conjugated diene copolymer satisfying the following requirements (a) to (d): Requirement (a): The content of structural units derived from aromatic vinyl monomers is 6% by mass or more and 80% by mass or less, based on the total amount of the conjugated diene copolymer. Requirement (b): The amount of 1,2-vinyl bonds in the conjugated diene copolymer is 10 mol% or more and 60 mol% or less. Requirement (c): The ratio of the amount of 1,4-cis bonds to the amount of 1,4-trans bonds in the conjugated diene copolymer is 30:70 to 50:50. Requirement (d): The weight-average molecular weight is 100,000 or more and 2,000,000 or less.

[0020] Furthermore, a component generally used as a non-aqueous binder for a positive electrode may be used in combination with the conjugated diene copolymer, such as polyvinylidene fluoride, polyacrylonitrile, polyimideamide, or polymethyl methacrylate.

[0021] The lower limit of the content of the non-aqueous binder is preferably 1 part by mass or more, more preferably 1.5 parts by mass or more, and even more preferably 2 parts by mass or more, per 100 parts by mass of the active material layer, from the viewpoint of adhesion between the active material and the current collector, whereas the upper limit of the content of the non-aqueous binder is preferably 12 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 9 parts by mass or less, from the viewpoint of improving electronic conductivity and ionic conductivity.

[0022] [Conjugated Diene Copolymer] In the lithium secondary battery positive electrode of this embodiment, the conjugated diene copolymer contained in the non-aqueous binder satisfies the above requirements (a) to (d). The conjugated diene copolymer includes a polymer having a structural unit (hereinafter also referred to as a "conjugated diene monomer unit") having a chemical structure derived from a conjugated diene compound (monomer), a polymer having a structural unit (hereinafter also referred to as a "aromatic vinyl monomer unit") having a chemical structure derived from an aromatic vinyl compound (monomer), and a copolymer containing a conjugated diene monomer unit and an aromatic vinyl monomer unit. It may further contain a hydrogenated product thereof.

[0023] Conjugated diene compounds include, but are not limited to, 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, 3-methyl-1,3-pentadiene, 1,3-hexadiene, and 1,3-heptadiene. Among these, from the viewpoint of ease of industrial availability, 1,3-butadiene and isoprene are preferred, and 1,3-butadiene is particularly preferred. These compounds may be used alone or in combination of two or more.

[0024] Examples of aromatic vinyl compounds include, but are not limited to, styrene, p-methylstyrene, α-methylstyrene, vinylethylbenzene, vinylxylene, vinylnaphthalene, and diphenylethylene. Among these, styrene is preferred from the viewpoint of industrial availability. These compounds may be used alone or in combination of two or more.

[0025] The conjugated diene copolymer of the present embodiment is characterized in that the so-called microstructure (content of aromatic vinyl monomer units, amount of 1,2-vinyl bonds, etc.) of the copolymer of a conjugated diene compound and an aromatic vinyl compound, as well as the molecular weight, viscosity, and hydrogenation rate are controlled.

[0026] (Aromatic vinyl monomer unit content) The lower limit of the aromatic vinyl monomer unit content of the conjugated diene copolymer of this embodiment is 6% by mass or more, preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and particularly preferably 35% by mass or more, from the viewpoint of the binding strength between the active material and the current collector. Furthermore, when dissolving in an organic solvent and applying, if the organic solvent used is highly polar, for example, when N-methylpyrrolidone or tetralin is used, from the viewpoint of solubility, the content of the aromatic vinyl monomer unit is preferably 24% by mass or more, more preferably 26% by mass or more, even more preferably 28% by mass or more, and particularly preferably 30% by mass or more. On the other hand, the upper limit of the aromatic vinyl monomer unit content of the conjugated diene copolymer of this embodiment is 80% by mass or less, preferably 70% by mass or less, more preferably 66% by mass or less, even more preferably 60% by mass or less, and particularly preferably 55% by mass or less, from the viewpoint of bale crushability and binder flexibility. The content of the aromatic vinyl monomer unit can be measured by 1H-NMR. The content of the aromatic vinyl monomer unit can be controlled within the above-mentioned range by adjusting the amount of the aromatic vinyl compound added during polymerization.

[0027] (1,2-Vinyl Bond Content) The 1,2-vinyl bond content in the conjugated diene copolymer of this embodiment refers to the molar ratio of 1,2-vinyl bonds based on the content of conjugated diene monomer units. From the viewpoints of polymerization stability and flexibility when used as a binder, it is 10 mol% or more, preferably 11 mol% or more, more preferably 12 mol% or more, and even more preferably 13 mol% or more. On the other hand, from the viewpoint of durability when used as a binder, it is 60 mol% or less, preferably 55 mol% or less, more preferably 50 mol% or less, and even more preferably 45 mol% or less. The 1,2-vinyl bond content can be measured by 1H-NMR. The 1,2-vinyl bond content can be controlled within the above numerical range by adjusting the reaction initiation temperature, reaction termination temperature, and the type and amount of polar substance added during polymerization.

[0028] (Ratio of 1,4 cis bonds to 1,4 trans bonds) The ratio of 1,4 cis bonds to 1,4 trans bonds in the conjugated diene copolymer of this embodiment is the molar ratio of the amount of 1,4 cis bonds to the amount of 1,4 trans bonds when the total content of 1,4 cis bonds and 1,4 trans bonds in the conjugated diene monomer units is taken as 100% by mass. From the viewpoint of the balance of the adhesive strength between the active material and the binder and the solubility in polar solvents, the ratio of 1,4 cis bonds to 1,4 trans bonds is preferably 30:70 to 50:50, more preferably 32:68 to 48:52, and even more preferably 35:65 to 46:54. The content and ratio of 1,4 cis bonds to 1,4 trans bonds can be measured by C-NMR. In order to control the ratio of 1,4 cis bonds to 1,4 trans bonds within the above range, for example, a method of obtaining a conjugated diene copolymer by living anionic polymerization using a lithium-based polymerization initiator can be mentioned. On the other hand, conjugated diene polymers obtained by coordination polymerization tend to have a high content ratio of 1,4 cis bonds, while conjugated diene copolymers obtained by emulsion polymerization tend to have a low content ratio of 1,4 cis bonds.

[0029] When the conjugated diene copolymer of the present embodiment is a copolymer composed of butadiene and styrene, the 1,2-vinyl bond amount and the styrene content can be measured by the method described in ISO21561-2005, and specifically, can be measured by the method described in the examples.

[0030] The conjugated diene copolymer of this embodiment has an aromatic vinyl monomer block content, measured according to the osmium tetroxide decomposition method described in I. M. Kolthoff, et al., J. Polym. Sci. 1, 429 (1946), of preferably 5% by mass or more, more preferably 8% by mass or more, even more preferably 11% by mass or more, and particularly preferably 20% by mass or more, from the viewpoint of the adhesive strength between the active material and the binder and the durability of the binder. On the other hand, from the viewpoint of bale crushability, the content is preferably 40% by mass or less, more preferably 37% by mass or less, and particularly preferably 35% by mass or less. The aromatic vinyl monomer block content can be controlled by the timing of addition of the aromatic vinyl compound during polymerization, the amount and type of polar substance added, etc.

[0031] The blocking ratio of the aromatic vinyl monomer block in the conjugated diene copolymer of this embodiment is not particularly limited, but is preferably 15% to 85%, more preferably 20% to 80%, and even more preferably 25% to 75%. Having the blocking ratio of the aromatic vinyl monomer block within the above range tends to result in excellent adhesion suppression and molding processability. The blocking ratio can be calculated by dividing the content of the aromatic vinyl monomer block by the aromatic vinyl monomer content in the conjugated diene polymer, which can be measured by 1H-NMR as described in the Examples. Here, the aromatic vinyl monomer block refers to the content of the aromatic vinyl monomer block measured by the method described above. Here, the blocking ratio of the aromatic vinyl monomer block refers to the ratio of the content of the vinyl aromatic monomer block in the conjugated diene copolymer to the content of all vinyl aromatic monomer units in the conjugated diene copolymer. As described in the Examples, the blocking ratio can be calculated by dividing the content of the aromatic vinyl monomer block by the aromatic vinyl monomer content in the conjugated diene copolymer, which can be measured by 1H-NMR as described in the Examples. 1 It can be measured by H-NMR.

[0032] In the conjugated diene copolymer of this embodiment, the aliphatic double bonds based on the conjugated diene monomer units may be hydrogenated. In the hydrogenated copolymer composition of this embodiment, the conjugated diene monomer units are incorporated into the copolymer in a 1,2-bond, 3,4-bond, or 1,4-bond bonding manner. The total hydrogenation rate of the conjugated diene copolymer is not particularly limited, but from the viewpoint of crosslinkability, it is preferably 10% to 99%, more preferably 15 to 95%, and even more preferably 20 to 90%. Here, the total hydrogenation rate refers to the proportion of hydrogen-bonded aliphatic double bonds (1,2-bonds, 3,4-bonds, and 1,4-bonds) based on the conjugated diene monomer units in the conjugated diene copolymer. The total hydrogenation rate is 1 It can be calculated from the spectral reduction rate of the unsaturated bond portion of the spectrum obtained by measuring H-NMR. Specifically, it can be measured by the method described in the Examples below.

[0033] The 1,2 hydrogenation rate of the conjugated diene copolymer of the present embodiment is not particularly limited, but is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. The upper limit of the 1,2 hydrogenation rate is not particularly limited, and may be 100% or less. The 1,2 hydrogenation rate refers to the ratio of hydrogen-bonded 1,2-bonds based on the conjugated diene monomer units in the conjugated diene copolymer. The 1,2 hydrogenation rate is 1 It can be calculated from the spectral reduction rate of the unsaturated bond portion of the spectrum obtained by measuring H-NMR.

[0034] (Weight-average molecular weight) From the viewpoint of adhesive strength and active material retention performance, the weight-average molecular weight (Mw) of the conjugated diene copolymer of this embodiment is preferably 100,000 or more, more preferably 120,000 or more, even more preferably 140,000 or more, and particularly preferably 200,000 or more. On the other hand, from the viewpoint of solubility and coatability, it is 2,000,000 or less, more preferably 1,800,000 or less, even more preferably 1,600,000 or less, and particularly preferably 1,200,000 or less. The weight-average molecular weight (Mw) of the conjugated diene copolymer can be measured using gel permeation chromatography (GPC).

[0035] The molecular weight distribution (Mw / Mn) based on the weight average molecular weight (Mw) and number average molecular weight (Mn) of the conjugated diene copolymer of this embodiment is not particularly limited, but from the viewpoint of bale crushability, the lower limit of the molecular weight distribution (Mw / Mn) is preferably 1.02 or more, more preferably 1.04 or more, and even more preferably 1.06 or more. On the other hand, from the viewpoint of solution viscosity and coatability, the upper limit of the molecular weight distribution (Mw / Mn) is preferably 3.0 or less, more preferably 2.5 or less, even more preferably 2.0 or less, and particularly preferably 1.8 or less. The number average molecular weight (Mn) of the conjugated diene copolymer can be measured using gel permeation chromatography (GPC).

[0036] (Mooney Viscosity) The Mooney viscosity of the conjugated diene copolymer of this embodiment measured at 100°C is not particularly limited, but from the viewpoint of bale handleability, the lower limit of the Mooney viscosity measured at 100°C is preferably 30 or more, more preferably 35 or more, and even more preferably 40 or more. On the other hand, from the viewpoint of bale crushability and solubility in polar solvents, the upper limit of the Mooney viscosity measured at 100°C is preferably 200 or less, more preferably 180 or less, and particularly preferably 160 or less. Specifically, the Mooney viscosity can be measured by the method described in the Examples below.

[0037] (Coupling) The conjugated diene copolymer of the present embodiment may be a conjugated diene copolymer obtained by subjecting an active terminal of the conjugated diene copolymer to a coupling reaction using a bifunctional or higher reactive compound (hereinafter also referred to as a "coupling agent"). In the coupling reaction step, one active terminal of the conjugated diene copolymer is subjected to a coupling reaction using the coupling agent to obtain a conjugated diene copolymer.

[0038] The coupling agent is not particularly limited, and examples thereof include coupling agents having one or more functional groups such as an epoxy group, a carbonyl group, a carboxylic acid ester group, a carboxylic acid amide group, an acid anhydride group, a phosphate ester group, a phosphite ester group, an epithio group, a thiocarbonyl group, a thiocarboxylic acid ester group, a dithiocarboxylic acid ester group, a thiocarboxylic acid amide group, an imino group, an ethyleneimino group, a halogen group, an alkoxysilyl group, an isocyanate group, a thioisocyanate group, a conjugated diene group, and an arylvinyl group. Among the coupling agents, nitrogen-containing coupling agents can also be used as modifiers, which will be described later.

[0039] Examples of coupling agents include, but are not limited to, halogenated silane compounds such as silicon tetrachloride, silicon tetrabromide, silicon tetraiodide, monomethyltrichlorosilicon, monoethyltrichlorosilicon, monobutyltrichlorosilicon, monohexyltrichlorosilicon, monomethyltribromosilicon, and bistrichlorosilylethane; and halogenated silane compounds such as monochlorotrimethoxysilane, monobromotrimethoxysilane, dichlorodimethoxysilane, dibromodimethoxysilane, trichloromethoxysilane, and tribromomethoxysilane.

[0040] Further, examples include, but are not limited to, alkoxysilane compounds such as tetramethoxysilane, tetraethoxysilane, tetrabutoxysilane, and alkyltriphenoxysilane; and compounds having an imino group and an alkoxysilyl group such as tristrimethoxysilylpropylamine, triethoxysilylpropylamine, N-(1,3-dimethylbutylidene)-3-(triethoxysilyl)-1-propanamine, N-(1,3-dimethylbutylidene)-3-(tributoxysilyl)-1-propanamine, N-(1-methylpropylidene)-3-(triethoxysilyl)-1-propanamine, N-ethylidene-3-(triethoxysilyl)-1-propanamine, and N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole.

[0041] Furthermore, examples thereof include, but are not limited to, 2-[3-(trimethoxysilyl)propyl]-1,3-dimethylimidazolidine, 2-[3-(trimethoxysilyl)propyl]-1,3-(bistrimethylsilyl)imidazolidine, 2-(diethoxydiethylsilyl)-1,3-diethylimidazolidine, 2-(triethoxysilyl)-1,4-diethylpiperazine, 2-(dimethoxymethylsilyl)-1,4-dimethylpiperazine, 5-(triethoxysilyl)-1,3-dipropylhexahydropyrimidine, 5-(diethoxyethylsilyl)-1,3 -diethylhexahydropyrimidine, {2-[3-(2-dimethylaminoethyl)-2-(ethyldimethoxysilyl)-imidazolidin-1-yl]-ethyl}-dimethylamine, 5-(trimethoxysilyl)-1,3-bis-(2-methoxyethyl)-hexahydropyrimidine, 5-(ethyldimethoxysilyl)-1,3-bis-(2-trimethylsilylethyl)-hexahydropyrimidine-1,3-dimethylimidazolidine, 2-(3-diethoxyethylsilyl-propyl)-1,3-diethylimidazolidine, 2-(3-triethoxysilyl-propyl)-1,3-diethylimidazolidine, propyl)-1,4-diethylpiperazine, 2-(3-dimethoxymethylsilyl-propyl)-1,4-dimethylpiperazine, 5-(3-triethoxysilyl-propyl)-1,3-dipropylhexahydropyrimidine, 5-(3-diethoxyethylsilyl-propyl)-1,3-diethylhexahydropyrimidine, {2-[3-(2-dimethylaminoethyl)-2-(3-ethyldimethoxysilyl-propyl)-imidazolidin-1-yl]-ethyl}-dimethylamine, 5-(3-trimethoxysilyl-propyl)-1,3-bis-(2-methoxyethyl 2-(triethoxysilyl)-1,4-bis(trimethylsilyl)piperazine, 2-(dimethoxymethylsilyl)-1,4-bis(trimethylsilyl)piperazine, 5-(3-ethyldimethoxysilyl-propyl)-1,3-bis-(2-trimethylsilylethyl)-hexahydropyrimidine, 2-[3-(trimethoxysilyl)propyl]-1,3-bis(trimethylsilyl)imidazolidine, 2-(diethoxyethylsilyl)-1,3-bis(triethylsilyl)imidazolidine, 2-(triethoxysilyl)-1,4-bis(trimethylsilyl)piperazine, 2-(dimethoxymethylsilyl)-1,4-bis(trimethylsilyl)piperazine, 5-(triethoxysilyl)-1,3-bis(tripropylsilyl)hexahydropyrimidine, etc.

[0042] Furthermore, examples of the silanes include, but are not limited to, [3-(1-hexamethyleneimino)propyl]triethoxysilane, [3-(1-hexamethyleneimino)propyl]trimethoxysilane, [2-(1-hexamethyleneimino)ethyl]triethoxysilane, [2-(1-hexamethyleneimino)ethyl]trimethoxysilane, [3-(1-pyrrolidinyl)propyl]triethoxysilane, [3-(1-pyrrolidinyl)propyl]trimethoxysilane, [3-(1-heptamethyleneimino)propyl]triethoxysilane, [3-(1-do [3-(1-hexamethyleneimino)propyl]triethoxysilane, [3-(1-hexamethyleneimino)propyl]diethoxymethylsilane, [3-(1-hexamethyleneimino)propyl]diethoxyethylsilane, N-[3-(triethoxysilyl)propyl]-N,N'-diethyl-N'-trimethylsilyl-ethane-1,2-diamine, N-[2-(trimethoxysilanyl)ethyl]-N,N',N'-trimethylethane-1,2-diamine, N,N-bis(trimethylsilyl)aminopropylmethyldiethoxysilane, and the like.

[0043] Furthermore, examples of the diisocyanate include, but are not limited to, tetraglycidyl meta-xylene diamine, tetraglycidyl aminodiphenylmethane, tetraglycidyl-p-phenylenediamine, diglycidyl aminomethylcyclohexane, tetraglycidyl-1,3-bisaminomethylcyclohexane, etc. Furthermore, examples of the diisocyanate include, but are not limited to, isocyanate compounds such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, diphenylmethane diisocyanate, diphenylethane diisocyanate, 1,3,5-benzene triisocyanate, etc.

[0044] In addition, examples of the silane derivatives include, but are not limited to, 3-(4-methylpiperazin-1-yl)propyltriethoxysilane, 1-[3-(diethoxyethylsilyl)propyl]-4-methylpiperazine, 1-[3-(trimethoxysilyl)propyl]-3-methylimidazolidine, 1-[3-(diethoxysilyl)propyl]-3-ethylimidazolidine, 1-[3-(triethoxysilyl)propyl]-3-methylhexahydropyrimidine, 1-[3-(di 3-[3-(trimethoxysilyl)propyl]-1-methyl-1,2,3,4-tetrahydropyrimidine, 3-[3-(dimethoxymethylsilyl)propyl]-1-ethyl-1,2,3,4-tetrahydropyrimidine, 1-(2-ethoxyethyl)-3-[3-(trimethoxysilyl)propyl]imidazolidine, (2-{3-[3-(trimethylsilyl)propyl]tetra hydropyrimidin-yl}ethyl)dimethylamine, 1-[3-(triethoxysilyl)propyl]-4-(trimethylsilyl)piperazine, 1-[3-(dimethoxymethylsilyl)propyl]-4-(trimethylsilyl)piperazine, 1-[3-(tributoxysilyl)propyl]-4-(trimethylsilyl)piperazine, 1-[3-(diethoxyethylsilyl)propyl]-3-(triethylsilyl)imidazolidine, 2-(trimethoxysilanilide

[0033] Examples of such compounds include 1-[3-(triethoxysilyl)propyl]-1,3-dimethylimidazolidine, 1-[3-(triethoxysilyl)propyl]-3-(trimethylsilyl)imidazolidine, 1-[3-(dimethoxymethylsilyl)propyl]-3-(trimethylsilyl)hexahydropyrimidine, 1-[3-(triethoxysilyl)propyl]-3-(trimethylsilyl)hexahydropyrimidine, and 1-[4-(triethoxysilyl)propyl]-4-(trimethylsilyl)piperazine.

[0045] (Modification) The conjugated diene copolymer of this embodiment may be modified. The term "modification" refers to modifying a conjugated diene copolymer with a nitrogen-containing compound. The modification method is not particularly limited, but examples include a method using a polymerization initiator containing a nitrogen-containing compound, a method using a nitrogen-containing compound as a polymerization monomer, a method using the aforementioned nitrogen atom-containing coupling agent, a method reacting a non-coupling nitrogen-containing compound with the reaction terminal, and a method of modifying the double bond of a conjugated diene copolymer after polymerization by reacting a nitrogen-containing compound with the double bond.

[0046] Examples of the polymerization initiator containing a nitrogen-containing compound include, but are not limited to, reaction products of nitrogen-containing compounds such as dimethylamine, diethylamine, dibutylamine, dipropylamine, diheptylamine, dihexylamine, dioctylamine, di(2-ethylhexyl)amine, didecylamine, ethylpropylamine, ethylbutylamine, ethylbenzylamine, methylphenethylamine, piperidine, hexamethyleneimine, azacyclooctane, 1,3,3-trimethyl-6-azabicyclo[3.2.1]octane, 1,2,3,6-tetrahydropyridine, and 3,5-dimethylpiperidine with organolithium compounds such as n-butyllithium, sec-butyllithium, t-butyllithium, n-propyllithium, and i-propyllithium. Furthermore, examples of non-coupling nitrogen-containing compounds include, but are not limited to, 1,3-diethyl-2-imidazolinone, 1,3-dimethyl-2-imidazolinone, 1,3-dipropyl-2-imidazolinone, 1-methyl-3-ethyl-2-imidazolinone, 1-methyl-3-propyl-2-imidazolinone, 1-methyl-3-butyl-2-imidazolinone, and 1,3-dihydro-1,3-dimethyl-2H-imidazol-2-one.

[0047] (Modification rate) In this specification, unless otherwise specified, the "modification rate" refers to the mass ratio of the polymer having a nitrogen atom-containing functional group to the total amount of the conjugated diene copolymer. For example, when a nitrogen atom-containing modifier is reacted with the terminal end of a polymer, the mass ratio of the polymer having a nitrogen atom-containing functional group due to the nitrogen atom-containing modifier to the total amount of the polymer is expressed as the modification rate. As mentioned above, nitrogen atom-containing coupling agents are also included in the nitrogen atom-containing modifier. On the other hand, when a polymer is branched using a nitrogen atom-containing branching agent, the resulting copolymer will have a nitrogen atom-containing functional group, so this branched polymer will also be counted as a polymer having a nitrogen atom-containing functional group when calculating the modification rate. In other words, in this specification, the polymer having a nitrogen atom-containing functional group refers to a polymer having a nitrogen atom-containing functional group due to a nitrogen atom-containing modifier and a branched polymer using a branching agent having a nitrogen atom-containing functional group, and the total mass ratio of these is the "modification rate". The conjugated diene copolymer of this embodiment may be modified from the viewpoint of dispersibility of the active material and adhesion between the binder and the active material. In the case of a modified conjugated diene copolymer, it is preferable that the modification rate measured by a column adsorption GPC method (hereinafter also simply referred to as "modification rate") is 60% or more and 99% or less.

[0048] The modification rate of the conjugated diene copolymer of this embodiment can be measured, for example, by chromatography, which can separate functional group-containing modified components from unmodified components. Examples of methods using chromatography include a method using a gel permeation chromatography column packed with a polar substance such as silica that adsorbs specific functional groups, and quantifying the unadsorbed components using an internal standard for comparison (column adsorption GPC method). More specifically, the modification rate can be determined by measuring the amount of adsorption to the silica column from the difference between a chromatogram measured on a polystyrene gel column and a chromatogram measured on a silica column for a sample solution containing a sample and a low-molecular-weight internal standard polystyrene. More specifically, the modification rate can be measured by the method described in the Examples. The modification rate of the conjugated diene copolymer of this embodiment can be controlled within the above-mentioned numerical range, for example, by adjusting the amount of the modifier added and the reaction method.

[0049] The conjugated diene copolymer of this embodiment may contain zinc, aluminum, copper, and / or iron. Examples of the zinc, aluminum, copper, and / or iron include those derived from a polymerization catalyst or a hydrogenation catalyst. The contents of each of the zinc, aluminum, copper, and iron in the conjugated diene copolymer are not particularly limited, but from the viewpoint of the cycle characteristics of the secondary battery, they are preferably 50 ppm or less, more preferably 40 ppm, and even more preferably 30 ppm or less, relative to the total amount of the conjugated diene copolymer. The lower limits of the contents of each of the zinc, aluminum, copper, and iron in the conjugated diene copolymer may be below the detection limit, and are preferably 0 ppm or more. These metals can be measured by the method described in the Examples below, and can be controlled to fall within the above-mentioned numerical ranges by adjusting the type and amount of the hydrogenation catalyst, the deashing, or the conditions of the solvent removal process described below.

[0050] The total content of zinc, aluminum, copper, and iron in the conjugated diene copolymer of this embodiment is not particularly limited, but from the viewpoint of the cycle characteristics of the secondary battery, it is preferably 50 ppm or less, more preferably 40 ppm, and even more preferably 30 ppm or less, relative to the total amount of the conjugated diene copolymer. The lower limit of each of the contents of zinc, aluminum, copper, and iron in the conjugated diene copolymer may be below the detection limit, and is preferably 0 ppm or more.

[0051] Examples of other metals contained in the conjugated diene copolymer of this embodiment include lithium and titanium. The respective contents of lithium and titanium in the conjugated diene copolymer are not particularly limited, but from the viewpoint of the cycle characteristics of the secondary battery, they are preferably 200 ppm or less, more preferably 150 ppm, and even more preferably 100 ppm or less, relative to the total amount of the conjugated diene copolymer. The lower limits of the respective contents of lithium and titanium in the conjugated diene copolymer may be below the detection limit, and are preferably 0 ppm or more.

[0052] [Method for Producing Conjugated Diene Copolymer] The conjugated diene copolymer of this embodiment is obtained by carrying out a polymerization step in which an aromatic vinyl compound and a conjugated diene compound are polymerized using a predetermined polymerization initiator. Preferably, a coupling reaction step and / or a modification reaction step may be carried out using the above-mentioned coupling agent or modifying agent, and a hydrogenation step may be carried out thereafter. A branching step may be carried out using a branching agent before the coupling reaction step or the modification step.

[0053] (Polymerization Step) At least an organic monolithium compound can be used as the polymerization initiator used in the polymerization step. Examples of organic monolithium compounds include, but are not limited to, low molecular weight compounds and solubilized oligomeric organic monolithium compounds. Furthermore, examples of organic monolithium compounds include compounds having a carbon-lithium bond, a nitrogen-lithium bond, and a tin-lithium bond in terms of the bonding mode between the organic group and the lithium. The amount of the organic monolithium compound used as the polymerization initiator is preferably determined based on the target structure of the conjugated diene copolymer and the molecular weight of the conjugated diene copolymer. The amount of monomer, such as a conjugated diene compound, used relative to the amount of polymerization initiator used is related to the degree of polymerization. Therefore, to increase the molecular weight, it is advisable to reduce the amount of polymerization initiator used, and to decrease the molecular weight, it is advisable to increase the amount of polymerization initiator used.

[0054] The organic monolithium compound may be an alkyllithium compound having a substituted amino group or a dialkylaminolithium compound, from the viewpoint of being used as a method for introducing nitrogen atoms into a conjugated diene copolymer. In this case, a conjugated diene copolymer having a nitrogen atom consisting of an amino group at the polymerization initiation terminal can be obtained.

[0055] The substituted amino group is an amino group that does not have an active hydrogen or has a structure in which the active hydrogen is protected. Examples of alkyllithium compounds having an amino group that does not have an active hydrogen include, but are not limited to, 3-dimethylaminopropyllithium, 3-diethylaminopropyllithium, 4-(methylpropylamino)butyllithium, and 4-hexamethyleneiminobutyllithium. Examples of alkyllithium compounds having an amino group with a structure in which the active hydrogen is protected include, but are not limited to, 3-bistrimethylsilylaminopropyllithium and 4-trimethylsilylmethylaminobutyllithium.

[0056] Examples of dialkylaminolithiums include, but are not limited to, lithium dimethylamide, lithium diethylamide, lithium dipropylamide, lithium dibutylamide, lithium di-n-hexylamide, lithium diheptylamide, lithium diisopropylamide, lithium dioctylamide, lithium-di-2-ethylhexylamide, lithium didecylamide, lithium ethylpropylamide, lithium ethylbutylamide, lithium ethylbenzylamide, lithium methylphenethylamide, lithium hexamethyleneimide, lithium pyrrolidide, lithium piperidide, lithium heptamethyleneimide, lithium morpholide, 1-lithioazacyclooctane, 6-lithio-1,3,3-trimethyl-6-azabicyclo[3.2.1]octane, and 1-lithio-1,2,3,6-tetrahydropyridine.

[0057] These organomonolithium compounds having a substituted amino group can also be used as solubilized oligomeric organomonolithium compounds by reacting them with a small amount of a polymerizable monomer, such as 1,3-butadiene, isoprene, or styrene.

[0058] The organic monolithium compound is preferably an alkyllithium compound from the viewpoints of industrial availability and ease of control of the polymerization reaction. In this case, a copolymer having an alkyl group at the polymerization initiation terminal can be obtained. Examples of the alkyllithium compound include, but are not limited to, n-butyllithium, sec-butyllithium, tert-butyllithium, n-hexyllithium, benzyllithium, phenyllithium, and stilbenelithium. From the viewpoints of industrial availability and ease of control of the polymerization reaction, n-butyllithium and sec-butyllithium are preferred as the alkyllithium compound. These organic monolithium compounds may be used alone or in combination of two or more. They may also be used in combination with other organometallic compounds.

[0059] Examples of the other organometallic compounds include alkaline earth metal compounds, other alkali metal compounds, and other organometallic compounds. Examples of alkaline earth metal compounds include, but are not limited to, organomagnesium compounds, organocalcium compounds, and organostrontium compounds. Examples of the other organometallic compounds include alkoxides, sulfonates, carbonates, and amides of alkaline earth metals. Examples of the organomagnesium compounds include dibutylmagnesium and ethylbutylmagnesium. Examples of the other organometallic compounds include organoaluminum compounds.

[0060] In the polymerization step, the polymerization reaction mode may be, but is not limited to, a batchwise mode (also referred to as a "batch mode") or a continuous mode. In a continuous mode, one or two or more connected reactors may be used. As a continuous reactor, for example, a tank-type or tubular reactor equipped with a stirrer is used. In a continuous mode, preferably, a monomer, an inert solvent, and a polymerization initiator are continuously fed into the reactor, a polymer solution containing a polymer is obtained in the reactor, and the polymer solution is continuously discharged. As a batch reactor, for example, a tank-type reactor equipped with a stirrer is used. In a batch mode, preferably, a monomer, an inert solvent, and a polymerization initiator are fed, and if necessary, a monomer is continuously or intermittently added during polymerization, a polymer solution containing a polymer is obtained in the reactor, and the polymer solution is discharged after completion of polymerization.

[0061] In the polymerization step of the conjugated diene copolymer of this embodiment, polymerization is preferably carried out in an inert solvent. Examples of the inert solvent include, but are not limited to, hydrocarbon solvents such as saturated hydrocarbons and aromatic hydrocarbons. Specific hydrocarbon solvents include, but are not limited to, aliphatic hydrocarbons such as butane, pentane, hexane, and heptane; alicyclic hydrocarbons such as cyclopentane, cyclohexane, methylcyclopentane, and methylcyclohexane; aromatic hydrocarbons such as benzene, toluene, and xylene, and hydrocarbons consisting of mixtures thereof. Treating impurities such as allenes and acetylenes with an organometallic compound before subjecting the mixture to the polymerization reaction tends to produce a conjugated diene copolymer having a high concentration of active terminals, and thus tends to produce a conjugated diene copolymer with a high coupling rate and modification rate, which is preferable.

[0062] A polar substance (polar compound) may be added in the polymerization step. This allows the aromatic vinyl compound to be randomly copolymerized with the conjugated diene compound, and the polar substance tends to be useful as a vinylating agent for controlling the microstructure of the conjugated diene portion. It also tends to be effective in promoting the polymerization reaction. Examples of polar substances include, but are not limited to, ethers such as tetrahydrofuran, diethyl ether, dioxane, ethylene glycol dimethyl ether, ethylene glycol dibutyl ether, diethylene glycol dimethyl ether, diethylene glycol dibutyl ether, dimethoxybenzene, and 2,2-bis(2-oxolanyl)propane; tertiary amine compounds such as tetramethylethylenediamine, dipiperidinoethane, trimethylamine, triethylamine, pyridine, and quinuclidine; alkali metal alkoxide compounds such as potassium tert-amylate, potassium tert-butylate, sodium tert-butylate, and sodium tert-amylate; and phosphine compounds such as triphenylphosphine. These polar substances may be used alone or in combination of two or more.

[0063] The amount of polar substance used is not particularly limited and can be selected depending on the purpose, etc., but is preferably 0.01 mol or more and 10 mol or less per mol of polymerization initiator. Such polar substances (vinylating agents) can be used in an appropriate amount depending on the desired 1,2-vinyl bond content as an adjuster for the microstructure of the conjugated diene portion in the conjugated diene copolymer. Many polar substances also have an effective randomizing effect in the copolymerization of a conjugated diene compound and an aromatic vinyl compound, and tend to be used as an adjuster for the distribution of the aromatic vinyl compound or the amount of styrene blocks.

[0064] As a method for randomizing the conjugated diene compound and the aromatic vinyl compound, for example, as described in JP-A-59-140211, a copolymerization reaction may be initiated with the whole amount of styrene and a part of 1,3-butadiene, and the remaining 1,3-butadiene may be intermittently added during the copolymerization reaction.

[0065] The polymerization temperature in the polymerization step is preferably a temperature at which living anionic polymerization proceeds, and from the viewpoint of productivity, is more preferably 0° C. or higher, even more preferably 120° C. or lower, and even more preferably 30° C. or higher and 100° C. or lower. When the temperature is in such a range, it tends to be possible to ensure a sufficient amount of the modifying agent to react with the active terminals after the completion of polymerization.

[0066] (Coupling step, modification step, hydrogenation step) The active terminals of the conjugated diene copolymer obtained through the above-mentioned polymerization step and, if necessary, a branching step using a predetermined branching agent may be subjected to a coupling reaction using the above-mentioned coupling agent or a modification reaction using a modifier having a nitrogen atom-containing group. When a nitrogen atom-containing coupling agent is used, the coupling reaction and the modification reaction proceed simultaneously. In addition, a hydrogenation step in which a hydrogenation reaction is carried out as appropriate may be carried out.

[0067] (Deactivator Addition Step, Neutralizer Addition Step) In the method for producing a conjugated diene copolymer of this embodiment, a deactivator, a neutralizer, or the like may be added to the polymer solution as needed. Examples of deactivators include, but are not limited to, water; and alcohols such as methanol, ethanol, and isopropanol. Examples of neutralizers include, but are not limited to, carboxylic acids such as stearic acid, oleic acid, and versatic acid (a highly branched carboxylic acid mixture having 9 to 11 carbon atoms, with the majority being 10 carbon atoms); aqueous solutions of inorganic acids; and carbon dioxide gas.

[0068] (Rubber Stabilizer) In the method for producing the conjugated diene copolymer of this embodiment, it is preferable to add a rubber stabilizer from the viewpoint of preventing gel formation after polymerization and improving stability during processing. The rubber stabilizer is not limited to the following, and known stabilizers can be used. Examples include antioxidants such as 2,6-di-tert-butyl-4-hydroxytoluene (hereinafter also referred to as "BHT"), n-octadecyl-3-(4'-hydroxy-3',5'-di-tert-butylphenol)propionate, and 2-methyl-4,6-bis[(octylthio)methyl]phenol.

[0069] (Solvent Removal Step) In the method for producing the conjugated diene copolymer of the present embodiment, a known method can be used as a method for obtaining the obtained conjugated diene copolymer from the polymer solution. Examples of the method include, but are not limited to, a method of separating the solvent by steam stripping or the like, filtering the polymer, and then removing the solvent and drying it to obtain the polymer, a method of concentrating the polymer in a flashing tank and then removing the volatilization using a vent extruder or the like, and a method of directly removing the volatilization using a drum dryer or the like.

[0070] [Positive Electrode Binder Composition] The positive electrode binder composition of this embodiment contains at least the active material described above, a non-aqueous binder containing the conjugated diene copolymer described above, and a solvent, and preferably further contains a conductive additive. When the binder composition is taken as 100 parts by mass, the active material is preferably 80 parts by mass to 99 parts by mass, the conductive additive is preferably 1 part by mass to 10 parts by mass, and the non-aqueous binder is preferably 1 part by mass to 12 parts by mass. The positive electrode binder composition may be applied after dispersing the conjugated diene copolymer and the active material in a solvent. Polar solvents are preferred as the solvent, and N-methylpyrrolidone, γ-butyrolactone, tetrahydrofuran, and toluene are preferred from the standpoints of solubility and volatility. The active material layer of this embodiment can be obtained by drying the positive electrode binder composition. The excellent solubility of the conjugated diene polymer in the solvent reduces insoluble components in the composition and suppresses viscosity variation. As a result, the coating properties during coating are improved, making it easier to form a coating film of uniform thickness, and realizing excellent adhesion between the current collector and the active material.

[0071] (Molded bale) The conjugated diene copolymer of this embodiment can be made into a molded bale. The molded bale is a rectangular parallelepiped rubber, and from the viewpoint of transportability, the weight is preferably 20 kg or more, more preferably 23 kg or more, even more preferably 25 kg or more, and particularly preferably 30 kg or more. The molded bale is a mass of the conjugated diene copolymer obtained by compression molding, and can be obtained, for example, by extruding the conjugated diene copolymer with an extruder, cutting it to obtain crumbs, and compression molding the crumbs.

[0072] The moisture content of the bale of conjugated diene copolymer is not particularly limited, but is preferably 1.0% by mass, more preferably 0.8% by mass, and even more preferably 0.6% by mass or less. When the moisture content of the bale of conjugated diene copolymer is within the above range, the moisture content of the positive electrode active material layer containing the conjugated diene copolymer is controlled within a specific range, which tends to suppress corrosion of the positive electrode current collector due to a reaction between the alkali metal compound and moisture.

[0073] [Method for manufacturing a positive electrode for a lithium ion secondary battery] The method for manufacturing a positive electrode for a lithium ion secondary battery of this embodiment is not particularly limited, but for example, the positive electrode binder composition of this embodiment is applied to a positive electrode current collector, heated, and dried to form an active material layer on one or both sides of the current collector, thereby producing a positive electrode for a lithium ion secondary battery. The positive electrode current collector is not particularly limited, but for example, aluminum foil is used.

[0074] [Lithium-ion secondary battery] The lithium-ion secondary battery of this embodiment includes the lithium-ion battery positive electrode of this embodiment and an electrolyte. Typical components of the lithium-ion secondary battery of this embodiment include a negative electrode, a positive electrode, a separator, and an electrolyte. The lithium-ion secondary battery of this embodiment may include at least the lithium-ion battery positive electrode of this embodiment and an electrolyte. Various known negative electrodes, electrolytes, and other power storage device components can be used as appropriate depending on the type of power storage device.

[0075] [Method for Manufacturing Lithium-Ion Secondary Battery] The lithium-ion secondary battery of this embodiment may be manufactured by, but is not limited to, placing a negative electrode and a positive electrode of this embodiment opposite each other via a separator, injecting an electrolyte solution, and sealing the battery. The negative electrode and the electrolyte are not particularly limited, and any suitable electrolyte suitable for lithium-ion secondary batteries may be appropriately selected and used. For example, the electrolyte may be prepared by dissolving an electrolyte such as LiClO4, LiBF4, or LiPF6 in an organic solvent. The organic solvent may be, but is not limited to, ethers, ketones, lactones, nitriles, amines, amides, carbonates, chlorinated hydrocarbons, and the like. Representative examples include tetrahydrofuran, acetonitrile, butyronitrile, propylene carbonate, ethylene carbonate, and diethyl carbonate. These organic solvents may be used alone or in a mixture of two or more.

[0076] The lithium ion secondary battery of this embodiment can also be used as a battery pack in which a plurality of lithium ion secondary batteries are connected in series or in parallel. From the viewpoint of extending the life of the battery pack by suppressing deterioration of the binder layer, the range of operating voltage per battery is preferably 4.0 V or less, more preferably 3.8 V or less, and particularly preferably 3.6 V or less. The lower limit of the range of operating voltage per battery is not particularly limited, and is preferably 2.0 V or more.

[0077] Hereinafter, the present embodiment will be described in more detail with reference to specific examples and comparative examples, but the present embodiment is not limited to the following examples and comparative examples. Specific examples of conjugated diene copolymers are referred to as "Preparation Examples," and specific examples of positive electrodes for lithium ion secondary batteries using the conjugated diene copolymers and the like are referred to as "Examples" and "Comparative Examples." Various physical properties in the Preparation Examples, Examples, and Comparative Examples were measured by the methods shown below.

[0078] [Physical property measurement method] [Weight average molecular weight (Mw)] A chromatogram was measured using a GPC measurement device equipped with three connected columns packed with polystyrene gel, and the peak-top molecular weight of the peak with the largest area was determined based on a calibration curve using standard polystyrene. Specific measurement conditions are shown below. Measurement was performed by injecting 20 μL of the following measurement solution into the GPC measurement device. (Measurement conditions) Device: Tosoh Corporation, product name "HLC-8320GPC" Eluent: Tetrahydrofuran (THF) containing 5 mmol / L triethylamine Guard column: Tosoh Corporation, product name "TSKguardcolumn Super H-H" Separation column: Tosoh Corporation, product names "TSKgel Super H5000", "TSKgel Super H6000", and "TSKgel Super H7000" connected in this order. Oven temperature: 40°C Flow rate: 0.6 mL / min Detector: RI detector (trade name "HLC8020" manufactured by Tosoh Corporation) Measurement solution: Measurement solution prepared by dissolving 10 mg of a measurement sample in 20 mL of THF

[0079] [Modification Ratio] The modification ratio was measured by the column adsorption GPC method, taking advantage of the property of the modified polymer to adsorb to a column, as follows. A sample solution containing a sample and a low-molecular-weight internal standard polystyrene was measured using a column packed with polystyrene-based gel, and the amount of adsorption to the silica-based column was measured from the difference between the chromatogram measured using a column packed with silica-based gel, and the modification ratio was determined. (GPC Measurement Conditions Using a Polystyrene-Based Column) The GPC measurement conditions using a polystyrene-based column are shown below. Measurement was performed by injecting 20 μL of the following measurement solution into the GPC measurement device. Apparatus: Tosoh Corporation, trade name "HLC-8320GPC" Eluent: THF containing 5 mmol / L triethylamine Guard column: Tosoh Corporation, trade name "TSKguard column Super H-H" Column: Tosoh Corporation, trade names "TSKgel Super H5000", "TSKgel Super H6000", and "TSKgel Super H7000" connected in this order Oven temperature: 40°C Flow rate: 0.6 mL / min Detector: RI detector (Tosoh Corporation, HLC8020) Measurement solution: 10 mg of sample and 5 mg of standard polystyrene were dissolved in 20 mL of THF to prepare a sample solution. (GPC measurement conditions using a silica-based column) The GPC measurement conditions using a silica-based column are shown below. 50 μL of the following measurement solution was injected into the GPC measurement device and the measurement was performed. Apparatus: Tosoh Corporation, product name "HLC-8320GPC" Eluent: THF Guard column: GL Sciences, product name "DIOL 4.6 x 12.5 mm 5 micron" Separation column: Agilent Technologies, product names "Zorbax PSM-1000S", "PSM-300S", and "PSM-60S" connected in this order Oven temperature: 40°C Flow rate: 0.5 mL / min Detector: RI detector (Tosoh Corporation, HLC8020)

[0080] (Method of calculating the modification rate): The total peak area of ​​the chromatogram using the polystyrene column was set to 100, the peak area of ​​the sample was set to P1, the peak area of ​​the standard polystyrene was set to P2, the total peak area of ​​the chromatogram using the silica column was set to 100, the peak area of ​​the sample was set to P3, and the peak area of ​​the standard polystyrene was set to P4, and the modification rate (%) was calculated using the following formula: Modification rate (%) = [1 - (P2 x P3) / (P1 x P4)] x 100 (where P1 + P2 = P3 + P4 = 100)

[0081] [Mooney Viscosity] The Mooney viscosity of each polymer was measured using a Mooney viscometer (trade name "VR1132" manufactured by Ueshima Seisakusho Co., Ltd.) in accordance with JIS K6300 (ISO289-1) and ISO289-4. The measurement temperature was 100°C. Here, the sample was preheated for 1 minute, and then the rotor was rotated at 2 rpm. The torque after 4 minutes was measured and used as the Mooney viscosity (ML(1+4)).

[0082] [Aromatic vinyl monomer unit content (bound styrene amount), 1,2 vinyl bond amount, total hydrogenation rate, and 1,2 hydrogenation rate of conjugated diene copolymer] Using a conjugated diene copolymer as a sample, the relative intensities of the bound styrene amount and 1,2 vinyl bond amount were measured by 1H-NMR measurement. In addition, in the case of a hydrogenated conjugated diene copolymer, the hydrogenation rate was measured by a known method. The conditions for the 1H-NMR measurement are described below. <Measurement conditions> Measurement equipment: JNM-LA400 (manufactured by JEOL) Solvent: deuterated chloroform Measurement sample: conjugated diene copolymer Sample concentration: 50 mg / mL Observation frequency: 400 MHz Chemical shift reference: TMS (tetramethylsilane) contained at 0.05% by mass relative to deuterated chloroform Pulse delay: 2.904 seconds Number of scans: 64 Pulse width: 45° Measurement temperature: 26°C

[0083] [1,4 cis bond content, 1,4 trans bond content, and ratio of 1,4 cis bond content to 1,4 trans bond content of conjugated diene copolymer] Using a conjugated diene copolymer as a sample, the 1,4 cis bond content and 1,4 trans bond content in the conjugated diene monomer component in the conjugated diene copolymer were measured using a nuclear magnetic resonance spectrometer (C-NMR). Using these values, the ratio of 1,4 cis bond content to 1,4 trans bond content was calculated. The measurement conditions were the same as those for the above-mentioned H-NMR measurement conditions, except that the number of scans was changed to 256.

[0084] (Aromatic vinyl monomer block content (styrene block content), blocking ratio) The aromatic vinyl monomer block (styrene block) content was measured according to the osmium tetroxide decomposition method described in I. M. Kolthoff, et al., J. Polym. Sci. 1,429 (1946). More specifically, 0.050 g of a conjugated diene copolymer was dissolved in 10 ml of chloroform, to which 16 mL of a 69% by mass aqueous solution of tert-butyl hydroperoxide and 4.0 mL of a 0.050% by mass chloroform solution of osmium tetroxide were added, and the mixture was refluxed in a 90°C bath for 12 minutes to carry out an oxidative decomposition reaction. After completion of the reaction, the reaction solution was cooled, and 200 mL of methanol was added to the reaction solution with stirring to precipitate the styrene block component, which was then filtered off using a 5 μm glass filter. The mass of the obtained product was divided by the total mass of the conjugated diene copolymer to determine the content of the aromatic vinyl monomer block. Furthermore, the value obtained by dividing the content of the aromatic vinyl monomer block by the amount of bound styrene measured by NMR as described above is the blocking ratio, which is the ratio of the content of the aromatic vinyl monomer block to the content of the aromatic vinyl monomer unit.

[0085] (Metal Content) The conjugated diene copolymers obtained in the production examples described below were subjected to elemental analysis using an inductively coupled plasma (ICP, manufactured by Shimadzu Corporation, apparatus name: ICPS-7510) to measure the zinc content (Zn amount, unit: ppm), aluminum content (Al amount, unit: ppm), copper content (Cu amount, unit: ppm), iron content (Fe amount, unit: ppm), lithium content (Li amount, unit: ppm), and titanium content (Ti amount, unit: ppm). When each metal content was below the detection limit, it was recorded as ND.

[0086] (Bale Molding) Conjugated diene copolymers (polymers 1 to 21) described below were subjected to desolvation and drying treatment, and the resulting conjugated diene copolymers were filled into a rectangular container having dimensions of 102 mm in width, 204 mm in length, and 150 mm in depth, and compressed with a cylinder at a pressure of 3.5 MPa for 10 seconds to obtain bale-molded conjugated diene copolymers.

[0087] (Moisture Content) 50 g of the bale was placed in a hot air dryer heated to 150°C and dried for 3 hours, and the moisture content of the bale was determined by measuring the difference in mass of the bale before and after drying.

[0088] <Preparation Examples of Conjugated Diene Copolymers> [Preparation Example 1] A 40 L internal volume, temperature-controllable autoclave equipped with a stirrer and a jacket was used as a reactor, and impurities were removed in advance. 2,100 g of 1,3-butadiene, 900 g of styrene, 21,000 g of cyclohexane, and 0.46 mol of tetrahydrofuran (THF) as a polar substance were added to the reactor, and the internal temperature of the reactor was raised to and maintained at 42°C. Next, 30.6 mmol of n-butyllithium was added to the reactor as a polymerization initiator to initiate polymerization. Thereafter, 33.7 mmol of methanol was added as a reaction terminator to terminate the polymerization. The internal temperature of the reactor before termination of the polymerization was 83°C. Furthermore, 12.6 g of n-octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate and 3.0 g of 4,6-bis(octylthiomethyl)-o-cresol were added to the reactor as antioxidants, and the conjugated diene copolymer solution was then added dropwise to warm water to remove the solvent. The solution was then dried in a dryer to obtain a conjugated diene copolymer (Polymer 1). The analytical results of the obtained Polymer 1, which were analyzed by the above-mentioned methods, are shown in Table 4.

[0089] [Preparation Examples 2, 5, 8, 11 to 14, 16 to 18] Conjugated diene copolymers (polymers 2, 5, 8, 11 to 14, 16 to 18) were obtained in the same manner as in Preparation Example 1, except that the monomer composition, the type and amount of polar substance, and the amount of polymerization initiator and polymerization terminator added were changed as shown in Tables 1 and 2. The obtained polymers 2, 5, 8, 11 to 14, and 16 to 18 were analyzed by the above-mentioned methods, and the results are shown in Tables 4 and 5.

[0090] [Preparation Example 3] A 40 L internal volume, temperature-controllable autoclave equipped with a stirrer and jacket was used as a reactor, and impurities were removed in advance. 2,100 g of 1,3-butadiene, 900 g of styrene, 21,000 g of cyclohexane, and 0.33 mol of tetrahydrofuran (THF) as a polar substance were added to the reactor, and the internal temperature of the reactor was raised to and maintained at 41°C. Next, 22.2 mmol of n-butyllithium was added to the reactor as a polymerization initiator to initiate polymerization. After the polymerization reaction began, the temperature inside the reactor began to rise due to heat generated by the polymerization. When the temperature increase ceased, 4.4 mol of silicon tetrachloride as a coupling agent was added to the reactor and stirred for 5 minutes. Subsequently, 6.7 mmol of methanol as a reaction terminator was added to terminate the polymerization. Furthermore, 12.6 g of n-octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate and 3.0 g of 4,6-bis(octylthiomethyl)-o-cresol were added to the reactor as antioxidants, and the conjugated diene copolymer solution was then added dropwise to warm water to remove the solvent. The resulting solution was then dried in a dryer to obtain a conjugated diene copolymer (polymer 3). The analytical results of the obtained polymer 3, which were analyzed by the above-mentioned methods, are shown in Table 4.

[0091] [Preparation Example 4] Each conjugated diene copolymer (Polymer 4) was obtained in the same manner as in Preparation Example 3, except that the coupling agent was changed to 2,2-dimethoxy-1-(3-trimethoxysilylpropyl)-1-aza-2-silacyclopentane, as shown in Table 1. The obtained Polymer 4 was analyzed by the above-mentioned method, and the results are shown in Table 4.

[0092] [Preparation Example 6] A 40 L internal volume, temperature-controllable autoclave equipped with a stirrer and jacket was used as a reactor, and impurities were removed in advance. 1,800 g of 1,3-butadiene, 400 g of styrene, 21,000 g of cyclohexane, and 0.18 mol of tetrahydrofuran (THF) as a polar substance were added to the reactor, and the internal temperature of the reactor was raised to and maintained at 45°C. Next, 11.8 mmol of n-butyllithium was added as a polymerization initiator to the reactor to initiate polymerization. Thereafter, 3 minutes after the internal temperature of the reactor reached its peak, 800 g of styrene (additional styrene in the table) was added to the reactor, and the polymerization reaction was continued. Subsequently, 13.0 mmol of methanol was added as a reaction terminator to terminate the polymerization. The internal temperature of the reactor before termination of the polymerization was 80°C. Furthermore, 12.6 g of n-octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate and 3.0 g of 4,6-bis(octylthiomethyl)-o-cresol were added to the reactor as antioxidants, and the conjugated diene copolymer solution was then added dropwise to warm water to remove the solvent. The resulting solution was then dried in a dryer to obtain a conjugated diene copolymer (Polymer 6). The analytical results of the obtained Polymer 6, which were analyzed by the above-mentioned methods, are shown in Table 4.

[0093] [Preparation Examples 7, 9, 15] Conjugated diene copolymers (polymers 7, 9, and 15) were obtained in the same manner as in Preparation Example 6, except that the monomer composition, the type and amount of polar substance, and the amount of polymerization initiator and polymerization terminator added were changed as shown in Tables 1 and 2. The obtained polymers 7, 9, and 15 were analyzed by the above-mentioned methods, and the results are shown in Tables 4 and 5.

[0094] <Preparation of Hydrogenation Catalyst> [Preparation Example a] A nitrogen-purged reactor was charged with 2 liters of dried and purified cyclohexane, and 40 mmol of bis(η5-cyclopentadienyl)titanium di-(p-tolyl) and 150 g of 1,2-polybutadiene (1,2-vinyl bond content: approximately 85%) having a molecular weight of approximately 1,000 were added and dissolved. Thereafter, a cyclohexane solution containing 60 mmol of n-butyllithium was added to the reactor and reacted at room temperature for 5 minutes. Immediately after that, 40 mmol of n-butanol was added and stirred to obtain a hydrogenation catalyst (TC1).

[0095] Preparation Example 10: A polymerization reaction was carried out in the same manner as in Preparation Example 1, except that the monomer composition was changed as shown in Table 1. Subsequently, a hydrogenation catalyst (TC1) was added to the reactor in an amount of 50 ppm (based on titanium) per 100 parts by mass of the resulting conjugated diene copolymer solution, and the hydrogenation reaction was carried out at a hydrogen pressure of 0.85 MPa and an average temperature of 90°C until the predetermined amount of hydrogen was reacted to completion. Furthermore, 12.6 g of n-octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate (antioxidant 1) and 3.0 g of 4,6-bis(octylthiomethyl)-o-cresol (antioxidant 2) were added to the reactor as antioxidants, and the conjugated diene copolymer solution was then added dropwise to warm water to remove the solvent. The resulting solution was then dried to obtain a conjugated diene copolymer (polymer 10). The analytical results of the resulting polymer 10, analyzed by the above-described method, are shown in Table 5.

[0096] [Preparation Example 19] Two connected tank-type pressure vessels with an internal volume of 10 L, an internal height (L) to diameter (D) ratio (L / D) of 4.0, an inlet at the bottom, an outlet at the top, and a jacket for temperature control were used as reactors. 1,3-butadiene from which moisture had been removed in advance was added at flow rates of 20.1 g / min, styrene at 5.2 g / min, and n-hexane at 175.2 g / min, and mixed. In a static mixer installed in the middle of the piping supplying this mixed solution to the inlet of the reactor, n-butyllithium was added as a residual impurity deactivation agent at a flow rate of 0.105 mmol / min, and after mixing, the mixture was continuously supplied to the bottom of the reactor. Furthermore, 2,2-bis(2-oxolanyl)propane as a polar substance at a flow rate of 0.037 mmol / min and n-butyllithium as a polymerization initiator at a flow rate of 0.463 mmol / min were supplied to the bottom of the first reactor while vigorously mixing with an agitator, and the polymerization reaction was initiated. The internal temperature of the reactor at this time was maintained at 77°C. The conjugated diene copolymer solution was continuously extracted from the top of the first reactor and continuously supplied to the bottom of the second reactor, and styrene (additional styrene) was continuously added from the center of the second reactor at a flow rate of 3.5 g / min, and the polymerization reaction was continued. Further, the solution was supplied to a static mixer from the top of the second reactor. Next, methanol as a reaction terminator was continuously added at a flow rate of 0.509 mmol / min to the conjugated diene copolymer solution supplied to the static mixer from the top of the second reactor, and the mixture was mixed using a static mixer to terminate the polymerization reaction. Next, 0.1 g of n-octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate (antioxidant 1) and 0.1 g of 4,6-bis(octylthiomethyl)-o-cresol (antioxidant 2) were added per 100 g of the conjugated diene copolymer solution to the obtained conjugated diene copolymer solution, and the conjugated diene copolymer solution was then added dropwise to warm water to remove the solvent. The resulting solution was then dried in a dryer to obtain a conjugated diene copolymer (polymer 19). The analytical results of the obtained polymer 19, which was analyzed by the above-mentioned methods, are shown in Table 6.

[0097] [Preparation Example 20] A conjugated diene copolymer (polymer 20) was obtained in the same manner as in Preparation Example 19, except that the monomer composition was changed as shown in Table 3. The obtained polymer 20 was analyzed by the above-mentioned method, and the results are shown in Table 6.

[0098]

[0099]

[0100]

[0101] The abbreviations in Tables 1 to 3 are as follows: THF: tetrahydrofuran BOP: 2,2-bis(2-oxolanyl)propane Polymerization initiator: n-butyllithium Coupling agent 1: silicon tetrachloride Coupling agent 2: 2,2-dimethoxy-1-(3-trimethoxysilylpropyl)-1-aza-2-silacyclopentane Polymerization terminator: methanol TC1: hydrogenation catalyst of Preparation Example a

[0102]

[0103]

[0104] In Preparation Example 21, emulsion SBR (product name ESBR1502) manufactured by ENEOS Materials Corporation was used.

[0105]

[0106] <Preparation of Positive Electrode for Lithium-Ion Secondary Battery> [Example 1] 96 parts by mass of LiFePO4 (average particle size 1 μm) as the positive electrode active material, 2 parts by mass of acetylene black as a conductive additive, and 2 parts by mass of the conjugated diene copolymer (Polymer 1) described in Preparation Example 1 as a non-aqueous binder were added to a rotation-revolution mixer (manufactured by Thinky Corporation). N-methylpyrrolidone (NMP) was then added as a solvent to a solids concentration of 5%, and the mixture was mixed to obtain a positive electrode binder composition. The positive electrode binder composition was applied to a 20 μm thick aluminum thin film using a slot die coater, and then vacuum dried at 120°C for 8 hours. The positive electrode active material layer was then rolled to a porosity of 25%, thereby obtaining a positive electrode. The active material layer had an average thickness of 70 μm. The electrode obtained in this manner was used as a positive electrode for a secondary battery and subjected to various physical property evaluations described below. The results are shown in Table 7.

[0107] [Examples 2 to 16, Comparative Examples 1 to 5] As shown in Tables 7 to 9, positive electrodes for secondary batteries were obtained in the same manner as in Example 1, except that the conjugated diene copolymers used were changed. Each of the obtained electrodes was used as a positive electrode for a secondary battery and subjected to various physical property evaluations described below. The results are shown in Tables 7 to 9.

[0108] (Solubility of Conjugated Diene Copolymer) The solubility of each copolymer of Preparation Examples 1 to 21 in N-methylpyrrolidone (NMP) solvent was evaluated. The dissolution time when the solvent was adjusted to give a 3 wt % concentration of the conjugated diene copolymer was used as the standard for evaluation according to the following criteria. The dissolution time was defined as the time until the conjugated diene copolymer was visually completely gone. A shorter dissolution time provides superior productivity and is less likely to cause uneven film thickness due to residual dissolution, making it easier to form a uniform active material layer. ○: Dissolved 20% or more quickly. △: The difference in dissolution time was less than 20%. ×: Dissolved 20% or more slowly.

[0109] (Adhesion) A test piece measuring 2 cm wide x 12 cm long was cut from the resulting secondary battery positive electrode, and the current collector side of the test piece was attached to an aluminum plate with double-sided tape. According to JIS 1522, an 18 mm wide tape (product name: Cellotape (registered trademark) manufactured by Nichiban Co., Ltd.) was attached to the electrode layer side of the test piece. The strength when the tape was peeled off in a 180° direction at a rate of 100 mm / min was measured six times, and the average value (N / 18 mm) was calculated as the peel strength. Evaluation was performed using the measured value of the secondary battery positive electrode of Comparative Example 5 as the standard, according to the following evaluation criteria. The higher the peel strength, the better the adhesion between the current collector and the active material layer. ⊚: The peel strength was 25% or more higher. ◯: The peel strength was 10% or more higher. Δ: The difference in peel strength was less than 10%. ×: The peel strength was 10% or more lower.

[0110] (Crack Resistance) A test piece measuring 2 cm wide x 12 cm long was cut out from the obtained positive electrode for secondary battery, bent at a 45-degree angle at the center of the test piece, held for 1 minute, and then returned to its original state. The presence of cracks in the binder layer was then visually confirmed, and the test piece was evaluated according to the following evaluation criteria. The more flexible the positive electrode, the less likely it is that electrode cracks will occur during winding during battery production. ○: No cracks occurred. △: No cracks occurred, but bending marks remained. ×: Cracks occurred.

[0111] <Fabrication of Lithium-Ion Secondary Battery> A positive electrode and a negative electrode for a secondary battery were punched into a circle, and the positive electrode, separator, and negative electrode were stacked in this order so that the active material surfaces of the positive electrode and negative electrode faced each other. The resulting stack was then housed in a lidded stainless steel metal container. The container and the lid were insulated, and the container was placed so that it was in contact with the copper foil of the negative electrode, and the lid was placed so that it was in contact with the aluminum foil of the positive electrode. An electrolyte solution was then poured into the container, which was then sealed and left to stand at room temperature for one day to fabricate a secondary battery. The electrolyte solution used here was a mixture of ethylene carbonate / ethyl methyl carbonate = 1 / 2 (volume ratio) and lithium hexafluorophosphate (LiPF ) as a solute. ６) to a concentration of 1.0 mol / L. The separator was made of a polyethylene porous film, and the secondary battery positive electrodes obtained in Examples 1 to 16 and Comparative Examples 1 to 5 were used. The secondary battery negative electrode was prepared as follows: 1.5 parts by mass of styrene-butadiene latex (pH 7, 10% solids), 100 parts by mass of natural graphite as the negative electrode active material, and 1.0 parts by mass of carboxymethyl cellulose as a thickener were added and mixed to a solids content of 10%. Ion-exchanged water was added thereto, and the mixture was stirred with a mechanical stirrer to adjust the total solids content to 60%. This mixture was dispersed for 30 seconds using a thin-film rotary high-speed mixer at a peripheral speed of 20 m / s to prepare a coating solution for a secondary battery negative electrode. The coating solution was applied to one side of a copper foil using a die coater so that the thickness after drying would be 100 μm, and then dried for 60 minutes at 60° C. After further drying for 3 minutes at 120° C., the resultant was compression-molded using a roll press to obtain a negative electrode for a secondary battery.

[0112] (Cycle Characteristics) The secondary batteries prepared by the above method were subjected to a charge-discharge cycle in which they were charged at a constant current of 2C at 60°C until the battery reached 3.6V, then charged at a constant voltage, and then discharged at a constant current of 2C until the battery reached 2.6V. The cycle test was performed for 100 cycles, and the ratio of the discharge capacity at the 100th cycle to the initial discharge capacity was defined as the capacity retention rate. This was evaluated according to the following criteria, using the volume retention rate of the secondary battery using the secondary battery positive electrode of Comparative Example 5 as the standard. A larger value indicates less capacity loss due to repeated charge-discharge. ∘: The retention rate was 5% or more higher. Δ: The retention rate was high, less than 5%, or low, less than 10%. ×: The retention rate was 10% or more lower.

[0113]

[0114]

[0115]

[0116] It was found that the nonaqueous binders for positive electrodes and positive electrodes obtained in Examples 1 to 16 had superior solubility in solvents, superior balance of adhesion to the current collector and crack resistance, and also superior cycle characteristics when formed into a battery, compared to the nonaqueous binders for positive electrodes and positive electrodes obtained in Comparative Examples 1 to 5.

[0117] The disclosure of Japanese Patent Application No. 2024-147668, filed on August 29, 2024, is incorporated herein by reference in its entirety. In addition, all documents, patent applications, and technical standards mentioned in the specification are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A positive electrode for a lithium ion secondary battery comprising a current collector and an active material layer containing an active material and a non-aqueous binder, wherein the non-aqueous binder contains a conjugated diene copolymer satisfying the following requirements (a) to (d): Requirement (a): The content of aromatic vinyl monomer units is 6% by mass or more and 80% by mass or less relative to the total amount of the conjugated diene copolymer; Requirement (b): The amount of 1,2-vinyl bonds relative to the conjugated diene monomer units in the conjugated diene copolymer is 10 mol% or more and 60 mol% or less; Requirement (c): The ratio of 1,4-cis bonds to 1,4-trans bonds in the conjugated diene copolymer is 30:70 to 50:50; and Requirement (d): The weight-average molecular weight is 100,000 or more and 2,000,000 or less.

2. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the conjugated diene copolymer has a Mooney viscosity of 30 or more and 200 or less when measured at 100°C.

3. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the content of the aromatic vinyl monomer unit is 30% by mass or more and 70% by mass or less with respect to the total amount of the conjugated diene copolymer.

4. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the content of the aromatic vinyl monomer block is 5% by mass or more and 40% by mass or less relative to the total amount of the conjugated diene copolymer.

5. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the blocking ratio of the aromatic vinyl monomer block in the conjugated diene copolymer is 15% or more and 85% or less.

6. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the active material includes lithium iron phosphate.

7. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the content of each of zinc, aluminum, copper and iron is 50 ppm or less relative to the total amount of the conjugated diene copolymer.

8. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the total content of zinc, aluminum, copper and iron is 50 ppm or less relative to the total amount of the conjugated diene copolymer.

9. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the conjugated diene copolymer has a total hydrogenation rate of 10% to 99%.

10. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the conjugated diene copolymer has a 1,2-hydrogenation rate of 80% or more.

11. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the non-aqueous binder further contains polyvinylidene fluoride.

12. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the active material layer further contains a conductive additive, and the conductive additive contains carbon black.

13. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the current collector contains aluminum.

14. A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to claim 1 and an electrolyte solution.

15. The lithium ion secondary battery according to claim 14, wherein the operating voltage is 4.0 V or less.

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