Positive electrode for lithium-ion secondary batteries and lithium-ion secondary batteries

Abstract

To provide a positive electrode for lithium-ion secondary batteries with excellent adhesion to the current collector and suppressed cracking, and a lithium-ion secondary battery with excellent cycle characteristics. [Solution] A positive electrode for a lithium-ion secondary battery comprising a current collector, a coating layer containing a conductive material and a non-aqueous binder, and an active material layer, wherein the non-aqueous binder contains a conjugated diene copolymer having specific requirements, the content of the conductive material is 40.0 parts by mass or more and 99.9 parts by mass or less per 100 parts by mass of the coating layer, and the content of the non-aqueous binder is 0.1 parts by mass or more and 60.0 parts by mass or less per 100 parts by mass of the coating layer.

Description

Technical Field

[0001] The present invention relates to a positive electrode for a lithium-ion secondary battery and a lithium-ion secondary battery.

Background Art

[0002] Conventionally, as a method for manufacturing an electrode used in an electrochemical device such as a lithium-ion secondary battery, a liquid composition in which a binder, a thickener, etc. are added to an electrode active material is applied to the surface of a current collector and dried, whereby an electrode layer is formed on the current collector. Here, as a binder capable of forming an electrode layer having a high adhesive force with the metal constituting the current collector on the positive electrode side and high flexibility, polyvinylidene fluoride (PVDF) is known.

[0003] Conventionally, a lithium secondary battery is composed of a positive electrode, a negative electrode, a separator, and an electrolyte, and as the positive electrode, an active material layer is formed on a current collector. Here, in order to improve the adhesion and conductivity between the positive electrode current collector and the active material layer, the current collector may be coated with a conductive material such as carbon black. At this time, as a material having a role of holding a conductive material such as carbon black on the current collector, for example, styrene-butadiene latex and polyvinylidene fluoride are known.

[0004] For example, Patent Document 1 discloses a positive electrode for a lithium-ion secondary battery using styrene-butadiene latex as a binder for a carbon-coated foil.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Incidentally, when using a water-based binder to adhere the active material layer to the current collector, there are still issues with the adhesive strength to the current collector and the crack resistance of the electrodes. Furthermore, if water remains in the active material layer, there are issues with the cycle characteristics of the lithium-ion secondary battery. There is room for further improvement in these areas. On the other hand, even when using a non-water-based binder, if the solubility in the solvent is insufficient, the coating properties deteriorate, so it is necessary to use a binder material that is easily soluble in the solvent.

[0007] This invention has been made in view of the problems of the prior art described above, and aims to provide a positive electrode for a lithium-ion secondary battery with excellent adhesion to a current collector and crack suppression, and a lithium-ion secondary battery with excellent cycle characteristics, by using a non-aqueous binder containing a conjugated diene copolymer having specific requirements as a coating layer. [Means for solving the problem]

[0008] <1> A positive electrode for a lithium-ion secondary battery comprising a current collector, a coating layer containing a conductive material and a non-aqueous binder, and an active material layer, The non-aqueous binder comprises a conjugated diene copolymer having the following requirements (a) to (d): The content of the conductive material is 40.0 parts by mass or more and 99.9 parts by mass or less per 100 parts by mass of the coating layer. The content of the non-aqueous binder is 0.1 parts by mass or more and 60.0 parts by mass or less per 100 parts by mass of the coating layer. Positive electrode for lithium-ion secondary batteries. 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 bonded to the conjugated diene monomer unit 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. Requirement (d): The weight-average molecular weight is between 100,000 and 2,000,000. <2> The Mooney viscosity of the aforementioned conjugated diene copolymer at 100°C is 30 or more and 200 or less. <1> Positive electrode for lithium-ion secondary batteries as described above. <3> The content of the aromatic vinyl monomer units is 30% by mass or more and 70% by mass or less, relative to the total amount of the conjugated diene copolymer. <1> or <2> Positive electrode for lithium-ion secondary batteries as described above. <4> The content of aromatic vinyl monomer blocks is 5.0% by mass or more and 40.0% by mass or less, relative to the total amount of the conjugated diene copolymer. <1> ~ <3> A positive electrode for a lithium-ion secondary battery as described in any one of the items. <5> The blocking rate of aromatic vinyl monomer blocks in the conjugated diene copolymer is 15.0% or more and 85.0% or less. <1> ~ <4> A positive electrode for a lithium-ion secondary battery as described in any one of the items. <6> The content of each of the elements, zinc, aluminum, copper, and iron, is 50 ppm or less relative to the total amount of the conjugated diene copolymer. <1> ~ <5> A positive electrode for a lithium-ion secondary battery as described in any one of the items. <7> The total content of zinc, aluminum, copper, and iron is 50 ppm or less relative to the total amount of the conjugated diene copolymer. <1> ~ <6> A positive electrode for a lithium-ion secondary battery as described in any one of the items. <8> The total hydrogenation rate of the conjugated diene copolymer is 10% to 99%. <1> ~ <7> A positive electrode for a lithium-ion secondary battery as described in any one of the items. <9> The 1,2-hydrogenation rate of the aforementioned conjugated diene copolymer is 80% or more. The positive electrode for a lithium-ion secondary battery according to any one of <1> to <8>. <10> The thickness of the coating layer is 0.1 μm or more and 10 μm or less. The positive electrode for a lithium-ion secondary battery according to any one of <1> to <9>. <11> The conductive material contains at least one selected from the group consisting of carbon black, graphite, carbon nanotubes, and aluminum hydroxide. The positive electrode for a lithium-ion secondary battery according to any one of <1> to <10>. <12> The coating layer does not contain an active material for the positive electrode. The positive electrode for a lithium-ion secondary battery according to any one of <1> to <11>. <13> The current collector contains aluminum. The positive electrode for a lithium-ion secondary battery according to any one of <1> to <12>. <14> The active material layer contains a binder. The binder contains polyvinylidene fluoride. The positive electrode for a lithium-ion secondary battery according to any one of <1> to <13>. <15> The active material layer contains an active material. The active material contains lithium iron phosphate or lithium iron oxide. The positive electrode for a lithium-ion secondary battery according to any one of <1> to <14>. <16> A lithium-ion secondary battery including the positive electrode for a lithium-ion secondary battery according to any one of <1> to <15>. A lithium-ion secondary battery. <17> Furthermore, it contains an electrolytic solution. The lithium-ion secondary battery according to <16>.

Advantages of the Invention

[0009] According to the present invention, by using a non-aqueous binder containing a conjugated diene copolymer having specific requirements in the coating layer, it is possible to provide a positive electrode for a lithium-ion secondary battery with excellent adhesion to the current collector and suppressed cracking, as well as a lithium-ion secondary battery with excellent cycle characteristics. [Modes for carrying out the invention]

[0010] The following describes in detail embodiments for carrying out the present invention (hereinafter referred to as "this embodiment"). Note that this embodiment is illustrative for explaining the present invention, and the present invention is not limited to the embodiments shown below. The present invention can be implemented in various modifications within the scope of its gist.

[0011] <Positive electrode for lithium-ion secondary batteries> The positive electrode for a lithium-ion secondary battery of this embodiment includes a current collector, a coating layer containing a conductive material and a non-aqueous binder, and an active material layer containing an active material and a binder, wherein the non-aqueous binder contains a conjugated diene copolymer having the following requirements (a) to (d), the content of the conductive material is 40.0 parts by mass or more and 99.9 parts by mass or less per 100 parts by mass of the coating layer, and the content of the non-aqueous binder is 0.1 parts by mass or more and 60.0 parts by mass or less per 100 parts by mass of the coating layer. 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 bond relative to the conjugated diene monomer unit 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. Requirement (d): The weight-average molecular weight is between 100,000 and 2,000,000.

[0012] [Current collector] The current collector used in the positive electrode of the lithium-ion secondary battery in 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, materials surface-treated with carbon, nickel, titanium, or silver can also be used. Furthermore, the current collector may be in the form of a film, sheet, foil, net, porous material, foam, or nonwoven fabric. Adhesion to the active material can also be increased by creating fine irregularities on the surface to increase the surface area. The average thickness of the current collector can be appropriately applied from 3 to 500 μm, taking into account the conductivity and total thickness of the manufactured positive electrode. As a material for the current collector, it is preferable to include aluminum from the viewpoint of price, weight, and supply stability.

[0013] [Coating layer] The coating layer of this embodiment includes at least a conductive material and a non-aqueous binder.

[0014] The coating layer may also contain other components. These other components may include a positive electrode active material, but from the viewpoint of increasing the freedom of selection for the active material layer, it is preferable that the coating layer does not contain a positive electrode active material. Furthermore, from the viewpoint of conductivity, it may contain carboxymethylcellulose as a thickening agent, but from the viewpoint of increasing the ratio of conductive components in the coating layer and improving conductivity, it is preferable that it does not contain this material.

[0015] In this embodiment, the coating layer is formed on one or both sides of the current collector, and the coating layer is a dried coating layer composition containing a conductive material and a non-aqueous binder.

[0016] In this embodiment, the coating layer is preferably 0.05 μm to 20 μm thick, more preferably 0.1 μm to 10 μm thick, even more preferably 0.1 μm to 7 μm thick, and particularly preferably 0.1 μm to 5 μm thick, from the viewpoint of balancing capacitance, adhesion, and conductivity. The thickness of the coating layer can be measured using a film thickness gauge or by observing sections with an optical microscope or scanning electron microscope.

[0017] [Conductive materials] The coating layer of this embodiment includes a conductive material.

[0018] The conductive material is used to improve the electrical conductivity and other performance aspects of the positive electrode. It can be a conductive material or conductive additive sold in battery materials and is not particularly limited, but examples include carbon materials and aluminum materials. Among these, one or more can be selected from the group consisting of natural graphite, artificial graphite, carbon black including acetylene black and Ketjenblack, carbon nanotubes, carbon fibers, and aluminum hydroxide, and two or more conductive materials may be used in combination. From the viewpoint of conductivity, the conductive material preferably contains at least one selected from the group consisting of carbon black, graphite, carbon nanotubes, and aluminum hydroxide. Furthermore, the carbon black is preferably acetylene black.

[0019] The conductive material content is 40.0 parts by mass or more and 99.9 parts by mass or less per 100 parts by mass of the coating layer. From the viewpoint of improving conductivity, 45.0 parts by mass or more is preferred, 50.0 parts by mass or more is more preferred, 55.0 parts by mass or more is even more preferred, and 90.0 parts by mass or more is particularly preferred. On the other hand, from the viewpoint of adhesion between the current collector and the coating layer, 99.5 parts by mass or less is preferred, 99.0 parts by mass or less is more preferred, and 98.5 parts by mass or less is even more preferred.

[0020] [Non-aqueous binder] The coating layer of this embodiment includes a non-aqueous binder, the non-aqueous binder includes a conjugated diene copolymer having 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 bonded to the conjugated diene monomer unit 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. Requirement (d): The weight-average molecular weight is between 100,000 and 2,000,000.

[0021] Furthermore, components commonly used as non-aqueous binders for coating layers may be used in combination with conjugated diene copolymers, such as polyvinylidene fluoride, polyacrylonitrile, polyimidoamide, and polymethyl methacrylate.

[0022] The lower limit of the non-aqueous binder content is 0.1 parts by mass or more per 100 parts by mass of the coating layer. From the viewpoint of improving adhesion between the coating layer and the current collector and retention of the conductive material, it is more preferable to have 0.5 parts by mass or more, even more preferable to have 1.0 part by mass or more, even more preferable to have 1.5 parts by mass or more, and particularly preferable to have 2.0 parts by mass or more, per 100 parts by mass of the coating layer. On the other hand, the upper limit of the non-aqueous binder content is 60.0 parts by mass or less per 100 parts by mass of the coating layer. From the viewpoint of improving electronic conductivity and ionic conductivity, it is preferable to have 55.0 parts by mass or less, more preferable to have 50.0 parts by mass or less, even more preferable to have 45.0 parts by mass or less, and particularly preferable to have 10.0 parts by mass or less.

[0023] [Conjugated diene copolymer] In the positive electrode for a lithium-ion secondary battery of this embodiment, the conjugated diene copolymer contained in the non-aqueous binder has the above requirements (a) to (d). The conjugated diene copolymer includes a polymer having structural units having a chemical structure derived from a conjugated diene compound (monomer) (hereinafter also referred to as "conjugated diene monomer units"), a polymer having structural units having a chemical structure derived from an aromatic vinyl compound (monomer) (hereinafter also referred to as "aromatic vinyl monomer units"), and a copolymer containing conjugated diene monomer units and aromatic vinyl monomer units. Furthermore, these hydrogenated substances may also be included.

[0024] Examples of 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, 1,3-butadiene and isoprene are preferred from the viewpoint of ease of industrial availability, and 1,3-butadiene is particularly preferred. These may be used individually or in combination of two or more types.

[0025] 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 ease of industrial availability. These may be used individually or in combination of two or more types.

[0026] The conjugated diene copolymer of this embodiment is characterized in that the so-called microstructure (content of aromatic vinyl monomer units, amount of 1,2-vinyl bonds, etc.), molecular weight, viscosity, and hydrogenation rate of the copolymer of the conjugated diene compound and the aromatic vinyl compound are controlled.

[0027] (Content of aromatic vinyl monomer units) The lower limit of the aromatic vinyl monomer unit content in the conjugated diene copolymer of this embodiment is 6% by mass or more. From the viewpoint of adhesion between the coating layer and the current collector, 10% by mass or more is preferred, 20% by mass or more is more preferred, 25% by mass or more is even more preferred, 30% by mass or more is even more preferred, and 35% by mass or more is particularly preferred. Furthermore, when dissolving in an organic solvent for coating, if the organic solvent used has high polarity, for example, when using N-methylpyrrolidone or tetralin, from the viewpoint of solubility, the aromatic vinyl monomer unit content is preferably 24% by mass or more, more preferably 26% by mass or more, even more preferably 28% by mass or more, and particularly preferred to be 30% by mass or more. On the other hand, the upper limit of the aromatic vinyl monomer unit content in the conjugated diene copolymer of this embodiment is 80% by mass or less. From the viewpoint of bale pulverability and binder flexibility, 70% by mass or less is preferred, 66% by mass or less is more preferred, 60% by mass or less is even more preferred, and 55% by mass or less is particularly preferred. The content of aromatic vinyl monomer units can be measured by 1H-NMR. Furthermore, the content of aromatic vinyl monomer units can be controlled within the above numerical range by adjusting the amount of aromatic vinyl compound added during polymerization.

[0028] (Amount of 1,2 vinyl bond) In this embodiment, the amount of 1,2-vinyl bonds in the conjugated diene copolymer is the molar ratio of the amount of 1,2-vinyl bonds based on the content of conjugated diene monomer units. The lower limit of the amount of 1,2-vinyl bonds in the conjugated diene copolymer in this embodiment is 10 mol% or more. From the viewpoint of polymerization stability and flexibility when used as a binder, 11 mol% or more is preferred, 12 mol% or more is more preferred, and 13 mol% or more is even more preferred. On the other hand, the upper limit of the amount of 1,2-vinyl bonds in the conjugated diene copolymer in this embodiment is 60 mol% or less. From the viewpoint of durability when used as a binder, 55 mol% or less is preferred, 50 mol% or less is more preferred, and 45 mol% or less is even more preferred. The amount of 1,2-vinyl bond can be measured by 1H-NMR. The amount of 1,2-vinyl bond can be controlled within the above numerical range by adjusting the reaction start temperature, reaction end temperature, and the type and amount of polar substance added during polymerization.

[0029] (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 1,4-cis bonds to 1,4-trans bonds when the total content of 1,4-cis bonds and 1,4-trans bonds in the conjugated diene monomer unit is taken as 100% by mass. The ratio of 1,4-cis bonds to 1,4-trans bonds in the conjugated diene copolymer of this embodiment is 30:70 to 50:50. From the viewpoint of balancing the adhesive strength between the conductive material and the binder and solubility in polar solvents, the ratio of 1,4-cis bonds to 1,4-trans bonds is preferably 32:68 to 48:52, and more preferably 35:65 to 46:54. The content and ratio of 1,4-cis and 1,4-trans bonds can be measured by 13C-NMR. To control the ratio of 1,4-cis bonds to 1,4-trans bonds as described above, one method is to obtain a conjugated diene copolymer by living anionic polymerization using a lithium-based polymerization initiator. On the other hand, conjugated diene polymers obtained by coordination polymerization tend to have a high proportion of 1,4-cis bonds, while conjugated diene copolymers obtained by emulsion polymerization tend to have a low proportion of 1,4-cis bonds.

[0030] In the case where the conjugated diene copolymer of this embodiment is a copolymer consisting of butadiene and styrene, the amount of 1,2-vinyl bond and the styrene content can be measured by the method described in ISO 21561-2005, and specifically, by the method described in the examples.

[0031] In this embodiment, the content of aromatic vinyl monomer blocks in the conjugated diene copolymer, as measured according to the osmium tetroxide decomposition method described in IMKolthoff, et al., J. Polym. Sci. 1, 429 (1946), is preferably 5.0% by mass or more, more preferably 8.0% by mass or more, even more preferably 11.0% by mass or more, and particularly preferably 20.0% by mass or more, relative to the total amount of the conjugated diene copolymer, from the viewpoint of adhesive strength between the conductive material and the binder and durability when used as a binder. On the other hand, from the viewpoint of pulverability of the bale, it is preferably 40.0% by mass or less, even more preferably 37.0% by mass or less, and particularly preferably 35.0% by mass or less. The content of aromatic vinyl monomer blocks can be controlled by the timing of the addition of aromatic vinyl compounds during polymerization, the amount and type of polar substances added, and other factors.

[0032] The blocking rate of aromatic vinyl monomer blocks in the conjugated diene copolymer of this embodiment is not particularly limited, but is preferably 15.0% to 85.0%, more preferably 20.0% to 80.0%, and even more preferably 25.0% to 75.0%. When the blocking rate of aromatic vinyl monomer blocks is within the above range, it tends to exhibit excellent resistance to increased adhesion and moldability.

[0033] The blocking rate can be calculated by dividing the content of the aromatic vinyl monomer blocks mentioned above by the content of the aromatic vinyl monomer in the conjugated diene copolymer, which can be measured by 1H-NMR as described in the examples. Here, "aromatic vinyl monomer block" refers to the content of aromatic vinyl monomer blocks measured by the method described above. Here, the blocking rate of aromatic vinyl monomer blocks refers to the ratio of the content of vinyl aromatic monomer blocks in the conjugated diene copolymer to the total content of all vinyl aromatic monomer units in the conjugated diene copolymer. The blocking rate is as described in the examples. 1 It can be measured by 1H-NMR.

[0034] The conjugated diene copolymer of this embodiment may have aliphatic double bonds based on conjugated diene monomer units that have been hydrogenated. In the hydrogenated copolymer composition of this embodiment, the conjugated diene monomer units are incorporated into the copolymer in the form of 1,2-bonds, 3,4-bonds, or 1,4-bonds. 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, total hydrogenation rate refers to the proportion of hydrogen bonds formed to aliphatic double bonds (1,2-bonds, 3,4-bonds, and 1,4-bonds) based on conjugated diene monomer units in a conjugated diene copolymer. The total hydrogenation rate is, 1 It can be calculated from the spectral reduction rate of the unsaturated bond region of the spectrum obtained by measuring 1H-NMR. Specifically, it can be measured by the method described in the examples below.

[0035] The 1,2-hydrogenation ratio of the conjugated diene copolymer in this 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 ratio is not particularly limited and may be 100% or less. The 1,2-hydrogenation ratio refers to the ratio of hydrogen bonds to 1,2-bonds based on conjugated diene monomer units in a conjugated diene copolymer. 1,2 Hydrogenation ratio is 1 It can be calculated from the spectral reduction rate of the unsaturated bond region of the spectrum obtained by measuring 1H-NMR.

[0036] (Weight average molecular weight) The weight-average molecular weight (Mw) of the conjugated diene copolymer in this embodiment is between 100,000 and 2,000,000. From the viewpoint of adhesive strength and conductive material retention performance, it is preferably 120,000 or more, more preferably 140,000 or more, and even more preferably 200,000 or more. On the other hand, from the viewpoint of solubility and coating properties, it is preferably 1,800,000 or less, more preferably 1,600,000 or less, and even more preferably 1,200,000 or less. The weight-average molecular weight (Mw) of conjugated diene copolymers can be measured using gel permeation chromatography (GPC).

[0037] The molecular weight distribution (Mw / Mn) of the conjugated diene copolymer in this embodiment, based on the weight-average molecular weight (Mw) and number-average molecular weight (Mn), is not particularly limited. However, from the viewpoint of bale pulverability, the lower limit of the molecular weight distribution (Mw / Mn) is preferably 1.02 or higher, more preferably 1.04 or higher, and even more preferably 1.06 or higher. On the other hand, from the viewpoint of solution viscosity and coating properties, the upper limit of the molecular weight distribution (Mw / Mn) is preferably 3.0 or lower, more preferably 2.5 or lower, even more preferably 2.0 or lower, and particularly preferably 1.8 or lower. The number-average molecular weight (Mn) of conjugated diene copolymers can be measured using gel permeation chromatography (GPC).

[0038] (Moony viscosity) The Mooney viscosity of the conjugated diene copolymer of this embodiment at 100°C is not particularly limited, but the lower limit of the Mooney viscosity at 100°C is preferably 30 or higher, more preferably 35 or higher, and even more preferably 40 or higher, from the viewpoint of handling the bale. On the other hand, from the viewpoint of pulverability of the bale and solubility in polar solvents, the upper limit of the Mooney viscosity at 100°C is preferably 200 or lower, more preferably 180 or lower, and particularly preferably 160 or lower. Mooney viscosity can be measured specifically by the method described in the examples below.

[0039] (Coupling) The conjugated diene copolymer of this embodiment may be a conjugated diene copolymer obtained by performing a coupling reaction using a two- or more functional reactive compound (hereinafter also referred to as a "coupling agent") on the active end of the conjugated diene copolymer. In the coupling reaction step, a coupling agent is applied to one end of the active end of the conjugated diene copolymer to cause a coupling reaction and obtain the conjugated diene copolymer.

[0040] Examples of coupling agents include, but are not limited to, those having one or more functional groups such as epoxy groups, carbonyl groups, carboxylic acid ester groups, carboxylic acid amide groups, acid anhydride groups, phosphate ester groups, phosphite ester groups, epithio groups, thiocarbonyl groups, thiocarboxylic acid ester groups, dithiocarboxylic acid ester groups, thiocarboxylic acid amide groups, imino groups, ethyleneimino groups, halogen groups, alkoxysilyl groups, isocyanate groups, thioisocyanate groups, conjugated diene groups, and aryl vinyl groups. Among coupling agents, nitrogen atom-containing coupling agents can also be used as modifying agents as described later.

[0041] Furthermore, coupling agents are not limited to the following, but examples include halogenated silane compounds such as silicon tetrachloride, silicon tetrabromide, silicon tetraiodide, monomethyltrichlorosilicon, monoethyltrichlorosilicon, monobutyltrichlorosilicon, monohexyltrichlorosilicon, monomethyltribromosilicon, and bistrichlorosilylethane, as well as alkoxyhalogenated silane compounds such as monochlorotrimethoxysilane, monobromotrimethoxysilane, dichlorodimethoxysilane, dibromodimethoxysilane, trichloromethoxysilane, and tribromomethoxysilane.

[0042] Furthermore, although not limited to the following, examples include 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-propaneamine, N-(1,3-dimethylbutylidene)-3-(tributoxysilyl)-1-propaneamine, N-(1-methylpropyridene)-3-(triethoxysilyl)-1-propaneamine, N-ethylidene-3-(triethoxysilyl)-1-propaneamine, and N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole.

[0043] Furthermore, although not limited to the following, examples include 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)-imidazolidine-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) {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)-imidazolidine-1-yl]-ethyl}dimethylamine, 5-(3-trimethoxysilyl-propyl)-1,3-bis-(2-methoxyethyl (Lu)-Hexahydropyrimidine, 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,Examples include 3-bis(tripropylsilyl)hexahydropyrimidine.

[0044] Furthermore, although not limited to the following, for example, [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- Other examples include decamethyleneimino)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, and N,N-bis(trimethylsilyl)aminopropylmethyldiethoxysilane.

[0045] Furthermore, although not limited to the following, examples include tetraglycidylmetoxylendiamine, tetraglycidylaminodiphenylmethane, tetraglycidyl-p-phenylenediamine, diglycidylaminomethylcyclohexane, tetraglycidyl-1,3-bisaminomethylcyclohexane, etc. In addition, although not limited to the following, examples include isocyanate compounds such as 2,4-tolylenediisocyanate, 2,6-tolylenediisocyanate, diphenylmethanediisocyanate, diphenylethanediisocyanate, and 1,3,5-benzenetriisocyanate.

[0046] Furthermore, although not limited to the following, examples include 3-(4-methylpiperazine-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 [Methoxymethylsilyl)propyl]-3-methylhexahydropyrimidine, 3-[3-(tributoxysilyl)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 Hydropyrimidine-ylethyl)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-(trimethoxysilyl) Examples include 1-1,3-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.

[0047] (degeneration) The conjugated diene copolymer in this embodiment may be modified. Furthermore, modifying a conjugated diene copolymer with a nitrogen-containing compound is referred to as modification. Modification methods are not particularly limited, but include methods using polymerization initiators containing nitrogen-containing compounds, methods using nitrogen-containing compounds as polymerization monomers, methods using the aforementioned nitrogen atom-containing coupling agents, methods reacting non-coupling nitrogen-containing compounds at the reaction termination end, and methods modifying the double bond of the polymerized conjugated diene copolymer by reacting it with a nitrogen-containing compound.

[0048] Polymerization initiators containing nitrogen-containing compounds include, but are not limited to, the following, examples: 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-imidazole-2-one.

[0049] (Degeneration rate) In this specification, unless otherwise specified, "denaturation rate" refers to the mass ratio of the polymer having nitrogen atom-containing functional groups to the total amount of the conjugated diene copolymer. For example, when a nitrogen atom-containing modifying agent is reacted with the terminal end of a polymer, the mass ratio of the polymer having nitrogen atom-containing functional groups due to the nitrogen atom-containing modifying agent 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 nitrogen atom-containing modifiers. On the other hand, when a polymer is branched using a branching agent containing nitrogen atoms, the resulting copolymer will also have nitrogen-containing functional groups. Therefore, this branched polymer will also be counted as a polymer with nitrogen-containing functional groups when calculating the modification rate. In other words, in this specification, a 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 modifying agent and a branched polymer due to a nitrogen atom-containing branching agent, and the total mass ratio of these is the "modification rate". In this embodiment, the conjugated diene copolymer may be modified from the viewpoint of the dispersibility of the active material and the adhesion between the binder and the active material. In the case of a modified conjugated diene copolymer, it is preferable that the modification rate (hereinafter also simply referred to as "modification rate") measured by column adsorption GPC is 60% or more and 99% or less.

[0050] The denaturation rate of the conjugated diene copolymer in this embodiment can be measured, for example, by chromatography that can separate the functional group-containing denatured component from the undenatured component. One method using chromatography is to use a gel permeation chromatography column packed with a polar substance such as silica that adsorbs specific functional groups, and quantify the non-adsorbed components using an internal standard for comparison (column adsorption GPC method). More specifically, the denaturation rate can be determined by measuring the amount adsorbed onto the silica column from the difference between the chromatogram measured on a polystyrene gel column and the chromatogram measured on a silica column for a sample solution containing the sample and a low molecular weight internal standard polystyrene. More specifically, the rate of denaturation can be measured by the method described in the examples. The modification rate of the conjugated diene copolymer in this embodiment can be controlled to the above numerical range by, for example, adjusting the amount of modification agent added and the reaction method.

[0051] The conjugated diene copolymer of this embodiment may contain zinc, aluminum, copper, and / or iron. Examples of zinc, aluminum, copper, and / or iron may be derived from polymerization catalysts or hydrogenation catalysts.

[0052] The content of zinc, aluminum, copper, and iron in the conjugated diene copolymer is not particularly limited, but from the viewpoint of the cycle characteristics of secondary batteries, 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 the content of zinc, aluminum, copper, and iron in the conjugated diene copolymer may be below the detection limit, and is preferably 0 ppm or more. These metals can be measured by the method described in the examples below, and can be controlled to the above numerical range by adjusting the type and amount of hydrogenation catalyst, deashification, or the conditions of the solvent removal process described below.

[0053] 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 the content of each of the zinc, aluminum, copper, and iron in the conjugated diene copolymer may be below the detection limit, and is preferably 0 ppm or more.

[0054] Other metals included in the conjugated diene copolymer of this embodiment include lithium and titanium. The respective content of lithium and titanium in the conjugated diene copolymer is not particularly limited, but from the viewpoint of the cycle characteristics of the secondary battery, it is 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 limit of the respective content of lithium and titanium in the conjugated diene copolymer may be below the detection limit, and is preferably 0 ppm or more.

[0055] [Method for producing conjugated diene copolymers] 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 coupling agent and modification agent described above, 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 and modification step.

[0056] (Polymerization process) At least an organic monolithium compound can be used as the polymerization initiator in the polymerization process. 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, compounds having a nitrogen-lithium bond, and compounds having a tin-lithium bond, depending on the bonding mode between the organic group and its lithium. The amount of organic monolithium compound used as a polymerization initiator is preferably determined by the structure of the target conjugated diene copolymer and the molecular weight of the conjugated diene copolymer. The degree of polymerization is related to the amount of monomers used, such as conjugated diene compounds, relative to the amount of polymerization initiator used. Therefore, to increase the molecular weight, it is best to adjust the amount of polymerization initiator used to decrease it, and to decrease the molecular weight, it is best to adjust the amount of polymerization initiator used to increase it.

[0057] As for the organic monolithium compound, from the viewpoint of being used as one method for introducing a nitrogen atom into a conjugated diene copolymer, it may be an alkyllithium compound having a substituted amino group, or a dialkylaminolithium compound. In this case, a conjugated diene copolymer having a nitrogen atom consisting of an amino group at the polymerization initiation end can be obtained.

[0058] A substituted amino group is an amino group that either lacks active hydrogen or has a structure in which active hydrogen is protected. Alkyl lithium compounds having an amino group that does not possess active hydrogen include, but are not limited to, 3-dimethylaminopropyllithium, 3-diethylaminopropyllithium, 4-(methylpropylamino)butyllithium, and 4-hexamethyleneiminobutyllithium. Alkyllithium compounds having an amino group with a structure that protects active hydrogen include, but are not limited to, 3-bistrimethylsilylaminopropyllithium and 4-trimethylsilylmethylaminobutyllithium.

[0059] Examples of dialkylaminolithium 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-lithiazacyclooctane, 6-lithio-1,3,3-trimethyl-6-azabicyclo[3.2.1]octane, and 1-lithio-1,2,3,6-tetrahydropyridine.

[0060] These organomonolithium compounds having substituted amino groups can also be used as solubilized oligomeric organomonolithium compounds by reacting small amounts of polymerizable monomers, such as 1,3-butadiene, isoprene, and styrene.

[0061] From the viewpoint of ease of industrial availability and ease of control of polymerization reactions, alkyllithium compounds are preferred as organic monolithium compounds. In this case, copolymers having an alkyl group at the polymerization initiation end can be obtained. The alkyllithium compound is not limited to the following, but examples include n-butyllithium, sec-butyllithium, tert-butyllithium, n-hexyllithium, benzyllithium, phenyllithium, and stilbenilithium. As the alkyllithium compounds, n-butyllithium and sec-butyllithium are preferred from the viewpoint of ease of industrial availability and ease of control of the polymerization reaction. These organic monolithium compounds may be used individually or in combination of two or more. They may also be used in combination with other organometallic compounds.

[0062] Examples of the aforementioned 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. Compounds of alkaline earth metal alkoxides, sulfonates, carbonates, and amides are also included. Examples of organomagnesium compounds include dibutylmagnesium and ethylbutylmagnesium. Examples of other organometallic compounds include organoaluminum compounds.

[0063] In the polymerization process, the polymerization reaction mode is not limited to the following, but examples include batch mode (also called "batch reaction") and continuous polymerization reaction mode. In a continuous reactor, one or more connected reactors can be used. Continuous reactors include, for example, tank-type or tubular-type reactors equipped with stirrers. Preferably, monomers, inert solvents, and polymerization initiators are continuously fed into the reactor, a polymer solution containing the polymer is obtained within the reactor, and the polymer solution is continuously discharged. Batch reactors, for example, are tank-type reactors equipped with stirrers. In a batch reactor, monomers, an inert solvent, and a polymerization initiator are preferably fed into the reactor, and monomers are added continuously or intermittently during polymerization as needed, to obtain a polymer solution containing the polymer in the reactor, and the polymer solution is discharged after polymerization is complete.

[0064] In this embodiment, the polymerization step of the conjugated diene copolymer is preferably carried out in an inert solvent. The inert solvent is not limited to the following, but examples include hydrocarbon solvents such as saturated hydrocarbons and aromatic hydrocarbons. Specific hydrocarbon solvents are not limited to the following, but examples include aliphatic hydrocarbons such as butane, pentane, hexane, and heptane; alicyclic hydrocarbons such as cyclopentane, cyclohexane, methylcyclopentane, and methylcyclohexane; and hydrocarbons consisting of aromatic hydrocarbons such as benzene, toluene, and xylene, and mixtures thereof. Treating the impurities, such as allenes and acetylenes, with organometallic compounds before the polymerization reaction tends to yield conjugated diene copolymers with high concentrations of active ends, and is preferable because it tends to yield conjugated diene copolymers with high coupling and modification rates.

[0065] In the polymerization process, polar substances (polar compounds) may be added. This allows for random copolymerization of aromatic vinyl compounds with conjugated diene compounds, and polar substances tend to be used as vinylizing agents to control the microstructure of the conjugated diene portion. They also tend to be effective in accelerating 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-butyrate, sodium-tert-butyrate, and sodium amylate; and phosphine compounds such as triphenylphosphine. These polar substances may be used individually or in combination of two or more.

[0066] The amount of polar substance used is not particularly limited and can be selected according to the purpose, but it is preferable to use 0.01 moles or more and 10 moles or less per mole of polymerization initiator. Such polar substances (vinylating agents) can be used in appropriate amounts depending on the desired amount of 1,2-vinyl bonds as modifiers of the microstructure of the conjugated diene moiety in conjugated diene copolymers. Many polar substances also have an effective randomization effect in copolymerization of conjugated diene compounds and aromatic vinyl compounds, and tend to be used as modifiers for adjusting the distribution of aromatic vinyl compounds and the amount of styrene block.

[0067] As a method for randomizing the conjugated diene compound and aromatic vinyl compound, for example, as described in Japanese Patent Publication No. 59-140211, a copolymerization reaction may be initiated with the entire amount of styrene and a portion of 1,3-butadiene, and the remaining 1,3-butadiene may be added intermittently during the copolymerization reaction.

[0068] The polymerization temperature in the polymerization process is preferably the temperature at which living anionic polymerization proceeds. From the viewpoint of productivity, it is more preferably 0°C or higher, even more preferably 120°C or lower, and even more preferably 30°C to 100°C. Within this range, it tends to be possible to ensure a sufficient amount of denaturing agent reacting with the active ends after polymerization is complete.

[0069] (Coupling process, modification process, hydrogenation process) The active ends of the conjugated diene copolymer obtained through the polymerization step described above, and optionally a branching step using a predetermined branching agent, may be subjected to a coupling reaction using the aforementioned coupling agent or a modification reaction using a modification agent containing a nitrogen atom group. When a nitrogen atom-containing coupling agent is used, the coupling reaction and modification reaction proceed simultaneously. Furthermore, a hydrogenation step may be performed as appropriate.

[0070] (Inactivator addition process, neutralizing agent addition process) In the method for producing the conjugated diene copolymer of this embodiment, a deactivator, neutralizing agent, etc. may be added to the polymer solution as needed. Examples of inactivators include, but are not limited to, water; and alcohols such as methanol, ethanol, and isopropanol. Examples of neutralizing agents include, but are not limited to, carboxylic acids such as stearic acid, oleic acid, and versatic acid (a highly branched mixture of carboxylic acids with 9 to 11 carbon atoms, mainly around 10); aqueous solutions of inorganic acids; and carbon dioxide.

[0071] (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 stabilizers are not limited to those listed below, but any known ones can be used. For example, 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)propinate, and 2-methyl-4,6-bis[(octylthio)methyl]phenol can be used.

[0072] (Solvent removal process) In the method for producing the conjugated diene copolymer of this embodiment, known methods can be used to obtain the conjugated diene copolymer from the polymer solution. These methods are not limited to the following, but include, for example, a method in which the solvent is separated by steam stripping or the like, the polymer is filtered off, and then the solvent is removed and the polymer is dried to obtain the polymer; a method in which the solution is concentrated in a flushing tank and then defolatable using a vent extruder or the like; and a method in which the solution is directly defolatable using a drum dryer or the like.

[0073] (Bale molded body) The conjugated diene copolymer of this embodiment can be formed into a bale. A bale molded body is a rectangular parallelepiped-shaped rubber, and in terms of weight, it 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, from the viewpoint of transportability. A bale molded body is a mass of conjugated diene copolymer obtained by compression molding. For example, it can be obtained by extruding a conjugated diene copolymer with an extruder, cutting it to obtain crumbs, and then compression molding the crumbs.

[0074] The moisture content of the bale molded body of the 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. Because the moisture content of the bale molded body of the 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, and therefore corrosion of the positive electrode current collector due to the reaction between alkali metal compounds and moisture tends to be suppressed.

[0075] [Binder composition for coating layer] The binder composition for the coating layer in this embodiment comprises a conductive material, a non-aqueous binder, and a solvent, and may also contain other components to the extent that they do not impair adhesion and conductivity. From the viewpoint of improving the freedom of selection of the active material layer, it is preferable that the binder composition for the coating layer does not contain the positive electrode active material. Furthermore, from the viewpoint of conductivity, it may contain carboxymethylcellulose as a thickening agent, but from the viewpoint of increasing the ratio of conductive components in the binder composition for the coating layer and improving the conductivity when it is used as a coating layer, it is preferable that it is not included. The conductive materials mentioned above can be used. As a non-aqueous binder, the ones mentioned above can be used.

[0076] When the solid content of the coating layer binder composition is 100 parts by mass, it is preferable that the conductive material is 80 parts by mass or more and 99 parts by mass or less, and the non-aqueous binder is 1 part by mass or more and 20 parts by mass or less.

[0077] The binder composition for the coating layer may be applied after dispersing a non-aqueous binder and a conductive material in a solvent. A polar solvent is preferred as the solvent, and N-methylpyrrolidone, γ-butyrolactone, tetrahydrofuran, and toluene are preferred from the viewpoint of solubility and volatility.

[0078] The coating layer of this embodiment can be obtained by applying the above-mentioned binder composition for the coating layer to the current collector and then drying it.

[0079] [Active material layer] The active material layer of this embodiment preferably comprises an active material and a binder for the active material layer. The active material layer may further contain a conductive additive.

[0080] In this embodiment, the active material layer is formed on one surface of the coating layer, and the active material layer is formed from a binder composition for the active material layer, which includes the active material and the binder for the active material layer, after drying. Therefore, when a coating layer is formed on one surface of the current collector, the layers are formed in the order of current collector / coating layer / active material layer.

[0081] [Active material] All commonly used positive electrode active materials can be used as the active material for the positive electrode of the lithium-ion secondary battery in this embodiment. The positive electrode active material is not limited to the following, but is preferably inorganic particles. For example, lithium manganese oxide represented by LiMnO2, LiMn2O4, and Li2Mn2O4; lithium cobalt oxide represented by LiCoO2; lithium nickel oxide represented by LiNiO2; lithium nickel manganese cobalt oxide represented by NMC532 and NMC811; lithium iron oxide represented by Li5FeO4; lithium iron phosphate represented by LiFePO4 and LiMnFePO4, or lithium composite oxides combining these. Among these, it is preferable that the active material contains at least one selected from the group consisting of lithium iron oxide, lithium magnesium iron phosphate, and lithium iron phosphate.

[0082] The lower limit of the active material content 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, per 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 conductive additives described later, the upper limit of the active material content 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.

[0083] [Conductive additive] In the positive electrode for the lithium-ion secondary battery of this embodiment, the active material layer may further contain a conductive additive. The conductive additive can be used to improve the electrical conductivity and other performance of the positive electrode and is not particularly limited, but examples include carbon materials and aluminum materials. Among these, one or more can be used selected from the group consisting of natural graphite, artificial graphite, carbon black including acetylene black and Ketjenblack, carbon nanotubes, carbon fibers, and aluminum hydroxide, and two or more conductive additives may be used in combination. From the viewpoint of conductivity, it is preferable that the conductive additive contains at least one selected from the group consisting of carbon black, graphite and carbon nanotubes, and aluminum hydroxide. Furthermore, it is preferable that the carbon black is acetylene black.

[0084] The amount of conductive additive may be 1 to 10 parts by mass per 100 parts by mass of the active material layer, preferably 1 to 8 parts by mass, and more preferably 1 to 6 parts by mass, from the viewpoint of improving electrical contact between positive electrode active materials.

[0085] [Binder for the active material layer] In the positive electrode for lithium-ion secondary batteries of this embodiment, the binder for the active material layer can be any binder used for the positive electrode active material layer and is not particularly limited. For example, fluorine-based binder materials such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and their derivatives, as well as the conjugated diene copolymers, hydrogenated nitrile rubber, and nitrile rubber mentioned above, can be used. From the viewpoint of balancing adhesion, crack resistance, and conductivity as a binder for the active material layer, it is preferable to include at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene and its derivatives, and the conjugated diene copolymers mentioned above. More preferably, it includes polyvinylidene fluoride.

[0086] [Binder composition for active material layer] In this embodiment, the binder composition for the positive electrode active material layer preferably contains the above-mentioned active material, the above-mentioned binder for the active material layer, a solvent, and a conductive material. When the binder composition is 100 parts by mass, it is preferable that the active material is 80 parts by mass or more and 99 parts by mass or less, the conductive additive is 1 part by mass or more and 10 parts by mass or less, and the binder for the active material layer is 1 part by mass or more and 12 parts by mass or less. The binder composition for the positive electrode active material layer may be applied after dispersing the binder for the active material layer, the active material, and the conductive material in a solvent. A polar solvent is preferred as the solvent, and N-methylpyrrolidone, γ-butyrolactone, tetrahydrofuran, and toluene are preferred from the viewpoint of solubility and volatility. The active material layer of this embodiment can be obtained by coating the above-mentioned binder composition for the active material layer and then drying it.

[0087] [Manufacturing method for positive electrodes for lithium-ion secondary batteries] The positive electrode for the lithium-ion secondary battery of this embodiment is not particularly limited, but can be obtained, for example, by applying the coating layer binder composition of this embodiment to a current collector for the positive electrode, heating and drying it, and then applying the active material layer binder composition on the coating layer, heating and drying it, and press molding it. In the positive electrode for the lithium-ion secondary battery of this embodiment, the coating layer is formed on one or both sides of the current collector, and the active material layer is formed on one or both sides of the coating layer.

[0088] [Lithium-ion rechargeable battery] The lithium-ion secondary battery of this embodiment includes at least the positive electrode for the lithium-ion battery of this embodiment. It may also include 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 solution. The lithium-ion secondary battery of this embodiment only needs to include at least the positive electrode for the lithium-ion battery of this embodiment. Various known components can be used as appropriate for the negative electrode, electrolyte solution, and other energy storage device components depending on the type of energy storage device. The lithium-ion secondary battery of this embodiment preferably further includes an electrolyte, and a solid electrolyte may be used as the electrolyte, but it is preferable to use an electrolyte solution.

[0089] [Method of manufacturing lithium-ion secondary batteries] The method for manufacturing the lithium-ion secondary battery of this embodiment is not particularly limited, but examples include a method in which the negative electrode and the positive electrode of this embodiment are placed facing each other via a separator, an electrolyte is injected, and then sealed. The negative electrode and electrolyte are not particularly limited, and those applicable to lithium-ion secondary batteries can be appropriately selected and used. For example, as the electrolyte, an electrolyte such as LiClO4, LiBF4, or LiPF6 dissolved in an organic solvent can be used. The organic solvent is not particularly limited, but examples include ethers, ketones, lactones, nitriles, amines, amides, carbonates, chlorinated hydrocarbons, etc. Representative examples include tetrahydrofuran, acetonitrile, butyronitrile, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, dipropyl carbonate, dimethoxyethane, diethoxyethane, dimethyl sulfoxide, vinylene carbonate Examples include sulfolane, gamma-butyrolactone, propylene sulfite, N-methyl-2-pyrrolidinone, 2-methyltetrahydrofuran, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, methyl propionate, ethyl propionate, etc., which can be used individually or as a mixture of two or more.

[0090] The lithium-ion secondary battery in this embodiment can also be used as a battery pack in which multiple lithium-ion secondary batteries are connected in series or parallel. From the viewpoint of extending the lifespan of the battery pack by suppressing the degradation of the binder layer, the operating voltage range per battery is preferably 4.0V or less, more preferably 3.8V or less, and particularly preferably 3.6V or less. The lower limit of the operating voltage range per battery is not particularly limited, but is preferably 2.0V or more. [Examples]

[0091] The embodiment will be described in more detail below with reference to specific examples and comparative examples, but this embodiment is not limited in any way to the following examples and comparative examples. Here, specific examples of conjugated diene copolymers are referred to as "Preparation Examples," and specific examples of positive electrodes for lithium-ion secondary batteries using conjugated diene copolymers, etc., are referred to as "Examples" and "Comparative Examples." The various physical properties in the preparation examples, examples, and comparative examples were measured by the methods described below.

[0092] [Physical property measurement method] [Weight average molecular weight (Mw)] Using a GPC analyzer with three columns packed with polystyrene gel, chromatograms were measured, and the peak top molecular weight of the peak with the largest area was determined based on a calibration curve using standard polystyrene. The specific measurement conditions are shown below. The following 20 μL of measurement solution was injected into the GPC measuring device and measurements were performed. (Measurement conditions) Device: Tosoh Corporation product name "HLC-8320GPC" Eluent: 5 mmol / L tetrahydrofuran (THF) containing triethylamine Guard column: Product name "TSKguardcolumn SuperH-H" manufactured by Tosoh Corporation. Separation column: A combination of TSKgel SuperH5000, TSKgel SuperH6000, and TSKgel SuperH7000, manufactured by Tosoh Corporation, linked together in that order. Oven temperature: 40℃ Flow rate: 0.6mL / min Detector: RI detector (product name "HLC8020" manufactured by Tosoh Corporation) Measurement solution: A measurement solution prepared by dissolving 10 mg of the sample in 20 mL of THF.

[0093] [Degeneration rate] The denaturation rate was measured using the column adsorption GPC method, taking advantage of the property that the denatured polymer is adsorbed onto the column, as follows. The amount of adsorption onto the silica-based column was determined by the difference between the chromatogram obtained by measuring the sample and a sample solution containing low molecular weight internal standard polystyrene using a polystyrene-based gel column and the chromatogram obtained by measuring it using a silica-based gel column, and the denaturation rate was calculated. (GPC measurement conditions using polystyrene columns) The GPC measurement conditions using a polystyrene column are shown below. 20 μL of the measurement solution listed below was injected into the GPC measuring device and the measurement was performed. Device: Tosoh Corporation product name "HLC-8320GPC" Eluent: THF containing 5 mmol / L triethylamine Guard column: Product name "TSKguardcolumn SuperH-H" manufactured by Tosoh Corporation. Column: A combination of the product names "TSKgel SuperH5000", "TSKgel SuperH6000", and "TSKgel SuperH7000" manufactured by Tosoh Corporation, in that order. Oven temperature: 40℃ Flow rate: 0.6mL / min Detector: RI detector (Tosoh Corporation HLC8020) Measurement solution: 10 mg of the sample and 5 mg of standard polystyrene were dissolved in 20 mL of THF to prepare the sample solution. (GPC measurement conditions using silica-based columns) The GPC measurement conditions using a silica-based column are shown below. 50 μL of the measurement solution listed below was injected into the GPC measuring device and the measurement was performed. Device: Tosoh Corporation product name "HLC-8320GPC" Eluent:THF Guard column: DIOL 4.6×12.5mm 5micron, manufactured by GL Sciences Co., Ltd. Separation column: Agilent Technologies' Zorbax PSM-1000S, PSM-300S, and PSM-60S columns linked together in that order. Oven temperature: 40℃ Flow rate: 0.5mL / min Detector: RI detector (Tosoh Corporation HLC8020)

[0094] (Method for calculating the degeneration rate): The denaturation rate (%) was calculated using the following formula, with the total peak area of ​​the chromatogram using a polystyrene column set to 100, the peak area of ​​the sample being P1, and the peak area of ​​standard polystyrene being P2. The total peak area of ​​the chromatogram using a silica column was also set to 100, with the peak area of ​​the sample being P3 and the peak area of ​​standard polystyrene being P4. Degeneration rate (%) = [1 - (P2 × P3) / (P1 × P4)] × 100 (However, P1+P2=P3+P4=100)

[0095] [Moony viscosity] The Mooney viscosity of each polymer was measured using a Mooney viscometer (product 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, then the rotor was rotated at 2 rpm, and the torque after 4 minutes was measured and defined as Mooney viscosity (ML(1+4)).

[0096] [Aromatic vinyl monomer unit content (amount of bound styrene), amount of 1,2-vinyl bonds, total hydrogenation rate, 1,2-hydrogenation rate of conjugated diene copolymers] Using conjugated diene copolymers as samples, the relative intensities of the bound styrene and 1,2-vinyl bonds were measured by 1H-NMR. In the case of hydrogenated conjugated diene copolymers, the hydrogenation rate was measured using a known method. The conditions for 1H-NMR measurement are described below. <Measurement conditions> Measuring instrument: JNM-LA400 (manufactured by JEOL) Solvent: Deuterated chloroform Measurement sample: Conjugated diene copolymer Sample concentration: 50 mg / mL Observation frequency: 400MHz Chemical shift standard: Contains 0.05% by mass of TMS (tetramethylsilane) relative to deuterated chloroform. Pulse delay: 2.904 seconds Number of scans: 64 Pulse width: 45° Measurement temperature: 26℃

[0097] [Amount of 1,4-cis bonds, amount of 1,4-trans bonds, and ratio of 1,4-cis bonds to 1,4-trans bonds in conjugated diene copolymers] Using conjugated diene copolymers as samples, the amounts of 1,4-cis and 1,4-trans bonds in the conjugated diene monomer components were measured using nuclear magnetic resonance (13C-NMR). The ratio of 1,4-cis to 1,4-trans bonds was calculated using these values. The measurement conditions were the same as those for the 1H-NMR measurement described above, except that the number of scans was increased to 256.

[0098] (Content of aromatic vinyl monomer blocks (styrene block content), blocking rate) The content of aromatic vinyl monomer blocks (styrene blocks) was measured according to the osmium tetroxide decomposition method described in IMKolthoff, et al., J. Polym. Sci. 1, 429 (1946). More specifically, 0.050 g of conjugated diene copolymer was dissolved in 10 ml of chloroform, and 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. The reaction was carried out under reflux in a 90°C bath for 12 minutes. After the reaction was complete, the reaction solution was cooled, and 200 ml of methanol was added to the reaction solution while stirring to precipitate the styrene block component, which was then filtered off through a 5 μm glass filter. The content of aromatic vinyl monomer blocks was determined by dividing the mass of the obtained material by the total mass of the conjugated diene copolymer. Furthermore, the blocking rate, as measured by the aforementioned NMR, is the value obtained by dividing the amount of aromatic vinyl monomer blocks by the amount of bound styrene, and is the ratio of the amount of aromatic vinyl monomer blocks to the amount of aromatic vinyl monomer units.

[0099] (metal content) The conjugated diene copolymers obtained in the manufacturing examples described later were subjected to elemental analysis using inductively coupled plasma (ICP, Shimadzu Corporation, instrument name: ICPS-7510) to determine the zinc content (Zn amount, in ppm), aluminum content (Al amount, in ppm), copper content (Cu amount, in ppm), iron content (Fe amount, in ppm), lithium content (Li amount, in ppm), and titanium content (Ti amount, in ppm). If the metal content was below the detection limit, it was indicated as ND (Not Detected).

[0100] (Bale forming) In the conjugated diene copolymers (polymers 1 to 20) described later, the conjugated diene copolymers, after desolvation and drying, were filled into a rectangular container with 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 a bale molded body of the conjugated diene copolymer.

[0101] (moisture content) A 50g bale molded body was placed in a hot air dryer heated to 150°C and dried for 3 hours. The moisture content of the bale molded body was determined by measuring the difference in mass before and after drying.

[0102] <Examples of preparation of conjugated diene copolymers> [Preparation Example 1] A temperature-controlled autoclave with an internal volume of 40 L, equipped with a stirrer and jacket, was used as the reactor to remove impurities beforehand. 2100 g of 1,3-butadiene, 900 g of styrene, 21000 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 42°C and maintained thereafter. Next, 30.6 mmol of n-butyllithium was added to the reactor as a polymerization initiator, and polymerization was started. Subsequently, 33.7 mmol of methanol was added as a reaction stopper to halt the polymerization. The internal temperature of the reactor before halting the polymerization was 83°C. Furthermore, 12.6 g of n-octadecyl-3-(3,5-di-t-butyl-4-hydrooxyphenyl)-propionate and 3.0 g of 4,6-bis(octylthiomethyl)-o-cresol were added to the reactor as antioxidants, and the solvent was removed by dropping the conjugated diene copolymer solution into warm water. Subsequently, the solution was dried in a dryer to obtain the conjugated diene copolymer (polymer 1). Table 4 shows the analysis results obtained by analyzing Polymer 1 using the method described above.

[0103] [Preparation Examples 2, 5, 8, 11-14, 16-18] As shown in Tables 1 and 2, each conjugated diene copolymer (polymers 2, 5, 8, 11-14, and 16-18) was obtained in the same manner as in Preparation Example 1, except that the monomer composition, the type and amount of polar substance added, and the amounts of polymerization initiator and polymerization inhibitor added were changed. Tables 4 and 5 show the results of analyzing the obtained polymers 2, 5, 8, 11-14, and 16-18 using the method described above.

[0104] [Preparation Example 3] A temperature-controlled autoclave with an internal volume of 40 L, equipped with a stirrer and jacket, was used as the reactor to remove impurities beforehand. 2100 g of 1,3-butadiene, 900 g of styrene, 21000 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 41°C and maintained therein. Next, 22.2 mmol of n-butyllithium was added to the reactor as a polymerization initiator, and polymerization was started. After the polymerization reaction started, the temperature inside the reactor began to rise due to the heat generated by polymerization. When the temperature stopped rising, 4.4 mol of silicon tetrachloride was added to the reactor as a coupling agent, and the mixture was stirred for 5 minutes. Subsequently, 6.7 mmol of methanol was added as a reaction stopper to halt 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 solvent was removed by dropping the conjugated diene copolymer solution into warm water. Subsequently, the mixture was dried in a dryer to obtain the conjugated diene copolymer (polymer 3). Table 4 shows the results of the analysis of the obtained polymer 3 using the method described above.

[0105] [Preparation Example 4] As shown in Table 1, 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. Table 4 shows the results of the analysis of the obtained polymer 4 using the method described above.

[0106] [Preparation Example 6] A temperature-controlled autoclave with an internal volume of 40 L, equipped with a stirrer and jacket, was used as the reactor to remove impurities beforehand. 1800 g of 1,3-butadiene, 400 g of styrene, 21000 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 45°C and maintained therein. Next, 11.8 mmol of n-butyllithium was added to the reactor as a polymerization initiator, and polymerization was started. Subsequently, 800g of styrene (additional styrene in the table) was added to the reactor 3 minutes after the internal temperature of the reactor reached its peak, and the polymerization reaction was continued. Subsequently, 13.0 mmol of methanol was added as a reaction stopper to halt the polymerization. The internal temperature of the reactor before halting the polymerization was 80°C. 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. The conjugated diene copolymer solution was then removed by dropping it into warm water. After that, the solution was dried in a dryer to obtain the conjugated diene copolymer (polymer 6). Table 4 shows the analysis results obtained by analyzing the obtained polymer 6 using the method described above.

[0107] [Preparation Examples 7, 9, 15] As shown in Tables 1 and 2, each conjugated diene copolymer (polymers 7, 9, and 15) was obtained in the same manner as in Preparation Example 6, except that the monomer composition, the type and amount of polar substance added, and the amounts of polymerization initiator and polymerization inhibitor added were changed. Tables 4 and 5 show the analytical results obtained by analyzing polymers 7, 9, and 15 using the method described above.

[0108] <Preparation of hydrogenation catalyst> [Preparation example a] Two liters of dried and purified cyclohexane were charged into a nitrogen-purged reactor. 40 mmol of bis(η5-cyclopentadienyl)titanium di-(p-tolyl) and 150 g of 1,2-polybutadiene (approximately 85% 1,2-vinyl bond) with a molecular weight of about 1,000 were added and dissolved. Subsequently, a cyclohexane solution containing 60 mmol of n-butyllithium was added to the reactor and reacted at room temperature for 5 minutes. Immediately afterward, 40 mmol of n-butanol was added and stirred to obtain a hydrogenation catalyst (TC1).

[0109] [Preparation Example 10] As shown in Table 1, the polymerization reaction was carried out in the same manner as in Preparation Example 1, except that the monomer composition was changed. Subsequently, hydrogenation catalyst (TC1) was added to the reactor at a concentration of 50 ppm (based on titanium) per 100 parts by mass of the obtained 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 had been added. 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 solvent was removed by dropping the conjugated diene copolymer solution into warm water. After that, the solution was dried to obtain the conjugated diene copolymer (polymer 10). Table 5 shows the analysis results obtained by analyzing the polymer 10 using the method described above.

[0110] [Preparation Example 19] Two tank-type pressure vessels, each 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 and an outlet at the top, and equipped with a stirrer and a jacket for temperature control, were connected and used as reactors. 1,3-butadiene, from which water had been removed, was added at a flow rate 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 reaction group, n-butyllithium was added at a flow rate of 0.105 mmol / min as a residual impurity inactivation agent, mixed, and then continuously supplied to the bottom of the reaction group. Furthermore, 2,2-bis(2-oxolanil)propane was supplied at a flow rate of 0.037 mmol / min as a polar substance, and n-butyllithium was supplied at a flow rate of 0.463 mmol / min as a polymerization initiator, and the polymerization reaction was started by supplying these to the bottom of the first reactor, which was vigorously mixed with a stirrer. The internal temperature of the reactor was maintained at 77°C at this time. The conjugated diene copolymer solution was continuously withdrawn from the top of the first reactor and continuously supplied to the bottom of the second reactor. At the same time, styrene (additional styrene) was continuously added from the center of the second reactor at a flow rate of 3.5 g / min to continue the polymerization reaction. Furthermore, the solution was supplied to a static mixer from the top of the second reactor. Next, methanol was continuously added at a flow rate of 0.509 mmol / min to the conjugated diene copolymer solution supplied from the top of the second reactor to the static mixer as a reaction terminating agent, and the mixture was mixed using the static mixer to stop the polymerization reaction. Next, to the obtained conjugated diene copolymer solution, 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 as antioxidants. Then, the solvent was removed by dropping the conjugated diene copolymer solution into warm water. After that, the solution was dried in a dryer to obtain the conjugated diene copolymer (polymer 19). Table 6 shows the analysis results obtained by analyzing the obtained polymer 19 using the method described above.

[0111] [Preparation Example 20] As shown in Table 3, a conjugated diene copolymer (polymer 20) was obtained in the same manner as in Preparation Example 19, except that the monomer composition was changed. Table 6 shows the results of the analysis of the obtained polymer 20 using the method described above.

[0112] [Table 1]

[0113] [Table 2]

[0114] [Table 3]

[0115] The abbreviations used in Tables 1 to 3 above are explained below. THF: Tetrahydrofuran BOP: 2,2-Bis(2-oxolanil)propane Polymerization initiator: n-butyllithium Coupling agent 1: Silicon tetrachloride Coupling agent 2: 2,2-dimethoxy-1-(3-trimethoxysilylpropyl)-1-aza-2-silacyclopentane Polymerization inhibitor: methanol TC1: Hydrogenation catalyst for preparation example a

[0116] [Table 4]

[0117] [Table 5]

[0118] [Table 6]

[0119] <Fabrication of positive electrodes for lithium-ion secondary batteries> [Example 1] In a rotation-orbit mixer (manufactured by Thinky Co., Ltd.), 98 parts by mass of acetylene black as a conductive material and 2 parts by mass of the conjugated diene copolymer (polymer 1) described in Preparation Example 1 as a non-aqueous binder were added. N-methylpyrrolidone (NMP) was then added as a solvent to achieve a solid content concentration of 5%, and the mixture was combined to obtain a binder composition for a coating layer. A 20 μm thick aluminum thin film was coated with the aforementioned coating layer binder composition using a wireless bar coater, and then vacuum-dried at 120°C for 5 hours to obtain a current collector having a coating layer containing a conductive material and a non-aqueous system (coated current collector). The average thickness of the coating layer was 3 μm.

[0120] Next, 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 polyvinylidene fluoride (PVDF) as a binder for the positive electrode active material layer were added to a rotation-orbital mixer (manufactured by Thinky Co., Ltd.). N-methylpyrrolidone (NMP) was then added as a solvent to achieve a solid content concentration of 5%, and the mixture was combined to obtain the binder composition for the active material layer. The coated current collector was coated with the binder composition for the positive electrode active material layer using a slot die coater, and then vacuum-dried at 120°C for 8 hours. Afterward, the positive electrode was obtained by rolling the positive electrode active material layer to a porosity of 25%. The average thickness of the active material layer was 70 μm. The electrode thus obtained was used as a positive electrode for a lithium-ion secondary battery and subjected to various physical property evaluations described later. The results are shown in Table 7.

[0121] The thickness of the coating layer on the coated current collector and the active material layer on the positive electrode of the lithium-ion secondary battery were measured by visually observing sections of them with an optical microscope.

[0122] [Examples 2-17, Comparative Examples 1-4, 7] As shown in Tables 7-10, current collectors and positive electrodes for lithium-ion secondary batteries coated in the same manner as in Example 1 were obtained, except that the conjugated diene copolymer used in the binder composition for the coating layer was changed.

[0123] [Comparative Example 5] In Comparative Example 5, a coated current collector and a positive electrode for a lithium-ion secondary battery were obtained in the same manner as in Example 1, except that the following changes were made to the binder composition for the coating layer. The non-aqueous binder was changed to an aqueous binder. In a rotation-orbit mixer (manufactured by Thinky Co., Ltd.), 97.5 parts by mass of acetylene black was used as a conductive material, 2.0 parts by mass of styrene-butadiene latex (DL-612, manufactured by Asahi Kasei Corporation, cis:trans ratio = 14:86) was used as an aqueous binder, and 0.5 parts by mass of carboxymethylcellulose was used as a thickener. An aqueous binder composition with a solid content of 30% by mass was prepared using water as the aqueous solvent. The aqueous binder composition was applied to a 20 μm thick aluminum thin film, and the drying conditions were 60°C for 8 hours.

[0124] [Comparative Example 6] In Comparative Example 6, a positive electrode for a lithium-ion secondary battery was obtained in the same manner as in Example 1, except that the binder composition for the coating layer was not applied to the aluminum thin film, but rather the binder composition for the active material layer was applied directly to the aluminum thin film.

[0125] The coated current collectors and electrodes obtained were used as positive electrodes for lithium-ion secondary batteries and subjected to various physical property evaluations described later. The results are shown in Tables 7 to 10. For the lithium-ion secondary battery positive electrode of Comparative Example 6, only the cycle characteristics test described later was performed.

[0126] (Solubility of conjugated diene copolymers) The solubility of each conjugated diene copolymer from Preparation Examples 1 to 20 in N-methylpyrrolidone (NMP) solvent was evaluated. The dissolution time for a solution adjusted to a 3% by mass concentration of conjugated diene copolymer was measured by shaking the solution in a shaker for up to 8 hours, and the time at which the conjugated diene copolymer was visually eliminated was used as the dissolution time. Shorter dissolution times result in better productivity and reduce the occurrence of uneven film thickness due to undissolved material, making it easier to form a uniform active material layer. ○: Dissolved within 6 hours. △: Dissolved in 6-8 hours. ×: It did not dissolve after 8 hours, and undissolved residue remained.

[0127] (Adhesiveness) A test piece measuring 2 cm wide x 12 cm long was cut from the obtained coated current collector, and the current collector side of this test piece was attached to an aluminum plate with double-sided tape. In accordance with JIS 1522, an 18 mm wide tape (product name: Cellotape®, manufactured by Nichiban Co., Ltd.) was attached to the electrode layer side of the test piece, and the strength of peeling the tape at a speed of 100 mm / min in a 180° direction was measured six times, and the average value (N / 18 mm) was calculated as the peel strength. The current collector coated in Comparative Example 5 was evaluated using the following criteria, based on the measured values. The higher the peel strength, the better the adhesion between the current collector and the coating layer. ◎: Peel strength was 25% or more higher. ○: Peel strength was 10% or more higher. △: The difference in peel strength was less than 10%. ×: Peel strength was 10% or more lower.

[0128] (Crack resistance) A 2cm wide x 12cm long test piece was cut from the obtained coated current collector. This test piece was bent at a 45-degree angle in the center, held for 1 minute, and then returned to its original state. After that, the binder layer was visually inspected for cracks and evaluated according to the following criteria. The more flexible the positive electrode, the less likely it is that electrode cracking will occur during winding due to differences in battery manufacturing. ○: No cracks occurred. △: No cracks appeared, but a bend mark remained. ×: A crack has occurred.

[0129] <Manufacturing of lithium-ion secondary batteries> The positive and negative electrodes for the secondary battery were punched out in a circular shape, and the positive electrode, separator, and negative electrode were stacked in that order so that the active material surfaces of the positive and negative electrodes faced each other. The stacked components were then placed in a stainless steel container with a lid. The container and lid were insulated from each other, and the container was positioned so that it was in contact with the copper foil of the negative electrode, and the lid was in contact with the aluminum foil of the positive electrode. Then, the electrolyte was poured into the container and sealed, and left at room temperature for one day to produce a lithium-ion secondary battery. The electrolyte used here was prepared by dissolving lithium hexafluoride phosphate (LiPF6) as a solute in a mixed solvent of ethylene carbonate / ethyl methyl carbonate = 1 / 2 (by volume ratio) to a concentration of 1.0 mol / L. Furthermore, the separator used was made of a porous polyethylene membrane, and the positive electrode for the secondary battery used was the lithium-ion secondary battery positive electrode obtained in Examples 1 to 17 and Comparative Examples 1 to 7. Furthermore, the negative electrode for the secondary battery described above was manufactured as follows. 1.5 parts by mass of styrene-butadiene latex (pH 7, 10% solids), 100 parts by mass of natural graphite as a negative electrode active material, and 1.0 part by mass of carboxymethylcellulose as a thickener were added and mixed with styrene-butadiene latex at a solid content of 10%. Deionized water was then added and the mixture was stirred with a mechanical stirrer to adjust the total solids content to 60%. This mixture was dispersed for 30 seconds at a peripheral speed of 20 m / s using a thin-film swirling high-speed mixer to obtain a coating solution for the negative electrode of a secondary battery. Using this coating solution, one side of a copper foil was coated with a die coater to a thickness of 100 μm after drying, and then dried at 60°C for 60 minutes. After further drying for 3 minutes at 120 minutes, it was compressed and molded with a roll press to obtain a negative electrode for a lithium-ion secondary battery.

[0130] (Cycle characteristics) For the lithium-ion secondary batteries prepared using the method described above, a charge-discharge cycle was performed at 60°C using a constant current / constant voltage charging method at 2C, charging with a constant current until the voltage reached 3.6V, then charging with a constant voltage, and finally discharging to 2.6V with a constant current of 2C. The cycle test was performed up to 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 using the volume retention rate of the secondary battery using the lithium-ion secondary battery positive electrode of Comparative Example 5 as the baseline, according to the following evaluation criteria. A larger value indicates less capacity loss due to repeated charge-discharge. ○: The maintenance rate was 5% or more higher. △: The maintenance rate was either high or low within the range of less than 5%. ×: Maintenance rate was 5% or more lower.

[0131] [Table 7]

[0132] [Table 8]

[0133] [Table 9]

[0134] [Table 10]

[0135] The non-aqueous binders, current collectors, and positive electrodes obtained in Examples 1-17, which have a coating layer containing a conductive material and a non-aqueous binder, were found to have superior solubility in solvents, a superior balance of adhesion to the current collector and crack resistance, and superior cycle characteristics when used in lithium-ion secondary batteries, compared to the non-aqueous binders obtained in Comparative Examples 1-4 and 7, and the aqueous binder, conductive material, and coating layer of Comparative Example 5.

Claims

1. A positive electrode for a lithium-ion secondary battery comprising a current collector, a coating layer containing a conductive material and a non-aqueous binder, and an active material layer, The non-aqueous binder comprises a conjugated diene copolymer having the following requirements (a) to (d): The content of the conductive material is 40.0 parts by mass or more and 99.9 parts by mass or less per 100 parts by mass of the coating layer. The content of the non-aqueous binder is 0.1 parts by mass or more and 60.0 parts by mass or less per 100 parts by mass of the coating layer. Positive electrode for lithium-ion secondary batteries. 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 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. Requirement (d): The weight-average molecular weight is between 100,000 and 2,000,000.

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

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

4. The content of aromatic vinyl monomer blocks is 5.0% by mass or more and 40.0% by mass or less, relative to the total amount of the conjugated diene copolymer. The positive electrode for a lithium-ion secondary battery according to claim 1.

5. The blocking rate of aromatic vinyl monomer blocks in the aforementioned conjugated diene copolymer is 15.0% or more and 85.0% or less. The positive electrode for a lithium-ion secondary battery according to claim 1.

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

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

8. The total hydrogenation rate of the conjugated diene copolymer is 10% to 99%. The positive electrode for a lithium-ion secondary battery according to claim 1.

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

10. The thickness of the coating layer is 0.1 μm or more and 10 μm or less. The positive electrode for a lithium-ion secondary battery according to claim 1.

11. The conductive material includes at least one selected from the group consisting of carbon black, graphite, carbon nanotubes, and aluminum hydroxide. The positive electrode for a lithium-ion secondary battery according to claim 1.

12. The coating layer does not contain positive electrode active material. The positive electrode for a lithium-ion secondary battery according to claim 1.

13. The aforementioned current collector includes aluminum, The positive electrode for a lithium-ion secondary battery according to claim 1.

14. The active material layer includes a binder, The binder contains polyvinylidene fluoride, The positive electrode for a lithium-ion secondary battery according to claim 1.

15. The active material layer contains an active material, The active material comprises at least one selected from the group consisting of lithium iron phosphate, lithium magnesium iron phosphate, and lithium iron oxide. The positive electrode for a lithium-ion secondary battery according to claim 1.

16. A positive electrode for a lithium-ion secondary battery as described in claim 1, Lithium-ion rechargeable battery.

17. Furthermore, including the electrolyte, The lithium-ion secondary battery according to claim 16.

Images