Binder for positive electrode

Abstract

Provided is a binder for a positive electrode excellent in viscosity characteristics (dispersibility) and stability of a conductive material dispersion when manufacturing an electrode for an electrochemical element, and capable of increasing peel strength of the electrode manufactured, output characteristics of the electrochemical element, cycle characteristics, effects of suppressing cracks in an electrode active material after a cycle test and the like. This binder for a positive electrode contains a hydrogenated nitrile rubber dissolved or dispersed in N-methylpyrrolidone (NMP), the hydrogenated nitrile rubber comprising a nitrile group-containing monomer unit and a conjugated diene monomer unit and / or an alkylene structural unit, having a weight average molecular weight (Mw) in the range of 1,000-1,000,000 and an iodine value in the range of 0.1-100 mg / 100 mg, containing a phenolic antioxidant, and having a palladium (Pd) content of 50 ppm or less and a calcium (Ca) content in the range of 1-2,000 ppm.

Description

Positive electrode binder

[0001] The present invention relates to a positive electrode binder and a method for producing the same, and more specifically to a positive electrode binder that provides a conductive material dispersion excellent in viscosity characteristics (dispersibility) and stability, improves the peel strength of the produced positive electrode, and the effect of suppressing cracking of the active material after a cycle test, and can improve the output characteristics and cycle characteristics of an electrochemical element, and a method for producing the same.

[0002] Electrochemical devices such as lithium ion secondary batteries, lithium ion capacitors, and electric double layer capacitors are small, lightweight, have high energy density, and can be repeatedly charged and discharged, and are therefore used in a wide range of applications. Therefore, in recent years, improvements in battery components such as electrodes have been investigated with the aim of further improving the performance of electrochemical devices.

[0003] Here, an electrode used in an electrochemical element typically includes a current collector and an electrode mixture layer formed on the current collector. This electrode mixture layer is formed, for example, by applying a slurry containing an electrode active material, a conductive material, a binder, etc., onto the current collector and then drying the applied slurry.

[0004] Therefore, in recent years, attempts have been made to improve the binder used in forming the electrode mixture layer in order to achieve further improvements in the performance of electrochemical elements. For example, technical studies have been conducted on positive electrode binders that use polymers containing nitrile group-containing monomer units as the binder.

[0005] For example, Patent Document 1 (JP 2018-160421 A) discloses a binder composition that contains a polymer containing a conjugated diene monomer unit and / or an alkylene structural unit and a nitrile group-containing monomer unit, and an organic solvent, and that has a solution turbidity of 1 to 80 at a solid content concentration of 10% by mass and has an excellent balance between peel strength and secondary battery output characteristics. Specifically, nitrile rubber latex, obtained by emulsion polymerization of acrylonitrile and 1,3-butadiene, was added to a 1% by mass aqueous solution of calcium chloride to coagulate, washed with water, filtered, and then vacuum-dried to obtain nitrile rubber. The resulting nitrile rubber was then metathesized with a ruthenium-based Grubbs catalyst and ethylene as a coolefin, followed by hydrogenation using a rhodium-based Wilkinson catalyst with an iodine value of 5 mg / 100 mg as the endpoint. 0.2 parts of activated carbon with an average diameter of 15 μm was then added and stirred for 30 minutes to obtain a hydrogenated polymer with a calcium ion concentration of 1000 ppm and an iodine value of 5 mg / 100 mg. N-methylpyrrolidone (NMP) was then added, and water was evaporated under reduced pressure to obtain an NMP solution of the hydrogenated polymer with a solution turbidity of 20. Electrodes were fabricated using a binder composition consisting of 1.6 parts polyvinylidene fluoride (solution viscosity 5 kPa s) and 0.4 parts of the hydrogenated polymer as a binder composition. However, the stability of the conductive material dispersion is poor, making it difficult to prepare electrochemical element electrodes. In addition, there has been a demand for further improvement in the peel strength of the electrode and the effect of suppressing cracking of the electrode active material after cycle testing.

[0006] Patent Document 2 (Japanese Patent No. 6911985) discloses an electrochemical device having excellent rate characteristics and high-temperature storage stability, which includes a polymer containing α,β-ethylenically unsaturated nitrile monomer units and specific alkylene structural units, and which has a volume average particle diameter D of 50 to 800 nm when subjected to dynamic light scattering measurement, and at least one peak detected in the range of 5 μm to 30 μm, and a conductive carbon material. Specifically, acrylonitrile and 1,3-butadiene are copolymerized, and dibutylhydroxytoluene (BHT) is added as an antioxidant. Then, a 25% by mass aqueous solution of calcium chloride is added in an amount of 3 parts per 100 parts of the polymer to coagulate the polymer. The polymer is washed with 50 times the amount of ion-exchanged water, and then hydrogenated using a palladium catalyst. The palladium catalyst is then filtered off to obtain polymers with iodine values ​​of 5 mg / 100 mg and 55 mg / 100 mg. These polymers are then dissolved in NMP and used as a positive electrode binder. However, it is desired that the positive electrode binder used here improves the dispersibility of the dispersion liquid and the peel strength of the electrode, and further improves the output characteristics and cycle characteristics of the electrochemical element, and the effect of suppressing cracking of the electrode active material after a cycle test.

[0007] Patent Document 3 (WO 2022 / 163769) discloses a nonaqueous secondary battery with excellent electrolyte mobility, including a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode comprises a current collector and a positive electrode composite layer formed thereon, the positive electrode comprising a positive electrode active material, a conductive material, and a polymer A containing nitrile group-containing monomer units and at least one of conjugated diene units and alkylene structural units. A specific example of polymer A is a polymer obtained by emulsion polymerizing acrylonitrile and 1,3-butadiene, adding dibutylhydroxytoluene (BHT) to the resulting emulsion, followed by the addition of a 25% by mass aqueous solution of calcium chloride to coagulate the resulting solution. The resulting solution is then hydrogenated using a palladium catalyst, and the palladium catalyst is filtered off to obtain a hydrogenated polymer with an iodine value of 20 mg / 100 mg. However, a positive electrode binder containing such a hydrogenated polymer exhibits poor dispersibility in the conductive material dispersion, resulting in problems with the output characteristics, cycle characteristics, and cracking of the active material after cycle testing of the resulting electrochemical device.

[0008] Patent Document 4 (WO 2007 / 049651) discloses a carboxyl group-containing nitrile rubber which provides a crosslinked product with low compression set and dynamic heat generation and has excellent processability, and which has an α,β-ethylenically unsaturated nitrile monomer unit content of 10 to 60% by weight, an iodine value of 120 or less, and a total amount of magnesium, calcium, and aluminum of 2,000 ppm or less. A specific carboxyl group-containing nitrile rubber is obtained by adding an aqueous magnesium sulfate solution to a latex of a nitrile group-containing saturated copolymer rubber obtained by emulsion polymerizing acrylonitrile, mono-n-butyl fumarate, and butadiene and then hydrogenating the copolymer with a palladium catalyst, coagulating the resulting mixture, filtering and washing with water three times, removing the aqueous magnesium sulfate solution contained in the rubber through a centrifuge, and vacuum drying at 60°C for 12 hours to obtain a nitrile rubber having a magnesium content of 1 to 2 ppm, a calcium content of 1 ppm, and an aluminum content of 1 ppm. However, the nitrile rubber obtained here is desired to be further improved in terms of the stability of the conductive material dispersion, and in terms of the electrode peel strength and cracking of the active material layer after cycle testing when used in an electrochemical element.

[0009] Japanese Patent Publication No. 2018-160421 Japanese Patent No. 6911985 WO2022 / 163769 WO2007 / 049651

[0010] The present invention has been made in view of the above circumstances, and aims to provide a positive electrode binder that provides excellent viscosity characteristics (dispersibility) and stability of a conductive material dispersion when producing an electrode for an electrochemical element, and that can improve the peel strength of the produced electrode, the output characteristics and cycle characteristics of the electrochemical element, and the effect of suppressing cracking of the electrode active material after a cycle test, and a method for producing the same.

[0011] The present inventors have conducted extensive research in light of the above-mentioned problems and have found that a positive electrode binder obtained by dissolving or dispersing hydrogenated nitrile rubber in N-methylpyrrolidone (NMP) which is composed of nitrile group-containing monomer units and conjugated diene monomer units and / or alkylene structural units, has a specific amount of iodine value and weight-average molecular weight (Mw), contains a phenolic antioxidant, and has a palladium (Pd) content and calcium (Ca) content within specific ranges, when mixed with a conductive material in the production of an electrochemical element, enhances the viscosity characteristics and stability of the conductive material dispersion, suppresses the peel strength of the produced electrode, and suppresses cracking of the active material after a cycle test, thereby improving the output characteristics and cycle characteristics of the electrochemical element.

[0012] The present inventors have also found that the above characteristics can be further improved by using, as a positive electrode binder, a hydrogenated nitrile rubber in which the contents of Pd, Rh, and Ru derived from the hydrogenation catalyst and Mg and Na derived from other reaction secondary materials are limited.

[0013] The present inventors have also found that hydrogenated nitrile rubber constituting a binder for a positive electrode, which uses calcium chloride as a coagulant, is particularly excellent in electrode peeling properties of electrochemical elements such as lithium ion secondary batteries, and can prevent deterioration of cycle characteristics and cracking of the active material layer after cycle testing.

[0014] The present inventors have found that by using hydrogenated nitrile rubber, which is obtained by emulsion polymerizing a nitrile group-containing monomer and a conjugated diene monomer and then hydrogenating the polymer (nitrile rubber) coagulated with calcium chloride, as a binder for a positive electrode, the viscosity characteristics (dispersibility) of a conductive material dispersion liquid can be improved when the conductive material dispersion liquid is prepared, and cracking of the electrode active material in the obtained electrode after a cycle test can be suppressed, thereby significantly improving the cycle characteristics of an electrochemical element.

[0015] The present inventors have found that hydrogenated nitrile rubber is obtained by emulsion polymerizing a nitrile group-containing monomer and a conjugated diene monomer, adding a specific antioxidant to the emulsion polymerization liquid, coagulating the polymer with calcium chloride to obtain a polymer, and then hydrogenating the polymer. When an electrical material dispersion is prepared from the hydrogenated nitrile rubber, the viscosity characteristics and stability of the electrical conductive material dispersion can be significantly improved, and cracking of the electrode active material in the resulting electrode after a cycle test can be suppressed, thereby significantly improving the output characteristics and cycle characteristics of an electrochemical element.

[0016] The present inventors have further found that a positive electrode binder excellent for producing such electrochemical devices can be easily produced by mixing hydrogenated nitrile rubber having specific properties and metal contents with NMP.

[0017] The present inventors have completed the present invention based on these findings.

[0018] Thus, according to the present invention, there is provided a positive electrode binder obtained by dissolving or dispersing in N-methylpyrrolidone (NMP) a hydrogenated nitrile rubber which contains nitrile group-containing monomer units and conjugated diene monomer units and / or alkylene structural units, has a weight average molecular weight (Mw) of 1,000 to 1,000,000, an iodine value in the range of 0.1 to 100 mg / 100 mg, contains a phenolic antioxidant, and has a palladium (Pd) content of 50 ppm or less and a calcium (Ca) content in the range of 1 to 2,000 ppm.

[0019] In the binder for a positive electrode of the present invention, the palladium (Pd) content in the hydrogenated nitrile rubber is preferably 10 ppm or less.

[0020] In the binder for a positive electrode of the present invention, it is further preferable that the content of rhodium (Rh) and / or ruthenium (Ru) in the hydrogenated nitrile rubber is in the range of 0.5 to 50 ppm.

[0021] In the binder for a positive electrode of the present invention, it is further preferred that the sodium (Na) content in the hydrogenated nitrile rubber is 300 ppm or less and the magnesium (Mg) content is 50 ppm or less.

[0022] In the positive electrode binder of the present invention, the hydrogenated nitrile rubber is preferably one obtained by emulsion polymerizing a nitrile group-containing monomer and a conjugated diene monomer, coagulating the polymer with calcium chloride, and hydrogenating the polymer.

[0023] In the binder for a positive electrode of the present invention, the hydrogenated nitrile rubber is preferably one obtained by emulsion polymerizing a nitrile group-containing monomer and a conjugated diene monomer, adding a phenolic antioxidant, and then hydrogenating the polymer coagulated with calcium chloride.

[0024] In the binder for a positive electrode of the present invention, the ratio of the nitrile group-containing monomer units of the hydrogenated nitrile rubber and other repeating units other than the conjugated diene monomer units and / or alkylene structural units is preferably 0% by mass or more and 5% by mass or less.

[0025] In the binder for a positive electrode of the present invention, the content of the phenol-based antioxidant is preferably in the range of 0.001 to 2% by mass.

[0026] In the binder for a positive electrode of the present invention, the solid content concentration is preferably in the range of 0.1 to 40 mass %.

[0027] The present invention also provides a method for producing a positive electrode binder, which comprises dissolving or dispersing in N-methylpyrrolidone (NMP) a hydrogenated nitrile rubber that contains nitrile group-containing monomer units, conjugated diene monomer units and / or alkylene structural units, has a weight average molecular weight (Mw) of 1,000 to 1,000,000, an iodine value in the range of 0.1 to 100 mg / 100 mg, contains a phenolic antioxidant, and has a palladium (Pd) content of 50 ppm or less and a calcium (Ca) content in the range of 1 to 2,000 ppm.

[0028] In the method for producing a binder for a positive electrode of the present invention, the hydrogenated nitrile rubber is preferably obtained by dividing hydrogenated nitrile rubber bale.

[0029] According to the present invention, a binder for a positive electrode, which has excellent viscosity characteristics and stability of a conductive material dispersion, can improve the peel strength of a produced electrode, can suppress cracking of the electrode active material after a cycle test of an electrochemical element, and can improve cycle characteristics and output characteristics, and a method for producing the same are provided.

[0030] Hereinafter, embodiments of the present invention will be described in detail.

[0031] [Hydrogenated Nitrile Rubber] The hydrogenated nitrile rubber used contains nitrile group-containing monomer units, conjugated diene monomer units and / or alkylene structural units, and is characterized by having a weight average molecular weight (Mw) of 1,000 to 1,000,000, an iodine value in the range of 0.1 to 100 mg / 100 mg, containing a phenol-based antioxidant, and having a Pd content of 50 ppm or less and a Ca content in the range of 1 to 2,000 ppm.

[0032] The properties and production method of the hydrogenated nitrile rubber used will be described in detail below.

[0033] <Monomer Component> The hydrogenated nitrile rubber used contains a nitrile group-containing monomer unit and a conjugated diene monomer unit and / or an alkylene structural unit. Such hydrogenated nitrile rubber is a component that can function as a binder in an electrode mixture layer formed in an electrochemical element, holding an electrode active material and the like from a current collector without causing the electrode active material to detach from the current collector. Furthermore, such hydrogenated nitrile rubber can also function as a dispersant that can disperse a conductive material in a conductive material dispersion liquid containing a conductive material.

[0034] Specifically, the hydrogenated nitrile rubber may be a hydrogenated polymer obtained by hydrogenating a polymer containing conjugated diene monomer units and nitrile group-containing monomer units. When the polymer containing conjugated diene monomer units and nitrile group-containing monomer units is completely hydrogenated, the polymer becomes a polymer containing alkylene structural units and nitrile group-containing monomer units, and when the hydrogenation is partially carried out, the polymer becomes a polymer containing conjugated diene monomer units, alkylene structural units, and nitrile group-containing monomer units.

[0035] (Nitrile Group-Containing Monomer Unit) Examples of nitrile group-containing monomers that can form nitrile group-containing monomer units include α,β-ethylenically unsaturated nitrile monomers. Specifically, the α,β-ethylenically unsaturated nitrile monomer is not particularly limited as long as it is an α,β-ethylenically unsaturated compound having a nitrile group, and examples include acrylonitrile; α-halogenoacrylonitriles such as α-chloroacrylonitrile and α-bromoacrylonitrile; and α-alkylacrylonitriles such as methacrylonitrile and α-ethylacrylonitrile. Among these, acrylonitrile is preferred. One type of nitrile group-containing monomer may be used alone, or two or more types may be used in combination at any ratio.

[0036] The content of the nitrile group-containing monomer units in the hydrogenated nitrile rubber is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, and is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less, based on 100% by mass of all repeating units in the hydrogenated nitrile rubber. If the content of the nitrile group-containing monomer units is within the above-mentioned range, the dispersion stability of the conductive material dispersion can be increased, and the peel strength of the obtained electrode can be further improved.

[0037] (Conjugated Diene Monomer Unit) Here, examples of the conjugated diene monomer capable of forming the conjugated diene monomer unit include conjugated diene compounds having 4 or more carbon atoms, such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, and 1,3-pentadiene. Of these, isoprene and 1,3-butadiene are preferred, and 1,3-butadiene is particularly preferred.

[0038] (Alkylene structural unit) Here, the alkylene structural unit is a unit represented by the general formula: -C ｎ H ２ｎ- [where n is an integer of 2 or greater]. The alkylene structural unit may be linear or branched, but from the viewpoint of reducing the resistance of the electrode and improving the rate characteristics of the electrochemical device, the alkylene structural unit is preferably linear, i.e., a linear alkylene structural unit. Furthermore, the alkylene structural unit preferably has 4 or more carbon atoms (i.e., n in the above general formula is an integer of 4 or greater).

[0039] The alkylene structural unit may be linear or branched, but is preferably a linear alkylene structural unit, and preferably has 4 or more carbon atoms (i.e., n in the general formula above is an integer of 4 or more).

[0040] The total content of the conjugated diene monomer units and the alkylene structural units in the hydrogenated nitrile rubber is preferably 50% by mass or more, more preferably 55% by mass or more, and particularly preferably 60% by mass or more, and is preferably 90% by mass or less, more preferably 85% by mass or less, and particularly preferably 80% by mass or less, when the total repeating units in the hydrogenated nitrile rubber (the total of the structural units and the monomer units) is taken as 100% by mass. If the total content of the conjugated diene monomer units and the alkylene structural units in the polymer is within the above range, the dispersion stability of the conductive material dispersion and the cycle characteristics of the electrochemical device can be improved.

[0041] When the hydrogenated nitrile rubber has only one of the content ratio of alkylene structural units and the content ratio of conjugated diene monomer units, it is preferable that the content ratio of the alkylene structural units and the conjugated diene monomer units satisfy the above range.

[0042] (Other repeating units) The other repeating units are not particularly limited, but include aromatic vinyl monomer units, acidic group-containing monomer units, and (meth)acrylic acid ester monomer units. The hydrogenated nitrile rubber may contain one type of other repeating unit, or may contain two or more types of other repeating units.

[0043] The content ratio of other repeating units in the hydrogenated nitrile rubber is preferably 0% by mass or more and 30% by mass or less, more preferably 0% by mass or more and 20% by mass or less, even more preferably 0% by mass or more and 10% by mass or less, and particularly preferably 0% by mass or more and 5% by mass or less, based on 100% by mass of all repeating units in the hydrogenated nitrile rubber.

[0044] <Antioxidant> The hydrogenated nitrile rubber used is characterized by containing a phenol-based antioxidant.

[0045] The phenolic antioxidant is not particularly limited as long as it is a commonly used one, but the melting point of the phenolic antioxidant is usually 150°C or lower, preferably 100°C or lower, and more preferably 90°C or lower, and the molecular weight of the phenolic antioxidant is usually in the range of 100 to 1,000, preferably 150 to 500, and more preferably 200 to 300.

[0046] Among phenol-based antioxidants, hindered phenol-based antioxidants are particularly preferred because they provide a highly stable conductive material dispersion. In the present invention, the hindered phenol-based antioxidant refers to a phenol-based antioxidant having bulky substituents on both sides of a phenol group, and the bulky substituents on both sides are an isobutyl group or a t-butyl group, preferably a t-butyl group.

[0047] Examples of the hindered phenol-based antioxidants include 2,6-di-t-butylphenol, 2,6-di-t-butyl-4-methylphenol, 2,6-di-t-pentenyl-4-methylphenol, 2,2-methylenebis(4-methyl-6-t-butylphenol), 2,4-bis(octylthiomethyl)-6-methylphenol, 2,6-di-t-butyl-α-dimethylamino-p-cresol, 3-(4-hydroxy-3, 3-(4-hydroxy-3,5-di-t-butylphenyl)butyl propionate, 3-(4-hydroxy-3,5-di-t-butylphenyl)hexyl propionate, 3-(4-hydroxy-3,5-di-t-butylphenyl)octyl propionate, 3-(4-hydroxy-3,5-di-t-butylphenyl)decyl propionate, 3-(4-hydroxy-3,5-di-t-butylphenyl)dodecyl propionate, 3-(4-hydroxy-3,5-di-t-butylphenyl)butyl propionate butylphenyl)propionate, octadecyl 3-(4-hydroxy-3,5-diisopropylphenyl)propionate, heptyl 3-(4-hydroxy-3,5-diisopropylphenyl)propionate, octyl 3-(4-hydroxy-3,5-diisopropylphenyl)propionate, dodecyl 3-(4-hydroxy-3,5-diisopropylphenyl)propionate, octadecyl 3-(4-hydroxy-3,5-diisopropylphenyl)propionate, 4,4'-methylenebis(2,6-dibutylphenol), 2,6-di-t-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol, and the like, preferably 2,6-di-t-butyl-4-methylphenol (dibutylhydroxytoluene = BHT), 2,2-methylenebis(4-methyl-6-t-butylphenol), 2,4-bis(octylthiomethyl)-6-methylphenol, and most preferably BHT.

[0048] Examples of phenol-based antiaging agents other than the hindered phenol-based antiaging agents include styrenated phenols such as butylhydroxyanisole, mono(or di-, or tri)(α-methylbenzyl)phenol, 2,2′-methylene-bis(4-methyl-6-t-butylphenol), alkylated bisphenols, 2,4-bis[(octylthio)methyl]-6-methylphenol, 2,2′-thiobis-(4-methyl-6-t-butylphenol), and 4,4′-thiobis-(6-t-butyl-o-cresol), and preferred are styrenated phenol and 2,4-bis[(octylthio)methyl]-6-methylphenol.

[0049] These phenolic antioxidants can be used alone or in combination of two or more. The content of the phenolic antioxidant in the hydrogenated nitrile rubber is not particularly limited, but is usually in the range of 0.001 to 2 mass%, preferably 0.01 to 1 mass%, and more preferably 0.1 to 0.5 mass%.

[0050] The hydrogenated nitrile rubber used may contain antioxidants other than the phenol-based antioxidants, as long as the object of the present invention is not impaired. As the other antioxidants, for example, amine-based antioxidants such as styrenated diphenylamine and bis(4-t-octylphenyl)amine are preferably used.

[0051] <Hydrogenated Nitrile Rubber> The hydrogenated nitrile rubber used is characterized by being composed of the repeating units described above, containing the antioxidant described above, and also containing specific amounts of weight average molecular weight (Mw), iodine value, and specific amounts of Ca and Pd.

[0052] The weight average molecular weight (Mw) of the hydrogenated nitrile rubber used is in the range of 1,000 to 1,000,000, preferably 2,000 to 500,000, more preferably 4,000 to 400,000, and most preferably 5,000 to 250,000. If the weight average molecular weight (Mw) of the hydrogenated nitrile rubber is in this range, the dispersion stability of the conductive material dispersion can be improved and the peel strength of the obtained electrode can be increased, which is preferable.

[0053] The iodine value of the hydrogenated nitrile rubber used is in the range of 0.1 to 100 mg / 100 mg, preferably 1 to 80 mg / 100 mg, more preferably 5 to 60 mg / 100 mg, and even more preferably 10 to 40 mg / 100 mg. When the iodine value of the hydrogenated nitrile rubber is in this range, the viscosity characteristics of the conductive material dispersion, the peel strength of the electrode, and the output characteristics of the electrochemical element are suitably well balanced.

[0054] The Pd content in the hydrogenated nitrile rubber used is 50 ppm or less, preferably 30 ppm or less, and more preferably 10 ppm or less. When the Pd content in the hydrogenated nitrile rubber is in this range, cracking of the active material in the electrode is suppressed and the cycle characteristics of the electrochemical device are improved, which is preferable.

[0055] The Ca content in the hydrogenated nitrile rubber used is in the range of 1 to 2000 ppm, preferably 5 to 1500 ppm, more preferably 50 to 800 ppm, and most preferably 100 to 600 ppm. When the Ca content in the hydrogenated nitrile rubber is in this range, cracking of the electrode active material in the resulting electrode can be effectively suppressed, and the peel strength of the electrode can be increased. Furthermore, when the Ca content in the hydrogenated nitrile rubber is in this range, the viscosity characteristics of the conductive material dispersion can be increased when it is prepared, and the cycle characteristics of the resulting electrochemical element and cracking of the electrode active material after a cycle test can be suppressed, which is preferable.

[0056] The Rh and / or Ru content (total amount of Rh and Ru) in the hydrogenated nitrile rubber used is not particularly limited, but when it is in the range of usually 0.5 to 50 ppm, preferably 1 to 25 ppm, more preferably 2 to 20 ppm, cracking of the active material in the produced electrode can be suppressed, which is suitable.

[0057] The Na content in the hydrogenated nitrile rubber used is not particularly limited, but is usually 300 ppm or less, preferably 250 ppm or less, more preferably 100 ppm or less, and most preferably 50 ppm or less. When the Na content in the hydrogenated nitrile rubber is in this range, cracking of the active material in the electrode is suppressed and the cycle characteristics of the electrochemical element are improved, which is preferable.

[0058] The Mg content in the hydrogenated nitrile rubber used is not particularly limited, but is usually 50 ppm or less, preferably 30 ppm or less, and more preferably 10 ppm or less. When the Mg content in the hydrogenated nitrile rubber is in this range, cracking of the active material in the electrode is suppressed and the cycle characteristics of the electrochemical element are improved, which is preferable.

[0059] The hydrogenated nitrile rubber used is not particularly limited, but is preferably one obtained by emulsion polymerization of a nitrile group-containing monomer and a conjugated diene monomer, followed by hydrogenation of the polymer coagulated with calcium chloride. When obtaining a polymer by coagulating an emulsion polymerization solution, the size, shape, and properties of the resulting hydrous crumbs vary depending on the type of coagulant used, resulting in different residues of various secondary materials from the polymerization and coagulation processes that can be removed in the subsequent washing and dehydration processes. It is impossible to verify all traces of these secondary materials and their reaction products, and these trace residues affect various properties, particularly when used in the electrochemical device of the present invention. Therefore, for electrochemical device applications, it is preferable to use calcium chloride as a coagulant and then hydrogenate the isolated polymer. Furthermore, among calcium (Ca) compounds, those using calcium chloride are preferred because they are easily removed in the subsequent washing and purification processes, reducing the residue in the hydrogenated nitrile rubber.

[0060] The hydrogenated nitrile rubber to be used is preferably one obtained by emulsion polymerizing a nitrile group-containing monomer and a conjugated diene monomer, adding the phenolic antioxidant, and then coagulating the polymer with calcium chloride, followed by hydrogenating the resulting polymer.

[0061] The effect of the above specific antioxidant is far greater and more preferable when the hydrogenated nitrile rubber is produced by adding it to an emulsion polymerization liquid obtained by emulsion polymerizing a nitrile group-containing monomer and a conjugated diene monomer, rather than mixing it with the hydrogenated nitrile rubber after production. Although it is not possible to measure this, it is believed that by coagulating the emulsion polymerization liquid in which the phenolic antioxidant is uniformly dispersed with calcium chloride, the phenolic antioxidant is uniformly and finely dispersed in the hydrogenated nitrile rubber obtained as is, thereby showing a high effect.

[0062] <Method for Producing Hydrogenated Nitrile Rubber> The method for producing the hydrogenated nitrile rubber used is not particularly limited, but for example, the hydrogenated nitrile rubber can be produced by emulsion polymerizing a nitrile group-containing monomer and a conjugated diene monomer, adding the phenolic antioxidant, and then hydrogenating the polymer coagulated with calcium chloride.

[0063] (Emulsion Polymerization) The monomer components used are the same as those described above, and the amounts used may be appropriately selected so as to obtain the monomer composition of the hydrogenated nitrile rubber.

[0064] The emulsifier is not particularly limited, but examples thereof include anionic emulsifiers, cationic emulsifiers, and nonionic emulsifiers, and preferably includes anionic emulsifiers.

[0065] These emulsifiers can be used alone or in combination of two or more. The amount used is usually in the range of 0.01 to 10 parts by mass, preferably 0.1 to 5 parts by mass, and more preferably 1 to 3 parts by mass, per 100 parts by mass of the monomer component.

[0066] The polymerization catalyst used in emulsion polymerization is not particularly limited as long as it is one that is commonly used in emulsion polymerization, but for example, a redox catalyst consisting of a radical generator and a reducing agent can be used. Examples of the radical generator include peroxides and azo compounds, and peroxides are preferred. As the peroxide, inorganic peroxides and organic peroxides can be used.

[0067] The emulsion polymerization method may be a conventional method, and may be any of batch, semi-batch, and continuous. The polymerization temperature and polymerization time are not particularly limited and can be appropriately selected depending on the type of polymerization initiator used, etc. The polymerization temperature is usually in the range of 0 to 100°C, preferably 5 to 80°C, and more preferably 10 to 50°C, and the polymerization time is usually 0.5 to 100 hours, and preferably 1 to 10 hours.

[0068] (Addition of Antiaging Agent) The phenolic antioxidant used is the same as the exemplified phenolic antioxidant contained in the hydrogenated nitrile rubber, and the method of adding the phenolic antioxidant to the emulsion polymerization liquid is not particularly limited and may be in accordance with a conventional method. For example, the phenolic antioxidant may be added as is, or may be emulsified with an emulsifier and then added.

[0069] The amount of the phenolic antioxidant used may be appropriately selected so as to achieve the content of the antioxidant in the hydrogenated nitrile rubber of the present invention, and is usually in the range of 0.001 to 15 parts by mass, preferably 0.01 to 10 parts by mass, more preferably 0.1 to 5 parts by mass, particularly preferably 0.3 to 3 parts by mass, and most preferably 0.5 to 2 parts by mass, per 100 parts by mass of the monomer component.

[0070] (Coagulation step) The nitrile rubber to be hydrogenated is isolated by coagulating the emulsion polymerization liquid to which the antioxidant has been added with calcium chloride. In particular, by using calcium chloride as a coagulant and setting the Ca content in the hydrogenated nitrile rubber to a specific range, it is possible to contribute to the dispersibility (viscosity characteristics) of the conductive material dispersion liquid, thereby extremely increasing the peel strength of the produced electrode, and further to improve the cycle characteristics of the electrochemical element and the effect of suppressing cracking of the electrode active material after a cycle test, which is preferable.

[0071] The calcium chloride used is usually used as an aqueous solution, and the coagulant concentration of the aqueous solution is usually in the range of 0.1 to 70 mass%, preferably 1 to 60 mass%, more preferably 5 to 50 mass%, and particularly preferably 10 to 30 mass%, which is suitable because the particle size of the generated hydrous crumbs can be uniformly concentrated in a specific region.

[0072] The temperature in the coagulation reaction is not particularly limited, but a temperature of 40°C or higher, preferably 40 to 90°C, more preferably 50 to 80°C, is suitable for producing uniform water-containing crumbs.

[0073] (Washing and Drying Step) After the coagulation reaction, the coagulated product is washed with water and then dried to obtain a polymer to be hydrogenated.

[0074] The washing method is not particularly limited and may be a conventional method, but it is effective to wash the hydrous crumbs coagulated with the high-concentration aqueous calcium chloride solution with a large amount of water. The amount of water used is usually 10 to 500 times, preferably 25 to 250 times, more preferably 50 to 100 times, based on 100 parts by mass of the nitrile rubber polymer.

[0075] The temperature of the water used for washing is not particularly limited, but it is preferable to use warm water, and washing efficiency can be significantly improved when the temperature is usually 40° C. or higher, preferably 40 to 100° C., more preferably 50 to 90° C., and most preferably 60 to 80° C. By setting the temperature of the washing water to the above lower limit or higher, the emulsifier and coagulant are released from the water-containing crumbs, and washing efficiency is further improved.

[0076] The washing time is not particularly limited, but is usually in the range of 1 to 120 minutes, preferably 2 to 60 minutes, and more preferably 3 to 30 minutes.

[0077] The washed hydrous crumbs may be dried in a conventional manner, for example, using a dryer such as a hot air dryer, a reduced pressure dryer, an expander dryer, a kneader dryer, or a screw extruder.

[0078] (Hydrogenation Step) The polymer obtained above can be dissolved in a solvent and then hydrogenated using a hydrogenation catalyst. The hydrogenation reaction can be carried out according to a conventional method, for example, the method described in Japanese Patent No. 4509792, in which a metathesis reaction is carried out using a Grubbs catalyst and then hydrogenated using a Wilkinson catalyst.

[0079] The hydrogenation step is a step in which the polymer obtained above is hydrogenated using a hydrogenation catalyst, and may be carried out after the polymer obtained is subjected to metathesis using a ruthenium-based catalyst and a coolefin.

[0080] Metathesis Reaction The metathesis reaction can be carried out using the method described in Japanese Patent No. 4,509,792, for example.

[0081] As a catalyst for the metathesis reaction, known ruthenium-based catalysts can be used, among which Grubbs catalysts such as bis(tricyclohexylphosphine)benzylidene ruthenium dichloride and 1,3-bis(2,4,6-trimethylphenyl)-2-(imidazolidinylidene)(dichlorophenylmethylene)(tricyclohexylphosphine)ruthenium are preferred.

[0082] The metathesis reaction is carried out in the presence of a coolefin.

[0083] Examples of the coolefin include olefins having 2 to 16 carbon atoms, such as ethylene, isobutane, styrene, and 1-hexane, and functional group-containing unsaturated compounds, such as cis-2-butene-1,4-diol, 3-butene-1-amine, vinyltrimethoxysilane, methoxypolyalkylene glycol methacrylate, and 2-(methacryloyloxy)ethanesulfonic acid, with the functional group-containing unsaturated compounds being preferred.

[0084] The amount of coolefin used is usually in the range of 0.1 to 20 parts by mass, preferably 0.5 to 10 parts by mass, more preferably 1 to 5 parts by mass, per 100 parts by mass of the polymer.

[0085] The solvent may be any solvent that does not generally inactivate the catalyst or interfere with the reaction. Preferred solvents include, but are not limited to, dichloromethane, benzene, toluene, tetrahydrofuran, cyclohexane, monochlorobenzene (MCB), etc., and MCB is the most preferred. In some cases, the coolefin itself can act as the solvent, in which case no other solvent is required.

[0086] The polymer concentration in the metathesis reaction is not critical, but is usually in the range of 1 to 20% by weight, preferably 6 to 15% by weight.

[0087] The reaction solution in the metathesis reaction is usually stirred vigorously, for example, at a speed in the range of 200 to 1000 rpm, preferably 300 to 900 rpm, more preferably 500 to 800 rpm.

[0088] The temperature of the metathesis reaction is usually in the range of 20 to 140°C, preferably 60 to 120°C. The reaction time depends on many factors, such as the concentration of cement, the amount of catalyst used, and the reaction temperature, but is usually complete within 2 hours. The progress of the metathesis reaction can be monitored using standard analytical methods, such as GPC or solution viscosity.

[0089] Hydrogenation Reaction The hydrogenation reaction can be carried out by dissolving the polymer in a solvent and then adding a hydrogenation catalyst, but the hydrogenation reaction after the metathesis is preferably carried out in the same reaction vessel as that used for the metathesis reaction, by adding a hydrogenation catalyst to the vessel without isolating the metathesis product, and then carrying out hydrogenation treatment to produce hydrogenated nitrile rubber.

[0090] The solvent for the hydrogenation reaction may be either an aqueous solvent or an organic solvent, but is preferably an organic solvent. Suitable organic solvents include, for example, acetone, methyl ethyl ketone, ethyl acetate, tetrahydrofuran, 1,3-dioxane, benzene, toluene, methylene chloride, chloroform, monochlorobenzene (MCB), and dichlorobenzene. Among these, MCB is particularly suitable as it is a good solvent for both the nitrile group-containing nitrile rubber before hydrogenation and the hydrogenated nitrile rubber after hydrogenation.

[0091] The hydrogenation catalyst can be any method known in the art, without any particular limitations. For example, the method described in Japanese Patent No. 6309634 can be used, and in particular, Wilkinson's catalyst (PPh ３ ) ３ It is preferred to use known homogeneous hydrogenation catalysts such as RhCl. Also, the Grubbs catalyst is a dihydride complex (PR) which is itself an olefin hydrogenation catalyst in the presence of hydrogen. ３ ) ２ RuCl ２ H ２ The hydrogenation reaction can be carried out without adding a Wilkinson catalyst, but the hydrogenation rate tends to be slow. Specific examples of the hydrogenation catalyst that can be used in the hydrogenation step include ruthenium-based catalysts and rhodium-based catalysts.

[0092] The amount of the hydrogenation catalyst used may be appropriately selected depending on the purpose of use and the iodine value, but is usually in the range of 0.001 to 0.5 parts by mass, preferably 0.005 to 0.1 parts by mass, and more preferably 0.01 to 0.05 parts by mass, based on 100 parts by mass of the polymer before hydrogenation.

[0093] The hydrogenation reaction can use a co-catalyst as needed. Examples of the co-catalyst for the Wilkinson catalyst include phosphine, diphosphine, and triphenylphosphine, with triphenylphosphine being preferred. These co-catalysts can be used alone or in combination of two or more. The amount used is usually 0.01 to 15 parts by mass, preferably 0.1 to 10 parts by mass, and more preferably 0.5 to 5 parts by mass, based on 100 parts by mass of the polymer to be hydrogenated.

[0094] The polymer concentration in the hydrogenation reaction is not particularly limited as long as it is within a range in which the polymer can be dissolved, but is usually in the range of 1 to 30 mass %, preferably 5 to 25 mass %, more preferably 7 to 20 mass %.

[0095] The pressure of the hydrogenation reaction is not particularly limited, but is usually in the range of 0.1 to 30 MPa, preferably 1 to 20 MPa, and more preferably 5 to 15 MPa.

[0096] The hydrogenation reaction temperature is usually in the range of 30 to 200° C., preferably 50 to 170° C., more preferably 100 to 150° C. The reaction time is usually 1 to 50 hours, preferably 2 to 25 hours.

[0097] The hydrogenation reaction can be stopped once the desired hydrogenation level is reached by depressurizing or cooling the reactor. Residual hydrogen is typically removed with a nitrogen purge. The hydrogenation catalyst can also be removed prior to removal of the solvent and isolation of the hydrogenated nitrile rubber from the organic layer.

[0098] After the hydrogenation reaction, impurities in the hydrogenated nitrile rubber are removed, and the resulting product can be used as the positive electrode binder of the present invention.

[0099] (Purification Step) In the method for producing hydrogenated nitrile rubber bale of the present invention, a purification step is provided after the hydrogenation step to remove impurities from the hydrogenated polymer (hydrogenated nitrile rubber).

[0100] The method for removing impurities in the hydrogenated nitrile rubber is not particularly limited, but it is preferable to carry out a treatment with an adsorbent, which can easily adjust the calcium (Ca) content in the hydrogenated nitrile rubber to a value of 1 to 2000 ppm or reduce the hydrogenation catalyst.

[0101] The adsorbent is not particularly limited, but examples thereof include activated carbon, ion exchange resin, and synthetic zeolite, with activated carbon being preferred.

[0102] The adsorbent treatment can be carried out by adding an adsorbent to a reaction liquid containing hydrogenated nitrile rubber after hydrogenation and mixing. There are no particular limitations on the amount of adsorbent added, but it is usually in the range of 0.001 to 1 part by mass, preferably 0.05 to 0.5 parts by mass, and more preferably 0.01 to 0.4 parts by mass per 100 parts by mass of hydrogenated nitrile rubber. The mixing temperature is usually room temperature to 80°C, preferably room temperature to 60°C, and the mixing time is usually 1 minute to 1 hour, preferably 20 to 40 minutes.

[0103] After the treatment with the adsorbent, the adsorbent is removed by filtration or decantation, and the filtrate is dried to obtain the hydrogenated nitrile rubber.

[0104] <Hydrogenated Nitrile Rubber Bale> The hydrogenated nitrile rubber bale used is the above-mentioned hydrogenated nitrile rubber baled. Baling is preferable because it allows the effects of the positive electrode binder of the present invention to be exhibited evenly.

[0105] The shape of the hydrogenated nitrile rubber bale is not particularly limited, but is usually a rectangular parallelepiped. The size is also not particularly limited, but the width is usually in the range of 100 to 800 mm, preferably 200 to 500 mm, and more preferably 250 to 450 mm, the length is usually in the range of 300 to 1200 mm, preferably 400 to 1000 mm, and more preferably 500 to 800 mm, and the height is usually in the range of 50 to 500 mm, preferably 100 to 300 mm, and more preferably 150 to 250 mm.

[0106] Baling of hydrogenated nitrile rubber can be carried out according to conventional methods, for example, by placing the dried rubber in a baler and compressing it. The compression pressure is selected appropriately depending on the intended use, but is usually in the range of 0.1 to 15 MPa, preferably 0.5 to 10 MPa, and more preferably 1 to 5 MPa. The compression time is not particularly limited, but is usually in the range of 1 to 60 seconds, preferably 5 to 50 seconds, and more preferably 10 to 40 seconds. Alternatively, dried rubber can be produced in sheet form and layered to form bales. Baling by layering sheets is easy to produce, produces bales with few bubbles (high specific gravity), and is therefore preferable because it has excellent storage stability.

[0107] When the hydrogenated nitrile rubber veil is used as a binder for a positive electrode, it is finely divided and dissolved or dispersed in a solvent.

[0108] [Positive Electrode Binder] The positive electrode binder of the present invention is obtained by dissolving or dispersing the hydrogenated nitrile rubber or the hydrogenated nitrile rubber veil in N-methylpyrrolidone (NMP), and is suitable as a material for producing a positive electrode of an electrochemical element. Specifically, the positive electrode binder of the present invention. When mixed with a conductive material to form a conductive material dispersion, the conductive material dispersion has excellent viscosity characteristics (dispersibility) and stability. Therefore, the produced electrode is smooth and has improved electrical conductivity and peel strength. This is also effective in suppressing the output characteristics and cycle characteristics of the electrochemical element and cracking of the electrode active material after cycle testing.

[0109] The positive electrode binder of the present invention can be combined with other components as needed, in addition to hydrogenated nitrile rubber and NMP. Examples of other components include, but are not limited to, binders other than hydrogenated nitrile rubber (such as polyvinylidene fluoride and polyacrylate), reinforcing materials, leveling agents, viscosity modifiers, and electrolyte additives. These components are not particularly limited as long as they do not affect the battery reaction, and known components can be used. The positive electrode binder of the present invention may also contain a solvent other than NMP to the extent that the properties of the present invention are not impaired. These other components may be used alone, or two or more may be combined in any ratio.

[0110] The solid content of the binder for a positive electrode of the present invention is not particularly limited, but is usually in the range of 0.1 to 40 mass%, preferably 0.5 to 20 mass%, more preferably 1 to 10 mass%. The hydrogenated nitrile rubber, NMP, and other components used as needed may be mixed in a conventional manner.

[0111] [Electrochemical element] The positive electrode binder of the present invention can be used in the production of a positive electrode by mixing it with a conductive material and then mixing it with a positive electrode active material to obtain a positive electrode mixture layer slurry, which can be applied to a current collector and dried.

[0112] (Conductive Material) The conductive material is a component that functions to ensure electrical contact between electrode active materials. Carbonaceous materials can be suitably used as the conductive material. Examples of such carbonaceous materials include carbon black (e.g., acetylene black, Ketjen Black (registered trademark), furnace black, etc.), single-walled or multi-walled carbon nanotubes (multi-walled carbon nanotubes include cup-stacked types), carbon nanohorns, vapor-grown carbon fibers, milled carbon fibers obtained by calcining and then crushing polymer fibers, single-walled or multi-walled graphene, and carbon nonwoven fabric sheets obtained by calcining nonwoven fabrics made of polymer fibers. These materials may be used alone or in combination of two or more types in any ratio. Among these, carbon nanotubes (CNTs) are preferred from the viewpoint of forming good conductive paths.

[0113] When mixing the positive electrode binder of the present invention with the conductive material, the ratio of the conductive material to the positive electrode binder is not particularly limited. For example, the conductive material and the positive electrode binder may be mixed so that the resulting conductive material dispersion contains typically 1 to 100 parts by mass, preferably 5 to 50 parts by mass, and more preferably 10 to 30 parts by mass of hydrogenated nitrile rubber per 100 parts by mass of the conductive material. Furthermore, when mixing the positive electrode binder and the conductive material, NMP can be added as needed to adjust the viscosity.

[0114] The method for mixing the positive electrode binder of the present invention and the conductive material is not particularly limited, and for example, they can be mixed using a known mixing device.

[0115] (Positive Electrode Active Material) The positive electrode active material is not particularly limited, but when the electrochemical element is a lithium ion secondary battery, a metal oxide containing lithium (Li) can be used. The positive electrode active material is preferably a positive electrode active material containing at least one selected from the group consisting of cobalt (Co), nickel (Ni), manganese (Mn), and iron (Fe) in addition to lithium (Li). Such a positive electrode active material is preferably a lithium-containing cobalt oxide (LiCoO ２ ), lithium manganese oxide (LiMn ２ O ４ ), lithium-containing nickel oxide (LiNiO ２ ), Co—Ni—Mn lithium-containing composite oxide, Ni—Mn—Al lithium-containing composite oxide, Ni—Co—Al lithium-containing composite oxide, olivine-type lithium manganese phosphate (LiMnPO ４ ), olivine-type lithium iron phosphate (LiFePO ４ ), Li １＋ｘ Mn ２－ｘ O ４ (0<X<2) Lithium-excess spinel compound represented by Li[Ni ０．１７ Li ０．２ Co ０．０７ Mn ０．５６ ]O ２ , LiNi ０．５ Mn １．５ O ４ , Li[Ni ０．５ Co ０．２ Mn ０．３ ]O ２The particle size of the positive electrode active material is not particularly limited and can be the same as that of a conventionally used electrode active material. The positive electrode active material may be used alone or in combination of two or more types in any ratio.

[0116] The positive electrode active material can be mixed with the positive electrode binder and conductive material to form a positive electrode composite layer slurry. The mixing method is not particularly limited and can be performed using a known mixing device. The amount of the positive electrode active material is not particularly limited and can be within a range that has been conventionally used.

[0117] <Electrode for electrochemical element> A positive electrode using the positive electrode binder of the present invention can be obtained by applying the above-mentioned positive electrode mixture layer slurry onto a current collector and drying it. Since the positive electrode mixture layer is formed containing the above-mentioned hydrogenated nitrile rubber, it has excellent flexibility and electrode peel strength, and is less likely to crack the active material.

[0118] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, the units "%", "ppm" and "parts" that represent amounts are by mass unless otherwise specified.

[0119] Furthermore, in a polymer produced by copolymerizing a plurality of types of monomers, the ratio of a monomer unit formed by polymerizing a certain monomer in the polymer usually coincides with the ratio (feed ratio) of the certain monomer to all the monomers used in the polymerization of the polymer, unless otherwise specified. Furthermore, when the polymer is a hydrogenated polymer obtained by hydrogenating a polymer containing conjugated diene monomer units, the total content of unhydrogenated conjugated diene monomer units and alkylene structural units as hydrogenated conjugated diene monomer units in the hydrogenated polymer coincides with the ratio (feed ratio) of the conjugated diene monomer to all the monomers used in the polymerization of the polymer.

[0120] In the examples and comparative examples, various measurements and evaluations were carried out according to the following methods.

[0121] <Iodine Value> The iodine value of the hydrogenated nitrile rubber was measured in accordance with JIS K 6235.

[0122] <Weight-average molecular weight> The weight-average molecular weight (Mw) of the hydrogenated nitrile rubber was measured by gel permeation chromatography (GPC) using a 10 mM LiBr-dimethylformamide (DMF) solution under the following measurement conditions: Separation column: Shodex KD-806M (manufactured by Showa Denko K.K.) Detector: Differential refractometer detector RID-10A (manufactured by Shimadzu Corporation) Eluent flow rate: 0.3 mL / min Column temperature: 40°C Standard polymer: TSK standard polystyrene (manufactured by Tosoh Corporation)

[0123] <Metal Content> Approximately 0.5 g of hydrogenated nitrile rubber, calculated as solid content, was collected and dissolved in approximately 5 mL of concentrated sulfuric acid. The sample was then carbonized on a heater and then incinerated in an electric furnace at 1000°C for approximately 3 hours. After cooling, approximately 2 mL of concentrated nitric acid was gradually added to the sample for wet decomposition. After decomposition, the acid was concentrated and adjusted to a constant volume of 10 mL with ultrapure water, and the metal ion concentration was measured using an ICP-MS (ICP-MS 7900, manufactured by Agilent). The metal ion concentration contained in the hydrogenated nitrile rubber was then calculated from the measured value. The metal ions measured were calcium ions, rhodium ions, ruthenium ions, palladium ions, iron ions, sodium ions, and magnesium ions.

[0124] <Viscosity of Conductive Material Dispersion> The viscosity of the conductive material dispersion obtained in each Example and Comparative Example was measured for 120 seconds at a temperature of 25°C and a shear rate of 10 (1 / s) using a rheometer ("MCR302" manufactured by Anton Paar), and the average viscosity measured from 61 seconds to 120 seconds was evaluated according to the following criteria: A: 1 Pa s or less B: More than 1 Pa s and 5 Pa s or less C: More than 5 Pa s and 10 Pa s or less D: More than 10 Pa s

[0125] <Conductive material dispersion stability> After measuring the dispersion viscosity (η0) of the conductive material dispersion obtained in each example and comparative example, the dispersion was stored in a sealed container at 25°C for 10 days. After that, the dispersion viscosity was measured again (η1), and the viscosity change rate was calculated according to the following formula, which was used as the conductive material dispersion stability. Evaluation was performed according to the following criteria. Viscosity change rate Δη = η1 / η0 × 100 A: Viscosity change rate Δη is 90% or more and 110% or less B: Viscosity change rate Δη is 80% or more and less than 90% C: Viscosity change rate Δη is 70% or more and less than 80% D: Viscosity change rate Δη is less than 70% or more than 110%

[0126] <Peel Strength> The positive electrodes prepared in the Examples and Comparative Examples were cut into rectangular specimens measuring 100 mm in length and 10 mm in width to prepare test pieces. Cellophane tape (compliant with JIS Z1522) was attached to the surface of the positive electrode composite layer with the side having the positive electrode composite layer facing down, and the stress when peeled off was measured by pulling one end of the current collector vertically at a pulling rate of 100 mm / min (note that the cellophane tape was fixed to the test table). Three measurements were performed, and the average value was calculated as the peel strength, which was evaluated according to the following criteria. A: Peel strength of 15 N / m or more; B: Peel strength of 10 N / m or more but less than 15 N / m; C: Peel strength of 5 N / m or more but less than 10 N / m; D: Peel strength of less than 5 N / m.

[0127] <Cycle Characteristics> After injecting the electrolyte, the lithium-ion secondary batteries prepared in the Examples and Comparative Examples were left standing at 25°C for 5 hours. Next, they were charged to a cell voltage of 3.65 V at 25°C using a constant current of 0.2 C, and then aged for 12 hours at 60°C. Then, they were discharged to a cell voltage of 3.00 V using a constant current of 0.2 C at 25°C. Subsequently, they were subjected to CC-CV charging (upper cell voltage 4.20 V) using a constant current of 0.2 C, and CC discharging to 3.00 V using a constant current of 0.2 C. This 0.2 C charge-discharge cycle was repeated three times. Next, 300 cycles of charge-discharge were performed at a charge-discharge rate of 1.0 C at a cell voltage of 4.20-3.00 V in a 45°C environment. The discharge capacity at the first cycle was defined as X1, and the discharge capacity at the 300th cycle as X2. Using the discharge capacities X1 and X2, the capacity retention rate = (X2 / X1) x 100 (%) was calculated and evaluated according to the following criteria. A larger capacity retention rate indicates that the lithium ion secondary battery has better cycle characteristics. A: Capacity retention rate is 85% or more B: Capacity retention rate is 80% or more but less than 85% C: Capacity retention rate is 75% or more but less than 80% D: Capacity retention rate is less than 75%

[0128] <Presence or absence of cracks in active material after cycling> The lithium ion secondary batteries produced in Examples and Comparative Examples were disassembled after the cycle test, and cross-sectional SEM images of the removed positive electrodes were observed at 1000x magnification. The number of cracks observed in the active material was evaluated as follows according to the number of cracks observed in the active material: A: Less than 2 cracked active material pieces B: 2 or more but less than 5 cracked active material pieces C: 5 or more but less than 9 cracked active material pieces D: 9 or more cracked active material pieces

[0129] <Output Characteristics> The output characteristics of the lithium-ion secondary batteries prepared in the Examples and Comparative Examples were evaluated by measuring the I-V resistance using the following method. As with the cycle characteristics, the lithium-ion secondary batteries were subjected to aging treatment and three charge / discharge cycles three times. They were then charged at 25°C until the charge rate reached 50% (i.e., until the charge capacity reached 350 mAh for a 700 mAh cell capacity). The resistance was measured using the following procedure: (1) Determine the voltage change ΔV(0.2) after 30 seconds of discharge using a 0.2 C constant current method (I0.2). (2) Charge the battery for 30 seconds using a 0.2 C constant current method. (3) Determine the voltage change ΔV(1.0) after 30 seconds of discharge using a 1.0 C constant current method (I1.0). I-V resistance (Ω) = {ΔV(1.0) - ΔV(0.2)} / (I1.0 - I0.2). Evaluation was performed based on the calculated I-V resistance values ​​as follows: A: 0.25Ω or less B: More than 0.25Ω and less than 0.30Ω C: More than 0.30Ω and less than 0.35 D: More than 0.35Ω

[0130] Example 1 <Production of Hydrogenated Nitrile Rubber> - Emulsion Polymerization Step to Washing and Drying Step - A reactor having an internal volume of 10 liters was charged with 100 parts of ion-exchanged water, 33 parts of acrylonitrile as an α,β-ethylenically unsaturated nitrile monomer, and 67 parts of 1,3-butadiene as a conjugated diene monomer, and 2 parts of potassium oleate as an emulsifier, 0.1 part of potassium phosphate as a stabilizer, and further, 0.3 parts of tert-dodecyl mercaptan (TDM) as a molecular weight modifier. Emulsion polymerization was carried out at a temperature of 5°C in the presence of 0.35 parts of potassium persulfate as a polymerization initiator, to copolymerize acrylonitrile and 1,3-butadiene.

[0131] When the polymerization conversion rate reached 89%, 0.2 parts of hydroxylamine sulfate per 100 parts of monomer was added to terminate the polymerization. Subsequently, the mixture was heated and subjected to steam distillation at about 90°C under reduced pressure to recover the residual monomer, and then 0.1 parts of dibutylhydroxytoluene (BHT), a phenolic antioxidant, was added to obtain an emulsion polymerization liquid.

[0132] Calcium chloride (CaCl) was used as a coagulant in an amount of 3 parts per 100 parts of polymer solids in the resulting emulsion polymerization liquid. ２A 25% aqueous solution of 2,4-dimethylaminobenzoate (2,4-dimethylaminobenzoate) was added with stirring to coagulate the polymer in the emulsion polymerization solution. The polymer was then filtered, washed with ion-exchanged water in an amount 50 times the amount of the obtained polymer, and then dried at a temperature of 90° C. under reduced pressure to obtain a polymer.

[0133] - Hydrogenation Step I (Metathesis Reaction) - Next, 9 parts of the obtained polymer were dissolved in 141 parts of monochlorobenzene, a halogenated hydrocarbon, and the solution was charged into a reactor. The reactor was then heated to 80°C, and 2 L of a monochlorobenzene solution containing bis(tricyclohexylphosphine)benzylidene ruthenium dichloride as a Grubbs catalyst was added so that the amount of Grubbs catalyst relative to the polymer was 1,000 ppm. Then, 4.4 parts of cis-2-butene-1,4-diol as a coolefin was added per 100 parts of Precursor 1, and the metathesis reaction of the polymer was carried out at a stirring speed of 600 rpm. During the reaction, the temperature was maintained constant using a cooling coil connected to a temperature control device and a heat sensor.

[0134] - Hydrogenation step II (hydrogenation reaction) - After that, the inside of the reactor was pressurized with 0.7 MPa of H ２ The reactor was degassed three times at 100°C. The temperature of the reactor was then raised to 130°C, and 1 L of a monochlorobenzene solution containing a Wilkinson catalyst and triphenylphosphine was added to the reactor. The amount of Wilkinson catalyst was 0.03 parts and the amount of triphenylphosphine was 1 part per 100 parts of the polymer. The temperature was then raised to 138°C, and the polymer was hydrogenated under a hydrogen pressure (gauge pressure) of 8.4 MPa.

[0135] - Purification Step - After completion of the hydrogenation reaction, 0.2 parts of activated carbon with an average diameter of 15 μm was added to the reactor and stirred for 30 minutes to adjust the concentration of divalent or higher metal ions, including calcium ions. The mixture was then filtered through a filter with a pore size of 5 μm. The filtered solution was then heated and dried until the monochlorobenzene solvent concentration reached 50 ppm, and hydrogenated nitrile rubber A was isolated. The iodine value, weight average molecular weight, and metal content of the obtained hydrogenated nitrile rubber A were measured. The results are shown in Table 1. Furthermore, 20 parts (20 kg) of the isolated nitrile rubber A were loaded into a 300 × 650 × 300 mm baler and compacted at a pressure of 3 MPa for 30 seconds to obtain nitrile rubber bale A. When hydrogenated nitrile rubber bale A was subjected to the same measurements as hydrogenated nitrile rubber A, the measured values ​​were the same.

[0136] <Preparation of positive electrode binder> 920 parts of NMP was weighed into a 2 L vessel equipped with a stirring blade and heated to 80° C. Next, 80 parts of hydrogenated nitrile rubber A cut into pieces of about 1 cm square from the hydrogenated nitrile rubber bale A produced above was added and dissolved with continued stirring for 5 hours to prepare a positive electrode binder A with a solids concentration of 8%.

[0137] <Preparation of Conductive Material Dispersion> Multi-walled carbon nanotubes (BET specific surface area: 250 m) were used as the conductive material. ２ 5 parts of a positive electrode binder A having a solids concentration of 8% obtained as described above (corresponding to 1 part as solids), and 82.5 parts of NMP as an organic solvent were added, and the mixture was stirred using a disper (3000 rpm, 10 minutes). Thereafter, a bead mill ("LMZ015" manufactured by Ashizawa Fine Tech) using zirconia beads having a diameter of 1 mm was used to mix the mixture at a peripheral speed of 8 m / s for 1 hour to prepare a conductive material dispersion, and the viscosity of the dispersion was measured, and the results are shown in Table 1. The stability of the conductive material dispersion was also evaluated, and the results are shown in Table 1.

[0138] <Preparation of slurry for secondary battery positive electrode> A ternary active material (LiNi) having a layered structure was added as a positive electrode active material to the above-mentioned conductive material dispersion. ０．５ Co ０．２ Mn ０．３ O ２98.0 parts of 10 μm (average particle size), 1.0 part of polyvinylidene fluoride as a binder, 1.0 part of the conductive material dispersion (solid content equivalent), and NMP were added and mixed with a planetary mixer (60 rpm, 30 minutes) to prepare a positive electrode slurry. The amount of NMP added was adjusted so that the viscosity of the resulting positive electrode slurry (measured with a single cylindrical rotational viscometer in accordance with JIS Z8803:1991, temperature: 25°C, rotation speed: 60 rpm) was within the range of 4000 to 5000 mPa s.

[0139] <Preparation of Positive Electrode> An aluminum foil having a thickness of 20 μm was prepared as a current collector. The positive electrode slurry was applied to the aluminum foil using a comma coater so that the coating weight after drying was 20 mg / cm. ２ The coating was applied so that the density became 3.5 g / cm. The coating was dried at 120°C for 5 minutes and at 130°C for 5 minutes, and then heat-treated at 60°C for 10 hours to obtain a positive electrode blank. The positive electrode blank was rolled using a roll press to obtain a positive electrode blank having a density of 3.5 g / cm. ３ A sheet-shaped positive electrode was fabricated from the positive electrode mixture layer and aluminum foil. This sheet-shaped positive electrode was cut into a width of 4.8 cm and a length of 50 cm to prepare a positive electrode for a lithium ion secondary battery.

[0140] The peel strength of the obtained positive electrode for a lithium ion secondary battery was evaluated, and the results are shown in Table 1.

[0141] <Preparation of a negative electrode for a lithium ion secondary battery> 33 parts of 1,3-butadiene as an aliphatic conjugated diene monomer, 3.5 parts of itaconic acid as a carboxylic acid group-containing monomer, 63.5 parts of styrene as an aromatic vinyl monomer, 0.4 parts of sodium dodecylbenzenesulfonate as an emulsifier, 150 parts of ion-exchanged water, and 0.5 parts of potassium persulfate as a polymerization initiator were placed in a 5 MPa pressure vessel equipped with a stirrer, and after thorough stirring, the mixture was heated to 50 ° C to initiate polymerization. When the polymerization conversion rate reached 96%, the mixture was cooled to terminate the polymerization reaction, yielding a mixture containing a particulate binder (styrene-butadiene copolymer). A 5% aqueous sodium hydroxide solution was added to the mixture to adjust the pH to 8, and unreacted monomer was removed by heated vacuum distillation. The mixture was then cooled to 30 ° C or below to obtain an aqueous dispersion containing a negative electrode binder.

[0142] Next, 48.75 parts of artificial graphite and 48.75 parts of natural graphite as negative electrode active materials, and 1 part of carboxymethyl cellulose as a thickener were added to a planetary mixer. The mixture was then diluted with ion-exchanged water to a solids concentration of 60%, and then kneaded for 60 minutes at a rotation speed of 45 rpm. Then, 1.5 parts (solids equivalent) of the aqueous dispersion containing the negative electrode binder obtained as described above was added, and the mixture was kneaded for 40 minutes at a rotation speed of 40 rpm. Then, ion-exchanged water was added to the mixture to a viscosity of 3000±500 mPa s (measured with a Brookfield viscometer at 25°C and 60 rpm), thereby preparing a negative electrode composite layer slurry.

[0143] Next, a copper foil having a thickness of 15 μm was prepared as a current collector. The negative electrode slurry was applied to both sides of the copper foil in a coated amount of 10 mg / cm after drying. ２ The negative electrode blank was rolled using a roll press to a density of 1.6 g / cm. ３ A sheet-shaped negative electrode was fabricated consisting of the negative electrode mixture layers (both sides) and copper foil. The sheet-shaped negative electrode was then cut into a piece 5.0 cm wide and 52 cm long to prepare a negative electrode for a lithium ion secondary battery.

[0144] <Fabrication of Lithium-Ion Secondary Battery> The fabricated lithium-ion secondary battery positive electrode and lithium-ion secondary battery negative electrode were arranged with the electrode mixture layers facing each other, and a 15 μm thick separator (a polyethylene microporous film) was interposed between them. The resulting wound body was then wound around a core with a diameter of 20 mm to obtain a wound body. The resulting wound body was then compressed in one direction at a speed of 10 mm / sec until it reached a thickness of 4.5 mm. The compressed wound body had an elliptical shape in plan view, and the ratio of its major axis to its minor axis (major axis / minor axis) was 7.7.

[0145] In addition, the electrolyte is LiPF with a concentration of 1.0 M. ６ A solution (solvent: a mixed solvent of ethylene carbonate (EC) / diethyl carbonate (DEC)=3 / 7 (volume ratio), additive: containing 2 vol % (solvent ratio) of vinylene carbonate) was prepared.

[0146] The compressed wound body was then housed in an aluminum laminate case together with 3.2 g of non-aqueous electrolyte. A nickel lead wire was connected to a predetermined location on the negative electrode, and an aluminum lead wire was connected to a predetermined location on the positive electrode. The opening of the case was then thermally sealed to obtain a lithium-ion secondary battery. This lithium-ion secondary battery was a pouch-shaped battery of a predetermined size capable of housing the wound body, and had a nominal capacity of 700 mAh.

[0147] The resulting lithium ion secondary battery was evaluated for output characteristics, cycle characteristics, and cracking of the electrode active material after cycling. The results are shown in Table 1.

[0148] Example 2 Hydrogenated nitrile rubber B was obtained and positive electrode binder B was produced in the same manner as in Example 1, except that the amount of activated carbon used in the purification step was changed to 0.5 parts. The properties of electrochemical devices using hydrogenated nitrile rubber B and positive electrode binder B were evaluated, and the results are shown in Table 1.

[0149] Example 3 Hydrogenated nitrile rubber C was obtained and positive electrode binder C was produced in the same manner as in Example 1, except that the amount of activated carbon used in the purification step was changed to 0.08 parts. The properties of electrochemical devices using hydrogenated nitrile rubber C and positive electrode binder C were evaluated, and the results are shown in Table 1.

[0150] (Example 4) Hydrogenated nitrile rubber D was obtained and positive electrode binder D was produced in the same manner as in Example 1, except that the amount of activated carbon used in the purification step was changed to 0.07 parts. The properties of electrochemical devices using hydrogenated nitrile rubber D and positive electrode binder D were evaluated, and the results are shown in Table 1. (Example 5)

[0151] Except for changing the amount of activated carbon used in the purification step to 0.05 parts, hydrogenated nitrile rubber E was obtained and positive electrode binder E was produced in the same manner as in Example 1. The properties of electrochemical devices using hydrogenated nitrile rubber E and positive electrode binder E were evaluated, and the results are shown in Table 1.

[0152] Example 6 Hydrogenated nitrile rubber F was obtained and positive electrode binder F was produced in the same manner as in Example 1, except that the amount of activated carbon used in the purification step was changed to 0.02 parts. The properties of electrochemical devices using hydrogenated nitrile rubber F and positive electrode binder F were evaluated, and the results are shown in Table 1.

[0153] (Example 7) Except for changing the amount of Grubbs catalyst in the metathesis reaction to 3000 ppm, hydrogenated nitrile rubber G was obtained in the same manner as in Example 1. The properties of an electrochemical device using hydrogenated nitrile rubber G and positive electrode binder G were evaluated, and the results are shown in Table 1.

[0154] (Example 8) Hydrogenated nitrile rubber H was obtained in the same manner as in Example 1, except that the amount of Grubbs catalyst in the metathesis reaction was changed to 500 ppm. The properties of an electrochemical device using hydrogenated nitrile rubber H and positive electrode binder H were evaluated, and the results are shown in Table 1.

[0155] (Example 9) Except for changing the amount of Grubbs catalyst in the metathesis reaction to 400 ppm, hydrogenated nitrile rubber I was obtained in the same manner as in Example 1. The properties of an electrochemical device using hydrogenated nitrile rubber I and positive electrode binder I were evaluated, and the results are shown in Table 1.

[0156] (Example 10) Except for changing the amount of Grubbs catalyst in the metathesis reaction to 200 ppm, hydrogenated nitrile rubber J was obtained in the same manner as in Example 1. The properties of an electrochemical device using hydrogenated nitrile rubber J and positive electrode binder J were evaluated, and the results are shown in Table 2.

[0157] (Comparative Example 1) Except for not adding an antioxidant, hydrogenated nitrile rubber K was obtained in the same manner as in Example 1. The properties of an electrochemical device using hydrogenated nitrile rubber K and positive electrode binder K were evaluated, and the results are shown in Table 1.

[0158] Comparative Example 2: No antioxidant was added, and 1 L of a monochlorobenzene solution containing Wilkinson's catalyst and triphenylphosphine was not added to the reactor in the hydrogenation step II. ２) was placed in an autoclave so that the Pd content relative to the polymer was 1000 ppm, and a hydrogenation reaction was carried out under conditions of a temperature of 90°C and a hydrogen pressure of 3.0 MPa. Hydrogenated nitrile rubber L was obtained in the same manner as in Example 1, except that in the purification step, activated carbon was added and 1 part of aminopropyl group-modified silica (trade name "QuadraSil AP", manufactured by SIGMA-ALDRICH Corporation, silica with aminopropyl groups introduced on the surface, average particle size 54 μm) was added. The characteristics of an electrochemical device using hydrogenated nitrile rubber L and positive electrode binder L were evaluated, and the results are shown in Table 1.

[0159]

[0160] From Table 1, it can be seen that the positive electrode binders A to J of the present invention are prepared by dissolving or dispersing specific hydrogenated nitrile rubber in N-methylpyrrolidone (NMP), and improve the viscosity characteristics and stability of the conductive material dispersion liquid, thereby improving the peel strength of the produced electrode, the output characteristics and cycle characteristics of the electrochemical element, and the effect of suppressing cracking of the electrode active material after cycle testing, and that each property is highly balanced (comparison between Examples 1 to 10 and Comparative Examples 1 and 2).

[0161] The viscosity characteristics (dispersibility) of the conductive material dispersion correlate with the Ca content in the hydrogenated nitrile rubber, and the higher the Ca content, the higher the viscosity and the worse the dispersibility tends to be (comparison among Examples 1 to 6). It is also clear that the dispersibility of the conductive material dispersion is greatly affected by the iodine value of the hydrogenated nitrile rubber, and that the viscosity of the dispersion rises sharply as the iodine value increases (comparison among Examples 7 to 10). Table 1 shows that the viscosity characteristics (dispersibility) of the conductive material dispersion are excellent when the iodine value of the hydrogenated nitrile rubber is small and the Ca content in the hydrogenated nitrile rubber is small.

[0162] The stability of the conductive material dispersion is greatly affected by the presence or absence of dibutylhydroxytoluene (BHT), a phenolic antioxidant, and Table 1 shows that the stability is extremely deteriorated when BHT is not present (Comparative Examples 1 and 2). The stability of the conductive material dispersion is also affected by the iodine value of the hydrogenated nitrile rubber, and it is found that the smaller the iodine value, the better the conductive material dispersion (Examples 7 to 10).

[0163] Table 1 shows that the peel strength of the electrode tends to deteriorate as the iodine value of the hydrogenated nitrile rubber decreases (Example 7). It is also clear that the peel strength is affected by the calcium (Ca) content in the hydrogenated nitrile rubber, with a higher content being preferable (comparison between Examples 1 to 6).

[0164] As can be seen from Table 1, the output characteristics of the electrochemical element tend to deteriorate as the iodine value of the hydrogenated nitrile rubber increases, but otherwise all of the Examples show good results.

[0165] It can be seen from Table 1 that the cycle characteristics of the electrochemical element are greatly affected by the amount of Ca in the hydrogenated nitrile rubber, with a smaller amount being preferable (comparison among Examples 1 to 6).

[0166] Regarding the prevention of cracking of the electrode active material after the cycle test of the electrochemical element, it is clear from Table 1 that a larger amount of Ca in the hydrogenated nitrile rubber is preferable (comparison among Examples 1 to 6). Furthermore, Table 1 also shows that the prevention of cracking of the electrode active material after the cycle test of the electrochemical element is rapidly deteriorated when a large amount of Pd, which is a hydrogenation catalyst residue, remains in the hydrogenated nitrile rubber (Comparative Example 2).

[0167] On the other hand, although not shown in the comparative examples in this specification, those with excessively high Rh and / or Ru contents, Mg contents, and Na contents have poor cycle characteristics of electrochemical elements and cause cracking of the electrode active material after cycle testing. However, Table 1 shows that, as long as the contents of these metals are in the amounts contained in the hydrogenated nitrile rubber of the positive electrode binders A to J of the present invention, they do not affect any of the properties of the viscosity characteristics and stability of the conductive material dispersion, peel strength, output characteristics, cycle characteristics, and the effect of suppressing cracking of the electrode active material after cycle testing.

[0168] Furthermore, Table 1 shows that the evaluation of the six characteristics of an electrochemical device and its manufacturing process is greatly affected by the magnitude or slight differences in the hydrogenated nitrile rubber factors, often resulting in conflicting results, making it difficult to balance these six characteristics. Among these, it can be seen that the use of positive electrode binders A to J of the present invention allows these characteristics to be highly balanced. In particular, positive electrode binders C and D of the present invention achieved an A rating in all evaluation items, indicating that the electrochemical device is optimized when the Ca content in the hydrogenated nitrile rubber is within a specific range.

[0169] Furthermore, from the examples and Table 1, it is clear that the novel hydrogenated nitrile rubbers A to J used can be produced by a combination of known methods, such as that [Mw] can be adjusted by the amount of catalyst for metathesis reaction, that [iodine value] can be adjusted by the amount of hydrogenation catalyst, the molecular weight of the polymer after metathesis reaction, and monitoring of the hydrogenation rate, that the [Mg, Na] content can be adjusted without using any auxiliary materials that would be a cause in the production steps such as the emulsion polymerization step and the hydrogenation step, and that [Ca] used in the coagulant and [Ru, Rh] used in the hydrogenation catalyst can be adjusted by washing with water and treating with an adsorbent.

[0170] According to the present invention, it is possible to provide a positive electrode binder that can improve the viscosity characteristics and stability of a conductive material dispersion, as well as increase the electrode peel strength and the ability to prevent cracking of the active material, thereby improving the cycle characteristics and output characteristics of an electrochemical element.

Claims

1. A positive electrode binder obtained by dissolving or dispersing in N-methylpyrrolidone (NMP) a hydrogenated nitrile rubber which contains a nitrile group-containing monomer unit, a conjugated diene monomer unit and / or an alkylene structural unit, has a weight average molecular weight (Mw) of 1,000 to 1,000,000, an iodine value in the range of 0.1 to 100 mg / 100 mg, contains a phenol-based antioxidant, and has a palladium (Pd) content of 50 ppm or less and a calcium (Ca) content in the range of 1 to 2000 ppm.

2. The positive electrode binder according to claim 1, wherein the hydrogenated nitrile rubber has a palladium (Pd) content of 10 ppm or less.

3. The positive electrode binder according to claim 1, wherein the hydrogenated nitrile rubber contains rhodium (Rh) and / or ruthenium (Ru) in an amount ranging from 0.5 to 50 ppm.

4. The positive electrode binder according to claim 1, wherein the hydrogenated nitrile rubber has a sodium (Na) content of 300 ppm or less and a magnesium (Mg) content of 50 ppm or less.

5. The positive electrode binder according to claim 1, wherein said hydrogenated nitrile rubber is obtained by emulsion polymerizing a nitrile group-containing monomer and a conjugated diene monomer, coagulating the polymer with calcium chloride, and hydrogenating the polymer.

6. The positive electrode binder according to claim 1, wherein the hydrogenated nitrile rubber is obtained by emulsion polymerizing a nitrile group-containing monomer and a conjugated diene monomer, adding a phenolic antioxidant, coagulating the polymer with calcium chloride, and hydrogenating the polymer.

7. The positive electrode binder according to claim 1, wherein the ratio of the nitrile group-containing monomer units and the other repeating units other than the conjugated diene monomer units and / or alkylene structural units in the hydrogenated nitrile rubber is 0% by mass or more and 5% by mass or less.

8. The positive electrode binder according to claim 1, wherein the content of the phenol-based antioxidant is in the range of 0.001 to 2 mass %.

9. The positive electrode binder according to claim 1, wherein the solid content is in the range of 0.1 to 40 mass %.

10. A method for producing a positive electrode binder, comprising a step of dissolving or dispersing in N-methylpyrrolidone (NMP) a hydrogenated nitrile rubber which contains a nitrile group-containing monomer unit, a conjugated diene monomer unit and / or an alkylene structural unit, has a weight average molecular weight (Mw) of 1,000 to 1,000,000, an iodine value in the range of 0.1 to 100 mg / 100 mg, contains a phenol-based antioxidant, and has a palladium (Pd) content of 50 ppm or less and a calcium (Ca) content in the range of 1 to 2000 ppm.

11. The method for producing a positive electrode binder according to claim 10, wherein the hydrogenated nitrile rubber is obtained by dividing hydrogenated nitrile rubber veil.

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