Binder suitable for electricity storage device electrodes, binder solution for electricity storage device electrodes, electricity storage device electrode slurry, electricity storage device electrode, and electricity storage device

Abstract

The present invention provides a binder which has a peel strength that is high enough to suppress particle fall-off during cutting of an electrode, and which is suitable for the achievement of an electrode having a low resistance. The present invention discloses a binder for electricity storage device electrodes, said binder containing a polyvinyl alcohol resin and an electrolyte solution-swellable resin that has an electrolyte solution swelling ratio of 10% by mass or more, said electrolyte solution swelling ratio being represented by formula (1). (1): (Electrolyte solution swelling ratio) = ((W2 - W1) / W1 × 100) (In formula (1), W1 represents the mass (g) of the resin before immersion in the electrolyte solution; and W2 represents the mass (g) of the resin after being immersed in diethyl carbonate at 25°C for 24 hours.) With respect to this binder for electricity storage device electrodes, the polyvinyl alcohol resin has a viscosity of 4 Pa·s or more as measured in the form of an aqueous solution having a solid content concentration of 10% by mass at 25°C at a shear rate of 10 s-1.

Description

[Technical Field]

[0001] This patent application claims priority under the Paris Convention to Japanese Patent Application No. 2020-103476 (filing date: June 16, 2020), the entire contents of which are incorporated herein by reference. The present invention relates to a binder suitable for electricity storage device electrodes, a binder solution for electricity storage device electrodes, an electricity storage device electrode slurry, an electricity storage device electrode, and an electricity storage device. [Background technology]

[0002] In recent years, the use of mobile devices such as mobile phones, notebook computers, and pad-type information terminals has become increasingly widespread. As mobile devices are rapidly becoming smaller, thinner, lighter, and more powerful due to the demand for more comfortable portability, batteries used in these devices are also being required to be smaller, thinner, lighter, and more powerful. Lithium-ion secondary batteries are widely used as power storage devices for such mobile devices. Power storage devices such as lithium-ion secondary batteries have a structure in which a positive electrode and a negative electrode are placed with a separator between them, and these electrodes are housed in a container together with an electrolyte solution in which a lithium salt such as LiPF6, LiBF4, LiTFSI (lithium bistrifluoromethylsulfonylimide), or LiFSI (lithium bisfluorosulfonylimide) is dissolved in an organic liquid such as ethylene carbonate.

[0003] The negative and positive electrodes that constitute the electricity storage device are usually formed by dissolving or dispersing a binder and a thickener in water or a solvent, mixing this with an active material, a conductive aid (conductivity-imparting agent), etc. to obtain an electrode slurry, which is then applied to a current collector, and then drying the water or solvent to bind the resulting mixture into a mixed layer.

[0004] From the viewpoints of reducing the burden on the environment and simplifying device manufacturing, there has been a rapid trend toward using aqueous media for electrode slurries, particularly in the manufacture of negative electrodes. Known binders for such aqueous electrode slurries include vinyl alcohol-based polymers, acrylic polymers such as acrylic acid, and amide / imide polymer binders (e.g., Patent Documents 1 and 2).

[0005] On the other hand, in the production of positive electrodes, electrode slurries using solvents are generally used. Examples of such solvents include organic solvents such as N-methyl-2-pyrrolidone, dimethylformamide, N,N-dimethylacetamide, N,N-dimethylmethanesulfonamide, and hexamethylphosphoric triamide. Known binders used in such electrode slurries for organic solvents include vinylidene fluoride polymers, tetrafluoroethylene polymers, and fluororubbers (e.g., Patent Documents 3 and 4). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 11-250915 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-59527 [Patent Document 3] Japanese Patent Application Publication No. 2017-107827 [Patent Document 4] Japanese Patent Application Laid-Open No. 2013-37955 Summary of the Invention [Problem to be solved by the invention]

[0007] However, to meet the demand for further miniaturization, thinning, and weight reduction, improvements in energy density are required for both negative and positive electrodes. This has led to an increasing demand for electrodes with increased amounts of active material and reduced amounts of binder. Therefore, there is a need for electrodes with high peel strength that can suppress powder falling off when cutting electrodes, even with compositions containing a small amount of binder.

[0008] Furthermore, from the viewpoint of shortening charging time, there is a demand for batteries that can suppress capacity loss during high-speed charging compared to low-speed charging. The capacity loss during high-speed charging is thought to be mainly due to energy loss associated with internal resistance, and therefore electrodes with low resistance are required.

[0009] In view of the above problems, an object of the present invention is to provide a binder that, when used in an electrode for an electricity storage device, has high peel strength that can suppress powder falling off when the electrode is cut, and is suitable for obtaining an electrode with low resistance. [Means for solving the problem]

[0010] As a result of extensive research, the present inventors have found that when a binder having a predetermined composition is used, an electrode having excellent peel strength and low resistance can be obtained.

[0011] That is, the present invention includes the following preferred embodiments. [1] A polyvinyl alcohol resin and a compound represented by the following formula (1) Electrolyte swelling ratio = ((W2 - W1) / W1 × 100) (1) [In formula (1), W1 is the mass (g) of the resin before immersion in the electrolyte, and W2 is the mass (g) of this resin after immersion in diethyl carbonate at 25°C for 24 hours.] and an electrolyte-swellable resin having an electrolyte swelling rate of 10 mass % or more, The polyvinyl alcohol resin is in an aqueous solution state with a solid content of 10% by mass, and the aqueous solution is heated at 25°C with a shear rate of 10 s -1 A binder having a viscosity of 4 Pa·s or more. [2] The polyvinyl alcohol resin, in an aqueous solution state having a solid content of 10% by mass, is dissolved at a shear rate of 10 s at 25°C. -1 The viscosity of the aqueous solution is 4 Pa·s or more and 30 Pa·s or less at a shear rate of 10 s at 25°C. -1 Viscosity and shear rate at 100 s -1 The thixotropic index, defined as the ratio of the viscosity at the Noba Inder. [3] The polyvinyl alcohol resin is in an N-methyl-2-pyrrolidone solution state having a solid content of 7.5% by mass, and the solution is heated at a shear rate of 10 s at 25°C. -1 The viscosity of the solution is 4 Pa·s or more and 35 Pa·s or less at a shear rate of 10 s at 25°C. -1 Viscosity and shear rate at 100 s -1 The thixotropic index, defined as the ratio of the viscosity at the temperature to the viscosity at the temperature of 2 or more and 6 or less, is as described in [1] or [2]. Noba Inder. [4] The polyvinyl alcohol-based resin according to any one of [1] to [3], which is a vinyl alcohol-based polymer having a crosslinked structure. Noba Inder. [5] The method according to any one of [1] to [4], wherein the modification rate of the polyvinyl alcohol resin is 0.02 mol % or more and 5 mol % or less based on the number of moles of all monomer units constituting the polyvinyl alcohol resin. Noba Inder. [6] The electrolyte-swellable resin is represented by the following formula (2): Electrolyte elution rate=((W1−W3) / W1×100) (2) [In formula (2), W1 is the mass (g) of the resin before immersion in the electrolyte, and W3 (g) is the mass of the resin after immersing it in diethyl carbonate for 24 hours at 25°C and then drying it in a hot air dryer at 80°C for 3 hours.] [1] to [5], wherein the electrolyte elution rate represented by the formula (I) is 5 mass % or less. Noba Inder. [7] An electrode for an electricity storage device, comprising the binder according to any one of [1] to [6]. [8][1]~[ 6

[0023] A binder solution for an electrode of an electricity storage device, comprising the binder according to any one of [1] to

[10] and a solvent. [9] An electrode slurry for a storage battery device, comprising the binder solution for a storage battery device electrode according to [8] and an active material. [10 ]Ba The electricity storage device electrode slurry according to [9], wherein the binder content is 0.1 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the active material.

[11] An electricity storage device electrode comprising a cured product of the electricity storage device electrode slurry according to [9] or

[10] and a current collector.

[12] An electricity storage device comprising the electricity storage device electrode according to

[11] . [Effects of the Invention]

[0012] According to the present invention, when used in an electrode for an electricity storage device, an electrode having high peel strength and low resistance can be obtained, which allows the amount of binder added to be reduced in the electrode, and as a result, an electrode is obtained which does not powder off when cut and can obtain high capacity even during high-rate charge and discharge.

[0013] Hereinafter, embodiments of the present invention will be described in detail, but it is not intended that the present invention be limited to the following embodiments.

[0014] <Binders (electrode binders for power storage devices)> The binder of the present invention, preferably an electricity storage device electrode binder (hereinafter also simply referred to as "the binder of the present invention" or "binder") contains a polyvinyl alcohol-based resin and an electrolyte-swellable resin having an electrolyte swelling property of 10 mass % or more, as measured by the method described below.

[0015] The polyvinyl alcohol resin, which is one of the constituent elements of the binder of the present invention, includes a vinyl alcohol polymer and its derivatives. Vinyl alcohol polymers (also referred to herein as "polyvinyl alcohol" or simply "PVA") have good affinity with active materials used in electricity storage devices, such as carbon materials, metals, and metal oxides, and therefore, when used as a binder, electrodes with high peel strength can be obtained.

[0016] In the present invention, PVA is used in the form of an aqueous solution having a solid content of 10% by mass at 25°C and a shear rate of 10 s -1 A viscosity of 4 Pa ​​s or more is preferable because, as will be described later, in the state of an electrode slurry for an electricity storage device (hereinafter simply referred to as "slurry of the present invention" or "slurry"), the adhesion between the cured product, particularly the active material and optional conductive additives that may be contained in the cured product, and the current collector is improved, allowing for the formation of an electrode with excellent peel strength. The viscosity is more preferably 4.5 Pa s or more, even more preferably 5 Pa s or more, still more preferably 6 Pa s or more, and even more preferably 9 Pa s or more.

[0017] In the present invention, PVA is used in the form of an aqueous solution having a solid content of 10% by mass at 25°C and a shear rate of 10 s -1 It is preferable that the viscosity is 30 Pa s or less, since this maintains the dispersibility of the active material, etc. in the slurry, thereby enhancing the effect of suppressing aggregation or sedimentation of the active material, etc. The viscosity is more preferably 28.5 Pa s or less, even more preferably 28 Pa s or less, and even more preferably 27.5 Pa s or less.

[0018] In the present invention, PVA is used in a predetermined aqueous solution state, and the aqueous solution is heated at a shear rate of 10 s at 25°C. -1 Viscosity and shear rate at 100 s -1 Viscosity ratio at shear rate 10 s -1 Viscosity at shear rate 100s -1A thixotropic index (hereinafter also simply referred to as "TI") defined as the viscosity at room temperature (at 2000 K) is preferably 1.8 or higher, since a uniform electrode film can be formed when the slurry is applied to a current collector. The thixotropic index is more preferably 2 or higher, even more preferably 2.2 or higher, and even more preferably 2.22 or higher. A TI of 5 or lower is also preferred, since the fluidity of the slurry is maintained when left standing, resulting in good coatability, and is more preferably 4 or lower, even more preferably 3.5 or lower.

[0019] In the present invention, PVA is a polymer that, in an N-methyl-2-pyrrolidone (hereinafter simply referred to as "NMP") solution state having a solid content of 7.5 mass % (hereinafter also referred to as "predetermined NMP solution state"), is heated at a shear rate of 10 s at 25°C. -1 If the viscosity is 4 Pa·s or more in the slurry state using NMP as a solvent, the adhesiveness between the cured product, particularly the active material and optionally the conductive additive, that may be contained in the cured product, and the current collector is improved, and an electrode with extremely excellent peel strength can be formed. The viscosity is more preferably 5 Pa·s or more, even more preferably 8.5 Pa·s or more, and even more preferably 15 Pa·s or more. In addition, when the viscosity is 25°C in the above-mentioned predetermined NMP solution state at a shear rate of 10 s -1 A viscosity of 35 Pa s or less in NMP-based slurry is preferable because it maintains the dispersibility of the active material, etc. in the slurry, and as a result, it is possible to enhance the effect of suppressing aggregation or sedimentation of the active material, etc. The viscosity is more preferably 33 Pa s or less, even more preferably 27 Pa s or less, and even more preferably 24 Pa s or less.

[0020] It is preferable that the thixotropic index (TI) of PVA in the above-mentioned predetermined NMP solution state is 2 or more, because a uniform electrode film can be formed when the slurry is applied to a current collector, and it is more preferably 2.5 or more, and even more preferably 2.7 or more. Furthermore, it is preferable that the TI of PVA in the above-mentioned predetermined NMP solution state is 6 or less, because the fluidity of the slurry is maintained when left standing and the coatability is good, and it is more preferably 5 or less, and even more preferably 4 or less.

[0021] In this specification, the shear rate of PVA in a predetermined aqueous solution state or a predetermined NMP solution state is 10 s at 25°C. -1 and 100s -1 The viscosity at 1000 kJ / min can be measured using each solution as a measurement sample, for example, with an E-type viscometer.

[0022] As the PVA, an uncrosslinked, unmodified PVA may be used, and the viscosity may be adjusted by using a thickener or the like in combination. However, it is preferable to use a vinyl alcohol polymer having a crosslinked structure, since the viscosity and TI in the above-mentioned specified aqueous solution state and NMP solution state can be easily controlled within the above-mentioned specified ranges.

[0023] The method for obtaining a vinyl alcohol polymer having a crosslinked structure is not particularly limited. Examples of the method include a method of heat-treating PVA under a nitrogen or air atmosphere, a method of treating unmodified PVA with an acid, and a method of chemically crosslinking with a multifunctional additive. Alternatively, a modified PVA capable of forming a crosslinked structure may be used.

[0024] Examples of acidic substances that can be used in the acid treatment of unmodified PVA include inorganic acids such as acetic acid, hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid; organic acids such as formic acid, acetic acid, oxalic acid, and p-toluenesulfonic acid; salts such as pyridinium p-toluenesulfonate and ammonium chloride; and Lewis acids such as zinc chloride, aluminum chloride, iron trichloride, tin dichloride, tin tetrachloride, and boron trifluoride diethyl ether complex. One or more of these may be used in combination. The amount of these acidic substances added is preferably 0.0001 to 5 parts by weight per 100 parts by weight of PVA.

[0025] Examples of polyfunctional additives that can be used in the method of chemical crosslinking with a polyfunctional additive include dials such as glyoxal and 1,4-butanedial, diepoxides such as ethylene glycol diglycidyl ether and diethylene glycol diglycidyl ether, and diisocyanates such as hexamethylene diisocyanate and toluene diisocyanate. The amount of these polyfunctional additives added is preferably 0.0001 to 5 parts by mass per 100 parts by mass of PVA.

[0026] As the PVA, one obtained by saponifying a vinyl ester polymer can be used.

[0027] The vinyl ester monomer used in the production of the vinyl ester polymer is not particularly limited, but examples thereof include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl versatate, vinyl caproate, vinyl caprylate, vinyl laurate, vinyl palmitate, vinyl stearate, vinyl oleate, vinyl benzoate, etc. Of these, vinyl acetate is preferred from an economical viewpoint.

[0028] The PVA may be an unmodified PVA consisting only of vinyl alcohol units, but preferably a modified PVA containing units derived from a monomer other than vinyl alcohol units, i.e., units derived from monomer (a), as a modifying component, can be used to construct a crosslinked structure. Specifically, a PVA having a crosslinked structure can be obtained by heating a polymer composed of vinyl alcohol monomer and monomer (a) to crosslink the polymer by forming an ester bond.

[0029] Such a monomer (a) may be at least one monomer selected from the group consisting of a carboxylic acid having an unsaturated double bond, an alkyl ester of the carboxylic acid, an acid anhydride of the carboxylic acid, a salt of the carboxylic acid, and a silyl compound having an unsaturated double bond.

[0030] Examples of carboxylic acids having an unsaturated double bond, alkyl esters of the carboxylic acids, acid anhydrides of the carboxylic acids, and salts of the carboxylic acids include maleic acid, monomethyl maleate, dimethyl maleate, monoethyl maleate, diethyl maleate, maleic anhydride, citraconic acid, monomethyl citraconic acid, dimethyl citraconic acid, diethyl citraconic acid, citraconic anhydride, fumaric acid, monomethyl fumarate, dimethyl fumarate, monoethyl fumarate, diethyl fumarate, itaconic acid, monomethyl itaconate, dimethyl itaconate, monoethyl itaconate, diethyl itaconate, itaconic anhydride, acrylic acid, methyl acrylate, ethyl acrylate, methacrylic acid, methyl methacrylate, and ethyl methacrylate.

[0031] Examples of silyl compounds having an unsaturated double bond include compounds having an unsaturated double bond and a trialkoxysilyl group, such as vinyltrimethoxysilane and vinyltriethoxysilane.

[0032] Among these monomers (a), monomethyl maleate, dimethyl maleate, monomethyl citraconic acid, monomethyl fumarate, monomethyl itaconate, itaconic anhydride, methyl acrylate, methyl methacrylate, and vinyltrimethoxysilane are preferred, as they can form a crosslinked structure that makes it easy to control the viscosity, and monomethyl maleate, dimethyl maleate, monomethyl fumarate, itaconic anhydride, methyl acrylate, methyl methacrylate, and vinyltrimethoxysilane are more preferred.

[0033] The modification rate of PVA is not particularly limited, but is preferably 0.02 mol % or more and 5 mol % or less based on the number of moles of all monomer units constituting the PVA. The modification rate of PVA is the content of units derived from monomer (a) based on the number of moles of all monomer units of the PVA.

[0034] The lower limit of the modification rate is more preferably 0.05 mol% or more, even more preferably 0.1 mol% or more, and even more preferably 0.2 mol% or more. The upper limit of the modification rate is more preferably 4.5 mol% or less, even more preferably 3 mol% or less, and even more preferably 1.1 mol% or less. By setting the modification rate within the above range, it becomes easier to set the viscosity range and TI range described above.

[0035] The modification rate of PVA was measured by using a vinyl ester polymer, which is a precursor of PVA, as in the examples described below. 1 It can be determined by a method using H-NMR.

[0036] The PVA may contain units (structural units) derived from other monomers other than vinyl alcohol units and units derived from monomer (a), provided that the effects of the present invention are not impaired. Examples of units derived from other monomers include units derived from α-olefins such as ethylene, propylene, n-butene, and isobutylene; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, and 2,3-diacetoxy-1-vinyloxypropane; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride and vinyl fluoride; vinylidene halides such as vinylidene chloride and vinylidene fluoride; allyl compounds such as allyl acetate, 2,3-diacetoxy-1-allyloxypropane, and allyl chloride; and isopropenyl acetate. The content of units derived from the other monomers in the PVA can be, for example, 15 mol % or less based on the number of moles of all monomer units constituting the PVA.

[0037] There is no particular limitation on the arrangement order of the vinyl alcohol units, the units derived from the monomer (a) and the units derived from the other monomers in the PVA, and they may be random, block or alternating.

[0038] The lower limit of the saponification degree of PVA (the molar fraction of hydroxyl groups relative to the total of hydroxyl groups and ester bonds in PVA) is preferably 20 mol% or more, more preferably 60 mol% or more, even more preferably 70 mol% or more, still more preferably 75 mol% or more, and even more preferably 80 mol% or more. The upper limit of the saponification degree of PVA may be 100 mol%, but is preferably 99.99 mol% or less, more preferably 99 mol% or less. The saponification degree can be measured in accordance with JIS-K6726:1994.

[0039] The upper limit of the viscosity-average degree of polymerization of the PVA is preferably 5,000 or less, more preferably 4,000 or less. Meanwhile, the lower limit of the viscosity-average degree of polymerization of the PVA is preferably 100 or more, more preferably 500 or more, and even more preferably 1,000 or more. When the PVA has a crosslinked structure, it is preferable that the viscosity-average degree of polymerization of the PVA before crosslinking be within the aforementioned range. A high viscosity-average degree of polymerization of the PVA increases the viscosity of the solution in the above-mentioned predetermined aqueous solution state or the above-mentioned predetermined NMP solution state. Furthermore, a high viscosity-average degree of polymerization of the PVA makes it easy to adjust the TI of the solution in the aqueous solution state or the NMP solution state to within the above-mentioned predetermined range. By setting the viscosity-average degree of polymerization of the PVA to 100 or more, the viscosity and TI of the PVA-containing binder in the aqueous solution or NMP solution state can be easily adjusted to within the above-mentioned predetermined range without excessively decreasing, thereby improving the adhesiveness of the adhesive layer formed from the binder-containing slurry. On the other hand, when the viscosity-average degree of polymerization of PVA is 5000 or less, the viscosity and TI in the aqueous solution or NMP solution state do not increase too much, and further, the productivity of PVA is improved, making it possible to produce PVA at lower cost.

[0040] The viscosity-average degree of polymerization (P) of PVA can be calculated by completely saponifying and purifying the PVA, and then measuring the intrinsic viscosity [η] (unit: liters / g) in a sodium chloride aqueous solution (0.5 mol / L) at 30°C for PVA containing units derived from monomer (a), or by measuring the intrinsic viscosity [η] (unit: liters / g) in an aqueous solution at 30°C for PVA not containing units derived from monomer (a). The viscosity-average degree of polymerization (P) of PVA can be calculated from the intrinsic viscosity [η] using the following formula: P = ([η] × 104 / 8.29)(1 / 0.62)

[0041] The binder of the present invention is suitable for use in an electrode of an electricity storage device, and by containing an electrolyte-swellable resin in addition to PVA, the ionic conductivity of the electrode is easily improved. Specifically, the binder of the present invention contains an electrolyte-swellable resin having an electrolyte swelling ratio, represented by the following formula (1), of 10% by mass or more, preferably 15% by mass or more, and more preferably 20% by mass or more: Electrolyte swelling ratio = ((W2 - W1) / W1 × 100) (1) [In formula (1), W1 is the mass (g) of the resin before immersion in the electrolyte, and W2 is the mass (g) of this resin after immersion in diethyl carbonate at 25°C for 24 hours.]

[0042] Such electrolyte-swellable resins are polymers that swell in an electrolyte solution, conduct lithium ions, and do not conduct electrons. In the present invention, methods for imparting electrolyte-swellable properties to resins include introducing functional groups with large steric hindrance, such as acetal groups or halogen functional groups, to increase the free volume between polymer chains, or introducing functional groups or monomer units with high affinity for carbonate ester solvents. Examples of such electrolyte-swellable resins include, but are not limited to, fluorine-based resins, polyolefin-based resins, polyester-based resins, polyacrylic resins, polyamide-based resins, polyurethane-based resins, cellulose-based resins, carbohydrate-based resins, polyol-based resins, polyvinyl acetal-based resins, polyvinyl formal-based resins, polyvinyl acetate-based resins, and mixtures of two or more of these. Specifically, polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), polyvinylidene fluoride-trichloroethylene, polymethyl methacrylate (PMMA), polybutyl acrylate, polyacrylonitrile (PAN), polyvinylpyrrolidone, polyvinyl acetate (PVAc), ethylene-vinyl acetate copolymer, polyethylene oxide (PEO), polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose (CMC), polyvinyl acetal, polyvinyl formal, and the like can be used, but are not limited to these.

[0043] The polyvinyl acetal resin mainly has a structural unit derived from vinyl alcohol and a structural unit derived from vinyl ester, but may contain a structural unit derived from a monomer other than these structural units within a range that does not impair the effects of the present invention. Examples of the other monomer include α-olefins such as ethylene, propylene, 1-butene, isobutene, and 1-hexene; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, phthalic acid, phthalic anhydride, maleic acid, maleic anhydride, itaconic acid, and itaconic anhydride, and salts thereof or alkyl esters thereof having 1 to 18 carbon atoms; acrylamide, N-alkylacrylamide having 1 to 18 carbon atoms, N,N-dimethylacrylamide, 2-acrylamidopropanesulfonic acid and salts thereof, acrylic acid, methacrylic acid, crotonic acid, phthalic acid, phthalic anhydride, maleic acid, maleic anhydride, itaconic acid, and itaconic anhydride; acrylamides such as propylamidopropyldimethylamine and its acid salts or quaternary salts; methacrylamide, N-alkylmethacrylamides having 1 to 18 carbon atoms, N,N-dimethylmethacrylamide, 2-methacrylamidopropanesulfonic acid and its salts, methacrylamidopropyldimethylamine and its acid salts or quaternary salts; N-vinylamides such as N-vinylpyrrolidone, N-vinylformamide, and N-vinylacetamide; acrylonitrile, methacrylonitrile vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, and n-butyl vinyl ether; allyl acetate; allyl ethers such as propyl allyl ether, butyl allyl ether, and hexyl allyl ether; vinyl halides such as vinyl chloride, vinyl fluoride, and vinyl bromide; vinylidene halides such as vinylidene chloride and vinylidene fluoride; vinyl silanes such as trimethoxyvinylsilane; compounds having an oxyalkylene group such as polyoxyalkylene allyl ether; isopropenyl acetate; α-olefins containing a hydroxy group such as 3-buten-1-ol, 4-penten-1-ol, 5-hexen-1-ol, 7-octen-1-ol, 9-decen-1-ol, and 3-methyl-3-buten-1-ol; compounds having a carboxyl group derived from fumaric acid, maleic acid, itaconic acid, maleic anhydride, phthalic anhydride, trimellitic anhydride, etc.Examples of the monomer include a monomer having a sulfonic acid group derived from ethylene sulfonic acid, allyl sulfonic acid, methallyl sulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, etc.; and a compound having a cationic group derived from vinyloxyethyl trimethylammonium chloride, vinyloxybutyl trimethylammonium chloride, vinyloxyethyl dimethylamine, vinyloxymethyl diethylamine, N-acrylamidomethyl trimethylammonium chloride, N-acrylamidoethyl trimethylammonium chloride, N-acrylamidodimethylamine, allyl trimethylammonium chloride, methallyl trimethylammonium chloride, dimethylallylamine, allylethylamine, etc. Among these, from the viewpoints of availability and copolymerizability, preferred are α-olefins such as ethylene, propylene, 1-butene, isobutene, and 1-hexene; N-vinylamides such as N-vinylpyrrolidone, N-vinylformamide, and N-vinylacetamide; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, and n-butyl vinyl ether; allyl acetate; allyl ethers such as propyl allyl ether, butyl allyl ether, and hexyl allyl ether; monomers having an oxyalkylene group such as polyoxyalkylene allyl ether; and α-olefins containing a hydroxy group such as 3-buten-1-ol, 4-penten-1-ol, 5-hexen-1-ol, 7-octen-1-ol, 9-decen-1-ol, and 3-methyl-3-buten-1-ol. These monomers can be used alone or in combination.

[0044] The degree of polymerization, degree of acetalization, degree of saponification, etc. of the polyvinyl acetal resin are not particularly limited, and any polyvinyl acetal resin can be used. Examples of aldehydes used for acetalization include aliphatic aldehydes such as formaldehyde, acetaldehyde, propyl aldehyde, n-butyl aldehyde (1-butanol), sec-butyl aldehyde, octyl aldehyde, and dodecyl aldehyde; alicyclic aldehydes such as cyclohexane carbaldehyde, cyclooctane carbaldehyde, trimethylcyclohexane carbaldehyde, cyclopentyl aldehyde, dimethylcyclohexane carbaldehyde, methylcyclohexane carbaldehyde, and methylcyclopentyl aldehyde; α-camphorene aldehyde, phellandral, cyclocitral, trimethyltetrahydrobenzol, and the like. Examples of suitable aldehydes include terpene aldehydes such as azaldehyde, α-pyronenaldehyde, myrtenal, dihydromyrtenal, and camphenylaldehyde; aromatic aldehydes such as benzaldehyde, naphthaldehyde, anthraldehyde, phenylacetaldehyde, tolualdehyde, dimethylbenzaldehyde, cuminaldehyde, and benzylaldehyde; unsaturated aldehydes such as cyclohexene aldehyde, dimethylcyclohexene aldehyde, and acrolein; aldehydes having heterocycles such as furfural and 5-methylfurfural; hemiacetals such as glucose and glucosamine; and aldehydes having amino groups such as 4-aminobutyraldehyde. These aldehydes can be used alone or in combination. Among these, aliphatic aldehydes such as n-butylaldehyde (1-butanol) are preferred from the viewpoint of easily increasing the capacity retention rate of batteries. In addition, aliphatic ketones such as 2-propanone, methyl ethyl ketone, 3-pentanone, and 2-hexanone; alicyclic ketones such as cyclopentanone and cyclohexanone; and aromatic ketones such as acetophenone and benzophenone can also be used in place of or in combination with the aldehyde.

[0045] Furthermore, it is preferable that the electrolyte-swellable resin has a low elution rate in the electrolyte, since this can prevent a decrease in durability due to elution of the binder during use of the electricity storage device. From this perspective, the electrolyte elution rate represented by the following formula (2) is preferably 5 mass% or less. Electrolyte elution rate=((W1−W3) / W1×100) (2) [In formula (2), W1 is the mass (g) of the resin before immersion in the electrolyte, and W3 (g) is the mass of the resin after immersing it in diethyl carbonate for 24 hours at 25°C and then drying it in a hot air dryer at 80°C for 3 hours.]

[0046] The higher the content of the electrolyte-swellable resin in the binder of the present invention, the lower the resistance. From this viewpoint, the content of the electrolyte-swellable resin per 100 parts by mass of PVA is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and particularly preferably 15 parts by mass or more. On the other hand, the lower the content of the electrolyte-swellable resin in the binder of the present invention, the higher the peel strength of the electrode. From this viewpoint, the content of the electrolyte-swellable resin per 100 parts by mass of PVA is preferably 900 parts by mass or less, more preferably 600 parts by mass or less, and particularly preferably 300 parts by mass or less.

[0047] The binder of the present invention may further contain a material that adjusts the viscosity of the binder in an aqueous solution or NMP solution. Examples of viscosity-adjusting materials include polybasic acids such as citric acid, tartaric acid, and aspartic acid, their salts, condensates thereof, and inorganic substances such as fumed silica and alumina. The amount of these added is not particularly limited, but is typically preferably 0.01 to 10 parts by mass, more preferably 0.02 to 8 parts by mass, and even more preferably 0.05 to 5 parts by mass, per 100 parts by mass of the binder. The more of such viscosity-adjusting materials added, the more the viscosity and TI of the binder in a solution state of the present invention can be increased, making it easier to adjust them within a specified range. The smaller the particle size of the inorganic substance added, the more easily the viscosity and TI of the binder in an aqueous solution or NMP solution state can be increased.

[0048] The binder of the present invention or the power storage device electrode binder solution of the present invention described below can further contain additives to the extent that the effects of the present invention are not impaired. Examples of additives include light stabilizers, UV absorbers, freeze stabilizers, thickeners, leveling agents, rheology stabilizers, thixotropic agents, antifoaming agents, plasticizers, lubricants, preservatives, rust inhibitors, antistatic agents, anti-yellowing agents, pH adjusters, film-forming aids, curing catalysts, crosslinking reaction catalysts, crosslinking agents (e.g., glyoxal, urea resins, melamine resins, polyvalent metal salts, polyvalent isocyanates, polyamide epichlorohydrin), and dispersants. These additives can be selected or combined according to the intended purpose. The content of the additives is, for example, 10% by mass or less, preferably 5% by mass or less, and more preferably 1% by mass or less, based on the total amount of the binder or power storage device electrode binder solution.

[0049] The binder of the present invention may be obtained by dissolving PVA and, if necessary, other components in a solvent (water or NMP) to form a solution, and then removing the solvent. Alternatively, the solution may be used directly as the binder solution for an electrode of the present invention, as described below, in the subsequent preparation of a slurry. In this case, the composition of the components in the binder solution other than the solvent is the binder of the present invention. The binder of the present invention is contained in a cured product of the slurry composition of the present invention in a state where it is mixed with components such as an active material.

[0050] <Electrode binder solution for energy storage devices> The binder of the present invention is dissolved in at least one solvent to obtain an electrode binder solution for an electricity storage device (hereinafter, simply referred to as "binder solution" or "binder solution of the present invention"). The solvent is not particularly limited, but water or NMP is preferred. Water is preferable as the solvent from the viewpoint of reducing the environmental load and simplifying the equipment. On the other hand, NMP is preferable as the solvent because it does not deteriorate the active material in the slurry, particularly when used as a positive electrode slurry.

[0051] In addition to the binder of the present invention described above, the binder solution may contain an additive (referred to as additive A) that can be dissolved in a solvent, provided that the effects of the present invention are not impaired. Examples of additive A include polyethylene glycol, polyethylene glycol dimethyl ether, polyethylene glycol diglycidyl ether, and polyethyleneimine. The content of additive A is, for example, 10% by mass or less, preferably 5% by mass or less, and more preferably 1% by mass or less, based on the total amount of the binder solution. In particular, it is preferable that additive A is not contained.

[0052] The binder solution is obtained by mixing the binder of the present invention, a solvent (water or NMP), and optional components other than the binder, such as those described above, by a known method, such as stirring. The mixing temperature and mixing time can be adjusted appropriately depending on the type of solvent. The binder solution refers to a solution in which the binder described above is dissolved in a solvent. The "dissolved state" refers to a state in which the mass of the binder completely dissolved in the solvent is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, even more preferably 99% by mass or more, and even more preferably 100% by mass, relative to the total mass (100% by mass) of the binder used to prepare the binder solution.

[0053] The binder content in the binder solution of the present invention is preferably 1% by mass or more and 30% by mass or less, more preferably 3% by mass or more and 20% by mass or less, and particularly preferably 5% by mass or more and 15% by mass or less, based on the total amount of the binder solution. When the binder content is 1% by mass or more, the adhesion of the active material to the current collector when forming an electrode is easily improved. When the binder content is 30% by mass or less, rapid aggregation of the active material when forming an electrode can be suppressed.

[0054] <Electrode slurry for power storage devices> An electricity storage device electrode slurry as one embodiment of the present invention contains the binder solution of the present invention described above and an active material.

[0055] The slurry of the present invention may be used for either a positive electrode or a negative electrode. It may also be used for both a positive electrode and a negative electrode. Therefore, the active material may be either a positive electrode active material or a negative electrode active material. Preferably, when the binder solution of the present invention contains water (when the solvent is water), it contains a negative electrode active material and is used as a negative electrode slurry. Preferably, when the binder solution of the present invention contains NMP (when the solvent is NMP), it contains a positive electrode active material and is used as a positive electrode slurry.

[0056] As the negative electrode active material, for example, a material that has been conventionally used as a negative electrode active material for power storage devices can be used. Examples of such a material include carbonaceous materials such as amorphous carbon, artificial graphite, natural graphite (graphite), mesocarbon microbeads (MCMB), pitch-based carbon fiber, carbon black, activated carbon, carbon fiber, hard carbon, soft carbon, mesoporous carbon, and conductive polymers such as polyacene; SiO x , SnO x and LiTiO x Examples of the negative electrode active material include composite metal oxides represented by the formula (I) and other metal oxides, lithium metal, lithium-based metals such as lithium alloys, metal compounds such as TiS2 and LiTiS2, and composite materials of metal oxides and carbonaceous materials. Among these, from the viewpoints of economy and battery capacity, graphite is preferred, and spherical natural graphite is particularly preferred. These negative electrode active materials can be used alone or in combination of two or more.

[0057] The positive electrode active material can be, for example, a material that has been conventionally used as a positive electrode active material in power storage devices, such as TiS2, TiS3, amorphous MoS3, Cu2V2O3, amorphous VO-P2O5, MoO3, VO5, and VO. 13 and lithium-containing composite metal oxides such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, etc. These positive electrode active materials can be used alone or in combination of two or more.

[0058] The slurry may contain a conductive additive. The conductive additive is used to increase the output of the electricity storage device and can be appropriately selected depending on whether it is used for the positive electrode or the negative electrode. Examples of conductive additives include graphite, acetylene black, carbon black, ketjen black, vapor-grown carbon fiber, etc. Among these, acetylene black is preferred because it facilitates increasing the output of the resulting electricity storage device.

[0059] When the slurry contains a conductive additive, the content of the conductive additive is preferably 0.1 to 15 parts by mass, more preferably 1 to 10 parts by mass, and even more preferably 3 to 10 parts by mass, relative to 100 parts by mass of the active material. When the content of the conductive additive is within this range, a sufficient conductive additive effect can be obtained without reducing the capacity of the battery to which the slurry is applied.

[0060] Preferably, the binder content in the slurry is 0.1 to 20 parts by mass per 100 parts by mass of the active material. A content of 0.1 parts by mass or more improves the adhesion of the active material to the current collector, which is advantageous from the viewpoint of maintaining the durability of the battery to which it is applied. Furthermore, a content of 20 parts by mass or less tends to improve the discharge capacity. From this viewpoint, the content range is more preferably 0.2 to 18 parts by mass, even more preferably 0.5 to 16 parts by mass, and even more preferably 1 to 12 parts by mass.

[0061] In addition to the binder, active material, conductive aid, and solvent, the slurry may contain, as necessary, additives such as a flame retardant aid, a thickener, an antifoaming agent, a leveling agent, an adhesion promoter, etc. When these additives are contained, the content of the additives is preferably about 0.1% by mass or more and 10% by mass or less based on the total amount of the slurry.

[0062] The slurry can be obtained by mixing the binder, the active material, and, if necessary, the conductive aid, the solvent, and the additives by a conventional method, for example, using a mixer such as a ball mill, a blender mill, or a three-roll mill.

[0063] <Electrodes for power storage devices> In this specification, the term "electricity storage device electrode" refers to a current collector and a cured product of the above-described slurry of the present invention. The cured product of the slurry is a cured product obtained by removing the solvent in the slurry by drying or the like.

[0064] Electrodes (positive and negative electrodes) using the binder of the present invention exhibit excellent adhesion of the active material to the current collector. Therefore, the peel strength of the electrode, before immersion in the electrolyte, is preferably 300 N / m or more, more preferably 350 N / m or more, even more preferably 400 N / m or more, and particularly preferably 450 N / m or more. The upper limit of the electrode peel strength is typically 1000 N / m. If an electrode in which the active material has peeled off from the current collector is used, lithium will precipitate during charging and discharging, causing a short circuit. Therefore, high adhesion between the active material and the current collector is preferable. A peel strength within this range is advantageous because the active material is less likely to peel off from the current collector when the electrode is punched or cut.

[0065] The electrode can be obtained by applying the slurry of the present invention to a current collector and removing the solvent by drying, etc. Alternatively, the electrode may be subjected to a rolling treatment after drying.

[0066] The current collector is not particularly limited as long as it is made of a conductive material. Examples include metal materials such as iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, and platinum. These current collectors can be used alone or in combination. Among current collectors, copper is preferred for the negative electrode current collector and aluminum is preferred for the positive electrode current collector from the viewpoints of adhesion of the active material and discharge capacity.

[0067] The method for applying the slurry to the current collector is not particularly limited, and examples thereof include methods using an extrusion coater, a reverse roller, a doctor blade, an applicator, etc. The amount of the slurry to be applied is appropriately selected depending on the desired thickness of the cured product derived from the slurry composition.

[0068] Examples of the method for rolling the electrode include die pressing, roll pressing, etc. The pressing pressure is preferably 1 MPa or more and 40 MPa or less, from the viewpoint of facilitating an increase in battery capacity.

[0069] In the electrode, the thickness of the current collector is preferably 1 μm or more and 20 μm or less, more preferably 2 μm or more and 15 μm or less. The thickness of the cured body is preferably 10 μm or more and 400 μm or less, more preferably 20 μm or more and 300 μm. The thickness of the electrode is preferably 20 μm or more and 200 μm.

[0070] <Electricity storage device> An electricity storage device according to one embodiment of the present invention includes the electricity storage device electrode described above as a negative electrode and / or a positive electrode.

[0071] Examples of the power storage device include lithium ion secondary batteries, sodium ion batteries, lithium sulfur batteries, all-solid-state batteries, lithium ion capacitors, lithium batteries, nickel-metal hydride batteries, and alkaline dry batteries.

[0072] The electrolyte contained in the battery is a solution in which an electrolyte is dissolved in a solvent. The electrolyte may be liquid or gel, as long as it is used in a normal power storage device, and may be selected appropriately to function as a battery depending on the type of negative electrode active material and positive electrode active material. Specific examples of suitable electrolytes include known lithium salts, such as LiClO4, LiBF6, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, and LiB 10 Cl 10 , LiAlCl4, LiCl, LiBr, LiB(C2H5)4, CF3SO3Li, CH3SO3Li, LiCF3SO3, LiC4F9SO3, Li(CF3SO2)2N, lower aliphatic lithium carboxylates, etc.

[0073] The solvent contained in the electrolytic solution is not particularly limited. Specific examples thereof include carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and vinylene carbonate, lactones such as γ-butyl lactone, ethers such as trimethoxymethane, 1,2-dimethoxyethane, diethyl ether, 2-ethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran, sulfoxides such as dimethyl sulfoxide, 1,3-dioxolane, and 4-methyl-1, Examples of suitable electrolytes include oxolanes such as 3-dioxolane, nitrogen-containing compounds such as acetonitrile and nitromethane, organic acid esters such as methyl formate, methyl acetate, ethyl acetate, butyl acetate, methyl propionate, and ethyl propionate, inorganic acid esters such as triethyl phosphate, dimethyl carbonate, and diethyl carbonate, diglymes, triglymes, sulfolanes, oxazolidinones such as 3-methyl-2-oxazolidinone, and sultones such as 1,3-propane sultone, 1,4-butane sultone, and naphthasultone. These may be used alone or in combination. When using a gel-like electrolyte, a gelling agent such as a nitrile polymer, an acrylic polymer, a fluorine-containing polymer, or an alkylene oxide polymer may be added.

[0074] When the binder of the present invention is used in either the positive electrode or the negative electrode, a conventional electrode can be used for the electrode not using the binder of the present invention.

[0075] In one preferred embodiment, the battery of the present invention comprises an electrode using the binder of the present invention as a negative electrode and a conventional electrode as a positive electrode. The positive electrode is not particularly limited as long as it is a positive electrode commonly used in electricity storage devices.

[0076] Alternatively, in another preferred embodiment, the battery of the present invention comprises an electrode using the binder of the present invention as a positive electrode and a conventional electrode as a negative electrode. The negative electrode is not particularly limited as long as it is a negative electrode that is normally used in electricity storage devices. In this case, the binder solution of the present invention contained in the slurry of the present invention used to form the positive electrode is a binder solution containing the binder of the present invention and N-methyl-2-pyrrolidone, and as described above, the binder solution is a binder solution containing the binder of the present invention and N-methyl-2-pyrrolidone at a shear rate of 10 s at 25°C. -1 The viscosity of the solution is 4 Pa·s or more and 35 Pa·s or less at a shear rate of 10 s at 25°C. -1 Viscosity and shear rate at 100 s -1 The TI, defined as the viscosity ratio to the viscosity in a slurry containing NMP, is preferably 2 or more and 6 or less. This is because the solvent in the slurry is NMP, which prevents deterioration of the positive electrode active material in the slurry, and the viscosity and TI of the binder NMP solution under the above-mentioned conditions are each within the above-mentioned specified ranges, which further increases the peel strength of the positive electrode.

[0077] Furthermore, both the positive electrode and the negative electrode may contain the binder of the present invention.

[0078] The method for producing the battery of the present invention is not particularly limited, but can be, for example, as follows. That is, the negative electrode and the positive electrode are stacked with a separator such as a porous polypropylene film interposed therebetween, and the stack is rolled and / or folded depending on the shape of the battery, placed in a battery container, and the electrolyte is poured in and sealed. The shape of the battery may be any of the known types such as coin type, button type, sheet type, cylindrical type, prismatic type, flat type, etc.

[0079] The battery of the present invention is useful for various applications. For example, it is very useful as a battery for use in portable terminals that require small size, thinness, light weight, and high performance. It can also be suitably used as a battery for devices that require flexibility, such as wound dry batteries and laminated batteries. [Example]

[0080] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited thereto. % in the embodiments is based on mass unless otherwise specified. First, the measurement method and evaluation method are shown below. The physical property values (or evaluation values) described in this specification are based on the values obtained by the following methods.

[0081] Regarding the physical property values of each PVA used in each of the following examples and comparative examples, the evaluation of the binder aqueous solution and NMP solution containing each PVA, the evaluation in electrode application, and the evaluation in battery application, they were measured according to the following methods.

[0082] <Modification rate of PVA> The modification rate of PVA (content rate of units derived from monomer (a) based on the number of moles of all monomer units of each PVA) used in each of the following examples and comparative examples was determined using the vinyl ester polymer, which is the precursor of each PVA, 1 by a method using 1H-NMR.

[0083] <Saponification degree of PVA> The saponification degree of PVA used in each of the following examples and comparative examples was determined according to JIS-K6726:1994.

[0084] <Degree of polymerization of PVA> The degree of polymerization (viscosity average degree of polymerization) of PVA used in each of the following examples and comparative examples was determined by the method described in JIS-K6726:1994.

[0085] For example, when monomethyl maleate is used as monomer (a), the above content rate is determined by the following procedure. That is, after sufficiently performing reprecipitation purification of the vinyl ester polymer, which is the precursor of PVA, three or more times using n-hexane / acetone as the solvent, the obtained purified product is dried under reduced pressure at 50°C for 2 days to prepare a sample for analysis. This sample is dissolved in CDCl3, 1The measurement is performed at room temperature using H-NMR. The content S of units derived from monomer (a) can be calculated using the following formula from peak α (4.7 to 5.2 ppm) derived from the methine structure of the vinyl ester unit in the vinyl ester polymer and peak β (3.6 to 3.8 ppm) derived from the methyl group of the methyl ester moiety of the unit derived from monomer (a). S (mol%) = {(number of protons of β / 3) / (number of protons of α+(number of protons of β / 3))} × 100

[0086] <Viscosity measurement and TI calculation in binder aqueous solution and NMP solution> Either a binder aqueous solution with a solid content of 10% by mass or a binder NMP solution with a solid content of 7.5% by mass prepared in each of the Examples and Comparative Examples described below was used as a measurement sample, and the viscosity was measured at a shear rate of 10 s at 25°C using an E-type viscometer (manufactured by Brookfield). -1 and 100s -1 Furthermore, from the measured values, the viscosity at a shear rate of 10 s at 25 °C was -1 Viscosity and shear rate at 100 s -1 The TI, defined as the viscosity ratio of the viscosity at

[0087] <Measurement of peel strength (N / m) of lithium-ion secondary batteries when applied to the positive electrode> For the lithium secondary battery electrodes (cathode) produced in each of the Examples and Comparative Examples described below, the strength was measured when the cured body (the portion derived from the slurry prepared in the Examples and Comparative Examples) was peeled off from the aluminum foil (cathode) serving as the current collector. Specifically, the slurry-coated surface of the obtained lithium ion secondary battery electrode was attached to a stainless steel plate using double-sided tape (double-sided tape manufactured by Nichiban), and the 180° peel strength (peel width 10 mm, peel speed 100 mm / min) was measured using a 50 N load cell (manufactured by Imada Co., Ltd.).

[0088] <Initial charge / discharge efficiency and DC resistance of lithium-ion secondary batteries when used as positive electrodes> Tests were carried out on the coin batteries produced in each of the examples and comparative examples described below using a commercially available charge-discharge tester (TOSCAT3100, manufactured by Toyo Systems). The resistance value when a current of 0.1 mA was passed for 3 seconds after the initial charge was taken as the DC resistance. Charging was carried out at 0.2 C (approximately 1 mA / cm) up to 4.2 V relative to the lithium potential. 2 ) was charged at a constant current of 0.2 C (approximately 0.5 mA / cm) relative to the lithium potential. 2 ) was discharged at a constant current down to 3 V. The coin battery was placed in a thermostatic chamber at 25°C and subjected to initial charge / discharge under the conditions described above, and the charge capacity, discharge capacity, and DC resistance were measured. The initial charge / discharge efficiency (%) was calculated using the formula: (discharge capacity) / (charge capacity) x 100.

[0089] <Measurement of discharge capacity retention rate (%) of lithium-ion secondary battery when using positive electrode> A rate test was carried out on the coin batteries fabricated in each of the examples and comparative examples described below using a commercially available charge-discharge tester (TOSCAT3100, manufactured by Toyo Systems). Charging was carried out at 0.2 C (approximately 1 mA / cm) up to 4.2 V relative to the lithium potential. 2 ) was charged at a constant current of 0.2 C (approximately 0.5 mA / cm) relative to the lithium potential. 2 ) was discharged at a constant current of 3 V down to 3 V. The coin battery was placed in a thermostatic chamber at 25°C and subjected to three initial charge-discharge cycles under the above conditions. After that, the discharge rate was changed to 5 C and one charge-discharge cycle was performed. The ratio of the discharge capacity at 5 C to the discharge capacity at 0.2 C was taken as the discharge capacity retention rate (%).

[0090] <Evaluation of the number of chips occurring in lithium-ion secondary battery electrodes when used as positive electrodes> Ten electrodes (positive electrodes) for lithium ion secondary batteries prepared in each of the examples and comparative examples described below were punched out using a punching machine with a diameter of 14 mm, and the number of electrodes in which the active material had peeled off from the current collector was counted.

[0091] (PVA production) [PVA-1] A reactor equipped with a stirrer, reflux condenser, nitrogen inlet, comonomer inlet, and polymerization initiator inlet was charged with 920 parts by mass of vinyl acetate and 80 parts by mass of methanol, and the system was purged with nitrogen for 30 minutes while nitrogen was bubbling through. Itaconic anhydride was selected as monomer (a), and a methanol solution of itaconic anhydride (20% concentration) was purged with nitrogen by bubbling nitrogen gas through it. The reactor was heated, and when the internal temperature reached 60°C, 0.25 parts by mass of 2,2'-azobisisobutyronitrile (AIBN) was added to initiate polymerization. The methanol solution of itaconic anhydride was added dropwise to the reactor, and polymerization was carried out at 60°C for 3 hours while maintaining a constant monomer composition ratio in the polymerization solution. The polymerization was then terminated by cooling. The total amount of monomer (a) added up to the termination of polymerization was 0.7 parts by mass, and the solids concentration at the time of termination was 33.3%. Subsequently, unreacted monomer was removed at 30°C under reduced pressure while occasionally adding methanol, yielding a methanol solution of vinyl ester polymer (concentration 35%). Next, 790.8 parts by mass of a methanol solution of vinyl ester polymer (200.0 parts by mass of the polymer in the solution) was prepared by further adding methanol to the methanol solution, and 9.2 parts by mass of a 10% methanol solution of sodium hydroxide was added to the resulting solution, followed by saponification at 40°C. The polymer concentration in the saponification solution was 25%, and the molar ratio of sodium hydroxide to vinyl acetate units in the polymer was 0.007. After the addition of the methanol solution of sodium hydroxide, a gel-like substance was formed within approximately 15 minutes. This was crushed in a grinder and allowed to stand for an additional hour at 40°C to allow saponification to proceed. Subsequently, 500 parts by mass of methyl acetate was added to neutralize the remaining alkali. After confirming completion of neutralization using a phenolphthalein indicator, the solution was filtered to obtain a white solid. 2000 parts by mass of methanol was added to the white solid, which was then left at room temperature for 3 hours and washed. This washing procedure was repeated three times, and the resulting white solid was then centrifuged and dewatered. The resulting solid was then heated in a dryer at 120°C for 4.5 hours to obtain PVA-1. The various manufacturing materials for PVA-1 and their physical properties are summarized in Table 1.

[0092] [PVA-2~PVA―6] Various PVAs were produced in the same manner as PVA-1, except that the polymerization conditions, such as the amounts of vinyl acetate and methanol charged, the type and amount of monomer (a) used during polymerization, and the polymerization rate (reaction rate (%) of monomer (vinyl acetate and monomer (a)) at the time of polymerization termination, calculated as 100 × mass of vinyl ester polymer at the time of polymerization termination / amount of monomer charged), and the saponification conditions, such as the molar ratio of sodium hydroxide, were changed as shown in Table 1 below. The physical properties of each PVA are summarized in Table 1 below.

[0093] [Table 1]

[0094] (Production of polyvinyl acetal resin) [Polyvinyl acetal resin-1] A three-neck flask equipped with a reflux condenser and thermometer was charged with 150 g of acetone, 100 g of water, and 10 g of 1-butanal. While stirring with a magnetic stirrer, 50 g of polyvinyl alcohol (saponification degree 99 mol%, average degree of polymerization 1700) was added over 1 minute. A mixed solution of 50 g of water and 21.2 g of 47% sulfuric acid was added dropwise from a dropping funnel over 5 minutes, and the mixture was heated to 30 °C and reacted for 5 hours. A 1 mol / L aqueous sodium hydroxide solution was added until the pH reached 8, and the solid was filtered off. The solid was washed five times with a 1:1 acetone / water mixture and then dried at 120 °C and 0.005 MPa for 6 hours to obtain a polyvinyl acetal resin with a hydroxyl group content of 74. The polymers used in the binder solution described below are summarized in Table 2.

[0095] [Polyvinyl acetal resin-2] Polyvinyl acetal resin-2 was produced in the same manner as [Polyvinyl acetal resin-1], except that 1-nonanal was used instead of 1-butanal and polyvinyl alcohol (saponification degree 99 mol%, average polymerization degree 1700) was used instead of polyvinyl alcohol (saponification degree 99 mol%, average polymerization degree 2400). The polymers used as the binder solution described below are summarized in Table 2 below.

[0096] In the following Examples and Comparative Examples, the PVA-1 to PVA-6 produced above are used to form a positive electrode.

[0097] Preparation of PVA aqueous solution PVA-1 was used as the PVA, one of the binder components. First, the modification rate, saponification degree, and polymerization degree of PVA-1 were determined using the methods described above. Next, water was added to the PVA-1 and the mixture was heated and mixed at 80°C for 1 hour to obtain a PVA aqueous solution containing a vinyl alcohol polymer and having a solids concentration of approximately 10% by mass. The solids concentration was calculated from the mass of the remaining solids after weighing 3 g of the PVA aqueous solution into an aluminum cup and drying it in a hot air dryer at 105°C for 3 hours. The viscosity of the PVA aqueous solution was measured and the TI was calculated using the methods described above. The physical properties of the PVA (modification rate, saponification degree, and polymerization degree) and the physical properties of the PVA aqueous solution (viscosity and TI) are summarized in Table 3 below.

[0098] Example 1 Preparation of PVA NMP solution In Example 1, PVA-1 was used as PVA (hereinafter also referred to as "resin a"), one of the binder components. 7.5 parts by mass of PVA-1 were added with 92.5 parts by mass of NMP (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and the mixture was heated to 80°C while stirring. Further heating and stirring were continued until complete dissolution was visually confirmed, yielding a vinyl alcohol polymer-containing NMP solution of PVA with a solids concentration of approximately 7.5% by mass. The solids concentration was calculated from the mass of the remaining solids after weighing 3 g of the PVA solution into an aluminum cup and drying it in a hot air dryer at 120°C for 4 hours. The viscosity and TI of the NMP solution of PVA were measured using the methods described above. Furthermore, the NMP solubility of the prepared PVA NMP solution was evaluated using the methods described above. The physical properties of the PVA (modification rate, degree of saponification, and degree of polymerization) and the physical properties of the PVA NMP solution (viscosity and TI) are summarized in Table 3 below.

[0099] Preparation of binder solution The binder solution was prepared by mixing the above-mentioned PVA in NMP solution with KYNAR (registered trademark) HSV900 (PVDF, manufactured by ARKEMA) as an electrolyte-swellable resin in NMP solution at a PVA:electrolyte-swellable resin ratio of 1:9. The non-PVA resin containing the electrolyte-swellable resin is also referred to as "resin b."

[0100] Preparation of positive electrode slurry Furthermore, the above binder solution, NCM (CellSeed C-5H, manufactured by Nippon Chemical Industry Co., Ltd.) as the positive electrode active material, and Super-P (manufactured by Timcal) as the conductive additive (conductivity imparting agent) were placed in a dedicated container and kneaded using a planetary mixer (ARE-250, manufactured by Thinky Corporation) to prepare a positive electrode slurry. The solid content of the binder solution was 3 parts by mass, the NCM was 95 parts by mass, and the Super-P solid content was 2 parts by mass. That is, the composition ratio of the active material, conductive additive, and binder in the positive electrode slurry was NCM powder:conductive additive:binder = 95:2:3 (mass ratio) in terms of solid content.

[0101] - Fabrication of positive electrodes for lithium-ion secondary batteries The positive electrode slurry obtained as described above was applied to a current collector made of aluminum foil (CST8G, manufactured by Fukuda Metal Foil & Powder Co., Ltd.) using a bar coater (T101, manufactured by Matsuo Sangyo Co., Ltd.). After primary drying in a hot air dryer at 80°C for 30 minutes, the coating was rolled using a roll press (manufactured by Hosen Co., Ltd.). Subsequently, battery electrodes (φ14 mm) were punched out, and secondary drying was performed under reduced pressure at 140°C for 3 hours to prepare positive electrodes for coin batteries. Ten punched electrodes with a diameter of 14 mm were prepared, and the number of chips was counted. Electrodes without chips were selected for use in coin batteries. The peel strength of the prepared positive electrodes for coin batteries was measured using the method described above. The results are summarized in Table 4 below.

[0102] - Fabrication of lithium-ion secondary batteries The battery positive electrode obtained as described above was transferred to a glove box (Miwa Manufacturing Co., Ltd.) under an argon gas atmosphere. Metallic lithium foil (0.2 mm thick, φ16 mm) was used as the negative electrode, a polypropylene separator (Celgard #2400, Polypore) was used as the separator, and a mixed solvent system of lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with vinylene carbonate (VC) added (1M-LiPF6, EC / EMC = 3 / 7 vol%, VC) was used as the electrolyte. 2 % by mass was used. A coin battery (2032 type) was fabricated using this configuration. The fabricated coin battery was subjected to measurements of the initial charge-discharge efficiency, 5C discharge capacity retention rate, and DC resistance using the methods described above. The results are summarized in Table 4 below.

[0103] Example 2-22 (Examples 21-22 are reference examples) Resin a and resin b were selected from those listed in Table 2, and the ratio of resin a:resin b was changed as listed in Table 4. Except for this, a binder solution was prepared, a positive electrode slurry was prepared, a positive electrode for a lithium ion secondary battery was fabricated, and a lithium ion secondary battery was fabricated, and measurements and evaluations were carried out in the same manner as in Example 1. The results are summarized in Tables 3 and 4 below.

[0104] Comparative Example 1-17 Resin a and resin b were selected from those listed in Table 2, and the ratio of resin a:resin b was changed as listed in Table 4. Except for this, a binder solution was prepared, a positive electrode slurry was prepared, a positive electrode for a lithium ion secondary battery was fabricated, and a lithium ion secondary battery was fabricated, and measurements and evaluations were carried out in the same manner as in Example 1. The results are summarized in Tables 3 and 4 below.

[0105] [Table 2]

[0106] [Table 3]

[0107] [Table 4]

[0108] As shown in Table 4 above, compared to the comparative examples, the examples had superior peel strength, were less likely to chip when the electrodes were cut, and had excellent initial charge-discharge efficiency, 5C discharge capacity retention rate, and low DC resistance.

Claims

1. A polyvinyl alcohol-based resin and a copolymer represented by the following formula (1) Electrolyte swelling rate = ((W ２ -W １ ) / W １ ×100) (1) [In formula (1), W １ is the mass (g) of the resin before immersion in the electrolyte, and the mass (g) of this resin after immersing it in diethyl carbonate at 25°C for 24 hours is W ２ ] and an electrolyte-swellable resin having an electrolyte swelling rate of 10 mass% or more, a modification rate of the polyvinyl alcohol-based resin is 0.02 mol % or more and 5 mol % or less based on the number of moles of all monomer units constituting the polyvinyl alcohol-based resin, The polyvinyl alcohol resin is in an aqueous solution state having a solid content of 10% by mass, and the aqueous solution is heated at 25°C at a shear rate of 10 s －１ The binder has a viscosity of 4 Pa·s or more at

2. The polyvinyl alcohol resin is in an aqueous solution state having a solid content of 10% by mass, and the aqueous solution is heated at a shear rate of 10 s at 25°C. －１ The viscosity of the aqueous solution is 4 Pa ​​s or more and 30 Pa s or less at a shear rate of 10 s at 25 ° C. －１ Viscosity and shear rate at 100 s －１ 2. The binder according to claim 1, wherein the binder has a thixotropic index, defined as the ratio of the viscosity at a temperature of 1.8 to the viscosity at a temperature of 1.

5.

3. The polyvinyl alcohol resin was dissolved in N-methyl-2-pyrrolidone at a solid content of 7.5% by mass, and the solution was heated at a shear rate of 10 s at 25°C. －１ The viscosity of the solution is 4 Pa ​​s or more and 35 Pa s or less at a shear rate of 10 s at 25 ° C. －１ Viscosity and shear rate at 100 s －１ 3. The binder according to claim 1, wherein the binder has a thixotropic index, defined as the ratio of the viscosity at a temperature of 1000° C. to the viscosity at a temperature of 1000° C., of 2 or more and 6 or less.

4. 4. The binder according to claim 1, wherein the polyvinyl alcohol resin is a vinyl alcohol polymer having a crosslinked structure.

5. The electrolyte-swellable resin is represented by the following formula (2): Electrolyte dissolution rate = ((W) １ ―W ３ ) / W １ (×100) (2) [In formula (2), W １ is the mass (g) of the resin before immersion in the electrolyte, and the mass after immersing this resin in diethyl carbonate at 25°C for 24 hours and then drying in a hot air dryer at 80°C for 3 hours is W ３ (g) The binder according to any one of claims 1 to 4, wherein the electrolyte elution rate represented by the formula (I) is 5 mass% or less.

6. An electrode for an electricity storage device, comprising the binder according to any one of claims 1 to 5.

7. A binder solution for an electrode of an electricity storage device, comprising the binder according to any one of claims 1 to 5 and a solvent.

8. A power storage device electrode slurry comprising the binder solution for a power storage device electrode according to claim 7 and an active material.

9. The electricity storage device electrode slurry according to claim 8 , wherein the content of the binder is 0.1 parts by mass or more and 20 parts by mass or less relative to 100 parts by mass of the active material.

10. An electricity storage device electrode comprising a current collector and a hardened product of the electricity storage device electrode slurry according to claim 8 .

11. An electricity storage device comprising the electricity storage device electrode according to claim 10.

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