Positive electrode composition for secondary battery and method for producing positive electrode for secondary battery

Abstract

To provide a positive electrode composition for a secondary battery that exhibits excellent adhesion to a current collector, has flexibility making it possible to accommodate volume changes of an active material layer during electrode winding and repeated charge-discharge cycles, and does not require rolling processing under severe conditions.SOLUTION: A positive electrode composition for a secondary battery comprises a binder resin, a conductive additive, and a positive electrode active material. The binder resin is a polymer having, as essential monomer units, (a1) a vinyl monomer having a carboxyl group, and (a2) a vinyl monomer represented by the general formula (1), CH2=C(R1)COOR2. The glass transition temperature of the binder resin is -41°C to -5°C, and the degree of crystallinity of the binder resin is less than 20.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode composition for a secondary battery and a method for producing a positive electrode for a secondary battery. [Background technology]

[0002] Due to their high voltage and high energy density, secondary batteries, including lithium-ion batteries, are widely used in the field of portable information devices, such as mobile phones and laptop computers. As the uses of secondary batteries continue to expand, their application in hybrid and electric vehicles, in addition to their traditional uses, is also being considered, and some of them have already been put into practical use.

[0003] A known method for manufacturing a positive electrode for a lithium-ion battery involves using polyvinylidene fluoride (PVDF) as a binder resin, mixing it with a conductive additive and a positive electrode active material, adding an organic solvent such as N-methylpyrrolidone to form a paste, and then applying and drying this on a current collector (Patent Document 1). However, the electrodes obtained by these methods do not have sufficient adhesive strength or flexibility, and there is a problem that the active material layer peels off from the current collector due to volume changes during winding of the electrode or due to repeated charging and discharging, resulting in a decrease in battery performance. Although styrene-butadiene rubber (SBR) and other materials have been considered as binder resins to replace PVDF, their poor oxidation resistance makes them difficult to use.

[0004] Furthermore, in recent years, in order to increase the capacity of batteries, the conditions for rolling processes such as roll pressing have become stricter in order to reduce the porosity of electrodes. In particular, for positive electrodes, due to various constraints, crystalline resins with poor mechanical deformability, such as PVDF, are often used as binder resins, making the conditions for rolling a major issue. A method for manufacturing lithium-ion battery electrode plates has been proposed that omits or simplifies the rolling process by chemically bonding the binder resin to the surface of the active material or conductive agent particles (Patent Document 2), but this method is not sufficient due to the significant constraints on the selection of battery materials. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-36889 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-100360 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above-mentioned circumstances, and aims to provide a positive electrode composition for a secondary battery that has excellent adhesion to a current collector, flexibility that can follow the volumetric changes of an active material layer that occur when the electrode is wound or due to repeated charge and discharge, and that does not require rolling treatment under severe conditions. [Means for solving the problem]

[0007] The present inventors have made extensive studies and arrived at the present invention. The present invention provides a positive electrode composition for a secondary battery, comprising a binder resin, a conductive additive, and a positive electrode active material, wherein the binder resin comprises a vinyl monomer (a1) having a carboxyl group and a compound represented by the following general formula (1): CH2=C(R 1 )COOR 2 (1) [In formula (1), R 1 is a hydrogen atom or a methyl group, and R 2 is a branched alkyl group having 4 to 36 carbon atoms. the binder resin has a glass transition temperature of −41 to −5° C. and a degree of crystallinity of less than 20; and a method for producing a positive electrode for a secondary battery, the method comprising: a mixing step of preparing a slurry containing the binder resin, a conductive additive, a positive electrode active material, and an aqueous solvent; a coating step of coating the slurry onto a current collector; and a drying step of drying the slurry after the coating step to form a positive electrode active material layer on the current collector. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a positive electrode composition for a secondary battery that has excellent adhesion to a current collector, flexibility that can follow the volumetric changes of an active material layer that occur when an electrode is wound or due to repeated charge and discharge, and that does not require rolling treatment under severe conditions. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present invention will be described in detail below. The present invention relates to a positive electrode composition for a secondary battery, a positive electrode for a secondary battery, and a secondary battery. The electrode composition for a secondary battery of the present invention may be an electrode composition for a lithium ion battery or an electrode composition for a sodium ion battery. In this specification, when lithium ion batteries and sodium ion batteries are mentioned, these concepts also include lithium ion secondary batteries and sodium ion secondary batteries, respectively. Hereinafter, the "positive electrode composition for a secondary battery" will also be referred to as the "positive electrode composition." The positive electrode composition refers to an electrode composition for a positive electrode.

[0010] [Cathode composition for secondary batteries] The positive electrode composition for a secondary battery of the present invention is a positive electrode composition for a secondary battery containing a binder resin, a conductive additive, and a positive electrode active material. The binder resin comprises a vinyl monomer (a1) having a carboxyl group and a copolymer represented by the following general formula (1): CH2=C(R 1 )COOR 2 (1) [In formula (1), R 1is a hydrogen atom or a methyl group, and R 2 is a branched alkyl group having 4 to 36 carbon atoms. The polymer contains a vinyl monomer (a2) represented by the following formula (1) as an essential constituent monomer.

[0011] Examples of the vinyl monomer (a1) having a carboxyl group include monocarboxylic acids having 3 to 15 carbon atoms, such as (meth)acrylic acid, crotonic acid, and cinnamic acid; dicarboxylic acids having 4 to 24 carbon atoms, such as maleic acid (anhydride), fumaric acid, itaconic acid (anhydride), citraconic acid, and mesaconic acid; and tri-, tetra-, or higher-valent polycarboxylic acids having 6 to 24 carbon atoms, such as aconitic acid. Of these, (meth)acrylic acid is preferred.

[0012] In the vinyl monomer (a2) represented by the general formula (1), R 1 represents a hydrogen atom or a methyl group. R 2 is a branched alkyl group having 4 to 36 carbon atoms, and R 2Specific examples of the alkyl group include 1-alkylalkyl groups (1-methylpropyl group (sec-butyl group), 1,1-dimethylethyl group (tert-butyl group), 1-methylbutyl group, 1-ethylpropyl group, 1,1-dimethylpropyl group, 1-methylpentyl group, 1-ethylbutyl group, 1-methylhexyl group, 1-ethylpentyl group, 1-methylheptyl group, 1-ethylhexyl group, 1-methyloctyl group, 1-ethylheptyl group, 1-methylnonyl group, 1-ethyloctyl group, 1-methyldecyl group, 1-ethylnonyl group, and the like). group, 1-butyleicosyl group, 1-hexyloctadecyl group, 1-octylhexadecyl group, 1-decyltetradecyl group, 1-undecyltridecyl group, etc.), 2-alkylalkyl group (2-methylpropyl group (isobutyl group), 2-methylbutyl group, 2,2-dimethylpropyl group, 2-methylpentyl group, 2-ethylbutyl group, 2-methylhexyl group, 2-ethylpentyl group, 2-methylheptyl group, 2-ethylhexyl group, 2-methyloctyl group, 2-ethylheptyl group, 2-methylnonyl group, 2-ethylhexyl group, 2-octyl, 2-methyldecyl, 2-ethylnonyl, 2-hexyloctadecyl, 2-octylhexadecyl, 2-decyltetradecyl, 2-undecyltridecyl, 2-dodecylhexadecyl, 2-tridecylpentadecyl, 2-decyloctadecyl, 2-tetradecyloctadecyl, 2-hexadecyloctadecyl, 2-tetradecyleicosyl, 2-hexadecyleicosyl, etc.), 3-alkylalkyl groups (3-methylbutyl, etc.), 4-34-alkylalkyl groups and mixed alkyl groups containing one or more branched alkyl groups such as alkyl residues of oxoalcohols corresponding to propylene oligomers (hepta-to-octamer), ethylene / propylene (molar ratio 16 / 1 to 1 / 11) oligomers, isobutylene oligomers (hepta-to-octamer), and α-olefin (carbon number 5 to 20) oligomers (tetra-to-octamer). Of these, from the viewpoint of achieving both flexibility of the binder resin and strength of the electrode, branched alkyl groups having 4 to 10 carbon atoms are preferred, and 1-alkylalkyl groups having 4 to 10 carbon atoms, 2-alkylalkyl groups having 4 to 10 carbon atoms, and 3-alkylalkyl groups having 4 to 10 carbon atoms are more preferred, and 2-alkylalkyl groups having 4 to 10 carbon atoms are even more preferred, and 2-ethylhexyl groups are the most preferred.

[0013] The monomers constituting the binder resin may include, in addition to the vinyl monomer (a1) and the vinyl monomer (a2) represented by the general formula (1), a copolymerizable vinyl monomer (a3) ​​that does not contain active hydrogen. Examples of the copolymerizable vinyl monomer (a3) ​​containing no active hydrogen include the following (a31) to (a35). (a31) Carbyl (meth)acrylate formed from a monool having 1 to 20 carbon atoms and (meth)acrylic acid Examples of the monools include (i) aliphatic monools [methanol, ethanol, n- or i-propyl alcohol, n-butyl alcohol, n-pentyl alcohol, n-octyl alcohol, nonyl alcohol, decyl alcohol, lauryl alcohol, tridecyl alcohol, myristyl alcohol, cetyl alcohol, and stearyl alcohol], (ii) alicyclic monools [cyclohexyl alcohol, and the like], and (iii) aromatic aliphatic monools [benzyl alcohol, and the like], and mixtures of two or more of these.

[0014] (a32) Poly(n=2-30) oxyalkylene (carbon number 2-4) alkyl (carbon number 1-18) ether (meth)acrylate [(meth)acrylate) adduct of 10 moles of ethylene oxide (hereinafter abbreviated as EO) with methanol, (meth)acrylate) adduct of 10 moles of propylene oxide (hereinafter abbreviated as PO) with methanol, etc.]

[0015] (a33) Nitrogen-containing vinyl compounds (a33-1) Amide group-containing vinyl compound (i) (meth)acrylamide compounds having 3 to 30 carbon atoms, such as N,N-dialkyl (having 1 to 6 carbon atoms) or diaralkyl (having 7 to 15 carbon atoms) (meth)acrylamides [N,N-dimethylacrylamide, N,N-dibenzylacrylamide, etc.], and diacetone acrylamide (ii) Amide group-containing vinyl compounds having 4 to 20 carbon atoms, excluding the above (meth)acrylamide compounds, such as N-methyl-N-vinylacetamide and cyclic amides (pyrrolidone compounds (having 6 to 13 carbon atoms, for example, N-vinylpyrrolidone, etc.))

[0016] (a33-2) (Meth)acrylate compounds (i) Dialkyl (C1-4) aminoalkyl (C1-4) (meth)acrylates [N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, t-butylaminoethyl (meth)acrylate, morpholinoethyl (meth)acrylate, etc.] (ii) Quaternary ammonium group-containing (meth)acrylates [quaternized products of tertiary amino group-containing (meth)acrylates [N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, etc.] (those quaternized using the above-mentioned quaternizing agents), etc.]

[0017] (a33-3) Heterocycle-containing vinyl compounds Pyridine compounds (having 7 to 14 carbon atoms, for example, 2- or 4-vinylpyridine), imidazole compounds (having 5 to 12 carbon atoms, for example, N-vinylimidazole), pyrrole compounds (having 6 to 13 carbon atoms, for example, N-vinylpyrrole), pyrrolidone compounds (having 6 to 13 carbon atoms, for example, N-vinyl-2-pyrrolidone)

[0018] (a33-4) Nitrile group-containing vinyl compounds Nitrile group-containing vinyl compounds having 3 to 15 carbon atoms, such as (meth)acrylonitrile, cyanostyrene, and cyanoalkyl (carbon number 1 to 4) acrylates

[0019] (a33-5) Other vinyl compounds Nitro group-containing vinyl compounds (carbon numbers 8 to 16, e.g., nitrostyrene), etc.

[0020] (a34) Vinyl hydrocarbons (a34-1) Aliphatic vinyl hydrocarbons Olefins with 2 to 18 or more carbon atoms [ethylene, propylene, butene, isobutylene, pentene, heptene, diisobutylene, octene, dodecene, octadecene, etc.], dienes with 4 to 10 or more carbon atoms [butadiene, isoprene, 1,4-pentadiene, 1,5-hexadiene, 1,7-octadiene, etc.], etc.

[0021] (a34-2) Alicyclic vinyl hydrocarbons Cyclic unsaturated compounds having 4 to 18 carbon atoms or more, such as cycloalkenes (e.g., cyclohexene), (di)cycloalkadienes [e.g., (di)cyclopentadiene], and terpenes (e.g., pinene, limonene, and indene).

[0022] (a34-3) Aromatic vinyl hydrocarbons Aromatic unsaturated compounds having 8 to 20 carbon atoms or more and their derivatives, such as styrene, α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene, isopropylstyrene, butylstyrene, phenylstyrene, cyclohexylstyrene, benzylstyrene, and lithium styrenesulfonate

[0023] (a35) Vinyl esters, vinyl ethers, vinyl ketones, unsaturated dicarboxylic acid diesters (a35-1) Vinyl ester Aliphatic vinyl esters [having 4 to 15 carbon atoms, for example, alkenyl esters of aliphatic carboxylic acids (mono- or dicarboxylic acids) (for example, vinyl acetate, vinyl propionate, vinyl butyrate, diallyl adipate, isopropenyl acetate, vinyl methoxyacetate)] Aromatic vinyl esters [having 9 to 20 carbon atoms, for example, alkenyl esters of aromatic carboxylic acids (mono- or dicarboxylic acids) (e.g., vinyl benzoate, diallyl phthalate, methyl-4-vinylbenzoate), aromatic ring-containing esters of aliphatic carboxylic acids (e.g., acetoxystyrene)]

[0024] (a35-2) Vinyl ether Aliphatic vinyl ethers (C3-15, for example, vinyl alkyl (C1-10) ethers [vinyl methyl ether, vinyl butyl ether, vinyl 2-ethylhexyl ether, etc.], vinyl alkoxy (C1-6) alkyl (C1-4) ethers [vinyl-2-methoxyethyl ether, methoxybutadiene, 3,4-dihydro-1,2-pyran, 2-butoxy-2'-vinyloxydiethyl ether, vinyl-2-ethylmercaptoethyl ether, etc.], poly(2-4)(meth)allyloxyalkanes (C2-6) [diallyloxyethane, triallyloxyethane, tetraallyloxybutane, tetramethallyloxyethane, etc.]) Aromatic vinyl ethers (8 to 20 carbon atoms, e.g., vinyl phenyl ether, phenoxystyrene)

[0025] (a35-3) Vinyl ketone Aliphatic vinyl ketones (carbon numbers 4 to 25, e.g., vinyl methyl ketone, vinyl ethyl ketone) Aromatic vinyl ketones (carbon numbers 9 to 21, e.g., vinyl phenyl ketone)

[0026] (a35-4) Unsaturated dicarboxylic acid diester Unsaturated dicarboxylic acid diesters having 4 to 34 carbon atoms, such as dialkyl fumarates (wherein the two alkyl groups are linear, branched, or alicyclic groups having 1 to 22 carbon atoms), and dialkyl maleates (wherein the two alkyl groups are linear, branched, or alicyclic groups having 1 to 22 carbon atoms).

[0027] Among the examples of (a3) ​​above, (a31), (a32), (a33), and (a34) are preferred from the viewpoint of electrolyte absorption and voltage resistance, and more preferred are methyl(meth)acrylate, ethyl(meth)acrylate, and n-butyl(meth)acrylate among (a31), and lithium styrenesulfonate among (a34). In addition to the above (a31) to (a35), a polyfunctional vinyl monomer (a4) having two or more polymerizable unsaturated double bonds that functions as a crosslinking agent can also be used as (a3). When using the polyfunctional vinyl monomer (a4), the content of the polyfunctional vinyl monomer (a4) is preferably 1.0 wt % or less, and more preferably 0.6 wt % or less, based on the weight of the binder resin.

[0028] Examples of the polyfunctional vinyl monomer (a4) include a di(meth)acrylate (a41) having two (meth)acryloyl groups as polymerizable unsaturated double bonds and a poly(meth)acrylate having three or more (meth)acryloyl groups as polymerizable unsaturated double bonds. In this specification, "(meth)acryloyl group" means "acryloyl group and / or methacryloyl group." "(meth)acrylate" means "methacrylate and / or acrylate." "Di(meth)acrylate" means a compound having a total of two methacryloyl and acryloyl groups, and the two (meth)acrylates may be the same or different. "Poly(meth)acrylate" means a compound having a total of three or more methacryloyl and acryloyl groups, and the three or more (meth)acrylates may be the same or different.

[0029] Examples of the di(meth)acrylate (a41) include a diester of a polyhydric (preferably di- to octahydric) alcohol having 2 to 30 carbon atoms and (meth)acrylic acid, a diester of an adduct of a polyhydric (preferably di- to octahydric) alcohol having 2 to 30 carbon atoms with 1 to 30 moles of alkylene oxide (the alkylene group has 2 to 4 carbon atoms) and (meth)acrylic acid, a diester of a diglycidyl ether and (meth)acrylic acid, a di(meth)acrylate of an ethylene oxide adduct of bisphenol A, and a di(meth)acrylate of an ethylene oxide adduct of fluorene. Among the di(meth)acrylates (a41), diesters of dihydric alcohols and (meth)acrylic acid are preferred from the viewpoint of achieving both resin strength and flexibility. Particularly preferred are 1,4-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 3-methyl-1,5-pentanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, and 1,10-decanediol di(meth)acrylate.

[0030] In the binder resin, the content of the vinyl monomer (a2) based on the weight of the binder resin is preferably 50 to 99.9% by weight, more preferably 65 to 98.9% by weight, and even more preferably 75 to 96% by weight, from the viewpoints of flexibility of the binder resin and electrode strength. In the binder resin, the contents of the vinyl monomer (a1) having a carboxyl group, the vinyl monomer (a2) represented by the general formula (1) above, and the copolymerizable vinyl monomer (a3) ​​not containing active hydrogen are preferably 0.1 to 50 wt % for (a1), 50 to 99.9 wt % for (a2), and 0 to 49.9 wt % for (a3), based on the weight of the binder resin. When the content of the monomer is within the above range, the flexibility of the binder resin and the electrode strength are good. More preferred contents are 1 to 30% by weight for (a1), 65 to 98.9% by weight for (a2), and 0.1 to 34% by weight for (a3), and even more preferred contents are 1 to 20% by weight for (a1), 75 to 96% by weight for (a2), and 0.5 to 24% by weight for (a3).

[0031] The binder resin has a glass transition temperature (Tg) of -41 to -5°C. If the Tg of the binder resin is less than -41°C, the strength of the resulting electrode will be weak, and if it exceeds -5°C, the flexibility of the resulting electrode will be poor. From the viewpoint of flexibility and strength of the resulting electrode, the Tg of the binder resin is preferably from -35 to -15°C. The Tg of the binder resin can be adjusted by the types and ratio of the monomers that make up the binder resin.

[0032] The Tg of the binder resin is measured by the method specified in ASTM D3418-82 (DSC method) using DSC20 and SSC / 580 manufactured by Seiko Instruments Inc. <Measurement conditions> (1) Heat from 30°C to 150°C at 20°C / min (2) Hold at 150°C for 10 minutes (3) Cool to -60°C at 20°C / min (4) Keep at -60°C for 10 minutes (5) Heat up to 150°C at 20°C / min (6) The differential scanning calorimetry curve measured in the step (5) is analyzed, and the position of the inflection point is determined as the glass transition temperature.

[0033] The binder resin has a crystallinity of less than 20. If the crystallinity of the binder resin is 20 or more, the flexibility of the resulting electrode deteriorates. From the viewpoint of flexibility of the resulting electrode, the crystallinity of the binder resin is preferably 15 or less, and more preferably 10 or less. The crystallinity of the binder resin can be adjusted by the monomers constituting the binder resin. For example, by making the content of the vinyl monomer (a2) in the binder resin 50% by weight or more based on the weight of the binder resin, the crystallinity of the binder resin can be made closer to less than 20. In addition, the crystallinity can be increased by using a monomer having a functional group with large intermolecular interactions (carboxylic acid, amine, etc.) or a monomer having large polarizability, while the crystallinity can be decreased by using a monomer having a long-chain hydrocarbon.

[0034] The crystallinity of the binder resin is determined by determining the measured heat of fusion (J / g) from the area of ​​the endothermic peak in a differential scanning calorimetry (DSC) measurement in accordance with "JIS K7122-1987 Method for measuring heat of transition of plastics," and calculating the crystallinity based on the measured heat of fusion using the following formula. Crystallinity = (measured heat of fusion / heat of fusion of completely crystalline body) x 100 In the above formula, the measured heat of fusion is the value obtained by subtracting the heat of crystallization (J / g) from the heat of fusion (J / g) when a 10 mg sample weighed in an aluminum container is heated in a nitrogen atmosphere from 25°C to 200°C at a heating rate of 20°C / min, then cooled to 25°C at a cooling rate of 20°C / min, and a second scan is performed under the same conditions. In the above formula, the heat of fusion of a completely crystalline material is the heat of fusion (J / g) when a 10 mg sample weighed in an aluminum container is heated in a nitrogen atmosphere from 25 to 200°C at a heating rate of 10°C / min, then cooled to 25°C at a cooling rate of 5°C / min, and then heated and cooled again at a rate of 10°C / min.

[0035] The weight average molecular weight (Mw) of the binder resin is preferably 50,000 to 200,000. When the weight average molecular weight of the binder resin is 50,000 to 200,000, the strength of the resulting electrode is excellent. The weight average molecular weight of the binder resin is more preferably 90,000 to 150,000.

[0036] The weight-average molecular weight of the binder resin can be determined by GPC (gel permeation chromatography) measurement under the following conditions: A sample polymer is dissolved in a solvent such as orthodichlorobenzene, N'-dimethylformamide (DMF), or tetrahydrofuran (THF) to prepare a 0.25 wt % solution, and the insoluble matter is filtered through a PTFE filter with a pore size of 1 μm to obtain a sample solution. Apparatus: Alliance GPC V2000 (Waters) Solvent: orthodichlorobenzene, DMF, THF Standard: Polystyrene sample Concentration: 3mg / ml Column stationary phase: PLgel 10um, MIXED-B, two columns in series (Polymer Laboratories) Column temperature: 135℃

[0037] The tensile elongation at break of the binder resin is preferably 1000% or more. When the tensile elongation at break of the binder resin is 1000% or more, the flexibility of the electrode is improved. From the viewpoint of achieving both flexibility and strength of the electrode, the tensile elongation at break of the binder resin is more preferably 1000 to 2000%.

[0038] A test piece for measuring tensile elongation at break is prepared as follows. A binder resin solution with a resin concentration of 25% by weight was poured into a 10cm x 10cm x 2cm mold and dried under reduced pressure (gauge pressure -100 to -90kPa) at 100°C for 3 hours to obtain a resin sheet. This resin sheet was sandwiched between 15cm x 15cm release papers and pressed at 15MPa for 30 seconds in a pressure press adjusted to 120°C to produce a resin film with a thickness of 100µm. This resin film was punched out using a punching blade [Super Straight Cutter SSK-1000, manufactured by Dumbbell Co., Ltd.] into a length of 10 cm and a width of 1 cm to prepare a test piece for measuring tensile elongation at break. If the obtained film is soft and adheres to the punching blade and cannot be punched out properly, talc or the like is evenly spread on the surface of the film before punching out to obtain a test piece.

[0039] The tensile elongation at break is a value calculated by the following formula using the test piece as the elongation until the test piece breaks in a tensile test in accordance with ASTM D882. Tensile elongation at break (%) = [(gauge length of test specimen at break - gage length of test specimen before test) / gage length of test specimen before test] x 100

[0040] The tensile elongation at break of the binder resin can be adjusted by the amount of polyfunctional monomer contained in the monomers constituting the binder resin. Increasing the amount of polyfunctional monomer reduces the tensile elongation at break, while decreasing the amount of polyfunctional monomer increases the tensile elongation at break. It can also be adjusted by the content of crystalline resin in the binder resin. Increasing the content of crystalline resin reduces the tensile elongation at break, while decreasing the content of crystalline resin increases the tensile elongation at break. The term "crystalline resin" refers to a resin that has a maximum and an endothermic peak in the differential scanning calorimetry (DSC) curve during the temperature rise process of the DSC curve obtained by differential scanning calorimetry (DSC measurement). The higher the content of the vinyl monomer (a2) based on the weight of the binder resin, the lower the crystalline resin content in the binder resin tends to be.

[0041] The average peel strength is preferably 4 to 10 N when a 180-degree peel test specified in JIS K6854-2 is carried out using a test piece having a length of 125 mm and a width of 25 mm, in which the binder resin and aluminum foil are bonded together. When a 180-degree peel test according to JIS K6854-2 is performed using a test piece having a length of 125 mm and a width of 25 mm, in which the binder resin and aluminum foil are bonded together, the adhesion between the positive electrode active material layer and the current collector is improved if the average peel force is 4 to 10 N. From the viewpoint of the adhesion between the positive electrode active material layer and the current collector, the average peel force is more preferably 5 to 8 N.

[0042] The test piece for measuring the average peel force is prepared as follows. A binder resin solution with a resin concentration of 25% by weight was poured into a 10cm x 10cm x 2cm mold and dried under reduced pressure (gauge pressure -100 to -90kPa) at 100°C for 3 hours to remove the solvent used for dilution, yielding a resin sheet. This resin sheet was sandwiched between 15cm x 15cm release papers and pressed at 15MPa for 30 seconds in a pressure press regulated at 120°C to produce a 100µm thick resin film.

[0043] The average peel strength is measured by performing a 180-degree peel strength test (based on JIS K6854-2:1999 adhesive - peel adhesion strength test method) on a test piece obtained by overlapping the resin film with 20 μm thick aluminum foil and pressing it at 100°C and 10 MPa for 30 seconds to obtain an adhesive film. The test piece is cut into a strip of 125 mm long and 25 mm wide. The average peel strength (N) is the average peel strength determined from the force-grip travel distance curve over the peel length (100 mm) excluding the first 25 mm in accordance with JIS K6854-2.

[0044] The average peel strength can be adjusted by the Tg of the binder resin, etc. Specifically, by setting the Tg of the binder resin to -35 to -15°C, the average peel strength often falls within the preferred range.

[0045] The positive electrode composition for a secondary battery of the present invention contains a positive electrode active material, a conductive additive, and the above-mentioned binder resin. As the positive electrode active material, a lithium ion-containing compound, a sodium ion-containing compound, or the like can be used. When the positive electrode active material contains a lithium ion-containing compound, the secondary battery is a lithium ion battery, and the positive electrode composition for a secondary battery is a positive electrode composition for a lithium ion battery. When the positive electrode active material contains a sodium ion-containing compound, the secondary battery is a sodium ion battery, and the positive electrode composition for a secondary battery is a positive electrode composition for a sodium ion battery.

[0046] The lithium ion-containing compounds include composite oxides of lithium and transition metals {composite oxides containing one type of transition metal (e.g., LiCoO2, LiNiO2, LiAlMnO4, LiMnO2, and LiMn2O4), composite oxides containing two types of transition metal elements (e.g., LiFeMnO4, LiNi 1-x Co x O2, LiMn 1-y Co y O2, LiNi 1 / 3 Co 1 / 3 Al 1 / 3 O2 and LiNi 0.8 Co 0.15 Al 0.05 O2) and complex oxides containing three or more metal elements [e.g., LiM a M' b M'' c O2 (M, M' and M'' are different transition metal elements, and a + b + c = 1 is satisfied. For example, LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2), etc.}, lithium-containing transition metal phosphates (for example, LiFePO4, LiCoPO4, LiMnPO4, and LiNiPO4), transition metal oxides (for example, MnO2 and V2O5), transition metal sulfides (for example, MoS2 and TiS2), and conductive polymers (for example, polyaniline, polypyrrole, polythiophene, polyacetylene, poly-p-phenylene, and polyvinylcarbazole), and two or more of them may be used in combination. The lithium-containing transition metal phosphate may have some of the transition metal sites substituted with other transition metals.

[0047] The sodium ion-containing compound is not particularly limited as long as it can be used in a sodium ion battery. Specific examples include layered active materials, spinel-type active materials, and oxoacid salt active materials, such as NaFeO2, NaNiO2, NaCoO2, NaCrO2, NaMnO2, NaVO2, and Na(Ni X Mn 1-X )O2(0 <X<1)、Na(Fe X Mn 1-X)O2 (0 < X < 1), NaVPO4F, Na2FePO4F, Na3V2(PO4)3, etc. can be mentioned. Particularly preferred is NaCrO2.

[0048] The positive electrode composition for a secondary battery of the present invention contains a conductive assistant. As the conductive assistant, there is no particular limitation as long as it is a material having conductivity. Examples of the conductive assistant include metals [such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium], carbons [such as graphite (flake graphite (UP)), carbon black (acetylene black, ketjen black, furnace black, channel black, and thermal lamp black, etc.), and carbon nanofibers (CNF), etc.], and mixtures thereof. These conductive assistants may be used alone or in combination of two or more. Also, these alloys or metal oxides may be used. From the viewpoint of electrical stability, carbon black and carbon nanotubes are preferably used, and acetylene black is more preferable.

[0049] The content of the conductive assistant is preferably 0.1 to 6% by weight, and more preferably 1 to 6% by weight, based on the solid content weight of the positive electrode composition.

[0050] From the viewpoints of binding performance and electrical characteristics, the weight ratio of the binder resin in the positive electrode composition for a secondary battery of the present invention is preferably 1 to 5% by weight based on the weight of the positive electrode composition for a secondary battery. Also, from the viewpoint of battery performance, the weight ratio of the positive electrode active material in the positive electrode composition for a secondary battery is preferably 90 to 98% by weight based on the weight of the positive electrode composition for a secondary battery.

[0051] In the positive electrode composition for a secondary battery of the present invention, the content of the binder resin is preferably 50 to 200% by weight based on the weight of the conductive assistant. Being within this range makes it easier to balance the internal resistance and capacity of the cell within a preferable range, which is preferable.

[0052] [Method of manufacturing a positive electrode for a secondary battery] The present invention is a method for producing a positive electrode for a secondary battery, the method including: a mixing step of preparing a slurry containing the binder resin, a conductive additive, a positive electrode active material, and an aqueous solvent; a coating step of coating the slurry onto a current collector; and a drying step of drying the slurry after the coating step to form a positive electrode active material layer on the current collector. As the binder resin, the conductive additive, and the positive electrode active material, the above-mentioned binder resin, conductive additive, and positive electrode active material can be used, respectively.

[0053] (Mixing process) The present invention includes a mixing step of preparing a slurry containing the binder resin, the conductive additive, the positive electrode active material, and an aqueous solvent. In the mixing step, a slurry containing the binder resin, the conductive additive, the positive electrode active material, and the aqueous solvent is prepared. The method for mixing the positive electrode composition for a secondary battery and the aqueous solvent is not particularly limited, and any known method can be used. There is no limitation on the order of mixing, and the binder resin, the conductive additive, the positive electrode active material, and the aqueous solvent may be mixed in any order.

[0054] As the aqueous solvent, any liquid containing water as an essential component can be used without limitation, and examples that can be used include water, an aqueous solution of an organic solvent, an aqueous solution of a surfactant, an aqueous solution of a water-soluble polymer, and mixtures of two or more of these, as described below.

[0055] (coating process) The present invention includes a coating step of coating the slurry obtained in the mixing step onto a current collector. The method of application is not particularly limited, and a known coating device such as a bar coater can be used.

[0056] Examples of materials that can be used to form the current collector include metal materials such as copper, aluminum, titanium, stainless steel, nickel, and alloys thereof, as well as baked carbon, conductive polymer materials, and conductive glass. The shape of the current collector is not particularly limited, and may be a film- or sheet-shaped current collector made of the above-mentioned material, or a deposition layer made of fine particles made of the above-mentioned material. The thickness of the current collector is not particularly limited, but is preferably 10 to 200 μm.

[0057] (drying process) The present invention includes a drying step of drying the slurry after the coating step to form a positive electrode active material layer on the current collector. The aqueous solvent contained in the slurry is preferably removed in a drying step. The method for removing the aqueous solvent is preferably drying using an oven or a vacuum oven. The atmosphere for removing the aqueous solvent may be air, an inert gas, or a vacuum. The temperature for removing the aqueous solvent is preferably 60 to 250°C.

[0058] The present specification discloses the following:

[0059] The present disclosure (1) provides a positive electrode composition for a secondary battery, comprising a binder resin, a conductive additive, and a positive electrode active material, The binder resin comprises a vinyl monomer (a1) having a carboxyl group and a copolymer represented by the following general formula (1): CH2=C(R 1 )COOR 2 (1) [In formula (1), R 1 is a hydrogen atom or a methyl group, and R 2 is a branched alkyl group having 4 to 36 carbon atoms.] It is a polymer containing, as an essential constituent monomer, a vinyl monomer (a2) represented by the following formula: the binder resin has a glass transition temperature of −41 to −5° C., The positive electrode composition for a secondary battery is characterized in that the crystallinity of the binder resin is less than 20.

[0060] The present disclosure (2) is the positive electrode composition for a secondary battery according to the present disclosure (1), in which the weight average molecular weight of the binder resin is 50,000 to 200,000.

[0061] The present disclosure (3) is characterized in that the tensile elongation at break of the binder resin is 1000% or more, In the positive electrode composition for secondary batteries according to the present disclosure (1) or (2), the tensile elongation at break is a value calculated by the following formula from the elongation until a test piece breaks in a tensile test in accordance with ASTM D882. Tensile elongation at break (%) = [(gauge length of test specimen at break - gage length of test specimen before test) / gage length of test specimen before test] x 100

[0062] The present disclosure (4) is the positive electrode composition for a secondary battery according to any one of the present disclosures (1) to (3), in which an average peel strength is 4 to 10 N when a 180-degree peel test specified in JIS K6854-2 is performed using a test piece having a length of 125 mm and a width of 25 mm, in which the binder resin and aluminum foil are bonded together.

[0063] The present disclosure (5) is the positive electrode composition for a secondary battery according to any one of the present disclosures (1) to (4), wherein the weight proportion of the binder resin in the positive electrode composition for a secondary battery is 1 to 5 wt % based on the weight of the positive electrode composition for a secondary battery, and the weight proportion of the positive electrode active material in the positive electrode composition for a secondary battery is 90 to 98 wt % based on the weight of the positive electrode composition for a secondary battery.

[0064] The present disclosure (6) is the positive electrode composition for a secondary battery according to any one of the present disclosures (1) to (5), wherein the content of the vinyl monomer (a2) based on the weight of the binder resin is 75 to 96 wt %.

[0065] The present disclosure (7) is a method for producing a positive electrode for a secondary battery, the method including: a mixing step of preparing a slurry containing a binder resin, a conductive additive, a positive electrode active material, and an aqueous solvent; a coating step of coating the slurry onto a current collector; and a drying step of drying the slurry after the coating step to form a positive electrode active material layer on the current collector, wherein the binder resin is a polymer containing, as essential constituent monomers, a vinyl monomer (a1) having a carboxyl group and a vinyl monomer (a2) represented by the following general formula (1), the glass transition temperature of the binder resin is −41 to −5° C., and the degree of crystallinity of the binder resin is less than 20: CH2=C(R 1 )COOR 2 (1) [In formula (1), R 1 is a hydrogen atom or a methyl group, and R 2 is a branched alkyl group having 4 to 36 carbon atoms. [Example]

[0066] The present invention will now be described in detail with reference to examples, but the present invention is not limited to these examples as long as they do not depart from the gist of the present invention.

[0067] (Production Example 1: Production of binder resin (A-1)) A four-neck flask equipped with a stirrer, thermometer, reflux condenser, dropping funnel, and nitrogen gas inlet tube was charged with 65.0 parts by weight of 2-ethylhexyl methacrylate, 30.0 parts by weight of 2-ethylhexyl acrylate, 4.6 parts by weight of acrylic acid, 0.4 parts by weight of 1,6-hexanediol dimethacrylate, and 70 parts by weight of toluene as a polymerization solvent, and the temperature was raised to 65°C. Furthermore, 10 parts by weight of polymerization solvent (toluene) and 0.200 parts by weight of 2,2'-azobis(2,4-dimethylvaleronitrile) as an initiator were mixed. The resulting monomer mixture was subjected to radical polymerization by continuously adding the polymerization initiator mixture dropwise using the dropping funnel over 4 hours while blowing nitrogen into the flask. The temperature was raised to 75°C, polymerization was continued for 2 hours, and then the polymerization solvent (toluene) was added to obtain a binder resin (A-1) solution with a resin concentration of 25% by weight.

[0068] (Production Examples 2 to 28: Production of Binder Resins (A-2) to (A-20) and (AX-2) to (AX-9)) Binder resins (A-2) to (A-20) and (AX-2) to (AX-9) were produced in the same manner as in Production Example 1, except that the type of monomer, the number of parts by weight charged, the amount of initiator, and the polymerization solvent were changed as shown in Tables 1 and 2.

[0069] [Table 1]

[0070] [Table 2]

[0071] (Production of resin sheet) The binder resin solution with a resin concentration of 25% by weight obtained in the Production Example was poured into a 10 cm x 10 cm x 2 cm mold and dried under reduced pressure (gauge pressure: -100 to -90 kPa) at 100°C for 3 hours to completely remove the polymerization solvent and obtain a binder resin sheet. This resin sheet was sandwiched between 15 cm x 15 cm release paper and pressed at 15 MPa for 30 seconds in a pressure press controlled at 120°C to produce a 100 μm thick resin film, which was then used to measure the tensile elongation at break and the average peel strength from the aluminum foil. The results are shown in Tables 3 and 4. Regarding the tensile elongation at break shown in Tables 3 and 4, for binder resins (A-8), (A-9), and (AX-4), the test pieces did not break until the point at which the safety device of the autograph was activated and stopped the test (elongation of 2400%) in the tensile test, making it impossible to measure specific values. However, it can be said that the tensile elongation at break of binder resins (A-8), (A-9), and (AX-4) was at least greater than 2400%. The Tg and crystallinity of each binder resin were measured by the above-mentioned method, and the results are shown in Tables 3 and 4. The test specimens used for measuring Tg and crystallinity were cut from the remainder of the resin sheet from which the test pieces were cut when preparing the test pieces for measuring the tensile elongation at break. As the binder resin (AX-1), polyvinylidene fluoride (hereinafter referred to as PVDF) [Kishida Chemical Co., Ltd., trademark #1100] was used.

[0072] [Table 3]

[0073] [Table 4]

[0074] (Examples 1 to 20, Comparative Examples 1 to 9: Preparation of Positive Electrode Composition for Lithium Ion Battery and Preparation of Positive Electrode for Lithium Ion Battery) Cathode active material particles (LiNi 0.8 Co 0.15 Al 0.05 90 parts by weight of O2 powder (volume average particle diameter 4 μm), 5 parts by weight of acetylene black (AB) [Denka Black Li-100, Denka Corporation] as a conductive additive, 5 parts by weight of a 25 wt% binder resin solution (obtained in each Production Example) of the binder resin listed in Table 5 (1.25 parts by weight as binder resin), and 100 parts by weight of the polymerization solvent for each binder resin solution were charged and stirred at 2000 rpm in a mixer for 1 minute to obtain a positive electrode composition slurry for a lithium-ion battery. This positive electrode composition slurry was applied to a 20 μm-thick aluminum foil with an applicator to a film thickness of 100 μm after drying, and then dried in a circulating air dryer at 100 °C for 20 minutes to form a positive electrode layer on the aluminum foil. (Hereinafter, the aluminum foil and positive electrode layer will be collectively referred to as the positive electrode sheet.) The center of the obtained positive electrode sheet was punched out to a diameter of 15 mm, dried in a vacuum dryer at 100°C for 3 hours, and then pressed twice for 3 seconds at 1.5 MPa in a press to produce a positive electrode for a lithium-ion battery. In addition, in the comparative examples (Comparative Examples 2, 3, and 7) in which the polymerization solvent of the binder resin solution was DMF, the drying temperature in the reduced pressure dryer was changed to 120°C.

[0075] (Examples 21 to 27 and Comparative Examples 10 to 13: Preparation of Positive Electrode Composition for Sodium Ion Battery and Preparation of Positive Electrode for Sodium Ion Battery) A mixture of 94.0 parts by weight of NaCrO2 (sodium chromite, manufactured by Kojundo Chemical Laboratory Co., Ltd.) as a positive electrode active material, 3.0 parts by weight of acetylene black (AB) (Denka Black Li-100, manufactured by Denka Corporation) as a conductive additive, 12.0 parts by weight of a 25 wt% resin solution (obtained in each manufacturing example) of the binder resin listed in Table 6 (3.0 parts by weight as binder resin), and 91.0 parts by weight of the polymerization solvent for each binder resin solution was charged and stirred at 2000 rpm in a mixer for 1 minute to obtain a positive electrode composition slurry for a sodium-ion battery. The resulting positive electrode composition slurry was applied to one side of a current collector (aluminum foil, 20 μm thick) with an applicator to a film thickness of 100 μm after drying, and then dried in a circulating air dryer at 100 °C for 20 minutes to form a positive electrode layer on the aluminum foil. (Hereinafter, the aluminum foil and positive electrode layer will be collectively referred to as the positive electrode sheet.) The center of the obtained positive electrode sheet was punched out to a diameter of 15 mm, dried in a vacuum dryer at 100°C for 3 hours, and then pressed twice for 3 seconds at 1.5 MPa in a press to produce a positive electrode for a sodium-ion battery. In the comparative examples (Comparative Examples 10 and 13) in which the polymerization solvent of the binder resin solution was DMF, the drying temperature in the reduced pressure dryer was changed to 120°C.

[0076] <Peeling test of electrodes for secondary batteries (evaluation of adhesive strength between current collector and positive electrode (positive electrode layer))> Test specimens were prepared by attaching double-sided tape (Nichiban Nicetack: model number NW-K15) to the electrode composition surface of each of the lithium ion battery electrodes (positive electrodes) prepared in Examples 1 to 20 and Comparative Examples 1 to 9, and the sodium ion battery electrodes (positive electrodes) prepared in Examples 21 to 27 and Comparative Examples 10 to 13, and then bonding the electrode composition surfaces to a SUS plate (thickness 1.2 mm) so that they faced each other. The metal foil (aluminum foil) on one end of the test piece was peeled off from the SUS plate, and an interfacial fracture test was performed to peel the electrode composition from the current collector using a 20 N test jig at a tensile speed of 150 mm / min, with the SUS plate held in the chuck of the lower test jig and the metal foil held in the chuck of the upper jig of a bench-top precision universal testing machine (Autograph AGS-X, manufactured by Shimadzu Corporation). The measurement results were read in accordance with JIS K 6854-2:1999 to determine the average peel strength (N). The average peel strength was divided by the width of the test piece (0.025 m) and recorded as the interfacial fracture strength (N / m) in Tables 5 and 6. The greater the interfacial destructive force, the higher the adhesive strength between the current collector and the positive electrode (positive electrode layer).

[0077] <Peel test of secondary battery electrodes (evaluation of adhesive strength between current collector and positive electrode (positive electrode layer))> Double-sided tape (Nichiban Nice Tack: model number NW-K15) was attached to the surface of the sample from which the current collector had been peeled off, and similarly, the sample was attached to a SUS plate (thickness 1.2 mm) with the electrode composition surface facing each other, to prepare a test specimen sandwiched between the SUS plates. The cohesive strength (N) of the electrode composition sandwiched between the SUS plates was determined using the same method as the adhesive strength test method described above. The average peel force was divided by the width of the test piece (0.025 m) and recorded as the cohesive fracture strength (N / m) in Tables 5 and 6. The greater the cohesive failure strength, the stronger the electrode composition.

[0078] <Grid test> The cross-cut test was carried out by the following method. The lithium-ion battery positive electrode composition slurries of Examples 1-20 and Comparative Examples 1-9 and the sodium-ion battery positive electrode composition slurries of Examples 21-27 and Comparative Examples 10-13, which were prepared during the preparation of the positive electrodes, were applied to a 0.5 mm thick aluminum plate with an applicator so that the dried film thickness was 180 μm, and the plate was dried at 100°C for 2 hours to obtain an electrode sheet. This electrode sheet was tested in accordance with JIS K5400-5-6 to measure the peel area. The results are shown in Tables 5 and 6. In addition, in the comparative examples in which the polymerization solvent of the binder resin solution was DMF (Comparative Examples 2, 3, 7, 10, and 13), the drying temperature was changed to 120°C. [Evaluation criteria] ◎: No peeling at all ○: Peeling is less than 10% △: Peeling is 10% or more but less than 30% ×: Peeling is 30% or more

[0079] <Bending test> The bending test indicates the flexibility of the electrode and was observed by the following method. The lithium ion battery positive electrode composition slurries of Examples 1 to 20 and Comparative Examples 1 to 9 and the sodium ion battery positive electrode composition slurries of Examples 21 to 27 and Comparative Examples 10 to 13, which were prepared when producing the above positive electrodes, were applied to aluminum foil with a thickness of 20 μm using an applicator so that the film thickness after drying would be 180 μm, and then dried at 100° C. for 2 hours to obtain test samples. In addition, in the comparative examples in which the polymerization solvent of the binder resin solution was DMF (Comparative Examples 2, 3, 7, 10, and 13), the drying temperature was changed to 120°C. The bending test was carried out as follows using metal rods with different diameters ranging from 0.5 mm to 3.0 mm. 1) The test sample is placed over the metal rod, with the metal rod positioned in the center of the sample in the longitudinal direction. 2) After arranging it as in 1), the sample was wrapped around the metal rod with the electrode layer facing outward (the sample was bent using the metal rod as the axis. In this case, rather than folding it, it was bent in a way that created an arc that matched the size of the φ of the metal rod). 3) After wrapping the sample around the metal rod, both ends of the sample were joined together, and a clip with a 100 g weight attached was used to hang the weight from both ends of the sample where the two ends met. 4) The test was carried out starting with the largest diameter, and the diameter (mm) of the metal rod at which the electrode began to crack was observed. The results are shown in Tables 5 and 6.

[0080] <Measurement of molding pressure> The molding pressure indicates the moldability of the electrode and was measured by the following method. The pressing pressure (kN) required to set the porosity of the positive electrodes fabricated in Examples 1 to 27 and Comparative Examples 1 to 13 to 20% was measured. Five electrodes were tested for each Example and Comparative Example, and the arithmetic mean value of the five measurements was recorded as the pressing pressure (kN). The porosity (%) was calculated using the following formula: The results are shown in Tables 5 and 6. The smaller the pressing pressure (kN) required to achieve a void ratio of 20%, the better the moldability, and from the viewpoint of production efficiency, it is preferable that the pressure be 6 kN or less. Porosity [%] = Electrode void volume [cm 3 ] / (electrode film thickness after pressing [μm] / 10 4 × Electrode area [cm 2 ]) The electrode gap volume [cm 3 ] was calculated using the following formula: Electrode gap volume [cm 3 ] = battery capacity [mAh / cm 2 ] based on the total amount of lithium ions in the non-aqueous electrolyte present in the positive electrode active material layer 2 ]× Electrode area [cm 2 ] / (electrolyte concentration of non-aqueous electrolyte [mol / L] / 10 3 × capacity conversion constant [mAh / mol]) The battery capacity based on the total amount of the positive electrode active material is calculated according to the following formula. Battery capacity [mAh / cm 2 ] = positive electrode active material capacity [mAh / g] × positive electrode active material basis weight [mg / cm 2 ] / 10 3 The positive electrode active material capacity [mAh / g] was determined by mixing the positive electrode active material with a nonaqueous electrolyte prepared by dissolving 1 mol / L of LiPF6 in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1:1) to form a slurry, which was then applied to one side of a separator [Celgard 3501] and pressed at a pressure of 10 MPa for 10 seconds to form an electrode. The electrode was then assembled into a battery pack with the separator interposed between it and a counter electrode (metallic lithium), and the discharge capacity when discharged from 4.2 V to 2.5 V (discharge rate: 1 / 20C) was measured using a charge / discharge measuring device such as the Battery Analyzer Model 1470 [manufactured by Toyo Corporation].

[0081] <Preparation of a battery for battery performance evaluation> (Production of negative electrodes for lithium-ion batteries for battery performance evaluation) 97 parts by weight of graphite, 1.5 parts by weight of sodium carboxymethylcellulose, 1.5 parts by weight of SBR [Zeon Corporation, BM-400B, solids content 40% by weight], and 100 parts by weight of ion-exchanged water were charged and stirred at 2000 rpm for 1 minute using a mixer to prepare a negative electrode slurry composition. This negative electrode slurry composition was applied to a 30 μm-thick copper foil with an applicator to a film thickness of approximately 100 μm after drying, and then dried at 100°C for 2 hours to obtain an electrode sheet. This electrode sheet was punched out to a diameter of 16 mm to prepare a negative electrode for a lithium-ion battery.

[0082] (Preparation of electrolyte for lithium-ion batteries) The electrolyte for the lithium-ion battery used to evaluate battery performance was prepared by dissolving LiPF6 at a ratio of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 1:1.

[0083] (Production of lithium-ion batteries for battery performance evaluation) The positive electrodes for lithium ion batteries prepared in Examples 1 to 20 and Comparative Examples 1 to 9 were placed on both ends of a 2032-type coin cell together with the negative electrodes for lithium ion batteries for evaluating battery performance punched to a diameter of 16 mm. As the current collector on the positive electrode side, aluminum foil with a thickness of 20 μm was used. A separator (Celgard 3501) was inserted between the electrodes to prepare a battery cell for evaluating battery performance. 30 μL of the above lithium ion battery electrolyte was poured into a battery cell for evaluating battery performance and sealed, to prepare a lithium ion battery for evaluating battery performance.

[0084] (Preparation of negative electrodes for sodium ion batteries for battery performance evaluation) 95.5 parts by weight of non-graphitizable carbon (Carbotron® PS(F) manufactured by Kureha Battery Materials Japan Co., Ltd., volume average particle diameter 9 μm) as the negative electrode active material, 1.5 parts by weight of acetylene black (AB) (Denka Corporation, DENKA BLACK Li-100) as the conductive additive, 1.5 parts by weight of sodium carboxymethyl cellulose (SBR) (Zeon Corporation, BM-400B, solids content 40 wt%) as the binder resin, and 3.75 parts by weight of SBR (Zeon Corporation, BM-400B, solids content 40 wt%) were mixed with 97.75 parts by weight of ion-exchanged water and stirred at 2000 rpm for 1 minute to obtain a negative electrode composition slurry. The resulting negative electrode composition slurry was applied to one side of a current collector (copper foil: 30 μm thick) with an applicator to a film thickness of 100 μm after drying, and dried at 100 °C for 2 hours to obtain a negative electrode sheet for sodium-ion batteries. This negative electrode sheet was punched out to a diameter of 16 mm to prepare a negative electrode for a sodium ion battery.

[0085] (Preparation of electrolyte for sodium ion batteries) The electrolyte for the sodium-ion battery used to evaluate battery performance was prepared by dissolving NaPF6 at a ratio of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 1:1.

[0086] (Fabrication of sodium ion batteries for battery performance evaluation) The sodium ion battery positive electrodes prepared in Examples 21 to 27 and Comparative Examples 10 to 13 were placed on both ends of a 2032-type coin cell together with the above-mentioned sodium ion battery negative electrode for battery performance evaluation punched to a diameter of 16 mm. As the current collector on the positive electrode side, aluminum foil with a thickness of 20 μm was used. A separator (Celgard 3501) was inserted between the electrodes to prepare a battery cell for evaluating battery performance. 30 μL of the above sodium ion battery electrolyte was poured into a battery cell for evaluating battery performance and sealed, to prepare a sodium ion battery for evaluating battery performance.

[0087] <Lithium-ion battery performance evaluation> At room temperature, using a charge / discharge measuring device [HJ0501SM8A] [manufactured by Hokuto Denko Corporation], the battery was charged at 0.1 C up to 4.2 C by CC-CV (cutoff current 0.01 C), and after a 1-hour pause, discharged at 0.1 C down to 2.5 V. The charge capacity at this time was designated as the initial charge capacity X0, and the discharge capacity was designated as the initial capacity Y0. The above charge / discharge cycle was then repeated, and the discharge capacity Y1 at the 50th cycle was obtained. The initial coulombic efficiency was calculated using the following formula and is shown in Table 5. Initial coulomb efficiency (%) = Initial discharge capacity (Y0) / Initial charge capacity (X0) × 100

[0088] Furthermore, the 50-cycle discharge capacity retention rate was calculated using the following formula, and the cycle characteristics were evaluated according to the following criteria. The results are shown in Table 5. 50-cycle discharge capacity retention rate (%) = Y1 / Y0 × 100 [Evaluation criteria] ◎: Discharge capacity retention rate is 91% or more ○: Discharge capacity retention rate is 89% or more and less than 91% △: Discharge capacity retention rate is 85% or more but less than 89% ×: Discharge capacity retention rate is less than 85%

[0089] <Sodium-ion battery performance evaluation> At room temperature, using a charge / discharge measuring device [HJ0501SM8A] [manufactured by Hokuto Denko Corporation], the battery was charged at 0.1 C up to 3.7 C by CC-CV (cutoff current 0.01 C), and after a 1-hour pause, discharged at 0.1 C down to 2.5 V. The charge capacity at this time was designated as the initial charge capacity X0, and the discharge capacity was designated as the initial capacity Y0. The above charge / discharge cycle was then repeated, and the discharge capacity Y1 at the 50th cycle was obtained. The initial coulombic efficiency was calculated using the following formula and is shown in Table 6. Initial coulomb efficiency (%) = initial discharge capacity (Y0) / initial charge capacity (X0) × 100.

[0090] Furthermore, the 50-cycle discharge capacity retention rate was calculated using the following formula, and the cycle characteristics were evaluated according to the following criteria. The results are shown in Table 6. Furthermore, because sodium ions have a larger ionic radius than lithium ions and are less efficient at moving in and out of electrode materials than lithium ions, the initial coulomb efficiency and capacity retention rate of sodium ion batteries are generally slightly lower than those of lithium ion batteries. For this reason, the evaluation criteria for lithium ion batteries and sodium ion batteries are different. 50-cycle discharge capacity retention rate (%) = Y1 / Y0 × 100 [Evaluation criteria] ◎: Discharge capacity retention rate is 89% or more ○: Discharge capacity retention rate is 87% or more and less than 89% △: Discharge capacity retention rate is 85% or more and less than 87% ×: Discharge capacity retention rate is less than 85%

[0091] [Table 5]

[0092] [Table 6]

[0093] Tables 5 and 6 show that the positive electrodes of the examples have superior adhesion to the current collector compared to the positive electrodes of the comparative examples, and have flexibility that can accommodate changes in the volume of the active material layer when the electrode is wound or during repeated charge and discharge, eliminating the need for rolling treatment under severe conditions. [Industrial Applicability]

[0094] Secondary batteries such as lithium ion batteries and sodium ion batteries obtained from the positive electrode composition for secondary batteries of the present invention are particularly useful as secondary batteries for use in mobile phones, personal computers, hybrid vehicles, and electric vehicles.

Claims

1. A positive electrode composition for a secondary battery, comprising a binder resin, a conductive additive, and a positive electrode active material, The binder resin comprises a vinyl monomer (a1) having a carboxyl group and a copolymer represented by the following general formula (1): CH ２ =C(R １ )COOR ２ (1) [In formula (1), R １ is a hydrogen atom or a methyl group, and R ２ is a branched alkyl group having 4 to 36 carbon atoms. It is a polymer containing, as an essential constituent monomer, a vinyl monomer (a2) represented by the following formula: the binder resin has a glass transition temperature of −41 to −5° C., A positive electrode composition for a secondary battery, wherein the binder resin has a crystallinity of less than 20.

2. 2. The positive electrode composition for a secondary battery according to claim 1, wherein the weight average molecular weight of the binder resin is 50,000 to 200,000.

3. The binder resin has a tensile elongation at break of 1000% or more, 2. The positive electrode composition for a secondary battery according to claim 1, wherein the tensile elongation at break is a value calculated from the elongation until a test piece breaks in a tensile test according to ASTM D882 using the following formula: Tensile elongation at break (%) = [(gauge length of test specimen at break - gage length of test specimen before test) / gage length of test specimen before test] x 100

4. 2. The positive electrode composition for a secondary battery according to claim 1, wherein the average peel strength is 4 to 10 N when a 180-degree peel test specified in JIS K6854-2 is performed using a test piece having a length of 125 mm and a width of 25 mm obtained by bonding the binder resin and aluminum foil.

5. the weight ratio of the binder resin in the positive electrode composition for secondary batteries is 1 to 5 wt % based on the weight of the positive electrode composition for secondary batteries; 2. The positive electrode composition for secondary batteries according to claim 1, wherein the weight ratio of the positive electrode active material in the positive electrode composition for secondary batteries is 90 to 98 wt % based on the weight of the positive electrode composition for secondary batteries.

6. 2. The positive electrode composition for secondary batteries according to claim 1, wherein the content of the vinyl monomer (a2) is 75 to 96% by weight based on the weight of the binder resin.

7. A method for manufacturing a positive electrode for a secondary battery, the method comprising: a mixing step of preparing a slurry containing a binder resin, a conductive additive, a positive electrode active material, and an aqueous solvent; a coating step of coating the slurry onto a current collector; and a drying step of drying the slurry after the coating step to form a positive electrode active material layer on the current collector, The binder resin comprises a vinyl monomer (a1) having a carboxyl group and a copolymer represented by the following general formula (1): CH ２ =C(R １ )COOR ２ (1) [In formula (1), R １ is a hydrogen atom or a methyl group, and R ２ is a branched alkyl group having 4 to 36 carbon atoms. It is a polymer containing, as an essential constituent monomer, a vinyl monomer (a2) represented by the following formula: the binder resin has a glass transition temperature of −41 to −5° C., The method for producing a positive electrode for a secondary battery, wherein the degree of crystallinity of the binder resin is less than 20.

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