Binder composition for nonaqueous secondary battery positive electrodes, composition for nonaqueous secondary battery positive electrodes, positive electrode for nonaqueous secondary batteries, and nonaqueous secondary battery

Abstract

Provided are: a binder composition for nonaqueous secondary battery positive electrodes which is excellent in durability against an electrolytic solution and in dispersibility of a positive electrode active material and is also compliant with organic fluorine compound regulations, and with which a secondary battery exhibiting excellent battery properties such as charging / discharging efficiency and a capacity retention rate can be formed; a composition for nonaqueous secondary battery positive electrodes which contains the binder composition for positive electrodes; a positive electrode formed from the composition for positive electrodes; and a secondary battery including the positive electrode.　The binder composition for nonaqueous secondary battery positive electrodes contains a urethane resin and a nonaqueous solvent. The urethane resin is obtained by reacting an aromatic polyisocyanate, a chain extender having a molecular weight of 50-500, and a polyol having a molecular weight of 500-5000.

Description

Binder composition for positive electrode of non-aqueous secondary battery, composition for positive electrode of non-aqueous secondary battery, positive electrode for non-aqueous secondary battery, and non-aqueous secondary battery

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

[0002] Non-aqueous secondary batteries (non-aqueous electrolyte secondary batteries), such as lithium-ion secondary batteries, have been rapidly deployed in hybrid vehicles, electric vehicles, home storage batteries, and other applications, taking advantage of their high energy density and ability to be repeatedly charged and discharged. In recent years, with the advancement of high performance and miniaturization of various portable electronic devices and communication devices, there has been an increasing demand for secondary batteries that are small, lightweight, have higher capacity, and have improved battery characteristics such as cycle performance and discharge rate performance. To further enhance the performance of non-aqueous secondary batteries, improvements in various battery components, such as electrodes, have been investigated. A positive electrode for a non-aqueous secondary battery typically includes a current collector and an electrode layer (positive electrode material layer) formed on the current collector. This positive electrode material layer is formed, for example, from a slurry-like positive electrode composition in which a composition containing a positive electrode active material and a binder (binding agent) is dispersed in a dispersion medium. Lithium-cobalt composite oxide (LCO) and lithium-nickel-manganese-cobalt composite oxide (NMC), which have high working voltages and excellent capacity characteristics, have been put to practical use as positive electrode active materials for lithium-ion secondary batteries. However, LCO and NMC have poor thermal properties due to the instability of their crystal structures caused by delithiation, and the high cost of cobalt makes mass production of lithium-ion secondary batteries difficult. In recent years, compared to lithium, LCO and NMC have been developed with working voltages of up to 3.5 V and a capacity of approximately 3.6 g / cm. ３Lithium iron phosphate (LFP)-based compounds have attracted attention as positive electrode active materials for lithium-ion secondary batteries because they have a high volume density, a theoretical capacity of approximately 170 mAh / g, and superior high-temperature stability and are inexpensive compared to cobalt. LFP-based compounds are structurally stable positive electrode active materials, but have low electrical and ionic conductivities. Therefore, LFP-based compounds are used by coating their surfaces with carbon to improve electrical conductivity and by reducing the particle size of the LFP-based compounds to improve ionic conductivity. However, reducing the particle size increases the specific surface area, which causes the particles of the LFP-based compounds to aggregate together, resulting in a problem of poor dispersion stability.

[0003] On the other hand, for the positive electrodes of lithium-ion secondary batteries, binders other than polyvinylidene fluoride (PVDF) or fluoropolymers with similar compositions have rarely been used. PVDF has a well-balanced combination of properties, such as oxidation resistance, heat resistance, adhesion, and electrolyte resistance, making it easy to obtain a positive electrode composition. For example, Patent Document 1 proposes a binder composition containing vinylidene fluoride and a polymer having structural units derived from a specific fluorine-based compound. However, fluorine-based compounds have recently become subject to international restrictions. For example, in the EU, perfluorobutanesulfonic acid and its metal salts (PFBS) are restricted under REACH, and legal restrictions on perfluorocompounds and polyfluoroalkyl compounds (so-called PFAS restrictions) are being implemented primarily in the EU and the United States. As these restrictions become more stringent in the future, it is conceivable that PVDF will be banned. As a binder other than PVDF for the positive electrode, for example, Patent Document 2 discloses an electrode binder composition containing a poly(dialkylene ester) thermoplastic polyurethane composition and a conductive material, which is said to provide a secondary battery with high initial discharge capacity, high capacity retention, and high charge / discharge capacity, and with an excellent balance of these properties. Patent Document 3 discloses an electrode binder for lithium secondary batteries, which contains an aqueous dispersion of a polyurethane resin with an acid value or amine value within a predetermined range, obtained using polyisocyanate, a compound having one or more active hydrogen groups and a hydrophilic group, and a chain extender. The binder is said to have high adhesion to the current collector, excellent binding properties and electrolyte resistance, and to reduce the internal resistance of the battery without reducing the content of the binder component.

[0004] JP 2022-061830 A JP 2015-501519 A International Publication No. 2016 / 132589

[0005] In Patent Document 2, a poly(dialkylene ester) thermoplastic polyurethane composition is obtained by melt polymerization (bulk polymerization), which makes it difficult to control the molecular weight of the polyurethane, resulting in a broad molecular weight distribution and a large amount of low-molecular-weight components. As a result, the physical properties, such as viscosity, of an electrode binder composition containing such a polyurethane composition and a conductive material are difficult to stabilize, resulting in inconsistent battery characteristics of the resulting positive electrode. In Patent Document 3, a polyurethane resin is used as an aqueous dispersion, but aqueous solvents result in poor dispersibility of the positive electrode active material, making it difficult to ensure the dispersion stability of the slurry-like positive electrode composition used to form the positive electrode. Therefore, there is a strong demand for a positive electrode binder other than PVDF that has battery characteristics such as capacity retention while also complying with legal restrictions on fluorine-based compounds. The inventors have investigated a positive electrode binder other than PVDF that can comply with legal restrictions on fluorine-based compounds and that can suppress aggregation even when a small-particle-sized LFP-based compound is used as the positive electrode active material, thereby improving dispersion stability in the positive electrode composition. As a result, the inventors discovered that when a composition containing a urethane resin having a specific structural unit and a non-aqueous solvent is used as a binder composition for a positive electrode, the resulting positive electrode material layer has excellent durability against the electrolyte and is effective in improving battery characteristics such as charge / discharge efficiency and capacity retention. The present invention has been completed based on this discovery. An object of the present invention is to provide a binder composition for a positive electrode of a non-aqueous secondary battery that complies with legal regulations on organic fluorine compounds and that can form a secondary battery that has excellent durability against the electrolyte, excellent dispersibility of the positive electrode active material, and excellent battery characteristics such as charge / discharge efficiency and capacity retention; a composition for a positive electrode of a non-aqueous secondary battery that contains such a binder composition for a positive electrode; a positive electrode for a non-aqueous secondary battery formed from such a positive electrode composition; and a secondary battery having such a positive electrode.

[0006] The present invention has the following aspects. [1] A binder composition for a non-aqueous secondary battery positive electrode, comprising a urethane resin and a non-aqueous solvent, the urethane resin being a urethane resin obtained by reacting an aromatic polyisocyanate, a chain extender having a molecular weight of 50 to 500, and a polyol having a molecular weight of 500 to 5,000. [2] The binder composition for a non-aqueous secondary battery positive electrode according to [1], in which, relative to the total amount of units constituting the urethane resin, a content of structural units based on the aromatic polyisocyanate is 35% by mass to 80% by mass, and a content of structural units based on the chain extender is 1% by mass to 40% by mass. [3] The binder composition for a non-aqueous secondary battery positive electrode according to [1] or [2], in which the aromatic polyisocyanate is a compound having a structure in which an isocyanate group is directly bonded to a carbon atom constituting an aromatic ring. [4] The binder composition for a non-aqueous secondary battery positive electrode according to any one of [1] to [3], wherein the chain extender is a polyol or a polyamine. [5] The binder composition for a non-aqueous secondary battery positive electrode according to any one of [1] to [4], wherein the polyol is at least one selected from the group consisting of polyester polyols, polycarbonate polyols, and polyether polyols. [6] The binder composition for a non-aqueous secondary battery positive electrode according to any one of [1] to [5], wherein the urethane resin is a urethane resin obtained by reacting the aromatic polyisocyanate, the chain extender, and the polyol in the non-aqueous solvent. [7] The binder composition for a non-aqueous secondary battery positive electrode according to any one of [1] to [6], wherein the non-aqueous solvent is an aprotic polar solvent. [8] The binder composition for a non-aqueous secondary battery positive electrode according to any one of [1] to [7], wherein the urethane resin has a weight average molecular weight of 10,000 or more and 500,000 or less. [9] A binder composition for a non-aqueous secondary battery positive electrode containing a urethane resin, wherein the urethane resin has a dispersion term (δd) in Hansen solubility parameters of 17 MPa. ０．５ Over 19 MPa ０．５ Less than 12 MPa, polar term (δp) ０．５ Over 17 MPa ０．５ less than 7 MPa, hydrogen bond term (δh) ０．５ Over 12 MPa ０．５and a film formed from the urethane resin is dissolved in an organic solvent S1 having a dispersion term (δd) of 15 MPa in the Hansen solubility parameter. ０．５ Over 21 MPa ０．５ Less than 6 MPa, polar term (δp) ０．５ Over 22 MPa ０．５ less than 3 MPa, hydrogen bond term (δh) ０．５ More than 10 MPa ０．５

[10] The binder composition for a non-aqueous secondary battery positive electrode according to [9], further comprising a non-aqueous solvent.

[11] The binder composition for a non-aqueous secondary battery positive electrode according to [9] or

[10] , wherein the urethane resin is a urethane resin obtained by reacting an aromatic polyisocyanate, a chain extender having a molecular weight of 50 to 500, and a polyol having a molecular weight of 500 to 5,000.

[12] The binder composition for a non-aqueous secondary battery positive electrode according to any of [9] to

[11] , wherein the content of structural units based on the aromatic polyisocyanate is 35 to 80% by mass and the content of structural units based on the chain extender is 1 to 40% by mass, relative to the total amount of units constituting the urethane resin.

[13] The binder composition for a non-aqueous secondary battery positive electrode according to any one of

[10] to

[12] , wherein the urethane resin is a urethane resin obtained by reacting the aromatic polyisocyanate, the chain extender, and the polyol in the non-aqueous solvent.

[14] A composition for a non-aqueous secondary battery positive electrode, comprising a positive electrode active material, a conductive material, and the binder composition for a non-aqueous secondary battery positive electrode according to any one of [1] to

[13] .

[15] The composition for a non-aqueous secondary battery positive electrode according to

[14] , wherein the positive electrode active material is a lithium iron phosphate compound.

[16] A non-aqueous secondary battery positive electrode comprising a positive electrode material layer formed using the composition for a non-aqueous secondary battery positive electrode according to

[14] or

[15] .

[17] A non-aqueous secondary battery comprising the non-aqueous secondary battery positive electrode according to

[16] , a negative electrode, an electrolyte, and a separator.

[0007] According to the present invention, it is possible to provide a binder composition for a positive electrode of a non-aqueous secondary battery that is excellent in durability against an electrolytic solution, dispersibility of a positive electrode active material, and can form a secondary battery that is excellent in battery properties such as charge / discharge efficiency and capacity retention rate, and that complies with legal regulations on organic fluorine compounds; a composition for a positive electrode of a non-aqueous secondary battery that contains such a binder composition for a positive electrode; a positive electrode formed from such a composition for a positive electrode; and a secondary battery that has such a positive electrode.

[0008] The present invention provides a binder composition for a non-aqueous secondary battery positive electrode (hereinafter also referred to simply as "positive electrode binder composition 1 of the present invention"), which contains a urethane resin and a non-aqueous solvent, and the urethane resin is a urethane resin obtained by reacting an aromatic polyisocyanate, a chain extender having a molecular weight of 50 to 500 (hereinafter also referred to simply as "chain extender"), and a polyol having a molecular weight of 500 to 5,000 (hereinafter also referred to simply as "polyol"). Hereinafter, embodiments of the present invention will be described in detail. In this specification, numerical ranges indicated using "to" indicate ranges that include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. First, each component of the positive electrode binder composition 1 of the present invention will be described.

[0009] In this specification, aromatic polyisocyanate refers to a polyisocyanate having an aromatic ring skeleton in its structure. Examples of aromatic polyisocyanates include phenylene diisocyanate, tolylene diisocyanate, diphenylmethane diisocyanate, naphthalene diisocyanate, xylylene diisocyanate, and tetramethylxylylene diisocyanate. These aromatic polyisocyanates may be used alone or in combination of two or more.

[0010] In particular, in the positive electrode binder composition 1 of the present invention, the aromatic polyisocyanate is preferably a polyisocyanate compound having a structure in which an isocyanate group is directly bonded to a carbon atom constituting an aromatic ring, and more preferably a diisocyanate compound having a structure in which an isocyanate group is directly bonded to a carbon atom constituting an aromatic ring. Examples of diisocyanates having a structure in which an isocyanate group is directly bonded to a carbon atom constituting an aromatic ring include diphenylmethane diisocyanate, naphthalene diisocyanate, phenylene diisocyanate, and tolylene diisocyanate. In this case, the urethane resin contained in the positive electrode binder composition 1 of the present invention is likely to have a linear structure without crosslinking, and in particular, the weight average molecular weight of the urethane resin obtained by solution polymerization, which will be described later, is easily controlled. In a positive electrode material layer formed from a positive electrode composition containing the positive electrode binder composition 1 of the present invention, which contains such a urethane resin, the isocyanate group has a structure in which it is directly bonded to a carbon atom constituting an aromatic ring. This is thought to facilitate the development of interactions due to the promotion of π-π stacking between aromatic rings in the diisocyanate-based structural units constituting the urethane resin. As a result, the crystallinity of the hard segment portion of the urethane resin is enhanced, and the urethane resin is soluble in the solvent for solution polymerization described below, preferably in the nonaqueous solvent described below. It is also thought that this facilitates the development of effects such as improved resistance to electrolytes and moderate swelling in electrolytes. Furthermore, it is thought that the battery performance, such as the charge / discharge characteristics and discharge capacity retention rate, of a secondary battery including such a positive electrode is improved.

[0011] The chain extender is a compound containing two or more active hydrogen atoms. A preferred chain extender is a polyol having two or more hydroxyl groups and a molecular weight of 50 to 500, or a polyamine having two or more amino groups and a molecular weight of 50 to 500. Examples of such chain extenders include ethylene glycol, 1,2-propanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 3,3'-dimethylolheptane, 1,4-cyclohexanedimethanol, neopentyl glycol, 3,3-bis(hydroxymethyl)heptane, diethylene glycol, dipropylene glycol, glycerin, trimethylolpropane, sol. Examples of suitable chain extenders include polyols such as butylol and hydroquinone diethylol ether, and polyamines such as ethylenediamine, propylenediamine, hexamethylenediamine, trimethylhexamethylenediamine, isophoronediamine, 4,4'-dicyclohexylmethanediamine, diaminocyclohexane, methyldiaminocyclohexane, norbornenediamine, diaminodiphenylmethane, piperazine, adipic acid dihydrazide, sebacic acid dihydrazide, and isophthalic acid dihydrazide. These chain extenders may be used alone or in combination of two or more.

[0012] In particular, in the positive electrode binder composition 1 of the present invention, a diol compound or diamine compound having a molecular weight of 50 to 500 is more preferred as the chain extender, with a diol compound being even more preferred. In this case, the resulting urethane resin is likely to have a linear, non-crosslinked structure. In particular, the weight-average molecular weight of the urethane resin obtained by solution polymerization, as described below, is easily controlled. Furthermore, a positive electrode material layer formed from a positive electrode composition containing the positive electrode binder composition 1 of the present invention, which contains such a urethane resin, is thought to have improved resistance to the electrolyte and to be moderately prone to swelling in the electrolyte due to the presumed mechanism described above, thereby improving the battery performance, such as the charge / discharge characteristics and discharge capacity retention rate, of a secondary battery equipped with such a positive electrode. Examples of diol compounds having a molecular weight of 50 to 500 include ethylene glycol, 1,4-butanediol, 1,6-hexanediol, and 1,4-cyclohexanedimethanol. Preferred examples of diamine compounds having a molecular weight of 50 to 500 include ethylenediamine, propylenediamine, isophoronediamine, 4,4'-dicyclohexylmethanediamine, diaminodiphenylmethane, and piperazine.

[0013] Examples of polyols having a molecular weight of 500 or more and 5,000 or less include polyolefin polyols, polyester polyols, polycarbonate polyols, polyether polyols, polyether ester polyols, etc. These may be used alone or in combination of two or more.

[0014] Examples of polyolefin polyols include polyethylene polyols, polypropylene polyols, polyisobutene polyols, hydrogenated butadiene polyols (hydrogenated polybutadiene containing hydroxyl groups at both ends), and hydrogenated polyisoprene polyols.

[0015] Examples of polyether polyols include polytetramethylene glycol obtained by ring-opening polymerization of tetrahydrofuran; modified polytetramethylene glycol obtained by copolymerizing tetrahydrofuran with alkyl-substituted tetrahydrofuran or neopentyl glycol; polyether polyols having an alicyclic structure; and products obtained by addition polymerization of one or more alkylene oxides, such as ethylene oxide, propylene oxide, and butylene oxide, with a compound having two or more active hydrogens, such as ethylene glycol, diethylene glycol, trimethylene glycol, propanediol, butanediol, hexanediol, neopentyl glycol, octanediol, glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, and sorbitol.

[0016] Examples of polyester polyols include condensation polyester polyols, lactone polyester polyols, etc. Condensation polyester polyols are, for example, reaction products of low molecular weight polyhydric alcohols (low molecular weight polyols such as ethylene glycol, propylene glycol, butanediol, pentanediol, neopentyl glycol, hexanediol, cyclohexanedimethanol, glycerin, 1,1,1-trimethylolpropane, 1,2,5-hexanetriol, pentaerythritol, 1,4-cyclohexanedimethanol, and 1,3-propanediol, and sugars such as sorbitol) with polybasic carboxylic acids (glutaric acid, adipic acid, azelaic acid, fumaric acid, maleic acid, pimelic acid, suberic acid, sebacic acid, phthalic acid, terephthalic acid, isophthalic acid, dimer acid, pyromellitic acid, oligomer acid, hexahydrophthalic anhydride, and 1,4-cyclohexanedicarboxylic acid), or anhydrides or ester-forming derivatives thereof. Examples of lactone polyester polyols include polycaprolactone polyols obtained by ring-opening polymerization of lactones such as ε-caprolactone, α-methyl-ε-caprolactone, and ε-methyl-ε-caprolactone.

[0017] Examples of polycarbonate polyols include polycarbonate polyols obtained by reacting a carbonate (carbonic acid ester) such as a dialkyl carbonate (dimethyl carbonate, diethyl carbonate, etc.), ethylene carbonate, or diphenyl carbonate with a glycol such as diethylene glycol, ethylene glycol, triethylene glycol propanediol, propanediol, butanediol, neopentyl glycol, pentanediol, 3-methyl-1,5-pentanediol, octanediol, 1,4-butynediol, dipropylene glycol, tripropylene glycol, polytetramethylene ether glycol, 2-methyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 1,4-cyclohexanediglycol, or 1,4-cyclohexanedimethanol.

[0018] The polyether ester polyol is a reaction product of a low-molecular-weight polyhydric alkylene glycol (e.g., dialkylene glycols such as oxydimethanol, diethylene glycol, dipropylene glycol, 3,3-oxydipropan-1-ol, and dibutylene glycol) with the above-mentioned polybasic carboxylic acid, and examples thereof include poly(dialkylene ester) polyols such as poly(diethylene glycol) adipate.

[0019] In particular, in the positive electrode binder composition 1 of the present invention, the polyol having a molecular weight of 500 to 5,000 is preferably one or more selected from the group consisting of polyester polyols, polycarbonate polyols, and polyether polyols, more preferably polyester polyols or polycarbonate polyols, and even more preferably polyester diols or polycarbonate diols. In this case, the resulting urethane resin is likely to have a linear, uncrosslinked structure, and in particular, the weight-average molecular weight of the urethane resin obtained by solution polymerization, as described below, is easily controlled. Furthermore, a positive electrode material layer formed from a positive electrode composition containing the positive electrode binder composition 1 of the present invention containing such a urethane resin is thought to have improved resistance to an electrolyte and to be easily swollen appropriately in an electrolyte due to the presumed mechanism described above, thereby improving the battery performance, such as the charge / discharge characteristics and discharge capacity retention rate, of a secondary battery including such a positive electrode.

[0020] The molecular weight of the polyol is 500 or more and 5,000 or less. The molecular weight of the polyol is preferably 800 or more, more preferably 1,000 or more. The molecular weight of the polyol is preferably 4,000 or less, more preferably 3,000 or less. When the molecular weight of the polyol is within the above range, the crystallinity of the hard segment portion of the urethane resin is increased in the positive electrode material layer formed from the positive electrode composition containing the positive electrode binder composition 1 of the present invention, while the soft segment is likely to exhibit an effect equivalent to that of the amorphous portion. The molecular weight of the polyol may be determined as a hydroxyl group-equivalent number average molecular weight from a hydroxyl value measured in accordance with JIS K1557-1, or it may be measured as a weight average molecular weight in terms of standard polystyrene using gel permeation chromatography (GPC).

[0021] The urethane resin contained in the positive electrode binder composition 1 of the present invention preferably has a content of constituent units based on aromatic polyisocyanate of 35% by mass to 80% by mass and a content of constituent units based on chain extenders of 1% by mass to 40% by mass, relative to the total amount of units constituting the urethane resin. The content of constituent units based on aromatic polyisocyanate is more preferably 40% by mass to 75% by mass, and the content of constituent units based on chain extenders is more preferably 5% by mass to 30% by mass. Furthermore, the content of constituent units based on polyols having a molecular weight of 500 to 5,000, relative to the total amount of units constituting the urethane resin, is preferably 1% by mass to 60% by mass, more preferably 5% by mass to 50% by mass. In other words, the sum of the content of constituent units based on aromatic polyisocyanate and the content of constituent units based on chain extenders is the content (% by mass) of hard segment components in the urethane resin. That is, the hard segment content of the urethane resin contained in the positive electrode binder composition 1 of the present invention is preferably 36% by mass or more and 99% by mass or less, and more preferably 45% by mass or less and 95% by mass or less. It is presumed that when the content of the hard segment constituents in the urethane resin is within the above-mentioned range, the hard segment constituents (units based on aromatic polyisocyanate and chain extender) and soft segment constituents (polyol units) of the urethane resin are more likely to substitute for the crystalline and amorphous portions of PVDF when used as a positive electrode binder, respectively. Furthermore, it is presumed that the aforementioned interaction between aromatic rings in the diisocyanate-based structural units constituting the urethane resin is more likely to occur due to the promotion of π-π stacking, thereby increasing the crystallinity of the hard segment portions of the urethane resin. As a result, the resistance to electrolyte is improved in a positive electrode material layer formed from a positive electrode composition containing the positive electrode binder composition 1 of the present invention, and as a result, the battery performance of a nonaqueous secondary battery equipped with a positive electrode having such a positive electrode material layer is improved.Furthermore, when the hard segment of the urethane resin is formed from an aromatic diisocyanate having a structure in which an isocyanate group is directly bonded to a carbon atom constituting an aromatic ring, and a chain extender that is a diol compound having a molecular weight of 50 to 500, the improvement in the resistance to the electrolyte and the battery performance described above becomes more significant.

[0022] The nonaqueous solvent contained in the positive electrode binder composition 1 of the present invention is preferably a nonaqueous solvent capable of dissolving the urethane resin obtained by reacting the aromatic polyisocyanate, the chain extender, and the polyol, and is preferably an aprotic solvent, more preferably an aprotic polar solvent. Examples of such aprotic polar solvents include acetone, tetrahydrofuran, acetonitrile, propionitrile, N-methylpyrrolidone, N-ethylpyrrolidone, dimethylformamide, dimethylacetamide, diethylformamide, dimethyl sulfoxide, sulfolane, 1,3-dimethyl-2-imidazolidinone, and hexamethylphosphoric triamide, with N-methylpyrrolidone, dimethylformamide, and dimethyl sulfoxide being preferred.

[0023] The content of the urethane resin in the entire positive electrode binder composition 1 of the present invention is preferably in the range of 1 to 70 mass %, more preferably in the range of 5 to 60 mass %, and even more preferably in the range of 10 to 50 mass %.

[0024] The positive electrode binder composition 1 of the present invention may further contain a resin other than the urethane resin (hereinafter referred to as "other resin") as necessary, as long as the effects of the present invention are not impaired. When the binder composition 1 of the present invention further contains the other resin, the content of the other resin is preferably in the range of 50% by mass or less, more preferably in the range of 40% by mass or less, and even more preferably in the range of 30% by mass or less, relative to 100% by mass of the nonvolatile content of the positive electrode binder composition 1.

[0025] The positive electrode binder composition 1 of the present invention may further contain various additives (hereinafter referred to as "various additives") such as surfactants, antioxidants, light stabilizers, plasticizers, viscosity modifiers, organic or inorganic fillers, etc., as necessary, within a range that does not impair the effects of the present invention. When the positive electrode binder composition 1 of the present invention further contains various additives, the content of the various additives is preferably in the range of 10% by mass or less, more preferably in the range of 5% by mass or less, and even more preferably in the range of 2% by mass or less, relative to 100% by mass of the nonvolatile content of the positive electrode binder composition 1.

[0026] The urethane resin contained in the positive electrode binder composition 1 of the present invention is preferably a urethane resin obtained by reacting the aromatic polyisocyanate, the chain extender, and the polyol in the non-aqueous solvent. That is, the urethane resin contained in the positive electrode binder composition 1 of the present invention is preferably obtained by solution polymerization of the aromatic polyisocyanate, the chain extender, and the polyol in a non-aqueous solvent. The weight-average molecular weight of the urethane resin obtained by solution polymerization is easily controlled, and in a positive electrode material layer formed from a positive electrode composition containing the positive electrode binder composition 1 of the present invention containing such a urethane resin, the above-mentioned π-π stacking interaction is easily exhibited, and the effects of improved resistance to the electrolyte and appropriate swelling in the electrolyte are easily exhibited. Furthermore, the battery performance, such as the charge / discharge characteristics and discharge capacity retention rate, of a secondary battery including such a positive electrode is easily improved. The resulting urethane resin solution can be used as the positive electrode binder composition 1 of the present invention as is, or may be diluted with a non-aqueous solvent as necessary before being used as the positive electrode binder composition 1 of the present invention.

[0027] During the reaction, the equivalent ratio of the isocyanate groups of the aromatic polyisocyanate to the hydroxyl groups of the chain extender (preferably a diol compound) and polyol is preferably in the range of 0.95 to 1.05, more preferably in the range of 0.98 to 1.02. The reaction temperature of the aromatic polyisocyanate, chain extender, and polyol in a non-aqueous solvent is usually preferably in the range of 50 to 150°C.

[0028] The weight-average molecular weight (Mw) of the urethane resin is preferably 10,000 or more and 500,000 or less, and more preferably 50,000 or more and 400,000 or less. Having the Mw of the urethane resin within the above-mentioned range is preferable from the viewpoint that a nonaqueous secondary battery having a positive electrode formed from the positive electrode binder composition 1 of the present invention is likely to have better battery characteristics, such as charge / discharge characteristics and capacity retention. Furthermore, it is preferable that the urethane resin has substantially no crosslinked structure. In other words, the urethane resin preferably has a linear structure. If the urethane resin does not substantially have a crosslinked structure, the urethane resin is easily soluble in the nonaqueous solvent described above, enabling solution polymerization. Furthermore, in a positive electrode material layer formed from a positive electrode composition containing the positive electrode binder composition 1 of the present invention, it is easy to control the durability to the electrolyte and the degree of swelling in the electrolyte.

[0029] The present invention also provides a binder composition for a non-aqueous secondary battery positive electrode containing a urethane resin, wherein the urethane resin has a dispersion term (δd) in the Hansen solubility parameter of 17 MPa. ０．５ Over 19 MPa ０．５ Less than 12 MPa, polar term (δp) ０．５ Over 17 MPa ０．５ less than 7 MPa, hydrogen bond term (δh) ０．５ Over 12 MPa ０．５ The urethane resin is dissolved in an organic solvent S1 (hereinafter simply referred to as "organic solvent S1") having a dispersion term (δd) of 15 MPa in the Hansen solubility parameter. ０．５ Over 21 MPa ０．５ Less than 6 MPa, polar term (δp) ０．５ Over 22 MPa ０．５ less than 3 MPa, hydrogen bond term (δh) ０．５ More than 10 MPa ０．５and a binder composition for a positive electrode of a non-aqueous secondary battery (hereinafter also referred to as "positive electrode binder composition 2 of the present invention"), in which the swelling degree after immersion in an organic solvent S2 (hereinafter simply referred to as "organic solvent S2") having a viscosity of less than 100% by mass at 60°C for 12 hours is 5% by mass or more and 100% by mass or less. As used herein, "the urethane resin dissolves in the organic solvent S1" means that the urethane resin dissolves in the organic solvent S1 as a homogeneous solution without leaving any undissolved matter or gel-like foreign matter. The swelling degree after immersion of a film formed from the urethane resin in the organic solvent S2 at 60°C for 12 hours is more preferably 10% by mass or more and 90% by mass or less. Details of the method for measuring the swelling degree will be described later in the Examples section. The positive electrode binder composition 2 of the present invention, which contains a urethane resin satisfying the above-mentioned characteristics, has durability against an electrolyte solution and moderate swelling property, and the battery performance of a non-aqueous secondary battery including a positive electrode formed from the positive electrode binder composition 2 of the present invention is likely to be improved.

[0030] Here, the dispersion term (δd), polar term (δp), and hydrogen bond term (δh) in the Hansen solubility parameter are parameters that take into account the polarity of a substance, and are expressed in three-dimensional space by dividing the solubility parameter (SP value: δ) introduced by Hildebrand into three components: the dispersion term δd, the polar term δp, and the hydrogen bond term δh, and the relationship of the following formula holds: δ [(cal / cm ３ ) ０．５ ]]=(δd ２ +δp ２ +δh ２ ) ０．５The dispersion term δd, polar term δp, and hydrogen bond term δh have been calculated by Hansen and subsequent researchers, and are listed, for example, in VII-698 to 711 of the Polymer Handbook (4th edition). Hansen solubility parameters for many solvents and resins have also been investigated, and these solubility parameters are listed, for example, in the Industrial Solvents Handbook (by Wesley L. Archer). These solubility parameters can also be calculated using Hansen Solubility Parameters in Practice (HSPiP) software.

[0031] Examples of organic solvents S1 and S2 that satisfy the above-mentioned requirements based on the Hansen solubility parameters δd, δp, and δh calculated using HSPiP software (6th Edition, 6.0.03) are shown below.

[0032]

[0033] In addition, the organic solvent S2 has a Hansen solubility parameter δd of 15 MPa. ０．５ Over 17 MPa ０．５ Less than δp is 10 MPa ０．５ Over 14 MPa ０．５ Less than δh is 3 MPa ０．５ Over 8 MPa ０．５ From the viewpoint of enabling a more practical evaluation of the swelling property and durability of a cathode material layer formed from a cathode composition containing the cathode binder composition 2 of the present invention in an electrolyte solution, an organic solvent S2 having a δd, δp, and δh range is more preferred. The organic solvent S2 satisfying these δd, δp, and δh ranges may be a single solvent or a mixed solvent containing two or more solvents. For example, a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in an EC:EMC ratio of 30:70 (volume ratio), which is a carbonate-based solvent used as an electrolyte solution for non-aqueous secondary batteries, has Hansen solubility parameters of δd = 16.2, δp = 12.6, and δh = 6.2, satisfying the above-mentioned more preferred δd, δp, and δh for the organic solvent S2.

[0034] The urethane resin having the above-described properties with respect to organic solvents S1 and S2 is preferably a urethane resin obtained by reacting an aromatic polyisocyanate, a chain extender having a molecular weight of 50 to 500, and a polyol having a molecular weight of 500 to 5,000. Details and suitable compounds of the aromatic polyisocyanate, the chain extender having a molecular weight of 50 to 500, and the polyol having a molecular weight of 500 to 5,000 are the same as those described above for the aromatic polyisocyanate, chain extender, and polyol constituting the urethane resin contained in positive electrode binder composition 1 of the present invention. Furthermore, it is preferable that the content of the constituent units based on the aromatic polyisocyanate is 35% by mass to 80% by mass, and the content of the constituent units based on the chain extender is 1% by mass to 40% by mass, relative to the total amount of units constituting the urethane resin contained in positive electrode binder composition 2 of the present invention. The content of structural units based on aromatic polyisocyanate is more preferably 40% by mass or more and 75% by mass or less, and the content of structural units based on chain extenders is more preferably 5% by mass or more and 30% by mass or less. Furthermore, the content of structural units based on polyols having a molecular weight of 500 to 5,000 relative to the total amount of units constituting the urethane resin is preferably 1% by mass or more and 60% by mass or less, more preferably 5% by mass or more and 50% by mass or less. The sum of the content of structural units based on aromatic polyisocyanate and the content of structural units based on chain extenders is, in other words, the content (% by mass) of hard segment components in the urethane resin. That is, the hard segment content of the urethane resin contained in the positive electrode binder composition 2 of the present invention is preferably 36% by mass or more and 99% by mass or less, more preferably in the range of 45% to 95% by mass or less. The effects of the content of the hard segment constituent part in the urethane resin being within the above range are the same as those described above for the urethane resin contained in the positive electrode binder composition 1 of the present invention.

[0035] The positive electrode binder composition 2 of the present invention may further contain a non-aqueous solvent. Details of the non-aqueous solvent are the same as those described above for the non-aqueous solvent contained in the positive electrode binder composition 1 of the present invention. In addition, the non-aqueous solvent is preferably the same type as the organic solvent S1 described above, and N-methylpyrrolidone, dimethylformamide, and dimethyl sulfoxide are more preferred.

[0036] The preferred range of the content of the urethane resin relative to the entire positive electrode binder composition 2 of the present invention is the same as that described above for the positive electrode binder composition 1 of the present invention.

[0037] The positive electrode binder composition 2 of the present invention may further contain a resin other than the urethane resin (hereinafter referred to as "other resin") as necessary, as long as the effects of the present invention are not impaired. When the binder composition 2 of the present invention further contains the other resin, the preferred range of the content of the other resin is the same as that described above for the positive electrode binder composition 1 of the present invention.

[0038] The positive electrode binder composition 2 of the present invention may further contain various additives (hereinafter referred to as "various additives") such as surfactants, antioxidants, light stabilizers, plasticizers, viscosity modifiers, organic or inorganic fillers, etc., as necessary, within the range that does not impair the effects of the present invention. When the positive electrode binder composition 2 of the present invention further contains various additives, the preferred ranges of the contents of the various additives are the same as those described above for the positive electrode binder composition 1 of the present invention.

[0039] When the positive electrode binder composition 2 of the present invention further contains a non-aqueous solvent, the urethane resin is preferably a urethane resin obtained by reacting the aromatic polyisocyanate, the chain extender, and the polyol in the non-aqueous solvent. That is, the urethane resin contained in the positive electrode binder composition 2 of the present invention is preferably obtained by solution polymerization of the aromatic polyisocyanate, the chain extender, and the polyol in a non-aqueous solvent. The weight-average molecular weight of the urethane resin obtained by solution polymerization is easily controlled. Furthermore, in a positive electrode material layer formed from a positive electrode composition containing the positive electrode binder composition 2 of the present invention, which contains such a urethane resin, the aforementioned π-π stacking interaction between aromatic rings in the diisocyanate-based structural units constituting the urethane resin is more likely to occur, thereby improving resistance to the electrolyte and promoting appropriate swelling in the electrolyte. Furthermore, it is believed that the battery performance, such as the charge / discharge characteristics and discharge capacity retention rate, of a secondary battery including such a positive electrode is improved. The resulting urethane resin solution can be used as the positive electrode binder composition 2 of the present invention as is, or may be diluted with a non-aqueous solvent as necessary before being used as the positive electrode binder composition 2 of the present invention.

[0040] During the reaction, the equivalent ratio of the isocyanate groups of the aromatic polyisocyanate to the hydroxyl groups of the chain extender (preferably a diol compound) and polyol is preferably in the range of 0.95 to 1.05, more preferably in the range of 0.98 to 1.02. The reaction temperature of the aromatic polyisocyanate, chain extender, and polyol in a non-aqueous solvent is usually preferably in the range of 50 to 150°C.

[0041] The weight-average molecular weight (Mw) of the urethane resin is preferably 10,000 or more and 500,000 or less, and more preferably 50,000 or more and 400,000 or less. Having the Mw of the urethane resin within the above-mentioned range is preferable from the viewpoint that a nonaqueous secondary battery having a positive electrode formed from the positive electrode binder composition 2 of the present invention is likely to have better battery characteristics, such as charge / discharge characteristics and capacity retention. Furthermore, it is preferable that the urethane resin is substantially free of a crosslinked structure. In other words, the urethane resin preferably has a linear structure. If the urethane resin is substantially free of a crosslinked structure, the urethane resin is easily soluble in the nonaqueous solvent described above, enabling solution polymerization. Furthermore, in a positive electrode material layer formed from a positive electrode composition containing the positive electrode binder composition 2 of the present invention, it is easy to control the durability to the electrolyte and the degree of swelling in the electrolyte, as described above.

[0042] The present invention also relates to a composition for a positive electrode of a non-aqueous secondary battery (hereinafter also simply referred to as "the positive electrode composition of the present invention") containing a positive electrode active material, a conductive material, and the above-described binder composition for a positive electrode of a non-aqueous secondary battery of the present invention (i.e., the positive electrode binder composition 1 of the present invention or the positive electrode binder composition 2 of the present invention). The positive electrode active material is not particularly limited, and when producing a non-aqueous electrolyte secondary battery, for example, a lithium ion secondary battery, examples of the positive electrode active material include metal compounds, metal oxides, metal sulfides, and conductive polymers capable of doping or intercalating lithium ions. Specifically, lithium cobalt oxide (LiCoO ２ Lithium cobalt composite oxides (LCO) such as lithium nickel oxide (LiNiO ２ ), lithium manganese oxide (LiMnO ２ ) and their composite oxides (LiCoxNiyMnzO ２ , x + y + z = 1; lithium nickel manganese cobalt composite oxide (NMC); lithium manganese spinel (LiMn ２ O ４ ), lithium vanadium compounds, V ２ O ５ , V ６ O １３ , V.O. ２ , MnO ２ , TiO２ , MoV ２ O ８ , TiS ２ , V ２ S ５ , V.S. ２ , MoS ２ , MoS ３ , Cr ３ O ８ , Cr ２ O ５ , olivine-type LiMPO ４ (wherein M is Co, Ni, Mn, or Fe); conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene, and porous carbon are examples. These may be used alone or in combination of two or more. Among them, from the viewpoints of working voltage, volume density, theoretical capacity, high-temperature stability, and economical efficiency, olivine-type LiFePO ４ It is more preferable to use lithium iron phosphate compounds (LFP) such as:

[0043] The average particle size of the positive electrode active material is not particularly limited. For example, when a lithium-cobalt composite oxide (LCO) or a lithium-nickel-manganese-cobalt composite oxide (NMC) is used as the positive electrode active material, the average particle size is usually preferably 1 μm or more and 100 μm or less, more preferably 5 μm or more and 50 μm or less. Furthermore, when a lithium iron phosphate compound (LFP) is used as the positive electrode active material, the average particle size is usually preferably 0.01 μm or more and 5 μm or less, more preferably 0.1 μm or more and 1 μm or less. When the average particle size of the positive electrode active material is within the above range, when a secondary battery is formed, the positive electrode expansion rate during charge and discharge is small, and a decrease in the reversible charge and discharge capacity per unit volume is easily prevented. Furthermore, peeling of the electrode film (positive electrode layer) from the current collector during electrode film production is easily suppressed. The average particle size of the positive electrode active material is the particle size (D50) at which the cumulative volume distribution curve reaches 50% of the total when the cumulative volume distribution curve is plotted from the smallest diameter side in the particle size distribution measured by dynamic light scattering using a laser diffraction particle size analyzer or the like.

[0044] At least a portion of the surface of the positive electrode active material may be coated with a coating material. The coating material is preferably a material that can be expected to have electronic conductivity, lithium ion conductivity, and the effect of inhibiting decomposition of the electrolyte, and examples of such coating materials include electron-conductive materials such as carbon, titanium, and nickel. Among these, carbon is preferred, and low-crystalline carbon is more preferred, from the viewpoint of improving the chemical stability and thermal stability of the positive electrode active material and tending to inhibit deterioration in the charge / discharge performance of the resulting secondary battery. When at least a portion of the surface of the positive electrode active material is coated with a coating material, the average thickness of the coating layer is preferably 10 nm or more and 300 nm or less, more preferably 20 nm or more and 200 nm or less. The content of the coating material is preferably 1 to 30 mass% based on the total amount of the components of the positive electrode active material and the coating material.

[0045] Examples of the conductive material include conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene, and porous carbon. As described above, these can also be used as the positive electrode active material, and one type may be used alone, or two or more types may be used in combination.

[0046] The positive electrode composition of the present invention can be obtained by mixing and dispersing the above-described positive electrode active material, conductive material, and non-aqueous secondary battery positive electrode binder composition of the present invention. The order of addition during mixing is not particularly limited. A non-aqueous solvent may be added as needed to adjust the viscosity of the resulting positive electrode composition of the present invention and enhance dispersion stability. Dispersion can be performed using a dispersing device such as a stirrer, planetary mixer, ball mill, super sand mill, or pressure kneader.

[0047] <Secondary Battery> The present invention also relates to a non-aqueous secondary battery positive electrode comprising a positive electrode material layer formed using the above-described positive electrode composition of the present invention. The present invention also relates to a non-aqueous secondary battery comprising such a non-aqueous secondary battery positive electrode, a negative electrode, an electrolyte, and a separator. The binder composition for a non-aqueous secondary battery positive electrode of the present invention has excellent swelling properties in an electrolyte. Therefore, a positive electrode material layer formed from a positive electrode composition containing the binder composition for a non-aqueous secondary battery positive electrode of the present invention has excellent charge / discharge capacity and capacity retention. In other words, a non-aqueous secondary battery of the present invention having a positive electrode comprising such a positive electrode material layer exhibits good charge / discharge characteristics and an excellent capacity retention. Non-aqueous electrolyte secondary batteries and solid electrolyte secondary batteries are preferred as non-aqueous secondary batteries of the present invention, and non-aqueous electrolyte secondary batteries comprising a positive electrode material layer formed using the non-aqueous secondary battery positive electrode composition of the present invention are particularly likely to exhibit excellent performance. For example, when the secondary battery of the present invention is a wet electrolyte secondary battery, it can be constructed by arranging a positive electrode containing the present negative electrode active material and a negative electrode facing each other with a separator interposed therebetween and injecting an electrolyte solution.

[0048] The positive electrode of the present invention can be obtained, for example, by applying the above-described nonaqueous secondary battery positive electrode composition of the present invention to a current collector to form a thin film positive electrode layer. Alternatively, the positive electrode composition of the present invention can be formed into a sheet, pellet, or other shape and integrated with a current collector to obtain a positive electrode.

[0049] Examples of materials for the current collector include copper, nickel, titanium, and stainless steel. The current collector is preferably in the form of a strip, such as a foil, a perforated foil, or a mesh. Porous materials such as porous metal (foamed metal) and carbon paper can also be used as the current collector. Examples of methods for applying the positive electrode composition to the current collector include metal mask printing, electrostatic coating, dip coating, spray coating, roll coating, doctor blade coating, gravure coating, and screen printing. After application, it is preferable to perform a rolling treatment using a flat plate press, a calendar roll, or the like, as needed. Alternatively, the positive electrode material layer may be obtained by forming the paste-like positive electrode composition into a sheet or pellet, and then integrating it with the current collector using a roll, a press, or a combination thereof.

[0050] The positive electrode layer formed on the current collector or the positive electrode layer integrated with the current collector is preferably subjected to heat treatment. Such heat treatment can remove the solvent derived from the binder composition for a positive electrode of a nonaqueous secondary battery of the present invention, promote high strength due to interactions based on π-π stacking in the hard segment components of the urethane resin, and improve adhesion between the positive electrode active materials and between the positive electrode active material and the current collector. The heat treatment temperature is preferably in the range of 50 to 220°C, more preferably 100 to 200°C. There is no particular limitation on the heat treatment time, which is typically in the range of 1 minute to 20 hours. It is preferable to perform the heat treatment in a non-oxidizing gas atmosphere such as helium, argon, or nitrogen, or in a vacuum atmosphere, from the viewpoint of preventing oxidation of the current collector during the heat treatment. Furthermore, after the heat treatment, the positive electrode composed of the positive electrode layer formed on the current collector or the positive electrode layer integrated with the current collector is preferably subjected to pressure treatment from the viewpoint of adjusting the electrode density. The electrode density of the positive electrode is usually 1 to 1.8 g / cm ３ is preferably 1.1 to 1.7 g / cm ３ More preferably, it is 1.2 to 1.6 g / cm ３It is more preferable that the electrode density is 100% or less. The higher the electrode density, the better the adhesion and the volumetric capacity density of the electrode tend to be. However, if the electrode density is too high, the voids in the electrode decrease, making it difficult to suppress the positive electrode expansion rate and resulting in a decrease in the capacity retention rate. Therefore, an optimal range of the electrode density is selected.

[0051] The negative electrode can be obtained by forming a negative electrode material layer on the surface of a current collector in the same manner as the positive electrode. For example, a negative electrode active material and an organic binder are mixed together with a solvent using a dispersing device such as a mixer, ball mill, super sand mill, or pressure kneader to prepare a negative electrode material slurry. This negative electrode material slurry can be applied to a current collector (e.g., copper foil) to form a negative electrode material layer. Examples of the organic binder include styrene-butadiene rubber copolymers (hereinafter also referred to as "SBR"); unsaturated carboxylic acid copolymers such as ethylenically unsaturated carboxylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, (meth)acrylonitrile, and hydroxyethyl (meth)acrylate, and (meth)acrylic copolymers composed of ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid; and polymeric compounds such as polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, polyamideimide, and carboxymethyl cellulose (hereinafter also referred to as "CMC"). Depending on their physical properties, these organic binders may be dispersed or dissolved in water or dissolved in an organic solvent such as N-methyl-2-pyrrolidone.

[0052] The content of the organic binder in the negative electrode material layer of the lithium ion secondary battery negative electrode is preferably 1 to 30% by mass, more preferably 2 to 20% by mass, and even more preferably 3 to 15% by mass. When the content of the organic binder is 1% by mass or more, adhesion is improved and destruction of the negative electrode structure due to expansion or contraction during charge and discharge is easily suppressed. On the other hand, when the content is 30% by mass or less, an increase in electrode resistance is more easily suppressed.

[0053] The negative electrode material slurry may further contain a conductive additive as needed. Examples of conductive additives include carbon black, graphite, acetylene black, and conductive oxides and nitrides. When the negative electrode material slurry further contains a conductive additive, the amount thereof is preferably in the range of 1 to 15 mass % relative to the negative electrode active material.

[0054] Examples of materials for the current collector include copper, nickel, titanium, and stainless steel. The current collector is preferably in the form of a strip, such as a foil, a perforated foil, or a mesh. Porous materials such as porous metal (foamed metal) and carbon paper can also be used as the current collector. Examples of methods for applying the negative electrode material slurry to the current collector include metal mask printing, electrostatic coating, dip coating, spray coating, roll coating, doctor blade coating, gravure coating, and screen printing. After application, it is preferable to perform a rolling process using a flat plate press, a calendar roll, or the like, as needed.

[0055] Alternatively, the negative electrode material layer may be obtained by forming the paste-like negative electrode material slurry into a sheet or pellet, and then integrating it with a current collector by rolling, pressing, or a combination thereof, etc. Furthermore, the negative electrode material layer can also be produced by adding a carbon material, such as natural graphite, artificial graphite, or amorphous carbon, such as hard carbon or soft carbon, to the negative electrode material slurry.

[0056] The negative electrode material layer formed on the current collector or the negative electrode material layer integrated with the current collector is preferably heat-treated depending on the type of organic binder. For example, when an aqueous styrene-butadiene rubber copolymer (SBR) or the like is used, the heat treatment is preferably performed at 100 to 130°C, and when an organic binder with a polyimide or polyamideimide main skeleton is used, the heat treatment is preferably performed at 150 to 450°C. This heat treatment not only removes the solvent derived from the organic binder, but also promotes strength by hardening the organic binder, improving adhesion between particles and between the particles and the current collector. It is preferable to perform the heat treatment in a non-oxidizing gas atmosphere such as helium, argon, or nitrogen, or in a vacuum atmosphere, in order to prevent oxidation of the current collector during the heat treatment.

[0057] After the heat treatment, the negative electrode material layer formed on the current collector or the negative electrode material layer integrated with the current collector is preferably subjected to pressure treatment in order to adjust the electrode density. The electrode density of the negative electrode is usually 1 to 1.8 g / cm. ３ is preferably 1.1 to 1.7 g / cm ３ More preferably, it is 1.2 to 1.6 g / cm ３ The higher the electrode density, the better the adhesion and the volumetric capacity density of the electrode tend to be. However, if the electrode density is too high, the voids in the electrode decrease, making it difficult to suppress the negative electrode expansion rate and resulting in a decrease in the capacity retention rate. Therefore, an optimal range of the electrode density is selected.

[0058] The separator may be a nonwoven fabric, cloth, microporous film, or a combination thereof, whose main component is a polyolefin such as polyethylene or polypropylene. If the nonaqueous electrolyte secondary battery to be fabricated is configured so that the positive electrode and the negative electrode are not in direct contact with each other, it is not necessary to use a separator.

[0059] The electrolyte may be, for example, LiClO ４ , LiPF ６ , LiAsF ６ , LiBF ４ , LiSO ３ CF ３ A so-called organic electrolyte solution can be used in which a lithium salt such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, or the like is dissolved in one kind of non-aqueous solvent alone or in a mixture of two or more kinds thereof.

[0060] The structure of a secondary battery using the binder composition for a non-aqueous secondary battery positive electrode of the present invention is not particularly limited, but typically, a positive electrode, a negative electrode, and an optional separator are wound into a flat spiral to form a wound electrode assembly, or these are stacked as flat plates to form a stacked electrode assembly, and the electrode assembly is sealed in an outer casing. Secondary batteries using the binder composition for a non-aqueous secondary battery positive electrode of the present invention are used, for example, as paper batteries, button batteries, coin batteries, stacked batteries, cylindrical batteries, and prismatic batteries. The binder composition for a non-aqueous secondary battery positive electrode of the present invention can also be used in general electrochemical devices that use lithium ion insertion and removal as a charge / discharge mechanism, such as hybrid capacitors and solid-state lithium secondary batteries.

[0061] Although the binder composition for a positive electrode of a nonaqueous secondary battery, the composition for a positive electrode, the positive electrode, and the secondary battery having the positive electrode of the present invention have been described above, the present invention is not limited to the configurations of the above-described embodiments. For example, the binder composition for a positive electrode of a nonaqueous secondary battery, the composition for a positive electrode, the positive electrode, and the secondary battery having the positive electrode of the present invention may each have any other optional components in addition to the configurations of the above-described embodiments, or may be substituted with any optional components that exhibit similar functions.

[0062] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the following examples. The raw materials used in each example and comparative example are described below.

[0063] <Polyisocyanate> MDI: Diphenylmethane diisocyanate NDI: Naphthalene-1,5-diisocyanate TDI: Orthotolidine diisocyanate PPDI: 1,4-phenylene diisocyanate HMDI: Dicyclohexylmethane diisocyanate IPDI: Isophorone diisocyanate mXDI: m-xylylene diisocyanate

[0064] <Chain extender> EG: Ethylene glycol BD: 1,4-butanediol HD: 1,6-hexanediol CHD: 1,4-cyclohexanedimethanol

[0065] <Polyols> Polyol 1: Polycarbonate diol (UH-200 (trade name) manufactured by UBE, number average molecular weight (Mn) 2000) Polyol 2: Polycarbonate diol (UH-100 (trade name) manufactured by UBE, Mn 1000) Polyol 3: Polycarbonate diol (UH-50 (trade name) manufactured by UBE, Mn 500) Polyol 4: Polyester diol (Mn 2000) obtained by condensation polymerization of adipic acid and ethylene glycol Polyol 5: Polyester diol (Mn 3000) obtained by condensation polymerization of adipic acid and ethylene glycol. Polyol 6: Polyether ester diol (Mn 2000) obtained by condensation polymerization of adipic acid and diethylene glycol. Polyol 7: Polytetramethylene ether glycol ("PTMG2000 (trade name)" manufactured by Mitsubishi Chemical Corporation, Mn 2000). Polyol 8: Polyether polycarbonate diol ("PEPCD NT2002 (trade name)" manufactured by Mitsubishi Chemical Corporation, Mn 2000). Polyol 9: Hydrogenated polybutadiene ("NISSO-PB BI-2000 (trade name)" manufactured by Nippon Soda Co., Ltd., Mn 2200).

[0066] Example 1 (1) Preparation of Positive Electrode Binder Composition A reaction vessel equipped with a stirrer, thermometer, cooling tube, and nitrogen gas inlet was charged with 400 parts by mass of N-methylpyrrolidone (NMP) as a solvent, 0.5 parts by mass of Polyol 1 as a polyol, 19.8 parts by mass of EG as a chain extender, and 79.7 parts by mass of MDI as a polyisocyanate. The mixture was heated to 80°C with stirring and reacted at 80°C for 10 hours. NMP was then added to adjust the nonvolatile content to 8% by mass, thereby obtaining a positive electrode binder composition 1. The weight average molecular weight (standard polystyrene equivalent) of the urethane resin constituting the positive electrode binder composition 1 was measured using the following apparatus and under the following measurement conditions, and was found to be 110,000. Apparatus: "HLC-8320 GPC (trade name)" manufactured by Tosoh Corporation Guard column: "HXL-L (trade name)" manufactured by Tosoh Corporation Column: "TSK-GEL G2000HXL", "TSK-GEL G3000HXL", "TSK-GEL G4000HXL", "TSK-GEL G5000HXL" and "TSK-GEL G6000HXL" connected in series in this order (all manufactured by Tosoh Corporation, trade names) Column temperature: 40°C Developing solvent: N,N-dimethylformamide (containing 0.1% by mass of lithium bromide) Developing solvent flow rate: 1.0 ml / min Detector: RI (differential refractometer) Data processing: "GPC workstation" manufactured by Tosoh Corporation A calibration curve was created using the EcoSEC-WorkStation and standard polystyrenes with known molecular weights manufactured by Tosoh Corporation: "A-500," "A-1000," "A-2500," "A-5000," "F-1," "F-2," "F-4," "F-10," "F-20," "F-40," "F-80," and "F-128" (all trade names).

[0067] (2) Preparation of a composition for a positive electrode of a non-aqueous secondary battery. Lithium iron phosphate (LiFePO) of olivine type was used as a positive electrode active material. ４) 94.0 parts by mass, and 3.0 parts by mass of acetylene black as a conductive material were weighed out and stirred for 30 seconds using a planetary centrifugal mixer ("ARE-310 (trade name)" manufactured by Thinky Corporation) at a rotation speed of 1000 rpm and a revolution speed of 2000 rpm. Thereafter, stirring using a planetary centrifugal mixer was performed using the same equipment and conditions unless otherwise specified. Next, 27.0 parts by mass (2.16 parts by mass in terms of solids content) of the positive electrode binder composition 1 obtained in (1) above and 19.0 parts by mass of NMP were added and mixed until the whole became a paste, and then stirred for 2 minutes using a planetary centrifugal mixer. Since heat was generated by stirring, the mixture was cooled to room temperature with ice water, stirred again with a planetary centrifugal mixer for 2 minutes, and cooled to room temperature with ice water. Next, 10.5 parts by mass (0.84 parts by mass in terms of solid content) of the positive electrode binder composition 1 obtained in (1) above and 5 parts by mass of NMP were added to this mixture and mixed until the entire mixture was uniform. While measuring the viscosity of the resulting slurry using a Brookfield viscometer at 25°C and 30 rpm, NMP was added so that the viscosity was in the range of 2000 to 4000 mPa s. Finally, the mixture was stirred for 30 seconds using a planetary centrifugal mixer to prepare a slurry of positive electrode composition 1.

[0068] (3) Preparation of Positive Electrode After drying, the coating amount (area density) of the positive electrode composition 1 was 25.0 mg / cm ２ The gap of the bar coater was adjusted so that the positive electrode composition 1 obtained above was applied to a carbon-coated aluminum foil current collector using this bar coater, and the coated electrode was dried for 10 minutes in a fan-type dryer set at 80°C. The dried electrode was cut into a width of 40 mm and pressed into a sheet having a layer density of 2.5 g / cm using a roll press ("Small Tabletop Roll Press SA-602" manufactured by Tester Sangyo Co., Ltd.). ３ After pressing the electrode so that the electrode was in a state of 100° C., the electrode was dried in vacuum at 110° C. for 10 hours to obtain a positive electrode 1.

[0069] (4) Preparation of Negative Electrode Composition 48.75 parts by mass of artificial graphite and 48.75 parts by mass of natural graphite were weighed as negative electrode active materials and stirred for 30 seconds with a planetary centrifugal mixer. Next, carboxymethylcellulose Na salt (Nippon Paper Industries Co., Ltd.'s "Sunrose MAC350HC") was dissolved in distilled water, and 48.0 parts by mass (0.96 parts by mass in terms of solids) of an aqueous solution adjusted to a non-volatile concentration of 2% (hereinafter referred to as "CMC solution") was added and mixed until the whole was paste-like, and then stirred with a planetary centrifugal mixer for 2 minutes. Since heat was generated by stirring, the mixture was cooled to room temperature with ice water, stirred again with a planetary centrifugal mixer for 2 minutes, and then cooled to room temperature with ice water. Subsequently, 27.0 parts by mass of the CMC solution (0.54 parts by mass in terms of solids) was added to this mixture, mixed until the whole was uniform, stirred with a planetary centrifugal mixer for 2 minutes, and then cooled to room temperature with ice water. 10 parts by weight of distilled water and 2.95 parts by mass (1.5 parts by mass in terms of solids content) of a styrene-butadiene copolymer ("DS407H (trade name)" manufactured by DIC Corporation, solids concentration 50.8% by mass) were added, and the mixture was stirred again for 2 minutes with a planetary centrifugal mixer and cooled to room temperature with ice water. The viscosity of the resulting slurry was measured with a Brookfield viscometer at 25°C and 30 rpm, and distilled water was added so that the viscosity was in the range of 2000 to 4000 mPa s. Finally, the mixture was stirred for 30 seconds with a planetary centrifugal mixer to prepare a slurry negative electrode composition.

[0070] (5) Preparation of negative electrode After drying, the coating amount (area density) of the negative electrode composition was 8.8 mg / cm ２ The gap of the bar coater was adjusted so that the negative electrode composition was applied to a copper foil current collector using the bar coater, and the negative electrode composition was dried for 8 minutes in a fan dryer set at 80°C. The dried electrode was cut into a width of 40 mm and pressed using a roll press ("Small Tabletop Roll Press SA-602" manufactured by Tester Sangyo Co., Ltd.) to a layer density of 1.55 g / cm. ３ After pressing the mixture so that it became a negative electrode, the mixture was dried in vacuum at 110° C. for 10 hours to obtain a negative electrode.

[0071] (6) Fabrication of Secondary Battery The negative electrode fabricated above was cut into a 24 mm x 24 mm square with a tab, and the positive electrode 1 fabricated above was cut into a 22 mm x 22 mm square with a tab, each using a Thomson blade. A nickel tab lead was welded to the tab portion of the cut electrode for the negative electrode, and an aluminum tab lead was welded to the tab portion of the cut electrode. Meanwhile, a 25 μm thick polyethylene microporous film was cut into a 28 mm x 3.8 cm rectangle using a Thomson blade as a separator. The positive electrode 1 and the negative electrode were placed opposite each other via the separator, packaged in a laminate film, and the tab portion was fixed by thermocompression. Then, LiPF ６ The above was dissolved in a mixed solution of ethylene carbonate / dimethyl carbonate / methyl ethyl carbonate = 30 / 30 / 40 (volume ratio) at a concentration of 1 mol / L, and 300 μL of a non-aqueous electrolyte solution obtained by further adding 1 vol % of vinyl carbonate and 5 vol % of fluoroethylene carbonate was added thereto, and the mixture was vacuum laminated to completely seal the battery, thereby producing a laminate-type secondary battery 1.

[0072] Examples 2 to 25 and Comparative Examples 1 to 5 Positive electrode binder compositions 2 to 30 were obtained in the same manner as in Example 1 (1), except that in Example 1 (1), the types and amounts of polyol, chain extender, and polyisocyanate used were changed as shown in Table 2. Subsequently, the same operations as in Examples 1 (2) to (6) were performed, except that positive electrode binder compositions 2 to 30 were used instead of positive electrode binder composition 1 in Example 1 (2), to obtain positive electrode compositions 2 to 30, positive electrodes 2 to 30, and laminate-type secondary batteries 2 to 30 using each positive electrode binder composition. The weight-average molecular weight of the urethane resin constituting each positive electrode binder composition was measured in the same manner as in Synthesis Example 1. The results are summarized in Table 2.

[0073] Comparative Example 6 The same operations as in (2) to (6) of Example 1 were performed, except that an NMP solution of PVDF ("Kureha KF Polymer W#7200" manufactured by Kureha Corporation) with a solid content concentration of 20 mass% was used instead of the positive electrode binder composition 1 in (2) of Example 1, to obtain a positive electrode composition, a positive electrode, and a laminate-type secondary battery.

[0074] 2. Evaluation 2-1. Swelling degree of urethane resin constituting the positive electrode binder composition in the electrolyte solution The positive electrode binder composition prepared in each example and comparative example was applied to a release PET film, dried at 140°C for 2 hours and then at 110°C for 10 hours to prepare a film with a thickness of 40 μm, and a test piece of 1 cm x 1 cm was cut out and its mass was measured (M1). ６ The specimen was immersed in a non-aqueous electrolyte solution prepared by dissolving the specimen in a 30 / 70 volumetric ratio of ethylene carbonate / ethyl methyl carbonate solution at a concentration of 1.2 mol / L at 60°C for 12 hours, then removed, the non-aqueous electrolyte solution on the surface of the specimen was wiped off, and the mass was measured again (M2). The swelling degree was calculated using the following formula: Swelling degree (%) = 100 × (M2 - M1) / M1

[0075] 2-2. Dispersion Stability of Positive Electrode Composition The viscosity (initial viscosity) of the positive electrode compositions prepared in each Example and Comparative Example immediately after preparation was measured using a Brookfield viscometer at 25°C and 30 rpm. The positive electrode compositions were placed in glass screw tubes, sealed, and stored in an incubator at 30°C for two weeks, after which the viscosity was measured using a Brookfield viscometer at 25°C and 30 rpm (viscosity after storage). The viscosity change rate of the positive electrode composition was calculated using the following formula and evaluated according to the following criteria: Viscosity change rate = 100 × (viscosity after storage - initial viscosity) / initial viscosity <Evaluation criteria> ◎: Viscosity change rate is less than 5% ○: Viscosity change rate is 5% or more but less than 10% △: Viscosity change rate is 10% or more but less than 20% ×: Viscosity change rate is 20% or more

[0076] 2-3. Positive Electrode Peel Strength Test pieces measuring 25 mm wide x 100 mm long were cut out from the positive electrodes prepared in each Example and Comparative Example, and the positive electrode active material side of the test piece was attached to a stainless steel plate using double-sided tape (Nitto Denko Corporation's "No. 5015"). Meanwhile, approximately 10 mm of the edge of the carbon-coated aluminum foil was peeled off, and polyimide tape was attached to this area to attach it to a peel tester (Shimadzu Corporation's "Autograph AG-XPlus"). A 180° peel test was performed using the peel tester to measure the peel strength.

[0077] 2-4. Battery Characteristics (Initial Charge-Discharge Efficiency and Capacity Retention Rate) The secondary batteries prepared in each Example and Comparative Example were attached to a charge-discharge device, left at 25°C for 3 hours, and then charged and discharged once at 0.1 C to measure the initial charge-discharge efficiency. Next, charge-discharge at 60°C and 0.2 C was repeated 50 times, and the discharge capacity retention rate after 50 cycles at 60°C (after 50 cycles) relative to the first discharge capacity at 0.2 C (initial discharge capacity) was measured using the following formula: Capacity retention rate (%) = 100 × 50th discharge capacity (mAh / g) / initial discharge capacity (mAh / g)

[0078] The above results are summarized in Table 2.

[0079] The results in Table 2 show that the positive electrode binder composition of the present invention has excellent dispersibility of the positive electrode active material, and that the positive electrode formed from the positive electrode binder composition of the present invention has excellent durability against the electrolyte, and that the secondary battery including such a positive electrode has a high initial charge-discharge efficiency of 97% or more and a capacity retention rate of 90% or more, resulting in an excellent balance of battery characteristics.

[0080] The positive electrode binder composition of the present invention complies with legal regulations regarding organic fluorine compounds, and can form a positive electrode and a secondary battery that exhibit excellent durability against an electrolyte, excellent dispersibility of a positive electrode active material, and excellent battery characteristics such as charge / discharge efficiency and capacity retention. Secondary batteries equipped with such positive electrodes exhibit excellent battery characteristics such as charge / discharge characteristics, and can be effectively used in portable electronic devices and the like as, for example, paper-type batteries, button-type batteries, coin-type batteries, laminated batteries, cylindrical batteries, and prismatic batteries.

Claims

1. A binder composition for a non-aqueous secondary battery positive electrode, comprising a urethane resin and a non-aqueous solvent, the urethane resin being obtained by reacting an aromatic polyisocyanate, a chain extender having a molecular weight of 50 or more and 500 or less, and a polyol having a molecular weight of 500 or more and 5,000 or less.

2. The binder composition for a non-aqueous secondary battery positive electrode according to claim 1, wherein the content of structural units based on the aromatic polyisocyanate is 35% by mass or more and 80% by mass or less, and the content of structural units based on the chain extender is 1% by mass or more and 40% by mass or less, relative to the total amount of units constituting the urethane resin.

3. The binder composition for a non-aqueous secondary battery positive electrode according to claim 1, wherein the aromatic polyisocyanate is a compound having a structure in which an isocyanate group is directly bonded to a carbon atom constituting an aromatic ring.

4. The binder composition for a non-aqueous secondary battery positive electrode according to claim 1, wherein the chain extender is a polyol or a polyamine.

5. The binder composition for a non-aqueous secondary battery positive electrode according to claim 1, wherein the polyol is at least one selected from the group consisting of polyester polyols, polycarbonate polyols, and polyether polyols.

6. The binder composition for a non-aqueous secondary battery positive electrode according to claim 1, wherein the urethane resin is obtained by reacting the aromatic polyisocyanate, the chain extender, and the polyol in the non-aqueous solvent.

7. The binder composition for a non-aqueous secondary battery positive electrode according to claim 1, wherein the non-aqueous solvent is an aprotic polar solvent.

8. The binder composition for a non-aqueous secondary battery positive electrode according to claim 1, wherein the weight average molecular weight of the urethane resin is 10,000 or more and 500,000 or less.

9. A binder composition for a non-aqueous secondary battery positive electrode containing a urethane resin, wherein the urethane resin has a dispersion term (δd) of 17 MPa in the Hansen solubility parameter. ０．５ Over 19 MPa ０．５ Less than 12 MPa, polar term (δp) ０．５ Over 17 MPa ０．５ less than 7 MPa, hydrogen bond term (δh) ０．５ Over 12 MPa ０．５ and a film formed from the urethane resin is dissolved in an organic solvent S1 having a dispersion term (δd) of 15 MPa in the Hansen solubility parameter. ０．５ Over 21 MPa ０．５ Less than 6 MPa, polar term (δp) ０．５ Over 22 MPa ０．５ less than 3 MPa, hydrogen bond term (δh) ０．５ More than 10 MPa ０．５ The binder composition for a non-aqueous secondary battery positive electrode has a swelling degree of 5 mass % or more and 100 mass % or less after immersion in an organic solvent S2 having a molecular weight of less than 10 ...

10. The binder composition for a non-aqueous secondary battery positive electrode according to claim 9, further comprising a non-aqueous solvent.

11. The binder composition for a non-aqueous secondary battery positive electrode according to claim 9, wherein the urethane resin is obtained by reacting an aromatic polyisocyanate, a chain extender having a molecular weight of 50 or more and 500 or less, and a polyol having a molecular weight of 500 or more and 5,000 or less.

12. The binder composition for a non-aqueous secondary battery positive electrode according to claim 9, wherein the content of constituent units based on the aromatic polyisocyanate is 35% by mass or more and 80% by mass or less, and the content of constituent units based on the chain extender is 1% by mass or more and 40% by mass or less, relative to the total amount of units constituting the urethane resin.

13. The binder composition for a non-aqueous secondary battery positive electrode according to claim 10, wherein the urethane resin is obtained by reacting the aromatic polyisocyanate, the chain extender, and the polyol in the non-aqueous solvent.

14. A composition for a positive electrode of a non-aqueous secondary battery, comprising a positive electrode active material, a conductive material, and the binder composition for a positive electrode of a non-aqueous secondary battery according to any one of claims 1 to 13.

15. The positive electrode composition for a non-aqueous secondary battery according to claim 14, wherein the positive electrode active material is a lithium iron phosphate compound.

16. A positive electrode for a non-aqueous secondary battery, comprising a positive electrode material layer formed using the composition for a positive electrode for a non-aqueous secondary battery according to claim 14 or 15.

17. A non-aqueous secondary battery comprising the non-aqueous secondary battery positive electrode according to claim 16, a negative electrode, an electrolyte, and a separator.

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