Powdery binder for secondary battery positive electrode and use of same

Abstract

The present invention provides a powdery binder which is for a secondary battery positive electrode and which enables improvement in productivity of a secondary battery positive electrode, and improvement in dispersiblity of the binder in an active material and adhesiveness of the binder to the active material. Further, the present invention also provides a powdery particle composite body containing said powdery binder.　The powdery binder for a secondary battery positive electrode contains an uncrosslinked polymer having a glass transition temperature of 60-150℃.　The powdery particle composite body contains a positive electrode active material and the powdery binder for a secondary battery positive electrode.

Description

Powdered binder for secondary battery positive electrode and use thereof

[0001] The present specification relates to a powdered binder for a secondary battery positive electrode and its use.

[0002] Various secondary batteries, such as nickel-metal hydride secondary batteries, lithium-ion secondary batteries, and electric double layer capacitors, have been put to practical use. The electrodes used in these secondary batteries are prepared by applying a composition for forming an electrode mixture layer containing an active material and a binder to a current collector, followed by drying. For example, in lithium-ion secondary batteries, an aqueous binder containing styrene butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC) is used as the binder for the negative electrode mixture layer. On the other hand, a solution of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) is widely used as the binder for the positive electrode mixture layer.

[0003] In recent years, as the applications of various secondary batteries have expanded, there has been an increasing demand for improvements in the energy density, reliability, durability, and productivity of secondary batteries. Under these circumstances, electrodes for secondary batteries are also required to achieve both higher performance and productivity.

[0004] A secondary battery electrode is formed by laminating an electrode mixture layer, in which an active material and a conductive additive are bound with a binder, on a current collector foil. The electrode is typically manufactured by coating an electrode slurry containing the active material, conductive additive, binder, etc., on a current collector and then drying the slurry. For example, in the manufacturing process of a secondary battery positive electrode, an electrode slurry containing a positive electrode active material, conductive additive, binder, and organic solvent is used.

[0005] However, when removing the organic solvent from the electrode slurry, a large amount of heat energy is required, which increases the drying time and makes it difficult to improve productivity.

[0006] Therefore, a method for manufacturing a positive electrode for a secondary battery using a mixed powder containing an active material (hereinafter simply referred to as "mixed powder") instead of an electrode slurry (so-called "dry blending") has been proposed. For example, Patent Document 1 describes a mixed powder containing an active material powder and a binder powder, and in the examples, specifically discloses a mixed powder containing lithium manganate powder (a positive electrode active material) as the active material powder and PVDF powder as the binder powder. It has been shown that by attaching the mixed powder to a current collector by powder coating and heating the mixed powder attached to the current collector to a temperature above the softening temperature of the binder (150°C) to fuse the powder, an electrode mixture layer (active material layer) with high film thickness accuracy and excellent load characteristics can be formed without using an electrode slurry (active material paste).

[0007] Patent Document 2 describes a mixed powder (powdered composite particles) that is made of a polymer having a glass transition temperature of 35 to 80°C and a primary particle volume-based D50 average particle size of 80 to 1000 nm, and that has a volatile content of less than 1% by weight at 120°C. In the examples, NMC powder (LiNi) is used as the active material powder. １／３ Co １／３ Mn １／３ O ２、 The document specifically discloses a mixed powder containing a cathode active material and a binder powder containing "a cross-linked polymer having structural units derived from a non-cross-linkable monomer (main component: ethyl methacrylate) and a cross-linkable monomer (allyl methacrylate)." Since no electrode slurry is used when forming the electrode mixture layer, the productivity of secondary battery electrodes is excellent, and since no water-soluble polymer component is required as a dispersant, low resistance is possible, and the resulting electrode has excellent thickness precision and flexibility.

[0008] JP 2001-351616 A International Publication No. 2014 / 192652

[0009] The powdery binder (PVDF) disclosed in Patent Document 1 makes it possible to produce secondary battery electrodes by dry blending without using an electrode slurry. However, after the mixed powder is attached to the surface of the current collector, the heat fusion temperature must be 150°C or higher (200°C in the examples), which poses a problem in terms of productivity and sometimes results in insufficient adhesion of the powdery binder to the active material.

[0010] On the other hand, the powdery binder (the above-mentioned crosslinked polymer) disclosed in Patent Document 2 can be pressure-molded (compressed) at a lower temperature than the above-mentioned PVDF after the mixed powder is attached to the surface of the current collector, but the binder is prone to fusion with other binders, and the dispersibility of the binder in the active material and the adhesion to the active material are sometimes insufficient.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a powdery binder for a secondary battery positive electrode that can improve the productivity of secondary battery positive electrodes and the dispersibility of the binder in and adhesion to the active material. It is also an object of the present invention to provide a powdery particle composite containing the powdery binder, and a secondary battery positive electrode and a secondary battery obtained using the powdery particle composite.

[0012] As a result of intensive research to solve the above-mentioned problems, the present inventors have found that by using a powdered binder for secondary battery positive electrodes containing a non-crosslinked polymer having a glass transition temperature within a specific range, it is possible to improve the productivity of secondary battery positive electrodes, as well as to improve the dispersibility of the binder in and the adhesion to the active material, and have completed the present invention.

[0013] The present invention is as follows: [1] A powdered binder for a secondary battery positive electrode, comprising a non-crosslinked polymer having a glass transition temperature of 60°C or higher and 150°C or lower. [2] The powdered binder for a secondary battery positive electrode according to [1], wherein the non-crosslinked polymer has structural units derived from a non-crosslinkable ethylenically unsaturated monomer. [3] The powdered binder for a secondary battery positive electrode according to [2], wherein the non-crosslinkable ethylenically unsaturated monomer includes a non-crosslinkable aromatic vinyl monomer or a non-crosslinkable ethylenically unsaturated carboxylic acid ester monomer. [4] The powdered binder for a secondary battery positive electrode according to any one of [1] to [3], wherein the non-crosslinked polymer has structural units derived from a non-crosslinkable ethylenically unsaturated carboxylic acid monomer in an amount of 5 mass% or less relative to the total structural units of the non-crosslinked polymer. [5] The powdered binder for a secondary battery positive electrode according to any one of [1] to [4], wherein the particle diameter of the non-crosslinked polymer is 80 nm to 800 nm as a volume-based median diameter (D50) measured by dynamic light scattering. [6] A powdered particle composite comprising a positive electrode active material and the powdered binder for a secondary battery positive electrode according to any one of [1] to [5]. [7] A secondary battery positive electrode comprising, on a current collector surface, a mixture layer formed from the powdered particle composite according to [6]. [8] A secondary battery comprising the secondary battery positive electrode according to [7].

[0014] The powdered binder for secondary battery positive electrodes of the present invention can improve productivity of secondary battery positive electrodes, and can also improve the dispersibility of the binder in the active material and the adhesion to the active material.

[0015] The powdered binder for a secondary battery positive electrode of the present invention (hereinafter also referred to as "the binder") contains a non-crosslinked polymer (hereinafter also referred to as "the non-crosslinked polymer") having a glass transition temperature of 60° C. or higher and 150° C. or lower. Furthermore, the binder is used as a powdered particle composite containing a positive electrode active material, and a positive electrode mixture layer is formed from the powdered particle composite on the surface of a current collector such as aluminum foil, thereby obtaining the secondary battery positive electrode of the present invention.

[0016] Here, the term "powdered" in this binder means that the solid content concentration is 85% by mass or more, and the method for measuring the solid content concentration will be described below. In particular, when a positive electrode for a secondary battery is produced by dry blending, the solid content concentration is more preferably 90% by mass or more, even more preferably 95% by mass or more, even more preferably 97% by mass or more, and even more preferably 98% by mass or more. (Method for measuring solid content concentration) Approximately 0.5 g of polymer is placed in a weighing bottle whose weight has been measured in advance [weight of weighing bottle = B (g)], and the weighing bottle is accurately weighed together with the polymer [W ０ (g)], the polymer together with the weighing bottle was placed in a ventilated dryer and dried at 155°C for 45 minutes, and the weight of the polymer together with the weighing bottle at that time was measured [W １ (g)], and the solid content concentration was calculated using the following formula (1): Solid content concentration (%) = (W １ -B) / (W ０ -B)×100...(1)

[0017] The binder will be described below together with its constituent elements. Furthermore, a powdered particle composite, a secondary battery positive electrode, and a secondary battery each containing the binder will also be described in detail. In this specification, "(meth)acrylic" means acrylic and / or methacrylic, and "(meth)acrylate" means acrylate and / or methacrylate. Furthermore, "(meth)acryloyl group" means acryloyl group and / or methacryloyl group. In the numerical ranges described in stages in this specification, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages, and the upper or lower limit value of that numerical range may be replaced with a value shown in the examples.

[0018] 1. The Present Binder The present binder contains the present non-crosslinked polymer, which melts and coats the positive electrode active material under compression treatment conditions at relatively high temperatures. This allows the binder to have excellent adhesion to the active material. Meanwhile, the glass transition temperature of the present non-crosslinked polymer is 60°C or higher and 150°C or lower, preventing fusion between binder particles during the preparation of a powdered particle composite, allowing the binder to have excellent dispersibility in the active material.

[0019] The glass transition temperature (hereinafter also simply referred to as "Tg") of the present non-crosslinked polymer is preferably 65°C or higher and 150°C or lower, more preferably 70°C or higher and 140°C or lower, even more preferably 75°C or higher and 130°C or lower, still more preferably 80°C or higher and 130°C or lower, even more preferably 85°C or higher and 120°C or lower, and even more preferably 90°C or higher and 110°C or lower, in order to improve the dispersibility of the binder in the active material. In this specification, Tg can be measured by a differential scanning calorimeter (DSC) as described in the examples.

[0020] <Structural Units of the Non-Crosslinked Polymer> The structural units of the non-crosslinked polymer are substantially free of structural units derived from crosslinkable monomers, and have structural units derived from monomers other than crosslinkable monomers (hereinafter also referred to as "non-crosslinkable monomers"). The non-crosslinkable monomer is not particularly limited, but preferably has a structural unit derived from a non-crosslinkable ethylenically unsaturated monomer, and the content of the structural unit is preferably 50% by mass or more and 100% by mass or less, more preferably 60% by mass or more and 100% by mass or less, even more preferably 70% by mass or more and 100% by mass or less, and even more preferably 80% by mass or more and 100% by mass or less, based on the total structural units of the non-crosslinked polymer. Examples of the non-crosslinkable ethylenically unsaturated monomer include a non-crosslinkable aromatic vinyl monomer (hereinafter also referred to as "monomer (a1)"), a non-crosslinkable ethylenically unsaturated carboxylic acid ester monomer (hereinafter also referred to as "monomer (a2)"), a non-crosslinkable ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "monomer (a3)"), a non-crosslinkable nitrile group-containing ethylenically unsaturated monomer, a non-crosslinkable (meth)acrylamide and its derivatives, a non-crosslinkable maleimide compound, etc. Among these, it is preferable to contain a structural unit derived from monomer (a1) or monomer (a2) in order to improve the dispersibility of the binder in the active material and the adhesion to the active material.

[0021] <Structural units derived from monomer (a1) and monomer (a2)>

[0022] Examples of the monomer (a1) include styrene, α-methylstyrene, vinylnaphthalene, isopropenylnaphthalene, etc., and one of these may be used alone or two or more may be used in combination. The content of the monomer (a1) component in the present non-crosslinked polymer is not particularly limited, but is preferably 50% by mass or more and 100% by mass or less, more preferably 60% by mass or more and 100% by mass or less, even more preferably 70% by mass or more and 100% by mass or less, and even more preferably 80% by mass or more and 100% by mass or less, based on all structural units of the present non-crosslinked polymer.

[0023] The monomer (a2) is preferably a (meth)acrylic acid ester monomer, and examples thereof include (meth)acrylic acid alkyl ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; aromatic (meth)acrylic acid ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, phenylethyl (meth)acrylate, and phenoxyethyl (meth)acrylate; (meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate; and (meth)acrylic acid hydroxyalkyl ester compounds such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate. These may be used alone or in combination of two or more.

[0024] The content of the monomer (a2) component in the present non-crosslinked polymer is not particularly limited, but is preferably 1 mass % or more and 100 mass % or less, more preferably 1 mass % or more and 50 mass % or less, even more preferably 1 mass % or more and 30 mass % or less, and even more preferably 1 mass % or more and 20 mass % or less, based on the total structural units of the present non-crosslinked polymer.

[0025] <Structural Units Derived from Monomer (a3)> Examples of the monomer (a3) ​​include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; (meth)acrylamidoalkylcarboxylic acids such as (meth)acrylamidohexanoic acid and (meth)acrylamidododecanoic acid; monohydroxyethyl succinate (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, β-carboxyethyl (meth)acrylate, etc., and one of these may be used alone or two or more may be used in combination. The amount of structural units derived from monomer (a3) ​​in the non-crosslinked polymer is not particularly limited, but may be, for example, 15% by mass or less of the total structural units of the non-crosslinked polymer. By containing the (a3) ​​component in this range, the mechanical stability of the non-crosslinked polymer can be improved. It is preferably 13% by mass or less, more preferably 11% by mass or less, even more preferably 9% by mass or less, even more preferably 7% by mass or less, even more preferably 5% by mass or less, and even more preferably 3% by mass or less.

[0026] <Other structural units> Examples of other structural units in the present non-crosslinked polymer include structural units derived from non-crosslinkable nitrile group-containing ethylenically unsaturated monomers, non-crosslinkable (meth)acrylamide and derivatives thereof, non-crosslinkable maleimide compounds, etc. The amount of the other structural units is, for example, 50% by mass or less, for example, 30% by mass or less, for example, 10% by mass or less, for example, 5% by mass or less, or for example, 1% by mass or less, relative to the total amount of monomers constituting the non-crosslinked polymer.

[0027] Examples of the non-crosslinkable nitrile group-containing ethylenically unsaturated monomer include (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as cyanomethyl (meth)acrylate and cyanoethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; and vinylidene cyanide. One of these may be used alone, or two or more may be used in combination.

[0028] Examples of non-crosslinkable (meth)acrylamide derivatives include N-alkyl(meth)acrylamide compounds such as N-isopropyl(meth)acrylamide and N-t-butyl(meth)acrylamide; N-alkoxyalkyl(meth)acrylamide compounds such as N-n-butoxymethyl(meth)acrylamide and N-isobutoxymethyl(meth)acrylamide; N,N-dialkyl(meth)acrylamide compounds such as N,N-dimethyl(meth)acrylamide and N,N-diethyl(meth)acrylamide; and cyclic (meth)acrylamide compounds such as 4-acryloylmorpholine. These may be used singly or in combination of two or more.

[0029] Non-crosslinkable maleimide compounds include maleimide and N-substituted maleimide compounds. Examples of N-substituted maleimide compounds include N-alkyl-substituted maleimide compounds such as N-methylmaleimide, N-ethylmaleimide, N-n-propylmaleimide, N-isopropylmaleimide, N-n-butylmaleimide, N-isobutylmaleimide, N-tert-butylmaleimide, N-pentylmaleimide, N-hexylmaleimide, N-heptylmaleimide, N-octylmaleimide, N-laurylmaleimide, and N-stearylmaleimide; N-cyclopentylmaleimide, N-cyclohexylmaleimide, and the like. N-cycloalkyl-substituted maleimide compounds such as N-cycloalkylphenyl)maleimide; and N-aryl-substituted maleimide compounds such as N-phenylmaleimide, N-(4-hydroxyphenyl)maleimide, N-(4-acetylphenyl)maleimide, N-(4-methoxyphenyl)maleimide, N-(4-ethoxyphenyl)maleimide, N-(4-chlorophenyl)maleimide, N-(4-bromophenyl)maleimide, and N-benzylmaleimide. These may be used singly or in combination of two or more.

[0030] <Particle diameter of the present non-crosslinked polymer> The particle diameter of the present non-crosslinked polymer is, in terms of excellent adhesion to the active material, for example, 80 nm to 950 nm, preferably 80 nm to 800 nm, more preferably 80 nm to 750 nm, even more preferably 80 nm to 700 nm, still more preferably 80 nm to 650 nm, and even more preferably 80 nm to 600 nm, as measured by dynamic light scattering. In this specification, the particle diameter can be measured by dynamic light scattering as described in the examples.

[0031] <Method for producing the present non-crosslinked polymer> The present non-crosslinked polymer can be produced by known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization, and can be appropriately selected based on the molecular weight, composition, etc. The polymerization initiator can be any known polymerization initiator such as an azo compound, organic peroxide, or inorganic peroxide, but is not particularly limited. The conditions for use can be adjusted to generate an appropriate amount of radicals using known methods such as thermal initiation, redox initiation in combination with a reducing agent, and UV initiation. Furthermore, known chain transfer agents may be used as needed for the purpose of adjusting the molecular weight, etc.

[0032] Among the polymerization methods, emulsion polymerization is preferred because it can produce a non-crosslinked polymer having the above particle size and provides a significant effect of the present invention. Examples of emulsion polymerization methods include a batch reaction in which a reaction vessel is charged with a monomer, a surfactant, and water and the monomer is reacted therein, and a dropping reaction in which the monomer is gradually added dropwise to a reaction vessel. Among these, a dropping reaction is preferred because it is easy to control the heat generated during the polymerization reaction. Furthermore, to further improve polymerization stability, the dropping reaction is preferably carried out by mixing and stirring the monomer, water, and surfactant to form an emulsified monomer pre-emulsion and then dropping the monomer. At this time, a solvent mainly composed of water is present in the reaction vessel. It is preferable to mix a surfactant with the solvent. Furthermore, it is preferable to preheat the solvent, and the heating conditions are, for example, 50 to 120°C, or 70 to 100°C.

[0033] The emulsion polymerization is preferably carried out in the presence of at least one of a surfactant and a protective colloid. The surfactant is preferably an anionic, cationic, or nonionic surfactant, more preferably an anionic or nonionic surfactant. Polymerizable surfactants having an ethylenically unsaturated double bond can also be used.

[0034] An anionic surfactant is a surfactant that can become ions in an aqueous solution and whose hydrophilic moiety becomes an anion. A nonionic surfactant is a surfactant that exhibits surface activity without dissociating into ions in an aqueous solution. A cationic surfactant is a surfactant that can become ions in an aqueous solution and whose hydrophilic moiety becomes a cation. A polymerizable surfactant is an anionic or nonionic surfactant that has one or more radically polymerizable unsaturated double bonds in the molecule.

[0035] The surfactants can be used alone or in combination of two or more. The amount of surfactant is not particularly limited, but is preferably 0.1 to 20 parts by mass per 100 parts by mass of the monomer mixture (in this specification, "monomer mixture" refers to a mixture containing a monomer and a chain transfer agent). Using an appropriate amount of surfactant further improves the mechanical stability of the resin particles, and using an appropriate amount of polymerizable surfactant further improves the mechanical stability. If the amount is less than 0.1 part by mass, it becomes difficult to ensure emulsion stability. Furthermore, if it exceeds 20 parts by mass, water resistance is significantly reduced.

[0036] For emulsion polymerization, it is preferable to use a radical polymerization initiator (hereinafter also referred to as "polymerization initiator"). Known oil-soluble polymerization initiators and water-soluble polymerization initiators can be used as the polymerization initiator. Examples of oil-soluble initiators include organic peroxides such as benzoyl peroxide, tertiary butyloxybenzoate, tertiary butyl hydroperoxide, tertiary butylperoxy-2-ethylhexanoate, tertiary butylperoxy-3,5,5,trimethylhexanoate, ditertiary butyl peroxide, cumene hydroperoxide, and p-menthane hydroperoxide; and azobis compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis-2,4-dimethylvaleronitrile, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), and 1,1'-azobis-cyclohexane-1-carbonitrile. Examples of the water-soluble polymerization initiator include ammonium persulfate, sodium persulfate, potassium persulfate, hydrogen peroxide, and 2,2'-azobis(2-methylpropionamidine) dihydrochloride.

[0037] In emulsion polymerization, a reducing agent can be used in combination with a polymerization initiator to accelerate the polymerization reaction. Examples of such reducing agents include reducing organic compounds such as ascorbic acid, erythorbic acid, tartaric acid, citric acid, glucose, and metal salts of formaldehyde sulfoxylate; reducing inorganic compounds such as sodium sulfite, sodium bisulfite, sodium metabisulfite (SMBS), and sodium hyposulfite; ferrous chloride, Rongalit, and thiourea dioxide.

[0038] For emulsion polymerization, it is preferable to use a water-soluble polymerization initiator. The polymerization initiator is preferably used in an amount of 0.05 to 5% by mass relative to 100 parts by mass of the monomer mixture. The reducing agent is preferably used in an amount of 0.01 to 2.5% by mass relative to 100 parts by mass of the monomer mixture.

[0039] During emulsion polymerization, a buffer, a chain transfer agent, a basic compound, etc. can be used as necessary. Examples of buffers include sodium acetate, sodium citrate, and sodium bicarbonate. Examples of chain transfer agents include 2-mercaptoethanol, octyl mercaptan, tertiary dodecyl mercaptan, lauryl mercaptan, stearyl mercaptan, 2-ethylhexyl mercaptoacetate, octyl mercaptoacetate, 2-ethylhexyl mercaptopropionate, and octyl mercaptopropionate.

[0040] Furthermore, the binder can be obtained by drying the non-crosslinked polymer obtained by the above polymerization method. The drying method is not particularly limited as long as it can dry the non-crosslinked polymer in a redispersible state without excessive fusion of the non-crosslinked polymer molecules, and examples thereof include a method of drying an aqueous dispersion in a forced air dryer (e.g., 70°C), a method of spray-drying an aqueous dispersion of the non-crosslinked polymer, and a method of drying in a rotary evaporator. Furthermore, it is more preferable to dry the non-crosslinked polymer under vacuum after spray-drying or drying in a rotary evaporator.

[0041] The drying temperature is preferably lower than the minimum film-forming temperature of the non-crosslinked polymers, from the viewpoint of removing moisture in a redispersible state without excessive fusion of the non-crosslinked polymers. If the drying temperature is too high, the non-crosslinked polymers will form a film, making it difficult to redisperse them.

[0042] Furthermore, the minimum film-forming temperature of the present non-crosslinked polymer is preferably 60° C. or higher, more preferably 70° C. or higher, and even more preferably 80° C. or higher, in order to enable drying of the non-crosslinked polymer in a redispersible state. If the minimum film-forming temperature is too low, it becomes difficult to dry the non-crosslinked polymer without excessive fusion bonding between the non-crosslinked polymer particles.

[0043] Here, the minimum film-forming temperature is the lowest temperature at which a film of the non-crosslinked polymer is formed. The minimum film-forming temperature can be measured in accordance with JIS K6828-2 (2003). Specifically, an aqueous dispersion of a non-crosslinked polymer is applied to a flat plate, such as an iron plate, having an appropriate temperature gradient to a thickness of about 100 μm and dried, and the boundary temperature between the filmed portion and the non-filmed portion is measured. Here, the filmed portion becomes transparent and the non-filmed portion becomes cloudy, so the boundary between the filmed portion and the non-filmed portion can be visually confirmed. Furthermore, when the flat plate after application and drying of the aqueous dispersion of the non-crosslinked polymer is rubbed, the non-filmed portion powders off, so the boundary between the filmed portion and the non-filmed portion can also be confirmed by the presence or absence of powder falling off.

[0044] 2. Powdered Particle Composite The powdered particle composite of the present invention comprises a positive electrode active material and the present binder. The amount of the present binder used in the present powdered particle composite is, for example, 0.1 parts by mass or more and 10 parts by mass or less, relative to 100 parts by mass of the total amount of the positive electrode active material. The amount used is, for example, 0.2 parts by mass or more and 8 parts by mass or less, such as 0.3 parts by mass or more and 5 parts by mass or less, or such as 0.4 parts by mass or more and 5 parts by mass or less. When the amount of the present binder used is 0.1 parts by mass or more, sufficient adhesion can be obtained. Furthermore, when the amount of the present binder used is 10 parts by mass or less, the binder is uniformly dispersed in the positive electrode active material, thereby forming a mixture layer having a uniform and smooth surface, which is also preferable from the viewpoint of the energy density and electrical resistance of the secondary battery.

[0045] Among the above active materials, lithium salts of transition metal oxides can be used as the positive electrode active material. For example, layered rock salt type and spinel type lithium-containing metal oxides can be used. Specific compounds of the layered rock salt type positive electrode active material include lithium cobalt oxide, lithium nickel oxide, and ternary NCMs {Li(Ni ｘ , Co ｙ , Mn ｚ ), x+y+z=1} and NCA{Li(Ni １－ａ－ｂ Co ａ Al b)}, etc. Examples of spinel-type positive electrode active materials include lithium manganate, etc. In addition to oxides, phosphates, silicates, sulfur, etc. are also used, and examples of phosphates include olivine-type lithium iron phosphate, etc. As the positive electrode active material, one of the above may be used alone, or two or more may be combined and used as a mixture or composite.

[0046] From the viewpoint of increasing the energy density of the secondary battery, the amount of active material used in the present powder particle composite is preferably in the range of 70 to 99.9 mass %, more preferably in the range of 80 to 99.9 mass %, and even more preferably in the range of 90 to 99.9 mass %, based on the total amount of the present powder particle composite.

[0047] Because all positive electrode active materials have low electrical conductivity, they are generally used with the addition of a conductive additive. Examples of conductive additives include carbon-based materials such as carbon black, carbon nanotubes, carbon fiber, graphite powder, and carbon fiber. Of these, carbon black, carbon nanotubes, and carbon fiber are preferred because they are more likely to provide excellent conductivity. Furthermore, ketjen black and acetylene black are preferred as carbon black. The conductive additives may be used alone or in combination with two or more of the above. The amount of conductive additive used may be, for example, 0.2 to 20 parts by mass, or, for example, 0.2 to 10 parts by mass, per 100 parts by mass of the total active material, from the viewpoint of achieving both electrical conductivity and energy density. Furthermore, the positive electrode active material may be surface-coated with a conductive carbon-based material.

[0048] The powdered particle composite may further contain other binder components such as styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), and polyvinylidene fluoride. When other binder components are used in combination, the amount used may be, for example, 0.1 to 5% by mass or less, or, for example, 0.1 to 2% by mass or less, or, for example, 0.1 to 1% by mass or less, relative to the active material. If the amount of other binder components used exceeds 5% by mass, resistance increases, and high-rate characteristics may become insufficient. Among the above, SBR and CMC are preferred in terms of their excellent balance of binding strength and flex resistance, and a combination of SBR and CMC is even more preferred.

[0049] The SBR refers to a copolymer having structural units derived from an aromatic vinyl monomer such as styrene and structural units derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Examples of the aromatic vinyl monomer include α-methylstyrene, vinyltoluene, and divinylbenzene, in addition to styrene, and one or more of these can be used. The structural units derived from the aromatic vinyl monomer in the copolymer can be, for example, in the range of 20 to 70% by mass, or, for example, in the range of 30 to 60% by mass, mainly from the viewpoint of binding properties. Examples of the aliphatic conjugated diene monomer include, in addition to 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-butadiene, and one or more of these can be used. The structural units derived from the aliphatic conjugated diene monomer in the copolymer can be, for example, in the range of 20 to 70% by mass, or, for example, in the range of 30 to 60% by mass, in order to improve the binding properties of the binder and the flexibility of the resulting electrode. In addition to the above-mentioned monomers, SBR may also contain other monomers as copolymerizable monomers, such as nitrile group-containing monomers such as (meth)acrylonitrile, carboxyl group-containing monomers such as (meth)acrylic acid, itanconic acid, and maleic acid, and ester group-containing monomers such as methyl (meth)acrylate, in order to further improve performance such as binding properties. The structural units derived from the other monomers in the copolymer can be, for example, in the range of 0 to 30% by mass, or, for example, in the range of 0 to 20% by mass.

[0050] The CMC refers to a nonionic cellulose-based semisynthetic polymer compound substituted with a carboxymethyl group and its salt. Examples of the nonionic cellulose-based semisynthetic polymer compound include alkyl celluloses such as methyl cellulose, methyl ethyl cellulose, ethyl cellulose, and microcrystalline cellulose; hydroxyethyl cellulose, hydroxybutyl methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose stearoxy ether, carboxymethyl hydroxyethyl cellulose, alkyl hydroxyethyl cellulose, and nonoxynyl hydroxyethyl cellulose.

[0051] The powdered particle composite of the present invention comprises the above-described positive electrode active material and the present binder as essential components. The method for mixing the components is not particularly limited, and known methods can be used. However, a method of dry-blending the components and, if necessary, powder components such as a conductive additive is preferred. The powdered particle composite of the present invention may be mixed with a dispersion medium such as water, followed by dispersion and kneading to produce an electrode slurry. Examples of dispersion media include, in addition to water, lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, and water-soluble organic solvents such as tetrahydrofuran and N-methylpyrrolidone.

[0052] 3. Secondary Battery Positive Electrode The secondary battery positive electrode of the present invention comprises a mixture layer formed from the powdery particle composite of the present invention on the surface of a current collector such as aluminum. The mixture layer is formed by applying the powdery particle composite to the surface of the current collector and then compressing the mixture layer using a mold press, roll press, or the like. The compression process brings the active material and binder into close contact, improving the strength of the mixture layer and its adhesion to the current collector. The compression process can adjust the thickness of the mixture layer to, for example, about 30 to 80% of the thickness before the compression process, and the thickness of the mixture layer after the compression process is generally about 4 to 200 μm.

[0053] 4. Secondary Battery A secondary battery can be fabricated by providing the secondary battery positive electrode of the present invention with a secondary battery negative electrode, a separator, and an electrolyte. The electrolyte may be liquid or gel-like. The separator is disposed between the positive and negative electrodes of the battery and serves to prevent short circuits due to contact between the two electrodes and to retain the electrolyte to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane that has good ion permeability and mechanical strength. Specific materials that can be used include polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene.

[0054] Examples of negative electrode active materials used in the secondary battery negative electrode include carbon-based materials, lithium metal, lithium alloys, and metal oxides. One or more of these materials can be used in combination. Among these, negative electrode active materials (hereinafter also referred to as "carbon-based negative electrode active materials") made of carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon are preferred, with graphite such as natural graphite and artificial graphite, and hard carbon being more preferred. In the case of graphite, spherical graphite is preferably used from the perspective of battery performance, and its particle size preferably ranges from 1 to 20 μm, for example, to 5 to 15 μm. Furthermore, to increase energy density, metals or metal oxides capable of absorbing lithium, such as silicon and tin, can also be used as negative electrode active materials. Among these, silicon has a higher capacity than graphite, and negative electrode active materials (hereinafter also referred to as "silicon-based negative electrode active materials") made of silicon-based materials such as silicon, silicon alloys, and silicon oxides such as silicon monoxide (SiO) can be used. However, while the silicon-based negative electrode active material has a high capacity, it undergoes a large volume change during charging and discharging. Therefore, it is preferable to use it in combination with the carbon-based negative electrode active material. In this case, a large amount of silicon-based negative electrode active material may cause the electrode material to collapse, significantly reducing the cycle characteristics (durability). From this perspective, when a silicon-based negative electrode active material is used in combination, the amount of silicon-based negative electrode active material used is, for example, 60% by mass or less, or, for example, 30% by mass or less, relative to the carbon-based negative electrode active material.

[0055] Since the carbon-based negative electrode active material itself has good electrical conductivity, it is not necessarily required to add a conductive additive. When a conductive additive is added for the purpose of further reducing resistance, etc., the amount used is, from the viewpoint of energy density, for example, 10 mass % or less, or, for example, 5 mass % or less, relative to the total amount of the negative electrode active material.

[0056] The electrolyte may be a known, commonly used one depending on the type of active material. Specific examples of the solvent for lithium ion secondary batteries include cyclic carbonates with high dielectric constants and high electrolyte dissolving ability, such as propylene carbonate and ethylene carbonate, and chain carbonates with low viscosity, such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. These may be used alone or as a mixed solvent. The electrolyte may be prepared by dissolving LiPF in these solvents. ６ , LiSbF ６ , LiBF ４ , LiClO ４ , LiAlO ４ In nickel-metal hydride secondary batteries, an aqueous solution of potassium hydroxide can be used as the electrolyte. A secondary battery is obtained by spirally or stacking positive and negative electrode plates separated by a separator and housing them in a case or the like.

[0057] As described above, a secondary battery having an electrode with a mixture layer formed from the powdered particle composite disclosed in this specification is expected to exhibit good durability (cycle characteristics) even after repeated charge and discharge, and is therefore suitable for use as an in-vehicle secondary battery, etc.

[0058] The present disclosure will be specifically described below based on examples. However, the present disclosure is not limited to these examples. In the following, "parts" and "%" mean parts by mass and % by mass unless otherwise specified. In the following examples, the polymers were evaluated by the following methods.

[0059] <Measurement of particle size> 0.02 g of the aqueous dispersion containing the polymer was diluted approximately 1000 times with 20 g of pure water, and then the particle size distribution was measured using a particle size measuring device (nanoSAQLA, manufactured by Otsuka Electronics Co., Ltd.) using a dynamic light scattering method, and the volume-based median diameter (D50) was obtained as a representative value of the particle size.

[0060] <Measurement of Glass Transition Temperature> Using a differential scanning calorimeter (manufactured by NETZSCH, DSC 214 Polymer, standard material: alumina), the polymer was heated from −50° C. to 150° C. at a rate of 10° C. / min, and the point at which the calorific value changes was taken as the glass transition temperature.

[0061] <Fusion property of polymer after drying at 70°C> A: After drying, the polymer does not fuse and becomes powdery. B: After drying, the polymer fuses and becomes film-like, and does not become powdery.

[0062] <Measurement of solid content concentration> Approximately 0.5 g of polymer was placed in a weighing bottle whose weight had been measured in advance [weight of weighing bottle = B (g)], and the weighing bottle was accurately weighed, and then [W ０ (g)], the polymer together with the weighing bottle was placed in a ventilated dryer and dried at 155°C for 45 minutes, and the weight of the polymer together with the weighing bottle at that time was measured [W １ (g)], and the solid content concentration was calculated using the following formula (1): Solid content concentration (%) = (W １ -B) / (W ０ -B)×100...(1)

[0063] <<Production of Polymers>> (Production Example 1: Production of Polymer R-1) For polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser, and a nitrogen inlet tube was used. Under a nitrogen atmosphere, 50 parts of water was charged into the reactor and heated to 70°C. Next, 40 parts of water, 2.0 parts of sodium lauryl sulfate (manufactured by Kao Corporation, product name "EMAL 2F-30") as a surfactant (solids equivalent), 95 parts of styrene, 4 parts of 2-ethylhexyl acrylate, and 1 part of methacrylic acid were added to a separate vessel equipped with a stirring blade to prepare an emulsified mixture. Furthermore, 0.2 parts of ammonium persulfate (hereinafter also referred to as "APS") as a polymerization initiator was added to the reactor, and the above-mentioned emulsified mixture was then added to the reactor at a constant rate over 3 hours. Furthermore, a mixture of 0.2 parts of APS and 10 parts of water, which had been prepared in advance in a separate vessel, was added to the reaction solution at a constant rate over 3 hours. The reaction was continued until the polymerization conversion rate exceeded 98%, yielding an aqueous dispersion of polymer R-1, which had a particle size of 260 nm.

[0064] (Production Examples 2 to 15 and Comparative Production Examples 1 and 2: Production of Polymers R-2 to R-17) Aqueous dispersions of polymers R-2 to R-17 were obtained in the same manner as in Production Example 1, except that the amounts of each raw material charged were as shown in Table 1. The measurement results of the particle diameters of polymers R-2 to R-17 are shown in Table 1.

[0065] <<Production of Powdered Binder for Secondary Battery Positive Electrode>> (Production Example 1: Production of Powdered Binder R-1 for Secondary Battery Positive Electrode) After the aqueous dispersion of polymer R-1 obtained in Production Example 1 was dried in a forced air dryer at 70°C, the polymer did not fuse (rating A). The polymer was further dried in a vacuum dryer at 2 kPa and 60°C for 4 hours. The obtained solid was pulverized to obtain powdered binder R-1 for secondary battery positive electrode containing polymer R-1. The glass transition temperature of polymer R-1 was 92°C, and the solid content was 99.2% by mass.

[0066] (Production Examples 2 to 15 and Comparative Production Examples 1 and 2: Production of Powdered Binders R-2 to R-17 for Secondary Battery Positive Electrodes) The aqueous dispersions of polymers R-2 to R-17 obtained in Production Examples 2 to 15 and Comparative Production Examples 1 and 2 were dried in the same manner as in Production Example 1 to obtain powdered binders R-2 to R-17 for secondary battery positive electrodes containing polymers R-2 to R-17. The glass transition temperatures, evaluation results of fusibility after drying at 70°C, and solids concentrations of polymers R-2 to R-17 are shown in Table 1.

[0067]

[0068] Details of the compounds used in Table 1 are shown below. St: styrene HA: 2-ethylhexyl acrylate BA: n-butyl acrylate MMA: methyl methacrylate IBOMA: isobornyl methacrylate MAA: methacrylic acid AN: acrylonitrile Surfactant 1: 30% aqueous solution of sodium lauryl sulfate (manufactured by Kao Corporation, trade name "EMAL 2F-30") Surfactant 2: 25% aqueous solution of polyoxyethylene styrenated propenyl phenyl ether sulfate ester ammonium (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., trade name "Aqualon AR-1025") AMA: allyl methacrylate (crosslinkable monomer) APS: ammonium persulfate

[0069] Example 1 <Evaluation of dispersibility of powdered binder in active material> LiNi as a positive electrode active material was placed in a container. ０．８ Co ０．１５ Al ０．０５ O ２ 99 parts of (NCA), 1 part of powdered binder (R-1) containing polymer R-1, and 100 parts of zirconia beads (φ=1 mm) were added and mixed for 60 minutes using a paint shaker to adhere the binder to the positive electrode active material and uniformly disperse it, thereby obtaining a powdered particle composite. After this, the dispersion state of the binder was evaluated visually. The evaluation results are shown in Table 2.

[0070] (Criteria for evaluation of dispersibility in active material) A: The white color of the binder is not visible after the positive electrode active material is dispersed. B: The white color of the binder is visible after the positive electrode active material is dispersed, but no lumps remain. C: Large lumps of the binder remain after the positive electrode active material is dispersed. Note that a rating of B or higher is considered a passing level.

[0071] <Evaluation of Adhesion of Powdered Binder to Active Material> The powdered particle composite obtained above was sandwiched between two sheets of aluminum foil (20 μm, manufactured by UACJ Corporation) and compressed at 120°C for 10 minutes with a pressure of 3 MPa, or at 150°C for 10 minutes with a pressure of 3 MPa. After compression, the aluminum foil was peeled off and the adhesion to the positive electrode active material was evaluated. The adhesion was evaluated by brushing off unbound active material from the aluminum foil after compression 10 times and measuring the amount of active material that slid off. The evaluation results are shown in Table 2. (Criteria for Determining Adhesion to Active Material) A: Less than 10% of the active material slid off. B: 10% or more but less than 30% of the active material slid off. C: 30% or more of the active material slid off. The smaller the amount of active material that slid off, the better the binder adhered, and a rating of B or higher was considered acceptable.

[0072] Examples 2 to 18 and Comparative Examples 1 and 2 Powdery particle composites were obtained by the same procedure as in Example 1, except that the formulations were as shown in Table 2, and the dispersibility of the powdery binder in the active material and the adhesion to the active material were evaluated. The results are shown in Table 2.

[0073]

[0074] The details of the compounds used in Table 2 are as follows: NCA: LiNi ０．８ Co ０．１５ Al ０．０５ O ２ (Manufactured by BASF Toda Battery Materials, product name "NCA7051") NMC: LiNi １／３ Co １／３ Mn １／３ O ２ (manufactured by BASF Toda Battery Materials, product name "NCM111 1040") AB: acetylene black (manufactured by Denka Company, product name "Denka Black Li-400")

[0075] <Evaluation Results> As is clear from the results of Examples 1 to 18, by using the powdered binder for secondary battery positive electrodes of the present invention, positive electrodes can be produced by dry blending instead of using an electrode slurry, which results in excellent productivity, and excellent dispersibility of the powdered binder in the active material and adhesion to the active material. Among these, focusing on the glass transition temperature, when the glass transition temperature was 70 ° C or higher (Examples 1, 14, 16, and 17), the dispersibility of the powdered binder in the active material was superior to when the glass transition temperature was 63 ° C (Example 15). Furthermore, when the glass transition temperature was in the range of 73 ° C to 109 ° C, the adhesion of the powdered binder to the active material was rated B or higher, regardless of whether the compression treatment was performed at 120 ° C or 150 ° C (Examples 1, 14, and 16). Furthermore, when focusing on the amount of structural units derived from ethylenically unsaturated carboxylic acid monomers, when the amount of structural units derived from methacrylic acid was 5% by mass or less (Examples 1 and 10), the adhesion to the active material was superior to when the amount was 10% by mass (Example 11). Furthermore, when focusing on particle size, when the particle size was in the range of 90 nm to 700 nm, the adhesion to the active material was excellent under compression treatment conditions of 150°C (Examples 1 and 5 to 8), and when the particle size was in the range of 90 nm to 510 nm, the adhesion to the active material was excellent even under compression treatment conditions of 120°C (Examples 1 and 5 to 7).

[0076] In contrast, powdery binders containing cross-linked polymers having structural units derived from cross-linkable monomers exhibited significantly poorer adhesion to the active material (Comparative Example 1). This is thought to be because the cross-linked polymers did not melt and did not cover the positive electrode active material under high-temperature (120°C and 150°C) compression treatment conditions. Furthermore, when the glass transition temperature of the non-cross-linked polymer was below 60°C (Comparative Example 2), fusion occurred between powdery binders, resulting in significantly poor dispersibility of the powdery binder in the active material.

[0077] The powdered binder for secondary battery positive electrodes disclosed herein allows secondary battery positive electrodes to be produced by dry blending without using an electrode slurry, resulting in excellent productivity and excellent dispersibility of the powdered binder in the active material and adhesion to the active material. Furthermore, secondary batteries equipped with secondary battery positive electrodes obtained using the powdered binder are expected to ensure good integrity and exhibit good durability (cycling characteristics) even after repeated charge and discharge, thereby contributing to the development of high-capacity automotive secondary batteries, etc. The powdered binder for secondary battery positive electrodes of the present invention is particularly suitable for use in non-aqueous electrolyte secondary battery positive electrodes, and is particularly useful for non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

Claims

1. A powdered binder for a secondary battery positive electrode, comprising a non-crosslinked polymer having a glass transition temperature of 60°C or higher and 150°C or lower.

2. 2. The powdered binder for a secondary battery positive electrode according to claim 1, wherein the non-crosslinked polymer has a structural unit derived from a non-crosslinkable ethylenically unsaturated monomer.

3. 3. The powdered binder for a secondary battery positive electrode according to claim 2, wherein the non-crosslinkable ethylenically unsaturated monomer includes a non-crosslinkable aromatic vinyl monomer or a non-crosslinkable ethylenically unsaturated carboxylic acid ester monomer.

4. 2. The powdered binder for a secondary battery positive electrode according to claim 1, wherein the non-crosslinked polymer has 5 mass% or less of structural units derived from a non-crosslinkable ethylenically unsaturated carboxylic acid monomer relative to all structural units of the non-crosslinked polymer.

5. 2. The powdered binder for a secondary battery positive electrode according to claim 1, wherein the particle size of the non-crosslinked polymer is 80 nm to 800 nm as a volume-based median diameter (D50) measured by a dynamic light scattering method.

6. A powdered particle composite comprising a positive electrode active material and the powdered binder for a secondary battery positive electrode according to any one of claims 1 to 5.

7. A positive electrode for a secondary battery, comprising a current collector having a mixture layer formed from the powdered particle composite according to claim 6 on a surface thereof.

8. A secondary battery comprising the secondary battery positive electrode according to claim 7.