Active material particles for coated positive electrode of lithium-ion batteries

Abstract

To provide coated positive electrode active material particles for a lithium ion battery capable of maintaining secondary particles of the coated positive electrode active material particles when a positive electrode active material is LiFePO4, and obtaining a battery excellent in electric performance (a discharge capacitance maintenance rate).SOLUTION: In coated positive electrode active material particles for a lithium ion battery, at least part of a surface of positive electrode active material particles is coated with a coating layer containing a polymer compound and conductive filler. The positive electrode active material particles are LiFePO4. A ratio of loose bulk density measured according to JIS K 6219-2 (2005) to tamped bulk density measured according to JIS K 5101-12-2 (2004) with a falling height of 5 mm and a number of times of tamping of 2000 (loose bulk density / tamped bulk density) is 0.50 to 0.90 using a cylindrical container having a volume of 100 cm3 and a diameter of 30 mm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to coated positive electrode active material particles for lithium-ion batteries. [Background technology]

[0002] Lithium-ion batteries, as rechargeable batteries capable of achieving high energy density and high power density, have been widely used in various applications in recent years, and various studies are being conducted to develop inexpensive and higher-performance lithium-ion batteries.

[0003] The use of LiFePO4 as a positive electrode active material for lithium-ion batteries, which can be expected to have a stable supply, has been considered. Because LiFePO4 has lower electronic conductivity compared to other positive electrode active materials, the conductivity of the electrode has been ensured by coating it with a resin binder and a carbon-based conductive additive to reduce resistance using an organic solvent and then drying it (Patent Document 1). Furthermore, in order to obtain a high-capacity battery and to produce a thicker electrode film, a method has been proposed in which the positive electrode active material is coated with a coating layer containing a polymer compound and a conductive additive, and the resulting coated positive electrode active material is compressed and molded to produce the electrode (Patent Document 2). It was thought that this method could also be applied to LiFePO4. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2002-117833 [Patent Document 2] Japanese Patent Publication No. 2016-189325 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, as we continued our investigation, we found that when the positive electrode active material is LiFePO4 and the coating active material is prepared using the method described in Patent Document 2, the electrical performance (discharge capacity retention rate) of the resulting battery deteriorates. From the viewpoint of reducing electrical resistance and improving the density of the active material, it is preferable for coated positive electrode active material particles to exist as secondary particles formed by the aggregation of primary particles. However, LiFePO4's secondary particles are easily broken down and readily revert to primary particles during the coated active material manufacturing process (coating process). The particle structure of the coated positive electrode active material particles is maintained both in the positive electrode composition and in the electrode after compression molding. Although this is purely speculation, we believe that the deterioration in battery performance when the positive electrode active material is LiFePO4 is due to the reversion of the coated positive electrode active material particles to primary particles.

[0006] The present invention aims to solve the above problems and to provide coated positive electrode active material particles for lithium-ion batteries that maintain secondary particles of the coated positive electrode active material particles when the positive electrode active material is LiFePO4, thereby obtaining a battery with excellent electrical performance (discharge capacity retention rate). [Means for solving the problem]

[0007] The present invention relates to coated positive electrode active material particles for lithium-ion batteries, wherein at least a portion of the surface of the positive electrode active material particles is coated with a coating layer containing a polymer compound and a conductive filler, and the positive electrode active material particles are LiFePO4, with a capacity of 100 cm³. 3 These are lithium-ion battery coating positive electrode active material particles in which the ratio (loose bulk density / hard bulk density) between the loose bulk density measured in accordance with JIS K 6219-2 (2005) using a cylindrical container with a diameter of 30 mm and the hard bulk density measured in accordance with JIS K 5101-12-2 (2004) with a drop height of 5 mm and 2000 tamping cycles is 0.50 to 0.90. [Effects of the Invention]

[0008] According to the present invention, when the positive electrode active material is LiFePO4, it is possible to provide coated positive electrode active material particles that maintain the secondary particles of the coated positive electrode active material particles and result in a battery with excellent electrical performance (discharge capacity retention rate).

Embodiments for Carrying Out the Invention

[0009] Hereinafter, the present invention will be described in detail. The present invention relates to coated positive electrode active material particles for a lithium ion battery. Hereinafter, "coated positive electrode active material particles for a lithium ion battery" may be referred to as "coated positive electrode active material particles". In this specification, when referring to a lithium ion battery, it is a concept that also includes a lithium ion secondary battery.

[0010] The coated positive electrode active material particles for a lithium ion battery of the present invention are coated with a coating layer containing a polymer compound and a conductive filler on at least a part of the surface of the positive electrode active material particles. The positive electrode active material particles are LiFePO4, and LiFePO4 is one kind of lithium-containing transition metal phosphate. LiFePO4 may be one in which a part of the transition metal site is substituted with another transition metal. Examples of the other transition metals include Co, Mn, Ni, V, Mo, and Ti.

[0011] From the viewpoint of the electrical characteristics of the battery, the volume average particle diameter of the positive electrode active material particles is preferably 0.01 to 100 μm, more preferably 0.1 to 35 μm, and still more preferably 2 to 30 μm. In this specification, the volume average particle diameter means the particle diameter (Dv50) at the integrated value of 50% in the particle size distribution obtained by the Microtrac method (laser diffraction / scattering method). The Microtrac method is a method for obtaining the particle size distribution by using the scattered light obtained by irradiating particles with laser light. For measuring the volume average particle diameter, Microtrac manufactured by Nikkiso Co., Ltd. etc. can be used.

[0012] The coated positive electrode active material particles for a lithium ion battery of the present invention have a capacity of 100 cm 3Using a cylindrical container with a diameter of 30 mm, the ratio of the loose bulk density measured in accordance with JIS K 6219-2 (2005) to the hard bulk density measured in accordance with JIS K 5101-12-2 (2004) with a drop height of 5 mm and 2000 tamping cycles (loose bulk density / hard bulk density) is 0.50 to 0.90. When the ratio of loose bulk density to hard bulk density is within the above range, secondary particles of the coated positive electrode active material are maintained, and a lithium-ion battery with excellent electrical performance such as discharge capacity retention rate can be obtained. The ratio of loose bulk density to hard bulk density is preferably 0.50 to 0.85, and more preferably 0.50 to 0.80. Note that the average values ​​of five measurements are used for both the loose bulk density and the firm density.

[0013] The coating layer contains a polymer compound and a conductive filler. The polymer compound is preferably a resin containing a polymer in which acrylic monomer (a) is an essential constituent monomer. Specifically, the polymer compound constituting the coating layer is preferably a polymer of a monomer composition containing acrylic acid (a0) as the acrylic monomer (a). In the above monomer composition, the content of acrylic acid (a0) is preferably more than 90% by weight and 98% by weight or less, based on the total weight of the monomer. From the viewpoint of the flexibility of the coating layer, the content of acrylic acid (a0) is more preferably 93.0 to 97.5% by weight, and even more preferably 95.0 to 97.0% by weight, based on the total weight of the monomer.

[0014] The polymer compound constituting the coating layer may contain, as the acrylic monomer (a), a monomer (a1) having a carboxyl group or an acid anhydride group other than acrylic acid (a0).

[0015] Examples of the monomer (a1) having a carboxyl group or acid anhydride group other than acrylic acid (a0) include monocarboxylic acids having 3 to 15 carbon atoms such as methacrylic acid, crotonic acid, and cinnamic acid; dicarboxylic acids having 4 to 24 carbon atoms such as (anhydrous) maleic acid, fumaric acid, (anhydrous) itaconic acid, citraconic acid, and mesaconic acid; polycarboxylic acids having 6 to 24 carbon atoms and trivalent to tetravalent or higher valences such as aconitic acid, and the like.

[0016] The polymer compound constituting the coating layer may contain, as the acrylic monomer (a), a monomer (a2) represented by the following general formula (1). CH2=C(R 1 )COOR 2 (1) [In formula (1), R 1 is a hydrogen atom or a methyl group, and R 2 is a linear alkyl group having 4 to 12 carbon atoms or a branched alkyl group having 3 to 36 carbon atoms.]

[0017] In the monomer (a2) represented by the above general formula (1), R 1 represents a hydrogen atom or a methyl group. It is preferable that R 1 is a methyl group. R 2 is preferably a linear or branched alkyl group having 4 to 12 carbon atoms, or a branched alkyl group having 13 to 36 carbon atoms.

[0018] (a21) An ester compound in which R 2 is a linear or branched alkyl group having 4 to 12 carbon atoms Examples of the linear alkyl group having 4 to 12 carbon atoms include a butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, and a dodecyl group. Examples of branched alkyl groups with 4 to 12 carbon atoms include: 1-methylpropyl group (sec-butyl group), 2-methylpropyl group, 1,1-dimethylethyl group (tert-butyl group), 1-methylbutyl group, 1,1-dimethylpropyl group, 1,2-dimethylpropyl group, 2,2-dimethylpropyl group (neopentyl group), 1-methylpentyl group, 2-methylpentyl group, 3-methylpentyl group, 4-methylpentyl group, 1,1-dimethylbutyl group, 1,2-dimethylbutyl group, 1,3-dimethylbutyl group, and 2,2-dimethylbutyl group. , 2,3-dimethylbutyl group, 1-ethylbutyl group, 2-ethylbutyl group, 1-methylhexyl group, 2-methylhexyl group, 3-methylhexyl group, 4-methylhexyl group, 5-methylhexyl group, 1-ethylpentyl group, 2-ethylpentyl group, 3-ethylpentyl group, 1,1-dimethylpentyl group, 1,2-dimethylpentyl group, 1,3-dimethylpentyl group, 2,2-dimethylpentyl group, 2,3-dimethylpentyl group, 1-methylheptyl group, 2-methylheptyl group, 3-methylheptyl group, 4-methylheptyl 1,1-dimethylhexyl group, 5-methylheptyl group, 6-methylheptyl group, 1,1-dimethylhexyl group, 1,2-dimethylhexyl group, 1,3-dimethylhexyl group, 1,4-dimethylhexyl group, 1,5-dimethylhexyl group, 1-ethylhexyl group, 2-ethylhexyl group, 1-methyloctyl group, 2-methyloctyl group, 3-methyloctyl group, 4-methyloctyl group, 5-methyloctyl group, 6-methyloctyl group, 7-methyloctyl group, 1,1-dimethylheptyl group, 1,2-dimethylheptyl group, 1,3-dimethylheptyl group 1,4-dimethylheptyl group, 1,5-dimethylheptyl group, 1,6-dimethylheptyl group, 1-ethylheptyl group, 2-ethylheptyl group, 1-methylnonyl group, 2-methylnonyl group, 3-methylnonyl group, 4-methylnonyl group, 5-methylnonyl group, 6-methylnonyl group, 7-methylnonyl group, 8-methylnonyl group, 1,1-dimethyloctyl group, 1,2-dimethyloctyl group, 1,3-dimethyloctyl group, 1,4-dimethyloctyl group, 1,5-dimethyloctyl group, 1,6-dimethyloctyl group, 1,7-dimethyloctyl group, 1-ethyloctyl group, 2-ethyloctyl group, 1-methyldecyl group, 2-methyldecyl group, 3-methyldecyl group, 4-methyldecyl group, 5-methyldecyl group, 6-methyldecyl group, 7-methyldecyl group, 8-methyldecyl group, 9-methyldecyl group, 1,1-dimethylnonyl group, 1,2-dimethylnonyl group, 1,3-dimethylnonyl group, 1,4-dimethylnonyl group, 1,5-dimethylnonyl group, 1,6-dimethylnonyl group, 1,7-dimethylnonyl group, 1,8-dimethylnonyl group, 1-ethylnonyl group, 2-ethylnonyl group, 1-methylundyl Examples include syl groups, 2-methylundecyl groups, 3-methylundecyl groups, 4-methylundecyl groups, 5-methylundecyl groups, 6-methylundecyl groups, 7-methylundecyl groups, 8-methylundecyl groups, 9-methylundecyl groups, 10-methylundecyl groups, 1,1-dimethyldecyl groups, 1,2-dimethyldecyl groups, 1,3-dimethyldecyl groups, 1,4-dimethyldecyl groups, 1,5-dimethyldecyl groups, 1,6-dimethyldecyl groups, 1,7-dimethyldecyl groups, 1,8-dimethyldecyl groups, 1,9-dimethyldecyl groups, 1-ethyldecyl groups, and 2-ethyldecyl groups. Among these, the 2-ethylhexyl group is particularly preferred.

[0019] (a22)R 2 Ester compounds in which the branched alkyl group has 13 to 36 carbon atoms. Branched alkyl groups with 13 to 36 carbon atoms include 1-alkylalkyl groups [1-methyldodecyl group, 1-butyleicosyl group, 1-hexyloctadecyl group, 1-octylhexadecyl group, 1-decyltetradecyl group, 1-undecyltridecyl group, etc.], and 2-alkylalkyl groups [2-methyldodecyl group, 2-hexyloctadecyl group, 2-octylhexadecyl group, 2-decyltetradecyl group, 2-undecyltridecyl group, 2-dodecylhexadecyl group, 2-tridecylpentadecyl group, 2-decyloctadecyl group, 2-tetradecyloctadecyl group, 2-hexadecyloctadecyl group, 2-tetradecyleicosyl Examples include groups such as 2-hexadecyleicosyl groups, 3-34-alkylalkyl groups (3-alkylalkyl groups, 4-alkylalkyl groups, 5-alkylalkyl groups, 32-alkylalkyl groups, 33-alkylalkyl groups, and 34-alkylalkyl groups), and mixed alkyl groups containing one or more branched alkyl groups, such as residues obtained by removing hydroxyl groups from oxo alcohols obtained from propylene oligomers (7-11 units), ethylene / propylene (molar ratio 16 / 1-1 / 11) oligomers, isobutylene oligomers (7-8 units), and α-olefin (5-10 carbon atoms) oligomers (4-8 units).

[0020] The polymer compound constituting the coating layer may contain, as acrylic monomer (a), an ester compound (a3) ​​of a monovalent aliphatic alcohol having 1 to 3 carbon atoms and (meth)acrylic acid. Examples of monovalent aliphatic alcohols having 1 to 3 carbon atoms that constitute the ester compound (a3) ​​include methanol, ethanol, 1-propanol, and 2-propanol. Note that (meth)acrylic acid refers to acrylic acid or methacrylic acid.

[0021] The polymer compound constituting the coating layer is preferably a polymer of a monomer composition comprising acrylic acid (a0) and at least one of monomer (a1), monomer (a2), and ester compound (a3); more preferably a polymer of a monomer composition comprising acrylic acid (a0) and at least one of monomer (a1), ester compound (a21), and ester compound (a3); even more preferably a polymer of a monomer composition comprising acrylic acid (a0) and any one of monomer (a1), monomer (a2), and ester compound (a3); and most preferably a polymer of a monomer composition comprising acrylic acid (a0) and any one of monomer (a1), ester compound (a21), and ester compound (a3). Examples of polymer compounds constituting the coating layer include a copolymer of acrylic acid and maleic acid using maleic acid as monomer (a1), a copolymer of acrylic acid and 2-ethylhexyl methacrylate using 2-ethylhexyl methacrylate as monomer (a2), and a copolymer of acrylic acid and methyl methacrylate using methyl methacrylate as ester compound (a3).

[0022] The total content of monomer (a1), monomer (a2), and ester compound (a3) ​​is preferably 2.0 to 9.9% by weight, and more preferably 2.5 to 7.0% by weight, based on the total weight of the monomers, from the viewpoint of suppressing volume changes of positive electrode active material particles.

[0023] The polymer compound constituting the coating layer preferably does not contain an anionic monomer salt (a4) having a polymerizable unsaturated double bond and an anionic group as the acrylic monomer (a).

[0024] Examples of structures having polymerizable unsaturated double bonds include vinyl groups, allyl groups, styrenyl groups, and (meth)acryloyl groups. Examples of anionic groups include sulfonic acid groups and carboxyl groups. Anionic monomers, which have polymerizable unsaturated double bonds and anionic groups, are compounds obtained by combining these, and examples include vinyl sulfonic acid, allyl sulfonic acid, styrene sulfonic acid, and (meth)acrylic acid. Note that (meth)acryloyl group refers to either an acryloyl group or a methacryloyl group. Examples of cations constituting the salt (a4) of anionic monomers include lithium ions, sodium ions, potassium ions, and ammonium ions.

[0025] Furthermore, the polymer compound constituting the coating layer may contain, to the extent that it does not impair the physical properties, a radical polymerizable monomer (a5) as the acrylic monomer (a), which can copolymerize with acrylic acid (a0), monomer (a1), monomer (a2), and ester compound (a3). As the radical polymerizable monomer (a5), monomers that do not contain active hydrogen are preferred, and the monomers listed below (a51) to (a58) can be used.

[0026] (a51) Hydrocarbyl (meth)acrylate formed from at least one monool from among linear aliphatic monools with 13 to 20 carbon atoms, alicyclic monools with 5 to 20 carbon atoms, and aromatic aliphatic monools with 7 to 20 carbon atoms, and (meth)acrylic acid. Examples of the above monools include (i) linear aliphatic monools (tridecyl alcohol, myristyl alcohol, pentadecyl alcohol, cetyl alcohol, heptadecyl alcohol, stearyl alcohol, nonadecyl alcohol, arachidyl alcohol, etc.), (ii) alicyclic monools (cyclopentyl alcohol, cyclohexyl alcohol, cycloheptyl alcohol, cyclooctyl alcohol, etc.), (iii) aromatic aliphatic monools (benzyl alcohol, etc.), and mixtures of two or more of these.

[0027] (a52) Poly(n=2~30) oxyalkylene (2~4 carbon atoms) alkyl (1~18 carbon atoms) ether (meth)acrylate [meth)acrylate of methanol ethylene oxide (hereinafter abbreviated as EO), meth)acrylate of methanol propylene oxide (hereinafter abbreviated as PO), etc.]

[0028] (a53) Nitrogen-containing vinyl compounds (a53-1) Amide group-containing vinyl compound (i) (meth)acrylamide compounds having 3 to 30 carbon atoms, such as N,N-dialkyl (1 to 6 carbon atoms) or dialkyl (7 to 15 carbon atoms) (meth)acrylamide (N,N-dimethylacrylamide, N,N-dibenzylacrylamide, etc.), and diacetone acrylamide. (ii) Vinyl compounds containing amide groups with 4 to 20 carbon atoms, excluding the (meth)acrylamide compounds mentioned above, such as N-methyl-N-vinylacetamide and cyclic amides [pyrrolidone compounds (with 6 to 13 carbon atoms, such as N-vinylpyrrolidone)].

[0029] (a53-2) (meth)acrylate compound (i) Dialkyl (C1-C4) aminoalkyl (C1-C4) (meth)acrylate [N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, t-butylaminoethyl (meth)acrylate, morpholinoethyl (meth)acrylate, etc.] (ii) Quaternary ammonium group-containing (meth)acrylates {quaternary amino group-containing (meth)acrylates [N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, etc.] (quaternary products obtained by quaternizing using quaternizing agents such as methyl chloride, dimethyl sulfate, benzyl chloride, dimethyl carbonate, etc.)}

[0030] (a53-3) Heterocyclic vinyl compounds Pyridine compounds (7-14 carbon atoms, e.g., 2- or 4-vinylpyridine), imidazole compounds (5-12 carbon atoms, e.g., N-vinylimidazole), pyrrole compounds (6-13 carbon atoms, e.g., N-vinylpyrrole), pyrrolidone compounds (6-13 carbon atoms, e.g., N-vinyl-2-pyrrolidone)

[0031] (a53-4) Nitrile group-containing vinyl compound Vinyl compounds containing nitrile groups with 3 to 15 carbon atoms, such as (meth)acrylonitrile, cyanostyrene, and cyanoalkyl (1 to 4 carbon atoms) acrylates.

[0032] (a53-5) Other nitrogen-containing vinyl compounds Nitro group-containing vinyl compounds (8-16 carbon atoms, e.g., nitrostyrene), etc.

[0033] (a54) Vinyl hydrocarbons (a54-1) Aliphatic vinyl hydrocarbons Olefins with 2 to 18 or more carbon atoms (ethylene, propylene, butene, isobutylene, pentene, heptene, diisobutylene, octene, dodecene, octadecene, etc.), dienes with 4 to 10 or more carbon atoms (butadiene, isoprene, 1,4-pentadiene, 1,5-hexadiene, 1,7-octadiene, etc.), etc.

[0034] (a54-2) Alicyclic vinyl hydrocarbons Cyclic unsaturated compounds with 4 to 18 or more carbon atoms, such as cycloalkenes (e.g., cyclohexene), (di)cycloalkadienes [e.g., (di)cyclopentadiene], terpenes (e.g., pinene and limonene), and indene.

[0035] (a54-3) Aromatic vinyl hydrocarbons Aromatic unsaturated compounds with 8 to 20 or more carbon atoms, such as styrene, α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene, isopropylstyrene, butylstyrene, phenylstyrene, cyclohexylstyrene, and benzylstyrene.

[0036] (a55) Vinyl ester Aliphatic vinyl esters [C4-C15, e.g., alkenyl esters of aliphatic carboxylic acids (mono- or dicarboxylic acids) (e.g., vinyl acetate, vinyl propionate, vinyl butyrate, diallyl adipate, isopropenyl acetate, vinyl methoxyacetate)] Aromatic vinyl esters [C9-C20, e.g., alkenyl esters of aromatic carboxylic acids (mono- or dicarboxylic acids) (e.g., vinyl benzoate, diallyl phthalate, methyl-4-vinyl benzoate), aromatic ring-containing esters of aliphatic carboxylic acids (e.g., acetoxystyrene)]

[0037] (a56) vinyl ether Aliphatic vinyl ethers [3-15 carbon atoms, e.g., vinyl alkyl (1-10 carbon atoms) ethers (vinyl methyl ether, vinyl butyl ether, vinyl 2-ethylhexyl ether, etc.), vinyl alkoxy (1-6 carbon atoms) alkyl (1-4 carbon atoms) ethers (vinyl-2-methoxyethyl ether, methoxybutadiene, 3,4-dihydro-1,2-pyran, 2-butoxy-2'-vinyloxydiethyl ether, vinyl-2-ethyl mercaptoethyl ether, etc.), poly(2-4)(meth)allyloxyalkanes (2-6 carbon atoms) (diallyloxyethane, triallyloxyethane, tetraallyloxybutane, tetramethallyloxyethane, etc.)], aromatic vinyl ethers [8-20 carbon atoms, e.g., vinyl phenyl ether, phenoxystyrene]

[0038] (a57) Vinyl ketone Aliphatic vinyl ketones (4-25 carbon atoms, e.g., vinyl methyl ketone, vinyl ethyl ketone), aromatic vinyl ketones (9-21 carbon atoms, e.g., vinyl phenyl ketone)

[0039] (a58) Unsaturated dicarboxylic acid diester Unsaturated dicarboxylic acid diesters with 4 to 34 carbon atoms, for example, dialkyl fumarates (two alkyl groups consisting of linear, branched, or alicyclic groups with 1 to 22 carbon atoms), dialkyl maleates (two alkyl groups consisting of linear, branched, or alicyclic groups with 1 to 22 carbon atoms).

[0040] If a radical polymerizable monomer (a5) is included, its content is preferably 0.1 to 3.0% by weight based on the total weight of the monomer.

[0041] The preferred lower limit for the weight-average molecular weight of the polymer compound constituting the coating layer is 3,000, the more preferred lower limit is 5,000, and the still preferred lower limit is 7,000. On the other hand, the preferred upper limit for the weight-average molecular weight of the above polymer compound is 100,000, and the more preferred upper limit is 70,000.

[0042] The weight-average molecular weight of the polymer compounds constituting the coating layer can be determined by gel permeation chromatography (hereinafter abbreviated as GPC) under the following conditions. Equipment: Alliance GPC V2000 (Waters Corporation) Solvents: Orthodichlorobenzene, DMF, THF Standard material: Polystyrene Sample concentration: 3 mg / ml Column stationary phase: PLgel 10 μm, MIXED-B x 2 in series (Polymer Laboratories, Inc.) Column temperature: 135℃

[0043] The polymer compounds constituting the coating layer can be produced by known polymerization methods (bulk polymerization, solution polymerization, emulsion polymerization, suspension polymerization, etc.) using known polymerization initiators {azo-based initiators [2,2'-azobis(2-methylpropionitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2-methylbutyronitrile), etc.], peroxide-based initiators (benzoyl peroxide, di-t-butyl peroxide, lauryl peroxide, etc.), etc.}. The amount of polymerization initiator used is preferably 0.01 to 5% by weight, more preferably 0.05 to 2% by weight, and even more preferably 0.1 to 1.5% by weight, based on the total weight of the monomer, from the viewpoint of adjusting the weight-average molecular weight to a preferred range. The polymerization temperature and polymerization time are adjusted according to the type of polymerization initiator, but the polymerization temperature is preferably -5 to 150°C (more preferably 30 to 120°C), and the reaction time is preferably 0.1 to 50 hours (more preferably 2 to 24 hours).

[0044] Examples of solvents used in solution polymerization include esters (2-8 carbon atoms, e.g., ethyl acetate and butyl acetate), alcohols (1-8 carbon atoms, e.g., methanol, ethanol, and octanol), hydrocarbons (4-8 carbon atoms, e.g., n-butane, cyclohexane, and toluene), amides (e.g., N,N-dimethylformamide (hereinafter abbreviated as DMF)), and ketones (3-9 carbon atoms, e.g., methyl ethyl ketone). From the viewpoint of adjusting the weight-average molecular weight to a preferred range, the amount used is preferably 5-900% by weight, more preferably 10-400% by weight, and even more preferably 30-300% by weight, based on the total weight of the monomers. The monomer concentration is preferably 10-95% by weight, more preferably 20-90% by weight, and even more preferably 30-80% by weight.

[0045] Examples of dispersion media in emulsion polymerization and suspension polymerization include water, alcohol (e.g., ethanol), ester (e.g., ethyl propionate), and light naphtha. Examples of emulsifiers include metal salts of higher fatty acids (10-24 carbon atoms) (e.g., sodium oleate and sodium stearate), metal ester salts of higher alcohols (10-24 carbon atoms) (e.g., sodium lauryl sulfate), ethoxylated tetramethyldecinediol, sodium sulfoethyl methacrylate, and dimethylaminomethyl methacrylate. Furthermore, stabilizers such as polyvinyl alcohol and polyvinylpyrrolidone may be added. The monomer concentration of the solution or dispersion is preferably 5 to 95% by weight, more preferably 10 to 90% by weight, and even more preferably 15 to 85% by weight, and the amount of polymerization initiator used is preferably 0.01 to 5% by weight, more preferably 0.05 to 2% by weight, based on the total weight of the monomer. During polymerization, known chain transfer agents, such as mercapto compounds (dodecyl mercaptan, n-butyl mercaptan, etc.) and / or halogenated hydrocarbons (carbon tetrachloride, carbon tetrabromide, benzyl chloride, etc.), can be used.

[0046] The polymer compound constituting the coating layer may be a crosslinked polymer obtained by crosslinking the polymer compound with a crosslinking agent (A') {preferably a polyepoxy compound (a'1) [such as polyglycidyl ether (bisphenol A diglycidyl ether, propylene glycol diglycidyl ether, and glycerin triglycidyl ether) and polyglycidylamine (N,N-diglycidylaniline and 1,3-bis(N,N-diglycidylaminomethyl))] and / or a polyol compound (a'2) (such as ethylene glycol)} which has a reactive functional group that reacts with a carboxyl group.

[0047] One method for crosslinking the polymer compound constituting the coating layer using a crosslinking agent (A') is to coat the positive electrode active material particles with the polymer compound constituting the coating layer and then crosslink them. Specifically, one method involves mixing positive electrode active material particles with a resin solution containing the polymer compound constituting the coating layer and desolventing it to produce coated active material particles. Then, a solution containing the crosslinking agent (A') is mixed with the coated active material particles and heated to induce desolventing and a crosslinking reaction, causing a reaction on the surface of the positive electrode active material particles in which the polymer compound constituting the coating layer is crosslinked by the crosslinking agent (A'). The heating temperature is adjusted according to the type of crosslinking agent, but is preferably 70°C or higher when using a polyepoxy compound (a'1) as the crosslinking agent, and preferably 120°C or higher when using a polyol compound (a'2).

[0048] The weight percentage of the polymer compound in the coated positive electrode active material particles for lithium-ion batteries of the present invention is preferably 0.1 to 11% by weight, based on the weight of the coated positive electrode active material particles for lithium-ion batteries. When the weight percentage of the polymer compound is within this range, secondary particles of the coated positive electrode active material particles are maintained, and a battery with excellent discharge capacity retention can be obtained. More preferably, it is 1.0 to 10.0% by weight. The weight percentage of polymer compounds is the weight percentage of the polymer compounds in terms of solid content.

[0049] The conductive filler is preferably selected from materials that have electrical conductivity. Preferred conductive fillers include metals [aluminum, stainless steel (SUS), silver, gold, copper, and titanium, etc.], carbon [graphite and carbon black (acetylene black, Ketjen black, furnace black, channel black, and thermal lamp black, etc.)], and mixtures thereof. These conductive fillers may be used individually or in combination of two or more. They may also be used as alloys or metal oxides. In particular, from the viewpoint of electrical stability, aluminum, stainless steel, carbon, silver, gold, copper, titanium, and mixtures thereof are more preferred, even more preferably silver, gold, aluminum, stainless steel, and carbon, and especially preferably carbon. These conductive fillers may also be made by coating a particulate ceramic material or resin material with a conductive material [preferably a metallic conductive filler from the above-mentioned conductive fillers] by plating or the like.

[0050] The shape (form) of the conductive filler is not limited to particle form; it may also be in a form other than particle form, such as carbon nanofibers or carbon nanotubes, which are commonly used as so-called filler-based conductive fillers.

[0051] The average particle size of the conductive filler is not particularly limited, but from the viewpoint of the electrical characteristics of the battery, it is preferably about 0.01 to 10 μm. In this specification, "particle diameter of conductive filler" means the maximum distance L between any two points on the contour line of the conductive filler. The value of "average particle diameter" shall be the value calculated as the average of the particle diameters of 30 particles using observation methods such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0052] The weight percentage of the conductive filler is preferably 2 to 14% by weight, based on the weight of the coated positive electrode active material particles for lithium-ion batteries. When the weight percentage of the conductive filler is within this range, secondary particles of the coated positive electrode active material particles are maintained, and a battery with excellent discharge capacity retention can be obtained. More preferably, it is 3 to 11% by weight. In the coated positive electrode active material particles for lithium-ion batteries of the present invention, it is preferable that the weight percentage of the polymer compound is 0.1 to 11% by weight based on the weight of the coated positive electrode active material particles for lithium-ion batteries, and the weight percentage of the conductive filler is 2 to 14% by weight based on the weight of the coated positive electrode active material particles for lithium-ion batteries.

[0053] The ratio of the polymer compound to the conductive filler constituting the coating layer is not particularly limited, but from the viewpoint of battery discharge capacity maintenance rate, the weight ratio of the polymer compound (resin solids weight) to the conductive filler constituting the coating layer is preferably 1:0.01 to 1:50, and more preferably 1:0.2 to 1:4.0.

[0054] The coating layer may optionally contain ceramic particles. Examples of ceramic particles include metal carbide particles, metal oxide particles, and glass ceramic particles.

[0055] Examples of metal carbide particles include silicon carbide (SiC), tungsten carbide (WC), molybdenum carbide (Mo2C), titanium carbide (TiC), tantalum carbide (TaC), niobium carbide (NbC), vanadium carbide (VC), and zirconium carbide (ZrC).

[0056] Examples of metal oxide particles include zinc oxide (ZnO), aluminum oxide (Al2O3), silicon dioxide (SiO2), tin oxide (SnO2), titania (TiO2), zirconia (ZrO2), indium oxide (In2O3), Li2B4O7, and Li4Ti5O. 12 Examples include perovskite-type oxide particles represented as Li2Ti2O5, LiTaO3, LiNbO3, LiAlO2, Li2ZrO3, Li2WO4, Li2TiO3, Li3PO4, Li2MoO4, Li3BO3, LiBO2, Li2CO3, Li2SiO3, and ABO3 (where A is at least one selected from the group consisting of Ca, Sr, Ba, La, Pr, and Y, and B is at least one selected from the group consisting of Ni, Ti, V, Cr, Mn, Fe, Co, Mo, Ru, Rh, Pd, and Re). As metal oxide particles, zinc oxide (ZnO), aluminum oxide (Al2O3), silicon dioxide (SiO2), and lithium tetraborate (Li2B4O7) are preferred from the viewpoint of effectively suppressing side reactions that occur between the electrolyte and the coated positive electrode active material particles.

[0057] As for the ceramic particles, glass ceramic particles are preferable from the viewpoint of effectively suppressing side reactions that occur between the electrolyte and the coated positive electrode active material particles. These may be used individually or in combination of two or more types.

[0058] The glass ceramic particles are preferably lithium-containing phosphate compounds having a rhombohedral crystal system, and their chemical formula is Li x M"2P3O 12 It can be expressed as (X = 1 to 1.7). Here, M'' is one or more elements selected from the group consisting of Zr, Ti, Fe, Mn, Co, Cr, Ca, Mg, Sr, Y, Sc, Sn, La, Ge, Nb, and Al. Furthermore, some of the P may be substituted with Si or B, and some of the O may be substituted with F, Cl, etc. For example, Li 1.15 Ti 1.85 Al 0.15 Si 0.05 P 2.95 O 12 Li 1.2 Ti 1.8 Al 0.1 Ge 0.1 Si 0.05 P 2.95 O 12 The following can be used. Furthermore, materials of different compositions may be mixed or compounded, and the surface may be coated with a glass electrolyte or the like. Alternatively, it is preferable to use glass ceramic particles that precipitate a crystalline phase of a lithium-containing phosphate compound having a NASICON-type structure by heat treatment. Examples of glass electrolytes include the glass electrolyte described in Japanese Patent Publication No. 2019-96478.

[0059] Here, it is preferable that the proportion of Li2O in the glass ceramic particles is 8% by mass or less in terms of oxide. Even if it is not a NASICON-type structure, it is composed of Li, La, Mg, Ca, Fe, Co, Cr, Mn, Ti, Zr, Sn, Y, Sc, P, Si, O, In, Nb, and F, and has liSICON-type, perovskite-type, β-Fe2(SO4)3-type, and Li3In2(PO4)3-type crystal structures, and Li ions can be released at room temperature in a quantity of 1 × 10⁻¹⁶. -5 A solid electrolyte with conductivity of S / cm or higher may also be used.

[0060] The ceramic particles described above may be used individually or in combination of two or more types.

[0061] The volume-average particle diameter of the ceramic particles is preferably 1 to 1000 nm, more preferably 1 to 500 nm, and even more preferably 1 to 150 nm, from the viewpoint of energy density and electrical resistance. The volume-average particle size of ceramic particles refers to the value obtained using the microtrac method, similar to the cathode active material particles.

[0062] The weight percentage of ceramic particles is preferably 0.5 to 5.0% by weight, based on the weight of the coated positive electrode active material particles. By including ceramic particles within the above range, side reactions occurring between the electrolyte and the coated positive electrode active material particles can be effectively suppressed. The weight percentage of ceramic particles is more preferably 2.0 to 4.0% by weight, based on the weight of the coated positive electrode active material particles.

[0063] The mixing ratio of positive electrode active material particles to the resin composition containing the polymer compound and conductive filler that constitute the coating layer is not particularly limited, but it is preferable that the weight ratio of positive electrode active material particles to resin composition is 1:0.001 to 0.2.

[0064] In the coated positive electrode active material particles for lithium-ion batteries of the present invention, at least a portion of the surface of the positive electrode active material particles is covered with a coating layer. From the viewpoint of cycle characteristics, the coverage of the positive electrode active material particles is preferably 30 to 95%, as calculated by the following formula. Coverage rate (%) = {1 - [BET specific surface area of ​​coated positive electrode active material particles / (BET specific surface area of ​​uncoated positive electrode active material particles × weight ratio of positive electrode active material particles contained in coated positive electrode active material + BET specific surface area of ​​conductive filler × weight ratio of conductive filler contained in coated positive electrode active material particles + BET specific surface area of ​​optionally included ceramic particles × weight ratio of optionally included ceramic particles in coated positive electrode active material particles)]} × 100

[0065] [Method for manufacturing coated positive electrode active material particles for lithium-ion batteries] The method for producing coated positive electrode active material particles for lithium-ion batteries of the present invention (hereinafter also simply referred to as "method for producing coated positive electrode active material particles") is not particularly limited, but one embodiment includes a stirring step in which positive electrode active material particles, a polymer compound, a conductive filler, and an organic solvent are mixed and stirred in a sealed container equipped with an impeller that rotates at a peripheral speed of 4.0 to 12.0 m / s.

[0066] The coated positive electrode active material particles of the present invention can be obtained by coating positive electrode active material particles with a coating layer containing a polymer compound and a conductive filler. In the above method for producing coated positive electrode active material particles, positive electrode active material particles, polymer compound, conductive filler, and organic solvent are mixed in a sealed container equipped with an impeller. The organic solvent is not particularly limited as long as it is an organic solvent capable of dissolving the polymer compound, and known organic solvents can be appropriately selected and used. The amount of organic solvent used is, for example, 50 to 500% by weight relative to the polymer compound.

[0067] The order in which the positive electrode active material particles, polymer compound, and conductive filler are mixed is not particularly limited. For example, a resin composition consisting of a pre-mixed polymer compound and conductive filler may be further mixed with the positive electrode active material particles, or the positive electrode active material particles, polymer compound, and conductive filler may be mixed simultaneously, or the polymer compound may be mixed with the positive electrode active material particles and then the conductive filler may be mixed. If ceramic particles are optionally used in the coating layer, they may be mixed together with the conductive filler. It is preferable that the polymer compound be dissolved in an organic solvent beforehand before mixing with the positive electrode active material particles and conductive filler.

[0068] Examples of sealed containers equipped with an impeller include the universal mixer and the high-speed mixer FS25 [(manufactured by Earth Technica Co., Ltd.)].

[0069] The peripheral speed of the impeller is 4.0 to 12.0 m / s, preferably 4.5 to 11.0 m / s, and more preferably 5.0 to 10.0 m / s, in order to keep the ratio of loose bulk density to hard bulk density of the coated positive electrode active material particles within the range described above. Peripheral speed refers to the speed of the outermost end of the impeller (rotating stirring blade) and can be expressed by the following formula (1). The stirring time can also be varied by the peripheral speed of the impeller, such that it becomes longer as the peripheral speed of the impeller decreases. Impeller peripheral speed (m / s) = Impeller radius (m) × 2 × π × rotational speed (rpm) ÷ 60 (1)

[0070] The stirring time needs to be adjusted as appropriate depending on the total amount of positive electrode active material particles, polymer compound, conductive filler, and organic solvent placed in the sealed container, but is, for example, 1 to 90 minutes, preferably 10 to 60 minutes.

[0071] When the above stirring process is carried out for 1 to 90 minutes in a sealed container equipped with an impeller that rotates at a peripheral speed of 4.0 to 12.0 m / s, the total amount of positive electrode active material particles, polymer compound, conductive filler, and organic solvent to be placed in the sealed container is 1 cm³ of the sealed container. 3 The amount per serving is preferably 0.1 to 0.6 g, and more preferably 0.2 to 0.5 g.

[0072] As an example of a method for producing coated positive electrode active material particles for lithium-ion batteries according to the present invention, coated positive electrode active material particles can be produced by placing positive electrode active material particles in a sealed container equipped with an impeller, stirring at a peripheral speed of 4.0 to 12.0 m / s, adding a resin solution containing a polymer compound dropwise over 1 to 90 minutes, and then mixing in a conductive filler.

[0073] In the method for producing coated positive electrode active material particles for lithium-ion batteries of the present invention, it is preferable to desolvent the mixture after the stirring step. The method of desolventing is not particularly limited, and can be carried out by stirring the positive electrode active material particles, polymer compound, conductive filler, and organic solvent and raising the temperature to, for example, 50 to 200°C, then reducing the pressure to 0.007 to 0.04 MPa and holding it for 10 to 150 minutes.

[0074] This specification also discloses a method for producing coated positive electrode active material particles for lithium-ion batteries, comprising a stirring step of mixing and stirring the above-mentioned positive electrode active material particles, polymer compound, conductive filler, and organic solvent in a sealed container equipped with an impeller rotating at a peripheral speed of 4.0 to 12.0 m / s.

[0075] <Positive electrode for lithium-ion batteries> The coated positive electrode active material particles for lithium-ion batteries of the present invention can be used in the manufacture of positive electrodes for lithium-ion batteries. The positive electrode for a lithium-ion battery comprises a positive electrode active material layer containing coated positive electrode active material particles for lithium-ion batteries according to the present invention and an electrolyte solution containing an electrolyte and a solvent. Known electrolytes and solvents may be used.

[0076] The coated positive electrode active material particles contained in the positive electrode active material layer are preferably 40 to 95% by weight, and more preferably 60 to 90% by weight, based on the weight of the positive electrode active material layer, from the viewpoint of dispersibility of the positive electrode active material particles and electrode moldability.

[0077] From the viewpoint of battery performance, the thickness of the positive electrode active material layer is preferably 150 to 550 μm, and more preferably 200 to 540 μm.

[0078] A positive electrode for a lithium-ion battery can be manufactured, for example, by applying a powder mixture (composition for the positive electrode active material layer) of the lithium-ion battery coated positive electrode active material particles of the present invention and, if necessary, a conductive additive, to a current collector, pressing it with a press machine to form a positive electrode active material layer, and then pouring in an electrolyte solution. Alternatively, the composition for the positive electrode active material layer may be applied to a release film and pressed to form the positive electrode active material layer, the positive electrode active material layer may be transferred to the current collector, and then the electrolyte may be injected. Alternatively, a positive electrode for a lithium-ion battery may be manufactured by placing a frame-shaped member on top of the current collector and filling the inside of the frame-shaped member with a composition for the positive electrode active material layer to the same thickness as the frame-shaped member.

[0079] [Lithium-ion battery] A lithium-ion battery can be obtained by combining the above-mentioned positive electrode for lithium-ion batteries with a counter electrode, placing them together with a separator in a cell container, injecting an electrolyte, and sealing the cell container. Alternatively, a lithium-ion battery can be obtained by forming a positive electrode for a lithium-ion battery on one side of a current collector and a negative electrode on the other side to create a bipolar electrode, stacking the bipolar electrode with a separator and housing it in a cell container, injecting an electrolyte, and sealing the cell container.

[0080] Examples of separators include known separators for lithium-ion batteries such as porous films made of polyethylene or polypropylene, laminated films of porous polyethylene film and porous polypropylene, nonwoven fabrics made of synthetic fibers (polyester fibers and aramid fibers, etc.) or glass fibers, and those on which ceramic fine particles such as silica, alumina, and titania are attached to the surface. [Examples]

[0081] The present invention will now be specifically described with reference to examples, but the present invention is not limited to these examples unless it deviates from the spirit of the invention. Unless otherwise specified, parts refer to parts by weight, and % refers to % by weight.

[0082] <Preparation of polymer compounds> 150 parts of DMF were placed in a four-necked flask equipped with a stirrer, thermometer, reflux condenser, dropping funnel, and nitrogen gas inlet tube, and the temperature was raised to 75°C. Next, a monomer composition containing 91 parts acrylic acid, 9 parts methyl methacrylate, and 50 parts DMF, along with an initiator solution containing 0.3 parts 2,2'-azobis(2,4-dimethylvaleronitrile) and 0.8 parts 2,2'-azobis(2-methylbutyronitrile) dissolved in 30 parts DMF, were continuously added dropwise over 2 hours using a dropping funnel while blowing nitrogen into the four-necked flask under stirring to carry out radical polymerization. After the dropwise addition was complete, the reaction was continued at 75°C for 3 hours. Next, the temperature was raised to 80°C and the reaction was continued for 3 hours to obtain a copolymer solution with a resin concentration of 30%. The obtained copolymer solution was transferred to a Teflon® vat and dried under reduced pressure at 150°C and 0.01 MPa for 3 hours, and the DMF was removed by distillation to obtain the copolymer. This copolymer was coarsely ground with a hammer, and then further ground in a mortar to obtain a powdered polymer compound. The weight-average molecular weight of the obtained polymer compound was 58,000.

[0083] <Preparation of electrolyte solution> An electrolyte was prepared by dissolving LiN(FSO2)2 at a ratio of 2.0 mol / L in a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (volume ratio 1:1).

[0084] <Manufacturing of resin current collectors> A resin mixture was obtained by melt-kneading 70 parts of polypropylene [product name "Sun Allomer PL500A", manufactured by Sun Allomer Co., Ltd.], 25 parts of carbon nanotubes [product name: "FloTube9000", manufactured by CNano], and 5 parts of dispersant [product name "Yumex 1001", manufactured by Sanyo Chemical Industries, Ltd.] in a twin-screw extruder at 200°C and 200 rpm. The obtained resin mixture was passed through a T-die extrusion film molding machine and stretched and rolled to obtain a conductive film for resin current collectors with a thickness of 100 μm. Next, the obtained conductive film for resin current collectors was cut to 17.0 cm × 17.0 cm, nickel deposition was applied to one side, and then a current extraction terminal (5 mm × 3 cm) was attached to obtain a resin current collector.

[0085] <Example 1> [Fabrication of coated cathode active material particles A] One part of the polymer compound was dissolved in four parts of DMF to obtain a polymer compound solution. 90.0 parts of positive electrode active material particles (LiFePO4, volume average particle size 13 μm) were placed in a universal mixer high-speed mixer FS25 [(manufactured by Earth Technica Co., Ltd.)], and while stirring at room temperature and a peripheral speed of 5.2 m / s, 25.0 parts of polymer compound solution (5.0 parts of resin solids) were added dropwise over 2 minutes, and the mixture was stirred for a further 5 minutes. Next, while stirring, 5.0 parts of acetylene black [Denka Black®, manufactured by Denka Co., Ltd., average particle size 35 nm], a conductive filler, were added in portions over 2 minutes, and stirring was continued for 30 minutes. Subsequently, the pressure was reduced to 0.01 MPa while maintaining stirring, and then the temperature was raised to 140°C while maintaining stirring and reduced pressure. The stirring, reduced pressure, and temperature were maintained for 8 hours to remove volatile components by distillation. The obtained powder was classified using a sieve with a mesh size of 200 μm to obtain coated positive electrode active material particles A.

[0086] <Example 2> [Fabrication of coated cathode active material particles B] Coated positive electrode active material particles B were obtained in the same manner as in Example 1, except that the positive electrode active material particles were changed to 92.0 parts and the polymer compound solution was changed to 15.0 parts (3.0 parts resin solids).

[0087] <Example 3> [Fabrication of coated cathode active material particles C] Coated positive electrode active material particles C were obtained in the same manner as in Example 1, except that the peripheral speed was changed from 5.2 m / s to 9.1 m / s.

[0088] <Example 4> [Fabrication of coated cathode active material particles D] Coated positive electrode active material particles D were obtained in the same manner as in Example 3, except that the positive electrode active material particles were changed to 86.0 parts, the polymer compound solution was changed to 20.0 parts (4.0 parts resin solids), and the conductive filler was changed to 10.0 parts.

[0089] <Example 5> [Fabrication of coated cathode active material particles E] Coated positive electrode active material particles E were obtained in the same manner as in Example 3, except that the positive electrode active material particles were changed to 88.0 parts, the polymer compound solution was changed to 45.0 parts (resin solids 9.0 parts), and the conductive filler was changed to 3.0 parts.

[0090] <Example 6> [Fabrication of coated cathode active material particles F] Coated positive electrode active material particles F were obtained in the same manner as in Example 1, except that the positive electrode active material particles were changed to 93.3 parts, the polymer compound solution was changed to 7.5 parts (1.5 parts resin solids), and the conductive filler was changed to 5.2 parts.

[0091] <Example 7> [Fabrication of coated cathode active material particles G] Coated positive electrode active material particles G were obtained in the same manner as in Example 1, except that the peripheral speed was changed from 5.2 m / s to 7.3 m / s.

[0092] <Comparative Example 1> [Fabrication of coated cathode active material particles H] Coated positive electrode active material particles H were obtained in the same manner as in Example 1, except that the peripheral speed was changed from 5.2 m / s to 15.2 m / s.

[0093] <Comparative Example 2> [Fabrication of coated cathode active material particles I] Coated positive electrode active material particles I were obtained in the same manner as in Comparative Example 1, except that the positive electrode active material particles were changed to 91.9 parts, the polymer compound solution was changed to 15.0 parts (3.0 parts resin solids), and the conductive filler was changed to 5.1 parts.

[0094] <Fabrication of positive electrodes for lithium-ion batteries> Each of the prepared lithium-ion battery coating positive electrode active material particles A to I was placed on a Φ15 mold at a basis weight of 100 mg / cm². 2 Fill to this size and press with a press machine (HANDTAB-100T15, manufactured by Ichihashi Seiki Co., Ltd.) at a rate of 1 ton / cm². 2A positive electrode active material layer (520 μm thick) was formed by tablet molding under pressure, and this layer was laminated onto one side of the resin current collector to produce positive electrodes (circular, 15 mm in diameter) A to I for lithium-ion batteries.

[0095] <Fabrication of coated negative electrode active material particles> One part of the above-mentioned coating polymer compound was dissolved in three parts of DMF to obtain a coating polymer compound solution. 80.04 parts of negative electrode active material particles (hard carbon powder, volume average particle size 25 μm) were placed in a universal mixer, High Speed ​​Mixer FS25 [manufactured by Earth Technica Co., Ltd.], and while stirring at room temperature and 720 rpm, 37.92 parts of the coating polymer compound solution were added dropwise over 2 minutes, and stirring continued for another 5 minutes. Next, while stirring continued, 9.48 parts of acetylene black [Denka Black® manufactured by Denka Co., Ltd.], a conductive additive, were added in installments over 2 minutes, and stirring continued for 30 minutes. After that, while maintaining stirring, the pressure was reduced to 0.01 MPa, and then, while maintaining stirring and reduced pressure, the temperature was raised to 140°C, and stirring, reduced pressure, and temperature were maintained for 8 hours to remove volatile components by distillation. The obtained powder was classified using a sieve with a mesh size of 200 μm to obtain coated negative electrode active material particles.

[0096] <Fabrication of negative electrodes for lithium-ion batteries> A negative electrode precursor was prepared by mixing 99 parts of the fabricated coated negative electrode active material particles with 1 part of carbon fiber [Donacarbo Milled S-243 manufactured by Osaka Gas Chemical Co., Ltd.: average fiber length 500 μm, average fiber diameter 13 μm: electrical conductivity 200 mS / cm]. The prepared negative electrode precursor was placed on a Φ16 mold with a basis weight of 41.6 mg / cm³ of negative electrode active material particles. 2 Fill to this size and press with a press machine (HANDTAB-100T15, manufactured by Ichihashi Seiki Co., Ltd.) at a rate of 1 ton / cm². 2 A negative electrode active material layer (450 μm thick) was formed by tablet molding under pressure, and this layer was laminated onto one side of the resin current collector to produce a negative electrode for a lithium-ion battery (circular with a diameter of 16 mm).

[0097] <Manufacturing of lithium-ion batteries> The lithium-ion battery positive electrodes A to I obtained above were combined with a lithium-ion battery negative electrode via a separator (Cellguard #3501) to fabricate a laminate cell.

[0098] The weights of the positive electrode active material particles, polymer compound solution, and conductive filler placed in the sealed container in each example and comparative example, and the weight of the sealed container 1 cm 3 Table 1 shows the weight distribution per unit. The loose and hard bulk densities of the coated positive electrode active material particles for lithium-ion batteries obtained in each example and comparative example were measured using the method described herein. The results are shown in Table 2. Furthermore, the discharge capacity of lithium-ion batteries prepared using the coated positive electrode active material particles obtained in each example and comparative example was measured, and the discharge capacity retention rate was calculated. The results are shown in Table 2.

[0099] <Measurement of discharge capacity maintenance rate> For each example and comparative example, the lithium-ion batteries were charged to 3.7V with a current of 0.1C at 25°C using the charge / discharge measurement device "HJ-SD8" [manufactured by Hokuto Denko Co., Ltd.], then discharged to 2.0V with a current of 0.1C after a 2-hour rest period, and this charge / discharge cycle was repeated 21 times. The battery capacity at the time of the first charge (discharge capacity at the first cycle) and the battery capacity at the time of the 21st charge (discharge capacity at the 21st cycle) were measured. The discharge capacity retention rate was calculated using the following formula. The results are shown in Table 2. Note that a higher value indicates less battery degradation. Discharge capacity retention rate (%) = (Discharge capacity at 21st cycle / Discharge capacity at 1st cycle) × 100

[0100] [Table 1]

[0101] [Table 2]

[0102] The coated positive electrode active material particles for lithium-ion batteries in Examples 1 to 7 had a ratio of loose bulk density to hard bulk density (loose bulk density / hard bulk density) of 0.51 to 0.79, while the coated positive electrode active material particles for lithium-ion batteries in Comparative Examples 1 to 2 had a ratio of loose bulk density to hard bulk density (loose bulk density / hard bulk density) of 0.45 to 0.46. The lithium-ion batteries obtained using the coated positive electrode active material particles for lithium-ion batteries of Examples 1 to 7 showed a higher discharge capacity retention rate at 21 cycles compared to the lithium-ion batteries obtained using the coated positive electrode active material particles for lithium-ion batteries of Comparative Examples 1 to 2. Comparing Examples 1, 3, and 7, it was found that the lower the peripheral speed, the higher the discharge capacity retention rate at the 21st cycle. Comparing Examples 1, 2, and 6, it was found that the lower the amount of positive electrode active material particles and the higher the amount of polymer compound, the higher the discharge capacity maintenance rate at the 21st cycle. Comparing Examples 3, 4, and 5, it was found that the lower the amount of polymer compound and the higher the amount of conductive filler, the higher the discharge capacity retention rate at the 21st cycle. [Industrial applicability]

[0103] The lithium-ion battery coated positive electrode active material particles of the present invention are particularly useful for manufacturing lithium-ion batteries used in stationary power supplies, mobile phones, personal computers, hybrid vehicles, and electric vehicles, among others.

Claims

1. Coated positive electrode active material particles for lithium-ion batteries, wherein at least a portion of the surface of the positive electrode active material particles is coated with a coating layer containing a polymer compound and a conductive filler, The positive electrode active material particles are LiFePO ４ And, Capacity 100cm ３ Lithium-ion battery coated positive electrode active material particles, wherein the ratio (loose bulk density / hard bulk density) between the loose bulk density measured in accordance with JIS K 6219-2 (2005) using a cylindrical container with a diameter of 30 mm and the hard bulk density measured in accordance with JIS K 5101-12-2 (2004) with a drop height of 5 mm and 2000 tamping cycles is 0.50 to 0.

90.

2. The weight percentage of the polymer compound is 0.1 to 11% by weight, based on the weight of the coated positive electrode active material particles for the lithium-ion battery. The coated positive electrode active material particle for lithium-ion battery according to claim 1, wherein the weight percentage of the conductive filler is 2 to 14% by weight based on the weight of the coated positive electrode active material particle for lithium-ion battery.

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