JP7809462B2 - Coated positive electrode active material particles for lithium ion batteries, positive electrode for lithium ion batteries, and method for producing coated positive electrode active material particles for lithium ion batteries - Google Patents

Description

The present invention relates to coated positive electrode active material particles for lithium ion batteries, positive electrodes for lithium ion batteries, and a method for producing coated positive electrode active material particles for lithium ion batteries.

Lithium-ion (secondary) batteries have been widely used in recent years for a variety of applications as secondary batteries that can achieve high energy density and high power density, and various materials are being investigated to develop higher performance lithium-ion batteries.

For example, Patent Document 1 discloses a resin composition for coating an active material, which comprises a polymer of a monomer composition containing an ester compound of a monohydric aliphatic alcohol having 1 to 12 carbon atoms with (meth)acrylic acid and an anionic monomer, the polymer having an acid value of 30 to 700, and a coated active material having a coating layer comprising the resin composition for coating an active material on at least a portion of the surface of the active material.

JP 2017-160294 A

Lithium-ion batteries are widely used in a variety of applications, including in high-temperature environments.
Lithium-ion batteries using conventional coated active materials have the problem that when used in high-temperature environments, side reactions can occur between the electrolyte and the coated active material particles, causing the lithium-ion battery to deteriorate (specifically, an increase in internal resistance), leaving room for improvement.

The present invention aims to provide coated positive electrode active material particles for lithium ion batteries that can suppress side reactions that occur between an electrolyte and coated positive electrode active material particles, thereby suppressing an increase in the internal resistance of the lithium ion battery. The present invention also aims to provide a positive electrode for lithium ion batteries that includes the coated positive electrode active material particles for lithium ion batteries, and a method for producing the coated positive electrode active material particles for lithium ion batteries.

As a result of extensive research to solve the above-mentioned problems, the inventors discovered that by forming a coating layer containing a polymer compound, a conductive additive, and ceramic particles with a specific BET specific surface area on the surface of positive electrode active material particles, it is possible to suppress side reactions that occur between the electrolyte and the coated positive electrode active material particles, and to prevent an increase in the internal resistance of the lithium-ion battery, leading to the present invention.

That is, the present invention provides coated positive electrode active material particles for lithium ion batteries, at least a portion of the surface of which is coated with a coating layer, the coating layer comprising a polymer compound, a conductive additive, and ceramic particles, the ceramic particles having a BET specific surface area of 70 to 300 m ² /g; a positive electrode for lithium ion batteries, comprising a positive electrode active material layer comprising the coated positive electrode active material particles for lithium ion batteries and an electrolytic solution containing an electrolyte and a solvent, the positive electrode active material layer being composed of a non-bound body of the coated positive electrode active material particles for lithium ion batteries; and a method for producing coated positive electrode active material particles for lithium ion batteries, comprising a step of mixing positive electrode active material particles, a polymer compound, a conductive additive, ceramic particles, and an organic solvent, and then removing the solvent.

According to the present invention, it is possible to obtain coated positive electrode active material particles for lithium ion batteries that can suppress side reactions that occur between the electrolyte and the coated positive electrode active material particles, thereby suppressing an increase in the internal resistance value of the lithium ion battery.

[Coated positive electrode active material particles for lithium ion batteries]
The coated positive electrode active material particles for lithium ion batteries of the present invention (hereinafter also simply referred to as "coated positive electrode active material particles") are coated positive electrode active material particles in which at least a portion of the surface of the positive electrode active material particles is coated with a coating layer, and the coating layer contains a polymer compound, a conductive additive, and ceramic particles.
The coated cathode active material particles of the present invention have a coating layer containing a polymer compound, a conductive additive, and ceramic particles having a specific BET specific surface area. The ceramic particles having the specific BET specific surface area contained in the coating layer can reduce the contact area between the cathode active material particles and the electrolyte, thereby suppressing side reactions that occur between the electrolyte and the coated cathode active material particles and preventing an increase in the internal resistance of the lithium-ion battery.

Positive electrode active material particles include composite oxides of lithium and transition metals {composite oxides containing one type of transition metal (e.g., LiCoO ₂ , LiNiO ₂ , LiAlMnO ₄ , LiMnO ₂ and LiMn ₂ O ₄ ), composite oxides containing two types of transition metal elements (e.g., LiFeMnO ₄ , LiNi _1-x Co _x O ₂ , LiMn _1-y Co _y O ₂ , LiNi _1/3 Co _1/3 Al _1/3 O ₂ and LiNi _0.8 Co _0.15 Al _0.05 O ₂ ), and composite oxides containing three or more types of metal elements [e.g., LiM _a M' _b M'' _c O ₂ (M, M', and M'' are different transition metal elements satisfying a+b+c=1, for example, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ), etc.}, lithium-containing transition metal phosphates (for example, LiFePO ₄ , LiCoPO ₄ , LiMnPO ₄ , and LiNiPO ₄ ), transition metal oxides (for example, MnO ₂ and V ₂ O ₅ ), transition metal sulfides (for example, MoS ₂ and TiS ₂ ), and conductive polymers (for example, polyaniline, polypyrrole, polythiophene, polyacetylene, poly-p-phenylene, and polyvinylcarbazole), and two or more of these may be used in combination.
The lithium-containing transition metal phosphate may have some of the transition metal sites substituted with other transition metals.

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 even more preferably 2 to 30 μm.

The coating layer contains a polymer compound, a conductive additive, and ceramic particles.
The polymer compound is preferably, for example, a resin containing a polymer having an acrylic monomer (a) as 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 monomer composition, the content of acrylic acid (a0) is preferably more than 90 wt% and not more than 98 wt% based on the weight of the total monomers. From the viewpoint of flexibility of the coating layer, the content of acrylic acid (a0) is more preferably 93.0 to 97.5 wt%, and even more preferably 95.0 to 97.0 wt%, based on the weight of the total monomers.

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).

Examples of monomers (a1) having a carboxyl group or an 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 maleic acid (anhydride), fumaric acid, itaconic acid (anhydride), citraconic acid, and mesaconic acid; and trivalent, tetravalent, or higher polycarboxylic acids having 6 to 24 carbon atoms, such as aconitic acid.

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

In the monomer (a2) represented by the general formula (1), R ¹ represents a hydrogen atom or a methyl group, and R ¹ is preferably a methyl group.
^R2 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.

(a21) Ester compounds in which ^R2 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 having 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, 2,2-dimethylbutyl group, 2,3-dimethylbutyl group, 1-ethylbutyl group, 2-ethylbutyl group, 1-methylhexyl group, 2-methylhexyl group, 2-methylhexyl group, 4-methylhexyl group, 5-methylhexyl group, 1-ethylpentyl group, 2-ethylpentyl group, 3-ethylpentyl group, 1,1-dimethylpentyl group, 2-ethylpent ... butyl group, 1,2-dimethylpentyl group, 1,3-dimethylpentyl group, 2,2-dimethylpentyl group, 2,3-dimethylpentyl group, 2-ethylpentyl group, 1-methylheptyl group, 2-methylheptyl group, 3-methylheptyl group, 4-methylheptyl 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 octyl 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-methylundecyl group, 2-methylundecyl group, 3-methylundecyl group, 4-methylundecyl group, 5-methylundecyl group, 6-methylundecyl group, 7-methylundecyl group , 8-methylundecyl group, 9-methylundecyl group, 10-methylundecyl group, 1,1-dimethyldecyl group, 1,2-dimethyldecyl group, 1,3-dimethyldecyl group, 1,4-dimethyldecyl group, 1,5-dimethyldecyl group, 1,6-dimethyldecyl group, 1,7-dimethyldecyl group, 1,8-dimethyldecyl group, 1,9-dimethyldecyl group, 1-ethyldecyl group, 2-ethyldecyl group, etc. Of these, 2-ethylhexyl group is particularly preferred.

(a22) Ester compounds in which R ² is a branched alkyl group having 13 to 36 carbon atoms. Examples of the branched alkyl group having 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.], 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, etc.]. , 2-tetradecyleicosyl group, 2-hexadecyleicosyl group, etc.], 3 to 34-alkylalkyl groups (3-alkylalkyl group, 4-alkylalkyl group, 5-alkylalkyl group, 32-alkylalkyl group, 33-alkylalkyl group, 34-alkylalkyl group, etc.), and mixed alkyl groups containing one or more branched alkyl groups such as residues obtained by removing hydroxyl groups from oxoalcohols obtained from propylene oligomers (hepta-octamer), ethylene/propylene (molar ratio 16/1 to 1/11) oligomers, isobutylene oligomers (hepta-octamer), and α-olefin (carbon number 5 to 20) oligomers (tetra-octamer).

The polymer compound constituting the coating layer may contain, as the acrylic monomer (a), an ester compound (a3) of a monohydric aliphatic alcohol having 1 to 3 carbon atoms and (meth)acrylic acid.
Examples of the monohydric aliphatic alcohol having 1 to 3 carbon atoms that constitutes the ester compound (a3) include methanol, ethanol, 1-propanol, and 2-propanol.
The (meth)acrylic acid means acrylic acid or methacrylic acid.

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

From the viewpoint of suppressing volumetric changes in the positive electrode active material particles, the total content of monomer (a1), monomer (a2), and ester compound (a3) is preferably 2.0 to 9.9 wt %, and more preferably 2.5 to 7.0 wt %, based on the total weight of the monomers.

It is preferable that the polymer compound constituting the coating layer does not contain, as the acrylic monomer (a), a salt of an anionic monomer (a4) having a polymerizable unsaturated double bond and an anionic group.

Examples of the structure having a polymerizable unsaturated double bond include a vinyl group, an allyl group, a styrenyl group, and a (meth)acryloyl group.
Examples of the anionic group include a sulfonic acid group and a carboxyl group.
Anionic monomers having a polymerizable unsaturated double bond and an anionic group are compounds obtained by combining these, and examples thereof include vinyl sulfonic acid, allyl sulfonic acid, styrene sulfonic acid, and (meth)acrylic acid.
The (meth)acryloyl group means an acryloyl group or a methacryloyl group.
Examples of the cation constituting the salt (a4) of the anionic monomer include a lithium ion, a sodium ion, a potassium ion, and an ammonium ion.

Furthermore, the polymer compound constituting the coating layer may contain, as the acrylic monomer (a), a radical polymerizable monomer (a5) that is copolymerizable with acrylic acid (a0), the monomer (a1), the monomer (a2), and the ester compound (a3), within a range that does not impair the physical properties.
The radical polymerizable monomer (a5) is preferably a monomer not containing active hydrogen, and the following monomers (a51) to (a58) can be used.

(a51) Hydrocarbyl (meth)acrylates formed from a linear aliphatic monool having 13 to 20 carbon atoms, an alicyclic monool having 5 to 20 carbon atoms, or an aromatic aliphatic monool having 7 to 20 carbon atoms and (meth)acrylic acid. Examples of the monool 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.

(a52) Poly(n = 2-30) oxyalkylene (carbon number 2-4) alkyl (carbon number 1-18) ether (meth)acrylate [(meth)acrylate) of 10 moles of ethylene oxide (hereinafter abbreviated as EO) adduct of methanol, (meth)acrylate) of 10 moles of propylene oxide (hereinafter abbreviated as PO) adduct of methanol, etc.]

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

(a53-2) (meth)acrylate compounds (i) dialkyl (carbon number 1 to 4) aminoalkyl (carbon number 1 to 4) (meth)acrylates [N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, t-butylaminoethyl (meth)acrylate, morpholinoethyl (meth)acrylate, etc.]
(ii) Quaternary ammonium group-containing (meth)acrylates {quaternized products of tertiary amino group-containing (meth)acrylates [N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, etc.] (those quaternized using a quaternizing agent such as methyl chloride, dimethyl sulfate, benzyl chloride, dimethyl carbonate, etc.), etc.}

(a53-3) Heterocycle-containing vinyl compounds: pyridine compounds (having 7 to 14 carbon atoms, for example, 2- or 4-vinylpyridine), imidazole compounds (having 5 to 12 carbon atoms, for example, N-vinylimidazole), pyrrole compounds (having 6 to 13 carbon atoms, for example, N-vinylpyrrole), and pyrrolidone compounds (having 6 to 13 carbon atoms, for example, N-vinyl-2-pyrrolidone).

(a53-4) Nitrile group-containing vinyl compounds Nitrile group-containing vinyl compounds having 3 to 15 carbon atoms, such as (meth)acrylonitrile, cyanostyrene, and cyanoalkyl (carbon number 1 to 4) acrylates

(a53-5) Other nitrogen-containing vinyl compounds, nitro group-containing vinyl compounds (having 8 to 16 carbon atoms, for example, nitrostyrene), etc.

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

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

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

(a55) Vinyl esters: Aliphatic vinyl esters [having 4 to 15 carbon atoms, for example, alkenyl esters of aliphatic carboxylic acids (mono- or dicarboxylic acids) (for example, vinyl acetate, vinyl propionate, vinyl butyrate, diallyl adipate, isopropenyl acetate, vinyl methoxyacetate)]
Aromatic vinyl esters [having 9 to 20 carbon atoms, for example, alkenyl esters of aromatic carboxylic acids (mono- or dicarboxylic acids) (e.g., vinyl benzoate, diallyl phthalate, methyl-4-vinylbenzoate), aromatic ring-containing esters of aliphatic carboxylic acids (e.g., acetoxystyrene)]

(a56) vinyl ethers; aliphatic vinyl ethers [having 3 to 15 carbon atoms, for example, vinyl alkyl (having 1 to 10 carbon atoms) ethers (vinyl methyl ether, vinyl butyl ether, vinyl 2-ethylhexyl ether, etc.), vinyl alkoxy (having 1 to 6 carbon atoms) alkyl (having 1 to 4 carbon atoms) ethers (vinyl-2-methoxyethyl ether, methoxybutadiene, 3,4-dihydro-1,2-pyran, 2-butoxy-2'-vinyloxydiethyl ether, vinyl-2-ethylmercaptoethyl ether, etc.), poly(2-4)(meth)allyloxyalkanes (having 2 to 6 carbon atoms) (diallyloxyethane, triallyloxyethane, tetraallyloxybutane, tetramethallyloxyethane, etc.)], aromatic vinyl ethers (having 8 to 20 carbon atoms, for example, vinyl phenyl ether, phenoxystyrene)

(a57) Vinyl ketones: Aliphatic vinyl ketones (having 4 to 25 carbon atoms, for example, vinyl methyl ketone, vinyl ethyl ketone), aromatic vinyl ketones (having 9 to 21 carbon atoms, for example, vinyl phenyl ketone)

(a58) Unsaturated dicarboxylic acid diesters: Unsaturated dicarboxylic acid diesters having 4 to 34 carbon atoms, such as dialkyl fumarate (wherein the two alkyl groups are linear, branched, or alicyclic groups having 1 to 22 carbon atoms), dialkyl maleate (wherein the two alkyl groups are linear, branched, or alicyclic groups having 1 to 22 carbon atoms).

When the radical polymerizable monomer (a5) is contained, its content is preferably 0.1 to 3.0% by weight based on the total weight of the monomers.

The preferred lower limit of the weight-average molecular weight of the polymer compound constituting the coating layer is 3,000, more preferably 5,000, and even more preferably 7,000. On the other hand, the preferred upper limit of the weight-average molecular weight of the polymer compound is 100,000, and more preferably 70,000.

The weight average molecular weight of the polymer compound constituting the coating layer can be determined by gel permeation chromatography (hereinafter abbreviated as GPC) measurement under the following conditions.
Apparatus: Alliance GPC V2000 (manufactured by Waters)
Solvent: orthodichlorobenzene, DMF, THF
Standard material: polystyrene Sample concentration: 3 mg/ml
Column stationary phase: PLgel 10 μm, MIXED-B, two columns in series (Polymer Laboratories)
Column temperature: 135°C

The polymer compound constituting the coating layer can be produced by a known polymerization method (bulk polymerization, solution polymerization, emulsion polymerization, suspension polymerization, etc.) using a known polymerization initiator {azo initiators [2,2'-azobis(2-methylpropionitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2-methylbutyronitrile), etc.], peroxide initiators (benzoyl peroxide, di-t-butyl peroxide, lauryl peroxide, etc.), etc.}.
The amount of the 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 monomers, from the viewpoint of adjusting the weight-average molecular weight within a preferred range, and the polymerization temperature and polymerization time are adjusted depending on the type of polymerization initiator, etc., 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).

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

Examples of dispersion media for emulsion polymerization and suspension polymerization include water, alcohols (e.g., ethanol), esters (e.g., ethyl propionate), light naphtha, etc. Examples of emulsifiers include metal salts of higher fatty acids (having 10 to 24 carbon atoms) (e.g., sodium oleate and sodium stearate), metal salts of higher alcohol (having 10 to 24 carbon atoms) sulfates (e.g., sodium lauryl sulfate), ethoxylated tetramethyldecynediol, sodium sulfoethyl methacrylate, dimethylaminomethyl methacrylate, etc. Furthermore, polyvinyl alcohol, polyvinylpyrrolidone, etc. may be added as a stabilizer.
The monomer concentration in 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 the 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 monomers.
In the 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.

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

As the method of crosslinking the polymer compound constituting the coating layer using crosslinking agent (A'), there can be mentioned a method of crosslinking after coating the positive electrode active material particles with the polymer compound constituting the coating layer.Specifically, there can be mentioned a method of mixing the resin solution containing the positive electrode active material particles and the polymer compound constituting the coating layer, removing the solvent, thereby producing coated active material particles, and then mixing the solution containing crosslinking agent (A') with the coated active material particles and heating, thereby causing desolvation and crosslinking reaction, and causing the reaction of the polymer compound constituting the coating layer to be crosslinked by the crosslinking agent (A') on the surface of the positive electrode active material particles.
The heating temperature is adjusted depending on the type of crosslinking agent, but is preferably 70°C or higher when a polyepoxy compound (a'1) is used as the crosslinking agent, and is preferably 120°C or higher when a polyol compound (a'2) is used.

The conductive additive is preferably selected from materials having electrical conductivity.
Preferred examples of the conductive additive include metals [aluminum, stainless steel (SUS), silver, gold, copper, titanium, etc.], carbon [graphite and carbon black (acetylene black, ketjen black, furnace black, channel black, thermal lamp black, etc.)], and mixtures thereof.
These conductive additives may be used alone or in combination of two or more, and may also be used as alloys or metal oxides thereof.
Among these, from the viewpoint of electrical stability, aluminum, stainless steel, carbon, silver, gold, copper, titanium, and mixtures thereof are more preferable, silver, gold, aluminum, stainless steel, and carbon are even more preferable, and carbon is particularly preferable.
Furthermore, these conductive additives may be obtained by coating a particulate ceramic material or a resin material with a conductive material (preferably a metal one of the conductive additives described above) by plating or the like.

The shape (form) of the conductive additive is not limited to particulate form, and may be in a form other than particulate form, such as carbon nanofibers, carbon nanotubes, or other forms that are currently in practical use as so-called filler-based conductive additives.

The average particle size of the conductive additive is not particularly limited, but is preferably about 0.01 to 10 μm from the viewpoint of the electrical characteristics of the battery.
In this specification, the "particle diameter of the conductive additive" refers to the maximum distance L between any two points on the contour line of the conductive additive. The value of the "average particle diameter" is calculated as the average value of particle diameters of particles observed in several to several tens of fields of view using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

The ratio of the polymer compound and conductive additive that make up the coating layer is not particularly limited, but from the perspective of the battery's internal resistance value, etc., the weight ratio of polymer compound (resin solid weight) that makes up the coating layer to conductive additive is preferably 1:0.01 to 1:50, and more preferably 1:0.2 to 1:3.0.

The ceramic particles have a BET specific surface area of 70 to 300 m ² /g.
If the BET specific surface area of the ceramic particles is less than 70 m ² /g, it is not possible to sufficiently suppress side reactions that occur between the electrolyte and the coated positive electrode active material particles, and it is not possible to sufficiently suppress an increase in the internal resistance value of the lithium ion battery.
On the other hand, it is technically difficult to prepare ceramic particles having a BET specific surface area exceeding 300 m ² /g.
The ceramic particles preferably have a BET specific surface area of 110 m ² /g or more, more preferably 125 m ² /g or more, even more preferably 140 m ² /g or more, and particularly preferably 150 m ² /g or more.
The BET specific surface area of the ceramic particles can be measured based on "JIS Z 8830:2013 Method for measuring specific surface area of powder (solid) by gas adsorption" using, for example, the following device and measurement conditions.
Measuring device: Mountec Co., Ltd. Macsorb (registered trademark) HMmodel-1201
Adsorption gas: _N2
Dead volume measurement gas: mixed gas (N ₂ 30% + He 70%)
Adsorption temperature: 77K
Pretreatment before measurement: Drying at 100°C for 5 minutes under nitrogen atmosphere

Ceramic particles include metal carbide particles, metal oxide particles, glass ceramic particles, etc.

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).

Examples of metal oxide particles include zinc oxide (ZnO), aluminum oxide (Al ₂ O ₃ ), silicon dioxide (SiO ₂ ), tin oxide (SnO ₂ ), titania (TiO ₂ ), zirconia (ZrO ₂ ), indium oxide (In ₂ O ₃ ), Li ₂ B ₄ O ₇ , Li ₄ Ti ₅ O ₁₂ , Li ₂ Ti ₂ O ₅ , LiTaO ₃ , LiNbO ₃ , LiAlO ₂ , Li ₂ ZrO ₃ , Li ₂ WO ₄ , Li ₂ TiO ₃ , Li ₃ PO ₄ , Li ₂ MoO ₄ , Li ₃ BO ₃ , LiBO ₂ , Li ₂ CO ₃ , and Li ₂ Examples of the oxide particles include _SiO3 and perovskite oxide particles represented by _ABO3 (wherein 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 the metal oxide particles, from the viewpoint of suitably suppressing side reactions that occur between the electrolyte and the coated positive electrode active material particles, aluminum oxide (Al ₂ O ₃ ), silicon dioxide (SiO ₂ ), and titania (TiO ₂ ) are preferred, with silicon dioxide (SiO ₂ ) being more preferred.

The ceramic particles may be glass ceramic particles from the viewpoint of suitably suppressing side reactions that occur between the electrolyte and the coated positive electrode active material particles.
These may be used alone or in combination of two or more.

The glass ceramic particles are preferably a lithium-containing phosphate compound having a rhombohedral crystal system, and the chemical formula thereof is Li _x M" ₂ P ₃ O ₁₂ (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. In addition, a portion of P may be substituted with Si or B, _and a portion of O may be substituted with F, Cl _{, or the like} . For _example , _{Li1.15Ti1.85Al0.15Si0.05P2.95O12} , _{Li1.2Ti1.8Al0.1Ge0.1Si0.05P2.95O12 , etc. can} be used.
Materials of different compositions may be mixed or composited, or the surface may be coated with a glass electrolyte, etc. Alternatively, it is preferable to use glass ceramic particles that precipitate a crystalline phase of a lithium-containing phosphate compound having a NASICON structure by heat treatment.
Examples of the glass electrolyte include the glass electrolyte described in JP-A-2019-96478.

Here, the blending ratio of Li ₂ O in the glass ceramic particles is preferably 8 mass % or less in terms of oxide.
Instead of a NASICON structure, a solid electrolyte may be used that is made of Li, La, Mg, Ca, Fe, Co, Cr, Mn, Ti, Zr, Sn, Y, Sc, P, Si, O, In, Nb, and F, has a LISICON, perovskite, β-Fe ₂ (SO ₄ ) ₃ type, or Li ₃ In ₂ (PO ₄ ) ₃ type crystal structure, and conducts Li ions at room temperature at 1×10 ⁻⁵ S/cm or more.

The above-mentioned ceramic particles may be used alone or in combination of two or more types.

From the viewpoints of energy density and electrical resistance, the volume average particle size of the ceramic particles is preferably 1 to 1000 nm, more preferably 1 to 500 nm, and even more preferably 1 to 150 nm.
In this specification, the volume average particle size refers to the particle size (Dv50) at 50% cumulative value in the particle size distribution determined by the Microtrac method (laser diffraction/scattering method). The Microtrac method is a method for determining particle size distribution by irradiating particles with laser light and utilizing scattered light obtained. Note that the volume average particle size can be measured using a Microtrac manufactured by Nikkiso Co., Ltd.

The weight ratio of the ceramic particles is preferably 1.0 to 5.0% by weight based on the weight of the coated positive electrode active material particles for lithium ion batteries.
By including the ceramic particles in the above range, it is possible to suitably suppress side reactions that occur between the electrolyte solution and the coated positive electrode active material particles. In addition, since the coating layer of the coated positive electrode active material particles has excellent flexibility, when the coated positive electrode active material particles are pressed to form a positive electrode active material layer, which will be described later, it is possible to form a positive electrode active material layer with high energy density.
The weight ratio of the ceramic particles is more preferably 2.0 to 4.0% by weight based on the weight of the coated positive electrode active material particles for lithium ion batteries.

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 positive electrode active material particles preferably have a coverage rate calculated by the following formula of 30 to 95%.
Coverage (%) = {1 - [BET specific surface area of coated positive electrode active material particles / (BET specific surface area of positive electrode active material particles × weight proportion of positive electrode active material particles contained in coated positive electrode active material + BET specific surface area of conductive additive × weight proportion of conductive additive contained in coated positive electrode active material particles + BET specific surface area of ceramic particles × weight proportion of ceramic particles contained in coated positive electrode active material particles)]} × 100

[Method of 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 the "method for producing coated positive electrode active material particles") includes a step of mixing positive electrode active material particles, a polymer compound, a conductive additive, ceramic particles, and an organic solvent, and then removing the solvent.

The organic solvent is not particularly limited as long as it is an organic solvent that can dissolve the polymer compound, and any known organic solvent can be appropriately selected and used.

In the method for producing coated positive electrode active material particles, first, the positive electrode active material particles, the polymer compound constituting the coating layer, the conductive additive, and the ceramic particles are mixed in an organic solvent.
The order in which the positive electrode active material particles, the polymer compound constituting the coating layer, the conductive additive, and the ceramic particles are mixed is not particularly limited. For example, a resin composition consisting of a premixed polymer compound constituting the coating layer, a conductive additive, and ceramic particles may be further mixed with the positive electrode active material particles; the positive electrode active material particles, the polymer compound constituting the coating layer, the conductive additive, and the ceramic particles may be mixed simultaneously; or the positive electrode active material particles may be mixed with the polymer compound constituting the coating layer, and then the conductive additive and the ceramic particles may be mixed.

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, a conductive additive, and ceramic particles. For example, the positive electrode active material particles are placed in a universal mixer and stirred at 30 to 500 rpm, and a resin solution containing the polymer compound that constitutes the coating layer is added dropwise over 1 to 90 minutes to mix, and the conductive additive and ceramic particles are mixed in. The mixture is then heated to 50 to 200°C while still stirring, and the pressure is reduced to 0.007 to 0.04 MPa, after which the mixture is held for 10 to 150 minutes to remove the solvent.

The blending ratio of the positive electrode active material particles to the resin composition containing the polymer compound, conductive additive, and ceramic particles that make up the coating layer is not particularly limited, but it is preferable that the weight ratio of positive electrode active material particles:resin composition be 1:0.001 to 0.1.

[Positive electrode for lithium-ion batteries]
The positive electrode for a lithium ion battery of the present invention (hereinafter also simply referred to as "positive electrode") comprises a positive electrode active material layer containing the coated positive electrode active material particles of the present invention and an electrolytic solution containing an electrolyte and a solvent.

From the viewpoint of dispersibility of the positive electrode active material particles and electrode formability, the amount of coated positive electrode active material particles contained in the positive electrode active material layer is preferably 40 to 95 wt %, and more preferably 60 to 90 wt %, based on the weight of the positive electrode active material layer.

As the electrolyte, electrolytes used in known electrolytic solutions can be used, such as lithium salts of inorganic anions such as _LiPF6 , _LiBF4 , _LiSbF6 , _LiAsF6 , _LiClO4 , and LiN( _FSO2 ) ₂ , and lithium salts of organic anions such as LiN( _CF3SO2 ) _{2 ,} LiN( _{C2F5SO2 ) 2 ,} and LiC( _CF3SO2 ) ₃ . Of these, LiN( _FSO2 ) ₂ is preferred from the viewpoint of battery output and charge _/ discharge cycle characteristics.

The solvent can be a non-aqueous solvent used in known electrolytic solutions, such as lactone compounds, cyclic or chain carbonate esters, chain carboxylic acid esters, cyclic or chain ethers, phosphate esters, nitrile compounds, amide compounds, sulfones, sulfolane, and mixtures thereof.

Lactone compounds include lactone compounds with five-membered rings (such as gamma-butyrolactone and gamma-valerolactone) and six-membered rings (such as delta-valerolactone).

Examples of cyclic carbonates include propylene carbonate, ethylene carbonate (EC), and butylene carbonate (BC).
Examples of the chain carbonate ester include dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), methyl-n-propyl carbonate, ethyl-n-propyl carbonate, and di-n-propyl carbonate.

Examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, and methyl propionate.

Cyclic ethers include tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, and 1,4-dioxane. Chain ethers include dimethoxymethane and 1,2-dimethoxyethane.

Examples of phosphate esters include trimethyl phosphate, triethyl phosphate, ethyl dimethyl phosphate, diethylmethyl phosphate, tripropyl phosphate, tributyl phosphate, tri(trifluoromethyl phosphate), tri(trichloromethyl phosphate), tri(trifluoroethyl phosphate), tri(triperfluoroethyl phosphate), 2-ethoxy-1,3,2-dioxaphospholan-2-one, 2-trifluoroethoxy-1,3,2-dioxaphospholan-2-one, and 2-methoxyethoxy-1,3,2-dioxaphospholan-2-one.

Nitrile compounds include acetonitrile, etc. Amide compounds include DMF, etc. Sulfones include dimethyl sulfone and diethyl sulfone, etc.

These solvents may be used alone or in combination of two or more.

The concentration of the electrolyte in the electrolytic solution is preferably 1.2 to 5.0 mol/L, more preferably 1.5 to 4.5 mol/L, even more preferably 1.8 to 4.0 mol/L, and particularly preferably 2.0 to 3.5 mol/L.
Such an electrolyte solution has an appropriate viscosity, and therefore can form a liquid film between the coated positive electrode active material particles, thereby imparting a lubricating effect (the ability to adjust the position of the coated active material particles) to the coated positive electrode active material particles.

The positive electrode active material layer may further contain a conductive additive in addition to the conductive additive contained as needed in the coating layer of the coated positive electrode active material particles described above. The conductive additive contained as needed in the coating layer is integral with the coated positive electrode active material particles, whereas the conductive additive contained in the positive electrode active material layer is contained separately from the coated positive electrode active material particles.
As the conductive additive that may be contained in the positive electrode active material layer, those described in [Coated positive electrode active material particles for lithium ion batteries] can be used.

When the positive electrode active material layer contains a conductive additive, the total content of the conductive additive contained in the positive electrode and the coating layer is preferably less than 4 wt %, and more preferably less than 3 wt %, based on the weight of the positive electrode active material layer excluding the electrolyte. On the other hand, the total content of the conductive additive contained in the positive electrode and the coating layer is preferably 2.5 wt % or more based on the weight of the positive electrode active material layer excluding the electrolyte.

The positive electrode active material layer preferably does not contain a binder.
In this specification, the term "binder" refers to an agent that cannot reversibly fix positive electrode active material particles to each other or to a current collector, and examples thereof include known solvent-dried binders for lithium ion batteries, such as starch, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, polyvinylpyrrolidone, tetrafluoroethylene, styrene-butadiene rubber, polyethylene, and polypropylene.
These binders are used by dissolving or dispersing them in a solvent, and are solidified by volatilizing or distilling off the solvent, thereby irreversibly fixing the positive electrode active material particles to each other and to the current collector.

The positive electrode active material layer may contain an adhesive resin. The adhesive resin refers to a resin that remains adhesive even after drying by volatilizing the solvent component, and is a material different from a binder and is therefore distinguished from the binder.
Furthermore, the coating layer constituting the coated positive electrode active material particles is fixed to the surface of the positive electrode active material particles, whereas the adhesive resin reversibly fixes the surfaces of the positive electrode active material particles together. While the adhesive resin can be easily separated from the surface of the positive electrode active material particles, the coating layer cannot be easily separated. Therefore, the coating layer and the adhesive resin are different materials.

The adhesive resin may be a polymer containing, as an essential constituent monomer, at least one low Tg monomer selected from the group consisting of vinyl acetate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, butyl acrylate, and butyl methacrylate, in which the total weight proportion of the low Tg monomer is 45% by weight or more based on the total weight of the constituent monomers.
When an adhesive resin is used, it is preferable to use 0.01 to 10% by weight of the adhesive resin based on the total weight of the positive electrode active material particles.

In the lithium ion battery positive electrode of the present invention, the weight ratio of the polymer compound contained in the lithium ion battery positive electrode is preferably 1 to 10% by weight based on the weight of the lithium ion battery positive electrode.
Here, the term "polymer compound" refers to the polymer compound, binder, and adhesive resin that constitute the coating layer, and in the positive electrode for a lithium ion battery of the present invention, the total weight ratio of the polymer compound and adhesive resin that constitute the coating layer is equal to the above-mentioned "weight ratio of polymer compound," and does not contain any binder (0 wt %).

In the positive electrode for a lithium ion battery of the present invention, the positive electrode active material layer is made of a non-bound body of coated positive electrode active material particles for a lithium ion battery.
Here, the term "non-bound material" means that the positions of the positive electrode active material particles are not fixed in the positive electrode active material layer, and the positive electrode active material particles are not irreversibly fixed to each other or to the current collector.
When the positive electrode active material layer is a non-binding material, the positive electrode active material particles are not irreversibly fixed to each other, and therefore can be separated without causing damage at the interface between the positive electrode active material particles, and even when stress is applied to the positive electrode active material layer, the positive electrode active material particles move, thereby preventing damage to the positive electrode active material layer, which is preferable.
The non-binding positive electrode active material layer can be obtained by forming a positive electrode active material layer slurry containing positive electrode active material particles, an electrolyte solution, etc. but not containing a binder.

From the viewpoint of battery performance, the thickness of the positive electrode active material layer is preferably 150 to 600 μm, and more preferably 200 to 470 μm.

The positive electrode for a lithium ion battery of the present invention can be produced, for example, by applying a powder (positive electrode precursor) obtained by mixing the coated positive electrode active material particles for a lithium ion battery of the present invention and, if necessary, a conductive additive or the like, to a current collector, pressing the powder with a press to form a positive electrode active material layer, and then injecting an electrolyte solution.
Alternatively, the positive electrode precursor may be applied to a release film and pressed to form a positive electrode active material layer, and the positive electrode active material layer may be transferred to a current collector, after which the electrolyte solution may be poured thereinto.
Alternatively, for example, the positive electrode active material layer may be produced by applying a positive electrode active material layer slurry containing the coated positive electrode active material particles of the present invention, an electrolytic solution containing an electrolyte and a solvent, and, if necessary, a conductive additive, to a current collector, followed by drying. Specifically, the positive electrode for a lithium ion battery may be produced by applying the positive electrode active material layer slurry to a current collector using a coating device such as a bar coater, removing the solvent by placing a nonwoven fabric on the positive electrode active material particles to absorb the liquid, and pressing the resulting mixture with a press, if necessary.

Examples of materials that can be used to form the current collector include metal materials such as copper, aluminum, titanium, stainless steel, nickel, and alloys thereof, as well as baked carbon, conductive polymer materials, and conductive glass.
The shape of the current collector is not particularly limited, and may be a sheet-shaped current collector made of the above-mentioned material, or a deposition layer made of fine particles made of the above-mentioned material.
The thickness of the current collector is not particularly limited, but is preferably 50 to 500 μm.

It is preferable that the positive electrode for a lithium-ion battery further includes a current collector, and that the positive electrode active material layer is provided on the surface of the current collector. For example, it is preferable that the positive electrode of the present invention includes a resin current collector made of a conductive polymer material, and that the positive electrode active material layer is provided on the surface of the resin current collector.

The conductive polymer material constituting the resin current collector may be, for example, a resin to which a conductive agent has been added.
As the conductive agent constituting the conductive polymer material, the same conductive assistant as the optional component of the coating layer can be suitably used.
Examples of resins that constitute the conductive polymer material include polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polycycloolefin (PCO), polyethylene terephthalate (PET), polyethernitrile (PEN), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVdF), epoxy resin, silicone resin, and mixtures thereof.
From the viewpoint of electrical stability, polyethylene (PE), polypropylene (PP), polymethylpentene (PMP) and polycycloolefin (PCO) are preferred, and polyethylene (PE), polypropylene (PP) and polymethylpentene (PMP) are more preferred.
The resin current collector can be obtained by a known method described in JP-A-2012-150905 and WO 2015/005116, for example.

[Lithium-ion battery]
A lithium ion battery can be obtained by combining the positive electrode of the present invention with a counter electrode, housing the electrode together with a separator in a cell container, injecting an electrolyte, and sealing the cell container.
Alternatively, the positive electrode of the present invention may be formed on one surface of a current collector and a negative electrode on the other surface to prepare a bipolar electrode, which may then be stacked with a separator and housed in a cell container, into which an electrolyte solution is poured, and the cell container may be sealed.

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 (such as polyester fibers and aramid fibers) or glass fibers, and those with ceramic particles such as silica, alumina, or titania attached to their surfaces.

Next, the present invention will be explained in detail using examples, but the present invention is not limited to these examples as long as it does not deviate from the gist of the present invention. Unless otherwise specified, parts mean parts by weight and % means % by weight.

<Preparation of Coating Polymer Compound>
A four-necked flask equipped with a stirrer, thermometer, reflux condenser, dropping funnel, and nitrogen gas inlet tube was charged with 150 parts of DMF and heated to 75 ° C. Next, a monomer composition containing 91 parts of acrylic acid, 9 parts of methyl methacrylate, and 50 parts of DMF, and an initiator solution containing 0.3 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) and 0.8 parts of 2,2'-azobis(2-methylbutyronitrile) dissolved in 30 parts of DMF were added dropwise to the four-necked flask over a period of 2 hours using a dropping funnel while blowing nitrogen into the flask, and radical polymerization was carried out. After completion of the addition, the reaction was continued for 3 hours at 75 ° C. The temperature was then raised to 80 ° C. and the reaction was continued for 3 hours, yielding a copolymer solution with a resin concentration of 30%. The obtained copolymer solution was transferred to a Teflon (registered trademark) tray and dried under reduced pressure at 150°C and 0.01 MPa for 3 hours, and the DMF was distilled off to obtain a copolymer. This copolymer was coarsely pulverized with a hammer and then further pulverized in a mortar to obtain a powdery coating polymer compound.

<Preparation of electrolyte solution>
An electrolyte solution was prepared by dissolving LiN(FSO ₂ ) 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).

<Ceramic particles>
The following materials were prepared as ceramic particles.
SiO ₂ (silicon dioxide particles, BET specific surface area 72.5 ^{m 2} /g, product name SiO ₂ , manufactured by Kanto Chemical Co., Ltd.)
AEROSIL R972 (silicon dioxide, BET specific surface area 110 m ² /g, product name "AEROSIL R972", manufactured by Nippon Aerosil Co., Ltd.)
REOLOSIL DM-10 (silicon dioxide, BET specific surface area 115 m ² /g, product name "REOLOSIL DM-10", manufactured by Tokuyama Corporation)
REOLOSIL MT-10 (silicon dioxide, BET specific surface area 126 m ² /g, product name "REOLOSIL MT-10", manufactured by Tokuyama Corporation)
NIPSIL NA (silicon dioxide, BET specific surface area 140 m ² /g, product name "NIPSIL NA", manufactured by Tosoh Corporation)
NIPSIL NS-T (silicon dioxide, BET specific surface area 160 m ² /g, product name "NIPSIL NS-T", manufactured by Tosoh Corporation)
AEROSIL R974 (silicon dioxide, BET specific surface area 170 m ² /g, product name "AEROSIL R974", manufactured by Nippon Aerosil Co., Ltd.)
ULTRASIL VN3 (silicon dioxide, BET specific surface area 170 m ² /g, product name "ULTRASIL VN3", manufactured by Evonik)
AEROSIL 200 (silicon dioxide, BET specific surface area 200 m ² /g, product name "AEROSIL 200", manufactured by Nippon Aerosil Co., Ltd.)
AEROSIL 300 (silicon dioxide, BET specific surface area 300 m ² /g, product name "AEROSIL 300", manufactured by Nippon Aerosil Co., Ltd.)
Al ₂ O ₃ (aluminum oxide, BET specific surface area 71.2 ^{m 2} /g, product name Al ₂ O ₃ , manufactured by Kanto Chemical Co., Inc.)
TiO ₂ (titania, BET specific surface area 73.6 ^{m 2} /g, product name TiO ₂ , manufactured by Kanto Chemical Co., Ltd.)
AEROSIL 50 (silicon dioxide, BET specific surface area 50 m ² /g, product name "AEROSIL 50", manufactured by Nippon Aerosil Co., Ltd.)
The BET specific surface area of the ceramic particles was measured based on "JIS Z 8830:2013 Method for measuring specific surface area of powder (solid) by gas adsorption" using, for example, the following device and measurement conditions.
Measuring device: Mountec Co., Ltd. Macsorb (registered trademark) HMmodel-1201
Adsorption gas: _N2
Dead volume measurement gas: mixed gas (N ₂ 30% + He 70%)
Adsorption temperature: 77K
Pre-treatment before measurement: Drying in a nitrogen atmosphere at 100°C for 5 minutes

Example 1
[Preparation of Coated Positive Electrode Active Material Particles]
One part of the coating polymer compound was dissolved in three parts of DMF to obtain a coating polymer compound solution.
90.12 parts of positive _electrode active material particles ( _{LiNi0.8Co0.15Al0.05O2 powder} , volume average particle diameter _: 4 μm) were placed in a universal mixer, high-speed mixer FS25 [manufactured by EarthTechnica Corporation], and while stirring at room temperature and 720 rpm, 12.56 parts of the coating polymer compound solution was added dropwise over 2 minutes, followed by stirring for an additional 5 minutes.
Next, while stirring, 3.14 parts of acetylene black (Denka Black (registered trademark) manufactured by Denka Co., Ltd.) as a conductive additive and 2.10 parts of ceramic particles (SiO ₂ ) were added in portions over 2 minutes, and stirring was continued for 30 minutes.
Thereafter, the pressure was reduced to 0.01 MPa while stirring was continued, and then the temperature was raised to 140°C while stirring and the degree of vacuum were maintained. The stirring, degree of vacuum and temperature were maintained for 8 hours to distill off the volatile matter.
The obtained powder was classified using a sieve with 200 μm openings to obtain coated positive electrode active material particles.

[Preparation of resin current collector]
A resin mixture was obtained by melt-kneading 70 parts of polypropylene (trade name "SunAllomer PL500A", manufactured by SunAllomer Co., Ltd.), 25 parts of carbon nanotubes (trade name "FloTube 9000", manufactured by CNano Corporation), and 5 parts of a dispersant (trade name "UMEX 1001", manufactured by Sanyo Chemical Industries, Ltd.) in a twin-screw extruder at 200°C and 200 rpm.
The resin mixture was passed through a T-die extrusion film molding machine and stretched and rolled to obtain a conductive film for a resin current collector having a thickness of 100 μm. The conductive film for a resin current collector was then cut into circles with a diameter of 15 mm or 16 mm, and nickel was vapor-deposited on one side, followed by connecting a terminal (5 mm × 3 cm) for current extraction to obtain a resin current collector.
A circular resin current collector having a diameter of 15 mm was used as the positive electrode resin current collector, and a circular resin current collector having a diameter of 16 mm was used as the negative electrode resin current collector.

[Preparation of positive electrode for lithium ion battery]
A positive electrode precursor was prepared by mixing 98.50 parts of the prepared coated positive electrode active material particles, 2.06 parts of carbon fiber [DonaCarbo Milled S-243 manufactured by Osaka Gas Chemicals Co., Ltd.: average fiber length 500 μm, average fiber diameter 13 μm: electrical conductivity 200 mS/cm], and 1.03 parts of Ketjen Black [EC300J manufactured by Lion Specialty Chemicals Co., Ltd.].
The prepared positive electrode precursor was filled into a Φ15 mold so that the amount of positive electrode active material per unit area was 50 mg/ ^cm² , and the mixture was tableted using a press (HANDTAB-100T15, manufactured by Ichihashi Seiki Co., Ltd.) at a pressure of ¹ ton/cm² to form a positive electrode active material layer (thickness: 213 µm). This was then laminated on one side of the resin current collector to prepare a positive electrode for a lithium ion battery (circular, 15 mm in diameter) according to Example 1.

[Preparation of Coated Negative Electrode Active Material Particles]
One part of the 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 diameter 25 μm) were placed in a universal mixer, high-speed mixer FS25 [manufactured by EarthTechnica Corporation], and while stirring at room temperature and 720 rpm, 37.92 parts of the coating polymer compound solution was added dropwise over 2 minutes, followed by further stirring for 5 minutes.
Next, while stirring, 9.48 parts of acetylene black (Denka Black (registered trademark) manufactured by Denka Co., Ltd.) as a conductive additive was added in portions over 2 minutes, and stirring was continued for 30 minutes.
Thereafter, the pressure was reduced to 0.01 MPa while stirring was continued, and then the temperature was raised to 140°C while stirring and the degree of vacuum were maintained. The stirring, degree of vacuum and temperature were maintained for 8 hours to distill off the volatile matter.
The obtained powder was classified using a sieve with 200 μm openings to obtain coated negative electrode active material particles.

[Preparation of negative electrode for lithium ion battery]
99 parts of the prepared coated negative electrode active material particles and 1 part of carbon fiber [DonaCarbo Milled S-243, manufactured by Osaka Gas Chemicals Co., Ltd.: average fiber length 500 μm, average fiber diameter 13 μm: electrical conductivity 200 mS/cm] were mixed to prepare a negative electrode precursor.
The prepared negative electrode precursor was filled into a Φ16 mold so that the amount of negative electrode active material per unit area was 23.4 mg/ ^cm² , and the mixture was tableted using a press (HANDTAB-100T15, manufactured by Ichihashi Seiki Co., Ltd.) at a pressure of ¹ ton/cm² to form a negative electrode active material layer (thickness: 300 µm). This was then laminated on one side of the resin current collector to prepare a lithium ion battery negative electrode (circular, diameter: 16 mm).

[Fabrication of lithium-ion battery]
The prepared positive electrode for a lithium ion battery and the negative electrode for a lithium ion battery were combined with a separator (#3501 manufactured by Celgard) interposed therebetween to prepare a lithium ion battery.

<Examples 2 to 10>
Coated positive electrode active material particles were prepared in the same manner as in Example 1 except that the ceramic particles were changed to those shown in Table 1, and positive electrodes for lithium ion batteries and lithium ion batteries were prepared in the same manner as in Example 1.

Example 11
[Preparation of Coated Positive Electrode Active Material Particles]
One part of the coating polymer compound was dissolved in three parts of DMF to obtain a coating polymer compound solution.
90.21 parts of positive _electrode active material particles ( _{LiNi0.8Co0.15Al0.05O2 powder} , volume average particle diameter _: 4 μm) were placed in a universal mixer, high-speed mixer FS25 [manufactured by EarthTechnica Corporation], and while stirring at room temperature and 720 rpm, 12.60 parts of the coating polymer compound solution were added dropwise over 2 minutes, followed by stirring for an additional 5 minutes.
Next, while stirring, 3.15 parts of acetylene black (Denka Black (registered trademark) manufactured by Denka Co., Ltd.) as a conductive additive and 2.00 parts of ceramic particles (SiO ₂ ) were added in portions over 2 minutes, and stirring was continued for 30 minutes.
Thereafter, the pressure was reduced to 0.01 MPa while stirring was continued, and then the temperature was raised to 140°C while stirring and the degree of vacuum were maintained. The stirring, degree of vacuum and temperature were maintained for 8 hours to distill off the volatile matter.
The obtained powder was classified using a sieve with 200 μm openings to obtain coated positive electrode active material particles.
Coated positive electrode active material particles were produced in the same manner as in Example 1 except that the above coated positive electrode active material particles were used, and a positive electrode for a lithium ion battery and a lithium ion battery were produced in the same manner as in Example 1.

Example 12
[Preparation of Coated Positive Electrode Active Material Particles]
One part of the coating polymer compound was dissolved in three parts of DMF to obtain a coating polymer compound solution.
87.33 parts of positive _electrode active material particles ( _{LiNi0.8Co0.15Al0.05O2 powder} , volume average particle diameter _: 4 μm) were placed in a universal mixer, high-speed mixer FS25 [manufactured by EarthTechnica Corporation], and while stirring at room temperature and 720 rpm, 12.20 parts of the coating polymer compound solution were added dropwise over 2 minutes, followed by stirring for an additional 5 minutes.
Next, while stirring, 3.05 parts of acetylene black (Denka Black (registered trademark) manufactured by Denka Co., Ltd.) as a conductive additive and 5.08 parts of ceramic particles (SiO ₂ ) were added in portions over 2 minutes, and stirring was continued for 30 minutes.
Thereafter, the pressure was reduced to 0.01 MPa while stirring was continued, and then the temperature was raised to 140°C while stirring and the degree of vacuum were maintained. The stirring, degree of vacuum and temperature were maintained for 8 hours to distill off the volatile matter.
The obtained powder was classified using a sieve with 200 μm openings to obtain coated positive electrode active material particles.
Coated positive electrode active material particles were produced in the same manner as in Example 1 except that the above coated positive electrode active material particles were used, and a positive electrode for a lithium ion battery and a lithium ion battery were produced in the same manner as in Example 1.

Example 13
[Preparation of Coated Positive Electrode Active Material Particles]
One part of the coating polymer compound was dissolved in three parts of DMF to obtain a coating polymer compound solution.
82.33 parts of positive _electrode active material particles ( _{LiNi0.8Co0.15Al0.05O2 powder} , volume average particle diameter _: 4 μm) were placed in a universal mixer, high-speed mixer FS25 [manufactured by EarthTechnica Corporation], and while stirring at room temperature and 720 rpm, 11.56 parts of the coating polymer compound solution were added dropwise over 2 minutes, followed by stirring for an additional 5 minutes.
Next, while stirring, 2.89 parts of acetylene black (Denka Black (registered trademark) manufactured by Denka Co., Ltd.) as a conductive additive and 10.00 parts of ceramic particles (SiO ₂ ) were added in portions over 2 minutes, and stirring was continued for 30 minutes.
Thereafter, the pressure was reduced to 0.01 MPa while stirring was continued, and then the temperature was raised to 140°C while stirring and the degree of vacuum were maintained. The stirring, degree of vacuum and temperature were maintained for 8 hours to distill off the volatile matter.
The obtained powder was classified using a sieve with 200 μm openings to obtain coated positive electrode active material particles.
Coated positive electrode active material particles were produced in the same manner as in Example 1 except that the above coated positive electrode active material particles were used, and a positive electrode for a lithium ion battery and a lithium ion battery were produced in the same manner as in Example 1.

Example 14
[Preparation of Coated Positive Electrode Active Material Particles]
One part of the coating polymer compound was dissolved in three parts of DMF to obtain a coating polymer compound solution.
87.33 parts of positive _electrode active material particles ( _{LiNi0.8Co0.15Al0.05O2 powder} , volume average particle diameter _: 4 μm) were placed in a universal mixer, high-speed mixer FS25 [manufactured by EarthTechnica Corporation], and while stirring at room temperature and 720 rpm, 12.20 parts of the coating polymer compound solution were added dropwise over 2 minutes, followed by stirring for an additional 5 minutes.
Next, while stirring, 3.05 parts of acetylene black (Denka Black (registered trademark) manufactured by Denka Co., Ltd.) as a conductive additive and 5.08 parts of ceramic particles (Al ₂ O ₃ ) were added in portions over 2 minutes, and stirring was continued for 30 minutes.
Thereafter, the pressure was reduced to 0.01 MPa while stirring was continued, and then the temperature was raised to 140°C while stirring and the degree of vacuum were maintained. The stirring, degree of vacuum and temperature were maintained for 8 hours to distill off the volatile matter.
The obtained powder was classified using a sieve with 200 μm openings to obtain coated positive electrode active material particles.
Coated positive electrode active material particles were produced in the same manner as in Example 1 except that the above coated positive electrode active material particles were used, and a positive electrode for a lithium ion battery and a lithium ion battery were produced in the same manner as in Example 1.

Example 15
[Preparation of Coated Positive Electrode Active Material Particles]
One part of the coating polymer compound was dissolved in three parts of DMF to obtain a coating polymer compound solution.
87.33 parts of positive _electrode active material particles ( _{LiNi0.8Co0.15Al0.05O2 powder} , volume average particle diameter _: 4 μm) were placed in a universal mixer, high-speed mixer FS25 [manufactured by EarthTechnica Corporation], and while stirring at room temperature and 720 rpm, 12.20 parts of the coating polymer compound solution were added dropwise over 2 minutes, followed by stirring for an additional 5 minutes.
Next, while stirring, 3.05 parts of acetylene black (Denka Black (registered trademark) manufactured by Denka Co., Ltd.) as a conductive additive and 5.08 parts of ceramic particles (TiO ₂ ) were added in portions over 2 minutes, and stirring was continued for 30 minutes.
Thereafter, the pressure was reduced to 0.01 MPa while stirring was continued, and then the temperature was raised to 140°C while stirring and the degree of vacuum were maintained. The stirring, degree of vacuum and temperature were maintained for 8 hours to distill off the volatile matter.
The obtained powder was classified using a sieve with 200 μm openings to obtain coated positive electrode active material particles.
Coated positive electrode active material particles were produced in the same manner as in Example 1 except that the above coated positive electrode active material particles were used, and a positive electrode for a lithium ion battery and a lithium ion battery were produced in the same manner as in Example 1.

<Comparative Example 1>
[Preparation of Coated Positive Electrode Active Material Particles]
One part of the coating polymer compound was dissolved in three parts of DMF to obtain a coating polymer compound solution.
92.22 parts of positive _electrode active material particles ( _{LiNi0.8Co0.15Al0.05O2 powder} , volume average particle diameter _: 4 μm) were placed in a universal mixer, high-speed mixer FS25 [manufactured by EarthTechnica Corporation], and while stirring at room temperature and 720 rpm, 12.56 parts of the coating polymer compound solution was added dropwise over 2 minutes, followed by stirring for an additional 5 minutes.
Next, while stirring, 3.14 parts of acetylene black (Denka Black (registered trademark) manufactured by Denka Co., Ltd.) as a conductive additive was added in portions over 2 minutes, and stirring was continued for 30 minutes.
Thereafter, the pressure was reduced to 0.01 MPa while stirring was continued, and then the temperature was raised to 140°C while stirring and the degree of vacuum were maintained. The stirring, degree of vacuum and temperature were maintained for 8 hours to distill off the volatile matter.
The obtained powder was classified using a sieve with 200 μm openings to obtain coated positive electrode active material particles.
Coated positive electrode active material particles were produced in the same manner as in Example 1 except that the above coated positive electrode active material particles were used, and a positive electrode for a lithium ion battery and a lithium ion battery were produced in the same manner as in Example 1.

<Comparative Example 2>
Coated positive electrode active material particles were prepared in the same manner as in Example 1 except that the ceramic particles were changed to those shown in Table 1, and positive electrodes for lithium ion batteries and lithium ion batteries were prepared in the same manner as in Example 1.

<Measurement of internal resistance>
The lithium ion batteries obtained in each of the examples and comparative examples were charged and discharged once at 25° C. Then, they were fully charged and stored in a 60° C. environment.
Using an impedance measuring device (Chemical Impedance Analyzer IM3590, manufactured by Hioki E.E. Corporation), the internal resistance values at a frequency of 1100 Hz were measured after 0 days (immediately after full charge), after 7 days of storage, after 14 days of storage, and after 21 days of storage, and the rate of increase in the internal resistance value after 21 days of storage relative to after 0 days, <[(internal resistance value after 21 days of storage−internal resistance value after 0 days)/internal resistance value after 0 days)]×100(%)> was calculated.
The results are shown in Table 1.

<Thickness of the positive electrode for lithium-ion batteries>
For the lithium ion batteries obtained in each of the Examples and Comparative Examples, the thickness of the positive electrode active material layer for lithium ion batteries was measured using a digital film thickness meter [Digimatic indicator: ID-C112CXB (manufactured by Mitutoyo Corporation), stand: 7007-10 (manufactured by Mitutoyo Corporation)].
From the viewpoint of the energy density of the positive electrode for a lithium ion battery, it was determined that the thickness of the positive electrode for a lithium ion battery is preferably 230 μm or less. The results are shown in Table 1.

From Table 1, it was confirmed that in the examples in which the coating layer contained ceramic particles having a predetermined BET specific surface area, an increase in the internal resistance value of the lithium ion battery could be prevented.
Furthermore, by comparing Examples 11 to 13, it was confirmed that a positive electrode for a lithium ion battery having a high energy density can be obtained by setting the weight ratio of the ceramic particles in the range of 1.0 to 5.0 wt % based on the weight of the coated positive electrode active material particles for a lithium ion battery.

The coated positive electrode active material particles of the present invention are particularly useful as positive electrode active materials for lithium ion batteries used in mobile phones, personal computers, hybrid vehicles, and electric vehicles.

Claims (5)

Coated positive electrode active material particles for lithium ion batteries, in which at least a portion of the surface of the positive electrode active material particles is coated with a coating layer,
the coating layer contains a polymer compound, a conductive additive, and ceramic particles,
The coated positive electrode active material particles for a lithium ion battery, wherein the ceramic particles have a BET specific surface area of 110 to 300 m ² /g. The coated positive electrode active material particles for a lithium ion battery according to claim ₁ , wherein the ceramic particles are SiO2. The coated positive electrode active material particles for lithium ion batteries according to claim 1 or 2, wherein the weight proportion of the ceramic particles is 1.0 to 5.0 wt % based on the weight of the coated positive electrode active material particles for lithium ion batteries. A positive electrode for a lithium ion battery comprising a positive electrode active material layer containing the coated positive electrode active material particles for a lithium ion battery according to any one of claims 1 to 3 and an electrolytic solution containing an electrolyte and a solvent,
The positive electrode active material layer is a positive electrode for a lithium ion battery comprising a non-bound body of the coated positive electrode active material particles for a lithium ion battery. A method for producing coated positive electrode active material particles for lithium ion batteries according to any one of claims 1 to 3, comprising a step of mixing positive electrode active material particles, a polymer compound, a conductive additive, ceramic particles, and an organic solvent, and then removing the solvent.