JP2020047549A - Coated positive electrode active material for lithium ion battery, positive electrode slurry for lithium ion battery, positive electrode for lithium ion battery, and lithium ion battery - Google Patents

Abstract

To provide a coated positive electrode active material which makes it possible to obtain a lithium ion battery having good cycle characteristics and a high capacity density.SOLUTION: A coated positive electrode active material for a lithium ion battery comprises a positive electrode active material, of which at least part of the surface is covered with a coating film containing a high-molecular weight compound. The positive electrode active material comprises a composite oxide represented by the structural formula, LiNiMO(where x is 0.3-1.0, and M is at least one element selected from a group consisting of Co, Mn and Al). The high-molecular weight compound is a polymer of monomer compositions, which contains acrylic acid of over 90 wt% and less than or equal to 98 wt% to a total weight of monomers. The content of the high-molecular compound is 0.01-1 wt% to a weight of the positive electrode active material.SELECTED DRAWING: None

Description

The present invention relates to a coated positive electrode active material for a lithium ion battery, a positive electrode slurry for a lithium ion battery, a positive electrode for a lithium ion battery, and a lithium ion battery.

In recent years, reduction of carbon dioxide emission has been urgently required for environmental protection. The automobile industry is expected to reduce carbon dioxide emissions through the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs), and is keenly developing motor-driven secondary batteries that are key to their practical use. Is being done. As a secondary battery, attention is being paid to a lithium ion battery that can achieve high energy density and high output density.

For example, Patent Document 1 discloses a polymer of a monomer composition comprising an ester compound of a monovalent aliphatic alcohol having 1 to 12 carbon atoms and (meth) acrylic acid and an anionic monomer. An active material coating resin composition containing a polymer having an acid value of 30 to 700, and a coating layer containing the active material coating resin composition on at least a part of the surface of the active material. A coated active material is disclosed.

JP 2017-160294 A

According to Patent Literature 1, by covering the surface of the active material with a specific resin, the internal resistance of the battery can be suppressed, and as a result, cycle characteristics are improved. However, on the other hand, if the amount of the coating resin is large, the amount of the active material per active material layer constituting the electrode is relatively reduced, so that there is a problem that it is difficult to improve the battery capacity.

Further, in order to meet the demand for higher electric capacity of the battery and higher safety, there is a tendency to use an active material having a higher electric capacity and an electrolytic solution having a higher boiling point. These materials are often highly polar materials, and conventional active material coating resins may not have sufficient affinity with these highly polar materials. Therefore, when a material having a high polarity is used, it is necessary to take measures such as increasing the amount of the conductive additive to maintain the battery performance. However, when the amount of the conductive additive increases, the amount of the active material per active material layer constituting the electrode relatively decreases, and thus there is a problem that it is difficult to improve the battery capacity.

Thus, when an active material having high polarity is used, it can be said that there is room for improvement in obtaining a lithium ion battery having good cycle characteristics and high capacity density.

An object of the present invention is to provide a coated positive electrode active material that can provide a lithium ion battery having good cycle characteristics and high capacity density. The present invention also provides a positive electrode slurry for a lithium ion battery including the coated positive electrode active material, a positive electrode for a lithium ion battery including the positive electrode active material layer including the coated positive electrode active material, and a lithium ion battery including the positive electrode. The purpose is to:

The present inventors have conducted intensive studies to solve the above problems, and as a result, by using a polymer compound that is optimized when the positive electrode active material is highly polar as a coating resin, the content of the polymer compound is reduced. The inventors have found that the battery performance is excellent, and have reached the present invention.

That is, the present invention is a coated positive electrode active material for a lithium ion battery in which at least a part of the surface of the positive electrode active material is coated with a coating film containing a polymer compound, wherein the positive electrode active material has a structural formula of LiNi _x A composite oxide represented by M _1-x O ₂ (where x is 0.3 to 1.0, and M is at least one element selected from the group consisting of Co, Mn and Al) Wherein the polymer compound is a polymer of a monomer composition containing acrylic acid in an amount of more than 90% by weight and 98% by weight or less based on the weight of the entire monomer, and the content of the polymer compound Wherein the content is 0.01 to 1% by weight based on the weight of the positive electrode active material; containing the coated positive electrode active material for a lithium ion battery, an electrolyte, and a solvent. Electrolyte containing A positive electrode slurry for a lithium ion battery, comprising: a positive electrode active material layer containing the coated positive electrode active material for a lithium ion battery and an electrolytic solution containing an electrolyte and a solvent; The material layer is a positive electrode for a lithium ion battery, comprising a non-binding body of the coated positive electrode active material for a lithium ion battery; a lithium ion battery, comprising the positive electrode for a lithium ion battery.

According to the present invention, a lithium ion battery having good cycle characteristics and high capacity density can be obtained.

[Coated positive electrode active material for lithium ion batteries]
The coated positive electrode active material for a lithium ion battery of the present invention (hereinafter, also simply referred to as “coated positive electrode active material”) is a coated positive electrode in which at least a part of the surface of the positive electrode active material is coated with a coating film containing a polymer compound. An active material, wherein the positive electrode active material has a structural formula of LiNi _x M _1-x O ₂ (where x is 0.3 to 1.0, and M is selected from the group consisting of Co, Mn, and Al). At least one element selected from the group consisting of a composite oxide represented by the general formula (1), and the polymer compound contains acrylic acid in an amount of more than 90% by weight and not more than 98% by weight based on the weight of the whole monomer. A polymer of the body composition, wherein the content of the polymer compound is 0.01 to 1% by weight based on the weight of the positive electrode active material.

In the coated positive electrode active material of the present invention, by increasing the polarity of the polymer compound constituting the coating film, the affinity with the highly polar positive electrode active material and the electrolytic solution is increased. In addition, the carboxyl group of the polymer compound constituting the coating film increases the affinity with the conductive additive. Therefore, even when the content of the polymer compound is as small as 0.01 to 1% by weight, the positive electrode active material is uniformly coated to suppress side reactions, and the swelling is moderately caused by the electrolyte so that the ionic conductivity is reduced. It also guarantees sex. Furthermore, since the dispersibility of the conductive additive is also improved, an increase in electrical resistance due to charge and discharge can be suppressed even when the amount of the conductive additive is reduced. As a result, the cycle characteristics of the lithium ion battery are improved.
Further, in the coated positive electrode active material of the present invention, since the amount of the positive electrode active material is relatively increased, the capacity density of the lithium ion battery is increased.

In the coated positive electrode active material of the present invention, the content of the polymer compound forming the coating film is 0.01 to 1% by weight based on the weight of the positive electrode active material. From the viewpoint of the energy density of the battery, the content of the polymer compound constituting the coating film is preferably 0.05 to 0.75% by weight based on the weight of the positive electrode active material, and 0.1 to 0%. More preferably, it is 0.5% by weight.

The polymer compound constituting the coating film is a resin containing a polymer having an acrylic monomer (a) as an essential constituent monomer.
Specifically, the polymer compound constituting the coating film is 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 more than 90% by weight and not more than 98% by weight based on the weight of the whole monomer. In light of the flexibility of the coating layer, the content of acrylic acid (a0) is preferably 93.0 to 97.5% by weight, and 95.0 to 97.0% by weight based on the weight of the entire monomer. More preferably, it is% by weight.

The polymer compound constituting the coating film 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 the monomer (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; C4 to C24 dicarboxylic acids such as acid, (anhydrous) itaconic acid, citraconic acid and mesaconic acid; C3 to C4 trivalent to tetravalent or higher valent polycarboxylic acids such as aconitic acid Is mentioned.

The polymer compound constituting the coating film may contain a monomer (a2) represented by the following general formula (1) as the acrylic monomer (a).
CH ₂ ＣC (R ¹ ) COOR ² (1)
[In the formula (1), R ¹ is a hydrogen atom or a methyl group, and R ² is a straight chain 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. R ¹ is preferably 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.

(A21) an ester compound wherein 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, Nonyl, decyl, undecyl and dodecyl groups are exemplified.
Examples of the branched alkyl group 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 1,1,1-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, 2-methylhexyl, 2-methylhexyl, 4-methylhexyl, 5-methylhexyl, 1-ethylpentyl, 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, 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, 2-ethylheptyl, 1-methylnonyl, 2-methylnonyl, 3-methylnonyl, 4-methylnonyl, 5-methylnonyl, 6-methylnonyl, 7-methylnonyl, 8-methylnonyl 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-methyl Rudecyl 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 1,3-dimethylnonyl, 1,4-dimethylnonyl, 1,5-dimethylnonyl, 1,6-dimethylnonyl, 1,7-dimethylnonyl, 1,8-dimethylnonyl, 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 and the like. Among these, a 2-ethylhexyl group is particularly preferred.

(A22) an ester compound 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 a 1-alkylalkyl group [1-methyldodecyl group, 1-butyleicosyl group, 1-hexyloctadecyl group, 1-octylhexadecyl group, 1-decyltetradecyl group, 1-undecyltridecyl group, etc.], 2-alkylalkyl group [2-methyldodecyl group, 2-hexyloctadecyl group, 2- Octyl hexadecyl group, 2-decyl tetradecyl group, 2-undecyl tridecyl group, 2-dodecyl hexadecyl group, 2-tridecyl pentadecyl group, 2-decyl octadecyl group, 2-tetradecyl octadecyl group, 2- Hexadecyl octadecyl group, 2-tetradecyl eicosyl group, 2-hexadecyl eicosyl group, etc.], 3 to 34-alkylalkyl groups (such as 3-alkylalkyl groups, 4-alkylalkyl groups, 5-alkylalkyl groups, 32-alkylalkyl groups, 33-alkylalkyl groups and 34-alkylalkyl groups), and propylene oligomers (7 To 11-mers), ethylene / propylene (molar ratio 16/1 to 1/11) oligomer, isobutylene oligomer (7 to 8 mer) and α-olefin (5 to 20 carbon atoms) oligomer (4 to 8 mer) And the like, and a mixed alkyl group containing one or more branched alkyl groups such as a residue obtained by removing a hydroxyl group from an oxo alcohol obtained from the above.

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

The polymer compound constituting the coating film is a polymer of a monomer composition containing acrylic acid (a0) and at least one of a monomer (a1), a monomer (a2) and an ester compound (a3). More preferably, it is 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). More preferably, the monomer is 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). Most preferably, it is a polymer of a monomer composition containing (a0) and any one of the monomer (a1), the ester compound (a21), and the ester compound (a3). Arbitrariness.
Examples of the polymer compound constituting the coating film include, for example, a copolymer of acrylic acid and maleic acid using maleic acid as the monomer (a1), and acrylic using 2-ethylhexyl methacrylate as the monomer (a2). A copolymer of acid and 2-ethylhexyl methacrylate, a copolymer of acrylic acid and methyl methacrylate using methyl methacrylate as the ester compound (a3), and the like can be given.

The total content of the monomer (a1), the monomer (a2) and the ester compound (a3) is from 2.0 to 9.9% by weight based on the weight of the entire monomer from the viewpoint of suppressing the volume change of the electrode active material. %, More preferably 2.5 to 7.0% by weight.

It is preferable that the polymer compound constituting the coating film does not contain, as the acrylic monomer (a), a salt (a4) of an anionic monomer 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.
The anionic monomer having a polymerizable unsaturated double bond and an anionic group is a compound obtained by a combination thereof, and examples thereof include vinyl sulfonic acid, allyl sulfonic acid, styrene sulfonic acid, and (meth) acrylic acid. Can be
In addition, a (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.

Further, the polymer composing the coating film is used as an acrylic monomer (a) together with acrylic acid (a0), monomer (a1), monomer (a2) and ester compound (a3) as long as the physical properties are not impaired. It may contain a polymerizable radical polymerizable monomer (a5).
As the radical polymerizable monomer (a5), a monomer containing no active hydrogen is preferable, and the following monomers (a51) to (a58) can be used.

(A51) C13-20 linear aliphatic monool, C5-20 alicyclic monol or C7-20 araliphatic monol and hydro formed from (meth) acrylic acid The carbyl (meth) acrylate monools include (i) linear aliphatic monols (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) araliphatic monols (benzyl alcohol, etc.) and mixtures of two or more of these. No.

(A52) poly (n = 2 to 30) oxyalkylene (C2 to C4) alkyl (C1 to C18) ether (meth) acrylate [methanol ethylene oxide (hereinafter abbreviated as EO) 10 mol adduct (meth) ) Acrylate, propylene oxide of methanol (hereinafter abbreviated as PO) 10 mol adduct (meth) acrylate, etc.]

(A53) Nitrogen-containing vinyl compound (a53-1) Amide group-containing vinyl compound (i) (meth) acrylamide compound having 3 to 30 carbon atoms, for example, N, N-dialkyl (1 to 6 carbon atoms) or diaralkyl (carbon number 7-15) (meth) acrylamide (N, N-dimethylacrylamide, N, N-dibenzylacrylamide, etc.), diacetone acrylamide (ii) Excluding the above (meth) acrylamide compound, containing an amide group having 4 to 20 carbon atoms Vinyl compound, for example, N-methyl-N-vinylacetamide, cyclic amide [pyrrolidone compound (C6-13, for example, N-vinylpyrrolidone)]

(A53-2) (meth) acrylate compound (i) dialkyl (C1-4) aminoalkyl (C1-4) (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) acrylate ｛tertiary amino group-containing (meth) acrylate [N, N-dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, etc.] (Quaternized with a quaternizing agent such as methyl chloride, dimethyl sulfate, benzyl chloride, dimethyl carbonate), etc.

(A53-3) Heterocycle-containing vinyl compound Pyridine compound (C7-14, for example, 2- or 4-vinylpyridine), imidazole compound (C5-12, for example, N-vinylimidazole), pyrrole compound (C5 6-13, for example, N-vinylpyrrole), pyrrolidone compound (C6-C13, for example, N-vinyl-2-pyrrolidone)

(A53-4) Nitrile group-containing vinyl compound Nitrile group-containing vinyl compound having 3 to 15 carbon atoms, for example, (meth) acrylonitrile, cyanostyrene, cyanoalkyl (1 to 4 carbon atoms) acrylate

(A53-5) Other nitrogen-containing vinyl compounds, nitro group-containing vinyl compounds (C8 to C16, for example, nitrostyrene) and the like

(A54) vinyl hydrocarbon (a54-1) aliphatic vinyl hydrocarbon olefin having 2 to 18 or more carbon atoms (ethylene, propylene, butene, isobutylene, pentene, heptene, diisobutylene, octene, dodecene, octadecene, etc.), Diene having 4 to 10 or more carbon atoms (butadiene, isoprene, 1,4-pentadiene, 1,5-hexadiene, 1,7-octadiene, etc.), etc.

(A54-2) Alicyclic vinyl hydrocarbon cyclic unsaturated compound having 4 to 18 or more carbon atoms, for example, cycloalkene (for example, cyclohexene), (di) cycloalkadiene [for example, (di) cyclopentadiene], terpene ( For example pinene and limonene), indene

(A54-3) Aromatic vinyl hydrocarbon aromatic unsaturated compound having 8 to 20 or more carbon atoms, for example, styrene, α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene, isopropylstyrene, butyl Styrene, phenylstyrene, cyclohexylstyrene, benzylstyrene

(A55) Vinyl ester Aliphatic vinyl ester [C4 to C15, for example, alkenyl ester of aliphatic carboxylic acid (mono- or dicarboxylic acid) (for example, vinyl acetate, vinyl propionate, vinyl butyrate, diallyl adipate, isopropenyl acetate, Vinyl methoxy acetate)]
Aromatic vinyl ester [9 to 20 carbon atoms, for example, alkenyl ester of aromatic carboxylic acid (mono- or dicarboxylic acid) (for example, vinyl benzoate, diallyl phthalate, methyl-4-vinyl benzoate), and aromatic ring containing aliphatic carboxylic acid Esters (eg, acetoxystyrene)]

(A56) Vinyl ether aliphatic vinyl ether [3 to 15 carbon atoms, for example, vinyl alkyl (1 to 10 carbon atoms) ether (vinyl methyl ether, vinyl butyl ether, vinyl 2-ethylhexyl ether, etc.), vinyl alkoxy (1 to 6 carbon atoms) Alkyl (C 1-4) ether (vinyl-2-methoxyethyl ether, methoxybutadiene, 3,4-dihydro-1,2-pyran, 2-butoxy-2′-vinyloxydiethylether, vinyl-2-ethyl Mercaptoethyl ether, etc.), poly (2-4) (meth) allyloxyalkanes (2-6 carbon atoms) (diaryloxyethane, triaryloxyethane, tetraallyloxybutane, tetramethallyloxyethane, etc.)], Aromatic vinyl ether (8 to 20 carbon atoms, for example, vinyl phenyl ether , Phenoxystyrene)

(A57) Vinyl ketone Aliphatic vinyl ketone (C4 to C25, for example, vinyl methyl ketone, vinylethyl ketone), aromatic vinyl ketone (C9 to C21, for example, vinyl phenyl ketone)

(A58) Unsaturated dicarboxylic acid diester An unsaturated dicarboxylic acid diester having 4 to 34 carbon atoms, for example, dialkyl fumarate (two alkyl groups each having 1 to 22 carbon atoms are a straight-chain, branched-chain or alicyclic group) ), Dialkyl maleate (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 weight of the entire monomer.

A preferred lower limit of the weight average molecular weight of the polymer compound constituting the coating film is 3,000, a more preferred lower limit is 5,000, and a still more preferred lower limit is 7,000. On the other hand, a preferable upper limit of the weight average molecular weight of the polymer compound is 100,000, and a more preferable upper limit is 70,000.

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

The polymer compound constituting the coating film may be a known polymerization initiator ｛azo-based initiator [2,2′-azobis (2-methylpropionitrile), 2,2′-azobis (2,4-dimethylvalero). Nitrile), 2,2'-azobis (2-methylbutyronitrile), etc., and peroxide initiators (benzoyl peroxide, di-t-butyl peroxide, lauryl peroxide, etc.) and the like. (Bulk polymerization, solution polymerization, emulsion polymerization, suspension polymerization, etc.).
The amount of the polymerization initiator to be 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, from the viewpoint of adjusting the weight average molecular weight to a preferable range. More preferably 0.1 to 1.5% by weight, the polymerization temperature and the polymerization time are adjusted according to the type of the polymerization initiator and the like, 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 the case of solution polymerization include, for example, esters (2 to 8 carbon atoms, for example, ethyl acetate and butyl acetate), alcohols (1 to 8 carbon atoms, for example, methanol, ethanol and octanol), and hydrocarbons (carbon number 4-8, for example, n-butane, cyclohexane and toluene, amides (for example, N, N-dimethylformamide (hereinafter abbreviated as DMF)) and ketones (3-9 carbon atoms, for example, methyl ethyl ketone). From the viewpoint of adjusting the molecular weight to a preferred range, the amount used is preferably 5 to 900% by weight, more preferably 10 to 400% by weight, and still 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, Preferably 30 to 80 wt%.

Examples of the dispersion medium in the emulsion polymerization and suspension polymerization include water, alcohol (eg, ethanol), ester (eg, ethyl propionate), and light naphtha. As the emulsifier, a metal salt of a higher fatty acid (10 to 24 carbon atoms) is used. (For example, sodium oleate and sodium stearate), higher alcohol (10 to 24 carbon atoms) sulfate metal salt (for example, sodium lauryl sulfate), ethoxylated tetramethyldecinediol, sodium sulfoethyl methacrylate, dimethylaminomethyl methacrylate, and the like. Is mentioned. Further, polyvinyl alcohol, polyvinyl pyrrolidone, or the like may be added as a stabilizer.
The monomer concentration of the solution or dispersion is preferably 5 to 95% by weight, more preferably 10 to 90% by weight, still more preferably 15 to 85% by weight, and the amount of the polymerization initiator used is based on the total weight of the monomer. And preferably 0.01 to 5% by weight, more preferably 0.05 to 2% by weight.
In the polymerization, a known chain transfer agent, for example, a mercapto compound (dodecyl mercaptan, n-butyl mercaptan, etc.) and / or a halogenated hydrocarbon (carbon tetrachloride, carbon tetrabromide, benzyl chloride, etc.) can be used. .

The polymer compound constituting the coating film may be a cross-linking agent (A ′) having a reactive functional group which reacts the polymer compound 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 and 1,3-bis (N, N-diglycidylaminomethyl)), etc.] and / or Alternatively, a crosslinked polymer formed by crosslinking with a polyol compound (a′2) (such as ethylene glycol) may be used.

As a method of crosslinking the polymer compound constituting the coating film by using the crosslinking agent (A ′), a method of crosslinking the electrode active material after coating it with the polymer compound constituting the coating film may be mentioned. Specifically, after the electrode active material and a resin solution containing the polymer compound constituting the coating film are mixed and the solvent is removed, the solution containing the crosslinking agent (A ′) is produced after the production of the coating active material. By mixing with the coating active material and heating, a solvent removal and a cross-linking reaction are caused to cause a reaction in which the polymer compound constituting the coating film is cross-linked by the cross-linking agent (A ′) on the surface of the electrode active material. There is a way to wake up.
The heating temperature is adjusted according to the type of the cross-linking agent. When the polyepoxy compound (a′1) is used as the cross-linking agent, the heating temperature is preferably 70 ° C. or higher, and when the polyol compound (a′2) is used, Preferably it is 120 ° C. or higher.

The acid value of the polymer compound forming the coating film is preferably from 600 to 800, more preferably from 650 to 790, and further preferably from 680 to 780.
The acid value in the present specification means an acid value measured based on “JIS K 0070-1992 Test method for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable product of chemical products”. I do.

Examples of the method for adjusting the acid value include a method for adjusting the number of carboxyl groups per unit weight of the constituent monomer, and a method for adjusting the constituent ratio of the carboxyl group-containing monomer in the polymer. From the viewpoint of increasing the acid value, in the former adjustment method, it is desirable to include monomers such as acrylic acid and methacrylic acid, and in the latter adjustment method, it is desirable to increase the constituent ratio of acrylic acid and methacrylic acid in the polymer. .

As for the polymer compound constituting the coating film, LiN (FSO ₂ ) ₂ was dissolved at a ratio of 2 mol / L in a mixed solvent in which the volume ratio of ethylene carbonate (EC) and propylene carbonate (PC) was 1: 1. It is preferable that the degree of swelling with respect to the electrolytic solution is 100 to 150%. The swelling degree is more preferably from 105 to 140%, further preferably from 105 to 120%. When the swelling degree of the polymer compound constituting the coating film is 100 to 150%, appropriate wettability can be imparted to the coating active material.

The degree of swelling can be measured by the following method.
The polymer compound solution in which the polymer compound forming the coating film is dissolved is applied on a horizontal glass plate and air-dried at room temperature for half a day. Next, the sample was allowed to stand in a vacuum drier heated to 150 ° C. for 3 hours, cooled to room temperature, and then cut into a polymer compound cut into a size of 10 × 40 × 0.2 mm as a test piece. It is immersed in an electrolytic solution at 50 ° C. for 3 days to obtain a saturated liquid absorbing state.
Thereafter, the degree of swelling can be determined by the following equation from the change in weight of the test piece before and after liquid absorption.
Swelling degree [%] = [(weight of test piece after liquid absorption−weight of test piece before liquid absorption) / weight of test piece before liquid absorption] × 100

The polymer compound constituting the coating film preferably has a static contact angle with the electrolyte of 45 ° or less. When the static contact angle is 45 ° or less, the coating active material is sufficiently compatible with the electrolytic solution, so that a liquid film can be suitably formed.

The static contact angle of the polymer compound constituting the coating film is measured in accordance with “JIS R 3257: 1999” by leaving an electrolyte solution instead of a water droplet.

The coated positive electrode active material of the present invention can be obtained by binding a polymer compound constituting a coating film to at least a part of the surface of the positive electrode active material.
From the viewpoint of the internal resistance of the battery and the like, the coating film preferably contains a conductive auxiliary.

When the coating film contains a conductive aid, the conductive aid is preferably selected from materials having conductivity.
Preferred conductive assistants include metals [aluminum, stainless steel (SUS), silver, gold, copper and titanium, etc.], carbons [graphite and carbon black (acetylene black, Ketjen black, furnace black, channel black and thermal lamps). Black, etc.), and mixtures thereof.
These conductive assistants may be used alone or in combination of two or more. Also, these alloys or metal oxides may be used.
Among them, from the viewpoint of electrical stability, more preferably aluminum, stainless steel, carbon, silver, gold, copper, titanium and mixtures thereof, further preferably silver, gold, aluminum, stainless steel and carbon, particularly Preferably it is carbon.
Further, as these conductive aids, those obtained by coating a conductive material [preferably, a metal among the above-described conductive aids] around a particle-based ceramic material or a resin material by plating or the like may be used.

The shape (form) of the conductive assistant is not limited to the particle form, and may be a form other than the particle form, such as carbon nanofiber or carbon nanotube, which is a form practically used as a so-called filler-based conductive aid. You may.

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 the present specification, the “particle size of the conductive additive” means the maximum distance L among any two points on the contour of the conductive additive. As the value of the “average particle diameter”, the average value of the particle diameter of particles observed in several to several tens of visual fields using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM) is used. It is assumed that the calculated value is adopted.

When the coating film contains a conductive auxiliary, the ratio of the polymer compound and the conductive auxiliary forming the coating film is not particularly limited, but from the viewpoint of the internal resistance of the battery and the like, the coating is performed in a weight ratio. High molecular compound (weight of resin solids) constituting the film: The conductive auxiliary agent is preferably in the range of 1: 0.01 to 1:50, more preferably 1: 0.2 to 1: 3.0. .

In the coated positive electrode active material of the present invention, the positive electrode active material is composed of a composite oxide represented by the structural formula LiNi _x M _1-x O ₂ .
In the above formula, x is from 0.3 to 1.0, preferably from 0.5 to 0.8. M is at least one element selected from the group consisting of Co, Mn and Al, preferably two or more elements, and more preferably Co and Mn, Co and Al.
Examples of such a positive electrode active material include LiNi _0.8 Co _0.15 Al _0.05 O ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , and LiNi _0.5 Co _0.2 Mn. _0.3 O _{2 and the} like.

The volume average particle diameter of the positive electrode active material is preferably from 0.1 to 100 μm, more preferably from 1 to 50 μm, and still more preferably from 2 to 20 μm, from the viewpoint of the electrical characteristics of the battery.
The volume average particle size of the positive electrode active material means a particle size (Dv50) at an integrated value of 50% in a particle size distribution obtained by a laser diffraction / scattering method (also referred to as a microtrack method). The laser diffraction / scattering method is a method of obtaining a particle size distribution using scattered light obtained by irradiating a particle with laser light. For measuring the volume average particle diameter, a micro track manufactured by Nikkiso Co., Ltd. is used. Can be used.

The coated positive electrode active material of the present invention can be produced by mixing a polymer compound constituting the coating film, the positive electrode active material, and a conductive auxiliary used as needed.
The order in which the polymer compound, the positive electrode active material, and the conductive additive forming the coating film are mixed is not particularly limited. For example, a resin composition including the polymer compound forming the coating film mixed in advance and the conductive auxiliary agent is used. The material may be further mixed with the positive electrode active material, the polymer compound constituting the coating film, the positive electrode active material and the conductive auxiliary may be mixed simultaneously, or the coating film may be formed on the positive electrode active material. A high molecular compound may be mixed, and further a conductive auxiliary may be mixed.

The coated positive electrode active material of the present invention can be obtained by coating the positive electrode active material with a polymer compound constituting the coating film. For example, the positive electrode active material is put into a universal mixer and stirred at 30 to 500 rpm. In this state, the resin solution containing the polymer compound constituting the coating film is dropped and mixed over 1 to 90 minutes, and further, if necessary, a conductive additive is mixed, and the temperature is raised to 50 to 200 ° C. with stirring. It can be obtained by reducing the pressure to 0.007 to 0.04 MPa and holding for 10 to 150 minutes.

[Positive electrode slurry for lithium ion batteries]
The positive electrode slurry for a lithium ion battery of the present invention (hereinafter, also simply referred to as “positive electrode slurry”) is characterized by containing the coated positive electrode active material of the present invention, and an electrolytic solution containing an electrolyte and a solvent.

The coated positive electrode active material contained in the positive electrode slurry for a lithium ion battery of the present invention is preferably 40 to 80% by weight, based on the weight of the positive electrode slurry, from the viewpoint of the dispersibility of the active material and the electrode formability. More preferably, it is about 75% by weight.

The positive electrode slurry of the present invention can be produced, for example, by dispersing the coated positive electrode active material of the present invention in an electrolytic solution.

In the positive electrode slurry of the present invention, the concentration of the electrolyte in the electrolyte is preferably 1.2 to 5.0 mol / L, more preferably 1.5 to 4.5 mol / L, and 1.8. It is more preferably from 4.0 to 4.0 mol / L, particularly preferably from 2.0 to 3.5 mol / L.
Since such an electrolytic solution has an appropriate viscosity, a liquid film can be formed between the coated positive electrode active materials, and a lubricating effect (position adjustment ability of the coated active material) can be imparted to the coated positive electrode active material. it can.

As the electrolyte, an electrolyte used in a known electrolytic solution can be used. For example, lithium salts of inorganic anions such as LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ and LiN (FSO ₂ ) _2, and LiN ₂ Lithium salts of organic anions such as (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ and LiC (CF ₃ SO ₂ ) ₃ are mentioned. Among them, LiN (FSO ₂ ) ₂ is preferable from the viewpoint of battery output and charge / discharge cycle characteristics.

As the solvent, non-aqueous solvents used in known electrolytic solutions can be used, for example, lactone compounds, cyclic or chain carbonates, chain carboxylic esters, cyclic or chain ethers, phosphates, nitrile compounds Amide compounds, sulfones, sulfolane and mixtures thereof.

Examples of the lactone compound include a 5-membered ring (such as γ-butyrolactone and γ-valerolactone) and a 6-membered ring (such as δ-valerolactone).

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

Examples of the chain carboxylate include methyl acetate, ethyl acetate, propyl acetate, and methyl propionate.

Examples of the cyclic ether include tetrahydrofuran, tetrahydropyran, 1,3-dioxolan, 1,4-dioxane and the like. Examples of the chain ether include dimethoxymethane and 1,2-dimethoxyethane.

Examples of the phosphate ester include trimethyl phosphate, triethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, tripropyl phosphate, tributyl phosphate, tri (trifluoromethyl) phosphate, tri (trichloromethyl) phosphate, Tri (trifluoroethyl) phosphate, tri (tripperfluoroethyl) phosphate, 2-ethoxy-1,3,2-dioxaphosphoran-2-one, 2-trifluoroethoxy-1,3,2- Dioxaphospholane-2-one and 2-methoxyethoxy-1,3,2-dioxaphospholane-2-one.

Examples of the nitrile compound include acetonitrile. Examples of the amide compound include DMF. Examples of the sulfone include dimethyl sulfone and diethyl sulfone.

One of these solvents may be used alone, or two or more thereof may be used in combination.

In the positive electrode slurry of the present invention, the flash point of the solvent in the electrolytic solution is preferably 100 ° C. or higher, more preferably 120 ° C. or higher, from the viewpoint of preventing heat generation and expansion of the lithium ion battery. It is more preferable that the temperature is not lower than ° C. On the other hand, the flash point of the solvent in the electrolytic solution may be 160 ° C. or lower.

In the present specification, the flash point of a solvent is a temperature measured by a tag sealing method specified in JIS K2265-1-2007.

Among these solvents, propylene carbonate and / or ethylene carbonate are preferable from the viewpoint of ion conductivity.

The positive electrode slurry of the present invention contains the above-mentioned coated positive electrode active material and an electrolytic solution containing an electrolyte and a solvent, but in addition to the conductive auxiliary that the coating film contains as necessary, may further contain a conductive auxiliary. Good. The conductive additive contained in the coating film as necessary is integral with the coated positive electrode active material, whereas the conductive additive contained in the positive electrode slurry is separately contained from the coated positive electrode active material.
As the conductive assistant that may be contained in the positive electrode slurry, those described in [Coated positive electrode active material for lithium ion battery] can be used. When the positive electrode slurry of the present invention contains a conductive auxiliary, the conductive auxiliary contained in the positive electrode slurry may be the same as or different from the conductive auxiliary contained in the coating film.

When the positive electrode slurry of the present invention contains a conductive auxiliary, the total content of the conductive auxiliary contained in the positive electrode slurry and the conductive auxiliary contained in the coating film is based on the weight of the positive electrode slurry excluding the electrolytic solution. Is preferably less than 4% by weight, more preferably less than 3% by weight. On the other hand, the total content of the conductive auxiliary contained in the positive electrode slurry and the conductive auxiliary contained in the coating film may be at least 2.5% by weight based on the weight of the positive electrode slurry excluding the electrolyte. preferable.

It is preferable that the positive electrode slurry of the present invention does not contain a solvent drying type binder described in [Positive electrode for lithium ion battery] described later.

[Positive electrode for lithium ion battery]
The positive electrode for a lithium ion battery of the present invention (hereinafter, also simply referred to as “positive electrode”) is a positive electrode including a positive electrode active material layer containing the coated positive electrode active material of the present invention and an electrolyte solution containing an electrolyte and a solvent. The positive electrode active material layer comprises a non-binding body of the coated positive electrode active material.

In the positive electrode of the present invention, the positive electrode active material layer is made of a non-binding body of the coated positive electrode active material.
Here, the term “non-binder” means that the position of the coated positive electrode active material is not fixed by a binder (also referred to as a binder). That is, each of the coated positive electrode active materials is in a state where it can move according to an external force.

When the positive electrode active material layer is made of a non-binder, the positive electrode active materials are not irreversibly fixed by the binder. Irreversible fixing means that the positive electrode active materials are bonded and fixed to each other by the following known solvent-drying type lithium ion battery binder. It is necessary to mechanically break the interface between the positive electrode active materials. On the other hand, in the case of a non-bound material, since the positive electrode active materials are not irreversibly bonded and fixed, they can be separated from each other without mechanically breaking the interface between the positive electrode active materials.

In the positive electrode of the present invention, it is preferable that the positive electrode active material layer does not include a solvent drying type binder.
Solvent-drying binders include known 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 in a solvent, the solvent is volatilized, and the surface is solidified without showing tackiness by evaporating, and the positive electrode active materials and the positive electrode active material and the current collector are solidified. Is fixed firmly.

In the positive electrode of the present invention, as the electrolyte and the solvent contained in the electrolytic solution, those described in [Positive electrode slurry for lithium ion battery] can be used.

In the positive electrode of the present invention, the flash point of the solvent in the electrolytic solution is preferably 100 ° C. or higher, more preferably 120 ° C. or higher, even more preferably 130 ° C. or higher. On the other hand, the flash point of the solvent in the electrolytic solution may be 160 ° C. or lower.

In the positive electrode of the present invention, the positive electrode active material layer may further include a conductive auxiliary in addition to the conductive auxiliary contained as necessary in the coating film of the coated positive electrode active material. Whereas the conductive auxiliary contained as necessary in the coating film is integrated with the coated positive electrode active material, the conductive auxiliary contained in the positive electrode active material layer is included separately from the coated positive electrode active material. Can be distinguished.
As the conductive additive that may be contained in the positive electrode active material layer, those described in [Coated positive electrode active material for lithium ion battery] can be used.

In the positive electrode of the present invention, when the positive electrode active material layer contains a conductive auxiliary, the total content of the conductive auxiliary contained in the positive electrode and the conductive auxiliary contained in the coating film is determined from the positive electrode active material layer to the electrolytic solution. Is preferably less than 4% by weight, more preferably less than 3% by weight, based on the weight excluding. On the other hand, the total content of the conductive additive contained in the positive electrode and the conductive additive contained in the coating film is 2.5% by weight or more based on the weight of the positive electrode active material layer excluding the electrolyte. Is preferred.

In the positive electrode of the present invention, the thickness of the positive electrode active material layer is preferably from 150 to 600 μm, and more preferably from 200 to 450 μm, from the viewpoint of battery performance.

The positive electrode of the present invention can be produced, for example, by applying the positive electrode slurry of the present invention to a current collector and then drying it. Specifically, after the positive electrode slurry of the present invention is applied to the current collector with a coating device such as a bar coater, the solvent is removed by leaving the nonwoven fabric on the active material and absorbing the liquid, If necessary, a method of pressing with a press machine and the like can be mentioned.

In the positive electrode of the present invention, as a material constituting the current collector, metal materials such as copper, aluminum, titanium, stainless steel, nickel and alloys thereof, and calcined carbon, a conductive polymer material, and a conductive glass Is mentioned.
The shape of the current collector is not particularly limited, and may be a sheet-shaped current collector made of the above material, or a deposited layer made of fine particles made of the above material.
The thickness of the current collector is not particularly limited, but is preferably 50 to 500 μm.

As described above, the positive electrode of the present invention preferably further includes a current collector, and the positive electrode active material layer is preferably provided on a surface of the current collector. For example, the positive electrode of the present invention preferably includes a resin current collector made of a conductive polymer material, and the positive electrode active material layer is preferably provided on a surface of the resin current collector.

As the conductive polymer material constituting the resin current collector, for example, a resin obtained by adding a conductive agent to a resin can be used.
As the conductive agent constituting the conductive polymer material, the same conductive agent as an optional component of the coating film can be suitably used.
Examples of the resin constituting the conductive polymer material include polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polycycloolefin (PCO), polyethylene terephthalate (PET), polyether nitrile (PEN), and poly (ethylene terephthalate). Tetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVdF), epoxy resin, silicone resin or a mixture thereof And the like.
From the viewpoint of electrical stability, polyethylene (PE), polypropylene (PP), polymethylpentene (PMP) and polycycloolefin (PCO) are preferred, and polyethylene (PE), polypropylene (PP) and polymethylpentene are more preferred. (PMP).
The resin current collector can be obtained by a known method described in JP-A-2012-150905, WO 2015/005116, and the like.

[Lithium ion battery]
A lithium ion battery according to the present invention includes the positive electrode according to the present invention.

The lithium ion battery of the present invention is obtained by combining electrodes serving as counter electrodes, storing the battery together with the separator in a cell container, injecting an electrolytic solution, and sealing the cell container.
In addition, a positive electrode is formed on one surface of the current collector, and a negative electrode is formed on the other surface to produce a bipolar (bipolar) electrode. The bipolar (bipolar) electrode is laminated with a separator to form a cell container. It can also be obtained by storing, injecting an electrolytic solution, and sealing the cell container.
By using the positive electrode of the present invention, the lithium ion battery of the present invention can be obtained.

Examples of the separator include a porous film made of polyethylene or polypropylene, a laminated film of a porous polyethylene film and porous polypropylene, a nonwoven fabric made of synthetic fibers (such as polyester fibers and aramid fibers) or glass fibers, and silica on the surface thereof. And known separators for lithium ion batteries, such as those to which ceramic fine particles such as alumina and titania are adhered.

Next, the present invention will be described specifically with reference to examples, but the present invention is not limited to the examples unless departing from the gist of the present invention. Unless otherwise specified, parts mean parts by weight and% means weight%.

<Preparation of polymer compound constituting coating film>
150 parts of DMF was charged into a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen gas inlet tube, and the temperature was raised to 75 ° C. Next, a monomer composition containing 91 parts of acrylic acid, 9 parts of methyl methacrylate and 50 parts of DMF, 0.3 part of 2,2′-azobis (2,4-dimethylvaleronitrile) and 2,2′- An initiator solution obtained by dissolving 0.8 part of azobis (2-methylbutyronitrile) in 30 parts of DMF was continuously dropped by a dropping funnel over 2 hours with stirring while blowing nitrogen into a four-necked flask. To perform radical polymerization. After completion of the dropwise addition, 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 having a resin concentration of 30%. The obtained copolymer solution was transferred to a Teflon (registered trademark) vat, dried under reduced pressure at 150 ° C. and 0.01 MPa for 3 hours, and DMF was distilled off to obtain a copolymer. The copolymer was roughly pulverized with a hammer and further pulverized in a mortar to obtain a powdery polymer compound (P-1). Table 1 shows the types and amounts of the monomers used for the polymer compound (P-1). Table 1 shows the acid value, weight average molecular weight, and swelling degree of the obtained polymer compound.

<Acid value measurement conditions>
Apparatus: Automatic titrator COM-1700 (manufactured by Hiranuma Sangyo Co., Ltd.)
The measurement was carried out using an automatic titrator [COM-1700 (manufactured by Hiranuma Sangyo Co.)] according to the potentiometric titration method described in JIS K 0070-1922.

<Measurement conditions of weight average molecular weight>
Apparatus: Alliance GPC V2000 (Waters)
Solvent: orthodichlorobenzene, DMF, THF
Standard substance: polystyrene Sample concentration: 3 mg / ml
Column stationary phase: PL gel 10um, MIXED-B two in series (Polymer Laboratories)
Column temperature: 135 ° C

<Measurement conditions of swelling degree>
The polymer compound was dissolved in DMF to obtain a polymer compound solution. Thereafter, the obtained polymer compound solution was applied on a horizontal glass plate, and air-dried at room temperature for half a day. Next, after leaving to stand in a vacuum dryer heated to 150 ° C. for 3 hours, and then cooling to room temperature, a polymer compound cut into a size of 10 × 40 × 0.2 mm was used as a test piece. The polymer compound was immersed in an electrolyte solution (X-1) described below at 50 ° C. for 3 days to prepare a polymer compound in a saturated liquid absorbing state.
The degree of swelling was determined from the weight change of the test piece before and after liquid absorption by the following equation.
Swelling degree [%] = [(weight of test piece after liquid absorption−weight of test piece before liquid absorption) / weight of test piece before liquid absorption] × 100

Except that the kind and amount of the monomer were changed as described in Table 1, the same procedure was performed as for the polymer compound (P-1), and the polymer compounds (P-2), (P-3), and ( (P-4), (P'-1), (P'-2), (P'-3) and (P'-4) were obtained.

<Preparation of electrolyte>
LiN (FSO ₂ ) ₂ was dissolved at a ratio of 2.0 mol / L in a mixed solvent (volume ratio 1: 1) of ethylene carbonate (EC) and propylene carbonate (PC) to prepare an electrolyte solution (X-1). .

<Example 1>
[Preparation of coated positive electrode active material]
0.1 part of the polymer compound (P-1) was dissolved in 5.3 parts of DMF to obtain a polymer compound solution.
96.9 parts of LiNi _0.8 Co _0.15 Al _0.05 O ₂ [manufactured by Toda Kogyo Co., Ltd., volume average particle diameter 6.4 μm] as a positive electrode active material is put into a universal mixer, high speed mixer FS25 [manufactured by Earth Technica]. While stirring at 720 rpm at room temperature, the polymer compound solution was added dropwise over 2 minutes, and the mixture was further stirred for 5 minutes.
Next, 3 parts of acetylene black [Denka Black (registered trademark) manufactured by Denki Kagaku Kogyo Co., Ltd.] was added as a conductive assistant in 2 minutes while stirring, and the stirring was continued for 30 minutes. Thereafter, the pressure was reduced to 0.01 MPa while maintaining the stirring, then the temperature was increased to 150 ° C. while maintaining the stirring and the degree of vacuum, and the volatile components were distilled off while maintaining the stirring, the degree of vacuum and the temperature for 8 hours. . The obtained powder was classified with a sieve having an opening of 212 μm to obtain a coated positive electrode active material according to Example 1.

[Production of resin current collector]
In a twin-screw extruder, 70 parts of polypropylene [trade name "Sun Allomer PL500A", manufactured by Sun Allomer Co., Ltd.], 25 parts of carbon nanotubes [trade name: "FloTube 9000", manufactured by CNano], and dispersant [trade name "UMEX 1001" 5 parts manufactured by Sanyo Chemical Industries, Ltd.] were melt-kneaded at 200 ° C. and 200 rpm to obtain a resin mixture.
The obtained resin mixture was passed through a T-die extrusion film forming machine and stretched and rolled to obtain a 100 μm-thick conductive film for a resin current collector. Next, the obtained conductive film for a resin current collector was cut so as to have a size of 17.0 cm × 17.0 cm, nickel was deposited on one side, and a terminal (5 mm × 3 cm) for current extraction was connected. A resin current collector was obtained.

[Production of positive electrode for lithium ion battery]
42 parts of electrolytic solution (X-1) and 4.2 parts of carbon fiber [Donacarbo Milled S-243: average fiber length 500 μm, average fiber diameter 13 μm: electric conductivity 200 mS / cm] manufactured by Osaka Gas Chemical Co., Ltd. After mixing at 2000 rpm for 5 minutes using a planetary stirring type mixing and kneading apparatus {Awatori Rintaro (manufactured by Shinky Co.)}, 30 parts of the electrolytic solution and 206 parts of the coated positive electrode active material were added. Further, the mixture was further mixed with Nawataro Neritaro at 2000 rpm for 2 minutes, and 20 parts of the above-mentioned electrolyte solution was further added. Then, the mixture was stirred with Nawataro Neritaro at 2000 rpm for 1 minute, and the electrolyte solution (X-1) was further 2.3. After further adding a portion, stirring by Nawataro Nawataro was mixed at 2000 rpm for 2 minutes to prepare a positive electrode slurry. The obtained positive electrode slurry was applied to one surface of the above-mentioned resin current collector so that the basis weight of the active material was 80 mg / cm ^2, and was pressed at a pressure of 1.4 MPa for about 10 seconds to obtain a 340 μm thick Example 1. (16.2 cm × 16.2 cm) was manufactured.

[Production of lithium ion battery]
The obtained positive electrode was combined with a counter electrode Li metal via a separator (# 3501 manufactured by Celgard) to produce a laminate cell.

<Example 2>
Except that the polymer compound (P-1) was changed to the polymer compound (P-2), a positive electrode for a lithium ion battery according to Example 2 was prepared in the same manner as in Example 1 to obtain a lithium ion battery. Was.

<Example 3>
Except that the polymer compound (P-1) was changed to the polymer compound (P-3), a positive electrode for a lithium ion battery according to Example 3 was produced in the same manner as in Example 1 to obtain a lithium ion battery. Was.

<Example 4>
The polymer compound (P-1) was changed to a polymer compound (P-4), and LiNi _0.5 Co _0.2 instead of LiNi _0.8 Co _0.15 Al _0.05 O ₂ as a positive electrode active material A positive electrode for a lithium ion battery according to Example 4 was prepared in the same manner as in Example 1 except that Mn _0.3 O ₂ [manufactured by Toda Kogyo, volume average particle diameter 10.7 μm] was used. I got

<Example 5>
A positive electrode for a lithium ion battery according to Example 5 was prepared in the same manner as in Example 1 except that the polymer compound (P-1) was changed from 0.1 part to 0.01 part, and a lithium ion battery was manufactured. Obtained.

<Example 6>
A positive electrode for a lithium ion battery according to Example 6 was prepared in the same manner as in Example 1 except that the polymer compound (P-1) was changed from 0.1 part to 1.0 part, and a lithium ion battery was manufactured. Obtained.

<Comparative Example 1>
Except that the polymer compound (P-1) was changed to the polymer compound (P'-1), a positive electrode for a lithium ion battery according to Comparative Example 1 was prepared in the same manner as in Example 1, and a lithium ion battery was manufactured. Obtained.

<Comparative Example 2>
Except that the polymer compound (P-1) was changed to the polymer compound (P'-2), a positive electrode for a lithium ion battery according to Comparative Example 2 was prepared in the same manner as in Example 1, and a lithium ion battery was manufactured. Obtained.

<Comparative Example 3>
Except that the polymer compound (P-1) was changed to the polymer compound (P'-3), a positive electrode for a lithium ion battery according to Comparative Example 3 was produced in the same manner as in Example 1, and a lithium ion battery was manufactured. Obtained.

<Comparative Example 4>
Except that the polymer compound (P-1) was changed to the polymer compound (P'-4), a positive electrode for a lithium ion battery according to Comparative Example 4 was produced in the same manner as in Example 1, and a lithium ion battery was manufactured. Obtained.

<First discharge capacity>
The initial performance of the lithium ion battery was evaluated at 25 ° C. using a charge / discharge measurement apparatus “HJ-SD8” (manufactured by Hokuto Denko Corporation) by the following method.
After charging to 4.2 V with a current of 0.1 C by a constant current constant voltage charging method (also referred to as CCCV mode), the battery was charged until the current value reached 0.01 C while maintaining 4.2 V. After a rest for 10 minutes, the battery was discharged to 2.5 V at a current of 0.1 C.
The capacity charged at this time was defined as [initial charge capacity (Ah)], and the discharged capacity was defined as [initial discharge capacity (Ah)].

<Rise increase rate>
After the first charge / discharge, the charge / discharge was repeated four times with a current of 0.1 C. The internal resistance R was calculated using the relationship of ΔV = IR using the voltage drop ΔV for 10 seconds from the start of the discharge in the fifth cycle. The resistance increase rate (%) was calculated from the ratio of the resistance at the first and fifth cycles.

From Table 2, in Examples 1 to 6, it is confirmed that the initial discharge capacity is high and the resistance increase rate is low. Therefore, a lithium ion battery having good cycle characteristics and high capacity density has been obtained.

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

Claims (10)

At least a part of the surface of the positive electrode active material is a coated positive electrode active material for a lithium ion battery coated with a coating film containing a polymer compound,
The positive electrode active material has a structural formula of LiNi _x M _1-x O ₂ (where x is 0.3 to 1.0, and M is at least one element selected from the group consisting of Co, Mn and Al). Is a composite oxide represented by
The polymer compound is a polymer of a monomer composition containing acrylic acid in an amount of more than 90% by weight and 98% by weight or less based on the weight of the whole monomer,
The coated positive electrode active material for a lithium ion battery, wherein the content of the polymer compound is 0.01 to 1% by weight based on the weight of the positive electrode active material. The polymer compound has a swelling degree of 100 to 150% with respect to an electrolyte obtained by dissolving LiN (FSO ₂ ) ₂ at a ratio of 2 mol / L in a mixed solvent in which a volume ratio of ethylene carbonate and propylene carbonate is 1: 1. The coated positive electrode active material for a lithium ion battery according to claim 1. A coated positive electrode active material for a lithium ion battery according to claim 1 or 2,
A positive electrode slurry for a lithium ion battery, comprising: an electrolyte solution containing an electrolyte and a solvent. The positive electrode slurry for a lithium ion battery according to claim 3, wherein the concentration of the electrolyte in the electrolyte is 1.2 to 5.0 mol / L. A positive electrode for a lithium ion battery comprising a coated positive electrode active material for a lithium ion battery according to claim 1 or 2, and a positive electrode active material layer including an electrolyte solution containing an electrolyte and a solvent,
The positive electrode for a lithium ion battery, wherein the positive electrode active material layer is made of a non-binding material of the coated positive electrode active material for a lithium ion battery. The positive electrode active material layer further includes a conductive auxiliary,
The positive electrode for a lithium ion battery according to claim 5, wherein the content of the conductive additive is less than 4% by weight based on the weight of the positive electrode active material layer excluding the electrolyte. The positive electrode for a lithium ion battery according to claim 5, wherein a flash point of the solvent in the electrolyte is 100 ° C. or higher. The positive electrode for a lithium ion battery according to any one of claims 5 to 7, wherein the thickness of the positive electrode active material layer is 150 to 600 µm. Further comprising a resin current collector,
The positive electrode for a lithium ion battery according to any one of claims 5 to 8, wherein the positive electrode active material layer is provided on a surface of the resin current collector. A lithium ion battery comprising the positive electrode for a lithium ion battery according to claim 5.