JP7097271B2 - Coated positive electrode active material for lithium ion batteries, positive electrode slurry for lithium ion batteries, positive electrode for lithium ion batteries, and lithium ion batteries - Google Patents

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, there has been an urgent need to reduce carbon dioxide emissions in order to protect the environment. In the automobile industry, expectations are high for the reduction of carbon dioxide emissions through the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs), and the development of secondary batteries for driving motors, which holds the key to their practical application, is earnest. It is done. As a secondary battery, a lithium ion battery that can achieve high energy density and high output density is attracting attention.

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

JP-A-2017-160294

According to Patent Document 1, by coating the surface of the active material with a specific resin, the internal resistance of the battery can be suppressed, and as a result, the 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 a larger electric capacity and higher safety of a battery, there is a tendency to use an active material having a large electric capacity or an electrolytic solution having a high 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 highly polar material is used, it is necessary to take measures such as increasing the amount of the conductive auxiliary agent added in order to maintain the battery performance. However, if the amount of the conductive auxiliary agent 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.

As described above, when a highly polar active material is used, it can be said that there is room for improvement in obtaining a lithium ion battery having good cycle characteristics and a high capacity density.

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

As a result of diligent studies to solve the above problems, the present inventors have reduced the content of the polymer compound by using the polymer compound optimized when the positive electrode active material is highly polar as the coating resin. However, they have found that they are excellent in battery performance 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 positive active material is coated with a coating film containing a polymer compound, and the positive positive active material is structural formula _LiNix . Composite oxide represented by M _1-x O ₂ (in the formula, x is 0.3 to 1.0, and M is at least one element selected from the group consisting of Co, Mn and Al). The above-mentioned 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 total weight of the monomer, and the content of the above-mentioned polymer compound. However, the coated positive electrode active material for a lithium ion battery is characterized by being 0.01 to 1% by weight based on the weight of the positive electrode active material; the coated positive electrode active material for a lithium ion battery, an electrolyte and a solvent are contained. Positive positive slurry for lithium ion batteries, characterized by containing an electrolytic solution; for a lithium ion battery including 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 positive electrode is a positive electrode for a lithium ion battery, wherein the positive electrode active material layer is composed of a non-bonded body of the coated positive electrode active material for a lithium ion battery; the positive electrode for a lithium ion battery is provided. It is a lithium ion battery.

According to the present invention, it is possible to obtain a lithium ion battery having good cycle characteristics and a high capacity density.

[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. The active material, the positive positive active material, is structural formula LiNi _x M _1-x O ₂ (in the formula, x is 0.3 to 1.0, and M is selected from the group consisting of Co, Mn and Al. It is composed of a composite oxide represented by (at least one element), and the above-mentioned polymer compound is a single amount containing acrylic acid in an amount of more than 90% by weight and 98% by weight or less based on the total weight of the monomer. It is a polymer of a body composition, and is characterized in that 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 enhances the affinity with the conductive auxiliary agent. 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 it is appropriately swollen by the electrolytic solution to conduct ion conduction. It also guarantees sex. Further, since the dispersibility of the conductive auxiliary agent is also improved, it is possible to suppress an increase in electrical resistance due to charging and discharging even if the amount of the conductive auxiliary agent added is small. 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 constituting 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 is preferably 0.1 to 0%. It is more preferably 5.5% by weight.

The polymer compound constituting the coating film is a resin containing a polymer containing the 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 above-mentioned monomer composition, the content of acrylic acid (a0) is more than 90% by weight and 98% by weight or less based on the weight of the whole monomer. From the viewpoint of the flexibility of the coating layer, the content of acrylic acid (a0) is preferably 93.0 to 97.5% by weight based on the weight of the entire monomer, and is 95.0 to 97.0. 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; (anhydrous) maleic acid and fumal. Dicarboxylic acids having 4 to 24 carbon atoms such as acids, (anhydrous) itaconic acid, citraconic acid, and mesaconic acid; trivalent to tetravalent to tetravalent or higher valent polycarboxylic acids having 6 to 24 carbon atoms such as aconitic acid, etc. Can be mentioned.

The polymer compound constituting the coating film may contain the monomer (a2) represented by the following general formula (1) as the acrylic monomer (a).
CH ₂ = C (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.
^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) An ester compound in which R ² is a linear or branched alkyl group having 4 to 12 carbon atoms. Examples of the linear alkyl group having 4 to 12 carbon atoms include a butyl group, a pentyl group, a hexyl group, a heptyl group and an octyl group. Examples thereof include a nonyl group, a decyl group, an undecyl group and a dodecyl group.
As the branched alkyl group having 4 to 12 carbon atoms, 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, 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, 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-Mech 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 , 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-methylundesyl group, 3-methylundesyl group, 4-methylundesyl group, 5-methylundesyl 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 can be mentioned. Of these, the 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, and the like. 1-hexyl octadecyl group, 1-octyl hexadecyl group, 1-decyl tetradecyl group, 1-undecyl tridecyl group, etc.], 2-alkylalkyl group [2-methyldodecyl group, 2-hexyl octadecyl group, 2- Octylhexadecyl group, 2-decyltetradecyl group, 2-undecyltridecyl group, 2-dodecylhexadecyl group, 2-tridecylpentadecyl group, 2-decyl octadecyl group, 2-tetradecyl octadecyl group, 2- Hexadecyl octadecyl group, 2-tetradecyl eicosyl group, 2-hexadecyl eicosyl group, etc.] 3-34-alkylalkyl group (3-alkylalkyl group, 4-alkylalkyl group, 5-alkylalkyl group, 32) -Alkylalkyl group, 33-alkylalkyl group, 34-alkylalkyl group, etc.), as well as propylene oligomer (7-11 mer), ethylene / propylene (molar ratio 16/1 to 1/11) oligomer, isobutylene oligomer ( A branched alkyl group of 1 or more such as a residue obtained by removing a hydroxyl group from an oxoalcohol obtained from a 7 to 8 mer), an α-olefin (5 to 20 carbon atoms) oligomer (4 to 8 mer), or the like. Examples thereof include a mixed alkyl group contained therein.

The polymer compound constituting the coating film 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 fatty alcohol having 1 to 3 carbon atoms constituting 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 an acrylic acid (a0) and at least one of a monomer (a1), a monomer (a2) and an ester compound (a3). It is preferably a polymer of a monomer composition containing an acrylic acid (a0) and at least one of a monomer (a1), an ester compound (a21) and an ester compound (a3). , Acrylic acid (a0) and a polymer of a monomer composition containing any one of a monomer (a1), a monomer (a2) and an ester compound (a3) are more preferable. Most preferably, it is a polymer of a monomer composition containing a0) and any one of a monomer (a1), an ester compound (a21) and an ester compound (a3).
As the polymer compound constituting the coating film, for example, acrylic acid using maleic acid as the monomer (a1), a copolymer of acrylic acid and maleic acid, and acrylic using 2-ethylhexyl methacrylate as the monomer (a2). Examples thereof include 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.

The total content of the monomer (a1), the monomer (a2) and the ester compound (a3) is 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.

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

Examples of the structure having a polymerizable unsaturated double bond include a vinyl group, an allyl group, a styrenyl group, a (meth) acryloyl group and the like.
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. Be done.
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 lithium ion, sodium ion, potassium ion, ammonium ion and the like.

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

(A51) Hydro 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. Calvir (meth) acrylate Examples of the monool include (i) linear aliphatic monool (tridecyl alcohol, myristyl alcohol, pentadecyl alcohol, cetyl alcohol, heptadecyl alcohol, stearyl alcohol, nonadecil alcohol, arachidyl alcohol). Etc.), (ii) alicyclic monool (cyclopentyl alcohol, cyclohexyl alcohol, cycloheptyl alcohol, cyclooctyl alcohol, etc.), (iii) aromatic aliphatic monool (benzyl alcohol, etc.) and a mixture of two or more thereof. Can be mentioned.

(A52) Poly (n = 2 to 30) Oxyalkylene (2 to 4 carbon atoms) Alkyl (1 to 18 carbon atoms) Ether (meth) Acrylate [Methanol ethylene oxide (hereinafter abbreviated as EO) 10 mol adduct (meth) ) Acrylate, methanol propylene oxide (hereinafter abbreviated as PO) 10 mol adduct (meth) acrylate, etc.]

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

(A53-2) (Meta) Acrylate Compound (i) Dialkyl (1 to 4 carbon atoms) Aminoalkyl (1 to 4 carbon atoms) (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.] Compounds (quaternized with a quaternary agent such as methyl chloride, dimethyl sulfate, benzyl chloride, dimethyl carbonate), etc.}

(A53-3) Heterocycle-containing vinyl compound Pyridine compound (7 to 14 carbon atoms, for example 2- or 4-vinylpyridine), imidazole compound (5 to 12 carbon atoms, for example N-vinylimidazole), pyrrole compound (carbon number) 6 to 13, for example N-vinylpyrrole), pyrrolidone compound (6 to 13 carbon atoms, for example N-vinyl-2-pyrrolidone)

(A53-4) Nitrile Group-Containing Vinyl Compound A 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 Nitrogen-containing vinyl compounds (8 to 16 carbon atoms, for example, nitrostyrene) and the like.

(A54) Vinyl Hydrocarbons (a54-1) aliphatic Vinyl Hydrocarbons Olefins having 2 to 18 or more carbon atoms (ethylene, propylene, butene, isopylene, pentene, heptene, diisoprene, octene, dodecene, octadecene, etc.), Dienes with 4 to 10 or more carbon atoms (butadiene, isoprene, 1,4-pentadiene, 1,5-hexadiene, 1,7-octadiene, etc.), etc.

(A54-2) Alicyclic vinyl hydrocarbons Cyclic unsaturated compounds having 4 to 18 or more carbon atoms, such as cycloalkene (eg cyclohexene), (di) cycloalkane [eg (di) cyclopentadiene], terpenes (eg, (di) cyclopentadiene] For example, pinen and limonene), inden

(A54-3) Aromatic Vinyl Hydrocarbons Aromatic unsaturated compounds having 8 to 20 or more carbon atoms, such as styrene, α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene, isopropylstyrene, and butyl. Styrene, phenylstyrene, cyclohexylstyrene, benzylstyrene

(A55) Vinyl Esters aliphatic vinyl esters [for example, alkenyl esters of aliphatic carboxylic acids (mono- or dicarboxylic acids) having 4 to 15 carbon atoms (for example, vinyl acetate, vinyl propionate, vinyl butyrate, diallyl adipate, isopropenyl acetate, etc.) Vinyl methoxyacetate)]
Aromatic vinyl ester [containing 9 to 20 carbon atoms, for example, an alkenyl ester of an aromatic carboxylic acid (mono- or dicarboxylic acid) (for example, vinylbenzoate, diallylphthalate, methyl-4-vinylbenzoate), an aromatic ring of an aliphatic carboxylic acid. Ester (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 (1 to 4 carbon atoms) ether (vinyl-2-methoxyethyl ether, methoxybutadiene, 3,4-dihydro-1,2-pyran, 2-butoxy-2'-vinyloxydiethyl ether, vinyl-2-ethyl Mercaptoethyl ether, etc.), Poly (2-4) (meth) allyloxyalkane (2 to 6 carbon atoms) (dialyloxyetane, trialiloxietan, tetraaryroxybutane, tetramethalyloxyetane, etc.)], Aromatic vinyl ether (8 to 20 carbon atoms, for example vinyl phenyl ether, phenoxystyrene)

(A57) Vinyl Ketone Aliper Vinyl Ketone (4 to 25 carbon atoms, for example, vinyl methyl ketone, vinyl ethyl ketone), aromatic vinyl ketone (9 to 21, carbon number, for example, vinyl phenyl ketone)

(A58) Unsaturated Dicarboxylic Acid Diester An unsaturated dicarboxylic acid diester having 4 to 34 carbon atoms, for example, a dialkyl fumarate (two alkyl groups are linear, branched or alicyclic groups having 1 to 22 carbon atoms). ), Dialkylmaleate (two alkyl groups are linear, branched or alicyclic groups with 1-22 carbon atoms).

When the radically polymerizable monomer (a5) is contained, the content thereof is preferably 0.1 to 3.0% by weight based on the weight of the entire monomer.

The preferable lower limit of the weight average molecular weight of the polymer compound constituting the coating film is 3,000, the more preferable lower limit is 5,000, and the further preferable lower limit is 7,000. On the other hand, the preferable upper limit of the weight average molecular weight of the polymer compound is 100,000, and the 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.
Equipment: Alliance GPC V2000 (manufactured by Waters)
Solvents: orthodichlorobenzene, DMF, THF
Standard substance: Polystyrene Sample concentration: 3 mg / ml
Column stationary phase: PLgel 10 μm, MIXED-B 2 in series (manufactured by Polymer Laboratories)
Column temperature: 135 ° C

The polymer compounds constituting the coating film are known polymerization initiators {azo-based initiators [2,2'-azobis (2-methylpropionitrile), 2,2'-azobis (2,4-dimethylvalero). Nitrile), 2,2'-azobis (2-methylbutyronitrile), etc.], known using a peroxide-based initiator (benzoyl peroxide, di-t-butyl peroxide, lauryl peroxide, etc.)}. Can be produced by the above-mentioned polymerization method (lumpy polymerization, solution polymerization, emulsion polymerization, suspension polymerization, etc.).
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 from the viewpoint of adjusting the weight average molecular weight to a preferable range. More preferably, it is 0.1 to 1.5% by weight, and 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, such as ethyl acetate and butyl acetate), alcohols (1 to 8 carbon atoms, such as methanol, ethanol and octanol), and hydrocarbons (carbon atoms). Examples include 4-8, such as n-butane, cyclohexane and toluene), amides (eg, N, N-dimethylformamide (hereinafter abbreviated as DMF)) and ketones (3-9 carbon atoms, eg methylethylketone), weight average. From the viewpoint of adjusting the molecular weight to a preferable range, the amount used is preferably 5 to 900% by weight, more preferably 10 to 400% by weight, 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, still more preferably 30 to 80% by weight.

Examples of the dispersion medium in emulsification polymerization and suspension polymerization include water, alcohol (for example, ethanol), ester (for example, ethyl propionate), light naphtha and the like, and examples of the emulsifier include higher fatty acid (10 to 24 carbon atoms) metal salt. (For example, sodium oleate and sodium stearate), higher alcohol (10 to 24 carbon atoms) sulfate ester metal salt (for example, sodium lauryl sulfate), tetramethyldecine ethoxylated tetramethyldecine diol, sodium sulfoethyl methacrylate, dimethylaminomethyl methacrylate, etc. Can be mentioned. Further, polyvinyl alcohol, polyvinylpyrrolidone and the like may be added as stabilizers.
The monomer concentration of the solution or dispersion is preferably 5 to 95% by weight, more preferably 10 to 90% by weight, even more preferably 15 to 85% by weight, and the amount of the polymerization initiator used is based on the total weight of the monomer. It is preferably 0.01 to 5% by weight, more preferably 0.05 to 2% by weight.
For 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 constituting the coating film is a cross-linking agent (A') having a reactive functional group that 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 / Alternatively, it may be a crosslinked polymer crosslinked with a polyol compound (a'2) (ethylene glycol or the like)}.

Examples of the method of cross-linking the polymer compound constituting the coating film using the cross-linking agent (A') include a method of coating the electrode active material with the polymer compound constituting the coating film and then cross-linking. Specifically, a resin solution containing an electrode active material and a polymer compound constituting a coating film is mixed and desolvated to produce a coating active material, and then a solution containing a cross-linking agent (A') is prepared. By mixing with the coating active material and heating, a reaction of removing the solvent and cross-linking occurs, and the reaction in which the polymer compound constituting the coating film is cross-linked by the cross-linking agent (A') is carried out on the surface of the electrode active material. There is a way to wake it up.
The heating temperature is adjusted according to the type of the cross-linking agent, but is preferably 70 ° C. or higher when the polyepoxy compound (a'1) is used as the cross-linking agent, and is preferably 70 ° C. or higher when the polyol compound (a'2) is used. It is preferably 120 ° C. or higher.

The acid value of the polymer compound constituting the coating film is preferably 600 to 800, more preferably 650 to 790, and even more preferably 680 to 780.
The acid value in the present specification means the acid value measured based on "the acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable matter test method of JIS K 0070-1992 chemical products". 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, a method for adjusting the constituent ratio of the carboxyl group-containing monomer in the polymer, and the like. From the viewpoint of increasing the acid value, it is desirable that the former adjusting method contains monomers such as acrylic acid and methacrylic acid, and in the latter adjusting method, it is desirable to increase the composition ratio of acrylic acid and methacrylic acid in the polymer. ..

The polymer compound constituting the coating film was prepared by dissolving LiN (FSO ₂ ) ₂ at a ratio of 2 mol / L in a mixed solvent having a volume ratio of ethylene carbonate (EC) and propylene carbonate (PC) of 1: 1. The degree of swelling with respect to the electrolytic solution is preferably 100 to 150%. The degree of swelling is more preferably 105 to 140%, and even more preferably 105 to 120%. When the degree of swelling of the polymer compound constituting the coating film is 100 to 150%, it is possible to impart appropriate wettability to the coating active material.

The degree of swelling can be measured by the following method.
A polymer compound solution in which the polymer compound constituting the coating film is dissolved is applied on a horizontal glass plate and naturally dried at room temperature for half a day. Next, the polymer compound was allowed to stand in a vacuum dryer heated to 150 ° C. for 3 hours, cooled to room temperature, and then cut into a size of 10 × 40 × 0.2 mm as a test piece, and this test piece was used as the above-mentioned test piece. Immerse it in the electrolytic solution at 50 ° C. for 3 days to bring it into a saturated liquid absorption state.
After that, the degree of swelling can be obtained by the following formula from the change in weight of the test piece before and after absorbing the liquid.
Swelling degree [%] = [(Weight of test piece after liquid absorption-Weight of test piece before liquid absorption) / Weight of test piece before liquid absorption] x 100

The polymer compound constituting the coating film preferably has a static contact angle of 45 ° or less with the electrolytic solution. 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 by allowing an electrolytic solution to stand in place of water droplets in accordance with "JIS R 3257: 1999".

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 agent.

When the coating film contains a conductive auxiliary agent, the conductive auxiliary agent is preferably selected from materials having conductivity.
Preferred conductive aids are metals [aluminum, stainless steel (SUS), silver, gold, copper and titanium, etc.], carbon [graphite and carbon black (acetylene black, ketjen black, furnace black, channel black and thermal lamps). Black, etc.), etc.], and mixtures thereof, etc. may be mentioned.
These conductive aids may be used alone or in combination of two or more. Further, it may be used as these alloys or metal oxides.
Among them, from the viewpoint of electrical stability, aluminum, stainless steel, carbon, silver, gold, copper, titanium and mixtures thereof are more preferable, and silver, gold, aluminum, stainless steel and carbon are particularly preferable. It is preferably carbon.
Further, as these conductive auxiliaries, a conductive material [preferably a metal one among the above-mentioned conductive auxiliaries] may be coated around a particle-based ceramic material or a resin material by plating or the like.

The shape (form) of the conductive auxiliary agent is not limited to the particle form, and may be a form other than the particle form, and is a form practically used as a so-called filler-based conductive auxiliary agent such as carbon nanofibers and carbon nanotubes. You may.

The average particle size of the conductive auxiliary agent 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 diameter of the conductive auxiliary agent" means the maximum distance L among the distances between arbitrary two points on the contour line of the conductive auxiliary agent. As the value of the "average particle size", the average value of the particle size of the particles observed in several to several tens of fields using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The calculated value shall be adopted.

When the coating film contains a conductive auxiliary agent, the ratio of the polymer compound constituting the coating film to the conductive auxiliary agent is not particularly limited, but it is coated by the weight ratio from the viewpoint of the internal resistance of the battery and the like. Polymer compound (resin solid content weight) constituting the membrane: The conductive auxiliary agent is preferably 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 0.3 to 1.0, preferably 0.5 to 0.8. Further, M is at least one element selected from the group consisting of Co, Mn and Al, preferably two or more kinds of elements, and more preferably Co and Mn, Co and Al.
Examples of such positive electrode active materials 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. Examples include _0.3 O ₂ .

The volume average particle size of the positive electrode active material is preferably 0.1 to 100 μm, more preferably 1 to 50 μm, and even more preferably 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 the particle size (Dv50) at an integrated value of 50% in the particle size distribution obtained by the laser diffraction / scattering method (also referred to as the microtrack method). The laser diffraction / scattering method is a method for obtaining a particle size distribution using scattered light obtained by irradiating particles with laser light, and for measuring the volume average particle size, a microtrack manufactured by Nikkiso Co., Ltd., etc. Can be used.

The coated positive electrode active material of the present invention can be produced by mixing a polymer compound constituting a coating film, a positive electrode active material, and a conductive additive used if necessary.
The order in which the polymer compound constituting the coating film, the positive electrode active material, and the conductive auxiliary agent are mixed is not particularly limited, and for example, a resin composition composed of the polymer compound constituting the coating film mixed in advance and the conductive auxiliary agent. The substance 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 agent may be mixed at the same time, or the positive electrode active material may form a coating film. A polymer compound may be mixed, and a conductive auxiliary agent may be further 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 placed in a universal mixer and stirred at 30 to 500 rpm. In this state, a resin solution containing the polymer compound constituting the coating film is added dropwise over 1 to 90 minutes, and 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 then holding it for 10 to 150 minutes.

[Positive 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 dispersibility of the active material and electrode moldability. More preferably, it is ~ 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 electrolytic solution 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 to 4.0 mol / L, and particularly preferably 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 adjusting ability of the coated positive electrode) can be imparted to the coated positive electrode active material. can.

As the electrolyte, an electrolyte used in a known electrolytic solution can be used, for example, a lithium salt of an inorganic anion such as LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ and LiN (FSO ₂ ) ₂ , LiN. Examples thereof include 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 . Of these, LiN (FSO ₂ ) ₂ is preferable from the viewpoint of battery output and charge / discharge cycle characteristics.

As the solvent, a non-aqueous solvent used in a known electrolytic solution can be used, and for example, a lactone compound, a cyclic or chain carbonate ester, a chain carboxylic acid ester, a cyclic or chain ether, a phosphoric acid ester, or a nitrile compound can be used. , Amid compounds, sulfones, sulfolanes and mixtures thereof can be used.

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

Examples of the cyclic carbonic acid ester 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, di-n-propyl carbonate and the like. ..

Examples of the chain carboxylic acid ester include methyl acetate, ethyl acetate, propyl acetate, methyl propionate and the like.

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

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

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

One of these solvents may be used alone, or two or more of them 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, and more preferably 130 ° C., from the viewpoint of preventing heat generation and expansion of the lithium ion battery. It is more preferably ℃ or higher. 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 the solvent is the temperature measured by the tag sealing method specified in JIS K 2265-1-2007.

Of these solvents, propylene carbonate and / or ethylene carbonate are preferable from the viewpoint of ionic 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 may further contain a conductive auxiliary agent in addition to the conductive auxiliary agent contained in the coating film as required. good. The conductive auxiliary agent contained in the coating film as needed is integrated with the coated positive electrode active material, whereas the conductive auxiliary agent contained in the positive electrode slurry is separately contained from the coated positive electrode active material.
As the conductive auxiliary agent that may be contained in the positive electrode slurry, the one 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 agent, the conductive auxiliary agent contained in the positive electrode slurry may be the same as or different from the conductive auxiliary agent contained in the coating film.

When the positive electrode slurry of the present invention contains a conductive auxiliary agent, the total content of the conductive auxiliary agent contained in the positive electrode slurry and the conductive auxiliary agent contained in the coating film is based on the weight of the positive electrode slurry excluding the electrolytic solution. It 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 agent contained in the positive electrode slurry and the conductive auxiliary agent contained in the coating film may be 2.5% by weight or more based on the weight of the positive electrode slurry excluding the electrolytic solution. preferable.

The positive electrode slurry of the present invention preferably does not contain the solvent-drying binder described in [Positive electrode for lithium ion battery] described later.

[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”) is a positive electrode including a positive electrode active material layer containing the coated positive electrode active material of the present invention and an electrolytic solution containing an electrolyte and a solvent. The positive electrode active material layer is characterized by being composed of a non-bonded body of the coated positive electrode active material.

In the positive electrode of the present invention, the positive electrode active material layer is composed of a non-bonded body of the coated positive electrode active material.
Here, the non-binding body 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 of being able to move according to an external force.

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

In the positive electrode of the present invention, the positive electrode active material layer preferably does not contain a solvent-drying binder.
Examples of the solvent-drying binder 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, and by volatilizing and distilling off the solvent, the surface is solidified without showing stickiness, and the positive electrode active substances and the positive electrode active material and the current collector are formed. Is firmly fixed.

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, and 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 contain a conductive auxiliary agent in addition to the conductive auxiliary agent contained in the coating film of the coated positive electrode active material as needed. The conductive auxiliary agent contained in the coating film as needed is integrated with the coated positive electrode active material, whereas the conductive auxiliary agent contained in the positive electrode active material layer is contained separately from the coated positive electrode active material. Can be distinguished.
As the conductive auxiliary agent 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 agent, the total content of the conductive auxiliary agent contained in the positive electrode and the conductive auxiliary agent contained in the coating film is determined from the positive electrode active material layer to the electrolytic solution. It 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 auxiliary agent contained in the positive electrode and the conductive auxiliary agent contained in the coating film shall be 2.5% by weight or more based on the weight of the positive electrode active material layer excluding the electrolytic solution. Is preferable.

In the positive electrode of the present invention, the thickness of the positive electrode active material layer is preferably 150 to 600 μm, more preferably 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, the positive electrode slurry of the present invention is applied onto a current collector with a coating device such as a bar coater, and then the non-woven fabric is allowed to stand on an active material to absorb liquid to remove the solvent. If necessary, a method of pressing with a press machine and the like can be mentioned.

In the positive electrode of the present invention, the materials constituting the current collector include metal materials such as copper, aluminum, titanium, stainless steel, nickel and alloys thereof, as well as calcined carbon, conductive polymer materials, conductive glass and the like. Can be mentioned.
The shape of the current collector is not particularly limited, and may be a sheet-shaped current collector made of the above-mentioned material and a deposited 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.

As described above, it is preferable that the positive electrode of the present invention further includes a current collector, and 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 the positive electrode active material layer is provided on the surface of the resin current collector.

As the conductive polymer material constituting the resin current collector, for example, a resin to which a conductive agent is added can be used.
As the conductive agent constituting the conductive polymer material, the same one as the conductive auxiliary agent which is an optional component of the coating film can be preferably used.
Resins constituting the conductive polymer material include polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polycycloolefin (PCO), polyethylene terephthalate (PET), polyether nitrile (PEN), and poly. 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 so on.
From the viewpoint of electrical stability, polyethylene (PE), polypropylene (PP), polymethylpentene (PMP) and polycycloolefin (PCO) are preferable, and polyethylene (PE), polypropylene (PP) and polymethylpentene are more preferable. (PMP).
The resin current collector can be obtained by a known method described in JP-A-2012-150905 and International Publication No. 2015/005116.

[Lithium-ion battery]
The lithium ion battery of the present invention is characterized by comprising the positive electrode of the present invention.

The lithium ion battery of the present invention can be obtained by combining electrodes to be opposite electrodes, storing the battery together with a 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 prepare a bipolar (bipolar) type electrode, and the bipolar (bipolar) type 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.

As the separator, a porous film made of polyethylene or polypropylene, a laminated film of a porous polyethylene film and a porous polypropylene, a non-woven fabric made of synthetic fibers (polyester fiber, aramid fiber, etc.) or glass fiber, etc., and silica on the surface thereof. , Known separators for lithium ion batteries such as those to which ceramic fine particles such as alumina and titania are attached can be mentioned.

Next, the present invention will be specifically described with reference to Examples, but the present invention is not limited to the 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 parts by weight.

<Preparation of polymer compounds constituting the coating film>
150 parts of DMF was placed in a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel and a nitrogen gas introduction 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, and 0.3 parts and 2,2'-azobis (2,4-dimethylvaleronitrile) of 2,2'-azobis (2,4-dimethylvaleronitrile). A solution of 0.8 parts of azobis (2-methylbutyronitrile) dissolved in 30 parts of DMF was continuously added dropwise over 2 hours with a dropping funnel while blowing nitrogen into a four-necked flask. Radical polymerization was carried out. After completion of the dropping, the reaction was continued at 75 ° C. for 3 hours. Then, 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 and dried under reduced pressure at 150 ° C. and 0.01 MPa for 3 hours, and DMF was distilled off to obtain a copolymer. This copolymer was roughly pulverized with a hammer and then additionally pulverized in a mortar to obtain a powdery polymer compound (P-1). Table 1 shows the types and amounts of the monomers used in the polymer compound (P-1). Table 1 shows the acid value, the weight average molecular weight, and the degree of swelling measured for the obtained polymer compound.

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

<Measurement conditions for weight average molecular weight>
Equipment: Alliance GPC V2000 (manufactured by Waters)
Solvents: orthodichlorobenzene, DMF, THF
Standard substance: Polystyrene Sample concentration: 3 mg / ml
Column stationary phase: PL gel 10um, MIXED-B 2 in series (manufactured by Polymer Laboratories)
Column temperature: 135 ° C

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

The same procedure as for the polymer compound (P-1) was carried out except that the type and amount of the monomer were changed as shown in Table 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 electrolytic solution>
An electrolytic solution (X-1) was prepared by dissolving LiN (FSO ₂ ) ₂ in a mixed solvent (volume ratio 1: 1) of ethylene carbonate (EC) and propylene carbonate (PC) at a ratio of 2.0 mol / L. ..

<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.
LiNi _0.8 Co _0.15 Al _0.05 O ₂ [manufactured by Toda Kogyo Co., Ltd., volume average particle diameter 6.4 μm] 96.9 parts was put into the universal mixer high speed mixer FS25 [manufactured by Artecnica] as the positive electrode active material. The above polymer compound solution was added dropwise over 2 minutes with stirring at room temperature and 720 rpm, and the mixture was further stirred for 5 minutes.
Then, in a stirred state, 3 parts of acetylene black [Denka Black (registered trademark) manufactured by Denki Kagaku Kogyo Co., Ltd.] was added as a conductive auxiliary agent in 2 minutes while being divided, and stirring was continued for 30 minutes. Then, the pressure was reduced to 0.01 MPa while maintaining the stirring, then the temperature was raised to 150 ° C. while maintaining the stirring and the degree of decompression, and the stirring, the degree of decompression and the temperature were maintained for 8 hours to distill off the volatile components. .. The obtained powder was classified by a sieve having an opening of 212 μm to obtain a coated positive electrode active material according to Example 1.

[Preparation of resin current collector]
With a twin-screw extruder, 70 parts of polypropylene [trade name "SunAllomer PL500A", manufactured by SunAllomer Ltd.], 25 parts of carbon nanotubes [trade name: "FloTube9000", manufactured by CNano] and dispersant [trade name "Umex 1001" , Sanyo Kasei Kogyo Co., Ltd.] 5 parts were melt-kneaded under the conditions of 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 conductive film for a resin collector having a film thickness of 100 μm. Next, the obtained conductive film for a resin current collector was cut to a size of 17.0 cm × 17.0 cm, nickel was vapor-deposited on one side, and then a terminal for current extraction (5 mm × 3 cm) was connected. A resin current collector was obtained.

[Manufacturing positive electrodes for lithium-ion batteries]
42 parts of electrolytic solution (X-1) and 4.2 parts of carbon fiber [Donacabo Milled S-243 manufactured by Osaka Gas Chemical Co., Ltd .: average fiber length 500 μm, average fiber diameter 13 μm: electrical conductivity 200 mS / cm] Using a planetary stirring type mixing and kneading device {Awatori Rentaro [manufactured by Shinky Co., Ltd.]}, the mixture was mixed at 2000 rpm for 5 minutes, and then 30 parts of the electrolytic solution and 206 parts of the coated positive electrode active material were added. Further, the mixture was mixed with Awatori Kentarou at 2000 rpm for 2 minutes, and after adding 20 parts of the above electrolytic solution, stirring by Awatori Kentarou was performed at 2000 rpm for 1 minute, and the electrolytic solution (X-1) was further added 2.3. After further addition of the parts, stirring by Awatori Rentaro was mixed at 2000 rpm for 2 minutes to prepare a positive electrode slurry. Example 1 in which the obtained positive electrode slurry was applied to one side of the above resin current collector so that the amount of active material was 80 mg / cm ² , pressed at a pressure of 1.4 MPa for about 10 seconds, and had a thickness of 340 μm. A positive electrode for a lithium ion battery (16.2 cm × 16.2 cm) according to the above was prepared.

[Manufacturing of lithium-ion batteries]
The obtained positive electrode was combined with a counter electrode Li metal via a separator (Celguard # 3501) to prepare a laminated cell.

<Example 2>
A positive electrode for a lithium ion battery according to Example 2 was produced in the same manner as in Example 1 except that the polymer compound (P-1) was changed to the polymer compound (P-2) to obtain a lithium ion battery. rice field.

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

<Example 4>
The polymer compound (P-1) was changed to the polymer compound (P-4), and LiNi _0.5 Co _0.2 was used instead of LiNi _0.8 Co _0.15 Al _0.05 O ₂ as the 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, and a lithium ion battery was prepared. 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 the lithium ion battery was prepared. 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 the lithium ion battery was prepared. Obtained.

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

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

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

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

<Initial discharge capacity>
The initial performance of the lithium ion battery was evaluated by the following method using a charge / discharge measuring device "HJ-SD8" [manufactured by Hokuto Denko Co., Ltd.] at 25 ° C.
After charging to 4.2V with a current of 0.1C by a constant current constant voltage charging method (also referred to as CCCV mode), charging was performed until the current value reached 0.01C while maintaining 4.2V. After a 10-minute rest, the current was discharged to 2.5 V with 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)].

<Ratio of resistance increase>
After the initial charge / discharge, charging / discharging was repeated 4 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 10 seconds from the start of the discharge in the 5th cycle. The resistance increase rate (%) was calculated from the ratio of the resistance in the first cycle and the resistance in the fifth cycle.

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

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.
The positive electrode active material is structural formula LiNi _x M _1-x O ₂ (in the formula, x is 0.3 to 1.0, and M is at least one element selected from the group consisting of Co, Mn and Al). It consists of 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 total weight of the monomer.
A 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 electrolytic solution in which LiN (FSO ₂ ) ₂ is dissolved at a ratio of 2 mol / L in a mixed solvent having a volume ratio of ethylene carbonate and propylene carbonate of 1: 1. The coated positive electrode active material for a lithium ion battery according to claim 1. The 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, which comprises an electrolytic 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 electrolytic solution is 1.2 to 5.0 mol / L. A positive electrode for a lithium ion battery comprising a positive electrode active material layer containing the coated positive electrode active material for a lithium ion battery according to claim 1 or 2 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, which is made of a non-bonded body of the coated positive electrode active material for the lithium ion battery. The positive electrode active material layer further contains a conductive auxiliary agent and contains.
The positive electrode for a lithium ion battery according to claim 5, wherein the content of the conductive auxiliary agent is less than 4% by weight based on the weight of the positive electrode active material layer excluding the electrolytic solution. The positive electrode for a lithium ion battery according to claim 5 or 6, wherein the flash point of the solvent in the electrolytic solution is 100 ° C. or higher. The positive electrode for a lithium ion battery according to any one of claims 5 to 7, wherein the positive electrode active material layer has a thickness of 150 to 600 μm. Further equipped with 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 the surface of the resin current collector. A lithium ion battery comprising the positive electrode for a lithium ion battery according to any one of claims 5 to 9.