JP5707847B2 - Positive electrode composition for lithium ion secondary battery, positive electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a positive electrode composition for a lithium ion secondary battery, a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery using the same.

  A positive electrode of a lithium ion secondary battery usually includes a current collector and a positive electrode active material layer provided on the surface of the current collector. The positive electrode active material layer generally contains a positive electrode active material and a binder, and is produced by applying a fluid positive electrode composition containing the positive electrode active material and the binder to the surface of the current collector. As the binder, a polymer is often used, and various studies have been made conventionally (see, for example, Patent Documents 1 to 3).

Japanese Patent Laid-Open No. 11-283630 Japanese Patent Laid-Open No. 10-55798 JP-A-7-201364

  Currently, cobalt-based positive electrode active materials containing Co are widely used as the positive electrode active material. However, from the viewpoint of safety, it is preferable to produce a positive electrode for a lithium ion secondary battery using a positive electrode active material containing Mn, Ni, or Fe. However, a positive electrode active material containing Mn, Ni, or Fe has a smaller capacity per unit weight and a lower specific gravity of the active material than a Co-based positive electrode active material. For this reason, when the positive electrode active material containing Mn, Ni, or Fe is used, the discharge capacity of the lithium ion secondary battery tends to be small. Therefore, development of a technique for increasing the discharge capacity is required in a lithium ion secondary battery using a positive electrode active material containing Mn, Ni, or Fe.

  One means for increasing the discharge capacity is to increase the thickness of the positive electrode active material layer. However, in the case where water-soluble celluloses are included in the positive electrode active material layer, the positive electrode active material layer is easily broken when the thickness of the positive electrode active material layer is increased. If the positive electrode active material layer is cracked, the power transmission path (power transmission path) in the positive electrode active material layer is lost, so it is difficult to increase the discharge capacity. On the other hand, since water-soluble celluloses are components that are required to be included in the positive electrode active material from the viewpoint of improving the dispersibility of the positive electrode active material, the case where a positive electrode active material containing Mn, Ni, or Fe is used. However, it is required to be included in the positive electrode active material layer.

From the above situation, even when a positive electrode active material containing Mn, Ni, or Fe is used, and a water-soluble cellulose is further included in the positive electrode active material layer, a lithium ion secondary battery having a large discharge capacity is obtained. A technology that can be easily manufactured has been demanded.
The present invention was devised in view of the above-described problems, and includes a positive electrode active material layer containing a positive electrode active material containing Mn, Ni or Fe and a water-soluble cellulose, and a lithium ion secondary battery having a large discharge capacity, And it aims at providing the positive electrode composition for lithium ion secondary batteries which can manufacture the said lithium ion secondary battery, and the positive electrode for lithium ion secondary batteries.

As a result of intensive studies to solve the above-mentioned problems, the present inventor has developed a positive electrode active material containing Mn, Ni, or Fe and a water-soluble substance by including a hydroxyl group-containing organic compound in the positive electrode composition for a lithium ion secondary battery. It was found that the positive electrode active material layer containing celluloses can be prevented from cracking, so that the thickness of the positive electrode active material layer can be increased. However, as a result of investigations, the positive electrode composition for a lithium ion secondary battery containing a hydroxyl group-containing organic compound has low stability when it contains a solvent, and the solid content contained in the composition tends to settle over time. There was found. If the solid content of the positive electrode composition for a lithium ion secondary battery settles, the coatability and the discharge capacity may be reduced. Then, this inventor further examined and discovered that sedimentation of the said solid content could be prevented by including a chelating agent in the positive electrode composition for lithium ion secondary batteries. From these findings, the present inventor, as a technology capable of achieving both the increase of the discharge capacity of the lithium ion secondary battery and the improvement of the stability of the positive electrode composition for the lithium ion secondary battery, Completed the invention.
That is, the gist of the present invention is the following [1] to [5].

[1] a positive electrode active material containing at least one metal selected from the group consisting of manganese, nickel and iron;
An acrylate polymer;
Water-soluble celluloses;
A hydroxyl-containing organic compound having a melting point of 30 ° C. or lower and a vapor pressure at 20 ° C. of less than 10 Pa;
A positive electrode composition for a lithium ion secondary battery, comprising a chelating agent.
[2] 0.5 parts by weight of the hydroxyl group-containing organic compound with respect to 100 parts by weight of the total amount of the positive electrode active material, the acrylate polymer, the water-soluble cellulose, the hydroxyl group-containing organic compound, and the chelating agent The positive electrode composition for a lithium ion secondary battery according to [1], containing 20 parts by weight or less.
[3] The chelating agent is an aminocarboxylic acid chelate compound, a phosphonic acid chelate compound, gluconic acid, alkali gluconate, citric acid, alkali citrate, malic acid, alkali malate, tartaric acid, or alkali tartrate A compound selected from the group consisting of salts,
0.005 part by weight or more and 3 parts by weight or less of the chelating agent with respect to 100 parts by weight of the total amount of the positive electrode active material, the acrylate polymer, the water-soluble cellulose, the hydroxyl group-containing organic compound, and the chelating agent. The positive electrode composition for lithium ion secondary batteries according to [1] or [2].
[4] A current collector, and a positive electrode active material layer provided on the surface of the current collector,
The positive electrode active material layer includes the positive electrode composition for a lithium ion secondary battery according to any one of [1] to [3],
The positive electrode for lithium ion secondary batteries whose weight of the said positive electrode active material layer is 10 mg / cm < ² > or more per unit area of the surface of the said collector.
[5] A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to [4].

  The positive electrode composition for a lithium ion secondary battery of the present invention has high stability in a fluid state containing a solvent. Further, if the positive electrode composition for a lithium ion secondary battery of the present invention is used, a thick positive electrode active material layer can be formed without cracking, so that conventional lithium using a positive electrode active material containing Mn, Ni or Fe is used. A lithium ion secondary battery having a larger discharge capacity than that of the ion secondary battery can be realized.

  In the positive electrode for a lithium ion secondary battery of the present invention, the thickness of the positive electrode active material layer can be increased. Therefore, if the positive electrode for a lithium ion secondary battery of the present invention is used, a lithium ion secondary battery having a larger discharge capacity than a conventional lithium ion secondary battery using a positive electrode active material containing Mn, Ni or Fe can be realized. .

  The lithium ion secondary battery of the present invention has a larger discharge capacity than a conventional lithium ion secondary battery using a positive electrode active material containing Mn, Ni, or Fe.

  Hereinafter, the present invention will be described in detail with reference to embodiments and examples, but the present invention is not limited to the embodiments and examples described below, and the gist of the present invention and its equivalent scope. Any change can be made without departing from the scope of the invention. In the following description, (meth) acryl means acryl and methacryl, and (meth) acrylate means acrylate and methacrylate.

[1. Positive electrode composition for lithium ion secondary battery]
The positive electrode composition for a lithium ion secondary battery of the present invention (hereinafter sometimes referred to as “the positive electrode composition of the present invention” as appropriate) contains at least one metal selected from the group consisting of manganese, nickel and iron. A positive electrode active material, an acrylate polymer, a water-soluble cellulose, a hydroxyl-containing organic compound having a melting point of 30 ° C. or lower and a vapor pressure at 20 ° C. of less than 10 Pa, a chelating agent (“chelating reagent”) Also called).

  Furthermore, the positive electrode composition of the present invention may contain a solvent. When the positive electrode composition of the present invention contains a solvent, the positive electrode composition of the present invention usually exists as a fluid composition. Hereinafter, the positive electrode composition of the present invention having such a fluid composition may be referred to as “the positive electrode slurry of the present invention”.

  When producing the positive electrode for lithium ion secondary batteries of the present invention (hereinafter sometimes referred to as “the positive electrode of the present invention” as appropriate), the positive electrode slurry of the present invention is usually applied to the surface of the current collector. By drying as necessary, a positive electrode active material layer is formed on the surface of the current collector to obtain the positive electrode of the present invention. Therefore, the positive electrode active material layer becomes a layer containing a positive electrode active material, an acrylate polymer, a water-soluble cellulose, a hydroxyl group-containing organic compound and a chelating agent, that is, a layer containing the positive electrode composition of the present invention. By using the positive electrode composition of the present invention, the above-mentioned positive electrode active material layer can be made thick, so that the lithium ion secondary battery of the present invention having a large discharge capacity (hereinafter sometimes referred to as “the battery of the present invention” as appropriate). Can be realized.

[1-1. Positive electrode active material)
As the positive electrode active material, a material containing at least one metal selected from the group consisting of manganese, nickel and iron and capable of reversibly releasing and occluding lithium ions is used. Especially, as a positive electrode active material, a material with high electron conductivity is preferable so that electron transport can be performed easily. Examples of such a positive electrode active material include an oxide mainly containing at least one metal selected from the group consisting of manganese, nickel, and iron, and lithium. Examples of the oxide include a lithium-containing composite metal oxide having a layered structure, a lithium-containing composite oxide having a spinel structure, and a lithium-containing composite metal oxide having an olivine structure.

Examples of the lithium-containing composite metal oxide having a layered structure include lithium-containing nickel oxide (LiNiO ₂ ), Ni—Mn—Al lithium composite oxide, and the like.
Examples of the lithium-containing composite metal oxide having a spinel structure include lithium manganate (LiMn ₂ O ₄ ) and Li [Mn _3/2 M ^A _1/2 ] O in which a part of Mn is substituted with another transition metal. ₄ (where ^{M A} is, Cr, Fe, Ni, Cu, etc.) and the like.
The lithium-containing composite metal oxide having an olivine structure, for _example, Li ^X M B PO ₄ (wherein, ^{M B} is, Mn, Fe, Ni, Cu , Mg, Zn, V, Ca, Sr, Ba, Ti, are those Al, Si, selected from the group consisting of B and Mo, at least a portion of the oxide total amount represents those containing Mn, Fe, or Ni as ^{M B,} X is 0 ≦ X ≦ 2 Olivine type lithium phosphate compound represented by the following formula: An iron-based oxide having poor electrical conductivity may be used as an electrode active material covered with a carbon material by allowing a carbon source material to be present during reduction firing. These compounds may be partially element-substituted.

  Among the compounds described above, as the positive electrode active material, a positive electrode active material containing manganese or iron is preferable, and a positive electrode active material containing manganese is particularly preferable. This is because the positive electrode active material containing manganese has a high reaction potential, and a high-energy lithium ion secondary battery can be obtained.

Moreover, unless the effect of this invention is impaired remarkably, the positive electrode composition of this invention may contain the positive electrode active material which does not contain any of manganese, nickel, and iron. However, from the viewpoint of remarkably exhibiting the effect of the present invention that a lithium ion secondary battery having a large discharge capacity can be realized while using a positive electrode active material containing manganese, nickel, or iron, the positive electrode composition of the present invention contains manganese. It is preferable that no positive electrode active material containing neither nickel nor iron is contained.
In addition, a positive electrode active material may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The positive electrode active material is usually present in the form of particles. The 50% volume cumulative diameter D50 of the positive electrode active material particles is usually 0.1 μm or more, preferably 1 μm or more, usually 50 μm or less, preferably 20 μm or less from the viewpoint of improving battery characteristics such as rate characteristics and cycle characteristics. It is. When the 50% volume cumulative diameter D50 of the positive electrode active material is in the above range, a battery having excellent rate characteristics and cycle characteristics can be realized, and handling in manufacturing the positive electrode composition and the positive electrode is easy. The 50% volume cumulative diameter D50 can be obtained as a particle diameter at which the cumulative volume calculated from the small diameter side in the measured particle size distribution is 50% by measuring the particle size distribution by a laser diffraction method.

  The amount of the positive electrode active material is usually 10 parts by weight or more, preferably 50 parts by weight or more with respect to 100 parts by weight of the total amount of the positive electrode active material, acrylate polymer, water-soluble cellulose, hydroxyl group-containing organic compound and chelating agent. It is usually 95 parts by weight or less. The discharge capacity of the battery of the present invention can be increased by setting the amount of the positive electrode active material to be equal to or higher than the lower limit value of the range, and the output of the battery of the present invention can be prevented from being lowered by setting the amount of the positive electrode active material to be lower than the upper limit value of the range. Further, detachment of the positive electrode active material from the positive electrode active material layer due to a shortage of the binder can be prevented.

[1-2. Acrylate polymer (binder)]
The acrylate polymer is usually a component that functions as a binder (binder). As the acrylate polymer binds the positive electrode active material, the positive electrode active material layer is fixed to the surface of the current collector, and the positive electrode active material is held by the positive electrode active material layer. It has become. Furthermore, an acrylate polymer is preferable as a binder for the positive electrode because it is excellent in oxidation resistance.

An acrylate polymer is a polymer containing a monomer unit derived from one or both of an acrylate ester and a methacrylate ester. Examples of acrylic acid esters and methacrylic acid esters include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, and 2-ethylhexyl. Acrylic acid alkyl esters such as acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, stearyl acrylate; methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, Pentyl methacrylate, hexyl methacrylate, Heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n- tetradecyl methacrylate, methacrylic acid alkyl esters such as stearyl methacrylate. Among these, non-carbonylic oxygen is shown because it exhibits lithium ion conductivity by moderate swelling into the electrolyte without eluting into the electrolyte, and it is difficult to cause bridging aggregation by the polymer in the dispersion of the positive electrode active material. Acrylic acid alkyl ester having 7 to 13 carbon atoms in the alkyl group bonded to the atom is preferred, and specifically heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate and lauryl acrylate are preferred. Furthermore, among them, those having 8 to 10 carbon atoms in the alkyl group bonded to the non-carbonyl oxygen atom are preferable, and specifically, octyl acrylate, 2-ethylhexyl acrylate, and nonyl acrylate are more preferable.
In addition, acrylic acid ester and methacrylic acid ester may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The proportion of monomer units derived from one or both of acrylic acid ester and methacrylic acid ester in the acrylate polymer is preferably 50% by weight or more, more preferably 60% by weight or more, and preferably 95% by weight. % Or less, more preferably 90% by weight or less. By making the ratio of monomer units derived from one or both of acrylic acid ester and methacrylic acid ester equal to or higher than the lower limit of the above range, the flexibility of the acrylate polymer is increased and the positive electrode active material layer is hardly broken. The strength of the acrylate polymer can be increased and the binding force can be increased by setting the amount to be not more than the upper limit of the above range.

  The acrylate polymer preferably includes a monomer unit derived from an α, β-unsaturated nitrile compound. Thereby, the intensity | strength of an acrylate-type polymer can be strengthened and binding force can be raised. Examples of the α, β-unsaturated nitrile compound include acrylonitrile and methacrylonitrile. Among them, methacrylonitrile is preferable. In addition, an alpha, beta-unsaturated nitrile compound may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The proportion of monomer units derived from the α, β-unsaturated nitrile compound in the acrylate polymer is preferably 3.0% by weight or more, more preferably 5.0% by weight or more, preferably 40%. % By weight or less, more preferably 30% by weight or less. By setting the ratio of the monomer units derived from the α, β-unsaturated nitrile compound to be equal to or higher than the lower limit of the above range, the strength of the acrylate polymer can be increased and the binding force can be increased. By setting it to the upper limit or less, the flexibility of the acrylate polymer can be increased and the positive electrode active material layer can be made difficult to break.

  The acrylate polymer preferably includes a monomer unit derived from unsaturated carboxylic acids. As a result, the stability of the acrylate polymer in the positive electrode composition can be improved, and the binding property of the acrylate polymer can be increased to further improve the output characteristics of the battery of the present invention.

Examples of unsaturated carboxylic acids include unsaturated monocarboxylic acids and derivatives thereof; unsaturated dicarboxylic acids and derivatives thereof.
Examples of the unsaturated monocarboxylic acid include acrylic acid, methacrylic acid, and crotonic acid. Examples of the unsaturated monocarboxylic acid derivative include 2-ethyl acrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E-methoxyacrylic acid, β-diamino Examples include acrylic acid.

Examples of the unsaturated dicarboxylic acid include maleic acid, fumaric acid, itaconic acid and the like. Examples of the unsaturated dicarboxylic acid derivatives include mono-substituted maleic acids such as methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, and fluoromaleic acid; monomethyl maleate, monophenyl maleate, malein Maleic acid monoesters such as monobutyl acid, monononyl maleate, monodecyl maleate, monododecyl maleate, monooctadecyl maleate, monofluoroalkyl maleate; and the like.
Among these, acrylic acid, methacrylic acid, maleic acid, fumaric acid and itaconic acid are preferable, and acrylic acid, methacrylic acid and itaconic acid are more preferable.
In addition, unsaturated carboxylic acids may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The proportion of monomer units derived from unsaturated carboxylic acids in the acrylate polymer is preferably 1% by weight or more, and preferably 3% by weight or less. By setting the proportion of monomer units derived from unsaturated carboxylic acids to be equal to or higher than the lower limit of the above range, the binding power of the acrylate polymer can be increased and the output characteristics of the battery of the present invention can be improved. Even when the amount is not more than the upper limit of the range, the binding force of the acrylate polymer can be increased and the output characteristics of the battery of the present invention can be improved.

  The acrylate polymer preferably includes a monomer unit derived from a monomer having a crosslinkable group (hereinafter, referred to as “crosslinkable group-containing monomer” as appropriate). By including a monomer unit having crosslinkability, the crosslink density of the binder can be increased and the electrolyte swellability can be decreased with a small amount, and the life characteristics of the secondary battery obtained as a result. Can be improved.

Examples of the crosslinkable group-containing monomer include a monofunctional monomer having one olefinic double bond having a heat-crosslinkable crosslinkable group, and a polyfunctional having at least two olefinic double bonds. And monomers.
The heat-crosslinkable crosslinkable group contained in the monofunctional monomer having one olefinic double bond is selected from the group consisting of, for example, an epoxy group, an N-methylolamide group, an oxetanyl group, and an oxazoline group. And a monomer containing at least one kind. Among these, an epoxy group-containing monomer is more preferable in terms of easy crosslinking and adjustment of the crosslinking density.

Examples of the monomer containing an epoxy group include a monomer containing a carbon-carbon double bond and an epoxy group, and a monomer containing a halogen atom and an epoxy group.
Examples of the monomer containing a carbon-carbon double bond and an epoxy group include unsaturated glycidyl ethers such as vinyl glycidyl ether, allyl glycidyl ether, butenyl glycidyl ether, o-allylphenyl glycidyl ether; butadiene monoepoxide, Diene or polyene monoepoxides such as chloroprene monoepoxide, 4,5-epoxy-2-pentene, 3,4-epoxy-1-vinylcyclohexene, 1,2-epoxy-5,9-cyclododecadiene; -Alkenyl epoxides such as epoxy-1-butene, 1,2-epoxy-5-hexene, 1,2-epoxy-9-decene; glycidyl acrylate, glycidyl methacrylate, glycidyl crotonate, glycidyl-4-heptenoate, glycidyl Glycidyl esters of unsaturated carboxylic acids such as sorbate, glycidyl linoleate, glycidyl-4-methyl-3-pentenoate, glycidyl ester of 3-cyclohexene carboxylic acid, glycidyl ester of 4-methyl-3-cyclohexene carboxylic acid; Can be mentioned.

  Examples of the monomer having a halogen atom and an epoxy group include epihalohydrins such as epichlorohydrin, epibromohydrin, epiiodohydrin, epifluorohydrin, β-methylepichlorohydrin; p-chlorostyrene oxide; dibromo Phenyl glycidyl ether; and the like.

  Examples of the monomer containing an N-methylolamide group include (meth) acrylamides having a methylol group such as N-methylol (meth) acrylamide.

  Examples of the monomer containing an oxetanyl group include 3-((meth) acryloyloxymethyl) oxetane, 3-((meth) acryloyloxymethyl) -2-trifluoromethyloxetane, and 3-((meth) acryloyl. And oxymethyl) -2-phenyloxetane, 2-((meth) acryloyloxymethyl) oxetane, 2-((meth) acryloyloxymethyl) -4-trifluoromethyloxetane, and the like.

  Examples of the monomer containing an oxazoline group include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl- Examples include 2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-methyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline.

  Examples of the polyfunctional monomer having at least two olefinic double bonds include allyl acrylate, allyl methacrylate, ethylene diacrylate, ethylene dimethacrylate, trimethylolpropane-triacrylate, trimethylolpropane-methacrylate, and dipropylene. Glycol diallyl ether, polyglycol diallyl ether, triethylene glycol divinyl ether, hydroquinone diallyl ether, tetraallyloxyethane, other allyl or vinyl ethers of polyfunctional alcohols, tetraethylene glycol diacrylate, triallylamine, trimethylolpropane-diallyl ether , Methylenebisacrylamide, divinylbenzene and the like.

  Among these, a polyfunctional monomer having at least two olefinic double bonds is preferable because the crosslinking density is easily improved and the binding property and the electrolytic solution resistance are excellent. Allyl acrylate and allyl methacrylate are preferable from the viewpoint of high polymerizability, and allyl methacrylate is more preferable.

  The lower limit of the proportion of monomer units derived from the monomer having a crosslinkable group in the acrylate polymer is preferably 0.05% by weight or more, more preferably 0.1% by weight or more. The upper limit is preferably 2.0% by weight or less, and more preferably 1.0% by weight or less. By making the ratio of the monomer unit derived from the monomer having a crosslinkable group equal to or higher than the lower limit of the above range, the function of the crosslinker can be sufficiently exhibited to effectively improve the life of the battery of the present invention. It can extend, and it can prevent that the polymerization stability of an acrylate polymer falls by excessive bridge | crosslinking by setting it as below the upper limit of the said range.

  An acrylate polymer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The glass transition temperature (Tg) of the acrylate polymer is usually −150 ° C. or higher, preferably −50 ° C. or higher, more preferably −35 ° C. or higher, usually + 100 ° C. or lower, preferably + 25 ° C. or lower, more preferably. + 5 ° C or lower. When the glass transition temperature Tg of the acrylate polymer is within this range, characteristics such as flexibility, binding property and winding property of the positive electrode of the present invention, and adhesion between the positive electrode active material layer and the current collector can be obtained. The balance is highly favorable, which is preferable.

  When the acrylate polymer is present as particles, the 50% volume cumulative diameter D50 of the acrylate polymer particles is preferably 50 nm or more, more preferably 70 nm or more, preferably 500 nm or less, more preferably 400 nm or less. It is. When the 50% volume cumulative diameter D50 is within this range, the strength and flexibility of the positive electrode of the present invention are good.

  The amount of the acrylate polymer is preferably 0.1 parts by weight or more, more preferably 100 parts by weight based on the total amount of the positive electrode active material, the acrylate polymer, the water-soluble cellulose, the hydroxyl group-containing organic compound, and the chelating agent. Is 0.5 parts by weight or more, preferably 10 parts by weight or less, more preferably 5 parts by weight or less. Since the positive electrode active material can be stably held in the positive electrode active material layer by setting the amount of the acrylate polymer to the lower limit value or more of the above range, the output characteristics of the battery of the present invention can be improved. By setting the amount to be not more than the upper limit of the above range, the discharge capacity can be increased by relatively increasing the amount of the positive electrode active material.

  The method for producing the acrylate polymer is not particularly limited, and any method such as a solution polymerization method, a dispersion polymerization method, a suspension polymerization method, a bulk polymerization method, and an emulsion polymerization method may be used. As the polymerization reaction, for example, any reaction such as ionic polymerization, radical polymerization, and living radical polymerization may be used. Examples of the polymerization initiator used for polymerization include lauroyl peroxide, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, t-butyl peroxypivalate, 3,3,5-trimethylhexanoyl peroxide, and the like. Organic peroxides, azo compounds such as α, α′-azobisisobutyronitrile, ammonium persulfate, potassium persulfate, and the like. Among these, since the acrylate polymer is preferably in a particle dispersion state, a dispersion polymerization method, an emulsion polymerization method, and a suspension polymerization method in an aqueous solvent are preferable.

[1-3. Water-soluble celluloses (thickeners)]
Water-soluble celluloses are usually components that function as thickeners. When the water-soluble cellulose functions as a thickener, the dispersibility of the positive electrode active material in the positive electrode composition of the present invention can be improved. Moreover, stability of the slurry for positive electrodes of this invention can be improved, or coating property can also be improved.

  The water-soluble cellulose means cellulose or a derivative of cellulose having water solubility. The term “water-soluble” as used herein means that when 0.5 g of the compound is dissolved in 100 g of water at 25 ° C., the insoluble content is less than 0.5% by weight. On the other hand, water-insoluble means that when 0.5 g of the compound is dissolved in 100 g of water at 25 ° C., the insoluble content is 90% by weight or more.

  Water-soluble celluloses can be classified into nonionic, anionic and cationic.

  Nonionic celluloses include, for example, alkylcelluloses such as methylcellulose, methylethylcellulose, ethylcellulose, and microcrystalline cellulose; hydroxyethylcellulose, hydroxybutylmethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, hydroxypropylmethylcellulose stearoxy ether, And hydroxyalkyl celluloses such as carboxymethyl hydroxyethyl cellulose, alkyl hydroxyethyl cellulose, and nonoxynyl hydroxyethyl cellulose;

  Examples of anionic celluloses include alkyl celluloses obtained by substituting the above nonionic cellulose semisynthetic polymers with various derivative groups, and sodium salts and ammonium salts thereof. Specific examples include sodium cellulose sulfate, methyl cellulose, methyl ethyl cellulose, ethyl cellulose, carboxymethyl cellulose (CMC), and sodium and ammonium salts thereof.

  Examples of cationic celluloses include low nitrogen hydroxyethyl cellulose dimethyl diallyl ammonium chloride (polyquaternium-4), chloride O- [2-hydroxy-3- (trimethylammonio) propyl] hydroxyethylcellulose (polyquaternium-10), and chloride O. -[2-hydroxy-3- (lauryldimethylammonio) propyl] hydroxyethylcellulose (polyquaternium-24) and the like.

Among these, anionic water-soluble celluloses are more preferable, and carboxymethyl cellulose and its sodium salt and ammonium salt are particularly preferable.
In addition, water-soluble celluloses may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The degree of etherification of the water-soluble celluloses is preferably 0.5 or more, more preferably 0.6 or more, preferably 1.0 or less, more preferably 0.8 or less. Here, the degree of etherification means the degree of substitution of a hydroxyl group (three) per anhydroglucose unit in cellulose with a substituent such as a carboxymethyl group. The degree of etherification can theoretically take a value of 0-3. When the degree of etherification is in the above range, the water-soluble celluloses are water-soluble while adsorbing on the surface of the positive electrode active material, so that the positive electrode active material can be uniformly dispersed.

  The amount of water-soluble cellulose is preferably 0.1 parts by weight or more, more preferably 100 parts by weight based on the total amount of positive electrode active material, acrylate polymer, water-soluble cellulose, hydroxyl group-containing organic compound and chelating agent. Is 0.3 parts by weight or more, preferably 5 parts by weight or less, more preferably 2 parts by weight or less. By setting the amount of the water-soluble cellulose to be not less than the lower limit value of the above range, the viscosity of the positive electrode slurry of the present invention can be increased and the stability can be improved. In particular, the discharge capacity can be increased by increasing the amount of the positive electrode active material.

[1-4. Hydroxyl-containing organic compound]
The positive electrode composition of the present invention contains a hydroxyl group-containing organic compound having a melting point of 30 ° C. or lower and a vapor pressure at 20 ° C. of less than 10 Pa. Thereby, even if the positive electrode active material layer is thickened, the positive electrode active material layer can be prevented from cracking. The reason why the positive electrode active material layer can be prevented from cracking is not clear, but is presumed as follows.
Although the water-soluble celluloses contained in the positive electrode active material layer are considered to adhere to the positive electrode active material and function as a binder in the positive electrode active material layer, ordinary water-soluble celluloses are hard and brittle. For this reason, when stress arises in a positive electrode active material layer, since water-soluble celluloses will be cracked by the stress, it is thought that the positive electrode active material layer is easy to break. Specifically, when the cathode active material layer is manufactured, a residual stress is generated in the cathode active material layer in the drying process, and the residual stress easily causes fine cracking (electrode plate cracking) in the cathode active material layer. It is thought that it was. On the other hand, since the hydroxyl-containing organic compound swells and plasticizes the water-soluble celluloses, the residual stress can be alleviated and the water-soluble celluloses can be prevented from cracking as described above. For this reason, even if the positive electrode active material layer is thickened, it is considered that the positive electrode active material layer can be prevented from cracking.

  The hydroxyl-containing organic compound can swell water-soluble celluloses, but preferably does not swell the acrylate polymer. By preventing the acrylate polymer from swelling, it is possible to prevent a decrease in the binding force of the acrylate polymer due to swelling, so that the positive electrode active material can be stably held in the positive electrode active material layer.

  A hydroxyl-containing organic compound is an organic compound containing a hydroxyl group. The number of hydroxyl groups contained in one molecule of the hydroxyl group-containing organic compound may be one or more, but preferably two or more. By having a hydroxyl group, the hydroxyl-containing organic compound can swell water-soluble celluloses. In addition, from the viewpoint of stably preventing swelling of the acrylate polymer, the number of hydroxyl groups contained in one molecule of the hydroxyl group-containing organic compound is preferably 6 or less.

  The melting point of the hydroxyl-containing organic compound is usually 30 ° C. or lower, preferably 25 ° C. or lower, more preferably 20 ° C. or lower. If the melting point of the hydroxyl group-containing organic compound is too high, the hydroxyl group-containing organic compound precipitates when the coating film is dried after the positive electrode slurry of the present invention is applied to the current collector, and the battery characteristics of the battery of the present invention are improved. May be exacerbated. In addition, although there is no restriction | limiting in the minimum of melting | fusing point of a hydroxyl-containing organic compound, Usually, it is -100 degreeC or more.

  The vapor pressure of the hydroxyl-containing organic compound at 20 ° C. is usually less than 10 Pa, preferably 5 Pa or less, more preferably 2 Pa or less, and particularly preferably 1 Pa or less. If the vapor pressure of the hydroxyl-containing organic compound at 20 ° C. is too high, foaming occurs when the coating film is dried after applying the positive electrode slurry of the present invention to the current collector, and the output of the battery of the present invention is reduced. There is a risk. In addition, there is no restriction | limiting in the minimum of the vapor pressure in 20 degreeC of a hydroxyl-containing organic compound, and becomes a value higher than zero.

  Examples of hydroxyl-containing organic compounds include glycols (alkanediols) and their condensates, alkanetriols and their condensates, and sugar alcohols.

  Examples of glycols (alkanediols) and condensates thereof include ethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, heptanediol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, dibutane Examples thereof include diol and tributanediol.

  Examples of alkanetriols and condensates thereof include glycerin, diglycerin, triglycerin, butanetriol, pentanetriol, hexanetriol, heptanetriol, trimethylolmethane, trimethylolethane, trimethylolpropane, trimethylolbutane, trimethylol. Examples include methylol pentane and trimethylol hexane.

Examples of sugar alcohols include erythritol, lactitol, maltitol, mannitol, sorbitol, xylitol and the like.
In addition, a hydroxyl-containing organic compound may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The amount of the hydroxyl-containing organic compound is preferably 0.5 parts by weight or more, more preferably 100 parts by weight based on the total amount of the positive electrode active material, the acrylate polymer, the water-soluble cellulose, the hydroxyl-containing organic compound and the chelating agent. Is 2 parts by weight or more, particularly preferably 5 parts by weight or more, and preferably 20 parts by weight or less. By making the amount of the hydroxyl-containing organic compound not less than the lower limit value of the above range, the positive electrode active material layer can be prevented from cracking, and the output characteristics of the positive electrode of the present invention can be improved. Thus, the discharge capacity can be increased by relatively increasing the amount of the positive electrode active material.

[1-5. (Chelating agent)
A chelating agent is a compound containing a ligand (usually a multidentate ligand) that binds to a metal ion to form a chelate compound. When the chelating agent is not included, the solid content (components other than the solvent) easily settles in the positive electrode slurry. However, by including the chelating agent, the solid content is prevented from being settled as described above, and the positive electrode of the present invention. The stability of the slurry can be improved. Thus, although the reason which can improve the stability of the slurry for positive electrodes of this invention is not certain, it estimates as follows.
Precipitation of the solid content in the positive electrode slurry as described above causes manganese, nickel, or iron contained in the positive electrode active material to be released into the solvent as metal ions, and the metal ions serve as a catalyst to produce an acrylate polymer. It is thought that this was caused by the progress of the polymerization reaction or the crosslinking reaction of the water-soluble celluloses. On the other hand, since the chelating agent provides a metal ion with a ligand to stabilize it, the released metal ion can be prevented from becoming a catalyst. Therefore, polymerization reaction and crosslinking reaction using free metal ions as a catalyst are difficult to proceed. Therefore, gelation and aggregation of the acrylate polymer and water-soluble cellulose are prevented, and the positive electrode slurry of the present invention is increased. It is assumed that sedimentation of viscosity and solid content can be suppressed. In addition, since the chelating agent functions as a dispersant for the positive electrode active material, the dispersibility of the positive electrode active material itself can also be improved, thereby improving the stability of the positive electrode slurry of the present invention. It is inferred that

  Examples of suitable chelating agents include aminocarboxylic acid chelating compounds, phosphonic acid chelating compounds, gluconic acid, gluconic acid alkali salts, citric acid, citric acid alkali salts, malic acid, malic acid alkali salts, tartaric acid. , And alkali tartaric acid salts.

Examples of aminocarboxylic acid-based chelate compounds include ethylenediaminetetraacetic acid, nitrilotriacetic acid, trans-1,2-diaminocyclohexanetetraacetic acid, diethylene-triaminepentaacetic acid, bis- (aminoethyl) glycol ether-N, N, N. Examples include ', N'-tetraacetic acid, N- (2-hydroxyethyl) ethylenediamine-N, N', N'-triacetic acid, and dihydroxyethylglycine.
Examples of the phosphonic acid-based chelate compound include 1-hydroxyethane-1,1-diphosphonic acid, nitrotrismethylenephosphonic acid, N, N, N ′, N′-ethylenediaminetetrakis (methylenephosphonic acid), 2-phosphonobutane- 1,2,4-tricarboxylic acid and the like.
Examples of the alkali salt of gluconic acid, alkali salt of citric acid, alkali salt of malate, and alkali salt of tartaric acid include the aforementioned salts of gluconic acid, citric acid, malic acid or tartaric acid and an alkali metal. Here, examples of the alkali metal include sodium and potassium, and sodium is preferable.

Among these, ethylenediaminetetraacetic acid, citric acid, sodium citrate, tartaric acid, sodium tartrate, and nitrotrismethylenephosphonic acid are more preferable as the chelating agent. The valence of the chelating agent is preferably 3 or more. This is because the chelating effect is excellent, and the thickening of the positive electrode composition of the present invention and the solid content can be stably prevented.
In addition, a chelating agent may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The amount of the chelating agent is preferably 0.005 parts by weight or more with respect to the total amount of 100 parts by weight of the positive electrode active material, acrylate polymer, water-soluble cellulose, hydroxyl group-containing organic compound and chelating agent, preferably It is 3 parts by weight or less, more preferably 1 part by weight or less, and particularly preferably 0.1 part by weight or less. By making the amount of the chelating agent not less than the lower limit of the above range, the positive electrode slurry of the present invention can be effectively stabilized, and by making it not more than the upper limit of the above range, the amount of the positive electrode active material can be relatively increased. To increase the discharge capacity. In addition, when the amount of the chelating agent is within the above range, crystal formation of the chelating agent in the positive electrode active material layer can be suppressed.

[1-6. solvent〕
As described above, when the positive electrode of the present invention is produced, the positive electrode composition of the present invention is often prepared as a fluid positive electrode slurry containing a solvent. Under the present circumstances, the solvent which the slurry for positive electrodes of this invention contains should just melt | dissolve or disperse | distribute an acrylate polymer and water-soluble cellulose. When a solvent that dissolves the acrylate polymer and water-soluble cellulose is used, the dispersion of the positive electrode active material is stabilized by the adsorption of the acrylate polymer and the water-soluble cellulose to the surface. It is preferable to select a specific type of solvent from the viewpoint of drying speed and environment.

  As the solvent, either water or an organic solvent may be used. Examples of the organic solvent include cycloaliphatic hydrocarbons such as cyclopentane and cyclohexane; aromatic hydrocarbons such as toluene and xylene; ketones such as ethyl methyl ketone and cyclohexanone; ethyl acetate, butyl acetate, and γ-butyrolactone Esters such as ε-caprolactone; Acylonitriles such as acetonitrile and propionitrile; Ethers such as tetrahydrofuran and ethylene glycol diethyl ether: Alcohols such as methanol, ethanol, isopropanol, ethylene glycol, and ethylene glycol monomethyl ether; N Amides such as -methylpyrrolidone and N, N-dimethylformamide; among them, N-methylpyrrolidone (NMP) is preferred. In addition, a solvent may be used individually by 1 type and may be used combining two or more types by arbitrary ratios. Of these, water is preferably used as the solvent.

  The amount of the solvent may be adjusted so that the viscosity of the positive electrode slurry of the present invention is suitable for coating. Specifically, the solid content concentration of the positive electrode slurry of the present invention is preferably 30% by weight or more, more preferably 40% by weight or more, preferably 90% by weight or less, more preferably 80% by weight or less. The amount is adjusted to be used.

[1-7. Other ingredients
The positive electrode composition of the present invention contains other components in addition to the positive electrode active material, the acrylate polymer, the water-soluble cellulose, the hydroxyl group-containing organic compound, the chelating agent, and the solvent unless the effects of the present invention are significantly impaired. Also good. Moreover, the positive electrode composition of this invention may contain only one type of other components, and may contain two or more types.

  For example, the positive electrode composition of the present invention may contain a conductive agent (also referred to as a conductivity imparting material). Examples of the conductive agent include conductive carbon such as acetylene black, ketjen black, carbon black, graphite, vapor-grown carbon fiber, and carbon nanotube; carbon powder such as graphite; fiber and foil of various metals; . By using the conductive agent, the electrical contact between the positive electrode active materials can be improved, and the discharge rate characteristics can be improved.

For example, the positive electrode composition of the present invention may contain a reinforcing material. Examples of the reinforcing material include various inorganic and organic spherical, plate-like, rod-like, or fibrous fillers.
The amount of the conductive agent and reinforcing agent used is usually 0 part by weight or more, preferably 1 part by weight or more, and usually 20 parts by weight or less, preferably 10 parts by weight or less, with respect to 100 parts by weight of the positive electrode active material. is there.

  In addition to the above components, the positive electrode composition of the present invention includes, for example, trifluoropropylene carbonate, vinylene carbonate, catechol carbonate, 1,6-dioxaspiro [4] in order to increase the stability and life of the battery of the present invention. , 4] nonane-2,7-dione, 12-crown-4-ether and the like.

[1-8. Method for producing positive electrode composition for lithium ion secondary battery]
The positive electrode composition of the present invention is, for example, a mixture of a positive electrode active material, an acrylate polymer, a water-soluble cellulose, a hydroxyl group-containing organic compound, a chelating agent, and a solvent and other components as necessary. Is obtained. The order of mixing is not particularly limited, and for example, the above-described components may be supplied to the mixer all at once and mixed at the same time. However, when the positive electrode composition of the present invention contains a conductive agent, the conductive agent and water-soluble celluloses are mixed in a solvent to disperse the conductive agent in the form of fine particles, and then this is combined with the remaining components. Mixing is preferable because the dispersibility of the resulting positive electrode composition of the present invention is improved.

  Examples of the mixer include a ball mill, a sand mill, a pigment disperser, a pulverizer, an ultrasonic disperser, a homogenizer, a planetary mixer, and a Hobart mixer.

[2. (Positive electrode for lithium ion secondary battery)
The positive electrode for lithium ion secondary batteries of the present invention (that is, the positive electrode of the present invention) includes a current collector and a positive electrode active material layer provided on the surface of the current collector. The positive electrode active material layer only needs to be provided on at least one side of the current collector, but is preferably provided on both sides.

[2-1. Current collector
The material of the current collector is not particularly limited as long as it has electrical conductivity and is electrochemically durable, but from the viewpoint of having heat resistance, for example, iron, copper, aluminum, nickel Metal materials such as stainless steel, titanium, tantalum, gold and platinum are preferred. Among these, aluminum is particularly preferable as a material for the current collector for the positive electrode.

The shape of the current collector is not particularly limited, and examples thereof include a film shape, a sheet shape, a net shape, a punching metal shape, a lath shape, a porous shape, and a foam shape. It is preferable.
The thickness of the current collector is preferably 0.001 mm to 0.5 mm.

The current collector may be used after the surface is previously roughened in order to increase the adhesive strength with the positive electrode active material layer. Examples of the roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method. In the mechanical polishing method, an abrasive cloth paper with a fixed abrasive particle, a grindstone, an emery buff, a wire brush provided with a steel wire or the like is used.
Further, an intermediate layer may be formed on the surface of the current collector in order to increase the adhesive strength and conductivity of the positive electrode active material layer.

[2-2. Positive electrode active material layer]
The positive electrode active material layer is a layer containing the positive electrode composition of the present invention. Therefore, the positive electrode active material layer includes a positive electrode active material, an acrylate polymer, water-soluble celluloses, a hydroxyl group-containing organic compound, and a chelating agent. Such a positive electrode active material layer can be formed by applying the positive electrode slurry of the present invention, which is a fluid positive electrode composition, to the surface of the current collector and drying it as necessary.

  The method for applying the positive electrode slurry of the present invention to the current collector is not particularly limited. Examples thereof include a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method. By applying the positive electrode slurry of the present invention to the surface of the current collector, the solid content of the positive electrode slurry of the present invention (positive electrode active material, acrylate polymer, water-soluble cellulose, hydroxyl group-containing organic compound, chelating agent, etc. ) Adheres in layers.

  After applying the positive electrode slurry of the present invention, the positive electrode slurry of the present invention adhering to a layer is dried to remove the solvent. Examples of the drying method include methods such as drying with warm air, hot air, low-humidity air, etc .; vacuum drying; Thereby, the positive electrode active material layer containing the positive electrode composition of the present invention is formed on the surface of the current collector.

  Moreover, you may heat-process, after apply | coating the slurry for positive electrodes of this invention as needed. The heat treatment is usually performed at a temperature of 120 ° C. or higher for 1 hour or longer.

  Thereafter, the positive electrode active material layer is preferably subjected to pressure treatment using, for example, a mold press or a roll press. By performing the pressure treatment, the porosity of the positive electrode active material layer can be lowered. The porosity is preferably 5% or more, more preferably 7% or more, preferably 15% or less, more preferably 13% or less. If the porosity is too low, the volume capacity may be difficult to increase, or the positive electrode active material layer may be easily peeled off and defects may be easily generated. Moreover, when the porosity is too high, the charging efficiency and the discharging efficiency may be lowered.

The weight of the positive electrode active material layer is preferably 10 mg / cm ² or more, more preferably 15 mg / cm ² or more, and particularly preferably 18 mg / cm ² or more per unit area of the surface of the current collector. The discharge capacity of the battery of the present invention can be increased by forming the positive electrode active material layer thick and increasing the weight as described above. Moreover, even if the positive electrode active material layer is thickened to such an extent that it has a large weight as described above, the positive electrode active material layer using the positive electrode composition of the present invention is hardly broken. The upper limit of the weight range of the positive electrode active material layer is not particularly limited, but is usually 70 mg / cm ² or less.

[3. Lithium ion secondary battery)
The lithium ion secondary battery of the present invention (that is, the battery of the present invention) includes the positive electrode of the present invention. Since the positive electrode of the present invention can be thickened while using a positive electrode active material containing manganese, nickel or iron, the battery of the present invention is a conventional lithium ion using a positive electrode active material containing manganese, nickel or iron. The discharge capacity can be made larger than that of the secondary battery.
The battery of the present invention usually includes a positive electrode, a negative electrode, and an electrolytic solution, and a separator as necessary. As the positive electrode, the positive electrode of the present invention is used.

[3-1. Negative electrode)
Examples of the negative electrode include those including a current collector and a negative electrode active material layer provided on the surface of the current collector. Examples of the current collector include those similar to the positive electrode current collector, and among them, a current collector made of copper is preferable. The negative electrode active material layer is usually a layer containing a negative electrode active material and a binder.

  Examples of the negative electrode active material include carbonaceous materials such as amorphous carbon, graphite, natural graphite, mesocarbon microbeads, pitch-based carbon fibers, and conductive polymer compounds such as polyacene. Further, metals such as silicon, tin, zinc, manganese, iron and nickel, and alloys thereof; oxides of the metals or alloys; sulfates of the metals or alloys; Moreover, metallic lithium; lithium alloys, such as Li-Al, Li-Bi-Cd, Li-Sn-Cd; lithium transition metal nitride, etc. are mentioned. Furthermore, as the negative electrode active material, a material obtained by attaching a conductive agent to the surface by a mechanical modification method may be used. In addition, a negative electrode active material may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  As the binder for the negative electrode active material layer, a diene polymer is preferable because of excellent reduction resistance and a strong binding force. The diene polymer is a polymer including monomer units formed by polymerizing conjugated dienes such as butadiene and isoprene. The proportion of the monomer unit obtained by polymerizing conjugated diene in the diene polymer is usually 40% by weight or more, preferably 50% by weight or more, more preferably 60% by weight or more. Examples of diene polymers include homopolymers of conjugated dienes such as polybutadiene and polyisoprene; copolymers of different types of common diene; copolymers of conjugated dienes and monomers copolymerizable therewith. Polymer; and the like. Examples of the copolymerizable monomer include α, β-unsaturated nitrile compounds such as acrylonitrile and methacrylonitrile; unsaturated carboxylic acids such as acrylic acid and methacrylic acid; styrene, chlorostyrene, vinyltoluene, styrene monomers such as t-butyl styrene, vinyl benzoic acid, methyl vinyl benzoate, vinyl naphthalene, chloromethyl styrene, hydroxymethyl styrene, α-methyl styrene, divinylbenzene; olefins such as ethylene and propylene; vinyl chloride And halogen atom-containing monomers such as vinylidene chloride; vinyl esters such as vinyl acetate, vinyl propionate, vinyl butyrate and vinyl benzoate; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether and butyl biether; methyl vinyl ketone and ethyl Niruketon, butyl vinyl ketone, hexyl vinyl ketone, such as isopropenyl vinyl ketone; N- vinylpyrrolidone, vinylpyridine, heterocycle-containing vinyl compounds such as vinyl imidazole; and the like. In addition, as for the conjugated diene and the copolymerizable monomer, one type may be used alone, or two or more types may be used in combination at any ratio. Moreover, the binder of a negative electrode active material layer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The amount of the binder is preferably 0.1 parts by weight or more, more preferably 0.2 parts by weight or more, particularly preferably 0.5 parts by weight or more, preferably 100 parts by weight or less of the negative electrode active material. 5 parts by weight or less, more preferably 4 parts by weight or less, and particularly preferably 3 parts by weight or less. When the amount of the binder is within the above range, it is possible to stably prevent the negative electrode active material from dropping from the negative electrode without inhibiting the battery reaction.

  The negative electrode active material layer may contain other components in addition to the negative electrode active material and the binder. Examples thereof include a conductive agent and a reinforcing material. In addition, the other component may be contained individually by 1 type, and may be contained combining two or more types by arbitrary ratios.

[3-2. Electrolyte)
The electrolyte solution may be liquid or gel as long as it is used for a normal lithium ion secondary battery. Usually, an electrolytic solution containing an electrolyte and a solvent is used.

As the electrolytic solution, an organic electrolytic solution in which a supporting electrolyte is dissolved in an organic solvent is usually used. For example, a lithium salt is used as the supporting electrolyte. Examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, C ₄ F ₉ SO ₃ Li, CF ₃ COOLi, (CF ₃ CO) ₂ NLi , (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) NLi, and the like. Among them, LiPF ₆ , LiClO ₄ , and CF ₃ SO ₃ Li are preferable because they are easily soluble in a solvent and exhibit a high degree of dissociation. In addition, electrolyte may be used individually by 1 type and may be used combining two or more types by arbitrary ratios. Usually, the lithium ion conductivity tends to increase as the supporting electrolyte having a higher degree of dissociation is used, so that the lithium ion conductivity can be adjusted depending on the type of the supporting electrolyte.

  The organic solvent used in the electrolytic solution is not particularly limited as long as it can dissolve the supporting electrolyte. For example, dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), Carbonates such as butylene carbonate (BC) and methyl ethyl carbonate (MEC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfur-containing compounds such as sulfolane and dimethyl sulfoxide Etc. are preferably used. Moreover, you may use the liquid mixture of these solvents. Among these, carbonates are preferable because they have a high dielectric constant and a wide stable potential region. Usually, the lower the viscosity of the solvent used, the higher the lithium ion conductivity tends to be, so the lithium ion conductivity can be adjusted depending on the type of solvent.

  The density | concentration of the supporting electrolyte in electrolyte solution is 1 mass% or more normally, Preferably it is 5 mass% or more, and is 30 mass% or less normally, Preferably it is 20 mass% or less. Further, depending on the type of the supporting electrolyte, it may be used usually at a concentration of 0.5 mol / L to 2.5 mol / L. Even if the concentration of the supporting electrolyte is too low or too high, the ionic conductivity tends to decrease. Usually, the lower the concentration of the electrolytic solution, the higher the degree of swelling of the polymer particles such as the binder, so that the lithium ion conductivity can be adjusted by adjusting the concentration of the electrolytic solution.

  Furthermore, you may include an additive etc. in electrolyte solution as needed.

[3-3. separator〕
The separator is a member provided between the positive electrode and the negative electrode in order to prevent a short circuit of the electrode. As this separator, a porous substrate having a pore portion is usually used. Examples of separators include (a) a porous separator having pores, (b) a porous separator having a polymer coating layer formed on one or both sides, and (c) a porous material containing an inorganic filler or an organic filler. And a porous separator having a coating layer formed thereon.

  (A) As a porous separator having a pore portion, for example, a porous membrane having a fine pore diameter that has no electron conductivity and ion conductivity and high resistance to an organic solvent is used. Specific examples include microporous membranes made of polyolefin polymers (eg, polyethylene, polypropylene, polybutene, polyvinyl chloride), and mixtures or copolymers thereof; polyethylene terephthalate, polycycloolefin, polyether sulfone. , Polyamide, polyimide, polyimide amide, polyaramid, polycycloolefin, nylon, polytetrafluoroethylene, etc .; microporous membrane; polyolefin fiber woven or non-woven fabric thereof; aggregate of insulating substance particles; Etc.

  (B) Examples of the porous separator having a polymer coat layer formed on one side or both sides include solid polymers such as polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, and polyvinylidene fluoride hexafluoropropylene copolymer Examples include a polymer film for an electrolyte or a gel polymer electrolyte; a gelled polymer coat layer.

(C) As a porous separator in which a porous coat layer containing an inorganic filler or an organic filler is formed, for example, a separator coated with a porous film layer composed of an inorganic filler or an organic filler and the filler dispersant; Etc.
Among these, a separator coated with a porous membrane layer composed of an inorganic filler or an organic filler and the dispersant for the filler reduces the overall thickness of the separator and increases the active material ratio in the battery to increase the capacity per volume. It is preferable because it can be raised.

  The thickness of the separator is usually 0.5 μm or more, preferably 1 μm or more, and is usually 40 μm or less, preferably 30 μm or less, more preferably 10 μm or less. Within this range, the resistance due to the separator in the battery is reduced, and the workability during battery production is excellent.

[3-4. Method for producing lithium ion secondary battery]
The method for producing the battery of the present invention is not particularly limited. For example, a negative electrode and a positive electrode may be overlapped via a separator, and this may be wound or folded according to the shape of the battery and placed in the battery container, and the electrolytic solution may be injected into the battery container and sealed. Furthermore, if necessary, an overcurrent prevention element such as an expanded metal, a fuse, or a PTC element; a lead plate or the like may be provided to prevent an increase in pressure inside the battery and overcharge / discharge. The shape of the battery may be any of a laminated cell type, a coin type, a button type, a sheet type, a cylindrical type, a square shape, a flat type, and the like.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the examples shown below, and may be arbitrarily changed without departing from the gist of the present invention and its equivalent scope. May be implemented. In the following description, “parts” and “%” representing amounts are based on weight unless otherwise specified. Further, in the following description, the operation was performed in an environment of normal temperature and pressure unless otherwise specified regarding temperature and pressure.

[Evaluation method]
[Method for evaluating stability of positive electrode slurry]
The prepared slurry for positive electrode is put into a 10 mmφ test tube with a liquid volume of 10 cm ³ or more. After allowing the test tube to stand for one day and three days, the state of the positive electrode slurry was visually observed to confirm the presence or absence of sediment at the bottom of the test tube. When the stability of the positive electrode slurry is low, sediment is observed at the bottom of the test tube. The presence or absence of sediment was determined according to the following criteria.
A: There was no sediment after both standing for one day and standing for three days.
B: There is no sediment after standing for one day, but there is sediment after standing for three days.
C: There is a sediment after standing for one day.

[Method for evaluating cracking of positive electrode]
Before rolling the produced positive electrode fabric with a roll press, the presence or absence of cracks and cracks in the positive electrode fabric was evaluated. When the plastic effect of water-soluble celluloses by the hydrous compound is small, cracks and cracks are observed in the positive electrode raw material. Based on the evaluation criteria of JISK5600-8-4, the state of cracks and cracks in the positive electrode fabric was evaluated as follows.
0: Cannot be seen even if it is magnified 10 times.
1: You can see it by magnifying it twice.
2: Recognizable at last with normal corrected vision.
3: Recognized clearly with normally corrected visual acuity.
4: Large crack generally reaching 1 mm wide.
5: Very large crack generally exceeding 1 mm in width.

[Method for evaluating battery characteristics (output characteristics)]
The produced battery is charged to 4.3 V by a constant current method of 0.1 C, and then discharged to 3.0 V at 0.1 C to obtain a 0.1 C discharge capacity a.
Thereafter, the battery is charged to 4.3 V at 0.1 C, and then discharged to 3.0 V at 10 C to obtain a 10 C discharge capacity b.
Using the average value of 10 cells as a measured value, a capacity retention represented by the ratio of discharge capacity between 10C discharge capacity b and 0.1C discharge capacity a (b / a (%)) was obtained, and this was determined as the output characteristic. Use the following criteria for evaluation. The higher the capacity retention rate, the better the output characteristics.
A: 70% or more B: 60% or more and less than 70% C: 40% or more and less than 60% D: Less than 40%

[Example 1]
(Production of binder composition)
70 parts of ion-exchanged water and 0.3 part of sodium dodecylbenzenesulfonate were respectively supplied to a reactor equipped with a stirrer and mixed sufficiently, and the gas phase part was replaced with nitrogen gas, and the temperature was raised to 70 ° C. .
On the other hand, in a separate container, 50 parts of ion-exchanged water, 0.5 part of sodium dodecylbenzenesulfonate, and 78 parts of 2-ethylhexyl acrylate, 20 parts of acrylonitrile, 1.8 parts of methacrylic acid, and allyl as polymerizable monomers A monomer mixture was obtained by mixing 0.2 parts of methacrylate. This monomer mixture was continuously added to the reactor over 4 hours to conduct polymerization. The reaction was started by adding 0.5 part of potassium persulfate as a 3% aqueous solution of potassium persulfate to the reactor when the addition of the monomer mixture was started. Moreover, reaction was performed at 60 degreeC during addition of a monomer mixture. After the addition of the monomer mixture was completed, the reaction was further terminated by stirring at 80 ° C. for 3 hours to obtain an aqueous dispersion (binder composition) containing an acrylate polymer. The polymerization conversion rate was 98.5%.

[Production of slurry for positive electrode]
93 parts of spinel manganese (LiMn ₂ O ₄ ; Mn content 60%) as a positive electrode active material, 3.5 parts of acetylene black (HS-100: Electrochemical Industry) as a conductive agent, and the binder composition 2.5 Part (solid content concentration 40%), 40 parts aqueous solution containing carboxymethylcellulose having a degree of etherification of 0.8 as a thickener (solid content concentration 2%), and 10 parts triethylene glycol as the hydroxyl-containing organic compound Then, 0.01 part of sodium citrate as a chelating agent and an appropriate amount of water were stirred with a planetary mixer to produce a positive electrode slurry as a fluid positive electrode composition for a lithium ion secondary battery. Triethylene glycol has a melting point of −7.2 ° C. and a vapor pressure at 20 ° C. of 0.02 Pa. A part of the produced positive electrode slurry was taken and its stability was evaluated. The results are shown in Table 1.

[Production of positive electrode]
The positive electrode slurry was applied on a 20 μm thick aluminum foil with a comma coater so that the weight of the positive electrode active material layer after drying was 20 mg / cm ² . After drying at 60 ° C. for 20 minutes, heat treatment was performed at 150 ° C. for 2 hours to obtain an electrode raw material. The obtained electrode fabric was evaluated for cracks. The results are shown in Table 1. Moreover, this electrode raw fabric was rolled by the roll press, and the positive electrode plate was manufactured as a positive electrode for lithium ion secondary batteries.

[Production of battery]
The positive electrode plate was cut into a disk shape with a diameter of 16 mm to prepare a positive electrode. A separator made of a disk-shaped polypropylene porous film having a diameter of 18 mm and a thickness of 25 μm, a lithium metal used as a negative electrode, and an expanded metal are sequentially laminated on the positive electrode active material layer surface side of this positive electrode, and this is installed with a polypropylene packing. It was stored in a stainless steel coin-type outer container (diameter 20 mm, height 1.8 mm, stainless steel thickness 0.25 mm). The electrolyte was poured into the container so that no air remained. After that, a stainless steel cap having a thickness of 0.2 mm was put on and fixed to the exterior container through a polypropylene packing, and the battery can was sealed. This produced a coin battery as a lithium ion secondary battery having a diameter of 20 mm and a thickness of about 2 mm.

In addition, as an electrolyte, LiPF ₆ is used as an electrolyte in a mixed solvent obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at EC: DEC = 1: 2 (volume ratio at 20 ° C.). A solution dissolved at a concentration of mol / liter was used.
The output characteristics were evaluated using the coin battery thus obtained. The results are shown in Table 1.

[Example 2]
A coin battery was manufactured in the same manner as in Example 1 except that the amount of triethylene glycol was changed to 15 parts. Table 1 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics.

[Example 3]
A coin battery was manufactured in the same manner as in Example 1 except that the amount of triethylene glycol was changed to 6 parts. Table 1 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics.

[Example 4]
A coin battery was manufactured in the same manner as in Example 1 except that the amount of triethylene glycol was changed to 1 part. Table 1 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics.

[Example 5]
A coin battery was manufactured in the same manner as in Example 1 except that the amount of sodium citrate was changed to 2 parts. Table 1 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics.

[Example 6]
A coin battery was manufactured in the same manner as in Example 1 except that ethylenediaminetetraacetic acid was used instead of sodium citrate as the chelating agent. Table 1 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics.

[Example 7]
A coin battery was manufactured in the same manner as in Example 1 except that hydroxyethyl cellulose was used instead of carboxymethyl cellulose as a thickener. Table 2 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics.

[Example 8]
A coin battery was manufactured in the same manner as in Example 1 except that LiFePO ₄ was used instead of LiMn ₂ O ₄ as the positive electrode active material. Table 2 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics.

[Example 9]
A coin battery was manufactured in the same manner as in Example 1 except that glycerin was used instead of triethylene glycol as the hydroxyl-containing organic compound. Table 2 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics. Glycerin has a melting point of 18 ° C., a vapor pressure at 50 ° C. of 0.3 Pa, and a vapor pressure at 25 ° C. of 0.01 Pa. Although the vapor pressure at 20 ° C. was not measured for glycerin, the vapor pressure at 25 ° C. is 0.01 Pa, so it is clear that the vapor pressure at 20 ° C. is 0.01 Pa or less.

[Example 10]
A coin battery was manufactured in the same manner as in Example 1 except that hexanetriol was used in place of triethylene glycol as the hydroxyl-containing organic compound. Table 2 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics. In addition, hexanetriol has a melting point of 30 ° C. and a vapor pressure at 20 ° C. of 1.3 Pa.

[Example 11]
A coin battery was manufactured in the same manner as in Example 1 except that tripropylene glycol was used instead of triethylene glycol as the hydroxyl-containing organic compound. Table 2 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics. Tripropylene glycol has a melting point of −54 ° C. and a vapor pressure at 20 ° C. of 0.67 Pa.

[Example 12]
A coin battery was manufactured in the same manner as in Example 1 except that tartaric acid was used instead of sodium citrate as the chelating agent. Table 2 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics.

[Comparative Example 1]
When producing the positive electrode slurry, a coin battery was produced in the same manner as in Example 1 except that triethylene glycol was not used. Table 3 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics.

[Comparative Example 2]
A coin battery was manufactured in the same manner as in Example 1 except that sodium citrate was not used when manufacturing the positive electrode slurry. Table 3 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics.

[Comparative Example 3]
A coin battery was manufactured in the same manner as in Example 1 except that butanediol was used instead of triethylene glycol. Table 3 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics. Butanediol has a melting point of 20 ° C. and a vapor pressure at 20 ° C. of 10 Pa.

[Comparative Example 4]
A coin battery was manufactured in the same manner as in Example 1 except that polyvinyl alcohol was used instead of carboxymethylcellulose. Table 3 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics.

[Comparative Example 5]
A coin battery was manufactured in the same manner as in Example 1 except that isosorbide was used instead of triethylene glycol. Table 3 shows the evaluation results of the stability of the positive electrode slurry, the positive electrode cracking, and the battery characteristics. Note that isosorbide had a melting point of 60 ° C. and decomposed before vaporization, so the vapor pressure could not be measured.

[Consideration]
In the examples, all the positive electrode slurries were stable with little or no sediment. Further, in the examples, cracks and cracks of the positive electrode were not generated, or even if they were generated, only very small ones were generated. Furthermore, in the examples, the output characteristics were high and the discharge capacity was excellent. Therefore, according to the present invention, it was confirmed that both the stability of the positive electrode slurry and the large discharge capacity can be achieved while using the positive electrode active material containing manganese, nickel, or iron.
On the other hand, those containing no hydroxyl-containing organic compound (Comparative Example 1), those containing no chelating agent (Comparative Example 2), and containing a hydroxyl-containing organic compound having a vapor pressure of 10 Pa or higher at 20 ° C. (Comparative Example 3), those containing no water-soluble cellulose (Comparative Example 4), those containing a hydroxyl-containing organic compound having a melting point exceeding 30 ° C. (Comparative Example 5) are slurries. The stability of the battery was inferior, the positive electrode was cracked or cracked, and the output characteristics were inferior.

  The present invention is suitable for use in a lithium ion secondary battery. Moreover, the lithium ion secondary battery of the present invention can be used, for example, as a power source for electric devices such as mobile phones and notebook computers, and vehicles such as electric vehicles.

Claims (5)

A positive electrode active material containing at least one metal selected from the group consisting of manganese, nickel and iron;
An acrylate polymer;
Water-soluble celluloses;
A hydroxyl-containing organic compound having a melting point of 30 ° C. or lower and a vapor pressure at 20 ° C. of less than 10 Pa;
Only contains a chelating agent,
The hydroxyl-containing organic compound is a glycol or an alkanetriol;
The acrylate-based polymer, including a positive electrode composition for a lithium ion secondary battery as a binder.   0.5 parts by weight or more and 20 parts by weight of the hydroxyl group-containing organic compound with respect to 100 parts by weight of the total amount of the positive electrode active material, the acrylate polymer, the water-soluble cellulose, the hydroxyl group-containing organic compound, and the chelating agent. The positive electrode composition for a lithium ion secondary battery according to claim 1, comprising at most parts. The chelating agent comprises an aminocarboxylic acid chelate compound, a phosphonic acid chelate compound, gluconic acid, alkali gluconate, citric acid, alkali citrate, malic acid, alkali malate, tartaric acid, and alkali tartaric acid. A compound selected from the group,
0.005 part by weight or more and 3 parts by weight or less of the chelating agent with respect to 100 parts by weight of the total amount of the positive electrode active material, the acrylate polymer, the water-soluble cellulose, the hydroxyl group-containing organic compound, and the chelating agent. The positive electrode composition for lithium ion secondary batteries according to claim 1 or 2, further comprising: A current collector, and a positive electrode active material layer provided on the surface of the current collector,
The positive electrode active material layer includes the positive electrode composition for a lithium ion secondary battery according to any one of claims 1 to 3,
The positive electrode for lithium ion secondary batteries whose weight of the said positive electrode active material layer is 10 mg / cm < ² > or more per unit area of the surface of the said collector.   A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to claim 4.