JP2014063676A - Aqueous binder liquid for secondary battery positive electrode, aqueous paste for secondary battery positive electrode produced with aqueous binder liquid, secondary battery positive electrode, and secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a binder liquid which can constitute a paste capable of coating without corrosion of metal collector while containing an active material containing a nickel atom, and furthermore to provide an aqueous binder liquid which is superior in adhesiveness with the active material and the metal collector, and can manufacture a positive electrode with less deterioration even if charge/discharge is repeated.SOLUTION: An aqueous binder liquid for a secondary battery positive electrode is a binder liquid for forming the secondary battery positive electrode having an active material containing a nickel atom and includes an acid-modified polyolefin resin (A) containing 0.1 to 10 mass% of an unsaturated carboxylic acid component, and an aqueous medium.

Description

  The present invention relates to an aqueous binder liquid for a secondary battery positive electrode, and further to a paste, a positive electrode and a secondary battery using the binder liquid.

  In recent years, with the demand for smaller and lighter portable electronic devices such as mobile phones and digital cameras, high-performance batteries have been actively developed, and the demand for secondary batteries with high energy density has increased. It is growing. In particular, lithium-ion batteries are being developed for use in electric vehicles in addition to applications such as mobile phones and notebook computers, and the range of their use is expanding greatly.

  In a lithium ion battery, an electrode having a configuration in which a separator is disposed between a positive electrode and a negative electrode is placed in a container together with an electrolytic solution (in the case of a lithium ion polymer battery, a gel or all solid electrolyte instead of a liquid electrolytic solution). It has a structure housed in.

  An electrode of a lithium ion battery is formed by layering an active material and, if necessary, a conductive material mainly made of a carbon material on a metal current collector such as an aluminum foil or a copper foil using a binder. . As the positive electrode active material, a lithium composite oxide containing a transition metal such as lithium cobaltate is used, and as the negative electrode active material, a carbon material or the like is mainly used. Such an electrode of a lithium ion battery is usually kneaded and prepared in the presence of a solvent such as N-methyl-2-pyrrolidone (NMP) by adding a binder or, if necessary, a conductive material to the active material. The obtained electrode paste is applied on a metal current collector by a doctor blade or the like and dried. Here, the binder is used for bonding the active material and the conductive material, and further to the metal current collector.

Conventionally, as a binder for a secondary battery positive electrode, a fluororesin such as polyvinylidene fluoride (PVDF) resin is mainly used, and a solution obtained by dissolving this in NMP is often used.
However, when PVDF is used as a binder, it is inferior in adhesiveness between active materials and between conductive materials, and also inferior in adhesiveness at the interface between the active material and the conductive material and the metal current collector. During the manufacturing process such as the process or the winding process, a part of the active material or the conductive material is peeled off from the metal current collector, which causes a micro short circuit and a variation in battery capacity.
Furthermore, since PVDF is inferior in swelling resistance to the electrolytic solution, the binder swells in the electrolytic solution by repeating charging and discharging, and between the active material and the conductive material, and between the active material and the conductive material and the metal current collector. The contact resistance between the active material and the conductive material may be partly peeled off from the metal current collector, resulting in inferior battery characteristics and further safety problems.
Moreover, since NMP used as a solvent for dissolving PVDF evaporates when the electrode paste is applied and dried on the metal current collector, it must be recovered safely. In addition, due to recent environmental laws and regulations, there are many places where it is not possible to use organic solvents that may affect the environment depending on the processing site.

Conventionally, as an active material for a positive electrode of a secondary battery, since an operating voltage exceeds 4 V, lithium cobaltate has been intensively studied, and lithium cobaltate is mainly used for small portable devices. Lithium cobalt oxide is widely used as a positive electrode active material in today's lithium ion secondary batteries because it exhibits good characteristics in terms of total performance such as potential flatness, capacity, discharge potential, and cycle characteristics.
However, since cobalt is unevenly distributed on the earth and is a scarce resource, there are problems that it is expensive and stable supply is difficult.

  Therefore, as a resource to replace cobalt, abundantly available on the earth and inexpensive nickel and manganese, based on these positive electrode active materials such as lithium nickelate, lithium manganate, etc. Cathode materials using transition metal oxides have been proposed and put into practical use. Among these, lithium nickelate has a capacity higher than that of lithium cobaltate, and thus is promising as a future alternative material.

  As described above, the secondary battery positive electrode is formed by coating a metal current collector with a paste containing a positive electrode active material and a PVDF binder. However, when an active material containing nickel atoms and a PVDF binder are mixed, a paste having a high basicity is obtained, and there is a problem that the stability of a paste containing a PVDF binder weak to a base is remarkably lowered due to a small amount of moisture mixed therein. It was.

In order to solve these problems, the following techniques have been disclosed for secondary battery electrode binders.
That is, Patent Documents 1 to 3 disclose that aqueous polytetrafluoroethylene (PTFE) is used as a binder. Since the PTFE binder is superior in base resistance as compared with the PVDF binder, the problem of deterioration in paste stability is solved. However, since this paste is a highly basic water-based one, there is a problem that the aluminum foil is corroded when this paste is applied to an aluminum foil that is a metal current collector.

JP 2000-149954 A JP 2001-216957 A JP 2001-266854 A

  The present invention solves the above problems, and is a binder liquid that can constitute a paste that contains an active material containing nickel atoms but can be applied without corrosion of the metal current collector. Furthermore, it is intended to provide a water-based binder liquid that can produce a positive electrode that is excellent in adhesiveness between an active material and a metal current collector and that is less deteriorated even after repeated charge and discharge. is there. Furthermore, it aims at providing the paste, positive electrode, and secondary battery using this binder liquid.

As a result of examining the above problems, the present inventors have reached the present invention. That is, the gist of the present invention is as follows.
(1) A binder liquid for forming a secondary battery positive electrode having an active material containing nickel atoms, an acid-modified polyolefin resin (A) containing 0.1 to 10% by mass of an unsaturated carboxylic acid component, and an aqueous solution A water-based binder liquid for a secondary battery positive electrode, comprising a medium.
(2) The acid-modified polyolefin resin (A) contains 50 to 98% by mass of an acid-modified polyolefin resin (a) containing 50 to 98% by mass of an ethylene component and / or at least one of a propylene component and a 1-butene component. The aqueous binder solution for a secondary battery positive electrode according to (1), which is an acid-modified polyolefin resin (b).
(3) The unsaturated carboxylic acid component of the acid-modified polyolefin resin (A) is at least one selected from the group consisting of maleic anhydride, maleic acid, acrylic acid and methacrylic acid (1) or ( 2) The aqueous binder liquid for secondary battery positive electrode of description.
(4) The secondary battery positive electrode aqueous binder liquid according to any one of (1) to (3) above, an active material containing nickel atoms, and a thickener that is an aqueous solution of polyacrylic acid (B). An aqueous paste for a secondary battery positive electrode, which is a paste to be contained and has a pH of 7 to 11.
(5) The aqueous paste for secondary battery positive electrode according to (4), wherein the polyacrylic acid (B) has a weight average molecular weight of 10,000 to 10,000,000.
(6) A secondary battery positive electrode formed using the aqueous paste for secondary battery positive electrode according to (4) or (5).
(7) A secondary battery formed using the secondary battery positive electrode described in (6) above.

  The aqueous binder solution for a positive electrode of the secondary battery of the present invention is excellent in adhesion between the active material containing nickel atoms and the metal current collector. A secondary battery positive electrode paste having a pH of 7 to 11 formed using this binder liquid, an active material containing nickel atoms, and a thickener containing polyacrylic acid (B) has a metal current collector. Coating can be performed without corrosion, and a positive electrode with little deterioration can be produced even when charging and discharging are repeated. Therefore, a secondary battery excellent in cycle characteristics can be obtained. Moreover, since the aqueous binder liquid for secondary battery positive electrodes of this invention uses an aqueous medium as a medium, it can solve the problem of organic solvents, such as an organic solvent evaporating at the time of electrode manufacture.

Hereinafter, the present invention will be described in detail.
The aqueous binder solution for a secondary battery positive electrode of the present invention contains an acid-modified polyolefin resin (A) and an aqueous medium.

In the present invention, the acid-modified polyolefin resin (A) needs to have an unsaturated carboxylic acid component content of 0.1 to 10% by mass from the viewpoint of satisfying dispersion of the resin and resistance to the electrolytic solution. Yes, it is preferably 0.3 to 8% by mass, more preferably 0.5 to 6% by mass, and even more preferably 1 to 5% by mass.
If the content of the unsaturated carboxylic acid component is less than 0.1% by mass, the adhesion between the active material and the conductive material and the metal current collector may be lowered, and the resin is dispersed in the aqueous medium as described later. In such a case, the dispersion tends to be difficult. When the content of the unsaturated carboxylic acid component exceeds 10% by mass, the resistance to the electrolytic solution may decrease.
Examples of the unsaturated carboxylic acid component include unsaturated carboxylic acids and anhydrides thereof. Specifically, acrylic acid, methacrylic acid, maleic acid, maleic anhydride, itaconic acid, itaconic anhydride, fumaric acid, croton In addition to acids, examples include half esters and half amides of unsaturated dicarboxylic acids. Of these, acrylic acid, methacrylic acid, maleic acid, and maleic anhydride are preferable, and acrylic acid and maleic anhydride are particularly preferable.
The form of the unsaturated carboxylic acid component is not particularly limited as long as it is copolymerized in the acid-modified polyolefin resin (A) by, for example, random copolymerization, block copolymerization, graft copolymerization, thermal degradation method or the like.
Incidentally, the acid anhydride introduced into the resin forms an acid anhydride structure in which the adjacent carboxyl group is dehydrated and cyclized in the dry state of the resin. May open to take the structure of a carboxylic acid or salt thereof.

Moreover, it is preferable that acid-modified polyolefin resin (A) contains 50-98 mass% of C2-C6 unsaturated hydrocarbon components from a viewpoint of dispersion | distribution of resin and the tolerance with respect to electrolyte solution, The content is more preferably 98% by mass, still more preferably 70 to 98% by mass, and most preferably 75 to 95% by mass.
When the content of the unsaturated hydrocarbon component having 2 to 6 carbon atoms is less than 50% by mass, the resistance to the electrolytic solution may be reduced. When the content exceeds 98% by mass, the content of the unsaturated carboxylic acid component is relatively high. Since it will fall, it may become difficult to make the resin water-based.

  Examples of unsaturated hydrocarbons having 2 to 6 carbon atoms include alkenes such as ethylene, propylene, 1-butene, isobutene, 1-pentene, 4-methyl-1-pentene, 3-methyl-1-pentene and 1-hexene. And dienes such as butadiene and isoprene. From the viewpoint of ease of resin production, ease of aqueous formation, resistance to electrolytes, etc., ethylene, propylene or butene components (1-butene, isobutene, etc.) ) Is preferable, and these may be used in combination.

  The acid-modified polyolefin resin (A) is an acid-modified polyolefin resin (a) containing 50 to 98% by mass of an ethylene component, or an acid modification containing 50 to 98% by mass of at least one of a propylene component and a 1-butene component. The polyolefin resin (b) is preferable, and the acid-modified polyolefin resins (a) and (b) can be used in combination.

  In addition to the propylene component and the 1-butene component, the acid-modified polyolefin resin (b) preferably further contains an ethylene component in an amount of 1.5 to 45% by mass, and more preferably 2 to 30% by mass. More preferred. By containing an ethylene component, the flexibility of the resin increases and it becomes easy to be aqueous.

As a copolymerization component contained in acid-modified polyolefin resin (A), the following components may be contained within the range which does not impair the effect of this invention. That is, examples of such components include acrylic acid esters such as methyl acrylate, methyl methacrylate, and ethyl acrylate, and methacrylic acid esters, 1-pentene, 4-methyl-1-pentene, and 3-methyl-1. -Alkenes and dienes such as pentene, 1-hexene and 1-octene, maleates such as dimethyl maleate, diethyl maleate and dibutyl maleate, (meth) acrylic acid amides, methyl vinyl ether, ethyl vinyl ether, etc. Alkyl vinyl ethers, vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl pivalate, vinyl versatate, vinyl alcohol, 2-hydroxyethyl acrylate, glycidyl (meth) acrylate, (meth) acrylo Tolyl, styrene, substituted styrene, vinyl halides, vinylidene halides such, carbon monoxide, sulfur dioxide, and mixtures thereof.
Such a component is preferably 20% by mass or less of the acid-modified polyolefin resin (A). When it exceeds 20 mass%, when it is used as a binder, it tends to be difficult to achieve the effects of the present invention as described above.

  Specific examples of the acid-modified polyolefin resin (a) include ethylene- (meth) acrylic acid copolymer, ethylene-acrylic acid ester-maleic anhydride terpolymer, or ethylene-methacrylic acid ester-maleic anhydride terpolymer. An original copolymer is most preferred. Such resins include Lexpearl EAA series (Nippon Polyethylene), Primacol (Dow Chemical Japan), Nuclel series (Mitsui / DuPont Polychemical), Bondine series (Arkema), Lexpearl ET series. (Nippon Polyethylene Co., Ltd.).

  Specific examples of the acid-modified polyolefin resin (b) include, for example, REXTAC (Rexene, USA), Bestplast 408, Bestplast 708 (Huls, Germany), Ubetak APAO ( Ube Lexen) and the like, and polyolefin resins in which an unsaturated carboxylic acid component is introduced into these commercially available resins by the above-mentioned method, Yumex series (Sanyo Kasei Co., Ltd.), and the like. Of the above-mentioned commercially available resins, it is preferable to use Bestplast 408, Bestplast 708, and Umex series.

  The acid-modified polyolefin resin (A) is prepared by mixing two or more arbitrary polyolefin resins so that the unsaturated carboxylic acid component and the unsaturated hydrocarbon component having 2 to 6 carbon atoms have the constituent component ratio of the present invention. It is good.

  The molecular weight of the acid-modified polyolefin resin (A) is not particularly limited, but the weight average molecular weight is preferably 10,000 or more, more preferably 10,000 to 150,000, and 12,000 to 120,000. Is more preferable, 15,000 to 100,000 is particularly preferable, and 20,000 to 90,000 is most preferable. When the weight average molecular weight is less than 10,000, the electrodes are fragile, and the adhesion between the active materials and the adhesion between the active material and the metal current collector may be reduced. When the weight average molecular weight exceeds 150,000, it tends to be difficult to make the resin water-based. In addition, the weight average molecular weight of resin can be calculated | required on the basis of polystyrene resin using a gel permeation chromatography (GPC).

  The binder liquid of the present invention needs to contain an aqueous medium. The medium of the binder liquid preferably contains water as a main component from the viewpoint of safety to workers and work environments. For the purpose of making the acid-modified polyolefin resin (A) aqueous, dissolving, reducing the drying load, the medium may contain an organic solvent in addition to water. The amount of the organic solvent in the total amount of the medium is preferably 5% by mass or less, more preferably 3% by mass or less, and further preferably 1% by mass or less. The medium may contain a water-soluble basic compound that is added when the acid-modified polyolefin resin (A) is made aqueous.

  In the binder liquid of the present invention, the solid content concentration of the acid-modified polyolefin resin (A) is preferably 1 to 50% by mass, more preferably 3 to 40% by mass, and 5 to 30% by mass. It is particularly preferred. When solid content concentration exceeds 50 mass%, there exists a tendency for a handleability to fall by the remarkable viscosity increase or solidification of a binder liquid. On the other hand, when the solid content concentration is less than 1% by mass, the handleability tends to be reduced due to a significant decrease in the viscosity of the binder liquid.

  The method for dispersing the acid-modified polyolefin resin (A) in the aqueous medium is not particularly limited. For example, the acid-modified polyolefin resin (A), the basic compound and the aqueous medium are heated and stirred in a sealed container under pressure. Can be used. The aqueous medium of the acid-modified polyolefin resin (A) is a medium mainly composed of water, and may contain a water-soluble organic solvent or a basic compound.

By adding a basic compound during dispersion of the acid-modified polyolefin resin (A) in the aqueous medium, the acid-modified polyolefin resin (A) can be uniformly dispersed in the aqueous medium, thereby improving the dispersion stability. An excellent binder liquid can be obtained.
Examples of basic compounds include ammonia and amines such as organic amine compounds. Specific examples of the organic amine compound include triethylamine, N, N-dimethylethanolamine, isopropylamine, iminobispropylamine, ethylamine, diethylamine, 3-ethoxypropylamine, 3-diethylaminopropylamine, sec-butylamine, propylamine, Examples thereof include methylaminopropylamine, methyliminobispropylamine, 3-methoxypropylamine, monoethanolamine, morpholine, N-methylmorpholine, N-ethylmorpholine and the like. Of these, triethylamine, N, N-dimethylethanolamine is preferably used.

  Moreover, it is preferable that content of the basic compound in the aqueous dispersion of acid-modified polyolefin resin (A) is 0.01-100 mass% with respect to resin solid content, and 0.05-40 mass% especially among them. It is preferable that it is 0.1-15 mass%. When the content of the basic compound is less than 0.01% by mass, the effect of adding the basic compound is poor, and it is difficult to obtain an aqueous dispersion excellent in dispersion stability. On the other hand, when the content of the basic compound exceeds 100% by mass, the binder is likely to be colored or gelled.

  By adding an organic solvent when the acid-modified polyolefin resin (A) is dispersed in an aqueous medium, the dispersion can be promoted and the dispersed particle size can be reduced. The amount of the organic solvent to be used is preferably 50% by mass or less in the aqueous medium, more preferably 1 to 45% by mass, further preferably 2 to 40% by mass, and 3 to 35% by mass. It is particularly preferred that When the amount of the organic solvent exceeds 50% by mass, the stability of the aqueous dispersion may be lowered depending on the organic solvent used.

  As the organic solvent, those having a solubility in water at 20 ° C. of 10 g / L or more are preferably used from the viewpoint of obtaining a good aqueous dispersion. More preferably, it is 20 g / L or more, Most preferably, it is 50 g / L or more.

  As the organic solvent, those having a boiling point of less than 250 ° C. at normal pressure are preferred from the viewpoint of easy removal, and those having a temperature of 50 ° C. or more and less than 185 ° C. are particularly preferred. An organic solvent having a boiling point of 250 ° C. or higher is difficult to be scattered from the resin coating film by drying, and may deteriorate the adhesion between the materials. Specific examples of the organic solvent used include, for example, methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol, tert-butanol, n-amyl alcohol, isoamyl alcohol, sec-amyl alcohol. , Tert-amyl alcohol, 1-ethyl-1-propanol, 2-methyl-1-butanol, n-hexanol, cyclohexanol and other alcohols, methyl ethyl ketone, methyl isobutyl ketone, ethyl butyl ketone, cyclohexanone and other ketones, tetrahydrofuran , Ethers such as dioxane, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, sec-butyl acetate, 3-methoxybutyl acetate, Esters such as methyl lopionate, ethyl propionate, diethyl carbonate, dimethyl carbonate, glycol derivatives such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, ethylene glycol ethyl ether acetate, Furthermore, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, 3-methoxy-3-methyl-1-butanol, methoxybutanol, acetonitrile, dimethylformamide, dimethylacetamide, diacetone alcohol, ethyl acetoacetate, 1,2-dimethylglycerol, 1,3-dimethylglycerol, trimethylglycerol, etc. are mentioned. These organic solvents may be used in combination of two or more.

  Among the above organic solvents, ethanol, n-propanol, isopropanol, n-butanol, methyl ethyl ketone, cyclohexanone, tetrahydrofuran, dioxane, ethylene glycol monoethyl ether, ethylene glycol monopropyl are effective because they are highly effective in promoting aqueous formation of resins. Ether, ethylene glycol monobutyl ether, and diethylene glycol monomethyl ether are preferable. Among these, an organic solvent having one hydroxyl group in the molecule is more preferable, and n-propanol, isopropanol, tetrahydrofuran, ethylene is preferable because the resin can be made aqueous with a small amount of addition. More preferred are glycol alkyl ethers.

  When the above-mentioned organic solvent is used for dispersing the acid-modified polyolefin resin (A), a part of the organic solvent is distilled out of the system by a solvent removal process generally called “stripping” after the dispersion. The amount of organic solvent can be reduced. By stripping, the content of the organic solvent in the aqueous dispersion can be 10% by mass or less, and if it is 5% by mass or less, it is environmentally preferable.

The acid-modified polyolefin resin (A) can be used after being hydrophilized. Examples of the hydrophilic treatment include sulfonation treatment, fluorine gas treatment, graft polymerization treatment, or discharge treatment, and one or more of these hydrophilic treatments can be performed. The sulfonation treatment is not particularly limited, but for example, a treatment immersed in a solution such as fuming sulfuric acid, sulfuric acid, chlorosulfuric acid, or sulfuryl chloride, a treatment in contact with SO ₃ gas, or SO gas and / or or SO ₂ discharge in the presence of gas can be cited a process to act. Among these, the sulfonation treatment with fuming sulfuric acid is preferable because it has high reactivity and can be sulfonated relatively easily. In this case, sulfonic acid groups are mainly introduced.

  The aqueous dispersion of the acid-modified polyolefin resin (A) prepared as described above contains the acid-modified polyolefin resin (A), a basic compound, and an aqueous medium, and contains a non-volatile aqueous dispersion aid. It is preferable not to contain.

  Moreover, you may add the aqueous dispersion of another polymer to the aqueous dispersion of acid-modified polyolefin resin (A) as needed within the range which does not impair the effect of this invention. For example, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride resin, styrene-maleic acid resin, styrene-butadiene resin, butadiene resin, acrylonitrile-butadiene resin, poly (meth) Examples include aqueous dispersions such as acrylonitrile resin, (meth) acrylamide resin, chlorinated polyethylene resin, chlorinated polypropylene resin, polyester resin, modified nylon resin, phenol resin, silicone resin, epoxy resin, urethane resin, and mixtures thereof. .

Next, the aqueous paste for secondary battery positive electrode of the present invention will be described.
The aqueous paste for secondary battery positive electrode of the present invention contains the binder liquid of the present invention, an active material containing nickel atoms, and a thickener that is an aqueous solution of polyacrylic acid (B), and has a pH of 7 to 7. 11 things. Moreover, the paste may contain a conductive material as necessary, and these active materials and conductive materials may contain two or more of each.

Examples of the active material containing nickel atoms used in the aqueous battery positive electrode paste of the present invention include lithium nickelate and Li—Ni composite oxide. Note that the Li—Ni composite oxide may be substituted with a plurality of different elements.
The average particle size of the active material is preferably 0.1 to 20 μm. Further, the surface of the active material may be subjected to a surface treatment with carbon or the like in order to increase conductivity.

  As the conductive material, a carbon material, a metal, or a compound thereof can be used. Examples of the carbon material include ketjen black, acetylene black, furnace black, graphite, and carbon fiber. Examples of the metal or compound thereof include nickel, cobalt, titanium, cobalt oxide, and titanium oxide. However, it is not limited to these.

  The thickener used for the aqueous paste for secondary battery positive electrode of the present invention needs to contain polyacrylic acid (B). The weight average molecular weight of the polyacrylic acid (B) is preferably 10,000 to 10,000,000, and more preferably 100,000 to 5,000,000. If the polyacrylic acid (B) is in the above range, the paste can have a high viscosity, and a stable paste can be obtained in which the active material does not settle. Here, the weight average molecular weight is a value of the weight average molecular weight measured by liquid chromatography.

  Moreover, the thickener containing polyacrylic acid (B) needs to be aqueous solution, and it is more preferable that pH is 2-5. If the thickener stays in the acidic range, the polyacrylic acid (B) may be partially neutralized with NaOH, LiOH, KOH or the like. By using an acidic thickener, the pH of the paste containing nickel atoms as the active material can be adjusted to a range of 7 to 11 described later.

Moreover, in addition to the thickener containing polyacrylic acid (B), a water-soluble polymer is added as a thickener to the aqueous battery positive electrode paste, as long as the effect of the present invention is not impaired. Also good. Specific examples of the water-soluble polymer include cellulosic polymers such as carboxymethyl cellulose, methyl cellulose, and hydroxypropyl cellulose, polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, acrylic acid or a copolymer of acrylate and vinyl alcohol, maleic anhydride or Examples include maleic acid or a copolymer of fumaric acid and vinyl alcohol, modified polyvinyl alcohol, and modified polyacrylic acid.
Of these, cellulose derivatives such as methylcellulose, carboxymethylcellulose, and hydroxymethylcellulose are effective. These celluloses improve the wettability between the metal current collector, the active material, and the conductive material.
As the blending amount of the water-soluble polymer, the mass ratio of the water-soluble polymer and the polyacrylic acid (B) is preferably 7: 3 to 1: 9, more preferably 6: 4 to 1: 9. preferable.

The total of the content of the acid-modified polyolefin resin (A) and the content of the polyacrylic acid (B) in the solid content of the aqueous paste for secondary battery positive electrode is preferably 0.3 to 8% by mass. If the total content exceeds 8% by mass, the electrical resistance value of the obtained electrode tends to increase. Moreover, when the total content is less than 0.3% by mass, it is difficult to obtain sufficient adhesion between the active material, the conductive material, and the metal current collector.
Moreover, it is preferable that content of acid-modified polyolefin resin (A) in solid content of the aqueous paste for secondary battery positive electrodes is 0.1-7.8 mass%. When the content is less than 0.1% by mass, sufficient adhesion between the active material, the conductive material and the metal current collector cannot be obtained, and when the content exceeds 7.8% by mass, a slurry is produced. It is difficult to adjust the viscosity at the time and to adjust the pH of the slurry.

  The pH of the aqueous paste for secondary battery positive electrode of the present invention needs to be 7 to 11, preferably 7 to 10, and more preferably 7 to 9. When the pH of the paste exceeds 11, the metal current collector may be corroded when the paste is applied, and when the pH of the paste is less than 7, the binder component and the active material may be aggregated. is there.

  In the present invention, the conditions and method for producing the secondary battery positive electrode aqueous paste are not particularly limited, and the paste controls the binder liquid, the active material, the thickener, and, if necessary, the conductive material at room temperature or appropriately controlled. After mixing at a given temperature, it can be produced by a method such as mechanical dispersion treatment or ultrasonic dispersion treatment. About a mixing order, it is preferable to add the aqueous | water-based binder liquid for secondary battery positive electrodes of this invention last. Moreover, the other component mentioned above, a solvent, etc. can also be added as needed.

The secondary battery positive electrode of the present invention can be formed using the secondary battery positive electrode aqueous binder liquid of the present invention. For example, the secondary battery positive electrode aqueous paste produced as described above is applied to the metal current collector. The secondary battery positive electrode can be formed by applying and drying.
The metal current collector may be any material having conductivity, and examples thereof include aluminum, nickel, and copper. Although there is no restriction | limiting in particular in the thickness of a metal electrical power collector, Usually, a 5-50 micrometers thin film is used.
Examples of the method for applying the paste on the metal current collector include a method using a doctor blade, and the method for removing the aqueous medium is, for example, 60 to 150 ° C., preferably 70 to 130 ° C. for 5 to 120 minutes. Examples of the method include drying and further drying under reduced pressure at 120 ° C. for 12 hours, for example. The thickness of the electrode after coating and drying is preferably 30 to 150 μm. In order to control the thickness and density of the electrode, it is preferable to press, for example, with a roll press.

  The secondary battery of the present invention is formed using the secondary battery positive electrode of the present invention. For example, the secondary battery positive electrode manufactured as described above is combined with the secondary battery negative electrode, separator and electrolyte. A secondary battery can be formed by enclosing in a container according to a conventional method.

  Examples of the active material for the negative electrode constituting the secondary battery include graphite, and examples of the separator include a polyethylene microporous membrane, a polypropylene microporous membrane, a polyethylene nonwoven fabric, a polypropylene nonwoven fabric, a polyamide nonwoven fabric, and a glass fiber. As the electrolytic solution, a supporting electrolytic salt such as lithium hexafluorophosphate and lithium perchlorate was added to a mixed solvent obtained by mixing one kind or two or more kinds of nonaqueous solvents such as ethylene carbonate, diethyl carbonate, and propylene carbonate. Things. Further, a solid electrolyte or a gel electrolyte may be used instead of the separator. Examples of the electrolyte include tetraethylammonium tetrafluoroborate, triethylmonomethylammonium tetrafluoroborate, tetraethylammonium hexafluorophosphate, ionic liquid, aqueous sulfuric acid solution, and aqueous potassium hydroxide solution. The solvent for dissolving the electrolyte (electrolytic solution solvent) is not particularly limited as long as it is generally used as an electrolytic solution solvent. Specific examples include carbonates such as propylene carbonate, ethylene carbonate, and butylene carbonate; lactones such as γ-butyrolactone; sulfolanes; nitriles such as acetonitrile; ionic liquids and the like. It can be used as a mixed solvent.

Hereinafter, the present invention will be described in detail with reference to examples.
Various properties described below were measured or evaluated by the following methods.

(1) Unsaturated carboxylic acid content of acid-modified polyolefin resin (A) The acid value of the acid-modified polyolefin resin (A) was measured according to the method described in JIS K5407, and the acid-modified polyolefin resin (A ) Content of unsaturated carboxylic acid.

(2) Structure of Resin of Acid-Modified Polyolefin Resin (A) ¹ H-NMR analysis (manufactured by Varian, 300 MHz) was performed at 120 ° C. in orthodichlorobenzene (d ₄ ).

(3) Molecular weight of acid-modified polyolefin resin (A) Using GPC analysis (HLC-8020 manufactured by Tosoh Corporation, column is TSK-GEL), the sample was dissolved in tetrahydrofuran and measured at 40 ° C. At this time, the weight average molecular weight was calculated | required from the calibration curve created with the polystyrene standard sample. When the sample was difficult to dissolve in tetrahydrofuran, orthodichlorobenzene was used.

(4) Electrode state The electrode state of the secondary battery positive electrode formed by applying an aqueous paste for a secondary battery positive electrode on an aluminum foil was visually observed. As the gas was generated due to corrosion of the aluminum foil, the case where the electrode had a hole was marked as x, and the case where the electrode did not have a hole was marked as ◯.

(5) Electrode adhesive strength From a secondary battery positive electrode formed by applying an aqueous paste for a secondary battery positive electrode on an aluminum foil, a test sample having a width of 2.5 cm and a length of 10 cm was cut out, and the aluminum foil side was sufficient. It bonded together with the double-sided tape to the steel plate which has thickness. Cellophane tape (Nichiban Co., Ltd., CT-18, 18 mm width) was affixed to the active material layer of the test sample, and the stress was measured when it was peeled off at a rate of 50 mm / min from one side in the direction of 180 °. The measurement was carried out three times for each sample, and the average value was taken as the adhesive strength.

(6) Cycle characteristics A charge / discharge test was performed using a coin-type battery produced by combining a secondary battery positive electrode and a graphite electrode. The cycle characteristics were evaluated by repeatedly performing 1C-3.0V constant current discharge after 1C-4.2V constant current constant voltage charging in a 50 ° C environment. The discharge capacity at the first cycle was set to 100%, the discharge capacity after 50 cycles was obtained, and the discharge capacity retention rate was calculated as cycle characteristics.

Reference example 1
<Production of aqueous dispersion “E-1” of acid-modified polyolefin resin>
Using a stirrer equipped with a 1 L glass container that can be sealed with a heater, acid-modified polyolefin resin (a) 60.0 g Bondine TX-8030 (manufactured by Arkema), 48.0 g isopropanol, basic compound 3.9 g of N, N-dimethylethanolamine and 188.1 g of distilled water were charged into a glass container and stirred at a rotation speed of a stirring blade of 300 rpm. No resin precipitation was observed at the bottom of the container, It was confirmed that it was floating. Therefore, while maintaining this state, the heater was turned on and heated after 10 minutes. Then, the system temperature was kept at 140 ° C. and further stirred for 60 minutes. Then, it cooled to room temperature (about 25 degreeC), stirring with air-cooling with the rotational speed of 300 rpm, and obtained the milky white uniform dispersion (solid content concentration 20 mass%).
290 g of the dispersion and 80 g of distilled water were placed in a 1 L eggplant flask, placed in an evaporator, and the aqueous medium was distilled off by reducing the pressure at 60 ° C. When about 87 g of the aqueous medium was distilled off, the heating was terminated, and the pressure was returned to normal pressure and cooled to room temperature. After cooling, the liquid component in the flask was pressure filtered (air pressure 0.2 MPa) with a 300 mesh stainless steel filter (wire diameter 0.035 mm, plain weave) to obtain a milky white uniform aqueous dispersion “E-1” (solid A partial concentration of 20% by mass) was obtained.

Reference example 2
<Production of aqueous dispersion “E-2” of acid-modified polyolefin resin>
In a sealable pressure-resistant 1 liter glass container equipped with a stirrer and a heater, 90 g of Umex 1001 (manufactured by Sanyo Chemical Co., Ltd.) as acid-modified polyolefin resin (b), and 8 g of N, N-dimethylethanolamine as a basic compound Then, 240 g of organic solvent tetrahydrofuran and 260 g of distilled water were charged, sealed, and then heated to 130 ° C. (internal temperature) with stirring at 300 rpm. After being kept at 130 ° C. for 1 hour under stirring, the heater was turned off and naturally cooled to 60 ° C. to obtain a milky white uniform dispersion (solid content concentration 15% by mass).
290 g of the dispersion and 40 g of distilled water were placed in a 1 L eggplant flask, placed in an evaporator, and the aqueous medium was distilled off by reducing the pressure at 60 ° C. When about 160 g of the aqueous medium was distilled off, the heating was terminated, and the pressure was returned to normal pressure and cooled to room temperature. After cooling, the liquid component in the flask was subjected to pressure filtration (air pressure 0.2 MPa) with a 300 mesh stainless steel filter (wire diameter 0.035 mm, plain weave) to obtain a milky white uniform aqueous dispersion “E-2” (solid A partial concentration of 25% by weight).

Reference example 3
<Production of aqueous dispersion “E-3” of acid-modified polyolefin resin>
280 g of propylene-butene-ethylene terpolymer (manufactured by Huls Japan, Bestplast 708, propylene / butene / ethylene = 64.8 / 23.9 / 11.3 mass%) in a four-necked flask And heated and melted in a nitrogen atmosphere. Thereafter, while maintaining the system temperature at 170 ° C., 32.0 g of maleic anhydride as an unsaturated carboxylic acid and 6.0 g of dicumyl peroxide as a radical generator were added over 1 hour with stirring. The reaction was carried out for 1 hour. After completion of the reaction, the obtained reaction product was put into a large amount of acetone to precipitate a resin. This resin was further washed several times with acetone to remove unreacted maleic anhydride, and then dried under reduced pressure in a vacuum dryer to obtain an acid-modified polyolefin resin (b) “P-1”.
As the acid-modified polyolefin resin (b), a milky white uniform aqueous dispersion “E-3” (solid content concentration: 25 mass%) was prepared in the same manner as in Reference Example 2 except that “P-1” was used. Obtained.

  Table 1 shows the composition and molecular weight of the acid-modified polyolefin resin (A) used for the production of the aqueous dispersions “E-1” to “E-3”, and the solid content concentration of the obtained aqueous dispersion.

The following were used as a thickener containing polyacrylic acid (B).
<Polyacrylic acid thickener “A-1”>
A 2% by mass aqueous solution of Jurimer AC-10L (polyacrylic acid, weight average molecular weight 20,000) manufactured by Toa Gosei Co., Ltd. was used. The pH of the aqueous solution was 2.0.
<Polyacrylic acid thickener “A-2”>
A 2% by mass aqueous solution of Jurimer AC-10H (polyacrylic acid, weight average molecular weight 150,000) manufactured by Toa Gosei Co., Ltd. was used. The pH of the aqueous solution was 2.5.
<Polyacrylic acid thickener “A-3”>
A 2% by weight aqueous solution of Jurimer AC-10SH (polyacrylic acid, weight average molecular weight 1,000,000) manufactured by Toa Gosei Co., Ltd. was used. The pH of the aqueous solution was 2.5.
<Polyacrylic acid thickener “A-4”>
A 2% by weight aqueous solution of AC-10SH, which was partially neutralized with NaOH, was used. The pH of the aqueous solution was 4.6.
<Polyacrylic acid thickener “A-5”>
A 2% by weight aqueous solution of AC-10SH neutralized 80% with NaOH was used. The pH of the aqueous solution was 6.0.
<Polyacrylic acid thickener “A-6”>
A 2% by weight aqueous solution of AC-10SH completely neutralized with NaOH was used. The pH of the aqueous solution was 8.1.

The following were used as water-soluble polymers used as a thickener.
<"C-1">
A 2% by weight aqueous solution of Serogen BSH-6 (carboxymethylcellulose) manufactured by Daiichi Kogyo Seiyaku Co., Ltd. was used. The pH of the aqueous solution was 6.8.

Example 1
An aqueous dispersion “E-1” of an acid-modified polyolefin resin produced by the method described in Reference Example 1 and having the composition shown in Table 1 was used as an aqueous binder liquid for a secondary battery positive electrode.
96 parts by mass of NMC (manufactured by Nippon Chemical Industry Co., Ltd., CELLSEED NMC (Ni: Mn: Co = 1: 1: 1)) as an active material containing nickel atoms, and acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd., Denka Black) as a conductive material ) 1 part by mass, polyacrylic acid thickener “A-2” (2% by mass aqueous solution) 50 parts by mass (containing 1 part by mass of polyacrylic acid (B)), and an aqueous binder liquid for secondary battery positive electrode “ E-1 "(solid content concentration 20% by mass) 10 parts by mass (containing 2 parts by mass of acid-modified polyolefin resin (A)) and further diluted with distilled water so that the solid content concentration becomes 55% by mass. Thereafter, an aqueous paste for a secondary battery positive electrode was prepared by sufficiently kneading.
Using a 20 μm thick aluminum foil as the metal current collector, the obtained paste was applied to one side of the aluminum foil using a film applicator so that the thickness after drying was about 120 μm, and heated at 80 ° C. for 30 minutes. After drying, in order to further remove moisture, it was vacuum-dried at 120 ° C. for 15 hours to form an active material layer on the aluminum foil to prepare a secondary battery positive electrode.
Using the positive electrode for the secondary battery obtained by the above preparation method, the active material layer formed on the aluminum foil was cut into a circular shape with an area of 2 cm ² , and the electrode thickness was 80 μm and the electrode density was 2 by pressing. A coin-type battery was fabricated by molding the composite electrode so as to be ³ g / cm ³ and combining it with a graphite electrode, and sandwiching a polypropylene membrane separator (Cell Guard # 2400, manufactured by Celgard) between both electrodes. Table 2 shows the characteristic values and evaluation results of the obtained positive electrode and battery.

Examples 2-14, Comparative Examples 1-2
Aqueous dispersions of acid-modified polyolefin resins “E-1” to “E-3” produced by the methods described in Reference Examples 1 to 3 as aqueous battery binder solutions for secondary battery positive electrodes and having the compositions shown in Table 1. Was used.
The same as in Example 1 except that the type of the aqueous binder liquid for the secondary battery positive electrode, the type and amount of the polyacrylic acid thickener, and the amount of the water-soluble polymer used as the thickener were changed as shown in Table 2. By this method, an aqueous paste for a secondary battery positive electrode, a secondary battery positive electrode, and a coin-type battery were produced. Table 2 shows the characteristic values and evaluation results of the obtained positive electrode and battery.

As is apparent from Table 2, in Examples 1 to 14, the aqueous binder liquid for the secondary battery positive electrode includes an acid-modified polyolefin resin (A) containing 0.1 to 10% by mass of an unsaturated carboxylic acid component, and an aqueous solution. The aqueous paste for a secondary battery positive electrode contains a thickener that is an aqueous solution of polyacrylic acid (B) and this binder liquid, and is formed because the pH is 7 to 11. The positive electrode of the secondary battery had no adhesive holes due to gas generation due to corrosion of the aluminum foil, and had high adhesive strength. Moreover, the obtained secondary battery was a thing with little deterioration accompanying a charging / discharging cycle.
On the other hand, the secondary battery positive electrode aqueous paste obtained in Comparative Examples 1 and 2 does not contain a thickener that is an aqueous solution of polyacrylic acid (B), and is also an aqueous solution of polyacrylic acid (B). However, since the neutralization degree is high and it contains a basic thickener, the pH of the paste becomes 12, and the formed secondary battery positive electrode has a hole in the electrode due to gas generation due to corrosion of the aluminum foil. There was a crack and the adhesive strength was low. Further, the obtained secondary battery was greatly deteriorated due to the charge / discharge cycle.

Claims (7)

  A binder liquid for forming a secondary battery positive electrode having an active material containing nickel atoms, an acid-modified polyolefin resin (A) containing 0.1 to 10% by mass of an unsaturated carboxylic acid component, and an aqueous medium A water-based binder liquid for a positive electrode of a secondary battery, comprising:   Acid-modified polyolefin resin (A) contains 50 to 98% by mass of an acid-modified polyolefin resin (a) containing 50 to 98% by mass of an ethylene component and / or 50 to 98% by mass of at least one of a propylene component and a 1-butene component. The aqueous binder solution for a secondary battery positive electrode according to claim 1, which is a polyolefin resin (b).   The unsaturated carboxylic acid component of the acid-modified polyolefin resin (A) is at least one selected from the group consisting of maleic anhydride, maleic acid, acrylic acid and methacrylic acid. Aqueous binder solution for secondary battery positive electrode.   A paste containing an aqueous binder liquid for a secondary battery positive electrode according to any one of claims 1 to 3, an active material containing nickel atoms, and a thickener that is an aqueous solution of polyacrylic acid (B), An aqueous paste for a secondary battery positive electrode, wherein the pH of the paste is 7-11.   The aqueous paste for secondary battery positive electrode according to claim 4, wherein the weight average molecular weight of the polyacrylic acid (B) is 10,000 to 10,000,000.   A secondary battery positive electrode formed using the aqueous paste for a secondary battery positive electrode according to claim 4 or 5. A secondary battery formed using the secondary battery positive electrode according to claim 6.