Electrode binder for lithium secondary batteries, electrode binder composition, negative electrode, and lithium secondary battery

Abstract

It exhibits excellent bonding properties, reduces electrode expansion due to charging and discharging, and has superior charge-discharge cycle characteristics. [Solution] The binder for the electrodes of the lithium secondary battery according to the embodiment comprises an aqueous polyurethane dispersion obtained by reacting (A) a polyisocyanate, (B) a polyol, (C) a compound having two or more hydroxyl groups and a carboxyl group, and (D) a chain extender. Component (A) comprises an aromatic polyisocyanate. Component (B) comprises a polybutadiene polyol, a hydrogenated polybutadiene polyol, a polyisoprene polyol, and / or a hydrogenated polyisoprene polyol. The ratio (y / x) of the number of moles (y) of amino groups of component (D) to the number of moles (x) of free isocyanate groups in the urethane prepolymer obtained by reacting components (A) to (C) is 0.1 to 0.8.

Description

Technical Field

[0001] Embodiments of the present invention relate to a binder for electrodes of a lithium secondary battery, a binder composition for electrodes, a negative electrode, and a lithium secondary battery.

Background Art

[0002] A lithium secondary battery, also called a lithium-ion secondary battery, is used as a power storage device with a high voltage and high energy density, for example, as a driving power source for electronic devices. Electrodes of a lithium secondary battery are usually manufactured by applying and drying a mixture of an electrode active material, a conductive agent, and a binder onto the surface of a current collector. The binder is used to impart an adhesive force between the electrode active material and the current collector. Therefore, the influence of the binder on the characteristics of the lithium secondary battery is great.

[0003] For example, Patent Document 1 discloses an electrode binder capable of reducing the expansion of an electrode due to charge and discharge of a lithium secondary battery, which contains an aqueous dispersion of a sodium salt of a polyurethane obtained by reacting an aliphatic and / or alicyclic polyisocyanate, a hydrogenated polybutadiene polyol, an active hydrogen group-containing carboxylic acid, and a chain extender.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the binder for electrodes of a lithium secondary battery, it is of course necessary to have binding properties. For example, when the negative electrode active material contains a silicon-based active material such as SiO, it is required to reduce the expansion of the electrode after repeated charge and discharge. Further, it is required to increase the capacity retention rate after repeated charge and discharge, that is, to improve the charge-discharge cycle characteristics.

[0006] The embodiment of the present invention aims to provide an electrode binder for lithium secondary batteries that exhibits excellent bonding properties, can reduce electrode expansion due to charging and discharging, and has excellent charge-discharge cycle characteristics. [Means for solving the problem]

[0007] The present invention includes embodiments shown below. [1] A binder for electrodes of lithium secondary batteries comprising a polyurethane aqueous dispersion obtained by reacting (A) a polyisocyanate, (B) a polyol (excluding component (C)), (C) a compound having two or more hydroxyl groups and carboxyl groups, and (D) a chain extender, wherein component (A) includes an aromatic polyisocyanate, and component (B) includes at least one selected from the group consisting of polybutadiene polyol, hydrogenated polybutadiene polyol, polyisoprene polyol, and hydrogenated polyisoprene polyol, and the ratio (y / x) of the number of moles of amino groups (y) of component (D) to the number of moles of free isocyanate groups (x) of the urethane prepolymer obtained by reacting component (A), component (B), and component (C) is 0.1 or more and 0.8 or less. [2] The electrode binder described in [1], which is for use as the negative electrode of a lithium secondary battery and contains a silicon-based active material as the negative electrode active material. [3] A binder composition for electrodes of a lithium secondary battery, comprising polyurethane, a conductive agent, and water, obtained by reacting (A) a polyisocyanate, (B) a polyol (excluding component (C)), (C) a compound having two or more hydroxyl groups and carboxyl groups, and (D) a chain extender, wherein component (A) includes an aromatic polyisocyanate, and component (B) includes at least one selected from the group consisting of polybutadiene polyol, hydrogenated polybutadiene polyol, polyisoprene polyol, and hydrogenated polyisoprene polyol, and the ratio (y / x) of the number of moles of amino groups (y) of component (D) to the number of moles of free isocyanate groups (x) of the urethane prepolymer obtained by reacting component (A), component (B), and component (C) is 0.1 or more and 0.8 or less. [4] A negative electrode for a lithium secondary battery comprising the solid component of the electrode binder described in [1] or [2] or the electrode binder composition described in [3], and a negative electrode active material containing a silicon-based active material. A lithium secondary battery having the negative electrode described in [5] [4]. [Effects of the Invention]

[0008] According to embodiments of the present invention, it is possible to provide a binder for lithium secondary battery electrodes that exhibits excellent binding properties, reduces electrode expansion due to charging and discharging, and has excellent charge-discharge cycle characteristics. [Modes for carrying out the invention]

[0009] The electrode binder for the lithium secondary battery according to this embodiment includes an aqueous dispersion of polyurethane obtained by reacting a polyisocyanate as component (A), a polyol as component (B), a compound having two or more hydroxyl groups and carboxyl groups as component (C), and a chain extender as component (D). Therefore, the polyurethane includes a structure derived from component (A), a structure derived from component (B), a structure derived from component (C), and a structure derived from component (D).

[0010] [(A) component] Component (A) is a polyisocyanate, and in this embodiment, it includes an aromatic polyisocyanate. In this specification, an aromatic polyisocyanate means a polyisocyanate having an aromatic ring in its molecule.

[0011] Examples of aromatic polyisocyanates include tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), 1,3-phenylenediisocyanate, 1,4-phenylenediisocyanate, 1,5-diisocyanatonaphthalene, xylylene diisocyanate (XDI), 1,3-bis(1-isocyanato-1-methylethyl)benzene, and their modified and polynuclear compounds. These can be used individually or in combination of two or more.

[0012] The polyisocyanate of component (A) may consist only of aromatic polyisocyanates, but may also contain aliphatic polyisocyanates and / or alicyclic polyisocyanates as long as the effects of this embodiment are not impaired. Component (A) is preferably mainly composed of aromatic polyisocyanates, and more preferably substantially composed only of aromatic polyisocyanates. The amount of aromatic polyisocyanate in component (A) is preferably 60% by mass or more, more preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and may be 100% by mass.

[0013] The amount of component (A) is not particularly limited. For example, the amount of component (A) may be 5 to 40 parts by mass, 10 to 35 parts by mass, or 15 to 30 parts by mass, relative to the total amount of components (A), (B), and (C) of 100 parts by mass (where the amount of component (C) is the amount in acid form).

[0014] [(B) Component] Component (B) is a polyol. However, compounds having two or more hydroxyl groups and carboxyl groups, which are component (C), are not included in component (B).

[0015] In this embodiment, component (B) comprises at least one selected from the group consisting of polybutadiene polyol, hydrogenated polybutadiene polyol, polyisoprene polyol, and hydrogenated polyisoprene polyol (hereinafter referred to as polyolefin polyol (b1)).

[0016] As the polyolefin polyol (b1), one having an average of 1.3 or more hydroxyl groups (-OH) per molecule is preferably used, more preferably having an average of 1.5 to 2.5 hydroxyl groups, and one having an average of 1.7 to 2.2 hydroxyl groups is also acceptable. In this specification, the number of hydroxyl groups (number of functional groups) per molecule is the value calculated by the following formula. Number of functional groups = {(Hydroxyl value) × (Mn)} / (56.1 × 1000)

[0017] The molecular weight of the polyolefin polyol (b1) is not particularly limited. For example, the number average molecular weight (Mn) may be 600 to 10,000, may be 800 to 6,000, may be 1,000 to 5,000, or may be 1,200 to 4,000.

[0018] In this specification, the number average molecular weight (Mn) is a value measured by the GPC method (gel permeation chromatography method) and calculated using a calibration curve with standard polystyrene. Specifically, as the GPC conditions, the column: "TSKgel Hxl" manufactured by Tosoh Corporation, the mobile phase: THF (tetrahydrofuran), the mobile phase flow rate: 1.0 mL / min, the column temperature: 40 °C, the sample injection volume: 50 μL, and the sample concentration: 0.2 mass% can be used for measurement.

[0019] The hydroxyl value of the polyolefin polyol (b1) is not particularly limited. For example, it may be 10 to 200 mgKOH / g, may be 20 to 120 mgKOH / g, may be 30 to 100 mgKOH / g, or may be 40 to 90 mgKOH / g. In this specification, the hydroxyl value is measured in accordance with Method A of JIS K1557-1:2007.

[0020] As the polybutadiene polyol, those having a 1,4-bond type, 1,2-bond type or a mixed structure thereof in the molecule and having a hydroxyl group at the molecular end are preferred. The hydrogenated polybutadiene polyol has a structure hydrogenated (hydrogenated) with respect to the polybutadiene polyol, and a part or all of the unsaturated double bonds contained in the polybutadiene polyol are hydrogenated. The degree of hydrogenation of the hydrogenated polybutadiene polyol is not particularly limited. For example, the iodine value may be 40 g / 100 g or less, may be 30 g / 100 g or less, may be 25 g / 100 g or less, or may be 15 g / 100 g or less. In this specification, the iodine value is measured in accordance with JIS K0070:1992.

[0021] Preferably, the polyisoprene polyol has a polyisoprene structure in which 1,4-bonded, 1,2-bonded, or a mixture thereof is present in the molecule, and has hydroxyl groups at its molecular ends. Hydrogenated polyisoprene polyol has a structure obtained by hydrogenation (hydrogenation) of the polyisoprene polyol, in which some or all of the unsaturated double bonds contained in the polyisoprene polyol are hydrogenated. The degree of hydrogenation of the hydrogenated polyisoprene polyol is not particularly limited; for example, the iodine value may be 40 g / 100 g or less, 30 g / 100 g or less, 25 g / 100 g or less, or 15 g / 100 g or less.

[0022] Component (B) may consist solely of a polyolefin polyol (b1), but may also contain other polyols. Examples of other polyols include polyether polyols, polyester polyols, and polycarbonate polyols, as well as low molecular weight polyhydric alcohols such as ethylene glycol, propylene glycol, diethylene glycol, trimethylolpropane, and glycerin.

[0023] In one embodiment, component (B) preferably contains a trihydric alcohol (b2) together with a polyolefin polyol (b1). The trihydric alcohol (b2) acts as a crosslinking agent to introduce a crosslinked structure into the polyurethane. The trihydric alcohol (b2) is preferably a trihydric alcohol having 3 to 8 carbon atoms, such as trimethylolpropane, glycerin, 1,2,4-butanetriol, 1,2,3-butanetriol, etc. Any one or two or more of these can be used in combination.

[0024] The amount of polyolefin polyol (b1) in component (B) is not particularly limited and may be, for example, 80 to 100% by mass, 90 to 99.5% by mass, or 95 to 99% by mass. The amount of other polyol (preferably trihydric alcohol (b2)) in component (B) is not particularly limited and may be, for example, 0 to 20% by mass, 0.5 to 10% by mass, or 1 to 5% by mass.

[0025] The amount of component (B) is not particularly limited. For example, the amount of component (B) may be 40 to 90 parts by mass, 50 to 85 parts by mass, or 60 to 80 parts by mass, relative to 100 parts by mass of the total amount of components (A), (B), and (C) (where the amount of component (C) is the amount in acid form). The amount of polyolefin polyol (b1) is not particularly limited, for example, it may be 40 to 90 parts by mass, 50 to 85 parts by mass, or 60 to 80 parts by mass, relative to the above total amount of 100 parts by mass. In one embodiment, if a trihydric alcohol (b2) is included, the amount of trihydric alcohol (b2) is not particularly limited, and may be 0.5 to 5 parts by mass or 1 to 3 parts by mass, relative to the above total amount of 100 parts by mass.

[0026] [(C) component] In this embodiment, a compound having two or more hydroxyl groups and carboxyl groups (hereinafter referred to as compound (C)) is used as component (C). The number of hydroxyl groups in one molecule of compound (C) is preferably two or three, more preferably two. The number of carboxyl groups in one molecule of compound (C) is preferably one or two, more preferably one.

[0027] Here, the carboxyl group is a concept that includes not only the acidic form (-COOH) but also the salt form, i.e., carboxylic acid bases (-COOX, where X is a cation that forms a salt with a carboxylic acid), and acidic and saltic forms may coexist. Examples of salts of carboxylic acid bases include alkali metal salts such as lithium salts, sodium salts, and potassium salts, alkaline earth metal salts such as magnesium salts and calcium salts, ammonium salts, amine salts (primary amine salts, secondary amine salts, and tertiary amine salts), and quaternary ammonium salts.

[0028] Examples of compound (C) include hydroxy acids and their derivatives such as dimethylolpropionic acid, dimethylolbutanoic acid, dimethylolvaleric acid, dihydroxymaleic acid, dihydroxybenzoic acid, and tartaric acid, as well as their salts. Any one of these may be used, or two or more may be used in combination.

[0029] The carboxyl groups mentioned above can be neutralized to form salts, which can make the resulting polyurethane water-dispersible. Examples of bases used to neutralize the carboxyl groups include non-volatile bases such as lithium hydroxide, sodium hydroxide, and potassium hydroxide; tertiary amines such as trimethylamine, triethylamine, dimethylethanolamine, methyldiethanolamine, and triethanolamine; and volatile bases such as ammonia. Neutralization can be carried out before, during, or after the urethane formation reaction.

[0030] The amount of component (C) (amount in acid form) is not particularly limited. For example, it may be 1 to 15 parts by mass, 2 to 10 parts by mass, or 3 to 7 parts by mass, relative to 100 parts by mass of the total amount of components (A), (B), and (C) (where the amount of component (C) is in acid form).

[0031] [(D) component] As the chain extender for component (D), an amine-based chain extender commonly used in the art can be used. That is, it is preferable that component (D) contains an amine-based chain extender. As the amine-based chain extender, a polyamine having two or more (preferably 2 to 4) primary amino groups and / or secondary amino groups per molecule is used, and is preferably a diamine (bifunctional amine) and / or a triamine (trifunctional amine).

[0032] Specific examples of amine-based chain extenders include aliphatic polyamines such as ethylenediamine, trimethylenediamine, diethylenetriamine, dipropylenetriamine, and triethylenetetramine; alicyclic polyamines such as piperazine and isophoronediamine; m-xylylenediamine, p-xylylenediamine, o-xylylenediamine, m-phenylenediamine, p-phenylenediamine, o-phenylenediamine, diethyltoluenediamine, 4,4'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, 2,4'-diaminodiphenylmethane, 4,4'-oxydianiline, 3,4'-oxydianiline, and 2,4'- Examples of aromatic polyamines include oxydianiline, diaminodimethyldiphenylmethane, 4,4'-diaminobenzophenone, 4,4'-methylenebis(2-ethyl-6-methylaniline), 4,4'-methylenebis(2,6-diethylaniline), 4,4'-methylenebis(2-isopropyl-6-methylaniline), 4,4'-methylenebis(2-methylaniline), 4,4'-diaminobiphenyl, 2,2'-dimethylbenzidine, naphthylenediamine, 1,3,5-benzenetriamine, 2,4,6-triaminotoluene, 1,3,5-tris(4-aminophenyl)benzene, and 3,3'-diaminobenzidine. These can be used individually or in combination of two or more.

[0033] The chain extender in component (D) may consist solely of an amine-based chain extender, but may also contain other chain extenders as long as the effects of this embodiment are not impaired. Furthermore, when chain extension is performed in an emulsified state, water can also act as a chain extender. That is, water reacts with the free isocyanate groups of the urethane prepolymer to form an amino group via carbamic acid, and chain extension occurs when another free isocyanate group reacts with this amino group. Therefore, component (D) may contain water along with the amine-based chain extender.

[0034] The amount of component (D) is not particularly limited as long as it satisfies the amine equivalent (y / x) described later. For example, with respect to a total amount of 100 parts by mass of components (A), (B), and (C) (where the amount of component (C) is in acid form), the amount of the amine chain extender may be 0.05 to 5 parts by mass, 0.08 to 3 parts by mass, or 0.1 to 2.5 parts by mass.

[0035] [Electrode binder] The electrode binder according to this embodiment includes an aqueous dispersion of polyurethane obtained by reacting components (A) to (D) above. In the polyurethane obtained by reacting components (A) to (D), the carboxyl groups derived from component (C) are dispersed in water as salts. Therefore, the electrode binder includes water and polyurethane dispersed in water.

[0036] Polyurethane is obtained by reacting component (D) with a urethane prepolymer obtained by reacting components (A), (B), and (C) to extend the molecular chain. The urethane prepolymer is a prepolymer having free isocyanate groups (isocyanate-terminated urethane prepolymer), and a urea bond is formed by reaction with the amino group of component (D).

[0037] In this embodiment, the ratio (y / x) of the number of moles (y) of amino groups in component (D) to the number of moles (x) of free isocyanate groups in the urethane prepolymer is set to 0.1 to 0.8. Hereinafter, this ratio (y / x) will also be referred to as the amine equivalent. The amine equivalent (y / x) is preferably 0.2 to 0.7, more preferably 0.3 to 0.6, even more preferably 0.4 to 0.6, and may also be 0.4 to 0.5.

[0038] (A) The aromatic polyisocyanate used as component (A) is highly reactive with water, and in the emulsified state, it reacts competitively with water and the amine-based chain extender. Specifically, with respect to the amine-based chain extender, its multiple amino groups react with the free isocyanate groups of the urethane prepolymer to generate urea bonds, thereby forming a crosslinked structure. With respect to water, it reacts with the free isocyanate groups of the urethane prepolymer, releasing carbon dioxide via carbamic acid to form an amino group. Another free isocyanate group reacts with this amino group to generate a urea bond, thereby forming a crosslinked structure. In this case, if the amine equivalent (y / x) is high, there will be an excess of the amine-based chain extender relative to the free isocyanate groups, and it will not perform its role as a chain extender but will act like a blocking agent. According to this embodiment, by keeping the amine equivalent (y / x) at 0.8 or less, it is possible to suppress the excess of the amine-based chain extender, improve binding properties, reduce electrode expansion due to charging and discharging, and improve charge-discharge cycle characteristics.

[0039] In the amine equivalent (y / x), the number of moles of free isocyanate groups (x) is calculated using the isocyanate group content (mass%) measured according to Method B of JIS K1603-1:2007 (back titration is indicator titration method), and the formula weight of the isocyanate group (42.02). The number of moles of amino groups in component (D) (y) is the number of moles of amino groups of the amine chain extender contained in component (D), and does not include the number of moles of amino groups produced by the reaction of free isocyanate groups with water.

[0040] When synthesizing polyurethane, the molar ratio (NCO / OH) of isocyanate groups in component (A) to the hydroxyl groups in components (B) and (C) (i.e., the total amount of hydroxyl groups in component (B) and component (C)) is not particularly limited and may be, for example, 1.10 to 1.60, 1.15 to 1.50, or 1.16 to 1.30.

[0041] The acid value of polyurethane is not particularly limited and may be, for example, 5 to 50 mg KOH / g or 5 to 45 mg KOH / g. Here, the acid value can be determined from the amount of KOH (mg) required to neutralize the free carboxyl groups contained in 1 g of solid content of the polyurethane aqueous dispersion, in accordance with JIS K0070-1992.

[0042] The polyurethane may consist only of components (A) to (D), but may also contain other components as long as they do not impair the effects of this embodiment.

[0043] In the electrode binder, the concentration of polyurethane is not particularly limited; for example, it may be 5 to 50% by mass or 10 to 40% by mass.

[0044] The method for producing an aqueous dispersion of polyurethane is not particularly limited, and for example, the following method can be used: Component (A) is reacted with components (B) and (C) without a solvent or in an organic solvent that does not contain active hydrogen groups to synthesize an isocyanate-terminated urethane prepolymer. After the synthesis of the urethane prepolymer, the carboxyl groups of component (C) are neutralized with a neutralizing agent, and then dispersed and emulsified in water. Subsequently, component (D) is added such that the molar ratio of amino groups to free isocyanate groups of the urethane prepolymer is the above amine equivalent (y / x), and the isocyanate groups in the emulsified micelles and component (D) undergo an interfacial polymerization reaction to generate urea bonds. This improves the crosslinking density within the emulsified micelles and forms a three-dimensional crosslinked structure. After that, the organic solvent may be removed as needed to obtain an aqueous dispersion of polyurethane.

[0045] The organic solvents optionally used in the synthesis of the above-mentioned urethane prepolymer are solvents that are inert with isocyanate groups and capable of dissolving the resulting urethane prepolymer. Examples of such organic solvents include dioxane, methyl ethyl ketone, acetone, dimethylformamide, tetrahydrofuran, N-methyl-2-pyrrolidone, toluene, and propylene glycol monomethyl ether acetate. In the electrode binder according to this embodiment, the polyurethane aqueous dispersion medium may or may not contain these organic solvents along with water. It is preferable that these organic solvents are ultimately removed.

[0046] In this embodiment, the average particle size of the polyurethane aqueous dispersion is not particularly limited and may be in the range of, for example, 0.005 to 0.5 μm. Also, the number-average molecular weight (Mn) of the polyurethane is not particularly limited and may be, for example, 10,000 or more, or 50,000 or more.

[0047] [Electrode Binding Composition] The electrode binder composition for a lithium secondary battery according to this embodiment comprises polyurethane obtained by reacting components (A) to (D) above, a conductive agent, and water. More specifically, the electrode binder composition contains the polyurethane and conductive agent dispersed in water. The electrode binder composition may be prepared, for example, by mixing an aqueous dispersion of the polyurethane and an aqueous dispersion of the conductive agent.

[0048] As conductive agents, electronically conductive materials that do not adversely affect battery performance can be used. Specific examples of conductive agents include fibrous nanocarbon, carbon black such as acetylene black and kicken black, natural graphite (scaly graphite, flake graphite, earthy graphite, etc.), artificial graphite, carbon whiskers, carbon fibers, metal powders (copper, nickel, aluminum, silver, gold, etc.), metal fibers, and conductive ceramic materials. These can be used individually or in combination of two or more.

[0049] Preferably, fibrous nanocarbon is used as the conductive agent. Fibrous nanocarbon can follow the expansion of the electrode when the electrode active material contains a silicon-based active material, and can suppress damage to the conductive path during repeated charging and discharging.

[0050] Fibrous nanocarbons are fibrous carbon fibers with a fiber diameter on the order of nanometers. The average fiber length of fibrous nanocarbons is not particularly limited, but is preferably, for example, 50 nm to 10 mm, and more preferably 0.5 to 100 μm. The average fiber diameter of fibrous nanocarbons is not particularly limited, but is preferably, for example, 0.5 to 200 nm, and more preferably 1 to 100 nm. The average fiber length and average fiber diameter can be determined by measuring the dimensions of 50 randomly selected fibrous nanocarbons in atomic force microscope (AFM) images and taking their arithmetic mean. Lengths on the order of millimeters that cannot be measured by atomic force microscopy can be measured using images taken with a microscope.

[0051] Specific examples of fibrous nanocarbons include single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), and nanocarbon fibers, and these can be used individually or in combination of two or more. Among these, carbon nanotubes are preferred, and SWCNTs are more preferred.

[0052] In one embodiment, the binder composition preferably contains the polyurethane (X) and fibrous nanocarbon (Y) in a mass ratio of (X):(Y) = 60:40 to 99.6:0.4. This enhances the effect of improving the charge-discharge cycle characteristics of lithium secondary batteries.

[0053] In one embodiment, the binder composition preferably contains carbon black together with fibrous nanocarbon as a conductive agent. In this case, the carbon black acts as a conductive additive, contributing to conductivity around the electrode active material, and allows the conductivity of the fibrous nanocarbon, which follows the expansion of the electrode, to be exhibited more effectively. The content ratio of fibrous nanocarbon to carbon black is not particularly limited; for example, the amount of carbon black may be 100 to 2000 parts by mass per 100 parts by mass of fibrous nanocarbon.

[0054] The conductive agent is preferably used in a dispersed state in a medium. While water is typically used as the medium, polar organic solvents such as alcohols and ketone solvents, or a mixture of these polar organic solvents and water, may also be used. Examples of dispersion devices for dispersing the conductive agent in the medium include jet mills, high-pressure dispersion devices, and ultrasonic homogenizers.

[0055] When preparing an aqueous dispersion in which conductive agents such as fibrous nanocarbon or carbon black are dispersed in water, a dispersant may be added. Examples of dispersants include hydroxymethylcellulose, carboxymethylcellulose and their alkali metal salts, celluloses such as methylcellulose, ethylcellulose, hydroxypropylmethylcellulose, and hydroxyethylmethylcellulose, cellulose nanofibers, polycarboxylic acid compounds such as polyacrylic acid and sodium polyacrylate, compounds having a vinylpyrrolidone structure such as polyvinylpyrrolidone, polyvinyl alcohol, sodium alginate, xanthan gum, carrageenan, guar gum, agar, and starch. Among these, carboxymethylcellulose salts can be preferably used.

[0056] In the electrode binder composition, the concentration of the polyurethane is not particularly limited and may be, for example, 5 to 50% by mass or 10 to 40% by mass. The concentration of the conductive agent is also not particularly limited and may be, for example, 1 to 35% by mass or 5 to 30% by mass.

[0057] [Electrode coating liquid composition] The electrode binder and electrode binder composition according to this embodiment are used to prepare an electrode coating liquid composition for manufacturing electrodes of lithium secondary batteries. When an electrode binder is used, the electrode coating liquid composition (1) includes an electrode binder, an electrode active material, and a conductive agent. When an electrode binder composition is used, the electrode coating liquid composition (2) includes an electrode binder composition and an electrode active material.

[0058] These electrode coating compositions are preferably used to manufacture the negative electrode of a lithium secondary battery. Therefore, the electrode active material is preferably a negative electrode active material. As the negative electrode active material, metallic lithium or a material capable of inserting / deinserting lithium ions can be used. Specific examples of negative electrode active materials include carbon materials such as natural graphite, artificial graphite, non-graphitizable carbon, and easily graphitizable carbon; metallic materials such as metallic lithium and alloys, tin compounds; lithium transition metal nitrides; crystalline metal oxides; amorphous metal oxides; silicon compounds; conductive polymers, etc. Any one of these may be used, or two or more may be used in combination.

[0059] Among these, the negative electrode active material preferably contains silicon-based active materials such as SiO (silicon monoxide) and SiC (silicon carbide) because it allows for increased capacity. The negative electrode active material may be a mixture of silicon-based active material and graphite, but it is preferable to use only silicon-based active material as the negative electrode active material, and more preferably to use SiO and / or SiC.

[0060] In the electrode coating liquid composition, the polyurethane content is not particularly limited, but is preferably 1 to 20% by mass relative to the content of the electrode active material (negative electrode active material), and more preferably 2 to 13% by mass.

[0061] In the electrode coating liquid composition (1) described above, the conductive agent is as described above in the electrode binder composition, including specific examples and preferred compositions, and therefore no further explanation is provided.

[0062] In the electrode coating liquid composition, the content of the conductive agent is not particularly limited, but is preferably 0.1 to 20% by mass relative to the content of the electrode active material (negative electrode active material), and more preferably 0.2 to 10% by mass.

[0063] The electrode coating liquid composition may contain a viscosity modifier, such as a water-soluble polymer, to form a slurry. Examples of thickeners include celluloses such as carboxymethylcellulose salt, methylcellulose, ethylcellulose, hydroxymethylcellulose, hydroxypropylmethylcellulose, and hydroxyethylmethylcellulose; polycarboxylic acid compounds such as polyacrylic acid and sodium polyacrylate; compounds having a vinylpyrrolidone structure such as polyvinylpyrrolidone; polyacrylamide, polyethylene oxide, polyvinyl alcohol, sodium alginate, xanthan gum, carrageenan, guar gum, agar, and starch. Any one or more of these can be used in combination. Among these, carboxymethylcellulose salt is preferred.

[0064] The content of electrode active material (negative electrode active material) in the solid content of the electrode coating liquid composition is not particularly limited and may be, for example, 65 to 99% by mass or 75 to 97% by mass. The content of dispersion medium such as water in the electrode coating liquid composition is not particularly limited and may be, for example, 20 to 80% by mass or 40 to 70% by mass.

[0065] The method for preparing the electrode coating liquid composition is not particularly limited. When mixing the above components, for example, a mortar and pestle, a mill mixer, a ball mill such as a planetary ball mill or a shaker ball mill, a mechanofusion, etc., can be used.

[0066] [Negative electrode of lithium secondary battery] The negative electrode of the lithium secondary battery according to the embodiment can be manufactured by applying the electrode coating liquid composition to a current collector and evaporating the dispersion medium. That is, the negative electrode comprises a current collector and a negative electrode composite layer (also referred to as an active material layer) formed on the current collector, and the negative electrode composite layer consists of the solid components of the electrode coating liquid composition. In one embodiment, the electrode coating liquid composition includes a silicon-based active material such as SiO and / or SiC as a negative electrode active material, together with the electrode binder or electrode binder composition. Therefore, the negative electrode of the lithium secondary battery according to one embodiment comprises the solid components of the electrode binder or electrode binder composition and a negative electrode active material containing a silicon-based active material such as SiO and / or SiC.

[0067] As the current collector, an electron conductor that does not adversely affect the constructed battery can be used. Examples of current collectors for the negative electrode include copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymers, conductive glass, Al-Cd alloys, and copper or other materials whose surfaces are treated with carbon, nickel, titanium, or silver to improve adhesion, conductivity, and oxidation resistance. The surfaces of these current collector materials may also be oxidized. Examples of current collector shapes include foil, film, sheet, net, punched or expanded materials, lath, porous materials, foam, and other molded bodies. The thickness of the current collector is not particularly limited, but those with a thickness of 1 to 100 μm are commonly used.

[0068] The thickness of the negative electrode composite layer is not particularly limited and may be, for example, 15 to 150 μm.

[0069] [Lithium-ion rechargeable battery] The lithium secondary battery according to this embodiment includes the negative electrode described above. More specifically, the lithium secondary battery comprises a negative electrode, a positive electrode, a separator disposed between the negative electrode and the positive electrode, and an electrolyte, wherein the negative electrode is an electrode made using the electrode coating liquid composition described above. Known configurations can be used for the positive electrode, separator, and electrolyte, and are not particularly limited.

[0070] The lithium secondary battery according to this embodiment can be formed into cylindrical, coin-type, prismatic, or any other arbitrary shape. The basic structure of the battery is the same regardless of the shape, and it can be designed and implemented according to the purpose. For example, in the case of a cylindrical battery, a negative electrode, which is made by coating a negative electrode active material onto a negative electrode current collector, and a positive electrode, which is made by coating a positive electrode active material onto a positive electrode current collector, are wound together with a separator in between. This wound body is then housed in a battery case, a non-aqueous electrolyte is injected, and insulating plates are placed on the top and bottom before sealing. In the case of a coin-type lithium secondary battery, a disc-shaped negative electrode, a separator, a disc-shaped positive electrode, and a stainless steel plate are stacked and housed in a coin-type battery case, a non-aqueous electrolyte is injected, and it is sealed. [Examples]

[0071] The present invention will be described in more detail below based on examples and comparative examples, but it is not limited thereto.

[0072] Details of the polyol, fibrous nanocarbon aqueous dispersion, acetylene black aqueous dispersion, and carboxymethylcellulose sodium salt used in the examples are as follows.

[0073] • Hydrogenated polybutadiene polyol (B1): "NISSO-PB GI-1000" manufactured by Nippon Soda Co., Ltd. (Mn: 1500, iodine value 11g / 100g, hydroxyl value 69mgKOH / g, number of functional groups: 1.84)

[0074] • Polybutadiene polyol (B2): "NISSO-PB G-1000" manufactured by Nippon Soda Co., Ltd. (Mn: 1400, hydroxyl value 72 mg KOH / g, number of functional groups: 1.8)

[0075] • Hydrogenated polybutadiene polyol (B3): Clay Valley's "Krasol HLBH-P2000" (Mn: 2000, hydroxyl value 53 mg KOH / g, number of functional groups: 1.9)

[0076] • Hydrogenated polyisoprene polyol (B4): "EPOL" manufactured by Idemitsu Kosan Co., Ltd. (Mn: 2500, hydroxyl value 52 mg KOH / g, number of functional groups: 2.3)

[0077] • Polybutadiene polyol (B5): Evonik "POLYVEST HT" (Mn: 2900, hydroxyl value 47 mg KOH / g, number of functional groups: 2.45)

[0078] • Polycarbonate polyol: "ETERNACOLL UH-300" manufactured by UBE Corporation (Mn: 3000, hydroxyl value 37 mg KOH / g, number of functional groups: 1.98)

[0079] • Fibrous nanocarbon aqueous dispersion: Using OCSiAl's "TUBALL BATT" (CNT purity >93%, average diameter 1.6±0.5 nm) as single-wall carbon nanotubes (SWCNTs), a fibrous nanocarbon aqueous dispersion with a fibrous nanocarbon concentration of 1% by mass was prepared according to the following manufacturing procedure. The average fiber diameter, average fiber length, and aspect ratio of the fibrous nanocarbons in the obtained aqueous dispersion were measured using the following measurement method. The average fiber diameter was 3 nm, the average fiber length was 3000 nm, and the aspect ratio was 1000.

[0080] (Manufacturing procedure for fibrous nanocarbon aqueous dispersions) 0.5 g of SWCNTs were mixed in a beaker with 50 g of a 1% by mass aqueous solution of carboxymethylcellulose salt ("Selogen 7A" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), stirred, and then dispersed for 90 minutes at an output of 100 μA while circulating the slurry using the beaker, an ultrasonic homogenizer (US-600T manufactured by Nippon Seiki Seisakusho Co., Ltd.), a circulation unit, and a tube pump to obtain a fibrous nanocarbon aqueous dispersion.

[0081] (Measurement method for fibrous nanocarbons) The average fiber diameter and average fiber length of fibrous nanocarbon were measured using a scanning probe microscope (SPM) (JEOL Ltd., AFM-5300E). Specifically, a fibrous nanocarbon aqueous dispersion was diluted with water until the concentration of fibrous nanocarbon was 0.01% by mass, then spread onto a mica substrate and the solvent was evaporated. The AFM image of this sample was then observed, and the average fiber diameter and average fiber length were calculated according to the method described above. Furthermore, the aspect ratio was calculated using these values ​​according to (Equation 1) below. Aspect ratio = Average fiber length (nm) / Average fiber diameter (nm) ... (Equation 1)

[0082] • Acetylene black aqueous dispersion: Denka Co., Ltd.'s "Li400" was used as the acetylene black. 100 g of acetylene black was added to 300 g of a 1% by mass aqueous solution of carboxymethylcellulose salt (Daiichi Kogyo Seiyaku Co., Ltd.'s "Selogen 7A") using a high-speed dispenser while stirring, and stirred until homogeneous. This yielded an acetylene black aqueous dispersion with an acetylene black concentration of 25% by mass.

[0083] • Carboxymethylcellulose sodium salt: "Selogen WS-C" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.

[0084] [Synthesis of polyurethane aqueous dispersions] (Manufacturing example 1: Binding agent 1) In a four-necked flask equipped with a stirrer, reflux condenser, thermometer, and nitrogen inlet, 74.09 parts by mass of hydrogenated polybutadiene polyol (B1), 1.80 parts by mass of trimethylolpropane, 4.20 parts by mass of dimethylolpropionic acid, 19.91 parts by mass of tolylene diisocyanate (2,4-TDI / 2,6-TDI=80 / 20 mixture), and 115 parts by mass of methyl ethyl ketone were added and reacted at 75°C for 4 hours to obtain a methyl ethyl ketone solution of urethane prepolymer with a free isocyanate group content of 2.28% by mass relative to nonvolatile matter. This solution was cooled to 45°C, and an aqueous sodium hydroxide solution consisting of 1.20 parts by mass of sodium hydroxide and 233 parts by mass of water was gradually added and emulsified and dispersed using a homogenizer. Subsequently, an aqueous solution of 1.12 parts by mass of diethylenetriamine diluted with 23.8 parts by mass of water was added, and a chain extension reaction was carried out for 1 hour. This was subjected to desolvation under reduced pressure and heating at 50°C to obtain an aqueous dispersion of a sodium salt of polyurethane (binding agent 1) with a non-volatile content of approximately 32% by mass.

[0085] In Production Example 1, the amount of diethylenetriamine, which is a chain extender, was set so that the amine equivalent (y / x) was 0.6. Specifically, since the isocyanate group content is 2.28% by mass, the amount of isocyanate groups in 100g of urethane prepolymer is 2.28g. Therefore, the number of moles of free isocyanate groups (x) is 0.5426 (=2.28 / 42.02) moles. Since the molecular weight of diethylenetriamine is 103.17 and the number of functional groups is 3, the amount of diethylenetriamine that results in an amine equivalent (y / x) of 0.6 is 0.05426 × (103.17 / 3) × 0.6 = 1.12 parts by mass.

[0086] (Manufacturing examples 2-3: Binding agents 2-3) Aqueous dispersions of lithium salts or triethylamine salts of polyurethane (binding agents 2-3) were obtained by substituting sodium hydroxide with lithium hydroxide or triethylamine in the type and amount of neutralizing agent as shown in Table 1, and otherwise proceeding in the same manner as in Production Example 1.

[0087] (Manufacturing examples 4-15: Binding agents 4-15) Aqueous dispersions of sodium polyurethane salts (binding agents 4-15) were obtained by changing the polyisocyanate, polyol, and / or chain extender as shown in Tables 1 and 2, and otherwise proceeding in the same manner as in Production Example 1. In this process, the amine equivalent (y / x) was set to 0.3 in Production Example 10, 0.4 in Production Example 11, 0.5 in Production Examples 12 and 13, 0.1 in Production Example 14, and 0.8 in Production Example 15.

[0088] (Comparative manufacturing examples 1-9: Binding agents C1-C9) Aqueous dispersions of sodium polyurethane salts (binding agents C1-C9) were obtained by changing the polyisocyanate, polyol, and / or chain extender as shown in Table 3, and otherwise proceeding in the same manner as in Production Example 1. In this process, the amine equivalent (y / x) was set to 0.5 for Comparative Production Examples 1-6, 0.9 for Comparative Production Example 7, 0.0 for Comparative Production Example 8, and 0.6 for Comparative Production Example 9.

[0089] Tables 1-3 show the polyurethane components for the obtained binders 1-15 and C1-9. Tables 1-3 also show the isocyanate group content (free F-NCO) of the urethane prepolymer obtained by reacting components (A)-(C), the molar ratio of isocyanate groups in component (A) to hydroxyl groups in components (B) and (C) (NCO / OH), and the acid value of the polyurethane. The methods for measuring free F-NCO and acid value are as described above.

[0090] [Table 1]

[0091] [Table 2]

[0092] [Table 3]

[0093] [Manufacturing of lithium secondary batteries] (Example 1) (Fabrication of the negative electrode) SiO(average particle size 4.5 μm, specific surface area 5.5 m²) is used as the negative electrode active material. 2 A negative electrode slurry was prepared by mixing 88.85 parts by mass of (1 / g), 40 parts by mass of fibrous nanocarbon aqueous dispersion and 4.0 parts by mass of acetylene black aqueous dispersion as conductive agents, 43.3 parts by mass of a 1.5% by mass aqueous solution of carboxymethylcellulose sodium salt as a thickener, 30 parts by mass of polyurethane aqueous dispersion of binder 1, and 14 parts by mass of ion-exchanged water using a planetary mixer to obtain a solid content of 49% by mass. An electrolytic copper foil with a thickness of 10 μm was used as a current collector, and a negative electrode composite layer consisting of the above negative electrode slurry was formed on the electrolytic copper foil. Specifically, the above negative electrode slurry was coated onto the electrolytic copper foil using a coating machine, roll-pressed, and then dried under reduced pressure at 130°C to obtain a negative electrode active material of 2.7 mg / cm³. 2 The negative electrode was obtained.

[0094] (Preparation of the positive electrode) 92 parts by mass of LiNiCoAlO2(NCA), the positive electrode active material, 4 parts by mass of acetylene black (Denka Co., Ltd.'s "Li-400") as a conductive agent, 4 parts by mass of polyvinylidene fluoride as a binder, and 49.2 parts by mass of N-methyl-2-pyrrolidone as a dispersion medium were mixed in a planetary mixer to prepare a positive electrode slurry with a solid content of 67% by mass. This positive electrode slurry was coated onto 15 μm thick aluminum foil using a coating machine, dried at 130°C, and then roll-pressed to obtain a positive electrode active material concentration of 16.6 mg / cm³. 2 The positive electrode was obtained.

[0095] (Battery Fabrication) The negative and positive electrodes obtained above were combined and laminated with a polyolefin-based (PE / PP / PE) separator sandwiched between the electrodes. The positive and negative electrode terminals were ultrasonically welded to each electrode. This laminate was placed in an aluminum laminate packaging and heat-sealed, leaving an opening for liquid injection. Positive electrode area: 18 cm² 2 , negative electrode area 19.8cm 2A battery was prepared before electrolyte injection. Next, an electrolyte solution prepared by dissolving LiPF6 (1.0 mol / L) in a solvent mixture of ethylene carbonate and diethyl carbonate (30 / 70 vol ratio) was injected, the opening was heat-sealed, and a battery for evaluation was obtained.

[0096] (Example 2) When preparing the negative electrode slurry, SiC (average particle size 11.4 μm, specific surface area 2.5 m²) is used instead of SiO as the negative electrode active material. 2 Using ( / g), and otherwise in the same manner as in Example 1, a negative electrode was prepared, and then an evaluation battery was fabricated.

[0097] (Examples 3-11, 13-17 and Comparative Examples 1-9) When preparing the negative electrode slurry, binders 2-15 and C1-9 as listed in Table 4 below were used as binders, and the negative electrode was prepared in the same manner as in Example 1. An evaluation battery was then prepared.

[0098] (Example 12) When preparing the negative electrode slurry, SiC (average particle size 11.4 μm, specific surface area 2.5 m²) is used instead of SiO as the negative electrode active material. 2 Using ( / g), and otherwise in the same manner as in Example 11, a negative electrode was prepared, and then an evaluation battery was fabricated.

[0099] [evaluation] The bonding properties of the fabricated negative electrode were evaluated. Furthermore, the electrode expansion rate of the evaluation battery was measured after repeated charge-discharge cycles to confirm the effect of reducing electrode expansion. The charge-discharge cycle characteristics of the evaluation battery were also evaluated. The test method is as follows:

[0100] (Binding properties) The bonding properties were evaluated by a peel test. In the peel test, the negative electrode was cut into 2 x 8 cm pieces, and the coated surface of the cut test piece was attached to a SUS steel plate using double-sided tape. The test piece was bent 180°, and the SUS steel plate was fixed to the lower jig of the testing machine. Single-sided adhesive tape was attached to the bent test piece and fixed to the upper jig. The 180° peel test was performed by raising the upper jig at a speed of 50 mm / min, and the average value of the peel strength was calculated as the peel strength (N / cm), which was evaluated according to the evaluation criteria below. Evaluation criteria: A: Peel strength of 0.9 N / cm or higher B: Peel strength of 0.3 N / cm or more and less than 0.9 N / cm C: Peel strength less than 0.3 N / cm

[0101] (Electrode expansion coefficient) The evaluation battery underwent repeated charging and discharging using a charge / discharge device under the following conditions: At 25°C, constant current (CC) charging was performed up to 4.2V at a current density equivalent to 1C, followed by switching to constant voltage (CV) charging at 4.2V. After charging for 1.5 hours or until the current density reached 0.1C, the battery was CC discharged up to 2.7V at a current density equivalent to 1C. This cycle was repeated 500 times at 25°C. A 10-minute pause was allowed between charging and discharging cycles. The cell thickness at the first fully charged state (cell thickness before the start of the cycle) and the cell thickness at the 500th fully charged state (cell thickness after the end of the cycle) were measured with a micrometer and calculated using the following formula. Electrode expansion ratio = {(Cell thickness after cycle completion - Cell thickness before cycle start) / Anode composite layer thickness} × 100% Here, the thickness of the negative electrode composite layer = the thickness of the negative electrode - the thickness of the copper foil

[0102] (Charge / Discharge Cycle Characteristics) The evaluation batteries underwent charge-discharge cycle testing using a charge-discharge device under the following conditions: At 25°C, the batteries were charged using CC (constant current) charging at a current density equivalent to 1C up to 4.2V, then switched to CV (constant voltage) charging at 4.2V for 1.5 hours or until the current density reached 0.1C, and then discharged using CC charging at a current density equivalent to 1C up to 2.7V. This cycle was repeated 500 times at 25°C. The ratio of the 1C discharge capacity after 500 cycles to the initial 1C discharge capacity was calculated as the charge-discharge cycle retention rate (%). A 10-minute pause was allowed between charging and discharging cycles.

[0103] [Table 4]

[0104] The results are shown in Table 4. In Comparative Examples 1 to 6, the binders C1 to C6 used to prepare the negative electrodes were synthesized using alicyclic polyisocyanates or aliphatic polyisocyanates instead of aromatic polyisocyanates, resulting in inferior electrode expansion suppression effect and charge-discharge cycle characteristics.

[0105] In Comparative Example 7, the polyurethane constituting the binder C7 was synthesized using an aromatic polyisocyanate, but the amine equivalent (y / x) was 0.9, which is higher than the specified range, resulting in poor binding properties, electrode expansion suppression effect, and charge-discharge cycle characteristics. In Comparative Example 8, no amine-based chain extender was used, and the amine equivalent (y / x) was 0, resulting in poor binding properties, electrode expansion suppression effect, and charge-discharge cycle characteristics.

[0106] In Comparative Example 9, a polycarbonate polyol was used instead of a polyolefin-based polyol (b1) as the polyol constituting the binder C4, resulting in inferior binding properties, electrode expansion suppression effect, and charge-discharge cycle characteristics.

[0107] In contrast, in Examples 1 to 17, by combining aromatic polyisocyanate with polyolefin-based polyol (b1) and setting the amine equivalent (y / x) to 0.1 to 0.8, the results were superior to Comparative Examples 1 to 9 in terms of binding properties, electrode expansion suppression effect, and charge-discharge cycle characteristics.

[0108] Furthermore, the various numerical ranges described in this specification can be any combination of their upper and lower limits, and all such combinations are described herein as preferred numerical ranges. Also, the description of a numerical range as "X~Y" means X or greater and Y or less.

[0109] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their omissions, substitutions, and modifications are included in the scope and spirit of the invention, as well as in the claims and their equivalents.

Claims

1. (A) polyisocyanate, (B) polyol (excluding component (C)), (C) a compound having two or more hydroxyl groups and carboxyl groups, and (D) a chain extender containing an amine-based chain extender are reacted to obtain the following: The aforementioned component (A) includes an aromatic polyisocyanate, The aforementioned component (B) comprises at least one selected from the group consisting of polybutadiene polyol, hydrogenated polybutadiene polyol, polyisoprene polyol, and hydrogenated polyisoprene polyol. The ratio (y / x) of the number of moles (y) of amino groups in component (D) to the number of moles (x) of free isocyanate groups in the urethane prepolymer obtained by reacting component (A), component (B), and component (C) is 0.3 or more and 0.8 or less. A binder for electrodes of lithium secondary batteries, containing a polyurethane aqueous dispersion.

2. The electrode binder according to claim 1, which is for use as the negative electrode of a lithium secondary battery, comprising a silicon-based active material as the negative electrode active material.

3. A polyurethane obtained by reacting (A) a polyisocyanate, (B) a polyol (excluding component (C)), (C) a compound having two or more hydroxyl groups and carboxyl groups, and (D) a chain extender containing an amine-based chain extender, wherein component (A) includes an aromatic polyisocyanate, and component (B) includes at least one selected from the group consisting of polybutadiene polyol, hydrogenated polybutadiene polyol, polyisoprene polyol, and hydrogenated polyisoprene polyol, and the ratio (y / x) of the number of moles of amino groups (y) of component (D) to the number of moles of free isocyanate groups (x) of the urethane prepolymer obtained by reacting component (A), component (B), and component (C) is 0.3 or more and 0.8 or less, Conductive agents, and, A binder composition for electrodes of lithium secondary batteries, containing water.

4. A negative electrode for a lithium secondary battery, comprising the solid component of the electrode binder described in claim 1 or the electrode binder composition described in claim 3, and a negative electrode active material containing a silicon-based active material.

5. A lithium secondary battery comprising the negative electrode described in claim 4.

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