JP7748595B1 - Electrode binder for lithium secondary battery, electrode binder composition, negative electrode, and lithium secondary battery - Google Patents

Abstract

The present invention provides a binder for electrodes of lithium secondary batteries that can reduce electrode expansion due to charge and discharge and has excellent charge and discharge cycle characteristics.
[Solution] According to an embodiment, the electrode binder for a lithium secondary battery includes an aqueous dispersion of polyurethane obtained by reacting (A) a polyisocyanate, (B) a polyol (excluding component (C)), (C) a compound having two or more hydroxyl groups and a carboxy group, and (D) a chain extender. Component (A) includes at least one selected from the group consisting of an aliphatic polyisocyanate and an alicyclic polyisocyanate. Component (B) includes at least one selected from the group consisting of a polybutadiene polyol, a hydrogenated polybutadiene polyol, a polyisoprene polyol, and a hydrogenated polyisoprene polyol. Component (D) includes an aromatic polyamine.
[Selection diagram] None

Description

Embodiments of the present invention relate to an electrode binder for a lithium secondary battery, an electrode binder composition, a negative electrode, and a lithium secondary battery.

Lithium secondary batteries, also known as lithium-ion secondary batteries, are high-voltage, high-energy density power storage devices used, for example, as power sources for electronic devices. Electrodes for lithium secondary batteries are typically manufactured by applying a mixture of an electrode active material, a conductive agent, and a binder to the surface of a current collector and drying the mixture. The binder is used to provide adhesion between the electrode active material and the current collector. Therefore, the binder has a significant impact on the characteristics of lithium secondary batteries.

For example, Patent Document 1 discloses an electrode binder that can reduce electrode expansion due to charging and discharging of lithium secondary batteries, and that contains an aqueous dispersion of a sodium salt of 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.

Patent No. 7161078

As mentioned above, electrode binders for lithium secondary batteries have a significant impact on battery characteristics. For example, when the negative electrode active material contains a silicon-based active material such as SiO, it is necessary to reduce electrode expansion after repeated charge and discharge. Furthermore, it is necessary to increase the capacity retention rate after repeated charge and discharge, i.e., to improve charge and discharge cycle characteristics.

An embodiment of the present invention aims to provide an electrode binder for lithium secondary batteries that can reduce electrode expansion due to charge and discharge and has excellent charge and discharge cycle characteristics.

The present invention includes the embodiments shown below.
[1] A binder for electrodes of a lithium secondary battery, comprising an aqueous dispersion of polyurethane obtained by reacting (A) a polyisocyanate, (B) a polyol (excluding component (C)), (C) a compound having two or more hydroxyl groups and a carboxy group, and (D) a chain extender, wherein the component (A) comprises at least one selected from the group consisting of an aliphatic polyisocyanate and an alicyclic polyisocyanate, the component (B) comprises at least one selected from the group consisting of a polybutadiene polyol, a hydrogenated polybutadiene polyol, a polyisoprene polyol, and a hydrogenated polyisoprene polyol, and the component (D) comprises an aromatic polyamine.
[2] The electrode binder according to [1], which is for use in the negative electrode of a lithium secondary battery containing a silicon-based active material as a negative electrode active material.
[3] A binder composition for an electrode of a lithium secondary battery, the binder composition comprising: a polyurethane obtained by reacting (A) a polyisocyanate, (B) a polyol (excluding component (C)), (C) a compound having two or more hydroxyl groups and a carboxy group, and (D) a chain extender, wherein the component (A) comprises at least one selected from the group consisting of an aliphatic polyisocyanate and an alicyclic polyisocyanate, the component (B) comprises at least one selected from the group consisting of a polybutadiene polyol, a hydrogenated polybutadiene polyol, a polyisoprene polyol, and a hydrogenated polyisoprene polyol, and the component (D) comprises an aromatic polyamine; a conductive agent; and water.
[4] A negative electrode of a lithium secondary battery, comprising the solid content of the electrode binder according to [1] or [2] or the electrode binder composition according to [3], and a negative electrode active material containing a silicon-based active material.
[5] A lithium secondary battery comprising the negative electrode according to [4].

Embodiments of the present invention provide a binder for electrodes in lithium secondary batteries that can reduce electrode expansion due to charge and discharge and has excellent charge and discharge cycle characteristics.

The electrode binder for the lithium secondary battery according to this embodiment includes an aqueous polyurethane dispersion obtained by reacting a polyisocyanate as component (A), a polyol as component (B), a compound having two or more hydroxyl groups and a carboxyl group 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). The electrode binder according to this embodiment can reduce electrode expansion even when the electrode active material contains a silicon-based active material such as SiO, and exhibits excellent charge-discharge cycle characteristics.

[Component (A)]
Component (A) is a polyisocyanate, and in this embodiment, contains at least one selected from the group consisting of aliphatic polyisocyanates and alicyclic polyisocyanates (hereinafter also referred to as an aliphatic/alicyclic polyisocyanate). That is, component (A) may contain an aliphatic polyisocyanate, an alicyclic polyisocyanate, or both an aliphatic polyisocyanate and an alicyclic polyisocyanate.

Examples of aliphatic polyisocyanates include aliphatic diisocyanates such as tetramethylene diisocyanate, dodecamethylene diisocyanate, hexamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, lysine diisocyanate, 2-methylpentane-1,5-diisocyanate, and 3-methylpentane-1,5-diisocyanate, as well as modified and polynuclear diisocyanates thereof. These can be used alone or in combination of two or more.

Examples of alicyclic polyisocyanates include alicyclic diisocyanates such as isophorone diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), hydrogenated xylylene diisocyanate, 1,4-cyclohexane diisocyanate, methylcyclohexylene diisocyanate, and 1,3-bis(isocyanatomethyl)cyclohexane, as well as modified and polynuclear products thereof. These can be used alone or in combination of two or more.

The polyisocyanate of component (A) may be composed solely of an aliphatic/alicyclic polyisocyanate, but may also contain an aromatic polyisocyanate as long as the effects of this embodiment are not impaired. Component (A) preferably contains an aliphatic/alicyclic polyisocyanate as the main component, and more preferably consists essentially of an aliphatic/alicyclic polyisocyanate. The amount of aliphatic/alicyclic 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, and even more preferably 90% by mass or more, and may even be 100% by mass.

The amount of component (A) is not particularly limited. For example, the amount of component (A) may be 5 to 45 parts by mass, 10 to 40 parts by mass, or 15 to 35 parts by mass relative to 100 parts by mass of the total amount of components (A), (B), and (C) (however, the amount of component (C) is the amount in acid form).

[(B) Component]
Component (B) is a polyol, provided that component (C), a compound having two or more hydroxyl groups and a carboxyl group, is not included in component (B).

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

The polyolefin polyol (b1) preferably has an average of 1.3 or more hydroxyl groups (—OH) per molecule, more preferably an average of 1.5 to 2.5 hydroxyl groups, and may also have an average of 1.7 to 2.2 hydroxyl groups. In this specification, the number of hydroxyl groups per molecule (number of functional groups) is a value calculated by the following formula:
Number of functional groups={(hydroxyl value)×(Mn)}/(56.1×1000)

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

In this specification, the number average molecular weight (Mn) is a value measured by gel permeation chromatography (GPC) and calculated using a calibration curve based on standard polystyrene. Specifically, the GPC conditions are as follows: column: "TSKgel Hx1" manufactured by Tosoh Corporation; mobile phase: THF (tetrahydrofuran); mobile phase flow rate: 1.0 mL/min; column temperature: 40°C; sample injection volume: 50 μL; sample concentration: 0.2% by mass.

The hydroxyl value of polyolefin polyol (b1) is not particularly limited and may be, for example, 10 to 200 mgKOH/g, 15 to 120 mgKOH/g, 20 to 100 mgKOH/g, or 50 to 90 mgKOH/g. In this specification, the hydroxyl value is measured in accordance with Method A of JIS K1557-1:2007.

Preferred polybutadiene polyols are those having a polybutadiene structure with 1,4-bonds, 1,2-bonds, or a mixture thereof in the molecule, and having hydroxyl groups at the molecular terminals. Hydrogenated polybutadiene polyols have a structure in which the polybutadiene polyol has been hydrogenated (hydrogenated), and some or all of the unsaturated double bonds contained in the polybutadiene polyol have been 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, 30 g/100 g or less, 25 g/100 g or less, or 15 g/100 g or less. In this specification, the iodine value is measured in accordance with JIS K0070:1992.

Preferably, the polyisoprene polyol has a polyisoprene structure with 1,4-bonds, 1,2-bonds, or a mixture thereof in the molecule, and has hydroxyl groups at the molecular terminals. Hydrogenated polyisoprene polyol has a structure obtained by hydrogenating (hydrogenating) 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.

Component (B) may consist solely of 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.

In one embodiment, component (B) preferably contains a trihydric alcohol (b2) along with a polyolefin polyol (b1). The trihydric alcohol (b2) acts as a crosslinking agent that introduces a crosslinked structure into the polyurethane. Trihydric alcohols (b2) preferably have 3 to 8 carbon atoms, such as trimethylolpropane, glycerin, 1,2,4-butanetriol, and 1,2,3-butanetriol. These can be used alone or in combination.

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.

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 per 100 parts by mass of the total of components (A), (B), and (C) (provided that the amount of component (C) is the amount in acid form). The amount of polyolefin-based polyol (b1) is not particularly limited, and may be, for example, 40 to 90 parts by mass, 50 to 85 parts by mass, or 60 to 80 parts by mass per 100 parts by mass of the total. In one embodiment, when trihydric alcohol (b2) is contained, 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 per 100 parts by mass of the total.

[(C) Component]
In this embodiment, a compound having two or more hydroxyl groups and a carboxy group (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, and more preferably two. The number of carboxy groups in one molecule of compound (C) is preferably one or two, and more preferably one.

The term "carboxyl group" as used herein refers not only to the acid form (-COOH) but also to the salt form, i.e., carboxylic acid salt group (-COOX, where X is a cation that forms a salt with carboxylic acid), and a mixture of the acid form and the salt form may be present. Examples of salts of carboxylic acid salts include alkali metal salts such as lithium salt, sodium salt, and potassium salt, alkaline earth metal salts such as magnesium salt and calcium salt, ammonium salt, amine salts (primary amine salt, secondary amine salt, tertiary amine salt), and quaternary ammonium salt.

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

By neutralizing the carboxyl groups to form salts, the final polyurethane can be made water-dispersible. Examples of bases that can be 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-forming reaction.

The amount of component (C) (amount in acid form) is not particularly limited, and may be, for example, 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) (however, the amount of component (C) in acid form).

[(D) Component]
Component (D) is a chain extender, and in this embodiment, contains an aromatic polyamine. By using an aromatic polyamine as a chain extender, it is possible to reduce electrode expansion due to charge and discharge while maintaining or improving binding strength, and improve charge and discharge cycle characteristics.

Aromatic polyamines are aromatic compounds with two or more (preferably two to four) amino groups per molecule. Examples of aromatic polyamines include 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, 2,4'-oxydianiline, diaminodimethyldiphenylmethane, and 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, 3,3'-diaminobenzidine, etc. These can be used alone or in combination of two or more.

In one embodiment, the aromatic polyamine is preferably an aromatic compound having two amino groups in one molecule, i.e., an aromatic bifunctional amine.

The chain extender of component (D) may consist solely of aromatic polyamine, but may also contain other chain extenders such as aliphatic polyamines or alicyclic polyamines, as long as the effects of this embodiment are not impaired. Furthermore, when chain extension is performed in an emulsified state, water can also serve as a chain extender. That is, water reacts with free isocyanate groups in the urethane prepolymer to form amino groups via carbamic acid, and chain extension occurs when these amino groups react with other free isocyanate groups. Therefore, component (D) may contain water in addition to aromatic polyamine. The amount of aromatic polyamine in component (D) is preferably 70% by mass or more, more preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more.

The amount of component (D) is not particularly limited. For example, the amount of component (D) may be 0.5 to 12 parts by mass, 1 to 10 parts by mass, or 2 to 9 parts by mass, relative to 100 parts by mass of the total amount of components (A), (B), and (C) (however, the amount of component (C) is the amount in acid form). The amount of aromatic polyamine may be 0.5 to 12 parts by mass, 1 to 10 parts by mass, or 2 to 9 parts by mass, relative to 100 parts by mass of the total amount.

[Electrode binder]
The electrode binder according to this embodiment contains an aqueous dispersion of polyurethane obtained by reacting the above components (A) to (D). In the polyurethane obtained by reacting the components (A) to (D), the carboxyl group derived from the component (C) is converted into a salt and dispersed in water. Therefore, the electrode binder contains water and polyurethane dispersed in water.

When synthesizing the polyurethane, the molar ratio (NCO/OH) of the isocyanate groups in component (A) to the hydroxyl groups in components (B) and (C) (i.e., the total number 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.

The acid value of the 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 in accordance with JIS K0070-1992 from the amount (mg) of KOH required to neutralize the free carboxyl groups contained in 1 g of solids in the polyurethane aqueous dispersion.

The polyurethane may be composed only of components (A) to (D), but may also contain other components as long as the effects of this embodiment are not impaired.

The concentration of polyurethane in the electrode binder is not particularly limited and may be, for example, 5 to 50% by mass, or 10 to 40% by mass.

The method for producing an aqueous polyurethane dispersion is not particularly limited, and examples include the following: Component (A) is reacted with components (B) and (C) without a solvent or in an organic solvent lacking active hydrogen groups to synthesize an isocyanate-terminated urethane prepolymer. After synthesis of the urethane prepolymer, the carboxy groups of component (C) are neutralized with a neutralizing agent and then dispersed and emulsified in water. Component (D) is then added in an equivalent amount less than the free isocyanate groups of the urethane prepolymer (e.g., a molar ratio of isocyanate groups to amino groups of the chain extender of 1:0.50-0.95), and the isocyanate groups in the emulsion micelles and the chain extender undergo an interfacial polymerization reaction to form urea bonds. This improves the crosslink density within the emulsion micelles and forms a three-dimensional crosslinked structure. The organic solvent can then be removed as needed, yielding an aqueous polyurethane dispersion.

The organic solvent optionally used in the synthesis of the urethane prepolymer is a solvent that is inert to 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 medium for the polyurethane aqueous dispersion may or may not contain these organic solvents along with water. It is preferable that these organic solvents are ultimately removed.

In this embodiment, the average particle size of the polyurethane aqueous dispersion is not particularly limited and may be, for example, in the range of 0.005 to 0.5 μm. Furthermore, 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.

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

The conductive agent can be any electronically conductive material that does not adversely affect battery performance. Specific examples of conductive agents include fibrous nanocarbon, carbon black such as acetylene black and ketjen black, natural graphite (scale graphite, flake graphite, earthy graphite, etc.), artificial graphite, carbon whiskers, carbon fibers, metal (copper, nickel, aluminum, silver, gold, etc.) powder, metal fibers, conductive ceramic materials, and other conductive materials. These can be used alone or in combination of two or more.

Fiber-shaped nanocarbon is preferably used as the conductive agent. When the electrode active material contains a silicon-based active material, the fibrous nanocarbon can accommodate the expansion of the electrode, preventing damage to the conductive path during repeated charge and discharge.

Fibrous nanocarbon is fibrous carbon with a fiber diameter on the order of nanometers. The average fiber length of the fibrous nanocarbon is not particularly limited, but is preferably 50 nm to 10 mm, and more preferably 0.5 to 100 μm. The average fiber diameter of the fibrous nanocarbon is not particularly limited, but is preferably 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 an atomic force microscope (AFM) image and taking the arithmetic mean. Lengths on the order of millimeters that cannot be measured using an atomic force microscope can be measured using a microscope image.

Specific examples of fibrous nanocarbons include single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), and nanocarbon fibers, and any one of these can be used alone or in combination of two or more. Of these, carbon nanotubes are preferred, and SWCNTs are more preferred.

The binder composition according to one embodiment preferably contains the polyurethane (X) and the fibrous nanocarbon (Y) in a mass ratio of (X):(Y) of 60:40 to 99.6:0.4. This can enhance the effect of improving the charge/discharge cycle characteristics of lithium secondary batteries.

The binder composition according to one embodiment preferably contains carbon black as a conductive agent together with the fibrous nanocarbon. In this case, the carbon black acts as a conductive aid, providing electrical continuity around the electrode active material, allowing the fibrous nanocarbon to more effectively demonstrate its electrical conductivity in response to the expansion of the electrode. The content ratio of the fibrous nanocarbon to the carbon black is not particularly limited; for example, the carbon black may be 100 to 2,000 parts by mass per 100 parts by mass of the fibrous nanocarbon.

The conductive agent is preferably used in a state dispersed in a medium. Water is typically used as the medium, but polar organic solvents such as alcohols and ketone-based solvents, or mixtures of these polar organic solvents with water, may also be used. Examples of dispersing devices used to disperse the conductive agent in the medium include jet mills, high-pressure dispersing devices, and ultrasonic homogenizers.

When preparing an aqueous dispersion in which a conductive agent such as fibrous nanocarbon or carbon black is dispersed in water, a dispersant may be added. Examples of dispersants include celluloses such as hydroxymethyl cellulose, carboxymethyl cellulose, and their alkali metal salts; methyl cellulose, ethyl cellulose, hydroxypropyl methyl cellulose, and hydroxyethyl methyl cellulose; cellulose nanofiber; 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, carboxymethyl cellulose salts are preferred.

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.

[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 producing an electrode for a lithium secondary battery. When an electrode binder is used, the electrode coating liquid composition (1) contains an electrode binder, an electrode active material, and a conductive agent. When an electrode binder composition is used, the electrode coating liquid composition (2) contains an electrode binder composition and an electrode active material.

These electrode coating liquid compositions are preferably used to manufacture negative electrodes for lithium secondary batteries. Therefore, the electrode active material is preferably a negative electrode active material. Materials capable of inserting/extracting metallic lithium or lithium ions can be used as negative electrode active materials. Specific examples of negative electrode active materials include carbon materials such as natural graphite, artificial graphite, non-graphitizable carbon, and graphitizable carbon; metallic lithium, alloys, and metal materials such as tin compounds; lithium transition metal nitrides; crystalline metal oxides; amorphous metal oxides; silicon compounds; and conductive polymers. These materials may be used alone or in combination.

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

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

In the above-mentioned electrode coating liquid composition (1), the conductive agent, including specific examples and preferred compositions, is as described above in the electrode binder composition, and therefore further explanation is omitted.

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

The electrode coating liquid composition may contain a thickener such as a water-soluble polymer as a viscosity adjuster to form a slurry of the composition. Examples of thickeners include celluloses such as carboxymethyl cellulose salts, methyl cellulose, ethyl cellulose, hydroxymethyl cellulose, hydroxypropyl methyl cellulose, and hydroxyethyl methyl cellulose; 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. These may be used alone or in combination of two or more. Of these, carboxymethyl cellulose salts are preferred.

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

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

[Negative electrode of lithium secondary battery]
The negative electrode of the lithium secondary battery according to the embodiment can be produced by applying the electrode coating composition to a current collector and evaporating the dispersion medium. That is, the negative electrode comprises a current collector and a negative electrode mixture layer (also referred to as an active material layer) formed on the current collector, the negative electrode mixture layer being composed of a solid of the electrode coating composition. In one embodiment, the electrode coating composition contains a silicon-based active material such as SiO and/or SiC as a negative electrode active material, in addition to the electrode binder or electrode binder composition. Therefore, the negative electrode of the lithium secondary battery according to the embodiment comprises a solid content 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.

The current collector can be any electronic conductor that does not adversely affect the constructed battery. Examples of current collectors for the negative electrode include copper, stainless steel, nickel, aluminum, titanium, baked carbon, conductive polymers, conductive glass, and Al-Cd alloys, as well as copper or other materials whose surfaces have been 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. Current collector shapes include, for example, foil, film, sheet, net, punched or expanded materials, lath, porous materials, foams, and other molded products. There are no particular limitations on the thickness of the current collector, but collectors of 1 to 100 μm are typically used.

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

[Lithium secondary battery]
The lithium secondary battery according to the embodiment includes the negative electrode. Specifically, the lithium secondary battery includes a negative electrode, a positive electrode, a separator disposed between the negative electrode and the positive electrode, and an electrolyte, and the negative electrode is an electrode prepared using the electrode coating liquid composition. Known configurations can be adopted for the positive electrode, separator, and electrolyte, and there are no particular limitations.

The lithium secondary battery according to the embodiment can be formed into any shape, including cylindrical, coin, and prismatic. The basic configuration of the battery is the same regardless of the shape, and the design can be modified to suit the purpose. For example, in the case of a cylindrical battery, a negative electrode formed by applying a negative electrode active material to a negative electrode current collector and a positive electrode formed by applying a positive electrode active material to a positive electrode current collector are wound together with a separator interposed between them, and the resulting wound body is then housed in a battery can, which is then filled with a non-aqueous electrolyte and sealed with insulating plates placed above and below. In addition, when applied to 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 can, which is then filled with a non-aqueous electrolyte and sealed.

The present invention will be explained in more detail below based on examples and comparative examples, but the present invention is not limited to these.

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

Hydrogenated polybutadiene polyol (B1): "NISSO-PB GI-1000" manufactured by Nippon Soda Co., Ltd. (Mn: 1500, iodine value: 11 g/100 g, hydroxyl value: 69 mg KOH/g, functionality: 1.84)

Hydrogenated polybutadiene polyol (B2): "NISSO-PB GI-3000" manufactured by Nippon Soda Co., Ltd. (Mn: 3100, iodine value: 12 g/100 g, hydroxyl value: 28 mg KOH/g, functionality: 1.55)

Polybutadiene polyol (B3): "NISSO-PB G-1000" manufactured by Nippon Soda Co., Ltd. (Mn: 1400, hydroxyl value: 72 mg KOH/g, functionality: 1.8)

Hydrogenated polybutadiene polyol (B4): Krasol HLBH-P3000 manufactured by Cray Valley (Mn: 3100, hydroxyl value: 34 mg KOH/g, functionality: 1.9)

Hydrogenated polybutadiene polyol (B5): Krasol HLBH-P2000 manufactured by Cray Valley (Mn: 2000, hydroxyl value: 53 mg KOH/g, functionality: 1.9)

Hydrogenated polyisoprene polyol (B6): "EPOL" manufactured by Idemitsu Kosan Co., Ltd. (Mn: 2500, hydroxyl value: 52 mg KOH/g, functionality: 2.3)

Polybutadiene polyol (B7): Evonik "POLYVEST HT" (Mn: 2900, hydroxyl value: 47 mg KOH/g, functionality: 2.45)

Polycarbonate polyol: UBE Corporation's "ETERNACOLL UH-300" (Mn: 3000, hydroxyl value: 37 mg KOH/g, functionality: 1.98)

- Fibrous nanocarbon aqueous dispersion: Using OCSiAl's "TUBALL BATT" single-walled carbon nanotubes (SWCNTs) (CNT purity >93%, average diameter 1.6±0.5 nm), an aqueous dispersion of fibrous nanocarbon 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 nanocarbon in the resulting aqueous dispersion were measured using the following measurement methods. The average fiber diameter was 3 nm, the average fiber length was 3,000 nm, and the aspect ratio was 1,000.

(Procedure for producing fibrous nanocarbon water dispersion)
0.5 g of SWCNT was mixed with 50 g of a 1 mass % aqueous solution of carboxymethyl cellulose salt (Cellogen 7A, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) in a beaker and stirred. After that, the slurry was circulated using the beaker, an ultrasonic homogenizer (US-600T, manufactured by Nippon Seiki Seisakusho Co., Ltd.), a circulation unit, and a tube pump while being dispersed at an output of 100 μA for 90 minutes, thereby obtaining a fibrous nanocarbon aqueous dispersion.

(Method for measuring fibrous nanocarbon)
The average fiber diameter and average fiber length of the fibrous nanocarbon were measured using a scanning probe microscope (SPM) (AFM-5300E, manufactured by JEOL Ltd.). That is, the fibrous nanocarbon aqueous dispersion was diluted with water until the concentration of the fibrous nanocarbon was 0.01% by mass, and then spread on a mica substrate to evaporate the solvent. Then, an AFM image of this sample was observed, and the average fiber diameter and average fiber length were calculated according to the method described above. Furthermore, using these values, the aspect ratio was calculated according to the following (Equation 1).
Aspect ratio = average fiber length (nm) / average fiber diameter (nm) (Equation 1)

- Acetylene black aqueous dispersion: Denka Corporation's "Li400" acetylene black was used. 100 g of acetylene black was added to 300 g of a 1% by mass aqueous solution of carboxymethyl cellulose salt (Dai-ichi Kogyo Seiyaku Co., Ltd.'s "Cellogen 7A") while stirring with a high-speed dispenser, and the mixture was stirred until homogenous. This resulted in an acetylene black aqueous dispersion with an acetylene black concentration of 25% by mass.

Carboxymethylcellulose sodium salt: "Cellogen WS-C" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.

[Synthesis of aqueous polyurethane dispersion]
(Production Example 1: Binder 1)
A four-necked flask equipped with a stirrer, reflux condenser, thermometer, and nitrogen inlet tube was charged with 62.30 parts by mass of hydrogenated polybutadiene polyol (B1), 1.80 parts by mass of trimethylolpropane, 4.20 parts by mass of dimethylolpropionic acid, 31.70 parts by mass of 4,4'-methylenebis(cyclohexyl isocyanate), and 115 parts by mass of methyl ethyl ketone. The reaction was carried out at 75 ° C. for 4 hours to obtain a methyl ethyl ketone solution of a urethane prepolymer having a free isocyanate group content of 3.39% by mass relative to the non-volatile content. This solution was cooled to 45 ° C., and an aqueous sodium hydroxide solution consisting of 1.20 parts by mass of sodium hydroxide and 203 parts by mass of water was gradually added and emulsified and dispersed using a homogenizer. Subsequently, an aqueous solution of 4.94 parts by mass of m-xylylenediamine diluted with 30.3 parts by mass of water was added, and the chain extension reaction was carried out for 1 hour. The solvent was removed from this by heating at 50° C. under reduced pressure, to obtain an aqueous dispersion of sodium salt of polyurethane (binder 1) with a nonvolatile content of about 32% by mass.

(Production Examples 2-3: Binders 2-3)
As shown in Table 1, the type and amount of the neutralizing agent were changed from sodium hydroxide to lithium hydroxide or triethylamine, and the rest was the same as in Production Example 1, to obtain aqueous dispersions of lithium salt or triethylamine salt of polyurethane (binders 2 and 3).

(Production Examples 4 to 23: Binders 4 to 23)
The polyisocyanate, polyol, and/or chain extender were changed as shown in Tables 1 to 3, and the other procedures were the same as in Production Example 1 to obtain aqueous dispersions of sodium salts of polyurethane (binders 4 to 23).

(Comparative Production Examples 1 to 4: Binders C1 to C4)
Aqueous dispersions of sodium salts of polyurethane (binders C1 to C4) were obtained in the same manner as in Production Example 1, except that the polyisocyanate, polyol, and/or chain extender were changed as shown in Table 4.

The polyurethane components for the resulting binders 1-23 and binders C1-4 are shown in Tables 1-4. In the tables, "hydrogenated MDI" represents 4,4'-methylenebis(cyclohexyl isocyanate), "HDI" represents hexamethylene diisocyanate, "IPDI" represents isophorone diisocyanate, and "1,3-H6XDI" represents 1,3-bis(isocyanatomethyl)cyclohexane.

Tables 1 to 4 also show the free isocyanate group content (free F-NCO) of the urethane prepolymer obtained by reacting components (A) to (C). Free F-NCO was measured in accordance with Method B of JIS K1603-1:2007 (back titration was performed using indicator titration).

[Fabrication of Lithium Secondary Battery]
Example 1
(Preparation of Negative Electrode) 88.85 parts by mass of SiO (average particle size 4.5 μm, specific surface area 5.5 m ² /g) as the negative electrode active material, 40 parts by mass of a fibrous nanocarbon aqueous dispersion and 4.0 parts by mass of an acetylene black aqueous dispersion as a conductive agent, 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 a polyurethane aqueous dispersion as binder 1, and 14 parts by mass of ion-exchanged water were used, and these were mixed in a planetary mixer to prepare a negative electrode slurry to a solids content of 49% by mass. A 10 μm thick electrolytic copper foil was used as a current collector, and a negative electrode mixture layer composed of the negative electrode slurry was formed on the electrolytic copper foil. In detail, the negative electrode slurry was coated on electrolytic copper foil using a coating machine, roll-pressed, and then dried under reduced pressure at 130 ° C. to obtain a negative electrode with a negative electrode active material of 2.7 mg / cm ² .

(Preparation of Positive Electrode) 92 parts by mass of LiNiCoAlO ₂ (NCA) as a positive electrode active material, 4 parts by mass of acetylene black ("Li-400" manufactured by Denka Co., Ltd.) 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 so as to have a solid content of 67% by mass. This positive electrode slurry was coated on aluminum foil with a thickness of 15 μm using a coating machine, dried at 130°C, and then subjected to a roll press treatment to obtain a positive electrode with a positive electrode active material of 16.6 mg/cm ² .

(Battery Fabrication) The negative and positive electrodes obtained above were combined and stacked with a polyolefin (PE/PP/PE) separator sandwiched between the electrodes. A positive electrode terminal and a negative electrode terminal were ultrasonically welded to each positive and negative electrode. This stack was placed in an aluminum laminate packaging material and heat-sealed, leaving an opening for electrolyte injection. A pre-filled battery with a positive electrode area of 18 cm ² and a negative electrode area of 19.8 cm ² was fabricated. Next, an electrolyte solution prepared by dissolving LiPF ₆ (1.0 mol/L) in a solvent mixture of ethylene carbonate and diethyl carbonate (30/70 vol ratio) was injected, and the opening was heat-sealed to obtain a battery for evaluation.

Example 2
When preparing the negative electrode slurry, SiC (average particle size 11.4 μm, specific surface area 2.5 m ² /g) was used instead of SiO as the negative electrode active material, and other procedures were the same as in Example 1 to prepare a negative electrode and further prepare a battery for evaluation.

(Examples 3 to 5, 7 to 25 and Comparative Examples 1 to 4)
When preparing the negative electrode slurry, binders 2 to 23 and C1 to 4 as shown in Table 5 below were used as binders, and the rest was the same as in Example 1 to prepare a negative electrode and further prepare an evaluation battery.

Example 6
When preparing the negative electrode slurry, SiC (average particle size 11.4 μm, specific surface area 2.5 m ² /g) was used instead of SiO as the negative electrode active material, and other procedures were the same as in Example 5 to prepare a negative electrode and further prepare a battery for evaluation.

[evaluation]
The binding property of the prepared negative electrode was evaluated. Furthermore, the electrode expansion rate of the evaluation battery was measured after repeated charge and discharge to confirm the electrode expansion reduction effect. Furthermore, the charge and discharge cycle characteristics of the evaluation battery were evaluated. The test method was as follows.

(Adhesion)
The adhesiveness was evaluated by a peel test. In the peel test, the negative electrode was cut into a size of 2 × 8 cm, 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. A 180° peel test was performed by raising the upper jig at a rate of 50 mm/min, and the average value of the peel strength was calculated as the peel strength (N/cm).

(electrode expansion rate)
The evaluation battery was repeatedly charged and discharged using a charge/discharge device under the following conditions: CC (constant current) charging at a current density equivalent to 1 C to 4.2 V in an environment of 25°C, followed by switching to CV (constant voltage) charging at 4.2 V, charging for 1.5 hours or until the current density reached 0.1 C, and then CC discharging to 2.7 V at a current density equivalent to 1 C. This cycle was repeated 500 times at 25°C. A 10-minute break was allowed between charges and discharges. The cell thickness at the first fully charged state (cell thickness before the start of cycling) and the cell thickness at the 500th fully charged state (cell thickness after the end of cycling) were measured with a micrometer and calculated using the following formula:
Electrode expansion rate = {(cell thickness after cycling - cell thickness before cycling) / negative electrode composite layer thickness} × 100%
Here, negative electrode composite layer thickness = negative electrode thickness - copper foil thickness

(Charge/discharge cycle characteristics)
The evaluation battery was subjected to a charge-discharge cycle test using a charge-discharge device under the following conditions: CC (constant current) charging at a current density equivalent to 1 C to 4.2 V in an environment of 25°C, followed by switching to CV (constant voltage) charging at 4.2 V, charging for 1.5 hours or until the current density reached 0.1 C, and then CC discharging at a current density equivalent to 1 C to 2.7 V. This cycle was repeated 500 times at 25°C. The ratio of the 1 C discharge capacity after 500 cycles to the initial 1 C discharge capacity was calculated as the charge-discharge cycle retention rate (%). A 10-minute break was allowed between charge and discharge.

The results are shown in Table 5. In Comparative Example 1, the binder C1 used to prepare the negative electrode was synthesized using an aliphatic polyamine (1,2-propanediamine) as a chain extender, and although it had excellent binding properties, it was poor in its ability to suppress electrode expansion and in its charge-discharge cycle characteristics. In Comparative Examples 2 and 3, binders C2 and C3 were synthesized using the aliphatic polyamines triethylenetetramine or tetraethylenepentamine, and were poor in its binding properties, as well as in its ability to suppress electrode expansion and in its charge-discharge cycle characteristics.

In Comparative Example 4, polycarbonate polyol was used instead of polyolefin polyol (b1) as the polyol constituting binder C4, and the binding properties, electrode expansion suppression effect, and charge/discharge cycle characteristics were inferior.

In contrast, in Examples 1 to 25, a combination of aliphatic/alicyclic polyisocyanate and polyolefin polyol (b1) was used as the binder, and an aromatic polyamine was used as the chain extender. Compared to Comparative Examples 1 to 4, this combination maintained or improved the binding properties while reducing electrode expansion and significantly improving charge/discharge cycle characteristics.

The various numerical ranges described in this specification can each be combined with any upper and lower limit, and all such combinations are considered to be preferred numerical ranges and are described in this specification. Furthermore, a numerical range described as "X to Y" means greater than or equal to X and less than or equal to Y.

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 embodied in a variety of 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, modifications, etc. are included within the scope and spirit of the invention, as well as within the scope of the inventions and their equivalents as set forth in the claims.

Claims (5)

(A) a polyisocyanate, (B) a polyol (excluding component (C)), (C) a compound having two or more hydroxyl groups and a carboxy group, and (D) a chain extender;
the component (A) contains at least one selected from the group consisting of aliphatic polyisocyanates and alicyclic polyisocyanates,
the component (B) comprises at least one selected from the group consisting of polybutadiene polyol, hydrogenated polybutadiene polyol, polyisoprene polyol, and hydrogenated polyisoprene polyol,
The component (D) contains an aromatic polyamine.
A binder for electrodes of lithium secondary batteries, comprising an aqueous dispersion of polyurethane. The electrode binder according to claim 1, which is for use in the negative electrode of a lithium secondary battery containing a silicon-based active material as the negative electrode active material. A polyurethane obtained by reacting (A) a polyisocyanate, (B) a polyol (excluding component (C)), (C) a compound having two or more hydroxyl groups and a carboxy group, and (D) a chain extender, wherein the component (A) contains at least one selected from the group consisting of an aliphatic polyisocyanate and an alicyclic polyisocyanate, the component (B) contains at least one selected from the group consisting of a polybutadiene polyol, a hydrogenated polybutadiene polyol, a polyisoprene polyol, and a hydrogenated polyisoprene polyol, and the component (D) contains an aromatic polyamine;
a conductive agent, and
A binder composition for an electrode of a lithium secondary battery, which contains water. A negative electrode for a lithium secondary battery, comprising the solid content 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. A lithium secondary battery comprising the negative electrode according to claim 4.