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

Abstract

Provided is a binder for electrodes of lithium secondary batteries that has excellent binding properties, can reduce electrode expansion due to charge and discharge, and has excellent charge and discharge cycle characteristics. [Solution] The electrode binder of the lithium secondary battery according to the embodiment includes an aqueous dispersion of polyurethane obtained by reacting (A) at least one of an aliphatic polyisocyanate and an alicyclic polyisocyanate, (B) a polyol, (C) a compound having two or more hydroxyl groups and a carboxyl group, and (D) a chain extender. The (B) component includes a hydrogenated polybutadiene polyol. The molar ratio (NCO / OH) of the isocyanate groups of the (A) component to the hydroxyl groups of the (B) component and the (C) component is 1.15 or more and less than 1.35.

Description

[Technical field]

[0001] An embodiment of the present invention relates to an electrode binder for a lithium secondary battery, an electrode binder composition, a negative electrode, and a lithium secondary battery. [Background technology]

[0002] Lithium secondary batteries, also called lithium ion secondary batteries, are used as high-voltage, high-energy density power storage devices, for example, as power sources for driving electronic devices. Electrodes for lithium secondary batteries are usually 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 impart adhesive strength between the electrode active material and the current collector. Therefore, the binder has a large effect on the characteristics of lithium secondary batteries.

[0003] For example, Patent Document 1 discloses an electrode binder capable of reducing electrode expansion due to charging and discharging of a lithium secondary battery, which 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. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 7161078 Summary of the Invention [Problem to be solved by the invention]

[0005] Binders for electrodes of lithium secondary batteries are required to have not only binding properties, but also to reduce the expansion of the electrode due to charging and discharging, for example, when the negative electrode active material contains a silicon-based active material such as SiO, and further to improve the charge-discharge cycle characteristics.

[0006] An object of an embodiment of the present invention is to provide an electrode binder for a lithium secondary battery that has excellent binding properties, can reduce electrode expansion due to charge and discharge, and has excellent charge and discharge cycle characteristics. [Means for solving the problem]

[0007] The present inventors have discovered that in an aqueous dispersion of polyurethane obtained by reacting an aliphatic polyisocyanate and / or an alicyclic polyisocyanate, a polyol including a hydrogenated polybutadiene polyol, a compound having two or more hydroxyl groups and a carboxy group, and a chain extender, by setting the molar ratio of isocyanate groups to hydroxyl groups (NCO / OH) within a predetermined range, it is possible to improve the charge / discharge cycle characteristics and solve the above-mentioned problems.

[0008] The present invention includes the embodiments set forth below. [1] An electrode binder for a lithium secondary battery, comprising an aqueous dispersion of polyurethane obtained by reacting (A) at least one selected from the group consisting of an aliphatic polyisocyanate and an alicyclic 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 (B) comprises a hydrogenated polybutadiene polyol, and the molar ratio (NCO / OH) of the isocyanate groups in the component (A) to the hydroxyl groups in the component (B) and the component (C) is 1.15 or more and less than 1.35. [2] The electrode binder according to [1], which is for use in a 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, comprising: a polyurethane, a conductive agent, and water, the polyurethane being obtained by reacting (A) at least one selected from the group consisting of an aliphatic polyisocyanate and an alicyclic 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 (B) comprises a hydrogenated polybutadiene polyol, and the molar ratio (NCO / OH) of the isocyanate groups contained in the component (A) to the hydroxyl groups contained in the components (B) and (C) is 1.15 or more and less than 1.35. [4] A negative electrode of a lithium secondary battery, comprising: a solid content of the electrode binder according to [1] 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]. Effect of the Invention

[0009] According to an embodiment of the present invention, it is possible to provide a binder for electrodes of lithium secondary batteries that has excellent binding properties, can reduce electrode expansion due to charge and discharge, and has excellent charge and discharge cycle characteristics. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The electrode binder of the lithium secondary battery according to this embodiment includes an aqueous dispersion of polyurethane obtained by reacting at least one selected from the group consisting of aliphatic polyisocyanates and alicyclic polyisocyanates 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). The electrode binder according to this embodiment has excellent binding properties, and can reduce the expansion of the electrode even when the electrode active material contains a silicon-based active material such as SiO, and has excellent charge-discharge cycle characteristics.

[0011] [Component (A)] In this embodiment, at least one selected from the group consisting of aliphatic polyisocyanates and alicyclic polyisocyanates is used as the polyisocyanate. That is, the component (A) may be an aliphatic polyisocyanate alone, an alicyclic polyisocyanate alone, or a combination of an aliphatic polyisocyanate and an alicyclic polyisocyanate.

[0012] Examples of the aliphatic polyisocyanate include aliphatic diisocyanates such as tetramethylene diisocyanate, dodecamethylene diisocyanate, hexamethylene diisocyanate (HDI), 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, lysine diisocyanate, 2-methylpentane-1,5-diisocyanate, 3-methylpentane-1,5-diisocyanate, etc. These can be used alone or in combination of two or more.

[0013] Examples of alicyclic polyisocyanates include alicyclic diisocyanates such as isophorone diisocyanate (IPDI), dicyclohexylmethane 4,4'-diisocyanate (hydrogenated MDI), hydrogenated xylylene diisocyanate, 1,4-cyclohexane diisocyanate, methylcyclohexylene diisocyanate, 1,3-bis(isocyanatemethyl)cyclohexane, etc. These can be used alone or in combination of two or more.

[0014] The amount of the component (A) may be, for example, 5 to 40% by mass, 10 to 35% by mass, or 15 to 30% by mass relative to 100% by mass of the polyurethane.

[0015] [(B) Component] Component (B) is a polyol, provided, however, that component (C), a compound having two or more hydroxyl groups and a carboxy group, is not included in component (B).

[0016] In this embodiment, the component (B) contains a hydrogenated polybutadiene polyol. As the hydrogenated polybutadiene polyol, one having an average of 1.3 or more hydroxyl groups (-OH) per molecule is preferably used, more preferably one having an average of 1.5 to 2.5 hydroxyl groups, and even more preferably one having an average of 1.7 to 2.2 hydroxyl groups. In this specification, the number of hydroxyl groups (number of functional groups) per molecule is a value calculated by the following formula. Functional group number = {(hydroxyl value) x (Mn)} / (56.1 x 1000)

[0017] The hydrogenated polybutadiene polyol preferably has a structure obtained by hydrogenating (hydrogenating) a polybutadiene polyol having a 1,4-bond type, a 1,2-bond type, or a mixture of them in the molecule and having a hydroxyl group at the end.More preferably, the hydrogenated polybutadiene polyol has a structure obtained by hydrogenating a polybutadiene polyol having a hydroxyl group at each end of the polybutadiene structure.

[0018] The hydrogenated polybutadiene polyol is a polybutadiene polyol in which a part or all of the unsaturated double bonds contained therein are hydrogenated. The degree of hydrogenation of the hydrogenated polybutadiene polyol is not particularly limited, and for example, the iodine value may be 40g / 100g or less, 30g / 100g or less, 25g / 100g or less, or 15g / 100g or less. In this specification, the iodine value is measured in accordance with JIS K0070:1992.

[0019] The molecular weight of the hydrogenated polybutadiene polyol is not particularly limited, and may be, for example, 600-10,000, 800-6,000, 1,000-5,000, or 1,200-4,000 in number average molecular weight (Mn).

[0020] In this specification, the number average molecular weight (Mn) is a value measured by GPC (gel permeation chromatography) and calculated using a calibration curve of standard polystyrene. In detail, the GPC conditions are as follows: column: "TSKgel Hxl" manufactured by Tosoh Corporation, mobile phase: THF (tetrahydrofuran), mobile phase flow rate: 1.0 mL / min, column temperature: 40°C, sample injection amount: 50 μL, and sample concentration: 0.2 mass%.

[0021] The hydroxyl value of the hydrogenated polybutadiene polyol 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 JIS K1557-1:2007, Method A.

[0022] The (B) component may be composed of only hydrogenated polybutadiene polyol, but may also contain other polyols, such as 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, the (B) component preferably contains a hydrogenated polybutadiene polyol and a trihydric alcohol. The trihydric alcohol acts as a crosslinking agent that introduces a crosslinked structure into the polyurethane. The trihydric alcohol is preferably a polyhydric alcohol having 3 to 8 carbon atoms, and examples of the trihydric alcohol include trimethylolpropane, glycerin, 1,2,4-butanetriol, and 1,2,3-butanetriol. These can be used alone or in combination of two or more.

[0024] The amount of the polymer (B) component may be, for example, 40 to 90% by mass, 50 to 80% by mass, or 60 to 75% by mass, relative to 100% by mass of the polyurethane. The amount of the hydrogenated polybutadiene polyol may be, for example, 40 to 90% by mass, 50 to 80% by mass, or 60 to 75% by mass, relative to 100% by mass of the polyurethane. In one embodiment, when a trihydric alcohol is contained, the amount of the trihydric alcohol may be 0.5 to 5% by mass, or 1 to 3% by mass, relative to 100% by mass of the polyurethane.

[0025] [(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 2 or 3, and more preferably 2. The number of carboxy groups in one molecule of compound (C) is preferably 1 or 2, and more preferably 1.

[0026] Here, the carboxyl group is a concept that includes not only the acid type (-COOH) but also the salt type, i.e., carboxylate group (-COOX, where X is a cation that forms a salt with a carboxylic acid), and the acid type and the salt type may be mixed. Examples of salts of carboxylate groups 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.

[0027] Examples of the compound (C) include hydroxy acids such as dimethylolpropionic acid, dimethylolbutanoic acid, dimethylolvaleric acid, dihydroxymaleic acid, dihydroxybenzoic acid, and tartaric acid, and their derivatives and salts thereof. These may be used alone or in combination of two or more.

[0028] The above-mentioned carboxyl group can be neutralized to form a salt to make the polyurethane finally obtained water dispersible. Examples of bases for neutralizing the carboxyl group 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 reaction.

[0029] The amount of the component (C) may be, for example, 1 to 15% by mass, 2 to 10% by mass, or 3 to 7% by mass relative to 100% by mass of the polyurethane.

[0030] [(D) component] The chain extender of component (D) may be a chain extender generally used in this technical field. Specific examples of the chain extender include amines, i.e., diamines and polyamines. Examples of diamines include ethylenediamine, trimethylenediamine, piperazine, and isophoronediamine. Examples of polyamines, i.e., hydrocarbon compounds having three or more amino groups in one molecule, include diethylenetriamine, dipropylenetriamine, and triethylenetetramine. Any one of these may be used, or two or more may be used in combination. It is preferable to use a diamine and / or a triamine as component (D).

[0031] The amount of the component (D) may be, for example, 0.3 to 10 mass %, 0.5 to 5 mass %, or 0.7 to 3 mass % relative to 100 mass % of the polyurethane.

[0032] [Electrode binder] The electrode binder according to this embodiment contains an aqueous dispersion of polyurethane obtained by reacting the above-mentioned 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.

[0033] When synthesizing the polyurethane, the molar ratio (NCO / OH) of the isocyanate groups in the (A) component to the hydroxyl groups in the (B) and (C) components (i.e., the total amount of hydroxyl groups in the (B) component and the (C) component) is set to 1.15 or more and less than 1.35. By setting the NCO / OH ratio within the above range, the charge / discharge cycle characteristics can be improved. The NCO / OH ratio is more preferably 1.16 to 1.30, more preferably 1.17 to 1.29, more preferably 1.18 to 1.25, and even more preferably 1.19 to 1.24.

[0034] 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. The acid value of the polyurethane can be adjusted by the amount of compound (C). Here, the acid value can be calculated from the amount (mg) of KOH required to neutralize the free carboxyl groups contained in 1 g of the solid content of the polyurethane water dispersion in accordance with JIS K0070-1992.

[0035] The polyurethane may be composed of only the components (A) to (D), but may also contain other components as long as the effects of the present embodiment are not impaired. For example, the polyurethane may contain a polyisocyanate other than the component (A), such as an aromatic polyisocyanate or an araliphatic polyisocyanate.

[0036] In the electrode binder, the concentration of polyurethane is not particularly limited and may be, for example, 5 to 50% by mass, or 10 to 40% by mass.

[0037] The method for producing the aqueous dispersion of polyurethane is not particularly limited, and examples thereof include the following methods. The (A) component is reacted with the (B) component and the (C) component without a solvent or in an organic solvent having no active hydrogen group to synthesize an isocyanate-terminated urethane prepolymer. After the synthesis of the urethane prepolymer, the carboxy group of the (C) component is neutralized with a neutralizing agent, and then dispersed and emulsified in water. Then, the (D) component is added in an equivalent amount less than the remaining isocyanate group (for example, an equivalent ratio of the isocyanate group to the active hydrogen group of the chain extender is 1:0.50 to 0.95), and the isocyanate group in the emulsion micelle and the chain extender are subjected to an interfacial polymerization reaction to generate urea bonds. This improves the crosslink density in the emulsion micelle, and a three-dimensional crosslinked structure is formed. Then, the solvent used is removed as necessary to obtain an aqueous dispersion of polyurethane.

[0038] In the synthesis of the urethane prepolymer, an organic solvent that is inactive to the isocyanate group and can dissolve the urethane prepolymer to be produced may be used. 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 the present embodiment, the medium for dispersing the polyurethane may or may not contain these organic solvents together with water. It is preferable that these organic solvents are finally removed.

[0039] 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. 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.

[0040] [Electrode binder composition] The electrode binder composition of the lithium secondary battery according to this embodiment contains polyurethane obtained by reacting the above (A) to (D) components, a conductive agent, and water. In detail, the electrode binder composition contains the above polyurethane and the conductive agent in a state of being dispersed in water. The electrode binder composition may be prepared, for example, by mixing the above aqueous dispersion of polyurethane with an aqueous dispersion of the conductive agent.

[0041] As the conductive agent, an electronic conductive material that does not adversely affect the battery performance can be used. Specific examples of the conductive agent include fibrous nanocarbon, carbon black such as acetylene black and kettin black, natural graphite (scale graphite, flake graphite, clay 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.

[0042] As the conductive agent, it is preferable to use fibrous nanocarbon, which 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.

[0043] The fibrous nanocarbon is a fibrous carbon having a fiber diameter on the order of nanometers. The average fiber length of the fibrous nanocarbon is not particularly limited, and is preferably, for example, 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, and 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 obtained 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 mm that cannot be measured by an atomic force microscope may be measured using an image taken by a microscope.

[0044] Specific examples of fibrous nanocarbon include single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), nanocarbon fibers, etc., and any one or more of these can be used in combination. Among these, carbon nanotubes are preferred, and SWCNTs are more preferred.

[0045] The binder composition according to one embodiment preferably contains the polyurethane (X) and the fibrous nanocarbon (Y) in a mass ratio of (X):(Y)=60:40 to 99.6:0.4, thereby enhancing the effect of improving the charge-discharge cycle characteristics of a lithium secondary battery.

[0046] The binder composition according to one embodiment preferably contains carbon black together with the fibrous nanocarbon as a conductive agent. In this case, the carbon black serves as a conductive assistant to provide electrical continuity around the electrode active material, and can more effectively exert the electrical conductivity of the fibrous nanocarbon that follows the expansion of the electrode. The content ratio of the fibrous nanocarbon to the carbon black is not particularly limited, and may be, for example, 100 to 2000 parts by mass of carbon black per 100 parts by mass of the fibrous nanocarbon.

[0047] The conductive agent is preferably used in a state of being dispersed in a medium. As the medium, water is usually used, but a polar organic solvent such as an alcohol or a ketone solvent, or a mixed solvent of such a polar organic solvent and water may also be used. Examples of a dispersing device for dispersing the conductive agent in the medium include a jet mill, a high-pressure dispersing device, and an ultrasonic homogenizer.

[0048] 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 hydroxymethylcellulose, carboxymethylcellulose, and alkali metal salts thereof, methylcellulose, ethylcellulose, hydroxypropylmethylcellulose, and hydroxyethylmethylcellulose, cellulose nanofibers, polyacrylic acid, polyacrylic acid sodium salt, and other polycarboxylic acid compounds, polyvinylpyrrolidone structure compounds such as polyvinylpyrrolidone, polyvinyl alcohol, alginate sodium, xanthan gum, carrageenan, guar gum, agar, and starch. Among these, carboxymethylcellulose salts can be preferably used.

[0049] In the electrode binder composition, the concentration of the polyurethane is not particularly limited and may be, for example, 5 to 50 mass % or 10 to 40 mass %. The concentration of the conductive agent is also not particularly limited and may be, for example, 5 to 35 mass % or 15 to 30 mass %.

[0050] [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.

[0051] These electrode coating liquid 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, a material capable of inserting / desorbing metallic lithium or lithium ions can be used. Specific examples of the negative electrode active material include carbon materials such as natural graphite, artificial graphite, non-graphitizable carbon, and easily graphitizable carbon, metallic lithium and alloys, metallic materials such as tin compounds, lithium transition metal nitrides, crystalline metal oxides, amorphous metal oxides, silicon compounds, conductive polymers, and the like. Any one of these may be used, or two or more may be used in combination.

[0052] Among these, the negative electrode active material preferably contains a silicon-based active material such as SiO (silicon monoxide) or SiC (silicon carbide) because it can increase the 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.

[0053] 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 %.

[0054] 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 description thereof will be omitted.

[0055] 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 %.

[0056] The electrode coating liquid composition may contain a thickener such as a water-soluble polymer as a viscosity adjusting agent for making the composition into a slurry. Examples of the thickener 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. These can be used alone or in combination of two or more. Among these, carboxymethylcellulose salt is preferred.

[0057] The content of the 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 mass % or 75 to 97 mass %. The content of the dispersion medium such as water in the electrode coating liquid composition is not particularly limited and may be, for example, 20 to 80 mass % or 40 to 70 mass %.

[0058] The method for preparing the electrode coating liquid composition is not particularly limited. When mixing the above-mentioned components, for example, a mortar, a mill mixer, a ball mill such as a planetary ball mill or a shaker ball mill, mechanofusion, etc. can be used.

[0059] [Anode 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 includes a current collector and a negative electrode mixture layer (also referred to as an active material layer) formed on the current collector, and the negative electrode mixture layer is made of a solid of the electrode coating liquid composition. In one embodiment, the electrode coating liquid composition contains 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 the embodiment includes 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.

[0060] As the current collector, an electronic conductor that does not adversely affect the constructed battery can be used. Examples of the current collector for the negative electrode include copper, stainless steel, nickel, aluminum, titanium, baked carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., as well as copper or the like whose surface is treated with carbon, nickel, titanium, silver, etc. for the purpose of improving adhesion, conductivity, and oxidation resistance. The surface of these current collector materials may be oxidized. Examples of the shape of the current collector include a foil, film, sheet, net, punched or expanded object, lath, porous body, foam, and other molded body. The thickness of the current collector is not particularly limited, but a current collector having a thickness of 1 to 100 μm is usually used.

[0061] The thickness of the negative electrode mixture layer is not particularly limited and may be, for example, 15 to 150 μm.

[0062] [Lithium secondary battery] The lithium secondary battery according to the embodiment includes the negative electrode. In detail, 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 an electrode prepared using the electrode coating liquid composition is used for the negative electrode. The positive electrode, the separator, and the electrolyte may be of any known configuration, for example, as described in Patent Document 1, and are not particularly limited.

[0063] The lithium secondary battery according to the embodiment can be formed into a cylindrical type, a coin type, a square type, or any other shape, and the basic configuration of the battery is the same regardless of the shape, and can be modified in design according to the purpose. For example, in the case of a cylindrical type, a negative electrode formed by applying a negative electrode active material to a negative electrode collector and a positive electrode formed by applying a positive electrode active material to a positive electrode collector are wound with a separator interposed between them, and the wound body is stored in a battery can, and a nonaqueous electrolyte is injected 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 stored in a coin-type battery can, and a nonaqueous electrolyte is injected and sealed. EXAMPLES

[0064] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited thereto.

[0065] Details of the polyol, fibrous nanocarbon water dispersion, acetylene black water dispersion, and carboxymethylcellulose sodium salt used in the examples are as follows.

[0066] Hydrogenated polybutadiene polyol (B1): "NISSO-PB GI-1000" manufactured by Nippon Soda Co., Ltd. (Mn: 1500, iodine value 11g / 100g, hydroxyl value 69KOHmg / g, functionality: 1.84)

[0067] Hydrogenated polybutadiene polyol (B2): "NISSO-PB GI-3000" manufactured by Nippon Soda Co., Ltd. (Mn: 3100, iodine value 12g / 100g, hydroxyl value 28KOHmg / g, functionality: 1.55)

[0068] Polybutadiene polyol (B3): "NISSO-PB G-1000" manufactured by Nippon Soda Co., Ltd. (Mn: 1400, hydroxyl value 70 KOHmg / g, functionality: 1.8)

[0069] Polybutadiene polyol (B4): Evonic's "POLYVEST HT" (Mn: 2900, functionality: 2.45)

[0070] Fibrous nanocarbon aqueous dispersion: Using OCSiAl's "TUBALL BATT" (CNT purity >93%, average diameter 1.6±0.5nm) as single-walled carbon nanotubes (SWCNT), an aqueous dispersion of fibrous nanocarbon with a 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 obtained aqueous dispersion were measured according to the following measurement method, and the average fiber diameter was 3nm, the average fiber length was 3000nm and the aspect ratio was 1000.

[0071] (Procedure for producing fibrous nanocarbon water dispersion) 0.5 g of SWCNT was mixed with 50 g of a 1 mass% aqueous solution of carboxymethylcellulose salt ("Cellogen 7A" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) in a beaker and stirred. After that, the slurry was circulated using a 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 to obtain a fibrous nanocarbon aqueous dispersion.

[0072] (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 water dispersion was diluted with water until the concentration of the fibrous nanocarbon became 0.01% by mass, and then spread on a mica substrate to evaporate the solvent. Then, the AFM image of this sample was observed, and the average fiber diameter and average fiber length were calculated according to the method described above. In addition, the aspect ratio was calculated using these values ​​according to the following (Equation 1). Aspect ratio = average fiber length (nm) / average fiber diameter (nm) ... (Equation 1)

[0073] Acetylene black aqueous dispersion: Denka Li400 was used as acetylene black. 100g of acetylene black was added to 300g of 1% by mass aqueous solution of carboxymethylcellulose salt (Dai-ichi Kogyo Seiyaku Cellogen 7A) with stirring using a high-speed dispenser, and stirred until homogenous. This produced an acetylene black aqueous dispersion with an acetylene black concentration of 25% by mass.

[0074] Carboxymethylcellulose sodium salt: "Cellogen WS-C" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.

[0075] [Synthesis of aqueous polyurethane dispersion] (Production Example 1: Binder 1) A four-neck flask equipped with a stirrer, reflux condenser, thermometer and nitrogen blowing tube was charged with 65.40 parts by mass of hydrogenated polybutadiene polyol (B1), 1.80 parts by mass of trimethylolpropane, 4.20 parts by mass of dimethylolpropionic acid, 28.60 parts by mass of dicyclohexylmethane 4,4'-diisocyanate (hydrogenated MDI), and 115 parts by mass of methyl ethyl ketone, and reacted at 75°C for 4 hours to obtain a methyl ethyl ketone solution of a urethane prepolymer having a free isocyanate group content of 2.3% by mass relative to the non-volatile content. The solution was cooled to 45°C, and an aqueous sodium hydroxide solution consisting of 1.20 parts by mass of sodium hydroxide and 80.80 parts by mass of water was gradually added and emulsified and dispersed using a homogenizer. Subsequently, an aqueous solution of 1.73 parts by mass of diethylenetriamine diluted with 30.30 parts by mass of water was added, and a 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) having a non-volatile content of about 32% by mass.

[0076] (Production Example 2: Binder 2) The amount of hydrogenated polybutadiene polyol (B1) was 65.98 parts by mass, the amount of hydrogenated MDI was 28.02 parts by mass, and the amount of diethylenetriamine was 1.57 parts by mass. The rest of the procedure was the same as in Production Example 1, and a water dispersion of sodium salt of polyurethane (binder 2) with a non-volatile content of approximately 32% by mass was obtained.

[0077] (Production Example 3: Binder 3) A water dispersion of sodium salt of polyurethane (binder 3) having a nonvolatile content of about 32% by mass was obtained in the same manner as in Production Example 1, except that 1.74 parts by mass of ethylenediamine was used instead of 1.73 parts by mass of diethylenetriamine.

[0078] (Production Example 4: Binder 4) The amount of hydrogenated polybutadiene polyol (B1) was 64.45 parts by mass, the amount of hydrogenated MDI was 29.55 parts by mass, and the amount of diethylenetriamine was 1.99 parts by mass. The rest of the procedure was the same as in Production Example 1, and a water dispersion of sodium salt of polyurethane (binder 4) with a non-volatile content of approximately 32% by mass was obtained.

[0079] (Production Example 5: Binder 5) The amount of hydrogenated polybutadiene polyol (B1) was 66.14 parts by mass, the amount of hydrogenated MDI was 28.03 parts by mass, the amount of trimethylolpropane was 1.63 parts by mass, and 1.46 parts by mass of ethylenediamine was used instead of 1.73 parts by mass of diethylenetriamine. The rest of the procedure was the same as in Production Example 1, and an aqueous dispersion of sodium salt of polyurethane (binder 5) with a non-volatile content of approximately 32% by mass was obtained.

[0080] (Production Example 6: Binder 6) The amount of hydrogenated polybutadiene polyol (B1) was 62.60 parts by mass, the amount of hydrogenated MDI was 31.20 parts by mass, the amount of trimethylolpropane was 2.00 parts by mass, and the amount of diethylenetriamine was 2.32 parts by mass. The rest of the procedure was the same as in Production Example 1, and an aqueous dispersion of sodium salt of polyurethane (binder 6) with a non-volatile content of approximately 32% by mass was obtained.

[0081] (Production Example 7: Binder 7) A water dispersion of sodium salt of polyurethane (binder 7) having a non-volatile content of about 32% by mass was obtained in the same manner as in Production Example 1, except that 71.60 parts by mass of hydrogenated polybutadiene polyol (B2) was used instead of 65.40 parts by mass of hydrogenated polybutadiene polyol (B1), the amount of hydrogenated MDI was 22.40 parts by mass, and the amount of diethylenetriamine was 1.45 parts by mass.

[0082] (Production Example 8: Binder 8) A water dispersion of a sodium salt of polyurethane (binder 8) having a non-volatile content of about 32% by mass was obtained in the same manner as in Production Example 1, except that 19.30 parts by mass of hexamethylene diisocyanate (HDI) was used instead of 28.60 parts by mass of hydrogenated MDI and the amount of hydrogenated polybutadiene polyol (B1) was 74.70 parts by mass.

[0083] (Production Example 9: Binder 9) A water dispersion of lithium salt of polyurethane (binder 9) having a nonvolatile content of about 32% by mass was obtained in the same manner as in Production Example 1, except that 0.72 parts by mass of lithium hydroxide was used instead of 1.20 parts by mass of sodium hydroxide.

[0084] (Production Example 10: Binder 10) A water dispersion of polyurethane triethylamine salt (binder 10) having a nonvolatile content of about 32% by mass was obtained in the same manner as in Production Example 1, except that 3.04 parts by mass of triethylamine was used instead of 1.20 parts by mass of sodium hydroxide.

[0085] (Comparative Manufacturing Example 1: Binder C1) A four-neck flask equipped with a stirrer, reflux condenser, thermometer and nitrogen blowing tube was charged with 69.17 parts by mass of hydrogenated polybutadiene polyol (B1), 4.18 parts by mass of dimethylolpropionic acid, 25.97 parts by mass of hydrogenated MDI and 115 parts by mass of methyl ethyl ketone, 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 3.5% by mass relative to the non-volatile content. The solution was cooled to 45°C, and an aqueous sodium hydroxide solution consisting of 1.25 parts by mass of sodium hydroxide and 80.80 parts by mass of water was gradually added and emulsified and dispersed using a homogenizer. Subsequently, an aqueous solution of 1.48 parts by mass of diethylenetriamine diluted with 30.30 parts by mass of water was added, and a chain extension reaction was carried out for 1 hour. The solvent was removed under reduced pressure and heating at 50°C to obtain an aqueous dispersion of sodium salt of polyurethane with a non-volatile content of about 32% by mass (binder C1).

[0086] (Comparative Manufacturing Example 2: Binder C2) The amount of hydrogenated polybutadiene polyol (B1) was 64.90 parts by mass, the amount of hydrogenated MDI was 28.70 parts by mass, the amount of trimethylolpropane was 2.20 parts by mass, and the amount of diethylenetriamine was 2.17 parts by mass. The rest of the procedure was the same as in Production Example 1, and a water dispersion of sodium salt of polyurethane (binder C2) with a non-volatile content of approximately 32% by mass was obtained.

[0087] (Comparative Manufacturing Example 3: Binder C3) Instead of 65.40 parts by mass of hydrogenated polybutadiene polyol (B1), 65.40 parts by mass of polybutadiene polyol (B3) was used, and the amount of diethylenetriamine was changed to 1.63 parts by mass. The rest of the procedure was the same as in Production Example 1, and an aqueous dispersion of sodium salt of polyurethane (binder C3) with a non-volatile content of approximately 32% by mass was obtained.

[0088] (Comparative Manufacturing Example 4: Binder C4) A water dispersion of a sodium salt of polyurethane (binder C4) having a nonvolatile content of approximately 32% by mass was obtained in the same manner as in Comparative Production Example 1, except that 74.10 parts by mass of polybutadiene polyol (B4) was used instead of 69.17 parts by mass of hydrogenated polybutadiene polyol (B1), the amount of hydrogenated MDI was 21.70 parts by mass, the amount of dimethylolpropionic acid was 4.20 parts by mass, and the amount of diethylenetriamine was 1.64 parts by mass.

[0089] (Comparative Manufacturing Example 5: Binder C5) A water dispersion of a sodium salt of polyurethane (binder C5) having a non-volatile content of about 32% by mass was obtained in the same manner as in Production Example 1, except that 21.00 parts by mass of toluene diisocyanate (TDI) was used instead of 28.60 parts by mass of hydrogenated MDI, the amount of hydrogenated polybutadiene polyol (B1) was 73.00 parts by mass, and the amount of diethylenetriamine was 2.17 parts by mass.

[0090] (Comparative Manufacturing Example 6: Binder C6) Instead of 28.60 parts by mass of hydrogenated MDI, 21.00 parts by mass of xylylene diisocyanate (XDI) was used, the amount of hydrogenated polybutadiene polyol (B1) was 73.00 parts by mass, the amount of diethylenetriamine was 1.62 parts by mass, and the rest was the same as in Production Example 1, to obtain an aqueous dispersion of sodium salt of polyurethane (binder C6) with a non-volatile content of approximately 32% by mass.

[0091] [Evaluation of polyurethane water dispersion] Constituent components of polyurethane for the obtained Binders 1 to 10 and Binders C1 to 6 are shown in Tables 1 and 2. Tables 1 and 2 also show the molar ratio (NCO / OH) of the isocyanate group in component (A) to the hydroxyl group in component (B) and component (C).

[0092] [Table 1]

[0093] [Table 2]

[0094] [Preparation of lithium secondary batteries] Example 1 (Preparation of negative electrode) SiO (average particle size 4.5 μm, specific surface area 5.5 m) was used as the negative electrode active material. 2 88.85 parts by mass of cellulose acetate / g), 40 parts by mass of fibrous nanocarbon water dispersion as a conductive agent, 4.0 parts by mass of acetylene black water dispersion, 43.3 parts by mass of 1.5% by mass aqueous solution of sodium salt of carboxymethylcellulose as a thickener, 30 parts by mass of aqueous dispersion of sodium salt of polyurethane 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 so that the solid content was 49% by mass. An electrolytic copper foil having a thickness of 10 μm was used as a current collector, and a negative electrode mixture layer made of the negative electrode slurry was formed on the electrolytic copper foil. In detail, the negative electrode slurry was coated on the electrolytic copper foil with a coater, roll-pressed, and then dried under reduced pressure at 130° C. to obtain a negative electrode active material of 2.7 mg / cm 2 . 2 A negative electrode of 1000 nm was obtained.

[0095] (Preparation of Positive Electrode) 92 parts by mass of LiNiCoAlO2 (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 an aluminum foil having a thickness of 15 μm using a coating machine, dried at 130° C., and then subjected to a roll press process to obtain a positive electrode active material of 16.6 mg / cm. 2 A positive electrode of 1000 nm was obtained.

[0096] (Production of battery) The negative and positive electrodes obtained above were combined and laminated with a polyolefin (PE / PP / PE) separator sandwiched between the electrodes as a separator, and the positive and negative terminals were ultrasonically welded to each positive and negative electrode. This laminate was placed in an aluminum laminate packaging material and heat sealed, leaving an opening for injection. Positive electrode area: 18 cm 2 , negative electrode area 19.8cm 2 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 poured into the battery, and the opening was heat sealed to obtain a battery for evaluation.

[0097] Example 2 When preparing the negative electrode slurry, SiC (average particle size 11.4 μm, specific surface area 2.5 m) was used instead of SiO as the negative electrode active material. 2 / g) and otherwise in the same manner as in Example 1, a negative electrode was produced, and further a battery for evaluation was produced.

[0098] (Examples 3 to 10 and Comparative Examples 1 to 6) When preparing the negative electrode slurry, binders 2 to 10 and C1 to 6 as shown in Table 2 below were used as binders, and the rest of the procedure was the same as in Example 1 to prepare negative electrodes and further prepare evaluation batteries.

[0099] [evaluation] The binding property of the prepared negative electrode was evaluated. In addition, the electrode expansion rate after charging and discharging was measured for the evaluation battery in order to confirm the effect of reducing the expansion of the electrode. In addition, the charge and discharge cycle characteristics of the evaluation battery were evaluated. The test method is as follows.

[0100] (Adhesiveness) The negative electrode was cut into a size of 3 x 6 cm, folded 180 degrees so that the coated surface was inward to a size of 3 x 3 cm, and then returned to its original position. The extent to which the active material on the coated surface had fallen off (the length of the fallen part relative to the folded part) was visually judged. The evaluation criteria were as follows: Evaluation criteria: A: 0% dropout (no dropout) B: Less than 5% loss (very little electrolytic copper foil is visible) C: Dropout rate: More than 5% but less than 25% D: Dropout: More than 25% but less than 50% E: More than 50% loss (some loss from the periphery of the bend)

[0101] (electrode expansion rate) The negative electrode thickness when the evaluation battery was assembled, the thickness of the battery before charging (cell thickness before charging), and the thickness of the battery after charging and discharging (cell thickness after evaluation) were measured with a micrometer and calculated using the following formula. Electrode expansion rate = (cell thickness after evaluation - cell thickness before charging) / negative electrode composite layer thickness × 100% Negative electrode composite layer thickness = negative electrode thickness - copper foil thickness The charge / discharge conditions were as follows, and the electrode expansion rate was measured after the first charge / discharge. Charge / discharge conditions: CC (constant current) charging was performed at a current density equivalent to 0.2 C up to 4.2 V, then switched to CV (constant voltage) charging at 4.2 V and charged up to a current density equivalent to 0.02 C. Discharge was performed at a current density equivalent to 0.2 C up to 2.7 V with CC (constant current) discharging.

[0102] (Charge / discharge cycle characteristics) A charge-discharge cycle test was performed on the evaluation battery using a charge-discharge device under the following conditions: CC (constant current) charging was performed at a current density equivalent to 1C up to 4.2V in an environment of 25°C, followed by switching to CV (constant voltage) charging at 4.2V, charging for 1.5 hours or until the current density reached 0.1C, and then CC discharging at a current density equivalent to 1C up to 2.7V was performed 500 times at 20°C, and the ratio of the 1C discharge capacity after 500 times to the initial 1C discharge capacity was calculated as the charge-discharge cycle retention rate (%). The rest between charging and discharging was 10 minutes.

[0103] [Table 3]

[0104] The results are shown in Table 3. Comparative Example 1 is an example in which an aqueous dispersion of polyurethane having an NCO / OH ratio of 1.35 was used as the binder. Comparative Example 2 is an example in which an aqueous dispersion of polyurethane having an NCO / OH ratio of 1.14 was used as the binder. In Comparative Examples 1 and 2, the NCO / OH ratio was outside the specified range, and therefore the charge / discharge cycle characteristics were poor.

[0105] Comparative Examples 3 and 4 are examples in which an aqueous dispersion of polyurethane synthesized using non-hydrogenated polybutadiene polyol was used as a binder. In Comparative Examples 3 and 4, the NCO / OH ratio was within the specified range, but since the polyurethane was not made using hydrogenated polybutadiene polyol, the binding property was poor, the expansion rate of the negative electrode during charging and discharging was high, and the charge and discharge cycle characteristics were poor.

[0106] Comparative Examples 5 and 6 are examples in which an aqueous dispersion of polyurethane synthesized using an aromatic or araliphatic polyisocyanate, which is not aliphatic or alicyclic, was used as a binder. Comparative Examples 5 and 6 had poor binding properties, a high expansion rate of the negative electrode during charging and discharging, and poor charge and discharge cycle characteristics.

[0107] In contrast, in Examples 1 to 11, which used a combination of hydrogenated polybutadiene polyol and aliphatic or alicyclic polyisocyanate as a binder and an aqueous dispersion of polyurethane with an NCO / OH ratio within a specified range, the binder had superior binding properties compared to Comparative Examples 2 to 6, had a lower expansion rate of the negative electrode during charge and discharge compared to Comparative Examples 3 to 6, and was superior in charge and discharge cycle characteristics compared to Comparative Examples 1 to 6.

[0108] In addition, the various numerical ranges described in the specification can be arbitrarily combined with their upper and lower limits, and all such combinations are considered to be preferred numerical ranges described in the specification. In addition, a numerical range described as "X to Y" means from X to Y.

[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 gist of the invention. These embodiments and their omissions, substitutions, modifications, etc. are included in the scope and gist of the invention as well as the invention and its equivalents described in the claims.

Claims

1. The polyisocyanate composition is obtained by reacting (A) at least one selected from the group consisting of an aliphatic polyisocyanate and an alicyclic 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 (B) contains a hydrogenated polybutadiene polyol, The binder for an electrode of a lithium secondary battery comprises an aqueous dispersion of polyurethane, in which a molar ratio (NCO / OH) of an isocyanate group in the component (A) to a hydroxyl group in the component (B) and the component (C) is 1.15 or more and 1.30 or less.

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

3. A polyurethane obtained by reacting (A) at least one selected from the group consisting of an aliphatic polyisocyanate and an alicyclic 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 (B) contains a hydrogenated polybutadiene polyol, and the molar ratio (NCO / OH) of the isocyanate groups contained in the component (A) to the hydroxyl groups contained in the components (B) and (C) is 1.15 or more and 1.30 or less. A conductive agent, and A binder composition for an electrode of a lithium secondary battery, comprising water.

4. 4. A negative electrode for a lithium secondary battery, comprising: the electrode binder according to claim 1 or the solid content of the electrode binder composition according to claim 3; and a negative electrode active material containing a silicon-based active material.

5. A lithium secondary battery comprising the negative electrode according to claim 4.

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