Binder for electrode of lithium secondary battery, binder composition for electrode, negative electrode, and lithium secondary battery

Abstract

Provided is a binder for an electrode of a lithium secondary battery, the binder having excellent binding properties, being capable of reducing electrode swelling caused by charge / discharge, and bringing about excellent charge / discharge cycling characteristics.　A binder for an electrode of a lithium secondary battery according to an embodiment comprises an aqueous dispersion of a polyurethane obtained by reacting (A) an aliphatic polyisocyanate and / or an alicyclic polyisocyanate, (B) one or more polyols, (C) a compound having two or more hydroxyl groups and a carboxy 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 (C) component is 1.15 or higher but less than 1.35.

Description

Binder for electrodes of lithium secondary batteries, electrode binder composition, negative electrode, and lithium secondary battery

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

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

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

[0004] Japanese Patent No. 7161078

[0005] In the binder for electrodes of lithium secondary batteries, it is of course necessary to have binding properties. For example, when the negative electrode active material contains a silicon-based active material such as SiO, it is required to reduce the expansion of the electrode due to charge and discharge, and further improve the charge and discharge cycle characteristics.

[0006] An object of embodiments of the present invention is to provide a binder for electrodes of lithium secondary batteries that is excellent in binding properties, can reduce the expansion of electrodes due to charge and discharge, and is excellent in charge and discharge cycle characteristics.

[0007] The inventors have found that the above-mentioned problems can be solved by improving the charge-discharge cycle characteristics of an aqueous polyurethane dispersion obtained by reacting an aliphatic polyisocyanate and / or alicyclic polyisocyanate with a polyol containing a hydrogenated polybutadiene polyol, a compound having two or more hydroxyl groups and carboxyl groups, and a chain extender, by setting the molar ratio of isocyanate groups to hydroxyl groups (NCO / OH) within a predetermined range.

[0008] The present invention includes the embodiments shown below: [1] A binder for electrodes of a lithium secondary battery, comprising a polyurethane aqueous dispersion obtained by reacting (A) at least one selected from the group consisting of aliphatic polyisocyanates and alicyclic polyisocyanates, (B) a polyol (excluding component (C)), (C) a compound having two or more hydroxyl groups and a carboxyl group, and (D) a chain extender, wherein component (B) includes a hydrogenated polybutadiene polyol, and the molar ratio (NCO / OH) of isocyanate groups of component (A) to hydroxyl groups of component (B) and component (C) is 1.15 or more and less than 1.35. [2] The electrode binder according to [1], for use as a negative electrode of a lithium secondary battery, comprising a silicon-based active material as the negative electrode active material. [3] A binder composition for electrodes of a lithium secondary battery, comprising polyurethane, a conductive agent, and water, obtained by reacting (A) at least one selected from the group consisting of aliphatic polyisocyanates and alicyclic polyisocyanates, (B) a polyol (except for component (C)), (C) a compound having two or more hydroxyl groups and a carboxyl group, and (D) a chain extender, wherein component (B) includes a hydrogenated polybutadiene polyol, and the molar ratio (NCO / OH) of isocyanate groups contained in component (A) to hydroxyl groups contained in component (B) and component (C) is 1.15 or more and less than 1.35. [4] A negative electrode for a lithium secondary battery, comprising the 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].

[0009] According to embodiments of the present invention, it is possible to provide a binder for lithium secondary battery electrodes that exhibits excellent binding properties, reduces electrode expansion due to charging and discharging, and has excellent charge-discharge cycle characteristics.

[0010] The electrode binder for the lithium secondary battery according to this embodiment includes an aqueous polyurethane dispersion 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, can reduce electrode expansion even when the electrode active material contains 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, component (A) may be an aliphatic polyisocyanate alone, an alicyclic polyisocyanate alone, or an aliphatic polyisocyanate and an alicyclic polyisocyanate in combination.

[0012] Examples of aliphatic polyisocyanates include tetramethylene diisocyanate, dodecamethylene diisocyanate, hexamethylene diisocyanate (HDI), 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, lysine diisocyanate, 2-methylpentane-1,5-diisocyanate, and 3-methylpentane-1,5-diisocyanate. These can be used individually or in combination of two or more.

[0013] Examples of alicyclic polyisocyanates include isophorone diisocyanate (IPDI), dicyclohexylmethane 4,4'-diisocyanate (hydrogenated MDI), hydrogenated xylylene diisocyanate, 1,4-cyclohexane diisocyanate, methylcyclohexylene diisocyanate, and 1,3-bis(isocyanate-methyl)cyclohexane. These can be used individually or in combination of two or more.

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

[0015] [Component (B)] Component (B) is a polyol. However, compounds having two or more hydroxyl groups and carboxyl groups, which are component (C), are not included in component (B).

[0016] In this embodiment, component (B) comprises a hydrogenated polybutadiene polyol. Preferably, the hydrogenated polybutadiene polyol 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 even more preferably 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 calculated using the following formula: Number of functional groups = {(Hydroxyl value) × (Mn)} / (56.1 × 1000)

[0017] Preferably, hydrogenated polybutadiene polyols are those obtained by hydrogenating (adding hydrogen) a polybutadiene polyol having a 1,4-linked, 1,2-linked, or a mixture thereof in its molecule, and having hydroxyl groups at its terminals. More preferably, hydrogenated polybutadiene polyols are those obtained by hydrogenating a polybutadiene polyol having hydroxyl groups at both ends of its polybutadiene structure.

[0018] Hydrogenated polybutadiene polyols are polybutadiene polyols in which some or all of the unsaturated double bonds contained in the polybutadiene polyol are hydrogenated. The degree of hydrogenation of a 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.

[0019] The molecular weight of the hydrogenated polybutadiene polyol is not particularly limited; for example, the number-average molecular weight (Mn) may be 600 to 10000, 800 to 6000, 1000 to 5000, or 1200 to 4000.

[0020] In this specification, the number-average molecular weight (Mn) is measured by GPC (gel permeation chromatography) and calculated using a calibration curve with standard polystyrene. Specifically, 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 volume: 50 μL, sample concentration: 0.2% by mass.

[0021] The hydroxyl value of hydrogenated polybutadiene polyol is not particularly limited and may be, for example, 10 to 200 mg KOH / g, 15 to 120 mg KOH / g, 20 to 100 mg KOH / g, or 50 to 90 mg KOH / g. In this specification, the hydroxyl value is measured in accordance with Method A of JIS K1557-1:2007.

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

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

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

[0025] [Component (C)] In this embodiment, a compound having two or more hydroxyl groups and carboxyl groups (hereinafter referred to as compound (C)) is used as component (C). The number of hydroxyl groups in one molecule of compound (C) is preferably two or three, and more preferably two. The number of carboxyl groups in one molecule of compound (C) is preferably one or two, and more preferably one.

[0026] Here, the carboxyl group is a concept that includes not only the acidic form (-COOH) but also the salt form, i.e., carboxylic acid bases (-COOX, where X is a cation that forms a salt with a carboxylic acid), and acidic and saltic forms may coexist. Examples of salts of carboxylic acid bases include alkali metal salts such as lithium salts, sodium salts, and potassium salts, alkaline earth metal salts such as magnesium salts and calcium salts, ammonium salts, amine salts (primary amine salts, secondary amine salts, and tertiary amine salts), and quaternary ammonium salts.

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

[0028] The carboxyl groups mentioned above can be neutralized to form salts, which can make the resulting polyurethane water-dispersible. Examples of bases used to neutralize the carboxyl groups include non-volatile bases such as lithium hydroxide, sodium hydroxide, and potassium hydroxide; tertiary amines such as trimethylamine, triethylamine, dimethylethanolamine, methyldiethanolamine, and triethanolamine; and volatile bases such as ammonia. Neutralization can be carried out before, during, or after the urethane formation reaction.

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

[0030] [Component (D)] As the chain extender for component (D), chain extenders commonly used in the art can be used. Specifically, amines, i.e., diamines and polyamines, are preferably used as chain extenders. Examples of diamines include ethylenediamine, trimethylenediamine, piperazine, and isophoronediamine. Examples of polyamines, i.e., hydrogen carbonate 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 triamine as component (D).

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

[0032] [Electrode Binding Agent] The electrode binding agent according to this embodiment includes an aqueous dispersion of polyurethane obtained by reacting components (A) to (D) above. In the polyurethane obtained by reacting components (A) to (D), the carboxyl groups derived from component (C) are dispersed in water as salts. Therefore, the electrode binding agent includes water and polyurethane dispersed in water.

[0033] When synthesizing the polyurethane described above, the molar ratio (NCO / OH) of isocyanate groups in component (A) to hydroxyl groups in component (B) and component (C) (i.e., the total amount of hydroxyl groups in component (B) and component (C)) is set to 1.15 or more and less than 1.35. By setting NCO / OH within the above range, the charge-discharge cycle characteristics can be improved. More preferably, NCO / OH is 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 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 polyurethane can be adjusted by the amount of compound (C). Here, the acid value can be determined from the amount of KOH (mg) required to neutralize the free carboxyl groups contained in 1 g of solid content of the polyurethane aqueous dispersion, in accordance with JIS K0070-1992.

[0035] The polyurethane may consist only of components (A) to (D), but may also contain other components as long as the effects of this embodiment are not impaired. For example, it may contain polyisocyanates other than component (A), such as aromatic polyisocyanates or aromatic aliphatic polyisocyanates.

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

[0037] The method for producing a polyurethane aqueous dispersion is not particularly limited, and for example, the following method can be used: Component (A) is reacted with components (B) and (C) without a solvent or in an organic solvent that does not contain active hydrogen groups to synthesize an isocyanate-terminated urethane prepolymer. After the synthesis of the urethane prepolymer, the carboxyl groups of component (C) are neutralized with a neutralizing agent, and then dispersed and emulsified in water. Subsequently, component (D) is added in an equivalent amount less than the remaining isocyanate groups (for example, an equivalent ratio of isocyanate groups to active hydrogen groups of the chain extender of 1:0.50 to 0.95) to cause an interfacial polymerization reaction between the isocyanate groups in the emulsion micelles and the chain extender to generate urea bonds. This improves the crosslinking density within the emulsion micelles and forms a three-dimensional crosslinked structure. Subsequently, the polyurethane aqueous dispersion can be obtained by removing the solvent used as needed.

[0038] In the synthesis of the above-mentioned urethane prepolymer, an organic solvent that is inert to the isocyanate group and capable of dissolving the resulting urethane prepolymer 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 this embodiment, the medium for dispersing the polyurethane may or may not contain these organic solvents along with water. It is preferable that these organic solvents are ultimately removed.

[0039] In this embodiment, the average particle size of the polyurethane aqueous dispersion is not particularly limited and may be in the range of, for example, 0.005 to 0.5 μm. Also, the number-average molecular weight (Mn) of the polyurethane is not particularly limited and may be, for example, 10,000 or more, or 50,000 or more.

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

[0041] As the conductive agent, an electron conductive material that does not adversely affect battery performance can be used. Specific examples of the conductive agent include fibrous nanocarbon, carbon black such as acetylene black and ketjen black, natural graphite (scaly graphite, flaky graphite, earthy graphite, etc.), artificial graphite, carbon whiskers, carbon fibers, metal (copper, nickel, aluminum, silver, gold, etc.) powders, metal fibers, and conductive materials such as conductive ceramic materials. Any one of these or a combination of two or more thereof can be used.

[0042] Preferably, fibrous nanocarbon is used as the conductive agent. Fibrous nanocarbon can follow the expansion of the electrode when the electrode active material contains a silicon-based active material, and can suppress the breakage of the conductive path during repeated charge and discharge.

[0043] Fibrous nanocarbon is fibrous carbon having a fiber diameter on the order of nanometers. The average fiber length of fibrous nanocarbon is not particularly limited, and is preferably, for example, 50 nm to 10 mm, more preferably 0.5 to 100 μm. The average fiber diameter of fibrous nanocarbon is not particularly limited, and is preferably, for example, 0.5 to 200 nm, more preferably 1 to 100 nm. The average fiber length and the 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 thereof. For lengths on the order of mm that cannot be measured by an atomic force microscope, measurement may be performed using an image taken by a microscope.

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

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

[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 aid to ensure conductivity around the electrode active material and can more effectively exert the conductivity of the fibrous nanocarbon that follows the expansion of the electrode. The content ratio of the fibrous nanocarbon and carbon black is not particularly limited. For example, the carbon black may be 100 to 2000 parts by mass with respect to 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, usually water is used, but polar organic solvents such as alcohol and ketone solvents, or a mixed solvent of these polar organic solvents and water may also be used. Examples of the dispersing device for dispersing the conductive agent in the medium include a jet mill, a high-pressure dispersing device, an ultrasonic homogenizer, etc.

[0048] When preparing an aqueous dispersion in which conductive agents such as fibrous nanocarbon or carbon black are dispersed in water, a dispersant may be added. Examples of dispersants include hydroxymethylcellulose, carboxymethylcellulose and their alkali metal salts, celluloses such as methylcellulose, ethylcellulose, hydroxypropylmethylcellulose, and hydroxyethylmethylcellulose, cellulose nanofibers, polycarboxylic acid compounds such as polyacrylic acid and sodium polyacrylate, compounds having a vinylpyrrolidone structure such as polyvinylpyrrolidone, polyvinyl alcohol, sodium alginate, xanthan gum, carrageenan, guar gum, agar, and starch. Among these, carboxymethylcellulose salts can be preferably used.

[0049] 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, 5 to 35% by mass or 15 to 30% by 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 manufacturing electrodes of lithium secondary batteries. When an electrode binder is used, the electrode coating liquid composition (1) includes an electrode binder, an electrode active material, and a conductive agent. When an electrode binder composition is used, the electrode coating liquid composition (2) includes an electrode binder composition and an electrode active material.

[0051] These electrode coating compositions are preferably used to manufacture the negative electrode of a lithium secondary battery. Therefore, the electrode active material is preferably a negative electrode active material. As the negative electrode active material, metallic lithium or a material capable of inserting / deinserting lithium ions can be used. Specific examples of negative electrode active materials include carbon materials such as natural graphite, artificial graphite, non-graphitizable carbon, and easily graphitizable carbon; metallic materials such as metallic lithium and alloys, tin compounds; lithium transition metal nitrides; crystalline metal oxides; amorphous metal oxides; silicon compounds; conductive polymers, etc. Any one of these may be used, or two or more may be used in combination.

[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 allows for increased 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 polyurethane content is not particularly limited, but is preferably 1 to 20% by mass relative to the content of the electrode active material (negative electrode active material), and more preferably 2 to 13% by mass.

[0054] In the electrode coating liquid composition (1) described above, the conductive agent is as described above in the electrode binder composition, including specific examples and preferred compositions, and therefore no further explanation is provided.

[0055] In the electrode coating liquid composition, the content of the conductive agent is not particularly limited, but is preferably 0.1 to 20% by mass, and more preferably 0.2 to 10% by mass, relative to the content of the electrode active material (negative electrode active material).

[0056] The electrode coating liquid composition may contain a thickening agent, such as a water-soluble polymer, as a viscosity modifier for forming the composition into a slurry. Examples of thickening agents include celluloses such as carboxymethylcellulose salt, methylcellulose, ethylcellulose, hydroxymethylcellulose, hydroxypropylmethylcellulose, and hydroxyethylmethylcellulose; polycarboxylic acid compounds such as polyacrylic acid and sodium polyacrylate; compounds having a vinylpyrrolidone structure such as polyvinylpyrrolidone; polyacrylamide, polyethylene oxide, polyvinyl alcohol, sodium alginate, xanthan gum, carrageenan, guar gum, agar, and starch. Any one or more of these can be used in combination. Among these, carboxymethylcellulose salt is preferred.

[0057] The content of electrode active material (negative electrode active material) in the solid content of the electrode coating liquid composition is not particularly limited and may be, for example, 65 to 99% by mass or 75 to 97% by mass. The content of dispersion medium such as water in the electrode coating liquid composition is not particularly limited and may be, for example, 20 to 80% by mass or 40 to 70% by mass.

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

[0059] [Negative electrode of lithium secondary battery] The negative electrode of the lithium secondary battery according to the embodiment can be manufactured by applying the electrode coating liquid composition to a current collector and evaporating the dispersion medium. That is, the negative electrode comprises a current collector and a negative electrode composite layer (also referred to as an active material layer) formed on the current collector, and the negative electrode composite layer consists of the solid components of the electrode coating liquid composition. In one embodiment, the electrode coating liquid composition includes a silicon-based active material such as SiO and / or SiC as a negative electrode active material, together with the electrode binder or electrode binder composition. Therefore, the negative electrode of the lithium secondary battery according to one embodiment comprises the solid components of the electrode binder or electrode binder composition and a negative electrode active material containing a silicon-based active material such as SiO and / or SiC.

[0060] As the current collector, an electron conductor that does not adversely affect the constructed battery can be used. Examples of current collectors for the negative electrode include copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymers, conductive glass, Al-Cd alloys, and copper or other materials whose surfaces are treated with carbon, nickel, titanium, or silver for the purpose of improving adhesion, conductivity, and oxidation resistance. The surfaces of these current collector materials may also be oxidized. Examples of current collector shapes include foil, film, sheet, net, punched or expanded materials, lath, porous materials, foam, and other molded bodies. The thickness of the current collector is not particularly limited, but a thickness of 1 to 100 μm is commonly used.

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

[0062] [Lithium Secondary Battery] The lithium secondary battery according to this embodiment includes the negative electrode described above. More specifically, the lithium secondary battery comprises a negative electrode, a positive electrode, a separator disposed between the negative electrode and the positive electrode, and an electrolyte, wherein the negative electrode is an electrode made using the electrode coating liquid composition described above. The positive electrode, separator, and electrolyte are not particularly limited and can be known configurations such as those described in Patent Document 1 described above.

[0063] The lithium secondary battery according to this embodiment can be formed into cylindrical, coin-type, prismatic, or any other arbitrary shape. The basic structure of the battery is the same regardless of the shape, and it can be designed and implemented according to the purpose. For example, in the case of a cylindrical battery, a negative electrode, which is made by coating a negative electrode active material onto a negative electrode current collector, and a positive electrode, which is made by coating a positive electrode active material onto a positive electrode current collector, are wound together with a separator in between. This wound body is then housed in a battery case, a non-aqueous electrolyte is injected, and insulating plates are placed on the top and bottom before sealing. In the case of a coin-type lithium secondary battery, a disc-shaped negative electrode, a separator, a disc-shaped positive electrode, and a stainless steel plate are stacked and housed in a coin-type battery case, a non-aqueous electrolyte is injected, and it is sealed.

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

[0065] Details of the polyol, fibrous nanocarbon aqueous dispersion, acetylene black aqueous 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, number of functional groups: 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, number of functional groups: 1.55)

[0068] • Polybutadiene polyol (B3): "NISSO-PB G-1000" manufactured by Nippon Soda Co., Ltd. (Mn: 1400, hydroxyl value 70 KOH mg / g, number of functional groups: 1.8)

[0069] • Polybutadiene polyol (B4): Evonic's "POLYVEST HT" (Mn: 2900, Number of functional groups: 2.45)

[0070] - Fibrous nanocarbon aqueous dispersion: Using OCSiAl's "TUBALL BATT" (CNT purity > 93%, average diameter 1.6 ± 0.5 nm) as single-walled carbon nanotubes (SWCNTs), a fibrous nanocarbon aqueous dispersion with a fibrous nanocarbon concentration of 1% by mass was prepared according to the following manufacturing procedure. The average fiber diameter, average fiber length, and aspect ratio of the fibrous nanocarbons in the obtained aqueous dispersion were measured using the following measurement method. The average fiber diameter was 3 nm, the average fiber length was 3000 nm, and the aspect ratio was 1000.

[0071] (Procedure for manufacturing fibrous nanocarbon aqueous dispersion) 0.5 g of SWCNT was mixed in a beaker with 50 g of a 1% by mass aqueous solution of carboxymethylcellulose salt ("Selogen 7A" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), stirred, and then dispersed for 90 minutes at an output of 100 μA while circulating the slurry using the beaker, an ultrasonic homogenizer (US-600T manufactured by Nippon Seiki Seisakusho Co., Ltd.), a circulation unit, and a tube pump to obtain a fibrous nanocarbon aqueous dispersion.

[0072] (Measurement Method for Fibrous Nanocarbons) The average fiber diameter and average fiber length of fibrous nanocarbons were measured using a scanning probe microscope (SPM) (AFM-5300E, JEOL Ltd.). Specifically, a fibrous nanocarbon aqueous dispersion was diluted with water until the concentration of fibrous nanocarbons was 0.01% by mass, then spread onto a mica substrate and the solvent was evaporated. The AFM image of this sample was then observed, and the average fiber diameter and average fiber length were calculated according to the method described above. Furthermore, the aspect ratio was calculated using these values ​​according to the following (Equation 1). Aspect ratio = Average fiber length (nm) / Average fiber diameter (nm) ... (Equation 1)

[0073] - Acetylene black aqueous dispersion: Denka Co., Ltd.'s "Li400" was used as the acetylene black. 100 g of acetylene black was added to 300 g of a 1% by mass aqueous solution of carboxymethylcellulose salt (Daiichi Kogyo Seiyaku Co., Ltd.'s "Selogen 7A") using a high-speed dispenser while stirring, and stirred until homogeneous. This yielded an acetylene black aqueous dispersion with an acetylene black concentration of 25% by mass.

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

[0075] [Synthesis of Polyurethane Aqueous Dispersion] (Production Example 1: Binding Agent 1) 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 were added to a four-necked flask equipped with a stirrer, reflux condenser, thermometer, and nitrogen blowing tube. The mixture was reacted at 75°C for 4 hours to obtain a methyl ethyl ketone solution of urethane prepolymer with a free isocyanate group content of 2.3% by mass relative to nonvolatile matter. This solution was cooled to 45°C, and an aqueous sodium hydroxide solution consisting of 1.20 parts by mass of sodium hydroxide and 80.80 parts by mass of water was gradually added and the mixture was emulsified and dispersed using a homogenizer. Next, an aqueous solution of 1.73 parts by mass of diethylenetriamine diluted with 30.30 parts by mass of water was added, and the chain extension reaction was carried out for 1 hour. This was then desolvated under reduced pressure and heating at 50°C to obtain an aqueous dispersion of sodium polyurethane salt (binding agent 1) with a non-volatile content of approximately 32% by mass.

[0076] (Production Example 2: Binding Agent 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, and the rest was carried out in the same manner as in Production Example 1 to obtain an aqueous dispersion of sodium polyurethane salt (binding agent 2) with a non-volatile content of approximately 32% by mass.

[0077] (Production Example 3: Binding Agent 3) By using 1.74 parts by mass of ethylenediamine instead of 1.73 parts by mass of diethylenetriamine, and otherwise proceeding in the same manner as in Production Example 1, an aqueous dispersion of sodium polyurethane salt (binding agent 3) with a non-volatile content of approximately 32% by mass was obtained.

[0078] (Production Example 4: Binding Agent 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, and the rest was the same as in Production Example 1 to obtain an aqueous dispersion of sodium polyurethane salt (binding agent 4) with a non-volatile content of approximately 32% by mass.

[0079] (Production Example 5: Binding Agent 5) The amount of hydrogenated polybutadiene polyol (B1) was set to 66.14 parts by mass, the amount of hydrogenated MDI to 28.03 parts by mass, the amount of trimethylolpropane to 1.63 parts by mass, and ethylenediamine to 1.46 parts by mass instead of 1.73 parts by mass of diethylenetriamine. The rest of the preparation was carried out in the same manner as in Production Example 1 to obtain an aqueous dispersion of sodium polyurethane salt (binding agent 5) with a non-volatile content of approximately 32% by mass.

[0080] (Production Example 6: Binding Agent 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, and the rest was the same as in Production Example 1 to obtain an aqueous dispersion of sodium polyurethane salt (binding agent 6) with a non-volatile content of approximately 32% by mass.

[0081] (Production Example 7: Binding Agent 7) Hydrogenated polybutadiene polyol (B2) was used in place of 65.40 parts by mass of hydrogenated polybutadiene polyol (B1) with 71.60 parts by mass of hydrogenated polybutadiene polyol (B2), and the amount of hydrogenated MDI was set to 22.40 parts by mass, and the amount of diethylenetriamine was set to 1.45 parts by mass. The rest of the preparation was carried out in the same manner as in Production Example 1 to obtain an aqueous dispersion of sodium salt of polyurethane (binding agent 7) with a non-volatile content of approximately 32% by mass.

[0082] (Production Example 8: Binding Agent 8) Instead of 28.60 parts by mass of hydrogenated MDI, 19.30 parts by mass of hexamethylene diisocyanate (HDI) was used, and the amount of hydrogenated polybutadiene polyol (B1) was set to 74.70 parts by mass. The rest of the preparation was carried out in the same manner as in Production Example 1 to obtain an aqueous dispersion of sodium polyurethane salt (binding agent 8) with a non-volatile content of approximately 32% by mass.

[0083] (Production Example 9: Binding Agent 9) By using 0.72 parts by mass of lithium hydroxide instead of 1.20 parts by mass of sodium hydroxide, and otherwise proceeding in the same manner as in Production Example 1, an aqueous dispersion of lithium salt of polyurethane (binding agent 9) with a non-volatile content of approximately 32% by mass was obtained.

[0084] (Production Example 10: Binding Agent 10) By using 3.04 parts by mass of triethylamine instead of 1.20 parts by mass of sodium hydroxide, and otherwise proceeding in the same manner as in Production Example 1, an aqueous dispersion of polyurethane triethylamine salt (binding agent 10) with a non-volatile content of approximately 32% by mass was obtained.

[0085] (Comparative Production Example 1: Binding Agent C1) 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 were added to a four-necked flask equipped with a stirrer, reflux condenser, thermometer, and nitrogen blowing tube. The mixture was 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 nonvolatile matter. This 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 the mixture was 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. This was subjected to desolvation under reduced pressure and heating at 50°C to obtain an aqueous dispersion of a sodium salt of polyurethane (binding agent C1) with a non-volatile content of approximately 32% by mass.

[0086] (Comparative Production Example 2: Binding Agent 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, and the rest was the same as in Production Example 1 to obtain an aqueous dispersion of sodium polyurethane salt (binding agent C2) with a non-volatile content of approximately 32% by mass.

[0087] (Comparative Production Example 3: Binding Agent C3) By using 65.40 parts by mass of polybutadiene polyol (B3) instead of hydrogenated polybutadiene polyol (B1), and by using 1.63 parts by mass of diethylenetriamine, and otherwise proceeding in the same manner as in Production Example 1, an aqueous dispersion of sodium salt of polyurethane (binding agent C3) with a non-volatile content of approximately 32% by mass was obtained.

[0088] (Comparative Production Example 4: Binding Agent C4) 74.10 parts by mass of polybutadiene polyol (B4) was used instead of 69.17 parts by mass of hydrogenated polybutadiene polyol (B1), and the amount of hydrogenated MDI was set to 21.70 parts by mass, the amount of dimethylolpropionic acid to 4.20 parts by mass, and the amount of diethylenetriamine to 1.64 parts by mass. The rest of the preparation was the same as in Comparative Production Example 1 to obtain an aqueous dispersion of sodium polyurethane salt (binding agent C4) with a non-volatile content of approximately 32% by mass.

[0089] (Comparative Production Example 5: Binding Agent C5) Instead of 28.60 parts by mass of hydrogenated MDI, 21.00 parts by mass of toluene diisocyanate (TDI) was used, the amount of hydrogenated polybutadiene polyol (B1) was set to 73.00 parts by mass, and the amount of diethylenetriamine was set to 2.17 parts by mass. The rest of the process was carried out in the same manner as in Production Example 1 to obtain an aqueous dispersion of sodium polyurethane salt (binding agent C5) with a non-volatile content of approximately 32% by mass.

[0090] (Comparative Production Example 6: Binding Agent C6) Xylylene diisocyanate (XDI) was used in place of 28.60 parts by mass of hydrogenated MDI, and the amount of hydrogenated polybutadiene polyol (B1) was set to 73.00 parts by mass, and the amount of diethylenetriamine was set to 1.62 parts by mass. The rest of the process was carried out in the same manner as in Production Example 1 to obtain an aqueous dispersion of sodium polyurethane salt (binding agent C6) with a non-volatile content of approximately 32% by mass.

[0091] [Evaluation of Polyurethane Aqueous Dispersions] The polyurethane components of 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 isocyanate groups in component (A) to hydroxyl groups in components (B) and (C).

[0092]

[0093]

[0094] [Fabrication of Lithium Secondary Battery] (Example 1) (Fabrication of Negative Electrode) Negative electrode active material: SiO (average particle size 4.5 μm, specific surface area 5.5 m²) ２ A negative electrode slurry was prepared by mixing 88.85 parts by mass of (1 / g), 40 parts by mass of fibrous nanocarbon aqueous dispersion and 4.0 parts by mass of acetylene black aqueous dispersion as conductive agents, 43.3 parts by mass of a 1.5% by mass aqueous solution of carboxymethylcellulose sodium salt as a thickener, 30 parts by mass of an aqueous dispersion of polyurethane sodium salt (binding agent 1), and 14 parts by mass of ion-exchanged water using a planetary mixer to obtain a solid content of 49% by mass. An electrolytic copper foil with a thickness of 10 μm was used as a current collector, and a negative electrode composite layer consisting of the above negative electrode slurry was formed on the electrolytic copper foil. Specifically, the above negative electrode slurry was coated onto the electrolytic copper foil using a coating machine, roll-pressed, and then dried under reduced pressure at 130°C to obtain a negative electrode active material of 2.7 mg / cm³. ２ The negative electrode was obtained.

[0095] (Fabrication of the positive electrode) The positive electrode active material is LiNiCoAlO ２ A positive electrode slurry was prepared by mixing 92 parts by mass of (NCA), 4 parts by mass of acetylene black (Denka Co., Ltd. "Li-400") as a conductive agent, 4 parts by mass of polyvinylidene fluoride as a binder, and 49.2 parts by mass of N-methyl-2-pyrrolidone as a dispersion medium in a planetary mixer to obtain a solid content of 67% by mass. This positive electrode slurry was coated onto 15 μm thick aluminum foil using a coating machine, dried at 130°C, and then roll-pressed to obtain a positive electrode active material of 16.6 mg / cm². ２ The positive electrode was obtained.

[0096] (Battery Fabrication) The negative and positive electrodes obtained above were combined and laminated with a polyolefin-based (PE / PP / PE) separator sandwiched between the electrodes. The positive and negative electrode terminals were ultrasonically welded to each electrode. This laminate was placed in an aluminum laminate packaging and heat-sealed, leaving an opening for liquid injection. Positive electrode area: 18 cm² ２ , negative electrode area 19.8cm ２ A battery was prepared before liquid injection. Next, LiPF was added to a solvent mixture of ethylene carbonate and diethyl carbonate (30 / 70 vol ratio). ６ An electrolyte solution containing (1.0 mol / L) was poured in, and the opening was heat-sealed to obtain an evaluation battery.

[0097] (Example 2) When preparing the negative electrode slurry, SiC was used instead of SiO as the negative electrode active material (average particle size 11.4 μm, specific surface area 2.5 m²). ２ Using ( / g), the negative electrode was prepared in the same manner as in Example 1, and then an evaluation battery was prepared.

[0098] (Examples 3-10 and Comparative Examples 1-6) When preparing the negative electrode slurry, binders 2-10 and C1-6 as shown in Table 2 below were used as binders, and the negative electrode was prepared in the same manner as in Example 1, and then an evaluation battery was prepared.

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

[0100] (Bonding properties) The negative electrode was cut to 3 x 6 cm, folded 180° so that the coated surface was 3 x 3 cm inward, and then unfolded. The degree of detachment of the active material from the coated surface (length of the detached area relative to the folded area) was visually assessed. The evaluation criteria are as follows: Evaluation criteria: A: 0% detachment (no detachment) B: 5% or less detachment (a very small amount of electrolytic copper foil is visible) C: More than 5% but less than 25% detachment D: More than 25% but less than 50% detachment E: 50% or more detachment (detachment also present around the folded area)

[0101] (Electrode Expansion Rate) The negative electrode thickness during battery assembly for evaluation, the battery thickness before charging (cell thickness before charging), and the battery thickness 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 charging and discharging conditions were as follows, and the electrode expansion rate was measured after the first charging and discharging cycle. Charging and discharging conditions: CC (constant current) charging was performed up to 4.2V at a current density equivalent to 0.2C, then switched to CV (constant voltage) charging at 4.2V and charged up to a current density equivalent to 0.02C. Discharging was performed CC (constant current) discharge up to 2.7V at a current density equivalent to 0.2C.

[0102] (Charge-Discharge Cycle Characteristics) The evaluation battery underwent charge-discharge cycle testing using a charge-discharge device under the following conditions. At 25°C, the battery was charged with CC (constant current) at a current density equivalent to 1C up to 4.2V, then switched to CV (constant voltage) charging at 4.2V, charged for 1.5 hours or until the current density reached 0.1C, and then discharged with CC at a current density equivalent to 1C up to 2.7V. This cycle was repeated 500 times at 20°C, and the ratio of the 1C discharge capacity after 500 cycles to the initial 1C discharge capacity was calculated as the charge-discharge cycle retention rate (%). A 10-minute pause was allowed between charging and discharging.

[0103]

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

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

[0106] Comparative Examples 5 and 6 are examples in which aqueous dispersions of polyurethane synthesized using non-aliphatic or non-alicyclic aromatic or fraoliphatic polyisocyanates were used as binders. Comparative Examples 5 and 6 exhibited poor binding properties, high expansion rate of the negative electrode during charge and discharge, and poor charge-discharge cycle characteristics.

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

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

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

Claims

1. A binder for electrodes of a lithium secondary battery, comprising a polyurethane aqueous dispersion obtained by reacting (A) at least one selected from the group consisting of aliphatic polyisocyanates and alicyclic polyisocyanates, (B) a polyol (excluding component (C)), (C) a compound having two or more hydroxyl groups and a carboxyl group, and (D) a chain extender, wherein component (B) includes a hydrogenated polybutadiene polyol, and the molar ratio (NCO / OH) of isocyanate groups of component (A) to hydroxyl groups of component (B) and component (C) is 1.15 or more and less than 1.

35.

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

3. A binder composition for electrodes of a lithium secondary battery, comprising a polyurethane, a conductive agent, and water, obtained by reacting (A) at least one selected from the group consisting of aliphatic polyisocyanates and alicyclic polyisocyanates, (B) a polyol (excluding component (C)), (C) a compound having two or more hydroxyl groups and a carboxyl group, and (D) a chain extender, wherein component (B) includes a hydrogenated polybutadiene polyol, and the molar ratio (NCO / OH) of isocyanate groups contained in component (A) to hydroxyl groups contained in component (B) and component (C) is 1.15 or more and less than 1.

35.

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

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

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