Patent Application
20230307653
The present invention relates to a carbon nanotube dispersion, and more specifically, relates to a carbon nanotube dispersion, a resin composition containing a carbon nanotube dispersion and a binder, a mixture slurry containing a carbon nanotube dispersion, a binder and an active material, and an electrode film obtained by forming a mixture slurry into a film, and a nonaqueous electrolyte secondary battery containing an electrode film and an electrolyte.
With the spread of electric vehicles and miniaturization, weight reduction and performance improvement of portable devices, there are demands to provide secondary batteries with high energy densities and to increase the capacity of such secondary batteries. In view of this, nonaqueous electrolyte secondary batteries using a nonaqueous electrolytic solution, particularly, lithium ion secondary batteries, have come to be used in many devices because of their high energy density and high voltage.
As a negative electrode material used in these lithium ion secondary batteries, a carbon material represented by graphite, which has a base potential close to that of lithium (Li) and has a large charging and discharging capacity per unit mass, is used. However, these electrode materials are used with the charging and discharging capacity per mass close to the theoretical value, and the energy density per mass of the battery is approaching its limit. Therefore, in order to increase the utilization rate of the electrode, there have been attempts to reduce the amount of conductivity aids or binders that do not contribute to the discharging capacity.
Carbon black, ketjen black, fullerene, graphene, fine carbon materials and the like are used as conductivity aids. Particularly, carbon nanotubes, which are a type of fine carbon fibers, are often used. For example, it is known that, when carbon nanotubes are added to graphite or silicon negative electrodes, the conductivity of the electrode, the strength of the electrode such as adhesion and expansion and contraction, and the rate property and the cycle property of the lithium ion secondary battery are improved (for example, refer to Patent Literature 1). In addition, studies are being conducted to reduce the electrode resistance by adding carbon nanotubes to the positive electrode (for example, refer to Patent Literature 2 and Patent Literature 3). Among these, multi-walled carbon nanotubes having an outer diameter of 10 nm to several tens of 10 nm are relatively inexpensive and are expected to be put into practice.
When carbon nanotubes having a small average outer diameter are used, a conductive network can be efficiently formed with a small amount of carbon nanotubes, and the amount of conductivity aid contained in the positive electrode and negative electrode for the lithium ion secondary battery can be reduced. In addition, it is known that the same effects can be obtained when carbon nanotubes having a long fiber length are used (for example, refer to Patent Literature 4).
In addition, methods of stabilizing dispersion of carbon nanotubes using various dispersing agents have been proposed. For example, dispersion in water and N-methyl-2-pyrrolidone (NMP) using a polymer-based dispersing agent such as a water-soluble polymer has been proposed (refer to Patent Literature 1, Patent Literature 5 and Patent Literature 6). In addition, a method of stabilizing dispersion of multi-walled carbon nanotubes using nitrile rubber as a dispersing agent has been proposed (refer to Patent Literature 7).
However, carbon nanotubes having features of a small average outer diameter and a large fiber length have a strong cohesive force and are unlikely to disperse, and therefore a carbon nanotube dispersion having sufficient dispersibility cannot be obtained. In Patent Literature 1, single-walled carbon nanotubes are dispersed in an NMP solvent containing polyvinylpyrrolidone using zirconia beads, but there are problems such as a long dispersion time and a small dispersed particle size of carbon nanotubes, and although the conductivity of the electrode is improved, the electrode strength cannot be more sufficiently improved. In Patent Literature 5, oxidized double-walled carbon nanotubes are dispersed in a carboxymethyl cellulose aqueous solution using an ultrasonic homogenizer, but it is difficult to disperse carbon nanotubes in a solvent at a high concentration. In addition, in Patent Literature 6, single-walled carbon nanotubes are dispersed in an NMP solvent containing polyvinylpyrrolidone using ultrasonic waves, but it is difficult to disperse carbon nanotubes in a solvent at a high concentration. In Patent Literature 7, it has been proposed that, when a multi-walled carbon nanotube dispersion having a specific complex elastic modulus is produced, the output property of the electrode is improved. However, in multi-walled carbon nanotubes having an outer diameter of 10 nm or more, improvement in the electrode strength was insufficient, and improvement in the cycle property of the lithium ion secondary battery was difficult. Therefore, obtaining a carbon nanotube dispersion in which carbon nanotubes, which are fine carbon fibers, particularly, single-walled carbon nanotubes, are uniformly dispersed in a dispersion medium at a high concentration is an important issue for more applications.
An objective achieved by the present invention is to provide a carbon nanotube dispersion having a high dispersibility and elastic modulus, a carbon nanotube resin composition and a mixture slurry in order to obtain an electrode film having excellent electrode strength and conductivity, and more specifically, to provide a nonaqueous electrolyte secondary battery having an excellent rate property and cycle property.
That is, the present invention relates to a carbon nanotube dispersion containing carbon nanotubes, a dispersing agent, and a solvent, which is a carbon nanotube dispersion that satisfies the following (1) to (4):
In addition, the present invention relates to the carbon nanotube dispersion in which, in Raman spectrums of carbon nanotubes, when the maximum peak intensity in a range of 1,560 to 1,600 cm−1 is G, and the maximum peak intensity in a range of 1,310 to 1,350 cm−1 is D, the G/D ratio of the carbon nanotubes is 10 to 50.
In addition, the present invention relates to the carbon nanotube dispersion in which, when the shear rate of 1 (s−1) of the carbon nanotube dispersion at 25° C. is measured using a rheometer, it is 5 P·as or more and less than 40 P·as.
In addition, the present invention relates to the carbon nanotube dispersion in which the cumulative particle size D10 measured by a dynamic light scattering method is 200 nm or more and less than 500 nm.
In addition, the present invention relates to the carbon nanotube dispersion in which the volume resistivity of the carbon nanotubes is 1.0×10−3 Ω·cm to 1.0×10−2 Ω·cm.
In addition, the present invention relates to the carbon nanotube dispersion in which the cumulative particle size D50 measured by a dynamic light scattering method is 500 nm or more and less than 3,000 nm.
In addition, the present invention relates to the carbon nanotube dispersion in which the weight average molecular weight of the dispersing agent is 10,000 to 100,000.
In addition, the present invention relates to the carbon nanotube dispersion in which the solvent includes water.
In addition, the present invention relates to a carbon nanotube resin composition containing the carbon nanotube dispersion and a binder.
In addition, the present invention relates to a mixture slurry containing the carbon nanotube resin composition and an active material.
In addition, the present invention relates to an electrode film which is a coating film of the mixture slurry.
In addition, the present invention relates to a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and an electrolyte, which is a nonaqueous electrolyte secondary battery in which at least one of the positive electrode and the negative electrode includes the electrode film.
A resin composition, a mixture slurry, and an electrode film which have an excellent electrode strength and adhesion can be obtained when the carbon nanotube dispersion of the present invention is used. In addition, a nonaqueous electrolyte secondary battery having an excellent rate property and cycle property can be obtained. Therefore, the carbon nanotube dispersion of the present invention can be used in various application fields for which high conductivity and durability are required.
Hereinafter, a carbon nanotube dispersion, a resin composition, a mixture slurry, an electrode film as a coating film thereof, and a nonaqueous electrolyte secondary battery of the present invention will be described in detail.
The carbon nanotube of the present embodiment is preferably a single-walled carbon nanotube. A mixture of a single-walled carbon nanotube and a multi-walled carbon nanotube may be used. The single-walled carbon nanotube has a structure in which one graphite layer is wound, and the multi-walled carbon nanotube has a structure in which two, three or more graphite layers are wound.
The average outer diameter of the carbon nanotubes of the present embodiment is 0.5 nm to 5 nm, preferably 1 nm to 3 nm, and more preferably 1 nm to 2 nm. The average outer diameter of the carbon nanotubes can be obtained by observing the morphology of carbon nanotubes using a transmission electron microscope (commercially available from JEOL Ltd.), measuring the lengths of 100 minor axes, and calculating the number average value thereof.
The BET specific surface area of the carbon nanotubes of the present embodiment is 550 m2/g to 1,200 m2/g, preferably 600 to 1,200 m2/g, more preferably 800 m2/g to 1,200 m2/g, and still more preferably 800 m2/g to 1,000 m2/g.
In the carbon nanotubes of the present embodiment, in the Raman spectrum, the G/D ratio is 5 to 100, more preferably 10 to 50, and still more preferably 20 to 50 when the maximum peak intensity in a range of 1,560 to 1,600 cm−1 is G, and the maximum peak intensity in a range of 1,310 to 1,350 cm−1 is D. The Raman spectrum can be measured using a laser beam with a wavelength of 532 nm according to Raman spectroscopy.
The volume resistivity of the carbon nanotubes of the present embodiment is preferably 1.0×10−3 Ω·cm to 3.0×10−2 Ω·cm and more preferably 1.0×10−3 Ω·cm to 1.0×10−2 Ω·cm. The volume resistivity of the carbon nanotubes can be measured using a powder resistivity measure device (Loresta GP powder resistivity measurement system MCP-PD-51, commercially available from Mitsubishi Chemical Analytech Co., Ltd.).
The carbon purity of the carbon nanotubes of the present embodiment is expressed by the content (%) of carbon atoms in the carbon nanotubes. The carbon purity with respect to 100 mass % of the carbon nanotubes is preferably 80 mass % or more, more preferably 90 mass % or more, and still more preferably 95 mass % or more.
The amount of metals contained in the carbon nanotubes of the present embodiment with respect to 100 mass % of the carbon nanotubes is preferably less than 20 mass %, more preferably less than 10 mass %, and still more preferably less than 5 mass %. Examples of metals contained in carbon nanotubes include metals and metal oxides used as a catalyst when carbon nanotubes are synthesized. Specific examples thereof include metals such as cobalt, nickel, aluminum, magnesium, silica, manganese, and molybdenum, alloys of these metals, metal oxides of these metals, and composite oxides of these metals.
The carbon nanotubes of the present embodiment may be surface-treated carbon nanotubes. In addition, the carbon nanotubes may be carbon nanotube derivatives to which a functional group represented by a carboxyl group is added. In addition, carbon nanotubes containing substances represented by organic compounds, metal atoms, or fullerenes can be used.
The carbon nanotubes of the present embodiment may be pulverized carbon nanotubes. The pulverization treatment is pulverization of carbon nanotubes without substantial intervention of a liquid substance using a pulverizer with a built-in pulverization medium such as beads and steel balls, and is also called dry pulverization. Pulverization is performed by utilizing a pulverizing force and a destructive force due to collision between pulverization media. Pulverization mainly has an effect of reducing the number of secondary particles of carbon nanotubes, and can improve the dispersibility of carbon nanotubes. As a dry pulverization device, a known method using a dry attritor, a ball mill, a vibration mill, a bead mill or the like can be used, and the pulverization time can be arbitrarily set depending on the device.
The carbon nanotubes of the present embodiment may be carbon nanotubes produced by any method. Carbon nanotubes can be generally produced by a laser ablation method, an arc discharging method, a thermal CVD method, a plasma CVD method and a combustion method, but the present invention is not limited thereto.
The dispersing agent of the present embodiment is not particularly limited as long as it can stabilize dispersion of carbon nanotubes, and a surfactant and a resin type dispersing agent can be used. Surfactants are mainly classified into anionic surfactants, cationic surfactants, nonionic surfactants and amphoteric surfactants. A suitable type of dispersing agent can be appropriately used in a suitable blending amount depending on properties required for dispersing carbon nanotubes.
When an anionic surfactant is selected, the type thereof is not particularly limited. Specific examples thereof include fatty acid salts, polysulfonates, polycarboxylates, alkyl sulfate ester salts, alkyl aryl sulfonates, alkyl naphthalene sulfonates, dialkyl sulfonates, dialkyl sulfosuccinates, alkyl phosphates, polyoxyethylene alkyl ether sulfates, polyoxyethylene alkyl aryl ether sulfates, naphthalenesulfonic acid formalin condensates, polyoxyethylene alkyl phosphate sulfonates, glycerol borate fatty acid esters and polyoxyethylene glycerol fatty acid esters, but the present invention is not limited thereto. More specific examples thereof include sodium dodecylbenzenesulfonate, sodium lauryl sulfate, sodium polyoxyethylene lauryl ether sulfate, polyoxyethylene nonylphenyl ether sulfate salts and sodium salts of β-naphthalenesulfonic acid formalin condensate, but the present invention is not limited thereto.
In addition, cationic surfactants include alkylamine salts and quaternary ammonium salts. Specific examples thereof include stearylamine acetate, coconut trimethyl ammonium chloride, tallow trimethyl ammonium chloride, dimethyl dioleyl ammonium chloride, methyloleyldiethanol chloride, tetramethyl ammonium chloride, lauryl pyridinium chloride, lauryl pyridinium bromide, lauryl pyridinium disulfate, cetylpyridinium bromide, 4-alkylmercaptopyridine, poly(vinylpyridine)-dodecyl bromide and dodecylbenzyltriethylammonium chloride, but the present invention is not limited thereto. In addition, examples of amphoteric surfactants include aminocarboxylates, but the present invention is not limited thereto.
In addition, examples of nonionic surfactants include polyoxyethylene alkyl ethers, polyoxyalkylene derivatives, polyoxyethylene phenyl ethers, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters and alkyl allyl ethers, but the present invention is not limited thereto. Specific examples thereof include polyoxyethylene lauryl ethers, sorbitan fatty acid esters and polyoxyethylene octylphenyl ethers, but the present invention is not limited thereto.
The selected surfactant is not limited to a single surfactant. Therefore, it is possible to use a combination of two or more surfactants. For example, a combination of an anionic surfactant and a nonionic surfactant or a combination of a cationic surfactant and a nonionic surfactant can be used. The blending amount in this case is preferably a blending amount suitable for each surfactant component. As the combination, a combination of an anionic surfactant and a nonionic surfactant is preferable. The anionic surfactant is preferably a polycarboxylate. The nonionic surfactant is preferably polyoxyethylene phenyl ether.
In addition, specific examples of resin type dispersing agents include cellulose derivatives (cellulose acetate, cellulose acetate butyrate, cellulose butyrate, cyanoethyl cellulose, ethyl hydroxyethyl cellulose, nitro cellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxymethyl cellulose, etc.), polyvinyl alcohol, polyvinyl butyral, polyvinylpyrrolidone, and polyacrylonitrile polymers. Particularly, methyl cellulose, ethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinylpyrrolidone, and polyacrylonitrile polymers are preferable.
Carboxymethyl cellulose as a resin type dispersing agent can be used in the form of a salt such as a sodium salt of carboxymethyl cellulose in which hydroxy groups of carboxymethyl cellulose are replaced with carboxymethyl sodium groups. The degree of etherification of carboxymethyl cellulose as a resin type dispersing agent is preferably 0.5 to 1.5 and more preferably 0.6 to 1.0. The degree of etherification of carboxymethyl cellulose can be measured according to a general method, and specifically, can be measured according to the method described in examples.
The dispersing agent of the present embodiment has a weight average molecular weight in terms of pullulan that is preferably 5,000 or more and 300,000 or less, more preferably 10,000 or more and 100,000 or less, and still more preferably 10,000 or more and 50,000 or less. When a dispersing agent having an appropriate weight average molecular weight is used, the adsorption to carbon nanotubes is improved, and the stability of the carbon nanotube dispersion is further improved. In addition, when a dispersing agent having a weight average molecular weight exceeding the above range is used, the viscosity of the carbon nanotube dispersion increases, and when a disperser such as a nozzle type high pressure homogenizer in which a liquid to be dispersed passes through a narrow flow path is used, the dispersion efficiency may decrease. Here, the resin type dispersing agent may have a binding ability in addition to the dispersing ability, and the above resin type dispersing agent can be used as a binder, and, the same type of resin as the resin type dispersing agent may be used as the binder. When the same type of resin as the resin type dispersing agent is used as the binder, it is preferable to use a resin having a weight average molecular weight larger than the weight average molecular weight of the resin type dispersing agent.
Here, the weight average molecular weight (Mw) of the dispersing agent can be measured using gel permeation chromatography (GPC) with a differential refractive index (RI) detector, and is a pullulan conversion value.
In addition, in addition to the dispersing agent of the present embodiment, an inorganic base and/or inorganic metal salt may be included. The inorganic bases and inorganic metal salts are preferably compounds containing at least one of alkali metals and alkaline earth metals, and specific examples thereof include chlorides, hydroxides, carbonates, nitrates, sulfates, phosphates, tungstates, vanadates, molybdates, niobates, and borates of alkali metals and alkaline earth metals. In addition, among these, chlorides, hydroxides, and carbonates of alkali metals and alkaline earth metals are preferable because they can easily supply cations. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides include calcium hydroxide and magnesium hydroxide. Examples of alkali metal carbonates include lithium carbonate, lithium hydrogencarbonate, sodium carbonate, sodium hydrogen carbonate, potassium carbonate, and potassium hydrogen carbonate. Examples of alkaline earth metal carbonates include calcium carbonate and magnesium carbonate. Among these, lithium hydroxide, sodium hydroxide, lithium carbonate, and sodium carbonate are more preferable.
In addition, in addition to the dispersing agent of the present embodiment, an acid may be included. When an acid is added, the charging state in the dispersion system and the balance between the hydrophilic part and the hydrophobic part change, and the dispersibility may be improved. The type of acid is not particularly limited, and one type may be used or a plurality of types may be used in combination. Examples of acids include oxalic acid, lactic acid, citric acid, polyacrylic acid, polystyrene sulfonic acid, acetic acid, malonic acid, hydrochloric acid, nitric acid, sulfuric acid, boric acid, and phosphoric acid.
In addition, in addition to the dispersing agent of the present embodiment, a defoaming agent may be included. The defoaming agent can be arbitrarily used as long as it has a defoaming effect such as commercially available defoaming agents, wetting agents, hydrophilic organic solvents, and water-soluble organic solvents, and one type thereof may be used or a plurality thereof may be used in combination.
Examples thereof include alcohols such as ethanol, propanol, isopropanol, butanol, octyl alcohol, hexadecyl alcohol, acetylene alcohol, ethylene glycol monobutyl ether, methyl cellosolve, butyl cellosolve, propylene glycol monomethyl ether, acetylene glycol, polyoxyalkylene glycol, propylene glycol, and other glycols, fatty acid esters such as diethylene glycol laurate, glycerin monoricinoleate, alkenylsuccinic acid derivatives, sorbitol monolaurate, sorbitol trioleate, polyoxyethylene monolaurate, polyoxyethylene sorbitol monolaurate, and natural wax, amides such as polyoxyalkyleneamide and acrylate polyamine, phosphate esters such as tributyl phosphate and sodium octyl phosphate, metallic soaps such as aluminum stearate and calcium oleate, fats and oils such as animal and vegetable oils, sesame oil, and castor oil mineral oils such as kerosene and paraffin, and silicones such as dimethyl silicone oil, silicone paste, silicone emulsion, organic modified polysiloxane, and fluorosilicone oil.
The solvent of the present embodiment is not particularly limited as long as carbon nanotubes can be dispersed, but any one selected from the group consisting of water and a water-soluble organic solvent or a mixed solvent containing two or more selected from the groups is preferable, and it is more preferable to contain water. When water is contained, its content with respect to 100 mass % of the solvent is preferably 95 mass % or more and more preferably 98 mass % or more, and a single water solvent may be used.
Examples of water-soluble organic solvents include alcohols (methanol, ethanol, propanol, isopropanol, butanol, isobutanol, secondary butanol, tertiary butanol, benzyl alcohol, etc.), polyhydric alcohols (ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, butylene glycol, hexanediol, pentanediol, glycerin, hexanetriol, thiodiglycol, etc.), polyhydric alcohol ethers (ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monobutyl ether, ethylene glycol monomethyl ether acetate, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, ethylene glycol monophenyl ether, propylene glycol monophenyl ether, etc.), amines (ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, tetramethyl propylene diamine, etc.), amides (N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylcaprolactam, etc.), heterocycles (cyclohexylpyrrolidone, 2-oxazolidone, 1,3-dimethyl-2-imidazolidinone, γ-butyrolactone, etc.), sulfoxides (dimethyl sulfoxide, etc.), sulfones (hexamethylphosphorotriamide, sulfolane, etc.), lower ketones (acetone, methyl ethyl ketone, etc.), and in addition, tetrahydrofuran, urea, acetonitrile and the like can be used. Among these, an amide-based water-soluble organic solvent is preferable, and N-methyl-2-pyrrolidone (NMP) is more preferable.
The carbon nanotube dispersion of the present embodiment contains carbon nanotubes, a dispersing agent and a solvent.
The complex elastic modulus of the carbon nanotube dispersion of the present embodiment at 25° C. and a frequency of 1 Hz is 5 Pa or more and less than 650 Pa, preferably 5 Pa or more and less than 400 Pa, and more preferably 10 Pa or more and less than 400 Pa. The complex elastic modulus of the carbon nanotube dispersion indicates the hardness of the carbon nanotube dispersion, and tends to be smaller as the dispersibility of carbon nanotubes is better and the viscosity of the carbon nanotube dispersion is lower. On the other hand, when the fiber length of carbon nanotubes is long, even if dispersion is favorable, the complex elastic modulus may become a high numerical value due to the structural viscosity of the carbon nanotube itself.
The phase angle of the carbon nanotube dispersion of the present embodiment at 25° C. and a frequency of 1 Hz is 5° or more and less than 50°, and more preferably 10° or more and less than 50°. The phase angle means the phase shift of the stress wave when the strain applied to the carbon nanotube dispersion is a sine wave. In the case of a purely elastic component, since the sine wave has the same phase as the applied strain, the phase angle is 0°. On the other hand, in a purely viscous component, the stress wave advances by 90°. A carbon nanotube dispersion component having complex elastic modulus and phase angle values within the above ranges has a favorable dispersed particle size and dispersion state of carbon nanotubes, and is suitable as a carbon nanotube dispersion for improving the electrode strength and conductivity.
The complex elastic modulus and the phase angle of the carbon nanotube dispersion can be obtained by performing dynamic viscoelasticity measurement using a rheometer with a cone of a diameter of 35 mm and 2° at 25° C. and a frequency of 1 Hz with a strain rate in the range of 0.01% to 5%. When the measured value contains a decimal point, it is rounded to an integer according to the rule B of JIS 28401: 1999. Here, when the measured value is one decimal place, the complex elastic modulus of the carbon nanotube dispersion at 25° C. and a frequency of 1 Hz is preferably 4.5 Pa or more and less than 650.4 Pa, and the phase angle of the carbon nanotube dispersion at 25° C. and a frequency of 1 Hz is preferably 4.5° or more and less than 50.4°.
A developed conductive network is formed by uniformly and favorably dispersing the carbon nanotubes while maintaining a certain length or longer so that the fiber length of the carbon nanotubes is not shortened by breakage. Therefore, it is not enough that the viscosity of the conductive material dispersion component is simply low and the (apparent) dispersibility is favorable, but it is particularly effective to determine the dispersion state by combining the complex elastic modulus and/or the phase angle with a conventional index such as viscosity. When the complex elastic modulus and/or the phase angle are within the above range, a conductive material dispersion component having favorable conductivity and electrode strength can be obtained.
The viscosity of the carbon nanotube dispersion of the present embodiment when measured using a rheometer at 25° C. and a shear rate of 1 (s−1) is preferably 5 P·as or more and less than 60 P·as, more preferably 10 P·as or more and less than 40 P·as, and still more preferably 20 P·as or more and less than 40 P·as. In addition, when the viscosity is measured using a rheometer at 25° C. and a shear rate of 10 (s−1), it is preferably 1 P·as or more and less than 10 P·as. When the shear viscosity at a shear rate of 1 (s−1) is measured, the dispersibility of the carbon nanotube dispersion can be determined, the carbon nanotube dispersion within the above range has a favorable dispersed particle size and dispersion state of the carbon nanotubes, and is suitable as a carbon nanotube dispersion for improving the electrode strength and conductivity.
The viscosity of the carbon nanotube dispersion can be obtained by leaving the carbon nanotube dispersion in a thermostatic chamber at 25° C. for 1 hour or longer, then sufficiently stirring the carbon nanotube dispersion, and measuring the shear viscosity at 25° C., a shear rate of 1 s−1 and 10 s−1 using a rheometer with a cone of a diameter of 35 mm and 2°. When the measured value contains a decimal point, it is rounded to an integer according to the rule B of JIS 28401: 1999.
The cumulative particle size D10 of the carbon nanotube dispersion of the present embodiment measured by a dynamic light scattering method is preferably 200 nm or more and less than 500 nm, more preferably 200 nm or more and less than 400 nm, and still more preferably 300 nm or more and less than 400 nm. In addition, the cumulative particle size D50 of the carbon nanotube dispersion measured by a dynamic light scattering method is preferably 500 nm or more and less than 3,000 nm, more preferably 500 nm or more and less than 2,000 nm, and still more preferably 500 nm or more and less than 1,500 nm. The cumulative particle sizes D10 and D50 of the carbon nanotube dispersion can be measured using a particle size distribution meter (Nanotrac UPA, model UPA-EX, commercially available from MicrotracBel Corp.). The particle size measured by a dynamic light scattering method has a correlation with the fiber length of the carbon nanotubes, and the carbon nanotube dispersion having a cumulative particle size D10 within the above range has a favorable dispersion state of the carbon nanotubes in the dispersion.
In order to obtain the carbon nanotube dispersion of the present embodiment, it is preferable to perform a treatment of dispersing carbon nanotubes in a solvent. The dispersing device used for such a treatment is not particularly limited.
As the dispersing device, a disperser that is generally used for pigment dispersion or the like can be used. Examples thereof include mixers such as a disper, a homomixer, and a planetary mixer, media type dispersers such as homogenizers (Advanced Digital Sonifer (registered trademark), MODEL 450DA, commercially available from BRANSON, “ClearMix,” commercially available from M Technique Co., Ltd., “Filmix” and the like, commercially available from PRIMIX, “Abramix” and the like, commercially available from Silverson Co., Ltd.), paint conditioners (commercially available from Red Devil), Colloid Mill (“PUC Colloid Mill,” commercially available from PUC, “Colloid Mill MK,” commercially available from IKA), Cone Mill (“Cone Mill MKO” and the like, commercially available from IKA), ball mill, sand mill (“Dyno-Mill,” commercially available from Shinmaru Enterprises Corporation), attritor, pearl mill (“DCP mill” and the like, commercially available from Nippon Eirich Co., Ltd.), and coball mill, medialess dispersers such as wet jet mills (“Genas PY,” commercially available from Genas, “Starburst,” commercially available from Sugino Machine Ltd., and “Nanomizer” and the like, commercially available from Nanomizer Inc.), “Clear SS-5,” commercially available from M Technique Co., Ltd. and MICROS,” commercially available from Nara Machinery Co., Ltd., and other roll mills, but the present invention is not limited thereto.
Thea amount of carbon nanotubes in the carbon nanotube dispersion of the present embodiment with respect to 100 parts by mass of the carbon nanotube dispersion is preferably 0.2 parts by mass to 1.5 parts by mass, more preferably 0.4 parts by mass to 1.2 parts by mass, and still more preferably 0.4 parts by mass to 1.0 part by mass.
The amount of the dispersing agent used in the carbon nanotube dispersion of the present embodiment with respect to 100 parts by mass of the carbon nanotubes is preferably 30 parts by mass to 250 parts by mass use, more preferably 50 parts by mass to 150 parts by mass use, and still more preferably 50 parts by mass to 100 parts by mass.
The pH of the carbon nanotube dispersion of the present embodiment is preferably 6 to 11, more preferably 7 to 11, still more preferably 8 to 11, and particularly preferably 9 to 11. The pH of the carbon nanotube dispersion can be measured using a pH meter (pH METER F-52, commercially available from HORIBA, Ltd.).
The binder is a resin that binds substances such as carbon nanotubes.
Examples of binders of the present embodiment include polymers or copolymers containing ethylene, propylene, vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid ester, methacrylic acid, methacrylic acid ester, acrylonitrile, styrene, vinyl butyral, vinyl acetal, vinylpyrrolidone or the like as a constituent unit; polyurethane resins, polyester resins, phenol resins, epoxy resins, phenoxy resins, urea resins, melamine resins, alkyd resins, acrylic resins, formaldehyde resins, silicone resins, and fluorine resins; cellulose resins such as carboxymethyl cellulose; rubbers such as styrene butadiene rubber and fluorine rubber; and conductive resins such as polyaniline and polyacetylene. In addition, modified products, mixtures, and copolymers of these resins may be used. Among these, polyvinylidene fluoride, polyvinyl fluoride, tetrafluoroethylene, carboxymethyl cellulose, styrene butadiene rubber, and polyacrylic acid are preferable.
Carboxymethyl cellulose as a binder resin preferably has a high viscosity, and for example, when a 1% aqueous solution is produced, the viscosity is preferably 500 to 6,000 m P·as, and more preferably 1,000 to 3,000 m P·as. The viscosity of a 1% aqueous solution containing carboxymethyl cellulose can be measured under a condition at 25° C. using a B type viscometer rotor at a rotation speed of 60 rpm.
Carboxymethyl cellulose as a binder resin preferably has a high degree of etherification. For example, the degree of etherification is preferably 0.6 to 1.5, more preferably 0.6 to 1.2, and still more preferably 0.8 to 1.2.
When the mass of the active material is set as 100 mass %, the amount of the binder in the mixture slurry of the present embodiment is preferably 0.5 to 30 mass %, more preferably 1 to 25 mass %, and particularly preferably 2 to 20 mass %. In addition, the type and amount ratio of the binders are appropriately selected according to properties of coexisting substances such as carbon nanotubes and active materials. For example, in the mixture slurry, regarding the amount of carboxymethyl cellulose used as a binder, when the mass of the active material is set as 100 mass %, the proportion of carboxymethyl cellulose is preferably 0.5 to 3.0 mass %, and more preferably 1.0 to 2.0 mass %.
Styrene butadiene rubber that is generally used as a binding material for an electrode can be used as long as it is an oil-in-water emulsion. In the mixture slurry, regarding the amount of styrene butadiene rubber used as a binder, when the mass of the active material is set as 100 mass %, the proportion of styrene butadiene rubber is preferably 0.5 to 3.0 mass % and more preferably 1.0 to 2.0 mass %.
In the mixture slurry, regarding the amount of polyacrylic acid used as a binder, when the mass of the active material is set as 100 mass %, the proportion of polyacrylic acid is preferably 1 to 25 mass %, and more preferably 5 to 20 mass %.
In the mixture slurry, regarding the amount of polyvinylidene fluoride used as a binder, when the mass of the active material is set as 100 mass %, the proportion of polyacrylic acid is preferably 1 to 10 mass %, and more preferably 1 to 5 mass %.
The carbon nanotube resin composition of the present embodiment contains carbon nanotubes, a dispersing agent, a solvent and a binder.
In order to obtain the carbon nanotube resin composition of the present embodiment, it is preferable to mix the carbon nanotube dispersion and the binder and homogenize the mixture. As a mixing method, various conventionally known methods can be used. The carbon nanotube resin composition can be produced using the dispersing device described above for the carbon nanotube dispersion.
The mixture slurry of the present embodiment contains carbon nanotubes, a dispersing agent, a solvent, a binder and an active material.
The active material of the present embodiment is a material that serves as a basis for the battery reaction. Active materials are classified into a positive electrode active material and a negative electrode active material according to the electromotive force.
The positive electrode active material is not particularly limited, but metal compounds such as metal oxides and metal sulfides that can dope or intercalate lithium ions, conductive polymers and the like can be used. Examples thereof include oxides of transition metals such as Fe, Co, Ni, and Mn, composite oxides with lithium, and inorganic compounds such as transition metal sulfides. Specific examples thereof include transition metal oxide powders such as MnO, V2O5, V6O13, and TiO2, composite oxide powders of lithium such as layered-structure lithium nickelate, lithium cobaltate, lithium manganate, and spinel-structure lithium manganite and transition metals, and transition metal sulfide powders such as a lithium iron phosphate material which is an olivine-structure phosphate compound, TiS2, and FeS. In addition, conductive polymers such as polyaniline, polyacetylene, polypyrrole, and polythiophene can be used. In addition, mixtures of the above inorganic compounds or organic compounds may be used.
The negative electrode active material is not particularly limited as long as it can dope or intercalate lithium ions. Examples thereof include metal Li, and its alloys such as tin alloys, silicon alloys, and lead alloys, metal oxide materials such as LixFe2O3, LixFe3O4, LixWO2 (x is a number of 0<x<1), lithium titanate, lithium vanadate, and lithium silicate, conductive polymer materials such as polyacetylene and poly-p-phenylene, amorphous carbonaceous materials such as soft carbon and hard carbon, carbonaceous powders such as artificial graphite such as highly-graphitized carbon materials and natural graphite, and carbon-based materials such as carbon black, mesophase carbon black, resin-baked carbon materials, air-grown carbon fibers, and carbon fibers. These negative electrode active materials can be used alone or a plurality thereof can be used in combination.
A silicon-based negative electrode active material is preferable as the negative electrode active material of the present embodiment. Specifically, a negative electrode active material containing silicon such as a silicon alloy and a lithium silicate is preferable.
As the silicon-based negative electrode active material, examples thereof include so-called metallurgical grade silicon produced by reducing silicon dioxide with carbon, industrial grade silicon obtained by reducing impurities from metallurgical grade silicon by an acid treatment, unidirectional coagulation or the like, high-purity silicon with different crystalline states such as high-purity single crystal, polycrystalline, and amorphous states, produced from silane obtained by reacting silicon, and silicone obtained by making industrial grade silicon have a high purity by a sputtering method or an electron beam vapor deposition (EB vapor deposition) method, and adjusting the crystalline state or precipitation state at the same time.
In addition, examples thereof include silicon oxide, which is a compound of silicon and oxygen, and silicon compounds containing silicon and various alloys and of which the crystalline state is adjusted by a rapid cooling method or the like. Among these, a silicon-based negative electrode active material having a structure of which the outside is coated with a carbon film and in which silicon nanoparticles are dispersed in silicon oxide is preferable.
For the negative electrode active material of the present embodiment, in addition to the silicon-based negative electrode active material, it is preferable to use amorphous carbonaceous materials such as soft carbon or hard carbon, artificial graphite such as highly-graphitized carbon materials, or carbon powder such as natural graphite. Among these, it is preferable to use carbon powder such as artificial graphite and natural graphite.
The amount of the silicon-based negative electrode active material is preferably 3 to 50 mass % and more preferably 5 to 25 mass % when the amount of carbon powder such as artificial graphite or natural graphite is set as 100 mass %.
The BET specific surface area of the active material of the present embodiment is preferably 0.1 to 10 m2/g, more preferably 0.2 to 5 m2/g, and still more preferably 0.3 to 3 m2/g.
The average particle size of the active material of the present embodiment is preferably in a range of 0.5 to 50 μm and more preferably 2 to 20 μm. The average particle size of the active material referred to in this specification is an average value of the particle sizes of the active material measured with an electron microscope.
The mixture slurry of the present embodiment can be produced by various conventionally known methods. For example, a production method of adding an active material to a carbon nanotube resin composition and a production method of adding an active material to a carbon nanotube dispersion and then adding a binder may be exemplified.
In order to obtain a mixture slurry of the present embodiment, it is preferable to add an active material to a carbon nanotube resin composition and then perform a treatment for dispersion. A dispersing device used for performing such a treatment is not particularly limited. For the mixture slurry, a mixture slurry can be obtained using the dispersing device described in the above carbon nanotube dispersion.
The amount of the active material in the mixture slurry of the present embodiment with respect to 100 parts by mass of the mixture slurry is preferably 20 to 85 parts by mass, more preferably 30 to 75 parts by mass, and still more preferably 40 to 70 parts by mass.
The amount of the carbon nanotubes in the mixture slurry of the present embodiment with respect to 100 parts by mass of the active material is preferably 0.01 to 10 parts by mass, more preferably 0.02 to 5 parts by mass, and still more preferably 0.03 to 1 part by mass.
The amount of the solid content in the mixture slurry of the present embodiment with respect to 100 mass % of the mixture slurry is preferably 30 to 90 mass %, more preferably 30 to 80 mass %, and still more preferably 40 to 75 mass %.
The electrode film of the present embodiment is formed by forming a mixture slurry. For example, it is a coating film in which an electrode mixture layer is formed by applying and drying a mixture slurry on a current collector.
The material and shape of the current collector used for the electrode film of the present embodiment are not particularly limited, and those suitable for various secondary batteries can be appropriately selected. For example, examples of materials of current collectors include metals such as aluminum, copper, nickel, titanium, and stainless steel, and alloys of these metals. In addition, regarding the shape, a foil on a flat plate is generally used, but a current collector with a roughened surface, a current collector having a perforated foil shape, and a current collector having a mesh shape can be used.
A method of applying a mixture slurry on the current collector is not particularly limited, and known methods can be used. Specific examples thereof include a die coating method, a dip coating method, a roll coating method, a doctor coating method, a knife coating method, a spray coating method, a gravure coating method, a screen printing method, and an electrostatic coating method. Regarding the drying method, standing drying, a fan dryer, a warm air dryer, an infrared heater, a far infrared heater and the like can be used, but the method is not particularly limited thereto.
In addition, after coating, rolling may be performed using a planographic press, a calendar roll or the like. The thickness of the electrode mixture layer is generally 1 μm or more and 500 μm or less, and preferably 10 μm or more and 300 μm or less.
The nonaqueous electrolyte secondary battery of the present embodiment includes a positive electrode, a negative electrode, and an electrolyte. At least one of the positive electrode and the negative electrode preferably includes the electrode film of the present embodiment.
Regarding the positive electrode, those obtained by applying and drying a mixture slurry containing a positive electrode active material on a current collector to produce an electrode film can be used.
Regarding the negative electrode, those obtained by applying and drying a mixture slurry containing a negative electrode active material on a current collector to produce an electrode film can be used.
Regarding the electrolyte, various conventionally known electrolytes in which ions can move can be used. Examples thereof include those containing lithium salts such as LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, Li(CF3SO2)3C, LiI, LiBr, LiCl, LiAlCl, LiHF2, LiSCN, and LiBPh4 (where, Ph is a phenyl group), but the present invention is not limited thereto, and those containing sodium salts or calcium salts can be used. The electrolyte is preferably dissolved in a nonaqueous solvent and used as an electrolytic solution.
The nonaqueous solvent is not particularly limited, and examples thereof include carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as γ-butyrolactone, γ-valerolactone, and γ-octanoic lactone; glymes such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-methoxyethane, 1,2-ethoxyethane and 1,2-dibutoxyethane; esters such as methylformate, methylacetate, and methylpropionate; sulfoxides such as dimethyl sulfoxide and sulfolane; and nitriles such as acetonitrile. These solvents may be used alone or two or more thereof may be used in combination.
The nonaqueous electrolyte secondary battery of the present embodiment preferably contains a separator. Examples of separators include a polyethylene non-woven fabric, a polypropylene non-woven fabric, a polyamide non-woven fabric and those obtained by subjecting them to a hydrophilic treatment, but the present invention is not particularly limited thereto.
The structure of the nonaqueous electrolyte secondary battery of the present embodiment is not particularly limited, but is generally composed of a positive electrode and a negative electrode, and a separator provided as necessary, and various shapes such as a paper shape, a cylindrical shape, a button shape, and a laminate shape can be used according to the purpose of use.
The present invention will be described below in more detail with reference to examples. The present invention is not limited to the following examples as long as other examples do not depart from the gist thereof. In examples, “carbon nanotube” may be abbreviated as “CNT.” Here, unless otherwise specified, “parts” represents “parts by mass,” and “%” represents “mass %.”
Physical properties of CNTs used in examples and comparative examples to be described below were measured by the following methods.
CNTs were placed on a Raman microscope (XploRA, commercially available from HORIBA, Ltd.) and measurement was performed using a laser wavelength of 532 nm. The measurement conditions were a fetching time of 60 seconds, a number of times of integrations of 2, a neutral density filter of 10%, an objective lens magnification of 20, a confocal hole of 500, a slit width of 100 μm, and a measurement wavelength of 100 to 3,000 cm−1. CNTs for measurement were dispensed on a slide glass, and flattened using a spatula. Among the obtained peaks, in the spectrum, the maximum peak intensity within a range of 1,560 to 1,600 cm−1 was G, the maximum peak intensity within a range of 1,310 to 1,350 cm−1 was D, and the ratio of G/D was used as the G/D ratio of CNTs.
0.03 g of CNTs was weighed out using an electronic balance (MSA225S100DI, commercially available from Sartorius), and then dried at 110° C. for 15 minutes while degassing. Then, the BET specific surface area of CNTs was measured using a fully automatic specific surface area measuring device (HM-model1208, commercially available from MOUNTECH).
0.2 g of CNTs was weighed out in a 450 mL SM sample bottle (commercially available from Sansyo Co., Ltd.) using an electronic balance (MSA225S100DI, commercially available from Sartorius), 200 mL of toluene was added thereto, and the mixture was dispersed using an ultrasonic homogenizer (Advanced Digital Sonifer (registered trademark), MODEL 450DA, commercially available from BRANSON) at an amplitude of 50% for 5 minutes under ice cooling to prepare a CNT dispersion. Then, the CNT dispersion was appropriately diluted, several μL thereof was added dropwise into a form of a collodion film and dried at room temperature, and observation was then performed using a direct transmission electron microscope (H-7650, commercially available from Hitachi, Ltd.). Observation was performed at a magnification of 50,000, a plurality of images including 10 or more CNTs within the field of view were captured, the outer diameters of 300 randomly extracted CNTs were measured, and the average value thereof was used as the average outer diameter (nm) of the CNTs.
Using a powder resistivity measuring device (Loresta GP powder resistivity measurement system MCP-PD-51, commercially available from Mitsubishi Chemical Analytech Co., Ltd.), 1.2 g of a sample mass was used, and using a powder probe unit (four-point probe/ring electrode, electrode interval of 5.0 mm, electrode radius of 1.0 mm, sample radius of 12.5 mm), an applied voltage limiter was set to 90 V, and the volume resistivity [Ω·cm] of the conductive powder was measured under various pressures. The value of the volume resistivity of CNTs at a density of 1 g/cm3 was evaluated.
The CNT dispersion was left in a thermostatic chamber at 25° C. for 1 hour or longer, the CNT dispersion was then sufficiently stirred and diluted, and the cumulative particle sizes D10 and D50 of the CNT dispersion were measured using a particle size distribution meter (Nanotrac UPA, model UPA-EX, commercially available from MicrotracBel Corp.). The permeability was absorption, the CNT density was 1.8, and the shape was non-spherical. The refractive index of the solvent was 1.333. Measurement was performed by diluting the concentration of the CNT dispersion so that the numerical value of the loading index was in a range of 0.8 to 1.2.
The complex elastic modulus and phase angle of the CNT dispersion were evaluated by using a rheometer (RheoStress1 rotational rheometer, commercially available from Thermo Fisher Scientific) with a cone of a diameter of 35 mm and 2°, and measuring the dynamic viscoelasticity at 25° C. and a frequency of 1 Hz with a strain rate in the range of 0.01% to 5%.
The CNT dispersion was left in a thermostatic chamber at 25° C. for 1 hour or longer, the CNT dispersion was then sufficiently stirred, and the viscosity was evaluated by measuring the shear viscosity at 25° C. and a shear rate of 1 s−1 and 10 s−1 using a rheometer (RheoStress1 rotational rheometer, commercially available from Thermo Fisher Scientific) with a cone of a diameter of 35 mm and 2°.
A negative electrode mixture slurry was applied onto a copper foil using an applicator so that the basis weight per unit of the electrode was 8 mg/cm2 and the coating film was then dried in an electric oven at 120° C.±5° C. for 25 minutes. Then, the film was cut into two 90 mm×20 mm rectangles with the coating direction as the major axis. The peeling strength was measured using a desktop tensile tester (Strograph E3, commercially available from Toyo Seiki Co., Ltd.) and evaluated according to the 180 degree peeling test method. Specifically, a double-sided tape with a size of 100 mm×30 mm (No. 5,000NS, commercially available from Nitoms Inc.) was attached to a stainless steel plate, the produced battery electrode mixture layer was brought into close contact with the other surface of the double-sided tape, peeling off was performed while pulling from the bottom to the top at a certain speed (50 mm/min), and the average value of stress at this time was used as the peeling strength.
A positive electrode mixture slurry was applied onto an aluminum foil using an applicator so that the basis weight per unit of the electrode was 20 mg/cm2 and the coating film was then dried in an electric oven at 120° C.±5° C. for 25 minutes. Then, the film was cut into two 90 mm×20 mm rectangles with the coating direction as the major axis. The peeling strength was measured using a desktop tensile tester (Strograph E3, commercially available from Toyo Seiki Co., Ltd.) and evaluated according to the 180 degree peeling test method. Specifically, a double-sided tape with a size of 100 mm×30 mm (No. 5,000NS, commercially available from Nitoms Inc.) was attached to a stainless steel plate, the produced battery electrode mixture layer was brought into close contact with the other surface of the double-sided tape, peeling off was performed while pulling from the bottom to the top at a certain speed (50 mm/min), and the average value of stress at this time was used as the peeling strength.
First, 93 parts by mass of the positive electrode active material (HED (registered trademark) NCM-111 1100, commercially available from BASF TODA Battery Materials LLC), 4 parts by mass of acetylene black (Denka Black (registered trademark) HS100, commercially available from Denka Co., Ltd.), and 3 parts by mass of PVDF (Kureha KF polymer W #1300, commercially available from Kureha Battery Materials Japan Co., Ltd.) were put into a plastic container having a volume of 150 cm3 and then mixed with a spatula until powder was uniform. Then, 20.5 parts by mass of NMP was added thereto, and the mixture was stirred at 2,000 rpm for 30 seconds using a rotation/revolution mixer (Thinky Mixer, ARE-310, commercially available from Thinky Corporation). Then, the mixture in the plastic container was mixed with a spatula until it was uniform, and stirred at 2,000 rpm for 30 seconds using the rotation/revolution mixer. In addition, 14.6 parts by mass of NMP was then added thereto, and the mixture was stirred at 2,000 rpm for 30 seconds using the rotation/revolution mixer. Finally, the sample was stirred at 3,000 rpm for 10 minutes using a high-speed stirrer to obtain a positive electrode mixture slurry. Then, the positive electrode mixture slurry was applied onto an aluminum foil having a thickness of 20 μm as a current collector using an applicator and then dried in an electric oven at 120° C.±5° C. for 25 minutes, and the basis weight per unit area of the electrode was adjusted to 20 mg/cm2. In addition, the sample was rolled by a roll press (3 t hydraulic roll press, commercially available from Thank-Metal Co., Ltd.) to produce a standard positive electrode having a mixture layer density of 3.1 g/cm3.
0.5 parts by mass of acetylene black (Denka Black (registered trademark) HS-100, commercially available from Denka Co., Ltd.), 1 part by mass of MAC500LC (carboxymethyl cellulose sodium salt Sunrose special type MAC500L, commercially available from Nippon Paper Industries Co., Ltd., non-volatile content of 100%), and 98.4 parts by mass of water were put into a plastic container having a volume of 150 ml, and then stirred using a rotation/revolution mixer (Thinky Mixer, ARE-310, commercially available from Thinky Corporation) at 2,000 rpm for 30 seconds. In addition, 87 parts by mass of artificial graphite (CGB-20, commercially available from Nippon Graphite Industries, Co., Ltd.) as an active material and 10 parts by mass of silicon were added, and the mixture was stirred using a high-speed stirrer at 3,000 rpm for 10 minutes. Subsequently, 3.1 parts by mass of SBR (TRD2001, commercially available from JSR) was added thereto, and the mixture was stirred at 2,000 rpm for 30 seconds using the rotation/revolution mixer to obtain a negative electrode mixture slurry. Then, the negative electrode mixture slurry was applied onto a copper foil using an applicator so that the basis weight per unit of the electrode was 8 mg/cm2 and the coating film was then dried in an electric oven at 120° C.±5° C. for 25 minutes. In addition, the sample was rolled by a roll press (3 t hydraulic roll press, commercially available from Thank-Metal Co., Ltd.) to produce a standard negative electrode having a mixture layer density of 1.7 g/cm3.
A laminate type lithium ion secondary battery was placed in a thermostatic chamber at 25° C., and charging and discharging measurement was performed using a charging and discharging device (SM-8, commercially available from Hokuto Denko Corporation). Constant current and constant voltage charging (a cutoff current of 1.1 mA (0.02 C)) was performed at a charging current of 11 mA (0.2 C) and a charge final voltage of 4.2 V, and constant current discharging was then performed at a discharging current of 11 mA (0.2 C) and a discharge final voltage of 2.5 V. This operation was repeated three times and constant current and constant voltage charging (cutoff current (1.1 mA 0.02 C)) was then performed at a charging current of 11 mA (0.2 C) and a charge final voltage of 4.2 V, and constant current discharging was performed at a discharging current of 0.2 C and 3 C until the discharge final voltage reached 2.5 V, and each discharging capacity was determined. The rate property can be expressed as a ratio between the 0.2 C discharging capacity and the 3 C discharging capacity according to the following Formula 1.
Rate property=3 C discharging capacity/3rd 0.2 C discharging capacity×100(%) (Formula 1)
The laminate type lithium ion secondary battery was placed in a thermostatic chamber at 25° C., and charging and discharging measurement was performed using a charging and discharging device (SM-8, commercially available from Hokuto Denko Corporation). Constant current and constant voltage charging (a cutoff current of 1.38 mA (0.025 C)) was performed at a charging current of 55 mA (1 C) and a charge final voltage of 4.2 V and constant current discharging was then performed at a discharging current of 55 mA (1 C) and a discharge final voltage of 2.5 V. This operation was repeated 200 times. 1 C was the current value at which the theoretical capacity of the positive electrode was discharged in 1 hour. The cycle property can be expressed as a ratio between the 3rd 1 C discharging capacity and the 200th 1 C discharging capacity at 25° C. according to the following Formula 2.
Cycle property=3rd 1 C discharging capacity/200th 1 C discharging capacity×100(%) (Formula 2)
<Synthesis of dispersing agent (A)>
100 parts of acetonitrile was put into a reaction container including a gas inlet pipe, a thermometer, a condenser, and a stirrer, and the inside was purged with nitrogen gas. The inside of the reaction container was heated to 70° C., and a mixture containing 85.0 parts of acrylonitrile, 15.0 parts of acrylic acid, and 5.0 parts of 2,2′-azobis(2,4-dimethylvaleronitrile) (V-65, commercially available from NOF Corporation) was added dropwise over 2 hours and a polymerization reaction was performed. After dropwise addition was completed, the reaction was additionally performed at 70° C. for 1 hour, 0.5 parts of perbutyl O was then added, and the reaction was additionally continued at 70° C. for 1 hour. Then, the non-volatile content was measured, and it was confirmed that the conversion ratio exceeded 98%, and the dispersion medium was completely removed by concentration under a reduced pressure to obtain a dispersing agent (A). The weight average molecular weight (Mw) of the dispersing agent (A) was 38,000.
The weight average molecular weight (Mw) of the produced dispersing agent (A) was measured through gel permeation chromatography (GPC) with an RI detector under the following conditions. The molecular weight was a pullulan conversion value.
2.0 g of carboxymethyl cellulose sodium salt and 100 mL of methanol nitrate were put into a 300 mL stoppered Erlenmeyer flask and shaken for 2 hours to replace sodium carboxymethylcellulose with carboxymethyl cellulose. Then, carboxymethyl cellulose was subjected to suction filtration using a glass filter and washed with 200 mL of 80% methanol. Then, it was replaced with 50 mL of anhydrous methanol and subjected to suction filtration and then dried at 105° C. for 2 hours. 1.0 to 1.5 g of dried carboxymethyl cellulose was weighed out, put into a 300 mL stoppered Erlenmeyer flask, and moistened with 15 mL of 80% methanol, 50 mL of 1/10 N sodium hydroxide was added thereto and the mixture was shaken for 2 hours. Then, using phenolphthalein as an indicator, excess sodium hydroxide was back-titrated with 1/10 N sulfuric acid, and the degree of etherification was calculated according to (Formula 3) and (Formula 4).
A=(50×F1−X×F2)/(Y×10) (Formula 3)
Degree of etherification=0.162A/(1−0.058A) (Formula 4)
Table 1 shows CNTs used in examples and comparative examples, the outer diameter of the CNTs, the specific surface area of the CNTs, the G/D ratio, and the volume resistivity.
Table 2 shows dispersing agents used in examples, comparative examples and reference examples.
98.25 parts of deionized water was put into a stainless steel container, 0.75 parts of the dispersing agent (A) was added while stirring was performed with a disper, and the mixture was stirred with a disper until it was uniform. Then, 1 part of the CNT (A) was weighed out and added while stirring was performed with a disper, a square hole high shear screen was installed in a high shear mixer (L5M-A, commercially available from SILVERSON), and batch type dispersion was performed at a rate of 8,600 rpm until the entire sample was made uniform. Subsequently, a liquid to be dispersed was supplied from the stainless steel container to a high pressure homogenizer (Star Burst Labo HJP-17007, commercially available from Sugino Machine Ltd.) through a pipe and a pass-type dispersion treatment was performed 5 times to obtain a CNT dispersion (WA1). The dispersion treatment was performed using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa.
CNT dispersions (WA2 to WF4) were obtained in the same method as in Example 1 except that the type of CNT, the amount of CNT added, the type of dispersing agent, the amount of dispersing agent added, the amount of deionized water added, and the number of passes were changed as listed in Table 3.
4 parts by mass of the CNT dispersion (WA1) produced in Example 1 and 6 parts by mass of deionized water were weighed out in a plastic container having a volume of 150 cm3. Then, the sample was stirred using a rotation/revolution mixer (Thinky Mixer, ARE-310, commercially available from Thinky Corporation) at 2,000 rpm for 30 seconds to obtain a CNT dispersion (WA13).
A CNT dispersion (WA14) was obtained in the same method as in Example 16 except that the CNT dispersion (WA3) produced in Example 3 was used.
A CNT dispersion (WA15) was obtained in the same method as in Example 16 except that the CNT dispersion (WA11) produced in Example 11 was used.
20 parts of the CNT (C), and 480 parts of zirconia beads with a diameter of 8 mm as pulverization media were put into a polypropylene bottle container, and a pulverization treatment was performed with a paint conditioner (commercially available from Red Devil) for 40 minutes. Then, the zirconia beads were separated to collect the CNT (C). Next, 98.38 parts of deionized water was added to a stainless steel container, and 1.13 parts of the dispersing agent (C) was added while stirring was performed with a disper, and the sample was stirred using a disper until it was uniform. Then, 1.5 parts of the collected CNT (C) was weighed out and added while stirring was performed with a disper, a square hole high shear screen was installed in a high shear mixer (L5M-A, commercially available from SILVERSON), and batch type dispersion was performed at a rate of 8,600 rpm until the entire sample was made uniform. Subsequently, a liquid to be dispersed was supplied from the stainless steel container to a high pressure homogenizer (Star Burst Labo HJP-17007, commercially available from Sugino Machine Ltd.) through a pipe, and a pass-type dispersion treatment was performed 20 times to obtain a CNT dispersion (WC27). The dispersion treatment was performed using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa.
98.40 parts of deionized water was put into a stainless steel container, 0.50 parts of the dispersing agent (C) and 0.10 parts of polyacrylic acid (commercially available from FUJIFILM Wako Pure Chemical Corporation, a molecular weight of 25,000) were added while stirring was performed with a disper, and the sample was stirred using a disper until it was uniform. Then, 1.0 part of the CNT (A) was weighed out and added while stirring was performed with a disper, a square hole high shear screen was installed in a high shear mixer (L5M-A, commercially available from SILVERSON), and batch type dispersion was performed at a rate of 8,600 rpm until the entire sample was made uniform. Subsequently, a liquid to be dispersed was supplied from the stainless steel container to a high pressure homogenizer (Star Burst Labo HJP-17007, commercially available from Sugino Machine Ltd.) through a pipe, and a pass-type dispersion treatment was performed 20 times to obtain a CNT dispersion (WA28). The dispersion treatment was performed using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa.
1 part of the CNT (A), 0.75 parts of the dispersing agent (A), 98.25 parts of deionized water and 120 parts of zirconia beads (a bead diameter of 1.25 mmφ) were put into a glass bottle (M-140, commercially available from Hakuyo Glass Co., Ltd.), a dispersion treatment was performed using a paint conditioner (commercially available from Red Devil) for 8 hours, and separation of the zirconia beads was then attempted, but the viscosity was high, and the CNT dispersion could not be obtained.
A dispersion treatment was performed in the same method as in Comparative Example 3 except that the amount of CNT added, the dispersion time, and the bead diameter were changed as listed in Table 3, the zirconia beads were then separated to obtain CNT dispersions (WA17 to WA19).
99.3 parts of NMP was put into a stainless steel container, 0.3 parts of the dispersing agent (E) was added while stirring was performed with a disper, and the mixture was stirred with a disper until the dispersing agent (E) was dissolved. Then, 0.4 parts of the CNT (A) was weighed out and added while stirring was performed with a disper, a square hole high shear screen was installed in a high shear mixer (L5M-A, commercially available from SILVERSON), and batch type dispersion was performed at a rate of 8,600 rpm until the entire sample was made uniform. Subsequently, a liquid to be dispersed was supplied from the stainless steel container to a high pressure homogenizer (Star Burst Labo HJP-17007, commercially available from Sugino Machine Ltd.) through a pipe, and a pass-type dispersion treatment was performed 20 times to obtain a CNT dispersion (A20). The dispersion treatment was performed using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa.
CNT dispersions (A21 to A23) were obtained in the same method as in Example 23 except that the number of passes was changed as shown in Table 4.
0.4 parts of the CNT (A), 0.3 parts of the dispersing agent (E), 99.3 parts of NMP and 120 parts of zirconia beads (a bead diameter of 1.25 mmφ) were put into a glass bottle (M-140, commercially available from Hakuyo Glass Co., Ltd.), a dispersion treatment was performed using a paint conditioner (commercially available from Red Devil) for 8 hours, and the zirconia beads were then separated to obtain a CNT dispersion (A24).
Table 5 shows the evaluation results of the CNT dispersions produced in Examples 1 to 26 and Comparative Examples 1 to 7. The phase angle of the CNT dispersion at 25° C. and a frequency of 1 Hz was evaluated as O (good) when it was 10 or more and less than 50, Δ (acceptable) when it was 5 or more and less than 10, and x (poor) when it was less than 5 or 50 or more. The complex elastic modulus of the CNT dispersion at 25° C. and a frequency of 1 Hz was evaluated as O (good) when it was 5 or more and less than 400, Δ (acceptable) when it was 400 or more and less than 650, and x (poor) when it was less than 5. The viscosity of the CNT dispersion was evaluated as ⊚ (very good) when the shear viscosity at a shear rate of 1 was 20 or more and less than 40, O (good) when the shear viscosity was 10 or more and less than 20 or 40 or more and less than 60, Δ (acceptable) when the shear viscosity was 5 or more and less than 10, and x (poor) when the shear viscosity was less than 5. The particle size of the CNT dispersion was evaluated as ⊚ (very good) when the particle size distribution in the particle size distribution D10 was 200 or more and less than 300, O (good) when the particle size distribution was 300 or more and less than 500, and x (poor) when the particle size distribution was less than 200.
0.63 parts by mass of the CNT dispersion (WA1), 12.5 parts by mass of an aqueous solution in which 2 mass % of CMC (#1190, commercially available from Daicel FineChem Co., Ltd.) was dissolved, and 13.8 parts by mass of deionized water were weighed out in a plastic container having a volume of 150 cm3. Then, the sample was stirred using a rotation/revolution mixer (Thinky Mixer, ARE-310, commercially available from Thinky Corporation) at 2,000 rpm for 30 seconds to obtain a CNT resin composition (WA1). Then, 2.92 parts by mass of silicon monoxide (SILICON MONOOXIDE, SiO 1.3 C 5 μm, commercially available from Osaka Titanium Technologies Co., Ltd.,) was added thereto, and the mixture was stirred at 2,000 rpm for 30 seconds using the rotation/revolution mixer. In addition, 21.44 parts by mass of artificial graphite (CGB-20, commercially available from Nippon Graphite Industries, Co., Ltd.) was added, and the mixture was stirred at 2,000 rpm for 30 seconds using the rotation/revolution mixer. In addition, 0.78 parts by mass of a styrene butadiene emulsion (TRD2001, commercially available from JSR Corporation) was then added, and the mixture was stirred at 2,000 rpm for 30 seconds using the rotation/revolution mixer to obtain a negative electrode mixture slurry (WA1).
CNT resin compositions (WA2 to WA19) and negative electrode mixture slurries (WA2 to WA19) were obtained in the same method as in Example 28 except that the CNT dispersion was changed as listed in Table 6, and the amounts of the CNT dispersion and deionized water added were adjusted so that the amount of the CNT in 100 parts by mass of the mixture slurry was 0.025 parts by mass. The non-volatile content of the negative electrode mixture slurry was set as 48 mass %.
7.0 parts by mass of NMP in which 8 mass % of PVDF (Solef #5130, commercially available from Solvey) was dissolved was weighed out in a plastic container having a volume of 150 cm3. Then, 0.19 parts by mass of the CNT dispersion (A20) was added, and the mixture was stirred at 2,000 rpm for 30 seconds using the rotation/revolution mixer (Thinky Mixer, ARE-310) to obtain a CNT resin composition (A20). In addition, 36.9 parts of the positive electrode active material (HED (registered trademark) NCM-111 1100, commercially available from BASF TODA Battery Materials LLC) was then added, and the mixture was stirred at 2,000 rpm for 2.5 minutes using the rotation/revolution mixer to obtain a positive electrode mixture slurry (A20).
CNT resin compositions (A21 to A24) and positive electrode mixture slurries (A21 to A24) were obtained in the same method as in Example 50 except that the CNT dispersion was changed as listed in Table 6.
The negative electrode mixture slurry (WA1) was applied onto a copper foil using an applicator so that the basis weight per unit of the electrode was 8 mg/cm2 and the coating film was then dried in an electric oven at 120° C.±5° C. for 25 minutes to obtain an electrode film (WA1).
Electrode films (WA2) to (WA19) were obtained in the same method as in Example 54 except that the negative electrode mixture slurry was changed as listed in Table 7.
The positive electrode mixture slurry (A20) was applied onto a copper foil using an applicator so that the basis weight per unit of the electrode was 20 mg/cm2 and the coating film was then dried in an electric oven at 120° C.±5° C. for 25 minutes to obtain an electrode film (A20).
Electrode films (A21) to (A24) were obtained in the same method as in Example 76 except that the positive electrode mixture slurry was changed as listed in Table 7.
Table 7 shows the evaluation results of the electrode films produced in Examples 54 to 79 and Comparative Examples 14 to 19. The adhesion was evaluated as ⊚ (very good) when the peeling strength (Ω·cm) was 0.5 or more, O (good) when the peeling strength (Ω·cm) was 0.3 or more and less than 0.5, Δ (acceptable) when the peeling strength (Ω·cm) was 0.1 or more and less than 0.3, and x (poor) when the peeling strength (Ω·cm) was less than 0.1.
The electrode films (WA1 to WA19) were rolled by a roll press (3 t hydraulic roll press, commercially available from Thank-Metal Co., Ltd.) to produce negative electrodes having a mixture layer density of 1.7 g/cm3.
The electrode films (A20 to A24) were rolled by a roll press (3 t hydraulic roll press, commercially available from Thank-Metal Co., Ltd.) to produce positive electrodes having a mixture layer density of 3.2 g/cm3.
Table 8 shows the negative electrodes and positive electrodes produced in Examples 80 to 105 and Comparative Examples 20 to 25.
The negative electrode (WA1) and the standard positive electrode were punch out into 50 mm×45 mm, and 45 mm×40 mm, respectively, and a separator (porous polypropylene film) inserted therebetween was inserted into an aluminum laminated bag and dried in an electric oven at 60° C. for 1 hour. Then, 2 mL of an electrolytic solution (a nonaqueous electrolytic solution obtained by preparing a mixed solvent obtained by mixing ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate at a ratio of 3:5:2 (volume ratio), additionally adding 1 part by mass of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) as additives with respect to 100 parts by mass of the mixed solvent and then dissolving LiPF6 at a concentration of 1 M) was injected into a glove box filled with argon gas, and the aluminum laminate was then sealed to produce a laminate type lithium ion secondary battery (WA1).
Laminate type lithium ion secondary batteries (WA2 to WA19) were produced in the same method except that the negative electrode was changed as listed in Table 9.
The standard negative electrode and the positive electrode (A20) were punch out into 50 mm×45 mm and 45 mm×40 mm, respectively, and a separator (porous polypropylene film) inserted therebetween was inserted into an aluminum laminated bag and dried in an electric oven at 60° C. for 1 hour. Then, 2 mL of an electrolytic solution (a nonaqueous electrolytic solution obtained by preparing a mixed solvent obtained by mixing ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate at a ratio of 3:5:2 (volume ratio), additionally adding 1 part by mass of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) as additives with respect to 100 parts by mass of the mixed solvent and then dissolving LiPF6 at a concentration of 1 M) was injected into a glove box filled with argon gas, and the aluminum laminate was then sealed to produce a laminate type lithium ion secondary battery (A20).
Laminate type lithium ion secondary batteries (A21 to A24) were produced in the same method except that the positive electrode was changed as listed in Table 9.
Table 10 shows the evaluation results of the laminate type lithium ion secondary batteries produced in Examples 106 to 131, and Comparative Examples 26 to 31. Regarding the rate property, the rate property was evaluated as @ (very good) when it was 80% or more, O (good) when it was 70% or more and less than 80%, Δ (acceptable) when it was 60% or more and less than 70%, and x (poor) when it was less than 60%. Regarding the cycle property, the cycle property was evaluated as @ (very good) when it was 90% or more, O (good) when it was 85% or more and less than 90%, Δ (acceptable) when it was 80% or more and less than 85%, and − (poor) when it was less than 80%.
In the above examples, the carbon nanotube dispersion containing carbon nanotubes, a dispersing agent, and a solvent, which is a carbon nanotube dispersion in which the G/D ratio of the carbon nanotubes was 5 to 100, and the content of the dispersing agent with respect to 100 parts by mass of the carbon nanotubes was 30 parts by mass or more and less than 250 parts by mass, and which is a carbon nanotube dispersion in which the complex elastic modulus of the carbon nanotube dispersion at 25° C. and a frequency of 1 Hz was 5 Pa or more and less than 650 Pa, and the phase angle was 5° or more and less than 50° was used. In the examples, the adhesion of the electrode tended to be improved as compared with the comparative examples. In addition, a lithium ion secondary battery having an excellent rate property and cycle property was obtained according to improved conductivity and electrode strength. Therefore, it can be clearly understood that the present invention can provide a lithium ion secondary battery having a high capacity, high output and high durability that cannot be realized with conventional carbon nanotube dispersions.
While the present invention has been described above with reference to the embodiments, the present invention is not limited to the above description. For the configuration and details of the present invention, various changes that can be understood by those skilled in the art can be made within the scope of the invention.
The disclosure of the present application relates to Japanese Patent Application No. 2020-171017, filed Oct. 9, 2020, and Japanese Patent Application No. 2021-160281, filed Sep. 30, 2021, the content of which is incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-171017 | Oct 2020 | JP | national |
| 2021-160281 | Sep 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/037078 | 10/7/2021 | WO |
