Carbon nanotube slurry and electrode slurry

Abstract

Provided is a carbon nanotube dispersion suitable for the production of an electrode for a high-performance lithium ion secondary battery or the like. A carbon nanotube slurry containing at least carbon nanotubes, a dispersant, and a non-aqueous solvent, the carbon nanotubes having a BET specific surface area of 70 m2 / g to 180 m2 / g and a peak intensity ratio (G / D) in Raman spectroscopy of 1.0 to 2.5. (In the Raman spectrum, when the maximum intensity of a G-band scattered light peak in the range of 1570 cm <-1 > to 1620 cm <-1 > is represented by G and the maximum intensity of a D-band scattered light peak in the range of 1320 cm <-1 > to 1370 cm <-1 > is represented by D, the peak intensity ratio G / D represents the ratio thereof.

Description

Technical Field

[0001] This invention relates to non-aqueous carbon nanotube dispersion slurries and electrode slurries. Background Technology

[0002] In recent years, the lithium-ion battery market has attracted attention with the popularization of electronic devices and environmentally friendly mobile tools. Conductive additives are used in lithium-ion batteries to reduce electrode resistance, but the development of dispersions of carbon nanotubes (hereinafter, sometimes simply referred to as CNTs) is underway, which can reduce resistance in small amounts compared to conventional carbon materials.

[0003] Patent document 1 discloses a slurry for forming electrodes in lithium-ion batteries, characterized in that it contains an electrode active material, a conductive additive, a binder and a polar solvent, and the average particle size of the conductive additive is less than 500 nm when it is dispersed.

[0004] Patent document 2 discloses a carbon nanotube dispersion, characterized in that it comprises carbon nanotubes (A), a solvent (B), and a dispersant (C). In powder X-ray diffraction analysis, carbon nanotubes (A) exhibit a peak at a diffraction angle of 2θ = 25° ± 2°, with a half-width of 2°–6°, and will appear in Raman spectra at 1560–1600 cm⁻¹. -1 The maximum peak intensity within the range is set as G, 1310–1350 cm⁻¹. -1 When the maximum peak intensity within the range is set to D, the G / D ratio is 0.5 to 5.0, and the dispersant (C) is a resin containing an ethylene alcohol skeleton.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2006-309958 (claims, embodiments, etc.)

[0008] Patent Document 2: Japanese Patent Application Publication No. 2020-11873 (claims, embodiments, etc.) Summary of the Invention

[0009] The problem the invention aims to solve

[0010] When carbon nanotube slurry is used to form electrodes for lithium-ion secondary batteries, it is typically coated onto a current collector such as aluminum foil and then dried. At this point, it is desirable to reduce the amount of solvent in the slurry; however, reducing the amount of solvent usually increases the viscosity of the carbon nanotube slurry, making uniform coating difficult.

[0011] Furthermore, the typical procedure for dispersing carbon nanotubes is to first mix the powdered carbon nanotubes into a solvent to achieve uniform premixing (hereinafter referred to as "premixing"), and then disperse the premixed product using a disperser. In this case, depending on the carbon nanotubes used, the carbon nanotube slurry becomes very viscous, which sometimes makes premixing difficult, and consequently, makes feeding the disperser difficult, leading to excessive load on the disperser in the early stages of dispersion, and thus sometimes making the disperser difficult to operate.

[0012] This invention was made to overcome the shortcomings of the aforementioned carbon nanotube slurries, and its objective is to provide a slurry that is easily prepared at low viscosity. Furthermore, this invention aims to provide a carbon nanotube slurry suitable for manufacturing electrodes for high-efficiency lithium-ion secondary batteries, etc.

[0013] Solution for solving the problem

[0014] The inventors conducted in-depth research on the aforementioned prior art and found that carbon nanotube slurries with specific properties are suitable for solving the problem. Based on this insight, the present invention was completed.

[0015] That is, the present invention is a carbon nanotube slurry, which contains at least carbon nanotubes, a dispersant, and a non-aqueous solvent, wherein the aforementioned carbon nanotubes have a diameter of 70 μm. 2 / g~180m 2 The BET specific surface area is / g, and the peak intensity ratio G / D in Raman spectroscopy is 1.0 to 2.5.

[0016] (Among them, the one located at 1570 cm⁻¹ in the Raman spectrum) -1 ~1620cm -1 The maximum intensity of the G-band scattering peak within the range is set as G, at 1320 cm⁻¹. -1 ~1370cm -1 When the maximum intensity of the D-band scattered light peak within the range is defined as D, the aforementioned peak intensity ratio G / D represents this ratio.

[0017] Furthermore, the present invention preferably uses a carbon nanotube slurry in which the content of the aforementioned carbon nanotubes is 0.1% to 3.0% by mass relative to the total amount of carbon nanotube slurry.

[0018] The effects of the invention

[0019] The carbon nanotube slurry of the present invention is a slurry in which carbon nanotube material is uniformly dispersed and has low viscosity. Therefore, premixing is easy, and the feeding to the disperser and the dispersion operation in the disperser can be carried out without applying a load. Thus, the carbon nanotube slurry of the present invention is easy to prepare.

[0020] Furthermore, electrodes made from electrode pastes containing the carbon nanotube paste of the present invention exhibit very good conductivity. Therefore, the carbon nanotube paste of the present invention can be used to manufacture electrode pastes for lithium secondary batteries. Detailed Implementation

[0021] Carbon nanotubes

[0022] The carbon nanotubes used in this invention are cylindrical materials with a diameter of nanometers, composed solely of carbon. In addition to single-layer carbon nanotubes, multilayer carbon nanotubes are also included, consisting of two or more carbon nanotubes of different diameters stacked together.

[0023] The carbon nanotubes used in this invention have a BET specific surface area of ​​70 m². 2 / g or more and less than 180m 2 / g, preferably exceeding 100m 2 / g and less than 170m 2 / g, more preferably exceeding 110m 2 / g and less than 160m 2 / g. The BET specific surface area of ​​carbon nanotubes can be measured using a specific surface area measuring device.

[0024] If carbon nanotubes with a BET specific surface area within the aforementioned range are used, the amount of solvent encapsulated by the carbon nanotubes during premixing is small, and the swelling of the carbon nanotubes is suppressed. Therefore, it is easy to add solvent to the carbon nanotubes for premixing. Consequently, the premixed slurry is less affected by the fiber length of the carbon nanotubes and has a low viscosity, thus improving the liquid delivery to the disperser. The load on the disperser is also small in the initial stage of dispersion, and the operation of the disperser does not become difficult.

[0025] On the other hand, when carbon nanotubes with a BET specific surface area exceeding the aforementioned range are premixed, premixing can sometimes become difficult because the solvent is encapsulated by the carbon nanotubes or because the carbon nanotube slurry becomes very viscous. Consequently, feeding the liquid to the disperser can sometimes become difficult, and excessive load can be applied to the disperser in the early stages of dispersion, making the disperser difficult to operate.

[0026] One characteristic of the carbon nanotubes used in this invention is their small BET specific surface area. The mechanism by which the carbon nanotubes used in this invention achieve their effects is hypothesized as follows: Carbon nanotubes typically form bundles in a powder state. When these bundles have gaps or are non-uniform and sparse, the BET specific surface area increases. The carbon nanotubes used in this invention, with their small BET specific surface area, have very few gaps within the bundles and are in a uniform and dense state, thus suppressing swelling caused by solvent intrusion and inhibiting the increase in slurry viscosity.

[0027] The carbon nanotubes used in this invention are those with a peak intensity ratio (G / D) of 1.0–2.5 in Raman spectroscopy. Here, the peak intensity at 1570 cm⁻¹ will be considered. -1 ~1620cm -1 The maximum intensity of the G-band scattering peak within the range is set as G, at 1320 cm⁻¹. -1 ~1370cm -1 When the maximum intensity of the D-band scattered light peak within the range is defined as D, the aforementioned peak intensity ratio G / D represents this ratio. Raman spectra can be determined by detecting Raman scattered light using a Raman spectrometer.

[0028] The aforementioned peak intensity ratio G / D is related to the crystallinity of the carbonaceous material. If the crystallinity of the carbonaceous material is too high, the carbon edges are reduced due to the developed graphite structure, resulting in fewer coordination sites for the electrolyte and problems such as decreased performance at low temperatures or increased resistance. Conversely, if the crystallinity of the carbonaceous material is too low, there is more amorphous material, leading to higher resistance and reduced utilization efficiency of the electric double layer at the electrolyte-electrode interface. Using carbon nanotubes with a G / D value within the aforementioned range offers the following advantages: electrodes with fewer defects, higher tolerance to high voltage driving, and higher long-term conductivity can be obtained during electrode fabrication.

[0029] The carbon nanotubes used in this invention preferably have a fiber length of 30 μm to 1000 μm and a fiber width of 3 nm to 20 nm, on average. The fiber length and fiber width of the carbon nanotubes can be measured using an electron microscope.

[0030] If carbon nanotubes with fiber length and fiber width within the aforementioned range are used, the carbon nanotubes, being in a slender fibrous state, have the advantage of good entanglement with the active material, thereby enabling the production of electrodes with low resistance.

[0031] On the other hand, if carbon nanotubes with shorter fiber lengths are used, the conductive path becomes easier to cut during electrode fabrication. If carbon nanotubes with fiber lengths exceeding the aforementioned range are used, premixing the carbon nanotube slurry can become difficult due to fiber entanglement or the slurry becoming very viscous. Consequently, feeding the slurry to the disperser can become difficult, leading to excessive load on the disperser during the initial stages of dispersion and hindering its operation. Alternatively, the slurry may become too viscous, resulting in the same adverse effects.

[0032] Furthermore, the carbon nanotubes used in this invention preferably exhibit a peak at a diffraction angle of 2θ = 25° ± 2° in powder X-ray diffraction, with a half-width of 1.3° to 2.9°. Powder X-ray diffraction can be performed using an X-ray diffraction apparatus.

[0033] The half-width at half-maximum (WWHM) of the 2θ peak, detected by powder X-ray diffraction analysis, is an indicator of the layer composition of carbon nanotubes. A smaller WWHM generally indicates a higher number of layers in the multilayered carbon nanotubes, while a larger WWHM indicates fewer layers. Using carbon nanotubes with WWHMs within the aforementioned range offers the advantage of balancing conductivity and dispersibility.

[0034] The carbon nanotubes used in this invention are manufactured, for example, by a flow method in which a feed gas is introduced into a high-temperature chamber containing a powdered catalyst, and the nanotubes are grown on the catalyst surface in a flowing state. Alternatively, they can be manufactured by a substrate method in which a feed gas is introduced into a high-temperature chamber containing a substrate that has been placed at rest, and the nanotubes are chemically grown on the substrate. Carbon nanotubes manufactured by the above methods or other methods can be used as long as they have the properties specified in this invention.

[0035] The carbon nanotubes used in this invention are preferably treated with one or a combination of pulverization, grading, and demetallization. These operations allow for the adjustment of various properties of the carbon nanotubes.

[0036] The pulverization process involves breaking carbon nanotubes into appropriate sizes. Examples of applicable pulverization methods include dry pulverization using pin mills, pulverizers, hammer mills, spray mills, ball mills, Henschel mixers, or grinding mills; and wet pulverization using ultrasonic dispersers, dispersers, homogenizers, rotary mixers, planetary mixers, high-pressure homogenizers, coating conditioners, colloid mills, bead mills, cone mills, wet spray mills, or thin-film rotary high-speed mixers.

[0037] Grading is a process of adjusting the size of carbon nanotubes. Grading can be implemented in dry or wet processes using devices that utilize gravity, inertia, or centrifugal force, or devices that utilize filters.

[0038] Carbon nanotubes sometimes contain heavy metals such as Fe, Co, and Ni (Groups VIII, VIIA, and VIA) from their manufacturing process. Demetallization is the process of removing heavy metals from carbon nanotubes to adjust their content. Suitable demetallization methods include one or more combinations of acid treatment, alkali treatment, or calcination in an inactive atmosphere. The amount of heavy metals in carbon nanotubes can be determined by extracting them with an acidic aqueous solution after calcination and using an ICP emission spectrometer.

[0039] The total content of heavy metals in the carbon nanotubes after the demetallization operation is preferably less than 5000 ppm, more preferably less than 3000 ppm, and even more preferably less than 1000 ppm.

[0040] When the total content of heavy metals in carbon nanotubes exceeds 5000 ppm, the heavy metals dissolve from the electrode into the electrolyte in the secondary battery, reducing the charge-discharge cycle characteristics and making it impossible to achieve a long battery life. Therefore, this method is not preferred.

[0041] <Carbon Nanotube Slurry>

[0042] The carbon nanotube slurry of the present invention is prepared by mixing and homogenizing at least carbon nanotubes having the aforementioned properties, a dispersant, and a non-aqueous solvent.

[0043] In the carbon nanotube slurry, the content of carbon nanotubes relative to the total amount of carbon nanotube slurry is preferably 0.1% to 3.0% by mass, more preferably 0.2% to 1.5% by mass, and even more preferably 0.5% to 1.0% by mass. If the content of carbon nanotubes is within this range, the slurry can be uniformly coated on the current collector, and the performance of the electrode for secondary batteries made from the slurry can be ensured.

[0044] In the carbon nanotube slurry of the present invention, the LD viscosity (X) is preferably in a specific relationship with the d90 particle size distribution (Y) of the carbon nanotubes.

[0045] Here, the LD viscosity (X) is measured using an E-type viscometer at a shear rate of 38.3 s⁻¹. -1 The viscosity of a 0.4% by mass carbon nanotube non-aqueous solvent slurry was measured at a sample temperature of 25°C. When the carbon nanotube concentration in the dispersed carbon nanotube slurry is higher than 0.4% by mass, the slurry is diluted with the same solvent (containing dispersant, etc.) used during dispersion to bring the carbon nanotube concentration to 0.4% by mass, and the viscosity is then measured. When the carbon nanotube concentration in the carbon nanotube slurry is lower than 0.4% by mass, the viscosity is obtained by evaporating the solvent from the slurry to a concentration of 0.4% by mass.

[0046] In addition, in carbon nanotube slurry, the particle size distribution d90(Y) of carbon nanotubes is the particle size value (μm) corresponding to the 90% cumulative distribution percentage of the carbon nanotube particle size measured using a laser diffraction / scattering particle size distribution measuring device.

[0047] One of the features of the carbon nanotube slurry of the present invention is that the product of the aforementioned X and Y is preferably less than 300,000 [(mPa·s)·μm], more preferably less than 150,000 [(mPa·s)·μm].

[0048] The inventors have discovered that when using carbon nanotubes with a viscosity X equal to or less than 300,000 for the product of the aforementioned viscosity X and 90% particle size Y, the surface resistance of the electrode fabricated using an electrode slurry containing such carbon nanotubes becomes lower. This is a useful insight for fabricating electrodes with low surface resistance. By using an electrode slurry containing carbon nanotubes having the aforementioned properties, the degree of contact and entanglement between the carbon nanotubes in the fabricated electrode is optimized, thereby presumably increasing the electrode's conductivity.

[0049] The carbon nanotube slurry of the present invention uses a substantially anhydrous non-aqueous solvent. The acceptable water content in the non-aqueous solvent is preferably less than 5000 mg / L, more preferably less than 3000 mg / L, and even more preferably less than 1000 mg / L. If the water content is within this range, the flowability of the carbon nanotube slurry can be ensured. The water content in the carbon nanotube slurry can be determined using a Karl Fischer moisture meter (electroelectric titration).

[0050] The amount of non-aqueous solvent mixed in the carbon nanotube slurry of the present invention is 88.0 to 99.9% by mass of the total slurry, preferably 95.0 to 99.8% by mass.

[0051] Examples of usable non-aqueous solvents include: methanol, ethanol, isopropanol, butanol, ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol dimethyl ether, dipropylene glycol methyl n-propyl ether, diethylene glycol diethyl ether, propylene glycol monomethyl ether, methyl monoglycidyl ether, ethyl monoglycidyl ether, butyl monoglycidyl ether, phenyl monoglycidyl ether, methyl diglycidyl ether, ethyl diglycidyl ether, butyl diglycidyl ether, phenyl diglycidyl ether, methylphenol monoglycidyl ether, ethylphenol monoglycidyl ether, butylphenol monoglycidyl ether, butylphenyl ether, dioxane, dioxolane, anisole, ethyl benzyl ether, tolyl methyl ether, diphenyl ether, dibenzyl ether, phenethyl ether.

[0052] Ethylene glycol monoacetate, ethylene glycol monomethyl ether acetate, propylene glycol diacetate, dipropylene glycol methyl ether acetate, 1,4-butanediol diacetate, 1,3-butanediol diacetate, 1,6-hexanediol diacetate, propylene glycol diacetate, ethylene glycol monoethyl ether acetate, diethylene glycol monoacetate, propylene glycol monoacetate, dipropylene glycol monoacetate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate

[0053] Methyl acetate, ethyl acetate, propyl acetate, butyl acetate, octyl acetate, cyclohexyl acetate, methyl pyruvate, ethyl pyruvate, methyl methoxypropionate, ethyl ethoxypropionate, methyl butyrate, ethyl butyrate, propyl butyrate, butyl butyrate, hexyl butyrate, methyl lactate, ethyl lactate, γ-butyrolactone, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, 4-vinylpyridine

[0054] Methyl methacrylate, 2-hydroxyethyl acrylate, tetrahydrofurfuryl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, glycidyl methacrylate, neopentyl glycol diacrylate, hexanediol diacrylate, trimethylolpropane triacrylate, acetone, methyl ethyl ketone, methyl n-pentyl ketone, methyl isobutyl ketone, methyl isopentyl ketone, 2-heptanone, cycloheptanone, cyclohexanone

[0055] Hexane, cyclohexane, methylcyclohexane, ethylcyclohexane, heptane, octane, benzene, toluene, xylene, isopropyltoluene, mesitylene, styrene, ethylbenzene, diethylbenzene, isopropylbenzene, pentylbenzene, mineral oil, perfluorocarbon, hydrofluoroether, hydrochlorofluorocarbon, hydrofluorocarbon, perfluoropolyether, various silicone oils, various cycloalkane solvents, various alkanes solvents, and various isoalkane solvents. One solvent selected from the group consisting of the above non-aqueous solvents, or a mixture containing two or more solvents, may be used.

[0056] Among these non-aqueous solvents, dioxolane, butyl butyrate, N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, methyl ethyl ketone, cyclohexanone, and isoparaffin solvents are preferred, with N-methyl-2-pyrrolidone (NMP) being particularly preferred.

[0057] The dispersant plays a role in ensuring that carbon nanotubes are well dispersed in the solvent, resulting in a stable slurry, and is preferably a polymer.

[0058] In this invention, preferred dispersants include polyvinylpyrrolidone, polyvinyl butyral, acrylate polymers, styrene-acrylate copolymers, polyvinylidene fluoride, and hydrogenated NBR.

[0059] As dispersants, polyvinylpyrrolidone (PVP) includes PVP K-12 (manufactured by Ashland Corporation), PVP K-30, 85, and 90 (manufactured by Nippon Shokubai Corporation). Polyvinyl butyral (PVB) includes BL-1, BL-S, and BL-5Z (manufactured by Sekisui Chemicals Co., Ltd.).

[0060] The content of the dispersant in the carbon nanotube slurry of the present invention is preferably 3 to 300 parts by weight relative to 100 parts by weight of carbon nanotube material, more preferably 5 to 200 parts by weight, further preferably 8 to 150 parts by weight, and particularly preferably 10 to 100 parts by weight. If the amount of dispersant is within this range, the dispersibility of carbon nanotubes in the slurry can be utilized, and the performance of the electrode for secondary batteries made from the slurry can be ensured.

[0061] In the carbon nanotube slurry of the present invention, in addition to the aforementioned carbon nanotubes, dispersant, and non-aqueous solvent, conductive materials other than carbon nanotubes may also be incorporated. By incorporating conductive materials, the conductivity of electrodes for secondary batteries made from the slurry can be improved.

[0062] The content of the conductive material in the mixture is preferably 0.5 to 10% by mass relative to the total amount of carbon nanotube slurry, more preferably 0.5 to 7% by mass, and particularly preferably 0.5 to 5% by mass.

[0063] The conductive materials that can be used include conductive carbon particles composed of graphite-type carbonaceous materials and conductive carbon fibers.

[0064] Examples of usable conductive carbon particles include acetylene black and Ketjen black. Examples of conductive carbon fibers include carbon nanofibers other than carbon nanotubes.

[0065] The specific gravity difference between conductive carbon particles and the aforementioned carbon nanotube materials is ±0.2 g / cm³. 3 When the particle size is within a certain range, separation due to gravity differences is less likely to occur when it is stored as a carbon nanotube slurry or electrode slurry, therefore it is preferred. Furthermore, from the perspective of conductivity and stability, it is more preferable that the primary particle size of the conductive material particles is 1 to 70 nm, preferably 1 to 50 nm, and even more preferably 1 to 40 nm. The primary particle size of the conductive carbon particles refers to the arithmetic mean of the outer diameters of a sufficient number n, measured using an electron microscope image.

[0066] From the perspective of conductivity and stability, the fiber width of conductive carbon fiber is preferably 1–500 nm, more preferably 1–400 nm, and even more preferably 1–200 nm. Particularly preferred are 1 nm or more and 90 nm or less, 3 nm or more and 30 nm or less, and even more preferably 3 nm or more and 15 nm or less. The fiber width of conductive carbon fiber refers to the arithmetic mean of a sufficient number of fiber widths (n) measured using an electron microscope image.

[0067] The carbon nanotube slurry of the present invention can be manufactured by a dispersion step involving at least the addition of the aforementioned carbon nanotubes, dispersant, and non-aqueous solvent to a mixer for stirring / mixing. Other components, such as conductive materials, can be added to the slurry during the aforementioned dispersion step, or they can be added separately from the aforementioned dispersion step.

[0068] The dispersion process in the manufacturing of carbon nanotube slurry can be carried out by using various dispersion devices, such as ultrasonic dispersers, dispersers, homogenizers, rotary mixers, Henschel mixers, planetary mixers, coating conditioners, colloid mills, bead mills, ball mills, sand mills, grinding mills, pearl mills, coball mills, (high pressure) homogenizers, wet spray mills, wet cavitation mills, thin film rotary high-speed mixers, cone mills, and other media-free dispersers, as well as roller mills.

[0069] From the perspectives of dispersion stability and dispersion efficiency, the preferred dispersion devices are (high pressure) homogenizers, wet cavitation mills, and bead mills.

[0070] The carbon nanotube slurry of this invention is a slurry with excellent flowability. Specifically, it can achieve a shear rate of 38.3 s⁻¹. -1 The viscosity at 25°C is preferably 5–500,000 mPa·s, more preferably 10–10,000 mPa·s, even more preferably 20–1,000 mPa·s, and even more preferably 30–700 mPa·s. The viscosity of the carbon nanotube slurry can be measured using an E-type rotational viscometer (manufactured by Toki Sangyo Co., Ltd., TV-22 type).

[0071] <Electrode Paste>

[0072] The carbon nanotube slurry of the present invention, by adding active substances, is suitable for use as an electrode slurry for manufacturing electrodes of secondary batteries such as lithium-ion batteries.

[0073] That is, one way to utilize the carbon nanotube slurry of the present invention is as an electrode slurry containing the aforementioned carbon nanotubes, dispersant, non-aqueous solvent and active material.

[0074] The electrode paste of the present invention is particularly suitable for manufacturing positive electrode.

[0075] That is, one aspect of the present invention is a slurry for making a positive electrode comprising a carbon nanotube slurry having the aforementioned properties and at least a positive electrode active material.

[0076] The cathode slurry of the present invention is prepared by mixing at least a cathode active material into the carbon nanotube slurry as described above. The cathode active material can be a substance that facilitates the reversible entry and exit of lithium ions into the cathode of a lithium-ion secondary battery.

[0077] Examples of active materials used in positive electrodes include lithium-nickel composite oxides, lithium-cobalt composite oxides, lithium-manganese composite oxides, lithium-nickel-manganese composite oxides, lithium-nickel-cobalt composite oxides, lithium-nickel-aluminum composite oxides, lithium-nickel-cobalt-aluminum composite oxides, lithium-nickel-manganese-cobalt composite oxides, lithium-nickel-manganese-aluminum composite oxides, lithium-nickel-cobalt-manganese-aluminum composite oxides, lithium-nickel-cobalt-manganese-aluminum composite oxides, lithium-nickel-cobalt-manganese-aluminum composite oxides, lithium-nickel-cobalt-manganese-aluminum composite oxides, and other lithium-transition metal composite oxides; transition metal sulfides such as TiS2, FeS, and MoS2; and MnO, V2O5, and V6O. 13 Transition metal oxides such as TiO2 and olivine-type lithium phosphorus oxides.

[0078] Olivine-type lithium phosphorus oxides are compounds comprising at least one element from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, Nb, and Fe, lithium, phosphorus, and oxygen. Olivine-type lithium phosphorus oxides can also be compounds in which some of the aforementioned elements are replaced with other elements to improve their properties.

[0079] The preferred active material for the positive electrode is a lithium-nickel composite oxide, and more preferably, it is of the formula: LiNi X M1 Y M2 Z O2 (M1 and M2 are at least one of the following metallic elements: Al, B, alkali metals, alkaline earth metals, and transition metals; 0.8≤X≤1.0, 0≤Y≤0.2, 0≤Z≤0.2) indicates a lithium-nickel composite oxide or lithium phosphate.

[0080] These positive electrode active materials can be used alone or in combination of two or more.

[0081] In the positive electrode slurry of the present invention, the content of the above-mentioned positive electrode active material is preferably 50 to 70% by mass relative to the total amount of the positive electrode slurry, and more preferably 50 to 63% by mass. If the content of the positive electrode active material in the positive electrode slurry is within this range, the performance of the manufactured electrode can be ensured, and the fluidity of the slurry can be maintained.

[0082] In addition, the content of carbon nanotube material in the positive electrode slurry is preferably 0.05 to 5 parts by mass relative to 100 parts by mass of the positive electrode active material, more preferably 0.05 to 3 parts by mass, and even more preferably 0.05 to 1 part by mass.

[0083] The positive electrode slurry of the present invention comprises the carbon nanotube slurry and the positive electrode active material as described above, and may contain solid electrolytes such as sulfide solid electrolyte, oxide solid electrolyte, dry polymer electrolyte, gel polymer electrolyte, and quasi-solid electrolyte as needed.

[0084] <Adhesive Materials>

[0085] In addition to carbon nanotube slurry and active material, the above-mentioned electrode slurry preferably also includes a binder.

[0086] Examples of usable adhesive materials include polyimide resins, polyvinylidene fluoride (PVdF), polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, tetrafluoroethylene-perfluorovinyl ether copolymers, and other fluoropolymers; polyolefin resins such as polyethylene and polypropylene; polyvinylpyrrolidone; polyvinyl alcohol; styrene-butadiene rubber (SBR); and acrylic resins. Combinations of two or more adhesive materials are also permitted.

[0087] The amount of binder material added is preferably 0.05 to 5 parts by mass relative to 100 parts by mass of active material in the electrode slurry for secondary batteries, more preferably 0.1 to 4.5 parts by mass, and even more preferably 0.5 to 4.5 parts by mass. When the amount of binder material added is within this range, it will not adversely affect the battery capacity or charge / discharge characteristics, and electrodes with high adhesion to the current collector can be obtained.

[0088] From the perspective of needing appropriate viscosity when coating the electrode paste onto the current collector, the amount of solvent contained in the electrode paste is preferably 0.5 to 80% by mass, more preferably 1 to 70% by mass, relative to the total amount of the electrode paste.

[0089] In addition to carbon nanotube slurry and active material, the electrode slurry can also be appropriately mixed with leveling agents, solid electrolytes, preservatives, etc.

[0090] Electrode slurries can be prepared by appropriately adding carbon nanotube slurry, active materials for the positive or negative electrode of a secondary battery, and, as needed, binder, non-aqueous solvent, and other components, and then mixing them. The mixing process can be performed, for example, using a twin-screw mixer.

[0091] By coating the electrode slurry of the present invention onto the current collector, which serves as a conductive component in a secondary battery such as a lithium-ion secondary battery, and then drying it, an electrode is manufactured as a positive or negative electrode. The resulting positive or negative electrode, when used as an electrode in a secondary battery, achieves high output battery performance and can withstand long-term repeated charge and discharge cycles.

[0092] That is, by using the electrode slurry of the present invention, it is possible to obtain a secondary battery electrode with high output and the ability to withstand long-term repeated charge and discharge.

[0093] The carbon nanotube slurry of the present invention is a slurry that uniformly disperses carbon nanotubes and the like and has low viscosity, thus enabling the uniform coating of carbon nanotubes and the like on current collectors at a high concentration.

[0094] Electrodes can be fabricated from the electrode slurry of the present invention as described below.

[0095] First, an electrode paste is applied to the current collector. The current collector is a conductive component that forms the electrode substrate of a secondary battery, such as a lithium-ion secondary battery. There are no particular limitations on the material and shape of the current collector used as the electrode substrate; a material and shape suitable for the secondary battery being used can be appropriately selected. Examples of materials for the current collector include metals and / or alloys such as aluminum, copper, nickel, titanium, or stainless steel. Furthermore, while planar metal foil is commonly used for the shape of the current collector, surface-roughened foil, perforated foil, and mesh foil can also be used.

[0096] Methods for coating electrode paste onto current collectors include molding, dip coating, roller coating, doctor coating, knife coating, spraying, gravure coating, screen printing, and electrostatic coating. Additionally, surface smoothing treatment using methods such as lithography or calendering rollers can be performed after coating.

[0097] Next, the current collector coated with the above-mentioned electrode paste is dried. Thus, an electrode with an electrode film formed on the current collector is produced.

[0098] As a method for drying the coated electrode with slurry, natural drying, blower dryer, hot air dryer, infrared heater, far-infrared heater, etc. can be used.

[0099] The thickness of the fabricated electrode, including the thickness of the current collector, is typically 1 μm or more and 500 μm or less, preferably 10 μm or more and 300 μm or less.

[0100] Electrodes made from electrode pastes using the carbon nanotube paste of the present invention have low resistance and are suitable for use as electrodes in lithium-ion secondary batteries, etc.

[0101] Lithium-ion secondary batteries

[0102] The following describes a lithium-ion secondary battery constructed using an electrode made from the electrode slurry of the present invention.

[0103] Secondary batteries typically consist of a positive electrode, a negative electrode, an electrolyte, a non-aqueous solvent, and separators as needed. They can be manufactured in various shapes, such as cylindrical, prismatic, gum-shaped, coin-shaped, button-shaped, needle-shaped, and paper-shaped, depending on their intended use. The positive or negative electrode of a lithium-ion secondary battery can be an electrode made by coating it with the aforementioned electrode paste.

[0104] Electrolytes can be Li salts in which ions can migrate. Examples include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, Li(CF3SO2)3C, LiI, LiBr, LiCl, LiAlCl, LiHF2, LiSCN, and LiBPh4 (where Ph represents phenyl).

[0105] Non-aqueous solvents include carbonates such as ethylene carbonate, propylene carbonate, butyl carbonate, dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate; lactones such as γ-butyrolactone, γ-valerolactone, and γ-octyl lactone; ethers 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 methyl formate, methyl acetate, and methyl propionate; sulfoxides such as dimethyl sulfoxide and sulfolane; and nitriles such as acetonitrile, which are aprotic polar solvents. These solvents can be used alone or in combination of two or more.

[0106] Examples of separators include polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and separators made by hydrophilic treatment of these materials.

[0107] Example

[0108] Examples of embodiments of the present invention are illustrated in the examples. It should be noted that the properties of the carbon nanotubes (CNTs) used in the preparation of the carbon nanotube slurries in the following examples and comparative examples were determined by the following methods. The results of the determination of the properties of the carbon nanotubes are shown in Table 1.

[0109] (Properties of carbon nanotubes)

[0110] [BET specific surface area]

[0111] Carbon nanotubes were collected, weighed precisely using an electronic balance, and dried at 110°C for 30 minutes while being degassed. The BET specific surface area was then determined using a fully automated specific surface area measuring device (Mountech Macsorb model HM-1208) via the BET one-point method.

[0112] [Raman spectroscopy]

[0113] Carbon nanotubes were collected, dispersed in N-methylpyrrolidone, and then coated onto a test piece using a spreader. After drying at 80°C, Raman spectra were measured using a Raman spectroscopy apparatus (Thermo Fisher Scientific, DXR2xi) over a 100 μm square region. The Raman spectra were measured in the range of 1560–1600 cm⁻¹. -1 The absorbance G at the peak position within the range of 1310–1350 cm⁻¹ -1 Calculate the G / D ratio by taking the absorbance D at the peak position within the range.

[0114] [Fiber length / fiber width]

[0115] Carbon nanotubes were collected and observed using a scanning electron microscope (SEM, Hitachi High-Tech Corporation, S-3400N). The arithmetic mean of the fiber lengths of 10 carbon nanotubes, measured using images at 1000x magnification, was calculated. Additionally, carbon nanotubes were observed using a transmission electron microscope (TEM, Hitachi High-Tech Corporation, H-7650). The arithmetic mean of the fiber widths of 10 carbon nanotubes, measured using images at 50,000x magnification, was calculated.

[0116] [Peak half-width]

[0117] Carbon nanotubes were collected and observed using a powder X-ray diffraction analysis system (Made by Rigaku Corporation, miniflex600) to determine the average half-width of the 2θ peak of 10 carbon nanotubes.

[0118] [Table 1]

[0119]

[0120] (Example 1)

[0121] <Preparation of Carbon Nanotube Slurry>

[0122] In a glass bottle (manufactured by Kashiwa Glass Co., Ltd., M-140), 0.4 parts by weight of commercially available carbon nanotubes A, 0.1 parts by weight of polyvinylpyrrolidone (manufactured by Nippon Shokubai Co., Ltd., K-30) as a dispersant, and 99.5 parts by weight of N-methylpyrrolidone as a solvent were added. The mixture was stirred for 1 hour using a disperser at a stirring speed (300 rpm) to avoid entrapment of bubbles (premixing) to obtain a premixed solution.

[0123] The flowability of the premixed carbon nanotube slurry was evaluated visually as follows.

[0124] A: The slurry flows well.

[0125] B: The slurry is difficult to flow.

[0126] C: The slurry does not flow.

[0127] Next, 300 parts by weight of zirconia microspheres (0.5 mm in diameter) were added, the glass bottle cap was tightened, and the mixture was vigorously shaken by hand to fuse the premix with the microspheres. The movement of the microspheres was then monitored, and the flowability of the slurry was evaluated. The slurry in the above state was then mechanically dispersed for 2 hours using a paint mixer (manufactured by SEIWA GIKEN CO., LTD.). The movement of the microspheres was then monitored again, and the aforementioned mechanical dispersion process was repeated, resulting in two more microsphere separation operations, yielding a carbon nanotube slurry (CNT dispersion) with a total of three mechanical dispersion treatments.

[0128] The flowability of carbon nanotube slurry after mechanical dispersion is evaluated visually, using the ease of movement of microspheres in the slurry as an indicator.

[0129] A: The microbeads move well.

[0130] B: Microbeads are difficult to move

[0131] C: Microbeads are very difficult to move.

[0132] D: Microspheres do not move

[0133] [LD viscosity of carbon nanotubes: X]

[0134] Using an E-type viscometer (TV-22 model, manufactured by Toki Sangyo Co., Ltd.), a 1°34' cone plate was subjected to a shear rate of 38.3 s⁻¹. -1 The sample was rotated, and the carbon nanotube slurry was measured at a sample temperature of 25°C. When the carbon nanotube concentration in the slurry was higher than 0.4% by mass, the slurry was diluted with a non-aqueous solvent containing the same concentration of the dispersant used in its preparation, so that the carbon nanotube concentration was 0.4% by mass, and the viscosity was measured as described above.

[0135] The measured viscosity value is expressed as LD viscosity (X).

[0136] [Particle size distribution d90 of carbon nanotubes: Y]

[0137] Carbon nanotube slurry obtained by mechanical dispersion was collected, and the particle size distribution of the carbon nanotubes was determined using a laser diffraction / scattering particle size distribution measuring device LA-960S. The particle size value (μm) corresponding to the 90% cumulative distribution percentage of the particle size distribution on a volume basis was calculated as the particle size distribution d90(Y).

[0138] Calculate the product X·Y[(mPa·s)·μm] of the aforementioned LD viscosity (X) and particle size distribution d90 (Y).

[0139] [Time-bound stability of carbon nanotube slurry]

[0140] Carbon nanotube slurry obtained through mechanical dispersion was collected and added to a capped glass bottle (manufactured by Kashiwagawa Glass Co., Ltd., M-140) at 100 parts by weight. The bottle was then stored at 50°C for 3 months. The state of the slurry in the glass bottle after storage was visually confirmed. The long-term stability of the slurry was evaluated according to the following criteria.

[0141] A: The slurry is uniform, and no separation was observed.

[0142] B: The slurry is slightly uneven, and a few carbon nanotubes are visible separated.

[0143] C: The slurry was uneven, and significant separation of carbon nanotubes was observed.

[0144] <Preparation of Electrode Paste>

[0145] In the carbon nanotube slurry prepared as described above, polyvinylidene fluoride (PVdF) (manufactured by Kishida Chemical Co., Ltd., GE51308, 8% NMP solution) and positive electrode active material (manufactured by Beijing DangSheng Materials Technology Co., Ltd., 5E-12D) as binders are mixed together using a mixer (manufactured by THINKYCORPORATION, Awatori Rentaro ARE-310) until homogeneous, to obtain the electrode (positive electrode) slurry.

[0146] <Electrode Fabrication>

[0147] Using a slurry applicator, the film thickness was set to 50 μm, and the prepared electrode was coated onto a soda-lime glass slide. The coated glass slide was then dried on a hot plate at 90°C for 10 minutes to fabricate a simulated electrode (positive electrode) for a lithium battery.

[0148] [Surface resistivity of the electrode]

[0149] The surface resistivity (Ω / □) of the obtained electrodes was measured using a resistivity meter (Mitsubishi Chemical Analytech Co., Ltd., Loresta GP, MCP-T610, four-probe probe, ASP tip spacing 5 mm).

[0150] (Examples 2-10, Comparative Examples 1-10)

[0151] Carbon nanotubes A were used directly in their A state or replaced with other grades of carbon nanotubes listed in Table 1. The number of mechanical dispersion treatments was increased or decreased, or the dispersant or solvent was changed, except that carbon nanotube slurries were prepared using the same method as in Example 1. The flowability and other properties of the obtained carbon nanotube slurries were evaluated using the same method as in Example 1.

[0152] Then, using the prepared carbon nanotube slurry, positive electrode materials were fabricated using the same method as in Example 1. The surface resistivity (Ω / □) of each electrode was measured using the same method as in Example 1.

[0153] The composition and evaluation results of the carbon nanotube slurries of Examples 1-10 and Comparative Examples 1-10 are shown in Table 2.

[0154] [Table 2]

[0155]

[0156] As shown in Table 2, the carbon nanotube slurries of Examples 1 to 10 within the scope of the present invention exhibit low viscosity and can be easily prepared.

[0157] In particular, it was confirmed that the carbon nanotube slurries of Examples 1 to 10 within the scope of the present invention exhibited good movement of microspheres after premixing and mechanical dispersion treatment using a paint mixer, resulting in carbon nanotube slurries with good operability.

[0158] As shown in Table 2, when electrodes are made using the electrode paste prepared from the carbon nanotube pastes of Examples 1 to 10 within the scope of the present invention, the coating operation is easy, and the electrodes made from the electrode pastes of Examples 1 to 10 exhibit low resistivity.

[0159] In contrast, Comparative Examples 1-6 used carbon nanotubes whose BET surface area or Raman spectroscopy peak intensity ratio (G / D) did not fall within the specific range of this invention. The viscosity of the carbon nanotube slurry was too high, making electrode fabrication difficult, or the fabricated electrode had high surface resistivity. Furthermore, Comparative Examples 7-10 used slurries containing carbon nanotubes whose product of LD viscosity (X) and d90 particle size distribution (Y) did not fall within the specific range of this invention (D-H). The fabricated electrodes had high surface resistivity.

[0160] Industrial availability

[0161] The carbon nanotube slurry of the present invention is suitable for use in the manufacture of electrodes for lithium secondary batteries.

Claims

1. A carbon nanotube slurry comprising at least carbon nanotubes, a dispersant, and a non-aqueous solvent, The carbon nanotube has a BET specific surface area of 70 m 2 / g ~ 180 m 2 / g, and a peak intensity ratio G / D in Raman spectroscopy of 1.0 ~ 2.5, wherein, The maximum intensity of the G-band scattering light peak in the Raman spectrum in the range of 1570 cm -1 ~ 1620 cm -1 is set as G, and the maximum intensity of the D-band scattering light peak in the range of 1320 cm -1 ~ 1370 cm -1 is set as D, the peak intensity ratio G / D is represented thereby.

2. The carbon nanotube paste of claim 1, wherein, the carbon nanotubes having a fiber length of 30 μm to 1000 μm and a fiber width of 3 nm to 20 nm.

3. The carbon nanotube paste of claim 1, wherein, the carbon nanotubes being contained in an amount of 0.1 mass% to 3.0 mass%.

4. The carbon nanotube paste of claim 1, wherein, The carbon nanotubes were dispersed in the non-aqueous solvent at 0.4 mass% and the dispersant at the same concentration as the carbon nanotube slurry to obtain a dispersion sample. The viscosity of the dispersion sample measured at 25°C and a shear rate of 38.3 s -1 -1 was set to X (mPa-s), and the d90 particle size distribution of the carbon nanotubes contained in the carbon nanotube slurry was set to Y (μm), the value of the product of X and Y was 300,000 [(mPa-s) μm] or less.

5. An electrode slurry comprising at least the carbon nanotube slurry according to any one of claims 1 to 4 and an active material.

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