Carbon nanotube slurry and electrode slurry

Abstract

Provided are: a carbon nanotube dispersion which is easy to produce; and a carbon nanotube dispersion which is suitable for producing 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 an 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. Wherein, when G is the maximum intensity of the G-band scattered light peak in the range of 1570-1620 cm <-1 > in the Raman spectrum, and D is the maximum intensity of the D-band scattered light peak in the range of 1320-1370 cm <-1 >, the peak intensity ratio G / D represents the ratio thereof.

Description

TECHNICAL FIELD

[0001] The present application relates to an electrode slurry suitable for the production of an electrode for a secondary battery such as a lithium ion secondary battery, and a carbon nanotube slurry used in the production thereof. BACKGROUND

[0002] With the popularization of electric vehicles, the miniaturization and high performance of portable devices such as mobile phones, notebook personal computers, and the like, secondary batteries having a high energy density, and the high output and long life of the secondary batteries are required. In this background, from the characteristics of high energy density and high voltage, non-aqueous secondary batteries including lithium ion batteries using non-aqueous electrolytes are used in many devices, and development is prevalent.

[0003] Patent Literature 1 discloses an electrode forming slurry for a lithium ion battery, characterized by containing an electrode active material, a conductive aid, a binder, and a polar solvent, and the average particle diameter at the time of dispersing the aforementioned conductive aid is 500 nm or less.

[0004] Patent Literature 2 discloses a carbon nanotube dispersion liquid, characterized by containing carbon nanotubes (A), a solvent (B), and a dispersant (C), the carbon nanotubes (A) have a peak at a diffraction angle 2θ = 25° ± 2° in powder X-ray diffraction analysis, the half-value width of the peak is 2° to 6°, and when the maximum peak intensity in the range of 1560 to 1600 cm -1 in Raman spectroscopy is set as G, and the maximum peak intensity in the range of 1310 to 1350 cm -1 is set as D, the G / D ratio is 0.5 to 5.0, and the dispersant (C) is a resin containing a vinyl alcohol skeleton.

[0005] Prior Art Documents

[0006] Patent Literature

[0007] Patent Literature 1: Japanese Patent Application Laid-Open No. 2006-309958 (Claims, Examples, etc.)

[0008] Patent Literature 2: Japanese Patent Application Laid-Open No. 2020-11873 (Claims, Examples, etc.) SUMMARY

[0009] Problems to be Solved by the Invention

[0010] When a carbon nanotube slurry is used for the formation of an electrode for a lithium ion secondary battery or the like, it is generally coated on a current collector such as an aluminum foil and dried. At this time, it is desirable to reduce the amount of solvent contained in the slurry, but if the amount of solvent is reduced, the viscosity of the carbon nanotube slurry generally becomes high, and thus uniform coating is difficult.

[0011] In addition, in dispersing carbon nanotubes, generally, a powder of carbon nanotubes is mixed in a solvent, and the mixture is uniformly premixed (hereinafter referred to as "premixing"), and then, the resulting premixed product is dispersed using a disperser. In this case, depending on the carbon nanotubes used, the carbon nanotube slurry becomes very high in viscosity, and thus, sometimes, the premixing becomes difficult, and further, the liquid feeding to the disperser becomes difficult, and at the initial stage of the dispersion, an excessive load is applied to the disperser, and thus, sometimes, the operation of the disperser becomes difficult.

[0012] The present application has been made to solve the above-described problems, and an object of the present application is to provide a carbon nanotube slurry which uniformly disperses carbon nanotube materials and is low in viscosity.

[0013] In addition, the carbon nanotube slurry is used for an electrode slurry, but it is required that an electrode produced from the electrode slurry has high conductivity.

[0014] Means for solving the problems

[0015] The present inventors have intensively studied the above-described conventional problems, and as a result, have found that a slurry containing carbon nanotubes (CNT) having specific properties is suitable, and based on this insight, the present application has been completed.

[0016] That is, the present application is a carbon nanotube slurry containing at least carbon nanotubes, a dispersant, and an aqueous solvent, the aforementioned carbon nanotubes having a BET specific surface area of 70 m 2 / g to 180 m 2 / g, and a peak intensity ratio G / D in Raman spectroscopy of 1.0 to 2.5.

[0017] (wherein, the maximum intensity of a G-band scattered light peak in Raman spectroscopy in the range of 1570 cm -1 to 1620 cm -1 is set as G, and the maximum intensity of a D-band scattered light peak in the range of 1320 cm -1 to 1370 cm -1 is set as D, and the aforementioned peak intensity ratio G / D represents the ratio thereof.)

[0018] In addition, the present application is preferably the aforementioned carbon nanotube slurry, wherein the viscosity of a slurry containing the aforementioned carbon nanotubes at 0.4 mass% at 25°C, 38.3 s -1 is set as A (mPa-s), and the particle size distribution d90 of the carbon nanotubes contained in the aforementioned carbon nanotube slurry is set as B (μm), and the value of A / B is 5.0 (mPa-s / μm) or more.

[0019] Effects of the Invention

[0020] The carbon nanotube slurry of the present application is a slurry in which carbon nanotube material is uniformly dispersed and which has a low viscosity. Therefore, premixing is easy, and liquid feeding to a disperser and dispersing operation in the disperser can be performed without applying a load. As described above, the carbon nanotube slurry of the present application is easy to prepare.

[0021] In particular, using the slurry for electrodes containing carbon nanotubes used in the present application, an electrode having a low resistance and very good electrical conductivity can be produced. As a result, the slurry for electrodes of the present application is good as a material for electrodes of lithium ion secondary batteries and the like. DETAILED DESCRIPTION

[0022] 〈Carbon nanotube material〉

[0023] The carbon nanotube used in the present application is a cylindrical substance having a diameter of nanometer size composed of only carbon. In addition to single-layer carbon nanotubes, multi-layer carbon nanotubes in which two or more carbon nanotubes having different diameters are overlapped in a layered manner are also included.

[0024] The BET specific surface area of the carbon nanotube used in the present application is 70 m 2 / g or more and less than 180 m 2 / g, preferably more than 100 m 2 / g and less than 170 m 2 / g, more preferably more than 110 m 2 / g and less than 160 m 2 / g. The BET specific surface area of the carbon nanotube can be measured using a specific surface area measuring device.

[0025] If a carbon nanotube having a BET specific surface area within the aforementioned range is used, since the BET specific surface area is relatively small, the amount of solvent wrapped by the carbon nanotube is small at the time of premixing, and swelling of the carbon nanotube is suppressed. Therefore, it is easy to add solvent to the carbon nanotube to perform premixing, and further, the slurry after premixing is not easily affected by the fiber length of the carbon nanotube and has a low viscosity, so liquid feeding to the disperser also becomes good, and the load on the disperser at the initial stage of dispersion is also small, and the operation of the disperser does not become difficult.

[0026] On the other hand, a carbon nanotube having a BET specific surface area exceeding the aforementioned range, when premixing is performed, since the solvent is wrapped by the carbon nanotube, or since the carbon nanotube slurry becomes a very high viscosity, sometimes premixing becomes difficult, and further, sometimes liquid feeding to the disperser becomes difficult, and the disperser becomes difficult to operate due to excessive load on the disperser at the initial stage of dispersion.

[0027] One of the characteristics of the carbon nanotube used in the present application is that the BET specific surface area is within a small range. The mechanism by which the carbon nanotube used in the present application exerts the effect of the present application is presumed as follows. That is, the carbon nanotube generally forms a state in which the tubes are bundled in a state of a bundle in the state of a powder. In the case where the bundle has a gap or is in a state of being uneven and sparse, the BET specific surface area becomes large. The carbon nanotube used in the present application, in which the BET specific surface area is within a small range, is in a state in which the gap in the bundle is very small and is uniform and dense, and thus the entry of the solvent into the resulting swelling can be suppressed, and the increase in the viscosity of the slurry can be suppressed.

[0028] The carbon nanotube used in the present application is a carbon nanotube in which the peak intensity ratio G / D in Raman spectroscopy is 1.0 to 2.5. Here, the maximum intensity of the G-band scattered light peak in the range of 1570 cm -1 to 1620 cm -1 in Raman spectroscopy is set as G, and the maximum intensity of the D-band scattered light peak in the range of 1320 cm -1 to 1370 cm -1 is set as D, and the aforementioned peak intensity ratio G / D represents the ratio thereof. The Raman spectroscopy can be measured by detecting Raman scattered light using a Raman spectroscopy device.

[0029] The 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 edge is reduced due to the development of the graphite structure, and the coordination sites of the electrolyte are reduced, and problems such as a decrease in the characteristics at low temperatures or an increase in the resistance occur. In addition, if the crystallinity of the carbonaceous material is too low, the amorphous phase is increased, the resistance is increased, and the utilization efficiency of the double electric layer of the interface between the electrolyte and the electrode material is decreased. If the carbon nanotube having a value of G / D within the aforementioned range is used, the following advantages are obtained: in the production of an electrode, an electrode having few defects, high resistance to high-voltage driving, and high long-term conductivity can be obtained.

[0030] The carbon nanotube used in the present application preferably has a fiber length of 30 μm to 1000 μm and a fiber width of 3 nm to 20 nm. The fiber length and the fiber width of the carbon nanotube can be measured using an electron microscope.

[0031] If the carbon nanotube having a fiber length and a fiber width within the aforementioned range is used, since the carbon nanotube is in a state of a fine and long fiber, the following advantages are obtained: the entanglement with the active material becomes good, and thus an electrode having low resistance can be obtained.

[0032] On the other hand, if a carbon nanotube having a fiber length shorter than the above range is used, the conductive path becomes easy to be cut off when the electrode is produced. If a carbon nanotube having a fiber length longer than the above range is used, when the pre-mixing of the carbon nanotube paste is performed, the pre-mixing becomes difficult because the fibers are entangled with each other or the carbon nanotube paste becomes very high in viscosity, and further, the liquid feeding to the disperser becomes difficult, the disperser is excessively loaded at the initial stage of the dispersion, and the operation of the disperser becomes difficult. Or, the same disadvantageous situation occurs because of the very high viscosity.

[0033] In addition, the carbon nanotube used in the present application preferably has a peak at a diffraction angle 2Θ = 25° ± 2° in powder X-ray diffraction method, and the peak half-value width is 1.3° to 2.9°. The powder X-ray diffraction method can be performed using an X-ray diffractometer.

[0034] The half-value width of the 2Θ peak detected by the powder X-ray diffraction analysis is an index of the layer constitution of the carbon nanotube. It is considered that the smaller the half-value width of the peak, the more the number of layers of the multi-walled carbon nanotube, and on the contrary, the larger the half-value width of the peak, the less the number of layers of the carbon nanotube. If a carbon nanotube having a peak half-value width within the above range is used, it has an advantage that the conductivity and the dispersibility can be taken into account.

[0035] The carbon nanotube used in the present application is manufactured, for example, by a flow method in which a raw material gas is introduced into a high-temperature chamber into which a powdered catalyst is charged, and grown on the surface of the catalyst in a flow state. Or, by a substrate method in which a raw material gas is introduced into a high-temperature chamber in which a substrate with a catalyst is left, and chemically grown on the substrate. As long as it is a carbon nanotube having the properties prescribed in the present application, a carbon nanotube manufactured by the above method or other methods can be used.

[0036] The carbon nanotube used in the present application is preferably subjected to one or a combination of a plurality of treatment operations of a pulverization operation, a classification operation, and a demetallization operation. By these operations, various properties of the carbon nanotube can be adjusted.

[0037] The pulverization operation is an operation of pulverizing the carbon nanotube into an appropriate size. As the pulverization operation that can be applied, there can be listed a dry pulverization using a pin mill, a pulverizer, a hammer mill, a jet mill, a ball mill, a Henschel mixer, or a grinder, etc.; a wet pulverization using an ultrasonic disperser, a disperser, a homomixer, a regular-rotary mixer, a planetary mixer, a high-pressure homogenizer, a paint conditioner, a colloid mill, a bead mill, a conical mill, a wet-type jet mill, or a thin-film rotary type high-speed mixer, etc.

[0038] The classification operation is an operation for adjusting the size of the carbon nanotubes. As the classification operation that can be applied, a device that utilizes gravity, inertial force, or centrifugal force, or a device that utilizes a filter can be implemented in a dry method or a wet method.

[0039] The carbon nanotubes sometimes contain Fe, Co, Ni, or the like, Group VIII, Group VIIA, Group VIA, or the like, heavy metals derived from the manufacturing process thereof. The demetallization operation is an operation for adjusting the amount of the heavy metals contained in the carbon nanotubes. As the demetallization operation that can be applied, one or a combination of two or more of acid treatment, alkali treatment, or calcination treatment in a non-active atmosphere can be implemented. The amount of the heavy metals contained in the carbon nanotubes can be measured using an ICP emission spectrometry device after calcination of the carbon nanotubes, with extraction using an acidic aqueous solution.

[0040] The total amount of the heavy metals contained in the carbon nanotubes after the demetallization operation is preferably less than 5000 ppm, more preferably less than 3000 ppm, and further preferably less than 1000 ppm.

[0041] In a case where the total amount of the heavy metals contained in the carbon nanotubes is 5000 ppm or more, in a secondary battery produced using the carbon nanotubes, the heavy metals are dissolved from the electrode into the electrolyte, which reduces the cycle characteristics of charge and discharge, and the long life of the battery cannot be achieved, and thus is not preferable.

[0042] In the carbon nanotube slurry, the content of the carbon nanotubes is preferably 0.1 to 3.0 mass%, more preferably 0.2 to 1.5 mass%, and further preferably 0.4 to 1.0 mass%, with respect to the total amount of the carbon nanotube slurry. If the content of the carbon nanotubes is within this range, the slurry can be uniformly applied to the current collector, and the performance of the electrode for a secondary battery produced using the slurry can be ensured.

[0043] 〈Carbon nanotube slurry〉

[0044] The carbon nanotube slurry of the present application is produced by mixing and homogenizing at least the carbon nanotubes having the aforementioned properties, the dispersant, and the water-based solvent.

[0045] The amount of the water-based solvent compounded in the carbon nanotube slurry is 88.0 to 99.9 mass%, and preferably 95.0 to 99.8 mass%, with respect to the total amount of the slurry.

[0046] The carbon nanotube slurry of the present application is a slurry in which the carbon nanotubes are uniformly dispersed in the water-based solvent and have low viscosity. The viscosity of the carbon nanotube slurry of the present application is preferably 5 to 500000 mPa-s, more preferably 10 to 10000 mPa-s, further preferably 20 to 1000 mPa-s, and further preferably 30 to 700 mPa-s at 25°C and a shear rate of 38.3 s -1 -1.

[0047] In the present application, the viscosity of the aforementioned carbon nanotube slurry at 0.4 mass% of the aforementioned carbon nanotube at 25°C, 38.3 s -1 -1 is set to A (mPa-s), and the particle size distribution d90 of the carbon nanotube contained in the aforementioned carbon nanotube slurry is set to B (μm), the value of A / B is 5.0 (mPa-s / μm) or more.

[0048] Here, the aforementioned slurry viscosity (A) refers to the viscosity value of the carbon nanotube aqueous slurry at a concentration of 0.4 mass% measured using an E-type viscometer at a shear rate of 38.3 s -1 -1 and a sample temperature of 25°C. In the case where the carbon nanotube concentration in the carbon nanotube slurry in which carbon nanotubes are dispersed is higher than 0.4 mass%, the viscosity is measured after diluting the slurry with the aqueous solvent used at the time of preparation to make the carbon nanotube concentration 0.4 mass%. In the case where the carbon nanotube concentration in the carbon nanotube slurry is lower than 0.4 mass%, the viscosity is measured after volatilizing the solvent from the slurry to a concentration of 0.4 mass%. In the present application, this measured value is referred to as the LD viscosity (mPa-s) of the carbon nanotube.

[0049] In addition, the aforementioned particle size distribution d90 (B) of the carbon nanotube is the value of the particle size (μm) corresponding to the 90% cumulative distribution percentage of the particle size of the carbon nanotube measured using a laser diffraction / scattering type particle size distribution measuring device.

[0050] The present inventors found that when the ratio of the aforementioned slurry viscosity (A) of the carbon nanotube slurry to the particle size distribution d90 (B) is 5.0 or more, the surface resistance of the electrode produced using the slurry for electrodes containing the carbon nanotube slurry is small. This is a useful insight in producing an electrode with small surface resistance. The mechanism of this phenomenon is not clear, but it is presumed that the degree of entanglement of the carbon nanotube with the active material becomes high, the electrical conductivity becomes high, and the value of the surface resistivity becomes low.

[0051] As the aqueous solvent used in the carbon nanotube slurry of the present application, water (for example, ion exchange water, distilled water, tap water), or a mixed solvent containing an aqueous solvent and a water-soluble solvent can be used.

[0052] As the water-soluble solvent which can be used as the mixed solvent of the water-based system, ethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 2,3-butanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 1,5-pentanediol, 2,5-hexanediol, 3-methyl-1,3-butanediol, 2-methylpentane-2,4-diol, 3-methylpentane-1,3,5-triol, 1,2,3-hexanetriol, and the like alkylene glycols, polyethylene glycol, polypropylene glycol, and the like polyalkylene glycols, glycerol, diglycerol, triglycerol, and the like glycerols,

[0053] glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol mono-n-butyl ether, and the like lower alkyl ethers of diols, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, N,N-dimethylacetamide, N,N-dimethylformamide, and the like amides, acetone, and the like ketones.

[0054] The content of the water-soluble solvent which is compounded in the mixed solvent of the water-based system is preferably 0.1 to 30% by mass, and further preferably 1 to 20% by mass, relative to the total amount of the mixed solvent. In this range, good flowability of the carbon nanotube paste can be obtained.

[0055] The dispersant is a polymer which allows the carbon nanotubes to be well dispersed in the solvent, thereby bringing about a stable paste.

[0056] As the preferred dispersant in the carbon nanotube paste of the present application, polyvinylpyrrolidone, carboxymethyl cellulose or its salt (alkali metal salt of Li, Na, K, and the like), cellulose nanofiber, modified polyvinyl butyral, and an acrylic acid salt polymer can be exemplified.

[0057] The content of the dispersant in the carbon nanotube paste of the present application is preferably 3 to 300 parts by mass, more preferably 5 to 200 parts by mass, further preferably 8 to 150 parts by mass, and particularly preferably 10 to 100 parts by mass, relative to 100 parts by mass of the carbon nanotube material. If the compounding amount of the dispersant is in this range, the dispersibility of the carbon nanotubes in the paste can be improved, and the performance of the electrode for secondary batteries produced from the paste can be ensured.

[0058] In the carbon nanotube paste of the present application, in addition to the above-mentioned carbon nanotubes, dispersant, and water-based solvent, a conductive material other than the carbon nanotubes can be compounded. By compounding the conductive material, the conductivity of the electrode for secondary batteries produced from the paste can be improved.

[0059] The content of the compounded conductive material is preferably 0.5 to 10% by mass, further preferably 0.5 to 7% by mass, and particularly preferably 0.5 to 5% by mass, relative to the total amount of the carbon nanotube paste.

[0060] The conductive material that can be used includes conductive carbon particles composed of carbonaceous substances of graphite type and conductive carbon fibers.

[0061] As the conductive carbon particles that can be used, carbon black particles such as acetylene black and Ketjen black can be listed. As the conductive carbon fibers, carbon nanofibers other than carbon nanotubes can be listed.

[0062] When the specific gravity difference of the conductive carbon particles from the aforementioned carbon nanotube material is within the range of ±0.2 g / cm 3 When the specific gravity difference of the conductive carbon particles from the aforementioned carbon nanotube material is within the range of ±0.2 g / cm

[0063] From the aspects of conductivity and stability, the fiber width of the conductive carbon fibers is preferably 1 to 500 nm, more preferably 1 to 400 nm, and further preferably 1 to 200 nm. Particularly preferably, it is 1 nm to 90 nm, 3 nm to 30 nm, and further 3 nm to 15 nm. The fiber width of the conductive carbon fibers refers to the arithmetic average of the fiber widths of a sufficient number of n measured using an image of an electron microscope.

[0064] The carbon nanotube paste of the present application can be manufactured by at least a dispersion process in which the aforementioned carbon nanotube, dispersant, and water-based solvent are put into a mixing machine and stirred / mixed. Other ingredients such as the conductive material can be added to prepare the paste in the aforementioned dispersion process, or can be separately added to the paste.

[0065] The dispersion process in the manufacture of the carbon nanotube paste can be performed by, for example, dispersing treatment using a disperser such as an ultrasonic disperser, disperser, homomixer, revolution and rotation mixer, Henschel mixer, planetary mixer, and the like, a media-type disperser such as a paint conditioner, colloid mill, bead mill, ball mill, sand mill, attritor, pearl mill, coball mill, and the like, a (high pressure) homogenizer, wet-type jet mill, wet-type cavitation mill, thin-film rotary high-speed mixer, conical mill, and the like, and other roll mill.

[0066] From the aspects of stability of the dispersion action and dispersion efficiency, the preferred dispersing device is a (high pressure) homogenizer, wet-type cavitation mill, and bead mill.

[0067] The carbon nanotube paste of the present application is a paste having excellent fluidity. Specifically, the viscosity value at 25°C under a shear rate of 38.3 s -1 -1 is preferably 5 to 500,000 mPa-s, more preferably 10 to 10,000 mPa-s, further preferably 20 to 1,000 mPa-s, and further preferably 30 to 700 mPa-s.

[0068] In the production of the carbon nanotube paste of the present application, the viscosity of the paste can be adjusted to the above range by appropriately selecting the dispersion conditions (dispersion time, dispersion intensity) or the kind or concentration of the dispersant. Note that the viscosity value of the carbon nanotube paste can be measured using an E-type rotational viscometer (manufactured by Tokimec, Inc., Model TV-22).

[0069]

[0070] The carbon nanotube paste of the present application can be used as an electrode paste for producing an electrode for a secondary battery such as a lithium ion battery by adding an active material.

[0071] That is, one of the present application is an electrode paste containing the aforementioned carbon nanotube, dispersant, water-based solvent, and active material.

[0072] The electrode paste of the present application can be used for the production of a negative electrode, and can also be used for the production of a positive electrode, and is particularly suitable for the production of a negative electrode.

[0073] That is, one of the present application is a negative electrode paste containing the aforementioned carbon nanotube, dispersant, water-based solvent, and negative electrode active material.

[0074] The negative electrode paste of the present application contains the carbon nanotube paste configured as described above and at least a negative electrode active material. The negative electrode active material can use metal oxide-based active material particles, silicon-based active material particles, and spherical graphite, and is particularly preferably metal oxide-based negative electrode active material particles.

[0075] As the metal oxide-based negative electrode active material particles, for example, titanium oxide can be used. As the titanium oxide, there is no particular limitation as long as it can occlude and release lithium, and for example, spinel-type lithium titanate, orthorhombic-type lithium titanate, titanium-containing metal complex oxide, titanium dioxide (TiO2(B)) having a crystal structure of monoclinic system, and anatase-type titanium dioxide, etc. can be used.

[0076] As the spinel-type lithium titanate, for example, Li 4+x Ti5O 12 (x changes in the range of -1 ≤ x ≤ 3 through charge and discharge reactions) and the like can be used. As the orthorhombic-type lithium titanate, for example, Li 2+y ​Ti3O7 (y is changed in the range of -1 ≤ y ≤ 3 by charge-discharge reaction) and the like. As the TiO2(B) and anatase titanium dioxide, Li 1+z TiO2(z is changed in the range of -1 ≤ z ≤ 0 by charge-discharge reaction) and the like.

[0077] As the titanium-containing metal composite oxide, a metal composite oxide containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni and Fe and the like can be exemplified. As the metal composite oxide containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni and Fe, for example, TiO2-P2O5, TiO2-V2O5, TiO2-P2O5-SnO2, TiO2-P2O5-MeO (Me is at least one element selected from the group consisting of Cu, Ni and Fe) and the like can be exemplified.

[0078] Such a metal composite oxide is preferably a microstructure in which crystallinity is low, a crystalline phase and an amorphous phase coexist, or an amorphous phase exists alone. By being a microstructure, the cycle performance can be further improved.

[0079] In the slurry for negative electrode of the present application, the content of the above-mentioned active material for negative electrode is preferably 30 to 60 mass% and further preferably 35 to 55 mass% with respect to the total amount of the slurry for negative electrode. If the content of the active material for negative electrode in the slurry for negative electrode is in this range, the performance of the electrode produced can be ensured, and the fluidity of the slurry is maintained.

[0080] In addition, the content of the carbon nanotube material in the slurry for negative electrode is preferably 0.05 to 5 mass parts and more preferably 0.05 to 3 mass parts and further preferably 0.05 to 1 mass part with respect to 100 mass parts of the active material for negative electrode. If the content of the active material for negative electrode in the slurry for negative electrode is in this range, the performance of the electrode produced can be ensured, and the fluidity of the slurry is maintained.

[0081] The slurry for negative electrode of the present application contains the carbon nanotube slurry composed as described above and the active material for negative electrode, and can contain a solid electrolyte such as a sulfide solid electrolyte, an oxide solid electrolyte, a dry polymer electrolyte, a gel polymer electrolyte, a quasi-solid electrolyte and the like as needed.

[0082] 〈Binder>

[0083] In the above electrode slurry, a binding material (binder) is preferably further included. As the binding material, various emulsion-type polymers can be used, and specifically, a polyimide-based resin; a polyvinylidene fluoride (PVdF), a polytetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride-based copolymer, a hexafluoropropylene-vinylidene fluoride-based copolymer, a tetrafluoroethylene-perfluoro vinyl ether-based copolymer, and the like fluorine-based resins; an ethylene-propylene-diene copolymer (EPDM), a nitrile rubber (NBR), a styrene butadiene rubber (SBR), and the like elastomer-based resins; and an acrylic resin can be used as emulsion types.

[0084] In addition, as a solution-type polymer, a solution of an acrylic resin, a cellulose-based resin, or the like can be used. Two or more kinds of binding materials can be used in combination.

[0085] The amount of the binding material used is preferably 0.05 to 5 parts by mass, more preferably 0.1 to 4.5 parts by mass, and further preferably 0.5 to 4.5 parts by mass, relative to 100 parts by mass of the active material in each electrode slurry for secondary batteries. When the amount of the binding material is within this range, the battery capacity and the charge-discharge characteristics are not adversely affected, and an electrode having high adhesion to the current collector can be obtained.

[0086] The amount of the solvent contained in the electrode slurry is preferably 0.5 to 80% by mass, and more preferably 1 to 70% by mass, relative to the total amount of the electrode slurry, from the viewpoint of achieving appropriate viscosity when the electrode slurry is applied to the current collector.

[0087] In the electrode slurry, a leveling agent, a solid electrolyte, a preservative, and the like can be appropriately blended in addition to the above components.

[0088] The electrode slurry can be prepared by appropriately adding a carbon nanotube slurry, an active material for a secondary battery, and a binding material (binder) as needed, an aqueous solvent, and other components, and mixing them. The mixing operation can be performed using, for example, a twin-screw type kneader.

[0089] An electrode is produced by applying the electrode slurry of the present application to a current collector, which is a conductive member of a secondary battery such as a lithium ion secondary battery, and drying it. The obtained electrode realizes high output and a battery performance capable of withstanding long-term repeated charge and discharge as an electrode for a secondary battery.

[0090] That is, by using the electrode slurry of the present application, an electrode for a secondary battery having high output and a battery performance capable of withstanding long-term repeated charge and discharge can be obtained.

[0091] The carbon nanotube slurry of the present application is a slurry in which carbon nanotubes and the like are uniformly dispersed and have low viscosity, and thus carbon nanotubes and the like can be uniformly applied to a current collector at a high concentration.

[0092] The electrode can be produced from the electrode slurry of the present application as described below.

[0093] First, the electrode slurry is applied to a current collector. The current collector is a conductive member that becomes an electrode substrate of a secondary battery such as a lithium ion secondary battery. The material and shape of the current collector used as the substrate of the electrode are not particularly limited, and can be appropriately selected depending on the material and shape of the secondary battery to be used. As the material of the current collector, metals such as aluminum, copper, nickel, titanium, or stainless steel, and / or alloys can be exemplified. In addition, as the shape of the current collector, a planar metal foil is generally used, but a foil having a roughened surface, a perforated foil, and a meshed foil can also be used.

[0094] As the method of applying the electrode slurry to the current collector, die coating, dip coating, roll coating, doctor coating, knife coating, spray coating, gravure coating, screen printing, or electrostatic coating can be exemplified. In addition, surface smoothing treatment using a planographic press, a calender roll, or the like can be performed after the application.

[0095] Next, the current collector on which the electrode slurry is applied is dried. Thus, an electrode having an electrode film formed on the current collector is produced.

[0096] As the method of drying the electrode slurry after the application, natural drying, a forced air dryer, a hot air dryer, an infrared heater, a far infrared heater, or the like can be used.

[0097] The thickness of the electrode produced using the electrode slurry of the present application is generally 1 μm or more and 500 μm or less, and preferably 10 μm or more and 300 μm or less, including the thickness of the current collector.

[0098] The electrode produced using the electrode slurry of the present application has a low resistance value, and is suitable for use as an electrode of a lithium ion secondary battery or the like.

[0099] < Lithium ion secondary battery >

[0100] A secondary battery is generally composed of a positive electrode, a negative electrode, an electrolyte, a non-aqueous solvent, and a separator provided as needed, and can be produced in various shapes such as a cylindrical shape, a prismatic shape, a gum shape, a coin shape, a button shape, a needle shape, a paper shape, and the like according to the purpose of use. The positive electrode or the negative electrode of a lithium ion secondary battery can use an electrode produced by applying the electrode slurry described above.

[0101] Hereinafter, a lithium ion secondary battery composed of an electrode produced using the electrode slurry of the present application will be described.

[0102] As the electrolyte, a Li salt from which ions can migrate can be used. Examples include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, Li(CF3SO2)3C, LiI, LiBr, LiCl, LiAlCl, LiHF2, LiSCN, and LiBPh4(wherein Ph represents a phenyl group).

[0103] As the non-aqueous solvent, aprotic polar solvents such as carbonic acid esters 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-octanolactone; ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-l,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 can be used. These solvents can be used singly or in combination of two or more.

[0104] As the separator, polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and separators subjected to hydrophilic treatment can be used.

[0105] Examples

[0106] Examples of embodiments of the present application are described in the examples. Note that the properties of the carbon nanotubes (CNT) used in the preparation of the carbon nanotube paste in the following examples and comparative examples were measured by the following methods. The results of the measurement of the properties of the carbon nanotubes are shown in Table 1.

[0107] (Properties of carbon nanotubes)

[0108] [BET specific surface area]

[0109] The carbon nanotubes were collected and accurately weighed using an electronic balance. After being degassed and dried at 110°C for 30 minutes, the BET specific surface area was measured by the BET one-point method using a full-automatic specific surface area measuring device (Macsorb model HM-1208 manufactured by Mountech).

[0110] [Raman spectrum]

[0111] The carbon nanotubes were collected, dispersed in water, and applied to a test piece using an applicator. After drying at 80°C, the Raman spectrum was measured by mapping a 100-μm square area using a Raman spectrometer (manufactured by Thermo Fisher Scientific, DXR2xi). The absorbance G of the peak top position in the range of 1560 to 1600 cm -1 and the absorbance D of the peak top position in the range of 1310 to 1350 cm -1 were measured, and the G / D ratio was calculated.

[0112] [Length of fiber / width of fiber]

[0113] The carbon nanotubes were collected and observed using a scanning electron microscope (manufactured by Hitachi High-Tech Corporation, S-3400N; SEM). The arithmetic mean of the lengths of 10 carbon nanotubes measured from an image taken at a magnification of 1000 was calculated. In addition, the carbon nanotubes were observed using a transmission electron microscope (manufactured by Hitachi High-Tech Corporation, H-7650; TEM). The arithmetic mean of the widths of 10 carbon nanotubes measured from an image taken at a magnification of 50,000 was calculated.

[0114] [Half-value width of peak]

[0115] The carbon nanotubes were collected and observed using a powder X-ray diffraction analyzer (manufactured by Rigaku Corporation, miniflex600). The average of the 2θ half-value widths of 10 carbon nanotubes was calculated.

[0116] [Table 1]

[0117]

[0118] (Example 1)

[0119] [Preparation of carbon nanotube slurry]

[0120] A glass bottle (manufactured by Pyrex Glass Co., Ltd., M-140) was charged with 0.4 parts by mass of commercially available carbon nanotubes A as the carbon nanotubes, 0.2 parts by mass of sodium carboxymethylcellulose (CMC, manufactured by Daicel Miraizu Ltd., 1190) as the dispersant, and 99.4 parts by mass of water as the solvent. The mixture was stirred (pre-mixed) at a stirring speed (300 rpm) that did not entrain air bubbles for 1 hour using a disperser, and a pre-mixed liquid in the form of a slurry was obtained.

[0121] The flowability of the pre-mixed liquid was evaluated by visual observation.

[0122] A: The premixed liquid flows well.

[0123] B: The premixed liquid is difficult to flow.

[0124] C: The premixed liquid does not flow.

[0125] Next, 300 parts by weight of zirconia microspheres (0.5 mm in diameter) were added to the bottle, the cap was tightened, and the bottle was vigorously shaken by hand to fuse the premix with the microspheres. Then, the premix was mechanically dispersed for 2 hours using a paint mixer (manufactured by SEIWA GIKEN CO., LTD.), and then removed. After confirming the movement of the microspheres, the aforementioned mechanical dispersion process was repeated to separate the microspheres, yielding a mechanically dispersed carbon nanotube slurry (CNT dispersion).

[0126] 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.

[0127] A: The microspheres in the slurry move well.

[0128] B: The microspheres in the slurry are difficult to move.

[0129] C: The microspheres in the slurry are very difficult to move.

[0130] D: The microspheres in the slurry do not move.

[0131] [LD viscosity A of carbon nanotube slurry]

[0132] For a carbon nanotube slurry with a carbon nanotube concentration of 0.4% by mass, using an E-type viscometer (Toki Sangyo Co., Ltd., TV-22 type), a 1°34' cone plate was subjected to a shear rate of 38.3 s⁻¹. -1 Rotate the sample and measure the viscosity at 25°C. The obtained viscosity value is expressed as LD viscosity (A). It should be noted that when the concentration of carbon nanotubes dispersed in the carbon nanotube slurry is higher than 0.4% by mass, the slurry is diluted with water, and the viscosity is measured at the adjusted concentration of 0.4% by mass. When the concentration of carbon nanotubes in the carbon nanotube slurry is lower than 0.4% by mass, the solvent is evaporated from the slurry, and the viscosity is measured at a concentration of 0.4% by mass.

[0133] [Particle size distribution of carbon nanotubes d90]

[0134] 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(B).

[0135] The ratio A / B (mPa-s / μm) of the aforementioned LD viscosity (A) to the particle size distribution d90 (B) was calculated.

[0136] 〔Time-dependent stability of carbon nanotube paste〕

[0137] The carbon nanotube paste obtained by the mechanical dispersion treatment was collected, 100 parts by mass was added to a glass bottle with a lid (manufactured by Hayashi Kako Glass Co., Ltd., M-140), and then stored for 3 months in a thermostat set to a temperature of 50°C. The state of the paste in the glass bottle after the time-dependent storage was confirmed visually. The state of the carbon nanotube paste was evaluated visually according to the following criteria.

[0138] A: The carbon nanotube paste was uniform, and no separation was observed

[0139] B: The carbon nanotube paste was slightly non-uniform, and a small amount of separation of carbon nanotubes was observed

[0140] C: The carbon nanotube paste was non-uniform, and significant separation of carbon nanotubes was observed

[0141] <Preparation of electrode paste>

[0142] In the carbon nanotube paste prepared by the mechanical dispersion treatment as described above, 0.4 parts by mass of carboxymethyl cellulose (CMC; manufactured by Daicel Miraizu Ltd., 1190) and 100 parts by mass of an active material for a negative electrode (manufactured by BTR New Energy Materials Co., Ltd., LTO-2S) were mixed in the amounts of 0.2 parts by mass and 100 parts by mass, respectively, relative to 0.4 parts by mass of carbon nanotubes contained in the paste, as a binder material and an active material for a negative electrode, respectively, using a mixer (manufactured by THINKY CORPORATION, Awatori Rentaro ARE-310) to knead until uniform, and an electrode paste was prepared.

[0143] <Manufacture of electrode>

[0144] The prepared electrode paste was coated on a soda-lime glass sheet using an applicator with a film thickness set to 50 μm. Next, the coated glass sheet was dried on a hot plate at 90°C for 10 minutes, and a model electrode for a lithium battery negative electrode was manufactured.

[0145] 〔Surface resistivity of electrode〕

[0146] The surface resistivity (Ω / □) was measured using a resistivity meter (manufactured by Mitsubishi Chemical Analytech Co., Ltd., Loresta GP, MCP-T610, four-probe probe, ASP needle tip distance 5 mm) for the obtained electrode.

[0147] (Examples 2 to 6, Comparative Examples 1 to 6)

[0148] Carbon nanotube A was used as it was in the state of A or was changed to other kinds of carbon nanotubes described in Table 1, the dispersant was changed, or the number of times of mechanical dispersion treatment was increased or decreased as appropriate, and, other than that, carbon nanotube slurries were prepared by the same method as in Example 1. The flowability and the like of the obtained carbon nanotube slurries were evaluated by the same method as in Example 1.

[0149] Further, electrode slurries were prepared using the prepared carbon nanotube slurries, respectively, and model electrodes for negative electrodes were produced by the same method as in Example 1. The surface resistivity (Ω / D) of each of the obtained model electrodes was measured by the same method as in Example 1.

[0150] The composition and evaluation results of the carbon nanotube slurries of Examples 1 to 6 and Comparative Examples 1 to 6 are shown in Table 2.

[0151] [Table 2]

[0152]

[0153] As is clear from Table 2, the carbon nanotube slurries of Examples 1 to 6 within the scope of the present application exhibited low viscosity and could be easily prepared.

[0154] In particular, in the case of using the carbon nanotube of Examples 1 to 6 within the scope of the present application, it was confirmed that the flowability of the slurry at the time of pre-mixing and after pre-mixing was good, the movement of the beads before and at the end of the mechanical dispersion treatment using a paint shaker was also good, and a carbon nanotube slurry was obtained as a dispersion with good workability.

[0155] The coating operation at the time of producing an electrode from an electrode slurry prepared using the carbon nanotube slurry of Examples 1 to 6 within the scope of the present application was easy. As is clear from Table 2, the electrodes of Examples 1 to 6 produced from an electrode slurry using a carbon nanotube slurry within the scope of the present application exhibited low resistivity.

[0156] On the other hand, the carbon nanotube slurries obtained in Comparative Examples 1 to 3 and Comparative Examples 5 to 6, in which the ratio A / B of the LD viscosity A (mPa-s) of the slurry to the d90 particle size distribution B (μm) of the carbon nanotube was less than 5.0 (mPa-s / μm), had low stability over time and were difficult to handle. Further, the electrodes produced from the carbon nanotube slurries in Comparative Examples 1 to 3 and Comparative Examples 5 to 6 had high surface resistivity. Note that, in Comparative Example 4, the viscosity of the carbon nanotube slurry was too high to be dispersed, and thus the preparation of an electrode slurry and the production of an electrode could not be performed.

[0157] Industrial applicability

[0158] The carbon nanotube paste of the present application can be suitably used for an electrode paste. An electrode for a lithium secondary battery can be easily manufactured from the electrode paste.

Claims

1. A carbon nanotube slurry comprising at least carbon nanotubes, a dispersant, and an 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. 2 2 2. The carbon nanotube slurry according to claim 1, wherein the carbon nanotubes have a peak intensity ratio G / D in Raman spectroscopy of 1.0 to 2.0.​ 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 nanotube has 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 content of the carbon nanotube is 0.1 mass% to 3.0 mass%.

4. The carbon nanotube paste of claim 1, wherein, In the carbon nanotube paste, the viscosity of the carbon nanotube paste at 25°C, 38.3 s -1 -1 is set to A (mPa-s), and the particle size distribution d90 of the carbon nanotubes contained in the carbon nanotube paste is set to B (μm), the value of A / B is 5.0 (mPa-s / μm) or more.

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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