Carbon nanotube-dispersed paste, method for producing carbon nanotube-dispersed paste, quality control method for carbon nanotube-dispersed paste, composite paste, electrode for positive electrode of lithium ion battery, and lithium ion battery

Abstract

To provide carbon nanotube dispersion paste of which the viscosity is low, in which a carbon nanotube is appropriately dispersed and by which an electrode for cathode improved in conductivity can be formed, a manufacturing method of the carbon nanotube dispersion paste, a quality management method for the carbon nanotube dispersion paste, mixture paste, an electrode for lithium ion battery cathode and a lithium ion battery.SOLUTION: The present invention relates to a carbon nanotube dispersion paste containing a carbon nanotube (B) and an organic solvent (C). In the carbon nanotube dispersion paste, a content of the carbon nanotube (B) is 1 to 10 mass% with respect to a total mass of the carbon nanotube dispersion paste. In a Bode plot which is obtained by measuring impedance and plotting reactance on a longitudinal axis and a frequency on a lateral axis, a minimal value of the reactance ranges from 50 to 250 kHz.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a carbon nanotube dispersed paste, a method for producing a carbon nanotube dispersed paste, a quality control method for a carbon nanotube dispersed paste, a composite paste, a positive electrode for a lithium ion battery, and a lithium ion battery. [Background technology]

[0002] Lithium-ion batteries are a type of non-aqueous electrolyte secondary battery in which lithium ions in the electrolyte are responsible for electrical conduction. Lithium-ion batteries have excellent properties, such as high energy density, excellent charge energy retention, and minimal memory effect, which is the apparent decrease in capacity. Therefore, lithium-ion batteries are used in a wide range of applications, including mobile phones, smartphones, personal computers, hybrid vehicles, and electric vehicles.

[0003] Here, a lithium-ion battery mainly includes a positive electrode, a negative electrode, a separator for insulating the positive electrode from the negative electrode, a non-aqueous electrolyte, etc. The positive electrode is formed by forming a positive electrode composite layer on the surface of a positive electrode core material. The positive electrode composite layer can be produced by applying a positive electrode composite paste, which is obtained by mixing an electrode active material with a conductive paste (also referred to as a "conductive additive dispersion paste") containing a conductive additive such as carbon nanotubes, a binder, and a solvent, onto the surface of the positive electrode core material, and then drying the paste.

[0004] For example, Patent Document 1 discloses a carbon nanotube dispersion slurry containing carbon nanotubes and a dispersion medium, having a viscosity of 100 to 5000 mPa·s, a maximum particle size of 20 μm or less, and a concentration dependency of admittance obtained by AC impedance measurement of 30 μS / mass% or less. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-107191 Summary of the Invention [Problem to be solved by the invention]

[0006] The conductive additive dispersion paste preferably has a low viscosity in consideration of workability, stability and coatability of the positive electrode composite paste, and the like. In recent years, further improvements in the performance of lithium-ion batteries have been required. The conductivity of the conductive additives present around the electrode active material varies significantly depending on the dispersion state in the positive electrode composite layer, which also affects battery performance. The conductive additive forms a structure in which multiple primary particles are connected in a chain (hereinafter also referred to as a "chain structure" or "structure"). For example, as shown in FIG. 1(a), if the interaction between primary particles 10 of a conductive pigment, which is an example of a conductive additive, is too strong and the primary particles of the conductive pigment are excessively aggregated in the conductive paste, an efficient conductive path is not formed when a positive electrode composite layer is formed, and electrons do not flow sufficiently within the electrode. On the other hand, as shown in FIG. 1(c), if the interaction between primary particles 10 of the conductive pigment is too weak, the structure is broken, a conductive path is not formed, and electrons do not flow sufficiently within the electrode. Therefore, conductive additives such as conductive pigments are required to be adequately dispersed in the conductive paste so that conductive paths can be efficiently formed, as shown in Figure 1(b). In FIG. 1, reference numeral 20 denotes an active material.

[0007] The dispersion state of the conductive additive in the positive electrode composite layer is reflected in the dispersion state of the conductive additive in the conductive paste. In other words, by using a conductive paste in which the conductive additive is appropriately dispersed, a positive electrode composite layer in which conductive paths are efficiently formed can be formed. Because conductive paste contains a high concentration of conductive additive, it is difficult to measure the dispersion state of the conductive additive, such as particle size and interparticle distance distribution, in its original state. Therefore, it is common to measure the particle size by, for example, diluting the conductive paste and measuring the particle size distribution. However, diluting the conductive paste changes the interaction between the primary particles of the conductive additive, so the dispersion state of the conductive additive in the conductive paste may not be reflected in the measurement results.

[0008] The present invention aims to provide a carbon nanotube-dispersed paste that has low viscosity, allows carbon nanotubes to be appropriately dispersed, and is capable of forming a positive electrode with excellent conductivity; a method for producing the carbon nanotube-dispersed paste; a quality control method for the carbon nanotube-dispersed paste; a composite paste; an electrode for a lithium-ion battery positive electrode; and a lithium-ion battery. [Means for solving the problem]

[0009] The present invention has the following aspects. [1] A carbon nanotube dispersion paste containing carbon nanotubes (B) and an organic solvent (C), the content of the carbon nanotubes (B) is 1 to 10 mass % relative to the total mass of the carbon nanotube dispersion paste, A carbon nanotube dispersion paste in which a Bode plot obtained by impedance measurement, in which reactance is plotted on the vertical axis and frequency is plotted on the horizontal axis, has a minimum reactance value in the frequency range of 50 to 250 kHz. [2] The carbon nanotube dispersion paste according to [1], wherein the organic solvent (C) is N-methyl-2-pyrrolidone. [3] Further containing a pigment dispersing resin (A), The carbon nanotube dispersion paste according to [1] or [2] above, wherein the content of the pigment dispersion resin (A) is 10 to 100 parts by mass per 100 parts by mass of the carbon nanotubes (B). [4] A carbon nanotube dispersion paste according to any one of [1] to [3], wherein in the Bode plot, the minimum reactance value in the frequency range of 50 to 250 kHz is at least five times the reactance value at a frequency of 1 kHz. [5] Shear rate 1.0 sec -1The carbon nanotube dispersion paste according to any one of [1] to [4] above, having a viscosity of 10 Pa·s or less when measured at 200°C. [6] A method for producing a carbon nanotube dispersion paste containing carbon nanotubes (B) and an organic solvent (C), comprising: the content of the carbon nanotubes (B) is 1 to 10 mass % relative to the total mass of the carbon nanotube dispersion paste, A method for producing a carbon nanotube dispersion paste, comprising mixing the carbon nanotubes (B) and an organic solvent (C) so that a minimum value of the reactance exists in a frequency range of 50 to 250 kHz in a Bode plot obtained by impedance measurement, in which reactance is plotted on the vertical axis and frequency is plotted on the horizontal axis, and dispersing the carbon nanotubes (B). [7] A quality control method for a carbon nanotube dispersion paste containing carbon nanotubes (B) and an organic solvent (C), comprising: the content of the carbon nanotubes (B) is 1 to 10 mass % relative to the total mass of the carbon nanotube dispersion paste, A quality control method for carbon nanotube-dispersed paste, in which the carbon nanotube-dispersed paste is controlled so that a minimum value of the reactance exists in the frequency range of 50 to 250 kHz in a Bode plot obtained by impedance measurement, where the reactance is plotted on the vertical axis and the frequency is plotted on the horizontal axis. [8] A composite paste containing the carbon nanotube dispersion paste according to any one of [1] to [5] above and an electrode active material. [9] A positive electrode for a lithium ion battery, comprising a positive electrode core material and a layer formed by applying the composite paste of [8] to the surface of the positive electrode core material.

[10] A lithium-ion battery having the lithium-ion battery positive electrode according to [9]. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a carbon nanotube-dispersed paste that has low viscosity, allows carbon nanotubes to be appropriately dispersed, and is capable of forming a positive electrode with excellent conductivity; a method for producing a carbon nanotube-dispersed paste; a quality control method for a carbon nanotube-dispersed paste; a composite paste; an electrode for a lithium-ion battery positive electrode; and a lithium-ion battery. [Brief explanation of the drawings]

[0011] [Figure 1] 1A and 1B are diagrams showing an example of the dispersion state of a conductive additive in a conductive paste and a positive electrode composite layer, in which FIG. 1A is a diagram showing an example of the dispersion state of a conductive additive when the interaction between primary particles of the conductive additive is too strong, FIG. 1B is a diagram showing an example of the dispersion state of a conductive additive when the interaction between primary particles of the conductive additive is appropriate, and FIG. 1C is a diagram showing an example of the dispersion state of a conductive additive when the interaction between primary particles of the conductive additive is too weak. [Figure 2] 1A and 1B are diagrams showing examples of Bode plots obtained by impedance measurement, where (a) is a diagram showing an example of a Bode plot when the degree of growth of the carbon nanotube structure is high, (b) is a diagram showing an example of a Bode plot when the degree of growth of the carbon nanotube structure is medium, and (c) is a diagram showing an example of a Bode plot when the degree of growth of the carbon nanotube structure is low. [Figure 3] 1A and 1B are diagrams showing examples of Bode plots obtained by impedance measurement, where (a) is a diagram showing an example of a Bode plot when the particle diameter of aggregates of primary particles of carbon nanotubes (B) is uniform, and (b) is a diagram showing an example of a Bode plot when the particle diameter of aggregates of primary particles of carbon nanotubes (B) is non-uniform. [Figure 4] FIG. 1 is a diagram showing Bode plots for Example 4 and Comparative Examples 1 and 3. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described in detail. It should be understood that the present invention is not limited to the following embodiments, but also includes various modifications that are implemented within the scope of the present invention. Furthermore, in this specification, "(meth)acrylate" is a general term for acrylate and methacrylate. "(meth)acrylic acid" is a general term for acrylic acid and methacrylic acid. "(meth)acryloyl" is a general term for acryloyl and methacryloyl. "(meth)acrylamide" is a general term for acrylamide and methacrylamide. "Polymerizable unsaturated monomer" refers to a monomer having a polymerizable unsaturated group capable of radical polymerization, and examples of such polymerizable unsaturated groups include a (meth)acryloyl group, an acrylamide group, a vinyl group, an allyl group, a (meth)acryloyloxy group, and a vinyl ether group. "Derivative" refers to a compound obtained by changing a small portion (or multiple portions) in the molecule of a certain compound by introducing a functional group, substituting an atom, or other chemical reaction. For example, a compound in which one or more functional groups such as an alkyl group, an alkoxy group, a hydroxyl group, a sulfonic acid group, a carboxyl group, an amino group, a nitro group, a halogen atom, an aryloxy group, an alkylthio group, or an arylthio group have been introduced into naphthalene is a naphthalene derivative.

[0013] [Carbon nanotube dispersion paste] The carbon nanotube dispersion paste of the present invention is a conductive paste for a lithium ion battery positive electrode, which contains carbon nanotubes (B) and an organic solvent (C). The carbon nanotube dispersion paste of the present invention preferably further contains a pigment dispersing resin (A). The carbon nanotube dispersion paste of the present invention may further contain components (optional components) other than the pigment dispersion resin (A), the carbon nanotubes (B), and the organic solvent (C), as needed, within a range that does not impair the effects of the present invention.

[0014] <Carbon nanotubes (B)> Examples of the carbon nanotubes (B) include single-walled carbon nanotubes and multi-walled carbon nanotubes, among which multi-walled carbon nanotubes are preferred from the viewpoint of an excellent balance between the viscosity and conductivity of the carbon nanotube dispersion paste and costs. These carbon nanotubes (B) may be used singly or in combination of two or more kinds.

[0015] The average outer diameter of the carbon nanotubes (B) is preferably 1 to 25 nm, more preferably 3 to 20 nm, and even more preferably 5 to 15 nm, from the viewpoint of further improving the dispersibility of the carbon nanotubes (B) and the conductivity of the carbon nanotube-dispersed paste. When a commercially available product is used as the carbon nanotubes (B), the average outer diameter of the carbon nanotubes (B) may be the value listed in the manufacturer's catalog.

[0016] The average length of the carbon nanotubes (B) is preferably 0.1 to 100 μm, more preferably 0.5 to 80 μm, and even more preferably 1 to 60 μm, from the viewpoint of further improving the dispersibility of the carbon nanotubes (B) and the conductivity of the carbon nanotube-dispersed paste. When a commercially available product is used as the carbon nanotubes (B), the average length of the carbon nanotubes (B) may be the value listed in the manufacturer's catalog.

[0017] The specific surface area of ​​carbon nanotubes (B) is 1 to 1000 m 2 / g is preferred, and 10 to 500m 2 When the specific surface area of ​​the carbon nanotubes (B) is within the above range, the conductivity of the carbon nanotube dispersion paste is further improved, and the viscosity is appropriately reduced. The specific surface area of ​​the carbon nanotubes (B) is a BET specific surface area measured by the BET (Brunauer-Emmet-Teller) method. In the BET method, a specific surface area measuring device using the gas adsorption method is used to adsorb nitrogen as an adsorption gas onto the surface of the carbon nanotube (B), and the amount of adsorption is measured using the BET equation based on the relationship between the pressure and the amount of adsorption, and the specific surface area is calculated.

[0018] <Organic solvent (C)> As the organic solvent (C), the organic solvent used in the polymerization of the resin (A2) or the solvent used for solvent substitution after polymerization can be suitably used. Specific examples of preferred organic solvents (C) include N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, propylene glycol monomethyl ether, and methanol. Among these, N-methyl-2-pyrrolidone is preferred. These organic solvents (C) may be used alone or in combination of two or more.

[0019] <Pigment dispersing resin (A)> The pigment dispersing resin (A) is a resin that disperses the carbon nanotubes (B). The pigment dispersing resin (A) can be any resin that improves the dispersibility of the carbon nanotubes (B). Specific examples of preferred pigment dispersing resins (A) include acrylic resins, polyester resins, epoxy resins, polyether resins, alkyd resins, urethane resins, silicone resins, polycarbonate resins, silicate resins, chlorine-based resins, fluorine-based resins, polyvinylpyrrolidone resins (PVP), polyvinyl alcohol resins (PVA), polyvinyl acetal resins, and composite resins thereof. Among these, non-crosslinked resins are preferred for the pigment dispersing resin (A), as they further improve the dispersibility of the carbon nanotubes (B) and appropriately reduce the viscosity of the carbon nanotube-dispersed paste. Among these, polyvinyl alcohol resins are particularly preferred as the pigment dispersing resin (A), as they further improve the dispersibility of the carbon nanotubes (B) and the storage stability of the carbon nanotube-dispersed paste. These pigment dispersing resins (A) may be used singly or in combination of two or more.

[0020] The polyvinyl alcohol resin may or may not have a modifying group. Hereinafter, in this specification, a polyvinyl alcohol resin that does not have a modifying group, i.e., an unmodified polyvinyl alcohol resin, will also be referred to as a "polyvinyl alcohol resin (a1)." A polyvinyl alcohol resin that has a modifying group will also be referred to as a "modified polyvinyl alcohol resin (a2)." As the polyvinyl alcohol resin, either the polyvinyl alcohol resin (a1) or the modified polyvinyl alcohol resin (a2) may be used, or the polyvinyl alcohol resin (a1) and the modified polyvinyl alcohol resin (a2) may be used in combination.

[0021] The polyvinyl alcohol resin (a1) can be obtained by polymerizing a fatty acid vinyl ester such as vinyl acetate and then saponifying the resulting polymer. The modified polyvinyl alcohol resin (a2) can be produced by any of the following methods (1) to (4). (1) A method in which a fatty acid vinyl ester such as vinyl acetate is copolymerized with other monomers containing a polymerizable unsaturated group, and the resulting polymer is then saponified. (2) A method of subjecting a polymerizable unsaturated group-containing monomer to Michael addition to polyvinyl alcohol. (3) A method of acetalizing polyvinyl alcohol with an aldehyde compound. (4) A method of polymerizing polyvinyl alcohol in the presence of a compound having a functional group such as alcohol, aldehyde, or thiol as a chain transfer agent.

[0022] The modified polyvinyl alcohol resin (a2) preferably contains a side chain having one or more functional groups (modifying groups) selected from the group consisting of an ester group, an amide group, an imide group, an ether group, a hydroxyl group, a carboxyl group, a sulfonic acid group, a phosphate group, a silanol group, and an amino group.

[0023] The polyvinyl alcohol resin (a1) and the modified polyvinyl alcohol resin (a2) preferably have a high degree of saponification (high polarity) from the viewpoint of further improving the dispersibility of the carbon nanotubes (B) and the storage stability of the carbon nanotube-dispersed paste. Specifically, the degree of saponification is preferably 85 mol% or more and less than 100 mol%, more preferably 90 mol% or more and less than 100 mol%, even more preferably 95 mol% or more and less than 100 mol%, and particularly preferably 96 mol% or more and less than 100 mol%.

[0024] Furthermore, the higher the polarity of the pigment dispersing resin (A), the better its adsorption to the carbon nanotubes (B). In particular, the higher the degree of saponification of the polyvinyl alcohol resin (a1) and the modified polyvinyl alcohol resin (a2), the better its adsorption to the carbon nanotubes (B).

[0025] From the viewpoint of further improving the dispersibility of the carbon nanotubes (B) and the storage stability of the carbon nanotube-dispersed paste, the polyvinyl alcohol resin (a1) and the modified polyvinyl alcohol resin (a2) each preferably have an average degree of polymerization of 50 to 3,000, more preferably 100 to 2,000, and even more preferably 100 to 1,500.

[0026] <Optional ingredients> Examples of optional components include a binder, resin components (other resin components) other than the pigment dispersion resin (A) and the binder, conductive pigments (other conductive pigments) other than the carbon nanotubes (B), neutralizers, antifoaming agents, preservatives, rust inhibitors, plasticizers, etc.

[0027] The binder is a resin that has the effect of adhering the film obtained by applying the paste to the substrate, and examples of such resins include acrylic resins other than the pigment-dispersing resin (A), polyester resins, epoxy resins, polyether resins, alkyd resins, urethane resins, silicone resins, polycarbonate resins, silicate resins, chlorine-based resins, fluorine-based resins, polyvinylpyrrolidone resins, and composite resins thereof. Among these, fluorine-based resins are preferred, and polyvinylidene fluoride (PVDF) is more preferred. These binders may be used alone or in combination of two or more.

[0028] Examples of other conductive pigments include conductive carbons other than carbon nanotubes (B) (other conductive carbons). Other conductive carbons include, for example, acetylene black, ketjen black, furnace black, thermal black, graphene, and graphite. These other conductive carbons may be used singly or in combination of two or more.

[0029] The average primary particle diameter of the other conductive carbon is preferably 10 to 80 nm, more preferably 20 to 50 nm. When the average primary particle diameter of the other conductive carbon is within the above range, the conductivity of the carbon nanotube dispersion paste is further improved, and the viscosity is appropriately reduced.

[0030] The specific surface area of ​​other conductive carbons ranges from 1 to 500 m 2 / g is preferred, and 30 to 150m 2 When the specific surface area of ​​the other conductive carbon is within the above range, the conductivity of the carbon nanotube dispersion paste is further improved, and the viscosity is appropriately reduced.

[0031] The other conductive carbon is preferably basic from the viewpoint of improving pigment dispersibility, and specifically, the pH is preferably 7.5 or higher, more preferably 8.0 to 12.0, and even more preferably 8.5 to 11.0.

[0032] Furthermore, from the viewpoint of increasing conductivity, the other conductive carbon preferably has a structure formed by a plurality of primary particles, and the structure index is more preferably 1.5 to 4.0, and particularly preferably 1.7 to 3.2. The structure itself can be observed relatively easily in images taken with an electron microscope, but the structure index is a numerical value that quantifies the degree of structure. The structure index is generally calculated by multiplying the DBP oil absorption (mL / 100g) by the specific surface area (m 2 / g). If the structure index is 1.5 or higher, the structure is well developed, making it easier to obtain sufficient conductivity. If the structure index is 4.0 or lower, the particle size is less likely to become large relative to the DBP oil absorption, which prevents the reduction in conductive paths and makes it easier to obtain sufficient conductivity. In addition, it is possible to prevent the viscosity of the carbon nanotube dispersion paste from becoming too high.

[0033] <Content> The content of carbon nanotubes (B) is 1 to 10 mass % relative to the total mass of the carbon nanotube dispersion paste, preferably 1.2 to 8 mass %, and more preferably 1.5 to 6 mass %. When the content of carbon nanotubes (B) is equal to or greater than the above lower limit, the conductivity of the carbon nanotube dispersion paste is increased. When the content of carbon nanotubes (B) is equal to or less than the above upper limit, the dispersibility of carbon nanotubes (B) can be maintained well.

[0034] The content of carbon nanotubes (B) is preferably 10 to 100 mass % relative to the total mass of the solid content of the carbon nanotube dispersion paste, more preferably 30 to 95 mass %, even more preferably 50 to 90 mass %, and particularly preferably 60 to 90 mass %. When the content of carbon nanotubes (B) is equal to or greater than the above lower limit, the conductivity of the carbon nanotube dispersion paste is further increased. When the content of carbon nanotubes (B) is equal to or less than the above upper limit, the dispersibility of carbon nanotubes (B) can be maintained better. The "solid content of the carbon nanotube dispersion paste" refers to all components contained in the carbon nanotube dispersion paste other than the solvent components (organic solvent (C) and water).

[0035] The content of the organic solvent (C) is preferably 90 to 99 mass %, more preferably 92 to 98.8 mass %, and even more preferably 94 to 98.5 mass %, based on the total mass of the carbon nanotube dispersion paste. When the content of the organic solvent (C) is equal to or greater than the above lower limit, the dispersibility of the carbon nanotubes (B) can be maintained better. When the content of the organic solvent (C) is equal to or less than the above upper limit, the conductivity of the carbon nanotube dispersion paste is further increased.

[0036] When the carbon nanotube dispersion paste contains the pigment dispersion resin (A), the content thereof is preferably 10 to 100 parts by mass, more preferably 12 to 70 parts by mass, and even more preferably 15 to 50 parts by mass, per 100 parts by mass of the carbon nanotubes (B). When the content of the pigment dispersion resin (A) is equal to or greater than the above lower limit, the dispersibility of the carbon nanotubes (B) can be maintained better. When the content of the pigment dispersion resin (A) is equal to or less than the above upper limit, the conductivity of the carbon nanotube dispersion paste is further increased.

[0037] The water content of the carbon nanotube dispersed paste is preferably less than 1 mass %, more preferably less than 0.7 mass %, and even more preferably less than 0.5 mass %, relative to the total mass of the carbon nanotube dispersed paste. The lower the water content, the easier it is to maintain good battery performance. A carbon nanotube dispersion paste having a water content within the above range can be said to be a substantially non-aqueous conductive paste. The water content of the carbon nanotube dispersion paste can be measured by Karl Fischer coulometric titration. Specifically, the water content can be measured using a Karl Fischer moisture meter (e.g., product name "MKC-610" manufactured by Kyoto Electronics Manufacturing Co., Ltd.) with the temperature of a moisture vaporizer (e.g., product name "ADP-611" manufactured by Kyoto Electronics Manufacturing Co., Ltd.) set to 130°C.

[0038] <Impedance spectrum> The carbon nanotube dispersion paste of the present invention is characterized in that in a Bode plot obtained by impedance measurement, in which reactance is plotted on the vertical axis and frequency is plotted on the horizontal axis, the reactance has a minimum value in the frequency range of 50 to 250 kHz. The reactance minimum value is preferably in the frequency range of 60 to 200 kHz, and more preferably in the frequency range of 70 to 150 kHz. In the present invention, reactance means the imaginary part of complex impedance. In a Bode plot, there may be two or more minimum reactance values. This tendency is particularly pronounced when two or more types of carbon nanotubes (B) are used in combination, when carbon nanotubes (B) are used in combination with other conductive carbons, or when the dispersion time of the carbon nanotube dispersion paste is extremely short. In addition, there may be two or more minimum reactance values ​​depending on the type of carbon nanotubes (B) used. When there are two or more minimum reactance values, it is sufficient that at least one of the minimum reactance values ​​is in the frequency range of 50 to 250 kHz.

[0039] The carbon nanotubes (B) form a structure in the carbon nanotube dispersion paste, and the minimum value of the reactance in the Bode plot shifts depending on the degree of growth of the carbon nanotube (B) structure, i.e., the size of the aggregates of the primary particles of the carbon nanotubes (B). For example, as shown in Figure 2, the stronger the interaction between the primary particles 10 and the higher the degree of growth of the structure, i.e., the larger the aggregates 30 of the primary particles 10 of the carbon nanotubes (B), the more likely the minimum value of the reactance in the Bode plot tends to shift to the low-frequency region. Note that Figure 2(a) is a Bode plot for a high degree of structure growth, Figure 2(b) is a Bode plot for a medium degree of structure growth, and Figure 2(c) is a Bode plot for a low degree of structure growth.

[0040] If the Bode plot shows a minimum reactance value in the frequency range of 50 to 250 kHz, this means that the structure of the carbon nanotubes (B) has grown appropriately, i.e., the primary particles of the carbon nanotubes (B) are dispersed in the carbon nanotube dispersion paste while maintaining their structure appropriately. Therefore, when a positive electrode composite layer is formed using a composite paste containing the carbon nanotube dispersion paste of the present invention, the primary particles of the carbon nanotubes (B) are dispersed in the positive electrode composite layer while maintaining their structure appropriately. Therefore, a conductive path is efficiently formed, allowing the formation of a positive electrode with excellent conductivity. In addition, the viscosity of the carbon nanotube dispersion paste also tends to be low.

[0041] The Bode plot can be obtained, for example, by the impedance measurement shown below. (Preparing the measurement cell) Two gold-plated copper plates with a thickness of 0.3 mm are placed opposite each other, and a two-electrode electrode with a distance of 9 mm is used. The size of the electrode is 100 mm. 2 A 20 ml cylindrical container is filled with 15 ml of carbon nanotube dispersion paste, and the electrode is inserted so that it is completely buried in the paste.

[0042] (Impedance measurement) At 25°C, an impedance analyzer is used to apply a sinusoidal AC voltage with a peak-to-peak voltage of 0.1 V to the carbon nanotube dispersion paste, and the complex impedance and phase difference are measured at 500 points while sweeping the frequency between 100 Hz and 100 MHz. From the obtained data, a Bode plot is created by plotting reactance on the vertical axis and frequency on the horizontal axis.

[0043] In the Bode plot, the minimum reactance value (X) in the frequency range of 50 to 250 kHz is preferably 5 times or more, more preferably 7.5 to 30 times, and even more preferably 10 to 30 times the reactance value (Y) at a frequency of 1 kHz. As mentioned above, there may be two or more minimum reactance values ​​in a Bode plot. Even if there are multiple minimum values ​​in the range of 50 to 250 kHz, the calculation (X / Y) should be performed using the minimum value in the range of 50 to 250 kHz. The ratio of the minimum value (X) to the value (Y) is affected by the particle size distribution of the aggregates of primary particles of carbon nanotubes (B). For example, as shown in Figure 3(a), the more uniform the particle diameter of the aggregates 30 of primary particles 10 of carbon nanotubes (B), i.e., the more uniform the particle diameter, the larger the ratio of the minimum value (X) to the value (Y). On the other hand, as shown in Figure 3(b), the more non-uniform the particle diameter of the aggregates 30 of primary particles 10 of carbon nanotubes (B), the smaller the ratio of the minimum value (X) to the value (Y).

[0044] If the minimum value (X) is 5 times or more the value (Y), this means that the particle diameter of the aggregates of primary particles of carbon nanotubes (B) in the carbon nanotube dispersion paste is uniform. Therefore, when a positive electrode mixture layer is formed using a mixture paste containing the carbon nanotube dispersion paste of the present invention, the primary particles of carbon nanotubes (B) are more uniformly dispersed in the positive electrode mixture layer. Therefore, a conductive path is formed more efficiently, and a positive electrode with superior conductivity can be formed.

[0045] <Viscosity> The carbon nanotube dispersion paste is designed to have excellent workability and coating properties, and is applied at a shear rate of 1.0 sec. -1 The viscosity at 2000 kJ / s is preferably 10 Pa·s or less, and more preferably 5 Pa·s or less. When the viscosity of the carbon nanotube dispersion paste is equal to or less than the upper limit, the viscosity of the composite paste produced using this paste can be prevented from increasing. The viscosity of the carbon nanotube dispersion paste is a value measured at 25°C using a cone and plate type viscometer.

[0046] <Manufacturing method> The carbon nanotube dispersion paste can be obtained by uniformly mixing carbon nanotubes (B) and an organic solvent (C), and optionally a pigment dispersion resin (A) and one or more optional components, using a disperser, and dispersing (dispersing) the carbon nanotubes (B) so that a minimum reactance value exists in the frequency range of 50 to 250 kHz in a Bode plot obtained by impedance measurement, for example. In this case, it is preferable to mix carbon nanotubes (B) and an organic solvent (C), and optionally a pigment dispersion resin (A) and one or more optional components, and disperse the carbon nanotubes (B) so that the minimum value (X) is at least five times the value (Y). The frequency at which the reactance minimum appears and the value (Y) can be adjusted by combining the amount of carbon nanotubes (B) blended and the degree of dispersion (dispersion time, etc.).

[0047] Examples of dispersing machines include, but are not limited to, bead mills, paint shakers, sand mills, ball mills, pebble mills, DCP pearl mills, planetary ball mills, homogenizers, twin-screw kneaders, thin film rotary high-speed mixers, etc. Among dispersing machines, bead mills and ball mills are preferred because they can obtain the desired impedance spectrum in a short time. Examples of bead mills include LMZ mills and Dynomills, with the LMZ mill being preferred.

[0048] <Action and effect> The carbon nanotube dispersion paste of the present invention described above has a minimum reactance value in the frequency range of 50 to 250 kHz in a Bode plot obtained by impedance measurement, and therefore has low viscosity, and the primary particles of carbon nanotubes (B) are dispersed while maintaining a moderate structure. Therefore, when a positive electrode mixture layer is formed using a mixture paste containing the carbon nanotube dispersion paste of the present invention, the primary particles of carbon nanotubes (B) are dispersed in the positive electrode mixture layer while maintaining a moderate structure. As a result, a conductive path is efficiently formed, electrons flow easily, and a positive electrode with excellent conductivity can be formed.

[0049] As described above, because conductive pastes contain a high concentration of conductive additive, it is difficult to measure the dispersion state of the conductive additive, such as the particle size and interparticle distance distribution, in its original state. For this reason, it is common to measure the particle size distribution, for example, by diluting the conductive paste. However, diluting the conductive paste changes the interaction between the primary particles of the conductive additive, and the measurement results may not reflect the dispersion state of the conductive additive in the conductive paste. However, in the present invention, the dispersion state of the carbon nanotubes (B) can be determined by impedance measurement without diluting the carbon nanotube dispersion paste, and the dispersion state of the conductive additive in the conductive paste is reflected in the measurement results.

[0050] The carbon nanotube dispersion paste of the present invention is suitable as a material for a composite paste for forming a positive electrode composite layer, which is a part of an electrode.

[0051] [Quality control method for carbon nanotube dispersion paste] The quality of the carbon nanotube dispersion paste is controlled so that, for example, in a Bode plot obtained by impedance measurement, a minimum value of reactance exists in a frequency range of 50 to 250 kHz. In this case, it is preferable to control the quality of the carbon nanotube dispersion paste so that the minimum value (X) is 5 times or more the value (Y). According to the present invention, the dispersion state of the carbon nanotubes (B) can be determined by impedance measurement without diluting the carbon nanotube-dispersed paste, and therefore the quality of the carbon nanotube-dispersed paste can be easily controlled. There are no particular limitations on the quality control method, but the quality of the carbon nanotube dispersion paste can be controlled, for example, by adjusting the frequency at which the reactance minimum appears by combining the amount of carbon nanotubes (B) and the degree of dispersion.

[0052] [Composite paste] The composite paste of the present invention is a composite paste for a lithium ion battery positive electrode, which contains the carbon nanotube dispersion paste of the present invention described above and an electrode active material. The composite paste of the present invention may further contain components (optional components) other than the carbon nanotube dispersion paste of the present invention and the electrode active material, as needed, within a range that does not impair the effects of the present invention.

[0053] <Electrode active material> Examples of electrode active materials include lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), lithium cobalt oxide (LiCoO2), and LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 Examples include lithium composite oxides such as O2. These electrode active materials may be used alone or in combination of two or more.

[0054] <Optional ingredients> Examples of optional components contained in the composite paste include resins, pigments, neutralizing agents, pigment dispersants, antifoaming agents, preservatives, rust inhibitors, plasticizers, antioxidants, and viscosity adjusters.

[0055] <Content> From the viewpoint of further improving electrical conductivity and lithium ion conductivity, the content of the carbon nanotube-dispersed paste in the composite paste is preferably such that the content of carbon nanotubes (B) contained in the carbon nanotube-dispersed paste is 0.1 to 10 mass %, more preferably 0.5 to 8 mass %, and even more preferably 1 to 5 mass %, relative to the total mass of the solid content of the composite paste. The "solid content of the composite paste" refers to all components contained in the composite paste other than the solvent component.

[0056] The content of the electrode active material is preferably 40 to 70 mass % relative to the total mass of the composite paste, more preferably 50 to 65 mass %, and even more preferably 55 to 65 mass %. The content of the electrode active material is preferably 80 to 99.5 mass % relative to the total mass of the solid content of the composite paste, more preferably 90 to 99 mass %, and even more preferably 95 to 98 mass %.

[0057] The content of the solvent component (organic solvent (C), water, etc.) in the composite paste is preferably 30 to 60 mass %, more preferably 35 to 50 mass %, and even more preferably 35 to 45 mass %, based on the total mass of the composite paste.

[0058] <Manufacturing method> The composite paste can be obtained, for example, by uniformly mixing and dispersing the carbon nanotube dispersion paste, the electrode active material, and, if necessary, one or more optional components and organic solvents using a disperser. The organic solvent used in producing the composite paste may be the organic solvent (C) exemplified above in the description of the carbon nanotube dispersion paste. The organic solvent used in producing the composite paste and the organic solvent (C) contained in the carbon nanotube dispersion paste may be the same type or different types, but are preferably the same type. Examples of the dispersing machine include the dispersing machines exemplified above in the description of the carbon nanotube dispersion paste.

[0059] <Action and effect> The composite paste of the present invention described above contains the carbon nanotube dispersion paste of the present invention, and therefore, when a positive electrode composite layer is formed, the primary particles of the carbon nanotubes (B) are dispersed in the positive electrode composite layer while maintaining an appropriate structure. As a result, a conductive path is efficiently formed, electrons can flow easily, and a positive electrode with excellent conductivity can be formed. The composite paste of the present invention is used to form a positive electrode composite layer, which is a part of an electrode, but the composite paste of the present invention may also be used to form a primer layer provided between the positive electrode core material and the positive electrode composite layer.

[0060] [Positive electrode for lithium-ion batteries] The positive electrode for a lithium ion battery of the present invention comprises a positive electrode core material and a layer formed by applying the above-described composite paste of the present invention to the surface of the positive electrode core material. The lithium-ion battery positive electrode of the present invention can be obtained, for example, by applying the composite paste of the present invention to the surface of a positive electrode core material, drying the applied composite paste, and forming a positive electrode composite layer on the surface of the composite paste. The lithium-ion battery positive electrode obtained in this manner has a positive electrode core material and a positive electrode composite layer formed on the surface of the positive electrode core material, and this positive electrode composite layer corresponds to the layer formed by applying the composite paste of the present invention.

[0061] The positive electrode core material is not particularly limited as long as it is a conductive substance, but metals are preferred, and specific examples include aluminum, copper, nickel, iron, titanium, vanadium, chromium, manganese, and alloys thereof. The positive electrode core material may be in the form of a thin film, a net, a fiber, etc. Among these, the thin film form is preferred.

[0062] The composite paste can be applied by a known method using a die coater or the like. The amount of the mixture paste to be applied is not particularly limited, but is preferably set so that the thickness of the positive electrode mixture layer after drying is 0.04 to 0.30 mm, and more preferably 0.06 to 0.24 mm.

[0063] The method for drying the composite paste is not particularly limited, and examples thereof include vacuum drying, pressure drying, heat drying, and air drying. The temperature when drying the composite paste (drying temperature) is, for example, preferably 80 to 200°C, and more preferably 100 to 180°C. The temperature (drying time) when drying the composite paste is, for example, preferably 5 to 120 seconds, more preferably 5 to 60 seconds.

[0064] The lithium ion battery positive electrode of the present invention described above has a layer formed by applying the composite paste of the present invention to the surface of a positive electrode core material, so that primary particles of carbon nanotubes (B) are dispersed in the positive electrode composite layer while maintaining a moderate structure. Therefore, the lithium ion battery positive electrode of the present invention efficiently forms a conductive path, facilitates sufficient electron flow, and has excellent conductivity.

[0065] [Lithium-ion battery] The lithium ion battery of the present invention has the above-described lithium ion battery positive electrode of the present invention. The lithium ion battery of the present invention can be obtained, for example, by arranging the positive electrode for a lithium ion battery of the present invention and the negative electrode facing each other with a permeable separator interposed therebetween, and winding these into a roll (spiral shape) to form a wound electrode body, which is then housed in a battery case and an electrolyte solution is poured into the battery case. The lithium ion battery obtained in this manner comprises the positive electrode for a lithium ion battery of the present invention, the negative electrode, the electrolyte solution, the separator, and a battery case that houses these.

[0066] As the negative electrode, a known negative electrode used in a lithium ion battery can be used. Examples of the separator include porous films made of polyolefins such as polyethylene and polypropylene, laminated films made by laminating these, and nonwoven fabrics.

[0067] The electrolyte solution may be a non-aqueous electrolyte solution, which is a solution in which an electrolyte is dissolved in an organic solvent. Examples of organic solvents include carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, lactones such as γ-butyrolactone, nitriles such as acetonitrile, esters such as methyl formate, methyl acetate, butyl acetate, methyl propionate, and ethyl propionate, ketones such as acetone and methyl ethyl ketone, and amides such as N-methylformamide, N,N-dimethylformamide, and N-methylacetamide. These organic solvents may be used alone or in combination of two or more. Examples of the electrolyte include LiClO4, LiBF4, LiI, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiCl, LiBr, LiB(C2H5)4, LiCH3SO3, LiC4F9SO3, Li(CF3SO2)2N, Li[(CO2)2]2B, etc. These electrolytes may be used alone or in combination of two or more. As the non-aqueous electrolyte, a solution in which LiPF6 is dissolved in a carbonate is preferred, and this solution is particularly suitable as an electrolyte for a lithium ion battery.

[0068] The lithium ion battery of the present invention described above has the lithium ion battery positive electrode of the present invention described above, and therefore an electrically conductive path is efficiently formed, allowing electrons to flow easily and resulting in excellent battery performance. [Example]

[0069] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0070] [Method for producing pigment dispersing resin (A)] <Production Example 1: Production of pigment dispersing resin (A1)> A reaction vessel equipped with a thermometer, reflux condenser, nitrogen gas inlet tube, and stirrer was charged with 100 parts by mass of vinyl acetate as a polymerizable monomer, methanol as a solvent, and azobisisobutyronitrile as a polymerization initiator, and a copolymerization reaction was carried out at a temperature of approximately 60°C. Unreacted monomers were then removed under reduced pressure to obtain a resin solution. Next, a methanol solution of sodium hydroxide was added to the resulting resin solution to carry out a saponification reaction, and the resulting resin solution was thoroughly washed and then dried in a hot air dryer to obtain polyvinyl alcohol (pigment-dispersed resin (A1)). The resulting pigment dispersing resin (A1) had a degree of saponification of 99 mol % and a degree of polymerization of 500.

[0071] <Production Example 2: Production of pigment dispersing resin (A2)> A reaction vessel equipped with a thermometer, reflux condenser, nitrogen gas inlet tube, and stirrer was charged with 100 parts by mass of vinyl acetate as a polymerizable monomer, methanol as a solvent, and azobisisobutyronitrile as a polymerization initiator, and a copolymerization reaction was carried out at a temperature of approximately 60°C. After that, unreacted monomer was removed under reduced pressure to obtain a resin solution. Next, a methanol solution of sodium hydroxide was added to the obtained resin solution to carry out a saponification reaction, and after thorough washing, the resin was dried in a hot air dryer to obtain polyvinyl alcohol (pigment-dispersed resin (A2)). The resulting pigment dispersing resin (A2) had a degree of saponification of 90 mol % and a degree of polymerization of 500.

[0072] <Production Example 3: Production of pigment dispersing resin (A3)> A reaction vessel equipped with a thermometer, reflux condenser, nitrogen gas inlet tube, and stirrer was charged with 97 parts by mass of vinyl acetate and 3 parts by mass of vinyl sulfonic acid as polymerizable monomers, methanol as a solvent, and azobisisobutyronitrile as a polymerization initiator, and a copolymerization reaction was carried out at a temperature of approximately 60°C. Unreacted monomers were then removed under reduced pressure to obtain a resin solution. Next, a methanol solution of sodium hydroxide was added to the resulting resin solution to carry out a saponification reaction, and the resulting resin solution was thoroughly washed and then dried in a hot air dryer to obtain a sulfonic acid-modified polyvinyl alcohol (pigment dispersion resin (A3)). The resulting pigment dispersing resin (A3) had a degree of saponification of 90 mol % and a degree of polymerization of 300.

[0073] <Production Example 4: Production of pigment dispersion resin (A4)> A reaction vessel equipped with a thermometer, reflux condenser, nitrogen gas inlet tube, and stirrer was charged with 90 parts by mass of vinyl acetate and 10 parts by mass of 1-pentene-4,5-diol as polymerizable monomers, methanol as a solvent, and azobisisobutyronitrile as a polymerization initiator, and a copolymerization reaction was carried out at a temperature of approximately 60°C. Unreacted monomers were then removed under reduced pressure to obtain a resin solution. Next, a methanol solution of sodium hydroxide was added to the resulting resin solution to carry out a saponification reaction, and the resulting resin solution was thoroughly washed and then dried in a hot air dryer to obtain a diol-modified polyvinyl alcohol (pigment dispersion resin (A4)). The resulting pigment dispersing resin (A4) had a degree of saponification of 90 mol % and a degree of polymerization of 300.

[0074] [Measurement and evaluation method] <Impedance measurement> (Preparing the measurement cell) Two gold-plated copper plates with a thickness of 0.3 mm were placed opposite each other, and a two-electrode electrode with a distance of 9 mm was used. The size of the electrode was 100 mm. 2 A 20 ml cylindrical container was filled with 15 ml of each of the carbon nanotube dispersed pastes obtained in the examples and comparative examples, and the electrodes were inserted so that they were completely buried in the paste.

[0075] (Impedance measurement) A sinusoidal AC voltage with a peak-to-peak voltage of 0.1 V was applied to each of the carbon nanotube dispersion pastes obtained in the Examples and Comparative Examples at 25°C using an impedance analyzer (Keysight Corporation, product name "4294A"), and the complex impedance and phase difference were measured at 500 points while sweeping the frequency between 100 Hz and 100 MHz. From the obtained data, a Bode plot was created by plotting reactance on the vertical axis and frequency on the horizontal axis.

[0076] <Viscosity measurement> Each of the carbon nanotube dispersion pastes obtained in the examples and comparative examples was measured at a shear rate of 1.0 sec using a cone and plate viscometer (manufactured by Thermo Fisher Scientific, product name "HAAKE Mars3", diameter 35 mm, cone and plate tilted at 2°). -1 The viscosity was measured at 25°C and evaluated according to the following criteria: S, A, and B are considered acceptable. S: Viscosity is less than 1.0 Pa·s. A: The viscosity is 1.0 Pa·s or more and less than 5.0 Pa·s. B: Viscosity is 5.0 Pa·s or more and less than 10.0 Pa·s. C: Viscosity is 10.0 Pa·s or more.

[0077] <Volume resistivity measurement> In measuring the volume resistivity, a 5 mass % solution of polyvinylidene fluoride (manufactured by Kureha Corporation, trade name "KF Polymer L#7305", solvent: N-methyl-2-pyrrolidone) was used as a binder. Each carbon nanotube dispersion paste and binder (KF Polymer L#7305) obtained in the examples and comparative examples was weighed out so that the ratio of the mass of carbon nanotubes (B) in the carbon nanotube dispersion paste to the total mass of the pigment dispersion resin (A) in the carbon nanotube dispersion paste and polyvinylidene fluoride in the binder (KF Polymer L#7305) was 1:9, and the mixture was mixed for 2 minutes using an ultrasonic homogenizer to obtain a coating material. The coating material was applied to a glass plate (2 mm × 100 mm × 150 mm) using the doctor blade method and dried by heating at 80°C for 60 minutes to form a coating film on the glass plate. The film thickness of the resulting coating film was measured, and then the resistance value was measured with an ASP probe (manufactured by Mitsubishi Chemical Analytech Co., Ltd., product name "MCP-TP03P") and a resistivity meter (manufactured by Mitsubishi Chemical Analytech Co., Ltd., product name "Loresta-GP MCP-T610"). The obtained resistance value was multiplied by a resistivity correction factor (RCF) of 4.532 and the film thickness of the coating film to calculate the volume resistivity. The volume resistivity was evaluated according to the following criteria: S, A, and B were considered acceptable. S: The volume resistivity is less than 5 Ω·cm, and the conductivity is very good. A: The volume resistivity is 5 Ω·cm or more and less than 10 Ω·cm, and the conductivity is good. B: The volume resistivity is 10 Ω·cm or more and less than 15 Ω·cm, and the conductivity is normal. C: The volume resistivity was 15 Ω·cm or more, and the conductivity was poor. Or, a smooth coating film could not be produced.

[0078] [Examples 1 to 15, Comparative Examples 1 to 11] According to the formulations shown in Tables 1 to 4, pigment dispersion resin (A), carbon nanotubes (B), and organic solvent (C) were mixed, and the carbon nanotubes (B) were dispersed in a ball mill for the dispersion times shown in Tables 1 to 4 to obtain carbon nanotube dispersion pastes (X-1) to (X-15) and carbon nanotube dispersion pastes (Y-1) to (Y-11). Note that the blending amounts of pigment dispersion resin (A) and carbon nanotubes (B) in the tables are solid content values. The impedance was measured using each of the carbon nanotube dispersion pastes obtained, and the frequency at which the minimum reactance value (X) existed, the minimum reactance value (X), the reactance value (Y) at a frequency of 1 kHz, and the ratio (X / Y) of the minimum value (X) to the value (Y) were determined from the obtained Bode plot. The results are shown in Tables 1 to 4. Note that the Bode plots for Example 4 and Comparative Examples 1 and 3 are shown in Figure 4. Furthermore, the viscosity and volume resistivity of each of the carbon nanotube dispersion pastes obtained were measured. The results are shown in Tables 1 to 4.

[0079] [Table 1]

[0080] [Table 2]

[0081] [Table 3]

[0082] [Table 4]

[0083] The abbreviations in the table are as follows: A blank cell in the table means that the component is not blended (amount blended: 0 parts). A1: Polyvinyl alcohol obtained in Production Example 1 (saponification degree 99 mol%, polymerization degree 500). A2: Polyvinyl alcohol obtained in Production Example 2 (saponification degree 90 mol%, polymerization degree 500). A3: Sulfonic acid-modified polyvinyl alcohol obtained in Production Example 3 (saponification degree 90 mol%, polymerization degree 300). A4: Diol-modified polyvinyl alcohol obtained in Production Example 4 (saponification degree 90 mol%, polymerization degree 300). A5: Polyacrylic acid (weight average molecular weight: 15,000). A6: Polyvinylpyrrolidone (weight average molecular weight: 30,000). B1: ENERMAX 61 (CABOT, multi-walled carbon nanotubes, average outer diameter: 4-16 nm, specific surface area: 230-350 cm 3 / g). B2: ENERMAX31 (CABOT, multi-walled carbon nanotubes, average outer diameter: 10-20 nm, specific surface area: 200-260 cm 3 / g). B3: ENERMAX12 (CABOT, multi-walled carbon nanotubes, average outer diameter: 30-50 nm, specific surface area: 85-110 cm 3 / g).

[0084] As is clear from Tables 1 and 2, the carbon nanotube-dispersed paste obtained in each Example had low viscosity, the carbon nanotubes were suitably dispersed, the volume resistivity was low, and the conductivity was excellent. On the other hand, as is clear from Tables 3 and 4, the carbon nanotube dispersed pastes obtained in the comparative examples did not satisfy both the viscosity and the conductivity. [Explanation of symbols]

[0085] 10 Primary particles 20 Active material 30 aggregates

Claims

1. A carbon nanotube dispersion paste containing carbon nanotubes (B) and an organic solvent (C), the content of the carbon nanotubes (B) is 1 to 10 mass % relative to the total mass of the carbon nanotube dispersion paste, A carbon nanotube dispersion paste, in which a Bode plot obtained by impedance measurement, in which reactance is plotted on the vertical axis and frequency is plotted on the horizontal axis, has a minimum value of the reactance in the frequency range of 50 to 250 kHz.

2. 2. The carbon nanotube dispersion paste according to claim 1, wherein the organic solvent (C) is N-methyl-2-pyrrolidone.

3. Further containing a pigment dispersing resin (A), 3. The carbon nanotube dispersion paste according to claim 1, wherein the content of the pigment dispersion resin (A) is 10 to 100 parts by mass per 100 parts by mass of the carbon nanotubes (B).

4. 4. The carbon nanotube dispersion paste according to claim 1, wherein in the Bode plot, the minimum reactance value in the frequency range of 50 to 250 kHz is 5 times or more the reactance value at a frequency of 1 kHz.

5. Shear rate 1.0 sec －１ The carbon nanotube dispersion paste according to any one of claims 1 to 4, wherein the viscosity at 2000 kJ / min is 10 Pa·s or less.

6. A method for producing a carbon nanotube dispersion paste containing carbon nanotubes (B) and an organic solvent (C), comprising: the content of the carbon nanotubes (B) is 1 to 10 mass % relative to the total mass of the carbon nanotube dispersion paste, A method for producing a carbon nanotube dispersion paste, comprising: mixing the carbon nanotubes (B) and an organic solvent (C) so that a minimum value of the reactance exists in a frequency range of 50 to 250 kHz in a Bode plot obtained by impedance measurement, in which reactance is plotted on the vertical axis and frequency is plotted on the horizontal axis; and dispersing the carbon nanotubes (B).

7. A quality control method for a carbon nanotube dispersion paste containing carbon nanotubes (B) and an organic solvent (C), comprising: the content of the carbon nanotubes (B) is 1 to 10 mass % relative to the total mass of the carbon nanotube dispersion paste, A quality control method for a carbon nanotube-dispersed paste, comprising: controlling the carbon nanotube-dispersed paste so that a minimum value of the reactance exists in a frequency range of 50 to 250 kHz in a Bode plot obtained by impedance measurement, the Bode plot having reactance on the vertical axis and frequency on the horizontal axis.

8. A composite paste comprising the carbon nanotube dispersion paste according to any one of claims 1 to 5 and an electrode active material.

9. A positive electrode for a lithium ion battery, comprising: a positive electrode core material; and a layer formed by applying the composite paste according to claim 8 to a surface of the positive electrode core material.

10. A lithium ion battery comprising the electrode for a positive electrode of a lithium ion battery according to claim 9.

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