Carbon nanotubes, carbon nanotube dispersion liquid, secondary battery using same, and method for producing carbon nanotubes

Abstract

The present invention provides carbon nanotubes which have high dispersibility and storage stability. The carbon nanotubes have a bulk density X of 0.095 g / cm3 to 0.125 g / cm3 inclusive, a Zr content of 5 ppm to 500 ppm inclusive, and a maximum solvent absorption capacity Y as defined by the formula (5) below of 8.0 or more.　Formula (5): Y = W / V (In formula (5), V represents the mass (g) of the carbon nanotubes, and W represents the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotubes when N-methyl-2-pyrrolidone is dropped onto the V (g) of the carbon nanotubes in an environment at 25°C.)

Description

Carbon nanotubes, carbon nanotube dispersion, secondary battery using the same, and method for producing carbon nanotubes

[0001] The present disclosure relates to carbon nanotubes and a method for producing carbon nanotubes. More specifically, the present disclosure relates to a carbon nanotube dispersion, a carbon nanotube resin composition containing the carbon nanotube dispersion and a binder resin, a composite slurry containing the carbon nanotube dispersion, a binder resin, and an active material, an electrode film formed from the composite slurry, and a secondary battery including the electrode film.

[0002] Carbon nanotubes have a cylindrical structure of graphite layers and are chemically stable, electrically conductive, and mechanically strong, and therefore are used in a variety of applications, such as electronic materials, structural materials, and paints. Specific applications of carbon nanotubes include electronic materials, semiconductor materials, filler materials, short needle probes for microscopes, adsorption materials, and filter materials. In particular, electrode materials using carbon nanotubes have excellent electrical conductivity, and are therefore expected to be applied to secondary batteries, fuel cells, electric double layer capacitors, and the like.

[0003] With the widespread use of electric vehicles and the trend toward smaller, lighter, and more powerful mobile phones, secondary batteries with high energy density and even higher capacity are required. Against this background, non-aqueous electrolyte secondary batteries, particularly lithium-ion secondary batteries, which use non-aqueous electrolytes, are increasingly being used in many devices due to their high energy density and high voltage characteristics.

[0004] Electrodes for secondary batteries are fabricated by coating a current collector with a composite slurry containing a positive electrode active material or a negative electrode active material, a conductive material, a binder resin, and the like. While carbon black, graphene, fine carbon materials, and the like have been used as conductive agents, the use of carbon nanotubes, a type of fine carbon fiber, has been investigated to further improve conductivity and thereby increase battery capacity. For example, adding carbon nanotubes to a positive electrode can improve the conductivity of the electrode film and reduce electrode resistance. Multiwalled carbon nanotubes with outer diameters of several nanometers to several tens of nanometers are relatively inexpensive and are increasingly being put to practical use. Using carbon nanotubes with small average outer diameters and long fiber lengths allows for efficient formation of a conductive network even in small amounts, thereby enabling the development of high-capacity secondary batteries. However, carbon nanotubes with these characteristics exhibit strong cohesion, making it difficult to further enhance the dispersibility of carbon nanotube dispersions.

[0005] Patent Document 1 proposes providing a high-concentration, low-viscosity carbon nanotube dispersion containing 2% by mass or more of carbon nanotubes by specifying a wettability index, which is the mass of absorbed solvent relative to the mass of carbon nanotubes. In Patent Document 1, the wettability index is controlled by dry-pulverizing carbon nanotubes using a milling device containing metal or ceramic balls. In a specific example, batch-type dry pulverization is performed using an attrition mill for 10 to 140 minutes. Patent Document 2 also proposes providing a carbon nanotube paint that imparts jet black and gloss to resin molded products, automobile bodies, and the like, using carbon nanotubes with specified average outer diameters and surface oxygen concentrations. In Patent Document 2, the surface oxygen concentration of carbon nanotubes is controlled by pulverizing untreated carbon nanotubes using a grinding medium without the use of a liquid medium.

[0006] Patent Document 3 discloses a conductive material dispersion containing a dispersant containing a specific copolymer, a conductive material, and a dispersion medium. Patent Document 3 proposes providing a conductive material dispersion in which even high concentrations of carbon fibers are well dispersed in a dispersion medium by appropriately controlling the dispersion state so that the phase angle at a frequency of 1 Hz measured by dynamic viscoelasticity measurement of the conductive material dispersion falls within a predetermined range. Patent Document 4 also discloses a conductive material dispersion containing a conductive material, a dispersant, and an amide-based organic solvent. Patent Document 4 proposes providing a conductive material dispersion that achieves all of good dispersibility, stability, and handleability by appropriately controlling the dispersion state so that the pH of the conductive material dispersion, and the phase angle and complex modulus at a frequency of 1 Hz measured by dynamic viscoelasticity measurement fall within predetermined ranges.

[0007] Korean Patent No. 10-2125933 Japanese Patent Application Publication No. 2020-029372 Japanese Patent Application Publication No. 2022-042689 Japanese Patent Application Publication No. 2022-042691

[0008] When carbon nanotubes are dispersed in a dispersion medium, their high aspect ratios and strong physical bonds due to van der Waals forces make them difficult to entangle and disentangle, making them difficult to spread in the dispersion medium. Furthermore, dispersing carbon nanotubes in a dispersion medium increases the viscosity of the dispersion, making it difficult to obtain a dispersion containing uniformly dispersed, high-concentration carbon nanotubes. Obtaining a high-concentration dispersion that combines fluidity and storage stability is particularly challenging. To overcome the inherent dispersion difficulties of carbon nanotubes due to their high aspect ratio, techniques such as those described in Patent Documents 1 and 2 have been proposed that dry-treat carbon nanotubes using grinding media to improve the wettability of carbon nanotubes to solvents, thereby obtaining a carbon nanotube dispersion containing 2% or more by mass but with low viscosity. However, there remains room for further research into the formation of a fully developed conductive network in electrode films using carbon nanotubes, which is due to the inherent high aspect ratio of carbon nanotubes.

[0009] Patent Documents 3 and 4 disclose techniques for dispersing carbon nanotubes, which are conductive materials, at high concentrations by controlling the physical properties of the conductive material dispersion, such as the phase angle and complex modulus, in addition to the composition (dispersant type, etc.) and pH of the conductive material dispersion. However, there is room for further study regarding the method for processing the carbon nanotubes themselves and the stability of the conductive material dispersion over time.

[0010] The present invention aims to provide a carbon nanotube dispersion having high dispersibility and storage stability, a composite slurry having excellent fluidity and dispersion stability, and good dispersibility even when mixed with an active material. More specifically, the present invention aims to provide an electrode film having excellent adhesion to a current collector, a secondary battery having high output, high capacity, and long life, and an electrode film for use therein.

[0011] To achieve both the fluidity and storage stability of a carbon nanotube dispersion, it is important to eliminate the high aspect ratio of the carbon nanotubes and improve their wettability. On the other hand, to fully form a well-developed conductive network in an electrode film using carbon nanotubes, it is important to disperse the carbon nanotubes while maintaining a certain level of their inherent high aspect ratio. There is a trade-off between the fluidity, storage stability, and electrical conductivity of the dispersion. The inventors conducted extensive research aimed at solving the above-mentioned problems and found that by adjusting the bulk density of the carbon nanotubes and the Zr content of the carbon nanotubes to appropriate ranges, it is possible to maintain the wettability of the CNTs to the dispersion medium and sufficiently form a conductive network when a conductive film is formed. Furthermore, by carefully controlling the milling conditions to achieve both the bulk density of the carbon nanotubes and the Zr content of the carbon nanotubes within appropriate ranges during dry processing, it is possible to simultaneously achieve the fluidity, storage stability, and electrical conductivity of the resulting CNT-containing dispersion.

[0012] The carbon nanotubes according to some embodiments of the present invention can be used to provide a carbon nanotube dispersion, a carbon nanotube resin composition, and a composite slurry that are both fluid and stable in storage. Furthermore, the use of these compositions with good carbon nanotube dispersibility can provide an electrode film with a well-developed conductive network, thereby providing a secondary battery with high output, high capacity, and long life.

[0013] Furthermore, due to the high dispersibility of the carbon nanotubes according to some embodiments of the present invention, even when a dispersion containing high concentrations of carbon nanotubes is prepared, it is possible to maintain fluidity and storage stability. This allows the carbon nanotubes to be contained uniformly at high concentrations in electrode films and secondary batteries, thereby providing secondary batteries with higher output and capacity.

[0014] That is, the present invention includes the following embodiments. The embodiments of the present invention are not limited to the following. [1] Bulk density X is 0.095 g / cm ３ 0.125g / cm or more ３ Hereinafter, carbon nanotubes having a Zr content of 5 ppm or more and 500 ppm or less, and a maximum solvent absorption capacity Y of the carbon nanotube defined by the following formula (5) of 8.0 or more: Y=W / V (5) (In formula (5), V is the mass (g) of the carbon nanotube, and W is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotube when N-methyl-2-pyrrolidone is added dropwise to V (g) of carbon nanotube in a 25°C environment.)

[0015] [2] Bulk density X (g / cm ３ and the carbon nanotube's maximum solvent absorption capacity Y satisfies all of the following formulas (1) to (4): Y≦−200X+37 (1), Y≧200X−17 (2), Y≧−200X+27 (3), and Y≦200X+7 (4).

[0016] [3] The carbon nanotube according to [1] or [2], having an average fiber length of 0.2 μm or more and 1.1 μm or less. [4] The carbon nanotube according to [1] or [2], having an average fiber length of 0.2 μm or more and 1.0 μm or less and a Zr content of 15 ppm or more and 200 ppm or less.

[0017] [5] A carbon nanotube dispersion liquid comprising the carbon nanotubes according to any one of [1] to [4] above, a dispersant, and a non-aqueous dispersion medium. [6] A coefficient of determination R of a regression line represented by the following formula (I): ２ The carbon nanotube dispersion according to [5], wherein the y-intercept is 0.85 or more, the slope a is 2≦a<20, and the y-intercept b is −5≦b<10. y=ax+b Formula (I) (Formula (1) is a regression line obtained by the least squares method from a scatter diagram plotting x on the horizontal axis and y on the vertical axis, measured using a carbon nanotube dispersion having a static storage temperature of 40°C for 1 day, 40°C for 7 days, and 40°C for 14 days, respectively, and wherein the linear regression line is a regression line obtained by the least squares method from a scatter diagram plotting x on the horizontal axis and y on the vertical axis, measured using a carbon nanotube dispersion having a static storage temperature of 40°C for 1 day, 40°C for 7 days, and －１ The shear viscosity at η ｉ (Pa s), the complex modulus of elasticity at 25 ° C. and a frequency of 1 Hz obtained by dynamic viscoelasticity measurement is G ＊ ｉ (Pa), x is 25°C, shear rate 1.0 s －１ The shear viscosity η ｉ The natural logarithm of (Pa s) (ln(η ｉ )), y is the complex elastic modulus G at 25 ° C. and a frequency of 1 Hz ＊ ｉ (Pa), and i is 1, 7, and 14, with the production date of the carbon nanotube dispersion being day 0.

[0018] [7] A carbon nanotube resin composition comprising the carbon nanotube dispersion liquid according to [5] or [6] above and a binder resin. [8] A composite slurry comprising the carbon nanotube dispersion liquid according to any one of [5] to [7] above, a binder resin, and an active material. [9] An electrode film formed from the composite slurry according to [8] above.

[10] A secondary battery comprising a positive electrode and a negative electrode, at least one of the positive electrode and the negative electrode having the electrode film according to [9].

[0019]

[11] A method for producing a carbon nanotube powder comprising: dry-pulverizing carbon nanotubes using a milling device including a milling medium made of a metal or ceramic material; wherein the dry-pulverizing is a pass-type milling or a circulation-type milling; and wherein the carbon nanotubes after the dry-pulverization have a bulk density X of 0.095 g / cm. ３ 0.125g / cm or more ３The following is a method for producing carbon nanotubes, wherein the Zr content is 5 ppm or more and 500 ppm or less, and the maximum solvent absorption capacity Y of the carbon nanotubes, as defined by the following formula (5), is 8.0 or more: Y=W / V formula (5) (In formula (5), V is the mass (g) of the carbon nanotubes, and W is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotubes when N-methyl-2-pyrrolidone is added dropwise to V (g) of carbon nanotubes in an environment of 25°C.)

[12] The method for producing carbon nanotubes according to

[11] , wherein the rate of change between the bulk density X after dry-pulverization and the bulk density X' before dry-pulverization is -60% or more and 1000% or less. Bulk density change rate (%) = (X / X' - 1) x 100

[13] The method for producing carbon nanotubes according to

[11] or

[12] , wherein the change rate between the powder resistivity Re after dry-pulverization and the powder resistivity Re' before dry-pulverization is 1% or more and 20% or less. Powder resistivity change rate (%) = (Re / Re' - 1) x 100

[14] The method for producing carbon nanotubes according to any of

[11] to

[13] , wherein the grinding media are zirconia beads, and the zirconia beads have a bead diameter of more than 5 mm and less than 10 mm.

[15] The method for producing carbon nanotubes according to any of

[11] to

[14] , wherein the carbon nanotubes after dry-pulverization have an average fiber length of 0.2 μm or more and 1.1 μm or less.

[16] The method for producing carbon nanotubes according to any one of

[11] to

[15] , wherein the carbon nanotubes after dry-pulverization have an average fiber length of 0.2 μm or more and 1.0 μm or less and a Zr content of 15 ppm or more and 200 ppm or less.

[17] A method for producing a carbon nanotube dispersion liquid, comprising a step of mixing the carbon nanotubes according to any one of

[11] to

[16] above, a dispersant, and a non-aqueous dispersion medium, and dispersing the mixture.

[0020] According to an embodiment of the present invention, carbon nanotubes exhibiting excellent dispersibility can be provided, and by using the carbon nanotubes, a carbon nanotube dispersion liquid having excellent fluidity and high dispersion and storage stability can be provided. Furthermore, according to another embodiment of the present invention, a carbon nanotube resin composition exhibiting excellent fluidity and storage stability even when mixed with a binder resin can be provided, and since the carbon nanotubes exhibit excellent dispersibility even after being mixed with an active material, it is possible to provide a secondary battery electrode composite slurry that is less likely to produce carbon nanotube aggregates or precipitates and that exhibits high conductivity during electrode film formation. According to yet another embodiment of the present invention, a secondary battery with high output, high capacity, and long life can be provided.

[0021] The natural logarithm (ln(η)) of the shear viscosity η (Pa s) measured using the carbon nanotube dispersion liquid of the Examples or Comparative Examples, and the complex elastic modulus G * 1 is a scatter diagram plotting (Pa) and an example of a regression line expressed from the scatter diagram by the least squares method.

[0022] The following describes in detail the carbon nanotubes, carbon nanotube dispersion, carbon nanotube resin composition containing the carbon nanotube dispersion and a binder resin, composite slurry containing the carbon nanotube dispersion, a binder resin, and an active material, an electrode film formed from the composite slurry, a secondary battery including the electrode film, and a method for producing carbon nanotubes, which are embodiments of the present invention. The present invention is not limited to the following embodiments, and includes embodiments that are implemented within the scope of the present invention.

[0023] The carbon nanotube dispersion and the carbon nanotube resin composition containing the carbon nanotube dispersion and the binder resin refer to the state before the active material is added. In this respect, the carbon nanotube dispersion and the carbon nanotube resin composition are distinguished from a composite slurry containing an active material. That is, the carbon nanotube dispersion and the carbon nanotube resin composition are substantially free of active material. This concept excludes a state in which an active material is intentionally added to the carbon nanotube dispersion and the carbon nanotube resin composition, and the active material may be present in an amount of 1% by mass or less, 0.5% by mass or less, or 0.1% by mass or less, relative to the total mass of the carbon nanotube dispersion and the carbon nanotube resin composition, or even 0% by mass.

[0024] In this specification, carbon nanotubes may be referred to as "CNT" and N-methyl-2-pyrrolidone may be referred to as "NMP." In this specification, a carbon nanotube dispersion may be referred to as a "CNT dispersion" or simply as a "dispersion."

[0025] <<Carbon Nanotubes>> The carbon nanotubes of this embodiment have a bulk density X of 0.045 g / cm ３ 0.125g / cm or more ３ The carbon nanotubes satisfy the following conditions, and the Zr content is 3 ppm or more and 500 ppm or less. This results in carbon nanotubes with excellent dispersibility, and a carbon nanotube dispersion using these carbon nanotubes can achieve both fluidity and storage stability. More preferably, the carbon nanotubes have a bulk density X of 0.095 g / cm ３ 0.125g / cm or more ３ The carbon nanotube dispersion preferably satisfies the following conditions, has a Zr content of 5 ppm or more and 500 ppm or less, and has a maximum solvent absorption capacity Y of 8.0 or more as defined by the formula (5) described below. This allows the filterability and acid stability of the carbon nanotube dispersion produced using the carbon nanotubes to be further improved. From another viewpoint, the carbon nanotubes preferably have a bulk density X of 0.045 g / cm ３ 0.125g / cm or more ３The following conditions are preferably satisfied, the Zr content is 3 ppm or more and 500 ppm or less, and the maximum solvent absorption capacity Y of the carbon nanotubes defined by the formula (5) described below is 14.0 or more. This allows for dispersibility in the CNT dispersion, and the battery performance in the secondary battery can be further improved by the conductive network. In this case, the average fiber length of the CNTs is preferably 1.0 μm or more, 1.5 μm or more, or 1.9 μm or more, and preferably 10 μm or less, 5 μm or less, or 2.0 μm or less. Within these ranges, the conductivity and adhesion can be further improved by the conductive network in the secondary battery.

[0026] The filterability of CNT dispersions tends to decrease due to small amounts of aggregates originating from CNTs and other resin components in the CNT dispersion. Therefore, improving filterability can also improve the degree of CNT disentanglement. This allows for better development of conductive networks in secondary batteries. Furthermore, improved filterability also reduces filter clogging during the manufacturing processes of CNT dispersions, CNT resin compositions, and composite slurries. Regarding the acid stability of CNT dispersions, if the CNT dispersion maintains good stability even after the addition of acid, it tends to maintain good viscosity over time. One possible cause of this is thought to be the action of acid on CNTs that do not have a dispersant adsorbed thereon, causing aggregation. Therefore, improving acid stability can also improve the stability of CNT dispersions over time. This allows for better development of conductive networks in secondary batteries.

[0027] The bulk density X of CNT contributes to the wettability of the liquid medium, and can also be used as an index of the degree of processing of CNT when processing CNT by dry grinding or the like.

[0028] In particular, when using CNT as a conductive material in battery applications, one of the most important factors is to utilize the high aspect ratio unique to CNT in a CNT dispersion. When improving the wettability of CNT in a liquid medium by dry processing or the like, if the processing intensity is too strong, the formation of CNT agglomerates is promoted, resulting in the CNT not being sufficiently dissolved in a non-aqueous solvent, and the aspect ratio is reduced, which may result in a decrease in conductivity. In addition, it is possible to disperse the CNT agglomerates at high intensity without performing CNT processing such as dry milling, but the resulting dispersion may have poor storage stability due to breakage of CNT fibers, etc. When the bulk density of the CNT is 0.045 g / cm ３ 0.125g / cm or more ３ By ensuring that the dispersion is equal to or less than 100%, it is possible to appropriately control the strength of CNT aggregation, the size of the aggregates, etc. Furthermore, the CNT fibers can be dispersed appropriately in the non-aqueous solvent without breaking, and the required conductivity can be maintained, so that by forming an electrode film using this dispersion, a well-developed conductive network can be formed in the electrode film.

[0029] The bulk density X of the CNT is 0.045 g / cm ３ or more, preferably 0.050 g / cm ３ or more, more preferably 0.080 g / cm ３ or more. Also, 0.125 g / cm ３ or less, preferably 0.125 g / cm ３ or less, more preferably 0.120 g / cm ３ or less, and more preferably 0.110 g / cm ３ or less, and particularly preferably 0.100 g / cm ３ The bulk density X of the CNTs is set to 0.090 g / cm from the viewpoint of further improving filterability by appropriately controlling the strength of aggregation of the CNTs, the size of the aggregates, etc. ３ More preferably, 0.095 g / cm ３ More preferably, 0.097 g / cm ３ Above, 0.099g / cm ３ or more, or 0.100 g / cm ３For example, the bulk density X of the CNTs may be 0.090 g / cm or more. ３ 0.125g / cm or more ３ More preferably, 0.095 g / cm or less ３ 0.120g / cm or more ３ The following ranges are more preferable: Within these ranges, the filterability and acid stability of the CNT dispersion can be further improved.

[0030] The bulk density X of CNTs in this disclosure refers to the loose bulk density when loosely packed. Bulk density X can be determined by allowing CNTs to freely fall into an open-top container of a specified capacity until they overflow, leaving the container to stand, leveling off the CNT powder that has risen on the upper surface with a leveling plate, measuring the mass of the CNTs, and dividing the mass by the volume of the container.

[0031] A specific method for measuring bulk density X is, for example, to allow CNT powder to freely fall into a 30 ml stainless steel cylindrical container, and after leveling off any bulging portion on the top surface of the container, determine the mass of the CNT powder and divide it by the volume of the container. Agglomerates formed during storage of the CNT powder are crushed, and a sample of a quantity sufficient for testing is passed through a 0.5 mm sieve to allow the obtained sample to flow freely into the measurement container until it overflows.

[0032] The Zr content contained in the CNT of this embodiment is 3 ppm or more and 500 ppm or less. It is preferably 400 ppm or less, more preferably 300 ppm or less, and even more preferably 200 ppm or less. It is also preferably 4 ppm or more and more preferably 5 ppm or more. From the viewpoint of weakening the van der Waals forces between CNTs and further improving the dispersion stability of the CNTs, the Zr content contained in the CNTs is more preferably 5 ppm or more, even more preferably 10 ppm or more, even more preferably 15 ppm or more, and may be 20 ppm or more, 30 ppm or more, or 40 ppm or more. For example, the Zr content contained in the CNTs is preferably 5 ppm or more and 500 ppm or less, more preferably 10 ppm or more and 300 ppm or less, and even more preferably 15 ppm or more and 200 ppm or less. Within these ranges, the dispersion stability of the CNT dispersion can be further improved, and battery performance can be further improved.

[0033] The Zr content of the CNTs can be measured by inductively coupled plasma (ICP) optical emission spectroscopy.

[0034] Furthermore, the Zr contained in CNT, detected as zirconium element by ICP optical emission spectrometry, may be a substance (e.g., grinding media) used in a pretreatment process such as dry processing of CNT, or may be from wear of disperser components, or may be introduced by adding a zirconium-containing compound such as zirconia (zirconium oxide) to CNT. The Zr content of CNT can be adjusted by adjusting the type of dry grinding media, additives / coating agents, and dry grinding processing conditions (time, intensity, processing temperature, etc.). Furthermore, the Zr contained in CNT can be partially reduced by centrifugation, utilizing the difference in specific gravity between CNT and Zr. Zr has excellent toughness and heat resistance, and the majority of zirconia exists at a size that cannot be classified from CNT. Therefore, it is difficult to completely remove all Zr once it has been mixed in any of the manufacturing processes, from CNT pretreatment to CNT dispersions, CNT resin compositions containing the dispersions, composite slurries, and ultimately secondary batteries. Therefore, it is important to appropriately control the Zr content of CNTs by the above-mentioned method, particularly in secondary battery applications. Carbon nanotubes according to some embodiments of the present invention have excellent effects because they are carbon nanotube compositions containing Zr.

[0035] Although the reason why the dispersibility of CNTs is improved by setting both the bulk density X and the Zr content within the specified ranges is unclear, it is speculated as follows. It is speculated that the presence of Zr weakens the van der Waals forces between CNTs, thereby improving the dispersion stability of the CNTs. A Zr content of 3 ppm or more does not require the application of strong force to uniformly disperse CNTs that form bundle structures due to van der Waals forces, and increases in the powder resistivity of the CNTs and the volume resistivity of the electrode containing CNTs can be suppressed. Furthermore, a Zr content of 500 ppm or less prevents excessive Zr from inhibiting the adsorption of the dispersant to the CNTs, resulting in an unstable dispersion state, and also suppresses a decrease in discharge capacity due to Zr adhering to the active material. More preferably, the Zr content is 5 ppm or more and 500 ppm or less. On the other hand, as mentioned above, the bulk density contributes to the wettability of the liquid medium to the carbon nanotubes, and by appropriately controlling the bulk density, it is possible to uniformly and satisfactorily disperse the CNTs while maintaining their length at a certain level or more, thereby forming a well-developed conductive network. By keeping both the bulk density and the Zr content within the specified ranges, it is possible to maximize the inherent conductivity of the CNTs while achieving both CNT dispersibility and stability over time.

[0036] The CNTs of this embodiment further have a bulk density X (g / cm 3 ) from the viewpoint of more precisely controlling the defibrillation state of the CNTs and microscopically controlling the dispersibility in the CNT dispersion. ３ ) and the maximum solvent absorption capacity Y of the carbon nanotube defined by the following formula (5) preferably satisfies all of the following formulas (1) to (4): Y≦−200X+37 (1) Y≧200X−17 (2) Y≧−200X+27 (3) Y≦200X+7 (4) Y=W / V (5) (In formula (5), V is the mass (g) of the carbon nanotube, and W is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotube when N-methyl-2-pyrrolidone is added dropwise to V (g) of carbon nanotube in a 25° C. environment.)

[0037] The maximum solvent absorption capacity Y of CNT is defined by the above formula (5). The maximum solvent absorption capacity Y is the maximum mass of N-methyl-2-pyrrolidone absorbed by CNT per mass of CNT when N-methyl-2-pyrrolidone is absorbed by CNT. The maximum mass of N-methyl-2-pyrrolidone absorbed by CNT is the total mass of N-methyl-2-pyrrolidone added dropwise to CNT powder up to the point just before N-methyl-2-pyrrolidone starts to flow out of the CNT powder.

[0038] Specifically, in a 25°C environment, 5 g (V (g)) of CNT powder is placed in a cylindrical polypropylene container with a diameter of 10 mm by gravity, and while the container is left to stand, 0.5 g of N-methyl-2-pyrrolidone is dropped onto the surface of the CNT powder at intervals of 1 minute. It is observed whether the droplets of N-methyl-2-pyrrolidone begin to flow out onto the surface of the CNT powder without being absorbed by the CNT powder. The total mass (g) of N-methyl-2-pyrrolidone dropped up to just before the droplets of N-methyl-2-pyrrolidone begin to flow out onto the surface of the CNT powder is defined as W (g).

[0039] Formulas (1) to (4) are explained below. When the maximum solvent absorption capacity Y and bulk density X calculated above are within the ranges specified by the formulas (1) to (4), the dispersibility and storage stability of the CNTs can be further improved.

[0040] The maximum solvent absorption capacity Y is the maximum mass of N-methyl-2-pyrrolidone absorbed by CNT, but is not limited to the type of dispersion medium. A smaller maximum solvent absorption capacity tends to suppress the increase in the initial viscosity of CNT in the dispersion medium and contribute to improved dispersibility. On the other hand, the bulk density X is a factor that affects the dispersibility and storage stability of CNT in the dispersion medium. While this cannot be generalized, a larger bulk density makes it more likely that CNT will sediment in the dispersion medium, while a smaller bulk density reduces the wettability of CNT in the dispersion medium, making it more likely that dispersibility will decrease.

[0041] Focusing on the fact that the maximum solvent absorption capacity Y and bulk density X interact to affect the dispersibility and storage stability of carbon nanotubes in a dispersion medium, carbon nanotubes can be specified by the above formulas (1) to (4). When improving the dispersibility and storage stability of carbon nanotubes, it is more preferable to defibrate the carbon nanotubes while maintaining a high aspect ratio and finely disperse them in a dispersion medium.

[0042] If the maximum solvent absorption capacity Y and the bulk density X do not satisfy formula (1), i.e., if Y > -200X + 37, the powder resistivity and electrode resistivity of the CNTs will increase, and battery performance may deteriorate. Preferably, Y ≤ -200X + 35, and more preferably, Y ≤ -200X + 33.

[0043] If the maximum solvent absorption capacity Y and the bulk density X do not satisfy formula (2), i.e., if Y<200X-17, aggregation and sedimentation of CNTs may occur, resulting in an increase in electrode resistivity and a decrease in battery performance. Preferably, Y≧200X-13, and more preferably, Y≧200X-9.

[0044] If the maximum solvent absorption capacity Y and the bulk density X do not satisfy formula (3), i.e., if Y<-200X+27, the CNTs will have poor dispersibility, which may result in aggregation and sedimentation, leading to a decrease in battery performance. Preferably, Y≧-200X+29, and more preferably, Y≧-200X+31.

[0045] If the maximum solvent absorption capacity Y and the bulk density X do not satisfy formula (4), i.e., if Y > 200X + 7, the dispersibility of the CNTs will be poor, the fluidity and storage stability of the CNT dispersion will be reduced, and battery performance may be reduced. Preferably, Y ≦ 200X + 3, and more preferably, Y ≦ 200X − 1.

[0046] The numerical range of the maximum solvent absorption capacity Y is not particularly limited. For example, the maximum solvent absorption capacity may be 5 to 22, 5 to 18, or 14 to 18. In other examples, the maximum solvent absorption capacity may be 5 to 12 or 14 to 22. From the viewpoint of further improving the filterability and acid stability of the CNT dispersion, the maximum solvent absorption capacity Y is preferably 8.0 or greater. It is more preferably 8.4 or greater, even more preferably 8.5 or greater, and even more preferably 11.8 or greater. For example, the maximum solvent absorption capacity Y may be 8.4 or greater and 22 or less, 8.5 or greater and 18 or less, or 11.8 or greater and 12.7 or less. From another viewpoint, the maximum solvent absorption capacity Y is preferably 14 or greater. For example, it may be 14 or greater and 22 or less, 16 or greater and 18 or less, or 17.1 or greater and 17.5 or less. Within these ranges, the dispersion stability of the CNT dispersion can be further enhanced, and battery performance can be further improved.

[0047] CNTs include single-walled CNTs and multi-walled CNTs, and these may be mixed. Single-walled CNTs have a structure in which one layer of graphite is wrapped around them, while multi-walled CNTs have a structure in which two or more layers of graphite are wrapped around them. Furthermore, the sidewalls of CNTs do not have to have a graphite structure. For example, CNTs with sidewalls having an amorphous structure are also considered CNTs in this specification. CNTs may be produced by any method. CNTs can generally be produced by laser ablation, arc discharge, thermal CVD, plasma CVD, and combustion methods, but are not limited to these. Furthermore, CNTs may be surface-treated CNTs. CNTs may be CNT derivatives to which functional groups, such as carboxyl groups, have been added. CNTs encapsulating organic compounds, metal atoms, or substances, such as fullerenes, may also be used.

[0048] The average outer diameter of the CNTs is preferably 1 nm or more, more preferably 3 nm or more, and even more preferably 5 nm or more. It is also preferably 30 nm or less, more preferably 20 nm or less, and even more preferably 15 nm or less. The average outer diameter of the CNTs can be calculated by first observing and photographing the CNTs using a transmission electron microscope, randomly selecting 300 CNTs from the photograph, and measuring the outer diameter of each.

[0049] The average fiber length of the CNTs is preferably 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.5 μm or more. It is also preferably 5.0 μm or less, more preferably 2.0 μm or less, and even more preferably 1.0 μm or less. The average fiber length of the CNTs can be calculated by first observing and photographing the CNTs using a scanning electron microscope, randomly selecting 300 CNTs from the photograph, and measuring the fiber length of each. From the viewpoint of further improving battery performance through the conductive network of the secondary battery, the average fiber length of the CNTs is more preferably 0.2 μm or more, even more preferably 0.5 μm or more, and even more preferably 0.8 μm or more. From the viewpoint of further improving filterability and acid stability in the CNT dispersion, the average fiber length of the CNTs is more preferably 1.5 μm or less, even more preferably 1.1 μm or less, and even more preferably 1.0 μm or less. For example, it may be 0.2 μm or more and 1.5 μm or less, 0.5 μm or more and 1.1 μm or less, or 0.8 μm or more and 1.0 μm or less, more preferably 0.2 μm or more and 1.1 μm or less, and even more preferably 0.2 μm or more and 1.0 μm or less.

[0050] The aspect ratio is the value obtained by dividing the fiber length of a CNT by its outer diameter. A typical aspect ratio can be calculated using the average fiber length and the average outer diameter. A conductive material with a higher aspect ratio can provide higher conductivity when used to form an electrode. The aspect ratio of a CNT is preferably 30 or more, more preferably 50 or more, and even more preferably 80 or more. It is also preferably 10,000 or less, more preferably 3,000 or less, and even more preferably 1,000 or less.

[0051] The specific surface area of ​​CNT is 100m ２ / g or more, and ２ / g or more is more preferable, and 200m ２ / g or more is more preferable. ２ / g or less, and ２ / g or less is more preferable, and ２ / g or less is even more preferable. The specific surface area of ​​the CNTs is calculated by the BET method using nitrogen adsorption measurements. When the average outer diameter, average fiber length, aspect ratio, and specific surface area are within the above ranges, a well-developed conductive path is easily formed in the electrode.

[0052] The carbon purity of CNT is expressed as the content (mass%) of carbon atoms in the CNT. The carbon purity is preferably 80 mass% or more, more preferably 90 mass% or more, even more preferably 95 mass% or more, and particularly preferably 98 mass% or more, relative to 100 mass% of CNT. By keeping the carbon purity within the above range, problems such as short circuits caused by the formation of dendrites due to impurities such as metal catalysts can be prevented. CNTs that have undergone a high-purity treatment may be used in order to remove or reduce impurities such as metal catalysts and increase the carbon purity. The method of the high-purity treatment is not particularly limited, and known methods can be used. CNTs according to some embodiments have a bulk density X of 0.095 g / cm ３ 0.125g / cm or more ３It is preferable that the following conditions are satisfied, the Zr content is 5 ppm or more and 500 ppm or less, and the maximum solvent absorption capacity Y of the carbon nanotubes defined by the formula (5) below is 8.0 or more, and that the following conditions (1) or (2) are satisfied. This makes it possible to further improve the filterability and acid stability of a carbon nanotube dispersion produced using the carbon nanotubes. (1) The average fiber length is 0.2 μm or more and 1.1 μm or less. (2) The average fiber length is 0.2 μm or more and 1.0 μm or less, and the Zr content is 15 ppm or more and 200 ppm or less.

[0053] <Method for Producing Carbon Nanotubes> The CNTs of this embodiment are not limited to a specific manufacturing method and can be specified by their physical property values. They can be prepared by pretreating CNTs manufactured according to a standard manufacturing method or conventionally known CNTs. The pretreatment method for CNTs is not particularly limited, and may include surface treatment using chemicals or heat treatment. Furthermore, one or more pretreatment methods may be used in combination, but it is preferable to at least perform a pulverization treatment. The physical properties of the carbon nanotubes can be controlled by optimizing the pulverization conditions.

[0054] For example, the CNT of this embodiment can be obtained by pulverizing CNTs having readily available physical properties. The pulverization may be either wet pulverization or dry pulverization, and pulverization media may be used. Dry pulverization is preferred because it is easier to control the physical properties of the CNTs, and dry pulverization using pulverization media is more preferred. The pulverization media may be made of metal or ceramic materials, and is not particularly limited. The components input into the pulverization device may be only untreated CNTs, but optional components such as lubricants, dispersants, and surface treatment agents may also be added. Dry pulverization is preferred, but if necessary, a liquid medium may be input into the pulverization device together with the untreated CNTs.

[0055] When a milling device is used, the milling process may be performed using any of the batch, pass, and circulation methods. However, due to the ease of control of the physical properties of the CNTs, the pass or circulation method is preferred, with the circulation method being more preferred. The batch method is a method in which processing is performed using only the dispersion device itself, without using piping or the like. The pass method is a method in which the milling device itself is equipped with a tank that supplies CNTs via piping and a tank that receives CNTs, and the CNTs are passed through the milling device itself. The circulation method is a method in which the CNTs that have passed through the dispersion device itself are returned to the tank that supplies CNTs and are circulated while being processed. In both cases, the longer the processing time, the more the milling process progresses, so the pass or circulation can be repeated until the desired state is reached, and the processing volume can be increased by changing the tank size or processing time. Specific milling devices include, for example, bead mills, ball mills, and attrition mills, with ball mills and attrition mills being considered batch processing. As a pass-type or circulation-type milling device, for example, a dry continuous milling device equipped with a high-speed rotating arm in a horizontal cylindrical tank and stirring the milling media can be used. When zirconia beads are used as the milling media, it is preferable to use a pass-type or circulation-type dry continuous milling device from the viewpoint of adjusting the Zr content of the CNTs to an appropriate range.

[0056] In a pass-type or circulation-type milling process, the desired CNTs can be obtained by controlling the mass of CNTs fed into the milling device per unit time, the type, diameter, and amount of milling media, the milling temperature, and other factors. In one example, a circulation-type bead mill is used to feed the CNT raw material and perform dry milling. Methods for controlling the milling temperature include cooling with a cooling jacket attached to the milling chamber body, cooling outside the bead mill system, and a combination of these. From the perspective of cooling efficiency, cooling with a cooling jacket attached to the milling chamber body is preferred. The milling temperature can be measured by installing a thermometer in any piping. Specifically, it can be confirmed by measuring the powder outlet temperature using a thermometer installed at the outlet of the bead mill.

[0057] When dry milling is performed using milling media, ceramic materials are preferred for the milling media, in order to prevent the incorporation of metal impurities such as iron into the CNTs and to enhance milling process strength, with zirconia being more preferred. The milling media diameter is preferably 1 mm or more, more preferably 3 mm or more, and even more preferably 5 mm or more. Having a milling media diameter equal to or greater than the above range increases the collision energy with the CNTs, thereby improving dry milling efficiency. Furthermore, the milling media diameter is preferably 15 mm or less, more preferably 10 mm or less, and even more preferably 8 mm or less. Having a milling media diameter equal to or less than the above range reduces wear due to collisions between milling media. Repeated use of milling media used in dry milling of CNTs can cause wear and cracking, which can lead to poor dispersion and contamination. Therefore, it is preferable to classify and clean the milling media before use. When using zirconia beads as milling media, the bead diameter should be greater than 5 mm and less than 10 mm to more appropriately adjust the Zr content of the CNTs. When two or more types of beads with significantly different diameters are used in combination, the beads collide with each other and wear, which can easily lead to a large amount of Zr being mixed into the CNTs, so it is advisable to keep the bead diameter within ±1 mm. This tendency is accentuated in bead mills for dry grinding, so when a bead mill is used, it is preferable to use one type of zirconia beads, and it is even more preferable to keep the bead diameter of the zirconia beads within ±1 mm.

[0058] Here, the necessity and effects of CNT pretreatment, particularly dry treatment, will be described in detail. If a CNT dispersion is prepared while maintaining the CNT length, i.e., while maintaining a high aspect ratio, the bulkiness of the CNTs results in poor wettability to the dispersion medium, which can lead to poor handling, increased viscosity, and reduced uniformity of the dispersion. Furthermore, the composite slurry produced using such a CNT dispersion also becomes highly viscous and non-uniform, which can lead to reduced coatability and reduced uniformity of the electrode film. On the other hand, if the bulk density becomes too high due to pretreatment, etc., the preparation of the CNT dispersion (specifically, ease of loading and wettability to the dispersion medium, etc.) becomes easier and the finished viscosity of the dispersion tends to be lower. However, the increased specific surface area of ​​the CNTs requires a large amount of dispersant to achieve sufficient stability, which is undesirable from the perspective of electrode film design. Furthermore, CNTs in an agglomerated state with a high bulk density, or CNTs that are dispersed with high strength to break down the agglomerates and then broken into short pieces, are less likely to form a well-developed conductive network, potentially resulting in poor conductivity. Furthermore, if the viscosity of the composite slurry is too low, migration may occur during the drying process during electrode film production, potentially resulting in non-uniform electrode films. Furthermore, if the CNTs are pretreated to such an extent that the bulk density becomes too high, Zr may inhibit the adsorption of the dispersant to the CNTs, and there is also the risk of Zr adhering to the active material, resulting in a decrease in discharge capacity. In light of these issues, by obtaining CNTs with appropriately controlled bulk density and Zr content, it is possible to realize a CNT dispersion with excellent dispersibility, stability, and conductivity. Furthermore, it is possible to obtain an electrode film with a well-developed conductive network, thereby providing a secondary battery with high output, high capacity, and long life.

[0059] The bulk density X of the CNT is 0.045 g / cm ３ ~0.125g / cm ３ and the Zr content of the CNT is adjusted to a range of 3 ppm to 500 ppm, thereby making it possible to obtain CNTs with excellent performance. More preferably, the bulk density X of the CNT is adjusted to 0.095 g / cm ３ ~0.125g / cm ３It is preferable to adjust the Zr content of the CNT to a range of 5 ppm to 500 ppm, and to adjust the maximum solvent absorption capacity Y to 8.0 or more.

[0060] Furthermore, the present inventors have found that when dry processing is used as a pretreatment for CNT, it is necessary to improve the wettability to a certain level or above while maintaining a high aspect ratio of the CNT to a certain extent. Therefore, they have found that by setting the bulk density X of the CNT and the Zr content of the CNT within the above ranges as indicators for determining the degree of CNT processing, in other words, the endpoint of the dry processing of CNT, CNT with excellent dispersibility and electrical conductivity can be obtained.

[0061] That is, when dry processing is employed as a pretreatment for CNTs, the pretreatment is carried out while controlling the bulk density X of the CNTs and the Zr content of the CNTs. It is preferable to appropriately adjust the dry processing conditions for CNTs by evaluating the dispersibility of the CNT dispersion, and even the conductivity of the composite slurry and electrode film using the same, but for the reasons mentioned above, it is also possible to measure the bulk density X and the Zr content during dry processing and directly use these physical properties as indicators for obtaining a desirable processing state.

[0062] The rate of change between the bulk density X after dry milling and the bulk density X' before dry milling is preferably -60% or more and 1000% or less, more preferably -20% or more and 500% or less, and even more preferably -5% or more and 250% or less. When the rate of change in bulk density is within the above range, CNTs exhibiting better dispersibility and higher conductivity can be obtained. The rate of change in bulk density can be calculated using the following formula: Rate of change in bulk density (%) = (X / X' - 1) x 100

[0063] The rate of change between the powder resistivity Re after dry milling and the powder resistivity Re' before dry milling is preferably 1% or more and 20% or less, more preferably 2% or more and 18% or less, and even more preferably 3% or more and 15% or less. When the rate of change in powder resistivity is within the above range, CNTs exhibiting better dispersibility and high conductivity can be obtained. The rate of change in powder resistivity can be calculated using the following formula: Rate of change in powder resistivity (%) = (Re / Re' - 1) x 100

[0064] The average fiber length of the CNTs after dry milling is preferably 0.1 μm or more and 5 μm or less, and may be 0.2 μm or more and 2 μm or less, or 0.5 μm or more and 1 μm or less. Carbon nanotubes with a fiber length of 0.1 μm or more can further enhance their electrical conductivity and toughness. Furthermore, carbon nanotubes with a fiber length of 1 μm or less can suppress the occurrence of aggregation even when defibration progresses during dispersion of CNTs in a non-aqueous dispersion medium, thereby more effectively preventing an increase in the viscosity of the dispersion. The average fiber length of the CNTs before dry milling is not particularly limited, but is preferably 0.6 μm or more, more preferably 0.9 μm or more, and even more preferably 1.2 μm or more, because it is preferable to appropriately control the fiber length distribution of the CNTs by performing a dry milling process. Furthermore, it is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 60 μm or less. The average fiber length of the CNTs can be calculated using the method described above. In addition, since it is preferable to appropriately adjust the dry processing conditions, etc., depending on the average outer diameter and average fiber length of the CNTs before dry processing, there are no particular limitations on the rate of change between the average fiber length of the CNTs after dry grinding and the average fiber length of the CNTs before dry grinding.

[0065] The untreated CNTs are not particularly limited and may be CNTs produced by any method. CNTs can generally be produced by, but are not limited to, laser ablation, arc discharge, thermal CVD, plasma CVD, and combustion. For example, CNTs can be produced by catalytically reacting a carbon source with a catalyst at 500 to 1000°C in an atmosphere with an oxygen concentration of 1% by volume or less. Furthermore, any conventionally known source gas can be used as the carbon source for untreated CNTs. For example, carbon-containing source gases include, but are not limited to, hydrocarbons such as methane, ethylene, propane, butane, and acetylene, carbon monoxide, and alcohols. From the standpoint of ease of use, it is desirable to use at least one of hydrocarbons and alcohols as the source gas.

[0066] Furthermore, untreated CNTs usually exist as secondary particles. The shape of these secondary particles may be, for example, a state in which CNTs, which are general primary particles, are intricately entangled. They may also be an aggregate of linear CNTs. Secondary particles, which are an aggregate of linear CNTs, are easier to disentangle than entangled CNTs. Furthermore, linear CNTs have better dispersibility than entangled CNTs, and are therefore suitable for use as CNTs. It is preferable to subject these untreated CNTs to a pulverization process so that they satisfy the desired physical properties.

[0067] Furthermore, the untreated CNT may be surface-treated CNT. Furthermore, the CNT may be a CNT derivative to which a functional group, such as a carboxyl group, has been added. When a crushing treatment is performed, these treatments may be performed before, after, or simultaneously with the crushing treatment, or a combination thereof.

[0068] In addition, the distribution of fiber length, pore size, etc., of untreated CNT varies depending on the type. Therefore, the compression and bending of CNTs due to processes such as dry milling vary depending on the type of CNT. Since the distribution of various properties of the resulting CNTs varies depending on the type of CNT and the processing method, it is advisable to apply an appropriate processing method depending on the type of CNT. For example, the processing method can be adjusted by selecting a milling processing device, processing time, etc.

[0069] <<Carbon Nanotube Dispersion>> The CNT dispersion of this embodiment contains the above-described CNTs, a dispersant, and a non-aqueous dispersion medium. Details of the CNTs are as described above.

[0070] <Dispersant> The CNT dispersion contains a dispersant. There are no particular limitations on the dispersant, but one that can stabilize the dispersion of CNTs in the CNT dispersion is preferred. Both resin-type dispersants and surfactants can be used as the dispersant, but resin-type dispersants are preferred because they have strong adsorption power to CNTs and provide good dispersion stability. Depending on the properties required for dispersing CNTs, an appropriate type of dispersant can be used in an appropriate amount.

[0071] As the resin-type dispersant, a (meth)acrylic polymer, a polymer derived from an ethylenically unsaturated hydrocarbon, a cellulose derivative, a copolymer thereof, or the like can be used.

[0072] Examples of polymers derived from ethylenically unsaturated hydrocarbons include polyvinyl alcohol resins, polyvinylpyrrolidone resins, polyacrylonitrile resins, nitrile rubbers, etc. Examples of polyvinyl alcohol resins include polyvinyl alcohol, modified polyvinyl alcohols having functional groups other than hydroxyl groups (e.g., acetyl groups, sulfo groups, carboxy groups, carbonyl groups, amino groups), polyvinyl alcohols modified with various salts, other anion- or cation-modified polyvinyl alcohols, and polyvinyl acetals (polyvinyl acetoacetal, polyvinyl butyral, etc.) modified with aldehydes (acetoacetal-modified, butyral-modified, etc.). The polyacrylonitrile resin may be a polyacrylonitrile homopolymer, a polyacrylonitrile copolymer, or a modified product thereof. Preferred examples include polyacrylonitrile resins having at least one selected from the group consisting of active hydrogen groups such as hydroxyl groups, carboxy groups, primary amino groups, secondary amino groups, and mercapto groups, basic groups, and alkyl groups derived from (meth)acrylic acid alkyl esters or α-olefins. For example, the acrylonitrile copolymer described in JP 2020-163362 A can be used. Nitrile rubbers include acrylonitrile butadiene rubber and hydrogenated acrylonitrile butadiene rubber. Cellulose derivatives include cellulose acetate, cellulose butyrate, cyanoethyl cellulose, ethylhydroxyethyl cellulose, nitrocellulose, methylcellulose, ethylcellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropylmethylcellulose, carboxymethylcellulose, and copolymers thereof. Further, dispersants described in International Publication No. 2008 / 108360, JP 2018-192379, JP 2019-087304, JP 2020-011934, and JP 2009-026744 may be used, but are not limited to these.Particularly preferred are methyl cellulose, ethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, polyacrylonitrile homopolymers, polyacrylonitrile copolymers, and hydrogenated acrylonitrile butadiene rubber. Polymers in which other substituents have been introduced into a portion of these polymers, modified polymers, etc. may also be used.

[0073] The weight-average molecular weight of the resin-type dispersant is preferably 500,000 or less, more preferably 300,000 or less, from the viewpoint of the affinity balance between the CNT to be dispersed and the non-aqueous dispersion medium, and from the viewpoint of resistance to the electrolyte. It is also preferably 3,000 or more, more preferably 5,000 or more. The resin-type dispersant may be used alone or in combination of two or more. The weight-average molecular weight is the weight-average molecular weight in terms of polystyrene determined by gel permeation chromatography (GPC) measurement.

[0074] Commercially available polyvinyl alcohol resins include Kuraray Poval (a polyvinyl alcohol resin manufactured by Kuraray), Gohsenol, Gohsenex (polyvinyl alcohol resins manufactured by Nippon Synthetic Chemical Industry Co., Ltd.), Denka Poval (a polyvinyl alcohol resin manufactured by Denka Co., Ltd.), and J-Poval (a polyvinyl alcohol resin manufactured by Nippon Vinyl Acetate & Poval Co., Ltd.), and various grades are available. Modified polyvinyl alcohols having various functional groups are also available. Polyvinyl alcohols synthesized by known synthesis methods may also be used. Specific examples of commercially available polyvinylpyrrolidone resins include Luvitec K17 (K value: 15.0 to 19.0, low molecular weight), K30 (K value: 27.0 to 33.0), K90 (K value: 88.0 to 92.0), K90HM (K value: 92.0 to 96.0, high molecular weight) (manufactured by BASF Japan), K15, K30, K90, K120 (manufactured by ISP), polyvinylpyrrolidone K30 (K value: 27.0 to 33.0), K90 (K value: 88.0 to 96.0) (manufactured by Nippon Shokubai), and PVP. Examples include K12 (K value 10-14), K15 (K value 13-19), K30 (K26-K35), K60 (K value 50-62), and K90 (K value 88-100) (manufactured by DSP Gokei Food & Chemical). From the viewpoint of preventing an increase in viscosity, the polyvinylpyrrolidone preferably has a K value of 150 or less, more preferably a K value of 100 or less, and even more preferably a K value of 85 or less. Commercially available nitrile rubbers include Therban (hydrogenated nitrile rubber manufactured by Arlanxeo), Baymod (nitrile rubber manufactured by Arlanxeo), Zetpole (hydrogenated nitrile rubber manufactured by Nippon Zeon), and Nipole NBR (nitrile rubber manufactured by Nippon Zeon), and various grades with different nitrile ratios, hydrogenation rates, molecular weights, etc. are available. Furthermore, those synthesized by known synthesis methods may also be used.

[0075] Surfactants may be used in place of or in addition to the resin-type dispersants described above. Surfactants are classified into anionic, cationic, amphoteric ionic surfactants and nonionic surfactants.

[0076] The CNT dispersion may further contain a base. The inclusion of a base in the CNT dispersion is preferable because it enhances the wettability of the CNTs in the dispersion medium, improving dispersibility or dispersion stability. The base to be added can be at least one selected from the group consisting of inorganic bases, inorganic metal salts, organic bases, and organic metal salts. The total amount of these is preferably 0.001 to 0.1 mass%, more preferably 0.005 to 0.05 mass%, based on the total amount of the CNT dispersion. Adding too much base can cause corrosion of the dispersion device and / or the inside of the battery. The water content of the added base is preferably less than 5 mass%, more preferably less than 3 mass%, and even more preferably less than 2 mass%. If the added base contains a large amount of water, the adsorption of the dispersant to the CNTs may be reduced, making it difficult to maintain the CNTs in a stable state in the non-aqueous dispersion medium. By setting the water content within the above range, it is possible to prevent the problem of gelling of the composite slurry produced by mixing with the active material, or the problem of moisture remaining in the positive electrode causing deterioration of the positive electrode due to the generation of acids such as HF. The problem of moisture remaining in the positive electrode causing deterioration of the positive electrode due to the generation of acids such as HF is also posed. Furthermore, the presence of moisture in an alkali metal hydroxide such as LiOH can easily cause gelling of the paste containing the positive electrode active material during positive electrode preparation, making electrode formation difficult.

[0077] Examples of inorganic bases and inorganic metal salts include chlorides, hydroxides, carbonates, nitrates, sulfates, phosphates, borates, and ammonium hydroxide of alkali metals or alkaline earth metals. Among these, hydroxides or alkoxides of alkali metals or alkaline earth metals are preferred from the viewpoint of easy supply of cations. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides include calcium hydroxide and magnesium hydroxide. Among these, it is more preferable to use at least one selected from the group consisting of lithium hydroxide, sodium hydroxide, and potassium hydroxide. The metal contained in the inorganic base may be a transition metal.

[0078] Examples of organic bases include primary, secondary, and tertiary amine compounds (e.g., alkylamines and aminoalcohols) having 1 to 40 carbon atoms, which may have a substituent, and organic hydroxides. Examples of primary alkylamines having 1 to 40 carbon atoms, which may have a substituent, include alkylamines such as propylamine, butylamine, isobutylamine, octylamine, 2-ethylhexylamine, and laurylamine; aminoalcohols such as 2-aminoethanol and 3-aminopropanol; and 3-ethoxypropylamine. Examples of secondary alkylamines having 1 to 40 carbon atoms, which may have a substituent, include alkylamines such as dibutylamine, diisobutylamine, N-methylhexylamine, and dioctylamine, and aminoalcohols such as 2-methylaminoethanol. Examples of tertiary alkylamines having 1 to 40 carbon atoms, which may have a substituent, include alkylamines such as triethylamine, tributylamine, N,N-dimethylbutylamine, N,N-diisopropylethylamine, dimethyloctylamine, trioctylamine, and dimethyldecylamine, triethanolamine, and 2-(dimethylamino)ethanol. The organic hydroxide is a salt containing an organic cation and a hydroxide ion. Examples of the organic hydroxide include trimethyl-2-hydroxyethylammonium hydroxide, tetramethylammonium hydroxide, trimethylphenylammonium hydroxide, 3-trifluoromethyl-phenyltrimethylammonium hydroxide, and benzyltrimethylammonium hydroxide. Among these, from the viewpoint of the action on CNTs, it is more preferable to use at least one selected from the group consisting of 2-aminoethanol, 3-aminopropanol, triethanolamine, and trimethyl-2-hydroxyethylammonium hydroxide.

[0079] Examples of organic metal salts include alkali metal alkoxides and alkali metal acetates. Examples of alkali metal alkoxides include lithium methoxide, lithium ethoxide, lithium propoxide, lithium t-butoxide, lithium n-butoxide, sodium methoxide, sodium ethoxide, sodium propoxide, sodium t-butoxide, sodium n-butoxide, potassium methoxide, potassium ethoxide, potassium propoxide, potassium t-butoxide, and potassium n-butoxide. Among these, sodium t-butoxide is preferred from the viewpoint of ease of supplying cations. The metal contained in the organic base may be a transition metal.

[0080] The CNT dispersion may further contain an acid. The inclusion of an acid in the CNT dispersion is preferable because it can suppress the polymerization of the resin and the resulting increase in viscosity and gelation of the lithium-ion battery positive electrode composite slurry. The acid to be added may be either an inorganic acid or an organic acid. The total amount of these is preferably 0.001 to 1.0 mass %, more preferably 0.005 to 0.5 mass %, based on the total amount of the CNT dispersion. Adding too much acid may reduce the adsorption and dispersion ability of the dispersant to the CNTs.

[0081] Examples of inorganic acids include hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid. Examples of organic acids include carboxylic acid compounds and sulfonic acid compounds. Examples of carboxylic acid compounds include formic acid, acetic acid, propionic acid, butyric acid, tartaric acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, (meth)acrylic acid, crotonic acid, fumaric acid, maleic acid, itaconic acid, citraconic acid, and fluoroacetic acid. Examples of sulfonic acid compounds include methanesulfonic acid, paratoluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, and dinonylnaphthalenedisulfonic acid. In addition, anhydrides, hydrates, or partial salts of the above acids can also be used. These can be used alone or in combination of two or more.

[0082] <Non-aqueous dispersion medium> The CNT dispersion liquid contains a non-aqueous dispersion medium as a dispersion medium. The non-aqueous dispersion medium is not particularly limited, but is preferably a high-dielectric constant solvent, and preferably contains a solvent consisting of any one of high-dielectric constant solvents or a mixed solvent consisting of two or more of high-dielectric constant solvents. Furthermore, the high-dielectric constant solvent may be mixed with one or more other solvents.

[0083] Examples of high-dielectric-constant solvents that can be used include amides (N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylcaprolactam, etc.), heterocyclics (cyclohexylpyrrolidone, γ-butyrolactone, etc.), sulfoxides (dimethyl sulfoxide, etc.), sulfones (hexamethylphosphorotriamide, sulfolane, etc.), lower ketones (acetone, methyl ethyl ketone, etc.), carbonates (diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, fluoroethylene carbonate, propylene carbonate, ethylene carbonate), and others, such as tetrahydrofuran, urea, and acetonitrile. The relative dielectric constant of the high-dielectric-constant solvent can be a value described in a solvent handbook or the like, and is preferably 2.5 or more at 20°C.

[0084] From the viewpoint of the solubility of the dispersant and binder resin, or the wettability of the CNTs to the dispersion medium, the non-aqueous dispersion medium preferably contains an amide-based organic solvent, and more preferably contains at least one selected from the group consisting of N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone.

[0085] The content of the amide organic solvent is preferably 60% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more, based on the total mass of the non-aqueous dispersion medium. From the viewpoint of handleability, it is more preferable to use N-methyl-2-pyrrolidone alone.

[0086] The CNT dispersion may contain optional components such as wetting agents, wetting penetrants, leveling agents, and other additives as needed, as long as the objectives of the present invention are not impaired. The optional components can be added at any timing, such as before preparing the CNT dispersion, during mixing, after mixing, or a combination of these.

[0087] Even if optional components are included, they are within a range that does not impair the object of the present invention, and since the CNT dispersion is substantially free of active materials, the content of CNTs and dispersant in the CNT dispersion is 60 mass % or more, preferably 70 mass % or more, more preferably 80 mass % or more, even more preferably 90 mass % or more, and even more preferably 95 mass % or more, based on the non-volatile components of the CNT dispersion, i.e., the solid content of the CNT dispersion, as long as it is 100 mass % or less.

[0088] Other conductive materials besides CNTs may include carbon black. Carbon black is preferably included in the CNT dispersion within a range that does not impair the effects of the present invention, and the CNT content is preferably 1 to 80 mass% and more preferably 1 to 50 mass% relative to the total mass of the carbon black and CNT. If the above range is satisfied, the dispersion state of the CNTs can be controlled and high dispersibility and fluidity can be maintained even when carbon black is included. Furthermore, the carbon black content is preferably 20 mass% or less and more preferably 15 mass% or less relative to the total mass of the CNT dispersion.

[0089] The step of adding carbon black to the CNT dispersion is not particularly limited, and may be before, after, or simultaneously with the addition of CNTs, or a combination thereof. Another example of a method for producing a CNT dispersion containing carbon black is a method in which carbon black is added before, after, or simultaneously with the addition of a binder resin to the CNT dispersion, or a combination thereof. In this method, it is preferable that carbon black is added to the CNT dispersion in a state in which CNTs are dispersed. Alternatively, a combination of these methods may be used. That is, carbon black may be further added before, after, or simultaneously with the addition of a binder resin to the CNT dispersion containing carbon black, or a combination thereof.

[0090] <Method for producing CNT dispersion> The CNT dispersion of this embodiment is characterized by comprising a step of mixing and dispersing the above-mentioned CNTs, a dispersant, and a non-aqueous dispersion medium. For example, it is preferable to produce the CNT dispersion by finely dispersing the CNTs, dispersant, and non-aqueous dispersion medium using a dispersion device. The dispersion device used for this process is not particularly limited. The dispersion process may be a multi-stage process of two or more steps, with the timing of addition of the materials used being adjusted as desired.

[0091] Examples of dispersing devices used to disperse CNTs include kneaders, two-roll mills, three-roll mills, planetary mixers, ball mills, horizontal sand mills, vertical sand mills, annular bead mills, attritors, high-shear mixers, high-pressure homogenizers, and ultrasonic homogenizers. Among these, a high-shear mixer, a high-pressure homogenizer, an ultrasonic homogenizer, or a combination of these is preferred to finely disperse CNTs in a CNT dispersion and achieve favorable dispersibility. In particular, a high-shear mixer is preferably used in the initial dispersion step to promote wetting of the CNTs and disintegrate coarse particles, followed by a high-pressure homogenizer to disperse the CNTs while maintaining their aspect ratio. Using a high-pressure homogenizer in multiple stages via circulatory dispersion can further enhance the dispersibility of the CNTs. Furthermore, by further dispersing the CNTs using a high-pressure homogenizer after dispersion using a high-pressure homogenizer, the dispersion state can be made uniform while maintaining the fiber length. Furthermore, in some embodiments of the present invention, CNTs with improved wettability due to dry processing or the like are dispersed using a high shear mixer in the initial dispersion step, followed by dispersion in a bead mill and further dispersion in a high-pressure homogenizer, which allows the CNTs aggregated by the dry processing or the like to be sufficiently decomposed with a certain force and the dispersion state to be made uniform. When using a high-pressure homogenizer, the pressure in the homogenizing section of the high-pressure homogenizer is preferably 60 to 150 MPa, and more preferably 60 to 120 MPa.

[0092] Dispersion methods using a dispersing device include batch dispersion, pass dispersion, and circulation dispersion. Any of these methods may be used, or two or more methods may be combined. Batch dispersion is a method in which dispersion is performed using only the dispersing device itself, without using piping or the like. Because handling is simple, it is preferred for small-scale production. Pass dispersion is a dispersion method in which the dispersing device itself is equipped with a tank that supplies the dispersion liquid via piping and a tank that receives the dispersion liquid, and the dispersion passes through the dispersing device itself. Furthermore, circulation dispersion is a method in which the dispersion liquid that has passed through the dispersing device itself is returned to the tank that supplies the dispersion liquid, and dispersion is performed while circulating. In both methods, the longer the processing time, the more the dispersion progresses; therefore, the pass or circulation can be repeated until the desired dispersion state is achieved, and the processing volume can be increased by changing the tank size or processing time. Pass dispersion is preferred over circulation dispersion because it is easier to achieve a uniform dispersion state. Circulation dispersion is preferred over pass dispersion because the operation and production equipment are simpler. In the dispersion step, the disintegration of aggregated particles, the loosening, wetting, stabilization, etc. of the conductive material proceed sequentially or simultaneously, and the final dispersion state differs depending on how these steps proceed, so it is preferable to control the dispersion state in each dispersion step by using various evaluation methods. For example, the control can be performed by the methods described in the Examples.

[0093] The dispersing device used when dispersing the CNT dispersion may be equipped with a heat exchanger or a coolant supply mechanism for cooling the CNT dispersion. By pre-cooling the CNT dispersion or crude dispersion that has reached a high temperature after dispersion, it is possible to suppress the generation of bubbles in the CNT dispersion and also to suppress the remaining solid content on the wall surface of the dispersion tank, etc. The liquid temperature of the CNT dispersion is not particularly limited, but is preferably controlled to 25 to 75°C, and more preferably 40 to 65°C.

[0094] In the CNT dispersion of this embodiment, from the viewpoint of microscopically controlling the dispersibility and fluidity of CNTs in the CNT dispersion, the regression line formula (I) is further ２is preferably 0.85 or more, the slope a is 2≦a<20, and the y-intercept b is preferably −5≦b<10. y=ax+b...Equation (I) (Equation (I) is a regression line obtained by the least squares method from a scatter diagram plotting x on the horizontal axis and y on the vertical axis, measured using carbon nanotube dispersions stored at a static storage temperature of 40°C for 1 day, 40°C for 7 days, and 40°C for 14 days, respectively. The regression line is obtained by the least squares method from a scatter diagram plotting x on the horizontal axis and y on the vertical axis, measured using carbon nanotube dispersions stored at a static storage temperature of 40°C for 1 day, 40°C for 7 days, and 40°C for 14 days. The regression line is obtained by the least squares method from a scatter diagram plotting x on the horizontal axis and y on the vertical axis, measured using carbon nanotube dispersions stored at a static storage temperature of 40°C for 25°C and a shear rate of 1.0 s －１ The shear viscosity at η ｉ (Pa s), the complex modulus of elasticity at 25 ° C. and a frequency of 1 Hz obtained by dynamic viscoelasticity measurement is G ＊ ｉ (Pa), x is the shear viscosity η ｉ The natural logarithm of (Pa s) (ln(η ｉ )) and y is the complex elastic modulus G at 25 ° C. and a frequency of 1 Hz. ＊ ｉ (Pa), and i is 1, 7, and 14, with the production date of the carbon nanotube dispersion being day 0.)

[0095] By satisfying the specific conditions in formula (I), the CNT dispersion exhibits excellent dispersion stability of the CNTs, and the dispersibility and fluidity of the CNT dispersion are maintained even over time. Furthermore, by using this CNT dispersion, the dispersibility and fluidity are maintained even after the addition of a binder resin, and a well-developed conductive network can be formed in the electrode film when the electrode film is produced. This results in an electrode film with excellent adhesion to the current collector, and a secondary battery with high output, high capacity, and long life can be produced.

[0096] A CNT dispersion satisfying the specific conditions in formula (I) can be obtained by appropriately controlling the types and contents of CNT, dispersant, and non-aqueous dispersion medium, as well as their mixing conditions, dispersion conditions, etc. In particular, it is effective to appropriately control the bulk density of the CNT and the Zr content in the CNT. For example, using CNT whose pretreatment conditions have been optimized to satisfy formula (I) can serve as an indicator for determining a good dispersion state in a CNT dispersion. More specifically, CNTs whose bulk density and Zr content are within appropriate ranges have a moderate aspect ratio and form an aggregated state, and when dispersed, a CNT dispersion in which the CNTs are sufficiently detangled can be obtained. Furthermore, by not containing excessive Zr, a CNT dispersion with excellent storage stability can be obtained.

[0097] In this way, the dispersibility of CNTs in a CNT dispersion can be evaluated by the shear viscosity and the complex modulus in dynamic viscoelasticity measurements, and by adjusting the CNT pretreatment method so that the shear viscosity and complex modulus of the CNT dispersion under specific conditions satisfy the above relationship, high dispersibility and fluidity can be maintained even in a CNT dispersion containing a high concentration of CNTs. The regression line of the present disclosure can be determined by measurement using a CNT dispersion after storage at 40°C for 1, 7, and 14 days.

[0098] Coefficient of determination R ２ If R is less than 0.85, CNT aggregates will be present in the CNT dispersion, and aggregation and sedimentation of the CNTs will occur over time, which may result in a decrease in the adhesion of the electrode film and a decrease in battery performance. ２ is preferably 0.90 or more, more preferably 0.95 or more. Here, the coefficient of determination R ２ can be calculated from the square of the Pearson product-moment correlation coefficient.

[0099] The slope a can be 2≦a<20. If the slope a is less than 2, the CNTs in the CNT dispersion will be present in a non-uniform state, which may result in a decrease in the adhesion of the electrode film and a decrease in battery performance. If the slope a is 20 or more, aggregation or sedimentation of the CNTs will occur in the CNT dispersion, which may result in an increase in electrode resistance and a decrease in battery performance. The slope a is preferably 2 or more, more preferably 5 or more, and even more preferably 7 or more. The slope a is preferably less than 20, more preferably less than 15, and even more preferably less than 14.

[0100] The y-intercept b can be -5≦b<10. If the y-intercept b is less than -5, the CNTs in the CNT dispersion may be present in a non-uniform state, which may result in a decrease in the adhesion of the electrode film and a decrease in battery performance. If the y-intercept b is 10 or more, aggregation or sedimentation of the CNTs may occur in the CNT dispersion, which may result in an increase in electrode resistance and a decrease in battery performance. The y-intercept b is preferably -5 or more, more preferably -3 or more, and even more preferably greater than 0. The y-intercept b is preferably less than 10, more preferably less than 8, and even more preferably less than 6.

[0101] By controlling the dispersion state of CNTs and preparing a CNT dispersion liquid such that formula (I) satisfies specific conditions, it is possible to simultaneously achieve the dispersion stability, stability over time, adhesion between the electrode film and the current collector, and battery characteristics of the CNT dispersion liquid.

[0102] i is the number of days that have passed since static storage, with the production date of the CNT dispersion being day 0, and is 1, 7, and 14. That is, for formula (I), the shear viscosity and complex modulus are measured using the CNT dispersion after static storage at 40°C for 1, 7, and 14 days from the production date, and a regression line can be found from the three points where x and y are plotted.

[0103] x is 25°C, shear rate 1.0 s －１ Shear viscosity η at ｉ The natural logarithm of (Pa s) (ln(η ｉ ) where the shear viscosity η ｉ(Pa·s) represents the shear viscosity η (Pa·s) measured using a CNT dispersion after storing it at 40° C. for i days.

[0104] y is the complex modulus G at 25°C and a frequency of 1 Hz obtained by dynamic viscoelasticity measurement. ＊ ｉ (Pa). Here, the complex elastic modulus G ＊ ｉ (Pa) is the complex modulus G measured using a CNT dispersion after storing at 40 ° C for 1 day ＊ (Pa).

[0105] The shear viscosity η (Pa s) of the CNT dispersion was measured at 25°C and a shear rate of 1.0 s －１ is the measured value at ＊ (Pa) is a measurement value at 25°C and a frequency of 1 Hz. For details, it can be measured by the method described in the Examples. The shear rate of a CNT dispersion tends to decrease as the dispersibility of the CNT improves, but when the CNT fiber length is long, the value itself may be high even if the CNT is uniformly and stably dissolved in the medium due to the structural viscosity of the CNT itself. In addition to the dispersion state of the CNT, it also changes due to the influence of entanglement of the CNT, dispersant, and other resin components, or the intermolecular forces between them.

[0106] The complex modulus indicates the hardness of the CNT dispersion, and tends to decrease as the CNT dispersibility improves and the viscosity of the CNT dispersion decreases. However, similar to the shear viscosity, if the CNT itself has structural viscosity, the CNT dispersion may exhibit a high value even if the CNT is uniformly and stably dissolved in the medium. In addition to the dispersion state of the CNT, it also changes due to the entanglement of the CNT, dispersant, and other resin components, or the influence of their intermolecular forces.

[0107] CNT dispersion temperature: 25°C, shear rate: 1.0 s －１The shear viscosity η (Pa s) in this case is preferably 0.05 or more, more preferably 0.20 or more, and even more preferably 0.80 or more. When the shear viscosity η is within the above range, the CNTs are sufficiently disintegrated, and a CNT dispersion in which the CNTs are uniformly dispersed is obtained, resulting in high composite adhesion and excellent battery performance. Furthermore, the shear viscosity η is preferably 3.0 or less, more preferably 2.5 or less, and even more preferably 2.0 or less. When the shear viscosity η is within the above range, a CNT dispersion in which the CNTs are stably dispersed without aggregation or sedimentation of the CNTs is obtained, resulting in excellent battery performance.

[0108] Complex modulus G of CNT dispersion at a temperature of 25°C and a frequency of 1 Hz ＊ (Pa) is preferably 0.80 or more, more preferably 2.0 or more, and even more preferably 4.0 or more. ＊ When the complex modulus G is within the above range, the CNTs are sufficiently disintegrated, and a uniformly dispersed CNT dispersion liquid is obtained, resulting in high composite adhesion and excellent battery performance. ＊ (Pa) is preferably 100.0 or less, more preferably 50.0 or less, and even more preferably 10.0 or less. ＊ When the content of CNT is within the above range, a CNT dispersion liquid can be obtained in which the CNTs are stably dispersed without aggregation or sedimentation of the CNTs, and excellent battery performance can be obtained.

[0109] The degree of CNT disintegration in a CNT dispersion can also be evaluated by the filtration rate of the CNT dispersion. More specifically, this can be evaluated by measuring the time it takes for the entire CNT dispersion to pass through the filter when it is diluted three times with a non-aqueous dispersion medium and filtered under reduced pressure. This is thought to be because, if a small amount of aggregates derived from CNTs or other resin components is present in the CNT dispersion, the aggregates may not be able to pass through the filter or the filtration rate may be slowed. Furthermore, if the dispersion state of the CNT dispersion is unstable, diluting the CNT dispersion with a non-aqueous dispersion medium is likely to cause dispersion shock, and the aggregates will be significantly observed, which will similarly result in the inability to pass through the filter or the filtration rate being slowed. For the above reasons, the filtration rate can be used as an index for evaluating the dispersion state and dispersion stability of CNTs in the CNT dispersion, as well as the fluidity of the CNT dispersion. More specifically, it can be measured by the method described in the Examples.

[0110] The fluidity and dispersion stability of a CNT dispersion can be evaluated by confirming its acid stability. More specifically, acid is added to the CNT dispersion, and the initial viscosity of the acid-containing CNT dispersion and the viscosity after storage at 40°C for 7 days are measured. The rate of change in viscosity after storage at 40°C for 7 days relative to the initial viscosity can be evaluated. If the CNTs are stably dispersed, the addition of acid to the CNT dispersion will not disrupt the CNT dispersion, and good viscosity will be maintained over time. On the other hand, even if the CNTs appear to be well dispersed in the initial stage, if the CNTs are not sufficiently dispersible, the addition of acid will cause CNTs that do not adsorb dispersant to aggregate, resulting in poor dispersion and an increase in viscosity over time. By evaluating under these conditions, the dispersion state and dispersion stability of the CNTs in the CNT dispersion can be evaluated, taking into account changes over time. More specifically, measurements can be performed using the methods described in the Examples.

[0111] The average fiber length of the CNTs in the CNT dispersion is preferably 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.3 μm or more. It is also preferably 20 μm or less, and more preferably 10 μm or less. The average fiber length of the CNTs in the CNT dispersion can be calculated by diluting the CNT dispersion 50 times with a non-aqueous dispersion medium such as NMP, dropping it onto a substrate, drying the resulting sample, observing the sample with a scanning electron microscope, randomly selecting 300 CNTs from the observation photograph, and measuring and averaging the fiber lengths of each. Here, if the dispersion state of the CNTs is unstable, diluting the CNT dispersion with a non-aqueous dispersion medium is likely to cause dispersion shock, resulting in the formation of aggregates, which tends to reduce the average fiber length. For the above reasons, dispersion stability can be evaluated based on the average fiber length of the CNTs in the CNT dispersion.

[0112] The CNT content in the CNT dispersion is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more, relative to the total amount of the CNT dispersion. It is also preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and even more preferably 8% by mass or less. By keeping the content within the above range, the CNTs can be present in a good and stable manner. It is more preferably 0.1% by mass or more and 20% by mass or less, and even more preferably 0.5% by mass or more and 10% by mass or less.

[0113] In the CNT dispersion, the mass ratio of dispersant to CNT (dispersant / CNT) is preferably 0.01 or more, more preferably 0.05 or more, and even more preferably 0.1 or more. It is also preferably 1.5 or less, more preferably 1 or less, even more preferably 0.5 or less, and particularly preferably 0.40 or less. By setting the ratio within the above range, the dispersion stability of the CNT can be further improved. It is more preferably 0.01 or more and 1.5 or less, and even more preferably 0.1 or more and 1 or less.

[0114] The Zr content in the CNT dispersion is preferably 0.01 ppm or more, more preferably 0.1 ppm or more, and even more preferably 1 ppm or more. It is also preferably 300 ppm or less, more preferably 150 ppm or less, even more preferably 100 ppm or less, and particularly preferably 50 ppm or less. By setting the Zr content within the above range, the dispersion stability of the CNTs in the CNT dispersion can be further improved. It is more preferably 0.01 ppm or more and 300 ppm or less, and even more preferably 0.1 ppm or more and 100 ppm or less.

[0115] The CNTs in the CNT dispersion function as a conductive material. The CNT dispersion may contain a conductive material other than CNT. Examples of other conductive materials include carbon materials such as carbon black, graphene, multilayer graphene, and graphite. When a conductive material other than CNT is used, carbon black is preferred from the viewpoint of the adsorption performance of the dispersant, and examples of such carbon black include acetylene black, furnace black, and ketjen black. These carbon blacks may be neutral, acidic, or basic, and oxidation-treated carbon black or graphitization-treated carbon black may also be used.

[0116] The CNT dispersion may be prepared by separately preparing two or more types of CNT with different average outer diameters and adding them to a dispersion medium. When two or more types of CNT with different average outer diameters are used as the CNT, the average outer diameter of the first CNT is preferably 0.5 nm or more and less than 5 nm. The average outer diameter of the second CNT is preferably 1 nm or more and 20 nm or less. For example, single-walled CNT may be used as the first CNT, and multi-walled CNT may be used as the second CNT. When two or more types of CNT with different average outer diameters are used as the CNT, the mass ratio of the first CNT to the second CNT is preferably 1:50 to 50:1, more preferably 1:10 to 10:1, and even more preferably 1:5 to 5:1. By setting the mass ratio of the first CNT to the second CNT within the above range, entanglement between the CNTs is suppressed, resulting in a dispersion with excellent fluidity.

[0117] <<Carbon nanotube resin composition>> The carbon nanotube resin composition of this embodiment may contain the carbon nanotube dispersion and a binder resin. Details of the carbon nanotube dispersion are as described above. That is, the carbon nanotube resin composition may contain carbon nanotubes, a dispersant, a non-aqueous dispersion medium, and a binder resin. The carbon nanotube resin composition may further contain an optional component. Details are as described above. The carbon nanotube resin composition can be used as a resin composition for a secondary battery.

[0118] The binder resin is not particularly limited as long as it is typically used as a binder resin for paints, and can be appropriately selected depending on the purpose. The binder resin may be a resin that bonds between substances such as active materials, CNTs, and other conductive materials. Examples of binder resins include homopolymers or copolymers containing ethylene, propylene, vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid esters, methacrylic acid, methacrylic acid esters, acrylonitrile, styrene, vinyl butyral, vinyl acetal, and vinyl pyrrolidone as structural units; polyurethane resins, polyester resins, phenolic resins, epoxy resins, phenoxy resins, urea resins, melamine resins, alkyd resins, acrylic resins, formaldehyde resins, silicone resins, and fluororesins; cellulose resins such as carboxymethyl cellulose or salts thereof; elastomers such as hydrogenated or non-hydrogenated styrene-butadiene rubber and fluororubber; and conductive resins such as polyaniline and polyacetylene. Modified versions of these resins and copolymers of these resins may also be used. The binder resin may be used alone or in combination of two or more. Among these, when used as a binder resin for the positive electrode of a secondary battery, homopolymers or copolymers having fluorine atoms in the molecule, such as polyvinylidene fluoride, polyvinyl fluoride, polytetrafluoroethylene, and modified products and copolymers thereof, are preferred from the viewpoint of durability. Furthermore, when used as a binder resin for the negative electrode of a secondary battery, CMC (carboxymethyl cellulose or a salt thereof), hydrogenated or non-hydrogenated styrene-butadiene rubber, polyacrylic acid, and the like, which have good adhesion, are preferred.

[0119] In the CNT resin composition, the binder resin preferably accounts for 0.5 to 30 mass %, and more preferably 0.5 to 25 mass %, of the total amount of nonvolatile components of the CNT resin composition.

[0120] The CNT resin composition is preferably obtained by adding a binder resin to a CNT dispersion and mixing. The binder resin may be used in the form of a solution (varnish) in which the binder resin is dispersed or dissolved in a dispersion medium or the like, but is preferably used in a state that does not contain a dispersion medium, more preferably in a solid state, and particularly preferably in powder form. By adding a powdered binder resin to a CNT dispersion, the amount of non-aqueous dispersion medium does not increase when the binder resin is added, so a CNT resin composition with a high binder resin concentration and non-volatile content concentration can be provided. Here, the form of the binder resin is not particularly limited and may be, for example, pellets, powder, granules, flakes, chunks, chopped fibers, or the like.

[0121] The method for adding the binder resin to the CNT dispersion is not particularly limited, but it is preferable to add the binder resin while stirring the CNT dispersion using a stirring device. The stirring device is not particularly limited, but a disper (stirring blade) or the like is generally used. There are various types of stirrers, such as propeller-type and turbine-type, and there is no particular limitation as long as they can stir the CNT resin composition homogeneously. The stirring speed can be adjusted appropriately by the size and rotation speed of the stirring blade. The stirring speed is not particularly limited as long as it is within the range allowed for in the actual operating process.

[0122] <<Composite Slurry for Secondary Battery Electrodes>> The composite slurry of this embodiment may contain the carbon nanotube dispersion, a binder resin, and an active material. The composite slurry of another embodiment may contain the carbon nanotube resin composition and an active material. Details of the carbon nanotube resin composition are as described above. That is, the composite slurry may contain carbon nanotubes, a dispersant, a non-aqueous dispersion medium, a binder resin, and an active material. The composite slurry may further contain optional components. Details are as described above. The active material may be either a positive electrode active material or a negative electrode active material. The composite slurry can be used as a composite slurry for a secondary battery electrode, and may be either a positive electrode composite slurry for a secondary battery or a negative electrode composite slurry for a secondary battery.

[0123] The composite slurry may contain other optional components as needed, as long as they do not impair the object of the present invention. The optional components can be added at any time, such as before preparing the composite slurry, during mixing, after mixing, or a combination thereof. The optional components may be those described above for the CNT resin composition.

[0124] The active material may be a positive electrode active material or a negative electrode active material. In this specification, the positive electrode active material and the negative electrode active material may be simply referred to as "active material." Active materials are materials that are the basis of battery reactions and are broadly classified into positive electrode active materials and negative electrode active materials based on electromotive force. The composite slurry is preferably in a slurry state to improve uniformity and processability.

[0125] <Positive Electrode Active Material> The positive electrode active material is not particularly limited, but for example, metal compounds such as metal oxides and metal sulfides that can reversibly dope or intercalate lithium ions can be used. For example, lithium manganese composite oxides (e.g., Li ｘ Mn ２ O ４ or Li ｘ MnO ２ ), lithium nickel composite oxide (e.g., Li ｘ NiO ２ ), lithium cobalt composite oxide (Li ｘ CoO ２ ), lithium nickel cobalt composite oxide (e.g., Li ｘ Ni １－ｙ Co ｙ O ２ ), lithium manganese cobalt composite oxide (e.g., Li ｘ Mn ｙ Co １－ｙ O ２ ), lithium nickel manganese cobalt composite oxide (e.g., Li ｘ Ni ｙ Co ｚ Mn １－ｙ－ｚ O ２ ), spinel-type lithium manganese nickel composite oxide (e.g., Li ｘ Mn ２－ｙ Ni ｙ O４ ), lithium phosphate powder having an olivine structure (e.g., Li ｘ FePO ４ , Li ｘ Fe １－ｙ Mn ｙ P.O. ４ , Li ｘ CoPO ４ etc.), manganese oxide, iron oxide, copper oxide, nickel oxide, vanadium oxide (e.g., V ２ O ５ , V ６ O １３ ), transition metal oxide powder such as titanium oxide, iron sulfate (Fe ２ (SO ４ ) ３ ), TiS ２ and transition metal sulfide powders such as FeS. Here, x, y, and z are numbers, and 0<x<1, 0<y<1, 0<z<1, and 0<y+z<1. These positive electrode active materials can be used singly or in combination. Among these active materials, active materials containing Ni and / or Mn (especially when the total amount of Ni and / or Mn in the transition metals is 50 mol% or more) tend to become highly basic due to the elution of raw material-derived components or metal ions, which can easily cause gelation of the binder resin or deterioration of the dispersion state. Therefore, this embodiment is particularly effective for batteries containing active materials containing Ni and / or Mn.

[0126] <Negative electrode active material> The negative electrode active material is not particularly limited, but examples thereof include metal Li, or its alloy, tin alloy, silicon alloy negative electrode, Li Ｘ TiO ２ , Li Ｘ Fe ２ O ３ , Li Ｘ Fe ３ O ４ , Li Ｘ WO ２Examples of materials that can be used include metal oxides such as those listed above, conductive polymers such as polyacetylene and poly-p-phenylene, artificial graphite such as highly graphitized carbon materials, carbonaceous powders such as natural graphite, and resin-baked carbon materials. Here, x is a number, 0<x<1. These negative electrode active materials can be used alone or in combination. In particular, when a silicon alloy negative electrode is used, the theoretical capacity is large but the volume expansion is extremely large. Therefore, it is preferable to use it in combination with artificial graphite such as highly graphitized carbon materials, carbonaceous powders such as natural graphite, resin-baked carbon materials, or the like.

[0127] The CNT content in the composite slurry is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and even more preferably 0.05% by mass or more, based on the mass of the active material (the mass of the active material being 100% by mass). It is also preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less. If the content exceeds the above range, the amount of active material filled in the electrode decreases, resulting in a lower battery capacity. If the content falls below the above range, the conductivity of the electrode and battery may be insufficient.

[0128] The content of the dispersant in the composite slurry is preferably 0.01% by mass or more, more preferably 0.02% by mass or more, based on the mass of the active material (the mass of the active material being 100% by mass), and is preferably 10% by mass or less, more preferably 5% by mass or less.

[0129] The content of the binder resin in the composite slurry is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, based on the mass of the active material (the mass of the active material being 100% by mass), and is preferably 20% by mass or less, more preferably 10% by mass or less.

[0130] The nonvolatile content in the composite slurry is preferably 40% by mass or more, more preferably 50% by mass or more, based on the mass of the composite slurry (the mass of the composite slurry being 100% by mass), and is preferably 90% by mass or less, more preferably 85% by mass or less.

[0131] A preferred method for preparing the composite slurry is to add an active material to the CNT resin composition and stir the mixture. The stirring device used for stirring is not particularly limited. Examples of suitable stirring devices include a disperser and a homogenizer.

[0132] The composite slurry may further contain carbon black, which functions as a conductive material. Carbon black is preferably included in the composite slurry within a range that does not impair the effects of the present invention. The CNT content is preferably 1 to 80 mass%, more preferably 1 to 50 mass%, of the total amount of conductive material containing carbon black and CNT. Within these ranges, the dispersion state of the CNTs can be controlled, and high dispersibility and fluidity can be better maintained in a state containing a binder resin. Furthermore, the carbon black content is preferably 10 mass% or less, more preferably 5 mass% or less, of the total mass of the composite slurry. The method for producing a composite slurry includes a step of adding an active material to a CNT resin composition, but may further include adding carbon black before, after, simultaneously with, or a combination thereof, adding the active material to the CNT resin composition. Alternatively, the active material may be added to a CNT resin composition that already contains carbon black, or the active material and carbon black may be further added to a CNT resin composition that already contains carbon black.

[0133] <<Electrode Film>> The electrode film of this embodiment is a coated film formed from the composite slurry. That is, the electrode film may include CNTs, a dispersant, a binder resin, and an active material. The active material may be either a positive electrode active material or a negative electrode active material. The electrode film may further include optional components. Details are as described above. The electrode film can be used as an electrode film for a secondary battery electrode and may be either an electrode film for a secondary battery positive electrode or a secondary battery negative electrode. The electrode film may be a coated film obtained using the composite slurry, and can be formed, for example, by coating and drying the composite slurry. An electrode can be provided using the electrode film. The electrode may include a current collector and an electrode film. The electrode can be obtained by coating and drying the composite slurry on a current collector to form a coated film. A positive electrode can be provided using an electrode film containing a positive electrode active material, and a negative electrode can be provided using an electrode film containing a negative electrode active material, and these positive and negative electrodes can be provided as the positive and negative electrodes of a secondary battery, respectively.

[0134] The material and shape of the current collector are not particularly limited and can be appropriately selected from those suitable for various secondary batteries. Examples of current collector materials include metals and alloys such as aluminum, copper, nickel, titanium, and stainless steel. While flat foils are generally used, current collectors with roughened surfaces, perforated foils, and mesh-shaped current collectors can also be used. The thickness of the current collector is preferably about 0.5 to 30 μm.

[0135] The method for applying the composite slurry to the current collector is not particularly limited, and known methods can be used. Specific examples include die coating, dip coating, roll coating, doctor coating, knife coating, spray coating, gravure coating, screen printing, and electrostatic coating. Drying methods after application include, but are not limited to, standing drying, a blower dryer, a hot air dryer, an infrared heater, and a far-infrared heater. Furthermore, after application of the composite slurry, rolling may be performed using a lithographic press, a calendar roll, or the like. The thickness of the electrode film is, for example, 1 μm or more and 500 μm or less, and preferably 10 μm or more and 300 μm or less.

[0136] <<Secondary Battery>> The secondary battery of this embodiment includes a positive electrode, a negative electrode, and an electrolyte, and at least one of the positive electrode and the negative electrode includes the above-mentioned electrode film. The positive electrode and the negative electrode may each include a current collector and an electrode film. Details are as described above. The secondary battery is more preferably a lithium-ion secondary battery.

[0137] The positive electrode can be prepared by coating a current collector with a composite slurry containing a positive electrode active material and drying the coated film. The negative electrode can be prepared by coating a current collector with a composite slurry containing a negative electrode active material and drying the coated film. The positive electrode active material and the negative electrode active material can be prepared by the above-mentioned methods. The composite slurry can be prepared by the above-mentioned methods.

[0138] The electrolyte may be any of a liquid electrolyte, a gel electrolyte, and a solid electrolyte. For example, the liquid electrolyte may contain an electrolyte salt such as a lithium salt and a non-aqueous dispersion medium.

[0139] As the electrolyte salt, various known salts in which ions are movable can be used. For example, LiBF ４ , LiClO ４ , LiPF ６ , LiAsF ６ , LiSbF ６ , LiCF ３ SO３ , Li(CF ３ SO ２ ) ２ N, LiC ４ F ９ SO ３ , Li(CF ３ SO ２ ) ３ C, LiI, LiBr, LiCl, LiAlCl, LiHF ２ , LiSCN, or LiBPh ４ (wherein Ph is a phenyl group), etc. The electrolyte salt is preferably dissolved in a non-aqueous dispersion medium and used as an electrolytic solution.

[0140] The non-aqueous dispersion medium is not particularly limited, but examples thereof include carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as γ-butyrolactone, γ-valerolactone, and γ-octanoic lactone; glymes such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-methoxyethane, 1,2-ethoxyethane, and 1,2-dibutoxyethane; esters such as methyl formate, methyl acetate, and methyl propionate; sulfoxides such as dimethyl sulfoxide and sulfolane; and nitriles such as acetonitrile. These solvents may be used alone or in combination of two or more.

[0141] The secondary battery preferably includes a separator, such as, but not limited to, polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and nonwoven fabrics obtained by subjecting these to hydrophilic treatment.

[0142] The structure of the secondary battery is not particularly limited, but it usually comprises a positive electrode, a negative electrode, and a separator that is provided as needed, and can be in various shapes depending on the purpose of use, such as a paper type, a cylindrical type, a button type, or a laminated type.

[0143] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples as long as it does not depart from the gist of the invention. Unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass".

[0144] In the examples and comparative examples, the following CNTs were used: BT1003M: LUCAN BT1003M, manufactured by LG Chem Ltd., multi-walled CNT, outer diameter 10 to 15 nm JENOTUBE10B: manufactured by JEIO, multi-walled CNT, outer diameter 7 to 12 nm

[0145] In the examples and comparative examples, the following dispersants were used: H-NBR1: Therban® AT 3404 (manufactured by ARLANXEO, hydrogenated acrylonitrile-butadiene rubber) H-NBR2: Zetpole 2000L (manufactured by Nippon Zeon, hydrogenated acrylonitrile-butadiene rubber) PVB: BL-10 (manufactured by Sekisui Chemical Co., Ltd., polyvinyl butyral) PVP: K-30 (manufactured by ISP, polyvinylpyrrolidone) PVA: Kuraray Poval 3-86SD (manufactured by Kuraray, modified polyvinyl alcohol)

[0146] In the examples and comparative examples, the following binder resins were used: PVDF: KF Polymer W#7300 (manufactured by Kureha, polyvinylidene fluoride resin) PTFE: PTFED-210C (manufactured by Daikin Industries, polytetrafluoroethylene)

[0147] <CNT> The types of CNT and the production conditions are shown in Table 1. Note that the endpoint indicator for the dry pulverization of the CNT in the example is a bulk density of 0.045 g / cm ３ ~0.125g / cm ３ The Zr content was set to be within the range of 3 ppm to 500 ppm.

[0148] (Example 1: CNT1) A 60 L dynamic mill was charged with zirconia beads with a diameter of 8 mm as milling media at a filling rate of 70%, and 100 kg of BT1003M as raw CNT was supplied at a flow rate of 120±20 kg / h (hour). The mill was then processed for 2 hours using a circulation milling method at a peripheral speed of 4 m / s (seconds), yielding CNT1. A sample was taken out and found to have a bulk density of 0.062 g / cm.３ The Zr content was 6 ppm, and it was confirmed that both the bulk density and the Zr content were within the specified ranges, and the treatment process was terminated. In the treatment process, cooling water (10°C) was flowed into the vessel (flow rate 10 L / min) to cool the powder so that the outlet powder temperature was 45±10°C.

[0149] (Examples 2 to 9: CNT2 to 9) CNT2 to 9 were obtained by dry pulverization of CNT in the same manner as in Example 1, using the raw CNT, processing time, processing flow rate, and dry dispersion method listed in Table 1. In each processing step, cooling was performed by flowing cooling water (10°C) into the vessel (flow rate 10 L / min) so that the outlet powder temperature was 45±10°C.

[0150] Comparative Example 1: Comparative CNT1 BT1003M in an untreated state without being subjected to a crushing treatment was used as comparative CNT1.

[0151] (Comparative Example 2: Comparative CNT2) A 60 L dynamic mill was charged with 8 mm diameter zirconia beads as milling media at a filling rate of 70%, and 100 kg of BT1003M as raw CNT was supplied at a flow rate of 120±20 kg / h and processed for 15 hours using a circulation milling method at a peripheral speed of 4 m / s to obtain Comparative CNT2. During the processing step, cooling was performed by flowing cooling water (10°C) into the vessel (flow rate of 10 L / min) so that the outlet powder temperature was 45±10°C.

[0152] (Comparative Example 3: Comparative CNT3) A 60 L dynamic mill was charged with 8 mm diameter zirconia beads as milling media at a filling rate of 70%, and 100 kg of BT1003M as raw CNT was supplied at a flow rate of 120±20 kg / h, and the milling was carried out for 15 hours using a circulation milling method at a peripheral speed of 4 m / s to obtain Comparative CNT3. During the milling process, room temperature water was run through the vessel to prevent equipment failure due to temperature rise.

[0153] (Comparative Example 4: Comparative CNT4) In a ball mill with a capacity of 250 L, zirconia beads with a diameter of 20 mm were charged as grinding media at a filling rate of 35%, and 5 kg of BT1003M was charged as raw CNT. The mixture was processed for 20 hours using a batch grinding method at a peripheral speed of 10 m / s to obtain Comparative CNT4.

[0154] (Comparative Example 5: Comparative CNT5) In a 150 L attrition mill, zirconia beads with a diameter of 8 mm were charged as grinding media at a filling rate of 70%, and 3 kg of BT1003M was charged as raw CNT. The mixture was processed for 10 hours using a batch grinding method at a rotation speed of 150 min-1 to obtain Comparative CNT5.

[0155] <<Physical Properties and Evaluation of CNTs>> The physical properties of CNTs were measured and evaluated by the following methods. The results are shown in Table 1.

[0156] <Zr Content> The Zr content of the CNT was measured using the following procedure. First, 0.1±0.02 g of CNT was placed in a decomposition vial, and nitric acid was added. The CNT was decomposed using a microwave sample decomposition device. The product was added to the decomposition solution, adjusted to 25 ml, and filtered through a membrane filter. The filtrate was analyzed using an ICP analyzer to determine the Zr content (ppm).

[0157] <Bulk Density X of CNT> Bulk density was measured in accordance with JIS K 5101-12-1 using the following procedure. First, prior to measurement, the CNTs were pretreated by vacuum drying at a pretreatment temperature of 305°C for 18 to 21 hours. Next, agglomerates formed during storage of the CNT powder were crushed, and a sufficient amount of sample for the test was passed through a 0.5 mm sieve to allow the obtained sample to flow freely into the measurement container until it overflowed. The prepared CNT powder was allowed to fall freely into a 30 ml stainless steel cylindrical container, and the portion that had risen to the top of the container was then leveled off. The mass of the CNT powder was then determined and divided by the volume of the container to obtain the bulk density (g / cm ３ ) was sought.

[0158] <Maximum Solvent Absorption Capacity Y of CNT> The maximum solvent absorption capacity Y is the maximum mass of solvent absorbed by CNT per mass of CNT when CNT absorbs a solvent. The maximum mass of solvent absorbed by CNT is the total mass of solvent added dropwise to the CNT powder up to the point where the solvent starts to flow out of the CNT powder. The maximum solvent absorption capacity Y was measured using the following procedure. First, prior to measurement, the CNTs are pretreated by vacuum drying at a pretreatment temperature of 305°C for 18 to 21 hours. In a 25°C environment, 5 g (V(g)) of CNT powder was placed into a cylindrical polypropylene container with a diameter of 10 cm by gravity, and while the container was left to stand, 0.5 g of N-methyl-2-pyrrolidone was dropped onto the surface of the CNT powder at intervals of 1 minute. The total mass (W(g)) of N-methyl-2-pyrrolidone dropped was measured just before the N-methyl-2-pyrrolidone liquid began to flow out onto the surface of the CNT powder without being absorbed by the CNT powder. The maximum solvent absorption capacity was calculated using the following formula (5): Y=W / V (5) (In formula (5), V is the mass of CNT, and W is the maximum mass of N-methyl-2-pyrrolidone absorbed by the CNT when N-methyl-2-pyrrolidone was dropped onto V(g) of CNT in a 25°C environment.)

[0159] <Powder Resistivity of CNT> Using a powder resistivity measuring device (Loresta GP Powder Resistivity Measuring System MCP-PD-51, manufactured by Nitto Seiko Analytech Co., Ltd.), the volume resistivity [Ω cm] of the conductive powder under various pressures was measured using a powder probe unit (four-point ring electrode, electrode spacing 5.0 mm, electrode radius 1.0 mm, sample radius 12.5 mm) with a sample mass of 0.8 g and an applied voltage limiter of 90 V. ３ The powder resistivity values ​​of the CNTs at the density of 1000 MPa were evaluated.

[0160]

[0161]

[0162]

[0163] In Table 1, the rate of change in bulk density is a value calculated from the bulk density X after dry-pulverization and the bulk density X' before dry-pulverization using the following formula, and the rate of change in powder resistivity is a value calculated from the powder resistivity Re after dry-pulverization and the powder resistivity Re' before dry-pulverization using the following formula: Rate of change in bulk density (%) = (X / X' - 1) x 100 Rate of change in powder resistivity (%) = (Re / Re' - 1) x 100

[0164] <CNT Dispersion> (Example 1-1) A CNT dispersion was prepared as follows by sequentially adding materials according to the materials and composition shown in Table 2. First, NMP was placed in a stainless steel jacketed tank and heated to 50°C. While stirring with a disperser, a dispersant (H-NBR1) and an additive (NaOH) were added, followed by stirring for 1 hour to dissolve the dispersant (H-NBR1). Next, while stirring with a disperser, 25 parts by mass of CNT was added per 100 parts by mass of CNT (CNT1), and a high-shear mixer (L5M-A, manufactured by SILVERSON) equipped with a square-hole high-shear screen was used, and batch dispersion was performed for 5 minutes at a speed of 8,000 rpm. The above CNT addition and dispersion process was repeated four times to add the entire amount of CNT. The entire mixture was homogenized, and batch dispersion was performed with the high-shear mixer until the dispersion particle size reached 250 μm or less as measured by a grind gauge with a maximum groove depth of 300 μm. Subsequently, the dispersion liquid was supplied from the jacketed tank to a bead mill (SC100, manufactured by Nippon Coke) via piping, and the dispersion liquid was subjected to a 5-minute circulation dispersion under the conditions of a bead diameter of 1.25 mm, a bead filling rate of 60%, a peripheral speed of 10 m / s, and a supply rate of 800 g / min. Furthermore, the piping was switched, and the dispersion liquid was supplied to a high-pressure homogenizer (Starburst Lab HJP-17007, manufactured by Sugino Machine), and a circulation dispersion process was carried out. The dispersion process was carried out using a single nozzle chamber, with a nozzle diameter of 0.25 mm and a pressure of 100 MPa. After dispersing the dispersion liquid until the viscosity at 60 rpm measured with a B-type viscometer (manufactured by TOKI SANGYO, VISCOMETER, MODEL: BL) was 3,000 mPa s or less, the dispersion liquid was subjected to 15 passes through the high-pressure homogenizer, and CNT dispersion liquid 1 was obtained.

[0165] Examples 1-2 to 1-14 CNT dispersions 2 to 13 were obtained in the same manner as in Example 1-1, except that the materials and compositions were changed according to Table 2.

[0166] (Example 1-15) Materials were added sequentially according to the materials and composition shown in Table 2, and a CNT dispersion was prepared as follows. First, NMP was placed in a stainless steel jacketed tank and heated to 50°C. A dispersant (H-NBR1) and an additive (NaOH) were added while stirring with a disperser, and then the mixture was stirred for 1 hour to dissolve the dispersant (H-NBR1). Next, 25 parts by mass of CNT was added to 100 parts by mass of CNT (CNT1) while stirring with a disperser, and a high-shear mixer (L5M-A, manufactured by SILVERSON) equipped with a square-hole high-shear screen was used, and batch dispersion was performed for 5 minutes at a speed of 8,000 rpm. The above CNT addition and dispersion process was repeated four times to add the entire amount of CNT. The mixture was then uniformly dispersed, and batch dispersion was performed with the high-shear mixer until the dispersion particle size reached 250 μm or less as measured by a grind gauge with a maximum groove depth of 300 μm. Subsequently, the dispersion liquid was supplied from the jacketed tank to a bead mill (SC100, manufactured by Nippon Coke) via piping, and the dispersion liquid was subjected to a 10-minute circulation dispersion under the conditions of a bead diameter of 1.25 mm, a bead filling rate of 60%, a peripheral speed of 10 m / s, and a supply rate of 800 g / min. Furthermore, the piping was switched, and the dispersion liquid was supplied to a high-pressure homogenizer (Starburst Lab HJP-17007, manufactured by Sugino Machine), and a circulation dispersion process was carried out. The dispersion process was carried out using a single nozzle chamber, with a nozzle diameter of 0.25 mm and a pressure of 100 MPa. After dispersing the dispersion liquid until the viscosity at 60 rpm measured with a B-type viscometer (manufactured by TOKI SANGYO, VISCOMETER, MODEL: BL) was 3,000 mPa s or less, the dispersion liquid was subjected to 15 passes in a high-pressure homogenizer, and CNT dispersion liquid 15 was obtained.

[0167] Comparative Examples 1-1 to 1-5 Comparative CNT dispersions 1 to 5 were obtained in the same manner as in Example 1-1, except that the materials and compositions were changed according to Table 2.

[0168] The additives and acids listed in Table 2 are as follows: NaOH: sodium hydroxide (manufactured by Tokyo Chemical Industry Co., Ltd., purity >98.0%, granular) Aminoethanol: 2-aminoethanol (manufactured by Tokyo Chemical Industry Co., Ltd., purity >99.0%)

[0169] <<Measurement and Evaluation of Physical Properties of CNT Dispersion>> The physical properties of the CNT dispersion were measured and evaluated by the following methods. The results are shown in Tables 2 and 3.

[0170] (Shear viscosity of CNT dispersion) Shear rate of CNT dispersion: 1.0 s －１ The viscosity at 25°C was measured using a rheometer (RheoStressl rotational rheometer manufactured by Thermo Fishers Scientific Co., Ltd.) with a diameter of 60 mm and a 2° cone, with a shear rate of 0.01 to 1000 S. －１ The shear rate over time was evaluated by performing the above flow curve measurement on the CNT dispersion liquid in a stationary state with the liquid temperature adjusted to 25°C.

[0171] (Complex Elastic Modulus of CNT Dispersion) The complex elastic modulus of the CNT dispersion was evaluated by performing dynamic viscoelasticity measurement at 25°C, a frequency of 1 Hz, and a strain rate range of 0.01% to 5% using a rheometer (RheoStressl rotational rheometer manufactured by Thermo Fisher Scientific Co., Ltd.) with a diameter of 60 mm and a 2° cone. The shear rate over time was evaluated by performing the above-mentioned dynamic viscoelasticity measurement on the CNT dispersion in a stationary state with the liquid temperature adjusted to 25°C.

[0172] Table 3 shows the shear viscosity η (Pa s) and complex modulus G measured using the CNT dispersions of Examples 1-2, 1-8, and Comparative Examples 1-3. ＊ The measurement results of (Pa) are shown.

[0173] Table 4 shows the natural logarithm (ln(η)) of shear viscosity η (Pa s) and complex modulus G * A regression line expressed by the least squares method was calculated from the scatter diagram on which (Pa) was plotted, and the coefficient of determination R ２ , the slope a, and the y-intercept b are written.

[0174] 1 shows an example of a scatter diagram plotting the results of measurements using the CNT dispersions of Examples 1-2, 1-8, and Comparative Example 1-3 after static storage at 40°C for 1 day, 7 days, and 14 days. For example, the coefficient of determination R ２ is 1.00, the slope a is 8.4, the y-intercept b is 4.1, and the coefficient of determination R ２ was 0.79, the slope a was 20.8, and the y-intercept b was −0.5.

[0175] The criteria in Table 4 are as follows: [coefficient of determination R ２ [Criteria for judgment of slope a] A: 0.95 or more B: 0.90 or more and less than 0.95 C: 0.85 or more and less than 0.90 D: less than 0.85 [Criteria for judgment of slope a] A: 7 or more and less than 14 B: 5 or more and less than 7, or 14 or more and less than 15 C: 2 or more and less than 5, or 15 or more and less than 20 D: less than 2, or 20 or more [Criteria for judgment of y-intercept b] A: 0 or more and less than 6 B: -3 or more and less than 0, or 6 or more and less than 8 C: -5 or more and less than -3, or 8 or more and less than 10 D: less than -5, or 10 or more

[0176] (Filtration rate) The dispersibility of the CNT dispersion was evaluated by the filtration rate. The filtration rate test was carried out by adding 80 g of NMP to 40 g of CNT dispersion and thoroughly mixing by hand. The entire amount was placed in a 300 mL vacuum filtration filter holder, and a scale was placed under the receiving container in the suction filter bell to measure the weight relative to the time it took to pass through a 10 μm mesh. The shorter the time it took to filter the entire diluted CNT dispersion, the better the dispersibility. Furthermore, poorly dispersed CNTs contained aggregated CNTs, and the entire amount could not be filtered. If the evaluation standard was + or higher, the dispersion was usable. [Evaluation standard] +++: Less than 30 seconds ++: 30 to 100 seconds +: 100 seconds or more NG: The entire amount could not be filtered

[0177] (Acid Stability) The fluidity and dispersion stability of the CNT dispersion were evaluated by acid stability. To evaluate acid stability, 5 parts by mass of succinic acid (succinic acid: Fujifilm Wako Pure Chemical Industries, purity >99.5%) was added to 100 parts by mass of the CNT dispersion. The initial viscosity of the acid-containing CNT dispersion and the viscosity after storage at 40°C for 7 days were measured, and the rate of change in viscosity after storage at 40°C for 7 days relative to the initial viscosity was calculated and evaluated. The rate of change in viscosity is expressed as a percentage by dividing the viscosity value after storage at 40°C for 7 days by the initial viscosity value. The viscosity of the CNT dispersion was measured using a Brookfield viscometer (manufactured by Toki Sangyo Co., Ltd., "BL") at 25°C, after thorough stirring with a spatula, immediately at a Brookfield viscometer rotor rotation speed of 60 rpm. A smaller rate of change is preferable, and a rating of + or higher indicates practical use. [Evaluation criteria] +++: Less than 200% ++: 200% or more and less than 500% +: 500% or more and less than 1000% NG: 1000% or more (gelled)

[0178]

[0179]

[0180]

[0181]

[0182]

[0183] <Preparation of Carbon Nanotube Resin Composition, Cathode Composite Slurry, and Cathode> (Examples 2-1 to 2-19, Comparative Examples 2-1 to 2-5) Carbon nanotube resin compositions, cathode composite slurries, and cathodes were prepared as follows according to the combinations and composition ratios shown in Table 5. A CNT dispersion was placed in a stainless steel jacketed tank, and the temperature of the jacketed tank was heated to 60°C so that the liquid temperature measured with a thermocouple thermometer reached 60°C. The binder resin was added in its entirety at a rate of 0.8 parts by mass / min while stirring with a disperser. The temperature was adjusted to maintain the liquid temperature of 60°C measured with a thermocouple thermometer, and the binder resin was completely dissolved by stirring for 3 hours using a disperser, thereby obtaining a carbon nanotube resin composition.

[0184] Capacity 150cm ３ The CNT resin composition and the positive electrode active material were added to the plastic container, and the mixture was stirred at 2,000 rpm for 5 minutes using a centrifugal mixer (Thinky, Awatori Rentaro, ARE-310) to obtain a positive electrode mixture slurry. The non-volatile content of the positive electrode mixture slurry was 78 mass%.

[0185] The positive electrode composite slurry was applied to a 20 μm thick aluminum foil using an applicator, and then heated and dried in an electric oven at 120±5°C for 25 minutes to prepare an electrode film. The electrode film was then rolled using a roll press (Thank Metal, 3 ton hydraulic roll press) to obtain positive electrodes (positive electrodes 1 to 18, comparative positive electrodes 1 to 4). The weight per unit area of ​​the composite layer was 20 mg / cm. ２ The density of the composite layer after rolling was 3.2 g / cm ３ It was.

[0186] In the examples and comparative examples, the following positive electrode active materials were used: NMC1: CellSeed NMC (LiNi ０．６ Co ０．２ Mn ０．２ O ２ , manufactured by Nihon Kagaku Kogyo) ・NMC2:S800 (LiNi ０．８ Mn ０．１ Co ０．１ O ２ , manufactured by Kinwa) ・NCA: NAT-7050 (LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , manufactured by BASF Toda Battery Materials) LFP: HED (trademark) LFP-400 (lithium iron phosphate, manufactured by BASF)

[0187] <Evaluation of Positive Electrode> (Conductivity of Positive Electrode) The surface resistivity (Ω / □) of the composite layer of the obtained positive electrode was measured using a Loresta GP, MCP-T610 manufactured by Mitsubishi Chemical Analytech. After measurement, the surface resistivity was multiplied by the thickness of the composite layer to obtain the volume resistivity (Ω cm) of the positive electrode. The thickness of the composite layer was measured at three points in the electrode using a film thickness meter (DIGIMICRO MH-15M manufactured by NIKON), and the film thickness of the aluminum foil was subtracted from the average value to obtain the volume resistivity (Ω cm) of the positive electrode. The smaller the volume resistivity, the more desirable it is. [Evaluation Criteria] ++++: Less than 10 Ω cm +++: 10 Ω cm or more and less than 20 Ω cm ++: 20 Ω cm or more and less than 30 Ω cm +: 30 Ω cm or more

[0188] (Adhesion of Positive Electrode) The obtained positive electrode was cut into two 90 mm x 20 mm rectangles with the coating direction as the long axis. Peel strength was measured using a desktop tensile tester (Strograph E3, manufactured by Toyo Seiki Seisakusho) and evaluated by a 180-degree peel test method. Specifically, a 100 mm x 30 mm double-sided tape (No. 5000NS, manufactured by Nitoms) was attached to a stainless steel plate, and the composite layer side of the prepared positive electrode was adhered to the other side of the double-sided tape to form a test sample. Next, the test sample was fixed vertically so that the short sides of the rectangle were at the top and bottom, and the end of the aluminum foil was peeled from below to above at a constant speed (50 mm / min). The average value of the stress at this time was taken as the peel strength. The higher the peel strength, the more desirable it is. [Evaluation criteria] ++++: 0.8 N / cm or more +++: 0.5 N / cm or more and less than 0.8 N / cm ++: 0.3 N / cm or more and less than 0.5 N / cm +: Less than 0.3 N / cm

[0189]

[0190] <Preparation of Secondary Battery> (Preparation of Standard Negative Electrode) 0.5 parts of acetylene black (Denka Black (registered trademark) HS-100, manufactured by Denka) and 1 part of MAC500LC (carboxymethylcellulose sodium salt Sunrose Special Type MAC500L, manufactured by Nippon Paper Industries Co., Ltd., non-volatile content 100%) and 98.4 parts of water were added to a 150 ml plastic container, and then the mixture was stirred at 2,000 rpm for 30 seconds using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310). Furthermore, 97 parts by mass of artificial graphite (CGB-20, manufactured by Nippon Graphite Industries Co., Ltd.) was added as an active material, and the mixture was stirred at 2,000 rpm for 150 seconds using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310). Subsequently, 3.1 parts of SBR (styrene butadiene rubber, TRD2001, non-volatile content 48%, manufactured by JSR) was added, and the mixture was stirred at 2,000 rpm for 30 seconds using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310) to obtain a standard negative electrode composite slurry. The non-volatile content of the standard negative electrode composite slurry was set to 50% by mass.

[0191] The standard negative electrode composite slurry was applied to a copper foil having a thickness of 20 μm as a current collector using an applicator, and then dried in an electric oven at 80° C.±5° C. for 25 minutes to obtain an electrode with a coating weight per unit area of ​​10 mg / cm. ２ Further, a rolling treatment was carried out using a roll press (a 3-ton hydraulic roll press manufactured by Thank Metal Co., Ltd.) to adjust the density of the composite layer to 1.6 g / cm. ３ A standard negative electrode was fabricated.

[0192] (Examples 3-1 to 3-19, Comparative Examples 3-1 to 3-5) The positive electrodes and standard negative electrodes listed in Table 8 were punched out to 50 mm x 45 mm and 45 mm x 40 mm, respectively. The punched positive electrodes and standard negative electrodes, along with a separator (porous polypropylene film) inserted therebetween, were inserted into an aluminum laminate bag and dried in an electric oven at 70°C for 1 hour. Thereafter, in a glove box filled with argon gas, an electrolyte solution (a mixed solvent of ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a volume ratio of 1:1:1 was prepared, and 1 part by mass of vinylene carbonate was further added to 100 parts by mass of the electrolyte solution) was added.６ After 2 mL of a non-aqueous electrolyte solution prepared by dissolving 1M of PEG-14 in the aluminum laminate was poured into each battery, the aluminum laminate was sealed to prepare a secondary battery.

[0193] <Battery Evaluation> (Rate Characteristics of Secondary Battery) The obtained secondary battery was placed in a thermostatic chamber at 25°C, and charge / discharge measurements were performed using a charge / discharge device (SM-8, manufactured by Hokuto Denko Corporation). Constant-current, constant-voltage charging (cutoff current 1 mA (0.02 C)) was performed at a charge current of 10 mA (0.2 C) with a charge end voltage of 4.3 V, followed by constant-current discharging at a discharge current of 10 mA (0.2 C) with a discharge end voltage of 3 V. This operation was repeated three times, and then constant-current, constant-voltage charging (cutoff current (1 mA 0.02 C)) was performed at a charge current of 10 mA (0.2 C) with a charge end voltage of 4.3 V, followed by constant-current discharging at discharge currents of 0.2 C and 3 C until the discharge end voltage reached 3.0 V, and the discharge capacity was determined for each. The rate characteristics can be expressed as the ratio of the 0.2 C discharge capacity to the 3 C discharge capacity, using the following formula 1: (Formula 1) Rate characteristic = 3C discharge capacity / third 0.2C discharge capacity × 100 (%) The higher the rate characteristic, the more desirable it is. [Evaluation criteria] ++++: 80% or more +++: 60% or more and less than 80% ++: 40% or more and less than 60% +: Less than 40%

[0194] (Cycle Characteristics of Secondary Battery) The obtained secondary battery was placed in a thermostatic chamber at 25°C, and charge / discharge measurements were performed using a charge / discharge device (SM-8, manufactured by Hokuto Denko Corporation). Constant-current, constant-voltage charging (cutoff current 2.5 mA (0.05 C)) was performed at a charge current of 25 mA (0.5 C) with a charge cut-off voltage of 4.3 V, followed by constant-current discharging at a discharge current of 25 mA (0.5 C) with a discharge cut-off voltage of 3 V. This operation was repeated 200 times. The cycle characteristics can be expressed by the ratio of the third 0.5 C discharge capacity to the 200th 0.5 C discharge capacity at 25°C, as shown in Equation 2 below: (Equation 2) Cycle characteristics = 200th 0.5 C discharge capacity / third 0.5 C discharge capacity × 100 (%). Higher cycle characteristics are desirable. [Evaluation criteria] ++++: 85% or more +++: 80% or more but less than 85% ++: 50% or more but less than 80% +: Less than 50%

[0195]

[0196] As shown in Tables 2 to 6, carbon nanotubes satisfying the ranges of the present invention exhibited excellent conductivity and dispersibility, and carbon nanotube dispersions containing such carbon nanotubes were confirmed to have excellent fluidity and high dispersion and storage stability. Furthermore, the secondary battery electrode composite slurries, when containing active material, exhibited excellent CNT dispersibility, and electrode films using such slurries exhibited high conductivity and adhesion. Furthermore, the resulting secondary batteries exhibited excellent rate and cycle characteristics. This confirmed the high-power, high-capacity, and long-life secondary batteries. CNTs 2 to 4 in Examples 2 to 4 and CNTs 6 to 9 in Examples 6 to 9 each satisfied the ranges of the present invention. The CNT dispersions using these slurries exhibited excellent filterability and acid stability, resulting in improved battery performance for secondary batteries using these slurries. CNT1 in Example 1 and CNT5 in Example 5 are reference examples.

[0197] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the invention.

[0198] The disclosure of this application is related to the subject matter described in Japanese Patent Application No. 2023-105931 filed on June 28, 2023 and the subject matter described in Japanese Patent Application No. 2024-089114 filed on May 31, 2024, the entire disclosures of which are incorporated herein by reference.

Claims

1. Bulk density X is 0.095 g / cm ３ 0.125g / cm or more ３ The following carbon nanotubes have a Zr content of 5 ppm or more and 500 ppm or less, and a maximum solvent absorption capacity Y of the carbon nanotube defined by the following formula (5) of 8.0 or more: Y=W / V (5) (In formula (5), V is the mass (g) of the carbon nanotube, and W is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotube when N-methyl-2-pyrrolidone is dropped onto a carbon nanotube of V (g) in a 25° C. environment.) 2. Bulk density X (g / cm ３ 2. The carbon nanotube according to claim 1, wherein the maximum solvent absorption capacity Y of the carbon nanotube satisfies all of the following formulas (1) to (4): Y≦−200X+37 (1) Y≧200X−17 (2) Y≧−200X+27 (3) Y≦200X+7 (4) 3. The carbon nanotubes according to claim 1 or 2, having an average fiber length of 0.2 μm or more and 1.1 μm or less.

4. The carbon nanotubes according to claim 1 or 2, having an average fiber length of 0.2 μm or more and 1.0 μm or less, and a Zr content of 15 ppm or more and 200 ppm or less.

5. A carbon nanotube dispersion comprising the carbon nanotubes according to claim 1 or 2, a dispersant, and a non-aqueous dispersion medium.

6. The coefficient of determination R of the regression line represented by the following formula (I) ２ 6. The carbon nanotube dispersion according to claim 5, wherein the slope a is 2≦a<20 and the y-intercept b is −5≦b<10. y=ax+b Formula (I) (Formula (1) is a regression line determined by the least squares method from a scatter diagram plotting x on the horizontal axis and y on the vertical axis, measured using carbon nanotube dispersions whose static storage temperatures and days were 40° C. for 1 day, 40° C. for 7 days, and 40° C. for 14 days, respectively, and wherein the linear regression line is a regression line obtained by the least squares method from a scatter diagram plotting x on the horizontal axis and y on the vertical axis, measured using carbon nanotube dispersions whose static storage temperatures and days were 40° C. for 1 day, 40° C. for 7 days, and 40° C. for 14 days, and －１ The shear viscosity at η ｉ (Pa s), the complex modulus of elasticity at 25 ° C. and a frequency of 1 Hz obtained by dynamic viscoelasticity measurement is G ＊ ｉ (Pa), x is 25°C, shear rate 1.0 s －１ The shear viscosity η ｉ (Pa s) natural logarithm (ln(η ｉ )), y is the complex elastic modulus G at 25 ° C. and a frequency of 1 Hz ＊ ｉ (Pa), and i is 1, 7, and 14, with the production date of the carbon nanotube dispersion being day 0.

7. A carbon nanotube resin composition comprising the carbon nanotube dispersion liquid according to claim 5 and a binder resin.

8. A composite slurry comprising the carbon nanotube dispersion liquid according to claim 5, a binder resin, and an active material.

9. An electrode film formed from the composite slurry according to claim 8.

10. A secondary battery comprising a positive electrode and a negative electrode, at least one of the positive electrode and the negative electrode having the electrode film according to claim 9.

11. A method for producing a carbon nanotube having a bulk density X of 0.095 g / cm3, comprising the steps of: dry-pulverizing carbon nanotubes using a milling device including a grinding medium made of a metal or ceramic material; and wherein the dry-pulverizing is a pass-type grinding or a circulation-type grinding. ３ 0.125g / cm or more ３ The following describes a method for producing carbon nanotubes having a Zr content of 5 ppm or more and 500 ppm or less, and a maximum solvent absorption capacity Y of the carbon nanotubes defined by the following formula (5) of 8.0 or more: Y=W / V (5) (In formula (5), V is the mass (g) of the carbon nanotubes, and W is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotubes when N-methyl-2-pyrrolidone is dropped onto carbon nanotubes of V (g) in an environment of 25° C.) 12. The method for producing carbon nanotubes according to claim 11, wherein the rate of change between the bulk density X after dry-pulverization and the bulk density X' before dry-pulverization is -60% or more and 1000% or less. Bulk density change rate (%) = (X / X' - 1) x 100 13. The method for producing carbon nanotubes according to claim 11, wherein the rate of change between the powder resistivity Re after dry-milling and the powder resistivity Re' before dry-milling is 1% or more and 20% or less. Powder resistivity change rate (%)=(Re / Re'-1)×100 14. The method for producing carbon nanotubes according to claim 11, wherein the grinding media is zirconia beads, and the diameter of the zirconia beads is greater than 5 mm and less than 10 mm.

15. The method for producing carbon nanotubes according to claim 11, wherein the carbon nanotubes after the dry grinding have an average fiber length of 0.2 μm or more and 1.1 μm or less.

16. The method for producing carbon nanotubes according to claim 11, wherein the carbon nanotubes after the dry milling have an average fiber length of 0.2 μm or more and 1.0 μm or less and a Zr content of 15 ppm or more and 200 ppm or less.

17. A method for producing a carbon nanotube dispersion liquid, comprising the steps of mixing and dispersing the carbon nanotubes according to any one of claims 11 to 16, a dispersant, and a non-aqueous dispersion medium.

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