JP7790132B2 - Carbon nanotube dispersion liquid, carbon nanotube dispersion composition, slurry for electrode film, electrode film, and secondary battery - Google Patents

Description

Embodiments of the present invention relate to a carbon nanotube dispersion liquid, a carbon nanotube dispersion composition, a slurry for an electrode film, an electrode film, and a secondary battery.

Lithium-ion secondary batteries are widely used as batteries for electric vehicles, portable devices, and other devices. As the performance of electric vehicles and portable devices improves, demand for lithium-ion secondary batteries with higher capacity, higher output, and smaller size and lighter weight is increasing year by year.

Since the capacity of a lithium-ion secondary battery depends heavily on the main materials used, the positive and negative electrode active materials, various materials for use in these electrode active materials are being actively researched. However, the charge capacity achieved using commercially available electrode active materials is close to the theoretical value, and improvement is close to its limit. Therefore, since the charge capacity can simply be increased by increasing the amount of electrode active material filled in the electrode film, attempts are being made to reduce the amount of conductive material and binder resin added, which do not directly contribute to charge capacity.

The conductive material plays a role in forming conductive paths within the electrode film and connecting particles of the electrode active material. It is necessary for the conductive paths and connections between particles to be resistant to disconnection due to the expansion and contraction of the electrode film. To maintain the conductive paths and connections between particles with a small amount of additive, it is effective to use nanocarbons with a large specific surface area, particularly carbon nanotubes (CNTs), as the conductive material to form an efficient conductive network. However, nanocarbons with a large specific surface area have strong cohesive forces, making it difficult to disperse the nanocarbons well in the electrode film slurry and/or the electrode film.

In light of this background, methods have been proposed in which a conductive material dispersion is prepared using various dispersants, and then a slurry for electrode films is produced via the conductive material dispersion, or methods in which the physical properties of the carbon nanotubes themselves are devised (see Patent Documents 1 to 5).

JP 2011-070908 A Special table 2018-534731 publication International Publication No. 2019/054173 JP 2010-238575 A Japanese Patent Application Laid-Open No. 2016-025077

Patent Document 1 proposes using non-ionic dispersants such as polyvinylpyrrolidone and polyvinyl alcohol as dispersants to effectively disperse conductive materials in solvents and improve battery output. However, while polyvinylpyrrolidone and polyvinyl alcohol can be used to produce conductive material dispersions, the dispersion state becomes poor during the electrode film formation process, resulting in a deterioration in conductivity, which is an issue.

Patent Document 2 proposes a conductive material dispersion using hydrogenated nitrile rubber as a dispersant. However, these hydrogenated nitrile rubbers have poor dispersibility and are insufficient for forming a good conductive network. Furthermore, the resulting conductive material dispersions have high viscosity and strong solid-like properties, which means that production of the conductive material dispersions takes a long time, and they also have poor fluidity, which reduces coatability and makes them prone to gelation.

Patent Document 3 proposes using hydrogenated nitrile rubber and polyvinylpyrrolidone in combination to improve the dispersibility of the conductive material and prevent uneven distribution of the conductive material in the resulting electrode mixture layer. However, carbon nanotubes, which have a small outer diameter and a large specific surface area, cannot be dispersed at high concentrations like acetylene black, and the resulting dispersion fails to form a good conductive network. Furthermore, high-concentration carbon nanotube dispersions are prone to forming aggregates and precipitates during storage, posing a challenge in terms of storage stability.

On the other hand, Patent Document 4 proposes suppressing the aggregation of conductive materials during dispersion and drying by combining carbon nanotubes with outer diameters less than 100 nm with carbon nanotubes with outer diameters of 100 nm or greater. However, this is still insufficient to form a good conductive network with a small amount of conductive material added. Furthermore, Patent Document 5 also proposes the preparation of electrodes for lithium-ion batteries by combining carbon nanotubes with different outer diameters, thereby reducing the amount of conductive material and binder resin added. Generally, the smaller the outer diameter of carbon nanotubes, the larger their specific surface area, which results in poorer wettability to solvents and makes it difficult to obtain a high-concentration, good-quality dispersion. However, carbon nanotubes with smaller outer diameters and higher specific surface areas ideally form an efficient conductive network. Therefore, there was an urgent need to obtain a well-dispersed dispersion of carbon nanotubes with small outer diameters and high specific surface areas, while retaining the inherent conductivity of carbon nanotubes and, more specifically, utilizing their fiber length without breaking them. The carbon nanotubes in the dispersion obtained in Patent Document 5 have short fiber lengths, making it impossible to stably disperse the carbon nanotubes at a high concentration while maintaining electrical conductivity. Furthermore, dispersions with low carbon nanotube concentrations present problems such as reduced design freedom when blending materials such as active materials and binders, and high transportation costs per unit of carbon nanotube solid content. Therefore, there was a need for a high-concentration dispersion of carbon nanotubes with small outer diameters and large specific surface areas.

The problem to be solved by the present invention is to provide a carbon nanotube dispersion liquid and a carbon nanotube dispersion composition that have a high concentration and high dispersibility in order to obtain a highly conductive electrode film. More specifically, the problem to be solved by the present invention is to provide a nonaqueous electrolyte secondary battery that has excellent rate characteristics and cycle characteristics.

An object of an embodiment of the present invention is to provide a carbon nanotube dispersion liquid having a high concentration and high dispersibility. Another object of an embodiment of the present invention is to provide a carbon nanotube dispersion composition. A further object of an embodiment of the present invention is to provide a slurry for an electrode film. A further object of an embodiment of the present invention is to provide an electrode film that can improve the output and cycle life of a secondary battery, and a secondary battery that has high output and good cycle life.

The inventors' intensive research has revealed that by including a carbon nanotube dispersion containing a polymer containing specific structural units and a polymer containing nitrile group-containing monomer units, and by including carbon nanotubes of different fiber diameters in the carbon nanotube dispersion, it is possible to disperse a high concentration of carbon nanotubes well in the solvent, and to maintain this good dispersion state during the preparation of the electrode film slurry and during the production of the electrode film, making it possible to form a good conductive network in the electrode.

That is, the present invention includes the following embodiments:

[1] A carbon nanotube dispersion comprising carbon nanotubes, a polymer (A) containing at least one selected from the group consisting of hydroxyl group-containing structural units and heterocycle-containing structural units and not containing a nitrile group-containing structural unit, a polymer (B) containing a nitrile group-containing structural unit, and a solvent, wherein the carbon nanotubes contain at least two types of carbon nanotubes that differ in fiber diameter distribution within a range of 100 nm or less in a fiber diameter distribution measured with a scanning electron microscope, and the two types of carbon nanotubes contain a component (I) exhibiting a first fiber diameter distribution and a component (II) exhibiting a second fiber diameter distribution.

[2] The carbon nanotube dispersion liquid described in [1], wherein the first fiber diameter distribution range is 2 nm or more and less than 30 nm, and the second fiber diameter distribution range is 30 nm or more and 100 nm or less.

[3] The carbon nanotube dispersion liquid according to [1] or [2], wherein the content of component (I) having the first fiber diameter distribution is 40% or more and 99% or less on a number basis based on the total amount of carbon nanotubes.

[4] The carbon nanotube dispersion liquid described in any one of [1] to [3], wherein the content of carbon nanotubes having a fiber length of 0.3 μm or less is 75% or less by number based on the total amount of carbon nanotubes, and the average fiber length of the carbon nanotubes in the carbon nanotube dispersion liquid is 0.1 μm or more and 1.0 μm or less.

[5] The carbon nanotube dispersion liquid according to any one of [1] to [4], wherein the content of polymer (A) is 30% by mass or more and 70% by mass or less, based on the total mass of polymer (A) and polymer (B).

[6] A carbon nanotube dispersion liquid according to any one of [1] to [5], wherein the particle size distribution measured by a laser diffraction/scattering particle size distribution measurement method under conditions of a laser light transmittance of 50% has at least two peaks, and the mode diameter of the first peak, which has the smallest particle diameter among the two or more peaks, is 0.2 μm or less.

[7] A carbon nanotube dispersion liquid according to any one of [1] to [6], wherein the solvent is substantially free of water and has a pH of 9.0 or more and 12.0 or less.

[8] The carbon nanotube dispersion liquid according to any one of [1] to [7], wherein the polymer (B) further contains an aliphatic hydrocarbon structural unit.

[9] A carbon nanotube dispersion composition comprising the carbon nanotube dispersion liquid described in any one of [1] to [8] and a fluorine-based resin.

[10] A slurry for an electrode film comprising the carbon nanotube dispersion liquid described in any one of [1] to [8] or the carbon nanotube dispersion composition described in [9], and an electrode active material.

[11] An electrode film comprising a coating film of the carbon nanotube dispersion liquid described in any one of [1] to [8], the carbon nanotube dispersion composition described in [9], or the electrode film slurry described in [10].

[12] A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, in which the electrode film described in [11] is used in at least one of the positive electrode or negative electrode.

Embodiments of the present invention make it possible to provide a carbon nanotube dispersion liquid, a carbon nanotube dispersion composition, and a slurry for an electrode film that have a high concentration and high dispersibility. Furthermore, embodiments of the present invention make it possible to provide an electrode film that can improve the output and cycle life of a secondary battery, and a secondary battery that has high output and a good cycle life.

FIG. 1 is a graph showing a distribution of fiber diameters of a carbon nanotube dispersion (dispersion 1) measured with a scanning electron microscope. FIG. 2 is a graph showing the particle size distribution of the carbon nanotube dispersion (dispersion 1) at a laser light transmittance of 50% in a laser diffraction/scattering particle size distribution measurement method.

The following provides a detailed description of embodiments of the present invention, including carbon nanotubes, polymers, carbon nanotube dispersions, carbon nanotube dispersion compositions, slurries for electrode films, electrode films, and secondary batteries. The present invention is not limited to the following embodiments, and includes embodiments that are implemented within the scope of the present invention.

In this specification, conductive materials such as carbon nanotubes and carbon black may be referred to as "conductive materials." The fiber diameter of carbon nanotubes may be referred to as "outer diameter." In this specification, carbon nanotube dispersions may be referred to as "carbon nanotube dispersions," or simply as "dispersions," or "dispersions." Furthermore, polymer (A), polymer (B), and "polymer (A) and polymer (B)" may be referred to as "dispersants."

In this specification, numerical ranges indicated using "to" indicate ranges that include the numerical values before and after "to" as the minimum and maximum values, respectively. In numerical ranges described in stages in this specification, the upper or lower limit of a numerical range in one stage can be arbitrarily combined with the upper or lower limit of a numerical range in another stage.

The carbon nanotube dispersion liquid of an embodiment of the present invention contains at least carbon nanotubes, a dispersant (polymer (A) and polymer (B)), and a solvent. The carbon nanotubes, dispersant, and solvent are described in detail below.

<Carbon nanotubes>
Carbon nanotubes have a cylindrical shape formed by rolling up planar graphite, and include single-walled carbon nanotubes and multi-walled carbon nanotubes, or a mixture of these. Single-walled carbon nanotubes have a structure in which one layer of graphite is rolled up. Multi-walled carbon nanotubes have a structure in which two or more layers of graphite are rolled up. Furthermore, the sidewalls of carbon nanotubes do not have to have a graphite structure. Furthermore, for example, carbon nanotubes having sidewalls with an amorphous structure are also considered to be carbon nanotubes in this specification.

The shape of the carbon nanotubes is not limited. Examples of such shapes include needles, cylindrical tubes, fishbone shapes (fishbone or cup stacked shapes), playing cards (platelets), and coil shapes. In this embodiment, the shape of the carbon nanotubes is preferably needles or cylindrical tubes. The carbon nanotubes may have a single shape or a combination of two or more shapes.

Examples of carbon nanotube forms include graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers. Carbon nanotubes may have any of these forms alone or a combination of two or more of them.

The average fiber diameter of carbon nanotubes is 100 nm or less, more preferably 80 nm or less, and even more preferably 60 nm or less. It is preferably 2 nm or more, and more preferably 5 nm or more. The average fiber diameter can be confirmed using a scanning electron microscope, as described in the Examples. The average fiber diameter can be confirmed as follows: First, carbon nanotubes are observed and photographed using a scanning electron microscope. Next, 300 carbon nanotubes are randomly selected from the photograph, and the outer diameter of each is measured. The average value is taken as the average fiber diameter.

The carbon nanotubes include two types of carbon nanotubes that differ in fiber diameter distribution within a range of 100 nm or less in fiber diameter distributions measured with a scanning electron microscope. The two types of carbon nanotubes include a component (I) that exhibits a first fiber diameter distribution and a component (II) that exhibits a second fiber diameter distribution. Here, the fiber diameter of component (I) is preferably smaller than that of component (II). In this specification, "different fiber diameter distributions" means that there are at least two peaks in the fiber diameter distribution. It is preferable that the two peaks do not overlap. When the two peaks partially overlap, the region below the fiber diameter at which the inflection point of the frequency (%) curve of the overlapping portion of the peaks becomes the smallest is defined as the fiber diameter distribution of component (I), and the region above that fiber diameter is defined as the fiber diameter distribution of component (II).

The first fiber diameter distribution range of component (I) is preferably 2 nm or more, more preferably 5 nm or more. It is also preferably less than 30 nm, more preferably 20 nm or less. The second fiber diameter distribution range of component (II) is preferably 30 nm or more, more preferably 40 nm or more. It is also 100 nm or less, preferably 80 nm or less, more preferably 60 nm or less. By including component (I) and component (II) having fiber diameter distributions within the above ranges, a dispersion liquid in which the carbon nanotubes are uniformly dispersed without forming entangled aggregates is obtained. Furthermore, they do not aggregate even during drying, such as in the formation of a coating film, and can remain uniformly dispersed even in the dried electrode, improving the output and cycle life of the secondary battery.

The content of component (I) having the first fiber diameter distribution is preferably 40% or more, more preferably 50% or more, and even more preferably 60% or more, based on the total amount of carbon nanotubes, on a number basis. It is also preferably 99% or less, and more preferably 98.5% or less. The content of component (II) having the second fiber diameter distribution is preferably 1% or more, more preferably 1.5% or more, based on the total amount of carbon nanotubes, on a number basis. It is also preferably 60% or less, more preferably 50% or less, and even more preferably 40% or less. By containing component (I) and component (II) in amounts equal to or greater than the above lower limits, a carbon nanotube dispersion liquid can be prepared that exhibits both good dispersibility and conductive path formation.

The specific surface area of the carbon nanotubes is preferably 100 m ² /g or more, more preferably 150 m ² /g or more, and even more preferably 200 m ² /g or more. It is also preferably 1200 m ² /g or less, more preferably 1000 m ² /g or less, and particularly preferably 800 m ² /g or less. The specific surface area of the carbon nanotubes is calculated by the BET method using nitrogen adsorption measurement.

The average fiber length of the carbon nanotubes in the carbon nanotube dispersion is preferably 0.05 μm or more, more preferably 0.1 μm or more, and even more preferably 0.2 μm or more. It is also preferably 1.0 μm or less, more preferably 0.8 μm or less, and even more preferably 0.5 μm or less. The longer the fiber length of the carbon nanotubes in the carbon nanotube dispersion, the more efficiently a conductive network can be formed with a small amount, thereby reducing the amount of conductive material required in the battery electrode. However, carbon nanotubes with long fiber lengths have strong cohesion and are difficult to disperse. Furthermore, the carbon nanotubes are prone to breakage during the dispersion process, making it difficult to control the fiber length of the carbon nanotubes. In an embodiment of the present invention, by setting the average fiber length of the carbon nanotubes in the carbon nanotube dispersion within the above range, carbon nanotubes with different fiber diameter distributions can be dispersed at a high concentration without agglomeration. This allows for the efficient formation of a conductive network with a small amount, thereby reducing the amount of conductive material required in the battery electrode. Furthermore, both dispersibility and stability can be achieved, and good dispersion can be maintained in the electrode film slurry and/or electrode film.

In the carbon nanotube dispersion of an embodiment of the present invention, the content of carbon nanotubes with a fiber length of 0.3 μm or less is preferably 75% or less, and more preferably 70% or less, by number, of the total amount of carbon nanotubes. Furthermore, in the carbon nanotube dispersion of an embodiment of the present invention, the content of carbon nanotubes with a fiber length greater than 0.3 μm is preferably 10% or more, and more preferably 25% or more, by number, of the total amount of carbon nanotubes. When the fiber length satisfies the above conditions, carbon nanotubes with different fiber diameter distributions can be dispersed at high concentrations without aggregation, and good dispersion can be maintained in the electrode film slurry and/or electrode film. The fiber length and average fiber length of carbon nanotubes in the carbon nanotube dispersion can be measured using the methods described in the Examples. The fiber length can be confirmed as follows: First, the carbon nanotubes are observed and photographed using a scanning electron microscope. Next, the fiber length of the carbon nanotubes can be confirmed by measuring the observed photograph. The average fiber length was determined by randomly selecting 300 carbon nanotubes from the observation photograph, measuring the fiber length of each, and averaging the measured values. Note that the fiber length of carbon nanotubes in this embodiment of the present invention refers to the fiber length in the carbon nanotube dispersion, i.e., the fiber length after the carbon nanotube dispersion has been prepared.

The carbon purity of carbon nanotubes is expressed as the carbon atom content (mass%) in the carbon nanotubes. The carbon purity is preferably 80 mass% relative to 100 mass% of the carbon nanotubes, more preferably 90 mass% or more, even more preferably 95 mass% or more, and particularly preferably 98 mass% or more. By keeping the carbon purity within the above range, problems such as short circuits caused by the formation of dendrites due to impurities can be prevented.

The carbon purity of carbon nanotubes can be adjusted by purification treatment. Various conventionally known methods can be used to purify carbon nanotubes. Examples include acid treatment, graphitization treatment, and chlorination treatment.

The acid used to treat carbon nanotubes with acid may be any acid capable of dissolving the metals and metal oxides contained in the carbon nanotubes. For example, inorganic acids and carboxylic acids are preferred, and among inorganic acids, hydrochloric acid, sulfuric acid, and nitric acid are particularly preferred. The acid treatment of carbon nanotubes is preferably carried out in a liquid phase, and it is more preferred to disperse and/or mix the carbon nanotubes in the liquid phase. After acid treatment, the carbon nanotubes are preferably washed with water and dried.

The graphitization process for carbon nanotubes is not particularly limited, but can be carried out by heating the carbon nanotubes at 1500°C to 3500°C in an inert atmosphere with an oxygen concentration of 0.1% or less.

The chlorination treatment of carbon nanotubes is not particularly limited, but can be carried out, for example, by introducing chlorine gas into an inert atmosphere with an oxygen concentration of 0.1% or less and heating the carbon nanotubes at 800°C to 2000°C.

The carbon nanotubes may be surface or terminally modified with functional groups or alkyl groups, or may be doped with alkali metals or halogens. For example, they may be functionalized with carboxyl groups, sulfo groups, or hydroxyl groups by heating in acid. Because polymer (A) and polymer (B) allow the resulting carbon nanotube dispersion to have both good stability and fluidity, it is preferable to use carbon nanotubes that do not have acidic functional groups such as carboxyl groups or sulfo groups.

As the conductive material, carbon materials such as carbon black and graphite may be used alone or in combination. Of these conductive materials, carbon black is preferred from the perspective of dispersant adsorption performance. Examples of carbon black include acetylene black, furnace black, hollow carbon black, channel black, thermal black, and ketjen black. Furthermore, carbon black may be neutral, acidic, or basic, and oxidation-treated carbon black and graphitization-treated carbon black may also be used.

The amount of metal contained in the conductive material is preferably less than 10% by mass, more preferably less than 5% by mass, and even more preferably less than 2% by mass, based on 100% by mass of the conductive material. In particular, metals contained in carbon nanotubes include metals, metal oxides, and metal hydroxides used as catalysts in synthesizing carbon nanotubes. Specific examples include metals such as copper, iron, cobalt, nickel, aluminum, magnesium, silica, manganese, and molybdenum, as well as metal oxides, metal hydroxides, and composite oxides thereof. In particular, if the amount of metal hydroxide contained in the carbon nanotubes exceeds the above range, the wettability of the carbon nanotubes may decrease. Furthermore, the pH of the carbon nanotube dispersion may increase, potentially impairing the dispersibility of polymer (A) and polymer (B).

The conductive material may also contain 50 ppm or less, more specifically, 20 ppm or less, of iron metal element in the catalyst used in the manufacturing process, based on 100% by mass of the conductive material. By significantly reducing the iron content as an impurity remaining in the conductive material in this way, superior conductivity can be achieved without the risk of side reactions in the electrode. The content of metal impurities remaining in the conductive material can be analyzed using inductively coupled plasma (ICP). The conductive material does not necessarily need to contain iron metal element.

The carbon nanotube content is preferably 0.2% by mass or more, more preferably 0.4% by mass or more, and even more preferably 1% by mass or more, based on the non-volatile content of the carbon nanotube dispersion. It is also preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less. By maintaining the carbon nanotube content within the above range, the carbon nanotubes can be maintained in a stable state without sedimentation or gelation. The carbon nanotube content is preferably adjusted appropriately to obtain a carbon nanotube dispersion with appropriate fluidity or viscosity, taking into account the specific surface area of the carbon nanotubes, their affinity for the dispersion medium, the dispersibility of the dispersant, and other factors. For example, from the perspective of achieving both good stability and fluidity, the conductive material content may be 10% by mass or less, and preferably 8% by mass or less, based on 100% by mass of the carbon nanotube dispersion. In embodiments of the present invention, the conductive material is preferably in the above range, since carbon nanotubes have a high specific surface area. Alternatively, for example, even when the concentration of the conductive material is high, good dispersibility and stability can be achieved by polymer (A) and polymer (B), and therefore the content of the conductive material may be 5% by mass or more, or may be greater than 10% by mass, relative to 100% by mass of the carbon nanotube dispersion liquid.

<Polymer (A)>
The polymer (A) contains at least one of a hydroxyl group-containing structural unit and a heterocycle-containing structural unit. The use of the polymer (A) together with the polymer (B) described below improves the dispersibility of carbon nanotubes, thereby suppressing uneven distribution of the conductive material containing carbon nanotubes in the resulting electrode mixture layer. Furthermore, although the reason is unclear, the presence of the polymer (A) controls the adsorption of the polymer (B) described below to the dispersed material and the affinity for the dispersion medium, allowing the dispersed material to be stably present in the dispersion medium.

(Hydroxyl Group-Containing Structural Unit)
When the polymer (A) contains a hydroxyl group-containing structural unit, it may optionally contain a structural unit other than the hydroxyl group-containing structural unit (other structural unit). The hydroxyl group-containing structural unit is a structural unit containing a hydroxyl group, and preferably contains a structural unit containing an alkylene structure substituted with a substituent containing a hydroxyl group. The alkylene structure is preferably a linear or branched alkylene structure. The number of hydroxyl groups contained in the hydroxyl group-containing structural unit is preferably one or two, more preferably one.
The method for introducing a hydroxyl group-containing structural unit into polymer (A) is not particularly limited, and examples thereof include a method of preparing a polymer by polymerization of a monomer containing a hydroxyl group, or a method of preparing a polymer by polymerization of a monomer containing a functional group other than a hydroxyl group and then modifying the polymer to have a hydroxyl group, etc. A reasonable method can be selected from the viewpoints of reactivity and raw material cost.

Examples of monomers containing a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, glycerol mono(meth)acrylate, 4-hydroxyvinylbenzene, 2-hydroxy-3-phenoxypropyl acrylate, and caprolactone adducts of these monomers (number of moles added: 1 to 5). The hydroxyl group-containing monomer is preferably a hydroxyalkyl (meth)acrylate, more preferably 2-hydroxyethyl (meth)acrylate, and even more preferably 2-hydroxyethyl acrylate.

One method for preparing a polymer by polymerization of monomers containing functional groups other than hydroxyl groups and then modifying them to hydroxyl groups is to saponify the acetyl groups of polyvinyl acetate obtained by polymerizing vinyl acetate with an alkali such as sodium hydroxide to convert them to hydroxyl groups (saponification reaction). The saponification reaction rate (degree of saponification) can be controlled as desired by varying the sodium hydroxide concentration and treatment time.

In addition, to increase the affinity between the carbon-based conductive material and the medium, the hydroxyl groups in the polymer may be reacted with an aldehyde compound to modify them into acetal groups (acetalization). Aldehyde compounds used in the acetalization reaction may include, but are not limited to, linear, branched, cyclic, saturated, unsaturated, or aromatic aldehydes and other compounds having 1 to 15 carbon atoms. Specific examples include formaldehyde, acetaldehyde, propionyl aldehyde, n-butyraldehyde, isobutyraldehyde, tert-butyraldehyde, benzaldehyde, and cyclohexyl aldehyde. These aldehyde compounds may be used alone or in combination. With the exception of formaldehyde, these aldehyde compounds may have one or more hydrogen atoms substituted with a halogen atom or the like. From the perspective of versatility, linear, branched, cyclic saturated, unsaturated, or aromatic aldehyde compounds having 1 to 10 carbon atoms are preferred, with linear aldehyde compounds having 1 to 4 carbon atoms being more preferred. By changing the aldehyde compound and treatment time, the acetalization reaction rate (degree of acetalization) can be controlled as desired.

The content of the hydroxyl group-containing structural unit is preferably 80% by mass or more, more preferably 85% by mass or more, and preferably 99.8% by mass or less, based on the mass of the polymer (i.e., when the mass of the polymer is 100% by mass). Setting it within the above range strengthens polarization and increases affinity with the carbon-based conductive material and medium. It is also preferable from the standpoint of electrolyte resistance. For the same reasons as for the content of the hydroxyl group-containing structural unit, the content of the acetal group is preferably within the preferred range of the content of the hydroxyl group-containing structural unit.

For example, polyvinyl alcohol resins and polyvinyl acetal resins contain hydroxyl group-containing structural units based on vinyl alcohol or the like in their molecules, and therefore can be used as polymer (A). One embodiment of polymer (A) is one or more polymers selected from the group consisting of polyvinyl alcohol resins and polyvinyl acetal resins including polyvinyl butyral and polyvinyl formal.

(Heterocycle-containing structural unit)
When the polymer (A) contains a heterocyclic ring-containing structural unit, it may optionally contain a structural unit other than the heterocyclic ring-containing structural unit (other structural unit). The heterocyclic ring of the heterocyclic ring-containing structural unit may be, for example, a monocyclic structure or a condensed ring structure, but is preferably a monocyclic structure. The heterocyclic ring contains atoms other than carbon in the ring-constituting atoms, and most of them contain one or more nitrogen, oxygen, and sulfur atoms. It is believed that by including a heterocyclic ring-containing structural unit as the polymer (A), polarization occurs within the heterocyclic ring, which acts strongly on the dispersed material and further enhances the adsorptivity and affinity for the dispersion medium.

Heterocycle-containing structural units are not particularly limited, but examples include structures based on heterocycle-containing monomers such as pyrrole, pyrrolidine, piperidine, imidazole, α-pyrone, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, 2-pyrrolidone, 5-methyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, N-vinyl-ε-caprolactam, N-vinyl-2-piperidone, N-vinyl-3-morpholinone, N-vinyl-1,3-oxazin-2-one, N-vinyl-3,5-morpholinedione, furan, tetrahydrofuran, and pyran. Among these, structural units based on an N-vinyl cyclic amide structure, such as N-vinyl-2-pyrrolidone, N-vinyl-ε-caprolactam, N-vinyl-2-piperidone, N-vinyl-3-morpholinone, N-vinyl-1,3-oxazin-2-one, and N-vinyl-3,5-morpholinedione, are preferred, with N-vinyl-2-pyrrolidone being particularly preferred from the perspective of improving battery characteristics. These can be used alone or in combination of two or more types.

The content of heterocycle-containing structural units in polymer (A) is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more, when the total mass of polymer (A) is 100% by mass. If the heterocycle-containing structural units in polymer (A) are 70% by mass or more, battery performance, particularly low-temperature characteristics, can be further improved. The content of heterocycle-containing structural units in polymer (A) is not particularly limited, and can be 100% by mass or less.

The content of polymer (A) is preferably 30% by mass or more and 70% by mass or less, based on the total mass of polymer (A) and polymer (B), which will be described later. The weight-average molecular weight of polymer (A), calculated as polystyrene, is preferably 5,000 or more, more preferably 6,000 or more, and even more preferably 8,000 or more. It is also preferably 450,000 or less, more preferably 300,000 or less, and even more preferably 150,000 or less. By adjusting the weight-average molecular weight to an appropriate level, it is possible to improve the adsorption to the dispersed material and further improve the stability of the dispersion.

<Polymer (B)>
The polymer (B) contains a nitrile group-containing structural unit. Because the polymer (B) contains the nitrile group-containing structural unit, it can exhibit excellent flexibility and stability due to its binding strength. This effect is believed to maintain a good conductive network even in the electrode film formed by mixing the carbon nanotube dispersion with an electrode active material to form a film of electrode film slurry. By using the polymer (B) together with the aforementioned polymer (A), it is possible to dramatically improve the adhesion (peel strength) between carbon nanotubes and between the carbon nanotubes and the active material and current collector. When the polymer (A) contains a hydroxyl group-containing structural unit, strong intermolecular forces such as hydrogen bonds act between the polymer (A) and the polymer (B), forming a crosslinked structure between the polymer (A) and the polymer (B), allowing three-dimensional adsorption to the dispersed material. This is believed to result in a dispersion that is not only highly dispersible but also highly stable. When polymer (A) contains a heterocycle-containing structural unit, polarization occurs not only within the heterocycle of polymer (A) but also in the nitrile group contained in the nitrile group-containing structural unit of polymer (B), and these exert a synergistic effect, acting strongly on the dispersed material, and it is believed that high binding strength can be exerted while maintaining a good dispersion state.

The nitrile group-containing structural unit is a structural unit containing a nitrile group, preferably a structural unit containing an alkylene structure substituted with a nitrile group, more preferably a structural unit consisting of only an alkylene structure substituted with a nitrile group. The alkylene structure is preferably a linear or branched alkylene structure. The nitrile group-containing structural unit may further contain a structural unit containing (or consisting of) an alkyl structure substituted with a nitrile group. The number of nitrile groups contained in the nitrile group-containing structural unit is preferably one.
The method for introducing the nitrile group-containing structural unit into the polymer (B) is not particularly limited, but for example, a method for preparing a polymer by polymerization reaction of a monomer containing a nitrile group can be preferably used.

Examples of monomers containing a nitrile group include acrylonitrile, methacrylonitrile, and fumaronitrile, and these can be used alone or in combination of two or more. When the monomer containing a nitrile group is acrylonitrile, the polymer is less likely to bend, and adjacent cyano groups are oriented to form a partial structure with strong polarization, which is thought to increase the intermolecular forces between polymers and between the polymer and the carbon-based conductive material. From the perspectives of increasing intermolecular forces, ease of raw material availability, and reactivity, it is preferable that the monomer containing a nitrile group be acrylonitrile.

From the viewpoint of enhancing intermolecular forces, the content of the nitrile group-containing structural unit is preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, based on the mass of polymer (B) (i.e., when the mass of polymer (B) is 100% by mass). Furthermore, the content of the nitrile group-containing structural unit is preferably 90% by mass or less, more preferably 70% by mass or less, even more preferably 50% by mass or less, and particularly preferably 40% by mass or less, based on the mass of polymer (B). By setting the content of the nitrile group-containing structural unit within the above range, it is possible to control the adsorption to the dispersed substance and the affinity to the dispersion medium, thereby enabling the dispersed substance to be stably present in the dispersion medium. Furthermore, it is possible to control the affinity of polymer (B) to the electrolyte solution, preventing problems such as dissolution of polymer (B) in the electrolyte solution within the battery and an increase in the resistance of the electrolyte solution. Furthermore, the arrangement of nitrile groups, which have high polarization, increases the dielectric constant and reduces the resistance of the electrolyte solution, thereby improving rate characteristics and cycle characteristics.

Furthermore, polymer (B) preferably contains an aliphatic hydrocarbon structural unit. The aliphatic hydrocarbon structural unit is a structural unit containing an aliphatic hydrocarbon structure, and preferably a structural unit consisting solely of an aliphatic hydrocarbon structure. The aliphatic hydrocarbon structure contains at least a saturated aliphatic hydrocarbon structure and may further contain an unsaturated aliphatic hydrocarbon structure. The aliphatic hydrocarbon structure preferably contains at least a linear aliphatic hydrocarbon structure and may further contain a branched aliphatic hydrocarbon structure.

Examples of aliphatic hydrocarbon structural units include alkylene structural units, alkenylene structural units, alkyl structural units, alkanetriyl structural units, and alkanetetrayl structural units. The aliphatic hydrocarbon structural units preferably contain at least an alkylene structural unit. The alkylene structural unit is a structural unit containing an alkylene structure, and is preferably a structural unit consisting solely of an alkylene structure. The alkylene structure is preferably a linear alkylene structure or a branched alkylene structure.

In the aliphatic hydrocarbon structural units, the content of alkylene structural units is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more, based on the total mass of the aliphatic hydrocarbon structural units (i.e., when the mass of the aliphatic hydrocarbon structural units is 100% by mass). The content of alkylene structural units may be, for example, less than 100% by mass, 99.5% by mass or less, 99% by mass or less, or 98% by mass or less, based on the total mass of the aliphatic hydrocarbon structural units. The content of alkylene structural units may be 100% by mass.

The method for introducing aliphatic hydrocarbon structural units into polymer (B) is not particularly limited, but examples include a method in which a polymer is prepared by polymerization using a monomer composition containing a conjugated diene monomer to contain monomer units derived from the conjugated diene monomer, and then the conjugated diene monomer units are hydrogenated (hydrogenated) to convert at least a portion of the conjugated diene monomer units into alkylene structural units, or a method in which a polymer is prepared by polymerization using a monomer composition containing an α-olefin monomer. In particular, a method in which a polymer is prepared by polymerization using a monomer composition containing a conjugated diene monomer, and then hydrogenated, is preferably used.

The conjugated diene monomer has four or more carbon atoms, preferably four to six. Examples of conjugated diene monomers include conjugated diene compounds such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. Of these, 1,3-butadiene is preferred. The alkylene structural unit preferably contains a structural unit (hydrogenated conjugated diene monomer unit) obtained by hydrogenating a conjugated diene monomer unit, and more preferably contains a structural unit (hydrogenated 1,3-butadiene monomer unit) obtained by hydrogenating a 1,3-butadiene monomer unit. One type of conjugated diene monomer can be used alone, or two or more types can be used in combination.

Preferably, the hydrogenation is carried out by a method that can selectively hydrogenate the conjugated diene monomer units. Examples of hydrogenation methods include well-known methods such as oil phase hydrogenation and aqueous phase hydrogenation. Hydrogenation can be carried out by a conventional method. For example, hydrogenation can be carried out by treating a copolymer having conjugated diene monomer units dissolved in a suitable solvent with hydrogen gas in the presence of a hydrogenation catalyst. Examples of hydrogenation catalysts include iron, nickel, palladium, platinum, and copper.

The carbon number of the α-olefin monomer is 2 or more, preferably 3 or more, and more preferably 4 or more. The carbon number of the α-olefin monomer is preferably 6 or less, and more preferably 5 or less. Examples of α-olefin monomers include α-olefin compounds such as ethylene, propylene, 1-butene, and 1-hexene. α-olefin monomers can be used alone or in combination of two or more.

The content of aliphatic hydrocarbon structural units is preferably 40% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more, based on the mass of polymer (B) (i.e., when the mass of polymer (B) is 100% by mass). The content of aliphatic hydrocarbon structural units is preferably less than 85% by mass, more preferably 75% by mass or less, and even more preferably 70% by mass or less, based on the mass of polymer (B).

Furthermore, polymer (B) may contain any structural unit. Examples of the optional structural unit include structural units containing a branch point, such as an amide group-containing structural unit; a carboxyl group-containing structural unit; an alkenylene structural unit; an alkyl structural unit; an alkanetriyl structural unit; and an alkanetetrayl structural unit. A structural unit containing a branch point is a structural unit different from a structural unit containing a branched alkylene structure and a structural unit containing a branched alkyl structure.

The amide group-containing structural unit is a structural unit containing an amide group, preferably a structural unit containing an alkylene structure substituted with an amide group, and more preferably a structural unit consisting solely of an alkylene structure substituted with an amide group. The alkylene structure is preferably a linear or branched alkylene structure. The amide group-containing structural unit may further contain a structural unit containing (or consisting solely of) an alkyl structure substituted with an amide group. The number of amide groups contained in the amide group-containing structural unit is preferably one.

The method for introducing amide group-containing structural units into polymer (B) is not particularly limited. For example, a polymer can be prepared by polymerization using a monomer composition containing an amide group-containing monomer. The prepared copolymer contains amide group-containing monomer units. The final polymer (B) contains the amide group-containing monomer units as amide group-containing structural units.

Examples of amide group-containing monomers include monoalkyl(meth)acrylamides such as (meth)acrylamide, N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N-propyl(meth)acrylamide, and N-isopropyl(meth)acrylamide; dialkyl(meth)acrylamides such as N,N-dimethyl(meth)acrylamide and N,N-diethyl(meth)acrylamide; N-(hydroxyalkyl)(meth)acrylamides such as N-(2-hydroxyethyl)(meth)acrylamide, N-(2-hydroxypropyl)(meth)acrylamide, and N-(2-hydroxybutyl)(meth)acrylamide; diacetone(meth)acrylamide; and acryloylmorpholine. In this specification, "(meth)acryl" refers to acrylic or methacrylic. The amide group-containing monomer preferably includes at least one selected from the group consisting of acrylamide, methacrylamide, and N,N-dimethylacrylamide. Amide group-containing monomers can be used singly or in combination.

The content of the amide group-containing structural units is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0.5% by mass or less, based on the mass of polymer (B) (i.e., when the mass of copolymer (B) is 100% by mass). When the content of the amide group-containing structural units is within the above range, it is possible to prevent the problem of gelation of the carbon nanotube dispersion during storage, which can occur when the hydrogen bonds between polymers (B) become too strong.

The carboxyl group-containing structural unit is a structural unit containing a carboxyl group, preferably a structural unit containing an alkylene structure substituted with a substituent containing a carboxyl group, and more preferably a structural unit consisting solely of an alkylene structure substituted with a carboxyl group. The alkylene structure is preferably a linear or branched alkylene structure. The carboxyl group-containing structural unit may further contain a structural unit containing (or consisting solely of) an alkyl structure substituted with a carboxyl group. The number of carboxyl groups contained in the carboxyl group-containing structural unit is preferably one or two. By including a carboxyl group-containing structural unit in polymer (B), the adsorption force to the dispersed material can be improved, the viscosity of the carbon nanotube dispersion can be reduced, and dispersion efficiency can be improved.

The method for introducing the carboxyl group-containing structural unit is not particularly limited, but examples include a method of preparing a polymer by the polymerization reaction of a monomer containing a carboxyl group, or a method of preparing a polymer by the polymerization reaction of a monomer containing a functional group other than a carboxyl group and then modifying it to a carboxyl group. In particular, a method of preparing a polymer by the polymerization reaction of a monomer containing a carboxyl group is preferably used.

Examples of monomers containing a carboxyl group include unsaturated fatty acids such as (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid; and carboxyl group-containing (meth)acrylates such as 2-(meth)acryloyloxyethyl phthalate, 2-(meth)acryloyloxypropyl phthalate, 2-(meth)acryloyloxyethyl hexahydrophthalate, 2-(meth)acryloyloxypropyl hexahydrophthalate, ethylene oxide-modified succinic acid (meth)acrylate, and β-carboxyethyl (meth)acrylate. Also included are acid anhydride group-containing monomers such as maleic anhydride, itaconic anhydride, and citraconic anhydride, which are polymers of the above carboxyl group-containing monomers, and monofunctional alcohol adducts thereof. Acid anhydride groups, which have a structure in which two carboxyl groups are dehydrated and condensed, also form carboxyl groups upon hydrolysis, and are therefore included in the term "carboxyl group" in this specification. Alternatively, a carboxyl group-containing monomer may be obtained by hydrolyzing the carbamoyl group of a polymer obtained by polymerization of a carbamoyl group-containing monomer such as (meth)acrylamide. The carboxyl group-containing monomer is preferably an unsaturated fatty acid, more preferably (meth)acrylic acid, and even more preferably acrylic acid.

The content of the carboxyl group-containing structural unit is preferably less than 1% by mass, more preferably 0.5% by mass or less, and even more preferably 0.3% by mass or less, based on the mass of polymer (B) (i.e., when the mass of polymer (B) is 100% by mass). If the content of the carboxyl group-containing structural unit is less than (or less than) the above range, it is possible to prevent the problem of gelation of the carbon nanotube dispersion liquid during storage, which can occur when the hydrogen bonds between polymers (B) become too strong, as described below.

An alkenylene structural unit is a structural unit containing an alkenylene structure, and preferably a structural unit consisting solely of an alkenylene structure. The alkenylene structure is preferably a linear alkenylene structure or a branched alkenylene structure, and preferably contains at least one type selected from the group consisting of structural units containing a linear alkenylene structure and structural units containing a branched alkenylene structure, and more preferably contains at least one type selected from the group consisting of structural units consisting solely of a linear alkenylene structure and structural units consisting solely of a branched alkenylene structure.

The alkyl structural unit is a structural unit containing an alkyl structure (however, it is a structural unit that does not fall under other aliphatic hydrocarbon structural units such as branched alkylene structural units, nitrile group-containing structural units, amide group-containing structural units, or carboxyl group-containing structural units), and is preferably a structural unit consisting solely of an alkyl structure. The alkyl structure is preferably a linear alkyl structure or a branched alkyl structure. The alkyl structural unit preferably includes at least one type selected from the group consisting of structural units containing a linear alkyl structure and structural units containing a branched alkyl structure.

An alkanetriyl structural unit is a structural unit containing an alkanetriyl structure, and preferably a structural unit consisting solely of an alkanetriyl structure. An alkanetetrayl structural unit is a structural unit containing an alkanetetrayl structure, and preferably a structural unit consisting solely of an alkanetetrayl structure.

Preferred embodiments of the polymer (B) include the following.
Polymer (B) in which the total content of aliphatic hydrocarbon structural units and nitrile group-containing structural units is 80% by mass or more and 100% by mass or less, based on the mass of polymer (B). The total content is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more.
Polymer (B) in which the total content of aliphatic hydrocarbon structural units, nitrile group-containing structural units, and amide group-containing structural units is 80% by mass or more and 100% by mass or less, based on the mass of polymer (B). The total content is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more.
Polymer (B) in which the total content of aliphatic hydrocarbon structural units, nitrile group-containing structural units, amide group-containing structural units, and carboxyl group-containing structural units is 80% by mass or more and 100% by mass or less, based on the mass of polymer (B). The total content is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more.

In this specification, the content of structural units can be determined using the amount of monomer used, NMR (nuclear magnetic resonance) and/or IR (infrared spectroscopy) measurements.

The polymerization reaction used to prepare polymer (B) is not particularly limited, and any method such as solution polymerization, suspension polymerization, or emulsion polymerization can be used. Among these, emulsion polymerization is preferred, and conventional emulsion polymerization methods can be used. The polymerization agents used in emulsion polymerization, such as emulsifiers (surfactants), polymerization initiators, chelating agents, oxygen scavengers, and molecular weight modifiers, are not particularly limited and may be any conventionally known agent. For example, anionic emulsifiers or anionic and nonionic emulsifiers are typically used.

Examples of anionic emulsifiers include fatty acid salts such as potassium tallow fatty acid, partially hydrogenated potassium tallow fatty acid, potassium oleate, and sodium oleate; resin acid salts such as potassium rosinate, sodium rosinate, hydrogenated potassium rosinate, and hydrogenated sodium rosinate; and alkylbenzene sulfonates such as sodium dodecylbenzene sulfonate. Examples of nonionic emulsifiers include emulsifiers such as polyethylene glycol ester type, polypropylene glycol ester type, and Pluronic (registered trademark) type emulsifiers such as block copolymers of ethylene oxide and propylene oxide.

Examples of polymerization initiators include thermal decomposition initiators such as persulfates, including potassium persulfate and ammonium persulfate; organic peroxides, including t-butyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, octanoyl peroxide, and 3,5,5-trimethylhexanoyl peroxide; azo compounds, including azobisisobutyronitrile; and redox initiators formed from these and a reducing agent, such as divalent iron ions. Among these, redox initiators are preferred. The amount of initiator used is, for example, in the range of 0.01 to 10% by mass based on the total amount of monomers.

The emulsion polymerization reaction may be either continuous or batchwise. The polymerization temperature may be either low-temperature or high-temperature, but is preferably 0 to 50°C, and more preferably 0 to 35°C. There are also no particular limitations on the method of monomer addition (lump addition, divided addition, etc.), polymerization time, or polymerization conversion rate. A conversion rate of 85% by mass or higher is preferred, and 90% by mass or higher is even more preferred.

The weight-average molecular weight of polymer (B) is preferably 5,000 or more, more preferably 10,000 or more, and even more preferably 50,000 or more. The weight-average molecular weight of polymer (B) is preferably 400,000 or less, more preferably 350,000 or less, and even more preferably 300,000 or less. When the weight-average molecular weight of polymer (B) is 5,000 or more and 400,000 or less, the adsorption to the dispersed substance and affinity to the dispersion medium are improved, and the stability of the dispersion tends to be improved. The weight-average molecular weight is the weight-average molecular weight in terms of polystyrene and can be measured by gel permeation chromatography (GPC).

The dispersion solvent contained in the carbon nanotube dispersion liquid preferably consists essentially of an amide-based organic solvent. There are no particular restrictions on the solvent as long as it is an amide-based organic solvent, but it is preferably a solvent that can dissolve polymer (B), and is preferably a solvent consisting of one type of amide-based organic solvent or a mixed solvent consisting of two or more types of amide-based organic solvents.

Examples of amide-based organic solvents include N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, and N-methylcaprolactam. It is particularly preferable to use at least one solvent selected from the group consisting of N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone.

It is preferable that the carbon nanotube dispersion liquid is substantially free of water. In the embodiments of the present invention, "substantially free of water" means that water is not intentionally added. Based on the mass of polymer (B), the water content is preferably less than 5 mass%, more preferably less than 1 mass%, and even more preferably less than 0.5 mass%. If a large amount of water is contained, the adsorption of polymer (B) to the dispersed material may decrease, and the dispersed material may not be able to exist stably in the dispersion solvent. By keeping the water content within the above range, the problem of gelling of the carbon nanotube dispersion liquid during storage can be reduced.

As described above, the carbon nanotube dispersion according to the embodiment of the present invention is substantially free of water. The "pH" of the carbon nanotube dispersion according to the embodiment of the present invention refers to a value measured using a general pH meter after adding water to the carbon nanotube dispersion so that the solids concentration after adding water is 50%, relative to the solids concentration before adding water, which is taken as 100%. For example, the pH can be measured by the following method.
While stirring a carbon nanotube dispersion liquid with a solids concentration of 5% using a disper or the like, water is added so that the solids concentration of the carbon nanotube dispersion liquid becomes 2.5%. After stirring uniformly, the pH of the carbon nanotube dispersion liquid can be measured at 25°C using a benchtop pH meter (Seven Compact S220 Expert Pro, manufactured by Mettler Toledo).
The pH of the carbon nanotube dispersion according to the embodiment of the present invention is preferably 9.0 or higher. It is also preferably 12.0 or lower, more preferably 11.0 or lower. By adjusting the pH within the above range, the wettability of the carbon nanotubes can be improved, and the function as a dispersant for the polymer (A) and the polymer (B) can be enhanced. If the pH exceeds the above range, problems such as corrosion of various raw materials and exterior materials in the battery, or gelation of the binder, are likely to occur.

The pH of the carbon nanotube dispersion can be adjusted by (1) the amount of metal hydroxide contained in the carbon nanotubes, (2) the type and amount of functional groups on the carbon nanotube surface, and (3) the type and amount of base added. By adjusting the pH by taking all of the above factors (1) to (3) into consideration, not only can the wettability of the carbon nanotubes be improved, but the combined effect of polymer (A) and polymer (B) can also be enhanced, resulting in a carbon nanotube dispersion that is excellent not only in dispersibility but also in stability.

As mentioned above, the metals, metal oxides, and metal hydroxides used as catalysts in the carbon nanotube production process remain in the system. The pH can be adjusted by adjusting the amount of metals remaining in the carbon nanotubes, particularly the content of metal hydroxides (see (1) above). Furthermore, the remaining metals, carbon purity, and pH of the carbon nanotube dispersion can be adjusted using conventional purification methods.

The functional groups on the carbon nanotube surface described in (2) above are not particularly limited, but examples include carboxyl groups, sulfo groups, and hydroxyl groups. The method for introducing functional groups into carbon nanotubes is also not particularly limited. For example, carboxyl groups can be introduced into carbon nanotubes by heating them with an oxidizing acid. This procedure is relatively easy and is preferable because it allows the addition of highly reactive carboxyl groups. Examples of oxidizing acids include concentrated nitric acid, hydrogen peroxide, a mixture of sulfuric acid and nitric acid, and aqua regia. When using concentrated nitric acid, its concentration is preferably 5% by mass or more, and more preferably 60% by mass or more. Heating can be performed using conventional methods, but the temperature should preferably be below the boiling point of the acid used. For example, a range of 50 to 130°C is preferred for concentrated nitric acid. The heating time is preferably between 30 minutes and 20 hours, and more preferably between 1 and 8 hours. In an embodiment of the present invention, since polymer (A) and polymer (B) can achieve both good dispersibility and stability, it is preferable that the carbon nanotubes do not have acidic functional groups such as carboxyl groups and sulfo groups. If the carbon nanotube dispersion contains a large number of acidic functional groups, there is a risk that the carbon nanotube dispersion will gel during storage.

The base added to adjust the pH in (3) above is not particularly limited, and specifically, for example, at least one base selected from the group consisting of inorganic bases, inorganic metal salts, organic hydroxides, and other organic bases can be used.

Examples of inorganic bases and inorganic metal salts include chlorides, hydroxides, carbonates, alkoxides, nitrates, sulfates, phosphates, tungstates, vanadates, molybdates, niobates, or borates of alkali metals or alkaline earth metals; and ammonium hydroxide. Among these, chlorides, hydroxides, carbonates, and alkoxides of alkali metals or alkaline earth metals are preferred from the viewpoint of easily supplying cations. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides include calcium hydroxide and magnesium hydroxide. Examples of alkali metal carbonates include lithium carbonate, lithium bicarbonate, sodium carbonate, sodium bicarbonate, potassium carbonate, and potassium bicarbonate. Examples of alkali metal alkoxides include lithium methoxide, lithium ethoxide, lithium n-butoxide, lithium t-butoxide, potassium methoxide, potassium ethoxide, potassium n-butoxide, potassium t-butoxide, sodium methoxide, sodium ethoxide, sodium n-butoxide, and sodium t-butoxide. The alkoxide may have 5 or more carbon atoms. Examples of alkaline earth metal alkoxides include magnesium methoxide, magnesium ethoxide, magnesium n-butoxide, and magnesium t-butoxide. The alkoxide may have 5 or more carbon atoms.
Among these, lithium hydroxide, sodium hydroxide, potassium hydroxide, lithium carbonate, sodium carbonate, lithium t-butoxide, potassium t-butoxide, and sodium t-butoxide are more preferable. Note that the metal contained in the inorganic base according to the embodiment of the present invention may be a transition metal.

Organic hydroxides are salts containing an organic cation and hydroxide ions. Examples of organic hydroxides include trimethyl-2-hydroxyethylammonium hydroxide, tetramethylammonium hydroxide, cetyltrimethylammonium hydroxide, hexadecyltrimethylammonium hydroxide, trimethylphenylammonium hydroxide, 3-trifluoromethyl-phenyltrimethylammonium hydroxide, and benzyltrimethylammonium hydroxide. Of these, trimethyl-2-hydroxyethylammonium hydroxide and tetramethylammonium hydroxide are particularly preferred.

Other organic bases include methylamine, ethylamine, propylamine, butylamine, hexylamine, octylamine, dioctylamine, trioctylamine, aminoethanol, aminopropanol, aminobutanol, and 2-methoxyethylamine. These organic bases have high solubility in the electrolyte, so using too much can reduce battery performance. Furthermore, because these compounds decompose easily, the decomposed products may remain in the coating film, and their presence in the battery can reduce the initial capacity.

The amount of base used is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, based on the total mass of polymer (A) and polymer (B). The amount of base used is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less, based on the total mass of polymer (A) and polymer (B). If the amount used is too large, the stability of the resulting carbon nanotube dispersion may be poor. Furthermore, it may cause corrosion of the dispersion device and/or the inside of the battery.

The reasons why dispersibility is improved by adjusting the pH to a predetermined value are thought to be as follows: Note that the reasons why dispersibility is improved are not limited to the factors (1) to (4) listed below.
(1) Improving the dispersibility of polymer (B) By adjusting the pH to a predetermined value, the nitrile group contained in the nitrile group-containing structural unit of polymer (B) is hydrolyzed to form an amide group. By incorporating an amide group-containing structural unit into polymer (B), the adsorption force to the dispersed material can be increased. Furthermore, since amide groups can form strong hydrogen bonds, by incorporating an amide group-containing structural unit into polymer (B), a crosslinked structure due to hydrogen bonds is introduced into the molecule of polymer (B), allowing the polymer to be three-dimensionally adsorbed to the dispersed material, and it is believed that a dispersion liquid excellent not only in dispersibility but also in stability can be obtained.
(2) Enhancing the combined effect of polymer (A) and polymer (B) By adjusting the pH to a predetermined value, polymer (B) can contain an amide group-containing structural unit as described above, and by using polymer (A) in combination with polymer (B), it is possible to specifically increase the adhesion (peel strength) between carbon nanotubes and between carbon nanotubes and active material and current collector. In particular, when polymer (A) contains a hydroxyl group-containing structural unit, strong intermolecular forces such as hydrogen bonds act between polymer (A) and polymer (B), forming a crosslinked structure between polymer (A) and polymer (B), which allows three-dimensional adsorption to the dispersed material, and it is thought that a dispersion liquid excellent not only in dispersibility but also in stability can be obtained.
(3) Decreasing the Solution Viscosity of Polymer (B) When polymer (B) is dissolved in a solvent and used, if the viscosity of the polymer (B) solution is low, it is believed that the dispersant can easily penetrate into the interior of the carbon nanotubes, which have strong cohesive forces, and a uniform dispersion can be obtained.
(4) Improving the wettability of carbon nanotubes. When dispersing carbon nanotubes, the carbon nanotubes are wetted with a solvent to reduce their cohesive force, and then crushed and stabilized to form a dispersion. Carbon nanotubes have significantly lower wettability than other conductive materials such as carbon black. Therefore, pretreatments such as chemical or mechanical crushing of carbon nanotubes are required to improve their wettability. However, these treatments may result in a decrease in conductivity. It appears that adjusting the pH to a predetermined value can dramatically improve the wettability of carbon nanotubes without impairing their conductivity.

The flow rate of the carbon nanotube dispersion is preferably 30 cm/min or less, more preferably 20 cm/min or less. Also, it is preferably 1 cm/min or more, more preferably 5 cm/min or more. By adjusting the flow rate of the carbon nanotube dispersion within the above range, the dispersibility of the carbon nanotube dispersion can be improved, and the preparation of the electrode film slurry can be facilitated. Furthermore, the good dispersion state can be maintained during the preparation of the electrode film slurry and during the production of the electrode film, and the active material and the conductive material can be kept in a uniform state without bias, allowing the formation of a good conductive network in the electrode.
In this specification, "flow rate" is one of the indicators that indicates the fluidity of a carbon nanotube dispersion. Since shear stress can cause the microstructure of the dispersed system of a carbon nanotube dispersion to collapse, resulting in a change in rheology, the "flow rate" in this specification is defined as the fluidity measured without applying shear stress to the dispersion using the method described below.

It is preferable that the particle size distribution of the carbon nanotube dispersion liquid measured by a laser diffraction/scattering particle size distribution measurement method under the condition of a laser light transmittance of 50% has at least two peaks as shown in Figure 2, and that the mode diameter of the first peak, which has the smallest particle diameter among these peaks, is 0.2 μm or less.
The laser diffraction/scattering particle size distribution measurement method can be used as an index of dispersed particle size and dispersion state, and is a measurement method that calculates particle size distribution from information on scattered (diffracted) light from particles (here, carbon nanotubes) when they are irradiated with laser light. Generally, the scattered light intensity of a particle is proportional to the particle diameter (perimeter) and inversely proportional to the wavelength of the incident laser light. It also varies depending on the particle's inherent refractive index. When the particle diameter is large, the scattered light is concentrated forward, but when the particle diameter is smaller than the incident wavelength of the laser light, the scattered light is scattered in all directions, including sideways and backward. This scattered light is detected with a detector, and a continuous particle size distribution curve can be measured by Fourier transform, with the horizontal axis representing particle diameter and the vertical axis representing volume-based frequency.
The laser light transmittance in this specification correlates with the concentration of the sample at the time of measurement, and the more dilute the sample, the higher the transmittance.
In this specification, the mode diameter refers to the particle diameter with the highest frequency within the peak in the particle size distribution obtained on a volume basis.

Typically, when measuring particle size distribution using a laser diffraction/scattering particle size analyzer, it is necessary to set conditions under which light scattered from particles does not undergo multiple scattering by adjusting the concentration of the measurement sample so that the laser light transmittance is 80% or more. Here, multiple scattering is a phenomenon in which light scattered from particles in a sample is further scattered by other particles before being detected by a detector, and becomes more pronounced as the laser light transmittance decreases (the sample concentration increases during measurement). When multiple scattering occurs, the intensity of scattered light to the side and back increases, and peaks are detected in a particle size distribution region smaller than the original particle size distribution of the particles.
However, in an embodiment of the present invention, the mode diameter of the peak due to multiple scattering (i.e., the first peak with a small particle diameter) measured under a condition of 50% laser light transmittance is used as an indicator of the dispersion state of a dispersion containing two types of carbon nanotubes with different fiber diameter distributions. When two or more types of carbon nanotubes are contained in a dispersion, it is believed that the appearance of the peak due to multiple scattering varies depending on the difference in the dispersed particle diameter distribution of each carbon nanotube. The greater the difference in particle diameter distribution between the two types of carbon nanotubes and the better the dispersion state of each carbon nanotube, the more easily the peak due to multiple scattering is detected and tends to be detected in a smaller particle diameter range. In an embodiment of the present invention, the mode diameter is preferably 0.2 μm or less. Furthermore, when only one type of carbon nanotube is contained in a dispersion, the peak due to multiple scattering is not observed, or even if a peak due to multiple scattering is detected, the mode diameter tends to be larger than 0.2 μm. This is presumably because the particle size distribution is relatively uniform and the refractive index of the carbon nanotubes in the dispersion is similar (due to the single type), resulting in little interference due to multiple scattering.

In an embodiment of the present invention, carbon nanotubes are used as conductive materials, and their physical properties may vary depending on the structure, crystallinity, and morphology of the unit layers that make them up, the structure and shape of the carbon nanotubes made up of the unit layers, and the content of metal elements contained therein. However, by including two types of carbon nanotubes with different fiber diameter distributions in the range of 100 nm or less in terms of fiber diameters observed with a scanning electron microscope, and further using polymer (A) and polymer (B), the desired physical properties can be achieved.

The method for producing the carbon nanotube dispersion is not particularly limited. The carbon nanotube dispersion can be obtained, for example, by mixing polymer (A) and polymer (B), a solvent, and carbon nanotubes, and dispersing the carbon nanotubes in the solvent. In addition to polymer (A), polymer (B), solvent, and carbon nanotubes, optional components may be mixed. Alternatively, the carbon nanotube dispersion can be obtained, for example, by dissolving polymer (A) and polymer (B) in a solvent, mixing the polymer with carbon nanotubes, and dispersing the carbon nanotubes in the solvent. In addition to polymer (A), polymer (B), solvent, and carbon nanotubes, optional components such as an additional solvent may be mixed. If a solvent is mixed, it is preferable that the solvent used to dissolve polymer (A) and polymer (B) is the same as the solvent used to dissolve polymer (A) and polymer (B). The order in which polymer (A), polymer (B), and carbon nanotubes are added to a container is not particularly limited. It is preferable that polymer (A) and polymer (B) are present together with the carbon nanotubes at some point during the carbon nanotube dispersion process. The dispersion process may be carried out in two or more stages by adjusting the timing of addition of the materials used as desired.

The carbon nanotube dispersion contains two types of carbon nanotubes with different fiber diameter distributions, i.e., component (I) having a first fiber diameter distribution and component (II) having a second fiber diameter distribution, but the production method is not particularly limited. When the fiber diameter of component (I) is smaller than the fiber diameter of component (II), the carbon nanotube dispersion can be obtained, for example, by dissolving polymer (A) and polymer (B) in a solvent, mixing component (I) having the first fiber diameter distribution, and then mixing and dispersing component (II) having the second fiber diameter distribution in the solvent. Alternatively, the carbon nanotube dispersion can be obtained by dissolving polymer (A) and polymer (B) in a solvent, mixing and dispersing component (I) having the first fiber diameter distribution, adding component (II) having the second fiber diameter distribution, and further dispersing. The order of addition of component (I) having the first fiber diameter distribution and component (II) having the second fiber diameter distribution is not particularly limited. It is preferable that at some point during the carbon nanotube dispersion process, polymer (A) and polymer (B) are present together with two types of carbon nanotubes having different fiber diameter distributions.

Dispersion methods include, for example, methods using various dispersing means such as a disperser (disperser), homogenizer, Silverson mixer, kneader, two-roll mill, three-roll mill, ball mill, horizontal sand mill, vertical sand mill, annular bead mill, attritor, planetary mixer, or high-pressure homogenizer. While there are no particular limitations on the dispersing means, it is preferable to use a high-pressure homogenizer to adjust the fiber length of the carbon nanotubes in the carbon nanotube dispersion to a desired range. It is also more preferable to select and combine multiple dispersing means. For example, it is most preferable to use a high-shear mixer in the initial dispersion step to promote wetting of the carbon nanotubes and break down coarse particles and agglomerates, followed by a high-pressure homogenizer to maintain the fiber length of the carbon nanotubes. Furthermore, by further dispersing using a bead mill after dispersing using a high-pressure homogenizer, it is possible to achieve a uniform dispersion state while maintaining the fiber length. There are no particular restrictions on the pressure when using a high-pressure homogenizer, but a pressure of 60 to 150 MPa is preferred, and 60 to 120 MPa is more preferred.

It is preferable to include a process for removing metal contaminants and other contaminants during the production of carbon nanotube dispersions. Conductive materials containing carbon nanotubes, polymer (A), and polymer (B) often contain metal contaminants derived from their production processes (as line contamination or catalysts). Removing these metal contaminants is extremely important for preventing battery short circuits. Metal contaminants refer to iron, chromium, and other particles present in the carbon nanotube dispersion, but do not include dissolved metal ions. In an embodiment of the present invention, the combined use of polymer (A) and polymer (B) breaks down carbon nanotube aggregates, allowing for more efficient removal of metal contaminants, even when the carbon nanotube concentration in the carbon nanotube dispersion is high, compared to when polymer (A) and polymer (B) are not used in combination. The method for removing particulate metal contaminants from the carbon nanotube dispersion in the metal contaminant removal process is not particularly limited, and examples include filtration using a filter, vibrating sieve removal, centrifugation, and magnetic removal. Among these, metal foreign matter such as iron and chromium is magnetic, so a method of removing it using magnetic force is preferred, and a method of combining a step of removing it using magnetic force with a step of removing it by filtration using a filter is even more preferred.

The type of metallic foreign matter contained in the carbon nanotube dispersion is not particularly limited, but specific examples include metals such as copper, iron, cobalt, nickel, chromium, aluminum, magnesium, silica, manganese, and molybdenum, metal oxides, and composite oxides of these metals.

The method of removal by magnetic force is not particularly limited as long as it can remove metal foreign matter, but considering productivity and removal efficiency, a method of removing the metal foreign matter by passing the carbon nanotube dispersion through a magnetic filter placed in the carbon nanotube dispersion production line is preferred.
The step of removing metallic foreign matter from a carbon nanotube dispersion using a magnetic filter is preferably carried out by passing the dispersion through a magnetic filter that forms a magnetic field with a magnetic flux density of 1,000 gauss or more. Since a low magnetic flux density reduces the efficiency of removing metal components, the magnetic flux density is preferably 5,000 gauss or more, more preferably 10,000 gauss or more in consideration of removing stainless steel, which has low magnetic properties, and most preferably 12,000 gauss or more.
When a magnetic filter is installed in a production line, it is preferable to include a process upstream of the magnetic filter that uses a filter such as a cartridge filter to remove coarse foreign matter or metal particles. This is because coarse metal particles may pass through the magnetic filter depending on the filtration flow rate. Furthermore, although a magnetic filter is effective even when used for a single filtration, a circulating type is more preferable. This is because a circulating type improves the efficiency of metal particle removal.
When a magnetic filter is placed in a production line for a carbon nanotube dispersion, the location of the magnetic filter is not particularly limited, but it is preferably placed immediately before filling a container with the carbon nanotube dispersion, or before the filter if a filtration step using a filtration filter is performed before filling the container, in order to prevent metal components from being mixed into the product if they are detached from the magnetic filter.

The amount of metal contaminants in a carbon nanotube dispersion can be analyzed using inductively coupled plasma (ICP) after drying the carbon nanotube dispersion. The content of metals such as copper, iron, and chromium detected by ICP analysis includes metal contaminants present in particulate form and those present in dissolved metal ion form. In other words, the amount of metal contaminants in a carbon nanotube dispersion that has undergone the metal contaminant removal process includes metal contaminants that were not completely removed and those present in dissolved metal ion form.

The metal content of iron and chromium contained in the carbon nanotube dispersion is preferably 50 ppm or less, and more preferably 20 ppm or less, based on 100% by mass of the carbon nanotube dispersion. By keeping the metal content within the above range, better conductivity can be achieved without the risk of side reactions within the electrode. The metal content can be measured, for example, by ICP atomic emission spectroscopy.

[Carbon nanotube dispersion composition]
The carbon nanotube dispersion composition contains a carbon nanotube dispersion liquid and a fluorine-based resin. That is, the carbon nanotube dispersion liquid does not contain a fluorine-based resin, and the carbon nanotube dispersion composition contains at least a polymer (A), a polymer (B), a solvent, carbon nanotubes, and a fluorine-based resin. In other words, the carbon nanotube dispersion composition contains at least a polymer (A), a polymer (B), a solvent, two types of carbon nanotubes having different fiber diameter distributions in the range of 100 nm or less in the fiber diameter of the carbon nanotubes observed with a scanning electron microscope, and a fluorine-based resin, and may further contain optional components such as a base or an acid. The carbon nanotube dispersion composition can be produced by mixing a fluorine-based resin and a carbon nanotube dispersion liquid. Optional components may be further mixed with the fluorine-based resin and the carbon nanotube dispersion liquid. When producing the carbon nanotube dispersion composition, a process of removing contaminants such as metal foreign matter described above for the carbon nanotube dispersion liquid may be included.
In this specification, the above-mentioned "carbon nanotube dispersion liquid" and "carbon nanotube dispersion composition" may be collectively referred to as "carbon nanotube dispersion composition".

Fluorine-based resins are binder resins that can bond materials such as electrode active materials and conductive materials. Fluorine-based resins are polymers or copolymers that contain fluorine atoms in their molecules, and are particularly preferred as binder resins for use in positive electrodes due to their durability. Examples of fluorine-based resins include polyvinylidene fluoride, polyvinyl fluoride, and tetrafluoroethylene.

The weight-average molecular weight of the fluororesin, calculated as polystyrene, is preferably 10,000 or more, more preferably 100,000 or more, and even more preferably 200,000 or more. It is also preferably 2,000,000 or less, and more preferably 1,000,000 or less.

The total content of polymer (A) and polymer (B) contained in the carbon nanotube dispersion composition 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, based on the mass of the carbon nanotube dispersion (taking the carbon nanotube dispersion as 100% by mass). It is also preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 25% by mass or less.

The content of carbon nanotubes in the carbon nanotube dispersion composition is preferably 0.05% by mass or more, and more preferably 0.1% by mass or more, based on the mass of the carbon nanotube dispersion liquid (the mass of the carbon nanotube dispersion liquid being 100% by mass). It is also preferably 30% by mass or less, and more preferably 20% by mass or less.

The content of the fluororesin contained in the carbon nanotube dispersion composition is preferably 0.05% by mass or more, and more preferably 0.1% by mass or more, based on the mass of the carbon nanotube dispersion liquid (the mass of the carbon nanotube dispersion liquid being 100% by mass). It is also preferably 25% by mass or less, and more preferably 15% by mass or less.

The carbon nanotube dispersion composition contains a solvent. The solvent is not particularly limited, but for example, the solvents exemplified in the description of the carbon nanotube dispersion liquid can be used. Furthermore, it is preferable that the carbon nanotube dispersion composition does not substantially contain water. If water is contained, there is a risk that the carbon nanotube dispersion liquid will gel during storage.

[Slurry for electrode membrane]
The electrode film slurry contains a carbon nanotube dispersion liquid or a carbon nanotube dispersion composition containing a fluorine-based resin, and an electrode active material. That is, the electrode film slurry contains at least a carbon nanotube dispersion liquid and an electrode active material, or at least a carbon nanotube dispersion composition and an electrode active material. In other words, the electrode film slurry contains at least a polymer (A), a polymer (B), two types of carbon nanotubes having different fiber diameter distributions in the range of 100 nm or less in terms of the fiber diameter of the carbon nanotubes observed with a scanning electron microscope, a solvent, and an electrode active material, and may further contain optional components such as a fluorine-based resin, other binder resins, bases, and acids. In this specification, "slurry" may also be referred to as "composite slurry."

Electrode active materials are materials that form the basis of battery reactions. Electrode active materials are divided into positive electrode active materials and negative electrode active materials based on their electromotive force.

The positive electrode active material is not particularly limited, but may be a material capable of reversibly doping or intercalating lithium ions. Examples include metal compounds such as metal oxides and metal sulfides. Specific examples include oxides of transition metals such as Fe, Co, Ni, and Mn; composite oxides with lithium; and inorganic compounds such as transition metal sulfides. Specific examples include transition metal oxide powders such as MnO, V ₂ O ₅ , V ₆ O ₁₃ , and TiO ₂ ; composite oxide powders of lithium and transition metals such as layered lithium nickel oxide, lithium cobalt oxide, lithium manganate, and spinel lithium manganate; lithium iron phosphate-based materials, which are phosphate compounds with an olivine structure; and transition metal sulfide powders such as TiS ₂ and FeS. The positive electrode active material used in the embodiment of the present invention is preferably a composite oxide of lithium containing a transition metal such as Al, Fe, Co, Ni, or Mn, more preferably a composite oxide of lithium containing any of Al, Co, Ni, or Mn, and particularly preferably a composite oxide of lithium containing Ni and/or Mn. When these positive electrode active materials are used, particularly good effects can be obtained. The positive electrode active materials can be used alone or in combination.

The negative electrode active material may be a material capable of reversibly doping or intercalating lithium ions. Examples include alloys such as metal Li and its alloys, such _{as tin alloys, silicon alloys, and lead alloys; metal oxides such as LiXFe2O3 , LiXFe3O4 , and LiXWO2} (where x is a number in the range of 0<x< ₁ ), _lithium titanate, lithium vanadate, and lithium silicate; conductive polymers such as polyacetylene and poly-p-phenylene; carbonaceous powders such as artificial graphite and natural graphite, including highly graphitized carbon materials; and carbon materials such as resin-baked carbon materials. The negative electrode active material may be used alone or in combination.

The total content of polymer (A) and polymer (B) in the electrode membrane slurry is preferably 0.01 to 10 mass%, and more preferably 0.05 to 5 mass%, based on the mass of the electrode active material (the mass of the electrode active material being 100 mass%).

The content of carbon nanotubes in the electrode film slurry is preferably 0.01 to 10 mass%, more preferably 0.02 to 5 mass%, and even more preferably 0.03 to 3 mass%, based on the mass of the electrode active material (the mass of the electrode active material being 100 mass%).

When the electrode film slurry further contains a fluororesin as a binder resin, the content of the fluororesin in the electrode film slurry is preferably 0.1 to 30 mass%, more preferably 0.5 to 20 mass%, and even more preferably 1 to 10 mass%, based on the mass of the electrode active material (the mass of the electrode active material being 100 mass%).

The solid content in the electrode membrane slurry is preferably 30 to 90 mass%, more preferably 30 to 80 mass%, and even more preferably 40 to 75 mass%, based on the mass of the electrode membrane slurry (the mass of the electrode membrane slurry being 100 mass%).

Slurry for electrode films can be prepared by a variety of conventional methods. Examples include a method of adding an electrode active material to a carbon nanotube dispersion; a method of adding a binder resin to a carbon nanotube dispersion and then adding an electrode active material; a method of adding an electrode active material to a carbon nanotube dispersion and then adding a binder resin; and a method of adding an electrode active material to a carbon nanotube dispersion composition.

A preferred method for preparing a slurry for an electrode film is to add a fluororesin as a binder resin to a carbon nanotube dispersion, and then add and disperse the electrode active material. The dispersing device used for dispersion is not particularly limited. The dispersion means described in the description of the carbon nanotube dispersion can be used to obtain a slurry for an electrode film. Because polymer (B) also functions as a binder, a slurry for an electrode film can be obtained without adding another binder resin. Therefore, a preferred method for preparing a slurry for an electrode film is to add and disperse an electrode active material to a carbon nanotube dispersion without adding a fluororesin as a binder resin.

[Electrode film]
The electrode film includes at least one selected from the group consisting of a film formed using a carbon nanotube dispersion liquid, a film formed using a carbon nanotube dispersion composition, and a film formed using a slurry for an electrode film. The electrode film may further include a current collector. For example, the electrode film can be obtained by applying the slurry for an electrode film onto a current collector and drying it, and includes the current collector and the film. In this specification, the "film formed using the slurry for an electrode film" may be referred to as an "electrode mixture layer."

The material and shape of the current collector used to form the electrode film are not particularly limited, and can be selected appropriately to suit various types of secondary batteries. Examples of current collector materials include metals or alloys such as aluminum, copper, nickel, titanium, and stainless steel. Furthermore, while flat foil is generally used as the shape, current collectors with a roughened surface, perforated foil current collectors, and mesh-shaped current collectors can also be used.

The method for applying the carbon nanotube dispersion liquid, carbon nanotube dispersion composition, or slurry to the current collector is not particularly limited, and any known method can be used. Specific examples include die coating, dip coating, roll coating, doctor coating, knife coating, spray coating, gravure coating, screen printing, and electrostatic painting. Drying methods include, but are not limited to, leaving the coating to dry, or drying using a blower dryer, warm air dryer, infrared heater, or far-infrared heater.

After coating, a rolling process using a lithographic press, calendar roll, or the like may be performed. The thickness of the formed film is, for example, from 1 μm to 500 μm, and preferably from 10 μm to 300 μm.

Films formed using carbon nanotube dispersion liquid or carbon nanotube dispersion composition can also be used as underlayers for electrode mixture layers to improve adhesion between the electrode mixture layer and the current collector, or to improve the conductivity of the electrode film.

[Secondary battery]
The secondary battery includes a positive electrode, a negative electrode, and an electrolyte, and at least one selected from the group consisting of the positive electrode and the negative electrode includes an electrode film.

For example, an electrode film can be used as the positive electrode, which is made by applying an electrode film slurry containing a positive electrode active material onto a current collector and then drying it to form a film.

For example, an electrode film can be used as the negative electrode, which is made by applying an electrode film slurry containing a negative electrode active material onto a current collector and then drying it to form a film.

As the electrolyte, various conventionally known materials capable of ion mobility can be used. For example, lithium salts such as LiBF ₄ , LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiCF ₃ SO ₃ , 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 (Ph is a phenyl group) can be included, but are not limited to these. The electrolyte is preferably dissolved in a non-aqueous solvent and used as a non-aqueous electrolyte solution. A secondary battery using a non-aqueous electrolyte solution is also called a non-aqueous electrolyte secondary battery.

Non-aqueous solvents include, but are not limited to, carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as gamma-butyrolactone, gamma-valerolactone, and gamma-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.

The secondary battery preferably includes a separator. Examples of separators include, but are not limited to, polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and nonwoven fabrics that have been hydrophilically treated.

The structure of the secondary battery of this embodiment is not particularly limited, but typically comprises a positive electrode, a negative electrode, and a separator, if necessary, and can be in a variety of shapes depending on the intended use, such as paper, cylindrical, button, or laminated.

The present invention will be explained in more detail below with reference to the following examples. The present invention is not limited to the following examples, provided it does not depart from the gist of the invention. Unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass." Furthermore, in the examples, "polymer (A)," "polymer (B)," and "polymer (A) and polymer (B)" may be referred to as "polymer" or "dispersant." Furthermore, carbon nanotubes may be referred to as "CNT."

<Carbon nanotube fiber diameter distribution and fiber length>
A few μL of carbon nanotube dispersion was dropped onto a mica substrate and then dried in an electric oven at 120°C to prepare a substrate for carbon nanotube observation. The surface of the prepared substrate was then sputtered with platinum. The substrate was then observed using a scanning electron microscope (SEM). Observations were performed at a magnification of 5,000x or 20,000x, depending on the carbon nanotubes, and multiple photographs were taken containing 10 or more carbon nanotubes within the field of view. The fiber diameter and fiber length of 300 randomly selected carbon nanotubes were measured.
The fiber diameters of 300 randomly extracted carbon nanotubes were plotted with the frequency (%) based on the number on the vertical axis and the fiber diameter (nm) on the horizontal axis to confirm the fiber diameter distribution. Figure 1 is a graph showing the distribution of fiber diameters in dispersion 1, where the frequency (%) on the vertical axis corresponds to the "content based on the number of carbon nanotubes relative to the total amount of carbon nanotubes." The average fiber length of the measured 300 carbon nanotubes was taken as the average fiber length (μm) of the carbon nanotubes, and the ratio of the number of carbon nanotubes with a fiber length of less than 0.3 μm to the number of carbon nanotubes with a fiber length of 0.3 μm or more was calculated.

<Measurement of Hydrogenation Ratio of Copolymer>
The hydrogenation rate was determined by IR measurement using a method similar to that used for infrared spectroscopy by total reflection measurement. The double bond derived from the conjugated diene monomer unit exhibits a peak at 970 cm ^-1 , and the hydrogenated single bond exhibits a peak at 723 cm ^-1 . The hydrogenation rate was calculated from the ratio of the heights of these two peaks.

<pH of carbon nanotube dispersion>
The carbon nanotube dispersion was left to stand in a thermostatic bath at 25°C for at least 1 hour, and then thoroughly stirred to adjust the solids concentration of the carbon nanotube dispersion to 100%, and water was added to the carbon nanotube dispersion while stirring with a disper so that the solids concentration became 50%. After stirring uniformly, the pH was measured at 25°C using a benchtop pH meter (Seven Compact S220 Expert Pro, manufactured by Mettler Toledo).

<Laser diffraction/scattering particle size distribution measurement of carbon nanotube dispersion liquid>
Particle size distribution was measured using a laser diffraction/scattering particle size distribution analyzer (Horiba, Ltd.; Partical LA-960V2). The laser wavelength of this analyzer is 650 nm, and the detectors are one ring-shaped 64-segment silicon photodiode, five 4-channel array detectors, and three silicon photodetectors. The measurement section uses a flow-type cell (sample cell) made of synthetic quartz.
First, NMP, the same solvent as the dispersion, was introduced into a sample bath containing a sample cell, and circulation/ultrasonic cleaning was performed. The operating modes were: circulation speed: 3, ultrasonic intensity: 7, ultrasonic time: 1 minute, stirring speed: 7, and stirring mode: continuous. Next, to remove air, ultrasonic operation was performed at ultrasonic intensity: 7 and ultrasonic time: 5 seconds, and then a blank (background) measurement was performed. The particle size was based on volume, and the particle refractive index was set to 1.920-0.522i (carbon material), and the solvent refractive index was set to 1.468 (NMP). The dispersion was added dropwise so that the laser light transmittance during measurement was 50% ± 1%, and sample preparation was performed. The operating modes during measurement were: circulation speed: 3, stirring speed: 7, and stirring mode: continuous.
Criterion A: At least two or more particle size distribution peaks are detected, and the mode diameter of the first peak, which has the smallest particle diameter among the particle size distribution peaks, is 0.2 μm or less. C: Two or more particle size distribution peaks are not detected, or two or more particle size distribution peaks are detected, but the mode diameter of the first peak, which has the smallest particle diameter among the particle size distribution peaks, is greater than 0.2 μm.

<Measurement of Initial Viscosity of Carbon Nanotube Dispersion>
The viscosity value was measured using a Brookfield viscometer ("BL" manufactured by Toki Sangyo Co., Ltd.) by leaving the carbon nanotube dispersion in a thermostatic chamber at 25°C for at least one hour, thoroughly stirring the carbon nanotube dispersion, and then immediately measuring the viscosity at a Brookfield viscometer rotor rotation speed of 60 rpm. The types of rotors used for the measurements were: No. 1 for viscosity values less than 100 mPa·s, No. 2 for viscosity values between 100 and 500 mPa·s, No. 3 for viscosity values between 500 and 2,000 mPa·s, and No. 4 for viscosity values between 2,000 and 10,000 mPa·s. The lower the viscosity, the better the dispersibility, and the higher the viscosity, the worse the dispersibility. The carbon nanotube dispersion obtained was judged to have poor dispersibility when it clearly separated or precipitated.
Judgment criteria AA: Less than 2,000 mPa·s (excellent)
A: 2,000 mPa·s or more and less than 6,000 mPa·s (good)
B: 6,000 mPa·s or more and less than 10,000 mPa·s (acceptable)
C: 10,000 mPa·s or more, settling or separation (poor)

<Evaluation of the stability of carbon nanotube dispersion liquid>
The storage stability was evaluated based on the change in the liquid properties after storing the carbon nanotube dispersion at 50° C. for 7 days. The change in the liquid properties was judged based on the ease of stirring when stirred with a spatula.
Criterion A: No problem (good)
B: Viscosity has increased but gelation has not occurred (acceptable)
C: Gelled (stirring marks do not disappear) (very poor)

<Flow Rate of Carbon Nanotube Dispersion Liquid>
The carbon nanotube dispersion was left standing in a thermostatic bath at 25°C for at least 1 hour, and then thoroughly stirred. 10 mL of the carbon nanotube dispersion was then poured onto a horizontal metal plate (made of brass) at a designated hemispherical position. Immediately afterwards, the metal plate was turned upright, and the vertical distance that the dispersion flowed down from that point in 1 minute was measured.
Criterion A: 5 cm/min or more and less than 20 cm/min (good)
B: 20cm/min or more and less than 30cm/min (acceptable)
C: Less than 5 cm/min, 30 cm/min or more (poor)

<Evaluation of the conductivity of the positive electrode mixture layer>
The positive electrode composite slurry was applied to a PET film (100 μm thick) using an applicator with a gap of 175 μm, and then dried for 10 minutes in a hot air oven at 70 °C and 15 minutes in a hot air oven at 120 °C to obtain a positive electrode film for conductivity evaluation. The surface resistivity (Ω/□) of the positive electrode composite layer was measured using a Loresta GP, MCP-T610 manufactured by Mitsubishi Chemical Analytech Co., Ltd. After measurement, the volume resistivity (Ω cm) was calculated by multiplying the thickness of the positive electrode composite layer formed on the PET film. The thickness of the positive electrode composite layer was measured using a film thickness meter (DIGIMICRO MH-15M manufactured by NIKON Corporation) at three points in the positive electrode film to determine the average value of the positive electrode film, and the difference between the average value of the positive electrode film and the film thickness of the PET film was calculated.
Criterion AA: The volume resistivity (Ω cm) of the positive electrode mixture layer is less than 10 (excellent)
A: The volume resistivity (Ω cm) of the positive electrode mixture layer is 10 or more and less than 20 (good)
C: The volume resistivity (Ω cm) of the positive electrode mixture layer is 20 or more (poor)

<Evaluation of rate characteristics of non-aqueous electrolyte secondary batteries>
The nonaqueous electrolyte secondary battery was placed in a thermostatic chamber at 25°C and subjected to charge/discharge measurements using a charge/discharge device (SM-8, manufactured by Hokuto Denko Corporation). A constant-current, constant-voltage charge (cutoff current 1 mA (0.02 C)) was performed at a charge current of 10 mA (0.2 C) with a charge cutoff voltage of 4.3 V, followed by a constant-current discharge at a discharge current of 10 mA (0.2 C) with a discharge cutoff voltage of 3 V. This operation was repeated three times, followed by a constant-current, constant-voltage charge (cutoff current 1 mA (0.02 C)) at a charge current of 10 mA (0.2 C) with a charge cutoff voltage of 4.3 V, followed by constant-current discharge at discharge currents of 0.2 C and 3 C until the discharge cutoff voltage reached 3.0 V, and the discharge capacity was calculated for each. The rate characteristic can be expressed as the ratio of the 0.2 C discharge capacity to the 3 C discharge capacity, using the following equation:

(Formula 1)
Rate characteristic = 3C discharge capacity / third 0.2C discharge capacity × 100 (%)

Criteria AA: Rate characteristics are 80% or more (excellent)
A: Rate characteristics are 60% or more and less than 80% (good)
C: Rate characteristics are less than 60% (poor)

<Method for Evaluating Cycle Characteristics of Non-Aqueous Electrolyte Secondary Batteries>
The nonaqueous electrolyte 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 as the ratio of the third 0.5 C discharge capacity to the 200th 0.5 C discharge capacity at 25°C, using the following equation:

(Formula 2)
Cycle characteristics = 3rd 0.5C discharge capacity / 200th 0.5C discharge capacity × 100 (%)

Criterion AA: Cycle characteristics are 85% or more (excellent)
A: Cycle characteristics are 80% or more and less than 85% (good)
C: Cycle characteristics are less than 80% (poor)

Details of the materials used in this example are as follows:
<Polymer (A)>
PVA-1: Kuraray Poval PVA-403 (manufactured by Kuraray Co., Ltd.): polyvinyl alcohol (average polymerization degree 300, saponification degree 87.0 to 89.0 mol%)
PVA-2: Polyvinyl alcohol (average polymerization degree 500, saponification degree 90 mol%) obtained by saponifying polyvinyl acetate with sodium hydroxide using a method known in the art
PVB-1: Polyvinyl acetal (acetalization degree 15 mol%)
A 10% aqueous solution of polyvinyl alcohol (degree of polymerization 300, degree of saponification 98 to 99 mol%) was prepared, and 0.2 parts by mass of hydrochloric acid and 2 parts by mass of butylaldehyde were added dropwise to 100 parts by mass of the aqueous solution while stirring. The temperature was then raised to 80°C and maintained for 1 hour, after which the solution was allowed to cool. The resulting solution was dried and pulverized to obtain polyvinyl acetal.
PVP-1: Polyvinylpyrrolidone K-30 (manufactured by Nippon Shokubai Co., Ltd.)

<Polymer (B)>
Synthesis Example 1: Preparation of Polymer (B-1)
A stainless steel polymerization reactor was charged with 40 parts of acrylonitrile, 60 parts of 1,3-butadiene, 3 parts of potassium soap oleate, 0.3 parts of azobisisobutyronitrile, 0.6 parts of t-dodecyl mercaptan, and 200 parts of ion-exchanged water. Under a nitrogen atmosphere, polymerization was carried out at 45°C for 20 hours with stirring, and the polymerization was terminated at a conversion rate of 90%. Unreacted monomers were removed by vacuum stripping to obtain an acrylonitrile-conjugated diene rubber latex with a solids concentration of approximately 30%. Subsequently, ion-exchanged water was added to the latex to adjust the total solids concentration to 12%, and the latex was placed in a 1 L autoclave equipped with a stirrer. Nitrogen gas was passed through the autoclave for 10 minutes to remove dissolved oxygen from the contents. A catalyst solution prepared by dissolving 75 mg of palladium acetate as a hydrogenation catalyst in 180 mL of ion-exchanged water containing 4 times the molar amount of nitric acid relative to the palladium was added to the autoclave. The autoclave was purged twice with hydrogen gas, and then the contents of the autoclave were heated to 50°C under a pressure of 3 MPa with hydrogen gas, and a hydrogenation reaction was carried out for 6 hours. The contents were then returned to room temperature, a nitrogen atmosphere was created inside the autoclave, and the solids were dried to recover polymer (B-1). The hydrogenation rate of polymer (B-1) was 99.5%, and the weight average molecular weight (Mw) was 190,000. In the acrylonitrile-conjugated diene rubber, the content of conjugated diene monomer units was 60%, and the content of nitrile group-containing monomer units was 40%, based on the mass of the acrylonitrile-conjugated diene rubber. Furthermore, in polymer (B-1), the content of aliphatic hydrocarbon structural units including alkylene structural units was 60%, and the content of nitrile group-containing monomer units was 40%, based on the mass of polymer (B-1). The content of these monomer units and the content of the structural units were determined from the amounts of the monomers used (the same applies hereinafter).

<Synthesis Examples 2 to 4: Preparation of Polymers (B-2) to (B-4)>
The monomer compositions used were changed according to Table 1 to prepare polymers (B-2) to (B-4).

The abbreviations shown in the monomer column in Table 1 have the following meanings.
BD: 1,3-butadiene AN: acrylonitrile MAN: methacrylonitrile AAm: acrylamide BA: butyl acrylate

In addition to the above, the following products were used as polymer (B).
H-NBR1: Therban® 4307 (manufactured by ARLANXEO Co., Ltd., acrylonitrile content 43.0%)
H-NBR2: Zetpole® 3300 (manufactured by Zeon Corporation, acrylonitrile content 23.6%)

<Production Example 1: Production of a catalyst for CNT synthesis>
The CNT synthesis catalyst used in the production of carbon nanotubes in each of the examples and comparative examples was prepared by the following method.

(Production Example 1-1)
[CNT synthesis catalyst (A)]
60 parts of cobalt hydroxide, 138 parts of magnesium acetate tetrahydrate, and 16.2 parts of manganese acetate were weighed into heat-resistant containers and dried in an electric oven at 170±5°C for 1 hour to evaporate the water, and then pulverized using a pulverizer (Wonder Crusher WC-3, manufactured by Osaka Chemical Co., Ltd.). The pulverized powders were then mixed in the pulverizer to produce a CNT synthesis catalyst precursor (A). The CNT synthesis catalyst precursor (A) was then transferred to a heat-resistant container and calcined in a muffle furnace (FO510, manufactured by Yamato Scientific Co., Ltd.) under conditions of an air atmosphere and 450±5°C for 30 minutes, and then pulverized in a mortar to obtain a CNT synthesis catalyst (A).

(Production Example 1-2)
[CNT synthesis catalyst (B)]
60 parts of cobalt hydroxide, 138 parts of magnesium acetate tetrahydrate, 16.2 parts of manganese carbonate, and 4.0 parts of zeolite (HSZ-940HOA, manufactured by Tosoh Corporation) were weighed into heat-resistant containers, dried in an electric oven at 170±5°C for 1 hour to evaporate the water, and then pulverized using a pulverizer. The pulverized powders were then mixed in a pulverizer to produce a CNT synthesis catalyst precursor (B). The CNT synthesis catalyst precursor (B) was then transferred to a heat-resistant container and calcined in a muffle furnace in an air atmosphere at 450±5°C for 30 minutes, after which it was pulverized in a mortar to obtain a CNT synthesis catalyst (B).

(Production Example 1-3)
[CNT synthesis catalyst (C)]
200 parts of cobalt acetate tetrahydrate and 78 parts of aluminum hydroxide acetate hydrate (alumina content 13% by mass) were weighed into a beaker and mixed uniformly using a mortar to obtain a CNT synthesis catalyst precursor (C). 300 parts of the obtained CNT synthesis catalyst precursor (C) were weighed into a heat-resistant container and baked in a muffle furnace in an air atmosphere at 470°C ± 5°C for 60 minutes, and then pulverized in a mortar to obtain a CNT synthesis catalyst (C).

(Production Example 1-4)
[CNT synthesis catalyst (D)]
200 parts of iron (II) acetate and 107 parts of aluminum hydroxide acetate hydrate (alumina content 13% by mass) were weighed into a beaker and mixed uniformly using a mortar to obtain CNT synthesis catalyst precursor (D). 300 parts of the obtained CNT synthesis catalyst precursor (D) were weighed into a heat-resistant container and baked in a muffle furnace in an air atmosphere at 470°C ± 5°C for 60 minutes, and then pulverized in a mortar to obtain CNT synthesis catalyst (D).

<Production Example 2: Production of carbon nanotubes>
The carbon nanotubes used in each of the examples and comparative examples were produced by the following method.

(Production Example 2-1)
[CNT(A)]
The CNT synthesis catalyst (A) prepared in Production Example 1-1 and the CNT synthesis catalyst (C) prepared in Production Example 1-3 were mixed in a pulverizer at a weight ratio of 9.5:0.5 to obtain a CNT synthesis catalyst (AC). A quartz glass heat-resistant dish onto which 1.0 g of the CNT synthesis catalyst (AC) was dispersed was placed in the center of a horizontal reactor tube with an internal volume of 10 L, which could be pressurized and heated with an external heater. The reactor was evacuated while nitrogen gas was injected, and the air inside the reactor tube was replaced with nitrogen gas. The horizontal reactor tube was heated until the ambient temperature reached 600°C. After reaching 600°C, propane gas was introduced into the reactor tube as a hydrocarbon at a flow rate of 2 L per minute, and a catalytic reaction was carried out for 60 minutes. After the reaction was completed, the gas inside the reactor tube was replaced with nitrogen gas, and the reactor tube was cooled to below 100°C and removed, yielding CNT (A).

(Production Example 2-2)
[CNT(B)]
The CNT synthesis catalyst (B) produced in Production Example 1-2 and the CNT synthesis catalyst (C) produced in Production Example 1-3 were mixed in a pulverizer at a weight ratio of 9.2:0.8 to obtain a CNT synthesis catalyst (BC). CNT (B) was produced in the same manner as in Production Example 2-1, except that 1.0 g of the CNT synthesis catalyst (BC) was used as the CNT synthesis catalyst.

(Production Example 2-3)
[CNT(C)]
A quartz glass heat-resistant dish on which 1.0 g of the CNT synthesis catalyst (C) prepared in Production Example 1-3 was dispersed was placed in the center of a horizontal reaction tube with an internal volume of 10 L, which could be pressurized and heated with an external heater. The reaction tube was evacuated while nitrogen gas was injected, and the air inside the reaction tube was replaced with nitrogen gas. The horizontal reaction tube was heated until the ambient temperature reached 750°C. After reaching 750°C, methane gas was introduced into the reaction tube as a hydrocarbon at a flow rate of 2 L per minute, and a catalytic reaction was carried out for 60 minutes. After the reaction was completed, the gas inside the reaction tube was replaced with nitrogen gas, and the reaction tube was cooled to below 100°C and removed, yielding CNTs (C).

(Production Example 2-4)
[CNT(D)]
The CNT synthesis catalyst (A) produced in Production Example 1-1 and the CNT synthesis catalyst (D) produced in Production Example 1-4 were mixed in a pulverizer at a weight ratio of 8.7:1.3 to obtain a CNT synthesis catalyst (AD). CNT (D) was produced in the same manner as in Production Example 2-1, except that 1.0 g of the CNT synthesis catalyst (AD) was used as the CNT synthesis catalyst.

(Production Example 2-5)
[CNT(E)]
The CNT synthesis catalyst (A) produced in Production Example 1-1 and the CNT synthesis catalyst (C) produced in Production Example 1-3 were mixed in a pulverizer at a weight ratio of 5.5:4.5 to obtain a CNT synthesis catalyst (AC2). CNT (E) was produced in the same manner as in Production Example 2-1, except that 1.0 g of the CNT synthesis catalyst (AC2) was used as the CNT synthesis catalyst.

In addition to the above, the following carbon nanotubes were used:
・ 100T: K-Nanos 100T (manufactured by Kumho Petrochemical Co., Ltd., multi-walled CNT, fiber diameter 10 to 15 nm)
BT1003M: LUCAN BT1003M (LG Chem Ltd., multi-walled CNT, fiber diameter 10 to 15 nm)
・8S: JENOTUBE8S (manufactured by JEIO Corporation, multi-walled CNT, fiber diameter 6 to 9 nm)
VGCF: VGCF (Showa Denko K.K., carbon nanofiber, fiber diameter 150 nm, fiber length 8 μm)
TUBALL (80%): made of OCSiAl, single-walled carbon nanotubes, average outer diameter: 1.5 nm, carbon content: 80%, specific surface area: 490 m ² /g

<Preparation of carbon nanotube dispersion liquid>
(Example 1-1)
According to the composition shown in Table 2-1, PVA-1, polymer (B-1), and NMP were added to a stainless steel container and stirred with a disperser until uniform. Subsequently, CNT (A) was added while stirring with a disperser, and a square-hole high-shear screen was attached to a high-shear mixer (L5M-A, manufactured by SILVERSON). A batch dispersion was carried out at a speed of 8,500 rpm until the entire mixture was uniform. Subsequently, the dispersion liquid was supplied from the stainless steel container to a high-pressure homogenizer (Starburst Lab HJP-17007, manufactured by Sugino Machine) via piping, and a five-pass 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. The mixture was then passed through a nylon mesh with 48 μm openings three times, and then filtered through a magnetic filter (manufactured by TOK Engineering) at room temperature and a magnetic flux density of 12,000 gauss to obtain a carbon nanotube dispersion (Dispersion 1). After filtration, magnetic granular metal particles were found to be attached to the magnetic filter. As shown in Table 3, Dispersion 1 had good stability. The pH of Dispersion 1 was measured by the above-mentioned method and was found to be 8.9.

(Examples 1-2 to 1-26, Comparative Examples 1-1 to 1-7)
Each dispersion (Dispersions 2 to 26, Comparative Dispersions 1 to 7) was prepared in the same manner as in Example 1-1 according to the composition shown in Table 2-1. In Examples 1-11 to 1-26, the base shown in Table 2-1 was added as an additive simultaneously with the polymer (B) to adjust the pH to the value shown in Table 2-2, and the other procedures were the same as in Example 1-1 to prepare the dispersion.

As shown in Table 3, all of the carbon nanotube dispersions of the examples (Dispersions 1 to 26) had good storage stability. They were also confirmed to have very excellent fluidity. On the other hand, comparative dispersions 1 to 6 had high viscosity or poor storage stability, and comparative dispersions 4 to 6 in particular had extremely poor initial viscosity and storage stability. Furthermore, all of the comparative dispersions had inferior fluidity compared to the carbon nanotube dispersions of the examples.

When a dispersion was prepared using only polymer (B), as in Comparative Example 1-4, the viscosity of the polymer (B) solution was high, and the viscosity of the resulting carbon nanotube dispersion was also very high. Furthermore, in Comparative Examples 1-1 and 1-2, which did not contain two types of carbon nanotubes with different fiber diameter distributions in the 100 nm or less range, a dispersion could be prepared using both polymer (A) and polymer (B), but the fluidity of the resulting dispersion was low.

Comparing Example 1-7 with Examples 1-11 to 13, dispersions with a pH adjusted to 9.0 or higher were obtained with lower viscosity. By adjusting the pH of the carbon nanotube dispersion to 9.0 or higher, not only was the solution viscosity of polymer (B) reduced, but the adsorption of polymer (A) and polymer (B) to the carbon nanotubes was improved, making it easier to obtain dispersions with good dispersibility. Similar low-viscosity dispersions were obtained in Examples 1-19 to 21, confirming that effects could be obtained not only by adding a base, but also by adjusting the pH of the dispersion.

<Preparation of Positive Electrode Composite Slurry and Positive Electrode Film>
<Example 2-1>
According to the composition shown in Table 4, a carbon nanotube dispersion (Dispersion 1) and 8% by mass PVDF dissolved in NMP were added to a 150 mL plastic container, and then the mixture was stirred at 2,000 rpm for 30 seconds using a rotation and revolution mixer (Thinky Corporation, Awatori Rentaro, ARE-310) to obtain a carbon nanotube dispersion composition. Thereafter, NMC was added as an electrode active material, and the mixture was stirred at 2,000 rpm for 20 minutes using a rotation and revolution mixer. Further, NMP was added, and the mixture was stirred at 2,000 rpm for 30 seconds using a rotation and revolution mixer to obtain a positive electrode composite slurry. The solids content of the positive electrode composite slurry was 70% by mass.

The positive electrode composite slurry was applied to a 20 μm thick aluminum foil current collector using an applicator, and then dried in an electric oven at 120°C ± 5°C for 25 minutes to adjust the electrode weight per unit area to 20 mg/ ^cm2 . Further, a rolling process was performed using a roll press (Thank Metal Co., Ltd., 3 ton hydraulic roll press) to produce a positive electrode film 1a with a positive electrode composite layer density of 3.0 g/ ^cm3 .

<Examples 2-2 to 2-26, Comparative Examples 2-1 to 2-7>
As shown in Table 4, positive electrode films 2a to 26a and comparative positive electrode films 1a to 7a were produced in the same manner as in Example 2-1, except that the type of carbon nanotube dispersion liquid was changed.

<Examples 3-1 to 3-3, Comparative Examples 3-1 to 3-3>
As shown in Table 4, positive electrode membranes 1b to 3b and comparative positive electrode membranes 1b to 3b were prepared in the same manner as in Example 2-1, except that the electrode active material was changed to NCA.

The abbreviations in Table 4 have the following meanings:
・NMC: NCM523 (manufactured by Nihon Kagaku Kogyo Co., Ltd., composition: LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , solid content 100%)
NCA: HED (registered trademark) NAT-7050 (manufactured by BASF Toda Battery Materials LLC, composition: LiNi _0.8 Co _0.15 Al _0.05 O ₂ ), solid content 100%
PVDF: Polyvinylidene fluoride (Solef #5130 (manufactured by Solvey Co., Ltd.), solid content 100%)

Table 5 shows the electrode evaluation results. All of the electrode films created using the carbon nanotube dispersion, which has good stability, showed good resistance values. Compared to the comparative example, it is believed that the carbon nanotubes were able to efficiently form a conductive network.

<Fabrication of Non-Aqueous Electrolyte Secondary Battery>
<Examples 4-1 to 4-26, Comparative Examples 4-1 to 4-7>
<Examples 5-1 to 5-3, Comparative Examples 5-1 to 5-3>
The standard negative electrode and the positive electrode film shown in Table 5 were punched out to 50 mm x 45 mm and 45 mm x 40 mm, respectively. The separator (porous polypropylene film) inserted between them was inserted into an aluminum laminate bag and dried in an electric oven at 70 °C for 1 hour. Subsequently, 2 mL of electrolyte was poured into a glove box filled with argon gas, and the aluminum laminate bag was sealed to produce Batteries 1a to 26a, Batteries 1b to 3b, Comparative Batteries 1a to 7a, and Comparative Batteries 1b to 3b. The electrolyte was a nonaqueous electrolyte prepared by mixing ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a 1:1:1 (volume ratio) mixed solvent. Furthermore, 1 part of VC (vinylene carbonate) was added as an additive to 100 parts of the electrolyte solution, and then _LiPF6 was dissolved in the nonaqueous electrolyte to a concentration of 1 M.

<Production Example 1: Preparation of standard negative electrode composite slurry>
Acetylene black (Denka Black (registered trademark) HS-100, manufactured by Denka Co., Ltd.), CMC, and water were added to a 150 mL plastic container, and the mixture was stirred at 2,000 rpm for 30 seconds using a centrifugal mixer (Thinky Corporation, ARE-310, Awatori Rentaro). Furthermore, artificial graphite was added as a negative electrode active material, and the mixture was stirred at 2,000 rpm for 150 seconds using a centrifugal mixer (Thinky Corporation, ARE-310, Awatori Rentaro). Subsequently, SBR was added, and the mixture was stirred at 2,000 rpm for 30 seconds using a centrifugal mixer (Thinky Corporation, ARE-310, Awatori Rentaro). A standard negative electrode composite slurry was obtained. The solids content of the standard negative electrode composite slurry was 48% by mass. The solid content ratio of the negative electrode active material:conductive material:CMC:SBR in the standard negative electrode mixture slurry was 97:0.5:1:1.5.

The abbreviations listed above have the following meanings:
HS-100: Denka Black HS-100 (manufactured by Denka Company Ltd., acetylene black, average primary particle diameter 48 nm, specific surface area 39 m ² /g)
Artificial graphite: CGB-20 (manufactured by Nippon Graphite Industries Co., Ltd.), solid content 100%
CMC: #1190 (manufactured by Daicel FineChem Co., Ltd.), solid content 100%
SBR: TRD2001 (manufactured by JSR Corporation), solid content 48%

<Production Example 2: Preparation of standard negative electrode>
The negative electrode composite slurry was applied to a 20 μm thick copper foil current collector using an applicator, and then dried in an electric oven at 80°C ± 5°C for 25 minutes to adjust the electrode weight per unit area to 10 mg/ ^cm2 . The resultant was then rolled using a roll press (Thank Metal Co., Ltd., 3 ton hydraulic roll press) to produce a negative electrode with a negative electrode composite layer density of 1.6 g/ ^cm3 .

<Rate test and cycle test results>
As shown in Table 5, batteries with a carbon nanotube dispersion liquid with good stability and fluidity in the positive electrode film had good rate and cycle characteristics, while batteries with a carbon nanotube dispersion liquid with poor fluidity in the positive electrode film had poor characteristics in both cases. A low-resistance positive electrode film is thought to have low resistance as a battery and improve rate characteristics. Furthermore, since the cycle load is concentrated on the electrode active material particles, which have relatively low resistance, deterioration is accelerated, whereas if a good conductive network is formed throughout, the load is distributed and deterioration is less likely to occur.

As described above, by achieving both dispersibility, stability, and fluidity, it was possible to maintain a good dispersion state in the electrode film and form an efficient conductive network, enabling the production of a battery with good rate and cycle characteristics.

The present invention has been described with reference to the above embodiments, but the present invention is not limited to the above. The configuration and details of the present invention may be modified in various ways that are understandable to those skilled in the art and are within the scope of the present invention.

1: First peak 2: Mode diameter of first peak

Claims (11)

A carbon nanotube dispersion liquid containing carbon nanotubes, a first polymer containing at least one selected from the group consisting of a hydroxyl group-containing structural unit and a heterocycle-containing structural unit and not containing a nitrile group-containing structural unit, a second polymer containing a nitrile group-containing structural unit, and a solvent,
the first polymer is at least one of polyvinyl alcohol, polyvinyl acetal, and polyvinylpyrrolidone;
the second polymer comprises a nitrile group-containing structural unit and an aliphatic hydrocarbon structural unit,
the content of the nitrile group-containing structural unit is 15% by mass or more and the content of the aliphatic hydrocarbon structural unit is 40% by mass or more, based on the mass of the second polymer;
Furthermore, the total content of the nitrile group-containing structural unit and the aliphatic hydrocarbon structural unit is 80% by mass or more and 100% by mass or less,
the solvent is an amide-based organic solvent,
the carbon nanotubes include at least two types of carbon nanotubes having different fiber diameter distributions in a range of 100 nm or less as measured by a scanning electron microscope;
The two types of carbon nanotubes include a component exhibiting a first fiber diameter distribution and a component exhibiting a second fiber diameter distribution. The carbon nanotube dispersion liquid according to claim 1, wherein the first fiber diameter distribution range is 2 nm or more and less than 30 nm, and the second fiber diameter distribution range is 30 nm or more and 100 nm or less. The carbon nanotube dispersion liquid according to claim 1 or 2, wherein the content of the component having the first fiber diameter distribution is 40% or more and 99% or less on a number basis relative to the total amount of the carbon nanotubes. The carbon nanotube dispersion liquid according to any one of claims 1 to 3, wherein the content of carbon nanotubes having a fiber length of 0.3 μm or less is 75% or less by number based on the total amount of the carbon nanotubes, and the average fiber length of the carbon nanotubes in the carbon nanotube dispersion liquid is 0.1 μm or more and 1.0 μm or less. The carbon nanotube dispersion liquid according to any one of claims 1 to 4, wherein the content of the first polymer is 30% by mass or more and 70% by mass or less relative to the total mass of the first polymer and the second polymer. The particle size distribution measured by a laser diffraction/scattering particle size distribution measurement method under the condition of a laser light transmittance of 50% has at least two peaks,
6. The carbon nanotube dispersion liquid according to claim 1, wherein a mode diameter of a first peak having the smallest particle diameter among the two or more peaks is 0.2 μm or less. The carbon nanotube dispersion liquid described in any one of claims 1 to 6, wherein the solvent is substantially free of water and has a pH of 9.0 or more and 12.0 or less. A carbon nanotube dispersion composition comprising the carbon nanotube dispersion liquid according to any one of claims 1 to 7 and a fluorine-based resin. A slurry for an electrode film, comprising the carbon nanotube dispersion liquid according to any one of claims 1 to 7 or the carbon nanotube dispersion composition according to claim 8 , and an electrode active material. 10. An electrode film comprising a coating film of the carbon nanotube dispersion liquid according to any one of claims 1 to 7 , the carbon nanotube dispersion composition according to claim 8 , or the slurry for an electrode film according to claim 9 . 11. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the electrode film according to claim 10 is used in at least one of the positive electrode and the negative electrode.