Carbon nanotube dispersed liquid, slurry for nonaqueous secondary battery negative electrodes, negative electrode for nonaqueous secondary batteries, and nonaqueous secondary battery

Abstract

The purpose of the present invention is to provide a carbon nanotube dispersed liquid which is capable of preparing a slurry for nonaqueous secondary battery negative electrodes, the slurry having excellent thickening inhibition properties, and which enables a nonaqueous secondary battery to exhibit excellent cycle characteristics. The present invention provides a carbon nanotube dispersed liquid which contains carbon nanotubes, a water-soluble polymer and water, wherein the carbon nanotubes have an average length of 5 µm to 100 µm, while having an average diameter of 20 nm to 100 nm.

Description

Carbon nanotube dispersion, slurry for non-aqueous secondary battery negative electrode, non-aqueous secondary battery negative electrode, and non-aqueous secondary battery

[0001] The present invention relates to a carbon nanotube dispersion, a slurry for a non-aqueous secondary battery negative electrode, a non-aqueous secondary battery negative electrode, and a non-aqueous secondary battery.

[0002] Carbon nanotubes (hereinafter, sometimes referred to as "CNTs") are excellent in various properties such as mechanical strength, optical properties, electrical properties, thermal properties, and molecular adsorption ability, and are therefore used in a variety of electronic engineering products, such as electronic circuits such as logic circuits, memories such as DRAM, SRAM, and NRAM, semiconductor devices, interconnects, electronic components such as complementary MOS and bipolar transistors, chemical sensors such as detectors for trace gases, etc., and biosensors such as measuring devices for DNA, proteins, etc.

[0003] Furthermore, in recent years, CNTs have also been used as a conductive material in secondary battery electrode slurries used to form electrode composite layers in electrodes of non-aqueous secondary batteries such as lithium-ion secondary batteries (hereinafter sometimes simply referred to as "secondary batteries"). Secondary battery electrodes undergo significant expansion and contraction due to charging and discharging, which reduces the conductivity of the electrodes and the cycle characteristics of the secondary battery. However, when CNTs are used as a conductive material in secondary battery electrode slurries, the decrease in conductivity is suppressed, and a secondary battery with excellent cycle characteristics can be obtained.

[0004] Here, when using CNTs, they need to be dispersed in a solvent in order to fully utilize their properties, and a dispersion containing CNTs has been proposed. For example, in Patent Document 1, CNTs are first dispersed in an N-methylpyrrolidone (NMP) solution of an acrylic binder to prepare a dispersion, and then the resulting dispersion is mixed with a positive electrode active material, etc. to prepare a positive electrode mixture slurry.

[0005] On the other hand, from the viewpoints of environmental impact and operability, it is desirable to use water instead of the organic solvents such as NMP as the dispersion medium for dispersing CNTs. However, because CNTs have low affinity for water, it is very difficult to obtain a CNT dispersion in which CNTs are dispersed in water.

[0006] Therefore, in recent years, there has been progress in the development of CNT dispersions containing water that have excellent CNT dispersibility. For example, Patent Document 2 describes such a CNT dispersion in which the average outer diameter of carbon nanotubes is more than 3 nm and 25 nm or less, and the BET specific surface area of ​​the carbon nanotubes is 150 to 800 m. ２ / g and the fiber length of the carbon nanotubes in the carbon nanotube dispersion is 0.8 to 3.5 μm. Also, Patent Document 3 proposes an aqueous dispersion using multi-walled carbon nanotubes characterized by having a length of 2 μm or more and an aspect ratio of length to diameter in the range of 200 to 1000.

[0007] Japanese Patent Application Laid-Open No. 2014-238944 Japanese Patent Application Laid-Open No. 2021-072279 Special Publication No. 2022-527707

[0008] However, when a negative electrode slurry is prepared using the CNT dispersion liquid of the above-mentioned conventional technology, there is room for improvement in suppressing an increase in the viscosity of the negative electrode slurry, i.e., in suppressing an increase in the viscosity of the negative electrode slurry.Furthermore, when a negative electrode slurry is prepared using the CNT dispersion liquid of the above-mentioned conventional technology, there is also room for improvement in enabling a secondary battery including a negative electrode formed using the negative electrode slurry to exhibit excellent cycle characteristics.

[0009] Therefore, an object of the present invention is to provide a carbon nanotube dispersion that can prepare a slurry for a non-aqueous secondary battery negative electrode that has excellent viscosity increase suppression properties and that can enable the non-aqueous secondary battery to exhibit excellent cycle characteristics. Another object of the present invention is to provide a slurry for a non-aqueous secondary battery negative electrode that has excellent viscosity increase suppression properties and that can enable the non-aqueous secondary battery to exhibit excellent cycle characteristics. Another object of the present invention is to provide a non-aqueous secondary battery negative electrode that can enable the non-aqueous secondary battery to exhibit excellent cycle characteristics. Another object of the present invention is to provide a non-aqueous secondary battery that can exhibit excellent cycle characteristics.

[0010] The present inventors have conducted extensive research to solve the above problems, and have newly discovered that a CNT dispersion containing CNTs each having an average length and an average diameter within a predetermined range, a water-soluble polymer, and water can prepare a slurry for a nonaqueous secondary battery negative electrode that is excellent in suppressing thickening, and that a nonaqueous secondary battery equipped with a negative electrode fabricated using the negative electrode slurry can exhibit excellent cycle characteristics, thereby completing the present invention.

[0011] The present invention aims to advantageously solve the above-mentioned problems. The present invention provides [1] a carbon nanotube dispersion containing carbon nanotubes, a water-soluble polymer, and water, wherein the carbon nanotubes have an average length of 5 μm to 100 μm and an average diameter of 20 nm to 100 nm. Thus, a carbon nanotube dispersion containing CNTs, a water-soluble polymer, and water, each having an average length and average diameter within a predetermined range, can prepare a slurry for a nonaqueous secondary battery negative electrode that exhibits excellent viscosity suppression, and can enable a nonaqueous secondary battery equipped with a negative electrode fabricated using the negative electrode slurry to exhibit excellent cycle performance. In the present invention, the average length and average diameter of the CNTs can be measured using the method described in the Examples section of this specification. In the present invention, a polymer being "water-soluble" means that when 0.5 g of the polymer is dissolved in 100 g of water at 25°C, the insoluble content is less than 1.0 mass%.

[0012] [2] In the carbon nanotube dispersion liquid of the above [1], the water-soluble polymer preferably has an acid functional group. Use of a water-soluble polymer having an acid functional group can improve the dispersibility and storage stability of the CNT dispersion liquid.

[0013] [3] In the carbon nanotube dispersion liquid of the above [1] or [2], it is preferable that at least a portion of the acid functional groups of the water-soluble polymer are alkali metal bases or ammonium bases. If at least a portion of the acid functional groups of the water-soluble polymer are in the form of a salt such as an alkali metal salt or an ammonium salt, the dispersibility and storage stability of the CNT dispersion liquid can be improved, and the viscosity increase of a negative electrode slurry containing the CNT dispersion liquid can be suppressed, and the cycle characteristics of a secondary battery including a negative electrode produced using the negative electrode slurry can be further improved.

[0014] [4] In the carbon nanotube dispersion liquid of any one of [1] to [3] above, it is preferable that the mass ratio of the carbon nanotubes to the water-soluble polymer is 0.1 or more and 10 or less. When the mass ratio of CNTs to the water-soluble polymer in the CNT dispersion liquid (CNT / water-soluble polymer) is within the above-mentioned predetermined range, the storage stability of the CNT dispersion liquid can be improved.

[0015] [5] The carbon nanotube dispersion liquid according to any one of [1] to [4] above preferably has a pH of 6 or more and 10 or less. If the pH of the CNT dispersion liquid is within the above-mentioned predetermined range, the viscosity increase suppression of the negative electrode slurry containing the CNT dispersion liquid can be further improved, and the cycle characteristics of a secondary battery including a negative electrode produced using the negative electrode slurry can be further improved.

[0016] The present invention also aims to advantageously solve the above-mentioned problems, and provides a slurry for a negative electrode of a non-aqueous secondary battery, comprising [6] a negative electrode active material and any one of the carbon nanotube dispersions described above in [1] to [5]. In this way, a slurry for a negative electrode of a non-aqueous secondary battery, comprising a negative electrode active material and the above-mentioned CNT dispersion, is excellent in suppressing thickening, and allows a non-aqueous secondary battery including a negative electrode produced using the negative electrode slurry to exhibit excellent cycle characteristics.

[0017] [7] In the slurry for a non-aqueous secondary battery negative electrode according to [6] above, the negative electrode active material preferably includes a silicon-based negative electrode active material. Using a silicon-based negative electrode active material (a silicon-containing negative electrode active material) as the negative electrode active material can increase the capacity of a non-aqueous secondary battery equipped with a negative electrode prepared using the slurry for a non-aqueous secondary battery negative electrode. While silicon-based negative electrode active materials are particularly susceptible to large expansion and contraction during charge and discharge, the slurry for a non-aqueous secondary battery negative electrode of the present invention is prepared using the carbon nanotube dispersion liquid of the present invention described above. Therefore, even when a silicon-based negative electrode active material is used as the negative electrode active material, a decrease in the conductivity of the electrode mixture layer is suppressed, and a negative electrode that can exhibit excellent cycle characteristics in a non-aqueous secondary battery can be formed.

[0018] [8] The slurry for a non-aqueous secondary battery negative electrode according to [6] or [7] above preferably further comprises a particulate polymer, the particulate polymer comprising carboxylic acid group-containing monomer units, aromatic vinyl monomer units, and conjugated diene monomer units. When the slurry for a non-aqueous secondary battery negative electrode further comprises the above-mentioned specific particulate polymer, the cycle characteristics of a non-aqueous secondary battery including a negative electrode produced using the negative electrode slurry can be further improved. In the present invention, the phrase "comprises monomer units" in a polymer means that "a polymer obtained using the monomer contains repeating units derived from the monomer."

[0019] [9] In the slurry for a non-aqueous secondary battery negative electrode according to [8] above, it is preferable that the content of the carboxylic acid group-containing monomer units in the particulate polymer is 3% by mass or more and 30% by mass or less, with the total repeating units contained in the particulate polymer being 100% by mass. If the content of the carboxylic acid group-containing monomer units in the particulate polymer is within the above-mentioned range, the cycle characteristics of a non-aqueous secondary battery including a negative electrode prepared using the slurry for a non-aqueous secondary battery negative electrode can be further improved. In the present invention, the content of the repeating units (monomer units) in the polymer is １ H-NMR and １３ It can be measured using a nuclear magnetic resonance (NMR) method such as C-NMR.

[0020] Another object of the present invention is to advantageously solve the above-mentioned problems, and the present invention provides

[10] a negative electrode for a nonaqueous secondary battery, comprising a negative electrode composite layer formed using the slurry for a nonaqueous secondary battery negative electrode according to any one of [6] to [9] above. In this way, a negative electrode for a nonaqueous secondary battery comprising a negative electrode composite layer formed using the above-mentioned slurry for a nonaqueous secondary battery negative electrode can enable the nonaqueous secondary battery to exhibit excellent cycle characteristics.

[0021] Another object of the present invention is to advantageously solve the above-mentioned problems, and the present invention provides

[11] a nonaqueous secondary battery comprising the negative electrode for a nonaqueous secondary battery according to

[10] . Thus, a nonaqueous secondary battery comprising the above-mentioned negative electrode for a nonaqueous secondary battery can exhibit excellent cycle characteristics.

[0022] According to the present invention, it is possible to prepare a slurry for a non-aqueous secondary battery negative electrode that exhibits excellent viscosity increase suppression, and it is possible to provide a carbon nanotube dispersion that can enable the non-aqueous secondary battery to exhibit excellent cycle characteristics. Furthermore, according to the present invention, it is possible to provide a slurry for a non-aqueous secondary battery negative electrode that exhibits excellent viscosity increase suppression, and it is possible to enable the non-aqueous secondary battery to exhibit excellent cycle characteristics. Furthermore, according to the present invention, it is possible to provide a non-aqueous secondary battery negative electrode that can enable the non-aqueous secondary battery to exhibit excellent cycle characteristics. Furthermore, according to the present invention, it is possible to provide a non-aqueous secondary battery that can exhibit excellent cycle characteristics.

[0023] Hereinafter, embodiments of the present invention will be described in detail. The CNT dispersion of the present invention is not particularly limited and can be used, for example, as a material for producing a slurry for a non-aqueous secondary battery negative electrode (hereinafter, sometimes simply referred to as "negative electrode slurry"). The negative electrode slurry of the present invention is prepared using the CNT dispersion of the present invention. Additionally, the non-aqueous secondary battery negative electrode (hereinafter, sometimes simply referred to as "negative electrode") of the present invention is characterized by comprising a negative electrode composite layer formed using the negative electrode slurry of the present invention. Furthermore, the non-aqueous secondary battery of the present invention is characterized by comprising the negative electrode of the present invention. The CNT dispersion of the present invention can be used as a raw material for a composite material containing a resin and CNTs, or for producing electronic products, etc.

[0024] (CNT Dispersion) The CNT dispersion of the present invention contains CNTs having an average length and an average diameter each within a predetermined range, a water-soluble polymer, water, and optionally other components. Note that the CNT dispersion usually does not contain electrode active materials (positive electrode active material, negative electrode active material).

[0025] Furthermore, the CNT dispersion liquid of the present invention makes it possible to prepare a negative electrode slurry that is excellent in suppressing thickening, and also makes it possible to make a secondary battery equipped with a negative electrode prepared using the negative electrode slurry exhibit excellent cycle characteristics.

[0026] <CNT> The CNT may be a single-walled CNT or a multi-walled CNT. Furthermore, the CNT may be a combination of single-walled CNT and multi-walled CNT in any ratio. From the viewpoint of improving the dispersibility and storage stability of the CNT dispersion, it is preferable to use a multi-walled CNT.

[0027] The CNTs are not particularly limited, and those synthesized using known CNT synthesis methods such as arc discharge, laser ablation, and chemical vapor deposition (CVD) can be used. Furthermore, after CNTs are synthesized by the above-described synthesis method, they may be subjected to a dispersion treatment as necessary to adjust the average length to within a predetermined range described below. By subjecting the CNTs to a dispersion treatment, some of the CNT fibers are cut, so that the average length of the CNTs after the dispersion treatment can be adjusted to within the predetermined range described above. Details of the dispersion treatment will be described later in the section "Preparation of CNT Dispersion."

[0028] <<Average Length>> The average length of the CNTs in the CNT dispersion must be 5 μm or more, preferably 6 μm or more, more preferably 7 μm or more, and even more preferably 8 μm or more. It must be 100 μm or less, preferably 80 μm or less, more preferably 60 μm or less, more preferably 40 μm or less, and even more preferably 20 μm or less. When the average length of the CNTs in the CNT dispersion is 5 μm or more, a good conductive path can be formed in the negative electrode composite layer of the negative electrode prepared using the negative electrode slurry containing the CNT dispersion, thereby sufficiently improving the cycle characteristics of a secondary battery including the negative electrode. Furthermore, when the average length of the CNTs in the CNT dispersion is equal to or greater than the above lower limit, the peel strength of the negative electrode can be improved. On the other hand, when the average length of the CNTs in the CNT dispersion is 100 μm or less, the viscosity increase suppression of the negative electrode slurry containing the CNT dispersion can be sufficiently improved. The average length of the CNTs in the CNT dispersion may be 10 μm or more or 10 μm or less. The average length of the CNTs can be controlled by the CNT synthesis method and the dispersion treatment of the synthesized CNTs.

[0029] <<Average Diameter>> The average diameter of the CNTs in the CNT dispersion must be 20 nm or more, preferably 30 nm or more, more preferably 40 nm or more, and must be 100 nm or less, preferably 80 nm or less, and more preferably 60 nm or less. When the average diameter of the CNTs in the CNT dispersion is 20 nm or more, the viscosity increase suppression of the negative electrode slurry containing the CNT dispersion can be sufficiently improved. On the other hand, when the average diameter of the CNTs in the CNT dispersion is 100 nm or less, the number of CNTs per unit mass can be sufficiently increased, thereby forming a good conductive path in the negative electrode composite layer of the negative electrode prepared using the negative electrode slurry containing the CNT dispersion, and therefore the cycle characteristics of the secondary battery including the negative electrode can be sufficiently improved. Furthermore, when the average diameter of the CNTs in the CNT dispersion is within the above upper limit, the number of CNTs per unit mass can be sufficiently increased, thereby improving the peel strength of the negative electrode. The average diameter of the CNTs in the CNT dispersion may be 50 nm or more, or may be 50 nm or less. The average diameter of the CNTs can be controlled by the CNT synthesis method, etc.

[0030] <<G / D Ratio>> The CNT preferably has a ratio of the G band peak intensity to the D band peak intensity in the Raman spectrum (G / D ratio) of 0.4 or more, more preferably 0.5 or more, and even more preferably 0.6 or more. If the G / D ratio of the CNT is equal to or higher than the above lower limit, the cycle characteristics of the secondary battery can be further improved. The upper limit of the G / D ratio of the CNT is not particularly limited, but is, for example, 200 or less. In this specification, the "G / D ratio" of the CNT is determined by measuring the Raman spectrum of the CNT using a microscopic laser Raman spectrophotometer (Nicolet Almega XR manufactured by Thermo Fisher Scientific Co., Ltd.) and calculating the G / D ratio at 1590 cm for the obtained Raman spectrum. －１ The intensity of the G band peak observed near 1340 cm －１ The intensity of the D band peak observed in the vicinity of the peak is then determined, and the ratio of these can be calculated.

[0031] <<Content in CNT Dispersion>> The content of CNT in the CNT dispersion is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and even more preferably 0.3% by mass or more, and is preferably 10.0% by mass or less, more preferably 5.0% by mass or less, even more preferably 2.0% by mass or less, and even more preferably 1.0% by mass or less, based on the total mass of the CNT dispersion being 100% by mass. If the content of CNT in the CNT dispersion is equal to or greater than the above lower limit, the productivity of products such as a negative electrode slurry prepared using the CNT dispersion can be improved. On the other hand, if the content of CNT in the CNT dispersion is equal to or less than the above upper limit, the dispersibility and storage stability of the CNT dispersion can be ensured to be sufficiently high.

[0032] <Water-soluble polymer> The water-soluble polymer is a component that can function as a dispersant in the CNT dispersion. As the water-soluble polymer, a natural polymer compound may be used, or a semi-synthetic polymer compound obtained by modifying the natural polymer compound using a chemical reaction may be used, or a synthetic polymer compound artificially produced using a chemical reaction may be used. Specific examples of natural polymer compounds and semi-synthetic polymer compounds include those described in JP 2017-010822 A. Furthermore, as the synthetic polymer compound, for example, a water-soluble polymer obtained by polymerizing one or more monomers through a polymerization reaction may be used.

[0033] From the viewpoint of improving the dispersibility and storage stability of the CNT dispersion, it is preferable to use a water-soluble polymer having an acid functional group as the water-soluble polymer, it is more preferable to use a water-soluble polymer having an acid functional group, and a cellulose derivative having an acid functional group such as carboxymethyl cellulose and its salts, and it is particularly preferable to use a water-soluble polymer having an acid functional group.

[0034] Examples of the acid functional group possessed by the water-soluble polymers such as the water-soluble polymers and cellulose derivatives described above include a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, etc. The water-soluble polymers and cellulose derivatives may have only one type of these acid functional groups or may have two or more types in combination at any ratio.

[0035] The "acid functional group" of the water-soluble polymer includes a salt neutralized with a base. That is, at least a part of the "acid functional group" of the water-soluble polymer may be in the form of a salt. For example, in the case of a carboxylic acid group (-COOH), the acid functional group may include a lithium carboxylate group (-COOH) in addition to the carboxylic acid group (-COOH). － Li ＋ ) and other neutralized types are also included. From the viewpoint of further improving the dispersibility and storage stability of the CNT dispersion, the ability to suppress thickening of the negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery equipped with a negative electrode produced using the negative electrode slurry, it is preferable that at least a portion of the acid functional groups possessed by the water-soluble polymer be an alkali metal base or an ammonium base. That is, it is preferable that at least a portion of the acid functional groups possessed by the water-soluble polymer be in the form of a salt, such as an alkali metal salt or an ammonium salt. All of the acid functional groups may be alkali metal bases or ammonium bases. Examples of alkali metal bases include lithium metal bases, sodium metal bases, and potassium metal bases. The water-soluble polymer may have only one of these alkali metal bases and ammonium bases as the acid functional group, or two or more of them may be combined in any ratio.

[0036] <<Water-Soluble Polymer Having Acid Functional Group>> In one embodiment, the water-soluble polymer serving as the water-soluble polymer preferably has a carboxylic acid group as the acid functional group. If the acid functional group of the water-soluble polymer is a carboxylic acid group, the dispersibility and storage stability of the CNT dispersion, the ability to suppress thickening of a negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery including a negative electrode fabricated using the negative electrode slurry can be further improved. In addition, the dispersibility and storage stability of the CNT dispersion can be maintained at a good level regardless of the type of CNT. In another embodiment, the water-soluble polymer serving as the water-soluble polymer preferably has a sulfonic acid group as the acid functional group. If the acid functional group of the water-soluble polymer is a sulfonic acid group, the dispersibility and storage stability of the CNT dispersion, the ability to suppress thickening of a negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery including a negative electrode fabricated using the negative electrode slurry can be further improved.

[0037] Furthermore, a water-soluble polymer that can be preferably used as the water-soluble polymer preferably contains an ether group. The "ether group" referred to here is a group represented by "-R'OR-". R and R' represent a linear or branched hydrocarbon group, preferably an alkyl group, having from 1 to 10 carbon atoms, and may be the same or different. The number of carbon atoms in R and R' is preferably from 2 to 5, more preferably 2 or 3, and even more preferably 2. If the water-soluble polymer contains an ether group, it is possible to further improve the dispersibility and storage stability of the CNT dispersion, the ability to suppress thickening of a negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery including a negative electrode produced using the negative electrode slurry.

[0038] Hereinafter, the water-soluble polymer contained in the CNT dispersion liquid of the present invention will be described using first and second embodiments as examples, but the water-soluble polymer is not limited to these.

[0039] [Water-soluble polymer of first embodiment] The water-soluble polymer of the first embodiment contains a carboxylic acid group-containing monomer unit and a conjugated diene monomer unit. The water-soluble polymer of the first embodiment may contain a repeating unit (another repeating unit) other than the carboxylic acid group-containing monomer unit and the conjugated diene monomer unit.

[0040] [Carboxylic Acid Group-Containing Monomer Unit] The carboxylic acid group-containing monomer unit is a repeating unit containing a carboxylic acid group (—COOH). In the CNT dispersion, some or all of the carboxylic acid groups of the carboxylic acid group-containing monomer unit are replaced with sodium carboxylate groups (—COOH). － Na ＋ ), lithium carboxylate group (—COO － Li ＋ ), and ammonium carboxylate groups (—COO － NH ４ ＋ When the carboxylic acid group takes the form of at least one of the above-mentioned carboxylate salt groups, it is possible to further improve the dispersibility and storage stability of the CNT dispersion, the ability to suppress thickening of the negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery including a negative electrode produced using the negative electrode slurry.

[0041] Carboxylic acid group-containing monomers capable of forming the carboxylic acid group-containing monomer units of the water-soluble polymer of the first embodiment include monocarboxylic acids and their derivatives, dicarboxylic acids and their acid anhydrides, and derivatives thereof. Examples of monocarboxylic acids include acrylic acid, methacrylic acid, and crotonic acid. Examples of monocarboxylic acid derivatives include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, and α-chloro-β-E-methoxyacrylic acid. Examples of dicarboxylic acids include maleic acid, fumaric acid, and itaconic acid. Examples of dicarboxylic acid derivatives include methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid, and maleic acid monoesters such as nonyl maleate, decyl maleate, dodecyl maleate, octadecyl maleate, and fluoroalkyl maleates. Examples of dicarboxylic acid anhydrides include maleic anhydride, acrylic anhydride, methylmaleic anhydride, and dimethylmaleic anhydride. Furthermore, as the carboxylic acid group-containing monomer, an acid anhydride that generates a carboxylic acid group upon hydrolysis can also be used.

[0042] The carboxylic acid group-containing monomer may be used alone or in combination of two or more. From the viewpoint of further improving the cycle characteristics of the secondary battery, acrylic acid and methacrylic acid are preferred as the carboxylic acid group-containing monomer. That is, the water-soluble polymer of the first embodiment preferably contains at least one of an acrylic acid unit and a methacrylic acid unit as the carboxylic acid group-containing monomer unit.

[0043] The content of the carboxylic acid group-containing monomer units in the water-soluble polymer of the first embodiment is preferably 30% by mass or more, more preferably 40% by mass or more, and even more preferably 50% by mass or more, and preferably 95% by mass or less, more preferably 90% by mass or less, even more preferably 80% by mass or less, and even more preferably 70% by mass or less, based on 100 mol% of all repeating units contained in the water-soluble polymer of the first embodiment. The content of the carboxylic acid group-containing monomer units in the water-soluble polymer of the first embodiment is preferably 30 mol% or more, more preferably 40 mol% or more, and even more preferably 50 mol% or more, and preferably 90 mol% or less, more preferably 80 mol% or less, even more preferably 70 mol% or less, based on 100 mol% of all repeating units contained in the water-soluble polymer of the first embodiment. When the content ratio of the carboxylic acid group-containing monomer unit in the water-soluble polymer of the first embodiment is within the above-mentioned predetermined range, it is possible to further improve the dispersibility and storage stability of the CNT dispersion, the ability to suppress thickening of the negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery including a negative electrode produced using the negative electrode slurry.

[0044] [Conjugated Diene Monomer Unit] Examples of conjugated diene monomers that can form the conjugated diene monomer units of the water-soluble polymer of the first embodiment include 1,3-butadiene, isoprene (2-methyl-1,3-butadiene), 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. These may be used alone or in combination of two or more. Among these, 1,3-butadiene and isoprene are preferred, with isoprene being more preferred. That is, the water-soluble polymer of the first embodiment preferably contains, as the conjugated diene monomer units of the first embodiment, at least one of a 1,3-butadiene unit and an isoprene unit, and more preferably contains an isoprene unit.

[0045] The content of the conjugated diene monomer units in the water-soluble polymer of the first embodiment is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 20% by mass or more, still more preferably 30% by mass or more, and preferably 70% by mass or less, more preferably 60% by mass or less, and even more preferably 50% by mass or less, based on 100 mol% of all repeating units contained in the water-soluble polymer of the first embodiment. The content of the conjugated diene monomer units in the water-soluble polymer of the first embodiment is preferably 10 mol% or more, more preferably 20 mol% or more, even more preferably 30 mol% or more, and even more preferably 40 mol% or more, preferably 70 mol% or less, more preferably 60 mol% or less, and even more preferably 50 mol% or less, based on 100 mol% of all repeating units contained in the water-soluble polymer of the first embodiment. When the content ratio of the conjugated diene monomer units in the water-soluble polymer of the first embodiment is within the above-mentioned predetermined range, it is possible to further improve the dispersibility and storage stability of the CNT dispersion, the ability to suppress thickening of the negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery equipped with a negative electrode produced using the negative electrode slurry.

[0046] [Other Repeating Units] The other repeating units that the water-soluble polymer of the first embodiment may contain, other than the above-mentioned carboxylic acid group-containing monomer units and conjugated diene monomer units, are not particularly limited, and examples thereof include monomer units derived from known monomers (other monomers) copolymerizable with the above-mentioned carboxylic acid group-containing monomers and conjugated diene monomers. The other monomers may be used alone or in combination of two or more.

[0047] However, from the viewpoint of further improving the cycle characteristics of the secondary battery, the content of the other repeating units in the water-soluble polymer of the first embodiment is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0% by mass, based on 100% by mass of all repeating units contained in the water-soluble polymer of the first embodiment. Furthermore, the content of the other repeating units in the water-soluble polymer of the first embodiment is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0% by mass, based on 100% by mass of all repeating units contained in the water-soluble polymer of the first embodiment. In other words, the total content of the carboxylic acid group-containing monomer units and the conjugated diene monomer units in the water-soluble polymer of the first embodiment is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, even more preferably 99% by mass or more, and particularly preferably 100% by mass, where the total repeating units contained in the water-soluble polymer of the first embodiment are taken as 100% by mass. Furthermore, the total content of the carboxylic acid group-containing monomer units and the conjugated diene monomer units in the water-soluble polymer of the first embodiment is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, even more preferably 99% by mass or more, and particularly preferably 100% by mass, where the total repeating units contained in the water-soluble polymer of the first embodiment are taken as 100% by mass.

[0048] [Water-soluble polymer of second embodiment] The water-soluble polymer of the second embodiment contains a sulfonic acid group-containing monomer unit and an alkylene oxide structure-containing monomer unit. The water-soluble polymer of the second embodiment may contain repeating units (other repeating units) other than the sulfonic acid group-containing monomer unit and the alkylene oxide structure-containing monomer unit.

[0049] [Sulfonic Acid Group-Containing Monomer Unit] The sulfonic acid group-containing monomer unit is a monomer unit containing a sulfonic acid group (—SO ３In the CNT dispersion, some or all of the sulfonic acid groups of the sulfonic acid group-containing monomer unit are converted to sodium sulfonate groups (—SO ３ － Na ＋ ), lithium sulfonate group (—SO ３ － Li ＋ ), and ammonium sulfonate groups (—SO ３ － NH ４ ＋ When the sulfonic acid group takes the form of at least one of the above-mentioned sulfonate salt groups, it is possible to further improve the dispersibility and storage stability of the CNT dispersion, the ability to suppress thickening of the negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery including a negative electrode produced using the negative electrode slurry.

[0050] Examples of sulfonic acid group-containing monomers capable of forming the sulfonic acid group-containing monomer units of the water-soluble polymer of the second embodiment include vinyl sulfonic acid, methyl vinyl sulfonic acid, (meth)allyl sulfonic acid, styrene sulfonic acid, (meth)acrylic acid-2-ethyl sulfonate, 2-acrylamido-2-methylpropanesulfonic acid, 3-allyloxy-2-hydroxypropanesulfonic acid, and salts thereof. In this specification, "(meth)allyl" means allyl and / or methallyl.

[0051] The sulfonic acid group-containing monomer may be used alone or in combination of two or more. From the viewpoint of further improving the cycle characteristics of the secondary battery, styrene sulfonic acid is preferred as the sulfonic acid group-containing monomer. That is, the water-soluble polymer of the second embodiment preferably contains a styrene sulfonic acid unit as the sulfonic acid group-containing monomer unit.

[0052] The content of the sulfonic acid group-containing monomer units in the water-soluble polymer of the second embodiment is preferably 40% by mass or more, more preferably 50% by mass or more, and even more preferably 55% by mass or more, and preferably 95% by mass or less, more preferably 90% by mass or less, even more preferably 80% by mass or less, and even more preferably 70% by mass or less, based on 100 mol% of all repeating units contained in the water-soluble polymer of the second embodiment. The content of the sulfonic acid group-containing monomer units in the water-soluble polymer of the second embodiment is preferably 40 mol% or more, more preferably 50 mol% or more, and even more preferably 55 mol% or more, and preferably 95 mol% or less, more preferably 90 mol% or less, even more preferably 80 mol% or less, and even more preferably 70 mol% or less, based on 100 mol% of all repeating units contained in the water-soluble polymer of the second embodiment. When the content ratio of the sulfonic acid group-containing monomer unit in the water-soluble polymer of the second embodiment is within the above-mentioned predetermined range, it is possible to further improve the dispersibility and storage stability of the CNT dispersion, the ability to suppress thickening of the negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery including a negative electrode produced using the negative electrode slurry.

[0053] [Alkylene oxide structure-containing monomer unit] The alkylene oxide structure-containing monomer unit of the water-soluble polymer of the second embodiment is a monomer unit containing a structure that can be represented by the following general formula (I). [In formula (I), m is an integer of 1 or more, and n is an integer of 1 or more.]

[0054] Incorporation of an alkylene oxide structure-containing monomer unit into the water-soluble polymer of the second embodiment can further improve the dispersibility and storage stability of a CNT dispersion, the suppression of thickening of a negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery equipped with a negative electrode produced using the negative electrode slurry. In the above formula (I), the integer m is preferably 2 or more and 5 or less, more preferably 2 or 3, and even more preferably 2. When the integer m is 2, a monomer unit containing a structural unit represented by general formula (I) is referred to as an ethylene oxide structure-containing monomer unit. When the integer m is 3, a monomer unit containing a structural unit represented by general formula (I) is referred to as a propylene oxide structure-containing monomer unit. When the integer m is equal to or less than the upper limit, the dispersibility and storage stability of the CNT dispersion, the suppression of thickening of the negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery including a negative electrode fabricated using the negative electrode slurry can be further improved. In particular, when the integer m is 2, i.e., when the water-soluble polymer of the second form contains an ethylene oxide structure-containing monomer unit, the water-soluble polymer of the second form can be imparted with appropriate hydrophilicity, thereby increasing the affinity of the water-soluble polymer of the second form for water. As a result, when the water-soluble polymer of the second form contains an ethylene oxide structure-containing monomer unit, the dispersibility and storage stability of the CNT dispersion, the suppression of thickening of the negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery including a negative electrode fabricated using the negative electrode slurry can be particularly improved.

[0055] The water-soluble polymer of the second embodiment may contain multiple types of alkylene oxide structure-containing monomer units. In other words, for example, the water-soluble polymer of the second embodiment may contain both an ethylene oxide structure-containing monomer unit and a propylene oxide structure-containing monomer unit.

[0056] The integer n defining the repeating number of the monomer unit containing the structure that can be represented by the above formula (I) is preferably 10 or less, more preferably 5 or less, even more preferably 3 or less, and preferably 2 or more. That is, the alkylene oxide structure-containing monomer unit contained in the water-soluble polymer of the second embodiment preferably contains a polyalkylene oxide structural unit formed by n repeats of the alkylene oxide structural unit. Furthermore, some or all of the hydrogen atoms of the alkylene oxide structural unit may be substituted with any substituent.

[0057] When the water-soluble polymer of the second embodiment contains a plurality of alkylene oxide structure-containing monomer units, the repeat number n for each alkylene oxide structure-containing monomer unit may be the same or different. In this case, it is preferable that the number average value of all the repeat numbers n is within the above-mentioned preferred range, and it is more preferable that all the repeat numbers n are within the above-mentioned preferred range.

[0058] Examples of the alkylene oxide structure-containing monomer capable of forming the alkylene oxide structure-containing monomer unit of the water-soluble polymer of the second embodiment include monomers represented by the following general formula (II). [In general formula (II), R １ is a (meth)acryloyl group, and R ２represents a hydrogen atom or a linear or branched alkyl group having from 1 to 10 carbon atoms. The number of carbon atoms in the alkyl group is preferably from 2 to 5, preferably 2 or 3, and more preferably 2. In general formula (II), m and n are the same as m and n in general formula (I).] Examples of linear or branched alkyl groups having from 1 to 10 carbon atoms include a methyl group, an ethyl group, and a propyl group. More specifically, the monomer represented by general formula (II) is not particularly limited, and examples thereof include methoxypolyethylene glycol (meth)acrylate, ethoxypolyethylene glycol (meth)acrylate such as ethoxydiethylene glycol (meth)acrylate, polypropylene glycol mono(meth)acrylate, and methoxypolypropylene glycol (meth)acrylate. Among them, as the monomer represented by general formula (II), ethoxypolyethylene glycol (meth)acrylate is preferred, ethoxydiethylene glycol (meth)acrylate is more preferred, and ethoxydiethylene glycol acrylate is particularly preferred. In this specification, "(meth)acrylate" means acrylate or methacrylate, and "(meth)acryloyl" means acryloyl or methacryloyl.

[0059] The content of the alkylene oxide structure-containing monomer units in the water-soluble polymer of the second embodiment is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 20% by mass or more, even more preferably 30% by mass or more, preferably 60% by mass or less, more preferably 50% by mass or less, and even more preferably 45% by mass or less, based on 100 mol% of all repeating units contained in the water-soluble polymer of the second embodiment. The content of the alkylene oxide structure-containing monomer units in the water-soluble polymer of the second embodiment is preferably 5 mol% or more, more preferably 10 mol% or more, even more preferably 20 mol% or more, even more preferably 30 mol% or more, preferably 60 mol% or less, more preferably 50 mol% or less, and even more preferably 45 mol% or less, based on 100 mol% of all repeating units contained in the water-soluble polymer of the second embodiment. When the content ratio of the alkylene oxide structure-containing monomer unit in the water-soluble polymer of the second embodiment is within the above-mentioned predetermined range, it is possible to further improve the dispersibility and storage stability of the CNT dispersion, the ability to suppress thickening of the negative electrode slurry containing the CNT dispersion, and the cycle characteristics of a secondary battery equipped with a negative electrode produced using the negative electrode slurry.

[0060] [Other repeating units] The other repeating units that the water-soluble polymer of the second embodiment may contain, other than the sulfonic acid group-containing monomer units and alkylene oxide structure-containing monomer units described above, are not particularly limited, and include monomer units derived from known monomers (other monomers) copolymerizable with the sulfonic acid group-containing monomer units and alkylene oxide structure-containing monomer units described above. The other monomers may be used alone or in combination of two or more.

[0061] However, from the viewpoint of further improving the cycle characteristics of the secondary battery, the content of the other repeating units in the water-soluble polymer of the second embodiment is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0% by mass, where the total amount of repeating units in the water-soluble polymer of the second embodiment is 100% by mass. The content of the other repeating units in the water-soluble polymer of the second embodiment is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0% by mass, where the total amount of repeating units in the water-soluble polymer of the second embodiment is 100% by mass. In other words, the total content of the sulfonic acid group-containing monomer units and the alkylene oxide structure-containing monomer units in the water-soluble polymer of the second embodiment is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, even more preferably 99% by mass or more, and particularly preferably 100% by mass, based on 100% by mass of all repeating units contained in the water-soluble polymer of the second embodiment. Furthermore, the total content of the sulfonic acid group-containing monomer units and the alkylene oxide structure-containing monomer units in the water-soluble polymer of the second embodiment is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, even more preferably 99% by mass or more, and particularly preferably 100% by mass, based on 100% by mass of all repeating units contained in the water-soluble polymer of the second embodiment.

[0062] [Preparation Method] The preparation method of the water-soluble polymer is not particularly limited. The water-soluble polymer can be obtained, for example, by polymerizing a monomer composition containing one or more types of monomers in an aqueous solvent. The obtained polymer may be optionally hydrogenated. The content ratio of each monomer in the monomer composition can be determined based on the content ratio of the desired repeating unit (monomer unit) in the polymer. The polymerization method is not particularly limited, and any method such as solution polymerization, suspension polymerization, bulk polymerization, or emulsion polymerization can be used. Furthermore, any reaction such as ionic polymerization, radical polymerization, living radical polymerization, various condensation polymerizations, or addition polymerization can be used as the polymerization reaction. During polymerization, known emulsifiers and polymerization initiators can be used as needed. Furthermore, hydrogenation can be performed by known methods. Furthermore, after polymerization, neutralization can be performed with aqueous sodium hydroxide solution, aqueous lithium hydroxide solution, aqueous ammonia, or the like, as needed, to prepare the water-soluble polymer having the neutralized acid functional group described above.

[0063] <<Content Ratio>> The content ratio of the water-soluble polymer in the CNT dispersion is not particularly limited, but is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, and preferably 5.0% by mass or less, more preferably 2.0% by mass or less, even more preferably 1.0% by mass or less, and even more preferably 0.8% by mass or less, where the mass of the entire CNT dispersion is 100% by mass. If the content ratio of the water-soluble polymer in the CNT dispersion is equal to or greater than the above-mentioned lower limit, the dispersibility of the CNT dispersion can be improved. On the other hand, if the content ratio of the water-soluble polymer in the CNT dispersion is equal to or less than the above-mentioned upper limit, the storage stability of the CNT dispersion can be improved.

[0064] <<Mass Ratio (CNT / Water-Soluble Polymer)>> The mass ratio of CNT to water-soluble polymer in the CNT dispersion (CNT / Water-Soluble Polymer) is preferably 0.1 or more, more preferably 0.3 or more, even more preferably 0.5 or more, still more preferably 0.7 or more, and preferably 10 or less, more preferably 8 or less, even more preferably 5 or less, and still more preferably 3 or less. When the mass ratio of CNT to water-soluble polymer in the CNT dispersion (CNT / Water-Soluble Polymer) is within the above-mentioned specified range, the storage stability of the CNT dispersion can be improved.

[0065] <Other Components> The other components that the CNT dispersion may contain in addition to CNT, water-soluble polymer, and water are not particularly limited, but include conductive materials other than CNT, dispersion media other than water, and components other than the negative electrode active material described below in the section "Slurry for Non-Aqueous Secondary Battery Negative Electrodes." The conductive materials other than CNT are not particularly limited, but include, for example, carbon black (acetylene black, Ketjen Black (registered trademark), furnace black, etc.), graphite, carbon flakes, and carbon nanofibers. As the dispersion media other than water, known organic solvents that are compatible with water can be used. The other components may be used alone or in combination of two or more.

[0066] <Method for Preparing CNT Dispersion> The method for preparing the CNT dispersion is not particularly limited. The CNT dispersion can be prepared by mixing CNTs, a water-soluble polymer, water, and other components used as needed. Note that, for mixing, a known mixing device such as a disper, a homomixer, a planetary mixer, or a kneader can be used.

[0067] The CNTs used to prepare the CNT dispersion can be subjected to a dispersion treatment to adjust the average length within the above-mentioned predetermined range. By subjecting the CNTs to a dispersion treatment, some of the CNT fibers are cut, making it possible to adjust the average length of the CNTs after the dispersion treatment within the above-mentioned predetermined range. For the dispersion treatment, for example, a dispersion treatment device such as a ball mill, a bead mill, or a jet mill can be used. Note that these dispersion treatment devices may be wet or dry. From the viewpoint of easily adjusting the CNT length within the above-mentioned predetermined range, it is preferable to use a wet jet mill for the dispersion treatment.

[0068] When performing dispersion treatment to adjust the average length of CNTs, a pretreatment may involve performing dispersion treatment on CNTs alone or on a mixture of CNTs and a dispersion medium such as water, and then mixing the CNTs after the average length adjustment with other components such as a water-soluble polymer and water to prepare a CNT dispersion, or a dispersion treatment may be performed on a mixture of CNTs, a water-soluble polymer, and water as a treatment that also serves as the preparation of the CNT dispersion itself. From the perspective of improving the production efficiency of the CNT dispersion, the latter treatment, i.e., performing dispersion treatment on a mixture of CNTs, a water-soluble polymer, and water as a treatment that also serves as the preparation of the CNT dispersion itself, is preferred.

[0069] When a dispersion treatment using a wet jet mill is performed as a treatment that also serves as the preparation of a CNT dispersion, the CNT dispersion can be prepared, for example, by the following procedure and conditions. First, a mixture is obtained by mixing CNTs before the dispersion treatment, a water-soluble polymer, water, and other components used as needed. The mixing device described above can be used for mixing. The average length of the CNTs before the dispersion treatment is, for example, 500 μm or more and 2000 μm or less. The average diameter of the CNTs before the dispersion treatment is within the same range as the preferred range of the average diameter of the CNTs described in the "CNT" section. Next, the resulting mixture is subjected to a dispersion treatment using a wet jet mill, and the liquid after the dispersion treatment is obtained as a CNT dispersion. The pressure applied during the dispersion treatment of the mixture using a wet jet mill is preferably 100 MPa or more, more preferably 125 MPa or more, and preferably 200 MPa or less, more preferably 175 MPa or less. The number of passes (number of passes) of dispersion treatment using a wet jet mill is preferably at least one, and is preferably 5 to 20 passes. The temperature during dispersion treatment is preferably 0°C or higher and 80°C or lower. The minimum flow path diameter of the wet jet mill is preferably 100 μm or higher from the viewpoint of preventing clogging, and is preferably 500 μm or lower, more preferably 300 μm or lower from the viewpoint of effective pressure dispersion. Wet jet mills that can be used in dispersion treatment include "NanoVeita (registered trademark)" (manufactured by Yoshida Kikai Kogyo Co., Ltd.), "BERYU SYSTEM PRO" (manufactured by Biryu Co., Ltd.), ultra-high pressure wet atomization device (manufactured by Yoshida Kogyo Co., Ltd.), "Nanomizer (registered trademark)" (manufactured by Nanomizer Co., Ltd.), and "Starburst (registered trademark)" (manufactured by Sugino Machine Co., Ltd.).

[0070] <pH of CNT Dispersion> The pH of the CNT dispersion is preferably 6 or more, more preferably 7 or more, and even more preferably 7.5 or more, and is preferably 10 or less, more preferably 9 or less, and even more preferably 8.5 or less. If the pH of the CNT dispersion is within the above-mentioned predetermined range, the ability to suppress thickening of the negative electrode slurry containing the CNT dispersion can be further improved, and the cycle characteristics of a secondary battery including a negative electrode produced using the negative electrode slurry can be further improved.

[0071] (Non-aqueous secondary battery negative electrode slurry) The negative electrode slurry of the present invention contains the above-described CNT dispersion and a negative electrode active material, and optionally contains optional components such as a binder. In other words, the negative electrode slurry of the present invention contains CNTs whose average length and average diameter are each within the above-described predetermined ranges, a water-soluble polymer, and water, and optionally contains optional components such as a binder. In this way, the negative electrode slurry containing the above-described CNT dispersion is excellent in suppressing thickening and can form a negative electrode that can exhibit excellent cycle characteristics in a secondary battery.

[0072] <Negative Electrode Active Material> The negative electrode active material to be mixed into the negative electrode slurry is not particularly limited, and known negative electrode active materials can be used.

[0073] For example, the negative electrode active material used in the lithium ion secondary battery is not particularly limited, but examples thereof include carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials that are a combination of these.

[0074] Here, the carbon-based negative electrode active material refers to an active material having carbon as the main skeleton into which lithium can be inserted (also referred to as "doped"). Examples of the carbon-based negative electrode active material include carbonaceous materials and graphite materials.

[0075] Examples of carbonaceous materials include graphitizable carbon and non-graphitizable carbon, which has a structure similar to an amorphous structure, such as glassy carbon. Examples of graphitizable carbon include carbon materials made from tar pitch obtained from petroleum or coal. Specific examples include coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fiber, and pyrolytic vapor-grown carbon fiber. Examples of non-graphitizable carbon include phenolic resin calcined bodies, polyacrylonitrile-based carbon fibers, pseudoisotropic carbon, furfuryl alcohol resin calcined bodies (PFA), and hard carbon. Examples of graphitizable materials include natural graphite and artificial graphite. Examples of artificial graphite include artificial graphite obtained by heat-treating carbon containing easily graphitized carbon mainly at 2800°C or higher, graphitized MCMB obtained by heat-treating MCMB at 2000°C or higher, and graphitized mesophase pitch-based carbon fiber obtained by heat-treating mesophase pitch-based carbon fiber at 2000°C or higher.

[0076] Furthermore, the term "metal-based negative electrode active material" refers to an active material containing a metal, typically containing an element capable of inserting lithium in its structure, and having a theoretical electrical capacity per unit mass of 500 mAh / g or more when lithium is inserted. Examples of metal-based active materials include lithium metal, elemental metals capable of forming lithium alloys (e.g., Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, Ti, etc.), and alloys thereof, as well as oxides, sulfides, nitrides, silicides, carbides, phosphides, etc., thereof. Among these, silicon-based negative electrode active materials (negative electrode active materials containing silicon) are preferred as metal-based negative electrode active materials. This is because the use of silicon-based negative electrode active materials can increase the capacity of secondary batteries. Silicon-based negative electrode active materials undergo particularly large expansion and contraction during charge and discharge. However, since the negative electrode slurry of the present invention is prepared using the CNT dispersion liquid of the present invention described above, even when a silicon-based negative electrode active material is used as the negative electrode active material, a decrease in the conductivity of the negative electrode composite layer is suppressed, and a negative electrode that can exhibit excellent cycle characteristics in a secondary battery can be formed.

[0077] Examples of silicon-based negative electrode active materials include silicon (Si), silicon-containing alloys, SiO, and SiO ｘ and a composite of a Si-containing material and conductive carbon, which is obtained by coating or compounding a Si-containing material with conductive carbon.

[0078] The proportion of the silicon-based negative electrode active material in the negative electrode active material is preferably 1% by mass or more, more preferably 3% by mass or more, and preferably 20% by mass or less, and more preferably 15% by mass or less, based on 100% by mass of the entire negative electrode active material. If the proportion of the silicon-based negative electrode active material is 1% by mass or more, the capacity of the secondary battery can be sufficiently increased, and if it is 20% by mass or less, the cycle characteristics of the secondary battery can be further improved.

[0079] The particle size of the negative electrode active material is not particularly limited and can be the same as that of a conventionally used negative electrode active material. The amount of the negative electrode active material in the negative electrode slurry is also not particularly limited and can be within a range that is conventionally used. The negative electrode active material may be used alone or in combination with two or more materials. However, from the viewpoint of further improving the cycle characteristics while sufficiently increasing the capacity of the secondary battery, it is preferable that the negative electrode slurry contains both a carbon-based negative electrode active material made of a graphite material and a silicon-based negative electrode active material.

[0080] <CNT dispersion liquid> The CNT dispersion liquid of the present invention described above can be used as the CNT dispersion liquid. Here, the content of CNT in the negative electrode slurry is preferably 0.01 parts by mass or more, more preferably 0.05 parts by mass or more, and even more preferably 0.08 parts by mass or more, and preferably 0.5 parts by mass or less, more preferably 0.3 parts by mass or less, and even more preferably 0.15 parts by mass or less, when the content of the negative electrode active material is 100 parts by mass. Furthermore, the content of the water-soluble polymer in the negative electrode slurry is preferably 0.01 parts by mass or more, more preferably 0.05 parts by mass or more, and even more preferably 0.08 parts by mass or more, and preferably 0.5 parts by mass or less, more preferably 0.3 parts by mass or less, and even more preferably 0.15 parts by mass or less, when the content of the negative electrode active material is 100 parts by mass.

[0081] <Optional Components> Optional components that can be contained in the negative electrode slurry include, for example, a binder, a viscosity modifier, a reinforcing material, an antioxidant, and an electrolyte additive that has the function of suppressing decomposition of the electrolyte. These optional components may be used alone or in combination of two or more. Among the optional components described above, the negative electrode slurry preferably contains a binder from the viewpoint of improving the cycle characteristics of the secondary battery.

[0082] [Binder] The binder is not particularly limited, and any binder that can be used as a binder for a negative electrode can be used. In the present invention, from the viewpoint of further improving the cycle characteristics of a secondary battery equipped with a negative electrode prepared using the negative electrode slurry, it is preferable to use a particulate polymer containing at least a carboxylic acid group-containing monomer unit, an aromatic vinyl monomer unit, and a conjugated diene monomer unit as the binder. Note that the particulate polymer that can be used as the binder can be prepared by a known method.

[0083] [Carboxylic acid group-containing monomer unit] Examples of the carboxylic acid group-containing monomer capable of forming the carboxylic acid group-containing monomer unit of the particulate polymer include the same as the carboxylic acid group-containing monomer described above in the section "Water-soluble polymer of the first embodiment". The carboxylic acid group-containing monomer may be used alone or in combination of two or more.

[0084] Here, the content of the carboxylic acid group-containing monomer units in the particulate polymer is preferably 3% by mass or more, more preferably 5% by mass or more, and preferably 30% by mass or less, and more preferably 20% by mass or less, based on 100% by mass of all repeating units contained in the particulate polymer. If the content of the carboxylic acid group-containing monomer units in the particulate polymer is within the above-mentioned range, the cycle characteristics of a secondary battery including a negative electrode prepared using the negative electrode slurry can be further improved.

[0085] [Aromatic vinyl monomer unit] Examples of aromatic vinyl monomers that can form the aromatic vinyl monomer unit of the particulate polymer include styrene, styrene sulfonic acid and its salts, α-methylstyrene, p-t-butylstyrene, butoxystyrene, vinyltoluene, chlorostyrene, and vinylnaphthalene. Aromatic vinyl monomers may be used alone or in combination of two or more. Styrene is preferred as the aromatic vinyl monomer from the viewpoint of further improving the cycle characteristics of a secondary battery equipped with a negative electrode prepared using the negative electrode slurry. That is, the particulate polymer preferably contains a styrene unit as the aromatic vinyl monomer unit.

[0086] Here, the content of the aromatic vinyl monomer units in the particulate polymer is preferably 20% by mass or more, more preferably 25% by mass or more, and preferably 80% by mass or less, and more preferably 75% by mass or less, based on 100% by mass of all repeating units contained in the particulate polymer. If the content of the aromatic vinyl monomer units in the particulate polymer is within the above-mentioned range, the cycle characteristics of a secondary battery including a negative electrode prepared using the negative electrode slurry can be further improved.

[0087] [Conjugated diene monomer unit] Examples of the conjugated diene monomer capable of forming the conjugated diene monomer unit of the particulate polymer include the same conjugated diene monomers as those described above in the section "Water-soluble polymer of the first embodiment." One type of conjugated diene monomer may be used alone, or two or more types may be used in combination. From the viewpoint of further improving the cycle characteristics of a secondary battery equipped with a negative electrode produced using the negative electrode slurry, 1,3-butadiene is preferred as the conjugated diene monomer. That is, the particulate polymer preferably contains a 1,3-butadiene unit as the conjugated diene monomer unit.

[0088] Here, the content of the conjugated diene monomer units in the particulate polymer is preferably 15% by mass or more, more preferably 20% by mass or more, and preferably 50% by mass or less, and more preferably 45% by mass or less, based on 100% by mass of all repeating units contained in the particulate polymer. If the content of the conjugated diene monomer units in the particulate polymer is within the above range, the cycle characteristics of a secondary battery equipped with a negative electrode prepared using the negative electrode slurry can be further improved.

[0089] [Other repeating units] The other repeating units that the above-mentioned particulate polymer may contain other than the carboxylic acid group-containing monomer unit, aromatic vinyl monomer unit, and conjugated diene monomer unit are not particularly limited, and include monomer units derived from known monomers (other monomers) that are copolymerizable with the above-mentioned carboxylic acid group-containing monomer, aromatic vinyl monomer, and conjugated diene monomer.The other monomers may be used alone or in combination of two or more.

[0090] The content of other repeating units in the particulate polymer is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, still more preferably 1% by mass or less, and particularly preferably 0% by mass, based on 100% by mass of all repeating units contained in the particulate polymer.

[0091] The content of the particulate polymer in the negative electrode slurry is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 0.8 parts by mass or more, and preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and even more preferably 1.5 parts by mass or less, based on 100 parts by mass of the negative electrode active material. If the content of the particulate polymer is within the above-mentioned range, the cycle characteristics of a secondary battery including a negative electrode prepared using the negative electrode slurry can be further improved. Furthermore, the content of the particulate polymer in the negative electrode slurry is preferably 100 parts by mass or more, more preferably 500 parts by mass or more, and preferably 5,000 parts by mass or less, and even more preferably 2,000 parts by mass or less, per 100 parts by mass of CNTs. If the content of the particulate polymer is within the above-mentioned range, the cycle characteristics of a secondary battery including a negative electrode prepared using the negative electrode slurry can be further improved.

[0092] <Method for Preparing Slurry for Negative Electrode> When the above-described components are mixed to obtain the slurry for the negative electrode, the mixing method is not particularly limited, and the known mixing device described above in the "Method for Preparing CNT Dispersion Liquid" can be used.

[0093] <pH of negative electrode slurry> The pH of the negative electrode slurry is preferably 6 or more, more preferably 7 or more, and preferably 10 or less, and more preferably 9 or less. When the pH of the negative electrode slurry is within the above-mentioned predetermined range, it is possible to further improve the ability to suppress thickening of the negative electrode slurry and the cycle characteristics of a secondary battery including a negative electrode produced using the negative electrode slurry.

[0094] (Negative electrode for non-aqueous secondary battery) The negative electrode of the present invention includes a negative electrode composite layer formed using the above-described negative electrode slurry. The negative electrode of the present invention typically includes a current collector and a negative electrode composite layer formed on the current collector using the above-described negative electrode slurry. Here, the negative electrode composite layer includes a negative electrode active material, CNTs having an average length and an average diameter each within the above-described ranges, a water-soluble polymer, and optionally a binder, etc. The negative electrode of the present invention includes a negative electrode composite layer formed using the above-described negative electrode slurry, and therefore can provide excellent cycle characteristics for the secondary battery.

[0095] <Current Collector> The current collector is made of a material that is electrically conductive and electrochemically durable. Specifically, current collectors made of, for example, iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, etc. can be used as the current collector. Among them, copper foil is preferred as the current collector used in the negative electrode of a lithium ion secondary battery from the viewpoint of improving the peel strength of the negative electrode. Note that the above materials constituting the current collector may be used alone or in combination of two or more.

[0096] <Method for Manufacturing Negative Electrode> The method for manufacturing the negative electrode of the present invention is not particularly limited. For example, the negative electrode of the present invention can be manufactured by applying the above-described negative electrode slurry of the present invention to at least one surface of a current collector and drying the slurry to form a negative electrode composite layer. More specifically, the manufacturing method includes a step of applying the negative electrode slurry to at least one surface of the current collector (application step), and a step of drying the negative electrode slurry applied to at least one surface of the current collector to form a negative electrode composite layer on the current collector (drying step).

[0097] [Coating Step] The method for applying the negative electrode slurry onto the current collector is not particularly limited, and known methods can be used. Specifically, examples of the coating method include a doctor blade method, a dipping method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method. In this case, the negative electrode slurry may be applied to only one side of the current collector, or may be applied to both sides. The thickness of the slurry film on the current collector after application and before drying can be appropriately set depending on the thickness of the negative electrode composite layer obtained by drying.

[0098] [Drying Step] The method for drying the negative electrode slurry on the current collector is not particularly limited and any known method can be used, for example, drying with warm air, hot air, or low-humidity air, vacuum drying, or drying by irradiation with infrared rays or electron beams, etc. By drying the negative electrode slurry on the current collector in this manner, a negative electrode composite layer can be formed on the current collector, and a negative electrode including the current collector and the negative electrode composite layer can be obtained.

[0099] After the drying step, the negative electrode mixture layer may be subjected to a pressure treatment using a mold press, a roll press, or the like. The pressure treatment allows the negative electrode mixture layer to be well adhered to the current collector. Furthermore, when the negative electrode mixture layer contains a curable polymer, the polymer may be cured after the formation of the negative electrode mixture layer.

[0100] (Nonaqueous secondary battery) The secondary battery of the present invention includes the above-described negative electrode of the present invention. Since the secondary battery of the present invention includes the negative electrode of the present invention, it has excellent cycle characteristics. The secondary battery of the present invention is preferably, for example, a lithium-ion secondary battery.

[0101] Hereinafter, the configuration of a lithium ion secondary battery will be described as an example of the secondary battery of the present invention. This lithium ion secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode is the above-described negative electrode for a nonaqueous secondary battery of the present invention.

[0102] <Positive Electrode> The positive electrode is not particularly limited, and any known positive electrode can be used.

[0103] <Electrolyte> As the electrolyte, an organic electrolyte solution in which a supporting electrolyte is dissolved in an organic solvent is usually used. As the supporting electrolyte, for example, a lithium salt is used. As the lithium salt, for example, LiPF ６ , LiAsF ６ , LiBF ４ , LiSbF ６ , LiAlCl ４ , LiClO ４ , C.F. ３ SO ３ Li, C ４ F ９ SO ３ Li, CF ３ COOLi, (CF ３ CO) ２ NLi, (CF ３ SO ２ ) ２ NLi, (C ２ F ５ SO ２ Among them, LiPF is preferred because it is easily soluble in solvents and shows a high degree of dissociation. ６ , LiClO ４ , C.F. ３ SO ３ Li is preferred, and LiPF ６ is particularly preferred. Note that one type of electrolyte may be used alone, or two or more types may be used in combination in any ratio. Generally, the lithium ion conductivity tends to increase as the supporting electrolyte with a higher degree of dissociation is used, so the lithium ion conductivity can be adjusted by the type of supporting electrolyte.

[0104] The organic solvent used in the electrolyte is not particularly limited as long as it can dissolve the supporting electrolyte. For example, carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), and methyl ethyl carbonate (EMC) are preferably used. Other suitable solvents include carbonates such as γ-butyrolactone and methyl formate, ethers such as 1,2-dimethoxyethane and tetrahydrofuran, and sulfur-containing compounds such as sulfolane and dimethyl sulfoxide. Mixtures of these solvents may also be used. Among these, carbonates are preferred because of their high dielectric constant and wide stable potential range, and a mixture of ethylene carbonate and ethyl methyl carbonate is even more preferred. The concentration of the electrolyte in the electrolyte can be adjusted as appropriate. For example, it is preferably 0.5 to 15% by mass, more preferably 2 to 13% by mass, and even more preferably 5 to 10% by mass. The electrolyte may also contain known additives such as fluoroethylene carbonate and ethyl methyl sulfone.

[0105] <Separator> The separator is not particularly limited, and for example, those described in JP 2012-204303 A can be used. Among these, a microporous film made of a polyolefin resin (polyethylene, polypropylene, polybutene, polyvinyl chloride) is preferred because it allows the thickness of the entire separator to be thin, thereby increasing the proportion of electrode active material in the lithium ion secondary battery and increasing the capacity per volume.

[0106] <Method for manufacturing lithium-ion secondary battery> The lithium-ion secondary battery according to the present invention can be manufactured, for example, by stacking a positive electrode and a negative electrode with a separator interposed therebetween, rolling or folding the resulting assembly as necessary according to the battery shape, placing it in a battery container, injecting an electrolyte into the battery container, and sealing it. To prevent internal pressure rise, overcharge / discharge, and the like, a fuse, an overcurrent prevention element such as a PTC element, an expanded metal, a lead plate, or the like may be provided as necessary. The shape of the secondary battery may be any of a coin type, a button type, a sheet type, a cylindrical type, a rectangular type, a flat type, and the like.

[0107] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples. In the following description, the terms "%" and "parts" used to represent amounts are based on mass unless otherwise specified. In addition, in a polymer produced by copolymerizing multiple types of monomers, the proportion of a monomer unit formed by polymerizing a certain monomer in the polymer usually corresponds to the ratio (feed ratio) of that certain monomer to all the monomers used in the polymerization of the polymer, unless otherwise specified. In the examples and comparative examples, the average length and average diameter of CNTs, the dispersibility and storage stability of CNT dispersions, the viscosity suppression of negative electrode slurries, the peel strength of negative electrode composite layers, and the cycle characteristics of secondary batteries were evaluated using the following methods.

[0108] <Average Length and Average Diameter of CNTs> A coating film formed by applying a CNT dispersion to a copper foil as a measurement sample was observed using a scanning electron microscope (SEM). 100 CNTs were randomly selected from the obtained SEM image, and their lengths and diameters (outer diameters) were measured. The average values ​​of the lengths and diameters (outer diameters) of the 100 CNTs were then used as the average length and average diameter of the CNTs in the CNT dispersion (i.e., after dispersion treatment). Furthermore, measurements were performed using the same method as above, except that a mixed liquid obtained by mixing CNTs before dispersion treatment using a wet jet mill with an appropriate amount of ion-exchanged water was used as the measurement sample instead of the CNT dispersion, and the average length and average diameter of the CNTs before dispersion treatment were determined.

[0109] <Dispersibility of CNT Dispersion> The volume average particle diameter D50 of the CNT dispersion was measured in a wet state using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT-3300EXII, manufactured by Microtrac Bell) in accordance with JIS Z8825:2013. The smaller the volume average particle diameter D50 value, the better the dispersibility. A: Volume average particle diameter D50 is less than 15 μm B: Volume average particle diameter D50 is 15 μm or more and less than 50 μm C: Volume average particle diameter D50 is 50 μm or more

[0110] <Storage Stability of CNT Dispersion> The viscosity η1 of the CNT dispersion immediately after preparation was measured using a Brookfield viscometer at a temperature of 25°C and a spindle rotation speed of 60 rpm, 60 seconds after the start of spindle rotation. After measuring η1, the CNT dispersion was stored at 25°C for 10 days under static conditions, and the viscosity η2 after storage was measured in the same manner as for viscosity η1. The ratio of η2 to η1 (η2 / η1) was taken as the viscosity ratio of the dispersion, and was evaluated according to the following criteria. The closer the viscosity ratio of the dispersion is to 1.0, the more the viscosity increase of the CNT dispersion is suppressed and the better the storage stability is. A: Viscosity ratio of the dispersion is less than 1.15 B: Viscosity ratio of the dispersion is 1.15 or more but less than 1.6 C: Viscosity ratio of the dispersion is 1.6 or more

[0111] <Thickening suppression of negative electrode slurry> As measurement samples, a negative electrode slurry and a control slurry obtained in the same manner as the preparation of the negative electrode slurry except that no CNT dispersion liquid was added were prepared. The viscosity η3 of the control slurry immediately after preparation was measured using a B-type viscometer at a temperature of 25°C and a spindle rotation speed of 60 rpm, 60 seconds after the start of spindle rotation. The viscosity η4 of the negative electrode slurry immediately after preparation was measured using the same procedures and conditions as above. The ratio of η4 to η3 (η4 / η3) was then defined as the slurry viscosity ratio, and evaluated according to the following criteria. The closer the slurry viscosity ratio value is to 1.0, the more the viscosity increase of the negative electrode slurry due to the addition of the CNT dispersion liquid is suppressed, indicating that the negative electrode slurry has excellent thickening suppression properties. A: Slurry viscosity ratio is less than 1.5 B: Slurry viscosity ratio is 1.5 or more but less than 3 C: Slurry viscosity ratio is 3 or more

[0112] <Negative Electrode Peel Strength> The prepared negative electrode was cut into a rectangle 100 mm long and 10 mm wide to prepare a test specimen. This test specimen was placed with the surface of the negative electrode composite layer facing down, and cellophane tape was attached to the surface of the negative electrode composite layer. The cellophane tape used was specified in JIS Z1522. The cellophane tape was fixed to a test table. One end of the current collector was then pulled vertically upward at a pulling rate of 150 mm / min to measure the stress when peeled off. This measurement was performed three times, and the average value was calculated. This average value was used as the peel strength of the negative electrode and evaluated according to the following criteria. A higher peel strength of the negative electrode indicates a stronger adhesive force of the negative electrode composite layer to the current collector, i.e., a higher adhesion strength. A: Peel strength of 8 N / m or more B: Peel strength of 5 N / m or more but less than 8 N / m C: Peel strength of less than 5 N / m

[0113] <Cycle Characteristics of Secondary Battery> After injecting the electrolyte, the secondary battery was left to stand for 24 hours in an environment at 25°C. Next, a charge / discharge operation was performed in which the cell voltage was charged to 4.35 V by constant current-constant voltage charging at 0.2 C (cutoff 0.02 C) and then discharged at a constant current to 2.75 V, and the initial capacity C0 was measured. Furthermore, in an environment at 25°C, the battery was charged to 4.35 V by constant current-constant voltage charging at 1.0 C (cutoff 0.02 C) and then discharged at a constant current to 2.75 V, and the capacity C1 after 50 cycles was measured. The capacity retention rate (%) = C1 / C0 × 100 was calculated and evaluated according to the following criteria. A higher capacity retention rate indicates better cycle characteristics of the secondary battery. A: Capacity retention rate is 90% or more B: Capacity retention rate is 85% or more but less than 90% C: Capacity retention rate is less than 85%

[0114] Example 1 Preparation of Water-Soluble Polymer (Dispersant) A reactor was charged with 473 parts of ion-exchanged water, 58 parts of methacrylic acid (a carboxylic acid group-containing monomer), 0.6 parts of t-dodecyl mercaptan, and 3.0 parts of sodium dodecylbenzenesulfonate diluted with ion-exchanged water to a solids concentration of 10%. The reactor was then sealed, and nitrogen purge was performed twice while stirring with a stirring blade. After completion of the nitrogen purge, 42 parts of nitrogen-purged isoprene (a conjugated diene monomer) were charged into the reactor. The temperature inside the reactor was then controlled to 5°C. After confirming that the temperature inside the reactor was controlled to 5°C, 0.01 parts of hydrosulfite was dissolved in ion-exchanged water and added to the reactor. Five minutes after the addition of hydrosulfite, 0.1 parts of cumene hydroperoxide (first addition) was added. Furthermore, using another container, a solution of 0.04 parts (first addition) of sodium formaldehyde sulfoxylate (manufactured by Mitsubishi Gas Chemical Company, Inc., product name "SFS"), 0.003 parts (first addition) of ferrous sulfate (manufactured by Chubu Chelest Co., Ltd., product name "Frost Fe"), and 0.03 parts of ethylenediaminetetraacetic acid (manufactured by Chubu Chelest Co., Ltd., product name "Chelest 400G") in 9.0 parts of ion-exchanged water was added to the reactor. When the polymerization conversion rate reached 40%, the temperature inside the reactor was raised to 10°C. Thereafter, after the polymerization conversion rate reached 60%, the temperature inside the reactor was raised to 18°C. Thereafter, when the polymerization conversion rate reached 70%, 0.09 parts (second addition) of cumene hydroperoxide was added to the reactor. Furthermore, using another container, a solution of 0.04 parts (second time) of sodium formaldehyde sulfoxylate (manufactured by Mitsubishi Gas Chemical Company, Inc., product name "SFS"), 0.003 parts (second time) of ferrous sulfate (manufactured by Chubu Chelest Co., Ltd., product name "Frost Fe"), and 0.03 parts of ethylenediaminetetraacetic acid (manufactured by Chubu Chelest Co., Ltd., product name "Chelest 400G") dissolved in 9.0 parts of ion-exchanged water was added to the reactor. After the polymerization conversion rate reached 93%, 0.12 parts of 2,2,6,6-tetramethylpiperidine-1-oxyl diluted with 10.35 parts of ion-exchanged water was added to the reactor to terminate the reaction. After the reaction was terminated, the mixture was deodorized using an evaporator until the residual isoprene level was 300 ppm or less.After the deodorization was completed, the pH was adjusted to 8 with a 5% aqueous sodium hydroxide solution while stirring, to obtain an aqueous solution of a water-soluble polymer (dispersant).

[0115] <Preparation of CNT Dispersion> 0.4 parts of multi-walled CNT (1) (average length before dispersion treatment: 1000 μm, average diameter before dispersion treatment: 50 nm, G / D ratio: 2.8), 0.4 parts of the water-soluble polymer (solid content equivalent) prepared above as a dispersant, and an appropriate amount of ion-exchanged water were stirred in a disperser (3000 rpm, 60 minutes), and then dispersed using a wet jet mill (manufactured by Yoshida Kikai Kogyo Co., Ltd., product name "NanoVata"). The conditions for the dispersion treatment using the wet jet mill were a pressure of 150 MPa and 10 treatments. The pH was then adjusted to 8 using a 5% aqueous sodium hydroxide solution to produce a CNT dispersion (CNT content = 0.4%, water-soluble polymer content = 0.4%). The average length and average diameter of the CNTs in the resulting CNT dispersion (i.e., after dispersion treatment) were measured, and the average length and average diameter of the CNTs were found to be 10 μm and 50 nm, respectively. The resulting CNT dispersion was also evaluated for dispersibility and storage stability. The results are shown in Table 1.

[0116] <Preparation of Particulate Polymer (Binder)> 3.15 parts of styrene, 1.66 parts of 1,3-butadiene, 0.2 parts of sodium lauryl sulfate as an emulsifier, 20 parts of ion-exchanged water, and 0.03 parts of potassium persulfate as a polymerization initiator were placed in a 5 MPa pressure vessel A equipped with a stirrer, and after thorough stirring, the mixture was heated to 60 ° C. to initiate polymerization and reacted for 6 hours to obtain seed particles. After the above reaction, the mixture was heated to 75 ° C., and from another vessel B containing 53.85 parts of styrene, 31.34 parts of 1,3-butadiene, 10.0 parts of acrylic acid, 0.25 parts of tert-dodecyl mercaptan as a chain transfer agent, and 0.35 parts of sodium lauryl sulfate as an emulsifier, the addition of this mixture to pressure vessel A was initiated. At the same time, the addition of 1 part of potassium persulfate as a polymerization initiator to pressure vessel A was initiated to initiate the second-stage polymerization. That is, the total monomer composition used was 57 parts of styrene, 33 parts of 1,3-butadiene, and 10 parts of acrylic acid. Five and a half hours after the start of the second-stage polymerization, the addition of the entire mixture containing these monomer compositions was completed, and then the mixture was further heated to 85°C and reacted for 6 hours. When the polymerization conversion rate reached 97%, the mixture was cooled to stop the reaction. A 5% aqueous solution of sodium hydroxide was added to the mixture containing this polymer to adjust the pH to 8. Thereafter, unreacted monomers were removed by heating and distillation under reduced pressure. Thereafter, the mixture was further cooled to obtain an aqueous dispersion of a water-insoluble particulate polymer.

[0117] <Preparation of negative electrode slurry> Artificial graphite (volume average particle diameter: 24.5 μm, specific surface area: 3.5 m) was added as a carbon-based negative electrode active material to a planetary mixer equipped with a disperser. ２ / g) 90 parts, and SiO as a silicon-based negative electrode active material ｘ10 parts of the CNT dispersion liquid and 2.0 parts (solids equivalent) of an aqueous solution of carboxymethyl cellulose as a viscosity modifier were added, and the solids concentration was adjusted to 58% with ion-exchanged water. The mixture was then mixed at room temperature for 60 minutes. After mixing, the CNT dispersion liquid obtained as described above was added to the planetary mixer so that the multi-walled CNTs accounted for 0.1 parts (solids equivalent) and mixed. The solids concentration was then adjusted to 50% with ion-exchanged water, and 1.0 part (solids equivalent) of the aqueous dispersion of particulate polymer (binder) obtained as described above was added to obtain a mixed liquid. The resulting mixed liquid was degassed under reduced pressure to obtain a negative electrode slurry with good fluidity. The pH of the resulting negative electrode slurry was measured and found to be 8. The thickening suppression ability of the resulting negative electrode slurry was evaluated. The results are shown in Table 1.

[0118] <Production of Negative Electrode> The negative electrode slurry obtained as described above was coated on a copper foil having a thickness of 16 μm as a current collector using a comma coater so that the film thickness after drying was 105 μm and the coating amount was 10 mg / cm. ２ The negative electrode slurry was applied so that the thickness of the negative electrode slurry was 100°C. The copper foil coated with the negative electrode slurry was transported at a speed of 0.5 m / min through an oven at 100°C for 2 minutes, and then through an oven at 120°C for 2 minutes to dry the negative electrode slurry on the copper foil, thereby obtaining a negative electrode blank. The negative electrode blank was rolled using a roll press to obtain a negative electrode having a negative electrode composite layer thickness of 80 μm. The peel strength of the obtained negative electrode was evaluated. The results are shown in Table 1.

[0119] <Production of Positive Electrode> A planetary mixer was used to prepare a positive electrode active material, LiCoO ２95 parts of the above, 3 parts of PVDF (polyvinylidene fluoride) as a binder in terms of solids equivalent, 2 parts of acetylene black as a conductive material, and 20 parts of N-methylpyrrolidone as a solvent were added and mixed to obtain a positive electrode slurry. The obtained positive electrode slurry was applied to a 20 μm thick aluminum foil (current collector) using a comma coater so that the film thickness after drying would be about 100 μm. The aluminum foil coated with the positive electrode slurry was conveyed at a speed of 0.5 m / min in an oven at a temperature of 60 ° C. for 2 minutes, and then in an oven at a temperature of 120 ° C. for 2 minutes, thereby drying the positive electrode slurry on the aluminum foil and obtaining a positive electrode raw sheet. This positive electrode raw sheet was rolled using a roll press to obtain a positive electrode with a positive electrode composite layer thickness of 70 μm.

[0120] <Preparation of Separator> A single-layer polypropylene separator (manufactured by a dry method, width 65 mm, length 500 mm, thickness 25 μm, porosity 55%) was prepared. This separator was cut into a 5 cm × 5 cm square and used in the production of a secondary battery.

[0121] <Manufacture of Secondary Battery> An aluminum packaging material was prepared as the battery packaging. The positive electrode was cut into a 4 cm x 4 cm square and placed so that the surface on the current collector side was in contact with the aluminum packaging material. Next, the square separator was placed on the surface of the positive electrode composite layer of the positive electrode. Furthermore, the negative electrode was cut into a 4.2 cm x 4.2 cm square and placed on the separator so that the surface on the negative electrode composite layer side faced the separator. Then, 1.0 M LiPF 6 was used as the electrolyte. ６ A solution (the solvent was a mixed solvent of ethylene carbonate / diethyl carbonate = 1 / 2 (volume ratio), containing 2 volume % each of fluoroethylene carbonate and vinylene carbonate (solvent ratio) as additives) was filled into the battery. Furthermore, in order to seal the opening of the aluminum packaging, the aluminum packaging exterior was closed by heat sealing at 150°C, thereby producing a laminate cell type lithium ion secondary battery. The cycle characteristics of the obtained lithium ion secondary battery were evaluated. The results are shown in Table 1.

[0122] (Example 2) Various operations, measurements, and evaluations were carried out in the same manner as in Example 1, except that 0.4 parts of carboxymethylcellulose sodium (sodium salt of carboxymethylcellulose) was used as the dispersant instead of 0.4 parts of the water-soluble polymer when preparing the CNT dispersion. The results are shown in Table 1.

[0123] (Example 3) When preparing the CNT dispersion, the number of treatments in the wet jet mill was changed from 10 to 20, thereby adjusting the average length of the CNTs in the resulting CNT dispersion from 10 μm to 5 μm. Except for this, various operations, measurements, and evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

[0124] (Example 4) When preparing the CNT dispersion, the number of treatments in the wet jet mill was changed from 10 to 5, thereby adjusting the average length of the CNTs in the resulting CNT dispersion (i.e., after the dispersion treatment) from 10 μm to 100 μm. Except for this, various operations, measurements, and evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

[0125] (Example 5) When preparing a CNT dispersion, multi-walled CNT (2) (average length before dispersion: 1000 μm, average diameter before dispersion: 20 nm, G / D ratio: 2.8) was used instead of multi-walled CNT (1) (average length before dispersion: 1000 μm, average diameter before dispersion: 50 nm, G / D ratio: 2.8), thereby adjusting the average diameter of the CNTs in the resulting CNT dispersion from 50 nm to 20 nm. Except for this, various operations, measurements, and evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

[0126] (Example 6) When preparing a CNT dispersion, multi-walled CNT (3) (average length before dispersion: 1000 μm, average diameter before dispersion: 100 nm, G / D ratio: 2.8) was used instead of multi-walled CNT (1) (average length before dispersion: 1000 μm, average diameter before dispersion: 50 nm, G / D ratio: 2.8), thereby adjusting the average diameter of the CNTs in the resulting CNT dispersion from 50 nm to 100 nm. Various operations, measurements, and evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

[0127] (Example 7) In the preparation of the water-soluble polymer and the preparation of the CNT dispersion, the pH was adjusted using a 5% aqueous solution of lithium hydroxide instead of a 5% aqueous solution of sodium hydroxide, and the other operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0128] (Example 8) In preparing the water-soluble polymer, various operations, measurements, and evaluations were carried out in the same manner as in Example 1, except that 59 parts of sodium styrenesulfonate (a sulfonic acid group-containing monomer) and 41 parts of ethoxydiethylene glycol acrylate (an alkylene oxide structure-containing monomer) were used instead of isoprene and methacrylic acid. The results are shown in Table 1.

[0129] (Comparative Example 1) When preparing a CNT dispersion, multi-walled CNT (4) (average length before dispersion: 60 μm, average diameter before dispersion: 12 nm, G / D ratio: 1.3) was used instead of multi-walled CNT (1) (average length before dispersion: 1000 μm, average diameter before dispersion: 50 nm, G / D ratio: 2.8), thereby adjusting the average length of the CNTs in the resulting CNT dispersion from 10 μm to 1 μm and the average diameter from 50 nm to 12 nm. Except for this, various operations, measurements, and evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

[0130] (Comparative Example 2) When preparing a CNT dispersion, multi-walled CNT (3) (average length before dispersion: 100 μm, average diameter before dispersion: 50 nm, G / D ratio: 2.8) was used instead of multi-walled CNT (1) (average length before dispersion: 1000 μm, average diameter before dispersion: 50 nm, G / D ratio: 2.8), thereby adjusting the average length of the CNTs in the resulting CNT dispersion from 10 μm to 1 μm. Various operations, measurements, and evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

[0131] Comparative Example 3 When preparing a CNT dispersion, single-walled CNTs (average length before dispersion: 5 μm, average diameter before dispersion: 2 nm, G / D ratio: 84) were used instead of multi-walled CNTs (1) (average length before dispersion: 1000 μm, average diameter before dispersion: 50 nm, G / D ratio: 2.8), thereby adjusting the average length of the CNTs in the resulting CNT dispersion from 10 μm to 1.2 μm and the average diameter from 50 nm to 2 nm. Except for this, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0132] (Comparative Example 4) When preparing a CNT dispersion, multi-walled CNT (5) (average length before dispersion: 1000 μm, average diameter before dispersion: 150 nm, G / D ratio: 2.8) was used instead of multi-walled CNT (1) (average length before dispersion: 1000 μm, average diameter before dispersion: 50 nm, G / D ratio: 2.8), thereby adjusting the average diameter of the CNTs in the resulting CNT dispersion from 50 nm to 150 nm. Except for this, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0133] Comparative Example 5 When preparing a CNT dispersion, multi-walled CNT (4) (average length before dispersion: 60 μm, average diameter before dispersion: 12 nm, G / D ratio: 1.3) was used instead of multi-walled CNT (1) (average length before dispersion: 1000 μm, average diameter before dispersion: 50 nm, G / D ratio: 2.8), and the number of treatments in the wet jet mill was changed from 10 to 5, thereby adjusting the average length of the CNTs in the resulting CNT dispersion from 10 μm to 5 μm and the average diameter from 50 nm to 12 nm. Except for this, various operations, measurements, and evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

[0134] (Comparative Example 6) When preparing the CNT dispersion, the number of treatments in the wet jet mill was changed from 10 to 3, thereby adjusting the average length of the CNTs in the resulting CNT dispersion from 10 μm to 200 μm. Except for this, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0135] In Table 1, "IP" represents an isoprene unit, "MAA" represents a methacrylic acid unit, "CMC" represents a sodium salt of carboxymethyl cellulose, "SS" represents a sodium styrene sulfonate unit, "EC-A" represents an ethoxydiethylene glycol acrylate unit, "Li" represents a lithium salt, and "Na" represents a sodium salt.

[0136]

[0137] Table 1 shows that the CNT dispersions of Examples 1 to 8, which contain CNTs with average lengths and average diameters within the specified ranges, a water-soluble polymer, and water, can prepare slurries for nonaqueous secondary battery negative electrodes with excellent viscosity suppression, and can also provide excellent cycle characteristics for nonaqueous secondary batteries. On the other hand, the CNT dispersions of Comparative Examples 1 and 3, which contain CNTs with average lengths and average diameters that are below the specified ranges, produce slurries for nonaqueous secondary battery negative electrodes with poor viscosity suppression. Furthermore, the CNT dispersion of Comparative Example 2, which contains CNTs with average lengths below the specified ranges, produces nonaqueous secondary batteries with poor cycle characteristics. Furthermore, the CNT dispersion of Comparative Example 4, which contains CNTs with average lengths within the specified ranges but average diameters that exceed the specified ranges, produces nonaqueous secondary batteries with poor cycle characteristics. Furthermore, it is found that the prepared non-aqueous secondary battery negative electrode slurry is inferior in viscosity increase suppression property in the case of the CNT dispersion of Comparative Example 5, which contains CNTs whose average length is within a predetermined range but whose average diameter is below the predetermined range. Furthermore, it is found that the prepared non-aqueous secondary battery negative electrode slurry is inferior in viscosity increase suppression property in the case of the CNT dispersion of Comparative Example 6, which contains CNTs whose average diameter is within a predetermined range but whose average length exceeds the predetermined range.

[0138] According to the present invention, it is possible to prepare a slurry for a non-aqueous secondary battery negative electrode that exhibits excellent viscosity increase suppression, and it is possible to provide a carbon nanotube dispersion that can enable the non-aqueous secondary battery to exhibit excellent cycle characteristics. Furthermore, according to the present invention, it is possible to provide a slurry for a non-aqueous secondary battery negative electrode that exhibits excellent viscosity increase suppression, and it is possible to enable the non-aqueous secondary battery to exhibit excellent cycle characteristics. Furthermore, according to the present invention, it is possible to provide a non-aqueous secondary battery negative electrode that can enable the non-aqueous secondary battery to exhibit excellent cycle characteristics. Furthermore, according to the present invention, it is possible to provide a non-aqueous secondary battery that can exhibit excellent cycle characteristics.

Claims

1. A carbon nanotube dispersion containing carbon nanotubes, a water-soluble polymer, and water, The carbon nanotube dispersion is characterized by having an average length of 5 μm or more and an average diameter of 20 nm or more and 100 nm or less.

2. The carbon nanotube dispersion according to claim 1, wherein the water-soluble polymer has an acidic functional group.

3. The carbon nanotube dispersion according to claim 2, wherein at least a portion of the acidic functional groups of the water-soluble polymer are alkali metal bases or ammonium bases.

4. The carbon nanotube dispersion according to claim 1, wherein the mass ratio of the carbon nanotubes to the water-soluble polymer is 0.1 or more and 10 or less.

5. A carbon nanotube dispersion according to claim 1, wherein the pH is 6 or higher and 10 or lower.

6. A slurry for a negative electrode of a non-aqueous secondary battery, comprising a negative electrode active material and a carbon nanotube dispersion according to any one of claims 1 to 5.

7. The slurry for a non-aqueous secondary battery anode according to claim 6, wherein the anode active material includes a silicon-based anode active material.

8. It further contains particulate polymer, The slurry for a negative electrode of a non-aqueous secondary battery according to claim 6, wherein the particulate polymer comprises carboxylic acid group-containing monomer units, aromatic vinyl monomer units, and conjugated diene monomer units.

9. The slurry for a negative electrode of a non-aqueous secondary battery according to claim 8, wherein the content ratio of the carboxylic acid group-containing monomer units in the particulate polymer is 3% by mass or more and 30% by mass or less, with the total repeating units contained in the particulate polymer being 100% by mass.

10. A negative electrode for a non-aqueous secondary battery, comprising a negative electrode composite layer formed using the slurry for a non-aqueous secondary battery negative electrode described in claim 6.

11. A non-aqueous secondary battery comprising the negative electrode for a non-aqueous secondary battery described in claim 10.