Carbon nanotube dispersion composition, composite slurry, electrode film, secondary battery, and vehicle

Abstract

To provide: a carbon nanotube dispersion composition with excellent dispersibility and low initial viscosity, where the carbon nanotube dispersion composition enables provision of a composite slurry for forming an electrode film with high conductivity and adhesion; and more specifically, a secondary battery exhibiting excellent rate characteristics and high-temperature cycle characteristics, and a vehicle comprising the secondary battery thereby having high safety and improved fuel efficiency.SOLUTION: A carbon nanotube dispersion composition comprises carbon nanotubes, a copolymer, and an amide-based polar solvent, where the content of metallic foreign particles measured under a specific condition 1 is 1.0 mg or less. The copolymer comprises: 50 mass% or more and 75 mass% or less of alkylene structural units; and 25 mass% or more and 50 mass% or less of monomer units comprising a nitrile group.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a carbon nanotube dispersion composition. More specifically, the present invention relates to a carbon nanotube dispersion composition containing carbon nanotubes, a copolymer containing monomer units having an alkylene structural unit and a nitrile group, and an amide-based organic solvent, a composite material slurry containing the carbon nanotube dispersion composition and an active material, an electrode film formed therefrom, a secondary battery provided with the electrode film, and a vehicle provided with the secondary battery.

Background Art

[0002] With the spread of electric vehicles and the miniaturization, light weight, and high performance of portable devices, there is a demand for secondary batteries having a high energy density, and further, an increase in the capacity of such secondary batteries. Under such circumstances, non-aqueous electrolyte secondary batteries using non-aqueous electrolytes, particularly lithium ion secondary batteries, have come to be used in many devices due to their characteristics of high energy density and high voltage.

[0003] As a negative electrode material used in these lithium ion secondary batteries, carbon materials typified by graphite, which have a low potential close to lithium (Li) and a large charge-discharge capacity per unit mass, are used. However, these electrode materials are used up to a point where the charge-discharge capacity per mass is close to the theoretical value, and the energy density per mass of the battery is approaching its limit. Therefore, in order to increase the utilization rate of the electrode, studies are underway to reduce conductive aids and binders that do not contribute to the discharge capacity.

[0004] The conductive aid plays a role of forming a conductive path inside the electrode, and it is required that the conductive aid is not easily cut due to the expansion and contraction of the electrode film. In order to maintain the conductive path with a small amount of conductive aid, it is effective to use a carbon material having a large specific surface area, particularly carbon nanotubes, which are one type of nanocarbon. However, although batteries using carbon nanotubes have excellent output characteristics, metal foreign substances derived from the metal catalyst used in the synthesis of carbon nanotubes often remain, and there has been a problem of voltage drop failure.

[0005] In addition, the metal foreign particles contained in the carbon nanotube dispersion composition may dissolve inside the secondary battery, deposit in a dendrite shape, pierce through the separator, and cause internal short circuits. Furthermore, when the content of metal foreign particles is high, deterioration of the electrolyte and the active material is likely to occur due to high-temperature charge and discharge of the secondary battery, and deterioration of the high-temperature cycle characteristics becomes a problem. Therefore, it is required to reduce the amount of metal foreign particles contained in the carbon nanotube dispersion composition.

[0006] Therefore, in Patent Document 1, a method of crushing carbon nanotubes containing metal foreign particles such as iron, cobalt, and nickel and removing the metal foreign particles of the carbon nanotubes using an electromagnet has been studied.

[0007] In Patent Documents 2 and 3, techniques for removing metal foreign particles of carbon nanotubes by dispersing the carbon nanotubes, crushing them, and then attaching the metal foreign particles to a magnet are disclosed.

[0008] In addition, in Patent Document 4, a method of removing metal foreign particles is proposed by circulating a dispersion containing a conductive auxiliary agent through a primary filter a plurality of times in a circulation method and then arranging at least two filters in series and passing them through once.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0010] However, in these conventional methods such as Patent Documents 1 to 4, there is a limit to reducing the metal content in the carbon nanotube dispersion composition. There is a risk that problems may arise, such as an increase in viscosity due to inhibition of dispersion stability by metal foreign particles and insufficient characteristics when used as a secondary battery.

[0011] Therefore, the problem to be solved by the present invention is to provide a carbon nanotube dispersion composition having good dispersibility, a low initial viscosity, and by using this carbon nanotube dispersion composition, a composite slurry capable of obtaining an electrode film with high conductivity and adhesion. More specifically, it is to provide a secondary battery having excellent rate characteristics and high-temperature cycle characteristics, and by having such a secondary battery, to provide a vehicle with high safety and improved fuel efficiency.

Means for Solving the Problems

[0012] The inventors of the present invention intensively studied to solve the above problems. The inventors found that a carbon nanotube dispersion composition containing carbon nanotubes, an amide-based polar solvent, and a copolymer having a specific structural unit in a specific ratio, and having a metal foreign particle content of 1.0 mg or less determined by Condition 1, has good dispersibility, a low initial viscosity, and by using such a dispersion composition, an electrode with high conductivity and adhesion can be obtained. Furthermore, it was found that a secondary battery obtained using this has excellent rate characteristics and high-temperature cycle characteristics. Based on such findings, the inventors have made the present invention.

[0013] That is, the present invention includes the following embodiments. The embodiments of the present invention are not limited to the following. 〔1〕A carbon nanotube dispersion composition containing carbon nanotubes, a copolymer, and an amide-based polar solvent, wherein the content of metal foreign particles determined by the following Condition 1 is 1.0 mg or less, The copolymer has an alkylene structural unit with a content of 50% by mass or more and 75% by mass or less, and a nitrile group-containing structural unit with a content of 25% by mass or more and 50% by mass or less. Carbon nanotube dispersion composition. <Condition 1> The metal foreign matter particles in 20 kg of the carbon nanotube dispersion composition are recovered with an electromagnet (an electromagnet having 31 grid screens with a magnetic flux density of 16,000 gauss, a space volume of 1.7 L, a diameter of 10 cm, and a thickness of 1.3 cm), washed with an amide-based polar solvent, and the obtained metal foreign matter particles are deposited on a filter with a disk diameter of 47 mm and a mesh opening of 5 μm, and the weight of the metal foreign matter particles on the filter is measured. 〔2〕The carbon nanotube dispersion composition according to 〔1〕, wherein the total content of iron, cobalt, nickel, chromium, molybdenum, and copper is 100 ppm or less. 〔3〕The carbon nanotube dispersion composition according to 〔1〕 or 〔2〕, wherein the angle of repose of the carbon nanotubes is 40° or more. 〔4〕The viscosity at 25 °C measured by a B-type viscometer is less than 2,000 mPa·s. 〔1〕~〔3〕The carbon nanotube dispersion composition according to any one of 〔1〕 to 〔3〕. 〔5〕A composite material slurry containing the carbon nanotube dispersion composition according to any one of 〔1〕 to 〔4〕 and an active material. 〔6〕An electrode film formed from the composite material slurry according to 〔5〕. 〔7〕A secondary battery including a positive electrode and a negative electrode, wherein at least one of the positive electrode and the negative electrode has the electrode film according to 〔6〕. 〔8〕A vehicle equipped with the secondary battery according to 〔7〕. 〔9〕A method for producing the carbon nanotube dispersion composition according to any one of 〔1〕 to 〔4〕, including all of the following steps (1) to (3). [Step (1): Crushing step] A step of applying a shear stress to carbon nanotubes to crush the carbon nanotubes [Step (2): Magnetic separation step] A step of removing metal foreign particles using an electromagnet with a magnetic flux density of 10,000 Gauss or more and 20,000 Gauss or less [Step (3): Filtration step] A step of filtering using a depth filter with a filtration accuracy of 5 μm or more and 50 μm or less

Advantages of the Invention

[0014] The carbon nanotube dispersion composition of the present invention has good dispersibility and a low initial viscosity. By using this carbon nanotube dispersion composition, an electrode film excellent in conductivity and adhesion can be obtained. In addition, a secondary battery excellent in rate characteristics and high-temperature cycle characteristics can be obtained. As a result, it can be suitably used even in vehicle applications such as hybrid vehicles, plug-in hybrid vehicles, and electric vehicles, where high capacity, high output, and high durability are required for the secondary batteries to be mounted.

Modes for Carrying Out the Invention

[0015] Hereinafter, the carbon nanotube dispersion composition, composite slurry, electrode film, and secondary battery of the present invention will be described in detail, but the present invention is not limited thereto. The numerical values specified in this specification are values obtained by the methods disclosed in the embodiments or examples.

[0016] In addition, in this specification, the numerical range specified using "~" shall include the numerical values described before and after "~" as the range of the lower limit value and the upper limit value. In this specification, carbon nanotubes may be referred to as "CNT", and N-methyl-2-pyrrolidone may be referred to as "NMP". In this specification, the carbon nanotube dispersion composition may be referred to as "CNT dispersion composition" or simply "dispersion composition". In the present invention, metal foreign particles are not related to the size or shape, and include fine metal powders and the like, and do not include those existing in a dissolved state as metal ions. Unless otherwise noted, the various components appearing in this specification may each be used alone or in combination of two or more.

[0017] ≪Carbon Nanotube Dispersion Composition≫ A carbon nanotube dispersion composition which is an embodiment of the present invention is a carbon nanotube dispersion composition containing carbon nanotubes, a copolymer, and an amide-based polar solvent, and the content of metal foreign matter particles determined by the following Condition 1 is 1.0 mg or less, the copolymer has an alkylene structural unit with a content of 50% by mass or more and 75% by mass or less, and a nitrile group-containing structural unit with a content of 25% by mass or more and 50% by mass or less. <Condition 1> After collecting metal foreign matter particles in 20 kg of the carbon nanotube dispersion composition with an electromagnet (an electromagnet having 31 grid screens with a magnetic flux density of 16,000 gauss, a space volume of 1.7 L, a diameter of 10 cm, and a thickness of 1.3 cm), washing with an amide-based polar solvent, and the obtained metal foreign matter particles are deposited on a filter with a disk diameter of 47 mm and a mesh opening of 5 μm, and the weight of the metal foreign matter particles on the filter is measured. Specifically, for example, the content of metal foreign matter particles can be determined by the method described in the examples.

[0018] Specifically, for example, the content of metal foreign matter particles can be determined by the method described in the examples.

[0019] The content of metal foreign matter particles contained in the carbon nanotube dispersion composition determined by Condition 1 is 1.0 mg or less, and from the viewpoints of the storage stability and high-temperature cycle characteristics of the secondary battery, 0.4 mg or less is preferable, and 0.1 mg or less is more preferable. Thereby, a carbon nanotube dispersion composition having good dispersibility and a low initial viscosity can be obtained. Further, by using this carbon nanotube dispersion composition, a secondary battery having excellent rate characteristics and high-temperature cycle characteristics can be obtained.

[0020] The carbon nanotube dispersion composition of the present invention is characterized in that, under condition 1, the content of metal foreign matter particles collected by a magnet and recovered by a filter with an opening of 5 μm is 1.0 mg or less. When the content of metal foreign matter particles of 5 μm or more collected by a magnet is large, the metal foreign matter particles may be exposed from the electrode surface, causing the secondary battery to short-circuit and not function as a secondary battery. In addition, metal foreign matter particles may elute into the electrolyte, increasing the self-discharge of the secondary battery.

[0021] The inventors of the present invention have found that simply reducing the content of metal ions contained in the carbon nanotube dispersion composition is not sufficient to fully satisfy the electrical characteristics when used as a secondary battery, and the content of metal foreign matter particles of 5 μm or more collected by a magnet is also important.

[0022] The reason is presumed as follows. When carbon nanotubes are synthesized, a metal catalyst is used during production. Therefore, metals derived from the metal catalyst remain, and in some cases, the metal catalyst is encapsulated. When the metals derived from the metal catalyst encapsulated in the carbon nanotubes and the wear powder that may be included when dispersing the carbon nanotubes are refined by dispersion, it is difficult to capture only the metal foreign matter particles even when using a filter with high filtration accuracy.

[0023] Furthermore, as metal foreign matter particles, there may be included wear powder due to wear of the inner wall, piping, and in some cases, the dispersion medium and stirring blades of the disperser used in the dispersion process of carbon nanotubes. Wear powder derived from a part of the piping and tank containing SUS304 and SUS316 used in the disperser and piping, etc., changes its crystal structure due to external stress applied to SUS304 and SUS316, becoming metal foreign matter particles with magnetism. However, since the magnetism is weak, it is difficult to remove.

[0024] These metallic foreign particles deteriorate the dispersion stability in the carbon nanotube dispersion composition and cause an increase in viscosity. Further, when the metallic foreign particles are larger than the thickness of the separator, for example, 20 μm or more, they may pierce through the separator that separates the positive electrode and the negative electrode, causing an internal short circuit and resulting in a voltage failure.

[0025] Furthermore, metallic foreign particles often have orientation. When filtering the carbon nanotube dispersion composition, even if a filter with a filtration accuracy of 20 μm is used, for example, needle-shaped metallic foreign particles of 20 μm or more may mix into the carbon nanotube dispersion composition. Such metallic foreign particles are weakly magnetic, and only the metallic foreign particles close to the magnetic pole part can be removed by a magnet, and it is difficult to reduce the metallic foreign particles only by using a magnet with a high magnetic force and increasing the number of magnetic separation treatments. However, by using a magnet such as an electromagnet that has a magnetic force equal to or higher than a certain level and has a narrow gap between magnetic poles to remove weakly magnetic metallic foreign particles, and making the content of metallic foreign particles of 5 μm or more required by Condition 1 1.0 mg or less, it is possible to fabricate a secondary battery that not only has high safety but also has good rate characteristics and high-temperature cycle characteristics.

[0026] Note that the carbon nanotube dispersion composition means the one in the state before the active material is added. In this regard, the carbon nanotube dispersion composition is distinguished from the composite slurry containing the active material. That is, the carbon nanotube dispersion composition does not substantially contain the active material. This is a concept excluding the state where the active material is intentionally added to the carbon nanotube dispersion composition. The active material may be 1 mass% or less, 0.5 mass% or less, or 0.1 mass% or less, or 0 mass% with respect to the total mass of the carbon nanotube dispersion composition. The active material will be described later.

[0027] The carbon nanotube dispersion composition of the present invention contains carbon nanotubes, and when used as a conductive aid for an electrode of a secondary battery, even a small amount can form a conductive path, enabling excellent rate characteristics and high-temperature cycle characteristics.

[0028] As described above, the carbon nanotubes contain and / or enclose metals derived from the metals used during production. If the metals are present as they are, voltage drop or the like may occur. Since the encapsulated metal powders are exposed and refined by being dispersed, from the perspective of the metal foreign matter removal efficiency, it is desirable to remove and reduce them before they are refined, that is, before the dispersion step or during the dispersion step.

[0029] In addition, metal foreign matter particles that can be mixed as wear powders derived from parts of pipes and tanks containing SUS304 or SUS316 in the carbon nanotube dispersion step change their crystal structure due to external stresses such as dispersion and have magnetism, and when used in a secondary battery, voltage drop or the like may occur. Therefore, it is desirable to reduce these metal foreign matter particles derived from wear powders before they are refined by the dispersion step. Preferably, removal by a magnet is performed before the dispersion step or simultaneously with the dispersion. This not only prevents it from being difficult to sufficiently reduce the metal foreign matter particles due to the decrease in the magnetic adhesion area of the metal foreign matter particles and the resulting reduction in the foreign matter removal efficiency by the magnet, but also suppresses the decrease in the stability of the carbon nanotube dispersion liquid caused by the refined metal foreign matter particles.

[0030] The carbon nanotube dispersion composition of the present invention controls the content of metal foreign matter particles, which is the sum of metal foreign matter particles caused by the production of such carbon nanotubes and metal foreign matter particles such as wear powders of the disperser during the dispersion step, to be 1.0 mg or less according to Condition 1, thereby making it possible to achieve excellent dispersibility and high-temperature cycle characteristics of the carbon nanotube dispersion composition. As a method for reducing the content of metal foreign particles determined by Condition 1 to 1.0 mg or less, it is possible to control by reducing the introduction of metal foreign substances derived from raw materials as much as possible, removing metal foreign substances derived from dispersion equipment and the dispersion process during the process, filter-filtering the dispersed carbon nanotube dispersion composition, and the like. Further, the content of metal foreign particles can be further reduced by a magnetic separation treatment using a magnet with a narrow gap between magnetic poles such as an electromagnet. Since metal foreign particles often have orientation, it is preferable to pass through a magnet or a filter a plurality of times. By these methods, it is possible to achieve dispersion stability that cannot be achieved only by reducing the metal element content in the carbon nanotube dispersion composition and excellent battery characteristics.

[0031] Further, the total content of iron, cobalt, nickel, chromium, molybdenum and copper in the carbon nanotube dispersion composition is preferably as low as possible, and preferably 100 ppm or less. More preferably, it is 50 ppm or less, and still more preferably, it is 10 ppm or less. When the content of metal foreign particles determined by Condition 1 is 1.0 mg or less and the total content of iron, cobalt, nickel, chromium, molybdenum and copper is within the above range, the voltage failure of the secondary battery can be more suppressed. The content of iron, cobalt, nickel, chromium, and molybdenum is preferably 10 ppm or less, preferably 5 ppm or less, and still more preferably 1 ppm or less. Further, the content of copper is preferably 5 ppm or less, preferably 1 ppm or less, and still more preferably 0.1 ppm or less. Iron, cobalt, nickel, and molybdenum are used as catalyst raw materials in the production of carbon nanotubes. Since carbon nanotubes are produced in a reducing atmosphere, the above-mentioned iron, cobalt, nickel, and molybdenum exist in the form of pure metals encapsulated in the carbon nanotubes. Due to their low oxidation potential, they dissolve in the electrolyte, precipitate on the surface of the negative electrode, and there is a risk of voltage failure and capacity reduction of the secondary battery. In addition, iron, nickel, and chromium may be mixed from the wear powder of tanks and stirrers for producing the carbon nanotube dispersion composition, and copper may be mixed from motors of dispersion equipment, etc. Among these, copper has a low oxidation-reduction potential, dissolves in the electrolyte, and precipitates dendritically on the surface of the negative electrode, which may lead to voltage failure of the secondary battery. The amounts of these metal elements can be measured, for example, by ICP emission spectrometry.

[0032] The content of carbon nanotubes contained in the carbon nanotube dispersion composition is preferably 1% by mass or more and 10% by mass or less, more preferably 2% by mass or more and 8% by mass or less, and even more preferably 3% by mass or more and 6% by mass or less, based on the mass of the carbon nanotube dispersion composition (taking the mass of the carbon nanotube dispersion composition as 100% by mass).

[0033] The content of the copolymer is preferably 5 parts by mass or more and 200 parts by mass or less, more preferably 10 parts by mass or more and 100 parts by mass or less, and even more preferably 20 parts by mass or more and 50 parts by mass or less, based on 100 parts by mass of the carbon nanotubes. When the amount of the copolymer containing an alkylene structural unit and a nitrile group-containing structural unit is within the above range, the dispersion stability of the carbon nanotubes in the carbon nanotube dispersion composition tends to be good. Also, the peel strength of the electrode for the secondary battery becomes good.

[0034] The content rate of the amide-based polar solvent is preferably 90% by mass to 99% by mass, more preferably 92% by mass to 98% by mass, based on the carbon nanotube dispersion composition (100% by mass). When it is within the above range, a carbon nanotube dispersion composition excellent in fluidity and dispersion stability is easily obtained. By using a carbon nanotube dispersion composition excellent in fluidity and dispersion stability, it is easy to remove metal foreign substances, an electrode film having stable conductivity can be obtained, and the quality of the secondary battery is likely to be stable.

[0035] The carbon nanotube dispersion composition preferably has a water content of 100 ppm or more and 1500 ppm or less, more preferably 200 ppm or more and 1000 ppm or less. When the water content of the carbon nanotube dispersion composition is within the above range, gelation of the composite slurry described later is suppressed, and a composite slurry and an electrode film of stable quality are easily obtained. When the water content exceeds the above range, the metal encapsulated in the carbon nanotubes may dissolve during dispersion, making it difficult to remove metal foreign substances in the manufacturing process.

[0036] The initial viscosity of the carbon nanotube dispersion composition preferably has a viscosity measured at 100 rpm and 25 °C using a B-type viscometer of less than 2,000 mPa·s. More preferably it is less than 1,000 mPa·s, and even more preferably less than 500 mPa·s. Note that it may be 100 mPa·s or more. When the initial viscosity is within the above range, the dispersion state of the carbon nanotubes contained in the carbon nanotube dispersion composition is appropriate, and it is easy to remove metal foreign substances using a filter or an electromagnet. The carbon nanotube dispersion composition with an initial viscosity within the above range is considered to have an appropriate composition ratio and dispersion process of carbon nanotubes, a copolymer, and an amide-based polar solvent, and good dispersion stability. Specifically, for example, it can be measured by the method described in the examples.

[0037] <Carbon Nanotubes> Carbon nanotubes have a structure in which planar graphite is wound into a cylindrical shape, including single-walled carbon nanotubes and multi-walled carbon nanotubes, and these may be mixed. Among them, it is preferable to include multi-walled carbon nanotubes. Multi-walled carbon nanotubes have a structure in which two or more layers of graphite are wound, and single-walled carbon nanotubes have a structure in which one layer of graphite is wound. The side walls of the carbon nanotubes do not necessarily have to be a graphite structure. For example, carbon nanotubes having side walls with an amorphous structure can also be used as the carbon material.

[0038] The average outer diameter of the carbon nanotubes is preferably 1 nm or more and 25 nm or less, more preferably 3 nm or more and 20 nm or less, and even more preferably 4 nm or more and 15 nm or less. When the average outer diameter is within the above range, it is easy to form a good conductive network in the electrode, and during charge and discharge, the active material inside the secondary battery is uniformly utilized, so the deterioration of the active material is suppressed, and the cycle characteristics of the secondary battery are further improved.

[0039] The BET specific surface area of the carbon nanotubes is 2 100 m 2 / g or more and 1000 m 2 / g or less, more preferably 170 m 2 / g or more and 600 m 2 / g or less, and even more preferably 200 m 2 / g or more and 500 m / g or less. When the BET specific surface area is within the above range, an efficient conductive network can be formed with a small amount, and the amount of conductive material in the electrode can be reduced. Thereby, the degree of freedom in battery design, such as increasing the amount of active material and binder resin, is increased. Furthermore, during the preparation of the composite slurry, the composite of the active material and the carbon nanotubes proceeds easily, so an electrode film having a homogeneous conductive network in which the surface of the active material is coated with the carbon nanotubes can be obtained, suppressing the electrolyte decomposition reaction at the interface between the electrolyte and the active material, and improving the cycle characteristics of the battery.The BET specific surface area can be measured by the BET method described in JIS Z 8830.

[0040] The G / D ratio (peak ratio of G-band and D-band) of the carbon nanotube is 1560 cm in the Raman spectrum -1 ~1600 cm -1 The maximum peak intensity within the range is G, and 1310 cm -1 ~1350 cm -1 When the maximum peak intensity within the range is D, the G / D ratio is preferably 0.5 to 10, and more preferably 0.7 to 4.5. When the G / D ratio of the carbon nanotube is within the above range, the contact resistance between the carbon nanotubes is considered to be small, and good conductivity is likely to be obtained. Also, it is presumed that the amount of functional groups on the surface of the multi-walled carbon nanotube is appropriate, the affinity with the solvent is good, and the dispersibility is better.

[0041] The volume resistivity of the carbon nanotube is preferably 1.0×10 -2 Ω·cm to 3.0×10 -2 Ω·cm, and more preferably 1.0×10 -2 Ω·cm to 2.0×10 -2 Ω·cm. The volume resistivity of the carbon nanotube can be measured using a powder resistivity measuring device (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: Loresta GP Powder Resistivity Measuring System MCP-PD-51). When the volume resistivity is within the above range, the conductivity of the electrode film is good, and a secondary battery having excellent rate characteristics and cycle characteristics can be obtained.

[0042] The angle of repose of the carbon nanotubes is preferably 40° or more. More preferably, it is 45° or more, and even more preferably, it is 50° or more. Also, it is preferably 85° or less, and more preferably, it is 70° or less. The angle of repose is an index representing the fluidity of the powder. Carbon nanotubes having an angle of repose within the above range are such that metal foreign particles encapsulated in the carbon nanotubes can be easily removed, and a carbon nanotube dispersion composition with a low content of metal foreign particles can be easily obtained. The angle of repose can be measured by the injection method. The injection method is a method of measuring by depositing the powder on a table having an upper surface in a disk shape. It is hardly affected by the material of the table, and the angle formed by the powder deposited in a conical shape and the horizontal plane can be measured using a protractor or the like. Also, the angle of repose can be measured using a commercially available measuring instrument.

[0043] The angle of repose of the carbon nanotubes can be controlled by treating them using conventionally known pulverization equipment. As the pulverization equipment, for example, a Henschel mixer, a super mixer, a Nauta mixer, a Trimix, a high-speed mixer, a mortar, a pin mill, a hammer mill, a pulpizer, an attritor, a jet mill, a cutter mill, a ball mill, a bead mill, a colloid mill, a conical mill, a disk mill, an edge mill, a wonder crusher, a vibration mill, an ultrasonic homogenizer, etc. can be used. Particularly preferred are an attritor, a pin mill, a hammer mill, a jet mill, a cutter mill, a ball mill, a bead mill, and a vibration mill.

[0044] The carbon nanotubes are preferably those from which metal foreign particles have been removed using an electromagnet and magnetic force. For example, it is preferable to set an electromagnet in the pulverization process or filling process of the carbon nanotubes and pass the carbon nanotubes through it to remove the metal foreign particles. The higher the carbon purity of the carbon nanotubes, the more preferable it is. Among 100% by mass of the carbon nanotubes, it is preferably 98.0% by mass or more, more preferably 99.5% by mass or more, even more preferably 99.8% by mass or more, and particularly preferably 99.9% by mass or more. That is, the content rate of metallic foreign particles is preferably as low as possible, and is preferably 2.0% by mass or less, more preferably 0.5% by mass or less, still more preferably 0.2% by mass or less, and particularly preferably 0.1% by mass or less in 100% by mass of the carbon nanotubes. By using carbon nanotubes produced by a production method not using a metal catalyst as a nucleus or carbon nanotubes obtained by a purification treatment method such as a conventionally known acid treatment, the content rate of metallic foreign particles is made 2.0% by mass or less with respect to 100% by mass of the carbon nanotubes, whereby the content of metallic foreign particles contained in the carbon nanotube dispersion composition can be reduced, and various characteristics of the secondary battery can be improved. The carbon purity of the carbon nanotubes can be determined by the method described in the examples using an ICP emission spectroscopic analyzer.

[0045] <Copolymer> The copolymer of the present invention has an alkylene structural unit with a content rate of 50% by mass or more and 75% by mass or less, and a nitrile group-containing structural unit with a content rate of 25% by mass or more and 50% by mass or less. By using such a copolymer, a carbon nanotube composition excellent in dispersibility and oxidation resistance can be obtained, and since the solubility in the electrolyte is also low, the ionic conductivity of the electrolyte is less likely to decrease. A secondary battery using this can be made excellent in rate characteristics and high-temperature cycle characteristics. The copolymer can be obtained, for example, by copolymerizing a monomer mixture containing a conjugated diene monomer and a nitrile group-containing monomer, and partially hydrogenating the carbon-carbon double bond of the structural unit derived from the conjugated diene monomer by hydrogenation to obtain a copolymer having an alkylene structural unit and a nitrile group-containing structural unit. Specifically, it can be produced by the method described later.

[0046] The Mooney viscosity of the copolymer is preferably 65 or less, more preferably 60 or less, still more preferably 50 or less, and particularly preferably 35 or less. Also, it is preferably 20 or more, more preferably 35 or more.

[0047] The "Mooney viscosity (ML" in the present invention 1+4, "(at 100 °C)" can be measured at a temperature of 100 °C in accordance with JIS K6300-1. By setting the Mooney viscosity within the above range, it is presumed that an appropriate repulsive force can be provided in a state where the copolymer is adsorbed on the carbon nanotubes, thereby enhancing the dispersion stability. When it is 20 or more, the balance between the solubility in the amide-based polar solvent used as the solvent and the adsorptivity to the carbon nanotubes becomes good, and the dispersibility of the carbon nanotubes is improved. Further, when the Mooney viscosity is 65 or less, the viscosity of the carbon nanotube dispersion composition does not become too high, the decrease in the energy transfer efficiency of the dispersing machine is suppressed, and metal foreign substances mixed in from the raw materials can be efficiently removed by methods such as magnets, filtration, and centrifugation. Therefore, it is preferable because it can prevent voltage defects in the secondary battery caused by metal foreign particles.

[0048] The copolymer of the present invention may have an appropriate fluidity at room temperature if it has an alkylene structural unit with a content of 50% by mass or more and 75% by mass or less, and a nitrile group-containing structural unit with a content of 25% by mass or more and 50% by mass or less.

[0049] The weight average molecular weight (Mw) of the copolymer is preferably 20,000 or more and 200,000 or less, and more preferably 20,000 or more and 150,000 or less. When the weight average molecular weight is within the above range, the adsorptivity to the carbon nanotubes and the affinity to the dispersion medium tend to be good. The weight average molecular weight is the weight average molecular weight in terms of polystyrene and can be measured by gel permeation chromatography (GPC).

[0050] The method for adjusting the Mooney viscosity is not particularly limited. For example, it can be adjusted by changing the composition of the polymer (type and content of structural units, hydrogenation rate, etc.), structure (linearity rate, etc.), molecular weight, preparation conditions (polymerization temperature, amount of molecular weight regulator, etc.), and the like.

[0051] In addition, the copolymer to be used is preferably one from which metal foreign particles have been removed by a filtering step using a filter and / or a magnetic separation step using a magnetic force of an electromagnet. For example, it is preferable to remove metal foreign particles by setting a filter or an electromagnet in a step of pulverizing or filling the copolymer and passing the copolymer therethrough. The magnetic separation step of the copolymer may be carried out after dissolving the copolymer in an amide-based polar solvent or the like to obtain a copolymer solution.

[0052] [Alkylene structural unit] The alkylene structural unit is a structural unit containing an alkylene structure, and is preferably a structural unit consisting only of an alkylene structure. The alkylene structure is preferably a linear alkylene structure or a branched alkylene structure. However, this does not apply to a structural unit having a nitrile group.

[0053] The alkylene structural unit preferably includes a structural unit represented by the following general formula (1A).

[0054] General formula (1A) [Chemical formula]

[0055] In the general formula (1A), n represents an integer of 1 or more. n is preferably an integer of 2 or more, more preferably an integer of 3 or more. n is preferably an integer of 5 or less, more preferably an integer of 4 or less. In particular, n is preferably 3.

[0056] The alkylene structural unit preferably includes a structural unit represented by the following general formula (1B).

[0057] General formula (1B) [Chemical formula]

[0058] In the general formula (1B), n represents an integer of 1 or more. n is preferably an integer of 4 or less, more preferably an integer of 3 or less, and still more preferably an integer of 2 or less. In particular, n is preferably 2.

[0059] The method for introducing an alkylene structural unit into the copolymer is not particularly limited, and examples thereof include the following methods (1a) or (1b).

[0060] In the method of (1a), a copolymer is prepared by a polymerization reaction using a monomer composition containing a conjugated diene monomer. The prepared copolymer contains monomer units derived from the conjugated diene monomer. In the present invention, the "monomer units derived from the conjugated diene monomer" may be referred to as "conjugated diene monomer units", and the same may be omitted for monomer units derived from other monomers. Next, by hydrogenating the conjugated diene monomer units, at least a part of the conjugated diene monomer units is converted into alkylene structural units. Hereinafter, "hydrogenation" may be referred to as "hydrogenation". The finally obtained copolymer contains units obtained by hydrogenating conjugated diene monomer units as alkylene structural units.

[0061] Note that the conjugated diene monomer units include at least monomer units having one carbon-carbon double bond. For example, 1,3-butadiene monomer units, which are conjugated diene monomer units, include at least one monomer unit selected from the group consisting of monomer units having a cis-1,4 structure, monomer units having a trans-1,4 structure, and monomer units having a 1,2 structure, and may include two or more monomer units. Further, the conjugated diene monomer units may further include monomer units having no carbon-carbon double bond and including a branching point. In the present specification, the "branching point" refers to the branching point in a branched polymer. When the conjugated diene monomer units include monomer units including a branching point, the above-prepared copolymer and copolymer are branched polymers. The monomer units having no carbon-carbon double bond and including a branching point. In the present specification, the "branching point" refers to the branching point in a branched polymer. When the conjugated diene monomer units include monomer units including a branching point, the above-prepared copolymer and copolymer are branched polymers.

[0062] In the method of (1b), a copolymer is prepared by a polymerization reaction using a monomer composition containing an α-olefin monomer. The prepared copolymer contains α-olefin monomer units. The finally obtained copolymer contains α-olefin monomer units as alkylene structural units.

[0063] Among these, the method of (1a) is preferred because the production of the copolymer is easy. The number of carbon atoms of the conjugated diene monomer is 4 or more, preferably 4 or more and 6 or less. Examples of the conjugated diene monomer include conjugated diene compounds such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. Among them, 1,3-butadiene is preferred. The alkylene structural unit preferably contains a structural unit obtained by hydrogenating a conjugated diene monomer unit (hydrogenated conjugated diene monomer unit), and more preferably contains a structural unit obtained by hydrogenating a 1,3-butadiene monomer unit (hydrogenated 1,3-butadiene monomer unit). The conjugated diene monomer can be used alone or in combination of two or more. The structural unit obtained by hydrogenating the 1,3-butadiene monomer unit (hydrogenated 1,3-butadiene monomer unit) is more preferably included. The hydrogenation is preferably a method capable of selectively hydrogenating the conjugated diene monomer unit. Examples of the hydrogenation method include known methods such as an oil-phase hydrogenation method or an aqueous-phase hydrogenation method.

[0064] The hydrogenation can be carried out by a usual method. For example, the hydrogenation can be carried out by treating a copolymer having a conjugated diene monomer unit with hydrogen gas in the presence of a hydrogenation catalyst in a state where it is dissolved in an appropriate solvent. Examples of the hydrogenation catalyst include nickel, palladium, platinum, copper, etc.

[0065] The hydrogenation can be carried out by a usual method. For example, the hydrogenation can be carried out by treating a copolymer having a conjugated diene monomer unit with hydrogen gas in the presence of a hydrogenation catalyst in a state where it is dissolved in an appropriate solvent. Examples of the hydrogenation catalyst include nickel, palladium, platinum, copper, etc.

[0066] In the method of (1b), the number of carbon atoms of the α-olefin monomer is 2 or more, preferably 3 or more, more preferably 4 or more. The number of carbon atoms of the α-olefin monomer is preferably 6 or less, more preferably 5 or less. Examples of the α-olefin monomer include α-olefin compounds such as ethylene, propylene, 1-butene, and 1-hexene. The α-olefin monomer can be used alone or in combination of two or more.

[0067] The alkylene structural unit preferably contains at least one selected from the group consisting of a structural unit containing a linear alkylene structure and a structural unit containing a branched alkylene structure, more preferably contains at least one selected from the group consisting of a structural unit consisting only of a linear alkylene structure and a structural unit consisting only of a branched alkylene structure, and still more preferably contains at least one selected from the group consisting of the structural unit represented by the above formula (1A) and the structural unit represented by the above formula (1B).

[0068] The content of the alkylene structural unit is 50% by mass or more and 75% by mass or less based on the mass of the copolymer (that is, when the mass of the copolymer is 100% by mass). It is preferably 55% by mass or more. Also, it is preferably 70% by mass or less, more preferably 65% by mass or less. By setting the content of the alkylene structural unit within the above range, the adsorptivity to carbon nanotubes and the affinity to the dispersion medium can be controlled, and the carbon nanotubes can be stably present in the dispersion medium, so that the stability of the dispersion composition is improved. Also, the affinity of the copolymer to the electrolyte can be controlled, and problems such as the copolymer dissolving in the electrolyte in the battery and increasing the resistance of the electrolyte can be prevented. liquid can be prevented.

[0069] [Nitrile group-containing structural unit] The nitrile group-containing structural unit is a structural unit having a nitrile group. A structural unit having an alkylene structure substituted by a nitrile group is preferred, and a structural unit consisting only of an alkylene structure substituted by a nitrile group is more preferred. The alkylene structure is preferably a linear or branched alkylene structure. The nitrile group-containing structural unit may further contain (or consist only of) a structural unit containing an alkyl structure substituted by a nitrile group. The number of nitrile groups contained in the nitrile group-containing structural unit is preferably one.

[0070] The nitrile group-containing structural unit preferably has a structural unit represented by the following general formula (2A).

[0071] General formula (2A)

Chemical formula

[0072] In the general formula (2A), n represents an integer of 2 or more. n is preferably an integer of 6 or less, more preferably an integer of 4 or less, and still more preferably an integer of 3 or less. In particular, n is preferably 2.

[0073] The nitrile group-containing structural unit may contain a structural unit represented by the following general formula (2B).

[0074] General formula (2B)

Chemical formula

[0075] In the general formula (2B), R represents a methyl group.

[0076] The method for introducing a nitrile group-containing structural unit into the copolymer is not particularly limited, but a method of preparing a copolymer by a polymerization reaction using a monomer composition containing a nitrile group-containing monomer (the method of (2a)) can be preferably used. The finally obtained copolymer has a nitrile group-containing single structural unit as a nitrile group-containing structural unit. To form a nitrile group-containing structural unit Examples of the nitrile group-containing monomer that can form include monomers having a polymerizable carbon-carbon double bond and a nitrile group. For example, α,β-ethylenically unsaturated group-containing compounds having a nitrile group, and specifically, acrylonitrile, methacrylonitrile, etc. are mentioned. In particular, from the viewpoint of enhancing the intermolecular force between copolymers and / or between a copolymer and a dispersed substance (adsorbed substance), the nitrile group-containing monomer preferably contains acrylonitrile. The nitrile group-containing monomer can be used alone or in combination of two or more.

[0077] The content of the nitrile group-containing structural unit is 25% by mass or more and 50% by mass or less based on the mass of the copolymer (that is, when the mass of the copolymer is 100% by mass). It is preferably 30% by mass or more. Also, it is preferably 45% by mass or less, and more preferably 40% by mass or less. By setting the content of the nitrile group-containing structural unit within the above range, the adsorptivity to carbon nanotubes and the affinity to the dispersion medium can be controlled, and the carbon nanotubes can be stably present in the dispersion medium, so that the stability of the dispersion composition is improved. Also, the affinity of the copolymer to the electrolyte can be controlled, and problems such as the copolymer dissolving in the electrolyte and increasing the resistance of the electrolyte in the battery can be prevented.

[0078] [Other structural units] Within a range that does not hinder the effects of the present invention, structural units other than the alkylene structural unit and the nitrile group-containing structural unit may be provided as needed. Examples of the other structural units include amide group-containing monomers, carboxyl group-containing monomers, etc.

[0079] [Amide-based polar solvent] Examples of amide-based polar solvents include N-methyl-2-pyrrolidone (NMP), N-ethyl- 2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylcaprolactam, and the like. Among these, it is more preferable to contain at least one selected from the group consisting of N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone.

[0080] The water content of the amide-based polar solvent is preferably 100 ppm or more and 1500 ppm or less, and more preferably 100 ppm or more and 1000 ppm or less. When within the above range, the alkali metal encapsulated in the carbon nanotube may dissolve in the carbon nanotube dispersion composition, and the dispersion stability of the carbon nanotube dispersion composition may be good.

[0081] <Optional Component> The carbon nanotube dispersion composition may optionally contain other additives such as alkali metal hydroxides, wetting agents, pH adjusters, wetting and penetrating agents, leveling agents, etc., and optional components such as other conductive materials other than carbon nanotubes, within a range that does not inhibit the object of the present invention. The optional components can be added at any timing, such as before the preparation of the carbon nanotube dispersion composition, during mixing, after mixing, or a combination thereof.

[0082] Examples of the alkali metal hydroxide include lithium hydroxide, sodium hydroxide, potassium hydroxide, and the like. When using an alkali metal hydroxide, its content is preferably 0.5 part by mass or more and 4 parts by mass or less, more preferably 1 part by mass or more and 3 parts by mass or less, and even more preferably 1.5 parts by mass or more and 2.5 parts by mass or less, based on 100 parts by mass of the copolymer. When using an alkali metal hydroxide, the wettability of the carbon nanotube is improved, and the dispersibility is further improved. In addition, since the dispersion proceeds to the target viscosity in a short time, there are few defects in the carbon nanotube, and it is easy to obtain an electrode film and a secondary battery with good conductivity.

[0083] <Method for Producing Carbon Nanotube Dispersion Composition> The carbon nanotube dispersion composition of this embodiment is obtained by dispersing a mixture containing carbon nanotubes, a copolymer, and an amide-based polar solvent with a disperser. Also, although the production method is not particularly limited, a step of applying shear stress to the carbon nanotubes to crush the carbon nanotubes, and a magnetic separation step, a filtration step using a filter, and a metal foreign matter removal step such as a centrifugation step for removing magnetic foreign matter in order to make the content of metal foreign matter particles in the carbon nanotube dispersion composition obtained under Condition 1 1.0 mg or less. As these metal foreign matter removal steps, at least one of a magnetic separation step and a filtration step is preferable. The metal foreign matter removal step may be at any timing, such as before, during, or after the dispersion of the carbon nanotube dispersion composition. It is preferable to remove the metal foreign matter particles before they are refined by the dispersion step, and it is preferable to include a metal foreign matter removal step before and / or during the dispersion of the carbon nanotube dispersion composition.

[0084] Note that a method for producing a carbon nanotube dispersion composition including the following steps (1) to (3) is preferable. [Step (1): Crushing Step] A step of applying shear stress to the carbon nanotubes to crush the carbon nanotubes [Step (2): Magnetic Separation Step] A step of removing metal foreign matter particles using an electromagnet having a magnetic flux density of 10,000 Gauss or more and 20,000 Gauss or less [Step (3): Filtration Step] A step of filtering using a depth filter having a filtration accuracy of 5 μm or more and 50 μm or less

[0085] The order of Step (1), Step (2), and Step (3) is not particularly limited, and the metal foreign matter removal steps of Step (2) and Step (3) may be before, after, simultaneous with, or a combination of these with respect to Step (1). Preferably, each step is provided in the order of step (1), step (2), and step (3), or step (1) and step (2) are performed simultaneously, and then step (3) is provided.

[0086] By this, in [step (1): crushing step], the metal foreign matter particles encapsulated in the carbon nanotubes are exposed, and then or simultaneously, in [step (2): magnetic separation step], the magnetic metal foreign matter particles can be removed by contacting with a magnet. Subsequently, in [step (3): filtration separation step], the weakly magnetic metal foreign matter particles (such as those with a large particle size but weak magnetism, or those with a particle size too small to have a sufficient magnetic attachment area and flow away with the liquid without being captured by the magnet) that cannot be completely removed by the magnet can be removed.

[0087] [Step of crushing carbon nanotubes] In the step of applying a shear stress to the carbon nanotubes to crush the carbon nanotubes, the carbon nanotubes can be crushed by a dispersion device or the like in a dry and / or wet manner.

[0088] The dispersion device used for crushing the carbon nanotubes is not particularly limited. As the dispersion device, a disperser commonly used for pigment dispersion or the like can be used. For example, mixers such as a disper, a homomixer, and a planetary mixer, homogenizers (Advanced Digital Sonifer (registered trademark), MODEL 450DA manufactured by BRANSON, "Claremix" manufactured by M. TECHNIC, "Filmix" manufactured by PRIMIX, "Abramix" manufactured by SILVERSON, etc.), paint con Dishoners (manufactured by Red Building Co., Ltd.), colloid mills (such as "PUC Colloid Mill" manufactured by PUC Co., Ltd. and "Colloid Mill MK" manufactured by IKA Co., Ltd.), cone mills (such as "Cone Mill MKO" manufactured by IKA Co., Ltd.), ball mills, sand mills (such as "Dynomill" manufactured by Simar Enterprises Co., Ltd.), attritors, pearl mills (such as "DCP Mill" manufactured by Ehrlich Co., Ltd.), coball mills and other media-type dispersers, high-pressure homogenizers (such as "Genius PY" manufactured by Genius Co., Ltd., "Starburst" manufactured by Sugino Machine Co., Ltd., "Nanomizer" manufactured by Nanomizer Co., Ltd.), "Clear SS-5" manufactured by M-Technique Co., Ltd., "MICROS" manufactured by Nara Machinery Co., Ltd. and other media-less dispersers, and other roll mills, etc. may be mentioned, but are not limited thereto. Since it is desirable to have less contamination of abrasion powder during the crushing process, a media-less disperser is preferred. The rotor and stator of the disperser are preferably made of ceramics. When using a bead mill in wet dispersion, the dispersion medium is also preferably made of ceramics. When using a bead mill as a wet disperser, it is preferably used after applying shear stress to carbon nanotubes in advance using a media-less disperser to crush the carbon nanotubes before the dispersion treatment by the bead mill. This is because when the bead mill is operated at a high circumferential speed with respect to a low-viscosity dispersion material in which the crushing of carbon nanotubes has not progressed, the beads are likely to wear. Also, when using a high-pressure homogenizer as a disperser, if there is abrasion powder of beads during the dispersion process, there is a risk of nozzle clogging or valve breakage of the high-pressure homogenizer.

[0089] In order to make the content of metal foreign particles required under Condition 1 1.0 mg or less, the carbon nanotube dispersion composition of the present embodiment preferably uses a product obtained by removing metal foreign particles by a filter or magnetic force after crushing the carbon nanotubes.

[0090] [Step of removing metal foreign particles] The method for removing metal foreign particles is not particularly limited, and examples include a filtration step of filtering by a filter and a magnetic separation step such as a magnetic separation treatment using an electromagnet. Preferably, a filtration step and a magnetic separation step are provided. By the magnetic separation step, metal foreign particles in carbon nanotubes grown with a metal catalyst as a core can be removed. By the filtration step, metal foreign particles that cannot be removed by a magnet can be recovered. Furthermore, it is more preferable to perform the magnetic separation step after the filtration step. By performing the filtration step at the end before shipping the CNT dispersion composition, metal foreign particles from pipes and the like can also be removed. As a result, by reducing the metal foreign particles, the characteristics as a battery can be made more excellent.

[0091] (Magnetic Separation Step) As a method for removing metal foreign particles using magnetic force by the magnetic separation step, various conventionally known methods can be used. For example, during the manufacturing process of the carbon nanotube dispersion composition, it is preferable to set an electromagnet and pass the carbon nanotube composition through it to remove metal foreign particles.

[0092] The magnetic flux density of the electromagnet is preferably 5000 Gauss or more and 20000 Gauss or less, and more preferably 10000 Gauss or more and 20000 Gauss or less. By using an electromagnet within the above range, not only metal foreign particles contained in the carbon nanotubes but also metal foreign particles mixed in the manufacturing process can be removed.

[0093] Specifically, for example, CS-150HHH, CS-250HHH, CS-300HHH manufactured by Nippon Magnetics Co., Ltd., DVF-50-6, DVF-50-9, DVF-50-12 manufactured by Nippon Elyz Magnetics Co., Ltd., EMF-100S, EMF-150S, EMF-250S, EMF-300S manufactured by Daiho Magnetic Co., Ltd. etc. can be used.

[0094] The flow rate when the carbon nanotube dispersion composition contacts the electromagnet is preferably 1 L / min or more and 300 L / min or less, and more preferably 30 L / min or more and 100 L / min or less.

[0095] The number of times the carbon nanotube dispersion composition passes through the electromagnet is preferably 3 or more. If the number of passes is small, there is a possibility that metal foreign particles cannot be removed. When passing through the electromagnet in a circulating manner, considering the uniformity in the tank used in the manufacturing process, it is preferable to pass through more times.

[0096] (Filtration step) As the filter for filtering metal foreign particles, it may be a surface filter such as a membrane filter or a depth filter, but a depth filter is more preferable. Since metal foreign particles are not spherical and many have orientation, by using a depth filter, the metal foreign particles in the carbon nanotube dispersion composition can be efficiently removed.

[0097] Different from a surface filter (a filter that mainly captures particulate matter in a fluid on the filter surface), a depth filter is a filter that mainly captures particulate matter in a fluid inside the filter medium, and has the characteristics of high particle retention performance and being less likely to clog. By using a depth filter, the metal foreign particles in the carbon nanotube dispersion composition can be removed more selectively.

[0098] As the depth filter, for example, a 3M(TM) PP non-woven fabric depth cartridge NT-T series can be used.

[0099] The filtration accuracy of the filter is preferably 5 μm or more and 50 μm or less, and more preferably 10 μm or more and 40 μm or less. When a filter with a small pore diameter is used for the purpose of increasing the removal efficiency of metal foreign particles in the carbon nanotube dispersion composition, the removal efficiency of metal foreign particles may decrease due to clogging of the carbon nanotubes. By using a filter with a filtration accuracy within the above range, metal foreign substances can be efficiently removed, and a carbon nanotube dispersion composition with few metal foreign particles can be obtained.

[0100] Even if a filtering process is performed using a filter with a filtration accuracy of 5 μm, metal foreign particles may have an orientation due to their needle-like shape or the like, and it is impossible to completely remove the metal foreign particles, which will remain in the carbon nanotube dispersion composition. Therefore, it is important to appropriately control the composition, viscosity, dispersion process, or the method of the metal foreign particle removal process of the carbon nanotube dispersion composition to reduce the content of the metal foreign particles required under Condition 1.

[0101] ≪Composite Material Slurry≫ The composite material slurry of this embodiment contains at least a carbon nanotube dispersion composition and an active material. That is, it preferably contains at least carbon nanotubes, a copolymer containing an alkylene structural unit and a nitrile group-containing structural unit, an amide-based polar solvent, an active material, and a binder resin.

[0102] A binder resin is a resin used to bind between substances. There is no particular limitation on the binder resin. For example, polymers or copolymers containing, as constituent units, fluororesins, ethylene, propylene, vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid esters, methacrylic acid, methacrylic acid esters, acrylonitrile, styrene, vinyl butyral, vinyl acetal, vinyl pyrrolidone, etc.; polyurethane resins, polyester resins, phenol resins, epoxy resins, phenoxy resins, urea resins, melamine resins, alkyd resins, acrylic resins, formaldehyde resins, silicone resins, fluororesins; cellulose resins such as carboxymethyl cellulose; rubbers such as styrene-butadiene rubber; conductive resins such as polyaniline and polyacetylene, etc. Among them, from the viewpoint of electrochemical oxidation-reduction resistance, it is preferable to use a fluororesin as the binder resin.

[0103] As the fluororesin of this embodiment, for example, polyvinylidene fluoride, polyvinyl fluoride, and tetrafluoroethylene are preferable.

[0104] The weight average molecular weight of the fluororesin is preferably 10,000 or more and 2,000,000 or less, more preferably 100,000 or more and 1,000,000 or less, and particularly preferably 200,000 or more and 1,000,000 or less.

[0105] The active material is a material that serves as the basis of the battery reaction. The active material is divided into a positive electrode active material and a negative electrode active material based on the electromotive force. In this specification, the positive electrode active material and the negative electrode active material may be simply referred to as "active material". The active material is a material that serves as the basis of the battery reaction. The active material is divided into a positive electrode active material and a negative electrode active material based on the electromotive force.

[0106] The positive electrode active material is not particularly limited, but metal compounds such as metal oxides and metal sulfides capable of doping or intercalating lithium ions, and conductive polymers can be used. For example, oxides of transition metals such as Fe, Co, Ni, and Mn, composite oxides with lithium, inorganic compounds such as transition metal sulfides, etc. can be mentioned. Specifically, transition metal oxide powders such as MnO, V2O5, V6O 13 , TiO2, etc., composite oxide powders of lithium and transition metals such as layered lithium nickelate, lithium cobaltate, lithium manganate, spinel-structured lithium manganate, lithium iron phosphate-based materials which are olivine-structured phosphate compounds, transition metal sulfide powders such as TiS2 and FeS, etc. can be mentioned. Also, conductive polymers such as polyaniline, polyacetylene, polypyrrole, and polythiophene can be used. Further, the above inorganic compounds and organic compounds may be mixed and used.

[0107] The positive electrode active material is preferably a composite oxide of lithium containing transition metals such as Al, Fe, Co, Ni, and Mn, more preferably a composite oxide of lithium containing any of Al, Co, Ni, and Mn, and particularly preferably a composite oxide of lithium containing Ni and / or Mn. When these active materials are used, particularly good effects can be obtained on the battery characteristics.

[0108] The negative electrode active material is not particularly limited as long as it can be doped or intercalated with lithium ions. For example, metallic Li, alloy systems such as tin alloys, silicon alloys, and lead alloys which are its alloys, metal oxide systems such as LiXFe2O3, LiXFe3O4, LiXWO2 (x is a number between 0 < x < 1), lithium titanate, lithium vanadate, lithium silicate, conductive polymer systems such as polyacetylene and poly-p-phenylene, amorphous carbonaceous materials such as soft carbon and hard carbon, artificial graphite such as highly graphitized carbon materials, or carbonaceous powders such as natural graphite, carbon black, mesophase carbon black, resin-fired carbon materials, gas-phase grown carbon fibers, carbon fibers and other carbon-based materials can be mentioned. These negative electrode active materials can be used alone or in combination of two or more.

[0109] The BET specific surface area of the active material is preferably 0.1 m 2 / g or more and 10 m 2 / g or less, more preferably 0.2 m 2 / g or more and 5 m 2 / g or less, still more preferably 0.3 m 2 / g or more and 3 m 2 / g or less.

[0110] The average particle diameter of the active material is preferably in the range of 0.05 μm to 100 μm, and more preferably in the range of 0.1 μm to 50 μm. The average particle diameter of the active material as referred to in this specification is the average value of the particle diameters measured by an electron microscope for the active material.

[0111] In order to obtain the composite material slurry of this embodiment, it is preferable to perform a dispersion treatment after adding the active material to the carbon nanotube dispersion composition. The dispersion device used for performing such treatment is not particularly limited. The composite material slurry can be obtained using the dispersion device described for the carbon nanotube dispersion composition.

[0112] The content rate of the active material in the composite material slurry is preferably 20% by mass to 85% by mass, more preferably 40% by mass to 85% by mass, based on 100% by mass of the composite material slurry.

[0113] The content of carbon nanotubes in the composite material slurry is preferably 0.05 parts by mass to 10 parts by mass, more preferably 0.1 parts by mass to 5 parts by mass, and particularly preferably 0.1 parts by mass to 3 parts by mass, based on 100 parts by mass of the active material.

[0114] The content of the binder resin in the composite material slurry is preferably 0.5 parts by mass to 20 parts by mass, more preferably 1 part by mass to 10 parts by mass, and particularly preferably 1 part by mass to 5 parts by mass, based on 100 parts by mass of the active material.

[0115] The solid content concentration of the composite material slurry is preferably 30% by mass to 90% by mass, more preferably 40% by mass to 85% by mass, based on 100% by mass of the composite material slurry.

[0116] The water content in the composite material slurry is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

[0117] ≪Electrode≫ The electrode of this embodiment includes a current collector and an electrode film formed from the composite material slurry. The electrode film is a coating film of the composite material slurry. For example, it is a coating film in which an electrode composite layer is formed by coating and drying the composite material slurry on the current collector.

[0118] The material and shape of the current collector used for the electrode film of this embodiment are not particularly limited, and those suitable for various secondary batteries can be appropriately selected. For example, examples of the material of the current collector include metals and alloys such as aluminum, copper, nickel, titanium, or stainless steel. Also, generally, a foil on a flat plate is used as the shape, but those with a roughened surface, perforated foil-shaped ones, and mesh-shaped current collectors can also be used.

[0119] As a method of applying a composite slurry onto a current collector to form an electrode film, there are no particular limitations, and known methods can be used. Specifically, die coating method, dip coating method, roll coating method, doctor coating method, knife coating method, spray coating method, gravure coating method, screen printing method, electrostatic coating method, etc. can be mentioned. As the drying method, air drying, hot air drying, infrared heating, far-infrared heating, etc. can be used, but it is not particularly limited to these.

[0120] Also, after coating, rolling treatment may be performed using a flat plate press, calender roll, or the like. The thickness of the electrode composite layer is generally 1 μm or more and 500 μm or less, preferably 10 μm or more and 300 μm or less.

[0121] ≪Secondary battery≫ The secondary battery of this embodiment includes an electrode having the electrode film of the present invention and an electrolyte. The carbon nanotube dispersion composition of this embodiment forms a good conductive network in the secondary battery electrode, so it has excellent rate characteristics. During charge and discharge, the active material is uniformly utilized, so the deterioration of the active material is less likely to progress. Furthermore, overcharge and overdischarge during charge and discharge are suppressed. In addition, since there are few metal foreign substances derived from the carbon nanotube dispersion composition, deterioration of battery characteristics due to electrolyte decomposition and metal precipitation is less likely to occur, and it has excellent high-temperature cycle characteristics. As the positive electrode, one obtained by applying and drying a composite slurry containing a positive electrode active material on a current collector to produce an electrode film can be used.

[0122] As the negative electrode, one obtained by applying and drying a composite slurry containing a negative electrode active material on a current collector to produce an electrode film can be used.

[0123]

[0124] ​As the electrolyte, various conventionally known ones in which ions can move can be used. For example, those containing lithium salts such as LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, Li(CF3SO2)3C, LiI, LiBr, LiCl, LiAlCl, LiHF2, LiSCN, or LiBPh4 (where Ph is a phenyl group) etc. can be mentioned, but it is not limited thereto, and those containing sodium salts can also be used. The electrolyte is preferably dissolved in a non-aqueous solvent and used as an electrolytic solution.

[0125] The non-aqueous solvent is not particularly limited. For example, carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as γ-butyrolactone, γ-valerolactone, and γ-octanoic lactone; glymes such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-methoxyethane, 1,2-ethoxyethane, and 1,2-dibutoxyethane; esters such as methyl formate, methyl acetate, and methyl propionate; sulfoxides such as dimethyl sulfoxide and sulfolane; and nitriles such as acetonitrile etc. can be mentioned. These solvents may be used alone respectively, or may be used as a mixture of two or more kinds.

[0126] The secondary battery of this embodiment preferably includes a separator. Examples of the separator include, but are not particularly limited to, polyethylene non-woven fabric, polypropylene non-woven fabric, polyamide non-woven fabric, and those obtained by subjecting these to hydrophilic treatment.

[0127] The structure of the secondary battery of this embodiment is not particularly limited, but usually, it is composed of a positive electrode and a negative electrode, and a separator provided as necessary, and can be made into various shapes according to the purpose of use, such as a paper type, a cylindrical type, a button type, a laminated type, etc.

[0128] The use of the secondary battery of this embodiment is not particularly limited. Specifically, it can be used as a power source for consumer devices such as mobile phones, notebook computers, digital cameras, etc., as an emergency power source for hospitals, factories, buildings, etc., and for vehicles such as hybrid vehicles, plug-in hybrid vehicles, electric vehicles, assist bicycles, railway vehicles, etc. The secondary battery is, for example, one that recovers the regenerative energy of the vehicle's power.

[0129] Among them, since it is a secondary battery having high charge-discharge performance and excellent cycle characteristics, it can be suitably used for vehicles, and a vehicle with high safety and expected fuel consumption improvement can be obtained. Furthermore, excellent effects can also be exhibited in the case of vehicle applications where charge and discharge at a large current are desired.

[0130] The mounting position of the secondary battery in the vehicle of this embodiment is not particularly limited. For example, when mounting the secondary battery in an automobile, the secondary battery can be mounted in the engine room of the vehicle, behind the vehicle body, or under the seat. It can be mounted below.

Examples

[0131] Examples are given below to explain the present invention more specifically. The present invention is not limited to the following examples as long as it does not exceed the gist thereof. Unless otherwise specified, "parts" represents "parts by mass" and "%" represents "% by mass". Also, the compounding amounts in the table are in parts by mass, and except for the solvent, they are values in terms of non-volatile content. Note that the blanks in the table indicate that they are not compounded.

[0132] The materials used in the examples and comparative examples are shown below. · Hydrogenated nitrile butadiene rubber (manufactured by Zannan Scitech, ZN35052, Mooney viscosity 20, weight average molecular weight 110,000, alkylene structural unit 66% by mass, content of nitrile group-containing structural unit 34% by mass) was used as the dispersant (A) below. · Hydrogenated nitrile butadiene rubber (manufactured by Zannan Scitech, ZN35053, Mooney viscosity 35, weight average molecular weight 130,000, alkylene structural unit 64% by mass, content of nitrile group-containing structural unit 36% by mass) or less was used as dispersant (B). · Hydrogenated nitrile butadiene rubber (manufactured by Zannan Scitech, ZN28255, Mooney viscosity 50, weight average molecular weight 190,000, alkylene structural unit 72% by mass, content of nitrile group-containing structural unit 28% by mass) or less was used as dispersant (C). · Hydrogenated nitrile butadiene rubber (manufactured by Zannan Scitech, ZN35056, Mooney viscosity 65, weight average molecular weight 180,000, alkylene structural unit 64% by mass, content of nitrile group-containing structural unit 36% by mass) or less was used as dispersant (D). · Hydrogenated nitrile butadiene rubber (manufactured by Zannan Scitech, ZN35058, Mooney viscosity 85, weight average molecular weight 220,000, alkylene structural unit 64% by mass, content of nitrile group-containing structural unit 36% by mass) or less was used as dispersant (E). · Hydrogenated nitrile butadiene rubber (manufactured by ARLANXEO, Therban(R) 3406, Mooney viscosity 63, weight average molecular weight 200,000, alkylene structural unit 66% by mass, content of nitrile group-containing structural unit 34% by mass) or less was used as dispersant (F). · Hydrogenated nitrile butadiene rubber (manufactured by Zannan Scitech, liquid hydrogenated nitrile butadiene rubber, weight average molecular weight 30,000, alkylene structural unit 66% by mass, content of nitrile group-containing structural unit 34% by mass) or less was used as dispersant (G). · Polyvinylpyrrolidone (manufactured by Nippon Shokubai Co., Ltd., K-30, weight average molecular weight 40,000) or less was used as dispersant (H). · Carbon nanotubes (manufactured by JEIO, JENOTUBE10B) or less were used as carbon nanotubes (A1). · Carbon nanotubes (manufactured by JEIO, JENOTUBE6A) or less were used as carbon nanotubes (B1). · The purified CNT described in paragraph 0098 of Patent 6586197 was used as carbon nanotubes (I1).

[0133] <Production of Carbon Nanotube (C1)> Weighed 1 kg of carbon nanotube (A1) into a 100 L glass container, added 50 kg of 20% hydrochloric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and then stirred well using a stirrer under the condition of 25 °C. Then, it was sufficiently diluted with ion-exchanged water and vacuum filtration was performed using a membrane filter. After repeating the dilution and filtration operations, the CNT was transferred to a PTFE vat and then dried at 80 °C using an oven to obtain carbon nanotube (C1).

[0134] <Production of Carbon Nanotube (C2)> Carbon nanotube (C1) was charged into a dynamic mill (manufactured by Nippon Coke & Engineering Co., Ltd.) with 8 mm diameter zirconia beads as the grinding medium, and supplied at an operating condition of 10.0 kg / h and processed at a peripheral speed of 5.0 m / s to obtain carbon nanotube (C2).

[0135] <Production of Carbon Nanotube (D1)> Weighed 10 kg of carbon nanotube (A1) into a 120 L heat-resistant container, and installed the heat-resistant container containing the carbon nanotube in the furnace. Then, nitrogen gas was introduced into the furnace to discharge the air in the furnace while maintaining a positive pressure. After the oxygen concentration in the furnace became 0.1% or less, it was heated to 1600 °C over 30 hours. While maintaining the furnace temperature at 1600 °C, chlorine gas was introduced at a rate of 50 L / min for 50 hours. Then, nitrogen gas was introduced at 50 L / min and cooled while maintaining a positive pressure to obtain carbon nanotube (D1).

[0136] <Production of Carbon Nanotube (E1)> 10 kg of carbon nanotubes (A1) were weighed into a heat-resistant container of 120 L, and the heat-resistant container containing the carbon nanotubes was placed in a furnace. Then, nitrogen gas was introduced into the furnace, and while maintaining a positive pressure, the air in the furnace was discharged. After the oxygen concentration in the furnace reached 0.1% or less, it was heated to 1800 °C over 30 hours. While maintaining the furnace temperature at 1800 °C, chlorine gas was introduced at a rate of 50 L / min for 50 hours. Then, nitrogen gas was introduced at 50 L / min and cooled while maintaining a positive pressure to obtain carbon nanotubes (E1).

[0137] <Production of carbon nanotubes (E2)> The carbon nanotubes (E1) were passed through an electromagnet (CG-150HHH manufactured by Nippon Magnetics Co., Ltd.) three times to remove magnetic foreign substances in the raw material, and carbon nanotubes (E2) were obtained. An electromagnetic separator with a screen opening of 10 mm was used.

[0138] <Production of carbon nanotubes (E3)> The carbon nanotubes (E1) were charged into a dynamic mill (manufactured by Nippon Coke & Engineering Co., Ltd.) with zirconia beads having a diameter of 8 mm as a grinding medium, supplied at an operating condition of 10.0 kg / h, processed at a peripheral speed of 5.0 m / s, and then passed through an electromagnet (CG-150HHH manufactured by Nippon Magnetics Co., Ltd.) three times to remove magnetic foreign substances in the raw material, and carbon nanotubes (E3) were obtained. An electromagnet with a screen opening of 10 mm was used.

[0139] <Production of carbon nanotubes (F1)> 10 kg of carbon nanotubes (A1) were weighed into a heat-resistant container of 120 L, and the heat-resistant container containing the carbon nanotubes was placed in a furnace. Then, nitrogen gas was introduced into the furnace, and while maintaining a positive pressure, the air in the furnace was discharged. After the oxygen concentration in the furnace reached 0.1% or less, it was heated to 2000 °C over 30 hours. While maintaining the furnace temperature at 2000 °C, chlorine gas was introduced at a rate of 50 L / min for 50 hours. Then, nitrogen gas was introduced at 50 L / min and cooled while maintaining a positive pressure to obtain carbon nanotubes (F1).

[0140] <Production of Carbon Nanotubes (G1)> 10 kg of carbon nanotubes (A1) was weighed into a heat-resistant container of 120 L, and the heat-resistant container containing the carbon nanotubes was placed in a furnace. Then, nitrogen gas was introduced into the furnace, and while maintaining a positive pressure, the air in the furnace was discharged. After the oxygen concentration in the furnace became 0.1% or less, it was heated to 3000 °C over 30 hours. While maintaining the furnace temperature at 3000 °C, chlorine gas was introduced at a rate of 50 L / min for 50 hours. Then, nitrogen gas was introduced at 50 L / min, and it was cooled while maintaining a positive pressure to obtain carbon nanotubes (G1).

[0141] <Production of Carbon Nanotubes (H1)> 10 kg of carbon nanotubes (B1) was weighed into a heat-resistant container of 120 L, and the heat-resistant container containing the carbon nanotubes was placed in a furnace. Then, nitrogen gas was introduced into the furnace, and while maintaining a positive pressure, the air in the furnace was discharged. After the oxygen concentration in the furnace became 0.1% or less later, it was heated to 1800 °C over 30 hours. While maintaining the furnace temperature at 1800 °C, chlorine gas was introduced at a rate of 50 L / min for 50 hours. Then, nitrogen gas was introduced at 50 L / min, and after cooling while maintaining a positive pressure, the carbon nanotubes were taken out. The carbon nanotubes were charged into a dynamic mill (manufactured by Nippon Coke & Engineering Co., Ltd.) with zirconia beads having a diameter of 8 mm as a grinding medium, supplied at an operating condition of 10.0 kg / h, processed at a peripheral speed of 5.0 m / s, and then passed through an electromagnet (CG-150HHH manufactured by Nippon Magnetics Co., Ltd.) three times to remove metal foreign particles in the raw material, thereby obtaining carbon nanotubes (H1). An electromagnet with a screen opening of 10 mm was used.

[0142] Table 1 shows the physical properties of the carbon nanotubes used in the examples and comparative examples.

[0143]

Table 1

[0144] <Measurement of Weight-Average Molecular Weight of Dispersant> The weight-average molecular weight (Mw) of the dispersant was measured by gel permeation chromatography (GPC) equipped with an RI detector. Using HLC-8320GPC (manufactured by Tosoh Corporation) as the apparatus, three separation columns were connected in series, and the packing materials used were "TSK-GEL SUPER AW-4000", "AW-3000", and "AW-2500", all manufactured by Tosoh Corporation, in order. The oven temperature was 40 °C, and a solution of 30 mM triethylamine and 10 mM LiBr in N,N-dimethylformamide was used as the eluent, and the measurement was carried out at a flow rate of 0.6 mL / min. The measurement sample was adjusted to a concentration of 1% using the solvent consisting of the above eluent and 20 microliters were injected. The weight-average molecular weight is a polystyrene conversion value.

[0145] <Fabrication of Standard Negative Electrode> To a plastic container with a volume of 150 ml, 0.5 parts by mass of acetylene black (Denka Black (registered trademark) HS-100, manufactured by Denka), 1 part by mass of MAC500LC (sodium carboxymethyl cellulose salt Sunrose special type MAC500L, manufactured by Nippon Paper Industries Co., Ltd., non-volatile content 100%), and 98.4 parts by mass of water were added. Then, using a planetary mixer (Sinkei, Awatori Rentaro, ARE-310), it was stirred at 2000 rpm for 30 seconds. Further, 92 parts by mass of artificial graphite (manufactured by Nippon Graphite Industries, CGB-20) and 5 parts by mass of silicon (manufactured by Osaka Titanium Technology Co., Ltd., SILICON MONOOXIDE SiO 1.3C 5μm, non-volatile content 100%) were added as active materials, and using a high-speed stirrer, it was stirred at 3000 rpm for 10 minutes. Subsequently, 3.1 parts by mass of SBR (TRD2001, manufactured by JSR Corporation) was added, and using the above planetary mixer, it was stirred at 2000 rpm for 30 seconds to obtain a negative electrode composite material slurry. Thereafter, the negative electrode composite material slurry was applied using an applicator so that the basis weight per unit of the electrode was 8 mg / cm 2After coating on a copper foil so as to achieve the following, the coating film was dried in an electric oven at 120°C ± 5°C for 25 minutes. Further, rolling treatment was performed using a roll press (manufactured by Sanku Metal Co., Ltd., 3t hydraulic roll press), and a standard negative electrode with a density of the composite material layer of 1.6 g / cm 3 was produced.

[0146] ≪Physical Property Measurement and Evaluation Method≫ The physical property measurement and evaluation methods for the carbon nanotubes, carbon nanotube dispersion compositions, electrode films, and secondary batteries used in the following Examples and Comparative Examples are as follows.

[0147] <Outer Diameter of Carbon Nanotubes> Using an electronic balance (manufactured by Sartorius, MSA225S100DI), 0.2 g of carbon nanotubes was weighed into a 450 mL SM sample bottle (manufactured by Sansho Co., Ltd.). 200 mL of toluene was added, and using an ultrasonic homogenizer (Advanced Digital Sonifer (registered trademark), MODEL 450DA, BR manufactured by ANSON), dispersion treatment was performed under ice cooling at an amplitude of 30% for 5 minutes to prepare a carbon nanotube dispersion composition. Then, the carbon nanotube dispersion composition was appropriately diluted, several μL was dropped in the form of a collodion film, dried at room temperature, and then observed using a transmission electron microscope (H-7650, manufactured by Hitachi, Ltd.). Observation was performed at a magnification of 50,000 times, and multiple photos containing 10 or more carbon nanotubes in the visual field were taken. The average outer diameter of 300 arbitrarily extracted carbon nanotubes was measured, and the average value was taken as the outer diameter (nm) of the carbon nanotubes.

[0148] <BET Specific Surface Area of Carbon Nanotubes> Using an electronic balance (manufactured by Sartorius, MSA225S100DI), 0.03 g of carbon nanotubes was weighed, and then dried while degassing at 110°C for 15 minutes. Then, using a fully automatic specific surface area measuring device (manufactured by MOUNTECH, HM-model1208), the BET specific surface area of the carbon nanotubes was measured.

[0149] <G / D ratio of carbon nanotubes> The carbon nanotubes were placed on a Raman microscope (XploRA, manufactured by Horiba, Ltd.) and measured using a laser wavelength of 532 nm. The measurement conditions were an integration time of 60 seconds, an integration number of 2 times, a neutral density filter of 10%, an objective lens magnification of 20 times, a confocal hole of 500, a slit width of 100 μm, and the measurement wavelength was 100 cm -1 ~3000 cm -1 The carbon nanotubes for measurement were separated on a slide glass and flattened using a spatula. Among the obtained peaks, the maximum peak intensity within the spectrum range of 1560 cm -1 ~1600 cm -1 was defined as G, and the maximum peak intensity within the range of 1310 cm -1 ~1350 cm -1 was defined as D, and the ratio of G / D was defined as the G / D ratio of the carbon nanotubes.

[0150] <Angle of repose of carbon nanotubes> The angle of repose of the carbon nanotubes was measured using a bulk density measuring device (manufactured by Tsutsui Rikagaku Kikai Co., Ltd., JIS Kasa specific gravity measuring device). First, the mass of the receiver was measured, and then the bulk density measuring device was leveled, a funnel was attached to the funnel stand, a sieve was placed on the funnel, and the receiver was placed on the receiver stand. Then, the carbon nanotubes were placed on the sieve using a spatula, and the carbon nanotubes were gently and evenly wiped across the entire surface of the sieve with a brush, and the sample passing through the sieve was received by the receiver. This operation was repeated until the carbon nanotubes formed a heap in the receiver. The angle formed between the conically deposited powder at the time of forming a heap and the horizontal plane was measured using a protractor and defined as the angle of repose of the carbon nanotubes.

[0151] <Carbon purity of carbon nanotubes> Carbon nanotubes were acid-digested using a microwave sample pretreatment apparatus (ETHOS1, manufactured by Milestone General) to extract the metals contained in the carbon nanotubes. Subsequently, analysis was performed using a multi-type ICP emission spectrometer (720-ES, manufactured by Agilent), and the amount of metals (total amount of iron, cobalt, nickel, copper, nickel, and molybdenum) contained in the extract was calculated. The carbon purity of the carbon nanotubes was calculated as follows. Carbon purity (%) of carbon nanotubes = ((carbon nanotube mass - metal mass) ÷ carbon nanotube mass) × 100

[0152] <Initial viscosity of carbon nanotube dispersion composition> After allowing the carbon nanotube dispersion composition to stand in a thermostat at 25°C for 1 hour or more, the carbon nanotube dispersion composition was immediately measured at a B-type viscometer rotor rotation speed of 100 rpm. A No. 4 rotor was used for the measurement. The evaluation of the initial viscosity was as follows: less than 500 mPa·s: ◎ (excellent), 500 mPa·s or more and less than 1000 mPa·s: 〇 (good), 1000 mPa·s or more and less than 2000 mPa·s: △ (acceptable), 2000 mPa·s or more: × (poor).

[0153] <Content of metal foreign particles in carbon nanotube dispersion composition> 20 kg of the carbon nanotube dispersion composition was passed through an electromagnet (manufactured by Daiho Magnetic Co., Ltd., EMF-100S, magnetic flux density 16,000 gauss) via a hose pump at a flow rate of 30 L / min five times. Thereafter, N-methyl-2-pyrrolidone (NMP) was passed through the electromagnet (manufactured by Daiho Magnetic Co., Ltd., EMF-100S, magnetic flux density 16,000 gauss), and the carbon nanotube dispersion composition in the hose pump and the electromagnet was extruded and washed until the solid content was substantially eliminated. Further thereafter, after turning off the power supply of the electromagnet and confirming that the magnetic force of the filter disappeared, 10 kg of NMP was passed through the electromagnet at a flow rate of 30 L / min to obtain 10 kg of NMP containing metal foreign particles. Thereafter, 10 kg of NMP containing metal foreign particles was dispersed by ultrasonic treatment for 2 minutes under the conditions of an output of 300 W and a frequency of 28 kHz using an ultrasonic processor (manufactured by Iwamedical Industry Co., Ltd., ultrasonic cleaner), and then the entire amount was passed through a filter (weight (W1), material: polyester, disk diameter: 47 mm, aperture: 5 μm) using a filtering bell. Thereafter, 100 g of ethanol was used to wash the metal foreign particles deposited on the filter. Further thereafter, the filter on which the metal foreign particles were deposited was removed, and the filter was dried in a hot air oven at 60°C for 10 minutes, and the weight (W2) was measured. The filter weight (W1) was subtracted from the weight (W2) of the dried filter on which the metal foreign particles were deposited to calculate the content (weight of metal foreign particles) of the metal foreign particles in the carbon nanotube dispersion composition. The measurement results of the amount of metal foreign particles are as follows: 0.1 mg or less: A, more than 0.1 mg and 0.4 mg or less: B, more than 0.4 mg and 1.0 mg or less: C, more than 1.0 mg: D.

[0154] <Metal element amount of carbon nanotube dispersion composition> The carbon nanotube dispersion composition was dried using a hot air oven, and then acid decomposition was carried out using a microwave sample pretreatment apparatus (ETHOS1, manufactured by Milestone General) to extract the metals contained in the carbon nanotubes. Thereafter, analysis was performed using a multi-type ICP emission spectrometer (720-ES, manufactured by Agilent), and the amount of metal elements (total content of iron, cobalt, nickel, chromium, molybdenum, and copper) contained in the extract was calculated. The smaller the amount of metal elements, the more effectively the voltage failure of the secondary battery can be suppressed. The evaluation criteria for the amount of metal elements were set as follows: ◎ (excellent) for 30 ppm or less, 〇 (good) for more than 30 ppm and 50 ppm or less, △ (acceptable) for more than 50 ppm and 100 ppm or less, and × (unacceptable) for more than 100 ppm.

[0155] <Conductivity> Conductivity was evaluated based on the volume resistivity of the electrode film. The composite slurry was applied onto an aluminum foil using an applicator so that the basis weight per unit of the electrode was 20 mg / cm 2 After that, the coating film was dried in an electric oven at 120 °C ± 5 °C for 25 minutes. Thereafter, the surface resistivity (Ω / □) of the dried coating film was measured using a resistivity meter (Loresta GP (MCP-T610), manufactured by Mitsubishi Chemical Analytech Co., Ltd.; probe: AP2 probe (RMH333)). After the measurement, the volume resistivity (Ω·cm) of the electrode film was obtained by multiplying the thickness of the electrode composite layer formed on the aluminum foil. The thickness of the electrode composite layer was determined by subtracting the thickness of the aluminum foil from the average value measured at three points in the electrode film using a film thickness meter (DIGIMICRO MH-15M, manufactured by NIKON), and was used as the volume resistivity (Ω·cm) of the electrode film. The evaluation criteria for the volume resistivity were set as follows: ◎ (excellent) for less than 8 Ω·cm, 〇 (good) for 8 Ω·cm or more and less than 12 Ω·cm, △ (acceptable) for 12 Ω·cm or more and less than 15 Ω·cm, and (defective) for 15 Ω·cm or more.

[0156] <Adhesion> Adhesion was evaluated based on the peel strength of the electrode film. The composite slurry was applied onto an aluminum foil using an applicator so that the basis weight per unit of the electrode was 20 mg / cm2 After coating on the aluminum foil so as to achieve this, the coating film was dried in an electric oven at 120 °C ± 5 °C for 25 minutes. Thereafter, two cuts were made into rectangles measuring 90 mm × 20 mm with the coating direction as the major axis. For the measurement of the peel strength, a tabletop tensile testing machine (manufactured by Toyo Seiki Seisakusho, Strograph E3) was used and evaluated by the 180-degree peel test method. Specifically, a double-sided tape (No. 5000NS, manufactured by Nitto Denko Corporation) with a size of 100 mm × 30 mm was pasted onto a stainless steel plate, and the prepared battery electrode composite material layer was adhered to the other side of the double-sided tape. While pulling upward from below at a constant speed (50 mm / min), it was peeled off, and the average value of the stress at this time was taken as the peel strength. The evaluation criteria for the peel strength were as follows: 0.7 N / cm or more: ◎ (excellent), 0.5 N / cm or more and less than 0.7 N / cm: 〇 (good), 0.3 N / cm or more and less than 0.5 N / cm: △ (fair), less than 0.3 N / cm: × (poor).

[0157] <Evaluation of the rate characteristics of a lithium-ion secondary battery> The laminated lithium-ion secondary battery was placed in a thermostatic chamber at 25 °C, and charge and discharge measurements were performed using a charge and discharge device (manufactured by Hokuto Denko Corporation, SM-8). After performing constant current and constant voltage charging (cut-off current 1.0 mA (0.02C)) at a charging current of 10 mA (0.2C) and a charging cut-off voltage of 4.2V, constant current discharge was performed at a discharge current of 10 mA (0.2C) and a discharge cut-off voltage of 2.5V. After repeating this operation 3 times, constant current and constant voltage charging (cut-off current (1.0 mA 0.02C)) was performed at a charging current of 10 mA (0.2C) and a charging cut-off voltage of 4.2V, and constant current discharge was performed at discharge currents of 0.2C and 3C until the discharge cut-off voltage of 2.5V was reached, and the discharge capacities were determined respectively. The rate characteristics can be represented by the ratio of the 0.2C discharge capacity to the 3C discharge capacity, as shown in the following formula 2. (Formula 2) Rate characteristics = 3C discharge capacity / 0.2C discharge capacity of the third cycle × 100 (%) The evaluation criteria for the rate characteristics were as follows: those with a rate characteristic of 80% or more were rated ◎ (excellent), those with a rate characteristic of 70% or more and less than 80% were rated 〇 (good), those with a rate characteristic of 60% or more and less than 70% were rated △ (fair), and those with a rate characteristic of less than 60% were rated × (poor).

[0158] <Evaluation of High-Temperature Cycle Characteristics of Lithium-Ion Secondary Batteries> The laminated lithium-ion secondary battery was placed in a thermostatic chamber at 45°C, and charge-discharge measurements were performed using a charge-discharge device (manufactured by Hokuto Denko Corporation, SM-8). After performing constant-current constant-voltage charging (cutoff current: 1.25 mA (0.025C)) at a charging current of 50 mA (1C) and a charging termination voltage of 4.2V, constant-current discharging was performed at a discharging current of 50 mA (1C) and a discharging termination voltage of 2.5V. This operation was repeated 200 times. 1C is the current value at which the theoretical capacity of the positive electrode is discharged in 1 hour, and The high-temperature cycle characteristics can be represented by the ratio of the 1C discharge capacity at the third cycle to the 1C discharge capacity at the 100th cycle at 45°C, as shown in Equation 3 below. (Equation 3) High-temperature cycle characteristics = 1C discharge capacity at the 100th cycle / 1C discharge capacity at the third cycle × 100 (%) For the evaluation of high-temperature cycle characteristics, when the high-temperature cycle characteristics are 90% or more, it is rated as ◎ (excellent); when it is 85% or more and less than 90%, it is rated as 〇 (good); when it is 80% or more and less than 85%, it is rated as △ (fair); when it is less than 80%, it is rated as × (poor).

[0159] (Example 1-1) 92 parts of N-methyl-2-pyrrolidone (NMP) and 8 parts of dispersant (A) were added to a stainless steel container, and the mixture was stirred at 80°C using a disperser until the dispersant (A) was completely dissolved. After passing through a nylon mesh with an opening size of 48 μm, an 8% solution of dispersant (A) was prepared through a high magnetic force mag filter (manufactured by Aisin, surface magnetic flux density: 17,000 gauss). Subsequently, 89.5 parts of N-methyl-2-pyrrolidone (NMP) and 7.5 parts of an 8% solution of dispersant (A) were added to a stainless steel container and stirred with a disper until uniform. Then, 3 parts of carbon nanotubes (C1) were taken and added while stirring with a disper. A fine emulsifier screen was attached to a high shear mixer (L5M-A, manufactured by SILVERSON), and batch dispersion was carried out at a speed of 9000 rpm until the whole became uniform and the dispersion particle size became 200 μm or less as measured by a grind gauge. After that, a carbon nanotube pre-dispersion (C1) was prepared through a high magnetic force mag filter (manufactured by Aisin, surface magnetic flux density 17000 gauss). Further, the carbon nanotube pre-dispersion was fed, and a circulation type dispersion treatment with a residence time of 15 minutes (bead filling rate 80%, peripheral speed 12 m / s) was carried out using a bead mill (manufactured by Ashizawa Fine Tek Co., Ltd., Mugen Flow (registered trademark)) filled with zirconia beads having a diameter of 1.0 mmφ. Subsequently, the liquid to be dispersed was supplied to a high-pressure homogenizer (manufactured by Sugino Machine, Starburst Turbo), and a 15-pass type dispersion treatment was carried out. The dispersion treatment was carried out using a single nozzle chamber at a nozzle diameter of 0.25 mm and a pressure of 100 Mpa. After that, the liquid to be dispersed was supplied to an electromagnet (manufactured by Daiho Magnetic Co., Ltd., EMF-100S, magnetic flux density 16000 gauss, spatial volume 1.7 L, diameter: 10 cm, thickness 1.3 cm, equipped with 31 grid screens), and a 3-pass type treatment was carried out. Then, it was passed through depth filters (manufactured by 3M, PP non-woven fabric depth cartridge NT-T series, filtration accuracy 20 μm) installed in series, and a carbon nanotube dispersion composition 1 was prepared.

[0160] (Examples 1-2 to 1-20), (Comparative Examples 1-1 to 1-13) Dispersion compositions 1-1 to 1-12 and comparative dispersion compositions 1-1 to 1-9 were obtained in the same manner as in Example 1-1, except that the dispersion conditions, carbon nanotubes, dispersants, and NMP listed in Table 2 were changed. The depth filters in the examples and comparative examples were 3M-made, PP non-woven depth cartridge NT-T series, and the surface filters used nylon mesh. When using nylon mesh, instead of passing through two depth filters (3M-made, PP non-woven depth cartridge NT-T series, filtration accuracy 20 μm) installed in series, a filtration bell was used to pass through the nylon mesh to obtain the carbon nanotube dispersion compositions of the examples and comparative examples. The dispersion compositions of Comparative Examples 1-4 and 1-11 could not be filtered through a filter with a filtration accuracy of 3 μm and could not be prepared.

[0161]

Table 2

[0162] Table 3 shows the evaluation results of the dispersion compositions prepared in (Examples 1-1 to 1-20) and (Comparative Examples 1-1 to 1-13).

[0163]

Table 3

[0164] (Example 2-1) Volume 150 cm 3To a plastic container, 18.8 parts by mass of an NMP solution in which 8% by mass of PVDF (polyvinylidene fluoride, manufactured by Solvey, Solef#5130) was dissolved and 5.8 parts by mass of NMP were weighed. Then, 13.3 parts by mass of a carbon nanotube dispersion composition (dispersion composition 1) was added, and using a planetary mixer (Avatar Kenjiro, ARE-310), it was stirred at 2000 rpm for 30 seconds. Subsequently, 98.1 parts by mass of a positive electrode active material (manufactured by BASF Toda Battery Materials Co., Ltd., HED (registered trademark) NCM-111 1100) was added, and using a planetary mixer (Avatar Kenjiro, ARE-310), it was stirred at 2000 rpm for 2.5 minutes to obtain a composite material slurry (composite material slurry 1).

[0165] Subsequently, the composite material slurry (composite material slurry 1) was applied onto an aluminum foil using an applicator so that the basis weight per unit of the electrode was 20 mg / cm 2 Then, after drying the coating film in an electric oven at 120 °C ± 5 °C for 25 minutes, an electrode film (electrode film 1) was obtained. Then, the electrode film (electrode film 1) was subjected to rolling treatment using a roll press (manufactured by Sanku Metal, 3t hydraulic roll press) to obtain a positive electrode (positive electrode 1). The basis weight per unit of the composite material layer was 20 mg / cm 2 and the density of the composite material layer after the rolling treatment was set to 3.1 g / cc.

[0166] (Examples 2-2 to 2-20), (Comparative Examples 2-1 to 2-13) As shown in Table 4, except that dispersion compositions 2 to comparative dispersion compositions 13 were used instead of dispersion composition 1, composite material slurries 2 to comparative composite material slurries 13, electrode films 2 to comparative electrode films 13, and positive electrodes 2 to comparative positive electrodes 13 were obtained by the same method as in Example 2-1.

[0167] Table 4 shows the evaluation results of the electrode films prepared in (Examples 2-1 to 2-20), (Comparative Examples 2-1 to 2-13).

[0168]

Table 4

[0169] (Example 3-1) The positive electrode (positive electrode 1) and the standard negative electrode were each punched out to 45 mm × 40 mm and 50 mm × 45 mm, and a separator (porous polypropylene film) inserted therebetween was inserted into an aluminum laminate bag and dried in an electric oven at 60°C for 1 hour. Thereafter, in a glove box filled with argon gas, an electrolytic solution (a mixed solvent prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate at a ratio of 1:1:1 (volume ratio), and further, as an additive, VC (vinylene carbonate) was added at 2 parts by mass per 100 parts by mass of the mixed solvent, and then LiPF6 was dissolved at a concentration of 1 M non-aqueous electrolytic solution) was injected in an amount of 2 mL, and then the aluminum laminate was sealed to fabricate a laminate-type lithium ion secondary battery (secondary battery 1).

[0170] (Examples 3-2 to 3-20), (Comparative Examples 3-1 to 3-13) Laminate-type lithium ion secondary batteries (secondary batteries 2) to (Comparative secondary battery 13) were fabricated in the same manner as the fabrication of the laminate-type lithium ion secondary battery (secondary battery 1), except that the positive electrode listed in Table 5 was changed.

[0171]

Table 5

[0172] In the above example, a carbon nanotube dispersion composition containing a carbon nanotube, a copolymer, and an amide-based polar solvent, wherein the content of metal foreign particles required under Condition 1 is 1.0 mg or less, and the copolymer has an alkylene structural unit with a content of 50% by mass or more and 75% by mass or less, and a nitrile group-containing structural unit with a content of 25% by mass or more and 50% by mass or less It was used. In the examples, compared with the comparative examples, a lithium ion secondary battery was obtained in which the initial viscosity of the carbon nanotube dispersion composition was low and the secondary battery characteristics, particularly the high-temperature cycle characteristics, were excellent. Therefore, it has become clear that the present invention can provide a lithium ion secondary battery having high capacity, high output, and high durability, which is difficult to achieve with conventional carbon nanotube dispersion compositions. A vehicle having the lithium ion secondary battery of the present invention has high charge and discharge performance and excellent high-temperature cycle characteristics, so that a vehicle with high safety and improved fuel efficiency can be obtained.

[0173] As described above, the present invention has been described with reference to the embodiments, but the present invention is not limited thereto. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the invention.

Claims

1. A carbon nanotube dispersion composition comprising carbon nanotubes, a copolymer, and an amide-based polar solvent, wherein the content of metal foreign particles determined by the following Condition 1 is 1.0 mg or less, the copolymer has an alkylene structural unit with a content of 50% by mass or more and 75% by mass or less, and a nitrile group-containing structural unit with a content of 25% by mass or more and 50% by mass or less, a carbon nanotube dispersion composition. <Condition 1> The metal foreign particles in 20 kg of the carbon nanotube dispersion composition are recovered using an electromagnet (an electromagnet having 31 grid screens with a magnetic flux density of 16,000 gauss, a spatial volume of 1.7 L, a diameter of 10 cm, and a thickness of 1.3 cm), washed with an amide-based polar solvent, and the obtained metal foreign particles are deposited on a filter with a disk diameter of 47 mm and a mesh opening of 5 μm, and the weight of the metal foreign particles on the filter is measured.

2. The carbon nanotube dispersion composition according to Claim 1, wherein the total content of iron, cobalt, nickel, chromium, molybdenum, and copper is 100 ppm or less.

3. The carbon nanotube dispersion composition according to Claim 1, wherein the angle of repose of the carbon nanotubes is 40° or more.

4. The carbon nanotube dispersion composition according to Claim 1, wherein the viscosity at 25°C measured by a B-type viscometer is less than 2,000 mPa·s.

5. A composite material slurry comprising the carbon nanotube dispersion composition according to any one of Claims 1 to 4 and an active material.

6. An electrode film formed from the composite material slurry according to Claim 5.

7. A secondary battery comprising a positive electrode and a negative electrode, wherein at least one of the positive electrode and the negative electrode has the electrode film according to Claim 6.

8. A vehicle equipped with the secondary battery according to Claim 7.

9. A method for producing the carbon nanotube dispersion composition according to any one of Claims 1 to 4, comprising all of the following steps (1) to (3): [Step (1): Crushing step] A step of applying a shear stress to carbon nanotubes to crush the carbon nanotubes [Step (2): Magnetic separation step] A step of removing metal foreign particles using an electromagnet having a magnetic flux density of 10,000 gauss or more and 20,000 gauss or less [Step (3): Filtration step] A step of filtering using a depth filter with a filtration accuracy of 5 μm or more and 50 μm or less ​ ​

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