Carbon nanotubes, carbon nanotube dispersion liquid, binder composition, composition for electrode, and secondary battery

Abstract

Disclosed are carbon nanotubes which satisfy the following requirements (1) to (3) and include multi-walled carbon nanotubes.　(1) If the maximum peak intensity within the range of 1,560-1,600 cm-1 is defined as G and the maximum peak intensity within the range of 1,310-1,350 cm-1 in the Raman spectrum is defined as D, the G / D ratio is 0.5 to 3.0 inclusive.　(2) The wetting index represented by formula (I) is 10 or less.　Formula (I): (wetting index) = (X / Y) (In formula (I), Y represents the mass (g) of the carbon nanotubes, and X represents the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotubes when N-methyl-2-pyrrolidone is dropped onto Y (g) of the carbon nanotubes in an environment at 25°C.)　(3) The aluminum content is 3,000 ppm or less.

Description

Carbon nanotubes, carbon nanotube dispersion, binder composition, electrode composition, and secondary battery

[0001] FIELD OF THE INVENTION The present invention relates to carbon nanotubes, a carbon nanotube dispersion, a binder composition, an electrode composition, and a secondary battery.

[0002] With the spread of electric vehicles and the trend toward smaller, lighter, and more powerful portable devices, secondary batteries with high energy density and even higher capacity are required. Under these circumstances, lithium-ion secondary batteries, in particular, are being used in many devices.

[0003] Carbon black, ketjen black, graphene, fine carbon materials, and the like are used as conductive additives in secondary batteries. Among these, carbon nanotubes (hereinafter also referred to as "CNTs"), a type of fibrous fine carbon material, are widely used. For example, adding CNTs to electrode active materials reduces electrode resistance, improves battery load resistance, increases electrode material strength, and increases the resistance of electrodes to expansion and contraction, thereby improving the rate characteristics and cycle life of secondary batteries. Among these, multi-walled CNTs with an outer diameter of 5 nm to several tens of nm are relatively inexpensive and are becoming widely used.

[0004] CNTs can generally be produced by methods such as arc discharge, laser ablation, and chemical vapor deposition. Among these, chemical vapor deposition is the most suitable for mass production in terms of productivity and economy, and is widely used. In chemical vapor deposition, CNTs are produced by reacting a carbon source gas with a catalyst containing a metal such as iron, cobalt, or nickel. Therefore, CNTs obtained by chemical vapor deposition contain particles of the catalyst containing a metal such as iron, cobalt, or nickel, or carbides or oxides derived from the catalyst. Metal-containing catalysts (hereinafter also referred to as catalytic metals) are essential for some CNT production methods. However, metals derived from the catalytic metal remaining in the CNTs after CNT production can become impurities. Furthermore, regardless of the CNT production method, metals can be contaminated into CNTs due to wear of metals used in synthesis equipment, filling equipment, piping, etc. during production. Such contaminated metals can become impurities. To obtain desired properties from CNTs, it is desirable for the metal content in CNTs to be low. For example, when CNTs containing metals derived from catalytic metals or the like are used in secondary batteries, the metals may dissolve and precipitate, causing problems such as short-circuiting the battery. A short-circuiting of the battery can lead to serious accidents such as fire or explosion. Therefore, in order to suppress problems caused by the metals contained in CNTs and further improve safety, several methods have been proposed for purifying CNTs to remove metals derived from catalytic metals or the like.

[0005] Patent Document 1 describes a method for purifying a carbon material containing CNTs through a carbon material preparation step in which a raw material containing at least carbon and a catalytic metal is used as an anode to prepare a carbon material containing CNTs by an arc discharge method, and a halogen treatment step in which the carbon material is brought into contact with a gas containing a halogen and / or a halogen compound, thereby removing the catalytic metal impurity while preventing the CNTs from being damaged or cut or from solidifying into clumps.

[0006] Patent Document 2 describes that when CNTs having a G band to D band intensity ratio (G / D ratio) of 50 or more in Raman spectroscopic analysis are subjected to liquid-phase oxidation with nitric acid, CNTs of higher quality are obtained that are free of catalyst residues, have high heat resistance, and produce fewer carbon by-products.

[0007] International Publication No. WO 2008 / 126534 International Publication No. WO 2018 / 043487

[0008] However, when CNTs are subjected to a halogen treatment process, the catalytic metal residue is removed, but the CNTs are baked at high temperatures for a long time, which increases the crystallinity of the CNTs. Highly crystalline CNTs are hard and therefore prone to problems due to breakage or damage during use. Therefore, when CNTs are used as a conductive additive in electrode films of secondary batteries, etc., the CNTs may break during the manufacturing process leading up to the formation of the electrode film, increasing the contact resistance between the CNTs. As a result, the conductivity of the electrode film may decrease, and the performance of the secondary battery containing CNTs may be reduced.

[0009] Furthermore, by performing liquid-phase oxidation of CNTs with nitric acid, the amount of catalytic metal residue can be reduced, but because nitric acid has a strong oxidizing power, the surface of the CNTs may be oxidized, potentially reducing the conductivity of the electrode film using CNTs.

[0010] As described above, it is difficult to reduce metals derived from catalytic metals and the like contained in CNTs without degrading CNT properties. Therefore, further improvements in CNTs are desired to improve the safety and performance of secondary batteries. In light of these circumstances, the present invention provides carbon nanotubes that can be used to form electrode films that are safer and have good electrical conductivity. Other embodiments provide carbon nanotube dispersions, binder compositions, electrode compositions, and secondary batteries that contain the carbon nanotubes.

[0011] In order to solve the above problems, the present inventors have conducted extensive research and have found carbon nanotubes that satisfy the following conditions, thereby completing the present invention. That is, one embodiment of the present invention relates to carbon nanotubes, including multi-walled carbon nanotubes, that satisfy the following conditions (1) to (3): (1) A peak in the Raman spectrum of 1560 to 1600 cm －１ The maximum peak intensity in the range of 1310 to 1350 cm －１ When the maximum peak intensity within this range is defined as D, the G / D ratio is 0.5 or more and 3.0 or less. (2) The wetting index represented by the following formula (I) is 10 or less. Formula (I): Wetting index = (X / Y) [In formula (I), Y is the mass (g) of the carbon nanotubes, and X is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotubes when N-methyl-2-pyrrolidone is dropped onto Y (g) of carbon nanotubes in a 25°C environment.] (3) The aluminum content is 3000 ppm or less.

[0012] One embodiment of the present invention relates to carbon nanotubes, including multi-walled carbon nanotubes, that satisfy the following (1) to (3): (1) In differential thermal analysis when the temperature is increased from 200°C to 1000°C at a rate of 10°C / min, the carbon nanotubes have an exothermic peak between 600°C and 800°C. (2) In Raman spectroscopy, the carbon nanotubes have an exothermic peak between 1560 and 1600cm. －１ The maximum peak intensity in the range of 1310 to 1350 cm －１ (3) The aluminum content is 3000 ppm or less, and the G / D ratio is 0.5 or more and 3.0 or less, where D is the maximum peak intensity within the range.

[0013] One embodiment of the present invention relates to a carbon nanotube dispersion liquid containing the carbon nanotubes of the above embodiment, a dispersant, and a dispersion medium.

[0014] One embodiment of the present invention relates to a binder composition containing the carbon nanotube dispersion of the above embodiment and a binder.

[0015] One embodiment of the present invention relates to a composition for an electrode, which comprises the carbon nanotube dispersion liquid of the above embodiment and an electrode active material.

[0016] One embodiment of the present invention relates to a composition for an electrode, which comprises the carbon nanotube dispersion liquid of the above embodiment and an electrode active material.

[0017] One embodiment of the present invention relates to a secondary battery including an electrode film, wherein the electrode film is obtained using a carbon nanotube dispersion containing the carbon nanotubes of the above embodiment, a dispersant, and a dispersion medium, a binder composition containing the above carbon nanotube dispersion and a binder, or an electrode composition containing the above carbon nanotube dispersion and an electrode active material.

[0018] According to the present invention, it is possible to provide carbon nanotubes that can be suitably used to form electrode films that are safe and have good conductivity. Furthermore, it is possible to provide a carbon nanotube dispersion, a binder composition, and an electrode composition that can be suitably used in secondary battery applications using the carbon nanotubes. Furthermore, it is possible to provide a secondary battery that is excellent in safety and high in performance.

[0019] Hereinafter, embodiments of the present invention will be described in detail. However, it goes without saying that the present invention is not limited to the embodiments described below, and various modifications can be made without departing from the spirit of the present invention.

[0020] In this specification, numerical ranges indicated using "to" indicate ranges that include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In numerical ranges described in stages in this specification, the upper limit or lower limit of a numerical range in one stage can be arbitrarily combined with the upper limit or lower limit of a numerical range in another stage.

[0021] <1> Carbon Nanotubes (CNTs) One embodiment of the present invention relates to carbon nanotubes (hereinafter also referred to as CNTs). The first CNT according to this embodiment satisfies the following (I-1) to (I-3) and includes at least multi-walled carbon nanotubes: (I-1) A peak in the Raman spectrum of 1560 to 1600 cm －１ The maximum peak intensity in the range of 1310 to 1350 cm －１ When the maximum peak intensity within this range is defined as D, the G / D ratio is 0.5 or more and 3.0 or less. (I-2) The wetting index represented by the following formula (I) is 10 or less. Formula (I): Wetting index = (X / Y) [In formula (I), Y is the mass (g) of the carbon nanotubes, and X is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotubes when N-methyl-2-pyrrolidone is dropped onto Y (g) of carbon nanotubes in a 25°C environment.] (I-3) The aluminum content is 3000 ppm or less.

[0022] The second CNT of this embodiment satisfies the following (II-1) to (II-3) and contains multi-walled carbon nanotubes: (II-1) In differential thermal analysis when the temperature is increased from 200°C to 1000°C at a rate of 10°C / min, the CNT has an exothermic peak between 600°C and 800°C; (II-2) In Raman spectrum, the CNT has an exothermic peak between 1560 and 1600cm －１ The maximum peak intensity in the range of 1310 to 1350 cm －１ When the maximum peak intensity within the range is defined as D, the G / D ratio is 0.5 or more and 3.0 or less. (II-3) The aluminum content is 3000 ppm or less.

[0023] The first CNT according to this embodiment is characterized by satisfying all of the above requirements (I-1) to (I-3). That is, the first CNT has a specific G / D ratio, a specific wettability index, and a limited aluminum content, thereby suppressing the occurrence of defects due to metal impurities and enabling the formation of an electrode film with excellent conductivity. A secondary battery containing such CNT is highly safe and can exhibit good performance. When the CNT includes multi-walled carbon nanotubes, it becomes easy to satisfy all of the above requirements (I-1) to (I-3).

[0024] Furthermore, the second CNT according to this embodiment is characterized by satisfying all of the above requirements (II-1) to (II-3). That is, the second CNT exhibits an exothermic peak in a specific range in differential thermal analysis, has a specific G / D ratio, and further has a limited aluminum content, thereby suppressing the occurrence of defects due to metal impurities and improving safety. Furthermore, an electrode film with excellent conductivity can be formed. A secondary battery including such CNT can exhibit excellent performance. When the CNT includes multi-walled carbon nanotubes, it becomes easy to satisfy all of the above requirements (II-1) to (II-3).

[0025] Each requirement will be described in more detail below. In the following description, unless otherwise specified, the CNT will be collectively referred to as the first CNT and the second CNT, and will be simply referred to as CNT. <G / D ratio> The G / D ratio (peak ratio between the G-band and the D-band) of the CNT of this embodiment is determined by Raman spectroscopy. Various laser light wavelengths are used in Raman spectroscopy, but in this embodiment, wavelengths of 532 nm and 632 nm are used. In the Raman spectrum, －１ The Raman shift observed around 1,350 cm is called the G band derived from graphite. －１ The Raman shift observed around 1000 Hz is called the D band, which is derived from defects in amorphous carbon and graphite. The wavenumber in Raman spectroscopy may vary depending on the measurement conditions, so the wavenumber specified here is ±10 cm. －１The higher the G / D ratio of carbon nanotubes, the higher the crystallinity. Furthermore, when carbon nanotubes are fired at high temperatures, the G / D ratio tends to increase, and the longer the firing time, the higher the G / D ratio tends to increase.

[0026] CNTs have a Raman spectrum of 1560-1600 cm －１ The maximum peak intensity in the range of 1310 to 1350 cm －１ When the maximum peak intensity within this range is defined as D, the G / D ratio is preferably 0.5 or more and 3.0 or less. The G / D ratio is more preferably 0.5 or more and 2.5 or less, even more preferably 0.5 or more and 2.0 or less, particularly preferably 0.5 or more and 1.5 or less, and even more preferably 0.5 or more and 1.3 or less. If the G / D ratio of the CNT exceeds the above range, the CNT becomes hard, and therefore the CNT is easily damaged during dispersion, which may increase the contact resistance. On the other hand, if the G / D ratio of the CNT is below the above range, the conductivity of the CNT itself is likely to be low. For these reasons, when the G / D ratio of the CNT is within the above range, the rate characteristics and cycle characteristics of a secondary battery using an electrode film made from a CNT dispersion are improved.

[0027] Conventionally, when raw CNTs are purified by a halogen treatment process, the CNTs are fired at high temperatures for long periods of time, which tends to increase the crystallinity of the CNTs. This state can also be confirmed by a high G / D ratio. On the other hand, according to this embodiment, the CNTs can be heat-treated in an inert atmosphere under reduced pressure and vacuum during the CNT purification process. This allows for a low heat treatment temperature and a short heat treatment time, thereby suppressing high crystallinity of the CNTs. In other words, an increase in the G / D ratio of the CNTs can be suppressed. When low-crystalline CNTs are used, bending of the CNTs can be suppressed by a dispersion process or the like during the electrode film production process, thereby suppressing an increase in contact resistance between CNTs in the resulting electrode film. As a result, low-crystalline CNTs can achieve good conductivity as an electrode film, and secondary batteries containing CNTs can exhibit good performance.

[0028] <Wetting Index> The wetting index is the ratio of the maximum mass of solvent absorbed by CNT to the mass of CNT, and is one of the indicators of CNT dispersibility, particularly the initial viscosity when preparing a dispersion containing a dispersion medium and CNT. More specifically, the CNT of this embodiment has a wetting index represented by the following formula (I) of 10 or less. The wetting index may be preferably 9.5 or less, more preferably 9.0 or less, and even more preferably 8.5 or less. When the wetting index is 10 or less, it becomes easy to obtain a suitable initial viscosity when preparing a dispersion containing a dispersion medium and CNT. When the wetting index exceeds 10, the initial viscosity becomes high when preparing the dispersion, which makes stirring with a stirrer and pumping difficult, and poor dispersion is likely to occur. Furthermore, when the initial viscosity is high, the conductivity of the CNT is likely to decrease.

[0029] Formula (I): Wetting index = (X / Y)

[0030] In formula (I), Y is the mass (g) of carbon nanotubes (CNTs), and X is the maximum mass (g) of N-methyl-2-pyrrolidone (NMP) absorbed by the CNTs when NMP is dropped onto Y (g) of CNTs in an environment of 25° C. The maximum mass (g) of NMP absorbed by the CNTs is the total mass of NMP dropped onto the CNT powder up to just before the NMP starts to flow out of the CNT powder.

[0031] The wettability index can be determined, for example, according to the following procedure. First, Y (g) of CNT powder is placed in a container by gravity in a 25°C environment. With the container standing still, 5 g of NMP is dropped onto the surface of the CNT powder in the container at 1-minute intervals. Next, it is observed whether the NMP droplets begin to flow out onto the surface of the CNT powder without being absorbed by the CNT powder. The total mass (g) of NMP dropped up to just before the NMP droplets begin to flow out onto the surface of the CNT powder is defined as X (g). From the values ​​of X (g) and Y (g) obtained in this way, the wettability index shown in formula (I) is calculated.

[0032] <Metal Content> From the viewpoint of safety, it is desirable that the content of impurities such as metals remaining in the purified CNT is small. Here, the metals contained in the CNT mainly refer to metals and metal oxides derived from the catalyst metal used during CNT production, but also include metals that are mixed in during CNT production. For example, metals such as stainless steel used in synthesis equipment, filling equipment, or piping may be mixed into the CNT due to wear or the like. In some embodiments, from the viewpoint of further enhancing safety, the total content of metals contained in the CNT is preferably 13,000 ppm or less, more preferably 10,000 ppm or less, even more preferably 9,000 ppm or less, and even more preferably 8,000 ppm or less.

[0033] The CNT of this embodiment has an aluminum content of 3000 ppm or less. The aluminum content of the CNT may be preferably 2000 ppm or less, more preferably 1100 ppm or less, and even more preferably 500 ppm or less. In some embodiments, the content may be more preferably 100 ppm or less, and even more preferably 50 ppm or less. The aluminum content may be 0 ppm. When aluminum oxide is used as a catalyst support during CNT synthesis, aluminum oxide nanoparticles, which are insulating components, may remain in the CNT, impairing conductivity. When the aluminum content of the CNT is within the above range, the amount of aluminum oxide nanoparticles, which are insulating components, is low, and the carbon nanotubes can be used to form electrode films with good conductivity. Furthermore, when the aluminum content of the CNT is within the above range, a conductive paste with excellent conductivity can be produced.

[0034] Representative CNTs also contain metals other than aluminum (hereinafter referred to as other metals). The content of other metals is preferably 10,000 ppm or less, more preferably 7,000 ppm or less, and particularly preferably 5,000 ppm or less.

[0035] The other metals may preferably be magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, or molybdenum. In the CNT, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be included as being derived from the catalytic metal. In addition to the metals and metal oxides used as catalytic metals, metals such as stainless steel used in synthesis equipment, filling equipment, or piping may be mixed into the CNT due to wear or the like. Therefore, in the CNT, the metals may be aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, or molybdenum, but these may also be metals not derived from the catalytic metal.

[0036] In some embodiments, the total content of magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum contained in the first CNT is preferably 10,000 ppm or less, more preferably 7,000 ppm or less, and particularly preferably 5,000 ppm or less. The total content is more preferably 3,000 ppm or less, even more preferably 2,500 ppm or less, even more preferably 1,850 ppm or less, and even more preferably 700 ppm or less. When the total content of these metals is within the above range, it is easier to improve the safety of the secondary battery. In the following description, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be collectively referred to simply as other metals.

[0037] In some embodiments, the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum contained in the second CNTs is preferably 10,000 ppm or less. The total content may be 8,000 ppm or less, or 7,000 ppm or less. In some embodiments, the total content is more preferably 5,000 ppm or less, even more preferably 3,000 ppm or less, even more preferably 2,100 ppm or less, and even more preferably 830 ppm or less. When the total content of these metals is within the above range, it is easier to improve the safety of the secondary battery. In the following description, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be collectively referred to simply as other metals.

[0038] Here, the aluminum content and other metal content contained in CNT are masses converted into elemental metals. CNT may contain metals as elemental metals, metal oxides, metal composite oxides, etc., and these are converted into elemental metals to determine the metal content. In CNT, aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may each be contained as elemental metals, metal oxides, composite oxides thereof, etc. These metals can cause short circuits, so it is desirable to reduce their total content.

[0039] In some embodiments, from the viewpoint of further enhancing safety, it is preferable to limit the content of cobalt and / or iron among the other metals. In some embodiments, the total cobalt content in the first CNT is preferably 10,000 ppm or less, more preferably 7,000 ppm or less, particularly preferably 6,500 ppm or less, more preferably 3,000 ppm or less, even more preferably 1,700 ppm or less, and even more preferably 800 ppm or less. Also, in some embodiments, the total iron content in the first CNT is preferably 7,000 ppm or less, more preferably 3,000 ppm or less, even more preferably 2,500 ppm or less, and even more preferably 1,000 ppm or less. Furthermore, in some embodiments, the total content of cobalt and iron in the first CNT is preferably 7000 ppm or less, more preferably 3000 ppm or less, even more preferably 2000 ppm or less, and even more preferably 1000 ppm or less.

[0040] In some embodiments, the total cobalt content in the second CNT is preferably 5000 ppm or less, more preferably 3000 ppm or less, even more preferably 2000 ppm or less, and even more preferably 1250 ppm or less. Also, in some embodiments, the total iron content in the second CNT is preferably 5000 ppm or less, more preferably 3000 ppm or less, even more preferably 2500 ppm or less, and even more preferably 1000 ppm or less. Furthermore, in some embodiments, the total cobalt and iron content in the second CNT is preferably 5000 ppm or less, more preferably 3000 ppm or less, even more preferably 2500 ppm or less, and even more preferably 1300 ppm or less.

[0041] In this embodiment, as in the CNT purification method described below, the total content of metals such as aluminum contained in the CNT can be reduced by calcining the CNT in an inert atmosphere under reduced pressure vacuum. By reducing the aluminum content and the total content of other metals such as cobalt and iron contained in the CNT, the exothermic peak temperature can be increased, resulting in CNT with improved safety. Secondary batteries containing such CNT can exhibit good performance.

[0042] The aluminum content and other metal content in CNT can be calculated, for example, by acid decomposing CNT, extracting the metals contained in the CNT, and analyzing the extract using inductively coupled plasma (ICP). The total content of aluminum and other metals such as magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum contained in CNT is expressed as the mass ratio (ppm) of the total content of extracted aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum, relative to the mass of the CNT before metal extraction. Here, the total content of aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is calculated by calculating the mass of each metal when converted to its simple metal form and adding them together.

[0043] <Exothermic Peak> From the viewpoint of safety, the CNTs of this embodiment preferably have an exothermic peak between 600°C and 800°C in differential thermal analysis (DTA) when heated from 200°C to 1000°C at a rate of 10°C / min. The exothermic peak can be measured by subjecting the CNTs to differential thermal analysis in an air atmosphere. DTA is a method in which the temperature difference between a sample and a reference material is measured as a function of temperature while the temperatures of the sample and the reference material are changed under certain conditions, and conforms to JIS K 0129. In a DTA curve created based on changes in the temperature difference between the sample and the reference material, the largest peak is taken as the exothermic peak.

[0044] Heat is generated as CNTs burn. As the CNT combustion initiation temperature increases, the exothermic peak temperature also increases. Factors that affect the change in CNT combustion initiation temperature include the content of metals derived from the catalytic metal, the degree of oxidation of the CNT surface, and the crystallinity of the CNT. When the catalytic metal (metal) contained in CNTs has a high heat storage capacity, a low catalytic metal content reduces the total heat storage capacity of all catalytic metals. Because the total heat storage capacity of the catalytic metals is low, the temperature required to burn CNTs may be higher than when the catalytic metal content is high. In addition to catalytic metals, metals may be mixed into CNTs during the CNT production process, and such metals may also affect the total heat storage capacity. Furthermore, since sites on the CNT surface that have oxygen-containing functional groups are more combustible than sites without functional groups, the lower the amount of oxygen-containing functional groups (i.e., the lower the surface oxygen content), the more difficult CNTs are to burn. The lower the degree of oxidation of the CNT surface, the higher the temperature required to burn CNTs. Furthermore, the higher the crystallinity of CNTs, the higher the combustion initiation temperature of CNTs. The crystallinity of CNTs can be expressed by the G / D ratio described above.

[0045] When the combustion start temperature of CNT is within an appropriate temperature range, the impurities contained in the CNT are reduced, resulting in safer CNT. The CNT of this embodiment preferably has an exothermic peak temperature of 600°C or higher, more preferably 650°C or higher. Furthermore, the exothermic peak temperature is preferably 800°C or lower, more preferably 740°C or lower. When the exothermic peak temperature is 600°C or higher, the metal content is low, improving the safety of the battery. Alternatively, the surface oxygen content is low, resulting in excellent conductivity. When the exothermic peak temperature is 800°C or lower, the crystallinity of the CNT is not too high, preventing the CNT from breaking and suppressing a decrease in the performance of the secondary battery. For example, a temperature of 600°C or higher and 800°C or lower, 630°C or higher and 750°C or lower, or 650°C or higher and 740°C or lower is preferred.

[0046] When the CNT is in the form of a powder before dispersion, the exothermic peak can be measured as is. Alternatively, when the CNT is present in a CNT dispersion, the exothermic peak can be identified from the shape of the exothermic peak after removing the dispersion medium by heat drying. The heat drying is preferably performed at a temperature at which the CNT does not oxidize (for example, 140°C or lower). When the CNT dispersion contains components other than the CNT and the dispersion medium (additives, etc.), the exothermic peak of the additive can be measured in advance, and the exothermic peak derived from the additive can be identified by determining that the remaining exothermic peak is derived from the CNT.

[0047] <Volume Resistivity> The volume resistivity of the CNT of this embodiment is 2.0 × 10 －２ In some embodiments, the volume resistivity is preferably 1.0×10 Ω·cm or less. －２ ~2.0 x 10 －２ In some embodiments, the volume resistivity is preferably 1.0×10 －２ ~1.9 x 10 －２ More preferably, it is 1.0×10 －２ ~1.8 x 10 －２ More preferably, it is 1.2×10 －２ ~1.7 × 10 －２ It is particularly preferable that the volume resistivity is Ω cm. When the volume resistivity of the CNT is within the above range, the volume resistivity of the electrode film is reduced, and the performance of the secondary battery is improved. The volume resistivity of the CNT can be measured using a powder resistivity measuring device (for example, Loresta-GP Powder Resistivity Measuring System MCP-PD-51 manufactured by Nitto Seiko Analytech Co., Ltd.). More specifically, the above measurement can be carried out according to the method exemplified in the Examples.

[0048] <Aggregation Force> The weakness of the aggregating force of CNT, i.e., the ease with which CNT aggregates crumble, directly affects the initial dispersibility. From this perspective, the aggregating force of CNT can be used as an index for evaluating and controlling the initial dispersibility of CNT. In some embodiments, the aggregating force of CNT is preferably 7.5 kPa or less, and more preferably 7.0 kPa or less. When the aggregating force is within the above range, the initial dispersibility of CNT is easily improved. Furthermore, excellent viscosity stability can be easily obtained in the CNT dispersion.

[0049] In this specification, the cohesion force is a value defined by the maximum peak value of the torque measured when a rotating jig penetrates the carbon nanotube and applies shear force to the compressed CNT. For example, an MCR302e (Anton Paar) can be used as the measurement device. The specific measurement method is as follows. First, the CNT is placed in a dedicated aluminum container (C-CC27 / D / Al), and the carbon nanotube is compressed at 12 kPa using a compression cylinder jig. The jig is then replaced with a blade-shaped jig for measuring the cohesion force, and the jig is moved at 125 μm / s and 0.1 revolutions per minute to penetrate the carbon nanotube. The torque measured when the shear force is applied is measured, and the maximum peak value is taken as the cohesion force of the CNT. A C-PTD200 can be used as the temperature control device, and measurements can be performed at a temperature of 25°C.

[0050] <Surface Oxygen Content> In some embodiments, from the viewpoint of CNT conductivity, the CNT of this embodiment preferably has a surface oxygen content of less than 1.0 atm%. The surface oxygen content is preferably 0.9 atm% or less, and more preferably 0.8 atm% or less. The surface oxygen content may be 0.7 atm% or less, or 0.6 atm% or less. In some embodiments, the surface oxygen content may be 0.1 to 0.8 atm%, more preferably 0.2 to 0.7 atm%, and even more preferably 0.2 to 0.7 atm% or less. When the surface oxygen content is less than 1.0 atm%, excellent conductivity can be easily obtained as an electrode film by using CNTs. In this specification, the "surface oxygen content" refers to a value expressed as the ratio (atm%) of oxygen atoms to carbon atoms on the surface of the CNT determined by X-ray photoelectron spectroscopy.

[0051] When nitric acid is used to purify raw CNTs, the oxidizing power of nitric acid results in oxidation of the CNT surfaces. In contrast, in this embodiment, as in the CNT purification method described below, the raw CNTs are heat-treated in an inert atmosphere under reduced pressure and vacuum, thereby reducing the content of metals such as aluminum, and acid treatment is not necessarily required. Therefore, oxidation of the CNT surface can be suppressed, reducing the surface oxygen content of the CNTs. This increases the combustion initiation temperature of the CNTs, and the CNTs can achieve good conductivity as an electrode film, allowing secondary batteries containing CNTs to exhibit good performance.

[0052] <Other Characteristics of CNT> CNTs have a cylindrical shape formed by rolling planar graphite. The CNTs may be a mixture of single-walled CNTs and multi-walled CNTs. Single-walled CNTs have a structure in which one layer of graphite is rolled into a cylinder. Multi-walled CNTs have a structure in which two or three or more layers of graphite are rolled into a cylinder. The CNTs in the present disclosure may not contain single-walled CNTs, and are preferably multi-walled CNTs. Alternatively, the CNTs may be a mixture of single-walled CNTs and multi-walled CNTs. Even in such cases, CNTs in which multi-walled CNTs account for 90% by mass or more are preferred, and CNTs in which multi-walled CNTs account for 99% by mass or more are more preferred. The multi-walled CNTs in the CNTs may account for 100% by mass. Using such CNTs makes it easy to achieve a G / D ratio in a preferred range, for example, from 0.5 to 3.0. Furthermore, the sidewalls of the CNTs do not need to have a graphite structure. For example, CNTs having sidewalls with an amorphous structure can also be used.

[0053] The CNT of this embodiment is preferably a multi-walled CNT, and the number of walls of the CNT is preferably 3 to 30, more preferably 3 to 20, and even more preferably 3 to 10.

[0054] The purity of CNT is expressed as a value (% by mass) obtained by subtracting the ash content (% by mass) from the mass of the CNT. The ash content (% by mass) of CNT can be measured, for example, in accordance with JIS K 6218-2. The ash content of CNT is a non-flammable component containing metals and the like. From the viewpoint of electrical conductivity, the purity of CNT is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more, based on the mass of the CNT. Furthermore, the content of non-flammable components contained in CNT is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less.

[0055] The BET specific surface area of ​​the CNT of this embodiment is 150 m ２ / g or more, and ２ / g or more. The BET specific surface area of ​​the CNT is preferably 800 m ２ / g or less, and ２ / g or less is more preferable, and 500m ２ / g or less is more preferable, and 450m ２ / g or less. The BET specific surface area of ​​CNT can be calculated by the BET method using nitrogen adsorption measurement. The specific surface area of ​​CNT often correlates with the average outer diameter of CNT, and the smaller the specific surface area, the larger the outer diameter of CNT and the fewer the number of CNTs per mass. On the other hand, the larger the specific surface area of ​​CNT, the smaller the outer diameter of CNT and the more the number of CNTs per mass. When the specific surface area of ​​CNT is 150 m ２ When the specific surface area of ​​the CNT is 800 m / g or more, the number of carbon nanotubes per mass can be secured and a conductive network can be efficiently formed, thereby achieving excellent rate characteristics and cycle characteristics of the battery. ２ When the CNT content is 1 / g or less, the CNTs are well dispersed, and a good conductive network can be formed in the electrode film.

[0056] The average outer diameter of the CNTs in this embodiment is preferably 3 nm or more, more preferably 5 nm or more. Furthermore, the average outer diameter of the CNTs is preferably 15 nm or less, more preferably 13 nm or less, and even more preferably 11 nm or less. When the average outer diameter of the CNTs is 15 nm or less, the number of carbon nanotubes per mass can be ensured, and a conductive network can be efficiently formed. When the average outer diameter of the CNTs is 3 nm or more, the CNTs are well dispersed, and a good conductive network can be formed in the electrode film.

[0057] The standard deviation of the average outer diameter of the CNTs is preferably 1 nm or more and 8 nm or less, and more preferably 1 nm or more and 6 nm or less. If the standard deviation of the average outer diameter of the CNTs is large, it may be difficult to efficiently form a conductive network, and the CNTs may become entangled and aggregate in the CNT dispersion liquid or composite slurry and / or electrode film, making it impossible to form a good conductive network.

[0058] The outer diameter and average outer diameter of CNTs are determined as follows. First, CNTs are observed and photographed using a transmission electron microscope. Next, 300 CNTs are randomly selected from the photograph and the outer diameter of each is measured. Next, the average outer diameter (nm) of the CNTs is calculated as the number average of the outer diameters.

[0059] CNTs usually exist as aggregates. This shape may be, for example, a state in which a single CNT is intricately entangled (entangled). It may also be an aggregate of linear CNTs (bundle-like). A bundle-like CNT aggregate is easier to disentangle than an entangled CNT aggregate. Furthermore, a bundle-like CNT aggregate has better dispersibility than an entangled CNT aggregate, and therefore can be suitably used as a CNT.

[0060] <Method for Producing Carbon Nanotubes (CNTs)> The CNTs of this embodiment can be produced by, for example, laser ablation, arc discharge, thermal CVD, plasma CVD, and combustion methods, but are not limited to these. For example, CNTs can be produced by catalytically reacting a carbon source with a catalytic metal at 500 to 1000°C in an atmosphere with an oxygen concentration of 1% by volume or less. The carbon source may be at least one of a hydrocarbon and an alcohol.

[0061] Any conventionally known source gas can be used as the carbon source for CNTs. For example, hydrocarbons such as methane, ethylene, propane, butane, and acetylene, carbon monoxide, and alcohols can be used as carbon-containing source gases, but they are not limited to these. From the viewpoint of ease of use, it is preferable to use at least one of hydrocarbons and alcohols as the source gas.

[0062] <Method for Purifying Carbon Nanotubes (CNTs)> A method for purifying carbon nanotubes (CNTs) will be described below. Note that the CNTs of this embodiment are not limited to those produced through the purification method described below, but the CNTs of this embodiment can be easily obtained by following the purification method described below.

[0063] Typical catalytic metals used when producing CNTs by chemical vapor deposition (thermal CVD) include catalytic metals in which active components such as iron, cobalt, and nickel are fixed to support components such as aluminum and magnesium. Therefore, metals such as aluminum, magnesium, iron, cobalt, and nickel, which are derived from the catalytic metal, tend to remain in the CNTs obtained by the above production. One example of a method for purifying CNTs is a method of treating them with an acid such as nitric acid, and it is known that acid treatment can remove iron, cobalt, nickel, and the like. However, it is difficult to remove metals such as aluminum and magnesium using acid treatment.

[0064] In contrast, the CNT purification method of this embodiment includes a step (I) of heat-treating the carbon nanotubes used as raw material in an inert atmosphere under reduced pressure vacuum. According to the CNT purification method of this embodiment, the content of metals such as aluminum and magnesium in the CNT can be easily reduced. The CNT purification method of this embodiment can also reduce the content of other metals, such as iron and cobalt, in addition to aluminum and magnesium.

[0065] In the purification method of the above embodiment, the inert atmosphere may be, for example, a nitrogen atmosphere, an argon atmosphere, a vacuum atmosphere, or a combination thereof. For example, an inert atmosphere can be obtained by introducing an inert gas such as nitrogen gas into an apparatus used for heat treatment and replacing the atmosphere inside the apparatus with the inert gas. Subsequently, the pressure inside the apparatus is reduced, and heat treatment is performed while maintaining a vacuum state (hereinafter referred to as reduced vacuum).

[0066] In this specification, "reduced pressure vacuum" means that a state (vacuum state) reduced in pressure below atmospheric pressure is maintained by a decompression pump. Specifically, an oil rotary pump, a booster pump, a roots pump, an oil diffusion pump, or the like is used as the decompression pump to reduce the pressure inside the device and maintain a vacuum state. A single decompression pump or a combination of multiple decompression pumps may be used. By reducing the pressure inside the device, pressure increases due to factors such as leakage from the device or gas expansion caused by heating can be countered, and the air pressure inside the device (internal air pressure) can be controlled to a target pressure or lower.

[0067] The internal pressure in the reduced pressure vacuum is preferably 10 Pa or less, more preferably 0.1 Pa or less, and even more preferably 0.05 Pa or less. In some embodiments, the internal pressure may be 0.03 Pa or less. To achieve an internal pressure in the above range, it is preferable to reduce the pressure stepwise. In one embodiment, the pressure reduction is preferably carried out in two stages. For example, in the first stage, the internal pressure is preferably adjusted to 10 Pa or less, more preferably 9.8 Pa or less, and even more preferably 9.6 Pa or less. In the first stage, the internal pressure may be in the range of 9.5 to 9.8 Pa. After adjusting the internal pressure to the above range and maintaining it for a certain period of time, in the second stage, the internal pressure can be adjusted to preferably 1 Pa or less, more preferably 0.1 Pa or less, and even more preferably 0.05 Pa or less. In some embodiments, the internal pressure in the second stage may be 0.03 Pa or less. The internal pressure may increase due to the pyrolysis gas of the CNTs and the sublimation of metals, making it difficult to reduce the internal pressure stepwise. In such cases, various methods can be applied to maintain the desired reduced pressure vacuum state, not limited to the method of performing the decompression in two stages as described above. For example, to produce the first CNTs, a method of performing only the decompression in the first stage as a purification step, or a method of performing the decompression in a third stage following the decompression in the second stage, may be applied. For example, in the decompression in the third stage, the internal pressure is preferably adjusted to 10 Pa or less, more preferably 9.8 Pa or less, and even more preferably 9.6 Pa or less. In this way, the internal pressure may be adjusted through multiple stages, three or more stages, and the number of stages is not limited. In some embodiments, the number of stages may be six or more, or seven or more.

[0068] In some embodiments, the heat treatment step (I) under reduced pressure is preferably carried out in stages, for example, in two stages. By carrying out the heat treatment in two stages, it is possible to suppress the increase in internal pressure due to the decomposition of the minor components (organic substances) contained in the CNT, and to easily maintain the degree of vacuum (reduced pressure). Furthermore, by carrying out the heat treatment at a temperature that does not impair the physical properties of the CNT, the by-products contained in the CNT are decomposed, and then the CNT can be purified by raising the temperature in a short period of time. Furthermore, sintering of the CNT can be suppressed, and CNT with excellent conductivity tends to be easily obtained.

[0069] Although not particularly limited, in the second CNT production, step (I) can be suitably carried out, for example, via the first heat treatment (Ia) and second heat treatment (Ib) described below. In the first heat treatment (Ia), the internal pressure may be preferably 10 Pa or less, more preferably 0.1 Pa or less, and even more preferably 0.05 Pa or less. The heating temperature (internal temperature) is preferably adjusted in the range of 500 to 2000°C, and this temperature is preferably maintained for 1 to 100 hours. In the first heat treatment (Ia), the internal temperature may more preferably be 900 to 1800°C, and even more preferably 1200 to 1500°C. The holding time may more preferably be 1 to 50 hours, and even more preferably 2 to 10 hours. The second heat treatment (Ib) is preferably carried out while maintaining the internal pressure of the first heat treatment (Ia). Therefore, in the second heat treatment (Ib), the internal pressure may be preferably 10 Pa or less, more preferably 0.1 Pa or less, and even more preferably 0.05 Pa or less. In the second heat treatment (Ib), the internal temperature is preferably adjusted in the range of 1000 to 2000°C, and the temperature is preferably maintained for 1 to 100 hours. In the second heat treatment (Ib), the internal temperature may more preferably be 1600 to 1800°C, and even more preferably be 1600 to 1700°C. The maintenance time may more preferably be 1 to 4 hours, and even more preferably 2 to 4 hours.

[0070] Heat treatment conditions such as the heat treatment temperature and heat treatment time may be appropriately set depending on the type of CNT and the type of metal derived from the catalytic metal contained in the CNT. The heat treatment temperature is preferably a temperature at which all metals, including aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum, begin to melt. Furthermore, since the catalytic metal used in CNT production is at the nano-level and has a lower melting temperature than bulk metal due to the nanosize effect, the heat treatment temperature may be lower than the melting temperature of bulk metal. From this perspective, the heat treatment temperature is preferably 1000°C or higher and 2000°C or lower. From the perspective of suppressing high crystallization of CNT, it may be 1800°C or lower, 1700°C or lower, or 1600°C or lower. In some embodiments, the heat treatment temperature is preferably 1600°C or lower, and may be 1500°C or lower. Note that the heat treatment temperature refers to the temperature within the apparatus (internal temperature). Typical methods for measuring the temperature inside the device include a measurement method using a thermocouple and a measurement method using a radiation thermometer, but the method is not limited to these, and any method that can measure the temperature inside the device may be applied.

[0071] The heat treatment time may be set appropriately depending on the calcination apparatus, calcination scale, etc. However, calcining CNTs at a high temperature for a long time increases the crystallinity of the CNTs. CNTs with high crystallinity become hard and are more likely to break when preparing a CNT dispersion. From this perspective, in some embodiments, the heat treatment time may be, for example, 10 hours or less, 8 hours or less, or 6 hours or less. In some embodiments, the heat treatment time may be 1 to 3 hours.

[0072] In some embodiments, the heat treatment under reduced pressure vacuum is preferably carried out by adjusting the temperature inside the apparatus (internal temperature) to a range of 500 to 2000°C and maintaining that temperature for 1 to 100 hours. In the heat treatment, the internal temperature may more preferably be 900 to 1600°C, and even more preferably be 1200 to 1400°C. The maintenance time may more preferably be 1 to 50 hours, and even more preferably be 2 to 10 hours.

[0073] Typically, when CNTs are purified by heat treatment, they are heated to 500°C or higher in an air atmosphere, where they undergo oxidation and combustion. In contrast, the heat treatment of this embodiment is performed in an inert atmosphere and under reduced pressure vacuum. This makes it possible to suppress combustion of the CNTs themselves. Furthermore, because the CNTs are heat treated in an inert atmosphere, oxidation of the CNT surfaces can be suppressed. Furthermore, it is possible to shorten the heat treatment time, which further suppresses high crystallization of the CNTs.

[0074] To further reduce the amount of metal contained in the CNTs, the purification method of this embodiment may be performed two or more times as needed. Furthermore, other processing steps may be added as needed, as long as they do not degrade the properties of the CNTs.

[0075] <Dry Milling of Carbon Nanotubes (CNTs)> The CNTs of this embodiment may be CNTs that have been dry-milled to crush the particles and improve dispersibility. Dry milling refers to a process of milling CNTs without the use of a liquid substance. Dry milling may be media milling, media-free milling, or a combination of two or more dry milling methods. For example, media milling uses a mill containing milling media such as beads or steel balls, and utilizes the crushing or destructive force caused by the collision of the milling media to mill the particles. Known methods such as dry attritors, ball mills, vibration mills, and bead mills can be used as dry milling devices, and the milling time can be set as desired depending on the device or the state of the milled particles.

[0076] <2> Carbon nanotube (CNT) dispersion One embodiment of the present invention relates to a CNT dispersion. The CNT dispersion according to this embodiment contains the CNTs of the above embodiment, a dispersant, and a dispersion medium. The CNT dispersion in this specification does not contain an electrode active material.

[0077] <Dispersant> The dispersant is not particularly limited as long as it can stabilize the dispersion of CNTs, and for example, surfactants and resin-type dispersants can be used. Surfactants are mainly classified into anionic, cationic, nonionic, and amphoteric. Depending on the properties required for dispersing CNTs, an appropriate type of dispersant can be used in an appropriate amount.

[0078] Examples of resin-type dispersants include cellulose derivatives (cellulose acetate, cellulose acetate butyrate, cellulose butyrate, cyanoethyl cellulose, ethylhydroxyethyl cellulose, nitrocellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropylmethyl cellulose, carboxymethyl cellulose, etc.), polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, hydrogenated nitrile butadiene rubber, polyacrylonitrile polymers, etc. Methyl cellulose, ethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, hydrogenated nitrile butadiene rubber, and polyacrylonitrile polymers are particularly preferred. The molecular weight of the resin-type dispersant is preferably 10,000 to 300,000, and more preferably 10,000 to 150,000.

[0079] In addition to the dispersant, it is preferable to add an amine compound or an inorganic base. As the amine compound, a primary amine (primary amine), a secondary amine (secondary amine), or a tertiary amine (tertiary amine) is used, and ammonia and quaternary ammonium compounds are not included. As the amine compound, in addition to monoamines, amine compounds such as diamines, triamines, and tetramines having multiple amino groups in the molecule can be used. Specific examples of inorganic bases include, but are not limited to, aliphatic primary amines such as methylamine, ethylamine, butylamine, and octylamine; aliphatic secondary amines such as dimethylamine, diethylamine, and dibutylamine; aliphatic tertiary amines such as trimethylamine, triethylamine, and dimethyloctylamine; amino acids such as alanine, methionine, proline, serine, asparagine, glutamine, lysine, arginine, histidine, aspartic acid, glutamic acid, and cysteine; alkanolamines such as dimethylaminoethanol, monoethanolamine, diethanolamine, methylethanolamine, and triethanolamine; and alicyclic nitrogen-containing heterocyclic compounds such as hexamethylenetetramine, morpholine, and piperidine. Examples of inorganic bases include alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates, alkali metal phosphates, and alkaline earth metal phosphates.

[0080] <Dispersion Medium> The dispersion medium is not particularly limited as long as it is capable of dispersing CNTs, but it is preferable that the dispersion medium is one or more of water and water-soluble organic solvents.

[0081] Examples of water-soluble organic solvents include alcohols, polyhydric alcohols, polyhydric alcohol ethers, amines, amides (N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylcaprolactam, etc.); heterocyclics, sulfoxides, sulfones, lower ketones, and others, such as tetrahydrofuran, urea, and acetonitrile. Of these, water or amide organic solvents are more preferred, and of the amide organic solvents, N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone are particularly preferred.

[0082] When only an amide-based organic solvent is used as the dispersion medium, the water content in the dispersion medium is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

[0083] (Method for Producing CNT Dispersion) The CNT dispersion can be produced, for example, by dispersing CNTs in a dispersion medium. The raw materials to be used may be added in one or more batches at any timing during the dispersion process. The dispersion method for such a process is not particularly limited. The CNT dispersion of this embodiment can be produced by dispersing at least the CNTs of the above-described embodiment, a dispersant, and a dispersion medium. One embodiment of the present invention relates to a method for producing a CNT dispersion. In some embodiments, the production method includes a step of preparing the CNTs of the above-described embodiment, and a step of dispersing the CNTs, the dispersant, the dispersion medium, and, if necessary, other raw materials. The step of preparing the CNTs of the above-described embodiment may include the step of producing the first CNTs and / or the second CNTs described above, or a step of purifying commercially available CNTs according to the purification method described above, if necessary.

[0084] The dispersion process can be carried out using a dispersion method that uses a dispersing device. The dispersion device is not particularly limited and may be one known in the art. For example, a dispersing device commonly used for pigment dispersion can be used as a dispersion device for producing a CNT dispersion. The dispersion process can be carried out using, for example, one or more dispersion devices selected from the group consisting of mixers, homogenizers, paint shakers, media-type dispersers, and media-less dispersers. The homogenizers are classified into ultrasonic, stirring, and high-pressure types depending on the crushing method, and any of these may be used. In some embodiments, it is preferable to carry out the dispersion process using a dispersion device that is at least classified as a high-pressure homogenizer (hereinafter also referred to as a high-pressure homogenizer). Dispersing devices classified as high-pressure homogenizers include nozzle-type and valve-type, and either may be used. A valve-type high-pressure homogenizer (hereinafter also referred to as a valve-type homogenizer) is more preferably used.

[0085] Although not particularly limited, specific examples of the dispersing device include the following. Mixers: Disper, Homomixer, Planetary mixer, High Shear mixer, etc. Ultrasonic homogenizers: BRANSON's "Advanced Digital Sonifer (registered trademark), MODEL 450DA", etc. Agitation homogenizers: M-Technique's "Clearmix", PRIMIX's "Filmix", Silverson's "Abramix", etc. High-pressure homogenizers: Genus's "Genus PY", Sugino Machine's "Starburst", Nanomizer's "Nanomizer", Sanmaru Machinery's "HC3 series", Izumi Food Machinery's "HV-H series" Paint shakers: Paint Mixer Fast & Fluid's "SO400", Red Devil's "Paint Conditioner", etc. Media-type dispersers: colloid mills (PUC Colloid Mill, manufactured by PUC; Colloid Mill MK, manufactured by IKA); cone mills (Cone Mill MKO, manufactured by IKA, etc.); ball mills; sand mills (Dyno Mill, manufactured by Shinmaru Enterprises, etc.; Star Mill (registered trademark) LMZ, manufactured by Ashizawa Finetech, etc.); Nano Bead Mill (NT-V, etc.) manufactured by Longly; attritors; pearl mills (DCP Mill, manufactured by Eirich); Coball mills, etc. Media-less dispersers: R-Model (Rannie 315-45.175, etc.) manufactured by SPX Flow, Claire SS-5 manufactured by M Technique, MICROS, etc. manufactured by Nara Machinery Co., Ltd. Others: two-roll mills, three-roll mills, etc.

[0086] In some embodiments, the dispersion device (disperser) can be selected from the group consisting of a disperser (disperser), homogenizers, high-shear mixers, kneaders, two-roll mills, three-roll mills, ball mills, horizontal sand mills, vertical sand mills, annular bead mills, paint shakers, attritors, planetary mixers, and high-pressure homogenizers. The homogenizers may be ultrasonic, agitator, or high-pressure homogenizers, with high-pressure homogenizers being more preferred, and valve-type homogenizers being more preferred. The stator / screen of the high-shear mixer can be a round-hole stator, a square-hole high-shear screen, a slotted stator, an emulsifier screen, or the like, or a combination of these. In some embodiments, the high-shear mixer can be a double stator, for example, equipped with a square-hole high-shear screen on the inside and a fine emulsifier screen on the outside. The dispersion devices (dispersers) can be used alone or in combination of two or more types. It is preferable to use two or more types of dispersing devices in combination, as this tends to make it easier to adjust the dispersion state of the CNTs.

[0087] The disperser is not particularly limited. However, for example, a high-pressure homogenizer is preferred from the viewpoint of adjusting the fiber length of the CNTs in the CNT dispersion to a desired range. A high-shear mixer is preferred from the viewpoint of promoting CNT wetting and breaking down coarse particles and agglomerates. A media-type disperser such as a bead mill is preferred from the viewpoint of crushing agglomerated particles. It is also more preferred to select and combine multiple dispersers as described above to perform the dispersion process, and the order of the dispersers can be changed as desired. The pressure when using a high-pressure homogenizer is not particularly limited. For example, when preparing the first CNT dispersion using a high-pressure homogenizer, the pressure is preferably 40 to 150 MPa, more preferably 40 to 120 MPa. As another example, when preparing the second CNT dispersion using a high-pressure homogenizer, the pressure is preferably 60 to 150 MPa, more preferably 60 to 120 MPa.

[0088] Dispersion methods using a dispersing device include batch dispersion, pass dispersion, and circulation dispersion. Any of these methods may be used, or two or more methods may be combined. Batch dispersion is a method in which dispersion is performed using only the dispersing device itself, without using piping or the like. Because handling is simple, it is preferred for small-scale production. Pass dispersion is a dispersion method in which the dispersing device itself is equipped with a tank that supplies the dispersion liquid via piping and a tank that receives the dispersion liquid, and the dispersion is passed through the dispersing device itself. Furthermore, circulation dispersion is a method in which the dispersion liquid that has passed through the dispersing device itself is returned to the tank that supplies the dispersion liquid, and dispersion is performed while circulating. In both methods, the longer the processing time, the more the dispersion progresses; therefore, the pass or circulation can be repeated until the desired dispersion state is achieved, and the processing volume can be increased by changing the tank size or processing time. Pass dispersion is preferred over circulation dispersion because it is easier to achieve a uniform dispersion state. Circulation dispersion is preferred over pass dispersion because the operation and production equipment are simpler. In the dispersion step, the disintegration of agglomerated particles, the loosening, wetting, stabilization, etc. of the conductive material proceed sequentially or simultaneously, and the final dispersion state varies depending on how these steps proceed. Therefore, it is preferable to control the dispersion state in each dispersion step by using various evaluation methods. For example, it can be controlled by the method described in the examples.

[0089] In the method for producing a CNT dispersion according to this embodiment, the dispersion treatment step uses at least the CNTs of the embodiment, a dispersant, and a dispersion medium as raw materials, and disperses these raw materials using a dispersion device. In one embodiment, the dispersion treatment step may be performed by first mixing the dispersant and the dispersion medium and dispersing them to prepare a dispersant dispersion, and then adding the CNTs to the dispersant dispersion and dispersing them. In preparing the dispersant dispersion, a portion of the dispersant may be dissolved in the dispersion medium, or the entire dispersant may be dissolved in the dispersion medium. In another embodiment, the dispersion treatment step may be performed by first dispersing the CNTs and the dispersion medium to prepare a CNT dispersion, and then adding the dispersant to the CNT dispersion and dispersing them. In the method for producing a CNT dispersion according to this embodiment, components other than the raw materials may be added at any time as needed. For example, a basic compound such as NaOH may be added to prepare the dispersant dispersion.

[0090] The method for producing a CNT dispersion according to the above embodiment preferably further includes a step of crushing the CNTs. By providing a step of crushing the CNTs, it tends to be easier to improve the dispersibility of the CNTs when preparing a CNT dispersion. The step of crushing the CNTs may be carried out before the step of dispersing the CNTs or simultaneously with the step of dispersing the CNTs. In some embodiments, the step of crushing the CNTs is preferably carried out simultaneously with the step of dispersing the CNTs.

[0091] Although abrasion powder may be generated during the disintegration of CNTs, it is desirable to minimize the inclusion of abrasion powder in the production of a CNT dispersion. From this perspective, a media-less disperser can be preferably used in the dispersion treatment process. The rotor and stator of the disperser are preferably made of ceramic. Furthermore, when a bead mill is used in wet dispersion, it is preferable to use ceramic dispersion media.

[0092] When a bead mill is used as a wet disperser, it is preferable to perform a CNT crushing process before dispersion using the bead mill. Specifically, a preferred method is to crush the CNT by applying shear stress to the CNT using a media-less disperser, and then disperse the crushed CNT using a bead mill. By applying this method, it is possible to improve the problem of beads being easily worn when a bead mill is operated at a high peripheral speed on a low-viscosity dispersion material in which CNT crushing has not progressed. Furthermore, when a high-pressure homogenizer is used in combination as a disperser, the presence of bead wear powder during the dispersion process can cause problems such as nozzle clogging and valve damage in the high-pressure homogenizer. In contrast, by applying the above method, the occurrence of these problems can be easily suppressed.

[0093] The solid content of the CNT dispersion is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, and particularly preferably 2% by mass or more, relative to 100% by mass of the CNT dispersion. Furthermore, the solid content of the CNT dispersion is preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 10% by mass or less, and particularly preferably 8% by mass or less, relative to 100% by mass of the CNT dispersion.

[0094] From the viewpoints of CNT feedability, dispersibility, and dispersion stability, the content of the dispersant in the CNT dispersion is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more, relative to 100% by mass of CNT. Furthermore, from the viewpoint of electrical conductivity, the content of the dispersant in the CNT dispersion is preferably 300% by mass or less, more preferably 100% by mass or less, and even more preferably 50% by mass or less, relative to 100% by mass of CNT.

[0095] The CNT dispersion of this embodiment is constructed using CNTs with a low content of metals such as aluminum. Therefore, the metal content in the CNT dispersion can be easily reduced. However, the CNT dispersion may contain metal particles and dissolved metal ions. Metal particles are metals present in particulate form in the CNT dispersion, specifically, the metals contained in the CNTs described above. CNTs, dispersants, and other materials may contain metal particles derived from their respective manufacturing processes, and metal particles may also be mixed in during the manufacturing process of the CNT dispersion. The presence of foreign metal particles (hereinafter also referred to as foreign metal particles) inside a battery makes the battery more susceptible to short-circuiting, so removing the metal particles is extremely important from a safety perspective. Therefore, it is preferable to include a process of removing contaminants such as metal particles (foreign metal removal process) at any time during the manufacturing process of the CNT dispersion. From the perspective of efficiency, the foreign metal removal process is preferably performed during and / or at the end of the dispersion process of the CNT dispersion. The metal foreign matter removal step may be carried out multiple times.

[0096] In the metal foreign matter removal step, the method for removing metal particles from the CNT dispersion liquid is not particularly limited, and examples thereof include a method of removal by filtration using a filter, a method of removal by a vibrating sieve, a method of removal by centrifugation, a method of removal by magnetic force, etc. Among these, since metal particles such as iron and chromium are magnetic, a method of removal by magnetic force is preferred, and a method of combining a step of removal by magnetic force and a step of removal by filtration using a filter is more preferred.

[0097] The method of removing metal particles using magnetic force is not particularly limited as long as it can remove metal particles. In some embodiments, from the viewpoints of productivity and removal efficiency, a method of removing metal particles by passing the CNT dispersion through a magnetic filter disposed in the CNT dispersion production line is preferably applied. The process of removing metal particles from the CNT dispersion using a magnetic filter is preferably carried out by passing the CNT dispersion through the magnetic filter, which generates a magnetic field with a magnetic flux density of 1,000 gauss or more. Since low magnetic flux densities reduce the metal particle removal efficiency, the magnetic flux density is preferably 5,000 gauss or more, more preferably 10,000 gauss or more when considering the removal of stainless steel, which has low magnetic properties, and most preferably 12,000 gauss or more. Since coarse metal particles may pass through the magnetic filter depending on the filtration flow rate, when a magnetic filter is disposed in the production line, it is preferable to include a process of removing coarse foreign matter or metal particles using a filter such as a cartridge filter upstream of the magnetic filter. Furthermore, although a single filtration of the magnetic filter is effective, a circulating type is more preferable. The circulation system improves the efficiency of removing metal particles. When a magnetic filter is installed in a CNT dispersion production line, the location of the magnetic filter is not particularly limited. For example, the magnetic filter is preferably installed immediately before filling the carbon nanotube dispersion into a container. When a filtration process using a filtration filter is performed before filling the container, the magnetic filter is preferably installed before the filtration filter. By installing a magnetic filter and a filtration filter in this manner, even if metal detaches from the magnetic filter (even if the magnetic filter is unable to remove all of the metal particles), it is possible to prevent metal particles from being mixed into the product by recovering them using the filtration filter.

[0098] The metal content in the CNT dispersion can be calculated by drying the CNT dispersion to remove the solvent and then analyzing it using ICP. The metal content detected by ICP analysis includes metal particles and dissolved metal ions. That is, the metal content of the CNT dispersion that has undergone the metal foreign matter removal step includes metal particles that were not completely removed and dissolved metal ions.

[0099] The content of the metals aluminum, iron, and chromium contained in the CNT dispersion is preferably 100 ppm or less, more preferably 50 ppm or less, and even more preferably 10 ppm or less, relative to 100 mass% of the CNT dispersion. By setting the metal content within the above range, side reactions in the electrode film are less likely to occur, and a secondary battery with better conductivity can be obtained.

[0100] The CNT dispersion may further contain, as a conductive material, one or more carbon materials such as carbon black, graphite, etc. Among these conductive materials, carbon black is preferred from the viewpoint of the adsorption performance of the dispersant.

[0101] Although not particularly limited, the CNT dispersion of this embodiment can be easily obtained by the production method exemplified below. The first production method includes the steps of preparing a dispersant dispersion, adding CNTs to the resulting dispersant dispersion to perform a dispersion treatment, and removing foreign matter from the resulting CNT dispersion. The dispersion treatment preferably involves sequentially performing a batch dispersion treatment, a circulation dispersion treatment, and a pass dispersion treatment. Specific examples include the production methods corresponding to Examples I and II described below. The batch dispersion treatment can use a disper and a high-shear mixer. The circulation dispersion treatment can use a bead mill filled with zirconia beads. The pass dispersion treatment can use a high-pressure homogenizer equipped with a single nozzle chamber. Furthermore, the step of removing foreign matter can use a depth filter made of nonwoven fabric.

[0102] The second manufacturing method includes the steps of preparing a dispersant dispersion, adding CNTs to the resulting dispersant dispersion to perform a dispersion treatment, and removing foreign matter from the resulting CNT dispersion. In the dispersion treatment, a circulation-type dispersion treatment is preferably repeatedly performed using multiple tanks connected via piping. The above-described circulation-type dispersion treatment makes it possible to easily obtain a CNT dispersion with excellent dispersibility even when the amount of dispersant used is reduced. As a result, it is easy to improve conductivity and further improve battery performance. Furthermore, since each dispersion treatment step is performed in a separate tank, each step can be performed more uniformly. The multiple tanks used in the dispersion treatment may be stainless steel jacketed tanks, and each piping connecting the tanks is equipped with a rotary pump. To prevent the introduction of metal foreign matter during production, the contact portion of the rotary pump with the dispersion liquid is preferably made of ceramic or zirconia. To prevent the introduction of moisture during production, it is preferable to blow nitrogen gas into the tanks and piping to create a positive pressure environment. Although not particularly limited, according to the embodiment described as the second manufacturing method, it is possible to easily provide a CNT dispersion liquid in which the content of metallic foreign matter is reduced and the water content is also reduced.

[0103] <3> Binder Composition One embodiment of the present invention relates to a binder composition. The binder composition according to this embodiment includes the above-described CNT dispersion and a binder. The binder composition in this specification does not contain an electrode active material. Furthermore, this embodiment can provide a method for producing a binder composition including the above-described CNTs, a dispersant, a dispersion medium, and a binder.

[0104] <Binder> A binder is a resin that binds various substances together in an electrode film. As the binder, binders known for use in batteries can be used. Examples include cellulose resins such as carboxymethyl cellulose; and rubbers such as styrene-butadiene rubber and fluororubber. Modified products, mixtures, and copolymers of these resins may also be used. In particular, from the standpoint of durability, it is preferable to use polymer compounds having fluorine atoms in the molecule, such as polyvinylidene fluoride, polyvinyl fluoride, and tetrafluoroethylene.

[0105] The weight-average molecular weight of the binder is preferably 10,000 or more, more preferably 100,000 or more, and particularly preferably 200,000 or more. Furthermore, the weight-average molecular weight of the binder is preferably 2,000,000 or less, more preferably 1,500,000 or less, and particularly preferably 1,000,000 or less. When the weight-average molecular weight is 10,000 or more, a decrease in the resistance and adhesion of the binder can be suppressed. When the weight-average molecular weight is 2,000,000 or less, the resistance and adhesion of the binder can be improved, while a decrease in workability due to an increase in the viscosity of the binder itself can be suppressed, and significant aggregation of dispersed particles can be suppressed.

[0106] The binder composition is preferably produced by mixing and homogenizing the CNT dispersion liquid and the binder, or the binder may be dissolved in advance. The binder may also be added at any timing during the process of producing the CNT dispersion liquid. The mixing method may be any of various conventionally known methods. The binder composition can be produced using the dispersing device described above for the CNT dispersion liquid. The binder composition may contain one type of binder, or two or more types may be used in combination. Furthermore, the process of producing the binder composition may include the above-mentioned metal foreign matter removal process.

[0107] <4> Electrode Composition One embodiment of the present invention relates to an electrode composition. The electrode composition according to this embodiment includes the above-described CNT dispersion and an electrode active material. The electrode composition can be further mixed with a binder to produce a composite slurry. Furthermore, this embodiment can provide a method for producing a binder composition including the above-described CNT, a dispersant, a dispersion medium, and an electrode active material, and a method for producing a binder composition including the above-described CNT, a dispersant, a dispersion medium, an electrode active material, and a composite slurry.

[0108] <Electrode active material> An electrode active material is a material that is the basis of a battery reaction. Active materials are divided into positive electrode active materials and negative electrode active materials based on electromotive force. The positive electrode active material is not particularly limited, but metal compounds such as metal oxides and metal sulfides that can dope or intercalate 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, and inorganic compounds such as transition metal sulfides can be used. Specifically, MnO, V ２ O ５ , V ６ O １３ , TiO ２ Examples of the positive electrode active material include transition metal oxide powders such as lithium nickel oxide, lithium cobalt oxide, lithium manganate, and lithium manganate with a spinel structure; composite oxide powders of lithium and transition metals such as lithium nickel oxide, lithium cobalt oxide, lithium manganate, and lithium manganate with a spinel structure; and lithium iron phosphate-based materials, which are phosphate compounds with an olivine structure. These positive electrode active materials can be used alone or in combination. The above inorganic and organic compounds may also be used as a mixture. The negative electrode active material is not particularly limited, but can be one capable of doping or intercalating lithium ions. For example, metallic Li and its alloys such as tin alloys, silicon alloys, and lead alloys; Li ｘ Fe ２ O ３ , Li ｘ Fe ３ O ４ , Li ｘ WO ２ (x is a number between 0 and 1), metal oxides such as lithium titanate, lithium vanadate, and lithium silicate; amorphous carbonaceous materials such as soft carbon and hard carbon, artificial graphite such as highly graphitized carbon materials, and carbonaceous powders such as natural graphite. These negative electrode active materials can be used alone or in combination. In particular, using a highly graphitized carbonaceous material in combination with lithium silicate is preferred from the viewpoints of capacity and life.

[0109] The BET specific surface area of ​​the electrode active material is 0.1 to 10 m ２ / g is preferred, and 0.2 to 5m ２ / g is more preferable, and 0.3 to 3m ２ / g is more preferable. The average particle size of the electrode active material is preferably 0.05 to 100 μm, and more preferably 0.1 to 50 μm. In this specification, the "average particle size of the electrode active material" refers to the average value of particle sizes measured by an electron microscope.

[0110] The electrode composition is preferably produced by mixing and homogenizing a CNT dispersion and an electrode active material, and the binder may be dissolved in the CNT dispersion in advance. The electrode active material may be added at any timing during the process of producing the CNT dispersion. The dispersing device used for the treatment to disperse the electrode active material is not particularly limited, and the dispersing devices exemplified for producing the CNT dispersion can be used.

[0111] In the case of a composite slurry containing an electrode composition, the content of the electrode active material contained in the composite slurry is preferably 20% by mass or more, more preferably 40% by mass or more, relative to 100% by mass of the composite slurry. Furthermore, the content of the electrode active material contained in the composite slurry is preferably 99% by mass or less, more preferably 97% by mass or less, relative to 100% by mass of the composite slurry. A content within the above range is preferable from the viewpoints of coatability, productivity, and uniformity of the electrode film. The content of CNT contained in the composite slurry is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, relative to 100% by mass of the electrode active material. Furthermore, the content of CNT contained in the composite slurry is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, relative to 100% by mass of the electrode active material. In some embodiments, the content of CNT is preferably 1% by mass or less. The content of the binder contained in the composite slurry is preferably 0.3% by mass or more, and more preferably 0.7% by mass or more, relative to 100% by mass of the electrode active material. The content of the binder contained in the composite slurry is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, relative to 100% by mass of the electrode active material. In some embodiments, the content of the CNTs is preferably 1% by mass or less. The solid content of the composite slurry is preferably 30% by mass or more, and more preferably 40% by mass or more, relative to 100% by mass of the composite slurry. The solid content of the composite slurry is preferably 90% by mass or less, and even more preferably 85% by mass or less, relative to 100% by mass of the composite slurry. The moisture content of the composite slurry is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

[0112] <5> Electrode Film One embodiment of the present invention relates to an electrode film. The electrode film according to this embodiment is an electrode film obtained using (i) a carbon nanotube dispersion containing carbon nanotubes, a dispersant, and a dispersion medium, (ii) a binder composition containing the carbon nanotube dispersion and a binder, or (iii) an electrode composition containing the carbon nanotube dispersion and an electrode active material. The carbon nanotubes used are the carbon nanotubes of this embodiment described above. The electrode film may also be an electrode film obtained using (iv) a composite slurry. The (iv) composite slurry can be obtained using the aforementioned (i) carbon nanotube dispersion, (ii) a binder composition, or (iii) an electrode composition.

[0113] For example, the electrode film is a coating film formed by coating a current collector with the above-described composite slurry and drying it. The material and shape of the current collector used for the electrode film are not particularly limited, and can be appropriately selected from those suitable for various secondary batteries. For example, the material of the current collector can be metals and alloys such as aluminum, copper, nickel, titanium, or stainless steel.

[0114] The method for applying the composite slurry onto the current collector is not particularly limited, and any known method can be used.

[0115] After coating and drying, the coating may be rolled using a lithographic press or a calender roll, etc. The thickness of the electrode film is generally 1 μm or more and 500 μm or less, and preferably 10 μm or more and 300 μm or less.

[0116] <6> Secondary Battery One embodiment of the present invention relates to a secondary battery. The secondary battery according to this embodiment includes the electrode film described above. The electrode film can be used as an electrode for a secondary battery, and is particularly preferably used as an electrode for a non-aqueous electrolyte secondary battery using an organic electrolyte solution. A non-aqueous electrolyte secondary battery is a battery including a positive electrode, a negative electrode, and an electrolyte containing an organic electrolyte solution. The electrode film can be used for either the positive electrode, the negative electrode, or both. In one embodiment, for example, an electrode film obtained by coating a current collector with an electrode composition containing a positive electrode active material and drying the electrode film can be used as a positive electrode. In another embodiment, for example, an electrode film obtained by coating a current collector with an electrode composition containing a negative electrode active material and drying the electrode film can be used as a negative electrode. In another embodiment, an electrode film obtained by coating a current collector with a CNT dispersion or a binder composition and drying the electrode film can be used as a current collector with an underlayer. In particular, from the viewpoint of safety, it is preferably used as a positive electrode.

[0117] As the electrolyte, various known materials in which ions can move can be used. For example, LiBF ４ , LiClO ４ , LiPF ６ , LiAsF ６ , LiSbF ６ , LiCF ３ SO ３ , Li(CF ３ SO ２ ) ２ N, LiC ４ F ９ SO ３ , Li(CF ３ SO ２ ) ３ C, LiI, LiBr, LiCl, LiAlCl, LiHF ２ , LiSCN, or LiBPh ４ Examples of suitable electrolytes include those containing lithium salts such as phenyl groups (where Ph is a phenyl group), but are not limited to these, 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. An all-solid-state electrolyte or a polymer electrolyte may also be used.

[0118] The non-aqueous solvent is not particularly limited, and various solvents suitable for secondary batteries can be used. Examples include carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, lactones, glymes, esters, sulfoxides, and nitriles. These solvents may be used alone or in combination of two or more.

[0119] The secondary battery preferably includes a separator, and examples of the separator include, but are not limited to, polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and those that have been subjected to a hydrophilic treatment.

[0120] The structure of the secondary battery is not particularly limited. The secondary battery may generally be composed of a positive electrode, a negative electrode, and a separator, if necessary. The shape of the secondary battery may be various, such as a paper type, a cylindrical type, a button type, or a laminated type, depending on the purpose of use.

[0121] Hereinafter, examples of embodiments of the present invention will be described. However, the embodiments of the present invention are not limited to the following and include various embodiments.

[0122] Embodiments of the present invention relate to carbon nanotubes described in the following items <1> to <4>, a carbon nanotube dispersion described in the following item <5> or <6>, a binder composition described in the following item <7>, an electrode composition described in the following item <8>, and a secondary battery described in the following item <9>. <1> Carbon nanotubes that satisfy the following items (1) to (3) and contain multi-walled carbon nanotubes. (1) A carbon nanotube having a peak at 1560 to 1600 cm in a Raman spectrum. －１ The maximum peak intensity in the range of 1310 to 1350 cm －１(2) The carbon nanotube has a wettability index of 10 or less. Equation (I): Wettability index = (X / Y) [In equation (I), Y is the mass (g) of the carbon nanotube, and X is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotube when N-methyl-2-pyrrolidone is dropped onto Y (g) of carbon nanotube in a 25°C environment.] (3) The aluminum content is 3000 ppm or less. <2> The carbon nanotube has a volume resistivity of 2.0 x 10 －２<1> The carbon nanotubes according to <1> above, having a viscosity of Ω·cm or less. <3> The carbon nanotubes according to <1> or <2> above, having a cohesive strength of 7.5 kPa or less. <4> The carbon nanotubes according to any one of <1> to <3> above, having an exothermic peak of 600°C or more and 800°C or less in differential thermal analysis when heated from 200°C to 1000°C at a rate of 10°C / min. <5> A carbon nanotube dispersion comprising the carbon nanotubes according to any one of <1> to <4> above, a dispersant, and a dispersion medium. <6> The carbon nanotube dispersion according to <5> above, having a viscosity of 5000 mPa·s or less at 25°C, measured with a Brookfield viscometer after being left to stand at 40°C for one week. <7> A binder composition comprising a carbon nanotube dispersion and a binder, wherein the carbon nanotube dispersion contains the carbon nanotubes according to any one of <1> to <4> above, a dispersant, and a dispersion medium. <8> A composition for an electrode, comprising a carbon nanotube dispersion and an electrode active material, wherein the carbon nanotube dispersion contains the carbon nanotubes according to any one of <1> to <4> above, a dispersant, and a dispersion medium. <9> A secondary battery including an electrode film, wherein the electrode film is obtained using: the carbon nanotube dispersion containing the carbon nanotubes according to any one of <1> to <4> above, a dispersant, and a dispersion medium; a binder composition containing the carbon nanotube dispersion and a binder; or a composition for an electrode containing the carbon nanotube dispersion and an electrode active material.

[0123] Other embodiments of the present invention relate to carbon nanotubes described in the following items <1> to <3>, a carbon nanotube dispersion described in the following item <4> or <5>, a binder composition described in the following item <6>, an electrode composition described in the following item <7>, and a secondary battery described in the following item <8>. <1> Carbon nanotubes that satisfy the following items (1) to (3) and contain multi-walled carbon nanotubes. (1) In differential thermal analysis when the temperature is increased from 200°C to 1000°C at a rate of 10°C / min, the carbon nanotubes have an exothermic peak between 600°C and 800°C. (2) In Raman spectroscopy, the carbon nanotubes have an exothermic peak between 1560 and 1600cm. －１ The maximum peak intensity in the range of 1310 to 1350 cm －１(2) The carbon nanotube dispersion according to any one of (1) to (3), wherein the maximum peak intensity within the range of (A) is D, has a G / D ratio of 0.5 or more and 3.0 or less. (3) The aluminum content is 3,000 ppm or less. <2> The carbon nanotubes according to claim 1, further satisfying the following condition (i): The total content of aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is 13,000 ppm or less. <3> The carbon nanotubes according to <1>, further satisfying the following condition (ii): (ii) The surface oxygen content is less than 1.0 atm%. <4> A carbon nanotube dispersion comprising the carbon nanotubes according to any one of (1) to (3), a dispersant, and a dispersion medium. <5> The carbon nanotube dispersion according to <4>, wherein the viscosity at 25°C, as measured with a Brookfield viscometer after storage at 60°C for one week, is 5,000 mPa·s or less. <6> A binder composition comprising a carbon nanotube dispersion and a binder, wherein the carbon nanotube dispersion contains the carbon nanotubes according to any one of <1> to <3> above, a dispersant, and a dispersion medium. <7> A composition for an electrode, comprising a carbon nanotube dispersion and an electrode active material, wherein the carbon nanotube dispersion contains the carbon nanotubes according to any one of <1> to <3> above, a dispersant, and a dispersion medium. <8> A secondary battery including an electrode film, wherein the electrode film is obtained using: the carbon nanotube dispersion containing the carbon nanotubes according to any one of <1> to <3> above, a dispersant, and a dispersion medium; a binder composition containing the carbon nanotube dispersion and a binder; or a composition for an electrode containing the carbon nanotube dispersion and an electrode active material.

[0124] The disclosure of this application is related to the subject matter described in Japanese Patent Application Nos. 2024-089038 and 2024-089206 filed on May 31, 2024, and Japanese Patent Application No. 2024-199927 filed on November 15, 2024, the disclosures of all of which are incorporated herein by reference.

[0125] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the present invention.

[0126] [Example I] I-1. Production Examples of CNT and CNT Dispersions Using the Same [Example I-1] <Production of CNT> 60 parts of CNT (JENOTUBE10B, manufactured by JEIO Corporation) were charged into a graphite crucible with a diameter of 10 cm and a height of 10 cm, and then heated under reduced pressure and vacuum using a multipurpose high-temperature furnace (Hi-Multi 5000, manufactured by Fuji Radio Industrial Co., Ltd.). After performing nitrogen gas replacement twice, the pressure was reduced using an oil rotary pump, and the furnace pressure was adjusted to 9.8 to 9.5 Pa. Subsequently, the pressure was further reduced using an oil diffusion pump, and the furnace pressure was adjusted to 0.03 Pa or less. Subsequently, while maintaining the reduced pressure using the oil diffusion pump, the temperature was increased to 1200 ° C at a rate of 20 ° C / min and held at 1200 ° C for 6 hours. After that, the furnace temperature was naturally cooled to 50 ° C or less, and CNT (I-A) was obtained.

[0127] <Preparation of CNT Dispersion> An NMP solution containing a hydrogenated nitrile butadiene rubber polymer (Zetpole 2000L, solids content 8% by mass, manufactured by Zeon Corporation) and NMP were added to a stainless steel container, and the polymer was adjusted to 0.6 parts by mass and the total amount of NMP was adjusted to 96.4 parts by mass. Furthermore, 0.015 parts by mass of NaOH was added to this solution. 3.0 parts by mass of CNT (I-A) were weighed and added to the solution obtained above while stirring with a disperser, and a fine emulsion screen was attached to a high shear mixer (L5M-A, manufactured by Silverson Corporation), and batch dispersion was performed at a speed of 7,000 rpm until the entire mixture was uniform. Next, the contents of the stainless steel container were transferred to a bead mill (Ashizawa Finetech Co., Ltd., Star Mill LMZ) filled with zirconia beads having a diameter of 0.5 mm, and a circulation dispersion treatment (bead filling rate 80%, peripheral speed 12 m / s) was carried out for a residence time of 10 minutes. Subsequently, the dispersion liquid was supplied to a high-pressure homogenizer (Sugino Machine Co., Ltd., Starburst Lab), and a 10-pass dispersion treatment was carried out. The dispersion treatment was carried out using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa. After the dispersion treatment, the mixture was passed through a depth filter (3M, PP nonwoven fabric depth cartridge NT-T series, filtration accuracy 40 μm). In this way, a carbon nanotube dispersion (I-A) was prepared.

[0128] <Preparation of binder composition> Capacity 150 cm ３ The CNT dispersion (I-A) and PVdF (polyvinylidene fluoride, Solef5130, manufactured by Solvay, non-volatile content 100%), which had been dissolved in advance in NMP (N-methyl-2-pyrrolidone) to a solid content of 8 mass%, were added to the plastic container. The mixture was then stirred at 2000 rpm for 30 seconds using a planetary centrifugal mixer (Awatori Rentaro, ARE-310). In this way, a binder composition (I-A) was obtained.

[0129] <Preparation of electrode composition> A binder composition (IA) was mixed with NMC (S800, LiNi) as a positive electrode active material. ０．８ Mn ０．１ Co ０．１ O ２, manufactured by Kinwa) was added, and the mixture was stirred for 30 seconds at 2,000 rpm using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310). Subsequently, the lumps were loosened with a spatula, and the mixture was stirred for 300 seconds at 2,000 rpm using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310) to obtain electrode composition (IA). The nonvolatile content of the electrode composition was 72.0% by mass. The nonvolatile content ratio of NMC:CNT:PVdF in the nonvolatile content of the electrode composition was 98.1:0.4:1.5.

[0130] <Preparation of Electrode Film> The electrode composition (IA) was applied to the electrode using an applicator so that the coating weight per unit area of ​​the electrode was 20 mg / cm ２ After coating, the coating was dried in an electric oven at 120°C ± 5°C for 25 minutes to obtain an electrode film (I-A). The electrode film (I-A) was then rolled using a roll press (Thank Metal, 3 ton hydraulic roll press) to obtain a positive electrode (I-A). The weight per unit area of ​​the composite layer was 20 mg / cm. ２ The density of the composite layer after rolling was 3.1 g / cc.

[0131] <Preparation of Secondary Battery> A positive electrode (I-A) and a standard negative electrode were punched out to 45 mm x 40 mm and 50 mm x 45 mm, respectively. These electrodes and a separator (porous polypropylene film) inserted therebetween were placed in an aluminum laminate bag and dried in an electric oven at 60°C for 1 hour. Then, in a glove box filled with argon gas, 2 mL of electrolyte (non-aqueous electrolyte) was poured into the bag, and the aluminum laminate bag was sealed to prepare a laminated secondary battery (I-A). The standard negative electrode and non-aqueous electrolyte were prepared as follows. (Standard negative electrode) 0.5 parts by mass of acetylene black (Denka Black (registered trademark) HS-100, manufactured by Denka) and 1 part by mass of MAC500LC (carboxymethyl cellulose sodium salt Sunrose special type MAC500LC, manufactured by Nippon Paper Industries Co., Ltd., non-volatile content 100%) and 98.4 parts by mass of water were added to a 150 ml plastic container, and then a rotation and revolution mixer (Thinky Awatori Rentaro, ARE-310) was used, and the mixture 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 oxide (manufactured by Osaka Titanium Technology Co., Ltd., SILICON MONOOXIDE SiO 1.3C 5 μm, non-volatile content 100%) were added as active materials, and the mixture was stirred at 3000 rpm for 10 minutes using a high-speed stirrer. Subsequently, 3.1 parts by mass of styrene-butadiene rubber (SBR) (TRD2001, manufactured by JSR Corporation) was added, and the mixture was stirred for 30 seconds at 2000 rpm using the rotation / revolution mixer to obtain a negative electrode composite slurry. Thereafter, the negative electrode composite slurry was applied using an applicator to an electrode with a basis weight per unit area of ​​8 mg / cm. ２ After coating on copper foil so that the density of the composite layer became 1.6 g / cm, the coating film was dried in an electric oven at 120°C ± 5°C for 25 minutes. Further, rolling treatment was carried out using a roll press (3 ton hydraulic roll press, manufactured by Thank Metal Co., Ltd.) to obtain a composite layer having a density of 1.6 g / cm. ３A standard negative electrode was prepared. (Nonaqueous electrolyte) First, a mixed solvent was prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 1:1:1. Next, 2 parts by mass of VC (vinylene carbonate) was added as an additive to 100 parts by mass of this mixed solvent, and LiPF ６ was dissolved at a concentration of 1 M to obtain a non-aqueous electrolyte solution.

[0132] Examples I-2 to I-9 and Comparative Examples I-1 to I-4 CNT (IB) to CNT (I-Y1) were obtained by the same method as in Example 1, except that the CNT species, holding temperature, and treatment time were changed as shown in Table I-1. Furthermore, using each of the obtained CNTs, CNT dispersions, binder compositions, electrode compositions, electrode films, and secondary batteries were produced according to the same methods as in Example 1.

[0133] Example I-10: 15 parts of CNT and 40 parts of zirconia beads with a diameter of 2 mm were added to a glass bottle (M-225, manufactured by Kakuyo Glass Co., Ltd.), and dry-treated for 1 minute using an automatic shaker (SK450, manufactured by Fast and Fluid Management). This procedure was then repeated to produce 60 parts of CNT, which were then used to obtain CNT (I-J) in the same manner as in Example 9. Furthermore, using the CNT (I-J) obtained as described above, a CNT dispersion, a binder composition, an electrode composition, an electrode film, and a secondary battery were produced in the same manner as in Example 1.

[0134] [Comparative Example I-5] Using an electronic balance (MSA225S100DI, manufactured by Sartorius), 50 parts of CNT (JENOTUBE10B, manufactured by JEIO Corporation) were weighed into an alumina crucible SSA-HB4 (manufactured by Nikkato), and the crucible containing the CNT was placed in a multipurpose high-temperature furnace (manufactured by Fuji Radio Industrial Co., Ltd., Hi-Multi 5000). Next, under a nitrogen atmosphere with a nitrogen flow rate of 2.0 L / min, the temperature inside the furnace was increased to 1200 ° C. at a heating rate of 20 ° C. / min, and then held at 1200 ° C. for 6 hours, after which the furnace temperature was allowed to cool naturally to 50 ° C. Thereafter, 10 parts of the heat-treated CNT were weighed into a 1 L glass container, and 500 parts of 10% hydrochloric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added, followed by thorough stirring using a stirrer. The mixture was then thoroughly diluted with ion-exchanged water and filtered under reduced pressure using a membrane filter. The dilution and filtration process was repeated until the pH of the filtrate reached 4 or higher, after which the CNTs were transferred to a PTFE tray. The mixture was then dried at 140°C using an oven. Thus, CNT (I-X4) was obtained. Furthermore, using the CNT (I-X4) obtained as described above, a CNT dispersion, a binder composition, an electrode composition, an electrode film, and a secondary battery were produced according to the same method as in Example 1.

[0135] I-2. Evaluation I-2-1. Evaluation of CNT Properties Measurements were carried out on the carbon nanotubes (CNTs) of the above-mentioned Examples and Comparative Examples as follows. Unless otherwise specified, measurements were carried out using purified CNTs. The respective results are shown in Table I-1.

[0136] <Metal Content of CNT> Using a microwave sample pretreatment device (ETHOS, manufactured by Milestone General Co., Ltd.), CNT were acid-decomposed to extract the metals contained in the CNT. The extracted metals were analyzed using a multi-type ICP optical emission spectrometer (720-ES, manufactured by Agilent), and the metal content of the CNT was calculated. Table I-1, described below, shows the contents of aluminum, iron, cobalt, and magnesium as the metal content. In addition, the total contents of aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum were determined from the calculated metal contents. The total contents of magnesium, aluminum, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum in the CNT are expressed as the mass ratio (ppm) of the total content of extracted magnesium, aluminum, iron, copper, zinc, nickel, chromium, manganese, and molybdenum to the mass of the CNT before metal extraction.

[0137] <G / D ratio of CNT> CNT was placed in a Raman microscope (XploRA, manufactured by Horiba, Ltd.) and measured using a laser light wavelength of 532 nm. The measurement conditions were: acquisition time 60 seconds, number of integrations 2, neutral density filter 10%, objective lens magnification 20x, confocus hole 500, slit width 100 μm, measurement wavelength 100 to 3000 cm －１ The CNTs for measurement were separated onto a slide glass and flattened using a spatula. Among the peaks obtained, the peak at 1560 to 1600 cm in the spectrum －１ The maximum peak intensity in the range of 1310 to 1350 cm －１ The maximum peak intensity within this range was defined as D, and the G / D ratio was calculated to obtain the G / D ratio of the CNT.

[0138] <CNT Wetting Index> In a 25°C environment, 5 g (Y (g)) of CNT powder was placed into a cylindrical polypropylene container with a diameter of 10 cm by gravity, and while the container was left to stand, 5 g of N-methyl-2-pyrrolidone (NMP) was dropped onto the surface of the CNT powder at intervals of 1 minute. The total mass (X (g)) of NMP dropped was measured just before the NMP began to flow onto the surface of the CNT powder without being absorbed by the CNT powder. The CNT wetting index was calculated using the following formula (I): Formula (I): Wetting index = (X / Y) [In formula (I), Y is the mass (g) of CNT, and X is the maximum mass (g) of NMP absorbed by the CNT when NMP was dropped onto Y (g) of CNT in a 25°C environment.]

[0139] <CNT particle size> The particle size of the CNT was measured using a laser diffraction method (for example, a MasterSizer 3000 manufactured by Malvern). The average particle size D50 is the particle size at which the volume-based integrated value in the particle size distribution is 50%. D10 and D90 are the particle sizes at which the volume-based integrated value is 10% and 90%, respectively.

[0140] <Exothermic Peak Temperature of CNT> Using a thermogravimetric differential thermal analyzer (Tg-DTA 8122 Thermo plus EVO2, manufactured by Rigaku Corporation), a sample mass of 1.0 mg was placed in an alumina pan and heated from 25°C to 1000°C at a heating rate of 10°C / min in an air atmosphere. For the obtained DTA curve, the temperature at the peak apex in the temperature range from 200°C to 1000°C was taken as the exothermic peak temperature.

[0141] <Surface oxygen content of CNT> The surface oxygen content of CNT was measured using an X-ray photoelectron spectrometer (XPS, manufactured by ThermoFisher Scientific, K-Alpha). After pelletizing the CNT, the sample was fixed to a sample stage with double-sided tape and subjected to measurement. Carbon atoms and oxygen atoms on the surface of the CNT sample were detected by XPS. Here, the ratio (atm %) of oxygen atoms to carbon atoms was calculated as the surface oxygen content.

[0142] <Volume Resistivity of CNT> Using a powder resistivity measuring device (Loresta-GP Powder Resistivity Measuring System MCP-PD-51 manufactured by Nitto Seiko Analytech Co., Ltd.), the volume resistivity [Ω cm] of the CNT powder under various pressures was measured using a powder probe unit (four-point ring electrode, electrode spacing 5.0 mm, electrode radius 1.0 mm, sample radius 12.5 mm) with a sample mass of 1.2 g and an applied voltage limiter of 90 V. ３ The volume resistivity of the CNTs at a density of 1000 MPa was evaluated.

[0143] <CNT Cohesion Force> The cohesion force of CNT was measured using a rheometer (MCR302e, manufactured by Anton Paar). First, 4.0 g of CNT was placed in a dedicated aluminum container (C-CC27 / D / Al), and the carbon nanotubes were compressed at 12 kPa using a cylinder jig for compression. Then, the jig was replaced with a wing-type jig for cohesion force measurement, and the jig was penetrated into the carbon nanotubes while moving at 125 μm / s and 0.1 rpm, and the torque when a shear force was applied was measured, and the maximum peak value was taken as the cohesion force of the CNT. A C-PTD200 temperature control device was used, and measurements were taken at a temperature of 25°C.

[0144] I-2-2. Evaluation of Other Properties The CNT dispersions, binder compositions, electrode compositions, electrode films, and secondary batteries prepared in the above-mentioned Examples and Comparative Examples were measured and their properties evaluated as follows. The respective evaluation results are shown in Table I-1. <Viscosity Stability of CNT Dispersion> After storing the CNT dispersion in a thermostatic chamber at 40°C for one week, the CNT dispersion was cooled to 25°C and then immediately measured using a Brookfield viscometer at a rotor rotation speed of 30 rpm. The evaluation criteria for viscosity stability are as follows: (Evaluation Criteria) ◎ (Excellent): 3000 mPa·s or less ○ (Good): More than 3000 mPa·s to 5000 mPa·s or less △ (Fair): More than 5000 mPa·s to 10000 mPa·s or less × (Poor): More than 10000 mPa

[0145] <Particle diameter (D90) of CNT dispersion> The particle diameter (D90) was measured using a particle size distribution analyzer (Partica LA-960V2, manufactured by HORIBA). The circulation / ultrasonic operating conditions were: circulation speed: 3, ultrasonic intensity: 7, ultrasonic time: 1 minute, stirring speed: 1, and stirring mode: continuous. During air evacuation, ultrasonic operation was performed with an ultrasonic intensity of 7 and an ultrasonic time of 5 seconds. The refractive index of NMP was 1.468, and the refractive index of CNT was 1.920. The measurement was performed after diluting the measurement sample so that the transmittance of the red laser diode was 60 to 70%, and the particle diameter was measured by volume. The particle diameter evaluation criteria are as follows: (Evaluation criteria) ◎ (Excellent): 0.6 μm or more and less than 2.0 μm ○ (Good): 2.0 μm or more and less than 5.0 μm − (Poor): Less than 0.6 μm or 5.0 μm or more

[0146] <Cycle Characteristics of Secondary Battery> The secondary battery was placed in a thermostatic chamber at 25°C, and charge / discharge measurements were performed using a charge / discharge device (SM-8, manufactured by Hokuto Denko Corporation). Constant-current, constant-voltage charging (cutoff current 1.25 mA (0.025 C)) was performed at a charge current of 50 mA (1 C) with a charge cut-off voltage of 4.2 V, followed by constant-current discharging at a discharge current of 50 mA (1 C) with a discharge cut-off voltage of 2.5 V. This operation was repeated 200 times. 1 C was defined as the current value required to discharge the theoretical capacity of the positive electrode in 1 hour. The cycle characteristics can be expressed by the ratio of the 3rd 1 C discharge capacity to the 200th 1 C discharge capacity at 25°C, as shown in Equation 3 below. (Equation 3): Cycle characteristics = 200th 1 C discharge capacity / 3rd 1 C discharge capacity × 100 (%). The evaluation criteria for the cycle characteristics of the secondary battery were as follows: (Evaluation criteria) ◎ (Excellent): 90% or more 〇 (Good): 85% or more but less than 90% △ (Fair): 80% or more but less than 85% × (Poor): Less than 80%

[0147] <Evaluation of Volume Resistivity of Electrode> The electrode compositions prepared in the Examples and Comparative Examples were applied to an electrode using an applicator so that the coating weight per unit area of ​​the electrode was 20 mg / cm. ２The composite coating was applied to a 100 μm thick PET foil so that the resistivity was 100 μm, and then dried in an electric oven at 120°C ± 5°C for 30 minutes to produce a composite coating film. The surface resistivity (Ω / □) of the composite layer of the prepared composite coating film was measured using a Mitsubishi Chemical Analytech Loresta GP, MCP-T610. After measurement, the surface resistivity was multiplied by the thickness of the composite layer to obtain the volume resistivity (Ω cm) of the electrode. The thickness of the composite layer was determined by subtracting the film thickness of the PET foil from the average value measured at three points in the electrode using a film thickness meter (NIKON, DIGIMICRO MH-15M). The evaluation criteria for the volume resistivity of the electrode are as follows: (Evaluation Criteria) ◎ (Excellent): Less than 7 Ω cm ○ (Good): 7 Ω cm or more but less than 15 Ω cm △ (Fair): 15 Ω cm or more but less than 20 Ω cm × (Poor): 20 Ω cm or more

[0148] Table I-1 shows the evaluation results of the CNTs, CNT dispersions, electrode films, and secondary batteries produced in Examples I-1 to I-10 and Comparative Examples I-1 to I-5.

[0149] In Table I-1, 10B and 6A used as CNT seeds are as follows: 10B: multi-walled carbon nanotube (manufactured by JEIO, JENOTUBE 10B) 6A: multi-walled carbon nanotube (manufactured by JEIO, JENOTUBE 6A)

[0150]

[0151] As shown in Table I-1, the CNTs of this embodiment (Examples I-1 to I-10) satisfy all of the requirements for (1) G / D ratio, (2) wettability, and (3) aluminum content described above. A comparison with comparative examples using CNTs that do not satisfy the above requirements (1) to (3) shows that the use of the CNTs of this embodiment can improve the properties of the CNT dispersion, electrode film, and secondary battery. Thus, the CNTs of this embodiment can form an electrode film with excellent conductivity, enabling the safety of secondary batteries to be improved.

[0152] [Example II] II-1. Production Examples of CNT and CNT Dispersions Using the Same [Example II-1] <Production of CNT> 60 parts of CNT (JENOTUBE 6A, manufactured by JEIO Corporation) were placed in a graphite crucible with a diameter of 10 cm and a height of 10 cm. The crucible containing the CNT was placed in a multipurpose high-temperature furnace (Hi-Multi 5000, manufactured by Fuji Denpa Kogyo Co., Ltd.) and subjected to heat treatment under reduced pressure and vacuum as follows. First, nitrogen gas was introduced into the multipurpose high-temperature furnace, and the nitrogen gas replacement operation was performed twice. Next, the pressure inside the furnace was reduced using an oil rotary pump, and the pressure inside the furnace was adjusted to 9.8-9.5 Pa. Subsequently, the pressure inside the furnace was further reduced using an oil diffusion pump, and the pressure inside the furnace was adjusted to 0.03 Pa or less. Next, while maintaining the reduced pressure using the oil diffusion pump, the temperature was increased to 1200°C at a rate of 20°C / min and held at 1200°C for 10 hours. Thereafter, the furnace was allowed to cool naturally until the temperature inside the furnace became 50° C. or less, and CNT (II-A) was obtained.

[0153] <Preparation of CNT Dispersion> An NMP solution containing a 7% concentration of hydrogenated nitrile butadiene rubber polymer (Zeon Corporation, Zetpole 2000L) and NMP were added to a stainless steel container, and the polymer was adjusted to 1.25 parts by mass and the total amount of NMP was adjusted to 96.25 parts by mass. Furthermore, 0.031 parts by mass of NaOH was added to this solution. 2.5 parts by mass of CNT (II-A) were weighed out and added to the solution obtained above while stirring with a disperser. A fine emulsion screen was attached to a high shear mixer (L5M-A, Silverson) and a batch dispersion treatment was carried out at a speed of 9,000 rpm. This batch dispersion treatment was carried out until the entire solution became uniform and the dispersion particle size measured by a grind gauge was 200 μm or less. Next, the contents of the stainless steel container were transferred to a bead mill (Ashizawa Finetech Co., Ltd., Star Mill LMZ) filled with zirconia beads having a diameter of 1.0 mm, and a circulation dispersion treatment (bead filling rate 80%, peripheral speed 12 m / s) was performed for a residence time of 10 minutes. Subsequently, the dispersion liquid was supplied to a high-pressure homogenizer via piping, and a 15-pass dispersion treatment was performed. Subsequently, the dispersion liquid was supplied to a high-pressure homogenizer (Sugino Machine Co., Ltd., Star Burst Lab), and a 15-pass dispersion treatment was performed. This dispersion treatment was performed using a single nozzle chamber, with a nozzle diameter of 0.25 mm and a pressure of 100 MPa. The solution after the dispersion treatment was passed through a depth filter (3M, PP nonwoven fabric depth cartridge NT-T series, filtration accuracy 20 μm). In this way, a carbon nanotube dispersion liquid (II-A) was obtained.

[0154] <Preparation of binder composition> Capacity 150 cm ３ The CNT dispersion (II-A) and PVdF (polyvinylidene fluoride, Solef5130, manufactured by Solvay, non-volatile content 100%), which had been dissolved in NMP (N-methyl-2-pyrrolidone) to a concentration of 8%, were added to the plastic container. The mixture was then stirred at 2000 rpm for 30 seconds using a planetary centrifugal mixer (Awatori Rentaro, ARE-310). Thus, a binder composition (II-A) was obtained.

[0155] <Preparation of electrode composition> A binder composition (II-A) was mixed with NMC (S800, LiNi) as a positive electrode active material. ０．８ Mn ０．１ Co ０．１ O ２ , manufactured by Kinwa) was added, and the mixture was stirred for 30 seconds at 2,000 rpm using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310). Subsequently, the lumps were loosened with a spatula, and the mixture was stirred for 300 seconds at 2,000 rpm using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310) to obtain an electrode composition (II-A). The nonvolatile content of the electrode composition was 73.5%. The nonvolatile content ratio of NMC:CNT:PVdF in the nonvolatile content of the electrode composition was 98.1:0.4:1.5.

[0156] <Preparation of Electrode Film> The electrode composition (II-A) was applied to the electrode using an applicator so that the coating weight per unit area of ​​the electrode was 20 mg / cm. ２ After coating, the coating film was dried in an electric oven at 120°C ± 5°C for 25 minutes to obtain an electrode film (II-A). The electrode film (II-A) was then rolled using a roll press (Thank Metal, 3 ton hydraulic roll press) to obtain a positive electrode (A). The weight per unit area of ​​the composite layer was 20 mg / cm. ２ The density of the composite layer after rolling was 3.1 g / cc.

[0157] <Preparation of Secondary Battery> A positive electrode (II-A) and a standard negative electrode were punched out to 45 mm x 40 mm and 50 mm x 45 mm, respectively. These electrodes and a separator (porous polypropylene film) inserted therebetween were placed in an aluminum laminate bag and dried in an electric oven at 60°C for 1 hour. Then, in a glove box filled with argon gas, 2 mL of electrolyte (non-aqueous electrolyte) was poured into the bag, and the aluminum laminate bag was sealed to prepare a laminated secondary battery (II-A). The standard negative electrode and non-aqueous electrolyte were prepared as follows. (Standard negative electrode) A 150 ml plastic container was charged with 0.5 parts by mass of acetylene black (Denka Black (registered trademark) HS-100, manufactured by Denka), 1 part by mass of MAC500LC (carboxymethyl cellulose sodium salt Sunrose special type MAC500LC, manufactured by Nippon Paper Industries Co., Ltd., non-volatile content 100%), and 98.4 parts by mass of water. Then, a centrifugal mixer (Thinky Awatori Rentaro, ARE-310) was used, and the mixture was stirred at 2000 rpm for 30 seconds. 92 parts by mass of artificial graphite (manufactured by Nippon Graphite Industries, CGB-20) and 5 parts by mass of silicon oxide (manufactured by Osaka Titanium Technology Co., Ltd., SILICON MONOOXIDE SiO 1.3C 5 μm, non-volatile content 100%) were added, and the mixture was stirred at 3000 rpm for 10 minutes using a high-speed stirrer. Subsequently, 3.1 parts by mass of styrene-butadiene rubber (SBR) (TRD2001, manufactured by JSR Corporation) was added, and the mixture was stirred for 30 seconds at 2000 rpm using the rotation / revolution mixer to obtain a negative electrode composite slurry. Thereafter, the negative electrode composite slurry was applied using an applicator to an electrode with a coating weight per unit area of ​​8 mg / cm. ２ After coating on copper foil so that the density of the composite layer became 1.6 g / cm, the coating film was dried in an electric oven at 120°C ± 5°C for 25 minutes. Further, rolling treatment was carried out using a roll press (3 ton hydraulic roll press, manufactured by Thank Metal Co., Ltd.) to obtain a composite layer having a density of 1.6 g / cm. ３A standard negative electrode was prepared. (Nonaqueous electrolyte) First, a mixed solvent was prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 1:1:1. Next, 2 parts by mass of VC (vinylene carbonate) was added as an additive to 100 parts by mass of this mixed solvent, and LiPF ６ was dissolved at a concentration of 1 M to obtain a non-aqueous electrolyte solution.

[0158] Examples II-2, II-4 to II-6 CNT (II-B), CNT (II-D) to CNT (II-F) of Examples II-2, II-4 to II-6 were obtained by the same method as in Example II-1, except that the first holding temperature and treatment time were changed to those listed in Table II-1. Furthermore, using each of the obtained CNTs, CNT dispersions, binder compositions, electrode compositions, electrode films, and secondary batteries were produced according to the same methods as in Example 1.

[0159] Example II-3: 15 parts of CNT and 40 parts of zirconia beads with a diameter of 2 mm were added to a glass bottle (M-225, manufactured by Kakuyo Glass Co., Ltd.), and dry-treated for 1 minute using an automatic shaker (SK450, manufactured by Fast and Fluid Management). This procedure was then repeated to produce 60 parts of CNT, which was then subjected to the same procedure as in Example II-2 to obtain CNT (C). Furthermore, using the CNT (II-C) obtained as described above, a CNT dispersion, a binder composition, an electrode composition, an electrode film, and a secondary battery were produced according to the same procedures as in Example II-1.

[0160] Example II-7: 60 parts of CNTs (JENOTUBE 6A, manufactured by JEIO Corporation) were placed in a graphite crucible with a diameter of 10 cm and a height of 10 cm. The crucible containing the CNTs was placed in a multipurpose high-temperature furnace (Hi-Multi 5000, manufactured by Fuji Radio Industrial Co., Ltd.) and subjected to heat treatment under reduced pressure and vacuum as follows. First, nitrogen gas was introduced into the multipurpose high-temperature furnace, and the nitrogen gas replacement operation was performed twice. Next, the furnace pressure was reduced using an oil rotary pump, adjusting the furnace pressure to 9.8-9.5 Pa. Subsequently, the furnace pressure was further reduced using an oil diffusion pump, adjusting the furnace pressure to 0.03 Pa or less. While maintaining the furnace pressure, the furnace temperature was increased to 900°C at a rate of 20°C / min and held at 900°C for 2 hours. The furnace temperature was then further increased to 1800°C at a rate of 20°C / min and held at 1800°C for 1 hour. The furnace was then allowed to cool naturally until the temperature inside the furnace reached 50°C or below, yielding CNT (G). The heating operation was carried out while maintaining the reduced pressure using the oil diffusion pump. Furthermore, using the CNT (II-G) obtained as described above, a CNT dispersion, a binder composition, an electrode composition, an electrode film, and a secondary battery were produced according to the same methods as in Example 1.

[0161] Examples II-8 to II-12 and Comparative Examples II-1 to II-4: CNT (II-H) to CNT (II-L) and CNT (II-X1) to (II-X3) were obtained by the same method as in Example II-7, except that the CNT species, first holding temperature, treatment time, second holding temperature, treatment time, and pulverization conditions were changed to those listed in Table II-1. The CNT pulverization was performed by the same method as in Example II-3. Furthermore, using each of the CNTs obtained as described above, CNT dispersions, binder compositions, electrode compositions, electrode films, and secondary batteries were produced by the same methods as in Example II-1.

[0162] [Comparative Example II-5] Using an electronic balance (MSA225S100DI, manufactured by Sartorius), 10 parts of CNT (JENOTUBE6A, manufactured by JEIO Corporation) were weighed into an alumina crucible SSA-HB4 (manufactured by Nikkato), and the crucible containing the CNT was placed in a muffle furnace (FO510, manufactured by Yamato Scientific Co., Ltd.). Next, under air, the temperature inside the furnace was increased to 330 ° C at a rate of 60 ° C / min, and maintained at 330 ° C for 18 hours (calcination), to obtain oxidation-treated CNT (6A). 10 parts of the oxidation-treated CNT were weighed into a 1 L glass container, and 500 parts of 10% nitric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added. The mixture was then heated to 90 ° C in a hot water bath and thoroughly stirred using a stirrer. The mixture was then thoroughly diluted with ion-exchanged water and filtered under reduced pressure using a membrane filter. After repeating the dilution and filtration process, the CNTs were transferred to a PTFE tray. The CNTs were then dried at 140°C using an oven. Nitric acid-treated CNTs (X4) were thus obtained. Furthermore, using the CNTs (X4) obtained as described above, a CNT dispersion, a binder composition, an electrode composition, an electrode film, and a secondary battery were produced according to the same method as in Example 1.

[0163] II-2. Evaluation II-2-1. Evaluation of CNT Properties For the carbon nanotubes (CNT) of the above-mentioned Examples and Comparative Examples, the metal content of the CNT, the G / D ratio of the CNT, the exothermic peak temperature of the CNT, the surface oxygen content of the CNT, the volume resistivity of the CNT, and the cohesion force of the CNT were measured. The respective measurement methods were the same as those described above in [Example I]. The respective results are shown in Table II-1.

[0164] II-2-2. Evaluation of Other Properties The CNT dispersions, binder compositions, electrode compositions, electrode films, and secondary batteries prepared in the above-mentioned Examples and Comparative Examples were measured and their properties evaluated as follows. The respective evaluation results are shown in Table II-1. <Viscosity Stability of CNT Dispersion> After leaving the CNT dispersion in a thermostatic chamber at 60°C for one week, the CNT dispersion was cooled to 25°C and then immediately measured using a Brookfield viscometer at a rotor rotation speed of 100 rpm. The evaluation criteria for viscosity stability are as follows: (Evaluation Criteria) ◎ (Excellent): 2500 mPa·s or less ○ (Good): More than 2500 mPa·s to 5000 mPa·s or less △ (Fair): More than 5000 mPa·s to 6000 mPa·s or less × (Poor): More than 6000 mPa

[0165] <Particle diameter (D90) of CNT dispersion> The particle diameter (D90) was measured using a particle size distribution analyzer (Partial LA-960V2, manufactured by HORIBA). The circulation / ultrasonic operating conditions were circulation speed: 3, ultrasonic intensity: 7, ultrasonic time: 1 minute, stirring speed: 1, and stirring mode: continuous. During air evacuation, ultrasonic operation was performed with an ultrasonic intensity of 7 and an ultrasonic time of 5 seconds. The refractive index of NMP was 1.468, and the refractive index of CNT was 1.920. The measurement was performed after diluting the measurement sample so that the transmittance of the red laser diode was 60 to 70%, and the particle diameter was measured by volume. The particle diameter evaluation criteria are as follows: (Evaluation criteria) ◎ (Excellent): 1.0 μm or more and less than 3.0 μm ○ (Good): 3.0 μm or more and less than 5.0 μm − (Poor): Less than 1.0 μm or 5.0 μm or more

[0166] <Cycle Characteristics of Secondary Battery> The secondary battery was placed in a thermostatic chamber at 45°C, and charge / discharge measurements were performed using a charge / discharge device (SM-8, manufactured by Hokuto Denko Corporation). Constant-current, constant-voltage charging (cutoff current 1.25 mA (0.025 C)) was performed at a charge current of 50 mA (1 C) with a charge cut-off voltage of 4.2 V, followed by constant-current discharging at a discharge current of 50 mA (1 C) with a discharge cut-off voltage of 2.5 V. This operation was repeated 200 times. 1 C was defined as the current value required to discharge the theoretical capacity of the positive electrode in 1 hour. The cycle characteristics can be expressed by the ratio of the 3rd 1 C discharge capacity to the 200th 1 C discharge capacity at 45°C, as shown in Equation 3 below. (Equation 3): Cycle characteristics = 200th 1 C discharge capacity / 3rd 1 C discharge capacity × 100 (%). The evaluation criteria for the cycle characteristics of the secondary battery were as follows: (Evaluation criteria) ◎ (Excellent): Cycle characteristics are 90% or more ○ (Good): 85% or more and less than 90% △ (Fair): 80% or more and less than 85% × (Poor): Less than 80%

[0167] <Evaluation of Volume Resistivity of Electrode> The electrode compositions prepared in the Examples and Comparative Examples were applied to an electrode using an applicator so that the coating weight per unit area of ​​the electrode was 20 mg / cm. ２ The composite coating was applied to a 100 μm thick PET foil so that the resistivity was 100 μm, and then dried in an electric oven at 120°C ± 5°C for 30 minutes to produce a composite coating film. The surface resistivity (Ω / □) of the composite layer of the prepared composite coating film was measured using a Mitsubishi Chemical Analytech Loresta-GP, MCP-T610. After measurement, the surface resistivity was multiplied by the thickness of the composite layer to obtain the volume resistivity (Ω·cm) of the electrode. The thickness of the composite layer was determined by subtracting the film thickness of the PET foil from the average value measured at three points in the electrode using a film thickness meter (NIKON, DIGIMICRO MH-15M). The evaluation criteria for the volume resistivity of the electrode are as follows: (Evaluation Criteria) ◎ (Excellent): Less than 4 Ω·cm ○ (Good): 4 Ω·cm or more and less than 10 Ω·cm △ (Fair): 10 Ω·cm or more and less than 15 Ω·cm × (Poor): 15 Ω·cm or more

[0168] Table II-1 shows the evaluation results of the CNTs, CNT dispersions, electrode films, and secondary batteries produced in Examples II-1 to II-12 and Comparative Examples II-1 to II-5.

[0169] In Table II-1, 10B and 6A used as CNT seeds are as follows: 10B: multi-walled carbon nanotube (manufactured by JEIO, JENOTUBE 10B) 6A: multi-walled carbon nanotube (manufactured by JEIO, JENOTUBE 6A)

[0170]

[0171] As shown in Table II-1, the CNTs of this embodiment (Examples II-1 to II-12) satisfy all of the requirements described above: (1) exothermic peak in differential thermal analysis, (2) G / D ratio, and (3) aluminum content. Comparison with comparative examples using CNTs that do not satisfy the above requirements (1) to (3) reveals that the use of the CNTs of this embodiment can improve the properties of CNT dispersions, electrode films, and secondary batteries. Thus, the CNTs of this embodiment can form electrode films with excellent conductivity, enabling the safety of secondary batteries to be improved.

Claims

1. Carbon nanotubes that satisfy the following (I-1) to (I-3) and include multi-walled carbon nanotubes: (I-1) A Raman spectrum of 1560 to 1600 cm －１ The maximum peak intensity in the range of 1310 to 1350 cm －１ When the maximum peak intensity within this range is defined as D, the G / D ratio is 0.5 or more and 3.0 or less. (I-2) The wetting index represented by the following formula (I) is 10 or less. Formula (I): Wetting index = (X / Y) [In formula (I), Y is the mass (g) of the carbon nanotubes, and X is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotubes when N-methyl-2-pyrrolidone is dropped onto Y (g) of carbon nanotubes in a 25°C environment.] (I-3) The aluminum content is 3000 ppm or less.

2. The carbon nanotube has a volume resistivity of 2.0 × 10 －２ The carbon nanotubes according to claim 1 , which have a resistivity of Ω·cm or less.

3. The carbon nanotubes according to claim 1, wherein the carbon nanotubes have a cohesive strength of 7.5 kPa or less.

4. The carbon nanotubes according to claim 1, which have an exothermic peak between 600°C and 800°C in differential thermal analysis when heated from 200°C to 1000°C at a rate of 10°C / min.

5. A carbon nanotube dispersion liquid comprising the carbon nanotubes according to any one of claims 1 to 4, a dispersant, and a dispersion medium.

6. The carbon nanotube dispersion liquid according to claim 5, which has a viscosity of 5000 mPa·s or less at 25°C as measured by a Brookfield viscometer after being left to stand at 40°C for one week.

7. Carbon nanotubes that satisfy the following (II-1) to (II-3), including multi-walled carbon nanotubes: (II-1) In differential thermal analysis when the temperature is increased from 200°C to 1000°C at a rate of 10°C / min, the carbon nanotubes have an exothermic peak between 600°C and 800°C; (II-2) In Raman spectroscopy, the carbon nanotubes have an exothermic peak between 1560 and 1600cm －１ The maximum peak intensity in the range of 1310 to 1350 cm －１ When the maximum peak intensity within the range is defined as D, the G / D ratio is 0.5 or more and 3.0 or less. (II-3) The aluminum content is 3000 ppm or less.

8. The carbon nanotubes according to claim 7, further satisfying the following requirement (i): (i) the total content of aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is 13,000 ppm or less.

9. The carbon nanotubes according to claim 7, further satisfying the following condition (ii): (ii) the surface oxygen content is less than 1.0 atm %.

10. A carbon nanotube dispersion liquid comprising the carbon nanotubes according to any one of claims 7 to 9, a dispersant, and a dispersion medium.

11. The carbon nanotube dispersion liquid according to claim 10, which has a viscosity of 5000 mPa·s or less at 25°C as measured with a Brookfield viscometer after being left to stand at 60°C for one week.

12. A binder composition comprising a carbon nanotube dispersion and a binder, wherein the carbon nanotube dispersion comprises the carbon nanotubes according to any one of claims 1 to 4 and 7 to 9, a dispersant, and a dispersion medium.

13. A composition for an electrode, comprising a carbon nanotube dispersion and an electrode active material, wherein the carbon nanotube dispersion comprises the carbon nanotubes according to any one of claims 1 to 4 and 7 to 9, a dispersant, and a dispersion medium.

14. A secondary battery including an electrode film, wherein the electrode film is obtained using: a carbon nanotube dispersion containing the carbon nanotubes according to any one of claims 1 to 4 and 7 to 9, a dispersant, and a dispersion medium; a binder composition containing the carbon nanotube dispersion and a binder; or an electrode composition containing the carbon nanotube dispersion and an electrode active material.

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