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

Abstract

To provide: carbon nanotubes that can be suitably used for forming an electrode film capable of improving safety and having good electrical conductivity; and a carbon nanotube dispersion liquid, a binder composition, a composition for an electrode, and a secondary battery, each containing the carbon nanotubes.SOLUTION: Disclosed are carbon nanotubes which satisfy the following requirements (1) to (3) and include multi-walled carbon nanotubes: (1) in the Raman spectrum, 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 is defined as D, the G / D ratio is 0.5 to 3.0 inclusive; (2) the wetting index represented by formula (I), (wetting index)=(X / Y), is 10 or less (where 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); and (3) the aluminum content is 3,000 ppm or less.

Description

[Technical Field]

[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. [Background technology]

[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 electrode resistance to expansion and contraction, thereby improving the rate characteristics and cycle life of secondary batteries. Among these, multi-walled CNTs with outer diameters 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 therefore 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, or carbides or oxides derived from the catalyst. Metal-containing catalysts (hereinafter also referred to as catalytic metals) are essential for several CNT production methods. However, metals derived from the catalytic metal remaining in 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, or piping during production. Such contaminated metals can become impurities. To achieve desired properties from CNTs, it is desirable for CNTs to have a low metal content. For example, when CNTs containing metals derived from catalytic metals are used in secondary batteries, the metals can 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, to prevent problems caused by the metals contained in CNTs and to improve safety, several methods have been proposed for purifying CNTs to remove metals derived from catalytic metals.

[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 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. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2008 / 126534 [Patent Document 2] International Publication No. 2018 / 043487 Summary of the Invention [Problem to be solved by the invention]

[0008] However, when CNTs are subjected to a halogen treatment process, although the catalytic metal residue is removed, the CNTs become highly crystalline due to the long-term baking at high temperatures. Highly crystalline CNTs become hard and are prone to breakage or damage during use. Therefore, when CNTs are used as a conductive additive in electrode films for 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. This can result in a decrease in the conductivity of the electrode film and a decrease in the performance of secondary batteries containing CNTs.

[0009] In addition, by performing liquid-phase oxidation of CNTs with nitric acid, it is possible to reduce catalytic metal residues, but because nitric acid has a strong oxidizing power, the surface of the CNTs may be oxidized, which may reduce 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 with improved safety and good conductivity. Other embodiments provide carbon nanotube dispersions, binder compositions, electrode compositions, and secondary batteries that contain the carbon nanotubes. [Means for solving the problem]

[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, embodiments of the present invention include the following. However, the present invention is not limited to the embodiments described below.

[0012] <1> Carbon nanotubes that satisfy the following (1) to (3) and include multi-walled carbon nanotubes: (1) 1560-1600 cm in the Raman spectrum -1 The maximum peak intensity in the range of 1310 to 1350 cm is G. -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 wettability 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 nanotube, and X is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotube when N-methyl-2-pyrrolidone is added dropwise to Y (g) of carbon nanotube in a 25°C environment.] (3) The aluminum content is 3000 ppm or less.

[0013] <2> The carbon nanotubes have a volume resistivity of 2.0×10 -2Ω·cm or less, above <1> The carbon nanotube according to claim 1.

[0014] <3> The carbon nanotubes have a cohesive strength of 7.5 kPa or less. <1> or <2> The carbon nanotube according to claim 1.

[0015] <4> The carbon nanotubes have an exothermic peak of 600°C or higher and 800°C or lower in a differential thermal analysis when heated from 200°C to 1000°C at a rate of 10°C / min. <1> ~ <3> 10. The carbon nanotube according to any one of the above.

[0016] <5> the above <1> ~ <4> 1. A carbon nanotube dispersion comprising the carbon nanotubes according to any one of 1 to 8, a dispersant, and a dispersion medium.

[0017] <6> After storing at 40°C for one week, the viscosity at 25°C measured with a Brookfield viscometer is 5000 mPa·s or less. <5> The carbon nanotube dispersion liquid according to claim 1.

[0018] <7> A carbon nanotube dispersion liquid and a binder are included, The carbon nanotube dispersion liquid is <1> ~ <4> 10. A binder composition comprising the carbon nanotubes according to any one of 1 to 9, a dispersant, and a dispersion medium.

[0019] <8> A carbon nanotube dispersion liquid and an electrode active material are included, The carbon nanotube dispersion liquid is <1> ~ <4> 10. A composition for an electrode comprising the carbon nanotubes according to any one of 1 to 9, a dispersant, and a dispersion medium.

[0020] <9> A secondary battery including an electrode film, the electrode film comprising: the above <1> ~ <4> a carbon nanotube dispersion liquid containing the carbon nanotubes according to any one of the above items, a dispersant, and a dispersion medium; a binder composition containing the carbon nanotube dispersion and a binder; or A secondary battery obtained by using an electrode composition containing the carbon nanotube dispersion and an electrode active material. [Effects of the Invention]

[0021] 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. DETAILED DESCRIPTION OF THE INVENTION

[0022] 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.

[0023] In this specification, a numerical range indicated using "to" indicates a range that includes 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.

[0024] <1> Carbon nanotubes (CNTs) One embodiment of the present invention relates to carbon nanotubes (hereinafter also referred to as CNTs). The CNTs according to this embodiment satisfy the following (1) to (3) and include at least multi-walled carbon nanotubes. (1) 1560-1600 cm in the Raman spectrum -1 The maximum peak intensity in the range of 1310 to 1350 cm is G. -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 wettability index represented by the following formula (I) is 10 or less. Formula (I): Wettability 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 under an environment of 25°C.] (3) The aluminum content is 3000 ppm or less.

[0025] The CNT according to this embodiment is characterized by satisfying all of the above requirements (1) to (3). That is, the CNT according to this embodiment has a specific G / D ratio, a specific wettability index, and a further restricted aluminum content, thereby suppressing the occurrence of problems caused by metals as impurities, and can form an electrode film having excellent conductivity. A secondary battery containing such CNTs is excellent in safety and can exhibit good performance. When the CNT contains multi-walled carbon nanotubes, it becomes easy to satisfy all of the above requirements (1) to (3). Hereinafter, each requirement will be described more specifically.

[0026] <G / D ratio> The G / D ratio (peak ratio of G-band and D-band) of the CNT of this embodiment is obtained by Raman spectroscopy. Although there are various laser wavelengths used in Raman spectroscopy, in this embodiment, wavelengths of 532 nm and 632 nm are used. The Raman shift observed around 1,590 cm -1 in the Raman spectrum is called the G-band derived from graphite, and the Raman shift observed around 1,350 cm -1 is called the D-band derived from amorphous carbon and defects in graphite. Since the wave number of Raman spectroscopy may vary depending on the measurement conditions, the wave numbers defined here are defined as wave number ±10 cm -1 . The higher the G / D ratio of the carbon nanotubes, the higher the crystallinity. Also, when the carbon nanotubes are fired at a high temperature, the G / D ratio tends to increase, and the longer the firing time, the higher the G / D ratio tends to be.

[0027] CNTs exhibit a Raman spectrum of 1560 to 1600 cm -1 The maximum peak intensity in the range of 1310 to 1350 cm is G. -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 is easily damaged during dispersion, which may increase 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 decrease. For these reasons, if 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 liquid are improved.

[0028] Conventionally, when raw CNTs are purified by a halogen treatment process, the CNTs are fired at high temperatures for a long period 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 the 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.

[0029] <Wetting index> The wetting index is the ratio of the maximum mass of solvent absorbed by CNTs to the mass of CNTs, and is an index of CNT dispersibility, particularly the initial viscosity when preparing a dispersion containing a dispersion medium and CNTs. More specifically, the CNTs of this embodiment have 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 is easy to obtain a suitable initial viscosity when preparing a dispersion containing a dispersion medium and CNTs. 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 dispersion failure is likely to occur. Furthermore, when the initial viscosity is high, the conductivity of the CNTs is likely to decrease.

[0030] Formula (I): Wetting index = (X / Y)

[0031] 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 added dropwise to Y (g) of CNTs at 25°C. The maximum mass (g) of NMP absorbed by CNT is the total mass of NMP added dropwise to the CNT powder up to the point just before the NMP starts to flow out of the CNT powder.

[0032] 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. While the container is left stationary, NMP is dropped onto the surface of the CNT powder in the container, 5 g at a time, at intervals of 1 minute. Next, it is observed whether the NMP droplets begin to flow 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 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 wetting index shown in formula (I) is calculated.

[0033] <Metal content> From a safety perspective, it is desirable that the amount of impurities, such as metals, remaining in the purified CNTs be as low as possible. Here, the metals contained in the CNTs are primarily metals and metal oxides derived from the catalytic metal used in 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 CNTs due to wear or other reasons. 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.

[0034] The CNTs of this embodiment have an aluminum content of 3000 ppm or less. The aluminum content of the CNTs 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. If aluminum oxide is used as a catalyst support during CNT synthesis, aluminum oxide nanoparticles, which are insulating components, may remain in the CNT, impairing electrical conductivity. When the aluminum content in the CNT is within the above range, the amount of aluminum oxide nanoparticles, which are insulating components, is low, and the resulting carbon nanotubes can be used to form electrode films with good electrical conductivity.

[0035] 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.

[0036] The other metals may preferably be magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, or molybdenum. In CNTs, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be contained as catalyst metals. In addition to the metals and metal oxides used as catalyst metals, metals such as stainless steel used in synthesis equipment, filling equipment, or piping may be contaminated into CNTs due to wear and tear. Therefore, in CNTs, aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may also be metals that are not derived from catalyst metals.

[0037] In some embodiments, the total content of magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum in the CNTs is preferably 10,000 ppm or less, more preferably 7,000 ppm or less, and particularly preferably 5,000 ppm or less, 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.

[0038] Here, the aluminum content and other metal content in CNTs are masses converted into elemental metals. CNTs 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 CNTs, aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum can be contained as simple metals, metal oxides, composite oxides of these, 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 CNTs 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 CNTs is preferably 7000 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 CNTs 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 this embodiment, as in the CNT purification method described below, the total content of metals such as aluminum contained in the CNTs can be reduced by firing the CNTs in an inert atmosphere under reduced pressure and vacuum. By reducing the aluminum content and the total content of other metals such as cobalt and iron contained in the CNTs, the exothermic peak temperature can be increased, resulting in CNTs with improved safety. Secondary batteries containing such CNTs can exhibit excellent performance.

[0041] The aluminum content and other metal content in CNTs can be calculated, for example, by acid decomposing CNTs, extracting the metals contained in the CNTs, 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 CNTs is expressed as the mass ratio (ppm) of the total content of extracted aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum to the mass of the CNTs 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 converted to its elemental metal and adding them together.

[0042] <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. The largest peak in a DTA curve created based on the change in the temperature difference between the sample and the reference material is taken as the exothermic peak.

[0043] Heat is generated as CNTs burn. As the CNT combustion initiation temperature increases, the peak heat generation temperature also increases. Factors that affect the CNT combustion initiation temperature include the catalytic metal content, the degree of oxidation of the CNT surface, and the crystallinity of the CNTs. If the catalytic metal contained in the CNTs has a high heat storage capacity, a low catalytic metal content will result in a small total heat storage capacity for all catalytic metals. Because the total heat storage capacity of the catalytic metals is small, the temperature required to burn the CNTs may be higher than when the catalytic metal content is high. In addition to catalytic metals, metals may be mixed into the CNTs during the CNT manufacturing process, and these metals may also affect the total heat storage capacity. Furthermore, since the portions of the CNT surface that have oxygen-containing functional groups are more easily combusted than the portions that do not have functional groups, the fewer the amount of oxygen-containing functional groups (i.e., the less surface oxygen), the more difficult it is for the CNT to burn, and the lower the degree of oxidation of the CNT surface, the higher the temperature required to burn the CNT. Furthermore, the higher the crystallinity of the CNT, the higher the temperature at which the CNT begins to burn. The crystallinity of the CNT can be expressed by the G / D ratio mentioned above.

[0044] When the combustion start temperature of CNTs is within an appropriate temperature range, the impurities contained in the CNTs are reduced, resulting in safer CNTs. The CNTs of this embodiment preferably have 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 battery safety. 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 CNTs is not too high, preventing CNT breakage and suppressing deterioration in secondary battery performance. For example, the temperature is preferably 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.

[0045] When the CNTs are in the form of powder before dispersion, the exothermic peak can be measured as is. Alternatively, when the CNTs are 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 CNTs do not oxidize (for example, 140°C or lower). When the CNT dispersion contains components other than CNTs 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 peaks are derived from the CNTs.

[0046] <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 ~2.0×10 -2 In some embodiments, the volume resistivity is preferably 1.0×10 Ω·cm. -2 ~1.9×10 -2 Ω·cm is more preferable, and 1.0×10 -2 ~1.8×10 -2 Ω·cm is more preferable, and 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, improving the performance of the secondary battery. The volume resistivity of the CNT can be measured using a powder resistivity measuring device (Loresta-GP Powder Resistivity Measuring System MCP-PD-51, manufactured by Nitto Seiko Analytech Co., Ltd.).

[0047] <Cohesive force> The weakness of the cohesive force of CNTs, i.e., the ease with which CNT aggregates crumble, directly affects the initial dispersibility. From this perspective, the cohesive force of CNTs can be used as an index for evaluating and controlling the initial dispersibility of CNTs. In some embodiments, the cohesive force of CNTs is preferably 7.5 kPa or less, and more preferably 7.0 kPa or less. When the cohesive force is within the above range, the initial dispersibility of CNTs is easily improved. Furthermore, excellent viscosity stability can be easily obtained in the CNT dispersion.

[0048] In this specification, the cohesion force is defined as the maximum peak torque measured when a rotating jig penetrates the compressed CNTs and applies shear force. For example, an MCR302e (Anton Paar) can be used as a measuring device. The specific measurement method is as follows: First, the CNTs are placed in a dedicated aluminum container (C-CC27 / D / Al), and the carbon nanotubes are compressed at 12 kPa using a compression cylinder jig. The jig is then replaced with a blade-shaped jig for measuring cohesion force, and the jig is rotated at 125 μm / s and 0.1 revolutions per minute to penetrate the carbon nanotubes. The torque measured when the shear force is applied is measured, and the maximum peak torque is defined as the cohesion force of the CNTs. A C-PTD200 can be used as a temperature control device, and measurements can be performed at 25°C.

[0049] <Surface oxygen content> In some embodiments, from the perspective of the conductivity of CNT, it is preferable that the surface oxygen content of the CNT in this embodiment is less than 1.0 atm%. The above surface oxygen content is preferably 0.9 atm% or less, and more preferably 0.8 atm% or less. In some embodiments, the above 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 CNT. In this specification, the "surface oxygen content" is a value represented by the ratio (atm%) of oxygen atoms to carbon atoms on the surface of CNT determined by X-ray photoelectron spectroscopy.

[0050] When nitric acid is used for the purification of CNT as a raw material, since the oxidizing power of nitric acid is strong, the surface of CNT will be oxidized. On the other hand, in this embodiment, like the CNT purification method described later, by heat-treating the CNT as a raw material in an inert atmosphere under reduced pressure and vacuum, the metal content such as aluminum can be reduced, and acid treatment is not necessarily required. Therefore, the oxidation of the CNT surface can be suppressed and the surface oxygen content of CNT can be reduced. As a result, the combustion start temperature of CNT becomes higher, and also, CNT can obtain good conductivity as an electrode film, and the secondary battery containing CNT can exhibit good performance.

[0051] <Other properties 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 cylindrical structure formed by rolling one layer of graphite. Multi-walled CNTs have a cylindrical structure formed by rolling two or three or more layers of graphite. 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 containing 90% or more multi-walled CNTs are preferred, and CNTs containing 99% or more multi-walled CNTs are more preferred. The multi-walled CNTs in the CNTs may be 100% by mass. Using such CNTs facilitates achieving a G / D ratio within 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 with sidewalls having an amorphous structure can also be used.

[0052] 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.

[0053] The purity of CNT is expressed as the value (mass%) obtained by subtracting the ash content (mass%) from the mass of the CNT. The ash content (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.

[0054] The BET specific surface area of ​​the CNT of this embodiment is 150 m 2 / g or more is preferable, and 180m 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 450m 2 The BET specific surface area of ​​the CNTs can be calculated by the BET method using nitrogen adsorption measurements. The specific surface area of ​​CNTs is often correlated with the average outer diameter of CNTs; the smaller the specific surface area, the larger the outer diameter of CNTs, and the fewer the number of CNTs per mass. On the other hand, the larger the specific surface area of ​​CNTs, the smaller the outer diameter of CNTs, and the more CNTs per mass. 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, resulting in 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.

[0055] The average outer diameter of the CNTs in this embodiment is preferably 3 nm or more, more preferably 5 nm or more. 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.

[0056] 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 or composite slurry and / or electrode film, making it impossible to form a good conductive network.

[0057] The outer diameter and average outer diameter of CNTs are determined as follows: First, the 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.

[0058] 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). Bundle-like CNT aggregates are easier to disentangle than entangled CNT aggregates. Furthermore, bundle-like CNT aggregates have better dispersibility than entangled CNT aggregates, and are therefore suitable for use as CNTs.

[0059] <Method for manufacturing carbon nanotubes (CNTs)> The CNTs of this embodiment can be produced by, but are not limited to, laser ablation, arc discharge, thermal CVD, plasma CVD, and combustion. For example, CNTs can be produced by catalytically reacting a carbon source with a 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.

[0060] 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.

[0061] <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.

[0062] Typical catalytic metals used in producing CNTs by chemical vapor deposition (thermal CVD) include catalytic metals in which active components such as iron, cobalt, or nickel are fixed to support components such as aluminum and magnesium. Therefore, metals such as aluminum, magnesium, iron, cobalt, and nickel, which originate from the catalytic metal, tend to remain in the CNTs obtained by this production. One example of a method for purifying CNTs is a method using an acid such as nitric acid, which is known to be able to remove iron, cobalt, nickel, and other metals. However, it is difficult to remove metals such as aluminum and magnesium using acid treatment.

[0063] 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 and vacuum. According to the CNT purification method of this embodiment, the content of metals such as aluminum and magnesium in the CNTs 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.

[0064] 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).

[0065] In this specification, "reduced pressure vacuum" means maintaining a state (vacuum state) that is reduced in pressure below atmospheric pressure using a vacuum pump. Specifically, an oil rotary pump, a booster pump, a roots pump, an oil diffusion pump, or the like is used as the vacuum pump to reduce the pressure inside the device and maintain a vacuum state. A single vacuum pump or a combination of multiple vacuum pumps may be used. By reducing the pressure inside the device, pressure increases due to factors such as leaks 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.

[0066] The internal pressure in the reduced 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, it is preferable to reduce the pressure in two stages. For example, in the first stage, the internal pressure is adjusted to preferably 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 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 two-stage decompression method described above. For example, a method of reducing the pressure only in the first stage, or a method of reducing the pressure in a third stage following the second stage, may be applied. For example, 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 in 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.

[0067] The heat treatment conditions, such as the heat treatment temperature and heat treatment time, can be set appropriately 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 of the metals, 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 viewpoint, the heat treatment temperature is preferably 1000°C or higher and 2000°C or lower. From the viewpoint of suppressing high crystallization of CNTs, the heat treatment temperature 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. The heat treatment temperature refers to the temperature inside the apparatus (internal temperature).

[0068] 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. Highly crystalline CNTs become hard and are prone to breaking 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.

[0069] 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 1200 to 1400°C. The maintenance time may more preferably be 1 to 50 hours, and even more preferably 2 to 10 hours.

[0070] Typically, when CNTs are purified by heat treatment, they are heated to 500°C or higher in an air atmosphere to undergo oxidation and combustion. In contrast, the heat treatment of this embodiment is performed in an inert atmosphere under reduced pressure and 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 surface can be suppressed. Furthermore, the heat treatment time can be shortened, which further suppresses high crystallization of the CNTs.

[0071] To further reduce the metal content of 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.

[0072] <Dry grinding of carbon nanotubes (CNTs)> The CNTs of this embodiment may be dry-pulverized CNTs to crush the particles and improve dispersibility. Dry pulverization refers to a process of pulverizing CNTs without the use of a liquid substance. Dry pulverization may be media pulverization, media-free pulverization, or a combination of two or more dry pulverization methods. For example, media pulverization uses a pulverizer equipped with pulverizing media such as beads or steel balls, and utilizes the pulverizing or destructive force generated by the collision of the pulverizing media to pulverize the particles. Known dry pulverization methods, such as a dry attritor, ball mill, vibration mill, or bead mill, can be used. The pulverization time can be set as desired depending on the device or the pulverized state of the particles.

[0073] <2> Carbon nanotube (CNT) dispersion One embodiment of the present invention relates to a CNT dispersion. The CNT dispersion according to this embodiment includes 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. Furthermore, this embodiment can provide a method for producing a CNT dispersion including the CNTs of the above embodiment, a dispersant, and a dispersion medium.

[0074] <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.

[0075] 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.

[0076] In addition to the dispersant, it is preferable to add an amine compound or an inorganic base. As the amine compound, primary amines (primary amines), secondary amines (secondary amines), and tertiary amines (tertiary amines) are 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.

[0077] <Dispersion medium> The dispersion medium is not particularly limited as long as it is capable of dispersing CNTs, but is preferably one or more of water and water-soluble organic solvents.

[0078] 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.

[0079] 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.

[0080] The CNT dispersion can be produced, for example, by dispersing CNTs in a dispersion medium. During the dispersion process, the raw materials to be used may be added in one or more batches at any timing. The dispersion method for carrying out such a process is not particularly limited.

[0081] Dispersion methods include, for example, methods using various dispersers such as a disperser (disperser), homogenizer, high-shear mixer, kneader, two-roll mill, three-roll mill, ball mill, horizontal sand mill, vertical sand mill, annular bead mill, paint conditioner, attritor, planetary mixer, or high-pressure homogenizer. While the disperser is not particularly limited, for example, a high-pressure homogenizer is preferred from the viewpoint of adjusting the CNT fiber length 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 aggregated particles. It is also more preferable to select and combine multiple dispersers described above for dispersion, and the order of the dispersers can be arbitrarily changed. The pressure when using a high-pressure homogenizer is not particularly limited, but is preferably 40 to 150 MPa, more preferably 40 to 120 MPa.

[0082] 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 it is easy to handle, it is preferred for small-scale production. Pass dispersion is a dispersion method in which the dispersing device itself is equipped with a tank that supplies the dispersion liquid via piping and a tank that receives the dispersion liquid, and the dispersion passes through the dispersing device itself. Furthermore, circulation dispersion is a method in which the dispersion liquid that has passed through the dispersing device itself is returned to the tank that supplies the dispersion liquid, and dispersion is performed while circulating. In both methods, the longer the processing time, the more the dispersion progresses; therefore, the pass or circulation can be repeated until the desired dispersion state is achieved, and the processing volume can be increased by changing the tank size or processing time. Pass dispersion is preferred over circulation dispersion because it is easier to achieve a uniform dispersion state. Circulation dispersion is preferred over pass dispersion because it requires simpler operations and manufacturing equipment. 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.

[0083] 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. 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.

[0084] 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 CNTs. Also, from the viewpoint of 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 CNTs.

[0085] The CNT dispersion of this embodiment is made using CNTs that contain a small amount of metals such as aluminum, etc. Therefore, the metal content in the CNT dispersion can be easily reduced. However, the CNT dispersion may contain metal particles and dissolved metal ions as metals. Metal particles are metals present in particulate form in the CNT dispersion, and specific examples include 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. If metal particles are present inside a battery, the battery is more likely to short-circuit, so removing the metal particles is extremely important from a safety perspective. Therefore, it is preferable to include a step of removing contaminants such as metal particles (metal foreign matter removal step) at any timing in the process of producing a CNT dispersion. From the viewpoint of efficiency, the metal foreign matter removal step is preferably performed during and / or at the end of the dispersion step of the CNT dispersion. The metal foreign matter removal step may be performed multiple times.

[0086] In the metal foreign matter removal step, the method for removing metal particles from the CNT dispersion 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.

[0087] The method of removing metal particles by magnetic force is not particularly limited as long as it can remove metal particles. However, from the viewpoints of productivity and removal efficiency, a method of removing metal particles by passing the CNT dispersion through a magnetic filter placed in a CNT dispersion production line is preferred. The step of removing metal particles from a CNT dispersion using a magnetic filter is preferably carried out by passing the particles through a magnetic filter that forms a magnetic field with a magnetic flux density of 1,000 gauss or more. Because a low magnetic flux density reduces the efficiency of removing metal particles, the magnetic flux density is preferably 5,000 gauss or more, more preferably 10,000 gauss or more in consideration of removing stainless steel, which has weak magnetism, and most preferably 12,000 gauss or more. Depending on the filtration flow rate, coarse metal particles may pass through the magnetic filter. Therefore, when a magnetic filter is installed in a production line, it is preferable to include a process for removing coarse foreign matter or metal particles using a filter such as a cartridge filter upstream of the magnetic filter. Although a magnetic filter is effective even when used for a single filtration, a circulating type is more preferable. The circulating type improves the efficiency of removing metal particles. When a magnetic filter is disposed in a production line for a CNT dispersion, the location of the magnetic filter is not particularly limited, but it is preferably disposed immediately before filling a container with the carbon nanotube dispersion, or before the filter if a filtration step using a filtration filter is performed before filling the container. By disposing the magnetic filter in this manner, it is possible to prevent metals from being mixed into the product if they are detached from the magnetic filter.

[0088] 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.

[0089] The content of aluminum, iron, and chromium metals 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.

[0090] 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.

[0091] <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.

[0092] <Binder> The binder is a resin that binds various substances together in the 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 polymeric compounds having fluorine atoms in the molecule, such as polyvinylidene fluoride, polyvinyl fluoride, and tetrafluoroethylene.

[0093] 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. A weight-average molecular weight of 10,000 or more can prevent a decrease in the resistance and adhesion of the binder. A weight-average molecular weight of 2,000,000 or less can improve the resistance and adhesion of the binder while preventing a decrease in workability due to an increase in the viscosity of the binder itself and preventing significant aggregation of dispersed particles.

[0094] The binder composition is preferably produced by mixing and homogenizing a CNT dispersion and a binder, or the binder may be dissolved in advance. The binder may be added at any timing during the process of producing the CNT dispersion. The mixing method may be any of various conventionally known methods. The binder composition can be produced using the dispersion device described above for the CNT dispersion. 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.

[0095] <4> Composition for electrodes 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. This embodiment also provides a method for producing a binder composition including the above-described CNTs, a dispersant, a dispersion medium, and an electrode active material, and a method for producing a binder composition including the above-described CNTs, a dispersant, a dispersion medium, an electrode active material, and a composite slurry.

[0096] <Electrode active material> Electrode active materials are the materials that form the basis of battery reactions. Active materials are divided into positive electrode active materials and negative electrode active materials based on their electromotive force. The positive electrode active material is not particularly limited, and metal compounds such as metal oxides and metal sulfides capable of doping or intercalating lithium ions, and conductive polymers can be used. For example, oxides of transition metals such as Fe, Co, Ni, and Mn, composite oxides with lithium, inorganic compounds such as transition metal sulfides, etc. can be mentioned. Specifically, transition metal oxide powders such as MnO, V2O5, V6O 13 , TiO2; composite oxide powders of lithium and transition metals such as lithium nickelate, lithium cobaltate, lithium manganate having a layered structure, and lithium manganate having a spinel structure; lithium iron phosphate-based materials which are olivine structure phosphate compounds, etc. can be mentioned. These positive electrode active materials can also be used alone or in combination of two or more. Further, the above inorganic compounds and organic compounds may be mixed and used. The negative electrode active material is not particularly limited, and those capable of doping or intercalating lithium ions can be used. For example, alloy systems such as metallic Li, its alloys such as tin alloys, silicon alloys, and lead alloys; Li x Fe2O3, Li x Fe3O4, Li x WO2 (x is a number where 0 < x < 1), metal oxide systems such as lithium titanate, lithium vanadate, and lithium silicate; amorphous carbonaceous materials such as soft carbon and hard carbon, or carbonaceous powders such as artificial graphite with a high degree of graphitization or natural graphite can be mentioned. These negative electrode active materials can also be used alone or in combination of two or more. In particular, it is preferable to use a combination of a highly graphitized carbon material and lithium silicate from the viewpoints of capacity and lifespan.

[0097] The BET specific surface area of the electrode active material is preferably 0.1 to 10 m 2 / g, more preferably 0.2 to 5 m 2 / g, and even more preferably 0.3 to 3 m 2 / g. 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.

[0098] 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.

[0099] 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, and 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, and 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 or productivity, and the uniformity of the electrode film. The CNT content in the composite slurry is preferably 0.01% by mass or more, and more preferably 0.05% by mass or more, relative to 100% by mass of the electrode active material. The CNT content 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. The content of the binder 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 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. The solid content of the composite slurry is preferably 30% by mass or more, more preferably 40% by mass or more, based on 100% by mass of the composite slurry, and is preferably 90% by mass or less, more preferably 85% by mass or less, based on 100% by mass of the composite slurry. The water content of the composite slurry is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

[0100] <5> electrode membrane 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. 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) the binder composition, or (iii) the electrode composition.

[0101] 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.

[0102] The method for applying the composite slurry onto the current collector is not particularly limited, and any known method can be used.

[0103] After coating and drying, the coating may be rolled using a lithographic press or a calendar 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.

[0104] <6> secondary battery One embodiment of the present invention relates to a secondary battery. The secondary battery according to this embodiment includes the above-described electrode film. The electrode film can be used as an electrode of the secondary battery, and is particularly preferably used as an electrode of a non-aqueous electrolyte secondary battery that uses an organic electrolyte solution. A non-aqueous electrolyte secondary battery is a battery that includes 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 coating can be used as a positive electrode. In one embodiment, for example, an electrode film obtained by applying an electrode composition containing a negative electrode active material to a current collector and drying the applied composition can be used as a negative electrode. In one embodiment, an electrode film obtained by coating a current collector with a CNT dispersion or a binder composition and drying the coating can be used as a current collector with an underlayer. In particular, from the viewpoint of safety, it is preferable to use it as a positive electrode.

[0105] Various conventionally known electrolytes capable of ion mobility can be used. Examples include, but are not limited to, lithium salts such as LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, Li(CF3SO2)3C, LiI, LiBr, LiCl, LiAlCl, LiHF2, LiSCN, and LiBPh4 (where Ph is a phenyl group). 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.

[0106] The non-aqueous solvent is not particularly limited, and various solvents suitable for secondary batteries can be used. For example, carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, lactones, glymes, esters, sulfoxides, and nitriles can be mentioned. These solvents may be used alone or in combination of two or more.

[0107] The secondary battery preferably includes a separator. Examples of the separator include, but are not particularly limited to, polyethylene non-woven fabric, polypropylene non-woven fabric, polyamide non-woven fabric, and those obtained by subjecting these to hydrophilic treatment.

[0108] The structure of the secondary battery is not particularly limited. The secondary battery may usually be composed of a positive electrode, a negative electrode, and a separator provided as necessary. The shape of the secondary battery can be various shapes such as a paper type, a cylindrical type, a button type, a laminated type, etc., according to the purpose of use.

Examples

[0109] The present invention will be described more specifically with reference to the following examples. The present invention is not limited to the following examples as long as the gist thereof is not exceeded.

[0110] 1. Production examples of CNT and CNT dispersion liquid using the same [Example 1] [Production of CNT] A graphite crucible with a diameter of 10 cm and a height of 10 cm was charged with 60 parts of CNT (manufactured by JEIO Co., Ltd., JENOTUBE10B), and vacuum heating under reduced pressure was carried out using a multi-purpose high-temperature furnace (manufactured by Fuji Denpa Kogyo Co., Ltd., Himulti 5000). After performing the nitrogen gas replacement operation twice, the pressure was reduced using an oil rotary pump, and after adjusting the furnace internal pressure to 9.8 - 9.5 Pa, subsequently, the pressure was further reduced using an oil diffusion pump, and the furnace internal pressure was adjusted to 0.03 Pa or less. Subsequently, while maintaining the pressure reduction by the oil diffusion pump, the temperature was raised to 1200 °C at a rate of 20 °C / min and held at 1200 °C for 6 hours. Then, it was allowed to cool naturally until the furnace internal temperature reached 50 °C or lower, and CNT (A) was obtained.

[0111] <Preparation of CNT Dispersion Liquid> An NMP solution containing a hydrogenated nitrile butadiene rubber polymer (manufactured by Nippon Zeon Co., Ltd., Zetpole2000L, solid content 8 mass%) and NMP were added to a stainless steel container, and the adjustment was made so that the polymer was 0.6 mass parts and the total amount of NMP was 96.4 mass parts. 3.0 mass parts of CNT (A) was weighed into this solution and added while stirring with a disper. A fine emulsifier screen was attached to a high-shear mixer (L5M-A, manufactured by SILVERSON), and batch dispersion was performed at a speed of 7,000 rpm until the whole became uniform. Next, the content of the stainless steel container was fed, and a circulation type dispersion treatment (bead filling amount 80%, peripheral speed 12 m / s) with a residence time of 10 minutes was performed using a bead mill (manufactured by Asazawa Fine Tech Co., Ltd., Star Mill LMZ) filled with zirconia beads having a diameter of 0.5 mmφ. Subsequently, the dispersion liquid was supplied to a high-pressure homogenizer (manufactured by Sugino Machine, Ltd., Starburst Labo), and a 10-pass type dispersion treatment was performed. The dispersion treatment was carried out using a single nozzle chamber at a nozzle diameter of 0.25 mm and a pressure of 100 MPa. After the dispersion treatment, it was passed through a depth filter (manufactured by 3M, PP non-woven fabric deep cartridge NT-T series, filtration accuracy 40 μm). In this way, a carbon nanotube dispersion liquid (A) was prepared.

[0112] <Preparation of Binder Composition> Volume 150 cm3 The CNT dispersion (A) and PVdF (polyvinylidene fluoride, Solef5130, Solvay, non-volatile content 100%), which had been dissolved in NMP (N-methyl-2-pyrrolidone) to a solid content of 8% by 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). Thus, binder composition (A) was obtained.

[0113] <Preparation of electrode composition> The binder composition (A) contains NMC (S800, LiNi 0.8 Mn 0.1 Co 0.1 After adding 02 (manufactured by Kinwa), the mixture was stirred at 2,000 rpm for 30 seconds using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310). Subsequently, the lumps were broken up with a spatula, and the mixture was stirred at 2,000 rpm for 300 seconds using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310) to obtain electrode composition (A). The nonvolatile content of the electrode composition was 72.0 mass%. The nonvolatile content ratio of NMC:CNT:PVdF in the nonvolatile portion of the electrode composition was 98.1:0.4:1.5.

[0114] <Preparation of electrode film> The electrode composition (A) was applied to the electrode using an applicator so that the amount 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 (A). The electrode film (A) was then rolled using a roll press (Thank Metal, 3 ton hydraulic roll press) to obtain a positive electrode (A). The weight of the composite layer per unit area was 20 mg / cm. 2 The density of the composite layer after the rolling treatment was 3.1 g / cc.

[0115] <Preparation of secondary battery> The positive electrode (A) and the standard negative electrode were punched out to 45 mm × 40 mm and 50 mm × 45 mm, respectively. These electrodes and a separator (porous polypropylene film) inserted between them were placed in an aluminum laminate bag and dried in an electric oven at 60°C for 1 hour. Thereafter, 2 mL of electrolyte (non-aqueous electrolyte) was poured into the bag in a glove box filled with argon gas, and the aluminum laminated bag was then sealed to prepare a laminated secondary battery (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 weight of acetylene black (Denka Black® HS-100, manufactured by Denka), 1 part by weight of MAC500LC (carboxymethylcellulose sodium salt, Sunrose Special Type MAC500LC, manufactured by Nippon Paper Industries Co., Ltd., 100% nonvolatile content), and 98.4 parts by weight of water, and then stirred at 2000 rpm for 30 seconds using a centrifugal mixer (Thinky Awatori Rentaro, ARE-310). Furthermore, 92 parts by weight of artificial graphite (Nippon Graphite Industries, CGB-20) and 5 parts by weight of silicon oxide (Osaka Titanium Technology, SILICON MONOOXIDE SiO 1.3C 5 μm, 100% nonvolatile content) were added as active materials, and the mixture was stirred at 3000 rpm for 10 minutes using a high-speed mixer. Next, 3.1 parts by mass of styrene butadiene rubber (SBR) (TRD2001, manufactured by JSR Corporation) was added, and the mixture was stirred at 2000 rpm for 30 seconds using the planetary centrifugal mixer to obtain a negative electrode composite slurry. The negative electrode composite slurry was then applied to the electrode using an applicator so that the weight per unit area of ​​the electrode was 8 mg / cm. 2 After coating on copper foil so that the density of the composite layer became 1.6 g / cm, the coating was dried in an electric oven at 120°C ± 5°C for 25 minutes. Then, the coating was rolled using a roll press (Thank Metal Co., Ltd., 3 ton hydraulic roll press) until the density of the composite layer became 1.6 g / cm. 3 A standard negative electrode was fabricated. (Non-aqueous 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 LiPF6 was further dissolved in the mixed solvent at a concentration of 1 M to obtain a non-aqueous electrolyte solution.

[0116] [Examples 2 to 9], [Comparative Examples 1 to 4] CNTs (B) to (Y1) were obtained in the same manner as in Example 1, except that the CNT species, holding temperature, and treatment time were changed as shown in Table 1. Furthermore, using each of the obtained CNTs, 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.

[0117] [Example 10] 15 parts of CNTs 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 Co., Ltd.). This procedure was then repeated to produce 60 parts of CNTs, which were then used to obtain CNTs (J) in the same manner as in Example 9. Furthermore, using the CNT(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.

[0118] Comparative Example 5 Using an electronic balance (MSA225S100DI, manufactured by Sartorius), 50 parts of CNTs (JENOTUBE10B, manufactured by JEIO) were weighed into an alumina crucible SSA-HB4 (manufactured by Nikkato), and the crucible containing the CNTs was placed in a multipurpose high-temperature furnace (Hi-Multi 5000, manufactured by Fuji Dempa Kogyo Co., Ltd.). Next, in 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 rate of 20°C / min, and held at 1200°C for 6 hours, after which the furnace was allowed to cool naturally to 50°C. Subsequently, 10 parts of the heat-treated CNT were weighed into a 1 L glass container, 500 parts of 10% hydrochloric acid (manufactured by Fujifilm Wako Pure Chemical Corporation) were added, and then it was sufficiently stirred using a stirrer. Thereafter, it was sufficiently diluted with ion-exchanged water, and vacuum filtration was performed using a membrane filter. The operations of dilution and filtration were repeated, and after confirming that the pH of the filtrate became 4 or more, the CNT was transferred to a PTFE vat. Next, it was dried at 140 °C using an oven. Thus, CNT (X4) was obtained. Furthermore, using the CNT (X4) obtained as described above, in accordance with the same method as in Example 1, a CNT dispersion, a binder composition, an electrode composition, an electrode film, and a secondary battery were produced.

[0119] 2. Evaluation 2-1. Evaluation of CNT Properties Regarding the carbon nanotubes (CNT) of the above-described Examples and Comparative Examples, measurements were performed as follows. Unless otherwise specified, the measurements were performed using the purified CNT. The respective results are shown in Table 1.

[0120] <Content of Metals in CNT> Using a microwave sample pretreatment device (ETHOS, manufactured by Milestone General Co., Ltd.), the CNT was acid-decomposed to extract the metals contained in the CNT. The analysis of the extracted metals was performed using a multi-type ICP emission spectroscopic analyzer (720-ES, manufactured by Agilent), and the content of the metals contained in the CNT was calculated. In Table 1 described later, the contents of aluminum, iron, cobalt, and magnesium are shown as the metal content. Also, from the calculated metal contents, the total contents of aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum were determined. 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 contents of the extracted magnesium, aluminum, iron, copper, zinc, nickel, chromium, manganese, and molybdenum to the mass of the CNT before metal extraction.

[0121] <G / D ratio of CNT> CNT was placed on a Raman microscope (manufactured by Horiba, Ltd., XploRA), and measurements were taken using a laser wavelength of 532 nm. The measurement conditions were an acquisition time of 60 seconds, an integration number of 2 times, a dimming filter of 10%, an objective lens magnification of 20 times, a confocal hole of 500, a slit width of 100 μm, and a measurement wavelength of 100 - 3000 cm -1 The CNT for measurement was separated on a slide glass and flattened using a spatula. Among the obtained peaks, within the spectrum range of 1560 - 1600 cm -1 the maximum peak intensity was defined as G, and within the range of 1310 - 1350 cm -1 the maximum peak intensity was defined as D. The ratio of G / D was calculated and used as the G / D ratio of CNT.

[0122] <Wetting index of CNT> In a 25°C environment, 5 g (Y (g)) of CNT powder was naturally dropped and contained in a cylindrical polypropylene container with a diameter of 10 cm. While in a static state, N-methyl-2-pyrrolidone (NMP) was dropped onto the surface of the CNT powder at 5 g per time and at intervals of 1 minute. The total mass (X (g)) of the NMP dropped until just before it began to flow out on the surface of the CNT powder without being absorbed by the CNT powder was measured. The wetting index of CNT was calculated from 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 CNT when NMP is dropped onto Y (g) of CNT in a 25°C environment.]

[0123] <Particle size of CNT> The particle size of CNT was measured using the laser diffraction method (for example, MasterSizer3000 manufactured by Malvern). The average particle size D50 is the particle size at which the integrated value based on volume in the particle size distribution is 50%. For D10 and D90, they are the particle sizes at which the integrated values based on volume are 10% and 90% respectively.

[0124] <Temperature of the exothermic peak of CNT> Using a thermogravimetric differential thermal analyzer (manufactured by Rigaku Corporation, Tg-DTA 8122 Thermo plus EVO2), with a sample mass of 1.0 mg, placed in an alumina pan container, the temperature was raised from 25 °C to 1000 °C at a heating rate of 10 °C / min in an air atmosphere. For the obtained DTA curve, in the temperature range from 200 °C to 1000 °C, the temperature at the peak apex was taken as the temperature of the exothermic peak.

[0125] <Amount of surface oxygen of CNT> The amount of surface oxygen of CNT was measured using an X-ray photoelectron spectrometer (XPS, manufactured by ThermoFisher Scientific, K-Alpha). After pelletizing the CNT, this sample was fixed to the sample stage with double-sided tape for measurement. The 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 amount of surface oxygen.

[0126] <Volume resistivity of CNT> Using a powder resistivity measuring device (manufactured by Nitto Seiko Analytic Co., Ltd.: Loresta-GP Powder Resistivity Measuring System MCP-PD-51), with a sample mass of 1.2 g, using a powder probe unit (four-probe · ring electrode, electrode interval 5.0 mm, electrode radius 1.0 mm, sample radius 12.5 mm), with the applied voltage limiter set to 90 V, the volume resistivity [Ω·cm] of the CNT powder under various pressures was measured. The value of the volume resistivity of CNT at a density of 1 g / cm 3 was evaluated.

[0127] <Cohesion force of CNT> The cohesion of CNTs was measured using a rheometer (MCR302e, manufactured by Anton Paar). First, 4.0 g of CNTs were 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, it was replaced with a blade jig for measuring cohesion, and while moving the jig at 125 μm / s and 0.1 rotation / min, it was penetrated into the carbon nanotubes, and the torque when applying a shear force was measured, and the maximum peak value was taken as the cohesion of the CNTs. The temperature control device used was C-PTD200, and the measurement was carried out at a temperature of 25°C.

[0128] 2-2. Evaluation of Other Properties For the CNT dispersion, binder composition, electrode composition, electrode film, and secondary battery prepared in the above-mentioned examples and comparative examples, measurements were carried out and properties were evaluated as follows. The evaluation results are shown in Table 1. <Viscosity Stability of CNT Dispersion> After the CNT dispersion was left standing in a constant temperature bath at 40°C for 1 week, after cooling the CNT dispersion to 25°C, it was immediately carried out using a B-type 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 〇 (Good): Exceeding 3000 mPa·s and 5000 mPa·s or less △ (Fair): Exceeding 5000 mPa·s and 10000 mPa·s or less × (Poor): Exceeding 10000 mPa

[0129] <Particle Size (D90) of CNT Dispersion> The particle size (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. Measurements were performed after diluting the measurement sample so that the transmittance of the red laser diode was 60-70%, and the particle size was measured by volume. The particle size evaluation criteria were as follows: (Evaluation criteria) ◎ (Excellent): 0.6 μm or more and less than 2.0 μm 〇 (Good): 2.0 μm or more and less than 5.0 μm - (Poor): Less than 0.6 μm or 5.0 μm or more It was decided.

[0130] <Cycle characteristics of secondary batteries> The secondary battery was placed in a thermostatic chamber at 25°C and subjected to charge / discharge measurements using a charge / discharge device (Hokuto Denko Corporation, SM-8). A constant-current, constant-voltage charge (cutoff current 1.25 mA (0.025 C)) was performed at a charge current of 50 mA (1 C) with a charge cutoff voltage of 4.2 V, followed by a constant-current discharge at a discharge current of 50 mA (1 C) with a discharge cutoff voltage of 2.5 V. This procedure 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 as the ratio of the 3rd 1 C discharge capacity to the 200th 1 C discharge capacity at 25°C, using the following equation: (Formula 3): Cycle characteristics = 200th 1C discharge capacity / 3rd 1C discharge capacity × 100 (%) The evaluation criteria for the cycle characteristics of the secondary battery are as follows. (Evaluation criteria) ◎ (Excellent): 90% or more 〇 (Good): 85% to less than 90% △ (Acceptable): 80% or more but less than 85% × (bad): Less than 80%

[0131] <Evaluation of electrode volume resistivity> The electrode compositions prepared in the examples and comparative examples were applied to the electrodes using an applicator so that the coating weight per unit area of ​​the electrodes was 20 mg / cm. 2 The mixture was then dried in an electric oven at 120°C ± 5°C for 30 minutes to prepare a composite coating film. The surface resistivity (Ω / □) of the composite layer of the prepared composite coating was measured using a Mitsubishi Chemical Analytech Loresta GP, MCP-T610. After measurement, this 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 on 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 7Ω·cm Good: 7 Ω·cm or more and less than 15 Ω·cm △ (Acceptable): 15Ω·cm or more and less than 20Ω·cm × (defective): 20Ω cm or more

[0132] Table 1 shows the evaluation results of the CNTs, CNT dispersions, electrode films, and secondary batteries produced in Examples 1 to 10 and Comparative Examples 1 to 5.

[0133] In Table 1, 10B and 6A used as CNT seeds are as follows: 10B: Multi-walled carbon nanotube (JEIO, JENOTUBE10B) 6A: Multi-walled carbon nanotubes (JEIO, JENOTUBE6A)

[0134] [Table 1]

[0135] As shown in Table 1, the CNTs of this embodiment (Examples 1 to 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) reveals 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.

Claims

1. Carbon nanotubes that satisfy the following (1) to (3) and include multi-walled carbon nanotubes: (1) 1560 to 1600 cm in the Raman spectrum －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. (2) The wettability 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.

2. The carbon nanotubes have 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. 2. The carbon nanotube according to claim 1, wherein the carbon nanotube has 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 comprising the carbon nanotubes according to any one of claims 1 to 4, a dispersant, and a dispersion medium.

6. 6. The carbon nanotube dispersion according to claim 5, wherein the viscosity at 25°C measured with a Brookfield viscometer after being left to stand at 40°C for one week is 5000 mPa·s or less.

7. A carbon nanotube dispersion liquid and a binder are included, The carbon nanotube dispersion liquid is a binder composition comprising the carbon nanotubes according to any one of claims 1 to 4, a dispersant, and a dispersion medium.

8. A carbon nanotube dispersion liquid and an electrode active material are included, 5. A composition for an electrode, wherein the carbon nanotube dispersion liquid comprises the carbon nanotubes according to claim 1, a dispersant, and a dispersion medium.

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

Images