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

Abstract

To provide a carbon nanotube that can improve safety and can be suitably used for forming an electrode film having good electrical conductivity, and a carbon nanotube dispersion, binder composition, electrode composition, and secondary battery comprising the carbon nanotube.SOLUTION: A carbon nanotube comprises multi-walled carbon nanotubes and satisfies the following (1) to (3). (1) The carbon nanotube has an exothermic peak at 600°C or higher and 800°C or lower in differential thermal analysis when heated from 200°C to 1000°C at 10°C / min. (2) When the maximum peak intensity within a range of 1560 to 1600 cm-1 in a Raman spectrum is G and the maximum peak intensity within a range of 1310 to 1350 cm-1 is D, the G / D ratio is 0.5 or more and 3.0 or less. (3) The content of aluminum is 3000 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 the electrode's 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. Of these, chemical vapor deposition is the most suitable for mass production from the standpoint 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 metals such as iron, cobalt, and 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 some 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, piping, etc. during production. These 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 with 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 catalytic metal 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 are baked at high temperatures for a long time, which increases their crystallinity. Highly crystalline CNTs are hard and 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 can 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) In a differential thermal analysis when the temperature is increased from 200°C to 1000°C at a rate of 10°C / minute, an exothermic peak is observed between 600°C and 800°C. (2) 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. (3) The aluminum content is 3000 ppm or less.

[0013] <2> Furthermore, the above-mentioned product satisfies the following (i): <1> The carbon nanotube according to claim 1. (i) The total content of aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is not more than 13,000 ppm.

[0014] <3> Furthermore, the above-mentioned product satisfies the following (ii): <1> or <2> The carbon nanotube according to claim 1. (ii) The surface oxygen content is less than 1.0 atm %.

[0015] <4> the above <1> ~ <3> 10. A carbon nanotube dispersion comprising the carbon nanotubes according to any one of 1 to 9, a dispersant, and a dispersion medium.

[0016] <5> After storing at 60°C for one week, the viscosity at 25°C measured with a Brookfield viscometer is 5000 mPa·s or less. <1> ~ <4> 1. The carbon nanotube dispersion liquid according to any one of the above.

[0017] <6> the above <4> or <5> 2. A binder composition comprising the carbon nanotube dispersion liquid according to claim 1, and a binder.

[0018] <7> the above <4> or <5> 10. A composition for an electrode, comprising the carbon nanotube dispersion liquid according to claim 1, and an electrode active material.

[0019] <8> A secondary battery including an electrode film, the electrode film comprising: the above <4> or <5> The carbon nanotube dispersion liquid described in <6> or the binder composition described above. <7> A secondary battery obtained by using the electrode composition described in 1. [Effects of the Invention]

[0020] 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

[0021] 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.

[0022] 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.

[0023] <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) In a differential thermal analysis when the temperature is increased from 200°C to 1000°C at a rate of 10°C / minute, an exothermic peak is observed between 600°C and 800°C. (2) 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. (3) The aluminum content is 3000 ppm or less.

[0024] The CNTs according to this embodiment are characterized by satisfying all of the above requirements (1) to (3). That is, the CNTs exhibit an exothermic peak in a specific range in differential thermal analysis, have a specific G / D ratio, and further have a limited aluminum content, thereby suppressing the occurrence of defects due to metal impurities and improving safety. Furthermore, an electrode film with excellent conductivity can be formed. A secondary battery containing such CNTs can exhibit good performance. When the CNTs include multi-walled carbon nanotubes, it becomes easy to satisfy all of the above requirements (1) to (3). Each requirement will be explained in more detail below.

[0025] <Exothermic peak> The CNTs of this embodiment 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 changes in the temperature difference between the sample and the reference material is taken as the exothermic peak.

[0026] Heat is generated as CNTs burn. As the CNT combustion initiation temperature increases, so does the peak heat generation temperature. Factors that affect the CNT combustion initiation temperature include the content of metal components derived from catalytic metals, the degree of oxidation of the CNT surface, and the crystallinity of the CNTs. If the metal components contained in CNTs have a high heat storage capacity, a low metal component content reduces the total heat storage capacity of all catalytic metals. Because the total heat storage capacity of the metal components is low, the temperature required to burn the CNTs may be higher than when the metal component content is high. In addition to catalytic metals, metal components may be mixed into CNTs during the CNT manufacturing process, and these metal components may also affect the total heat storage capacity. In addition, the sites on the CNT surface having oxygen-containing functional groups are more likely to burn compared to the sites without functional groups. Therefore, the less the amount of oxygen-containing functional groups (i.e., the less the surface oxygen amount), the more difficult it is for CNT to burn, and the smaller 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 combustion initiation temperature of the CNT. The crystallinity of the CNT can be represented by the G / D ratio described later.

[0027] When the combustion initiation temperature of the CNT is within an appropriate temperature range, the impurities contained in the CNT are reduced, so that safer CNT can be obtained. The CNT of this embodiment preferably has a heat generation peak temperature of 600 °C or higher, more preferably 650 °C or higher. Also, the heat generation peak temperature is preferably 800 °C or lower, more preferably 740 °C or lower. When the heat generation peak temperature is 600 °C or higher, the metal content is low, and the safety of the battery can be improved. Or, the surface oxygen amount is small and it has excellent conductivity. When the heat generation peak temperature is 800 °C or lower, the crystallinity of the CNT is not too high, the breakage of the CNT can be suppressed, and the degradation of the performance of the secondary battery can be suppressed. For example, it 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.

[0028] When the CNT is a powder before dispersion, the heat generation peak can be measured as it is. Also, when the CNT is present in the CNT dispersion, after removing the dispersion medium by heating and drying, it can be measured and specified from the shape of the heat generation peak. The temperature of heating and drying is preferably carried out at a temperature at which the CNT is not oxidized (for example, 140 °C or lower). When the CNT dispersion contains components other than CNT and the dispersion medium (additives, etc.), the heat generation peak of the additive can be measured in advance, and by specifying the heat generation peak derived from the additive, the remaining heat generation peak can be determined to be derived from the CNT and the heat generation peak can be specified.

[0029] <G / D ratio> The G / D ratio (peak ratio between the G-band and the D-band) of the CNTs in this embodiment is determined by Raman spectroscopy. Various laser wavelengths are used in Raman spectroscopy, but in this embodiment, 532 nm and 632 nm are used. In the Raman spectrum, the peaks at 1,590 cm -1 The Raman shift observed around 1,350 cm is called the G band derived from graphite. -1 The Raman shift observed around this band is called the D band, which is derived from defects in amorphous carbon and graphite. The wavenumber in Raman spectroscopy analysis may vary depending on the measurement conditions, so the wavenumber specified here is ±10 cm. -1 The higher the G / D ratio, the higher the crystallinity of the carbon nanotube. Furthermore, when carbon nanotubes are fired at high temperatures, the G / D ratio tends to increase, and the longer the firing time, the higher the G / D ratio tends to increase.

[0030] 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 0.5 to 3.0, more preferably 0.5 to 2.5, even more preferably 0.5 to 2.0, particularly preferably 0.5 to 1.5, and even more preferably 0.5 to 1.3. If the G / D ratio of the CNT exceeds the above range, the CNT becomes hard and is therefore more likely to be damaged during dispersion, which may increase contact resistance. On the other hand, if the G / D ratio of the CNT falls 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 the CNT dispersion are improved.

[0031] 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, CNT purification can be performed by heat treatment in an inert atmosphere under reduced pressure and vacuum. This allows the heat treatment temperature to be kept low and the heat treatment to be performed for a short period of 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, CNT breakage can be suppressed by a dispersion process or the like in the process of producing an electrode film, thereby suppressing an increase in contact resistance between CNTs in the resulting electrode film. As a result, low-crystalline CNTs can achieve good conductivity as an electrode film, and secondary batteries containing CNTs can exhibit good performance.

[0032] <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.

[0033] The CNTs of this embodiment have an aluminum content of 3000 ppm or less. The aluminum content contained in the CNTs may be preferably 2000 ppm or less, more preferably 1100 ppm or less, and even more preferably 500 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 of the CNT is within the above range, the amount of aluminum oxide nanoparticles, which are insulating components, is low, and carbon nanotubes capable of forming electrode films with good electrical conductivity can be obtained. Furthermore, when the aluminum content of the CNT is within the above range, a conductive paste with excellent electrical conductivity can be produced.

[0034] Representative CNTs also contain metals other than aluminum (hereinafter referred to as other metals) derived from the catalyst metal. The other metals may be magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum. In addition to the metals and metal oxides derived from the catalyst metal, metals such as stainless steel used in synthesis equipment, filling equipment, or piping may be mixed into the CNTs due to wear and tear. Therefore, the metal components in the CNTs may be aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum, but these may also be metals not derived from the catalyst metal. From this perspective, the total content of aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum in the CNTs 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.

[0035] In some embodiments, the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum contained in the CNTs is preferably 10,000 ppm or less. The total content may be 8,000 ppm or less, or 7,000 ppm or less. In some embodiments, the total content is more preferably 5,000 ppm or less, even more preferably 3,000 ppm or less, even more preferably 2,100 ppm or less, and even more preferably 830 ppm or less. When the total content of these metals is within the above range, it is easier to improve the safety of the secondary battery. In the following description, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be collectively referred to simply as other metals.

[0036] 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, 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.

[0037] 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 5000 ppm or less, more preferably 3000 ppm or less, even more preferably 2000 ppm or less, and even more preferably 1250 ppm or less. Also, in some embodiments, the total iron content in the CNTs is preferably 5000 ppm or less, more preferably 3000 ppm or less, even more preferably 2500 ppm or less, and even more preferably 1000 ppm or less. Furthermore, in some embodiments, the total cobalt and iron content in the CNTs is preferably 5000 ppm or less, more preferably 3000 ppm or less, even more preferably 2500 ppm or less, and even more preferably 1300 ppm or less.

[0038] 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.

[0039] 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 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, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum to the mass of the CNTs before metal extraction. Here, the total content of aluminum, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is calculated by calculating the mass of each metal when converted to its elemental metal form and adding them together.

[0040] <Surface oxygen content> From the viewpoint of the electrical conductivity of the CNTs, the CNTs of this embodiment preferably have a surface oxygen content of less than 1.0 atm%. The surface oxygen content is preferably 0.9 atm% or less, more preferably 0.8 atm% or less, even more preferably 0.7 atm% or less, and particularly preferably 0.6 atm% or less. When the surface oxygen content is 1.0 atm% or less, excellent electrical conductivity as an electrode film can be easily obtained by using CNTs. In this specification, the "surface oxygen content" refers to a value expressed as the ratio (atm%) of oxygen atoms to carbon atoms on the surface of the CNTs determined by X-ray photoelectron spectroscopy.

[0041] When nitric acid is used for purifying CNT as a raw material, since the oxidizing power of nitric acid is strong, the surface of the CNT will be oxidized. On the other hand, in this embodiment, as in 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 content of metal components such as aluminum can be reduced, and it is not always necessary to perform treatment with an acid. Therefore, oxidation of the CNT surface can be suppressed and the amount of surface oxygen of the CNT can be reduced. As a result, the combustion start temperature of the CNT becomes higher, and the CNT can obtain good conductivity as an electrode film, and the secondary battery containing the CNT can exhibit good performance.

[0042] <Other properties of CNT> CNT has a shape in which planar graphite is wound into a cylindrical shape. The CNT may be a mixture of single-walled CNT and multi-walled CNT. The single-walled CNT has a structure in which one layer of graphite is wound into a cylindrical shape. The multi-walled CNT has a structure in which two or three or more layers of graphite are wound into a cylindrical shape. The CNT in the present disclosure may not contain single-walled CNT and is preferably multi-walled CNT. Alternatively, the CNT may be a mixture of single-walled CNT and multi-walled CNT, but even in such a case, CNT in which multi-walled CNT accounts for 90% by mass or more is preferred, and CNT in which multi-walled CNT accounts for 99% by mass or more is more preferred. The multi-walled CNT in the CNT may be 100% by mass. By using such CNT, it becomes easy to set the G / D ratio within a suitable range, for example, 0.5 or more and 3.0 or less. Also, the side wall of the CNT does not have to be a graphite structure. For example, CNT having a side wall with an amorphous structure can also be used as the CNT.

[0043] The CNT of this embodiment is preferably multi-walled CNT, and preferably has 3 or more and 30 or less layers, more preferably 3 or more and 20 or less layers, and still more preferably 3 or more and 10 or less layers.

[0044] 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.

[0045] The volume resistivity of the CNT of this embodiment is 1.0 × 10 -2 ~2.5×10 -2 Ω·cm is preferred, and 1.0×10 -2 ~2.2×10 -2 Ω·cm is more preferable, and 1.0×10 -2 ~2.0×10 -2 Ω·cm is more preferable, and 1.2×10 -2 ~1.8×10 -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.).

[0046] 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 ​​CNT is 800 m 2 / g or less, and 2 / g or less is more preferable, and 500m 2 It is more preferable that the saturation coefficient is 1 / g or less. The BET specific surface area of ​​CNTs can be calculated using the BET method based on nitrogen adsorption measurements. There is often a correlation between the specific surface area of ​​CNTs and 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. When the specific surface area of ​​CNTs is 150m 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 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.

[0047] 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.

[0048] The standard deviation of the average outer diameter of the CNTs is preferably 1 nm to 8 nm, more preferably 1 nm to 4 nm. 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.

[0049] The outer diameter and average outer diameter of CNTs can be 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.

[0050] 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.

[0051] <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.

[0052] 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.

[0053] <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.

[0054] 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, metal components such as aluminum, magnesium, iron, cobalt, and nickel derived 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 metal components such as aluminum and magnesium using acid treatment.

[0055] 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. The CNT purification method of this embodiment can easily reduce the content of metal components such as aluminum and magnesium in the CNTs. The CNT purification method of this embodiment can also reduce the content of other metal components, such as iron and cobalt, in addition to aluminum and magnesium.

[0056] 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).

[0057] 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. One or more vacuum pumps may be used in combination. 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.

[0058] The internal pressure in the reduced pressure vacuum is preferably 10 Pa or less, more preferably 0.1 Pa or less, and even more preferably 0.05 Pa or less. In some embodiments, the internal pressure may be 0.03 Pa or less. In some embodiments, it is preferable to reduce the pressure in stages to achieve an internal pressure in the above range. In one embodiment, the pressure reduction can be carried out in two stages. For example, in the first stage, the internal pressure is preferably adjusted to 10 Pa or less, more preferably 9.8 Pa or less, and even more preferably 9.6 Pa or less. In the first stage, the internal pressure may be in the range of 9.5 to 9.8 Pa. After adjusting the internal pressure to the above range and maintaining it for a certain period of time, in the second stage, the internal pressure can be adjusted to preferably 1 Pa or less, more preferably 0.1 Pa or less, and even more preferably 0.05 Pa or less. In some embodiments, the internal pressure in the second stage may be 0.03 Pa or less.

[0059] The heat treatment conditions, such as the heat treatment temperature and heat treatment time, can be determined appropriately depending on the type of CNT and the type of metal components 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. The heat treatment temperature refers to the temperature inside the apparatus (internal temperature).

[0060] 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.

[0061] In some embodiments, step (I) of heat treatment under reduced pressure is preferably carried out stepwise, for example, in two stages. By carrying out the heat treatment in two stages, it is possible to suppress an increase in internal pressure due to decomposition of the subcomponents (organic substances) contained in the CNTs and to easily maintain the degree of vacuum (reduced pressure). Furthermore, by carrying out the heat treatment at a temperature that does not impair the physical properties of the CNTs, the by-products contained in the CNTs are decomposed, and then the CNTs can be purified by increasing the temperature in a short period of time. Furthermore, sintering of the CNTs can be suppressed, and CNTs with excellent electrical conductivity tend to be easily obtained.

[0062] Although not particularly limited, step (I) can be suitably carried out, for example, via a first heat treatment (Ia) and a second heat treatment (Ib) described below. In the first heat treatment (Ia), the internal pressure may be preferably 10 Pa or less, more preferably 0.1 Pa or less, and even more preferably 0.05 Pa or less. The heating temperature (internal temperature) is preferably adjusted in the range of 500 to 2000°C, and this temperature is preferably maintained for 1 to 100 hours. In the first heat treatment (Ia), the internal temperature may more preferably be 900 to 1800°C, and even more preferably 1200 to 1500°C. The maintenance time may more preferably be 1 to 50 hours, and even more preferably 2 to 10 hours. The second heat treatment (Ib) is preferably carried out while maintaining the internal pressure of the first heat treatment (Ia). Therefore, in the second heat treatment (Ib), the internal pressure may be preferably 10 Pa or less, more preferably 0.1 Pa or less, and even more preferably 0.05 Pa or less. In the second heat treatment (Ib), the internal temperature is preferably adjusted in the range of 1000 to 2000°C, and this temperature is preferably maintained for 1 to 100 hours. In the second heat treatment (Ib), the internal temperature may more preferably be 1600 to 1800°C, and even more preferably 1600 to 1700°C. The maintenance time may more preferably be 1 to 4 hours, and even more preferably 2 to 4 hours.

[0063] 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.

[0064] To further reduce the amount of metal contained in the CNTs, the purification method of this embodiment may be performed two or more times as needed. Furthermore, other treatment steps may be added as needed, as long as they do not degrade the properties of the CNTs. For example, the CNT purification method of this embodiment may include other treatment steps, such as an acid treatment, in addition to the heat treatment step (I).

[0065] <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.

[0066] <2> Carbon nanotube (CNT) dispersion One embodiment of the present invention relates to a CNT dispersion. The CNT dispersion according to this embodiment contains the above-described CNTs, 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 containing the above-described CNTs, a dispersant, and a dispersion medium.

[0067] <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.

[0068] 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.

[0069] 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.

[0070] <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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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 to adjust the CNT fiber length in the CNT dispersion to a desired range. A high-shear mixer is preferred to promote CNT wetting and break down coarse particles and agglomerates. A media-type disperser such as a bead mill is preferred to crush agglomerated particles. It is more preferable to select and combine multiple dispersers described above for dispersion, and the order of the dispersers can be changed as desired. The pressure when using a high-pressure homogenizer is not particularly limited, but is preferably 60 to 150 MPa, more preferably 60 to 120 MPa.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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, CNT dispersions may contain metal particles and dissolved metal ions as metals. Metal particles are metals present in particulate form in CNT dispersions, specifically, the metals contained in the CNTs described above. CNTs, dispersants, and other materials may contain metal particles resulting from their respective manufacturing processes, and metal foreign matter may also be mixed in during the manufacturing process of CNT dispersions. If metal foreign matter is present inside a battery, the battery is more likely to short-circuit, so removing metal particles is extremely important from a safety perspective. The process for producing a CNT dispersion preferably includes a step of removing contaminants such as metallic foreign matter at any timing (metallic foreign matter removal step). From the viewpoint of efficiency, the metallic foreign matter removal step is preferably carried out during the dispersion step of the CNT dispersion and / or at the end of the dispersion step. The metallic foreign matter removal step may be carried out multiple times.

[0079] 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.

[0080] 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 the 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.

[0081] The metal content in the CNT dispersion liquid was determined by drying the CNT dispersion liquid to remove the solvent. It can be calculated by analysis 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 liquid that has undergone the metal foreign matter removal step includes metal particles that were not completely removed and dissolved metal ions.

[0082] 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 secondary battery are less likely to occur, and a secondary battery with better conductivity can be obtained.

[0083] 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.

[0084] <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.

[0085] <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.

[0086] 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.

[0087] 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.

[0088] <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 an electrode composition including the above-described CNTs, a dispersant, a dispersion medium, an electrode active material, and a composite slurry.

[0089] <Electrode active material> An electrode active material is a material that serves as the basis for a battery reaction. The active material can be divided into a positive electrode active material and a negative electrode active material based on the 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, etc.; composite oxide powders of lithium and transition metals such as lithium nickelate, lithium cobaltate, lithium manganate with a layered structure, and lithium manganate with a spinel structure; lithium iron phosphate-based materials which are olivine structure phosphate compounds, etc. can be mentioned. These positive electrode active materials can be used alone or in combination of two or more. Also, the above inorganic compounds and organic compounds may be mixed and used. The negative electrode active material is not particularly limited, but 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 highly graphitized carbon materials or natural graphite. These negative electrode active materials can be used alone or in combination of two or more. In particular, using a combination of a highly graphitized carbon material and lithium silicate is preferable from the viewpoints of capacity and lifespan.

[0090] 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. The average particle size of the electrode active material as used herein refers to the average value of particle sizes measured by an electron microscope.

[0091] 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.

[0092] 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 0.05% by mass or more, more preferably 0.1% by mass or more, relative to 100% by mass of the electrode active material, and is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% 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, and is preferably 20% 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 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.

[0093] <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.

[0094] 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.

[0095] The method for applying the composite slurry onto the current collector is not particularly limited, and any known method can be used.

[0096] After coating and drying, the coating may be rolled using a lithographic press or a calender roll, etc. The thickness of the electrode film is generally 1 μm or more and 500 μm or less, and preferably 10 μm or more and 300 μm or less.

[0097] <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.

[0098] 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.

[0099] 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 used. These solvents may be used alone or in combination of two or more.

[0100] 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 a hydrophilic treatment.

[0101] The structure of the secondary battery is not particularly limited, but it is usually composed of a positive electrode and a negative electrode, and a separator provided as necessary, and can have various shapes according to the purpose of use, such as a paper type, a cylindrical type, a button type, a laminated type, etc.

Examples

[0102] 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.

[0103] 1. Production examples of CNT and CNT dispersion liquid using the same [Example 1] [Production of CNT] 60 parts of CNT (manufactured by JEIO Co., Ltd., JENOTUBE6A) were placed in a graphite crucible having a diameter of 10 cm and a height of 10 cm. The crucible containing the above CNT was placed in a multi-purpose high-temperature furnace (manufactured by Fuji Denpa Kogyo Co., Ltd., Himulti 5000), and heat treatment was performed under reduced pressure and vacuum as follows. First, nitrogen gas was introduced into the above multi-purpose high-temperature furnace, and the substitution operation with nitrogen gas was performed twice. Next, 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 heating rate of 20 °C / min and held at 1200 °C for 10 hours. Then, it was naturally cooled until the furnace internal temperature reached 50 °C or less, and CNT(A) was obtained.

[0104] <Preparation of CNT Dispersion Liquid> In a stainless steel container, an NMP solution containing a hydrogenated nitrile butadiene rubber polymer (manufactured by Nippon Zeon Co., Ltd., Zetpole2000L) with a concentration of 7% and NMP were added, and adjusted so that the polymer was 1.25 parts by mass and the total amount of NMP was 96.25 parts by mass. 2.5 parts by mass of CNT(A) was weighed into this solution, added while stirring with a disper, and a fine emulsifier screen was attached to a high shear mixer (L5M - A, manufactured by SILVERSON), and batch-type dispersion treatment was performed at a speed of 9,000 rpm. This batch-type dispersion treatment was carried out until the entire solution became uniform and the dispersion particle size became 200 μm or less using a grind gauge. Next, the content of the above stainless steel container was pumped, 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 1.0 mmφ. Subsequently, the dispersion liquid was supplied to a high-pressure homogenizer via a pipe, and 15-pass type dispersion treatment was performed. Subsequently, the dispersion liquid was supplied to a high-pressure homogenizer (manufactured by Sugino Machine Ltd., Starburst Turbo), and 15-pass type dispersion treatment was performed. This dispersion treatment was carried out using a single nozzle chamber at a nozzle diameter of 0.25 mm and a pressure of a 100 Mpa. The solution after the dispersion treatment was passed through a depth filter (manufactured by 3M, PP non-woven fabric depth cartridge NT-T series, filtration accuracy 20 μm). Thus, a carbon nanotube dispersion liquid (A) was obtained.

[0105] <Preparation of Binder Composition> Capacity 150cm 3 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 concentration of 8%, were added to the plastic container. The mixture was then stirred at 2000 rpm for 30 seconds using a planetary centrifugal mixer (Awatori Rentaro, ARE-310). Thus, binder composition (A) was obtained.

[0106] <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 73.5%. The nonvolatile content ratio of NMC:CNT:PVdF in the nonvolatile portion of the electrode composition was 98.1:0.4:1.5.

[0107] <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.

[0108] <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 a glove box filled with argon gas, and the aluminum laminate was then sealed to prepare a laminate-type 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 MAC500L, 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). Further added were 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) 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.

[0109] [Examples 2, 4 to 6] CNT(B) and CNT(D) to CNT(F) of Examples 2, 4 to 6 were obtained in the same manner as in Example 1, except that the first holding temperature and treatment time were changed to those 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.

[0110] [Example 3] 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 (C) in the same manner as in Example 2. Furthermore, using the CNTs (C) 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.

[0111] [Example 7] 60 parts of CNTs (JENOTUBE6A, manufactured by JEIO Corporation) were placed in a graphite crucible with a diameter of 10 cm and a height of 10 cm. The crucible containing the CNTs was placed in a multipurpose high-temperature furnace (Hi-Multi 5000, manufactured by Fuji Dempa Kogyo Co., Ltd.) and subjected to heat treatment under reduced pressure vacuum as follows. First, nitrogen gas was introduced into the multipurpose high-temperature furnace, and the nitrogen gas replacement operation was performed twice. Next, the pressure inside the furnace was reduced using an oil rotary pump to adjust the pressure inside the furnace to 9.8 to 9.5 Pa. Subsequently, the pressure inside the furnace was further reduced using an oil diffusion pump to adjust the pressure inside the furnace to 0.03 Pa or less. While maintaining the pressure inside the furnace, the temperature inside the furnace was increased to 900 °C at a rate of 20 °C / min and held at 900 °C for 2 hours. Thereafter, the temperature inside the furnace was further increased to 1800 °C at a rate of 20 °C / min and held at 1800 °C for 1 hour. After that, the furnace was allowed to cool naturally until the temperature inside the furnace reached 50 °C or less, and CNT (G) was obtained. Note that the above heating operation was performed while maintaining the pressure reduction operation using the oil diffusion pump. Furthermore, using the CNTs (G) 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.

[0112] [Examples 8 to 12], [Comparative Examples 1 to 4] CNT(H) to CNT(L) and CNT(X1) to (X3) were obtained in the same manner as in Example 7, except that the CNT species, first holding temperature, treatment time, second holding temperature, treatment time, and pulverization conditions were changed as shown in Table 1. The CNT pulverization treatment was carried out in the same manner as in Example 3. Furthermore, using each of the CNTs 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.

[0113] Comparative Example 5 Using an electronic balance (MSA225S100DI, manufactured by Sartorius), 10 parts of CNT (JENOTUBE6A, manufactured by JEIO) were weighed into an alumina crucible SSA-HB4 (manufactured by Nikkato), and the crucible containing the CNT was placed in a muffle furnace (FO510, manufactured by Yamato Scientific Co., Ltd.). Next, the temperature inside the furnace was increased to 330°C at a rate of 60°C / min in an air atmosphere, and maintained at 330°C for 18 hours (calcination), yielding oxidized CNT (6A). 10 parts of the CNT thus oxidized were weighed into a 1-L glass container, 500 parts of 10% nitric acid (manufactured by FUJIFILM Wako Pure Chemical Corporation) were added, and then the mixture was sufficiently stirred using a stirrer while heating it to 90 °C by a water bath. Thereafter, it was sufficiently diluted using ion-exchanged water, and vacuum filtration was performed using a membrane filter. After repeating the dilution and filtration operations, the CNT was transferred to a PTFE vat. It was dried at 140 °C using an oven. Thus, the nitric acid-treated 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.

[0114] 2. Evaluation 2-1. Evaluation of CNT Characteristics Regarding the carbon nanotubes (CNT) of the above-described Examples and Comparative Examples, measurements were performed as follows. The respective results are shown in Table 1.

[0115] <Content of Metals in CNT> Using a microwave sample pretreatment apparatus (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 content of metals. Also, from the calculated contents of the metals, 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 represented by the mass ratio (ppm) of the total content mass of the extracted magnesium, aluminum, iron, copper, zinc, nickel, chromium, manganese, and molybdenum to the mass of the CNT before metal extraction.

[0116] <G / D Ratio of CNT> The CNT was placed on a Raman microscope (manufactured by Horiba, Ltd., XploRA), and measurements were carried out 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 to 3000 cm -1 The CNT for measurement was separated on a slide glass and flattened using a spatula. Among the obtained peaks, the maximum peak intensity within the range of 1560 to 1600 cm -1 in the spectrum was defined as G, and the maximum peak intensity within the range of 1310 to 1350 cm -1 was defined as D. The ratio of G / D was calculated and used as the G / D ratio of the CNT.

[0117] <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, and heated from 25°C to 1000°C at a heating rate of 10°C / min in an air atmosphere. For the obtained DTA curve, the temperature at the peak apex within the temperature range of 200°C to 1000°C was defined as the temperature of the exothermic peak.

[0118] <Amount of surface oxygen of CNT> The amount of surface oxygen of the CNT was measured using an X-ray photoelectron spectrometer (XPS, manufactured by Thermo Fisher 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 as the sample were detected by XPS. Here, the ratio (atm%) of oxygen atoms to carbon atoms was calculated as the amount of surface oxygen.

[0119] <Volume resistivity of CNT> Using a powder resistivity measurement device (manufactured by Nitto Seiko Analytic Co., Ltd.: Loresta-GP Powder Resistivity Measurement System MCP-PD-51), with a sample mass of 1.2 g, and a powder probe unit (four-probe ring electrode, electrode spacing 5.0 mm, electrode radius 1.0 mm, sample radius 12.5 mm), with the applied voltage limiter set at 90 V, the volume resistivity [Ω·cm] of CNT powder under various pressures was measured. At a density of 1 g / cm 3 The value of the volume resistivity of CNT at this density was evaluated.

[0120] <Cohesion of CNT> Using a rheometer (manufactured by Anton Paar, MCR302e), the cohesion of CNT was measured. First, 4.0 g of CNT was placed in a dedicated aluminum container (C-CC27 / D / Al), and the carbon nanotubes were compressed at 12 kPa using a compression cylinder jig. Then, it was replaced with a wing-shaped jig for cohesion measurement, 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 CNT. The temperature control device used was C-PTD200, and the measurement was carried out at a temperature of 25°C.

[0121] 2-2. Evaluation of Other Characteristics 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 as follows to evaluate the characteristics. The evaluation results are shown in Table 1. <Viscosity Stability of CNT Dispersion> After leaving the CNT dispersion in a constant temperature bath at 60°C for one week, the CNT dispersion was cooled to 25°C, and immediately measured using a B-type viscometer at a rotor rotation speed of 100 rpm. The evaluation criteria for viscosity stability are as follows. (Evaluation Criteria) ◎ (Excellent): 2500 mPa·s or less 〇 (Good): Exceeding 2500 mPa·s and 5000 mPa·s or less △ (Fair): Exceeding 5000 mPa·s and 6000 mPa·s or less × (Poor): Exceeding 6000 mPa

[0122] <Particle diameter (D90) of CNT dispersion> The particle diameter (D90) was measured using a particle size distribution measuring device (Partical LA-960V2, manufactured by HORIBA). The operating conditions of circulation / ultrasonic wave were as follows: circulation speed: 3, ultrasonic wave intensity: 7, ultrasonic wave time: 1 minute, stirring speed: 1, stirring mode: continuous. Also, during air venting, ultrasonic wave operation was performed at an ultrasonic wave intensity of 7 and an ultrasonic wave time of 5 seconds. The refractive index of NMP was 1.468 and that of CNT was 1.920. The measurement was carried out after diluting the measurement sample so that the transmittance of the red laser diode was 60 - 70%, and the particle diameter standard was volume. The evaluation criteria for the particle diameter are as follows. (Evaluation criteria) ◎ (Excellent): 1.0 μm or more and less than 3.0 μm 〇 (Good): 3.0 μm or more and less than 5.0 μm - (Poor): Less than 1.0 μm or 5.0 μm or more

[0123] <Cycle characteristics of secondary battery> The secondary battery was placed in a constant temperature chamber at 45°C, and charge-discharge measurement was carried out using a charge-discharge device (manufactured by Hokuto Denko Corporation, SM-8). After performing constant current constant voltage charging (cut-off current 1.25 mA (0.025C)) at a charging current of 50 mA (1C) and a charging end voltage of 4.2V, constant current discharge was carried out at a discharge current of 50 mA (1C) and a discharge end voltage of 2.5V. This operation was repeated 200 times. 1C was defined as the current value for discharging the theoretical capacity of the positive electrode in 1 hour. The cycle characteristics can be expressed by the ratio of the 3rd 1C discharge capacity to the 200th 1C discharge capacity at 45°C, as shown in the following formula 3. (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): Cycle characteristics are 90% or more 〇 (Good): 85% or more and less than 90% △ (Fair): 80% or more and less than 85% × (Poor): Less than 80%

[0124] <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 determine 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 4 Ω·cm Good: 4 Ω·cm or more and less than 10 Ω·cm △ (Acceptable): 10Ω·cm or more and less than 15Ω·cm × (defective): 15Ω cm or more

[0125] Table 1 shows the evaluation results of the CNTs, CNT dispersions, electrode films, and secondary batteries produced in Examples 1 to 12 and Comparative Examples 1 to 5.

[0126] 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)

[0127] [Table 1]

[0128] As shown in Table 1, the CNTs of this embodiment (Examples 1 to 12) satisfy all of the requirements for (1) the exothermic peak in differential thermal analysis, (2) the G / D ratio, and (3) the aluminum content, as explained 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) In a differential thermal analysis when the temperature is increased from 200°C to 1000°C at a rate of 10°C / minute, an exothermic peak is observed between 600°C and 800°C. (2) 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. (3) The aluminum content is 3000 ppm or less.

2. The carbon nanotube according to claim 1, further satisfying the following (i): (i) The total content of aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum is 13,000 ppm or less.

3. The carbon nanotube according to claim 1, further satisfying the following condition (ii): (ii) The surface oxygen content is less than 1.0 atm %.

4. A carbon nanotube dispersion comprising the carbon nanotubes according to any one of claims 1 to 3, a dispersant, and a dispersion medium.

5. After storing the mixture at 60°C for one week, the viscosity at 25°C measured by a Brookfield viscometer was: The carbon nanotube dispersion liquid according to claim 4, having a viscosity of 5000 mPa·s or less.

6. 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 3, a dispersant, and a dispersion medium.

7. A carbon nanotube dispersion liquid and an electrode active material are included, 4. 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.

8. 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 3, 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.

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