Carbon nanotube, method for purifying same, carbon nanotube dispersion, binder composition, electrode composition, and secondary battery

Abstract

In the present disclosure, a carbon nanotube capable of forming an electrode film having further improved safety and good conductivity, a method for purifying the same, a carbon nanotube dispersion containing the carbon nanotube, a binder composition, a composition for electrodes, and a secondary battery are provided.

Description

Technical Field

[0001] One embodiment of the present invention relates to carbon nanotubes and a purification method thereof, a carbon nanotube dispersion, a binder composition, an electrode composition, and a secondary battery. Background Art

[0002] With the popularization of electric vehicles and the reduction in size, weight, and performance of portable devices, there is a demand for secondary batteries with high energy density and, in turn, for higher capacity. In this context, lithium-ion secondary batteries are particularly used in a wide range of devices.

[0003] In secondary batteries, carbon black, Ketjen black, graphene, fine carbon materials, etc. are used as conductive additives. In particular, carbon nanotubes (hereinafter also referred to as "CNTs"), which are a type of fine carbon fiber, are used in large quantities. For example, by adding CNTs to the electrode active material, the electrode resistance is reduced, or the load resistance of the battery is improved, or the material strength of the electrode is increased, or the electrode's resistance to expansion and contraction is increased, thereby improving the rate characteristics and cycle life of the secondary battery. Among them, multilayer CNTs with an outer diameter of 5nm to several tens of nm are relatively cheap and are being widely used.

[0004] CNTs can generally be manufactured by arc discharge, laser evaporation, chemical vapor deposition, etc. Among these, chemical vapor deposition is most suitable for mass production from the perspective of productivity and economy and is widely used. In the chemical vapor deposition method, catalyst particles containing metals such as iron, cobalt, and nickel are used to react with a gas serving as a carbon source to generate CNTs. Therefore, the CNTs obtained by the chemical vapor deposition method contain catalyst particles containing metals such as iron, cobalt, and nickel, or particles such as carbides or oxides derived from the catalyst particles. If CNTs containing catalyst particles containing metals are used in secondary batteries, metal dissolution and precipitation may sometimes occur, causing the battery to short-circuit. If the battery short-circuits, it may sometimes cause major accidents such as fire or explosion. Therefore, in order to further improve safety, several methods for refining CNTs and removing catalyst particles containing metals have been proposed.

[0005] Patent document 1 describes a method of refining a carbon material containing CNTs through a process including a carbon material production process in which a raw material containing at least carbon and a catalyst metal is used as an anode and an arc discharge method is used to produce a carbon material containing CNTs, and a halogen treatment process in which the carbon material is brought into contact with a gas containing a halogen and / or a halogen compound, thereby suppressing damage or cutting of the CNTs or solidification of the CNTs into blocks, and removing the catalyst metal as an impurity.

[0006] Patent Document 2 states that liquid-phase oxidation of CNTs having a G-band to D-band intensity ratio (G / D ratio) of 50 or greater in Raman spectroscopy using nitric acid yields higher-quality CNTs with no catalyst residue, high heat resistance, and low carbon byproducts.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: International Publication No. 2008 / 126534

[0010] Patent Document 2: International Publication No. 2018 / 043487 Summary of the Invention

[0011] Problems to be solved by the invention

[0012] However, when CNTs are subjected to a halogen treatment process, the catalyst metal is removed. However, due to the long-term calcination of CNTs at high temperatures, the crystallinity of the CNTs increases. CNTs with high crystallinity become hard and therefore easily break. Therefore, when CNTs are used as conductive additives in electrode films of secondary batteries, the contact resistance between CNTs increases due to CNT breakage, resulting in a decrease in the conductivity of the electrode film and the performance of the secondary battery containing CNTs.

[0013] Furthermore, while liquid-phase oxidation of CNTs with nitric acid can reduce catalyst residues, nitric acid has a strong oxidizing power, which may cause surface oxidation of the CNTs and reduce the conductivity of an electrode film using the CNTs.

[0014] One embodiment of the present invention is made in view of the above-mentioned situation, and its purpose is to provide a carbon nanotube and a purification method thereof that can form an electrode film with further improved safety and good conductivity, a carbon nanotube dispersion containing the carbon nanotube, a binder composition, an electrode composition, and a secondary battery.

[0015] Technical means to solve the problem

[0016] The present inventors have conducted intensive studies to solve the above-mentioned problems and, as a result, have discovered carbon nanotubes that satisfy the following conditions, thereby completing the present invention.

[0017] That is, several aspects of the present invention are as follows.

[0018] <1> A carbon nanotube satisfying the following (1) to (3).

[0019] (1) In differential thermal analysis when the temperature is increased from 200°C to 1000°C at a rate of 10°C / min, an exothermic peak is present at 600°C or higher and 800°C or lower.

[0020] (2) When 1560 cm -1 ~1600cm -1 The maximum peak intensity within the range is defined as G, and the peak intensity at 1310 cm -1 ~1350cm -1 When the maximum peak intensity within the range of is defined as D, the G / D ratio is greater than or equal to 0.5 and less than or equal to 3.0.

[0021] (3) The total content of cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum is less than 5000 ppm.

[0022] <2> The carbon nanotube according to <1>, satisfying the following (4).

[0023] (4) The surface oxygen content is below 2.5 atm%.

[0024] <3> The carbon nanotube according to <1> or <2>, further satisfying (i) a total content of cobalt and iron of 5000 ppm or less.

[0025] <4> The carbon nanotube according to <3>, further satisfying (ii) a total content of cobalt and iron of 1000 ppm or less.

[0026] <5> A carbon nanotube dispersion comprising the carbon nanotube according to any one of <1> to <4>, a dispersant, and a dispersion medium.

[0027] <6> An adhesive composition comprising a carbon nanotube dispersion and an adhesive,

[0028] The carbon nanotube dispersion liquid includes the carbon nanotube according to any one of <1> to <4>, a dispersant, and a dispersion medium.

[0029] <7> A composition for an electrode, comprising a carbon nanotube dispersion and an electrode active material,

[0030] The carbon nanotube dispersion liquid includes the carbon nanotube according to any one of <1> to <4>, a dispersant, and a dispersion medium.

[0031] <8> A secondary battery comprising an electrode film, wherein the electrode film is

[0032] A carbon nanotube dispersion comprising the carbon nanotube according to any one of <1> to <4>, a dispersant, and a dispersion medium;

[0033] An adhesive composition comprising the carbon nanotube dispersion and an adhesive, or

[0034] The present invention can obtain an electrode composition comprising the carbon nanotube dispersion and an electrode active material.

[0035] <9> A method for purifying carbon nanotubes, comprising: a first step of heat-treating the carbon nanotubes at a temperature of 1000° C. to 2000° C. in an inert atmosphere; and a second step of contacting the carbon nanotubes heat-treated in the first step with an acid.

[0036] The first step and the second step are each performed once or twice or more.

[0037] <10> The method for purifying carbon nanotubes according to <9>, wherein in the second step, a standard electrode potential of the acid is 0.8 V vs. SHE or less.

[0038] Effects of the Invention

[0039] According to one embodiment of the present invention, there are provided carbon nanotubes capable of forming an electrode film having improved safety and good conductivity, and a purification method thereof. Furthermore, there are provided a carbon nanotube dispersion, a binder composition, an electrode composition, and a secondary battery containing the carbon nanotubes. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] [ Figure 1 ] Figure 1 This is a graph showing DTA curves of CNTs prepared in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0041] Hereinafter, the carbon nanotubes, the carbon nanotube dispersion, the binder composition, the electrode composition, and the secondary battery according to one embodiment of the present invention will be described in detail.

[0042] Hereinafter, carbon nanotubes are also referred to as CNTs.

[0043] Carbon Nanotubes (CNTs)

[0044] The CNT of this embodiment satisfies the following (1) to (3).

[0045] (1) In differential thermal analysis when the temperature is increased from 200°C to 1000°C at a rate of 10°C / min, an exothermic peak is present at 600°C or higher and 800°C or lower.

[0046] (2) When 1560 cm -1 ~1600cm -1 The maximum peak intensity within the range is defined as G, and the peak intensity at 1310 cm -1 ~1350cm -1 When the maximum peak intensity within the range of is defined as D, the G / D ratio is greater than or equal to 0.5 and less than or equal to 3.0.

[0047] (3) The total content of cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum is less than 5000 ppm.

[0048] Furthermore, the CNT of this embodiment can also satisfy the following (4).

[0049] (4) The surface oxygen content is below 2.5 atm%.

[0050] The CNT of this embodiment satisfies at least the above conditions (1) to (3), but preferably satisfies all of the above conditions (1) to (4). Each condition will be described below.

[0051] <(1) Fever Peak>

[0052] The CNTs of this embodiment have an exothermic peak at 600°C or higher and 800°C or lower in differential thermal analysis (DTA) when the temperature is increased from 200°C to 1000°C at a rate of 10°C / min. The exothermic peak can be measured by performing differential thermal analysis on the CNTs in an atmospheric environment. DTA is a method for measuring the temperature difference between the sample and the reference substance as a function of temperature while changing the temperature of the sample and the reference substance according to certain conditions, and is in accordance with Japanese Industrial Standards (JIS) K 0129. In the DTA curve produced based on the change in the temperature difference between the sample and the reference substance, the largest peak is set as the exothermic peak.

[0053] Heat is generated as CNTs burn. As the CNT combustion start temperature increases, the exothermic peak temperature also increases. Factors that affect the CNT combustion start temperature include the catalyst metal content, the degree of oxidation on the CNT surface, and the CNT crystallinity. If the catalyst metal contained in the CNT has a high heat storage capacity, if the catalyst metal content is low, the total heat storage capacity of all the catalyst metals will decrease. Because the total heat storage capacity of the catalyst metal is low, the temperature required to burn the CNT may be higher than when the catalyst metal content is high. In addition to the catalyst metal, metal impurities may also be mixed into the CNT during the CNT manufacturing process, and these metal impurities may also affect the total heat storage capacity.

[0054] Furthermore, CNT surfaces with oxygen-containing functional groups are more susceptible to combustion than those without functional groups. Therefore, the fewer oxygen-containing functional groups (i.e., the lower the surface oxygen content), the harder the CNTs burn. This reduces the degree of CNT surface oxidation, leading to higher temperatures for CNT combustion. Furthermore, the higher the CNT crystallinity, the higher the combustion initiation temperature. CNT crystallinity can be expressed by the G / D ratio, described below.

[0055] If the combustion start temperature of the CNT is within an appropriate temperature range, the impurities contained in the CNT are reduced, and thus a safer CNT can be obtained. The temperature of the exothermic peak of the CNT of this embodiment is preferably 600°C or higher, and more preferably 650°C or higher. In addition, the temperature of the exothermic peak is preferably 800°C or lower, and more preferably 740°C or lower. If the temperature of the exothermic peak is 600°C or higher, the metal content is low, which can improve the safety of the battery. Alternatively, the surface oxygen content is low, and it has excellent conductivity. If the temperature of the exothermic peak is 800°C or lower, the crystallinity of the CNT will not be too high, the breakage of the CNT can be suppressed, and the reduction in 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.

[0056] When CNT is a powder before dispersion, the exothermic peak can be directly measured. In addition, when CNT is present in a CNT dispersion, it can be measured after removing the dispersion medium by heating and drying, and determined based on the shape of the exothermic peak. The temperature of heating and drying is preferably carried out at a temperature at which the CNT is not oxidized (for example, below 140°C). When the CNT dispersion contains components other than CNT and the dispersion medium (additives, etc.), the exothermic peak of the additive can also be measured in advance, and the exothermic peak derived from the additive can be determined, thereby judging the remaining exothermic peak to be derived from CNT, thereby determining the exothermic peak.

[0057] (2) G / D ratio

[0058] The G / D ratio (peak ratio of the G-band to the D-band) of the CNT of this embodiment is determined by Raman spectroscopy. There are various laser wavelengths used in Raman spectroscopy, but in this embodiment, 532 nm and 632 nm are used. In the Raman spectrum, the wavelength at 1,590 cm -1 The Raman shift observed near 1,350 cm is known as the G band originating from graphite. -1 The Raman shift observed near the D band is called the D band originating from defects in amorphous carbon and graphite. Since the wave number of Raman spectroscopy sometimes varies depending on the measurement conditions, the wave number specified here is ±10cm -1 The higher the G / D ratio, the higher the crystallinity of the carbon nanotubes. Furthermore, if carbon nanotubes are calcined at high temperatures, the G / D ratio tends to increase, and the longer the calcination time, the higher the G / D ratio tends to be.

[0059] Regarding CNTs, when the 1560 cm -1 ~1600cm -1 The maximum peak intensity within the range is defined as G, and the peak intensity at 1310 cm -1~1350cm -1 When the maximum peak intensity within the range is set to D, the G / D ratio is 0.5 or more and 3.0 or less, more preferably 0.5 or more and 2.5 or less, and further preferably 0.5 or more and 2.0 or less, particularly preferably 0.5 or more and 1.5 or less, and further preferably 0.5 or more and 1.3 or less. If the G / D ratio of CNT exceeds the range, the CNT becomes hard, so the CNT is easily damaged during dispersion, and the contact resistance sometimes increases. On the other hand, if the G / D ratio of CNT is lower than the range, the conductivity of the CNT itself is easily reduced. Thus, if the G / D ratio of CNT is within the range, in a secondary battery using an electrode film utilizing a CNT dispersion, rate characteristics and cycle characteristics are improved.

[0060] As in the past, if a halogen treatment process is performed on CNTs as raw materials and the CNTs are refined, the crystallinity of the CNTs tends to increase due to the long-term calcination of the CNTs at high temperatures. This state can also be confirmed by the increase in the G / D ratio. On the other hand, in the present embodiment, in the CNT refining process, the high crystallization of the CNTs can be suppressed by suppressing the upper limit temperature to a low level under an inert environment and performing a heat treatment of the CNTs in a short period of time. That is, the G / D ratio of the CNTs can be suppressed from increasing. When using CNTs with low crystallinity, in the process of manufacturing the electrode film, the breakage of the CNTs can be suppressed by dispersion treatment, etc., and the increase in the contact resistance between the CNTs in the obtained electrode film can be suppressed. As a result, the CNTs with low crystallinity can obtain good conductivity as an electrode film, and the secondary battery containing the CNTs can exhibit good performance.

[0061] <(3)Total metal content>

[0062] The CNTs of this embodiment preferably have a total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum of 5000 ppm or less, more preferably 3000 ppm or less, further preferably 2500 ppm or less, further preferably 1000 ppm or less, and even more preferably 830 ppm or less. If the total content of these metals is within the above range, the safety of the secondary battery is improved. In the following description, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum are sometimes simply collectively referred to as metals.

[0063] Here, the metal content in CNTs is the mass when converted to a single metal. CNTs may contain metal in the form of a single metal, metal oxide, or metal composite oxide. The metal content is calculated by converting these into a single metal.

[0064] Cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be contained in CNTs in the form of simple metals, metal oxides, and composite oxides thereof. Since these metals may cause short circuits, it is desirable to reduce their total content.

[0065] More preferably, from the viewpoint of stricter safety, the total content of cobalt and iron in CNTs is preferably 5000 ppm or less, more preferably 3000 ppm or less, further preferably 2500 ppm or less, and even more preferably 1000 ppm or less.

[0066] More preferably, the total cobalt content in the CNT is preferably 5000 ppm or less, more preferably 3000 ppm or less, further preferably 2500 ppm or less, and even more preferably 1000 ppm or less.

[0067] Furthermore, the total iron content in the CNT is preferably 5000 ppm or less, more preferably 3000 ppm or less, further preferably 2500 ppm or less, and even more preferably 1000 ppm or less.

[0068] Cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be contained in CNTs derived from the catalyst metal. In addition to the metals and metal oxides used as catalyst metals, metals such as stainless steel used in synthesis equipment, filling equipment, and piping may sometimes be incorporated into CNTs due to wear and tear. Therefore, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may also be contained in CNTs that are not derived from the catalyst metal.

[0069] In this embodiment, as described in the CNT purification method below, the total metal content of the CNTs can be reduced by dissolving the metals exposed by calcining the CNTs with acid. By reducing the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum in the CNTs, the exothermic peak temperature can be increased, resulting in safer CNTs. Secondary batteries containing such CNTs can exhibit excellent performance.

[0070] The metal content in CNTs can be calculated, for example, by subjecting CNTs to acid decomposition, extracting the metals contained in the CNTs, and analyzing the extract using high-frequency inductively coupled plasma (ICP). The total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum contained in CNTs is expressed as the mass ratio (ppm) of the total amount of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum extracted relative to the mass of the CNTs before metal extraction. Here, the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is calculated by calculating the mass of each metal when converted to its elemental form, and this is used as the total amount.

[0071] <(4) Surface oxygen content>

[0072] From the perspective of CNT conductivity, the surface oxygen content of the CNTs of this embodiment is preferably 2.5 atm% or less, more preferably 1.9 atm% or less, even more preferably 1.4 atm% or less, and particularly preferably 1.2 atm% or less. A surface oxygen content of 2.5 atm% or less allows the CNTs to exhibit excellent conductivity as electrode films. In this specification, the term "surface oxygen content" refers to the ratio (atm%) of oxygen atoms to carbon atoms on the CNT surface, as determined by X-ray photoelectron spectroscopy.

[0073] If nitric acid is used to purify the raw material CNTs, the CNT surface will oxidize due to its strong oxidizing power. On the other hand, in this embodiment, as in the CNT purification method described later, by using a strong acid with low oxidizing power to dissolve the metal exposed by heat-treating the raw material CNTs, oxidation of the CNT surface can be suppressed, thereby reducing the surface oxygen content of the CNTs. As a result, the combustion start temperature of the CNTs is increased, and the CNTs can be used as electrode films to obtain excellent conductivity, thereby achieving excellent performance of secondary batteries containing CNTs.

[0074] <(5)Other>

[0075] CNTs have a cylindrical shape formed by winding planar graphite. CNTs may be a mixture of single-layer CNTs and multi-layer CNTs. Single-layer CNTs have a structure in which one layer of graphite is wound into a cylindrical shape. Multi-layer CNTs have a structure in which two or three or more layers of graphite are wound into a cylindrical shape. The CNTs in the present disclosure may not include single-layer CNTs, but are preferably multi-layer CNTs. Alternatively, the CNTs may be a mixture of single-layer CNTs and multi-layer CNTs, but in this case, CNTs containing multi-layer CNTs accounting for more than 90% by mass are preferred, and more preferably CNTs containing multi-layer CNTs accounting for more than 99% by mass are more preferred. By using such CNTs, it is easy to achieve a G / D ratio within a preferred range, for example, greater than 0.5 and less than 3.0. In addition, the sidewalls of the CNTs do not have to be graphite structures. For example, CNTs having sidewalls having an amorphous structure may also be used as CNTs.

[0076] The CNT of the present embodiment is preferably a multilayer CNT, and the number of layers of the CNT is preferably 3 or more and 30 or less, more preferably 3 or more and 20 or less, and even more preferably 3 or more and 10 or less.

[0077] The purity of CNT is represented by the value (mass %) after deducting the ash (mass %) from the mass of CNT. The ash (mass %) of CNT can be measured, for example, in accordance with JIS K 6218-2. The ash of CNT is a non-combustible component containing metals and the like. From the viewpoint of conductivity, the purity of CNT is preferably 90% by mass or more, more preferably 95% by mass or more, and further preferably 99% by mass or more, based on the mass of CNT. In addition, the non-combustible component contained in CNT is preferably 10% by mass or less, more preferably 5% by mass or less, and further preferably 1% by mass or less.

[0078] The volume resistivity of the CNT of this embodiment is preferably 1.0×10 -2 Ω·cm~2.5×10 -2 Ω·cm, more preferably 1.0×10 -2 Ω·cm~2.2×10 -2 Ω·cm, more preferably 1.0×10 -2 Ω·cm~2.0×10 -2 Ω·cm, particularly preferably 1.2×10 -2 Ω·cm~1.8×10 -2 Ω·cm. When the volume resistivity of CNTs is within this range, the volume resistivity of the electrode film decreases, improving the performance of the secondary battery. The volume resistivity of CNTs can be measured using a powder resistivity measuring device (Nittoseiko Analytech Co., Ltd.: Loresta GP Powder Resistivity Measuring System MCP-PD-51).

[0079] The Brunauer-Emmett-Teller (BET) specific surface area of ​​the CNT of this embodiment is preferably 150 m 2 / g or more, more preferably 180m 2 / g or more. In addition, the BET specific surface area of ​​CNT is preferably 800m 2 / g or less, more preferably 600m 2 / g or less, more preferably 400m 2 / g or less. The BET specific surface area of ​​CNT can be calculated by the BET method using nitrogen adsorption measurement. The specific surface area of ​​CNT is often correlated with the average outer diameter of CNT. The smaller the specific surface area, the larger the outer diameter of CNT becomes, and the fewer the number of CNTs per mass becomes. On the other hand, the larger the specific surface area of ​​CNT, the smaller the outer diameter of CNT becomes, and the more the number of CNTs per mass becomes. If the specific surface area of ​​CNT is 150m 2 / g or more, the number of carbon nanotubes per mass can be ensured, and a conductive network can be formed efficiently, thereby obtaining excellent rate characteristics and cycle characteristics of the battery. In addition, if the specific surface area of ​​CNT is 800m 2 / g or less, the CNTs are well dispersed and can form a good conductive network in the electrode film.

[0080] The average outer diameter of the CNTs of this embodiment is preferably 3 nm or more, more preferably 5 nm or more. In addition, the average outer diameter of the CNTs is preferably 15 nm or less, more preferably 13 nm or less, and further preferably 11 nm or less. If 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. If 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.

[0081] The standard deviation of the average outer diameter of CNTs is preferably from 2 nm to 8 nm, more preferably from 3 nm to 6 nm. A large standard deviation of the average outer diameter of CNTs may make it difficult to efficiently form a conductive network, or may cause CNTs to entangle and aggregate in the CNT dispersion or composite slurry and / or electrode film, preventing the formation of a good conductive network.

[0082] The outer diameter and average outer diameter of CNTs are calculated as follows. First, observe and photograph the CNTs using a transmission electron microscope. Next, randomly select 300 CNTs from the photograph and measure their outer diameters. Next, calculate the average outer diameter (nm) of the CNTs as the number average of the outer diameters.

[0083] CNTs typically exist as aggregates. For example, the shape may be a complexly entangled CNT (twisted shape). Alternatively, the shape may be a linear aggregate (bundled shape). Bundled CNT aggregates are easier to untangle than entangled CNT aggregates. Furthermore, bundled CNT aggregates have better dispersibility than entangled CNT aggregates and are therefore preferably used as CNTs.

[0084] According to this embodiment, by satisfying at least the above (1) to (3), CNTs capable of forming an electrode film having further improved safety and good conductivity can be obtained, and a secondary battery including such CNTs can exhibit good performance.

[0085] <Method for producing carbon nanotubes (CNTs)>

[0086] The CNTs of this embodiment can be produced, for example, by laser ablation, arc discharge, thermal chemical vapor deposition (CVD), plasma CVD, and combustion, but are not limited to these methods. For example, CNTs can be produced by contacting and reacting a carbon source with a catalyst metal at 500°C to 1000°C in an atmosphere with an oxygen concentration of 1% by volume or less. The carbon source can be at least one of a hydrocarbon and an alcohol.

[0087] Any known gas can be used as a raw material gas for the carbon source of CNTs. For example, hydrocarbons such as methane, ethylene, propane, butane, and acetylene, carbon monoxide, and alcohols can be used as raw material gases, but the gases are not limited to these. In particular, from the perspective of ease of use, it is preferred to use at least one of a hydrocarbon and an alcohol as the raw material gas.

[0088] <Carbon Nanotube (CNT) Purification Method>

[0089] Hereinafter, a method for purifying carbon nanotubes (CNTs) will be described. The CNTs of this embodiment are not limited to those produced by the purification method described below, but the CNTs of this embodiment can be obtained by following the purification method described below.

[0090] The CNT purification method of this embodiment includes a first step of heat-treating raw carbon nanotubes at a temperature of 1000°C to 2000°C in an inert environment; and a second step of contacting the carbon nanotubes heat-treated in the first step with an acid. The first and second steps can each be performed once or twice or more.

[0091] (First process)

[0092] First, the raw material CNTs are heat-treated in an inert environment to calcine the CNTs. Examples of the inert environment include a nitrogen environment, an argon environment, a vacuum environment, and combinations thereof.

[0093] The heat treatment conditions such as heat treatment temperature and heat treatment time can be appropriately determined according to the type of CNT and the type of metal such as the catalyst metal contained in the CNT. The heat treatment temperature is preferably the temperature at which all metals of cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum begin to melt. In addition, the catalyst metal used in the manufacture of CNT is nanoscale, and due to the nanoscale effect, the melting temperature is lower than that of the bulk metal, so it can also be a heat treatment temperature lower than the melting temperature of the bulk metal. From this point of view, the heat treatment temperature is preferably above 1000°C and below 2000°C. From the perspective of suppressing the high crystallization of CNT, it can be below 1800°C, below 1400°C, or below 1200°C. In addition, the heat treatment time can be appropriately set according to the calcination device and calcination scale, for example, preferably 1 hour to 3 hours.

[0094] The heat treatment melts the metal contained in the CNTs and removes the amorphous carbon on the CNT surface, exposing the metal on the CNT surface. This promotes contact between the metal and the acid in the second step, described later. Furthermore, while CNTs typically oxidize and combust at temperatures above 500°C in an atmospheric environment, the heat treatment in the first step is performed in an inert environment and at a relatively low temperature, thus suppressing combustion of the CNTs themselves. Furthermore, since the CNTs are heat treated in an inert environment, oxidation of the CNT surface is suppressed. Furthermore, the heat treatment time can be shortened, which can further suppress the excessive crystallization of the CNTs.

[0095] (Second process)

[0096] Next, the CNTs that have been heat-treated in the first step are brought into contact with an acid. This allows the metal contained in the CNTs to be dissolved. In order to dissolve the metal contained in the CNTs, a strong acid is preferably used. As a strong acid, from the perspective of acidity, an acid having an acid dissociation constant (pKa) of 3 or less in an aqueous medium is more preferably used. In the first step, heat treatment is performed under an inert environment to suppress the surface oxygen content of the CNTs, and further, an acid having a low oxidizing power to a certain extent is preferably used as the acid. As an acid with low oxidizing power, a standard electrode potential (E°) in an aqueous solution is more preferably 0.80V vs.SHE or less, more preferably 0.50V vs.SHE or less, and further preferably 0.20V vs.SHE or less. Since the ability of an acid with low oxidizing power to oxidize the surface of the CNT is low, the surface oxygen content of the CNT can be further suppressed when the acid is treated with a low oxidizing power acid. In this specification, the standard electrode potential (E°) represents the potential relative to the standard hydrogen electrode (SHE) at 25°C.

[0097] From this perspective, acids with strong acidity and low oxidizing power include hydrochloric acid (acid dissociation constant pKa = -3.7, E° = 0 V vs. SHE), hydrobromic acid (pKa = -4.1, E° = 0 V vs. SHE), and dilute sulfuric acid (pKa = 2.0, E° = 0.16 V vs. SHE). (Both pKa and E° are values ​​in aqueous media.) Hydrochloric acid is industrially preferred. Furthermore, hydrochloric acid tends to have a high ability to remove metals contained in CNTs.

[0098] On the other hand, examples of strong acids with high oxidizing power include nitric acid (pKa = -1.3, E° = 0.84 V vs. SHE), hot concentrated sulfuric acid (fuming sulfuric acid), perchloric acid (pKa = -10, E° = 1.20 V vs. SHE), chloric acid (pKa = -1.0, E° = 1.18 V vs. SHE), and chlorous acid (pKa = -2.0, E° = 1.67 V vs. SHE). Because these acids oxidize the CNT surface and increase the number of acidic groups introduced to the CNT surface, CNTs treated with nitric acid or the like tend to have an increased surface oxygen concentration. From the perspective of suppressing the surface oxygen content of CNTs, it is also preferable to use an acid with such strong acidity and low oxidizing power. In another embodiment, in the first step, it is preferable to use an acid with a lower standard electrode potential (E°) relative to SHE than nitric acid. In another embodiment, in the first step, it is preferred to use an acid having a standard electrode potential (E°) with respect to SHE that is the same as or lower than that of dilute sulfuric acid.

[0099] Preferably, by exposing the CNTs, which have been heat-treated in an inert environment in the first step, to a low-oxidizing acid in the second step, surface oxidation of the resulting CNTs can be suppressed, reducing the number of surface acidic groups and further suppressing the surface oxygen content of the resulting CNTs. This, by suppressing the amount of oxygen, which contributes to the increased resistance of CNTs, allows for the production of CNTs with high electrical conductivity.

[0100] In the second step, as a method for contacting the heat-treated CNTs with an acid, either a gaseous acid or a liquid acid can be used. From the perspective of metal solubility, a liquid acid is preferably used. For example, there is a method of contacting the CNTs with an aqueous acid solution. The acid treatment conditions can be appropriately determined based on the type of acid, the type of CNTs, the type and amount of metal contained in the CNTs, etc. The acid treatment can also be carried out with heating. On the other hand, the acid treatment temperature is preferably below 80°C, more preferably below 50°C. If the acid treatment temperature is below 80°C, a higher metal solubility can be obtained. When hydrochloric acid is used, the vapor of hydrochloric acid is corrosive. From the perspectives of safety, equipment pollution, and environmental pollution, the acid treatment temperature is more preferably below 50°C, and can be 10°C to 40°C, or around room temperature (25°C). The acid treatment time can be appropriately set based on the acid treatment method, acid treatment scale, acid treatment temperature, etc., for example, it can be 1 hour to 3 hours.

[0101] By dissolving the metal exposed by heat-treating the CNTs at room temperature using a strong acid with low oxidizing power, the metal content in the CNTs can be reduced, and oxidation of the CNT surface and increase in the oxygen content on the CNT surface can be suppressed.

[0102] To further reduce the metal content in the CNTs, the first and second steps may be performed two or more times. Furthermore, the CNTs may be subjected to other treatment steps between the first and second steps, as long as the crystallinity and surface oxidation of the CNTs obtained by the first step are maintained.

[0103] The combustion starting temperature of the CNTs increases through the first and second steps, resulting in the production of CNTs capable of forming an electrode film having excellent conductivity. This allows a secondary battery including the CNTs to exhibit excellent performance.

[0104] <Dry grinding of carbon nanotubes (CNTs)>

[0105] From the perspective of crushing particles and improving dispersibility, the CNTs of this embodiment may also be CNTs that have been dry-crushed. So-called dry crushing refers to a process in which CNTs are crushed without the intervention of a liquid substance. As dry crushing, it can be medium crushing, it can be crushing without using a medium, or it can be a combination of two or more dry crushings. For example, in medium crushing, a crusher with built-in crushing media such as beads or steel balls is used to crush particles using the crushing force or destructive force generated by the collision of the crushing media with each other. As a dry crushing device, known methods such as a dry grinder, ball mill, vibration mill, and bead mill can be used, and the crushing time can be arbitrarily set according to the device or according to the crushing state of the particles.

[0106] [Carbon nanotube (CNT) dispersion]

[0107] The CNT dispersion of this embodiment includes the CNTs, a dispersant, and a dispersion medium. The CNT dispersion in this specification does not contain an electrode active material. In addition, according to this embodiment, a method for producing a CNT dispersion including the CNTs, a dispersant, and a dispersion medium can be provided.

[0108] Dispersants

[0109] Dispersants are not particularly limited as long as they stabilize the CNT dispersion. For example, surfactants and resin-type dispersants can be used. Surfactants are primarily classified into anionic, cationic, nonionic, and amphoteric types. Depending on the desired CNT dispersion properties, the preferred type of dispersant can be used in the preferred amount.

[0110] 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, hydroxypropyl methyl cellulose, carboxymethyl cellulose, etc.), polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, hydrogenated nitrile rubber, and polyacrylonitrile polymers. Methyl cellulose, ethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, hydrogenated nitrile rubber, and polyacrylonitrile polymers are particularly preferred. The molecular weight of the resin-type dispersant is preferably 10,000 to 300,000, more preferably 10,000 to 150,000.

[0111] In addition, it is preferred to add an amine compound or an inorganic base in addition to the dispersant. As the amine compound, a first amine (primary amine), a second amine (secondary amine), and a third amine (tertiary amine) are used, and ammonia and quaternary ammonium compounds are not included. In addition to monoamines, amine compounds such as diamines, triamines, and tetraamines having multiple amino groups in the molecule can also be used. Specifically, for example, there can be mentioned: 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, alicyclic nitrogen-containing heterocyclic compounds such as hexamethylenetetramine, morpholine, and piperidine, but are not limited to these. Examples of the inorganic base include alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates, alkali metal phosphates, and alkaline earth metal phosphates.

[0112] Dispersion medium

[0113] The dispersion medium is not particularly limited as long as it can disperse the CNTs, but preferably contains one or more of water and a water-soluble organic solvent.

[0114] Examples of water-soluble organic solvents include alcohols, polyols, polyol ethers, amines, and amides (N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, and N-methylcaprolactam); heterocyclic organic solvents, sulfoxide organic solvents, sulfone organic solvents, lower ketone organic solvents, and other organic solvents such as tetrahydrofuran, urea, and acetonitrile. Among these, water and amide organic solvents are more preferred, and N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone are particularly preferred among amide organic solvents.

[0115] 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.

[0116] The CNT dispersion can be prepared, for example, by dispersing CNTs in a dispersion medium. The raw materials used can be added once or in multiple additions at any time during the dispersion process. The dispersion method used for the above process is not particularly limited.

[0117] As a dispersion method, for example, the method using various dispersion machines such as disperser (dispersor), homogenizer, high shear mixer, kneader, two-roll mill, three-roll mill, ball mill, horizontal sand mill, vertical sand mill, annular bead mill, coating regulator, attritor, planetary mixer or high-pressure homogenizer can be listed.Dispersor is not particularly limited, for example, with respect to the viewpoint prepared in the manner of the fiber length of the CNT in the CNT dispersion liquid becoming a preferred range, preferably using a high-pressure homogenizer, with respect to promoting the wetting of CNT, unraveling rough particles and coagulation, preferably using a high-shear mixer, with respect to the viewpoint of crushing coagulation and solidifying particles, preferably using a medium type dispersion machine such as a bead mill. In addition, it is more preferred to select a plurality of the dispersion machines to be combined and dispersed, and the order of the dispersion machine can be arbitrarily changed. The pressure when using a high-pressure homogenizer is not particularly limited, for example, preferably 60MPa~150MPa, more preferably 60MPa~120MPa.

[0118] Among the dispersion methods using a dispersion device, there are batch dispersion, through dispersion, circulation dispersion, etc., which can be any one method or a combination of two or more methods. The so-called batch dispersion refers to a method of dispersing only in the dispersion device body without using piping, etc. Because it is easy to operate, it is preferred in the case of small-scale production. The so-called through dispersion refers to a dispersion method in which a tank for supplying a dispersed liquid via a piping and a tank for receiving the dispersed liquid are included on the dispersion device body, and the dispersed liquid passes through the dispersion device body. In addition, the so-called circulation dispersion refers to a method in which the dispersed liquid after passing through the dispersion device body is returned to the tank for supplying the dispersed liquid, and is dispersed while circulating it. In all of the above methods, the longer the processing time, the more the dispersion is promoted. Therefore, it is sufficient to repeatedly pass or circulate until the target dispersion state is reached. If the size of the tank or the processing time is changed, the processing volume can be increased. Compared with circulation dispersion, through dispersion is easy to make the dispersion state uniform, and is preferred in this respect. Compared with through dispersion, the operation or manufacturing equipment of circulation dispersion is simple, and is preferred in this respect. In the dispersion process, the disintegration of aggregated particles, the dispersion, wetting, and stabilization of the conductive material are performed sequentially or simultaneously. The final dispersion state varies depending on the method used. Therefore, it is preferable to manage the dispersion state in each dispersion process using various evaluation methods. For example, management can be performed using the methods described in the Examples.

[0119] The solid content of the CNT dispersion is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, further preferably 1% by mass or more, and particularly preferably 2% by mass or more, relative to 100% by mass of the CNT dispersion. Furthermore, the solid content of the CNT dispersion is preferably 30% by mass or less, more preferably 25% by mass or less, further preferably 10% by mass or less, and particularly preferably 8% by mass or less, relative to 100% by mass of the CNT dispersion.

[0120] From the perspectives of CNT loading, dispersibility, and dispersion stability, the content of the dispersant in the CNT dispersion is preferably 3% by mass or greater, more preferably 5% by mass or greater, and even more preferably 10% by mass or greater, relative to 100% by mass of the CNTs. Furthermore, from the perspective of electrical conductivity, the content of the dispersant in the CNT dispersion is preferably 300% by mass or less, more preferably 100% by mass or less, and even more preferably 50% by mass or less, relative to 100% by mass of the CNTs.

[0121] The CNT dispersion may contain particulate metallic foreign matter and dissolved metal ions as metal. Metallic foreign matter is a metal present in the CNT dispersion in particulate form. Specifically, examples include metals contained in the CNTs. CNTs, dispersants, and other materials sometimes contain metallic foreign matter from their respective manufacturing processes, and metallic foreign matter is sometimes mixed into the CNT dispersion manufacturing process. If metallic foreign matter exists inside the battery, the battery is prone to short circuiting, so removing metallic foreign matter is very important from a safety perspective.

[0122] The process of producing a CNT dispersion preferably includes a step for removing contaminants such as metallic foreign matter at an arbitrary time (a metallic foreign matter removal step). From the perspective of efficiency, the metallic foreign matter removal step is preferably performed midway through the CNT dispersion process and / or at the end of the dispersion process. The metallic foreign matter removal step may be performed multiple times.

[0123] The method for removing metallic foreign matter from the CNT dispersion in the metallic foreign matter removal step is not particularly limited, and examples thereof include removal by filtration using a filter, removal using a vibrating screen, removal by centrifugation, and removal using a magnetic force. Of these, metallic foreign matter such as iron and chromium is magnetic, and therefore removal using a magnetic force is preferred, with a method combining a magnetic removal step with a filtration step being more preferred.

[0124] The method of removing by magnetic force is not particularly limited as long as it is a method that can remove metal foreign matter. From the perspective of productivity and removal efficiency, a method of removing by placing a magnetic filter in the CNT dispersion production line and passing the CNT dispersion through it is preferred.

[0125] The step of removing metallic foreign matter from the CNT dispersion using a magnetic filter is preferably performed by passing the CNT dispersion through a magnetic filter that generates a magnetic field with a magnetic flux density of 1,000 gauss or higher. Since lower magnetic flux densities reduce the efficiency of metallic foreign matter removal, a magnetic flux density of 5,000 gauss or higher is preferred. Considering the removal of weakly magnetic stainless steel, a magnetic flux density of 10,000 gauss or higher is more preferred, and 12,000 gauss or higher is most preferred.

[0126] Depending on the filtration flow rate, there is a concern that coarse metal particles may pass through the magnetic filter. Therefore, when the magnetic filter is deployed in the 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. While a single filter can be effective, a circulating magnetic filter is more preferable. This improves the efficiency of metal particle removal.

[0127] When a magnetic filter is placed in a CNT dispersion production line, there are no particular restrictions on where the magnetic filter should be placed. However, it is preferably placed before the filter, if a filtration step is performed before the carbon nanotube dispersion is filled into the container. This placement can prevent metal from being mixed into the product if it escapes from the magnetic filter.

[0128] The metal content in a CNT dispersion can be calculated by ICP analysis after drying the CNT dispersion to remove the solvent. The metal content detected by ICP analysis includes both particulate metallic foreign matter and dissolved metal ions. In other words, the metal content of a CNT dispersion that has undergone the metal foreign matter removal process includes both partially removed metallic foreign matter and dissolved metal ions.

[0129] The metal content of iron and chromium 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% by mass of the CNT dispersion. By keeping the metal content within this range, side reactions within the electrode film are less likely to occur, resulting in a secondary battery with superior conductivity.

[0130] The CNT dispersion of this embodiment includes CNTs with reduced metal content, and therefore the metal content in the CNT dispersion can also be reduced.

[0131] The CNT dispersion may further contain one or more carbon materials such as carbon black and graphite as a conductive material. Among these conductive materials, carbon black is preferred from the viewpoint of the adsorption performance of the dispersant.

[0132] [Adhesive composition]

[0133] The adhesive composition of this embodiment includes the CNT dispersion and an adhesive. The adhesive composition in this specification does not contain an electrode active material. In addition, according to this embodiment, a method for producing an adhesive composition including the CNT, a dispersant, a dispersion medium, and an adhesive can be provided.

[0134] [Adhesive]

[0135] The binder is a resin that binds various substances together in the electrode film.

[0136] As the binder, any known binder for batteries can be used. Examples include cellulose resins such as carboxymethyl cellulose and rubbers such as styrene-butadiene rubber and fluororubber. Modified forms, mixtures, and copolymers of these resins are also possible. In particular, polymer compounds containing fluorine atoms in their molecules, such as polyvinylidene fluoride, polyvinyl fluoride, and tetrafluoroethylene, are preferred for their durability.

[0137] The weight average molecular weight of the adhesive is preferably 10,000 or more, more preferably 100,000 or more, and particularly preferably 200,000 or more. In addition, the weight average molecular weight of the adhesive is preferably 2,000,000 or less, more preferably 1,500,000 or less, and particularly preferably 1,000,000 or less. If the weight average molecular weight is 10,000 or more, the reduction in the tolerance and adhesion of the adhesive can be suppressed. If the weight average molecular weight is 2,000,000 or less, the reduction in the workability of the adhesive can be improved and the viscosity of the adhesive itself can be suppressed, thereby suppressing the significant aggregation of dispersed particles.

[0138] The adhesive composition is preferably manufactured by mixing and homogenizing a CNT dispersion with an adhesive, or the adhesive may be dissolved in advance for use. In addition, the adhesive may be added at any time during the process of manufacturing the CNT dispersion. The mixing method may be various methods known in the art. The adhesive composition may be manufactured using the dispersing device described in the CNT dispersion. The adhesive in the adhesive composition may be one type of adhesive or two or more types may be used in combination. Furthermore, the manufacturing process of the adhesive composition may include the metal foreign matter removal process.

[0139] [Electrode composition]

[0140] The electrode composition of this embodiment includes the CNT dispersion and an electrode active material. The electrode composition can be further mixed with a binder to produce a composite material slurry. In addition, according to this embodiment, a method for producing a binder composition including the CNTs, a dispersant, a dispersion medium, and an electrode active material, and a method for producing a binder composition including the CNTs, a dispersant, a dispersion medium, an electrode active material, and a composite material slurry can be provided.

[0141] [Electrode active material]

[0142] The so-called electrode active material refers to the material that forms the basis of the battery reaction. In terms of electromotive force, active materials are divided into positive electrode active materials and negative electrode active materials.

[0143] There are no particular limitations on the positive electrode active material, and metal compounds such as metal oxides and metal sulfides that can be doped or intercalated with lithium ions, and conductive polymers can be used. Examples include oxides of transition metals such as Fe, Co, Ni, and Mn, composite oxides with lithium, inorganic compounds such as transition metal sulfides, and the like. Specifically, MnO, V2O5, V6O 13Transition metal oxide powders such as TiO2; composite oxide powders of lithium and transition metals such as layered lithium nickelate, lithium cobaltate, lithium manganate, and spinel lithium manganate; and lithium iron phosphate-based materials such as olivine-structured phosphate compounds. These positive electrode active materials can be used alone or in combination. Furthermore, a mixture of the aforementioned inorganic and organic compounds can also be used.

[0144] There are no particular limitations on the negative electrode active material, and any negative electrode active material that can be doped or embedded with lithium ions can be used. Examples include metal Li, and alloys thereof such as tin alloys, silicon alloys, and lead alloys; Li x Fe2O3、Li x Fe3O4、Li x Metal oxides such as WO2 (x is a number where 0 < x < 1), lithium titanate, lithium vanadate, and lithium silicate; amorphous carbonaceous materials such as soft carbon and hard carbon; artificial graphite such as highly graphitized carbon materials; or carbonaceous powders such as natural graphite. These negative electrode active materials may be used alone or in combination. Combining highly graphitized carbon materials with lithium silicate is particularly preferred from the perspectives of capacity and battery life.

[0145] The BET specific surface area of ​​the electrode active material is preferably 0.1 m 2 / g~10m 2 / g, more preferably 0.2m 2 / g~5m 2 / g, more preferably 0.3m 2 / g~3m 2 / g.

[0146] The average particle size of the electrode active material is preferably 0.05 μm to 100 μm, more preferably 0.1 μm to 50 μm. The average particle size of the electrode active material referred to herein is the average value of the particle sizes of the electrode active material measured using an electron microscope.

[0147] The electrode composition is preferably manufactured by mixing a CNT dispersion with an electrode active material and homogenizing them. Alternatively, the binder may be dissolved in the CNT dispersion in advance for use. In addition, the electrode active material may be added at any time during the process of manufacturing the CNT dispersion. The dispersing device for dispersing the electrode active material is not particularly limited, and any dispersing device exemplified in the manufacture of the CNT dispersion may be used.

[0148] In the case of a composite material slurry comprising a composition for an electrode, the content of the electrode active material contained in the composite material slurry is preferably 20% by mass or more relative to 100% by mass of the composite material slurry, more preferably 40% by mass or more. In addition, the content of the electrode active material contained in the composite material slurry is preferably 99% by mass or less relative to 100% by mass of the composite material slurry, more preferably 97% by mass or less. If within the scope of the invention, it is preferred from the perspective of coatability or productivity and the uniformity of the electrode film.

[0149] The content of CNTs contained in the composite material slurry is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, relative to 100% by mass of the electrode active material. In addition, the content of CNTs contained in the composite material slurry is preferably 10% by mass or less, more preferably 5% by mass or less, and further preferably 3% by mass or less, relative to 100% by mass of the electrode active material.

[0150] The content of the binder contained in the composite material slurry is preferably 0.3% by mass or more, more preferably 0.7% by mass or more, relative to 100% by mass of the electrode active material. In addition, the content of the binder contained in the composite material slurry is preferably 20% by mass or less, more preferably 10% by mass or less, and further preferably 5% by mass or less, relative to 100% by mass of the electrode active material.

[0151] The solid content of the composite material slurry is preferably 30% by mass or more, more preferably 40% by mass or more, relative to 100% by mass of the composite material slurry. In addition, the solid content of the composite material slurry is preferably 90% by mass or less, and further preferably 85% by mass or less, relative to 100% by mass of the composite material slurry.

[0152] The amount of water contained in the composite material slurry is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

[0153] (Electrode membrane)

[0154] The electrode film of the present embodiment is an electrode film obtained using (i) a carbon nanotube dispersion comprising carbon nanotubes, a dispersant, and a dispersion medium, (ii) an adhesive composition comprising the carbon nanotube dispersion and an adhesive, or (iii) an electrode composition comprising the carbon nanotube dispersion and an electrode active material. As carbon nanotubes, the carbon nanotubes of the present embodiment are used. In addition, the electrode film may also be an electrode film obtained using (iv) a composite material slurry. The composite material slurry (iv) can be obtained using the (i) carbon nanotube dispersion, (ii) the adhesive composition, or (iii) the electrode composition.

[0155] For example, the electrode film is formed by applying the composite material slurry onto a current collector and drying it. The material and shape of the current collector used in the electrode film are not particularly limited, and a current collector suitable for various secondary batteries can be selected. Examples of the material of the current collector include metals and alloys such as aluminum, copper, nickel, titanium, and stainless steel.

[0156] The method for coating the composite material slurry on the current collector is not particularly limited, and a known method can be used.

[0157] Alternatively, after coating and drying, a rolling process using a plate pressing machine or a calender roll may be performed. The thickness of the electrode film is generally 1 μm to 500 μm, preferably 10 μm to 300 μm.

[0158] (Secondary Battery)

[0159] The secondary battery of this embodiment includes the electrode film. The electrode film can be used as an electrode of a secondary battery, and is particularly preferably used as an electrode of a non-aqueous electrolyte secondary battery using an organic electrolyte. A non-aqueous electrolyte secondary battery is a battery comprising a positive electrode, a negative electrode, and an electrolyte containing an organic electrolyte. The electrode film can be used for either or both of the positive and negative electrodes.

[0160] In one embodiment, for example, an electrode film obtained by applying an electrode composition containing a positive electrode active material onto a current collector and drying the coated electrode composition can be used as the positive electrode.

[0161] In one embodiment, for example, an electrode film obtained by applying an electrode composition containing a negative electrode active material onto a current collector and drying the resulting electrode film can be used as the negative electrode.

[0162] In one embodiment, an electrode film obtained by applying a CNT dispersion or a binder composition to a current collector and drying the resulting film can be used as a current collector with a base layer.

[0163] In particular, it is preferably used as a positive electrode from the viewpoint of safety.

[0164] As an electrolyte, various existing known electrolytes in which ions can move can be used. For example, LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, Li(CF3SO2)3C, LiI, LiBr, LiCl, LiAlCl, LiHF2, LiSCN, or LiBPh4 (wherein Ph is a phenyl group) etc. include electrolytes containing lithium salts, but are not limited to these, and electrolytes containing sodium salts can also be used. The electrolyte is preferably used as an electrolyte after being dissolved in a non-aqueous solvent. A fully solid electrolyte or a polymer electrolyte can also be used.

[0165] The non-aqueous solvent is not particularly limited, and various solvents preferred 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, glyme, esters, sulfoxides, and nitriles, etc. These solvents can be used alone or in combination of two or more.

[0166] The secondary battery preferably includes a separator. Examples of the separator include polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and those subjected to a hydrophilic treatment, but the separator is not particularly limited thereto.

[0167] The structure of a secondary battery is not particularly limited, but generally comprises a positive electrode, a negative electrode, and, if necessary, a separator. The secondary battery can be manufactured in various shapes, such as paper-shaped, cylindrical, button-shaped, and laminated, depending on the intended use.

[0168] In this specification, a numerical range expressed using "to" indicates a range including the numerical values ​​described 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.

[0169] Example

[0170] The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to the following Examples unless the scope of the invention exceeds the scope of the invention.

[0171] <Measurement methods of physical properties>

[0172] The physical properties of carbon nanotubes (CNTs) were measured by the following methods: Unless otherwise specified, the physical properties of CNTs were measured after the CNTs were purified.

[0173] Total metal content of CNTs

[0174] CNTs were subjected to acid decomposition using a microwave sample pretreatment system (ETHOS, manufactured by Milestone-General Co., Ltd.) to extract the metals contained in the CNTs. The extracted metals were analyzed using a hybrid ICP emission spectrometer (720-ES, manufactured by Agilent) to calculate the metal content in the CNTs. The total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum was determined from the calculated metal content.

[0175] The total content of cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum in CNTs is expressed as the mass ratio (ppm) of the total mass of extracted iron, copper, zinc, nickel, chromium, manganese and molybdenum relative to the mass of CNTs before metal extraction.

[0176] <G / D ratio of CNT>

[0177] CNTs were placed on a Raman microscope (XploRA, manufactured by Horiba, Ltd.) and measured using a laser wavelength of 532 nm. The measurement conditions were an acquisition time of 60 seconds, a cumulative count of 2 times, a 10% neutral density filter, a 20x objective magnification, a confocal aperture of 500, a slit width of 100 μm, and a measurement wavelength of 100 cm. -1 ~3000cm -1 The CNTs for measurement were separated and taken out onto a glass slide and flattened using a spatula. -1 ~1600cm -1 The maximum peak intensity within the range is defined as G, and the peak intensity at 1310 cm -1 ~1350cm -1 The maximum peak intensity within the range of is defined as D, and the G / D ratio is calculated as the G / D ratio of CNT.

[0178] <CNT exothermic peak temperature>

[0179] A thermogravimetric differential thermal analysis apparatus (Tg-DTA 8122 Thermoplus EVO2, manufactured by Rigaku Co., Ltd.) was used to analyze the sample, with a mass of 1.0 mg, placed in an alumina pan. The temperature was then raised from 25°C to 1000°C in an atmospheric atmosphere at a rate of 10°C / minute. For the resulting DTA curve, the temperature at the peak apex within the temperature range of 200°C to 1000°C was defined as the exothermic peak temperature.

[0180] <Surface oxygen content of CNT>

[0181] The surface oxygen content of CNTs was measured using an X-ray photoelectron spectroscopy (XPS), manufactured by ThermoFisher Scientific, K-Alpha. After pelletizing the CNTs, the sample was secured to a sample stand with double-sided tape for measurement. XPS detected carbon and oxygen atoms on the surface of the CNT sample. The ratio of oxygen atoms to carbon atoms (atm%) was calculated as the surface oxygen content.

[0182] Volume Resistivity of CNTs

[0183] The volume resistivity [Ω·cm] of CNT powders under various pressures was measured using a powder resistivity measuring device (Nittoseiko Analytech Co., Ltd.: Loresta GP Powder Resistivity Measuring 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 a voltage limiter set to 90 V. The results were as follows: 3 The volume resistivity of CNTs at a density of 100 nm was evaluated.

[0184] (Measurement conditions)

[0185] Excitation source: monochromated AlKα 15kV×10mA

[0186] Analysis size: 400 μm (elliptical shape)

[0187] Photoelectron extraction angle: 0° (perpendicular to the sample surface)

[0188] Import area

[0189] Wide spectrum scan (Survey scan): 0eV~1,350eV

[0190] Narrow scan: C1s, O1s, N1s, Cl2p

[0191] Pass Energy

[0192] Wide spectrum scan: 200eV

[0193] Narrow spectrum scan: 50eV

[0194] <Volume resistivity of electrode>

[0195] The volume resistivity of the electrode was evaluated by using the prepared CNT dispersion and positive electrode active material to create a positive electrode composite composition. This composite composition was then coated onto polyethylene terephthalate (PET) foil and dried to form a composite film. The surface resistivity of the composite layer was then measured and converted to volume resistivity. The CNT dispersion used was the one prepared in the Examples and Comparative Examples described below.

[0196] (Preparation of composite material composition for positive electrode)

[0197] In capacity 150cm 3 In a plastic container, CNT dispersion, polyvinylidene fluoride (PVdF, Solvay 5130, manufactured by Solvay, non-volatile content 100%), which was previously dissolved in NMP (N-methyl-2-pyrrolidone) to a concentration of 8%, and NMC (S800, LiNi) as the positive electrode active material were added. 0.8 Mn 0.1 Co 0.1 After adding O2 (manufactured by Jinhe), the mixture was stirred at 2,000 rpm for 30 seconds using a rotary / orbital mixer (Thinky Defoaming Stirring Taro, ARE-310). The solidified material was then loosened with a spatula and stirred at 2,000 rpm for 300 seconds using a rotary / orbital mixer (Thinky Defoaming Stirring Taro, ARE-310) to obtain a positive electrode composite material. The nonvolatile content of the positive electrode composite material was 78%. The nonvolatile content ratio of NMC:CNT:PVdF in the nonvolatile portion of the positive electrode composite material was 98:0.5:1.5.

[0198] (Production of composite coatings)

[0199] The composite material composition for positive electrode was applied using an applicator so that the weight per unit area of ​​the electrode became 20 mg / cm 2 After coating on a PET foil with a thickness of 100 μm, it was dried in an electric oven at 120°C ± 5°C for 30 minutes to produce a composite coating film.

[0200] (Evaluation of Volume Resistivity of Electrode)

[0201] The surface resistivity (Ω / γ) of the composite coating was measured using a Loresta GP MCP-T610 (Mitsubishi Chemical Analytech). The measured value was multiplied by the thickness of the composite layer to obtain the volume resistivity (Ω·cm) of the electrode. The thickness of the composite layer was calculated by subtracting the thickness of the PET foil from the average of three measurements taken at the electrode using a film thickness meter (NIKON DIGIMICRO MH-15M).

[0202] In the examples and comparative examples described below, the following CNTs were used.

[0203] 10B: Multi-walled carbon nanotube (JEIO, JENOTUBE 10B)

[0204] BT1001M: Multi-walled carbon nanotubes (LG Chem, BT1001M)

[0205] 6A: Multi-walled carbon nanotubes (JEIO, JENOTUBE 6A)

[0206] (Example 1)

[0207] Using an electronic balance (Sartorius MSA225S100DI), 50 g of 10B was weighed into a crucible and placed in a multi-purpose high-temperature furnace (Fuji Denpa Industries, Ltd. High Multi 5000). Under a nitrogen atmosphere (N2) with a nitrogen flow rate of 1.5 L / min, the temperature was raised at a rate of 20°C / min to 1200°C. After calcination at 1200°C for 3 hours, the crucible was naturally cooled to below 50°C to obtain calcined 10B.

[0208] 10g of the calcined 10B was measured into a 1L glass container, 500g of 10% hydrochloric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added, and the mixture was stirred thoroughly with a stirrer. Then, the mixture was diluted thoroughly with ion-exchanged water and filtered under reduced pressure using a membrane filter. After repeated dilution and filtration, the CNTs were transferred to a polytetrafluoroethylene (PTFE) tank. The treated 10B was dried in an oven at 140°C.

[0209] To prepare the CNT dispersion, a 7% hydrogenated nitrile rubber (Zetpole 2000L, manufactured by ZEON Japan) solution and NMP were added to a stainless steel container to a total of 0.6 parts by mass of polymer and 96.4 parts by mass of NMP. 3.0 parts by mass of the treated 10B were added while stirring with a disperser. The mixture was then batch-dispersed at 8,600 rpm in a high-shear mixer (L5M-A, manufactured by SILVERSON) equipped with a square-hole high-shear screen until uniformity was achieved. The dispersed particle size, as measured by a micrometer, was 250 μm or less. The dispersed particle size, as confirmed by the micrometer, was 180 μm. Next, the dispersion was transferred from the stainless steel container to a bead mill (Star Mill LMZ, manufactured by Ashizawa Finetech) filled with zirconia beads having a diameter of 1.00 mm, and subjected to a circulating dispersion treatment to obtain a CNT dispersion.

[0210] (Example 2)

[0211] Treated CNTs were obtained by treating the CNTs according to the method described in Example 1 except that the type of CNTs, calcination temperature, and calcination time described in Table 1 were changed.

[0212] In the preparation of the CNT dispersion, the method described in Example 1 was used to obtain the CNT dispersion.

[0213] (Example 3, Example 4)

[0214] Treated CNTs were obtained by treating the CNTs according to the method described in Example 1 except that the type of CNTs, calcination temperature, and calcination time described in Table 1 were changed.

[0215] To prepare the CNT dispersion, a 7% hydrogenated nitrile rubber (Zetpole 2000L, manufactured by ZEON Japan) solution and NMP were added to a stainless steel container to a total amount of 0.4 parts by mass of polymer and 97.6 parts by mass of NMP. 2.0 parts by mass of the treated 10B were added while stirring with a disperser. The mixture was then dispersed in batches at 8,600 rpm using a high-shear mixer (L5M-A, manufactured by SILVERSON) equipped with a square-hole high-shear screen until the mixture became uniform and the particle size, as measured by a micrometer, was 250 μm or less. The particle size, as confirmed by the micrometer, was 180 μm. Next, the dispersion was transferred from the stainless steel container to a bead mill (Star Mill LMZ, manufactured by Ashizawa Finetech) filled with zirconia beads having a diameter of 1.00 mm, and subjected to a circulating dispersion treatment to obtain a CNT dispersion.

[0216] (Example 5)

[0217] Treated CNTs were obtained by treating the CNTs according to the method described in Example 1 except that the type of CNTs, calcination temperature, and calcination time described in Table 1 were changed.

[0218] To prepare the CNT dispersion, a 7% hydrogenated nitrile rubber (Zetpole 2000L, manufactured by ZEON Japan) solution and NMP were added to a stainless steel container to a total amount of 0.4 parts by mass of polymer and 97.6 parts by mass of NMP. 2.0 parts by mass of the treated 10B were added while stirring with a disperser. The mixture was then dispersed in batches at 8,600 rpm using a high-shear mixer (L5M-A, manufactured by SILVERSON) equipped with a square-hole high-shear screen until the mixture became uniform and the particle size, as measured by a micrometer, was 250 μm or less. The particle size, as confirmed by the micrometer, was 180 μm. Then, the dispersed liquid was transferred from the stainless steel container to a bead mill (Star Mill LMZ, manufactured by Ashizawa Finetech) filled with zirconia beads having a diameter of 1.00 mm, and a cyclic dispersion process was performed. After confirming that the viscosity of the contents in the stainless steel container was reduced and that the contents had sufficient fluidity, the dispersed liquid was supplied to a high-pressure homogenizer (Star Burst Labo HJP-17007, manufactured by SUGINO MACHINE) and a cyclic dispersion process was performed to obtain a CNT dispersion. The dispersion process in the high-pressure homogenizer was performed using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa.

[0219] (Example 6)

[0220] Treated CNTs were obtained by the method described in Example 1. Subsequently, 10 parts of the treated CNTs and 200 parts of zirconia beads with a diameter of 2 mm were added as a grinding medium and dry-milled for 15 minutes using a paint conditioner (manufactured by Red Devil) to obtain dry-milled CNTs.

[0221] In the preparation of the CNT dispersion, the method described in Example 1 was used to obtain the CNT dispersion.

[0222] (Comparative Example 1: Untreated)

[0223] The sample 10B that was not subjected to any treatment was designated as the sample of Comparative Example 1.

[0224] To prepare the CNT dispersion, a 7% hydrogenated nitrile rubber (Zetpole 2000L, manufactured by ZEON Japan) solution and NMP were added to a stainless steel container to a total of 0.6 parts by mass of polymer and 96.4 parts by mass of NMP. 3.0 parts by mass of untreated 10B were added while stirring with a disperser. The mixture was then batch-dispersed at 8,600 rpm in a high-shear mixer (L5M-A, manufactured by SILVERSON) equipped with a square-hole high-shear screen until uniformity was achieved. The dispersed particle size, as measured by a micrometer, was 250 μm or less. The dispersed particle size, as confirmed by the micrometer, was 180 μm. The dispersion was then transferred from the stainless steel container to a high-pressure homogenizer (Star Burst Labo HJP-17007, manufactured by Sugino Machine) for a circulating dispersion process to obtain a CNT dispersion. The high-pressure homogenizer used a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa.

[0225] (Comparative Example 2: Nitric Acid Treatment)

[0226] Using an electronic balance (Sartorius MSA225S100DI), 10 g of 10B was weighed into an alumina crucible SSA-HB4 (Nikkato) and placed in a muffle furnace (Yamato Scientific Co., Ltd. FO510). In an atmospheric environment (air), the temperature was raised to 330°C at a rate of 60°C / min and calcined at 330°C for 18 hours to obtain oxidized 10B.

[0227] After oxidation treatment, 10g of 10B was weighed into a 1L glass container. 500g of 10% nitric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added. The mixture was then heated to 90°C in a water bath while being stirred thoroughly with a stirrer. The mixture was then diluted thoroughly with ion-exchanged water and filtered under reduced pressure using a membrane filter. After repeated dilution and filtration, the CNTs were transferred to a PTFE tank. After drying in an oven at 140°C, the nitric acid-treated 10B was obtained.

[0228] In the preparation of the CNT dispersion, the method described in Comparative Example 1 was used to obtain a CNT dispersion.

[0229] (Comparative Example 3: Halogen Purification)

[0230] 10 kg of 6A was weighed into a 120 L heat-resistant container, and the container containing 6A was placed in a furnace. Nitrogen was then introduced into the furnace, and while maintaining a positive pressure, the air inside the furnace was exhausted. After the oxygen concentration in the furnace reached 0.1% by volume or less, the furnace was heated to 1500°C over 30 hours. While maintaining the furnace temperature at 1500°C, chlorine gas was introduced at a rate of 50 L / min for 100 hours. Nitrogen was then introduced at 50 L / min, and the mixture was cooled while maintaining a positive pressure, yielding halogen-purified 6A.

[0231] In the preparation of the CNT dispersion, the method described in Example 3 was used to obtain the CNT dispersion.

[0232] Table 1 shows the physical properties of CNTs prepared in each example and comparative example. The "total metal content" in the table is the total content of iron, copper, zinc, nickel, chromium, manganese, and molybdenum.

[0233] in addition, Figure 1 The following table shows DTA curves of CNTs prepared in Example 1 and Comparative Example 1. The vertical axis of the DTA curve represents thermoelectromotive force [μV], and the horizontal axis represents temperature [°C]. The graph shows that the exothermic peak temperature in Example 1 is 726°C, and the exothermic peak temperature in Comparative Example 1 is 520°C. Although not shown, the exothermic peak temperatures were similarly determined for the other Examples and Comparative Examples.

[0234] [Table 1]

[0235]

[0236] As can be seen, the CNTs in each example have controlled at least the G / D ratio, total metal content, and exothermic peak temperature, resulting in a lower volume resistivity than the comparative example, leading to higher conductivity of the CNT powder. This indicates that using this CNT powder to fabricate an electrode film can provide a secondary battery having a low volume resistivity and improved conductivity.

[0237] Furthermore, it can be seen that the total metal content of the CNTs of each example is low, and therefore the safety of the electrode film and the secondary battery using the CNTs can be improved.

[0238] The present disclosure includes the following embodiments.

[0239] (Item 1)

[0240] A carbon nanotube that satisfies the following (1) to (3).

[0241] (1) In differential thermal analysis when the temperature is increased from 200°C to 1000°C at a rate of 10°C / min, an exothermic peak is present at 600°C or higher and 800°C or lower.

[0242] (2) When 1560 cm -1 ~1600cm -1 The maximum peak intensity within the range is defined as G, and the peak intensity at 1310 cm -1 ~1350cm -1 When the maximum peak intensity within the range of is defined as D, the G / D ratio is greater than or equal to 0.5 and less than or equal to 3.0.

[0243] (3) The total content of cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum is less than 5000 ppm.

[0244] (Item 2)

[0245] The carbon nanotube according to item 1, which satisfies the following (4).

[0246] (4) The surface oxygen content is below 2.5 atm%.

[0247] (Item 3)

[0248] The carbon nanotube according to item 1 or 2, wherein the carbon nanotube further satisfies (i) a total content of cobalt and iron of 5000 ppm or less.

[0249] (Item 4)

[0250] The carbon nanotube according to item 3, wherein the carbon nanotube further satisfies (ii) a total content of cobalt and iron of 1000 ppm or less.

[0251] (Item 5)

[0252] The carbon nanotube according to any one of items 1 to 4, wherein the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum in the carbon nanotube is 830 ppm or less.

[0253] (Item 6)

[0254] The carbon nanotube according to any one of items 1 to 5, wherein in the carbon nanotube, when 1560 cm -1 ~1600cm -1 The maximum peak intensity within the range is defined as G, and the peak intensity at 1310 cm -1 ~1350cm -1 When the maximum peak intensity within the range of is defined as D, the G / D ratio is greater than or equal to 0.5 and less than or equal to 2.5.

[0255] (Item 7)

[0256] A carbon nanotube dispersion comprising the carbon nanotube according to any one of items 1 to 6, a dispersant, and a dispersion medium.

[0257] (Item 8)

[0258] An adhesive composition comprising a carbon nanotube dispersion and an adhesive,

[0259] The carbon nanotube dispersion liquid contains the carbon nanotube according to any one of items 1 to 6, a dispersant, and a dispersion medium.

[0260] (Item 9)

[0261] A composition for an electrode, comprising a carbon nanotube dispersion and an electrode active material,

[0262] The carbon nanotube dispersion liquid contains the carbon nanotube according to any one of items 1 to 6, a dispersant, and a dispersion medium.

[0263] (Item 10)

[0264] A secondary battery comprising an electrode film, wherein the electrode film is

[0265] A carbon nanotube dispersion comprising the carbon nanotube according to any one of items 1 to 6, a dispersant, and a dispersion medium; an adhesive composition comprising the carbon nanotube dispersion and an adhesive; or

[0266] The present invention can obtain an electrode composition comprising the carbon nanotube dispersion and an electrode active material.

[0267] (Item 11)

[0268] A method for purifying carbon nanotubes, comprising: a first step of heat-treating the carbon nanotubes at a temperature of 1000° C. to 2000° C. in an inert environment; and a second step of contacting the carbon nanotubes heat-treated in the first step with an acid.

[0269] The first step and the second step are each performed once or twice or more.

[0270] (Item 12)

[0271] The method for purifying carbon nanotubes according to item 11, wherein in the second step, the standard electrode potential of the acid is 0.8 V vs. SHE or less.

[0272] The disclosure of this application is related to the subject matter described in Japanese Patent Application No. 2022-211319, filed on December 28, 2022, the entire disclosure of which is incorporated herein by reference.

Claims

1. A carbon nanotube, satisfying the following (1) to (3): (1) In differential thermal analysis when the temperature is increased from 200°C to 1000°C at a rate of 10°C / min, an exothermic peak is present at a temperature of 600°C or higher and 800°C or lower; (2) When 1560 cm -1 ~1600cm -1 The maximum peak intensity within the range is defined as G, and the peak intensity at 1310 cm -1 ~1350cm -1 When the maximum peak intensity within the range is set to D, the G / D ratio is 0.5 or more and 3.0 or less; (3) The total content of cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum is less than 5000 ppm.

2. The carbon nanotube according to claim 1, satisfying the following (4): (4) The surface oxygen content is below 2.5 atm%.

3. The carbon nanotube according to claim 1, wherein The carbon nanotubes further satisfy (i) a total content of cobalt and iron of 5000 ppm or less.

4. The carbon nanotube according to claim 3, wherein The carbon nanotubes further satisfy (ii) a total content of cobalt and iron of 1000 ppm or less.

5. The carbon nanotube according to claim 1, wherein The total content of cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum in the carbon nanotubes is less than 830 ppm. The carbon nanotube according to claim 1 , wherein: In the carbon nanotubes, when the 1560 cm -1 ~1600cm -1 The maximum peak intensity within the range is defined as G, and the peak intensity at 1310 cm -1 ~1350cm -1 When the maximum peak intensity within the range of is defined as D, the G / D ratio is greater than or equal to 0.5 and less than or equal to 2.

5. 7 . A carbon nanotube dispersion comprising the carbon nanotubes according to claim 1 , a dispersant, and a dispersion medium.

8. An adhesive composition comprising a carbon nanotube dispersion and an adhesive, The carbon nanotube dispersion liquid comprises the carbon nanotubes according to any one of claims 1 to 6, a dispersant, and a dispersion medium.

9. A composition for an electrode, comprising a carbon nanotube dispersion and an electrode active material. The carbon nanotube dispersion liquid comprises the carbon nanotubes according to any one of claims 1 to 6, a dispersant, and a dispersion medium.

10. A secondary battery comprising an electrode film, wherein the electrode film is A carbon nanotube dispersion comprising the carbon nanotube according to any one of claims 1 to 6, a dispersant, and a dispersion medium, An adhesive composition comprising the carbon nanotube dispersion and an adhesive, or The present invention can obtain an electrode composition comprising the carbon nanotube dispersion and an electrode active material.

11. A method for refining carbon nanotubes, comprising: The first step is to heat-treat the carbon nanotubes at a temperature of 1000° C. to 2000° C. in an inert environment; and the second step is to contact the carbon nanotubes heat-treated in the first step with an acid. The first step and the second step are each performed once or twice or more.

12. The method for purifying carbon nanotubes according to claim 11, wherein: In the second step, the standard electrode potential of the acid is 0.8 V or less with respect to a standard hydrogen electrode.

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