Carbon nanotubes and method for purifying the same, carbon nanotube dispersion, binder composition, electrode composition, and secondary cell

Abstract

To provide: carbon nanotubes capable of forming an electrode membrane having good electrical conductivity while further enhancing safety, and a method for purifying the carbon nanotubes; and a carbon nanotube dispersion, a binder composition, an electrode composition and a secondary cell that comprise the carbon nanotubes.SOLUTION: The carbon nanotubes satisfy (1) to (3) in the following: (1) in differential thermal analysis when the temperature is increased from 200°C to 1000°C at 10°C / minute, there is an exothermic peak from 600°C to 800°C inclusive; (2) the G / D ratio is from 0.5 to 3.0 inclusive, where G refers to the maximum peak intensity in the range from 1560 to 1600 cm-1 in the Raman spectrum, and D refers to the maximum peak intensity in the range from 1310 to 1350 cm-1; and (3) the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum is 5000 ppm or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] With the spread of electric vehicles and the miniaturization and weight reduction and high performance of portable devices, there is a demand for secondary batteries with high energy density and further for high capacity of the secondary batteries. Under these circumstances, lithium ion secondary batteries in particular are being used in many devices.

[0003] In secondary batteries, carbon black, ketjen black, graphene, fine carbon materials, etc. are used as conductive assistants. In particular, carbon nanotubes (hereinafter also referred to as "CNTs"), which are a type of fine carbon fiber, are widely used. For example, by adding CNTs to electrode active materials, the electrode resistance can be reduced, the load resistance of the battery can be improved, the material strength of the electrode can be increased, and the resistance of the electrode to expansion and contraction can be increased, thereby improving the rate characteristics and cycle life of the secondary battery. Among them, multi-walled CNTs with an outer diameter of 5 nm to several tens of nm are relatively inexpensive and are being widely used.

[0004] CNTs can generally be manufactured by arc discharge, laser evaporation, chemical vapor deposition, etc. Of these, chemical vapor deposition is the most suitable for mass production in terms of productivity and economy, and is widely used. In chemical vapor deposition, carbon source gas is reacted with catalyst particles containing metals such as iron, cobalt, and nickel to generate CNTs. Therefore, CNTs obtained by chemical vapor deposition contain catalyst particles containing metals such as iron, cobalt, and nickel, or carbides or oxides derived from the catalyst particles. When CNTs containing metal-containing catalyst particles are used in secondary batteries, the metal may dissolve and precipitate, causing problems such as short-circuiting the battery. If the battery shorts out, it may lead to serious accidents such as fire or explosion. Therefore, several methods have been proposed to purify CNTs and remove metal-containing catalyst particles to improve safety.

[0005] Patent Document 1 describes a method of purifying a carbon material containing CNTs through a carbon material preparation process 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 process 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 suppressing damage or cutting of the CNTs and solidification of the CNTs into clumps.

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

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

[0008] However, when the halogen treatment process is applied to CNTs, the catalytic metal is removed, but the CNTs are fired at high temperatures for a long time, which increases their crystallinity. CNTs with high crystallinity become hard and therefore more likely to break. For this reason, when CNTs are used as a conductive additive in the electrode film of a secondary battery, etc., the contact resistance between the CNTs increases due to the breakage of the CNTs, which results in a decrease in the conductivity of the electrode film and may result in a decrease in the performance of the secondary battery containing the CNTs.

[0009] In addition, catalyst residues can be reduced by performing liquid-phase oxidation of CNTs with nitric acid, but because nitric acid has a strong oxidizing power, the surface of the CNTs can be oxidized, which may reduce the conductivity of the electrode film using CNTs.

[0010] One embodiment of the present invention has been made in consideration of the above-mentioned circumstances, and aims to provide carbon nanotubes capable of forming an electrode film having good conductivity while improving safety, a purification method thereof, and a carbon nanotube dispersion, a binder composition, an electrode composition, and a secondary battery each containing the carbon nanotubes. [Means for solving the problem]

[0011] Means of the Invention In order to solve the above problems, the present inventors have conducted intensive research and have found carbon nanotubes that satisfy the following conditions, thereby completing the present invention.

[0012] That is, some aspects of the present invention are as follows. <1> A carbon nanotube that satisfies the following (1) to (3). (1) In a differential thermal analysis when the temperature is increased from 200° C. to 1,000° C. at 10° C. / min, an exothermic peak is observed between 600° C. and 800° C. (2) 1560-1600 cm in the Raman spectrum -1The maximum peak intensity in the range of 1310 to 1350 cm is G. -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 total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is not more than 5,000 ppm. <2> Satisfy the following (4) <1> The carbon nanotube according to claim 1. (4) The surface oxygen content is 2.5 atm % or less. <3> The carbon nanotubes further satisfy the following: (i) a total content of cobalt and iron is 5000 ppm or less; <1> or <2> The carbon nanotube according to claim 1. <4> The carbon nanotubes further satisfy (ii) a total content of cobalt and iron of 1000 ppm or less; <3> The carbon nanotube according to claim 1.

[0013] <5> the above <1> from <4> 2. A carbon nanotube dispersion comprising the carbon nanotubes according to any one of the above items 1 to 11, a dispersant, and a dispersion medium. <6> A carbon nanotube dispersion and a binder are included, The carbon nanotube dispersion liquid is <1> from <4> 2. A binder composition comprising the carbon nanotubes according to any one of claims 1 to 11, a dispersant, and a dispersion medium. <7> A carbon nanotube dispersion liquid and an electrode active material are included, The carbon nanotube dispersion liquid is <1> from <4> 2. A composition for an electrode comprising the carbon nanotubes according to any one of claims 1 to 11, a dispersant, and a dispersion medium. <8> The electrode film includes: the above <1> from <4> a carbon nanotube dispersion liquid comprising the carbon nanotubes according to any one of the above items, a dispersant, and a dispersion medium; A binder composition comprising the carbon nanotube dispersion and a binder, or A secondary battery obtained by using a composition for an electrode containing the carbon nanotube dispersion and an electrode active material.

[0014] <9> The method includes a first step of heat-treating carbon nanotubes at 1000° C. or more and 2000° C. or less in an inert atmosphere, and a second step of contacting the carbon nanotubes heat-treated in the first step with an acid, The first step and the second step are each carried out once or twice or more; A method for purifying carbon nanotubes. <10> In the second step, the acid has a standard electrode potential of 0.8 V vs. SHE or less. <9> 2. A method for purifying carbon nanotubes according to claim 1. Effect of the Invention

[0015] According to an embodiment of the present invention, it is possible to provide carbon nanotubes capable of forming an electrode film having good conductivity while improving safety, and a purification method thereof. It is also possible to provide a carbon nanotube dispersion, a binder composition, an electrode composition, and a secondary battery, each of which contains the carbon nanotubes. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a graph showing the DTA curves of the CNTs prepared in Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] 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. Hereinafter, carbon nanotubes are also referred to as CNTs.

[0018] <Carbon nanotubes (CNTs)> The CNT according to this embodiment satisfies the following (1) to (3). (1) In a differential thermal analysis when the temperature is increased from 200° C. to 1,000° C. at 10° C. / min, an exothermic peak is observed between 600° C. and 800° C. (2) 1560-1600 cm in the Raman spectrum -1The maximum peak intensity in the range of 1310 to 1350 cm is G. -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 total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is not more than 5,000 ppm. Furthermore, the CNT according to this embodiment may satisfy the following (4). (4) The surface oxygen content is 2.5 atm % or less.

[0019] The CNT according to this embodiment satisfies at least the above (1) to (3), and preferably satisfies all of the above (1) to (4). Each condition will be explained below.

[0020] <(1) Heat generation peak> The CNT of this embodiment has an exothermic peak at 600°C or more and 800°C or less in differential thermal analysis (DTA) when the temperature is increased from 200°C to 1000°C at 10°C / min. The exothermic peak can be measured by subjecting the CNT to differential thermal analysis in an air atmosphere. DTA is a method in which the temperature difference between a sample and a reference substance is measured as a function of temperature while the temperatures of the sample and the reference substance are changed under certain conditions, and is in accordance with JIS K 0129. In a DTA curve created based on the change in the temperature difference between the sample and the reference substance, the largest peak is regarded as the exothermic peak.

[0021] Heat is generated as the CNTs burn. As the CNT combustion start temperature increases, the heat peak temperature also increases. Factors that affect the change in the CNT combustion start temperature include the catalytic metal content, the degree of oxidation of the CNT surface, and the crystallinity of the CNTs. When the catalytic metal contained in the CNT has a high heat storage capacity, the total amount of heat stored in all catalytic metals decreases when the catalytic metal content is low. Since the total amount of heat stored in the catalytic metal is small, the temperature required to burn the CNTs may be higher than when the catalytic metal content is high. In addition to the catalytic metal, metal impurities may be mixed into the CNTs during the CNT manufacturing process, and such metal impurities may also affect the total amount of heat stored. In addition, since the sites on the CNT surface that have oxygen-containing functional groups are more easily combusted than sites that do not have functional groups, the fewer the amount of oxygen-containing functional groups (i.e., the less surface oxygen), the more difficult it is for the CNT to burn, and the lower the degree of oxidation of the CNT surface, the higher the temperature required to burn the CNT. Furthermore, the higher the crystallinity of the CNT, the higher the combustion initiation temperature of the CNT. The crystallinity of the CNT can be expressed by the G / D ratio, which will be described later.

[0022] When the combustion start temperature of CNT is within an appropriate temperature range, the impurities contained in CNT are reduced, and therefore, a safer CNT can be obtained. The CNT of this embodiment preferably has an exothermic peak temperature of 600°C or higher, more preferably 650°C or higher. In addition, the exothermic peak temperature is preferably 800°C or lower, more preferably 740°C or lower. When the exothermic peak temperature is 600°C or higher, the metal content is low, and the safety of the battery can be improved. Alternatively, the surface oxygen amount is small, and the electrical conductivity is excellent. When the exothermic peak temperature is 800°C or lower, the crystallinity of the CNT is not too high, and the bending of the CNT can be suppressed, and the deterioration of the performance of the secondary battery can be suppressed.

[0023] When CNTs are in the form of powder before dispersion, the exothermic peak can be measured as is. When CNTs are present in a CNT dispersion, the dispersion medium is removed by heating and drying, and then the measurement can be performed and the exothermic peak can be identified from the shape of the exothermic peak. The heating and drying temperature is preferably a temperature at which the CNTs are not oxidized (for example, 140°C or lower). When the CNT dispersion contains components other than CNTs and the dispersion medium (additives, etc.), the exothermic peak of the additives may be measured in advance, and the exothermic peak derived from the additives may be identified, so that the remaining exothermic peaks are determined to be derived from CNTs, and the exothermic peaks may be identified.

[0024] <(2)G / D ratio> The G / D ratio (peak ratio of G-band to 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. -1 The Raman shift seen around 1,350 cm is called the G band of graphite. -1 The Raman shift observed around 10 cm is called the D band, which is derived from defects in amorphous carbon and graphite. The wavenumber of Raman spectroscopy may vary depending on the measurement conditions, so the wavenumber specified here is ±10 cm. -1 The higher the G / D ratio of a carbon nanotube, the higher its crystallinity. In addition, 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 be.

[0025] CNTs have 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 the range is 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, so that the CNT is easily damaged during dispersion, and the contact resistance may increase. 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 be low. As a result, 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 using the CNT dispersion liquid are improved.

[0026] Conventionally, when raw CNT is subjected to a halogen treatment process to purify CNT, the crystallinity of CNT tends to increase because CNT is baked at high temperature for a long time. This state can also be confirmed by the fact that the G / D ratio increases. On the other hand, in the present embodiment, in the purification process of CNT, the heat treatment of CNT is performed in a short time with the upper limit temperature kept low in an inert atmosphere, so that high crystallization of CNT can be suppressed. In other words, it is possible to suppress the G / D ratio of CNT from increasing. When low-crystalline CNT is used, in the process of manufacturing the electrode film, the bending of CNT can be suppressed by a dispersion process or the like, and the increase in the contact resistance between CNTs in the obtained electrode film can be suppressed. As a result, low-crystalline CNT can obtain good conductivity as an electrode film, and a secondary battery including CNT can exhibit good performance.

[0027] <(3) Total metal content> In the CNT of this embodiment, the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum 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. When 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 may be collectively referred to simply as metal.

[0028] Here, the content of metal contained in CNT is the mass converted into the metal element. Metals may be contained in CNT as metal elements, metal oxides, metal composite oxides, etc., but these are converted into the metal element to determine the metal content. In CNTs, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be contained as elemental metals, metal oxides, and composite oxides thereof, etc. These metals can cause short circuits, so it is desirable to reduce their total content.

[0029] More preferably, from the standpoint of stricter safety, the total content of cobalt and iron in the CNT is preferably 5000 ppm or less, more preferably 3000 ppm or less, even more preferably 2500 ppm or less, and even more preferably 1000 ppm or less.

[0030] More preferably, the total cobalt content in the CNTs is 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 the CNTs, the total iron content 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.

[0031] In CNTs, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be included as catalyst metals. In addition to metals and metal oxides used as catalyst metals, metals such as stainless steel used in synthesis equipment, filling equipment, or piping may be mixed into CNTs due to wear, etc. Therefore, in CNTs, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be metals that are not catalyst metals.

[0032] In this embodiment, the total content of metals contained in the CNT can be reduced by dissolving the exposed metals by firing the CNT with acid, as in the CNT purification method described later. By reducing the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum contained in the CNT, the temperature of the heat generation peak can be increased, and CNT with improved safety can be obtained. A secondary battery containing such CNT can exhibit good performance.

[0033] The content of metals contained in CNTs can be calculated, for example, by acid decomposing CNTs, extracting the metals contained in CNTs, and analyzing the extracts using 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 content of extracted cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum to the mass of 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 into a metal element, and the total amount is the sum of the masses.

[0034] <(4) Surface oxygen content> In this embodiment, the surface oxygen content of the CNT is preferably 2.5 atm% or less from the viewpoint of the electrical conductivity of the CNT, more preferably 1.9 atm% or less, even more preferably 1.4 atm% or less, and particularly preferably 1.2 atm% or less. When the surface oxygen content is 2.5 atm% or less, the CNT has excellent electrical conductivity as an electrode film. In this specification, the "surface oxygen content" is a value represented by the ratio (atm%) of oxygen atoms to carbon atoms on the surface of the CNT determined by X-ray photoelectron spectroscopy.

[0035] When nitric acid is used to refine the raw CNT, the surface of the CNT is oxidized due to the strong oxidizing power of nitric acid. On the other hand, in this embodiment, as in the CNT refinement method described later, the raw CNT is heat-treated to expose the metal, and the metal is dissolved with an acid having a strong acidity and a low oxidizing power, so that the oxidation of the CNT surface can be suppressed and the amount of oxygen on the surface of the CNT can be reduced. As a result, the combustion start temperature of the CNT is increased, and the CNT can obtain good conductivity as an electrode film, and a secondary battery containing the CNT can exhibit good performance.

[0036] <(5)Other> CNTs have a shape in which planar graphite is wound into a cylindrical shape. The CNTs may be a mixture of single-walled CNTs and multi-walled CNTs. Single-walled CNTs have a structure in which a single layer of graphite is wound into a cylindrical shape. Multi-walled CNTs have a structure in which two or more layers of graphite are wound into a cylindrical shape. In addition, the sidewalls of the CNTs do not have to have a graphite structure. For example, CNTs with sidewalls having an amorphous structure can also be used as the CNTs.

[0037] The CNT of this embodiment is preferably a multi-walled CNT, and the number of walls of the CNT is preferably 3 to 30, more preferably 3 to 20, and even more preferably 3 to 10.

[0038] The purity of CNT is expressed as a value (mass%) obtained by subtracting the ash content (mass%) from the mass of 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 mass% or more, more preferably 95 mass% or more, and even more preferably 99 mass% or more, based on the mass of CNT. In addition, the non-flammable component contained in CNT is preferably 10 mass% or less, more preferably 5 mass% or less, and even more preferably 1 mass% or less.

[0039] 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 More preferably, it is 1.2×10 -2 ~1.8×10 -2It is particularly preferable that the volume resistivity is Ω·cm. When the volume resistivity of the CNT is within the above range, the volume resistivity of the electrode film is reduced, and the performance of the secondary battery is improved. The volume resistivity of the CNT can be measured using a powder resistivity measuring device (Loresta GP Powder Resistivity Measuring System MCP-PD-51, manufactured by Nitto Seiko Analytech Co., Ltd.).

[0040] The BET specific surface area of ​​the CNT of this embodiment is 150 m 2 / g or more, and 2 / g or more. The BET specific surface area of ​​CNTs is preferably 800 m 2 / g or less, and 2 / g or less is more preferable, and 2 / g or less is even more preferable. The BET specific surface area of ​​CNTs can be calculated by the BET method using nitrogen adsorption measurements. There is often a correlation between the specific surface area of ​​CNTs and the average outer diameter of CNTs, and 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 the number of CNTs per mass. When the specific surface area of ​​CNTs is 150 m 2 When the specific surface area of ​​the CNT is 800 m / g or more, the number of carbon nanotubes per mass can be secured, and a conductive network can be efficiently formed, resulting in excellent rate characteristics and cycle characteristics of the battery. 2 When the CNT content is 1 / g or less, the CNTs are well dispersed, and a good conductive network can be formed in the electrode film.

[0041] 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 secured, and a conductive network can be efficiently formed. When the average outer diameter of the CNTs is 3 nm or more, the dispersion of the CNTs is good, and a good conductive network can be formed in the electrode film.

[0042] The standard deviation of the average outer diameter of the CNTs is preferably 2 nm to 8 nm, more preferably 3 nm to 6 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.

[0043] The outer diameter and average outer diameter of CNTs are determined as follows. First, the CNTs are observed and photographed using a transmission electron microscope. Next, 300 CNTs are randomly selected from the photograph and the outer diameter of each is measured. Next, the average outer diameter (nm) of the CNTs is calculated as the number average of the outer diameters.

[0044] CNTs usually exist as aggregates. This shape may be, for example, a state in which a single CNT is intricately entangled (entangled), or an aggregate of linear CNTs (bundle-like). A bundle-like CNT aggregate is easier to disentangle than an entangled CNT aggregate. Also, a bundle-like CNT aggregate has better dispersibility than an entangled CNT aggregate, and can therefore be suitably used as a CNT.

[0045] According to this embodiment, by satisfying at least the above (1) to (3), it is possible to obtain CNTs capable of forming an electrode film having good electrical conductivity while improving safety, and a secondary battery including such CNTs can exhibit good performance.

[0046] <Method of manufacturing carbon nanotubes (CNTs)> The CNTs of this embodiment can be produced by, for example, a laser ablation method, an arc discharge method, a thermal CVD method, a plasma CVD method, and a combustion method, but are not limited to these. For example, CNTs can be produced by contacting 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.

[0047] The raw material gas that is the carbon source of CNTs can be any conventionally known gas. For example, the raw material gas containing carbon can be, but is not limited to, hydrocarbons such as methane, ethylene, propane, butane, and acetylene, carbon monoxide, and alcohol. In particular, from the viewpoint of ease of use, it is preferable to use at least one of hydrocarbons and alcohol as the raw material gas.

[0048] <Method of purifying carbon nanotubes (CNTs)> A method for purifying carbon nanotubes (CNTs) will be described below. 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 obtained by following the purification method described below.

[0049] The method for producing CNTs according to the present embodiment includes a first step of heat-treating raw carbon nanotubes at 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. Each of the first step and the second step may be performed once or twice or more.

[0050] (First step) First, the raw material CNTs are heat-treated in an inert atmosphere. This causes the CNTs to be sintered. Examples of the inert atmosphere include a nitrogen atmosphere, an argon atmosphere, a combination of these, and a vacuum atmosphere.

[0051] Heat treatment conditions such as heat treatment temperature and heat treatment time may be appropriately determined depending on the type of CNT and the type of metal derived from the catalytic metal contained in the CNT. The heat treatment temperature is preferably a temperature at which all metals, cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum, begin to melt. In addition, the catalytic metal used in the production of CNT is at the nano level, and the melting temperature is lower than that of bulk metal due to the nano-size effect, so 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 more and 2000°C or less. From the viewpoint of suppressing high crystallization of CNT, it may be 1800°C or less, 1400°C or less, or 1200°C or less. In addition, the heat treatment time may be appropriately set depending on the firing device and firing scale, and is preferably, for example, 1 to 3 hours.

[0052] The above-mentioned heat treatment melts the metal contained in the CNT, and also removes the amorphous carbon on the CNT surface, thereby exposing the metal on the surface of the CNT, and in the second step described below, the contact between the metal and the acid can be promoted. In addition, CNTs are usually oxidized and burned at 500°C or higher in the air atmosphere, but in the heat treatment in the first step, the heat treatment is performed in an inert atmosphere and at a relatively low heat treatment temperature, so that the combustion of the CNT itself can be suppressed. Furthermore, since the CNTs are heat-treated in an inert atmosphere, the oxidation of the surface of the CNTs can be suppressed. In addition, the heat treatment time can be shortened, in which case the high crystallization of the CNTs can be further suppressed.

[0053] (Second step) Next, the CNTs heat-treated in the first step are brought into contact with an acid. This allows the metal contained in the CNTs to dissolve. It is preferable to use a strong acid to dissolve the metal contained in the CNTs. As the strong acid, from the viewpoint of acidity, an acid dissociation constant (pKa) of 3 or less in an aqueous medium is more preferable. In the first step, the heat treatment is performed in an inert atmosphere to suppress the surface oxygen amount of the CNTs, and it is preferable to use an acid with a relatively low oxidizing power. As the acid with low oxidizing power, an acid with a standard electrode potential (E°) in an aqueous solution of 0.80 V vs. SHE or less is more preferable, an acid with a standard electrode potential (E°) of 0.50 V vs. SHE or less is more preferable, and an acid with a standard electrode potential (E°) of 0.20 V vs. SHE or less is even more preferable. Since an acid with low oxidizing power has a low ability to oxidize the surface of the CNTs, the surface oxygen amount of the CNTs can be further suppressed in a state where the CNTs are treated with an acid with low oxidizing power. In this specification, the standard electrode potential (E°) represents the potential with respect to the standard hydrogen electrode (SHE) at 25°C.

[0054] From this viewpoint, as acids with strong acidity and low oxidizing power, hydrochloric acid (acid dissociation constant pKa=-3.7, E°=0 V vs. SHE), hydrobromic acid (pKa=-4.1, E°=0 V vs. SHE), dilute sulfuric acid (pKa=2.0, E°=0.16 V vs. SHE) and the like can be preferably used (pKa and E° are values ​​in aqueous medium in all cases). From an industrial perspective, hydrochloric acid is preferred. Hydrochloric acid also tends to have a high ability to remove metals contained in CNTs.

[0055] 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). These oxidize the CNT surface and increase the acidic groups introduced to the CNT surface, so that the CNT treated with nitric acid or the like tends to have a high surface oxygen concentration. From the viewpoint of suppressing the surface oxygen amount of CNT, it is preferable to use the above-mentioned acid with strong acidity and low oxidizing power. In another embodiment, it is preferable to use an acid with a standard electrode potential (E°) against SHE lower than nitric acid in the first step. In yet another embodiment, it is preferable to use an acid with a standard electrode potential (E°) against SHE equal to or lower than dilute sulfuric acid in the first step.

[0056] Preferably, the CNTs heat-treated in an inert atmosphere in the first step are contacted with an acid having a low oxidizing power in the second step, whereby oxidation of the surface of the resulting CNTs is suppressed and the acidic groups on the surface are reduced, so that the amount of oxygen on the surface of the resulting CNTs can be further suppressed. In this way, by suppressing the amount of oxygen, which is one of the factors that increase the resistance of CNTs, CNTs with high electrical conductivity can be obtained.

[0057] In the second step, the method of contacting the heat-treated CNT with the acid may be either a gaseous acid or a liquid acid, but from the viewpoint of metal solubility, it is preferable to use a liquid acid. For example, there is a method of contacting the CNT with an aqueous acid solution. The acid treatment conditions may be appropriately determined depending on the type of acid, the type of CNT, the type and amount of metal contained in the CNT, and the like. The acid treatment may be performed by heating. On the other hand, the acid treatment temperature is preferably 80° C. or less, more preferably 50° C. or less. When the acid treatment temperature is 80° C. or less, a higher metal dissolving ability can be obtained. When hydrochloric acid is used, the steam of hydrochloric acid is corrosive, and from the viewpoints of safety, facility contamination, environmental pollution, and the like, the acid treatment temperature is more preferably 50° C. or less, and may be about room temperature (25° C.). The acid treatment time may be appropriately set depending on the acid treatment method, the acid treatment scale, the acid treatment temperature, and the like, and may be, for example, 1 to 3 hours.

[0058] By heat-treating CNTs at room temperature and dissolving the exposed metals with an acid that has strong acidity and low oxidizing power, it is possible to reduce the amount of metal contained in the CNTs while suppressing oxidation of the CNT surface and thereby suppressing an increase in the amount of oxygen on the CNT surface.

[0059] From the viewpoint of further reducing the metal contained in the CNTs, each of the first step and the second step may be performed two or more times. In addition, other treatment steps may be performed on the CNTs between the first step and the second step, as long as the crystallinity and the degree of surface oxidation of the CNTs performed in the first step can be maintained.

[0060] The first and second steps increase the combustion start temperature of the CNTs, resulting in CNTs capable of forming an electrode film with good electrical conductivity, and a secondary battery containing the CNTs can exhibit good performance.

[0061] <Dry grinding of carbon nanotubes (CNTs)> The CNTs of this embodiment may be CNTs that have been dry-pulverized in order to crush the particles and increase the dispersibility. Dry pulverization refers to a process of pulverizing CNTs without the use of a liquid substance. The dry pulverization may be media pulverization, pulverization without media, or a combination of two or more dry pulverizations. For example, in media pulverization, a pulverizer containing pulverization media such as beads or steel balls is used to pulverize particles by utilizing the pulverizing force or destructive force caused by the collision between the pulverization media. As the dry pulverization device, a known method such as a dry attritor, ball mill, vibration mill, or bead mill can be used, and the pulverization time can be set arbitrarily depending on the device or the pulverization state of the particles.

[0062] [Carbon nanotube (CNT) dispersion] The CNT dispersion according to this embodiment includes the above-mentioned CNT, a dispersant, and a dispersion medium. The CNT dispersion in this specification does not include an electrode active material. In addition, according to this embodiment, a method for producing the CNT dispersion including the above-mentioned CNT, a dispersant, and a dispersion medium can be provided.

[0063] <Dispersant> The dispersant is not particularly limited as long as it can disperse and stabilize the CNTs, and for example, a surfactant or a resin-type dispersant can be used. Surfactants are mainly classified into anionic, cationic, nonionic, and amphoteric. Depending on the characteristics required for dispersing the CNTs, a suitable type of dispersant can be used in a suitable amount.

[0064] Examples of resin-type dispersants include cellulose derivatives (cellulose acetate, cellulose acetate butyrate, cellulose butyrate, cyanoethyl cellulose, ethyl hydroxyethyl cellulose, nitrocellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxymethyl cellulose, etc.), polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, hydrogenated nitrile butadiene rubber, polyacrylonitrile polymers, etc. Particularly, methyl cellulose, ethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, hydrogenated nitrile butadiene rubber, and polyacrylonitrile polymers are preferred. The molecular weight of the resin-type dispersant is preferably 10,000 to 300,000, more preferably 10,000 to 150,000.

[0065] In addition to the dispersant, it is preferable to add an amine compound or an inorganic base. As the amine compound, a primary amine (primary amine), a secondary amine (secondary amine), or a tertiary amine (tertiary amine) is used, and ammonia and quaternary ammonium compounds are not included. As the amine compound, in addition to monoamine, amine compounds such as diamine, triamine, and tetramine having multiple amino groups in the molecule can be used. Specifically, examples of the inorganic base 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; alicyclic nitrogen-containing heterocyclic compounds such as hexamethylenetetramine, morpholine, and piperidine. Examples of the inorganic base include, but are not limited to, hydroxides of alkali metals, hydroxides of alkaline earth metals, carbonates of alkali metals, carbonates of alkaline earth metals, phosphates of alkali metals, and phosphates of alkaline earth metals.

[0066] <Dispersion medium> The dispersion medium is not particularly limited as long as it is capable of dispersing CNTs, but is preferably composed of one or more of water and water-soluble organic solvents.

[0067] 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 an amide-based organic solvent is more preferable, and of the amide-based organic solvents, N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone are particularly preferable.

[0068] 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.

[0069] The CNT dispersion can be produced, for example, by dispersing CNTs in a dispersion medium. The raw materials to be used may be added once or in multiple batches at any timing during the dispersion process. The dispersion method for carrying out such a process is not particularly limited.

[0070] Examples of the dispersion method include a method using various dispersers such as a disperser (dispersing machine), a homogenizer, a high shear mixer, a kneader, a two-roll mill, a three-roll mill, a ball mill, a horizontal sand mill, a vertical sand mill, an annular bead mill, a paint conditioner, an attritor, a planetary mixer, or a high-pressure homogenizer. The disperser is not particularly limited, but for example, from the viewpoint of adjusting the fiber length of the CNT in the CNT dispersion to be within a preferred range, it is preferable to use a high shear mixer from the viewpoint of promoting the wetting of the CNT and dissolving coarse particles and agglomerations, and it is preferable to use a media-type disperser such as a bead mill from the viewpoint of crushing aggregated and solidified particles. In addition, it is more preferable to select and combine a plurality of the above dispersers to disperse, and the order of the dispersers can be changed as desired. The pressure when using the high-pressure homogenizer is not particularly limited, and is preferably, for example, 60 to 150 MPa, more preferably 60 to 120 MPa.

[0071] Dispersion methods using a dispersion device include batch dispersion, pass dispersion, circulation dispersion, etc., and any of these methods may be used, or two or more methods may be combined. Dispersion methods using a dispersion device include batch dispersion, pass dispersion, circulation dispersion, etc., and 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 only using the dispersion device body without using piping or the like. It is easy to handle, so it is preferable for small-scale production. Pass dispersion is a dispersion method in which the dispersion device body is equipped with a tank that supplies the dispersion liquid through piping and a tank that receives the dispersion liquid, and the dispersion liquid is passed through the dispersion device body. In addition, circulation dispersion is a method in which the dispersion liquid that has passed through the dispersion device body is returned to the tank that supplies the dispersion liquid, and dispersion is performed while circulating. In either case, the longer the processing time, the more the dispersion progresses, so it is sufficient to repeat the pass or circulation until the desired dispersion state is reached, and the processing amount can be increased by changing the size of the tank or the processing time. Pass dispersion is preferable in that it is easier to uniformize the dispersion state compared to circulation dispersion. Circulation dispersion is preferred in that the operation and manufacturing equipment are simpler than pass dispersion. In the dispersion process, the disintegration of aggregated particles, the loosening, wetting, stabilization, etc. of the conductive material proceed sequentially or simultaneously, and the final dispersion state differs depending on the way of proceeding, so it is preferable to control the dispersion state in each dispersion process by using various evaluation methods. For example, it can be controlled by the method described in the examples.

[0072] 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, based on 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, based on 100% by mass of the CNT dispersion.

[0073] 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, based on 100% by mass of CNT, from the viewpoint of CNT feedability, dispersibility, and dispersion stability. Also, 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, based on 100% by mass of CNT, from the viewpoint of electrical conductivity.

[0074] The CNT dispersion may contain particulate foreign metal matter and dissolved metal ions as metals. Foreign metal matter is metal present in the CNT dispersion in the form of particles, and specifically includes the metals contained in the CNTs described above. CNTs, dispersants, and other materials may contain foreign metal matter derived from their respective manufacturing processes, and foreign metal matter may also be mixed in during the manufacturing process of the CNT dispersion. If foreign metal matter exists inside a battery, the battery is more likely to short-circuit, so removing foreign metal matter is very important from the perspective of safety. It is preferable to include a step of removing contaminants such as metallic foreign matter at any timing during the process of producing the CNT dispersion liquid (metallic foreign matter removal step). From the viewpoint of efficiency, the metallic foreign matter removal step is preferably performed during the dispersion step of the CNT dispersion liquid and / or at the end of the dispersion step. The metallic foreign matter removal step may be performed multiple times.

[0075] In the metal foreign matter removal step, the method for removing the metal foreign matter from the CNT dispersion liquid is not particularly limited, and examples thereof include a method of removing by filtration using a filter, a method of removing by a vibrating sieve, a method of removing by centrifugation, a method of removing by magnetic force, etc. Among these, since metal foreign matters such as iron and chromium are magnetic, a method of removing by magnetic force is preferred, and a method of combining a step of removing by magnetic force and a step of removing by filtration using a filter is more preferred.

[0076] The method of removal by magnetic force is not particularly limited as long as it can remove metallic foreign matter, but from the standpoint of productivity and removal efficiency, a method of removing the metallic foreign matter by passing the CNT dispersion through a magnetic filter placed in the CNT dispersion production line is preferred. The step of removing metallic foreign matter from the CNT dispersion liquid using a magnetic filter is preferably carried out by passing the liquid through a magnetic filter that forms a magnetic field with a magnetic flux density of 1,000 Gauss or more. Since a low magnetic flux density reduces the efficiency of removing metallic foreign matter, 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 a low magnetic property, and most preferably 12,000 Gauss or more. Depending on the flow rate of the filtration, coarse metal particles may pass through the magnetic filter, so when arranging the magnetic filter in the production line, it is preferable to include a process of removing coarse foreign matter or metal particles using a filter such as a cartridge filter upstream of the magnetic filter. Although the magnetic filter is effective even if it only filters once, it is more preferable that the magnetic filter is of a circulating type. By adopting a circulating type, the efficiency of removing metal particles is improved. When a magnetic filter is arranged in a production line for a CNT dispersion liquid, the location of the magnetic filter is not particularly limited, but it is preferable to arrange the magnetic filter immediately before filling a container with the carbon nanotube dispersion liquid, and in the case where a filtration process using a filtration filter is performed before filling the container, it is preferable to arrange the magnetic filter before the filtration filter. By arranging the magnetic filter in this way, it is possible to prevent metal from being mixed into the product when it is detached from the magnetic filter.

[0077] The metal content in the CNT dispersion can be calculated by drying the CNT dispersion to remove the solvent, and then analyzing it using ICP. The metal content detected by ICP analysis includes metallic foreign matter present in particulate form and dissolved metal ions. In other words, the metal content of the CNT dispersion that has undergone the metallic foreign matter removal process includes metallic foreign matter that has not been completely removed and dissolved metal ions.

[0078] The content of the metals iron and chromium contained in the CNT dispersion is preferably 100 ppm or less, more preferably 50 ppm or less, and even more preferably 10 ppm or less, relative to 100 mass% of the CNT dispersion. By setting the metal content within the above range, it is possible to make side reactions in the electrode film less likely to occur, and to obtain a secondary battery with better conductivity.

[0079] Since the CNT dispersion of the present embodiment contains CNTs with reduced metal content, the metal content in the CNT dispersion can also be reduced.

[0080] 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.

[0081] [Binder Composition] The binder composition according to the present embodiment includes the above-mentioned CNT dispersion and a binder. The binder composition in the present specification does not include an electrode active material. In addition, according to the present embodiment, a method for producing a binder composition including the above-mentioned CNT, a dispersant, a dispersion medium, and a binder can be provided.

[0082] [binder] The binder is a resin that binds various substances together in the electrode film. As the binder, binders known for batteries can be used. Examples include cellulose resins such as carboxymethyl cellulose; rubbers such as styrene-butadiene rubber and fluororubber. Modified bodies, mixtures, and copolymers of these resins may also be used. In particular, from the standpoint of resistance, it is preferable to use polyvinylidene fluoride, polyvinyl fluoride, tetrafluoroethylene, and the like, which are polymeric compounds having fluorine atoms in the molecule.

[0083] 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. The weight average molecular weight of the binder is preferably 2,000,000 or less, more preferably 1,000,000 or less, and particularly preferably 1,000,000 or less. When the weight average molecular weight is 10,000 or more, the deterioration of the resistance and adhesion of the binder can be suppressed. When the weight average molecular weight is 2,000,000 or less, the resistance and adhesion of the binder can be improved, while the deterioration of workability due to the increase in the viscosity of the binder itself can be suppressed, and the dispersed particles can be suppressed from being significantly aggregated.

[0084] The binder composition is preferably produced by mixing and homogenizing the CNT dispersion liquid and the binder, and the binder may be dissolved in advance. The binder may be added at any timing in the process of producing the CNT dispersion liquid. The mixing method may be any of various conventionally known methods. The binder composition can be produced using the dispersion device described above for the CNT dispersion liquid. The binder composition may contain one type of binder, or two or more types of binders. Furthermore, the process of producing the binder composition may include the above-mentioned metal foreign matter removal process.

[0085] [Composition for electrode] The electrode composition according to this embodiment includes the above-mentioned CNT dispersion liquid and an electrode active material. The electrode composition can be further mixed with a binder to produce a composite slurry. In addition, according to this embodiment, it is possible to provide a method for producing a binder composition including the above-mentioned CNT, a dispersant, a dispersion medium, and an electrode active material, and a method for producing a binder composition including the above-mentioned CNT, a dispersant, a dispersion medium, an electrode active material, and a composite slurry.

[0086] [Electrode active material] Electrode active materials are materials that are the basis of battery reactions. Active materials are divided into positive and negative electrode active materials based on their electromotive force. As the positive electrode active material, there is no particular limitation, but metal compounds such as metal oxides and metal sulfides capable of doping or intercalating lithium ions, and conductive polymers can be used. For example, oxides of transition metals such as Fe, Co, Ni, Mn, composite oxides with lithium, inorganic compounds such as transition metal sulfides, etc. can be mentioned. Specifically, transition metal oxide powders such as MnO, V2O5, V6O 13 , TiO2; composite oxide powders of lithium and transition metals such as lithium nickelate, lithium cobaltate, lithium manganate having a layered structure, and lithium manganate having a spinel structure; lithium iron phosphate-based materials which are phosphate compounds having an olivine structure, etc. can be mentioned. These positive electrode active materials can also be used alone or in combination of two or more. Further, the above inorganic compounds and organic compounds may be mixed and used. As the negative electrode active material, there is no particular limitation, 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 artificial graphite and natural graphite which are highly graphitized carbon materials can be mentioned. These negative electrode active materials can also be used alone or in combination of two or more. In particular, it is preferable to use a combination of a highly graphitized carbon material and lithium silicate from the viewpoints of capacity and life.

[0087] 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 in this specification refers to the average value of particle sizes measured by an electron microscope.

[0088] The electrode composition is preferably produced by mixing and homogenizing the CNT dispersion liquid and the electrode active material, and the binder may be dissolved in the CNT dispersion liquid beforehand. The electrode active material may be added at any timing in the process of producing the CNT dispersion liquid. The dispersing device used for the treatment of dispersing the electrode active material is not particularly limited, and the dispersing devices exemplified in the production of the CNT dispersion liquid may be used.

[0089] In the case of a composite slurry containing an electrode composition, the content of the electrode active material contained in the composite slurry is preferably 20% by mass or more, more preferably 40% by mass or more, based on 100% by mass of the composite slurry. The content of the electrode active material contained in the composite slurry is preferably 99% by mass or less, more preferably 97% by mass or less, based on 100% by mass of the composite slurry. The above range is preferable from the viewpoint of coatability or productivity, and from the viewpoint of uniformity of the electrode film. The CNT content in the composite slurry is preferably 0.01% by mass or more, and more preferably 0.05% by mass or more, relative to 100% by mass of the electrode active material. The CNT content in the composite slurry is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, relative to 100% by mass of the electrode active material. The content of the binder in the composite slurry is preferably 0.3% by mass or more, and more preferably 0.7% by mass or more, relative to 100% by mass of the electrode active material. The content of the binder in the composite slurry is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, relative to 100% by mass of the electrode active material. The solid content of the composite slurry is preferably 30% by mass or more, more preferably 40% by mass or more, based on 100% by mass of the composite slurry, and is preferably 90% by mass or less, more preferably 85% by mass or less, based on 100% by mass of the composite slurry. The water content in the composite slurry is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

[0090] (electrode film) The electrode film according to this embodiment is an electrode film obtained by using (i) a carbon nanotube dispersion liquid containing carbon nanotubes, a dispersant, and a dispersion medium, (ii) a binder composition containing the carbon nanotube dispersion liquid and a binder, or (iii) an electrode composition containing the carbon nanotube dispersion liquid and an electrode active material. The carbon nanotubes used are the carbon nanotubes of this embodiment described above. The electrode film may also be an electrode film obtained by using (iv) a composite slurry. The (iv) composite slurry can be obtained by using the above-mentioned (i) carbon nanotube dispersion liquid, (ii) the binder composition, or (iii) the electrode composition.

[0091] For example, the electrode film is a coating film formed by coating the above-mentioned composite slurry on a current collector 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.

[0092] The method for applying the composite slurry onto the current collector is not particularly limited, and any known method can be used.

[0093] After coating and drying, the coating may be rolled using a lithographic press or a calendar roll, etc. The thickness of the electrode film is generally 1 μm or more and 500 μm or less, and preferably 10 μm or more and 300 μm or less.

[0094] (Secondary battery) The secondary battery according to the present embodiment includes the above-mentioned electrode film. The electrode film can be used as an electrode of the secondary battery, and is preferably used as an electrode of a non-aqueous electrolyte secondary battery using an organic electrolyte. The non-aqueous electrolyte secondary battery is a battery including a positive electrode, a negative electrode, and an electrolyte containing an organic electrolyte. The electrode film can be used for either the positive electrode or the negative electrode, or for both. In one embodiment, for example, an electrode film obtained by coating a current collector with a composition for an electrode containing a positive electrode active material and drying the same 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 electrode film 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 same can be used as a current collector with an underlayer. In particular, from the viewpoint of safety, it is preferably used as the positive electrode.

[0095] As the electrolyte, various conventionally known electrolytes capable of moving ions can be used. For example, those containing lithium salts such as LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, Li(CF3SO2)3C, LiI, LiBr, LiCl, LiAlCl, LiHF2, LiSCN, or LiBPh4 (wherein Ph is a phenyl group) can be used, but are not limited thereto, and those containing sodium salts can also be used. The electrolyte is preferably dissolved in a non-aqueous solvent and used as an electrolytic solution. An all-solid electrolyte or a polymer electrolyte may be used.

[0096] The non-aqueous solvent is not particularly limited, and various solvents suitable for secondary batteries can be used. For example, carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, lactones, glymes, esters, sulfoxides, and nitriles can be mentioned. These solvents may be used alone or in combination of two or more.

[0097] The secondary battery preferably contains a separator. Examples of the separator include, but are not particularly limited to, polyethylene non-woven fabric, polypropylene non-woven fabric, polyamide non-woven fabric, and those obtained by subjecting these to hydrophilic treatment.

[0098] The structure of the secondary battery is not particularly limited. Usually, it is composed of a positive electrode and a negative electrode, and a separator provided as needed, 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.

[0099] In this specification, the numerical range indicated by "~" means a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. In the numerical ranges described stepwise in this specification, the upper limit value or the lower limit value of a certain stepwise numerical range can be arbitrarily combined with the upper limit value or the lower limit value of another stepwise numerical range.

Examples

[0100] 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 it does not exceed the gist thereof.

[0101] <Measurement method of physical properties> The physical properties of carbon nanotubes (CNTs) were measured by the following method. Unless otherwise specified, the physical properties of CNTs were measured in the state after purification of CNTs.

[0102] <Total metal content of CNTs> A microwave sample pretreatment apparatus (ETHOS, manufactured by Milestone General Co., Ltd.) was used to decompose CNT by acid and extract the metals contained in CNT. The extracted metals were analyzed using a multi-type ICP emission spectrometer (720-ES, manufactured by Agilent), and the metal content in CNT was calculated. From the calculated metal contents, the total contents of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum were determined. The total contents of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum in CNT are expressed as the mass ratio (ppm) of the total mass of the extracted iron, copper, zinc, nickel, chromium, manganese, and molybdenum to the mass of CNT before metal extraction.

[0103] <G / D ratio of CNT> CNT was placed on a Raman microscope (XploRA, manufactured by Horiba, Ltd.), and measurements were performed using a laser wavelength of 532 nm. The measurement conditions were an acquisition time of 60 seconds, an integration number of 2 times, a neutral density filter of 10%, an objective lens magnification of 20 times, a confocal hole of 500, a slit width of 100 μm, and a measurement wavelength of 100 to 3000 cm -1 The CNT for measurement was aliquoted onto a slide glass and flattened using a spatula. Among the obtained peaks, the maximum peak intensity within the range of 1560 - 1600 cm -1 in the spectrum was defined as G, and the maximum peak intensity within the range of 1310 - 1350 cm -1 in the spectrum was defined as D. The ratio of G / D was calculated and used as the G / D ratio of CNT.

[0104] <Temperature of the exothermic peak of CNT> Using a thermogravimetric differential thermal analyzer (Tg-DTA 8122 Thermo plus EVO2, manufactured by Rigaku Corporation), with a sample mass of 1.0 mg, placed in an alumina pan container, heated from 25°C to 1000°C at a heating rate of 10°C / min in an air atmosphere. For the obtained DTA curve, the temperature at the peak apex in the temperature range from 200°C to 1000°C was defined as the temperature of the exothermic peak.

[0105] <Amount of surface oxygen of CNT> The surface oxygen content of the CNTs was measured using an X-ray photoelectron spectrometer (XPS, manufactured by Thermo Fisher Scientific, K-Alpha). After pelletizing the CNTs, the sample was fixed to the sample stage with double-sided tape for measurement. The carbon atoms and oxygen atoms on the surface of the CNT sample were detected by XPS. Here, the ratio (atm%) of oxygen atoms to carbon atoms was calculated as the surface oxygen content.

[0106] <Volume resistivity of CNT> Using a powder resistivity measuring 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, a powder probe unit (four-probe ring electrode, electrode spacing 5.0 mm, electrode radius 1.0 mm, sample radius 12.5 mm), and an applied voltage limiter of 90 V, the volume resistivity [Ω·cm] of the CNT powder under various pressures was measured. At a density of 1 g / cm 3 the values of the volume resistivity of the CNTs were evaluated.

[0107] (Measurement conditions) Excitation source: Monochromatic AlKα 15 kV × 10 mA Analysis size: 400 μm (shape is elliptical) Photoelectron extraction angle: 0° (perpendicular to the sample surface) Capture region ·Survey scan: 0 - 1,350 eV ·Narrow scan: C1s, O1s, N1s, Cl2p ·Pass Energy Survey scan: 200 eV Narrow scan: 50 eV

[0108] <Volume resistivity of the electrode> The volume resistivity of the electrode was evaluated by preparing a composite composition for the positive electrode using the prepared CNT dispersion liquid and the positive electrode active material, applying the composite composition for the positive electrode onto a PET (polyethylene terephthalate) foil, drying it to prepare a composite coating film, measuring the surface resistivity of the composite layer, and converting it into a volume resistivity. The CNT dispersion liquid used was prepared in the examples and comparative examples described below.

[0109] (Preparation of Positive Electrode Mixture Composition) Capacity 150cm 3 The CNT dispersion liquid, PVdF (polyvinylidene fluoride, Sоlef5130, Sоlvay, non-volatile content 100%) that had been dissolved in NMP (N-methyl-2-pyrrolidone) to a concentration of 8%, and NMC (S800, LiNi 0.8 Mn 0.1 Co 0.1 After adding 02, Kinwa Kogyo Co., Ltd., the mixture was stirred at 2,000 rpm for 30 seconds using a planetary centrifugal mixer (Thinki Thinker, ARE-310). The lumps were then broken down with a spatula, and the mixture was stirred at 2,000 rpm for 300 seconds using a planetary centrifugal mixer (Thinki Thinker, ARE-310) to obtain a positive electrode composite composition. The non-volatile content of the positive electrode composite composition was 78%. The non-volatile content ratio of NMC:CNT:PVdF in the non-volatile portion of the positive electrode composite composition was 98:0.5:1.5.

[0110] (Preparation of composite coating film) The positive electrode mixture composition was applied to the electrode using an applicator so that the weight per unit area was 20 mg / cm 2 After coating on a PET foil having a thickness of 100 μm so as to obtain the above composition, the composition was dried in an electric oven at 120° C.±5° C. for 30 minutes to prepare a composite coating film.

[0111] (Evaluation of volume resistivity of electrodes) The surface resistivity (Ω / □) of the composite layer of the prepared composite coating was measured using Mitsubishi Chemical Analytech's Loresta GP, MCP-T610. After measurement, the thickness of the composite layer was multiplied 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 value measured at three points in the electrode using a film thickness meter (NIKON, DIGIMICRO MH-15M).

[0112] In the examples and comparative examples described below, the following CNTs were used. 10B: Multi-walled carbon nanotube (JEIO, JENOTUBE10B) BT1001M: Multi-walled carbon nanotubes (LG Chem, BT1001M) 6A: Multi-walled carbon nanotubes (JEIO, JENOTUBE6A)

[0113] Example 1 Using an electronic balance (MSA225S100DI, manufactured by Sartorius), 50 g of 10B was weighed into a crucible and placed in a multipurpose high-temperature furnace (Hi-Multi 5000, manufactured by Fuji Denpa Kogyo Co., Ltd.). The temperature was raised to 1200°C at a rate of 20°C / min in a nitrogen atmosphere (N2) with a nitrogen flow rate of 1.5 L / min, and the material was fired at 1200°C for 3 hours, after which it was allowed to cool naturally until the temperature was below 50°C, obtaining fired 10B.

[0114] 10g of the baked 10B was weighed into a 1L glass container, and 500g of 10% hydrochloric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added, followed by thorough stirring using a stirrer. The mixture was then thoroughly diluted with ion-exchanged water and subjected to vacuum filtration using a membrane filter. After repeating the dilution and filtration process, the CNTs were transferred to a polytetrafluoroethylene (PTFE) tray. The mixture was dried at 140°C in an oven to obtain treated 10B.

[0115] The CNT dispersion was prepared by adding an NMP solution containing 7% hydrogenated nitrile butadiene rubber (Zetpole 2000L, manufactured by Zeon Corporation) and NMP to a stainless steel container so that the polymer was 0.6 parts by mass and the total amount of NMP was 96.4 parts by mass, and then adding 3.0 parts by mass of the treated 10B while stirring with a disperser. A high shear mixer (L5M-A, manufactured by Silverson) was fitted with a square hole high shear screen, and the mixture was dispersed in a batch manner at a speed of 8,600 rpm until the entire mixture became uniform and the dispersion particle size was 250 μm or less using a grind gauge. At this time, the dispersion particle size confirmed by the grind gauge was 180 μm. Next, the liquid to be dispersed was transferred from the stainless steel container to a bead mill (Star Mill LMZ, manufactured by Ashizawa Finetech) filled with zirconia beads with a diameter of 1.00 mm, and a CNT dispersion was obtained by performing a circulation dispersion process.

[0116] Example 2 Treated CNTs were obtained by carrying out the treatment in the same manner as in Example 1, except that the type of CNTs, the firing temperature, and the firing time shown in Table 1 were changed. The CNT dispersion was prepared by the method described in Example 1 to obtain a CNT dispersion.

[0117] (Example 3 and Example 4) Treated CNTs were obtained by carrying out the treatment in the same manner as in Example 1, except that the type of CNTs, the firing temperature, and the firing time shown in Table 1 were changed.

[0118] The CNT dispersion was prepared by adding an NMP solution containing 7% hydrogenated nitrile butadiene rubber (Zetpole 2000L, manufactured by Zeon Corporation) and NMP to a stainless steel container so that the polymer was 0.4 parts by mass and the total amount of NMP was 97.6 parts by mass, and then adding 2.0 parts by mass of the treated 10B while stirring with a disperser. A high shear mixer (L5M-A, manufactured by Silverson) was fitted with a square hole high shear screen, and the mixture was dispersed in a batch manner at a speed of 8,600 rpm until the entire mixture became uniform and the dispersion particle size was 250 μm or less using a grind gauge. At this time, the dispersion particle size confirmed by the grind gauge was 180 μm. Next, the liquid to be dispersed was transferred from the stainless steel container to a bead mill (Star Mill LMZ, manufactured by Ashizawa Finetech) filled with zirconia beads with a diameter of 1.00 mm, and a CNT dispersion was obtained by performing a circulation dispersion process.

[0119] Example 5 Treated CNTs were obtained by carrying out the treatment in the same manner as in Example 1, except that the type of CNTs, the firing temperature, and the firing time shown in Table 1 were changed.

[0120] The CNT dispersion was prepared by adding an NMP solution containing 7% hydrogenated nitrile butadiene rubber (Zetpole 2000L, manufactured by Zeon Corporation) and NMP to a stainless steel container so that the polymer was 0.4 parts by mass and the total amount of NMP was 97.6 parts by mass, taking 2.0 parts by mass of the treated 10B, adding it while stirring with a disperser, and dispersing it in a batch manner until the entire mixture became uniform at a speed of 8,600 rpm and the dispersion particle size was 250 μm or less using a grind gauge. At this time, the dispersion particle size confirmed by the grind gauge was 180 μm. Next, the liquid to be dispersed was transferred from the stainless steel container to a bead mill (Star Mill LMZ, manufactured by Ashizawa Finetech) filled with zirconia beads with a diameter of 1.00 mm, and a circulation dispersion process was performed. After confirming that the viscosity of the contents in the stainless steel container had decreased and that it had sufficient fluidity, the liquid to be dispersed was then fed into a high-pressure homogenizer (Starburst Lab HJP-17007, manufactured by Sugino Machine) and a circulation-type dispersion process was carried out to obtain a CNT dispersion. The dispersion process with the high-pressure homogenizer was carried out using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa.

[0121] Example 6 The treatment was carried out according to the method described in Example 1 to obtain treated CNTs. 10 parts of the powder and 200 parts of zirconia beads having a diameter of 2 mm were charged as grinding media, and dry-ground for 15 minutes in a Paint Conditioner (manufactured by Red Devil) to obtain dry-ground CNTs. The CNT dispersion was prepared by the method described in Example 1 to obtain a CNT dispersion.

[0122] (Comparative Example 1: Untreated) The sample of Comparative Example 1 was 10B that had not been subjected to any treatment. The CNT dispersion was prepared by adding an NMP solution containing 7% hydrogenated nitrile butadiene rubber (Zetpole 2000L, manufactured by Zeon Corporation) and NMP to a stainless steel container so that the polymer was 0.6 parts by mass and the total amount of NMP was 96.4 parts by mass, taking 3.0 parts by mass of untreated 10B, adding it while stirring with a disperser, attaching a square hole high shear screen to a high shear mixer (L5M-A, manufactured by Silverson), and performing batch dispersion at a speed of 8,600 rpm until the entire mixture became uniform and the dispersion particle size was 250 μm or less using a grind gauge. At this time, the dispersion particle size confirmed by the grind gauge was 180 μm. Next, the liquid to be dispersed was supplied from the stainless steel container to a high pressure homogenizer (Starburst Lab HJP-17007, manufactured by Sugino Machine Co., Ltd.) and a circulating dispersion process was performed to obtain a CNT dispersion. The dispersion treatment with the high-pressure homogenizer was carried out using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa.

[0123] (Comparative Example 2: Nitric Acid Treatment) Using an electronic balance (MSA225S100DI, manufactured by Sartorius), 10 g of 10B was weighed into an alumina crucible SSA-HB4 (manufactured by Nikkato Corporation) and placed in a muffle furnace (FO510, manufactured by Yamato Scientific Co., Ltd.). The temperature was raised to 330°C at a rate of 60°C / min in an air atmosphere, and then fired at 330°C for 18 hours to obtain oxidized 10B. 10g of the oxidized 10B was weighed into a 1L glass container, and 500g of 10% nitric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added. The mixture was heated to 90°C in a hot water bath and thoroughly stirred using a stirrer. The mixture was then thoroughly diluted with ion-exchanged water and filtered under reduced pressure using a membrane filter. After repeating the dilution and filtration process, the CNTs were transferred to a PTFE tray. After drying at 140°C in an oven, nitric acid-treated 10B was obtained. The CNT dispersion was prepared by the method described in Comparative Example 1, to obtain a CNT dispersion.

[0124] (Comparative Example 3: Halogen Purification) 10 kg of 6A was weighed into a 120 L heat-resistant container, and the heat-resistant container containing 6A was placed in a furnace. Nitrogen gas was then introduced into the furnace, and the air in the furnace was discharged while maintaining positive pressure. After the oxygen concentration in the furnace became 0.1 vol% or less, it was heated to 1500°C over 30 hours. While maintaining the temperature in the furnace at 1500°C, chlorine gas was introduced at a rate of 50 L / min for 100 hours. Nitrogen gas was then introduced at 50 L / min to cool the material while maintaining positive pressure, and halogen-purified 6A was obtained. The CNT dispersion was prepared by the method described in Example 3 to obtain a CNT dispersion.

[0125] The physical properties of the CNTs prepared in each of the examples and comparative examples are shown in Table 1. In the table, the "total metal content" is the total content of iron, copper, zinc, nickel, chromium, manganese, and molybdenum.

[0126] 1 shows the DTA curves of the CNTs prepared in Example 1 and Comparative Example 1. In this DTA curve, the vertical axis indicates thermoelectromotive force [μV], and the horizontal axis indicates temperature [°C]. From this figure, the exothermic peak temperature in Example 1 was 726°C, and the exothermic peak temperature in Comparative Example 1 was 520°C. Although not shown, the exothermic peak temperatures were determined in the same manner for the other Examples and Comparative Examples.

[0127] [Table 1]

[0128] The CNTs in each example are controlled in at least the G / D ratio, the total metal content, and the exothermic peak temperature, and have a lower volume resistivity than the comparative examples, which indicates that the CNT powder has high conductivity. It is clear that by producing an electrode film using this CNT powder, it is possible to provide a secondary battery with a low electrode volume resistivity and improved conductivity. Moreover, since the CNTs of each example have a low total metal content, it is understood that the safety of the electrode film and secondary battery using these CNTs can be improved.

Claims

1. A carbon nanotube dispersion liquid that satisfies the following (1) to (3), and contains carbon nanotubes including multi-walled carbon nanotubes, a dispersant, and a dispersion medium, The carbon nanotube dispersion liquid, wherein the dispersant comprises at least one selected from the group consisting of methyl cellulose, ethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, hydrogenated nitrile butadiene rubber, and polyacrylonitrile-based polymers. (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 total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is 5,000 ppm or less.

2. The BET specific surface area of ​​the carbon nanotubes is 150 m ２ / g or more 800m ２ 2. The carbon nanotube dispersion according to claim 1, wherein the average molecular weight of the carbon nanotubes is 1 / g or less.

3. The volume resistivity of the carbon nanotube is 1.0×10 －２ Ω・cm~2.5×10 －２ 2. The carbon nanotube dispersion liquid according to claim 1, wherein the viscosity is Ω·cm.

4. 2. The carbon nanotube dispersion according to claim 1, wherein the carbon nanotubes have an average outer diameter of 3 nm or more and 15 nm or less.

5. 5. The carbon nanotube dispersion liquid according to claim 1, wherein the dispersion medium is an amide-based organic solvent and the water content in the dispersion medium is 500 ppm or less.

6. The carbon nanotube dispersion according to any one of claims 1 to 4, further comprising an amine compound or an inorganic base.

7. 5. The method for producing a carbon nanotube dispersion liquid according to claim 1, comprising the following steps (1) and (2): (1) Dispersion process using a high shear mixer (2) Dispersion process using a media-type disperser

8. The method for producing a carbon nanotube dispersion liquid according to claim 7, further comprising the following steps (3) and (4): (3) A step of removing metallic foreign matter using magnetic force (4) A step of removing metallic foreign matter by filtration using a filter.

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