Carbon nanotubes, carbon nanotube dispersions, binder compositions, electrode compositions, and secondary batteries

Abstract

Provided is a carbon nanotube that can improve safety and is suitably used for forming an electrode film having good conductivity. Another embodiment provides a carbon nanotube dispersion, an electrode composition, and a secondary battery containing the carbon nanotube. 【Solution means】It is solved by a carbon nanotube that satisfies the following (1) to (5). (1) In the Raman spectrum, when the maximum peak intensity in the range of 1560 to 1600 cm -1 is G, and the maximum peak intensity in the range of 1310 to 1350 cm -1 is D, the G / D ratio is 16 or more and 38 or less. (2) The BET specific surface area is 150 m 2 / g or more and 450 m 2 / g or less. (3) The volume resistivity is 1.0×10 -3 to 9.9×10 -3 Ω·cm. (4) The iron content is 7000 ppm or less. (5) The sulfur content exceeds 0 ppm and is 1600 ppm or less.

Description

[Technical Field]

[0001] Embodiments of the present invention relate to carbon nanotubes, carbon nanotube dispersions, binder compositions, electrode compositions, and secondary batteries. [Background technology]

[0002] With the spread of electric vehicles and the miniaturization, weight reduction, and increased performance of portable devices, there is a growing demand for rechargeable batteries with high energy density, as well as higher capacity rechargeable batteries. Against this backdrop, lithium-ion rechargeable batteries, in particular, are increasingly being used in a wide range of devices.

[0003] Secondary batteries utilize various conductive additives, including carbon black, Ketjenblack, graphene, and fine carbon materials. Among these, carbon nanotubes (hereinafter also referred to as "CNTs"), a type of fibrous fine carbon material, are widely used. For example, by adding CNTs to the electrode active material, electrode resistance can be reduced, the battery's load resistance improved, the electrode material strength increased, and the electrode's resistance to expansion and contraction increased, thereby improving the rate characteristics and cycle life of secondary batteries. In particular, multi-walled CNTs with an outer diameter of 5 nm to several tens of nm are relatively inexpensive and are becoming widely used.

[0004] Carbon nanotubes (CNTs) can generally be manufactured by methods such as arc discharge, laser evaporation, and chemical vapor deposition (CVD). Of these, CVD is the most suitable for mass production from a productivity and economic standpoint and is widely used. In CVD, a catalyst containing metals such as iron, cobalt, and nickel is used to react with a gas that serves as a carbon source to produce CNTs. Therefore, CNTs obtained by CVD contain catalysts containing metals such as iron, cobalt, and nickel, sulfur as a co-catalyst, or particles of carbides or oxides derived from the catalyst. Metal-containing catalysts (hereinafter also referred to as catalyst metals) and sulfur as a co-catalyst are indispensable in several methods of CNT production. In particular, when producing CNTs by FCCVD, using sulfur as a co-catalyst allows for control of the catalyst particle size, enabling the production of finer CNTs, and also increases the CNT growth rate by lowering the temperature required for CNT synthesis. As a result, highly conductive CNTs can be produced (Non-Patent Literature 1). However, after CNT production, residual metals and sulfur derived from the catalyst metal can become impurities in the CNTs. Furthermore, regardless of the CNT manufacturing method, metal contamination may occur due to wear and tear of metals used in synthesis equipment, filling equipment, or piping during production.

[0005] For example, if carbon nanotubes (CNTs) containing metals derived from catalyst metals or manufacturing processes are used in secondary batteries, the metals may dissolve and precipitate, potentially causing short circuits in the battery. Short circuits can lead to serious accidents such as fire or explosion. Therefore, several methods have been proposed to purify CNTs and remove metals derived from catalyst metals, etc., in order to suppress problems caused by metals contained in CNTs and to further enhance safety.

[0006] Patent Document 1 describes that a carbon material production process is used to produce a carbon material containing CNTs by using a raw material containing at least carbon and a catalytic metal as an anode and applying an arc discharge method, and a halogen treatment process of bringing the carbon material into contact with a gas containing a halogen and / or a halogen compound. By purifying the CNTs through these processes, it is possible to remove the catalytic metal, which is an impurity, while suppressing damage or cutting of the CNTs.

[0007] Patent Document 2 states that when liquid-phase oxidation is performed with nitric acid on CNTs having an intensity ratio of the G band to the D band (G / D ratio) of 50 or more in Raman spectroscopic analysis, CNTs with higher quality, no catalytic residues, high heat resistance, and few carbon by-products can be obtained.

[0008] Patent Document 3 exemplifies that single-walled CNTs (Meijo Nano Carbon Co., Ltd.) with a G / D ratio of 100 or more were heat-treated under vacuum.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0010] However, while halogen treatment of CNTs can remove residual catalyst metals, it requires prolonged firing at high temperatures, which tends to reduce the BET specific surface area of ​​the CNTs after treatment. Because such CNTs are fused together and exist in clumps, they are difficult to disperse, requiring significant energy, and prone to breakage or damage during dispersion. In the fabrication of electrode films for secondary batteries, the presence of broken or damaged CNTs increases contact resistance between them, potentially reducing the conductivity of the electrode film and degrading the performance of the secondary battery. Furthermore, shortening the halogen treatment process not only fails to remove residual catalyst metals but also risks corroding metal components used in the manufacturing process due to residual halogens.

[0011] Furthermore, while liquid-phase oxidation of CNTs with nitric acid can reduce catalyst metal residue, the strong oxidizing power of nitric acid can cause oxidation of the CNT surface, potentially reducing the conductivity of the electrode film using CNTs.

[0012] As described above, it is difficult to reduce the amount of metal derived from catalyst metals, etc., contained in CNTs without degrading their properties. Furthermore, if a large amount of sulfur is used, such as when sulfur is used as a co-catalyst during CNT production, it can corrode metals used in piping, etc., and there is a possibility that metals may be introduced as impurities. However, there has been room for consideration regarding methods to reduce the sulfur content in CNTs. Since it is expected that the safety of batteries can be improved by using CNTs in which the amount of metal and sulfur is adjusted to a specific range, further improvements to CNTs are desired to improve the safety and performance of secondary batteries. In view of these circumstances, the present invention provides carbon nanotubes that can be suitably used to form electrode films that have improved safety and good conductivity. Other embodiments provide a carbon nanotube dispersion, an electrode composition, and a secondary battery, all containing the above-mentioned carbon nanotubes. [Means for solving the problem]

[0013] In order to solve the above problems, the inventors of this invention diligently conducted research and, as a result, discovered carbon nanotubes that satisfy the following conditions, and completed the present invention. That is, embodiments of the present invention include the following. However, the present invention is not limited to the embodiments described below.

[0014] <1> Carbon nanotubes that satisfy the following conditions (1) to (5). (1) In the Raman spectrum, 1560-1600 cm -1 G represents the maximum peak intensity within the range of 1310-1350 cm. -1 When the maximum peak intensity within the specified range is denoted as D, the G / D ratio is between 16 and 38. (2) BET specific surface area is 150m 2 / g or more 450m 2 It is less than / g. (3) Volume resistivity is 1.0 × 10 -3 ~9.9×10 -3 It is Ω·cm. (4) The iron content is 7000 ppm or less. (5) The sulfur content is greater than 0 ppm and 1600 ppm or less. <2> The above (6) that satisfies the following conditions <1> Carbon nanotubes as described above. (6) The wettability index is 19.0 or higher and 25.0 or lower, as shown by the following formula (I). Equation (I): Wetting index = (X / Y) [In equation (I), Y is the mass (g) of the carbon nanotube, and X is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotube when N-methyl-2-pyrrolidone is dropped onto Y(g) of carbon nanotube at 25°C.] <3> The above (7) that satisfies the following <1> or <2> Carbon nanotubes as described above. (7) The iron content is 6000 ppm or less, and the aluminum content is 500 ppm or less. <4> the above <1> ~ <3> A carbon nanotube dispersion comprising a carbon nanotube as described in any one of the following, a dispersant, and a dispersion medium. <5> It contains a carbon nanotube dispersion and a binder. The carbon nanotube dispersion is as described above. <1> ~ <3> A binder composition comprising carbon nanotubes as described in any one of the above, a dispersant, and a dispersion medium. <6> The mixture contains a carbon nanotube dispersion and an electrode active material. The carbon nanotube dispersion is as described above. <1> ~ <3> An electrode composition comprising carbon nanotubes as described in any one of the following, a dispersant, and a dispersion medium. <7> A secondary battery comprising an electrode film, wherein the electrode film is the above <1> ~ <3> A carbon nanotube dispersion containing a carbon nanotube as described in any one of the following, a dispersant, and a dispersion medium. A secondary battery obtained using a composition comprising the carbon nanotube dispersion and a binder, or an electrode composition comprising the carbon nanotube dispersion and an electrode active material. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide carbon nanotubes that can be suitably used to form electrode films that have improved safety and good conductivity. Furthermore, it is possible to provide carbon nanotube dispersions and electrode compositions that can be suitably used in secondary battery applications using these carbon nanotubes. Moreover, it is possible to provide secondary batteries that are highly safe and high performance. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1 is a photograph of the CNT(B) produced in Example 2, observed at 50,000x magnification using a scanning electron microscope (SEM). [Figure 2] Figure 2 is a photograph of the CNT(Z3) produced in Comparative Example 5, observed at 50,000x magnification using a scanning electron microscope (SEM). [Modes for carrying out the invention]

[0017] Hereinafter, embodiments of the present invention will be described in detail. However, it should be noted that the present invention is not limited to the embodiments described below, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

[0018] In this specification, a numerical range indicated by "~" represents 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 numerical range can be arbitrarily combined with the upper limit value or the lower limit value of other numerical ranges. In addition, the numerical values described in this specification refer to the values obtained by the method described in [Examples] described later.

[0019] <(1) Carbon nanotube (CNT) One embodiment of the present invention relates to a carbon nanotube (hereinafter also referred to as CNT). The CNT according to this embodiment satisfies the following (1) to (5). (1) In the Raman spectrum, when the maximum peak intensity in the range of 1560 to 1600 cm -1 is G and the maximum peak intensity in the range of 1310 to 1350 cm -1 is D, the G / D ratio is 16 or more and 38 or less. (2) The BET specific surface area is 150 m 2 / g or more and 450 m 2 / g or less. (3) The volume resistivity is 1.0×10 -3 ~9.9×10 -3 Ω·cm. (4) The iron content is 7000 ppm or less. (5) The sulfur content exceeds 0 ppm and is 1600 ppm or less.

[0020] The CNT according to this embodiment is characterized by satisfying all of the above requirements (1) to (5). That is, the CNT according to this embodiment has a specific G / D ratio, a specific BET specific surface area, a specific volume resistivity, a limited iron content, and further controls the amount of sulfur used as a promoter within a specific range, thereby suppressing the occurrence of problems caused by metals as impurities and forming an electrode film having excellent conductivity. A secondary battery containing such CNTs is excellent in durability and safety and can exhibit good performance. When the CNT contains multi-walled carbon nanotubes, it is preferable because it becomes easy to satisfy all of the above requirements (1) to (5). Hereinafter, each requirement will be described more specifically.

[0021] <G / D ratio> The G / D ratio (peak ratio of G-band and D-band) of the CNT of this embodiment is determined by Raman spectroscopy. Although there are various laser wavelengths used in Raman spectroscopy, in this embodiment, a wavelength of 532 nm is utilized. In the Raman spectrum, the Raman shift observed around 1,590 cm -1 is called the G-band derived from graphite, and the Raman shift observed around 1,350 cm -1 is called the D-band derived from amorphous carbon and defects in graphite. Since the wave number of Raman spectroscopy may vary depending on the measurement conditions, the wave numbers defined here are defined within wave number ±10 cm -1 . The higher the G / D ratio of the carbon nanotube, the higher the crystallinity. Also, when the carbon nanotube is fired at a high temperature, the G / D ratio tends to increase, and the longer the firing time, the higher the G / D ratio tends to be.

[0022] For the CNT, the maximum peak intensity within the range of 1560 to 1600 cm -1 in the Raman spectrum is G, and 1310 to 1350 cm -1When the maximum peak intensity within the range is defined as D, the G / D ratio is 16 or more and 38 or less. The G / D ratio is more preferably 20 or more and 35 or less, and even more preferably 25 or more and 34 or less. When the G / D ratio of the CNT exceeds the above range, the CNT becomes hard, so the CNT is likely to be damaged when dispersed, and the contact resistance may increase. On the other hand, when the G / D ratio of the CNT is below the above range, the conductivity of the CNT itself tends to be low. Due to these, when the G / D ratio of the CNT is within the above range, in a secondary battery using an electrode film using a CNT dispersion, the decomposition of the electrolyte is suppressed, and the storage characteristics of the secondary battery are improved.

[0023] Conventionally, when the raw material CNT is purified by subjecting it to a halogen treatment process, the BET specific surface area of the CNT tends to decrease because the CNT is calcined at a high temperature for a long time. This state can also be confirmed from the fact that the volume resistivity increases. On the other hand, according to this embodiment, in the purification process of the CNT, after making an inert atmosphere, heat treatment of the CNT can be carried out under reduced pressure vacuum. Therefore, the heat treatment temperature can be kept low and heat treatment for a short time is possible, and a decrease in the BET specific surface area of the CNT can be suppressed. That is, an increase in the volume resistivity of the CNT can be suppressed. When using the CNT of this embodiment, in the process of manufacturing an electrode film, breakage of the CNT can be suppressed by dispersion treatment or the like, and an increase in the contact resistance between CNTs in the obtained electrode film can be suppressed. As a result, the CNT of this embodiment can obtain good conductivity as an electrode film, and the secondary battery containing the CNT of this embodiment can exhibit good performance.

[0024] <BET specific surface area> The BET specific surface area of the CNT of this embodiment is 150 m 2 / g or more and 450 m 2 / g or less. The above BET specific surface area is preferably 200 m 2 / g or more and 450 m 2 / g or less, more preferably 300 m 2 / g or more and 400 m 2 / g or less, and even more preferably 300 m 2 / g or more and 380 m2 It is even more preferable that the amount be less than or equal to / g. The BET specific surface area of ​​a carbon nanotube (CNT) tends to correlate with the average outer diameter of the CNT and the structure of the secondary aggregate (e.g., bundle structure). A smaller BET specific surface area of ​​a CNT corresponds to a larger outer diameter and fewer CNTs per unit mass. Conversely, a larger BET specific surface area of ​​a CNT corresponds to a smaller outer diameter and more CNTs per unit mass. 2 When the specific surface area of ​​the CNTs is 450 m² or more, the number of CNTs per unit 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 BET specific surface area of ​​the CNTs is less than or equal to / g, the dispersion of CNTs is good, and a good conductive network can be formed in the electrode film. If the BET specific surface area of ​​the CNTs exceeds the above range, the surface area of ​​the electrodes of the secondary battery will increase, which may accelerate the decomposition reaction of the electrolyte when the secondary battery is stored at high temperatures, potentially degrading the storage characteristics.

[0025] <Volume resistivity> The volume resistivity of the CNT in this embodiment is 1.0 × 10⁻⁶. -3 ~9.9×10 -3 It is Ω·cm, and 1.0 × 10⁻⁶ -3 ~8.4×10 -3 It is more 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 resistance measuring device (Rolestar GP Powder Resistivity Measurement System MCP-PD-51, manufactured by Nitto Seikou Analytech Co., Ltd.).

[0026] <Metal content> From a safety standpoint, it is desirable that the amount of impurities such as metals remaining in refined CNTs be low. Here, the metals contained in CNTs mainly consist of metals and metal oxides derived from the catalyst metals used during CNT production, but also include metals that are introduced during CNT production. For example, metals such as stainless steel used in synthesis equipment, filling equipment, or piping may be introduced into the CNTs due to wear or other factors.

[0027] The metal content is calculated based on the mass of the metal when converted to its elemental form. CNTs may contain metals as elemental metals, metal oxides, or metal composite oxides; the metal content is determined by converting these to their elemental form. In CNTs, iron, cobalt, aluminum, magnesium, copper, zinc, nickel, chromium, manganese, and molybdenum may be present as elemental metals, metal oxides, or composite oxides thereof, respectively. Since these metals can cause short circuits, it is desirable to reduce their respective content.

[0028] In this embodiment, as will be described later, by firing the CNTs under reduced pressure and vacuum after placing them in an inert atmosphere, the amount of metals such as iron, cobalt, aluminum, and magnesium contained in the CNTs can be reduced, and CNTs with improved dispersibility and safety can be obtained. A secondary battery containing such CNTs can exhibit good performance.

[0029] The metal content of carbon nanotubes (CNTs) can be calculated, for example, by acid-decomposing the CNTs, extracting the metals contained within them, and analyzing the extracts using inductively coupled plasma (ICP). The metal content of the CNTs is expressed as the mass ratio (ppm) of the extracted metal to the mass of the CNTs before metal extraction.

[0030] [Iron content] The iron content of the CNTs in this embodiment is 7000 ppm or less. Preferably, the iron content is 6000 ppm or less, more preferably 5000 ppm or less, even more preferably 3500 ppm or less, and may be 1500 ppm or less. Having the iron content in the CNTs within the above range makes it possible to achieve excellent storage stability of the battery.

[0031] [Cobalt content] The cobalt content of the CNTs in this embodiment is preferably 1000 ppm or less, more preferably 500 ppm or less, even more preferably 120 ppm or less, and may be 80 ppm or less. When cobalt is included during the synthesis of CNTs, there is a risk that the cobalt may leach out inside the secondary battery, degrading the storage characteristics of the secondary battery.

[0032] [Aluminum content] The aluminum content of the CNTs in this embodiment is preferably 500 ppm or less, more preferably 300 ppm or less, even more preferably 100 ppm or less, and may be 50 ppm or less. Preferably, the iron content is 6000 ppm or less, and the aluminum content is 500 ppm or less. When aluminum oxide is included as a catalyst support during the synthesis of carbon nanotubes (CNTs), insulating aluminum oxide nanoparticles may remain in the CNTs, potentially impairing conductivity. If the aluminum content in the CNTs is within the above range, the amount of insulating aluminum oxide nanoparticles is low, and the resulting carbon nanotubes can form electrode films with good conductivity.

[0033] [Magnesium content] The magnesium content of the CNTs in this embodiment is preferably 500 ppm or less, more preferably 20 ppm or less, and even more preferably 5 ppm or less. When magnesium oxide is included as a catalyst support during the synthesis of carbon nanotubes (CNTs), magnesium oxide nanoparticles, which are an insulating component, may remain in the CNTs, potentially impairing conductivity. If the magnesium content in the CNTs is within the above range, the amount of magnesium oxide nanoparticles, which are the insulating component, is low, and the resulting carbon nanotubes can form electrode films with good conductivity.

[0034] <Sulfur content> The sulfur content of the CNTs in this embodiment is greater than 0 ppm and less than or equal to 1600 ppm. Preferably, the sulfur content is 500 ppm or less, more preferably 200 ppm or less, and even more preferably 175 ppm or less. The sulfur contained in the CNTs in this embodiment often originates from the catalyst used during CNT synthesis. If a large amount of sulfur, which acts as a co-catalyst, is included during production, there is a risk of an increase in the number of CNT layers or a shortening of the CNT fiber length. Therefore, it is desirable that the CNTs do not contain a large amount of sulfur, which acts as a de-catalyst, during production. In addition, if the CNTs contain more sulfur than the above range, there is a risk of poor dispersibility. The refining of carbon nanotubes (CNTs) can reduce not only the amount of metals they contain, but also the amount of sulfur. The reason is not entirely clear, but it is speculated to be as follows: When sulfur is used as a co-catalyst, the sulfur strongly adsorbs to iron and coats its surface, forming an Fe-SC alloy or an FeS-Fe eutectic. As in the CNT of this embodiment, by including a process of heat treatment under reduced pressure and vacuum in an inert atmosphere as a raw material, it is possible to efficiently remove the catalyst in the form of an alloy or eutectic, and thus reduce both the iron and sulfur content contained in the CNT.

[0035] Within the above range, CNT dispersion is good, and a good conductive network can be formed in the electrode film. If the sulfur content is higher than the above range, the molecular weight of the dispersion resin and binder may decrease, which may reduce the adhesion of the electrode and the battery characteristics. In addition, corrosion of metal parts used during dispersion may cause metal foreign matter to be mixed into the CNT dispersion.

[0036] The sulfur content in CNTs can be determined, for example, in conjunction with the measurement of the metal content mentioned above. CNT powder is acid-decomposed using a microwave sample pretreatment device (Milestone General, ETHOS1) to extract the sulfur contained in the CNTs. Subsequently, analysis is performed using a multi-type ICP emission spectrometer (Agilant, 720-ES) to calculate the sulfur content in the extract, thereby determining the sulfur content of the CNT powder.

[0037] <Wetness Index> The wetting index is the ratio of the maximum mass of solvent absorbed by CNTs to the mass of CNTs, and is one of the indicators of the dispersibility of CNTs, particularly the initial viscosity when preparing a dispersion containing a dispersion medium and CNTs. In this embodiment, the wetting index of the CNTs, as shown by the following formula (I), is preferably 10.0 to 25.0, more preferably 17.0 to 25.0, and even more preferably 19.0 to 25.0. When the wetting index is within the above range, it is easy to obtain a suitable initial viscosity when preparing a dispersion containing a dispersion medium and CNTs. If the wetting index exceeds 25.0, the initial viscosity during dispersion preparation tends to increase, reducing ease of preparation and making dispersion failures that make stirring with a stirrer and pumping difficult. Also, if the initial viscosity is high, the conductivity of the CNTs tends to decrease during the dispersion treatment. On the other hand, when purification treatment by calcination of CNTs is performed, the wetting index tends to decrease. In that case, the ease of mixing during dispersion improves, but if it is excessively fired, as shown in Figure 2, the CNTs fuse together, making it difficult to dissolve the CNTs during dispersion, and the dispersed particle size tends to become larger. In addition, it tends to result in poor stability during the preparation of the asphalt slurry. Equation (I): Wetting index = (X / Y)

[0038] In equation (I), Y is the mass (g) of carbon nanotubes (CNTs), and X is the maximum mass (g) of N-methyl-2-pyrrolidone (NMP) absorbed by CNTs when NMP is added dropwise to Y(g) of CNTs at 25°C. The maximum mass (g) of NMP absorbed by CNTs is the total mass of NMP added to the CNT powder until just before it flows out of the CNT powder.

[0039] The wetting index can be determined, for example, by following the procedure below. First, at 25°C, Y(g) of CNT powder is collected in a container by gravity. With the container standing still, 5g of NMP is dropped onto the surface of the CNT powder inside the container at 1-minute intervals. Next, it is observed whether the NMP droplets are absorbed by the CNT powder and begin to flow out onto the surface of the CNT powder. Let X(g) be the total mass (g) of NMP dropped just before the droplets begin to flow out onto the surface of the CNT powder. From the values ​​of X(g) and Y(g) obtained in this way, the wetting index shown by equation (I) is calculated.

[0040] <Fever peak> From a safety standpoint, it is preferable that the CNTs of this embodiment have an exothermic peak between 600°C and 800°C 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 differential thermal analysis of the CNTs in an atmospheric environment. DTA is a method of measuring the temperature of a sample and a reference substance as a function of temperature while changing the temperatures of the sample and the reference substance under constant conditions, and conforms to JIS K 0129. In the DTA curve created based on the change in the temperature difference between the sample and the reference substance, the largest peak is defined as the exothermic peak.

[0041] Heat generation occurs during the combustion of carbon nanotubes (CNTs). As the combustion initiation temperature of CNTs increases, the peak heat generation temperature also increases. Factors influencing the combustion initiation temperature of CNTs include the content of catalytic metals, the degree of oxidation of the CNT surface, and the crystallinity of the CNTs. If the catalytic metals contained in the CNTs have high heat storage capacity, a low catalytic metal content will result in a smaller total heat storage capacity across all catalytic metals. Due to the smaller total heat storage capacity across the catalytic metals, the temperature required to burn the CNTs may be higher compared to cases with a high catalytic metal content. In addition to catalytic metals, other metals may be introduced into the CNTs during the CNT manufacturing process, and these metals can also affect the total heat storage capacity. Furthermore, since the parts of the CNT surface that have oxygen-containing functional groups are more easily combusted than the parts that do not have functional groups, the less oxygen-containing functional groups there are (i.e., the less surface oxygen there is), the less easily the CNT burns, and the lower the degree of oxidation on the CNT surface, the higher the temperature required to burn the CNT. In addition, the higher the crystallinity of the CNT, the higher the combustion start temperature of the CNT. The crystallinity of the CNT can be expressed by the G / D ratio mentioned above.

[0042] If the combustion start temperature of the CNTs is within an appropriate temperature range, the amount of impurities contained in the CNTs will be reduced, thus enabling the production of safer CNTs. In this embodiment, the CNTs preferably have an exothermic peak temperature of 600°C or higher, and more preferably 650°C or higher. Furthermore, the exothermic peak temperature is preferably 800°C or lower, and more preferably 740°C or lower. If the exothermic peak temperature 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, resulting in excellent conductivity. If the exothermic peak temperature is 800°C or lower, the crystallinity of the CNTs is not too high, which can suppress CNT breakage and prevent a decrease in the performance of the secondary battery. For example, it is preferable that the temperature is 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.

[0043] When the powder is the one before the CNTs are dispersed, the exothermic peak can be measured as it is. Also, when the CNTs are present in the CNT dispersion liquid, after removing the dispersion medium by heating and drying, the measurement can be carried out, and it can be specified from the shape of the exothermic peak. The temperature of heating and drying is preferably carried out at a temperature at which the CNTs are not oxidized (for example, 140 °C or lower). When the CNT dispersion liquid contains components other than CNTs and the dispersion medium (additives, etc.), the exothermic peak of the additive is measured in advance, and by specifying the exothermic peak derived from the additive, the remaining exothermic peak may be determined to be derived from CNTs and the exothermic peak may be specified.

[0044] <Other properties of CNT> 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. The single-walled CNTs have a structure in which a single layer of graphite is wound into a cylindrical shape. The multi-walled 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 contain single-walled CNTs and are preferably multi-walled CNTs. Alternatively, the CNTs may be a mixture of single-walled CNTs and multi-walled CNTs, but even in such a case, CNTs in which the multi-walled CNTs account for 50% by mass or more are preferred, and CNTs in which the multi-walled CNTs account for 70% by mass or more are more preferred. The multi-walled CNTs in the CNTs may be 100% by mass. By using such CNTs, it becomes easy to set the G / D ratio within a suitable range, for example, 16 or more and 38 or less. Also, the sidewalls of the CNTs do not have to be a graphite structure. For example, CNTs having sidewalls with an amorphous structure can also be used as the CNTs.

[0045] The CNTs of the present embodiment preferably contain multi-walled CNTs, preferably contain CNTs with the number of layers of the CNTs being 3 or more and 10 or less, and more preferably contain CNTs with the number of layers of 3 or more and 7 or less. The number of layers of the CNTs can be confirmed by TEM observation of the CNTs. For example, after performing a dispersion treatment on the CNTs in a solvent using an ultrasonic disperser, by using a sample dried on a microgrid and performing TEM observation at a magnification of 10 million times, it can be confirmed.

[0046] The purity of CNTs is expressed as the mass of the CNTs minus the ash content (mass%). The ash content (mass%) of CNTs can be measured, for example, in accordance with JIS K 6218-2. The ash content of CNTs is a non-combustible component containing metals, etc. From the viewpoint of conductivity, the purity of CNTs is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more, based on the mass of the CNTs. Furthermore, the non-combustible component contained in the CNTs is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less.

[0047] In this embodiment, the average outer diameter of the CNTs is preferably 3 nm or more, and more preferably 4 nm or more. Furthermore, the average outer diameter of the CNTs is preferably 10 nm or less, more preferably 8 nm or less, and even more preferably 6 nm or less. When the average outer diameter of the CNTs is 10 nm or less, the number of CNTs per unit 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 CNTs is good, and a good conductive network can be formed in the electrode film.

[0048] The standard deviation of the average outer diameter of the CNTs is preferably between 1 nm and 4 nm, and more preferably between 1 nm and 3 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 aggregate by becoming entangled with each other in the CNT dispersion or composite slurry and / or electrode film, preventing the formation of a good conductive network.

[0049] The outer diameter and average outer diameter of CNTs are determined as follows: First, CNTs are observed and imaged using a transmission electron microscope. Next, 100 CNTs are randomly selected from the observed images, and their outer diameters are measured. Then, the average outer diameter (nm) of the CNTs is calculated as the number average of the outer diameters.

[0050] Carbon nanotubes (CNTs) usually exist as aggregates. This shape can be, for example, a state in which individual CNTs are intricately intertwined (tangled), or an aggregate of straight CNTs (bundle). Bundle-shaped CNT aggregates are easier to unravel than tangled CNT aggregates. Furthermore, bundle-shaped CNT aggregates have better dispersibility than tangled CNT aggregates, making them suitable for use as CNTs.

[0051] <Method for manufacturing carbon nanotubes (CNTs)> The carbon nanotubes (CNTs) of this embodiment can be produced by, for example, laser ablation, arc discharge, thermal CVD, plasma CVD, and combustion. However, they are not limited to these methods. For example, CNTs can be produced by contacting a carbon source with a catalytic metal at 500 to 1200°C in an atmosphere with an oxygen concentration of 1 volume% or less. The carbon source may be at least one of hydrocarbons and alcohols.

[0052] Any conventionally known raw material gas can be used as the carbon source for CNTs. For example, hydrocarbons such as methane, ethylene, propane, butane, and acetylene, carbon monoxide, and alcohols can be used as carbon-containing raw material gases, but are not limited to these. Particularly from the viewpoint of ease of use, it is preferable to use at least one of hydrocarbons and alcohols as the raw material gas.

[0053] <Method for purifying carbon nanotubes (CNTs)> The following describes a method for purifying carbon nanotubes (CNTs). While the CNTs in this embodiment are not limited to those produced through the purification method described below, the purification method described below makes it easy to obtain the CNTs of this embodiment.

[0054] Typical catalyst metals used in the production of carbon nanotubes (CNTs) by chemical vapor deposition (thermal CVD) include catalyst metals in which active components such as iron, cobalt, and nickel are immobilized on supporting components such as aluminum and magnesium, and catalyst metals obtained by dissolving raw materials such as ferrocene (iron source) and thiophene (sulfur source) in benzene or toluene solution and vaporizing them. Therefore, CNTs obtained by the above production method tend to retain metals such as iron, cobalt, nickel, aluminum, and magnesium, as well as sulfur, derived from the catalyst metal. One example of a method for purifying CNTs is treatment with an acid such as nitric acid, and it is known that iron, cobalt, and nickel can be removed by acid treatment. However, it is difficult to remove metals such as aluminum and magnesium, as well as sulfur, by acid treatment.

[0055] In contrast, the CNT purification method of this embodiment includes a step of heat-treating the CNTs used as raw materials in an inert atmosphere under reduced pressure and vacuum. According to the CNT purification method of this embodiment, the content of metals such as aluminum and magnesium in the CNTs can be easily reduced. The CNT purification method of this embodiment can also reduce the content of other metals such as sulfur, iron, and cobalt in the CNTs, not just aluminum and magnesium.

[0056] In the purification method of the above embodiment, examples of inert atmospheres include a nitrogen atmosphere, an argon atmosphere, a vacuum atmosphere, and a combination thereof. For example, an inert atmosphere can be obtained by introducing an inert gas such as nitrogen gas into the apparatus used during heat treatment and replacing the inside of the apparatus with the inert gas. Subsequently, the heat treatment is carried out while maintaining a vacuum state (hereinafter referred to as reduced vacuum) by reducing the pressure inside the apparatus.

[0057] In this specification, "reduced pressure vacuum" means a state in which the pressure is lower than that of the atmosphere (a vacuum state) maintained by a pressure reducing pump. Specifically, a pressure reducing pump such as an oil rotary pump, booster pump, Roots pump, or oil diffusion pump is used to reduce the pressure inside the device and maintain a vacuum state. One or more pressure reducing pumps may be used. Reducing the pressure inside the device counteracts pressure increase factors such as leaks from the device or gas expansion due to heating, and means controlling the air pressure inside the device (internal pressure) to be below the target pressure.

[0058] The internal pressure in a reduced-pressure vacuum is preferably 10 Pa or less, more preferably 0.1 Pa or less, and even more preferably 0.05 Pa or less. In some embodiments, the internal pressure may be 0.03 Pa or less. To obtain the internal pressure within the above range, it is preferable to reduce the pressure in stages. In one embodiment, it is preferable to reduce the pressure in two stages. For example, in the first stage, the internal pressure is preferably adjusted to 10 Pa or less, more preferably 9.8 Pa or less, and even more preferably 9.6 Pa or less. In the first stage, the internal pressure may be in the range of 9.5 to 9.8 Pa. After adjusting the internal pressure to the above range and maintaining it for a certain period of time, in the second stage, the internal pressure can be adjusted to preferably 1 Pa or less, more preferably 0.1 Pa or less, and even more preferably 0.05 Pa or less. In some embodiments, the internal pressure in the second stage may be 0.03 Pa or less. The internal pressure of CNTs increases due to the pyrolysis gases and sublimation of metals, making it difficult to gradually reduce the internal pressure. In such cases, various methods can be applied to maintain the desired reduced vacuum state, rather than being limited to the two-stage reduction method described earlier. For example, a method of performing only the first stage of reduction may be applied, or a method of performing reduction by adding a third stage after the second stage of reduction may be applied. For example, in the third stage of reduction, the internal pressure is preferably adjusted to 10 Pa or less, more preferably 9.8 Pa or less, and even more preferably 9.6 Pa or less. The internal pressure may be adjusted through three or more stages in this way, and the number of stages is not limited. In some embodiments, the number of stages may be six or more, or even seven or more.

[0059] The heat treatment conditions, such as the heat treatment temperature and heat treatment time, can be appropriately set depending on the type of CNT and the type of metal derived from the catalyst metal contained in the CNT. The heat treatment temperature is preferably the temperature at which sulfur and all metals—iron, cobalt, aluminum, magnesium, copper, zinc, nickel, chromium, manganese, and molybdenum—begin to melt. Furthermore, since the catalyst metal used in the production of CNTs is at the nanoscale, and due to the nanoscale effect, its melting point is lower than that of bulk metals, a heat treatment temperature lower than that of bulk metals may also be acceptable. From this viewpoint, the heat treatment temperature is preferably 1000°C or higher, more preferably 1500°C or higher, and even more preferably 1600°C or higher. It may also be 2000°C or lower, but from the viewpoint of suppressing the fusion of CNTs due to heat treatment, it may be 1900°C or lower, or 1800°C or lower. In some embodiments, the heat treatment temperature is preferably 1800°C or lower, and may also be 1750°C or lower. Note that the heat treatment temperature refers to the temperature inside the apparatus (internal temperature).

[0060] Furthermore, the heat treatment time can be set appropriately according to the firing apparatus and firing scale. However, firing CNTs at high temperatures for a long time reduces the BET specific surface area of ​​the CNTs. CNTs whose BET specific surface area has decreased due to heat treatment tend to fuse together, resulting in a higher volume resistivity, making it difficult to obtain a CNT dispersion with excellent conductivity and dispersibility. From this viewpoint, in some embodiments, the heat treatment time may be, for example, 10 hours or less, 8 hours or less, or 6 hours or less. In some embodiments, the heat treatment time may be 1 to 3 hours.

[0061] In some embodiments, heat treatment under reduced pressure and vacuum is preferably carried out by adjusting the temperature inside the apparatus (internal temperature) to a range of 500 to 2000°C and maintaining that temperature for 1 to 100 hours. In the above heat treatment, the internal temperature may more preferably be 900 to 1800°C, and even more preferably 1400 to 1750°C. The holding time may more preferably be 1 to 50 hours, and even more preferably 2 to 12 hours.

[0062] Normally, when CNTs are purified by heat treatment, the process is carried out in an atmospheric environment by heating to over 500°C to induce oxidation and combustion. In contrast, the heat treatment in this embodiment is carried out under reduced pressure and vacuum after creating an inert atmosphere. Therefore, combustion of the CNTs themselves can be suppressed. Furthermore, since the CNTs are heat-treated in an inert atmosphere, oxidation of the CNT surface can be suppressed. In addition, the heat treatment time can be shortened, in which case the increase in the volume resistivity of the CNTs can be further suppressed.

[0063] From the viewpoint of further reducing the metals contained in the CNTs, the purification method of this embodiment may be performed two or more times as needed. In addition, other processing steps may be added as needed, provided that they do not degrade the properties of the CNTs.

[0064] <Dry grinding of carbon nanotubes (CNTs)> The CNTs in this embodiment may be CNTs that have been dry-milled from the viewpoint of crushing the particles and improving dispersibility. Dry milling refers to a process of pulverizing CNTs without the interposition of a liquid substance. Dry milling may be media milling, milling without media, or a combination of two or more dry milling methods. For example, in media milling, by using a milling machine that incorporates milling media such as beads or steel balls, the particles are pulverized by utilizing the milling force and destructive force caused by the collision of the milling media with each other. Known methods such as dry attritors, ball mills, vibratory mills, and bead mills can be used as dry milling devices, and the milling time can be arbitrarily set depending on the device or the state of particle milling. It is preferable to perform a grinding treatment before purifying the CNTs because it reduces the particle size of the CNTs and improves the metal removal effect.

[0065] <2> Carbon nanotube (CNT) dispersion One embodiment of the present invention relates to a CNT dispersion. The CNT dispersion according to this embodiment includes the CNTs of the above embodiment, a dispersant, and a dispersion medium. The CNT dispersion in this specification does not contain an electrode active material. Furthermore, according to this embodiment, a method for producing a CNT dispersion containing the CNTs of the above embodiment, a dispersant, and a dispersion medium can be provided.

[0066] The wet particle size of the CNT dispersion in this embodiment is the 90% particle size (D 90 The particle size (D) of the CNT dispersion is preferably 1 μm or more and less than 20 μm, and more preferably 1 μm or more and less than 10 μm. 90 When the above range is present, the viscosity stability of the electrode composition is good, the active material and CNTs are easily compounded, and the viscosity stability of the electrode composition and the volume resistivity of the electrode tend to be good. 90% particle size (D) of CNT dispersion 90 ) can be determined using a laser diffraction / scattering particle size distribution analyzer. Specifically, for example, measurements are taken using a particle size distribution analyzer (Partical LA-960V2, manufactured by HORIBA). The operating conditions for circulation / ultrasonics are: circulation speed: 3, ultrasonic intensity: 7, ultrasonic duration: 1 minute, stirring speed: 1, and stirring mode: continuous. During air removal, ultrasonic operation is performed at ultrasonic intensity 7 and ultrasonic duration 5 seconds. The refractive index of NMP is assumed to be 1.468, the refractive index of water is 1.333, and the refractive index of carbon material is 1.92. Measurements are performed after diluting the sample to be measured so that the transmittance of the red laser diode is 30-80%, and the particle size standard is volume-based.

[0067] In this embodiment, the phase angle of the CNT dispersion is preferably greater than 5°, more preferably greater than 10°, and even more preferably greater than 20°. When the phase angle of the CNT dispersion is within the above range, the dispersibility of the CNTs is good, and the storage characteristics of the battery tend to be good. The phase angle of the CNT dispersion was measured using a viscoelasticity measuring device (Anton Paar "MCR 102e") with a 2° cone plate, 25 mm diameter, a measurement temperature of 25°C, and a frequency of 1 Hz, within a strain range of 0.01% to 5%, and the value at a strain of 1% was defined as the phase angle.

[0068] <Dispersant> The dispersant is not particularly limited as long as it can disperse and stabilize the CNTs, and for example, surfactants and resin-type dispersants can be used. Surfactants are mainly classified into anionic, cationic, nonionic, and amphoteric. Depending on the properties required for the dispersion of CNTs, a suitable type of dispersant can be used in a suitable amount.

[0069] Examples of resin-type dispersants include cellulose derivatives (cellulose acetate, cellulose acetate butyrate, cellulose butyrate, cyanoethylcellulose, ethyl hydroxyethylcellulose, nitrocellulose, methylcellulose, ethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, carboxymethylcellulose, etc.), polyvinyl alcohol, polyvinyl butyral, polyvinylpyrrolidone, hydrogenated nitrile butadiene rubber, and polyacrylonitrile polymers. Methylcellulose, ethylcellulose, carboxymethylcellulose, polyvinyl alcohol, polyvinyl butyral, polyvinylpyrrolidone, hydrogenated nitrile butadiene rubber, and polyacrylonitrile polymers are particularly preferred. The molecular weight of the resin-type dispersant is preferably 10,000 to 300,000, and more preferably 10,000 to 150,000.

[0070] Furthermore, it is preferable to add an amine compound or an inorganic base in addition to the dispersant. As amine compounds, primary amines, secondary amines, and tertiary amines can be used, but ammonia and quaternary ammonium compounds are not included. In addition to monoamines, amine compounds such as diamines, triamines, and tetramines, which have multiple amino groups in their molecules, can also be used. Specifically, examples include, but are not limited to, primary aliphatic amines such as methylamine, ethylamine, butylamine, and octylamine; secondary aliphatic amines such as dimethylamine, diethylamine, and dibutylamine; tertiary aliphatic amines such as trimethylamine, triethylamine, and dimethyloctylamine; amino acids such as alanine, methionine, proline, serine, asparagine, glutamine, lysine, arginine, histidine, aspartic acid, glutamic acid, and cysteine; alkanolamines such as dimethylaminoethanol, monoethanolamine, diethanolamine, methylethanolamine, and triethanolamine; and alicyclic nitrogen-containing heterocyclic compounds such as hexamethylenetetramine, morpholine, and piperidine. Examples of inorganic bases include alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates, alkali metal phosphates, and alkaline earth metal phosphates.

[0071] <Dispersion medium> The dispersion medium is not particularly limited as long as it can disperse CNTs, but it is preferable that it consists of one or more of water and water-soluble organic solvents.

[0072] Examples of water-soluble organic solvents include alcohols, polyhydric alcohols, polyhydric alcohol ethers, amines, amides (such as N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, and N-methylcaprolactam); heterocyclic, sulfoxide, sulfone, lower ketone, and others such as tetrahydrofuran, urea, and acetonitrile. Among these, water or amide-based organic solvents are more preferred, and among amide-based organic solvents, N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone are particularly preferred.

[0073] When using only amide-based organic solvents 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.

[0074] A CNT dispersion can be produced, for example, by dispersing CNTs in a dispersion medium. The raw materials used may be added in one or more stages at any point during the dispersion process. The dispersion method for such a process is not particularly limited.

[0075] As for the dispersion method, examples include using various dispersers such as dispersers, homogenizers, high-shear mixers, kneaders, two-roll mills, three-roll mills, ball mills, horizontal sand mills, vertical sand mills, annular bead mills, paint conditioners, attritors, planetary mixers, or high-pressure homogenizers. There are no particular restrictions on the disperser, but for example, from the viewpoint of preparing the CNT dispersion so that the fiber length of the CNTs in the dispersion is within a desirable range, it is preferable to use a high-pressure homogenizer; from the viewpoint of promoting wetting of the CNTs and breaking down coarse particles and agglomerations, it is preferable to use a high-shear mixer; and from the viewpoint of crushing agglomerated and solidified particles, it is preferable to use a media-type disperser such as a bead mill. Furthermore, it is more preferable to select and combine multiple of the above dispersers for dispersion, and the order of the dispersers can be changed arbitrarily. The pressure when using a high-pressure homogenizer is not particularly limited, but for example, it is preferably 40 to 150 MPa, and more preferably 40 to 120 MPa.

[0076] Dispersion methods using a dispersion device include batch dispersion, pass dispersion, and circulating dispersion. Any of these methods may be used, or two or more methods may be combined. Batch dispersion is a method of dispersion using only the dispersion device itself, without the use of piping. Because it is easy to handle, it is preferable for small-scale production. Pass dispersion is a dispersion method in which the dispersion device is equipped with a tank to supply the liquid to be dispersed via piping and a tank to receive the liquid to be dispersed, and the liquid is dispersed by passing it through the dispersion device. Circulating dispersion is a method in which the liquid to be dispersed, after passing through the dispersion device, is returned to the tank to supply the liquid and dispersed while being circulated. In all cases, dispersion progresses as the processing time increases, so it is sufficient to repeat the pass or circulation until the desired dispersion state is achieved, and the processing volume can be increased by changing the size of the tanks or the processing time. Pass dispersion is preferable to circulating dispersion because it is easier to achieve a uniform dispersion state. Circulating dispersion is preferable to pass dispersion because the work and manufacturing equipment are simpler. In the dispersion process, the disintegration of aggregated particles, the breakdown of CNTs, wetting, stabilization, etc., proceed sequentially or simultaneously, and the final dispersion state differs depending on how these processes proceed. Therefore, 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.

[0077] The solid content of the CNT dispersion is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more, based on 100% by mass of the CNT dispersion. Furthermore, the solid content of the CNT dispersion is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 2% by mass or less, based on 100% by mass of the CNT dispersion.

[0078] From the viewpoint of ease of loading, dispersibility, and dispersion stability of CNTs, 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 CNTs. Furthermore, from the viewpoint of conductivity, the content of the dispersant in the CNT dispersion is preferably 300% by mass or less, more preferably 100% by mass or less, and even more preferably 75% by mass or less, based on 100% by mass of CNTs.

[0079] The CNT dispersion in this embodiment is prepared using CNTs with a low content of metals such as iron. Therefore, the metal content in the CNT dispersion can be easily reduced. However, CNT dispersions may contain metal particles and dissolved metal ions as metals. Metal particles are metals present in particulate form in the CNT dispersion, specifically the metals contained in the CNTs mentioned above. CNTs, dispersants, and other materials may contain metal particles originating from their respective manufacturing processes, and metal particles may also be introduced during the manufacturing process of the CNT dispersion. The presence of metal particles inside a battery increases the risk of short circuits; therefore, removing metal particles is extremely important from a safety standpoint. For this reason, it is preferable to include a step to remove contaminants such as metal particles (metal foreign matter removal step) at any arbitrary timing in the process of manufacturing the CNT dispersion. From the viewpoint of efficiency, it is preferable to perform the metal foreign matter removal step in the middle of the dispersion step of the CNT dispersion and / or at the end of the dispersion step. The metal foreign matter removal step may be performed multiple times.

[0080] In the metal foreign matter removal process, the method for removing metal particles from the CNT dispersion is not particularly limited. Examples include removal by filtration using a filter, removal by vibrating sieve, removal by centrifugal separation, and removal by magnetic force. Among these, since metal particles such as iron and chromium are magnetic, removal by magnetic force is preferred, and a method combining the removal process by magnetic force with the removal process by filtration using a filter is more preferred.

[0081] While there are no particular limitations on the method of removal using magnetism, as long as it can remove metal particles, from the viewpoint of productivity and removal efficiency, it is preferable to place a magnetic filter in the CNT dispersion manufacturing line and remove the particles by passing the CNT dispersion through it. The step of removing metal particles from the CNT dispersion using a magnetic filter is preferably carried out by passing the dispersion 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 metal particles, it is preferably 5,000 gauss or more, more preferably 10,000 gauss or more, and most preferably 12,000 gauss or more, considering the removal of weakly magnetic stainless steel. Coarse metal particles may pass through the magnetic filter depending on the filtration flow rate. Therefore, when placing a magnetic filter in the manufacturing line, it is preferable to include a process upstream of the magnetic filter to remove coarse foreign matter or metal particles using a cartridge filter or similar filter. Furthermore, while a single filtration by the magnetic filter is effective, a circulating system is more preferable. A circulating system improves the efficiency of metal particle removal. When a magnetic filter is placed in the manufacturing line for a CNT dispersion, there are no particular restrictions on where the magnetic filter is placed. However, it is preferable to place the magnetic filter immediately before filling the CNT dispersion into containers, or, if there is a filtration step using a filtration filter before filling the containers, before the filtration filter. By placing it in this manner, it is possible to prevent contamination of the product if metal detaches from the magnetic filter.

[0082] The metal content in a CNT dispersion can be calculated by drying the dispersion to remove the solvent and then analyzing it using ICP. The metal content detected by ICP analysis includes metal particles and dissolved metal ions. In other words, the metal content of a CNT dispersion that has undergone a metal impurity removal process includes metal particles that could not be completely removed and dissolved metal ions.

[0083] The metal content of iron, cobalt, aluminum, and magnesium in the CNT dispersion is preferably 100 ppm or less, more preferably 50 ppm or less, and even more preferably 10 ppm or less, based on 100% by mass of the CNT dispersion. By setting the metal content within the above range, side reactions within the electrode film are less likely to occur, and a conductive secondary battery with excellent storage characteristics can be obtained.

[0084] The sulfur content in the CNT dispersion is preferably 10 ppm or less, more preferably 5 ppm or less, more preferably 2 ppm or less, and may be 1 ppm or less, based on 100% by mass of the CNT dispersion.

[0085] The CNT dispersion may further contain one or more carbon materials such as carbon black and graphite as conductive materials. Among these conductive materials, carbon black is preferred from the viewpoint of dispersant adsorption performance.

[0086] <3> Binder composition One embodiment of the present invention relates to a binder composition. The binder composition according to this embodiment comprises the above-mentioned CNT dispersion and a binder. The binder composition described herein does not contain an electrode active material. Furthermore, according to this embodiment, a method for producing a binder composition comprising the above-mentioned CNT, dispersant, dispersion medium, and binder can be provided.

[0087] <Binder> A binder is a resin that binds various substances together in an electrode film. As a binder, binders known for battery applications can be used. Examples include cellulose resins such as carboxymethylcellulose; rubbers such as styrene-butadiene rubber and fluororubber. Modified forms, mixtures, and copolymers of these resins may also be used. In particular, from a resistance standpoint, the use of polymer compounds having fluorine atoms in the molecule, such as polyvinylidene fluoride, polyvinyl fluoride, and tetrafluoroethylene, is preferred.

[0088] The weight-average molecular weight of the binder is preferably 10,000 or more, more preferably 100,000 or more, and particularly preferably 200,000 or more. Furthermore, the weight-average molecular weight of the binder is preferably 2,000,000 or less, more preferably 1,500,000 or less, and particularly preferably 1,200,000 or less. When the weight-average molecular weight is 10,000 or more, a decrease in the binder's resistance and adhesion can be suppressed. When the weight-average molecular weight is 2,000,000 or less, while improving the binder's resistance and adhesion, a decrease in workability due to the increased viscosity of the binder itself can be suppressed, and significant aggregation of dispersed particles can be suppressed.

[0089] The binder composition is preferably manufactured by mixing and homogenizing a CNT dispersion with a binder, although the binder may be pre-dissolved before use. Alternatively, the binder may be added at any point during the manufacturing process of the CNT dispersion. Various conventionally known mixing methods are acceptable. The binder composition can be manufactured using the dispersion apparatus described above for the CNT dispersion. The binder composition may contain only one type of binder, or two or more types may be used in combination. Furthermore, the manufacturing process of the binder composition may include the metal foreign matter removal step described above.

[0090] <4> Composition for electrodes One embodiment of the present invention relates to an electrode composition. The electrode composition according to this embodiment comprises the above-mentioned CNT or CNT dispersion and an electrode active material. The electrode composition can be further mixed with a binder to produce a composite slurry. Furthermore, according to this embodiment, a method for producing a binder composition comprising the above-mentioned CNT, dispersant, dispersion medium, and electrode active material, and a method for producing a binder composition comprising the above-mentioned CNT, dispersant, dispersion medium, electrode active material, and composite slurry can be provided.

[0091] The viscosity stability of the electrode composition in this embodiment can be evaluated by the rate of change in the viscosity measurement value after standing at 25°C for 24 hours, with the viscosity measurement value after preparation of the electrode composition as the reference. Preferably, the absolute value of the rate of change in viscosity is 50% or less, more preferably 30% or less, and even more preferably 10% or less. When the viscosity stability of the electrode composition is within the above range, solid-liquid separation of the active material and solvent within the electrode composition can be suppressed, and by using such an electrode composition, an electrode film or secondary battery exhibiting excellent performance can be obtained.

[0092] The complex modulus of the electrode composition in this embodiment is preferably 5 Pa or more and less than 500 Pa, and more preferably 50 Pa or more and less than 150 Pa. When the complex modulus of the electrode composition is within the above range, the dispersibility of CNTs and active material is good, and an electrode film with excellent conductivity and strength is easily obtained. The complex modulus of elasticity can be measured by the method described in the examples.

[0093] <Electrode active material> Electrode active materials are the materials that form the basis of a battery reaction. Active materials can be divided into positive electrode active materials and negative electrode active materials based on their electromotive force. The positive electrode active material is not particularly limited, but 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, and inorganic compounds such as transition metal sulfides. Specifically, MnO, V2O5, V6O 13 Examples include transition 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-structured lithium manganate; and lithium iron phosphate-based materials, which are olivine-structured phosphoric acid compounds. These positive electrode active materials can be used individually or in combination. Furthermore, the above inorganic and organic compounds may be mixed and used. Although there is no particular limitation on the negative electrode active material, 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 artificial graphite such as highly graphitized carbon materials, or carbonaceous powders such as natural graphite. 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.

[0094] 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 diameter of the electrode active material is preferably 0.05 to 100 μm, and more preferably 0.1 to 50 μm. In this specification, the "average particle diameter of the electrode active material" is the average value of the particle diameters measured by an electron microscope for the electrode active material.

[0095] The electrode composition is preferably manufactured by mixing and homogenizing the CNT dispersion liquid and the electrode active material, and the binder may be dissolved in the CNT dispersion liquid in advance for use. Also, the electrode active material may be added at any timing in the process of manufacturing the CNT dispersion liquid. The dispersion device used for performing the treatment of dispersing the electrode active material is not particularly limited, and the dispersion devices exemplified in the production of the CNT dispersion liquid can be used.

[0096] In the case of an asphalt slurry containing an electrode composition, the content of electrode active material in the asphalt slurry is preferably 20% by mass or more, and more preferably 40% by mass or more, based on 100% by mass of the asphalt slurry. Furthermore, the content of electrode active material in the asphalt slurry is preferably 99% by mass or less, and more preferably 97% by mass or less, based on 100% by mass of the asphalt slurry. Within the above range is preferable from the viewpoint of coating properties, productivity, and uniformity of the electrode film. The CNT content in the asphalt slurry is preferably 0.01% by mass or more, and more preferably 0.05% by mass or more, based on 100% by mass of the electrode active material. Furthermore, the CNT content in the asphalt slurry is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, based on 100% by mass of the electrode active material. The binder content in the asphalt slurry is preferably 0.3% by mass or more, and more preferably 0.7% by mass or more, based on 100% by mass of the electrode active material. Furthermore, the binder content in the asphalt slurry is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, based on 100% by mass of the electrode active material. The solid content of the asphalt slurry is preferably 30% by mass or more, and more preferably 40% by mass or more, based on 100% by mass of the asphalt slurry. Furthermore, the solid content of the asphalt slurry is preferably 90% by mass or less, and more preferably 85% by mass or less, based on 100% by mass of the asphalt slurry. The water content in the asphalt slurry is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

[0097] <5> electrode membrane One embodiment of the present invention relates to an electrode film. The electrode film according to this embodiment is an electrode film obtained using (i) carbon nanotubes, (ii) a carbon nanotube dispersion containing carbon nanotubes, a dispersant, and a dispersion medium, (iii) a binder composition containing the carbon nanotube dispersion and a binder, or (iv) an electrode composition containing the carbon nanotube dispersion and an electrode active material. The carbon nanotubes of this embodiment described above are used as the carbon nanotubes. The electrode film may also be an electrode film obtained using (v) an asphalt slurry. The (v) asphalt slurry can be obtained using the aforementioned (i) carbon nanotubes, (ii) carbon nanotube dispersion, (iii) binder composition, or (iv) electrode composition.

[0098] For example, the electrode film is a coating obtained by coating the above-mentioned composite slurry onto a current collector and drying it. The material and shape of the current collector used for the electrode film are not particularly limited, and one suitable for various secondary batteries can be appropriately selected. For example, the material of the current collector can be a metal or alloy such as aluminum, copper, nickel, titanium, or stainless steel.

[0099] There are no particular restrictions on the method for applying the asphalt slurry onto the current collector; known methods can be used.

[0100] Furthermore, rolling may be performed using a flatbed press or calender roll after coating and drying. The thickness of the electrode film is generally 1 μm or more and 500 μm or less, preferably 10 μm or more and 300 μm or less.

[0101] <6> secondary battery One embodiment of the present invention relates to a secondary battery. The secondary battery according to this embodiment includes the electrode film described above. The electrode film can be used as an electrode for a secondary battery, and is particularly preferably used as an electrode for a non-aqueous electrolyte secondary battery using an organic electrolyte. 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 the positive electrode, the negative electrode, or both. In one embodiment, for example, an electrode film obtained by coating a current collector with an electrode composition containing a positive electrode active material and drying it can be used as the positive electrode. Furthermore, in one embodiment, for example, an electrode film obtained by coating a current collector with an electrode composition containing a negative electrode active material and drying it can be used as the negative electrode. In one embodiment, an electrode film obtained by coating a current collector with a CNT dispersion or binder composition and drying it can be used as a current collector with a base layer. In particular, from a safety standpoint, it is preferable to use it as the positive electrode.

[0102] Various conventionally known electrolytes that allow ion movement can be used. For example, 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 (where Ph is a phenyl group) can be used, but are not limited to these, and sodium salts can also be used. It is preferable to dissolve the electrolyte in a non-aqueous solvent and use it as an electrolyte solution. All-solid electrolytes or polymer electrolytes may also be used.

[0103] While not particularly limited, various non-aqueous solvents suitable for secondary batteries can be used. Examples include carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, lactones, glyces, esters, sulfoxides, and nitriles. These solvents may be used individually or in mixtures of two or more.

[0104] The secondary battery preferably includes a separator. Examples of the separator include, but are not particularly limited to, a polyethylene non-woven fabric, a polypropylene non-woven fabric, a polyamide non-woven fabric, and those obtained by subjecting these to a hydrophilic treatment.

[0105] The structure of the secondary battery is not particularly limited. The secondary battery may generally be composed of a positive electrode and a negative electrode, and a separator provided as needed. The shape of the secondary battery can be various shapes such as a paper type, a cylindrical type, a button type, a laminated type, etc., according to the purpose of use.

Examples

[0106] The present invention will be described more specifically with reference to the following examples. The present invention is not limited to the following examples as long as the gist thereof is not exceeded.

[0107] 1. Production examples of CNT and CNT dispersion liquid using the same [Example 1] <Production of CNT> 60 parts of CNT (manufactured by JEIO Co., Ltd., JENOTUBE3A) was charged into a graphite crucible with a diameter of 10 cm and a height of 10 cm, and vacuum heating under reduced pressure was performed using a multi-purpose high-temperature furnace (manufactured by Fuji Denpa Kogyo Co., Ltd., Hymult 5000). After performing the nitrogen gas replacement operation twice, the pressure was reduced using an oil rotary pump, and after adjusting the furnace internal pressure to 9.8 - 9.5 Pa, subsequently, the pressure was further reduced using an oil diffusion pump, and the furnace internal pressure was adjusted to 0.03 Pa or less. Subsequently, while maintaining the pressure reduction by the oil diffusion pump, the temperature was raised at a rate of 20 °C / min to 1450 °C and held at 1450 °C for 8 hours. Thereafter, it was naturally cooled until the furnace internal temperature became 50 °C or lower, and CNT (A) was obtained.

[0108] <Preparation of CNT dispersion liquid> An NMP solution containing a hydrogenated nitrile butadiene rubber polymer (manufactured by Nippon Zeon Co., Ltd., Zetpole2000L) with a concentration of 8% and NMP were added to a stainless steel container, and adjusted so that the polymer was 0.3 parts by mass and the total amount of NMP was 99.394 parts by mass. Further, 0.006 parts by mass of NaOH was added to this solution. To the solution obtained above, 0.3 parts by mass of CNT(A) were weighed and added while stirring with a disperser. A fine emulsion screen was attached to a high-shear mixer (L5M-A, manufactured by SILVERSON), and batch dispersion was performed at a speed of 7,000 rpm until the entire mixture was uniform. Next, the contents of the stainless steel container were transferred and subjected to a circulating dispersion treatment (80% bead filling, peripheral speed 8 m / s) with a residence time of 24 minutes using a bead mill (Star Mill LMZ, manufactured by Ashizawa Fine Tech Co., Ltd.) filled with 1.0 mm diameter zirconia beads. Subsequently, the dispersion liquid was supplied to a high-pressure homogenizer (Star Burst Lab, manufactured by Sugino Machine Co., Ltd.) and subjected to a 12-pass dispersion treatment. The dispersion treatment was performed using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa. After the dispersion treatment, the mixture was passed through a depth filter (3M, PP nonwoven fabric depth cartridge NT-T series, filtration accuracy 40 μm). In this manner, a CNT dispersion (A) was prepared.

[0109] <Preparation of asphalt slurry> Capacity 150cm 3 In a plastic container, a CNT dispersion (A), PVdF (polyvinylidene fluoride, Solef 5130, manufactured by Solvay, 100% non-volatile content) pre-dissolved in NMP (N-methyl-2-pyrrolidone) to a concentration of 8%, and a ternary active material (manufactured by Easpring, ME-88ED) as the positive electrode active material were added. The mixture was then stirred at 2,000 rpm for 30 seconds using a rotation / revolution mixer (Sinky Awatori Rentaro, ARE-310). Subsequently, the clumps were broken up with a spatula, and the mixture was stirred at 2,000 rpm for 300 seconds using the rotation / revolution mixer (Sinky Awatori Rentaro, ARE-310) to obtain a composite slurry (A) containing the electrode composition. The non-volatile content of the electrode composition was 66.5%. In the non-volatile portion of the asphalt slurry, the non-volatile content ratio of the ternary active material:CNT:PVdF was set to 98.4:0.1:1.5.

[0110] <Fabrication of electrode films> The asphalt slurry (A) is applied using an applicator, and the basis weight per unit area of ​​the electrode is 20 mg / cm³.2 The material was coated onto aluminum foil in the manner described above. After coating, the coating was dried in an electric oven at 120°C ± 5°C for 25 minutes to obtain electrode film (A). Subsequently, electrode film (A) was rolled using a roll press (Sankmetal, 3t hydraulic roll press) to obtain positive electrode (A). The basis weight per unit area of ​​the composite layer was 20 mg / cm². 2 The density of the asphalt layer after rolling was set to 3.1 g / cc.

[0111] <Manufacturing of secondary batteries> The positive electrode (A) and the standard negative electrode were punched out to 45mm x 40mm and 50mm x 45mm, respectively. These electrodes, along with the separator (porous polypropylene film) to be inserted between them, were placed in an aluminum laminate bag and dried in an electric oven at 60°C for 1 hour. Subsequently, 2 mL of electrolyte (non-aqueous electrolyte) was injected into a glove box filled with argon gas, and then an aluminum laminate bag was sealed to create a laminate-type secondary battery (A). The standard negative electrode and non-aqueous electrolyte were prepared as follows. (Standard negative electrode) In a 150ml plastic container, 0.5 parts by mass of acetylene black (Denka Black® HS-100, manufactured by Denka), 1 part by mass of MAC500LC (carboxymethylcellulose sodium salt, Sunrose special type MAC500LC, manufactured by Nippon Paper Industries, 100% non-volatile content), and 98.4 parts by mass of water were added. The mixture was then stirred at 2000 rpm for 30 seconds using a rotation / revolution mixer (Sinky Awatori Rentaro, ARE-310). Furthermore, 92 parts by mass of artificial graphite (manufactured by Nippon Graphite Industry, CGB-20) and 5 parts by mass of silicon oxide (manufactured by Osaka Titanium Technology, SILICON MONOOXIDE SiO 1.3C 5μm, 100% non-volatile content) were added as active materials, and the mixture was stirred at 3000 rpm for 10 minutes using a high-speed stirrer. Next, 3.1 parts by mass of styrene-butadiene rubber (SBR) (TRD2001, manufactured by JSR Corporation) was added, and the mixture was stirred at 2000 rpm for 30 seconds using the aforementioned rotating / revolving mixer to obtain a negative electrode mixture slurry. Subsequently, the negative electrode mixture slurry was measured using an applicator to obtain a basis weight of 8 mg / cm³ per unit area of ​​the electrode. 2 After coating the copper foil in this manner, the coating was dried in an electric oven at 120°C ± 5°C for 25 minutes. Furthermore, it was rolled using a roll press (manufactured by Sankumetal Co., Ltd., 3t hydraulic roll press) to obtain a density of 1.6 g / cm³ in the asphalt layer. 3 A standard negative electrode was fabricated. (Non-aqueous electrolyte) First, a mixed solvent was prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a 1:1:1 (volume ratio). Next, 2 parts by mass of VC (vinylene carbonate) was added as an additive to 100 parts by mass of this mixed solvent, and then LiPF6 was dissolved at a concentration of 1 M to obtain a non-aqueous electrolyte.

[0112] [Examples 2-4, 6, 7], [Comparative Examples 4-5] Except for changing the raw material CNT type, holding temperature, and processing time listed in the table, CNT(B) to CNT(D), CNT(F), CNT(G), CNT(Z2), and CNT(Z3) were obtained using the same method as in Example 1. Furthermore, using the obtained CNTs, a CNT dispersion, a composite slurry electrode film, and a secondary battery were fabricated in the same manner as in Example 1.

[0113] [Example 5] Into an iBOT (500 cm 3 , manufactured by Taiyo Co., Ltd.), 15 parts of CNTs and 474 parts of zirconia beads with a diameter of 8 mm were added, and dry processing was performed for 6 minutes using an automatic shaker (SK450, manufactured by Fast and Fluid Management). Then, 60 parts of CNTs prepared by repeating this operation were used to obtain CNT(E) in the same manner as described in Example 1. Furthermore, using CNT(E) obtained as described above, a CNT dispersion, an electrode composition, an electrode film, and a secondary battery were fabricated in the same manner as in Example 1.

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

[0115] <Content of Sulfur and Metals in CNT> Using a microwave sample pretreatment device (ETHOS, manufactured by Milestone General Co., Ltd.), the CNTs were decomposed by acid, and the sulfur and metals contained in the CNTs were extracted. The extracted metals were analyzed using a multi-type ICP emission spectroscopic analyzer (720-ES, manufactured by Agilent), and the contents of sulfur and metals contained in the CNTs were calculated. In Table 1 described later, the contents of sulfur, iron, cobalt, aluminum, and magnesium are shown as the contents of sulfur and metals. The contents of sulfur, iron, cobalt, aluminum, and magnesium in the CNTs are expressed as the mass ratio (ppm) of the contents of the extracted sulfur, iron, cobalt, aluminum, and magnesium to the mass of the CNTs before extraction of metals and the like.

[0116] <G / D Ratio of CNT> <000054CNTs were placed on a Raman microscope (XploRA, manufactured by Horiba, Ltd.) and measurements were carried out using a laser wavelength of 532 nm. The measurement conditions were as follows: acquisition time of 60 seconds, number of integrations of 2, dimming filter of 10%, objective lens magnification of 20x, confocal hole of 500, slit width of 100 μm, and measurement wavelength range of 100 - 3000 cm -1 −1. The CNTs for measurement were separated onto a slide glass and flattened using a spatula. Among the obtained peaks, the maximum peak intensity within the spectral range of 1560 - 1600 cm -1 −1 was designated as G, and the maximum peak intensity within the range of 1310 - 1350 cm -1 −1 was designated as D. The ratio of G / D was calculated and taken as the G / D ratio of the CNTs.

[0117] <Specific surface area of CNT> Using an electronic balance (MSA225S100DI, manufactured by Sartorius), 0.03 g of CNTs were weighed and then dried while degassing at 110 °C for 15 minutes. Subsequently, using a fully automatic specific surface area measuring device (HM - model1208, manufactured by MOUNTECH), the BET specific surface area of the CNTs was measured. The BET specific surface area was measured in accordance with the BET method of JIS Z 8830:2013.

[0118] <Wetting index of CNT> Under an environment of 25 °C, 5 g (Y (g)) of CNT powder was naturally dropped and contained in a cylindrical polypropylene container with a diameter of 10 cm. While in a static state, N - methyl - 2 - pyrrolidone (NMP) was dropped onto the surface of the CNT powder at 5 g per time and at intervals of 1 minute. The total mass (X (g)) of the NMP dropped until just before it started to flow out on the surface of the CNT powder without being absorbed by the CNT powder was measured. The wetting index of the CNTs was calculated from the following formula (I). Formula (I): Wetting index = (X / Y) [In formula (I), Y is the mass (g) of the CNTs, and X is the maximum mass (g) of the NMP absorbed by the CNTs when NMP is dropped onto Y (g) of the CNTs under an environment of 25 °C.]

[0119] <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, and a powder probe unit (four-probe · ring electrode, electrode spacing 5.0 mm, electrode radius 1.0 mm, sample radius 12.5 mm), with the applied voltage limiter set to 90 V, the volume resistivity [Ω·cm] of the CNT powder under pressures of 4 kN, 8 kN, 12 kN, 16 kN, and 20 kN was measured respectively. The values of the five measured points were obtained by exponential approximation, and the volume resistivity at a density of 1 g / cm 3 was calculated and taken as the volume resistivity of the CNT.

[0120] <Scanning Electron Microscope (SEM) Observation of CNT> Observation of CNT by SEM can be measured by the following method. Stick carbon tape on the pedestal for SEM measurement, and place the CNT on it. By observing with an SEM at a magnification of 50,000, images as shown in Figures 1 and 2 can be taken.

[0121] 2-2. Evaluation of Other Characteristics Regarding the CNT dispersion liquid, binder composition, electrode composition, electrode film, and secondary battery prepared in the above-mentioned examples and comparative examples, measurements were carried out as follows to evaluate the characteristics. The respective evaluation results are shown in Table 1.

[0122] <Fluidity of CNT Dispersion Liquid> After leaving the CNT dispersion liquid standing in a constant temperature bath at 25°C for 1 hour or more, using a rheometer (MCR302e, manufactured by Anton Paar) with a cone of diameter 50 mm and 2°, at 25°C and a frequency of 1 Hz, dynamic viscoelasticity measurement was carried out in the range of shear strain from 0.01% to 5%, and the value at a strain rate of 1% was taken as the phase angle to evaluate the fluidity of the CNT dispersion liquid. (Evaluation Criteria) ◎ (Excellent): The phase angle exceeds 20° ○ (Good): The phase angle exceeds 10° and is less than 20° △ (Fair): The phase angle exceeds 5° and is less than 10° × (Poor): The phase angle is less than 5°

[0123] <Particle size (D of CNT dispersion 90 )> The particle size (D 90 ) was measured using a particle size distribution measuring device (Partica LA-960V2, manufactured by HORIBA). The operating conditions of circulation / ultrasound were circulation rate: 3, ultrasound intensity: 7, ultrasound time: 1 minute, stirring rate: 1, and stirring mode: continuous. Also, during air evacuation, ultrasound was operated at an ultrasound intensity of 7 and an ultrasound time of 􀀁ve seconds. The refractive index of NMP was 1.468 and that of CNT was 1.920. The measurement was carried out after diluting the measurement sample so that the transmittance of the red laser diode was 35%, and the particle size standard was volume. The evaluation criteria for the particle size are as follows. (Evaluation criteria) ◎ (Excellent): 1 μm or more and less than 〇 (Good): 10 μm or more and less than △ (Fair): 15 μm or more and less than × (Poor): Less than 1 μm or 20 μm or more was adopted.

[0124] <Viscosity stability of composite slurry The viscosity stability of the composite slurry was evaluated by the change rate of the viscosity measurement value after standing at 25 °C for 24 hours based on the viscosity measurement value after preparing the composite slurry. The smaller the change, the better the stability. After standing the composite slurry in a thermostatic bath at 25 °C for 1 hour or more, using a rheometer (MCR302e, manufactured by Anton Paar) with a -1 cone with a diameter of 25 mm and an angle of 2°, the shear viscosity at 25 °C and a shear rate of 100 s (Evaluation criteria) ◎ (Excellent): The absolute value of the viscosity change rate is 10% or less 〇 (Good): The absolute value of the viscosity change rate exceeds 10% and is 30% or less △ (Fair): The absolute value of the viscosity change rate exceeds 30% and is 50% or less × (Poor): The absolute value of the viscosity change rate exceeds 50%

[0125] <Complex elastic modulus of composite slurry The complex modulus of elasticity of the asphalt slurry was evaluated by viscoelasticity measurement after standing at 25°C for 24 hours, with the slurry preparation as the baseline. For viscoelasticity measurement, the asphalt slurry was left to stand in a constant temperature bath at 25°C for more than 1 hour. Then, using a rheometer (MCR302e, manufactured by Anton Paar) with a 25 mm diameter, 2° cone, dynamic viscoelasticity measurements were performed at 25°C and a frequency of 1 Hz, within the range of shear strain from 0.01% to 5%. The value at a strain rate of 1% was used as the complex modulus for evaluation. (Evaluation Criteria) ○ (Good): 50 Pa or more and less than 150 Pa △ (Acceptable): 5 Pa or more but less than 50 Pa or 150 Pa or more but less than 500 Pa -(Poor): Less than 5 Pa or 500 Pa or more

[0126] <Evaluation of electrode volume resistivity> The mixture slurry prepared in the examples and comparative examples was measured using an applicator to obtain a basis weight of 20 mg / cm³ per unit area of ​​the electrode. 2 The material was coated onto a PET foil with a thickness of 100 μm, and then dried in an electric oven at 120°C ± 5°C for 30 minutes to produce a composite coating film. The surface resistivity (Ω / □) of the fabricated composite coating was measured using a Mitsubishi Chemical Analytec Rolester GP, MCP-T610. After measurement, the volume resistivity (Ω·cm) of the electrode was obtained by multiplying by the thickness of the composite layer. The thickness of the composite layer was determined by subtracting the thickness of the PET foil from the average value of measurements taken at three points in the electrode using a film thickness gauge (NIKON, DIGIMICRO MH-15M). The evaluation criteria for the volume resistivity of the electrode are as follows. (Evaluation Criteria) ◎ (Excellent): Less than 5Ω·cm ○ (Good): 5Ω·cm or higher, less than 8Ω·cm △ (Acceptable): 8Ω·cm or more and less than 10Ω·cm × (Defective): Exceeds 10Ω·cm

[0127] <Evaluation of storage characteristics of secondary batteries> The secondary battery was placed in a constant temperature chamber at 25°C, and charge / discharge measurements were performed using a charge / discharge device (SM-8, manufactured by Hokuto Denko Co., Ltd.). Constant current constant voltage charging (cutoff current 1.25mA (0.025C)) was performed with a charging current of 25mA (0.5C) and a charging termination voltage of 4.2V, followed by constant current discharge at a discharge current of 50mA (1C) and a discharge termination voltage of 2.5V. 1C was defined as the current value required to discharge the theoretical capacity of the positive electrode in one hour. Subsequently, initial charge and discharge were performed similarly, with charging at a charging current of 25mA (0.5C) followed by constant current discharge at a discharge current of 150mA (3C) and a discharge termination voltage of 2.5V. After charging at a charging current of 25mA (0.5C), the secondary battery was removed from the charge / discharge device and left to stand in a constant temperature chamber at 80°C for 24 hours for high-temperature storage. Subsequently, the secondary battery was placed in a constant temperature room at 25°C and left undisturbed for 24 hours. After that, charge and discharge measurements were performed using a charge / discharge device. Constant current constant voltage charging (cutoff current 1.25mA (0.025C)) was performed with a charging current of 25mA (0.5C) and a charging termination voltage of 4.2V, followed by constant current discharge at a discharge current of 50mA (1C) and a discharge termination voltage of 2.5V. After that, charging was performed similarly with a charging current of 25mA (0.5C), followed by constant current discharge at a discharge current of 150mA (3C) and a discharge termination voltage of 2.5V. The DCIR after high-temperature storage was measured to evaluate the storage characteristics of the secondary battery. The storage characteristics of the secondary battery can be calculated from the ratio of the voltage drop after 3 seconds during 3C discharge at 25°C to the applied current, using the following equation II. (Equation II): Storage characteristics of secondary battery = (Voltage at 0 seconds of 3C discharge - Voltage after 3 seconds of 3C discharge) / Applied current (Ω) The evaluation criteria for the storage characteristics of secondary batteries are as follows: (Evaluation Criteria) ◎(Excellent): 1.7Ω or less ○ (Good): Greater than 1.7Ω and less than 2.0Ω × (defective): 2.0Ω or more

[0128] Table 1 shows the evaluation results of the CNTs, CNT dispersions, electrode films, and secondary batteries prepared in Examples 1-10 and Comparative Examples 1-5.

[0129] In Table 1, the CNTs used, 10B, 3A, and 01RW03, are defined as follows: • 10B: Multiwall carbon nanotube (manufactured by JEIO, JENOTUBE10B) • 3A: Multi-walled carbon nanotube (manufactured by JEIO, JENOTUBE3A) • TUBALL01RW03: Single-walled carbon nanotube (manufactured by OCSiAl, TUBALL, 01RW03)

[0130] [Table 1]

[0131] Figure 1 shows carbon nanotubes (CNTs) that have been calcined under appropriate conditions, while Figure 2 shows CNTs that have been excessively calcined. It can be seen that the CNTs calcined under excessive conditions (Figure 2) exist in a state where they are more fused together than the CNTs treated under appropriate conditions (Figure 1).

[0132] As shown in Table 1, the CNTs of this embodiment (Examples 1-7) satisfy all of the requirements previously described: (1) G / D ratio, (2) BET specific surface area, (3) volume resistivity, (4) iron content, and (5) sulfur content. By comparing them with comparative examples that use CNTs that do not satisfy the above requirements (1) to (5), it can be seen that using the CNTs of this embodiment improves the properties of the CNT dispersion, electrode composition, electrode film, and secondary battery. Thus, the CNTs of this embodiment make it possible to form an electrode film with excellent conductivity, thereby improving the safety of the secondary battery.

Claims

1. A carbon nanotube that satisfies the following conditions (1) to (5). (1) In the Raman spectrum, 1560–1600 cm⁻¹ －１ The maximum peak intensity within the range is G, 1310-1350 cm. －１ When the maximum peak intensity within the specified range is denoted as D, the G / D ratio is between 16 and 38. (2) The BET specific surface area is 150 m ２ / g or more 450m ２ It is less than / g. (3) Volume resistivity is 1.0 × 10 －３ ~9.9 x 10 －３ It is Ω·cm. (4) The iron content is 7000 ppm or less. (5) The sulfur content is greater than 0 ppm and 1600 ppm or less.

2. A carbon nanotube according to claim 1, satisfying the following (6). (6) The wettability index shown by the following formula (I) is 19.0 or more and 25.0 or less. Equation (I): Wetting index = (X / Y) [In equation (I), Y is the mass (g) of the carbon nanotube, and X is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotube when N-methyl-2-pyrrolidone is dropped onto Y (g) of carbon nanotube at 25°C.]

3. A carbon nanotube according to claim 1, satisfying the following (7). (7) The iron content is 6,000 ppm or less, and the aluminum content is 500 ppm or less.

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

5. It contains a carbon nanotube dispersion and a binder. The carbon nanotube dispersion is a binder composition comprising the carbon nanotubes described in any one of claims 1 to 3, a dispersant, and a dispersion medium.

6. The mixture contains a carbon nanotube dispersion and an electrode active material. The carbon nanotube dispersion is an electrode composition comprising the carbon nanotubes described in any one of claims 1 to 3, a dispersant, and a dispersion medium.

7. A secondary battery comprising an electrode film, wherein the electrode film is A carbon nanotube dispersion comprising a carbon nanotube according to any one of claims 1 to 3, a dispersant, and a dispersion medium. A secondary battery obtained using a composition comprising the carbon nanotube dispersion and a binder, or an electrode composition comprising the carbon nanotube dispersion and an electrode active material.

Images