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

Abstract

Provided are a carbon nanotube that can improve safety and can be suitably used for forming an electrode film having good conductivity, and a carbon nanotube dispersion, a binder composition, an electrode composition, and a secondary battery containing the carbon nanotube. 【Solution means】A carbon nanotube that satisfies the following (1) to (3) and contains multi-walled carbon nanotubes. (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 0.5 or more and 3.0 or less. (2) The wetting index represented by the following formula (I) is 10 or less. Formula (I): Wetting index = (X / Y) [In formula (I), Y is the mass (g) of the carbon nanotube, and X is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotube when N-methyl-2-pyrrolidone is dropped onto Y (g) of the carbon nanotube in an environment at 25°C. ] (3) The aluminum content is 3000 ppm or less.

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE The present invention relates to a carbon nanotube, a carbon nanotube dispersion, a binder composition, an electrode composition, and a secondary battery. [Background technology]

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

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

[0004] CNTs can generally be manufactured by arc discharge, laser evaporation, chemical vapor deposition, and the like. Of these, the chemical vapor deposition method is the most suitable for mass production in terms of productivity and economy, and is widely used. In the chemical vapor deposition method, a carbon source gas is reacted with a catalyst containing a metal such as iron, cobalt, or nickel to generate CNTs. Therefore, the CNTs obtained by the chemical vapor deposition method contain particles such as catalysts containing metals such as iron, cobalt, or nickel, or carbides or oxides derived from the catalyst. Catalysts containing metals (hereinafter also referred to as catalytic metals) are indispensable in some methods for manufacturing CNTs. However, after the manufacture of CNTs, metals derived from the catalytic metals remaining in the CNTs can become impurities. In addition, regardless of the manufacturing method of CNTs, metals may be mixed into the CNTs due to wear of metals used in synthesis equipment, filling equipment, or piping, etc., used during the manufacture. The mixed metals can become impurities. From the viewpoint of obtaining desired properties from CNTs, it is desirable for the metal content in CNTs to be low. For example, when CNTs containing metals derived from catalytic metals are used in secondary batteries, the metals may dissolve and precipitate, causing problems such as short-circuiting the battery. If the battery shorts out, it may lead to serious accidents such as fire or explosion. Therefore, in order to suppress problems caused by the metals contained in CNTs and to further increase safety, several methods have been proposed for purifying CNTs and removing metals derived from catalytic metals.

[0005] Patent Document 1 describes a method of purifying a carbon material containing CNTs through a carbon material preparation process in which a raw material containing at least carbon and a catalytic metal is used as an anode to prepare a carbon material containing CNTs by an arc discharge method, and a halogen treatment process in which the carbon material is brought into contact with a gas containing a halogen and / or a halogen compound, thereby removing the catalytic metal impurity while suppressing damage or cutting of the CNTs and solidification of the CNTs into clumps.

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

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

[0008] However, when the halogen treatment process is performed on the CNTs, the catalytic metal residue is removed, but the CNTs are fired at high temperatures for a long time, which increases the crystallinity of the CNTs. Highly crystalline CNTs are hard and therefore prone to problems due to breakage or damage during use. For this reason, when CNTs are used as a conductive assistant in the electrode film of a secondary battery or the like, the CNTs may break during the manufacturing process before the electrode film is formed, which may increase the contact resistance between the CNTs, resulting in a decrease in the conductivity of the electrode film and a decrease in the performance of the secondary battery containing the CNTs.

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

[0010] As described above, it is difficult to reduce metals derived from catalytic metals and the like contained in CNTs without deteriorating the CNT characteristics. Therefore, further improvement of CNTs is desired to improve the safety and performance of secondary batteries. In view of such circumstances, the present invention provides carbon nanotubes that can be used to improve safety and can be suitably used to form an electrode film having good conductivity. In addition, other embodiments provide a carbon nanotube dispersion, a binder composition, an electrode composition, and a secondary battery, each of which includes the carbon nanotubes. [Means for solving the problem]

[0011] In order to solve the above problems, the present inventors have conducted intensive research and have found a carbon nanotube that satisfies the following conditions, and have completed the present invention. That is, the embodiments of the present invention include the following. However, the present invention is not limited to the embodiments described below.

[0012] <1> Carbon nanotubes that satisfy the following (1) to (3), including multi-walled carbon nanotubes: (1) 1560-1600 cm in the Raman spectrum -1 The maximum peak intensity in the range of 1310 to 1350 cm is G. -1 When the maximum peak intensity within the range is defined as D, the G / D ratio is 0.5 or more and 3.0 or less. (2) The wettability index represented by the following formula (I) is 10 or less. Formula (I): Wettability Index = (X / Y) [In formula (I), Y is the mass (g) of the carbon nanotube, and X is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotube when N-methyl-2-pyrrolidone is dropped onto the carbon nanotube of Y(g) in a 25°C environment.] (3) The aluminum content is 3000 ppm or less.

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

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

[0015] <4> The carbon nanotubes have an exothermic peak at 600° C. or more and 800° C. or less in a differential thermal analysis when heated from 200° C. to 1000° C. at a rate of 10° C. / min. <1> ~ <3> 13. The carbon nanotube according to claim 12,

[0016] <5> the above <1> ~ <4> 13. A carbon nanotube dispersion comprising the carbon nanotubes according to any one of 1 to 2 above, a dispersant, and a dispersion medium.

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

[0018] <7> A carbon nanotube dispersion and a binder are included, The carbon nanotube dispersion liquid is <1> ~ <4> 13. A binder composition comprising the carbon nanotubes according to any one of claims 1 to 12, a dispersant, and a dispersion medium.

[0019] <8> A carbon nanotube dispersion liquid and an electrode active material are included, The carbon nanotube dispersion liquid is <1> ~ <4> 13. A composition for an electrode comprising the carbon nanotubes according to any one of 1 to 2 above, a dispersant, and a dispersion medium.

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

[0021] According to the present invention, it is possible to provide carbon nanotubes that can improve safety and can be suitably used to form an electrode film having good electrical conductivity. It is also possible to provide a carbon nanotube dispersion, a binder composition, and an electrode composition that can be suitably used in secondary battery applications by using the carbon nanotubes. Furthermore, it is possible to provide a secondary battery that is excellent in safety and high in performance. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Hereinafter, the embodiments of the present invention will be described in detail. However, it goes without saying that the present invention is not limited to the embodiments described below, and various modifications can be made without departing from the gist of the present invention.

[0023] In this specification, a numerical range indicated using "~" indicates a range including the numerical values ​​before and after "~" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this specification, the upper limit or lower limit of a numerical range of a certain stage can be arbitrarily combined with the upper limit or lower limit of a numerical range of another stage.

[0024] <1> Carbon Nanotubes (CNTs) 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 (3) and includes at least a multi-walled carbon nanotube. (1) 1560-1600 cm in the Raman spectrum -1 The maximum peak intensity in the range of 1310 to 1350 cm is G. -1 When the maximum peak intensity within the range is defined as D, the G / D ratio is 0.5 or more and 3.0 or less. (2) The wettability index represented by the following formula (I) is 10 or less. Formula (I): Wettability index = (X / Y) [In formula (I), Y is the mass (g) of the carbon nanotubes, and X is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotubes when N-methyl-2-pyrrolidone is dropped onto Y (g) of the carbon nanotubes in an environment at 25°C.] (3) The aluminum content is 3000 ppm or less.

[0025] The CNT according to this embodiment is characterized by satisfying all of the above requirements (1) to (3). That is, the CNT according to this embodiment has a specific G / D ratio, a specific wettability index, and a further limited aluminum content, 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 safety and can exhibit good performance. When the CNT contains multi-walled carbon nanotubes, it becomes easy to satisfy all of the above requirements (1) to (3). Hereinafter, each requirement will be described more specifically.

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

[0027] CNTs have a Raman spectrum of 1560 to 1600 cm -1 The maximum peak intensity in the range of 1310 to 1350 cm is G. -1 When the maximum peak intensity within the range is D, the G / D ratio is preferably 0.5 to 3.0. The G / D ratio is more preferably 0.5 to 2.5, even more preferably 0.5 to 2.0, particularly preferably 0.5 to 1.5, and even more preferably 0.5 to 1.3. If the G / D ratio of the CNT exceeds the above range, the CNT becomes hard, so that the CNT is easily damaged during dispersion, and the contact resistance may increase. On the other hand, if the G / D ratio of the CNT falls below the above range, the conductivity of the CNT itself is likely to be low. As a result, if the G / D ratio of the CNT is within the above range, the rate characteristics and cycle characteristics are improved in a secondary battery using an electrode film using the CNT dispersion liquid.

[0028] Conventionally, when raw CNTs are subjected to a halogen treatment process to purify CNTs, the crystallinity of the CNTs tends to increase because the CNTs are fired at high temperatures for a long time. This state can also be confirmed by the fact that the G / D ratio increases. On the other hand, according to this embodiment, in the purification process of CNTs, heat treatment of CNTs can be performed in an inert atmosphere under reduced pressure vacuum. Therefore, the heat treatment temperature can be kept low and the heat treatment can be performed in a short time, and high crystallization of CNTs can be suppressed. In other words, it is possible to suppress the G / D ratio of CNTs from increasing. When low-crystalline CNTs are used, in the process of manufacturing an electrode film, bending of CNTs can be suppressed by dispersion processing or the like, and an increase in contact resistance between CNTs in the obtained electrode film can be suppressed. As a result, low-crystalline CNTs can obtain good conductivity as an electrode film, and secondary batteries containing CNTs can exhibit good performance.

[0029] <Wetting index> The wettability index is the ratio of the maximum mass of the solvent absorbed by the CNT to the mass of the CNT, and is one of the indicators of the dispersibility of the CNT, particularly the initial viscosity when preparing a dispersion containing a dispersion medium and CNT. More specifically, the CNT of this embodiment has a wettability index represented by the following formula (I) of 10 or less. The wettability index may be preferably 9.5 or less, more preferably 9.0 or less, and even more preferably 8.5 or less. When the wettability index is 10 or less, it is easy to obtain a suitable initial viscosity when preparing a dispersion containing a dispersion medium and CNT. When the wettability index exceeds 10, the initial viscosity becomes high when preparing the dispersion, and dispersion failure that makes stirring with a stirrer and pumping difficult is likely to occur. In addition, when the initial viscosity is high, the conductivity of the CNT is likely to decrease.

[0030] Formula (I): Wettability Index = (X / Y)

[0031] In formula (I), Y is the mass (g) of a carbon nanotube (CNT), and X is the maximum mass (g) of N-methyl-2-pyrrolidone (NMP) absorbed by the CNT when NMP is dropped onto a CNT of Y (g) in a 25°C environment. The maximum mass (g) of NMP absorbed by the CNTs is the total mass of NMP added dropwise to the CNT powder up to the point where the NMP starts to flow out of the CNT powder.

[0032] The wettability index can be determined, for example, according to the following procedure. First, Y(g) of CNT powder is placed in a container by gravity in a 25°C environment. With the container standing still, NMP is dropped onto the surface of the CNT powder in the container, 5 g per drop, at 1-minute intervals. Next, observe whether the NMP droplets begin to flow onto the surface of the CNT powder without being absorbed by the CNT powder. The total mass (g) of NMP dropped just before the NMP droplets begin to flow onto the surface of the CNT powder is defined as X(g). The wettability index shown in formula (I) is calculated from the values ​​of X(g) and Y(g) obtained in this way.

[0033] <Metal content> From the viewpoint of safety, it is desirable that the amount of impurities such as metals remaining in the purified CNTs is small. Here, the metals contained in the CNTs are mainly metals and metal oxides derived from the catalytic metal used in the CNT production, but also include metals that are mixed in during the CNT production. For example, metals such as stainless steel used in the synthesis equipment, filling equipment, or piping may be mixed into the CNTs due to wear or the like. In some embodiments, from the viewpoint of further enhancing safety, the total content of metals contained in the CNTs is preferably 13,000 ppm or less, more preferably 10,000 ppm or less, even more preferably 9,000 ppm or less, and even more preferably 8,000 ppm or less.

[0034] The CNT of this embodiment has an aluminum content of 3000 ppm or less. The aluminum content of the CNT may be preferably 2000 ppm or less, more preferably 1100 ppm or less, and even more preferably 500 ppm or less. In some embodiments, the content may be more preferably 100 ppm or less, and even more preferably 50 ppm or less. The aluminum content may be 0 ppm. When aluminum oxide is used as a catalyst carrier during synthesis of CNTs, aluminum oxide nanoparticles, which are insulating components, may remain in the CNTs and impair electrical conductivity. When the aluminum content in the CNTs is within the above range, the amount of aluminum oxide nanoparticles, which are insulating components, is small, and the carbon nanotubes can be used to form electrode films with good electrical conductivity.

[0035] Representative CNTs also contain metals other than aluminum (hereinafter, referred to as other metals). The content of other metals is preferably 10,000 ppm or less, more preferably 7,000 ppm or less, and particularly preferably 5,000 ppm or less.

[0036] The other metals may preferably be magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, molybdenum. In CNTs, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be included as derived from catalytic metals. In addition to metals and metal oxides used as catalytic metals, metals such as stainless steel used in synthesis equipment, filling equipment, or piping may be mixed into CNTs due to wear, etc. For this reason, in CNTs, aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be metals that are not derived from catalytic metals.

[0037] In some embodiments, the total content of magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum contained in the CNTs is preferably 10,000 ppm or less, more preferably 7,000 ppm or less, and particularly preferably 5,000 ppm or less. The total content is more preferably 3,000 ppm or less, even more preferably 2,500 ppm or less, even more preferably 1,850 ppm or less, and even more preferably 700 ppm or less. When the total content of these metals is within the above range, it is easier to improve the safety of the secondary battery. In the following description, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum may be collectively referred to simply as other metals.

[0038] Here, the aluminum content and other metal content in CNTs are masses converted into elemental metals. Metals may be contained in CNTs as elemental metals, metal oxides, metal composite oxides, etc., but these are converted into elemental metals to determine the metal content. In CNTs, aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum may be contained as simple metals, metal oxides, and composite oxides thereof, etc. These metals can cause short circuits, so it is desirable to reduce their total content.

[0039] In some embodiments, from the viewpoint of further enhancing safety, it is preferable to limit the content of cobalt and / or iron among the other metals. In some embodiments, the total cobalt content in the CNTs is preferably 10,000 ppm or less, more preferably 7,000 ppm or less, particularly preferably 6,500 ppm or less, more preferably 3,000 ppm or less, even more preferably 1,700 ppm or less, and even more preferably 800 ppm or less. Additionally, in some embodiments, the total iron content in the CNTs is preferably 7000 ppm or less, more preferably 3000 ppm or less, even more preferably 2500 ppm or less, and even more preferably 1000 ppm or less. Furthermore, in some embodiments, the total cobalt and iron content in the CNTs is preferably 7000 ppm or less, more preferably 3000 ppm or less, even more preferably 2000 ppm or less, and even more preferably 1000 ppm or less.

[0040] In this embodiment, as in the purification method of CNT described later, the total content of metals such as aluminum contained in CNT can be reduced by firing CNT in an inert atmosphere under reduced pressure vacuum. By reducing the aluminum content and the total content of other metals such as cobalt and iron contained in CNT, the temperature of the heat generation peak can be increased, and CNT with improved safety can be obtained. A secondary battery containing such CNT can exhibit good performance.

[0041] The aluminum content and other metal content in CNT can be calculated, for example, by decomposing CNT with an acid, extracting the metals contained in CNT, and analyzing the extract using inductively coupled plasma (ICP). The total content of aluminum and other metals such as magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum contained in CNT is expressed as the mass ratio (ppm) of the total content of extracted aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum, etc., relative to the mass of CNT before metal extraction. Here, the total content of aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is calculated by calculating the mass of each metal when converted into a metal element, and the total amount is the sum of the masses.

[0042] <Exothermic peak> From the viewpoint of safety, the CNT of this embodiment preferably has an exothermic peak at 600°C or more and 800°C or less in differential thermal analysis (DTA) when heated from 200°C to 1000°C at 10°C / min. The exothermic peak can be measured by subjecting the CNT to differential thermal analysis in an air atmosphere. DTA is a method in which the temperature difference between a sample and a reference substance is measured as a function of temperature while the temperatures of the sample and the reference substance are changed under certain conditions, and is in accordance with JIS K 0129. In a DTA curve created based on the change in the temperature difference between the sample and the reference substance, the largest peak is taken as the exothermic peak.

[0043] Heat generation occurs along with the combustion of CNTs. As the combustion start temperature of CNTs increases, the heat generation peak temperature also increases. The factors affecting the change in the combustion start temperature of CNTs include the content of the catalyst metal, the degree of oxidation on the CNT surface, and the crystallinity of CNTs. When the heat storage property of the catalyst metal contained in CNTs is high, if the content of the catalyst metal is small, the total heat storage amount in all the catalyst metals becomes small. Since the total heat storage amount in the catalyst metal is small, the temperature for burning CNTs may be higher compared to the case where the content of the catalyst metal is large. In addition to the catalyst metal, there is a possibility that metals may be mixed into CNTs during the CNT manufacturing process, and such metals can also affect the total heat storage amount. Also, the sites having oxygen-containing functional groups on the CNT surface are more likely to burn compared to the sites without functional groups. Therefore, the less the amount of oxygen-containing functional groups (that is, the less the surface oxygen amount), the more difficult it is for CNTs to burn, and the lower the degree of oxidation on the CNT surface, the higher the temperature for burning CNTs. Furthermore, the higher the crystallinity of CNTs, the higher the combustion start temperature of CNTs. The crystallinity of CNTs can be represented by the above-mentioned G / D ratio.

[0044] When the combustion start temperature of CNTs is within an appropriate temperature range, the impurities contained in CNTs are reduced, so that safer CNTs can be obtained. The CNTs of the present embodiment preferably have a heat generation peak temperature of 600 °C or higher, more preferably 650 °C or higher. Also, the heat generation peak temperature is preferably 800 °C or lower, more preferably 740 °C or lower. When the heat generation peak temperature is 600 °C or higher, the metal content is low, and the safety of the battery can be improved. Or, the surface oxygen amount is small and it has excellent conductivity. When the heat generation peak temperature is 800 °C or lower, the crystallinity of CNTs is not too high, the breakage of CNTs can be suppressed, and the deterioration of the performance of the secondary battery can be suppressed. For example, it is preferably 600 °C or higher and 800 °C or lower, 630 °C or higher and 750 °C or lower, or 650 °C or higher and 740 °C or lower.

[0045] When CNTs are in the form of powder before dispersion, the exothermic peak can be measured as is. When CNTs are present in a CNT dispersion, the dispersion medium is removed by heating and drying, and then the measurement can be performed and the exothermic peak can be identified from the shape of the exothermic peak. The heating and drying temperature is preferably a temperature at which the CNTs are not oxidized (for example, 140°C or lower). When the CNT dispersion contains components other than CNTs and the dispersion medium (additives, etc.), the exothermic peak of the additives may be measured in advance, and the exothermic peak derived from the additives may be identified, so that the remaining exothermic peaks are determined to be derived from CNTs, and the exothermic peaks may be identified.

[0046] <Volume resistivity> The volume resistivity of the CNT of this embodiment is 2.0×10 -2 In some embodiments, the volume resistivity is 1.0×10 -2 ~2.0×10 -2 In some embodiments, the volume resistivity is 1.0×10 -2 ~1.9×10 -2 Ω cm is more preferable, and 1.0×10 -2 ~1.8×10 -2 More preferably, it is 1.2×10 -2 ~1.7×10 -2 It is particularly preferable that the volume resistivity is Ω·cm. When the volume resistivity of the CNT is within the above range, the volume resistivity of the electrode film is reduced, and the performance of the secondary battery is improved. The volume resistivity of the CNT can be measured using a powder resistivity measuring device (Loresta-GP Powder Resistivity Measuring System MCP-PD-51, manufactured by Nitto Seiko Analytech Co., Ltd.).

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

[0048] In this specification, the cohesive force is a value defined by the maximum peak value of the torque measured when a shear force is applied to a compressed CNT by penetrating the carbon nanotube while rotating a jig. For example, MCR302e (Anton Paar) can be used as a measuring device. The specific measuring method is as follows. First, CNT is placed in a dedicated aluminum container (C-CC27 / D / Al), and the carbon nanotube is compressed at 12 kPa with a cylinder jig for compression. Then, the jig is replaced with a wing-shaped jig for measuring the cohesive force, and the jig is moved at 125 μm / s and 0.1 revolutions / minute to penetrate the carbon nanotube, and the torque when the shear force is applied is measured, and the maximum peak value is taken as the cohesive force of the CNT. C-PTD200 can be used as a temperature control device, and the measurement can be performed at a temperature of 25°C.

[0049] <Surface oxygen content> In some embodiments, from the perspective of the conductivity of CNT, the CNT of this embodiment preferably has a surface oxygen content of less than 1.0 atm%. The above surface oxygen content is preferably 0.9 atm% or less, and more preferably 0.8 atm% or less. In some embodiments, the above surface oxygen content may be 0.1 to 0.8 atm%, more preferably 0.2 to 0.7 atm%, and even more preferably 0.2 to 0.7 atm% or less. When the surface oxygen content is less than 1.0 atm%, excellent conductivity can be easily obtained as an electrode film by using CNT. In this specification, the "surface oxygen content" is a value represented by the ratio (atm%) of oxygen atoms to carbon atoms on the surface of CNT determined by X-ray photoelectron spectroscopy.

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

[0051] <Other properties of CNT> CNT has a shape in which planar graphite is wound into a cylindrical shape. CNT may be a mixture of single-walled CNT and multi-walled CNT. Single-walled CNT has a structure in which one layer of graphite is wound into a cylindrical shape. Multi-walled CNT has a structure in which two or three or more layers of graphite are wound into a cylindrical shape. CNT in the present disclosure may not contain single-walled CNT, and is preferably a multi-walled CNT. Alternatively, CNT may be a mixture of single-walled CNT and multi-walled CNT, but even in such a case, CNT in which multi-walled CNT accounts for 90% by mass or more is preferable, and CNT in which multi-walled CNT accounts for 99% by mass or more is more preferable. Multi-walled CNT in CNT may be 100% by mass. By using such CNT, it is easy to set the G / D ratio in a suitable range, for example, 0.5 to 3.0. In addition, the sidewall of CNT does not have to have a graphite structure. For example, CNT with a sidewall having an amorphous structure can be used as CNT.

[0052] The CNT of this embodiment is preferably a multi-walled CNT, and the number of walls of the CNT is preferably 3 to 30, more preferably 3 to 20, and even more preferably 3 to 10.

[0053] The purity of CNT is expressed as a value (mass%) obtained by subtracting the ash content (mass%) from the mass of CNT. The ash content (mass%) of CNT can be measured, for example, in accordance with JIS K 6218-2. The ash content of CNT is a non-flammable component containing metals and the like. From the viewpoint of electrical conductivity, the purity of CNT is preferably 90 mass% or more, more preferably 95 mass% or more, and even more preferably 99 mass% or more, based on the mass of CNT. In addition, the non-flammable component contained in CNT is preferably 10 mass% or less, more preferably 5 mass% or less, and even more preferably 1 mass% or less.

[0054] The BET specific surface area of ​​the CNT of this embodiment is 150 m 2 / g or more, and 2It is more preferable that the BET specific surface area of ​​the CNT is 800 m 2 / g or less, and 2 / g or less is more preferable, and 2 The BET specific surface area of ​​the CNTs can be calculated by the BET method using nitrogen adsorption measurement. The specific surface area of ​​CNTs is often correlated with the average outer diameter of CNTs. The smaller the specific surface area, the larger the outer diameter of CNTs and the fewer the number of CNTs per mass. On the other hand, the larger the specific surface area of ​​CNTs, the smaller the outer diameter of CNTs and the more the number of CNTs per mass. 2 When the specific surface area of ​​the CNT is 800 m / g or more, the number of carbon nanotubes per mass can be secured, and a conductive network can be efficiently formed, so that the battery can have excellent rate characteristics and cycle characteristics. 2 When the CNT content is 1 / g or less, the CNTs are well dispersed, and a good conductive network can be formed in the electrode film.

[0055] The average outer diameter of the CNTs in this embodiment is preferably 3 nm or more, more preferably 5 nm or more. The average outer diameter of the CNTs is preferably 15 nm or less, more preferably 13 nm or less, and even more preferably 11 nm or less. When the average outer diameter of the CNTs is 15 nm or less, the number of carbon nanotubes per mass can be secured, and a conductive network can be efficiently formed. When the average outer diameter of the CNTs is 3 nm or more, the dispersion of the CNTs is good, and a good conductive network can be formed in the electrode film.

[0056] The standard deviation of the average outer diameter of the CNTs is preferably 1 nm to 8 nm, more preferably 1 nm to 6 nm. If the standard deviation of the average outer diameter of the CNTs is large, it may be difficult to efficiently form a conductive network, and the CNTs may become entangled and aggregate in the CNT dispersion or composite slurry and / or electrode film, making it impossible to form a good conductive network.

[0057] The outer diameter and average outer diameter of CNTs are obtained as follows. First, the CNTs are observed and photographed using a transmission electron microscope. Next, 300 CNTs are randomly selected from the photograph and the outer diameter of each is measured. Next, the average outer diameter (nm) of the CNTs is calculated as the number average of the outer diameters.

[0058] CNTs usually exist as aggregates. This shape may be, for example, a state in which a single CNT is intricately entangled (entangled), or an aggregate of linear CNTs (bundle-like). A bundle-like CNT aggregate is easier to disentangle than an entangled CNT aggregate. Also, a bundle-like CNT aggregate has better dispersibility than an entangled CNT aggregate, and can therefore be suitably used as a CNT.

[0059] <Method of manufacturing carbon nanotubes (CNTs)> The CNTs of this embodiment can be produced by, for example, a laser ablation method, an arc discharge method, a thermal CVD method, a plasma CVD method, and a combustion method, but are not limited to these. For example, CNTs can be produced by contacting a carbon source with a catalytic metal at 500 to 1000°C in an atmosphere with an oxygen concentration of 1% by volume or less. The carbon source may be at least one of a hydrocarbon and an alcohol.

[0060] The raw material gas that is the carbon source of CNTs can be any conventionally known gas. For example, the raw material gas containing carbon can be, but is not limited to, hydrocarbons such as methane, ethylene, propane, butane, and acetylene, carbon monoxide, and alcohol. In particular, from the viewpoint of ease of use, it is preferable to use at least one of hydrocarbons and alcohol as the raw material gas.

[0061] <Method of purifying carbon nanotubes (CNTs)> A method for purifying carbon nanotubes (CNTs) will be described below. The CNTs of this embodiment are not limited to those produced through the purification method described below, but the CNTs of this embodiment can be easily obtained by following the purification method described below.

[0062] Representative catalytic metals used when manufacturing CNTs by chemical vapor deposition (thermal CVD) include catalytic metals in which active components such as iron, cobalt, and nickel are fixed to support components such as aluminum and magnesium. Therefore, metals such as aluminum, magnesium, iron, cobalt, and nickel are likely to remain in the CNTs obtained by the above manufacturing process due to the catalytic metals. One example of a method for purifying CNTs is a method of treating with an acid such as nitric acid, and it is known that iron, cobalt, nickel, and the like can be removed by acid treatment. However, it is difficult to remove metals such as aluminum and magnesium by the acid treatment method.

[0063] In contrast, the CNT purification method of this embodiment includes a step (I) of heat-treating the carbon nanotubes used as raw material in an inert atmosphere under reduced pressure vacuum. According to the CNT purification method of this embodiment, the content of metals such as aluminum and magnesium in the CNTs can be easily reduced. The CNT purification method of this embodiment can also reduce the content of other metals such as iron and cobalt in the CNTs, in addition to aluminum and magnesium.

[0064] In the purification method of the above embodiment, the inert atmosphere may be, for example, a nitrogen atmosphere, an argon atmosphere, a vacuum atmosphere, or a combination of these. For example, an inert gas such as nitrogen gas may be introduced into an apparatus used during heat treatment, and the inside of the apparatus may be replaced with the inert gas to obtain an inert atmosphere. Subsequently, the inside of the apparatus is depressurized to maintain a vacuum state (hereinafter referred to as reduced pressure vacuum) while the heat treatment is performed.

[0065] In this specification, "reduced pressure vacuum" means that a state (vacuum state) reduced below atmospheric pressure is maintained by a decompression pump. Specifically, an oil rotary pump, a booster pump, a roots pump, an oil diffusion pump, or the like is used as a decompression pump to reduce the pressure inside the device and maintain a vacuum state. A single decompression pump or a combination of multiple decompression pumps may be used. By reducing the pressure inside the device, it means that the pressure inside the device (internal pressure) is controlled to a target pressure or lower to counteract pressure rise factors caused by leakage from the device or gas expansion due to heating.

[0066] The internal pressure in the reduced vacuum is preferably 10 Pa or less, more preferably 0.1 Pa or less, and even more preferably 0.05 Pa or less. In some embodiments, the internal pressure may be 0.03 Pa or less. In order to obtain an internal pressure in the above range, it is preferable to perform the decompression stepwise. In one embodiment, it is preferable to perform the decompression in two steps. For example, in the first step, the internal pressure is adjusted to preferably 10 Pa or less, more preferably 9.8 Pa or less, and even more preferably 9.6 Pa or less. In the first step, 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 step, 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 step may be 0.03 Pa or less. The internal pressure may increase due to the sublimation of the pyrolysis gas of the CNT and the metal, and it may be difficult to reduce the internal pressure stepwise. In such a case, various methods can be applied to maintain the desired reduced pressure vacuum state, not limited to the method of performing the decompression in two stages as described above. For example, a method of performing only the first stage decompression, or a method of performing the decompression by providing a third stage following the second stage decompression may be applied. For example, in the third stage decompression, the internal air pressure is preferably adjusted to 10 Pa or less, more preferably 9.8 Pa or less, and even more preferably 9.6 Pa or less. In this way, the internal air pressure may be adjusted through a plurality of stages of three or more, and the number of stages is not limited. In some embodiments, the number of stages may be six or more, or seven or more.

[0067] The heat treatment conditions such as the heat treatment temperature and the heat treatment time may be appropriately set depending on the type of CNT and the type of metal derived from the catalytic metal contained in the CNT. The heat treatment temperature is preferably a temperature at which all metals, including aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum, begin to melt. In addition, the catalytic metal used in the production of CNT is at the nano level, and the melting temperature is lower than that of bulk metal due to the nano-size effect, so the heat treatment temperature may be lower than the melting temperature of bulk metal. From this viewpoint, the heat treatment temperature is preferably 1000° C. or higher and 2000° C. or lower. From the viewpoint of suppressing high crystallization of CNTs, the heat treatment temperature may be 1800° C. or lower, 1700° C. or lower, or 1600° C. or lower. In some embodiments, the heat treatment temperature is preferably 1600° C. or lower, and may be 1500° C. or lower. The heat treatment temperature means the temperature inside the apparatus (internal temperature).

[0068] The heat treatment time may be appropriately set depending on the calcination device, calcination scale, etc. However, if the CNTs are calcined at a high temperature for a long time, the crystallinity of the CNTs increases. CNTs with high crystallinity become hard and are easily broken when preparing a CNT dispersion. 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.

[0069] In some embodiments, the heat treatment under reduced pressure vacuum is preferably carried out by adjusting the temperature inside the apparatus (internal temperature) to a range of 500 to 2000° C. and maintaining that temperature for 1 to 100 hours. In the heat treatment, the internal temperature may more preferably be 900 to 1600° C., and further preferably be 1200 to 1400° C. The maintenance time may more preferably be 1 to 50 hours, and further preferably be 2 to 10 hours.

[0070] Usually, when CNTs are purified by heat treatment, they are heated to 500°C or higher in an air atmosphere to perform oxidation and combustion treatment. In contrast, the heat treatment of this embodiment is performed in an inert atmosphere and under reduced pressure vacuum. Therefore, it is possible to suppress combustion of the CNTs themselves. Furthermore, since the CNTs are heat treated in an inert atmosphere, it is possible to suppress oxidation of the surface of the CNTs. It is also possible to shorten the heat treatment time, in which case it is possible to further suppress high crystallization of the CNTs.

[0071] From the viewpoint of further reducing the metal contained in the CNTs, the purification method of the present embodiment may be performed two or more times as necessary. In addition, other treatment steps may be added as necessary within a range that does not deteriorate the properties of the CNTs.

[0072] <Dry grinding of carbon nanotubes (CNTs)> The CNTs of this embodiment may be CNTs that have been dry-pulverized in order to crush the particles and increase the dispersibility. Dry pulverization refers to a process of pulverizing CNTs without the use of a liquid substance. The dry pulverization may be media pulverization, pulverization without media, or a combination of two or more dry pulverizations. For example, in media pulverization, a pulverizer containing pulverization media such as beads or steel balls is used to pulverize particles by utilizing the pulverizing force or destructive force caused by the collision between the pulverization media. As the dry pulverization device, a known method such as a dry attritor, ball mill, vibration mill, or bead mill can be used, and the pulverization time can be set arbitrarily depending on the device or the pulverization state of the particles.

[0073] <2> Carbon nanotube (CNT) dispersion One embodiment of the present invention relates to a CNT dispersion. The CNT dispersion according to this embodiment includes the CNT of the above embodiment, a dispersant, and a dispersion medium. The CNT dispersion in this specification does not contain an electrode active material. In addition, according to this embodiment, a method for producing a CNT dispersion including the CNT of the above embodiment, a dispersant, and a dispersion medium can be provided.

[0074] <Dispersant> The dispersant is not particularly limited as long as it can disperse and stabilize the CNTs, and for example, a surfactant or a resin-type dispersant can be used. Surfactants are mainly classified into anionic, cationic, nonionic, and amphoteric. Depending on the characteristics required for dispersing the CNTs, a suitable type of dispersant can be used in a suitable amount.

[0075] Examples of resin-type dispersants include cellulose derivatives (cellulose acetate, cellulose acetate butyrate, cellulose butyrate, cyanoethyl cellulose, ethyl hydroxyethyl cellulose, nitrocellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxymethyl cellulose, etc.), polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, hydrogenated nitrile butadiene rubber, polyacrylonitrile polymers, etc. Particularly, methyl cellulose, ethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, hydrogenated nitrile butadiene rubber, and polyacrylonitrile polymers are preferred. The molecular weight of the resin-type dispersant is preferably 10,000 to 300,000, more preferably 10,000 to 150,000.

[0076] In addition to the dispersant, it is preferable to add an amine compound or an inorganic base. As the amine compound, a primary amine (primary amine), a secondary amine (secondary amine), or a tertiary amine (tertiary amine) is used, and ammonia and quaternary ammonium compounds are not included. As the amine compound, in addition to monoamine, amine compounds such as diamine, triamine, and tetramine having multiple amino groups in the molecule can be used. Specifically, examples of the inorganic base include, but are not limited to, aliphatic primary amines such as methylamine, ethylamine, butylamine, and octylamine; aliphatic secondary amines such as dimethylamine, diethylamine, and dibutylamine; aliphatic tertiary amines such as trimethylamine, triethylamine, and dimethyloctylamine; amino acids such as alanine, methionine, proline, serine, asparagine, glutamine, lysine, arginine, histidine, aspartic acid, glutamic acid, and cysteine; alkanolamines such as dimethylaminoethanol, monoethanolamine, diethanolamine, methylethanolamine, and triethanolamine; alicyclic nitrogen-containing heterocyclic compounds such as hexamethylenetetramine, morpholine, and piperidine. Examples of the inorganic base include, but are not limited to, hydroxides of alkali metals, hydroxides of alkaline earth metals, carbonates of alkali metals, carbonates of alkaline earth metals, phosphates of alkali metals, and phosphates of alkaline earth metals.

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

[0078] Examples of water-soluble organic solvents include alcohols, polyhydric alcohols, polyhydric alcohol ethers, amines, amides (N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylcaprolactam, etc.); heterocyclics, sulfoxides, sulfones, lower ketones, and others such as tetrahydrofuran, urea, and acetonitrile. Of these, water or an amide-based organic solvent is more preferable, and of the amide-based organic solvents, N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone are particularly preferable.

[0079] When only an amide-based organic solvent is used as the dispersion medium, the water content in the dispersion medium is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

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

[0081] Examples of the dispersion method include a method using various dispersers such as a disperser (dispersing machine), a homogenizer, a high shear mixer, a kneader, a two-roll mill, a three-roll mill, a ball mill, a horizontal sand mill, a vertical sand mill, an annular bead mill, a paint conditioner, an attritor, a planetary mixer, or a high-pressure homogenizer. The disperser is not particularly limited, but for example, from the viewpoint of adjusting the fiber length of the CNT in the CNT dispersion to be within a preferred range, it is preferable to use a high shear mixer from the viewpoint of promoting the wetting of the CNT and dissolving coarse particles and agglomerations, and it is preferable to use a media-type disperser such as a bead mill from the viewpoint of crushing aggregated and solidified particles. In addition, it is more preferable to select and combine a plurality of the above dispersers to disperse, and the order of the dispersers can be changed as desired. The pressure when using the high-pressure homogenizer is not particularly limited, and is preferably 40 to 150 MPa, more preferably 40 to 120 MPa, for example.

[0082] Dispersion methods using a dispersion device include batch dispersion, pass dispersion, circulation dispersion, etc., and any of these methods may be used, or two or more methods may be combined. Batch dispersion is a method in which dispersion is performed using only the dispersion device body without using piping or the like. It is easy to handle, so it is preferable for small-scale production. Pass dispersion is a dispersion method in which the dispersion device body is equipped with a tank that supplies the dispersion liquid through piping and a tank that receives the dispersion liquid, and the dispersion liquid is passed through the dispersion device body. In addition, circulation dispersion is a method in which the dispersion liquid that has passed through the dispersion device body is returned to the tank that supplies the dispersion liquid, and dispersion is performed while circulating. In either case, the longer the processing time, the more the dispersion progresses, so it is sufficient to repeat the pass or circulation until the desired dispersion state is reached, and the processing amount can be increased by changing the size of the tank or the processing time. Pass dispersion is preferable in that it is easier to homogenize the dispersion state than circulation dispersion. Circulation dispersion is preferable in that the work and manufacturing equipment are simpler than pass dispersion. In the dispersion process, the disintegration of aggregated particles, the loosening, wetting, stabilization, etc. of the conductive material proceed sequentially or simultaneously, and the final dispersion state differs depending on how the 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.

[0083] The solid content of the CNT dispersion is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, and particularly preferably 2% by mass or more, based on 100% by mass of the CNT dispersion. The solid content of the CNT dispersion is preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 10% by mass or less, and particularly preferably 8% by mass or less, based on 100% by mass of the CNT dispersion.

[0084] The content of the dispersant in the CNT dispersion is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more, based on 100% by mass of CNT, from the viewpoint of CNT feedability, dispersibility, and dispersion stability. Also, the content of the dispersant in the CNT dispersion is preferably 300% by mass or less, more preferably 100% by mass or less, and even more preferably 50% by mass or less, based on 100% by mass of CNT, from the viewpoint of electrical conductivity.

[0085] The CNT dispersion of the present embodiment is formed using CNTs that contain a small amount of metals such as aluminum, etc. Therefore, the metal content in the CNT dispersion can be easily reduced. However, the CNT dispersion may contain metal particles and dissolved metal ions as metals. Metal particles are metals present in particulate form in the CNT dispersion, and specific examples include the metals contained in the CNTs described above. CNTs, dispersants, and other materials may contain metal particles derived from their respective manufacturing processes, and metal particles may also be mixed in during the manufacturing process of the CNT dispersion. If metal particles are present inside a battery, the battery is likely to short-circuit, so removing the metal particles is very important from the viewpoint of safety. Therefore, in the process of producing a CNT dispersion, it is preferable to include a process of removing contaminants such as metal particles (metal foreign matter removal process) at any timing. From the viewpoint of efficiency, the metal foreign matter removal process is preferably performed during the dispersion process of the CNT dispersion and / or at the end of the dispersion process. The metal foreign matter removal process may be performed multiple times.

[0086] In the metal foreign matter removal step, the method of removing metal particles from the CNT dispersion liquid is not particularly limited, and examples thereof include a method of removing by filtration using a filter, a method of removing by a vibrating sieve, a method of removing by centrifugation, a method of removing by magnetic force, etc. Among these, since metal particles such as iron and chromium have magnetism, a method of removing by magnetic force is preferred, and a method of combining a step of removing by magnetic force and a step of removing by filtration using a filter is more preferred.

[0087] The method of removal by magnetic force is not particularly limited as long as it is a method that can remove metal particles. From the viewpoints of productivity and removal efficiency, however, a method in which a magnetic filter is placed in a production line for the CNT dispersion and the CNT dispersion is passed through the filter to remove the metal particles is preferred. The step of removing metal particles from a CNT dispersion liquid using a magnetic filter is preferably carried out by passing the particles through a magnetic filter that forms a magnetic field with a magnetic flux density of 1,000 Gauss or more. Since a low magnetic flux density reduces the efficiency of removing metal particles, the magnetic flux density is preferably 5,000 Gauss or more, more preferably 10,000 Gauss or more in consideration of removing stainless steel, which has a low magnetic property, and most preferably 12,000 Gauss or more. Depending on the flow rate of the filtration, coarse metal particles may pass through the magnetic filter, so when arranging the magnetic filter in the production line, it is preferable to include a process of removing coarse foreign matter or metal particles using a filter such as a cartridge filter upstream of the magnetic filter. Although the magnetic filter is effective even if it only filters once, it is more preferable that the magnetic filter is of a circulating type. By adopting a circulating type, the efficiency of removing metal particles is improved. When a magnetic filter is arranged in a production line for a CNT dispersion liquid, the location of the magnetic filter is not particularly limited, but it is preferable to arrange the magnetic filter immediately before filling a container with the carbon nanotube dispersion liquid, and in the case where a filtration process using a filtration filter is performed before filling the container, it is preferable to arrange the magnetic filter before the filtration filter. By arranging the magnetic filter in this way, it is possible to prevent metal from being mixed into the product when it is detached from the magnetic filter.

[0088] The metal content in the CNT dispersion can be calculated by drying the CNT dispersion to remove the solvent, and then analyzing it using ICP. The metal content detected by ICP analysis includes metal particles and dissolved metal ions. That is, the metal content of the CNT dispersion that has undergone the metal foreign matter removal process includes metal particles that have not been completely removed and dissolved metal ions.

[0089] The content of the metals aluminum, iron and chromium contained in the CNT dispersion is preferably 100 ppm or less, more preferably 50 ppm or less, and even more preferably 10 ppm or less, based on 100 mass% of the CNT dispersion. By setting the metal content within the above range, side reactions in the electrode film are less likely to occur, and a secondary battery with better conductivity can be obtained.

[0090] The CNT dispersion may further contain, as a conductive material, one or more carbon materials such as carbon black, graphite, etc. Among these conductive materials, carbon black is preferred from the viewpoint of the adsorption performance of the dispersant.

[0091] <3> Binder Composition One embodiment of the present invention relates to a binder composition. The binder composition according to this embodiment includes the above-mentioned CNT dispersion and a binder. The binder composition in this specification does not contain an electrode active material. In addition, according to this embodiment, a method for producing a binder composition including the above-mentioned CNT, a dispersant, a dispersion medium, and a binder can be provided.

[0092] <Binder> The binder is a resin that binds various substances together in the electrode film. As the binder, binders known for batteries can be used. Examples include cellulose resins such as carboxymethyl cellulose; and rubbers such as styrene-butadiene rubber and fluororubber. Modified bodies, mixtures, and copolymers of these resins may also be used. In particular, from the standpoint of resistance, it is preferable to use polymer compounds having fluorine atoms in the molecule, such as polyvinylidene fluoride, polyvinyl fluoride, and tetrafluoroethylene.

[0093] The weight-average molecular weight of the binder is preferably 10,000 or more, more preferably 100,000 or more, and particularly preferably 200,000 or more. Also, the weight-average molecular weight of the binder is preferably 2,000,000 or less, more preferably 1,500,000 or less, and particularly preferably 1,000,000 or less. When the weight-average molecular weight is 10,000 or more, it is possible to suppress a decrease in the resistance and adhesion of the binder. When the weight-average molecular weight is 2,000,000 or less, while improving the resistance and adhesion of the binder, it is possible to suppress a decrease in workability due to an increase in the viscosity of the binder itself, and to suppress significant aggregation of the dispersed particles.

[0094] The binder composition is preferably produced by mixing and homogenizing the CNT dispersion and the binder, and the binder may be pre-dissolved and used. Also, the binder may be added at any timing in the process of producing the CNT dispersion. The mixing method may be various conventionally known methods. The binder composition can be produced using the dispersion device described for the above CNT dispersion. The binder in the binder composition may be one type or two or more types may be used in combination. Furthermore, the manufacturing process of the binder composition may include the above-described metal foreign matter removal process.

[0095] <4> Composition for electrode One embodiment of the present invention relates to a composition for an electrode. The composition for an electrode according to this embodiment contains the above-described CNT dispersion and an electrode active material. The composition for an electrode can be further mixed with a binder to produce a composite slurry. Also, according to this embodiment, it is possible to provide a method for producing a binder composition containing the above-described CNT, dispersant, dispersion medium, and electrode active material, and a method for producing a binder composition containing the above-described CNT, dispersant, dispersion medium, electrode active material, and composite slurry.

[0096] <Electrode active material> An electrode active material is a material that serves as the basis for a battery reaction. The active material is divided into a positive electrode active material and a negative electrode active material based on the electromotive force. The positive electrode active material is not particularly limited, and metal compounds such as metal oxides and metal sulfides capable of doping or intercalating lithium ions, and conductive polymers can be used. For example, oxides of transition metals such as Fe, Co, Ni, and Mn, composite oxides with lithium, inorganic compounds such as transition metal sulfides, etc. can be mentioned. Specifically, transition metal oxide powders such as MnO, V 2 O 5 、V 6 O 13 、TiO 2 etc.; composite oxide powders of lithium and transition metals such as lithium nickelate, lithium cobaltate, lithium manganate with a layered structure, and lithium manganate with a spinel structure; lithium iron phosphate-based materials which are phosphate compounds with an olivine structure, etc. can be mentioned. These positive electrode active materials can also be used alone or in combination of two or more. Further, the above inorganic compounds and organic compounds may be mixed and used. The negative electrode active material is not particularly limited, and those capable of doping or intercalating lithium ions can be used. For example, alloy systems such as metallic Li, its alloys such as tin alloys, silicon alloys, and lead alloys; Li x Fe 2 O 3 、Li x Fe 3 O 4 、Li x WO 2 (x is a number where 0 < x < 1.), metal oxide systems such as lithium titanate, lithium vanadate, and lithium silicate; amorphous carbonaceous materials such as soft carbon and hard carbon, or carbonaceous powders such as artificial graphite or natural graphite which are highly graphitized carbon materials. These negative electrode active materials can also be used alone or in combination of two or more. In particular, using a combination of a highly graphitized carbon material and lithium silicate is preferable from the viewpoints of capacity and lifespan.

[0097] The BET specific surface area of the electrode active material is preferably 0.1 to 10 m 2 / g, more preferably 0.2 to 5 m 2 / g, and even more preferably 0.3 to 3 m 2 / g. The average particle size of the electrode active material is preferably 0.05 to 100 μm, and more preferably 0.1 to 50 μm. In this specification, the term "average particle size of the electrode active material" refers to the average value of particle sizes measured by an electron microscope.

[0098] The electrode composition is preferably produced by mixing and homogenizing the CNT dispersion liquid and the electrode active material, and the binder may be dissolved in the CNT dispersion liquid beforehand. The electrode active material may be added at any timing in the process of producing the CNT dispersion liquid. The dispersing device used for the treatment of dispersing the electrode active material is not particularly limited, and the dispersing devices exemplified in the production of the CNT dispersion liquid may be used.

[0099] In the case of a composite slurry containing an electrode composition, the content of the electrode active material contained in the composite slurry is preferably 20% by mass or more, more preferably 40% by mass or more, based on 100% by mass of the composite slurry. The content of the electrode active material contained in the composite slurry is preferably 99% by mass or less, more preferably 97% by mass or less, based on 100% by mass of the composite slurry. The above range is preferable from the viewpoint of coatability or productivity, and from the viewpoint of uniformity of the electrode film. The CNT content in the composite slurry is preferably 0.01% by mass or more, and more preferably 0.05% by mass or more, relative to 100% by mass of the electrode active material. The CNT content in the composite slurry is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, relative to 100% by mass of the electrode active material. The content of the binder in the composite slurry is preferably 0.3% by mass or more, and more preferably 0.7% by mass or more, relative to 100% by mass of the electrode active material. The content of the binder in the composite slurry is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, relative to 100% by mass of the electrode active material. The solid content of the composite material slurry is preferably 30% by mass or more, more preferably 40% by mass or more, based on 100% by mass of the composite material slurry. Also, the solid content of the composite material slurry is preferably 90% by mass or less, more preferably 85% by mass or less, based on 100% by mass of the composite material slurry. The water content contained in the composite material slurry is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

[0100] <5>Electrode film One embodiment of the present invention relates to an electrode film. The electrode film according to this embodiment is an electrode film obtained using (i) a carbon nanotube dispersion liquid containing carbon nanotubes, a dispersant, and a dispersion medium, (ii) a binder composition containing the carbon nanotube dispersion liquid and a binder, or (iii) a composition for an electrode containing the carbon nanotube dispersion liquid and an electrode active material. As the carbon nanotubes, the carbon nanotubes of the above-described embodiment are used. Also, the electrode film may be an electrode film obtained using (iv) a composite material slurry. (iv) The composite material slurry can be obtained using the aforementioned (i) carbon nanotube dispersion liquid, (ii) binder composition, or (iii) composition for an electrode.

[0101] For example, the electrode film is a coating film formed by coating and drying the above-described composite material slurry on a current collector. The material and shape of the current collector used for the electrode film are not particularly limited, and those suitable for various secondary batteries can be appropriately selected. For example, examples of the material of the current collector include metals and alloys such as aluminum, copper, nickel, titanium, or stainless steel.

[0102] There is no particular limitation on the method of coating the composite material slurry on the current collector, and known methods can be used.

[0103] After coating and drying, the coating may be rolled using a lithographic press or a calendar roll, etc. The thickness of the electrode film is generally 1 μm or more and 500 μm or less, and preferably 10 μm or more and 300 μm or less.

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

[0105] As the electrolyte, various known electrolytes in which ions can move can be used. For example, LiBF 4 , LiClO 4 , LiPF 6 , LiAsF 6 , LiSbF 6 , LiCF 3 SO 3 , Li(CF 3 SO 2 ) 2 N, LiC 4 F 9 SO 3 , Li(CF 3 SO 2 ) 3 C, LiI, LiBr, LiCl, LiAlCl, LiHF2 , LiSCN, or LiBPh 4 (where Ph is a phenyl group), etc. Those containing lithium salts can be mentioned, but are not limited thereto, and those containing sodium salts can also be used. The electrolyte is preferably dissolved in a non-aqueous solvent and used as an electrolytic solution. An all-solid electrolyte or a polymer electrolyte may be used.

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

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

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

Examples

[0109] Examples are given below to explain the present invention more specifically. The present invention is not limited to the following examples as long as it does not exceed the gist thereof.

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

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

[0112] <Preparation of Binder Composition> Volume 150 cm3 The CNT dispersion (A) and PVdF (polyvinylidene fluoride, Solef5130, Solvay, non-volatile content 100%), which had been dissolved in advance in NMP (N-methyl-2-pyrrolidone) to a solid content of 8 mass%, were added to the plastic container. After that, the mixture was stirred at 2000 rpm for 30 seconds using a planetary centrifugal mixer (Awatori Rentaro, ARE-310). In this way, the binder composition (A) was obtained.

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

[0114] <Preparation of electrode film> The electrode composition (A) was applied to the electrode using an applicator so that the amount of the electrode per unit area was 20 mg / cm 2 After coating, the coating was dried in an electric oven at 120°C ± 5°C for 25 minutes to obtain an electrode film (A). The electrode film (A) was then rolled using a roll press (3t hydraulic roll press, manufactured by Sun Metals) to obtain a positive electrode (A). The weight per unit area of ​​the composite layer was 20 mg / cm. 2 The density of the composite layer after rolling was 3.1 g / cc.

[0115] <Preparation of secondary battery> The positive electrode (A) and the standard negative electrode were punched out to 45 mm × 40 mm and 50 mm × 45 mm, respectively. These electrodes and a separator (porous polypropylene film) were inserted between them in an aluminum laminate bag and dried in an electric oven at 60°C for 1 hour. Thereafter, 2 mL of electrolyte (non-aqueous electrolyte) was injected into the glove box filled with argon gas, and the aluminum laminated bag was then sealed to prepare a laminated secondary battery (A). The above standard negative electrode and non-aqueous electrolyte were prepared as follows. (Standard negative electrode) 0.5 parts by mass of acetylene black (Denka Black (registered trademark) HS-100, manufactured by Denka), 1 part by mass of MAC500LC (carboxymethylcellulose sodium salt Sunrose special type MAC500LC, manufactured by Nippon Paper Industries Co., Ltd., non-volatile content 100%), and 98.4 parts by mass of water were added to a plastic container with a capacity of 150 ml, and then the mixture was stirred for 30 seconds at 2000 rpm using a centrifugal mixer (Thinky Awatori Rentaro, ARE-310). Furthermore, 92 parts by mass of artificial graphite (manufactured by Nippon Graphite Industry Co., Ltd., CGB-20) and 5 parts by mass of silicon oxide (manufactured by Osaka Titanium Technology Co., Ltd., SILICON MONOOXIDE SiO 1.3C 5 μm, non-volatile content 100%) were added as active materials, and the mixture was stirred for 10 minutes at 3000 rpm 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 centrifugal mixer to obtain a negative electrode composite slurry. Thereafter, the negative electrode composite slurry was spread using an applicator so that the weight per unit area of ​​the electrode was 8 mg / cm. 2 After coating on copper foil so that the thickness was 1.6g / cm, the coating was dried in an electric oven at 120°C ± 5°C for 25 minutes. Then, rolling was performed using a roll press (Thank Metal Co., Ltd., 3t hydraulic roll press) to reduce the density of the composite layer to 1.6g / cm. 3 A standard negative electrode was prepared. (Non-aqueous electrolyte) First, ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 1:1:1 to prepare a mixed solvent. Next, 2 parts by mass of VC (vinylene carbonate) was added as an additive to 100 parts by mass of this mixed solvent, and LiPF 6 was dissolved at a concentration of 1 M to obtain a non-aqueous electrolyte solution.

[0116] [Examples 2 to 9], [Comparative Examples 1 to 4] CNTs (B) to (Y1) were obtained in the same manner as in Example 1, except that the CNT type, holding temperature, and treatment time were changed as shown in Table 1. Furthermore, using each of the obtained CNTs, a CNT dispersion, a binder composition, a composition for an electrode, an electrode film, and a secondary battery were produced in the same manner as in Example 1.

[0117] [Example 10] 15 parts of CNTs and 40 parts of zirconia beads with a diameter of 2 mm were added to a glass bottle (M-225, manufactured by Kashiwa Glass Co., Ltd.), and dry-treated for 1 minute using an automatic shaker (SK450, manufactured by Fast and Fluid Management Co., Ltd.). This procedure was then repeated to produce 60 parts of CNTs, which were then used to obtain CNTs (J) in the same manner as in Example 9. Furthermore, using the CNT(J) obtained as described above, a CNT dispersion, a binder composition, a composition for an electrode, an electrode film, and a secondary battery were produced in the same manner as in Example 1.

[0118] [Comparative Example 5] Using an electronic balance (MSA225S100DI, manufactured by Sartorius), 50 parts of CNTs (JENOTUBE10B, manufactured by JEIO) were weighed into an alumina crucible SSA-HB4 (manufactured by Nikkato), and the crucible containing the CNTs was placed in a multipurpose high-temperature furnace (Hi-Multi 5000, manufactured by Fuji Denpa Kogyo Co., Ltd.). Next, in a nitrogen atmosphere with a nitrogen flow rate of 2.0 L / min, the temperature inside the furnace was raised to 1200°C at a heating rate of 20°C / min, and the temperature was maintained at 1200°C for 6 hours, after which the furnace was allowed to cool naturally to 50°C. Subsequently, 10 parts of the heat-treated CNT were weighed into a 1-L glass container, 500 parts of 10% hydrochloric acid (manufactured by FUJIFILM Wako Pure Chemical Corporation) were added, and then the mixture was sufficiently stirred using a stirrer. Subsequently, it was sufficiently diluted with ion-exchanged water, and vacuum filtration was performed using a membrane filter. The operations of dilution and filtration were repeated, and after confirming that the pH of the filtrate became 4 or higher, the CNT was transferred to a PTFE vat. Next, it was dried at 140 °C using an oven. Thus, CNT (X4) was obtained. Furthermore, using the CNT (X4) obtained as described above, in accordance with the same method as in Example 1, a CNT dispersion, a binder composition, an electrode composition, an electrode film, and a secondary battery were produced.

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

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

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

[0122] <Wettability index of CNT> Under a 25 °C environment, 5 g (Y (g)) of CNT powder was placed in a cylindrical polypropylene container with a diameter of 10 cm by natural dropping and allowed to stand. N-methyl-2-pyrrolidone (NMP) was dropped onto the surface of the CNT powder at 5 g per time and at 1-minute intervals until just before NMP began to flow out on the surface of the CNT powder without being absorbed by the CNT powder. The total mass (X (g)) of the dropped NMP was measured. The wettability index of CNT was calculated from the following formula (I). Formula (I): Wettability index = (X / Y) [In formula (I), Y is the mass (g) of CNT, and X is the maximum mass (g) of NMP absorbed by CNT when NMP is dropped onto Y (g) of CNT under a 25 °C environment.]

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

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

[0125] <Amount of surface oxygen of CNT> The amount of surface oxygen of CNT was measured using an X-ray photoelectron spectrometer (XPS, manufactured by ThermoFisher Scientific, K-Alpha). After pelletizing the CNT, this sample was fixed to the sample stage with double-sided tape for measurement. The carbon atoms and oxygen atoms on the surface of the CNT sample were detected by XPS. Here, the ratio (atm%) of oxygen atoms to carbon atoms was calculated as the amount of surface oxygen.

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

[0127] <Cohesion of CNT> The cohesive force of CNTs was measured using a rheometer (MCR302e, manufactured by Anton Paar). First, 4.0 g of CNTs were placed in a dedicated aluminum container (C-CC27 / D / Al), and the carbon nanotubes were compressed at 12 kPa using a cylinder jig for compression. Then, it was replaced with a blade jig for measuring the cohesive force, and while moving the jig at 125 μm / s and 0.1 rotation per minute, it was penetrated into the carbon nanotubes, and the torque when applying a shear force was measured, and the maximum peak value was taken as the cohesive force of the CNTs. A C-PTD200 temperature control device was used, and the measurement was carried out at a temperature of 25°C.

[0128] 2-2. Evaluation of Other Properties For the CNT dispersions, binder compositions, electrode compositions, electrode films, and secondary batteries prepared in the above-described examples and comparative examples, measurements were carried out and the properties were evaluated as follows. The evaluation results for each are shown in Table 1. <Viscosity Stability of CNT Dispersion> After the CNT dispersion was left standing in a constant temperature bath at 40°C for one week, the CNT dispersion was cooled to 25°C, and immediately measured using a B-type viscometer at a rotor rotation speed of 30 rpm. The evaluation criteria for viscosity stability are as follows. (Evaluation Criteria) ◎(Excellent): 3000 mPa·s or less 〇(Good): Exceeding 3000 mPa·s and 5000 mPa·s or less △(Fair): Exceeding 5000 mPa·s and 10000 mPa·s or less ×(Poor): Exceeding 10000 mPa

[0129] <Particle Size (D90) of CNT Dispersion> The particle size (D90) was measured using a particle size distribution analyzer (Partica LA-960V2, manufactured by HORIBA). The circulation / ultrasonic operation conditions were: circulation speed: 3, ultrasonic intensity: 7, ultrasonic time: 1 minute, stirring speed: 1, stirring mode: continuous. In addition, ultrasonic operation was performed with ultrasonic intensity 7 and ultrasonic time 5 seconds during air removal. The refractive index of NMP was 1.468, and the refractive index of CNT was 1.920. The measurement was performed after diluting the measurement sample so that the transmittance of the red laser diode was 60 to 70%, and the particle size standard was volume. The evaluation criteria for particle size are as follows. (Evaluation Criteria) ◎ (Excellent): 0.6 μm or more and less than 2.0 μm 〇 (Good): 2.0μm or more and less than 5.0μm - (Failure): Less than 0.6 μm or more than 5.0 μm It was decided.

[0130] <Cycle characteristics of secondary batteries> The secondary battery was placed in a constant temperature room at 25°C, and charge / discharge measurements were performed using a charge / discharge device (Hokuto Denko Corporation, SM-8). After constant current / constant voltage charging (cutoff current 1.25mA (0.025C)) was performed with a charge current of 50mA (1C) and a charge end voltage of 4.2V, constant current discharging was performed with a discharge current of 50mA (1C) and a discharge end voltage of 2.5V. This operation was repeated 200 times. 1C was the current value at which the theoretical capacity of the positive electrode was discharged in 1 hour. The cycle characteristics can be expressed as the ratio of the 3rd 1C discharge capacity at 25°C to the 200th 1C discharge capacity, as shown in the following formula 3. (Formula 3): Cycle characteristics = 200th 1C discharge capacity / 3rd 1C discharge capacity x 100 (%) The evaluation criteria for the cycle characteristics of the secondary battery are as follows. (Evaluation Criteria) ◎(Excellent): 90% or more 〇(Good): 85% to less than 90% △(Acceptable): 80% or more but less than 85% × (defective): Less than 80%

[0131] <Evaluation of volume resistivity of electrodes> The electrode compositions prepared in the Examples and Comparative Examples were applied to the electrodes using an applicator so that the coating weight per unit area was 20 mg / cm 2 The mixture was then dried in an electric oven at 120° C.±5° C. for 30 minutes to prepare a composite coating film. The surface resistivity (Ω / □) of the composite layer of the prepared composite coating was measured using Mitsubishi Chemical Analytech's Loresta GP, MCP-T610. After measurement, the thickness of the composite layer was multiplied to obtain the volume resistivity (Ω·cm) of the electrode. The thickness of the composite layer was calculated by subtracting the film thickness of the PET foil from the average value measured at three points in the electrode using a film thickness meter (NIKON, DIGIMICRO MH-15M). The evaluation criteria for the volume resistivity of the electrode are as follows: (Evaluation Criteria) ◎(Excellent): Less than 7Ω·cm Good: 7 Ω·cm or more and less than 15 Ω·cm △(Acceptable): 15Ω·cm or more and less than 20Ω·cm × (defective): 20Ω cm or more

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

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

[0134] [Table 1]

[0135] As shown in Table 1, the CNTs of this embodiment (Examples 1 to 10) satisfy all of the requirements of (1) G / D ratio, (2) wettability, and (3) aluminum content described above. In comparison with the comparative examples using CNTs that do not satisfy the above requirements (1) to (3), it is understood that the use of the CNTs of this embodiment can improve the characteristics of the CNT dispersion, electrode film, and secondary battery. Thus, the CNTs of this embodiment can form an electrode film with excellent conductivity, and the safety of the secondary battery can be improved.

Claims

1. Carbon nanotubes that satisfy the following (1) to (3) and include multi-walled carbon nanotubes: (1) 1560 to 1600 cm in Raman spectrum －１ The maximum peak intensity in the range of G, 1310 to 1350 cm －１ When the maximum peak intensity within the range is defined as D, the G / D ratio is 0.5 or more and 3.0 or less. (2) The wetting index represented by the following formula (I) is 10 or less. Formula (I): Wetting Index = (X / Y) [In formula (I), Y is the mass (g) of the carbon nanotube, and X is the maximum mass (g) of N-methyl-2-pyrrolidone absorbed by the carbon nanotube when N-methyl-2-pyrrolidone is dropped onto carbon nanotube of Y (g) in a 25° C. environment.] (3) The aluminum content is 3000 ppm or less.

2. The carbon nanotube has a volume resistivity of 2.0×10 －２ 2. The carbon nanotube according to claim 1, having a resistance of Ω·cm or less.

3. The carbon nanotubes according to claim 1 , wherein the carbon nanotubes have a cohesive strength of 7.5 kPa or less.

4. 2. The carbon nanotube according to claim 1, which has an exothermic peak at 600° C. or more and 800° C. or less in a differential thermal analysis when the carbon nanotube is heated from 200° C. to 1000° C. at a rate of 10° C. / min.

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

6. 6. The carbon nanotube dispersion according to claim 5, which has a viscosity of 5000 mPa·s or less at 25° C. as measured by a Brookfield viscometer after being left to stand at 40° C. for one week.

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

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

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

Images