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

Abstract

The present invention provides carbon nanotubes that can improve safety and can be suitably used for forming an electrode film having good electrical conductivity, as well as a carbon nanotube dispersion, a binder composition, an electrode composition, and a secondary battery that each contain the carbon nanotubes. SOLUTION: Carbon nanotubes that satisfy the following (1) to (3) and include multi-walled carbon nanotubes. (1) In a differential thermal analysis when the temperature is increased from 200° C. to 1,000° C. at 10° C. / min, an exothermic peak is observed between 600° C. and 800° C. (2) 1560-1600 cm in the Raman spectrum -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. (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, etc. 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 produced by methods such as arc discharge method, laser evaporation method, chemical vapor deposition method, etc. Among these, the chemical vapor deposition method is most suitable for mass production from the viewpoints of productivity and economy and is widely used. In the chemical vapor deposition method, a catalyst containing metals such as iron, cobalt, nickel, etc. is used to react a gas serving as a carbon source to generate CNTs. Therefore, the CNTs obtained by the chemical vapor deposition method contain particles such as a catalyst containing metals such as iron, cobalt, nickel, etc., or carbides or oxides derived from the catalyst. A catalyst containing a metal (hereinafter also referred to as a catalyst metal) is indispensable in some methods for producing CNTs. However, after the production of CNTs, the metal derived from the catalyst metal remaining in the CNTs can become impurities. Also, regardless of the production method of CNTs, metals may be mixed into the CNTs due to abrasion of metals used in synthesis apparatuses, filling apparatuses, or pipes, etc. used during production. The mixed metal can become an impurity. From the viewpoint of obtaining desired properties by CNTs, it is desirable that the metal content in the CNTs is low. For example, when using CNTs containing a metal derived from a catalyst metal or the like in a secondary battery, problems such as the metal dissolving and precipitating to short-circuit the battery may occur. When the battery is short-circuited, it may lead to serious accidents such as ignition or explosion. Therefore, in order to suppress problems caused by the metal contained in the CNTs and enhance safety more, several methods for purifying the CNTs to remove the metal derived from the catalyst metal or the like have been proposed.

[0005] Patent Document 1 describes that by using a raw material containing at least carbon and a catalyst metal as an anode, a carbon material production step of producing a carbon material containing CNTs by an arc discharge method, and a halogen treatment step of bringing the carbon material into contact with a gas containing a halogen and / or a halogen compound, the CNTs are purified while suppressing damage or cutting of the CNTs or solidification of the CNTs into lumps, and removing the catalyst metal which is an impurity.

[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 catalytic metal residues, have high heat resistance, and produce fewer carbon by-products. [Prior art documents] [Patent documents]

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

[0008] However, when 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 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) In a differential thermal analysis when the temperature is increased from 200° C. to 1,000° C. at 10° C. / min, an exothermic peak is observed between 600° C. and 800° C. (2) 1560-1600 cm in the Raman spectrum -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. (3) The aluminum content is 3000 ppm or less.

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

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

[0015] <4> the above <1> ~ <3> 13. A carbon nanotube dispersion comprising the carbon nanotubes according to any one of the above items, a dispersant, and a dispersion medium.

[0016] <5> After being stored at 60°C for one week, the viscosity at 25°C measured with a B-type viscometer is 5000 mPa s or less. <1> ~ <4> 13. A carbon nanotube dispersion liquid according to any one of the above.

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

[0018] <7> the above <4> or <5> 2. A composition for an electrode comprising the carbon nanotube dispersion liquid according to claim 1 and an electrode active material.

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

[0020] 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

[0021] 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.

[0022] 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.

[0023] <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) In a differential thermal analysis when the temperature is increased from 200° C. to 1,000° C. at 10° C. / min, an exothermic peak is observed between 600° C. and 800° C. (2) 1560-1600 cm in the Raman spectrum -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. (3) The aluminum content is 3000 ppm or less.

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

[0025] <Exothermic peak> The CNT of this embodiment has an exothermic peak at 600°C or more and 800°C or less in differential thermal analysis (DTA) when the temperature is increased from 200°C to 1000°C at 10°C / min. The exothermic peak can be measured by subjecting the CNT to differential thermal analysis in an air atmosphere. DTA is a method in which the temperature difference between a sample and a reference substance is measured as a function of temperature while the temperatures of the sample and the reference substance are changed under certain conditions, and is in accordance with JIS K 0129. In a DTA curve created based on the change in the temperature difference between the sample and the reference substance, the largest peak is regarded as the exothermic peak.

[0026] Heat is generated as the CNTs burn. As the CNT combustion start temperature increases, the heat generation peak temperature also increases. Factors that affect the change in the CNT combustion start temperature include the content of metal components derived from the catalyst metal, the degree of oxidation of the CNT surface, and the crystallinity of the CNT. When the heat storage capacity of the metal components contained in the CNT is high, the total heat storage capacity of all catalyst metals is small when the content of the metal components is low. Since the total heat storage capacity of the metal components is small, the temperature for burning the CNTs may be higher than when the content of the metal components is high. In addition to the catalyst metal, metal components may be mixed into the CNTs during the CNT manufacturing process, and such metal components may also affect the total heat storage capacity. In addition, at the CNT surface, the sites with oxygen-containing functional groups are more likely to burn compared to the sites without functional groups. Therefore, the less the amount of oxygen-containing functional groups (i.e., the less the surface oxygen amount), the more difficult it is for the CNT to burn, and the smaller the degree of oxidation of the CNT surface, the higher the temperature required to burn the CNT. Furthermore, the higher the crystallinity of the CNT, the higher the combustion start temperature of the CNT. The crystallinity of the CNT can be represented by the G / D ratio described later.

[0027] When the combustion start temperature of the CNT is within an appropriate temperature range, the impurities contained in the CNT are reduced, so that a safer CNT can be obtained. The CNT of this embodiment preferably has a heat generation peak temperature of 600 °C or higher, more preferably 650 °C or higher. Also, the heat generation peak temperature is preferably 800 °C or lower, more preferably 740 °C or lower. When the heat generation peak temperature is 600 °C or higher, the metal content is low, and the safety of the battery can be improved. Or, the surface oxygen amount is small and it has excellent conductivity. When the heat generation peak temperature is 800 °C or lower, the crystallinity of the CNT is not too high, the breakage of the CNT can be suppressed, and the degradation of the performance of the secondary battery can be suppressed. For example, it is preferably 600 °C or higher and 800 °C or lower, 630 °C or higher and 750 °C or lower, or 650 °C or higher and 740 °C or lower.

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

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

[0030] CNTs have a Raman spectrum of 1560 to 1600 cm -1 The maximum peak intensity in the range of 1310 to 1350 cm is G. -1 When the maximum peak intensity within the range is D, the G / D ratio is 0.5 to 3.0, more preferably 0.5 to 2.5, even more preferably 0.5 to 2.0, particularly preferably 0.5 to 1.5, and even more preferably 0.5 to 1.3. If the G / D ratio of the CNT exceeds the above range, the CNT becomes hard, so that the CNT is easily damaged during dispersion, and the contact resistance may increase. On the other hand, if the G / D ratio of the CNT falls below the above range, the conductivity of the CNT itself is likely to be low. As a result, if the G / D ratio of the CNT is within the above range, the rate characteristics and cycle characteristics of a secondary battery using an electrode film using the CNT dispersion liquid are improved.

[0031] Conventionally, when raw CNT is subjected to a halogen treatment process to purify CNT, the crystallinity of CNT tends to increase because CNT is baked at high temperature for a long time. This state can also be confirmed by the fact that the G / D ratio increases. On the other hand, according to the present embodiment, heat treatment can be performed in an inert atmosphere under reduced pressure vacuum to purify CNT. Therefore, the heat treatment temperature can be kept low and the heat treatment can be performed for a short time, and high crystallization of CNT can be suppressed. In other words, it is possible to suppress the G / D ratio of CNT from increasing. When low-crystalline CNT is used, the breakage of CNT can be suppressed by dispersion treatment or the like in the process of manufacturing the electrode film, and the increase in contact resistance between CNTs in the obtained electrode film can be suppressed. As a result, low-crystalline CNT can obtain good conductivity as an electrode film, and a secondary battery including CNT can exhibit good performance.

[0032] <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.

[0033] 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 further preferably 500 ppm or less. The aluminum content may be 0 ppm. When aluminum oxide is included as a catalyst carrier during synthesis of CNTs, aluminum oxide nanoparticles, which are insulating components, may remain in the CNTs, impairing electrical conductivity. When the aluminum content of the CNTs is within the above range, the carbon nanotubes can be formed into an electrode film having good electrical conductivity with few aluminum oxide nanoparticles, which are insulating components. In addition, when the aluminum content of the CNTs is within the above range, a conductive paste having excellent electrical conductivity can be produced.

[0034] Representative CNTs also contain metals other than aluminum (hereinafter referred to as other metals) that originate from the catalyst metal. The other metals may be magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum. In addition to the metals and metal oxides that originate from the catalyst metal, metals such as stainless steel used in synthesis equipment, filling equipment, or piping may be mixed into the CNTs due to wear and tear. For this reason, in the CNTs, the metal components may be aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum, but these may also be metals that do not originate from the catalyst metal. From this viewpoint, the total content of aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum in the CNT is preferably 13000 ppm or less, more preferably 10000 ppm or less, even more preferably 9000 ppm or less, and even more preferably 8000 ppm or less.

[0035] In some embodiments, the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum contained in the CNTs is preferably 10000 ppm or less. The total content may be 8000 ppm or less, or 7000 ppm or less. In some embodiments, the total content is more preferably 5000 ppm or less, even more preferably 3000 ppm or less, even more preferably 2100 ppm or less, and even more preferably 830 ppm or less. When the total content of these metals is within the above range, it is easier to improve the safety of the secondary battery. In the following description, cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum may be collectively referred to simply as other metals.

[0036] Here, the aluminum content and other metal content in CNTs are masses converted into elemental metals. 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, 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.

[0037] In some embodiments, from the viewpoint of further enhancing safety, it is preferable to limit the content of cobalt and / or iron among the other metals. In some embodiments, the total cobalt content in the CNTs is preferably 5000 ppm or less, more preferably 3000 ppm or less, even more preferably 2000 ppm or less, and even more preferably 1250 ppm or less. Also, in some embodiments, the total iron content in the CNTs is preferably 5000 ppm or less, more preferably 3000 ppm or less, even more preferably 2500 ppm or less, and even more preferably 1000 ppm or less. Furthermore, in some embodiments, the total cobalt and iron content in the CNTs is preferably 5000 ppm or less, more preferably 3000 ppm or less, even more preferably 2500 ppm or less, and even more preferably 1300 ppm or less.

[0038] In this embodiment, as in the 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.

[0039] The aluminum content and other metal content in CNT can be calculated, for example, by acid decomposing CNT, 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 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, 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, 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.

[0040] <Surface oxygen content> From the viewpoint of the electrical conductivity of the CNT, the CNT of this embodiment preferably has a surface oxygen content of less than 1.0 atm%. The surface oxygen content is preferably 0.9 atm% or less, more preferably 0.8 atm% or less, even more preferably 0.7 atm% or less, and particularly preferably 0.6 atm% or less. When the surface oxygen content is 1.0 atm% or less, excellent electrical conductivity as an electrode film can be easily obtained by using the 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 the CNT determined by X-ray photoelectron spectroscopy.

[0041] When nitric acid is used for purifying CNT as a raw material, since the oxidizing power of nitric acid is strong, the surface of CNT will be oxidized. On the other hand, in this embodiment, like the CNT purification method described later, by heat-treating the CNT as a raw material in an inert atmosphere under reduced pressure and vacuum, the content of metal components such as aluminum can be reduced, and treatment with an acid is not necessarily required. Therefore, oxidation of the CNT surface can be suppressed and the amount of surface oxygen of the CNT can be reduced. As a result, the combustion start temperature of the CNT becomes higher, and the CNT can obtain good conductivity as an electrode film, and the secondary battery containing the CNT can exhibit good performance.

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

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

[0044] The purity of the CNT is represented by the value (% by mass) obtained by subtracting the ash content (% by mass) from the mass of the CNT. The ash content (% by mass) of the CNT can be measured, for example, in accordance with JIS K 6218-2. The ash content of the CNT is a non-combustible component containing metals and the like. From the viewpoint of conductivity, the purity of the CNT is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more, based on the mass of the CNT. Further, the non-combustible component contained in the CNT is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less.

[0045] The volume resistivity of the CNT of the present embodiment is preferably from 1.0×10 -2 to 2.5×10 -2 Ω·cm, more preferably from 1.0×10 -2 to 2.2×10 -2 Ω·cm, even more preferably from 1.0×10 -2 to 2.0×10 -2 Ω·cm, and particularly preferably from 1.2×10 -2 to 1.8×10 -2 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 apparatus (manufactured by Nitto Seiko Analytic Co., Ltd.: Loresta GP Powder Resistivity Measuring System MCP-PD-51).

[0046] The BET specific surface area of the CNT of the present embodiment is preferably 150 m 2 / g or more, more preferably 180 m 2 / g or more. Also, the BET specific surface area of the CNT is preferably 800 m 2 / g or less, more preferably 600 m 2 / g or less, and even more preferably 500 m 2 / g or less. The BET specific surface area of ​​CNTs can be calculated by the BET method using nitrogen adsorption measurements. There is often a correlation between the specific surface area of ​​CNTs and the average outer diameter of CNTs; the smaller the specific surface area, the larger the outer diameter of the 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 the CNTs and the more CNTs per mass. When the specific surface area of ​​CNTs is 150 m 2 When the specific surface area of ​​the CNT is 800 m / g or more, the number of carbon nanotubes per mass can be secured, and a conductive network can be efficiently formed, resulting in excellent rate characteristics and cycle characteristics of the battery. 2 When the CNT content is 1 / g or less, the CNTs are well dispersed, and a good conductive network can be formed in the electrode film.

[0047] The average outer diameter of the CNTs in this embodiment is preferably 3 nm or more, more preferably 5 nm or more. The average outer diameter of the CNTs is preferably 15 nm or less, more preferably 13 nm or less, and even more preferably 11 nm or less. When the average outer diameter of the CNTs is 15 nm or less, the number of carbon nanotubes per mass can be 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.

[0048] The standard deviation of the average outer diameter of the CNTs is preferably 1 nm to 8 nm, more preferably 1 nm to 4 nm. If the standard deviation of the average outer diameter of the CNTs is large, it may be difficult to efficiently form a conductive network, and the CNTs may become entangled and aggregate in the CNT dispersion or composite slurry and / or electrode film, making it impossible to form a good conductive network.

[0049] The outer diameter and average outer diameter of CNTs are determined as follows. First, the CNTs are observed and photographed using a transmission electron microscope. Next, 300 CNTs are randomly selected from the photograph and the outer diameter of each is measured. Next, the average outer diameter (nm) of the CNTs is calculated as the number average of the outer diameters.

[0050] 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.

[0051] <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.

[0052] 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.

[0053] <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.

[0054] As typical catalytic metals used when manufacturing CNTs by chemical vapor deposition (thermal CVD method), catalytic metals in which active components such as iron, cobalt, and nickel are fixed to supporting components such as aluminum and magnesium can be mentioned. Therefore, in the CNTs obtained by the above manufacturing, metal components such as aluminum, magnesium, iron, cobalt, and nickel tend to remain due to the catalytic metal. As an example of a method for purifying CNTs, a method of treating with an acid such as nitric acid can be mentioned, and it is known that iron, cobalt, and nickel can be removed by the acid treatment. However, it is difficult to remove metal components such as aluminum and magnesium by the method using acid treatment.

[0055] In contrast, the method for purifying CNTs of the present embodiment includes a step (I) of heat-treating the carbon nanotubes used as a raw material in an inert atmosphere under reduced pressure and vacuum. According to the method for purifying CNTs of the present embodiment, the content of metal components such as aluminum and magnesium in the CNTs can be easily reduced. The method for purifying CNTs of the present embodiment can reduce the content of other metal components such as iron and cobalt in the CNTs, not limited to aluminum and magnesium.

[0056] In the purification method of the above embodiment, examples of the inert atmosphere include a nitrogen atmosphere, an argon atmosphere, a vacuum atmosphere, and an atmosphere combining these. For example, an inert gas such as nitrogen gas is introduced into the inside of the apparatus used during the heat treatment, and the inside of the apparatus is replaced with the inert gas to obtain an inert atmosphere. Subsequently, the heat treatment is carried out while reducing the pressure inside the apparatus to maintain a vacuum state (hereinafter referred to as reduced pressure and vacuum).

[0057] 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.

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

[0059] The heat treatment conditions such as the heat treatment temperature and the heat treatment time may be appropriately determined depending on the type of CNT and the type of metal components derived from the catalytic metal contained in the CNT. The heat treatment temperature is preferably a temperature at which all 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. The heat treatment temperature means the temperature inside the device (internal temperature).

[0060] 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.

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

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

[0063] Usually, when CNTs are purified by heat treatment, they are heated to 500°C or higher in an air atmosphere, and then oxidized and burned. In contrast, the heat treatment of this embodiment is performed in an inert atmosphere and under reduced pressure vacuum. Therefore, the burning of the CNTs themselves can be suppressed. Furthermore, since the CNTs are heat treated in an inert atmosphere, the oxidation of the surface of the CNTs can be suppressed. In addition, the heat treatment time can be shortened, and in this case, the high crystallization of the CNTs can be further suppressed.

[0064] From the viewpoint of further reducing the amount of metal contained in the CNTs, the purification method of this 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 characteristics of the CNTs. For example, the purification method of CNTs of this embodiment may include other treatment steps such as an acid treatment as necessary in addition to the step (I) of performing the heat treatment.

[0065] <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.

[0066] <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 above-mentioned CNT, 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 above-mentioned CNT, a dispersant, and a dispersion medium can be provided.

[0067] <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.

[0068] 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, and more preferably 10,000 to 150,000.

[0069] In addition to the dispersant, it is preferable to add an amine compound or an inorganic base. As the amine compound, 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.

[0070] <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.

[0071] Examples of water-soluble organic solvents include alcohols, polyhydric alcohols, polyhydric alcohol ethers, amines, amides (N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylcaprolactam, etc.); heterocyclics, sulfoxides, sulfones, lower ketones, and others such as tetrahydrofuran, urea, and acetonitrile. Of these, water or 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.

[0072] When only an amide-based organic solvent is used as the dispersion medium, the water content in the dispersion medium is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

[0073] The CNT dispersion can be produced, for example, by dispersing CNTs in a dispersion medium. 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.

[0074] Examples of the dispersion method include a method using various dispersers such as a disperser (dispersing machine), a homogenizer, a high shear mixer, a kneader, a two-roll mill, a three-roll mill, a ball mill, a horizontal sand mill, a vertical sand mill, an annular bead mill, a paint conditioner, an attritor, a planetary mixer, or a high-pressure homogenizer. The disperser is not particularly limited, but for example, from the viewpoint of adjusting the fiber length of the CNT in the CNT dispersion to be within a preferred range, it is preferable to use a high shear mixer from the viewpoint of promoting the wetting of the CNT and dissolving coarse particles and agglomerations, and it is preferable to use a media-type disperser such as a bead mill from the viewpoint of crushing aggregated and solidified particles. In addition, it is more preferable to select and combine a plurality of the above dispersers to disperse, and the order of the dispersers can be changed as desired. The pressure when using the high-pressure homogenizer is not particularly limited, and is preferably, for example, 60 to 150 MPa, more preferably 60 to 120 MPa.

[0075] 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.

[0076] The solid content of the CNT dispersion is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, and particularly preferably 2% by mass or more, 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.

[0077] 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.

[0078] 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 the form of particles in the CNT dispersion, and specifically, the metals contained in the CNTs described above may be mentioned. CNTs, dispersants, and other materials may contain metal particles derived from their respective manufacturing processes, and metal foreign matter may also be mixed in during the manufacturing process of the CNT dispersion. If metal foreign matter is present inside the battery, the battery is more likely to short-circuit, so removing the metal particles is very important from the viewpoint of safety. In the process of producing a CNT dispersion liquid, it is preferable to include a process of removing contaminants such as metallic foreign matter at any timing (metallic foreign matter removal process). From the viewpoint of efficiency, the metallic foreign matter removal process is preferably performed during the dispersion process of the CNT dispersion liquid and / or at the end of the dispersion process. The metallic foreign matter removal process may be performed multiple times.

[0079] 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.

[0080] The method of removal by magnetic force is not particularly limited as long as it is a method that can remove metal particles, but from the standpoint of productivity and removal efficiency, a method in which a magnetic filter is placed in the production line of the CNT dispersion and the CNT dispersion is passed through it 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, if metal is detached from the magnetic filter, it is possible to prevent the metal from being mixed into the product.

[0081] The metal content in the CNT dispersion liquid was measured after drying the CNT dispersion liquid to remove the solvent. It can be calculated by analyzing using ICP. The metal content detected by ICP analysis includes metal particles and dissolved metal ions. That is, the metal content of the CNT dispersion liquid that has been subjected to the metal foreign matter removal process includes metal particles that have not been completely removed and dissolved metal ions.

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

[0083] The CNT dispersion may further contain, as a conductive material, one or more carbon materials such as carbon black, graphite, etc. Among these conductive materials, carbon black is preferred from the viewpoint of the adsorption performance of the dispersant.

[0084] <3> Binder Composition One embodiment of the present invention relates to a binder composition. The binder composition according to this embodiment includes the above-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.

[0085] <Binder> The binder is a resin that binds various substances together in the electrode film. As the binder, binders known for batteries can be used. Examples include cellulose resins such as carboxymethyl cellulose; rubbers such as styrene-butadiene rubber and fluororubber. Modified bodies, mixtures, and copolymers of these resins may also be used. In particular, from the standpoint of resistance, it is preferable to use polyvinylidene fluoride, polyvinyl fluoride, tetrafluoroethylene, and the like, which are polymeric compounds having fluorine atoms in the molecule.

[0086] The weight average molecular weight of the binder is preferably 10,000 or more, more preferably 100,000 or more, and particularly preferably 200,000 or more. 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, the resistance and adhesion of the binder can be suppressed from decreasing. When the weight average molecular weight is 2,000,000 or less, the resistance and adhesion of the binder can be improved, while suppressing the decrease in workability caused by the increase in the viscosity of the binder itself, and suppressing the significant aggregation of dispersed particles.

[0087] The binder composition is preferably produced by mixing and homogenizing the CNT dispersion liquid and the binder, and the binder may be dissolved in advance. The binder may be added at any timing in the process of producing the CNT dispersion liquid. The mixing method may be any of various conventionally known methods. The binder composition can be produced using the dispersion device described above for the CNT dispersion liquid. The binder composition may contain one type of binder, or two or more types of binders. Furthermore, the process of producing the binder composition may include the above-mentioned metal foreign matter removal process.

[0088] <4> Composition for electrodes One embodiment of the present invention relates to a composition for electrodes. The composition for electrodes according to this embodiment includes the above-mentioned CNT dispersion liquid and an electrode active material. The composition for electrodes can be further mixed with a binder to produce a composite slurry. In addition, according to this embodiment, it is possible to provide a method for producing a binder composition including the above-mentioned CNT, a dispersant, a dispersion medium, and an electrode active material, and a method for producing a composition for electrodes including the above-mentioned CNT, a dispersant, a dispersion medium, an electrode active material, and a composite slurry.

[0089] <Electrode active material> An electrode active material is a material that serves as the basis for a battery reaction. The active material 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, MnO, V 2 O 5 、V 6 O 13 、TiO 2 and other transition metal oxide powders; composite oxide powders of lithium and transition metals such as lithium nickelate, lithium cobaltate, lithium manganate with a layered structure, and lithium manganate with a spinel structure; lithium iron phosphate-based materials which are olivine structure phosphate compounds, etc. can be mentioned. These positive electrode active materials can be used alone or in combination of two or more. Also, the above inorganic compounds and organic compounds may be mixed and used. The negative electrode active material is not particularly limited, 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 artificial graphite such as highly graphitized carbon materials, or carbonaceous powders such as natural graphite can be mentioned. These negative electrode active materials can be used alone or in combination of two or more. In particular, using a combination of a highly graphitized carbon material and lithium silicate is preferable from the viewpoints of capacity and lifespan.

[0090] The BET specific surface area of the electrode active material is 0.1 to 10 m 2 / g is preferable, and 0.2 to 5m 2 / g is more preferable, and 0.3 to 3m 2 / g is more preferred. The average particle size of the electrode active material is preferably 0.05 to 100 μm, and more preferably 0.1 to 50 μm. The average particle size of the electrode active material in this specification refers to the average value of particle sizes measured by an electron microscope.

[0091] 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.

[0092] In the case of a composite slurry containing an electrode composition, the content of the electrode active material contained in the composite slurry is preferably 20% by mass or more, 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 0.05% by mass or more, and more preferably 0.1% by mass or more, relative to 100% by mass of the electrode active material. The CNT content in the composite slurry is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less, relative to 100% by mass of the electrode active material. The content of the binder in the composite slurry is preferably 0.3% by mass or more, and more preferably 0.7% by mass or more, relative to 100% by mass of the electrode active material. The content of the binder in the composite slurry is preferably 20% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, relative to 100% by mass of the electrode active material. The solid content of the composite slurry is preferably 30% by mass or more, more preferably 40% by mass or more, based on 100% by mass of the composite slurry, and is preferably 90% by mass or less, more preferably 85% by mass or less, based on 100% by mass of the composite slurry. The water content in the composite slurry is preferably 500 ppm or less, more preferably 300 ppm or less, and particularly preferably 100 ppm or less.

[0093] <5> electrode membrane One embodiment of the present invention relates to an electrode film. The electrode film according to this embodiment is an electrode film obtained by using (i) a carbon nanotube dispersion liquid containing carbon nanotubes, a dispersant, and a dispersion medium, (ii) a binder composition containing the carbon nanotube dispersion liquid and a binder, or (iii) an electrode composition containing the carbon nanotube dispersion liquid and an electrode active material. The carbon nanotubes of this embodiment described above are used as the carbon nanotubes. The electrode film may also be an electrode film obtained by using (iv) a composite material slurry. The (iv) composite material slurry can be obtained by using the above-mentioned (i) carbon nanotube dispersion liquid, (ii) a binder composition, or (iii) an electrode composition.

[0094] For example, the electrode film is a coating film formed by coating the above-mentioned composite slurry on a current collector and drying it. The material and shape of the current collector used for the electrode film are not particularly limited, and can be appropriately selected from those suitable for various secondary batteries. For example, the material of the current collector can be metals and alloys such as aluminum, copper, nickel, titanium, or stainless steel.

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

[0096] After coating and drying, the coating may be rolled using a lithographic press or a 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.

[0097] <6> secondary battery One embodiment of the present invention relates to a secondary battery. The secondary battery according to this embodiment includes the above-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.

[0098] 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, LiC4 F 9 SO 3 , Li(CF 3 SO 2 ) 3 C, LiI, LiBr, LiCl, LiAlCl, LiHF 2 , LiSCN, or LiBPh 4 (wherein Ph is a phenyl group) and the like, but are not limited thereto, and those containing a sodium salt 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 also be used.

[0099] The non-aqueous solvent is not particularly limited, but various solvents suitable for secondary batteries can be used, for example, carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, lactones, glymes, esters, sulfoxides, and nitriles, etc. These solvents may be used alone or in combination of two or more.

[0100] The secondary battery preferably includes a separator, for example, polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and those which have been subjected to a hydrophilic treatment, but are not particularly limited to these.

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

[0102] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples as long as it does not depart from the gist of the present invention.

[0103] 1. Manufacturing Examples of CNT and CNT Dispersions Using the Same [Example 1] [Manufacture of CNT] 60 parts of CNT (manufactured by JEIO Co., Ltd., JENOTUBE6A) were placed in a graphite crucible with a diameter of 10 cm and a height of 10 cm. The crucible containing the above CNT was placed in a multi-purpose high-temperature furnace (manufactured by Fuji Denpa Kogyo Co., Ltd., Himulti 5000), and heat treatment was carried out under reduced pressure and vacuum as follows. First, nitrogen gas was introduced into the above multi-purpose high-temperature furnace, and the substitution operation with nitrogen gas was performed twice. Next, the pressure was reduced using an oil rotary pump, and after adjusting the furnace internal pressure to 9.8 - 9.5 Pa, subsequently, the pressure was further reduced using an oil diffusion pump, and the furnace internal pressure was adjusted to 0.03 Pa or less. Subsequently, while maintaining the pressure reduction by the oil diffusion pump, the temperature was raised to 1200 °C at a rate of 20 °C / min and held at 1200 °C for 10 hours. Then, it was naturally cooled until the furnace internal temperature reached 50 °C or less, and CNT (A) was obtained.

[0104] [Preparation of CNT Dispersion] An NMP solution containing a 7% hydrogenated nitrile butadiene rubber polymer (manufactured by Nippon Zeon Co., Ltd., Zetpole2000L) and NMP were added to a stainless steel container, and adjusted so that the polymer was 1.25 parts by mass and the total amount of NMP was 96.25 parts by mass. 2.5 parts by mass of CNT (A) was weighed into this solution and added while stirring with a disper. A fine emulsifier screen was attached to a high-shear mixer (L5M-A, manufactured by SILVERSON), and batch dispersion treatment was performed at a speed of 9,000 rpm. This batch dispersion treatment was carried out until the entire solution became uniform and the dispersion particle size became 200 μm or less as measured by a grind gauge. Next, the contents of the stainless steel container were transferred to a bead mill (Ashizawa Finetech Co., Ltd., Star Mill LMZ) filled with zirconia beads having a diameter of 1.0 mmφ, and a circulation type dispersion treatment (bead filling amount 80%, peripheral speed 12 m / s) was performed for a residence time of 10 minutes. Then, the liquid to be dispersed was supplied to a high-pressure homogenizer through a pipe, and a 15-pass type dispersion treatment was performed. Then, the liquid to be dispersed was supplied to a high-pressure homogenizer (Sugino Machine Co., Ltd., Star Burst Lab), and a 15-pass type dispersion treatment was performed. This dispersion treatment was performed using a single nozzle chamber, with a nozzle diameter of 0.25 mm and a pressure of 100 MPa. The solution after the dispersion treatment was passed through a depth filter (3M Co., Ltd., PP nonwoven fabric depth cartridge NT-T series, filtration accuracy 20 μm). In this way, a carbon nanotube dispersion liquid (A) was obtained.

[0105] <Preparation of Binder Composition> Capacity 150cm 3 The CNT dispersion (A) and PVdF (polyvinylidene fluoride, Solef5130, Solvay, non-volatile content 100%), which had been dissolved in advance in NMP (N-methyl-2-pyrrolidone) to a concentration of 8%, 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.

[0106] <Preparation of electrode composition> The binder composition (A) contains NMC (S800, LiNi 0.8 Mn 0.1 Co 0.1 O 2After 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 broken up with a spatula, and the mixture was stirred at 2,000 rpm for 300 seconds using a planetary centrifugal mixer (Thinky's Awatori Rentaro, ARE-310) to obtain electrode composition (A). The non-volatile content of the electrode composition was 73.5%. 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.

[0107] <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.

[0108] <Preparation of secondary battery> The positive electrode (A) and the standard negative electrode were punched out to 45 mm × 40 mm and 50 mm × 45 mm, respectively. These electrodes and a separator (porous polypropylene film) inserted between them were inserted into 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 laminate was then sealed to prepare a laminate type 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 MAC500L, 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 (Nippon Graphite Industry Co., Ltd., CGB-20) and 5 parts by mass of silicon oxide (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 for 30 seconds at 2000 rpm 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 became 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.

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

[0110] [Example 3] 15 parts of CNTs and 40 parts of zirconia beads with a diameter of 2 mm were added to a glass bottle (M-225, manufactured by 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 (C) in the same manner as in Example 2. Furthermore, using the CNT(C) 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.

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

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

[0113] [Comparative Example 5] Using an electronic balance (MSA225S100DI, Sartorius), 10 parts of CNTs (JENOTUBE6A, JEIO) were weighed into an alumina crucible SSA-HB4 (Nikkato), and the crucible containing the CNTs was placed in a muffle furnace (FO510, Yamato Scientific Co., Ltd.). Next, the temperature inside the furnace was raised to 330°C at a rate of 60°C / min in an air atmosphere (Air), and the temperature was maintained at 330°C for 18 hours (calcination), yielding oxidized CNTs (6A). 10 parts of the CNTs thus oxidized were weighed into a 1L glass container, 500 parts of 10% nitric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added, and the mixture was heated to 90°C in a hot water bath while thoroughly stirring using a stirrer. The mixture was then thoroughly diluted with ion-exchanged water and filtered under reduced pressure using a membrane filter. After repeating the dilution and filtration steps, the CNTs were transferred to a PTFE tray. The mixture was dried at 140°C using an oven. In this way, nitric acid-treated CNTs (X4) were obtained. Furthermore, using the CNTs (X4) 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.

[0114] 2. Evaluation 2-1.Evaluation of CNT characteristics The carbon nanotubes (CNTs) of the above-mentioned Examples and Comparative Examples were measured as follows. The results are shown in Table 1.

[0115] <Content of metal in CNT> Using a microwave sample pretreatment device (ETHOS, manufactured by Milestone General Co., Ltd.), the CNT was decomposed by acid to extract the metals contained in the CNT. The analysis of the extracted metals was performed using a multi-type ICP emission spectrometer (720-ES, manufactured by Agilent), and the content of the metals contained in the CNT was calculated. In Table 1 described later, the contents of aluminum, iron, cobalt, and magnesium are shown as the metal contents. 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 expressed as the mass ratio (ppm) of the total mass of the extracted magnesium, aluminum, iron, copper, zinc, nickel, chromium, manganese, and molybdenum to the mass of the CNT before metal extraction.

[0116] <G / D ratio of CNT> The CNT was placed on a Raman microscope (XploRA, manufactured by Horiba, Ltd.), and the measurement was performed using a laser wavelength of 532 nm. The measurement conditions were an acquisition time of 60 seconds, an integration number of 2 times, a neutral density filter of 10%, an objective lens magnification of 20 times, a confocal hole of 500, a slit width of 100 μm, and the measurement wavelength was 100 to 3000 cm -1 was used. The CNT for measurement was aliquoted onto a slide glass and flattened using a spatula. Among the obtained peaks, the maximum peak intensity within the range of 1560 to 1600 cm -1 in the spectrum was defined as G, and the maximum peak intensity within the range of 1310 to 1350 cm -1 in the spectrum was defined as D, and the ratio of G / D was calculated and used as the G / D ratio of the CNT.

[0117] <Temperature of the exothermic peak of CNT> Using a thermogravimetric differential thermal analyzer (manufactured by Rigaku Corporation, Tg-DTA 8122 Thermo plus EVO2), with a sample mass of 1.0 mg, placed in an alumina pan container, 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.

[0118] <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.

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

[0120] <Cohesion force of CNT> The cohesion of CNTs was measured using a rheometer (Anton Paar, MCR302e). First, 4.0 g of CNTs was 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 cohesion, and while moving the jig at 125 μm / s and 0.1 rotation / min., it was penetrated into the carbon nanotubes, and the torque when applying a shear force was measured. The maximum peak value was taken as the cohesion of the CNTs. A C-PTD200 temperature control device was used, and the measurement was performed at a temperature of 25°C.

[0121] 2-2. Evaluation of Other Properties Regarding the CNT dispersion, binder composition, electrode composition, electrode film, and secondary battery prepared in the above-mentioned examples and comparative examples, measurements were performed and properties were evaluated as follows. The evaluation results are shown in Table 1. <Viscosity Stability of CNT Dispersion> After leaving the CNT dispersion in a constant-temperature bath at 60°C for one week, the CNT dispersion was cooled to 25°C, and immediately measured using a B-type viscometer at a rotor rotation speed of 100 rpm. The evaluation criteria for viscosity stability are as follows. (Evaluation Criteria) ◎ (Excellent): 2500 mPa·s or less 〇 (Good): Exceeding 2500 mPa·s and 5000 mPa·s or less △ (Fair): Exceeding 5000 mPa·s and 6000 mPa·s or less × (Poor): Exceeding 6000 mPa

[0122] <Particle Size (D90) of CNT Dispersion> The particle size (D90) was measured using a particle size distribution analyzer (Partical 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. During air removal, ultrasonic operation was performed with ultrasonic intensity 7 and ultrasonic time 5 seconds. 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): 1.0 μm or more and less than 3.0 μm 〇 (Good): 3.0μm or more and less than 5.0μm - (Defective): Less than 1.0 μm or more than 5.0 μm

[0123] <Cycle characteristics of secondary batteries> The secondary battery was placed in a constant temperature room at 45°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 45°C to the 200th 1C discharge capacity, as shown in the following formula 3. (Formula 3) Cycle characteristic = 200th 1C discharge capacity / 3rd 1C discharge capacity × 100 (%) The evaluation criteria for the cycle characteristics of the secondary battery are as follows. (Evaluation Criteria) ◎ (Excellent): Cycle characteristics are 90% or more 〇(Good): 85% to less than 90% △(Acceptable): 80% or more but less than 85% × (defective): Less than 80%

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

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

[0126] In Table 1, 10B and 6A used as the CNT species are as follows. 10B: Multi-walled carbon nanotube (JEIO, JENOTUBE10B) 6A: Multi-walled carbon nanotubes (JEIO, JENOTUBE6A)

[0127] [Table 1]

[0128] As shown in Table 1, the CNTs of this embodiment (Examples 1 to 12) satisfy all of the requirements for (1) heat generation peak in differential thermal analysis, (2) G / D ratio, and (3) aluminum content described above. In comparison with 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, making it possible to improve the safety of the secondary battery.

Claims

1. Carbon nanotubes, including multi-walled carbon nanotubes, which satisfy the following (1) to (3) and further satisfy the following (ii): (1) In a differential thermal analysis when the temperature is increased from 200° C. to 1,000° C. at 10° C. / min, an exothermic peak is observed between 650° C. and 800° C. (2) 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. (3) The aluminum content is 3000 ppm or less. (ii) The surface oxygen content is less than 1.0 atm %.

2. The carbon nanotube according to claim 1 , further satisfying the following (i): (i) The total content of aluminum, magnesium, cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum is not more than 13,000 ppm.

3. The carbon nanotube according to claim 1 , further satisfying the following (iii): (iii) The outer diameter is 3 nm or more and 15 nm or less.

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

5. 5. The carbon nanotube dispersion according to claim 4, which has a viscosity of 5000 mPa·s or less at 25° C. as measured by a Brookfield viscometer after being left to stand for one week at 60° C.

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

7. A carbon nanotube dispersion liquid and an electrode active material are included, 4. A composition for an electrode, comprising the carbon nanotube dispersion liquid according to claim 1, a dispersant, and a dispersion medium.

8. A secondary battery including an electrode film, the electrode film comprising: A carbon nanotube dispersion liquid comprising the carbon nanotubes according to any one of claims 1 to 3, a dispersant, and a dispersion medium. A secondary battery obtained by using a binder composition containing the carbon nanotube dispersion and a binder, or an electrode composition containing the carbon nanotube dispersion and an electrode active material.

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