Carbon nanotubes and dispersions containing the same

Abstract

The present invention relates to carbon nanotubes that satisfy a specific formula. The carbon nanotubes of the present invention are particularly suitable for use as conductive materials in secondary batteries because they simultaneously exhibit excellent dispersibility and electrical conductivity when applied to a dispersion liquid.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0141851, filed October 28, 2022, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference. The present invention relates to carbon nanotubes that have low viscosity and low powder resistance when producing a dispersion, and to a dispersion containing the carbon nanotubes. [Background technology]

[0002] Carbon nanomaterials include fullerenes, carbon nanotubes (CNTs), graphene, and graphite nanoplates, depending on the shape of the material. Among them, carbon nanotubes are giant molecules consisting of hexagonal honeycomb-shaped graphite sheets, in which one carbon atom is bonded to three other carbon atoms, rolled up into a nano-sized diameter.

[0003] Carbon nanotubes are hollow and lightweight, have electrical conductivity as good as copper, thermal conductivity as good as diamond, and tensile strength comparable to steel. Depending on their rolled shape, they can be divided into single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), and rope carbon nanotubes.

[0004] In recent years, the field of lithium secondary batteries has been the most actively studied area for the application of carbon nanotubes. The ultimate goal of lithium secondary batteries is to store more electrical energy in a smaller size. Research is being conducted into the use of carbon nanotubes as a conductive material as one way to improve the electrode density of lithium secondary batteries and manufacture electrodes with higher energy density per unit volume. High-density electrodes are typically formed by high-pressure pressing electrode active material particles ranging in size from several micrometers to several tens of micrometers. During the molding process, the particles tend to deform, reducing the interparticle space and reducing electrolyte permeability. To address this issue, materials with excellent electrical conductivity and strength are used as conductive materials during electrode fabrication. Carbon nanotubes, with their excellent strength and electrical conductivity, are also widely used as such materials. When conductive materials are used in electrode fabrication, they are dispersed among the compressed electrode active material, maintaining micropores between the active material particles, facilitating electrolyte permeation. Furthermore, their excellent conductivity reduces internal electrode resistance.

[0005] However, the biggest problem with using carbon nanotubes as a conductive material is their low solubility and dispersibility. Due to the strong van der Waals attraction between carbon nanotubes, a stable dispersion state cannot be achieved in aqueous solutions, resulting in aggregation. Aggregation of carbon nanotubes in a dispersion reduces the processability of the dispersion, making it difficult to apply the dispersion uniformly. Therefore, it is preferable to apply the dispersion with its viscosity minimized.

[0006] Various methods have been proposed to improve the dispersibility of carbon nanotube dispersions. One example is a method in which carbon nanotubes are dispersed in a dispersion medium by mechanical dispersion treatment, such as ultrasonication. However, while this method provides excellent dispersibility during ultrasonic irradiation, aggregation of carbon nanotubes begins immediately upon termination of ultrasonic irradiation. This problem occurs even when the carbon nanotube concentration exceeds a certain level. Furthermore, methods for stabilizing the dispersion of carbon nanotubes using various dispersants have been proposed. However, these methods also suffer from the problem of difficulty in suppressing the increase in viscosity that can occur when carbon nanotubes are dispersed in a dispersion medium at high concentrations.

[0007] Therefore, there is a need for a novel method for producing carbon nanotubes that can further improve dispersibility by improving the physical properties of the carbon nanotubes themselves, rather than relying on mechanical dispersion treatments such as ultrasonication or auxiliary means such as the use of dispersants. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Korean Patent Publication No. 10-2017-0031061 (2017.03.20) Summary of the Invention [Problem to be solved by the invention]

[0009] The object of the present invention is to provide novel carbon nanotubes that are particularly suitable for use as conductive materials because they have low viscosity during dispersion production and low powder resistance when the slurry is produced, and a dispersion containing the carbon nanotubes. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention provides novel carbon nanotubes, a dispersion containing the carbon nanotubes, and a positive electrode slurry composition containing the dispersion.

[0011] (1) Specifically, the present invention provides carbon nanotubes characterized by satisfying the following formula 1:

[0012] [Formula 1] -0.004*A+0.0385≦R≦-0.004*A+0.0425

[0013] In the formula 1, R is the powder resistivity of the carbon nanotube (Ω·cm); A is ln{(purity of carbon nanotubes (wt%) * specific surface area (m 2 / g) / bulk density (kg / m 3 )}, The specific surface area of ​​the carbon nanotubes is 320m 2 / g or more, The bulk density of the carbon nanotubes is 30 kg / m 3 The following is the result.

[0014] (2) In the present invention, the specific surface area of ​​the carbon nanotubes is 320 m 2 / g~500m 2 / g.

[0015] (3) In the present invention, the bulk density of the carbon nanotubes is 15 kg / m 3 ~30kg / m 3 The carbon nanotube according to (1) or (2) above is provided.

[0016] (4) The present invention provides the carbon nanotube according to any one of (1) to (3) above, wherein R is 0.0125 Ω·cm or less.

[0017] (5) The present invention provides the carbon nanotube according to any one of (1) to (4) above, wherein R is 0.0080 Ω·cm to 0.0125 Ω·cm.

[0018] (6) The present invention provides a carbon nanotube dispersion liquid containing the carbon nanotubes according to any one of (1) to (5) above and a dispersion medium.

[0019] (7) The present invention provides the carbon nanotube dispersion liquid according to (6) above, wherein the dispersion medium is one or more selected from the group consisting of N-methylpyrrolidone, pyridine, dimethylaminobenzene, and diethylaminobenzene.

[0020] (8) The present invention provides the carbon nanotube dispersion liquid according to (6) or (7) above, wherein the carbon nanotube content in the dispersion liquid is 0.05% by weight to 5% by weight.

[0021] (9) The present invention provides a positive electrode slurry composition comprising the carbon nanotube dispersion liquid according to any one of (6) to (8) above, a positive electrode material, and a stabilizer.

[0022] (10) The present invention provides the positive electrode slurry composition according to (9), wherein the stabilizer is contained in an amount of 10% by weight to 100% by weight based on the content of carbon nanotubes in the positive electrode slurry composition. [Effects of the Invention]

[0023] The carbon nanotubes of the present invention have a low viscosity during the production of the dispersion, so the carbon nanotube content in the dispersion state can be made to a certain level or above, and the powder resistance of the slurry is also low, making them particularly suitable for use as a conductive material. DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention will now be described in more detail. The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, based on the principle that the inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0025] The term "carbon nanotube" as used herein refers to a secondary structure formed by the complete or partial assembly of carbon nanotube units into a bundle. The carbon nanotube units have graphite sheets in the shape of cylinders with nanometer-sized diameters and an sp2 bond structure. Depending on the angle and structure of the graphite sheets, they can exhibit conductive or semiconductive properties. Carbon nanotube units can be classified into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs) according to the number of bonds that make up the walls. The thinner the wall thickness, the lower the resistance. The carbon nanotubes of the present invention may include any one or more of single-walled, double-walled, and multi-walled carbon nanotube units.

[0026] carbon nanotubes The present invention provides a carbon nanotube characterized by satisfying the following formula 1:

[0027] [Formula 1] -0.004*A+0.0385≦R≦-0.004*A+0.0425

[0028] In the formula 1, R is the powder resistivity of the carbon nanotube (Ω·cm); A is ln{(purity of carbon nanotubes (wt%) * specific surface area (m 2 / g) / bulk density (kg / m 3 )}, The specific surface area of ​​the carbon nanotubes is 320m 2 / g or more, The bulk density of the carbon nanotubes is 30 kg / m 3 The following is the result.

[0029] The inventors of the present invention have studied the physical properties of carbon nanotubes that allow them to maintain excellent levels of both viscosity and conductivity during the preparation of a dispersion, and have found that the powder resistivity, purity, specific surface area, and bulk density of carbon nanotubes satisfy the above formula 1, and the specific surface area of ​​the carbon nanotubes is 320 m 2 / g or more and a bulk density of 30 kg / m 3 The inventors have confirmed that when the temperature is as follows, both the dispersibility and electrical conductivity of the carbon nanotubes can be maintained well, and have completed the present invention.

[0030] More specifically, when the production conditions of the catalyst used to produce carbon nanotubes, specifically the content of the active component, the ratio of the main catalyst component to the co-catalyst component, the content of the organic acid in the precursor solution, the calcination temperature, etc., are changed, the physical properties of the carbon nanotubes produced from the catalyst also change. Therefore, by changing the production conditions of the catalyst, it is possible to synthesize various carbon nanotubes with different physical properties. Furthermore, as a result of confirming the correlation between the dispersibility, electrical conductivity, and physical properties of various synthesized carbon nanotubes using various methods, it was found that carbon nanotubes satisfying the above formula 1 are characterized by simultaneously excellent dispersibility and electrical conductivity.

[0031] More specifically, the formula 1 indicates that there is a correlation between the powder resistivity of carbon nanotubes and their purity, specific surface area, and bulk density. The formula 1 was derived based on various data, and carbon nanotubes that satisfy the formula 1 have the characteristics of being excellent in both dispersibility and electrical conductivity.

[0032] The A and R values ​​in Equation 1 have different units, but in the present invention, the units of each value are ignored and each value is assumed to be a dimensionless number. However, since each value may vary depending on the units of the powder resistivity, purity, specific surface area, and bulk density of the carbon nanotubes, which are variables of each value, when applying Equation 1, the units of each variable are fixed as follows:

[0033] Carbon nanotube powder resistivity (R) unit: Ω cm Carbon nanotube purity unit: wt% The specific surface area of ​​carbon nanotubes is measured in m 2 / g Bulk density of carbon nanotubes in kg / m 3

[0034] On the other hand, in the carbon nanotubes provided by the present invention, the specific surface area of ​​the carbon nanotubes is 320 m 2 / g or more, preferably 320m 2 / g~500m 2 / g. If carbon nanotubes are defined solely by Equation 1 without any range restrictions on the specific surface area of ​​the carbon nanotubes, virtually an infinite number of carbon nanotubes can satisfy Equation 1, and this range may include not only carbon nanotubes that simultaneously exhibit excellent dispersibility and electrical conductivity, as the objective of the present invention, but also carbon nanotubes that do not exhibit excellent dispersibility and / or electrical conductivity. Therefore, the carbon nanotubes of the present invention must satisfy Equation 1 and have a specific surface area within the aforementioned range. Meanwhile, the specific surface area may be measured by the BET method, and more specifically, may be calculated by determining the nitrogen gas adsorption amount at liquid nitrogen temperature (77 K) using a BELSORP-mini II manufactured by BEL Japan.

[0035] Furthermore, the carbon nanotubes of the present invention have a bulk density of 30 kg / m 3 Less than 15 kg / m 3 ~30kg / m 3When the above-mentioned formula 1 and specific surface area conditions are satisfied and the bulk density of the carbon nanotubes is within the above-mentioned range, the carbon nanotubes have particularly excellent dispersibility and electrical conductivity, and sufficient electrical conductivity can be provided even with a low carbon nanotube content during the preparation of the dispersion. Meanwhile, the bulk density can be calculated by measuring the weight of the carbon nanotubes that have fallen into a container using a 25 ml SUS measuring cup under free fall and dividing the measured weight by the volume of the container.

[0036] Furthermore, the powder resistivity R of the carbon nanotubes of the present invention may be 0.0125 Ω·cm or less, and particularly preferably 0.0080 Ω·cm to 0.0125 Ω·cm. As with the bulk density described above, when the powder resistivity of the carbon nanotubes satisfies the above-mentioned formula 1 and the specific surface area condition and is within the above-mentioned range, the electrical conductivity can be particularly excellent. Meanwhile, the powder resistivity may be measured by measuring the resistance due to pressure at a compressed density of 1 g / cc using an MCP-PD51 device manufactured by Nittoseiko Analytech.

[0037] The carbon nanotubes of the present invention may have a purity of 80% by weight or more, and more preferably 83% by weight or more. The purity refers to the content of carbon nanotubes remaining after impurities have been removed from the carbon nanotubes, and can be calculated using the following formula: Purity = (carbon nanotube yield - catalyst input) / carbon nanotube yield * 100%

[0038] Carbon nanotube dispersion The present invention provides a dispersion containing the aforementioned carbon nanotubes. More specifically, the present invention provides a carbon nanotube dispersion containing the aforementioned carbon nanotubes and a dispersion medium.

[0039] In the carbon nanotube dispersion of the present invention, the dispersion medium may be one or more selected from the group consisting of N-methylpyrrolidone, pyridine, dimethylaminobenzene, and diethylaminobenzene, and may be preferably N-methylpyrrolidone. When one of the above-listed dispersion mediums is used as the dispersion medium, the carbon nanotubes can be smoothly dispersed, and the dispersion medium alone can be selectively and easily removed during the subsequent coating and baking of the dispersion.

[0040] In the carbon nanotube dispersion of the present invention, the carbon nanotube content in the dispersion may be 0.05 wt% to 5 wt%, preferably 0.5 wt% to 3 wt%. If the carbon nanotube content in the dispersion is lower than the above range, sufficient electrical conductivity cannot be achieved, and if the carbon nanotube content is higher than the above range, the excess carbon nanotubes may aggregate, increasing viscosity and significantly reducing the processability of the dispersion itself.

[0041] In the carbon nanotube dispersion of the present invention, the dispersion may contain dispersants such as polysaccharides and monosaccharides, such as carboxymethyl cellulose (CMC), hydroxyethyl cellulose, pectin, alginic acid, guar gum, locust bean gum, gum arabic, dextrin, altose, sorbitol, lactose, rice starch, and sucrose; sodium cholate, gelatin, and polyvinyl alcohol; anionic surfactants, such as naphthalene sulfonic acid-formaldehyde condensates and alkylbenzene sulfonates, cationic surfactants, nonionic surfactants, polyether-modified silicone surfactants, and hydrogenated nitrile butadiene rubber (HNBR). The dispersant may be HNBR. The dispersant may be present in an amount of 0.1 wt % to 5 wt %, preferably 0.3 wt % to 3 wt %, based on the total weight of the dispersion. Within the above-mentioned content ranges, the dispersion may have low viscosity and excellent viscosity stability.

[0042] Positive electrode slurry composition Since the carbon nanotube dispersion provided by the present invention has excellent electrical conductivity, it can be used as a conductive material for the positive electrode slurry composition. Accordingly, the present invention provides a positive electrode slurry composition containing the above-described dispersion.

[0043] Specifically, the present invention provides a positive electrode slurry composition containing the above-described carbon nanotube dispersion, a positive electrode material, and a stabilizer. The dispersion is as described above.

[0044] The positive electrode material is not particularly limited as long as it can be used as a positive electrode material for a lithium secondary battery. For example, known positive electrode materials such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li(Ni a Co b Mn c )O2 (where 0 < a < 1, 0 < b < 1, 0 < c < 1, a + b + c = 1), LiNi 1-Y Co Y O2, LiCo 1-Y Mn Y O2, LiNi 1-Y Mn Y O2 (where 0 < Y < 1), Li(Ni a Co b Mn c )O4 (0 < a < 2, 0 < b < 2, 0 < c < 2, a + b + c = 2), LiMn 2-Z Ni Z O4, LiMn 2-Z Co Z O4 (where 0 < Z < 2) may be used as the positive electrode material, either one selected from the group consisting of these or a mixture of two or more of these.

[0045] In the carbon nanotube positive electrode slurry composition of the present invention, the stabilizer may be polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), or a mixture thereof. The stabilizer may be included in an amount of 10 wt% to 100 wt%, preferably 30 wt% to 70 wt%, based on the carbon nanotube content in the positive electrode slurry composition. When the stabilizer content is within the above range, the stability during slurry preparation may be further improved.

[0046] Hereinafter, the present invention will be described in more detail with reference to examples and experimental examples, but the present invention is not limited to these examples and experimental examples. The examples according to the present invention may be modified in various different forms, and the scope of the present invention should not be construed as being limited to the examples described below. The examples of the present invention are provided to more specifically explain the present invention to those skilled in the art.

[0047] Catalyst manufacturing example Co(NO3)2·6H2O was used as the cobalt precursor and NH4VO3 was used as the vanadium precursor. The cobalt and vanadium precursors were dissolved in water, followed by anhydrous citric acid (CA) as a complexing agent to prepare a catalyst precursor composition. After thorough stirring, the catalyst precursor composition was added to a hydrotalcite support. The catalyst was then dried in an oven at 190°C for 5 hours and calcined in air at a specific calcination temperature for 4 hours to complete the catalyst. The amount of vanadium added during this process was adjusted to a molar ratio of 0.3 per mole of cobalt, and the amount of anhydrous citric acid added was adjusted to a molar ratio of 0.13 per mole of cobalt. Various catalysts were prepared by varying the cobalt content, cobalt to vanadium ratio, vanadium to anhydrous citric acid ratio, and calcination temperature in the catalyst precursor composition. The catalyst preparation conditions for each preparation example are summarized in Table 1 below.

[0048] [Table 1]

[0049] Examples and Comparative Examples Carbon nanotubes were synthesized using the catalyst used in the catalyst preparation example. Specifically, 0.3 g of the prepared catalyst was loaded into a fixed-bed reactor, and nitrogen gas was injected into the fixed-bed reactor at 1600 sccm, and the internal temperature of the reactor was heated to the reaction temperature. Ethylene gas was then injected as a carbon source gas at 400 sccm, and the reaction was continued for 90 minutes to synthesize carbon nanotubes. The catalysts and reaction temperature conditions used in each example and comparative example are summarized in Table 2 below.

[0050] [Table 2]

[0051] Experimental Example 1: Check whether the produced carbon nanotubes satisfy Equation 1 The purity, specific surface area, bulk density, and powder resistivity of the carbon nanotubes prepared in the examples and comparative examples were measured to confirm whether they satisfied Equation 1. Each physical property was measured by the following method.

[0052] 1) Purity: (carbon nanotube yield - catalyst input) / carbon nanotube yield * 100% 2) Specific surface area: Calculated by determining the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using a BELSORP-mini II manufactured by BEL Japan.

[0053] 3) Bulk density: Using a 25 ml SUS measuring cup, the weight of the carbon nanotubes that had fallen into the container was measured by free fall, and the measured weight was divided by the volume of the container to calculate the bulk density.

[0054] 4) Powder resistance: The resistance due to pressure when the compressed density was 1 g / cc was measured using an MCP-PD51 device manufactured by Nittoseiko Analytech. The measured results are summarized in Table 3 below.

[0055] [Table 3]

[0056] As can be seen from Table 3 above, the carbon nanotubes of the examples of the present invention satisfy formula 1. On the other hand, the carbon nanotubes of Comparative Examples 1-1, 1-2, and 2-1 satisfy formula 1 but do not satisfy the specific surface area range required by the present invention, the carbon nanotube of Comparative Example 3-1 satisfies formula 1 but does not satisfy the bulk density range required by the present invention, and the carbon nanotube of Comparative Example 3-2 does not satisfy formula 1.

[0057] Experimental Example 2: Measurement of viscosity of carbon nanotube dispersion and powder resistivity of slurry A dispersion was prepared using the carbon nanotubes prepared in the examples and comparative examples. 1.0 g of the prepared carbon nanotubes and 0.6 g of dispersant HNBR were added to 98.4 g of dispersion medium N-methylpyrrolidone. A dispersion was then prepared using a high-pressure homogenizer at 1500 bar and three passes. The prepared dispersion was mixed with a cathode material to achieve a carbon nanotube content of 0.5 wt %, and PVDF was further added as a stabilizer to achieve a carbon nanotube content of 60 wt % to prepare a slurry. The slurry was then dried in an oven at 130°C to remove the dispersion medium, yielding a powder, which was then measured for powder resistance.

[0058] The viscosity of the prepared dispersion and the powder resistance of the slurry were measured by the following methods. 1) Viscosity: The viscosity of the dispersion was measured at room temperature using a Brookfield DV2T device.

[0059] 2) Powder resistance of the slurry: The resistance due to pressure when the compressed density was 2.5 g / cc was measured using an MCP-PD51 device manufactured by Nittoseiko Analytech. The measurement results are summarized in Table 4 below.

[0060] [Table 4]

[0061] As can be seen from Table 4 above, when dispersions and slurries are produced using the carbon nanotubes of the examples of the present invention, the viscosity of the dispersion is maintained at an appropriate level, processability is excellent, the powder resistance of the slurry is low, and excellent performance can be demonstrated when used as a conductive material.

[0062] On the other hand, in the case of the carbon nanotubes of the comparative examples which do not satisfy the formula 1 of the present invention or which do not satisfy the appropriate range of specific surface area or bulk density, the powder resistance during slurry production is much higher than that of the examples, and this confirms that the carbon nanotubes are not suitable for use as conductive materials.

Claims

1. Carbon nanotubes characterized by satisfying the following formula 1: [Formula 1] -0.004*A+0.0385≦R≦-0.004*A+0.0425 In the formula 1, R is the powder resistivity (Ω cm) of the carbon nanotubes, which is measured using an MCP-PD51 device manufactured by Nittosei Analytech Co., Ltd., as the resistance due to pressure at a compressed density of 1 g / cc; A is ln {(purity of carbon nanotubes (wt%) * specific surface area (m ２ / g) / bulk density (kg / m ３ )}, The specific surface area of ​​the carbon nanotubes is 320 m 2 / g to 500 m ２ / g, The carbon nanotubes have a bulk density of 15 kg / m 3 to 30 kg / m 3 .

2. 2. The carbon nanotube according to claim 1, wherein the R is 0.0080 Ω·cm to 0.0125 Ω·cm.

3. The carbon nanotube according to claim 1 or 2, and A carbon nanotube dispersion containing a dispersion medium.

4. 4. The carbon nanotube dispersion liquid according to claim 3, wherein the dispersion medium is one or more selected from the group consisting of N-methylpyrrolidone, pyridine, dimethylaminobenzene, and diethylaminobenzene.

5. 4. The carbon nanotube dispersion according to claim 3, wherein the carbon nanotube content in the dispersion is 0.05% by weight to 5% by weight.

6. The carbon nanotube dispersion liquid according to claim 3 . cathode materials, and A positive electrode slurry composition comprising a stabilizer.

7. The positive electrode slurry composition of claim 6, wherein the stabilizer is contained in an amount of 10 to 100 wt % based on the content of the carbon nanotubes in the positive electrode slurry composition.

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