CARBON NANOTUBE AND DISPERSION CONTAINING THE SAME

Abstract

Carbon nanotubes satisfy Equation 1: 0.004*A+0.0385R0.004*A+0.0425, wherein R is the powder resistance of the carbon nanotubes (.Math.cm), A is ln{(purity of carbon nanotube (weight %)*specific surface area (m.sup.2/g))/bulk density (kg/m.sup.3)}, wherein the specific surface area of the carbon nanotube is 320 m.sup.2/g or more, and the bulk density of the carbon nanotube is 30 kg/m.sup.3 or less. The carbon nanotubes of the present invention have both excellent dispersibility and electrical conductivity when applied as a dispersion, and thus, are particularly suitable for use as a conductive material in secondary batteries.

Claims

1. A carbon nanotube characterized by satisfying Equation 1: $\begin{array}{cc}-0.004*A+0.0385R-0.004*A+0.0425& \mathrm{Equation}1\end{array}$ wherein: R is the powder resistance of the carbon nanotubes (.Math.cm), A is ln{(purity of carbon nanotube (weight %)*specific surface area (m.sup.2/g))/bulk density (kg/m.sup.3)}, wherein the specific surface area of the carbon nanotube is 320 m.sup.2/g or more, and the bulk density of the carbon nanotube is 30 kg/m.sup.3 or less.

2. The carbon nanotube according to claim 1, wherein the specific surface area of the carbon nanotube is 320 to 500 m.sup.2/g.

3. The carbon nanotube according to claim 1, wherein the bulk density of the carbon nanotube is 15 to 30 kg/m.sup.3.

4. The carbon nanotube according to claim 1, wherein the R is 0.0125 .Math.cm or less.

5. The carbon nanotube according to claim 4, wherein The R is 0.0080 to 0.0125 .Math.cm.

6. A carbon nanotube dispersion including: the carbon nanotube of claim 1; and a dispersion medium.

7. The carbon nanotube dispersion according to claim 6, wherein the dispersion medium is at least one selected from the group consisting of N-methylpyrrolidone, pyridine, dimethylaminobenzene, and diethylaminobenzene.

8. The carbon nanotube dispersion according to claim 6, wherein the carbon nanotube content in the dispersion is 0.05 to 5% by weight.

9. A positive electrode slurry composition including: the carbon nanotube dispersion of claim 6; a positive electrode material; and a stabilizer.

10. The positive electrode slurry composition according to claim 9, wherein the stabilizer is contained in an amount of 10 to 100% by weight based on the content of the carbon nanotube in the positive electrode slurry composition.

Description

DETAILED DESCRIPTION

[0028] Hereinafter, the present invention will be described in more detail.

[0029] The terms or words used in the specification and claims of the present application should not be construed as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts consistent with the technical spirit of the present invention, based on the principle that the inventor may adequately define the concepts of terms to best describe his invention.

[0030] The term carbon nanotube used in the present invention is a secondary structure formed by gathering units of carbon nanotubes to form a bundle entirely or partially, wherein the carbon nanotube unit has a graphite sheet in the shape of a cylinder with a nano-sized diameter, and has a sp.sup.2 bond structure. In this case, the characteristics of a conductor or semiconductor may appear depending on the angle and structure in which the graphite surface is rolled. Depending on the number of bonds forming the wall, the carbon nanotube unit may be divided into single-walled carbon nanotube (SWCNT), double-walled carbon nanotube (DWCNT), and multi-walled carbon nanotube (MWCNT), wherein the thinner the wall thickness, the lower the resistance. The carbon nanotubes of the present invention may include one or more of single-walled, double-walled, and multi-walled carbon nanotube units.

Carbon Nanotube

[0031] The present invention provides a carbon nanotube characterized by satisfying Equation 1 below:

[00002]$\begin{array}{cc}-0.004*A+0.0385R-0.004*A+0.0425& \mathrm{Equation}1\end{array}$

[0032] In equation 1 above: [0033] R is the powder resistance of the carbon nanotubes (.Math.cm), [0034] A is ln{(purity of carbon nanotube (weight %)*specific surface area (m.sup.2/g))/bulk density (kg/m.sup.3)}, [0035] wherein the specific surface area of the carbon nanotube is 320 m.sup.2/g or more, and [0036] the bulk density of the carbon nanotube is 30 kg/m.sup.3 or less.

[0037] The inventors of the present invention have studied the properties of carbon nanotubes that enable both viscosity and conductivity characteristics to be maintained at an excellent level when preparing a dispersion, and as a result, have confirmed that when the powder resistance, purity, specific surface area, and bulk density of the carbon nanotubes satisfy Equation 1 above, the specific surface area of the carbon nanotubes is 320 m.sup.2/g or more, and the bulk density is 30 kg/m.sup.3 or less, both the dispersibility and electrical conductivity of the corresponding carbon nanotubes can be excellently maintained, thereby completing the present invention.

[0038] More specifically, when changing the manufacturing conditions of the catalyst used to produce carbon nanotubes, specifically the active ingredient content, the ratio of the main catalyst component to the cocatalyst component, the content of organic acid in the precursor solution, the calcination temperature, or the like, the properties of the carbon nanotubes produced from the corresponding catalyst also change. Therefore, various carbon nanotubes with different properties can be synthesized by changing the catalyst preparation conditions. Furthermore, as a result of confirming the correlation between the dispersibility, electrical conductivity, and physical properties of various carbon nanotubes in synthesized various ways, it has been found that the carbon nanotubes satisfying Equation 1 have excellent dispersibility and electrical conductivity at the same time. More specifically, Equation 1 above means that there is a correlation between the powder resistance, purity, specific surface area, and bulk density of the carbon nanotubes. Equation 1 above was derived based on various data, and it can be confirmed that the carbon nanotubes satisfying Equation 1 have excellent dispersibility and electrical conductivity at the same time.

[0039] The A and R values in Equation 1 above have different units, respectively, 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 powder resistance, purity, specific surface area, and bulk density of the carbon nanotubes, which are variables of each value, the units of each variable are fixed as follows when applying Equation 1 above: [0040] Unit of powder resistance (R) of carbon nanotubes: .Math.cm [0041] Unit of purity of carbon nanotubes: % by weight [0042] Unit of specific surface area of carbon nanotubes: m.sup.2/g [0043] Unit of bulk density of carbon nanotubes: kg/m.sup.3

[0044] In the carbon nanotubes provided by the present invention, the specific surface area of the carbon nanotubes may be 320 m.sup.2/g or more, preferably 320 to 500 m.sup.2/g. If carbon nanotubes are defined only by Equation 1 above without limiting the range of the specific surface area value of carbon nanotubes, the carbon nanotubes satisfying Equation 1 may be substantially infinite, including not only carbon nanotubes that have both excellent dispersibility and electrical conductivity as desired by the present invention, but also carbon nanotubes that are not excellent in dispersibility and/or electrical conductivity. Therefore, the carbon nanotubes of the present invention must satisfy Equation 1 above and also have a specific surface area within the above-mentioned range. The specific surface area may be measured according to the BET method, and more specifically, it may be calculated by determining the amount of nitrogen gas adsorbed under liquid nitrogen temperature (77K) using BELSORP-mini II of BEL Japan.

[0045] Furthermore, the carbon nanotubes of the present invention may have a bulk density of 30 kg/m.sup.3 or less, preferably 15 to 30 kg/m.sup.3. When the bulk density of the carbon nanotubes is within the above-described range while satisfying Equation 1 and the specific surface area conditions described above, the carbon nanotubes can have particularly excellent dispersibility and electrical conductivity, and can further exhibit sufficient electrical conductivity even with a low carbon nanotube content when preparing a dispersion. The bulk density can be calculated by measuring the weight of carbon nanotubes contained in a container in free fall using a 25 ml SUS measuring cup and dividing the measured weight by the volume of the container.

[0046] In addition, R, which is the powder resistance of the carbon nanotubes of the present invention, may be 0.0125 .Math.cm or less, and particularly preferably 0.0080 to 0.0125 .Math.cm. As in the case of the bulk density described above, when the powder resistance of the carbon nanotube is within the above-described range while satisfying Equation 1 and the specific surface area conditions described above, the electrical conductivity may be particularly excellent. Meanwhile, the powder resistance may be measured by measuring the resistance according to pressure when the compressed density is 1 g/cc by using MCP-PD51 equipment of Nittoseiko Analytech.

[0047] In addition, the carbon nanotubes of the present invention may have a purity of 80% by weight or more, and particularly preferably 83% by weight or more. The purity refers to the content of carbon nanotubes remaining after the impurities in the carbon nanotubes are removed, and can be calculated using the following equation:

[00003]$\mathrm{Purity}=\left(\mathrm{Carbon}\mathrm{nanotube}\mathrm{yield}-\mathrm{catalyst}\mathrm{input}\mathrm{amount}\right)/\mathrm{carbon}\mathrm{nanotide}\mathrm{yield}*100\%$

Carbon Nanotube Dispersion

[0048] The present invention provides a dispersion including the carbon nanotubes described above. More specifically, the present invention provides a carbon nanotube dispersion including the above carbon nanotube and a dispersion medium.

[0049] In the carbon nanotube dispersion of the present invention, the dispersion medium may be at least one selected from the group consisting of N-methylpyrrolidone, pyridine, dimethylaminobenzene, and diethylaminobenzene, and preferably N-methylpyrrolidone. When the above-listed dispersion medium is used, the carbon nanotubes can be smoothly dispersed, and the dispersion medium can be easily and selectively removed during the subsequent application and calcination process of the dispersion.

[0050] In the carbon nanotube dispersion of the present invention, the carbon nanotube content in the dispersion may be 0.05 to 5% by weight, preferably 0.5 to 3% by weight. When the carbon nanotube content in the dispersion is lower than the above-mentioned range, sufficient electrical conductivity cannot be achieved, and if the carbon nanotube content is higher than the above-mentioned range, agglomeration of excessively added carbon nanotubes occurs, causing an increase in viscosity, and thus, the processability of the dispersion itself may be greatly reduced.

[0051] In the carbon nanotube dispersion of the present invention, the dispersion includes, as a dispersing agent, polysaccharides s and monosaccharides such as carboxymethylcellulose (CMC), hydroxyethylcellulose, 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 condensate and alkyl benzenesulfonate, cationic surfactants, nonionic surfactants, polyether-modified silicone surfactants, and hydrogenated nitrile butadiene rubber (HNBR) etc. Particularly preferably, the dispersing agent may be HNBR. The dispersing agent may be included in an amount of 0.1 to 5% by weight, preferably 0.3 to 3% by weight, based on the total weight of the dispersion. Within the above content range, the viscosity of the dispersion may be low, and the viscosity stability may be excellent.

Positive Electrode Slurry Composition

[0052] The carbon nanotube dispersion provided by the present invention has excellent electrical conductivity and thus can be used as a conductive material in a positive electrode slurry composition. Accordingly, the present invention provides a positive electrode slurry composition containing the dispersion.

[0053] Specifically, the present invention covers a positive electrode slurry composition including the above carbon nanotube dispersion, a positive electrode material, and a stabilizer. The dispersion was the same as previously described.

[0054] 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, the positive electrode material may be any one or a mixture of two or more selected from the group consisting of LiCoO.sub.2, LiNiO.sub.2, LiMnO.sub.2, LiMn.sub.2O.sub.4, Li(Ni.sub.aCo.sub.bMn.sub.c)O.sub.2 (here, 0<a<1, 0<b<1, 0<c<1, a+b+c=1), LiNi.sub.1-YCo.sub.YO.sub.2, LiCo.sub.1-YMn.sub.YO2, LiNi.sub.1-YMn.sub.YO2 (here, 0Y<1), Li(Ni.sub.aCo.sub.bMn.sub.c)O.sub.4 (0<a<2, 0<b<2, 0<c<2, a+b+c=2), LiMn.sub.2-zNi.sub.zO4, LiMn.sub.2-zCo.sub.zO.sub.4 (here, 0<Z<2), which are known positive electrode materials.

[0055] 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 to 100% by weight, preferably 30 to 70% by weight, based on the content of the carbon nanotube in the positive electrode slurry composition. Within the above content range, the stability during slurry production may be improved.

[0056] Hereinafter, the present invention will be described in more detail by way of examples and experimental examples to specifically illustrate the present invention, 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 in detail below. The examples of the present invention are provided to explain the present invention more completely to those skilled in the art.

EXAMPLES

Catalyst Preparation Example

[0057] Co(NO.sub.3).sub.2.Math.6H.sub.2O was used as a cobalt precursor, and NH.sub.4VO.sub.3 was used as a vanadium precursor. A catalyst precursor composition was prepared by dissolving the cobalt precursor and vanadium precursor in water along with citric anhydride (CA) as a complexing agent. The catalyst precursor composition was sufficiently stirred, and then added to hydrotalcite as a support. Afterwards, it was dried at 190 C. for 5 hours using an oven, and then calcined for 4 hours at a specific calcination temperature and in the atmosphere to complete the catalyst. In the above process, the added amount of vanadium was 0.3 mole ratio relative to 1 mole of cobalt, and the amount of the citric anhydride was 0.13 mole ratio relative to 1 mole of cobalt. In the above process, various catalysts were prepared by varying the cobalt content in the catalyst precursor composition, the ratio of cobalt to vanadium, the ratio of citric anhydride to vanadium, the calcination temperature, etc., and the catalyst preparation conditions in each preparation example are summarized in Table 1 below.

TABLE-US-00001 TABLE 1 Cobalt content in the Co/V Ca/V Calcination composition (molar (molar temperature (% by weight) ratio) ratio) ( C.) Catalyst Preparation 5.0 3.33 0.44 300 Example 1 Catalyst Preparation 5.5 3.33 0.44 300 Example 2 Catalyst Preparation 6.0 3.33 0.44 300 Example 3 Catalyst Preparation 6.5 3.33 0.44 300 Example 4 Catalyst Preparation 8.5 3.33 0.44 300 Example 5 Catalyst Preparation 11.0 3.33 0.44 300 Example 6 Catalyst Preparation 14.0 3.33 0.44 300 Example 7 Catalyst Preparation 6.0 3.33 0.44 250 Example 8 Catalyst Preparation 6.0 3.33 0.44 400 Example 9 Catalyst Preparation 6.0 3.33 0.44 500 Example 10 Catalyst Preparation 6.0 3.33 0.31 300 Example 11 Catalyst Preparation 6.0 3.33 0.43 300 Example 12 Catalyst Preparation 6.0 3.33 0.65 300 Example 13 Catalyst Preparation 6.0 3.33 0.87 300 Example 14 Catalyst Preparation 6.0 1.00 0.44 300 Example 15 Catalyst Preparation 6.0 1.70 0.44 300 Example 16 Catalyst Preparation 6.0 2.50 0.44 300 Example 17 Catalyst Preparation 6.0 2.90 0.44 300 Example 18 Catalyst Preparation 6.0 4.90 0.44 300 Example 19 Catalyst Preparation 6.0 10.00 0.44 300 Example 20

EXAMPLES AND COMPARATIVE EXAMPLES

[0058] Carbon nanotubes were synthesized using the catalyst used in the above catalyst preparation example. Specifically, after 0.3 g of the prepared catalyst was filled in the fixed bed reactor, nitrogen gas was injected into the fixed bed reactor at 1600 sccm, and the temperature inside the reactor was heated to the reaction temperature. Then, ethylene gas as a carbon source gas was injected 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.

TABLE-US-00002 TABLE 2 Catalyst Reaction temperature ( C.) Example 1-1 Catalyst 670 Preparation Example 1 Example 1-2 Catalyst 670 Preparation Example 2 Example 1-3 Catalyst 670 Preparation Example 3 Example 1-4 Catalyst 670 Preparation Example 4 Example 1-5 Catalyst 670 Preparation Example 5 Comparative Catalyst 670 Example 1-1 Preparation Example 6 Comparative Catalyst 670 Example 1-2 Preparation Example 7 Example 2-1 Catalyst 670 Preparation Example 8 Example 2-2 Catalyst 670 Preparation Example 9 Comparative Catalyst 670 Example 2-1 Preparation Example 10 Example 3-1 Catalyst 670 Preparation Example 11 Example 3-2 Catalyst 670 Preparation Example 12 Comparative Catalyst 670 Example 3-1 Preparation Example 13 Comparative Catalyst 670 Example 3-2 Preparation Example 14 Example 4-1 Catalyst 670 Preparation Example 15 Example 4-2 Catalyst 670 Preparation Example 16 Example 4-3 Catalyst 670 Preparation Example 17 Example 4-4 Catalyst 670 Preparation Example 18 Example 4-5 Catalyst 670 Preparation Example 19 Example 4-6 Catalyst 670 Preparation Example 20 Example 5-1 Catalyst 640 Preparation Example 3 Example 5-2 Catalyst 610 Preparation Example 3

Experimental Example 1. Check Whether the Manufactured Carbon Nanotubes Satisfy Equation 1

[0059] The purity, specific surface area, bulk density, and powder resistance of the carbon nanotubes prepared in the above examples and comparative examples were measured, and it was examined whether Equation 1 was satisfied. Each property was measured using the corresponding method below.

[00004]$1)\mathrm{Purity}=\left(\mathrm{obtained}\mathrm{amount}\mathrm{of}\mathrm{carbon}\mathrm{nanotubes}-\mathrm{added}\mathrm{amount}\mathrm{of}\mathrm{catalyst}\right)/\mathrm{obtained}\mathrm{amount}\mathrm{of}\mathrm{carbon}\mathrm{nanotubes}*100\%.$

[0060] 2) Specific surface area was calculated by determining the amount of nitrogen gas adsorbed under liquid nitrogen temperature (77K) using BELSORP-mini II of BEL Japan.

[0061] 3) Bulk density was calculated by measuring the weight of carbon nanotubes contained in a container in free fall using a 25 ml SUS measuring cup and dividing the measured weight by the volume of the container.

[0062] 4) Powder resistance was measured by measuring the resistance according to pressure when the compressed density is 1 g/cc by using MCP-PD51 equipment of Nittoseiko Analytech.

[0063] The measured results are summarized in Table 3 below.

TABLE-US-00003 TABLE 3 Spec. Whether Purity surface Powder Equation (% by area Bulk density resistance 1 is weight) (m.sup.2/g) (kg/m.sup.3) ( .Math. cm) satisfied Example 1-1 83.17 417 22.4 0.0110 Example 1-2 90.79 422 16.8 0.0095 Example 1-3 93.6 415 17.2 0.0097 Example 1-4 94.1 406 19.3 0.0100 Example 1-5 96.3 325 24.0 0.0110 Comparative 96.3 287 28.4 0.0121 Example 1-1 Comparative 96.3 240 33.0 0.0135 Example 1-2 Example 2-1 96.3 415 12.9 0.0090 Example 2-2 94.1 350 21.2 0.0115 Comparative 94.3 280 20.1 0.0125 Example 2-1 Example 3-1 90.1 391 23.3 0.0115 Example 3-2 89.7 393 23.1 0.0116 Comparative 85.5 368 32.1 0.0144 Example 3-1 Comparative 80.0 350 38.7 0.0180 X Example 3-2 Example 4-1 91.5 321 26.7 0.0125 Example 4-2 89.1 346 22.0 0.0123 Example 4-3 95.1 389 20.9 0.0105 Example 4-4 94.4 404 18.7 0.0100 Example 4-5 92.3 367 16.2 0.0108 Example 4-6 81.5 335 22.0 0.0122 Example 5-1 95.7 372 16.0 0.0089 Example 5-2 95.6 361 18.0 0.0091

[0064] As can be seen in Table 3, the carbon nanotubes according to the examples of the present invention satisfy Equation 1. Meanwhile, the carbon nanotubes according to Comparative Examples 1-1, 1-2 and 2-1 satisfy Equation 1, but do not satisfy the specific surface area range required by the present invention. The carbon nanotubes according to Comparative Example 3-1 satisfies Equation 1, but does not satisfy the bulk density range required by the present invention. The carbon nanotube according to Comparative Example 3-2 does not satisfy Equation 1.

Experimental Example 2. Measurement of Viscosity and Slurry Powder Resistance of Carbon Nanotube Dispersion

[0065] A dispersion was prepared by using the carbon nanotubes prepared in the above examples and comparative examples. 1.0 g of the prepared carbon nanotube and 0.6 g of HNBR as a dispersant were added to 98.4 g of N-methylpyrrolidone as a dispersion medium. Then, a dispersion was prepared under conditions of 1500 bar and 3 pass using a high-pressure homogenizer. The prepared dispersion was mixed with a positive electrode material so that the carbon nanotube content was 0.5% by weight, and further PVDF as a stabilizer was added at 60% by weight based on the carbon nanotube content to prepare a slurry. The slurry was dried in an oven at 130 C. to remove the dispersion medium and obtain powder to measure the powder resistance.

[0066] The viscosity and slurry powder resistance of the prepared dispersion were measured as follows:

[0067] 1) Viscosity: The viscosity of the dispersion was measured at room temperature using Brookfield's DV2T equipment.

[0068] 2) Slurry powder resistance: The resistance according to pressure was measured at a compressed density of 2.5 g/cc by using MCP-PD51 equipment of Nittoseiko Analytech.

[0069] The measured results are summarized in Table 4 below.

TABLE-US-00004 TABLE 4 Slurry powder resistance Viscosity (cP) (m .Math. cm) Example 1-1 900 6.9 Example 1-2 1050 7.3 Example 1-3 1000 6.2 Example 1-4 1000 7.3 Example 1-5 650 9.2 Comparative 300 16.7 Example 1-1 Comparative 250 23.2 Example 1-2 Example 2-1 1000 5.8 Example 2-2 700 7.9 Comparative 300 19.6 Example 2-1 Example 3-1 950 7.2 Example 3-2 950 7.3 Comparative 800 16.9 Example 3-1 Comparative 750 18.9 Example 3-2 Example 4-1 650 10.0 Example 4-2 700 9.5 Example 4-3 950 7.4 Example 4-4 1000 7.0 Example 4-5 800 7.5 Example 4-6 650 8.6 Example 5-1 900 6.1 Example 5-2 850 6.3

[0070] As can be seen in Table 4 above, it can be confirmed that when the dispersions and slurries are manufactured using the carbon nanotubes according to the examples of the present invention, the dispersion viscosity is maintained at an appropriate level, thereby providing excellent processability, and the slurry powder resistance is low, thereby exhibiting excellent performance when used as a conductive material.

[0071] Meanwhile, in the case of the carbon nanotubes of the comparative examples that do not satisfy Equation 1 of the present invention or do not satisfy the appropriate specific surface area or bulk density range, it can be confirmed that all of them have significantly higher powder resistance during slurry production compared to the examples, and thus, the corresponding carbon nanotubes are less suitable for use as a conductive material.