Carbon nanotube purification method and application and conductive paste

Abstract

The present invention discloses a carbon nanotube purification method, application, and conductive slurry. The carbon nanotube purification method comprises: (1) blending carbon nanotube raw powder, a non-thickening ionic dispersant, and a first solvent to perform a first dispersion treatment to obtain a carbon nanotube slurry; (2) freeze-drying the carbon nanotube slurry and then performing gas phase purification to obtain a carbon nanotube powder; and (3) acid-washing and water-washing the carbon nanotube powder to obtain carbon nanotubes. This method can significantly improve the purification effect of carbon nanotubes and reduce the amorphous carbon content and metal impurity content in the carbon nanotubes. The method is simple and easy to implement.

Description

Technical Field

[0001] The invention belongs to the technical field of conductive slurry preparation, and particularly relates to a carbon nanotube purification method and application and conductive slurry. Background Art

[0002] Carbon nanotubes are a type of carbon nanotube with perfect sp 2 A special one-dimensional nanocarbon material with a high aspect ratio hollow tubular structure composed of hybrid materials. Using carbon nanotubes as a conductive agent in lithium batteries significantly improves the conductivity of the electrode and interacts well with the active material, thereby inhibiting the expansion of the silicon-carbon negative electrode. As a result, carbon nanotubes are increasingly used in lithium batteries.

[0003] Currently, the most effective method for preparing carbon nanotubes is chemical vapor deposition (CVD), which uses a metal catalyst to crack a carbon source at a relatively high temperature. The chemical vapor deposition process inevitably introduces various impurities, such as amorphous carbon, metal catalyst particles, onion-like carbon nanoparticles, and residual catalyst support materials. During the CVD process of preparing carbon nanotubes, due to their flexibility, carbon nanotubes entangle themselves on the surfaces of catalyst supports, catalysts, and carbon impurities, often forming a dense structure of carbon nanotubes encapsulating impurities. Simple solid-phase mechanical grinding is difficult to destroy this structure, which affects the subsequent purification effect. As a result, the obtained carbon nanotubes will have a large amount of residual impurities, making it difficult to meet the requirements of battery-grade conductive agents. At the same time, the presence of impurities will affect the performance of lithium-ion batteries. In order to meet battery-grade requirements, carbon nanotubes must undergo a rigorous purification process before use.

[0004] Currently, industrial carbon nanotube purification methods rely on gas-phase oxidation and acid purification. Oxidizing gases such as carbon dioxide and oxygen react with amorphous carbon to form gases, which open up the carbon-coated iron structure, exposing metal impurities and improving pickling efficiency. However, due to the large size of some amorphous carbon, gas-phase purification efficiency is limited while avoiding oxidation losses of the carbon nanotubes, and some carbon-coated iron structures remain. Furthermore, some metal catalysts are tightly entangled and wrapped by surface carbon nanotube aggregates, significantly affecting pickling efficiency. Therefore, finding a convenient and effective method for carbon nanotube purification is of great significance. Summary of the Invention

[0005] The present invention aims to at least partially address one of the technical problems in the related art. To this end, one object of the present invention is to provide a carbon nanotube purification method, its application, and a conductive slurry. This method significantly improves carbon nanotube purification efficiency, reduces the amorphous carbon content and metallic impurity content in the carbon nanotubes, and is simple and easy to implement.

[0006] In one aspect of the present invention, a method for purifying carbon nanotubes is provided. According to an embodiment of the present invention, the method comprises:

[0007] (1) blending carbon nanotube raw powder, a non-thickening ionic dispersant, and a first solvent to perform a first dispersion treatment to obtain a carbon nanotube slurry;

[0008] (2) freeze-drying the carbon nanotube slurry and then performing gas phase purification to obtain carbon nanotube powder;

[0009] (3) The carbon nanotube powder is acid-washed and water-washed to obtain carbon nanotubes.

[0010] According to the carbon nanotube purification method of an embodiment of the present invention, first, carbon nanotube raw powder, a non-thickening ionic dispersant and a first solvent are blended to perform a first dispersion treatment. The non-thickening ionic dispersant can be adsorbed and wrapped on the surface of the carbon nanotube raw powder or intercalated into the gaps between the carbon nanotube raw powder, thereby preventing the carbon nanotube raw powder from shrinking and agglomerating through steric hindrance and electrostatic repulsion, greatly improving the dispersion of the carbon nanotube raw powder in the solution, and making the carbon nanotube raw powder uniformly suspended in the solution, significantly improving the effect of the first dispersion treatment, thereby effectively crushing large particle impurities, exposing more metal impurities and amorphous carbon, increasing the contact area between the amorphous carbon and the weak oxidant during gas phase purification, and increasing the contact area between the metal impurities and the acid solution, thereby effectively removing the amorphous carbon and metal impurities in the carbon nanotubes.

[0011] The carbon nanotube slurry is then freeze-dried and subjected to gas-phase purification. Freeze-drying produces a fluffy carbon nanotube mass, thereby resolving the issues of shrinkage and thermal agglomeration of the carbon nanotubes during thermal drying. This prevents the degradation of gas-phase purification and acid washing efficiency due to severe entanglement and difficulty in dispersing the carbon nanotubes. After freeze-drying, gas-phase purification is then used to remove amorphous carbon from the carbon nanotubes. Finally, the gas-phase purified carbon nanotube powder is acid-washed and then washed with water to remove metal impurities, thereby obtaining highly pure carbon nanotubes. This method significantly improves carbon nanotube purification efficiency, reduces the amorphous carbon content and metal impurity content in the carbon nanotubes, and is simple and easy to implement. Specifically, the metal content of the purified carbon nanotubes is no greater than 100 ppm, and the amorphous carbon content is no greater than 3 wt%.

[0012] In addition, the carbon nanotube purification method according to the above embodiment of the present invention may also have the following technical features:

[0013] In some embodiments of the present invention, in step (1), the viscosity of the carbon nanotube slurry is not greater than 200 mPa·s at 25° C. This can improve the pickling effect.

[0014] In some embodiments of the present invention, the non-thickening ionic dispersant includes at least one of sodium dodecylsulfonate, sodium dodecylbenzenesulfonate, sodium polystyrenesulfonate, lithium polystyrenesulfonate, sodium ligninsulfonate, alkali lignin, and sodium lauryl sulfate, thereby improving the purification effect of carbon nanotubes.

[0015] In some embodiments of the present invention, in step (1), the mass ratio of the non-thickening ionic dispersant to the carbon nanotube raw powder in the carbon nanotube slurry is (2.5-20): 1000. This can improve the purification effect of carbon nanotubes.

[0016] In some embodiments of the present invention, in step (1), the first dispersion treatment includes sand milling or a shearing treatment and sand milling, thereby improving the purification effect of carbon nanotubes.

[0017] In some embodiments of the present invention, the linear speed of the sanding treatment is 1200 rpm to 1600 rpm, and the sanding treatment time is 60 minutes to 360 minutes; the speed of the shearing treatment is 4000 rpm to 16000 rpm, and the shearing treatment time is 15 minutes to 360 minutes. This can improve the purification effect of carbon nanotubes.

[0018] In some embodiments of the present invention, in step (2), the gas phase purification includes: introducing air and an inert carrier gas, reacting at 350°C to 450°C for 30 to 120 minutes; shutting off the air, continuously introducing an inert carrier gas and H2, raising the temperature to 800°C to 950°C, and continuing for 10 to 30 minutes; shutting off the H2, and continuously introducing an inert carrier gas and a weak oxidizing gas for 30 to 180 minutes. This can improve the removal rate of amorphous carbon from carbon nanotubes.

[0019] In some embodiments of the present invention, in step (3), the concentration of the pickling acid solution is 25 wt% to 50 wt%, the pickling temperature is 30° C. to 80° C., and the pickling time is 2 to 8 hours. This can improve the metal removal rate of carbon nanotubes.

[0020] In a second aspect of the present invention, the present invention proposes the application of the above carbon nanotube purification method in the preparation of conductive paste.

[0021] According to an embodiment of the present invention, the application includes:

[0022] The carbon nanotubes are obtained by the purification method of the first aspect;

[0023] The carbon nanotubes, a thickening dispersant and a second solvent are mixed to perform a second dispersion treatment, so as to obtain a conductive paste.

[0024] According to the application of the above embodiment of the present invention, first, the purification method of the first aspect is used to obtain carbon nanotubes, and then the carbon nanotubes are mixed with a thickening dispersant and a second solvent to perform a second dispersion treatment. Through the second dispersion treatment, a small-particle-size carbon nanotube conductive paste is formed. The thickening dispersant can increase the viscosity of the solution. Under the synergistic effect of the second dispersion treatment, the carbon nanotubes can be evenly dispersed in the solution, thereby improving the conductive effect of the conductive paste. In this way, a conductive paste with excellent carbon nanotube dispersion and low carbon nanotube impurity content can be obtained, so that the conductive paste has an excellent conductive effect. Specifically, the metal content of the carbon nanotubes in the conductive paste is not greater than 100ppm, the amorphous carbon content in the carbon nanotubes is not greater than 3wt%, and the particle size D of the carbon nanotubes is less than 100ppm. V50 20nm~9μm.

[0025] In addition, the application of the carbon nanotube purification method in the preparation of conductive paste according to the present invention may also have the following technical features:

[0026] In some embodiments of the present invention, the second dispersion treatment includes shearing and homogenizing or shearing and sanding, thereby improving the dispersibility of the carbon nanotubes in the conductive paste and reducing the particle size of the carbon nanotubes.

[0027] In some embodiments of the present invention, the shearing process is performed at a rate of 3,000 to 16,000 rpm for a duration of 20 to 60 minutes; the homogenization process is performed at a pressure of 5,000 to 40,000 psi, and the homogenization chamber of the homogenizer has an aperture of 75 μm to 300 μm; the sanding process is performed at a linear speed of 1,200 to 1,600 rpm for a duration of 1 to 4 hours. This improves the dispersion of the carbon nanotubes in the conductive paste and reduces the particle size of the carbon nanotubes in the conductive paste.

[0028] In some embodiments of the present invention, the thickening dispersant includes at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium hexametaphosphate, and sodium alginate, thereby improving the dispersibility of the carbon nanotubes in the conductive paste.

[0029] In some embodiments of the present invention, the mass ratio of the thickening dispersant to the carbon nanotubes is (2.5-20): 10. This can improve the dispersibility of the carbon nanotubes in the conductive paste.

[0030] In a third aspect, the present invention provides a conductive paste. According to an embodiment of the present invention, the conductive paste comprises carbon nanotubes obtained using the aforementioned carbon nanotube purification method. As a result, the carbon nanotubes in the conductive paste have low metal and amorphous carbon contents, and the conductive paste exhibits excellent electrical conductivity.

[0031] Additional aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description which follows, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which:

[0033] Figure 1 This is a schematic flow chart of a method for purifying carbon nanotubes according to an embodiment of the present invention;

[0034] Figure 2 1 is a schematic flow chart of a method for preparing a conductive paste according to an embodiment of the present invention;

[0035] Figure 3 This is a SEM morphology diagram of the conductive paste sample after dispersion according to Example 1 of the present invention;

[0036] Figure 4 This is a SEM morphology diagram of the conductive paste sample after dispersion according to Example 2 of the present invention;

[0037] Figure 5 This is a SEM morphology diagram of the conductive paste sample after dispersion according to Example 4 of the present invention;

[0038] Figure 6 is a SEM morphology diagram of the conductive paste sample after dispersion according to Example 11 of the present invention;

[0039] Figure 7 This is a SEM morphology diagram of the conductive paste sample after dispersion in Comparative Example 1 of the present invention;

[0040] Figure 8 This is a SEM morphology diagram of the conductive paste sample of Comparative Example 2 of the present invention after dispersion. DETAILED DESCRIPTION

[0041] The embodiments of the present invention are described in detail below, which are intended to explain the present invention but are not to be construed as limiting the present invention.

[0042] In one aspect of the present invention, a method for purifying carbon nanotubes is provided. Figure 1 , the method comprising:

[0043] S100: Mixing carbon nanotube raw powder, non-thickening ionic dispersant and first solvent for first dispersion treatment

[0044] In this step, the carbon nanotube raw powder, the non-thickening ionic dispersant and the first solvent are mixed and subjected to a first dispersion treatment to obtain a carbon nanotube slurry, wherein the non-thickening ionic dispersant is adsorbed and wrapped on the surface of the carbon nanotube raw powder or intercalated into the gaps between the carbon nanotube raw powder and the carbon nanotube raw powder, and the carbon nanotube raw powder is prevented from shrinking and agglomerating through steric hindrance and electrostatic repulsion, thereby greatly improving the dispersion of the carbon nanotube raw powder in the solution, so that the carbon nanotube raw powder is uniformly suspended in the solution. Therefore, on the one hand, the efficiency of the first dispersion treatment can be improved, thereby significantly reducing the particle size of the impurity particles, exposing more metal impurities and amorphous carbon, and improving the gas phase in the later stage. The invention discloses a novel non-thickening ionic dispersant, which can effectively prevent the carbon nanotubes from agglomerating due to shrinkage in the later water washing process through steric hindrance. On the other hand, the non-thickening dispersant will not cause the solution viscosity to be too high, which is beneficial to the later pickling and thus improves the metal removal rate. The non-thickening dispersant has a certain acid resistance and will not react with the pickling acid to cause the dispersant to fail. In the entire later purification process (gas phase purification, pickling, water washing), the non-thickening dispersant will play a good dispersing role, which can reduce the agglomeration of carbon nanotubes, thereby significantly improving the purification effect of carbon nanotubes.

[0045] It should be noted that the carbon nanotube raw powder is prepared by conventional methods in the field, for example, the carbon nanotube raw powder is obtained by a fluidized bed method. The amorphous carbon content in the carbon nanotube raw powder is not less than 15wt%, and the metal content in the carbon nanotube raw powder is not less than 5000ppm. The first solvent is a conventional solvent in the field, and those skilled in the art can select it according to actual conditions. For example, the first solvent includes but is not limited to water, NMP, acetone or ethanol. Those skilled in the art will understand that the carbon nanotube raw powder includes single-walled carbon nanotube raw powder and multi-walled carbon nanotube raw powder, and the purification method of the present application is applicable to both single-walled carbon nanotube raw powder and multi-walled carbon nanotube raw powder.

[0046] According to an embodiment of the present invention, the viscosity of the carbon nanotube slurry is no greater than 200 mPa·s at 25°C. Non-thickening dispersants have excellent dispersing effects and do not cause excessive viscosity of the carbon nanotube slurry. The inventors have discovered that the viscosity of the carbon nanotube slurry is no greater than 200 mPa·s at 25°C. Therefore, after freeze-drying and gas-phase purification of the obtained carbon nanotube slurry, the acid solution is mixed with the acid solution for pickling without causing excessive viscosity. This increases the contact area between the acid and metal impurities, thereby improving the removal rate of metal impurities during the pickling process.

[0047] According to an embodiment of the present invention, the mass ratio of the non-thickening ionic dispersant to the raw carbon nanotube powder in the carbon nanotube slurry is (2.5-20):1000. The inventors have discovered that controlling the mass ratio of the non-thickening ionic dispersant to the raw carbon nanotube powder in the carbon nanotube slurry within this range can form a uniform dispersed phase of the carbon nanotubes in the first dispersion treatment and avoid problems such as excessive viscosity of the carbon nanotube slurry and excessive coating of the carbon nanotubes.

[0048] According to an embodiment of the present invention, the non-thickening ionic dispersant includes at least one of sodium dodecylsulfonate, sodium dodecylbenzenesulfonate, sodium polystyrenesulfonate, lithium polystyrenesulfonate, sodium ligninsulfonate, alkali lignin, and sodium lauryl sulfate, thereby improving the purification effect of carbon nanotubes.

[0049] According to an embodiment of the present invention, the first dispersion treatment includes shearing treatment and sanding treatment. The shearing treatment can shear and crush carbon nanotubes with larger particle sizes, which is conducive to sanding the sheared and crushed carbon nanotubes into carbon nanotubes with smaller particle sizes. Through shearing treatment and sanding treatment, carbon nanotube agglomerates can be effectively broken up to obtain carbon nanotubes with small particle sizes, exposing more amorphous carbon and metal impurities, thereby improving the later purification effect of carbon nanotubes. Furthermore, the linear speed of the sanding treatment is 1200rpm~1600rpm, and the time of the sanding treatment is 60min~360min; the rate of the shearing treatment is 4000rpm~16000rpm, and the time of the shearing treatment is 15min~360min.

[0050] According to an embodiment of the present invention, the first dispersion treatment includes a sanding treatment. The sanding treatment can be used to sand the carbon nanotubes into carbon nanotubes with small particle sizes, thereby improving the purification effect of the carbon nanotubes in the later stage. Specifically, the linear speed of the sanding treatment is 1200rpm to 1600rpm, and the sanding treatment time is 60min to 360min. It should be noted that when the carbon nanotubes themselves are seriously agglomerated and the particle size of the carbon nanotubes is relatively large, a shearing treatment can be used first to pre-crush the carbon nanotubes with large particle sizes, and then a sanding treatment can be performed to improve the dispersion treatment effect; when the particle size of the carbon nanotubes themselves is relatively small, a sanding treatment can be directly used to achieve a dispersion effect.

[0051] S200: Freeze-dry the carbon nanotube slurry and then perform gas phase purification

[0052] In this step, the carbon nanotube slurry is freeze-dried and then subjected to vapor-phase purification. Freeze-drying produces a fluffy carbon nanotube mass, thereby resolving the issues of shrinkage, entanglement, and thermal agglomeration of the carbon nanotubes during thermal drying. This also avoids the reduced effectiveness of vapor-phase purification and subsequent acid washing due to severe entanglement and difficulty in dispersing the carbon nanotubes. After freeze-drying, vapor-phase purification is then used to remove amorphous carbon from the carbon nanotubes. It should be noted that freeze-drying is a common practice in the art, and those skilled in the art can select freeze-drying conditions based on their specific circumstances. This will not be elaborated upon here.

[0053] According to an embodiment of the present invention, gas-phase purification includes: introducing air and an inert carrier gas, reacting at 350°C to 450°C for 30 to 120 minutes; shutting off the air, continuously introducing the inert carrier gas and H2, raising the temperature to 800°C to 950°C for 10 to 30 minutes; shutting off the H2, and continuously introducing the inert carrier gas and a weak oxidizing gas for 30 to 180 minutes. Amorphous carbon has lower oxidation resistance than carbon nanotubes, so a gradient oxidation process using air and a weak oxidizing gas can effectively remove the amorphous carbon from carbon nanotubes. Thus, the gas-phase purification method described above can improve the removal rate of amorphous carbon from carbon nanotubes. It should be noted that the inert carrier gas includes nitrogen, argon, etc.; the weak oxidizing gas includes at least one of carbon dioxide, sulfur dioxide, nitrogen dioxide, and water vapor.

[0054] According to an embodiment of the present invention, the volume ratio of air to inert carrier gas is 1:(0.75~2); the volume ratio of H2 to inert carrier gas is 1:(3~6); and the volume ratio of weak oxidizing gas to inert carrier gas is 1:(1~2), thereby effectively removing amorphous carbon from carbon nanotubes.

[0055] S300: Acid washing and water washing of carbon nanotube powder

[0056] In this step, the carbon nanotube powder obtained after the gas phase purification is sequentially subjected to acid washing and water washing. The acid washing removes metal impurities, thereby obtaining purified carbon nanotubes with a low impurity content. The water washing cleanses the carbon nanotubes to a neutral state. It should be noted that the acid used for the acid washing is conventional in the art and can be selected by those skilled in the art based on practical needs. For example, the acid solution used for the acid washing may include at least one of sulfuric acid, nitric acid, and hydrochloric acid.

[0057] According to an embodiment of the present invention, the concentration of the pickling acid solution is 25 wt% to 50 wt%, the pickling temperature is 30° C. to 80° C., and the pickling time is 2 to 8 hours, thereby improving the removal rate of metal impurities in carbon nanotubes.

[0058] This method significantly improves the purification of carbon nanotubes, reducing the amorphous carbon content and metal impurity content in the carbon nanotubes. The method is simple and easy to implement. Specifically, the metal content of the purified carbon nanotubes is no more than 100 ppm, and the amorphous carbon content is no more than 3 wt%.

[0059] In a second aspect of the present invention, the present invention proposes the application of the above purification method in the preparation of conductive paste.

[0060] According to an embodiment of the present invention, referring to Figure 2 , the application includes:

[0061] S10: Obtain carbon nanotubes using the above purification method

[0062] In this step, carbon nanotubes are obtained by the purification method of the first aspect. It should be noted that the obtained carbon nanotubes exist in two states: one is the carbon nanotube dry powder obtained by drying the washed carbon nanotubes, and the other is the carbon nanotube wet material obtained by washing and filtering. Regardless of which state of carbon nanotubes is used to prepare the conductive slurry, it does not affect the amorphous carbon content and metal impurity content in the carbon nanotubes. Taking into account the transportation and storage costs, if the carbon nanotube dry powder is used to prepare the conductive slurry, it is preferred that the washed carbon nanotubes be freeze-dried to obtain the carbon nanotube dry powder, thereby reducing the agglomeration of the carbon nanotubes during the drying process. Taking into account energy conservation and further reducing the dispersibility of the carbon nanotubes in the conductive slurry, it is preferred to use the carbon nanotube wet material obtained by washing and filtering in the first aspect. The inventors have found that the carbon nanotube wet material can significantly reduce the agglomeration of the carbon nanotubes without undergoing a drying process, thereby improving the dispersibility of the carbon nanotubes in the conductive slurry. In addition, directly using the wet material can also save energy consumption in the drying process and reduce production costs.

[0063] S20: Mixing the carbon nanotubes, the thickening dispersant, and the second solvent for a second dispersion treatment

[0064] In this step, the carbon nanotubes are mixed with a thickening dispersant and a second solvent for a second dispersion treatment. This second dispersion treatment forms a small-particle carbon nanotube conductive paste. The thickening dispersant increases the viscosity of the solution. The synergistic effect of the second dispersion treatment allows the carbon nanotubes to be evenly dispersed in the solution, thereby enhancing the conductive effect of the conductive paste. It should be noted that the second solvent is conventional in the art and can be selected by those skilled in the art based on practical needs. For example, the second solvent includes, but is not limited to, water, NMP, acetone, or ethanol.

[0065] According to an embodiment of the present invention, the second dispersion treatment includes a shearing treatment and a homogenizing treatment. The shearing treatment can shear and crush carbon nanotubes with larger particle sizes, thereby facilitating the homogenization treatment to obtain carbon nanotubes with smaller particle sizes. Through the shearing and homogenizing treatments, carbon nanotube agglomerates can be effectively crushed to obtain carbon nanotubes with small particle sizes, thereby improving the conductive effect of the conductive paste. Furthermore, the shearing treatment rate is 3000rpm to 16000rpm, and the shearing treatment time is 20min to 60min; the homogenizing treatment pressure is 5000psi to 40000psi, and the homogenizing cavity aperture of the homogenizer for the homogenizing treatment is 75μm to 300μm.

[0066] According to an embodiment of the present invention, the second dispersion treatment includes shearing and sanding. The shearing treatment can shear and crush carbon nanotubes with larger particle sizes, thereby facilitating the grinding of the sheared and crushed carbon nanotubes into smaller particle sizes, thereby improving the conductive effect of the conductive paste. Furthermore, the shearing treatment rate is 3000 rpm to 16000 rpm, and the shearing treatment time is 20 minutes to 60 minutes; the sanding treatment line speed is 1200 rpm to 1600 rpm, and the sanding treatment time is 1 hour to 4 hours.

[0067] According to an embodiment of the present invention, the thickening dispersant includes at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium hexametaphosphate and sodium alginate.

[0068] According to an embodiment of the present invention, the mass ratio of the thickening dispersant to the carbon nanotubes is (2.5-20):10. The inventors have found that controlling the mass ratio of the thickening dispersant to the carbon nanotubes within the above range can ensure an appropriate viscosity of the conductive paste, thereby improving the conductive performance of the conductive paste.

[0069] According to an embodiment of the present invention, the solid content of the conductive paste is 0.44% to 10%. The solid content of the conductive paste within the above range can ensure good dispersion of the carbon nanotubes in the conductive paste and improve the dispersibility of the carbon nanotubes.

[0070] Thus, a conductive paste with excellent carbon nanotube dispersion and low carbon nanotube impurity content can be obtained. Specifically, the metal content of the carbon nanotubes in the conductive paste is not more than 100 ppm, the amorphous carbon content in the carbon nanotubes is not more than 3 wt%, and the particle size D of the carbon nanotubes is less than 0. V50 20nm~9μm.

[0071] In a third aspect, the present invention provides a conductive paste. According to an embodiment of the present invention, the conductive paste comprises carbon nanotubes obtained using the aforementioned carbon nanotube purification method. As a result, the carbon nanotubes in the conductive paste have low metal and amorphous carbon contents, and the conductive paste exhibits excellent electrical conductivity.

[0072] The present invention is described below with reference to specific examples. It should be noted that these examples are merely illustrative and do not limit the present invention in any way.

[0073] Example 1

[0074] (1) Pre-dispersion: 1000 g of single-walled carbon nanotube powder was added to deionized water, and 10 g of sodium lignin sulfonate was added and stirred evenly. The mixture was sheared and dispersed in a high shear emulsifier at a speed of 10,000 rpm, and the stirring temperature was controlled below 55°C by circulating water. The shear treatment was performed for 1 hour.

[0075] (2) Secondary dispersion: The slurry obtained in step (1) was sand-milled at a linear speed of 1300 rpm, and the sand-milling temperature was controlled below 55°C by circulating water for 4 hours. The viscosity of the single-walled carbon nanotube slurry obtained after the secondary dispersion at 25°C was 110.1 mPa·s.

[0076] (3) Gas phase purification: The above-mentioned single-walled carbon nanotube slurry is freeze-dried to obtain a fluffy single-walled carbon nanotube body, which is filled in a fluidized bed reactor and introduced with air at a volume ratio of 1:1 to nitrogen. The reaction temperature is 350°C. After the air is continuously introduced for 2 hours, the air is turned off; the temperature is raised to 900°C, and H2 is introduced for 15 minutes, with a volume ratio of H2 to N2 of 1:5; H2 is turned off, and carbon dioxide is introduced for 3 hours, with a volume ratio of carbon dioxide to nitrogen of 1:1. The weak oxidizing gas is turned off, and the temperature is lowered to room temperature to obtain gas-phase purified single-walled carbon nanotube powder.

[0077] (4) Acid washing: Add 35 wt% concentrated sulfuric acid to the carbon nanotube powder obtained in step (3), maintain the temperature at 60°C, continue acid washing for 6 hours, wash with deionized water until neutral, and then filter to obtain a wet material.

[0078] (5) Tertiary dispersion: Calculate the solid content of the wet material obtained in step (4), calculate the amount of wet material to be added based on the mass proportion of carbon nanotubes in the conductive slurry being 0.4%, add the carbon nanotube wet material into deionized water, and then add sodium carboxymethyl cellulose and mix evenly, wherein the mass ratio of sodium carboxymethyl cellulose to carbon nanotubes is 1:1, and then place the above mixed slurry in a high shear emulsifier at a speed of 10,000 rpm for 20 minutes, and control the shear temperature below 55°C.

[0079] (6) Disperse the slurry obtained in step (5) four times, homogenize it in a high-pressure homogenizer, keep the discharge temperature below 40°C, and circulate the homogenization until the particle size of the carbon nanotubes in the slurry is D V50 Less than 40μm.

[0080] Example 2

[0081] (1) Pre-dispersion: 1000 g of single-walled carbon nanotube powder was added to deionized water, and 10 g of sodium lignin sulfonate was added and stirred evenly. The mixture was sheared and dispersed in a high shear emulsifier at a speed of 10,000 rpm, and the stirring temperature was controlled below 55°C by circulating water. The shear treatment was performed for 1 hour.

[0082] (2) Secondary dispersion: The slurry obtained in step (1) was sand-milled at a linear speed of 1300 rpm, and the sand-milling temperature was controlled below 55°C by circulating water for 4 hours. The viscosity of the single-walled carbon nanotube slurry obtained after the secondary dispersion was 109.8 mPa·s at 25°C.

[0083] (3) Gas phase purification: The above-mentioned single-walled carbon nanotube slurry is freeze-dried to obtain a fluffy single-walled carbon nanotube body, which is filled in a fluidized bed reactor and introduced with air at a volume ratio of 1:1 to nitrogen. The reaction temperature is 350°C, and the air is continuously introduced for 2 hours before the air is turned off. The temperature is raised to 900°C, and H2 is introduced for 15 minutes, with a volume ratio of H2 to N2 of 1:5. H2 is turned off, and carbon dioxide is introduced for 3 hours, with a volume ratio of carbon dioxide to nitrogen of 1:1. The weak oxidizing gas is turned off, and the temperature is lowered to room temperature to obtain gas-phase purified single-walled carbon nanotube powder;

[0084] (4) Acid washing: Add 35 wt% concentrated sulfuric acid to the carbon nanotube powder obtained in step (3), maintain the temperature at 60°C, continue acid washing for 4 hours, wash with deionized water until neutral, and then filter to obtain a wet material.

[0085] (5) Tertiary dispersion: Calculate the solid content of the wet material obtained in step (4), calculate the amount of wet material to be added based on the mass proportion of carbon nanotubes in the conductive slurry being 0.4%, add the carbon nanotube wet material into deionized water, and then add sodium carboxymethyl cellulose and mix evenly, wherein the mass ratio of sodium carboxymethyl cellulose to carbon nanotubes is 1:1, and then place the above mixed slurry in a high shear emulsifier at a speed of 10,000 rpm for 20 minutes, and control the shear temperature below 55°C.

[0086] (6) Disperse the slurry obtained in step (5) four times, homogenize it in a high-pressure homogenizer, keep the discharge temperature below 40°C, and circulate the homogenization until the particle size of the carbon nanotubes in the slurry is D V50 Less than 40μm.

[0087] Example 3

[0088] (1) Pre-dispersion: 1000 g of single-walled carbon nanotube powder was added to deionized water, and 10 g of sodium lignin sulfonate was added and stirred evenly. The mixture was sheared and dispersed in a high shear emulsifier at a speed of 10,000 rpm, and the stirring temperature was controlled below 55°C by circulating water. The shear treatment and dispersion were performed for 30 minutes.

[0089] (2) Secondary dispersion: The slurry obtained in step (1) was sand-milled at a linear speed of 1300 rpm, and the sand-milling temperature was controlled below 55°C by circulating water for 2 h. The viscosity of the single-walled carbon nanotube slurry obtained after the secondary dispersion at 25°C was 107.7 mPa·s.

[0090] (3) Gas phase purification: The above-mentioned single-walled carbon nanotube slurry is freeze-dried to obtain a fluffy single-walled carbon nanotube body, which is filled in a fluidized bed reactor and introduced with air at a volume ratio of 1:1 to nitrogen. The reaction temperature is 350°C, and the air is continuously introduced for 2 hours before the air is turned off. The temperature is raised to 900°C, and H2 is introduced for 15 minutes, with a volume ratio of H2 to N2 of 1:5. H2 is turned off, and carbon dioxide is introduced for 3 hours, with a volume ratio of carbon dioxide to nitrogen of 1:1. The weak oxidizing gas is turned off, and the temperature is lowered to room temperature to obtain gas-phase purified single-walled carbon nanotube powder.

[0091] (4) Acid washing: Add 35 wt% concentrated sulfuric acid to the carbon nanotube powder obtained in step (3), maintain the temperature at 60°C, continue acid washing for 6 hours, wash with deionized water until neutral, and then filter to obtain a wet material.

[0092] (5) Tertiary dispersion: Calculate the solid content of the wet material obtained in step (4), calculate the amount of wet material to be added based on the mass proportion of carbon nanotubes in the conductive slurry being 0.4%, add the carbon nanotube wet material into deionized water, and then add sodium carboxymethyl cellulose and mix evenly, wherein the mass ratio of sodium carboxymethyl cellulose to carbon nanotubes is 1:1, and then place the above mixed slurry in a high shear emulsifier at a speed of 10,000 rpm for 20 minutes, and control the shear temperature below 55°C.

[0093] (6) Disperse the slurry obtained in step (5) four times, homogenize it in a high-pressure homogenizer, keep the discharge temperature below 40°C, and circulate the homogenization until the particle size of the carbon nanotubes in the slurry is D V50 Less than 40μm.

[0094] Example 4

[0095] (1) Pre-dispersion: 1000 g of single-walled carbon nanotube powder was added to deionized water, and 20 g of sodium lignin sulfonate was added and stirred evenly. The mixture was sheared and dispersed in a high shear emulsifier at a speed of 10,000 rpm, and the stirring temperature was controlled below 55°C by circulating water. The shear treatment was performed for 1 hour.

[0096] (2) Secondary dispersion: The slurry obtained in step (1) was sand-milled at a linear speed of 1300 rpm, and the sand-milling temperature was controlled below 55°C by circulating water for 4 h. The viscosity of the single-walled carbon nanotube slurry obtained after the secondary dispersion at 25°C was 130.6 mPa·s.

[0097] (3) Gas Phase Purification: The above-mentioned single-walled carbon nanotube slurry was freeze-dried to obtain a fluffy single-walled carbon nanotube body, which was filled in a fluidized bed reactor and introduced with air at a volume ratio of 1:1 to nitrogen. The reaction temperature was 350°C. After the air was introduced for 2 hours, the air was turned off. The temperature was raised to 900°C, and H2 was introduced for 15 minutes at a volume ratio of H2 to N2 of 1:5. The H2 was turned off, and carbon dioxide was introduced for 3 hours at a volume ratio of carbon dioxide to nitrogen of 1:1. The weak oxidizing gas was turned off, and the temperature was lowered to room temperature to obtain gas-phase purified single-walled carbon nanotube powder.

[0098] (4) Acid washing: Add 35 wt% concentrated sulfuric acid to the carbon nanotube powder obtained in step (3), maintain the temperature at 60°C, continue acid washing for 6 hours, wash with deionized water until neutral, and then filter to obtain a wet material.

[0099] (5) Tertiary dispersion: Calculate the solid content of the wet material obtained in step (4), calculate the amount of wet material to be added based on the mass proportion of carbon nanotubes in the conductive slurry being 0.4%, add the carbon nanotube wet material into deionized water, and then add sodium carboxymethyl cellulose and mix evenly, wherein the mass ratio of sodium carboxymethyl cellulose to carbon nanotubes is 1:1, and then place the above mixed slurry in a high shear emulsifier at a speed of 10,000 rpm for 20 minutes, and control the shear temperature below 55°C.

[0100] (6) Disperse the slurry obtained in step (5) four times, homogenize it in a high-pressure homogenizer, keep the discharge temperature below 40°C, and circulate the homogenization until the particle size of the carbon nanotubes in the slurry is D V50 Less than 40μm.

[0101] Example 5

[0102] (1) Pre-dispersion: 1000 g of single-walled carbon nanotube powder was added to deionized water, and 10 g of sodium lignin sulfonate was added and stirred evenly. The mixture was sheared and dispersed in a high shear emulsifier at a speed of 10,000 rpm, and the stirring temperature was controlled below 55 ° C by circulating water. The shear treatment was dispersed for 1 hour.

[0103] (2) Secondary dispersion: The slurry obtained in step (1) was sand-milled at a linear speed of 1300 rpm, and the sand-milling temperature was controlled below 55°C by circulating water for 4 hours. The viscosity of the single-walled carbon nanotube slurry obtained after the secondary dispersion at 25°C was 111.2 mPa·s.

[0104] (3) Gas-phase purification: The above-mentioned single-walled tube slurry is freeze-dried to obtain a fluffy single-walled carbon nanotube body, which is filled in a fluidized bed reactor and introduced with air at a volume ratio of 1:1 to nitrogen. The reaction temperature is 350°C, and the air is continuously introduced for 1 hour, after which the air is turned off. The temperature is raised to 900°C, and H2 is introduced for 15 minutes, with a volume ratio of H2 to N2 of 1:5. The H2 is turned off, and carbon dioxide is introduced for 2 hours, with a volume ratio of carbon dioxide to nitrogen of 1:1. The weak oxidizing gas is turned off, and the temperature is lowered to room temperature to obtain gas-phase purified single-walled carbon nanotube powder.

[0105] (4) Acid washing: Add 35 wt% concentrated sulfuric acid to the carbon nanotube powder obtained in step (3), maintain the temperature at 60°C, continue acid washing for 6 hours, wash with deionized water until neutral, and then filter to obtain a wet material.

[0106] (5) Tertiary dispersion: Calculate the solid content of the wet material obtained in step (4), calculate the amount of wet material to be added based on the mass proportion of carbon nanotubes in the conductive slurry being 0.4%, add the carbon nanotube wet material into deionized water, and then add sodium carboxymethyl cellulose and mix evenly, wherein the mass ratio of sodium carboxymethyl cellulose to carbon nanotubes is 1:1, and then place the above mixed slurry in a high shear emulsifier at a speed of 10,000 rpm for 20 minutes, and control the shear temperature below 55°C.

[0107] (6) Disperse the slurry obtained in step (3) four times and homogenize it in a high-pressure homogenizer, keep the discharge temperature below 40°C, and circulate the homogenization until the particle size of the carbon nanotubes in the slurry is D V50 Less than 40μm.

[0108] Example 6

[0109] The main differences between Example 6 and Example 1 are:

[0110] (1) Pre-dispersion: 1000 g of single-walled carbon nanotube powder was added to deionized water, and 4 g of sodium lignin sulfonate was added and stirred evenly. The mixture was sheared and dispersed in a high shear emulsifier at a speed of 10,000 rpm, and the stirring temperature was controlled below 55°C by circulating water. The shear treatment was performed for 1 hour.

[0111] (2) Secondary dispersion: The slurry obtained in step (1) was sand-milled at a linear speed of 1300 rpm, and the sand-milling temperature was controlled below 55°C by circulating water for 4 hours. The viscosity of the single-walled carbon nanotube slurry obtained after the secondary dispersion at 25°C was 101.1 mPa·s.

[0112] Example 7

[0113] The main differences between Example 7 and Example 1 are:

[0114] (1) Pre-dispersion: 1000 g of single-walled carbon nanotube powder was added to deionized water, and 2 g of sodium lignin sulfonate was added and stirred evenly. The mixture was sheared and dispersed in a high shear emulsifier at a speed of 10,000 rpm, and the stirring temperature was controlled below 55°C by circulating water. The shear treatment was performed for 1 hour.

[0115] (2) Secondary dispersion: The slurry obtained in step (1) was sand-milled at a linear speed of 1300 rpm, and the sand-milling temperature was controlled below 55°C by circulating water for 4 hours. The viscosity of the single-walled carbon nanotube slurry obtained after the secondary dispersion at 25°C was 96.8 mPa·s.

[0116] Example 8

[0117] The main differences between Example 8 and Example 1 are:

[0118] (1) Pre-dispersion: 1000 g of single-walled carbon nanotube powder was added to deionized water, and 30 g of sodium lignin sulfonate was added and stirred evenly. The mixture was sheared and dispersed in a high shear emulsifier at a speed of 10,000 rpm, and the stirring temperature was controlled below 55°C by circulating water. The shear treatment was performed for 1 hour.

[0119] (2) Secondary dispersion: The slurry obtained in step (1) was sand-milled at a linear speed of 1300 rpm, and the sand-milling temperature was controlled below 55°C by circulating water for 4 hours. The viscosity of the single-walled carbon nanotube slurry obtained after the secondary dispersion at 25°C was 139.6 mPa·s.

[0120] Example 9

[0121] The main differences between Example 9 and Example 1 are:

[0122] (1) Pre-dispersion: 1000 g of single-walled carbon nanotube powder was added to deionized water, and 10 g of sodium dodecylbenzene sulfonate was added and stirred evenly. The mixture was sheared and dispersed in a high shear emulsifier at a speed of 10,000 rpm, and the stirring temperature was controlled below 55°C by circulating water. The shear treatment was carried out for 1 hour.

[0123] (2) Secondary dispersion: The slurry obtained in step (1) was sand-milled at a linear speed of 1300 rpm, and the sand-milling temperature was controlled below 55°C by circulating water for 4 hours. The viscosity of the single-walled carbon nanotube slurry obtained after the secondary dispersion at 25°C was 107.4 mPa·s.

[0124] (5) Tertiary dispersion: Calculate the solid content of the wet material obtained in step (4), calculate the amount of wet material to be added based on the mass proportion of carbon nanotubes in the conductive slurry being 0.4%, add the carbon nanotube wet material into deionized water, and then add sodium alginate and mix evenly, wherein the mass ratio of sodium alginate to carbon nanotubes is 1:1, and then place the above mixed slurry in a high shear emulsifier at a speed of 10,000 rpm for 20 minutes, and control the shear temperature below 55°C.

[0125] Example 10

[0126] (1) Pre-dispersion: 1000 g of single-walled carbon nanotube powder was added to deionized water, and 10 g of sodium lignin sulfonate was added and stirred evenly; shear dispersion was performed in a high shear emulsifier at a speed of 10,000 rpm, and the stirring temperature was controlled below 55°C by circulating water, and the shear treatment was dispersed for 1 hour.

[0127] (2) Secondary dispersion: The slurry obtained in step (1) was sand-milled at a linear speed of 1300 rpm, and the sand-milling temperature was controlled below 55°C by circulating water for 4 hours. The viscosity of the single-walled carbon nanotube slurry obtained after the secondary dispersion at 25°C was 106.7 mPa·s.

[0128] (3) Gas phase purification: The above-mentioned single-walled carbon nanotube slurry is freeze-dried to obtain a fluffy single-walled carbon nanotube body, which is filled in a fluidized bed reactor and introduced with air at a volume ratio of 1:1 to nitrogen. The reaction temperature is 350°C, and the air is continuously introduced for 2 hours and then the air is turned off; the temperature is raised to 900°C, and H2 is introduced for 15 minutes, and the volume ratio of H2 to N2 is 1:5; H2 is turned off, and carbon dioxide is introduced for 3 hours, and the volume ratio of carbon dioxide to nitrogen is 1:1; the weak oxidizing gas is turned off, and the temperature is lowered to room temperature to obtain gas-phase purified single-walled carbon nanotube powder.

[0129] (4) Acid washing: Add 35 wt % concentrated sulfuric acid to the carbon nanotube powder obtained in step (3), maintain the temperature at 60° C., continue acid washing for 6 h, wash with deionized water until neutral, and then dry in an oven.

[0130] (5) Tertiary dispersion: 0.8 g of the carbon nanotube powder obtained after drying in step (4) was selected and added to 198.4 ml of deionized water, and 0.8 g of sodium carboxymethyl cellulose was added and mixed evenly; 0.8 g of sodium carboxymethyl cellulose was added and mixed evenly, and the mixture was subjected to a high shear emulsifier at a speed of 10,000 rpm for 20 min, and the shear temperature was controlled below 55°C.

[0131] (6) Disperse the slurry obtained in step (5) four times and homogenize it in a high-pressure homogenizer, keeping the discharge temperature below 40°C, and circulate the homogenization until the slurry particle size D V50 Less than 40μm.

[0132] Comparative Example 1

[0133] (1) Gas phase purification: 1000 g of single-walled carbon nanotube powder was filled into a fluidized bed reactor, and air was introduced at a volume ratio of 1:1 to nitrogen. The reaction temperature was 350°C, and the air was continuously introduced for 2 hours, and then the air was turned off. The temperature was raised to 900°C, and H2 was introduced for 15 minutes, with a volume ratio of H2 to N2 of 1:5. H2 was turned off, and carbon dioxide was introduced for 3 hours, with a volume ratio of carbon dioxide to nitrogen of 1:1. The weak oxidizing gas was turned off, and the temperature was lowered to room temperature to obtain gas-phase purified single-walled carbon nanotube powder.

[0134] (2) Acid washing: Take the carbon nanotube powder obtained in step (1), add 35 wt% concentrated sulfuric acid, maintain the temperature at 60°C, continue acid washing for 6 hours, wash with deionized water until neutral, filter and dry in an oven to obtain carbon nanotube powder.

[0135] (3) Initial dispersion: 0.8 g of the carbon nanotube powder obtained after drying in step (2) was selected and added to 198.4 ml of deionized water. 0.8 g of sodium carboxymethyl cellulose was added and mixed evenly. The mixture was stirred in a high shear emulsifier at a speed of 1000 rpm for 1 h, and the shear temperature was controlled below 55°C.

[0136] (4) Secondary dispersion: homogenize the slurry obtained in step (3) in a high-pressure homogenizer, maintain the discharge temperature below 40°C, and circulate the homogenization until the slurry particle size D V50 Less than 40μm.

[0137] Comparative Example 2

[0138] The main difference between Comparative Example 2 and Example 1 is:

[0139] (1) Pre-dispersion: Add the raw single-walled carbon nanotube powder into deionized water, add 10g of sodium carboxymethyl cellulose and stir evenly, and shear disperse it in a high shear emulsifier at a speed of 10000rpm. The stirring temperature is controlled below 55℃ by circulating water, and the shear treatment is dispersed for 1h.

[0140] (2) Secondary dispersion: The slurry obtained in step (1) was sand-milled at a linear speed of 1300 rpm, and the sand-milling temperature was controlled below 55°C by circulating water for 4 hours. The viscosity of the single-walled carbon nanotube slurry obtained after the secondary dispersion at 25°C was 560.2 mPa·s.

[0141] Comparative Example 3

[0142] The main difference between Comparative Example 3 and Example 1 is:

[0143] (3) Gas phase purification: The above-mentioned single-walled carbon nanotube slurry is oven-dried to obtain single-walled carbon nanotube powder, which is then filled into a fluidized bed reactor for purification.

[0144] The metal content and amorphous carbon content of the purified carbon nanotubes of Examples 1-10 and Comparative Examples 1-3 were measured, and the conductive pastes prepared in Examples 1-10 and Comparative Examples 1-3 were characterized by SEM, and the particle size of the carbon nanotubes in the conductive pastes and the viscosity of the conductive pastes were measured. The specific methods are as follows:

[0145] (1) Determination of metal content: ICP was used to detect the metal (Fe) content of the purified carbon nanotubes.

[0146] (2) Determination of amorphous carbon content: The amorphous carbon content in the purified carbon nanotubes was analyzed using TGA. The temperature was raised to 150°C and kept constant for 30 min, then raised to 800°C at a rate of 10°C / min. The peak of amorphous carbon weight loss was from 200°C to 450°C, and the peak of carbon nanotube weight loss was from 550°C to 700°C.

[0147] (3) SEM: Take a drop of slurry and dilute it 100 times, then ultrasonicate it for 10 minutes. Then drop the ultrasonicated sample on the surface of the silicon wafer and observe it.

[0148] (4) Particle size analysis: The slurry was added dropwise into a laser particle size analyzer beaker filled with deionized water until the transmittance reached 5% to 10%.

[0149] (5) Viscosity measurement: Weigh two milliliters of slurry and place it on the sample table of the high and low temperature viscometer. Set the temperature to 25°C, adjust the gap, and measure the viscosity at 18.6 seconds. -1 The viscosity is measured at a shear rate of 1.

[0150] The specific measurement results are shown in Table 1.

[0151] Table 1

[0152]

[0153]

[0154] From the data in Table 1, it can be seen that the metal content, amorphous carbon content and particle size of the carbon nanotubes in the conductive paste of Examples 1-10 are significantly smaller than those in Comparative Examples 1-3, indicating that the purification method of the present application can effectively remove metal impurities and amorphous carbon in the carbon nanotubes. At the same time, the particle size of the carbon nanotubes in the conductive paste prepared by the present application is relatively small, indicating that the carbon nanotubes in the conductive paste of the present application have excellent dispersibility. For details, please refer to Figure 3-Figure 8 ,from Figure 3It can be seen that almost all carbon nanotubes can be monodispersed, with no obvious entanglement and particle agglomerates. Each tube bundle can be clearly seen, and metal and amorphous carbon impurity particles are almost invisible. Figure 4 It can be seen that almost all carbon nanotubes can be monodispersed without large-area entanglement and large particle agglomerates, but there are still a small amount of small particle impurities distributed between carbon nanotubes. Figure 5 It can be seen that almost all carbon nanotubes can be monodispersed without large-area entanglement and large particle agglomerates, but there are still a small amount of impurities distributed between carbon nanotubes, and the particle size of the impurity particles is larger than Figure 4 Impurity particle size. Figure 6 It can be seen that although the impurity content is small, the carbon nanotubes shrink and agglomerate after drying, and local entanglement and agglomeration occur between the tubes, resulting in uneven distribution, which makes the particle size larger and the dispersion average. Figure 7 It can be seen that the traditional method uses the raw powder to directly undergo gas phase purification. Most of the amorphous carbon is not removed completely, and there are still large particles of impurities, and the metal particles covered by the carbon still exist over a large area. At the same time, most of the carbon nanotubes are extremely entangled. Figure 8 It can be seen that due to the high content of thickening dispersant and the high viscosity, the dispersion efficiency of the slurry is reduced. Most of the carbon nanotubes coated with the dispersant exist in the form of tube bundles and are severely entangled, with almost no monodispersity. Most of the metals and amorphous carbon are severely coated by the dispersant sodium carboxymethyl cellulose, and the purification efficiency is reduced.

[0155] Compared to Example 10, Comparative Example 1 did not employ the preparation process of step (1) of the present application. The carbon nanotubes in Comparative Example 1 had a high metal content and a high amorphous carbon content, indicating that the traditional combined gas phase purification and acid washing method was not effective in removing metal impurities from the carbon nanotubes. Furthermore, the carbon nanotubes in Comparative Example 2 had a larger particle size, indicating that the absence of the preparation process of step (1) would affect the dispersibility of the carbon nanotubes in the conductive paste prepared later.

[0156] Compared with Example 1, Comparative Example 2 uses a thickening dispersant (sodium carboxymethyl cellulose) instead of a non-thickening dispersant in the purification step (1). The metal content of Comparative Example 2 is very high, and the viscosity of the conductive paste is also very high, indicating that the addition of the thickening dispersant causes the viscosity of the solution to be too high, which will affect the pickling effect, and the conductive paste finally prepared will also have a decreased electrical performance due to the excessive viscosity.

[0157] Compared with Example 1, Comparative Example 3 did not use freeze-drying before gas phase purification, but used thermal drying. The metal content of Comparative Example 3 increased significantly, indicating that using thermal drying before gas phase purification would aggravate the agglomeration of carbon nanotubes, thereby affecting the gas phase purification effect and the acid washing effect.

[0158] In the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0159] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A method for purifying carbon nanotubes, characterized in that: include: (1) blending carbon nanotube raw powder, a non-thickening ionic dispersant, and a first solvent to perform a first dispersion treatment to obtain a carbon nanotube slurry; (2) freeze-drying the carbon nanotube slurry and then performing gas phase purification to obtain carbon nanotube powder; (3) acid-washing and water-washing the carbon nanotube powder to obtain carbon nanotubes; In step (1), the mass ratio of the non-thickening ionic dispersant to the carbon nanotube raw powder in the carbon nanotube slurry is (2.5-20):1000.

2. The method according to claim 1, characterized in that In step (1), at 25° C., the viscosity of the carbon nanotube slurry is not greater than 200 mPa·s.

3. The method according to claim 1 or 2, characterized in that The non-thickening ionic dispersant includes at least one of sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, sodium polystyrene sulfonate, lithium polystyrene sulfonate, sodium lignin sulfonate, alkali lignin and sodium dodecyl sulfate.

4. The method according to claim 1 or 2, characterized in that In step (1), the first dispersion treatment includes a sand milling treatment or includes a shearing treatment and a sand milling treatment.

5. The method according to claim 4, characterized in that The linear speed of the sanding treatment is 1200 rpm to 1600 rpm, and the time of the sanding treatment is 60 min to 360 min; The shearing treatment rate is 4000 rpm to 16000 rpm, and the shearing treatment time is 15 min to 360 min.

6. The method according to claim 1 or 2, wherein In step (2), the gas phase purification includes: introducing air and an inert carrier gas, reacting at 350°C to 450°C for 30 minutes to 120 minutes; turning off the air, continuously introducing an inert carrier gas and H2, raising the temperature to 800°C to 950°C, and continuing for 10 minutes to 30 minutes; turning off H2, and continuously introducing an inert carrier gas and a weak oxidizing gas for 30 minutes to 180 minutes.

7. The method according to claim 1 or 2, characterized in that In step (3), the concentration of the pickling acid solution is 25wt%~50wt%; The pickling temperature is 30°C to 80°C; The pickling time is 2 to 8 hours.

8. Use of the purification method according to any one of claims 1 to 7 in the preparation of conductive paste.

9. The use according to claim 8, characterized in that include: Obtaining carbon nanotubes using the purification method according to any one of claims 1 to 7; The carbon nanotubes, a thickening dispersant and a second solvent are mixed to perform a second dispersion treatment, so as to obtain a conductive paste.

10. The use according to claim 9, characterized in that The second dispersing treatment includes a shearing treatment and a homogenizing treatment or includes a shearing treatment and a sand milling treatment.

11. The use according to claim 10, characterized in that The shearing treatment rate is 3000 rpm to 16000 rpm, and the shearing treatment time is 20 min to 60 min; The pressure of the homogenization process is 5000psi~40000psi, and the pore size of the homogenization cavity of the homogenizer is 75μm~300μm; The linear speed of the sand grinding process is 1200 rpm to 1600 rpm, and the time of the sand grinding process is 1 hour to 4 hours.

12. The use according to claim 9, characterized in that The thickening dispersant includes at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium hexametaphosphate and sodium alginate.

13. The use according to claim 9, characterized in that The mass ratio of the thickening dispersant to the carbon nanotubes is (2.5-20):

10.

14. A conductive paste, characterized in that: The carbon nanotubes are obtained by the method according to any one of claims 1 to 7.

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