Purification method of carbon nanotubes

Abstract

The invention provides a carbon nanotube purification method, which comprises: carrying out densification treatment on a carbon nanotube, and purifying the densified carbon nanotube at a temperature of 800-1400 DEG C by using ammonium chloride to obtain a purified carbon nanotube. According to the purification method provided by the invention, purification is carried out under the sub-high temperature condition of 800-1400 DEG C, the purification purity of the carbon nanotubes is high, and the content of metal impurities is low; pre-oxidation heat treatment is not needed, and the phenomenon that the structure of the carbon nano tube is damaged by high-temperature heat treatment at the temperature of 1700 DEG C or above is avoided; meanwhile, the purification efficiency is improved, and industrial amplification of the whole purification method is facilitated.

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon nanotubes, and particularly relates to a purification method for carbon nanotubes. Background Art

[0002] Carbon nanotubes are one-dimensional quantum materials with a special structure, mainly consisting of a coaxial hollow seamless tubular structure formed by curling single or multiple layers of graphite sheets around a center at a certain angle. Most of their tube walls are composed of hexagonal carbon atom grids. Carbon nanotubes have excellent properties in terms of mechanics, electricity, electrical conductivity, and heat conduction. For example, they have electrical conductivity and thermal conductivity far exceeding those of other metal materials, as well as extremely high mechanical strength. In addition, carbon nanotubes also have properties such as high elasticity, large specific surface area, good stability, and fatigue resistance. Carbon nanotubes have broad potential application prospects in many fields such as lithium-ion batteries, coatings, catalyst carriers, rubber and plastic composites, electrochemical materials, and optoelectronic sensing.

[0003] The main preparation methods of carbon nanotubes include CVD method, arc method, laser ablation method, etc. However, the catalysts and carriers introduced during the preparation process cause a large amount of impurities such as metals (Fe, Co, Ni, etc.) and amorphous carbon to remain in the carbon nanotubes. If such carbon nanotubes are directly used in lithium-ion batteries, it will cause risks such as spontaneous combustion of the battery. Therefore, it is necessary to purify the carbon nanotubes.

[0004] Currently, the main purification methods of carbon nanotubes are pre-oxidation pickling method and high-temperature purification method. The pre-oxidation pickling method is to pre-oxidize carbon nanotubes in air at a certain temperature, and then pickle, wash, and dry the carbon nanotubes with strong acids such as hydrochloric acid, sulfuric acid, and hydrofluoric acid. A large amount of acidic wastewater will be generated during the treatment process, greatly increasing the sewage treatment cost, and it is difficult for the metal content of the purified carbon nanotubes to reach below 200 ppm. The high-temperature purification method is to perform heat treatment on carbon nanotubes at above 1700 °C. Although it can make the metal content in the carbon nanotubes lower than 200 ppm or even lower than 50 ppm, the energy consumption is very high, and thus the cost is very high. More importantly, it destroys the structure of the carbon nanotubes, making the electrical conductivity of the carbon nanotubes worse.

[0005] CN115536005A discloses a method for purifying carbon nanotubes. Using ammonium chloride powder as a reaction reagent, it is mixed with ammonium chloride solid powder in a first container and reacted at 350 - 400 °C to remove metal impurities. Then it reacts with oxygen and carbon dioxide in a second container to remove impurity carbon. This method does not use strong acid pickling and does not damage the structure of carbon nanotubes. Yun Chen et al. published an article titled "Purification of double - walled carbon nanotube macro - films" in New Journal of Chemistry, 2012, 36(3): 542 - 545, which discloses a method for purifying carbon nanotubes. The carbon nanotube film is placed on a quartz sheet, put into a quartz tube, heated in air at 400 °C for 60 min, then an excessive amount of ammonium chloride powder is placed above and below the film, the inside of the quartz tube is evacuated, and heat - treated at 700 °C for 60 min. This method purifies carbon nanotubes through an air oxidation and ammonium chloride purification process, and the purity of the purified carbon nanotubes reaches 95.3%. In the above - mentioned methods, it is necessary to use oxygen or air for oxidative heat - treatment of carbon nanotubes to remove impurity carbon therein, which increases the purification steps, and if oxygen is used additionally, the cost is relatively high.

[0006] Therefore, it is necessary to provide a method for purifying carbon nanotubes with a simple process, without oxidative heat - treatment, and with good purification effect. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for purifying carbon nanotubes, which does not require high - temperature treatment above 1700 °C or oxidative heat - treatment, simplifies the purification process, and improves the purification effect of carbon nanotubes.

[0008] To achieve the purpose of this invention, the following technical solutions are adopted:

[0009] The present invention provides a method for purifying carbon nanotubes, and the purification method includes the following steps:

[0010] The carbon nanotubes are densified, and then ammonium chloride is used to purify the densified carbon nanotubes at 800 - 1400 °C to obtain purified carbon nanotubes.

[0011] In the purification method provided by the present invention, the carbon nanotubes are first densified to improve the purification efficiency, which is conducive to the industrial scale - up of the overall purification method; ammonium chloride is used to purify the carbon nanotubes under the sub - high - temperature condition of 800 - 1400 °C. At this temperature, the amorphous carbon around iron in the carbon nanotubes can dissolve in iron without pre - oxidative heat - treatment. Iron reacts with ammonium chloride to form chloride and sublimes to be removed. The purified carbon nanotubes have high purity and low metal impurity content. At the same time, it avoids the structure of carbon nanotubes being damaged by high - temperature heat - treatment above 1700 °C and affecting their performance.

[0012] Among them, the purification temperature is 800 - 1400 °C. For example, it can be 800 °C, 850 °C, 900 °C, 950 °C, 1000 °C, 1050 °C, 1100 °C, 1150 °C, 1200 °C, 1250 °C, 1300 °C, 1350 °C or 1400 °C, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0013] The carbon nanotubes include single-walled carbon nanotubes and / or multi-walled carbon nanotubes.

[0014] The bulk density of the carbon nanotubes is 0.003 - 0.1 g / cm 3 , for example, it can be 0.003 g / cm 3 , 0.01 g / cm 3 , 0.02 g / cm 3 , 0.03 g / cm 3 , 0.04 g / cm 3 , 0.05 g / cm 3 , 0.06 g / cm 3 , 0.07 g / cm 3 , 0.08 g / cm 3 , 0.09 g / cm 3 or 0.1 g / cm 3 , but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0015] Preferably, the densification treatment includes: mixing the carbon nanotubes with water, then separating and drying to obtain densified carbon nanotubes.

[0016] Preferably, the mass ratio of the carbon nanotubes to water is 1:(3 - 60). For example, it can be 1:3, 1:5, 1:10, 1:15, 1:20, 1:25, 1:30, 1:35, 1:40, 1:45, 1:50, 1:55 or 1:60, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0017] Preferably, the mass ratio of the densified carbon nanotubes to ammonium chloride is 1:(1.5 - 5). For example, it can be 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5 or 1:5, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0018] Preferably, the purification process includes: introducing an inert gas into a container, heating ammonium chloride in the first temperature zone of the container to 350 - 400 °C, heating the densified carbon nanotubes in the second temperature zone of the container to 800 - 1400 °C, and the inert gas introducing the gas generated by ammonium chloride into the second temperature zone to purify the densified carbon nanotubes.

[0019] Among them, the heating temperature of ammonium chloride is 350 - 400 °C. For example, it can be 350 °C, 360 °C, 370 °C, 380 °C, 390 °C or 400 °C, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0020] Preferably, the inert gas includes nitrogen and / or argon.

[0021] Preferably, the flow rate of the inert gas is 0.1 - 1 L / min. For example, it can be 0.1 L / min, 0.2 L / min, 0.3 L / min, 0.4 L / min, 0.5 L / min, 0.6 L / min, 0.7 L / min, 0.8 L / min, 0.9 L / min or 1 L / min, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0022] Preferably, the purification time is 0.5 - 2 h. For example, it can be 0.5 h, 0.6 h, 0.8 h, 1 h, 1.2 h, 1.4 h, 1.5 h, 1.6 h, 1.8 h or 2 h, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0023] Preferably, the purification method further includes washing with water and drying after purification.

[0024] As a preferred technical solution of the purification method provided by the present invention, the purification method includes the following steps:

[0025] (1) Soak the carbon nanotubes in water, where the mass ratio of carbon nanotubes to water is 1:(3 - 60), then perform suction filtration and drying to obtain densified carbon nanotubes;

[0026] (2) Introduce an inert gas into the reaction container, the flow rate of the inert gas is 0.1 - 1 L / min, heat ammonium chloride in the first temperature zone to 350 - 400 °C, heat the densified carbon nanotubes in the second temperature zone to 800 - 1400 °C, so that the first temperature zone and the second temperature zone reach the predetermined temperature simultaneously, and the inert gas introduces the gas generated by ammonium chloride into the second temperature zone to purify the densified carbon nanotubes, and the purification time is 0.5 - 2 h;

[0027] (3) Wash and dry the treated carbon nanotubes to obtain purified carbon nanotubes.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The purification method provided by the present invention is carried out under the sub-high temperature condition of 800 - 1400 °C, with high purification purity of carbon nanotubes, small content of metal impurities, and small loss rate; there is no need for pre-oxidation heat treatment, and the high-temperature heat treatment above 1700 °C is avoided from damaging the structure of carbon nanotubes; meanwhile, the purification efficiency is improved, which is conducive to the industrial scale-up of the overall purification method. Brief Description of the Drawings

[0030] Figure 1 It is the process flow chart of the purification process provided in Example 1.

[0031] Figure 2 It is the SEM image of the purified carbon nanotubes provided in Example 3.

[0032] Figure 3 It is the SEM image of the purified carbon nanotubes provided in Example 13.

[0033] Figure 4 It is the bar chart of the purity of the purified carbon nanotubes provided in the examples and comparative examples.

[0034] Figure 5 It is the bar chart of the metal impurity content of the purified carbon nanotubes provided in the examples and comparative examples. Detailed Embodiments

[0035] The technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the described embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0036] Example 1

[0037] This example provides a purification method for carbon nanotubes, and the purification method includes the following steps:

[0038] (1) Soak multi-walled carbon nanotubes in water. Among them, the bulk density of the multi-walled carbon nanotubes is 0.021 g / cm 3 , the mass ratio of the multi-walled carbon nanotubes to water is 1:50, then filter by suction and dry to obtain densified multi-walled carbon nanotubes, and the bulk density of the densified multi-walled carbon nanotubes is 0.082 g / cm 3 ;

[0039] (2) Take 10 g of densified multi-walled carbon nanotubes and put them into the front temperature zone of the tube furnace, and put 25 g of ammonium chloride powder into the rear temperature zone of the tube furnace, as Figure 1As shown, the front temperature zone is set at 800 °C, the rear temperature zone is set at 400 °C, nitrogen gas is introduced into the tubular furnace at a flow rate of 0.5 L / min. First, start the heating program for the front temperature zone, and then start the heating program for the rear temperature zone later, so that both temperature zones reach their respective set temperatures simultaneously. The inert gas brings the gas generated by ammonium chloride into the temperature zone where the carbon nanotubes are located to react with the impurities therein, and the reaction time is 0.5 h;

[0040] (3) Wash the multi-walled carbon nanotubes treated in step (2) twice with 500 g of water each time, then perform suction filtration and drying to obtain purified carbon nanotubes.

[0041] Example 2

[0042] This example provides a method for purifying carbon nanotubes, and the purification method includes the following steps:

[0043] (1) Soak the multi-walled carbon nanotubes in water. Among them, the bulk density of the multi-walled carbon nanotubes is 0.021 g / cm 3 , the mass ratio of the multi-walled carbon nanotubes to water is 1:50, then perform suction filtration and drying to obtain densified multi-walled carbon nanotubes, and the bulk density of the densified multi-walled carbon nanotubes is 0.082 g / cm 3 ;

[0044] (2) Take 10 g of densified multi-walled carbon nanotubes and put them into the front temperature zone of the tubular furnace, and put 20 g of ammonium chloride powder into the rear temperature zone of the tubular furnace. The front temperature zone is set at 1050 °C, the rear temperature zone is set at 350 °C, introduce nitrogen gas into the tubular furnace at a flow rate of 0.1 L / min. First, start the heating program for the front temperature zone, and then start the heating program for the rear temperature zone later, so that both temperature zones reach their respective set temperatures simultaneously. The inert gas brings the gas generated by ammonium chloride into the temperature zone where the carbon nanotubes are located to react with the impurities therein, and the reaction time is 1.5 h;

[0045] (3) Wash the multi-walled carbon nanotubes treated in step (2) twice with 500 g of water each time, then perform suction filtration and drying to obtain purified carbon nanotubes.

[0046] Example 3

[0047] This example provides a method for purifying carbon nanotubes, and the purification method includes the following steps:

[0048] (1) Soak the multi-walled carbon nanotubes in water. Among them, the bulk density of the multi-walled carbon nanotubes is 0.021 g / cm 3 , the mass ratio of the multi-walled carbon nanotubes to water is 1:50, then perform suction filtration and drying to obtain densified multi-walled carbon nanotubes, and the bulk density of the densified multi-walled carbon nanotubes is 0.082 g / cm 3 ;

[0049] (2) Put 10 g of densified multi-walled carbon nanotubes into the front temperature zone of the tube furnace, and 15 g of ammonium chloride powder into the rear temperature zone of the tube furnace. Set the front temperature zone to 1050 °C and the rear temperature zone to 400 °C. Introduce nitrogen into the tube furnace at a flow rate of 0.5 L / min. First, start the heating program for the front temperature zone, and then start the heating program for the rear temperature zone later, so that both temperature zones reach their respective set temperatures simultaneously. The inert gas brings the gas generated by ammonium chloride into the temperature zone where the carbon nanotubes are located to react with the impurities therein, and the reaction time is 2 h;

[0050] (3) Wash the multi-walled carbon nanotubes treated in step (2) twice with 500 g of water each time, and then perform suction filtration and drying to obtain purified carbon nanotubes.

[0051] Example 4

[0052] This example provides a method for purifying carbon nanotubes, and the purification method includes the following steps:

[0053] (1) Soak the multi-walled carbon nanotubes in water. Among them, the bulk density of the multi-walled carbon nanotubes is 0.021 g / cm 3 , the mass ratio of the multi-walled carbon nanotubes to water is 1:50, and then perform suction filtration and drying to obtain densified multi-walled carbon nanotubes. The bulk density of the densified multi-walled carbon nanotubes is 0.082 g / cm 3 ;

[0054] (2) Put 10 g of densified multi-walled carbon nanotubes into the front temperature zone of the tube furnace, and 22 g of ammonium chloride powder into the rear temperature zone of the tube furnace. Set the front temperature zone to 1250 °C and the rear temperature zone to 350 °C. Introduce nitrogen into the tube furnace at a flow rate of 0.3 L / min. First, start the heating program for the front temperature zone, and then start the heating program for the rear temperature zone later, so that both temperature zones reach their respective set temperatures simultaneously. The inert gas brings the gas generated by ammonium chloride into the temperature zone where the carbon nanotubes are located to react with the impurities therein, and the reaction time is 1 h;

[0055] (3) Wash the multi-walled carbon nanotubes treated in step (2) twice with 500 g of water each time, and then perform suction filtration and drying to obtain purified carbon nanotubes.

[0056] Example 5

[0057] This example provides a method for purifying carbon nanotubes, and the purification method includes the following steps:

[0058] (1) Soak the multi-walled carbon nanotubes in water. Among them, the bulk density of the multi-walled carbon nanotubes is 0.021 g / cm 3, the mass ratio of multi-walled carbon nanotubes to water is 1:50, and then it is subjected to suction filtration and drying to obtain densified multi-walled carbon nanotubes. The bulk density of the densified multi-walled carbon nanotubes is 0.082 g / cm 3 ;

[0059] (2) Take 10 g of densified multi-walled carbon nanotubes and place them in the front temperature zone of the tubular furnace, and 25 g of ammonium chloride powder in the rear temperature zone of the tubular furnace. The front temperature zone is set at 1400 °C, and the rear temperature zone is set at 400 °C. Nitrogen is introduced into the tubular furnace at a flow rate of 1 L / min. First, start the heating program of the front temperature zone, and then start the heating program of the rear temperature zone later, so that both temperature zones reach their respective set temperatures simultaneously. The inert gas brings the gas generated by ammonium chloride into the temperature zone where the carbon nanotubes are located to react with the impurities therein, and the reaction time is 2 h;

[0060] (3) Wash the multi-walled carbon nanotubes treated in step (2) twice with 500 g of water each time, and then perform suction filtration and drying to obtain purified carbon nanotubes.

[0061] Example 6

[0062] This example provides a method for purifying carbon nanotubes. The purification method includes the following steps:

[0063] (1) Soak the multi-walled carbon nanotubes in water. Among them, the bulk density of the multi-walled carbon nanotubes is 0.021 g / cm 3 , the mass ratio of multi-walled carbon nanotubes to water is 1:50, and then it is subjected to suction filtration and drying to obtain densified multi-walled carbon nanotubes. The bulk density of the densified multi-walled carbon nanotubes is 0.082 g / cm 3 ;

[0064] (2) Take 10 g of densified multi-walled carbon nanotubes and place them in the front temperature zone of the tubular furnace, and 25 g of ammonium chloride powder in the rear temperature zone of the tubular furnace. The front temperature zone is set at 1350 °C, and the rear temperature zone is set at 400 °C. Nitrogen is introduced into the tubular furnace at a flow rate of 1 L / min. First, start the heating program of the front temperature zone, and then start the heating program of the rear temperature zone later, so that both temperature zones reach their respective set temperatures simultaneously. The inert gas brings the gas generated by ammonium chloride into the temperature zone where the carbon nanotubes are located to react with the impurities therein, and the reaction time is 2 h;

[0065] (3) Wash the multi-walled carbon nanotubes treated in step (2) twice with 500 g of water each time, and then perform suction filtration and drying to obtain purified carbon nanotubes.

[0066] Example 7

[0067] This example provides a method for purifying carbon nanotubes. Compared with Example 5, in step (1), the mass ratio of multi-walled carbon nanotubes to water is controlled to be 1:3, and the rest are the same as in Example 5.

[0068] Example 8

[0069] This example provides a method for purifying carbon nanotubes. Compared with Example 5, in step (1), the mass ratio of multi-walled carbon nanotubes to water is controlled to be 1:60, and the rest are the same as in Example 5.

[0070] Example 9

[0071] This example provides a method for purifying carbon nanotubes. Compared with Example 5, in step (2), the mass of ammonium chloride is 5 g, so the mass ratio of densified carbon nanotubes to ammonium chloride is 1:0.5, and the rest are the same as in Example 5.

[0072] Example 10

[0073] This example provides a method for purifying carbon nanotubes. Compared with Example 5, in step (2), the densified multi-walled carbon nanotubes and ammonium chloride are mixed and heated together to 1400 °C, and the rest are the same as in Example 5.

[0074] Example 11

[0075] This comparative example provides a method for purifying carbon nanotubes. Compared with Example 5, step (3) is not carried out, and the rest are the same as in Example 5.

[0076] Example 12

[0077] This example provides a method for purifying carbon nanotubes, and the purification method includes the following steps:

[0078] (1) Soak single-walled carbon nanotubes in water. Among them, the bulk density of single-walled carbon nanotubes is 0.006 g / cm 3 , the mass ratio of single-walled carbon nanotubes to water is 1:50, then filter by suction and dry to obtain densified single-walled carbon nanotubes, and the bulk density of the densified single-walled carbon nanotubes is 0.0178 g / cm 3 ;

[0079] (2) Take 4 g of densified single-walled carbon nanotubes and put them in the front temperature zone of a tube furnace, and 10 g of ammonium chloride powder in the rear temperature zone of the tube furnace. The front temperature zone is set to 800 °C, and the rear temperature zone is set to 400 °C. Pass nitrogen into the tube furnace at a flow rate of 0.5 L / min. First, turn on the heating program of the front temperature zone, and then turn on the heating program of the rear temperature zone to make both temperature zones reach their set temperatures simultaneously. The inert gas will carry the gas generated by ammonium chloride into the temperature zone where the carbon nanotubes are located to react with the impurities therein, and the reaction time is 1.5 h;

[0080] (3) Wash the single-walled carbon nanotubes processed in step (2) twice with 500 g of water each time, then perform suction filtration and drying to obtain purified carbon nanotubes.

[0081] Example 13

[0082] This example provides a method for purifying carbon nanotubes, and the purification method includes the following steps:

[0083] (1) Soak the single-walled carbon nanotubes in water. Among them, the bulk density of the single-walled carbon nanotubes is 0.006 g / cm 3 , and the mass ratio of the single-walled carbon nanotubes to water is 1:50. Then perform suction filtration and drying to obtain densified single-walled carbon nanotubes, and the bulk density of the densified single-walled carbon nanotubes is 0.0178 g / cm 3 ;

[0084] (2) Take 4 g of densified single-walled carbon nanotubes and put them into the front temperature zone of the tube furnace, and put 20 g of ammonium chloride powder into the rear temperature zone of the tube furnace. The front temperature zone is set to 850 °C, and the rear temperature zone is set to 350 °C. Pass nitrogen into the tube furnace with a flow rate of 1 L / min. First, turn on the heating program of the front temperature zone, and then turn on the heating program of the rear temperature zone later to make both temperature zones reach their respective set temperatures simultaneously. The inert gas will bring the gas generated by ammonium chloride into the temperature zone where the carbon nanotubes are located to react with the impurities therein, and the reaction time is 2 h;

[0085] (3) Wash the single-walled carbon nanotubes processed in step (2) twice with 500 g of water each time, then perform suction filtration and drying to obtain purified carbon nanotubes.

[0086] Example 14

[0087] This example provides a method for purifying carbon nanotubes, and the purification method includes the following steps:

[0088] (1) Soak the single-walled carbon nanotubes in water. Among them, the bulk density of the single-walled carbon nanotubes is 0.006 g / cm 3 , and the mass ratio of the single-walled carbon nanotubes to water is 1:50. Then perform suction filtration and drying to obtain densified single-walled carbon nanotubes, and the bulk density of the densified single-walled carbon nanotubes is 0.0178 g / cm 3 ;

[0089] (2) Take 4 g of densified single-walled carbon nanotubes and place them in the front temperature zone of a tube furnace, and 16 g of ammonium chloride powder in the rear temperature zone of the tube furnace. Set the front temperature zone to 900 °C and the rear temperature zone to 400 °C. Pass nitrogen into the tube furnace at a flow rate of 1 L / min. First, start the heating program for the front temperature zone, and then start the heating program for the rear temperature zone later, so that both temperature zones reach their respective set temperatures simultaneously. The inert gas will carry the gas generated by ammonium chloride into the temperature zone where the carbon nanotubes are located to react with the impurities therein, and the reaction time is 2 h;

[0090] (3) Wash the single-walled carbon nanotubes treated in step (2) twice with 500 g of water each time, and then perform suction filtration and drying to obtain purified carbon nanotubes.

[0091] Comparative Example 1

[0092] This comparative example provides a method for purifying carbon nanotubes, and the purification method includes the following steps:

[0093] (1) Take 2.5 g of multi-walled carbon nanotubes and place them in the front temperature zone of a tube furnace. The bulk density of the multi-walled carbon nanotubes is 0.021 g / cm 3 , and 6.25 g of ammonium chloride powder in the rear temperature zone of the tube furnace. Set the front temperature zone to 1400 °C and the rear temperature zone to 400 °C. Pass nitrogen into the tube furnace at a flow rate of 1 L / min. First, start the heating program for the front temperature zone, and then start the heating program for the rear temperature zone later, so that both temperature zones reach their respective set temperatures simultaneously. The inert gas will carry the gas generated by ammonium chloride into the temperature zone where the carbon nanotubes are located to react with the impurities therein, and the reaction time is 2 h;

[0094] (2) Wash the multi-walled carbon nanotubes treated in step (1) twice with 500 g of water each time, and then perform suction filtration and drying to obtain purified carbon nanotubes.

[0095] Comparative Example 2

[0096] This comparative example provides a method for purifying carbon nanotubes. Compared with Example 5, in step (2), the front temperature zone where the multi-walled carbon nanotubes are located is set to 700 °C, and the rest are the same as in Example 5.

[0097] Comparative Example 3

[0098] This comparative example provides a method for purifying carbon nanotubes. Compared with Example 5, in step (2), the front temperature zone where the multi-walled carbon nanotubes are located is set to 1500 °C, and the rest are the same as in Example 5.

[0099] Performance Characterization

[0100] The unprocessed carbon nanotubes, the purified carbon nanotubes obtained in the examples and comparative examples were tested for their loss rate, purity, and metal impurity content, and the results are listed in Tables 1 and 2.

[0101] The unprocessed carbon nanotubes, the purified carbon nanotubes obtained in the examples and comparative examples were prepared into conductive slurries together with a dispersant, an additive, and N-methylpyrrolidone, and then the positive electrode of the single crystal ternary material was slurried. The viscosity of the slurry was tested, and then the slurried mixture was coated with a coater and dried in an oven, and the resistivity of the corresponding positive electrode precursor was tested. The results are listed in Tables 1 and 2. Among them, the slurry formulation for multi-walled carbon nanotubes is: NCM is 98.375%, CNT is 0.5%, dispersant is 0.125%, PVDF is 1%, and the solid content of the slurry is 74%; the slurry formulation for single-walled carbon nanotubes is: NCM is 97.9%, CNT is 0.05%, dispersant is 0.05%, PVDF is 2%, and the solid content of the slurry is 65%.

[0102] Table 1

[0103]

[0104]

[0105] Table 2

[0106]

[0107] In the table, " / " indicates no data.

[0108] From Figure 2 and Figure 3 it can be seen that there is no residual metal catalyst impurity in the purified multi-walled carbon nanotubes and single-walled carbon nanotubes of the present invention, and the purification effect is good.

[0109] From Tables 1 and 2, it can be seen that the carbon nanotubes purified by the purification method provided by the present invention have high purity, and the purity can reach more than 98.25%, especially more than 99.5%. Using the preferred process parameters of the present invention, when the purification reaction is carried out at 1050-1400 °C for 1-2 h, the purity can reach more than 99.9%, the metal content is lower than 50 ppm, the viscosity of the conductive slurry is significantly reduced, the volume resistivity is reduced, and the conductivity is good. Compared with Example 5, in Example 9, the mass of ammonium chloride is less, and the carbon nanotubes cannot be fully reacted and purified, resulting in a decrease in the purity of the carbon nanotubes and a higher impurity content; in Example 10, the carbon nanotubes and ammonium chloride are heated together, making it inconvenient to accurately control the heating temperature and difficult to control the actual reaction amount of ammonium chloride, which may lead to insufficient purification reaction and a decrease in the purity of the carbon nanotubes; in Example 11, no water washing is carried out after purification, resulting in a high metal content in the carbon nanotubes.

[0110] In Comparative Example 1, the carbon nanotubes were not densified, and the treatment amount during purification was only 2.5 g, which was 75% lower than that after densification compared with Example 5. In Comparative Example 2, the carbon nanotubes were purified under the condition of lower than 800 °C, and the purity of the carbon nanotubes was lower than 99%. In Comparative Example 3, the carbon nanotubes were purified under the condition of higher than 1400 °C. Although the purity of the carbon nanotubes could reach more than 99.5%, the volume resistivity increased, the electrical conductivity deteriorated, and a large loss rate was caused, which might have damaged the structure of the carbon nanotubes.

[0111] In summary, the purification method provided by the present invention is carried out under the sub-high temperature condition of 800-1400 °C, with high purification purity of carbon nanotubes, small metal impurity content, and small loss rate; without pre-oxidation heat treatment, and avoiding the destruction of the structure of carbon nanotubes by high temperature heat treatment above 1700 °C; at the same time, the purification efficiency is improved, which is beneficial to the industrial scale-up of the overall purification method.

[0112] The applicant declares that the above description is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A purification method of carbon nanotubes, characterized in that, The purification method includes the following steps: Densify the carbon nanotubes, and then use ammonium chloride to purify the densified carbon nanotubes at 800 - 1400 °C to obtain purified carbon nanotubes.

2. The purification method according to claim 1, wherein The purification process includes: introducing an inert gas into a container, heating ammonium chloride placed in the first temperature zone of the container to 350 - 400 °C, heating the densified carbon nanotubes placed in the second temperature zone of the container to 800 - 1400 °C, and the inert gas brings the gas generated by ammonium chloride into the second temperature zone to purify the densified carbon nanotubes.

3. The purification method according to claim 2, characterized in that, The inert gas includes nitrogen and / or argon.

4. The purification method according to claim 2 or 3, characterized in that The flow rate of the inert gas is 0.1 - 1 L / min.

5. The purification method according to any one of claims 1-4, characterized in that, The purification time is 0.5 - 2 h.

6. The purification method according to any one of claims 1-5, characterized in that, The mass ratio of the densified carbon nanotubes to ammonium chloride is 1:(1.5 - 5).

7. The purification method according to any one of claims 1-6, characterized in that, The densification treatment includes: mixing the carbon nanotubes with water, and then separating and drying them to obtain densified carbon nanotubes.

8. The purification method according to claim 7, characterized in that, The mass ratio of the carbon nanotubes to water is 1:(3 - 60).

9. The purification method according to any one of claims 1-8, characterized in that, The purification method further includes washing and drying after purification.

10. The purification method according to any one of claims 1-9, characterized in that, The purification method includes the following steps: (1) Soak the carbon nanotubes in water, where the mass ratio of the carbon nanotubes to water is 1:(3 - 60), and then perform suction filtration and drying to obtain densified carbon nanotubes; (2) Introduce an inert gas into a container, the flow rate of the inert gas is 0.1 - 1 L / min, heat ammonium chloride placed in the first temperature zone of the container to 350 - 400 °C, heat the densified carbon nanotubes placed in the second temperature zone of the container to 800 - 1400 °C, so that the first temperature zone and the second temperature zone reach the predetermined temperature simultaneously, and the inert gas brings the gas generated by ammonium chloride into the second temperature zone to purify the densified carbon nanotubes, and the purification time is 0.5 - 2 h; (3) Wash and dry the treated carbon nanotubes to obtain purified carbon nanotubes.

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