Preparation method of conductive paste for improving carbon nanotube conductivity and application thereof

Abstract

The application provides a preparation method of conductive paste for improving the conductivity of carbon nanotubes and application thereof. The method comprises the following steps: (1) preparing a catalyst by a sol-gel method, and placing the catalyst into a tube furnace and heating to 800 DEG C to nanometerize the catalyst particles; (2) placing a catalyst precursor into the tube furnace, heating to 680 DEG C under the protection of argon, and introducing a carbon source to grow carbon nanotubes by chemical vapor deposition; (3) placing the prepared carbon nanotubes into a graphitization furnace under the protection of argon, heating to a temperature range of 2000-2800 DEG C, keeping for 3 hours, cooling to room temperature, and taking out; and (4) taking out the carbon nanotubes, and preparing stable carbon nanotube conductive paste in a solution by ultrasonic oscillation and magnetic stirring. The application can effectively remove the metal catalyst and metal oxide carrier in the carbon nanotubes, and effectively improve the conductivity of the carbon nanotubes, and the application in lithium ion batteries is embodied by the first charge-discharge efficiency, cycle performance, rate performance and impedance performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion battery materials, in particular to a preparation method of conductive paste for improving the conductivity of carbon nanotubes and application thereof. BACKGROUND

[0002] Lithium ion batteries have been widely used in consumer electronics due to their high operating voltage, large specific energy, fast charging rate, large output power, long cycle life, and no memory effect. In recent years, with the rapid development of the electric vehicle industry, the energy density of power batteries has been exposed. However, as an important component of lithium ion batteries, although the proportion of conductive agent is small, it plays an important role in the transmission of ions and electrons. Traditional conductive agents such as carbon black and conductive graphite will not help the energy storage of the battery, and the polarization phenomenon is serious in the later stage of lithium ion battery cycle, and the battery internal loss is large. Therefore, it is necessary to find a conductive agent with a small amount and high conductivity.

[0003] At present, carbon nanotubes are a kind of carbon-based conductive agent with high prospects. Because of its large aspect ratio, only a small amount is needed to form a conductive path and improve the rate and cycle performance of the battery. However, carbon nanotubes have two major drawbacks. First, the mainstream method for preparing carbon nanotubes is chemical vapor deposition, which has low energy consumption, simple preparation process and can synthesize carbon nanotubes in batches. However, the carbon nanotubes synthesized by this method contain a large amount of metal impurities. The common method for removing impurities is chemical removal method, which uses acidic solutions such as hydrochloric acid and nitric acid to purify carbon nanotubes. However, this purification method often damages the structure of carbon nanotubes, and the impurity removal effect is not very good, which affects its performance in various fields, especially in lithium ion batteries. The metal impurities in carbon nanotubes can cause self-discharge of the battery, i.e. short circuit, which has a great influence on the performance and safety of lithium ion batteries. Second, due to the strong van der Waals force between carbon nanotubes and the one-dimensional morphology, carbon nanotubes are prone to agglomeration and difficult to disperse. If directly applied in lithium ion batteries, carbon nanotubes will be severely entangled and cannot be well dispersed between active materials to form a continuous conductive network. Moreover, the internal loss of the battery increases and the internal resistance rises during the cycle to the later stage or at high rate. The dispersion method of carbon nanotubes in the market is mainly physical and mechanical dispersion such as sand mill, which has high requirements for instruments and energy consumption, and the process is complex.

[0004] In summary, in view of the series of problems of carbon nanotubes, we urgently need a method that can remove the metal impurities of carbon nanotubes without damaging their structure, so as to improve their conductivity. SUMMARY

[0005] The carbon nanotube conductive agent applied to the positive electrode material of the lithium ion battery has many metal impurities and low purity, and with the increase of the negative electrode voltage in the lithium ion battery, the positive electrode will precipitate metal impurities such as lithium dendrite and iron, which will pierce the separator and cause short circuit of the battery, not only poor conductivity, but also reduce the performance of the battery and there is a great safety hazard, and the carbon nanotube treated by acid will change the structure and make the conductivity poor and the impurity removal efficiency low, which is not conducive to the application of the carbon nanotube as a conductive agent in the lithium ion battery.

[0006] Based on this, the application provides a preparation method of a conductive slurry for improving the conductivity of carbon nanotubes and application thereof.

[0007] In the first aspect, the application provides a preparation method of a conductive slurry for improving the conductivity of carbon nanotubes, comprising the following steps:

[0008] Step (1): a certain amount of mixed solvent is prepared, and a carbon nanotube catalyst is prepared by a sol-gel method using a water bath kettle or an oil bath kettle;

[0009] Step (2): the catalyst obtained in step (1) is placed in a tube furnace, heated to 200 DEG C under air atmosphere for 120 min, and then heated to 800 DEG C for 60 min to prepare a catalyst precursor;

[0010] Step (3): the catalyst precursor in step (2) is ground and sieved, then placed in a tube furnace, and inert gas is introduced, then reducing gas is introduced and heated to 680 DEG C, and then carbon source is introduced to synthesize carbon nanotubes by chemical vapor deposition method;

[0011] Step (4): the carbon nanotubes prepared in step (3) are placed in a graphitization furnace, vacuumized to make the pressure value in the furnace-0.1 MPa, then inert gas is introduced, the heating temperature range is 2000-2800 DEG C, and the temperature is kept for 3 h, then cooled to room temperature under inert gas protection, and taken out to obtain carbon nanotubes purified at different high temperatures.

[0012] Step (5): a certain mass of the purified carbon nanotubes in step (4) are weighed, dispersed into stable high-temperature purified carbon nanotube conductive slurry by ultrasonic oscillation and magnetic stirring.

[0013] Further, the solvent in step (1) is ferric nitrate, aluminum nitrate and citric acid, deionized water as solution, mixed stirring 8-12h; the mass ratio of ferric nitrate, aluminum nitrate and citric acid is 1:0.9:1.1; the deionized water solution is 30-40ml.

[0014] Further, the water bath or oil bath in step (1) is heated to above 90℃, and the heating time is above 12h.

[0015] Further, the sieve in step (3) uses a 300-mesh sieve.

[0016] Further, the inert gas in step (3) is Ar2; the reducing gas is H2; and the carbon source is propane.

[0017] Further, the 2000-2800℃ in step (4) are respectively 2000℃, 2200℃, 2400℃, 2600℃ and 2800℃.

[0018] Further, the vacuum in step (4) should meet the pressure that the metal catalyst can withstand is higher than the pressure saturation value of the graphitization furnace.

[0019] Further, in step (5), the ultrasonic dispersion has an ultrasonic power of 50W-150W, an ultrasonic frequency of 40KHz, an ultrasonic oscillation time of above 2h, a magnetic stirring power voltage of 220V / 50Hz, and a stirring time of above 3h, and the ultrasonic and magnetic stirring are alternately performed.

[0020] Further, in step (5), an NMP solution is used, wherein the N-methyl pyrrolidone content is 94%-96%, and the mass ratio of carbon nanotubes to PVP dispersant is 5:1.

[0021] In a second aspect, the application further provides the application of the carbon nanotube conductive paste prepared by the preparation method in a lithium ion battery.

[0022] The preparation method of the conductive paste for improving the conductivity of carbon nanotubes and the application thereof, high-temperature purification of carbon nanotubes is used to effectively remove metal impurities (Fe, Al) and metal oxides in the carbon nanotubes, and Fe-Al is used as a catalyst in the application, and the content of Fe-Al impurities decreases with the increase of temperature. The method overcomes the problems of high impurity content of traditional acid-treated carbon nanotubes and damage to the structure of carbon nanotubes after impurity removal. The method is simple to operate, has high impurity removal efficiency and reduces defects after impurity removal, and does not cause the safety hazard of piercing the diaphragm and short circuiting when applied to lithium ion batteries, and the prepared conductive paste has simple process, low energy consumption and does not reduce the aspect ratio of carbon nanotubes.

[0023] The application has the following advantages:

[0024] The traditional carbon nanotubes have high impurity content, and are usually treated by acid washing when used as a conductive agent in lithium ion batteries. This method has low impurity removal rate, is serious in waste and complex in process, and is not conducive to the structure and performance of carbon nanotubes. The treated carbon nanotubes in the application have high purity and few defects, high graphitization degree of carbon nanotubes and improved conductivity. The carbon nanotubes prepared by a simple dispersion process can be effectively used in the positive electrode material of lithium ion batteries, coated on active substances and connected to single isolated positive electrode particles to construct a continuous and stable conductive network.

[0025] The carbon nanotubes after high-temperature purification have low impurity content, and the impurity content decreases with the increase of temperature. The content of metal impurities (Fe, Al) is less than 100 ppm at 2800 DEG C.

[0026] The carbon nanotubes after high-temperature purification have few defects and high graphitization degree. In the XRD graph, the (002) peak is higher with the increase of temperature, the graphitization degree is higher, and the conductivity of the carbon nanotubes is better.

[0027] The carbon nanotubes after high-temperature purification have smooth walls, and high-temperature purification helps to improve the conductivity of the carbon nanotubes.

[0028] The carbon nanotube conductive paste prepared in the application can form a continuous and stable conductive network after being added to the positive electrode material, thereby improving the conductivity.

[0029] The carbon nanotube conductive paste prepared in the application has a higher first discharge specific capacity than the acid-treated carbon nanotubes after being added to the positive electrode material, and the discharge specific capacity is higher with the increase of purification temperature, and is the highest at 2800 DEG C and has the best conductivity.

[0030] The carbon nanotube conductive paste prepared in the application has better cycle performance and better conductivity with the increase of purification temperature after being added to the positive electrode material.

[0031] The carbon nanotube conductive slurry prepared by the application has a CV curve of five cycles which is basically coincident after adding the positive electrode material, which indicates that the conductive network constructed by the carbon nanotube as a conductive agent at the purification temperature is stable and continuous, and the carbon nanotube has optimal conductivity.

[0032] The carbon nanotube conductive slurry prepared by the application has a better rate performance with the increase of the purification temperature after adding the positive electrode material, and has a higher performance at a high rate after five cycles. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 ICP data and curve diagram of the high-temperature purified carbon nanotube of the application;

[0034] Figure 2 ICP curve diagram of the high-temperature purified carbon nanotube of the application;

[0035] Figure 3 XRD diagram of the high-temperature purified carbon nanotube of the application;

[0036] Figure 4 SEM diagram of the high-temperature purified carbon nanotube of the application;

[0037] Figure 5 SEM diagram of the high-temperature purified carbon nanotube conductive slurry applied to an electrode of the application;

[0038] Figure 6 First charge-discharge curve diagram of the high-temperature purified carbon nanotube conductive slurry added with the positive electrode material of the application;

[0039] Figure 7 Cycle curve diagram (2.75-4.2V, 0.2C, 25℃) of the high-temperature purified carbon nanotube conductive slurry added with the positive electrode material of the application;

[0040] Figure 8 CV curve diagram of the high-temperature purified carbon nanotube conductive slurry added with the positive electrode material of the application at 2800℃;

[0041] Figure 9 Rate curve diagram (2.75-4.2V, 0.2C, 0.5C, 1C, 2C, 3C, 4C, 5C, 0.2C, 25℃) of the high-temperature purified carbon nanotube conductive slurry added with the positive electrode material after five cycles of the application.

[0042] Figure 10 EIS curve diagram of the high-temperature purified carbon nanotube conductive slurry added with the positive electrode material of the application. DETAILED DESCRIPTION

[0043] For the purpose of facilitating the understanding of the present application, a more comprehensive description of the present application will be given below with reference to the relevant drawings. The preferred embodiments of the present application are shown in the drawings. However, the present application can be realized in many different forms and is not limited to the embodiments described herein. On the contrary, these embodiments are provided for the purpose of making the disclosure of the present application more thorough and comprehensive.

[0044] The basic condition of the present application is that, first, vacuum is drawn to make the pressure in the furnace -0.1 MPa, argon is introduced to restore the normal pressure, the carbon nanotubes are kept at high temperature for 3 h in the atmosphere filled with argon, the carbon impurities such as fullerenes and amorphous carbon in the wall of the carbon nanotubes and iron aluminum spinel (FeAl2O4) are removed, and the carbonized iron (Fe3C) particles inlaid in the interior of the carbon nanotubes can also be removed, the metal particles are gasified by high temperature, and thus the metal gas is discharged through the pipeline of the carbon nanotubes to achieve the effect of purification. A small amount of dispersant is added to the purified carbon nanotubes, and the effect of dispersion is achieved by alternately performing ultrasonic oscillation and magnetic stirring, the synthesized conductive paste of the carbon nanotubes is added to the positive electrode material to make an electrode, and the lithium ion battery assembled has good conductive performance, which helps to improve the performance of the active material.

[0045] Please refer to Figures 1-10 The present application provides a preparation method of a conductive paste for improving the conductivity of carbon nanotubes, comprising the following steps:

[0046] Step (1): a certain amount of mixed solvent is prepared, a carbon nanotube catalyst is prepared by a sol-gel method, and a water bath kettle or an oil bath kettle is used;

[0047] Step (2): the catalyst obtained in step (1) is put into a tube furnace, heated to 200℃ in an air atmosphere for 120 min, and then heated to 800℃ for 60 min to prepare a catalyst precursor;

[0048] Step (3): the catalyst precursor in step (2) is ground and sieved, then put into a tube furnace, inert gas is introduced, then reducing gas is introduced, heated to 680℃, and kept for 120 min, and finally carbon source is introduced to synthesize carbon nanotubes by chemical vapor deposition method;

[0049] Step (4): the carbon nanotubes prepared in step (3) are put into a graphitization furnace, vacuum is drawn to make the pressure value in the furnace -0.1 MPa, then inert gas is introduced, the heating temperature range is 2000-2800℃, and the temperature is kept for 3 h, and the temperature is cooled to room temperature under the protection of inert gas, and then taken out to obtain carbon nanotubes purified at different high temperatures.

[0050] Step (5): a certain mass of the carbon nanotubes purified in step (4) is weighed, and is dispersed into stable high-temperature purified carbon nanotube conductive paste by ultrasonic oscillation and magnetic stirring.

[0051] Further, the solvent in step (1) is ferric nitrate, aluminum nitrate and citric acid, and deionized water is used as a solution, mixed and stirred for 8-12 h; the mass ratio of ferric nitrate, aluminum nitrate and citric acid is 1:0.9:1.1; and the deionized water solution is 30-40 ml.

[0052] Further, the water bath or oil bath in step (1) is heated to above 90℃, and the heating time is above 12 h.

[0053] Further, the sieve in step (3) uses a 300-mesh sieve.

[0054] Further, the inert gas in step (3) is Ar2; the reducing gas is H2; and the carbon source is propane.

[0055] Further, the 2000-2800℃ in step (4) is respectively 2000℃, 2200℃, 2400℃, 2600℃ and 2800℃.

[0056] Further, the vacuum in step (4) should meet the pressure that the metal catalyst can withstand is higher than the pressure saturation value of the graphitization furnace.

[0057] Further, in step (5), the ultrasonic dispersion has an ultrasonic power of 50-150 W, an ultrasonic frequency of 40 KHz, an ultrasonic oscillation time of above 2 h, a magnetic stirring power voltage of 220 V / 50 Hz, and a stirring time of above 3 h; and the ultrasonic and magnetic stirring are alternately performed.

[0058] Further, in step (5), an NMP solution is used, wherein the content of N-methyl pyrrolidone is 94-96%, and the mass ratio of carbon nanotubes to PVP dispersant is 5:1.

[0059] In a second aspect, the application further provides an application of the carbon nanotube conductive paste prepared by the preparation method in a lithium ion battery.

[0060] Comparative Example 1

[0061] The carbon nanotube crude sample adopts Fe-Al catalyst citric acid as a binder, and multi-armed carbon nanotubes are prepared by propane.

[0062] Step (1): prepare a hydrochloric acid solution in a 400 ml beaker according to a ratio of concentrated hydrochloric acid to deionized water of 1:6;

[0063] Step (2): 5 g of carbon nanotubes were weighed, the diluted hydrochloric acid solution was poured into the carbon nanotubes, a stirring rod was added, and the water bath or oil bath was heated to 80°C. Stirring and heating were performed for 8 h.

[0064] Step (3): The filter bottle, Buchner funnel and SHZ-D(III) circulating water type multi-purpose vacuum pump were assembled in a certain order. Two pieces of filter paper were placed in the Buchner funnel and a small amount of deionized water was added to wet the filter paper. Then, the acid-treated carbon nanotubes were poured into the Buchner funnel. The circulating water type multi-purpose vacuum pump switch was turned on. A glass rod was used to stir while adding deionized water to prevent the carbon nanotubes from settling at the bottom of the funnel and blocking the filter holes. Finally, PH test paper was used to detect the acid-base level, and the PH was adjusted to neutral.

[0065] Step (4): The filtered carbon nanotubes were placed in a 120°C oven and dried for more than 12 h.

[0066] Step (5): The positive electrode material, PVDF and carbon nanotubes were weighed according to the mass ratio of 93:5:2. The ratio of carbon nanotubes to dispersant PVP was 5:1. A carbon nanotube solution was prepared with a solid content of 48%. The prepared carbon nanotube solution was subjected to 40 Hz ultrasonic oscillation for 2-4 h, followed by magnetic stirring for 1-2 h or more. The above steps were alternately performed for 5-6 times to obtain the carbon nanotube conductive slurry.

[0067] Example 1

[0068] The carbon nanotube sample was the same as that of Comparative Example 1.

[0069] Step (1): 5 g of crude carbon nanotubes were weighed and placed in a graphitization furnace. First, vacuum was applied to -0.1 MPa, then argon was introduced to normal pressure, and then the temperature was raised to 1100°C. The temperature was raised to 1100°C for 30 min. After 30 min, the temperature was calibrated by a double-color infrared thermometer (E1RH) aimed at the red dot in the furnace. Then, the temperature was raised to 2800°C for 120 min. The carbon nanotubes with a purification temperature of 2800°C were taken out when the temperature in the furnace cooled to room temperature, and were marked as CNT-2800°C.

[0070] Step (2): The carbon nanotubes with a purification temperature of 2800°C were taken out under the protection of argon when the temperature in the furnace cooled to room temperature, and were marked as CNT-2800°C.

[0071] Step (3): 0.03 g of dispersant PVP was weighed and poured into 7.8 g of N-methyl pyrrolidone solution. Stirring was performed for 1-2 h. Then, 0.15 g of CNT-2800°C sample was added and stirred for 2-3 h. Subsequently, ultrasonic oscillation was performed for 40 Hz for 2-3 h. Then, stirring was performed again for 1-2 h. Ultrasonic oscillation was performed again for the same time. The above steps were alternately performed for 4-5 times to make the carbon nanotubes fully and stably dispersed in the N-methyl pyrrolidone solution. The conductive slurry of CNT-2800°C was prepared.

[0072] The ICP curve of the carbon nanotubes prepared in Example 1 is shown in Figure 1 and Figure 2 It can be seen that the impurity content is the lowest, better than the carbon nanotubes without impurity removal and Comparative Example 1, and the impurity removal effect is obvious and excellent; as shown in Figure 3 The XRD curve of CNT-2800℃ is shown in Figure 4 It can be seen that the (002) peak is the highest, and the graphitization degree is improved. The SEM of the carbon nanotubes purified at 2800℃ is shown in

[0073] The conductive paste prepared in step (3) of Example 1 was made into an electrode. The positive electrode material was LiNi0.5Co0.2Mn0.3O2, the binder was PVDF, and the mass ratio of N-methyl pyrrolidone solution was 93:5:2 to prepare a mixture with a solid content of 48%. The mixture was put into a homogenizer, the stirring speed was 800 r / min, and the stirring time was 30 s. Then the stirring speed was adjusted to 2000 r / min, and the stirring time was 20 min to obtain a uniformly dispersed electrode paste. A 5×10 cm thick 20 μm aluminum foil was cut, and the cut aluminum foil was placed on an automatic film coating machine. The thickness of the knife pen was adjusted to 10 μm, and the uniformly dispersed electrode paste was poured onto one side of the aluminum foil. The switch of the coating machine was turned on, and the knife table was pushed at a constant speed. The paste was uniformly covered on the surface of the aluminum foil. Then the coated electrode was placed in a 120℃ oven for drying. After the roll was punched, a circular electrode with a diameter of 1.2 cm was obtained. The SEM of the electrode is shown in Figure 5 It can be seen that the carbon nanotubes are uniformly coated on the surface of the active material, and connect the positive electrode particles to form a continuous and stable conductive network. The battery was assembled in a glove box filled with argon and with water and oxygen contents less than 0.01 ppm. The order of assembling the battery was negative electrode shell, lithium sheet, separator, 40 μL electrolyte, electrode sheet, gasket, spring, positive electrode shell, and then the battery was sealed in a sealing machine with a pressure of 900-1100 N to obtain a CR2032 type button cell. The battery was formed at a voltage of 2.7-4.2 V, a rate of 0.1 C, and a temperature of 25℃. The first charge and discharge performance is shown in Figure 6 It can be seen that the first discharge specific capacity is 164.5 mAh / g, which is better than the actual capacity of LiNi0.5Co0.2Mn0.3O2 positive electrode material, indicating that the high-temperature purification temperature of 2800℃ improves the conductivity of the carbon nanotubes and helps to improve the performance of the lithium ion battery. The discharge retention rate is 97.81% after 50 cycles at a rate of 0.2 C, which is higher than that of Comparative Example 1 and other purification temperatures.

[0074] Figure 8For the cyclic voltammetry curve of Example 1, it can be seen that the curve thereof is approximately coincident after 5 cycles at a scan rate of 10 mV / s, and the shift of the redox peak is small, indicating that the battery performance is stable, a continuous stable conductive network is constructed, and the carbon nanotubes at this purification temperature have good conductive performance. Figure 9 For the rate curve of Example 1 cycled 5 times at 0.2C, 0.5C, 1C, 2C, 3C, 4C, 5C and 0.2C respectively, it can be seen that the rate performance thereof is best at 2800℃, indicating that at this purification temperature, the decrease of the content of metal impurities helps to improve the conductivity of the carbon nanotubes.

[0075] Figure 10 For the EIS curve of Example 1, the semicircle in the high-frequency region represents the charge transfer impedance, and the straight line in the low-frequency region represents the inherent impedance, and it can be seen from the figure that the charge transfer impedance is smallest at the purification temperature of 2800℃, indicating that the diffusion impedance of lithium ions between the active material and the carbon tube is smallest, while the charge transfer impedance of the acid washing in Comparative Example 1 is largest, because the high-temperature purification has less effect on the structure of the carbon nanotubes and the carbon nanotubes have the highest graphitization degree and the smallest defects and the best conductive performance at 2800℃, while the acid washing purification changes the structure of the carbon nanotubes, thereby reducing the crystallinity thereof and the conductive performance thereof.

[0076] Example 2

[0077] Example 2

[0078] Step (1): The crude sample of carbon nanotubes is the same as that in Example 1. 6 g of crude carbon nanotubes is placed in a graphitization furnace, vacuum is first extracted to -0.1 MPa, argon is then introduced to normal pressure, the temperature is first increased to 1100℃, the temperature increasing time is 30 min, after the temperature is increased for 30 min, a double-color infrared temperature measuring instrument (E1RH) is aimed at the red dot in the furnace, and then the temperature is increased to 2600℃ for 140 min, and the temperature is kept at 2600℃ for 3 h.

[0079] Step (2): The carbon nanotubes with a purification temperature of 2600℃ are taken out under the protection of argon when the temperature in the furnace cools to room temperature, and are recorded as CNT-2600℃.

[0080] Step (3): 0.04 g of dispersant PVP is weighed and poured into 7.8 g of N-methyl pyrrolidone solution, stirred for 1-2 h, 0.2 g of CNT-2600℃ sample is then added and stirred for 2-3 h, followed by 50 Hz ultrasonic oscillation for 2-3 h, then stirring for 1-2 h, and then ultrasonic oscillation for the same time, and the above steps are alternately performed for 3-4 times, so that the carbon nanotubes are fully and stably dispersed in the N-methyl pyrrolidone solution, and the conductive slurry of CNT-2600℃ is prepared.

[0081] As Figure 1 and Figure 2ICP curve is shown, it can be seen that its impurity content is higher than 2800℃, but still better than comparative example 1 and non-impurity-removed carbon nanotubes; its XRD curve is shown, the graphitization degree at 2600℃ is also higher than comparative example 1 and non-impurity-removed carbon nanotubes, the structure of carbon nanotubes is not destroyed and has certain improvement, it can be seen that high-temperature purification has certain reduction on defects of carbon nanotubes, which is helpful to improve its structure and conductive performance. Figure 3 ICP curve is shown, it can be seen that its impurity content is higher than 2800℃, but still better than comparative example 1 and non-impurity-removed carbon nanotubes; its XRD curve is shown, the graphitization degree at 2600℃ is also higher than comparative example 1 and non-impurity-removed carbon nanotubes, the structure of carbon nanotubes is not destroyed and has certain improvement, it can be seen that high-temperature purification has certain reduction on defects of carbon nanotubes, which is helpful to improve its structure and conductive performance.

[0082] The conductive slurry prepared in step (3) of example 2 is prepared into electrode slurry by the method in example 1, and assembled into a button cell, which is formed under the conditions of voltage 2.7-4.2V, rate 0.1C and temperature 25℃, and its first charge-discharge performance is shown in Figure 6 It can be seen that its first discharge specific capacity is 150.1mAh / g, and its discharge retention rate is 95.52% after 50 cycles under the conditions of voltage 2.75-4.3V, rate 0.2C and temperature 25℃. Figure 9 The rate performance of example 2 is slightly lower than that of example 1 but higher than that at other temperatures, which is also helpful to the conductivity of carbon nanotubes. Figure 10 The AC impedance curve of example 2 is shown in

[0083] Example 3

[0084] The carbon nanotube sample is the same as that in example 1. The temperature condition is 2400℃, and the holding time is 3h. The ICP curve is shown in Figure 1 and Figure 2 It can be seen that its impurity content is similar to that at 2600℃, but the XRD curve Figure 3 shows that the (002) peak intensity at this temperature is lower than that at 2600℃, which indicates that although the impurity removal effect is obvious at this temperature, it is not conducive to the graphitization of carbon nanotubes, and there are certain defects in carbon nanotubes.

[0085] The conductive slurry at this temperature (CNT-2400℃) is prepared into electrode slurry by the method in example 1, and assembled into a button cell, which is formed under the conditions of voltage 2.7-4.2V, rate 0.1C and temperature 25℃, and its first charge-discharge performance is shown in Figure 6 It can be seen that its first discharge specific capacity is 140.52mAh / g, and its discharge retention rate is 95.01% after 50 cycles under the conditions of voltage 2.75-4.3V, rate 0.2C and temperature 25℃; the rate curve is shown in Figure 9

[0086] Example 4

[0087] The carbon nanotube sample is the same as that in example 1. The temperature condition is 2200℃, and the holding time is 3h. The ICP curve is shown in Figure 1 and Figure 2 ​The XRD curve is shown in Figure 3 .

[0088] The conductive paste at this temperature (CNT-2200℃) was prepared into electrode paste by the method in Embodiment 1, assembled into a button cell, and subjected to formation under the condition of a voltage of 2.7-4.2V, a rate of 0.1C, and a temperature of 25℃, and the first charge-discharge performance is shown in Figure 6 It can be seen that the first discharge specific capacity is 153.04mAh / g, and the discharge retention rate is 94.72% after 50 cycles under the condition of a voltage of 2.75-4.3V, a rate of 0.2C, and a temperature of 25℃; the rate curve is shown in Figure 9 ; and the AC impedance curve is shown in Figure 10 .

[0089] Embodiment 5

[0090] The carbon nanotube sample is the same as in Embodiment 1. The temperature condition is 2000℃, and the holding time is 3h. The ICP curve is shown in Figure 1 and Figure 2 ; and the XRD curve is shown in Figure 3 .

[0091] The conductive paste at this temperature (CNT-2000℃) was prepared into electrode paste by the method in Embodiment 1, assembled into a button cell, and subjected to formation under the condition of a voltage of 2.7-4.2V, a rate of 0.1C, and a temperature of 25℃, and the first charge-discharge performance is shown in Figure 6 It can be seen that the first discharge specific capacity is 140.47mAh / g, and the discharge retention rate is 95.02% after 50 cycles under the condition of a voltage of 2.75-4.3V, a rate of 0.2C, and a temperature of 25℃; the rate curve is shown in Figure 9 .

[0092] The preparation method of the conductive paste for improving the conductivity of carbon nanotubes and the application thereof in the application can effectively remove metal impurities (Fe, Al) and metal oxides in the carbon nanotubes by high-temperature purification of the carbon nanotubes. In the application, Fe-Al is used as a catalyst, and the content of Fe-Al impurities decreases with the increase of temperature. The method overcomes the problems of high impurity content in traditional acid treatment of carbon nanotubes and damage to the structure of carbon nanotubes after impurity removal. The method is simple to operate, has high impurity removal efficiency, and reduces defects after impurity removal. When applied to lithium ion batteries, there is no safety hazard of piercing the separator and causing short circuit, and the prepared conductive paste has simple process, low energy consumption, and does not reduce the aspect ratio of carbon nanotubes.

[0093] The application has the following advantages:

[0094] The traditional carbon nanotube has high impurity content, and when used as a conductive agent in a lithium ion battery, is treated by an acid washing method, which has low impurity removal rate, is serious in waste, and has complex process, and is not conducive to the structure and performance of the carbon nanotube. The carbon nanotube treated by the method has high purity and few defects, the carbon nanotube has high desertification degree, and the conductive performance is improved; the carbon nanotube is effectively used in a positive material of a lithium ion battery, coated on an active material, and connected to single isolated positive particles to construct a continuous and stable conductive network through a simple dispersion process.

[0095] The carbon nanotube after high-temperature purification has low impurity content, and the impurity content is reduced in sequence with the increase of temperature, and the metal impurity (Fe, Al) content is lower than 100 ppm at 2800 DEG C.

[0096] The carbon nanotube after high-temperature purification has few defects and high graphitization degree, and the (002) peak is higher in the XRD diagram with the increase of temperature, the graphitization degree is higher, and the conductive performance of the carbon nanotube is better.

[0097] The carbon nanotube after high-temperature purification has a smooth tube wall, and the high-temperature purification has a certain effect on the dispersion of the carbon nanotube.

[0098] The carbon nanotube conductive slurry prepared by the method is added into the positive material to form a continuous and stable conductive network and improve the conductive performance.

[0099] The carbon nanotube conductive slurry prepared by the method is added into the positive material, and the first discharge specific capacity is higher than that of the carbon nanotube treated by acid, the discharge specific capacity is higher with the increase of purification temperature, and the discharge specific capacity is the highest at 2800 DEG C and the conductive performance is the best.

[0100] The carbon nanotube conductive slurry prepared by the method is added into the positive material, and the cycle performance is better with the increase of purification temperature and the conductive performance is also better.

[0101] The carbon nanotube conductive slurry prepared by the method is added into the positive material, and the cycle five circle curves are basically coincident, which indicates that the conductive network constructed by the carbon nanotube as a conductive agent at the purification temperature is stable and continuous, and the conductive performance of the carbon nanotube is optimal.

[0102] The above examples only express the embodiments of the application, and the description is more specific and detailed, but it cannot be understood as the limitation of the patent scope of the application. It should be pointed out that for ordinary skilled persons in the art, some modifications and improvements can be made without departing from the concept of the application, which belongs to the protection scope of the application. Therefore, the protection scope of the patent of the application should be subject to the appended claims.

Claims

1. A method for preparing a conductive paste for improving the electrical conductivity of carbon nanotubes, characterized by, The method comprises the following steps: Step (1): a certain amount of mixed solvent is prepared, and a carbon nanotube catalyst is prepared by a sol-gel method using a water bath kettle or an oil bath kettle; Step (2): the catalyst obtained in step (1) is placed in a tube furnace, heated to 200 DEG C in an air atmosphere for 120 min, and then heated to 800 DEG C for 60 min to obtain a catalyst precursor; Step (3): the catalyst precursor in step (2) is ground and sieved, then placed in a tube furnace, and inert gas and reducing gas are introduced and heated to 680 DEG C for 120 min, and finally carbon source is introduced to synthesize carbon nanotubes by chemical vapor deposition method; Step (4): the carbon nanotubes prepared in step (3) are placed in a graphitization furnace, vacuum is drawn to make the pressure value in the furnace-0.1 MPa, then inert gas is introduced, the heating temperature range is 2000-2800 DEG C, and the temperature is kept for 3 h, and the inert gas is cooled to room temperature under the protection of inert gas, and different high-temperature purified carbon nanotubes are obtained; Step (5): a certain amount of purified carbon nanotubes in step (4) are dispersed into stable high-temperature purified carbon nanotube conductive paste by ultrasonic vibration and magnetic stirring; the solvent in step (1) is iron nitrate, aluminum nitrate and citric acid, deionized water is used as solution, and mixing and stirring is carried out for 8-12 h; the mass ratio of iron nitrate, aluminum nitrate and citric acid is 1:0.9:1.1; the deionized water solution is 30-40 ml; the water bath kettle or oil bath kettle in step (1) is heated to above 90 DEG C, and the heating time is more than 12 h; the sieving in step (3) uses a 300 mesh sieve; the inert gas in step (3) is Ar2; the reducing gas is H2; the carbon source is propane; in step (4), 2000-2800 DEG C respectively refers to 2000 DEG C, 2200 DEG C, 2400 DEG C, 2600 DEG C and 2800 DEG C; in step (4), the vacuum drawing should meet the condition that the pressure that the metal catalyst can withstand is higher than the pressure saturation value of the graphitization furnace; in step (5), the ultrasonic dispersion is carried out at a power of 50 W-150 W and a frequency of 40 KHz; the ultrasonic vibration time is more than 2 h; the magnetic stirring power is 220 V / 50 Hz, and the stirring time is more than 3 h; the ultrasonic vibration and magnetic stirring are carried out alternately; in step (5), NMP solution is used, the content of N-methyl pyrrolidone is 94%-96%, and the mass ratio of carbon nanotubes to PVP dispersant is 5:

1.

2. The application of the carbon nanotube conductive paste prepared by the preparation method of claim 1 in lithium ion batteries.

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