Nitrogen-doped carbon nanotube and preparation method thereof, positive pole piece and lithium ion battery

Abstract

The invention provides a nitrogen-doped carbon nanotube and a preparation method thereof, a positive pole piece and a lithium ion battery, and the specific preparation process is as follows: firstly modifying some hydrophilic groups such as carboxyl and hydroxyl on the surface of the carbon nanotube in a pickling manner to reduce Van der Waals' force among the carbon nanotubes and improve dispersibility, and preparing the nitrogen-doped carbon nanotube by taking a nicotinamide compound as a nitrogen source; uniformly stirring and dispersing with the acid-washed carbon nanotubes in an aqueous solution, drying to form a precursor, and calcining the precursor at high temperature; carbon atoms on the surface of the doped carbon nanotube are replaced by nitrogen atoms to form pyridine nitrogen and graphite nitrogen, the electron and spin characteristics of a carbon skeleton can be destroyed, and the nitrogen atoms can be used as electron donors, so that the conductivity of the carbon nanotube and the intercalation capability of lithium ions can be regulated and controlled; according to the preparation method, the dispersibility of the nitrogen-doped carbon nanotubes and the conductivity of the carbon nanotubes can be improved, the internal resistance of the positive pole piece is reduced, and the rate capability and the cycle performance of the lithium ion battery are finally improved.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion battery preparation, and particularly to nitrogen-doped carbon nanotubes, a preparation method thereof, a positive electrode sheet, and a lithium-ion battery. Background Art

[0002] With the continuous progress of technology and the increasing energy demand, the importance of energy storage technology has become increasingly prominent. Among various energy storage technologies, lithium-ion batteries have been widely used in fields such as portable electronic devices and electric vehicles due to their advantages such as high energy density and long cycle life. However, existing cathode materials for lithium-ion batteries, such as lithium iron phosphate, have problems such as poor conductivity, large polarization, and poor rate performance, which limit the further improvement of their performance.

[0003] In order to improve the conductivity of lithium iron phosphate cathode materials, the following solutions are mainly adopted in the prior art:

[0004] (1) Increasing the amount of conductive carbon: By increasing the amount of conductive carbon in the cathode slurry, the conductivity of the cathode can be improved. However, this will lead to a decrease in the proportion of the main material lithium iron phosphate in the cathode slurry, thereby reducing the energy density of the battery.

[0005] (2) Using carbon nanotubes: Carbon nanotubes have excellent electrical conductivity and mechanical properties and can be added as a conductive agent to the cathode slurry. However, since carbon nanotubes are non-polar materials, they are prone to agglomeration and difficult to disperse into the cathode slurry, resulting in a significant reduction in their conductivity. Summary of the Invention

[0006] The present invention provides a nitrogen-doped carbon nanotube, a preparation method thereof, a positive electrode sheet, and a lithium-ion battery, which improve the conductivity and rate performance of the lithium-ion battery.

[0007] The present invention discloses a preparation method of a nitrogen-doped carbon nanotube, and the preparation method includes:

[0008] Providing acidified carbon nanotubes, mixing the acidified carbon nanotubes with a nicotinamide compound to obtain a precursor;

[0009] Calcinating the precursor under an anaerobic condition to obtain nitrogen-doped carbon nanotubes.

[0010] Further, providing acidified carbon nanotubes includes: performing pickling treatment on carbon nanotubes.

[0011] Further, performing pickling treatment on the carbon nanotubes includes:

[0012] Placing the carbon nanotubes in sulfuric acid; wherein, the mass fraction of the sulfuric acid is greater than or equal to 70%;

[0013] Heating and stirring the mixture of the carbon nanotubes and the sulfuric acid;

[0014] Dilute the mixture to neutral.

[0015] Furthermore, the temperature range for the heating and stirring is 60 - 90 °C; the time range for the heating and stirring is 2 - 5 h.

[0016] Furthermore, the mass ratio of the acidified carbon nanotubes to the nicotinamide compound is 1:(0.8 - 2); the mixing is stirring mixing.

[0017] Furthermore, the mass ratio of the acidified carbon nanotubes to the nicotinamide compound is 1:(0.5 - 2).

[0018] Furthermore, the temperature of the stirring mixing is 60 - 95 °C.

[0019] Furthermore, the nicotinamide compound is amino nicotinamide or nicotinamide;

[0020] The amino nicotinamide is selected from at least one of 2 - amino nicotinamide, 6 - amino nicotinamide, 5 - amino nicotinamide, 4 - amino nicotinamide, 2,6 - diamino nicotinamide, 2,5 - diamino nicotinamide, 2,4 - diamino nicotinamide, 5,6 - diamino nicotinamide, 4,6 - diamino nicotinamide, 4,5 - diamino nicotinamide, 2,5,6 - triamino nicotinamide, 2,4,6 - triamino nicotinamide, 2,4,5 - triamino nicotinamide, 4,5,6 - triamino nicotinamide, and 2,4,5,6 - tetranicotinamide.

[0021] Furthermore, obtaining the nitrogen - doped carbon nanotubes by calcining the precursor under an anaerobic condition includes:

[0022] Calcining under an inert atmosphere; the inert atmosphere is nitrogen.

[0023] Furthermore, the temperature range for the calcining is 800 - 1000 °C; the time range for the calcining is 90 - 300 min.

[0024] The present invention also discloses a nitrogen - doped carbon nanotube prepared by using the above - mentioned preparation method of the nitrogen - doped carbon nanotube.

[0025] The present invention also discloses a positive electrode plate of a lithium - ion battery, and the positive electrode plate includes the above - mentioned nitrogen - doped carbon nanotube.

[0026] The present invention also discloses a lithium - ion battery including the above - mentioned positive electrode plate.

[0027] Compared with the prior art, the present invention has at least the following technical effects:

[0028] In the preparation process of nitrogen-doped carbon nanotubes, first, some hydrophilic groups such as carboxyl and hydroxyl groups are modified on the surface of carbon nanotubes by pickling to reduce the van der Waals force between carbon nanotubes and improve the dispersibility; a nicotinamide molecule is used as a nitrogen source, and after being stirred and dispersed evenly with the pickled carbon nanotubes in an aqueous solution, it is dried to form a precursor, and the precursor is calcined at a high temperature; the carbon atoms on the surface of the calcined carbon nanotubes are replaced by nitrogen atoms to form pyridine nitrogen and graphitic nitrogen. This structure can destroy the electronic and spin properties of the carbon skeleton, and the nitrogen atoms, as electron donors, improve the conductivity and form more active sites on the surface. These active sites provide additional lithium-ion transport channels; at the same time, the surface-modified hydrophilic groups improve the wettability, thereby enhancing the electrolyte penetration ability. The synergistic effect of the above characteristics improves the conductivity of carbon nanotubes and reduces the internal resistance of the positive electrode plate. The higher conductivity of carbon nanotubes and the lower internal resistance of the positive electrode plate ultimately improve the rate performance and cycling performance of lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic flow chart of the preparation method of nitrogen-doped carbon nanotubes in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The following will describe a method for preparing nitrogen-doped carbon nanotubes and a lithium-ion battery according to the present invention with reference to the schematic diagrams, in which the preferred embodiments of the present invention are shown. It should be understood that those skilled in the art can modify the present invention described herein and still achieve the advantageous effects of the present invention. Therefore, the following description should be understood as a broad guidance for those skilled in the art and not as a limitation to the present invention.

[0031] In the following paragraphs, the present invention will be described more specifically by way of example with reference to the drawings. According to the following description, the advantages and features of the present invention will be clearer. It should be noted that the drawings are all in a very simplified form and use non-precise scales, only for the purpose of conveniently and clearly assisting in explaining the embodiments of the present invention.

[0032] Embodiment 1

[0033] This embodiment discloses a method for preparing nitrogen-doped carbon nanotubes, and the preparation method includes:

[0034] S1. Provide acidified carbon nanotubes, mix the acidified carbon nanotubes with a nicotinamide compound to obtain a precursor;

[0035] S2. Calcine the precursor under an anaerobic condition to obtain nitrogen-doped carbon nanotubes.

[0036] In this embodiment, first, some hydrophilic groups such as carboxyl groups and hydroxyl groups are modified on the surface of carbon nanotubes by pickling to reduce the van der Waals force between carbon nanotubes and improve the dispersibility. A nicotinamide compound is used as a nitrogen source and stirred and dispersed evenly with the pickled carbon nanotubes in an aqueous solution and then dried to form a precursor. The precursor is calcined at a high temperature. The carbon atoms on the surface of the doped carbon nanotubes are replaced by nitrogen atoms to form pyridine nitrogen and graphitic nitrogen, which can destroy the electronic and spin properties of the carbon skeleton. Moreover, the nitrogen atoms, as electron donors, improve the conductivity and form more active sites on the surface. These active sites provide additional lithium-ion transport channels. At the same time, the surface-modified hydrophilic groups improve the wettability, thereby enhancing the electrolyte penetration ability. The synergistic effect of the above characteristics improves the conductivity of carbon nanotubes and reduces the internal resistance of the positive electrode plate. The higher conductivity of carbon nanotubes and the lower internal resistance of the positive electrode plate ultimately improve the rate performance and cycling performance of lithium-ion batteries.

[0037] In step S1, the acidified carbon nanotubes can be obtained after pickling the carbon nanotubes. The pickling treatment of the carbon nanotubes includes the steps:

[0038] S11. Place the carbon nanotubes in sulfuric acid; wherein, the mass fraction of the sulfuric acid is greater than or equal to 70%.

[0039] S12. Heat and stir the mixture of the carbon nanotubes and the sulfuric acid.

[0040] S13. Dilute the mixture to neutral.

[0041] It can be understood that in step S1, both the heating and stirring time and the heating and stirring temperature will affect the internal structure of the carbon nanotubes. Too high a temperature and too long a stirring time will cause damage to the structure of the carbon nanotubes. Too low a temperature or too short a stirring time will result in insufficient dispersibility of the carbon nanotubes. Therefore, those skilled in the art need to select appropriate heating temperatures and stirring times according to the actual situation.

[0042] In a specific embodiment, the preferred heating temperature is: 60 - 90 °C, for example: 70 °C, 80 °C, 85 °C, and 88 °C.

[0043] In another specific embodiment, the preferred stirring time is: 2 - 5 h (hours), for example: 3 h, 4 h, and 4.5 h.

[0044] In another specific embodiment, deionized water is used to dilute and wash the mixture to neutral.

[0045] One possible specific operation of the above pickling treatment is as follows: Put a certain amount of carbon nanotubes into a beaker, add a certain amount of concentrated sulfuric acid to the beaker so that the liquid level covers the powder, heat and stir at 80 °C for 3 h, and then dilute and wash the mixture with deionized water until it is neutral to obtain pickled carbon nanotubes.

[0046] In this embodiment, the pickling treatment can introduce hydrophilic groups, reduce the van der Waals force between carbon nanotubes, improve the gel, and thus improve the dispersibility of carbon nanotubes in the slurry.

[0047] In step S1, the carbon nanotubes and the nicotinamide compound can be stirred and mixed in water at a mass ratio of 1:(0.2 - 2.5). However, if the addition amount of nicotinamide is too high, the content of nitrogen doping will be excessive, which will damage the conductive network of the carbon nanotubes themselves and cause a decrease in the conductivity of the carbon nanotubes. If the addition amount of nicotinamide is too low, the reduction of the film resistance is less, resulting in a decrease in the conductivity of the carbon nanotubes. According to the above principle, in this embodiment, the mass ratio of the carbon nanotubes to the nicotinamide compound is preferably 1:

[0048] (0.5 - 2) ， For example: 1:0.5, 1:0.7 or 1:0.75; within the above mass ratio range, the nitrogen doping content is moderate and the conductivity of the carbon nanotubes is relatively good. In addition, a more preferred mass ratio range is 1:(0.8 - 2), for example: 1:0.9

[0049] 、1:1.2 or 1:1.5. Within the above mass ratio range, the nitrogen doping content is relatively good and the conductivity of the carbon nanotubes is better.

[0050] In a specific embodiment, the stirring temperature in the above step S2 is 60 - 95 °C, for example 65 °C, 70 °C and 80 °C.

[0051] Furthermore, in step S1, the nicotinamide compound is amino nicotinamide or nicotinamide. In this embodiment, the carbon nanotubes are mixed with one or more nicotinamide compounds and then dried.

[0052] In a specific embodiment, the amino nicotinamide includes but is not limited to at least one of 2 - amino nicotinamide, 6 - amino nicotinamide, 5 - amino nicotinamide, 4 - amino nicotinamide, 2,6 - diamino nicotinamide, 2,5 - diamino nicotinamide, 2,4 - diamino nicotinamide, 5,6 - diamino nicotinamide, 4,6 - diamino nicotinamide, 4,5 - diamino nicotinamide, 2,5,6 - triamino nicotinamide, 2,4,6 - triamino nicotinamide, 2,4,5 - triamino nicotinamide, 4,5,6 - triamino nicotinamide and 2,4,5,6 - tetranicotinamide.

[0053] It can be understood that in this embodiment, the addition amount of nicotinamide provides a necessary nitrogen source for the synthesis of carbon nanotubes, and the increase of amino substituents promotes the incorporation of nitrogen atoms into the carbon nanotube structure, jointly enhancing the nitrogen doping level. However, the precise regulation of nitrogen content is crucial. Excessive nitrogen may lead to impaired conductivity and affect battery performance, while too little nitrogen is not sufficient to significantly improve performance. Therefore, technicians need to select appropriate ratios of amino nicotinamide and substituent types according to the actual situation to prepare carbon nanotubes with suitable nitrogen content.

[0054] In a specific embodiment, a possible specific operation for preparing the above precursor is as follows: Mix the acid-washed carbon nanotubes and 5-aminonicotinamide in an aqueous solution at a mass ratio of 1:0.5, stir at 95 °C, and obtain the precursor through freeze-drying.

[0055] In another specific embodiment, another possible specific operation for preparing the above precursor is as follows: Mix the acid-washed carbon nanotubes with a mixture of 4,6-diaminonicotinamide and 2,4,5-triaminonicotinamide in an aqueous solution at a mass ratio of 1:0.5, stir at 95 °C, and obtain the precursor through freeze-drying.

[0056] In step S2, the precursor calcination is carried out in an inert gas atmosphere. For example: The precursor is calcined at a high temperature in an inert atmosphere such as nitrogen, argon, or helium.

[0057] Specifically, during the high-temperature calcination process, the nicotinamide compound (nitrogen source) will decompose and release nitrogen atoms. These nitrogen atoms can chemically react with the carbon atoms on the surface of the carbon nanotubes to form a nitrogen-doped structure, such as pyridine nitrogen and graphitic nitrogen. In this embodiment, high temperature can promote the diffusion of nitrogen atoms into the interior of the carbon nanotubes, thereby increasing the degree of nitrogen doping. The higher the degree of nitrogen doping, the stronger the conductivity of the carbon nanotubes and the ability to embed lithium ions, which is more beneficial to improving the performance of lithium-ion batteries. Calcination can remove impurities on the surface of the carbon nanotubes, such as organic substances and metal ions, thereby improving its purity; calcination can also change the surface structure of the carbon nanotubes, making its surface rougher, thereby increasing its specific surface area and the number of active sites.

[0058] In a specific embodiment, the place for the high-temperature calcination can be a tube furnace.

[0059] Furthermore, in step S3, the temperature range for the high-temperature calcination is 800 - 1000 °C. For example: 820 °C, 900 °C, or 920 °C.

[0060] Furthermore, in step S3, the calcination time for the high-temperature calcination is 90 - 300 min. For example: 100 min, 150 min, 240 min, or 260 min.

[0061] It can be understood that by using the above calcination temperature, nitrogen atoms can be effectively doped into the carbon nanotubes to form active sites such as pyridine nitrogen and graphitic nitrogen. By using the above calcination time, it can ensure that the nitrogen doping reaction proceeds fully and avoid excessive graphitization of the carbon nanotubes.

[0062] A possible specific operation for nitrogen doping is as follows: Place the precursor in a tube furnace and heat it for 100 min under an inert nitrogen atmosphere at 820 °C to obtain nitrogen-doped carbon nanotubes.

[0063] Another possible specific operation for nitrogen doping is as follows: Place the precursor in a tube furnace and heat it for 200 min under an inert argon atmosphere at 920 °C to obtain nitrogen-doped carbon nanotubes.

[0064] In a specific embodiment, a complete preparation process of nitrogen-doped carbon nanotubes is as follows: Put a certain amount of carbon nanotubes into a beaker, add a certain amount of concentrated sulfuric acid into the beaker, and the liquid level should cover the powder. Heat and stir at 80 °C for 3 h. Then pour the mixture into deionized water for dilution and washing, and wash the mixture until it is neutral to obtain pickled carbon nanotubes; Mix the pickled carbon nanotubes and nicotinamide in a mass ratio of 1:0.2 in an aqueous solution, stir at 95 °C, and obtain the precursor through freeze-drying; Place the precursor in a tube furnace and heat it at 800 °C for 240 min under a nitrogen atmosphere to obtain the first group of nitrogen-doped carbon nanotubes.

[0065] Example Two

[0066] This embodiment provides a positive electrode plate for a lithium battery. Among them, the nitrogen-doped carbon nanotubes in the positive electrode plate are prepared by the preparation method of nitrogen-doped carbon nanotubes disclosed in Example One. The positive electrode plate for a lithium battery provided in this embodiment also has the advantages of the nitrogen-doped carbon nanotubes disclosed in Example One, which will not be elaborated here.

[0067] Specifically, the method for preparing the positive electrode plate for a lithium battery includes:

[0068] S100. Weigh lithium iron phosphate, polyvinylidene fluoride, nitrogen-doped carbon nanotubes, and conductive carbon accurately according to mass percentages;

[0069] S200. Put the weighed materials into a ball mill, add an appropriate amount of NMP as a solvent; turn on the ball mill for ball milling to obtain a uniformly mixed slurry;

[0070] S300. Uniformly coat the prepared positive electrode slurry on an aluminum foil current collector;

[0071] S400. After coating, put the aluminum foil into a vacuum oven and dry it at an appropriate temperature to remove the solvent.

[0072] S500. The dried positive electrode sheet is subjected to embossing roller pressing and slitting operations in sequence.

[0073] In a specific embodiment, the composition of the positive electrode slurry is: 96.5 wt% of lithium iron phosphate, 2 wt% of polyvinylidene fluoride, 0.3 wt% of the above-mentioned nitrogen-doped carbon nanotubes, and 1.2 wt% of conductive carbon.

[0074] In another specific embodiment, the temperature of the drying operation is 100 °C.

[0075] Example 3

[0076] This embodiment provides a lithium-ion battery, which includes a positive electrode sheet obtained by using the positive electrode sheet preparation method disclosed in Example 2. The advantages of the nitrogen-doped carbon nanotubes disclosed in Example 2 are also possessed by the lithium-ion battery provided in this embodiment, and will not be elaborated here.

[0077] Furthermore, the lithium-ion battery further includes a negative electrode sheet. The preparation process of the negative electrode sheet includes: mixing artificial graphite, conductive agent carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) according to a certain weight ratio, adding an appropriate amount of deionized water and stirring for an appropriate time to form a uniform negative electrode slurry; uniformly coating the negative electrode slurry on a copper foil and drying it in a vacuum oven at 100 °C, and then obtaining the negative electrode sheet through rolling and slitting.

[0078] Furthermore, the lithium-ion battery further includes an electrolyte. The preparation process of the electrolyte includes: in a glove box filled with argon (moisture < 10 ppm (0.01%), oxygen content < 10 ppm (0.01%)), mixing ethylene carbonate and ethyl methyl carbonate in a mass ratio of 3:7, slowly adding 3% by mass of vinylene carbonate and 12.5% of lithium hexafluorophosphate to the mixed solution, and stirring until it is completely dissolved to obtain the electrolyte.

[0079] Finally, the above-prepared positive and negative electrode sheets and polyethylene separator are assembled into a soft-pack battery cell and baked at 100 °C for 48 h to ensure that the water content of the positive and negative electrode sheets and the separator is all below 200 ppm (0.02%). Then, the battery cell is transferred to a glove box to inject the above electrolyte. After the electrolyte injection, the battery cell is further subjected to steps such as standing, formation, aging, shaping, and grading to obtain a complete lithium-ion battery.

[0080] In this embodiment, adding nitrogen-doped carbon nanotubes to the positive electrode can significantly improve the conductivity and performance of the lithium battery. Through the introduction and surface modification of nitrogen atoms, the dispersion and conductive network of the material are effectively improved, the internal resistance of the battery is reduced, thereby improving the rate performance, cycle life, and safety of the battery, and reducing the cost.

[0081] Example 4

[0082] This example is a comparative example. A plurality of groups of lithium-ion batteries are prepared by the method of Example 1. Each group of lithium-ion batteries contains nitrogen-doped carbon nanotubes with different material ratios, and the performance of the nitrogen-doped carbon nanotubes under different material ratios is compared. In addition, two control groups are set up in this example to compare the performance with different groups of lithium-ion batteries containing the nitrogen-doped carbon nanotubes prepared in Example 1.

[0083] Group 1 is a lithium-ion battery prepared by the preparation steps in Example 3. The specific preparation steps are as follows:

[0084] The first step is to prepare nitrogen-doped carbon nanotubes:

[0085] (1) Pretreatment of carbon nanotubes: Put a certain amount of carbon nanotubes into a beaker, add a certain amount of concentrated sulfuric acid to the beaker, and the liquid level covers the powder. Heat and stir at 80 °C for 3 h. Then pour the mixture into deionized water for dilution and washing, and wash the mixture until it is neutral to obtain pickled carbon nanotubes.

[0086] (2) Preparation of the precursor: Mix the pickled carbon nanotubes and nicotinamide in a mass ratio of 1:0.2 in an aqueous solution, stir at 95 °C, and obtain the precursor by freeze-drying.

[0087] (3) Nitrogen doping process: Place the precursor in a tube furnace and heat it at 800 °C for 240 min under a nitrogen atmosphere to obtain nitrogen-doped carbon nanotubes.

[0088] The second step is to prepare the positive electrode plate:

[0089] (1) Preparation of the positive electrode slurry: Mix 96.5 wt% of lithium iron phosphate, 2 wt% of polyvinylidene fluoride, 0.3 wt% of the above-mentioned nitrogen-doped carbon nanotubes, and 1.2 wt% of conductive carbon to form a mixed solution. Add an appropriate amount of N-methylpyrrolidone to the mixed solution and stir for an appropriate time to form a uniform positive electrode slurry.

[0090] (2) Preparation of the positive electrode plate: Coat the positive electrode slurry evenly on the aluminum foil and dry it in a vacuum oven at 100 °C, and then obtain the positive electrode plate after rolling and slitting operations.

[0091] The third step is to prepare the negative electrode plate:

[0092] (1) Preparation of the negative electrode slurry: Mix artificial graphite, conductive agent carbon black, binder styrene-butadiene rubber, and thickener sodium carboxymethyl cellulose in a certain weight ratio, add an appropriate amount of deionized water and stir for an appropriate time to form a uniform negative electrode slurry.

[0093] (2) Preparation of the negative electrode slurry: The negative electrode slurry was uniformly coated on a copper foil and dried in a vacuum oven at 100 °C, and then rolled and slit to obtain a negative electrode sheet.

[0094] Fourth step, preparation of the electrolyte: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 10 ppm), ethylene carbonate and ethyl methyl carbonate were mixed evenly at a mass ratio of 3:7. Lithium vinyl carbonate with a mass fraction of 3% and lithium hexafluorophosphate with a mass fraction of 12.5% were slowly added to the mixed solution and stirred until completely dissolved to obtain the electrolyte.

[0095] Fifth step, preparation of the lithium-ion battery: The positive and negative electrode sheets and the ethylene separator prepared above were assembled into a soft-pack battery cell and baked at 100 °C for 48 h to ensure that the water content of the positive and negative electrode sheets and the separator was below 200 ppm. Then, the battery cell was transferred to a glove box for liquid injection, and the electrolyte used was the above-mentioned electrolyte. After liquid injection, the battery cell was further subjected to steps such as standing, formation, aging, shaping, and grading to obtain a complete lithium-ion battery.

[0096] Group 2 was also a lithium-ion battery prepared by using the preparation steps in Example 3. The preparation process of the precursor in this group was: The acid-washed carbon nanotubes and nicotinamide were mixed in an aqueous solution at a mass ratio of 1:0.8, stirred at 95 °C, and freeze-dried to obtain the precursor. Compared with Group 1, except for the different way of preparing the precursor, the remaining preparation steps and the order of step implementation were the same as those in Group 1.

[0097] Group 3 was also a lithium-ion battery prepared by using the preparation steps in Example 3. The preparation process of the precursor in this group was: The acid-washed carbon nanotubes and nicotinamide were mixed in an aqueous solution at a mass ratio of 1:1.2, stirred at 95 °C, and freeze-dried to obtain the precursor. Compared with Group 1, except for the different way of preparing the precursor, the remaining preparation steps and the order of step implementation were the same as those in Group 1.

[0098] Group 4 was also a lithium-ion battery prepared by using the preparation steps in Example 3. The preparation process of the precursor in this group was: The acid-washed carbon nanotubes and nicotinamide were mixed in an aqueous solution at a mass ratio of 1:2.5, stirred at 95 °C, and freeze-dried to obtain the precursor. Compared with Group 1, except for the different way of preparing the precursor, the remaining preparation steps and the order of step implementation were the same as those in Group 1.

[0099] Group 5 was also a lithium-ion battery prepared by using the preparation steps in Example 3. The preparation process of the precursor in this group was: The acid-washed carbon nanotubes and 6-aminonicotinamide were mixed in an aqueous solution at a mass ratio of 1:0.8, stirred at 95 °C, and freeze-dried to obtain the precursor. Compared with Group 1, except for the different way of preparing the precursor, the remaining preparation steps and the order of step implementation were the same as those in Group 1.

[0100] Take the nitrogen-doped carbon nanotubes in the above-mentioned Group 1 - Group 5, and use scanning electron microscopy - energy dispersive spectrometer (SEM-EDS) to characterize the nitrogen doping content of each sample. The test results are as follows:

[0101] Table 1

[0102] Group Nitrogen content / % Group 1 0.85 Group 2 5.23 Group 3 8.64 Group 4 14.37 Group 5 6.53

[0103] From the results in Table 1, it is verified that the nitrogen content in the nitrogen-doped carbon nanotubes gradually increases with the increase of the niacinamide content as disclosed in Example 1. In addition, under the same conditions, the nitrogen doping amount in the carbon nanotubes prepared using 6-aminonicotinamide is higher than that prepared using niacinamide, which is mainly due to the more amino substitutions of 6-aminonicotinamide.

[0104] Furthermore, set Control Group 6. In Control Group 6, no precursor preparation and doping treatment are carried out, and the remaining preparation steps and the implementation order of the steps are the same as those in Group 1.

[0105] Set Control Group 7. In Control Group 7, no pretreatment of carbon nanotubes is carried out, and the remaining preparation steps and the implementation order of the steps are the same as those in Group 1.

[0106] Take the positive electrode slurries in the above-mentioned Group 1 - Group 7, and test the viscosity after standing for different times. When the viscosity > 30000 mPa·s, it is determined that the slurry gels, and the time at this time is defined as the anti-gel time. The test results are as follows:

[0107] Table 2

[0108] Group Anti-gel time / h Group 1 32 Group 2 35 Group 3 34 Group 4 30 Group 5 34 Group 6 (control group) 32 Group 7 (control group) 22

[0109] From the results in Table 2, compared with Group 7, the anti-gel time of the positive electrode slurry in Group 6 is longer. This is mainly because some hydrophilic groups such as carboxyl and hydroxyl groups can be modified on the surface of carbon nanotubes through pickling, reducing the van der Waals force between carbon nanotubes and improving the dispersibility. In addition, compared with Group 7, the anti-gel times of Group 1 - 6 are basically at the same level, proving that the improvement of gel by nitrogen-doped carbon nanotubes mainly stems from the pickling process.

[0110] Take the positive electrode plates in the above-mentioned Group 1 - Group 7, and use the four-probe method to test the sheet resistance of the positive electrode plates. The test results are shown in Table 3:

[0111] Table 3

[0112]

[0113]

[0114] From the results in Table 3, the diaphragm resistances of Group 1 and Group 2 are close, indicating that simple pickling cannot improve the conductivity of carbon nanotubes. Compared with Group 6 and Group 7, the reduction in diaphragm resistance of Group 1 is not obvious, which is related to the factor of too little nitrogen doping. The diaphragm resistances of Group 2, Group 3 and Group 5 are all significantly lower than that of Group 6, mainly due to the improvement of the conductivity of carbon nanotubes after nitrogen doping. The diaphragm resistance of Group 4 is slightly higher than that of Group 6, probably because excessive nitrogen doping destroys the conductive network of carbon nanotubes themselves.

[0115] Furthermore, the rate performance tests were carried out on the lithium-ion batteries prepared in the above Group 1 - Group 7:

[0116] The specific test method is as follows: The lithium-ion battery is subjected to 1 charge-discharge test at a rate of 0.5C / 0.5C, and the charge-discharge voltage range is 2.5 - 3.65V. Then, it is subjected to 1 charge-discharge test at a rate of 0.5C / 1C, and the rate discharge capacity retention rate is the 1C discharge capacity / 0.5C discharge capacity. The test results are shown in Table 4.

[0117] Table 4

[0118] Group Retention rate of discharge capacity at multiple rates Example 1 95.3％ Example 2 97.5％ Example 3 98.2％ Example 4 94.3％ Example 5 98.1％ Group 6 (control group) 95.2％ Group 7 (control group) 95.1％

[0119] It can be seen from Table 4 that compared with Group 6 and Group 7, the rate discharge capacity retention rates of Group 2, Group 3 and Group 5 are significantly improved, mainly due to the improvement of the conductive ability of carbon nanotubes after nitrogen doping, which reduces polarization. The rate discharge capacity retention rates of Group 1 and Group 6 are at the same level, because the nitrogen doping content is too small to have a great impact on conductivity. The rate discharge capacity retention rate of Group 4 is lower than that of Group 6, probably because excessive nitrogen doping destroys the conductive network of carbon nanotubes themselves, resulting in greater polarization.

[0120] In summary, at least the following can be proved through the above experiments:

[0121] (1) The nitrogen content in carbon nanotubes: is mainly related to the content of nicotinamide and the number of amino substituents. The greater the content of nicotinamide, the higher the nitrogen doping content. Under the same nicotinamide content, the higher the number of amino substituents, the higher the nitrogen doping content.

[0122] (1) The anti-gel time: is mainly related to whether pickling treatment is carried out. Pickling treatment can introduce hydrophilic groups such as carboxyl and hydroxyl groups on the surface of carbon nanotubes. Compared with carbon nanotubes without pickling treatment, the anti-gel time of the positive electrode slurry of pickled carbon nanotubes is longer, which indicates that pickling treatment can effectively improve the dispersibility of carbon nanotubes, prevent slurry gelation, and ensure the processing performance of the battery.

[0123] (2) Diaphragm resistance: It is mainly related to whether nitrogen doping is carried out and the content of nitrogen doping in carbon nanotubes. Compared with carbon nanotubes without nitrogen doping, the diaphragm resistance of the nitrogen-doped carbon nanotube positive electrode sheet is lower, which indicates that nitrogen doping can effectively improve the conductivity of carbon nanotubes and reduce the internal resistance of the battery. In addition, if the content of nitrogen doping is too low, the reduction of diaphragm resistance is less; if the content of nitrogen doping is moderate, it can significantly assist in reducing the diaphragm resistance and improving the conductivity of carbon nanotubes; if the content of nitrogen doping is excessive, it will damage the conductive network of carbon nanotubes themselves, resulting in a decrease in the conductivity of carbon nanotubes.

[0124] (3) Rate performance of lithium-ion batteries: It is mainly related to whether nitrogen doping is carried out and the content of nitrogen doping. Compared with carbon nanotubes without nitrogen doping, the retention rate of the rate discharge capacity of the lithium-ion battery with a nitrogen-doped carbon nanotube positive electrode is higher, which indicates that nitrogen doping can effectively improve the rate performance of the battery, enabling it to maintain a good capacity under fast charge and discharge conditions. In addition, if the content of nitrogen doping is too low, the change in rate performance is less; if the content of nitrogen doping is appropriate, it can significantly improve the rate performance; if the content of nitrogen doping is excessive, it will damage the conductive network of carbon nanotubes themselves, resulting in a greater polarization and causing damage to the rate performance.

[0125] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modifications and variations.

Claims

1. A method for preparing nitrogen-doped carbon nanotubes, characterized in that, The preparation method includes: providing acidified carbon nanotubes, mixing the acidified carbon nanotubes with a nicotinamide compound to obtain a precursor; calcining the precursor under an anaerobic condition to obtain nitrogen-doped carbon nanotubes.

2. The preparation method of the nitrogen-doped carbon nanotubes according to claim 1, wherein, Providing acidified carbon nanotubes includes: performing pickling treatment on carbon nanotubes.

3. The preparation method of the nitrogen-doped carbon nanotubes according to claim 2, wherein, Performing pickling treatment on carbon nanotubes includes: placing the carbon nanotubes in sulfuric acid; wherein, the mass fraction of the sulfuric acid is greater than or equal to 70%; heating and stirring the mixture of the carbon nanotubes and the sulfuric acid; diluting the mixture to neutrality.

4. The preparation method of the nitrogen-doped carbon nanotubes according to claim 3, characterized in that, The temperature range of the heating and stirring is 60-90°C; the time range of the heating and stirring is 2-5 h.

5. The preparation method of the nitrogen-doped carbon nanotubes according to claim 1, characterized in that, The mass ratio of the acidified carbon nanotubes to the nicotinamide compound is 1:(0.8-2); the mixing is stirring mixing.

6. The preparation method of the nitrogen-doped carbon nanotubes according to claim 5, wherein, The mass ratio of the acidified carbon nanotubes to the nicotinamide compound is 1:(0.5-2).

7. The method for preparing the nitrogen-doped carbon nanotubes according to claim 5 or 6, characterized in that, The temperature of the stirring mixing is 60-95°C.

8. The preparation method of the nitrogen-doped carbon nanotubes according to claim 1, characterized in that, The nicotinamide compound is amino nicotinamide or nicotinamide; The amino nicotinamide is selected from at least one of 2-amino nicotinamide, 6-amino nicotinamide, 5-amino nicotinamide, 4-amino nicotinamide, 2,6-diamino nicotinamide, 2,5-diamino nicotinamide, 2,4-diamino nicotinamide, 5,6-diamino nicotinamide, 4,6-diamino nicotinamide, 4,5-diamino nicotinamide, 2,5,6-triamino nicotinamide, 2,4,6-triamino nicotinamide, 2,4,5-triamino nicotinamide, 4,5,6-triamino nicotinamide, and 2,4,5,6-tetranicotinamide.

9. The preparation method of the nitrogen-doped carbon nanotubes according to claim 1, characterized in that, Calcining the precursor under an anaerobic condition to obtain nitrogen-doped carbon nanotubes includes: performing calcination in an inert atmosphere; the inert atmosphere is nitrogen.

10. The preparation method of the nitrogen-doped carbon nanotubes according to claim 1, characterized in that, The temperature range of the calcination is 800-1000°C; the time range of the calcination is 90-300 min.

11. A nitrogen-doped carbon nanotube, characterized in that, Prepared by using the preparation method of the nitrogen-doped carbon nanotubes as described in any one of claims 1-10.

12. A positive electrode sheet, characterized in that, The positive electrode sheet includes the nitrogen-doped carbon nanotubes as described in claim 11.

13. A lithium-ion battery, characterized in that, Including the positive electrode sheet as described in claim 12.

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