A nitrogen-containing multi-carbon-coated graphite-based composite material and its preparation method

Abstract

The present invention relates to the technical field of lithium-ion battery materials, and particularly relates to a nitrogen-containing multi-carbon-coated graphite-based composite material and a preparation method thereof. First, artificial graphite is mixed uniformly with a transition metal and its compound and then spray-dried, and activated by high-temperature steam to obtain porous graphite; carbon nanotubes, a surfactant and glucose are mixed uniformly in water and then subjected to a hydrothermal reaction to obtain carbon nanotubes coated with hard carbon; then, the porous graphite, the hard-carbon-coated carbon nanotubes and the surfactant are mixed uniformly and added to a dilute hydrochloric acid solution containing an oxidant, and pyrrole monomer is added in an ice-water bath to in-situ polymerize polypyrrole on the surfaces of the porous graphite and the carbon nanotubes; thereafter, high-temperature carbonization is carried out in an inert gas to make the polypyrrole into amorphous carbon, and a nitrogen-doped hard-carbon-coated carbon nanotube / porous graphite composite material with an amorphous carbon surface is obtained. It can effectively improve the intercalation and deintercalation ability of lithium ions, reduce the material impedance and improve the battery rate performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery materials, and particularly relates to a nitrogen-containing multi-carbon-coated graphite-based composite material and a preparation method thereof. Background Art

[0002] As a new type of power source, lithium-ion batteries have been widely studied and used in various electric devices due to their advantages such as high working voltage, high energy density, low self-discharge, and environmental friendliness. However, one of the main challenges currently faced by lithium-ion batteries is to meet the market requirements of fast charging and discharging. The rate performance of the battery mainly depends on the anode material. Therefore, the existing fast-charging batteries are mainly lithium titanate batteries and fast-charging graphite batteries. However, the relatively high voltage platform of lithium titanate (about 1.5V for Li + / Li) and the relatively small theoretical specific capacity (175 mAh / g) severely limit the energy density of battery devices.

[0003] Currently, the anode material widely used in the field of lithium-ion batteries is still graphite-based material. During battery charging, lithium ions are released from the positive electrode, and at the solid-liquid interface on the surface of the negative electrode, the solvent shell is removed, and Li + is embedded in graphite and diffuses over a long distance between the layers, especially showing worse kinetic characteristics during fast charging and low-temperature charging. Hard carbon materials are widely sourced, have a large interlayer spacing, and have excellent fast-charging performance, low-temperature performance, and zero expansion characteristics. However, due to its own porous structure and high specific surface area, the material has a low first efficiency (80%) and a low specific capacity (300 Ah / g). The charging rate of graphite is generally less than 2C, which is difficult to meet the fast-charging requirements. Currently, methods such as reducing the graphite particle size and coating graphite with hard carbon / soft carbon can be used to improve the rate performance. For example, patent (application number 201910812070.X) discloses a preparation method of a fast-charging graphite composite material. By coating porous titanium-containing hard carbon and lithium salt on the surface of graphite, the lithium ion extraction channel and the structural defects of hard carbon are improved, the irreversible capacity is reduced, and the first efficiency of the material is increased.

[0004] Most of the existing graphite composite materials mainly perform surface modification on them, including doping with metal / non-metal particles or coating with carbon materials with a larger interlayer spacing to improve the lithium ion insertion / extraction rate. However, the diffusion rate of lithium ions entering the graphite interlayer is still slow. Therefore, it is very necessary to accelerate the diffusion kinetics process of lithium ions inside graphite. The information disclosed in this background art section is only intended to deepen the understanding of the overall background art of the present invention, and should not be regarded as an admission or any form of implication that this information constitutes the prior art known to those skilled in the art. Summary of the Invention

[0005] To overcome the above-mentioned disadvantages of the prior art, one of the objectives of the present invention is to provide a method for preparing a nitrogen-containing multi-carbon-coated graphite-based composite material to improve the lithium ion diffusion rate and the initial Coulomb efficiency.

[0006] A method for preparing a nitrogen-containing multi-carbon-coated graphite-based composite material includes the following steps:

[0007] S01: Artificial graphite and transition metals and their compounds are uniformly mixed in deionized water and then spray-dried to obtain a porous graphite precursor.

[0008] S02: The porous graphite precursor is placed in a tube furnace, an inert gas is introduced, and it is heated to a high temperature at a certain rate and then water vapor is introduced to keep the temperature for several hours to obtain porous graphite.

[0009] In the present invention, the transition metals and their compounds are used to directionally etch and create pores on the artificial graphite with water vapor, and pores perpendicular to the graphite sheet layer can be formed. Among them, the transition metals and their compounds adhere to the graphite surface, which can effectively carry out catalytic activation, break the carbon-carbon bond, reduce the reaction activation energy, and after contacting with water vapor at high temperature, the reaction can preferentially occur at the contact area between the transition metals and their compounds and the graphite surface to form surface pores, enabling the water vapor to further contact the next layer of graphite layer and further etch, continuously penetrate, thereby forming through large pores. The reaction equation is:

[0010] C + H2O → CO + H2

[0011] In this step, without the transition metals and their compounds, the water vapor can only etch at the edge of the graphite layer, and effective pores cannot be formed, and the reaction rate is extremely slow. The present invention directionally etches and creates pores on the porous graphite, providing a channel for the rapid migration of lithium ions in the graphite material and improving the intrinsic rate performance of the graphite material;

[0012] S03: Carbon nanotubes, a surfactant, and glucose are ultrasonically mixed uniformly in water, transferred to a hydrothermal autoclave, and hydrothermal reaction is carried out to carbonize glucose in the hydrothermal environment to obtain amorphous hydrothermal carbon-coated carbon nanotubes. The product is centrifuged, washed 3 times with deionized water, and vacuum dried to obtain carbon nanotubes coated with a hard carbon precursor.

[0013] S04: The porous graphite, carbon nanotubes coated with a hard carbon precursor, and a surfactant are ultrasonically dispersed uniformly, and then added to a 0.1 mol / L dilute hydrochloric acid solution containing an oxidant. After stirring in an ice-water bath environment for 0.5 hours, pyrrole monomer is added, and it is kept at 0 - 4 °C for a period of time to uniformly coat and in-situ polymerize polypyrrole on the surfaces of the porous graphite and the carbon nanotubes coated with a hard carbon precursor. After centrifugal washing, it is vacuum dried at 80 °C for 12 hours to obtain a composite material precursor.

[0014] Due to the non-uniformity of the lithium insertion sites (surface and side surfaces) of the etched porous graphite, the polarization degree is higher than that of the original graphite at low temperature, during the first cycle, and at high rates. As a commonly used method to improve the electrochemical interface of materials, hard carbon coating is relatively compatible with this material and has an obvious improvement effect on this phenomenon. During this process, the carbon nanotubes coated with hard carbon are evenly distributed between and on the surface of the porous graphite flakes, effectively reducing the internal resistance of the electrode while improving the rate performance and mass specific capacity of the composite material; coating polypyrrole on the outer surface of the material and carbonizing it at high temperature to form amorphous carbon and undergoing in-situ N doping is beneficial to improving the electronic conductivity and energy storage capacity. In the present invention, the carbon nanotubes coated with hard carbon precursor are dispersed on the surface of the porous graphite and carbonized, and the formed polypyrrole has a good combination with the porous graphite and the carbon nanotubes coated with hard carbon precursor, promoting the formation of a three-dimensional conductive network structure of the composite material, and a graphite-hard carbon composite material with a three-dimensional conductive network structure can be obtained, which can have both high rate and high capacity. Compared with the carbon nanotube material coated with hard carbon alone or the porous graphite, the specific capacity is higher.

[0015] Under the protection of an inert gas, the above composite material precursor is subjected to high-temperature carbonization treatment. The polypyrrole is transformed into amorphous carbon, and the hard carbon precursor on the surface of the carbon nanotubes is transformed into hard carbon, finally obtaining a nitrogen-doped hard carbon-coated carbon nanotube / porous graphite composite material with amorphous carbon on the surface, which is a graphite-based composite material with nitrogen-containing multi-carbon coating. In step 3 of the present invention, a hard carbon precursor is formed on the surface of the carbon nanotubes. After high-temperature carbonization of this step, the precursor is transformed into hard carbon, forming carbon nanotubes coated with hard carbon. Incorporating nitrogen into the carbon material can form vacancies and dangling bonds around the nitrogen sites after carbonization, thereby improving the conductivity and reactivity of the nitrogen-doped carbon.

[0016] Preferably, in step S01, the transition metal and its compounds are one or more of nickel, nickel oxide, ammonium molybdate, nickel nitrate, and vanadium pentoxide, and the molar ratio of artificial graphite to the transition metal and its compounds is 40-70:1. 3: Compared with pure graphite, the transition metal-oxygen-graphite system requires a lower defect formation energy. Due to the existence of its d orbitals, the transition metal can provide d electrons or empty d orbitals for oxygen in the heating system at high temperature. The T-oxygen bond absorbs activation energy with the carbon-carbon single bond in graphite at high temperature to generate more stable carbon monoxide, thereby undergoing etching. After the etching reaction starts, due to the high mobility of a single metal on the graphite surface, the metal atoms diffuse through the surface and adsorb and migrate to the edge of the graphite with unstable defect energy to continue the reaction. The diffusion rate of lithium ions is increased.

[0017] In the present invention, porous graphite is directionally etched and perforated by using transition metals and their compounds and water vapor to provide channels for the rapid migration of lithium ions within the graphite material, thereby improving the intrinsic rate performance of the graphite material. If the ratio of artificial graphite to transition metals and their compounds is too low, the number of pores formed in the porous graphite will be too small, resulting in a reduction in the rapid transmission paths for lithium ions. On the other hand, if the ratio is too high, excessive etching will occur, leading to an overly high specific surface area of the porous graphite, and consequently, a too low initial Coulombic efficiency of the battery.

[0018] Preferably, in step S01, the concentration of the mixed solution is 25% - 55%, and the mixing method is at least one of ultrasonic mixing and mechanical stirring.

[0019] Preferably, in step S01, the temperature of the spray drying nozzle is 170°C - 190°C. When the temperature is too low, it is difficult for the moisture in the porous graphite precursor to completely volatilize, resulting in a reduced product yield. And 190°C is sufficient for spray drying to be completed, and no higher temperature is required.

[0020] Preferably, in step S02, the inert gas is argon, the heating rate is 2 - 0°C / min, the carbonization temperature is 700 - 1000°C, the heat preservation time is 3 - 7 hours, and the water vapor flow rate is 0.1 - 1 L / min.

[0021] Preferably, in step S03, the carbon nanotubes are at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes, with a required purity greater than 99%, a diameter of 50 - 200 nm, and a length of 5 - 20 μm. The length and diameter of the carbon nanotubes are appropriate. If the carbon nanotubes are too long and thin, they are prone to entanglement and difficult to ultrasonically disperse; if they are short and thick, it is not conducive to the formation of a three-dimensional conductive network structure. The surfactants include sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and cetyltrimethylammonium bromide. One end of the surfactant binds to the surface of the carbon nanotubes, and the other end is a hydrophilic group, which can promote the uniform dispersion of the carbon nanotubes in water.

[0022] Preferably, in step S03, the concentration of the glucose solution is 2 - 6%, and the mass ratio of the carbon nanotubes, surfactants, and glucose solution is 0.1 - 0.5:0.02 - 0.05:100.

[0023] Preferably, in step S03, the heating rate of the hydrothermal reaction is 2 - 5°C / min, the hydrothermal temperature is 170 - 190°C, the heat preservation time is 8 - 12 hours, and the vacuum drying conditions are drying at 80°C and -0.09 Mpa for 12 - 20 hours. The heating rate, hydrothermal temperature, and heat preservation time here are conducive to forming a hard carbon precursor coating layer with a consistent and uniform thickness on the surface of the carbon nanotubes, which is beneficial for ion transport and can prevent the carbon nanotubes from entangling and stacking.

[0024] Preferably, in step S04, the mass ratio of the porous graphite, the hard carbon-coated carbon nanotube precursor, and the surfactant is 50-70:35-50:1-5, the molar ratio of the surfactant to hydrochloric acid, the oxidant, and the pyrrole monomer is 1-4:20:10:2-5, and the heat preservation time is 7-15 hours. Wherein, a porous graphite ratio higher than 50% can ensure that the composite material has a high mass specific capacity; at the same time, the proportion of carbon nanotubes coated with the hard carbon precursor is moderate, which is beneficial to improving the battery rate without reducing the specific capacity; only a small amount of surfactant is needed to effectively disperse the porous graphite and the carbon nanotubes coated with the hard carbon precursor.

[0025] Preferably, in step S05, the high-temperature carbonization treatment temperature range is 800-1000 °C, the heat preservation time is 1-4 hours, and the heating rate is 1-5 °C / min.

[0026] The second object of the present invention is to provide a nitrogen-containing multi-carbon-coated graphite-based composite material, which can improve the electronic conductivity and energy storage capacity and achieve the goal of large-rate charge and discharge.

[0027] A nitrogen-containing multi-carbon-coated graphite-based composite material, the composite material includes a core and a shell coated on the surface of the core. The core includes hard carbon-coated carbon nanotubes and porous graphite. The hard carbon-coated carbon nanotubes are uniformly dispersed on the surface of the porous graphite and interspersed in the porous structure. The shell is nitrogen-containing amorphous carbon.

[0028] The third object of the present invention is to provide a button cell, which uses the above-mentioned nitrogen-containing multi-carbon-coated graphite-based composite material to meet the market requirements of rapid charge and discharge.

[0029] Assemble the nitrogen-containing multi-carbon-coated graphite-based composite material into a button cell. The preparation method of the button cell is as follows: Add a binder, a conductive agent, and a solvent to the nitrogen-containing multi-carbon-coated graphite-based composite material, stir and mix evenly to make a slurry, coat the obtained electrode slurry on a copper foil, and perform vacuum drying, rolling, and cutting to obtain the corresponding electrode sheet.

[0030] Preferably, the binder is CMC, SBR, the conductive agent is conductive agent Super-P, and the solvent is deionized water; and the mass ratio of the nitrogen-containing multi-carbon-coated graphite-based composite material, conductive agent Super-P, binder CMC, and SBR is 90:5:2.5:2.5, and the viscosity of the electrode slurry is 3000-5000 mPa·s.

[0031] Preferably, a metal lithium sheet is used as the counter electrode, a Celgard 2500 (PP material) membrane is used as the separator, and 1 mol / L LiPF6 / EC+DMC+EMC (1:1:1, volume ratio) is used as the electrolyte, and the button cell is assembled in a glove box filled with argon.

[0032] By adopting the above technical solutions, the beneficial effects of the present invention are as follows:

[0033] (1) The present invention uses transition metals and their compounds and water vapor to physically activate graphite. The obtained porous graphite has large and permeable pores and a slightly higher specific surface area, improving the liquid absorption and retention capabilities of the material. It avoids the reduction of the first Coulomb efficiency and effectively shortens the transmission path of lithium ions between graphite layers, improving the electrical conductivity of the material.

[0034] (2) The hard carbon-coated carbon nanotube composite material prepared by the present invention through ultrasonic dispersion and hydrothermal reaction has the characteristics of high consistency and high conductivity, and can avoid the re-agglomeration of carbon nanotubes, providing more insertion / extraction channels during charge and discharge.

[0035] (3) After ultrasonic mixing of porous graphite and the precursor of hard carbon-coated carbon nanotubes, the carbon nanotubes can be uniformly dispersed on the surface of the porous graphite block and interspersed in its large and permeable pores, realizing the point-line combination of carbon-carbon. Then, in-situ polymerized polypyrrole is used for coating. After high-temperature carbonization, the thickness of the amorphous carbon layer is uniform, forming an efficient conductive network with the carbon nanotubes and reducing the surface impedance of the material. The ammonia gas generated during pyrolysis induces in-situ nitrogen-doped hard carbon, which can improve the electronic conductivity and energy storage capacity, achieving the goal of high-rate charge and discharge. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 SEM image of the composite material prepared in Example 3.

[0037] Figure 2 SEM image of the porous graphite obtained in Step 2 of Example 3.

[0038] Figure 3 TEM image of the porous graphite obtained in Step 2 of Example 3.

[0039] Figure 4 Flow chart for the preparation of the nitrogen-containing multi-carbon-coated graphite-based composite material of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0040] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be described in detail below by combining the accompanying drawings and specific embodiments.

[0041] As used herein, "one embodiment" or "an embodiment" refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The appearances of "in one embodiment" in different places in this specification do not all refer to the same embodiment, nor are they separate or selectively exclusive embodiments from other embodiments.

[0042] In the embodiments of the present invention, the sources of commercially available materials are as follows:

[0043] Material composition Commercially available source location Commercial graphite Shenzhen Kejing Zhida Technology Co., Ltd. Nickel nitrate Sinopharm Group Ammonium heptamolybdate Sinopharm Group Vanadium pentoxide Sinopharm Group Ammonium persulfate Sinopharm Group Carbon nanotubes Sinopharm Group Polyvinylpyrrolidone Sinopharm Group Cetyltrimethylammonium bromide Sinopharm Group Glucose Sinopharm Group Dilute hydrochloric acid Sinopharm Group Pyrrole Sinopharm Group

[0044] Example 1

[0045] As Figure 4 shown, the specific preparation of the nitrogen-containing multi-carbon-coated graphite-based composite material in this example is as follows:

[0046] S01: Add 100 g of commercial graphite and 2.423 g of nickel nitrate hexahydrate to 1000 ml of water and stir for 10 h. Pump out the uniformly mixed liquid using a spray dryer at 180 °C to obtain 71.58 g of nickel nitrate / commercial graphite mixed material.

[0047] S02: Place 10 g of the nickel nitrate / commercial graphite mixed material in a tubular furnace. After purging the gas in the furnace with argon at 0.1 L / min, heat it to 900 °C at a rate of 10 °C / min. Subsequently, introduce steam at 0.5 L / min and maintain the temperature at 900 °C for 3 h to obtain porous graphite.

[0048] S03: Add 5 g of carbon nanotubes and 0.5 g of polyvinylpyrrolidone to 1000 ml of 5% glucose solution, ultrasonically mix for 3 h, transfer it to a hydrothermal autoclave, heat it to 180 °C at a rate of 5 °C / min, and keep it warm for 8 h. After centrifuging the obtained product and washing it three times with deionized water, place it in a vacuum drying oven, evacuate to -0.09 Mpa, and dry it at 80 °C for 12 h to obtain a hard carbon-coated carbon nanotube precursor;

[0049] S04: Add 5 g of porous graphite, 5 g of the hard carbon-coated carbon nanotube precursor, and 0.5 g of polyvinylpyrrolidone to 50 ml of water, ultrasonically disperse for 3 h, add 90 ml of 1 mol / L dilute hydrochloric acid solution containing 7.3 g of ferric chloride, stir for 0.5 h in an ice-water bath environment, then add 1.51 g of pyrrole monomer, keep it at 0 °C for 7 h, and then wash it three times by centrifugation with deionized water and dry it in vacuum at 80 °C for 12 h to obtain a hard carbon-coated carbon nanotube / porous graphite precursor with polypyrrole on the surface.

[0050] S05: In a tubular furnace, after purging the gas in the furnace with argon at 0.1 L / min, heat it to 900 °C at a rate of 5 °C / min under argon protection and keep it warm for 3 h to obtain a nitrogen-doped hard carbon-coated carbon nanotube / porous graphite composite material with amorphous carbon on the surface.

[0051] Example 2

[0052] As Figure 4 shown, the specific preparation of the nitrogen-containing multi-carbon-coated graphite-based composite material in this example is as follows:

[0053] S01: Add 100 g of commercial graphite and 1.94 g of ammonium heptamolybdate to 1000 ml of water and stir for 10 h. Pump out the evenly mixed solution through atomization at 180 °C using a spray dryer to obtain 75.45 g of ammonium heptamolybdate / commercial graphite mixed material.

[0054] S02: Place 10 g of ammonium heptamolybdate / commercial graphite mixed material in a tube furnace. After purging the gas in the furnace with argon at a rate of 0.1 L / min, heat it to 950 °C at a rate of 10 °C / min under argon protection. Subsequently, introduce steam at a rate of 1 L / min and maintain the temperature at 900 °C for 3 h to obtain 7.1 g of porous graphite.

[0055] S03: Add 5 g of carbon nanotubes and 0.5 g of polyvinylpyrrolidone to 1000 ml of 5% glucose solution, ultrasonically mix for 3 h, transfer it to a hydrothermal autoclave, heat it to 190 °C at a rate of 5 °C / min, and keep it warm for 8 h. After centrifuging the obtained product and washing it three times with deionized water, place it in a vacuum drying oven, evacuate to -0.09 Mpa, and dry it at 80 °C for 20 h to obtain a carbon nanotube precursor coated with hard carbon;

[0056] S04: Add 5 g of porous graphite, 5 g of carbon nanotube precursor coated with hard carbon, and 0.5 g of polyvinylpyrrolidone to 50 ml of water, ultrasonically disperse for 3 h, add 90 ml of 1 mol / L dilute hydrochloric acid solution containing 10.266 g of ammonium persulfate, stir for 0.5 h in an ice-water bath environment, then add 0.6 g of pyrrole monomer, keep it at 0 °C for 10 h, centrifuge and wash it three times with deionized water, and dry it in vacuum at 80 °C for 12 h to obtain a hard carbon-coated carbon nanotube / porous graphite precursor with polypyrrole on the surface.

[0057] S05: In a tube furnace, after purging the gas in the furnace with argon at a rate of 0.1 L / min, heat it to 1000 °C at a rate of 5 °C / min under argon protection and keep it warm for 3 h to obtain a nitrogen-doped hard carbon-coated carbon nanotube / porous graphite composite with amorphous carbon on the surface.

[0058] Example 3

[0059] As Figures 1 - 4 shown, the specific preparation of the nitrogen-containing multi-carbon-coated graphite-based composite material in this example is as follows:

[0060] S01: Add 100 g of commercial graphite and 0.76 g of vanadium pentoxide to 1000 ml of water and stir for 10 h. Pump out the evenly mixed solution through atomization at 180 °C using a spray dryer to obtain 68.7 g of vanadium pentoxide / commercial graphite mixed material.

[0061] S02: Place 10 g of ammonium heptamolybdate / commercial graphite mixed material in a tube furnace. After purging the gas in the furnace with argon at a rate of 0.1 L / min, heat it to 850 °C at a rate of 10 °C / min. Subsequently, introduce steam at a rate of 0.5 L / min and maintain the temperature of 850 °C for 3 h to obtain 8.5 g of porous graphite.

[0062] S03: Ultrasonically mix 5 g of carbon nanotubes, 0.5 g of cetyltrimethylammonium bromide, and 1000 ml of 5% glucose in water for 3 h. Transfer it to a hydrothermal autoclave and heat it to 180 °C at a rate of 5 °C / min, and keep it at this temperature for 8 h. After centrifuging the obtained product and washing it three times with deionized water, place it in a vacuum drying oven, evacuate it to -0.09 Mpa, and dry it at 80 °C for 12 h to obtain a carbon nanotube precursor coated with hard carbon;

[0063] S04: Add 5 g of porous graphite, 5 g of carbon nanotube precursor coated with hard carbon, and 0.5 g of cetyltrimethylammonium bromide to 50 ml of aqueous solution and ultrasonically disperse for 3 h. Add 27 ml of 1 mol / L dilute hydrochloric acid solution containing 3.13 g of ammonium persulfate. After stirring for 0.5 h in an ice-water bath environment, add 0.45 g of pyrrole monomer, keep it at 0 °C for a period of time, wash it three times by centrifugation with deionized water, and dry it in vacuum at 80 °C for 12 h to obtain hard carbon-coated carbon nanotubes / porous graphite with polypyrrole on the surface.

[0064] S05: In a tube furnace, after purging the gas in the furnace with argon at a rate of 0.1 L / min, heat it to 900 °C at a rate of 5 °C / min under argon protection and keep it at this temperature for 3 h to obtain a nitrogen-doped hard carbon-coated carbon nanotubes / porous graphite composite material with amorphous carbon on the surface.

[0065] Comparative Example 1

[0066] In this comparative example, the raw material (commercial graphite) is not subjected to directional etching treatment. Directly compound commercial graphite with the carbon nanotube precursor coated with hard carbon and in-situ wrap polypyrrole on the outer layer, and finally perform high-temperature carbonization to obtain a nitrogen-doped hard carbon-coated carbon nanotubes / graphite composite material with amorphous carbon on the surface. The specific experimental steps are as follows:

[0067] S01: Ultrasonically mix 5 g of carbon nanotubes, 0.5 g of cetyltrimethylammonium bromide, and 1000 ml of 5% glucose in water for 3 h. Transfer it to a hydrothermal autoclave and heat it to 180 °C at a rate of 5 °C / min, and keep it at this temperature for 8 h. After centrifuging the obtained product and washing it three times with deionized water, place it in a vacuum drying oven, evacuate it to -0.09 Mpa, and dry it at 80 °C for 12 h to obtain a carbon nanotube precursor coated with hard carbon;

[0068] S02: Add 5 g of commercial graphite, 5 g of hard carbon-coated carbon nanotube precursor, and 0.5 g of cetyltrimethylammonium bromide into 50 ml of aqueous solution, and ultrasonically disperse for 3 h. Then add 27 ml of 1 mol / L dilute hydrochloric acid solution containing 3.13 g of ammonium persulfate. After stirring for 0.5 h in an ice-water bath environment, add 0.45 g of pyrrole monomer, keep it at 0 °C for a period of time, wash it three times by centrifugation with deionized water, and vacuum dry at 80 °C for 12 h to obtain hard carbon-coated carbon nanotube / graphite with polypyrrole on the surface.

[0069] S03: In a tube furnace, after purging the gas in the furnace with argon at a rate of 0.1 L / min, heat it to 900 °C at a rate of 5 °C / min under argon protection and keep it for 3 h to obtain a nitrogen-doped hard carbon-coated carbon nanotube / graphite composite with amorphous carbon on the surface.

[0070] Comparative Example 2

[0071] In this comparative example, hard carbon is not wrapped on the surface of carbon nanotubes. Porous graphite and carbon nanotubes are uniformly dispersed, and polypyrrole is in-situ wrapped on the outer layer, and finally high-temperature carbonization is carried out to obtain a nitrogen-doped carbon nanotube / porous graphite composite with amorphous carbon on the surface. The specific experimental steps are as follows:

[0072] S01: Add 100 g of commercial graphite and 0.76 g of vanadium pentoxide into 1000 ml of water and stir for 10 h. Pump out the uniformly mixed liquid using a spray dryer at 180 °C to obtain 68.7 g of vanadium pentoxide / commercial graphite mixed material.

[0073] S02: Place 10 g of vanadium pentoxide / commercial graphite mixed material in a tube furnace. After purging the gas in the furnace with argon at a rate of 0.1 L / min, heat it to 850 °C at a rate of 10 °C / min. Then introduce water vapor at a rate of 0.5 L / min and keep the temperature at 850 °C for 3 h to obtain 8.5 g of porous graphite.

[0074] S03: Add 5 g of porous graphite, 5 g of carbon nanotubes, and 0.5 g of cetyltrimethylammonium bromide into 50 ml of aqueous solution, and ultrasonically disperse for 3 h. Then add 27 ml of 1 mol / L dilute hydrochloric acid solution containing 3.13 g of ammonium persulfate. After stirring for 0.5 h in an ice-water bath environment, add 0.45 g of pyrrole monomer, keep it at 0 °C for a period of time, and in-situ polymerize polypyrrole on the surface of porous graphite and carbon nanotubes. Then wash it three times by centrifugation with deionized water and vacuum dry at 80 °C for 12 h to obtain carbon nanotube / porous graphite with polypyrrole on the surface.

[0075] S04: In a tube furnace, after purging the gas in the furnace with argon at a rate of 0.1 L / min, heat it to 900 °C at a rate of 5 °C / min under argon protection and keep it for 3 h to obtain a nitrogen-doped carbon nanotube / porous graphite composite with amorphous carbon on the surface.

[0076] Comparative Example 3

[0077] In this comparative example, polypyrrole is not wrapped on the surface of the porous graphite / hard carbon precursor-coated carbon nanotubes. The porous graphite and the hard carbon precursor-coated carbon nanotubes are uniformly dispersed and subjected to high-temperature carbonization to obtain a hard carbon-coated carbon nanotube / porous graphite composite material. The specific experimental steps are as follows:

[0078] S01: Add 100 g of commercial graphite and 0.76 g of vanadium pentoxide to 1000 ml of water and stir for 10 h. The uniformly mixed liquid is atomized and pumped out at 180 °C using a spray dryer to obtain 68.7 g of vanadium pentoxide / commercial graphite mixed material.

[0079] S02: Place 10 g of ammonium heptamolybdate / commercial graphite mixed material in a tubular furnace. After purging the gas in the furnace with argon at a rate of 0.1 L / min, heat it to 850 °C at a rate of 10 °C / min. Subsequently, introduce water vapor at a rate of 0.5 L / min and maintain the temperature at 850 °C for 3 h to obtain 8.5 g of porous graphite.

[0080] S03: Ultrasonically disperse 5 g of carbon nanotubes and 0.5 g of cetyltrimethylammonium bromide in 1000 ml of 5% glucose solution for 3 h, transfer it to a hydrothermal autoclave, heat it to 180 °C at a rate of 5 °C / min, and keep it warm for 8 h. After centrifuging the obtained product and washing it three times with deionized water, put it into a vacuum drying oven, evacuate it to -0.09 Mpa, and dry it at 80 °C for 12 h to obtain a hard carbon-coated carbon nanotube precursor;

[0081] S04: Add 5 g of porous graphite, 5 g of hard carbon-coated carbon nanotube precursor, and 0.5 g of cetyltrimethylammonium bromide to 50 ml of water, ultrasonically disperse for 3 h, centrifugally wash with deionized water three times, and vacuum dry at 90 °C for 12 h.

[0082] S05: In a tubular furnace, after purging the gas in the furnace with argon at a rate of 0.1 L / min, heat it to 900 °C at a rate of 5 °C / min under argon protection and keep it warm for 3 h to obtain a hard carbon-coated carbon nanotube / porous graphite composite material.

[0083] Test data

[0084] Assemble the composite materials prepared in Examples 1 - 3 and Comparative Examples 1 - 3 of the present invention into coin cells, and mark them correspondingly as follows: The coin cell assembled with the composite material of Example 1 is marked as A1, the coin cell assembled with the composite material of Example 2 is marked as A2, the coin cell assembled with the composite material of Example 3 is marked as A3, the coin cell assembled with the composite material of Comparative Example 1 is marked as B1, the coin cell assembled with the composite material of Comparative Example 2 is marked as B2, and the coin cell assembled with the composite material of Comparative Example 3 is marked as B3.

[0085] The preparation method of the button battery is as follows: Add a binder, a conductive agent, and a solvent to a nitrogen-doped hard carbon-coated carbon nanotube / porous graphite composite material with an amorphous carbon surface, stir and mix evenly to make a slurry, coat the obtained electrode slurry on a copper foil, and perform vacuum drying, rolling, and cutting to obtain the corresponding electrode sheet.

[0086] As a preferred solution, the binder is CMC and SBR, the conductive agent is conductive agent Super-P, and the solvent is deionized water; and the mass ratio of the active material, conductive agent Super-P, binder CMC, and SBR is 90:5:2.5:2.5, and the viscosity of the electrode slurry is 3000 - 5000 mPa·s.

[0087] Using a lithium metal sheet as the counter electrode, a Celgard 2500 (PP material) membrane as the separator, and 1 mol / L LiPF6 / EC + DMC + EMC (1:1:1, volume ratio) as the electrolyte, assemble the button battery in a glove box filled with argon.

[0088] 1. Initial coulombic efficiency, rate performance, and cycle retention rate at room temperature

[0089] Place button batteries A1, A2, A3, B1, B2, and B3 on a Wuhan Blue Electric CT2001A battery tester for testing respectively. For the first charge and discharge, discharge at a rate of 0. I C and charge at a rate of 0.1 C. The initial coulombic efficiency is the ratio of the charge capacity to the discharge capacity. Perform constant current charge and discharge tests at different current rates, with the voltage range from 0.005 V to 1.5 V. The measured initial coulombic efficiency, capacities at different rates, capacity retention rate, and cycle stability are shown in Table 1:

[0090] Table 1 Performance of the composite materials in Examples 1 - 3 and Comparative Examples 1 - 3

[0091]

[0092] It can be seen from Table 1 that the lowest initial coulombic efficiency of the composite anode materials prepared in Examples 1 - 3 is 90.6%, and the lowest discharge capacity at a rate of 1 C is 410.3 mAh / g; while the highest discharge capacity at a rate of 1 C of the composite anode materials prepared in the comparative examples is 405.8 mAh / g, and the highest initial discharge efficiency is 94.2%.

[0093] The initial charge and discharge efficiency and the discharge capacity at a rate of 1 C of the lithium-ion battery with the nitrogen-doped hard carbon-coated carbon nanotube / porous graphite composite material with an amorphous carbon surface of the present invention are significantly higher than those of Comparative Examples 2 - 3; and the specific surface area and rate of the composite anode material prepared in the present invention are significantly higher than those of Comparative Examples 1 - 3.

[0094] That is, the composite material of the present invention creates pores by directionally etching porous graphite, providing channels for the rapid migration of lithium ions within the graphite material and improving the intrinsic rate performance of the graphite material; the carbon nanotubes coated with hard carbon are uniformly distributed between and on the surfaces of the porous graphite flakes, effectively improving the rate performance and mass specific capacity of the composite material while reducing the internal resistance of the electrode; polypyrrole is coated on the outer surface of the material and carbonized at high temperature to form amorphous carbon and in-situ nitrogen doping occurs, which is beneficial to improving the electronic conductivity and energy storage capacity.

[0095] 2. Liquid absorption and retention capacity of the electrode sheet

[0096] The negative electrode sheets obtained when preparing lithium-ion batteries in Examples 1-3 and Comparative Examples 1-3 were tested for liquid absorption rate according to the following method: In a glove box under an argon atmosphere, negative electrode sheets made of nitrogen-doped hard carbon-coated carbon nanotubes / porous graphite composites with amorphous carbon on the surface in Examples 1-3 and Comparative Examples 1-3 were respectively taken, with the specification: circular electrode sheet with a diameter of 14 mm. The electrolyte was sucked into a burette and titrated onto the electrode sheet respectively until there was no obvious electrolyte on the surface of the electrode sheet, and the time and the added amount of the electrolyte were recorded, and thus the liquid absorption rate was obtained.

[0097] The test method for the liquid retention rate: Calculate the theoretical liquid injection volume V1 according to the electrode sheet parameters, place the electrode sheet in the theoretical electrolyte, and let it stand for 24 h. Weigh the electrolyte V2 absorbed by the electrode sheet, and finally obtain the liquid retention rate = V2 / V1 * 100%; the test results are shown in Table 2:

[0098] Table 2 Liquid absorption and retention capacities of the negative electrode sheets in Examples 1-3 and Comparative Examples 1-3

[0099] Electrode material Liquid absorption rate (mL / min) Liquid retention rate (electrolyte volume at 24h / electrolyte volume at 0h) Example 1 9.0 94.5％ Example 2 8.7 95.2％ Example 3 8.8 94.7％ Comparative Example 1 4.9 85.1％ Comparative Example 2 8.1 88.6％ Comparative Example 3 7.8 87.9％

[0100] As can be seen from Table 2, the liquid absorption and retention capacities of the negative electrode sheets prepared in Examples 1-3 are significantly better than those in Comparative Examples 1-3. That is, the negative electrode sheets made of the nitrogen-doped hard carbon-coated carbon nanotubes / porous graphite composites with amorphous carbon on the surface of the present invention have more through large pore structures and a slightly higher specific surface area, improving the liquid absorption and retention capacities of the material.

[0101] Inspired by the ideal embodiments of the present invention as described above, through the above description, relevant staff can completely make various changes and modifications without departing from the technical idea of this invention. The technical scope of this invention is not limited to the content in the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A preparation method of a nitrogen-containing multi-carbon-coated graphite-based composite material, characterized in that, It includes the following steps: S01: Mix artificial graphite with transition metals and their compounds uniformly in deionized water, and then perform spray drying to obtain a porous graphite precursor; S02: Place the porous graphite precursor in a tube furnace, introduce an inert gas, heat it up to 700 - 1000 °C at a constant speed, then introduce water vapor for heat preservation, and use the transition metals and their compounds to etch and create pores directionally on the artificial graphite to obtain porous graphite; S03: Ultrasonically mix carbon nanotubes, a surfactant, and a glucose solution with a concentration of 2% - 6% in water according to a mass ratio of 0.1 - 0.5:0.02 - 0.05:100, transfer it to a hydrothermal autoclave for hydrothermal reaction, with a heating rate of 2 - 5 °C / min, a hydrothermal temperature of 170 - 190 °C, and a heat preservation time of 8 - 12 hours. After the product is centrifuged, washed, and dried in vacuum, a hard carbon-coated carbon nanotube precursor is obtained; S04: Mix the porous graphite, the hard carbon-coated carbon nanotube precursor, and a surfactant and ultrasonically disperse them uniformly. The hard carbon-coated carbon nanotubes are evenly distributed between and on the surfaces of the porous graphite flakes. Add a 0.1 mol / L dilute hydrochloric acid solution containing an oxidant, stir for 0.5 hours in an ice-water bath environment, then add pyrrole monomer, and keep it at 0 - 4 °C for 7 - 15 hours to in-situ polymerize polypyrrole on the surfaces of the porous graphite and the hard carbon-coated carbon nanotube precursor. After centrifugation and washing, dry it in vacuum to obtain a composite precursor; S05: Under the protection of an inert gas, perform high-temperature carbonization treatment on the composite precursor. The polypyrrole is transformed into amorphous carbon, and the hard carbon precursor on the surface of the carbon nanotubes is transformed into hard carbon to obtain a nitrogen-containing multi-carbon-coated graphite-based composite material.

2. The preparation method of a nitrogen-containing multi-carbon-coated graphite-based composite material according to claim 1, characterized in that, In step S01, the transition metals and their compounds are one or more of nickel, nickel oxide, ammonium molybdate, nickel nitrate, and vanadium pentoxide, and the molar ratio of the artificial graphite to the transition metals and their compounds is 40 - 70:

1.

3. The preparation method of a nitrogen-containing multi-carbon-coated graphite-based composite material according to claim 1, characterized in that, In step S01, the concentration of the mixed solution is 25% - 55%, and the mixing method is at least one of ultrasonic mixing and mechanical stirring; the temperature of the spray drying nozzle is 170 °C - 190 °C.

4. The preparation method of a nitrogen-containing multi-carbon-coated graphite-based composite material according to claim 1, characterized in that, In step S02, the inert gas is argon, the heating rate is 2 - 10 °C / min, carbonize at 700 - 1000 °C, the heat preservation time is 3 - 7 hours, and the water vapor flow rate is 0.1 - 1 L / min.

5. The preparation method of a nitrogen-containing multi-carbon-coated graphite-based composite material according to claim 1, characterized in that, In step S03, the carbon nanotubes are at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes, with a purity greater than 99%, a diameter of 50 - 200 nm, and a length of 5 - 20 μm. The surfactant is one or more of polyvinylpyrrolidone, sodium dodecyl sulfate, sodium dodecylbenzenesulfonate, and cetyltrimethylammonium bromide.

6. The preparation method of a nitrogen-containing multi-carbon-coated graphite-based composite material according to claim 1, characterized in that, In step S03, the vacuum drying conditions are: drying at 80 °C and -0.09 Mpa for 12 - 20 hours.

7. The preparation method of a nitrogen-containing multi-carbon-coated graphite-based composite material according to claim 1, characterized in that, In step S04, the mass ratio of the porous graphite, the hard carbon-coated carbon nanotube precursor, and the surfactant is 50-70:35-50:1-5, the molar ratio of the surfactant to hydrochloric acid, the oxidant, and the pyrrole monomer is 1-4:20:10:2-5, and the heat preservation time is 7-15 hours.

8. The preparation method of a nitrogen-containing multi-carbon-coated graphite-based composite material according to claim 1, characterized in that, In step S05, the temperature of the high-temperature carbonization treatment is 800-1000 °C, the heat preservation time is 1-4 hours, and the heating rate is 1-5 °C / min.

9. A nitrogen-containing multi-carbon-coated graphite-based composite material, prepared by using the preparation method according to any one of claims 1 to 8, characterized in that, The composite material includes a core and a shell coated on the surface of the core. The core includes hard carbon-coated carbon nanotubes and porous graphite. The hard carbon-coated carbon nanotubes are uniformly dispersed on the surface of the porous graphite and interspersed in the porous structure. The shell is nitrogen-containing amorphous carbon.

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