Nitrogen-doped hard carbon material, preparation method therefor, use thereof, electrode sheet, and electrochemical device

Abstract

The application discloses a kind of nitrogen-doped hard carbon materials and preparation method, application, pole piece and electrochemical device thereof, wherein the preparation method of nitrogen-doped hard carbon material includes the following steps: first heat treatment is carried out to mixture A, to obtain nitrogen-doped hard carbon material;Mixture A includes porous carbon and g-C3N4.The nitrogen-doped hard carbon material obtained by heat treatment of the mixture of porous carbon and g-C3N4 has a low specific surface area, and its use in lithium ion battery materials can improve the initial efficiency of the battery, while the nitrogen in the nitrogen-doped hard carbon material of the application is doped in carbon in the form of edge nitrogen, and the addition of g-C3N4 can promote the increase of graphite-like microcrystalline structure of carbon material, which is used in lithium ion battery materials to improve the capacity on the basis of ensuring better initial efficiency and improving the capacity.

Description

Technical Field

[0001] This invention relates to a nitrogen-doped hard carbon material, its preparation method, applications, electrodes, and electrochemical devices. Background Technology

[0002] With technological advancements and rapid societal development, the demands on lithium-ion batteries are increasing. In today's fast-paced lifestyle, people are increasingly seeking to improve the performance of lithium-ion batteries. Currently, the negative electrode material used in batteries is generally graphite, but its specific capacity has reached its limit, its structure lacks long-term stability, and it is difficult to maintain high-current discharge for extended periods. Therefore, there is a need to develop alternative materials with superior performance. Compared to graphite, hard carbon has an isotropic structure, an amorphous microstructure, and larger interlayer spacing, which facilitates rapid lithium-ion diffusion. It exhibits excellent cycle performance, rate performance, and safety, making it a focus again in the field of power lithium-ion batteries.

[0003] However, hard carbon also suffers from drawbacks such as low initial coulombic efficiency and low reversible capacity, requiring doping modification to achieve higher coulombic efficiency. Simultaneously, doping modification can increase the active sites of carbon materials, thereby improving their lithium storage capacity. CN109301220A discloses a method for preparing nitrogen-doped hard carbon materials, which involves acid washing of plant resources, followed by immersion in a nitrogen source aqueous solution to obtain a pre-product; then, under a protective atmosphere, the pre-product is heat-treated to obtain a nitrogen-doped hard carbon material with a porous structure. CN109768218A discloses a nitrogen-doped hard carbon lithium-ion battery anode material. This patent involves hydrothermally treating a biomass carbon source, then uniformly mixing it with a choline chloride-type ionic liquid in a specific ratio, pre-treating it, and then carbonizing it at high temperature to obtain a nitrogen-doped biomass carbon material, which can be applied to lithium-ion battery anode materials. Doping hard carbon materials with nitrogen can improve capacity to a certain extent, but excessive doping increases surface defects, leading to a decrease in initial efficiency. Summary of the Invention

[0004] The purpose of this invention is to address the problems of low capacity and first-efficiency of existing nitrogen-doped hard carbon materials, and to provide a nitrogen-doped hard carbon material, its preparation method, applications, electrodes, and electrochemical devices. The nitrogen-doped hard carbon material of this invention possesses both good capacity and high first-efficiency.

[0005] The present invention solves the above-mentioned technical problems through the following technical solutions:

[0006] This invention provides a method for preparing nitrogen-doped hard carbon material, which includes the following steps: subjecting mixture A to a first heat treatment to obtain nitrogen-doped hard carbon material;

[0007] The mixture A comprises porous carbon and g-C3N4.

[0008] In some embodiments of the present invention, the mass ratio of the porous carbon to the g-C3N4 is 1:(0.5-2), preferably 1:(0.8-1.2), for example 1:0.5, 1:1 or 1:2.

[0009] In some embodiments of the present invention, the temperature of the first heat treatment is 1000 to 1200°C, for example, 1000°C or 1200°C.

[0010] In some embodiments of the present invention, the first heat treatment time is 1 to 4 hours, for example 2 hours.

[0011] In some embodiments of the present invention, the preparation method of the mixture A includes: mixing the porous carbon and the g-C3N4 to obtain the mixture A.

[0012] In some embodiments of the present invention, the specific surface area of ​​the porous carbon is 150–800 m². 2 / g, preferably 300-400m 2 / g, for example 235.4m 2 / g, 373.6m 2 / g or 693.6m 2 / g.

[0013] In some embodiments of the present invention, the average pore size of the porous carbon is 1 to 15 nm, preferably 1 to 6 nm, more preferably 2 to 5 nm, for example 1.5 nm, 2.7 nm or 10.2 nm.

[0014] In some embodiments of the present invention, the total pore volume of the porous carbon is 0.1–1.0 cm³. 3 / g, preferably 0.2–0.5cm 3 / g, for example 0.12cm 3 / g, 0.46cm 3 / g or 0.6cm 3 / g.

[0015] In some embodiments of the invention, the volume of micropores in the porous carbon accounts for 20-95% of the total pore volume of the porous carbon, preferably 50-80%, for example 21%, 70% or 90%.

[0016] In this invention, the particle size of the porous carbon is the conventional market-demanded particle size in the art. In some embodiments, the D50 particle size of the porous carbon is 5 to 10 μm, for example, 5 μm.

[0017] In this invention, the porous carbon can be commercially available or prepared using conventional methods in the art.

[0018] In some embodiments of the present invention, the method for preparing the porous carbon includes the following steps:

[0019] (1) The asphalt is subjected to a second heat treatment in an oxygen-containing atmosphere to obtain a preliminary material, wherein the temperature of the second heat treatment is higher than the softening point temperature of the asphalt.

[0020] (2) The mixture B is subjected to a third heat treatment to obtain porous carbon; the mixture B includes the preliminary material and the zinc source; the temperature of the third heat treatment is higher than the evaporation temperature of the zinc source.

[0021] In this invention, the temperature of the second heat treatment is related to the softening point of the asphalt.

[0022] In some embodiments of the present invention, the temperature of the second heat treatment is 20 to 30°C higher than the softening point of the asphalt.

[0023] In some embodiments of the present invention, the softening point temperature of the asphalt is 180–300°C, for example, 200°C. If the softening point temperature of the asphalt is too low, the amount of light components increases, leading to a lower yield; if the temperature is too high, the cost increases. A temperature of 180–300°C is preferred.

[0024] In some embodiments of the present invention, the temperature of the second heat treatment is 150–300°C.

[0025] In some embodiments of the present invention, the second heat treatment time is 12 to 24 hours, for example 16 hours.

[0026] In this invention, an excessively large D50 particle size of the asphalt will result in incomplete internal oxidation, while an excessively small particle size will make the powder difficult to process. In some embodiments, the D50 particle size of the asphalt is 5–20 μm, for example, 10 μm.

[0027] In this invention, the oxygen-containing atmosphere is conventional in the art, and in some embodiments, the oxygen-containing atmosphere is an air atmosphere.

[0028] In some embodiments of the present invention, the gas flow rate of the oxygen-containing atmosphere is 1 to 5 L / min, for example 2 L / min.

[0029] In some embodiments of the present invention, the step of pulverizing the asphalt is further included before subjecting the asphalt to the second heat treatment.

[0030] The pulverization is conventional in the art, such as an air jet mill or a rod mill. Preferably, the air jet mill is a flat-type air jet mill, and the grading frequency of the flat-type air jet mill is preferably 25–40 Hz, for example, 30 Hz, and the fan frequency is preferably 20–30 Hz, for example, 25 Hz.

[0031] In this invention, the zinc source is a conventional zinc source used for pore formation in the art. In some embodiments, the zinc source is a zinc compound, preferably one or more of ZnCl2, zinc acetate, and zinc acetic acid.

[0032] In some embodiments of the present invention, the mass ratio of the preliminary material to the zinc source is 1:(0.2 to 2.5), preferably 1:(0.4 to 0.6), for example 1:0.5, 1:1 or 1:2.

[0033] In this invention, the temperature of the third heat treatment is related to the evaporation temperature of the zinc source.

[0034] In some embodiments of the present invention, the temperature of the third heat treatment is 850–950°C, for example 900°C.

[0035] In some embodiments of the present invention, the third heat treatment time is 2 to 6 hours, for example 4 hours.

[0036] In some embodiments of the invention, the third heat treatment is performed in an inert protective gas. The inert protective gas is conventional in the art, such as nitrogen and / or argon.

[0037] In some embodiments of the present invention, the mixture B further includes the steps of pulverizing and sieving the porous carbon after the third heat treatment.

[0038] The pulverization is conventional in the art, such as an air jet mill or a rod mill. The air jet mill is preferably a flat air jet mill, and the grading frequency of the flat air jet mill is preferably 50-70 Hz, for example 60 Hz, and the fan frequency is preferably 30-50 Hz, for example 40 Hz.

[0039] The sieving process uses conventional sieves in the art, and the particle size of the porous carbon after sieving is 5–10 μm.

[0040] In this invention, the particle size of the g-C3N4 is conventional in the art. In some embodiments, the D50 particle size of the g-C3N4 is 5 to 20 μm, for example, 5 μm.

[0041] The present invention also provides a nitrogen-doped hard carbon material prepared by the aforementioned method for preparing nitrogen-doped hard carbon material.

[0042] The present invention also provides a nitrogen-doped hard carbon material comprising porous hard carbon particles and nitrogen doped in the porous hard carbon particles, wherein the nitrogen includes edge nitrogen;

[0043] The nitrogen-doped hard carbon material has a specific surface area of ​​5–100 m². 2 / g;

[0044] The nitrogen-doped hard carbon material includes a graphite-like microcrystalline structure.

[0045] In some embodiments of the present invention, the nitrogen content is 2 to 7%, for example 2%, 3.2%, 5%, 5.6% or 7%, which refers to the mass fraction of nitrogen in the nitrogen-doped hard carbon material.

[0046] In some embodiments of the invention, the edge nitrogen is pyrrole nitrogen and / or pyridine nitrogen.

[0047] In some embodiments of the present invention, the nitrogen-doped hard carbon material has a specific surface area of ​​10–60 m². 2 / g, for example, 10.7m 2 / g, 12.1m 2 / g, 18.4m 2 / g, 21.3m 2 / g, 32.6m 2 / g or 54.5m 2 / g.

[0048] In some embodiments of the present invention, the total pore volume of the nitrogen-doped hard carbon material is 0.005–0.1 m³. 2 / g, preferably 0.008~0.05m 2 / g, for example 0.009m 2 / g, 0.012m 2 / g, 0.013m 2 / g, 0.022m 2 / g, 0.023m 2 / g or 0.034m 2 / g.

[0049] In some embodiments of the present invention, the nitrogen-doped hard carbon material has an average pore size of 1 to 3 nm, preferably 1.5 to 2 nm, for example 1.5 nm, 1.7 nm, 1.8 nm, 2.1 nm or 2.5 nm.

[0050] In some embodiments of the present invention, the volume of the micropores in the nitrogen-doped hard carbon material accounts for 35-70% of the total pore volume of the nitrogen-doped hard carbon material, preferably 50-60%, for example 42.7%, 44.7%, 50.1%, 52.1%, 52.3% or 54.3%.

[0051] In this invention, "micropore" refers to a pore with a diameter of less than 2 nm.

[0052] In some embodiments of the present invention, the D50 particle size of the porous hard carbon particles is 5 to 10 μm, for example 5 μm.

[0053] The present invention also provides an application of the aforementioned nitrogen-doped hard carbon material as a negative electrode material in lithium-ion batteries.

[0054] The present invention also provides an electrode comprising the aforementioned nitrogen-doped hard carbon material.

[0055] The present invention also provides an electrochemical device comprising the aforementioned electrode.

[0056] The positive and progressive effects of this invention are as follows:

[0057] This invention obtains a nitrogen-doped hard carbon material with a low specific surface area by heat-treating a mixture of porous carbon and g-C3N4. When used in lithium-ion battery materials, it can improve the initial efficiency of the battery. At the same time, the nitrogen in the nitrogen-doped hard carbon material of this invention is doped into the carbon in the form of edge nitrogen, and the addition of g-C3N4 can promote the increase of the graphite-like microcrystalline structure of the carbon material. When used in lithium-ion battery materials, it can improve the capacity while ensuring good initial efficiency. Attached Figure Description

[0058] Figure 1 The figures show the nitrogen adsorption-desorption isotherms of the nitrogen-doped hard carbon materials in Examples 1-3.

[0059] Figure 2 The first charge-discharge curve of the nitrogen-doped hard carbon material in Example 2 is shown.

[0060] Figure 3 The images show the XRD patterns of the nitrogen-doped hard carbon material of Example 2 and the hard carbon material of Comparative Example 1. Detailed Implementation

[0061] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0062] In the following examples, g-C3N4 was purchased from Maclean's Reagent Network, and the zinc chloride powder was industrial grade zinc chloride.

[0063] The nitrogen content in the following examples is tested using the EDS surface scanning method.

[0064] Example 1

[0065] (1) The LQ-200 (softening point is 200℃) asphalt was pulverized to a particle size D50 = 10μm using a flat air jet mill. The grading frequency of the flat air jet mill was 30Hz and the blower frequency was 25Hz.

[0066] (2) Add the crushed asphalt material to the drum furnace, the volume of the material is half of the volume of the drum furnace, under air atmosphere, the gas flow rate is 2L / min, the temperature is kept constant at 220~230℃ for 16h, and the preliminary material is obtained after cooling.

[0067] (3) Mix the initial material with zinc chloride powder at a mass ratio of 1:0.5 and load it into a graphite crucible. Use an atmosphere furnace to perform pre-carbonization treatment under a nitrogen atmosphere. The carbonization temperature is 900℃ and the temperature is kept at 900℃ for 4 hours to obtain porous carbon, named DK-BHC-0.5.

[0068] (4) The DK-BHC-0.5 obtained in step (3) is pulverized to a particle size D50 = 5 μm using a flat air jet mill. The grading frequency of the flat air jet mill is 60 Hz and the fan frequency is 40 Hz. Then, the pulverized material is mixed with g-C3N4 with a D50 particle size of 5 μm at a mass ratio of 1:1. The mixed material is loaded into a graphite crucible and kept at 1000℃ for 2 h in a nitrogen atmosphere. Then, it is cooled naturally to obtain nitrogen-doped hard carbon material.

[0069] The nitrogen content of the nitrogen-doped hard carbon material is 4.3%.

[0070] Example 2

[0071] (1) The LQ-200 (softening point is 200℃) asphalt was pulverized to a particle size D50 = 10μm using a flat air jet mill. The grading frequency of the flat air jet mill was 30Hz and the blower frequency was 25Hz.

[0072] (2) Add the crushed asphalt material to the drum furnace, the volume of the material is half of the volume of the drum furnace, under air atmosphere, the gas flow rate is 2L / min, the temperature is kept constant at 220~230℃ for 16h, and after cooling, the preliminary material is obtained.

[0073] (3) Mix the preliminary material with zinc chloride powder in a mass ratio of 1:1 and load it into a graphite crucible. Use an atmosphere furnace to perform pre-carbonization treatment under a nitrogen atmosphere. The carbonization temperature is 900℃ and the temperature is held at 900℃ for 4 hours to obtain porous carbon, named DK-BHC-1.

[0074] (4) The DK-BHC-1 obtained in step (3) is pulverized to a particle size D50 = 5 μm using a flat air jet mill. The grading frequency of the flat air jet mill is 60 Hz and the fan frequency is 40 Hz. Then, the pulverized material is mixed with g-C3N4 with a D50 particle size of 5 μm at a mass ratio of 1:1. The mixed material is loaded into a graphite crucible and kept at 1000℃ for 2 h in a nitrogen atmosphere. Then, it is cooled naturally to obtain nitrogen-doped hard carbon material.

[0075] The nitrogen content of nitrogen-doped hard carbon materials is 5%.

[0076] Example 3

[0077] (1) The LQ-200 (softening point is 200℃) asphalt was pulverized to a particle size D50 = 10μm using a flat air jet mill. The grading frequency of the flat air jet mill was 30Hz and the blower frequency was 25Hz.

[0078] (2) Add the crushed asphalt material to the drum furnace, the volume of the material is half of the volume of the drum furnace, under air atmosphere, the gas flow rate is 2L / min, the temperature is kept constant at 220~230℃ for 16h, and the preliminary material is obtained after cooling.

[0079] (3) Mix the preliminary material with zinc chloride powder at a mass ratio of 1:2 and load it into a graphite crucible. Use an atmosphere furnace to perform pre-carbonization treatment under a nitrogen atmosphere. The carbonization temperature is 900℃ and the temperature is kept at 900℃ for 4 hours to obtain porous carbon, named DK-BHC-2.

[0080] (4) The DK-BHC-2 obtained in step (3) is pulverized to a particle size D50 = 5 μm using a flat air jet mill. The grading frequency of the flat air jet mill is 60 Hz and the fan frequency is 40 Hz. Then, the pulverized material is mixed with g-C3N4 with a D50 particle size of 5 μm at a mass ratio of 1:1. The mixed material is loaded into a graphite crucible and kept at 1000℃ for 2 h in a nitrogen atmosphere. Then, it is cooled naturally to obtain nitrogen-doped hard carbon material.

[0081] The nitrogen content of the nitrogen-doped hard carbon material is 5.6%.

[0082] Example 4

[0083] This embodiment is basically the same as embodiment 2, except that in step (4), the mass ratio of the crushed material to g-C3N4 is 1:0.5.

[0084] The nitrogen content of nitrogen-doped hard carbon materials is 2%.

[0085] Example 5

[0086] This embodiment is basically the same as embodiment 2, except that in step (4), the mass ratio of the crushed material to g-C3N4 is 1:2.

[0087] The nitrogen content of nitrogen-doped hard carbon materials is 7%.

[0088] Example 6

[0089] This embodiment is basically the same as embodiment 2, except that the carbonization temperature in step (4) is 1200℃.

[0090] The nitrogen content of the nitrogen-doped hard carbon material is 3.2%.

[0091] Comparative Example 1

[0092] This comparative example is basically the same as Example 2, except that: the difference between this comparative example and Example 1 is that: in step (4), g-C3N4 is not added, and the crushed material is directly carbonized at 1000°C for 2 hours under a nitrogen atmosphere to obtain hard carbon material.

[0093] Effect Example

[0094] 1. Pore structure parameters of porous carbon

[0095] The specific surface area, average pore size, total pore volume, and micropore ratio of the porous carbon prepared in Examples 1 to 3 were tested, and the results are shown in Table 1.

[0096] The pore structure parameters of porous carbon were tested using a nitrogen adsorption-desorption method, specifically using an ASAP 2460 instrument. First, a certain amount of porous carbon sample was weighed into a sample tube, and the sample was degassed under vacuum at 300℃ for 4 hours. Then, the analysis was performed in a liquid nitrogen environment. After the test, the pore structure was analyzed using the HS-2D-NLDFT,Carbon,N2,77 analytical model to obtain the specific surface area, average pore diameter, total pore volume, and micropore ratio.

[0097] Figure 1 The figures shown are the porous carbon and nitrogen adsorption-desorption isotherms of Examples 1-3.

[0098] Table 1

[0099] Sample Name <![CDATA[Specific surface area (m 2 / g)]]> Average pore size (nm) <![CDATA[Total pore volume (cm 3 / g)]]> Micropore percentage (%) DK-BHC-0.5 235.4 1.5 0.12 90 DK-BHC-1 373.6 2.7 0.46 70 DK-BHC-2 693.6 10.2 0.6 21

[0100] 2. Pore structure parameters of nitrogen-doped hard carbon materials

[0101] The specific surface area, average pore size, total pore volume, and micropore ratio of the nitrogen-doped hard carbon material and the comparative hard carbon material in the above embodiments were tested, and the results are shown in Table 2. The testing method is the same as that for porous carbon.

[0102] Table 2

[0103] Sample Name <![CDATA[Specific surface area (m 2 / g)]]> Average pore size (nm) <![CDATA[Total pore volume (cm 3 / g)]]> Micropore percentage (%) Example 1 32.6 2.1 0.022 44.7 Example 2 12.1 1.7 0.013 54.3 Example 3 54.5 2.5 0.034 42.7 Example 4 21.3 1.8 0.023 52.1 Example 5 18.4 1.7 0.012 50.1 Example 6 10.7 1.5 0.009 52.3 Comparative Example 1 132.1 3.1 0.12 32.1

[0104] 3. Electrochemical performance testing

[0105] Electrodes were fabricated using the nitrogen-doped hard carbon materials from the above embodiments and the hard carbon materials obtained from the comparative examples, and then assembled into batteries. The battery fabrication method is as follows:

[0106] The nitrogen-doped hard carbon material of the above embodiment or the hard carbon material of the comparative example, binder (CMC), and superconducting carbon black (SP) were placed in a vacuum oven at a mass ratio of 8:1:1 and vacuum dried at 80°C for 24 hours. The mixture was then stirred with water as a solvent for 1 hour to disperse it until no obvious particles were observed, resulting in a negative electrode slurry. Each negative electrode slurry was uniformly coated onto a copper foil, dried at 80°C for 12 hours, and then punched into a negative electrode sheet with a diameter of 12 mm. The prepared negative electrode sheet was placed in the center of the negative electrode shell, and then a separator (a composite membrane of polyethylene PE, polypropylene PP, and polyethylene propylene PEP) was gently placed on top. An appropriate amount of electrolyte LPF6 (EC:DMC:EMC-1:1:1V%) was added, followed by the lithium metal sheet gasket and spring sheet. Finally, the positive electrode shell was covered. After assembly, a hydraulic sealing machine was used to seal the assembly and wipe off any excess electrolyte from the surface. The assembly process was completed in a glove box, where the water and oxygen content were controlled below 0.5 ppm. The assembled battery was placed in a battery box and left to stand in a constant temperature chamber for 12 hours to obtain a CR2032 button cell.

[0107] The charge / discharge test parameters are as follows: The test instrument used is the Shenzhen Xinwei Battery Tester. Test conditions: constant temperature (25℃), first discharge at 0.2C to 0V, let stand for 10 minutes, then discharge at 0.02C to 0V, and finally charge at 0.2C to 2V.

[0108] The test results are shown in Table 3.

[0109] Table 3 Electrochemical performance test results

[0110] Sample Name Initial discharge capacity (mAh / g) Initial charge capacity (mAh / g) First charge / discharge efficiency (%) Example 1 684 438 64 Example 2 713 563 79 Example 3 849 501 59 Example 4 621 454 73 Example 5 756 537 71 Example 6 601 463 77 Comparative Example 1 468 253 54

[0111] Test Result Analysis:

[0112] As can be seen from Example 2 and Comparative Example 1, doping g-C3N4 effectively reduces its specific surface area, and nitrogen atom doping provides more lithium storage sites, thus improving capacity. Figure 3 It can be seen that the interlayer spacing of porous hard carbon doped with g-C3N4 is reduced compared with that of porous hard carbon without g-C3N4. In Example 2, D002 = 0.36 nm, while in Comparative Example 1, D002 = 0.37 nm. The 002 peak of Example 2 is significantly shifted to the right, and the peak intensity and peak width of the 002 peak of Example 2 are increased, indicating that it has more graphite microcrystals than Comparative Example 1.

[0113] As can be seen from Examples 2, 1, and 3, more micropores and suitable pore volume porous carbon, after doping with g-C3N4, provide more active sites for lithium storage. In the first charge-discharge test, the first discharge capacity was 713 mAh / g, the first charge capacity was 563 mAh / g, and the first efficiency was 79%.

[0114] As can be seen from Examples 2, 4, and 5, an appropriate amount of g-C3N4 doping is more beneficial to capacity improvement. Too little g-C3N4 (such as in Example 4) cannot provide enough lithium storage sites, while too much g-C3N4 (such as in Example 5) can provide more lithium storage active sites, but will reduce the first efficiency.

[0115] As can be seen from Examples 2 and 6, excessively high carbonization temperatures of porous carbon and g-C3N4 can affect capacity and reduce first-efficiency.

Claims

1. A method for preparing a nitrogen-doped hard carbon material, characterized in that, It includes the following steps: Mixture A was subjected to a first heat treatment to obtain nitrogen-doped hard carbon material; The mixture A comprises porous carbon and g-C3N4; The average pore size of the porous carbon is 1~15 nm; The mass ratio of the porous carbon to the g-C3N4 is 1:(0.5~2). The temperature of the first heat treatment is 1000~1200℃; The nitrogen-doped hard carbon material has a specific surface area of ​​5~100 m². 2 / g.

2. The method for preparing nitrogen-doped hard carbon material as described in claim 1, characterized in that, The method for preparing the nitrogen-doped hard carbon material satisfies one or more of the following conditions: (a) The mass ratio of the porous carbon to the g-C3N4 is 1:(0.8~1.2); (b) The temperature of the first heat treatment is 1000°C or 1200°C; (c) The duration of the first heat treatment is 1 to 4 hours; (d) The preparation method of the mixture A includes: mixing the porous carbon and the g-C3N4 to obtain the mixture A.

3. The method for preparing nitrogen-doped hard carbon material as described in claim 1, characterized in that, The mass ratio of the porous carbon to the g-C3N4 is 1:0.5, 1:1, or 1:2; And / or, the duration of the first heat treatment is 2 hours.

4. The method for preparing nitrogen-doped hard carbon material as described in claim 1, characterized in that, The porous carbon satisfies one or more of the following conditions: (a) The specific surface area of ​​the porous carbon is 150~800 m². 2 / g; (b) The average pore size of the porous carbon is 1~6 nm; (c) The total pore volume of the porous carbon is 0.1~1.0 cm³. 3 / g; (d) The volume of micropores in the porous carbon accounts for 20-95% of the total pore volume of the porous carbon; (e) The D50 particle size of the porous carbon is 5~10 μm; (f) The D50 particle size of the g-C3N4 is 5~20μm.

5. The method for preparing nitrogen-doped hard carbon material as described in claim 1, characterized in that, The porous carbon satisfies one or more of the following conditions: (a) The specific surface area of ​​the porous carbon is 300~400 m². 2 / g; (b) The average pore size of the porous carbon is 2~5 nm; (c) The total pore volume of the porous carbon is 0.2~0.5 cm³. 3 / g; (d) The volume of micropores in the porous carbon accounts for 50-80% of the total pore volume of the porous carbon; (e) The D50 particle size of the porous carbon is 5 μm; (f) The D50 particle size of the g-C3N4 is 5 μm.

6. The method for preparing nitrogen-doped hard carbon material as described in claim 1, characterized in that, The porous carbon satisfies one or more of the following conditions: (a) The specific surface area of ​​the porous carbon is 235.4 m². 2 / g, 373.6m 2 / g or 693.6m 2 / g; (b) The average pore size of the porous carbon is 1.5 nm, 2.7 nm, or 10.2 nm; (c) The total pore volume of the porous carbon is 0.12 cm³. 3 / g, 0.46cm 3 / g or 0.6cm 3 / g; (d) The volume of micropores in the porous carbon accounts for 21%, 70% or 90% of the total pore volume of the porous carbon.

7. The method for preparing nitrogen-doped hard carbon material as described in claim 1, characterized in that, The method for preparing the porous carbon includes the following steps: (1) The asphalt is subjected to a second heat treatment in an oxygen-containing atmosphere to obtain a preliminary material, wherein the temperature of the second heat treatment is higher than the softening point temperature of the asphalt. (2) The mixture B is subjected to a third heat treatment to obtain porous carbon; the mixture B includes the preliminary material and the zinc source; the temperature of the third heat treatment is higher than the evaporation temperature of the zinc source.

8. The method for preparing nitrogen-doped hard carbon material as described in claim 7, characterized in that, The method for preparing porous carbon satisfies one or more of the following conditions: (a) The temperature of the second heat treatment is 20-30°C higher than the softening point of the asphalt; (b) The softening point temperature of the asphalt is 180~300℃; (c) The temperature of the second heat treatment is 150~300℃; (d) The second heat treatment time is 12~24h; (e) The D50 particle size of the asphalt is 5~20μm; (f) The oxygen-containing atmosphere is an air atmosphere; (g) The gas flow rate of the oxygen-containing atmosphere is 1~5 L / min; (h) The process further includes a step of pulverizing the asphalt before subjecting it to the second heat treatment; (i) The zinc source is a zinc compound; (j) The mass ratio of the preliminary material to the zinc source is 1:(0.2~2.5); (k) The temperature of the third heat treatment is 850~950℃; (l) The duration of the third heat treatment is 2-6 hours; (m) The third heat treatment is carried out in an inert protective gas; (n) The mixture B, after undergoing the third heat treatment, further includes the steps of crushing and sieving the porous carbon.

9. The method for preparing nitrogen-doped hard carbon material as described in claim 7, characterized in that, The method for preparing porous carbon satisfies one or more of the following conditions: (a) The softening point temperature of the asphalt is 200°C; (b) The second heat treatment lasts for 16 hours; (c) The D50 particle size of the asphalt is 10 μm; (d) The gas flow rate of the oxygen-containing atmosphere is 2 L / min; (e) Before subjecting the asphalt to the second heat treatment, the asphalt is further subjected to a crushing step, wherein the crushing is performed by an air jet mill or a rod mill, the air jet mill is a flat air jet mill device, the grading frequency of the flat air jet mill device is 25~40Hz, and the fan frequency is 20~30Hz; (f) The zinc source is a zinc compound, wherein the zinc compound is ZnCl2 and / or zinc acetate; (g) The mass ratio of the preliminary material to the zinc source is 1:(0.4~0.6); (h) The temperature of the third heat treatment is 900°C; (i) The duration of the third heat treatment is 4 hours; (j) The third heat treatment is carried out in an inert protective gas, wherein the inert protective gas is nitrogen and / or argon; (k) The mixture B, after undergoing the third heat treatment, further includes the steps of crushing and sieving the porous carbon; the crushing is carried out by an air jet mill or a rod mill, the air jet mill is a flat air jet mill, the grading frequency of the flat air jet mill is 50~70Hz, and the fan frequency is 30~50Hz; the particle size of the porous carbon after sieving is 5~10μm.

10. The method for preparing nitrogen-doped hard carbon material as described in claim 7, characterized in that, The method for preparing porous carbon satisfies one or more of the following conditions: (a) Before subjecting the asphalt to the second heat treatment, the asphalt is further subjected to a crushing step, wherein the crushing is performed by an air jet mill or a rod mill, wherein the air jet mill is a flat air jet mill device, and the grading frequency of the flat air jet mill device is 30 Hz and the fan frequency is 25 Hz. (b) The mass ratio of the preliminary material to the zinc source is 1:0.5, 1:1, or 1:2; (c) The mixture B, after undergoing the third heat treatment, further includes the steps of crushing and sieving the porous carbon; the crushing is carried out by an air jet mill or a rod mill, the air jet mill is a flat air jet mill device, the grading frequency of the flat air jet mill device is 60Hz, and the fan frequency is 40Hz.

11. A nitrogen-doped hard carbon material prepared by the method described in any one of claims 1 to 10.

12. The nitrogen-doped hard carbon material as described in claim 11, characterized in that, It comprises porous hard carbon particles and nitrogen doped in the porous hard carbon particles, the nitrogen including edge nitrogen; The nitrogen-doped hard carbon material has a specific surface area of ​​5~100 m². 2 / g; The nitrogen-doped hard carbon material includes a graphite-like microcrystalline structure.

13. The nitrogen-doped hard carbon material as described in claim 12, characterized in that, The nitrogen-doped hard carbon material satisfies one or more of the following conditions: (a) The nitrogen content is 2-7%, where % refers to the mass fraction of nitrogen in the nitrogen-doped hard carbon material; (b) The edge nitrogen is pyrrole nitrogen and / or pyridine nitrogen; (c) The specific surface area of ​​the nitrogen-doped hard carbon material is 10~60 m². 2 / g; (d) The total pore volume of the nitrogen-doped hard carbon material is 0.005~0.1m. 2 / g; (e) The average pore size of the nitrogen-doped hard carbon material is 1~3 nm; (f) The volume of the micropores in the nitrogen-doped hard carbon material accounts for 35-70% of the total pore volume of the nitrogen-doped hard carbon material; (g) The D50 particle size of the porous hard carbon particles is 5~10μm.

14. The nitrogen-doped hard carbon material as described in claim 12, characterized in that, The nitrogen-doped hard carbon material satisfies one or more of the following conditions: (a) The nitrogen content is 2%, 3.2%, 5%, 5.6% or 7%, where % refers to the mass fraction of nitrogen in the nitrogen-doped hard carbon material; (b) The specific surface area of ​​the nitrogen-doped hard carbon material is 10.7 m². 2 / g, 12.1m 2 / g, 18.4m 2 / g, 21.3m 2 / g, 32.6m 2 / g or 54.5m 2 / g; (c) The total pore volume of the nitrogen-doped hard carbon material is 0.008~0.05 m³. 2 / g; (d) The average pore size of the nitrogen-doped hard carbon material is 1.5~2 nm; (e) The volume of the micropores in the nitrogen-doped hard carbon material accounts for 50-60% of the total pore volume of the nitrogen-doped hard carbon material; (f) The D50 particle size of the porous hard carbon particles is 5 μm.

15. The nitrogen-doped hard carbon material as described in claim 12, characterized in that, The nitrogen-doped hard carbon material satisfies one or more of the following conditions: (a) The total pore volume of the nitrogen-doped hard carbon material is 0.009 m³. 2 / g, 0.012 m 2 / g, 0.013 m 2 / g, 0.022 m 2 / g, 0.023 m 2 / g or 0.034 m 2 / g; (b) The nitrogen-doped hard carbon material has an average pore size of 1.5 nm, 1.7 nm, 1.8 nm, 2.1 nm or 2.5 nm; (c) The volume of the micropores in the nitrogen-doped hard carbon material accounts for 42.7%, 44.7%, 50.1%, 52.1%, 52.3% or 54.3% of the total pore volume of the nitrogen-doped hard carbon material.

16. The application of a nitrogen-doped hard carbon material as described in any one of claims 11 to 15 as a negative electrode material in a lithium-ion battery.

17. An electrode sheet, characterized in that, It includes nitrogen-doped hard carbon materials as described in any one of claims 11 to 15.

18. An electrochemical device, characterized in that, It includes the electrode as described in claim 17.

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