Preparation method of potassium-nitrogen co-doped artificial graphite, artificial graphite and secondary battery

Abstract

The invention provides a preparation method of potassium-nitrogen co-doped artificial graphite, artificial graphite and a secondary battery. The preparation method of the potassium-nitrogen co-doped artificial graphite comprises the following steps: performing heat treatment on artificial graphite in a hydrogen-containing atmosphere to obtain a precursor; performing ultrasonic treatment on KOH, ammonium bicarbonate, urea, a dispersing agent, water and the precursor to obtain composite slurry; transferring the composite slurry into a reactor, and sequentially carrying out a first microwave reaction, a second microwave reaction and a third microwave reaction; pickling, washing, drying and annealing in sequence. According to the preparation method, through the combination of multi-step microwave reaction, multi-element doping and multi-component pore forming, the limitation of a traditional artificial graphite modification method is broken through, and collaborative optimization of the structure and performance of the artificial graphite material is achieved.

Description

Technical Field

[0001] This invention relates to the field of new energy material preparation technology, and in particular to a method for preparing potassium-nitrogen co-doped artificial graphite, artificial graphite, and secondary batteries. Background Technology

[0002] Artificial graphite is widely used in secondary batteries due to its excellent cycle stability, but its low theoretical specific capacity (372 mAh / g) and poor rate performance cannot meet the application requirements of high-energy-density secondary batteries for anode materials, thus limiting its further development. To promote its development, the industry has modified it to improve its performance, but existing modification technologies mainly suffer from the following problems: (1) Limited effect of single modification method: Traditional element doping or pore-forming methods can often only improve one aspect of performance and it is difficult to achieve a breakthrough improvement in comprehensive performance; (2) Difficulty in controlling the pore structure: The pores formed by conventional activation methods are mostly randomly distributed and lack hierarchical structure, which cannot simultaneously meet the requirements of high specific capacity and fast charging. (3) Uneven element distribution: Traditional element doping methods are prone to element segregation, which leads to local stress concentration and structural instability; (4) Complex process and high cost: Multi-step processing not only increases production costs, but may also cause damage to material structure.

[0003] The root causes of these technical bottlenecks are: the temperature field distribution of traditional heating methods is uneven, making it difficult to achieve precise spatiotemporal control; the pore-forming mechanism of a single activator is limited; and the solid solubility of dopant elements in graphite is low and their distribution is difficult to control. Summary of the Invention

[0004] In view of the above problems, the purpose of this invention is to provide a method for preparing potassium-nitrogen co-doped artificial graphite, artificial graphite, and a secondary battery. The preparation method of this invention, through a combination of multi-step microwave reaction, multi-element doping, and multi-component pore-forming, overcomes the limitations of traditional artificial graphite modification methods, achieving synergistic optimization of the structure and properties of artificial graphite materials.

[0005] To achieve the above objectives, the first aspect of the present invention provides a method for preparing potassium-nitrogen co-doped artificial graphite, comprising the steps of: (I) Activation The precursor was obtained by heat treatment of artificial graphite in an atmosphere containing hydrogen. (II) Preparation of composite slurry A composite slurry is obtained by ultrasonic treatment of KOH, ammonium bicarbonate, urea, dispersant, water, and the precursor. (III) Microwave reaction The composite slurry is transferred into a reactor, where a first microwave reaction, a second microwave reaction, and a third microwave reaction are carried out sequentially. The conditions for the first microwave reaction, the second microwave reaction, and the third microwave reaction are as follows: First microwave reaction: microwave power of 1.2~1.5KW, temperature increased from room temperature to T1℃, carried out under an inert atmosphere and pressure of 0.1~0.3MPa. The second microwave reaction: the microwave power is 0.8~1.0KW, the temperature rises from T1℃ to T2℃, and then gradually decreases to normal pressure. The third microwave reaction: the microwave power is 0.5~0.8KW, the temperature drops from T2℃ to T3℃, and it is carried out in a mixed atmosphere of inert atmosphere and hydrogen and the pressure is 0.05~0.10MPa; (IV) Post-processing The process involves pickling, washing, drying, and annealing in sequence.

[0006] In the preparation method of this invention, three-stage microwave control ensures the controllable formation of the pore structure and the uniform distribution of elements; the composite pore-forming agent system enables precise control of pore size and distribution; and supercritical drying technology maintains the integrity of the pore structure. The mechanism is as follows.

[0007] (1) This invention uses KOH, ammonium bicarbonate and urea as composite pore-forming agents, all of which produce gas when they decompose under microwave heating (KOH reacts with carbon to produce H2, ammonium bicarbonate decomposes to produce NH3 and CO2, urea decomposes to produce NH3 and CO2, etc.). This multi-component composite pore-forming agent that can generate gas can produce richer and more controllable pore-forming behavior over a wider temperature range under different microwave conditions and the synergistic effect of the gas-generating pore-forming agent, thereby achieving precise control over the pore size, distribution and pore shape.

[0008] (2) The present invention employs a three-stage microwave reaction.

[0009] The first stage (high temperature and pressure, inert atmosphere): The system is rapidly heated to the target temperature T1 under high power and pressure. The pressure environment helps suppress the violent vaporization of moisture or volatile components in the precursor, making the heating process more stable and creating stable initial conditions for subsequent reactions. The inert atmosphere prevents the material from oxidizing at high temperatures. The second stage (medium power, atmospheric pressure): Power decreases, and temperature continues to rise from T1 to a higher T2, while gradually decreasing to atmospheric pressure. During this stage, the pressure release causes potassium-containing compounds (such as KOH decomposition products), urea, and ammonium bicarbonate decomposition gases (such as NH3, CO2, H2O) embedded in the graphite precursor to escape. This controlled depressurization and gas release process is equivalent to "blowing" or "etching" pores into the graphite. By controlling the heating rate and pressure release rate, the intensity of pore formation can be adjusted, thereby affecting the pore size and distribution. The third stage (low power, low pressure, hydrogen-containing mixed atmosphere): This stage is carried out at lower power and temperature (T2 to T3), and the introduced hydrogen is the key reducing and doping atmosphere. The potassium-containing oxide or intermediate is reduced to the more reactive metallic potassium or K. + Compounds facilitate their insertion and doping into the graphite interlayer. The hydrogen atmosphere, combined with nitrogen-containing decomposition products (such as NH3), helps nitrogen to be doped into the graphite lattice in specific forms (such as pyridine nitrogen and graphitic nitrogen). Simultaneously, lower temperature and pressure help stabilize the porous structure formed in the second stage and promote the uniform distribution and fixation of the dopant elements.

[0010] As a technical solution of the present invention, the artificial graphite has a Dv50 of 10~14μm, a specific surface area of ​​2~4m² / g, and a graphitization degree of >88%.

[0011] As a technical solution of the present invention, the atmosphere containing hydrogen is a mixture of an inert atmosphere and hydrogen, and the volume ratio of the two is 90~98:2~10, and the flow rate is 100~200mL / min.

[0012] As one technical solution of the present invention, the heat treatment temperature is 400~800℃, the time is 0.5~4.0h, and the heating rate is 1~10℃ / min.

[0013] As one technical solution of the present invention, the ultrasonic treatment is carried out in an ultrasonic cell disruptor, using a pulse mode, with a power of 500~750W and a treatment time of 15~40min.

[0014] As a technical solution of the present invention, T1 is 500~700℃, the first microwave reaction time is 3~5min, and the stirring speed is 100~180rpm.

[0015] As one technical solution of the present invention, T2 is 750~900℃, the second microwave reaction time is 8~12min, and the stirring speed is 200~300rpm.

[0016] As one technical solution of the present invention, T3 is 650~800℃, the third microwave reaction time is 5~8min, and the stirring speed is 200~300rpm.

[0017] As a technical solution of the present invention, the product of the microwave reaction is cooled to below 200°C at a rate of 2~5°C / min before acid washing.

[0018] As one technical solution of the present invention, the acid washing is performed by washing with 0.5~1.5mol / L HCl and 0.1~1.0mol / L H2SO4 for 1~4 hours respectively.

[0019] As one technical solution of the present invention, after the water is washed to neutral, ethanol is used for solvent replacement.

[0020] As one technical solution of the present invention, the drying is carried out using supercritical CO2 drying, and the drying temperature is 35~50℃, the pressure is 5~10MPa, and the drying time is 2~10h.

[0021] As one technical solution of the present invention, the annealing treatment is carried out at an inert atmosphere of 650~900℃ for 0.5~2.0h.

[0022] As one technical solution of the present invention, the purity of KOH is 90~95%, the purity of ammonium bicarbonate is ≥99%, and the mass ratio of KOH to ammonium bicarbonate is 3~5:1.

[0023] As one technical solution of the present invention, the purity of the urea is ≥99.5%, and the mass ratio of the urea to the precursor is 1:5~8.

[0024] As one technical solution of the present invention, the concentration of KOH in the water is 20~40 wt.%.

[0025] As one technical solution of the present invention, the mass ratio of the graphite to the ammonium bicarbonate is 10:1~2.

[0026] As one technical solution of the present invention, the dispersant is a polyethylene glycol-based substance, and the dispersant accounts for 1 to 3% of the mass of the precursor.

[0027] The second aspect of this invention provides a potassium-nitrogen co-doped artificial graphite prepared by the aforementioned method. This potassium-nitrogen co-doped artificial graphite has a hierarchical pore structure consisting of micropores (0.8–1.2 nm), mesopores (10–20 nm), and macropores (50–200 nm), with the micropores comprising 15–25%, the mesopores 40–50%, and the macropores 25–40%. The potassium-nitrogen co-doped artificial graphite contains potassium and nitrogen dopants. Potassium doping can adjust the interlayer spacing, and the nitrogen gradient can improve the material's first-efficiency and rate performance. The artificial graphite has a hierarchical pore structure consisting of micropores, mesopores, and macropores. The micropores provide numerous lithium-ion storage sites, contributing additional capacity; the mesopores ensure rapid electrolyte penetration and ion transport; and the macropores effectively mitigate volume changes during cycling.

[0028] As one technical solution of the present invention, the total pore volume is 0.08~0.15cm. 3 / g.

[0029] As one technical solution of the present invention, the aperture distribution deviation is <15%.

[0030] As one technical solution of the present invention, the specific surface area is 8~15m² 2 / g.

[0031] As one technical solution of the present invention, the graphite interlayer spacing is 0.338~0.345nm.

[0032] As one technical solution of the present invention, the carbon content is ≥98.5wt.%, the potassium content is 0.3~0.8wt.%, the nitrogen content is 1.2~2.5wt.%, the oxygen content is 0.5~1.2wt.%, the metal impurities are <20ppm, the sulfur content is <50ppm, and the potassium element is distributed in a gradient inside, while the nitrogen element content on the surface is 1.5~2.0 times that inside.

[0033] As one technical solution of the present invention, the ash content is <0.2wt.%.

[0034] As one technical solution of the present invention, the degree of graphitization is 85-92%.

[0035] As one technical solution of the present invention, ID / IG is 0.15~0.18.

[0036] As one technical solution of the present invention, Dv50 is 10~15μm and Dv90<30μm.

[0037] As one technical solution of the present invention, the tap density is 1.0~1.2 g / cm³. 3 .

[0038] As one technical solution of the present invention, the compaction density is 1.6~1.8 g / cm³.3 .

[0039] As one technical solution of the present invention, the conductivity is ≥2.5S / cm.

[0040] As one technical solution of the present invention, the specific capacity at 0.1C is 450~480mAh / g, and the specific capacity at 0.5C is 360~380mAh / g.

[0041] As one technical solution of the present invention, the initial coulombic efficiency is 94.0~97.0%.

[0042] As a technical solution of the present invention, the capacity retention rate of 5C / 0.1C is ≥80%.

[0043] As one technical solution of the present invention, the capacity retention rate after 200 cycles is ≥90%.

[0044] A third aspect of the present invention provides a secondary battery comprising a positive electrode material, a separator, an electrolyte, and a negative electrode material, wherein the negative electrode material comprises the aforementioned potassium-nitrogen co-doped artificial graphite. Detailed Implementation

[0045] The potassium-nitrogen co-doped artificial graphite of this invention can be used alone or in combination with other negative electrode active materials (such as natural graphite, silicon-oxygen materials, silicon-carbon materials, soft carbon and / or hard carbon, etc.). The potassium-nitrogen co-doped artificial graphite can be applied in secondary batteries, which include a positive electrode material, a separator, an electrolyte, and a negative electrode material. The positive electrode material includes layered oxide series lithium-ion positive electrode materials or olivine-type lithium-ion positive electrode materials. Layered oxide series lithium-ion positive electrode materials can be lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, or lithium nickel cobalt aluminum oxide. Olivine-type lithium-ion positive electrode materials can be lithium iron phosphate or lithium manganese iron phosphate. The separator can be a polyethylene or polypropylene separator. The electrolyte may include lithium salts, non-aqueous organic solvents, and conventional additives. The lithium salt may be selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(oxalate-borate), lithium difluorophosphate, lithium bis(oxalate-borate), lithium difluorooxalate-borate, lithium lower aliphatic carboxylic acids, lithium difluorobis(oxalate-phosphate), lithium bis(fluorosulfonyl)imide, lithium chloroborane, and lithium tetraphenylborate. The non-aqueous organic solvent is selected from at least one of γ-butyrolactone, γ-valerolactone, δ-valerolactone, methyl acetate, ethyl acetate, ethyl propionate, butyl acetate, propyl propionate, butyl propionate, ethylene carbonate, propylene carbonate, butyl carbonate, methyl pentyl carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, propylene carbonate, 1,3-dioxane, 1,4-dioxane, crown ether, tetrahydrofuran, 2-methyltetrahydrofuran, 2-trifluoromethyltetrahydrofuran, dimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether. The additive may be at least one of vinylene carbonate, fluoroethylene carbonate, vinyl sulfite, 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, 1,4-butanesulfonate lactone, and vinyl sulfate.

[0046] The potassium-nitrogen co-doped artificial graphite of this invention can be used as a negative electrode active material in secondary batteries. Its specific capacity at 0.1C rate is 450~480 mAh / g, and examples, but not limited to, 450 mAh / g, 455 mAh / g, 460 mAh / g, 465 mAh / g, 470 mAh / g, 475 mAh / g, and 480 mAh / g. Its specific capacity at 0.5C rate is 360~380 mAh / g, and examples, but not limited to, 360 mAh / g, 365 mAh / g, 370 mAh / g, 375 mAh / g, and 380 mAh / g. Its initial coulombic efficiency is 94.0~97.0%, and examples, but not limited to, 94.0%, 94.5%, 95.0%, 95.5%, 96.0%, 96.5%, and 97.0%. Capacity retention at 5C / 0.1C ≥ 80%, for example, but not limited to 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%. Capacity retention after 200 cycles ≥ 90%, for example, but not limited to 90.0%, 90.5%, 91.0%, 91.5%, 92.0%, 92.5%, 93.0%, 93.5%, 94.0%, 94.5%, 95.0%.

[0047] The potassium-nitrogen co-doped artificial graphite of the present invention comprises a three-level pore structure consisting of micropores, mesopores, and macropores. The micropores have a pore size of 0.8–1.2 nm and a volume fraction of 15–25%. The mesopores have a pore size of 10–20 nm and a volume fraction of 40–50%. The macropores have a pore size of 50–200 nm and a volume fraction of 25–40%. The total pore volume of the potassium-nitrogen co-doped artificial graphite of the present invention is 0.08–0.15 cm³. 3 / g, as an example, can be, but is not limited to, 0.08cm 3 / g, 0.09cm 3 / g, 0.10cm 3 / g, 0.11cm 3 / g, 0.12cm 3 / g, 0.13cm 3 / g, 0.14cm 3 / g, 0.15cm 3 / g. Pore size distribution deviation <15%, for example, deviations may be, but are not limited to, 14%, 13%, 12%, 11%, 10%. Specific surface area is 8~15m². 2 / g, as an example, the specific surface area can be, but is not limited to, 8m². 2 / g、9m 2 / g, 10m 2 / g、11m 2 / g、12m2 / g、13m 2 / g、14m 2 / g, 15m 2 / g. The graphite interlayer spacing is 0.338~0.345nm, and for example, but not limited to, 0.338nm, 0.339nm, 0.340nm, 0.341nm, 0.342nm, 0.343nm, 0.344nm, and 0.345nm. Carbon content ≥98.5wt.%, potassium content 0.3~0.8wt.%, nitrogen content 1.2~2.5wt.%, oxygen content 0.5~1.2wt.%, metal impurities <20ppm, and sulfur content <50ppm. Potassium is distributed in a gradient internally, while nitrogen content on the surface is 1.5~2.0 times that internally. The interlayer gradient doping of potassium enables gradual adjustment of the interlayer spacing, reducing the lithium-ion insertion barrier. The surface gradient distribution of nitrogen improves interface characteristics, thereby improving first-time efficiency and rate performance. The ash content of the potassium-nitrogen co-doped artificial graphite of this invention is <0.2wt.%. The degree of graphitization is 85-92%, and for example, it may be, but is not limited to, 85%, 86%, 87%, 88%, 89%, 90%, 91%, and 92%. The ID / IG ratio is 0.15-0.18, and for example, it may be, but is not limited to, 0.15, 0.16, 0.17, and 0.18. The Dv50 is 10-15 μm, and for example, it may be, but is not limited to, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, and 15 μm. The Dv90 is <30 μm, and for example, it may be, but is not limited to, 29 μm, 28 μm, 27 μm, 26 μm, 25 μm, 24 μm, 23 μm, 22 μm, 21 μm, and 20 μm. The tap density is 1.0-1.2 g / cm³. 3 As an example, it can be, but is not limited to, 1.0 g / cm³. 3 1.1g / cm 3 1.2g / cm 3 The compacted density is 1.6~1.8 g / cm³. 3 For example, it can be, but is not limited to, 1.6 g / cm³. 3 1.7g / cm 3 1.8g / cm 3 The conductivity is ≥2.5 S / cm. For example, the conductivity may be, but is not limited to, 2.5 S / cm, 2.6 S / cm, 2.7 S / cm, 2.8 S / cm, 2.9 S / cm, and 3.0 S / cm.

[0048] The method for preparing potassium-nitrogen co-doped artificial graphite of the present invention may include the following steps: (I) activation, (II) preparation of composite slurry, (III) microwave reaction and (IV) post-treatment.

[0049] Step (I) activation involves heat-treating artificial graphite in an atmosphere containing hydrogen to obtain a precursor.

[0050] The Dv50 of artificial graphite is 10~14μm, and for example, it can be, but is not limited to, 10μm, 11μm, 12μm, 13μm, and 14μm. The specific surface area is 2~4m². 2 / g, as an example, can be, but is not limited to, 2m 2 / g、3m 2 / g、4m 2 / g. Graphitization degree >88%. For example, the graphitization degree may be, but is not limited to, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, or 97%. The atmosphere containing hydrogen is a mixture of an inert atmosphere and hydrogen, with a volume ratio of 90~98:2~10 and a flow rate of 100~200 mL / min. For example, the volume ratio may be, but is not limited to, 90:10, 91:9, 92:8, 93:7, 94:6, 95:5, 96:4, 97:3, or 98:2, and the flow rate may be, but is not limited to, 100 mL / min, 110 mL / min, 120 mL / min, 130 mL / min, 140 mL / min, 150 mL / min, 160 mL / min, 170 mL / min, 180 mL / min, 190 mL / min, or 200 mL / min. The heat treatment temperature is 400~800℃, the time is 0.5~4.0h, and the heating rate is 1~10℃ / min. For example, the temperature can be, but is not limited to, 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, or 800℃, the time can be, but is not limited to, 0.5h, 1.0h, 1.5h, 2.0h, 2.5h, 3.0h, 3.5h, or 4.0h, and the heating rate can be, but is not limited to, 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, or 10℃ / min.

[0051] Step (II) Preparation of composite slurry includes ultrasonic treatment of KOH, ammonium bicarbonate, urea, dispersant, water and precursor to obtain composite slurry.

[0052] The purity of KOH is 90-95%, and the purity of ammonium bicarbonate is ≥99%. The mass ratio of KOH to ammonium bicarbonate is 3-5:1. For example, the mass ratio can be, but is not limited to, 3:1, 4:1, or 5:1. The purity of urea is ≥99.5%, and the mass ratio of urea to the precursor is 1:5-8. For example, the mass ratio can be, but is not limited to, 1:5, 1:6, 1:7, or 1:8. The concentration of KOH in water is 20-40 wt.%. For example, the concentration can be, but is not limited to, 20 wt.%, 22 wt.%, 24 wt.%, 26 wt.%, 28 wt.%, 30 wt.%, 32 wt.%, 34 wt.%, 36 wt.%, 38 wt.%, or 40 wt.%. The mass ratio of graphite to ammonium bicarbonate is 10:1-2. For example, the mass ratio can be, but is not limited to, 10:1 or 10:2. The dispersant is a polyethylene glycol-based substance, and more specifically, PEG-200, PEG-800, PEG-1000, or PEG-2000. The dispersant accounts for 1-3% of the precursor mass; for example, the mass may be, but is not limited to, 1%, 2%, or 3%. Ultrasonic treatment is performed in an ultrasonic cell disruptor using a pulse mode and a power of 500-750W for 15-40 minutes; for example, the power may be, but is not limited to, 500W, 550W, 600W, 650W, 700W, or 750W, and the time may be, but is not limited to, 15 minutes, 20 minutes, 25 minutes, 30 minutes, 35 minutes, or 40 minutes.

[0053] Step (III) Microwave reaction includes transferring the composite slurry into the reactor and sequentially performing the first microwave reaction, the second microwave reaction, and the third microwave reaction.

[0054] The equipment used for the microwave reaction is a multi-cavity microwave reactor, which includes a quartz reactor with a stirring device, an infrared temperature measurement system, and an atmosphere control system. The maximum power is 2KW and the maximum frequency is 2.45GHz.

[0055] The conditions for the first microwave reaction may include: microwave power of 1.2~1.5KW, temperature increased from room temperature to T1℃, carried out under an inert atmosphere with a pressure of 0.1~0.3MPa, reaction time of 3~5min, and stirring speed of 100~180rpm. T1 is 500~700℃, and the inert atmosphere is selected from nitrogen, argon, helium, and neon.

[0056] The conditions for the second microwave reaction may include: microwave power of 0.8~1.0KW, temperature gradually decreasing from T1℃ to T2℃ and then to atmospheric pressure, reaction time of 8~12min, and stirring speed of 200~300rpm. T2 is 750~900℃.

[0057] The conditions for the third microwave reaction may include: microwave power of 0.5~0.8KW, temperature decreasing from T2℃ to T3℃, carried out in a mixed atmosphere of inert gas and hydrogen at a pressure of 0.05~0.10MPa, reaction time of 5~8min, and stirring speed of 200~300rpm. Specifically, T3 is 650~800℃, and the inert atmosphere is selected from nitrogen, argon, helium, and neon. The volume ratio of inert atmosphere to hydrogen is 90~98:2~10, and the flow rate is 100~200 mL / min. For example, the volume ratio can be, but is not limited to, 90:10, 91:9, 92:8, 93:7, 94:6, 95:5, 96:4, 97:3, 98:2, and the flow rate can be, but is not limited to, 100 mL / min, 110 mL / min, 120 mL / min, 130 mL / min, 140 mL / min, 150 mL / min, 160 mL / min, 170 mL / min, 180 mL / min, 190 mL / min, 200 mL / min.

[0058] Step (IV) post-treatment includes pickling, washing, drying and annealing in sequence.

[0059] Before acid washing, the product from the microwave reaction is cooled to below 200°C at a rate of 2–5°C / min. For example, the cooling rate can be, but is not limited to, 2°C / min, 3°C / min, 4°C / min, or 5°C / min. Acid washing is performed sequentially with 0.5–1.5 mol / L HCl and 0.1–1.0 mol / L H₂SO₄ for 1–4 h each. First, HCl is used to remove metallic impurities, then H₂SO₄ is used to remove residual alkali and passivate the material surface. After washing with water until neutral, solvent replacement is performed with ethanol. Drying is performed using supercritical CO₂ drying at a temperature of 35–50°C, a pressure of 5–10 MPa, and a drying time of 2–10 h. For example, the drying temperature may be, but is not limited to, 35℃, 37℃, 39℃, 40℃, 42℃, 44℃, 46℃, 48℃, or 50℃; the pressure may be, but is not limited to, 5MPa, 6MPa, 7MPa, 8MPa, 9MPa, or 10MPa; and the drying time may be, but is not limited to, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, or 10h. Annealing is performed at 650~900℃ in an inert atmosphere for 0.5~2.0h. For example, the annealing temperature may be, but is not limited to, 650℃, 700℃, 750℃, 800℃, 850℃, or 900℃; and the holding time may be, but is not limited to, 0.5h, 1.0h, 1.5h, or 2.0h.

[0060] To better illustrate the purpose, technical solution, and beneficial effects of this invention, the invention will be further described below in conjunction with specific embodiments. It should be noted that the methods described below are further explanations of this invention and should not be construed as limiting it. The embodiments of this invention include a description of the technical solution for potassium-nitrogen co-doped artificial graphite and a description of the technical solution for the preparation method of potassium-nitrogen co-doped artificial graphite.

[0061] Example 1 This embodiment describes a method for preparing potassium-nitrogen co-doped artificial graphite, which includes the following steps.

[0062] (I) Activation Artificial graphite (Dv50 of 12μm, specific surface area of ​​3.2m²) was used. 2 10.0 g of a corundum boat (with a graphitization degree of 89%) was placed in a tube furnace and a mixed gas with a volume ratio of Ar:H2 of 95:5 was introduced. The flow rate of the mixed gas was 200 mL / min. The temperature was increased to 600 °C at 5 °C / min and held for 2 h. The mixture was then naturally cooled to room temperature to obtain the precursor.

[0063] (II) Preparation of composite slurry Weigh 4.0g of analytical grade KOH and add it to deionized water to prepare a 30wt.% KOH solution. Stir magnetically until completely dissolved. Weigh 1.2g of ammonium bicarbonate and slowly add it to the KOH solution, stirring until dissolved. Add 1.8g of urea and 0.20g of PEG-400 sequentially to obtain a mixture. Add 10.0g of the precursor to the mixture and transfer it to a 100mL beaker. Place the beaker in an ultrasonic cell disruptor to obtain a composite slurry. Ultrasonic cell disruptor parameters: pulse mode, power 600W, working time 3s, interval time 2s, total processing time 30min, water temperature maintained at 25±2℃.

[0064] (III) Microwave reaction The composite slurry was transferred to a quartz reactor, the system was sealed, and a vacuum was applied to -0.095 MPa. Nitrogen gas was then introduced to 0.2 MPa, and stirring was started. The stirring speed was set to 150 rpm, and the microwave power was 1.5 kW. The temperature was raised from room temperature to 600°C and held for 4 minutes. The microwave power was then adjusted to 1.0 kW, the pressure was slowly released to atmospheric pressure, the stirring speed was increased to 250 rpm, and the temperature was raised to 800°C and held for 10 minutes. The nitrogen gas was then switched to a mixed gas (N2 and H2 volume ratio of 95:5), the system pressure was adjusted to 0.08 MPa, the microwave power was adjusted to 0.6 kW, the stirring speed was maintained at 250 rpm, and the temperature was lowered to 750°C and held for 6 minutes.

[0065] (IV) Post-processing Turn off the microwave power, start the programmed cooling system, set the cooling rate to 3℃ / min, and continue to cool naturally to room temperature after reaching 200℃. Take the product after the microwave reaction, add 100mL of 1mol / L HCl, heat in a water bath to 60℃ and stir for 2h, let it stand to precipitate, discard the supernatant, add 100mL of 0.5mol / L H2SO4, stir at room temperature for 1h, centrifuge and collect the solid. Wash repeatedly with deionized water until the pH of the filtrate is 6.8~7.2 and the conductivity is <10μS / cm, then wash three times with anhydrous ethanol, 50mL each time, soaking for 30min. Place in a supercritical drying vessel, inject liquid CO2, set the temperature to 40℃ and the pressure to 8MPa, maintain for 4h, then slowly depressurize and remove the sample. Place the dried sample in a corundum boat, put it in a tube furnace, introduce argon gas, heat to 800℃ at 3℃ / min and hold for 1h, then cool naturally to room temperature.

[0066] Example 2 This embodiment describes a method for preparing potassium-nitrogen co-doped artificial graphite, which includes the following steps.

[0067] (I) Activation Artificial graphite (Dv50 of 12μm, specific surface area of ​​3.2m²) was used. 2 10.0 g of a corundum boat (with a graphitization degree of 89%) was placed in a tube furnace and a mixed gas with a volume ratio of Ar:H2 of 95:5 was introduced. The flow rate of the mixed gas was 200 mL / min. The temperature was increased to 600 °C at 5 °C / min and held for 2 h. The mixture was then naturally cooled to room temperature to obtain the precursor.

[0068] (II) Preparation of composite slurry Weigh 5.0 g of analytical grade KOH and add it to deionized water to prepare a 40 wt.% KOH solution. Stir magnetically until completely dissolved. Weigh 1.5 g of ammonium bicarbonate and slowly add it to the KOH solution, stirring until dissolved. Add 2.0 g of urea and 0.30 g of PEG-400 sequentially to obtain a mixture. Add 10.0 g of the precursor to the mixture and transfer it to a 100 mL beaker. Place the beaker in an ultrasonic cell disruptor to obtain a composite slurry. The parameters of the ultrasonic cell disruptor are: pulse mode, power 600 W, working time 3 s, interval time 2 s, total processing time 30 min, and water temperature maintained at 25 ± 2℃.

[0069] (III) Microwave reaction The composite slurry was transferred to a quartz reactor, the system was sealed, and a vacuum was applied to -0.095 MPa. Nitrogen gas was then introduced to 0.2 MPa, and stirring was started. The stirring speed was set to 150 rpm, and the microwave power was 1.2 kW. The temperature was raised from room temperature to 600°C and held for 4 minutes. The microwave power was then adjusted to 1.0 kW, the pressure was slowly released to atmospheric pressure, the stirring speed was increased to 250 rpm, and the temperature was raised to 850°C and held for 8 minutes. The nitrogen gas was then switched to a mixed gas (N2 and H2 volume ratio of 95:5), the system pressure was adjusted to 0.08 MPa, the microwave power was adjusted to 0.8 kW, the stirring speed was maintained at 250 rpm, and the temperature was lowered to 780°C and held for 6 minutes.

[0070] (IV) Post-processing Turn off the microwave power, start the programmed cooling system, set the cooling rate to 3℃ / min, and continue to cool naturally to room temperature after reaching 200℃. Take the product after the microwave reaction, add 100mL of 1mol / L HCl, heat in a water bath to 60℃ and stir for 2h, let it stand to precipitate, discard the supernatant, add 100mL of 0.5mol / L H2SO4, stir at room temperature for 1h, centrifuge and collect the solid. Wash repeatedly with deionized water until the pH of the filtrate is 6.8~7.2 and the conductivity is <10μS / cm, then wash three times with anhydrous ethanol, 50mL each time, soaking for 30min. Place in a supercritical drying vessel, inject liquid CO2, set the temperature to 40℃ and the pressure to 8MPa, maintain for 4h, then slowly depressurize and remove the sample. Place the dried sample in a corundum boat, put it in a tube furnace, introduce argon gas, heat to 800℃ at 3℃ / min and hold for 1h, then cool naturally to room temperature.

[0071] Example 3 This embodiment describes a method for preparing potassium-nitrogen co-doped artificial graphite, which includes the following steps.

[0072] (I) Activation Artificial graphite (Dv50 of 12μm, specific surface area of ​​3.2m²) was used. 2 10.0 g of a corundum boat (with a graphitization degree of 89%) was placed in a tube furnace and a mixed gas with a volume ratio of Ar:H2 of 95:5 was introduced. The flow rate of the mixed gas was 200 mL / min. The temperature was increased to 600 °C at 5 °C / min and held for 2 h. The mixture was then naturally cooled to room temperature to obtain the precursor.

[0073] (II) Preparation of composite slurry Weigh 3.0 g of analytical grade KOH and add it to deionized water to prepare a 20 wt.% KOH solution. Stir magnetically until completely dissolved. Weigh 1.0 g of ammonium bicarbonate and slowly add it to the KOH solution, stirring until dissolved. Add 1.5 g of urea and 0.15 g of PEG-400 sequentially to obtain a mixture. Add 10.0 g of the precursor to the mixture and transfer it to a 100 mL beaker. Place the beaker in an ultrasonic cell disruptor to obtain a composite slurry. The parameters of the ultrasonic cell disruptor are: pulse mode, power 600 W, working time 3 s, interval time 2 s, total processing time 30 min, and water temperature maintained at 25 ± 2℃.

[0074] (III) Microwave reaction The composite slurry was transferred to a quartz reactor, the system was sealed, and a vacuum was applied to -0.095 MPa. Nitrogen gas was then introduced to 0.2 MPa, and stirring was started. The stirring speed was set to 150 rpm, and the microwave power to 1.2 kW. The temperature was raised from room temperature to 600°C and held for 5 minutes. The microwave power was then adjusted to 1.0 kW, and the pressure was slowly released to atmospheric pressure. The stirring speed was increased to 300 rpm, and the temperature was raised to 800°C and held for 12 minutes. The nitrogen gas was then switched to a mixed gas (N2 and H2 volume ratio of 95:5). The system pressure was adjusted to 0.08 MPa, and the microwave power was adjusted to 0.6 kW. The stirring speed was maintained at 250 rpm, and the temperature was lowered to 750°C and held for 6 minutes.

[0075] (IV) Post-processing Turn off the microwave power, start the programmed cooling system, set the cooling rate to 3℃ / min, and continue to cool naturally to room temperature after reaching 200℃. Take the product after the microwave reaction, add 100mL of 1mol / L HCl, heat in a water bath to 60℃ and stir for 2h, let it stand to precipitate, discard the supernatant, add 100mL of 0.5mol / L H2SO4, stir at room temperature for 1h, centrifuge and collect the solid. Wash repeatedly with deionized water until the pH of the filtrate is 6.8~7.2 and the conductivity is <10μS / cm, then wash three times with anhydrous ethanol, 50mL each time, soaking for 30min. Place in a supercritical drying vessel, inject liquid CO2, set the temperature to 40℃ and the pressure to 8MPa, maintain for 4h, then slowly depressurize and remove the sample. Place the dried sample in a corundum boat, put it in a tube furnace, introduce argon gas, heat to 800℃ at 3℃ / min and hold for 1h, then cool naturally to room temperature.

[0076] Example 4 This embodiment describes a method for preparing potassium-nitrogen co-doped artificial graphite, which includes the following steps.

[0077] (I) Activation Artificial graphite (Dv50 of 10 μm, specific surface area of ​​3.1 m²) was used. 210.0 g of a corundum boat (with a graphitization degree of 90%) was placed in a tube furnace and a N2:H2 mixture with a volume ratio of 90:10 was introduced at a flow rate of 220 mL / min. The temperature was increased to 750 °C at 8 °C / min and held for 0.5 h. The mixture was then allowed to cool naturally to room temperature to obtain the precursor.

[0078] (II) Preparation of composite slurry Weigh 4.0 g of analytical grade KOH and add it to deionized water to prepare a 30 wt.% KOH solution. Stir magnetically until completely dissolved. Weigh 1.2 g of ammonium bicarbonate and slowly add it to the KOH solution, stirring until dissolved. Add 1.8 g of urea and 0.20 g of PEG-400 sequentially to obtain a mixture. Add 10.0 g of the precursor to the mixture and transfer it to a 100 mL beaker. Place the beaker in an ultrasonic cell disruptor to obtain a composite slurry. The parameters of the ultrasonic cell disruptor are: pulse mode, power 750 W, working time 5 s, interval time 3 s, total processing time 45 min, and water temperature maintained at 25 ± 2℃.

[0079] (III) Microwave reaction The composite slurry was transferred to a quartz reactor, the system was sealed, and a vacuum was applied to -0.095 MPa. Argon gas was then introduced to 0.3 MPa, and stirring was started. The stirring speed was set to 120 rpm, and the microwave power was 1.2 kW. The temperature was raised from room temperature to 650°C and held for 5 minutes. The microwave power was then adjusted to 0.8 kW, the pressure was slowly released to atmospheric pressure, the stirring speed was increased to 300 rpm, and the temperature was raised to 800°C and held for 10 minutes. The argon gas was then switched to a mixed gas (N2 and H2 volume ratio of 95:5), the system pressure was adjusted to 0.10 MPa, the microwave power was adjusted to 0.5 kW, the stirring speed was maintained at 280 rpm, and the temperature was lowered to 750°C and held for 8 minutes.

[0080] (IV) Post-processing Turn off the microwave power, start the programmed cooling system, set the cooling rate to 5℃ / min, and continue to cool naturally to room temperature after reaching 200℃. Take the product after the microwave reaction, add 80mL of 1.5mol / L HCl, heat in a water bath to 55℃ and stir for 3h, let it stand to precipitate, discard the supernatant, add 100mL of 1.0mol / L H2SO4, stir at room temperature for 2h, centrifuge and collect the solid. Wash repeatedly with deionized water until the pH of the filtrate is 6.8~7.2 and the conductivity is <10μS / cm, then wash three times with anhydrous ethanol, 50mL each time, soaking for 25min. Place in a supercritical drying vessel, inject liquid CO2, set the temperature to 45℃ and the pressure to 8MPa, maintain for 6h, then slowly release the pressure and remove the sample. Place the dried sample in a corundum boat, put it in a tube furnace, introduce argon gas, heat to 900℃ at 5℃ / min and hold for 0.5h, then cool naturally to room temperature.

[0081] Example 5 This embodiment describes a method for preparing potassium-nitrogen co-doped artificial graphite, which includes the following steps.

[0082] (I) Activation Artificial graphite (Dv50 of 14 μm, specific surface area of ​​4.0 m²) was used. 2 10.0 g of a corundum boat (with a graphitization degree of 90%) was placed in a tube furnace and a mixed gas with a volume ratio of Ar:H2 of 97:3 was introduced. The flow rate of the mixed gas was 180 mL / min. The temperature was increased to 800 °C at 8 °C / min and held for 4 h. The mixture was then naturally cooled to room temperature to obtain the precursor.

[0083] (II) Preparation of composite slurry Weigh 5.0 g of analytical grade KOH and add it to deionized water to prepare a 35 wt.% KOH solution. Stir magnetically until completely dissolved. Weigh 1.5 g of ammonium bicarbonate and slowly add it to the KOH solution, stirring until dissolved. Add 1.5 g of urea and 0.40 g of PEG-400 sequentially to obtain a mixture. Add 10.0 g of the precursor to the mixture and transfer it to a 100 mL beaker. Place the beaker in an ultrasonic cell disruptor to obtain a composite slurry. The parameters of the ultrasonic cell disruptor are: pulse mode, power 700 W, working time 3 s, interval time 2 s, total processing time 25 min, and water temperature maintained at 25 ± 2℃.

[0084] (III) Microwave reaction The composite slurry was transferred to a quartz reactor, the system was sealed, and a vacuum was applied to -0.095 MPa. Nitrogen gas was then introduced to 0.3 MPa, and stirring was started. The stirring speed was set to 180 rpm, and the microwave power to 1.3 kW. The temperature was raised from room temperature to 650°C and held for 4 minutes. The microwave power was then adjusted to 0.9 kW, and the pressure was slowly released to atmospheric pressure. The stirring speed was increased to 280 rpm, and the temperature was raised to 850°C and held for 10 minutes. The nitrogen gas was then switched to a mixed gas (N2 and H2 volume ratio of 95:5). The system pressure was adjusted to 0.06 MPa, the microwave power to 0.7 kW, the stirring speed was controlled at 200 rpm, and the temperature was lowered to 700°C and held for 6 minutes.

[0085] (IV) Post-processing Turn off the microwave power, start the programmed cooling system, set the cooling rate to 8℃ / min, and continue to cool naturally to room temperature after reaching 150℃. Take the product after the microwave reaction, add 100mL of 1mol / L HCl, heat in a water bath to 55℃ and stir for 3h, let it stand to precipitate, discard the supernatant, add 100mL of 0.5mol / L H2SO4, stir at room temperature for 2h, centrifuge and collect the solid. Wash repeatedly with deionized water until the pH of the filtrate is 6.8~7.2 and the conductivity is <10μS / cm, then wash three times with anhydrous ethanol, 50mL each time, soaking for 30min. Place in a supercritical drying vessel, inject liquid CO2, set the temperature to 45℃ and the pressure to 8MPa, maintain for 3h, then slowly depressurize and remove the sample. Place the dried sample in a corundum boat, put it in a tube furnace, introduce argon gas, heat to 850℃ at 5℃ / min and hold for 2h, then cool naturally to room temperature.

[0086] Comparative Example 1 This comparative example illustrates a method for preparing artificial graphite, comprising the following steps.

[0087] (I) Activation Artificial graphite (Dv50 of 12μm, specific surface area of ​​3.2m²) was used. 2 10.0 g of a corundum boat (with a graphitization degree of 89%) was placed in a tube furnace and a mixed gas with a volume ratio of Ar:H2 of 95:5 was introduced. The flow rate of the mixed gas was 200 mL / min. The temperature was increased to 600 °C at 5 °C / min and held for 2 h. The mixture was then naturally cooled to room temperature to obtain the precursor.

[0088] (II) Thermal reaction The precursor was placed in a tube furnace and heated to 800°C at a rate of 10°C / min under a nitrogen atmosphere, and then held at that temperature for 2 hours.

[0089] (III) Post-processing The heat-treated product was cooled to room temperature, and 100 mL of 1 mol / L HCl was added. The mixture was heated in a water bath to 60°C and stirred for 2 hours. After settling, the supernatant was discarded. Then, 100 mL of 0.5 mol / L H₂SO₄ was added, and the mixture was stirred at room temperature for 1 hour. The solid was then centrifuged and collected. The solid was repeatedly washed with deionized water until the pH of the filtrate was 6.8–7.2 and the conductivity was <10 μS / cm. It was then washed three times with anhydrous ethanol (50 mL each time, soaking for 30 minutes). The solid was placed in a supercritical drying vessel, and liquid CO₂ was injected. The temperature was set to 40°C and the pressure to 8 MPa. After maintaining this temperature for 4 hours, the pressure was slowly released, and the sample was removed. The dried sample was placed in a corundum boat, placed in a tube furnace, and purged with argon gas. The temperature was increased to 800°C at a rate of 3°C / min and held for 1 hour. The sample was then allowed to cool naturally to room temperature.

[0090] Comparative Example 2 This comparative example illustrates a method for preparing artificial graphite, comprising the following steps.

[0091] (I) Activation Artificial graphite (Dv50 of 12μm, specific surface area of ​​3.2m²) was used. 2 10.0 g of a corundum boat (with a graphitization degree of 89%) was placed in a tube furnace and a mixed gas with a volume ratio of Ar:H2 of 95:5 was introduced. The flow rate of the mixed gas was 200 mL / min. The temperature was increased to 600 °C at 5 °C / min and held for 2 h. The mixture was then naturally cooled to room temperature to obtain the precursor.

[0092] (II) Preparation of composite slurry Weigh 4.0 g of analytical grade KOH and add it to deionized water to prepare a 30 wt.% KOH solution. Stir magnetically until completely dissolved. Add 1.8 g of urea and 0.20 g of PEG-400 sequentially to obtain a mixture. Add 10.0 g of the precursor to the mixture and transfer it to a 100 mL beaker. Place the beaker in an ultrasonic cell disruptor to obtain a composite slurry. Ultrasonic cell disruptor parameters: pulse mode, power 600 W, working time 3 s, interval time 2 s, total processing time 30 min, water temperature maintained at 25 ± 2℃.

[0093] (III) Microwave reaction The composite slurry was transferred to a quartz reactor, the system was sealed, and a vacuum was applied to -0.095 MPa. Nitrogen gas was then introduced to 0.2 MPa, and stirring was started. The stirring speed was set to 150 rpm, and the microwave power was 1.5 kW. The temperature was raised from room temperature to 600°C and held for 4 minutes. The microwave power was then adjusted to 1.0 kW, the pressure was slowly released to atmospheric pressure, the stirring speed was increased to 250 rpm, and the temperature was raised to 800°C and held for 10 minutes. The nitrogen gas was then switched to a mixed gas (N2 and H2 volume ratio of 95:5), the system pressure was adjusted to 0.08 MPa, the microwave power was adjusted to 0.6 kW, the stirring speed was maintained at 250 rpm, and the temperature was lowered to 750°C and held for 6 minutes.

[0094] (IV) Post-processing Turn off the microwave power, start the programmed cooling system, set the cooling rate to 3℃ / min, and continue to cool naturally to room temperature after reaching 200℃. Take the product after the microwave reaction, add 100mL of 1mol / L HCl, heat in a water bath to 60℃ and stir for 2h, let it stand to precipitate, discard the supernatant, add 100mL of 0.5mol / L H2SO4, stir at room temperature for 1h, centrifuge and collect the solid. Wash repeatedly with deionized water until the pH of the filtrate is 6.8~7.2 and the conductivity is <10μS / cm, then wash three times with anhydrous ethanol, 50mL each time, soaking for 30min. Place in a supercritical drying vessel, inject liquid CO2, set the temperature to 40℃ and the pressure to 8MPa, maintain for 4h, then slowly depressurize and remove the sample. Place the dried sample in a corundum boat, put it in a tube furnace, introduce argon gas, heat to 800℃ at 3℃ / min and hold for 1h, then cool naturally to room temperature.

[0095] Comparative Example 3 This comparative example illustrates a method for preparing artificial graphite, comprising the following steps.

[0096] (I) Activation Artificial graphite (Dv50 of 12μm, specific surface area of ​​3.2m²) was used. 2 10.0 g of a corundum boat (with a graphitization degree of 89%) was placed in a tube furnace and a mixed gas with a volume ratio of Ar:H2 of 95:5 was introduced. The flow rate of the mixed gas was 200 mL / min. The temperature was increased to 600 °C at 5 °C / min and held for 2 h. The mixture was then naturally cooled to room temperature to obtain the precursor.

[0097] (II) Preparation of composite slurry Weigh 1.2g of ammonium bicarbonate and add it to deionized water to prepare a 9wt.% ammonium bicarbonate solution. Add 1.8g of urea and 0.20g of PEG-400 sequentially to obtain a mixture. Add 10.0g of the precursor to the mixture and transfer it to a 100mL beaker. Place the beaker in an ultrasonic cell disruptor to obtain a composite slurry. Ultrasonic cell disruptor parameters: pulse mode, power 600W, working time 3s, interval time 2s, total processing time 30min, water temperature maintained at 25±2℃.

[0098] (III) Microwave reaction The composite slurry was transferred to a quartz reactor, the system was sealed, and a vacuum was applied to -0.095 MPa. Nitrogen gas was then introduced to 0.2 MPa, and stirring was started. The stirring speed was set to 150 rpm, and the microwave power was 1.5 kW. The temperature was raised from room temperature to 600°C and held for 4 minutes. The microwave power was then adjusted to 1.0 kW, the pressure was slowly released to atmospheric pressure, the stirring speed was increased to 250 rpm, and the temperature was raised to 800°C and held for 10 minutes. The nitrogen gas was then switched to a mixed gas (N2 and H2 volume ratio of 95:5), the system pressure was adjusted to 0.08 MPa, the microwave power was adjusted to 0.6 kW, the stirring speed was maintained at 250 rpm, and the temperature was lowered to 750°C and held for 6 minutes.

[0099] (IV) Post-processing Turn off the microwave power, start the programmed cooling system, set the cooling rate to 3℃ / min, and continue to cool naturally to room temperature after reaching 200℃. Take the product after the microwave reaction, add 100mL of 1mol / L HCl, heat in a water bath to 60℃ and stir for 2h, let it stand to precipitate, discard the supernatant, add 100mL of 0.5mol / L H2SO4, stir at room temperature for 1h, centrifuge and collect the solid. Wash repeatedly with deionized water until the pH of the filtrate is 6.8~7.2 and the conductivity is <10μS / cm, then wash three times with anhydrous ethanol, 50mL each time, soaking for 30min. Place in a supercritical drying vessel, inject liquid CO2, set the temperature to 40℃ and the pressure to 8MPa, maintain for 4h, then slowly depressurize and remove the sample. Place the dried sample in a corundum boat, put it in a tube furnace, introduce argon gas, heat to 800℃ at 3℃ / min and hold for 1h, then cool naturally to room temperature.

[0100] Comparative Example 4 This comparative example illustrates a method for preparing potassium-nitrogen co-doped artificial graphite, comprising the following steps.

[0101] (I) Activation Artificial graphite (Dv50 of 12μm, specific surface area of ​​3.2m²) was used. 2 10.0 g of a corundum boat (with a graphitization degree of 89%) was placed in a tube furnace and a mixed gas with a volume ratio of Ar:H2 of 95:5 was introduced. The flow rate of the mixed gas was 200 mL / min. The temperature was increased to 600 °C at 5 °C / min and held for 2 h. The mixture was then naturally cooled to room temperature to obtain the precursor.

[0102] (II) Preparation of composite slurry Weigh 4.0g of analytical grade KOH and add it to deionized water to prepare a 30wt.% KOH solution. Stir magnetically until completely dissolved. Weigh 1.2g of ammonium bicarbonate and slowly add it to the KOH solution, stirring until dissolved. Add 1.8g of urea and 0.20g of PEG-400 sequentially to obtain a mixture. Add 10.0g of the precursor to the mixture and transfer it to a 100mL beaker. Place the beaker in an ultrasonic cell disruptor to obtain a composite slurry. The parameters of the ultrasonic cell disruptor are: pulse mode, power 600W, working time 3s, interval time 2s, total processing time 30min, and water temperature maintained at 25±2℃.

[0103] (III) Microwave reaction The composite slurry was transferred into a quartz reactor, sealed, microwave power was 1.0KW, atmospheric pressure was maintained, stirring speed was 250rpm, the temperature was raised to 800℃ and held for 15min.

[0104] (IV) Post-processing Turn off the microwave power, start the programmed cooling system, set the cooling rate to 3℃ / min, and continue to cool naturally to room temperature after reaching 200℃. Take the product after the microwave reaction, add 100mL of 1mol / L HCl, heat in a water bath to 60℃ and stir for 2h, let it stand to precipitate, discard the supernatant, add 100mL of 0.5mol / L H2SO4, stir at room temperature for 1h, centrifuge and collect the solid. Wash repeatedly with deionized water until the pH of the filtrate is 6.8~7.2 and the conductivity is <10μS / cm, then wash three times with anhydrous ethanol, 50mL each time, soaking for 30min. Place in a supercritical drying vessel, inject liquid CO2, set the temperature to 40℃ and the pressure to 8MPa, maintain for 4h, then slowly depressurize and remove the sample. Place the dried sample in a corundum boat, put it in a tube furnace, introduce argon gas, heat to 800℃ at 3℃ / min and hold for 1h, then cool naturally to room temperature.

[0105] The artificial graphite prepared in Examples 1-5 and Comparative Examples 1-4 was tested for physical properties and chemical composition. The results are shown in Table 1. The test conditions are as follows.

[0106] Pore ​​size analysis was performed using the ASAP2460 multi-station fully automated surface area and pore size analyzer.

[0107] The tap density was determined using a BT-312 tap density tester. The sample height in the graduated cylinder was 21 mL to 25 mL, the vibration frequency was 250 times / min, and the vibration was repeated 5000 times.

[0108] The compaction density reference standard GB / T24533-2019 was used, and a UTM7305 battery powder compaction density meter was employed. The sample mass was 1g, the pressure was 10KN, and the displacement was 10mm / min.

[0109] The specific surface area was measured using a specific surface area analyzer.

[0110] The carbon content in artificial graphite was tested using a carbon-sulfur analyzer.

[0111] The oxygen and nitrogen content in artificial graphite was tested using an oxygen-nitrogen analyzer, and the metal content was tested using XRF.

[0112] The sulfur content in artificial graphite was tested using a sulfur analyzer.

[0113] Particle size was measured using a Malvern MS2000 laser particle size analyzer.

[0114] Conductivity was measured using a conductivity meter.

[0115] ID / IG was tested using a Raman spectrometer.

[0116] The ash content was determined according to the national standard GB / T 1429-2009 "Determination of Ash Content in Carbon Materials".

[0117] Interlayer spacing d002 test: The 002 and 100 diffraction peak patterns of graphite were automatically recorded using a Malvern Panaco X-ray diffractometer. The diffraction angles were read at the same time, and the diffraction angles (2θcor) after silicon powder correction were obtained by the internal standard method. The interlayer spacing d002 of graphite was calculated according to the Bragg formula d002=λ / 2sin(θcor(002)).

[0118] Graphitization degree: The graphitization degree is calculated from the interlayer spacing d002 according to the Franklin formula.

[0119] Degree of graphitization = (0.3440 - d002) / (0.3440 - 0.3354) * 100% 0.3440 represents the interlayer spacing of completely ungraphitized carbon, with units in nm.

[0120] 0.3354 represents the interlayer spacing of ideal graphite, with units in nm.

[0121] The artificial graphite prepared in Examples 1-5 and Comparative Examples 1-4 was used to make button cells for electrochemical performance testing. The results are shown in Table 2. The test conditions are as follows.

[0122] Electrochemical performance testing procedure: The artificial graphite prepared in Examples 1-5 and Comparative Examples 1-4 was dispersed in a beaker containing N-methylpyrrolidone (NMP) solvent at a mass ratio of 7:2:1 with conductive carbon black (SP) and polyvinylidene fluoride (PVDF). The mixture was heated and stirred until fully dissolved, and then stirred for 12 hours to prepare a slurry with good flowability. After uniform mixing, the slurry was uniformly coated onto copper foil to form an electrode. Subsequently, the electrode sheet was dried under vacuum at 60°C for 12 hours to ensure complete evaporation of NMP. Finally, the electrode was rolled and cut into working electrodes with a diameter of 12 mm. The active material mass loading of each electrode was approximately 1.5 mg / cm³. 2 CR2025 coin cells (including positive and negative electrode cases, spring contacts, and gaskets) were assembled. The process was carried out in a glove box filled with inert argon gas, where the water and oxygen content were both less than 0.1 ppm. The assembly sequence was: positive electrode case, working electrode, electrolyte (1M LiPF6, DEC and EC volume ratio 3:2), glass fiber separator, lithium metal sheet, gasket, spring contact, and negative electrode case. The cells were then packaged using a battery packaging machine. After packaging, the cells were placed in a 25°C constant temperature chamber for standby. The prepared lithium-ion batteries were tested using a LAND CT 3002A charge-discharge test cabinet at a voltage range of 0-3.0V and a current density of 2A / g. The 0.1C specific capacity, 0.5C specific capacity, initial coulombic efficiency, 5C / 0.1C capacity retention, and capacity retention after 200 cycles were obtained.

[0123] Table 1. Test results of the physical and chemical properties of artificial graphite in Examples 1-5 and Comparative Examples 1-4

[0124] Continued from the table above

[0125] Table 2. Electrochemical performance test results of artificial graphite in Examples 1-5 and Comparative Examples 1-4

[0126] The results in Tables 1 and 2 show that the potassium-nitrogen co-doped artificial graphite in Examples 1-5 has better physical and electrochemical properties. This indicates that the preparation method of the present invention can achieve better performance modification of graphite materials through composite pore formation of KOH and ammonium bicarbonate, doping of multiple potassium and nitrogen elements, and parameter control of multi-step microwave reaction.

[0127] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, it is not limited to those listed in the embodiments. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing potassium-nitrogen co-doped artificial graphite, characterized in that, Including the following steps: (I) Activation The precursor was obtained by heat treatment of artificial graphite in an atmosphere containing hydrogen. (II) Preparation of composite slurry A composite slurry is obtained by ultrasonic treatment of KOH, ammonium bicarbonate, urea, dispersant, water, and the precursor. (III) Microwave reaction The composite slurry is transferred into a reactor, where a first microwave reaction, a second microwave reaction, and a third microwave reaction are carried out sequentially. The conditions for the first microwave reaction, the second microwave reaction, and the third microwave reaction are as follows: First microwave reaction: microwave power of 1.2~1.5KW, temperature increased from room temperature to T1℃, carried out under an inert atmosphere and pressure of 0.1~0.3MPa. The second microwave reaction: the microwave power is 0.8~1.0KW, the temperature rises from T1℃ to T2℃, and then gradually decreases to normal pressure. The third microwave reaction: the microwave power is 0.5~0.8KW, the temperature drops from T2℃ to T3℃, and it is carried out in a mixed atmosphere of inert atmosphere and hydrogen and the pressure is 0.05~0.10MPa; (IV) Post-processing The process involves pickling, washing, drying, and annealing in sequence.

2. The method for preparing potassium-nitrogen co-doped artificial graphite according to claim 1, characterized in that, Includes at least one of the following features (1) to (12): (1) The artificial graphite has a Dv50 of 10~14μm, a specific surface area of ​​2~4m² / g, and a graphitization degree >88%; (2) The atmosphere containing hydrogen is a mixture of inert atmosphere and hydrogen, and the volume ratio of the two is 90~98:2~10, and the flow rate is 100~200mL / min; (3) The heat treatment temperature is 400~800℃, the time is 0.5~4.0h, and the heating rate is 1~10℃ / min; (4) The ultrasonic treatment is carried out in an ultrasonic cell disruptor, using pulse mode, with a power of 500~750W and a treatment time of 15~40min. (5) The temperature of T1 is 500~700℃, the time of the first microwave reaction is 3~5min, and the stirring speed is 100~180rpm; (6) The T2 is 750~900℃, the second microwave reaction time is 8~12min, and the stirring speed is 200~300rpm; (7) The T3 is 650~800℃, the third microwave reaction time is 5~8min, and the stirring speed is 200~300rpm; (8) Before the pickling, the product of the microwave reaction is cooled to below 200°C at a rate of 2~5°C / min; (9) The acid washing is performed by washing with 0.5~1.5mol / L HCl and 0.1~1.0mol / L H2SO4 for 1~4h respectively; (10) After washing with water until neutral, solvent replacement is performed with ethanol; (11) The drying is carried out using supercritical CO2 drying, and the drying temperature is 35~50℃, the pressure is 5~10MPa, and the drying time is 2~10h; (12) The annealing treatment is carried out in an inert atmosphere at 650~900℃ for 0.5~2.0h.

3. The method for preparing potassium-nitrogen co-doped artificial graphite according to claim 1, characterized in that, The purity of the KOH is 90-95%, the purity of the ammonium bicarbonate is ≥99%, and the mass ratio of the KOH to the ammonium bicarbonate is 3-5:

1.

4. The method for preparing potassium-nitrogen co-doped artificial graphite according to claim 1, characterized in that, The purity of the urea is ≥99.5%, and the mass ratio of the urea to the precursor is 1:5~8.

5. The method for preparing potassium-nitrogen co-doped artificial graphite according to claim 1, characterized in that, The concentration of KOH in the water is 20~40 wt.%.

6. The method for preparing potassium-nitrogen co-doped artificial graphite according to claim 1, characterized in that, The mass ratio of the graphite to the ammonium bicarbonate is 10:1~2.

7. The method for preparing potassium-nitrogen co-doped artificial graphite according to claim 1, characterized in that, The dispersant is a polyethylene glycol-based substance, and the dispersant accounts for 1 to 3% of the mass of the precursor.

8. The potassium-nitrogen co-doped artificial graphite prepared by the method for preparing potassium-nitrogen co-doped artificial graphite according to any one of claims 1 to 7, characterized in that, It has a three-level pore structure consisting of micropores of 0.8~1.2nm, mesopores of 10~20nm and macropores of 50~200nm, wherein the proportion of micropores is 15~25%, the proportion of mesopores is 40~50%, and the proportion of macropores is 25~40%.

9. The potassium-nitrogen co-doped artificial graphite according to claim 8, characterized in that, Includes at least one of the following features (i) to (xvi): (i) Total pore volume is 0.08~0.15cm 3 / g; (ii) Aperture distribution deviation <15%; (iii) Specific surface area is 8~15m² 2 / g; (iv) The interlayer spacing of graphite is 0.338~0.345 nm; (v) Carbon content ≥98.5 wt.%, potassium content 0.3~0.8 wt.%, nitrogen content 1.2~2.5 wt.%, oxygen content 0.5~1.2 wt.%, metal impurities <20 ppm, sulfur content <50 ppm, and potassium element is distributed in a gradient inside, while the nitrogen element content on the surface is 1.5~2.0 times that inside; (vi) Ash content <0.2wt.%; (vii) The degree of graphitization is 85-92%; (viii) ID / IG is 0.15~0.18; (ix) Dv50 is 10~15μm, Dv90 <30μm; (x) Tap density is 1.0~1.2 g / cm³ 3 ; (xi) The compacted density is 1.6~1.8 g / cm³. 3 ; (xii) Conductivity ≥ 2.5 S / cm; (xiii) The specific capacity at 0.1C is 450~480mAh / g, and the specific capacity at 0.5C is 360~380mAh / g; (xiv) The initial Coulomb efficiency is 94.0–97.0%; (xv) Capacity retention ≥80% at 5C / 0.1C; (xvi) Capacity retention ≥ 90% after 200 cycles.

10. A secondary battery, comprising a positive electrode material, a separator, an electrolyte, and a negative electrode material, characterized in that, The negative electrode material includes the potassium-nitrogen co-doped artificial graphite as described in claim 8 or 9.

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