Lithium ion battery graphite negative electrode material and preparation method thereof

Abstract

The application belongs to the field of secondary battery materials, and discloses a lithium ion battery graphite negative electrode material and a preparation method thereof, the preparation method comprising the following steps: S1, heat treating natural graphite; S2, forming a thick graphene quantum dot coating layer on the surface of graphite particles through carbon thermal reduction reaction by carrying graphene quantum dot precursors in inert gas; S3, placing the graphene quantum dot coated graphite particles in a supercritical CO2 reactor, and forming a gradient pore channel structure composed of mesopores and micropores in the graphite particles through supercritical reaction; S4, adding the graphite particles with the pore channel structure into P and N precursors, ultrasonic adsorbing, and then high-temperature treatment, so as to realize phosphorus and nitrogen double-element doping; and S5, post-treatment; the method constructs a conductive network through graphene quantum dots, shortens the ion diffusion path through gradient pore channels, and optimizes the electronic structure through heteroatom doping, so that the lithium ion migration rate and the rapid charging and discharging performance of the graphite negative electrode are improved.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery materials technology, specifically relating to a graphite anode material for lithium-ion batteries and its preparation method. Background Technology

[0002] Lithium-ion batteries, as chemical energy storage devices, possess high energy density and long cycle life, making them irreplaceable in consumer electronics products such as mobile phones. Simultaneously, their application in the new energy vehicle sector is gradually becoming mainstream. However, regardless of whether it's a mobile phone or an electric vehicle, the most noticeable user experience issues are twofold: firstly, the battery life or range is never long enough (for lithium-ion batteries, this translates to low energy density), and secondly, the replenishment speed after energy depletion is never fast enough (for lithium-ion batteries, this translates to slow charging speed).

[0003] Anode materials are one of the key factors restricting the energy density and high-current charge-discharge performance of lithium-ion batteries. To meet the requirements of driving range or distance, artificial graphite prepared from natural graphite or needle coke is currently commonly used as the anode material. The preparation process of natural graphite anode materials in the industry has formed a relatively mature system. The mainstream technical route is mainly based on the purification, crushing and shaping, high-temperature graphitization and surface coating of natural graphite. However, the existing technology still has obvious limitations: on the one hand, single-dimensional modification (such as coating or doping only) is difficult to balance electron conduction, ion diffusion and structural stability; on the other hand, the problems of structural defects caused by rapid heating during high-temperature graphitization and weak bonding between the coating layer and the graphite body have not been solved. As a result, the capacity decay of the material is serious at high rates (10C and above), which makes it difficult to meet the requirements of rapid charge-discharge and cycle stability of new energy vehicles. Summary of the Invention

[0004] To address the shortcomings mentioned in the background art, the present invention aims to provide a graphite anode material for lithium-ion batteries and its preparation method. Through the synergistic effect of constructing a conductive network with graphene quantum dots, shortening the ion diffusion path with gradient channels, and optimizing the electronic structure with heteroatom doping, the fast charge and discharge performance of the graphite anode can be significantly improved while maintaining its high specific capacity characteristics.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] A method for preparing a graphite anode material for lithium-ion batteries includes the following steps:

[0007] S1. High-purity graphite is obtained by purifying natural graphite and placing it in a high-temperature heat treatment furnace. Under the protection of the atmosphere, it is first slowly heated to 600-800℃ at a heating rate of 1-5℃ / min and held for 1-2 hours. Then, it is rapidly heated to 1000-1100℃ at a heating rate of 10-20℃ / min and sintered at a constant temperature for 2-3 hours.

[0008] S2. A thick graphene quantum dot coating layer is formed by a carbothermic reduction reaction on the surface of graphite particles using an inert gas to carry the graphene quantum dot precursor.

[0009] S3. Graphene quantum dot-coated graphite particles are placed in a supercritical CO2 reactor, and a gradient pore structure composed of mesopores and micropores is formed inside the graphite particles through supercritical reaction.

[0010] S4. The graphite particles forming the porous structure are added to the P and N precursors and then ultrasonically adsorbed and treated at high temperature to achieve phosphorus and nitrogen dual element doping.

[0011] S5. After the reaction is completed, the material is cooled to room temperature in the furnace and then mechanically dispersed, sieved, and magnetically separated to obtain the graphite anode material for lithium-ion batteries.

[0012] Preferably, step S1 further includes using an air jet mill or mechanical grinding equipment to crush natural graphite to the micron level and form spherical or near-spherical particles through interparticle collisions.

[0013] Preferably, the natural graphite has a carbon content of ≥99.9% and an ash content of ≤0.1%, and the high-temperature heat treatment furnace is a rotary drum furnace.

[0014] Preferably, step S2 specifically includes:

[0015] Citric acid and urea are mixed to obtain a precursor. The precursor is heated to 180-200℃ to vaporize it and obtain precursor vapor. The vapor is introduced into a high-temperature heat treatment furnace through nitrogen carrier gas and reacted at 1000-1100℃ for 20-40 minutes to form a graphene quantum dot coating layer on the surface of graphite particles. After the reaction is completed, nitrogen gas is continued to be introduced for 20-40 minutes, and the furnace is cooled to room temperature to obtain graphene quantum dot coated graphite particles.

[0016] Preferably, citric acid and urea are mixed at a mass ratio of 3:1, the nitrogen carrier gas flow rate is 50-80 mL / min, and the precursor vapor concentration is 5-8 g / m³. 3， The thickness of the graphene quantum dot coating is 2~5nm.

[0017] Preferably, step S3 specifically includes:

[0018] The graphite particles obtained in step S2 were transferred to a supercritical CO2 reactor, anhydrous ethanol was added as a co-solvent, the reactor was sealed, CO2 was introduced to a pressure of 13~14MPa, the temperature was raised to 45℃, the mixture was stirred at 300r / min for 60min, the pressure was slowly reduced to atmospheric pressure, the sample was taken out and vacuum dried at 60℃ for 8h, and a gradient pore structure was formed inside the graphite particles.

[0019] Preferably, the solid-liquid ratio of graphene quantum dot-coated graphite particles to anhydrous ethanol is 1:5, the pressure drop rate is ≤0.5MPa / min, and 5~20nm mesopores and 0.5~2nm micropores are formed inside the graphite.

[0020] Preferably, step S4 specifically includes:

[0021] Ammonium dihydrogen phosphate was dissolved in deionized water to obtain a precursor solution. The graphite particles obtained in step S3 were added to the solution and ultrasonically dispersed at 300W for 20-40 min. After filtration, the solution was dried at 100℃ for 6 h. The solution was then placed in a tube furnace and heated to 800℃ at 5℃ / min under a nitrogen atmosphere and held for 2 h to achieve nitrogen and phosphorus dual-element doping.

[0022] Preferably, the precursor solution has a mass concentration of 5 wt% ammonium dihydrogen phosphate and a solid-liquid ratio of graphite particles to the precursor solution of 1:8~10.

[0023] A graphite anode material for lithium-ion batteries is prepared by the above method.

[0024] The beneficial effects of this invention are:

[0025] This invention utilizes the synergistic effect of constructing a conductive network with graphene quantum dots, shortening the ion diffusion path with gradient channels, and optimizing the electronic structure through heteroatom doping to increase the lithium-ion diffusion coefficient of the material to 3.8 × 10⁻⁶. -9 cm 2The capacity retention rate at 10C rate reaches over 85%. In the heat treatment process, the graphene quantum dot precursor is carried into a drum furnace by an inert gas, where high temperatures cause a carbothermic reduction reaction on the graphite particle surface, forming a 2-5 nm thick graphene quantum dot coating. This coating not only constructs a three-dimensional conductive network but also reduces the lithium-ion migration barrier through quantum size effects while retaining the high crystallinity of the graphite bulk. This invention places graphite particles in a supercritical CO2 reactor and, by controlling the diffusion and desorption behavior of the supercritical fluid, forms a gradient pore structure composed of mesopores and micropores within the graphite particles. This structure shortens the lithium-ion diffusion path (increasing the migration rate) and increases the contact area between the electrode and the electrolyte, significantly improving ion transport dynamics during rapid charge and discharge. This invention introduces nitrogen and phosphorus doping into the graphite particles. Nitrogen atoms enhance the electronic conductivity of the material, while phosphorus atoms increase the interlayer spacing; their synergistic effect further reduces the lithium-ion insertion / extraction resistance. After this modification, the lithium-ion diffusion coefficient of the material can be improved by 1 to 2 orders of magnitude, and the capacity retention rate at 10C rate is improved by more than 25% compared with the original process.

[0026] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0028] In the description of this invention, it should be understood that the terms "opening", "upper", "lower", "thickness", "top", "middle", "length", "inner", "around", etc., which indicate orientation or positional relationship, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the components or elements referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting this invention.

[0029] Example 1

[0030] A method for preparing a graphite anode material for lithium-ion batteries includes the following steps:

[0031] S1. Natural graphite is coarsely crushed to a particle size ≤5mm using a jaw crusher, and then ground by an air jet mill to obtain primary particles with a D50 of 15~20μm. After washing with deionized water 3~5 times, it is vacuum dried at 80℃ for 12h. After purification, high-purity graphite is obtained. It is placed in a high-temperature heat treatment furnace and, under a protective atmosphere, is first slowly heated to 600℃ at a heating rate of 5℃ / min; the holding time is 2h; then it is rapidly heated to 1100℃ at a heating rate of 10℃ / min and sintered at a constant temperature for 2h.

[0032] S2. Citric acid and urea are mixed at a mass ratio of 3:1 to obtain a precursor. The precursor is heated to 200℃ to vaporize it, resulting in precursor vapor. This vapor is then introduced into a high-temperature heat treatment furnace using nitrogen as a carrier gas at a flow rate of 50 mL / min, with the precursor vapor concentration controlled at 8 g / m³. 3 The reaction was carried out at 1000℃ for 40 minutes to form a graphene quantum dot coating layer with a thickness of 2~5nm on the surface of the graphite particles. After the reaction was completed, nitrogen gas was continued to be introduced for 20 minutes, and the furnace was cooled to room temperature to obtain graphene quantum dot coated graphite particles.

[0033] S3. Transfer the graphite particles obtained in step S2 to a supercritical CO2 reactor, add anhydrous ethanol as a co-solvent, the solid-liquid ratio of graphite particles to anhydrous ethanol is 1:5, seal the reactor, introduce CO2 to a pressure of 14MPa, heat to 45℃, stir at 300r / min for 60min, slowly reduce the pressure to atmospheric pressure at a depressurization rate ≤0.5MPa / min, take out the sample and vacuum dry at 60℃ for 8h, forming a gradient pore structure of 5~20nm mesopores and 0.5~2nm micropores inside the graphite particles;

[0034] S4. Dissolve ammonium dihydrogen phosphate in deionized water to obtain a 5wt% precursor solution. Add the graphite particles obtained in step S3 to the solution at a solid-liquid ratio of 1:8. Disperse the solution by ultrasonication at 300W for 40 min. After filtration, dry at 100℃ for 6 h. Place the solution in a tube furnace and heat it to 800℃ at 5℃ / min under a nitrogen atmosphere. Hold the temperature for 2 h to achieve nitrogen and phosphorus dual-element doping.

[0035] S5. After the reaction is completed, the material is cooled to room temperature in the furnace and then mechanically dispersed, sieved, and magnetically separated to obtain the graphite anode material for lithium-ion batteries.

[0036] Example 2

[0037] A method for preparing a graphite anode material for lithium-ion batteries includes the following steps:

[0038] S1. Natural graphite is coarsely crushed to a particle size ≤5mm using a jaw crusher, and then ground by an air jet mill to obtain primary particles with a D50 of 15~20μm. After washing with deionized water 3~5 times, it is vacuum dried at 80℃ for 12h. After purification, high-purity graphite is obtained. It is placed in a high-temperature heat treatment furnace and, under a protective atmosphere, is first slowly heated to 800℃ at a heating rate of 1℃ / min; the holding time is 1h; then it is rapidly heated to 1000℃ at a heating rate of 20℃ / min and sintered at a constant temperature for 3h.

[0039] S2. Citric acid and urea are mixed at a mass ratio of 3:1 to obtain a precursor. The precursor is heated to 180°C to vaporize it, resulting in precursor vapor. This vapor is then introduced into a high-temperature heat treatment furnace using nitrogen as a carrier gas at a flow rate of 80 mL / min, with the precursor vapor concentration controlled at 5 g / m³. 3 The reaction was carried out at 1100℃ for 20 min to form a graphene quantum dot coating layer with a thickness of 2~5 nm on the surface of the graphite particles. After the reaction was completed, nitrogen gas was continuously purged for 40 min, and the furnace was cooled to room temperature to obtain graphene quantum dot coated graphite particles.

[0040] S3. Transfer the graphite particles obtained in step S2 to a supercritical CO2 reactor, add anhydrous ethanol as a co-solvent, the solid-liquid ratio of graphite particles to anhydrous ethanol is 1:5, seal the reactor, introduce CO2 to a pressure of 13MPa, heat to 45℃, stir at 300r / min for 60min, slowly reduce the pressure to atmospheric pressure at a depressurization rate ≤0.5MPa / min, take out the sample and vacuum dry at 60℃ for 8h, forming a gradient pore structure of 5~20nm mesopores and 0.5~2nm micropores inside the graphite particles;

[0041] S4. Dissolve ammonium dihydrogen phosphate in deionized water to obtain a 5wt% precursor solution. Add the graphite particles obtained in step S3 to the solution at a solid-liquid ratio of 1:10. Disperse the solution by ultrasonication at 300W for 20 min. After filtration, dry at 100℃ for 6 h. Place the solution in a tube furnace and heat it to 800℃ at 5℃ / min under a nitrogen atmosphere. Hold the temperature for 2 h to achieve nitrogen and phosphorus dual-element doping.

[0042] S5. After the reaction is completed, the material is cooled to room temperature in the furnace and then mechanically dispersed, sieved, and magnetically separated to obtain the graphite anode material for lithium-ion batteries.

[0043] Example 3

[0044] A method for preparing a graphite anode material for lithium-ion batteries includes the following steps:

[0045] S1. Natural graphite is coarsely crushed to a particle size ≤5mm using a jaw crusher, and then ground by an air jet mill to obtain primary particles with a D50 of 15~20μm. After washing with deionized water 3~5 times, it is vacuum dried at 80℃ for 12h. After purification, high-purity graphite is obtained. It is placed in a high-temperature heat treatment furnace and, under a protective atmosphere, is first slowly heated to 700℃ at a heating rate of 3℃ / min; the holding time is 1.5h; then it is rapidly heated to 1050℃ at a heating rate of 15℃ / min and sintered at a constant temperature for 2.5h.

[0046] S2. Citric acid and urea are mixed at a mass ratio of 3:1 to obtain a precursor. The precursor is heated to 190℃ to vaporize it, resulting in precursor vapor. This vapor is then introduced into a high-temperature heat treatment furnace via nitrogen carrier gas at a flow rate of 65 mL / min, controlling the precursor vapor concentration to be 6.5 g / m³. 3 The reaction was carried out at 1050℃ for 30 minutes to form a graphene quantum dot coating layer with a thickness of 2~5nm on the surface of the graphite particles. After the reaction was completed, nitrogen gas was continued to be introduced for 30 minutes, and the furnace was cooled to room temperature to obtain graphene quantum dot coated graphite particles.

[0047] S3. Transfer the graphite particles obtained in step S2 to a supercritical CO2 reactor, add anhydrous ethanol as a co-solvent, the solid-liquid ratio of graphite particles to anhydrous ethanol is 1:5, seal the reactor, introduce CO2 to a pressure of 13.5 MPa, heat to 45℃, stir at 300 r / min for 60 min, slowly reduce the pressure to atmospheric pressure at a depressurization rate ≤0.5 MPa / min, take out the sample and vacuum dry at 60℃ for 8 h, forming a gradient pore structure of 5~20 nm mesopores and 0.5~2 nm micropores inside the graphite particles;

[0048] S4. Dissolve ammonium dihydrogen phosphate in deionized water to obtain a 5wt% precursor solution. Add the graphite particles obtained in step S3 to the solution at a solid-liquid ratio of 1:9. Disperse the solution by ultrasonication at 300W for 30 min. After filtration, dry at 100℃ for 6 h. Place the solution in a tube furnace and heat it to 800℃ at 5℃ / min under a nitrogen atmosphere. Hold the temperature for 2 h to achieve nitrogen and phosphorus dual-element doping.

[0049] S5. After the reaction is completed, the material is cooled to room temperature in the furnace and then mechanically dispersed, sieved, and magnetically separated to obtain the graphite anode material for lithium-ion batteries.

[0050] Comparative Example 1

[0051] A method for preparing a graphite anode material for lithium-ion batteries includes the following steps:

[0052] S1. Natural graphite is coarsely crushed to a particle size ≤5mm using a jaw crusher, and then ground by an air jet mill to obtain primary particles with a D50 of 15~20μm. After washing with deionized water 3~5 times, it is vacuum dried at 80℃ for 12h. After purification, high-purity graphite is obtained. It is placed in a high-temperature heat treatment furnace and, under a protective atmosphere, is first slowly heated to 700℃ at a heating rate of 3℃ / min; the holding time is 1.5h; then it is rapidly heated to 1050℃ at a heating rate of 15℃ / min and sintered at a constant temperature for 2.5h.

[0053] S2. Transfer the graphite particles obtained in step S1 to a supercritical CO2 reactor, add anhydrous ethanol as a co-solvent, the solid-liquid ratio of graphite particles to anhydrous ethanol is 1:5, seal the reactor, introduce CO2 to a pressure of 13.5 MPa, heat to 45℃, stir at 300 r / min for 60 min, slowly reduce the pressure to atmospheric pressure at a depressurization rate ≤0.5 MPa / min, take out the sample and vacuum dry at 60℃ for 8 h, forming a gradient pore structure of 5~20 nm mesopores and 0.5~2 nm micropores inside the graphite particles;

[0054] S3. Dissolve ammonium dihydrogen phosphate in deionized water to obtain a 5wt% precursor solution. Add the graphite particles obtained in step S2 to the solution at a solid-liquid ratio of 1:9. Disperse the solution by ultrasonication at 300W for 30 min. After filtration, dry at 100℃ for 6 h. Place the solution in a tube furnace and heat it to 800℃ at 5℃ / min under a nitrogen atmosphere. Hold the temperature for 2 h to achieve nitrogen and phosphorus dual-element doping.

[0055] S4. After the reaction is completed, the material is cooled to room temperature in the furnace and then mechanically dispersed, sieved, and magnetically separated to obtain the graphite anode material for lithium-ion batteries.

[0056] Comparative Example 2

[0057] A method for preparing a graphite anode material for lithium-ion batteries includes the following steps:

[0058] S1. Natural graphite is coarsely crushed to a particle size ≤5mm using a jaw crusher, and then ground by an air jet mill to obtain primary particles with a D50 of 15~20μm. After washing with deionized water 3~5 times, it is vacuum dried at 80℃ for 12h. After purification, high-purity graphite is obtained. It is placed in a high-temperature heat treatment furnace and, under a protective atmosphere, is first slowly heated to 700℃ at a heating rate of 3℃ / min; the holding time is 1.5h; then it is rapidly heated to 1050℃ at a heating rate of 15℃ / min and sintered at a constant temperature for 2.5h.

[0059] S2. Citric acid and urea are mixed at a mass ratio of 3:1 to obtain a precursor. The precursor is heated to 190℃ to vaporize it, resulting in precursor vapor. This vapor is then introduced into a high-temperature heat treatment furnace via nitrogen carrier gas at a flow rate of 65 mL / min, controlling the precursor vapor concentration to be 6.5 g / m³. 3 The reaction was carried out at 1050℃ for 30 minutes to form a graphene quantum dot coating layer with a thickness of 2~5nm on the surface of the graphite particles. After the reaction was completed, nitrogen gas was continued to be introduced for 30 minutes, and the furnace was cooled to room temperature to obtain graphene quantum dot coated graphite particles.

[0060] S3. Dissolve ammonium dihydrogen phosphate in deionized water to obtain a 5wt% precursor solution. Add the graphite particles obtained in step S2 to the solution at a solid-liquid ratio of 1:9. Disperse the solution by ultrasonication at 300W for 30 min. After filtration, dry at 100℃ for 6 h. Place the solution in a tube furnace and heat it to 800℃ at 5℃ / min under a nitrogen atmosphere. Hold the temperature for 2 h to achieve nitrogen and phosphorus dual-element doping.

[0061] S4. After the reaction is completed, the material is cooled to room temperature in the furnace and then mechanically dispersed, sieved, and magnetically separated to obtain the graphite anode material for lithium-ion batteries.

[0062] Comparative Example 3

[0063] A method for preparing a graphite anode material for lithium-ion batteries includes the following steps:

[0064] S1. Natural graphite is coarsely crushed to a particle size ≤5mm using a jaw crusher, and then ground by an air jet mill to obtain primary particles with a D50 of 15~20μm. After washing with deionized water 3~5 times, it is vacuum dried at 80℃ for 12h. After purification, high-purity graphite is obtained. It is placed in a high-temperature heat treatment furnace and, under a protective atmosphere, is first slowly heated to 700℃ at a heating rate of 3℃ / min; the holding time is 1.5h; then it is rapidly heated to 1050℃ at a heating rate of 15℃ / min and sintered at a constant temperature for 2.5h.

[0065] S2. Citric acid and urea are mixed at a mass ratio of 3:1 to obtain a precursor. The precursor is heated to 190℃ to vaporize it, resulting in precursor vapor. This vapor is then introduced into a high-temperature heat treatment furnace via nitrogen carrier gas at a flow rate of 65 mL / min, controlling the precursor vapor concentration to be 6.5 g / m³. 3 The reaction was carried out at 1050℃ for 30 minutes to form a graphene quantum dot coating layer with a thickness of 2~5nm on the surface of the graphite particles. After the reaction was completed, nitrogen gas was continued to be introduced for 30 minutes, and the furnace was cooled to room temperature to obtain graphene quantum dot coated graphite particles.

[0066] S3. Transfer the graphite particles obtained in step S2 to a supercritical CO2 reactor, add anhydrous ethanol as a co-solvent, the solid-liquid ratio of graphite particles to anhydrous ethanol is 1:5, seal the reactor, introduce CO2 to a pressure of 13.5 MPa, heat to 45℃, stir at 300 r / min for 60 min, slowly reduce the pressure to atmospheric pressure at a depressurization rate ≤0.5 MPa / min, take out the sample and vacuum dry at 60℃ for 8 h, forming a gradient pore structure of 5~20 nm mesopores and 0.5~2 nm micropores inside the graphite particles;

[0067] S4. After the reaction is completed, the material is cooled to room temperature in the furnace and then mechanically dispersed, sieved, and magnetically separated to obtain the graphite anode material for lithium-ion batteries.

[0068] Performance testing

[0069] The lithium-ion battery graphite anode materials prepared in Example 3 and Comparative Examples 1-3 were subjected to the following performance tests:

[0070] (1) Lithium-ion diffusion coefficient test: Electrochemical impedance spectroscopy (EIS) combined with galvanostatic titration (GITT) was used. 80 mg of modified graphite sample, 10 mg of acetylene black (conductive agent), and 10 mg of PVDF were weighed, and N-methylpyrrolidone was added to make a paste. The paste was then uniformly coated onto a 12 μm copper foil (coating area 1.13 cm²). 2 After vacuum drying at 60℃ for 12 hours, the product was compressed into tablets using a tablet press at a pressure of 10 MPa, with the active substance loading controlled at 1.0-1.2 mg / cm³. 2 A negative electrode was obtained. CR2032 button-type half-cells were assembled in an argon glove box (H2O, O2 < 0.1ppm), using lithium metal sheets as the positive electrode, Celgard 2400 as the separator, and a 1 mol / L LiPF6 solution as the electrolyte. The solvent was a mixture of EC, DMC, and EMC in a 1:1:1 volume ratio. Electrochemical induction (EIS) tests were performed using an electrochemical workstation, and GITT tests were conducted using a LANDCT2001A battery testing system. The final lithium-ion diffusion coefficient was the average of the EIS and GITT calculations.

[0071] (2) Rate performance test: The LANDCT2001A battery test system was used. The voltage range was 0.01~2.0V. The cycle was 3 times at 0.1C (activation) → 1C → 2C → 5C → 10C → 20C, and 5 times at each rate. The specific capacity (mAh / g) and rate capacity retention rate (%) were calculated. Specific capacity = (discharge time × current) / (mass of active material). Rate capacity retention rate = (average discharge capacity at nC / third discharge capacity at 0.1C) × 100% (n is the rate coefficient, such as n=10 for 10C).

[0072] (3) Cyclic performance test: 500 cycles at 1C (1C=372mA / g), voltage range 0.01~2.0V, calculate the cycle capacity retention rate (%), cycle capacity retention rate = (500th discharge capacity / 3rd discharge capacity) × 100%;

[0073] (4) Physical performance test: The sample was first degassed at 200℃ for 4h, and the specific surface area (BET) was tested by a fully automatic specific surface area analyzer. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) equation. XRD analysis was performed by an X-ray diffractometer, and the interlayer spacing d was calculated by the Bragg equation.

[0074] The obtained data is shown in Table 1 below:

[0075] Table 1 Performance test results of graphite anode materials for lithium-ion batteries

[0076] Performance indicators Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Lithium-ion diffusion coefficient (cm² / s) <![CDATA[3.8×10 -9 ]]> <![CDATA[8.2×10 -10 ]]> <![CDATA[1.1×10 -9 ]]> <![CDATA[1.5×10 -9 ]]> 0.1C specific capacity (mAh / g) 365 362 358 360 10C capacity retention (%) 86 52 68 72 20C capacity retention (%) 65 28 42 45 Retention rate (%) after 500 cycles at 1C 92 78 83 85 <![CDATA[BET specific surface area (m 2 / g)]]> 18.6 8.2 9.1 17.9 Interlayer spacing d (nm) 0.340 0.336 0.341 0.337

[0077] As can be seen from the data in Table 1, the lithium-ion diffusion coefficient of the sample in Example 3 is significantly higher than that of the three comparative examples, indicating that the synergistic effect of the three steps can significantly reduce the lithium-ion migration resistance. The sample in Comparative Example 1 has the lowest diffusion coefficient, proving that the three-dimensional conductive network of graphene quantum dots is the key to improving ion transport. The diffusion coefficient of the sample in Comparative Example 2 is only 29% of that of the sample in Example 3, indicating that the gradient channel can effectively shorten the ion diffusion path. Due to the small interlayer spacing, the sample in Comparative Example 3 has a large lithium-ion insertion / extraction resistance, and the diffusion coefficient decreases by 60%.

[0078] At high rates (10C, 20C), the sample of Example 3 showed a significant advantage in capacity retention. The sample of Comparative Example 1 had a capacity retention of only 52% at 10C, which was due to the lack of a conductive coating layer, resulting in blocked electron conduction and severe polarization. Although the sample of Comparative Example 2 had a larger interlayer spacing, its low specific surface area resulted in insufficient electrode / electrolyte contact area, leading to a 35% decrease in capacity retention at 20C. The sample of Comparative Example 3 had an unoptimized interlayer spacing, making lithium-ion insertion difficult at high rates, and its capacity retention at 10C was 16% lower than that of the modified sample.

[0079] The sample in Example 3 achieved a 92% cycle retention rate after 500 cycles, thanks to the graphene quantum dot coating layer suppressing electrolyte decomposition, the gradient channels mitigating volume expansion, and heteroatom doping enhancing structural stability. The sample in Comparative Example 1, lacking coating, had a cycle retention rate of only 78%, while the sample in Comparative Example 2, lacking channel buffer, had a 9% decrease in cycle retention rate. The heteroatom doping enhanced structural stability, while the sample in Comparative Example 3, with weaker interlayer bonding, had a 7% decrease in cycle retention rate.

[0080] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0081] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for preparing a lithium ion battery graphite anode material, characterized in that, The method comprises the following steps: S1, high-purity graphite is obtained by purifying natural graphite, and the high-purity graphite is placed in a high-temperature heat treatment furnace, and is slowly heated to 600-800 DEG C at a heating rate of 1-5 DEG C / min under the action of a protective atmosphere; the holding time is 1-2 h, and then the graphite is rapidly heated to 1000-1100 DEG C at a heating rate of 10-20 DEG C / min, and is sintered at constant temperature for 2-3 h; S2, mixing citric acid and urea with a mass ratio of 3:1 to obtain a precursor, heating the precursor to 180~200℃ to vaporize it to obtain a precursor vapor, the precursor vapor concentration is 5~8g / m 3 The nitrogen carrier gas is introduced into a high-temperature heat treatment furnace, the nitrogen carrier gas flow is 50~80mL / min, the reaction is carried out at 1000~1100℃ for 20~40min, the surface of the graphite particles is coated with a graphene quantum dot coating layer, after the reaction, the nitrogen gas is continuously introduced for 20~40min, and the furnace is cooled to room temperature, to obtain graphene quantum dot coated graphite particles, the thickness of the graphene quantum dot coating layer is 2~5nm. S3, the graphite particles obtained in step S2 are transferred to a supercritical CO2 reactor, anhydrous ethanol is added as a cosolvent, the solid-liquid ratio of the graphite particles and the anhydrous ethanol is 1:5, the reactor is closed, CO2 is introduced until the pressure reaches 13-14 MPa, the temperature is raised to 45 DEG C, stirring is carried out at 300 r / min for 60 min, the pressure is slowly reduced to normal pressure at a rate of ≤0.5 MPa / min, the sample is taken out and dried in a vacuum at 60 DEG C for 8 h, and a gradient pore structure of 5-20 nm mesopores and 0.5-2 nm micropores is formed in the graphite particles; S4, ammonium dihydrogen phosphate is dissolved in deionized water to obtain a precursor solution, the graphite particles obtained in step S3 are added to the solution, ultrasonic dispersion is carried out at 300 W for 20-40 min, and then the solution is filtered and dried at 100 DEG C for 6 h; the graphite particles are placed in a tube furnace and heated to 800 DEG C at a rate of 5 DEG C / min under a nitrogen atmosphere, and the temperature is kept constant for 2 h to realize nitrogen and phosphorus dual-element doping; S5, after the reaction is completed, the furnace is cooled to room temperature, and the lithium ion battery graphite negative electrode material is obtained by mechanical dispersion, screening and magnetic separation after discharging.

2. The method for preparing the graphite anode material for lithium-ion batteries according to claim 1, characterized in that, The step S1 further comprises using an air flow mill or a mechanical grinding device to crush the natural graphite to a micron level, and forming spherical or spherical-like particles through particle-particle collision.

3. The method for preparing the graphite anode material for lithium-ion batteries according to claim 1, characterized in that, The natural graphite has a carbon content of ≥99.9% and an ash content of ≤0.1%, and the high-temperature heat treatment furnace is a roller furnace.

4. The method for preparing the graphite anode material for lithium-ion batteries according to claim 1, characterized in that, The mass concentration of ammonium dihydrogen phosphate in the precursor solution is 5 wt%, and the solid-liquid ratio of the graphite particles to the precursor solution is 1:8-10.

5. A lithium-ion battery graphite anode material, characterized in that, Prepared by the method of any one of claims 1-4.