Single-particle secondary-particle mixed high-energy-density graphite negative electrode material and preparation method thereof

Abstract

The application relates to the technical field of negative electrode materials, in particular to a single-particle and secondary-particle mixed high-energy-density graphite negative electrode material and a preparation method thereof, which comprises the following steps: mechanically shaping needle coke after airflow crushing; mixing and kneading the needle coke and a binder, and granulating; mixing single-particle graphite precursors and secondary-particle green balls in proportion, and performing graphitization treatment; performing surface chemical grafting coating on the mixed graphite matrix to obtain the high-energy-density graphite negative electrode material. Through the single-particle and secondary-particle mixing and surface chemical grafting coating treatment, the electrochemical performance of the graphite negative electrode material is effectively improved, including the first coulomb efficiency, the cycle capacity retention rate and the rate capacity retention rate.

Description

Technical Field

[0001] This invention relates to the field of anode material technology, and in particular to a high-energy-density graphite anode material composed of single-particle and secondary-particle mixtures and its preparation method. Background Technology

[0002] Based on their particle structure, graphite anode materials are mainly divided into two categories: single-particle graphite and secondary-particle graphite. Single-particle graphite consists of independent and complete graphite crystals with a regular crystal structure, high graphitization degree, and good structural stability. It exhibits efficient capacity utilization during lithium-ion intercalation and deintercalation, and its particle morphology and surface characteristics are beneficial for electrode processing and stable electrode interface construction. Secondary-particle graphite, on the other hand, is an aggregate of small-sized primary graphite particles prepared by granulation and graphitization processes using binders. It contains multi-level pores and abundant ion transport channels, exhibiting significant isotropic characteristics, which can effectively improve the structural stability and kinetic performance of the electrode during cycling. However, single-structure graphite particles cannot simultaneously achieve high compaction density, excellent ion transport efficiency, and stable interface characteristics. Therefore, blending single-particle and secondary-particle graphite has become an important technical route to improve the overall performance of graphite anodes. By adjusting the particle size distribution, packing method, and interfacial compatibility of the two, synergistic complementarity at the particle level can be achieved. While improving the overall tap density and compaction density of the material, it also takes into account the fast lithium-ion transport channel and the stable lithium insertion / extraction structure, thereby effectively improving the volumetric energy density and rate performance of the anode material.

[0003] Patent publication number CN105720258A discloses a lithium-ion battery anode material, its preparation method, and its application. The method involves spray-drying a silicon powder slurry with a binder to form primary particles. These primary particles are then added during the synthesis of asphalt resin, followed by sintering and crushing to obtain secondary particles. Finally, the secondary particles are mixed with graphite, modified with asphalt surface, and calcined to obtain tertiary finished particles. This method is simple, low-cost, and produces anode materials with high tap density, uniform silicon-carbon dispersion, and significantly improved initial specific capacity, coulombic efficiency, and cycle capacity retention.

[0004] In existing technologies, single-particle and secondary-particle structures exhibit significant differences in surface characteristics. Single-particle structures have regular crystals and few surface defects, forming only a thin, dense, and stable SEI film. Secondary-particle structures, on the other hand, are aggregates with a large specific surface area and numerous active sites, easily forming a thick, loose, and unstable SEI film. When the two are mixed, the SEI film grows unevenly. During charge-discharge cycles, the difference in particle volume expansion and contraction leads to repeated rupture and reconstruction of the SEI film, continuously consuming active lithium and electrolyte, resulting in problems such as high-temperature gas generation and severe self-discharge. Furthermore, existing physical or simple chemical coating methods have weak adhesion to the graphite matrix, easily detaching and cracking during long-term cycling, losing their interfacial protection, leading to increased electrode impedance and performance degradation, failing to meet the long-term use requirements of high-energy-density graphite anodes. Summary of the Invention

[0005] To address the problems mentioned in the background section, this invention provides a high-energy-density graphite anode material composed of single-particle and secondary-particle mixtures, and its preparation method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a high-energy-density graphite anode material consisting of a mixture of single-particle and secondary-particle materials includes the following steps: S1. After the needle coke is pulverized by airflow, it is mechanically shaped to obtain a single-particle graphite precursor. S2. Mix and granulate the needle coke with the binder to obtain spherical secondary particles; S3. Mix single-particle graphite precursors and secondary-particle green pellets in a certain proportion and perform graphitization treatment to obtain a mixed graphite matrix. S4. Surface chemical grafting and coating are performed on the mixed graphite matrix to obtain a high energy density graphite anode material; The surface chemical grafting coating includes: pretreatment of the mixed graphite matrix with allyl glycidyl ether, copolymerization with trifluoroethyl methacrylate, purification and cross-linking curing.

[0007] Furthermore, in step S1, the average particle size of the needle-shaped coke particles after airflow pulverization is controlled to be 8-18 μm; mechanical shaping is carried out using a mechanical fusion machine, with the mechanical fusion machine speed set at 3000-5000 rpm and the shaping time at 20-40 min.

[0008] Furthermore, in step S2, the binder is selected as high-temperature coal tar pitch, with a softening point of 250-280℃, a quinoline insoluble content of 5-10wt%, and a coking value of 45-55wt%. The amount of high-temperature coal tar pitch added is 8-12wt% of the mass of needle coke powder. The kneading is carried out at a constant temperature of 140-160℃ for 0.8-1.2h. Granulation is carried out using a disc granulator, with an average particle size of 14-26μm for the spherical secondary granules. After granulation, hot air drying is carried out at a temperature of 80-100℃ for 2-4h.

[0009] Further, in step S3, the mixing mass ratio of single-particle graphite precursor to secondary-particle green pellets is 40:(50-70); the mixing is carried out by stirring at a speed of 800-1200 rpm for 15-25 min; the graphitization treatment conditions are: heating to 2800-3000℃ and then holding at that temperature for 24-48 h, with a heating rate of 3-5℃ / min, and after the holding at that temperature, naturally cooling to room temperature.

[0010] Furthermore, the surface chemical grafting coating in step S4 specifically includes the following steps: S41. Disperse the mixed graphite matrix in an organic solvent, add allyl glycidyl ether and catalyst, and stir the reaction at a constant temperature under nitrogen protection. After the reaction is completed, centrifuge, wash and dry to obtain the pretreated mixed graphite matrix. S42. The pretreated mixed graphite matrix is ​​dispersed in a solvent containing trifluoroethyl methacrylate and an initiator. After purging with nitrogen to remove oxygen, the mixture is sealed and subjected to constant temperature copolymerization to obtain the crude product. S43. The crude product is purified by Soxhlet extraction using a mixed extractant, and then cross-linked and cured at a constant temperature to obtain the final product.

[0011] Further, in step S41, the organic solvent is selected from one or more of toluene, xylene, N-methylpyrrolidone, or N,N-dimethylformamide, and the dispersion concentration of the mixed graphite matrix in the organic solvent is 5-10 g / L; the dispersion is performed by ultrasonic dispersion, with an ultrasonic power of 200-300 W and an ultrasonic time of 30-60 min; the catalyst is selected from boron trifluoride diethyl ether complex, and its addition amount is 0.2-0.3% of the mass of the mixed graphite matrix; the constant temperature stirring reaction temperature is 75-85℃, and the reaction time is 5-7 h; the drying temperature is 80-100℃, and the drying time is 4-6 h.

[0012] Further, in step S42, the solvent is selected from one or more of butyl acetate, ethyl acetate, or acetone; the initiator is selected from azobisisobutyronitrile or azobisisoheptanenitrile, and its addition amount is 1.0-1.5% of the mass of trifluoroethyl methacrylate. The initiator is added after being dissolved in ethanol, and the mass ratio of ethanol to initiator is 10:1. After addition, the mixture is stirred for 10-15 min to ensure uniform dispersion; the temperature of the sealed constant temperature copolymerization reaction is 65-75℃, and the reaction time is 10-14 h.

[0013] Further, in step S43, the mixed extractant is a mixture of acetone and ethanol, with a volume ratio of 1:(1-2); the Soxhlet extraction and purification time is 10-14 h, and the temperature is 70-80℃; the isothermal crosslinking and curing adopts vacuum isothermal treatment, with a vacuum degree of 10-50 Pa, a curing temperature of 110-120℃, and a curing time of 5-7 h.

[0014] According to another aspect of the present invention, a high energy density graphite anode material prepared by the above-described preparation method is provided, which is a mixture of single-particle and secondary-particle materials.

[0015] The beneficial effects of this invention are: 1. In this invention, the single-particle graphite precursor, after mechanical shaping, has a regular particle shape and few surface defects. The secondary particle pellets are formed by agglomerating needle-shaped coke micropowder with a binder. After graphitization, they form spherical aggregates with a reasonable internal pore structure. The two are then mixed to form a gradient close-packed structure. In the preparation process, the mixed graphite matrix is ​​first pretreated with allyl glycidyl ether. The epoxy groups of allyl glycidyl ether react with the polar functional groups on the graphite surface to form stable covalent bonds, which firmly bind allyl glycidyl ether to the graphite surface. At the same time, the reactive double bonds are retained as anchor points for subsequent grafting. Subsequently, a polymer layer is grafted onto the graphite surface through the copolymerization reaction of trifluoroethyl methacrylate and the double bonds. After purification and cross-linking curing, a dense coating film resistant to electrolyte swelling is formed, which can effectively avoid the problem of coating layer peeling and cracking during long-term cycling, continuously play an interfacial protection role, and inhibit electrode impedance rebound and performance degradation.

[0016] 2. In this invention, the regular structure of single particles is beneficial to improving the compaction density and structural stability of the material, while the porous structure inside the secondary particles can provide relatively abundant ion transport channels, which helps to improve the kinetic performance of the material. This can, to a certain extent, solve the technical challenge of single-structure graphite simultaneously achieving high compaction density, excellent ion transport efficiency, and stable interface characteristics. Simultaneously, the surface covalent graft coating can further optimize the interfacial compatibility of the material, reduce the occurrence of interfacial side reactions, and enable the material to possess relatively good cycle stability while having high volumetric energy density and good rate performance. This better meets the long-term usage requirements of high-energy-density graphite anodes, providing support for the development of high-energy-density lithium batteries. Detailed Implementation

[0017] The technical solution 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Example 1 A method for preparing a high-energy-density graphite anode material consisting of a mixture of single-particle and secondary-particle materials includes the following steps: S1. Take 100g of needle coke, pulverize it with airflow, control the average particle size of the pulverized needle coke particles to be 8μm, and use a mechanical fusion machine for mechanical shaping. Set the speed of the mechanical fusion machine to 3000rpm and the shaping time to 20min to obtain a single-particle graphite precursor. S2. Take 80g of needle-shaped coke powder and 6.4g of high-temperature coal tar pitch, knead and granulate. Kneading is carried out at a constant temperature of 140℃ for 0.8h. Granulation is carried out using a disc granulator to obtain spherical secondary green pellets with an average particle size of 14μm. After granulation, the pellets are dried with hot air at 80℃ for 2h. S3. Take 40g of single-particle graphite precursor and mix it with 50g of green pellets. Mix by stirring at 800rpm for 15min. Then, perform graphitization treatment by heating to 2800℃ and holding at that temperature for 24h at a heating rate of 3℃ / min. After the holding at that temperature is completed, allow it to cool naturally to room temperature to obtain the mixed graphite matrix. S4. Perform surface chemical grafting coating on the mixed graphite matrix, with the following specific steps: S41. Take 88g of mixed graphite matrix and disperse it in 17600mL of toluene. Disperse it using ultrasound with an ultrasound power of 200W and an ultrasound time of 30min. Add 10g of allyl glycidyl ether and 0.176g of boron trifluoride diethyl ether complex. Under nitrogen protection, stir and react at 75℃ for 5h. After the reaction is completed, centrifuge, wash, and dry at 80℃ for 4h to obtain the pretreated mixed graphite matrix. S42. Take 96g of pretreated mixed graphite matrix and disperse it in 12000mL of butyl acetate. Add 0.2g of azobisisobutyronitrile to 2g of ethanol to obtain an initiator solution. Add 20g of trifluoroethyl methacrylate and the initiator solution. Stir for 10min after adding to ensure uniform dispersion. After purging with nitrogen to remove oxygen, seal and copolymerize at 65℃ for 10h to obtain crude product. S43. 114g of crude product was purified by Soxhlet extraction using a mixed extractant of 5000mL acetone and ethanol in a volume ratio of 1:1. The extraction temperature was 70℃ and the extraction time was 10h. Then, vacuum isothermal crosslinking and curing were performed at a vacuum degree of 10Pa, a curing temperature of 110℃ and a curing time of 5h to obtain high energy density graphite anode material.

[0019] Example 2 A method for preparing a high-energy-density graphite anode material consisting of a mixture of single-particle and secondary-particle materials includes the following steps: S1. Take 100g of needle coke, pulverize it with airflow, control the average particle size of the pulverized needle coke particles to be 13μm, and use a mechanical fusion machine for mechanical shaping. Set the speed of the mechanical fusion machine to 4000rpm and the shaping time to 30min to obtain a single-particle graphite precursor. S2. Take 80g of needle-shaped coke powder and 8g of high-temperature coal tar pitch, knead and granulate. Kneading is carried out at a constant temperature of 150℃ for 1.0h. Granulation is carried out using a disc granulator to obtain spherical secondary particles with an average particle size of 20μm. After granulation, the particles are dried with hot air at 90℃ for 3h to obtain the green pellets. S3. Take 40g of single-particle graphite precursor and mix it with 60g of green pellets. Mix by stirring at 1000rpm for 20min. Then, perform graphitization treatment by heating to 2900℃ and holding at that temperature for 36h at a heating rate of 4℃ / min. After the holding at that temperature is completed, allow it to cool naturally to room temperature to obtain the mixed graphite matrix. S4. Perform surface chemical grafting coating on the mixed graphite matrix, with the following specific steps: S41. Take 97g of mixed graphite matrix and disperse it in 12933mL of N-methylpyrrolidone. Disperse it using ultrasound with an ultrasonic power of 250W and an ultrasonic time of 45min. Add 12g of allyl glycidyl ether and 0.2425g of boron trifluoride diethyl ether complex. Under nitrogen protection, stir and react at 80℃ for 6h. After the reaction is completed, centrifuge, wash, and dry at 90℃ for 5h to obtain the pretreated mixed graphite matrix. S42. Take 107g of pretreated mixed graphite matrix and disperse it in 13375mL of ethyl acetate. Add 0.3125g of azobisisobutyronitrile to 3.125g of ethanol to obtain an initiator solution. Add 25g of trifluoroethyl methacrylate and the initiator solution. After adding, stir for 12min to ensure uniform dispersion. After purging with nitrogen to remove oxygen, seal and copolymerize at 70℃ for 12h to obtain the crude product. S43. 129g of crude product was purified by Soxhlet extraction using a mixed extractant of 6000mL acetone and ethanol in a volume ratio of 1:1.5. The extraction temperature was 75℃ and the extraction time was 12h. Vacuum constant temperature crosslinking and curing were then carried out at a vacuum degree of 30Pa, a curing temperature of 115℃ and a curing time of 6h to obtain high energy density graphite anode material.

[0020] Example 3 A method for preparing a high-energy-density graphite anode material consisting of a mixture of single-particle and secondary-particle materials includes the following steps: S1. Take 100g of needle coke, and after air jet milling, control the average particle size of the needle coke particles after milling to be 18μm; use a mechanical fusion machine for mechanical shaping, set the speed of the mechanical fusion machine to 5000rpm, and the shaping time to 40min to obtain a single-particle graphite precursor. S2. Take 80g of needle-shaped coke powder and 9.6g of high-temperature coal tar pitch, knead and granulate. Kneading is carried out at a constant temperature of 160℃ for 1.2h. Granulation is carried out using a disc granulator to obtain spherical secondary green pellets with an average particle size of 26μm. After granulation, the pellets are dried with hot air at 100℃ for 4h. S3. Take 40g of single-particle graphite precursor and mix it with 70g of green pellets. Mix by stirring at 1200rpm for 25min. Then, perform graphitization treatment by heating to 3000℃ and holding at that temperature for 48h at a heating rate of 5℃ / min. After the holding at that temperature is completed, allow it to cool naturally to room temperature to obtain the mixed graphite matrix. S4. Perform surface chemical grafting coating on the mixed graphite matrix, with the following specific steps: S41. Take 105g of mixed graphite matrix and disperse it in 10500mL of N,N-dimethylformamide. Disperse it using ultrasound with a power of 300W and a duration of 60min. Add 15g of allyl glycidyl ether and 0.315g of boron trifluoride diethyl ether complex. Under nitrogen protection, stir and react at 85℃ for 7h. After the reaction is completed, centrifuge, wash, and dry at 100℃ for 6h to obtain the pretreated mixed graphite matrix. S42. Take 118g of pretreated mixed graphite matrix and disperse it in 14750mL of acetone. Add 0.45g of azobisisobutyronitrile to 4.5g of ethanol to obtain an initiator solution. Add 30g of trifluoroethyl methacrylate and the initiator solution. After adding, stir for 15min to ensure uniform dispersion. After purging with nitrogen to remove oxygen, seal and copolymerize at 75℃ for 14h to obtain the crude product. S43. 145g of crude product was purified by Soxhlet extraction using a mixed extractant of 7000mL acetone and ethanol in a volume ratio of 1:2. The extraction temperature was 80℃ and the extraction time was 14h. Vacuum constant temperature crosslinking and curing were then carried out at a vacuum degree of 50Pa, a curing temperature of 120℃ and a curing time of 7h to obtain high energy density graphite anode material.

[0021] Example 4 A method for preparing a high-energy-density graphite anode material consisting of a mixture of single-particle and secondary-particle materials includes the following steps: S1. Take 100g of needle coke, pulverize it with airflow, control the average particle size of the pulverized needle coke particles to be 10μm, and use a mechanical fusion machine for mechanical shaping. Set the speed of the mechanical fusion machine to 3500rpm and the shaping time to 25min to obtain a single-particle graphite precursor. S2. Take 80g of needle-shaped coke powder and 7g of high-temperature coal tar pitch, knead and granulate. Kneading is carried out at a constant temperature of 145℃ for 0.9h. Granulation is carried out using a disc granulator to obtain spherical secondary green pellets with an average particle size of 16μm. After granulation, the pellets are dried with hot air at 85℃ for 2.5h. S3. Take 40g of single-particle graphite precursor and mix it with 55g of green pellets. Mix by stirring at 900rpm for 18min. Then, perform graphitization treatment by heating to 2850℃ and holding at that temperature for 30h at a heating rate of 3.5℃ / min. After the holding at that temperature is completed, allow it to cool naturally to room temperature to obtain the mixed graphite matrix. S4. Perform surface chemical grafting coating on the mixed graphite matrix, with the following specific steps: S41. Take 90g of mixed graphite matrix and disperse it in 12000mL of xylene. Disperse it using ultrasound with an ultrasound power of 220W and an ultrasound time of 35min. Add 11g of allyl glycidyl ether and 0.18g of boron trifluoride diethyl ether complex. Under nitrogen protection, stir and react at 78℃ for 5.5h. After the reaction is completed, centrifuge, wash, and dry at 85℃ for 4.5h to obtain the pretreated mixed graphite matrix. S42. Take 98g of pretreated mixed graphite matrix and disperse it in 12500mL of ethyl acetate. Add 0.22g of azobisisobutyronitrile to 2.2g of ethanol to obtain an initiator solution. Add 22g of trifluoroethyl methacrylate and the initiator solution. Stir for 11min after addition to ensure uniform dispersion. After purging with nitrogen to remove oxygen, seal and copolymerize at 68℃ for 11h to obtain crude product. S43. 118g of crude product was purified by Soxhlet extraction using 5200mL of a mixed extractant of acetone and ethanol in a volume ratio of 1:1.2. The extraction temperature was 72℃ and the extraction time was 11h. Vacuum constant temperature crosslinking and curing were then carried out at a vacuum degree of 20Pa, a curing temperature of 112℃ and a curing time of 5.5h to obtain a high energy density graphite anode material.

[0022] Example 5 A method for preparing a high-energy-density graphite anode material consisting of a mixture of single-particle and secondary-particle materials includes the following steps: S1. Take 100g of needle coke, pulverize it with airflow, control the average particle size of the pulverized needle coke particles to be 16μm, and use a mechanical fusion machine for mechanical shaping. Set the speed of the mechanical fusion machine to 4500rpm and the shaping time to 35min to obtain a single-particle graphite precursor. S2. Take 80g of needle-shaped coke powder and 9g of high-temperature coal tar pitch, knead and granulate. Kneading is carried out at a constant temperature of 155℃ for 1.1h. Granulation is carried out using a disc granulator to obtain spherical secondary green pellets with an average particle size of 24μm. After granulation, the pellets are dried with hot air at 95℃ for 3.5h. S3. Take 40g of single-particle graphite precursor and mix it with 65g of green pellets. Mix by stirring at 1100rpm for 22min. Then, perform graphitization treatment by heating to 2950℃ and holding at that temperature for 42h at a heating rate of 4.5℃ / min. After the holding at that temperature is completed, allow it to cool naturally to room temperature to obtain the mixed graphite matrix. S4. Perform surface chemical grafting coating on the mixed graphite matrix, with the following specific steps: S41. Take 102g of mixed graphite matrix and disperse it in 11333mL of N-methylpyrrolidone. Disperse it using ultrasound with an ultrasonic power of 280W and an ultrasonic time of 50min. Add 14g of allyl glycidyl ether and 0.3g of boron trifluoride diethyl ether complex. Under nitrogen protection, stir and react at 82℃ for 6.5h. After the reaction is completed, centrifuge, wash, and dry at 95℃ for 5.5h to obtain the pretreated mixed graphite matrix. S42. Take 115g of pretreated mixed graphite matrix and disperse it in 14000mL of acetone; add 0.4g of azobisisobutyronitrile to 4g of ethanol to obtain an initiator solution, add 28g of trifluoroethyl methacrylate and the initiator solution, stir for 14min after addition to ensure uniform dispersion, purge with nitrogen to remove oxygen and seal, and copolymerize at 73℃ for 13h to obtain crude product; S43. 140g of crude product was purified by Soxhlet extraction using a mixed extractant of 6800mL acetone and ethanol in a volume ratio of 1:1.8. The extraction temperature was 78℃ and the extraction time was 13h. Vacuum isothermal crosslinking and curing were then carried out at a vacuum degree of 40Pa, a curing temperature of 118℃ and a curing time of 6.5h to obtain a high energy density graphite anode material.

[0023] Example 6 A method for preparing a high-energy-density graphite anode material consisting of a mixture of single-particle and secondary-particle materials includes the following steps: S1. Take 100g of needle coke, pulverize it with airflow, control the average particle size of the pulverized needle coke particles to be 11μm, and use a mechanical fusion machine for mechanical shaping. Set the speed of the mechanical fusion machine to 3200rpm and the shaping time to 22min to obtain a single-particle graphite precursor. S2. Take 80g of needle-shaped coke powder and 7.5g of high-temperature coal tar pitch, knead and granulate. Kneading is carried out at a constant temperature of 142℃ for 0.85h. Granulation is carried out using a disc granulator to obtain spherical secondary green pellets with an average particle size of 18μm. After granulation, the pellets are dried with hot air at 82℃ for 2.2h. S3. Take 40g of single-particle graphite precursor and mix it with 52g of green pellets. Mix by stirring at 850rpm for 16min. Then, perform graphitization treatment by heating to 2820℃ and holding at that temperature for 28h at a heating rate of 3.2℃ / min. After the holding at that temperature is completed, allow it to cool naturally to room temperature to obtain the mixed graphite matrix. S4. Perform surface chemical grafting coating on the mixed graphite matrix, with the following specific steps: S41. Take 89g of mixed graphite matrix and disperse it in 11867mL of toluene. Disperse it using ultrasound with an ultrasonic power of 210W and an ultrasonic time of 32min. Add 10.5g of allyl glycidyl ether and 0.178g of boron trifluoride diethyl ether complex. Under nitrogen protection, stir and react at 76℃ for 5.2h. After the reaction is completed, centrifuge, wash, and dry at 82℃ for 4.2h to obtain the pretreated mixed graphite matrix. S42. Take 97g of pretreated mixed graphite matrix and disperse it in 12250mL of butyl acetate. Add 0.21g of azobisisobutyronitrile to 2.1g of ethanol to obtain an initiator solution. Add 21g of trifluoroethyl methacrylate and the initiator solution. After adding, stir for 10.5min to ensure uniform dispersion. After purging with nitrogen to remove oxygen, seal and copolymerize at 66℃ for 10.5h to obtain the crude product. S43. 115g of crude product was purified by Soxhlet extraction using a mixed extractant of 5100mL acetone and ethanol in a volume ratio of 1:1.1. The extraction temperature was 71℃ and the extraction time was 10.5h. Vacuum constant temperature crosslinking and curing were then carried out at a vacuum degree of 15Pa, a curing temperature of 111℃ and a curing time of 5.2h to obtain high energy density graphite anode material.

[0024] Comparative Example 1 The difference between this comparative example and Example 1 is that the surface chemical grafting coating in step S4 is omitted. That is, after the mixed graphite matrix is ​​graphitized, it is directly used as the final product without the coating operations in S41-S43. The remaining steps are the same as in Example 1.

[0025] Comparative Example 2 The difference between this comparative example and Example 2 is that allyl glycidyl ether in step S41 is not added; the remaining steps are the same as in Example 2.

[0026] Comparative Example 3 The difference between this comparative example and Example 3 is that the catalyst boron trifluoride diethyl ether complex in step S41 is not added; the remaining steps are the same as in Example 3.

[0027] Comparative Example 4 The difference between this comparative example and Example 4 is that trifluoroethyl methacrylate in step S42 is not added; the remaining steps are completely the same as in Example 4.

[0028] Comparative Example 5 The difference between this comparative example and Example 5 is that the mechanical shaping step in S1 is deleted, and the needle coke after air jet pulverization is directly used as a single-particle graphite precursor. The remaining steps are the same as in Example 5.

[0029] Accurately weigh 9.0 g each of the graphite anode material from Examples 1-6 and Comparative Examples 1-5, 0.5 g of polyvinylidene fluoride (PVDF) binder, and 0.5 g of acetylene black, respectively, and prepare the electrolyte (1 mol / L LiPF6, solvent: EC:DMC:EMC = 1:1:1, volume ratio). Place all the weighed materials into a 50 mL polytetrafluoroethylene ball mill jar. Add 15 mL of NMP solvent to the ball mill jar, place the jar in a planetary ball mill, set the ball mill speed to 300 r / min, and the stirring time to 30 min to prepare a uniform and fine anode slurry. Use a doctor blade coating machine to uniformly coat the anode slurry onto a 10 μm thick copper foil. Adjust the doctor blade gap to control the wet coating thickness to 200 μm, the coating speed to 5 cm / s, and the coating area to 5 cm × 5 cm. After coating, the electrode was placed in a vacuum drying oven at a temperature of 80℃ and a vacuum of -0.09MPa for 12 hours. After drying, the electrode was removed and allowed to cool to room temperature for 30 minutes. It was then pressed using a tablet press at a pressure of 10MPa for 30 seconds, with the thickness controlled at 110±5μm. Using a 12mm diameter circular die, the electrode was cut into standard round pieces, 10 pieces per sample. After cutting, the surface of the electrode was wiped with lint-free paper to remove any loose powder, yielding the negative electrode for later use.

[0030] Place the button cell casing, gasket, spring plate, lithium metal sheet, and Celgard 2400 separator in a glove box. Cut the lithium metal sheet to a diameter of 14 mm and a thickness of 0.3 mm, and let it stand for 1 hour. Place the prepared negative electrode disc and lithium metal sheet in a vacuum desiccator in the glove box and dry for 30 minutes. Cut the Celgard 2400 separator into a disc with a diameter of 16 mm and set aside. Assemble the button cell in the following order: positive electrode casing, spring plate, gasket, lithium metal sheet (counter electrode), separator, negative electrode sheet (working electrode), and button cell negative electrode casing. Add 80 μL of electrolyte to the separator, ensuring that the electrolyte completely wets the separator and both electrodes. After assembly, use a button cell sealing machine, set the sealing pressure to 5 MPa, and the pressure holding time to 10 seconds to complete the sealing. Assemble 3 parallel cells for each sample.

[0031] Referring to GB / T 31484-2015 "Graphite-based Anode Materials for Lithium-ion Batteries", the assembled button batteries were placed in a constant temperature test chamber at 25±0.5℃ and left to stand for 2 hours to ensure the electrolyte fully wetted the electrodes and separator. A Blue Battery testing system was used, with the following test parameters set: voltage range 0.01-3.0V, and constant current charge / discharge mode. The first charge and discharge cycle was performed at 0.1C (corresponding to a current of 0.372mA), recording the initial charge capacity and initial discharge capacity. The initial coulombic efficiency was calculated as: Initial coulombic efficiency = (Initial discharge capacity / Initial charge capacity) × 100%. After the initial charge and discharge cycle, 50 cycles were performed at 0.1C, recording the capacity for each cycle. The capacity retention rate after 50 cycles was calculated as: Capacity retention rate after 50 cycles = (50th discharge capacity / Initial discharge capacity) × 100%. After the cyclic test was completed, rate performance tests were performed sequentially at 0.2C, 0.5C, 1C, and 2C rates (corresponding to 0.744mA, 1.86mA, 3.72mA, and 7.44mA). Each rate was charged and discharged twice, and the discharge capacity of the second discharge was taken as the discharge capacity at that rate. Data were recorded, and the rate capacity retention rate was calculated as follows: (Rate capacity = Discharge capacity at that rate / Discharge capacity at 0.1C rate × 100%). The results are shown in Table 1. Table 1. Charge-discharge performance test results of Examples 1-6 and Comparative Examples 1-5 Referring to GB / T 31486-2015 "Test Methods for Electrochemical Performance of Anode Materials for Lithium-ion Batteries", an electrochemical workstation was used. Before testing, the button batteries were placed in a constant temperature environment of 25±0.5℃ for 1 hour. Cyclic voltammetry (CV) testing was performed with a scan rate of 0.1 mV / s, a voltage range of 0.01-3.0 V, and 3 scan cycles. The CV curve for each cycle was recorded to analyze the reversibility of the electrode reaction. After the CV test, electrochemical impedance spectroscopy (EIS) testing was performed with a frequency range of 10 Hz. -2 -10 5 The test frequency was set at Hz, amplitude was 5mV, and the test temperature was 25±0.5℃. The EIS curve was recorded, and the electrode interface impedance was calculated. The results are shown in Table 2. Table 2. Electrochemical performance test results of Examples 1-6 and Comparative Examples 1-5 As shown in Table 1, the initial coulombic efficiency of Examples 1-6 ranged from 92.8% to 94.1%, while that of Comparative Examples 1-5 was relatively lower, ranging from 88.2% to 91.5%. In these examples, surface chemical grafting coating treatment formed a dense, electrolyte-resistant coating film on the graphite surface, effectively reducing direct contact between the electrolyte and graphite, lowering the occurrence of side reactions, and thus reducing irreversible capacity loss during the first charge, thereby improving the initial coulombic efficiency. In contrast, Comparative Example 1 did not undergo coating treatment, and Comparative Examples 2-5 had missing key components or omitted steps during the coating process, resulting in poor coating effects, increased side reactions, and lower initial coulombic efficiency.

[0032] The capacity retention rates after 50 cycles in Examples 1-6 were between 96.5% and 98.0%, while those in Comparative Examples 1-5 were between 90.5% and 94.5%. The coating film in the examples continuously provides interfacial protection, suppressing electrode impedance rise and performance degradation. It effectively prevents coating layer detachment and cracking during long-term cycling, ensuring the structural stability of the graphite anode material and thus improving the capacity retention rate. In contrast, the comparative examples, due to imperfect coating, experienced intensified interfacial side reactions during cycling, leading to gradual material structure degradation and a decrease in capacity retention.

[0033] As the rate of increase increases, the rate capacity retention of the embodiments is higher than that of the comparative examples. For example, at a 2C rate, the rate capacity retention of embodiments 1-6 is between 74.6% and 77.5%, while that of comparative examples 1-5 is between 58.2% and 69.5%. The regular structure of the single particles in the embodiments is beneficial to improving the compaction density and structural stability of the material. The porous structure inside the secondary particles provides relatively abundant ion transport channels, while the surface covalent graft coating optimizes the interfacial compatibility of the material and reduces the occurrence of interfacial side reactions.

[0034] As shown in Table 2, the oxidation peak potentials of Examples 1-6 were between 0.208-0.221V, the reduction peak potentials were between 0.058-0.082V, and the peak potential difference was between 126-163mV; while the oxidation peak potentials of Comparative Examples 1-5 were between 0.235-0.268V, the reduction peak potentials were between 0.022-0.046V, and the peak potential difference was between 189-246mV. The coating treatment in the examples enhanced the reversibility of the electrode reaction. The dense coating film reduced side reactions on the electrode surface, making the redox reaction closer to the ideal state, the oxidation and reduction peak potentials closer, and the peak potential difference smaller. In the comparative examples, due to imperfect coating, the side reactions on the electrode surface increased, leading to poorer reversibility of the redox reaction and an increased peak potential difference.

[0035] The charge transfer impedance of Examples 1-6 ranged from 24.6 to 38.5 Ω, and the SEI film impedance ranged from 8.9 to 12.3 Ω. In Comparative Examples 1-5, the charge transfer impedance ranged from 55.8 to 98.7 Ω, and the SEI film impedance ranged from 16.9 to 28.5 Ω. The coating layers in the Examples optimized the interfacial compatibility of the materials, reduced interfacial side reactions, formed a stable and thin SEI film, and lowered the SEI film impedance. Simultaneously, the favorable coating structure facilitated ion transport and electron transfer, further reducing charge transfer impedance. In the Comparative Examples, due to poor coating performance, interfacial side reactions increased, the SEI film thickened and became unstable, leading to increased SEI film impedance and charge transfer impedance.

[0036] In summary, this invention effectively improves the electrochemical performance of graphite anode materials, including initial coulombic efficiency, cycle capacity retention, and rate capacity retention, through single-particle and secondary-particle mixing and surface chemical grafting coating. At the same time, it enhances the reversibility of electrode reactions and reduces charge transfer impedance and SEI film impedance.

[0037] In the description of this specification, the reference to terms such as "embodiment," "various embodiments," etc., indicates that a specific feature, structure, material, or characteristic described in connection with that embodiment or preparation example is included in at least one embodiment of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments.

[0038] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a high-energy-density graphite anode material composed of single-particle and secondary-particle mixtures, characterized in that, Includes the following steps: S1. After the needle coke is pulverized by airflow, it is mechanically shaped to obtain a single-particle graphite precursor. S2. Mix and granulate the needle coke with the binder to obtain spherical secondary particles; S3. Mix single-particle graphite precursors and secondary-particle green pellets in a certain proportion and perform graphitization treatment to obtain a mixed graphite matrix. S4. Surface chemical grafting and coating are performed on the mixed graphite matrix to obtain a high energy density graphite anode material; The surface chemical grafting coating includes: pretreatment of the mixed graphite matrix with allyl glycidyl ether, copolymerization with trifluoroethyl methacrylate, purification and cross-linking curing.

2. The preparation method according to claim 1, characterized in that, In step S1, the average particle size of the needle-shaped coke particles after air jet milling is controlled to be 8-18 μm; mechanical shaping is carried out using a mechanical fusion machine, with the speed of the mechanical fusion machine set to 3000-5000 rpm and the shaping time to 20-40 min.

3. The preparation method according to claim 1, characterized in that, In step S2, the binder is high-temperature coal tar pitch with a softening point of 250-280℃, and the addition amount is 8-12wt% of the mass of needle coke powder; the kneading is carried out at a constant temperature of 140-160℃ for 0.8-1.2h; the granulation is carried out using a disc granulator, and the average particle size of the spherical secondary granules is 14-26μm.

4. The preparation method according to claim 1, characterized in that, In step S3, the mixing mass ratio of single-particle graphite precursor to secondary-particle green pellets is 40:(50-70); the mixing is carried out by stirring at a speed of 800-1200 rpm for 15-25 min; the graphitization treatment conditions are heating to 2800-3000℃ and then holding at that temperature for 24-48 h, with a heating rate of 3-5℃ / min.

5. The preparation method according to claim 1, characterized in that, Step S4, surface chemical grafting and coating, specifically includes the following steps: S41. Disperse the mixed graphite matrix in an organic solvent, add allyl glycidyl ether and catalyst, and stir the reaction at a constant temperature under nitrogen protection. After the reaction is completed, centrifuge, wash and dry to obtain the pretreated mixed graphite matrix. S42. The pretreated mixed graphite matrix is ​​dispersed in a solvent containing trifluoroethyl methacrylate and an initiator. After purging with nitrogen to remove oxygen, the mixture is sealed and subjected to constant temperature copolymerization to obtain the crude product. S43. The crude product is purified by Soxhlet extraction using a mixed extractant, and then cross-linked and cured at a constant temperature to obtain the final product.

6. The preparation method according to claim 5, characterized in that, In step S41, the organic solvent is selected from one or more of toluene, xylene, N-methylpyrrolidone, or N,N-dimethylformamide; the catalyst is selected from boron trifluoride diethyl ether complex, and its addition amount is 0.2-0.3% of the mass of the mixed graphite matrix; the temperature of the constant temperature stirring reaction is 75-85℃, and the reaction time is 5-7h.

7. The preparation method according to claim 5, characterized in that, In step S42, the solvent is selected from one or more of butyl acetate, ethyl acetate, or acetone; the initiator is selected from azobisisobutyronitrile or azobisisoheptanenitrile, and its addition amount is 1.0-1.5% of the mass of trifluoroethyl methacrylate; the addition amount of allyl glycidyl ether is 5-10% of the mass of the mixed graphite matrix; the temperature of the sealed isothermal copolymerization reaction is 65-75℃, and the reaction time is 10-14h.

8. The preparation method according to claim 5, characterized in that, In step S43, the mixed extractant is a mixture of acetone and ethanol, with a volume ratio of 1:(1-2); the Soxhlet extraction and purification time is 10-14 h, and the temperature is 70-80℃; the constant temperature crosslinking and curing is carried out by vacuum constant temperature treatment, with a curing temperature of 110-120℃ and a curing time of 5-7 h.

9. A high-energy-density graphite anode material prepared by the preparation method according to any one of claims 1-8, consisting of a mixture of single-particle and secondary-particle materials.

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