CN111232971A - Long-circulation natural graphite-based modified composite material and preparation method and application thereof - Google Patents
Long-circulation natural graphite-based modified composite material and preparation method and application thereof Download PDFInfo
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Abstract
The invention discloses a long-circulating natural graphite-based modified composite material and a preparation method and application thereof. The method comprises the following steps: mixing natural crystalline flake graphite with molten asphalt to coat the asphalt on the surface of the natural crystalline flake graphite, and then cooling; then, sequentially crushing and shaping the natural crystalline flake graphite with the surface coated with the asphalt to obtain spherical graphite; graphitizing the spherical graphite, cooling, scattering and screening the graphitized material; then evenly mixing the graphite powder with asphalt to ensure that the asphalt is coated on the surface of the graphitized material to obtain a mixture; and carbonizing the mixture, cooling, scattering and screening to obtain the natural graphite-based modified composite material. When the composite material prepared by the method is used as a negative electrode material, the composite material has longer cycle performance and better electrochemical performance, and solves the problem of high expansion of a natural graphite electrode.
Description
Technical Field
The invention belongs to the technical field of lithium ion battery carbon cathode materials, and particularly relates to a long-cycle natural graphite-based modified composite material, and a preparation method and application thereof.
Background
Lithium ion batteries have become a new generation of secondary batteries following nickel-metal hydride batteries in the nineties of the last century because of their advantages of high operating voltage, high energy density, long cycle life, small self-discharge, no memory effect, etc. In the development process of the lithium ion battery technology, the battery quality is continuously improved, and the production cost is continuously reduced. The negative electrode material plays a great role in contributing to the technical progress of lithium ion batteries. At present, the cathode material of commercial lithium ion batteries is still the dominant graphite material, and can be divided into artificial graphite and natural graphite from the raw material source. The artificial graphite occupies most of the power battery market with good cycle stability, the manufacturing process of the artificial graphite not only needs a graphitization process with high price, but also the price of raw materials continuously rises, so that the price of the artificial graphite is not likely to be reduced, but can continue to rise, in order to further reduce the cost of a negative electrode material, people aim at natural graphite again, the natural graphite has the greatest advantages of no graphitization process and low price, but the poor cycle stability of the power battery offsets the price advantage, and therefore how to improve the cycle stability becomes an important research subject.
In order to improve the electrochemical performance of natural graphite, physical and chemical modification and surface modification are carried out on the natural graphite by various methods, and corresponding results are obtained. For example, patent CN101976735B discloses a shaping technique for rolling up a crystalline flake carbon layer of natural graphite into spherical graphite with a shape similar to a sphere, oval or potato, and then coating a layer of amorphous carbon on the outer surface of the spherical graphite to prevent the electrolyte from entering, but there are some gaps in the rolled carbon layer, organic molecules in the electrolyte will gradually penetrate into the spherical graphite through the gaps during charging and discharging, and react with the crystalline flake carbon layer not coated inside the spherical graphite to generate a new SEI film, so that the rolled carbon layer inside the spherical graphite is peeled off, and the cycle performance can only be improved to 300 weeks. In order to further improve the cycle performance, patent CN107814382B adopts isostatic pressing to press various surface modifiers into the voids of the spherical graphite, and patent application CN107814383A adopts vacuum and pressurization to press the modifiers into the voids of the spherical graphite. Both methods greatly improve the cycle performance of the natural graphite, but the production process is carried out under high pressure and the working procedure is complex.
Disclosure of Invention
In order to solve the problems, the invention aims to provide a long-circulating natural graphite-based modified composite material as well as a preparation method and application thereof.
The invention provides a preparation method of a natural graphite-based modified composite material, which comprises the following steps:
(1) coating: mixing natural crystalline flake graphite with molten asphalt to coat the asphalt on the surface of the natural crystalline flake graphite, and then cooling;
(2) crushing and shaping: sequentially crushing and shaping the natural crystalline flake graphite with the asphalt coated on the surface obtained in the step (1) to obtain spherical graphite;
(3) graphitization: graphitizing the spherical graphite obtained in the step (2), cooling, and scattering and screening the graphitized material;
(4) coating: uniformly mixing the graphitized material obtained in the step (3) with asphalt, so that asphalt powder is coated on the surface of the graphitized material to obtain a mixture;
(5) carbonizing: and (4) carbonizing the mixture obtained in the step (4), cooling, scattering and screening to obtain the natural graphite-based modified composite material.
According to the invention, in step (1), the natural crystalline flake graphite has a fixed carbon content of 80% or more, such as 85% or more, for example 85%, 88%, 90%, 92.5% or more. Wherein the particle size of the natural crystalline flake graphite is 150-2000 μm, such as 150-1000 μm, and further such as 150-500 μm, such as 150 μm, 160 μm, 180 μm, 200 μm.
According to the invention, the asphalt in the step (1) is coal asphalt or petroleum asphalt. Wherein the asphalt has a softening point of 150 to 200 ℃, for example 160 to 180 ℃, and exemplary temperatures of 150 ℃, 160 ℃, 180 ℃, and 200 ℃. Further, the pitch has a carbon residue value of 50% or more, such as 60% or more, illustratively 50%, 60%, 70%.
According to the invention, the molten bitumen of step (1) is obtained by heating the bitumen to a temperature of from 20 ℃ to 50 ℃ above the softening point of the bitumen, for example from 30 ℃ to 45 ℃ above the softening point of the bitumen, and illustratively to a temperature of from 20 ℃ to 30 ℃ above the softening point of the bitumen, 40 ℃.
According to the invention, in the step (1), the mass ratio of the natural crystalline flake graphite to the asphalt is 100 (8-15), and examples are 100:8, 100:10, 100:12 and 100: 15.
According to the present invention, in the step (2), the equipment used for the pulverization is not particularly limited, and equipment known in the art, such as a jet mill, a high-pressure mill or a bar-type mechanical pulverizer, may be selected. Further, D of the pulverized natural crystalline flake graphite50The particle size is 10 to 27 μm, for example 10 to 25 μm, exemplified by 13 μm, 17 μm, 18 μm, 27 μm.
According to the present invention, in the step (2), the shaping equipment is not particularly limited, and equipment known in the art, such as a mechanical shaper or an airflow shaper, may be selected. In one embodiment, hot air at 10-30 deg.C lower than the softening point of the coated asphalt (e.g., 15-25 deg.C lower, exemplary 10 deg.C, 15 deg.C, 20 deg.C lower) is introduced into the shaper during the shaping process.
According to the invention, in the step (2), the spherical graphite D50The particle size is 8 to 25 μm, for example 10 to 20 μm, exemplified by 12 μm, 16 μm, 17 μm, 25 μm.
According to the invention, in step (3), the graphitization temperature is controlled to be 2800-3000 ℃, for example 2850-2950 ℃, for example 2800 ℃, 2900 ℃, 3000 ℃. Further, the graphitization time is 2-10 hours, such as 3-8 hours, with 3 hours, 6 hours being exemplary. Wherein said graphitization may be by any means known in the art, such as by using a conventional graphitization process furnace.
According to the present invention, in the step (3), the equipment used for the scattering is not particularly limited, and equipment known in the art, such as a turbine type scattering machine or an air flow type scattering machine, can be selected. The apparatus used for the screening is not particularly limited, and may be selected from those known in the art, such as a vibratory screening machine.
According to the invention, in the step (4), the asphalt is isotropic high-softening-point asphalt. Wherein the pitch has a coking value of 60% or more, such as 70% or more, illustratively 60%, 64%, 68%, 72%. Wherein the asphalt has a softening point of 200-260 deg.C, such as 210-250 deg.C, exemplary 200 deg.C, 220 deg.C, 240 deg.C.
According to the invention, in step (4), the particle size of the pitch is 1 to 3 μm, such as 1.5 to 2.5 μm, for example 2 μm, 3 μm.
According to the invention, in the step (4), the mass ratio of the asphalt to the graphitized material is (0.8-1.5):100, and is exemplarily 0.8:100, 1.0:100, 1.2:100, and 1.5: 100.
According to the present invention, in the step (4), the mixing may be performed by a mixing method known in the art. For example, the asphalt powder and the graphitized material are placed in a mixer, the temperature is controlled to be 15-80 ℃, and the mixture is processed for 1-300 min at the rotating speed of 50-500 r/min to obtain a mixture. Wherein the mixer is a high-speed modified VC mixer, a conical mixer or a kneading machine.
According to the invention, in step (5), the temperature of the carbonization treatment is 800 to 1200 ℃, for example 1000 to 1200 ℃, exemplarily 1100 ℃ and 1200 ℃. Further, the time of the carbonization treatment is 1 to 6 hours, such as 2 to 5 hours, and illustratively 4 hours, 6 hours. Wherein the carbonization reaction is carried out under the protection of inert atmosphere, for example, the inert atmosphere is N2An atmosphere or an argon atmosphere. Further, after the carbonization treatment is finished, the obtained product is naturally cooled.
According to the invention, the cooling in steps (1), (3) and (5) is to cool the mass to room temperature. Wherein room temperature means a temperature of 15 to 40 ℃, for example 20 to 35 ℃.
The invention also provides a composite material prepared by the method.
In the method, firstly, the liquid asphalt is coated on the surface of the natural flake graphite, the natural flake graphite is curled into a sphere in the shaping process, and part of the asphalt coated on the surface of the natural flake graphite is also curled on the inner surface of a spherical graphite curling layer, namely the asphalt is coated in the spherical graphite. Hot air is introduced in the shaping process, so that the asphalt wrapped in the spherical graphite can be used as a binder to bind the graphite curled into spheres together, and no gap exists in the shaped spherical graphite. Then graphitizing to remove impurities in the natural graphite, converting the pitch into artificial graphite, then coating the graphitized material with pitch powder, and carbonizing to coat a layer of amorphous carbon on the surface of the spherical graphite. By the method, the inner surface and the outer surface of the natural graphite can be synchronously modified and integrated.
The invention also provides a natural graphite-based modified composite material which has a core-shell structure, wherein the core is spherical graphite and at least one artificial graphite layer coated on the inner surface of the spherical graphite, and the shell is an amorphous carbon layer. Optionally, the artificial graphite layer may be coated on the outer surface of the spheroidal graphite.
According to the invention, the spherical graphite is obtained by crushing and shaping natural crystalline flake graphite coated with asphalt.
According to the invention, the thickness of the artificial graphite layer is 0.2 to 0.5 μm, for example 0.3 to 0.4 μm.
According to the invention, the thickness of the amorphous carbon layer is 0.5 to 2 μm, for example 1.0 to 2 μm.
According to the invention, the modified composite has an average particle diameter D508-25 μm, e.g. 10-20 μm, exemplary 12 μm, 16 μm, 17 μm, 25 μm.
According to the invention, the first discharge capacity of the modified composite material is more than or equal to 360mAh/g, such as more than or equal to 362mAh/g, exemplary are 362.6mAh/g, 364.2mAh/g, 364.7mAh/g, 365.6 mAh/g.
According to the invention, the first charge-discharge efficiency of the modified composite material is equal to or greater than 92.5%, such as equal to or greater than 93%, illustratively 92.6%, 92.9%, 93.1%, 93.2%.
According to the invention, the capacity retention rate of the modified composite material is more than 90%, for example more than 90.5%, and exemplary values are 90.2%, 90.6% and 91.2% at normal temperature in a 1C charge-discharge cycle for 1000 weeks.
According to the invention, the capacity retention rate of the modified composite material is more than 85%, for example more than 85.5%, and is exemplified by 85.6%, 85.7%, 85.8% and 86.2% at 45 ℃ after 1000 cycles of 1C charge-discharge.
According to the invention, the modified composite material can be obtained by the preparation method.
The invention also provides application of the natural graphite-based modified composite material in a lithium ion battery, and the natural graphite-based modified composite material is preferably used as a lithium ion battery cathode material.
The invention has the beneficial effects that:
the invention provides a method for synchronously modifying the inner surface and the outer surface of natural graphite. Coating liquid asphalt on the surface of natural crystalline flake graphite, then crushing and shaping, wherein the crystalline flake carbon layer of the natural crystalline flake graphite is curled into a sphere in the shaping process, and part of asphalt coated on the surface of the natural crystalline flake graphite is curled into the carbon layer and filled in an inner gap of the curled carbon layer; and simultaneously, coating partial asphalt on the outer surface of the natural graphite particles, performing graphitization treatment, coating graphitized materials with asphalt powder, and carbonizing to prepare the natural graphite-based modified composite material. The method mainly aims at the bottleneck problems that in the prior art, when natural graphite is used as a negative electrode, electrolyte enters the inner pores of the carbon coating layer after the carbon coating layer falls off, so that the cycle performance is poor, the safety is poor and the like. When the composite material prepared by the method is used as a negative electrode material, the composite material has longer cycle performance and better electrochemical performance, and solves the problem of high expansion of a natural graphite electrode.
The battery prepared from the natural graphite-based modified composite material has the first discharge capacity of more than or equal to 360mAh/g, and the capacity retention rate of 1000 weeks of 1C charge-discharge circulation is more than 90%; the high-temperature cycle performance is excellent, the capacity retention rate is more than 85 percent after 1C charge-discharge cycle for 1000 weeks at 45 ℃, the material can replace artificial graphite to manufacture a battery cathode material, and the material is suitable for lithium ion batteries for mobile electronic equipment such as mobile phones and digital cameras and power lithium ion batteries for electric vehicles, so that the cost is greatly reduced.
Detailed Description
The technical solution of the present invention will be further described in detail with reference to specific embodiments. It is to be understood that the following examples are only illustrative and explanatory of the present invention and should not be construed as limiting the scope of the present invention. All the technologies realized based on the above-mentioned contents of the present invention are covered in the protection scope of the present invention.
Unless otherwise indicated, the raw materials and reagents used in the following examples are all commercially available products or can be prepared by known methods.
Example 1
Weighing 12kg of petroleum asphalt with physicochemical index of 60% of carbon residue and 180% of softening point, heating the asphalt to 220 ℃, weighing 100kg of natural crystalline flake graphite (150 μm) with the carbon content of 92.5%, adding the natural crystalline flake graphite into the molten petroleum asphalt, stirring for 30 minutes, cooling to room temperature, and pulverizing the cooled product to D in a jet mill5027 μm to obtain material A.
Introducing 160 ℃ hot air into the cavity of the shaping machine, and shaping the material A in the shaping machine to obtain a material D50Spherical graphite of 25 μm, graphitizing at 2800 deg.C for 3 hr, cooling to room temperature, scattering, and sieving to obtain material B.
Crushing isotropic asphalt (coking value 60%) with softening point of 240 ℃ to 3 mu m in an impact crusher, mixing with the prepared material B according to the mass ratio of 1.5:100, and adding into the mixture in the presence of N2Processing for 4 hours at 1200 ℃ under protection, cooling to room temperature, scattering, screening and demagnetizing to obtain the product.
Example 2
Weighing 10kg of coal tar pitchThe chemical index is 50% of carbon residue value and the softening point is 150 ℃, asphalt is heated to 180 ℃, 100kg of natural crystalline flake graphite (180 mu m) with the carbon content of 85.0% is weighed and added into molten coal asphalt, the mixture is stirred for 40 minutes and then cooled to room temperature, and then the cooled product is crushed to D in a jet mill5017 μm to obtain a material A.
Introducing 130 ℃ hot air into the cavity of the shaping machine, and shaping the material A in the shaping machine to obtain a material D50Is spherical graphite with the diameter of 16 mu m, is graphitized for 6 hours at 3000 ℃, is cooled to room temperature, is scattered and is screened to obtain a material B.
Crushing isotropic asphalt (coking value 64%) with softening point of 220 deg.C to 3 μm in impact crusher, mixing with the above prepared material B at a mass ratio of 1.0:100, and adding into the mixture2Processing for 6 hours at 1100 ℃ under protection, cooling to room temperature, scattering, screening and demagnetizing to obtain the product.
Example 3
Weighing 15kg of petroleum asphalt with the physicochemical index of 70% of carbon residue and the softening point of 200 ℃, heating the asphalt to 220 ℃, weighing 100kg of natural crystalline flake graphite (180 mu m) with the carbon content of 90.0%, adding the natural crystalline flake graphite into the molten petroleum asphalt, stirring for 60 minutes, cooling to room temperature, and crushing the cooled product to D in a jet mill50The particle size is 13 μm, and a material A is obtained.
Introducing hot air of 180 ℃ into the cavity of the shaping machine, and shaping the material A in the shaping machine to obtain a material D50Is spherical graphite with the particle size of 12 mu m, is graphitized for 3 hours at 3000 ℃, is cooled to room temperature, is scattered and is screened to obtain a material B.
Crushing isotropic asphalt (coking value 72%) with softening point of 200 deg.C to 3 μm in impact crusher, mixing with the above prepared material B at a mass ratio of 0.8:100, and adding into the mixture2Processing for 4 hours at 1200 ℃ under protection, cooling to room temperature, scattering, screening and demagnetizing to obtain the product.
Example 4
Weighing 10kg of petroleum asphalt with physicochemical index of 60% of carbon residue and softening point of 160 deg.C, heating asphalt to 180 deg.C, and weighing carbon100kg of natural crystalline flake graphite (160 mu m) with the mass content of 88.0 percent is added into molten petroleum asphalt, stirred for 30 minutes, cooled to room temperature, and then the cooled product is crushed to D in a jet mill5018 μm to obtain a material A.
Introducing hot air of 150 ℃ into the cavity of the shaping machine, and shaping the material A in the shaping machine to obtain a material D50Is spherical graphite with the particle size of 17 mu m, is graphitized for 6 hours at 3000 ℃, is cooled to room temperature, is scattered and is screened to obtain a material B.
Crushing isotropic asphalt (coking value 68%) with softening point of 220 deg.C to 3 μm in impact crusher, mixing with the above prepared material B at a mass ratio of 1.2:100, and adding into the mixture2Processing for 4 hours at 1200 ℃ under protection, cooling to room temperature, scattering, screening and demagnetizing to obtain the product.
Comparative example 1
Weighing 12kg of petroleum asphalt with physicochemical index of 60% of carbon residue and 180% of softening point, heating the asphalt to 220 ℃, weighing 100kg of natural crystalline flake graphite (150 microns) with the carbon content of 92.5%, adding the natural crystalline flake graphite into the molten petroleum asphalt, stirring for 30 minutes, cooling to room temperature, and crushing the cooled product to D in a jet mill5027 μm to obtain material A.
Shaping the material A in a shaping machine at room temperature to obtain material D50Spherical graphite of 25 μm, graphitizing at 3000 deg.C for 3 hr, cooling to room temperature, scattering, and sieving to obtain material B.
Crushing isotropic asphalt (coking value 60%) with softening point of 240 ℃ to 3 mu m in an impact crusher, mixing with the prepared material B according to the mass ratio of 1.5:100, and adding into the mixture in the presence of N2Processing for 4 hours at 1200 ℃ under protection, cooling to room temperature, scattering, screening and demagnetizing to obtain the product.
Comparative example 2
Weighing natural crystalline flake graphite (150 mu m) with the carbon content of 92.5 percent, and crushing the natural crystalline flake graphite to D by using a jet mill5027 μm to obtain material A.
Shaping the material A in a shaping machine at room temperature to obtain material D50Spherical graphite of 25 μm, graphitizing at 3000 deg.C for 3 hr, cooling to room temperature, scattering, and sieving to obtain material B.
Crushing isotropic asphalt (coking value 60%) with softening point of 240 ℃ to 3 mu m in an impact crusher, mixing with the prepared material B according to the mass ratio of 1.5:100, and adding into the mixture in the presence of N2Processing for 4 hours at 1200 ℃ under protection, cooling to room temperature, scattering, screening and demagnetizing to obtain the product.
Electrochemical performance test
The semi-electric test method comprises the following steps: the natural graphite-based modified composite materials prepared in examples 1 to 4 and comparative examples 1 to 2, which were prepared by uniformly mixing conductive carbon black (SP), carboxymethylcellulose (CMC), Styrene Butadiene Rubber (SBR) at a mass ratio of 95:1:1.5:2.5, coating the mixture on a copper foil, and drying the coated electrode sheet in a vacuum drying oven at 120 ℃ for 12 hours. Assembling a simulated battery in an argon-protected Braun glove box, wherein the electrolyte is 1M-LiPF6And (3) performing simulated battery tests on a 5V and 10mA New Wien battery test cabinet by using a metal lithium sheet as a counter electrode and DEC DMC (volume ratio of 1:1:1), wherein the charge-discharge voltage is 0.01-1.5V, the charge-discharge rate is 0.1C, and the first capacity and efficiency obtained by the tests are listed in Table 1.
The full battery test method comprises the following steps: the natural graphite-based modified composite materials prepared in examples 1 to 4 and comparative examples 1 to 2 were used as a negative electrode, lithium cobaltate was used as a positive electrode, and 1M-LiPF was used6And (3) preparing a full cell by using a solution of DEC and DMC (volume ratio of 1:1:1) as an electrolyte, performing charge and discharge at normal temperature and 45 ℃ at the multiplying power of 1C, wherein the voltage range is 3.0-4.2V, and the cycle performance obtained by testing is listed in Table 1.
TABLE 1 electrochemical Performance test results
As can be seen from Table 1, the modified composite material prepared by the present invention has higher cycle performance than the conventional product on the market (comparative example 2). The invention has simple preparation process, low cost and higher practicability, can replace artificial graphite to prepare the cathode material of the battery, and is suitable for lithium ion batteries for mobile electronic equipment such as mobile phones, digital cameras and the like and power lithium ion batteries for electric vehicles, thereby greatly reducing the cost.
The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiment. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. A preparation method of a natural graphite-based modified composite material is characterized by comprising the following steps:
(1) coating: mixing natural crystalline flake graphite with molten asphalt to coat the asphalt on the surface of the natural crystalline flake graphite, and then cooling;
(2) crushing and shaping: sequentially crushing and shaping the natural crystalline flake graphite with the asphalt coated on the surface obtained in the step (1) to obtain spherical graphite;
(3) graphitization: graphitizing the spherical graphite obtained in the step (2), cooling, and scattering and screening the graphitized material;
(4) coating: uniformly mixing the graphitized material obtained in the step (3) with asphalt, so that the asphalt is coated on the surface of the graphitized material to obtain a mixture;
(5) carbonizing: and (4) carbonizing the mixture obtained in the step (4), cooling, scattering and screening to obtain the natural graphite-based modified composite material.
2. The preparation method according to claim 1, wherein in the step (1), the mass content of the fixed carbon in the natural crystalline flake graphite is more than or equal to 80%;
preferably, the particle size of the natural crystalline flake graphite is 150-2000 μm;
preferably, the asphalt is coal asphalt or petroleum asphalt; preferably, the softening point of the asphalt is 150-200 ℃; preferably, the carbon residue value of the asphalt is more than or equal to 50 percent;
preferably, the molten asphalt is obtained by heating asphalt to 20 ℃ to 50 ℃ above the asphalt softening point;
preferably, the mass ratio of the natural crystalline flake graphite to the asphalt is 100 (8-15).
3. The production method according to claim 1 or 2, wherein in the step (2), the pulverization is carried out using an apparatus such as a jet mill, a high-pressure mill or a bar-type mechanical pulverizer;
preferably, D of the pulverized natural flake graphite50The particle size is 10-27 μm;
preferably, the shaping equipment is a mechanical shaping machine or an airflow shaping machine;
preferably, hot air with the temperature 10-30 ℃ lower than the softening point of the coated asphalt is introduced into the shaping machine in the shaping process;
preferably, said spheroidal graphite D50The particle size is 8 to 25 μm.
4. The method according to any one of claims 1 to 3, wherein in the step (3), the temperature for graphitization is controlled to be 2800 to 3000 ℃; preferably, the graphitization time is 2-10 hours;
preferably, the equipment adopted for scattering is a turbine type scattering machine or an air flow type scattering machine; preferably, the screening equipment is a vibration screening machine.
5. The production method according to any one of claims 1 to 4, wherein in the step (4), the pitch is an isotropic high-softening-point pitch;
preferably, the coking value of the asphalt is more than or equal to 60 percent;
preferably, the softening point of the asphalt is 200-260 ℃;
preferably, the particle size of the asphalt is 1-3 μm;
preferably, the mass ratio of the asphalt to the graphitized material is (0.8-1.5): 100;
preferably, the mixing comprises the steps of placing the asphalt and the graphitized material in a mixer, controlling the temperature to be 15-80 ℃, and processing for 1-300 min at the rotating speed of 50-500 r/min to obtain a mixture;
preferably, the mixer is a high speed modified VC mixer, a conical mixer or a kneader.
6. The production method according to any one of claims 1 to 5, wherein in the step (5), the temperature of the carbonization treatment is 800 to 1200 ℃, and preferably the time of the carbonization treatment is 1 to 6 hours;
preferably, the carbonization reaction is carried out under the protection of an inert atmosphere;
preferably, after the carbonization treatment is completed, the obtained product is naturally cooled.
7. The method according to any one of claims 1 to 6, wherein the cooling in steps (1), (3) and (5) is to cool the material to room temperature.
8. The natural graphite-based modified composite material is characterized by having a core-shell structure, wherein the core is spherical graphite and at least comprises an artificial graphite layer coated on the inner surface of the spherical graphite, and the shell is an amorphous carbon layer;
optionally, the artificial graphite layer is also coated on the outer surface of the spherical graphite.
9. The modified composite material of claim 8, wherein the spherical graphite is obtained by pulverizing and shaping natural crystalline flake graphite coated with asphalt;
preferably, the thickness of the artificial graphite layer is 0.2-0.5 μm;
preferably, the thickness of the amorphous carbon layer is 0.5-2 μm;
preferably, the modified composite material has an average particle diameter D508-25 μm;
preferably, the first discharge capacity of the modified composite material is more than or equal to 360 mAh/g;
preferably, the first charge-discharge efficiency of the modified composite material is more than or equal to 92.5%;
preferably, the capacity retention rate of the modified composite material is more than 90% at normal temperature in 1000 weeks of 1C charge-discharge cycle;
preferably, the capacity retention rate of the modified composite material is more than 85% at 45 ℃ in a 1C charge-discharge cycle for 1000 weeks;
preferably, the modified composite material is obtained by the preparation method of any one of claims 1 to 7.
10. Use of the natural graphite-based modified composite material prepared according to any one of claims 1 to 7 or the natural graphite-based modified composite material according to claim 8 or 9 in a lithium ion battery, preferably as a negative electrode material for a lithium ion battery.
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