Double-layer carbon-coated graphite negative electrode material as well as preparation method and application thereof

Abstract

The invention discloses a preparation method of a double-layer carbon-coated graphite negative electrode material, which comprises the following steps: (1) mixing artificial graphite particles, natural graphite micro powder and asphalt, and granulating to obtain a mixture; (2) carbonizing the mixture to obtain a single-layer carbon coated graphite material; (3) mixing the single-layer carbon-coated graphite material with a polymer solution, and carbonizing to obtain a double-layer carbon-coated graphite negative electrode material; wherein the mass ratio of the asphalt to the mixture is 0.02 to 0.15; the mass ratio of the single-layer carbon-coated graphite material to macromolecules in the macromolecular solution is 20 to 100. By constructing hard carbon / soft carbon double-layer carbon coating, natural graphite and artificial graphite are combined more tightly, a composite structure and a layering structure are designed more reasonably, action mechanisms are complemented, comprehensive performance such as electrochemical performance and stability of the graphite negative electrode is effectively improved, and meanwhile the production cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of carbon-separated materials, and in particular to a double-layer carbon-coated graphite negative electrode material and a preparation method and application thereof. Background Art

[0002] Graphite is the most common negative electrode material for commercial lithium-ion batteries. The development of lithium-ion batteries is inseparable from the continuous improvement of the structure of graphite negative electrode materials and the continuous improvement of their performance. Especially in recent years, with the rise of the electric vehicle industry and large-scale energy storage industry, higher requirements have been placed on the performance and cost of graphite negative electrode materials.

[0003] According to the type of raw materials, graphite negative electrode materials can be divided into natural graphite negative electrode materials and artificial graphite negative electrode materials. The raw materials of natural graphite negative electrode materials are natural flake graphite or natural microcrystalline graphite, generally based on natural flake graphite, which is obtained through crushing, purification, shaping, coating and other processes. Natural graphite negative electrode has the advantages of low cost and good rate performance. There are many types of raw materials for artificial graphite negative electrode materials, and the prices vary. The raw materials of high-end artificial graphite negative electrode materials are mostly needle coke, and the raw materials of ordinary artificial graphite negative electrode materials are mostly ordinary petroleum coke. The refined coke material is obtained after a series of treatments such as crushing, screening, graphitization and coating. Artificial graphite negative electrode has the advantages of high compaction density and good cycle performance. In comprehensive comparison, natural graphite negative electrode has rate performance and price advantages, and artificial graphite negative electrode has the advantages of high capacity and long life. The high-temperature graphitization treatment that is essential in the preparation process of artificial graphite negative electrode increases the manufacturing cost of the material.

[0004] In order to reduce the cost of graphite negative electrode materials, artificial graphite and natural graphite are usually used to form a composite negative electrode material. For example, Chinese patent document CN1808745A discloses a negative electrode active material for a lithium ion secondary battery, a negative electrode of the active material, and a lithium ion secondary battery including the negative electrode. Among them, the negative electrode active material includes a mixed graphite composed of flaky natural graphite, spherical natural graphite and flaky artificial graphite, and the lithium ion battery using the negative electrode active material has a higher reversible capacity and a better cycle life. However, this simple mixing of natural graphite and artificial graphite is prone to the problem of uneven mixing, and due to the differences in the structure and properties of the two, it is difficult to ensure the stability of product performance.

[0005] Chinese patent document CN116062745A discloses a high-capacity fast-charging composite negative electrode material for lithium-ion batteries and a preparation method. The method first polymerizes the primary particles after grinding and shaping of needle coke and the primary spherical natural graphite small particles to obtain a secondary particle precursor in which artificial graphite and natural graphite are bonded, and then the secondary particle precursor is graphitized to obtain composite graphite, and the graphitized secondary particles are mixed with asphalt in a reactor, and then carbonized, powdered, sieved, and demagnetized to obtain a composite graphite negative electrode material having both natural graphite and artificial graphite characteristics and a stable material structure. The needle coke selected in this scheme has excellent anisotropic properties and good rate performance. The selected spherical natural graphite has the characteristics of high capacity and low cost, and the gram capacity is about 370mAh / g, thereby improving the gram capacity performance of the composite material. However, this method re-graphitizes natural graphite, which already has a high degree of graphitization, which adds additional production costs; and the asphalt-based soft carbon coating tends to form a dense graphite-like structure, which is not conducive to the penetration of electrolytes and the rapid transmission of ions.

[0006] In order to improve the comprehensive performance of graphite negative electrode materials such as stability, rate performance and cycle performance during the electrochemical reaction process, the graphite material is usually subjected to surface carbon coating treatment. According to the type of coating precursor, it can be divided into soft carbon coating (such as asphalt as a precursor) and hard carbon coating (such as resin and sugar). The soft coating layer has good conductivity, but the dense and microstructured soft carbon coating layer has limited improvement on structural stability and rate performance. The hard carbon coating layer has a large carbon layer spacing and a rich carbon layer cross-linking structure, which is conducive to improving ion diffusion and structural stability, but it will consume too much electrolyte, resulting in a low first coulomb efficiency. For example, Chinese patent documents CN1304187A and Chinese patent documents CN1224251A use resin polymer pyrolytic carbon coating treatment, which can prevent the co-insertion of solvent molecules in the electrolyte, improve rate performance and cycle performance, but the specific surface area of ​​the material after resin coating is relatively large, resulting in low first efficiency and large irreversible capacity. Chinese patent document CN1585172A and Chinese patent document CN1624955A use asphalt to coat graphite materials and graphitize them at high temperature, which reduces the specific surface area of ​​the material, effectively prevents solvent co-insertion, and improves the initial efficiency, but its rate performance is poor and the cycle performance has not been completely improved.

[0007] Chinese patent document CN112820874A discloses a composite graphite negative electrode material and its preparation method and use, the method comprising: (1) mixing artificial graphite raw material, natural graphite and medium temperature asphalt to obtain a mixture; (2) granulating and graphitizing the mixture to obtain a composite of natural graphite and artificial graphite; (3) mixing the graphitized mixture with high temperature asphalt and carbonizing it to obtain the composite graphite negative electrode material. This method effectively mixes natural graphite and artificial graphite to form a mixture of single particles, which can effectively combine the advantages of the two. However, due to the high degree of graphitization of natural graphite itself, the second graphitization treatment has little effect on the development of the intrinsic structure of natural graphite, but increases the burden of the graphitization process and improves the production cost.

[0008] Chinese patent document CN115347176A discloses a method for preparing a graphite-based composite negative electrode material, the method comprising the following steps: (1) kneading and pressing natural graphite, artificial graphite precursor and asphalt 1 to obtain a graphite block; (2) heat-treating, crushing and shaping the graphite block of step (1) to obtain primary composite particles; (3) mixing and granulating the primary composite particles of step (2) with asphalt 2 to obtain secondary composite particles; (4) breaking up the secondary composite particles of step (3) and graphitizing them to obtain composite graphitized particles; (5) mixing asphalt 2, an organic solvent and the composite graphitized particles of step (4), removing the organic solvent, and then heat-treating them to obtain the graphite-based composite negative electrode material. However, the degree of graphitization of natural graphite is already very high. Graphitizing natural graphite and an artificial graphite precursor together will increase the cost of the graphitization process. Moreover, natural graphite and an artificial graphite precursor (such as needle coke) are graphitized together, and both are coated with asphalt before graphitization. In order to form a hard carbon structure, the coated asphalt is oxidized. During the subsequent graphitization process, the needle coke transforms into a denser graphite structure and thus shrinks in volume. However, the oxidized asphalt is difficult to undergo synchronous structural transformation and volume deformation due to the formation of a hard carbon layer, which will form structural defects between different phases under the action of internal stress.

[0009] In summary, the existing improvement methods or products do not significantly improve the comprehensive performance of graphite negative electrode materials. Therefore, how to obtain graphite negative electrode materials with excellent comprehensive performance and high cost performance to meet the application needs of graphite negative electrode materials in the existing electric vehicle industry and large-scale energy storage fields is a technical problem that needs to be solved urgently. Summary of the invention

[0010] The purpose of the present invention is to provide a method for preparing a double-layer carbon-coated graphite negative electrode material. By constructing a hard carbon / soft carbon double-layer carbon coating, the combination of natural graphite and artificial graphite is made closer, the composite structure and hierarchical structure design are more reasonable, and the action mechanism is complementary, which effectively improves the comprehensive performance of the graphite negative electrode such as the electrochemical performance and stability while reducing the production cost.

[0011] To achieve the above object, the present invention provides a method for preparing a double-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0012] (1) mixing and granulating artificial graphite particles, natural graphite powder and asphalt to obtain a mixture;

[0013] (2) carbonizing the mixture to obtain a single-layer carbon-coated graphite material;

[0014] (3) mixing the single-layer carbon-coated graphite material with a polymer solution and carbonizing the mixture to obtain a double-layer carbon-coated graphite negative electrode material;

[0015] Wherein, the mass ratio of the asphalt to the mixture is 0.02-0.15, preferably 0.05-0.1;

[0016] The mass ratio of the single-layer carbon-coated graphite material to the polymer in the polymer solution is 20-100.

[0017] The preparation method of the double-layer carbon-coated graphite negative electrode material provided by the present invention comprises the following steps: granulating artificial graphite particles and natural graphite powder by asphalt, tightly combining the artificial graphite particles and natural graphite powder, and dispersing them around each other, and forming composite particles with a secondary structure under the bonding and coating action of asphalt-based carbon; and then coating the secondary structure composite particles with a polymer, and converting the polymer into a hard carbon coating layer through carbonization treatment, and finally forming a double-layer carbon-coated natural graphite / artificial graphite composite graphite negative electrode material (such as Figure 1 As shown). Among them, the introduction of natural graphite components is conducive to reducing material costs and improving the rate performance of graphite negative electrodes; the introduction of artificial graphite components improves the cycle stability of graphite negative electrodes; the formation of asphalt-based soft carbon and polymer-based hard carbon and their hierarchical coating structure realizes the surface protection of natural graphite and artificial graphite primary particles and graphite / soft carbon secondary particles, which is helpful to build a conductive network, reduce surface side reactions, improve structural stabilization and ion transmission efficiency, thereby improving the electrochemical performance of graphite negative electrode materials.

[0018] Optionally, in the method for preparing the double-layer carbon-coated graphite negative electrode material provided by the present invention, the mass ratio of the artificial graphite particles to the natural graphite powder is 1:4-4:1, preferably 1:2-2:1.

[0019] Optionally, in the method for preparing a double-layer carbon-coated graphite negative electrode material provided by the present invention, the asphalt is an intermediate phase asphalt or an isotropic asphalt; preferably an intermediate phase asphalt; the softening point of the asphalt is 100-300°C, preferably 150-250°C, and more preferably 170-220°C.

[0020] Optionally, in the preparation method of the double-layer carbon-coated graphite negative electrode material provided by the present invention, in the polymer solution, the polymer is selected from any one of polyacrylonitrile, phenolic resin, polyvinyl pyrrolidone, polyvinyl alcohol, sucrose, glucose, starch and chitosan; and the solvent used to dissolve the polymer is selected from any one of dimethylformamide, ethanol, acetone and water.

[0021] Optionally, in the preparation method of the double-layer carbon-coated graphite negative electrode material provided by the present invention, the particle size D50 of the natural graphite powder is less than 15μm, preferably D50 is 5-12μm; the particle size D50 of the artificial graphite particles is less than 15μm, preferably D50 is 3-10μm.

[0022] Optionally, in the method for preparing the double-layer carbon-coated graphite negative electrode material provided by the present invention, the concentration of the polymer solution is 1wt%-5wt%.

[0023] Optionally, in step (1) of the method for preparing a double-layer carbon-coated graphite negative electrode material provided by the present invention, the granulation temperature is 300-450° C., preferably 350-400° C.; the time is 1-6 hours, preferably 2-3 hours;

[0024] In step (2) and step (3), the carbonization temperature is 800-1400°C, preferably 1000-1200°C; and the time is 5-30h, preferably 10-20h.

[0025] Optionally, the preparation method of the double-layer carbon-coated graphite negative electrode material provided by the present invention also includes the steps of crushing, screening and graphitizing the artificial graphite raw material to obtain artificial graphite particles. Preferably, the graphitization temperature is 2800-3200°C, preferably 2900-3100°C; the time is 5-20h, preferably 8-12h.

[0026] Optionally, the method for preparing the double-layer carbon-coated graphite negative electrode material provided by the present invention further includes the step of crushing and shaping the natural graphite raw material to obtain natural graphite powder.

[0027] Optionally, in the method for preparing the double-layer carbon-coated graphite negative electrode material provided by the present invention, the raw material of the artificial graphite particles is selected from needle coke and / or petroleum coke; preferably, the needle coke is selected from coal-based needle coke and / or petroleum-based needle coke;

[0028] The raw material of the natural graphite powder is selected from primary crushed and purified natural flake graphite.

[0029] Optionally, in step (2) and step (3) of the method for preparing the double-layer carbon-coated graphite negative electrode material provided by the present invention, after the carbonization, the steps of screening and demagnetization (that is, removing magnetic substances to achieve the purpose of demagnetization) are also included.

[0030] Specifically, the method for preparing the double-layer carbon-coated graphite negative electrode material provided by the present invention comprises the following steps:

[0031] (1) crushing the artificial graphite raw material into particles with a particle size of less than 15 μm (preferably 3-10 μm), and then graphitizing the particles at 2900-3100° C. for 8-12 h to obtain artificial graphite particles;

[0032] (2) crushing and shaping the purified natural flake graphite into fine powder with a particle size of less than 15 μm (preferably 5-12 μm);

[0033] (3) mixing the artificial graphite particles obtained in step (1) and the natural graphite powder obtained in step (2) with the mesophase asphalt, and stirring and granulating at 350-400° C. for 2-3 hours to obtain a mixture;

[0034] The mass ratio of the artificial graphite particles to the natural graphite powder is 1:2-2:1, and the mass ratio of the asphalt to the mixture is 0.05-0.1.

[0035] (4) The mixture obtained in step (3) is carbonized at 1000-1200° C. for 10-20 h, and then sieved and demagnetized to obtain a single-layer carbon-coated graphite material.

[0036] (5) fully mixing the single-layer carbon-coated graphite material obtained in step (4) with the polymer solution, drying to remove the solvent, and obtaining a polymer-coated graphite material;

[0037] The concentration of the organic polymer material solution is 1wt%-5wt%, and the mass ratio of the polymer to the single-layer carbon-coated graphite material is 1:100-5:100.

[0038] (6) The polymer-coated graphite material obtained in step (5) is carbonized at 1000-1200° C. for 10-20 h, and after screening and demagnetization, a double-layer carbon-coated graphite negative electrode material is obtained.

[0039] The present invention also provides a double-layer carbon-coated graphite negative electrode material, which is prepared by the above-mentioned preparation method of the double-layer carbon-coated graphite negative electrode material. The double-layer carbon-coated graphite negative electrode material has a core-shell structure and a hierarchical structure. The core part is composed of two main components of natural graphite and artificial graphite, and the shell part is composed of a soft carbon inner shell layer and a hard carbon outer shell layer. The soft carbon inner shell layer has a three-dimensional network structure feature. After coating natural graphite and artificial graphite, a complete secondary structure is formed, and the soft carbon inner shell layer is overall coated by the hard carbon outer shell layer.

[0040] The present invention also provides a negative electrode sheet, and the negative electrode sheet comprises the double-layer carbon-coated graphite negative electrode material prepared by the preparation method of the double-layer carbon-coated graphite negative electrode material.

[0041] The present invention also provides a lithium ion battery comprising the above-mentioned negative electrode sheet.

[0042] Compared with the prior art, the present invention has at least the following beneficial effects:

[0043] Beneficial effect 1: The preparation method of the double-layer carbon-coated graphite negative electrode material provided by the present invention is to mix natural graphite powder, artificial graphite particles and asphalt, and perform granulation and carbonization treatment; the artificial graphite particles and natural graphite powder are tightly combined to form composite particles with a secondary structure; the formation of asphalt-based carbon and its coating structure realizes the surface protection of natural graphite and artificial graphite primary particles, and at the same time forms a secondary structure, which helps to reduce surface side reactions, thereby improving the coulomb efficiency. By coating the surface of a single-layer carbon-coated graphite material with a polymer, and then subjecting it to a subsequent carbonization treatment, a surface hard carbon coating layer is formed for secondary coating, and a hard carbon coating layer is introduced on the surface of a single-layer carbon-coated graphite material (asphalt-based carbon / natural graphite / artificial graphite composite secondary particles), the structural stability and ion transmission performance of the material during the electrochemical reaction are improved. The amount of asphalt and polymer raw materials for forming soft carbon and hard carbon is further limited to avoid the situation where the amount of soft carbon and hard carbon is too small, the particle surface is not completely coated and protected, and the cycle stability and rate performance cannot be improved; or, although the particle surface coating and protection effect is good, which is beneficial to improving the cycle stability, the soft carbon and hard carbon coating layer is too thick, which significantly reduces the first coulomb efficiency and also causes a decrease in rate performance.

[0044] The preparation method of the double-layer carbon-coated graphite negative electrode material provided by the present invention, through the limitation of each step and the thickness of the soft carbon and hard carbon coating layers, cooperates with each other, significantly improves the comprehensive performance of the graphite negative electrode material such as rate performance, coulombic efficiency and stability, while reducing production costs.

[0045] Beneficial effect 2: The preparation method of the double-layer carbon-coated graphite negative electrode material provided by the present invention has a high degree of graphitization of natural graphite and does not need to be graphitized again. After the artificial graphite raw material is independently graphitized, the physical structure becomes stable. The two are then bonded and coated with asphalt and carbonized. The soft carbon formed can achieve dense and tight coating of the two, and there are no structural defects inside the secondary structure. Moreover, the polymer material does not need oxidation treatment before carbonization, and can be directly carbonized to form a hard carbon layer. Due to the low carbon yield of the precursor, a more significant amorphous structure and rich microporous structure can be formed. The present invention ensures the cycle life by using artificial graphite components and reduces costs by introducing natural graphite components; artificial graphite and natural graphite are linked and stacked into secondary structure particles through a soft carbon coating network, thereby improving the conductivity between primary particles and the structural stability of the graphite material itself; and further introducing a hard carbon coating layer on the surface of the secondary particles to further improve the structural stability and surface electrochemical activity of the composite negative electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 A schematic diagram of the structure of the double-layer carbon-coated graphite negative electrode material provided by the present invention. DETAILED DESCRIPTION

[0047] The present invention is described in detail below by way of examples. It is necessary to point out that the following examples are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Those skilled in the art in this field can make some non-essential improvements and adjustments to the present invention based on the above content of the present invention.

[0048] Example 1

[0049] This embodiment provides a method for preparing a double-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0050] (1) crushing needle coke into particles with a particle size D50 of 8 μm, and then graphitizing at 2900° C. for 11 h to obtain artificial graphite particles;

[0051] (2) crushing and shaping natural flake graphite into natural graphite powder with a particle size D50 of 10 μm;

[0052] (3) fully mixing the artificial graphite particles obtained in step (1) and the natural graphite powder obtained in step (2) with the mesophase asphalt, stirring and granulating at 380° C. for 3 h to obtain a mixture;

[0053] The mass ratio of artificial graphite particles to natural graphite powder is 1:1, the mass ratio of mesophase pitch to the mixture is 0.08, and the softening point of mesophase pitch is 220°C;

[0054] (4) carbonizing the mixture obtained in step (3) at 1200° C. for 16 h, screening and demagnetizing to obtain a single-layer carbon-coated graphite material;

[0055] (5) fully mixing the single-layer carbon-coated graphite material obtained in step (4) and the polyacrylonitrile solution, and drying at 90° C. for 3 h to remove the solvent to obtain a polyacrylonitrile-coated graphite material;

[0056] The mass ratio of polyacrylonitrile to the single-layer carbon-coated graphite material is 1:75, the concentration of the polyacrylonitrile solution is 3wt%, and the solvent is N,N-dimethylformamide;

[0057] (6) The polyacrylonitrile-coated graphite material obtained in step (5) is carbonized at 1200° C. for 10 h, and after screening and demagnetization, a double-layer carbon-coated graphite negative electrode material is obtained.

[0058] Figure 1 The structure diagram of the double-layer carbon-coated graphite negative electrode material is shown in FIG. Figure 1 It can be seen that under the bonding and coating action of asphalt-based carbon, composite particles with secondary structure are formed between artificial graphite and natural graphite through a soft carbon coating layer; the secondary structure composite particles are then surface coated with polymers, and the polymers are converted into hard carbon coating layers, ultimately forming a double-layer carbon-coated natural graphite / artificial graphite composite graphite negative electrode material.

[0059] Example 2

[0060] This embodiment provides a method for preparing a double-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0061] (1) crushing needle coke into particles with a particle size D50 of 7 μm, and then graphitizing at 2800° C. for 16 h to obtain artificial graphite particles;

[0062] (2) crushing and shaping natural flake graphite into natural graphite powder with a particle size D50 of 12 μm;

[0063] (3) fully mixing the artificial graphite particles obtained in step (1) and the natural graphite powder obtained in step (2) with the mesophase asphalt, stirring and granulating at 320° C. for 5.5 h to obtain a mixture;

[0064] The mass ratio of artificial graphite particles to natural graphite powder is 1:0.3, the mass ratio of mesophase pitch to the mixture is 0.03, and the softening point of mesophase pitch is 280°C;

[0065] (4) carbonizing the mixture obtained in step (3) at 850° C. for 25 h, sieving and demagnetizing to obtain a single-layer carbon-coated graphite material;

[0066] (5) fully mixing the single-layer carbon-coated graphite material obtained in step (4) and the phenolic resin solution, and drying at 90° C. for 3 h to remove the solvent, thereby obtaining a phenolic resin-coated graphite material;

[0067] The mass ratio of phenolic resin to the single-layer carbon-coated graphite material is 1:95, the concentration of the phenolic resin solution is 3wt%, and the solvent is acetone;

[0068] (6) The phenolic resin-coated graphite material obtained in step (5) is carbonized at 1300° C. for 6 h, and after screening and demagnetization, a double-layer carbon-coated graphite negative electrode material is obtained.

[0069] Example 3

[0070] This embodiment provides a method for preparing a double-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0071] (1) crushing needle coke into particles with a particle size D50 of 5 μm, and then graphitizing at 3200° C. for 7 h to obtain artificial graphite particles;

[0072] (2) crushing and shaping natural flake graphite into natural graphite powder with a particle size D50 of 5 μm;

[0073] (3) fully mixing the artificial graphite particles obtained in step (1) and the natural graphite powder obtained in step (2) with the mesophase asphalt, stirring and granulating at 430° C. for 1 h to obtain a mixture;

[0074] The mass ratio of artificial graphite particles to natural graphite powder is 1:2.5, the mass ratio of mesophase pitch to the mixture is 0.05, and the softening point of mesophase pitch is 245°C;

[0075] (4) carbonizing the mixture obtained in step (3) at 1300° C. for 10 h, screening and demagnetizing to obtain a single-layer carbon-coated graphite material;

[0076] (5) fully mixing the single-layer carbon-coated graphite material obtained in step (4) and the starch solution, and drying at 90° C. for 3 h to remove the solvent to obtain a starch-coated graphite material;

[0077] The mass ratio of starch to the single-layer carbon-coated graphite material is 1:60, the concentration of the starch solution is 1.5wt%, and the solvent is water;

[0078] (6) The polyacrylonitrile-coated graphite material obtained in step (5) is carbonized at 1200° C. for 15 h, and after screening and demagnetization, a double-layer carbon-coated graphite negative electrode material is obtained.

[0079] Example 4

[0080] This embodiment provides a method for preparing a double-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0081] (1) crushing needle coke into particles with a particle size D50 of 7 μm, and then graphitizing at 3000° C. for 10 h to obtain artificial graphite particles;

[0082] (2) crushing and shaping natural flake graphite into natural graphite powder with a particle size D50 of 10 μm;

[0083] (3) fully mixing the artificial graphite particles obtained in step (1) and the natural graphite powder obtained in step (2) with the mesophase asphalt, stirring and granulating at 340° C. for 4 h to obtain a mixture;

[0084] The mass ratio of artificial graphite particles to natural graphite powder is 1:4, the mass ratio of isotropic asphalt to the mixture is 0.1, and the softening point of isotropic asphalt is 150°C;

[0085] (4) carbonizing the mixture obtained in step (3) at 1000° C. for 5 h, sieving and demagnetizing to obtain a single-layer carbon-coated graphite material;

[0086] (5) fully mixing the single-layer carbon-coated graphite material obtained in step (4) and the glucose solution, and drying at 90° C. for 3 h to remove the solvent to obtain a glucose-coated graphite material;

[0087] The mass ratio of glucose to the single-layer carbon-coated graphite material is 1:40, the concentration of the glucose solution is 2wt%, and the solvent is water;

[0088] (6) The glucose-coated graphite material obtained in step (5) is carbonized at 1200° C. for 10 h, and after screening and demagnetization, a double-layer carbon-coated graphite negative electrode material is obtained.

[0089] Example 5

[0090] This embodiment provides a method for preparing a double-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0091] (1) crushing needle coke into particles with a particle size D50 of 7 μm, and then graphitizing at 3100° C. for 9 h to obtain artificial graphite particles;

[0092] (2) crushing and shaping natural flake graphite into natural graphite powder with a particle size D50 of 5 μm;

[0093] (3) fully mixing the artificial graphite particles obtained in step (1) and the natural graphite powder obtained in step (2) with the mesophase asphalt, stirring and granulating at 360° C. for 2 h to obtain a mixture;

[0094] The mass ratio of artificial graphite particles to natural graphite powder is 1:0.5, the mass ratio of isotropic asphalt to the mixture is 0.06, and the softening point of isotropic asphalt is 180°C;

[0095] (4) carbonizing the mixture obtained in step (3) at 1000° C. for 12 h, screening and demagnetizing to obtain a single-layer carbon-coated graphite material;

[0096] (5) fully mixing the single-layer carbon-coated graphite material obtained in step (4) and the polyvinyl alcohol solution, and drying at 90° C. for 3 h to remove the solvent, thereby obtaining a polyvinyl alcohol-coated graphite material;

[0097] The mass ratio of polyvinyl alcohol to the single-layer carbon-coated graphite material is 1:50, the concentration of the polyvinyl alcohol solution is 3wt%, and the solvent is water;

[0098] (6) The polyvinyl alcohol-coated graphite material obtained in step (5) is carbonized at 1200° C. for 10 h, and after screening and demagnetization, a double-layer carbon-coated graphite negative electrode material is obtained.

[0099] Example 6

[0100] This embodiment provides a method for preparing a double-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0101] (1) crushing petroleum coke into particles with a particle size D50 of 12 μm, and then graphitizing at 3000° C. for 10 h to obtain artificial graphite particles;

[0102] (2) crushing and shaping natural flake graphite into natural graphite powder with a particle size D50 of 10 μm;

[0103] (3) fully mixing the artificial graphite particles obtained in step (1) and the natural graphite powder obtained in step (2) with the mesophase asphalt, stirring and granulating at 390° C. for 2 h to obtain a mixture;

[0104] The mass ratio of artificial graphite particles to natural graphite powder is 1:0.8, the mass ratio of isotropic asphalt to the mixture is 0.12, and the softening point of isotropic asphalt is 120°C;

[0105] (4) carbonizing the mixture obtained in step (3) at 1100° C. for 15 h, sieving and demagnetizing to obtain a single-layer carbon-coated graphite material;

[0106] (5) fully mixing the single-layer carbon-coated graphite material obtained in step (4) and the polyvinyl pyrrolidone solution, and drying at 90° C. for 3 h to remove the solvent to obtain a polyvinyl pyrrolidone-coated graphite material;

[0107] The mass ratio of polyvinyl pyrrolidone to the single-layer carbon-coated graphite material is 1:25, the concentration of the polyvinyl pyrrolidone solution is 4wt%, and the solvent is ethanol;

[0108] (6) The polyvinyl pyrrolidone-coated graphite material obtained in step (5) is carbonized at 1200° C. for 10 h, and after screening and demagnetization, a double-layer carbon-coated graphite negative electrode material is obtained.

[0109] Comparative Example 1

[0110] This comparative example provides a method for preparing a single-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0111] (1) crushing needle coke into particles with a particle size D50 of 8 μm, and then graphitizing at 2900° C. for 11 h to obtain artificial graphite particles;

[0112] (2) fully mixing the artificial graphite particles obtained in step (1) and the mesophase pitch, stirring and granulating at 380° C. for 3 h to obtain a mixture;

[0113] The mass ratio of mesophase asphalt to the mixture is 0.08, and the softening point of mesophase asphalt is 220°C;

[0114] (3) The mixture obtained in step (2) is carbonized at 1200° C. for 16 h, sieved, and demagnetized to obtain a single-layer carbon-coated graphite negative electrode material.

[0115] Comparative Example 2

[0116] This comparative example provides a method for preparing a single-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0117] (1) crushing and shaping natural flake graphite into natural graphite powder with a particle size D50 of 10 μm;

[0118] (2) fully mixing the natural graphite powder obtained in step (1) with the mesophase asphalt, stirring and granulating at 380° C. for 3 h to obtain a mixture;

[0119] The mass ratio of mesophase asphalt to the mixture is 0.08, and the softening point of mesophase asphalt is 220°C;

[0120] (3) The mixture obtained in step (2) is carbonized at 1200° C. for 16 h, sieved, and demagnetized to obtain a single-layer carbon-coated graphite material.

[0121] Comparative Example 3

[0122] This comparative example provides a method for preparing a double-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0123] (1) crushing needle coke into particles with a particle size D50 of 8 μm, and then graphitizing at 2900° C. for 11 h to obtain artificial graphite particles;

[0124] (2) fully mixing the artificial graphite particles obtained in step (1) with the mesophase asphalt, stirring and granulating at 380° C. for 3 h to obtain a mixture;

[0125] The mass ratio of mesophase asphalt to the mixture is 0.08, and the softening point of mesophase asphalt is 220°C;

[0126] (3) carbonizing the mixture obtained in step (2) at 1200° C. for 16 h, screening and demagnetizing to obtain a single-layer carbon-coated graphite material;

[0127] (4) fully mixing the single-layer carbon-coated graphite material obtained in step (3) and the polyacrylonitrile solution, and drying at 90° C. for 3 h to remove the solvent to obtain a polyacrylonitrile-coated graphite material;

[0128] The mass ratio of polyacrylonitrile to the single-layer carbon-coated graphite material is 1:75, the concentration of the polyacrylonitrile solution is 3wt%, and the solvent is N,N-dimethylformamide;

[0129] (5) The polyacrylonitrile-coated graphite material obtained in step (4) is carbonized at 1200° C. for 10 h, and after screening and demagnetization, a double-layer carbon-coated graphite negative electrode material is obtained.

[0130] Comparative Example 4

[0131] This comparative example provides a method for preparing a single-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0132] (1) crushing needle coke into particles with a particle size D50 of 8 μm, and then graphitizing at 2900° C. for 11 h to obtain artificial graphite particles;

[0133] (2) crushing and shaping natural flake graphite into natural graphite powder with a particle size D50 of 10 μm;

[0134] (3) fully mixing the artificial graphite particles obtained in step (1) and the natural graphite powder obtained in step (2) with the mesophase asphalt, stirring and granulating at 380° C. for 3 h to obtain a mixture;

[0135] The mass ratio of artificial graphite particles to natural graphite powder is 1:1, the mass ratio of mesophase pitch to the mixture is 0.08, and the softening point of mesophase pitch is 220°C;

[0136] (4) The mixture obtained in step (3) is carbonized at 1200° C. for 16 h, sieved, and demagnetized to obtain a single-layer carbon-coated graphite negative electrode material.

[0137] Comparative Example 5

[0138] This comparative example provides a method for preparing a double-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0139] (1) crushing needle coke into particles with a particle size D50 of 8 μm, and then graphitizing at 2900° C. for 11 h to obtain artificial graphite particles;

[0140] (2) crushing and shaping natural flake graphite into natural graphite powder with a particle size D50 of 10 μm;

[0141] (3) fully mixing the artificial graphite particles obtained in step (1) and the natural graphite powder obtained in step (2) with the mesophase asphalt, stirring and granulating at 380° C. for 3 h to obtain a mixture;

[0142] The mass ratio of artificial graphite particles to natural graphite powder is 1:1, the mass ratio of mesophase pitch to the mixture is 0.01, and the softening point of mesophase pitch is 220°C;

[0143] (4) carbonizing the mixture obtained in step (3) at 1200° C. for 16 h, screening and demagnetizing to obtain a single-layer carbon-coated graphite material;

[0144] (5) fully mixing the single-layer carbon-coated graphite material obtained in step (4) and the polyacrylonitrile solution, and drying at 90° C. for 3 h to remove the solvent to obtain a polyacrylonitrile-coated graphite material;

[0145] The mass ratio of polyacrylonitrile to the single-layer carbon-coated graphite material is 1:110, the concentration of the polyacrylonitrile solution is 3wt%, and the solvent is N,N-dimethylformamide;

[0146] (6) The polyacrylonitrile-coated graphite material obtained in step (5) is carbonized at 1200° C. for 10 h, and after screening and demagnetization, a double-layer carbon-coated graphite negative electrode material is obtained.

[0147] Comparative Example 6

[0148] This comparative example provides a method for preparing a single-layer hard carbon-coated graphite negative electrode material, comprising the following steps:

[0149] (1) crushing needle coke into particles with a particle size D50 of 8 μm, and then graphitizing at 2900° C. for 11 h to obtain artificial graphite particles;

[0150] (2) crushing and shaping natural flake graphite into natural graphite powder with a particle size D50 of 10 μm;

[0151] (3) fully mixing the artificial graphite particles obtained in step (1) and the natural graphite powder obtained in step (2) with the polyacrylonitrile solution, drying at 90° C. for 3 h to remove the solvent, and obtaining a polyacrylonitrile-coated graphite material;

[0152] The mass ratio of artificial graphite particles to natural graphite powder is 1:1, the mass ratio of polyacrylonitrile to the mixture of artificial graphite particles and natural graphite powder is 0.08, the concentration of the polyacrylonitrile solution is 3wt%, and the solvent is N,N-dimethylformamide;

[0153] (4) The polyacrylonitrile-coated graphite material obtained in step (3) is carbonized at 1200° C. for 16 h, and after screening and demagnetization, a single-layer hard carbon-coated graphite negative electrode material is obtained.

[0154] Comparative Example 7

[0155] This comparative example provides a method for preparing a double-layer carbon-coated graphite negative electrode material, comprising the following steps:

[0156] (1) crushing needle coke into particles with a particle size D50 of 8 μm, and then graphitizing at 2900° C. for 11 h to obtain artificial graphite particles;

[0157] (2) crushing and shaping natural flake graphite into natural graphite powder with a particle size D50 of 10 μm;

[0158] (3) fully mixing the artificial graphite particles obtained in step (1) and the natural graphite powder obtained in step (2) with the polyacrylonitrile solution, drying at 90° C. for 3 h to remove the solvent, and obtaining a polyacrylonitrile-coated graphite material;

[0159] The mass ratio of artificial graphite particles to natural graphite powder is 1:1, the mass ratio of polyacrylonitrile to the mixture of artificial graphite particles and natural graphite powder is 0.08, the concentration of the polyacrylonitrile solution is 3wt%, and the solvent is N,N-dimethylformamide;

[0160] (4) carbonizing the polyacrylonitrile-coated graphite material obtained in step (3) at 1200° C. for 10 h, screening and demagnetizing to obtain a single-layer hard carbon-coated graphite negative electrode material;

[0161] (5) The single-layer hard carbon-coated graphite negative electrode material obtained in step (4) is fully mixed with the mesophase asphalt, and granulated at 380° C. for 3 h to obtain a mixture;

[0162] The mass ratio of the mesophase pitch to the single-layer hard carbon-coated graphite negative electrode material is 1:75, and the softening point of the mesophase pitch is 220°C;

[0163] (6) The mixture obtained in step (3) is carbonized at 1200° C. for 16 h, sieved, and demagnetized to obtain a double-layer carbon-coated graphite negative electrode material.

[0164] Comparative Example 8

[0165] This comparative example provides a method for preparing a double-layer hard carbon-coated graphite negative electrode material, comprising the following steps:

[0166] (1) crushing needle coke into particles with a particle size D50 of 8 μm, and then graphitizing at 2900° C. for 11 h to obtain artificial graphite particles;

[0167] (2) crushing and shaping natural flake graphite into natural graphite powder with a particle size D50 of 10 μm;

[0168] (3) fully mixing the artificial graphite particles obtained in step (1) and the natural graphite powder obtained in step (2) with the polyacrylonitrile solution, drying at 90° C. for 3 h to remove the solvent, and obtaining a polyacrylonitrile-coated graphite material;

[0169] The mass ratio of artificial graphite particles to natural graphite powder is 1:1, the mass ratio of glucose to the mixture of artificial graphite particles and natural graphite powder is 0.08, the concentration of the glucose solution is 3wt%, and the solvent is water;

[0170] (4) carbonizing the glucose-coated graphite material obtained in step (3) at 1200° C. for 16 h, screening and demagnetizing to obtain a single-layer hard carbon-coated graphite negative electrode material;

[0171] (5) fully mixing the single-layer hard carbon-coated graphite material obtained in step (4) and the polyacrylonitrile solution, and drying at 90° C. for 3 h to remove the solvent to obtain a polyacrylonitrile-coated graphite material;

[0172] (6) The mass ratio of polyacrylonitrile to the single-layer hard carbon-coated graphite material is 1:75, the concentration of the polyacrylonitrile solution is 3 wt %, and the solvent is N,N-dimethylformamide;

[0173] (7) The polyacrylonitrile-coated graphite material obtained in step (5) is carbonized at 1200° C. for 16 h, and after screening and demagnetization, a double-layer hard carbon-coated graphite negative electrode material is obtained.

[0174] Experimental example

[0175] The graphite negative electrode material obtained in each embodiment and comparative example was mixed with Super P conductive carbon black and PVDF in a mass ratio of 9:0.5:0.5 to prepare a negative electrode sheet, which was assembled into a half-cell with metal lithium foil and a separator, and the electrochemical performance was tested. The test method is as follows:

[0176] Lithium insertion capacity and first coulombic efficiency test: 0.05C discharge to 5mV, stand for 2min; 0.01C discharge to 5mV, stand for 2min; 0.1C charge to 1.5V, stand for 2min.

[0177] Cyclic performance test: 0.2C discharge to 5mV, stand for 2min; 0.2C charge to 1.5V, stand for 2min. Cycle 500 times.

[0178] Rate performance test: 0.5C discharge to 5mV, let stand for 2min; 0.5C charge to 1.5V, let stand for 2min.

[0179] The specific test results are shown in Table 1.

[0180] Table 1 Electrochemical properties of graphite anode materials

[0181]

[0182]

[0183] It can be seen from the data in the above table that in the present invention, artificial graphite particles and natural graphite powder are fully mixed, and the structural optimization combination is achieved through secondary granulation and double-layer carbon coating, thereby preparing a graphite negative electrode material with a hierarchical structure and dual main components; the uniformity of the composite of natural graphite and artificial graphite and the stability of the secondary composite structure are effectively improved, so that the negative electrode material has the advantages of both artificial graphite negative electrode and natural graphite negative electrode, and effectively overcomes their respective shortcomings, showing advantageous specific capacity, first coulomb efficiency, rate performance and cycle stability; at the same time, the manufacturing cost of high-performance graphite negative electrode materials is reduced.

[0184] Of course, the present invention may have many other embodiments and variations thereof. Without departing from the spirit and essence of the present invention, technicians familiar with the field may make various corresponding changes and variations based on the present invention, but these corresponding changes and variations should all fall within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a double-layer carbon-coated graphite negative electrode material, characterized in that: The following steps are involved: (1) mixing and granulating artificial graphite particles, natural graphite powder and asphalt to obtain a mixture; (2) carbonizing the mixture to obtain a single-layer carbon-coated graphite material; (3) mixing the single-layer carbon-coated graphite material with a polymer solution and carbonizing the mixture to obtain a double-layer carbon-coated graphite negative electrode material; Wherein, the mass ratio of the asphalt to the mixture is 0.02-0.15; The mass ratio of the single-layer carbon-coated graphite material to the polymer in the polymer solution is 20-100.

2. The preparation method according to claim 1, characterized in that The mass ratio of the artificial graphite particles to the natural graphite powder is 1:4-4:

1.

3. The preparation method according to claim 1, characterized in that: The asphalt is mesophase asphalt or isotropic asphalt; preferably mesophase asphalt; the softening point of the asphalt is 100-300°C.

4. The preparation method according to claim 1, characterized in that: In the polymer solution, the polymer is selected from any one of polyacrylonitrile, phenolic resin, polyvinyl pyrrolidone, polyvinyl alcohol, sucrose, glucose, starch and chitosan; and the solvent is selected from any one of dimethylformamide, ethanol, acetone and water.

5. The preparation method according to claim 1, characterized in that: The particle size D50 of the natural graphite powder is less than 15 μm, and the particle size D50 of the artificial graphite particles is less than 15 μm.

6. The preparation method according to claim 1, characterized in that: In step (1), the granulation temperature is 300-450°C and the time is 1-6h; In step (2) and step (3), the carbonization temperature is 800-1400° C. and the time is 5-30 hours.

7. The preparation method according to claim 1, characterized in that: The raw material of the artificial graphite particles is selected from needle coke and / or petroleum coke; The raw material of the natural graphite powder is selected from natural flake graphite.

8. A double-layer carbon-coated graphite negative electrode material, characterized in that: The negative electrode material is prepared by the method for preparing the double-layer carbon-coated graphite negative electrode material according to any one of claims 1 to 7.

9. A negative electrode sheet, characterized in that: The negative electrode sheet comprises a double-layer carbon-coated graphite negative electrode material prepared by the preparation method of a double-layer carbon-coated graphite negative electrode material according to any one of claims 1 to 7.

10. A lithium ion battery, characterized in that: A negative electrode sheet comprising the negative electrode sheet according to claim 9.

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