Carbon-coated graphite negative electrode material as well as preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a carbon-coated graphite negative electrode material as well as a preparation method and application thereof. The method comprises the following steps: (1) carrying out ball milling on a nitrogen source, a fluorine source and graphite to obtain a mixture; and (2) in the presence of inert gas, sequentially carrying out first heat treatment and second heat treatment on the mixture to obtain the carbon-coated graphite negative electrode material, the conditions of the first heat treatment are as follows: the temperature is 300-500 DEG C; the conditions of the second heat treatment are as follows: the temperature is 600-800 DEG C; the temperature of the second heat treatment is at least 200 DEG C higher than the temperature of the first heat treatment. The preparation method provided by the invention is simple in process and easy for industrial production. The carbon-coated graphite negative electrode material obtained by the preparation method provided by the invention has a good application prospect in the field of lithium ion batteries.

Description

Technical Field

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

[0002] Lithium-ion batteries have demonstrated great application value in the field of green energy storage due to their many outstanding advantages such as high energy density, low self-discharge rate, long cycle life and low cost.

[0003] Anode materials play a crucial role in the structure of lithium-ion batteries. Graphite, with its low operating voltage and well-defined layered structure, allows for reversible insertion and extraction of lithium ions, making it a dominant anode material due to its unique physical and chemical properties.

[0004] However, graphite still presents some significant challenges in practical applications. For example, during the charge and discharge process, graphite experiences a certain degree of irreversible capacity loss and the breakdown and reformation of the SEI film, resulting in suboptimal initial charge and discharge efficiency. Furthermore, as the number of charge and discharge cycles increases, graphite undergoes volume expansion and contraction, as well as co-intercalation of solvent molecules, leading to the shedding of graphite flakes. This not only affects structural stability but also increases internal resistance, shortening the battery's lifespan.

[0005] The above problems are like barriers that create resistance to the large-scale application of lithium-ion batteries and seriously restrict their further development.

[0006] By uniformly coating the graphite surface with functional materials, the direct contact between graphite and electrolyte can be effectively isolated, the co-intercalation of solvent molecules can be prevented, and the volume expansion of graphite particles during charging and discharging can be inhibited, thereby significantly improving the cycle stability.

[0007] The coating materials of commercial graphite negative electrode materials are often asphalt, phenolic resin, etc., and their production and preparation process has problems such as uneven coating, insufficient consistency, environmental pollution and high energy consumption.

[0008] Element doping can also be used as a core modification strategy for graphite negative electrodes. It can not only change the electronic structure and improve the conductivity of the material by introducing specific elements into the graphite lattice, but also regulate the formation and composition of the SEI film on the surface of the graphite negative electrode, thereby promoting the formation of a stable, uniform and ionically conductive SEI film, reducing irreversible capacity loss, and improving the initial charge and discharge efficiency and long-term cycle performance.

[0009] However, the element doping methods available in the prior art are usually costly, complex in process, and environmentally polluting, making them difficult to apply to industrial production.

[0010] Therefore, there is an urgent need for a method that is simple to prepare, economical, green, and easy to industrialize to optimize the performance of graphite negative electrode materials to meet the multiple technical requirements of the new generation of lithium-ion batteries for energy density, cycle stability, and fast charging performance. Summary of the Invention

[0011] The purpose of the present invention is to overcome the problems in the prior art of complex preparation process of element-doped graphite negative electrode materials and difficulty in industrial production.

[0012] In order to achieve the above object, the first aspect of the present invention provides a method for preparing a carbon-coated graphite negative electrode material, the method comprising:

[0013] (1) ball milling a nitrogen source, a fluorine source, and graphite to obtain a mixture;

[0014] (2) in the presence of an inert gas, sequentially subjecting the mixture to a first heat treatment and a second heat treatment to obtain the carbon-coated graphite negative electrode material;

[0015] The ball milling conditions are controlled so that the average particle diameter of the mixture is 10-20 nm;

[0016] The conditions of the first heat treatment include: a temperature of 300-500°C;

[0017] The conditions of the second heat treatment include: a temperature of 600-800°C;

[0018] The temperature of the second heat treatment is at least 200° C. higher than the temperature of the first heat treatment.

[0019] The second aspect of the present invention provides a carbon-coated graphite negative electrode material prepared by the method described in the first aspect.

[0020] The third aspect of the present invention provides the use of the carbon-coated graphite negative electrode material described in the second aspect in a lithium-ion battery.

[0021] The method for preparing a carbon-coated graphite negative electrode material provided by the present invention comprises sequentially subjecting a mixture (containing a nitrogen source, a fluorine source, and graphite) with an average particle diameter of 10-20 nm to a first heat treatment (300-500°C) and a second heat treatment (600-800°C), and requiring that the temperature of the second heat treatment be at least 200°C higher than that of the first heat treatment. The carbon-coated graphite negative electrode material obtained under such specific conditions can promote the formation of a LiF-rich SEI film on the graphite surface, effectively inhibit side reactions between the graphite negative electrode and the electrolyte, provide a fast channel for the migration of lithium ions, greatly improve the graphite negative electrode / electrolyte interface performance, and enhance the structural stability and electrochemical stability of the graphite.

[0022] The method for preparing carbon-coated graphite negative electrode materials provided in the present invention is simple and easy to operate, green and environmentally friendly, and has low cost, and provides new ideas and application support for the design and preparation of high-performance lithium-ion battery negative electrode materials.

[0023] The carbon-coated graphite negative electrode material prepared by the method provided by the present invention has a unique porous structure; the carbon-coated graphite negative electrode material is applied to lithium-ion batteries, so that the lithium-ion batteries have high specific capacity and good cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a SEM image of the carbon-coated graphite negative electrode material prepared in Example 1 of the present invention;

[0025] Figure 2 is an SEM image of the carbon-coated graphite negative electrode material prepared in Comparative Example 1 of the present invention;

[0026] Figure 3 is an SEM image of the carbon-coated graphite negative electrode material prepared in Comparative Example 2 of the present invention;

[0027] Figure 4 is an EDS image of the carbon-coated graphite negative electrode material prepared in Example 1 of the present invention;

[0028] Figure 5 1 is a Raman image of the carbon-coated graphite negative electrode material prepared in Example 1, Comparative Example 1, and Comparative Example 2 of the present invention;

[0029] Figure 6 1 is the XPS spectrum of the carbon-coated graphite negative electrode materials prepared in Example 1, Comparative Example 1, and Comparative Example 2 of the present invention;

[0030] Figure 7 This is a detailed spectrum of fluorine element in the carbon-coated graphite negative electrode material prepared in Example 1 of the present invention;

[0031] Figure 8 2 is a N2 adsorption / desorption isotherm diagram of the carbon-coated graphite negative electrode materials prepared in Example 1, Comparative Example 1, and Comparative Example 2 of the present invention;

[0032] Figure 9 This is a comparison chart of the cycle performance of the carbon-coated graphite negative electrode materials prepared in Example 1, Comparative Example 1, and Comparative Example 2 of the present invention when applied to lithium-ion batteries at a charge and discharge rate of 0.5C. DETAILED DESCRIPTION

[0033] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0034] In the present invention, D50 refers to the particle size corresponding to when the cumulative particle size distribution percentage of graphite reaches 50%.

[0035] As mentioned above, the first aspect of the present invention provides a method for preparing a carbon-coated graphite negative electrode material, the method comprising:

[0036] (1) ball milling a nitrogen source, a fluorine source, and graphite to obtain a mixture;

[0037] (2) in the presence of an inert gas, sequentially subjecting the mixture to a first heat treatment and a second heat treatment to obtain the carbon-coated graphite negative electrode material;

[0038] The ball milling conditions are controlled so that the average particle diameter of the mixture is 10-20 nm;

[0039] The conditions of the first heat treatment include: a temperature of 300-500°C;

[0040] The conditions of the second heat treatment include: a temperature of 600-800°C;

[0041] The temperature of the second heat treatment is at least 200° C. higher than the temperature of the first heat treatment.

[0042] Preferably, in step (1), the mass ratio of the nitrogen source, the fluorine source, and the graphite is 1:1-2:5-8. The inventors of the present invention have found that in this preferred embodiment, the carbon-coated graphite negative electrode material prepared has better cycle performance when used in lithium-ion batteries.

[0043] Preferably, in step (1), the nitrogen source is melamine; and the fluorine source is polyvinylidene fluoride.

[0044] Preferably, the weight average molecular weight of the polyvinylidene fluoride is 700,000 to 1,000,000. The inventors of the present invention have found that in this preferred embodiment, the carbon-coated graphite negative electrode material prepared has better cycle performance when used in lithium-ion batteries.

[0045] Preferably, in step (1), the D50 of the graphite is 7-15 μm.

[0046] According to a preferred embodiment, in step (1), the ball milling conditions are controlled so that the average particle diameter of the mixture is 12-15 nm; the inventors of the present invention found that under this preferred embodiment, the prepared carbon-coated graphite negative electrode material is applied to lithium-ion batteries to obtain better cycle stability.

[0047] Preferably, in step (1), the ratio of the sum of the mass of the nitrogen source, the fluorine source and the graphite to the mass of the grinding balls is controlled to be 1:1.5-2.5.

[0048] Preferably, in step (1), the diameter of each grinding ball is independently 4-10 mm.

[0049] Preferably, in step (1), the ball milling is performed under an argon atmosphere.

[0050] Preferably, in step (1), the ball milling is carried out in a ball milling jar, and the ball milling conditions include: a rotation speed of 200-600 rpm, more preferably 300-400 rpm; and a time of 4-8 h, more preferably 5-6 h.

[0051] Preferably, the ball milling conditions further include: alternating the ball milling direction every 8-12 minutes (forward ball milling for 8-12 minutes, then reverse ball milling for 8-12 minutes, and repeating the alternation).

[0052] According to a preferred embodiment, the method further comprises: in step (1), grinding, sieving and drying the material after ball milling in sequence to obtain the mixed material.

[0053] The present invention has no special requirements for the grinding and screening methods, and those skilled in the art can select them according to conventional means in the art.

[0054] Preferably, the drying treatment conditions include: a temperature of 50-70° C. and a time of 3-5 hours.

[0055] Preferably, in step (2), the conditions of the first heat treatment also include: a heating rate V1 of 1-2°C / min, and a holding time T1 of 1.5-2.5h.

[0056] Preferably, in step (2), the conditions of the second heat treatment also include: a heating rate V2 of 1-2°C / min, and a holding time T2 of 1-3h.

[0057] It should be noted that, in the present invention, the holding time does not include the time required for heating.

[0058] Preferably, the inert gas is argon.

[0059] As mentioned above, the second aspect of the present invention provides a carbon-coated graphite negative electrode material prepared by the method described in the first aspect.

[0060] As mentioned above, the third aspect of the present invention provides the use of the carbon-coated graphite negative electrode material described in the second aspect in a lithium-ion battery.

[0061] The present invention will be described in detail below by way of examples. In the following examples, unless otherwise specified, all instruments and raw materials used are commercially available.

[0062] Nitrogen source: melamine.

[0063] Fluorine source: polyvinylidene fluoride: weight average molecular weight of 1 million, purchased from Duoduo Chemical Reagent Company, brand HSV900.

[0064] Graphite: D50 is 10 μm, purchased from Hunan Rongli New Material Technology Co., Ltd., brand A08.

[0065] Grinding balls: 4-10 mm in diameter, purchased from Xinshang Agate Jade Factory Co., Ltd. in Heishan County.

[0066] Example 1

[0067] This example is used to illustrate the preparation of a carbon-coated graphite negative electrode material according to the following steps with reference to the formula and process parameters in Table 1:

[0068] (1) In a ball mill filled with argon, a nitrogen source, a fluorine source, and graphite are ball milled in the presence of grinding balls (the ball milling direction is changed every 10 minutes, with forward ball milling for 10 minutes and reverse ball milling for 10 minutes, and repeated alternately), and then the ball-milled materials are sequentially ground, sieved, and dried to obtain a mixed material;

[0069] (2) in the presence of argon, sequentially subjecting the mixture to a first heat treatment and a second heat treatment to obtain the carbon-coated graphite negative electrode material;

[0070] The drying temperature was 60°C and the drying time was 4 h.

[0071] Example 2

[0072] This embodiment is carried out using a method similar to that of embodiment 1, except that the formula and process parameters are shown in Table 1. The unlisted parts are the same as those of embodiment 1, and a carbon-coated graphite negative electrode material is prepared.

[0073] Example 3

[0074] This embodiment is carried out using a method similar to that of embodiment 1, except that: the formula and process parameters are shown in Table 1 for details, and the unlisted parts are the same as those of embodiment 1, to prepare a carbon-coated graphite negative electrode material.

[0075] Example 4

[0076] This embodiment is carried out using a method similar to that of embodiment 1, except that the process parameters are shown in Table 1. The unlisted parts are the same as those of embodiment 1, and a carbon-coated graphite negative electrode material is prepared.

[0077] Comparative Example 1

[0078] In this comparative example, graphite is directly used as the negative electrode material.

[0079] Comparative Example 2

[0080] This comparative example was carried out using a method similar to that of Example 1, except that no nitrogen source was added in step (1). The unlisted parts were the same as those of Example 1, and a carbon-coated graphite negative electrode material was prepared.

[0081] Comparative Example 3

[0082] This comparative example was carried out using a method similar to that of Example 1, except that the temperature of the first heat treatment in this comparative example was 500° C., and the unlisted parts were the same as those of Example 1, to prepare a carbon-coated graphite negative electrode material.

[0083] Comparative Example 4

[0084] This comparative example was carried out using a method similar to that of Example 1, except that the temperature of the second heat treatment in this comparative example was 1000° C., and the unlisted parts were the same as those of Example 1, to prepare a carbon-coated graphite negative electrode material.

[0085] Table 1

[0086]

[0087] Test Case

[0088] 1. The morphology of the carbon-coated graphite negative electrode materials prepared in Example 1, Comparative Example 1 and Comparative Example 2 was tested using a scanning electron microscope (SEM). The results were as follows: Figure 1 、 Figure 2 and Figure 3 As shown in:

[0089] pass Figure 1 It can be seen that the surface of the carbon-coated graphite negative electrode material prepared in Example 1 has a continuous and complete carbon coating layer, its surface becomes relatively smooth and neat, the edge contour becomes smooth, and more small carbon flakes are distributed on the surface;

[0090] pass Figure 2 It can be seen that the original graphite is in the form of irregular block particles with an uneven surface, many protrusions and particle fragments, and an average diameter of about 10μm. Its typical layered stacking structure can be clearly observed at the side edges of the original graphite.

[0091] pass Figure 3 It can be seen that the carbon-coated graphite negative electrode material prepared in Comparative Example 2 has a continuous and uniform carbon coating layer, forming a typical core-shell structure, and retains the structural morphology of the original graphite. The carbon coating layer is tightly attached to the graphite surface, giving the graphite surface a certain gloss and smoothness, and some small carbon flakes are scattered.

[0092] 2. The elements of the carbon-coated graphite negative electrode material prepared in Example 1 were analyzed using a scanning electron microscope (SEM) and an energy dispersive X-ray spectrometer (EDS). The results are as follows: Figure 4 As shown in:

[0093] pass Figure 4 It can be seen that the EDS energy spectrum of the carbon-coated graphite negative electrode material prepared in Example 1 detected signals of four elements: C, O, N, and F, and the F and N elements had a certain content; this indicates that during the coating and heat treatment process, the F and N elements were effectively retained, and the carbon layer containing F and N elements successfully coated the graphite surface.

[0094] 3. The surface structure of the carbon-coated graphite negative electrode materials obtained in Example 1, Comparative Example 1 and Comparative Example 2 was analyzed using a Raman spectrometer. The results are as follows: Figure 5 As shown in:

[0095] pass Figure 5 It can be seen that polyvinylidene fluoride and melamine are converted into amorphous carbon with low graphite order during the carbonization process, and the doping of F and N elements introduces additional defects, thereby increasing the D peak intensity and I D / I G By calculation, the carbon-coated graphite negative electrode material prepared in Example 1 has an I D / I G is 0.583, which is significantly higher than that of Comparative Example 1 (original graphite, 0.167) and Comparative Example 2 (F-doped carbon-coated graphite, 0.411). This shows that the carbon-coated graphite negative electrode material prepared in Example 1 has more defects and Li + Active sites are beneficial to improving its rate performance and specific capacity.

[0096] 4. The elements of the carbon-coated graphite negative electrode materials prepared in Example 1, Comparative Example 1 and Comparative Example 2 were analyzed using an X-ray photoelectron spectrometer. The results are as follows: Figure 6 As shown in:

[0097] pass Figure 6 It can be seen that the XPS full spectrum of the carbon-coated graphite negative electrode material prepared in Comparative Example 1 shows two elements, C and O. In contrast, the full spectrum of the carbon-coated graphite negative electrode material prepared in Comparative Example 2 shows three element signals of C, O and F, indicating that polyvinylidene fluoride successfully introduced the F element into graphite; while the full spectrum of the carbon-coated graphite negative electrode material prepared in Example 1 shows four element signals of C, N, O and F, which also confirms the presence of F / N co-doped carbon.

[0098] 5. The X-ray photoelectron spectrometer was used to analyze the fine spectrum of fluorine element in the carbon-coated graphite negative electrode material prepared in Example 1. The results are as follows: Figure 7 As shown in:

[0099] pass Figure 7 It can be seen that the F1s spectrum of the carbon-coated graphite negative electrode material prepared in Example 1 has two characteristic peaks, corresponding to the conductive semi-ion CF and the covalent CF bond, respectively; the covalent CF bond originates from the typical covalent CF bond in the polyvinylidene fluoride structure, while the conductive semi-ion C–F is formed by the interaction between F atoms and carbon atoms in graphite.

[0100] 6. The specific surface area and porosity tester were used to analyze the specific surface area of ​​the carbon-coated graphite negative electrode materials obtained in Example 1, Comparative Example 1 and Comparative Example 2. The results are as follows: Figure 8 As shown in:

[0101] pass Figure 8 It can be seen that the adsorption amount of the negative electrode material of Comparative Example 1 increases slowly when the relative pressure is low, and the adsorption amount increases significantly when the relative pressure is high, showing a typical type III adsorption / desorption isotherm; while the negative electrode materials of Example 1 and Comparative Example 2 show type IV adsorption / desorption isotherms, which indicates that the coating of pyrolytic carbon and the doping of F and N elements introduce more defects, making the mesoporous structure in the modified graphite dominant, increasing the Li + The diffusion path is conducive to Li + storage and diffusion.

[0102] 7. The carbon-coated graphite negative electrode materials prepared in the above examples were applied to lithium-ion batteries (CR2025 model). The specific operations are as follows:

[0103] Negative electrode preparation:

[0104] Among them, the mass ratio of active material (carbon-coated graphite negative electrode material), conductive carbon black and carboxymethyl cellulose is 8:1:1.

[0105] The active material and conductive carbon black powder were first ground in an agate mortar for 40 minutes to obtain a mixed powder. Simultaneously, carboxymethyl cellulose and deionized water were added sequentially to a weighing bottle, followed by the mixed powder and magnetically stirred for 8 hours to obtain a black slurry. The black slurry was then evenly coated onto copper foil using a 100μm blade spatula and dried in a vacuum drying oven at 100°C for 12 hours. The coated foil was then cut into 12mm-diameter circular electrodes using a tablet press. These were then weighed using an electronic balance with an accuracy of 1 / 10,000th, and the mass of the active material in the electrode was further calculated.

[0106] Button lithium-ion battery assembly:

[0107] The half-cell was assembled in an argon-filled glove box, using a 12 mm lithium metal sheet as the counter electrode; a CR2025 button cell shell was used, and the positive electrode shell, positive electrode, separator (Celgard 2400), negative electrode, nickel foam, and negative electrode shell were placed in the order of positive electrode shell; 50 μL of electrolyte (1.0 mol / L electrolyte) was added between the negative electrode sheet and the separator and between the positive electrode sheet and the separator. -1 LiPF6 is dissolved in a mixed solution of ethylene carbonate / diethyl carbonate / dimethyl carbonate in a volume ratio of 1:1:1, and 1 Vol% vinylene carbonate is added), and the button battery is sealed with a battery packaging machine.

[0108] (1) The performance of each of the assembled lithium-ion batteries was tested (test conditions: the assembled batteries were left at room temperature (25°C ± 2°C) for 12 hours before electrochemical performance testing, 1C = 360 mAh / g, the results are shown in Table 2:

[0109] Table 2

[0110] 0.1C first charge specific capacity (mAh / g) Capacity retention rate after 150 cycles at 0.5C (%) Example 1 402.05 96.92 Example 2 398.17 98.14 Example 3 388.68 95.95 Example 4 392.43 96.58 Comparative Example 1 356.86 93.38 Comparative Example 2 384.29 95.72 Comparative Example 3 364.57 84.76 Comparative Example 4 380.14 78.34

[0111] (2) The cycle performance of the carbon-coated graphite negative electrode materials prepared in Example 1, Comparative Example 1 and Comparative Example 2 after being assembled into lithium-ion batteries was tested (test conditions: cycle performance test at room temperature (25°C ± 2°C) and a charge-discharge rate of 0.5C). The results are as follows: Figure 9 As shown in:

[0112] pass Figure 9It can be seen that after 150 cycles, the negative electrode material of Comparative Example 1 rapidly dropped from the initial 267.63 mAh / g to 249.93 mAh / g, and the capacity retention rate was only 93.38%. This may be due to the instability of the original graphite structure. Compared with Comparative Example 1, the negative electrode material prepared in Comparative Example 2 has a relatively stable coating layer on the surface of the F-doped carbon-coated graphite negative electrode, and the capacity and cycle stability are enhanced. After 150 cycles, the charge capacity of the negative electrode material of Comparative Example 2 dropped from the initial 283.25 to 271.13 mAh / g, and the capacity retention rate was 95.72%. When the negative electrode material prepared in Example 1 simultaneously introduced F and N elements, it was possible to effectively increase the active sites and expand the interlayer spacing, thereby improving the lithium storage capacity of graphite. Its charge capacity dropped from the initial 310.98 to 301.43 mAh / g, and the capacity retention rate was 96.92%. Therefore, the carbon-coated graphite negative electrode material obtained by the method provided by the present invention exhibits higher specific capacity and better cycle stability.

[0113] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A method for preparing a carbon-coated graphite negative electrode material, characterized in that: The method includes: (1) ball milling a nitrogen source, a fluorine source, and graphite to obtain a mixture; (2) in the presence of an inert gas, sequentially subjecting the mixture to a first heat treatment and a second heat treatment to obtain the carbon-coated graphite negative electrode material; The ball milling conditions are controlled so that the average particle diameter of the mixture is 10-20 nm; The conditions of the first heat treatment include: a temperature of 300-500°C; The conditions of the second heat treatment include: a temperature of 600-800°C; The temperature of the second heat treatment is at least 200° C. higher than the temperature of the first heat treatment.

2. The method according to claim 1, characterized in that In step (1), the mass ratio of the nitrogen source, the fluorine source and the graphite is 1:1-2:5-8.

3. The method according to claim 1 or 2, characterized in that In step (1), the nitrogen source is melamine; the fluorine source is polyvinylidene fluoride; And / or, in step (1), the D50 of the graphite is 7-15 μm.

4. The method according to claim 1 or 2, characterized in that In step (1), the ratio of the sum of the mass of the nitrogen source, the fluorine source and the graphite to the mass of the grinding balls is controlled to be 1:1.5-2.

5.

5. The method according to claim 1 or 2, characterized in that In step (1), the diameter of each grinding ball is independently 4-10 mm; And / or, in step (1), the ball milling is carried out in a ball milling jar, and the ball milling conditions include: a rotation speed of 200-600 rpm and a time of 4-8 hours.

6. The method according to claim 1 or 2, characterized in that The method further comprises: in step (1), grinding, screening and drying the material after ball milling in sequence to obtain the mixed material.

7. The method according to claim 1 or 2, characterized in that In step (2), the conditions of the first heat treatment also include: a heating rate V1 of 1-2°C / min, and a holding time T1 of 1.5-2.5h.

8. The method according to claim 1 or 2, characterized in that In step (2), the conditions of the second heat treatment also include: a heating rate V2 of 1-2°C / min, and a holding time T2 of 1-3h.

9. A carbon-coated graphite negative electrode material prepared by the method according to any one of claims 1 to 8.

10. Use of the carbon-coated graphite negative electrode material according to claim 9 in lithium-ion batteries.

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