Graphite composite material, preparation method thereof, and secondary battery

Abstract

The present invention provides a graphite composite material, a preparation method thereof, and a secondary battery, relating to the technical field of negative electrode materials for lithium-ion batteries. The graphite composite material comprises a core and a shell layer, wherein the core comprises a metal element-doped metal oxide and graphite, wherein the metal element-doped metal oxide is embedded between the graphite sheets, and the shell layer comprises multi-walled carbon nanotubes and amorphous carbon, wherein the multi-walled carbon nanotubes are distributed in the amorphous carbon in a 3D network. The present invention increases the lithium insertion active sites of the metal oxide by introducing doping elements, thereby improving the lithium storage performance of the graphite composite material; and by constructing a core-shell and intercalation structure, the volume expansion of the metal oxide is suppressed, thereby improving the cyclic stability of the graphite composite material.

Description

Technical Field

[0001] The present invention provides a graphite composite material, a preparation method thereof and a secondary battery, and relates to the technical field of new material preparation. Background Art

[0002] Currently, graphite is widely used as a negative electrode material in lithium-ion batteries due to its stable chemical properties, high conductivity, and moderate specific capacity. However, with market development, the sole use of graphite as a negative electrode material has gradually failed to meet diverse demands. Therefore, the industry has begun to seek new negative electrode materials to improve the overall performance of lithium-ion batteries.

[0003] Among the emerging negative electrode materials, silicon-based materials, tin-based materials and metal compounds have attracted widespread attention due to their higher theoretical specific capacity. However, these new materials are also accompanied by significant disadvantages. For example, large volume effects and low electrical conductivity seriously affect the cycle and rate performance of the materials. In order to overcome these shortcomings, the mainstream method is to mix a small amount of new materials into graphite, aiming to obtain the advantages of both at the same time, thereby seeking a balance between the material in terms of capacity, cycle and rate performance. However, simple mechanical mixing has many shortcomings, such as poor dispersion between materials, loose interface bonding and lack of synergistic effects. This seriously affects battery consistency, ion transfer efficiency and cycle stability.

[0004] In response to the above problems, the industry urgently needs to develop more sophisticated and synergistic composite strategies. For example, through micro-nano-sized composite technology, the combination of graphite and new materials can be optimized, which can not only increase the contact area between materials and improve the transmission efficiency of lithium ions and electrons, but also suppress the negative effects of high-expansion materials by strengthening the interface. In addition, the development of surface modification technology is also an effective way to solve the stability problem of composite materials. By coating the surface of the composite material with highly conductive and high-tensile strength materials, the conductivity and structural integrity of the composite material can be effectively enhanced. In short, although simple mechanical mixing provides a certain solution to the improvement of battery performance, in order to give full play to the advantages of various negative electrode materials, it is also necessary to carry out systematic optimization and innovation in many aspects such as material size, structure and coating engineering, so as to meet the current battery technology's comprehensive requirements for high energy density, fast charging and long life. Summary of the Invention

[0005] To solve the above problems, the present invention prepares a graphite composite material by means of doping, intercalation and surface modification. When used as a negative electrode material in the preparation of secondary batteries, the graphite composite material exhibits high capacity and excellent cycle performance. The specific scheme is as follows:

[0006] First, the present invention provides a graphite composite material, including a core and a shell, the core including metal element doped metal oxide and graphite, the metal element doped metal oxide is embedded between graphite sheets, and the shell includes multi-walled carbon nanotubes and amorphous carbon.

[0007] Preferably, the graphite composite material comprises at least one of the following features (1) to (9):

[0008] (1) The mass ratio of graphite to metal oxide is 100:(10-100);

[0009] (2) The mass ratio of graphite to amorphous carbon is 100:(1-20);

[0010] (3) the mass ratio of amorphous carbon to multi-walled carbon nanotubes is 100:(1-15);

[0011] (4) The particle size of graphite is 3-30 μm;

[0012] (5) Shell thickness is 100 nm-2 μm;

[0013] (6) The graphite is artificial graphite;

[0014] (7) The metal doping element is one or more of manganese, cobalt, and nickel; the doping amount of the metal element is 1% to 15%;

[0015] (8) The metal oxide is one or more of the oxides of titanium, vanadium, chromium, zirconium, niobium, and molybdenum;

[0016] (9) Multi-walled carbon nanotubes have an outer diameter of less than 200 nm and a length of less than 10 μm.

[0017] Preferably, the graphite composite material comprises at least one of the following features (1) to (4):

[0018] (1) The initial reversible capacity of the graphite composite material is greater than 410 mAh / g;

[0019] (2) The first coulombic efficiency of the graphite composite material is greater than 83%;

[0020] (3) The 50-week capacity retention rate of the graphite composite material is greater than 96%;

[0021] (4) The expansion rate of the graphite composite material is less than 35%.

[0022] Preferably, in the shell layer of the graphite composite material, the multi-walled carbon nanotubes are distributed in the amorphous carbon in a 3D network, and the amorphous carbon is uniformly coated on the surface of the core.

[0023] Second, the present invention provides a method for preparing the above-mentioned graphite composite material, comprising the following steps:

[0024] S1. Intercalant pretreatment: preparing bromide / metal oxide mixed fine powder and coke aggregate;

[0025] S2. Preparation of intercalated graphite precursor: mixing bromide / metal oxide mixed fine powder with coke aggregate to obtain intercalated graphite precursor;

[0026] S3. Preparation of intercalated graphite: placing the intercalated graphite precursor into a crucible, performing a programmed heat treatment, and obtaining metal-doped metal oxide intercalated graphite after natural cooling;

[0027] S4, pretreatment of multi-walled carbon nanotubes: immersing the multi-walled carbon nanotubes in a nitric acid / concentrated sulfuric acid mixed solution, heating and stirring in a water bath, and first separating to obtain functionalized multi-walled carbon nanotubes;

[0028] S5. Preparation of coating layer precursor: dissolving pretreated multi-walled carbon nanotubes and polyvinyl pyrrolidone in N,N-dimethylformamide solution and ultrasonically mixing to obtain solution A; dissolving dopamine hydrochloride and tris(hydroxymethyl)aminomethane in deionized water and adjusting the pH to 8.0-9.0 to obtain brown-yellow solution B; rapidly adding solution B to solution A and stirring for the first time to obtain solution C; adding metal-doped metal oxide intercalated graphite to solution C and continuing stirring for the second time; and performing the second separation to obtain the coating layer precursor;

[0029] S6. Preparation of multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite: placing the product obtained in step S5 in a tube furnace, under argon protection, and naturally cooling to room temperature after heat treatment to obtain a multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite composite material.

[0030] Preferably, in S1, coke aggregate is prepared by crushing coke into aggregate with D50=1-25 μm by roller mill; bromide / metal oxide mixed fine powder is prepared by mixing metal oxide and bromide at a ratio of 100:

[0031] The mixture is placed in a ball mill with a mass ratio of (5-50) and ball milled at 100-400 rpm for 1-20 h to obtain the product.

[0032] Preferably, in S2, the mass ratio of coke aggregate to bromide / metal oxide mixed fine powder is 100:(10-125), and the batch mixing is carried out using a batch mixing machine for 2-3 hours.

[0033] Preferably, in S3, the programmed heat treatment includes: heating to 200°C in a medium frequency furnace at 5-8°C / min, keeping warm for 2-2.5 hours, then continuing to heat to 2000-3200°C at 8-15°C / min, and keeping warm for 1-1.5 hours.

[0034] Preferably, in S4, in the nitric acid / concentrated sulfuric acid mixed solution, the volume ratio of nitric acid / concentrated sulfuric acid is (2.5-3):1; the water bath temperature is 80-85°C, the stirring speed is 100-400 rpm, and the treatment time is 2-5h.

[0035] Preferably, the first separation in S4 refers to collecting the solid by suction filtration, washing the obtained solid with deionized water 3-5 times, and then drying it at 80° C. for 12 h.

[0036] Preferably, in S5, the ultrasonic mixing time is 0.5-2h, the first stirring is carried out at room temperature, the stirring speed is 300-800rpm, and the time is 0.5-2h; the second stirring is carried out at room temperature, the stirring speed is 50-200rpm, and the time is 12-48h.

[0037] Preferably, the second separation in S5 refers to collecting the solid by suction filtration, washing it with deionized water 3-5 times to a pH of 6.5-7.5, and drying it at 80-85° C. for 12 h.

[0038] Preferably, in S6, the heat treatment refers to heating the temperature to 300-500°C at a heating rate of 5°C / min and keeping the temperature for 3-3.5h.

[0039] Third, the present invention provides a secondary battery comprising a positive electrode material, a negative electrode material and an electrolyte, wherein the negative electrode material comprises the aforementioned graphite composite material or the graphite composite material prepared by the aforementioned graphite composite material preparation method.

[0040] Beneficial effects of the present invention:

[0041] The present invention provides a graphite composite material, a preparation method thereof, and a secondary battery. The composite material has a three-layer structure, the innermost layer of which is a metal oxide doped with a metal element, which is embedded between graphite sheets to form a special metal oxide intercalated graphite structure. The outermost layer is a multi-walled carbon nanotube / amorphous carbon layer, which is uniformly coated on the graphite surface to form a stable core-shell structure. The composite material prepared by the present invention combines the advantages of high capacity of metal oxides and high cycle stability of graphite. By introducing doping elements, the lithium insertion active sites of the metal oxide are increased, thereby improving the lithium storage performance of the graphite composite material. By constructing the core-shell and intercalated structures, the volume expansion of the metal oxide is suppressed, thereby improving the cycle stability of the graphite composite material.

[0042] Specifically:

[0043] 1) The present invention utilizes a high-temperature graphitization process to prepare a metal oxide / graphite composite material. During the heating phase, the water of crystallization in the coke / metal oxide / bromide mixture is removed. During the graphitization and heat preservation phase, the coke is graphitized, the metal oxide is transformed into a molten state, and the bromide decomposes to produce bromine gas and a metal compound. During the cooling phase, the metal compound fuses with the metal oxide, thereby transforming it into a metal-doped metal oxide. Simultaneously, bromine gas is dynamically intercalated and deintercalated within the graphitized coke, effectively expanding the interlayer spacing between graphite layers and assisting the intercalation of the molten metal-doped metal oxide. The entire process is completed in a single step, fully utilizing the energy consumption of the graphitization process, and has the advantages of low cost and favorable industrial production.

[0044] 2) The present invention uses polyvinyl pyrrolidone as a dispersant, and uniformly mixes the pretreated multi-walled carbon nanotubes, dopamine hydrochloride and intercalated graphite by ultrasound and stirring. Under appropriate pH conditions, dopamine undergoes a polymerization reaction, and the polydopamine wrapped around the multi-walled carbon nanotubes is uniformly coated on the surface of the intercalated graphite. Subsequently, a carbonization process is performed to prepare multi-walled carbon nanotubes / carbon-coated metal-doped metal oxide intercalated graphite. The multi-walled carbon nanotubes / carbon coating prepared by the present invention has the advantages of good coating uniformity and high tensile strength. The multi-walled carbon nanotubes and amorphous carbon form a stable structure similar to the steel bar / cement structure. The multi-walled carbon nanotubes are embedded in the amorphous carbon, providing the amorphous carbon layer with a good three-dimensional conductive network and mechanical properties, thereby improving the electrical conductivity of the material while alleviating the volume expansion of the intercalated graphite during the circulation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 Schematic diagram of the structure of the graphite composite material of the present invention;

[0046] Figure 2 This is the SEM image of the graphite composite material prepared in Example 1. DETAILED DESCRIPTION

[0047] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0048] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as those familiar to those skilled in the art. Furthermore, any methods and materials similar or equivalent to those described herein can be applied to the present invention. The preferred embodiments and materials described herein are for illustrative purposes only and are not intended to limit the scope of this application.

[0049] The experimental methods in the following examples, unless otherwise specified, are all conventional methods, and the experimental materials used in the following examples, unless otherwise specified, are all purchased from commercial channels.

[0050] Example 1 Multi-walled carbon nanotubes / carbon-coated cobalt-doped chromium oxide intercalated graphite (intermediate frequency furnace 2500°C)

[0051] (1) Raw material pretreatment: Use a roller mill to crush the petroleum coke into aggregate with a D50 of 8-9 μm. Place 30 g of CoBr2 and 150 g of Cr2O3 in a ball mill and mill at 300 rpm for 10 h to obtain a uniform CoBr2 / Cr2O3 mixed fine powder.

[0052] (2) Preparation of intercalated graphite precursor: The CoBr2 / Cr2O3 mixed fine powder obtained in step (1) and 400 g of needle coke aggregate were placed in a batch mixer and mixed for 2 h to obtain an intercalated graphite precursor;

[0053] (3) Preparation of intercalated graphite: The intercalated graphite precursor obtained in step (2) was placed in a crucible, heated to 200°C in a medium frequency furnace at 5°C / min, kept warm for 2 h, then heated to 2500°C at 10°C / min, kept warm for 1 h, and cooled naturally to obtain Co-doped Cr2O3 intercalated graphite;

[0054] (4) Pretreatment of multi-walled carbon nanotubes: 300 mg of multi-walled carbon nanotubes were immersed in 50 mL of a nitric acid / concentrated sulfuric acid (3:1) mixed solution at 80°C in a water bath and stirred at 200 rpm for 3 h. The resulting mixture was then collected by filtration, washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain functionalized multi-walled carbon nanotubes.

[0055] (5) Preparation of coating layer precursor: 1 mg of pretreated multi-walled carbon nanotubes and 6 mg of polyvinyl pyrrolidone were dissolved in 10 mL of N,N-dimethylformamide solution and ultrasonicated for 1 h. Solution A was obtained. 40 mg of dopamine hydrochloride and 60 mg of tris(hydroxymethyl)aminomethane were dissolved in 40 mL of deionized water, and the pH of the solution was adjusted to 8.5 using sodium hydroxide solution to obtain a brown-yellow solution B. Solution A was then quickly added to solution B, and stirred at 500 rpm for 1 h at room temperature to obtain solution C. 40 mg of intercalated graphite prepared in step (3) was then added to solution C, and stirred at 100 rpm for 24 h, then washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain a coating layer precursor;

[0056] (6) Preparation of multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite: The product obtained in step (5) was placed in a tube furnace, and under argon protection, the temperature was increased to 400°C at a heating rate of 5°C / min, kept at this temperature for 3 hours, and naturally cooled to room temperature to obtain multi-walled carbon nanotube / carbon-coated Co-doped Cr2O3 intercalated graphite.

[0057] Example 2 Multi-walled carbon nanotubes / carbon-coated cobalt-doped chromium oxide intercalated graphite (intermediate frequency furnace 2300°C)

[0058] (1) Raw material pretreatment: Use a roller mill to crush the petroleum coke into aggregate with a D50 of 8-9 μm. Place 30 g of CoBr2 and 150 g of Cr2O3 in a ball mill and mill at 300 rpm for 10 h to obtain a uniform CoBr2 / Cr2O3 mixed fine powder.

[0059] (2) Preparation of intercalated graphite precursor: The CoBr2 / Cr2O3 mixed fine powder obtained in step (1) and 400 g of petroleum coke aggregate were placed in a batch mixer and mixed for 2 h to obtain an intercalated graphite precursor;

[0060] (3) Preparation of intercalated graphite: The intercalated graphite precursor obtained in step (2) was placed in a crucible, heated to 200°C in a medium frequency furnace at 5°C / min, kept warm for 2 h, then heated to 2300°C at 10°C / min, kept warm for 1 h, and cooled naturally to obtain Co-doped Cr2O3 intercalated graphite;

[0061] (4) Pretreatment of multi-walled carbon nanotubes: 300 mg of multi-walled carbon nanotubes were immersed in 50 mL of a nitric acid / concentrated sulfuric acid (3:1) mixed solution at 80°C in a water bath and stirred at 200 rpm for 3 h. The resulting mixture was then collected by filtration, washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain functionalized multi-walled carbon nanotubes.

[0062] (5) Preparation of coating layer precursor: 1 mg of pretreated multi-walled carbon nanotubes and 6 mg of polyvinyl pyrrolidone were dissolved in 10 mL of N,N-dimethylformamide solution and ultrasonicated for 1 h. Solution A was obtained. 40 mg of dopamine hydrochloride and 60 mg of tris(hydroxymethyl)aminomethane were dissolved in 40 mL of deionized water, and the pH of the solution was adjusted to 8.5 using sodium hydroxide solution to obtain a brown-yellow solution B. Solution A was then quickly added to solution B, and stirred at 500 rpm for 1 h at room temperature to obtain solution C. 40 mg of intercalated graphite prepared in step (3) was then added to solution C, and stirred at 100 rpm for 24 h, then washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain a coating layer precursor;

[0063] (6) Preparation of multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite: The product obtained in step (5) was placed in a tube furnace, and under argon protection, the temperature was increased to 400°C at a heating rate of 5°C / min, kept at this temperature for 3 hours, and naturally cooled to room temperature to obtain multi-walled carbon nanotube / carbon-coated Co-doped Cr2O3 intercalated graphite.

[0064] Example 3 Multi-walled carbon nanotubes / carbon-coated cobalt-doped chromium oxide intercalated graphite (intermediate frequency furnace 2700°C)

[0065] (1) Raw material pretreatment: Use a roller mill to crush the petroleum coke into aggregate with a D50 of 8-9 μm. Place 30 g of CoBr2 and 150 g of Cr2O3 in a ball mill and mill at 300 rpm for 10 h to obtain a uniform CoBr2 / Cr2O3 mixed fine powder.

[0066] (2) Preparation of intercalated graphite precursor: The CoBr2 / Cr2O3 mixed fine powder obtained in step (1) and 400 g of petroleum coke aggregate were placed in a batch mixer and mixed for 2 h to obtain an intercalated graphite precursor;

[0067] (3) Preparation of intercalated graphite: The intercalated graphite precursor obtained in step (2) was placed in a crucible, heated to 200°C in a medium frequency furnace at 5°C / min, kept warm for 2 h, then heated to 2700°C at 10°C / min, kept warm for 1 h, and cooled naturally to obtain Co-doped Cr2O3 intercalated graphite;

[0068] (4) Pretreatment of multi-walled carbon nanotubes: 300 mg of multi-walled carbon nanotubes were immersed in 50 mL of a nitric acid / concentrated sulfuric acid (3:1) mixed solution at 80°C in a water bath and stirred at 200 rpm for 3 h. The resulting mixture was then collected by filtration, washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain functionalized multi-walled carbon nanotubes.

[0069] (5) Preparation of coating layer precursor: 1 mg of pretreated multi-walled carbon nanotubes and 6 mg of polyvinyl pyrrolidone were dissolved in 10 mL of N,N-dimethylformamide solution and ultrasonicated for 1 h. Solution A was obtained. 40 mg of dopamine hydrochloride and 60 mg of tris(hydroxymethyl)aminomethane were dissolved in 40 mL of deionized water, and the pH of the solution was adjusted to 8.5 using sodium hydroxide solution to obtain a brown-yellow solution B. Solution A was then quickly added to solution B, and stirred at 500 rpm for 1 h at room temperature to obtain solution C. 40 mg of intercalated graphite prepared in step (3) was then added to solution C, and stirred at 100 rpm for 24 h, then washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain a coating layer precursor;

[0070] (6) Preparation of multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite: The product obtained in step (5) was placed in a tube furnace, and under argon protection, the temperature was increased to 400°C at a heating rate of 5°C / min, kept at this temperature for 3 hours, and naturally cooled to room temperature to obtain multi-walled carbon nanotube / carbon-coated Co-doped Cr2O3 intercalated graphite.

[0071] Example 4 Multi-walled carbon nanotubes / carbon-coated cobalt-doped chromium oxide intercalated graphite (mass ratio of CoBr2 / Cr2O3 mixture to petroleum coke is 9:40)

[0072] (1) Raw material pretreatment: Use a roller mill to crush the petroleum coke into aggregate with a D50 of 8-9 μm. Place 15 g of CoBr2 and 75 g of Cr2O3 in a ball mill and mill at 300 rpm for 10 h to obtain a uniform CoBr2 / Cr2O3 mixed fine powder.

[0073] (2) Preparation of intercalated graphite precursor: The CoBr2 / Cr2O3 mixed fine powder obtained in step (1) and 400 g of petroleum coke aggregate were placed in a batch mixer and mixed for 2 h to obtain an intercalated graphite precursor;

[0074] (3) Preparation of intercalated graphite: The intercalated graphite precursor obtained in step (2) was placed in a crucible, heated to 200°C in a medium frequency furnace at 5°C / min, kept warm for 2 h, then heated to 2500°C at 10°C / min, kept warm for 1 h, and cooled naturally to obtain Co-doped Cr2O3 intercalated graphite;

[0075] (4) Pretreatment of multi-walled carbon nanotubes: 300 mg of multi-walled carbon nanotubes were immersed in 50 mL of a nitric acid / concentrated sulfuric acid (3:1) mixed solution at 80°C in a water bath and stirred at 200 rpm for 3 h. The resulting mixture was then collected by filtration, washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain functionalized multi-walled carbon nanotubes.

[0076] (5) Preparation of coating layer precursor: 1 mg of pretreated multi-walled carbon nanotubes and 6 mg of polyvinyl pyrrolidone were dissolved in 10 mL of N,N-dimethylformamide solution and ultrasonicated for 1 h. Solution A was obtained. 40 mg of dopamine hydrochloride and 60 mg of tris(hydroxymethyl)aminomethane were dissolved in 40 mL of deionized water, and the pH of the solution was adjusted to 8.5 using sodium hydroxide solution to obtain a brown-yellow solution B. Solution A was then quickly added to solution B, and stirred at 500 rpm for 1 h at room temperature to obtain solution C. 40 mg of intercalated graphite prepared in step (3) was then added to solution C, and stirred at 100 rpm for 24 h, then washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain a coating layer precursor;

[0077] (6) Preparation of multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite: The product obtained in step (5) was placed in a tube furnace, and under argon protection, the temperature was increased to 400°C at a heating rate of 5°C / min, kept at this temperature for 3 hours, and naturally cooled to room temperature to obtain multi-walled carbon nanotube / carbon-coated Co-doped Cr2O3 intercalated graphite.

[0078] Example 5 Multi-walled carbon nanotubes / carbon-coated cobalt-doped chromium oxide intercalated graphite (mass ratio of CoBr2 / Cr2O3 mixture to petroleum coke is 9:10)

[0079] (1) Raw material pretreatment: Use a roller mill to crush the petroleum coke into aggregate with a D50 of 8-9 μm. Place 60 g of CoBr2 and 300 g of Cr2O3 in a ball mill and mill at 300 rpm for 10 h to obtain a uniform CoBr2 / Cr2O3 mixed fine powder.

[0080] (2) Preparation of intercalated graphite precursor: The CoBr2 / Cr2O3 mixed fine powder obtained in step (1) and 400 g of petroleum coke aggregate were placed in a batch mixer and mixed for 2 h to obtain an intercalated graphite precursor;

[0081] (3) Preparation of intercalated graphite: The intercalated graphite precursor obtained in step (2) was placed in a crucible, heated to 200°C in a medium frequency furnace at 5°C / min, kept warm for 2 h, then heated to 2500°C at 10°C / min, kept warm for 1 h, and cooled naturally to obtain Co-doped Cr2O3 intercalated graphite;

[0082] (4) Pretreatment of multi-walled carbon nanotubes: 300 mg of multi-walled carbon nanotubes were immersed in 50 mL of a nitric acid / concentrated sulfuric acid (3:1) mixed solution at 80°C in a water bath and stirred at 200 rpm for 3 h. The resulting mixture was then collected by filtration, washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain functionalized multi-walled carbon nanotubes.

[0083] (5) Preparation of coating layer precursor: 1 mg of pretreated multi-walled carbon nanotubes and 6 mg of polyvinyl pyrrolidone were dissolved in 10 mL of N,N-dimethylformamide solution and ultrasonicated for 1 h. Solution A was obtained. 40 mg of dopamine hydrochloride and 60 mg of tris(hydroxymethyl)aminomethane were dissolved in 40 mL of deionized water, and the pH of the solution was adjusted to 8.5 using sodium hydroxide solution to obtain a brown-yellow solution B. Solution A was then quickly added to solution B, and stirred at 500 rpm for 1 h at room temperature to obtain solution C. 40 mg of intercalated graphite prepared in step (3) was then added to solution C, and stirred at 100 rpm for 24 h, then washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain a coating layer precursor;

[0084] (6) Preparation of multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite: The product obtained in step (5) was placed in a tube furnace, and under argon protection, the temperature was increased to 400°C at a heating rate of 5°C / min, kept at this temperature for 3 hours, and naturally cooled to room temperature to obtain multi-walled carbon nanotube / carbon-coated Co-doped Cr2O3 intercalated graphite.

[0085] Example 6 Multi-walled carbon nanotubes / carbon-coated cobalt-doped chromium oxide intercalated graphite (mass ratio of multi-walled carbon nanotubes to dopamine hydrochloride 1:20)

[0086] (1) Raw material pretreatment: Use a roller mill to crush the petroleum coke into aggregate with a D50 of 8-9 μm. Place 30 g of CoBr2 and 150 g of Cr2O3 in a ball mill and mill at 300 rpm for 10 h to obtain a uniform CoBr2 / Cr2O3 mixed fine powder.

[0087] (2) Preparation of intercalated graphite precursor: The CoBr2 / Cr2O3 mixed fine powder obtained in step (1) and 400 g of petroleum coke aggregate were placed in a batch mixer and mixed for 2 h to obtain an intercalated graphite precursor;

[0088] (3) Preparation of intercalated graphite: The intercalated graphite precursor obtained in step (2) was placed in a crucible, heated to 200°C in a medium frequency furnace at 5°C / min, kept warm for 2 h, then heated to 2500°C at 10°C / min, kept warm for 1 h, and cooled naturally to obtain Co-doped Cr2O3 intercalated graphite;

[0089] (4) Pretreatment of multi-walled carbon nanotubes: 300 mg of multi-walled carbon nanotubes were immersed in 50 mL of a nitric acid / concentrated sulfuric acid (3:1) mixed solution at 80°C in a water bath and stirred at 200 rpm for 3 h. The resulting mixture was then collected by filtration, washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain functionalized multi-walled carbon nanotubes.

[0090] (5) Preparation of coating layer precursor: 1 mg of pretreated multi-walled carbon nanotubes and 6 mg of polyvinyl pyrrolidone were dissolved in 10 mL of N,N-dimethylformamide solution and ultrasonicated for 1 h. Solution A was obtained. 20 mg of dopamine hydrochloride and 30 mg of tris(hydroxymethyl)aminomethane were dissolved in 20 mL of deionized water, and the pH of the solution was adjusted to 8.5 using sodium hydroxide solution to obtain a brown-yellow solution B. Solution A was then quickly added to solution B, and stirred at 500 rpm for 1 h at room temperature to obtain solution C. 20 mg of the intercalated graphite prepared in step (3) was then added to solution C, and stirring was continued at 100 rpm for 24 h. The mixture was then washed with deionized water for 3-5 times and dried at 80°C for 12 h to obtain a coating layer precursor.

[0091] (6) Preparation of multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite: The product obtained in step (5) was placed in a tube furnace, and under argon protection, the temperature was increased to 400°C at a heating rate of 5°C / min, kept at this temperature for 3 hours, and naturally cooled to room temperature to obtain multi-walled carbon nanotube / carbon-coated Co-doped Cr2O3 intercalated graphite.

[0092] Example 7 Multi-walled carbon nanotubes / carbon-coated cobalt-doped chromium oxide intercalated graphite (mass ratio of multi-walled carbon nanotubes to dopamine hydrochloride is 1:60)

[0093] (1) Raw material pretreatment: Use a roller mill to crush the petroleum coke into aggregate with a D50 of 8-9 μm. Place 30 g of CoBr2 and 150 g of Cr2O3 in a ball mill and mill at 300 rpm for 10 h to obtain a uniform CoBr2 / Cr2O3 mixed fine powder.

[0094] (2) Preparation of intercalated graphite precursor: The CoBr2 / Cr2O3 mixed fine powder obtained in step (1) and 400 g of petroleum coke aggregate were placed in a batch mixer and mixed for 2 h to obtain an intercalated graphite precursor;

[0095] (3) Preparation of intercalated graphite: The intercalated graphite precursor obtained in step (2) was placed in a crucible, heated to 200°C in a medium frequency furnace at 5°C / min, kept warm for 2 h, then heated to 2500°C at 10°C / min, kept warm for 1 h, and cooled naturally to obtain Co-doped Cr2O3 intercalated graphite;

[0096] (4) Pretreatment of multi-walled carbon nanotubes: 300 mg of multi-walled carbon nanotubes were immersed in 50 mL of a nitric acid / concentrated sulfuric acid (3:1) mixed solution at 80°C in a water bath and stirred at 200 rpm for 3 h. The resulting mixture was then collected by filtration, washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain functionalized multi-walled carbon nanotubes.

[0097] (5) Preparation of coating layer precursor: 1 mg of pretreated multi-walled carbon nanotubes and 6 mg of polyvinyl pyrrolidone were dissolved in 10 mL of N,N-dimethylformamide solution and ultrasonicated for 1 h. Solution A was obtained. 60 mg of dopamine hydrochloride and 90 mg of tris(hydroxymethyl)aminomethane were dissolved in 60 mL of deionized water, and the pH of the solution was adjusted to 8.5 using sodium hydroxide solution to obtain a brown-yellow solution B. Solution A was then quickly added to solution B, and stirred at 500 rpm for 1 h at room temperature to obtain solution C. 60 mg of the intercalated graphite prepared in step (3) was then added to solution C, and continued stirring at 100 rpm for 24 h. The mixture was then washed with deionized water for 3-5 times and dried at 80°C for 12 h to obtain a coating layer precursor.

[0098] (6) Preparation of multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite: The product obtained in step (5) was placed in a tube furnace, and under argon protection, the temperature was increased to 400°C at a heating rate of 5°C / min, kept at this temperature for 3 hours, and naturally cooled to room temperature to obtain multi-walled carbon nanotube / carbon-coated Co-doped Cr2O3 intercalated graphite.

[0099] Comparative Example 1 Multi-walled carbon nanotubes / carbon-coated cobalt-doped chromium oxide / graphite mixture (no intercalation structure)

[0100] (1) Raw material pretreatment: Use a roller mill to crush the petroleum coke into aggregate with a D50 of 8-9 μm. Place 30 g of CoBr2 and 150 g of Cr2O3 in a ball mill and mill at 300 rpm for 10 h to obtain a uniform CoBr2 / Cr2O3 mixed fine powder.

[0101] (2) Preparation of cobalt-doped chromium trioxide: The CoBr2 / Cr2O3 mixed fine powder obtained in step (1) was placed in a crucible, heated to 200°C in a medium frequency furnace at 5°C / min, kept warm for 2 hours, then heated to 2500°C at 10°C / min, kept warm for 1 hour, and cooled naturally to obtain cobalt-doped chromium trioxide;

[0102] (3) Preparation of graphite: The petroleum coke aggregate obtained in step (1) was placed in a crucible, heated to 200°C in a medium frequency furnace at 5°C / min, kept warm for 2 h, then heated to 2500°C at 10°C / min, kept warm for 1 h, and cooled naturally to obtain artificial graphite;

[0103] (4) Preparation of cobalt-doped chromium trioxide / graphite: The cobalt-doped chromium trioxide obtained in step (2) and the graphite obtained in step (3) were placed in a batch mixer at a mass ratio of 9:20 and mixed for 2 h to obtain a cobalt-doped chromium trioxide / graphite mixture;

[0104] (5) Pretreatment of multi-walled carbon nanotubes: 300 mg of multi-walled carbon nanotubes were immersed in 50 mL of a nitric acid / concentrated sulfuric acid (3:1) mixture solution at 80°C in a water bath and stirred at 200 rpm for 3 h. The resulting mixture was then collected by filtration, washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain functionalized multi-walled carbon nanotubes.

[0105] (6) Preparation of coating layer precursor: 1 mg of pretreated multi-walled carbon nanotubes and 6 mg of polyvinyl pyrrolidone were dissolved in 10 mL of N,N-dimethylformamide solution and ultrasonicated for 1 h. Solution A was obtained. 40 mg of dopamine hydrochloride and 60 mg of tris(hydroxymethyl)aminomethane were dissolved in 40 mL of deionized water, and the pH of the solution was adjusted to 8.5 using sodium hydroxide solution to obtain a brown-yellow solution B. Solution A was then quickly added to solution B, and stirred at 500 rpm for 1 h at room temperature to obtain solution C. 40 mg of the product obtained in step (4) was then added to solution C, and stirring was continued at 100 rpm for 24 h. The product was then washed with deionized water for 3-5 times and dried at 80°C for 12 h to obtain a coating layer precursor.

[0106] (7) Preparation of multi-walled carbon nanotube / carbon-coated metal-doped metal oxide / graphite mixture: The product obtained in step (6) was placed in a tube furnace, and under argon protection, the temperature was increased to 400°C at a heating rate of 5°C / min, kept at this temperature for 3 h, and naturally cooled to room temperature to obtain a multi-walled carbon nanotube / carbon-coated Co-doped Cr2O3 / graphite mixture.

[0107] Since the cobalt-doped chromium oxide and the artificial graphite were only mechanically mixed in step (4), no intercalation reaction occurred in this comparative example.

[0108] Comparative Example 2 Cobalt-doped chromium oxide intercalated graphite (without coating layer)

[0109] (1) Raw material pretreatment: Use a roller mill to crush the petroleum coke into aggregate with a D50 of 8-9 μm. Place 30 g of CoBr2 and 150 g of Cr2O3 in a ball mill and mill at 300 rpm for 10 h to obtain a uniform CoBr2 / Cr2O3 mixed fine powder.

[0110] (2) Preparation of intercalated graphite precursor: The CoBr2 / Cr2O3 mixed fine powder obtained in step (1) and 400 g of petroleum coke aggregate were placed in a batch mixer and mixed for 2 h to obtain an intercalated graphite precursor;

[0111] (3) Preparation of intercalated graphite: The intercalated graphite precursor obtained in step (2) was placed in a crucible, heated to 200°C at 5°C / min in a medium frequency furnace, kept warm for 2 h, then heated to 2500°C at 10°C / min, kept warm for 1 h, and cooled naturally to obtain Co-doped Cr2O3 intercalated graphite.

[0112] Comparative Example 3 Multi-walled carbon nanotubes / carbon-coated chromium oxide intercalated graphite (no element doping)

[0113] (1) Raw material pretreatment: Use a roller mill to crush the petroleum coke into aggregate with a D50 of 8-9 μm. Place 180 g of Cr2O3 in a ball mill and mill at 300 rpm for 10 h to obtain Cr2O3 fine powder.

[0114] (2) Preparation of intercalated graphite precursor: The Cr2O3 fine powder obtained in step (1) and 400 g of petroleum coke aggregate were placed in a batch mixer and mixed for 2 h to obtain an intercalated graphite precursor;

[0115] (3) Preparation of intercalated graphite: The intercalated graphite precursor obtained in step (2) was placed in a crucible, heated to 200°C in a medium frequency furnace at 5°C / min, kept warm for 2 h, then heated to 2500°C at 10°C / min, kept warm for 1 h, and cooled naturally to obtain Cr2O3 intercalated graphite;

[0116] (4) Pretreatment of multi-walled carbon nanotubes: 300 mg of multi-walled carbon nanotubes were immersed in 50 mL of a nitric acid / concentrated sulfuric acid (3:1) mixed solution at 80°C in a water bath and stirred at 200 rpm for 3 h. The resulting mixture was then collected by filtration, washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain functionalized multi-walled carbon nanotubes.

[0117] (5) Preparation of coating layer precursor: 1 mg of pretreated multi-walled carbon nanotubes and 6 mg of polyvinyl pyrrolidone were dissolved in 10 mL of N,N-dimethylformamide solution and ultrasonicated for 1 h. Solution A was obtained. 40 mg of dopamine hydrochloride and 60 mg of tris(hydroxymethyl)aminomethane were dissolved in 40 mL of deionized water, and the pH of the solution was adjusted to 8.5 using sodium hydroxide solution to obtain a brown-yellow solution B. Solution A was then quickly added to solution B, and stirred at 500 rpm for 1 h at room temperature to obtain solution C. 40 mg of intercalated graphite prepared in step (3) was then added to solution C, and stirred at 100 rpm for 24 h, then washed with deionized water 3-5 times, and dried at 80°C for 12 h to obtain a coating layer precursor;

[0118] (6) Preparation of multi-walled carbon nanotube / carbon-coated metal oxide intercalated graphite: The product obtained in step (5) was placed in a tube furnace, and under argon protection, the temperature was increased to 400°C at a heating rate of 5°C / min, kept at this temperature for 3 hours, and naturally cooled to room temperature to obtain multi-walled carbon nanotube / carbon-coated Cr2O3 intercalated graphite.

[0119] Electrochemical performance test:

[0120] 1. Pole piece preparation:

[0121] The final product obtained in the above embodiments and comparative examples was used as the active material, polyvinylidene fluoride was used as the binder, and carbon black Super-P was used as the conductive agent. The three were mixed in a mass ratio of 91:7:2, and an appropriate amount of N-methylpyrrolidone solvent was added to form a uniform slurry, which was then coated on copper foil. The obtained copper foil was placed in a 100°C oven and baked for 2 hours. After being taken out and stamped, an electrode sheet with a diameter of 14 mm was obtained. Finally, the obtained electrode sheet was baked in a vacuum box at 105°C for 4 hours, and then quickly placed in a glove box to wait for assembly into a button battery.

[0122] 2. Button battery assembly and testing:

[0123] The obtained electrode was used as the working electrode, the lithium sheet as the counter electrode, the polypropylene microporous membrane as the separator, and the electrolyte was 1MLiPF6+EC:DEC:DMC=1:1:1 (volume ratio). CR2430 button cells were assembled in an argon atmosphere glove box. The charge and discharge test of the button cell was carried out on the battery testing system of Wuhan Blue Electric Electronics Co., Ltd. Under room temperature conditions, 0.1C constant current charge and discharge were adopted, and the charge and discharge voltage was limited to 0.005V to 2.0V. Its first reversible specific capacity, first coulombic efficiency, 50-week capacity retention rate and electrode full charge expansion rate were tested. Among them, the 50-week cycle capacity retention rate = 50th week cycle discharge capacity / first week discharge capacity × 100%, the full charge expansion rate of the negative electrode = (thickness of the electrode after full charge - thickness of the electrode before uncharged) / thickness of the electrode before uncharged × 100%. The obtained data are shown in Table 1.

[0124] Table 1 Electrochemical performance test results of examples and comparative examples

[0125]

[0126] Table 1 shows the electrochemical performance test results of the samples obtained in the examples and comparative examples. It can be seen from Table 1 that the graphite composite materials prepared in Examples 1 to 7 have excellent performance in terms of capacity, first effect, capacity retention rate and full-charge expansion rate of the pole piece. In terms of the first reversible capacity, the first reversible capacity of Examples 1-7 is greater than 410mAh / g, far exceeding the first reversible capacity of the current mainstream artificial graphite negative electrode (340-360mAh / g), which is mainly due to the high capacity provided by the metal oxide embedded between the graphite layers. Among comparative examples 1-3, the first reversible capacity of comparative example 3 without element doping is the lowest (only 365.1mAh / g). This phenomenon shows that element doping has a positive effect on the improvement of the capacity of the metal oxide. The possible reason is that the lone pair electrons formed by element doping enhance the electronic conductivity of the metal oxide. In addition, the introduction of heterogeneous elements produces a large number of lattice defects inside the metal oxide, thereby increasing its lithium insertion active sites and significantly improving the lithium storage performance of the material. In terms of the first efficiency, the first efficiency of Examples 1-7 is higher than 83%, which is higher than that of Comparative Example 1 (72.8%) without intercalation structure. This can be attributed to the special intercalation structure of the embodiments, which significantly reduces the contact between the metal oxide and the electrolyte, thereby inhibiting the lithium loss caused by the side reaction between the metal oxide and the electrolyte. In terms of capacity retention, the capacity retention of Examples 1-7 is higher than 96% after 50 cycles at 1C, while that of Comparative Example 1 without intercalation structure is higher than 96%. The capacity retention rates of Comparative Example 1 with an intercalation structure and Comparative Example 2 without a coating layer are both lower than 76%, indicating that the limitations of the intercalation structure and the coating layer during the cycle effectively improve the capacity attenuation caused by the volume effect of the metal oxide or the leakage of graphite sheets; in terms of the full-charge expansion rate of the electrode, the expansion rates of Examples 1-7 are all lower than 35%, while the expansion rates of Comparative Example 1 without an intercalation structure and Comparative Example 2 without a coating layer are both higher than 50%. This result strongly demonstrates the inhibitory effect of the intercalation structure and the coating layer on the volume expansion of the composite material during the charge and discharge process, thereby effectively ensuring the cyclic stability of the material.

[0127] In summary, the multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite composite material prepared by the method provided by the present invention is a negative electrode material with high capacity and excellent cycle performance.

[0128] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

[0129] The above description of the present invention and its embodiments is non-limiting. The accompanying drawings illustrate only one embodiment of the present invention, and actual applications are not limited thereto. In short, if a person skilled in the art is inspired by the above and, without departing from the purpose of the present invention, devises methods and embodiments similar to the technical solution without inventive means, they shall fall within the scope of protection of the present invention.

Claims

1. A graphite composite material, characterized in that: The invention comprises a core and a shell, wherein the core comprises a metal element-doped metal oxide and graphite, the metal element-doped metal oxide is embedded between the graphite sheets, and the shell comprises multi-walled carbon nanotubes and amorphous carbon; The preparation method of the graphite composite material comprises the following steps: S1. Intercalant pretreatment: preparing bromide / metal oxide mixed fine powder and coke aggregate; S2. Preparation of intercalated graphite precursor: mixing bromide / metal oxide mixed fine powder with coke aggregate to obtain intercalated graphite precursor; S3. Preparation of intercalated graphite: placing the intercalated graphite precursor into a crucible, performing a programmed heat treatment, and obtaining metal-doped metal oxide intercalated graphite after natural cooling; S4, pretreatment of multi-walled carbon nanotubes: immersing the multi-walled carbon nanotubes in a nitric acid / concentrated sulfuric acid mixed solution, heating and stirring in a water bath, and first separating to obtain functionalized multi-walled carbon nanotubes; S5. Preparation of coating layer precursor: dissolving pretreated multi-walled carbon nanotubes and polyvinyl pyrrolidone in N,N-dimethylformamide solution and ultrasonically mixing to obtain solution A; dissolving dopamine hydrochloride and tris(hydroxymethyl)aminomethane in deionized water and adjusting the pH to 8.0-9.0 to obtain brown-yellow solution B; rapidly adding solution B to solution A and stirring for the first time to obtain solution C; adding metal-doped metal oxide intercalated graphite to solution C and continuing stirring for the second time; and performing the second separation to obtain the coating layer precursor; S6. Preparation of multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite: placing the product obtained in step S5 in a tube furnace, under argon protection, and naturally cooling to room temperature after heat treatment to obtain a multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite composite material.

2. The graphite composite material according to claim 1, characterized in that: Contains at least one of the following features (1) to (9): (1) the mass ratio of the graphite to the metal oxide is 100:(10-100); (2) the mass ratio of the graphite to the amorphous carbon is 100:(1-20); (3) the mass ratio of the amorphous carbon to the multi-walled carbon nanotubes is 100:(1-15); (4) the particle size of the graphite is 3-30 μm; (5) the shell layer has a thickness of 100 nm to 2 μm; (6) The graphite is artificial graphite; (7) The metal element doped metal oxide, wherein the doped metal element is one or more of manganese, cobalt, and nickel, and the doping amount of the metal element is 1% to 15%; (8) The metal oxide is one or more oxides of titanium, vanadium, chromium, zirconium, niobium, and molybdenum; (9) The multi-walled carbon nanotubes have an outer diameter of less than 200 nm and a length of less than 10 μm.

3. The graphite composite material according to claim 1, characterized in that: Contains at least one of the following features (1) to (4): (1) The graphite composite material has an initial reversible capacity greater than 410 mAh / g; (2) The first coulombic efficiency of the graphite composite material is greater than 83%; (3) The 50-week capacity retention rate of the graphite composite material is greater than 96%; (4) The expansion rate of the graphite composite material is less than 35%.

4. The graphite composite material according to claim 1, characterized in that: In the shell layer of the graphite composite material, multi-walled carbon nanotubes are distributed in amorphous carbon in a 3D network shape, and the amorphous carbon is evenly coated on the surface of the core.

5. A method for preparing the graphite composite material according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1. Intercalant pretreatment: preparing bromide / metal oxide mixed fine powder and coke aggregate; S2. Preparation of intercalated graphite precursor: mixing bromide / metal oxide mixed fine powder with coke aggregate to obtain intercalated graphite precursor; S3. Preparation of intercalated graphite: placing the intercalated graphite precursor into a crucible, performing a programmed heat treatment, and obtaining metal-doped metal oxide intercalated graphite after natural cooling; S4, pretreatment of multi-walled carbon nanotubes: immersing the multi-walled carbon nanotubes in a nitric acid / concentrated sulfuric acid mixed solution, heating and stirring in a water bath, and first separating to obtain functionalized multi-walled carbon nanotubes; S5. Preparation of coating layer precursor: dissolving pretreated multi-walled carbon nanotubes and polyvinyl pyrrolidone in N,N-dimethylformamide solution and ultrasonically mixing to obtain solution A; dissolving dopamine hydrochloride and tris(hydroxymethyl)aminomethane in deionized water and adjusting the pH to 8.0-9.0 to obtain brown-yellow solution B; Quickly add solution B to solution A and stir for the first time to obtain solution C; Adding the metal-doped metal oxide intercalated graphite to solution C and continuing stirring for the second time; The second separation yields the coating layer precursor; S6. Preparation of multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite: placing the product obtained in step S5 in a tube furnace, under argon protection, and naturally cooling to room temperature after heat treatment to obtain a multi-walled carbon nanotube / carbon-coated metal-doped metal oxide intercalated graphite composite material.

6. The method for preparing the graphite composite material according to claim 5, wherein: In S1, the method for preparing coke aggregate includes: using a roller mill to crush coke into aggregate with D50 = 1-25 μm; preparing bromide / metal oxide mixed fine powder: placing metal oxide and bromide in a ball mill according to a mass ratio of 100:(5-50) and ball milling at 100-400 rpm for 1-20 hours to obtain the obtained powder.

7. The method for preparing the graphite composite material according to claim 5, wherein: In S2, the mass ratio of coke aggregate to bromide / metal oxide mixed fine powder is 100:(10-125), and the batch mixing is carried out by a batch mixing machine for 2-3 hours; In S3, the programmed heat treatment includes: heating to 200°C in a medium frequency furnace at 5-8°C / min, keeping the temperature for 2-2.5 hours, then heating to 2000-3200°C at 8-15°C / min, and keeping the temperature for 1-1.5 hours.

8. The method for preparing the graphite composite material according to claim 5, wherein: In S4, the volume ratio of nitric acid / concentrated sulfuric acid in the nitric acid / concentrated sulfuric acid mixed solution is (2.5-3):1; the water bath temperature is 80-85°C, the stirring speed is 100-400 rpm, and the treatment time is 2-5 hours; the first separation in S4 refers to collecting the solid by filtration, washing the obtained solid with deionized water 3-5 times, and then drying at 80°C for 12 hours.

9. The method for preparing the graphite composite material according to claim 5, wherein: In S5, the ultrasonic mixing time is 0.5-2h, the first stirring is carried out at room temperature, the stirring speed is 300-800rpm, and the time is 0.5-2h; the second stirring is carried out at room temperature, the stirring speed is 50-200rpm, and the time is 12-48h; the second separation in S5 is to collect the solid by filtration, wash it with deionized water 3-5 times to a pH of 6.5-7.5, and dry it at 80-85°C for 12h; in S6, the heat treatment refers to heating to 300-500°C at a heating rate of 5°C / min and keeping it warm for 3-3.5h.

10. A secondary battery comprising a positive electrode material, a negative electrode material and an electrolyte, characterized in that: The negative electrode material includes the graphite composite material according to any one of claims 1 to 4 or the graphite composite material prepared by the method for preparing the graphite composite material according to any one of claims 5 to 9.

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