Composite graphite material, method for preparing the same, and use thereof

Abstract

The application relates to a preparation method and application of a composite graphite material. The preparation method comprises the following steps: grinding natural graphite to prepare primary particles; the primary particles are subjected to an oxidation reaction with an oxidizing agent to prepare oxidized particles; the oxidized particles, a binder and a conductive agent are mixed and ground to form a protective film of the binder and the conductive agent on the surfaces of the primary particles, thereby preparing secondary particles; the secondary particles are subjected to heat treatment at 500 DEG C to 1500 DEG C under a protective atmosphere to prepare reduced particles; and the reduced particles are immersed in molten iron to perform molten corrosion treatment on the reduced particles, thereby preparing the composite graphite material. Through reasonable process design, the composite graphite material prepared by the preparation method has good electron conduction performance and ion conduction performance, and has good structural stability; and the composite graphite material is used for secondary batteries and has good rate performance and cycle life at low temperatures.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a composite graphite material and a preparation method and application thereof. BACKGROUND

[0002] Lithium ion batteries have the advantages of high energy density and power density, high working voltage, light weight, small size, long cycle life, good safety, green environmental protection, etc., and have broad application prospects in portable electrical appliances, digital products, electric tools, large-scale energy storage, etc.

[0003] Among them, the four key materials of lithium ion batteries are positive electrode material, negative electrode material, electrolyte and separator. The negative electrode material mainly realizes energy storage and release through the migration and insertion / embedding of lithium ions. Carbon-based materials such as natural graphite and artificial graphite are the mainstream negative electrode materials.

[0004] Under low temperature conditions, the viscosity of the electrolyte in the traditional lithium ion battery increases, the electrolyte and electrode interface film impedance and charge transfer impedance increase, and the reaction activity of the active material decreases. Under the combined action of these factors, the rate performance and cycle stability of the traditional lithium ion battery under low temperature conditions are poor, and it is difficult to adapt to the application in harsh low temperature environments. SUMMARY

[0005] Based on the above problems, the present application provides a composite graphite material and a preparation method thereof, which can improve the rate performance and cycle performance of the secondary battery under low temperature.

[0006] In addition, a negative electrode sheet and a secondary battery containing the above-mentioned composite graphite material are also provided.

[0007] In one aspect of the present application, a preparation method of a composite graphite material is provided, comprising the following steps:

[0008] Ball milling natural graphite to prepare primary particles;

[0009] Oxidizing the primary particles with an oxidizing agent to prepare oxidized particles;

[0010] Mixing and ball milling the oxidized particles, a binder and a conductive agent to form a protective film of the binder and the conductive agent on the surface of the primary particles, to prepare secondary particles;

[0011] Heat treating the secondary particles at 500-1500℃ under a protective atmosphere to prepare reduced particles;

[0012] Immersion of the reduced particles in molten iron for molten corrosion treatment of the reduced particles to prepare the composite graphite material.

[0013] In some embodiments, the primary particles have a particle size of 1 μm to 10 μm.

[0014] In some embodiments, the step of preparing the oxidized particles satisfies at least one of (1) to (3):

[0015] (1) the oxidizing agent comprises concentrated nitric acid, sulfuric acid, or a mixture thereof;

[0016] (2) the temperature of the oxidation reaction is 25°C to 80°C;

[0017] (3) the time of the oxidation reaction is 1 hour to 24 hours.

[0018] In some embodiments, the step of preparing the secondary particles satisfies at least one of (1) to (5):

[0019] (1) the average particle size of the secondary particles is 10 μm to 20 μm;

[0020] (2) the thickness of the protective film is 0.05 μm to 1 μm;

[0021] (3) the mass ratio of the oxidized particles, the binder, and the conductive agent is (85 to 99) : (0.1 to 5) : (0.1 to 5);

[0022] (4) the binder comprises one or more of polyvinylidene fluoride, sodium polyacrylate, polyvinyl alcohol, and polyacrylonitrile;

[0023] (5) the conductive agent comprises one or more of conductive carbon black, graphene, and carbon nanotubes.

[0024] In some embodiments, the step of preparing the reduced particles satisfies at least one of (1) to (2):

[0025] (1) the protective atmosphere comprises one of nitrogen, argon, and hydrogen;

[0026] (2) the time of the heat treatment is 1 hour to 10 hours.

[0027] In some embodiments, the molten treatment satisfies at least one of (1) to (2):

[0028] (1) the temperature of the molten iron is 1500°C to 1600°C;

[0029] (2) the time of the molten treatment is 1 minute to 40 minutes.

[0030] In a second aspect, the present application also provides a composite graphite material prepared according to the method described above.

[0031] In some embodiments, the composite graphite material satisfies at least one of (1)-(3):

[0032] (1) the particle size D10 of the composite graphite material is 6 μm-10 μm; the particle size D50 of the composite graphite material is 12 μm-17 μm; and the particle size D90 of the composite graphite material is 20 μm-32 μm;

[0033] (2) the tap density of the composite graphite material is ≤1.05 g / cm 3 ;

[0034] (3) the specific surface area of the composite graphite material is ≥1.5 m 2 / g.

[0035] In a third aspect, the application further provides a negative electrode sheet comprising the composite graphite material described above.

[0036] In a fourth aspect, the application further provides a secondary battery comprising the negative electrode sheet described above.

[0037] The composite graphite material prepared by the preparation method provided in the embodiments of the application has good electron conduction performance and ion conduction performance, and has good structural stability. The secondary battery using the composite graphite material has good rate performance and cycle life at low temperature. DETAILED DESCRIPTION

[0038] In order to facilitate the understanding of the application, the application will be described more fully below. The application can be implemented in many different forms, and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the application more thorough and comprehensive.

[0039] In the present application, the selection range involving "and / or", "or / and", "and / or" includes any one of two or more related listed items, and also includes any and all combinations of related listed items, which includes any two related listed items, any more related listed items, or all related listed items. It should be noted that when at least two conjunctions selected from "and / or", "or / and", "and / or" are combined to connect at least three items, it should be understood that the technical solution undoubtedly includes the technical solution connected by "logical and", and also undoubtedly includes the technical solution connected by "logical or". For example, "A and / or B" includes three parallel schemes of A, B and A+B. For another example, the technical solution of "A, and / or, B, and / or, C, and / or, D" includes any one of A, B, C and D (i.e. the technical solution connected by "logical or"), and also includes any and all combinations of A, B, C and D, i.e. includes the combination of any two or any three of A, B, C and D, and also includes the four-item combination of A, B, C and D (i.e. the technical solution connected by "logical and").

[0040] In the present application, "multiple", "multiple", "multiple", "multiple" and the like are used without special limitation, which means more than two or equal to two in quantity. For example, "one or more" means one or more than two.

[0041] In the present application, "combination thereof", "any combination thereof", "any combination thereof" and the like include all suitable combinations of any two or more listed items.

[0042] In the present application, "suitable combination", "suitable manner", "any suitable manner" and the like are described as "suitable" which can implement the technical solutions of the present application, solve the technical problems of the present application, and achieve the expected technical effects of the present application.

[0043] In the present application, "preferably", "better", "better", "preferably" are only used to describe the better implementation or embodiment, and it should be understood that it does not constitute a limitation on the protection scope of the present application.

[0044] In the present application, "further", "further", "particularly" and the like are used for description purposes, indicating differences in content, but should not be understood as a limitation on the protection scope of the present application.

[0045] In the present application, "optionally", "optional", "optional" means optional, i.e. selected from "yes" or "no" two parallel schemes. If there are multiple "optional" in a technical solution, each "optional" is independent unless otherwise stated, without contradiction or mutual restriction.

[0046] In the present application, the terms "first", "second", "third", "fourth" and the like in the "first aspect", "second aspect", "third aspect", "fourth aspect" and the like are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or quantity, nor can they be understood as implicitly indicating the importance or quantity of the technical features indicated. Moreover, "first", "second", "third", "fourth" and the like only serve the purpose of non-exhaustive enumeration and description, and should be understood as not constituting a closed limitation on the quantity.

[0047] In the present application, among the technical features described in an open manner, both the closed technical solution consisting of the listed features and the open technical solution containing the listed features are included.

[0048] In the present application, with respect to a numerical interval (i.e. a numerical range), if no special instructions are given, it is considered continuous within the numerical interval and includes both numerical endpoints (i.e. the minimum value and the maximum value) of the numerical range and every numerical value between the two numerical endpoints. If no special instructions are given, when a numerical interval only refers to integers within the numerical interval, including both endpoint integers of the numerical range and every integer between the two endpoints, in this document, it is equivalent to directly listing each integer, for example, t is an integer selected from 1 to 10, which means that t is any one integer selected from the group consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10. In addition, when multiple ranges are provided to describe a feature or characteristic, these ranges can be combined. In other words, unless otherwise specified, the ranges disclosed in this document should be understood to include any and all sub-ranges encompassed therein.

[0049] In the present application, the temperature parameter, if not specifically limited, allows both constant temperature treatment and variation within a certain temperature range. It should be understood that the constant temperature treatment allows the temperature to fluctuate within the accuracy range controlled by the instrument. It is allowed to fluctuate within the range of, for example, ±5℃, ±4℃, ±3℃, ±2℃, ±1℃.

[0050] In the present application, with respect to the percentage content, if no special instructions are given, it refers to the mass percentage for solid-liquid mixing and solid-solid mixing, and refers to the volume percentage for liquid-liquid mixing.

[0051] In the present application, with respect to the percentage concentration, if no special instructions are given, it refers to the final concentration. The final concentration refers to the proportion of the added ingredient in the system after the ingredient is added.

[0052] In the present application, % (w / w) and wt% both represent weight percentage, % (v / v) refers to volume percentage, and % (w / v) refers to mass volume percentage.

[0053] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for describing particular embodiments only and is not intended to be limiting of the application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0054] The application provides a preparation method of a composite graphite material in a real-time manner, comprising the following steps:

[0055] Step S100: milling natural graphite to prepare primary particles. By milling the natural graphite, graphite particles with smaller particle size can be obtained to improve the uniformity of subsequent processing. The present application does not have specific limitations on the rotation speed and time of the milling, which can be adjusted according to the particle size of the primary particles.

[0056] In some embodiments, the particle size of the primary particles is 1 μm to 10 μm. For example, the particle size of the primary particles can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm or any range formed by any combination of the above values.

[0057] Step S200: oxidizing the primary particles with an oxidizing agent to prepare oxidized particles. By oxidizing the primary particles with an oxidizing agent, the surface of the primary particles can be oxidized to modify the functional groups, and the prepared oxidized particles have better binding capacity, which is beneficial to surface modification.

[0058] In some embodiments, the oxidizing agent comprises concentrated nitric acid, sulfuric acid or a mixture thereof.

[0059] In some embodiments, the temperature of the oxidation reaction is 25°C to 80°C. Alternatively, the temperature of the oxidation reaction is 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C or any range formed by any combination of the above values.

[0060] In some embodiments, the time of the oxidation reaction is 1 hour to 24 hours. Alternatively, the time of the oxidation reaction is 1 hour, 2 hours, 4 hours, 5 hours, 6 hours, 10 hours, 12 hours, 16 hours, 18 hours, 24 hours or any range formed by any combination of the above values.

[0061] In some embodiments, step S200 comprises:

[0062] Step S210: mixing the primary particles with an oxidizing agent to prepare a reaction system. The oxidizing agent comprises concentrated nitric acid, sulfuric acid or a mixture thereof.

[0063] Step S220: Stir the reaction system at 25-80°C for 1-24 hours to oxidize the surface of the primary particles.

[0064] Step S230: After step S220, add a neutralizing agent to the reaction system to neutralize the pH of the reaction system to 6-7.

[0065] Step S240: Filter the mixture obtained in step S230, wash the solid product with water multiple times, and dry to obtain the oxidized particles. Specifically, the drying temperature is 100-200°C, and the drying time is 2-12 hours. Alternatively, the drying temperature is 100°C, 120°C, 140°C, 150°C, 160°C, 180°C, 200°C, or any range formed by any of these values. The drying time is 2 hours, 4 hours, 6 hours, 8 hours, 10 hours, 12 hours, or any range formed by any of these values.

[0066] Step S300: Mix the oxidized particles, binder, and conductive agent and ball mill to form a protective film of the binder and conductive agent on the surface of the primary particles to prepare secondary particles. Through ball milling, the binder and conductive agent can be uniformly dispersed and adsorbed on the surface of the oxidized particles and further form a protective film, which not only improves the conductivity of the material but also obtains secondary particles with a compact structure. Since the surface of the oxidized particles is modified with functional groups, the binding force with the binder is better, the protective film has better compactness, and the protective film can protect the internal structure of the graphite and improve the structural stability of the material.

[0067] In some embodiments, the average particle size of the secondary particles is 10-20 μm. Alternatively, the average particle size of the secondary particles is 10 μm, 12 μm, 14 μm, 15 μm, 16 μm, 18 μm, 20 μm, or any range formed by any of these values.

[0068] In some embodiments, the thickness of the protective film is 0.05-1 μm. Alternatively, the thickness of the protective film is 0.05 μm, 0.1 μm, 0.2 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.8 μm, 1 μm, or any range formed by any of these values. Specifically, the thickness of the protective film can be adjusted by controlling the rotation speed and time of ball milling, which is not described here.

[0069] In some embodiments, the mass ratio of the oxidized particles, binder, and conductive agent is (85-99):(0.1-5):(0.1-5).

[0070] In some embodiments, the binder includes one or more of polyvinylidene fluoride (PVDF), polyacrylic acid sodium (PAA-Na), polyvinyl alcohol (PVA), and polyacrylonitrile (PAN).

[0071] In some embodiments, the conductive agent includes one or more of conductive carbon black, graphene, and carbon nanotubes.

[0072] Step S400: heat treating the secondary particles at 500-1500°C under a protective atmosphere to obtain reduced particles. By heat treating the secondary particles under a protective atmosphere, the oxidized particles in the secondary particles can be reduced, further improving the conductivity of the material. Optionally, the temperature of the heat treatment is 500°C, 600°C, 700°C, 800°C, 900°C, 1000°C, 1100°C, 1200°C, 1300°C, 1400°C, 1500°C, or any range defined by any two of the above values.

[0073] In some embodiments, the protective atmosphere includes one of nitrogen, argon, and hydrogen.

[0074] In some embodiments, the heat treatment is performed for 1-10 hours. Optionally, the heat treatment is performed for 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, or any range defined by any two of the above values.

[0075] In some embodiments, the heat treatment of step S400 is performed in a tube furnace or a muffle furnace.

[0076] In some embodiments, step S400 includes:

[0077] Step S410: placing the secondary particles in a tube furnace and heating the secondary particles from room temperature to 500-1000°C at a rate of 3-5°C / min under a protective atmosphere.

[0078] Step S420: holding the secondary particles at 500-1000°C for 1-10 hours.

[0079] Step S430: after the holding, cooling the secondary particles to room temperature at a rate of 3-5°C / min to obtain reduced particles.

[0080] The above steps can be used to control the heating and cooling rates during the heat treatment process, and the reduced particles obtained have good structural integrity and no obvious surface damage.

[0081] Step S500: immerging the reduced particles into the molten iron to make the molten iron erode the surface of the reduced particles, and preparing the composite graphite material. By immersing the reduced particles into the molten iron, the molten iron erodes the carbon of the reduced particles, thereby improving the structure of the particles and the electronic and ionic conductivity of the material.

[0082] In some embodiments, the temperature of the molten iron is 1500-1600℃.

[0083] In some embodiments, the time for the erosion is 1-40 minutes. Alternatively, the time for the erosion is 1 minute, 2 minutes, 5 minutes, 10 minutes, 20 minutes, 30 minutes, 40 minutes, or any range formed by any combination thereof.

[0084] In some embodiments, step S500 comprises:

[0085] Step S510: cleaning the surface of the reduced particles with deionized water or ethanol to remove dust, grease or other impurities on the surface.

[0086] Step S520: drying the cleaned reduced particles at 50-70℃ for 2-4 hours to completely dry the reduced particles.

[0087] Step S530: immersing the reduced particles obtained in step S520 into the molten iron at 1500-1600℃ for 1-40 minutes to make the reduced particles erode.

[0088] Step S530: taking out the reduced particles treated in step S530 and cooling in a cooling medium to obtain the composite graphite material. Specifically, the cooling medium can be water, silicone oil or air.

[0089] The composite graphite material prepared by the above preparation method has good electronic and ionic conductivity and good structural stability. When used in secondary batteries, the composite graphite material has good rate performance and cycle life at low temperature.

[0090] In the second aspect, the application further provides a composite graphite material prepared according to the above preparation method of the composite graphite material. The composite graphite material used in secondary batteries can maintain good rate performance and cycle life at low temperature.

[0091] In some embodiments, the particle size D10 of the composite graphite material is 6-10 μm; the particle size D50 of the composite graphite material is 12-17 μm; and the particle size D90 of the composite graphite material is 20-32 μm.

[0092] In some embodiments, the tap density of the composite graphite material is ≤1.05 g / cm 3.

[0093] In some embodiments, the specific surface area of the composite graphite material is ≥1.5 m 2 / g.

[0094] In a third aspect, the application further provides a negative electrode sheet comprising the composite graphite material described above. The negative electrode sheet has all the beneficial effects of the composite graphite material, which will not be repeated here.

[0095] In a fourth aspect, the application further provides a secondary battery comprising the negative electrode sheet described above. The secondary battery contains the composite graphite material described above, which can maintain good rate performance and cycle life at low temperatures, and is particularly suitable for application scenarios in harsh low-temperature environments. The secondary battery described above can be used in the fields of consumer electronics, power tools, new energy vehicles, energy storage equipment, etc.

[0096] In some embodiments, the secondary battery includes but is not limited to lithium ion batteries, lithium metal batteries, sodium ion batteries, solid-state batteries, etc.

[0097] In order to make the purpose, technical scheme and advantages of the application more concise and clear, the application is described by the following specific embodiments, but the application is not limited to these embodiments. The embodiments described below are only good embodiments of the application, which can be used to describe the application, and should not be understood as limiting the scope of the application. It should be noted that any modifications, equivalent replacements and improvements made within the spirit and principles of the application should be included in the protection scope of the application.

[0098] Example 1

[0099] The preparation of the composite graphite material of the present embodiment includes the following steps:

[0100] (1) Disperse the natural graphite in ethanol and ball mill, dry to obtain primary particles with an average particle size of 5 μm.

[0101] (2) Mix the primary particles and the concentrated nitric acid-sulfuric acid mixed solution, stir and react at 40°C for 4 hours to make the surface of the primary particles oxidize. Then add a neutralizing agent sodium carbonate solution to the reaction system to neutralize the pH value of the reaction system to 7, then filter to obtain a solid product, wash the obtained solid product with deionized water for 3 times, then filter and dry to obtain oxidized particles.

[0102] (3) Mix and ball mill the oxidized particles, the binder PVDF and the conductive agent graphene according to a mass ratio of 94:5:1 to make the PVDF and graphene form a protective film of 0.1 μm on the surface of the oxidized particles, to obtain secondary particles with an average particle size of 14 μm.

[0103] (4) The secondary particles prepared in step (3) were put into a tube furnace, and heated from room temperature to 800℃ at a rate of 3℃ / min under a protective atmosphere, then kept for 3 hours, and then decreased to room temperature at a rate of 3℃ / min, to obtain reduced particles.

[0104] (5) Hot metal with a temperature of 1500-1600℃ was prepared. The surface of the reduced particles was cleaned with deionized water, and then dried at 60℃ for 2 hours to obtain pretreated reduced particles. The reduced particles were immersed in the hot metal for 20 minutes for melting corrosion treatment. After the treatment, the reduced particles were taken out and cooled to room temperature in a water bath to obtain a composite graphite material.

[0105] The particle size D10 of the composite graphite material prepared in this example was 7.8 μm, D50 was 14.6 μm, and D90 was 24.9 μm.

[0106] Comparative Example 1

[0107] In this comparative example, commercially available artificial graphite was used as the negative electrode material.

[0108] Comparative Example 2

[0109] In this comparative example, the reduced particles prepared in step (4) of Example 1 were used as the negative electrode material.

[0110] Preparation of the negative electrode sheet: the negative electrode material, the conductive agent Super P and the binder PVDF of the above examples or comparative examples were prepared into a slurry with a mass ratio of 92:3:5. Then the slurry was coated on a copper foil to prepare a working electrode.

[0111] Preparation of the battery: LiCoO2 electrode sheets were used as positive electrode sheets, and the positive electrode sheets and the negative electrode sheets were assembled into a hard shell 2032 type button cell.

[0112] The above prepared battery was subjected to electrochemical test, and the test method was as follows. The test results are recorded in Table 1.

[0113] First charge-discharge efficiency test: the test battery was put into a thermostat, and was charged to a full state at 25℃ and -20℃ respectively, and the charge capacity was recorded. Then the first discharge was carried out, and the discharge capacity was recorded. The first charge-discharge efficiency = (first discharge capacity / first charge capacity) x 100%.

[0114] 3C discharge capacity retention rate: the test battery was charged to a full state at 25℃ at 0.5C and 3C rates respectively. Then it was discharged to the lower limit voltage at 0.5C and 3C rates respectively. The discharge capacities at 0.5C and 3C rates were recorded, and the 3C discharge capacity retention rate = 3C discharge capacity / 0.5C discharge capacity*100%.

[0115] Cycle life test: the test battery was charged and discharged at 1C rate at 25℃, and the cycle number was recorded when the discharge capacity was reduced to 80% of the initial discharge capacity, which was the cycle life.

[0116] -20℃ discharge capacity retention test: the test battery was placed in a constant temperature oven, and the test battery was charged to full state at 25℃ and -20℃ at 0.2C rate, and then discharged at 0.2C rate, and the discharge capacity was recorded. The -20℃ discharge capacity retention rate = -20℃ discharge capacity / 25℃ discharge capacity*100%.

[0117] -20℃ cycle capacity retention test: the test battery was placed in a constant temperature oven, and the test battery was charged and discharged at 0.2C rate at -20℃ for N times, and the discharge capacity after the Nth cycle was compared with the initial discharge capacity. The capacity retention rate = (discharge capacity after the Nth cycle / initial discharge capacity)*100%.

[0118] Table 1

[0119]

[0120] From the data in Table 1, it can be seen that the composite graphite material prepared by the preparation method of the application in Example 1 has higher initial charge-discharge efficiency, longer cycle life and rate performance at 25℃, and higher initial charge-discharge efficiency, discharge capacity retention rate and cycle capacity retention rate at -20℃ low temperature condition compared with Comparative Examples 1-2. It can be seen that the composite graphite material prepared by the preparation method provided by the application has good conductivity and structural stability at room temperature and low temperature conditions; and used as a negative material of a secondary battery, it can significantly improve the rate performance and cycle stability of the secondary battery at low temperature conditions.

[0121] Each technical feature of the above-described embodiments can be combined arbitrarily, and to make the description concise, not all possible combinations of each technical feature in the above-described embodiments are described, however, as long as the combination of these technical features does not exist contradictory, it should be considered as the scope of the present disclosure.

[0122] The above-described embodiments only express several implementation manners of the present application, facilitate specific and detailed understanding of the technical solutions of the present application, but cannot be understood as a limitation on the patent protection scope of the present application. It should be noted that, for ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. It should be understood that, on the basis of the technical solutions provided by the present application, the technical solutions obtained by logical analysis, reasoning or limited experiments by the skilled person in the art all belong to the protection scope of the appended claims of the present application. Therefore, the patent protection scope of the present application should be subject to the content of the appended claims, and the description can be used to explain the content of the claims.

Claims

1. A method for producing a composite graphite material, characterized by, The method comprises the following steps: ball-milling natural graphite to prepare primary particles; oxidizing the primary particles with an oxidizing agent to prepare oxidized particles; ball-milling the oxidized particles, a binder and a conductive agent to form a protective film of the binder and the conductive agent on surfaces of the primary particles, thereby preparing secondary particles; heat-treating the secondary particles at 500-1500°C in a protective atmosphere to prepare reduced particles; immersing the reduced particles in molten iron to perform a molten corrosion treatment on the reduced particles, thereby preparing the composite graphite material.

2. The method of claim 1, wherein the method further comprises the step of: The primary particles have a particle size of 1-10 μm.

3. The method of claim 1, wherein the method further comprises the step of: The step of preparing the oxidized particles satisfies at least one of conditions (1)-(3): (1) the oxidizing agent comprises concentrated nitric acid, sulfuric acid or a mixture thereof; (2) the temperature of the oxidation reaction is 25-80°C; (3) the time of the oxidation reaction is 1-24 hours.

4. The method of claim 1, wherein the method further comprises: The step of preparing the secondary particles satisfies at least one of conditions (1)-(5): (1) the secondary particles have an average particle size of 10-20 μm; (2) the protective film has a thickness of 0.05-1 μm; (3) the oxidized particles, the binder and the conductive agent have a mass ratio of (85-99):(0.1-5):(0.1-5); (4) the binder comprises one or more of polyvinylidene fluoride, sodium polyacrylate, polyvinyl alcohol and polyacrylonitrile; (5) the conductive agent comprises one or more of conductive carbon black, graphene and carbon nanotubes.

5. The method of claim 1, wherein the composite graphite material is prepared by a process comprising: mixing graphite particles with a binder to form a mixture; and compressing the mixture to form the composite graphite material. The step of preparing the reduced particles satisfies at least one of conditions (1)-(2): (1) the protective atmosphere comprises one of nitrogen, argon and hydrogen; (2) the heat treatment is performed for 1-10 hours.

6. The method of producing a composite graphite material according to any one of claims 1 to 5, characterized by, The molten corrosion treatment satisfies at least one of conditions (1)-(2): (1) the temperature of the molten iron is 1500-1600°C; (2) the molten corrosion is performed for 1-40 minutes.

7. A composite graphite material, characterized by, The composite graphite material is prepared according to the method of any one of claims 1-6.

8. The composite graphite material of claim 7, wherein, The composite graphite material satisfies at least one of conditions (1)-(3): (1) the particle size D10 of the composite graphite material is 6-10 μm; the particle size D50 of the composite graphite material is 12-17 μm; and the particle size D90 of the composite graphite material is 20-32 μm; (2) the tap density of the composite graphite material is ≤ 1.05 g / cm 3 ; (3) the specific surface area of the composite graphite material is ≥1.5 m 2 / g.

9. A negative electrode sheet characterized by comprising: The method comprises the composite graphite material of claim 7 or 8.

10. A secondary battery characterized by comprising: The method comprises the negative electrode sheet of claim 9.

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