Soft carbon-graphene composite material, preparation method thereof, negative electrode sheet and secondary battery

Abstract

The application provides a soft carbon-graphene composite material and a preparation method thereof, a negative plate and a secondary battery. The preparation method comprises the following steps: performing a hydrothermal reaction on coke and oxidized graphite at 180-220 DEG C to prepare an intermediate; and performing graphitization treatment on the intermediate at 800-1500 DEG C to prepare the soft carbon-graphene composite material. The coke and the oxidized graphite are subjected to the hydrothermal reaction at 180-220 DEG C, so that the oxidized graphite is reduced to form a graphene aerosol, and the coke is dispersed in the graphene aerosol; and then the graphitization treatment is performed at a temperature of 800-1500 DEG C. Compared with a traditional carbon negative electrode material, the soft carbon-graphene composite material prepared by the preparation method has good high-temperature storage performance, high-temperature cycle performance and a fast charging rate as a negative electrode material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of negative electrode materials, in particular to a soft carbon-graphene composite material, a preparation method thereof, a negative electrode sheet and a secondary battery. BACKGROUND

[0002] With the rapid development of new energy vehicles and large-scale energy storage equipment industry, lithium ion batteries are increasingly widely used. As one of the core components of lithium ion batteries, the performance of negative electrode materials directly affects the overall performance of the battery.

[0003] The negative active material of lithium ion batteries mainly includes carbon materials and non-carbon materials. Among them, carbon materials include natural graphite, artificial graphite, soft carbon, and hard carbon. Natural graphite is the earliest carbon material used in the negative electrode of lithium ion batteries, which has a layered structure, and the layers are connected by van der Waals force. Artificial graphite is a graphite material prepared by calcining easily graphitized carbon at a certain temperature, and then crushing, molding, grading, and high-temperature graphitization. Compared with natural graphite, artificial graphite has more uniform layer size and morphology, smaller interlayer spacing, and fewer lattice defects. Soft carbon refers to a carbon material with low carbon content and soft texture. Hard carbon refers to carbon that is difficult to graphitize, such as resin carbon and organic polymer carbon. Hard carbon can be obtained by thermal decomposition of cross-linked resin at about 1000°C. This type of carbon is also difficult to graphitize at high temperatures above 2500°C.

[0004] However, these traditional carbon negative electrode materials have problems such as poor high-temperature storage and high-temperature cycle performance, and poor rate performance, which limits the application scenarios of traditional carbon negative electrode materials. SUMMARY

[0005] Based on the above problems, the present application provides a soft carbon-graphene composite material with good high-temperature cycle performance and fast charging rate, a preparation method thereof, a negative electrode sheet and a secondary battery.

[0006] In one aspect of the present application, a preparation method of a soft carbon-graphene composite material is provided, comprising the following steps:

[0007] The coke and the oxidized graphite are subjected to a hydrothermal reaction at 180°C to 220°C to prepare an intermediate;

[0008] The intermediate is subjected to graphitization treatment at 800°C to 1500°C to prepare the soft carbon-graphene composite material.

[0009] In some embodiments, before the hydrothermal reaction, the oxidized graphite is subjected to a mechanical exfoliation treatment to prepare oxidized graphite with a layered structure.

[0010] In some embodiments, the hydrothermal reaction comprises:

[0011] The coke, the oxidized graphite and the surfactant are dispersed in water to prepare a reaction system;

[0012] The reaction system is heat treated at 180-220 DEG C to reduce the oxidized graphite to form a graphene gas aerosol, and the coke is dispersed in the graphene gas aerosol to prepare the intermediate.

[0013] In some embodiments, the mass percentage of the surfactant in the reaction system is 0.1-5%.

[0014] In some embodiments, the surfactant comprises one or more of polyethylene glycol, guar gum and sodium carboxymethyl cellulose.

[0015] In some embodiments, the preparation method satisfies at least one of (1)-(5):

[0016] (1) the hydrothermal reaction time is 6-12 h;

[0017] (2) the hydrothermal reaction pressure is 1.5-2.0 MPa;

[0018] (3) the graphitization treatment temperature is 800-1000 DEG C;

[0019] (4) the graphitization treatment time is 2-4 h;

[0020] (5) the graphitization treatment is carried out in a protective gas atmosphere.

[0021] In the second aspect, the application further provides a soft carbon-graphene composite material prepared according to the above preparation method of a soft carbon-graphene composite material.

[0022] In some embodiments, the soft carbon-graphene composite material has a particle size D10 of 2-5 μm, a particle size D50 of 6-9 μm and a particle size D90 of 12-15 μm.

[0023] In the third aspect, the application further provides a negative electrode sheet comprising the above soft carbon-graphene composite material.

[0024] In the fourth aspect, the application further provides a secondary battery comprising the above negative electrode sheet.

[0025] The preparation method of the soft carbon-graphene composite material provided by the embodiment of the application is that coke and graphite oxide are subjected to hydrothermal reaction at 180-220 DEG C, so that the graphite oxide is reduced to form graphene aerosol, and the coke is dispersed in the graphene aerosol; and then graphitization treatment is carried out at 800-1500 DEG C. Compared with the traditional carbon negative electrode material, the soft carbon-graphene composite material prepared by the preparation method of the application has good high-temperature storage performance, high-temperature cycle performance and fast charging rate as a negative electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 A scanning electron microscope photo of the soft carbon material prepared for the embodiment 1 of the application;

[0027] Figure 2 A scanning electron microscope photo of the soft carbon material prepared for the embodiment 1 of the application. DETAILED DESCRIPTION

[0028] In order to facilitate the understanding of the application, the application will be described more fully below with reference to the accompanying drawings. The preferred embodiments of the application are given in the accompanying drawings. However, the application can be realized 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.

[0029] In this document, the technical features described in an open form include both the closed technical solution consisting of the listed features and the open technical solution containing the listed features.

[0030] In this document, when referring to a numerical interval, the numerical interval is considered to be continuous and includes the minimum value and the maximum value of the range and each value between the minimum value and the maximum value, unless otherwise specified. Further, when the range is an integer, each integer between the minimum value and the maximum value of the range is included. In addition, when multiple ranges are provided to describe a feature or a characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all sub-ranges subsumed therein.

[0031] In this document, when referring to a data range, if only the unit is indicated after the right end point, it means that the units of the left end point and the right end point are the same. For example, 800-850 nm means that the units of the left end point "800" and the right end point "850" are both nm (nanometer).

[0032] Only some numerical ranges are specifically disclosed herein. However, any lower limit can be combined with any upper limit to form a range not explicitly recited; and any lower limit can be combined with any other lower limit to form a range not explicitly recited, as can any upper limit with any other upper limit to form a range not explicitly recited. Further, each individual disclosed point or single numerical value can be combined with any other point or single numerical value to form a range not explicitly recited, either as a lower or upper limit, or in combination with other lower or upper limits.

[0033] The temperature parameters herein, if not specifically limited, allow for both constant temperature processing as well as processing over a temperature range. The constant temperature processing allows for fluctuations within the precision of the instrument control.

[0034] In the present disclosure, the terms "first", "second", "third", "fourth", "fifth", "sixth", "seventh" and "eighth" are used only for the purpose of description, and are not to be interpreted as indicating or implying relative importance or a specific number of technical features indicated. Thus, the features defined with "first", "second", "third", "fourth", "fifth", "sixth", "seventh" and "eighth" can include at least one of the features, either explicitly or implicitly. In the description of the present disclosure, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise specifically limited. In the description of the present disclosure, the meaning of "several" is at least one, such as one, two, etc., unless otherwise specifically limited.

[0035] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0036] If not specifically stated, all steps of the present application can be performed in sequence or randomly, preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method also comprises step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0037] 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 of the application herein is for the purpose of 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.

[0038] In one aspect of the present application, a method for preparing a soft carbon-graphene composite material is provided, comprising the following steps:

[0039] Step S100: hydrothermal reaction of the coke and the graphite oxide at 180-220°C to prepare an intermediate. Through the hydrothermal reaction, the graphite oxide is reduced to form a graphene gas sol, and the coke is dispersed in the graphene gas sol to form the intermediate. The coke in the prepared intermediate is uniformly distributed, which is beneficial to the subsequent graphitization process. The graphene gas sol cube is a new type of graphene material. The graphene gas sol not only retains the original chemical properties of graphene, but also effectively prevents mutual adsorption of graphene nanosheets, provides a multi-dimensional electron transfer path and shortens the mass transfer distance.

[0040] Optionally, the temperature of the hydrothermal reaction is 180°C, 190°C, 200°C, 210°C, 220°C, or any range formed by any of the above values.

[0041] In some embodiments, the hydrothermal reaction is performed in a protective gas atmosphere. The pressure of the hydrothermal reaction is 1.5-2.0 MPa. Optionally, the pressure of the hydrothermal reaction is 1.5 MPa, 1.6 MPa, 1.7 MPa, 1.8 MPa, 1.9 MPa, 2.0 MPa, or any range formed by any of the above values. Controlling the pressure of the hydrothermal reaction in the above range can preliminarily graphitize the coke and form a graphene gas sol at a lower reaction temperature.

[0042] In some embodiments, the time of the hydrothermal reaction is 6-12 h. Optionally, the time of the hydrothermal reaction is 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, or any range formed by any of the above values.

[0043] In some embodiments, before the hydrothermal reaction, the graphite oxide is subjected to mechanical exfoliation treatment to prepare the graphite oxide with a lamellar structure. The graphite oxide with a lamellar structure obtained through the mechanical exfoliation treatment is beneficial to the generation of the graphene gas sol during the hydrothermal reaction.

[0044] In some embodiments, the mechanical exfoliation treatment is performed by a high-energy ball mill or ultrasonic waves.

[0045] In some embodiments, the step of preparing the graphite oxide with a lamellar structure through mechanical exfoliation treatment comprises dispersing the graphite oxide in an organic solvent and performing mechanical exfoliation treatment using a high-energy ball mill or ultrasonic waves. Then, deionized water is added to the liquid subjected to the mechanical exfoliation treatment, and stirred for 10-20 minutes to disperse the graphite oxide with a lamellar structure between the two-phase interface of the organic solvent and water. Finally, the graphite oxide with a lamellar structure is collected by filtration or centrifugation.

[0046] In some embodiments, the coke is subjected to a pulverization treatment to prepare a powder-like raw material before the hydrothermal reaction. Specifically, the pulverization treatment can be achieved by a high-energy ball mill. The powder-like coke raw material prepared by the pulverization treatment is beneficial to the uniform dispersion of the coke in the subsequent hydrothermal reaction, thereby preparing a coke-dispersed intermediate.

[0047] Specifically, in step S100, the hydrothermal reaction comprises:

[0048] Step S110: dispersing the coke, the graphite oxide and the surfactant in water to prepare a reaction system. The surfactant can help the coke and the graphite oxide to be well dispersed in water, forming a reaction system with good uniformity, and in particular, the surfactant can prevent the powder-like coke and the graphite oxide with a lamellar structure from accumulating or aggregating in water.

[0049] In some embodiments, the surfactant comprises one or more of polyethylene glycol, guar gum and sodium carboxymethyl cellulose. In some embodiments, the mass percentage of the surfactant in the reaction system is 0.1% to 5%. Alternatively, the mass percentage of the surfactant is 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5% or within a range consisting of any of the above values.

[0050] In some embodiments, in step S110, the coke, the graphite oxide and the surfactant are mixed and dispersed in water at 30°C to 40°C for 1h to 2h, so that the surfactant is not dispersed between the graphite oxide with a lamellar structure, and the interlayer distance of the lamellar graphite oxide is increased.

[0051] Step S120: heat-treating the reaction system at 180°C to 220°C to reduce the graphite oxide to form a graphene gas aerosol, and disperse the coke in the graphene gas aerosol to prepare an intermediate.

[0052] Step S200: graphitizing the intermediate at 800°C to 1500°C to prepare a soft carbon-graphene composite material. Under the given conditions, the coke in the intermediate will gradually be converted into a graphite structure. By controlling the temperature and time of the graphitization treatment, the graphitization degree of the intermediate can be controlled, and a soft carbon-graphene composite material with good electrical conductivity and stability can be prepared. Alternatively, the temperature of the graphitization treatment is 800°C, 900°C, 1000°C, 1100°C, 1200°C, 1300°C, 1400°C, 1500°C or within a range consisting of any of the above values. In some embodiments, the temperature of the graphitization treatment is 800°C to 1000°C.

[0053] In some embodiments, the time of the graphitization treatment is 2h to 4h. Alternatively, the time of the graphitization treatment is 2h, 2.5h, 3h, 3.5h, 4h or within a range consisting of any of the above values.

[0054] In some embodiments, the graphitization process is performed under a protective gas atmosphere. The graphitization process is performed under a protective gas atmosphere to avoid the reaction of the material with oxygen at high temperature. Optionally, the protective gas is nitrogen or argon.

[0055] In some embodiments, after step S200, the method further comprises:

[0056] Step S300: washing, drying and sieving the product of step S200 to obtain the soft carbon-graphene composite material.

[0057] In some embodiments, the sieving process in step S300 comprises: firstly mechanically crushing the dried product, then sieving using a sieve, and finally airflow sorting to obtain the soft carbon-graphene composite material with a suitable particle size. Sieving is a process of sorting materials using a sieve, which can remove materials with too small or too large particle sizes. Airflow sorting is a process of using airflow to drive materials with different particle sizes to separate in a classifier.

[0058] Specifically, the mechanical crushing is performed by a ball mill. The dried product is placed in the ball mill, and grinding balls are added for ball milling. Optionally, the grinding balls can be selected from, but are not limited to, stainless steel balls, zirconium balls, etc.

[0059] In some embodiments, in the sieving step, the mechanically crushed product is placed on a sieve, the sieve is turned on, and the sieved material is collected. In some embodiments, the sieving step specifically comprises a pretreatment step, a wet sieving step and an ultrasonic sieving step. Specifically, in the pretreatment step, the sieve is treated by ultrasonic waves, mechanical vibration or air blowing to reduce the possibility of sieve clogging. Optionally, the material of the sieve is copper or stainless steel, which can avoid the influence of static electricity on sieving. In the wet sieving step, the mechanically crushed material is mixed with a liquid for sieving, which can effectively reduce sieve clogging and improve the sieving efficiency. Specifically, the liquid can be deionized water. After the wet sieving step, ultrasonic sieving is performed. Ultrasonic sieving is a process of vibrating the sieve by ultrasonic waves, which can further sieve the residual material on the sieve and avoid sieve clogging.

[0060] The preparation method of the soft carbon-graphene composite material provided by the embodiments of the present application comprises: subjecting coke and graphite oxide to a hydrothermal reaction at 180-220 DEG C to reduce the graphite oxide to form a graphene gas aerosol, and disperse the coke in the graphene gas aerosol; and then graphitizing at a temperature of 800-1500 DEG C. Through the above preparation method, the soft carbon and the graphene are well dispersed in the soft carbon-graphene composite material prepared, the graphene serves as a three-dimensional skeleton to load soft carbon particles, and a good conductive network is formed among the soft carbon particles. Compared with traditional carbon negative electrode materials, the soft carbon-graphene composite material prepared by the preparation method of the present application has good high-temperature storage performance, high-temperature cycle performance and fast charging rate as a negative electrode material.

[0061] In a second aspect, the present application further provides a soft carbon-graphene composite material prepared according to the above preparation method of the soft carbon-graphene composite material.

[0062] In some embodiments, the particle size D10 of the soft carbon-graphene composite material is 2-5 μm; the particle size D50 of the soft carbon-graphene composite material is 6-9 μm; and the particle size D90 of the soft carbon-graphene composite material is 12-15 μm.

[0063] In some embodiments, the tap density of the soft carbon-graphene composite material is ≥0.9 g / cm 3 . Alternatively, the tap density of the soft carbon-graphene composite material is 0.9 g / cm 3 -1.1 g / cm 3 . Specifically, the tap density of the soft carbon-graphene composite material is 0.9 g / cm 3 , 0.93 g / cm 3 , 1.0 g / cm 3 , 1.05 g / cm 3 , 1.1 g / cm 3 , or a range consisting of any of the above values.

[0064] In some embodiments, the specific surface area of the soft carbon-graphene composite material is ≤3.5 m 2 / g. Alternatively, the specific surface area of the soft carbon-graphene composite material is 2.5 m 2 / g-3.5 m 2 / g. Specifically, the specific surface area of the soft carbon-graphene composite material is 2.5 m 2 / g, 2.8 m 2 / g, 3.0 m 2 / g, 3.2 m 2 / g, 3.4 m 2 / g, 3.5 m 2 / g, or a range consisting of any of the above values.

[0065] In some embodiments, the soft carbon-graphene composite has a gravimetric capacity ≥ 270 mAh / g.

[0066] In some embodiments, the soft carbon-graphene composite has a first coulombic efficiency ≥ 90%.

[0067] In some embodiments, the soft carbon-graphene composite has a gravimetric capacity ≥ 270 mAh / g.

[0068] In some embodiments, the soft carbon-graphene composite has a first coulombic efficiency ≥ 90%.

[0069] In order to make the purpose, technical scheme and advantages of the present application more concise and clear, the present application is described by the following specific embodiments, but the present application is not limited to these embodiments. The embodiments described below are only good embodiments of the present application, which can be used to describe the present application, and cannot be understood as limiting the scope of the present application. It should be noted that any modification, equivalent replacement and improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.

[0070] Embodiment 1

[0071] The preparation method of the soft carbon-graphene composite of the present embodiment includes the following steps:

[0072] (1) The coke is crushed into powder by a ball mill.

[0073] (2) The graphite oxide is dispersed in the organic solvent NMP, and then ultrasonic treatment is performed to obtain the graphite oxide with a sheet structure. Deionized water is added to the dispersion system, wherein the volume ratio of deionized water to NMP is 1.5:1. Then, the graphite oxide sheets are dispersed at the interface of the two phases under stirring at 250 rpm for 15 minutes. The graphite oxide sheets are filtered.

[0074] (3) Coke powder, graphite oxide sheets, and 1.5 wt% polyethylene glycol (PEG) solution were mixed and kept at 35°C for 1 hour to obtain a dispersion. The dispersion was then placed in a high-pressure reactor, and the reaction temperature was set to 200°C and the reaction pressure to 1.5 MPa for hydrothermal reaction for 10 hours. After the reaction was completed, the high-pressure reactor was allowed to cool naturally to room temperature, and then the reaction product was removed.

[0075] (4) Wash the reaction product obtained in step (3) with distilled water and dry it for later use.

[0076] (5) Place the product obtained in step (4) into a graphitization furnace, set the reaction temperature to 1000℃, and graphitize for 3 hours under a nitrogen atmosphere. After the reaction is completed, cool to room temperature and remove the graphitized product.

[0077] (6) The graphitized product obtained in step (5) is washed with distilled water, dried, and sieved to obtain a soft carbon-graphene composite material.

[0078] See Figure 1 , Figure 2 Table 1 shows a scanning electron microscope image of the soft carbon-graphene composite material prepared in this embodiment. The physicochemical parameters of the prepared soft carbon-graphene composite material are shown in Table 1.

[0079] Table 1

[0080]

[0081] Comparative Example 1

[0082] This comparative example uses commercially available natural graphite as the active material.

[0083] Comparative Example 2

[0084] This comparative example uses commercially available artificial graphite as the active material.

[0085] Working electrode preparation: A slurry was prepared by mixing the materials prepared in the above examples or comparative examples, the conductive agent SuperP, and the binder PVDF in a mass ratio of 92:3:5. The slurry was then coated onto copper foil to prepare the working electrode.

[0086] Battery fabrication: Lithium foil was used as the counter electrode; the electrolyte contained 1M LiPF6, and its solvent was a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1. The electrolyte also contained 1wt% of the additive vinylene carbonate (VC). The working electrode, separator, and counter electrode were stacked to prepare the cell, and then the electrolyte was injected to prepare the pouch battery.

[0087] The prepared pouch cells were subjected to electrochemical tests using the methods described below. The test results are recorded in Table 2.

[0088] Charge speed test method: discharge the battery to the rated minimum voltage, then charge the battery at different rates, record the time required from the discharge state to the charging cut-off voltage. Monitor the battery temperature and voltage changes during the entire charging process. Record the data during the charging process, including charging time, charging capacity and charging efficiency, to analyze and calculate the charging speed.

[0089] Cycle life: compare the discharge capacity after the nth cycle with the first discharge capacity by performing charge-discharge cycles on the battery. Capacity retention rate = (discharge capacity after the nth cycle / first discharge capacity) * 100%. The cycle number at which the capacity retention rate is 80% is the cycle life.

[0090] Capacity retention rate at 80°C (high temperature storage): charge the battery to the rated capacity, then place the fully charged battery in a temperature-controlled environmental test chamber, set the temperature to 80°C. Let the battery stand in this high temperature environment for a certain period of time, then take it out and place it in a room temperature environment for at least 2 hours to allow the temperature to stabilize. Use the battery test system to perform a discharge test on the battery, discharge to the set cut-off voltage. Record the capacity during discharge, compare with the initial capacity before exposure to high temperature, calculate the capacity retention rate. Capacity retention rate at 80°C = (battery discharge capacity after 80°C exposure / battery discharge capacity at room temperature) * 100%.

[0091] Capacity retention rate after 100 cycles at 60°C (high temperature cycle): place the battery in a test chamber with a temperature of 60°C. Perform 100 charge-discharge cycles, record the discharge capacity of each cycle. Analyze the change in discharge capacity as the number of cycles increases to evaluate the cycle stability at high temperature. High temperature (60°C) capacity retention rate = (discharge capacity after the 100th cycle / first discharge capacity) * 100%.

[0092] Table 2

[0093] Serial number Charge speed (C) Cycle life (times) High temperature storage (%) High temperature cycle (%) Example 1 2.5 1200 95 93 Comparative example 1 1.8 800 88 85 Comparative example 2 2.0 1000 90 87

[0094] As can be seen from the data in Table 2, compared with the conventional natural graphite or artificial graphite of Comparative Examples 1-2, the soft carbon-graphene composite material prepared in Example 1 has faster charging speed, longer cycle life, and better capacity and cycle stability under high temperature conditions. Therefore, the soft carbon-graphene composite material prepared by first hydrothermally reacting coke and oxidized graphite, and then graphitizing at 800-1500°C, has faster charging speed, longer cycle life, and better capacity and cycle stability under high temperature conditions, compared with the conventional artificial graphite prepared by graphitizing at more than 2000°C.

[0095] Any technical features in the above-described embodiments can be combined in any manner, and for the sake of brevity, not all possible combinations are described, however, any combination of the technical features is considered to be within the scope of the present disclosure.

[0096] 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 should not be understood as a limitation on the patent protection scope of the present application. It should be pointed out that, for ordinary skilled persons in the art, a number of modifications and improvements can be made without departing from the concept of the present application, and these are within the protection scope of the present application. It should be understood that, on the basis of the technical solutions provided by the present application, technical solutions obtained by logical analysis, reasoning or limited experiments by those skilled in the art are within the protection scope of the appended claims. Therefore, the patent protection scope of the present application should be subject to the contents of the appended claims, and the description and drawings can be used to explain the contents of the claims.

Claims

1. A method for preparing a soft carbon-graphene composite material, characterized by, The method comprises the following steps: The intermediate is prepared by hydrothermal reaction of coke and graphite oxide at 180-220 DEG C; The soft carbon-graphene composite material is prepared by graphitization treatment of the intermediate at 800-1500 DEG C; The graphite oxide is subjected to mechanical exfoliation treatment to prepare graphite oxide with lamellar structure before the hydrothermal reaction; The hydrothermal reaction comprises: The reaction system is prepared by dispersing coke, graphite oxide and surfactant in water; The reaction system is heat-treated at 180-220 DEG C to reduce the graphite oxide to form graphene aerosol, and the coke is dispersed in the graphene aerosol to obtain the intermediate.

2. The method of claim 1, wherein the soft carbon-graphene composite material is prepared by the steps of: mixing a carbon source and a graphene source to form a mixture; and heating the mixture to form the soft carbon-graphene composite material. The mass percentage of the surfactant in the reaction system is 0.1-5%.

3. The method of claim 1, wherein the soft carbon-graphene composite material is prepared by the steps of: mixing a carbon source and a graphene source to form a mixture; and heating the mixture to form the soft carbon-graphene composite material. The surfactant comprises one or more of polyethylene glycol, guar gum and sodium carboxymethyl cellulose.

4. The method of claim 1 to 3, wherein the soft carbon-graphene composite material is prepared by the following steps of: (1) mixing the carbon source and the graphene to form a mixture; (2) heating the mixture to form a soft carbon-graphene composite material. The preparation method satisfies at least one of the following conditions (1) to (5): (1) The time of the hydrothermal reaction is 6-12 h; (2) The pressure of the hydrothermal reaction is 1.5-2.0 MPa; (3) The temperature of the graphitization treatment is 800-1000 DEG C; (4) The time of the graphitization treatment is 2-4 h; (5) The graphitization treatment is carried out in a protective gas atmosphere.

5. A soft carbon-graphene composite material, characterized by, The soft carbon-graphene composite material is prepared according to the preparation method of any one of claims 1 to 4.

6. The soft carbon-graphene composite of claim 5, wherein, The particle size D10 of the soft carbon-graphene composite material is 2-5 μm, the particle size D50 is 6-9 μm, and the particle size D90 is 12-15 μm.

7. A negative electrode sheet characterized by comprising: The soft carbon-graphene composite material comprises the soft carbon-graphene composite material of any one of claims 5 or 6.

8. A secondary battery characterized by comprising: The negative electrode sheet comprises the soft carbon-graphene composite material of claim 7.

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