A hard carbon / graphene composite material and a preparation method thereof

Abstract

The application discloses a hard carbon / graphene composite material and a preparation method thereof. Graphene is introduced to the surface of paper, and microwave treatment is performed on the paper. In the process, the graphene absorbs microwave to generate high-frequency plasma electromagnetic waves, thereby promoting the rapid absorption of microwave energy by the whole sample, and an electric breakdown phenomenon is generated in the sample to complete carbonization. The microwave carbonization method of the composite graphene realizes the carbonization of the paper by the microwave, so that the composite joint structure is effectively formed. Compared with a conventional carbonization method, the electrode material obtained by the method has better rate performance and higher cycle stability. The method is extremely efficient, reasonable in design, and low in cost. Through waste in life, the graphene is used as an intermediate reactant to quickly realize the preparation of the hard carbon and graphene composite material.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of micro-nano material manufacturing, and relates to a hard carbon / graphene composite material and a preparation method thereof. BACKGROUND

[0002] Layered carbon materials, as a kind of two-dimensional material with highly adjustable structure, have been widely used in the field of energy storage. Although natural graphite has been used as a commercial negative electrode, it has poor performance in potassium ion storage. Therefore, researchers have used various methods to regulate the microcrystalline structure of layered carbon materials to improve the potassium storage performance, such as using various heat treatment techniques, physical or chemical methods to prepare carbon materials or composite carbon materials. However, these methods have problems in the practical application of preparing carbon materials, such as the inability to balance the preparation efficiency and potassium storage performance. For example, the products prepared by some heat treatment methods have high potassium storage performance, but the preparation time often takes several hours or even a day. Conventional experimental techniques are time-consuming and energy-consuming, and cannot meet the urgent need for high efficiency and energy saving. As an energy medium, microwave has the characteristics of efficient energy transmission, and microwave carbonization technology can be used to quickly prepare carbon materials. However, the microwave method has high requirements for the quality of carbon sources, and the performance of the products is not satisfactory at present. Therefore, it is still necessary to further design experimental processes to achieve the purpose of quickly and efficiently preparing carbon materials with excellent performance, so as to meet the actual application requirements. SUMMARY

[0003] In view of the problems in the prior art, the present application provides a hard carbon / graphene composite material and a preparation method thereof, thereby solving the technical problem that the preparation efficiency and potassium storage performance cannot be balanced.

[0004] The present application is realized by the following technical solutions:

[0005] A preparation method of a hard carbon / graphene composite material, comprising the following steps:

[0006] S1: graphite is used as an anode, platinum is used as a cathode, and the graphite and the platinum are placed in an electrolyte H2SO4 solution to perform electrolysis, so as to prepare a graphene nanosheet suspension;

[0007] S2: the graphene nanosheet suspension is centrifuged and washed to neutral, then emulsified, and then paper towels are immersed in the suspension and ultrasonicated, so as to obtain paper adsorbed with graphene nanosheets;

[0008] S3: the paper adsorbed with graphene nanosheets is dried, and then subjected to microwave irradiation treatment under the protection of an argon atmosphere, so as to prepare the hard carbon / graphene composite material.

[0009] Preferably, in step S1, the concentration of the H2SO4 solution is 0.5 mol / L.

[0010] Preferably, in step S1, when the electrolysis process is performed, the electrolysis current is 0.1-0.15 A, and the electrolysis time is 10-20 min; after electrolysis, the concentration of the graphene suspension is adjusted by centrifugal treatment.

[0011] Preferably, in step S2, the emulsification treatment is performed for 10-20 min, and the ultrasonic treatment is performed for 10-20 min.

[0012] Preferably, in step S2, the paper comprises lignin.

[0013] Preferably, in step S3, when the paper adsorbed with graphene nanosheets is dried, the drying temperature is 50-70 DEG C.

[0014] Preferably, in step S3, when the microwave irradiation treatment is performed, the microwave power is 500-1000 W, and the irradiation time is 5-15 s.

[0015] A hard carbon / graphene composite material is prepared by the above method; the hard carbon / graphene composite material comprises nitrogen and sulfur elements.

[0016] A negative electrode material comprises the above hard carbon / graphene composite material.

[0017] A potassium ion button cell comprises the above negative electrode material; the potassium ion button cell has a reversible capacity of 230-330 mAh g -1 after the first cycle activation under a current density of 100 mA g -1 .

[0018] Compared with the prior art, the present application has the following beneficial technical effects:

[0019] A preparation method of a hard carbon / graphene composite material, graphene suspension is prepared by electrolysis, and further emulsification treatment is performed to obtain thin-layer high-crystallinity graphene sheet. Then, graphene is introduced to the surface of paper, and microwave treatment is performed. In the process, graphene absorbs microwave to generate high-frequency plasma electromagnetic waves, thereby promoting the rapid absorption of microwave energy by the whole sample, generating an electric breakdown phenomenon in the sample to complete carbonization. The method of microwave carbonization by composite graphene realizes the carbonization of paper by microwave, so that the two effectively form a composite joint structure. At the same time, the thin-layer high-crystallinity graphene promotes the improvement of the potassium storage performance, and the obtained material has a capacity of 332.5 mAh g -1 under a current density of 0.1 Ag -1The reversible capacity of the prepared hard carbon / graphene composite material is 650mAh / g, which is higher than that of the traditional carbonization method, and the prepared hard carbon / graphene composite material has better rate performance and higher cycle stability than the electrode material prepared by the traditional carbonization method. The method has high efficiency, reasonable design and proper structure regulation, and thin-layer graphene is used as an intermediate reactor to quickly prepare the hard carbon / graphene composite material. The technical problem that the preparation efficiency and the potassium storage performance cannot be considered together is solved by being combined with thin-layer high-crystallinity graphene. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced as follows. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0021] Figure 1 It is a flowchart of the hard carbon / graphene composite material preparation method in the present application.

[0022] Figure 2 It is an atomic force microscope picture of the graphene prepared in Example 1 of the present application.

[0023] Figure 3 It is a scanning electron microscope picture of the hard carbon / graphene composite material prepared in Example 1 of the present application.

[0024] Figure 4 It is a transmission electron microscope picture of the hard carbon / graphene composite material prepared in Example 1 of the present application.

[0025] Figure 5 It is a high-magnification transmission electron microscope picture of the hard carbon / graphene composite material prepared in Example 1 of the present application.

[0026] Figure 6 It is a transmission electron microscope picture and element content data of the hard carbon / graphene composite material prepared in Example 1 of the present application.

[0027] Figure 7 It is a sulfur element X-ray photoelectron spectrum of the hard carbon / graphene composite material prepared in Example 1 of the present application.

[0028] Figure 8 It is a nitrogen element X-ray photoelectron spectrum of the hard carbon / graphene composite material prepared in Example 1 of the present application.

[0029] Figure 9 It is a rate performance picture of the hard carbon / graphene composite material prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0030] To enable persons skilled in the art to understand the features and effects of the present application, the following is a general description and definition of the terms and phrases mentioned in the specification and claims. Unless otherwise specified, all technical and scientific words used herein have their usual meanings to those skilled in the art of the present application, and in the event of conflict, the definitions contained herein shall prevail.

[0031] Theories or mechanisms described and disclosed herein, whether correct or not, should not be considered limiting on the scope of the present application, i.e., the present application can be practiced without regard to any particular theory or mechanism.

[0032] Herein, all features defined in the form of numerical ranges or percentage ranges, such as numerical values, amounts, contents and concentrations, are for the sake of brevity and convenience only. Accordingly, the description of numerical ranges or percentage ranges should be considered to have encompassed and specifically disclosed all possible sub-ranges and individual numerical values (including integers and fractions) within the range.

[0033] Herein, unless otherwise specified, "comprise", "include", "contain", "have" or similar words encompass the meaning of "consist of" and "consist essentially of", for example, "A comprises a" encompasses the meaning of "A comprises a and other" and "A comprises only a".

[0034] Herein, for the sake of brevity, all possible combinations of the various technical features in the various embodiments or examples are not described. Therefore, as long as the combinations of the technical features do not contradict each other, the technical features in the various embodiments or examples can be combined arbitrarily, and all possible combinations should be considered as the scope of the present specification.

[0035] As shown in Figure 1 The present application provides a preparation method of a hard carbon / graphene composite material, comprising the following steps:

[0036] S1: graphite is used as an anode, platinum is used as a cathode, and they are placed in an electrolyte H2SO4 solution to perform electrolysis, thereby obtaining a graphene nanosheet suspension; during the electrolysis process, the electrolysis current is 0.1-0.15 A, and the electrolysis time is 10-20 min, which can ensure that the graphene sheet layer is exfoliated.

[0037] S2: after the electrolysis, the concentration of the graphene suspension is adjusted by centrifugation and adjusted to neutral. Further emulsification treatment is performed for 10-20 min, which can ensure that the graphene sheet layer exfoliated by electrolysis is fully opened, which is conducive to the formation of high-crystallinity thin-layer graphene. Then, a paper towel is immersed in the suspension and ultrasonicated, thereby obtaining a paper fully adsorbed with graphene nanosheets; when the paper adsorbed with graphene nanosheets is subjected to drying treatment, the drying temperature is 50-70°C.

[0038] S3: The graphene-composite paper is placed in an argon-filled and sealed glass bottle, and microwave irradiation treatment is performed at a microwave power of 500-1000 W for 5-15 s. During the microwave treatment, the high-crystalline thin-layer graphene absorbs microwave energy and generates plasma resonance, producing excitation in the visible light band, thereby achieving rapid carbonization of the paper. The thin-layer high-crystalline graphene helps to rapidly convert microwave energy, thereby improving efficiency. At the same time, the thin-layer high-crystalline graphene helps to improve the potassium storage capacity and rate, thereby achieving a balance between preparation efficiency and potassium storage performance.

[0039] The application provides a preparation method of a high-potassium-storage-performance hard carbon / graphene composite material, which uses common paper in life to help form a product with impurity element doping. The thin-layer high-crystalline graphene prepared by electrochemical exfoliation and emulsification process can promote microwave energy conversion efficiency and improve the potassium storage performance.

[0040] The hard carbon / graphene composite material prepared by the application has a surface thin-layer high-crystalline graphene and a hard carbon with impurity element doping, mainly nitrogen doping and sulfur doping, and the structure is beneficial to potassium ion adsorption, thereby improving the energy storage capacity.

[0041] In addition, the hard carbon / graphene composite material prepared by the application is made into a negative electrode material and used in a potassium ion button cell. The potassium ion button cell has a reversible capacity of 230-330 mAh·g-1 at a current density of 100 mA·g-1 after the first cycle activation. -1 -1

[0042] The application will be further described in conjunction with specific examples. It should be understood that the examples are only used to illustrate the application but not to limit the scope of the application. Furthermore, it should be understood that those skilled in the art can make various modifications or changes to the application after reading the content of the application, and these equivalent forms also fall within the scope of the appended claims of the application.

[0043] The following examples use conventional instruments and equipment in the art. The experimental methods in the following examples not specifically indicated are usually carried out under conventional conditions or under conditions recommended by the manufacturer. Various raw materials are used in the following examples, and unless otherwise specified, conventional commercially available products are used, and the specifications are conventional specifications in the art. In the specification of the application and the following examples, unless otherwise specified, "%" means weight percent, "parts" means weight parts, and the ratio means weight ratio.

[0044] Example 1

[0045] ​​A method for preparing a hard carbon / graphene composite material includes the following steps: graphene nanosheets are prepared by electrochemical exfoliation using a constant current of 0.1 A. Then, the graphene is washed with ethanol and deionized water by centrifugation, followed by emulsification for 15 minutes to obtain a graphene nanosheet suspension. Waste paper is then immersed in 20 mL of the graphene suspension (2.5 mg / mL). -1 The graphene-adsorbed black waste paper was then ultrasonically treated for 10 minutes. Subsequently, it was dried overnight at 70°C. The graphene-containing waste paper was placed in a 500mL beaker, which was then filled with argon gas and sealed in a glove box. The beaker was removed and placed in a microwave oven, where it was treated at high power (800W) for 10 seconds. Under microwave radiation, strong flashes and smoke were observed. A fluffy black product was then obtained, representing the hard carbon / graphene composite material.

[0046] The graphene structure obtained in this embodiment is as follows: Figure 2 As shown, atomic force microscopy images indicate that it has fewer than 5 layers and a smooth surface, thus a thin layer of highly crystalline graphene material was obtained through electrolysis and emulsification.

[0047] The scanning electron micrograph of the hard carbon / graphene composite material prepared in this embodiment is shown in the figure below. Figures 3-5 As shown, by Figure 3 It can be seen that the hard carbon / graphene composite material exhibits a fibrous morphology. (From...) Figure 4 and 5 It can be seen that the surface of the hard carbon fiber is coated with a thin layer of graphene, and the graphene has no obvious wrinkles, indicating that the graphene still maintains high crystallinity after composite preparation. The doping of other elements in the hard carbon / graphene composite material prepared in this embodiment is as follows: Figures 6-8 As shown, the hard carbon and graphene composite material prepared in this embodiment contains impurity elements other than carbon, including oxygen, sulfur, nitrogen, etc., which are derived from other elements besides carbon in the paper precursor and introduced during the processing. Thus, the product is a carbon composite material doped with impurity elements.

[0048] The hard carbon / graphene composite material obtained in the examples was used as a negative electrode material, and then fabricated into a potassium-ion coin cell. The rate performance of this potassium-ion coin cell was tested as follows: Figure 9 As shown in the figure, this potassium-ion coin cell exhibits performance at 100 mA·g -1 At a current density of 330 mAh·g, the reversible capacity after the first activation cycle is 330 mAh·g. -1 After cycling at different current densities, at 1500 mA·g -1 The reversible capacity at the given current density is 122 mAh g. -1 .

[0049] Example 2

[0050] A method for preparing a hard carbon / graphene composite material includes the following steps: graphene nanosheets are prepared by an electrochemical exfoliation method at 0.15 A. Then, the graphene is washed with ethanol and deionized water and emulsified for 10 minutes to obtain a graphene nanosheet suspension. Waste paper is then immersed in 20 mL of the graphene suspension (2.5 mg / mL). -1 The graphene-adsorbed black waste paper was then ultrasonically treated for 20 minutes. Subsequently, it was dried overnight at 60°C. The graphene-containing waste paper was placed in a 500mL beaker, which was then filled with argon gas and sealed in a glove box. The beaker was removed and placed in a microwave oven, where it was treated at high power (1000W) for 5 seconds. Under microwave radiation, strong flashes and smoke were observed. A fluffy black product was then obtained, representing the hard carbon / graphene composite material.

[0051] The hard carbon / graphene composite material obtained in the examples was used as a negative electrode material and then fabricated into a potassium-ion coin cell. This potassium-ion coin cell exhibited a performance of 100 mA·g. -1 At a current density of 280 mAh·g, the reversible capacity after the first activation cycle is 280 mAh·g. -1 After cycling at different current densities, at 1500 mA·g -1 The reversible capacity at the current density is 75 mAh g. -1 .

[0052] Example 3

[0053] A method for preparing a hard carbon / graphene composite material includes the following steps: graphene nanosheets are prepared by an electrochemical exfoliation method at 0.1A. The graphene is washed with ethanol and deionized water, and then emulsified for 20 minutes to obtain a graphene nanosheet suspension. Waste paper is immersed in 20 mL of the graphene suspension (2.5 mg / mL). -1 The graphene-adsorbed black waste paper was then ultrasonically treated for 15 minutes. Subsequently, the black waste paper was dried overnight at 60°C. The graphene-containing waste paper was placed in a 500mL beaker, which was then filled with argon gas and sealed in a glove box. The beaker was removed and placed in a microwave oven, where it was treated at high power (800W) for 15 seconds. Under microwave radiation, strong flashes and smoke were observed. A fluffy black product was then obtained, which was the hard carbon / graphene composite material.

[0054] The hard carbon / graphene composite material obtained in the examples was used as a negative electrode material and then fabricated into a potassium-ion coin cell. This potassium-ion coin cell exhibited a performance of 100 mA·g. -1 At a current density of 296 mAh·g, the reversible capacity after the first activation cycle is 296 mAh·g. -1 After cycling at different current densities, at 1500 mA·g -1 The reversible capacity at the current density is 105 mAh g. -1.

[0055] Example 4

[0056] A method for preparing a hard carbon / graphene composite material includes the following steps: graphene nanosheets are prepared by an electrochemical exfoliation method at 0.12 A. Then, the graphene is washed with ethanol and deionized water to obtain a graphene nanosheet suspension. Waste paper is then immersed in 20 mL of the graphene suspension (2.5 mg / mL). -1 The graphene-adsorbed black waste paper was then ultrasonically treated for 10 minutes. Subsequently, it was dried overnight at 60°C. The graphene-containing waste paper was placed in a 500mL beaker, which was then filled with argon gas and sealed in a glove box. The beaker was removed and placed in a microwave oven, where it was treated at high power (800W) for 10 seconds. Under microwave radiation, strong flashes and smoke were observed. A fluffy black product was then obtained, representing the hard carbon / graphene composite material.

[0057] The hard carbon / graphene composite material obtained in the examples was used as a negative electrode material and then fabricated into a potassium-ion coin cell. This potassium-ion coin cell exhibited a performance of 100 mA·g. -1 At a current density of 308 mAh·g, the reversible capacity after the first activation cycle is 308 mAh·g. -1 After cycling at different current densities, at 1500 mA·g -1 The reversible capacity at the given current density is 113 mAh g. -1 .

[0058] Example 5

[0059] A method for preparing a hard carbon / graphene composite material includes the following steps: graphene nanosheets are prepared by an electrochemical exfoliation method at 0.1A. Then, the graphene is washed with ethanol and deionized water to obtain a graphene nanosheet suspension. Waste paper is then immersed in 20 mL of the graphene suspension (2.5 mg / mL). -1 The graphene-adsorbed black waste paper was then ultrasonically treated for 10 minutes. Subsequently, the black waste paper was dried overnight at 60°C. The graphene-containing waste paper was placed in a 500mL beaker, which was then filled with argon gas and sealed in a glove box. The beaker was removed and placed in a microwave oven, where it was treated at high power (500W) for 15 seconds. Under microwave radiation, strong flashes and smoke were observed. A fluffy black product was then obtained, which was the hard carbon / graphene composite material.

[0060] The hard carbon / graphene composite material obtained in the examples was used as a negative electrode material and then fabricated into a potassium-ion coin cell. This potassium-ion coin cell exhibited a performance of 100 mA·g. -1 At a current density of 230 mAh·g, the reversible capacity after the first activation cycle is 230 mAh·g. -1After cycling at different current densities, at 1500 mA·g -1 The reversible capacity at the current density is 70 mAh g. -1 .

[0061] Example 6

[0062] A method for preparing a hard carbon / graphene composite material includes the following steps:

[0063] S1: Using graphite as the anode and platinum sheet as the cathode, the solution is placed in a 0.5 mol / L H2SO4 electrolyte solution for electrolysis to obtain a graphene nanosheet suspension; the electrolysis current is 0.15 A and the electrolysis time is 10 min.

[0064] S2: After electrolysis, the pH of the graphene suspension is adjusted to neutral by centrifugation and washing, and then further emulsified for 10 minutes. The paper towel is then immersed in the suspension and sonicated to obtain paper with graphene nanosheets adsorbed.

[0065] S3: The paper with graphene nanosheets adsorbed is dried at 50°C, and then the paper with graphene composite is placed in a sealed glass bottle filled with argon gas and subjected to microwave irradiation treatment. The microwave power is 500W and the irradiation time is 15s to obtain hard carbon / graphene composite material.

[0066] The hard carbon / graphene composite material prepared in this embodiment was used as a negative electrode material in a potassium-ion coin cell. This potassium-ion coin cell achieved a performance of 100 mA·g. -1 At a current density of 330 mAh·g, the reversible capacity after the first activation cycle is 330 mAh·g. -1 .

[0067] Example 7

[0068] A method for preparing a hard carbon / graphene composite material includes the following steps:

[0069] S1: Using graphite as the anode and platinum sheet as the cathode, the solution is placed in a 0.5 mol / L H2SO4 electrolyte solution for electrolysis to obtain a graphene nanosheet suspension; the electrolysis current is 0.12 A and the electrolysis time is 15 min.

[0070] S2: After electrolysis, the pH of the graphene suspension is adjusted to neutral by centrifugation and washing, and further emulsification is carried out for 15 minutes. Then, the paper towel is immersed in the suspension and sonicated to obtain paper with graphene nanosheets adsorbed.

[0071] S3: The paper with graphene nanosheets adsorbed is dried at 60°C, and then the paper with graphene composite is placed in a sealed glass bottle filled with argon gas and subjected to microwave irradiation treatment. The microwave power is 800W and the irradiation time is 10s to obtain hard carbon / graphene composite material.

[0072] The hard carbon / graphene composite material prepared in this embodiment was used as a negative electrode material in a potassium-ion coin cell. This potassium-ion coin cell achieved a performance of 100 mA·g. -1 At a current density of 320 mAh·g, the reversible capacity after the first activation cycle is 320 mAh·g. -1 .

[0073] Example 8

[0074] A method for preparing a hard carbon / graphene composite material includes the following steps:

[0075] S1: Using graphite as the anode and platinum sheet as the cathode, the graphene nanosheet suspension is prepared by electrolysis in a 0.5 mol / L H2SO4 electrolyte solution; the electrolysis current is 0.1-0.15 A and the electrolysis time is 20 min.

[0076] S2: After electrolysis, the pH of the graphene suspension is adjusted to neutral by centrifugation and washing, and then further emulsified for 20 minutes. The paper towel is then immersed in the suspension and sonicated to obtain paper with graphene nanosheets adsorbed.

[0077] S3: The paper with graphene nanosheets adsorbed is dried at 50°C, and then the paper with graphene composite is placed in a sealed glass bottle filled with argon gas and subjected to microwave irradiation treatment. The microwave power is 1000W and the irradiation time is 5s to obtain hard carbon / graphene composite material.

[0078] The hard carbon / graphene composite material prepared in this embodiment was used as a negative electrode material in a potassium-ion coin cell. This potassium-ion coin cell achieved a performance of 100 mA·g. -1 At a current density of 220 mAh·g, the reversible capacity after the first activation cycle is 220 mAh·g. -1 .

[0079] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for producing a hard carbon / graphene composite material, characterized by, The method comprises the following steps: S1: graphite is used as an anode and platinum is used as a cathode, which are placed in an electrolyte H2SO4 solution to perform electrolysis, so as to obtain a graphene nanosheet suspension; S2: the graphene nanosheet suspension is centrifuged and washed to be neutral, then emulsification treatment is performed, then paper towels are immersed in the suspension and ultrasonic treatment is performed, so as to obtain paper adsorbed with graphene nanosheets; S3: the paper adsorbed with graphene nanosheets is dried, then microwave irradiation treatment is performed under the protection of an argon atmosphere, so as to obtain the hard carbon / graphene composite material; In step S1, when the electrolysis process is performed, the electrolysis current is 0.1-0.15 A, the electrolysis time is 10-20 min, and after the electrolysis is completed, the concentration of the graphene suspension is adjusted through centrifugal treatment; In the step S2, the emulsification treatment is performed for 10-20 min, and the ultrasonic treatment is performed for 10-20 min; In the step S3, when the microwave irradiation treatment is performed, the microwave power is 500-1000 W, and the irradiation time is 5-15 s.

2. The method for preparing the hard carbon / graphene composite material according to claim 1, characterized in that, In step S1, the concentration of the H2SO4 solution is 0.5 mol / L.

3. The method for preparing the hard carbon / graphene composite material according to claim 1, characterized in that, In step S2, the components of the paper include lignin.

4. The method for preparing the hard carbon / graphene composite material according to claim 1, characterized in that, In the step S3, when the paper adsorbed with graphene nanosheets is dried, the drying temperature is 50-70 DEG C.

5. A hard carbon / graphene composite material, characterized by, The hard carbon / graphene composite material is prepared by the method in any one of claims 1-4, and the hard carbon / graphene composite material contains nitrogen and sulfur elements.

6. A negative electrode material characterized by comprising: The hard carbon / graphene composite material in claim 5 is contained.

7. A potassium-ion button cell, characterized by The potassium ion button cell comprises the negative electrode material as claimed in claim 6, and has a reversible capacity of 230-330 mAh·g -1 at a current density of 100 mA·g -1 after the first circle of activation.

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