A cobalt sulfate / hard carbon / three-dimensional graphene composite material and its preparation method and application

Abstract

The present invention discloses a cobalt sulfate / hard carbon / three-dimensional graphene composite material and its preparation method and application. The preparation method includes acidifying GF to obtain GF modified by oxygen-containing functional groups; adding GF modified by oxygen-containing functional groups to a mixed solution of cobalt nitrate and 2-methylimidazole, and after standing, obtaining Co-MOF / GF composite material; pyrolysis treatment is performed on the obtained Co-MOF / GF composite material to obtain Co / C / GF composite material; the obtained Co / C / GF composite material and sulfur powder are placed in a mixed atmosphere of argon and air for high temperature treatment, and after the reaction is completed, cobalt sulfate / hard carbon / three-dimensional graphene composite material is obtained. The method is reasonably designed and easy to operate. The obtained composite material has a multi-level structure, which effectively avoids the agglomeration of nanoparticles and ensures the cycle stability of electrode materials. When the composite material is used as the negative electrode of a sodium ion battery, it has good charge and discharge performance and long cycle life.

Description

Technical Field

[0001] The invention belongs to the technical field of energy storage material development, and relates to a cobalt sulfate / hard carbon / three-dimensional graphene composite material and a preparation method and application thereof. Background Art

[0002] Cobalt sulfate has the advantages of high theoretical capacity, good physical stability, low cost and suitable redox potential, and can be developed as a negative electrode material for sodium ion batteries (SIBs). However, similar to metal compounds, these materials have disadvantages such as volume expansion and slow reaction kinetics during discharge / charge. Studies have shown that composite materials of carbon materials and nano-cobalt sulfate can effectively improve the conductivity of cobalt sulfate electrodes. At the same time, the construction of nanostructures helps to reduce the diffusion length of sodium ions, effectively transfer electrons, and enhance ion mobility to achieve rapid charging and discharging. However, due to the small size and high surface energy of nanomaterials, they are easy to agglomerate, resulting in poor cycle stability when the cobalt sulfate is used as the negative electrode of the battery, affecting the battery performance and limiting the application of cobalt sulfate materials in sodium ion batteries. Summary of the invention

[0003] In view of the problems existing in the prior art, the present invention provides a cobalt sulfate / hard carbon / three-dimensional graphene composite material and a preparation method and application thereof, thereby solving the technical problems in the prior art that cobalt sulfate is easy to agglomerate and has poor cycle stability when used as a negative electrode material for sodium ion batteries.

[0004] The present invention is achieved through the following technical solutions:

[0005] A method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material comprises the following steps:

[0006] S1: Acidification treatment of GF to obtain GF modified with oxygen-containing functional groups;

[0007] S2: adding the GF modified with oxygen-containing functional groups into a mixed solution of cobalt nitrate and 2-methylimidazole, and after standing, obtaining a Co-MOF / GF composite material;

[0008] S3: pyrolyzing the Co-MOF / GF composite material to obtain a Co / C / GF composite material; the carbon in the Co / C / GF composite material is a porous carbon nanosheet array;

[0009] S4: placing the Co / C / GF composite material and sulfur powder in a mixed atmosphere of argon and air for high temperature treatment, and after the reaction is completed, a cobalt sulfate / hard carbon / three-dimensional graphene composite material is obtained.

[0010] Preferably, the acidification process is specifically to place the three-dimensional graphene in a nitric acid aqueous solution with a concentration of 4 to 8 M, and perform hydrothermal treatment at a temperature of 80 to 90° C. for 8 to 12 hours.

[0011] Preferably, the molar concentration ratio of cobalt nitrate to 2-methylimidazole in step S2 is (0.025-0.05):0.4.

[0012] Preferably, the step S2 is left to stand for 2 to 4 hours.

[0013] Preferably, in step S3, the Co-MOF / GF composite material is pyrolyzed in an argon atmosphere at a pyrolysis temperature of 500-600°C.

[0014] Preferably, the temperature of the high temperature treatment in step S4 is 350-550°C.

[0015] Preferably, in step S4, the mass ratio of the Co / C / GF composite material to sulfur powder is 1:(1-3).

[0016] Preferably, in step S4, the sulfur powder is placed at one end of the argon gas inlet.

[0017] A cobalt sulfate / hard carbon / three-dimensional graphene composite material, prepared by the above-mentioned preparation method;

[0018] The cobalt sulfate / hard carbon / three-dimensional graphene composite material has a three-level structure, wherein the primary structure is a three-dimensional flaky graphene skeleton of 2 to 4 cm, the secondary structure is a cobalt sulfate / hard carbon composite sheet structure of 4 to 7 μm, and the tertiary structure is flaky hard carbon and flaky cobalt sulfate of 5 to 10 nm.

[0019] Application of the above-mentioned cobalt sulfate / hard carbon / three-dimensional graphene composite material in sodium ion battery negative electrode material.

[0020] Compared with the prior art, the present invention has the following beneficial technical effects:

[0021] A method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material, using three-dimensional graphene (GF) as a skeleton template, and performing an acid modification treatment on the three-dimensional graphene (GF), wherein the acid modification makes the GF surface carry oxygen-containing functional groups, so as to facilitate the loading of metal ions in the later stage. Then, Co-MOF is grafted on the surface of the GF modified with oxygen-containing functional groups, and then a Co / C / GF composite material is prepared by high-temperature thermal cracking, wherein the carbon is a porous carbon nanosheet array, and finally a cobalt sulfate / hard carbon / three-dimensional graphene (CHG) composite material is constructed by a solid phase sulfurization method. The method is reasonably designed and convenient to operate, and the obtained composite material is a self-fractal structure, that is, it has a multi-level structure, wherein the primary structure is a centimeter-scale (2-4 cm) three-dimensional flaky graphene skeleton, namely GF, the secondary structure is a micrometer-scale (4-7 μm) cobalt sulfate / hard carbon, namely CH, a composite sheet structure, and the tertiary structure is a nanometer-scale (5-10 nm) flaky hard carbon and flaky cobalt sulfate. The hard carbon-coated nanoparticle array structure in the composite material provides more active sites for sodium storage. The unique flake fractal structure is conducive to the migration of ions / electrons and improves the diffusion kinetics. At the same time, the gaps between different layers of the fractal structure provide sufficient space to overcome the large volume expansion during the sodium insertion / extraction process. At the same time, the multi-level structure of different scales effectively avoids the agglomeration of nanoparticles and ensures the cycle stability of the electrode material. When the composite material is used as the negative electrode of the sodium ion battery, it has good charge and discharge performance and long cycle life.

[0022] Furthermore, the acidification process specifically involves placing the three-dimensional graphene in a nitric acid aqueous solution with a concentration of 4 to 8 M, and hydrothermally treating it at a temperature of 80 to 90°C for 8 to 12 hours to ensure that active sites are created on the three-dimensional graphene. If the temperature is too high, an explosion will occur, and mild experimental conditions are selected to make the reaction process safer.

[0023] Furthermore, in step S2, the molar concentration ratio of cobalt nitrate to 2-methylimidazole is (0.025-0.05):0.4. According to the stoichiometric ratio, 2-methylimidazole is in excess in this ratio, which can effectively ensure the complete coordination of the cobalt element in the cobalt nitrate.

[0024] Furthermore, in step S2, the mixture is allowed to stand for 2 to 4 hours to allow the cobalt nitrate and 2-methylimidazole to react completely and fully. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0026] Figure 1 It is a schematic diagram of a process for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material in the present invention;

[0027] Figure 2 The XRD diagrams of CHG prepared in Example 1 of the present invention and CH prepared in the comparative example;

[0028] Figure 3 is the SEM image of CHG prepared in Example 1 of the present invention ( Figure 3 a, b) and TEM image (3c);

[0029] Figure 4 This is a SEM image of CH obtained in the comparative example of the present invention;

[0030] Figure 5 is a TEM image of CHG prepared in Example 1 of the present invention at another magnification;

[0031] Figure 6 It is a synchrotron radiation small angle (SAXS) image of CHG prepared in Example 1 of the present invention and CH prepared in the comparative example;

[0032] Figure 7 is a charge and discharge curve diagram of CHG prepared in Example 1 of the present invention;

[0033] Figure 8 It is a charge and discharge curve diagram of CH prepared in the comparative example of the present invention;

[0034] Fig. 9 The figure is a rate performance diagram of CHG prepared in Example 1 of the present invention and CH prepared in the comparative example at different current densities;

[0035] Fig.10 The graphs are cycle lifespans of CHG prepared in Example 1 of the present invention and CH prepared in the comparative example at different current densities. DETAILED DESCRIPTION

[0036] In order to enable those skilled in the art to understand the characteristics and effects of the present invention, the following is a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used in the text are the common meanings understood by those skilled in the art for the present invention. In the event of a conflict, the definition in this specification shall prevail.

[0037] The theories or mechanisms described and disclosed herein, whether correct or incorrect, should not limit the scope of the present invention in any way, that is, the present invention can be implemented without being limited by any specific theory or mechanism.

[0038] Herein, all features such as values, quantities, contents and concentrations defined in the form of numerical ranges or percentage ranges are for simplicity and convenience only. Accordingly, the description of numerical ranges or percentage ranges should be considered to have included and specifically disclosed all possible secondary ranges and individual values ​​within the range (including integers and fractions).

[0039] In this document, unless otherwise specified, “includes,” “including,” “contains,” “has,” or similar terms cover the meanings of “consisting of” and “mainly consisting of,” for example, “A includes a” covers the meanings of “A includes a and other” and “A only includes a.”

[0040] In this document, in order to make the description concise, not all possible combinations of various technical features in various embodiments or examples are described. Therefore, as long as there is no contradiction in the combination of these technical features, the various technical features in various embodiments or examples can be combined arbitrarily, and all possible combinations should be considered to be within the scope of this specification.

[0041] like Figure 1 As shown, the present invention provides a method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material, which specifically comprises the following steps:

[0042] S1: Acidification treatment is performed on GF to obtain GF modified with oxygen-containing functional groups; the acidification process specifically includes placing the three-dimensional graphene in a nitric acid aqueous solution with a concentration of 4 to 8M, hydrothermally treating it at a temperature of 80 to 90°C for 8 to 12 hours, then washing it with deionized water, and drying it for use. The acidification process allows the oxygen-containing functional groups to be combined on the surface of the GF, forming a hydrophilic structure on the surface, and forming metal ion attachment sites.

[0043] S2: Adding the GF modified with oxygen-containing functional groups to a mixed solution of cobalt nitrate and 2-methylimidazole, and letting it stand for 2 to 4 hours to obtain a Co-MOF / GF composite material; wherein the molar concentration ratio of cobalt nitrate to 2-methylimidazole is (0.025 to 0.05):0.4.

[0044] S3: pyrolyzing the Co-MOF / GF composite material obtained in step S2 at 500-600° C. to obtain a Co / C / GF composite material; the carbon in the Co / C / GF composite material obtained in this step is a porous carbon nanosheet array;

[0045] S4: Place the Co / C / GF composite material obtained in step S3 and sulfur powder in a mixed atmosphere of argon and air at a mass ratio of 1: (1-3) for high temperature treatment. After the reaction is completed, a cobalt sulfate / hard carbon / three-dimensional graphene composite material is obtained. Here, the test is carried out in a mixed atmosphere of argon and air, which effectively ensures the safety of the test. If the oxygen content is too high, the sulfur powder will burn and the test will be unsafe. The specific process is to place the Co / C / GF composite material and sulfur powder at both ends of the porcelain boat, respectively, and put them into a micro high-temperature tube furnace, wherein the sulfur powder is located at one end of the air inlet, and the micro high-temperature tube furnace is not vacuumed. After argon is introduced for 20 minutes, a mixed atmosphere of argon and air is formed, and the temperature is raised to 350-550°C at a heating rate of 2°C / min, and the temperature is kept. Take out the calcined product, and then use 50mL of CS2 solution to wash away the excess sulfur powder. The obtained product is placed in an oven at 60°C for drying to obtain a cobalt sulfate / hard carbon / three-dimensional graphene (CHG) material.

[0046] The preparation process of the three-dimensional graphene (GF) in the present invention is as follows: 4×10 cm of commercial nickel foam is cut, and a graphene oxide solution is directly added thereto until the nickel foam is completely covered. Then it is placed in a vacuum drying oven and dried at 80° C. for 12 hours. Then the dried graphene oxide / nickel mesh is placed in a tubular furnace, heated to 600-800° C. at a heating rate of 2° C. / min, and kept warm for 2-4 hours. After the insulation is completed, the temperature is reduced to 200° C. at a cooling rate of 2° C. / min, and then naturally cooled to room temperature to obtain reduced graphene oxide / nickel mesh (GF(Ni)). Then the obtained GF(Ni) is placed in a solution of 200 mL FeCl3(1M) and 20 mL HCl(1M) to remove the nickel mesh. Specifically, when the reaction solution changes from yellow to green, the solution is replaced, repeated at least three times, and then cleaned with deionized water to obtain a three-dimensional graphene (GF).

[0047] The present invention provides a method for preparing a self-fractal cobalt sulfate / hard carbon / three-dimensional graphene composite electrode material. The method firstly hydrophilizes GF, and then grafts Co-MOF onto the GF skeleton. The method has simple process and low cost, does not require high temperature and high pressure conditions, and saves time and energy. Then, a Co / C / GF composite material is prepared by high-temperature thermal cracking, wherein the carbon is a porous carbon nanosheet array, and finally, a cobalt sulfate / hard carbon / three-dimensional graphene (CHG) composite material is constructed by solid phase sulfurization. The self-supporting CHG composite electrode constructed by the present invention has obvious self-fractal characteristics. The structure is similar to the fractal structure in nature, wherein the primary structure is a three-dimensional flaky graphene skeleton of centimeter scale (2-4 cm), namely GF, the secondary structure is a composite sheet structure of micrometer-scale (4-7 μm) cobalt sulfate / hard carbon, namely CH, and the tertiary structure is a nanometer-scale (5-10 nm) flaky hard carbon and flaky cobalt sulfate. The hard carbon-coated nanoparticle array structure in the composite material provides more active sites for sodium storage. The unique fractal structure is conducive to the migration of ions / electrons and improves the diffusion kinetics. At the same time, the gaps between different layers of the fractal structure provide sufficient space to overcome the large volume expansion during the sodium insertion / extraction process. At the same time, the multi-level structure of different scales effectively avoids the agglomeration of nanoparticles and ensures the cycle stability of the electrode material. When the composite material is used as the negative electrode of the sodium ion battery, it has good charge and discharge performance and long cycle life, which significantly improves the electrochemical properties of the sodium ion battery. The excellent performance of the composite electrode shows that it has a good application prospect in sodium ion batteries.

[0048] The present invention also discloses the use of the obtained cobalt sulfate / hard carbon / three-dimensional graphene composite material in the negative electrode material of sodium ion battery, wherein the negative electrode material has a conductivity of 0.1 mA·g -1 At a current density of 1.5 %, after 50 cycles, the reversible capacity is 220-255 mAh g -1 .

[0049] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0050] The following examples use conventional instruments and equipment in the art. The experimental methods in the following examples where specific conditions are not specified are usually carried out under conventional conditions or under conditions recommended by the manufacturer. The various raw materials used in the following examples are conventional commercial products unless otherwise specified, and their specifications are conventional specifications in the art. In the specification of the present invention and the following examples, unless otherwise specified, "%" means weight percentage, "part" means weight part, and ratio means weight ratio.

[0051] Example 1

[0052] (1) The three-dimensional graphene (GF(Ni)) grown on the nickel mesh prepared in the present invention is cut into small pieces of 2×4 cm, and the cut GF(Ni) is placed in a solution of 200 mL FeCl3 (1M) and 20 ml HCl (1M) to remove the nickel mesh to obtain the three-dimensional graphene (GF). Subsequently, the GF is placed in a reactor containing 60 mL of dilute nitric acid aqueous solution (4M) and subjected to hydrothermal reaction at 90°C for 12 hours. After the acid treatment, the GF is washed with deionized water until it is neutral, and then placed in an oven to dry at 60°C;

[0053] (2) Weigh 0.583g of Co(NO3)2·6H2O and dissolve it in 40mL of deionized water. Weigh 1.314g of 2-methylimidazole and dissolve it in 40mL of deionized water. Stir and dissolve them separately. Then, quickly pour the 2-methylimidazole solution into the Co(NO3)2 solution and ultrasonicate for 1min to obtain a mixed solution. Then, insert the acidified GF obliquely into the mixed solution and let it stand at room temperature for 2h. Then, take out the GF and wash away the Co-MOF that is not grown on the GF surface with deionized water. Finally, put it in a vacuum oven and dry it at 60℃ for 6h to in-situ grow the Co-MOF nanosheet array and obtain the Co-MOF / GF composite material.

[0054] (3) placing the Co-MOF / GF composite material in a high-temperature tube furnace, introducing argon gas for 20 min without evacuating the high-temperature tube furnace under Ar gas, and then heating the temperature to 600 °C at a rate of 2 °C / min and keeping the temperature for 4 h to obtain a Co metal / hard carbon / three-dimensional graphene (Co / C / GF) composite material;

[0055] (4) Co / C / GF composite material and sulfur powder were placed at both ends of the porcelain boat in a mass ratio of 3:1, and then placed in a micro high-temperature tube furnace, where the sulfur powder was located at one end of the air inlet. The micro high-temperature tube furnace was not vacuumed, and argon was introduced for 20 minutes, then the temperature was raised to 450°C at a rate of 2°C / min and kept warm for 3 hours. The calcined product was taken out, and then 50mL of CS2 solution was used to wash away the excess sulfur powder, and then it was placed in an oven at 60°C for drying to obtain cobalt sulfate / hard carbon / three-dimensional graphene (CHG) material.

[0056] The XRD pattern of the cobalt sulfate / hard carbon / three-dimensional graphene material prepared in this embodiment is as follows: Figure 2 As shown, SEM Figure 3 As shown, TEM Figure 4 As shown, SAXS Figure 5 The electrochemical performance is shown in Figure 6 shown.

[0057] Example 2

[0058] (1) Using homemade three-dimensional graphene (GF(Ni)) grown on a nickel mesh, cut it into small pieces of 2×4 cm, and place the cut GF(Ni) in a solution of 200 mL FeCl3 (1M) and 20 mL HCl (1M) to remove the nickel mesh. Subsequently, place the GF in a reactor containing 60 mL of dilute nitric acid aqueous solution (6M) and perform a hydrothermal reaction at 90°C for 12 h. After the acid treatment, wash the GF with deionized water until it is neutral, and place it in an oven to dry at 60°C;

[0059] (2) Weigh 0.291g of Co(NO3)2·6H2O and dissolve it in 40mL of deionized water, which is called solution A; weigh 1.314g of 2-methylimidazole and dissolve it in 40mL of deionized water, which is called solution B. Then, pour solution B into solution A quickly, and ultrasonicate for 1min to obtain a mixed solution; then insert the acidified GF obliquely into the mixed solution, let it stand at room temperature for 4h, then take out the GF, and wash away the Co-MOF that is not grown on the GF surface with deionized water, and finally put it in a vacuum oven and dry it at 60℃ for 6h, so as to in situ grow the Co-MOF nanosheet array and obtain the Co-MOF / GF composite material;

[0060] (3) placing the Co-MOF / GF composite material in a high-temperature tube furnace, introducing argon gas for 20 min without evacuating the high-temperature tube furnace under Ar gas, and then heating the temperature to 600 °C at a rate of 2 °C / min and keeping the temperature for 2 h to obtain a Co metal / hard carbon / three-dimensional graphene (Co / C / GF) composite material;

[0061] (4) The mass ratio of sulfur powder to Co / C / GF is 2:1. The Co / C / GF composite material and sulfur powder are placed at both ends of the porcelain boat, respectively, and placed in a micro high-temperature tube furnace, wherein the sulfur powder is located at one end of the air inlet. The micro high-temperature tube furnace is not vacuumed, and argon is introduced for 20 minutes, then the temperature is increased to 450°C at a rate of 2°C / min, and the temperature is kept for at least 3 hours. The calcined product is taken out, and then 50mL of CS2 solution is used to wash away the excess sulfur powder, and then it is placed in an oven at 60°C for drying to obtain cobalt sulfate / hard carbon / three-dimensional graphene (CHG) material.

[0062] The cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment has a three-level structure, wherein the primary structure is a three-dimensional flake graphene skeleton with an average size of 2.3 cm, the secondary structure is a cobalt sulfate / hard carbon composite sheet structure with an average size of 4.8 μm, and the tertiary structure is an average size of 5.7 nm. When the cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment is used as a negative electrode material for a sodium ion battery, the negative electrode material has a high conductivity at 0.1 mA·g -1 At a current density of , after 50 cycles, the reversible capacity is 220.1 mAh g -1 .

[0063] Example 3

[0064] (1) Use homemade three-dimensional graphene (GF(Ni)) grown on a nickel mesh, cut it into small pieces of 2×4 cm, and place the cut GF(Ni) in a solution of 200 mL FeCl3 (1M) and 20 mL HCl (1M) to remove the nickel mesh. When the reaction solution turns from yellow to green, replace the solution, repeat at least three times, and then rinse it with deionized water. Subsequently, place the treated GF in a reactor containing 60 mL dilute nitric acid (4M) and perform a hydrothermal reaction at 80°C for 10 hours. After the acid treatment, wash the GF with deionized water until it is neutral, and place it in an oven to dry at 60°C;

[0065] (2) Weigh 0.583 g of Co(NO3)2·6H2O and dissolve it in 40 mL of deionized water, which is called solution A; weigh 1.314 g of 2-methylimidazole and dissolve it in 40 mL of deionized water, which is called solution B. Then, pour solution B into solution A quickly, and ultrasonicate for 1 min to obtain a mixed solution; then insert the acidified GF obliquely into the mixed solution, let it stand at room temperature for 2 h, then take out the GF, and wash away the Co-MOF that is not grown on the GF surface with deionized water, and finally put it in a vacuum oven and dry it at 60°C for 6 h, thereby in situ growing a Co-MOF nanosheet array to obtain a Co-MOF / GF composite material;

[0066] (3) placing the Co-MOF / GF composite material in a high-temperature tube furnace, and introducing argon gas for 20 min without evacuating the high-temperature tube furnace under Ar gas, then heating the temperature to 500 °C at a rate of 2 °C / min, and keeping the temperature for 4 h to obtain a Co metal / hard carbon / three-dimensional graphene (Co / C / GF) composite material;

[0067] (4) The mass ratio of sulfur powder to Co / C / GF is 1:1. The Co / C / GF composite material and sulfur powder are placed at both ends of the porcelain boat, respectively, and placed in a micro high-temperature tube furnace, wherein the sulfur powder is located at one end of the air inlet. The micro high-temperature tube furnace is not vacuumed, and argon is introduced for 20 minutes, then the temperature is increased to 450°C at a rate of 2°C / min and kept warm for 3 hours. The calcined product is taken out, and then 50mL of CS2 solution is used to wash away the excess sulfur powder, and then it is placed in an oven at 60°C for drying to obtain cobalt sulfate / hard carbon / three-dimensional graphene (CHG) material.

[0068] The cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment has a three-level structure, wherein the primary structure is a three-dimensional flake graphene skeleton with an average size of 2.5 cm, the secondary structure is a cobalt sulfate / hard carbon composite sheet structure with an average size of 5.3 μm, and the tertiary structure is an average size of 6.5 nm. When the cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment is used as a negative electrode material for a sodium ion battery, the negative electrode material has a high conductivity at 0.1 mA·g -1 At a current density of , after 50 cycles, the reversible capacity is 223.5 mAh g -1 .

[0069] Example 4

[0070] (1) Use homemade three-dimensional graphene (GF(Ni)) grown on a nickel mesh, cut it into small pieces of 2×4 cm, and place the cut GF(Ni) in a solution of 200 mL FeCl3 (1M) and 20 mL HCl (1M) to remove the nickel mesh. When the reaction solution turns from yellow to green, replace the solution, repeat at least three times, and then rinse it with deionized water. Subsequently, place the treated GF in a reactor containing 60 mL dilute nitric acid (4M) and perform a hydrothermal reaction at 85°C for 8 hours. After the acid treatment, wash the GF with deionized water until it is neutral, and place it in an oven to dry at 60°C;

[0071] (2) Weigh 0.583 g of Co(NO3)2·6H2O and dissolve it in 40 mL of deionized water, which is called solution A; weigh 1.314 g of 2-methylimidazole and dissolve it in 40 mL of deionized water, which is called solution B. Then, pour solution B into solution A quickly, and ultrasonicate for 1 min to obtain a mixed solution; then insert the acidified GF obliquely into the mixed solution, let it stand at room temperature for 2 h, then take out the GF, and wash away the Co-MOF that is not grown on the GF surface with deionized water, and finally put it in a vacuum oven and dry it at 60°C for 6 h, thereby in situ growing a Co-MOF nanosheet array to obtain a Co-MOF / GF composite material;

[0072] (3) placing the Co-MOF / GF composite material prepared in step (2) into a high-temperature tube furnace, introducing argon gas for 20 min without evacuating the high-temperature tube furnace, and then heating the temperature to 550° C. at a heating rate of 2° C. / min, and keeping the temperature for 4 h to obtain a Co metal / hard carbon / three-dimensional graphene (Co / C / GF) composite material;

[0073] (4) The mass ratio of sulfur powder to Co / C / GF is 3:1. The Co / C / GF composite material and sulfur powder are placed at both ends of the porcelain boat, respectively, and placed in a micro high-temperature tube furnace, wherein the sulfur powder is located at one end of the air inlet. The micro high-temperature tube furnace is not vacuumed, and argon is introduced for 20 minutes, then the temperature is increased to 450°C at a rate of 2°C / min and kept warm for 3 hours. The calcined product is taken out, and then 50mL of CS2 solution is used to wash away the excess sulfur powder, and then it is placed in an oven at 60°C for drying to obtain cobalt sulfate / hard carbon / three-dimensional graphene (CHG) material.

[0074] The cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment has a three-level structure, wherein the primary structure is a three-dimensional flake graphene skeleton with an average size of 3.0 cm, the secondary structure is a cobalt sulfate / hard carbon composite sheet structure with an average size of 5.8 μm, and the tertiary structure is an average size of 7.3 nm. When the cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment is used as a negative electrode material for a sodium ion battery, the negative electrode material has a high conductivity at 0.1 mA·g -1 At a current density of , after 50 cycles, the reversible capacity is 235.1 mAh g -1 .

[0075] Example 5

[0076] (1) Use homemade three-dimensional graphene (GF(Ni)) grown on a nickel mesh, cut it into small pieces of 2×4 cm, and place the cut GF(Ni) in a solution of 200 mL FeCl3 (1M) and 20 mL HCl (1M) to remove the nickel mesh. When the reaction solution turns from yellow to green, replace the solution, repeat at least three times, and then rinse it with deionized water. Subsequently, place the treated GF in a reactor containing 60 mL dilute nitric acid (8M) and perform a hydrothermal reaction at 90°C for 12 hours. After the acid treatment, wash the GF with deionized water until it is neutral, and place it in an oven to dry at 60°C;

[0077] (2) Weigh 0.583 g of Co(NO3)2·6H2O and dissolve it in 40 mL of deionized water, which is called solution A; weigh 1.314 g of 2-methylimidazole and dissolve it in 40 mL of deionized water, which is called solution B. Then, pour solution B into solution A quickly, and ultrasonicate for 1 min to obtain a mixed solution; then insert the acidified GF obliquely into the mixed solution, let it stand at room temperature for 2 h, then take out the GF, and wash away the Co-MOF that is not grown on the GF surface with deionized water, and finally put it in a vacuum oven and dry it at 60°C for 6 h, thereby in situ growing a Co-MOF nanosheet array to obtain a Co-MOF / GF composite material;

[0078] (3) placing the Co-MOF / GF composite material prepared in step (2) into a high-temperature tube furnace, introducing argon gas for 20 min without evacuating the high-temperature tube furnace, and then heating the temperature to 600 ° C at a rate of 2 ° C / min, and keeping the temperature for 4 h to obtain a Co metal / hard carbon / three-dimensional graphene (Co / C / GF) composite material;

[0079] (4) The mass ratio of sulfur powder to Co / C / GF is 1.5:1. The Co / C / GF composite material and sulfur powder are placed at both ends of the porcelain boat, respectively, and placed in a micro high-temperature tube furnace, wherein the sulfur powder is located at one end of the air inlet. The micro high-temperature tube furnace is not vacuumed, and argon is introduced for 20 minutes, then the temperature is increased to 450°C at a rate of 2°C / min and kept warm for 3 hours. The calcined product is taken out, and then 50mL of CS2 solution is used to wash away the excess sulfur powder, and then it is placed in an oven at 60°C for drying to obtain cobalt sulfate / hard carbon / three-dimensional graphene (CHG) material.

[0080] The cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment has a three-level structure, wherein the primary structure is a three-dimensional flake graphene skeleton with an average size of 3.7 cm, the secondary structure is a cobalt sulfate / hard carbon composite sheet structure with an average size of 6.3 μm, and the tertiary structure is an average size of 8.5 nm. When the cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment is used as a negative electrode material for a sodium ion battery, the negative electrode material has a high conductivity at 0.1 mA·g -1 At a current density of , after 50 cycles, the reversible capacity is 245.6 mAh g -1 .

[0081] Comparative Example

[0082] (1) First, weigh 3.638g of Co(NO3)2·6H2O and dissolve it in 250mL of deionized water, which is called solution A. Then weigh 8.210g of 2-methylimidazole and dissolve it in 250mL of deionized water, which is called solution B. Then, quickly pour solution B into solution A and ultrasonicate for 1min to obtain a mixed solution. After standing at room temperature for 2h, centrifuge with deionized water and ethanol three times (10000r, 10min), and then dry in an oven at 60℃ for 6h to obtain purple Co-MOF powder for later use.

[0083] (2) placing the Co-MOF powder composite material prepared in step (1) into a high-temperature tube furnace, without evacuating the high-temperature tube furnace, introducing argon for 20 minutes, and then heating the temperature to 600° C. at a rate of 2° C. / min, and keeping the temperature for 4 hours to obtain a Co metal / hard carbon (Co / C) composite material. Set aside;

[0084] (3) The mass ratio of sulfur powder to Co / C is 3:1. The Co / C composite material and sulfur powder are placed at both ends of a porcelain boat and placed in a high-temperature tube furnace, where the sulfur powder is located at one end of the air inlet. Solid phase sulfurization treatment is carried out under Ar gas, and the temperature is increased to 450°C at a heating rate of 2°C / min and kept warm for at least 3h. Then, 50mL of CS2 solution is used to wash away the excess sulfur powder, and the cobalt sulfate / hard carbon (CH) composite material is obtained by drying at 60°C in an oven.

[0085] The XRD patterns of CHG and CH prepared in Example 1 and Comparative Example are as follows: Figure 2 As shown. Figure 2 It can be seen that the diffraction peak of the comparative sample CH corresponds to the (1 1 -2)(-2 0 0)(0 2 -2) of the PDF card of cobalt sulfate, indicating the successful synthesis of cobalt sulfate. For the CHG composite material, the diffraction peak of the graphene (002) crystal plane appears, indicating the successful synthesis of CHG.

[0086] The SEM and TEM images of CHG prepared in Example 1 are as follows: Figure 3 As shown. Figure 3 It can be seen that the self-fractal CHG electrode material is a flaky layered three-level structure, in which the primary structure is a three-dimensional flaky graphene skeleton with an average size of 2 cm, the secondary structure is a cobalt sulfate / hard carbon composite flaky structure with an average size of 4 μm, and the tertiary structure is a flaky hard carbon and flaky cobalt sulfate with an average size of 5 nm. This structure will be beneficial to the improvement of battery performance.

[0087] The SEM of CH prepared in the comparative example is as follows Figure 4 As shown. Figure 4 It can be seen that the product of CH exhibits an irregular morphology during the preparation process because there is no graphene skeleton support.

[0088] TEM of CHG prepared in Example 1 is as follows Figure 5 As shown. Figure 5 It can be seen that the tertiary structure of CHG is composed of smaller cobalt sulfate sheets and hard carbon sheets.

[0089] The SAXS spectra of the CHG and CH composite materials obtained in Example 1 and the comparative example are as follows: Figure 6 As shown. Synchrotron radiation small angle X-ray scattering (SAXS) method is widely used to study the fractal structure of irregular objects and obtain quantitative structural parameters. If the system has a fractal structure, its structural characteristics must be reflected in the scattering intensity, and the fractal dimension can be easily obtained from the slope of the measured intensity as a function of the wave vector in the log-log coordinate system. If a material has obvious self-fractal characteristics, it will show a fractal dimension. Therefore, by Figure 5 It can be seen that compared with CH material, CHG has obvious self-fractal characteristics.

[0090] The multiple charge and discharge curves of the CHG electrode material prepared in Example 1 are as follows: Figure 7 As shown. At 0.05A g -1 At a current density of 1.34 Å, the first charge and discharge capacities of CHG were 357.5 and 366.9 mAh·g, respectively. -1 , the first coulombic efficiency (ICE) was as high as 97.4%. In the following two cycles, the discharge / charge curves were highly overlapped, and the coulombic efficiency immediately increased to more than 99%, indicating excellent reversibility. This is because the CHG electrode has many active sites and a multi-level fractal structure, which makes the material more stable and conducive to the reversible reaction.

[0091] The multiple charge and discharge curves of the CH electrode material prepared in the comparative example are as follows: Figure 8 As shown. Figure 8 It can be seen that compared with CHG, CH has only a first coulombic efficiency of 55.88%, and the capacity decays significantly in the first three cycles. This is because the structure of CH is collapsed and irregular, with fewer active stable points, poor conductivity, and cannot alleviate the volume expansion during the charge and discharge process.

[0092] The rate performance of CHG prepared in Example 1 and CH prepared in the comparative example is as follows: Fig. 9 As shown. Fig. 9 It can be seen that the CHG prepared in Example 1 has a current density of 0.05, 0.1, 0.2, 0.5 and 1.0 A·g -1 When the specific capacities of CHG are 345.5, 276.3, 249.6, 202.8 and 146.1 mAh·g -1 When the current density returns to 0.05A·g -1 When the discharge capacity is restored to 255.4 mAh g-1 This excellent rate performance is due to the unique flake-like fractal structure and porous channels of CHG, which are conducive to the migration of ions / electrons, improve the diffusion kinetics, and structural stability.

[0093] The cycle performance of CHG prepared in Example 1 and CH prepared in the comparative example is as follows: Fig.10 As shown. Fig.10 It can be seen that CHG at 0.1A·g -1 After 50 cycles at a current density of 1.34 W, the discharge capacity remained at 228.9 mAh g -1 , while the discharge capacity of CH electrode dropped to 32.5 mAh g -1 This is because the gaps between different layers of the fractal structure provide sufficient space to overcome the large volume expansion during the sodium insertion / extraction process. At the same time, the fractal structure makes the material more stable and not easy to agglomerate, thus ensuring the stability of the electrode material. When used as a negative electrode material for sodium ion batteries, CHG electrode materials have good charge and discharge performance and long cycle life. At the same time, Fig.10 It can be seen that the initial coulombic efficiency of CHG is 97.2%, and it remains at around 100% in the subsequent cycles. The coulombic efficiency of CH is 81.94%, and it even drops to 77.52% in the second cycle. In comparison, the high and stable coulombic efficiency of CHG is mainly due to the unique fractal structure and the addition of three-dimensional graphene. The fractal structure can ensure that CHG continues to charge and discharge without damage during the cycle, ensuring the stability of the coulombic efficiency during the entire cycle; secondly, the high coulombic efficiency of the three-dimensional graphene itself drives the coulombic efficiency of the CHG overall electrode, so compared with CH powder, CHG shows better coulombic efficiency.

[0094] Example 6

[0095] A method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material, specifically comprising the following steps:

[0096] S1: Acidification treatment is performed on GF to obtain GF modified with oxygen-containing functional groups; the acidification process is specifically to place the three-dimensional graphene in a 4M nitric acid aqueous solution, hydrothermally treat it at a temperature of 80° C. for 8 hours, then wash it with deionized water, and dry it for use.

[0097] S2: Adding GF modified with oxygen-containing functional groups to a mixed solution of cobalt nitrate and 2-methylimidazole, and letting it stand for 2 hours, a Co-MOF / GF composite material is obtained; wherein the molar concentration ratio of cobalt nitrate to 2-methylimidazole is 0.025:0.4.

[0098] S3: pyrolyzing the Co-MOF / GF composite material at 500°C to obtain a Co / C / GF composite material;

[0099] S4: The Co / C / GF composite material and sulfur powder were placed in a mixed atmosphere of argon and air in a mass ratio of 1:1 and treated at 350°C. After the reaction was completed, 50 mL of CS2 solution was used to wash away excess sulfur powder, and the obtained product was placed in an oven and dried at 60°C to obtain cobalt sulfate / hard carbon / three-dimensional graphene (CHG) material.

[0100] The cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment has a three-level structure, wherein the primary structure is a three-dimensional flake graphene skeleton with an average size of 4 cm, the secondary structure is a cobalt sulfate / hard carbon composite sheet structure with an average size of 6.5 μm, and the tertiary structure is an average size of 9.5 nm. When the cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment is used as a negative electrode material for a sodium ion battery, the negative electrode material has a high conductivity at 0.1 mA·g -1 At a current density of , after 50 cycles, the reversible capacity is 255.6 mAh g -1 .

[0101] Example 7

[0102] A method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material, specifically comprising the following steps:

[0103] S1: Acidification treatment is performed on GF to obtain GF modified with oxygen-containing functional groups; the acidification process is specifically to place the three-dimensional graphene in a 4.5M nitric acid aqueous solution, hydrothermally treat it at a temperature of 85°C for 9.5h, then wash it with deionized water, and dry it for use.

[0104] S2: Adding the GF modified with oxygen-containing functional groups to a mixed solution of cobalt nitrate and 2-methylimidazole, and letting it stand for 2.5 hours, a Co-MOF / GF composite material is obtained; wherein the molar concentration ratio of cobalt nitrate to 2-methylimidazole is 0.03:0.4.

[0105] S3: pyrolyzing the Co-MOF / GF composite material at 550°C to obtain a Co / C / GF composite material;

[0106] S4: The Co / C / GF composite material and sulfur powder were placed in a mixed atmosphere of argon and air at a mass ratio of 1:1.5 and treated at 370°C. After the reaction was completed, 50 mL of CS2 solution was used to wash away excess sulfur powder, and the obtained product was placed in an oven and dried at 60°C to obtain cobalt sulfate / hard carbon / three-dimensional graphene (CHG) material.

[0107] The cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment has a three-level structure, wherein the primary structure is a three-dimensional flake graphene skeleton with an average size of 4 cm, the secondary structure is a cobalt sulfate / hard carbon composite sheet structure with an average size of 7 μm, and the tertiary structure is an average size of 10 nm. When the cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment is used as a negative electrode material for a sodium ion battery, the negative electrode material has a high conductivity at 0.1 mA·g -1 At a current density of , after 50 cycles, the reversible capacity is 254.3 mAh g -1 .

[0108] Example 8

[0109] A method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material, specifically comprising the following steps:

[0110] S1: Acidification treatment is performed on GF to obtain GF modified with oxygen-containing functional groups; the acidification process is specifically to place the three-dimensional graphene in a 6M nitric acid aqueous solution, hydrothermally treat it at a temperature of 90° C. for 10 hours, then wash it with deionized water, and dry it for use.

[0111] S2: Adding the GF modified with oxygen-containing functional groups to a mixed solution of cobalt nitrate and 2-methylimidazole, and letting it stand for 2.5 hours, a Co-MOF / GF composite material is obtained; wherein the molar concentration ratio of cobalt nitrate to 2-methylimidazole is 0.035:0.4.

[0112] S3: pyrolyzing the Co-MOF / GF composite material at 570°C to obtain a Co / C / GF composite material;

[0113] S4: The Co / C / GF composite material and sulfur powder were placed in a mixed atmosphere of argon and air at a mass ratio of 1:2.3 and treated at 410°C. After the reaction was completed, 50 mL of CS2 solution was used to wash away excess sulfur powder, and the obtained product was placed in an oven and dried at 60°C to obtain cobalt sulfate / hard carbon / three-dimensional graphene (CHG) material.

[0114] When the cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment is used as the negative electrode material of a sodium ion battery, the negative electrode material has a conductivity of 0.1 mA·g -1 At a current density of , after 50 cycles, the reversible capacity is 242.3 mAh g -1 .

[0115] Example 9

[0116] A method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material, specifically comprising the following steps:

[0117] S1: Acidification treatment is performed on GF to obtain GF modified with oxygen-containing functional groups; the acidification process is specifically to place the three-dimensional graphene in an aqueous solution of 8M nitric acid, hydrothermally treat it at a temperature of 90°C for 12h, then wash it with deionized water, and dry it for use.

[0118] S2: Adding GF modified with oxygen-containing functional groups to a mixed solution of cobalt nitrate and 2-methylimidazole, and standing for 2.5 hours, a Co-MOF / GF composite material is obtained; wherein the molar concentration ratio of cobalt nitrate to 2-methylimidazole is 0.045:0.4.

[0119] S3: pyrolyzing the Co-MOF / GF composite material at 600°C to obtain a Co / C / GF composite material;

[0120] S4: The Co / C / GF composite material and sulfur powder were placed in a mixed atmosphere of argon and air at a mass ratio of 1:2.5 and treated at 450°C. After the reaction was completed, 50 mL of CS2 solution was used to wash away excess sulfur powder, and the obtained product was placed in an oven and dried at 60°C to obtain cobalt sulfate / hard carbon / three-dimensional graphene (CHG) material.

[0121] When the cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment is used as the negative electrode material of a sodium ion battery, the negative electrode material has a conductivity of 0.1 mA·g -1 At a current density of , after 50 cycles, the reversible capacity is 233.5 mAh g -1 .

[0122] Example 10

[0123] A method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material, specifically comprising the following steps:

[0124] S1: Acidification treatment is performed on GF to obtain GF modified with oxygen-containing functional groups; the acidification process is specifically to place the three-dimensional graphene in an aqueous solution of 8M nitric acid, hydrothermally treat it at a temperature of 90°C for 12h, then wash it with deionized water, and dry it for use.

[0125] S2: Adding the GF modified with oxygen-containing functional groups to a mixed solution of cobalt nitrate and 2-methylimidazole, and letting it stand for 2.5 hours, a Co-MOF / GF composite material is obtained; wherein the molar concentration ratio of cobalt nitrate to 2-methylimidazole is 0.05:0.4.

[0126] S3: pyrolyzing the Co-MOF / GF composite material at 600°C to obtain a Co / C / GF composite material;

[0127] S4: The Co / C / GF composite material and sulfur powder were placed in a mixed atmosphere of argon and air in a mass ratio of 1:3 and treated at 550°C. After the reaction was completed, 50 mL of CS2 solution was used to wash away excess sulfur powder, and the obtained product was placed in an oven and dried at 60°C to obtain cobalt sulfate / hard carbon / three-dimensional graphene (CHG) material.

[0128] When the cobalt sulfate / hard carbon / three-dimensional graphene composite material prepared in this embodiment is used as the negative electrode material of a sodium ion battery, the negative electrode material has a conductivity of 0.1 mA·g -1 At a current density of , after 50 cycles, the reversible capacity is 221.5 mAh g -1 .

[0129] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

[0130] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material, characterized in that: The following steps are involved: S1: Acidification treatment of GF to obtain GF modified with oxygen-containing functional groups; S2: adding the GF modified with oxygen-containing functional groups into a mixed solution of cobalt nitrate and 2-methylimidazole, and after standing, obtaining a Co-MOF / GF composite material; S3: pyrolyzing the Co-MOF / GF composite material to obtain a Co / C / GF composite material; the carbon in the Co / C / GF composite material is a porous carbon nanosheet array; S4: placing the Co / C / GF composite material and sulfur powder in a mixed atmosphere of argon and air for high temperature treatment, and after the reaction is completed, a cobalt sulfate / hard carbon / three-dimensional graphene composite material is obtained.

2. According to the preparation method of a cobalt sulfate / hard carbon / three-dimensional graphene composite material according to claim 1, the acidification process is specifically to place the three-dimensional graphene in a nitric acid aqueous solution with a concentration of 4 to 8M, and hydrothermally treat it at a temperature of 80 to 90°C for 8 to 12 hours.

3. The method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material according to claim 1, wherein the molar concentration ratio of cobalt nitrate to 2-methylimidazole in step S2 is (0.025-0.05):0.

4.

4. The method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material according to claim 1, wherein the step S2 is performed by standing for 2 to 4 hours.

5. The method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material according to claim 1, wherein in step S3, the Co-MOF / GF composite material is pyrolyzed under an argon atmosphere at a pyrolysis temperature of 500-600°C.

6. According to the method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material according to claim 1, the temperature of the high temperature treatment in step S4 is 350-550°C.

7. The method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material according to claim 1, wherein in step S4, the mass ratio of the Co / C / GF composite material to sulfur powder is 1:(1-3).

8. The method for preparing a cobalt sulfate / hard carbon / three-dimensional graphene composite material according to claim 1, wherein in step S4, the sulfur powder is placed at one end of the argon gas inlet.

9. A cobalt sulfate / hard carbon / three-dimensional graphene composite material, characterized in that: Prepared by the preparation method described in any one of claims 1 to 8; The cobalt sulfate / hard carbon / three-dimensional graphene composite material has a three-level structure, wherein the primary structure is a three-dimensional flaky graphene skeleton of 2 to 4 cm, the secondary structure is a cobalt sulfate / hard carbon composite sheet structure of 4 to 7 μm, and the tertiary structure is flaky hard carbon and flaky cobalt sulfate of 5 to 10 nm.

10. Use of the cobalt sulfate / hard carbon / three-dimensional graphene composite material of claim 9 in a negative electrode material for a sodium ion battery.

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