A graphene foam with single-atom-dispersed and in-situ grown nitrogen-doped carbon nanospheres, a preparation method thereof, and an application thereof

Abstract

The present invention relates to the technical field of new energy materials, and particularly relates to a graphene foam with single-atom-dispersed in-situ grown nitrogen-doped carbon nanospheres, a preparation method and an application thereof. The graphene foam has carbon nanospheres grown in-situ, and the carbon nanospheres are doped with nitrogen atoms and metal single atoms. The preparation method includes: preparing a graphene foam on the surface of a foam metal template by chemical vapor deposition, and then in-situ growing nitrogen-containing organic nanospheres on the surface of the graphene foam. After calcination and removal of the foam template, a graphene foam with in-situ grown nitrogen-doped and metal single-atom-doped carbon nanospheres is obtained. The obtained composite material is applied to a lithium-sulfur battery, which can greatly improve the utilization rate of active substances and has excellent overall electrochemical performance.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy materials, and particularly to a graphene foam with single-atom dispersed in-situ grown nitrogen-doped carbon nanospheres, a preparation method thereof, and an application thereof. Background Art

[0002] With the rapid development of society, higher requirements are put forward for the performance of electric energy storage systems, especially for the energy density of secondary battery systems. Although traditional lithium-ion batteries have advantages such as high specific energy, high working voltage, long cycle life, and no environmental pollution, the theoretical specific capacity of commercial lithium-ion battery cathode materials generally does not exceed 300 Wh·kg -1 , which cannot effectively meet the requirements of higher energy density. Therefore, developing a new generation of secondary battery systems with high theoretical specific capacity has become the focus of current research. In the new secondary battery system, the lithium-sulfur battery is a lithium secondary battery with metallic lithium as the anode material and elemental sulfur as the cathode material. The theoretical specific capacity of the sulfur cathode is as high as 1675 mAh / g, which is 3-5 times the actual specific capacity of current commercial lithium-ion batteries. In addition, elemental sulfur has rich reserves in nature and is environmentally friendly. Based on the above advantages, the lithium-sulfur battery is recognized as a high-energy-density energy storage system with great development potential.

[0003] However, due to some properties and reaction characteristics of the cathode active material sulfur in the lithium-sulfur battery, it has caused great obstacles to its practical application and industrialization. (1) At room temperature, elemental sulfur is non-conductive, with a conductivity of 5×10 -30 S / cm. Therefore, when elemental sulfur is used alone as the cathode of a lithium-sulfur battery, it will lead to difficult electron transfer, a decrease in the rate of the electrochemical reaction, and a consequent decrease in the utilization rate of the active material sulfur, thus affecting the electrochemical performance of the lithium-sulfur battery. (2) The soluble polysulfide intermediates generated during the charge and discharge process are easily soluble in the electrolyte, which will cause the loss of the active material sulfur. At the same time, the viscosity of the electrolyte increases, resulting in a continuous decrease in the ionic conductivity. (3) During the charge and discharge process, due to the different densities of the intermediates at different stages, the volume of the cathode material changes, and this volume change is extremely likely to cause the structure of the cathode material to change, resulting in the separation of the active material sulfur from the conductive network skeleton, thereby causing irreversible attenuation of the capacity of the lithium-sulfur battery.

[0004] To solve the problems of the sulfur cathode of the above lithium-sulfur battery, common measures are as follows: introducing a good conductive carrier to coat sulfur to improve the conductivity of the sulfur cathode material; designing voids or three-dimensional structures to limit the dissolution of polysulfide intermediates; constructing a stable porous structure to buffer the large volume changes caused during charge and discharge. Typical conductive carrier materials include conductive metals, metal oxides, covalent organic frameworks, polymers, and carbon matrix materials. Compared with other composite materials, carbon matrix materials have unique morphologies, high conductivity, and a relatively large specific surface area. Therefore, carbon matrix materials have been widely studied in the field of new energy.

[0005] Among them, graphene has become the most promising material in applied energy storage devices. It is a sheet structure composed of single-layer carbon atoms with sp2 hybrid orbitals, having excellent conductivity, high strength, and good flexibility itself. Moreover, the graphene sheets are intertwined and crosslinked with each other, which can form a three-dimensional structure with rich pores. This three-dimensional porous structure can not only increase the loading amount of active substances but also effectively alleviate the volume expansion effect of the cathode material. Therefore, constructing a three-dimensional structure based on graphene is more conducive to exerting its practical application value. Related technologies on the research of constructing a three-dimensional structure based on graphene have been reported:

[0006] CN201711265638.8 discloses a preparation method of a metal-free current collector and a self-supporting graphene-based lithium-sulfur battery cathode. This technology freeze-dries and reduces a graphene oxide / carbon nanotube mixed slurry to obtain a self-supporting graphene / carbon nanotube three-dimensional composite material, and then performs sulfur loading and tablet pressing on this composite material to obtain a metal-free current collector and a self-supporting graphene-based lithium-sulfur battery cathode material. CN201610092808.6 discloses a preparation method of a boron and nitrogen co-doped three-dimensional structure lithium-sulfur battery cathode material. This technology adds ammonia water to a graphene oxide suspension, then adds sodium borohydride, and then transfers it to a hydrothermal autoclave for hydrothermal reaction, and obtains three-dimensional boron and nitrogen co-doped graphene after freeze-drying. There are also reports on the prior art of metal single-atom dispersed graphene composite materials: CN202010559379.5 discloses a metal atom-non-metal atom co-doped graphene catalyst, its preparation method, cathode material and lithium-sulfur battery. This technology uniformly mixes graphene oxide powder with a single metal salt and prepares metal single-atom-non-metal atom co-doped graphene by high-temperature pyrolysis; then, in an atomic layer deposition mode, a metal organic precursor vapor is introduced to react with the metal single-atom-non-metal atom co-doped graphene to obtain a metal atom-non-metal atom co-doped graphene catalyst. There are also reports on the prior art of covalent organic framework composite materials: CN202010392549.5 discloses a preparation method of a composite aerogel material for a lithium-sulfur battery cathode. This technology etches and peels a layered MAX-phase ceramic powder with hydrochloric acid and lithium fluoride solution to obtain a stable suspension of two-dimensional transition metal carbide nanosheets. Then, a cross-linking agent and a covalent organic framework material are added to this suspension to guide its self-assembly to form a hydrogel, and a two-dimensional transition metal carbide / covalent organic framework composite aerogel material is obtained through vacuum freeze-drying. A two-dimensional transition metal carbide / covalent organic framework / sulfur composite aerogel material is obtained by doping sulfur through a thermal melting method. The common defect of the above prior art is that the utilization rate of the active substance in the composite cathode material is relatively low, and the ability to adsorb and convert polysulfides during the electrochemical reaction process is weak, resulting in a poor ability to inhibit the shuttle effect, and the specific capacity of the prepared lithium-sulfur battery is relatively low and the cycling performance is poor during charge and discharge.

[0007] In view of this, the present invention is specifically proposed. Summary of the Invention

[0008] The purpose of the present invention is to provide a graphene foam with single-atom dispersed in-situ grown nitrogen atom-doped carbon nanospheres to alleviate the technical problems of relatively low specific capacity and poor cycling performance widely existing in the cathode materials of lithium-sulfur batteries in the prior art.

[0009] The second object of the present invention is to provide a method for preparing the graphene foam with single-atom-dispersed in-situ grown nitrogen-doped carbon nanospheres, so as to successfully prepare the graphene foam with single-atom-dispersed in-situ grown nitrogen-doped carbon nanospheres.

[0010] The third object of the present invention is to provide the application of the graphene foam with single-atom-dispersed in-situ grown nitrogen-doped carbon nanospheres in lithium-sulfur batteries and a method for preparing a cathode material for lithium-sulfur batteries, so as to realize the popularization and application of the graphene foam with single-atom-dispersed in-situ grown nitrogen-doped carbon nanospheres provided by the present invention.

[0011] In order to solve the above technical problems and achieve the above objects, the present invention provides the following technical solutions:

[0012] In a first aspect, the present invention provides a graphene foam with single-atom-dispersed in-situ grown nitrogen-doped carbon nanospheres, wherein carbon nanospheres are in-situ grown on the graphene foam, and the carbon nanospheres are doped with nitrogen atoms and metal single atoms.

[0013] In a second aspect, the present invention provides a method for preparing the graphene foam according to the foregoing embodiment, including: preparing the graphene foam on the surface of a foam metal template by chemical vapor deposition, and then in-situ growing nitrogen-containing organic nanospheres on the surface of the graphene foam. After a first calcination step and a step of removing the foam metal template, a graphene foam with single-atom-dispersed in-situ grown nitrogen-doped carbon nanospheres is obtained.

[0014] In an alternative embodiment, the chemical vapor deposition method includes placing the foam metal template in a mixed atmosphere and reacting at a reaction temperature of 950-1050°C for 10-60 minutes. The mixed atmosphere includes an inert gas, hydrogen, and methane with a flow rate ratio of 3-5:1-3:1.

[0015] Preferably, the foam metal template includes nickel foam.

[0016] Preferably, the reaction temperature is 1000°C and the reaction duration is 30 minutes.

[0017] Preferably, the inert gas includes argon.

[0018] Preferably, the flow rate ratio of argon, hydrogen, and methane is 4:2:1.

[0019] Preferably, the flow rate of methane is 50-150 sccm.

[0020] Preferably, the flow rate of methane is 100 sccm.

[0021] In an alternative embodiment, before the chemical vapor deposition reaction, it further includes a step of evacuating the chemical vapor deposition reaction system, and / or a step of removing the oxide on the surface of the foamed metal template.

[0022] Preferably, the evacuating step includes repeating the step of evacuating and then introducing an inert gas until the atmospheric pressure reaches one standard atmosphere at least twice.

[0023] Preferably, the evacuating includes reducing the pressure of the chemical vapor deposition reaction system to 3-7 Pa, more preferably 5 Pa.

[0024] Preferably, the step of removing the oxide on the surface of the foamed metal template includes removing the oxide on the surface of the foamed metal template through a second calcination step under a reducing atmosphere.

[0025] Preferably, the reducing atmosphere includes a mixed atmosphere of an inert gas and hydrogen.

[0026] Preferably, the flow rate ratio of the inert gas to hydrogen is 0.5-1.5:1, more preferably 1:1.

[0027] Preferably, the fourth calcination step includes heating to 1000 °C within 30 min and annealing for 20 min.

[0028] Preferably, the product obtained from the chemical vapor deposition reaction is cooled to room temperature under a mixed atmosphere composed of an inert gas and a reducing gas. As used herein, "room temperature" generally refers to a comfortable indoor temperature for the convenience of operation by the operator, or it may refer to the usual storage temperature of the chemical vapor deposition product for the convenience of stable storage of the chemical vapor deposition product, including but not limited to 20-25 °C.

[0029] Preferably, the inert gas includes argon.

[0030] Preferably, the reducing gas includes hydrogen.

[0031] Preferably, the mixed atmosphere includes argon and hydrogen, and the flow rate ratio of argon to hydrogen is 0.5-1.5:1, more preferably 2:3.

[0032] Preferably, the flow rate of hydrogen is 300 sccm.

[0033] Preferably, the cooling rate is 5-20 °C / min.

[0034] In an alternative embodiment, the in-situ growth method of the nitrogen-containing organic nanospheres includes immersing the graphene foam in a nitrogen-containing organic solution and generating nitrogen-containing organic nanospheres on the surface of the graphene foam through an in-situ organic polycondensation reaction.

[0035] Preferably, the nitrogen-containing organic solution comprises an organic solution of 1,3,5-tris(4-aminophenyl)benzene and / or 1,3,5-benzenetricarbaldehyde.

[0036] Preferably, the solvent of the organic solution comprises methanol and / or acetone mixed in equal volumes.

[0037] Preferably, glacial acetic acid is further added to the organic solution.

[0038] Preferably, the organic solution is an organic solution of 1,3,5-tris(4-aminophenyl)benzene, wherein the concentration of 1,3,5-benzenetricarbaldehyde is 1-1.5 g / L, more preferably 1.3 g / L.

[0039] Preferably, the organic solution is an organic solution of 1,3,5-tris(4-aminophenyl)benzene and 1,3,5-benzenetricarbaldehyde, wherein the mass ratio of 1,3,5-tris(4-aminophenyl)benzene to 1,3,5-benzenetricarbaldehyde is 1-3:1, preferably 2.33:1.

[0040] Preferably, the volume ratio of the added glacial acetic acid to the mass of 1,3,5-benzenetricarbaldehyde is 1.8-1.5 L:10 g, more preferably 1 L:10 g.

[0041] In an alternative embodiment, the first calcination step comprises heating the graphene foam with in-situ grown nitrogen-containing organic nanospheres to 800-1000 °C and then holding for 2-4 h.

[0042] Preferably, the heating rate is 5-10 °C / min.

[0043] Preferably, the graphene foam with in-situ grown nitrogen-containing organic nanospheres is heated to 900 °C and then held for 2 h.

[0044] In an alternative embodiment, the step of removing the foam template comprises soaking the product after the first calcination step in an etching solution, removing the foam template, washing until pH is neutral, and drying to obtain graphene foam with in-situ grown nitrogen-doped carbon nanospheres.

[0045] Preferably, the etching solution comprises an acid solution.

[0046] Preferably, the acid solution comprises a hydrochloric acid solution.

[0047] Preferably, the concentration of the hydrochloric acid solution is 0.5-2 mol / L, the soaking temperature is 50-70 °C, and the soaking time is 2-4 h.

[0048] Preferably, the concentration of the hydrochloric acid solution is 1 mol / L, the soaking temperature is 60 °C, and the soaking time is 3 h.

[0049] Preferably, the foam metal template comprises foamed nickel, and the obtained graphene foam in situ grown nitrogen atom-doped carbon nanospheres is graphene foam in situ grown nitrogen atom-doped carbon nanospheres dispersed with single nickel atoms.

[0050] In a third aspect, the present invention also provides the use of the graphene foam of the single-atom-dispersed in-situ grown nitrogen-doped carbon nanospheres described in the aforementioned embodiment or the graphene foam of the single-atom-dispersed in-situ grown nitrogen-doped carbon nanospheres obtained by the preparation method described in the aforementioned embodiment in lithium-sulfur batteries.

[0051] Preferably, the application includes application in preparing positive electrode materials for lithium-sulfur batteries.

[0052] In a fourth aspect, the present invention provides a method for preparing a positive electrode material for a lithium-sulfur battery, the preparation method comprising coating the graphene foam of the single-atom-dispersed in-situ grown nitrogen-atom-doped carbon nanospheres described in the aforementioned embodiment or the graphene foam of the single-atom-dispersed in-situ grown nitrogen-atom-doped carbon nanospheres prepared by the preparation method described in the aforementioned embodiment with sulfur, and then tableting and drying in sequence to obtain a positive electrode material for a lithium-sulfur battery.

[0053] Preferably, the tabletting pressure is 3-5 MPa.

[0054] Preferably, the drying comprises vacuum drying, and further preferably, the vacuum drying temperature is 50 to 70° C., and the vacuum drying time is 12 to 24 hours.

[0055] In an optional embodiment, the sulfur coating step includes applying a carbon disulfide solution containing active sulfur to the surface of the graphene foam with in-situ grown nitrogen atom-doped carbon nanospheres, removing the solvent carbon disulfide, and then drying at a constant temperature in an inert atmosphere.

[0056] Preferably, the active sulfur comprises elemental sulfur.

[0057] Preferably, the size of the sulfur element is nanometer-scale.

[0058] Preferably, the concentration of active sulfur in the active sulfur carbon disulfide solution is 0.5 to 1 mol / L;

[0059] Preferably, the coating method comprises dripping.

[0060] Preferably, the method of removing the solvent carbon disulfide comprises volatilization in a fume hood.

[0061] Preferably, the inert atmosphere comprises a nitrogen atmosphere.

[0062] Preferably, the constant temperature heating temperature is 150-160°C;

[0063] Preferably, the constant temperature heating time is 6 - 12 h.

[0064] The beneficial effects of the present invention are as follows:

[0065] 1. The present invention uses graphene foam as the substrate, which is compounded with metal single - atom - dispersed and nitrogen - doped carbon nanospheres to construct a stable three - level hierarchical structure. When it is used as the cathode material of a lithium - sulfur battery by compounding with sulfur, it effectively solves the technical problems of the unstable three - level graphene foam structure and the poor adsorption and catalytic conversion effect of polar carbon materials on polysulfides in lithium - sulfur batteries. The composite material prepared by the present invention can not only give play to the advantages of the stable three - level hierarchical structure, effectively improve the utilization rate of the active materials of the lithium - sulfur battery, but also play the catalytic role of metal single atoms in the conversion of polysulfides, thereby effectively inhibiting the "shuttle effect" and promoting the redox kinetics of the cathode.

[0066] 2. By adjusting the time of chemical vapor deposition and the ratio of raw materials, the present invention realizes the growth of multi - layer graphene, solves the problem that the three - level graphene foam structure has an insignificant effect on alleviating the volume expansion effect of active substances during the charge - discharge process under the condition of high loading of active substances in lithium - sulfur batteries. At the same time, the multi - layer graphene also serves as a multifunctional screening material to realize the separation of metal particles, clusters and metal single atoms. Thus, it effectively buffers the volume expansion during the charge - discharge process and provides more active sites for sulfur loading, enabling the battery to have good cycle and rate performance while meeting the high loading of active substances.

[0067] 3. By in - situ growing covalent organic framework nanospheres on the surface of graphene foam and then forming nitrogen - doped carbon nanospheres through high - temperature annealing, the present invention effectively solves the problems of limited specific surface area and low volume density of the three - level graphene foam. The uniformly distributed three - level nitrogen - doped carbon nanospheres on the graphene surface can expand the specific surface area of the composite material in a limited space, increase the active sites and volume density of the composite material, thereby improving the energy density of the lithium - sulfur battery. At the same time, nitrogen doping can endow the composite material framework with better conductivity through coordination interactions.

[0068] 4. During the design process of the present invention, the structural problems of the sulfur / single-atom-dispersed nitrogen-doped carbon nanospheres / graphene foam composite material in the cathode material of the lithium-sulfur battery were fully considered, and the structure of graphene was modified before sulfur doping. The single-atom-dispersed nitrogen-doped carbon nanospheres / graphene foam composite material was prepared by chemical vapor deposition, in-situ organic polycondensation reaction, high-temperature pyrolysis, and acid etching methods. The nitrogen-doped carbon nanospheres / graphene foam have more excellent electrical conductivity. Its porous channels effectively increase the specific surface area of the composite material and are also beneficial to the rapid transmission of ions in the electrolyte. At the same time, it can physically block the outward diffusion of polysulfides, thereby encapsulating the polysulfides in the hollow interior; it can also provide sufficient buffer space to relieve the volume expansion effect during the electrochemical reaction process. In addition, metal single atoms are more uniformly dispersed on the nitrogen-doped carbon nanospheres, and the nitrogen-doped carbon nanospheres are uniformly dispersed inside the graphene foam. This hierarchical structure further increases the active sites of the specific surface area of the composite material. At the same time, the uniformly dispersed metal single atoms greatly improve the chemical adsorption ability of the composite material for polysulfides and the catalytic conversion ability of polysulfides, effectively inhibiting the shuttle effect of polysulfides, thereby improving the utilization rate of active substances. The excellent properties of two-dimensional graphene, the unique advantages of the three-level hierarchical structure, and the synergistic effect of the adsorption and catalytic ability of metal single atoms effectively improve the electrochemical performance of the lithium-sulfur battery.

[0069] 5. In a preferred embodiment, first, chemical vapor deposition is used to grow multiple layers of graphene on the surface of nickel foam. The growth of multiple layers of graphene wraps around the surface of nickel foam, mainly playing a filtering role. It can serve as an attachment site for the in-situ growth of covalent organic frameworks during the in-situ organic polycondensation reaction, and at the same time, it can maintain a stable three-level structure in the overall structure, increasing the specific surface area of the composite material. During the high-temperature pyrolysis process, the metal atoms on the nickel foam can diffuse and move through the multiple layers of graphene formed on the surface. Due to the different diffusion rates of metal atoms, metal particles, and clusters, the multiple layers of graphene can effectively screen out large metal particles and clusters, so that the metal atoms are captured by the nitrogen atoms in the covalent organic framework, forming a single-atom nickel-dispersed nitrogen-doped carbon nanospheres / graphene foam. Then, sulfur is doped by a simple solution titration method and hydrothermal method to uniformly introduce the active substance sulfur into the prepared structure of single-atom nickel-dispersed nitrogen-doped graphene foam. Brief Description of the Drawings

[0070] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0071] Figure 1 Scanning electron microscope photograph of the graphene foam composite material of single-atom nickel-dispersed in-situ grown nitrogen-doped atomic carbon nanospheres provided for Example 1;

[0072] Figure 2 X-ray diffraction pattern of the graphene foam composite material of single-atom nickel-dispersed in-situ grown nitrogen-doped atomic carbon nanospheres provided for Example 1;

[0073] Figure 3 Electrochemical cycling performance curve of the sulfur / single-atom dispersed nitrogen-doped carbon nanospheres / graphene foam composite materials prepared in Example 3, Example 5, and Example 7 at a current density of 0.2C;

[0074] Figure 4 Electrochemical rate performance curve of the sulfur / single-atom dispersed nitrogen-doped carbon nanospheres / graphene foam composite materials prepared in Example 3, Example 5, and Example 7;

[0075] Figure 5 Scanning electron microscope photograph of the graphene foam material provided for Comparative Example 1;

[0076] Figure 6 X-ray diffraction pattern of the graphene foam material provided for Comparative Example 1;

[0077] Figure 7 Electrochemical cycling performance curve of the sulfur / graphene foam composite material prepared in Comparative Example 3 at a current density of 0.2C;

[0078] Figure 8 Electrochemical rate performance curve of the sulfur / graphene foam composite material prepared in Comparative Example 3. Detailed Description of the Invention

[0079] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Usually, the components of the embodiments of the present invention described and illustrated herein can be arranged and designed in various different configurations.

[0080] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0081] It should be noted that similar reference numerals and letters denote similar items in the following figures. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. In the description of the present invention, it should be noted that the terms "first", "second", "third", etc. are only used for distinguishing descriptions and cannot be understood as indicating or implying relative importance.

[0082] The following will describe in detail some embodiments of the present invention with reference to the accompanying drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0083] Embodiment 1

[0084] This embodiment provides a graphene foam with single-atom nickel-dispersed in-situ grown nitrogen-doped carbon nanospheres. The scanning electron microscope photograph is as Figure 1 shown. It can be seen that the diameter of the carbon nanospheres is 200 - 400 nm, the surface is smooth, the dispersibility is good, and there are no impurities. The X-ray diffraction pattern is as Figure 2 shown. It can be seen from the figure that two broad diffraction peaks can be clearly observed at diffraction angles of 25° and 44°, which are attributed to the presence of carbon nanospheres; in addition, the sharp peak at 26° and the weak diffraction peaks at 44° and 54° are attributed to the graphene foam structure.

[0085] Embodiment 2:

[0086] This embodiment provides a preparation method of the graphene foam with single-atom nickel-dispersed in-situ grown nitrogen-doped carbon nanospheres provided in Embodiment 1, including the following steps:

[0087] The first step: Prepare a nickel foam / graphene composite material.

[0088] First, ultrasonically clean nickel foam in acetone, ethanol, and deionized water for 5 minutes each. After rinsing with deionized water, place it in a vacuum oven and dry it for 1 hour; put the cleaned nickel foam into the CVD system tube furnace, first remove the air in the tube, then use a vacuum pump to pump the air pressure in the tube to 5 Pa, and then fill it with argon to normal pressure, and repeat this twice; then, in a mixed gas flow of Ar:H2 (200 sccm:200 sccm), heat it to 1000 °C in 30 minutes; anneal it in an Ar:H2 (200 sccm:300 sccm) gas flow for 20 minutes to remove the oxide on the surface of the nickel foam; introduce CH4 as a carbon source and grow graphene for 30 minutes in an Ar:H2:CH4 (400 sccm:200 sccm:100 sccm) gas flow. Stop introducing CH4 and quickly cool it to room temperature in an Ar:H2 (200 sccm:300 sccm) gas flow to obtain a nickel foam / graphene composite material.

[0089] Step 2: Prepare covalently organic nanospheres grown in-situ on the surface of nickel foam / graphene.

[0090] Vigorously stir 10 mL of methanol and 10 mL of acetone at room temperature. Then, pour 70 mg of 1,3,5-tris(4-aminophenyl)benzene and 30 mg of 1,3,5-benzenetricarboxaldehyde into the above solution and dissolve it under ultrasonic action for several minutes. Add 3 mL of glacial acetic acid to the solution, stir rapidly for 2 min, and then immerse the nickel foam / graphene composite material prepared in the first step in the solution. Let it stand for 30 min, then take out the nickel foam and wash it twice with methanol to obtain covalently organic nanospheres grown in-situ on the surface of nickel foam / graphene.

[0091] Step 3: Prepare single-atom-dispersed nitrogen-doped carbon nanospheres / graphene foam.

[0092] Place the covalently organic nanospheres grown in-situ on the surface of nickel foam / graphene in a vacuum oven and dry it at 60 °C for 6 hours. Then transfer it to a tube furnace and heat it to 900 °C at a rate of 5 °C / min and hold for 2 h to obtain single-atom-dispersed nitrogen-doped carbon nanospheres / graphene / nickel foam. After cooling to room temperature, completely immerse the composite material in a hydrochloric acid solution with a concentration of 1 mol / L and soak it at 60 °C for 3 h to completely etch away the nickel foam. Take it out and wash it with deionized water and absolute ethanol in turn until neutral, and place it in a vacuum drying oven at 60 °C for two hours to finally obtain single-atom-dispersed nitrogen-doped carbon nanospheres / graphene foam.

[0093] Example 3

[0094] This example provides a cathode material for a lithium-sulfur battery, and its preparation method includes:

[0095] Weigh the active material sulfur and the single-atom-dispersed nitrogen-doped carbon nanospheres / graphene foam composite material prepared in Example 2 according to a ratio of 2:1. Dissolve the active material sulfur in carbon disulfide solution at room temperature and ultrasonicate for 0.5 h to ensure that the concentration of the active material sulfur is 0.5 mol / L. Then use a dropper to take a small amount of the solution containing the active material sulfur and titrate and coat sulfur on the single-atom-dispersed nitrogen-doped carbon nanospheres / graphene foam composite material in a fume hood. Let the titrated composite material stand in the fume hood for 3 h until all the carbon disulfide has evaporated. Then put the dried composite material into a stainless steel autoclave lined with polytetrafluoroethylene, perform gas replacement treatment in a glove box filled with nitrogen, and then place the autoclave in an electrothermal constant temperature drying oven and keep it at 155 °C for 12 h. After cooling, take it out, and further press it into a denser thin sheet under a pressure of 3 MPa with a tablet press, and then put it into a vacuum drying oven and dry it at 60 °C for 12 h to finally obtain a sulfur / single-atom-dispersed nitrogen-doped carbon nanospheres / graphene foam composite material.

[0096] Example 4

[0097] Step 1: Prepare nickel foam / graphene composite material.

[0098] First, ultrasonically clean nickel foam in acetone, ethanol, and deionized water for 5 minutes each. After rinsing with deionized water, place it in a vacuum oven and dry for 1 hour. Put the cleaned nickel foam into the tube furnace of the CVD system. First, evacuate the air in the tube, then use a vacuum pump to pump the air pressure in the tube to 5 Pa, and then fill it with argon to atmospheric pressure. Repeat this twice. Then, in a mixed gas flow of Ar:H2 (200 sccm:200 sccm), heat it to 950 °C in 30 minutes. Anneal it in an Ar:H2 (200 sccm:300 sccm) gas flow for 20 minutes to remove the oxides on the surface of nickel foam. Introduce CH4 as the carbon source and grow graphene for 10 minutes in an Ar:H2:CH4 (400 sccm:200 sccm:100 sccm) gas flow. Stop introducing CH4 and quickly cool it to room temperature in an Ar:H2 (200 sccm:300 sccm) gas flow to obtain the nickel foam / graphene composite material.

[0099] Step 2: Prepare covalently organic nanospheres grown in-situ on the surface of nickel foam / graphene.

[0100] Vigorously stir 10 mL of methanol and 10 mL of acetone at room temperature. Then, pour 70 mg of 1,3,5-tris(4-aminophenyl)benzene and 30 mg of 1,3,5-benzenetricarboxaldehyde into the above solution and dissolve it under ultrasonic action for several minutes. Add 3 ml of glacial acetic acid to the solution, stir quickly for 2 minutes, and then immerse the nickel foam / graphene composite material prepared in the first step in the solution. Let it stand for 30 minutes, then take out the nickel foam and wash it twice with methanol to obtain covalently organic nanospheres grown in-situ on the surface of nickel foam / graphene.

[0101] Step 3: Prepare single-atom dispersed nitrogen-doped carbon nanospheres / graphene foam.

[0102] Place the covalently organic nanospheres grown in-situ on the surface of nickel foam / graphene in a vacuum oven and dry it at 60 °C for 6 hours. Then transfer it to a tube furnace and heat it to 800 °C at a rate of 5 °C / min and hold for 2 hours to obtain single-atom dispersed nitrogen-doped carbon nanospheres / graphene / nickel foam. After cooling to room temperature, immerse the composite material completely in a hydrochloric acid solution with a concentration of 0.5 mol / L and soak it at 50 °C for 3 hours to completely etch away the nickel foam. Take it out, wash it with deionized water and absolute ethanol in turn until neutral, and place it in a 60 °C vacuum drying oven for two hours to finally obtain single-atom dispersed nitrogen-doped carbon nanospheres / graphene foam.

[0103] Example 5

[0104] This embodiment provides a cathode material for a lithium-sulfur battery, and its preparation method includes:

[0105] Weigh the active material sulfur and the single-atom dispersed nitrogen-doped carbon nanosphere / graphene foam composite material prepared in Example 4 according to a ratio of 2:1. At room temperature, dissolve the active material sulfur in carbon disulfide solution and ultrasonicate for 0.5 h to ensure that the concentration of the active material sulfur is 0.5 mol / L. Then, use a dropper to measure a small amount of the solution containing the active material sulfur and titrate and coat sulfur on the single-atom dispersed nitrogen-doped carbon nanosphere / graphene foam composite material in a fume hood. Let the titrated composite material stand in the fume hood for 3 h until all the carbon disulfide has evaporated. Then, put the dried composite material into a stainless steel autoclave with a polytetrafluoroethylene liner, perform gas replacement treatment in a glove box filled with nitrogen, and then place the autoclave in an electrothermal constant temperature drying oven and keep it at 155 °C for 12 h. After cooling, take it out, and further press it into a thin sheet with a higher density under a pressure of 3 MPa using a tablet press, and then put it into a vacuum drying oven and dry it at 60 °C for 12 h to finally obtain a sulfur / single-atom dispersed nitrogen-doped carbon nanosphere / graphene foam composite material.

[0106] Example 6

[0107] The first step: Prepare a nickel foam / graphene composite material.

[0108] First, ultrasonically clean nickel foam in acetone, ethanol, and deionized water for 5 min each. After rinsing with deionized water, place it in a vacuum oven and dry it for 1 h. Put the cleaned nickel foam into the tube furnace of the CVD system, first evacuate the air in the tube, then use a vacuum pump to pump the air pressure in the tube to 5 Pa, and then fill it with argon to normal pressure, and repeat this twice. Then, in a mixed gas flow of Ar:H2 (200 sccm:200 sccm), heat it to 1050 °C in 30 min. Anneal it for 20 min in an Ar:H2 (200 sccm:300 sccm) gas flow to remove the oxides on the surface of the nickel foam. Introduce CH4 as a carbon source and grow graphene for 1 h in an Ar:H2:CH4 (400 sccm:200 sccm:100 sccm) gas flow. Stop introducing CH4 and quickly cool it to room temperature in an Ar:H2 (200 sccm:300 sccm) gas flow to obtain a nickel foam / graphene composite material.

[0109] The second step: Prepare covalently organic nanospheres grown in-situ on the surface of nickel foam / graphene.

[0110] Stir 10 mL of methanol and 10 mL of acetone vigorously at room temperature. Then, pour 70 mg of 1,3,5-tris(4-aminophenyl)benzene and 30 mg of 1,3,5-benzenetricarbaldehyde into the above solution and dissolve it under ultrasonic wave for several minutes. Add 3 ml of glacial acetic acid to the solution, stir rapidly for 2 min, and then immerse the nickel foam / graphene composite material prepared in the first step into the solution. Let it stand for 30 min, then take out the nickel foam, wash it twice with methanol, and obtain nickel foam / graphene with covalently organic nanospheres grown in situ on the surface.

[0111] Step 3: Prepare single-atom-dispersed nitrogen-doped carbon nanospheres / graphene foam.

[0112] Place the nickel foam / graphene with covalently organic nanospheres grown in situ on the surface in a vacuum oven and dry it at 60 °C for 6 hours. Then transfer it to a tube furnace, heat it to 1000 °C at a rate of 5 °C / min, and hold for 2 h to obtain single-atom-dispersed nitrogen-doped carbon nanospheres / graphene / nickel foam. After cooling to room temperature, immerse the composite material completely in a hydrochloric acid solution with a concentration of 2 mol / L and soak it at 70 °C for 3 h to completely etch away the nickel foam. After taking it out, wash it with deionized water and absolute ethanol in turn until neutral, and place it in a 60 °C vacuum drying oven for two hours to finally obtain single-atom-dispersed nitrogen-doped carbon nanospheres / graphene foam.

[0113] Example 7

[0114] This example provides a cathode material for a lithium-sulfur battery, and its preparation method includes:

[0115] Weigh the active material sulfur and the single-atom-dispersed nitrogen-doped carbon nanospheres / graphene foam composite material prepared in Example 6 according to 2:1. Dissolve the active material sulfur in carbon disulfide solution at room temperature and ultrasonicate for 0.5 h to ensure that the concentration of the active material sulfur is 0.5 mol / L. Then use a dropper to measure a small amount of the solution containing the active material sulfur and titrate and coat sulfur on the single-atom-dispersed nitrogen-doped carbon nanospheres / graphene foam composite material in a fume hood. Let the titrated composite material stand in the fume hood for 3 h until all the carbon disulfide has volatilized. Then put the dried composite material into a stainless steel autoclave lined with polytetrafluoroethylene, perform gas replacement treatment in a glove box filled with nitrogen, and then place the autoclave in an electrothermal constant temperature drying oven and keep it at 155 °C for 12 h. After cooling, take it out, and further press it into a denser thin sheet under a pressure of 3 MPa with a tablet press, and then put it into a vacuum drying oven and dry it at 60 °C for 12 h to finally obtain a sulfur / single-atom-dispersed nitrogen-doped carbon nanospheres / graphene foam composite material.

[0116] Taking the sulfur / single-atom-dispersed nitrogen-doped carbon nanosphere / graphene foam composite materials obtained in the above Example 3, Example 5, and Example 7 as the cathode materials, their electrochemical properties were tested, and the results are as follows Figure 3 , 4 shown. As can be seen from Figure 3 , at a current density of 0.2C, the initial discharge capacity of the lithium-sulfur battery composed of the composite material prepared in Example 3 was 1164 mAh / g. After 100 cycles, the discharge capacity still remained at 896 mAh / g, and the Coulomb efficiency almost always remained at 99.5%. The initial discharge capacity of the lithium-sulfur battery composed of the composite material prepared in Example 5 was 1130 mAh / g. After 100 cycles, the discharge capacity remained at 704 mAh / g. The initial discharge capacity of the lithium-sulfur battery composed of the composite material prepared in Example 7 was 1123 mAh / g. After 100 cycles, the discharge capacity remained at 630 mAh / g.

[0117] As can be seen from Figure 4 , at different current densities, the lithium-sulfur battery assembled with the composite material prepared in Example 3 still maintained a more stable and higher-capacity discharge state, which were 0.2C (1118 mAh / g), 0.5C (859 mAh / g), 1C (726 mAh / g), 2C (599 mAh / g), and 3C (530 mAh / g) respectively. The electrochemical properties of the composite material prepared in Example 5 were 0.2C (1052 mAh / g), 0.5C (802 mAh / g), 1C (673 mAh / g), 2C (501 mAh / g), and 3C (419 mAh / g) respectively. The electrochemical properties of the composite material prepared in Example 7 were 0.2C (1043 mAh / g), 0.5C (665 mAh / g), 1C (559 mAh / g), 2C (430 mAh / g), and 3C (352 mAh / g) respectively.

[0118] Comparative Example 1

[0119] This comparative example provides a graphene foam, and its scanning electron microscope photograph is as shown in Figure 5 . As can be seen, the graphene sheets are evenly distributed, forming a three-dimensional foam structure. Its X-ray diffraction pattern is as shown in Figure 6 . It can be seen from the figure that the diffraction peaks at diffraction angles of 24°, 43°, and 52° are attributed to the graphene foam structure.

[0120] Comparative Example 2:

[0121] This comparative example provides a preparation method of the graphene foam provided in Comparative Example 1, including the following steps:

[0122] The first step: Prepare the graphene foam material.

[0123] First, ultrasonically clean nickel foam in acetone, ethanol, and deionized water for 5 minutes each. After rinsing with deionized water, place it in a vacuum oven and dry for 1 hour. Put the cleaned nickel foam into the tube furnace of the CVD system. First, evacuate the air in the tube, then use a vacuum pump to pump the air pressure in the tube down to 5 Pa, and then fill it with argon to atmospheric pressure. Repeat this twice. Then, in a mixed gas flow of Ar:H₂ (200 sccm:200 sccm), heat it to 1000 °C in 30 minutes. Anneal it for 20 minutes in an Ar:H₂ (200 sccm:300 sccm) gas flow to remove the oxides on the surface of the nickel foam. Introduce CH₄ as the carbon source and grow graphene for 30 minutes in an Ar:H₂:CH₄ (400 sccm:200 sccm:100 sccm) gas flow. Stop introducing CH₄ and quickly cool it to room temperature in an Ar:H₂ (200 sccm:300 sccm) gas flow to obtain the graphene foam material.

[0124] Step 2: Prepare the graphene foam material.

[0125] Completely immerse the nickel foam / graphene material in a hydrochloric acid solution with a concentration of 1 mol / L and soak it at 60 °C for 3 hours to completely etch away the nickel foam. After taking it out, wash it with deionized water and absolute ethanol in turn until it is neutral, and place it in a 60 °C vacuum drying oven for two hours to finally obtain the graphene foam material.

[0126] Comparative Example 3

[0127] This comparative example provides a cathode material for a lithium-sulfur battery, and its preparation method includes:

[0128] Weigh the active material sulfur and the graphene foam material prepared in Comparative Example 2 according to 2:1. Dissolve the active material sulfur in a carbon disulfide solution at room temperature and ultrasonically treat it for 0.5 hour to ensure that the concentration of the active material sulfur is 0.5 mol / L. Then, use a dropper to measure a small amount of the solution containing the active material sulfur and titrate and coat the graphene foam material in a fume hood. Let the titrated composite material stand in the fume hood for 3 hours until all the carbon disulfide has evaporated. Then, put the dried composite material into a stainless steel autoclave with a polytetrafluoroethylene liner, perform gas replacement treatment in a glove box filled with nitrogen, and then put the autoclave into an electrothermal constant temperature drying oven and keep it at 155 °C for 12 hours. After cooling, take it out, and further press it into a denser thin sheet under a pressure of 3 MPa with a tablet press, and then put it into a vacuum drying oven and dry it at 60 °C for 12 hours to finally obtain the sulfur / graphene foam composite material.

[0129] Use the sulfur / graphene foam composite material obtained above as the cathode material and detect its electrochemical performance. The results are as Figure 7 shown. From Figure 7It can be seen that at a current density of 0.2C, the initial discharge capacity of the lithium-sulfur battery composed of the composite material prepared in Comparative Example 3 is 958 mAh / g. After 100 cycles, the discharge capacity remains at 463 mAh / g, and the capacity decay is rapid.

[0130] Composed of Figure 8 It can be seen that at different current densities, the rate capacity of the composite material prepared in Comparative Example 3 is relatively low, which are 0.2C (910 mAh / g), 0.5C (603 mAh / g), 1C (451 mAh / g), 2C (380 mAh / g), and 3C (292 mAh / g), respectively.

[0131] 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 them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A graphene foam, characterized in that, Carbon nanospheres are in-situ grown on the graphene foam, and the carbon nanospheres are doped with nitrogen atoms and single metal nickel atoms; The preparation method of the graphene foam includes preparing graphene foam on the surface of a foam nickel template by chemical vapor deposition, then in-situ growing nitrogen-containing organic nanospheres on the surface of the graphene foam, and obtaining the graphene foam after a first calcination step and a step of removing the foam nickel template in sequence; The in-situ growth method of the nitrogen-containing organic nanospheres includes immersing the graphene foam in a nitrogen-containing organic solution, and generating nitrogen-containing organic nanospheres on the surface of the graphene foam through an in-situ organic polycondensation reaction; The nitrogen-containing organic solution includes 1,3,5-tris(4-aminophenyl)benzene and 1,3,5-benzenetricarboxaldehyde.

2. The graphene foam according to claim 1, wherein, The chemical vapor deposition method includes placing the foam nickel template in a first mixed atmosphere and reacting at a reaction temperature of 950-1050 °C for 10-60 min. The first mixed atmosphere includes an inert gas, hydrogen, and methane with a flow rate ratio of 3-5:1-3:

1.

3. The graphene foam according to claim 2, wherein The temperature of the reaction is 1000 °C, and the duration of the reaction is 30 min.

4. The graphene foam according to claim 2, wherein The inert gas in the first mixed atmosphere includes argon.

5. The graphene foam according to claim 4, wherein The first mixed atmosphere includes argon, hydrogen, and methane with a flow rate ratio of 4:2:

1.

6. The graphene foam according to claim 5, wherein, The flow rate of methane in the first mixed atmosphere is 50-150 sccm.

7. The graphene foam according to claim 6, wherein The flow rate of methane in the first mixed atmosphere is 100 sccm.

8. The graphene foam according to any one of claims 2 to 7, characterized in that, Before the chemical vapor deposition reaction, it further includes a step of evacuating the chemical vapor deposition reaction system, and / or a step of removing the oxide on the surface of the foam nickel template.

9. The graphene foam according to claim 8, wherein, The evacuating step includes repeating the step of evacuating and then introducing an inert gas to one standard atmosphere at least twice.

10. The graphene foam according to claim 9, wherein, The evacuating includes reducing the pressure of the chemical vapor deposition reaction system to 3-7 Pa.

11. The graphene foam according to claim 10, wherein, The evacuating includes reducing the pressure of the chemical vapor deposition reaction system to 5 Pa.

12. The graphene foam according to claim 8, wherein, The step of removing the oxide on the surface of the foam nickel template includes removing the oxide on the surface of the foam nickel template through a second calcination step in a reducing atmosphere.

13. The graphene foam according to claim 12, wherein, The reducing atmosphere includes a second mixed atmosphere of an inert gas and hydrogen.

14. The graphene foam according to claim 13, wherein, The flow rate ratio of the inert gas to hydrogen in the second mixed atmosphere is 0.5-1.5:

1.

15. The graphene foam according to claim 14, wherein The flow rate ratio of the inert gas to hydrogen in the second mixed atmosphere is 1:

1.

16. The graphene foam according to claim 14, wherein, The second calcination step includes heating to 1000 °C within 30 min and annealing for 20 min.

17. The graphene foam according to any one of claims 2 to 7, characterized in that, The product obtained from the chemical vapor deposition reaction is cooled to room temperature under a third mixed atmosphere composed of an inert gas and a reducing gas.

18. The graphene foam according to claim 17, wherein The inert gas in the third mixed atmosphere includes argon.

19. The graphene foam according to claim 17, wherein The reducing gas in the third mixed atmosphere includes hydrogen.

20. The graphene foam according to claim 17, wherein, The third mixed atmosphere includes argon and hydrogen, and the flow rate ratio of argon to hydrogen is 0.5-1.5:

1.

21. The graphene foam according to claim 20, wherein, The flow rate ratio of argon to hydrogen in the third mixed atmosphere is 2:

3.

22. The graphene foam according to claim 21, wherein The flow rate of hydrogen in the third mixed atmosphere is 300 sccm.

23. The graphene foam according to claim 17, wherein, The cooling rate is 5-20 °C / min.

24. The graphene foam according to claim 1, wherein The nitrogen-containing organic solution further includes methanol and acetone mixed in equal volume.

25. The graphene foam according to claim 1, wherein Glacial acetic acid is further added to the nitrogen-containing organic solution.

26. The graphene foam according to claim 1, wherein The mass ratio of the 1,3,5-tris(4-aminophenyl)benzene to the 1,3,5-benzenetricarbaldehyde is 1 to 3:

1.

27. The graphene foam according to claim 26, wherein The mass ratio of the 1,3,5-tris(4-aminophenyl)benzene to the 1,3,5-benzenetricarbaldehyde is 2.33:

1.

28. The graphene foam according to claim 25, wherein, The volume ratio of the added glacial acetic acid to the mass of the 1,3,5-benzenetricarbaldehyde is 0.8 to 1.5 L:10 g.

29. The graphene foam according to claim 28, wherein The volume ratio of the added glacial acetic acid to the mass of the 1,3,5-benzenetricarbaldehyde is 1 L:10 g.

30. The graphene foam according to claim 1, wherein The first calcination step includes heating the graphene foam with in-situ grown nitrogen-containing organic nanospheres to 800 to 1000 °C and then holding for 2 to 4 h.

31. The graphene foam according to claim 30, wherein, The heating rate is 5 to 10 °C / min.

32. The graphene foam according to claim 30, wherein, The first calcination step includes heating the graphene foam with in-situ grown nitrogen-containing organic nanospheres to 900 °C and then holding for 2 h.

33. The graphene foam according to claim 1, characterized in that, The step of removing the nickel foam template includes soaking the product after the first calcination step in an etching solution, removing the nickel foam template, washing until the pH is neutral, and drying to obtain graphene foam.

34. The graphene foam according to claim 33, wherein The etching solution includes an acid solution.

35. The graphene foam according to claim 34, wherein The acid solution includes a hydrochloric acid solution.

36. The graphene foam according to claim 35, characterized in that, The concentration of the hydrochloric acid solution is 0.5 to 2 mol / L, the soaking temperature is 50 to 70 °C, and the soaking time is 2 to 4 h.

37. The graphene foam according to claim 36, wherein The concentration of the hydrochloric acid solution is 1 mol / L, the soaking temperature is 60 °C, and the soaking time is 3 h.

38. The application of the graphene foam according to any one of claims 1 to 37 in a lithium-sulfur battery.

39. The application according to claim 38, wherein The application includes the application in the preparation of a positive electrode material for a lithium-sulfur battery.

40. A method for preparing a cathode material of a lithium-sulfur battery, characterized in that, The preparation method includes that after the graphene foam according to any one of claims 1 to 37 is sulfur-coated, it is successively tableted and dried to obtain a positive electrode material for a lithium-sulfur battery.

41. The preparation method according to claim 40, characterized in that, The pressure of the tableting is 3 to 5 MPa.

42. The preparation method according to claim 41, characterized in that, The drying includes vacuum drying.

43. The preparation method according to claim 42, characterized in that, The temperature of the vacuum drying is 50 to 70 °C, and the time of the vacuum drying is 12 to 24 h.

44. The preparation method according to any one of claims 40 to 43, characterized in that, The sulfur-coating step includes coating a carbon disulfide solution containing active sulfur on the surface of the graphene foam, removing the solvent carbon disulfide, and then performing constant-temperature drying in an inert atmosphere.

45. The preparation method according to claim 44, characterized in that, The active sulfur includes elemental sulfur.

46. The preparation method according to claim 45, characterized in that, The size of the elemental sulfur is nanoscale.

47. The preparation method according to claim 44, characterized in that, The concentration of the active sulfur in the carbon disulfide solution of the active sulfur is 0.5 to 1 mol / L.

48. The preparation method according to claim 44, characterized in that, The coating method includes dropping.

49. The preparation method according to claim 44, characterized in that, The method of removing the solvent carbon disulfide includes volatilization in a fume hood.

50. The preparation method according to claim 44, characterized in that, The inert atmosphere includes a nitrogen atmosphere.

51. The preparation method according to claim 44, characterized in that, The heating temperature of the constant temperature is 150 to 160 °C.

52. The preparation method according to claim 51, characterized in that, The heating time of the constant temperature is 6 to 12 h.

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