CN112838215A - Three-dimensional porous carbon nanosheet-sulfur material and preparation method and application thereof - Google Patents
Three-dimensional porous carbon nanosheet-sulfur material and preparation method and application thereof Download PDFInfo
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Abstract
The invention discloses a three-dimensional porous carbon nanosheet-sulfur material, which is prepared by using melamine, phytic acid and sulfur as raw materials, synthesizing the three-dimensional porous carbon nanosheet through a solution mixing method and a high-temperature pyrolysis method, and obtaining the three-dimensional porous carbon nanosheet-sulfur material through a melting method, wherein the sulfur content of the three-dimensional porous carbon nanosheet-sulfur material is 80-90%. The preparation method comprisesThe method comprises the following steps: 1) preparing a three-dimensional porous carbon nanosheet through a pyrolysis method; 2) preparing the three-dimensional porous carbon nanosheet-sulfur positive electrode material by a melting method. Wherein, two-stage pyrolysis and two-stage heat treatment are adopted. When the current density is 838 mA/cm, the lithium sulfur battery cathode can be used2In the process, after the charge and the discharge are carried out for 200 times in a circulating manner, the discharge specific capacity is 600-plus 700mAh/g, and the coulomb efficiency is more stable and approaches to 100 percent; when the current density is 1675 mA/g, after the charge and the discharge are carried out for 500 times in a circulating way, the discharge specific capacity is 400-600 mAh/g, and the average attenuation rate per time is 0.079%. The invention has the following advantages: has high sulfur content, inhibits polysulfide dissolution; the battery capacity attenuation is reduced, and the cycle performance is improved.
Description
Technical Field
The invention belongs to the technical field of batteries, and particularly relates to a three-dimensional porous carbon nanosheet-sulfur material and a preparation method and application thereof.
Background
As society develops, human needs for energy are gradually increasing. However, as fossil fuel resources such as coal, oil, and natural gas are continuously and rapidly exploited for nearly 200 years, the resources tend to be exhausted gradually. Therefore, energy problems and environmental problems become global concerns and are urgently needed to be solved. The development of secondary batteries with high energy, high density, high safety, environmental protection and low cost has great significance in the field of new energy. The lithium-sulfur battery is one of secondary battery systems with higher energy density, adopts elemental sulfur or a sulfur-containing material as a positive active material, has the theoretical energy density of 2600Wh/kg, and has the advantages of rich sulfur resources, environmental friendliness, low price and the like. The lithium-sulfur battery with high sulfur content has high capacity density and energy density, is favorable for the requirements of electric automobiles, and can solve the technical problem that the energy density of the lithium-ion battery cannot meet the requirements of the electric automobiles.
The defect that elemental sulfur is an insulator is the root of the main problems of the lithium-sulfur battery, so that the research on the cathode material becomes the key for developing the high-performance lithium-sulfur battery. In order to solve the above challenges, the problem of low sulfur conductivity can be greatly overcome by compounding the sulfur simple substance with good conductivity and a specific structural matrix, the volume expansion and the shuttle effect are reduced, and the effective way of improving the electrochemical activity is further improved, and meanwhile, the dissolution of an intermediate product can be effectively inhibited by utilizing the physical or chemical adsorption effect of the sulfur-carrying material on lithium polysulfide, so that the cycle stability of the composite anode is improved. For the composite modification of the sulfur positive electrode, researchers design and synthesize a series of composite sulfur positive electrode materials, so that the electrochemical performance of the lithium-sulfur battery is effectively improved, and the matrix materials for compounding the lithium-sulfur battery can be generally divided into three types: carbon materials, inorganic compounds, conductive polymers.
In the carbon material, the graphene derivative has a single-layer carbon atom thickness and no-mass dirac fermi, and becomes an effective substrate for loading sulfur in lithium-sulfur battery application due to high specific surface area, excellent electronic conductivity and mechanical property and good chemical thermal stability. Researchers have proved that wrapping sulfur in graphene sheets to form a two-dimensional graphene/sulfur composite material can avoid the insulativity of sulfur to a certain extent, and the mechanical strength of the two-dimensional graphene/sulfur composite material can inhibit the volume expansion of sulfur, thereby greatly improving the cycle performance. However, since it is a single-layer open structure, the storage amount of sulfur is not high, and polysulfide cannot be captured, resulting in low supported sulfur content, low coulombic efficiency, and limited cycle stability.
Therefore, the graphene nanosheets are assembled into the composite positive electrode which has a three-dimensional structure, has a large reaction space inside, inhibits polysulfide from being dissolved in electrolyte, is beneficial to the reaction and transmission of electrons and ions in the three-dimensional space, is a promising carrier material for wrapping or anchoring sulfur, and is used for preparing an excellent lithium-sulfur battery composite positive electrode.
Jiang et al synthesized a three-dimensional porous graphene aerogel/sulfur (GA-S) nanocomposite (Yong Jiang, Mengna Lu, Xuetao Ling, et al, "One-step hydrothermal synthesis of a three-dimensional porous graphene aerogels/sulfur nanocrystals for lithium-sulfur bases-scientific direct," Journal of Alloys and composites 645(2015): 509-. Under the condition that the current density is 1A/g, the specific capacity is 517.49 mA h/g after 50 times of circulation, and the sulfur carrying capacity of the material is 73.69%.
Wang et al introduced that in the solvothermal process, graphene oxide and nano-sulfur particles were ultrasonically mixed and freeze-dried to obtain a 3D-NG sulfur positive electrode composite material. (Wang C, Wang X, Wang Y, et al, microporosius free-standing Nano-sulfur/reduced graphene oxide paper as stable catalyst for lithium-sulfur battery [ J ] Nano Energy, 2015, 11:678-686.) the obtained three-dimensional nitrogen-doped graphene is a good electronic conductive framework and sulfur stabilizer, can wrap sulfur and improve the insulating property of sulfur. Under the condition that the current density is 1.5A/g, the specific capacity after 168 cycles is 592 mA h/g, and the sulfur carrying capacity of the material is 87.6 percent.
Although graphene can provide a firm electron transport network in a three-dimensional space and can adapt to volume expansion/contraction of sulfur, the preparation of graphene has the defects of long period, high cost, high risk, environmental pollution and the like; its thickness is only a monolayer of carbon atoms, with limited loading for elemental sulfur.
Based on the research results of the three-dimensional graphene related materials, how to prepare the three-dimensional structure-based multiple adjustable materials with high conductivity, high specific surface area and high porosity is a crucial step in the research field of design and synthesis of lithium-sulfur battery positive electrode composite materials. The method for synthesizing the N-doped three-dimensional porous carbon material can improve the conductivity of the carbon material and promote the electrochemical reaction in the charging and discharging processes by doping the heteroatom (N, P, B), most of the methods relate to the pyrolysis of nitrogen-rich carbon materials (such as melamine, urea and the like), and the phosphorus doping usually adopts the heating decomposition of hypophosphite, the synthesis of organic phosphorus sources, the reduction of phosphorus sources by hydrogen and the like, and has certain disadvantages. For example, the synthesis process is easy to catch fire, has high corrosion resistance requirement on equipment, generates highly toxic gas in the synthesis process, needs dangerous gas in the synthesis process and the like. Phytic acid, also known as phytic acid, is a green, non-toxic, environmentally friendly compound that can be widely extracted from plants. Further, the six phosphoric acids contained in the molecular structure thereof can be used not only as a source of phosphorus but also as a source of carbon atoms by mixing with melamine.
Based on the research of the three-dimensional carbon material and the research results of the inventor, the electrical conductivity, the specific surface area and the porosity of the composite material can be improved by the following methods:
1. the pyrolysis reaction is a reaction process in which substances are heated to decompose, and the high-temperature pyrolysis method under nitrogen is a feasible method for synthesizing a carbon material with high specific surface area and high porosity, so that the physical adsorption effect on sulfur is increased;
2. the phytic acid solution and melamine are fused to capture carbon atoms, a three-dimensional porous network structure is formed through high-temperature pyrolysis, and the limitation of electron transmission, high-sulfur-content storage and polysulfide is realized;
3. phosphorus doping is carried out through phytic acid, so that the limitation of dangerous factors and the generation of highly toxic gas in the synthesis process are avoided, and the method is an effective synthesis way for realizing green chemistry;
4. the three-dimensional carbon material is doped by the N source in the melamine and the P source in the phytic acid, so that the conductivity of the carbon material is improved, the adsorption effect of the material on polysulfide can be greatly improved, and the polysulfide is limited in the positive electrode area of the battery, so that the cycle stability and the coulombic efficiency of the battery are effectively improved.
Therefore, the preparation of the three-dimensional carbon material, the preparation of the graphene-like micro-morphology, and the doping with the specific heteroatom are feasible methods for improving the conductivity of the composite material, and have application prospects and commercial values in the field of lithium-sulfur batteries.
Disclosure of Invention
The invention aims to provide a three-dimensional porous carbon nanosheet-sulfur positive electrode material, which solves the following technical problems of a lithium-sulfur battery:
firstly, elemental sulfur is an electronic and ionic insulator at room temperature;
secondly, the sulfur carrying amount of the anode is low and generally not more than 70 percent;
thirdly, the polysulfide of the intermediate product of the sulfur in the electrochemical reduction is easily dissolved in the organic electrolyte to cause the problem of extremely large attenuation of specific capacity, and the cycle frequency is less.
In order to solve the problems, the low conductivity of elemental sulfur is compensated by preparing a high-conductivity carbon material, and the rapid transfer of lithium ions can be realized by compounding the nitrogen-doped nano flaky carbon material prepared by activation pyrolysis with sulfur, so that the high-rate charge-discharge efficiency is improved.
In order to achieve the purpose of the invention, the invention adopts the technical scheme that:
a three-dimensional porous carbon nanosheet-sulfur material is prepared by taking melamine, phytic acid and sulfur as raw materials, synthesizing the three-dimensional porous carbon nanosheet through a solution mixing method and a high-temperature pyrolysis method, and obtaining the three-dimensional porous carbon nanosheet-sulfur material through a melting method, wherein the sulfur content of the three-dimensional porous carbon nanosheet-sulfur material is 80-90%.
A preparation method of a three-dimensional porous carbon nanosheet-sulfur material comprises the following steps:
step 1) preparing a three-dimensional porous carbon nanosheet by a pyrolysis method, adding melamine into water to meet a certain mass ratio, stirring for the first time to obtain a melamine solution, dropwise adding the melamine solution into a phytic acid solution, stirring for the second time to obtain a mixed solution, drying the mixed solution to obtain mixed powder, and calcining and pyrolyzing the mixed powder under a certain condition to obtain the three-dimensional porous carbon nanosheet;
the mass ratio of the melamine to the phytic acid in the step 1 is 1:1, the first stirring time in the step 1 is 5-10 min, the second stirring time in the step 1 is 20-30 min, and the drying conditions in the step 1 are that the drying temperature is 60-100 ℃ and the drying time is 12-24 h; the calcination pyrolysis condition in the step 1 is two-stage pyrolysis, the temperature rise rate of the first-stage calcination pyrolysis is 5-10 ℃/min, the calcination pyrolysis temperature is 800-.
Step 2) preparing a three-dimensional porous carbon nanosheet-sulfur positive electrode material by a melting method, grinding and uniformly mixing the three-dimensional porous carbon nanosheet and sulfur according to the condition that the three-dimensional porous carbon nanosheet and sulfur obtained in the step 1) meet a certain mass ratio, and then carrying out heat treatment under certain conditions to obtain the three-dimensional porous carbon nanosheet-sulfur positive electrode material;
in the step 2, the mass ratio of the three-dimensional porous carbon nanosheet to the sulfur is 1 (4-9);
the heat treatment condition of the step 2 is two-stage heat treatment, the temperature rise rate of the first-stage heat treatment is 2-3 ℃/min, the heat treatment temperature is 150-.
Application of three-dimensional porous carbon nanosheet-sulfur positive electrode material as positive electrode of lithium-sulfur battery, when current density is 838 mA/cm2In the process, after the charge and the discharge are carried out for 200 times in a circulating manner, the discharge specific capacity is 600-plus 700mAh/g, and the coulomb efficiency is more stable and approaches to 100 percent; when the current density is 1675 mA/g, after the charge and the discharge are carried out for 500 times in a circulating way, the discharge specific capacity is 400-600 mAh/g, and the average attenuation rate per time is 0.079%.
The beneficial technical effects of the three-dimensional porous carbon nanosheet-based material obtained by the invention are as follows through detection results:
based on the three-dimensional porous carbon nanosheet material, through the scanning electron microscope test, the PCS900 of the three-dimensional porous carbon nanosheet material is in a three-dimensional lamellar structure.
Based on Thermogravimetric (TG) tests, it can be seen that the sulfur carrying amount of the three-dimensional porous carbon nanosheet material PCS900 is 83.5%, and the prepared three-dimensional porous carbon nanosheet material has high sulfur carrying amount.
Based on a nitrogen adsorption and desorption curve test, the specific surface area of the PCS900 of the three-dimensional porous carbon nanosheet material is 2816.12 m2G and pore volume of 2.82cm-3The material has a large specific surface area and a high pore volume.
Based on the electrochemical performance test of the prepared three-dimensional porous carbon nanosheet-sulfur positive electrode material, 949 mAh/g can be achieved under the current density of 838 mA/g, the cycle performance is greatly improved, after 200 times of cyclic charge and discharge, the discharge specific capacity is 652 mAh/g, and the coulomb efficiency is more stable and approaches 100%. The material shows good cycle performance in the charging and discharging process.
The prepared three-dimensional porous carbon nanosheet-sulfur positive electrode material still has stable electrochemical cycling stability in high current density and long cycling, when the current density is 1675 mA/g, the first discharge specific capacity is 738 mAh/g, after 500 times of cyclic charging and discharging, the discharge specific capacity is 447 mAh/g, the average attenuation rate per time is 0.079%, and the three-dimensional porous carbon nanosheet-sulfur positive electrode material has good cycling performance and can be widely applied to the field of energy storage.
Compared with the prior art, the invention has the following advantages:
1. according to the technical scheme, the three-dimensional porous carbon nanosheet prepared at 900 ℃ has 2816.12 m2Specific surface area per gram and 2.82cm-3Pore volume per gram; compared with a three-dimensional porous carbon nanosheet prepared at 800 ℃, the specific surface of the three-dimensional porous carbon nanosheet is only 125.77 m2Per g, pore volume of only 0.08cm-3The comparison shows that the invention obtains great and unexpected improvement of technical effect;
2. experiments prove that the sulfur content of the material is greatly improved and can reach 80-90%, the discharge specific capacity is also greatly improved and can reach 1000 mAh/g of 900-;
3. according to the material, the three-dimensional porous carbon nanosheet is prepared through a two-step pyrolysis method, and heterogeneous atom doping is realized by neutralizing a nitrogen source in melamine with phytic acid rich in a phosphorus source, so that a material with high specific surface area, high porosity and high conductivity is obtained, and the electrochemical cycle performance and stability of the lithium-sulfur battery are improved;
4. the three-dimensional porous carbon nanosheet-sulfur composite material prepared by the method disclosed by the invention is uniform in component, sulfur can fully enter mesoporous carbon, and the composition is very uniform. Thereby inhibiting the active substances of the electrode from gradually reducing, and also effectively inhibiting the phenomena of cathode corrosion and increase of the internal resistance of the battery caused by that the dissolved polysulfide penetrates through the diaphragm to reach the cathode lithium sheet of the battery due to the shuttle principle, further improving the cycle performance of the lithium-sulfur battery and reducing the capacity attenuation speed of the battery;
5. the invention uses the green and environment-friendly phytic acid and melamine as carriers, has low cost, good safety performance, high repeatability and high production efficiency, and can be commercially produced in a large scale.
Description of the drawings:
fig. 1 is a scanning electron microscope image of a three-dimensional porous carbon nanosheet PCS 800;
FIG. 2 is a scanning electron microscope image of a three-dimensional porous carbon nanosheet PCS 900;
FIG. 3 thermogravimetric plot of three-dimensional porous carbon nanosheet-sulfur (PCS 900-S);
FIG. 4N of three-dimensional porous carbon nanosheets (PCS 800 and PCS 900)2Adsorption and desorption curves;
fig. 5 pore size distribution of three-dimensional porous carbon nanoplates (PCS 800 and PCS 900);
FIG. 6 is a graph of the cycling performance of three-dimensional porous carbon nanosheet-sulfur (PCS 800-S and PCS 900-S) at 838 mA/g (0.5C);
FIG. 7 cycle performance plot of three-dimensional porous carbon nanosheet-sulfur (PCS 900-S) at 1675 mA/g (1C).
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings, which are given by way of examples, but are not intended to limit the present invention.
The positive electrode material (accounting for 80% of the positive electrode of the lithium-sulfur battery), the acetylene black conductive agent (accounting for 10% of the positive electrode of the lithium-sulfur battery), and the binder (accounting for 10% of the positive electrode of the lithium-sulfur battery, and the binder is 15wt% of polyvinylidene fluoride solution) in the embodiment are fully dispersed and uniformly ground to obtain positive electrode slurry, the prepared positive electrode slurry is coated on an aluminum foil current collector to prepare an electrode plate, and the electrode plate is dried to obtain the positive electrode of the lithium-sulfur battery.
The lithium-sulfur battery prepared in this example was assembled with the positive electrode, negative electrode (lithium metal sheet) and separator (polyethylene film) together, and the electrolyte solution filled in the battery was a mixed solution of 1, 3-dioxolane, ethylene glycol dimethyl ether, and lithium trifluoromethanesulfonate.
Example 1
A preparation method of a three-dimensional porous carbon nanosheet-sulfur positive electrode material comprises the following steps:
step 1) preparing a three-dimensional porous carbon nanosheet by a pyrolysis method, wherein the mass ratio of melamine to phytic acid is 1:1, firstly, 4 g of melamine is added into 80 mL of water, the first stirring is carried out for 30 min to obtain a melamine solution, then, the melamine solution is dropwise added into 4 g of phytic acid solution, the second stirring is carried out for 60 min to obtain a mixed solution, then, the mixed solution is transferred into a 60 ℃ oven to be dried for 24 h to obtain mixed powder, then, the temperature is raised to 800 ℃ at the heating rate of 5 ℃/min under the nitrogen atmosphere, the heat is preserved for 1 h, then, the temperature is raised to 900 ℃ at the heating rate of 5 ℃/min, the heat is preserved for 2 h, and;
step 2) preparing a three-dimensional porous carbon nanosheet-sulfur positive electrode material by a melting method, grinding and uniformly mixing the three-dimensional porous carbon nanosheet and sulfur obtained in the step 1) according to the mass ratio of 1:9, and then carrying out two-stage heat treatment, wherein the temperature rise rate of the first-stage heat treatment is 2 ℃/min, the heat treatment temperature is 155 ℃, the heat treatment time is 12 h, the temperature rise rate of the second-stage heat treatment is 2 ℃/min, the heat treatment temperature is 280 ℃, and the heat treatment time is 10 min. And obtaining the three-dimensional porous carbon nanosheet-sulfur cathode material.
In order to demonstrate the microscopic morphological structure of the prepared three-dimensional porous carbon nanosheet material, Scanning Electron Microscope (SEM) testing was performed. As shown in fig. 2, the three-dimensional porous carbon nanosheet material PCS900 of the present invention has a three-dimensional lamellar structure, which is beneficial to improving the conductivity of electrons and ions in a three-dimensional network space.
In order to prove that the prepared three-dimensional porous carbon nanosheet material has high sulfur carrying capacity, Thermogravimetry (TG) test is carried out. As a result, as shown in fig. 3, it can be seen that the sulfur loading of the three-dimensional porous carbon nanosheet material PCS900 is 83.5%.
In order to prove that the prepared three-dimensional porous carbon nanosheet material has large specific surface area and high pore volume, the specific surface area of PCS900 of the three-dimensional porous carbon nanosheet material is 2816.12 m as shown in figures 4 and 52G and pore volume of 2.82cm-3/g。
In order to prove the electrochemical cycling stability of the prepared three-dimensional porous carbon nanosheet material PCS900, the result is shown in figure 6, 949 mAh/g can be achieved under the current density of 838 mA/g, the cycling performance is greatly improved, after 200 times of cycling charge and discharge, the specific discharge capacity is 652 mAh/g, and the coulombic efficiency is more stable and approaches 100%. The material shows good cycle performance in the charging and discharging process, and can be widely applied to the field of energy storage.
In order to prove that the prepared three-dimensional porous carbon nanosheet material PCS900 still has stable electrochemical cycling stability in high current density long cycling, the result is shown in figure 7, when the current density is 1675 mA/g, the first discharge specific capacity is 738 mAh/g, after 500 times of cycling charge and discharge, the discharge specific capacity is 447 mAh/g, the average attenuation rate of each time is 0.079%, and the material has good cycling performance and can be widely applied to the field of energy storage.
To demonstrate the significant impact of calcination pyrolysis on material properties, comparative example 1 was provided, with a second stage of different step 1 calcination pyrolysis of the resulting three-dimensional porous carbon nanosheet material.
Comparative example 1
A preparation method of a three-dimensional porous carbon nanosheet material is the same as that in example 1 except that: the second stage calcination pyrolysis temperature of step 1 was 800 ℃, and the sample was named PCS 800.
In order to demonstrate the microscopic morphological structure of the prepared three-dimensional porous carbon nanosheet material, Scanning Electron Microscope (SEM) testing was performed. As a result, as shown in fig. 1, the three-dimensional porous carbon nanosheet material PCS800 of the present invention is a bulk structure and stacked.
To demonstrate the specific surface area and pore volume properties of the prepared three-dimensional porous carbon nanosheet material, the results are shown in fig. 4 and 5 with a specific surface area of 125.77 m2G and pore volume of 0.08cm-3(ii) in terms of/g. It can be concluded that the three-dimensional porous carbon nanosheet PCS800 has a very low specific surface area and very small pore volume.
In order to prove that the prepared three-dimensional porous carbon nanosheet material PCS800 still has stable electrochemical cycling stability in high current density long cycling, the result is shown in figure 7, when the current density is 838 mA/g, the first discharge specific capacity is 224 mAh/g, after 200 times of cyclic charging and discharging, the discharge specific capacity is 95 mAh/g, and the electrochemical cycling performance is poor.
Combining the experimental results of example 1 and comparative example 1, the following conclusions can be drawn:
1. the PCS900 of the three-dimensional porous carbon nanosheet material subjected to pyrolysis at 900 ℃ is a three-dimensional lamellar structure observed from a microscopic morphological structure, and the PCS800 not subjected to pyrolysis at 900 ℃ is a blocky structure.
2. The nitrogen adsorption and desorption curve test result shows that the PCS900 of the three-dimensional porous carbon nanosheet material has higher specific surface area and pore volume, and is more favorable for sulfur storage and electron transmission.
3. Electrochemical tests show that the initial capacity of the prepared three-dimensional porous carbon nanosheet material PCS900 can reach 949 mAh/g at the current density of 838 mA/g, the cycle performance is greatly improved, after 200 times of cyclic charge and discharge, the specific discharge capacity is 652 mAh/g, and the coulomb efficiency is stable and approaches 100%. When the current density is 1675 mA/g, the first discharge specific capacity is 738 mAh/g, after 500 times of cyclic charge and discharge, the discharge specific capacity is 447 mAh/g, and the average attenuation rate per time is 0.079%. The material shows good cycle performance in the charging and discharging process, and can be widely applied to the field of energy storage. And the first capacity of PCS800 is 224 mAh/g under the current density of 838 mA/g, and the discharge specific capacity is 95 mAh/g after 200 times of circulating charge and discharge. The battery performance is sharply decreased, and has poor electrochemical properties.
By combining various properties, the three-dimensional porous carbon nanosheet material PCS900 has good electrochemical advantages.
Claims (7)
1. A three-dimensional porous carbon nanosheet-sulfur material is characterized in that: the preparation method comprises the steps of taking melamine, phytic acid and sulfur as raw materials, synthesizing a three-dimensional porous carbon nanosheet through a solution mixing method and a high-temperature pyrolysis method, and obtaining a three-dimensional porous carbon nanosheet-sulfur material through a melting method, wherein the sulfur content of the three-dimensional porous carbon nanosheet-sulfur material is 80-90%.
2. A preparation method of a three-dimensional porous carbon nanosheet-sulfur material is characterized by comprising the following steps:
step 1) preparing a three-dimensional porous carbon nanosheet by a pyrolysis method, adding melamine into water to meet a certain mass ratio, stirring for the first time to obtain a melamine solution, dropwise adding the melamine solution into a phytic acid solution, stirring for the second time to obtain a mixed solution, drying the mixed solution to obtain mixed powder, and calcining and pyrolyzing the mixed powder under a certain condition to obtain the three-dimensional porous carbon nanosheet;
and 2) preparing the three-dimensional porous carbon nanosheet-sulfur positive electrode material by a melting method, grinding and uniformly mixing the three-dimensional porous carbon nanosheet and sulfur according to the condition that the three-dimensional porous carbon nanosheet and sulfur obtained in the step 1) meet a certain mass ratio, and then carrying out heat treatment under certain conditions to obtain the three-dimensional porous carbon nanosheet-sulfur positive electrode material.
3. The three-dimensional porous carbon nanosheet-sulfur positive electrode material of claim 2, characterized by: the mass ratio of the melamine to the phytic acid in the step 1 is 1 (1-2), the first stirring time in the step 1 is 5-10 min, the second stirring time in the step 1 is 20-30 min, and the drying conditions in the step 1 are that the drying temperature is 60-100 ℃ and the drying time is 12-24 h; the calcination pyrolysis condition in the step 1 is two-stage pyrolysis, the temperature rise rate of the first-stage calcination pyrolysis is 5-10 ℃/min, the calcination pyrolysis temperature is 800-.
4. The method of claim 2, wherein: and in the step 2, the mass ratio of the three-dimensional porous carbon nanosheet to the sulfur is 1 (4-9).
5. The method of claim 2, wherein: the heat treatment condition of the step 2 is two-stage heat treatment, the temperature rise rate of the first-stage heat treatment is 2-3 ℃/min, the heat treatment temperature is 150-.
6. The application of the three-dimensional porous carbon nanosheet-sulfur positive electrode material as the positive electrode of the lithium-sulfur battery is characterized in that: when the current density is 838 mA/cm2In the process, after the charge and the discharge are carried out for 200 times in a circulating way, the discharge specific capacity is 600-700mAh/g, and the coulomb efficiency is more stable and approaches to 100 percent.
7. The application of the three-dimensional porous carbon nanosheet-sulfur positive electrode material as the positive electrode of the lithium-sulfur battery is characterized in that: when the current density is 1675 mA/g, after the charge and the discharge are carried out for 500 times in a circulating way, the discharge specific capacity is 400-600 mAh/g, and the average attenuation rate per time is 0.079%.
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