CN110571418B - Lithium-sulfur battery positive electrode material and preparation method thereof - Google Patents
Lithium-sulfur battery positive electrode material and preparation method thereof Download PDFInfo
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Abstract
The invention discloses a lithium-sulfur battery positive electrode material and a preparation method thereof. The method comprises the following steps: providing a metal-organic framework material; dissolving a metal-organic framework material, a sulfur source and a polymer in a solvent, stirring for 1-12 h, and controlling the temperature at 0-30 ℃ to obtain a spinning solution; performing electrostatic spinning on the spinning solution to obtain a polymer/metal organic framework film; sequentially carrying out pre-oxidation, carbonization and acid solution treatment on the film to obtain sulfur and nitrogen co-doped porous carbon nanofiber; and mixing the porous carbon nanofiber and sulfur powder, and heating and preserving heat for 1-12 h in an inert atmosphere to obtain the lithium-sulfur battery positive electrode material. The sulfur and nitrogen co-doped porous carbon nanofiber prepared by the preparation method has a large specific surface area and a rich pore structure. After sulfur is loaded, the lithium sulfur battery anode is used as the anode of the lithium sulfur battery, so that the conductivity of a sulfur electrode is improved, the influence caused by volume expansion of sulfur is relieved, and the shuttle effect of sulfur can be effectively inhibited.
Description
Technical Field
The invention relates to the field of lithium-sulfur battery positive electrode materials, in particular to a lithium-sulfur battery positive electrode material and a preparation method thereof.
Background
Problems of environmental pollution and exhaustion of fossil energy are increasingly aggravated, and lithium ion batteries having a long cycle life and a high energy density are considered as effective means for solving these problems. However, the theoretical energy density of lithium ion batteries based on "intercalation" electrochemistry is only 387W h kg-1. In this case, lithium-sulfur batteries (Li-S) based on the "conversion" electrochemical mechanism are due to their high specific capacity (1675mA h g)-1) And energy density (2600W h. kg)-1) And received much attention. In addition, the sulfur also has the advantages of economy, environmental protection, rich resources and the like. However, the practical application of Li-S batteries is restricted: (i) the insulating properties of sulfur and its final discharge products reduce the availability of active materials, (ii) sulfur undergoes volumetric expansion and contraction during the electrochemical process, causing structural damage to the electrode, and (iii) lithium polysulfides (Li)2SnN is 4. ltoreq. n.ltoreq.6) is readily soluble in electrolytes and is accompanied by a "shuttling" effect, leading to loss of active material.
Researchers have proposed a number of approaches to solve the above problems, such as sulfur loading using porous carbon, sulfur coating with conductive polymers, construction of novel electrode structures, and electrolyte modification. Of these methods, sulfur loading onto porous carbon substrates is of most interest. The porous carbon has the unique properties of large specific surface area, excellent conductivity and the like, and can promote ion/electron transfer and relieve the dissolution of lithium polysulfide. Although the sulfur and carbon composite can significantly improve sulfur utilization and mitigate "shuttle effect," the sulfur/carbon composite electrode still does not achieve satisfactory electrochemical performance. The inventor researches and discovers that the heteroatom doping can effectively regulate and control the polarity of the carbon fiber, and is beneficial to improving the chemical adsorption effect of the carbon material on a lithium polysulfide intermediate, thereby improving the electrochemical performance of the carbon material. Doping nitrogen atoms into the carbon matrix can form nitrogen-containing groups such as pyrrole nitrogen, graphite nitrogen, and pyridine nitrogen, wherein the graphite nitrogen contributes to improving the conductivity of the carbon material, and the pyrrole nitrogen and pyridine nitrogen contribute to promoting the chemisorption of lithium polysulfide.
Disclosure of Invention
In view of the above-mentioned shortcomings of the prior art, the present invention aims to provide a lithium-sulfur battery cathode material and a preparation method thereof, which are used as a host material of sulfur and are aimed at improving the conductivity of a sulfur cathode, alleviating the problem of volume expansion during the charging and discharging processes of the sulfur cathode, and solving the influence caused by the shuttle effect.
The technical scheme of the invention is as follows:
a preparation method of a positive electrode material of a lithium-sulfur battery comprises the following steps:
A. providing a metal salt solution and an organic ligand solution, mixing the metal salt solution and the organic ligand solution, stirring for 0.5-10h, controlling the temperature at 0-30 ℃, and standing for 1-24 h to obtain a metal-organic framework material;
B. dissolving the metal-organic framework material, a sulfur source and a polymer in a solvent, stirring for 1-12 h, and controlling the temperature at 0-30 ℃ to obtain a spinning solution; performing electrostatic spinning on the spinning solution to obtain a polymer/metal organic framework film;
C. pre-oxidizing the polymer/metal organic framework film for 1-3 h under an air atmosphere, carbonizing the polymer/metal organic framework film under an inert atmosphere, and finally treating the carbonized polymer/metal organic framework film with an acid solution to obtain sulfur and nitrogen co-doped porous carbon nanofibers;
D. and mixing the sulfur and nitrogen co-doped porous carbon nanofiber with sulfur powder, and heating and preserving heat for 1-12 hours in an inert atmosphere to obtain the lithium-sulfur battery positive electrode material.
Further, step a comprises: respectively dissolving metal salt and an organic ligand in a solvent, stirring for 1-2 hours to respectively obtain a metal salt solution and an organic ligand solution, mixing the metal salt solution and the organic ligand solution, magnetically stirring for 0.5-10 hours at the temperature of 0-30 ℃, standing for 1-24 hours, separating, washing and drying to obtain the metal-organic framework material.
Further, step B comprises: dissolving the metal-organic framework material, a sulfur source and a polymer in a solvent, magnetically stirring for 1-12 hours, and controlling the temperature at 0-30 ℃ to obtain a spinning solution; and injecting the spinning solution into an electrostatic spinning device, performing electrostatic spinning to obtain a polymer/metal organic framework film, and finally performing vacuum drying at 50-100 ℃ for 1-10 hours to remove the residual solvent.
Further, in the step A, the metal salt is selected from one of zinc nitrate, zinc acetate, zinc sulfate, cobalt nitrate, cobalt acetate and cobalt sulfate; the organic ligand is selected from one of dimethyl imidazole, formic acid and fumaric acid.
Further, in the step B, the sulfur source is one of thiourea, thioacetamide and trithiouric acid; the polymer is selected from one of polyaniline, polyacrylonitrile and polyvinylpyrrolidone.
Further, in the step C, the pre-oxidation temperature is 200-300 ℃, and the carbonization temperature is 500-1000 ℃.
Further, in the step C, the acid solution is hydrochloric acid solution, and the concentration of the hydrochloric acid solution is 3 mol.L-1And the treatment time of the acid solution is 1-24 hours.
Further, in the step D, the mass ratio of the sulfur-nitrogen co-doped porous carbon nanofiber to the sulfur powder is 1: 1-10.
Further, in the step D, the heating and heat preservation temperature is 100-200 ℃.
The invention discloses a lithium-sulfur battery positive electrode material, which is prepared by the preparation method of the lithium-sulfur battery positive electrode material.
Has the advantages that: the sulfur and nitrogen co-doped porous carbon nanofiber prepared by the preparation method has a large specific surface area and a rich pore structure. After sulfur is loaded, the lithium sulfur battery anode is used as the anode of the lithium sulfur battery, so that the conductivity of a sulfur electrode is improved, the influence caused by volume expansion of sulfur is relieved, and more importantly, the shuttling effect of sulfur can be effectively inhibited. The lithium-sulfur battery shows better cycling stability and still has 554mA h g after being cycled for 150 circles under the current density of 0.1C-1The specific capacity of (A).
Drawings
FIG. 1 is an electron micrograph of ZIF-8 prepared in example 1.
FIG. 2 is an electron micrograph of SN-PCNF-1 prepared in example 2.
FIG. 3 is an electron micrograph of SN-PCNF-2 prepared in example 3.
FIG. 4 is an SEM image of N-PCNF prepared in example 4.
Fig. 5 is an electron micrograph of the CNF prepared in example 5.
Fig. 6 is a graph showing cycle performance of lithium sulfur batteries assembled by the positive electrode materials of the lithium sulfur batteries prepared in examples 6 and 7.
Detailed Description
The invention provides a lithium-sulfur battery positive electrode material and a preparation method thereof, and the invention is further described in detail below in order to make the purpose, technical scheme and effect of the invention clearer and more clear and definite. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The embodiment of the invention provides a preparation method of a lithium-sulfur battery positive electrode material, which comprises the following steps:
s100, providing a metal salt solution and an organic ligand solution, mixing the metal salt solution and the organic ligand solution, stirring for 0.5-10 hours, controlling the temperature at 0-30 ℃, and standing for 1-24 hours to obtain a metal-organic framework material;
s200, dissolving the metal-organic framework material, a sulfur source and a polymer in a solvent, stirring for 1-12 hours, and controlling the temperature at 0-30 ℃ to obtain a spinning solution; injecting the spinning solution into an electrostatic spinning device, and performing electrostatic spinning to obtain a polymer/metal organic framework film;
s300, pre-oxidizing the polymer/metal organic framework film for 1-3 hours in an air atmosphere, carbonizing the polymer/metal organic framework film in an inert atmosphere, and finally treating the carbonized polymer/metal organic framework film with an acid solution to obtain sulfur and nitrogen co-doped porous carbon nanofibers;
s400, mixing the sulfur and nitrogen co-doped porous carbon nanofiber with sulfur powder serving as a positive electrode material, and heating and preserving heat for 1-12 hours in an inert atmosphere to obtain the lithium-sulfur battery positive electrode material.
Compared with the prior art, the method has the following effects:
(1) the embodiment constructs the unique sulfur and nitrogen co-doped porous carbon nanofiber, so that the conductivity of the sulfur anode is effectively improved, the physical fixation effect on a lithium polysulfide intermediate is enhanced, micropores in a hierarchical pore structure can be used as a sulfur host, and mesopores and macropores are beneficial to electrolyte infiltration and ion diffusion.
(2) Nitrogen atoms (N) doped in a carbon skeleton have stronger electronegativity and can adsorb lithium ions with positive charges in lithium polysulfide, sulfur (S) has stronger electropositivity and can adsorb ions with negative charges, so the sulfur and nitrogen co-doping strategy greatly enhances the chemical adsorption effect of a carbon material on a lithium polysulfide intermediate and solves the influence caused by a shuttle effect;
(3) the material prepared by the electrostatic spinning method can directly support the electrode, does not need a conductive agent, an adhesive and a metal current collector, and simultaneously improves the energy density of the battery;
(4) the preparation method of the embodiment is simple in preparation process and strong in operability.
In one embodiment, step S100 includes: respectively dissolving metal salt and an organic ligand in a solvent, stirring for 1-2 hours to respectively obtain a metal salt solution and an organic ligand solution, mixing the metal salt solution and the organic ligand solution, magnetically stirring for 0.5-10 hours at the temperature of 0-30 ℃, standing for 1-24 hours, separating, washing and drying to obtain the metal-organic framework material.
In one embodiment, the metal salt is selected from zinc nitrate, zinc acetate, zinc sulfate, cobalt nitrate, cobalt acetate, cobalt sulfate, and the like, without being limited thereto. Preferably, the metal salt is zinc nitrate, most of metal zinc ions can form steam to be removed in the carbonization process, and the use amount of subsequent acid solution treatment is reduced.
In one embodiment, the organic ligand is selected from the group consisting of dimethylimidazole, formic acid, fumaric acid, and the like, without being limited thereto. Preferably, the organic ligand is dimethyl imidazole, and the abundant N atom in the structure can increase the content of nitrogen in the product.
In one embodiment, step S200 includes: dissolving the metal-organic framework material, a sulfur source and a polymer in a solvent according to a certain proportion, magnetically stirring for 1-12 h, and controlling the temperature at 0-30 ℃ to obtain a spinning solution; and injecting the spinning solution into an electrostatic spinning device, carrying out electrostatic spinning to obtain a polymer/metal organic framework film, and finally carrying out vacuum drying at 50-100 ℃ for 1-10 h to remove the residual solvent. Preferably, the mass ratio is 0.25: 1: 1.
in one embodiment, the sulfur source is thiourea, thioacetamide, trithiouric acid, and the like, without limitation. In this example, the sulfur source was used for doping into porous carbon nanofibers.
In one embodiment, the polymer is selected from polyaniline, polyacrylonitrile, polyvinylpyrrolidone, and the like, without being limited thereto. Preferably, the polymer is polyacrylonitrile, and the effect is optimal.
In one embodiment, the solvent is one of, but not limited to, water, acetonitrile, N-N dimethylformamide, and the like.
In the step S300, the polymer/metal organic framework film is pre-oxidized for 1-3 hours under an air atmosphere, so that polymer molecules are further crosslinked and cyclized, and the flexibility of the fiber and the survival rate after carbonization are improved. And carbonizing in an inert atmosphere, forming a carbon material by the polymer and the organic ligand, converting metal ions in the skeleton into simple substances, and finally removing residual metal simple substances in the metal organic ligand by using an acid solution to obtain the sulfur and nitrogen co-doped porous carbon nanofiber.
In one embodiment, in step S300, the pre-oxidation temperature is 200 to 300 ℃, the carbonization temperature is 500 to 1000 ℃, and the inert atmosphere is nitrogen or argon.
In one embodiment, in step S300, the acid solution is a hydrochloric acid solution with a concentration of 3mol · L-1And the acid solution treatment time is 1-24 h.
In one embodiment, in the step S400, the mass ratio of the sulfur-nitrogen co-doped porous carbon nanofiber to the sulfur powder is 1: 1-10. Preferably, when the mass ratio is 1:5, the sulfur carrying effect is optimal. In this example, the sulfur powder was used as a positive electrode material.
In one embodiment, in step S400, the heating and heat preservation temperature is 100-200 ℃.
In a specific embodiment, the preparation method of the lithium-sulfur battery positive electrode material comprises the following steps:
1. respectively dissolving zinc nitrate and 2-methylimidazole in methanol, stirring for 1-2 hours to obtain a stable solution, mixing the two solutions, magnetically stirring for 0.5-10 hours, controlling the temperature at 0-30 ℃, standing for 1-24 hours, separating, washing and drying to obtain a ZIF-8 material as a sacrificial template and a carbon precursor;
2. and (3) dissolving the ZIF-8 material obtained in the step (1) with trithiocyanuric acid and polyacrylonitrile in DMF (dimethyl formamide), magnetically stirring for 1-12 hours, and controlling the temperature to be 0-30 ℃ to obtain uniform spinning solution. Injecting the spinning solution into a device, carrying out electrostatic spinning to obtain a polyacrylonitrile/ZIF-8 film, and carrying out vacuum drying on the film at the temperature of 50-100 ℃ for 1-10 h to remove residual solvent;
3. placing the polyacrylonitrile/ZIF-8 film obtained in the step 2 in a tubular furnace, pre-oxidizing in an air atmosphere, preserving heat for 1-3 hours, carbonizing in an inert atmosphere, and finally treating with an acid solution to remove residual metal simple substance zinc to obtain sulfur and nitrogen co-doped porous carbon nanofiber;
4. mixing sulfur and nitrogen co-doped porous carbon nanofiber and sulfur powder together in a certain mass ratio, placing the mixture in a tubular furnace, and then heating and preserving the heat of the tubular furnace for 1-12 hours under an inert atmosphere to obtain the lithium-sulfur battery cathode material.
In this embodiment, a metal salt and an organic ligand are respectively dissolved in a suitable solvent, and the two solutions are mixed, stirred, stood, separated, washed and dried to obtain a metal-organic framework material; dissolving the obtained metal-organic framework material, a sulfur source and polymer powder in a proper amount of solvent, stirring to obtain uniform spinning solution, and performing electrostatic spinning to obtain a polymer/metal-organic framework film; pre-oxidizing the film in an air atmosphere, carbonizing the film in an inert atmosphere, and treating the film with an acid solution to obtain sulfur and nitrogen co-doped porous carbon nanofibers; mixing sulfur and nitrogen co-doped porous carbon nanofiber and sulfur powder according to a certain mass ratio, and heating and preserving heat in an inert atmosphere to obtain the lithium-sulfur battery positive electrode material. The lithium-sulfur battery cathode material prepared by the method provided by the embodiment improves the conductivity of a sulfur cathode, the volume expansion of sulfur in the cyclic process is buffered, and the co-doping strategy of sulfur atoms and nitrogen atoms obviously enhances the chemical adsorption of carbon materials on lithium polysulfide, thereby effectively inhibiting the shuttle effect.
The embodiment of the invention provides a lithium-sulfur battery cathode material, which is prepared by the preparation method of the lithium-sulfur battery cathode material.
The sulfur and nitrogen co-doped porous carbon nanofiber prepared by the preparation method has a large specific surface area and a rich pore structure. After sulfur is loaded, the lithium sulfur battery anode is used as a lithium sulfur battery anode, so that the conductivity of a sulfur electrode is improved, the influence caused by volume expansion of sulfur is relieved, and more importantly, the shuttling effect of sulfur can be effectively inhibited. The lithium-sulfur battery shows better cycling stability and still has 554mA h g after being cycled for 150 circles under the current density of 0.1C-1The specific capacity of (A).
The invention is further illustrated by the following specific examples.
Example 1
Respectively dissolving zinc nitrate hexahydrate and 2-methylimidazole in 100mL of methanol according to the molar ratio of 1:5, stirring at room temperature to obtain stable solution, mixing the two solutions, magnetically stirring for 0.5h at the rotation speed of 400 r.min-1And finally standing for 24h to obtain ZIF-8. The sample was washed several times with ethanol, transferred to a 75 ℃ oven and dried to obtain pure ZIF-8, as shown in fig. 1.
Example 2
The ZIF-8 obtained in the example 1 is adopted, and the mass ratio of trithiocyanuric acid to ZIF-8 to polyacrylonitrile is 0.25: 1:1, dissolved in 15mL of N-N Dimethylformamide (DMF) solution and stirred for 12h to obtain a uniform spinning solution. And (3) injecting the spinning solution into an electrostatic spinning device, setting voltage, temperature, distance from a needle head to a receiving plate and liquid feeding speed, and carrying out electrostatic spinning to obtain the polyacrylonitrile/ZIF-8 film. The residual DMF solvent was removed by drying in a vacuum oven at 75 ℃ for 2 h. Putting the polyacrylonitrile/ZIF-8 film into a tube furnace, keeping the temperature at 240 ℃ for 2h in the air atmosphere, and then carrying out N reaction2Keeping the temperature at 800 ℃ for 6h in the atmosphere. Soaking the sample in 3 mol.L-1Removing residual simple substance zinc in the sample from HCl solution to obtain SN-PCNF-1The morphology is shown in FIG. 2.
Example 3
Adopting the ZIF-8 obtained in the example 1, wherein the mass ratio of the trithiocyanuric acid to the ZIF-8 to the polyacrylonitrile is 0.25: 1: 2, dissolving in 15mL of N-N Dimethylformamide (DMF) solution, and stirring for 12h to obtain a uniform spinning solution. And then injecting the spinning solution into an electrostatic spinning device, setting the voltage, the temperature, the distance from the needle head to the receiving plate and the liquid feeding speed, and carrying out electrostatic spinning to obtain the polyacrylonitrile/ZIF-8 film. The residual DMF solvent was removed by drying in a vacuum oven at 75 ℃ for 2 h. Putting the polyacrylonitrile/ZIF-8 film into a tube furnace, keeping the temperature at 240 ℃ for 2h in the air atmosphere, and then carrying out N reaction2Keeping the temperature at 800 ℃ for 6h in the atmosphere. Soaking the sample in 3 mol.L-1Residual elemental zinc in the sample is removed from the HCl solution to obtain SN-PCNF-2, and the appearance is shown in figure 3.
Example 4
Adopting the ZIF-8 obtained in the example 1, wherein the mass ratio of the trithiocyanuric acid to the ZIF-8 to the polyacrylonitrile is 0: 1: 2, dissolving in 15mL of N-N Dimethylformamide (DMF) solution, and stirring for 12h to obtain a uniform spinning solution. And then injecting the spinning solution into an electrostatic spinning device, setting the voltage, the temperature, the distance from a needle head to a receiving plate and the liquid feeding speed, and carrying out electrostatic spinning to obtain the polyacrylonitrile/ZIF-8 film. The residual DMF solvent was removed by drying in a vacuum oven at 75 ℃ for 2 h. Putting the polyacrylonitrile/ZIF-8 film into a tube furnace, keeping the temperature at 240 ℃ for 2h in the air atmosphere, and then carrying out N reaction2Keeping the temperature at 800 ℃ for 6h in the atmosphere to obtain the N-PCNF, wherein the appearance is shown in figure 4.
Example 5
Dissolving polyacrylonitrile with a certain mass in 15mL of N-N Dimethylformamide (DMF) solution, and stirring for 12 hours to obtain a uniform spinning solution. And then injecting the spinning solution into an electrostatic spinning device, setting the voltage, the temperature, the distance from the needle head to the receiving plate and the liquid feeding speed, and carrying out electrostatic spinning to obtain the polyacrylonitrile film. The residual DMF solvent was removed by drying in a vacuum oven at 75 ℃ for 2 h. Placing polyacrylonitrile film in a tube furnace, keeping the temperature at 240 ℃ for 2h under the atmosphere of air, and then carrying out N2Keeping the temperature at 800 ℃ for 6h under the atmosphere to obtain CNF, wherein the appearance is shown in figure 5.
Example 6
Mixing the SN-PCNF-2 obtained in the embodiment 3 and sulfur powder together in a mass ratio of 1:5, placing the mixture in a tube furnace, preserving the heat at 155 ℃ in Ar atmosphere for 12h to obtain SN-PCNF @ S, cutting the prepared SN-PCNF @ S into round pieces with the diameter of 14mm, directly using the round pieces as a positive electrode material, using a lithium piece as a negative electrode, using a Cellgard2300 porous membrane as a diaphragm, and using a mixed solution of LiTFSI/DME + DOL (volume ratio of 1:1) in a volume ratio of 1mol/L as an electrolyte to assemble a 2032 button cell. And performing electrochemical performance test on a Land-CT2001A (Wuhanjinnuo electronic) program-controlled full-automatic electrochemical tester. Referring to FIG. 6, the charge and discharge test was carried out at a current of 0.1C, the voltage interval was set to 1.6-3.0V, and the initial discharge specific capacity of SN-PCNF @ S was 1133mA h g-1After the initial coulombic efficiency is 76.4 percent and the circulation is 150 circles, the capacity of SN-PCNF @ S is kept at 554mA h.g-1。
Example 7
Mixing the N-PCNF obtained in the example 4 and sulfur powder together in a mass ratio of 1:5, placing the mixture in a tubular furnace, preserving the heat at 155 ℃ for 12h in Ar atmosphere to obtain N-PCNF @ S, cutting the prepared N-PCNF @ S into round pieces with the diameter of 14mm, directly using the round pieces as a positive electrode material, using a lithium piece as a negative electrode, using a Cellgard2300 porous membrane as a diaphragm, and using a mixed solution of LiTFSI/DME + DOL (volume ratio of 1:1) in a volume ratio of 1mol/L as an electrolyte to assemble a 2032 button cell. And performing electrochemical performance test on a program-controlled full-automatic electrochemical tester of Land-CT2001A (Wuhanjinnuo electronic). Referring to FIG. 6, the charge and discharge test was carried out at a current of 0.1C, the voltage interval was set to 1.6-3.0V, and the initial discharge specific capacity of N-PCNF @ S was 1152mA h g-1The first coulombic efficiency is 74.6 percent, and after circulating for 150 circles, the capacity of the N-PCNF @ S is attenuated to 416mA h g-1。
In conclusion, the sulfur and nitrogen co-doped porous carbon nanofiber lithium sulfur battery positive electrode material prepared by the method improves the conductivity of a sulfur positive electrode, buffers the volume expansion of sulfur in the cyclic process, and obviously enhances the chemical adsorption of carbon materials on lithium polysulfide by a sulfur atom and nitrogen atom co-doping strategy, thereby effectively inhibiting the shuttle effect.
It is to be understood that the invention is not limited to the examples described above, but that modifications and variations may be effected thereto by those of ordinary skill in the art in light of the foregoing description, and that all such modifications and variations are intended to be within the scope of the invention as defined by the appended claims.
Claims (9)
1. A preparation method of a lithium-sulfur battery positive electrode material is characterized by comprising the following steps:
A. providing a metal salt solution and an organic ligand solution, mixing the metal salt solution and the organic ligand solution, stirring for 0.5-10h, controlling the temperature at 0-30 ℃, and standing for 1-24 h to obtain a metal-organic framework material;
B. dissolving the metal-organic framework material, a sulfur source and a polymer in a solvent, stirring for 1-12 h, and controlling the temperature at 0-30 ℃ to obtain a spinning solution; performing electrostatic spinning on the spinning solution to obtain a polymer/metal organic framework film;
C. pre-oxidizing the polymer/metal organic framework film for 1-3 h under an air atmosphere, carbonizing the polymer/metal organic framework film under an inert atmosphere, and finally treating the carbonized polymer/metal organic framework film with an acid solution to obtain sulfur and nitrogen co-doped porous carbon nanofibers;
D. mixing the sulfur and nitrogen co-doped porous carbon nanofiber with sulfur powder, and heating and insulating for 1-12 hours in an inert atmosphere to obtain a lithium-sulfur battery positive electrode material; in the step D, the heating and heat preservation temperature is 100-200 ℃.
2. The method for preparing a positive electrode material for a lithium-sulfur battery according to claim 1, wherein step a comprises: respectively dissolving metal salt and an organic ligand in a solvent, stirring for 1-2 hours to respectively obtain a metal salt solution and an organic ligand solution, mixing the metal salt solution and the organic ligand solution, magnetically stirring for 1-10 hours at the temperature of 0-30 ℃, standing for 1-24 hours, separating, washing and drying to obtain the metal-organic framework material.
3. The method for preparing a positive electrode material for a lithium-sulfur battery according to claim 1, wherein step B comprises: dissolving the metal-organic framework material, a sulfur source and a polymer in a solvent, magnetically stirring for 1-12 hours, and controlling the temperature at 0-30 ℃ to obtain a spinning solution; and injecting the spinning solution into an electrostatic spinning device, performing electrostatic spinning to obtain a polymer/metal organic framework film, and finally performing vacuum drying at 50-100 ℃ for 1-10 hours to remove the residual solvent.
4. The method for preparing a positive electrode material for a lithium-sulfur battery according to claim 1, wherein in the step a, the metal salt is one selected from the group consisting of zinc nitrate, zinc acetate, zinc sulfate, cobalt nitrate, cobalt acetate, and cobalt sulfate; the organic ligand is selected from one of dimethyl imidazole, formic acid and fumaric acid.
5. The method for preparing the positive electrode material of the lithium-sulfur battery according to claim 1, wherein in the step B, the sulfur source is one of thiourea, thioacetamide and trithiouric acid; the polymer is selected from one of polyaniline, polyacrylonitrile and polyvinylpyrrolidone.
6. The method for preparing a positive electrode material for a lithium-sulfur battery according to claim 1, wherein the pre-oxidation temperature in step C is 200 to 300 ℃ and the carbonization temperature is 500 to 1000 ℃.
7. The method of claim 1, wherein in step C, the acid solution is a hydrochloric acid solution having a concentration of 3 mol-L-1And the treatment time of the acid solution is 1-24 hours.
8. The preparation method of the positive electrode material of the lithium-sulfur battery according to claim 1, wherein in the step D, the mass ratio of the sulfur-nitrogen co-doped porous carbon nanofiber to the sulfur powder is 1: 1-10.
9. A lithium-sulfur battery positive electrode material, characterized by being prepared by the preparation method of the lithium-sulfur battery positive electrode material according to any one of claims 1 to 8.
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