CN109004205B - Preparation method of lithium-sulfur battery positive electrode material - Google Patents

Preparation method of lithium-sulfur battery positive electrode material Download PDF

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CN109004205B
CN109004205B CN201810889071.XA CN201810889071A CN109004205B CN 109004205 B CN109004205 B CN 109004205B CN 201810889071 A CN201810889071 A CN 201810889071A CN 109004205 B CN109004205 B CN 109004205B
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graphene oxide
sulfur
carbon fiber
doped carbon
reduced graphene
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CN109004205A (en
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张永光
贺禹森
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Synergy Innovation Institute Of Gdut Heyuan
Hebei University of Technology
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Hebei University of Technology
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Abstract

The invention relates to a preparation method of a lithium-sulfur battery anode material. The method comprises the steps of using N, N-Dimethylformamide (DMF) as a solvent, dissolving in-situ Polyacrylonitrile (PAN)/Graphene Oxide (GO) in a certain proportion in the DMF to prepare a spinning precursor solution, using an electrostatic spinning process to prepare a continuous nanofiber membrane, carrying out carbonization treatment on electrostatic spinning PAN/GO fibers to prepare an in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material, and finally carrying out a sulfur doping process to prepare a nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite positive electrode material.

Description

Preparation method of lithium-sulfur battery positive electrode material
Technical Field
The invention relates to a preparation method of a lithium-sulfur battery positive electrode material, in particular to an in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material prepared by an electrostatic spinning technology, and belongs to the field of material chemistry.
Background
With the increasing development of society, people have higher requirements on the high efficiency, portability and safety of energy. The theoretical specific capacity of the lithium-sulfur battery is up to 1672mAh/g (the theoretical energy density can reach 2600Wh/kg), which is several times of the specific capacity of the existing lithium ion anode material, and the elemental sulfur has the advantages of rich resources, low price, environmental protection and the like, thus becoming a novel lithium ion battery which is concerned about. However, the low conductivity of elemental sulfur and the shuttling effect of soluble polysulfide during charging and discharging result in the utilization rate of the cathode material being at a lower level all the time. At present, most of lithium-sulfur battery positive electrode materials adopt sulfur and carbon composite, and elemental sulfur and carbon materials with high specific surface area are compounded by filling, mixing or coating methods, so that the conductivity of the sulfur-based positive electrode materials and the cycle performance of batteries are improved.
The in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material is composed of reduced graphene oxide and nitrogen-doped carbon fiber, and finally nano sulfur is loaded into the carbon fiber and the graphene sheet layer. Wherein the nitrogen-doped carbon fiber has excellent electrical conductivity, and the nitrogen doping can enhance the surface absorption of the soluble polysulfide and improve the electronic conductivity of the carbon. The graphene oxide also has excellent conductivity, and can provide a strong electron transport framework. Graphene Oxide (GO) is reduced into reduced graphene oxide (rGO) by a high-temperature heat treatment method, most oxygen-containing functional groups are reduced, so that the surface of the rGO has defects, the rGO shows strong adsorption performance, and the shuttle effect is inhibited by adsorbing polysulfide. According to the invention, the polyacrylonitrile and the graphene oxide can be well combined by in-situ polymerization of acrylonitrile, and the in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material is obtained after low-temperature pre-heat treatment and high-temperature carbonization. Studies on nitrogen doped carbon materials have also been reported: CN105702937A discloses SnO2Method for producing/C fibres from SiO2Mixing the pore-forming agent with PAN and PVP for electrostatic spinning to form the composite fiber. After high-temperature calcination and carbonization, the obtained nitrogen-doped porous carbon fiber and SnCl in a certain proportion4·5H2Mixing O and urea, and preparing SnO by using microwave method2a/C fiber. Produced SnO2the/C fiber has a diameter of 1mm and an internal SiO2Removing the hollow structure. SnO prepared by the invention2the/C fiber not only has uniform surface distribution, but also has high specific capacity when being used as a lithium ion battery material. CN107633959A discloses a preparation method of an electrode material, which comprises the steps of dispersing a nano inorganic material in N, N-dimethylformamide to obtain a dispersion liquid, adding polyacrylonitrile, heating at 70 ℃ for 1 hour to obtain a mixed viscous liquid, carrying out electrostatic spinning on the mixed viscous liquid, and carrying out electrostatic spinning on the obtained original viscous liquid, wherein the mass fraction of the polyacrylonitrile in the mixed viscous liquid is 15 percentThe silk is subjected to air firing for 0.5 hour at the temperature of 250 ℃, the precursor after heat treatment is heated for 3 hours under the protection of nitrogen at the temperature of 800 ℃, and is cleaned and dried to obtain an in-situ nitrogen-doped carbon fiber/magnesium oxide composite material, and the obtained composite material is subjected to acid treatment to obtain the in-situ nitrogen-doped nano porous carbon fiber electrode material. However, the above-mentioned techniques have the general disadvantages that: the preparation process is relatively complex, thereby resulting in high production cost and affecting the wide application of the lithium sulfur battery. And the prepared composite material has poor conductivity, the microcosmic appearance of the composite material is difficult to control during preparation, and the electrode material still has the volume expansion phenomenon in the charge and discharge processes, so that the electrochemical performance of the electrode material is limited.
Disclosure of Invention
The invention aims to provide a preparation method of a positive electrode material applied to a lithium-sulfur battery, aiming at the defects existing in the current technical center. The method uses N, N-Dimethylformamide (DMF) as a solvent, dissolves in-situ Polyacrylonitrile (PAN)/Graphene Oxide (GO) with a certain proportion into the DMF to prepare a spinning precursor solution, uses an electrostatic spinning process to prepare a continuous nanofiber membrane, carbonizing the electrostatic spinning PAN/GO fibers to prepare an in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material, finally preparing a nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite anode material by a sulfur doping process, the composite material with excellent conductivity and polysulfide adsorption capacity not only can make up the defect of poor conductivity of elemental sulfur, but also can play a role in sulfur fixation, improve shuttle effect of polysulfide in the reaction process, thus, the utilization rate of the positive active material is improved, and the electrochemical performance of the lithium-sulfur battery is improved.
The purpose of the invention is realized by the following technical scheme:
a preparation method of a positive electrode material of a lithium-sulfur battery comprises the following steps:
preparing graphene oxide, and preparing a graphene oxide solution with the concentration of 1-4 mg/ml;
step two: in-situ polymerization acrylonitrile/graphene oxide composite material
Sequentially adding deionized water, acrylonitrile and GO solution into a beaker and stirring, then dripping concentrated sulfuric acid, sodium thiosulfate solution and potassium persulfate solution, continuously stirring, reacting at 50-100 ℃ for 0.5-1.5 h, carrying out vacuum filtration, washing the obtained solid with deionized water, and drying to obtain the PAN/GO composite material;
wherein, the volume ratio is deionized water: acrylonitrile and: GO solution: concentrated sulfuric acid: sodium thiosulfate solution: 50-150 parts of potassium persulfate solution: 5-20: 1-50: 0.5-2: 1-5: 5-25; the concentration of the GO solution is 2mg/ml, the mass fraction of the sodium thiosulfate solution is 1-10%, and the mass fraction of the potassium persulfate solution is 1-2%;
step three: preparation of polyacrylonitrile/graphene oxide fiber by electrostatic spinning
Sucking the spinning solution into an injector, and performing electrostatic spinning by adopting the following parameters: the distance between the needle point and the receiving cylinder is 25cm, the applied voltage is 18KV, the speed of the injection pump is 0.3mL/h, and the rotating speed of the collector is 500 rpm; obtaining polyacrylonitrile/graphene oxide fibers;
the spinning solution is composed of DMF and PAN/GO composite materials, and the mass ratio of the DMF to the PAN/GO: DMF 1:1 to 10;
step four: pre-heat treatment of polyacrylonitrile/graphene oxide fibers
Placing the polyacrylonitrile/graphene oxide fiber obtained in the previous step into a muffle furnace, and heating to 200-300 ℃ for stabilizing for 1-10 h to obtain oxidized polyacrylonitrile/graphene oxide fiber;
step five: high-temperature heat treatment for preparing in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material
Heating polyacrylonitrile/graphene oxide fibers to 500-1200 ℃ under the nitrogen atmosphere, and keeping the temperature for 1-5 hours; immersing the product into NaOH solution for 1-5 hours, washing the product with distilled water, and drying at 50-120 ℃ for 1-12 hours; obtaining an in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material;
step six: in-situ nitrogen-doped carbon fiber/reduced graphene oxide material sulfur doping
Mixing the in-situ nitrogen-doped carbon fiber/reduced graphene oxide and nano sulfur, grinding the mixture in an agate mortar until the mixture does not show yellow, drying the mixture in a vacuum drying oven for 1 to 20min, putting the dried mixture into a reaction kettle, sealing the reaction kettle in an argon atmosphere, and heating the reaction kettle at the temperature of between 100 and 160 ℃ for 1 to 12 hours to obtain an in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite material;
wherein, the mass ratio of the in-situ nitrogen-doped carbon fiber to the reduced graphene oxide is as follows: nano sulfur is 3: 1; dropwise adding carbon disulfide at grinding time intervals, wherein the total mass of the dropwise added carbon disulfide is 10-55% of the mass of the nano sulfur;
step seven: preparation of in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite electrode material
Mixing the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur positive electrode material obtained in the sixth step, a conductive agent and a binder polyvinylidene fluoride (PVDF), dripping NMP (N-methyl pyrrolidone) to prepare a slurry, coating the slurry on a current collector, and airing, rolling and cutting to obtain a lithium-sulfur battery positive electrode material;
wherein the material ratio is in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite material: conductive agent: 7-8.5 of binder: 0.5-2: 1; the total mass of the dropwise added NMP is 10-50% of the total mass of the materials. (the materials are in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite material, conductive agent and binder)
The conductive agent is acetylene black or Super P; the coating thickness of the slurry on the current collector is 0.01-0.1 mm; the current collector is aluminum foil, carbon-containing aluminum foil, foam nickel or carbon fiber cloth.
And the heating rates in the fourth step and the fifth step are both 1-10 ℃/min.
And the concentration of the NaOH solution in the fifth step is 0.5-2 mol/L.
The invention has the substantive characteristics that:
according to the invention, the PAN/GO composite material is synthesized in situ by the AN monomer, PAN and GO are not mechanically mixed, and the original N element in the PAN fiber is carbonized to form the in-situ N-doped carbon fiber, so that the PAN/GO composite material has excellent electrical conductivity, and experiments prove that the N doping can effectively inhibit and adsorb polysulfide, GO has a high specific surface area and a strong conductive network, rGO is generated by high-temperature heat treatment reduction, and the polysulfide can be adsorbed by the defects existing in the reduction sites, so that the electrochemical performance of the battery can be greatly improved by utilizing the synergistic effect of the PAN and GO.
Compared with the prior art, the invention has the following advantages and beneficial effects:
according to the invention, the composite material is prepared by in-situ polymerization of acrylonitrile/graphene oxide, the polyacrylonitrile/graphene oxide fiber is prepared by electrostatic spinning, the in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material is formed by carbonization treatment, and finally the sulfur doping process is carried out. The carbon-sulfur material has the characteristics of in-situ nitrogen doping, excellent conductivity and polysulfide adsorption performance, low cost, good repeatability and the like. The lithium sulfur battery anode material is applied to a lithium sulfur battery as an anode material, can effectively fix and adsorb shuttle of polysulfide in the charging and discharging process, improves the utilization rate of an anode active substance, is beneficial to reducing the polarization phenomenon in the electrochemical process of an electrode, thereby effectively relieving the volume expansion problem of the electrode material, avoids negative effects on the electrode material due to the volume expansion problem of the electrode material to a certain extent, improves the reaction reversibility, improves the cycle performance of the electrode, and further enhances the electrochemical performance of the lithium sulfur battery. Combining the electrochemical data in example 1, the initial discharge capacity of the battery at 0.1C rate reaches 1230 mAh/g; excellent rate performance is shown at current densities of 0.1C, 0.2C, 0.5C and 1C, wherein a specific discharge capacity of approximately 800mAh/g can be maintained at a high current density of 1C; it is worth noting that the discharging specific capacity of 1260mAh/g can still be kept after the electrode is cycled for 300 times under the multiplying power of 0.1C, the discharging specific capacity is gradually increased in the previous 200 cycles due to the activation effect of the electrode, and the highest discharging specific capacity reaches 1550mAh/g (which is close to the theoretical specific capacity of 1672 mAh/g).
Drawings
FIG. 1 is a micro-pore size distribution diagram of the in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material prepared in example 1;
FIG. 2 is an X-ray photoelectron spectroscopy (XPS) plot of the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite prepared in example 1; wherein, fig. 2a is a high resolution XPS spectrum of an in situ nitrogen doped carbon fiber/reduced graphene oxide/sulfur composite; FIG. 2b shows high resolution C1 s; FIG. 2c is a high resolution S2 p; FIG. 2d shows high resolution N1 s;
FIG. 3 is a graph of the infrared spectroscopy (FTIR) of the in situ nitrogen doped carbon fiber/reduced graphene oxide composite prepared in example 1;
fig. 4 is a constant current charging and discharging diagram of the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite material prepared in example 1 at 0.1C.
Fig. 5 is a charge-discharge curve diagram of the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite material prepared in example 1 at 0.1C, 0.2C, 0.5C and 1C rates;
fig. 6 is a long cycle plot at 0.1C rate of the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite prepared in example 1.
Detailed Description
The present invention will be further described with reference to the following specific examples and drawings, which are not intended to limit the invention in any manner. Reagents, methods and apparatus used in the present invention are conventional in the art unless otherwise indicated.
Unless otherwise indicated, reagents and materials used in the present invention are commercially available.
Example 1:
step one, preparing graphene oxide by improving a Hummers method:
1g of graphite and 27ml of H with the mass fraction of 95 percent2SO43ml of 0.1M H3PO4Placing the mixture into a three-neck flask, adding 6g of potassium permanganate in portions, stirring the mixture in an ice-water bath for 1 hour, raising the temperature to 50 ℃, and carrying out heat preservation reaction for 12 hours. And pouring the obtained product into ice water, adding 30% hydrogen peroxide by mass fraction while stirring until the color of the solution turns to be golden yellow, then filtering, and washing the product with 5% HCL by volume fraction and distilled water until the pH value is close to 7. Dispersing the obtained graphite oxide in water, performing ultrasonic treatment for 8 hours, and finally preparing into a graphene oxide solution of 2mg/ml for later use；
Step two: in-situ polymerization acrylonitrile/graphene oxide composite material
PAN/GO composites were prepared by in situ polymerization. Firstly, respectively adding 125ml of deionized water, 12.5ml of acrylonitrile and 50ml of GO solution with the concentration of 2mg/ml into a three-neck flask, stirring, then dropwise adding 1ml of concentrated sulfuric acid with the mass fraction of 95%, 5ml of sodium thiosulfate solution with the mass fraction of 10% and 25ml of potassium persulfate solution with the mass fraction of 2%, continuously stirring, heating by using an oil bath to maintain the temperature at 60 ℃, carrying out vacuum filtration and washing by using the deionized water for a plurality of times after reacting for 1h, finally placing the product in a surface dish and drying to obtain the PAN/GO composite material,
step three: preparation of polyacrylonitrile/graphene oxide fiber by electrostatic spinning
DMF is used as a solvent of the PAN/GO composite material, and the mass ratio of the DMF to the solvent is PAN/GO: and (4) preparing a spinning precursor solution by using DMF (dimethyl formamide) 1: 4. Sucking the spinning solution into an injector, and performing electrostatic spinning by adopting the following parameters: and adjusting the distance between the needle point and the receiving cylinder to a set distance of 25cm, applying a voltage of 18KV, the speed of an injection pump of 0.3mL/h and the rotating speed of a collector of 500rpm, and obtaining the PAN/GO electrospun fiber.
Step four: pre-heat treatment of polyacrylonitrile/graphene oxide fibers
And (3) placing the polyacrylonitrile/graphene oxide fiber material in a muffle furnace, and stabilizing for 4h at 260 ℃, wherein the heating rate is 1 ℃/min, so as to obtain the oxidized polyacrylonitrile/graphene oxide fiber material.
Step five: high-temperature heat treatment for preparing in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material
Carbonizing polyacrylonitrile/graphene oxide fibers in a nitrogen atmosphere, wherein the heating rate is as follows: 5 ℃/min, reaction temperature: keeping the temperature at 950 ℃ for 1h to carbonize the electrospun fibers. And (2) immersing the carbon fiber sample into 1mol/L NaOH solution for 5 hours to obtain micropores, washing the obtained sample with distilled water to remove residual potassium, and drying at 60 ℃ for 12 hours to obtain the in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material.
Step six: in-situ nitrogen-doped carbon fiber/reduced graphene oxide material sulfur doping
Weighing 3g of in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material and 1g of nano sulfur, mixing the materials in an agate mortar, grinding the materials, and dripping 0.2ml of carbon disulfide (CS) into the mixture in batches2) Grinding for 5min each time, and dripping CS2And (3) repeating the steps for grinding for 6 times until no obvious sulfur yellow exists, drying in a vacuum drying oven for 20min, putting the dried sample into a reaction kettle, shaking for several times in an argon filled glove box to remove oxygen in the reaction kettle, and finally putting the reaction kettle into an electric heating constant temperature drying oven to heat at 155 ℃ for 12h to prepare the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur positive electrode material.
Step seven: preparation of in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite electrode material
Mixing the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur positive electrode material obtained in the sixth step, a conductive agent and a binder polyvinylidene fluoride (PVDF), and dripping NMP (the total mass of the dripped NMP is 10% of the total mass of the material), wherein the material refers to the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite material, the conductive agent and the binder, the same as the embodiment is carried out below), preparing slurry, coating the slurry on a carbon-containing aluminum foil, airing, rolling (the pressure is 5Mpa), and cutting to obtain the lithium-sulfur battery positive electrode material;
wherein the material ratio is in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite material by mass ratio: super P: PVDF 8: 1:1, the coating thickness of the slurry on the carbon-containing aluminum foil current collector is 0.1 mm.
And C, assembling the positive plate and the lithium negative plate obtained in the step seven in a glove box filled with argon to obtain the lithium-sulfur battery. And assembling the C2025 button cell (namely assembling the positive electrode shell, the positive electrode sheet, the diaphragm, the negative electrode sheet, the gasket, the spring sheet and the negative electrode shell to obtain the lithium-sulfur cell).
Wherein, fig. 1 is a micropore size distribution diagram of the in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material prepared in example 1. It is seen from the figure that the pore diameter of the material is concentrated in 1-1.2nm, and the pore diameters are less than 2nm, which belongs to microporous materials, thus being beneficial to improving the sulfur carrying capacity of the material. FIG. 2 shows an embodiment1X-ray photoelectron spectroscopy (XPS) chart of the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite material prepared by the method. The existence of C-N/C-S bond, pyridine-N, pyrrolic-N and graphic-N bond is shown in the high-resolution C1S and N1S graphs, which indicates the existence of nitrogen-doped carbon in the composite material. As seen from the infrared spectrum of FIG. 3, a 1620cm spectrum appears in the spectrum due to the stretching vibration of the C-N bond-1This also indicates the formation of nitrogen-doped carbon during the carbonization process. Fig. 4 and 5 are a graph of the specific capacity voltage at 0.1C and a graph of the rate cycle at each current density, respectively, for the material made in example 1. It can be seen from the figure that there are two platforms for this in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur in the vicinity of 2.3V and 2.0V, which are two reduction peaks commonly found in lithium-sulfur batteries, and the initial discharge capacity reaches 1230mAh/g at 0.1C rate, and also shows excellent rate performance at current densities of 0.1C, 0.2C, 0.5C, and 1C. The in-situ nitrogen carbon fiber/reduced graphene oxide structure not only can provide a porous ion channel for a liquid electrolyte, but also can adsorb an intermediate polysulfide in a circulation process through the pore diameter in the charging and discharging processes, so that the performance of the battery is integrally improved. As shown in FIG. 6, the specific discharge capacity of 1260mAh/g is still maintained after cycling at a rate of 0.1C for 300 times.
Example 2:
step one, preparing graphene oxide by improving a Hummers method:
1g of graphite and 27ml of H with the mass fraction of 95 percent2SO43ml of 0.1M H3PO4Placing the mixture into a three-neck flask, adding 6g of potassium permanganate in portions, stirring the mixture in an ice-water bath for 1 hour, raising the temperature to 50 ℃, and carrying out heat preservation reaction for 12 hours. And pouring the obtained product into ice water, adding 30% hydrogen peroxide by mass fraction while stirring until the color of the solution turns to be golden yellow, then filtering, and washing the product with 5% HCL by volume fraction and distilled water until the pH value is close to 7. Dispersing the obtained graphite oxide in water, performing ultrasonic treatment for 8 hours, and finally preparing a graphene oxide solution of 2mg/ml for later use;
step two: in-situ polymerization acrylonitrile/graphene oxide composite material
PAN/GO composites were prepared by in situ polymerization. Firstly, respectively adding 125ml of deionized water, 12.5ml of acrylonitrile and 50ml of GO solution with the concentration of 2mg/ml into a three-neck flask, stirring, then dropwise adding 1ml of concentrated sulfuric acid with the mass fraction of 95%, 5ml of sodium thiosulfate solution with the mass fraction of 10% and 25ml of potassium persulfate solution with the mass fraction of 2%, continuously stirring, heating by using an oil bath to maintain the temperature at 60 ℃, carrying out vacuum filtration and washing by using the deionized water for a plurality of times after reacting for 1h, finally placing the product in a surface dish and drying to obtain the PAN/GO composite material,
step three: preparation of polyacrylonitrile/graphene oxide fiber by electrostatic spinning
DMF is used as a solvent of the PAN/GO composite material, and the mass ratio of the DMF to the solvent is PAN/GO: and (3) preparing a spinning precursor solution from DMF (1: 5). Sucking the spinning solution into an injector, and performing electrostatic spinning by adopting the following parameters: and adjusting the distance between the needle point and the receiving cylinder to a set distance of 25cm, applying a voltage of 18KV, the speed of an injection pump of 0.3mL/h and the rotating speed of a collector of 500rpm, and obtaining the PAN/GO electrospun fiber.
Step four: pre-heat treatment of polyacrylonitrile/graphene oxide fibers
And (3) placing the polyacrylonitrile/graphene oxide fiber material in a muffle furnace, and stabilizing for 4h at 280 ℃, wherein the heating rate is 1 ℃/min, so as to obtain the oxidized polyacrylonitrile/graphene oxide fiber material.
Step five: high-temperature heat treatment for preparing in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material
Carbonizing polyacrylonitrile/graphene oxide fibers in a nitrogen atmosphere, wherein the heating rate is as follows: 5 ℃/min, reaction temperature: keeping the temperature at 950 ℃ for 1h to carbonize the electrospun fibers. And (2) immersing the carbon fiber sample into 1mol/L NaOH solution for 5 hours to obtain micropores, washing the obtained sample with distilled water to remove residual potassium, and drying at 60 ℃ for 12 hours to obtain the in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material.
Step six: in-situ nitrogen-doped carbon fiber/reduced graphene oxide material sulfur doping
Weighing in-situ nitrogen-doped carbon fiber/reduced graphite oxide3g of alkene composite material and 1g of nano sulfur are mixed in an agate mortar for grinding, and 0.2ml of carbon disulfide (CS) is dripped in each time2) Grinding for 5min each time, and dripping CS2And (3) repeating the steps, grinding for a plurality of times until no obvious sulfur yellow exists, drying in a vacuum drying oven for 20min, putting the dried sample into a reaction kettle, shaking for a plurality of times in an argon-filled glove box to remove oxygen in the reaction kettle, and finally putting the reaction kettle into an electric heating constant-temperature drying oven to heat at 155 ℃ for 12h to prepare the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur positive electrode material.
Step seven: preparation of in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite electrode material
Mixing the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur positive electrode material obtained in the sixth step, a conductive agent and a binder polyvinylidene fluoride (PVDF), dripping NMP (N-methyl pyrrolidone) to prepare a slurry, coating the slurry on a carbon-containing aluminum foil, airing, rolling (pressure is 5Mpa), and cutting to obtain the lithium-sulfur battery positive electrode material;
wherein the material ratio is in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite material: super P: PVDF 7: 2: 1, the coating thickness of the slurry on the carbon-containing aluminum foil current collector is 0.1 mm.
The lithium-sulfur battery prepared by the embodiment has the first discharge specific capacity of 1097mAh/g under the multiplying power of 0.1C through the test of the charge-discharge cycle performance, and the discharge specific capacity can still keep 1002mAh/g after 300 cycles.
Example 3:
step one, preparing graphene oxide by improving a Hummers method:
1g of graphite and 27ml of H with the mass fraction of 95 percent2SO43ml of 0.1M H3PO4Placing the mixture into a three-neck flask, adding 6g of potassium permanganate in portions, stirring the mixture in an ice-water bath for 1 hour, raising the temperature to 50 ℃, and carrying out heat preservation reaction for 12 hours. And pouring the obtained product into ice water, adding 30% hydrogen peroxide by mass fraction while stirring until the color of the solution turns to be golden yellow, then filtering, and washing the product with 5% HCL by volume fraction and distilled water until the pH value is close to 7. Dispersing the obtained graphite oxide in water, and performing ultrasonic treatment for 8hPreparing a graphene oxide solution of 2mg/ml for later use;
step two: in-situ polymerization acrylonitrile/graphene oxide composite material
PAN/GO composites were prepared by in situ polymerization. Firstly, respectively adding 125ml of deionized water, 12.5ml of acrylonitrile and 50ml of GO solution with the concentration of 2mg/ml into a three-neck flask, stirring, then dropwise adding 1ml of concentrated sulfuric acid with the mass fraction of 95%, 5ml of sodium thiosulfate solution with the mass fraction of 10% and 25ml of potassium persulfate solution with the mass fraction of 2%, continuously stirring, heating by using an oil bath to maintain the temperature at 60 ℃, carrying out vacuum filtration and washing by using the deionized water for a plurality of times after reacting for 1h, finally placing the product in a surface dish and drying to obtain the PAN/GO composite material,
step three: preparation of polyacrylonitrile/graphene oxide fiber by electrostatic spinning
DMF is used as a solvent of the PAN/GO composite material, and the mass ratio of the DMF to the solvent is PAN/GO: and (3) preparing a spinning precursor solution from DMF (1: 6). Sucking the spinning solution into an injector, and performing electrostatic spinning by adopting the following parameters: and adjusting the distance between the needle point and the receiving cylinder to a set distance of 25cm, applying a voltage of 18KV, the speed of an injection pump of 0.3mL/h and the rotating speed of a collector of 500rpm, and obtaining the PAN/GO electrospun fiber.
Step four: pre-heat treatment of polyacrylonitrile/graphene oxide fibers
And (3) placing the polyacrylonitrile/graphene oxide fiber material in a muffle furnace, and stabilizing for 4h at 260 ℃, wherein the heating rate is 1 ℃/min, so as to obtain the oxidized polyacrylonitrile/graphene oxide fiber material.
Step five: high-temperature heat treatment for preparing in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material
Carbonizing polyacrylonitrile/graphene oxide fibers in a nitrogen atmosphere, wherein the heating rate is as follows: 5 ℃/min, reaction temperature: keeping the temperature at 1050 ℃ for 1h to carbonize the electrospun fibers. And (2) immersing the carbon fiber sample into 1mol/L NaOH solution for 5 hours to obtain micropores, washing the obtained sample with distilled water to remove residual potassium, and drying at 60 ℃ for 12 hours to obtain the in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material.
Step six: in-situ nitrogen-doped carbon fiber/reduced graphene oxide material sulfur doping
Weighing 3g of in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material and 1g of nano sulfur, mixing the materials in an agate mortar, grinding the materials, and dripping 0.2ml of carbon disulfide (CS) into the mixture in batches2) Grinding for 5min each time, and dripping CS2And (3) repeating the steps, grinding for a plurality of times until no obvious sulfur yellow exists, drying in a vacuum drying oven for 20min, putting the dried sample into a reaction kettle, shaking for a plurality of times in an argon-filled glove box to remove oxygen in the reaction kettle, and finally putting the reaction kettle into an electric heating constant-temperature drying oven to heat at 155 ℃ for 12h to prepare the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur positive electrode material.
Step seven: preparation of in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite electrode material
Mixing the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur positive electrode material obtained in the sixth step, a conductive agent and a binder polyvinylidene fluoride (PVDF), dripping NMP (N-methyl pyrrolidone) to prepare a slurry, coating the slurry on a carbon-containing aluminum foil, airing, rolling (pressure is 5Mpa), and cutting to obtain the lithium-sulfur battery positive electrode material;
wherein the material ratio is in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite material: super P: PVDF 8: 1:1, the coating thickness of the slurry on the carbon-containing aluminum foil current collector is 0.1 mm.
The lithium-sulfur battery prepared by the embodiment can achieve 1265mAh/g of specific discharge capacity at the first time under the multiplying power of 0.1C through the test of charge-discharge cycle performance, and the specific discharge capacity can still maintain 1237mAh/g after 300 cycles.
The invention is not the best known technology.

Claims (5)

1. A preparation method of a positive electrode material of a lithium-sulfur battery is characterized by comprising the following steps:
preparing graphene oxide, and preparing a graphene oxide solution with the concentration of 1-4 mg/mL;
step two: in-situ polymerization acrylonitrile/graphene oxide composite material
Sequentially adding deionized water, acrylonitrile and GO solution into a beaker and stirring, then dripping concentrated sulfuric acid, sodium thiosulfate solution and potassium persulfate solution, continuously stirring, reacting at 50-100 ℃ for 0.5-1.5 h, carrying out vacuum filtration, washing the obtained solid with deionized water, and drying to obtain the PAN/GO composite material;
wherein, the volume ratio is deionized water: acrylonitrile: GO solution: concentrated sulfuric acid: sodium thiosulfate solution: potassium persulfate solution = 50-150: 5-20: 1-50: 0.5-2: 1-5: 5-25; the mass fraction of the sodium thiosulfate solution is 1-10%, and the mass fraction of the potassium persulfate solution is 1-2%;
step three: preparation of polyacrylonitrile/graphene oxide fiber by electrostatic spinning
Sucking the spinning solution into an injector, and performing electrostatic spinning by adopting the following parameters: the distance between the needle point and the receiving cylinder is 25cm, the applied voltage is 18KV, the speed of the injection pump is 0.3mL/h, and the rotating speed of the collector is 500 rpm; obtaining polyacrylonitrile/graphene oxide fibers;
the spinning solution is composed of DMF and PAN/GO composite materials, and the mass ratio of the DMF to the PAN/GO: DMF =1: 1-10;
step four: pre-heat treatment of polyacrylonitrile/graphene oxide fibers
Placing the polyacrylonitrile/graphene oxide fiber obtained in the previous step into a muffle furnace, and heating to 200-300 ℃ for stabilizing for 1-10 h to obtain oxidized polyacrylonitrile/graphene oxide fiber;
step five: high-temperature heat treatment for preparing in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material
Heating oxidized polyacrylonitrile/graphene oxide fibers to 500-1200 ℃ under the nitrogen atmosphere, and keeping the temperature for 1-5 hours; immersing the product into NaOH solution for 1-5 hours, washing the product with distilled water, and drying at 50-120 ℃ for 1-12 hours; obtaining an in-situ nitrogen-doped carbon fiber/reduced graphene oxide composite material;
step six: in-situ nitrogen-doped carbon fiber/reduced graphene oxide material sulfur doping
Mixing the in-situ nitrogen-doped carbon fiber/reduced graphene oxide and nano sulfur, grinding the mixture in an agate mortar until the mixture does not show yellow, drying the mixture in a vacuum drying oven for 1 to 20min, putting the dried mixture into a reaction kettle, sealing the reaction kettle in an argon atmosphere, and heating the reaction kettle at the temperature of between 100 and 160 ℃ for 1 to 12 hours to obtain an in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite material;
wherein the mass ratio is in-situ nitrogen-doped carbon fiber/reduced graphene oxide: nano sulfur =3: 1; dropwise adding carbon disulfide at grinding time intervals, wherein the total mass of the dropwise added carbon disulfide is 10-55% of the mass of the nano sulfur;
step seven: preparation of in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite electrode material
Mixing the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur positive electrode material obtained in the sixth step, a conductive agent and a binder polyvinylidene fluoride (PVDF), dripping NMP (N-methyl pyrrolidone) to prepare a slurry, coating the slurry on a current collector, and airing, rolling and cutting to obtain a lithium-sulfur battery positive electrode material;
wherein the material ratio is in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur composite material: conductive agent: 7-8.5 of binder: 0.5-2: 1.
2. the method of claim 1, wherein the conductive agent is acetylene black or Super P; the coating thickness of the slurry on the current collector is 0.01-0.1 mm; the current collector is aluminum foil, foam nickel or carbon fiber cloth.
3. The method according to claim 1, wherein the temperature rise rate of the temperature rise in the fourth step and the temperature rise rate of the temperature rise in the fifth step are both 1 to 10 ℃/min.
4. The method of claim 1, wherein the concentration of the NaOH solution in step five is 0.5-2 mol/L.
5. The method for preparing the positive electrode material of the lithium-sulfur battery according to claim 1, wherein the total mass of the dropwise added NMP in the seventh step is 10-50% of the total mass of the in-situ nitrogen-doped carbon fiber/reduced graphene oxide/sulfur positive electrode material, the conductive agent and the binder polyvinylidene fluoride.
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