Preparation method of sulfur-polyionic liquid copolymer composite positive electrode and lithium-sulfur battery

Abstract

The application discloses a preparation method of a sulfur@poly ionic liquid copolymer composite positive electrode and a lithium-sulfur battery. The preparation method comprises the following steps: (1) mixing an ionic liquid and a solvent, adding lithium salt and an initiator, stirring until clear and transparent, and obtaining a precursor solution; (2) mixing the precursor solution and sulfur powder, putting the mixture into a beating box for ball milling, and then drying to obtain sulfur@poly ionic liquid copolymer; and (3) taking the sulfur@poly ionic liquid copolymer, conductive graphite, acetylene black and polyvinylidene fluoride as raw materials to prepare a positive electrode slurry; coating the positive electrode slurry on a carbon-coated aluminum foil, drying, and obtaining the sulfur@poly ionic liquid copolymer composite positive electrode. In the poly ionic liquid copolymer prepared by the application, the surfaces of part of the sulfur powder are coated with poly ionic liquid, so that the ion transmission rate and the electrochemical stability of the positive electrode can be enhanced; the surfaces of the remaining part of the sulfur powder are not coated with poly ionic liquid, and the remaining part of the sulfur powder is exposed, so that the remaining part of the sulfur powder can be combined with the conductive agent to enhance the conductivity of the positive electrode. The sulfur@poly ionic liquid copolymer composite positive electrode can make the lithium-sulfur battery have excellent conductivity and chemical stability, and can greatly improve the cycle performance and the rate performance of the lithium-sulfur battery under different current densities.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of batteries, and particularly relates to a preparation method of a sulfur@polyionic liquid copolymer composite cathode and a lithium-sulfur battery. BACKGROUND

[0002] The compounding of the high molecular polymer with the active material sulfur is still the key to realize the modification of the cathode due to the special physicochemical properties of the high molecular polymer. The high molecular polymer generally has rich functional groups, and the functional groups have strong chemical adsorption effect on polysulfides, and can well inhibit the shuttle effect of polysulfides. The conductive polymer is the most widely used high molecular polymer, and based on these advantages, the conductive polymer is used in the lithium-sulfur battery more and more in recent years. The compounding forms of the conductive polymer and S mainly include two kinds: one is to form a core-shell structure of “polymer wrapping S”, such as polythiophene (PTh) / S, polypyrrole (PPy) / S, polyvinylcarbazole (PVK) / S, polyaniline (PANi) / S and poly(3, 4-ethylenedioxythiophene) (PEDOT) / S composite materials, which mainly relies on the physical S limiting effect of the polymer shell to stabilize the cathode, and shows good electrochemical performance. The other is the core-shell structure of “S wrapping polymer”, such as the compounding of S to the surface of the conductive polymer (mainly including PPy nanowires and nanotubes, PANi nanotubes and hollow PANi spheres) matrix in a molten or chemical deposition manner, the polymer plays the role of conductive agent, dispersant, adsorbent and buffer volume change in the composite material, and provides a better carrier for the deposition of S.

[0003] Although the conductive polymer composite sulfur cathode material exhibits significant advantages in the modification of the lithium-sulfur battery cathode, it still faces the following challenges in practical application: (1) The conductive polymer generally has a non-localized π electron conjugated structure, so that its conductive ability is limited, and it still needs to be further strengthened by copolymerization or doping method. (2) The conductive polymer material generally has large rigidity, which is not conducive to the penetration of the electrolyte and the formation of the ion conductive network, and the inhibition effect on the volume expansion of the electrode is also not ideal. (3) The adsorption and fixation effect on polysulfides is still relatively weak, and more importantly, it cannot solve the problem of the insulation of sulfur and lithium sulfide ions, and this defect will greatly limit the practical application of the composite material in the modification of the lithium-sulfur battery cathode.

[0004] Polymeric ionic liquid (PIL) is a kind of special polymer polymerized from ionic liquid monomer, which has excellent physical and chemical stability, such as high temperature stability and oxidation resistance, which makes it excellent in high requirement environment. In addition, polyionic liquid also has good ionic conductivity, which makes it widely used in the field of electrochemistry, commonly used as electrolyte of battery, fuel cell and capacitor and other energy equipment. The present application utilizes the above advantages, for the first time tries to apply polymeric ionic liquid (PIL) in the modification of positive electrode material, and develops a preparation method of sulfur@polyionic liquid copolymer composite positive electrode and lithium-sulfur battery to solve part of the defects in the above technical background. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a preparation method of sulfur@polyionic liquid copolymer composite positive electrode and lithium-sulfur battery. In the polyionic liquid copolymer prepared by the present application, part of the surface of the sulfur powder is coated with polyionic liquid, which can enhance the ion transmission rate and electrochemical stability of the positive electrode; the remaining part of the surface of the sulfur powder is not coated with polyionic liquid, and the remaining part of the sulfur powder is exposed, which can be combined with the conductive agent to enhance the conductivity of the positive electrode. The sulfur@polyionic liquid copolymer composite positive electrode of the present application can make the lithium-sulfur battery have excellent conductivity and chemical stability, and can greatly improve the cycle performance and rate performance of the lithium-sulfur battery under different current densities.

[0006] Therefore, the present application provides a preparation method of sulfur@polyionic liquid copolymer composite positive electrode, comprising:

[0007] (1) mixing an ionic liquid and a solvent, adding a lithium salt and an initiator, stirring until clear and transparent to obtain a precursor solution;

[0008] (2) mixing the precursor solution with sulfur powder and putting it into a beater box for ball milling, and then drying to obtain a sulfur@polyionic liquid copolymer;

[0009] (3) taking the sulfur@polyionic liquid copolymer, conductive graphite, acetylene black and polyvinylidene fluoride PVDF as raw materials to prepare a positive electrode slurry; coating the positive electrode slurry on a carbon-coated aluminum foil, drying to obtain a sulfur@polyionic liquid copolymer composite positive electrode.

[0010] Preferably, in the step (1), the ionic liquid is at least one of 1-vinyl-3-butyl imidazole trifluoromethanesulfonylimide salt, 1-propenyl-3-methyl imidazole and 1,4-bis(3-vinylimidazole) dibromo butane.

[0011] Preferably, in the step (1), the solvent is N-methyl pyrrolidone NMP, and the mass of the ionic liquid to the volume of the solvent is (0.5-1.5) g:(1500-3000) uL.

[0012] Preferably, in the step (1), the lithium salt comprises lithium bis-trifluoromethanesulfonimide LiTFSI and / or lithium hexafluorophosphate LiPF6, and the mass of the lithium salt is 30%-50% of the mass of the ionic liquid.

[0013] Preferably, in the step (1), the initiator comprises azobisisobutyronitrile AIBN and / or azobisisoheptyl nitrile, and the mass of the initiator is 0.5%-3% of the mass of the ionic liquid.

[0014] Preferably, in the step (2), the mass ratio of the ionic liquid and the sulfur powder is (30-40):(70-150).

[0015] Preferably, in the step (2), the ball milling speed is 200-400 r / min, and the ball milling time is 40-55 h; the drying process comprises: drying in a blast drying oven for 10-15 h, and then drying in a vacuum drying oven for 10-15 h.

[0016] Preferably, in the step (3), the mass ratio of the sulfur@poly-ionic liquid copolymer, the conductive graphite, the acetylene black and the PVDF is (60-70):(10-20):(10-20):(3-9).

[0017] Preferably, in the step (3), the thickness of the sulfur@poly-ionic liquid copolymer composite positive electrode is 200-300 um, and the diameter is 7-15 mm.

[0018] The application further provides a lithium-sulfur battery, comprising a lithium negative electrode, a polyethylene diaphragm, and a positive electrode, wherein the positive electrode is the sulfur@poly-ionic liquid copolymer composite positive electrode prepared by the preparation method of the sulfur@poly-ionic liquid copolymer composite positive electrode.

[0019] Compared with the prior art, the application has the following advantages and positive effects:

[0020] (1) The preparation process of the sulfur@poly-ionic liquid copolymer is carried out at room temperature, has low requirements on the environment, and does not need to prepare a template in advance, and has the advantages of simplicity, easy operation, energy saving and the like.

[0021] (2) The ion liquid of the present application can be effectively and uniformly in-situ polymerized on the surface of sulfur powder, realizing controllable coating, and ensuring the formation of semi-coated morphology and core-shell structure of the poly-ion liquid copolymer. In the poly-ion liquid copolymer prepared by the present application, part of the surface of the sulfur powder is coated by the polymerized ion liquid, which can enhance the ion transmission rate and electrochemical stability of the positive electrode; the remaining part of the surface of the sulfur powder is not coated by the polymerized ion liquid, and the remaining part of the sulfur powder is exposed, which can be combined with the conductive agent to enhance the conductivity of the positive electrode. The sulfur@poly-ion liquid copolymer composite positive electrode of the present application can make the lithium-sulfur battery have excellent conductivity, chemical stability, and can greatly improve the cycle performance and rate performance of the lithium-sulfur battery under different current densities.

[0022] (3) The ion liquid of the present application is preferably 1-vinyl-3-butyl imidazole trifluoromethane sulfonimide salt, which has excellent ion conductivity, high thermal stability, excellent chemical stability, and low volatility, and can increase the ion conductivity of sulfur and effectively prevent the dissolution of sulfur and polysulfides; secondly, the polymerized ion liquid has excellent flexibility and mechanical stability, which is beneficial to the penetration of electrolyte and the formation of ion conduction network, and has good buffering and relieving effect on the volume expansion and / or volume shrinkage of sulfur during the charging and discharging process of the battery, which can ensure the structural stability of the positive electrode, and can greatly improve the cycle performance and rate performance of the lithium-sulfur battery under different current densities. In addition, the polymerized ion liquid contains rich C-F bonds, which have excellent fixation and absorption effect on polysulfides, and can effectively prevent the dissolution of polysulfides.

[0023] Other features and advantages of the present application will become more apparent after reading the specific embodiments of the present application in combination with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 SEM morphology diagram of the sulfur@poly-ion liquid copolymer composite positive electrode prepared in Example 1 of the present application;

[0025] Figure 2 SEM morphology diagram of the sulfur powder;

[0026] Figure 3 Rate performance test diagram of the lithium-sulfur battery of Example 2 and Comparative Example 1 of the present application;

[0027] Figure 4 Cycle performance test diagram of the lithium-sulfur battery of Example 2 and Comparative Example 1 of the present application;

[0028] Figure 5 Visual absorption test diagram of the polysulfide solution by the polymerized ion liquid. DETAILED DESCRIPTION

[0029] The application will be further described in connection with specific embodiments. These embodiments are only used to illustrate the application and not to limit the scope of the application. Meanwhile, after reading the content of the application, those skilled in the art can make various modifications or changes to the application, and these equivalent forms also fall within the scope defined by the appended claims.

[0030] The preparation method of the sulfur-polyionic liquid copolymer composite cathode of the application comprises:

[0031] (1) mixing the ionic liquid and the solvent, adding lithium salt and an initiator, stirring until clear and transparent to obtain a precursor solution;

[0032] The ionic liquid is at least one of 1-vinyl-3-butyl imidazole trifluoromethane sulfonimide salt, 1-propenyl-3-methyl imidazole, and 1,4-bis(3-vinylimidazole) dibromo butane.

[0033] The ionic liquid of the application is preferably 1-vinyl-3-butyl imidazole trifluoromethane sulfonimide salt, which has excellent ion conductivity, and also has high thermal stability and excellent chemical stability, low volatility, and the characteristics of preventing the dissolution of sulfur and polysulfides while increasing the ion conductivity of sulfur; secondly, the polymer ionic liquid has excellent flexibility and mechanical stability, which is beneficial to the penetration of electrolyte and the formation of ion conduction network, and has good buffering and relieving effect on the volume expansion and / or volume shrinkage of sulfur during the charging and discharging process of the battery, ensuring the structural stability of the positive electrode, and can greatly improve the cycle performance and rate performance of lithium-sulfur batteries under different current densities. In addition, the polymer ionic liquid contains rich C-F bonds, which have excellent fixation and absorption effect on polysulfides, and can effectively prevent the dissolution of polysulfides.

[0034] The solvent is N-methyl pyrrolidone NMP, and the mass of the ionic liquid to the volume of the solvent is (0.5-1.5) g:(1500-3000) uL; preferably, the mass of the ionic liquid to the volume of the solvent is (0.8-1.2) g:(1800-2200) uL.

[0035] The solvent of the application can make the precursor solution and the sulfur powder fully mixed and uniform, and can make the ionic liquid effectively and uniformly polymerize in situ on the surface of the sulfur powder to form a semi-coated morphology and a core-shell structure of the polyionic liquid copolymer.

[0036] The lithium salt includes lithium bis(trifluoromethanesulfonyl)imide LiTFSI, which has high electrochemical stability and electrical conductivity, and has a small probability of side reactions, and can provide the electrochemical stability and conductivity of the lithium-sulfur battery.

[0037] The mass of the lithium salt is 30-50% of the mass of the ionic liquid, preferably, the mass of the lithium salt is 35-45% of the mass of the ionic liquid; can make LiTFSI maximize the ionic conductivity and electrochemical stability of the polyionic liquid copolymer.

[0038] The initiator includes azobisisobutyronitrile AIBN and / or azobisisoheptyl nitrile, and the mass of the initiator is 0.5-3% of the mass of the ionic liquid. Preferably, the mass of the initiator is 0.5-1.5% of the mass of the ionic liquid.

[0039] The initiator of the present application can initiate complete polymerization of the ionic liquid, so that the ionic liquid is uniformly polymerized on the surface of the sulfur powder, while avoiding the generation of bubbles caused by excessive polymerization of the ionic liquid, and ensuring the formation of a polyionic liquid copolymer with a semi-coated morphology and a core-shell structure.

[0040] By adjusting the amount of lithium salt and initiator, the present application can enable effective and uniform in-situ polymerization of the ionic liquid on the surface of the sulfur powder, achieve controllability of coating, and ensure the formation of a polyionic liquid copolymer with a semi-coated morphology and a core-shell structure. In the polyionic liquid copolymer prepared by the present application, part of the surface of the sulfur powder is coated with a polymerized ionic liquid, which can enhance the ion transmission rate and electrochemical stability of the positive electrode; the remaining part of the surface of the sulfur powder is not coated with a polymerized ionic liquid, and the remaining part of the sulfur powder is exposed, which can be combined with a conductive agent to enhance the conductivity of the positive electrode.

[0041] (2) The precursor solution and the sulfur powder are mixed and put into a beater box for ball milling, and then dried to obtain a sulfur@polyionic liquid copolymer;

[0042] The mass ratio of the ionic liquid to the sulfur powder is (30-40):(70-150), preferably, the mass ratio of the ionic liquid to the sulfur powder is (30-35):(80-120); can enable effective and uniform in-situ polymerization of the ionic liquid on the surface of the sulfur powder, forming a polyionic liquid copolymer with a semi-coated morphology and a core-shell structure.

[0043] The ball milling speed is 200-400 r / min, and the ball milling time is 40-55 h; through ball milling, the ionic liquid can be effectively and uniformly in-situ polymerized on the surface of the sulfur powder, forming a sulfur@polyionic liquid copolymer with a semi-coated morphology and a core-shell structure.

[0044] The drying process includes: drying in a blast oven for 10-15 h, and then transferring to a vacuum oven for drying for 10-15 h.

[0045] (3) Using the sulfur@polyionic liquid copolymer, conductive graphite, acetylene black and polyvinylidene fluoride PVDF as raw materials, a positive electrode slurry is prepared; the positive electrode slurry is coated on a carbon-coated aluminum foil, dried, and a sulfur@polyionic liquid copolymer composite positive electrode is obtained.

[0046] The mass ratio of the sulfur@poly ionic liquid copolymer, the conductive graphite, the acetylene black and the PVDF is (60-70):(10-20):(10-20):(3-9), and the reaction is carried out in the above mass ratio range, so that the positive electrode with stable structure can be obtained, and the ion transmission rate, the conductivity and the electrochemical stability of the positive electrode can be improved.

[0047] The conductive graphite and the acetylene black of the application can effectively solve the problem of non-conductivity of sulfur and effectively enhance the conductivity of the positive electrode.

[0048] The binder PVDF of the application can ensure the effective adhesion of the sulfur@poly ionic liquid copolymer, the conductive graphite and the acetylene black to each other, prevent the sulfur@poly ionic liquid copolymer and / or the conductive graphite and / or the acetylene black from falling off after drying, and ensure the formation of the stable sulfur@poly ionic liquid copolymer composite positive electrode.

[0049] The thickness of the sulfur@poly ionic liquid copolymer composite positive electrode is 200-300 um, and the diameter is 7-15 mm.

[0050] The application further provides a lithium-sulfur battery, which is assembled into a button lithium-sulfur battery and uses a lithium negative electrode, a polyethylene diaphragm, a lithium-sulfur electrolyte and a sulfur@poly ether copolymer composite positive electrode prepared by the preparation method of the sulfur@poly ether copolymer composite positive electrode.

[0051] The advantages and positive effects of the application include:

[0052] (1) The preparation process of the sulfur@poly ionic liquid copolymer of the application is carried out at room temperature, has low requirements on the environment, and does not need to prepare a template in advance, and has the advantages of simplicity, easy operation, energy saving and the like. The production efficiency of the sulfur@poly ionic liquid copolymer is high, and batch production can be realized.

[0053] (2) The ionic liquid of the application can be effectively and uniformly polymerized in situ on the surface of the sulfur powder, realizes controllability of coating, and can ensure the formation of the poly ionic liquid copolymer with a semi-coating morphology and a core-shell structure. In the poly ionic liquid copolymer prepared by the application, part of the surface of the sulfur powder is coated by the ionic liquid, which can enhance the ion transmission rate and the electrochemical stability of the positive electrode; the remaining part of the surface of the sulfur powder is not coated by the ionic liquid, and the remaining part of the sulfur powder is exposed, which can be combined with the conductive agent to enhance the conductivity of the positive electrode.

[0054] (3) The ion liquid of the application is preferably 1-vinyl-3-butyl imidazole trifluoromethane sulfonimide salt, which has excellent ion conductivity, and also has high thermal stability and excellent chemical stability, low volatility, and can increase the ion conductivity of sulfur while effectively preventing the dissolution of sulfur and polysulfides; secondly, the polymeric ionic liquid has excellent flexibility and mechanical stability, which is beneficial to the penetration of electrolyte and the formation of ion conduction network, and has good buffering effect on the volume expansion and / or volume shrinkage of sulfur during the charging and discharging process of the battery, which ensures the structural stability of the positive electrode, and can greatly improve the cycle performance and rate performance of lithium-sulfur batteries under different current densities. In addition, the polymeric ionic liquid contains a large number of C-F bonds, which have excellent fixation and absorption effect on polysulfides, and can effectively prevent the dissolution of polysulfides. Example 1

[0055] The preparation method of the sulfur@poly ionic liquid copolymer composite positive electrode of the present embodiment comprises:

[0056] (1) 0.33 g of ion liquid 1-vinyl-3-butyl imidazole trifluoromethane sulfonimide salt is mixed with 2000 ul of NMP solution, 0.132 g of LiTFSI and 0.0033 g of AIBN are added, and stirred until clear and transparent to obtain a precursor solution.

[0057] (2) The precursor solution is mixed with 1 g of sulfur powder and placed in a beater box, and ball milled at a speed of 300 r / min for 48 h. Take out and dry in a blast oven for 12 h, then transfer to a vacuum oven and dry for 12 h, which is the sulfur@poly ionic liquid copolymer.

[0058] (3) A positive electrode slurry is prepared with polyvinylidene fluoride (PVDF) as a binder, in a ratio of sulfur@poly ionic liquid copolymer: conductive graphite: acetylene black: PVDF = 65:15:15:5. Then, a 250 um thick positive electrode slurry is coated on a carbon-coated aluminum foil, dried, and cut into a 10 mm diameter disc as a positive electrode.

[0059] Figure 1 The SEM morphology of the sulfur@poly ionic liquid copolymer composite positive electrode prepared in Example 1 of the present application is shown in Figure 1, Figure 2 The SEM morphology of the sulfur powder is shown in Figure 2; and Figure 1 And Figure 2 As can be seen from Figures 1 and 2, the surface of the blank sulfur is smooth and no substance is coated; while the surface of the sulfur@poly ionic liquid copolymer is rough and the naked eye can see that it is coated with a polymer, which verifies that the polymeric ionic liquid is successfully coated on the surface of the sulfur.

[0060] Figure 5 The visual absorption test of the polymeric ionic liquid 1-vinyl-3-butyl imidazole trifluoromethane sulfonimide salt polymer of the present application on the polysulfide solution is shown in Figure 3,Figure 5 (a) the photo of the individual polymeric ionic liquid; Figure 5 (b) the initial photo of the polymeric ionic liquid to which polysulfide solution was added; Figure 5 (c) the photo after 6 hours of soaking. Figure 5 It can be seen that the polymeric ionic liquid of the present application has excellent adsorption and fixation of polysulfide. Example 2

[0061] The lithium-sulfur battery of this example includes sulfur@polyether copolymer of Example 1 as the positive electrode, lithium sheet as the negative electrode, 19 mm polyethylene (PE) as the separator, and uses lithium-sulfur electrolyte to assemble a button cell.

[0062] Comparative Example 1

[0063] The lithium-sulfur battery of this example includes sulfur S as the positive electrode, lithium sheet as the negative electrode, 19 mm polyethylene (PE) as the separator, and uses lithium-sulfur electrolyte to assemble a button cell.

[0064] Figure 3 The rate performance test chart of the lithium-sulfur battery of Example 2 and Comparative Example 1 of the present application is the rate performance test chart of the lithium-sulfur battery cycled 10 times at different current densities (0.1C, 0.2C, 0.5C, 1C, 2C, 0.1C). It can be seen from the chart that Figure 3 It can be seen that the capacity of the lithium-sulfur battery of Example 2 is higher than that of the lithium-sulfur battery of Comparative Example 1 at each current density; and when the current density gradually increases and finally returns to 0.1C, the capacity recovery of the lithium-sulfur battery of Example 2 is good.

[0065] Figure 4 The cycle performance test chart of the lithium-sulfur battery of Example 2 and Comparative Example 1 of the present application is the cycle performance test chart of the battery cycled 200 times at a current density of 0.5C. It can be seen from the chart that Figure 4 It can be seen that the lithium-sulfur battery of Example 2 has high capacity, small attenuation, and high retention rate, and has excellent cycle performance.

[0066] The above examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, for those skilled in the art, the technical solutions recorded in the foregoing examples can be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions claimed by the present application.

Claims

1. A method for preparing a sulfur@polyionic liquid copolymer composite positive electrode, characterized in that: include: (1) Mixing the ionic liquid and the solvent, adding the lithium salt and the initiator, and stirring until the solution is clear and transparent to obtain a precursor solution; (2) Mixing the precursor solution and sulfur powder into a beating box for ball milling, and then drying to obtain sulfur@polyionic liquid copolymer; (3) A positive electrode slurry was prepared using sulfur@polyionic liquid copolymer, conductive graphite, acetylene black and polyvinylidene fluoride (PVDF) as raw materials; the positive electrode slurry was scraped onto carbon-coated aluminum foil and dried to obtain a sulfur@polyionic liquid copolymer composite positive electrode.

2. The method for preparing the sulfur@polyionic liquid copolymer composite positive electrode according to claim 1, characterized in that: In the step (1), the ionic liquid includes at least one of 1-vinyl-3-butylimidazole trifluoromethanesulfonyl imide salt, 1-propenyl-3-methylimidazole and 1,4-bis(3-vinylimidazole)dibromobutane.

3. The method for preparing the sulfur@polyionic liquid copolymer composite positive electrode according to claim 1, characterized in that: In the step (1), the solvent is N-methylpyrrolidone (NMP), and the ratio of the mass of the ionic liquid to the volume of the solvent is (0.5-1.5) g: (1500-3000) uL.

4. The method for preparing the sulfur@polyionic liquid copolymer composite positive electrode according to claim 1, characterized in that: In the step (1), the lithium salt includes lithium bis(trifluoromethanesulfonyl)imide LiTFSI and / or lithium hexafluorophosphate LiPF6, and the mass of the lithium salt is 30%-50% of the mass of the ionic liquid.

5. The method for preparing the sulfur@polyionic liquid copolymer composite positive electrode according to claim 1, characterized in that: In the step (1), the initiator includes azobisisobutyronitrile (AIBN) and / or azobisisoheptanenitrile, and the mass of the initiator is 0.5%-3% of the mass of the ionic liquid.

6. The method for preparing the sulfur@polyionic liquid copolymer composite positive electrode according to claim 1, characterized in that: In the step (2), the mass ratio of the ionic liquid to the sulfur powder is (30-40): (70-150).

7. The method for preparing the sulfur@polyionic liquid copolymer composite positive electrode according to claim 1, characterized in that: In the step (2), the ball milling speed is 200-400 r / min and the ball milling time is 40-55h; The drying process includes: drying in a forced air oven for 10-15 hours, and then drying in a vacuum oven for 10-15 hours.

8. The method for preparing the sulfur@polyionic liquid copolymer composite positive electrode according to claim 1, characterized in that: In the step (3), the mass ratio of sulfur@polyionic liquid copolymer: conductive graphite: acetylene black: PVDF is (60-70): (10-20): (10-20): (3-9).

9. The method for preparing the sulfur@polyionic liquid copolymer composite positive electrode according to claim 1, characterized in that: In the step (3), the thickness of the sulfur@polyionic liquid copolymer composite positive electrode is 200-300 um and the diameter is 7-15 mm.

10. A lithium-sulfur battery comprising a lithium negative electrode and a polyethylene separator, characterized in that: It also includes a positive electrode, which is a sulfur@polyionic liquid copolymer composite positive electrode prepared by the preparation method of the sulfur@polyionic liquid copolymer composite positive electrode according to any one of claims 1-9.

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