Self-supporting positive electrode material of lithium-sulfur battery as well as preparation method and application of self-supporting positive electrode material

Abstract

The invention belongs to the technical field of lithium-sulfur batteries, and particularly relates to a self-supporting positive electrode material of a lithium-sulfur battery as well as a preparation method and application of the self-supporting positive electrode material. The preparation method comprises the following steps: preparing zinc activated carbon cloth, anchoring zinc ions on the carbon cloth to induce ZIF8 to nucleate and ZIF67 to nucleate, embedding Se in ZIF67 and embedding Se in ZIF8, carbonizing to load a metal selenide carbon skeleton material derived from a bimetallic organic framework material on the carbon cloth, vulcanizing, and pouring sulfur into the metal selenide carbon skeleton material to prepare the bimetallic organic framework material-based composite material. The lithium-sulfur battery self-supporting positive electrode material is obtained. In the self-supporting positive electrode material of the lithium-sulfur battery prepared by the method, the metal selenide carbon framework material realizes high-quantity loading of sulfur; in addition, sulfur is loaded through a self-supporting electrode carbon cloth carbon porous framework, and the self-supporting electrode has a free conductive network, so that the use of a conductive agent is avoided, and the defects in the prior art are overcome.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-sulfur batteries, and specifically relates to a self-supporting cathode material for lithium-sulfur batteries, its preparation method and application. Background Art

[0002] The theoretical energy density of traditional lithium-ion battery cathode materials has approached its physical limit, which is 200 Wh / kg -1 ~250 Wh / kg -1 , and it is difficult to meet the requirements of electric vehicles and large-scale energy storage systems for high-energy-density energy storage devices. This technical bottleneck has promoted the basic research of new battery systems. Among them, lithium-sulfur batteries have become a key research direction due to their excellent theoretical energy density, and the theoretical energy density is 2600 Wh / kg -1 / 2800 Wh / L -1 .

[0003] The history of sulfur as a cathode active material can be traced back to the 1960s. Its theoretical specific capacity is as high as 1675 mAh / g -1 , which is significantly better than the traditional transition metal oxide system. Coupled with the advantages of sulfur element such as rich reserves, low cost and environmental friendliness, it has become the core material of the new generation of high specific energy batteries with the most potential for industrialization. However, the practical application of lithium-sulfur batteries still faces multiple technical obstacles: the intrinsic electron / ion conductivity of elemental sulfur and its reduction intermediate products is extremely low, and the chemical formula of the sulfur reduction intermediate product is Li2S x , where x is 4-8. At room temperature, the intrinsic electron / ion conductivity of elemental sulfur and its reduction intermediate products is less than 10 -30 S / cm -1 , resulting in problems of electrode reaction kinetic retardation and limited high-rate charge-discharge performance; in addition, the long-chain polysulfides generated during the electrochemical process are easily soluble in organic electrolytes, and the chemical formula of the long-chain polysulfides is Li2S x , 4 < x < 8, triggering a series of problems such as continuous loss of active substances, shuttle effect, increase in electrolyte viscosity and side reactions at the negative electrode interface. The migration of polysulfides between electrodes not only causes capacity attenuation, but also exacerbates the self-discharge phenomenon; the interface passivation layer formed by the end products Li2S / Li2S2 as insulating solid precipitates will further hinder the electron conduction network and reduce the utilization rate of active substances.

[0004] In response to the above challenges, the current research focus is on constructing a sulfur / carbon composite cathode system, and its core design needs to take into account both the construction of the conductive network and the optimization of the hierarchical pore structure. The conductivity of the high-conductive carbon matrix > 100 S / cm -1, which can establish three-dimensional electron transport channels to compensate for the intrinsic conductive defects of sulfur, while the hierarchical porous architecture realizes sulfur loading and volume expansion buffering through micropores, promotes electrolyte infiltration and lithium ion diffusion through mesopores, and constructs continuous ion transport channels through macropores. In this context, metal-organic framework materials have shown significant advantages due to their unique structural characteristics, which are specifically reflected in: the pore confinement effect effectively inhibits the dissolution and diffusion of polysulfides, the surface functional groups and metal sites are bonded through strong chemical bonds to achieve dynamic anchoring of polysulfides, and its structural adaptability allows the precise design of pore structure through precursor regulation, thereby optimizing sulfur loading and accommodating volume expansion. Therefore, metal-organic framework materials are a new type of porous skeleton with great practical prospects in lithium-sulfur batteries.

[0005] The typical preparation process of the existing technology for sulfur / carbon composite positive electrode systems based on metal-organic framework materials is the melt diffusion method, in which high-purity sulfur is infiltrated into the carbon skeleton derived from the metal-organic framework material under vacuum conditions to form a composite system with uniform sulfur distribution. This process achieves effective encapsulation and cyclic stability of sulfur, and improves Coulombic efficiency, providing an innovative solution for the practical application of lithium-sulfur batteries.

[0006] However, existing technologies that use carbon skeletons derived from metal-organic frameworks as direct host materials for sulfur face the following disadvantages: (1) limited sulfur loading. The high energy density of sulfur cathodes is achieved based on high loading, and it is difficult to achieve high sulfur loading using traditional technologies; (2) the use of conductive agents. Traditional cathodes often require the addition of a large amount of conductive agents to achieve conductivity within the cathode structure. Summary of the Invention

[0007] In response to the shortcomings of the above-mentioned prior art, the present invention provides a self-supporting positive electrode material for lithium-sulfur batteries, its preparation method, and its application. The present invention first prepares zinc-activated carbon cloth, anchors zinc ions on the carbon cloth to induce nucleation of ZIF8 first and ZIF67 later, and then embeds Se in ZIF67 and simultaneously embeds it in ZIF8. Carbonization is then performed to load a metal selenide carbon skeleton material derived from a bimetallic organic framework material onto the carbon cloth. Sulfurization is then performed to infuse sulfur into the metal selenide carbon skeleton material, thereby obtaining a self-supporting positive electrode material for lithium-sulfur batteries. In the self-supporting positive electrode material for lithium-sulfur batteries prepared using the present invention, the metal selenide carbon skeleton material achieves a high sulfur loading. Furthermore, sulfur is loaded via the porous carbon skeleton of the self-supporting electrode carbon cloth, resulting in a free conductive network for the self-supporting electrode, thereby avoiding the use of a conductive agent and overcoming the shortcomings of the prior art.

[0008] Based on the above technical problems, the present invention adopts the following technical solutions: The present invention provides a method for preparing a self-supporting positive electrode material for a lithium-sulfur battery, comprising the following steps: The carbon cloth is immersed in an alcohol solution of soluble zinc salt to anchor zinc ions on the carbon cloth to obtain zinc-activated carbon cloth; zinc ions are used as inducers. Without zinc ions, the purpose of ZIF8 nucleation first and ZIF67 nucleation later cannot be achieved, resulting in the inability to obtain uniform metal selenide carbon skeleton materials.

[0009] A soluble zinc salt and a soluble cobalt salt are placed together in an alcohol solvent, and stirred at 50°C~70°C for 5min~20min under ultraviolet light irradiation to obtain a metal solution; 2-methylimidazole and selenium powder are placed together in an alcohol solvent, and stirred at 50°C~70°C for 5min~20min under ultraviolet light irradiation to obtain a ligand solution; the metal solution and the ligand solution are mixed, and stirred at 50°C~70°C for 5min~20min under ultraviolet light irradiation to obtain a mixed solution; heating and stirring under ultraviolet light irradiation is one of the innovations of the present invention. Without the ultraviolet light irradiation, heating and stirring conditions, the soluble zinc salt, the soluble cobalt salt, 2-methylimidazole and selenium powder will be unevenly dispersed, the coating order of ZIF8 and ZIF67 will be disrupted, and the nucleation rate of ZIF8 and ZIF67 will be reduced.

[0010] The zinc activated carbon cloth was immersed in the mixed solution, and the coordination reaction was carried out in the dark to achieve slow nucleation of ZIF8 and ZIF67 in the dark. Under the induction of zinc ions on the zinc activated carbon cloth, the zinc ions of the soluble zinc salt first coordinated with the ligand 2-methylimidazole to form ZIF8 while coating the zinc ions on the zinc activated carbon cloth. Then, the cobalt ions of the soluble cobalt salt coordinated with the ligand 2-methylimidazole to form ZIF67 while coating ZIF8. Se was embedded in ZIF67 and embedded in ZIF8. ZIF8 and ZIF67 were jointly bonded to the zinc activated carbon cloth to obtain Se-ZIF67-ZIF8-Zn@C.

[0011] Se-ZIF67-ZIF8-Zn@C was carbonized, and ZIF67 and ZIF8 were carbonized together to form a double-layer structure consisting of an inner and outer carbon porous skeleton. Cobalt ions and zinc ions reacted with Se together to attach CoSe to the outer carbon porous skeleton and ZnSe to the inner carbon porous skeleton, obtaining CoSe-ZnSe-C-Zn@C.

[0012] CoSe-ZnSe-C-Zn@C is sulfurized, and sulfur is infused into a double-layer structure consisting of an inner and outer carbon porous skeleton to obtain a self-supporting positive electrode material for lithium-sulfur batteries.

[0013] Preferably, the method for anchoring zinc ions on the carbon cloth is: stirring at 50° C. to 60° C. for 3 h under ultraviolet light, and then standing for 1 h.

[0014] Preferably, the concentration of the alcohol solution of the soluble zinc salt is 1 mol / L~5 mol / L; if the zinc ion concentration is too high, the ZIF8 nucleation will be too fast and the uniformity will be poor; if the zinc ion concentration is too low, the ZIF8 nucleation will not be achieved.

[0015] Preferably, in the mixed solution, the molar ratio of soluble zinc salt, soluble cobalt salt, 2-methylimidazole and selenium powder is 1-2:1-2:4-8:2-4, and the ratio of the sum of the amount of zinc ions and cobalt ions to the amount of selenium powder is 1:1.

[0016] Preferably, the coordination reaction is carried out at 70° C. to 80° C. for 12 hours.

[0017] Preferably, the carbonization conditions are: in an inert atmosphere, first keeping the temperature at 200° C. to 350° C. for 1 h to 3 h, and then keeping the temperature at 700° C. to 850° C. for 4 h to 6 h.

[0018] Preferably, the sulfurization method is: mixing CoSe-ZnSe-C-Zn@C with sulfur powder and heat treating the mixture at 150° C. to 160° C. for 12 h to 24 h.

[0019] The present invention also provides a self-supporting positive electrode material for a lithium-sulfur battery, which is prepared by the above preparation method.

[0020] Preferably, the self-supporting positive electrode material of the lithium-sulfur battery is composed of CoSe, ZnSe, carbon cloth carbon porous skeleton and S, the mass percentage of CoSe is 5%~20%, the mass percentage of ZnSe is 5%~20%, the mass percentage of carbon cloth carbon porous skeleton is 10%~20%, the mass percentage of S is 50%~80%, and the sum of the mass percentages of CoSe, ZnSe, carbon cloth carbon porous skeleton and S is 100%, and the carbon cloth carbon porous skeleton is a self-supporting electrode.

[0021] The present invention also protects the application of the self-supporting positive electrode material of the lithium-sulfur battery in the preparation of the lithium-sulfur battery.

[0022] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention anchors zinc ions on carbon cloth to obtain zinc activated carbon cloth; a metal solution consisting of a soluble zinc salt, a soluble cobalt salt and an alcohol solvent is mixed with a ligand solution consisting of 2-methylimidazole, selenium powder and an alcohol solvent to obtain a mixed solution; the zinc activated carbon cloth is immersed in the mixed solution and a coordination reaction is carried out in the dark, wherein the zinc ions on the zinc activated carbon cloth serve as an inducer. Under the induction of the zinc ions, the zinc ions of the soluble zinc salt first coordinate with the 2-methylimidazole to form ZIF8 while coating the zinc ions on the zinc activated carbon cloth, and then the cobalt ions of the soluble cobalt salt coordinate with the 2-methylimidazole to form ZIF67 while coating ZIF8, Se is embedded in ZIF67 and embedded in ZIF8, ZIF67 and ZIF8 form an inside-outside structure, and ZIF8 and ZIF67 are jointly bonded to the zinc activated carbon cloth to obtain Se-ZIF67-ZIF8-Zn@C.

[0023] Se-ZIF67-ZIF8-Zn@C is carbonized. At this time, both ZIF67 and ZIF8 are carbonized, forming a double-layer structure consisting of inner and outer carbon porous skeletons. Cobalt ions and zinc ions are reduced, and while CoSe and ZnSe are obtained, CoSe is attached to the outer carbon porous skeleton and ZnSe is attached to the inner carbon porous skeleton, obtaining CoSe-ZnSe-C-Zn@C. Finally, CoSe-ZnSe-C-Zn@C is sulfurized. During the sulfurization process, sulfur is infused into the double-layer structure to obtain a self-supporting positive electrode material for lithium-sulfur batteries.

[0024] 2. In the self-supporting positive electrode material of the lithium-sulfur battery of the present invention, a metal selenide carbon skeleton material derived from a bimetallic organic framework material is loaded on the carbon cloth. The self-supporting positive electrode material of the lithium-sulfur battery is composed of CoSe, ZnSe, a carbon cloth carbon porous skeleton and S. It has a rich pore structure and a conductive network, which can enhance the conductive performance of the electrode. At the same time, the bimetallic selenide accelerates the catalytic conversion of lithium polysulfide in the positive electrode, avoiding the use of conductive agents and binders while increasing the sulfur loading capacity of the positive electrode.

[0025] The self-supporting cathode material for lithium-sulfur batteries of the present invention uses an inner and outer double-layer design of CoSe and ZnSe and a carbon porous framework to synergistically fix sulfur, improve sulfur utilization, enhance conductivity, and achieve high sulfur loading. At the same time, the double-layer structure of the self-supporting cathode material for lithium-sulfur batteries alleviates the volume change of sulfur during the cycle, and the electrochemical performance is stable.

[0026] 3. In the present invention, S is loaded through the carbon cloth porous skeleton of the self-supporting electrode. The self-supporting electrode has a free conductive network, thereby avoiding the use of a conductive agent and further increasing the sulfur loading capacity of the positive electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1This is a cycle curve diagram of the first 360 cycles of a lithium-sulfur battery assembled using the lithium-sulfur battery self-supporting positive electrode material of Example 1 at 0.5C.

[0028] Figure 2 This is a charge and discharge curve of the first cycle of a lithium-sulfur battery assembled using the lithium-sulfur battery self-supporting positive electrode material of Example 2 at 0.2C.

[0029] Figure 3 This is a transmission electron microscope image of CoSe-ZnSe-C-Zn in Example 1. CoSe-ZnSe-C-Zn was obtained by scraping off the surface of CoSe-ZnSe-C-Zn@C.

[0030] Figure 4 These are the electrochemical impedance spectroscopy diagrams of a lithium-sulfur battery assembled using the self-supporting positive electrode material for lithium-sulfur batteries of Example 1 before and after 100 cycles.

[0031] Figure 5 This is a transmission electron microscope image of CoSe-ZnSe-C-Zn in Example 3. CoSe-ZnSe-C-Zn was obtained by scraping off the surface of CoSe-ZnSe-C-Zn@C. DETAILED DESCRIPTION

[0032] The following will be combined with the specific embodiments of the present invention to provide a clearer and more complete explanation of the technical solution of the present invention. The described embodiments are only part of the embodiments of the present invention, not all of them. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

[0033] Considering that the existing technology uses carbon skeletons derived from metal-organic framework materials to directly serve as sulfur host materials, there are defects such as limited sulfur loading and the need to be used together with a conductive agent to be applied as a positive electrode material. The present invention proposes a new type of flexible carbon cloth-loaded metal selenide carbon skeleton material derived from a bimetallic organic framework material. The self-supporting positive electrode material for lithium-sulfur batteries of the present invention has a rich pore structure and a conductive network structure. At the same time, the derived bimetallic selenide acts synergistically to improve the adsorption and catalytic conversion effect of soluble lithium polysulfide, thereby improving the utilization rate of the self-supporting positive electrode material for lithium-sulfur batteries.

[0034] In the preparation of the self-supporting positive electrode material of the lithium-sulfur battery of the present invention, the raw materials involved are all obtained through commercial purchase. The ultra-thin carbon energy carbon cloth is the carbon energy carbon cloth WOS1002 from Taiwan, China. The reason for using the ultra-thin carbon energy carbon cloth as the raw material is: the ultra-thin carbon energy carbon cloth is light in weight, which reduces the weight of the self-supporting positive electrode material of the lithium-sulfur battery, thereby reducing the weight of the lithium-sulfur battery.

[0035] The technical solution of the present invention is studied using the following examples and comparative examples. The specific research methods and results are as follows: Example 1 A method for preparing a self-supporting positive electrode material for a lithium-sulfur battery comprises the following steps: (1) Preparation of zinc activated carbon cloth: The ultrathin carbon cloth was immersed in methanol containing zinc nitrate at a concentration of 1 mol / L and stirred at 50 °C for 3 h under ultraviolet light. The ultraviolet light combined with 50 °C can accelerate the dissociation of zinc ions and anchor them on the ultrathin carbon cloth. The solution was then allowed to stand for 1 h to obtain zinc activated carbon cloth, which was recorded as Zn@C.

[0036] (2) Preparation of self-supporting electrode precursor: S1. Place 10 mmol of zinc nitrate and 10 mmol of cobalt nitrate in 200 mL of methanol, and stir at 60°C for 10 min under ultraviolet light to obtain a metal solution; disperse 40 mmol of 2-methylimidazole and 20 mmol of selenium powder in 200 mL of methanol, and stir at 60°C for 10 min under ultraviolet light to obtain a ligand solution; mix the metal solution and the ligand solution, and stir at 60°C for 10 min under ultraviolet light to accelerate the dissolution of zinc ions and cobalt ions, while making the zinc ions, cobalt ions and selenium powder evenly dispersed, promote the nano-sizing of selenium powder, and make the selenium in the selenium powder easily agglomerate to obtain a mixed solution.

[0037] S2. Immerse the zinc activated carbon cloth in the mixed solution and let it stand at 70°C for 12 h in the dark to obtain a self-supporting electrode precursor, which is recorded as Se-ZIF67-ZIF8-Zn@C.

[0038] (3) Preparation of self-supporting electrodes: S1. Se-ZIF67-ZIF8-Zn@C was placed in a tube furnace and kept at 200°C for 1 h. Then the temperature was raised to 700°C and kept at this temperature for 4 h. The whole process was heated in an argon atmosphere at a heating rate of 1°C / min to obtain CoSe-ZnSe-C-Zn@C.

[0039] S2. Se-ZIF67-ZIF8-Zn@C and pure phase nano-sulfur powder were mixed, and then heat-treated at 155°C in a nitrogen-protected tube furnace for 18 hours to obtain a self-supporting positive electrode material for lithium-sulfur batteries, which was recorded as S-CoSe-ZnSe-C-Zn@C.

[0040] A method for preparing a lithium-sulfur battery comprises the following steps: CR2025 button cells were assembled in an argon-filled glove box with S-CoSe-ZnSe-C-Zn@C as the positive electrode, metallic lithium as the counter electrode and reference electrode, lithium-sulfur electrolyte as the electrolyte, and porous polypropylene as the separator.

[0041] Example 2 A method for preparing a self-supporting positive electrode material for a lithium-sulfur battery comprises the following steps: (1) Preparation of zinc activated carbon cloth: The ultrathin carbon cloth was immersed in methanol containing zinc nitrate at a concentration of 2 mol / L and stirred at 55 °C for 3 h under ultraviolet light. The ultraviolet light combined with 55 °C can accelerate the dissociation of zinc ions and anchor them on the ultrathin carbon cloth. The solution was then allowed to stand for 1 h to obtain zinc activated carbon cloth, which was recorded as Zn@C.

[0042] (2) Preparation of self-supporting electrode precursor: S1. Place 15 mmol of zinc nitrate and 15 mmol of cobalt nitrate in 300 mL of methanol, and stir at 65°C for 20 min under ultraviolet light to obtain a metal solution; disperse 50 mmol of 2-methylimidazole and 30 mmol of selenium powder in 300 mL of methanol, and stir at 65°C for 20 min under ultraviolet light to obtain a ligand solution; mix the metal solution and the ligand solution, and stir at 65°C for 20 min under ultraviolet light to accelerate the dissolution of zinc ions and cobalt ions, while making the zinc ions, cobalt ions and selenium powder evenly dispersed, promote the nano-sizing of selenium powder, and make the selenium in the selenium powder easily agglomerate to obtain a mixed solution.

[0043] S2. Immerse the zinc activated carbon cloth in the mixed solution and let it stand at 75°C for 12 h in the dark to obtain a self-supporting electrode precursor, which is recorded as Se-ZIF67-ZIF8-Zn@C.

[0044] (3) Preparation of self-supporting electrodes: S1. Se-ZIF67-ZIF8-Zn@C was placed in a tube furnace and kept at 250°C for 2 h. Then the temperature was raised to 750°C and kept at this temperature for 5 h. The whole process was heated in an argon atmosphere at a heating rate of 1.5°C / min to obtain CoSe-ZnSe-C-Zn@C.

[0045] S2. CoSe-ZnSe-C-Zn@C and pure phase nano-sulfur powder were mixed, and then heat-treated at 155°C for 18 hours in a tube furnace under nitrogen protection to obtain a self-supporting positive electrode material for lithium-sulfur batteries, which was recorded as S-CoSe-ZnSe-C-Zn@C.

[0046] A method for preparing a lithium-sulfur battery comprises the following steps: CR2025 button cells were assembled in an argon-filled glove box with S-CoSe-ZnSe-C-Zn@C as the positive electrode, metallic lithium as the counter electrode and reference electrode, lithium-sulfur electrolyte as the electrolyte, and porous polypropylene as the separator.

[0047] Example 3 A method for preparing a self-supporting positive electrode material for a lithium-sulfur battery comprises the following steps: (1) Preparation of zinc activated carbon cloth: The ultrathin carbon cloth was immersed in methanol containing zinc nitrate at a concentration of 5 mol / L and stirred at 60 °C for 3 h under ultraviolet light. The ultraviolet light combined with 60 °C can accelerate the dissociation of zinc ions and anchor them on the ultrathin carbon cloth. The solution was then allowed to stand for 1 h to obtain zinc activated carbon cloth, which was recorded as Zn@C.

[0048] (2) Preparation of self-supporting electrode precursor: S1. Place 20 mmol of zinc nitrate and 20 mmol of cobalt nitrate in 500 mL of methanol, and stir at 70°C for 5 min under ultraviolet light to obtain a metal solution; disperse 80 mmol of 2-methylimidazole and 40 mmol of selenium powder in 500 mL of methanol, and stir at 70°C for 5 min under ultraviolet light to obtain a ligand solution; mix the metal solution and the ligand solution, and stir at 70°C for 5 min under ultraviolet light to accelerate the dissolution of zinc ions and cobalt ions, while making the zinc ions, cobalt ions and selenium powder uniformly dispersed, promote the nano-sizing of selenium powder, and make the selenium in the selenium powder easily agglomerate to obtain a mixed solution.

[0049] S2. Immerse the zinc activated carbon cloth in the mixed solution and let it stand at 80°C for 12 h in the dark to obtain a self-supporting electrode precursor, which is recorded as Se-ZIF67-ZIF8-Zn@C.

[0050] (3) Preparation of self-supporting electrodes: S1. Se-ZIF67-ZIF8-Zn@C was placed in a tube furnace and kept at 350°C for 3 h, then heated to 850°C and kept at this temperature for 6 h. The whole process was heated in an argon atmosphere at a heating rate of 2°C / min to obtain CoSe-ZnSe-C-Zn@C. S2. CoSe-ZnSe-C-Zn@C and pure phase nano-sulfur powder were mixed, and then heat-treated at 155°C for 18 hours in a tube furnace under nitrogen protection to obtain a self-supporting positive electrode material for lithium-sulfur batteries, which was recorded as S-CoSe-ZnSe-C-Zn@C.

[0051] A method for preparing a lithium-sulfur battery comprises the following steps: CR2025 button cells were assembled in an argon-filled glove box with S-CoSe-ZnSe-C-Zn@C as the positive electrode, metallic lithium as the counter electrode and reference electrode, lithium-sulfur electrolyte as the electrolyte, and porous polypropylene as the separator.

[0052] Example 4 A method for preparing a self-supporting positive electrode material for a lithium-sulfur battery comprises the following steps: (1) Preparation of zinc activated carbon cloth: The ultrathin carbon cloth was immersed in methanol containing zinc nitrate at a concentration of 5 mol / L and stirred at 60 °C for 3 h under ultraviolet light. The ultraviolet light combined with 60 °C can accelerate the dissociation of zinc ions and anchor them on the ultrathin carbon cloth. The solution was then allowed to stand for 1 h to obtain zinc activated carbon cloth, which was recorded as Zn@C.

[0053] (2) Preparation of self-supporting electrode precursor: S1. Place 20 mmol of zinc nitrate and 20 mmol of cobalt nitrate in 500 mL of methanol, and stir at 50°C for 20 min under ultraviolet light to obtain a metal solution; disperse 80 mmol of 2-methylimidazole and 40 mmol of selenium powder in 500 mL of methanol, and stir at 50°C for 20 min under ultraviolet light to obtain a ligand solution; mix the metal solution and the ligand solution, and stir at 50°C for 20 min under ultraviolet light to accelerate the dissolution of zinc ions and cobalt ions, while making the zinc ions, cobalt ions and selenium powder uniformly dispersed, promote the nano-sizing of selenium powder, and make the selenium in the selenium powder easily agglomerate to obtain a mixed solution.

[0054] S2. Immerse the zinc activated carbon cloth in the mixed solution and let it stand at 80°C for 12 h in the dark to obtain a self-supporting electrode precursor, which is recorded as Se-ZIF67-ZIF8-Zn@C.

[0055] (3) Preparation of self-supporting electrodes: S1. Se-ZIF67-ZIF8-Zn@C was placed in a tube furnace and kept at 350°C for 2 h, then heated to 850°C and kept at this temperature for 5 h. The whole process was heated in an argon atmosphere at a heating rate of 2°C / min to obtain CoSe-ZnSe-C-Zn@C. S2. CoSe-ZnSe-C-Zn@C and pure phase nano-sulfur powder were mixed, and then heat-treated at 150°C in a tube furnace under nitrogen protection for 24 hours to obtain a self-supporting positive electrode material for lithium-sulfur batteries, which was recorded as S-CoSe-ZnSe-C-Zn@C.

[0056] A method for preparing a lithium-sulfur battery comprises the following steps: CR2025 button cells were assembled in an argon-filled glove box with S-CoSe-ZnSe-C-Zn@C as the positive electrode, metallic lithium as the counter electrode and reference electrode, lithium-sulfur electrolyte as the electrolyte, and porous polypropylene as the separator.

[0057] Example 5 A method for preparing a self-supporting positive electrode material for a lithium-sulfur battery comprises the following steps: (1) Preparation of zinc activated carbon cloth: The ultrathin carbon cloth was immersed in methanol containing zinc nitrate at a concentration of 4 mol / L and stirred at 60 °C for 3 h under ultraviolet light. The ultraviolet light combined with 60 °C can accelerate the dissociation of zinc ions and anchor them on the ultrathin carbon cloth. The solution was then allowed to stand for 1 h to obtain zinc activated carbon cloth, which was recorded as Zn@C.

[0058] (2) Preparation of self-supporting electrode precursor: S1. Place 20 mmol of zinc nitrate and 20 mmol of cobalt nitrate in 500 mL of methanol, and stir at 60°C for 15 min under ultraviolet light to obtain a metal solution; disperse 80 mmol of 2-methylimidazole and 40 mmol of selenium powder in 500 mL of methanol, and stir at 60°C for 15 min under ultraviolet light to obtain a ligand solution; mix the metal solution and the ligand solution, and stir at 60°C for 15 min under ultraviolet light to accelerate the dissolution of zinc ions and cobalt ions, while making the zinc ions, cobalt ions and selenium powder uniformly dispersed, promote the nano-sizing of selenium powder, and make the selenium in the selenium powder easily agglomerate to obtain a mixed solution.

[0059] S2. Immerse the zinc activated carbon cloth in the mixed solution and let it stand at 80°C for 12 h in the dark to obtain a self-supporting electrode precursor, which is recorded as Se-ZIF67-ZIF8-Zn@C.

[0060] (3) Preparation of self-supporting electrodes: S1. Se-ZIF67-ZIF8-Zn@C was placed in a tube furnace and kept at 250°C for 2 h, then heated to 800°C and kept at this temperature for 5 h. The whole process was heated in an argon atmosphere at a heating rate of 2°C / min to obtain CoSe-ZnSe-C-Zn@C. S2. CoSe-ZnSe-C-Zn@C and pure phase nano-sulfur powder were mixed, and then heat-treated at 160°C for 12 hours in a tube furnace under nitrogen protection to obtain a self-supporting positive electrode material for lithium-sulfur batteries, which was recorded as S-CoSe-ZnSe-C-Zn@C.

[0061] A method for preparing a lithium-sulfur battery comprises the following steps: CR2025 button cells were assembled in an argon-filled glove box with S-CoSe-ZnSe-C-Zn@C as the positive electrode, metallic lithium as the counter electrode and reference electrode, lithium-sulfur electrolyte as the electrolyte, and porous polypropylene as the separator.

[0062] Examples 1 to 5 of the present invention all produce self-supporting positive electrode materials for lithium-sulfur batteries with excellent electrochemical performance. The following electrochemical performance study is conducted using lithium-sulfur batteries assembled with the self-supporting positive electrode materials for lithium-sulfur batteries of Examples 1 and 2 as examples. The specific research methods and results are shown below: Figure 1The results show that the lithium-sulfur battery assembled using the self-supporting positive electrode material of Example 1 has good cycle stability. After 360 cycles, it still retains a discharge specific capacity of about 1020 mAh / mg. This means that S-CoSe-ZnSe-C-Zn@C can increase the energy density of the active material sulfur while increasing the electrochemical performance of the lithium-sulfur battery.

[0063] Taking the lithium-sulfur battery of Example 2 as an example, Figure 2 The results show that when the self-supporting positive electrode material of the lithium-sulfur battery in Example 2 is used as the positive electrode material of the lithium-sulfur battery, the first cycle specific capacity is 1200 mAh / mg, the charge and discharge platform is also obvious, the discharge voltage is 2.1 V, and the coulombic efficiency is close to 100%.

[0064] Taking the CoSe-ZnSe-C-Zn@C of Example 1 as an example, the material loaded on the CoSe-ZnSe-C-Zn@C was scraped off from the ultra-thin carbon cloth with a blade to obtain CoSe-ZnSe-C-Zn; Figure 3 The results show that CoSe-ZnSe-C-Zn has a regular nanocubic structure and retains the original structural properties of ZIF67 and ZIF8. This indicates that during the conversion process, the bulk structure and chemical properties of ZIF67 and ZIF8 are retained, and the structural stability is strong, which is conducive to improving the cycle performance of lithium-sulfur batteries.

[0065] Figure 4 The results show that after 100 cycles, the charge transfer impedance of the lithium-sulfur battery prepared using the self-supporting positive electrode material of the lithium-sulfur battery in Example 1 is significantly reduced, indicating that the transmission of ions and electrons in the self-supporting positive electrode material of the lithium-sulfur battery is accelerated, which is due to the excellent structural stability and carbon cross-linked network of the self-supporting positive electrode material of the lithium-sulfur battery.

[0066] Taking the CoSe-ZnSe-C-Zn@C of Example 3 as an example, the material loaded on the CoSe-ZnSe-C-Zn@C was scraped off from the ultra-thin carbon cloth with a blade to obtain CoSe-ZnSe-C-Zn; Figure 5 The results showed that the ZnSe and CoSe lattices could be clearly distinguished within the CoSe-ZnSe-C-Zn structure, confirming the completeness and sufficiency of the reaction between Se and the metal elements in ZIF67 and ZIF8 during the reaction process. This also verified the feasibility of the technical solution of the present invention.

[0067] The above descriptions are merely several embodiments of the present invention and do not constitute any form of limitation to the present invention. Although the present invention is disclosed as above in terms of preferred embodiments, they are not intended to limit the present invention. Any technician familiar with the present profession who, without departing from the scope of the technical solution of the present invention, makes slight changes or modifications using the technical contents disclosed above are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for preparing a self-supporting positive electrode material for a lithium-sulfur battery, characterized in that: The steps include: Immersing the carbon cloth in an alcohol solution of a soluble zinc salt to anchor zinc ions on the carbon cloth to obtain zinc-activated carbon cloth; Mixing a soluble zinc salt and a soluble cobalt salt in an alcohol solvent to obtain a metal solution; Mixing 2-methylimidazole and selenium powder in an alcohol solvent to obtain a ligand solution; mixing the metal solution and the ligand solution to obtain a mixed solution; The zinc activated carbon cloth was immersed in the mixed solution and the coordination reaction was carried out in the dark. Under the induction of the zinc ions on the zinc activated carbon cloth, the zinc ions of the soluble zinc salt first coordinated with the ligand 2-methylimidazole to form ZIF8 while coating the zinc ions on the zinc activated carbon cloth. Then, the cobalt ions of the soluble cobalt salt coordinated with the ligand 2-methylimidazole to form ZIF67 while coating ZIF8. Se was embedded in ZIF67 and embedded in ZIF8 to obtain Se-ZIF67-ZIF8-Zn@C. Se-ZIF67-ZIF8-Zn@C was carbonized, and ZIF67 and ZIF8 were carbonized together to form a double-layer structure consisting of an inner and outer carbon porous skeleton. CoSe was attached to the outer carbon porous skeleton, and ZnSe was attached to the inner carbon porous skeleton to obtain CoSe-ZnSe-C-Zn@C. CoSe-ZnSe-C-Zn@C is sulfurized and sulfur is infused into the double-layer structure to obtain a self-supporting positive electrode material for lithium-sulfur batteries.

2. The method for preparing a self-supporting positive electrode material for a lithium-sulfur battery according to claim 1, wherein: The method for anchoring zinc ions on carbon cloth is as follows: stirring at 50°C~60°C for 3 hours under ultraviolet light conditions, and then standing for 1 hour.

3. The method for preparing the self-supporting positive electrode material for lithium-sulfur batteries according to claim 1, wherein: The concentration of the alcohol solution of the soluble zinc salt is 1 mol / L~5 mol / L.

4. The method for preparing a self-supporting positive electrode material for a lithium-sulfur battery according to claim 1, wherein: In the mixed solution, the molar ratio of the soluble zinc salt, the soluble cobalt salt, the 2-methylimidazole and the selenium powder is 1-2:1-2:4-8:2-4.

5. The method for preparing the self-supporting positive electrode material for lithium-sulfur batteries according to claim 1, wherein: The conditions for preparing the metal solution, ligand solution or mixed solution are: stirring at 50°C~70°C for 5min~20min under ultraviolet light irradiation.

6. The method for preparing a self-supporting positive electrode material for a lithium-sulfur battery according to claim 1, wherein: The conditions for the coordination reaction are: standing at 70℃~80℃ for 12h.

7. The method for preparing a self-supporting positive electrode material for a lithium-sulfur battery according to claim 1, wherein: The carbonization conditions are: in an inert atmosphere, first keep the temperature at 200°C~350°C for 1h~3h, and then keep the temperature at 700°C~850°C for 4h-6h.

8. A self-supporting positive electrode material for a lithium-sulfur battery, characterized in that: Prepared by the preparation method according to any one of claims 1 to 7.

9. The self-supporting positive electrode material for lithium-sulfur batteries according to claim 8, characterized in that: The self-supporting positive electrode material of the lithium-sulfur battery is composed of CoSe, ZnSe, carbon cloth carbon porous skeleton and S, the mass percentage of CoSe is 5%~20%, the mass percentage of ZnSe is 5%~20%, the mass percentage of carbon cloth carbon porous skeleton is 10%~20%, the mass percentage of S is 50%~80%, and the sum of the mass percentages of CoSe, ZnSe, carbon cloth carbon porous skeleton and S is 100%.

10. Use of the self-supporting positive electrode material for lithium-sulfur batteries according to claim 8 in preparing lithium-sulfur batteries.

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