Zinc sulfide-porous carbon nanofiber composite membrane, preparation method thereof, electrode sheet and lithium ion battery

Abstract

The present invention provides a zinc sulfide-porous carbon nanofiber composite membrane, a preparation method thereof, an electrode sheet, and a lithium ion battery, relating to the fields of nanostructured materials and battery technologies. The preparation method of the zinc sulfide-porous carbon nanofiber composite membrane combines electrospinning and calcination sulfidation processes to prepare a zinc sulfide-porous carbon nanofiber composite membrane with a special morphology. The fiber composite membrane has a unique structure in which zinc sulfide is uniformly distributed inside lotus root-like porous carbon nanofibers. This composite structure provides a good electron transport network and improves the conductivity of the material. Moreover, due to the coating and confinement effects of the porous carbon nanofibers on zinc sulfide, the volume expansion effect during cycling can be effectively inhibited, and defects such as poor conductivity and large volume expansion of zinc sulfide are improved, thereby enhancing the cycle stability of the composite membrane. Since the fiber composite membrane has mechanical flexibility and self-supporting properties, it can be directly used as an electrode sheet, greatly simplifying the electrode sheet preparation process.

Description

Technical Field

[0001] The present invention belongs to the technical fields of nanostructured materials and battery technologies, and relates to a zinc sulfide-porous carbon nanofiber composite membrane, a preparation method thereof, an electrode sheet, and a lithium-ion battery. Background Art

[0002] Lithium-Ion Batteries (LIBs) are an indispensable power source in modern electronic devices and are favored for their high energy density, long lifespan, and low maintenance requirements. LIBs are widely used in multiple fields such as consumer electronics (e.g., mobile phones, laptops), electric vehicles, power tools, and energy storage systems. A lithium-ion battery consists of a positive electrode, a negative electrode, an electrolyte, a separator, and two current collectors (positive and negative electrodes). During charge and discharge processes, lithium ions move between the positive and negative electrodes and are transported through the electrolyte. During discharge, the negative electrode releases lithium ions, which are received by the positive electrode, while electrons flow from the negative electrode to the positive electrode through an external circuit, generating an electric current. During charging, the process is reversed, and lithium ions move from the positive electrode to the negative electrode. Currently, the graphite negative electrode material of commercial LIBs can no longer meet the requirements for the next generation of LIBs due to its low theoretical capacity. Therefore, one of the important steps to promote the development of the next generation of high-performance LIBs is to find a suitable negative electrode material.

[0003] Metal Organic Frameworks (MOFs) are porous materials with a periodic topological structure formed by metal ions (or metal clusters) and organic ligands through coordination bonds. These organic ligands are usually organic molecules containing multiple coordination groups, such as structures containing carboxyl groups, amino groups, aromatic rings, etc. The organic ligands form coordination bonds with metal ions through their coordination groups to construct a three-dimensional network structure, forming pore structures and channels. This unique structure endows MOFs materials with highly adjustable pore sizes, shapes, and surface chemical properties, thus giving MOFs materials rich functions and application values. As a member of the MOF series, ZIF-8 has also been widely studied in the field of energy materials due to its unique porous structure and high specific surface area. However, due to its disadvantages such as insufficient conductivity and poor cycle stability, these factors limit the application of ZIF-8 as a battery material. To solve these difficulties, ZIF-8-derived nanomaterials and nanocomposites have been developed, such as loading ZIF-8 onto other functional materials (such as graphene, nanofibers), but there are usually problems such as uneven dispersion of ZIF-8, easy agglomeration, and difficult morphology control. Once used as an electrode material in a battery, during the cycling process, due to insufficient structural stability, the material structure is prone to collapse, affecting the long-term stability and cycle life of the battery.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] Aiming at the deficiencies and defects existing in the prior art, the present invention aims to provide a zinc sulfide-porous carbon nanofiber composite membrane, a preparation method thereof, an electrode sheet, and a lithium-ion battery.

[0006] In order to achieve the above object, the following technical solutions are adopted:

[0007] The first object of the present invention is to provide a preparation method of a zinc sulfide-porous carbon nanofiber composite membrane, comprising the following steps:

[0008] S1. Electrospinning a slurry formed by mixing ZIF-8, polyacrylonitrile, and dimethylformamide to obtain a nanofiber membrane;

[0009] S2. After drying the nanofiber membrane prepared in step S1, pre-calcining it, then adding sulfur powder to the pre-calcined nanofiber membrane and making the sulfur powder evenly distributed on the surface of the nanofiber membrane, and then placing it in a protective atmosphere for high-temperature calcination to obtain a zinc sulfide-porous carbon nanofiber composite membrane with a lotus root-like structure;

[0010] Among them, the temperature of the high-temperature calcination is 1000-1300 °C, the high-temperature calcination time is 8-12 h, and the protective atmosphere includes at least one of nitrogen or argon;

[0011] The porous carbon nanofibers in the zinc sulfide-porous carbon nanofiber composite membrane have a lotus root-like structure, and zinc sulfide is at least partially distributed inside the lotus root-like porous carbon nanofibers.

[0012] Further, on the basis of the above technical solution of the present invention, in step S1, the mass ratio of ZIF-8, polyacrylonitrile, and dimethylformamide is (1-5):(1-5):(10-20);

[0013] and / or, the molecular weight of the polyacrylonitrile is 50000-200000.

[0014] Further, on the basis of the above technical solution of the present invention, in step S1, the electrospinning adopts at least one of the following process parameters:

[0015] The voltage of the electrospinning is 10-25 kV; and / or,

[0016] The injection speed of the syringe is 0.05-0.5 mL / h.

[0017] Further, on the basis of the above technical solution of the present invention, in step S1, the preparation method of the ZIF-8 includes the following steps:

[0018] Provide a mixed solution A formed by zinc nitrate hexahydrate and methanol, and a mixed solution B formed by 2-methylimidazole and methanol;

[0019] Mix and react the mixed solution A and the mixed solution B, perform solid-liquid separation on the obtained reaction product, wash and dry the separated solid product to obtain ZIF-8.

[0020] Furthermore, on the basis of the above technical solution of the present invention, the molar concentration of zinc nitrate hexahydrate in the mixed solution A is 0.4 - 0.6 mol / L;

[0021] and / or, the molar concentration of 2-methylimidazole in the mixed solution B is 4.0 - 5.0 mol / L;

[0022] and / or, the molar ratio of zinc nitrate hexahydrate in the mixed solution A to 2-methylimidazole in the mixed solution B is (0.4 - 0.6) : (4.0 - 5.0).

[0023] Furthermore, on the basis of the above technical solution of the present invention, in step S2, the drying is vacuum drying, the drying temperature is 60 - 120 °C, and the drying time is 8 - 24 h;

[0024] and / or, in step S2, put the nanofiber membrane into a graphite plate fixture and pre-calcine it in a muffle furnace;

[0025] and / or, in step S2, the pre-calcination temperature is 200 - 400 °C, and the pre-calcination time is 2 - 6 h.

[0026] Furthermore, on the basis of the above technical solution of the present invention, in step S2, the mass ratio of sulfur powder to the nanofiber membrane is (8 - 10) : 1.

[0027] The second object of the present invention is to provide a zinc sulfide-porous carbon nanofiber composite membrane, which is prepared by using the preparation method of the zinc sulfide-porous carbon nanofiber composite membrane provided by the first object of the present invention;

[0028] The porous carbon nanofibers in the zinc sulfide-porous carbon nanofiber composite membrane have a lotus root-like structure, and zinc sulfide is at least partially distributed inside the lotus root-like porous carbon nanofibers. The zinc sulfide-porous carbon nanofiber composite membrane is a flexible self-supporting material.

[0029] The third object of the present invention is to provide an electrode pole piece, which is made of the zinc sulfide-porous carbon nanofiber composite membrane prepared by using the preparation method provided by the first object of the present invention or the zinc sulfide-porous carbon nanofiber composite membrane provided by the second object of the present invention;

[0030] The electrode pole piece is a flexible self-supporting electrode pole piece.

[0031] The fourth object of the present invention is to provide a lithium-ion battery, including the electrode sheet provided by the third object of the present invention.

[0032] Compared with the prior art, the technical solution of the present invention has at least the following technical effects:

[0033] (1) The present invention provides a preparation method of a zinc sulfide-porous carbon nanofiber composite film. This method combines electrospinning and calcination sulfidation processes, and successfully prepares a zinc sulfide-porous carbon nanofiber composite film with a special lotus root-like structure.

[0034] (2) The present invention provides a zinc sulfide-porous carbon nanofiber composite film, which has a unique structure with uniformly distributed zinc sulfide inside the lotus root-like porous carbon nanofibers. This composite structure provides a good electron transport network, improving the conductivity of the material. And due to the coating and confinement effect of the porous carbon nanofibers on zinc sulfide, it can effectively inhibit the volume expansion effect during the cycle process, improving the defects such as poor conductivity and large volume expansion of zinc sulfide, and enhancing the cycle stability of the composite film. In addition, the composite film also has a high specific surface area and a rich pore structure, which can provide more adsorption and storage sites for lithium ions, significantly enhancing the specific capacity of the material. The large specific surface area is conducive to shortening the lithium ion transport path, improving the diffusion and transport rate of lithium ions. In addition, it also increases the contact area between the material and the electrolyte, facilitating the rapid diffusion and transport of lithium ions in the electrolyte.

[0035] (3) The present invention also provides an electrode sheet, which is made of the zinc sulfide-porous carbon nanofiber composite film provided by the present invention. The electrode sheet is a flexible self-supporting electrode sheet. Since the zinc sulfide-porous carbon nanofiber composite film has mechanical flexibility and self-supporting properties, and it is itself a film structure. At the same time, due to its good conductivity, it can be directly (or only need to be cut into the size of the electrode sheet) used as the electrode sheet (working electrode) of the battery without introducing conductive agents, binders, etc., and without the need for processes such as coating, greatly simplifying the electrode sheet preparation process. Brief Description of the Drawings

[0036] Figure 1 SEM image of the zinc sulfide-porous carbon nanofiber composite film prepared in Example 1 of the present invention;

[0037] Figure 2 Electrochemical performance graph of the button battery assembled from the zinc sulfide-porous carbon nanofiber composite film in Example 1 of the present invention at a current density of 0.2 mA / g. Detailed Description of the Invention

[0038] To make the objectives, technical solutions and advantages of the present invention more clear, the following will describe the technical solutions in the embodiments of the present invention clearly and completely in conjunction with the embodiments of the present invention. Those skilled in the art should understand that the described embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. The process parameters without specific conditions noted in the following embodiments are usually in accordance with conventional conditions.

[0039] In the ranges disclosed in the present invention, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0040] According to the first aspect of the present invention, a method for preparing a zinc sulfide-porous carbon nanofiber composite membrane is provided, including the following steps:

[0041] S1. Electrospinning a slurry formed by mixing ZIF-8, polyacrylonitrile and dimethylformamide to obtain a nanofiber membrane;

[0042] S2. After drying the nanofiber membrane prepared in step S1, pre-calcine it, then add sulfur powder to the pre-calcined nanofiber membrane and make the sulfur powder evenly distributed on the surface of the nanofiber membrane, and then place it in a protective atmosphere for high-temperature calcination to obtain a zinc sulfide-porous carbon nanofiber composite membrane with a lotus root-like structure (lotus root-like porous carbon@ZnS).

[0043] Specifically, step S1 of the present invention mainly manufactures a nanofiber membrane containing ZIF-8 through an electrospinning process.

[0044] Step S2 mainly first dries the nanofiber membrane containing ZIF-8 to remove residual solvents to obtain a fiber membrane with a stable structure. After drying, continue with pre-calcination. Pre-calcination can optimize the fiber morphology, making it more uniform, dense and stable, which is beneficial to subsequent carbonization treatment; at the same time, it can further remove organic impurities and residual solvents in the fibers.

[0045] The pre-calcined nanofiber membrane and sulfur powder are subjected to a high-temperature calcination sulfidation process. ZIF-8 gasifies at high temperature to create pores in the carbon nanofibers, and a porous carbon nanofiber with a special lotus root-like morphology is prepared. Moreover, zinc in ZIF-8 reacts with sulfur powder to form zinc sulfide (ZnS), and at least part of the zinc sulfide is evenly distributed inside the pores of the lotus root-like porous carbon nanofibers, thereby forming a zinc sulfide-porous carbon nanofiber composite membrane.

[0046] This lotus root-like zinc sulfide-porous carbon nanofiber composite membrane has advantages such as a high specific surface area, a controllable pore structure, excellent mechanical flexibility, outstanding electrical conductivity, and chemical stability. These characteristics are of great significance for improving battery performance. Moreover, due to its certain self-supporting toughness, it can be directly used as a working electrode for the lithium battery anode and exhibits good lithium storage performance.

[0047] Through the preparation method provided by the present invention, the controllable preparation of a zinc sulfide-porous carbon nanofiber composite membrane with a unique morphology can be achieved.

[0048] The raw materials and composition in the slurry for electrospinning in step S1 are further defined.

[0049] ZIF-8 is formed by the coordination bond of zinc ions ( ) and 2-methylimidazole ( ), and has a unique porous structure and a relatively high specific surface area. The source of ZIF-8 is not specifically limited and can be obtained by self-preparation or by purchase.

[0050] As an optional implementation manner of the technical solution of the present invention, in step S1, the preparation method of ZIF-8 includes the following steps:

[0051] Provide a mixed solution A formed by zinc nitrate hexahydrate and methanol, and a mixed solution B formed by 2-methylimidazole and methanol;

[0052] Mix and react the mixed solution A and the mixed solution B, perform solid-liquid separation on the obtained reaction product, wash and dry the separated solid product to obtain ZIF-8.

[0053] As an optional implementation manner of the technical solution of the present invention, the molar concentration of zinc nitrate hexahydrate in the mixed solution A is 0.4 - 0.6 mol / L, such as 0.4 mol / L, 0.45 mol / L, 0.5 mol / L, 0.55 mol / L or 0.6 mol / L, etc., and preferably 0.5 mol / L;

[0054] And / or, the molar concentration of 2-methylimidazole in the mixed solution B is 4.0 - 5.0 mol / L, such as 4.0 mol / L, 4.2 mol / L, 4.5 mol / L, 4.8 mol / L or 5.0 mol / L, etc., and preferably 4.5 mol / L;

[0055] And / or, the molar ratio of zinc nitrate hexahydrate in the mixed solution A to 2-methylimidazole in the mixed solution B is (0.4 - 0.6):(4.0 - 5.0), and preferably 0.4:4.5.

[0056] As an alternative embodiment of the technical solution of the present invention, in step S1, the molecular weight of polyacrylonitrile is 50,000 - 200,000, such as 50,000, 60,000, 80,000, 100,000, 120,000, 150,000, 180,000 or 200,000, and preferably 85,000.

[0057] As an alternative embodiment of the technical solution of the present invention, in step S1, the mass ratio of ZIF-8, polyacrylonitrile and dimethylformamide is (1 - 5):(1 - 5):(10 - 20), such as 1:1:10, 1:2:10, 1:5:10, 1:5:15, 1:5:20, 2:1:10, 2:2:10, 2:2:20, 2:5:10, 2:5:15, 2:5:20, 5:1:10, 5:1:10, 5:2:10, 5:5:10, 5:5:15 or 5:5:20, etc., and preferably 2:2:20;

[0058] By further limiting the mass ratio of ZIF-8, polyacrylonitrile and dimethylformamide, the viscosity of the spinning slurry is made moderate, and uniform fibers can be spun subsequently.

[0059] As an alternative embodiment of the technical solution of the present invention, in step S1, the electrospinning adopts at least one of the following process parameters:

[0060] The voltage of the electrospinning is 10 - 25 kV, such as 10 kV, 12 kV, 15 kV, 18 kV, 20 kV, 22 kV, 25 kV, etc., and preferably 18 kV; and / or,

[0061] The injection speed of the syringe is 0.05 - 0.5 mL / h, such as 0.05 mL / h, 0.1 mL / h, 0.2 mL / h, 0.3 mL / h, 0.4 mL / h or 0.5 mL / h, and preferably 0.1 mL / h.

[0062] As an optional implementation of the technical solution of the present invention, in step S2, drying is vacuum drying, the drying temperature is 60-120°C, for example, 60°C, 80°C, 100°C or 120°C, preferably 80°C, and the drying time is 8-24h, for example, 8h, 10h, 12h, 16h, 18h, 20h or 24h, etc., preferably 12h;

[0063] and / or, in step S2, placing the nanofiber membrane in a graphite plate fixture and pre-calcining it in a muffle furnace;

[0064] And / or, in step S2, the pre-calcination temperature is 200-400°C, for example, 200°C, 220°C, 250°C, 280°C, 300°C, 320°C, 350°C, 380°C or 400°C, etc., preferably 200°C, the pre-calcination time is 2-6h, for example, 2h, 4h, 5h or 6h, etc., preferably 2h; the pre-calcination atmosphere is air.

[0065] As an optional implementation of the technical solution of the present invention, in step S2, the mass ratio of sulfur powder to nanofiber membrane is (8-10): 1, for example, 8: 1, 9: 1 or 10: 1, etc., preferably 10: 1. Excess sulfur powder is used mainly to ensure that there is enough sulfur source to react with zinc in ZIF-8 to form sulfide.

[0066] As an optional implementation scheme of the technical solution of the present invention, in step S2, the temperature of high-temperature calcination is 1000-1300°C, for example, 1000°C, 1050°C, 1100°C, 1150°C, 1200°C, 1250°C or 1300°C, and the high-temperature calcination time is 8-12h, for example, 8h, 9h, 10h or 12h, and preferably 10h.

[0067] And / or, in step S2, the protective atmosphere includes at least one of nitrogen or argon.

[0068] The inventors have determined the optimal calcination process (such as calcination temperature, calcination time, etc.) conditions through a large number of experimental optimizations, so that the prepared zinc sulfide-porous carbon nanofiber composite membrane has a unique microscopic morphology, as well as good mechanical flexibility and conductivity, thereby achieving better electrochemical performance of the electrode plate made therefrom.

[0069] According to a second aspect of the present invention, there is also provided a zinc sulfide-porous carbon nanofiber composite membrane, which is prepared by the method for preparing the zinc sulfide-porous carbon nanofiber composite membrane provided by the first aspect of the present invention;

[0070] The porous carbon nanofibers in the zinc sulfide-porous carbon nanofiber composite membrane have a lotus root-like structure, and zinc sulfide is at least partially distributed inside the lotus root-like porous carbon nanofibers. The zinc sulfide-porous carbon nanofiber composite membrane is a flexible self-supporting material.

[0071] The zinc sulfide-porous carbon nanofiber composite membrane prepared by the present invention has a unique structure in which zinc sulfide is uniformly distributed inside the lotus root-like porous carbon nanofibers. This composite structure provides a good electron transport network and improves the conductivity of the material. Moreover, due to the coating and confinement effect of the porous carbon nanofibers on zinc sulfide, the volume expansion effect during the cycle can be effectively inhibited, and the defects such as poor conductivity and large volume expansion of zinc sulfide are improved, thereby enhancing the cycle stability of the composite membrane. In addition, the composite membrane also has a high specific surface area and a rich pore structure, which can provide more adsorption and storage sites for lithium ions, and can significantly improve the specific capacity of the material. The large specific surface area is beneficial to shortening the lithium ion transport path, increasing the diffusion and transport rate of lithium ions. In addition, it also increases the contact area between the material and the electrolyte, which is conducive to the rapid diffusion and transport of lithium ions in the electrolyte.

[0072] According to the third aspect of the present invention, an electrode plate is further provided, which is made of the zinc sulfide-porous carbon nanofiber composite membrane prepared by the preparation method provided in the first aspect of the present invention and the zinc sulfide-porous carbon nanofiber composite membrane provided in the second aspect of the present invention;

[0073] The electrode plate is a flexible self-supporting electrode plate.

[0074] The zinc sulfide-porous carbon nanofiber composite membrane provided by the present invention has mechanical flexibility and self-supporting property. It is itself a membrane structure. At the same time, due to its good conductivity, it can be directly (or only need to be cut into the size of the electrode plate) used as the electrode plate (working electrode) of the battery without introducing conductive agents, binders, etc., and without the need for processes such as coating, greatly simplifying the electrode plate preparation process.

[0075] According to the fourth aspect of the present invention, a lithium ion battery is further provided, including the electrode plate provided in the third aspect of the present invention.

[0076] In view of the above advantages of the electrode plate, the lithium ion battery containing it has the same advantages.

[0077] The present invention will be further described in detail below with specific examples and comparative examples.

[0078] Example 1

[0079] This example provides a preparation method of a zinc sulfide-porous carbon nanofiber composite membrane, including the following steps:

[0080] S1. Dissolve Zn(NO3)2·6H2O (0.05 mol) and 2-methylimidazole (0.45 mol) separately in 1000 mL of methanol to form solution A and solution B;

[0081] Then pour solution A into solution B, react at room temperature for 24 h, centrifuge the reaction solution, wash the obtained solid product and dry it under vacuum to obtain ZIF-8;

[0082] S2. Mix 2 g of ZIF-8, 2 g of polyacrylonitrile (molecular weight 85000) and 20 g of dimethylformamide and stir for 12 h to obtain an electrospinning slurry; transfer the slurry to a syringe and perform electrospinning at a voltage of 18 KV and an injection rate of 0.1 mL / h to obtain a nanofiber membrane;

[0083] S3. Vacuum-dry the electrospun fiber membrane at 80 °C for 12 h, then place the fiber membrane in a graphite plate fixture and pre-calcine it in a muffle furnace under an air atmosphere. The pre-calcination temperature is 200 °C and the pre-calcination time is 2 h. After the pre-calcination is completed, cool it to room temperature;

[0084] Add an excessive amount of sulfur powder to the pre-calcined nanofiber membrane (the mass ratio of sulfur powder to nanofiber membrane is 10:1) and make the sulfur powder evenly distributed on the surface of the nanofiber membrane, then place it in a tubular furnace and perform high-temperature calcination under a nitrogen atmosphere. The high-temperature calcination temperature is 1200 °C and the calcination time is 10 h to obtain a zinc sulfide-porous carbon nanofiber composite membrane (with a thickness of 60 - 80 μm).

[0085] Example 2

[0086] This example provides a method for preparing a zinc sulfide-porous carbon nanofiber composite membrane. Except for adjusting the high-temperature calcination temperature in step S3 from 1200 °C to 1100 °C, the other steps and process parameters are the same as those in Example 1.

[0087] Example 3

[0088] This example provides a method for preparing a zinc sulfide-porous carbon nanofiber composite membrane. Except for adjusting the high-temperature calcination temperature in step S3 from 1200 °C to 1300 °C, the other steps and process parameters are the same as those in Example 1.

[0089] Example 4

[0090] This example provides a method for preparing a zinc sulfide-porous carbon nanofiber composite membrane. Except for adjusting the mass of ZIF-8 in step S2 of Example 1 from 2 g to 4 g, the other steps and process parameters are the same as those in Example 1.

[0091] Example 5

[0092] This embodiment provides a method for preparing a zinc sulfide-porous carbon nanofiber composite membrane. Except that the mass ratio of sulfur powder to nanofiber membrane in step S3 of Example 1 is adjusted from 10:1 to 6:1, the remaining steps and process parameters are the same as those in Example 1.

[0093] Example 6

[0094] This embodiment provides a method for preparing a zinc sulfide-porous carbon nanofiber composite membrane, including the following steps:

[0095] S1. Dissolve Zn(NO3)2·6H2O (0.05 mol) and 2-methylimidazole (0.45 mol) in 1000 mL of methanol respectively to form solution A and solution B;

[0096] Then pour solution A into solution B, react at room temperature for 24 h, centrifuge the reaction solution, wash and vacuum-dry the obtained solid product to obtain ZIF-8;

[0097] S2. Mix 5 g of ZIF-8, 5 g of polyacrylonitrile and 20 g of dimethylformamide and stir for 12 h to obtain an electrospinning slurry; transfer the slurry to a syringe and perform electrospinning under the conditions of a voltage of 20 KV and an injection speed of 0.15 mL / h to obtain a nanofiber membrane;

[0098] S3. Vacuum-dry the electrospun fiber membrane at 100 °C for 10 h, then place the fiber membrane in a graphite plate fixture and pre-calcine it in a muffle furnace under an air atmosphere. The pre-calcination temperature is 350 °C and the pre-calcination time is 2 h. After the pre-calcination is completed, cool it to room temperature;

[0099] Add an excessive amount of sulfur powder to the pre-calcined nanofiber membrane (the mass ratio of sulfur powder to nanofiber membrane is 8:1) and make the sulfur powder evenly distributed on the surface of the nanofiber membrane, then place it in a tube furnace and perform high-temperature calcination under a nitrogen atmosphere. The high-temperature calcination temperature is 1000 °C and the calcination time is 12 h to obtain a zinc sulfide-porous carbon nanofiber composite membrane (with a thickness of 60-90 μm).

[0100] Comparative Example 1

[0101] This comparative example provides a method for preparing a zinc sulfide-carbon composite material, including the following steps:

[0102] S1. Dissolve Zn(NO3)2·6H2O (0.05 mol) and 2-methylimidazole (0.45 mol) in 1000 mL of methanol respectively to form solution A and solution B. Then pour solution A into solution B and react at room temperature for 24 h. Centrifuge the reaction solution, wash and vacuum-dry the obtained solid product to obtain ZIF-8.

[0103] Mix sulfur powder and ZIF-8 with a mass ratio of 10:1, place them in a tubular furnace, and perform high-temperature calcination under a nitrogen atmosphere. The temperature of the high-temperature calcination is 1200 °C, and the calcination time is 10 h to obtain a derivative material of ZIF-8, namely zinc sulfide-carbon composite material (abbreviated as ZnS@C).

[0104] Comparative Example 2

[0105] This comparative example provides a method for preparing a carbon nanofiber membrane, including the following steps:

[0106] S1. Mix 2 g of PAN and 20 g of DMF and stir for 12 h to obtain an electrospinning slurry; transfer the slurry to a syringe and perform electrospinning at a voltage of 18 KV and an injection speed of 0.1 mL / h to obtain a fiber membrane;

[0107] S2. Vacuum-dry the fiber membrane obtained by electrospinning at 80 °C for 12 h, then place the fiber membrane in a graphite plate fixture and perform pre-calcination in a muffle furnace under an air atmosphere. The pre-calcination temperature is 200 °C, and the pre-calcination time is 2 h. After the pre-calcination is completed, cool to room temperature;

[0108] Place the pre-calcined fiber membrane in a tubular furnace and perform high-temperature calcination under a nitrogen atmosphere. The temperature of the high-temperature calcination is 1200 °C, and the calcination time is 10 h to finally obtain a carbon nanofiber membrane.

[0109] Comparative Example 3

[0110] This comparative example provides a method for preparing a zinc sulfide-porous carbon nanofiber composite membrane. Except that pre-calcination is not performed in step S3, the other steps and process parameters are the same as those in Example 1.

[0111] Step S3 of this comparative example specifically includes the following steps:

[0112] S3. Vacuum-dry the fiber membrane obtained by electrospinning at 80 °C for 12 h;

[0113] Add an excessive amount of sulfur powder to the dried nanofiber membrane (the mass ratio of sulfur powder to nanofiber membrane is 10:1) and make the sulfur powder evenly distributed on the surface of the nanofiber membrane. Then place it in a tubular furnace and perform high-temperature calcination under a nitrogen atmosphere. The temperature of the high-temperature calcination is 1200 °C, and the calcination time is 10 h to obtain a zinc sulfide-porous carbon nanofiber composite membrane.

[0114] Comparative Example 4

[0115] This comparative example provides a method for preparing a porous carbon nanofiber membrane. Except that sulfur powder is not added in step S3, the other steps and process parameters are the same as those in Example 1.

[0116] Step S3 of this comparative example specifically includes the following steps:

[0117] S3. Vacuum-dry the fiber membrane obtained by spinning at 80 °C for 12 h, then place the fiber membrane in a graphite plate fixture and perform pre-calcination in a muffle furnace at a pre-calcination temperature of 200 °C for 2 h. After the pre-calcination is completed, cool it to room temperature;

[0118] Place the pre-calcined nanofiber membrane in a tube furnace and perform high-temperature calcination in a nitrogen atmosphere at a high-temperature calcination temperature of 1200 °C for 10 h to obtain a porous carbon nanofiber membrane.

[0119] Comparative Example 5

[0120] This comparative example provides a method for preparing a sulfur-carbon nanofiber composite membrane, including the following steps:

[0121] S1. Mix 2 g of polyacrylonitrile and 20 g of dimethylformamide and stir for 12 h to obtain an electrospinning slurry; transfer the slurry to a syringe and perform electrospinning at a voltage of 18 kV and an injection speed of 0.1 mL / h to obtain a nanofiber membrane;

[0122] S2. Vacuum-dry the fiber membrane obtained by spinning at 80 °C for 12 h, then place the fiber membrane in a graphite plate fixture and perform pre-calcination in a muffle furnace at a pre-calcination temperature of 200 °C for 2 h. After the pre-calcination is completed, cool it to room temperature;

[0123] Add an excessive amount of sulfur powder to the pre-calcined nanofiber membrane (the mass ratio of sulfur powder to nanofiber membrane is 10:1), and make the sulfur powder evenly distributed on the surface of the nanofiber membrane. Then place it in a tube furnace and perform high-temperature calcination in a nitrogen atmosphere at a high-temperature calcination temperature of 1200 °C for 10 h to obtain a sulfur-carbon nanofiber composite membrane.

[0124] Comparative Example 6

[0125] This comparative example provides a method for preparing a sulfur-carbon nanofiber composite membrane, including the following steps:

[0126] S1. Mix 2 g of polystyrene, 2 g of polyacrylonitrile and 20 g of dimethylformamide and stir for 12 h to obtain an electrospinning slurry; transfer the slurry to a syringe and perform electrospinning at a voltage of 18 kV and an injection speed of 0.1 mL / h to obtain a nanofiber membrane;

[0127] S2. The fiber membrane obtained by spinning is dried in vacuum at 80 °C for 12 h, and then the fiber membrane is placed in a graphite plate fixture and pre-calcined in a muffle furnace at a pre-calcination temperature of 200 °C for 2 h. After the pre-calcination is completed, it is cooled to room temperature;

[0128] Zinc sulfide is added to the pre-calcined nanofiber membrane (the mass ratio of zinc sulfide to the nanofiber membrane is 0.5:1), and the zinc sulfide powder is evenly distributed on the surface of the nanofiber membrane. Then it is placed in a tube furnace and calcined at a high temperature of 600 °C for 10 h in a nitrogen atmosphere to obtain a zinc sulfide-carbon nanofiber composite membrane.

[0129] In order to verify the technical effects that can be achieved by the examples and comparative examples of the present invention, the following experimental examples are specially set.

[0130] Experimental Example 1

[0131] The morphology of the zinc sulfide-porous carbon nanofiber composite membrane prepared in Example 1 is detected, specifically as Figure 1 shown.

[0132] As Figure 1 can be seen, the surface of the fiber composite membrane prepared by the preparation method of the present invention is wrinkled, with a diameter between 200-300 nm. Due to the gasification of some zinc ions inside, a uniform porous lotus root-like structure is formed, and there are no obvious agglomerated metal compound particles on its surface. And from the ports of the fibers in the SEM image, the pore diameters are similar and the pores are evenly distributed. It can be inferred that zinc sulfide should be evenly distributed inside the fibers.

[0133] Experimental Example 2

[0134] For the zinc sulfide-porous carbon nanofiber composite membranes prepared in Examples 1-6 and the fiber membranes or composite membranes prepared in Comparative Examples 2-6, the composite membranes or fiber membranes are directly cut into circular pole pieces with a diameter of 14 mm by a cutter. Then the above circular pole pieces are assembled into coin cells with lithium sheets, separators, electrolytes (1 M LiPF6, EC: DMC: DEC (1:1:1)), shrapnel, gaskets, and positive and negative electrode cases respectively.

[0135] The composite material obtained in Comparative Example 1 is used as the negative electrode active material. A uniform slurry is obtained by dissolving a mixture formed by the negative electrode active material, PVDF, and Super P (the mass ratio of the negative electrode active material, PVDF, and Super P is 80:10:10) in N-methylpyrrolidone (NMP); the slurry is evenly coated on a copper foil, and after drying, the electrode sheet is cut into a disc with a diameter of 14 mm, and the load density of the disc is about 1.2 mg / cm 2The above-mentioned pole piece, lithium piece, separator, electrolyte (1M LiPF6, EC: DMC: DEC (1:1:1)), shrapnel, gasket, and positive and negative electrode cases were assembled into a button cell.

[0136] The NEWARE BTS-5V battery test system was used to conduct constant current charge and discharge tests on the button cells prepared in each example and comparative example. The voltage range was 0.01 - 3V, and the current densities were 0.2A / g and 2A / g. The specific steps were as follows: First, let it stand for 5 mins, charge the battery at a current of 0.2A / g (or 2A / g) until the cut-off voltage was reached. Let it stand for 10 min, and then discharge it at a constant current of 0.2A / g (or 2A / g) until the cut-off voltage was reached. Repeat the above charging and discharging steps for multiple cycle tests. The relevant performance is shown in Table 1.

[0137] Table 1

[0138]

[0139] Combined with Figure 2 and the data in Table 1, it can be seen that the zinc sulfide-porous carbon nanofiber composite film prepared in Examples 1-6 of the present invention can be directly used as an electrode sheet and has good lithium storage performance. At a current density of 0.2A / g, after 100 cycles, its specific capacity > 690 mAh / g.

[0140] Comparative Examples 1-6 of the present invention are all comparative experiments of Example 1. Specifically, compared with adding sulfur powder to the pre-calcined nanofiber membrane for high-temperature sulfidation in Example 1, in Comparative Example 1, sulfur powder was directly mixed with ZIF-8 and then subjected to high-temperature sulfidation. Since electrospinning and film formation were not carried out in Comparative Example 1, the prepared zinc sulfide-carbon composite material did not exist in the form of a film. Therefore, when it was used as the negative electrode active material, it needed to be used in combination with a conductive agent and a binder, and a conventional coating process was required to form an electrode pole piece. And it can be seen from the data in Table 1 that the electrical performance of the battery prepared in Comparative Example 1 decreased significantly compared with Example 1.

[0141] Compared with Example 1, in Comparative Example 2, neither ZIF-8 nor sulfur powder was added during the preparation of the fiber composite membrane. In Comparative Example 4, sulfur powder was not added during the preparation of the fiber composite membrane. In Comparative Example 5, ZIF-8 was not added during the preparation of the fiber composite membrane. As can be seen from the data in Table 1, the electrical performance data of the battery prepared in Comparative Example 2 decreased significantly. This is mainly because the fiber composite membrane material obtained in Comparative Example 2 is a pure carbon nanofiber material, so its capacity is significantly lower than that of the carbon nanofiber and transition metal sulfide composite material in Example 1. The electrical performance data of the batteries prepared in Comparative Example 4 and Comparative Example 5 also decreased to a certain extent, but the degree of decrease was not as large as that in Comparative Example 2. This is mainly because in Comparative Example 4, a composite material of carbon nanofibers and zinc oxide was formed, and in Comparative Example 5, sulfur-doped carbon nanofibers were obtained. The composite of transition metal compounds and the doping of heteroelements both contribute to improving the lithium storage capacity of carbon nanofibers.

[0142] Compared with Example 1, in Comparative Example 6, ZIF-8 was not added during the preparation of the fiber composite membrane, and at the same time, sulfur powder was directly replaced with a small amount of zinc sulfide. As can be seen from the data in Table 1, the data of Comparative Example 6 decreased to a certain extent compared with Example 1. This is mainly because the mixing method of zinc sulfide and the spinning fiber membrane in Comparative Example 6 is a physical mixing, which is prone to metal particle agglomeration, affecting the material morphology and structural stability. In Example 1, zinc sulfide and carbon nanofibers are in-situ composite and uniformly dispersed, and its structural stability and cycling performance are better than those in Comparative Example 6.

[0143] The above experiments show that the fiber composite membrane prepared by the preparation method of the present invention not only realizes the uniform composite of transition metal sulfides and carbon materials, improves the conductivity of the materials, but also due to its good flexibility and mechanical properties, can be directly used as a working electrode of the battery, showing excellent lithium storage performance.

[0144] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention are within the scope of the claims of the present invention pending approval.

Claims

1. A method for preparing a zinc sulfide-porous carbon nanofiber composite membrane, characterized in that: The following steps are involved: S1. Electrospinning a slurry formed by mixing ZIF-8, polyacrylonitrile and dimethylformamide to obtain a nanofiber membrane; S2. The nanofiber membrane obtained in step S1 is dried and pre-calcined, and then sulfur powder is added to the pre-calcined nanofiber membrane so that the sulfur powder is evenly distributed on the surface of the nanofiber membrane, and then placed in a protective atmosphere for high-temperature calcination to obtain a zinc sulfide-porous carbon nanofiber composite membrane having a lotus root-like structure; Wherein, the mass ratio of the sulfur powder to the pre-calcined nanofiber membrane is (8-10): 1; The high temperature calcination temperature is 1000-1300°C, the high temperature calcination time is 8-12h, and the protective atmosphere includes at least one of nitrogen or argon; The porous carbon nanofibers in the zinc sulfide-porous carbon nanofiber composite membrane have a lotus root-like structure, and the zinc sulfide is at least partially distributed inside the lotus root-like porous carbon nanofibers.

2. The method for preparing the zinc sulfide-porous carbon nanofiber composite membrane according to claim 1, characterized in that: In step S1, the mass ratio of ZIF-8, polyacrylonitrile and dimethylformamide is (1-5): (1-5): (10-20); And / or, the molecular weight of the polyacrylonitrile is 50,000-200,000.

3. The method for preparing the zinc sulfide-porous carbon nanofiber composite membrane according to claim 1, characterized in that: In step S1, the electrospinning adopts at least one of the following process parameters: The voltage of electrospinning is 10-25 kV; and / or, The injection rate of the syringe is 0.05-0.5mL / h.

4. The method for preparing the zinc sulfide-porous carbon nanofiber composite membrane according to claim 1, characterized in that: In step S1, the preparation method of ZIF-8 comprises the following steps: Providing a mixed solution A formed by zinc nitrate hexahydrate and methanol, and a mixed solution B formed by 2-methylimidazole and methanol; The mixed solution A and the mixed solution B are mixed and reacted, the obtained reaction product is subjected to solid-liquid separation, and the separated solid product is washed and dried to obtain ZIF-8.

5. The method for preparing the zinc sulfide-porous carbon nanofiber composite membrane according to claim 4, characterized in that: The molar concentration of zinc nitrate hexahydrate in the mixed solution A is 0.4-0.6 mol / L; And / or, the molar concentration of 2-methylimidazole in the mixed solution B is 4.0-5.0 mol / L; And / or, the molar ratio of zinc nitrate hexahydrate in the mixed solution A to 2-methylimidazole in the mixed solution B is (0.4-0.6): (4.0-5.0).

6. The method for preparing the zinc sulfide-porous carbon nanofiber composite membrane according to claim 1, characterized in that: In step S2, the drying is vacuum drying, the drying temperature is 60-120° C., and the drying time is 8-24 h; and / or, in step S2, placing the nanofiber membrane in a graphite plate fixture and pre-calcining it in a muffle furnace; And / or, in step S2, the pre-calcination temperature is 200-400°C, and the pre-calcination time is 2-6h.

7. A zinc sulfide-porous carbon nanofiber composite membrane, characterized in that: The zinc sulfide-porous carbon nanofiber composite membrane is prepared by the preparation method of any one of claims 1 to 6; The porous carbon nanofibers in the zinc sulfide-porous carbon nanofiber composite membrane have a lotus root-like structure, zinc sulfide is at least partially distributed inside the lotus root-like porous carbon nanofibers, and the zinc sulfide-porous carbon nanofiber composite membrane is a flexible self-supporting material.

8. An electrode plate, characterized in that: Made of the zinc sulfide-porous carbon nanofiber composite membrane prepared by the preparation method according to any one of claims 1 to 6 or the zinc sulfide-porous carbon nanofiber composite membrane according to claim 7; The electrode plate is a flexible self-supporting electrode plate.

9. A lithium ion battery, characterized in that: Including the electrode plate as described in claim 8.

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