A method for preparing a metal-doped porous carbon fiber / graphene composite material having interconnected macropores

Abstract

The invention discloses a method for preparing a metal-doped porous carbon fiber / graphene composite material with interconnected macropores. The porous carbon fiber / graphene composite material is prepared by an electrostatic spinning method; graphene is dispersed in a PVP spinning solution, and then the uniform mixed solution is spun by an electrostatic spinning method, and finally the electrostatic spinning product is pre-oxidized and then sintered to obtain the porous carbon fiber / graphene composite material; in order to improve its electrochemical performance, cobalt salts, iron salts, nickel salts, etc. can also be added to the spinning solution to form single-atom doping to promote the catalytic conversion reaction of a lithium-sulfur battery.

Description

Technical Field

[0001] The invention relates to the technical field of sulfur host materials, and in particular to a method for preparing a metal-doped porous carbon fiber / graphene composite material with interconnected macropores. Background Art

[0002] As large electric vehicles have put forward new demands for high power density and high energy density secondary batteries, traditional lithium cobalt oxide and ternary cathode materials are far from meeting their requirements. As a cathode material for LSBs, elemental sulfur has a theoretical specific capacity of up to 1675 mAh g -1 , the theoretical specific energy can reach 2600Wh·kg -1 , and elemental sulfur has the advantages of abundant reserves, low cost, and environmental friendliness. Therefore, LSBs have been greatly developed in recent years.

[0003] However, there are still many problems that need to be solved when using sulfur as a positive electrode, such as the poor conductivity of sulfur, which leads to low sulfur utilization, and the dissolution and shuttle effect of polysulfides in the electrolyte, which leads to continuous sulfur loss.

[0004] The main method to solve the above problems is to combine carbon materials (porous carbon, graphene, etc.) with sulfur to improve the conductivity and adsorption of the material. However, as a non-polar material, carbon materials have low adsorption performance for polysulfides and the sulfur loading is limited by the specific surface area of ​​the carbon material. However, macroporous carbon materials that are usually interconnected show poor pore structure stability. Summary of the invention

[0005] The object of the present invention is to provide a method for preparing a metal-doped porous carbon fiber / graphene composite material with interconnected macropores, aiming to solve the problems of low sulfur loading and low polysulfide adsorption performance.

[0006] In order to improve the adsorption and conversion capacity of the load material for polysulfides, the present application prepares a porous metal-doped carbon nanofiber / graphene composite material with a large specific surface area by an electrospinning method. The porous carbon fiber increases the loading of sulfur, the metal single atom doping enhances the catalytic conversion reaction of polysulfides, and the graphene constructs a three-dimensional conductive network to improve the fiber conductivity and structural stability and play a physical barrier role for polysulfides.

[0007] To achieve the above object, the present invention provides the following technical solution: a method for preparing a metal-doped porous carbon fiber / graphene composite material having interconnected macropores, comprising the following specific steps:

[0008] S1 Carbon source dispersion: add graphene to treatment solution A, perform ultrasonic treatment, and obtain graphene dispersion by ultrasonic dispersion;

[0009] S2 Preparation of spinning solution: Add nitrate and melamine to the graphene dispersion obtained in step S1 and stir; then add PVP and stir for 3 to 6 hours, add polytetrafluoroethylene emulsion after it is fully dissolved and stir for 1 to 3 hours; finally, perform electrostatic spinning at a voltage of 18 to 25 kV and a pushing speed of 0.5 ml / h to 2 ml / h, and dry to obtain dry fibers;

[0010] S3 is the preparation of interconnected macroporous metal-doped porous carbon fiber / graphene composite materials: the dried fibers prepared in step S2 are pre-oxidized in a tubular furnace at 180-250°C to form a stable carbon fiber structure; then the temperature is raised to 800-1000°C and heated in an Ar atmosphere to volatilize the polytetrafluoroethylene at high temperature, leaving voids in the carbon fibers, and the metal atoms and nitrogen atoms form a stable chemical structure at high temperature to obtain a metal-doped porous carbon fiber / graphene composite material.

[0011] Preferably, the treatment liquid A in step S1 is one of deionized water and ethanol / DMF mixed solution.

[0012] Preferably, the volume ratio of ethanol / DMF is 1:1.

[0013] Preferably, 10 mg of graphene is added to 3-5 g of deionized water and ultrasonically treated for 2 hours for ultrasonic dispersion.

[0014] Preferably, the nitrate in step S2 is one or more of cobalt nitrate, nickel nitrate and iron nitrate.

[0015] Preferably, the molecular weight of the PVP is 1.3 million, 20 mg of cobalt nitrate and 100 mg of melamine are added to the graphene dispersion and stirred; then 1 g of PVP is added and stirred for 3 to 6 hours.

[0016] Compared with the prior art, the present invention has the following beneficial effects: the present invention prepares a porous carbon fiber / graphene composite material by an electrostatic spinning method. Graphene is dispersed in a PVP spinning solution, and then the uniform mixed solution is spun by an electrostatic spinning method, and finally the electrostatic spinning product is pre-oxidized and then fired to obtain a porous carbon fiber / graphene composite material. In order to improve its electrochemical performance, cobalt salts, iron salts, nickel salts, etc. can also be added to the spinning solution to form single-atom doping to promote the catalytic conversion reaction of lithium-sulfur batteries.

[0017] The preparation process of the present invention is controllable, and the specific surface area of ​​the porous carbon fiber can be controlled by controlling the amount of template added; the prepared carbon fiber has interconnected macropores, which can increase the loading amount of S; polytetrafluoroethylene is added as a pore-forming agent, and no subsequent treatment such as activation and etching is required, which is environmentally friendly and efficient and suitable for commercial production; a large specific surface area helps to increase the reaction active sites and adsorb polysulfides; graphene increases the stability of the porous carbon fiber structure and improves the conductivity; the single-atom doping of the metal-nitrogen-carbon structure of the present invention, the high conductivity and high mechanical property network formed by graphene, and the high specific surface area of ​​the porous carbon fiber can synergistically improve the performance of the lithium-sulfur battery.

[0018] (1) The present invention utilizes electrospinning technology to perform jet spinning of polymer solution or melt in a strong electric field. Under the action of the electric field, the droplet at the needle tip changes from a sphere to a cone, i.e., a "Taylor cone", and extends from the tip of the cone to obtain fiber filaments, i.e., polymer micro-jet, which can run for a considerable distance and eventually solidify into fibers. Electrospinning pulls out "filaments" rather than droplets from the needle tip, and finally utilizes electrospinning technology to prepare metal-doped carbon fiber / graphene composite materials with interconnected macroporous structures.

[0019] (2) The conductivity of the carbon fiber after pure PVP heat treatment in the present invention is poor. The present invention completely wraps the graphene in the carbon nanofiber. The graphene can connect the pores of the fiber inside the fiber to form a three-dimensional conductive network in the cavity to improve the conductivity and structural stability of the material. In addition, the excellent mechanical properties of graphene can also improve the overall structural stability of the carbon fiber.

[0020] (3) The various metal salts added can form metal doping and improve the catalytic conversion effect of lithium-sulfur batteries (LSBs). The addition of melamine to the spinning solution helps to form a metal-nitrogen-carbon structure and improve the stability of the metal site. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a SEM image of the Co-doped porous carbon fiber / graphene-0.5 composite material prepared in Example 1;

[0022] Figure 2 is a SEM image of the Co-doped porous carbon fiber / graphene-1 composite material prepared in Example 2;

[0023] Figure 3 is a SEM image of the Co-doped porous carbon fiber / graphene-2 ​​composite material prepared in Example 3;

[0024] Figure 4 is a SEM image of the Co-doped porous carbon fiber / graphene-3 composite material prepared in Example 4;

[0025] Figure 5This is the performance diagram of lithium-sulfur battery of Co-doped porous carbon fiber-3;

[0026] Figure 6 The performance of Co-doped porous carbon fiber / graphene-3 lithium-sulfur battery. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] The experimental methods described in the following examples are conventional methods unless otherwise specified; if no specific techniques or conditions are specified in the examples, they are carried out according to the techniques or conditions described in the literature in the field or according to the product instructions; the reagents and materials are all analytically pure reagents and can be obtained from commercial channels unless otherwise specified.

[0029] Example 1

[0030] A method for preparing a Co-doped porous carbon fiber / graphene composite material comprises the following steps:

[0031] S1: Graphene dispersion

[0032] Take 10 mg of graphene and add it to 3-5 g of deionized water and ultrasonically treat it for 2 hours for ultrasonic dispersion.

[0033] S2: Preparation of spinning solution

[0034] Take 20 mg of cobalt nitrate and 100 mg of melamine and add them to the graphene dispersion and stir. Then add 1 g of PVP with a molecular weight of 1.3 million and stir for 3 to 6 hours. After fully dissolved, add 3 g of polytetrafluoroethylene emulsion and stir for 2 hours. Finally, electrospinning is carried out at a voltage of 18 to 25 kV and a pushing speed of 0.5 ml / h to 2 ml / h.

[0035] S3: Preparation of Co-doped porous carbon fiber / graphene composites

[0036] The dried fiber is heated to 180-250°C in a tubular furnace at a heating rate of 2°C / min and pre-oxidized for 1 hour to form a stable carbon fiber structure; the temperature is raised to 800-1000°C at a heating rate of 5°C / min in an Ar atmosphere and heated for 3 hours to volatilize the polytetrafluoroethylene at high temperature and leave voids in the carbon fiber; and at high temperature, the metal atoms and nitrogen atoms form a stable chemical structure.

[0037] Embodiment 2-4

[0038] The method steps are consistent with those of Example 1, except that the amount of polytetrafluoroethylene is adjusted to 1 g, 2 g and 3 g.

[0039] Comparative Example 1

[0040] A method for preparing Co-doped porous carbon fiber comprises the following steps:

[0041] Take 20 mg of cobalt nitrate and 100 mg of melamine and add them to 5 g of water and stir, then add 1 g of PVP with a molecular weight of 1.3 million and stir for 3 to 6 hours. After fully dissolved, add it to 3 g of polytetrafluoroethylene emulsion and stir for 2 hours. Finally, electrospinning is carried out at a voltage of 18 to 25 kV and a pushing speed of 0.5 ml / h to 2 ml / h.

[0042] The dried fiber is heated to 180-250°C in a tubular furnace at a heating rate of 2°C / min and pre-oxidized for 1 hour to form a stable carbon fiber structure; the temperature is raised to 800-1000°C at a heating rate of 5°C / min in an Ar atmosphere and heated for 3 hours to volatilize the polytetrafluoroethylene at high temperature and leave voids in the carbon fiber; and at high temperature, the metal atoms and nitrogen atoms form a stable chemical structure.

[0043] like Figure 1-Figure 4 It can be seen that the present invention successfully prepared a metal-doped porous carbon fiber / graphene composite material with interconnected macropores. The prepared carbon fiber has relatively large pores, and the single-atom doping of the metal-nitrogen-carbon structure of the present invention and the high conductivity and high mechanical performance network formed by graphene maintain the fiber morphology.

[0044] Figure 5 The rate performance diagram of lithium-sulfur battery for Co-doped porous carbon fiber is shown in Figure 1. The discharge capacities at current densities of 0.1C, 0.3C, 0.5C, 1C, 3C, and 5C are 1086, 621, 254, 160, 129, and 96 mA h g -1 .

[0045] Figure 6 The rate performance diagram of lithium-sulfur battery for Co-doped porous carbon fiber / graphene-3 shows that the discharge capacities at current densities of 0.1C, 0.3C, 0.5C, 1C, 3C, and 5C are 1320, 908, 729, 502, 229, and 121 mA h g -1 .

[0046] Figure 6 Sample ratio Figure 5 More graphene is added, from Figure 5-Figure 6By comparison, as far as electrode materials are concerned, improving conductivity can enhance rate performance, so at the same current density, the rate performance of Co-doped porous carbon fiber / graphene-3 is better. Secondly, the addition of graphene also improves the utilization rate of S, so the first-cycle discharge capacity is also increased a lot.

[0047] In summary, the present invention can synergistically improve the performance of lithium-sulfur batteries through single-atom doping of metal-nitrogen-carbon structure, high-conductivity and high-mechanical performance network formed by graphene, and high specific surface area of ​​porous carbon fibers.

[0048] As an example, the graphene in the experimental step S1 of the present invention can be replaced by carbon nanotubes or MXene.

[0049] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments, and the same or similar parts between the embodiments can be referred to each other. The above description of the disclosed embodiments enables professionals and technicians in this field to implement or use the present invention. Various modifications to these embodiments will be obvious to professionals and technicians in this field, and the general principles defined herein can be implemented in other embodiments without departing from the principles or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown in this article, but will comply with the widest range consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a metal-doped porous carbon fiber / graphene composite material having interconnected macropores, characterized in that: The specific steps include: S1 Carbon source dispersion: adding graphene oxide to treatment solution A, ultrasonic treatment, and ultrasonic dispersion to obtain graphene oxide dispersion; the treatment solution A is a mixture of deionized water and ethanol / DMF; S2 Preparation of spinning solution: Add nitrate and melamine to the graphene oxide dispersion obtained in step S1 and stir; then add PVP and stir for 3-6 hours, add polytetrafluoroethylene emulsion after it is fully dissolved and stir for 1-3 hours; finally, perform electrospinning at a voltage of 18-25 kV and a pushing speed of 0.5 ml / h-2 ml / h, and dry to obtain dry fibers; The molecular weight of the PVP is 1.3 million. 20 mg of cobalt nitrate and 100 mg of melamine are added to the graphene oxide dispersion and stirred; then 1 g of PVP is added and stirred for 3-6 h; S3 Preparation of metal-doped porous carbon fiber / graphene composite material: pre-oxidize the dried fiber prepared in step S2 at 180-250°C in a tube furnace to form a stable carbon fiber structure; then heat to 800-1000°C in an Ar atmosphere to volatilize the polytetrafluoroethylene at high temperature, leaving cavities in the carbon fiber, and form a stable chemical structure with the metal atoms and nitrogen atoms at high temperature to obtain a metal-doped porous carbon fiber / graphene composite material; The metal-doped porous carbon fiber / graphene composite material is a single-atom doped structure of a metal-nitrogen-carbon structure; the high conductivity and high mechanical performance network formed by graphene and the high specific surface area of ​​the porous carbon fiber synergistically improve the performance of the lithium-sulfur battery.

2. The preparation method according to claim 1, characterized in that The volume ratio of ethanol / DMF is 1:

1.

3. The preparation method according to claim 1, characterized in that: 10 mg of graphene oxide was added to 3-5 g of deionized water and ultrasonically treated for 2 h for ultrasonic dispersion.

Images