Sulfur / nitrogen co-doped porous carbon nanofiber, composite material, and preparation method and application thereof

Abstract

The present invention discloses a sulfur / nitrogen co-doped porous carbon nanofiber, a composite material, and a preparation method and application thereof. The present invention first encapsulates metal organic framework nanoparticles into polymer nanofibers through electrospinning. The composite material is then subjected to pre-oxidation, solvent thermal vulcanization, high-temperature carbonization, and in-situ sulfide oxidation to obtain a sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material. The sulfur nanoparticles are then removed by calcination to obtain the sulfur / nitrogen co-doped porous carbon nanofiber. Elemental sulfur is then loaded to obtain the sulfur-loaded composite material. The sulfur / nitrogen co-doped porous carbon nanofiber prepared by the present invention can act as a sulfur carrier to synergistically electrocatalyze sulfur reduction reactions, reducing the reaction energy barrier and accelerating the reaction kinetics. Furthermore, the abundant nanopore structure in the carbon nanofiber can effectively load elemental sulfur, facilitating contact between the sulfur and the electrolyte, thereby improving charge transfer and accelerating sulfur reduction kinetics, thereby enhancing the performance of zinc-sulfur batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrochemical energy storage, and specifically relates to a sulfur / nitrogen co-doped porous carbon nanofiber, a composite material, and a preparation method and application thereof. Background Art

[0002] Aqueous zinc-ion batteries (Zn-ion batteries) offer the advantages of abundant resources, low cost, and high safety due to their ability to utilize a high theoretical capacity zinc anode (820 mAh / g) and non-flammable electrolyte. They are promising large-scale energy storage devices. However, conventional cathode materials, such as manganese-based materials, vanadium-based materials, Prussian blue analogs, and organic polymers, typically exhibit specific capacities of less than 500 mAh / g, resulting in a low energy density for Zn-ion batteries and limiting their large-scale application. Therefore, the development of novel high-energy-density cathode materials is crucial for the development of aqueous Zn-ion batteries. In recent years, low-cost conversion-type sulfur cathodes have attracted increasing attention due to their high theoretical specific capacity of 1675 mAh / g and moderate theoretical voltage of 1.04 V in aqueous Zn-ion batteries, resulting in a theoretical energy density of 577 Wh / kg for Zn-sulfur batteries. However, in aqueous electrolytes, the insulating nature of the sulfur cathode requires a high energy barrier for the multi-electron conversion reaction during charge and discharge, resulting in sluggish reaction kinetics, low discharge voltage, and low specific capacity, further limiting the energy density. In order to improve the performance of aqueous zinc-sulfur batteries, conductive materials such as carbon materials are usually used to load sulfur to improve the overall conductivity of the composite sulfur positive electrode. 4- Single Fe atoms have been used as electrocatalysts in aqueous sulfur cathodes (J.Am.Chem.Soc.2023,145,5384-5392; ACS Nano 2022,16,7344-7351; Adv.Funct.Mater.2023,33,2210899) to reduce the energy barrier and accelerate the reaction kinetics of the sulfur conversion reaction, thereby improving battery performance. However, research on cathode materials for aqueous zinc-sulfur batteries is still in its early stages, and the development of new technologies to prepare high-performance sulfur cathode materials is extremely challenging and of practical significance. Summary of the Invention

[0003] The present invention provides a sulfur / nitrogen co-doped porous carbon nanofiber, a composite material, and a preparation method and application thereof, the purpose of which is to reduce the energy barrier of the sulfur reduction reaction, accelerate the sulfur reduction reaction kinetics, and increase the discharge voltage and specific capacity of the sulfur positive electrode, thereby improving its energy density.

[0004] The present invention is achieved through the following technical solutions.

[0005] A first aspect of the present invention provides a method for preparing a sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material, comprising the following steps:

[0006] (1) adding metal organic framework nanoparticles and polymers to an organic solvent, stirring uniformly to form a spinning solution, and then electrospinning to obtain a polymer fiber encapsulated metal organic framework nanoparticle precursor;

[0007] (2) The precursor obtained in step (1) is pre-oxidized in an air atmosphere, and the obtained pre-oxidized product is added to a solvent containing a sulfur source. After solvent thermal vulcanization, the solid product is placed in an inert gas atmosphere for calcination and carbonization, and then the calcined product is subjected to an in-situ oxidation reaction. After acid washing, a sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material is obtained.

[0008] Preferably, the metal organic framework in step (1) is a zinc metal organic framework ZIF-8;

[0009] Preferably, the mass concentration of the metal organic framework nanoparticles in the spinning solution in step (1) is 5-20%;

[0010] Preferably, the polymer in step (1) is polyacrylonitrile; the mass concentration of the polymer in the spinning solution is 3-20%;

[0011] Preferably, the organic solvent in step (1) is N,N-dimethylformamide; the stirring is carried out at room temperature;

[0012] Preferably, the process parameters of the electrospinning in step (1) are: voltage of 10-25 kV, spinning solution injection speed of 0.1-0.5 mm / min, and spinning receiving distance of 10-20 cm.

[0013] Preferably, the pre-oxidation in step (2) has a heating rate of 0.5-5°C / min, a temperature of 200-300°C, and a constant temperature time of 1-5 hours;

[0014] Preferably, the sulfur source in step (2) is thioacetamide; the solvent is ethanol; the usage ratio of the pre-oxidation product, the sulfur source and the solvent is 1g:(1-10)g:(10-100)mL; the temperature of the solvent thermal vulcanization is 50-200°C, and the constant temperature time is 1-24 hours.

[0015] Preferably, the inert gas in step (2) is nitrogen; the heating rate of the calcination is 1-10°C / min, the temperature is 700-1000°C, and the constant temperature time is 0.5-5h.

[0016] Preferably, the in-situ oxidation reaction in step (2) is to immerse the calcined product into the Fe-containing 3+in an aqueous solution of iron salt;

[0017] Further preferably, the iron salt is ferric nitrate nonahydrate; the ratio of the calcined product, the iron salt and water is 1 g: (5-40) g: (30-150) mL;

[0018] Preferably, the acid solution for pickling in step (2) is a hydrochloric acid solution, and the molar concentration of the acid solution for pickling is 1-6 mol / L.

[0019] A second aspect of the present invention provides a sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material, which is prepared by the above-mentioned preparation method.

[0020] Preferably, the sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material is composed of sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticles (sulfur element); in the sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material, the mass percentage of sulfur nanoparticles is 10-80%; in the sulfur / nitrogen co-doped porous carbon nanofiber, the mass percentage of sulfur doping atoms is 1-15%, and the mass percentage of nitrogen doping atoms is 1-15%.

[0021] A third aspect of the present invention provides a method for preparing sulfur / nitrogen co-doped porous carbon nanofibers, comprising the following steps:

[0022] The sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material is calcined in an inert atmosphere to remove the sulfur nanoparticles (sulfur element) to obtain sulfur / nitrogen co-doped porous carbon nanofiber.

[0023] Preferably, the inert gas is nitrogen;

[0024] Preferably, the calcination temperature is 1-10°C / min, the temperature is 300-600°C, and the constant temperature time is 0.5-5h;

[0025] A fourth aspect of the present invention provides a sulfur / nitrogen co-doped porous carbon nanofiber, which is prepared using the above-mentioned preparation method.

[0026] A fifth aspect of the present invention provides a method for preparing a sulfur / nitrogen co-doped porous carbon nanofiber-loaded sulfur composite material, comprising the following steps:

[0027] The sulfur / nitrogen co-doped porous carbon nanofibers prepared by the above preparation method are compounded with elemental sulfur to obtain a sulfur / nitrogen co-doped porous carbon nanofiber-loaded sulfur composite material.

[0028] Preferably, the sulfur / nitrogen co-doped porous carbon nanofibers and elemental sulfur are composited by mixing and grinding for 0.5-10 hours;

[0029] Preferably, the mass ratio of the sulfur / nitrogen co-doped porous carbon nanofibers to elemental sulfur is 90-20%:10-80%.

[0030] Preferably, the elemental sulfur is commercial sulfur powder.

[0031] A sixth aspect of the present invention provides a sulfur / nitrogen co-doped porous carbon nanofiber-loaded sulfur composite material, which is prepared using the above-mentioned preparation method.

[0032] In a seventh aspect, the present invention provides an aqueous zinc-sulfur battery, wherein the positive electrode material is the above-mentioned sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material or the above-mentioned sulfur / nitrogen co-doped porous carbon nanofiber-loaded sulfur composite material.

[0033] The beneficial effects of the present invention are as follows:

[0034] (1) This invention combines electrocatalytic and nanostructure engineering strategies to prepare a composite material composed of sulfur / nitrogen co-doped porous carbon nanofibers coated with sulfur nanoparticles. When used as a cathode material for aqueous zinc-sulfur batteries, this composite material exhibits high discharge voltage and specific discharge capacity, good rate capability, and long cycle life, providing a new approach for the preparation of aqueous sulfur-based cathode materials.

[0035] (2) The rich nanoporous structure in the carbon nanofibers prepared by the present invention can effectively load sulfur, which is beneficial to the contact between sulfur and electrolyte, thereby improving charge transfer and accelerating sulfur reduction (SRR) kinetics; at the same time, sulfur / nitrogen co-doped porous carbon nanofibers as sulfur carriers can synergistically electrocatalyze the SRR reaction, reduce the reaction energy barrier and accelerate the SRR kinetics, thereby improving battery performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 These are a scanning electron microscope image (a) of the S@S,N-CNF prepared in Example 1 of the present invention, transmission electron microscope images at different magnifications (b, c), and a high-angle annular dark-field scanning transmission microscope image and element distribution image (d).

[0037] Figure 2 1 is a nitrogen adsorption-desorption curve (a) and a corresponding pore size distribution diagram (b) of S@S,N-CNF prepared in Example 1 of the present invention.

[0038] Figure 3 This is the thermogravimetric curve of S@S,N-CNF prepared in Example 1 of the present invention.

[0039] Figure 4 is the element content of S,N-CNF prepared in Example 2 of the present invention.

[0040] Figure 5The figures are the charge and discharge curves (a) of the sulfur / carbon composite materials prepared in Examples 1 and 2 of the present invention and Comparative Example 1 at a current density of 0.5 A / g and the rate performance (b) at different current densities.

[0041] Figure 6 This is the cycle performance of the sulfur / carbon composite material prepared in Examples 1 and 2 of the present invention at a current density of 4 A / g. DETAILED DESCRIPTION

[0042] For a better understanding of the present invention, the specific implementation of the present invention is further described in detail below with reference to the embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0043] Example 1

[0044] In this embodiment, the preparation process of sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material (S@S,N-CNF) is as follows:

[0045] Step 1: Weigh 2.97g of zinc nitrate hexahydrate and dissolve it in 200mL of anhydrous ethanol to obtain solution A; weigh 6.44g of 2-methylimidazole and dissolve it in 200mL of anhydrous ethanol to obtain solution B; pour solution B into solution A, stir evenly, let it stand at room temperature for 1 hour, and obtain metal-organic framework ZIF-8 nanoparticles after centrifugation, washing and drying; weigh 0.9g of ZIF-8 nanoparticles and 0.2g of polyacrylonitrile (PAN) and add them to 3.5g of N,N-dimethylformamide, stir at room temperature for 24 hours to obtain a spinning solution; then absorb the spinning solution into a syringe, use aluminum foil as a collector, the vertical distance between the syringe needle and the aluminum foil is 16cm, the spinning voltage is 23kV, the spinning rate is 0.25mm / min, and electrospinning obtains ZIF-8@PAN precursor fibers.

[0046] Step 2: Place the ZIF-8@PAN precursor fiber in a muffle furnace, heat it to 250°C at a heating rate of 1°C / min, and hold it for 2 hours; add 0.9g of the pre-oxidation product to 60mL of an ethanol solution containing 4.5g of thioacetamide, and react at 90°C for 20 hours; the product after the reaction is centrifuged, washed and dried, and the resulting brown powder is placed in a tube furnace, heated to 800°C at a heating rate of 5°C / min under a nitrogen atmosphere, and calcined for 2 hours; add 0.3g of the obtained black powder to 30mL of an aqueous solution containing 6g of ferric nitrate nonahydrate, stir at room temperature for 48 hours, and collect the precipitate by centrifugation; add the precipitate to 30mL of a hydrochloric acid solution (4mol / L), stir at room temperature for 1 hour, and finally obtain S@S,N-CNF after centrifugation, washing and drying.

[0047] Figure 1a in the figure is a scanning electron microscope image of the S@S,N-CNF prepared in this example, which shows that it has a nanofiber structure; Figure 1 b and Figure 1 Figure c is a transmission electron microscope image of the S@S,N-CNF prepared in this example at different magnifications, from which it can be seen that the nanofibers have rich pore structures; Figure 1 Figure d is a high-angle annular dark field scanning transmission microscopy image and element distribution image of the S@S,N-CNF prepared in this example. It can be seen that carbon, nitrogen and sulfur elements are evenly distributed on the nanofibers; Figure 2 a and Figure 2 b in the figure are the nitrogen adsorption-desorption curve and pore size distribution diagram of S@S,N-CNF prepared in this example, which further confirms that it has a rich mesoporous structure; Figure 3 This is the thermogravimetric curve of the S@S,N-CNF prepared in this example. According to the weight loss portion of the curve, it can be determined that the mass percentage of elemental sulfur is approximately 40%.

[0048] Example 2

[0049] In this embodiment, the preparation process of sulfur / nitrogen co-doped porous carbon nanofiber-supported sulfur composite material (CS / S,N-CNF) is as follows:

[0050] Step 1: Weigh 2.97g of zinc nitrate hexahydrate and dissolve it in 200mL of anhydrous ethanol to obtain solution A; weigh 6.44g of 2-methylimidazole and dissolve it in 200mL of anhydrous ethanol to obtain solution B; pour solution B into solution A, stir evenly, let it stand at room temperature for 1 hour, and obtain metal-organic framework ZIF-8 nanoparticles after centrifugation, washing and drying; weigh 0.9g of ZIF-8 nanoparticles and 0.2g of polyacrylonitrile (PAN) and add them to 3.5g of N,N-dimethylformamide, stir at room temperature for 24 hours to obtain a spinning solution; then absorb the spinning solution into a syringe, use aluminum foil as a collector, the vertical distance between the syringe needle and the aluminum foil is 16cm, the spinning voltage is 23kV, the spinning rate is 0.25mm / min, and electrospinning obtains ZIF-8@PAN precursor fibers.

[0051] Step 2: Place the ZIF-8@PAN precursor fiber in a muffle furnace, heat it to 250°C at a heating rate of 1°C / min, and hold it for 2 hours; add 0.9g of the pre-oxidation product to 60mL of an ethanol solution containing 4.5g of thioacetamide, and react at 90°C for 20 hours; the product after the reaction is centrifuged, washed and dried, and the resulting brown powder is placed in a tube furnace, heated to 800°C at a heating rate of 5°C / min under a nitrogen atmosphere, and calcined for 2 hours; add 0.3g of the obtained black powder to 30mL of an aqueous solution containing 6g of ferric nitrate nonahydrate, stir at room temperature for 48 hours, and collect the precipitate by centrifugation; add the precipitate to 30mL of a hydrochloric acid solution (4mol / L), stir at room temperature for 1 hour, and finally obtain S@S,N-CNF after centrifugation, washing and drying.

[0052] Step 3: The S@S,N-CNF was placed in a tube furnace and heated to 500°C under a nitrogen atmosphere at a heating rate of 5°C / min. Calcination was performed for 1.5 hours to obtain sulfur / nitrogen co-doped porous carbon nanofibers (S,N-CNF). Commercial elemental sulfur was then loaded onto the S,N-CNF, based on the elemental sulfur content of the S@S,N-CNF obtained in Example 1 (40%, i.e., the weight loss from calcination). The mixture was then ground to obtain CS / S,N-CNF.

[0053] Figure 4 The sulfur and nitrogen content in the S,N-CNF prepared in this example was obtained by energy spectrum EDS analysis, wherein the mass percentage of sulfur doping element was 2.8%, and the mass percentage of nitrogen doping element was 10.8%.

[0054] Comparative Example 1

[0055] In this comparative example, commercial elemental sulfur was loaded on multi-walled carbon nanotubes (CNTs) according to the mass percentage of elemental sulfur in the product S@S,N-CNF obtained in Example 1 (40%), and mixed and ground to obtain CS / CNTs.

[0056] Example 3

[0057] The sulfur / carbon composite materials prepared in Examples 1-2 and Comparative Example 1 were used in aqueous zinc-sulfur batteries. The sulfur / carbon composite material, conductive agent Super P, and binder polyvinylidene fluoride were dispersed in N-methylpyrrolidone at a mass ratio of 8:1:1 to form a uniform slurry. The slurry was then coated onto a carbon cloth current collector and vacuum-dried at 60°C to produce the aqueous zinc-sulfur battery positive electrode.

[0058] CR2032 batteries were assembled with Zn@In as the negative electrode, Glass fiber as the separator and 3M ZnSO4 aqueous solution containing ZnI2 additive as the electrolyte, and the charge and discharge tests were carried out. The preparation process of Zn@In negative electrode is as follows: polish and clean 10×10cm 2 The zinc sheet was immersed in 100 ml of an aqueous solution containing 0.59 g of indium chloride tetrahydrate, allowed to stand for 5 minutes, and then washed and dried to obtain a Zn@In negative electrode.

[0059] Figure 5 a and Figure 5 b in the figure is the charge-discharge curves of the sulfur / carbon composite positive electrode prepared in Examples 1 to 2 of the present invention and Comparative Example 1 at a current density of 0.5 A / g and the rate performance at different current densities. It can be seen that Examples 1 to 2 have higher discharge voltage and discharge specific capacity and better rate performance than Comparative Example 1, confirming that sulfur / nitrogen co-doped carbon nanofibers and sulfur@carbon core-shell structures with internal cavities can synergistically improve the performance of aqueous zinc-sulfur batteries.

[0060] Figure 6 The figure shows the cycling performance of the sulfur / carbon composite positive electrode prepared in Examples 1 and 2 of the present invention at a current density of 4 A / g. It can be seen that the S@S,N-CNF positive electrode prepared in Example 1 has better cycling stability, and the discharge specific capacity is still higher than 680 mAh / g after 300 cycles.

[0061] The above embodiments are used to illustrate the present invention, and the implementation methods of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material, characterized in that: The steps include: (1) adding metal organic framework nanoparticles and a polymer to an organic solvent, stirring uniformly to form a spinning solution, and then electrospinning to obtain a polymer fiber encapsulated metal organic framework nanoparticle precursor; the metal organic framework is zinc metal organic framework ZIF-8; and the polymer is polyacrylonitrile; (2) The precursor obtained in step (1) is pre-oxidized in an air atmosphere, and the obtained pre-oxidized product is added to a solvent containing a sulfur source, wherein the sulfur source is thioacetamide. After solvent thermal vulcanization, the solid product is placed in an inert gas atmosphere for calcination and carbonization, and then the calcined product is subjected to an in-situ oxidation reaction, and after acid washing, a sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material is obtained; the in-situ oxidation reaction is to immerse the calcined product in a solvent containing Fe 3+ in an aqueous solution of iron salt.

2. The preparation method according to claim 1, characterized in that The mass concentration of the metal organic framework nanoparticles in the spinning solution in step (1) is 5-20%; The mass concentration of the polymer in the spinning solution in step (1) is 3-20%; The organic solvent in step (1) is N,N-dimethylformamide; the stirring is carried out at room temperature; The electrospinning process parameters of step (1) are as follows: voltage of 10-25 kV, spinning solution injection speed of 0.1-0.5 mm / min, and spinning receiving distance of 10-20 cm; The pre-oxidation in step (2) is carried out at a heating rate of 0.5-5 °C / min, a temperature of 200-300 °C, and a constant temperature time of 1-5 hours; The solvent in step (2) is ethanol; the ratio of the pre-oxidation product, the sulfur source, and the solvent is 1 g: (1-10) g: (10-100) mL; the temperature of the solvent thermal vulcanization is 50-200 °C, and the constant temperature time is 1-24 hours; The inert gas in step (2) is nitrogen; the heating rate of the calcination is 1-10°C / min, the temperature is 700-1000°C, and the constant temperature time is 0.5-5 h; The iron salt in step (2) is ferric nitrate nonahydrate; the ratio of the calcined product, the iron salt, and water is 1 g: (5-40) g: (30-150) mL; The acid solution for pickling in step (2) is a hydrochloric acid solution, and the molar concentration of the acid solution for pickling is 1-6 mol / L.

3. A sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material, characterized in that: Prepared by the preparation method according to any one of claims 1-2.

4. The sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material according to claim 3, characterized in that: The invention is composed of sulfur / nitrogen co-doped porous carbon nanofibers coated with sulfur nanoparticles; in the sulfur / nitrogen co-doped porous carbon nanofibers coated with sulfur nanoparticles composite material, the mass percentage of sulfur nanoparticles is 10-80%; in the sulfur / nitrogen co-doped porous carbon nanofibers, the mass percentage of sulfur doping atoms is 1-15%, and the mass percentage of nitrogen doping atoms is 1-15%.

5. A method for preparing sulfur / nitrogen co-doped porous carbon nanofibers, characterized in that: The following steps are involved: The sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material according to any one of claims 3 or 4 is calcined in an inert atmosphere to remove the sulfur nanoparticles, thereby obtaining sulfur / nitrogen co-doped porous carbon nanofibers.

6. The preparation method according to claim 5, characterized in that The inert gas is nitrogen; the calcination temperature is 300-600 °C, the heating rate is 1-10 °C / min, and the holding time is 0.5-5 h.

7. A method for preparing a sulfur / nitrogen co-doped porous carbon nanofiber-loaded sulfur composite material, characterized in that: The following steps are involved: The sulfur / nitrogen co-doped porous carbon nanofibers prepared by the preparation method according to any one of claims 5 to 6 are compounded with elemental sulfur to obtain a sulfur / nitrogen co-doped porous carbon nanofiber-loaded sulfur composite material.

8. The preparation method according to claim 7, characterized in that The sulfur / nitrogen co-doped porous carbon nanofibers and elemental sulfur are composited by mixing and grinding for 0.5-10 h; the mass ratio of the sulfur / nitrogen co-doped porous carbon nanofibers to elemental sulfur is 90-20%:10-80%.

9. A sulfur / nitrogen co-doped porous carbon nanofiber-supported sulfur composite material, characterized in that: Prepared by the preparation method according to any one of claims 7-8.

10. An aqueous zinc-sulfur battery, characterized in that: The positive electrode material is the sulfur / nitrogen co-doped porous carbon nanofiber-coated sulfur nanoparticle composite material according to any one of claims 3 to 4 or the sulfur / nitrogen co-doped porous carbon nanofiber-loaded sulfur composite material according to claim 9.

Images