Preparation method and application of multi-channel carbon nanofiber composite material with high load of ferrocobalt sulfide

Abstract

The invention relates to the field of sodium ion batteries, and discloses a preparation method and application of a multi-channel carbon nanofiber composite material with high load of ferrocobalt sulfide. The method comprises the following steps: obtaining a cobalt-iron Prussian blue analogue precursor by adopting a ball milling method, preparing an electrostatic spinning solution from the cobalt-iron Prussian blue analogue precursor, polyacrylonitrile and a pore-forming additive, carrying out electrostatic spinning to obtain a uniformly distributed fiber membrane, and carrying out pre-oxidation, heat treatment and gas-phase vulcanization reaction to obtain the porous carbon nanofiber composite material with high cobalt-iron sulfide loading capacity. A heterojunction interface formed by compounding the porous three-dimensional network carbon nanofibers and the ferrocobalt sulfide effectively relieves volume fluctuation in the cycle process, the conductivity of the composite material is enhanced, and the diffusion rate of sodium ions and the cycle stability of the battery are improved. In addition, by improving the loading capacity of the sulfide in the carbon content, the energy density and the electrochemical performance of the battery are improved.

Description

Technical Field

[0001] The present invention relates to the field of sodium ion batteries, and in particular to a preparation method and application of a multi-porous carbon nanofiber composite material with high load of cobalt iron sulfide. Background Art

[0002] Sodium-ion batteries have become a strong competitor to lithium-ion batteries due to their abundant resources, low cost, high safety and excellent low-temperature performance. As a potential energy storage technology, the research progress of sodium-ion batteries' negative electrode materials is crucial to improving battery performance. Iron-based sulfides, as negative electrode materials for sodium-ion batteries, have the advantages of abundant resources, environmental friendliness and high theoretical specific capacity, but their electrical conductivity is low and their volume changes greatly during the cycle, resulting in poor cycle stability and rate performance. Methods such as composite with conductive carbon materials, nanostructured structure design, metal ion doping and heterostructure construction have been widely used to improve the electrochemical properties of iron-based sulfides.

[0003] Although these methods have shown potential in improving the performance of iron-based sulfide anode materials, they still have some limitations. For example, invention patent CN202311244127.3 discloses a carbon-coated FeS 2 Nanosphere negative electrode material, this negative electrode material has a unique core-shell nanostructure, with an ordered carbon structure as a coating shell, which improves the conductivity, but the preparation process is relatively complicated, the experimental conditions are harsh, and the cycle stability and rate performance after modification are still poor. Invention patent CN202310469850.5 discloses a cage-shaped cobalt iron sulfide carbon nanofiber composite material, the preparation method of which adopts a coprecipitation method combined with an electrostatic spinning method, which effectively improves the conductivity of the material and alleviates the volume change of the electrode material during the charge and discharge process, but the preparation of its precursor is time-consuming, the output is low, and the actual load of the derived sulfide on the carbon fiber is low, which limits the improvement of capacity and energy density.

[0004] In summary, it is necessary to continue to explore more effective methods to address the challenges of iron-based sulfide anode materials in order to achieve the commercialization and widespread application of sodium-ion batteries. Summary of the invention

[0005] In view of the technical problems that the iron-based sulfide has low electrical conductivity and large volume change during the cycle, the present invention provides a method for preparing a high-load cobalt iron sulfide multi-porous carbon nanofiber composite material and its application. The present invention prepares a cobalt iron Prussian blue analog precursor by a green and efficient ball milling method, and then prepares a high-load cobalt iron sulfide multi-porous carbon nanofiber composite material (CoS 2 / FeS 2@MCNFs), the three-dimensional network structure of the multi-porous carbon nanofibers in the composite material can effectively alleviate the volume change during the cycle, thereby improving the cycle stability, and at the same time increase the diffusion rate of sodium ions by improving the conductivity of the composite material. High-loaded cobalt iron sulfide can increase the amount of charge stored per unit mass, thereby significantly improving the energy density of the battery. In addition, CoS 2 and FeS 2 The heterojunction formed by the composite produces a built-in electric field effect, which can further improve the reaction kinetics.

[0006] The specific technical scheme of the present invention is:

[0007] First, the present invention provides a method for preparing a porous carbon nanofiber composite material with high cobalt iron sulfide loading and its application, which comprises the following steps:

[0008] (1) Ball milling iron salt, cobalt salt and a grinding aid, and washing to obtain a cobalt-iron Prussian blue analog precursor powder.

[0009] (2) Cobalt iron Prussian blue analog precursor powder, pore-forming additive, and polyacrylonitrile are mixed and dissolved in N,N-dimethylformamide to obtain an electrospinning solution.

[0010] (3) Obtain fiber membrane through electrospinning.

[0011] (4) The fiber membrane A is pre-oxidized to obtain the fiber membrane B, and the fiber membrane B is calcined under an inert atmosphere to obtain the intermediate product fiber membrane C.

[0012] (5) subjecting the intermediate product fiber membrane C to a vapor phase sulfurization reaction with a sulfur source under an inert atmosphere to obtain a multi-porous carbon nanofiber composite material with a high load of cobalt iron sulfide; the multi-porous carbon nanofiber composite material with a high load of cobalt iron sulfide is a nitrogen-doped multi-porous carbon nanofiber coated with CoS 2 / FeS 2 Heterojunction.

[0013] In the above-mentioned preparation process of the present invention, the main reactions are as follows: first, the reactants are mixed evenly under the action of strong mechanical force, and the cobalt salt reacts with the iron salt to generate a cobalt-iron Prussian blue analog precursor; then the precursor, pore-forming additive and polyacrylonitrile are dispersed in N,N-dimethylformamide to prepare an electrospinning solution, and a fiber membrane is obtained by electrospinning, and then the carbon fiber structure is stabilized by pre-oxidation; finally, heat treatment and gas phase vulcanization treatment are performed, and the sulfur source sublimates at high temperature to become sulfur vapor, which reacts with the heat-treated product to generate CoS 2 / FeS 2 @MCNFs, the excess sulfur is removed in the form of gas.

[0014] Prussian blue analogs are typical coordination framework materials with stable structures. The present invention adopts a ball milling method to prepare a cobalt-iron Prussian blue analog precursor. Compared with the traditional coprecipitation method and hydrothermal method, this method has the advantages of simple operation, low reaction activation energy, grain refinement, high uniformity of particle distribution, and large-scale production. Introducing the ball-milled Prussian blue analogs into the electrospinning solution can further increase the content of fiber-coated active substances and derive a uniformly distributed heterojunction interface.

[0015] The multi-porous carbon nanofiber composite material (CoS 2 / FeS 2 @MCNFs), its three-dimensional network structure of multi-porous carbon nanofibers can effectively alleviate the volume change during the cycle, thereby improving the cycle stability, and at the same time increase the diffusion rate of sodium ions by improving the conductivity of the composite material. High-loaded cobalt iron sulfide can increase the amount of charge stored per unit mass, thereby significantly improving the energy density of the battery. In addition, CoS 2 and FeS 2 The heterojunction formed by the composite produces a built-in electric field effect, which can further improve the reaction kinetics.

[0016] Preferably, in step (1), the molar ratio of iron to cobalt in the cobalt-iron Prussian blue analog precursor powder is 1:(0.5-2).

[0017] When the iron-cobalt ratio is too high or too low, the precursor mainly derives into a single FeS 2 or CoS 2 , resulting in the material structure being too simple and lacking the synergistic effect of the heterojunction. When the iron-cobalt ratio is too high, the derived FeS 2 The electrical conductivity is low, which is not as good as the heterojunction structure, reducing the rate performance of the material. When the iron-cobalt ratio is too low, the derived CoS 2 Large volume changes will occur during the cycle, causing the material's structure to pulverize and break, thereby reducing the material's cyclic stability.

[0018] Preferably, in step (1), the iron salt is one or more of potassium ferrocyanide and potassium ferrocyanide; the cobalt salt is one or more of cobalt acetate, cobalt chloride and cobalt nitrate; and the grinding aid is one or more of anhydrous ethanol, ethylene glycol and glycerol.

[0019] Potassium ferrocyanide and potassium ferrocyanide have high chemical stability, which allows them to remain stable during the ball milling process. Under appropriate conditions, they can participate in redox reactions to generate products with specific properties. Their stable chemical structure can be gradually activated during the ball milling process, thereby improving the controllability of the reaction and the uniformity of the product. Cobalt acetate, cobalt chloride and cobalt nitrate show good adaptability under different environmental conditions and can react effectively with other ingredients during the ball milling process to improve production efficiency.

[0020] Preferably, in step (1), the ball-to-material mass ratio of the ball milling is 1:(10-30), the ball milling speed is 200-400 rpm, and the ball milling time is 2-6 h.

[0021] Further preferably, in step (1), the ball milling includes the following stages:

[0022] The first stage: ball milling the iron salt and cobalt salt separately, with a ball-to-material mass ratio of 1: (20-30), a ball milling speed of 300-500 rpm, and a ball milling time of 1-2 h.

[0023] The second stage: the iron salt and the grinding aid obtained in the first stage are mixed and ball-milled, the ball-to-material mass ratio is 1:(10-20), the ball-milling speed is 200-400rpm, and the ball-milling time is 1-2h.

[0024] The third stage: the cobalt salt obtained in the first stage of ball milling and the product obtained in the second stage of ball milling are mixed and ball milled, the ball-to-material mass ratio is 1:(10-30), the ball milling speed is 200-500rpm, and the ball milling time is 2-4h.

[0025] In order to further improve the ball milling effect, the present invention specifically designs the ball milling into three stages. In the three stages, by changing the rotating speed, the ball-to-material ratio, the ball milling time and adding a grinding aid, the ball milling efficiency is improved, the particles of the material are made finer, more uniform, and more suitable for the subsequent electrostatic spinning technology. Specifically: in the first stage, the iron salt and the cobalt salt are respectively ball milled, which can ensure that each salt is subjected to a targeted refinement treatment in the initial stage, and avoid the uneven refinement or incomplete local reaction caused by the difference in physicochemical properties during the mixed ball milling process, and the uniform particle size distribution helps to improve the reaction rate and the uniformity of the product. In the second stage, the refined iron salt is mixed with a grinding aid for ball milling, and the addition of the grinding aid can reduce the friction between the particles, prevent the particles from agglomerating, and improve the ball milling efficiency and the dispersibility of the particles. By mixing the iron salt particles with the grinding aid, the iron salt particles can be further refined, and its surface properties are improved at the same time, so that it is easier to mix and react with other components. In the third stage, the iron salt and the cobalt salt processed in the first two stages are mixed for ball milling, and a chemical reaction occurs between the two at this time. The ball milling in this stage not only further refines the particles, but also promotes the chemical reaction between the iron salt and the cobalt salt through the local high temperature and high pressure environment induced by mechanical force to generate a cobalt-iron Prussian blue analog precursor. Compared with the conventional one-step ball milling, the three-stage differentiated ball milling process of the present invention significantly improves the reaction activity, product uniformity and production efficiency through staged refinement, mixing and reaction, thereby preparing a precursor material with better performance.

[0026] Preferably, in step (2), the mass ratio of the cobalt iron Prussian blue analog precursor to polyacrylonitrile is 1:(0.5-2); the mass ratio of the pore-forming additive to polyacrylonitrile is 1:(1-3).

[0027] If the mass ratio of the cobalt iron Prussian blue analog precursor to polyacrylonitrile is too high, the viscosity of the electrospinning solution will be too high. The high viscosity solution has poor fluidity and is difficult to eject smoothly from the spinneret to form a continuous jet, so that continuous fibers cannot be obtained. The formed fibers are prone to uneven morphology such as beads and spindles, which affects the performance and application of the fibers. If the mass ratio of the cobalt iron Prussian blue analog precursor to polyacrylonitrile is too low, the subsequent derived active substances will be loaded too little on the fiber, resulting in poor electrochemical performance.

[0028] If the mass ratio of pore-forming additives to polyacrylonitrile is too high, the fiber structure will become unstable and fiber collapse will occur, which is not conducive to the stability of the material during the cycle. If the mass ratio of pore-forming additives to polyacrylonitrile is too low, the pore structure produced after heat treatment will be reduced, the specific surface area of ​​the fiber will be significantly reduced, the ion transmission rate will be reduced, and the electrochemical performance will be affected.

[0029] Preferably, in step (2), the pore-forming additive is one or more of polystyrene and polyvinyl pyrrolidone.

[0030] Preferably, in step (3), the electrospinning parameters are: spinning voltage is 10-15 kV, the receiving distance from the needle tip to the receiver is 10-20 cm, and the pushing speed is 0.5-1.5 mL·h -1 .

[0031] Preferably, in step (3), the pre-oxidation temperature is 200-300° C., and the holding time is 1-3 hours.

[0032] Preferably, in step (4), the calcination temperature is 400-800° C., and the holding time is 1-3 hours.

[0033] Preferably, in step (5), the temperature of the gas phase vulcanization is 400-600° C., and the holding time is 1-3 hours.

[0034] Preferably, in step (5), the sulfur source is sulfur powder.

[0035] Preferably, in step (5), the mass ratio of the intermediate product fiber membrane C to the sulfur source is 1:(3-6).

[0036] Preferably, in step (5), the diameter distribution of the nanofibers in the multi-porous carbon nanofiber composite material with high cobalt iron sulfide loading is 1-2 μm, and the pore diameter distribution is 200-500 nm.

[0037] Secondly, the present invention provides the use of a high-load cobalt iron sulfide multi-porous carbon nanofiber composite material prepared by the above preparation method as a negative electrode material for a sodium ion battery.

[0038] Compared with the prior art, the present invention has the following advantages:

[0039] (1) The present invention adopts ball milling to prepare a cobalt-iron Prussian blue analog precursor. Compared with the traditional coprecipitation method and hydrothermal method, this method has the advantages of simple operation, reduced reaction activation energy, grain refinement, high uniformity of particle distribution, and large-scale production. The refined grains can be more effectively coated in carbon fibers in the subsequent electrospinning process, which not only improves the energy density of the battery, but also alleviates the volume change of the sulfide during the charge and discharge process. Furthermore, the present invention can design the ball milling into three stages, which can significantly improve the reaction activity, product uniformity and production efficiency.

[0040] (2) The present invention introduces a cobalt-iron Prussian blue analogue into a spinning solution for electrostatic spinning, and obtains a carbon nanofiber composite material with uniform distribution of cobalt-iron sulfide after pre-oxidation, heat treatment, and vapor phase sulfidation, thereby avoiding the problems of uneven distribution of heterojunctions and susceptibility to environmental influences caused by direct spinning of metal salts.

[0041] (3) The bimetallic sulfide derived from the cobalt-iron Prussian blue analogue prepared by the present invention forms a heterogeneous interface, generates a built-in electric field effect, accelerates the ion diffusion kinetics, reduces the ion diffusion barrier, and significantly improves the electrochemical performance. The multi-porous structure generated by the calcination of the additive increases the electrolyte contact area and effectively alleviates the volume fluctuation during the cycle, thereby improving the cycle stability of the electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is the X-ray diffraction pattern of the multi-porous carbon nanofiber composite cobalt iron sulfide prepared in Example 2;

[0043] Figure 2 is a scanning electron micrograph of the cobalt-iron Prussian blue analogue prepared in Example 4;

[0044] Figure 3 This is a scanning electron microscope image of the porous carbon nanofiber composite cobalt iron sulfide prepared in Example 5;

[0045] Figure 4 This is a cycle performance diagram of the multi-porous carbon nanofiber composite cobalt iron sulfide prepared in Example 1;

[0046] Figure 5 This is a rate performance diagram of the cobalt iron sulfide carbon nanofibers prepared in Comparative Example 2. DETAILED DESCRIPTION

[0047] The present invention will be further described below in conjunction with the embodiments.

[0048] Example 1

[0049] (1) 2 mmol of cobalt chloride, 1 mmol of potassium ferrocyanide, 2 mL of anhydrous ethanol, and 20 g of agate ball milling beads were added to an agate ball milling jar in sequence, and the ball milling jar was placed in a planetary ball mill. The ball milling time was 4 h and the rotation speed was 300 rpm to obtain a cobalt-iron Prussian blue analog precursor (Co-Fe PBA);

[0050] (2) 0.3375 g of the precursor was ultrasonically dispersed in 4.5 g of N,N-dimethylformamide (DMF), and then 0.45 g of polyacrylonitrile (PAN) and 0.15 g of polystyrene (PS) were added and stirred vigorously to obtain a blue-green spinning precursor solution;

[0051] (3) The spinning precursor solution was injected into a 5 mL syringe. The electrospinning process parameters were: the distance from the needle to the receiver was 15 cm, the applied voltage was 13 kV, and the pushing speed was 1 mL h -1 , spinning to obtain a Co-Fe PBA nanofiber composite fiber film;

[0052] (4) The fiber film obtained above was pre-oxidized in air atmosphere at 1°C min -1 The temperature was raised to 260°C at a rate of 1.5°C / min and kept at this temperature for 1 hour. After the pre-oxidation was completed, the -1 The temperature was raised to 600°C at a heating rate of 1000 °C and kept at that temperature for 2 h to obtain a carbonized film;

[0053] (5) The carbonized film obtained in step (4) and sulfur powder were mixed in a nitrogen atmosphere at a mass ratio of 1:6 at 2 °C min -1 The temperature was raised to 500°C at a heating rate of and kept at this temperature for 2 hours to obtain a porous carbon nanofiber composite cobalt iron sulfide composite material.

[0054] Example 2 (the difference from Example 1 is that the amount of pore-forming additive is increased)

[0055] (1) 2 mmol of cobalt chloride, 1 mmol of potassium ferrocyanide, 2 mL of anhydrous ethanol, and 20 g of agate ball milling beads were added to an agate ball milling jar in sequence, and the ball milling jar was placed in a planetary ball mill. The ball milling time was 4 h and the rotation speed was 300 rpm to obtain Co-FePBA;

[0056] (2) 0.3375 g of the precursor was ultrasonically dispersed in 4.5 g of DMF, and then 0.45 g of PAN and 0.45 g of PS were added and stirred vigorously to obtain a blue-green spinning precursor solution;

[0057] (3) The spinning precursor solution was injected into a 5 mL syringe. The electrospinning process parameters were: the distance from the needle to the receiver was 15 cm, the applied voltage was 13 kV, and the pushing speed was 1 mL h -1 , spinning to obtain a Co-Fe PBA nanofiber composite fiber film;

[0058] (4) The fiber film obtained above was pre-oxidized in air atmosphere at 1°C min -1 The temperature was raised to 260°C at a rate of 1.5°C / min and kept at this temperature for 1 hour. After the pre-oxidation was completed, the -1 The temperature was raised to 600°C at a heating rate of 1000 °C and kept at that temperature for 2 h to obtain a carbonized film;

[0059] (5) The carbonized film obtained in step (4) and sulfur powder were mixed in a nitrogen atmosphere at a mass ratio of 1:6 at 2 °C min -1 The temperature was raised to 500°C at a heating rate of and kept at this temperature for 2 hours to obtain a porous carbon nanofiber composite cobalt iron sulfide composite material.

[0060] Example 3 (the difference from Example 1 is that the amount of precursor added is reduced)

[0061] (1) 2 mmol of cobalt chloride, 1 mmol of potassium ferrocyanide, 2 mL of anhydrous ethanol, and 20 g of agate ball milling beads were added to an agate ball milling jar in sequence, and the ball milling jar was placed in a planetary ball mill. The ball milling time was 4 h and the rotation speed was 300 rpm to obtain Co-FePBA;

[0062] (2) 0.2 g of the precursor was ultrasonically dispersed in 4.5 g of DMF, and then 0.45 g of PAN and 0.15 g of PS were added and stirred vigorously to obtain a blue-green spinning precursor solution;

[0063] (3) The spinning precursor solution was injected into a 5 mL syringe. The electrospinning process parameters were: the distance from the needle to the receiver was 15 cm, the applied voltage was 13 kV, and the pushing speed was 1 mL h -1 , spinning to obtain a Co-Fe PBA nanofiber composite fiber film;

[0064] (4) The fiber film obtained above was pre-oxidized in air atmosphere at 1°C min -1 The temperature was raised to 260°C at a rate of 1.5°C / min and kept at this temperature for 1 hour. After the pre-oxidation was completed, the -1 The temperature was raised to 600°C at a heating rate of 1000 °C and kept at that temperature for 2 h to obtain a carbonized film;

[0065] (5) The carbonized film obtained in step (4) and sulfur powder were mixed in a nitrogen atmosphere at a mass ratio of 1:6 at 2 °C min -1 The temperature was raised to 500°C at a heating rate of and kept at this temperature for 2 hours to obtain a porous carbon nanofiber composite cobalt iron sulfide composite material.

[0066] Embodiment 4 (the difference from Embodiment 1 is that the amount of sulfur powder added is reduced)

[0067] (1) 2 mmol of cobalt chloride, 1 mmol of potassium ferrocyanide, 2 mL of anhydrous ethanol, and 20 g of agate ball milling beads were added to an agate ball milling jar in sequence, and the ball milling jar was placed in a planetary ball mill. The ball milling time was 4 h and the rotation speed was 300 rpm to obtain Co-FePBA;

[0068] (2) 0.3375 g of the precursor was ultrasonically dispersed in 4.5 g of DMF, and then 0.45 g of PAN and 0.15 g of PS were added and stirred vigorously to obtain a blue-green spinning precursor solution;

[0069] (3) The spinning precursor solution was injected into a 5 mL syringe. The electrospinning process parameters were: the distance from the needle to the receiver was 15 cm, the applied voltage was 13 kV, and the pushing speed was 1 mL h -1 , spinning to obtain a Co-Fe PBA nanofiber composite fiber film;

[0070] (4) The fiber film obtained above was pre-oxidized in air atmosphere at 1°C min -1 The temperature was raised to 260°C at a rate of 1.5°C / min and kept at this temperature for 1 hour. After the pre-oxidation was completed, the -1 The temperature was raised to 600°C at a heating rate of 1000 °C and kept at that temperature for 2 h to obtain a carbonized film;

[0071] (5) The carbonized film obtained in step (4) and sulfur powder were mixed in a nitrogen atmosphere at a mass ratio of 1:3 at 2 °C min -1 The temperature was raised to 500°C at a heating rate of and kept at this temperature for 2 hours to obtain a porous carbon nanofiber composite cobalt iron sulfide composite material.

[0072] Example 5 (the difference from Example 1 is that the feed of precursor is increased)

[0073] (1) 2 mmol of cobalt chloride, 1 mmol of potassium ferrocyanide, 2 mL of anhydrous ethanol, and 20 g of agate ball milling beads were added to an agate ball milling jar in sequence, and the ball milling jar was placed in a planetary ball mill. The ball milling time was 4 h and the rotation speed was 300 rpm to obtain Co-FePBA;

[0074] (2) 0.6 g of the precursor was ultrasonically dispersed in 4.5 g of DMF, and then 0.45 g of PAN and 0.15 g of PS were added and stirred vigorously to obtain a blue-green spinning precursor solution;

[0075] (3) The spinning precursor solution was injected into a 5 mL syringe. The electrospinning process parameters were: the distance from the needle to the receiver was 16 cm, the applied voltage was 11 kV, and the pushing speed was 0.5 mL h -1 , spinning to obtain a Co-Fe PBA nanofiber composite fiber film;

[0076] (4) The fiber film obtained above was pre-oxidized in air atmosphere at 1°C min -1 The temperature was raised to 270°C at a rate of 1.5°C and kept at this temperature for 3 hours. After the pre-oxidation was completed, the temperature was raised to 270°C at a rate of 2°C·min in a nitrogen atmosphere. -1 The temperature was raised to 400°C at a heating rate of 100°C and kept at that temperature for 2 hours to obtain a carbonized film;

[0077] (5) The carbonized film obtained in step (4) and sulfur powder were mixed in a nitrogen atmosphere at a mass ratio of 1:6 at 2 °C min -1 The temperature was raised to 500°C at a heating rate of and kept at this temperature for 2 hours to obtain a porous carbon nanofiber composite cobalt iron sulfide composite material.

[0078] Example 6 (the difference from Example 1 is that a three-stage differentiated ball milling process is adopted)

[0079] (1) 2 mmol of cobalt chloride and 1 mmol of potassium ferrocyanide were placed in two ball mills respectively, with a ball-to-material mass ratio of 1:30, a ball milling time of 1 h, and a ball milling speed of 400 rpm;

[0080] (2) After the ball milling process is completed, take out the powder from the ball mill. Place 2 mL of anhydrous ethanol and crushed potassium ferrocyanide in the ball mill, with a ball-to-material mass ratio of 1:15, a ball milling time of 1.5 h, and a ball milling speed of 300 rpm;

[0081] (3) After the ball milling process is completed, the crushed cobalt chloride is added to a ball mill, and an appropriate amount of ball milling beads are added. The total ball-to-material mass ratio is 1:20. The ball milling time is 3 h and the ball milling speed is 400 rpm to obtain Co-Fe PBA;

[0082] (4) 0.3375 g of the precursor was ultrasonically dispersed in 4.5 g of DMF, and then 0.45 g of PAN and 0.15 g of PS were added and stirred vigorously to obtain a blue-green spinning precursor solution;

[0083] (5) The spinning precursor solution was injected into a 5 mL syringe, and the electrospinning process parameters were: the distance from the needle to the receiver was 15 cm, the applied voltage was 13 kV, and the pushing speed was 1 mL·h-1, and the Co-Fe PBA nanofiber composite fiber film was obtained by spinning;

[0084] (6) Pre-oxidizing the fiber film obtained above in an air atmosphere, heating it to 260°C at a heating rate of 1°C·min-1, and keeping it warm for 1 hour. After the pre-oxidation is completed, heating it to 600°C at a heating rate of 2°C·min-1 in a nitrogen atmosphere, and keeping it warm for 2 hours to obtain a carbonized film;

[0085] (7) The carbonized film obtained in step (6) and sulfur powder were heated to 500° C. at a mass ratio of 1:6 under a nitrogen atmosphere at a heating rate of 2° C. min-1, and kept at this temperature for 2 h to obtain a porous carbon nanofiber composite cobalt iron sulfide composite material. Comparative Example 1 (the difference from Example 1 is that Co-Fe PBA was synthesized by coprecipitation method)

[0086] (1) 2 mmol of cobalt chloride and 1 mmol of potassium ferrocyanide were dissolved in 100 mL of water, allowed to stand at 80° C. for 20 h, centrifuged, washed, and dried to obtain Co-Fe PBA;

[0087] (2) 0.3375 g of the precursor was ultrasonically dispersed in 4.5 g of DMF, and then 0.45 g of PAN and 0.15 g of PS were added and stirred vigorously to obtain a blue-green spinning precursor solution;

[0088] (3) The spinning precursor solution was injected into a 5 mL syringe. The electrospinning process parameters were: the distance from the needle to the receiver was 15 cm, the applied voltage was 13 kV, and the pushing speed was 1 mL h -1 , spinning to obtain a Co-Fe PBA nanofiber composite fiber film;

[0089] (4) The fiber film obtained above was pre-oxidized in air atmosphere at 1°C min -1 The temperature was raised to 260°C at a rate of 1.5°C / min and kept at this temperature for 1 hour. After the pre-oxidation was completed, the -1 The temperature was raised to 600°C at a heating rate of 1000 °C and kept at that temperature for 2 h to obtain a carbonized film;

[0090] (5) The carbonized film obtained in step (4) and sulfur powder were mixed in a nitrogen atmosphere at a mass ratio of 1:6 at 2 °C min -1 The temperature was raised to 500° C. at a heating rate of 100° C. and kept at this temperature for 2 hours to obtain a porous carbon nanofiber composite cobalt iron sulfide composite material. Comparative Example 2 (the difference from Example 1 is that polystyrene is not added)

[0091] (1) 2 mmol of cobalt chloride, 1 mmol of potassium ferrocyanide, 2 mL of anhydrous ethanol, and 20 g of agate ball milling beads were added to an agate ball milling jar in sequence, and the ball milling jar was placed in a planetary ball mill. The ball milling time was 4 h and the rotation speed was 300 rpm to obtain Co-FePBA;

[0092] (2) 0.3375 g of the precursor was ultrasonically dispersed in 4.5 g of DMF, and then 0.45 g of PAN was added and stirred vigorously to obtain a blue-green spinning precursor solution;

[0093] (3) The spinning precursor solution was injected into a 5 mL syringe. The electrospinning process parameters were: the distance from the needle to the receiver was 15 cm, the applied voltage was 13 kV, and the pushing speed was 1 mL h -1 , spinning to obtain a Co-Fe PBA nanofiber composite fiber film;

[0094] (4) The fiber film obtained above was pre-oxidized in air atmosphere at 1°C min -1 The temperature was raised to 260°C at a rate of 1.5°C / min and kept at this temperature for 1 hour. After the pre-oxidation was completed, the -1 The temperature was raised to 600°C at a heating rate of 1000 °C and kept at that temperature for 2 h to obtain a carbonized film;

[0095] (5) The carbonized film obtained in step (4) and sulfur powder were mixed in a nitrogen atmosphere at a mass ratio of 1:6 at 2 °C min -1 The temperature was raised to 500°C at a heating rate of and kept at that temperature for 2 hours to obtain a cobalt iron sulfide carbon nanofiber composite material.

[0096] Comparative Example 3 (the difference from Example 1 is that the gas phase vulcanization temperature is lowered)

[0097] (1) 2 mmol of cobalt chloride, 1 mmol of potassium ferrocyanide, 2 mL of anhydrous ethanol, and 20 g of agate ball milling beads were added to an agate ball milling jar in sequence, and the ball milling jar was placed in a planetary ball mill. The ball milling time was 4 h and the rotation speed was 300 rpm to obtain Co-FePBA;

[0098] (2) 0.3375 g of the precursor was ultrasonically dispersed in 4.5 g of DMF, and then 0.45 g of PAN and 0.15 g of PS were added and stirred vigorously to obtain a blue-green spinning precursor solution;

[0099] (3) The spinning precursor solution was injected into a 5 mL syringe. The electrospinning process parameters were: the distance from the needle to the receiver was 15 cm, the applied voltage was 13 kV, and the pushing speed was 1 mL h -1 , spinning to obtain a Co-Fe PBA nanofiber composite fiber film;

[0100] (4) The fiber film obtained above was pre-oxidized in air atmosphere at 1°C min -1 The temperature was raised to 260°C at a rate of 1.5°C / min and kept at this temperature for 1 hour. After the pre-oxidation was completed, the -1 The temperature was raised to 600°C at a heating rate of 1000 °C and kept at that temperature for 2 h to obtain a carbonized film;

[0101] (5) The carbonized film obtained in step (4) and sulfur powder were mixed in a nitrogen atmosphere at a mass ratio of 1:6 at 2 °C min -1 The temperature was raised to 300°C at a heating rate of 100°C and kept at this temperature for 2 hours to obtain a porous carbon nanofiber composite cobalt iron sulfide composite material. Comparative Example 4 (the difference from Example 1 is that Co-Fe PBA is synthesized by a hydrothermal method)

[0102] (1) 2 mmol of cobalt chloride and 1 mmol of potassium ferrocyanide were dissolved in 100 mL of water, hydrothermally heated at 70° C. for 24 h, centrifuged, washed, and dried to obtain Co-Fe PBA;

[0103] (2) 0.3375 g of the precursor was ultrasonically dispersed in 4.5 g of DMF, and then 0.45 g of PAN and 0.15 g of PS were added and stirred vigorously to obtain a blue-green spinning precursor solution;

[0104] (3) The spinning precursor solution was injected into a 5 mL syringe. The electrospinning process parameters were: the distance from the needle to the receiver was 15 cm, the applied voltage was 13 kV, and the pushing speed was 1 mL h -1 , spinning to obtain a Co-Fe PBA nanofiber composite fiber film;

[0105] (4) The fiber film obtained above was pre-oxidized in air atmosphere at 1°C min -1 The temperature was raised to 260°C at a rate of 1.5°C / min and kept at this temperature for 1 hour. After the pre-oxidation was completed, the -1 The temperature was raised to 600°C at a heating rate of 1000 °C and kept at that temperature for 2 h to obtain a carbonized film;

[0106] (5) The carbonized film obtained in step (4) and sulfur powder were mixed in a nitrogen atmosphere at a mass ratio of 1:6 at 2 °C min -1 The temperature was raised to 500°C at a heating rate of and kept at this temperature for 2 hours to obtain a porous carbon nanofiber composite cobalt iron sulfide composite material.

[0107] Performance Testing

[0108] The final product material obtained in step (5) of Examples 1 to 6 and Comparative Examples 1 to 4 was mixed with a conductive agent (SuperP) and a binder (polyvinylidene fluoride) in a mass ratio of 7:2:1 and an appropriate amount of solvent N-methylpyrrolidone (NMP) to form a homogeneous slurry, which was then evenly coated on the copper current collector. After being dried at a constant temperature of 60°C for 12 hours, it was punched into pole pieces with a diameter of 1.2 cm as sodium ion battery electrodes. Glass fiber was used as the diaphragm and sodium sheet as the counter electrode. The electrolyte was 1.0 mol·L - 1 NeA 6 As the solute, diethylene glycol dimethyl ether was used as the solvent, and a CR2025 button cell was assembled in a glove box. The cells were placed in a constant temperature environment of 25°C for 24 hours, and then electrochemical tests were performed.

[0109] Table 1

[0110]

[0111] *At 0.5A g -1 The test was conducted under a current density of 1.5 V (voltage window of 0.01-3 V), and the data obtained were the capacity retention rate at the 100th cycle of the half-cell.

[0112] Table 1 summarizes the performance of various embodiments and comparative examples at 0.5A g -1 The first cycle discharge capacity of the half-cell and the capacity retention rate at the 100th cycle at a current density of 0.5A g -1 At a current density of 1.34 W, the first cycle discharge capacity of the half-cell is 737 mAh g -1 , 783mAh g -1 , 581mAh g -1 , 613mAh g -1 , 826mAh g-1 , 769mAh g -1 , 687mAh g -1 , 661mAh g -1 , 638mAh g -1 and 712mAh g -1 The capacity retention rates at the 100th cycle were 91%, 75%, 88%, 81%, 71%, 93%, 79%, 73%, 73% and 77%, respectively.

[0113] As can be seen from Table 1, the structure and performance of the composite material can be fully improved by reasonably controlling the feed ratio, gas phase vulcanization temperature and reaction time. The feed ratio directly affects the CoS content in the carbon fiber. 2 and FeS 2 The content of the precursor in Example 5 is too large compared with Example 1, which affects the viscosity of the spinning solution, resulting in fiber entanglement and serious precursor agglomeration, and the carbon coating cannot be effectively formed. Compared with Example 1, the precursor in Example 3 is too small, and the derived CoS 2 and FeS 2 Low content affects the performance of the battery. Compared with Example 1, Example 2 greatly increases the content of pore-forming additives, and a large number of pores are generated during the calcination process, and even collapse occurs, resulting in pulverization of the electrode material after cycling and a lower capacity retention rate. When the sulfur powder ratio (such as Example 4) and the sulfurization temperature (such as Comparative Example 3) are reduced, the material performance decreases accordingly, indicating that too low a sulfurization temperature and sulfur powder ratio will affect CoS 2 and FeS 2 The formation of physical phases, which in turn affects the electrochemical properties of the material. Compared with Example 1, the three-stage ball milling process introduced in Example 6 significantly improves the electrochemical performance due to its more uniform particle distribution and high crystallinity. The precursor materials synthesized in Comparative Examples 1 and 4 have large particle sizes and uneven distribution, which results in poor fiber coating of the precursor during the subsequent spinning process, resulting in poor cycle stability. In Comparative Example 2, since no pore-forming agent was added, no multi-porous structure was formed during the calcination process, which reduced the specific surface area of ​​the material and the transmission rate of sodium ions, resulting in poor electrochemical functions. It can be seen that the multi-porous structure produced by the calcination of the pore-forming agent can greatly improve battery performance.

[0114] Figure 1 The XRD spectrum of Example 2 shows that the synthesized multi-porous carbon nanofiber composite cobalt iron sulfide composite material shows a cubic structure CoS 2 (JCPDS 89-3059) and pyrite structure FeS 2 (JCPDS 42-1340).

[0115] Figure 2This is the SEM image of the cobalt-iron Prussian blue analogue in Example 4. Its morphology is not a traditional cubic block structure, and due to the small particle size, it appears as a cluster with no specific morphology. This ultrafine particle structure is very beneficial to increasing the content of active substances in carbon fibers.

[0116] Figure 3 This is the SEM image of the composite material in Example 5. It can be seen that the precursor feed is too large, which affects the viscosity of the spinning solution, resulting in fiber entanglement, severe precursor agglomeration, and ineffective carbon coating. A large number of lamellar CoS2+ growths on the fiber surface 2 / FeS 2 .

[0117] Figure 4 The cycle performance diagram of the sodium ion battery in Example 1 is shown in FIG. -1 The electrochemical performance was excellent at a current density of 1.34 Å, and it still maintained 658 mAh g after 200 cycles. -1 reversible specific capacity.

[0118] Figure 5 The rate performance diagram of the sodium ion battery in Comparative Example 2 is shown in FIG. -1 , 0.5Ag -1 , 1Ag -1 , 2Ag -1 , 5Ag -1 and returns 0.2Ag -1 The reversible specific capacities at current densities of 532 mAh g -1 , 499mAh g -1 , 486mAh g -1 , 464mAh g -1 , 429mAh g -1 and 537mAh g -1 , showing poor rate performance.

[0119] The raw materials and equipment used in the present invention, unless otherwise specified, are all commonly used raw materials and equipment in the art; the methods used in the present invention, unless otherwise specified, are all conventional methods in the art.

[0120] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent transformation made to the above embodiment based on the technical essence of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing a porous carbon nanofiber composite material with high cobalt iron sulfide loading, characterized in that The following steps are involved: (1) ball-milling an iron salt, a cobalt salt and a grinding aid, and washing to obtain a cobalt-iron Prussian blue analog precursor powder; the molar ratio of iron to cobalt in the cobalt-iron Prussian blue analog precursor powder is 1:(0.5-2); (2) mixing and dissolving cobalt iron Prussian blue analog precursor powder, pore-forming additive, and polyacrylonitrile in N,N-dimethylformamide to obtain an electrospinning solution; (3) Obtaining fiber membrane by electrospinning; (4) pre-oxidizing the fiber membrane A to obtain the fiber membrane B, and calcining the fiber membrane B under an inert atmosphere to obtain the intermediate product fiber membrane C; (5) subjecting the intermediate product fiber membrane C to a vapor phase sulfurization reaction with a sulfur source under an inert atmosphere to obtain a multiporous carbon nanofiber composite material with a high load of cobalt iron sulfide; the multiporous carbon nanofiber composite material with a high load of cobalt iron sulfide is a nitrogen-doped multiporous carbon nanofiber coated CoS2 / FeS2 heterojunction.

2. The preparation method according to claim 1, characterized in that: In step (1), The iron salt is one or more of potassium ferrocyanide and potassium ferrocyanide; The cobalt salt is one or more of cobalt acetate, cobalt chloride and cobalt nitrate; The grinding aid is one or more of anhydrous ethanol, ethylene glycol and glycerol.

3. The preparation method according to claim 1, characterized in that: In step (1), the ball-to-material mass ratio of the ball mill is 1:(10-30), the ball mill speed is 200-400 rpm, and the ball mill time is 2-6 hours.

4. The preparation method according to claim 1, characterized in that: In step (1), the ball milling includes the following stages: The first stage: ball milling the iron salt and cobalt salt separately, the ball-to-material mass ratio is 1: (20-30), the ball milling speed is 300-500rpm, and the ball milling time is 1-2h; The second stage: the iron salt obtained in the first stage of ball milling and the grinding aid are mixed and ball milled, the ball-to-material mass ratio is 1: (10-20), the ball milling speed is 200-400rpm, and the ball milling time is 1-2h; The third stage: the cobalt salt obtained in the first stage of ball milling and the product obtained in the second stage of ball milling are mixed and ball milled, the ball-to-material mass ratio is 1:(10-30), the ball milling speed is 200-500rpm, and the ball milling time is 2-4h.

5. The preparation method according to claim 1, characterized in that: In step (2), The mass ratio of the cobalt iron Prussian blue analog precursor to polyacrylonitrile is 1:(0.5-2); The mass ratio of the pore-forming additive to polyacrylonitrile is 1:(1-3); The pore-forming additive is one or more of polystyrene and polyvinyl pyrrolidone.

6. The preparation method according to claim 1, characterized in that: In step (3), The parameters of the electrospinning are: the spinning voltage is 10-15 kV, the receiving distance from the needle tip to the receiver is 10-20 cm, and the pushing speed is 0.5-1.5 mL·h -1 ; The pre-oxidation temperature is 200-300° C., and the heat preservation time is 1-3 hours.

7. The preparation method according to claim 1, characterized in that: In step (4), the calcination temperature is 400-800° C. and the insulation time is 1-3 hours.

8. The preparation method according to claim 1, characterized in that: In step (5), The temperature of the gas phase vulcanization is 400-600°C, and the holding time is 1-3h; The sulfur source is sulfur powder; The mass ratio of the intermediate product fiber membrane C to the sulfur source is 1:(3-6).

9. The preparation method according to claim 1, characterized in that: In step (5), the diameter distribution of nanofibers in the multi-porous carbon nanofiber composite material with high cobalt iron sulfide loading is 1-2 μm, and the pore diameter distribution is 200-500 nm.

10. Use of a porous carbon nanofiber composite material with high cobalt iron sulfide loading prepared by the preparation method according to any one of claims 1 to 9 as a negative electrode material for sodium ion batteries.

Images