A method for preparing hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires and its application

Abstract

The application relates to the field of sodium ion batteries, and discloses a preparation method of hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires and application thereof. The application takes NTA as a metal ion chelating agent, chelates zinc ions and iron ions through a solvothermal reaction, and prepares a ZnFe-NTA precursor; the precursor is pyrolyzed at high temperature under an inert atmosphere, metal ion reduction is induced through a carbon thermal reaction, low-boiling-point zinc particles are volatilized, nitrogen-doped carbon formed through in-situ conversion of an organic ligand is combined, and a hierarchical porous structure is generated; finally, a gas-phase sulfuration method is adopted to form Fe7S8, and the hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires are obtained. When the composite nanowires are used as a negative electrode of a sodium ion battery, the hierarchical porous structure can effectively promote full infiltration of electrolyte and rapid migration of sodium ions, and can improve cycle stability and rate performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion batteries, and more particularly to a method for preparing hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires and their applications. Background Technology

[0002] Against the backdrop of increasing environmental awareness, human society has placed higher demands on the storage of clean energy. Lithium-ion batteries are a typical example of rechargeable batteries; however, the scarcity and extremely uneven distribution of lithium resources can no longer meet the large-scale needs of future human society. Sodium, an element in the same group, is abundant and inexpensive, making it more conducive to the development of large-scale energy storage technologies. Compared to rechargeable batteries such as lithium-sulfur batteries, zinc-air batteries, and hydrogen fuel cells, sodium-ion batteries have a more similar supporting industrial chain, thus saving costs and enabling rapid industrial deployment.

[0003] Currently, the industrialization of sodium-ion batteries still faces many obstacles. Even hard carbon anodes, which theoretically possess excellent sodium storage performance, encounter problems in practical applications such as low initial coulombic efficiency, insufficient long-cycle stability, and poor rate performance. Therefore, research on sodium-storage anode materials remains crucial.

[0004] Iron-based sulfur compounds are inexpensive and environmentally friendly. Among them, Fe7S8 has a relatively high theoretical specific capacity (661 mAh g⁻¹). -1 Fe7S8 exhibits a high discharge specific capacity as a sodium-ion battery anode material, but its severe volume expansion and slow sodium-ion diffusion negatively impact cycle stability and rate performance. Preliminary studies indicate that while Fe7S8 possesses a high discharge specific capacity as a sodium-ion battery anode material, its severe volume expansion and slow sodium-ion diffusion affect cycle stability and rate performance.

[0005] To address these issues, researchers have conducted studies and explorations, discovering that micro / nano structure design and nitrogen-doped carbon composites can effectively enhance the sodium storage performance of Fe7S8. Specifically: (1) Nitrogen-doped carbon composites not only provide structural protection for the material, mitigating pulverization and shedding caused by volume changes during sodium insertion / extraction, but also enhance the overall conductivity and improve conductivity efficiency; (2) Micro / nano structure design alters the diffusion and reaction kinetics of sodium ions at the microscale, thereby stimulating the material's sodium storage potential. However, problems such as unstable carbon composite structures and slow sodium ion transport still exist. Therefore, further exploration and research are needed on these key aspects to truly realize the application of conversion materials. Summary of the Invention

[0006] To address the issues of volume change and slow sodium ion transport during sodium insertion / extraction in Fe7S8 materials, this invention provides a method for preparing hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires and their applications. When used as the anode in sodium-ion batteries, these composite nanowires effectively mitigate the volume change during sodium ion insertion / extraction, enhancing the material's cycle stability. They also improve the overall conductivity and sodium ion diffusion rate of the composite material. Furthermore, their hierarchical porous structure promotes thorough electrolyte wetting and rapid sodium ion migration, further enhancing cycle stability and rate performance.

[0007] The specific technical solution of this invention is as follows:

[0008] In a first aspect, the present invention provides a method for preparing hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires, comprising the following steps:

[0009] (1) Dissolve iron salt and zinc salt in alcohol solvent and stir until homogeneous to obtain a mixed solution.

[0010] (2) Add water solvent to the mixed solution obtained in step (1) and stir until uniform to obtain a solution.

[0011] (3) Add aminotriacetic acid to the mixed solution obtained in step (2) and stir thoroughly to obtain a suspension.

[0012] (4) The suspension obtained in step (3) is subjected to a solvothermal reaction. After the reaction is completed, it is washed and dried to obtain zinc iron aminotriacetate (ZnFe-NTA) precursor.

[0013] (5) The zinc iron aminotriacetate precursor obtained in step (4) is pyrolyzed under an inert atmosphere to obtain an intermediate product.

[0014] (6) The intermediate product obtained in step (5) is vapor-phase sulfided to obtain hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires.

[0015] In the above preparation process, this invention uses aminotriacetic acid (NTA), which is inexpensive and has abundant chelating sites, as a metal ion chelating agent. A certain proportion of zinc ions and iron ions are chelated through a solvothermal reaction to obtain an aminotriacetic acid zinc iron (ZnFe-NTA) precursor. Subsequently, the precursor is subjected to high-temperature pyrolysis and carbothermal reaction under an inert atmosphere to induce the reduction of metal ions, and the low-boiling-point zinc particles formed are further volatilized. At the same time, nitrogen-doped carbon formed by the in-situ transformation of organic ligands can produce a hierarchical porous structure (the pyrolysis of organic ligands produces smaller pores; the zinc particles will aggregate to varying degrees, thus obtaining a porous material with a non-uniform pore size). Finally, Fe7S8 is formed by gas-phase sulfidation to obtain hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires.

[0016] The material obtained by this invention has the following advantages: (1) The material inherits the one-dimensional structural characteristics of the precursor, which enables electrons to be conducted rapidly along the nanowires; (2) Compared with the introduced carbon, the nitrogen-doped carbon formed by the in-situ conversion of organic ligands can be tightly bound to Fe7S8, which can not only effectively alleviate the volume change caused by sodium ion insertion and extraction, effectively enhance the cycle stability of the material, but also improve the overall conductivity of the composite material and the diffusion rate of sodium ions; (3) When the composite nanowire is used as the negative electrode of sodium-ion battery, its hierarchical porous structure can effectively promote the full wetting of the electrolyte and the rapid migration of sodium ions, which can improve the cycle stability and rate performance.

[0017] Compared to existing reports on hierarchical porous materials, the method used in this invention to construct hierarchical porous structures is more advantageous. To introduce sufficient porosity, existing technologies widely employ template methods or activation methods, both of which exhibit drawbacks, such as the use of hazardous chemicals, cumbersome procedures (requiring additional activation or template removal steps), or corrosion of the host material. The method of this invention avoids these problems. It is simple (requiring no additional activation or template removal steps) and controllable (porosity and pore size can be altered by changing the amount of Zn added and the heat treatment time).

[0018] Furthermore, this invention selects zinc as the volatile porous metal for the following reasons: First, zinc can coordinate with the organic ligand NTA; second, zinc has a low boiling point (908°C). According to the Taman principle, the smaller the size, the less energy is required for surface atoms to escape. Therefore, the boiling point of small-particle-size Zn is much lower than that of bulk Zn. In summary, we find that Zn is the most suitable metal.

[0019] Preferably, in step (5), the pyrolysis temperature is 700-1000℃, the holding time is 1-3 hours, and the heating rate is 3-5℃ / min.

[0020] To better form a hierarchical porous structure, the pyrolysis temperature and heating rate need to be strictly controlled during the pyrolysis process. Our team discovered that the pyrolysis temperature should ideally be controlled between 700 and 1000°C. Too low a temperature hinders zinc volatilization, while too high a temperature leads to an unstable linear structure. On the other hand, a too slow heating rate causes excessive aggregation of zinc particles, resulting in excessively large pores; conversely, a too fast heating rate prevents zinc particles from aggregating sufficiently, resulting in smaller pores.

[0021] Preferably, in step (6), the temperature of the gas phase sulfidation is 400-700°C and the holding time is 1-4 hours.

[0022] To better form a hierarchical porous structure, strict temperature control is required during the gas-phase sulfidation process. The present invention has found that the higher the gas-phase sulfidation temperature, the shorter the holding time, thus preventing excessive growth of Fe7S8 and its occupation of pores.

[0023] Preferably, in steps (1)-(3), the molar ratio of the iron salt, zinc salt and aminotriacetic acid is 1:(0.1-0.4):(1.5-5); the volume ratio of the water solvent and the alcohol solvent is 1:(0.3-3); and the ratio of the total mass of the metal salt to the total mass of the solvent is 1:(10-500).

[0024] Preferably, in step (1), the iron salt is ferric chloride and / or ferric nitrate; the zinc salt is one or more of zinc chloride, zinc nitrate, zinc sulfate and zinc acetate.

[0025] Preferably, in step (4), the temperature of the solvothermal reaction is 120-200°C and the holding time is 12-60 hours.

[0026] Preferably, in step (5), the protective atmosphere for pyrolysis is nitrogen or argon.

[0027] Preferably, in step (6), the mass ratio of the intermediate product to the sulfur powder used for gas-phase sulfidation is 1:(3-5); the atmosphere for gas-phase sulfidation is an Ar / H2 mixture.

[0028] Preferably, the hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires prepared by the above method are composed of nitrogen-doped carbon and Fe7S8, and exhibit a core-shell structure with Fe7S8 coating nitrogen-doped carbon; the composite nanowires have a hierarchical porous structure, with a nanowire diameter distribution of 100-500 nanometers and a length of 1-30 micrometers.

[0029] Preferably, the well-developed pores in the hierarchical porous structure are mainly divided into micropores of 1.8–2.0 nm, small mesopores of 2.0–20 nm, and large mesopores of 20 nm and above.

[0030] By adjusting the size distribution, the hierarchical pore structure constructed on nanowires in this invention has two main peaks, consisting of micropores (1.8–2.0 nm), small mesopores (2.0–20 nm), and larger mesopores (above 20 nm). This combination of large and small pores is beneficial for improving the specific surface area and mass transfer efficiency of the material.

[0031] Secondly, this invention provides the application of the hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires prepared by the above method as a negative electrode material for sodium-ion batteries.

[0032] Compared with the prior art, the beneficial effects of the present invention are:

[0033] (1) The composite nanowires of the present invention have one-dimensional structural features, which enable electrons to conduct rapidly along the nanowires.

[0034] (2) Compared with introduced carbon, the nitrogen-doped carbon formed by the in-situ conversion of organic ligands in this invention can be tightly bound to Fe7S8, which can not only effectively alleviate the volume change caused by sodium ion insertion and extraction and effectively enhance the cycle stability of the material, but also improve the overall conductivity of the composite material and the diffusion rate of sodium ions.

[0035] (3) When the composite nanowires of the present invention are used as the negative electrode of sodium-ion batteries, their hierarchical porous structure can effectively promote the full wetting of the electrolyte and the rapid migration of sodium ions, thereby improving cycle stability and rate performance.

[0036] (4) Compared with existing pore-forming methods, the method of the present invention is simple (no additional activation or template removal steps are required) and controllable (the porosity and pore size can be changed by changing the amount of Zn added and the heat treatment time). Attached Figure Description

[0037] Figure 1 This is a scanning electron microscope image of the precursor prepared in Example 2;

[0038] Figure 2 This is a scanning electron microscope image of the final product prepared in Example 2;

[0039] Figure 3 This is a scanning electron microscope image of the final product prepared in Example 5;

[0040] Figure 4 The X-ray diffraction pattern of the final product prepared in Example 1;

[0041] Figure 5 The pore size distribution of the final product prepared in Example 1;

[0042] Figure 6 The pore size distribution of the final product prepared in Example 7;

[0043] Figure 7 The sodium-ion half-cell cycle performance of the negative electrode material prepared in Example 1;

[0044] Figure 8 The pore size distribution of the final product prepared in Comparative Example 1;

[0045] Figure 9 The image shows a scanning electron microscope (SEM) image of the final product prepared in Comparative Example 2.

[0046] Figure 10 This is a scanning electron microscope image of the final product prepared in Comparative Example 3. Detailed Implementation

[0047] The present invention will be further described below with reference to embodiments.

[0048] Example 1

[0049] (1) Take 2 mmol of ferric nitrate and 0.2 mmol of zinc nitrate and dissolve them in 40 mL of isopropanol and stir at a constant speed for 20 minutes. The ferric nitrate particles and zinc nitrate particles are completely dissolved to obtain a reddish-brown solution A.

[0050] (2) Add 40 mL of deionized water to the mixed solution A obtained in step (1) and stir at a constant speed for 15 minutes to obtain solution B;

[0051] (3) Add 9 mmol of aminotriacetic acid to the mixed solution B obtained in step (2) and stir continuously for 20 minutes to obtain suspension C;

[0052] (4) The suspension C obtained in step (3) was transferred to a reaction vessel for a solvothermal reaction at a temperature of 200°C for 36 hours. After the reaction was completed, the mixture was washed and dried to obtain a slightly yellow ZnFe-NTA precursor material.

[0053] (5) The ZnFe-NTA precursor obtained in step (4) was heated to 700°C at a heating rate of 3°C / min under nitrogen atmosphere protection and then pyrolyzed for 3 hours to obtain the intermediate product.

[0054] (6) The intermediate product obtained in step (5) is mixed with sulfur powder at a mass ratio of 1:4 and then subjected to gas phase sulfurization at 500°C for 2 hours under an Ar / H2 reducing atmosphere to obtain the final product.

[0055] Example 2

[0056] (1) Take 1 mmol of ferric nitrate and 0.2 mmol of zinc nitrate and dissolve them in 30 mL of isopropanol and stir at a constant speed for 20 minutes. The ferric nitrate particles and zinc nitrate particles are completely dissolved to obtain a reddish-brown solution A.

[0057] (2) Add 30 mL of deionized water to the mixed solution A obtained in step (1) and stir at a constant speed for 15 minutes to obtain solution B;

[0058] (3) Add 5 mmol of aminotriacetic acid to the mixed solution B obtained in step (2) and stir continuously for 20 minutes to obtain suspension C;

[0059] (4) The suspension C obtained in step (3) was transferred to a reaction vessel for a solvothermal reaction at a temperature of 180°C for 60 hours. After the reaction was completed, the mixture was washed and dried to obtain a slightly yellow ZnFe-NTA precursor material.

[0060] (5) The yellowish precursor material obtained in step (4) was heated to 700°C at a heating rate of 3°C / min under nitrogen atmosphere protection and then pyrolyzed for 3 hours to obtain the intermediate product.

[0061] (6) The intermediate product obtained in step (5) is mixed with sulfur powder at a mass ratio of 1:3 and then vapor-phase sulfurized at 500°C for 2 hours under an Ar / H2 reducing atmosphere to obtain the final product.

[0062] Example 3

[0063] (1) Take 1 mmol of ferric chloride and 0.1 mmol of zinc nitrate and dissolve them in 20 mL of isopropanol and stir at a constant speed for 20 minutes. The ferric chloride powder and zinc nitrate particles are completely dissolved to obtain a reddish-brown solution A.

[0064] (2) Add 10 mL of deionized water to the mixed solution A obtained in step (1) and stir at a constant speed for 15 minutes to obtain solution B;

[0065] (3) Add 4 mmol of aminotriacetic acid to the mixed solution B obtained in step (2) and stir continuously for 20 minutes to obtain suspension C;

[0066] (4) The suspension C obtained in step (3) was transferred to a reaction vessel for a solvothermal reaction at a temperature of 150°C for 24 hours. After the reaction was completed, the mixture was washed and dried to obtain a slightly yellow ZnFe-NTA precursor material.

[0067] (5) The ZnFe-NTA precursor material obtained in step (4) was heated to 800°C at a heating rate of 4°C / min under nitrogen atmosphere protection and then pyrolyzed for 2 hours to obtain the intermediate product.

[0068] (6) The intermediate product obtained in step (5) is mixed with sulfur powder at a mass ratio of 1:5 and then subjected to gas-phase sulfurization at 400°C for 3 hours under an Ar / H2 reducing atmosphere to obtain the final product.

[0069] Example 4

[0070] (1) Take 2 mmol of ferric nitrate and 0.4 mmol of zinc chloride and dissolve them in 10 mL of isopropanol. Stir at a constant speed for 20 minutes. The ferric nitrate particles and zinc chloride particles are completely dissolved to obtain a reddish-brown solution A.

[0071] (2) Add 30 mL of deionized water to the mixed solution A obtained in step (1) and stir at a constant speed for 15 minutes to obtain solution B;

[0072] (3) Add 9 mmol of aminotriacetic acid to the mixed solution B obtained in step (2) and stir continuously for 20 minutes to obtain suspension C;

[0073] (4) The suspension C obtained in step (3) was transferred to a reaction vessel for a solvothermal reaction at a temperature of 120°C for 42 hours. After the reaction was completed, the mixture was washed and dried to obtain a slightly yellow ZnFe-NTA precursor material.

[0074] (5) The ZnFe-NTA precursor material obtained in step (4) is heated to 900°C at a heating rate of 5°C / min under nitrogen atmosphere protection and then pyrolyzed for 1 hour to obtain the intermediate product.

[0075] (6) The intermediate product obtained in step (5) is mixed with sulfur powder at a mass ratio of 1:3 and then subjected to gas phase sulfidation at 400°C for 2 hours under an Ar / H2 reducing atmosphere to obtain the final product.

[0076] Example 5

[0077] (1) Take 1 mmol of ferric chloride and 0.2 mmol of zinc chloride and dissolve them in 30 mL of isopropanol and stir at a constant speed for 20 minutes. The ferric chloride powder and zinc chloride particles are completely dissolved to obtain a reddish-brown solution A.

[0078] (2) Add 10 mL of deionized water to the mixed solution A obtained in step (1) and stir at a constant speed for 15 minutes to obtain solution B;

[0079] (3) Add 5 mmol of aminotriacetic acid to the mixed solution B obtained in step (2) and stir continuously for 20 minutes to obtain suspension C;

[0080] (4) The suspension C obtained in step (3) was transferred to a reaction vessel for a solvothermal reaction at a temperature of 150°C for 60 hours. After the reaction was completed, the mixture was washed and dried to obtain a slightly yellow ZnFe-NTA precursor material.

[0081] (5) The ZnFe-NTA precursor material obtained in step (4) is heated to 700°C at a heating rate of 3°C / min under argon atmosphere protection and then pyrolyzed for 3 hours to obtain intermediate product.

[0082] (6) The intermediate product obtained in step (5) is mixed with sulfur powder at a mass ratio of 1:4 and then vapor-phase sulfurized at 600°C for 1 hour under an Ar / H2 reducing atmosphere to obtain the final product.

[0083] Example 6

[0084] (1) Take 2 mmol of ferric nitrate and 0.2 mmol of zinc acetate and dissolve them in 45 mL of isopropanol and stir at a constant speed for 20 minutes. The ferric nitrate particles and zinc acetate particles are completely dissolved to obtain a reddish-brown solution A.

[0085] (2) Add 15 mL of deionized water to the mixed solution A obtained in step (1) and stir at a constant speed for 15 minutes to obtain solution B;

[0086] (3) Add 7 mmol of aminotriacetic acid to the mixed solution B obtained in step (2) and stir continuously for 20 minutes to obtain suspension C;

[0087] (4) The suspension C obtained in step (3) was transferred to a reaction vessel for a solvothermal reaction at a temperature of 200°C for 24 hours. After the reaction was completed, the mixture was washed and dried to obtain a slightly yellow ZnFe-NTA precursor material.

[0088] (5) The ZnFe-NTA precursor material obtained in step (4) is heated to 800°C at a heating rate of 4°C / min under argon atmosphere protection and then pyrolyzed for 2 hours to obtain the intermediate product.

[0089] (6) The intermediate product obtained in step (5) is mixed with sulfur powder at a mass ratio of 1:5 and subjected to gas phase sulfurization at 400°C for 3 hours under an Ar / H2 reducing atmosphere to obtain the final product.

[0090] Example 7

[0091] (1) Take 1 mmol of ferric chloride and 0.3 mmol of zinc sulfate and dissolve them in 45 mL of isopropanol and stir at a constant speed for 20 minutes. The ferric chloride powder and zinc sulfate particles are completely dissolved to obtain a reddish-brown solution A.

[0092] (2) Add 15 mL of deionized water to the mixed solution A obtained in step (1) and stir at a constant speed for 15 minutes to obtain solution B;

[0093] (3) Add 5 mmol of aminotriacetic acid to the mixed solution B obtained in step (2) and stir continuously for 20 minutes to obtain suspension C;

[0094] (4) The suspension C obtained in step (3) was transferred to a reaction vessel for a solvothermal reaction at a temperature of 180°C for 42 hours. After the reaction was completed, the mixture was washed and dried to obtain a slightly yellow ZnFe-NTA precursor material.

[0095] (5) The ZnFe-NTA precursor material obtained in step (4) was heated to 800°C at a heating rate of 4°C / min under nitrogen atmosphere protection and then pyrolyzed for 2 hours to obtain the intermediate product.

[0096] (6) The intermediate product obtained in step (5) is mixed with sulfur powder at a mass ratio of 1:4 and then subjected to gas phase sulfurization at 500°C for 2 hours under an Ar / H2 reducing atmosphere to obtain the final product.

[0097] Comparative Example 1

[0098] (1) Dissolve 2 mmol of ferric chloride in 40 mL of isopropanol and stir at a constant speed for 20 minutes. The ferric chloride particles are completely dissolved, and a reddish-brown solution A is obtained.

[0099] (2) Add 40 mL of deionized water to the mixed solution A obtained in step (1) and stir at a constant speed for 15 minutes to obtain solution B;

[0100] (3) Add 11 mmol of aminotriacetic acid to the mixed solution B obtained in step (2) and stir continuously for 20 minutes to obtain suspension C;

[0101] (4) The suspension C obtained in step (3) was transferred to a reaction vessel for a solvothermal reaction at a temperature of 200°C for 36 hours. After the reaction was completed, the mixture was washed and dried to obtain a slightly yellow ZnFe-NTA precursor material.

[0102] (5) The ZnFe-NTA precursor obtained in step (4) was heated to 700°C at a heating rate of 4°C / min under nitrogen atmosphere protection and then pyrolyzed for 3 hours to obtain the intermediate product.

[0103] (6) The intermediate product obtained in step (5) is mixed with sulfur powder at a mass ratio of 1:3 and then vapor-phase sulfurized at 500°C for 2 hours under an Ar / H2 reducing atmosphere to obtain the final product.

[0104] Comparative Example 2

[0105] (1) Take 1 mmol of ferric chloride and 0.3 mmol of zinc sulfate and dissolve them in 45 mL of isopropanol and stir at a constant speed for 20 minutes. The ferric chloride powder and zinc sulfate particles are completely dissolved to obtain a reddish-brown solution A.

[0106] (2) Add 15 mL of deionized water to the mixed solution A obtained in step (1) and stir at a constant speed for 15 minutes to obtain solution B;

[0107] (3) Add 11 mmol of aminotriacetic acid to the mixed solution B obtained in step (2) and stir continuously for 20 minutes to obtain suspension C;

[0108] (4) The suspension C obtained in step (3) was transferred to a reaction vessel for a solvothermal reaction at a temperature of 200°C for 36 hours. After the reaction was completed, the mixture was washed and dried to obtain a slightly yellow ZnFe-NTA precursor material.

[0109] (5) The ZnFe-NTA precursor obtained in step (4) was heated to 900°C at a heating rate of 10°C / min under nitrogen atmosphere protection and then pyrolyzed at high temperature for 3 hours to obtain the intermediate product.

[0110] (6) The intermediate product obtained in step (5) is mixed with sulfur powder at a mass ratio of 1:3 and then vapor-phase sulfurized at 500°C for 2 hours under an Ar / H2 reducing atmosphere to obtain the final product.

[0111] Comparative Example 3

[0112] (1) Take 1 mmol of ferric chloride and 0.1 mmol of zinc chloride and dissolve them in 30 mL of isopropanol and stir at a constant speed for 20 minutes. The ferric chloride powder and zinc chloride particles are completely dissolved to obtain a reddish-brown solution A.

[0113] (2) Add 30 mL of deionized water to the mixed solution A obtained in step (1) and stir at a constant speed for 15 minutes to obtain solution B;

[0114] (3) Add 11 mmol of aminotriacetic acid to the mixed solution B obtained in step (2) and stir continuously for 20 minutes to obtain suspension C;

[0115] (4) The suspension C obtained in step (3) was transferred to a reaction vessel for a solvothermal reaction at a temperature of 200°C for 36 hours. After the reaction was completed, the mixture was washed and dried to obtain a slightly yellow ZnFe-NTA precursor material.

[0116] (5) The ZnFe-NTA precursor obtained in step (4) was heated to 800°C at a heating rate of 4°C / min under nitrogen atmosphere protection and then pyrolyzed at high temperature for 2 hours to obtain the intermediate product.

[0117] (6) The intermediate product obtained in step (5) is mixed with sulfur powder at a mass ratio of 1:3 and then vapor-phase sulfurized at 700°C for 1 hour under an Ar / H2 reducing atmosphere to obtain the final product.

[0118] Performance testing

[0119] The final product materials obtained in step (6) of Examples 1-7 and Comparative Examples 1-3 were mixed with conductive agent Super P and 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. This slurry was then smoothly coated onto a copper current collector and dried at 60°C for 12 hours. The slurry was then cut into 1.1 cm diameter discs to serve as sodium-ion battery electrodes. Next, glass fiber was used as a separator, and a solution of 1.0 mol·L⁻¹ was applied. -1 A half-cell was assembled using NaPF6 as the solute, diethylene glycol dimethyl ether as the solvent, and a sodium sheet as the counter electrode. The assembled sodium-ion half-cell was left to stand for 12 hours at a constant temperature of 25°C, and then electrochemical tests were performed. The results are shown in Table 1.

[0120] Table 1

[0121]

[0122] *In 1A g -1 The test was conducted at a current density (voltage window of 0.01–3V), and the data obtained is the capacity retention rate of the half-cell at the 100th cycle.

[0123] Table 1 summarizes the various embodiments in 1A g -1 The discharge capacity of the half-cell in the first cycle and the capacity retention rate at the 100th cycle at a current density of [value missing]. Examples 1, 2, 3, 4, 5, 6, 7, Comparative Examples 1, 2, and 3 were tested at 1 A g [value missing]. -1 At the given current density, the first discharge capacity of the half-cell is 781 mAh g. -1 890mAh g -1 737mAh g -1 921mAh g -1 722mAh g -1 765mAh g -1 905mAh g -1 452mAh g -1 805mAh g -1 and 402mAh g -1 The capacity retention rates at 100 laps were 80%, 91%, 75%, 65%, 72%, 78%, 69%, 82%, 46%, and 61%, respectively.

[0124] To investigate the factors influencing changes in the electrochemical performance of materials, further exploration was conducted using characterization techniques such as SEM, XRD, and BET. Taking the material in Example 2 as an example, Figure 1 , 2 The images show the synthesized ZnFe-NTA precursor material and the final product, respectively. The ZnFe-NTA precursor material exhibits a nanowire structure with a relatively uniform diameter distribution. After heat treatment, the final product still maintains the nanowire structure of the precursor well, and some small Fe7S8 particles grow on the surface. Figure 3 This is a scanning electron microscope (SEM) image of the final product prepared in Example 5. In Example 5, the diameter distribution is 100–500 nm, and the length is relatively short, ranging from 1–5 μm. To further illustrate the crystal structure of the examples, Example 1 is used as an example. Figure 4 The spectral lines of the final XRD product were recorded, and their peak positions matched well with the JCPDS standard card for Fe7S8.

[0125] Figure 5 and Figure 6The figures show the pore size distribution of the final products obtained in Examples 1 and 7. As can be seen from the figures, both contain two pore types, confirming that the material is hierarchically porous. Furthermore, the pore size of Example 7 is generally larger, and the pore content is higher. BET multi-point method testing shows that the specific surface area of ​​Examples 1 and 7 is 21.7 m². 2 / g and 65.47m 2 / g. Figure 8 The image shows the pore size distribution of the final product obtained in Comparative Example 1. Comparing the pore size distributions of Examples 1 and 7, it is easy to see that without the addition of zinc salt, the specific surface area of ​​the material (12.5 m²) is significantly higher. 2 Both the surface area ( / g) and pore size are very small, and the pores are mainly small-diameter. Therefore, changing the proportion of zinc salt added will change the specific surface area and pore size of the material, thereby affecting the initial discharge capacity and cycle stability of the material.

[0126] Figure 9 (Comparative Example 2) and Figure 10 (Comparative Example 3) When the heat treatment conditions exceeded the upper limit of the preferred range, the nanowire structure was severely damaged and excessive growth of Fe7S8 particles occurred.

[0127] Figure 7 The circuit performance diagram of the half-cell in Example 1 is shown at 1A g. -1 At the given current density, Example 1 exhibited relatively stable cycling performance and high reversible capacity.

[0128] In summary, the proportion of zinc salt added is a major factor affecting the first-cycle discharge capacity and cycle stability of the material. At the same time, adjusting the solvothermal synthesis and heat treatment conditions also has a significant impact on the electrochemical performance of the material.

[0129] Unless otherwise specified, the raw materials and equipment used in this invention are all commonly used in the field; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0130] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. The application of hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires as anode materials for sodium-ion batteries, characterized by: The preparation method of hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires includes the following steps: (1) Dissolve the iron salt and zinc salt in an alcohol solvent and stir until homogeneous to obtain a mixed solution; (2) Add water solvent to the mixed solution obtained in step (1), stir evenly to obtain a solution; the volume ratio of water solvent to alcohol solvent is 1:(0.3~3); (3) Add aminotriacetic acid to the mixed solution obtained in step (2) and stir thoroughly to obtain a suspension; the molar ratio of iron salt, zinc salt and aminotriacetic acid is 1:(0.1~0.4):(1.5~5); the ratio of the total mass of metal salt to the total mass of solvent is 1:(10~500); (4) The suspension obtained in step (3) is subjected to a solvothermal reaction. After the reaction is completed, it is washed and dried to obtain zinc iron aminotriacetate precursor. (5) The zinc iron aminotriacetate precursor obtained in step (4) is pyrolyzed under an inert atmosphere at a temperature of 700~1000℃ and a holding time of 1~3 hours; the heating rate is 3~5℃ / min; and an intermediate product is obtained. (6) The intermediate product obtained in step (5) is subjected to gas-phase sulfurization at 400~700℃ in an atmosphere of Ar / H2 mixed gas for 1~4 hours to obtain hierarchical porous Fe7S8 / nitrogen-doped carbon composite nanowires, which are composed of nitrogen-doped carbon and Fe7S8 and present a core-shell structure of Fe7S8 coated with nitrogen-doped carbon. The composite nanowires have a hierarchical porous structure, which is divided into micropores of 1.8~2.0 nm, small mesopores of 2.0~20 nm and large mesopores of more than 20 nm. The diameter of the nanowires is 100~500 nm and the length is 1~30 μm. The mass ratio of the intermediate product to the sulfur powder used for gas-phase sulfurization is 1:(3~5).

2. The application as described in claim 1, characterized in that: In step (1), the iron salt is ferric chloride and / or ferric nitrate.

3. The application as described in claim 1, characterized in that: In step (1), the zinc salt is one or more of zinc chloride, zinc nitrate, zinc sulfate and zinc acetate.

4. The application as described in claim 1, characterized in that: In step (4), the temperature of the solvothermal reaction is 120~200 ℃ and the holding time is 12~60 hours.

5. The application as described in claim 1 or 2, characterized in that: In step (5), the protective atmosphere for the pyrolysis is nitrogen or argon.

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