A Fe3C / hard carbon composite sodium-ion battery anode material, its preparation method and application

Abstract

The application discloses a Fe3C / hard carbon composite sodium ion battery negative electrode material and a preparation method and application thereof, and gradually constructs a three-dimensional hierarchical structure of a core-skeleton-surface layer through a staged process of ball milling premixing, hydrothermal growth and CVD modification, realizes nanoscale uniform dispersion of Fe3C nanoparticles and hard carbon through ball milling premixing, in-situ generates a three-dimensional hard carbon porous network skeleton to coat the Fe3C nanoparticles through hydrothermal growth, and finally deposits an ultrathin graphitized carbon layer on the surface of the material through CVD modification, forms a conductive network and stabilizes the interface, and obtains the Fe3C / hard carbon composite sodium ion battery negative electrode material which has high electronic / ion conduction efficiency, strong volume expansion buffering capacity and stable interface characteristics. The application realizes nanoscale uniform dispersion and close combination of Fe3C and hard carbon, and solves technical problems of uneven dispersion, high interface impedance, uncontrollable volume expansion and the like of traditional composite materials.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery anode material technology, specifically to an Fe3C / hard carbon composite sodium-ion battery anode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion battery anode materials mainly include carbon-based materials (such as hard carbon) and metal / metal compound materials. Among them, hard carbon has advantages such as large interlayer spacing, abundant sodium storage sites, and good electrochemical stability. However, it has problems such as limited actual sodium storage capacity (usually less than 300 mAh / g) and insufficient ion transport rate at high rates, making it difficult to meet the application requirements of high energy density batteries. Among metal carbides, Fe3C has a high theoretical sodium storage capacity of about 600 mAh / g. However, Fe3C itself has poor conductivity and undergoes significant volume expansion during sodium insertion / deintercalation cycles, which easily leads to particle pulverization and active phase shedding, resulting in rapid battery capacity decay. It cannot be used alone for practical applications.

[0003] To combine the structural stability of hard carbon with the high capacity advantage of Fe3C, existing technologies often employ composite processes such as mechanical mixing and single carbon coating to prepare Fe3C / hard carbon composites. However, these methods have significant technical drawbacks: mechanical mixing can only achieve macroscopic physical mixing of Fe3C and hard carbon, failing to achieve nanoscale uniform dispersion and easily leading to active phase agglomeration, resulting in a reduction of effective sodium storage sites. Furthermore, the interfacial bonding between the two phases is weak, making interfacial delamination during cycling a common occurrence. While single carbon coating can improve conductivity and alleviate volume expansion to some extent, it is difficult to simultaneously achieve multiple objectives such as constructing rapid ion transport channels, suppressing interfacial side reactions, and enhancing structural stability. This often results in uneven coating layers and weak bonding with the matrix, causing the actual sodium storage performance of the composite material to fall far short of theoretical expectations. Problems such as high interfacial impedance, uncontrollable volume expansion, and low initial efficiency remain prominent.

[0004] In addition, traditional Fe3C / hard carbon composite materials are prone to forming a thick and uneven solid electrolyte interphase (SEI) film during the first charge and discharge process due to the unstable interfacial structure. This not only consumes a large amount of active sodium, resulting in low initial coulombic efficiency, but also causes continuous interfacial side reactions due to repeated rupture and regeneration of the SEI film, further aggravating battery capacity decay and reducing cycle stability. Summary of the Invention

[0005] The purpose of this invention is to solve the above-mentioned technical problems and provide a Fe3C / hard carbon composite sodium-ion battery anode material, its preparation method and application, so as to achieve nanoscale uniform dispersion and tight bonding of Fe3C and hard carbon, and construct a Fe3C / hard carbon composite sodium-ion battery anode material with high electron / ion conduction efficiency, strong volume expansion buffering capacity and stable interface characteristics.

[0006] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0007] The first aspect of this invention provides a method for preparing a Fe3C / hard carbon composite sodium-ion battery anode material, comprising the following steps:

[0008] (1) After mixing Fe3O4 powder, biomass hard carbon precursor and reducing agent, the mixture is ball-milled at 300-500 rpm for 8-15 h under inert gas protection to obtain ball-milled product;

[0009] (2) Disperse the ball milling product obtained in step (1) in water, adjust the pH of the system to alkaline, and carry out a hydrothermal reaction at 150-300 °C for 20-30 h to obtain Fe(OH)3 / hard carbon precursor complex. Then, carbonize the Fe(OH)3 / hard carbon precursor complex at 700-900 °C under an inert atmosphere to obtain Fe3C / hard carbon complex.

[0010] (3) Using carbon source gas as a precursor and inert gas as a carrier gas, the Fe3C / hard carbon composite obtained in step (2) is subjected to chemical vapor deposition (CVD) in a tube furnace and deposited at 600-700 °C for 10-60 min to obtain the Fe3C / hard carbon composite sodium-ion battery anode material.

[0011] This invention constructs a gradient composite Fe3C / hard carbon composite sodium-ion battery anode material through a three-step process. The first step involves ball milling premixing to achieve mechanical dispersion and activation, using mechanochemical effects to uniformly mix Fe3O4 with the biomass hard carbon precursor, and pre-reducing Fe3O4 to elemental Fe. The second step involves hydrothermal growth to achieve in-situ carbonization and composite formation. First, a Fe(OH)3 / hard carbon precursor composite is generated via a hydrothermal reaction, then Fe(OH)3 is reduced to Fe3C through carbonization, simultaneously completing the carbonization of the hard carbon precursor and forming a porous hard carbon network framework. The third step involves CVD to achieve surface graphitization coating, coating the Fe3C / hard carbon composite surface with a 1-3 nm graphitized carbon layer, ultimately forming a Fe3C@hard carbon@C core-shell gradient composite structure.

[0012] Furthermore, in step (1), the Fe3O4 powder has a purity of >99% and a particle size of 50-100 nm.

[0013] Furthermore, in step (1), the biomass hard carbon precursor is selected from one or more of coconut shell powder, walnut shell, and bamboo fiber. Coconut shell powder, walnut shell, and bamboo fiber are all rich in lignin.

[0014] Further, in step (1), the reducing agent is glucose and / or ethylene glycol.

[0015] Further, in step (1), the mass ratio of the iron oxide powder, biomass hard carbon precursor and reducing agent is (15-40):(50-80):(5-20).

[0016] Furthermore, in step (1), the inert gas is argon (Ar).

[0017] Further, in step (1), the ball-to-material ratio of the ball milling process is 1:(8-12).

[0018] Furthermore, in step (2), the pH of the system is adjusted to 10-12 by adding a sodium hydroxide (NaOH) solution with a concentration of 0.5-1 M.

[0019] Furthermore, in step (2), the carbonization process takes 3-6 hours.

[0020] Further, in step (2), the carbonization process is carried out at a heating rate of 5-10 °C / min to 700-900 °C and held for 3-6 h.

[0021] Furthermore, in step (2), the inert atmosphere is argon.

[0022] Furthermore, in step (3), the carbon source gas is acetylene (C2H2).

[0023] Furthermore, in step (3), the inert gas is argon.

[0024] Furthermore, in step (3), the flow rate ratio of the carbon source gas to the inert gas is 1:(18-25).

[0025] Furthermore, in step (3), the flow rate of the carbon source gas is 5-15 sccm.

[0026] Further, in step (3), the temperature is increased to 600-700 ℃ at a heating rate of 10-15 ℃ / min and then deposited at a constant temperature for 10-60 min.

[0027] The second aspect of this invention provides a Fe3C / hard carbon composite sodium-ion battery anode material prepared by the preparation method described in the first aspect.

[0028] Furthermore, the Fe3C / hard carbon composite sodium-ion battery anode material provided by the present invention has a gradient composite structure, consisting of Fe3C nanoparticles as the core, a hard carbon porous network as the framework, and a graphitized carbon layer as the surface layer, from the inside out. The Fe3C nanoparticles are coated in the hard carbon porous network, and the graphitized carbon layer is deposited on the outside of the hard carbon porous network, forming a Fe3C@hard carbon@C core-shell structure.

[0029] In the Fe3C / hard carbon composite sodium-ion battery anode material provided by this invention, Fe3C nanoparticles obtained by ball milling and premixing serve as the active phase for sodium storage, and the hard carbon porous network generated by hydrothermal method can provide efficient ion transport channels and effectively suppress volume expansion. The ultrathin graphitized carbon layer deposited by CVD can significantly improve the overall electronic conductivity of the material and optimize the formation of the solid electrolyte interface (SEI) film.

[0030] Furthermore, the Fe3C nanoparticles have a particle size of 20-50 nm, the hard carbon porous network has a pore size of 2-10 nm, and the graphitized carbon layer has a thickness of 1-3 nm.

[0031] The third aspect of this invention provides the application of the Fe3C / hard carbon composite sodium-ion battery anode material described in the second aspect in the assembly of sodium-ion batteries.

[0032] Compared with the prior art, the above-described technical solution of the present invention has the following advantages:

[0033] 1. This invention constructs a three-dimensional hierarchical structure of "core-skeleton-surface layer" through a phased process of ball milling premixing, hydrothermal growth, and CVD modification. First, ball milling premixing achieves uniform nanoscale dispersion of Fe3C nanoparticles and hard carbon. Then, hydrothermal growth generates a three-dimensional porous hard carbon network skeleton in situ to coat the Fe3C nanoparticles. Finally, CVD modification deposits an ultrathin graphitized carbon layer on the material surface, forming a conductive network and stabilizing the interface. This invention solves the technical problems of uneven dispersion, high interfacial impedance, and uncontrollable volume expansion inherent in traditional composite materials through synergistic regulation of physical and chemical scales.

[0034] 2. Because the Fe3C / hard carbon composite sodium-ion battery anode material prepared by this invention has a unique three-dimensional hierarchical structure, its internal Fe3C nanoparticles are uniformly coated by a hard carbon porous network, and the surface is further modified by a graphitized carbon layer. This structure effectively suppresses the insertion plateau behavior of sodium ions at low potentials, so that the sodium storage potential of the material presents a slope shape without a "plateau region" close to 0 V, thereby significantly reducing the risk of sodium deposition and improving the safety and long-term cycle stability of the battery.

[0035] 3. The Fe3C / hard carbon composite sodium-ion battery anode material prepared by this invention exhibits significant performance advantages in sodium-ion battery applications. Compared with pure hard carbon materials, its sodium storage capacity is increased by more than 30%, and its volume expansion rate is significantly reduced to below 5%, which is far superior to traditional Fe3C materials with a volume expansion rate of over 20%. At the same time, the interfacial side reactions of the material are effectively suppressed, and the initial coulombic efficiency is increased to over 85%, resulting in a significant optimization of the overall electrochemical performance. Attached Figure Description

[0036] Figure 1 The first charge-discharge curve of the CR2032 coin cell sodium-ion battery assembled from the Fe3C / hard carbon composite sodium-ion battery anode material prepared in Example 1 is shown. Detailed Implementation

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0038] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0039] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, and the materials and reagents used are commercially available.

[0040] The Fe3O4 powder used in the following examples and comparative examples is a commercially available nano-sized product with a purity >99.5% and a particle size of 50-100 nm. It was vacuum dried at 120 °C for 12 h before use to remove adsorbed water from the powder surface. The biomass hard carbon precursor is coconut shell powder, which is first ball-milled to 200 mesh sieve particles, then acid-washed with 5% HNO3 solution at 60 °C for 6 h with stirring to remove metal impurities, followed by washing until neutral and drying. The reducing agent is analytical grade glucose, which is ground to a particle size <10 μm.

[0041] Example 1

[0042] A method for preparing an Fe3C / hard carbon composite sodium-ion battery anode material includes the following steps:

[0043] (1) Mix 10 g Fe3O4 powder, 20 g coconut shell powder and 5 g glucose, and use zirconia balls with diameters of 3 mm and 5 mm in a quantity ratio of 2:1 as the ball milling medium. Place them in a QM-3SP2 high-energy planetary ball mill at a ball-to-material ratio of 1:10. Under argon protection, the ball milling is carried out at a speed of 400 rpm for 12 h. During the ball milling process, pause for 10 min every 30 min to prevent the system from overheating. After the ball milling is completed, the obtained powder is sieved through a 200-mesh sieve and the uniform ball milling product is collected.

[0044] (2) The above ball-milled product was dispersed in 200 mL of deionized water and ultrasonically treated for 30 min. Then, 0.5 M NaOH solution was added to adjust the pH of the system to 11.5. The solution was then transferred to a polytetrafluoroethylene-lined reactor and hydrothermally reacted at 200 °C for 24 h. After the reaction was completed, the product was naturally cooled to room temperature. The obtained product was washed 5 times by alternating centrifugation with deionized water and ethanol. After vacuum drying at 80 °C for 12 h, it was placed in a tube furnace and heated to 800 °C at a heating rate of 5 °C / min under the protection of Ar atmosphere with a flow rate of 200 sccm and held for 4 h. Finally, it was cooled to room temperature with the furnace to obtain the Fe3C / hard carbon composite.

[0045] (3) A horizontal tube furnace with a diameter of 50 mm quartz tube was used as the reaction device. The Fe3C / hard carbon composite was placed in the center of the constant temperature zone of the furnace for CVD treatment. The deposition was carried out with C2H2 with a purity of 99.99% as the carbon source precursor and Ar gas as the carrier gas. The flow rate of C2H2 was 10 sccm and the flow rate of Ar gas was 200 sccm. The system pressure was maintained at 100 Pa. The temperature was increased to 650 ℃ at a heating rate of 10 ℃ / min and then deposited at a constant temperature for 30 min. After the deposition reaction was completed, the material was naturally cooled to room temperature under Ar atmosphere to obtain the Fe3C / hard carbon composite sodium ion battery anode material, named Fe3C@hard carbon@C.

[0046] Example 2

[0047] A method for preparing a Fe3C / hard carbon composite sodium-ion battery anode material is basically the same as that in Example 1, except that in step (1), the material is ball-milled at 300 rpm for 15 h under argon protection.

[0048] Example 3

[0049] A method for preparing a Fe3C / hard carbon composite sodium-ion battery anode material is basically the same as that in Example 1, except that in step (1), the material is ball-milled at 500 rpm for 8 h under argon protection.

[0050] Comparative Example 1

[0051] A method for preparing an Fe3C / hard carbon composite sodium-ion battery anode material includes the following steps:

[0052] (1) Mix 10 g Fe3O4 powder, 20 g coconut shell powder and 5 g glucose, and use zirconia balls with diameters of 3 mm and 5 mm in a quantity ratio of 2:1 as the ball milling medium. Place them in a QM-3SP2 high-energy planetary ball mill at a ball-to-material ratio of 1:10. Under argon protection, the ball milling is carried out at a speed of 400 rpm for 12 h. During the ball milling process, pause for 10 min every 30 min to prevent the system from overheating. After the ball milling is completed, the obtained powder is sieved through a 200-mesh sieve and the uniform ball milling product is collected.

[0053] (2) A horizontal tube furnace with a diameter of 50 mm quartz tube was used as the reaction device. The ball milled product was placed in the center of the constant temperature zone of the furnace for CVD treatment. C2H2 with a purity of 99.99% was used as the carbon source precursor and Ar gas was used as the carrier gas. The flow rate of C2H2 was 10 sccm and the flow rate of Ar gas was 200 sccm. The system pressure was maintained at 100 Pa. The temperature was increased to 650 ℃ at a heating rate of 10 ℃ / min and then deposited at a constant temperature for 30 min. After the deposition reaction was completed, the material was naturally cooled to room temperature under Ar atmosphere to obtain Fe3C / hard carbon composite sodium ion battery anode material.

[0054] Comparative Example 2

[0055] A method for preparing an Fe3C / hard carbon composite includes the following steps:

[0056] (1) Mix 10 g Fe3O4 powder, 20 g coconut shell powder and 5 g glucose, and use zirconia balls with diameters of 3 mm and 5 mm in a quantity ratio of 2:1 as the ball milling medium. Place them in a QM-3SP2 high-energy planetary ball mill at a ball-to-material ratio of 1:10. Under argon protection, the ball milling is carried out at a speed of 400 rpm for 12 h. During the ball milling process, pause for 10 min every 30 min to prevent the system from overheating. After the ball milling is completed, the obtained powder is sieved through a 200-mesh sieve and the uniform ball milling product is collected.

[0057] (2) The above ball-milled product was dispersed in 200 mL of deionized water and ultrasonically treated for 30 min. Then, 0.5 M NaOH solution was added to adjust the pH of the system to 11.5. The solution was then transferred to a polytetrafluoroethylene-lined reactor and hydrothermally reacted at 200 °C for 24 h. After the reaction was completed, the product was naturally cooled to room temperature. The obtained product was washed 5 times by alternating centrifugation with deionized water and ethanol. After vacuum drying at 80 °C for 12 h, it was placed in a tube furnace and heated to 800 °C at a heating rate of 5 °C / min under the protection of Ar atmosphere with a flow rate of 200 sccm and held for 4 h. Finally, it was cooled to room temperature with the furnace to obtain the Fe3C / hard carbon composite.

[0058] Comparative Example 3

[0059] A method for preparing a Fe3C / hard carbon composite sodium-ion battery anode material is basically the same as that in Example 1, except that in step (2), a hydrothermal reaction is carried out at a constant temperature of 200 °C for 15 h.

[0060] Comparative Example 4

[0061] A method for preparing a Fe3C / hard carbon composite sodium-ion battery anode material is basically the same as that in Example 1, except that in step (2), a hydrothermal reaction is carried out at a constant temperature of 200 °C for 35 h.

[0062] Comparative Example 5

[0063] A method for preparing a Fe3C / hard carbon composite sodium-ion battery anode material is basically the same as that in Example 1, except that in step (3), the temperature is raised to 550 ℃ at a heating rate of 10 ℃ / min and then deposited at a constant temperature for 30 min.

[0064] Comparative Example 6

[0065] A method for preparing a Fe3C / hard carbon composite sodium-ion battery anode material is basically the same as that in Example 1, except that in step (3), the temperature is raised to 750 ℃ ​​at a heating rate of 10 ℃ / min and then deposited at a constant temperature for 30 min.

[0066] Application Example 1

[0067] The Fe3C / hard carbon composite sodium-ion battery anode materials prepared in Examples 1-3, the Fe3C / hard carbon composite sodium-ion battery anode material prepared in Comparative Example 1, the Fe3C / hard carbon composite prepared in Comparative Example 2, and the Fe3C / hard carbon composite sodium-ion battery anode materials prepared in Comparative Examples 3-6 were used as anode active materials to assemble sodium-ion batteries. The assembly method was as follows: the anode active material, conductive agent acetylene black, and binder polyvinylidene fluoride were mixed in a mass ratio of 8:1:1, and N-methylpyrrolidone was added and stirred to prepare a uniform and stable anode slurry with a solid content of 40%. The anode slurry was coated onto the surface of a 6 μm thick current collector aluminum foil using a 200 μm doctor blade. After drying and cold pressing, a loading of 2.5 mg / cm³ was obtained. 2 The negative electrode is made of sodium metal, the counter electrode is made of 9 μm thick polyethylene film, and the electrolyte is a 1 mol / L sodium hexafluorophosphate polycarbonate solution. The negative electrode, the counter electrode, and the counter electrode are stacked in sequence to complete the assembly of the CR2032 coin cell sodium-ion battery.

[0068] Test Example 1

[0069] Electrochemical performance tests were conducted on CR2032 coin cells assembled from the Fe3C / hard carbon composite sodium-ion battery anode materials prepared in Examples 1-3 of Case 1, the Fe3C / hard carbon composite sodium-ion battery anode material prepared in Comparative Example 1, the Fe3C / hard carbon composite material prepared in Comparative Example 2, and the Fe3C / hard carbon composite sodium-ion battery anode materials prepared in Comparative Examples 3-6. The test methods are as follows:

[0070] (1) Initial Coulombic Efficiency Test: The sodium-ion battery was left to stand at 60 °C for 40 min, then discharged at a rate of 0.1 C to the activation voltage of 0 V and charged to the activation voltage of 1.5 V. The initial coulombic efficiency was calculated as the ratio of the initial charge capacity to the initial discharge capacity. Simultaneously, under these test conditions, the first charge-discharge curve (voltage-specific capacity curve) of the CR2032 coin-type sodium-ion battery assembled from the Fe3C / hard carbon composite sodium-ion battery anode material prepared in Example 1 was collected. The results are as follows: Figure 1 As shown.

[0071] (2) Discharge specific capacity test: Within the voltage range of 0-3 V, the battery is subjected to charge and discharge tests at rates of 0.1 C, 1 C, and 5 C, respectively, according to the formula C=Q. D / M is used to calculate the discharge specific capacity of the active material, where C is the discharge specific capacity and Q is the discharge specific capacity. D Where M is the discharge capacity and M is the mass of the active material.

[0072] (3) Cyclic stability test: First, the sodium-ion battery is calibrated at a 1 C rate, charged to 3 V and discharged to 0 V, and the discharge capacity at this time is recorded as C0; then, it is cycled at a 1 C rate, and after 500 cycles, the discharge capacity C of the battery on the 500th cycle is tested. 500 According to the formula R=C 500 / C0×100% is used to calculate the capacity retention rate, thereby evaluating the battery's cycle stability over 500 cycles.

[0073] The test results are shown in Table 1:

[0074] Table 1

[0075]

[0076] As shown in Table 1, the optimized parameter combination used in Example 1 was as follows: ball milling at 400 rpm for 12 h, hydrothermal reaction at 200 ℃ for 24 h, and chemical vapor deposition at 650 ℃. The ball milling process provided sufficient mechanical energy to ensure complete reduction of Fe3O4 to Fe3C, with no residual phase detected by XRD. The 24-h hydrothermal reaction promoted the full condensation of lignin, forming a three-dimensional hard carbon porous network framework, which effectively buffered the material's volume expansion. The chemical vapor deposition at 650 ℃ generated a 2.5 nm graphitized carbon layer, which reduced the material's interfacial impedance and synergistically improved the material's rate performance and cycle stability.

[0077] In Example 2, the material was ball-milled at 300 rpm for 15 h. When the ball mill speed is low, the mechanical energy can be compensated by extending the ball milling time. However, the activation energy of the Fe3O4 reduction reaction was not broken, and there were unreacted Fe3O4 residual phases, which ultimately resulted in the sodium storage capacity of the material being lower than that in Example 1.

[0078] In Example 3, the ball milling process was carried out at a speed of 500 rpm for 8 hours. When the ball mill speed is higher, the mechanical energy input is sufficient, and the ball milling time can be appropriately shortened. Under this parameter, it can ensure that Fe3O4 is fully reduced and avoid particle agglomeration caused by excessive ball milling.

[0079] Comparative Example 1 only underwent ball milling and CVD, without hydrothermal growth steps. As a result, the material lacked a hard carbon framework to provide support. The volume expansion of Fe3C during sodium insertion / deintercalation directly caused particle pulverization. Furthermore, Fe3C was directly exposed to the electrolyte, triggering continuous reduction side reactions, which led to a significant decrease in battery coulombic efficiency, and consequently, a decrease in rate performance and cycle performance.

[0080] Comparative Example 2 only underwent ball milling and hydrothermal growth without CVD modification, resulting in the absence of graphitized carbon layer modification. This led to high charge transfer impedance, severe interfacial polarization, and a significant decrease in rate performance. Furthermore, an uneven SEI film was directly formed on the Fe3C surface, and the film repeatedly ruptured and regenerated during cycling, resulting in reduced cycle performance of the battery.

[0081] Comparative Example 3 underwent a hydrothermal reaction at 200 °C for 15 h. The hydrothermal reaction time was less than 20 h, resulting in incomplete breakage of the COC bonds in the lignin, insufficient condensation reaction, and low crosslinking degree of the hard carbon porous network framework. The hard carbon porous network framework had a pore size <2 nm, limiting ion transport channels and reducing Na... + Stress concentration occurs during embedding, accelerating the pulverization of the material structure and reducing cycle stability.

[0082] Comparative Example 4 underwent a hydrothermal reaction at 200 °C for 35 h. The hydrothermal reaction time exceeded 30 h, which led to coarsening of Fe3C nanoparticles, a decrease in specific surface area, and a reduction in effective sodium storage active sites. There was a mismatch in the coefficient of thermal expansion between hard carbon and Fe3C, which caused interfacial peeling under long-term high temperature. During the cycling process, hard carbon pulverized, resulting in a rapid decrease in material capacity.

[0083] Comparative Example 5: A graphitized carbon layer of <1 nm was generated by chemical vapor deposition at 550 °C. The CVD temperature was below 600 °C, the carbon source gas was not completely decomposed, the degree of graphitization was insufficient, the deposited carbon layer had poor electrical conductivity and low electronic conductivity, which in turn affected the overall electrochemical performance of the material.

[0084] Comparative Example 6: A graphitized carbon layer >3 nm was generated by chemical vapor deposition at 750 °C. At high temperature, the carbon source gas was decomposed to produce H2 byproduct. H2 will trigger the reduction reaction of Fe3C (Fe3C + H2 → 3Fe + CH4), resulting in the loss of the active phase Fe3C, which ultimately causes a significant decrease in the sodium storage capacity of the material.

[0085] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art should understand that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing an Fe3C / hard carbon composite sodium-ion battery anode material, characterized in that, Includes the following steps: (1) After mixing iron oxide powder, biomass hard carbon precursor and reducing agent, the mixture is ball-milled at 300-500 rpm for 8-15 h under inert gas protection to obtain ball-milled product; (2) Disperse the ball milling product obtained in step (1) in water, adjust the pH of the system to alkaline, and carry out a hydrothermal reaction at 150-300 °C for 20-30 h to obtain Fe(OH)3 / hard carbon precursor complex. Then, carbonize the Fe(OH)3 / hard carbon precursor complex at 700-900 °C under an inert atmosphere to obtain Fe3C / hard carbon complex. (3) Using carbon source gas as precursor and inert gas as carrier gas, the Fe3C / hard carbon composite obtained in step (2) is subjected to chemical vapor deposition in a tube furnace for 10-60 min at 600-700 °C to obtain the Fe3C / hard carbon composite sodium-ion battery anode material.

2. The preparation method according to claim 1, characterized in that, In step (1), the biomass hard carbon precursor is selected from one or more of coconut shell powder, walnut shell and bamboo fiber; in step (1), the reducing agent is glucose and / or ethylene glycol.

3. The preparation method according to claim 1, characterized in that, In step (1), the mass ratio of the iron oxide powder, biomass hard carbon precursor and reducing agent is (15-40):(50-80):(5-20).

4. The preparation method according to claim 1, characterized in that, In step (2), the pH of the system is adjusted to 10-12 by adding a 0.5-1 M sodium hydroxide (NaOH) solution.

5. The preparation method according to claim 1, characterized in that, In step (2), the carbonization process takes 3-6 hours.

6. The preparation method according to claim 1, characterized in that, In step (3), the carbon source gas is acetylene; the flow rate ratio of the carbon source gas to the inert gas is 1:(18-25); and the flow rate of the carbon source gas is 5-15 sccm.

7. A Fe3C / hard carbon composite sodium-ion battery anode material prepared by the preparation method according to any one of claims 1-6.

8. A Fe3C / hard carbon composite sodium-ion battery anode material according to claim 7, characterized in that, The Fe3C / hard carbon composite sodium-ion battery anode material has a gradient composite structure, consisting of Fe3C nanoparticles as the core, a hard carbon porous network as the framework, and a graphitized carbon layer as the surface layer, from the inside out. The Fe3C nanoparticles are coated in the hard carbon porous network, and the graphitized carbon layer is deposited on the outside of the hard carbon porous network, forming a Fe3C@hard carbon@C core-shell structure.

9. A Fe3C / hard carbon composite sodium-ion battery anode material according to claim 8, characterized in that, The Fe3C nanoparticles have a particle size of 20-50 nm, the hard carbon porous network has a pore size of 2-10 nm, and the graphitized carbon layer has a thickness of 1-3 nm.

10. The application of the Fe3C / hard carbon composite sodium-ion battery anode material according to any one of claims 7-9 in the assembly of sodium-ion batteries.

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