Hard carbon negative electrode material, preparation method thereof and sodium ion battery

Abstract

The invention relates to the technical field of sodium-ion batteries, in particular to a hard carbon negative electrode material, a preparation method thereof and a sodium-ion battery. The preparation method comprises the following steps: preparing a porous carbon matrix by taking a carbon source as a raw material; crushing the porous carbon matrix, dipping the porous carbon matrix in a doping solution containing a first doping source, and carrying out heat treatment at 250-350 DEG C in an inert atmosphere to obtain a doped precursor; the first doping source is selected from at least one of a rubidium source and a cesium source; carrying out carbonization treatment on the doped precursor at the temperature of 1100-1700 DEG C to obtain a hard carbon intermediate; and carrying out carbon coating on the hard carbon intermediate to obtain the hard carbon negative electrode material. Large-radius ions (such as Rb < + > / Cs < + >) are introduced to be embedded into carbon layer gaps, the carbon layer gaps are enlarged through the strut effect of the ions, and the cycle performance is improved.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a hard carbon anode material, its preparation method, and a sodium-ion battery. Background Technology

[0002] With the rapid growth of global demand for renewable energy storage, the development of low-cost, high-performance rechargeable battery systems has become a research hotspot. While lithium-ion batteries are widely used, the uneven geographical distribution of lithium resources and long-term cost pressures have prompted the search for new alternative technologies. Meanwhile, sodium-ion batteries, due to the abundant and widely distributed nature of sodium resources and their low cost, are considered a highly promising candidate for large-scale energy storage.

[0003] In sodium-ion batteries, the anode material is one of the key factors determining its overall performance. Because the radius of sodium ions is larger than that of lithium ions, traditional graphite anode materials used in lithium-ion batteries have limited sodium storage capacity. Therefore, it is necessary to develop novel anode materials suitable for sodium-ion batteries.

[0004] Hard carbon materials, due to their abundant defects, large interlayer spacing, and tunable microstructure, can provide high reversible specific capacity and are considered one of the most promising anode materials for sodium-ion batteries.

[0005] Currently, porous structures are commonly used to improve the capacity of hard carbon anode materials. However, existing porous hard carbon anode materials are typically prepared through chemical activation or the use of pore-forming agents. These methods result in randomly distributed micropores and mesopores with a wide pore size range and irregular shapes. While this disordered pore structure increases sodium storage potential, it does not provide sufficient and effective channels for the rapid migration of sodium ions. During charge and discharge, sodium ions must repeatedly diffuse through these tortuous and obstructed channels. The tortuous channels lead to extremely uneven ion concentration and current density distribution, resulting in low diffusion efficiency. The material near the inlet or surface is subjected to intense and repeated insertion / extraction, easily leading to pulverization or failure due to stress concentration. This uneven reaction causes the solid electrolyte interphase (SEI) film to become excessively thick locally or repeatedly rupture and regenerate, continuously consuming active sodium and electrolyte, increasing overall impedance, accelerating material degradation, accelerating capacity decay, and resulting in poor cycle performance, thus limiting the development of sodium-ion batteries. Summary of the Invention

[0006] To address the poor cycle performance of porous hard carbon anode materials in existing technologies, this invention provides a method for preparing hard carbon anode materials. By embedding large ions into the carbon interlayer, the interlayer spacing is widened as a physical support, improving the uniformity of the material structure, accelerating sodium ion migration, slowing capacity decay, and improving cycle performance. This method solves the problem of poor cycle performance of porous hard carbon anode materials in existing technologies.

[0007] The technical solution adopted by this invention to solve its technical problem is: A method for preparing a hard carbon anode material includes the following steps: S1: Prepare porous carbon matrix using carbon source as raw material; S2: The porous carbon matrix is ​​crushed, impregnated in a doping solution containing the first doping source, and then heat-treated in an inert atmosphere at 250-350°C to obtain a doped precursor. The first doping source is selected from at least one of rubidium source and cesium source; S3: The doped precursor is carbonized at 1100-1700℃ to obtain a hard carbon intermediate; S4: Carbon coating is performed on the hard carbon intermediate to obtain a hard carbon anode material.

[0008] Optionally, the rubidium source is selected from at least one of rubidium acetate, rubidium carbonate, and rubidium citrate.

[0009] Optionally, the cesium source is selected from at least one of cesium acetate, cesium carbonate, and cesium citrate.

[0010] Optionally, the doping solution further includes a second doping source; the second doping source is selected from at least one of a nitrogen source and a phosphorus source.

[0011] Optionally, the nitrogen source is selected from at least one of melamine, urea, polyaniline, and polyacrylonitrile.

[0012] Optionally, the phosphorus source is selected from at least one of phytic acid, phosphoric acid, ammonium phosphate, and sodium hypophosphite.

[0013] Optionally, the preparation of a porous carbon matrix using a carbon source as raw material includes: mixing the carbon source with a potassium source, pyrolyzing the mixture in an inert atmosphere at 600-950°C, followed by acid washing and drying to obtain a porous carbon matrix.

[0014] Optionally, carbon coating of the hard carbon intermediate includes: using acetonitrile as the carbon source for carbon coating, and carbon coating of the hard carbon intermediate by chemical vapor deposition.

[0015] Another object of the present invention is to provide a hard carbon anode material, which is prepared by the preparation method of hard carbon anode material as described above.

[0016] Another object of the present invention is to provide a sodium-ion battery comprising the hard carbon anode material as described above.

[0017] The beneficial effects of this invention are: The method for preparing hard carbon anode materials provided by this invention involves introducing large-radius ions (such as Rb). + / Cs + The random layer structure intercalated into carbon expands the interlayer spacing of graphite microcrystals. The larger interlayer spacing is conducive to the rapid insertion / extraction and storage of sodium ions, and can buffer volume changes, reduce the collapse or peeling of the layered structure, reduce the risk of material pulverization during repeated sodium intercalation, help maintain capacity stability, and thus improve the cycling performance of the material. Detailed Implementation

[0018] The present invention will now be described in further detail. The embodiments described below are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0019] To address the poor cycle performance of porous hard carbon anode materials in existing technologies, this invention provides a method for preparing hard carbon anode materials, comprising the following steps: S1: Prepare porous carbon matrix using carbon source as raw material; This step forms the basic framework and main pore structure of the hard carbon anode material.

[0020] S2: The porous carbon matrix is ​​crushed, preferably to a particle size of 2-15 μm, and then impregnated in a doping solution containing the first doping source, and then heat-treated in an inert atmosphere at 250-350°C to obtain the doped precursor. Preferably, the doping solution containing the first doping source in this step is an aqueous solution or an ethanol solution; in this step, the porous carbon matrix after crushing is immersed in the doping solution containing the first doping source, and vacuum-assisted impregnation is performed (preferably the vacuum degree of vacuum-assisted impregnation is 0.01-0.5 atm, and the impregnation time is 1-4 hours), and then heat-treated at 250-350℃ for 2-5 hours in an inert atmosphere to obtain the doped precursor; The first doping source is selected from at least one of rubidium (Rb) source and cesium (Cs) source. The ionic radii of Rb and Cs are much larger than the covalent radius of carbon atoms. By introducing at least one of rubidium and cesium sources through this step, these metal atoms or ions can be intercalated into the disordered layer structure of carbon during the subsequent high-temperature carbonization process, which expands the interlayer spacing of graphite microcrystals. The larger interlayer spacing is conducive to the rapid insertion / extraction and storage of sodium ions, which can buffer volume changes, reduce the collapse or peeling of layered structures, reduce the risk of material pulverization during repeated sodium intercalation, help maintain capacity stability, and thus help improve the rate performance while improving the cycling performance of the material. In this invention, the preferred mass ratio of the porous carbon matrix to the first dopant source in this step is 5:(0.5-0.6).

[0021] S3: Carbonize the doped precursor at 1100-1700℃ to obtain a hard carbon intermediate; Preferably, in this step, the doped precursor is carbonized at a high temperature of 1100-1700℃ for 1-4 hours in a carbonization furnace to obtain a hard carbon intermediate.

[0022] During the heating process in this step, Rb / Cs elements are embedded in the carbon interlayer gaps, expanding the interlayer spacing and inhibiting the excessive growth of graphite microcrystals at high temperatures, thus obtaining a hard carbon intermediate.

[0023] S4: Carbon coating is performed on the hard carbon intermediate to obtain the hard carbon anode material; Through the carbon coating process in this step, a dense carbon layer is formed on the surface of the negative electrode material, which physically isolates the highly active core (especially the surface-enriched Rb / Cs) from the electrolyte, greatly suppressing side reactions and the dissolution and loss of active components, and constraining the volume change of the core during charging and discharging, thereby improving the structural integrity of the material and ensuring long-term cycle stability.

[0024] The method for preparing hard carbon anode materials provided by this invention involves introducing large-radius ions (such as Rb). + / Cs + The random layer structure intercalated into carbon expands the interlayer spacing of graphite microcrystals. The larger interlayer spacing is conducive to the rapid insertion / extraction and storage of sodium ions, and can buffer volume changes, reduce the collapse or peeling of the layered structure, reduce the risk of material pulverization during repeated sodium intercalation, help maintain capacity stability, and thus improve the cycling performance of the material.

[0025] The rubidium source of the present invention is preferably selected from at least one of rubidium acetate, rubidium carbonate, and rubidium citrate; the cesium source is preferably selected from at least one of cesium acetate, cesium carbonate, and cesium citrate.

[0026] To improve the cycling stability of the material, the present invention preferably further includes a second doping source in the doping solution; the second doping source is selected from at least one of nitrogen source and phosphorus source.

[0027] In addition to the traditional effects of significantly improving the electrocatalytic performance and thermal stability of materials (such as nitrogen-doped graphene helping to enhance the charge transport capacity of materials in the field of electrocatalysis, phosphorus doping helping to expand the carbon interlayer spacing, provide a strong electron donor effect, enhance structural stability and cycle life, and nitrogen-phosphorus co-doping helping to improve the capacitance performance of three-dimensional graphene), the introduction of N and P heteroatoms can also form MN / P coordination structures with Rb and Cs (where M is Rb or Cs). During the subsequent high-temperature carbonization process, the easily lost large-radius alkali metal ions Rb and Cs are anchored in the carbon framework, thereby improving the cycle stability of Rb and Cs.

[0028] The preferred mass ratio of porous carbon matrix to second dopant source in this invention is 5:(1-2).

[0029] The preferred nitrogen source is selected from at least one of melamine, urea, polyaniline, and polyacrylonitrile; the preferred phosphorus source is selected from at least one of phytic acid, phosphoric acid, ammonium phosphate, and sodium hypophosphite.

[0030] Specifically, in step S2, Rb + or Cs + It combines with N-containing groups in a nitrogen source or P-containing groups in a phosphorus source through coordination to form a soluble complex. Under vacuum assistance, this complex is forcibly drawn into the pores of a porous carbon matrix and adsorbed onto the pore walls and surface.

[0031] Furthermore, in an inert atmosphere at 250-350℃, N / P-containing organic precursors (nitrogen and phosphorus sources) decompose and cross-link, releasing N and P atoms that are incorporated into the carbon framework. Simultaneously, Rb atoms coordinated with these precursors... + or Cs + They are anchored in situ near the newly formed N / P doping sites.

[0032] By introducing a second doping source, during the heating process in step S3, Rb / Cs elements are embedded in the carbon interlayer gap in the form of MN / P coordination structure, which expands the interlayer gap and inhibits the excessive growth of graphite microcrystals at high temperature. At the same time, the easily lost large-radius alkali metal ions Rb and Cs are anchored in the carbon framework through N / P, resulting in a hard carbon intermediate.

[0033] The method for preparing hard carbon anode material provided by the present invention expands the interlayer spacing by introducing Rb / Cs, and uses N and P heteroatoms as chemical anchors to fix the easily lost Rb / Cs through strong bonding. This improves the rate performance and the cycle stability of the alkali metal Rb / Cs doped carbon material.

[0034] The present invention preferably uses a carbon source as raw material to prepare a porous carbon matrix, which includes: mixing a carbon source with a potassium source, pyrolyzing the mixture in an inert atmosphere at 600-800°C, acid washing, and drying to obtain a porous carbon matrix.

[0035] Preferably, in this step, the carbon source and potassium source are mixed at a mass ratio of 1:(0.5-2), pyrolyzed at 600-950℃ for 1-4 hours in an inert atmosphere, and then acid-washed and dried to obtain a porous carbon matrix.

[0036] In this step, the carbon source undergoes pyrolysis and carbonization at high temperature, while the potassium source acts as an activator, reacting with the carbon in a redox reaction to generate a large amount of gas, creating abundant micropores and mesopores in situ; and the generated K vapor or K + At high temperatures, potassium (K) can be inserted into the interlayer of forming carbon microcrystals. Some K elements are doped into the carbon framework through substitution or adsorption, existing stably in the carbon matrix in atomic or ionic states, thus constructing a conductive carbon framework with high specific surface area, well-developed porosity, and uniform K doping. Uniform K doping not only improves the electronic conductivity of the anode material and helps expand the interlayer spacing, but also lays the foundation for subsequent Rb... + or Cs + Doping provides a structurally stable carrier; specifically, uniform K doping provides basic conductivity and structural stability, while Rb / Cs surface enrichment provides a greater interlayer spacing expansion effect, achieving an optimized balance between performance and cost; that is, K achieves stable bulk conductivity, and surface enrichment of Rb / Cs further expands the interlayer spacing, thus enabling the anode material to balance stability and high ionic conductivity, while keeping costs under control.

[0037] The preferred carbon source of this invention is selected from at least one of glucose, sucrose, starch, cellulose, polyacrylonitrile, and phenolic resin; the preferred potassium source is selected from at least one of potassium hydroxide, potassium carbonate, potassium acetate, and potassium citrate.

[0038] The present invention preferably describes carbon coating of hard carbon intermediates by using acetonitrile as the carbon source and carbon coating of hard carbon intermediates by chemical vapor deposition.

[0039] Preferably, this step is carried out according to the following process: the hard carbon intermediate is placed in a CVD reactor, acetonitrile is used as the carbon source for coating, and deposition is carried out at 600-1000℃ and 0.01-0.07 MPa pressure for 10-60 minutes to obtain the hard carbon anode material.

[0040] Acetonitrile is preferably delivered via a bubbler at a temperature of 10-30°C, with nitrogen as the carrier gas and a flow rate of 50-200 mL / min.

[0041] This invention utilizes acetonitrile to achieve carbon deposition and nitrogen doping in one step. The moderately nitrogen-doped carbon shell has excellent electronic conductivity, providing a high-speed surface channel for rapid charge collection and transport, which helps to significantly reduce interfacial charge transfer impedance. In addition, the nitrogen functional groups in the shell help improve the wettability of the electrode / electrolyte interface, and can also provide fast surface pseudocapacitance, further improving rate performance.

[0042] Using acetonitrile as a single precursor, nitrogen-doped hard carbon surface layer deposition can be achieved in one step. The process is simple and controllable, and the carbon layer has both good electronic conductivity and chemical stability.

[0043] Another object of the present invention is to provide a hard carbon anode material, which is prepared by the preparation method of hard carbon anode material as described above.

[0044] The hard carbon anode material provided by this invention introduces large-radius ions (such as Rb) during its preparation process. + / Cs + The random layer structure intercalated into carbon expands the interlayer spacing of graphite microcrystals. The larger interlayer spacing is conducive to the rapid insertion / extraction and storage of sodium ions, and can buffer volume changes, reduce the collapse or peeling of the layered structure, reduce the risk of material pulverization during repeated sodium intercalation, help maintain capacity stability, and thus improve the cycling performance of the material.

[0045] Another object of the present invention is to provide a sodium-ion battery comprising the hard carbon anode material as described above.

[0046] The sodium-ion battery provided by this invention uses hard carbon anode material prepared by introducing large-radius ions (such as Rb) during the manufacturing process. + / Cs + The random layer structure intercalated into carbon expands the interlayer spacing of graphite microcrystals. The larger interlayer spacing is conducive to the rapid insertion / extraction and storage of sodium ions, and can buffer volume changes, reduce the collapse or peeling of the layered structure, reduce the risk of material pulverization during repeated sodium intercalation, help maintain capacity stability, and thus improve the cycling performance of the material.

[0047] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below.

[0048] Example 1 S1: Mix 10 g glucose with 15 g KOH, add 50 mL deionized water, stir to dissolve, spray dry, and then heat the resulting powder to 700 °C at 5 °C / min in an argon atmosphere for 2 hours. After cooling, wash with 1 M HCl, wash with deionized water until neutral, and dry at 80 °C to obtain a porous carbon matrix. S2: 5 g of porous carbon matrix was crushed and then immersed in 100 mL of an aqueous solution containing 0.6 g of cesium acetate and 2 g of ammonium phosphate for 2 hours under a vacuum of 0.1 atm. After filtration, the material was heat-treated at 300°C for 3 hours in an argon atmosphere to obtain the doped precursor. S3: The doped precursor is placed in a carbonization furnace and carbonized at 1400℃ for 2 hours under an argon atmosphere at a rate of 5℃ / min to obtain a hard carbon intermediate. S4: Place 2 g of hard carbon intermediate in a CVD reactor, use acetonitrile as the carbon source for coating, and deposit at 800℃ and 0.04MPa pressure for 30 minutes to carbon-coat the hard carbon intermediate and obtain hard carbon anode material.

[0049] Example 2 S1: Mix 10 g glucose with 20 g KOH, add 50 mL deionized water, stir to dissolve, spray dry, and then heat the resulting powder to 800 °C at 5 °C / min in an argon atmosphere for 1 hour. After cooling, wash with 1 M HCl, wash with deionized water until neutral, and dry at 80 °C to obtain a porous carbon matrix. S2: After crushing 5 g of porous carbon matrix, it was immersed in 100 mL of ethanol solution containing 0.5 g of rubidium acetate and 2 g of melamine. The solution was immersed for 2 hours under a vacuum of 0.1 atm. After filtration, the material was heat-treated at 300℃ for 3 hours in an argon atmosphere to obtain the doped precursor. S3: The doped precursor is placed in a carbonization furnace and carbonized at 1400℃ for 2 hours under an argon atmosphere at a rate of 5℃ / min to obtain a hard carbon intermediate. S4: Place 2 g of hard carbon intermediate in a CVD reactor, use acetonitrile as the carbon source for coating, and deposit at 800℃ and 0.04MPa pressure for 30 minutes to carbon-coat the hard carbon intermediate and obtain hard carbon anode material.

[0050] Example 3 S1: Mix 10 g glucose with 10 g KOH, add 50 mL deionized water, stir to dissolve, spray dry, and then heat the resulting powder to 600℃ at 5℃ / min in an argon atmosphere for 4 hours. After cooling, wash with 1 M HCl, wash with deionized water until neutral, and dry at 80℃ to obtain a porous carbon matrix. S2: After crushing 5 g of porous carbon matrix, it was immersed in 100 mL of ethanol solution containing 0.5 g of rubidium acetate and 2 g of ammonium phosphate. The solution was immersed for 2 hours under a vacuum of 0.1 atm. After filtration, the material was heat-treated at 300℃ for 3 hours in an argon atmosphere to obtain the doped precursor. S3: The doped precursor is placed in a carbonization furnace and carbonized at 1400℃ for 2 hours under an argon atmosphere at a rate of 5℃ / min to obtain a hard carbon intermediate. S4: Place 2 g of hard carbon intermediate in a CVD reactor, use acetonitrile as the carbon source for coating, and deposit at 800℃ and 0.04MPa pressure for 30 minutes to carbon-coat the hard carbon intermediate and obtain hard carbon anode material.

[0051] Example 4 S1: Mix 10 g glucose with 15 g KOH, add 50 mL deionized water, stir to dissolve, spray dry, and then heat the resulting powder to 700 °C at 5 °C / min in an argon atmosphere for 2 hours. After cooling, wash with 1 M HCl, wash with deionized water until neutral, and dry at 80 °C to obtain a porous carbon matrix. S2: 5 g of porous carbon matrix was crushed and then immersed in 100 mL of an aqueous solution containing 0.6 g of cesium acetate for 2 hours under a vacuum of 0.1 atm. After filtration, the material was heat-treated at 300°C for 3 hours in an argon atmosphere to obtain the doped precursor. S3: The doped precursor is placed in a carbonization furnace and carbonized at 1400℃ for 2 hours under an argon atmosphere at a rate of 5℃ / min to obtain a hard carbon intermediate. S4: Place 2 g of hard carbon intermediate in a CVD reactor, use acetonitrile as the carbon source for coating, and deposit at 800℃ and 0.04MPa pressure for 30 minutes to carbon-coat the hard carbon intermediate and obtain hard carbon anode material.

[0052] Each comparative example in this invention is compared with Example 1.

[0053] Comparative Example 1 S1: Mix 10 g glucose with 15 g KOH, add 50 mL deionized water, stir to dissolve, spray dry, and then heat the resulting powder to 700 °C at 5 °C / min in an argon atmosphere for 2 hours. After cooling, wash with 1 M HCl, wash with deionized water until neutral, and dry at 80 °C to obtain a porous carbon matrix. S2: 5 g of porous carbon matrix was crushed and then immersed in 100 mL of an aqueous solution containing 2 g of ammonium phosphate for 2 hours under a vacuum of 0.1 atm. After filtration, the material was heat-treated at 300℃ for 3 hours in an argon atmosphere to obtain the doped precursor. S3: The doped precursor is placed in a carbonization furnace and carbonized at 1400℃ for 2 hours under an argon atmosphere at a rate of 5℃ / min to obtain a hard carbon intermediate. S4: Place 2 g of hard carbon intermediate in a CVD reactor, use acetonitrile as the carbon source for coating, and deposit at 800℃ and 0.04MPa pressure for 30 minutes to carbon-coat the hard carbon intermediate and obtain hard carbon anode material.

[0054] Comparative Example 2 S1: Mix 10 g glucose with 15 g KOH, add 50 mL deionized water, stir to dissolve, spray dry, and then heat the resulting powder to 700 °C at 5 °C / min in an argon atmosphere for 2 hours. After cooling, wash with 1 M HCl, wash with deionized water until neutral, and dry at 80 °C to obtain a porous carbon matrix. S2: 5 g of porous carbon matrix was crushed and then immersed in 100 mL of an aqueous solution containing 0.6 g of cesium acetate and 2 g of ammonium phosphate for 2 hours under a vacuum of 0.1 atm. After filtration, the material was heat-treated at 300°C for 3 hours in an argon atmosphere to obtain the doped precursor. S3: The doped precursor is placed in a carbonization furnace and carbonized at 1400℃ for 2 hours under an argon atmosphere at a rate of 5℃ / min to obtain a hard carbon anode material.

[0055] Comparative Example 3 S1: Add 10 g glucose and 15 g KOH to 100 mL of an aqueous solution containing 0.6 g cesium acetate and 2 g ammonium phosphate. After stirring and dissolving, spray dry the solution. Heat the resulting powder at 300 °C for 3 hours in an argon atmosphere to obtain the doped precursor. S2: The doped precursor is placed in a carbonization furnace and carbonized at 1400℃ for 2 hours under an argon atmosphere at a rate of 5℃ / min to obtain a hard carbon intermediate. S3: Place 2 g of hard carbon intermediate in a CVD reactor, use acetonitrile as the carbon source for coating, and deposit at 800℃ and 0.04MPa pressure for 30 minutes to carbon-coat the hard carbon intermediate and obtain hard carbon anode material.

[0056] Comparative Example 4 S1: Add 5 g of glucose to 100 mL of an aqueous solution containing 0.6 g of cesium acetate and 2 g of ammonium phosphate, stir to dissolve, spray dry, and heat-treat the resulting powder at 300 °C for 3 hours in an argon atmosphere to obtain the doped precursor; S2: The doped precursor is placed in a carbonization furnace and carbonized at 1400℃ for 2 hours under an argon atmosphere at a rate of 5℃ / min to obtain a hard carbon intermediate. S3: Place 2 g of hard carbon intermediate in a CVD reactor, use acetonitrile as the carbon source for coating, and deposit at 800℃ and 0.04MPa pressure for 30 minutes to carbon-coat the hard carbon intermediate and obtain hard carbon anode material.

[0057] The electrochemical performance of the hard carbon anode materials prepared in the above embodiments and comparative examples was tested using the following methods: Hard carbon anode material (SP:CMC) was uniformly dispersed in an aqueous solution at a mass ratio of 90:5:5, then coated onto copper foil, dried, and assembled into a half-cell for electrochemical performance testing. Capacity and first-efficiency testing steps: Battery rested for 20 min; constant current discharge at 0.05C until V ≤ 0.005V; rested for 20 min; constant current discharge at 0.025C until V ≤ 0.005V; battery rested for 20 min; constant current discharge at 0.005C until V ≤ 0.005V; rested for 20 min; constant current charging at 0.05C until V ≥ 2.0V.

[0058] The test results are shown in Table 1: Table 1 Discharge specific capacity / mAh / g Charging capacity / mAh / g First Coulomb efficiency / % 100-cycle capacity retention rate / % Example 1 374.6 345.0 92.1 99 Example 2 406.0 371.9 91.6 96 Example 3 348.7 318.4 91.3 97 Example 4 371.9 336.9 90.6 93 Comparative Example 1 370.5 334.2 90.2 87 Comparative Example 2 368.6 307.8 83.5 81 Comparative Example 3 333.8 286.1 85.7 75 Comparative Example 4 329.2 298.3 90.6 85 The difference between Comparative Example 1 and Example 1 is that cesium acetate was not added in step S2. Because no large-diameter metal element was added, the interlayer spacing of the hard carbon was reduced, and the number of defects and active sites was reduced, resulting in a decrease in the capacity, first efficiency and cycle performance of the hard carbon material.

[0059] The difference between Comparative Example 2 and Example 1 is that no carbon coating was performed. Due to the lack of effective closed-pore structure and defective active sites formed by surface defects and microporous structure, sodium ions could not be properly extracted after insertion, resulting in reduced capacity and first-time efficiency of the hard carbon material and poorer cycle performance.

[0060] The difference between Comparative Example 3 and Example 1 is that no porous carbon matrix preparation step was performed. Glucose, KOH, cesium acetate, and ammonium phosphate were directly mixed and carbonized. Because no medium-to-high temperature pretreatment was performed, the KOH activity was insufficient to activate and create pores, resulting in fewer mesoporous and defective structures in the precursor. Furthermore, the lack of acid washing led to a higher impurity content in the hard carbon, resulting in a significant reduction in capacity and first-time efficiency. In addition, impurities reacted with the electrolyte during cycling, continuously consuming the SEI membrane and causing a deterioration in cycling performance.

[0061] The difference between Comparative Example 4 and Example 1 is that no porous carbon matrix preparation step was performed. Instead, glucose was directly mixed with cesium acetate and ammonium phosphate and then carbonized. Because no pore-forming process was performed, the precursor had fewer pore and defect structures, resulting in fewer sites available for sodium storage in the hard carbon material, which led to a significantly lower capacity of the hard carbon material compared to the Example.

[0062] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for preparing a hard carbon anode material, characterized in that, Includes the following steps: S1: Prepare porous carbon matrix using carbon source as raw material; S2: The porous carbon matrix is ​​crushed, impregnated in a doping solution containing the first doping source, and then heat-treated in an inert atmosphere at 250-350°C to obtain a doped precursor. The first doping source is selected from at least one of rubidium source and cesium source; S3: The doped precursor is carbonized at 1100-1700℃ to obtain a hard carbon intermediate; S4: Carbon coating is performed on the hard carbon intermediate to obtain a hard carbon anode material.

2. The method for preparing the hard carbon anode material as described in claim 1, characterized in that, The rubidium source is selected from at least one of rubidium acetate, rubidium carbonate, and rubidium citrate.

3. The method for preparing the hard carbon anode material as described in claim 1, characterized in that, The cesium source is selected from at least one of cesium acetate, cesium carbonate, and cesium citrate.

4. The method for preparing the hard carbon anode material as described in claim 1, characterized in that, The doping solution also includes a second doping source; the second doping source is selected from at least one of a nitrogen source and a phosphorus source.

5. The method for preparing the hard carbon anode material as described in claim 4, characterized in that, The nitrogen source is selected from at least one of melamine, urea, polyaniline, and polyacrylonitrile.

6. The method for preparing the hard carbon anode material as described in claim 1, characterized in that, The phosphorus source is selected from at least one of phytic acid, phosphoric acid, ammonium phosphate, and sodium hypophosphite.

7. The method for preparing the hard carbon anode material according to any one of claims 1-6, characterized in that, The preparation of a porous carbon matrix using a carbon source as raw material includes: mixing the carbon source with a potassium source, pyrolyzing the mixture in an inert atmosphere at 600-950°C, followed by acid washing and drying to obtain a porous carbon matrix.

8. The method for preparing the hard carbon anode material as described in claim 7, characterized in that, Carbon coating of the hard carbon intermediate includes: using acetonitrile as the carbon source for coating, and carbon coating the hard carbon intermediate by chemical vapor deposition.

9. A hard carbon anode material, characterized in that, The hard carbon anode material is prepared by the preparation method described in any one of claims 1-8.

10. A sodium-ion battery, characterized in that, Including the hard carbon anode material as described in claim 9.