A doped hard carbon anode material, its preparation method and application

Abstract

This invention discloses a doped hard carbon anode material, its preparation method, and its application. The preparation method of the doped hard carbon anode material includes the following steps: S1: Heat-treating a nitrogen-containing precursor to obtain graphitic carbon nitride; grinding it into powder, mixing it with acid, ultrasonically stirring, washing with water until neutral; drying and adding water to obtain a suspension; S2: Mixing phenolic compounds and aldehyde compounds with an alkaline aqueous solution, reacting them, then mixing them evenly with the suspension, and spray-drying to obtain dry powder; S3: Placing NaF in a high-frequency plasma reaction chamber to form vapor, depositing NaF on the surface of the dry powder; S4: Heat-treating the NaF-deposited dry powder in a protective gas atmosphere, and then performing carbon coating. The doped hard carbon anode material provided by this invention uses plasma technology to uniformly coat the surface and pores of hard carbon with NaF vapor, and then performs carbon coating through vapor phase deposition, increasing the binding force of NaF and reducing the specific surface area of ​​the hard carbon material, thereby exhibiting high initial efficiency and long cycle life.

Description

Technical Field

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

[0002] Sodium-ion batteries offer advantages such as good low-temperature performance and low cost, and are expected to capture a share of the lithium-ion battery market. Hard carbon materials, due to their unique layered structure, low sodium intercalation platform, and high capacity, are currently the most commercially mature sodium-ion anode material. However, hard carbon materials suffer from poor rate capability and low initial efficiency, which significantly limits their large-scale application.

[0003] Non-metallic atom doping (B, N, S, P, etc.) can alter the electronic structure and improve the conductivity of hard carbon materials. Simultaneously, heteroatomic doping can increase the interlayer spacing of hard carbon materials, thereby improving their rate performance. The reasons for low first-time efficiency are: 1) The abundant porosity of hard carbon materials results in a relatively large specific surface area, leading to the formation of a solid electrolyte interphase (SEI) film on the material surface during charging and discharging, causing irreversible consumption of sodium ions; 2) The excessively high binding energy between defect sites and sodium ions in hard carbon materials binds the sodium ions, preventing reversible reactions. Currently, effective strategies for solving the low first-time efficiency problem of hard carbon materials are carbon coating and pre-sodiumization.

[0004] Patent CN115347178A describes a process where hard carbon material and nitrogen-containing organic matter are sintered once to form a nitrogen-doped hard carbon material. This material is then mixed with pre-sodium material and sintered a second time to obtain a nitrogen-boron co-doped pre-sodiumized anode material. This method is simple, safe, and produces no byproducts. The co-doping of nitrogen and boron elements improves the material's capacity and rate performance, and pre-sodiumization enhances the material's initial coulombic efficiency. However, the solid-phase mixing process during preparation can lead to uneven mixing, making it difficult for the pre-sodium material to penetrate the pores, resulting in uneven pre-sodiumization. Furthermore, the lack of further carbon coating treatment results in poor adhesion between the pre-sodium material and the hard carbon surface, making it prone to cracking and detachment during sodium insertion / extraction, ultimately affecting the electrochemical performance of the hard carbon anode material.

[0005] Patent CN118439581A describes a process where organic sodium salts are doped into a hard carbon precursor and carbonized to obtain sodium compounds. Simultaneously, heteroatoms are doped into the precursor, improving the initial efficiency and electronic conductivity of the hard carbon material, thereby enhancing its rate and cycle performance. However, the use of chloroform as a solvent is highly toxic and volatile, making it environmentally unfriendly. Furthermore, the doping process creates pores in the core, and the lack of carbon coating results in a relatively large specific surface area, thus affecting the electrochemical performance of the hard carbon anode material. Summary of the Invention

[0006] The purpose of this invention is to provide a doped hard carbon anode material with high initial efficiency and long cycle performance; another purpose of this invention is to provide a method for preparing a doped hard carbon anode material that does not require the use of toxic solvents and has a more uniform pre-sodium treatment.

[0007] This invention discloses a method for preparing a doped hard carbon anode material, comprising the following steps:

[0008] S1: The nitrogen-containing precursor is heated to 400-600℃ in a protective gas atmosphere and kept at that temperature for 1-5 hours to obtain graphitic carbon nitride; the graphitic carbon nitride is ground into powder and mixed with acid solution, ultrasonically stirred for 1-5 hours, washed with water until neutral; after drying, it is dispersed in water to obtain a suspension;

[0009] S2: Phenolic compounds and aldehyde compounds are mixed and reacted with an alkaline aqueous solution, then mixed evenly with the suspension, and spray-dried to obtain dry powder;

[0010] S3: NaF is placed in a high-frequency plasma reaction chamber to form NaF vapor. Dry powder is sent into a condensation chamber. NaF vapor is introduced into the condensation chamber and comes into contact with the dry powder, depositing NaF on the surface and in the pores of the dry powder.

[0011] S4: In a protective gas atmosphere, the dry powder with deposited NaF is heat-treated, then a gaseous carbon source is introduced for carbon coating, cooled, and sieved to obtain doped hard carbon anode material.

[0012] By using acid to oxidize graphitic carbon nitride, the surface of graphitic carbon nitride is enriched with oxygen-containing functional groups, thus achieving oxygen doping of graphitic carbon nitride.

[0013] Using graphitic carbon nitride as a nitrogen dopant, the higher electronegativity of nitrogen atoms compared to carbon atoms causes them to attract electron clouds around carbon atoms during doping, resulting in a positively charged region around nitrogen atoms and a negatively charged region around carbon atoms. This non-uniform charge distribution leads to increased interlayer electrostatic repulsion, thereby widening the hard carbon interlayer spacing and improving the charge-discharge rate of the anode material.

[0014] The role of nitrogen-containing precursors is to obtain graphitic carbon nitride through heat treatment.

[0015] Phenolic and aldehyde compounds are cross-linked and polymerized into phenolic resins when mixed with an alkaline aqueous solution. The oxygen-containing functional groups of the added graphitic carbon nitride and the hydroxyl groups in the phenolic resin participate in the cross-linking polymerization, thereby achieving in-situ doping.

[0016] NaF vapor is formed using plasma technology and uniformly deposited on the surface and pores of hard carbon materials, thereby improving the uniformity of pre-sodiumization.

[0017] Protective gases include nitrogen and / or argon.

[0018] Furthermore, in step S1, the nitrogen-containing precursor includes one or at least two of urea, dicyandiamide, and melamine; the acid solution includes nitric acid and sulfuric acid; the mass ratio of the graphitic carbon nitride to the volume of the acid solution is 1 g:(300-500) mL; and the solid content of the suspension is 1-10%.

[0019] Furthermore, in step S2, the mass ratio of the phenolic compound to the aldehyde compound is 1:(1-2); the phenolic compound includes one or more of phenol, catechol, resorcinol, and hydroquinone; the aldehyde compound includes one or more of formaldehyde, acetaldehyde, propionaldehyde, trioxymethylene, and paraformaldehyde.

[0020] Furthermore, in step S2, the alkaline aqueous solution comprises one or more of NaOH, KOH, Na2CO3, NaHCO3, and ammonia dissolved in water; the mass ratio of the solute to the phenolic compound in the alkaline aqueous solution is (5-10):100; the reaction temperature is 30-90℃, and the reaction time is 1-10h; the volume of the added suspension accounts for 5-15% of the total liquid volume.

[0021] Total liquid volume refers to the sum of suspensions and reaction solutions of phenolic compounds, aldehyde compounds, and alkaline aqueous solutions.

[0022] Furthermore, in step S2, the atomization inlet temperature is 110-150℃, the feeding rate is 0.1-0.5L / h, and the D50 of the dry powder is 5-10μm.

[0023] Furthermore, in step S3, the discharge current in the plasma reaction chamber is 40-200A, and the working voltage is 60-180V; the mass ratio of NaF to the dry powder is (0.1-1):1.

[0024] Furthermore, in step S4, the protective gas is nitrogen, and the flow rate of nitrogen is 1-5 L / min; the heat treatment of the dry powder deposited with NaF is carried out in two stages; the first stage: the temperature is increased to 400-800℃ at a heating rate of 1-5℃ / min and held for 1-3 hours; the second stage: the temperature is increased to 850-1200℃ at a heating rate of 1-10℃ / min and held for 0.5-2 hours.

[0025] The first stage of heat treatment, at a temperature that facilitates the release of gases such as CO2 from oxygen-containing functional groups in the dry powder, creates defects and pores within the hard carbon. The second stage of heat treatment, involving high-temperature calcination, reduces the specific surface area of ​​the hard carbon material, promoting the formation of closed-cell structures and improving its electronic conductivity and capacitance. Carbon coating further reduces the specific surface area while simultaneously coating the NaF surface, preventing NaF from detaching during charging and discharging.

[0026] Furthermore, in step S4, the gaseous carbon source includes one of methane, ethane, ethylene, and acetylene; the flow rate of the gaseous carbon source is 0.1-1 L / min, and the duration is 0.5-2 h.

[0027] The present invention also discloses a doped hard carbon anode material, which is prepared by the preparation method described above.

[0028] The present invention also discloses a sodium-ion battery, comprising the doped hard carbon anode material as described above.

[0029] The sodium-ion battery described above has an initial efficiency of over 98%.

[0030] The present invention provides a doped hard carbon anode material that uses oxygen-doped carbon nitride as a nitrogen source to achieve in-situ doping, thereby reducing the intrinsic resistance of the material, increasing the interlayer spacing, and improving the charge-discharge rate of the material. By using plasma technology to make NaF vapor uniformly coat the surface and pores of hard carbon, and then using vapor phase deposition to coat carbon, the bonding force between NaF and hard carbon material is increased and the specific surface area is reduced, thus exhibiting high initial efficiency and long cycle life. Attached Figure Description

[0031] Figure 1 This is a graph showing the discharge rate test results of Example 1 and Comparative Examples 1-3 in this invention;

[0032] Figure 2 The graph shows the cycle performance test results of Example 1 and Comparative Examples 1-3 in this invention. Detailed Implementation

[0033] To make the technical solution of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] Example 1

[0035] Preparation of doped hard carbon anode materials:

[0036] S1: Place 10g of melamine in a tube furnace and heat it to 550℃ at a rate of 5℃ / min under nitrogen atmosphere. Hold the temperature for 2h to obtain graphitic carbon nitride. Grind the graphitic carbon nitride into powder and then place it in a mixed acid of sulfuric acid and nitric acid (the mass ratio of sulfuric acid and nitric acid is 1:1, and 400mL of mixed acid solution is added to 1g of graphitic carbon nitride) and ultrasonically stir for 5h. Finally, wash it with deionized water until neutral, dry it and disperse it in deionized water to obtain a suspension (solid content of 5%).

[0037] S2: Add m-diphenol and formaldehyde to a 28wt% NaOH aqueous solution at a mass ratio of 1:1.6 (the mass of NaOH solute is 5% of the mass of the phenolic compound). After stirring at 70℃ for 4.5h, slowly add the suspension obtained in step S1 (accounting for 10% of the total liquid volume). Continue stirring for 60min, then add pure water to adjust the viscosity of the mixture to 1000mpa·s. Spray dry the mixture at an inlet temperature of 120℃ and a feed rate of 0.35L / min to obtain dry powder (D50 controlled at 7-8μm).

[0038] S3: Place NaF powder in a high-frequency plasma reaction chamber, use argon as a protective gas, set the plasma generator operating current to 110A and the operating voltage to 120V, so that NaF in the reaction chamber forms vapor. Place dry powder (where the mass ratio of NaF powder to dry powder is 0.2:1) in a condensation chamber, with the condensation chamber rotating at 1r / min. NaF vapor is fed into the condensation chamber by 1.2L / min of argon gas to contact the dry powder and deposit NaF on its surface and in its pores.

[0039] S4: The NaF-deposited dry powder was placed in a CVD rotary furnace at a furnace tube rotation speed of 0.5 r / min. Nitrogen gas was introduced throughout step S4 at a flow rate of 1.5 L / min. The process involved two heat treatment stages: the first stage involved heating to 600℃ at a rate of 5℃ / min and holding for 2 hours; the second stage involved heating to 950℃ at a rate of 5℃ / min, holding for 1 hour, followed by introducing methane and holding for 2 hours at a methane flow rate of 0.45 L / min. After cooling, the resulting powder was passed through a 300-mesh sieve to obtain the doped hard carbon anode material.

[0040] Example 2

[0041] Preparation of doped hard carbon anode materials:

[0042] S1: Place 10g of urea in a tube furnace and heat it to 550℃ at a rate of 2℃ / min under nitrogen atmosphere. Hold the temperature for 2h to obtain graphitic carbon nitride. Grind the graphitic carbon nitride into powder and then place it in a mixed acid of sulfuric acid and nitric acid (the mass ratio of sulfuric acid and nitric acid is 1:1, and 400mL of mixed acid solution is added to 1g of graphitic carbon nitride) and ultrasonically stir for 4h. Finally, wash it with deionized water until neutral, dry it and disperse it in deionized water to obtain a suspension (solid content of 3%).

[0043] S2: Add m-diphenol and formaldehyde to a 25wt% Na2CO3 aqueous solution at a mass ratio of 1:1.4 (the mass of Na2CO3 is 8% of the mass of the phenolic compound). After stirring at 70℃ for 4 hours, slowly add the suspension obtained in step S1 (accounting for 10% of the total liquid volume). Continue stirring for 60 minutes, then add pure water to adjust the viscosity of the mixture to 1200 mPa·s. Spray dry the mixture at an inlet temperature of 130℃ and a feed rate of 0.5 L / min to obtain dry powder (D50 controlled at 7-8 μm).

[0044] S3: Place NaF powder in a high-frequency plasma reaction chamber, use argon as a protective gas, set the plasma generator operating current to 100A and the operating voltage to 130V, so that NaF in the reaction chamber forms vapor. Place dry powder (where the mass ratio of NaF powder to dry powder is 0.15:1) in a condensation chamber, with the condensation chamber rotating at 0.8r / min. NaF vapor is fed into the condensation chamber by 1.3L / min of argon gas to contact the dry powder, depositing NaF on its surface and in its pores.

[0045] S4: The NaF-deposited dry powder was placed in a CVD rotary furnace at a furnace tube rotation speed of 0.3 r / min. Nitrogen gas was introduced throughout step S4 at a flow rate of 1 L / min. The process involved two heat treatment stages: the first stage involved heating to 650℃ at a rate of 2℃ / min and holding for 1.5 h; the second stage involved heating to 900℃ at a rate of 5℃ / min, holding for 1 h, followed by acetylene introduction and holding for 2 h at a flow rate of 0.3 L / min. After cooling, the resulting powder was passed through a 300-mesh sieve to obtain the doped hard carbon anode material.

[0046] Example 3

[0047] Preparation of doped hard carbon anode materials:

[0048] S1: Place 10g of dicyandiamide in a tube furnace and heat it to 600℃ at a rate of 5℃ / min under nitrogen atmosphere, hold for 1.5h to obtain graphitic carbon nitride. Grind it into powder and place it in a mixed acid of sulfuric acid and nitric acid (the mass ratio of sulfuric acid and nitric acid is 1:1, and 400mL of mixed acid solution is added to 1g of graphitic carbon nitride) and ultrasonically stir for 5h. Finally, wash it with deionized water until neutral, dry it and disperse it in deionized water to obtain a suspension (solid content is 8%).

[0049] S2: Add parabens and trioxymethylene to a 32wt% KOH solution at a mass ratio of 1:1.6 (the mass of KOH is 9% of the mass of the phenolic compound). Stir at 80℃ for 5 hours, then slowly add the suspension obtained in step S1 (accounting for 8% of the total solution volume). Continue stirring for 60 minutes, then add pure water to adjust the viscosity of the mixture to 800 mPa·s. Spray dry the mixture at an inlet temperature of 130℃ and a feed rate of 0.5 L / min to obtain dry powder (D50 controlled at 8-9 μm).

[0050] S3: Place NaF powder in a high-frequency plasma reaction chamber, use argon as a protective gas, set the plasma generator operating current to 110A and the operating voltage to 110V, so that NaF in the reaction chamber forms vapor. Place dry powder (where the mass ratio of NaF powder to dry powder is 0.22:1) in a condensation chamber, with the condensation chamber rotating at 0.5r / min. NaF vapor is fed into the condensation chamber by 1.1L / min of argon gas to contact the dry powder and deposit NaF on its surface and in its pores.

[0051] S4: The NaF-deposited dry powder was placed in a CVD rotary furnace at a furnace tube rotation speed of 0.5 r / min. Nitrogen gas was introduced throughout step S4 at a flow rate of 1.2 L / min. The process involved two heat treatment stages: the first stage involved heating to 700℃ at a rate of 5℃ / min and holding for 1.5 h; the second stage involved heating to 950℃ at a rate of 5℃ / min, holding for 1 h, followed by acetylene introduction and holding for 2 h at a flow rate of 0.3 L / min. After cooling, the resulting powder was passed through a 300-mesh sieve to obtain the doped hard carbon anode material.

[0052] Example 4

[0053] Preparation of doped hard carbon anode materials:

[0054] S1: Place 10g of melamine in a tube furnace and heat it to 550℃ at a rate of 5℃ / min under nitrogen atmosphere. Hold the temperature for 2h to obtain graphitic carbon nitride. Grind the graphitic carbon nitride into powder and then place it in a mixed acid of sulfuric acid and nitric acid (the mass ratio of sulfuric acid and nitric acid is 1:1, and 400mL of mixed acid solution is added to 1g of graphitic carbon nitride) and ultrasonically stir for 5h. Finally, wash it with deionized water until neutral, dry it and disperse it in deionized water to obtain a suspension (solid content of 5%).

[0055] S2: Add m-diphenol and formaldehyde to a 28wt% NaOH aqueous solution at a mass ratio of 1:1.6 (the mass of NaOH solute is 5% of the mass of the phenolic compound). After stirring at 70℃ for 4.5h, slowly add the suspension obtained in step S1 (accounting for 10% of the total liquid volume). Continue stirring for 60min, then add pure water to adjust the viscosity of the mixture to 1000mpa·s. Spray dry the mixture at an inlet temperature of 120℃ and a feed rate of 0.35L / min to obtain dry powder (D50 controlled at 7-8μm).

[0056] S3: Place NaF powder in a high-frequency plasma reaction chamber, use argon as a protective gas, set the plasma generator operating current to 110A and the operating voltage to 120V, so that NaF in the reaction chamber forms vapor. Place dry powder (where the mass ratio of NaF powder to dry powder is 0.2:1) in a condensation chamber, with the condensation chamber rotating at 1r / min. NaF vapor is fed into the condensation chamber by 1.2L / min of argon gas to contact the dry powder and deposit NaF on its surface and in its pores.

[0057] S4: The NaF-deposited dry powder was placed in a CVD rotary furnace at a furnace tube rotation speed of 0.5 r / min. Nitrogen gas was purged throughout step S4 at a flow rate of 1.5 L / min. The temperature was increased to 950℃ at a rate of 5℃ / min and held for 1 h. Then, methane was introduced and held for 2 h at a flow rate of 0.45 L / min. After cooling, the obtained powder was passed through a 300-mesh sieve to obtain the doped hard carbon anode material.

[0058] Comparative Example 1

[0059] Preparation of hard carbon anode materials:

[0060] (1) Add m-diphenol and formaldehyde to a 28wt% NaOH aqueous solution at a mass ratio of 1:1.6 (the mass of NaOH is 5% of the mass of m-diphenol). Stir at 70℃ for 4.5h and then slowly add pure water to adjust the viscosity of the mixture to 1000mpa·s. Spray dry the mixture at an inlet temperature of 120℃ and a feed rate of 0.35L / min to obtain phenolic resin dry powder (D50 controlled at 7-8μm).

[0061] (2) Place NaF powder in a high-frequency plasma reaction chamber, use argon as a protective gas, set the working current of the plasma generator to 110A and the working voltage to 120V, so that NaF in the reaction chamber forms vapor. Place phenolic resin dry powder (where the mass ratio of NaF powder to phenolic resin dry powder is 0.2:1) in a condensation chamber, with the condensation chamber rotating at 1r / min. NaF vapor is sent into the condensation chamber under the drive of 1.2L / min argon to contact the phenolic resin dry powder, and deposit NaF on its surface and in its pores.

[0062] (3) The NaF-deposited phenolic resin powder was placed in a CVD rotary furnace at a furnace speed of 0.5 r / min. Nitrogen gas was introduced throughout the process at a flow rate of 1.5 L / min. The heat treatment was performed in two stages: the first stage involved heating to 600℃ at a rate of 5℃ / min and holding for 2 hours; the second stage involved heating to 950℃ at a rate of 5℃ / min, holding for 1 hour, followed by the introduction of methane and holding for 2 hours at a flow rate of 0.45 L / min. After cooling, the obtained powder was passed through a 300-mesh sieve to obtain the hard carbon anode material.

[0063] Comparative Example 2

[0064] Preparation of hard carbon anode materials:

[0065] (1) Place 10g of melamine in a tube furnace and heat it to 550℃ at a rate of 5℃ / min under nitrogen atmosphere. Hold the temperature for 2h to obtain graphitic carbon nitride. Grind it into powder and place it in a mixed acid (the mass ratio of sulfuric acid and nitric acid is 1:1, and 400mL of mixed acid solution is added to 1g of graphitic carbon nitride) and ultrasonically stir for 5h. Finally, wash it with deionized water until neutral, dry it and disperse it in deionized water to obtain a suspension (solid content is 5%).

[0066] (2) Add m-diphenol and formaldehyde to a 28wt% NaOH aqueous solution at a mass ratio of 1:1.6 (the mass of NaOH is 5% of the mass of m-diphenol). Stir at 70℃ for 4.5h and then slowly add the suspension obtained in step (1) (accounting for 10% of the total solution volume). After stirring for 60min, add pure water to adjust the viscosity of the mixture to 1000mpa·s. Spray dry the mixture at an inlet temperature of 120℃ and a feed rate of 0.35L / min to obtain dry powder (D50 controlled at 7-8μm).

[0067] (3) The obtained dry powder was placed in a CVD rotary furnace at a furnace tube speed of 0.5 r / min. Nitrogen gas was introduced throughout the process at a flow rate of 1.5 L / min. The heat treatment was performed in two stages: the first stage involved heating to 600℃ at a rate of 5℃ / min and holding for 2 hours; the second stage involved heating to 950℃ at a rate of 5℃ / min, holding for 1 hour, followed by the introduction of methane and holding for 2 hours at a flow rate of 0.45 L / min. After cooling, the obtained powder was passed through a 300-mesh sieve to obtain the hard carbon anode material.

[0068] Comparative Example 3

[0069] Preparation of hard carbon anode materials:

[0070] (1) Place 10g of melamine in a tube furnace and heat it to 550℃ at a rate of 5℃ / min under nitrogen atmosphere. Hold the temperature for 2h to obtain graphitic carbon nitride. Grind it into powder and place it in a mixed acid (the mass ratio of sulfuric acid and nitric acid is 1:1, and 400mL of mixed acid solution is added to 1g of graphitic carbon nitride) and ultrasonically stir for 5h. Finally, wash it with deionized water until neutral, dry it and disperse it in deionized water to obtain suspension A (concentration of 5%).

[0071] (2) Add m-diphenol and formaldehyde to a 28wt% NaOH aqueous solution at a mass ratio of 1:1.6 (the mass of NaOH is 5% of the mass of m-diphenol). Stir at 70℃ for 4.5h and then slowly add the suspension obtained in step (1) (the suspension accounts for 10% of the total solution volume). Continue stirring for 60min and then add pure water to adjust the viscosity of the mixture to 1000mpa·s. Spray dry the mixture at an inlet temperature of 120℃ and a feed rate of 0.35L / min to obtain dry powder (D50 controlled at 7-8μm).

[0072] (3) Place NaF powder in a high-frequency plasma reaction chamber, use argon as a protective gas, set the working current of the plasma generator to 110A and the working voltage to 120V, so that NaF in the reaction chamber forms vapor. Place dry powder (where the mass ratio of NaF powder to dry powder B is 0.2:1) in a condensation chamber, with the condensation chamber rotating at 1r / min. NaF vapor is sent into the condensation chamber under the drive of 1.2L / min argon to contact the dry powder and deposit NaF on its surface and in its pores.

[0073] (4) The dry powder with deposited NaF was placed in a CVD rotary furnace with a furnace tube rotation speed of 0.5 r / min. Nitrogen gas was passed through the entire process at a flow rate of 1.5 L / min. The heat treatment was carried out in two stages. The first stage heat treatment was carried out by heating to 600℃ at a heating rate of 5℃ / min and holding for 2 hours. The second stage heat treatment was carried out by heating to 950℃ at a heating rate of 5℃ / min and holding for 2 hours. After cooling, the obtained powder was passed through a 300-mesh sieve to obtain hard carbon anode material.

[0074] Performance testing:

[0075] Powder resistance tests were performed on Examples 1-3 and Comparative Examples 1-3, and the resulting finished materials were used to prepare Model 2032 button batteries for evaluation.

[0076] The specific method involves mixing the prepared negative electrode material, conductive agent SP (conductive carbon black), binder CMC (hydroxymethyl cellulose), and binder SBR (styrene-butadiene rubber) in a mass ratio of 94.5:2:1.2:2.3. Using water as a solvent, the slurry is coated onto copper foil. The counter electrode is a sodium sheet, and the separator is a sodium-electric glass fiber separator. The charge / discharge cutoff voltage is 0.005-2.0V. The discharge rate is first discharged at 0.1C to 0.005V, then discharged at 0.02C to 0.05C to ensure complete discharge. The charging rate is 0.1C to 2.0V.

[0077] The finished materials obtained in Example 1 and Comparative Examples 1-3 were used to prepare pouch cells for evaluation.

[0078] The specific method involves dispersing and slurrying the preparation materials, conductive agent SP, and binder CMC+SBR in a mass ratio of 94.5:2:1.2:2.3, and then combining them with sodium nickel iron manganese oxide cathode to form a soft-pack battery through cell preparation processes such as coating, rolling, and slitting.

[0079] The performance of hard carbon anode materials in pouch cells was tested.

[0080] The specific test items are as follows: 1. Rate performance test at 1.25℃ with a charge / discharge voltage range of 1.5-3.95V. First, capacity calibration is performed by constant current charge / discharge at 1 / 3C. While maintaining a charge rate of 1C, the discharge rates are 1C, 2C, 3C and 4C. The capacity of the cell at the higher rate is tested relative to that at 1 / 3C. Finally, the capacity recovery is tested by charge / discharge at 1 / 3C rate. 2. Cyclic performance test at 1C rate with a charge / discharge voltage range of 1.5-3.95V at 25℃.

[0081] The test results are shown in Tables 1-3 and Figures 1-2 As shown.

[0082] Table 1. Results of powder resistance and electrochemical performance tests in the examples and comparative examples.

[0083] serial number Powder resistivity / Ω / cm Initial sodium intercalation capacity / mAh / g Initial sodium removal capacity / mAh / g First-efficacy / % Example 1 2.3 360.8 358.6 100.6 Example 2 2.0 361.4 356.34 98.6 Example 3 2.4 357.5 355.71 99.5 Example 4 2.5 354.9 343.54 96.8 Comparative Example 1 4.3 342.1 319.52 93.4 Comparative Example 2 2.5 351.3 314.41 89.5 Comparative Example 3 2.6 353.8 338.23 95.6

[0084] As shown in Table 1, the powder resistivity of Examples 1-4 is lower than that of Comparative Examples 1-3, while the powder resistivity of Comparative Examples 2-3 is lower than that of Comparative Example 1. This is because in-situ doping is achieved by using oxygen-doped carbon nitride as the nitrogen source, thereby reducing the intrinsic resistance of the material. In-situ doping, plasma-formed pre-sodium layer, and carbon coating greatly improve the first-stage efficiency of hard carbon materials.

[0085] Table 2. Discharge capacity retention test results of the examples and comparative examples.

[0086] serial number 1C (Capacity retention rate / %) 2C (Capacity retention rate / %) 3C (Capacity Retention Rate / %) 4C (Capacity Retention Rate / %) Example 1 98.2 96.8 92.4 85.3 Example 2 98.7 97.2 93.1 86.5 Example 3 98.0 96.5 91.2 84.2 Example 4 97.6 94.6 88.7 80.8 Comparative Example 1 92.4 85.3 75.3 68.2 Comparative Example 2 93.6 87.4 80.6 73.4 Comparative Example 3 93.2 87.1 79.4 70.9

[0087] Table 3. Discharge capacity retention test results for the examples and comparative examples.

[0088] serial number Capacity retention rate after 200 cycles / % Capacity retention rate after 300 cycles / % Capacity retention rate after 400 cycles / % Example 1 100.5 99.9 98.2 Example 2 100.1 99.4 97.7 Example 3 99.9 99.2 97.4 Example 4 99.6 98.6 96.3 Comparative Example 1 99.7 97.5 93.6 Comparative Example 2 98.8 95.5 90.7 Comparative Example 3 98.4 96.0 92.0

[0089] As shown in Tables 2 and 3, Examples 1-4 exhibit higher rate performance and better cycle performance compared to Comparative Examples 1-3. This is mainly because the powder resistance of the examples is lower, while in-situ nitrogen doping accelerates electron transfer, and pre-sodiumization and carbon coating result in less lithium ion consumption, thus exhibiting better electrochemical performance.

[0090] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing a doped hard carbon anode material, characterized in that, Includes the following steps: S1: The nitrogen-containing precursor is heated to 400-600℃ in a protective gas atmosphere and kept at that temperature for 1-5 hours to obtain graphitic carbon nitride; the graphitic carbon nitride is ground into powder and mixed with acid solution, ultrasonically stirred for 1-5 hours, washed with water until neutral; after drying, it is dispersed in water to obtain a suspension; S2: Phenolic compounds and aldehyde compounds are mixed and reacted with an alkaline aqueous solution, then mixed evenly with the suspension, and spray-dried to obtain dry powder; S3: NaF is placed in a high-frequency plasma reaction chamber to form NaF vapor. Dry powder is sent into a condensation chamber. NaF vapor is introduced into the condensation chamber and comes into contact with the dry powder, depositing NaF on the surface and in the pores of the dry powder. S4: In a protective gas atmosphere, the dry powder with deposited NaF is heat-treated, then a gaseous carbon source is introduced for carbon coating, cooled, and sieved to obtain doped hard carbon anode material.

2. The method for preparing a doped hard carbon anode material according to claim 1, characterized in that, In step S1, the nitrogen-containing precursor includes one or at least two of urea, dicyandiamide, and melamine; the acid solution includes nitric acid and sulfuric acid; the mass ratio of the graphitic carbon nitride to the volume of the acid solution is 1g:300-500mL; and the solid content of the suspension is 1-10%.

3. The method for preparing a doped hard carbon anode material according to claim 1, characterized in that, In step S2, the mass ratio of the phenolic compound to the aldehyde compound is 1:1-2; the phenolic compound includes one or more of phenol, catechol, resorcinol, and hydroquinone; the aldehyde compound includes one or more of formaldehyde, acetaldehyde, propionaldehyde, trioxymethylene, and paraformaldehyde.

4. The method for preparing a doped hard carbon anode material according to claim 3, characterized in that, In step S2, the alkaline aqueous solution includes one or more of NaOH, KOH, Na2CO3, NaHCO3, and ammonia dissolved in water; the mass ratio of the solute to the phenolic compound in the alkaline aqueous solution is 5-10:100; the reaction temperature is 30-90℃, and the reaction time is 1-10h; the volume of the added suspension accounts for 5-15% of the total liquid volume.

5. The method for preparing a doped hard carbon anode material according to claim 4, characterized in that, In step S2, the atomization inlet temperature is 110℃-150℃, and the feed rate is 0.1L / h-0.5L / h; the D50 of the dry powder is 5-10μm.

6. The method for preparing a doped hard carbon anode material according to claim 5, characterized in that, In step S3, the discharge current in the plasma reaction chamber is 40-200A, and the working voltage is 60-180V; the mass ratio of NaF to the dry powder is 0.1-1:

1.

7. The method for preparing a doped hard carbon anode material according to claim 1, characterized in that, In step S4, the protective gas is nitrogen, and the flow rate of nitrogen is 1-5 L / min. The heat treatment of the dry powder deposited with NaF is carried out in two stages: the first stage: the temperature is increased to 400-800℃ at a heating rate of 1-5℃ / min and held for 1-3 hours; the second stage: the temperature is increased to 850-1200℃ at a heating rate of 1-10℃ / min and held for 0.5-2 hours.

8. The method for preparing a doped hard carbon anode material according to claim 7, characterized in that, In step S4, the gaseous carbon source includes one of methane, ethane, ethylene, and acetylene; the flow rate of the gaseous carbon source is 0.1-1 L / min, and the duration is 0.5-2 h.

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

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

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