Hard carbon-doped negative electrode material and preparation method and application thereof

Abstract

The invention discloses a hard-carbon-doped negative electrode material and a preparation method and application thereof, and the preparation method of the hard-carbon-doped negative electrode material comprises the following steps: S1, carrying out heat treatment on a nitrogen-containing precursor to obtain graphite-phase carbon nitride; grinding into powder, mixing with acid liquor, ultrasonically stirring, and washing to be neutral; drying and adding water to obtain a suspension; s2, mixing and reacting a phenolic compound and an aldehyde compound with an alkaline aqueous solution, then uniformly mixing with the suspension, and carrying out spray drying to obtain dry powder; s3, NaF is placed in a high-frequency plasma reaction bin to form steam, and NaF is deposited on the surface of the dry powder; and S4, in the atmosphere of protective gas, carrying out heat treatment on the dry powder deposited with the NaF, and then carrying out carbon coating. According to the hard carbon-doped negative electrode material provided by the invention, NaF forms steam through a plasma technology to uniformly coat the surface and pores of hard carbon, and then carbon coating is performed through vapor deposition, so that the binding force of NaF is increased, and the specific surface area of the hard carbon material is reduced, and therefore, the hard carbon-doped negative electrode material has high first efficiency and long cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium-ion batteries, and particularly to a doped hard carbon negative electrode material, a preparation method thereof, and an application thereof. Background Art

[0002] Sodium-ion batteries have advantages such as good low-temperature performance and low cost, and are expected to capture the application market of lithium-ion batteries. Due to its special layered structure, low sodium intercalation platform, high capacity and other advantages, hard carbon materials are currently the most commercially mature sodium-ion negative electrode materials. However, hard carbon materials have problems of poor rate performance and low initial efficiency, which greatly limit their large-scale application.

[0003] Doping with non-metallic atoms (such as B, N, S, P, etc.) can change the electronic structure and improve the conductivity of hard carbon materials. At the same time, heteroatom doping can increase the layer spacing of hard carbon materials, thereby improving their rate performance. The reasons for the low initial efficiency are as follows: 1) The rich pore structure of hard carbon materials leads to a relatively large specific surface area, and a solid electrolyte interface (SEI) is formed on the material surface during charge and discharge, resulting in irreversible consumption of sodium ions; 2) The large binding energy between the defect sites of hard carbon materials and sodium ions restricts sodium ions and makes them unable to undergo reversible reactions. Currently, effective strategies to solve the problem of low initial efficiency of hard carbon materials are carbon coating and pre-sodiation.

[0004] Patent CN115347178A forms a nitrogen-doped hard carbon material by sintering hard carbon materials and nitrogen-containing organic compounds once, and then mixing with a pre-sodium material for secondary sintering to obtain a nitrogen-boron co-doped pre-sodiated negative electrode material. This method is simple, has good safety performance, and no by-products are generated. The capacity and rate performance of the material are improved by co-doping with nitrogen and boron elements, and pre-sodiation improves the first Coulomb efficiency of the material. However, in the preparation process, solid-phase mixing is likely to cause uneven mixing, and it is difficult for the pre-sodium material to penetrate into the pores, resulting in uneven pre-sodiation. At the same time, no further carbon coating treatment is carried out, and the binding force between the pre-sodium material and the hard carbon surface is poor, which is easy to break and fall off during the process of sodium deintercalation and intercalation, ultimately affecting the electrochemical performance of the hard carbon negative electrode material.

[0005] Patent CN118439581A obtains sodium compounds by doping organic sodium salts in a hard carbon precursor and then carbonizing, and at the same time doping heteroatoms to improve the initial efficiency and electronic conductivity of the hard carbon material, thereby improving the rate and cycle performance. However, using chloroform as a solvent has high toxicity, is volatile, and is not friendly to the environment. At the same time, doping in the core will generate pores during the process, and without carbon coating treatment, the specific surface area will be relatively large, thus affecting the electrochemical performance of the hard carbon negative electrode material. Summary of the Invention

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

[0007] A preparation method of a doped hard carbon anode material disclosed by the present invention includes the following steps:

[0008] S1: Place a nitrogen-containing precursor in a protective gas atmosphere and heat it to 400 - 600 °C, keep it warm for 1 - 5 h to obtain graphitic carbon nitride; after grinding the graphitic carbon nitride into powder, mix it with an acid solution, stir ultrasonically for 1 - 5 h, wash it with water until neutral; after drying, disperse it in water to obtain a suspension;

[0009] S2: Mix a phenolic compound and an aldehyde compound with an alkaline aqueous solution for reaction, then mix it evenly with the suspension, and spray-dry to obtain a dry powder;

[0010] S3: Place NaF in a high-frequency plasma reaction chamber to form NaF vapor, send the dry powder into a condensation chamber, and introduce the NaF vapor into the condensation chamber to contact with the dry powder, and deposit NaF on the surface and pores of the dry powder;

[0011] S4: In a protective gas atmosphere, heat-treat the dry powder deposited with NaF, then introduce a gaseous carbon source for carbon coating, cool down, and sieve to obtain a doped hard carbon anode material.

[0012] Use an acid solution to oxidize graphitic carbon nitride, so that the surface of graphitic carbon nitride is rich in oxygen-containing functional groups, realizing the oxygen doping of graphitic carbon nitride.

[0013] Use graphitic carbon nitride as a nitrogen doping agent. The electronegativity of nitrogen atoms is greater than that of carbon atoms, which causes nitrogen atoms to attract the electron cloud around carbon atoms during the doping process, thus forming a positive charge region around nitrogen atoms and a negative charge region around carbon atoms. This non-uniform charge distribution leads to an increase in the interlayer electrostatic repulsion, thereby expanding the hard carbon layer spacing and being beneficial to improving the charge-discharge rate of the anode material.

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

[0015] After the phenolic compound and the aldehyde compound are mixed with the alkaline aqueous solution, they crosslink and polymerize into phenolic resin. The oxygen-containing functional groups of the added graphitic carbon nitride and groups such as hydroxyl groups in the phenolic resin jointly participate in the crosslinking polymerization, thereby realizing in-situ doping.

[0016] Use plasma technology to form NaF vapor, which is uniformly deposited on the surface and pores of the hard carbon material, thereby improving the uniformity of pre-sodiation.

[0017] The protective gas includes nitrogen and / or argon.

[0018] Further, in the 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, and the mass ratio of the graphitic carbon nitride to the volume of the acid solution is 1 g:(300 - 500) mL; the solid content of the suspension is 1 - 10%.

[0019] Further, in the 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] Further, in the step S2, the alkaline aqueous solution includes one or several of NaOH, KOH, Na 2 CO 3 、NaHCO 3 、ammonia dissolved in water; the mass ratio of the solute in the alkaline aqueous solution to the mass of the phenolic compound is (5 - 10):100; the reaction temperature is 30 - 90 °C, the reaction time is 1 - 10 h; the volume of the suspension added accounts for 5 - 15% of the total volume of the liquid.

[0021] The total liquid refers to the sum of the suspension and the reaction liquid of the phenolic compound, the aldehyde compound, and the alkaline aqueous solution.

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

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

[0024] Further, in the step S4, the protective gas is nitrogen, and the flow rate of nitrogen is 1 - 5 L / min; when heat-treating the dry powder deposited with NaF, it is carried out in two stages; the first stage: heating at a heating rate of 1 - 5 °C / min to 400 - 800 °C and holding for 1 - 3 h, the second stage: heating at a heating rate of 1 - 10 °C / min to 850 - 1200 °C and holding for 0.5 - 2 h.

[0025] The heat treatment temperature in the first stage facilitates the formation of CO from the oxygen-containing functional groups in the dry powder 2Gases such as etc. overflow, thus forming defect and pore structures inside the hard carbon; for the heat treatment temperature in the second stage, high-temperature calcination reduces the specific surface area of the hard carbon material, which is beneficial to form closed pore structures inside the hard carbon material, and improves the electronic conductivity and capacity of the material. The carbon coating further reduces the specific surface area while coating the NaF on the surface of the material, avoiding the shedding of the surface NaF during charge and discharge.

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

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

[0028] The present invention also discloses a sodium ion battery, including the doped hard carbon negative electrode material described above.

[0029] The first efficiency of the described sodium ion battery is greater than 98%.

[0030] A doped hard carbon negative electrode material provided by the present invention uses oxygen-doped carbon nitride as a nitrogen source to achieve in-situ doping, reduce the intrinsic resistance of the material, increase the layer spacing, which is beneficial to improving the rate charge and discharge of the material. Through plasma technology, NaF forms vapor to uniformly coat on the surface and pores of the hard carbon, and then carbon coating is carried out by chemical vapor deposition, increasing the binding force between NaF and the hard carbon material and reducing the specific surface area, thus having the performance of high first efficiency and long cycle life. Description of the Drawings

[0031] Figure 1 is the test result diagram of the discharge rate of Example 1 and Comparative Examples 1 - 3 in the present invention;

[0032] Figure 2 is the test result diagram of the cycle performance of Example 1 and Comparative Examples 1 - 3 in the present invention. Detailed Embodiments

[0033] To make the technical solutions of the present invention clearer, the following further describes the present invention in detail with reference to the drawings and specific embodiments.

[0034] Example 1

[0035] Prepare a doped hard carbon negative electrode material:

[0036] S1: Place 10 g of melamine in a tubular furnace. Under a nitrogen atmosphere, heat it to 550 °C at a rate of 5 °C / min, hold for 2 h to obtain graphitic carbon nitride. After grinding it into powder, place it in a mixed acid of sulfuric acid and nitric acid (the mass ratio of sulfuric acid to nitric acid is 1:1, and 400 mL of the mixed acid solution is added to 1 g of graphitic carbon nitride), and perform ultrasonic stirring for 5 h. Finally, wash it with deionized water until neutral, dry it, and disperse it in deionized water to obtain a suspension (the solid content is 5%).

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

[0038] S3: Place the NaF powder in a high-frequency plasma reaction chamber, with argon as the protective gas. Set the working current of the plasma generator to 110 A and the working voltage to 120 V to form NaF vapor in the reaction chamber. Place the dry powder (the mass ratio of NaF powder to dry powder is 0.2:1) in the condensation chamber, with the rotation speed of the condensation chamber being 1 r / min. The NaF vapor is sent into the condensation chamber by 1.2 L / min of argon to contact the dry powder, and NaF is deposited on its surface and in the pores.

[0039] S4: Place the dry powder deposited with NaF in a CVD rotary furnace, with the rotation speed of the furnace tube being 0.5 r / min. Nitrogen is passed throughout step S4, with a nitrogen flow rate of 1.5 L / min. Heat-treat in two stages. The first heat treatment: Heat it to 600 °C at a heating rate of 5 °C / min and hold for 2 h. The second heat treatment: Heat it to 950 °C at a heating rate of 5 °C / min, hold for 1 h, then pass methane, hold for 2 h, and the methane flow rate is 0.45 L / min. After cooling, sieve the obtained powder through a 300-mesh sieve to obtain the doped hard carbon anode material.

[0040] Example 2

[0041] Preparation of doped hard carbon anode material:

[0042] S1: Place 10 g of urea in a tubular furnace. Under a nitrogen atmosphere, heat it to 550 °C at a rate of 2 °C / min and hold for 2 h to obtain graphitic carbon nitride. After grinding it into powder, place it in a mixed acid of sulfuric acid and nitric acid (the mass ratio of sulfuric acid to nitric acid is 1:1, and 400 mL of the mixed acid solution is added to 1 g of graphitic carbon nitride), and perform ultrasonic stirring treatment for 4 h. Finally, wash it with deionized water until neutral, dry it, and disperse it in deionized water to obtain a suspension (solid content is 3%).

[0043] S2: Add resorcinol and formaldehyde in a mass ratio of 1:1.4 to a 25 wt% Na 2 CO 3 aqueous solution (the mass of the solute Na 2 CO 3 is 8% of the mass of the phenolic compound), stir at 70 °C for 4 h, then slowly add the suspension obtained in step S1 (accounting for 10% of the total liquid volume). After continuously stirring for 60 min, add pure water to adjust the viscosity of the mixed solution to 1200 mPa·s. Spray-dry the mixed solution, with an inlet temperature of 130 °C and a feeding rate of 0.5 L / min to obtain dry powder (D50 is controlled at 7 - 8 μm).

[0044] S3: Place the NaF powder in a high-frequency plasma reaction chamber, with argon as the protective gas. Set the working current of the plasma generator to 100 A and the working voltage to 130 V to form NaF vapor in the reaction chamber. Place the dry powder (the mass ratio of NaF powder to dry powder is 0.15:1) in the condensation chamber, with the rotation speed of the condensation chamber being 0.8 r / min. The NaF vapor is sent into the condensation chamber by 1.3 L / min of argon to contact the dry powder, and NaF is deposited on its surface and in the pores.

[0045] S4: Place the dry powder deposited with NaF in a CVD rotary furnace, with the rotation speed of the furnace tube being 0.3 r / min. Nitrogen is passed throughout step S4, with a nitrogen flow rate of 1 L / min. Heat-treat in two stages. The first heat treatment: Heat it to 650 °C at a heating rate of 2 °C / min and hold for 1.5 h. The second heat treatment: Heat it to 900 °C at a heating rate of 5 °C / min, hold for 1 h, then pass acetylene and hold for 2 h, with an acetylene flow rate of 0.3 L / min. After cooling, sieve the obtained powder through a 300-mesh sieve to obtain the doped hard carbon anode material.

[0046] Example 3

[0047] Preparation of doped hard carbon anode material:

[0048] S1: Place 10 g of dicyandiamide in a tubular furnace, heat it to 600 °C at a rate of 5 °C / min under a nitrogen atmosphere, hold for 1.5 h to obtain graphitic carbon nitride. After grinding it into powder, place it in a mixed acid of sulfuric acid and nitric acid (the mass ratio of sulfuric acid to nitric acid is 1:1, and 400 mL of the mixed acid solution is added to 1 g of graphitic carbon nitride), carry out ultrasonic stirring treatment for 5 h, and finally wash it with deionized water until neutral. After drying, disperse it in deionized water to obtain a suspension (solid content is 8%).

[0049] S2: Add hydroquinone and trioxymethylene in a mass ratio of 1:1.6 to 32 wt% KOH solution (the mass of solute KOH is 9% of the mass of phenolic compounds). Stir at 80 °C for 5 h, then slowly add the suspension obtained in step S1 (accounting for 8% of the total solution volume). After continuously stirring for 60 min, add pure water to adjust the viscosity of the mixed solution to 800 mPa·s. Spray-dry the mixed solution, with an inlet temperature of 130 °C and a feeding rate of 0.5 L / min to obtain dry powder (D50 is controlled at 8 - 9 μm).

[0050] S3: Place NaF powder in a high-frequency plasma reaction chamber, use argon as the protective gas, set the working current of the plasma generator to 110 A and the working voltage to 110 V to form NaF vapor in the reaction chamber. Place the dry powder (the mass ratio of NaF powder to dry powder is 0.22:1) in the condensation chamber, with the rotation speed of the condensation chamber being 0.5 r / min. The NaF vapor is sent into the condensation chamber by 1.1 L / min of argon to contact the dry powder, and NaF is deposited on its surface and pores.

[0051] S4: Place the dry powder deposited with NaF in a CVD rotary furnace, with the rotation speed of the furnace tube being 0.5 r / min. Nitrogen is passed throughout step S4, and the nitrogen flow rate is 1.2 L / min. Heat-treat in two stages. The first heat treatment: Heat it to 700 °C at a heating rate of 5 °C / min and hold for 1.5 h. The second heat treatment: Heat it to 950 °C at a heating rate of 5 °C / min, hold for 1 h, then pass acetylene and hold for 2 h, with the acetylene flow rate being 0.3 L / min. After cooling, sieve the obtained powder through a 300-mesh sieve to obtain the doped hard carbon negative electrode material.

[0052] Example 4

[0053] Preparation of doped hard carbon negative electrode material:

[0054] S1: Place 10 g of melamine in a tubular furnace, heat it to 550 °C at a rate of 5 °C / min under a nitrogen atmosphere, hold for 2 h to obtain graphitic carbon nitride. After grinding it into powder, place it in a mixed acid of sulfuric acid and nitric acid (the mass ratio of sulfuric acid to nitric acid is 1:1, and 400 mL of the mixed acid solution is added to 1 g of graphitic carbon nitride), perform ultrasonic stirring for 5 h, and finally wash it with deionized water until neutral. After drying, disperse it in deionized water to obtain a suspension (solid content is 5%).

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

[0056] S3: Place NaF powder in a high-frequency plasma reaction chamber, use argon as the protective gas, set the working current of the plasma generator to 110 A and the working voltage to 120 V to form NaF vapor in the reaction chamber. Place the dry powder (the mass ratio of NaF powder to dry powder is 0.2:1) in the condensation chamber, with the rotation speed of the condensation chamber being 1 r / min. The NaF vapor is sent into the condensation chamber by 1.2 L / min of argon to contact the dry powder, and NaF is deposited on its surface and in the pores.

[0057] S4: Place the dry powder deposited with NaF in a CVD rotary furnace, with the rotation speed of the furnace tube being 0.5 r / min. Nitrogen is passed throughout step S4, with a nitrogen flow rate of 1.5 L / min. Heat it to 950 °C at a rate of 5 °C / min, hold for 1 h, then pass methane, hold for 2 h, and the methane flow rate is 0.45 L / min. After cooling, sieve the obtained powder through a 300-mesh sieve to obtain a doped hard carbon anode material.

[0058] Comparative Example 1

[0059] Prepare a hard carbon anode material:

[0060] (1) Add resorcinol and formaldehyde in a mass ratio of 1:1.6 to 28 wt% aqueous NaOH solution (the mass of solute NaOH is 5% of the mass of resorcinol), stir at 70 °C for 4.5 h, then slowly add pure water to adjust the viscosity of the mixed solution to 1000 mPa·s. Spray-dry the mixed solution, with an inlet temperature of 120 °C and a feeding rate of 0.35 L / min to obtain a phenolic resin dry powder (D50 is controlled at 7 - 8 μm).

[0061] (2) Place the NaF powder in a high-frequency plasma reaction chamber with argon as the protective gas. Set the working current of the plasma generator to 110 A and the working voltage to 120 V to form NaF vapor in the reaction chamber. Place the phenolic resin dry powder (where the mass ratio of NaF powder to phenolic resin dry powder is 0.2:1) in the condensation chamber with a rotation speed of 1 r / min. The NaF vapor is sent into the condensation chamber by 1.2 L / min of argon to contact the phenolic resin dry powder, and NaF is deposited on its surface and in the pores.

[0062] (3) Place the phenolic resin dry powder deposited with NaF in a CVD rotary furnace with a furnace tube rotation speed of 0.5 r / min. Nitrogen is passed throughout the process with a nitrogen flow rate of 1.5 L / min. Heat treatment is carried out in two stages. The first-stage heat treatment: heat up to 600 °C at a heating rate of 5 °C / min and hold for 2 h. The second-stage heat treatment: heat up to 950 °C at a heating rate of 5 °C / min, introduce methane after holding for 1 h, hold for 2 h with a methane flow rate of 0.45 L / min. After cooling, sieve the obtained powder through a 300-mesh sieve to obtain the hard carbon anode material.

[0063] Comparative Example 2

[0064] Preparation of hard carbon anode material:

[0065] (1) Place 10 g of melamine in a tubular furnace and heat it up to 550 °C at a rate of 5 °C / min in a nitrogen environment, hold for 2 h to obtain graphitic carbon nitride. After grinding it into powder, place it in a mixed acid (the mass ratio of sulfuric acid to nitric acid is 1:1, and 400 mL of mixed acid solution is added to 1 g of graphitic carbon nitride) and perform ultrasonic stirring treatment for 5 h. 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 resorcinol and formaldehyde in a mass ratio of 1:1.6 to a 28 wt% NaOH aqueous solution (the mass of solute NaOH is 5% of the mass of resorcinol). Stir at 70 °C for 4.5 h, then slowly add the suspension obtained in step (1) (accounting for 10% of the total solution volume). After continuously stirring for 60 min, add pure water to adjust the viscosity of the mixed solution to 1000 mPa·s. Spray-dry the mixed solution with an inlet temperature of 120 °C and a feeding rate of 0.35 L / min to obtain a dry powder (D50 controlled at 7 - 8 μm).

[0067] (3) Place the obtained dry powder in a CVD rotary furnace with a furnace tube rotation speed of 0.5 r / min. Nitrogen is passed throughout the process with a nitrogen flow rate of 1.5 L / min. Heat treatment is carried out in two stages. The first-stage heat treatment: Heat up to 600 °C at a heating rate of 5 °C / min and hold for 2 h. The second-stage heat treatment: Heat up to 950 °C at a heating rate of 5 °C / min, pass methane after holding for 1 h, hold for 2 h, and the methane flow rate is 0.45 L / min. After cooling, sieve the obtained powder through a 300-mesh sieve to obtain the hard carbon anode material.

[0068] Comparative Example 3

[0069] Preparation of hard carbon anode material:

[0070] (1) Place 10 g of melamine in a tubular furnace. Under a nitrogen atmosphere, heat up to 550 °C at a rate of 5 °C / min and hold for 2 h to obtain graphitic carbon nitride. After grinding it into powder, place it in a mixed acid (the mass ratio of sulfuric acid to nitric acid is 1:1, and 400 mL of the mixed acid solution is added to 1 g of graphitic carbon nitride) and perform ultrasonic stirring treatment for 5 h. Finally, wash it with deionized water until neutral, dry it, and disperse it in deionized water to obtain suspension A (with a concentration of 5%).

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

[0072] (3) Place the NaF powder in a high-frequency plasma reaction chamber with argon as the protective gas. Set the working current of the plasma generator to 110 A and the working voltage to 120 V to form NaF vapor in the reaction chamber. Place the dry powder (the mass ratio of NaF powder to dry powder B is 0.2:1) in the condensation chamber with a condensation chamber rotation speed of 1 r / min. The NaF vapor is sent into the condensation chamber by 1.2 L / min of argon to contact the dry powder, and NaF is deposited on its surface and in the pores.

[0073] (4) Place the dry powder deposited with NaF in a CVD rotary furnace with a furnace tube rotation speed of 0.5 r / min. Nitrogen is passed throughout the process with a nitrogen flow rate of 1.5 L / min. Heat treatment is carried out in two stages. The first-stage heat treatment: Heat up to 600 °C at a heating rate of 5 °C / min and hold for 2 h. The second-stage heat treatment: Heat up to 950 °C at a heating rate of 5 °C / min and hold for 2 h. After cooling, sieve the obtained powder through a 300-mesh sieve to obtain the hard carbon anode material.

[0074] Performance test:

[0075] The negative electrode materials, conductive agent SP (conductive carbon black), binder CMC (carboxymethyl cellulose), and binder SBR (styrene-butadiene rubber) obtained in Examples 1-3 and Comparative Examples 1-3 were subjected to powder resistance testing, and the prepared finished materials were made into type 2032 button batteries for evaluation.

[0076] Specifically, the prepared negative electrode material, conductive agent SP, binder CMC, and binder SBR were mixed in a mass ratio of 94.5:2:1.2:2.3. Using water as a solvent, the slurry was coated on copper foil. The counter electrode was a sodium sheet, the separator was a sodium ion conductive glass fiber separator, the charge and discharge cut-off voltage was 0.005 - 2.0 V, the discharge rate was first discharged at 0.1 C to 0.005 V, and then discharged at 0.02 C to 0.05 C to ensure sufficient discharge. The charge rate was 0.1 C to charge to 2.0 V.

[0077] The finished materials obtained in Example 1 and Comparative Examples 1-3 were made into soft-pack batteries for evaluation.

[0078] Specifically, the prepared materials, conductive agent SP, and binder CMC + SBR were dispersed and pulped in a mass ratio of 94.5:2:1.2:2.3. After processes such as coating, rolling, and slitting for cell preparation, a soft-pack battery was made in combination with a sodium nickel manganese iron oxide positive electrode.

[0079] Test the performance of the hard carbon negative electrode material in a soft-pack battery.

[0080] Specific test items are as follows: 1. Perform rate performance testing at a charge and discharge voltage range of 1.5 - 3.95 V at 25 °C. First, perform capacity calibration at a constant current charge and discharge rate of 1 / 3 C. While maintaining a charge rate of 1 C, the discharge rates are 1 C, 2 C, 3 C, and 4 C respectively. Test the ratio of the capacity of the cell at high rates to that at 1 / 3 C. Finally, test the capacity recovery at a charge and discharge rate of 1 / 3 C. 2. Perform cycle performance testing at a 1 C rate at a charge and discharge voltage range of 1.5 - 3.95 V at 25 °C.

[0081] The test results are shown in Table 1 - Table 3 and Figure 1 - Figure 2 as follows.

[0082] Table 1 Test Results of Powder Resistance and Electrochemical Performance of Examples and Comparative Examples

[0083] Number Powder resistivity / Ω / cm Initial sodium intercalation capacity / mAh / g Initial sodium deintercalation capacity / mAh / g Initial efficiency / % 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 smaller than that of Comparative Examples 1-3. Compared with Comparative Example 1, the powder resistivity of the hard carbon negative electrode material in Comparative Examples 2-3 is smaller because in-situ doping is achieved by using oxygen-doped carbon nitride as the nitrogen source, reducing the intrinsic resistance of the material. In-situ doping, plasma formation of a pre-sodiation layer, and carbon coating greatly improve the first efficiency of the hard carbon material.

[0085] Table 2 Test results of discharge capacity retention rate of examples and comparative examples

[0086] 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 Table of test results of discharge capacity retention rate of examples and comparative examples

[0088] 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 Table 2 and Table 3, Examples 1-4 have higher rate performance and better cycling performance than Comparative Examples 1-3, mainly because the powder resistance of the examples is smaller, and at the same time, in-situ doping of nitrogen atoms accelerates electron transfer, and pre-sodiation and carbon coating consume less lithium ions, thus showing better electrochemical performance.

[0090] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the present invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A method for preparing a doped hard carbon negative electrode material, characterized in that: The following steps are involved: S1: placing a nitrogen-containing precursor in a protective gas atmosphere and heating it to 400-600°C, keeping the temperature for 1-5 hours, to obtain graphite phase carbon nitride; grinding the graphite phase carbon nitride into powder and mixing it with acid solution, ultrasonically stirring it for 1-5 hours, washing it with water until it is neutral; after drying, dispersing it in water to obtain a suspension; S2: mixing the phenolic compound and the aldehyde compound with the alkaline aqueous solution for reaction, then uniformly mixing with the suspension, and spray drying to obtain a dry powder; S3: NaF is placed in a high-frequency plasma reaction chamber to form NaF vapor, and the dry powder is sent to a condensation chamber. The NaF vapor enters the condensation chamber and contacts the dry powder, and NaF is deposited on the surface and pores of the dry powder; S4: In an atmosphere of protective gas, the dry powder on which NaF is deposited is heat treated, a gaseous carbon source is introduced for carbon coating, the temperature is reduced, and the powder is sieved to obtain a doped hard carbon negative electrode material.

2. The method for preparing a doped hard carbon negative electrode material according to claim 1, characterized in that: In the 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, and the ratio of the mass of the graphite phase 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 negative electrode material according to claim 1, characterized in that: In the 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 polyformaldehyde.

4. The method for preparing a doped hard carbon negative electrode 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 ratio of the mass of the solute in the alkaline aqueous solution to the mass of the phenolic compound is 5-10:100; the reaction temperature is 30-90°C, and the reaction time is 1-10h; the volume of the added suspension accounts for 5-15% of the volume of the total liquid.

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

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

1.

7. The method for preparing a doped hard carbon negative electrode material according to claim 1, characterized in that: In the step S4, the protective gas is nitrogen, and the flow rate of nitrogen is 1-5L / min; when the dry powder deposited with NaF is heat treated, it is carried out in two stages; the first stage: heating to 400-800°C at a heating rate of 1-5°C / min, and keeping warm for 1-3h, the second stage: heating to 850-1200°C at a heating rate of 1-10°C / min, and keeping warm for 0.5-2h.

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

9. A doped hard carbon negative electrode material, characterized in that: The invention is prepared by the preparation method according to any one of claims 1 to 8.

10. A sodium ion battery, characterized in that: Comprising the doped hard carbon negative electrode material as claimed in claim 9.

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