Preparation method of hard carbon anode material for double-coated sodium batteries

Abstract

This invention discloses a method for preparing a double-coated hard carbon anode material for sodium batteries, comprising the following steps: hard carbon pretreatment, preparation of hard carbon-carbon nanotube composite, preparation of composite dopant solution and composite oxidant solution, and in-situ polymerization coating of hard carbon. First, by adding a dispersant and combining it with ultrasonic dispersion, uniform loading of carbon nanotubes on the surface of hard carbon is achieved, preventing carbon nanotube aggregation. Then, in-situ polymerization is used to coat polypyrrole onto the surface of the hard carbon-carbon nanotube composite, forming a three-dimensional conductive network of polypyrrole-carbon nanotubes. This continuous conductive network significantly increases the transport channels for electrons and sodium ions.

Description

Technical Field

[0001] This invention relates to the field of sodium batteries, and in particular to a method for preparing a double-coated hard carbon anode material for sodium batteries. Background Technology

[0002] Sodium-ion batteries, due to the abundant and uniform distribution of sodium resources and their low cost, have become one of the most promising alternatives to lithium-ion batteries, with broad application prospects in fields such as power batteries and large-scale energy storage. Hard carbon, as the mainstream anode material for sodium-ion batteries, has advantages such as low price, large interlayer spacing, and good reversibility of sodium-ion intercalation / deintercalation. However, pure hard carbon suffers from problems such as low initial coulombic efficiency, poor conductivity, and a tendency for volume expansion during cycling, leading to SEI film rupture and reconstruction, which limits its application in high-power, long-life sodium-ion batteries.

[0003] Existing hard carbon anodes possess a large interlayer spacing, which is beneficial for sodium ion insertion / extraction, making them highly suitable for sodium-ion batteries. To further improve the performance of hard carbon anodes, most manufacturers modify the hard carbon through coating. The core drawbacks of existing hard carbon coating methods are concentrated in the coating layer, modification effect, preparation process, and industrial application: single polymer coating is prone to problems such as discontinuous conductive network, weak bonding with hard carbon, difficulty in controlling the polymerization process, resulting in uneven coating layer, and limited modification effect of single dopant. Therefore, it is necessary to propose a new solution to address these issues. Summary of the Invention

[0004] In view of this, the present invention addresses the deficiencies of the existing technology, and its main objective is to provide a method for preparing a double-coated hard carbon anode material for sodium batteries. This method can effectively solve the problems of discontinuous conductive network, weak bonding force with hard carbon, and difficulty in controlling the polymerization process of existing coated hard carbon anodes, which lead to uneven coating layers and limited performance improvement.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a double-coated hard carbon anode material for sodium batteries includes the following steps: (1) Hard carbon pretreatment Hard carbon was placed in a vacuum drying oven and dried at 110°C for 5 hours to obtain pretreated hard carbon. (2) Preparation of carbon nanotube dispersion Carbon nanotubes were added to deionized water, followed by sodium dodecylbenzenesulfonate. The mixture was then ultrasonically dispersed for 35 minutes at a power of 350 W to obtain a carbon nanotube dispersion. In the dispersion, the mass fraction of carbon nanotubes was 0.8%, and the mass fraction of sodium dodecylbenzenesulfonate was 0.8% of the carbon nanotubes. (3) Preparation of hard carbon-carbon nanotube composite Add 8.5-9.5g of the pretreated hard carbon obtained in step (1) to the carbon nanotube dispersion obtained in step (2). The carbon nanotube content in the carbon nanotube dispersion is 0.2-0.4g. Mix ultrasonically for 25min with an ultrasonic power of 350w. Stir at a speed of 250r / min for 1.5h. Filter the mixture and dry the resulting filter cake at 85℃ to constant weight to obtain the hard carbon-carbon nanotube composite. (4) Preparation of composite dopant solution and composite oxidant solution A composite dopant solution with a mass concentration of 1.2% was prepared by adding the composite dopant to deionized water and the mass of the composite oxidant was 0.108-0.22 g. A composite oxidant solution with a mass concentration of 1.0 mol / L was prepared by adding the composite oxidant to deionized water and the mass of the composite oxidant was 3.54-3.84 g. (5) In-situ polymerization coating of hard carbon Take 8.7-9.9g of the hard carbon-carbon nanotube composite obtained in step (3) and 0.5g of pyrrole and add it to deionized water. Mix it ultrasonically for 18min with an ultrasonic power of 300W. Then place it in a low temperature bath at 5℃. Add the composite dopant solution obtained in step (4) and stir for 30min. Then add the composite oxidant solution obtained in step (4) at a rate of 1.5 drops / second. After the addition is complete, stir the reaction at 5℃ for 10h. Filter the mixture and wash the resulting filter cake with deionized water until neutral. Then, vacuum dry it to constant weight at 60℃ and -0.09MPa to obtain the double-coated hard carbon anode material for sodium batteries.

[0006] As a preferred embodiment, in step (2), the carbon nanotube is a multi-walled carbon nanotube.

[0007] As a preferred embodiment, in step (4), the composite oxidant is composed of p-aminobenzenesulfonic acid and aminophosphoric acid, wherein the mass of p-aminobenzenesulfonic acid is 0.08-0.12g and the mass of aminophosphoric acid is 0.1g; As a preferred embodiment, in step (4), the composite oxidant is composed of ferric chloride and manganese sulfate, wherein the mass of ferric chloride is 3.04 g and the mass of manganese sulfate is 0.5-0.8 g.

[0008] Compared with the prior art, the present invention has obvious advantages and beneficial effects. Specifically, as can be seen from the above technical solution: First, by adding a dispersant and combining it with ultrasonic dispersion, uniform loading of carbon nanotubes on the hard carbon surface was achieved, avoiding the aggregation of carbon nanotubes. Then, in-situ polymerization was used to coat polypyrrole on the surface of the hard carbon-carbon nanotube composite, forming a three-dimensional conductive network of polypyrrole-carbon nanotubes. This made the conductive network continuous and significantly increased the transport channels for electrons and sodium ions.

[0009] Secondly, a composite dopant of p-aminobenzenesulfonic acid and aminophosphoric acid is used. The sulfonic acid and phosphate groups synergistically enhance the conductivity of polypyrrole. At the same time, the amino groups of both form chemical bonds with hard carbon and polypyrrole, which greatly improves the bonding force between the polypyrrole-carbon nanotube coating layer and the hard carbon substrate, effectively preventing the coating layer from falling off during cycling. Furthermore, a composite oxidant of ferric chloride and manganese sulfate can precisely control the polymerization rate of pyrrole, making the coating layer dense and uniform with a thickness controlled at 5-10 nm. This inhibits the excessive growth of the SEI film without hindering the insertion and extraction of sodium ions.

[0010] To more clearly illustrate the structural features and effects of the present invention, the present invention will be described in detail below with reference to specific embodiments: Detailed Implementation This invention discloses a method for preparing a double-coated hard carbon anode material for sodium batteries, comprising the following steps: (1) Hard carbon pretreatment Hard carbon was placed in a vacuum drying oven and dried at 110°C for 5 hours to obtain pretreated hard carbon. (2) Preparation of carbon nanotube dispersion Carbon nanotubes were added to deionized water, followed by sodium dodecylbenzenesulfonate. The mixture was then ultrasonically dispersed for 35 minutes at a power of 350 W to obtain a carbon nanotube dispersion. The carbon nanotubes in the dispersion had a mass fraction of 0.8%, and the sodium dodecylbenzenesulfonate had a mass fraction of 0.8% of the carbon nanotubes. The carbon nanotubes were multi-walled carbon nanotubes.

[0011] (3) Preparation of hard carbon-carbon nanotube composite Add 8.5-9.5g of the pretreated hard carbon obtained in step (1) to the carbon nanotube dispersion obtained in step (2). The carbon nanotube content in the carbon nanotube dispersion is 0.2-0.4g. Mix ultrasonically for 25min with an ultrasonic power of 350w. Stir at a speed of 250r / min for 1.5h. Filter the mixture and dry the resulting filter cake at 85℃ to constant weight to obtain the hard carbon-carbon nanotube composite. (4) Preparation of composite dopant solution and composite oxidant solution A composite dopant solution with a mass concentration of 1.2% was prepared by adding the composite dopant to deionized water, with the composite oxidant weighing 0.108-0.22 g. Another composite oxidant solution with a mass concentration of 1.0 mol / L was prepared by adding the composite oxidant to deionized water, with the composite oxidant weighing 3.54-3.84 g. The composite oxidant consisted of p-aminobenzenesulfonic acid and aminophosphoric acid, wherein the mass of p-aminobenzenesulfonic acid was 0.08-0.12 g and the mass of aminophosphoric acid was 0.1 g. The composite oxidant also consisted of ferric chloride and manganese sulfate, wherein the mass of ferric chloride was 3.04 g and the mass of manganese sulfate was 0.5-0.8 g.

[0012] (5) In-situ polymerization coating of hard carbon Take 8.7-9.9g of the hard carbon-carbon nanotube composite obtained in step (3) and 0.5g of pyrrole and add it to deionized water. Mix it ultrasonically for 18min with an ultrasonic power of 300W. Then place it in a low temperature bath at 5℃. Add the composite dopant solution obtained in step (4) and stir for 30min. Then add the composite oxidant solution obtained in step (4) at a rate of 1.5 drops / second. After the addition is complete, stir the reaction at 5℃ for 10h. Filter the mixture and wash the resulting filter cake with deionized water until neutral. Then, vacuum dry it to constant weight at 60℃ and -0.09MPa to obtain the double-coated hard carbon anode material for sodium batteries.

[0013] The following detailed description is provided in conjunction with several embodiments and comparative examples.

[0014] Example 1 (1) Hard carbon pretreatment Hard carbon was placed in a vacuum drying oven and dried at 110°C for 5 hours to obtain pretreated hard carbon. (2) Preparation of carbon nanotube dispersion Carbon nanotubes were added to deionized water, followed by sodium dodecylbenzenesulfonate. The mixture was then ultrasonically dispersed for 35 minutes at a power of 350 W to obtain a carbon nanotube dispersion. The carbon nanotubes in the dispersion had a mass fraction of 0.8%, and the sodium dodecylbenzenesulfonate had a mass fraction of 0.8% of the carbon nanotubes. The carbon nanotubes were multi-walled carbon nanotubes.

[0015] (3) Preparation of hard carbon-carbon nanotube composite Add 9.0g of the pretreated hard carbon obtained in step (1) to the carbon nanotube dispersion obtained in step (2). The carbon nanotube content in the carbon nanotube dispersion is 0.3g. Mix ultrasonically for 25min with an ultrasonic power of 350w. Stir at a speed of 250r / min for 1.5h. Filter and dry the obtained filter cake at 85℃ to constant weight to obtain the hard carbon-carbon nanotube composite. (4) Preparation of composite dopant solution and composite oxidant solution A composite dopant was added to deionized water to prepare a composite dopant solution with a mass concentration of 1.2%. A composite oxidant was added to deionized water to prepare a composite oxidant solution with a concentration of 1.0 mol / L. The composite oxidant is composed of p-aminobenzenesulfonic acid and aminophosphoric acid, wherein the mass of p-aminobenzenesulfonic acid is 0.1 g and the mass of aminophosphoric acid is 0.1 g. The composite oxidant is composed of ferric chloride and manganese sulfate, wherein the mass of ferric chloride is 3.04 g and the mass of manganese sulfate is 0.6 g.

[0016] (5) In-situ polymerization coating of hard carbon Take 9.3g of the hard carbon-carbon nanotube composite obtained in step (3) and 0.5g of pyrrole and add it to deionized water. Mix it ultrasonically for 18min with an ultrasonic power of 300W. Then place it in a low temperature bath at 5℃. Add the composite dopant solution obtained in step (4) and stir for 30min. Then add the composite oxidant solution obtained in step (4) at a rate of 1.5 drops / second. After the addition is complete, stir and react for 10h at 5℃. Filter the mixture and wash the resulting filter cake with deionized water until neutral. Then, vacuum dry it to constant weight at 60℃ and -0.09MPa to obtain the double-coated hard carbon anode material for sodium batteries.

[0017] Example 2 (1) Hard carbon pretreatment Hard carbon was placed in a vacuum drying oven and dried at 110°C for 5 hours to obtain pretreated hard carbon. (2) Preparation of carbon nanotube dispersion Carbon nanotubes were added to deionized water, followed by sodium dodecylbenzenesulfonate. The mixture was then ultrasonically dispersed for 35 minutes at a power of 350 W to obtain a carbon nanotube dispersion. The carbon nanotubes in the dispersion had a mass fraction of 0.8%, and the sodium dodecylbenzenesulfonate had a mass fraction of 0.8% of the carbon nanotubes. The carbon nanotubes were multi-walled carbon nanotubes.

[0018] (3) Preparation of hard carbon-carbon nanotube composite Add 8.5g of the pretreated hard carbon obtained in step (1) to the carbon nanotube dispersion obtained in step (2). The carbon nanotube content in the carbon nanotube dispersion is 0.2g. Mix ultrasonically for 25min with an ultrasonic power of 350w. Stir at 250r / min for 1.5h. Filter and dry the obtained filter cake at 85℃ to constant weight to obtain the hard carbon-carbon nanotube composite. (4) Preparation of composite dopant solution and composite oxidant solution A composite dopant was added to deionized water to prepare a composite dopant solution with a mass concentration of 1.2%. A composite oxidant was added to deionized water to prepare a composite oxidant solution with a concentration of 1.0 mol / L. The composite oxidant is composed of p-aminobenzenesulfonic acid and aminophosphoric acid, wherein the mass of p-aminobenzenesulfonic acid is 0.08 g and the mass of aminophosphoric acid is 0.1 g. The composite oxidant is composed of ferric chloride and manganese sulfate, wherein the mass of ferric chloride is 3.04 g and the mass of manganese sulfate is 0.5 g.

[0019] (5) In-situ polymerization coating of hard carbon Take 8.7g of the hard carbon-carbon nanotube composite obtained in step (3) and 0.5g of pyrrole and add it to deionized water. Mix it ultrasonically for 18min with an ultrasonic power of 300W. Then place it in a low temperature bath at 5℃. Add the composite dopant solution obtained in step (4) and stir for 30min. Then add the composite oxidant solution obtained in step (4) at a rate of 1.5 drops / second. After the addition is complete, stir the reaction at 5℃ for 10h. Filter the mixture and wash the resulting filter cake with deionized water until neutral. Then, vacuum dry it to constant weight at 60℃ and -0.09MPa to obtain the double-coated hard carbon anode material for sodium batteries.

[0020] Example 3 (1) Hard carbon pretreatment Hard carbon was placed in a vacuum drying oven and dried at 110°C for 5 hours to obtain pretreated hard carbon. (2) Preparation of carbon nanotube dispersion Carbon nanotubes were added to deionized water, followed by sodium dodecylbenzenesulfonate. The mixture was then ultrasonically dispersed for 35 minutes at a power of 350 W to obtain a carbon nanotube dispersion. The carbon nanotubes in the dispersion had a mass fraction of 0.8%, and the sodium dodecylbenzenesulfonate had a mass fraction of 0.8% of the carbon nanotubes. The carbon nanotubes were multi-walled carbon nanotubes.

[0021] (3) Preparation of hard carbon-carbon nanotube composite Add 9.5g of the pretreated hard carbon obtained in step (1) to the carbon nanotube dispersion obtained in step (2). The carbon nanotube content in the carbon nanotube dispersion is 0.4g. Mix ultrasonically for 25min with an ultrasonic power of 350w. Stir at a speed of 250r / min for 1.5h. Filter and dry the obtained filter cake at 85℃ to constant weight to obtain the hard carbon-carbon nanotube composite. (4) Preparation of composite dopant solution and composite oxidant solution A composite dopant was added to deionized water to prepare a composite dopant solution with a mass concentration of 1.2%. A composite oxidant was added to deionized water to prepare a composite oxidant solution with a concentration of 1.0 mol / L. The composite oxidant is composed of p-aminobenzenesulfonic acid and aminophosphoric acid, wherein the mass of p-aminobenzenesulfonic acid is 0.12 g and the mass of aminophosphoric acid is 0.1 g. The composite oxidant is composed of ferric chloride and manganese sulfate, wherein the mass of ferric chloride is 3.04 g and the mass of manganese sulfate is 0.8 g.

[0022] (5) In-situ polymerization coating of hard carbon Take 9.9g of the hard carbon-carbon nanotube composite obtained in step (3) and 0.5g of pyrrole and add it to deionized water. Mix it ultrasonically for 18min with an ultrasonic power of 300W. Then place it in a low temperature bath at 5℃. Add the composite dopant solution obtained in step (4) and stir for 30min. Then add the composite oxidant solution obtained in step (4) at a rate of 1.5 drops / second. After the addition is complete, stir the reaction at 5℃ for 10h. Filter the mixture and wash the resulting filter cake with deionized water until neutral. Then, vacuum dry it to constant weight at 60℃ and -0.09MPa to obtain the double-coated hard carbon anode material for sodium batteries.

[0023] Example 4 (1) Hard carbon pretreatment Hard carbon was placed in a vacuum drying oven and dried at 110°C for 5 hours to obtain pretreated hard carbon. (2) Preparation of carbon nanotube dispersion Carbon nanotubes were added to deionized water, followed by sodium dodecylbenzenesulfonate. The mixture was then ultrasonically dispersed for 35 minutes at a power of 350 W to obtain a carbon nanotube dispersion. The carbon nanotubes in the dispersion had a mass fraction of 0.8%, and the sodium dodecylbenzenesulfonate had a mass fraction of 0.8% of the carbon nanotubes. The carbon nanotubes were multi-walled carbon nanotubes.

[0024] (3) Preparation of hard carbon-carbon nanotube composite Add 8.8g of the pretreated hard carbon obtained in step (1) to the carbon nanotube dispersion obtained in step (2). The carbon nanotube content in the carbon nanotube dispersion is 0.3g. Mix ultrasonically for 25min with an ultrasonic power of 350w. Stir at a speed of 250r / min for 1.5h. Filter and dry the obtained filter cake at 85℃ to constant weight to obtain hard carbon-carbon nanotube composite. (4) Preparation of composite dopant solution and composite oxidant solution A composite dopant was added to deionized water to prepare a composite dopant solution with a mass concentration of 1.2%. A composite oxidant was added to deionized water to prepare a composite oxidant solution with a concentration of 1.0 mol / L. The composite oxidant is composed of p-aminobenzenesulfonic acid and aminophosphoric acid, wherein the mass of p-aminobenzenesulfonic acid is 0.09 g and the mass of aminophosphoric acid is 0.1 g. The composite oxidant is composed of ferric chloride and manganese sulfate, wherein the mass of ferric chloride is 3.04 g and the mass of manganese sulfate is 0.55 g.

[0025] (5) In-situ polymerization coating of hard carbon Take 9.3g of the hard carbon-carbon nanotube composite obtained in step (3) and 0.5g of pyrrole and add it to deionized water. Mix it ultrasonically for 18min with an ultrasonic power of 300W. Then place it in a low temperature bath at 5℃. Add the composite dopant solution obtained in step (4) and stir for 30min. Then add the composite oxidant solution obtained in step (4) at a rate of 1.5 drops / second. After the addition is complete, stir and react for 10h at 5℃. Filter the mixture and wash the resulting filter cake with deionized water until neutral. Then, vacuum dry it to constant weight at 60℃ and -0.09MPa to obtain the double-coated hard carbon anode material for sodium batteries.

[0026] Example 5 (1) Hard carbon pretreatment Hard carbon was placed in a vacuum drying oven and dried at 110°C for 5 hours to obtain pretreated hard carbon. (2) Preparation of carbon nanotube dispersion Carbon nanotubes were added to deionized water, followed by sodium dodecylbenzenesulfonate. The mixture was then ultrasonically dispersed for 35 minutes at a power of 350 W to obtain a carbon nanotube dispersion. The carbon nanotubes in the dispersion had a mass fraction of 0.8%, and the sodium dodecylbenzenesulfonate had a mass fraction of 0.8% of the carbon nanotubes. The carbon nanotubes were multi-walled carbon nanotubes.

[0027] (3) Preparation of hard carbon-carbon nanotube composite Add 9.2g of the pretreated hard carbon obtained in step (1) to the carbon nanotube dispersion obtained in step (2). The carbon nanotube content in the carbon nanotube dispersion is 0.3g. Mix ultrasonically for 25min with an ultrasonic power of 350w. Stir at a speed of 250r / min for 1.5h. Filter and dry the obtained filter cake at 85℃ to constant weight to obtain hard carbon-carbon nanotube composite. (4) Preparation of composite dopant solution and composite oxidant solution A composite dopant was added to deionized water to prepare a composite dopant solution with a mass concentration of 1.2%. A composite oxidant was added to deionized water to prepare a composite oxidant solution with a concentration of 1.0 mol / L. The composite oxidant is composed of p-aminobenzenesulfonic acid and aminophosphoric acid, wherein the mass of p-aminobenzenesulfonic acid is 0.1 g and the mass of aminophosphoric acid is 0.1 g. The composite oxidant is composed of ferric chloride and manganese sulfate, wherein the mass of ferric chloride is 3.04 g and the mass of manganese sulfate is 0.7 g.

[0028] (5) In-situ polymerization coating of hard carbon Take 9.3g of the hard carbon-carbon nanotube composite obtained in step (3) and 0.5g of pyrrole and add it to deionized water. Mix it ultrasonically for 18min with an ultrasonic power of 300W. Then place it in a low temperature bath at 5℃. Add the composite dopant solution obtained in step (4) and stir for 30min. Then add the composite oxidant solution obtained in step (4) at a rate of 1.5 drops / second. After the addition is complete, stir and react for 10h at 5℃. Filter the mixture and wash the resulting filter cake with deionized water until neutral. Then, vacuum dry it to constant weight at 60℃ and -0.09MPa to obtain the double-coated hard carbon anode material for sodium batteries.

[0029] Example 6 (1) Hard carbon pretreatment Hard carbon was placed in a vacuum drying oven and dried at 110°C for 5 hours to obtain pretreated hard carbon. (2) Preparation of carbon nanotube dispersion Carbon nanotubes were added to deionized water, followed by sodium dodecylbenzenesulfonate. The mixture was then ultrasonically dispersed for 35 minutes at a power of 350 W to obtain a carbon nanotube dispersion. The carbon nanotubes in the dispersion had a mass fraction of 0.8%, and the sodium dodecylbenzenesulfonate had a mass fraction of 0.8% of the carbon nanotubes. The carbon nanotubes were multi-walled carbon nanotubes.

[0030] (3) Preparation of hard carbon-carbon nanotube composite Add 9.5g of the pretreated hard carbon obtained in step (1) to the carbon nanotube dispersion obtained in step (2). The carbon nanotube content in the carbon nanotube dispersion is 0.2g. Mix ultrasonically for 25min with an ultrasonic power of 350w. Stir at a speed of 250r / min for 1.5h. Filter and dry the obtained filter cake at 85℃ to constant weight to obtain the hard carbon-carbon nanotube composite. (4) Preparation of composite dopant solution and composite oxidant solution A composite dopant was added to deionized water to prepare a composite dopant solution with a mass concentration of 1.2%. A composite oxidant was added to deionized water to prepare a composite oxidant solution with a concentration of 1.0 mol / L. The composite oxidant is composed of p-aminobenzenesulfonic acid and aminophosphoric acid, wherein the mass of p-aminobenzenesulfonic acid is 0.09 g and the mass of aminophosphoric acid is 0.1 g. The composite oxidant is composed of ferric chloride and manganese sulfate, wherein the mass of ferric chloride is 3.04 g and the mass of manganese sulfate is 0.7 g.

[0031] (5) In-situ polymerization coating of hard carbon Take 8.8g of the hard carbon-carbon nanotube composite obtained in step (3) and 0.5g of pyrrole and add it to deionized water. Mix it ultrasonically for 18min with an ultrasonic power of 300W. Then place it in a low temperature bath at 5℃. Add the composite dopant solution obtained in step (4) and stir for 30min. Then add the composite oxidant solution obtained in step (4) at a rate of 1.5 drops / second. After the addition is complete, stir the reaction at 5℃ for 10h. Filter the mixture and wash the resulting filter cake with deionized water until neutral. Then, vacuum dry it to constant weight at 60℃ and -0.09MPa to obtain the double-coated hard carbon anode material for sodium batteries.

[0032] Comparative Example 1 (1) Hard carbon pretreatment Hard carbon was placed in a vacuum drying oven and dried at 110°C for 5 hours to obtain pretreated hard carbon. (2) Preparation of carbon nanotube dispersion Carbon nanotubes were added to deionized water, followed by sodium dodecylbenzenesulfonate. The mixture was then ultrasonically dispersed for 35 minutes at a power of 350 W to obtain a carbon nanotube dispersion. The carbon nanotubes in the dispersion had a mass fraction of 0.8%, and the sodium dodecylbenzenesulfonate had a mass fraction of 0.8% of the carbon nanotubes. The carbon nanotubes were multi-walled carbon nanotubes.

[0033] (3) Preparation of hard carbon-carbon nanotube composite Add 9.0g of the pretreated hard carbon obtained in step (1) to the carbon nanotube dispersion obtained in step (2). The carbon nanotube content in the carbon nanotube dispersion is 0.3g. Mix ultrasonically for 25min with an ultrasonic power of 350w. Stir at a speed of 250r / min for 1.5h. Filter and dry the obtained filter cake at 85℃ to constant weight to obtain the hard carbon-carbon nanotube composite. (4) Preparation of dopant solution and composite oxidant solution A dopant was added to deionized water to prepare a dopant solution with a mass concentration of 1.2%. A composite oxidant was added to deionized water to prepare a composite oxidant solution with a concentration of 1.0 mol / L. The oxidant was p-aminobenzenesulfonic acid, with a mass of 0.1 g. The composite oxidant was composed of ferric chloride and manganese sulfate, with a mass of 3.04 g of ferric chloride and a mass of 0.6 g of manganese sulfate.

[0034] (5) In-situ polymerization coating of hard carbon Take 9.3g of the hard carbon-carbon nanotube composite obtained in step (3) and 0.5g of pyrrole and add it to deionized water. Mix it ultrasonically for 18min with an ultrasonic power of 300W. Then place it in a low temperature bath at 5℃. Add the dopant solution obtained in step (4) and stir for 30min. Then add the composite oxidant solution obtained in step (4) at a rate of 1.5 drops / second. After the addition is complete, stir the reaction at 5℃ for 10h. Filter the mixture and wash the resulting filter cake with deionized water until neutral. Then, vacuum dry it to constant weight at 60℃ and -0.09MPa to obtain the hard carbon anode material for sodium batteries.

[0035] Performance tests were conducted on the above-mentioned multiple examples and comparative examples. The test methods are as follows: A negative electrode slurry was prepared using the hard carbon negative electrode material for sodium batteries obtained in the above examples and comparative examples as the active material. The slurry consisted of 93 parts by weight of hard carbon negative electrode material for sodium batteries, 2 parts of conductive carbon black, 3 parts of sodium alginate, 0.8 parts of aldehyde-terminated hyperbranched polyester, and 250 parts of deionized water. After coating (120 μm thickness), drying at 95°C, rolling, and slitting, a negative electrode sheet was obtained. A CR2025 type button cell was assembled (electrolyte: 1 mol / L NaPF6 dimethyl ether solution; sodium sheet as counter electrode; glass fiber membrane as separator). Constant current charge-discharge tests were performed with a voltage window of 0.01-2.5V. Test indicators included 0.1C discharge specific capacity, initial coulombic efficiency, 1C discharge specific capacity, and 0.1C discharge specific capacity retention rate after 200 cycles. The test results are shown in Table 1.

[0036]

[0037] Table 1 Analyzing the above data, compared with Comparative Example 1, Comparative Example 1 used a single dopant, and all other conditions and parameters were the same as in Example 1. However, the electrochemical performance of Example 1 was far superior to that of Comparative Example 1. This is because Example 1 used a combination of aminobenzenesulfonic acid and aminophosphoric acid for doping. The synergistic doping of sulfonic acid groups and phosphate groups improved the conductivity of polypyrrole, making the conductivity of Example 1 nearly twice that of Comparative Example 1. Furthermore, the amino groups of aminobenzenesulfonic acid and aminophosphoric acid reacted chemically with hard carbon and polypyrrole to form chemical bonds, which greatly improved the bonding force between the coating layer and hard carbon, prevented the coating layer from falling off, and made the generated SEI film more stable and undamaged, thus greatly improving the cycle performance.

[0038] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the technical scope of the present invention. Therefore, any minor modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a double-coated hard carbon anode material for sodium batteries, characterized in that: It includes the following steps: (1) Hard carbon pretreatment Hard carbon was placed in a vacuum drying oven and dried at 110°C for 5 hours to obtain pretreated hard carbon. (2) Preparation of carbon nanotube dispersion Carbon nanotubes were added to deionized water, followed by sodium dodecylbenzenesulfonate. The mixture was then ultrasonically dispersed for 35 minutes at a power of 350 W to obtain a carbon nanotube dispersion. In the dispersion, the mass fraction of carbon nanotubes was 0.8%, and the mass fraction of sodium dodecylbenzenesulfonate was 0.8% of the carbon nanotubes. (3) Preparation of hard carbon-carbon nanotube composite Add 8.5-9.5g of the pretreated hard carbon obtained in step (1) to the carbon nanotube dispersion obtained in step (2). The carbon nanotube content in the carbon nanotube dispersion is 0.2-0.4g. Mix ultrasonically for 25min with an ultrasonic power of 350w. Stir at a speed of 250r / min for 1.5h. Filter the mixture and dry the resulting filter cake at 85℃ to constant weight to obtain the hard carbon-carbon nanotube composite. (4) Preparation of composite dopant solution and composite oxidant solution A composite dopant solution with a mass concentration of 1.2% was prepared by adding the composite dopant to deionized water and the mass of the composite oxidant was 0.108-0.22 g. A composite oxidant solution with a mass concentration of 1.0 mol / L was prepared by adding the composite oxidant to deionized water and the mass of the composite oxidant was 3.54-3.84 g. (5) In-situ polymerization coating of hard carbon Take 8.7-9.9g of the hard carbon-carbon nanotube composite obtained in step (3) and 0.5g of pyrrole and add it to deionized water. Mix it ultrasonically for 18min with an ultrasonic power of 300W. Then place it in a low temperature bath at 5℃. Add the composite dopant solution obtained in step (4) and stir for 30min. Then add the composite oxidant solution obtained in step (4) at a rate of 1.5 drops / second. After the addition is complete, stir the reaction at 5℃ for 10h. Filter the mixture and wash the resulting filter cake with deionized water until neutral. Then, vacuum dry it to constant weight at 60℃ and -0.09MPa to obtain the double-coated hard carbon anode material for sodium batteries.

2. The method for preparing the double-coated hard carbon anode material for sodium batteries according to claim 1, characterized in that: In step (2), the carbon nanotubes are multi-walled carbon nanotubes.

3. The method for preparing the double-coated hard carbon anode material for sodium batteries according to claim 1, characterized in that: In step (4), the composite oxidant is composed of p-aminobenzenesulfonic acid and aminophosphoric acid, wherein the mass of p-aminobenzenesulfonic acid is 0.08-0.12g and the mass of aminophosphoric acid is 0.1g.

4. The method for preparing the double-coated hard carbon anode material for sodium batteries according to claim 1, characterized in that: In step (4), the composite oxidant is composed of ferric chloride and manganese sulfate, wherein the mass of ferric chloride is 3.04 g and the mass of manganese sulfate is 0.5-0.8 g.

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