Preparation method of a hybrid soft and hard carbon material and its application in sodium ion batteries

Abstract

The present invention relates to the field of preparation of negative electrode materials for sodium ion batteries, and in particular to a method for preparing a hybrid soft and hard carbon material and its application in sodium ion batteries. The method comprises mixing polyphosphazene and asphalt in a tetrahydrofuran solution, dissolving the asphalt and filling it into the pores rich in polyphosphazene nanotubes, and converting it into closed pores during a subsequent carbonization process, thereby repairing the surface and internal defects of the hard carbon. The method has the advantages of simple operation, low cost, high output, and environmental protection. The prepared hybrid soft and hard carbon material has a smaller specific surface area, fewer surface defects, a larger number of closed pores, and excellent electrical conductivity. When used as a negative electrode material for sodium ion batteries, it has excellent electrochemical properties.

Description

Technical Field

[0001] The present invention relates to the field of preparation of negative electrode materials for sodium ion batteries, and in particular to a preparation method of a mixed soft and hard carbon material and application thereof in sodium ion batteries. Background Art

[0002] Sodium-ion batteries have attracted widespread attention in large-scale energy storage due to their abundant sodium resources and cost advantages. However, the current commercial graphite anode is not suitable for sodium ion storage due to its thermodynamic instability with sodium ions. Therefore, the development of high-performance negative electrode materials is crucial to promote the commercialization of sodium-ion batteries. Hard carbon (HC) is a form of non-graphitized carbon with randomly oriented carbon layers and complex and diverse nanodomains, as well as a high proportion of closed nanopores. It is considered to be the most suitable negative electrode material for commercial sodium-ion batteries (SIBs). A large number of studies have emphasized the role of closed pores as a material for accommodating Na + The importance of efficient active sites, which contribute to the enhanced low-voltage (≤0.1 V) plateau capacity.

[0003] Therefore, existing technologies often use methods such as chemical activation, the introduction of templates, and metal element catalysis to adjust the closed-pore structure. While these methods can effectively increase the number and size of closed pores in HC and increase the plateau capacity, they are subject to unresolved issues such as high costs, environmental pollution, and increased experimental steps, which seriously hinder the practical application of sodium-ion batteries. Summary of the Invention

[0004] In response to the problems existing in the background technology, the present invention proposes a method for preparing a mixed soft and hard carbon material, in which asphalt is dissolved and filled into the pores of polyphosphazene nanotubes, and then the mixed soft and hard carbon material is obtained by carbonization.

[0005] Preferably, the specific preparation method is:

[0006] S1, preparing polyphosphazene nanotubes;

[0007] S2. Preparation of a polyphosphazene nanotube@asphalt precursor material: dissolving a certain amount of asphalt in an appropriate amount of solvent; then stirring uniformly at room temperature and adding a certain mass ratio of polyphosphazene nanotubes; the mass ratio of asphalt to polyphosphazene nanotubes being controlled at 1:2-1:10; continuing stirring to allow the dissolved asphalt to diffuse into the pores of the polyphosphazene nanotubes and deposit, thereby obtaining the polyphosphazene nanotube@asphalt precursor material;

[0008] S3. Preparation of mixed soft and hard carbon materials: The polyphosphazene nanotubes@asphalt precursor material obtained in S2 is carbonized to obtain mixed soft and hard carbon material powder.

[0009] Preferably, the preparation method of polyphosphazene nanotubes in S1 is: dissolving a certain proportion of hexachlorocyclotriphosphazene and 4,4-dihydroxydiphenyl sulfone in an appropriate amount of tetrahydrofuran, stirring and dissolving into a transparent solution, then adding an appropriate amount of acid binding agent, and then ultrasonicating at 40°C for 10 hours. After the reaction is completed, washing with acetone and deionized water respectively, centrifuging, and then drying to obtain polyphosphazene nanotubes.

[0010] Preferably, the solvent in S2 is one or more of tetrahydrofuran, N-methylpyrrolidone, benzene, toluene or carbon tetrachloride, the amount of the solvent is 20-100 ml, and the solution concentration is 5 g / L-10 g / L.

[0011] Preferably, the solvent in S2 is tetrahydrofuran, and after the magnetic stirring is completed, the mixture is heated to 50° C.-100° C. in an oil bath to volatilize the tetrahydrofuran.

[0012] Preferably, the two stirring steps in S2 are both magnetic stirring for 10 h.

[0013] Preferably, the carbonization treatment method in S3 is: the polyphosphazene nanotube @ asphalt precursor material is ground evenly and placed in a corundum crucible, and then the crucible is placed in a high-temperature tube furnace and passed with high-purity nitrogen as a protective gas. According to the actual situation, the temperature is increased at a rate of 1-5°C / min to 1300-1600°C for carbonization and kept warm for 1-3h. After the reaction is completed, it is naturally cooled to room temperature. The entire reaction process is carried out in an inert atmosphere. After the reaction is completed, a mixed soft and hard carbon material powder is obtained.

[0014] The present invention further proposes the application of mixed soft and hard carbon materials in sodium ion batteries, and the mixed soft and hard carbon materials prepared by the above-mentioned preparation method of mixed soft and hard carbon materials are applied to the preparation of negative electrode materials for sodium ion batteries.

[0015] Preferably, the application includes: fully mixing and grinding the prepared mixed soft and hard carbon material, conductive carbon black and sodium alginate in a mass ratio of 7:2:1-8:1:1, then transferring them to a glass bottle, adding an appropriate amount of deionized water and stirring overnight to form a viscous and evenly dispersed slurry, coating it on a copper foil with a scraper of different thicknesses, then vacuum drying it at 80°C for 12 h, and punching it into 12 mm sheets as sodium ion battery negative electrode materials; cutting the above sodium ion battery negative electrode materials into circular pole pieces with a diameter of 12 mm, and then using the positive electrode material, electrolyte, diaphragm, electrode shell and negative electrode material to assemble a battery.

[0016] Preferably, the positive electrode material is a sodium sheet; the separator is Whatman GF / D; and the electrolyte is a 1 M NaPF6 solution in DIGLYME = 100 Vol%.

[0017] Compared with the prior art, the present invention has the following beneficial technical effects:

[0018] The present invention dissolves asphalt and fills it into the pores of polyphosphazene nanotubes, and then obtains a mixed soft and hard carbon material through carbonization. The asphalt acts as a "binder" to connect the dispersed nanotubes together. After carbonization, the three-dimensionally interconnected asphalt is carbonized into graphitized carbon, which improves conductivity, promotes the transmission of electrons in the negative electrode material, shortens the transmission distance of sodium ions, and is more conducive to the diffusion of sodium ions. At the same time, the asphalt has fewer surface defects. After high-temperature carbonization, the asphalt will repair the defects on the HC surface as a surface coating. At the same time, the shrinkage and rearrangement of the carbon skeleton will cause the contact surface between the asphalt and PZS to form closed pores, increase the active sites for storing sodium ions, and thus increase the platform capacity in the low-pressure area. The change in specific surface area before carbonization can be known through BET. After filling with asphalt, the specific surface area decreases and the number of micropores is significantly reduced, indicating that the asphalt diffuses into the pores of PZS and repairs surface defects. The specific surface area of ​​the agglomerate after carbonization also increases from the original 76.6 m 2 g -1 Lowered to 5.8 m 2 g -1 . The present invention can also adjust the sp2 / sp3 content distribution in HC-XP after carbonization by adjusting the asphalt content. As the asphalt content increases, the ratio of sp2 / sp3 hybridized graphite carbon on the surface changes. By adjusting the sp2 / sp3 structure to a coordinated equilibrium state, the optimal solid electrolyte interface (SEI) is obtained during the battery cycle, achieving the optimal ICE and sodium ion storage capacity. In addition, since the soft and hard carbon composite materials of the present invention have a large number of closed pores, sp2 / sp3 coordinated balance and surface coating modification, the specific capacity, rate performance and long cycle life of the sodium ion battery negative electrode material based on the material of the present invention are significantly improved, and excellent cycle performance is also obtained after assembling the full battery, demonstrating practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of nitrogen adsorption and desorption of polyphosphazene nanotubes@asphalt precursor material.

[0020] Figure 2 Schematic diagram of the pore size distribution of polyphosphazene nanotubes@asphalt precursor material.

[0021] Figure 3 This is the SEM image of mixed soft and hard carbon materials with different asphalt contents after carbonization.

[0022] Figure 4 Schematic diagram of nitrogen adsorption and desorption of HC, HC-30P, and PC after carbonization.

[0023] Figure 5 Schematic diagram of the pore size distribution of HC, HC-30P, and PC after carbonization.

[0024] Figure 6 XPS and Raman images of mixed soft and hard carbon materials with different asphalt contents.

[0025] Figure 7 TEM image of HC.

[0026] Figure 8 This is the TEM image of HC-30P.

[0027] Figure 9 Schematic diagram of directly mixing and carbonizing the solid phase.

[0028] Figure 10 Schematic diagram of carbonization after dissolution in tetrahydrofuran.

[0029] Figure 11 For different asphalt ratios at 0.05 A g -1 Schematic diagram of the electrochemical performance.

[0030] Figure 12 Schematic diagram of the proportion of low-pressure platform areas with different asphalt ratios.

[0031] Figure 13 For HC-30P at 3 A g -1 Schematic diagram of the electrochemical performance.

[0032] Figure 14 Schematic diagram of rate performance with different asphalt ratios. DETAILED DESCRIPTION

[0033] The present invention proposes a method for preparing a mixed soft and hard carbon material. The method involves dissolving pitch and filling the pores of polyphosphazene nanotubes, followed by carbonization to obtain the mixed soft and hard carbon material. The specific preparation method is as follows:

[0034] S1. Preparation of polyphosphazene nanotubes (PZS);

[0035] S2. Preparation of polyphosphazene nanotubes@asphalt precursor material (PZS@P): Dissolve a certain amount of pitch in an appropriate amount of solvent; then stir evenly at room temperature and add a certain mass ratio of polyphosphazene nanotubes; the mass ratio of pitch to polyphosphazene nanotubes is controlled to be 1:2-1:10; continue stirring to allow the dissolved pitch to diffuse into the pores of the polyphosphazene nanotubes and deposit, thereby obtaining the polyphosphazene nanotubes@asphalt precursor material;

[0036] S3. Preparation of a Hybrid Soft and Hard Carbon Material (HC-XP): The polyphosphazene nanotubes and pitch precursor obtained in S2 is carbonized to obtain a hybrid soft and hard carbon material powder. The carbonized PZS@P mixture is designated as HC-XP (where X represents the pitch content in the PZS@P precursor).

[0037] The self-template method is used in step (1) of the preparation method of the mixed soft and hard carbon material. The so-called self-template method is a process in which atoms or ions are deposited on the template by physical, chemical or biological methods during the reaction process to reduce the introduction of external variables. The template is removed by physical or chemical methods after the reaction to obtain the desired nanostructured material. The nanomaterial prepared by this method has structural characteristics similar to the template pores. The present invention uses the reaction intermediate product as a template to prepare polyphosphazene nanotubes. The reactants, namely hexachlorocyclotriphosphazene (HCCP), 4,4-dihydroxydiphenyl sulfone (BPS) and acid-binding agent triethylamine (TEA), are all known to those skilled in the art.

[0038] In step (2) of the preparation method of the mixed soft and hard carbon material, since the polyphosphazene nanotubes have a porous structure, they provide abundant storage space. However, the presence of these micropores increases the specific surface area after carbonization, which will greatly reduce the initial coulomb efficiency. By introducing asphalt, the asphalt can be dissolved into small molecules and diffuse into the pores. After carbonization, it can well fill these pores and shrink in the pores to produce closed pores of suitable size. The asphalt can also form a soft carbon coating on the hard carbon surface to modify surface defects and reduce the specific surface area. Figure 1 As shown, it is the change of specific surface area before carbonization. Figure 4 The change in specific surface area after carbonization shows that the number of 0.7-5 nm open pores in PZS@P and HC-30P is significantly reduced compared to PZS and HC, indicating that the open pores are filled with asphalt and graphitized carbon after asphalt carbonization, and the specific surface area is also significantly reduced. In addition, the mesoporous channels of the material are usually well interconnected. After being carbonized in the pores, these asphalts are interconnected to form a three-dimensional interconnected conductive network, which increases the conductivity of the material and can be used in Figure 5 The interconnected graphitized carbon structures are clearly seen.

[0039] However, the inventors found that when polyphosphazene and asphalt were directly mixed in solid phase, obvious two-phase separation occurred after carbonization, indicating that when direct solid phase mixing was performed, asphalt could not be diffused into the pores of polyphosphazene in the molten state. Figure 9 In order to improve the uniform dispersion of asphalt, the asphalt is dissolved into small molecules in the solution. These small molecules diffuse into the pores in the liquid and are dispersed more evenly. After carbonization, as shown in Figure 10 As shown, there is no particular limitation on the solution, as long as it can dissolve asphalt, such as tetrahydrofuran, N-methylpyrrolidone and toluene. In one embodiment of the present invention, tetrahydrofuran is used as the solvent to dissolve the asphalt and diffuse it into the pores, so that the distribution is more uniform (such as Figure 9 and Figure 10 shown).

[0040] In step (3) of the preparation method of the mixed soft and hard carbon material, PZS@P is carbonized. The carbonization temperature has a significant impact on the properties of the final material because the carbonization temperature affects the closed-pore structure and the degree of graphitization, thereby affecting the active sites of sodium ions. If the temperature is too low, the carbon skeleton cannot grow and rearrange, and fewer closed pores are generated; while if the temperature is too high, the disordered carbon will transform into a more ordered state, thereby reducing the closed-pore size and the interlayer spacing, affecting the transmission of sodium ions. Therefore, it is necessary to optimize the temperature. In the present invention, the carbonization temperature is 1300-1600℃.

[0041] The mixed soft and hard carbon material prepared by the above-mentioned preparation method can be used in the preparation of negative electrode materials for sodium ion batteries. The application includes: thoroughly mixing and grinding the prepared mixed soft and hard carbon material, conductive carbon black, and sodium alginate in a mass ratio of 7:2:1-8:1:1, then transferring the mixed soft and hard carbon material to a glass bottle, adding an appropriate amount of deionized water, and stirring overnight to form a viscous and evenly dispersed slurry. The slurry is then applied to copper foil with a scraper of varying thicknesses, followed by vacuum drying at 80°C for 12 hours, and punching into 12 mm sheets as negative electrode materials for sodium ion batteries; the above-mentioned negative electrode materials for sodium ion batteries are cut into circular electrode sheets with a diameter of 12 mm, and then a battery is assembled using a positive electrode material, electrolyte, separator, electrode shell, and negative electrode material. The positive electrode material is a sodium sheet; the separator is Whatman GF / D; and the electrolyte is a 1 M NaPF6 solution in DIGLYME = 100 vol%.

[0042] Example 1

[0043] The present invention proposes a method for preparing a hybrid soft and hard carbon material and its application in sodium ion batteries, the steps of which are as follows:

[0044] (1) Preparation of PZS: 0.5 g HCCP (1.44 mmol) and 1.08 g BPS (4.32 mmol) were dissolved in 100 mL tetrahydrofuran to obtain solution A. After magnetic stirring for 30 min, a transparent solution was obtained. Then, 1.31 g TEA (12.96 mmol) was added to solution A. The solution was then ultrasonicated at room temperature for 10 h at an ultrasonic power of 40 KW. After the reaction was completed, the solution was centrifuged and the centrifuged product was washed 3-4 times with acetone and deionized water, respectively, to obtain a white product. Finally, the white product was dried in a vacuum oven overnight to obtain PZS.

[0045] (2) Preparation of PZS@P: 0.1 g of asphalt was dissolved in 20 ml of tetrahydrofuran, followed by magnetic stirring at room temperature for 10 h. After stirring evenly, 1 g of PZS (mass ratio of asphalt to PZS 1:10) was added and magnetic stirring was continued for 10 h to allow the dissolved asphalt to fully diffuse into the pores of PZS and deposit. After stirring, the mixture was heated to 80 °C in an oil bath to volatilize the tetrahydrofuran, thereby obtaining PZS@P.

[0046] (3) Preparation of HC-10P: The PZS@P mixture in step (2) is carbonized and recorded as HC-10P (10P is the asphalt content in the PZS@P precursor). The precursor obtained in step (2) is ground evenly and placed in a corundum crucible. The crucible is then placed in a high-temperature tube furnace and filled with high-purity nitrogen as a protective gas. The temperature is raised at a rate of 2°C / min according to actual conditions to 1400°C for carbonization and kept at this temperature for 1 h. After the reaction is completed, the mixture is naturally cooled to room temperature. The entire reaction process is carried out in an inert atmosphere. HC-10P powder is obtained after the reaction is completed.

[0047] (4) Preparation of HC-10P negative electrode: The prepared HC-10P, conductive carbon black and sodium alginate (CMC) were thoroughly mixed and ground in a mass ratio of 8:1:1, then transferred to a glass bottle, added with an appropriate amount of deionized water and stirred overnight to form a viscous and evenly dispersed slurry. The slurry was coated on a copper foil with a 100 µm scraper, and then vacuum dried at 80 °C for 12 h. The electrode sheets were punched into 12 mm sheets as the negative electrode material for sodium ion batteries.

[0048] Example 2

[0049] The present invention proposes a method for preparing a hybrid soft and hard carbon material and its application in sodium ion batteries, the steps of which are as follows:

[0050] (1) Preparation of PZS: 0.5 g HCCP (1.44 mmol) and 1.08 g BPS (4.32 mmol) were dissolved in 100 mL tetrahydrofuran to obtain solution A. After magnetic stirring for 30 min, a transparent solution was obtained. Then, 1.31 g TEA (12.96 mmol) was added to solution A. The solution was then ultrasonicated at room temperature for 10 h at an ultrasonic power of 40 KW. After the reaction was completed, the solution was centrifuged and the centrifuged product was washed 3-4 times with acetone and deionized water, respectively, to obtain a white product. Finally, the white product was dried in a vacuum oven overnight to obtain PZS.

[0051] (2) Preparation of PZS@P: 0.3 g of asphalt was dissolved in 50 ml of tetrahydrofuran, followed by magnetic stirring at room temperature for 10 h. After stirring evenly, 1 g of PZS (mass ratio of asphalt to PZS 3:10) was added and magnetic stirring was continued for 10 h to allow the dissolved asphalt to fully diffuse into the pores of PZS and deposit. After stirring, the mixture was heated to 80 °C in an oil bath to volatilize the tetrahydrofuran, thereby obtaining PZS@P.

[0052] (3) Preparation of HC-30P: The PZS@P mixture in step (2) is carbonized and recorded as HC-30P. The obtained PZS@P precursor is ground evenly and placed in a corundum crucible. The crucible is then placed in a high-temperature tube furnace and filled with high-purity nitrogen as a protective gas. The temperature is raised at a rate of 2°C / min according to the actual situation to 1400°C for carbonization and kept at this temperature for 1 h. After the reaction is completed, it is naturally cooled to room temperature. The entire reaction process is carried out in an inert atmosphere. HC-30P powder is obtained after the reaction is completed.

[0053] (4) Preparation of HC-30P negative electrode: The prepared HC-30P, conductive carbon black and sodium alginate (CMC) were thoroughly mixed and ground in a mass ratio of 8:1:1, then transferred to a glass bottle, added with an appropriate amount of deionized water and stirred overnight to form a viscous and evenly dispersed slurry. The slurry was coated on a copper foil with a 100 µm scraper, and then vacuum dried at 80 °C for 12 h. The electrode sheets were punched into 12 mm sheets as the negative electrode material for sodium ion batteries.

[0054] Example 3

[0055] The present invention proposes a method for preparing a hybrid soft and hard carbon material and its application in sodium ion batteries, the steps of which are as follows:

[0056] (1) Preparation of PZS: 0.5 g HCCP (1.44 mmol) and 1.08 g BPS (4.32 mmol) were dissolved in 100 mL tetrahydrofuran to obtain solution A. After magnetic stirring for 30 min, a transparent solution was obtained. Then, 1.31 g TEA (12.96 mmol) was added to solution A. The solution was then ultrasonicated at room temperature for 10 h at an ultrasonic power of 40 KW. After the reaction was completed, the solution was centrifuged and the centrifuged product was washed 3-4 times with acetone and deionized water, respectively, to obtain a white product. Finally, the white product was dried in a vacuum oven overnight to obtain PZS.

[0057] (2) Preparation of PZS@P: 0.5 g of asphalt was dissolved in 70 ml of tetrahydrofuran, followed by magnetic stirring at room temperature for 10 h. After stirring evenly, 1 g of PZS (mass ratio of asphalt to PZS 1:2) was added and magnetic stirring was continued for 10 h to allow the dissolved asphalt to fully diffuse into the pores of PZS and deposit. After stirring, the mixture was heated to 80 °C in an oil bath to volatilize the tetrahydrofuran, thereby obtaining PZS@P.

[0058] (3) Preparation of HC-50P: The PZS@P mixture in step (2) was carbonized and recorded as HC-50P. The obtained PZS@P precursor was ground evenly and placed in a corundum crucible. The crucible was then placed in a high-temperature tube furnace and passed through with high-purity nitrogen as a protective gas. The temperature was raised at a rate of 2°C / min according to the actual situation to 1400°C for carbonization and kept at this temperature for 1 h. After the reaction was completed, it was naturally cooled to room temperature. The entire reaction process was carried out in an inert atmosphere. HC-50P powder was obtained after the reaction was completed.

[0059] (4) Preparation of HC-50P negative electrode: The prepared HC-50P, conductive carbon black and sodium alginate (CMC) were thoroughly mixed and ground in a mass ratio of 8:1:1, then transferred to a glass bottle, added with an appropriate amount of deionized water and stirred overnight to form a viscous and evenly dispersed slurry. The slurry was coated on a copper foil with a 100 µm scraper, and then vacuum dried at 80 °C for 12 h. The electrode sheets were punched into 12 mm sheets as the negative electrode material for sodium ion batteries.

[0060] Example 4

[0061] The present invention proposes a method for preparing a hybrid soft and hard carbon material and its application in sodium ion batteries, the steps of which are as follows:

[0062] (1) Preparation of PZS: 0.5 g HCCP (1.44 mmol) and 1.08 g BPS (4.32 mmol) were dissolved in 100 mL tetrahydrofuran to obtain solution A. After magnetic stirring for 30 min, a transparent solution was obtained. Then, 1.31 g TEA (12.96 mmol) was added to solution A. The solution was then ultrasonicated at room temperature for 10 h at an ultrasonic power of 40 KW. After the reaction was completed, the solution was centrifuged and the centrifuged product was washed 3-4 times with acetone and deionized water, respectively, to obtain a white product. Finally, the white product was dried in a vacuum oven overnight to obtain PZS.

[0063] (2) Preparation of PZS@P: 0.3 g of asphalt was dissolved in 50 ml of the mixture, followed by magnetic stirring at room temperature for 10 h. After stirring evenly, 1 g of PZS (mass ratio of asphalt to PZS 3:10) was added and magnetic stirring was continued for 10 h to allow the dissolved asphalt to fully diffuse into the pores of the PZS and to deposit. After stirring, the mixture was heated to 80 °C in an oil bath to volatilize the tetrahydrofuran, thereby obtaining PZS@P.

[0064] (3) Preparation of HC-30P-1300: The obtained PZS@P precursor was ground evenly and placed in a corundum crucible. The crucible was then placed in a high-temperature tube furnace and passed with high-purity nitrogen as a protective gas. The temperature was raised at a rate of 2°C / min according to the actual situation to 1300°C for carbonization and kept warm for 1 h. After the reaction was completed, it was naturally cooled to room temperature. The entire reaction process was carried out in an inert atmosphere. HC-30P-1300 powder was obtained after the reaction was completed.

[0065] (4) Preparation of HC-30P-1300 negative electrode: The prepared HC-30P-1300, conductive carbon black and sodium alginate (CMC) were thoroughly mixed and ground in a mass ratio of 8:1:1, then transferred to a glass bottle, added with an appropriate amount of deionized water and stirred overnight to form a viscous and evenly dispersed slurry. The slurry was coated on a copper foil with a 100 µm scraper, and then vacuum dried at 80 °C for 12 h. The electrode sheets were punched into 12 mm sheets as the negative electrode material for sodium ion batteries.

[0066] Comparative Example 1

[0067] (1) Preparation of PZS: 0.5 g HCCP (1.44 mmol) and 1.08 g BPS (4.32 mmol) were dissolved in 100 mL tetrahydrofuran to obtain solution A. After magnetic stirring for 30 min, a transparent solution was obtained. Then, 1.31 g TEA (12.96 mmol) was added to solution A. The solution was then ultrasonicated at room temperature for 10 h at an ultrasonic power of 40 KW. After the reaction was completed, the solution was centrifuged and the centrifuged product was washed 3-4 times with acetone and deionized water, respectively, to obtain a white product. Finally, the white product was dried in a vacuum oven overnight to obtain PZS.

[0068] (2) Preparation of HC: The obtained PZS precursor was ground evenly and placed in a corundum crucible. The crucible was then placed in a high-temperature tube furnace and passed with high-purity nitrogen as a protective gas. The temperature was raised at a rate of 2°C / min according to the actual situation. It was raised to 1400°C for carbonization and kept warm for 1 h. After the reaction was completed, it was naturally cooled to room temperature. The entire reaction process was carried out in an inert atmosphere. HC powder was obtained after the reaction was completed.

[0069] (3) Preparation of HC negative electrode: The prepared HC, conductive carbon black and sodium alginate (CMC) were thoroughly mixed and ground in a mass ratio of 8:1:1, then transferred to a glass bottle, added with an appropriate amount of deionized water and stirred overnight to form a viscous and evenly dispersed slurry. The slurry was coated on a copper foil with a 100 µm scraper, and then vacuum dried at 80 °C for 12 h. The electrode sheets were punched into 12 mm sheets as the negative electrode material for sodium ion batteries.

[0070] Comparative Example 2

[0071] (1) Preparation of PC: Place commercial asphalt precursor into a corundum crucible, then place the crucible into a high-temperature tube furnace and pass high-purity nitrogen as a protective gas. According to the actual situation, the temperature is raised at a rate of 2°C / min to 1400°C for carbonization and kept warm for 1 hour. After the reaction is completed, it is naturally cooled to room temperature. The entire reaction process is carried out in an inert atmosphere. PC powder is obtained after the reaction is completed.

[0072] (2) Preparation of PC negative electrode: The prepared PC, conductive carbon black and sodium alginate (CMC) were thoroughly mixed and ground in a mass ratio of 8:1:1, then transferred to a glass bottle, added with an appropriate amount of deionized water and stirred overnight to form a viscous and evenly dispersed slurry. The slurry was coated on a copper foil with a 100 µm scraper, and then vacuum dried at 80 °C for 12 h. The electrode sheets were punched into 12 mm sheets as the negative electrode material for sodium ion batteries.

[0073] Experimental example

[0074] Performance testing of HC-XP, HC, and PC materials

[0075] 1. Pore Size Distribution Test: Nitrogen adsorption and desorption tests were performed on the HC-30P in Example 1 and the HC and PC materials in Comparative Examples 1 and 2 to determine their specific surface area and pore size distribution. All samples were vacuum degassed at 200°C for 6 hours, cooled to room temperature under vacuum, and weighed. The sample tubes were then transferred to the analysis station, where liquid nitrogen was added to the Dewar flask to provide a low-temperature environment for adsorption testing. Nitrogen was used as the adsorption medium, and helium was used as the carrier gas. A mixture of nitrogen and helium was mixed in a predetermined ratio and adjusted to a predetermined relative pressure level. The equipment used was a Guoyi Precision Measurement V-Sorb 4800TP.

[0076] Figure 1 and Figure 4 The nitrogen adsorption and desorption curves of the HC-30P material of Example 3 and the HC and PC materials of Comparative Examples 1 and 2 are shown. The curves show that the specific surface area of ​​the HC-30P material of Example 3 according to the present invention is significantly reduced compared to the HC of Comparative Example 1, and the reduction is mainly in the specific surface area of ​​the micropore region. The specific surface area of ​​the material calculated by the BET method is 5.8 m 2 g -1The HC material of Comparative Example 1 is mainly microporous and has a specific surface area of ​​76.6 m 2 g -1 .

[0077] Figure 2 and Figure 5 Pore ​​size distribution curves for the HC-30P material of Example 3 and the HC and PC materials of Comparative Examples 1 and 2 are shown. The figure shows that the HC-30P material of Example 3 lacks a distinct pore size distribution, indicating a relatively low number of open pores within each pore size range. In contrast, the HC material of Comparative Example 1 exhibits a distinct micropore distribution, demonstrating that the dissolved pitch diffuses into the open pores of the polyphosphazene precursor. This asphalt is retained within the pores during carbonization, increasing the number and volume of closed pores.

[0078] 2. Morphology characterization: Figure 3 The SEM images of the HC-XP materials obtained in Examples 1-3 are shown. Figure 1 It can be seen that the HC-XP prepared according to this method agglomerates with the increase of pitch content, and the carbonized nanotubes are surrounded by pitch carbon and interconnected. Figure 8 It can be seen from the TEM image of Experimental Example 3 that interconnected short-range ordered graphitized stripes appear inside HC-30P, and the appearance of closed pores can be observed from the TEM, while the TEM image of Comparative Example 1 is as follows: Figure 7 As shown, there are no obvious graphite streaks, showing a more disordered state. However, when the asphalt content exceeds 50%, the asphalt will preferentially agglomerate and fail to form an effective surface coating, affecting the material's performance. Therefore, the optimal asphalt content is 10-50%.

[0079] 3. Structural test: Through the Raman and XPS analysis of the materials of Experimental Examples 1-3 and Comparative Examples 1 and 2, it can be seen that with the increase of asphalt content, the Raman D / I G The value of gradually becomes smaller, indicating a transition to ordering. This is because the pitch is in a relatively ordered state after carbonization, which can repair the defects in the HC structure. The same pattern is also seen in XPS, with the content of CC bonds decreasing and the content of C=C increasing. This is because CC generally corresponds to a defective sp3 carbon structure, while C=C corresponds to a more ordered sp2 carbon structure. Studies have shown that a suitable ratio of sp2 / sp3 hybridized graphitic carbon can form an optimal solid electrolyte interface (SEI) during battery cycling, and the HC-30P ratio is the most suitable. The SEI formed is thinner and more uniform, which is conducive to ion transport and reduces the consumption of irreversible sodium ions, thereby improving the first coulombic efficiency.

[0080] 4. Cycling performance test of sodium ion battery negative electrode: Cycling performance test was conducted on the HC-XP negative electrode in Examples 1 to 3 and the HC negative electrode in Comparative Example 1. The specific process is as follows: Battery cycling performance and constant current charge and discharge curve test were tested and analyzed on the Xinwei battery test system. The HC-XP and HC negative electrodes were used as working electrodes. When set, they were discharged first and then charged and then cycled in sequence. The voltage range was 0.01-2.5 V. The results of the cycling performance test of the HC-XP negative electrode in Examples 1 to 3 and the HC negative electrode in Comparative Example 1 are shown in Figures 11 to 14 Among them, HC-30P prepared based on the present invention has the highest specific capacity, at 0.05 A g -1 At a current density of 1.5 GHz, the reversible capacity reaches 413 mAh g -1 , and the platform area accounts for 67%, the highest platform ratio among all tested samples, and the coulombic efficiency is increased from 80% of HC to 88.7% of HC-30P for the first time. These indicate that it has great commercial prospects and further demonstrate the high performance of HC-30P at high current of 3 A g -1 Long cycle test is performed under Figure 13 As shown, even at 3 A g -1 Even at high current density, the capacity remains at 200 mAh g -1 Above, and the capacity retention rate after 2500 cycles is 87%. Rate tests were carried out at different current densities, such as Figure 14 As shown, HC-30P also has the best specific capacity and -1 At a current density of 181.5 mAh g -1 The excellent electrochemical performance of HC-30P mainly comes from the appropriate closed-pore size and the mixed distribution of soft and hard carbon. The appropriate closed-pore size can provide sufficient active sites, thereby increasing the storage capacity of sodium ions, while the mixed distribution of soft and hard carbon reduces surface defects and improves conductivity, promoting the diffusion of sodium ions while reducing the occurrence of side reactions. When the asphalt content is less than 10%, the open pores cannot be fully filled, resulting in insufficient closed pores to provide active sites, and due to insufficient asphalt, the distribution on the precursor surface is also uneven, resulting in a low first coulomb efficiency. When the asphalt content is greater than 50%, due to excessive asphalt, the asphalt tends to agglomerate itself during the carbonization process, and asphalt is a long-range ordered soft carbon with a small interlayer spacing and fewer closed pores, which cannot provide active sites, resulting in too low capacity. Therefore, an asphalt content of 10-50% is more suitable, and an asphalt content of 30% can provide the largest sodium ion storage active sites and the best electrochemical performance.

[0081] The present invention converts open pores into closed pores by preparing a composite carbon material. The synthesized polyphosphazene PZS is used as a precursor. PZS contains a large number of micropores and mesopores, and asphalt can be dissolved in tetrahydrofuran as small molecules. These small molecules can diffuse into the pores of PZS in the solvent and gradually deposit to repair the pores. After drying, the asphalt acts as a "binder" to connect the various nanotubes together. After carbonization, the three-dimensionally interconnected asphalt is carbonized into graphitized carbon, which improves conductivity and promotes the transmission of electrons in the negative electrode material. At the same time, the asphalt has fewer surface defects. After high-temperature carbonization, the asphalt will serve as a surface coating to repair the defects on the HC surface. At the same time, the shrinkage and rearrangement of the carbon skeleton cause the contact surface between the asphalt and PZS to form closed pores, thereby increasing the active sites for storing sodium ions, thereby increasing the platform capacity in the low-pressure area. The change in specific surface area after dissolving the asphalt before carbonization can be known through BET, such as Figure 1 As shown in Figure 3, the surface area and the number of micropores were significantly reduced after coating, indicating that the asphalt diffused into the pores of PZS and repaired the surface defects.

[0082] pass Figure 3 From the SEM images in the figure, it can be found that larger particles appear with the increase of asphalt content after carbonization. This is because asphalt has strong adhesion at room temperature, which agglomerates the individual PZS nanotubes (a, e, i are HC-10P; b, f, j are HC-30P; c, g, k are HC-50P; d, h, l are HC-80P). After carbonization, these asphalt-based graphite carbons can increase the conductivity and shorten the transmission distance of sodium ions, which is more conducive to the diffusion of sodium ions. The specific surface area after agglomeration is also increased from the original 76.6 m 2 g -1 Lowered to 5.8 m 2 g -1 ,

[0083] Adjusting the asphalt content also adjusts the sp2 / sp3 distribution of carbonized HC-XP. As the asphalt content increases, the ratio of sp2 / sp3 hybridized graphitic carbon on the surface changes. A higher ratio of sp2 hybridized graphitic carbon contributes to higher electron transport capabilities, but the capacity is limited due to fewer defects. A higher ratio of sp3 hybridized amorphous carbon allows for more defect structures, enabling greater storage capacity, but these defects can cause a decrease in the first coulombic efficiency (ICE). To overcome the trade-off between ICE and capacity, the sp2 / sp3 structure should be adjusted to a balanced state, achieving an optimal solid electrolyte interface (SEI) during battery cycling and achieving the optimal ICE and sodium ion storage capacity.

[0084] In addition, since the soft and hard carbon composite materials of the present invention have a high closed-pore content, sp2 / sp3 synergistic balance and surface coating modification, the specific capacity, rate performance and cycle life of the sodium ion battery negative electrode material based on the material of the present invention are significantly improved, and excellent cycle performance is also obtained after assembling the full battery, demonstrating practicality.

[0085] The embodiments of the present invention are described in detail above with reference to the accompanying drawings, but the present invention is not limited thereto. Various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention.

Claims

1. A method for preparing a mixed soft and hard carbon material, characterized in that: The asphalt is dissolved and filled into the pores of polyphosphazene nanotubes, and then carbonized to obtain a mixed soft and hard carbon material. The specific preparation method is as follows: S1, preparing polyphosphazene nanotubes; S2. Preparation of a polyphosphazene nanotube@asphalt precursor material: dissolving a certain amount of asphalt in an appropriate amount of solvent; then stirring uniformly at room temperature and adding a certain mass ratio of polyphosphazene nanotubes; the mass ratio of asphalt to polyphosphazene nanotubes being controlled at 1:2-1:10; continuing stirring to allow the dissolved asphalt to diffuse into the pores of the polyphosphazene nanotubes and deposit, thereby obtaining the polyphosphazene nanotube@asphalt precursor material; S3. Preparation of mixed soft and hard carbon materials: The polyphosphazene nanotubes@asphalt precursor material obtained in S2 is carbonized to obtain mixed soft and hard carbon material powder.

2. The method for preparing a mixed soft and hard carbon material according to claim 1, characterized in that: The preparation method of polyphosphazene nanotubes in S1 is as follows: a certain proportion of hexachlorocyclotriphosphazene and 4,4-dihydroxydiphenyl sulfone are dissolved in an appropriate amount of tetrahydrofuran, stirred to dissolve into a transparent solution, then an appropriate amount of acid binding agent is added, and then ultrasonicated at 40°C for 10 hours. After the reaction is completed, the polyphosphazene nanotubes are washed with acetone and deionized water respectively, centrifuged, and then dried to obtain the polyphosphazene nanotubes.

3. The method for preparing a mixed soft and hard carbon material according to claim 1, wherein: The solvent in S2 is one or more of tetrahydrofuran, N-methylpyrrolidone, benzene, toluene or carbon tetrachloride, the amount of the solvent is 20-100 ml, and the solution concentration is 5 g / L-10 g / L.

4. The method for preparing a mixed soft and hard carbon material according to claim 3, characterized in that: The solvent in S2 is tetrahydrofuran, which is heated to 50°C-100°C in an oil bath after magnetic stirring to volatilize the tetrahydrofuran.

5. The method for preparing a mixed soft and hard carbon material according to claim 1, wherein: In S2, both stirring steps were performed with magnetic stirring for 10 h.

6. The method for preparing a mixed soft and hard carbon material according to claim 1, characterized in that: The carbonization treatment method in S3 is: grind the polyphosphazene nanotube@asphalt precursor material evenly and put it into a corundum crucible, then put the crucible into a high-temperature tube furnace and pass high-purity nitrogen as a protective gas, and heat it at a rate of 1-5℃ / min according to actual conditions, raise it to 1300-1600℃ for carbonization and keep it warm for 1-3h. After the reaction is completed, it is naturally cooled to room temperature. The entire reaction process is carried out in an inert atmosphere. After the reaction is completed, a mixed soft and hard carbon material powder is obtained.

7. Application of sodium ion batteries using hybrid soft and hard carbon materials, characterized in that: The mixed soft and hard carbon material prepared by the method for preparing the mixed soft and hard carbon material according to claim 1 is used in the preparation of negative electrode materials for sodium ion batteries.

8. Application of the hybrid soft and hard carbon material in sodium ion batteries according to claim 7, characterized in that: The application includes: fully mixing and grinding the prepared mixed soft and hard carbon material, conductive carbon black and sodium alginate in a mass ratio of 7:2:1-8:1:1, then transferring the mixture into a glass bottle, adding an appropriate amount of deionized water and stirring overnight to form a viscous and evenly dispersed slurry, applying the slurry on a copper foil with a scraper of different thicknesses, then vacuum drying at 80°C for 12 hours, and punching the slurry into 12 mm sheets as a negative electrode material for a sodium ion battery; cutting the above-mentioned negative electrode material for a sodium ion battery into circular pole pieces with a diameter of 12 mm, and then assembling a battery using a positive electrode material, an electrolyte, a separator, an electrode shell and the negative electrode material.

9. Application of the hybrid soft and hard carbon material in sodium ion batteries according to claim 8, characterized in that: The positive electrode material is sodium sheet; the separator is Whatman GF / D; and the electrolyte is 1 M NaPF6 in DIGLYME = 100 Vol% solution.

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