Preparation method of hard carbon material with improved high rate capability for sodium ion battery

Abstract

The invention relates to a preparation method of a sodium ion battery hard carbon material with improved high rate performance, which comprises the following steps: (S1) uniformly mixing asphalt, a monomer and a solvent, heating, adding an initiator, and carrying out in-situ polymerization reaction to obtain modified asphalt; the monomer is a mixture of vinyl benzenesulfonic acid, vinyl imidazole, vinyl benzyl chloride and acrylonitrile, the modified asphalt (S2) and polyvinyl alcohol are ball-milled and mixed, the mixture is slowly heated to 240-330 DEG C for presintering in an inert atmosphere, then the mixture is heated to 1000-1300 DEG C for carbonization, and the hard carbon material for the sodium ion battery is obtained. According to the invention, the asphalt is subjected to in-situ polymerization through four monomers, modification of the asphalt is completed, and the prepared hard carbon material has excellent electrochemical performance, especially high-rate performance.

Description

Technical Field

[0001] The present invention belongs to the technical field, and particularly relates to a preparation method of a hard carbon material for improving the high-rate performance of a sodium-ion battery. Background Art

[0002] Due to the research and development of new energy, the current lithium-ion battery technology has been very mature and is widely used in fields such as automobiles and household appliances. However, there are currently problems such as the shortage of lithium resources and uneven distribution, which limit the development of large-scale energy storage of lithium-ion batteries. Sodium has a very high storage capacity and is generally considered the best choice to replace lithium-ion batteries in the new energy field. However, due to the much larger radius of Na ions than Li ions, it is difficult for them to be inserted and removed in graphite. In addition, the interaction between Na ions and the graphite layer is weak, and it is difficult to form a stable graphite intercalation compound, resulting in the reversible capacity and rate performance of current sodium-ion batteries not being ideal enough. Currently, hard carbon materials are recognized as suitable anode materials for sodium-ion batteries. Hard carbon materials are carbon materials that will not be converted into graphite even at temperatures above 3000 °C. The sources of hard carbon anode materials are extensive, including polymer resins, bio-based, and petroleum-based carbon precursors. Coal / petroleum pitch is rich in aromatic polymers, has a high carbon content, is abundant in sources, and is inexpensive, and is currently a suitable hard carbon source for commercial development. However, pitch is a soft carbon precursor. The excessive hydrogen in the aromatic molecular structure and the intermolecular interaction will cause it to be prone to graphitization to form a graphite-like structure during pyrolysis, reducing the interlayer spacing. This microscopic structure is not conducive to the insertion or removal of sodium ions and is not conducive to the exertion of electrochemical performance.

[0003] In order to inhibit the formation of a graphite-like structure during the pyrolysis carbonization process of the particle size, the prior art crosslinks and modifies the pitch through a crosslinking agent to change the microscopic structure and hinder the growth of graphite microcrystals during the pyrolysis carbonization process, such as CN119528114A, CN119100369A, and CN119191268A. Another method is to pre-oxidize the pitch carbon precursor to increase the oxygen content in the precursor. Due to the presence of oxygen heteroatoms, the pitch is not easily formed into an ordered structure during pyrolysis, such as CN119430148A, CN119551654A, and CN119528112. However, there is also a combination of the crosslinking modification and pre-oxidation processes, such as CN119461333A.

[0004] However, there are side reactions between the hard carbon materials obtained by the above-mentioned existing technologies and the electrolyte. Especially under high-rate charge and discharge conditions, the side reactions are severe, resulting in poor cycle stability at high rates. During the actual use of the battery, especially as a power source, the charge and discharge are carried out under high-rate conditions. A hard carbon material that shows excellent cycle stability under low-rate conditions such as 0.1C cannot reflect excellent electrochemical performance in actual applications. Therefore, the development of hard carbon materials with excellent performance at high rates is of great significance and commercial value. Summary of the Invention

[0005] In order to solve the problem that the hard carbon materials in the prior art perform poorly under rate charge and discharge conditions, the present invention proposes a preparation method of a hard carbon material for sodium-ion batteries with improved high-rate performance. The present invention uses a mixture of vinylbenzenesulfonic acid, vinylimidazole, vinylbenzyl chloride, and acrylonitrile as monomers to in-situ polymerize asphalt. The polymer fully enters the interior of the asphalt to complete the modification of the asphalt. The prepared hard carbon material has excellent electrochemical performance, especially rate performance, and can still have excellent capacity and cycle stability at a high rate of 10C. Specifically, the present invention provides the following technical solutions to solve the above technical problems:

[0006] A preparation method of a hard carbon material for sodium-ion batteries with improved high-rate performance, comprising the following steps:

[0007] (S1) Mix asphalt, monomers, and a solvent evenly, add an initiator after heating, and carry out an in-situ polymerization reaction to obtain modified asphalt; the monomers are a mixture of vinylbenzenesulfonic acid, vinylimidazole, vinylbenzyl chloride, and acrylonitrile,

[0008] (S2) After ball-milling and mixing the modified asphalt and polyvinyl alcohol, slowly heat it to 240 - 330°C for pre-burning under an inert atmosphere, and then heat it to 1000 - 1300°C for carbonization to obtain the hard carbon material for sodium-ion batteries as the product.

[0009] Further, in step (S1), the mass ratio of asphalt, monomers, and the solvent is 10:4 - 5:500 - 1000.

[0010] Further, in step (S1), the solvent is selected from at least one of tetrahydrofuran, dichloromethane, chloroform, and toluene. Preferably, it is a composite solvent of tetrahydrofuran and dichloromethane in a volume ratio of 1 - 3:1 - 3. The inventors found that in the above mixed solvent, in-situ polymerization of the monomers can be more conducive to obtaining a functional polymer incorporated into the asphalt matrix to complete the modification of the asphalt.

[0011] Further, in step (S1), in the preparation of the functional polymer, the molar ratio of vinylbenzenesulfonic acid, vinylimidazole, vinylbenzyl chloride, and acrylonitrile is 20-30:20-30:10-15:10-15. The inventors found that using the functional polymer in the above ratio as an asphalt modifier can obtain a hard carbon material with the optimal comprehensive electrochemical performance. The heteroatoms such as N, S, and O introduced by the functional polymer can prevent the ordered arrangement of graphitization during the pyrolysis process, facilitating the insertion and extraction of sodium ions by the hard carbon material. The functional polymer may also play a certain cross-linking role. The introduction of the functional copolymer can reduce the degree of graphitization and expand the interlayer spacing. However, the inventors found that the introduction of the functional copolymer needs to be in-situ polymerized with asphalt under suitable conditions. If the functional copolymer is obtained in advance and then physically blended with asphalt, it cannot effectively modify the asphalt.

[0012] Further, in step (S1), the in-situ polymerization includes the following steps: after the asphalt and the solvent are mixed evenly, the mixed monomers are added, and the initiator solution is added under stirring in an inert atmosphere, and the temperature is raised for reaction. After the reaction is completed, the precipitate is washed in an alcohol solvent, and the precipitate is dried to obtain the modified asphalt.

[0013] Furthermore, the initiator is selected from at least one of azobisisobutyronitrile, azobisisoheptonitrile, dimethyl azodicarboxylate, azobiscyclohexanecarbonitrile, and dibutyl azodicarboxylate. The dosage of the initiator is 0.5-2 wt% of the total mass of the monomers; the inert atmosphere is nitrogen and / or argon. The temperature-raising reaction is to raise the temperature to the decomposition temperature of the initiator. For example, for azobisisobutyronitrile, the decomposition temperature is 60-75 °C.

[0014] Further, in step (S2), the mass ratio of the modified asphalt to polyvinyl alcohol is 100:5-8.

[0015] Further, in step (S2), the weight-average molecular weight of polyvinyl alcohol is 20,000 to 50,000. The purpose of adding polyvinyl alcohol is to further oxidize the modified asphalt by using the active oxygen released by polyvinyl alcohol during the pyrolysis process.

[0016] Further, in step (S2), the slow heating is at a heating rate of 1-5 °C / min, and the pre-burning time is 2-3 h; the heating rate for heating to 1000-1300 °C is 10-20 °C / min, and the carbonization time is 10-15 h. The purpose of slow heating is that the modified asphalt and polyvinyl alcohol can fully react to further improve the internal microstructure of the asphalt.

[0017] The present invention also provides a sodium-ion battery, the negative electrode of which comprises the hard carbon material prepared by the above preparation method.

[0018] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0019] The polymer obtained by copolymerizing four monomers, namely vinylbenzenesulfonic acid, vinylimidazole, vinylbenzyl chloride, and acrylonitrile, is used as a modifier in the present invention. Moreover, the polymerization is carried out in situ in an asphalt solution, enabling the polymer to fully penetrate into the interior of the asphalt and complete the modification of the asphalt. The presence of the functional polymer, on the one hand, introduces heteroatoms such as O and S, inhibiting the degree of graphitization during the subsequent carbonization process. On the other hand, the functional polymer also acts as a crosslinking agent. The combined action of these two aspects promotes a decrease in the degree of graphitization and an increase in the interlayer spacing of the obtained modified asphalt during the subsequent carbonization process, which is beneficial to the deintercalation of sodium ions and improves the electrochemical performance of the hard carbon material, especially the electrochemical performance at high rates. Description of the Drawings

[0020] Figure 1 It is the SEM image of the hard carbon material prepared in Example 1.

[0021] Figure 2 It is the XRD pattern of the hard carbon material prepared in Example 1.

[0022] Figure 3 It is the Raman spectrum of the hard carbon material prepared in Example 1. Detailed Embodiments

[0023] The technical solution of the present invention will be further explained and illustrated by specific examples below.

[0024] Example 1

[0025] (S1) 100 g of petroleum asphalt, 50 g of a mixed monomer (a mixture of vinylbenzenesulfonic acid, vinylimidazole, vinylbenzyl chloride, and acrylonitrile in a molar ratio of 20:20:10:10), 8000 g of a composite solvent of tetrahydrofuran and dichloromethane in a volume ratio of 1:1 are mixed evenly, slowly heated to 75 °C, and a 10 wt% solution of azobisisobutyronitrile in THF is added dropwise as an initiator, with the amount of azobisisobutyronitrile being 0.5 g. The in-situ polymerization reaction is carried out under stirring and reflux conditions for 6 h, cooled and cooled down, and the product is put into absolute ethanol. The precipitate is washed 3 times with absolute ethanol and dried in vacuo to obtain modified asphalt;

[0026] (S2) 100 g of modified asphalt and 5 g of polyvinyl alcohol (number-average molecular weight of about 32000) are ball-milled and mixed, then transferred to a tubular furnace. Under a nitrogen atmosphere, it is heated to 310 °C at a rate of 3 °C / min and pre-burned for 3 h, and then heated to 1000 °C at a heating rate of 10 °C / min for carbonization for 10 h to obtain a hard carbon material for sodium-ion batteries.

[0027] Figure 1 It is the SEM image of the hard carbon material prepared in Example 1.

[0028] Figure 2 It is the XRD pattern of the hard carbon material prepared in Example 1. It can be calculated that d(002) = 0.387 nm. This shows that the hard carbon material obtained by the preparation method of the present invention has a relatively large interlayer spacing.

[0029] Figure 3 It is the Raman spectrum of the hard carbon material prepared in Example 1, I D / I G = 1.972, indicating that the hard carbon material prepared in Example 1 has more defects and low crystallinity, which is attributed to the modification of in-situ polymerization of asphalt. The introduction of heteroatoms can effectively inhibit the increase in graphitization degree caused by rearrangement during the carbonization process.

[0030] Example 2

[0031] Other conditions are the same as those in Example 1. The difference is that in step (S1), the amount of the mixed monomer is 40 g, and the mixed monomer is a mixture of vinylbenzenesulfonic acid, vinylimidazole, vinylbenzyl chloride, and acrylonitrile in a molar ratio of 30:20:15:10.

[0032] Example 3

[0033] Other conditions are the same as those in Example 1. The difference is that in step (S1), the mixed monomer is a mixture of vinylbenzenesulfonic acid, vinylimidazole, vinylbenzyl chloride, and acrylonitrile in a molar ratio of 30:20:10:15.

[0034] Example 4

[0035] Other conditions are the same as those in Example 1. The difference is that in step (S1), the mixed monomer is a mixture of vinylbenzenesulfonic acid, vinylimidazole, vinylbenzyl chloride, and acrylonitrile in a molar ratio of 20:30:15:10.

[0036] Example 5

[0037] Other conditions are the same as those in Example 1. The difference is that in step (S1), the solvent is tetrahydrofuran.

[0038] Example 6

[0039] Other conditions are the same as those in Example 1. The difference is that in step (S1), the solvent is dichloromethane.

[0040] Comparative Example 1

[0041] Other conditions are the same as those in Example 1. The difference is that in step (S1), the mixed monomer is a mixture of vinylbenzenesulfonic acid, vinylimidazole, and vinylbenzyl chloride in a molar ratio of 20:20:10, that is, the monomer does not contain acrylonitrile.

[0042] Comparative Example 2

[0043] Other conditions are the same as in Example 1, except that in step (S1), the mixed monomers are a mixture of vinylimidazole, vinylbenzyl chloride, and acrylonitrile in a molar ratio of 20:10:10, that is, the monomers do not contain vinylbenzenesulfonic acid.

[0044] Comparative Example 3

[0045] Other conditions are the same as in Example 1, except that in step (S1), the mixed monomers are a mixture of vinylbenzenesulfonic acid, vinylbenzyl chloride, and acrylonitrile in a molar ratio of 20:10:10, that is, the monomers do not contain vinylimidazole.

[0046] Comparative Example 4

[0047] Other conditions are the same as in Example 1, except that in step (S1), the mixed monomers are a mixture of vinylbenzenesulfonic acid, vinylimidazole, and acrylonitrile in a molar ratio of 20:20:10, that is, the monomers do not contain vinylbenzyl chloride.

[0048] Comparative Example 5

[0049] (S1) Vinylbenzenesulfonic acid, vinylimidazole, vinylbenzyl chloride, and acrylonitrile are formulated in a molar ratio of 20:20:10:10 to obtain a mixture of monomers. 100 g of the mixed monomers are added to 800 g of tetrahydrofuran, and the temperature is slowly raised to 75 °C. 10 parts by mass of a THF solution of 10 wt% azobisisobutyronitrile is added dropwise as an initiator, and the polymerization reaction is carried out under stirring and reflux conditions for 6 h. After cooling and lowering the temperature, the solvent is removed by rotary evaporation and vacuum dried to obtain a functional polymer;

[0050] (S2) 100 g of petroleum asphalt, 35 g of the functional polymer obtained in step (S1), 8000 g of a composite solvent of tetrahydrofuran and dichloromethane in a volume ratio of 1:1 are mixed evenly, heated to 90 °C and kept warm for 6 h, the solvent is evaporated, washed 3 times with absolute ethanol, and vacuum dried to obtain modified asphalt;

[0051] (S3) 100 g of modified asphalt and 5 g of polyvinyl alcohol (number average molecular weight of about 32000) are ball-milled and mixed, then transferred to a tube furnace. Under a nitrogen atmosphere, it is heated to 310 °C at a rate of 3 °C / min and pre-burned for 3 h, and then heated to 1000 °C at a heating rate of 10 °C / min for carbonization. The carbonization time is 10 h to obtain a hard carbon material for sodium ion batteries.

[0052] That is, compared with Example 1, the functional polymer in Comparative Example 5 is prepared in advance instead of in-situ polymerization.

[0053] Comparative Example 6

[0054] Other conditions are the same as in Example 1, except that in step (S1), vinylimidazole is replaced with an equimolar amount of vinylpyrrolidone.

[0055] Comparative Example 7

[0056] Other conditions were the same as in Example 1, except that in step (S1), vinylbenzyl chloride was replaced with an equimolar amount of styrene.

[0057] Comparative Example 8

[0058] Other conditions were the same as in Example 1, except that in step (S1), acrylonitrile was replaced with an equimolar amount of acrylamide.

[0059] Comparative Example 9

[0060] Other conditions were the same as in Example 1, except that in step (S2), polyvinyl alcohol was replaced with an equal mass of polyethylene glycol.

[0061] Application Example

[0062] The hard carbon materials prepared in the above examples and comparative examples were assembled into sodium ion batteries. Specifically, the hard carbon materials, PVDF, and carbon black of the examples and comparative examples were added to the solvent N-methylpyrrolidone in a mass ratio of 90:7:3 to prepare a slurry. The slurry was coated on a copper foil current collector so that the loading amount of the hard carbon material on the copper foil current collector was 1 mg / cm 2 , to make a negative electrode; a sodium sheet was used as the counter electrode, EC:DMC with a volume ratio of 1:1 was used as the electrolyte, 1 mol / L NaPF6 was used as the electrolyte, and a 2032 coin cell was assembled in a glove box filled with argon. After assembly, it was left standing for 8 h at 25±2 °C, and the electrochemical performance was tested. The results are shown in Table 1. The charge-discharge cycle voltage range was 0.01 V - 2.5 V.

[0063] Table 1 Electrochemical Performance Test of Hard Carbon Materials

[0064]

[0065]

[0066] It can be seen from the data in Table 1 that the hard carbon materials prepared by the preparation method of the present invention have excellent electrochemical performance, especially excellent capacity and cycle stability at high rates, which can play a promoting role in the practical application of sodium ion batteries. The purpose of the present invention is achieved by a process in which four monomers, vinylbenzenesulfonic acid, vinylimidazole, vinylbenzyl chloride, and acrylonitrile, are polymerized in situ in a solution of asphalt according to a certain ratio to obtain a modified asphalt, and then the modified asphalt is blended with polyvinyl alcohol, preheated first, and then carbonized at high temperature. The four monomers play a cooperative role. When one of the monomers is missing, or the monomer is changed to another monomer, or polyvinyl alcohol is missing, or the polymer is not prepared by in-situ polymerization, it is impossible to obtain a hard carbon material with excellent performance at high rates.

Claims

1. A preparation method of a hard carbon material for a sodium-ion battery with improved high-rate performance, comprising the following steps: (S1) Asphalt, monomers, and a solvent are mixed evenly, and after heating, an initiator is added to carry out an in-situ polymerization reaction to obtain modified asphalt; The monomers are a mixture of vinylbenzenesulfonic acid, vinylimidazole, vinylbenzyl chloride, and acrylonitrile. (S2) After the modified asphalt and polyvinyl alcohol are mixed by ball milling, under an inert atmosphere, the temperature is slowly raised to 240 - 330 °C for pre-burning, and then the temperature is raised to 1000 - 1300 °C for carbonization to obtain the hard carbon material for the sodium-ion battery product.

2. The preparation method according to claim 1, characterized in that, In step (S1), the mass ratio of asphalt, monomers, and the solvent is 10:4 - 5:500 - 1000.

3. The preparation method according to claim 1, wherein In step (S1), the solvent is selected from at least one of tetrahydrofuran, dichloromethane, chloroform, and toluene; preferably a composite solvent of tetrahydrofuran and dichloromethane in a volume ratio of 1 - 3:1 - 3.

4. The preparation method according to claim 1, wherein In step (S1), in the preparation of the functional polymer, the molar ratio of vinylbenzenesulfonic acid, vinylimidazole, vinylbenzyl chloride, and acrylonitrile is 20 - 30:20 - 30:10 - 15:10 - 15.

5. The preparation method according to claim 1, characterized in that, In step (S1), the in-situ polymerization comprises the following steps: after asphalt and the solvent are mixed evenly, the mixed monomers are added, and an initiator solution is added under stirring in an inert atmosphere, the temperature is raised for reaction, and after the reaction ends, the precipitate is washed in an alcohol solvent, and the precipitate is dried to obtain modified asphalt.

6. The preparation method according to claim 5, characterized in that, The initiator is selected from at least one of azobisisobutyronitrile, azobisisoheptonitrile, dimethyl azodicarboxylate, azobiscyclohexanecarbonitrile, and dibutyl azodicarboxylate. The amount of the initiator is 0.5 - 2 wt% of the total mass of the monomers; the inert atmosphere is nitrogen and / or argon.

7. The preparation method according to claim 1, wherein In step (S2), the mass ratio of the modified asphalt and polyvinyl alcohol is 100:5 - 8; 8. The preparation method according to claim 1, characterized in that, In step (S2), the molecular weight of polyvinyl alcohol is 20,000 to 50,000.

9. The preparation method according to claim 1, characterized in that, In step (S2), the slow heating is at a heating rate of 1 - 5 °C / min, and the pre-burning time is 2 - 3 h; the heating rate for raising the temperature to 1000 - 1300 °C is 10 - 20 °C / min, and the carbonization time is 10 - 15 h.

10. A sodium-ion battery, the negative electrode of which comprises the hard carbon material prepared by the preparation method according to any one of claims 1 - 9 above.

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