Reconstruction method and application of c=o functional groups on surface of coconut shell-based hard carbon material

Abstract

The present application relates to sodium ion battery negative electrode material related technology, especially a kind of reconstruction method and application of C=O functional group on the surface of coconut shell-based hard carbon material.The method described in the present application uses coconut shell as precursor, after pre-carbonization treatment, activation is carried out by gas-solid reaction of CO2 and carbon, then high-temperature carbonization is carried out, and original hard carbon material is obtained.A certain proportion of citric acid is mixed uniformly with the hard carbon material, and heat treatment is carried out at a specific temperature, so that hard carbon material with adjustable surface C=O functional group, defect density and pseudo-graphite phase ratio is obtained.The material can effectively optimize the sodium ion storage performance under ester electrolyte system, and improve the electrochemical performance of hard carbon negative electrode.

Description

Technical Field

[0001] This invention relates to technologies related to sodium-ion battery anode materials, and in particular to a method and application for reconstructing C=O functional groups on the surface of coconut shell-based hard carbon materials. Background Technology

[0002] Sodium-ion batteries have received widespread attention in recent years due to the abundance of sodium resources, significant cost advantages, and a working mechanism similar to lithium-ion batteries. They are considered an important complementary technology to lithium-ion batteries. Sodium-ion batteries show promising application prospects in applications where energy density requirements are relatively low but cost and safety requirements are high, such as large-scale energy storage systems and some power and energy storage devices.

[0003] Anode materials are a crucial component of sodium-ion batteries, and their structural characteristics and interfacial stability directly impact battery capacity, initial efficiency, and cycle life. Graphite anode materials, widely used in existing lithium-ion batteries, are unsuitable for sodium-ion battery systems due to their small interlayer spacing, which hinders efficient and reversible sodium ion insertion and extraction. This has prompted researchers to continuously explore anode material systems more suitable for sodium ion storage. Hard carbon materials, with their disordered layered structure, relatively large interlayer spacing, and internal defects and nanopores, can provide multiple storage sites for sodium ions and are considered a promising anode material for sodium-ion batteries. Furthermore, hard carbon materials can be prepared from various biomass resources, with abundant raw material sources and relatively low preparation costs, providing the foundation for large-scale application.

[0004] However, in electrolyte systems primarily composed of ester solvents, hard carbon anode materials are prone to complex interfacial reactions during the initial charge-discharge cycle, leading to the formation of a thick and heterogeneous solid electrolyte interphase (SEI) film on their surface. This type of SEI film typically lacks stability, not only consuming large amounts of active sodium source but also affecting interfacial kinetics during subsequent cycles, resulting in low initial coulombic efficiency and declining cycle performance. Therefore, effectively controlling the surface chemical state of hard carbon without significantly altering its bulk structural characteristics to improve the interfacial compatibility between the hard carbon anode and the electrolyte remains a pressing technical challenge in this field. Summary of the Invention

[0005] One objective of this invention is to provide a method for reconstructing C=O functional groups on the surface of coconut shell-based hard carbon materials. By controlling the chemical state of the hard carbon material surface, the resulting material can maintain the main structural characteristics of hard carbon while having adjustable C=O functional group content, defect degree, and pseudo-graphite phase ratio, thereby improving its sodium storage electrochemical behavior in ester-based electrolytes. This effectively solves the problems of complex interfacial reactions, low initial coulombic efficiency, and insufficient cycle stability of existing hard carbon anode materials in ester-based electrolyte systems.

[0006] Another objective of this invention is to provide coconut shell-based hard carbon materials prepared by the above method and their applications, so as to realize the effective utilization of such materials in the field of sodium-ion battery anodes.

[0007] To achieve the above objectives, this invention provides a method for reconstructing C=O functional groups on the surface of a coconut shell-based hard carbon material, comprising the following steps:

[0008] (1) Take coconut shell raw material and pre-treat it to obtain precursor;

[0009] (2) Under a protective atmosphere, the precursor is heated to 600-900℃ to complete the pre-carbonization process; then the atmosphere is switched to CO2 and the temperature is maintained at 600-900℃ to carry out the gas-solid phase reaction of CO2 and C to complete the activation treatment, and a porous carbon matrix with rich pore structure is obtained.

[0010] (3) Switch back to a protective atmosphere and heat the above CO2-activated porous carbon matrix to 900-1500℃ for carbonization treatment to complete the transformation from open-pore structure to closed-pore structure and obtain hard carbon matrix.

[0011] (4) Under a protective atmosphere, the hard carbon matrix is ​​mixed with citric acid and heated to 160-220°C for pyrolysis treatment, so that the citric acid undergoes a stepwise decarboxylation reaction. Under the condition that the specific surface area of ​​the hard carbon matrix is ​​not significantly increased, oxygen-containing functional groups mainly composed of C=O are directionally introduced on its surface. The C=O functional groups are reconstructed on the surface of the hard carbon matrix, thereby obtaining a coconut shell-based hard carbon material with C=O functional groups on the surface.

[0012] Specifically, in the method for reconstructing C=O functional groups on the surface of the coconut shell-based hard carbon material, in step (2), the heating rate of the heating step is 2-5℃ / min; and / or, the activation treatment time is 0.1-3h.

[0013] Specifically, in the method for reconstructing C=O functional groups on the surface of the coconut shell-based hard carbon material, in step (3), the heating rate of the heating step is 2-5℃ / min; and / or, the carbonization treatment time is 0.1-3h.

[0014] Specifically, in the method for reconstructing C=O functional groups on the surface of the coconut shell-based hard carbon material, in step (4), the heating rate of the heating step is 2-5℃ / min; and / or, the pyrolysis treatment time is 1-5h.

[0015] Specifically, in the method for reconstructing the C=O functional groups on the surface of the coconut shell-based hard carbon material, in step (4), the amount of citric acid added is 1-20 wt% of the mass of the hard carbon material.

[0016] Specifically, in the method for reconstructing C=O functional groups on the surface of the coconut shell-based hard carbon material, in step (4), the mixing step uses a planetary centrifugal mixer to mix the hard carbon matrix with citric acid. The mixing process includes: a first mixing at 100-200 rpm, a second mixing at 400-500 rpm, and a third mixing at 100-200 rpm.

[0017] The times for the first mixing, second mixing, and third mixing stages are independent of each other, ranging from 0.01 to 10 minutes.

[0018] Preferably, the mixing process also includes grinding the mixture for 10-60 minutes.

[0019] Specifically, in the method for reconstructing C=O functional groups on the surface of the coconut shell-based hard carbon material, in step (1), the pretreatment includes cutting, cleaning, drying, crushing and sieving the coconut shell raw material; wherein, preferably, the coconut shell fragments are soaked in deionized water and rinsed multiple times to remove inorganic impurities, and then washed with anhydrous ethanol to reduce organic residues, and after drying, crushing and sieving, the precursor powder is obtained.

[0020] More preferably, the coconut shell raw material is cut to a size of 0.1cm×0.1cm to 2.0cm×2.0cm; the deionized water soaking time is 0.1-24h; the drying temperature is 25-100℃ and the drying time is 0.1-24h; and the sieve used for screening is 50-300 mesh.

[0021] The present invention also relates to a coconut shell-based hard carbon material with surface C=O functional groups modified by the above preparation method.

[0022] Based on the above materials, the present invention further provides their application in the preparation of sodium-ion battery anode materials.

[0023] In addition, the present invention provides a sodium-ion battery negative electrode sheet, wherein the negative electrode material used in the negative electrode sheet comprises the above-mentioned coconut shell-based hard carbon material with surface C=O functional groups modified.

[0024] Furthermore, the present invention also relates to a sodium-ion secondary battery, which includes a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode, wherein the negative electrode is the aforementioned negative electrode.

[0025] Based on this, the present invention also provides a battery module comprising at least one of the above-described sodium-ion secondary batteries.

[0026] Furthermore, the present invention also relates to a battery pack comprising the battery module.

[0027] In addition, the present invention also provides an electrical device comprising at least one power supply unit selected from the above-mentioned sodium-ion secondary battery, battery module or battery pack.

[0028] Compared with existing technologies, this invention uses coconut shell as a precursor to obtain the original hard carbon structure through CO2 activation and high-temperature carbonization. Then, by utilizing the pyrolysis behavior of citric acid under relatively low-temperature conditions, the chemical state of the hard carbon surface is reconstructed, thereby achieving synergistic regulation of surface C=O functional groups, defect degree, and pseudo-graphite phase ratio.

[0029] The hard carbon materials modified in the above manner are beneficial to optimizing their electrode / electrolyte interface characteristics in ester electrolyte systems, promoting the formation of a more uniform and stable solid electrolyte interface structure, improving the stability of interfacial reactions, and helping to shorten the sodium ion diffusion path and enhance sodium ion migration kinetics, thereby improving the first coulombic efficiency, rate performance and cycle stability of hard carbon anode materials in ester-based electrolytes. Attached Figure Description

[0030] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein...

[0031] Figure 1 The FT-IR image of the sample obtained in Example 1;

[0032] Figure 2 The first charge-discharge diagram of the sodium-ion battery anode material obtained in Example 1 is shown.

[0033] Figure 3 The graph shows the rate test results obtained in Example 1 as a negative electrode material for sodium-ion batteries. Detailed Implementation

[0034] Example 1

[0035] Commercially obtained coconut shells were first cut into uniform fragments of approximately 2cm × 2cm, then soaked in deionized water for about 3 hours, and intermittently rinsed three times to remove inorganic impurities. Subsequently, the fragments were washed with 99.99% anhydrous ethanol to remove organic residues and dried in a convection oven at 80°C for 24 hours. The dried material was then pulverized and passed through a 100-mesh sieve to obtain a uniform precursor powder.

[0036] 10g of precursor powder was placed in a tube furnace. First, the temperature was increased to 750℃ at 5℃ / min under a nitrogen flow (80mL / min) to complete the pre-carbonization process. Then, the atmosphere was switched to CO2 and maintained for 2 hours to complete activation. After activation, nitrogen was restored, and the temperature was increased to 1100℃ and maintained for 3 hours to complete the carbonization process, promoting carbon structure rearrangement and partial graphitization. Subsequently, the temperature was reduced to 600℃ at 5℃ / min and allowed to cool naturally to room temperature. The resulting carbon material was ground and passed through a 300-mesh sieve to obtain the original hard carbon material.

[0037] A planetary centrifugal mixer was used to mix 1g of hard carbon material with 1wt% citric acid. The mixing process included: a first mixing at 150 rpm for 5 min; a second mixing at 500 rpm for 10 min; and a third mixing at 100 rpm for 5 min, until the system was thoroughly mixed and homogeneous. The mixture was placed in a corundum crucible and transferred to a vacuum tube furnace, where nitrogen was purged for 2 hours to remove oxygen. Subsequently, pyrolysis was performed under a nitrogen atmosphere at 200°C for 3 hours. After cooling to room temperature, the surface-modified C=O hard carbon material was obtained.

[0038] Hard carbon material, conductive carbon (Super P) and 5wt% PVDF are added to a homogenizing box in a ratio of 8:1:1. An appropriate amount of NMP solvent is added to adjust the viscosity of the slurry. Homogenize for 15 minutes, coat it onto aluminum foil, dry it at 80℃ for 3 hours, cut it into 14mm round pieces, roll and weigh them, vacuum dry at 100℃ for 12 hours, and then transfer it to a glove box for later use.

[0039] Assemble CR2032 coin cells in a glove box filled with argon (H2O and O2 < 0.01ppm). The assembly sequence is: negative electrode shell - sodium sheet - glass fiber separator (Whatman GF / D) - electrode - gasket - spring sheet - electrolyte - positive electrode shell. After encapsulation, let stand for 12 hours to ensure that the electrolyte is fully wetted.

[0040] In this embodiment, the prepared hard carbon material was characterized by FT-IR (see Appendix). Figure 1 The results showed that its surface was rich in oxygen-containing functional groups. Specifically, 3430 cm⁻¹ -1 The absorption peak at 1640 cm⁻¹ corresponds to the OH stretching vibration. -1 The peak at 1120 cm⁻¹ is attributed to the C=O bond vibration. -1The peak at that point is attributed to CO bond vibration.

[0041] The assembled sodium-ion battery underwent its first charge-discharge test at 20 mA / g (see attached). Figure 2 Its initial coulombic efficiency reached 81.66%, and its reversible specific capacity was 335.7 mAh / g.

[0042] In the rate performance test (see appendix) Figure 3 The battery exhibits good performance in the current density range of 50-1500 mA / g, with corresponding specific capacities of 335.7, 158.2, 130.8, 102.7, 85.7, 70.8, 62.5 and 305.4 mAh / g, indicating that the hard carbon material has excellent fast charge and discharge capabilities.

[0043] Example 2

[0044] Similar to Example 1, the difference is that the citric acid ratio is 3wt%, while other operations and parameters remain unchanged.

[0045] Example 3

[0046] Similar to Example 1, the citric acid ratio was 5 wt%, and other conditions were the same.

[0047] Example 4

[0048] The pretreatment was the same as before, with a heating rate of 2℃ / min for pre-carbonization and carbonization. After heating to 600℃, CO2 was switched and maintained for 2 hours, then nitrogen was restored and the temperature was raised to 900℃ and maintained for 3 hours, before being cooled naturally to 600℃. The hard carbon material was ground with 3wt% citric acid for 30 minutes, and the pyrolysis temperature was 160℃ for 3 hours. The remaining operations were the same as in Example 1.

[0049] Example 5

[0050] The pre-carbonization and carbonization heating rates were 4°C / min. After reaching 850°C, CO2 was switched and maintained for 2 hours, then nitrogen was restored and the temperature was raised to 1500°C and maintained for 3 hours, before being cooled naturally to 600°C. Hard carbon was ground with 3wt% citric acid for 30 minutes, and the pyrolysis temperature was 220°C for 3 hours. The remaining operations were the same as in Example 1.

[0051] Comparative Example 1

[0052] The pretreatment was the same as before. The pre-carbonization temperature was raised to 500°C, CO2 was switched and maintained for 2 hours, nitrogen was restored and the temperature was raised to 1100°C and maintained for 3 hours, and then the temperature was lowered to 600°C and allowed to cool naturally. Hard carbon was ground with 1 wt% citric acid for 30 minutes, and the pyrolysis temperature was 140°C for 3 hours. The remaining operations were the same as in Example 1.

[0053] Comparative Example 2

[0054] Pre-carbonization was carried out at 900°C, CO2 was switched and maintained for 2 hours, nitrogen was restored and the temperature was raised to 1100°C and maintained for 3 hours, and then the temperature was lowered to 600°C and allowed to cool naturally. Hard carbon was ground with 1 wt% citric acid for 30 minutes, and pyrolysis was carried out at 240°C for 3 hours. The remaining operations were the same as in Example 1.

[0055] Comparative Example 3

[0056] Pre-carbonization was carried out at 550°C, then CO2 was switched and maintained for 2 hours. Nitrogen was then restored and the temperature was raised to 1100°C and maintained for 3 hours. Finally, the temperature was lowered to 600°C and allowed to cool naturally. Hard carbon was then ground with 3 wt% citric acid for 30 minutes. The pyrolysis temperature was 260°C and the time was 3 hours. The remaining operations were the same as in Example 1.

[0057] Comparative Example 4

[0058] Direct carbonization: Heat to 1100℃ and hold for 3 hours without CO2 activation. Grind hard carbon with 3wt% citric acid for 30 minutes, pyrolyze at 260℃ for 3 hours, and perform the remaining operations as in Example 1.

[0059] Referring to the method in Example 1, the electrochemical performance test results of the batteries under the above Examples 1-4 and Comparative Examples 1-4 are shown in Table 1 below.

[0060] Table 1 Comparison of electrochemical performance between the examples and comparative examples

[0061] sample First Coulomb efficiency First recharge capacity comparison Ratio performance Example 1 81.66%@20mA / g 335.7mAh / g@20mA / g 62.5 mAh / g @ 1500mA / g Example 2 79.35%@20mA / g 288.9mAh / g@20mA / g 51.8 mAh / g@1500mA / g Example 3 78.94%@20mA / g 323.3mAh / g@20mA / g 60.5 mAh / g@1500mA / g Example 4 80.90%@20mA / g 295.9mAh / g@20mA / g 48.0 mAh / g@1500mA / g Comparative Example 1 77.29%@20mA / g 258.7mAh / g@20mA / g 33.1 mAh / g @ 1500mA / g Comparative Example 2 77.05%@20mA / g 270.3mAh / g@20mA / g 39.4 mAh / g @ 1500mA / g Comparative Example 3 75.29%@20mA / g 265.4mAh / g@20mA / g 18.5 mAh / g @ 1500mA / g Comparative Example 4 74.86%@20mA / g 227.4mAh / g@20mA / g 36.8 mAh / g@1500mA / g

[0062] In summary, this invention provides a method for reconstructing C=O functional groups on the surface of hard carbon materials based on coconut shells. The hard carbon materials prepared have tunable surface C=O functional groups, defect levels, and pseudo-graphite phase ratios. This method can be used to improve the sodium storage electrochemical performance of hard carbon anode materials in ester-based electrolytes, shorten the sodium ion diffusion path, and enhance Na+ absorption capacity. + Diffusion kinetics performance improves the initial coulombic efficiency, rate performance, and cycle stability of hard carbon in ester-based electrolytes.

[0063] The embodiments of the present invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method of restructuring C=0 functional groups on the surface of a coconut shell-based hard carbon material, characterized by, The method comprises the following steps: (1) taking coconut shell raw materials for pretreatment to obtain a precursor; (2) under a protective atmosphere, heating the precursor to 600-900℃ to complete a pre-carbonization process; then switching to a CO2 atmosphere and continuing to maintain the temperature at 600-900℃ to perform a gas-solid phase activation treatment of CO2 and C to obtain a porous carbon matrix; (3) switching back to the protective atmosphere, heating the porous carbon matrix to 900-1500℃ to perform a carbonization treatment to complete the conversion of the open pore structure to a closed pore structure to obtain a hard carbon matrix; (4) under a protective atmosphere, mixing the hard carbon matrix with citric acid and heating to 160-220℃ to perform a pyrolysis treatment to complete the reconstruction of C=O functional groups on the surface of the hard carbon matrix.

2. The method of claim 1, wherein, In the step (2): the heating step has a heating rate of 2-5℃ / min; and / or, the activation treatment has a time of 0.01-3h.

3. The method of claim 1, wherein, In the step (3): the heating step has a heating rate of 2-5℃ / min; and / or, the carbonization treatment has a time of 1-5h.

4. The method of claim 1, wherein, In the step (4), the amount of citric acid added is 1-20wt% of the mass of the hard carbon matrix.

5. The method of claim 1, wherein, In the step (4): the heating step has a heating rate of 2-5℃ / min; and / or, the pyrolysis treatment has a time of 1-5h.

6. The method of claim 1, wherein, In the step (4), the mixing step uses a planetary centrifugal mixer to mix the hard carbon matrix and citric acid, and the mixing process comprises: first mixing at 100-200rpm, then second mixing at 400-500rpm, and third mixing at 100-200rpm; the time of the first mixing, second mixing and third mixing stages are independently 0.01-10min.

7. The method according to any one of claims 1 to 6, characterized in that, The protective atmosphere is one or more of nitrogen, argon or other inert gases.

8. A coconut shell-based hard carbon material with a surface C=O functional group enrichment prepared by the method of any one of claims 1-7; The proportion of C=O functional groups in the surface oxygen-containing functional groups on the surface of the material is not less than 55%.

9. Use of the coconut shell-based hard carbon material of claim 8 for preparing a sodium ion battery negative electrode material.

10. A sodium-ion battery, characterized in that, The negative electrode material comprises the coconut shell-based hard carbon material of claim 8.

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