Hard carbon negative electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of sodium ion batteries, in particular to a hard carbon negative electrode material and a preparation method and application thereof. After pre-oxidation treatment and high-temperature carbonization treatment, the nitrogen element is doped into the structure of the hard carbon negative electrode material to form a standard short-range ordered long-range disordered structure, so that the carbon layer spacing of the nitrogen-doped hard carbon negative electrode material is increased, the surface active sites are increased, the transmission of sodium ions is facilitated, and the service life of the hard carbon negative electrode material is prolonged. Therefore, the sodium storage performance of the hard carbon negative electrode material can be improved, and the technical problems of low initial coulombic efficiency and poor reversible capacity of the existing pitch-based amorphous carbon as the negative electrode of the sodium-ion battery are solved.

Description

Technical Field

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

[0002] Hard carbon materials are often used as negative electrode materials in sodium-ion batteries. The synthesis precursors of hard carbon materials are generally resins, biomass, asphalt, etc. Due to the advantages of asphalt in terms of raw material price, carbon yield, production cost, etc., it has been widely studied. However, for the carbon negative electrode materials obtained by directly carbonizing asphalt, the storage capacity of sodium-ion batteries is generally not high, usually difficult to exceed 100 mAh g-1. In addition, the first Coulomb efficiency of such negative electrode materials often tends to be lower than 50%, showing unsatisfactory performance characteristics.

[0003] The disadvantages of the prior art are that asphalt is rich in a large amount of sp 2 hybrid structures. This structure makes the carbonaceous materials obtained by directly pyrolyzing asphalt exhibit highly graphitized characteristics, specifically manifested as a small interlayer spacing between carbon layers. However, this tight structural characteristic limits its sodium storage performance, because the small interlayer spacing is not conducive to the insertion and extraction of sodium ions, thus affecting the sodium storage capacity of the battery. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies of the prior art, and provide a hard carbon negative electrode material, a preparation method thereof, and an application thereof, which solve the technical problems of low first Coulomb efficiency and poor reversible capacity when the existing asphalt-based amorphous carbon is used as the negative electrode of sodium-ion batteries.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions:

[0006] The present invention provides a preparation method of a hard carbon negative electrode material, including the following steps: using asphalt and melamine as raw materials, preparing a precursor through ball milling and mixing; performing pre-oxidation treatment on the precursor to obtain an intermediate; and performing high-temperature carbonization treatment on the intermediate to obtain a hard carbon negative electrode material.

[0007] Optionally, the mass ratio of the asphalt to the melamine is 1-5:1.

[0008] Optionally, the heating temperature of the pre-oxidation treatment is 250 °C, and the heating time is 12 h.

[0009] Optionally, the high-temperature carbonization treatment specifically includes the steps of: under a protective atmosphere, slowly heating to 800 °C - 1800 °C at a heating rate of 2 °C per minute, and maintaining the temperature for 1.5 hours.

[0010] The present invention provides a hard carbon negative electrode material prepared by using the above preparation method of the hard carbon negative electrode material.

[0011] Optionally, the lattice spacing of the hard carbon negative electrode material is 0.35 nm to 0.45 nm.

[0012] Optionally, the hard carbon negative electrode material has a short-range ordered and long-range disordered structure.

[0013] The present invention provides an application of the above-mentioned hard carbon negative electrode material as a negative electrode material in the preparation of a sodium ion battery.

[0014] The beneficial effects of the present invention are as follows. Compared with the prior art, after pre-oxidation treatment and high-temperature carbonization treatment, nitrogen elements are doped into the structure of the hard carbon negative electrode material, forming a standard short-range ordered and long-range disordered structure, which increases the carbon layer spacing and surface active sites of the nitrogen-doped hard carbon negative electrode material, facilitating the transmission of sodium ions, thereby improving the sodium storage performance of the hard carbon negative electrode material and solving the technical problems of low initial Coulomb efficiency and poor reversible capacity of the existing asphalt-based amorphous carbon as the negative electrode of a sodium ion battery.

[0015] In addition, partial oxygen elements will be introduced during the pre-oxidation treatment, and the introduction of oxygen elements will affect the electrochemical performance of the hard carbon material, especially in improving the hydrophilicity of the material and promoting the adsorption and diffusion of sodium ions. Oxygen doping may also further improve the electrochemical performance of the material by enhancing the ion transport ability of the material.

[0016] The oxygen and nitrogen introduced during the preparation of the hard carbon negative electrode material will form a synergistic effect. Specifically, nitrogen doping can change the electronic structure of the material and improve the electronic conductivity, while the introduction of oxygen groups can improve the hydrophilicity and surface activity of the material. The combination of oxygen and nitrogen may form stable nitrogen-oxygen functional groups, such as pyridine nitrogen-oxygen, graphite nitrogen-oxygen, etc. These functional groups provide a more convenient path for the adsorption and diffusion of sodium ions, thereby improving the rate performance and cycle stability of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a preparation route diagram of the hard carbon negative electrode material provided by the present invention.

[0018] Figure 2Scanning electron micrographs of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention. Among them, (a) is the low-magnification image of the hard carbon anode material prepared in Comparative Example 1, (b) is the low-magnification image of the hard carbon anode material prepared in Example 2, (c) is the high-magnification image of the hard carbon anode material prepared in Comparative Example 1, (d) is the high-magnification image of the hard carbon anode material prepared in Example 2, (e) is the low-magnification image of the hard carbon anode material prepared in Example 1, (f) is the low-magnification image of the hard carbon anode material prepared in Example 3, (g) is the high-magnification image of the hard carbon anode material prepared in Example 1, and (h) is the high-magnification image of the hard carbon anode material prepared in Example 3.

[0019] Figure 3 Transmission electron micrographs of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention. Among them, (a) is the transmission electron micrograph of the hard carbon anode material prepared in Comparative Example 1, (b) is the transmission electron micrograph of the hard carbon anode material prepared in Example 1, (c) is the transmission electron micrograph of the hard carbon anode material prepared in Example 2, and (d) is the transmission electron micrograph of the hard carbon anode material prepared in Example 3.

[0020] Figure 4 X-ray diffraction patterns of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention.

[0021] Figure 5 Raman spectra of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention.

[0022] Figure 6 XPS full spectra of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention.

[0023] Figure 7 Surface element distribution maps of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention. Among them, (a) to (d) are the surface element distribution maps of different hard carbon anode materials, (e) to (h) are the C element distribution maps of different hard carbon anode materials, (i) to (l) are the O element distribution maps of different hard carbon anode materials, and (m) to (o) are the N element distribution maps of different hard carbon anode materials.

[0024] Figure 8 First charge / discharge curves of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention.

[0025] Figure 9 Cycling performance graphs of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention.

[0026] Figure 10 The rate performance graphs of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention.

[0027] Figure 11 The long cycle performance graphs of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention. Detailed implementation manners

[0028] To solve the above technical problems, the present invention provides a hard carbon anode material, its preparation method and application. Now, the technical solutions and embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0029] The present invention will be described in detail below through specific embodiments. The embodiments are exemplary and are intended to explain the present invention, but should not be construed as a limitation to the present invention.

[0030] Example 1

[0031] Refer to Figure 1 , this example provides a preparation method of a hard carbon anode material, including the following steps:

[0032] (1) Ball milling and mixing: Four grams of pitch and two grams of melamine are ball milled and mixed. During mixing, the equipment first rotates forward at a speed of 900 revolutions per minute for 30 minutes, and then rotates backward and runs at the same speed for another 30 minutes. This forward and backward rotation process is repeated five times to form a precursor.

[0033] (2) Pre-oxidation: The precursor is placed in a muffle furnace and heated to 250°C at a heating rate of 5°C per minute, and kept at this temperature for 12 hours. The purpose of this step is to perform pre-oxidation treatment to generate an intermediate.

[0034] (3) High-temperature carbonization: The intermediate is transferred to a tubular furnace, and argon is used as a protective atmosphere. It is slowly heated to 1100°C at a heating rate of 2°C per minute and kept at this high temperature for 1.5 hours to obtain the hard carbon anode material.

[0035] Example 2

[0036] This example provides a preparation method of a hard carbon anode material. The difference from Example 1 is that the addition amount of melamine is one gram.

[0037] Example 3

[0038] This example provides a preparation method of a hard carbon anode material. The difference from Example 1 is that the addition amount of melamine is four grams.

[0039] Example 4

[0040] This embodiment provides a method for preparing a hard carbon negative electrode material, which is different from that of Embodiment 1 in that the heat preservation time of pre-oxidation is 6 h.

[0041] Example 5

[0042] This embodiment provides a method for preparing a hard carbon negative electrode material, which is different from that of Embodiment 1 in that the temperature of high-temperature carbonization is 900 °C.

[0043] Example 6

[0044] This embodiment provides a method for preparing a hard carbon negative electrode material, which is different from that of Embodiment 1 in that the temperature of high-temperature carbonization is 1300 °C.

[0045] Comparative Example 1

[0046] This comparative example provides a method for preparing a hard carbon negative electrode material, which is different from that of Embodiment 1 in that no melamine is added.

[0047] Material characterization test:

[0048] The hard carbon negative electrode material prepared in Embodiment 1 was labeled as P-M12-N2-1100, the hard carbon negative electrode material prepared in Embodiment 2 was labeled as P-M12-N1-1100, the hard carbon negative electrode material prepared in Embodiment 3 was labeled as P-M12-N4-1100, and the hard carbon negative electrode material prepared in Comparative Example 1 was labeled as P-M12-1100. Among them, P represents pitch, M is the heat preservation time of pre-oxidation, N is the addition amount of melamine, and 1100 is the temperature of high-temperature carbonization.

[0049] The crystal structures and defect conditions of the hard carbon negative electrode materials prepared in Examples 1 to 3 and Comparative Example 1 were analyzed using a scanning electron microscope, a transmission electron microscope, an X-ray diffractometer, and a Raman spectrometer, and the surface element composition and the distribution of nitrogen doping sites of the hard carbon negative electrode materials prepared in Examples 1 to 3 and Comparative Example 1 were analyzed using an X-ray photoelectron spectrometer.

[0050] Electrochemical performance test:

[0051] The hard carbon negative electrode materials prepared in Examples 1 to 3 and Comparative Example 1 were used as the negative electrodes respectively, the counter electrode was a sodium sheet, and the electrolyte was 1 mol sodium trifluoromethanesulfonate dissolved in diglyme, and a sodium ion battery of model CR2032 was assembled.

[0052] The test results are as follows:

[0053] (1) Analysis of the test results of the scanning electron microscope

[0054] Figure 2Scanning electron microscope images of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention. It can be observed from Figure 2 that with different amounts of melamine incorporated, the surface morphology and particle structure of the hard carbon materials have changed significantly. P-M12-1100, the sample without nitrogen doping, exhibits a relatively regular particle structure with relatively close contact between particles. The particle morphology is relatively large and the surface is smooth. Although this structure may provide certain stability, due to the lack of a rich pore structure and reaction sites, the ion diffusion and charge transport capabilities are limited, which is also one of the reasons for its poor electrochemical performance.

[0055] In the nitrogen-doped samples, especially P-M12-N2-1100, the sample with 2 g of melamine doped with nitrogen, its scanning electron microscope image shows a significant change in particle morphology. The particles of this sample are relatively uniform, and more pores and microcracks appear on the surface, showing a relatively complex structure overall. These microcracks and pores can provide more reaction sites and diffusion channels for ions, contributing to the rapid insertion and extraction of sodium ions in the material, thereby improving the rate performance and cycling stability of the material. This phenomenon is consistent with the improvement of electrochemical performance, indicating that nitrogen doping optimizes the energy storage performance of the material by changing its microscopic morphology.

[0056] The scanning electron microscope images of P-M12-N1-1100 and P-M12-N4-1100 also reveal similar changes. The surface of the P-M12-N1-1100 sample is relatively flat. Although there are a small number of pores and microcracks, the overall particle morphology and pore structure are relatively simple, indicating that low nitrogen doping has limited improvement on the pore structure of hard carbon. On the contrary, the particles of the P-M12-N4-1100 sample show excessive aggregation and agglomeration, and the surface pore structure is relatively complex. However, due to excessive nitrogen doping, it may lead to unstable structure of the material, affecting the conductivity and ion diffusion performance of the material.

[0057] By comparing the scanning electron microscope images, it can be concluded that appropriate nitrogen doping, such as P-M12-N2-1100, can significantly improve the microscopic morphology of hard carbon materials, form more reaction sites and ion diffusion channels, thereby enhancing the electrochemical performance. Excessive or insufficient nitrogen doping will lead to unstable structure or unoptimized pore structure, thus affecting the improvement of electrochemical performance. Therefore, reasonably adjusting the amount of nitrogen doping is crucial for optimizing the microscopic structure and electrochemical performance of pitch-based hard carbon materials.

[0058] (2) Analysis of transmission electron microscope test results

[0059] Figure 3 Transmission electron microscope images of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention. It can be observed fromFigure 3 It can be clearly seen that nitrogen doping has a significant impact on the microstructure of hard carbon materials. In P-M12-1100, the sample without nitrogen doping, the TEM image shows that the structure of the material mainly exhibits amorphous characteristics, with carbon atoms arranged disorderly and almost no obvious ordered layered structure. This disordered structure may lead to a relatively complex charge transport path and poor electronic conductivity, thereby affecting the electrochemical performance. Carbon materials with a disordered structure usually lack stable conduction paths and sufficient active sites, which is also one of the reasons why the P-M12-1100 sample performs poorly in electrochemical performance tests.

[0060] In contrast, the TEM image of the P-M12-N2-1100 sample shows more significant changes. After nitrogen doping, more ordered layered structures are formed in the hard carbon material, with a lattice spacing of about 0.384 nm to 0.403 nm, indicating that nitrogen doping promotes the formation of ordered structures inside the material. The formation of layered structures not only provides a more stable conduction path for the material but also increases the migration channels for electrons and ions, which plays a positive role in improving the rate performance and cycle stability of the battery. In addition, nitrogen doping sites, especially active nitrogen sites such as pyridine nitrogen and graphitic nitrogen, are observed in the layered structures in the TEM image. These sites can effectively promote the conduction of electrons and ions, further enhancing the electrochemical performance.

[0061] The TEM images of P-M12-N1-1100 and P-M12-N4-1100 show different microstructural characteristics respectively. The nitrogen doping in the P-M12-N1-1100 sample is less. Although some layered structures can still be observed, the formed layer spacing is larger and there is a certain degree of disordered structure. Compared with the P-M12-N2-1100 sample, the layered structure of the P-M12-N1-1100 sample is relatively loose, and the conduction performance of electrons and ions is poor, which explains its mediocre performance in electrochemical performance tests. In the P-M12-N4-1100 sample, the layered structure is relatively dense, but excessive nitrogen doping leads to over-structuring of the material, with a large number of defects and distortions, which inhibits its conductivity and also hinders ion diffusion to a certain extent. Therefore, excessive nitrogen doping may lead to the instability of the material microstructure and affect its electrochemical performance.

[0062] Through the analysis of TEM images, the following conclusions can be drawn: Appropriate nitrogen doping, such as P-M12-N2-1100, can promote the formation of ordered structures in hard carbon materials, increase the migration channels of electrons and ions, thereby enhancing the conductivity and ion diffusion ability of the materials, and ultimately improving the rate performance and cycle stability of the battery. Excessive or insufficient nitrogen doping may lead to unstable material structures and affect the improvement of their electrochemical performance. Therefore, reasonable control of the nitrogen doping amount is the key to optimizing the microstructure and electrochemical performance of hard carbon materials.

[0063] (3) Analysis of X-ray diffraction test results

[0064] Figure 4 The X-ray diffraction patterns of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention are shown. From Figure 4 it can be clearly observed that all samples show a broad diffraction peak located at about 23°, which indicates that these materials mainly exhibit an amorphous or low-graphitized structure. The amorphous nature of hard carbon materials is an important difference from traditional graphite materials. Amorphous carbon materials usually have a high defect density and a rich pore structure, and these characteristics enable hard carbon materials to exhibit a high specific capacity and good cycle stability in sodium-ion batteries.

[0065] In the nitrogen-doped samples, especially P-M12-N2-1100, the X-ray diffraction pattern shows a slight broadening of its diffraction peak. Compared with P-M12-1100, the width of its diffraction peak increases, and the diffraction intensity also changes. This indicates that nitrogen doping may introduce more defects or promote the local graphitization process in the hard carbon structure. Although nitrogen doping does not significantly change the overall crystal structure of the material, its influence on the local structure indicates that nitrogen atoms may enter the carbon-based structure, forming nitrogen vacancies or other types of defect sites, thereby changing the microstructure of the material. These defect sites help to enhance the electrochemical activity and increase the charge storage capacity.

[0066] Further analyzing the X-ray diffraction patterns of P-M12-N4-1100 and P-M12-N1-1100, it is found that their diffraction peak morphologies are not significantly different from that of the P-M12-N2-1100 sample. However, the P-M12-N4-1100 sample shows a more obvious broadening of the diffraction peak, and the intensity of the diffraction peak is weaker. This may indicate that excessive nitrogen doping leads to unstable material structures, and even may result in crystal defects or local structural disorders, thereby affecting the conductivity and energy storage performance of the material. This phenomenon is consistent with the results of the electrochemical performance analysis. The poor performance of the P-M12-N4-1100 sample may be due to the unstable microstructure caused by excessive nitrogen doping.

[0067] In addition, the X-ray diffraction pattern of the P-M12-N2-1100 sample exhibits obvious amorphous carbon characteristics. However, compared with P-M12-1100, its diffraction peaks are slightly broader, indicating a moderate degree of amorphousness and possibly containing a small amount of graphitized regions. Nitrogen doping can promote the stability of the carbon skeleton and provide more electrochemically active sites, which plays an important role in improving the electrochemical performance of the material.

[0068] In summary, the X-ray diffraction analysis results show that although nitrogen doping does not change the overall crystal structure of the hard carbon material, it improves the microstructure of the material by introducing defects and local graphitization. These changes not only provide more electrochemically active sites for the hard carbon material but also may enhance ion diffusion, thereby improving the electrochemical performance of the material. Appropriate nitrogen doping, such as P-M12-N2-1100, can optimize the electrochemical performance of hard carbon without damaging the material structure, while excessive nitrogen doping, such as P-M12-N4-1100, may lead to structural instability and thus affect its battery performance. Therefore, reasonable regulation of the nitrogen doping amount is the key to improving the electrochemical performance of hard carbon materials.

[0069] (4) Analysis of Raman spectroscopy test results

[0070] Figure 5 This is the Raman spectrum of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention. From Figure 5 it can be observed that all samples show two main characteristic peaks: a D band at a wavelength of approximately 1350 cm -1 and a G band at a wavelength of approximately 1600 cm -1 . The D band is related to defects or disordered structures in the carbon material, while the G band is related to the ordered graphite structure in the carbon material. By analyzing the intensity ratio I D / I G of the D band and the G band, important information about the defect density and structural disorder of the material can be obtained.

[0071] In P-M12-1100, the intensity ratio of the D band to the G band is 1.12, indicating a low defect density and a relatively ordered structure for this sample. However, since hard carbon materials usually exhibit an amorphous or low-graphitized structure, the presence of defects and disordered structures is an important factor in their energy storage capacity. Although the I D / I G ratio of the P-M12-1100 sample is low, its specific capacity and electrochemical stability are relatively poor, which is closely related to its relatively regular structure, fewer defect sites, and poor ion diffusivity.

[0072] In P-M12-N2-1100, the Raman spectrum shows a significant increase in the intensity of the D band, and I D / I G The ratio increased to 1.33. This indicates that nitrogen doping introduced more defect sites, resulting in a more disordered structure of the material. The defect sites provided more reactive sites for the insertion and extraction of sodium ions, thus enhancing the energy storage capacity of the battery. In addition, the enhancement of the D band in the Raman spectrum may also indicate that nitrogen doping promoted the formation of active nitrogen sites such as graphitic nitrogen N-6 and pyridinic nitrogen N-Q in the carbon material. These sites can effectively improve the electronic conductivity and ion diffusion ability of the material, thereby improving the rate performance and cycle stability of the battery. The higher I D / I G ratio of the P-M12-N2-1100 sample is consistent with its better electrochemical performance, indicating that appropriate nitrogen doping can effectively improve the electrochemical activity of the material.

[0073] For P-M12-N1-1100 and P-M12-N4-1100, the Raman spectra show that their I D / I G ratios are 1.20 and 1.40 respectively, which are between those of P-M12-1100 and P-M12-N2-1100. P-M12-N1-1100 has less nitrogen doping and fewer defect sites, so the I D / I G ratio is lower. In contrast, P-M12-N4-1100 has a higher nitrogen doping amount, resulting in too many defect sites and a slightly higher I D / I G ratio, and the material structure is more disordered. This indicates that excessive nitrogen doping may lead to unstable structure of the material, even affecting its electronic conductivity and ion diffusivity. Excessive nitrogen doping may cause excessive structuring of the carbon material, thus forming more unstable regions and affecting the long-term stability of the battery.

[0074] Generally speaking, the Raman spectrum analysis results show that nitrogen doping can significantly affect the microstructure of the hard carbon material, especially the defect density and structural disorder. Appropriate nitrogen doping, such as P-M12-N2-1100, improves the electrochemical performance of the carbon material by introducing more defect sites and active nitrogen sites, while excessive nitrogen doping, such as P-M12-N4-1100, may lead to unstable structure and affect the conductivity and ion diffusion ability of the material. Therefore, reasonably adjusting the nitrogen doping amount is crucial for optimizing the electrochemical performance of the hard carbon material.

[0075] (5) Analysis of XPS full-spectrum test results

[0076] Figure 6 are the XPS full-spectrum diagrams of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention. See Figure 6 , and the specific analysis is as follows:

[0077] Analysis of C 1s spectrum: The C 1s spectrum is one of the key spectra in XPS analysis, which can provide important information about the chemical environment of carbon elements. In the C 1s spectra of all samples, the main peak positions can be observed, which are 284.8 eV, 285.5 eV, and 287.8 eV respectively. These peaks represent different types of carbon bonding environments. Specifically, the peak at 284.8 eV corresponds to sp 2 hybridized carbon atoms, usually representing carbon atoms in the graphite structure; the peak at 285.5 eV is usually related to C—C and C—H bonds, indicating organic carbon in the carbon material; while the peak at 287.8 eV represents C—O and C—N bonds, which are usually related to surface functional groups such as carboxyl groups and amide groups.

[0078] In P-M12-1100, the C 1s spectrum shows relatively obvious peaks at 285.5 eV and 284.8 eV, while the intensity of the peak at 287.8 eV is weak. This indicates that the surface of the nitrogen-free doped hard carbon material is mainly composed of C—C and C—H bonds, with fewer oxidation groups and nitrogen sites, suggesting that the surface activity of this sample is low and the electrochemical performance is relatively poor.

[0079] In contrast, in the C 1s spectrum of P-M12-N2-1100, the intensity of the peak at 287.8 eV increases significantly, indicating that nitrogen doping has successfully introduced C—N bonds and may also have formed nitrogen-carbon covalent bonds. The increase in C—N bonds means that more nitrogen doping sites appear on the surface of the hard carbon material. These sites provide more electrochemical active sites for the battery, which can promote the storage and deintercalation of sodium ions, thereby improving the specific capacity and cycle stability of the battery. In addition, the increase in surface functional groups of the nitrogen-doped material, such as amino groups and pyridine nitrogen, helps to improve the conductivity and ion diffusivity of the material.

[0080] The C 1s spectra of P-M12-N1-1100 and P-M12-N4-1100 show similar trends, but the intensity of the peak at 287.8 eV in P-M12-N4-1100 is more prominent, indicating that excessive nitrogen doping has led to more nitrogen-carbon bonding structures, which may cause instability on the material surface and thus affect its electrochemical performance. Therefore, appropriate nitrogen doping can optimize surface functional groups, improve conductivity and ion diffusion ability, while excessive nitrogen doping may lead to too many unstable structures, which instead inhibits electrochemical performance.

[0081] Oxygen element analysis: In XPS analysis, the distribution and chemical state of oxygen elements are also important characterization indicators. Because oxygen not only forms oxides on the surface of materials, but may also react chemically with other elements such as carbon and nitrogen, thereby affecting the surface chemical properties, conductivity and electrochemical performance of materials. In this study, XPS was used to analyze the oxygen element content of pitch-based hard carbon materials at different nitrogen doping levels, and the influence of the chemical state of oxygen on the performance of hard carbon materials was mainly analyzed.

[0082] Analysis of O 1s spectra: In the O 1s spectra in XPS analysis, several peaks related to oxygen elements can mainly be observed, and these peaks correspond to different oxides or oxygen functional groups respectively. Common oxides or functional groups include C—O at about 532.0 eV, C=O at about 531.0 eV, O—C=O at about 533.0 eV, etc. The O 1s spectra provide information about the oxygen content and its oxidation state on the material surface, and the introduction of oxygen may have a significant impact on the electrochemical performance of the material.

[0083] In the O 1s spectra of P-M12-1100, the main peak positions appear at 532.0 eV and 531.0 eV, namely the C—O bond and the C=O bond, indicating that there are a large number of oxidation groups on the surface of the nitrogen-free doped hard carbon material. These oxidation groups may provide hydrophilicity for the material, promoting the adsorption and diffusion of sodium ions, so they play a certain role in the electrochemical reaction. However, the existence of these oxygen functional groups may also lead to a decrease in electronic conductivity. Especially during the charge and discharge process of the battery, the oxygen groups may react with the electrolyte, thereby affecting the cycle stability of the material.

[0084] In contrast, in the O 1s spectra of P-M12-N2-1100, the intensity of the 532.0 eV peak is relatively enhanced, and at the same time, a new peak position appears at 533.0 eV, namely the O—C=O bond. This change indicates that nitrogen doping not only enhances the formation of oxides on the material surface, but also the interaction between nitrogen atoms and oxygen groups may promote the stability of the oxides. Especially the appearance of O—C=O may indicate the synergistic effect of nitrogen and oxygen, and this structure may play a positive role in the conductivity and energy storage capacity of the material. In addition, nitrogen doping may also promote the further optimization of oxygen sites, making the oxides less likely to have adverse reactions with sodium ions, thereby improving the cycle stability of the material.

[0085] The O1s spectra of P-M12-N1-1100 and P-M12-N4-1100 also showed similar changes, but the oxygen functional group signals of P-M12-N4-1100 were more significant, especially at the peak position of 533.0 eV. The stronger oxygen signal indicates that excessive nitrogen doping may lead to the over-generation of oxides, and these excessive oxides may affect the stability and electrochemical performance of the material. Excessive oxygen groups may cause the material surface to have too high hydrophilicity, affecting the ion diffusion path, and may even lead to the attenuation of the material after long-term cycling.

[0086] The influence of oxygen element on electrochemical performance: The presence of oxygen element in hard carbon materials has a profound impact on electrochemical performance. Oxygen groups can improve the hydrophilicity of the material, making it easier for sodium ions to adsorb onto the material surface, thereby enhancing the initial specific capacity of the material. However, the presence of oxides may also have an adverse effect on the conductivity of the material, because oxygen functional groups may reduce the electron migration rate, thus affecting the rate performance and cycling stability of the material.

[0087] In the P-M12-N2-1100 sample, the interaction between nitrogen doping and oxygen functional groups may have produced a favorable synergistic effect, enhancing the electrochemical stability of the material. The introduction of oxygen enhanced the hydrophilicity of the material, while the introduction of nitrogen atoms helped to improve the conductivity and ion diffusivity of the material. Therefore, the combined action of appropriate amounts of nitrogen and oxygen can improve the overall electrochemical performance of hard carbon materials, making them exhibit a higher specific capacity and better cycling stability in batteries.

[0088] However, in the P-M12-N4-1100 sample, the excessive introduction of oxygen may lead to the over-formation of oxides, and these oxides may have an adverse interaction with sodium ions, thereby reducing the electrochemical performance of the material. Especially during long-term cycling, oxygen groups may react with the electrolyte, resulting in capacity decay.

[0089] The synergistic effect of oxygen and nitrogen: The synergistic effect of oxygen and nitrogen plays a crucial role in hard carbon materials. Nitrogen doping can not only change the electronic structure of the material and enhance the electronic conductivity, but also the introduction of oxygen groups can improve the hydrophilicity and surface activity of the material. The combination of oxygen element and nitrogen may form stable nitrogen-oxygen functional groups, such as pyridine nitrogen-oxygen, graphite nitrogen-oxygen, etc. These functional groups provide a more convenient path for the adsorption and diffusion of sodium ions, thereby enhancing the rate performance and cycling stability of the material.

[0090] Overall, the presence of oxygen has a dual impact on the electrochemical performance of hard carbon materials. An appropriate amount of oxygen functional groups can increase the hydrophilicity and active sites of the materials, thereby enhancing the diffusion and storage capabilities of sodium ions. However, excessive oxygen groups may lead to unstable material structures, affecting their conductivity and long-term cycling performance. Therefore, rationally regulating the ratio of nitrogen and oxygen is the key to optimizing the electrochemical performance of hard carbon materials.

[0091] The XPS analysis results show that the combined action of nitrogen doping and oxygen can significantly improve the electrochemical performance of pitch-based hard carbon materials, especially in energy storage and battery applications, with great potential. By optimizing the ratio of nitrogen and oxygen, hard carbon materials with excellent electrochemical performance can be designed, thus promoting the wide application of sodium-ion batteries and other energy storage devices.

[0092] Analysis of the N 1s spectrum: The N 1s spectrum is another important spectrum in XPS analysis, which can provide information on the chemical state of nitrogen. In the N 1s spectra of all samples, three main peaks can be observed, located at approximately 398.0 eV, 399.5 eV, and 401.2 eV respectively. These peaks correspond to different types of nitrogen atoms: the 398.0 eV peak corresponds to pyridinic nitrogen N-Q, which usually exists at the edges or defect sites of carbon-based materials; the 399.5 eV peak corresponds to graphitic nitrogen N-5, which is usually related to the graphite structure in carbon materials; while the 401.2 eV peak corresponds to amino nitrogen N-6, and this type of nitrogen is usually introduced through nitrogen doping reactions.

[0093] In the N 1s spectrum of P-M12-1100, the nitrogen signal is very weak and there are almost no obvious peaks, indicating that this sample has no nitrogen doping, a very low nitrogen content, and poor electrochemical activity. In P-M12-N2-1100, three obvious nitrogen peaks appear in the N 1s spectrum, especially the relatively high intensity of the pyridinic nitrogen N-Q peak, indicating that nitrogen doping effectively introduces pyridinic nitrogen sites. These nitrogen sites help to improve the electronic conductivity and ion diffusivity of the material, thereby enhancing the rate performance and cycling stability of the battery.

[0094] The N 1s spectra of P-M12-N1-1100 and P-M12-N4-1100 also show similar nitrogen doping peaks, but the nitrogen signal in the P-M12-N4-1100 sample is stronger, especially the enhancement of the amino nitrogen N-6 peak. This may be due to the uneven distribution of nitrogen sites caused by excessive nitrogen doping, which in turn affects the stability and electrochemical performance of the material. Therefore, appropriate nitrogen doping, such as P-M12-N2-1100, can provide more nitrogen active sites, which is beneficial to improving the performance of the battery, while excessive nitrogen doping, such as P-M12-N4-1100, may lead to structural instability.

[0095] Analysis of N—C covalent bonds: Through the C1s and N1s spectra in XPS analysis, the covalent bonding between nitrogen atoms and carbon atoms can be further studied. The presence of C—N covalent bonds can promote the conduction of electrons in carbon-based materials and also increase the electrochemical active sites of carbon materials. In the P-M12-N2-1100 sample, the analysis of the C 1s and N1s spectra shows that nitrogen doping introduces a large number of C—N covalent bonds, which not only enhance the electronic conductivity of the material but also provide more electrochemical active sites, further improving the energy storage capacity and cycle stability of the battery.

[0096] In summary, the XPS analysis results show that nitrogen doping significantly optimizes the electrochemical performance of hard carbon materials by introducing various nitrogen sites such as pyridine nitrogen, graphitic nitrogen, and amino nitrogen. Appropriate nitrogen doping can play a key role in improving electronic conductivity and ion diffusivity, while excessive nitrogen doping may lead to surface instability of the material and affect its electrochemical performance. Therefore, reasonable control of the nitrogen doping amount is the key to optimizing the electrochemical performance of hard carbon materials.

[0097] (6) Analysis of EDS surface element test results

[0098] Figure 7 This is the surface element distribution map of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention.

[0099] As Figure 7 shown, specifically referring to Figures (m) to (o), EDS analysis shows that the distribution of nitrogen elements in nitrogen-doped samples is relatively uniform, and with the increase of nitrogen doping amount, the content of nitrogen elements increases significantly. Specifically, the nitrogen element content in the P-M12-N2-1100 sample is 2.56%, and the nitrogen elements are evenly distributed in the carbon-based structure, showing high doping uniformity.

[0100] The content of oxygen elements in all samples remains between 4% and 5%, which may be due to the oxygen introduced during the pre-oxidation treatment. The introduction of oxygen has a certain impact on the electrochemical performance of hard carbon materials, especially in enhancing the hydrophilicity of the material and promoting the adsorption and diffusion of sodium ions. Oxygen doping may also further improve the electrochemical performance of the material by enhancing its ion transport ability.

[0101] (7) Analysis of constant current charge and discharge test results

[0102] Figure 8 This is the first charge / discharge curve diagram of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention. Figure 9 This is the cycle performance diagram of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 14 provided by the present invention.

[0103] First, refer to Figure 8 and Figure 9 , the P-M12-N2-1100 sample exhibits excellent specific capacity at a low current density of 0.1 A / g. At a current density of 0.1 A / g, the first Coulombic efficiency can reach 90%, and the discharge capacity in the first cycle is as high as 322 mAh·g -1 . After 200 cycles, the initial specific capacity of P-M12-N2-1100 is about 300 mAh / g, and it still maintains 250 mAh / g after 200 cycles, with a capacity retention rate as high as 83.3%. This result indicates that nitrogen doping can not only improve the specific capacity of hard carbon materials but also effectively enhance the cycling stability of the materials.

[0104] At a relatively high current density of 1 A / g, the discharge capacity of the P-M12-N2-1100 sample also shows good stability. Even after 1000 cycles, the specific capacity of this sample still remains stable at about 170 mAh / g, showing excellent cycling stability. This proves that nitrogen doping can enhance the conductivity and ion diffusivity of the material, thereby improving the discharge performance of the battery at high current densities.

[0105] In contrast, the specific capacity of P-M12-1100 at a low current density of 0.1 A / g is significantly lower than that of the nitrogen-doped sample, and the capacity decay is more obvious after 200 cycles. This indicates that the hard carbon material lacking nitrogen doping has a lower ion diffusion rate during charge and discharge processes, resulting in poor cycling stability of the battery.

[0106] Through constant current charge-discharge tests, it can be concluded that nitrogen doping not only increases the specific capacity of the material but also significantly enhances its cycling stability and rate performance. Appropriate nitrogen doping, such as P-M12-N2-1100, can effectively improve the electrochemical performance of hard carbon materials and enhance the energy density and power density of sodium-ion batteries.

[0107] (8) Analysis of rate performance test results

[0108] Figure 10 This is the rate performance graph of the hard carbon negative electrode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention.

[0109] From Figure 10 , it can be seen that the rate performance of the P-M12-N2-1100 sample is particularly prominent at different current densities. At a high current density of 5 A / g, the discharge capacity of P-M12-N2-1100 can still maintain a relatively high level, showing strong rate performance. In contrast, the rate performance of the P-M12-1100 sample is poor, and the discharge capacity rapidly decays with the increase of the current density, proving its weak rate performance.

[0110] This result indicates that nitrogen doping can significantly improve the rate performance of hard carbon materials. Nitrogen doping optimizes the pore structure and ion diffusion channels by changing the microstructure of hard carbon, thereby effectively enhancing the rate performance of the materials. Especially at high current densities, nitrogen doping provides more active sites, enhancing the migration ability of ions in the materials and thus maintaining a high discharge capacity.

[0111] Further analysis shows that the P-M12-N2-1100 sample exhibits lower internal resistance and higher ion diffusion rate at high rates, demonstrating that nitrogen doping can optimize the electrical conductivity and ion conductivity of electrode materials, providing more energy storage space and faster ion migration channels for the battery at high rates.

[0112] (9) Analysis of the cyclic stability test results

[0113] Figure 11 This is the long-term cyclic performance graph of the hard carbon anode materials prepared in Examples 1 to 3 and Comparative Example 1 provided by the present invention.

[0114] Cyclic stability is a key indicator for evaluating the long-term performance of batteries. Referring to Figure 11 , during 1000 cycles of testing, the P-M12-N2-1100 sample demonstrated excellent long-term stability. After 1000 cycles, the discharge capacity of P-M12-N2-1100 remained at 170 mAh / g, indicating its good cyclic stability. However, the discharge capacity of the P-M12-1100 sample decreased significantly with the increase in the number of cycles, indicating its poor cyclic stability.

[0115] It can be seen from the cyclic stability test that nitrogen-doped hard carbon materials, such as P-M12-N2-1100, exhibit high capacity retention and low capacity decay, which is attributed to the improvement of the electrical conductivity and the stability of the pore structure of hard carbon by nitrogen doping, reducing the decay of the electrode material during long-term cycling.

[0116] The above-described are only the preferred embodiments of the present invention, and the above specific embodiments do not limit the present invention. Within the scope of the technical idea of the present invention, various deformations and modifications can occur. Any embellishment, modification, or equivalent replacement made by those of ordinary skill in the art based on the above description belongs to the scope protected by the present invention.

Claims

1. A method for preparing a hard carbon negative electrode material, characterized in that, It includes the following steps: Using asphalt and melamine as raw materials, through ball milling and mixing treatment, a precursor is prepared; the mass ratio of the asphalt to the melamine is 1-5:1; Performing pre-oxidation treatment on the precursor to obtain an intermediate; Performing high-temperature carbonization treatment on the intermediate to obtain a hard carbon negative electrode material.

2. The preparation method of the hard carbon negative electrode material according to claim 1, characterized in that, The mass ratio of the asphalt to the melamine is 2:

1.

3. The preparation method of the hard carbon negative electrode material according to claim 1, characterized in that, The heating temperature of the pre-oxidation treatment is 250°C - 255°C, and the heating time is 6h - 12h.

4. The preparation method of the hard carbon negative electrode material according to claim 3, wherein, The conditions of the pre-oxidation treatment are: heating to 250°C at a heating rate of 5°C / minute, and maintaining at this temperature for 12h.

5. The preparation method of the hard carbon negative electrode material according to claim 1, characterized in that, The high-temperature carbonization treatment specifically includes the steps: Under a protective atmosphere, heating to 800°C - 1800°C at a heating rate of 2°C / min, and maintaining at a constant temperature for 1.5h.

6. The preparation method of the hard carbon negative electrode material according to claim 5, characterized in that, Heating to 1100°C at a heating rate of 2°C / min.

7. A hard carbon negative electrode material, characterized in that, Prepared by using the preparation method of the hard carbon negative electrode material according to any one of claims 1-6.

8. The hard carbon negative electrode material according to claim 7, characterized in that The lattice spacing of the hard carbon negative electrode material is 0.35nm - 0.45nm.

9. The hard carbon negative electrode material according to claim 7, wherein The hard carbon negative electrode material has a structure with short-range order and long-range disorder.

10. Application of the hard carbon negative electrode material according to any one of claims 7-9 as a negative electrode material in the preparation of a sodium ion battery.

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