A nitrogen-self-doped phenolic resin-based hard carbon microsphere, its preparation method and application

Abstract

This invention belongs to the field of new energy materials and devices, specifically disclosing a nitrogen-self-doped phenolic resin-based hard carbon microsphere, its preparation method, and its application. The preparation method includes the following steps: mixing nitrogen-containing phenolic monomers, aldehyde monomers, and an alkaline catalyst in a certain order, prepolymerizing at low temperature, and then performing a high-temperature hydrothermal reaction to obtain phenolic resin microspheres; subsequently subjecting them to low-temperature carbonization and high-temperature carbonization treatments to obtain nitrogen-self-doped phenolic resin-based hard carbon microspheres. Compared to existing nitrogen-doped resin hard carbon preparation methods, the above-mentioned nitrogen-self-doped phenolic resin-based hard carbon microspheres, used in the anode of sodium-ion batteries, introduce nitrogen from the resin source, simplifying the production process and achieving uniform nitrogen distribution and atomic-level doping. This material, used in sodium-ion batteries, exhibits excellent long-cycle stability, good rate performance, and high specific capacity, showing promising prospects for industrial application.

Description

Technical Field

[0001] This invention relates to the field of new energy materials and devices, specifically to a nitrogen-self-doped phenolic resin-based hard carbon microsphere, its preparation method, and its application. Background Technology

[0002] In recent years, the imbalance between supply and demand has led to soaring global lithium prices, severely limiting the application of lithium-ion batteries in large-scale energy storage. Developing new electrochemical energy storage systems is therefore urgently needed. Sodium-ion batteries, with a similar structure to lithium-ion batteries but at a lower cost, have attracted widespread attention. Although sodium-ion batteries have bright application prospects, their commercialization process has been fraught with difficulties, the biggest challenge being the selection of anode materials. Among various anode materials, hard carbon from amorphous carbon is considered a promising anode material for sodium-ion batteries.

[0003] Hard carbon is a type of carbon that is difficult to graphitize even above 2500℃. After high-temperature carbonization, the bent graphitized carbon layers in hard carbon stack and connect to form a highly distorted structure with numerous nanopores. In addition, the hard carbon structure contains a large number of vacancies, edges, and defect sites. Due to the unique structure of hard carbon, integrating high capacity, high initial coulombic efficiency (ICE), and good durability remains challenging. In the modification of hard carbon materials, heteroatom doping (such as N, P, S, etc.) is one of the most frequently reported effective strategies. Taking nitrogen doping as an example, it can be divided into self-doping using nitrogen-containing precursors or introducing nitrogen sources from external sources. For example, Dong Xiaoqian from Hebei University of Geosciences prepared a nitrogen-doped phenolic resin-based porous carbon-supported manganese-cobalt composite material by calcining a mixture of phenolic resin-based porous carbon, polyvinylpyrrolidone, and MnCo2O4 nanospheres in Chinese patent CN117373839A. Xiao Wende from Shanghai Jiao Tong University obtained a nitrogen-doped hard carbon material by carbonizing a mixture of phenolic resin and melamine resin in Chinese patent CN116425140A. However, introducing an external nitrogen source and mixing it with phenolic resin presents the problem of uneven nitrogen doping, and the production process is relatively complex.

[0004] Therefore, it is necessary to propose methods to simplify the production process and improve the electrochemical performance of hard carbon microspheres. Summary of the Invention

[0005] In view of the above-mentioned shortcomings, this invention provides nitrogen-self-doped phenolic resin-based hard carbon microspheres, their preparation method, and their applications. The phenolic resin hard carbon of this application introduces nitrogen from the resin source, simplifying the production process and achieving uniform nitrogen distribution and atomic-level doping. This material, when used in sodium-ion batteries, exhibits excellent long-cycle stability, good rate performance, and high specific capacity, showing promising prospects for industrial application.

[0006] To achieve the above objectives, the present invention provides a method for preparing nitrogen-doped phenolic resin-based hard carbon microspheres, comprising the following steps:

[0007] S1. Adjust the pH of the alkaline catalyst to 8.5-9.5 with deionized water. At room temperature, add phenol monomer to form a phenol suspension A. Then add an aldehyde monomer solution to the phenol suspension A to obtain suspension B. The phenol monomer is aminophenol.

[0008] It should be noted that an alkaline environment is conducive to the formation of phenol oxide ions, thereby promoting the addition of phenol and aldehyde. Moreover, under relatively mild alkaline conditions, the addition reaction is 5 times more efficient than the condensation reaction.

[0009] It should be noted that the phenol monomer in this application is aminophenol. Such monomers inherently possess an amino group, which, compared to a single phenolic hydroxyl group, increases the addition active sites for phenol. Furthermore, after carbonization, the nitrogen atom remains in the hard carbon phase, achieving atomic-level doping. The electron-rich nitrogen atom can modulate the conductivity of hard carbon materials, increasing the electron transport rate and thus improving the rate performance of sodium-ion batteries. Secondly, nitrogen doping can also alter the structure and chemical properties of hard carbon materials. The presence of nitrogen atoms will affect the bending and folding of the carbon layer, increasing the active sites for sodium storage reactions and improving the sodium ion storage capacity.

[0010] S2. After the suspension B is reacted at temperature T1 for a certain time t1, an aldehyde monomer solution is added dropwise to it, and then it is reacted at temperature T2 for a certain time t2 to obtain a phenolic resin oligomer with a certain molecular weight.

[0011] S3. The phenolic resin oligomer and the reaction solution are placed together in a hydrothermal reactor and reacted at a temperature T3 for a certain time t3. After solid-liquid separation, phenolic resin microspheres are obtained.

[0012] S4. The phenolic resin microspheres are carbonized in an inert atmosphere in two steps to obtain nitrogen-doped phenolic resin-based hard carbon microspheres; wherein the two carbonization steps are low-temperature T4 carbonization and high-temperature T5 carbonization.

[0013] According to one aspect of the present invention, in step S1, the alkaline catalyst is one or more of NaOH, Na2CO3, Ba(OH)2 or NH3·H2O; the aldehyde monomer is one or more of formaldehyde, acetaldehyde, propionaldehyde, glutaraldehyde, and furfural; and the aminophenol is one or more of p-aminophenol and m-aminophenol.

[0014] According to one aspect of the present invention, in step S1, the molar ratio of the phenol monomer to the aldehyde monomer is 1:3-4.

[0015] It should be noted that phenol monomers have approximately 2-3 reaction sites. In the first step, the phenol monomer adds to formaldehyde to form hydroxymethylphenol. In the second step, formaldehyde further adds to hydroxymethylphenol. When the formaldehyde content is higher than 50%, the reaction between formaldehyde and hydroxymethylphenol is easier than the reaction between formaldehyde and phenol. Therefore, controlling the rate and amount of formaldehyde addition in the first step can promote the formation of more uniform polyhydroxymethyl compounds in the phenolic resin, providing a core for the subsequent elongation of the phenolic resin carbon chain.

[0016] According to one aspect of the present invention, in step S1, the dropping rate of the aldehyde monomer solution is 4-6 mL / min.

[0017] According to one aspect of the present invention, in step S2, the temperature T1 is 20-30°C and the time t1 is 30-60 min; the temperature T2 is 40-60°C and the time t2 is 60-120 min.

[0018] It should be noted that the above temperature T1 is a low-temperature reaction, which, together with the excess formaldehyde added in step S2, promotes the further growth of polyhydroxymethyl compounds into small resin molecules with linear or small branched structures, preparing for subsequent high-temperature cross-linking to form insoluble thermosetting resins.

[0019] According to one aspect of the present invention, in step S2, the molar ratio of the aldehyde monomer solution to the phenol monomer is 3-4:1; and the relative molecular weight of the phenolic resin oligomer is 300-1000 g / mol.

[0020] According to one aspect of the present invention, in step S3, the temperature T3 is 80-150°C and the time t3 is 6-24h.

[0021] It should be noted that in step S3, centrifugation is the preferred method for solid-liquid separation, with a rotation speed of 7000-9000 r / min. In addition to centrifugation, freeze drying, filtration followed by drying, and other methods can also be used.

[0022] According to one aspect of the present invention, in step S4, the low temperature T4 is 300-500°C, the high temperature T5 is 1000-1500°C, and the carbonization time for both steps is 60-120 min.

[0023] It should be noted that the decomposition temperature of amino resins is between 300 and 400°C. In step S4, introducing a low-temperature treatment before high-temperature carbonization can greatly alleviate the rapid removal of small molecular structures such as amino groups at high temperatures, thus mitigating structural defects in the material.

[0024] It should be noted that in step S4, the inert atmosphere is nitrogen or argon.

[0025] Based on the same inventive concept, the present invention also provides nitrogen-doped phenolic resin-based hard carbon microspheres prepared by the above preparation method.

[0026] It should be noted that the above-mentioned nitrogen-doped phenolic resin-based hard carbon microspheres are spherical, with a diameter of 2-5 μm and a layer spacing d. 002 The wavelength is 0.375-0.390 nm, I D / I G Between 1.10 and 1.60, the average width L of graphite domains along the a-axis direction is... a The size is 12-20nm, and nitrogen self-doping is achieved. The proportion of N atoms in the hard carbon microspheres is 0.5-1.5wt%, and the reversible specific capacity is 330-360mAh / g.

[0027] Based on the same inventive concept, the present invention also provides the application of nitrogen-doped phenolic resin-based hard carbon microspheres prepared by the above preparation method or the above nitrogen-doped phenolic resin-based hard carbon microspheres in carbon anode materials for sodium-ion batteries.

[0028] Based on the same inventive concept, the present invention also provides an electrode material, comprising a conductive additive, a binder, and the nitrogen-doped phenolic resin-based hard carbon microspheres of the present application as a sodium-ion battery carbon anode material.

[0029] The beneficial effects of this invention are:

[0030] (1) The resin synthesis steps in the preparation method of this application take into account the priority of addition and condensation reactions, and adjust the alkaline environment, the order and amount of aldehyde addition, etc., in order to form spherical resin particles of uniform size. In the resin carbonization process, a two-step carbonization is adopted to minimize the removal of nitrogen atoms and reduce surface defects.

[0031] (2) The spherical structure of the phenolic resin-based hard carbon microspheres in this application facilitates the insertion and extraction of sodium ions from all directions. Furthermore, the phenolic resin-based hard carbon microspheres achieve nitrogen self-doping, and the presence of nitrogen atoms expands the interlayer spacing. In addition, as an electron-rich group, it modulates the conductivity of the hard carbon material while increasing the abundance of sodium-storing active sites, thereby improving the hard carbon's ability to store sodium ions.

[0032] (3) This application achieves nitrogen doping directly from the source. Compared with recombining the resin with the nitrogen source, this method not only simplifies the production process, but also achieves atomic-level uniform doping of nitrogen. Attached Figure Description

[0033] Figure 1 This is a SEM image of the hard carbon material prepared in Example 1 of the present invention;

[0034] Figure 2The images shown are SEM images of the hard carbon materials prepared in the comparative examples of this invention; where (a) is the SEM image of the hard carbon material of Comparative Example 1; and (b) is the SEM image of the hard carbon material of Comparative Example 2.

[0035] Figure 3 EDS diagrams of the hard carbon material prepared in Example 1 of this invention are shown; where (a) is an EDS diagram; (b) shows the distribution of C atoms; (c) shows the distribution of N atoms; and (d) shows the distribution of O atoms.

[0036] Figure 4 The images show the XRD patterns of the hard carbon materials prepared in Examples 1-4 of this invention.

[0037] Figure 5 The images show the Raman diagrams of the hard carbon materials prepared in Examples 1-4 of this invention.

[0038] Figure 6 The constant current charge-discharge curves of sodium-ion batteries prepared from hard carbon materials in Examples 1-4 of this invention are shown.

[0039] Figure 7 The diagram shows the cycle performance of sodium-ion batteries prepared from hard carbon materials in Examples 1-4 of this invention. Detailed Implementation

[0040] To make the present invention easier to understand, specific embodiments are described below to further illustrate the invention. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical terms used below have the same meaning as understood by those skilled in the art; unless otherwise specified, the raw materials and reagents involved herein can be purchased commercially or obtained by known methods.

[0041] Example 1

[0042] This example first provides a method for preparing nitrogen-doped phenolic resin-based hard carbon microspheres, including the following steps:

[0043] Add an appropriate amount of NH3·H2O to deionized water (pH 8.5), totaling 15 ml. Then add 3.2 g of m-aminophenol, and while stirring, add 10 ml of formaldehyde solution (37% formaldehyde solution: deionized water volume ratio 3:2) to the phenol suspension at a dropping rate of 5 mL / min. After reacting the suspension at 25 °C for 30 min, continue to add 5 ml of formaldehyde solution (37% formaldehyde solution: deionized water volume ratio 2:3), and react at 40 °C for 120 min to obtain phenolic resin oligomers (relative molecular weight of about 500 g / mol). Then, place phenolic resin oligomer A together with its reaction solution into a hydrothermal reactor and react at 100 °C for 24 h. After solid-liquid separation, phenolic resin microspheres are obtained.

[0044] The nitrogen-doped phenolic resin microspheres were obtained by reacting them in argon at 400°C for 2 hours and then at 1400°C for 2 hours.

[0045] Example 2

[0046] This example first provides a method for preparing nitrogen-doped phenolic resin-based hard carbon microspheres, including the following steps:

[0047] Add an appropriate amount of NH3·H2O to deionized water (pH 8.5), totaling 15 ml. Then add 3.2 g of p-aminophenol, and while stirring, add 10 ml of formaldehyde solution (37% formaldehyde solution: deionized water volume ratio 3:2) to the phenol suspension at a rate of 4 mL / min. After reacting the suspension at 25 °C for 30 min, continue to add 5 ml of formaldehyde solution (37% formaldehyde solution: deionized water volume ratio 2:3), and react at 45 °C for 120 min to obtain phenolic resin oligomers (relative molecular weight of approximately 500 g / mol). Then, place the phenolic resin oligomers and its reaction solution together into a hydrothermal reactor and react at 120 °C for 24 h. After solid-liquid separation, phenolic resin microspheres are obtained.

[0048] The nitrogen-doped phenolic resin microspheres were obtained by reacting them in argon at 400°C for 2 hours and then at 1200°C for 2 hours.

[0049] Example 3

[0050] This example first provides a method for preparing nitrogen-doped phenolic resin-based hard carbon microspheres, including the following steps:

[0051] Add an appropriate amount of NH3·H2O to deionized water (pH 8.5), totaling 15 ml. Then add 3.2 g of m-aminophenol, and while stirring, add 10 ml of formaldehyde solution (37% formaldehyde solution: deionized water volume ratio 3:2) to the phenol suspension at a rate of 5 mL / min. After reacting the suspension at 25 °C for 30 min, continue to add 5 ml of formaldehyde solution (37% formaldehyde solution: deionized water volume ratio 2:3), and react at 40 °C for 180 min to obtain phenolic resin oligomers (relative molecular weight of approximately 500 g / mol). Then, place the phenolic resin oligomers and its reaction solution into a hydrothermal reactor and react at 120 °C for 24 h. After solid-liquid separation, phenolic resin microspheres are obtained.

[0052] The nitrogen-doped phenolic resin microspheres were obtained by reacting them in argon at 400°C for 2 hours and then at 1300°C for 2 hours.

[0053] Example 4

[0054] This example first provides a method for preparing nitrogen-doped phenolic resin-based hard carbon microspheres, including the following steps:

[0055] This example first provides a method for preparing a hard carbon microsphere anode material, including the following steps:

[0056] Add an appropriate amount of NH3·H2O to deionized water (pH 8.5), totaling 15 ml. Then add 3.2 g of p-aminophenol, and while stirring, add 10 ml of formaldehyde solution (37% formaldehyde solution: deionized water volume ratio 3:2) to the phenol suspension at a dropping rate of 5 mL / min. After reacting the suspension at 25 °C for 30 min, continue to add 5 ml of formaldehyde solution (37% formaldehyde solution: deionized water volume ratio 2:3), and react at 40 °C for 180 min to obtain phenolic resin oligomers (relative molecular weight of approximately 500 g / mol). Then, place the phenolic resin oligomers and its reaction solution into a hydrothermal reactor and react at 120 °C for 18 h. After solid-liquid separation, phenolic resin microspheres are obtained.

[0057] The nitrogen-doped phenolic resin microspheres were obtained by reacting them in argon at 450°C for 2 hours and then at 1400°C for 2 hours.

[0058] Comparative Example 1

[0059] The difference between this comparative example and Example 1 is that 3.2g of m-aminophenol and 15ml of formaldehyde solution (37% formaldehyde solution: deionized water volume ratio 3:2) were added together at one time to ammonia water with a pH of 8.5. Other steps and parameters were the same as in Example 1.

[0060] Comparative Example 2 (without nitrogen doping)

[0061] The difference between this comparative example and Example 1 is that m-aminophenol is replaced with phenol. All other steps and parameters are the same as in Example 1.

[0062] Structural characterization:

[0063] The hard carbon materials prepared in Example 1 and Comparative Examples 1-2 were analyzed by scanning electron microscopy (SEM), and the results are as follows: Figure 1-2 As shown, by Figure 1 It can be seen that the hard carbon prepared by the method of this application is spherical with a uniform particle size distribution, mostly 2-5 μm. Figure 2 (a) It can be seen that when aminophenol and formaldehyde are added to the alkaline catalyst at once, the resulting phenolic resin hard carbon balls tend to stick together. Figure 2 (b) It can be seen that the resin prepared from phenol monomers exhibits irregular blocky shapes and relatively large particles after carbonization. Excessively large particles are detrimental to the diffusion and storage of sodium ions within the resin.

[0064] The hard carbon material prepared in Example 1 was analyzed by EDS energy dispersive spectroscopy, and the results are as follows: Figure 3 As shown, by Figure 3 It is known that in the hard carbon material prepared in Example 1 of this application, nitrogen atoms are uniformly distributed in the hard carbon spheres.

[0065] The X-ray diffraction (XRD) spectra of the hard carbon materials prepared in Examples 1-4 are as follows: Figure 4 As shown, Figure 4 The curve shows two broad peaks near 23.5° and 43°, which are typical characteristics of hard carbon structures. From the Bragg equation 2dsinθ=nλ, the interlayer spacing d in hard carbon can be determined. 002 The wavelength (0.37–0.39 nm) is much larger than the interlayer spacing of graphite (0.334 nm), which is more conducive to the transport of sodium ions with larger radii.

[0066] The Raman spectra of the hard carbon materials prepared in Examples 1-4 are as follows: Figure 5 As shown, it can be calculated that I D / I G Between 1.10 and 1.60, the average width L of graphite domains along the a-axis is represented. a At 12-20nm.

[0067] Performance characterization:

[0068] The hard carbon powder (i.e., hard carbon microspheres) prepared in Examples 1-4 and Comparative Examples 1-2 were mixed with conductive agent Super P and PVDF at a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (NMP) was added and stirred to form a slurry. The slurry was then uniformly coated onto a current collector copper foil, dried at 120°C, and cut into circular electrode sheets with a diameter of 12 mm. The electrode sheets were dried at 70°C for 12 hours under vacuum and then transferred to a glove box for later use. Battery assembly was carried out in an Ar atmosphere glove box, using a sodium metal sheet as the counter electrode. The electrolyte was 1M NaPF6 dissolved in (ethylene carbonate and diethyl carbonate <Example 1>, dimethyl ether <Example 2>, ethylene carbonate and diethyl carbonate <Example 3>, ethylene carbonate and diethyl carbonate <Example 4>, ethylene carbonate and diethyl carbonate <Comparative Example 1>, ethylene carbonate and diethyl carbonate <Comparative Example 2>), and assembled into CR2025 button batteries. In Example 1, ethylene carbonate and diethyl carbonate are used to describe the process of making a CR2025 button battery from the hard carbon material of Example 1, where 1M NaPF6 is dissolved in ethylene carbonate and diethyl carbonate as the electrolyte. Other similar descriptions follow the same principle.

[0069] Electrochemical activation was performed using a constant current charge-discharge mode at a current density of 30 mA / g, and battery cycle charge-discharge performance was tested at a current density of 150 mA / g. The discharge cutoff voltage was 0V, and the charge cutoff voltage was 2.0V.

[0070] The sodium-ion batteries (CR2025 button batteries) prepared from the hard carbon materials in Examples 1-4 above were subjected to constant current charge-discharge, and their constant current charge-discharge curves are shown below. Figure 6 As shown, by Figure 6It can be seen that the sodium-ion battery prepared with the hard carbon material in Example 1 has a reversible specific capacity of 301.8 mAh / g, an initial coulombic efficiency (ICE) of 71.5%, a plateau region specific capacity of 188.7 mAh / g, and a plateau region specific capacity percentage of 62.5%; the sodium-ion battery prepared with the hard carbon material in Example 2 has a reversible specific capacity of 320.4 mAh / g, an initial coulombic efficiency (ICE) of 72.1%, a plateau region specific capacity of 202.2 mAh / g, and a plateau region specific capacity percentage of 63.1%. The sodium-ion battery prepared with hard carbon material in Example 3 had a reversible specific capacity of 320.9 mAh / g, an initial coulombic efficiency (ICE) of 73.2%, a plateau specific capacity of 207.5 mAh / g, and a plateau specific capacity percentage of 64.7%. The sodium-ion battery prepared with hard carbon material in Example 4 had a reversible specific capacity of 332.6 mAh / g, an initial coulombic efficiency (ICE) of 74.9%, a plateau specific capacity of 239.0 mAh / g, and a plateau specific capacity percentage of 71.9%. The sodium-ion batteries (CR2025 button cells) prepared with hard carbon material in Comparative Examples 1-2 were subjected to constant current charge-discharge tests. The results are shown in Table 1. It can be seen that the sodium-ion battery prepared with hard carbon material in Comparative Example 1 had a reversible specific capacity of 291.4 mAh / g and a plateau specific capacity of 172.7 mAh / g. Comparing this performance with the electrochemical performance of the sodium-ion battery prepared from the hard carbon material in Example 1, it can be seen that the order of phenolic resin addition has a significant impact on the formed phenolic resin and even the final hard carbon. Since m-aminophenol is poorly soluble in water at room temperature, this means that the contact between phenolic resin and formaldehyde is extremely uneven during the addition of m-aminophenol and formaldehyde. This inevitably affects the addition condensation and molecular chain elongation crosslinking of the phenolic resin in subsequent processes, thus affecting the uniformity of the phenolic resin sphere formation. Therefore, after the resin spheres are carbonized, the graphite domains inside the hard carbon are uneven, i.e., the interlayer spacing of each graphite domain is different, which affects the sodium storage in the plateau region, leading to a decrease in the overall sodium storage performance of the hard carbon. As shown in Table 1, the reversible specific capacity of the sodium-ion battery prepared from the hard carbon material in Comparative Example 1 is only 283.4 mAh / g, and the plateau region specific capacity is only 118.2 mAh / g. This indicates that the phenolic resin formed after phenol replaces m-aminophenol lacks nitrogen atom regulation in the ordered carbon layers during carbonization, and the resulting interlayer and closed-cell structure is not optimal for sodium storage. Therefore, the plateau region specific capacity and even the overall reversible specific capacity are greatly reduced.

[0071] Table 1:

[0072]

[0073] The sodium-ion battery (CR2025 button cell) prepared above was subjected to cycle stability testing, and its stability test curve is shown below. Figure 7 As shown. From Figure 7As can be seen, the sodium-ion battery prepared from the hard carbon material in Example 1 exhibits a reversible specific capacity of 271.4 mAh / g and a capacity retention rate of 91.9% after 100 charge-discharge cycles at a current density of 1.5 A / g, indicating that the hard carbon structure is stable and has good cycle stability. Similarly, the sodium-ion battery prepared from the hard carbon material in Example 2 exhibits a reversible specific capacity of 287.1 mAh / g and a capacity retention rate of 92.8% after 100 charge-discharge cycles at a current density of 1.5 A / g, indicating that the hard carbon structure is stable and has good cycle stability. The results are as follows: The sodium-ion battery prepared from the hard carbon material in Example 3 exhibited a reversible specific capacity of 283.5 mAh / g and a capacity retention rate of 89.7% after 100 charge-discharge cycles at a current density of 1.5 A / g, indicating that the hard carbon structure is stable and has good cycle stability. Similarly, the sodium-ion battery prepared from the hard carbon material in Example 4 exhibited a reversible specific capacity of 298.5 mAh / g and a capacity retention rate of 92.5% after 100 charge-discharge cycles at a current density of 1.5 A / g, indicating that the hard carbon structure is stable and has good cycle stability.

[0074] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing nitrogen self-doped phenolic resin-based hard carbon microspheres, characterized in that, The method comprises the following steps: S1, adjusting the pH of a basic catalyst to 8.5-9.5 with deionized water, adding a phenolic monomer to the basic catalyst at room temperature to form a phenolic suspension A; then adding an aldehyde monomer solution dropwise to the phenolic suspension A to obtain a suspension B; wherein the phenolic monomer is an aminophenol; the basic catalyst is one or more of NaOH, Na2CO3, Ba(OH)2 or NH3·H2O; the dropwise adding speed of the aldehyde monomer solution is 4-6 mL / min; the molar ratio of the phenolic monomer to the aldehyde monomer is 1:3-4; S2, after the suspension B is reacted at a temperature T1 for a time t1, an aldehyde monomer solution is added dropwise to the suspension B, and then the suspension B is reacted at a temperature T2 for a time t2 to obtain a phenolic aldehyde resin oligomer with a certain molecular weight; wherein the temperature T1 is 20-30℃, and the time t1 is 30-60 min; the temperature T2 is 40-60℃, and the time t2 is 60-120 min; the relative molecular weight of the phenolic aldehyde resin oligomer is 300-1000 g / mol; S3, the phenolic aldehyde resin oligomer is placed in a hydrothermal reaction kettle together with a reaction solution, and reacted at a temperature T3 for a time t3, and then the phenolic aldehyde resin oligomer is obtained after solid-liquid separation; S4, the phenolic aldehyde resin oligomer is subjected to two-step carbonization in an inert atmosphere to obtain a nitrogen self-doped phenolic aldehyde resin-based hard carbon microsphere; wherein the two-step carbonization comprises low-temperature T4 carbonization and high-temperature T5 carbonization.

2. The method for preparing nitrogen-self-doped phenolic resin-based hard carbon microspheres according to claim 1, characterized in that, In step S1, the aldehyde monomer is one or more of formaldehyde, acetaldehyde, propyl aldehyde, glutaraldehyde, and furfural; and the aminophenol is one or more of p-aminophenol and m-aminophenol.

3. The method for preparing nitrogen-self-doped phenolic resin-based hard carbon microspheres according to claim 1, characterized in that, In step S3, the temperature T3 is 80-150℃, and the time t3 is 6-24 h.

4. The method for preparing nitrogen-self-doped phenolic resin-based hard carbon microspheres according to claim 1, characterized in that, In step S4, the low temperature T4 is 300-500℃, the high temperature T5 is 1000-1500℃, and the time of the two-step carbonization is 60-120 min.

5. The nitrogen self-doped phenolic resin-based hard carbon microspheres prepared by the method according to any one of claims 1-4, characterized in that, The proportion of N atoms in the hard carbon microsphere is 0.5-1.5 wt%.

6. A nitrogen self-doped phenolic aldehyde resin-based hard carbon microsphere prepared by the preparation method of any one of claims 1-4 or the nitrogen self-doped phenolic aldehyde resin-based hard carbon microsphere of claim 5 is applied to a sodium ion battery carbon negative electrode material.

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