Hard carbon microspheres with double coating layers and preparation method and application thereof

Abstract

The present invention provides a hard carbon microsphere with a double coating layer and its preparation method and application. The preparation method of the hard carbon microsphere comprises: placing a hard carbon source in an autoclave, and hydrothermally reacting at 150-220°C for 12-24h; after the reaction is completed, filtering, collecting the precursor, dehydrating the precursor, and co-dispersing it in anhydrous toluene with an oxidant and a silane coupling agent, heating and refluxing for 1-10h to obtain a monomolecularly coated precursor; placing the monomolecularly coated precursor in an environment containing conductive polymer monomer vapor, performing in situ polymerization, obtaining an ordered polymer layer on the outer layer of the monomolecularly coated precursor, and then sintering in an inert atmosphere to obtain hard carbon microspheres with a double coating layer. The obtained hard carbon microspheres are used in the preparation of sodium ion batteries, which can reduce the defects on the surface of the hard carbon negative electrode of the sodium ion battery, increase the sites for sodium ion adsorption, improve the capacity of the hard carbon negative electrode to a certain extent, and optimize the rate performance and the long cycle and safety at high rates.

Description

Technical Field

[0001] The present invention belongs to the field of sodium ion batteries, and specifically comprises hard carbon microspheres with double coating layers, and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries (LIBs) are already widely used in consumer electronics and electric vehicles. However, for large-scale electrical energy storage (EES), the characteristics of electrochemical energy storage technologies make them ideal for grid-scale EES. In particular, LIBs offer rapid response to load changes, high energy density, good coulombic efficiency, and low standby losses. However, whether LIBs can meet the demands of large-scale EES remains uncertain given resource, cost, and environmental challenges. Given these demands and challenges, it is crucial to develop an alternative, scalable electrochemical battery technology composed of inexpensive, abundant, and environmentally friendly materials.

[0003] Given the abundance of sodium in the Earth's crust (2.75% vs. 0.0065% lithium), its low electrochemical potential (-2.71V vs. standard hydrogen electrode), which is only 330mV higher than lithium, and its lower raw material costs, sodium-ion batteries (SIBs) hold great promise for meeting the needs of large-scale EES. Anode materials are crucial for the excellent electrochemical performance of SIBs. Designing novel anode materials with superior performance and low cost could accelerate the commercialization of SIBs. Among the various SIB anode materials, hard carbon materials offer the advantages of high capacity, low cost, and low operating voltage. Their unique structure facilitates the adsorption and reversible insertion / extraction of sodium ions, demonstrating excellent sodium storage properties and making them the most promising anode materials for commercialization. However, the commercialization of hard carbon materials faces challenges such as relatively low first-cycle coulombic efficiency, poor rate capability, and poor cycling stability. In particular, the solvation of sodium ions causes irreversible volume changes in hard carbon during charge and discharge. Over the past decade, a large number of experimental and computational studies have been carried out to explore hard carbon negative electrodes from different carbon sources and modify them. For example, chemical vapor deposition, physical vapor deposition, and atomic deposition are used to coat or deposit a layer of modified material on the surface of the material after it is formed, so as to reduce the specific surface area of ​​the hard carbon and improve the initial efficiency. However, these methods all have certain problems. For example, physical vapor deposition and atomic deposition cannot be applied on a large scale due to cost issues. Although chemical vapor deposition can be used for large-scale production, it requires heating the raw materials, which limits its scope of application. In addition, the operation is relatively complicated in the industrial process, and its application is still subject to certain restrictions.

[0004] In view of the above reasons, it is very necessary to fundamentally study its ion storage mechanism and find a way to improve the performance of the negative electrode of sodium ion batteries. Summary of the Invention

[0005] In response to the aforementioned problems with the prior art, the first objective of the present invention is to provide a method for preparing hard carbon microspheres with a double coating. During this preparation process, the double-coated hard carbon microspheres are formed by utilizing the coupling effect of a silane coupling agent and in-situ polymerization of conductive polymer monomer vapor. This layered design allows for the construction of ordered sodium ion transport channels on the surface of the hard carbon microspheres, regulating the sodium ion desolvation and solvation processes, optimizing rate performance, long-term circulation at high rates, and safety. Furthermore, this preparation method is simple, convenient, and has low production costs.

[0006] The second object of the present invention is to provide hard carbon microspheres with double coating layers prepared by the method described above.

[0007] The third object of the present invention is to provide a negative electrode material comprising the hard carbon microspheres having a double coating layer as described above.

[0008] A fourth object of the present invention is to provide a sodium ion battery comprising the negative electrode material described above.

[0009] To achieve the above first purpose, the technical solution adopted by the present invention includes:

[0010] The present invention discloses a method for preparing hard carbon microspheres with double coating layers, comprising the following steps:

[0011] S1. Place the hard carbon source in an autoclave and perform a hydrothermal reaction at 150-220°C for 12-24 hours;

[0012] S2. After the reaction is completed, the precursor is filtered and collected, and the precursor is dehydrated and dispersed in anhydrous toluene together with an oxidant and a silane coupling agent, and heated under reflux for 1-10 hours to obtain a single molecule-coated precursor;

[0013] S3. The single-molecule coated precursor is placed in an environment containing conductive polymer monomer vapor to carry out in-situ polymerization reaction, and an ordered polymer layer is obtained on the outer layer of the single-molecule coated precursor, which is then sintered in an inert atmosphere to obtain hard carbon microspheres with double coating layers.

[0014] In response to the problems of relatively low first-cycle coulombic efficiency, poor rate performance, and poor cycle stability of existing hard carbon materials, the present invention constructs a double coating layer on the surface of hard carbon microspheres through a layered design. The inner layer is grafted on the surface of the hard carbon microspheres through a silane coupling agent to obtain an ordered single-molecule coating. The outer layer is in situ polymerized to form an ordered two-dimensional polymer layer on this basis, and then carbonization is performed to achieve the construction of an ordered two-dimensional polymer carbonization layer. In this way, an ordered sodium ion transmission channel can be constructed on the surface of the hard carbon microspheres, reducing the defects on the surface of the hard carbon negative electrode of the sodium ion battery, increasing the sites for sodium ion adsorption, and inhibiting the deposition of sodium ions on the negative electrode surface at high rates. While improving the capacity of the hard carbon negative electrode, the process of sodium ion embedding / de-embedding on the hard carbon surface is improved, the impedance and overpotential are reduced, the rate performance and long cycle and safety at high rates are optimized, and the short circuit caused by the generation of dendrites during high-rate charge and discharge of the hard carbon negative electrode is avoided, and it has high versatility.

[0015] Furthermore, the hard carbon source includes but is not limited to one or more of a biomass hard carbon source, a resin-based hard carbon source and an asphalt-based hard carbon source; illustratively, the biomass hard carbon source is derived from coconut shells, nut shells, corn cobs, sawdust, bamboo chips or sisal fibers, etc.; the resin-based hard carbon source is derived from one or more of phenolic resin, epoxy resin and polyfurfuryl alcohol; the asphalt-based hard carbon source is derived from one or more of coal tar asphalt, petroleum asphalt and natural asphalt. These various types of hard carbon sources are all commercially available products and can be purchased directly. Before using these hard carbon sources for hydrothermal reaction to prepare precursors, the hard carbon sources are also pretreated. Technicians in this field adopt different pretreatment methods according to the sources of the hard carbon sources. For example, for biomass hard carbon sources, the purchased biomass hard carbon sources are cleaned and then dried. For asphalt-based hard carbon sources, phenolic resins, epoxy resins, polyfurfuryl alcohol, etc. need to be solidified to form a solid before the subsequent hydrothermal reaction can be carried out. For asphalt-based hard carbon sources, coal tar asphalt, petroleum asphalt or natural asphalt needs to be dried and ground with sublimated sulfur, and then placed in an argon-protected reactor. After heating the reaction at 250-300°C, the subsequent hydrothermal reaction can be carried out.

[0016] Furthermore, when the hard carbon source is selected from a biomass hard carbon source, hydrochloric acid needs to be used as a solvent to participate in the hydrothermal reaction.

[0017] Furthermore, the silane coupling agent includes but is not limited to one or more of an aminosilane coupling agent, an epoxysilane coupling agent, a methacryloxysilane coupling agent and a vinylsilane coupling agent; illustratively, the aminosilane coupling agent includes but is not limited to one or more of KBM-602, KBM-603, and KBM-903; the epoxysilane coupling agent includes but is not limited to one or more of KBM-303, KBM-402, and KBM-403; the methacryloxysilane coupling agent includes but is not limited to one or more of KBM-502, KBM-503, and KBE-502; the vinylsilane coupling agent includes but is not limited to one or more of KA-1003, KBM-1003, and KBE-1003.

[0018] Furthermore, the mass ratio of the silane coupling agent to the precursor is 5-10:100; illustratively, the mass ratio of the silane coupling agent to the precursor can be 5:100, 6:100, 7:100, 8:100, 9:100, 10:100, and the like.

[0019] Furthermore, the oxidant includes but is not limited to ferric chloride and / or ammonium persulfate.

[0020] Furthermore, the concentration of the oxidant in toluene is 5-100 mmol / L; illustratively, the concentration of the oxidant in toluene can be 5 mmol / L, 10 mmol / L, 20 mmol / L, 30 mmol / L, 40 mmol / L, 50 mmol / L, 60 mmol / L, 70 mmol / L, 80 mmol / L, 90 mmol / L, 100 mmol / L, and the like.

[0021] Furthermore, the conductive polymer monomer includes, but is not limited to, one or more of pyrrole, aniline, thiophene, and 3,4-ethylenedioxythiophene.

[0022] Furthermore, in step S3, the volume concentration of the conductive polymer monomer vapor in the environment containing the conductive polymer monomer vapor is 20-30%.

[0023] Furthermore, in step S3, the reaction temperature of the in-situ polymerization is -4 to 4°C, and the reaction time is 12 to 36 hours. The conductive polymer monomer can volatilize at this temperature to produce gaseous conductive polymer monomer vapor. When the conductive polymer monomer vapor reaches a specified concentration, the conditions for in-situ polymerization are met.

[0024] Furthermore, in step S3, the sintering temperature is 800-1100° C., and the sintering time is 1-3 hours.

[0025] To achieve the above second purpose, the technical solutions adopted by the present invention include:

[0026] The invention discloses hard carbon microspheres with double coating layers which are prepared by the above-mentioned preparation method.

[0027] To achieve the third objective above, the technical solutions adopted by the present invention include:

[0028] The present invention discloses a negative electrode material comprising the hard carbon microspheres with double coating layers as described above.

[0029] To achieve the fourth objective above, the technical solutions adopted by the present invention include:

[0030] The present invention discloses a sodium ion battery, which comprises the above-mentioned negative electrode material, positive electrode material, a separator arranged between the positive electrode material and the negative electrode material, and an electrolyte.

[0031] Furthermore, the positive electrode material may be one or more of layered oxides, Prussian blue analogs, and polyanion compounds.

[0032] Beneficial effects of the present invention:

[0033] In response to the problems of relatively low first-cycle coulombic efficiency, poor rate performance, and poor cycle stability of existing hard carbon materials, the present invention constructs a double coating layer on the surface of hard carbon microspheres through a layered design. The inner layer is grafted on the surface of the hard carbon microspheres through a silane coupling agent to obtain an ordered single-molecule coating. The outer layer is in situ polymerized to form an ordered two-dimensional polymer layer on this basis, and then carbonization is performed to achieve the construction of an ordered two-dimensional polymer carbonization layer. In this way, an ordered sodium ion transmission channel can be constructed on the surface of the hard carbon microspheres, reducing the defects on the surface of the hard carbon negative electrode of the sodium ion battery, increasing the sites for sodium ion adsorption, and inhibiting the deposition of sodium ions on the negative electrode surface at high rates. While improving the capacity of the hard carbon negative electrode, the process of sodium ion embedding / de-embedding on the hard carbon surface is improved, the impedance and overpotential are reduced, the rate performance and long cycle and safety at high rates are optimized, and the short circuit caused by the generation of dendrites during high-rate charge and discharge of the hard carbon negative electrode is avoided, and it has high versatility. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0035] Figure 1 The transmission electron microscope image of the sisal hard carbon microspheres with double coating layers prepared in Example 1 is shown.

[0036] Figure 2 The selected area electron diffraction pattern of the precursor of sisal hard carbon microspheres grown with a polypyrrole (PPy) layer prepared in Example 1 is shown.

[0037] Figure 3The graph shows the cycle performance of the Na / / NaClO4 / / AHC-Si@C-2D PPy battery prepared in Example 1 after 10,000 charge and discharge cycles.

[0038] Figure 4 The graph shows the cycle performance of the Na / / NaClO4 / / AHC-2D PPy battery prepared in Comparative Example 1 after 10,000 charge and discharge cycles.

[0039] Figure 5 The figure shows the cycle performance of the Na / / NaClO4 / / AHC-Si@C battery prepared in Comparative Example 2 after 1000 charge and discharge cycles.

[0040] Figure 6 The graph shows the cycle performance of the Na / / NaClO4 / / AHC battery prepared in Comparative Example 3 after 370 charge and discharge cycles.

[0041] Figure 7 The rate performance diagrams of the hard carbon negative electrodes prepared in Example 1 and Comparative Examples 1-3 are shown.

[0042] Figure 8 The impedance spectra of the hard carbon negative electrodes prepared in Example 1 and Comparative Examples 1-3 are shown.

[0043] Figure 9 The graph shows the cycle performance of the Na / / NaClO4 / / EPNHC-Si@C-2D PANI battery prepared in Example 2 after 10,000 charge and discharge cycles.

[0044] Figure 10 The selected area electron diffraction pattern of the precursor of polyaniline-coated epoxy novolac resin hard carbon prepared in Comparative Example 4 is shown.

[0045] Figure 11 The graph shows the cycle performance of the Na / / NaClO4 / / EPNHC-PANI battery prepared in Comparative Example 4 after 1000 charge and discharge cycles.

[0046] Figure 12 The graph shows the cycle performance of the Na / / NaClO4 / / AHC-Si@C-2D Pth battery prepared in Example 3 after 10,000 charge and discharge cycles. DETAILED DESCRIPTION

[0047] In order to more clearly illustrate the present invention, the present invention will be further described below in conjunction with specific embodiments and drawings. Those skilled in the art should understand that the following specific description is illustrative rather than restrictive and should not be used to limit the scope of protection of the present invention.

[0048] Example 1

[0049] (1) Preparation of sisal hard carbon microspheres

[0050] Wash the sisal fiber and dry it. Take 5g of the clean and dry sisal fiber and put it into 100ml 2mol L -1 The hydrochloric acid solution was transferred to an autoclave and hydrothermally reacted at 180°C for 12 hours. After the reaction was completed, the product was naturally cooled to room temperature, filtered, and the precursor was collected and washed until neutral. The precursor was placed in an oven for drying and dehydration for 24 hours. 1g of the precursor, ferric chloride, and 0.05g of a silane coupling agent (KBM-503) were co-dispersed in 30ml of toluene dehydrated with 3A molecular sieves, where the concentration of ferric chloride in toluene was 5mmol / L. Ultrasonic dispersion was performed for 10 minutes and the product was heated to reflux. After 5 hours of reaction, the reaction was stopped and cooled, centrifuged at 10,000r / min, and the precipitate was washed several times with anhydrous ethanol. After drying, a single-molecule-encapsulated precursor was obtained.

[0051] The single-molecule coated precursor is placed in an environment containing pyrrole monomer vapor, the ambient temperature is 0°C, and the volume concentration of pyrrole monomer vapor in the environment is 20-30%. In this environment, an ordered polypyrrole (PPy) layer can be in situ grown within 24 hours. The precursor with the polypyrrole (PPy) layer grown on the surface is then placed in a crucible and sintered at 900°C in a nitrogen atmosphere in a tube furnace for 1 hour to obtain a black powder, which is the sisal hard carbon microspheres with double coating layers (AHC-Si@C-2D PPy) (see Figure 1 ).

[0052] Figure 2 This is the selected area electron diffraction pattern of a precursor with a polypyrrole (PPy) layer grown on the surface. The figure shows that the polypyrrole coated on the surface of the precursor presents polycrystalline and ordered diffraction rings.

[0053] (2) Preparation of sodium ion batteries

[0054] The prepared sisal hard carbon microspheres were uniformly mixed with conductive carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and then N-methyl-2-pyrrolidone (NMP) was used as a solvent to make a slurry, which was coated on aluminum foil (250 μm) and dried in a 90°C forced air oven for 12 hours to obtain a hard carbon negative electrode, in which the sisal hard carbon microspheres were approximately 1-1.1 mg.

[0055] Assemble the battery: Use the above hard carbon negative electrode as the negative electrode, dissolve NaClO4 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC), where the volume ratio of ethylene carbonate to dimethyl carbonate is 1:1, and the concentration of NaClO4 is 1 mol L -1, and added 5vol% fluoroethylene carbonate (FEC) as the electrolyte, and the sodium sheet as the positive electrode material to assemble a button battery. The constant current charge and discharge performance and cycle performance of the battery were tested at 25℃. The battery has a voltage window of 0.001-3V at 0.05Ag -1 The first cycle discharge capacity of the current density is 351.44 mAh g -1 , the first charge and discharge efficiency reached 89.89%. Figure 3 The battery has a voltage window of 0.001-3 V at 25°C and a charge of 0.05 Ag. -1 The battery still has a capacity retention rate of 93.8% after 10,000 cycles.

[0056] Comparative Example 1

[0057] The preparation of the sodium ion battery in this comparative example is the same as in Example 1, except that the sisal hard carbon microspheres used are only orderly coated with polypyrrole. The specific operation is as follows:

[0058] Wash the sisal fiber and dry it. Take 5g of clean and dry sisal fiber and put ferric chloride into 100ml 2mol L -1 The hydrochloric acid solution, in which the concentration of ferric chloride in the hydrochloric acid solution is 5 mmol / L, is transferred to an autoclave and subjected to a hydrothermal reaction at 180° C. for 12 hours. After the reaction is completed, the product is naturally cooled to room temperature, filtered, and the precursor is collected and washed to neutrality.

[0059] After drying, the precursor is placed in an environment containing pyrrole monomer vapor at a temperature of 0°C and a volume concentration of 20-30% in the environment. An ordered polypyrrole (PPy) layer is grown in situ in this environment for 24 hours. The precursor with the polypyrrole (PPy) layer grown on the surface is then placed in a crucible and sintered at 900°C in a nitrogen atmosphere in a tubular furnace for 1 hour to obtain a black powder, which is sisal hard carbon microspheres coated with an ordered two-dimensional polymer carbonization layer (AHC-2D PPy).

[0060] The assembled battery has a voltage window of 0.001-3 V at 25 °C and a charge capacity of 0.05 Ag. -1 The first cycle discharge capacity of the current density is 348.72 mAh g -1 , the first charge and discharge efficiency reached 82.31%. Figure 4 The battery has a voltage window of 0.001-3 V at 25 °C and a flow rate of 0.05 A g -1The battery maintains 96.6% capacity retention after 3,000 cycles, but only 72.5% after 10,000 cycles. The hard carbon anode, lacking pre-desolvation, retains a large radius upon reaching the hard carbon surface. As the number of charge and discharge cycles increases, the hard carbon anode undergoes irreversible volume expansion, ultimately leading to a decrease in cycling stability.

[0061] Comparative Example 2

[0062] The preparation of the sodium ion battery in this comparative example is the same as in Example 1, except that the sisal hard carbon used is only coated with Si@C. The specific operation is as follows:

[0063] Wash the sisal fiber and dry it. Take 5g of the clean and dry sisal fiber and put it into 100ml 2mol L -1 The hydrochloric acid solution was transferred to an autoclave and hydrothermally reacted at 180°C for 12 hours. After the reaction was completed, it was naturally cooled to room temperature, filtered, and the precursor was collected and washed until neutral. The precursor was placed in an oven for drying and dehydration for 24 hours. 1g of the precursor and 0.05g of silane coupling agent (KBM-503) were co-dispersed in 30ml of toluene dehydrated with 3A molecular sieves, ultrasonically dispersed for 10 minutes, and heated to reflux. After reacting for 5 hours, the reaction was stopped and cooled, centrifuged at 10000r / min, and the precipitate was washed several times with anhydrous ethanol. After drying, it was sintered at 900°C in a nitrogen atmosphere in a tube furnace for 1 hour to obtain a black powder, which is Si@C-coated sisal hard carbon microspheres (AHC-Si@C).

[0064] The assembled battery has a voltage window of 0.001-3 V at 25 °C and a charge capacity of 0.05 Ag. -1 The first cycle discharge capacity of the current density is 308.95 mAh g -1 , the first charge and discharge efficiency reached 66.16%. Figure 5 The battery has a voltage window of 0.001-3 V at 25°C and a charge of 0.05 Ag. -1 The current density charge and discharge cycle performance diagram shows that the battery has a capacity retention rate of 87.3% after 1000 cycles.

[0065] Comparative Example 3

[0066] The preparation of the sodium ion battery in this comparative example is the same as in Example 1, except that the sisal hard carbon microspheres used are not coated. The specific operation is as follows:

[0067] Wash the sisal fiber and dry it. Take 5g of the clean and dry sisal fiber and put it into 100ml 2mol L -1The product was transferred to a high-pressure autoclave and subjected to a hydrothermal reaction at 180°C for 12 hours. After the reaction was completed, the product was naturally cooled to room temperature, filtered, and the precursor was collected and washed to neutrality.

[0068] After drying, the precursor was placed in a crucible and sintered at 900° C. for 1 h in a nitrogen atmosphere in a tube furnace to obtain black powder, which was uncoated sisal hard carbon microspheres (AHC).

[0069] The assembled battery has a voltage window of 0.001-3 V at 25 °C and a charge capacity of 0.05 Ag. -1 The first cycle discharge capacity of the current density is 302.31 mAh g -1 , the first charge and discharge efficiency is 63.11%. Figure 6 The battery has a voltage window of 0.001-3 V at 25°C and a charge of 0.05 Ag. -1 The current density charge and discharge cycle performance diagram shows that the battery has only a capacity retention rate of 77.9% after 370 cycles. Subsequently, the battery short-circuited. The reason is that there are a large number of defects on the surface of sisal hard carbon, where sodium ions gather and adsorb in large quantities, resulting in the gradual formation of sodium metal microspheres on the surface in the cycle range below 0.1V and eventually piercing the separator. The surface of the negative electrode material prepared with AHC-2D PPy and AHC-Si@C-2D PPy as raw materials is coated with a uniform two-dimensional NC, which increases the sodium ion adsorption sites to a certain extent, makes them evenly distributed, reduces the dominant deposition of sodium ions, and improves the stability of the negative electrode while increasing the capacity of the negative electrode. On the contrary, with the increase of current density, the sisal microsphere hard carbon negative electrode that is not coated with an ordered two-dimensional polymer carbonization layer short-circuits faster, causing battery failure.

[0070] Figure 7 The rate performance diagram of the four negative electrode materials prepared in Example 1 and Comparative Examples 1-3 shows that after Si@C coating, the desolvation process of sodium ions on the negative electrode surface is improved, the surface overpotential is reduced, and the rate performance is better than that of the AHC negative electrode. Due to the ordered structure of the 2D PPy carbonized layer, the sodium ions are uniformly adsorbed on the negative electrode surface with lower interface transfer resistance, and the synergistic effect of the double coating layer further reduces its interface transfer resistance. In the impedance spectrum, the interface transfer resistance of the AHC-2D PPy negative electrode is larger than that of the AHC-Si@C-2D PPy, but both are much smaller than the interface transfer resistance of the AHC-Si@C and AHC negative electrodes ( Figure 8), so that the coated negative electrode has better rate performance. The double-layer coating has a synergistic effect. The ordered two-dimensional polymer carbonization layer on the outside forms an ordered sodium ion transmission channel for the mesoporous structure. The solvated sodium ions in the electrolyte are arranged in it and uniformly distributed through the synergistic layers. Then the Si@C with a small pore structure pre-desolvates the solvated sodium ions to reduce their radius, and completes the final desolvation when it reaches the hard carbon surface, and vice versa. This double coating alleviates the volume expansion of the hard carbon negative electrode during the charge and discharge process and reduces dendrite growth, ensuring long-term stability and safety while improving the first efficiency and rate performance.

[0071] Example 2

[0072] (1) Preparation of epoxy phenolic resin hard carbon microspheres

[0073] Epoxy novolac resin and maleic anhydride were mixed in a 5:2 mass ratio and hydrothermally treated in a reactor at 180°C for 12 hours. After the reaction, the cured epoxy novolac resin was placed in a crucible and sintered at 500°C for 1 hour in a tube furnace under high-purity argon atmosphere. After cooling to room temperature, the sample was removed and vibrated into a powder to complete the pretreatment. 5g of the powder was added to 100ml of water in an autoclave and hydrothermally reacted at 180°C for 12 hours. After the reaction, the mixture was naturally cooled to room temperature, filtered, collected, and washed until neutral. The precursor was crushed into 2-5μm particles and dried for 24 hours. 1g of the precursor, ferric chloride, and 0.05g of a silane coupling agent (KBM-503) were dispersed in 30ml of toluene dehydrated with 3A molecular sieves. The concentration of ferric chloride in toluene was 5mmol / L. Ultrasonic dispersion was performed for 10 minutes and the mixture was heated to reflux. After 5 h of reaction, the reaction was stopped and cooled, and the product was centrifuged at 10,000 r / min. The precipitate was washed several times with anhydrous ethanol and dried to obtain a single-molecule-encapsulated precursor.

[0074] The single-molecule-coated precursor is placed in an environment containing aniline monomer vapor at a temperature of 0°C and a volume concentration of aniline monomer vapor of 20-30%. In this environment, an ordered polyaniline (PANI) layer can be grown in situ within 24 hours. The precursor with the polyaniline (PANI) layer grown on the surface is then placed in a crucible and sintered at 900°C in a nitrogen atmosphere in a tubular furnace for 1 hour to obtain a black powder, which is the epoxy phenolic resin hard carbon microspheres with a double coating layer (EPNHC-Si@C-2DPANI).

[0075] (2) Preparation of sodium ion batteries

[0076] The prepared epoxy phenolic resin hard carbon microspheres were uniformly mixed with conductive carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and then N-methyl-2-pyrrolidone (NMP) was used as a solvent to make a slurry, which was coated on aluminum foil (250 μm) and dried in a forced air oven at 90°C for 12 hours to obtain a hard carbon negative electrode, in which the epoxy phenolic resin hard carbon microspheres were approximately 1-1.1 mg.

[0077] Assemble the battery: Use the above hard carbon negative electrode as the negative electrode, dissolve NaClO4 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC), where the volume ratio of ethylene carbonate to dimethyl carbonate is 1:1, and the concentration of NaClO4 is 1 mol L -1 , and added 5vol% fluoroethylene carbonate (FEC) as the electrolyte, and the sodium sheet as the positive electrode material to assemble a button battery. The constant current charge and discharge performance and cycle performance of the battery were tested at 25℃. The battery has a voltage window of 0.001-3V at 0.05Ag -1 The first cycle discharge capacity of the current density is 553.67 mAh g -1 , the first charge and discharge efficiency reached 87.44%. Figure 9 The battery has a voltage window of 0.001-3 V at 25°C and a charge of 0.05 Ag. -1 The battery still has a capacity retention rate of 91.6% after 10,000 cycles.

[0078] Comparative Example 4

[0079] The assembly of the battery in this comparative example is the same as that in Example 2, except that the epoxy phenolic resin hard carbon surface non-uniformly grows a polymer. The specific operation is as follows:

[0080] Epoxy phenolic resin and maleic anhydride were mixed in a mass ratio of 5:2 and hydrothermally treated at 180℃ in a reactor for 12h. After the reaction, the cured epoxy phenolic resin was placed in a crucible and sintered at 500℃ in a high-purity argon atmosphere in a tube furnace for 1h. After cooling to room temperature, the sample was taken out and vibrated into powder to complete the pretreatment. 0.5g of powder was mixed with 10ml of 0.05molL -1 An ammonium persulfate aqueous solution is mixed and stirred evenly, then freeze-dried, and then placed in an environment containing aniline monomer vapor, the temperature of the environment is 25°C, and the volume concentration of the aniline monomer vapor in the environment is 20-30%. A polyaniline layer is grown in situ in this environment for 5 hours. Then, the precursor with a polyaniline (PANI) layer grown on the surface is placed in a crucible and sintered at 900°C in a nitrogen atmosphere in a tubular furnace for 1 hour to obtain a black powder, which is an epoxy phenolic resin hard carbon coated with a polymer carbonization layer (EPNHC-PANI).

[0081] Figure 10 This is the selected area electron diffraction pattern of the precursor on which the polyaniline (PANI) layer is grown. The figure shows that the polyaniline coated on the surface of the precursor has poor crystalline order. This is due to the uneven distribution of the solid oxidant ammonium persulfate and the rapid in situ growth rate at room temperature, resulting in poor crystalline order of the coated polyaniline.

[0082] The assembled battery has a voltage window of 0.001-3 V at 25 °C and a charge capacity of 0.05 Ag. -1 The first cycle discharge capacity of the current density is 544.92 mAh g -1 , the first charge and discharge efficiency reached 79.2%. Figure 11 The battery has a voltage window of 0.001-3 V at 25°C and a charge of 0.05 Ag. -1 The battery's capacity retention after 1000 cycles is shown in the cyclic performance diagram of charge and discharge at a current density of 1000. The EPNHC-Si@C-PANI anode, with its surface coated with a layer of NC, increases the adsorption sites for sodium ions and reduces the preferential deposition of sodium ions, thereby increasing both capacity and stability. However, due to the uneven coating of NC, portions of the hard carbon surface remain exposed, resulting in capacity decay during cycling.

[0083] Example 3

[0084] (1) Preparation of sisal hard carbon microspheres

[0085] Wash the sisal fiber and dry it. Take 5g of the clean and dry sisal fiber and put it into 100ml 2mol L -1 The hydrothermal reaction was carried out at 180°C in a hydrochloric acid solution for 12 hours. After the reaction was completed, the product was naturally cooled to room temperature, filtered, and the precursor was collected and washed until neutral. The precursor was placed in an oven for drying and dehydration for 24 hours. 1g of the precursor, ferric chloride, and 0.05g of a silane coupling agent (KBM-503) were co-dispersed in 30ml of toluene dehydrated with 3A molecular sieves. The concentration of ferric chloride in toluene was 5mmol / L. Ultrasonic dispersion was performed for 10 minutes and the product was heated to reflux. After 5 hours of reaction, the reaction was stopped and cooled. The product was centrifuged at 10,000 rpm. The precipitate was washed several times with anhydrous ethanol and dried to obtain a single-molecule-encapsulated precursor.

[0086] The single-molecule-coated precursor is placed in an environment containing thiophene monomer vapor. The ambient temperature is 0°C and the volume concentration of thiophene monomer vapor in the environment is 20-30%. In this environment, an ordered polythiophene (Pth) layer can be grown in situ within 24 hours. The precursor with the polythiophene (Pth) layer grown on the surface is then placed in a crucible and sintered at 900°C in a nitrogen atmosphere in a tubular furnace for 1 hour to obtain a black powder, which is sisal hard carbon microspheres with a double coating layer (AHC-Si@C-2D Pth).

[0087] (2) Preparation of sodium ion batteries

[0088] The prepared sisal hard carbon microspheres were uniformly mixed with conductive carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and then N-methyl-2-pyrrolidone (NMP) was used as a solvent to make a slurry, which was coated on aluminum foil (250 μm) and dried in a 90°C forced air oven for 12 hours to obtain a hard carbon negative electrode, in which the sisal hard carbon microspheres were approximately 1-1.1 mg.

[0089] Assemble the battery: The hard carbon negative electrode is used as the negative electrode material. NaClO4 is dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC), where the volume ratio of ethylene carbonate to dimethyl carbonate is 1:1 and the concentration of NaClO4 is 1 mol L -1 , and added 5vol% fluoroethylene carbonate (FEC) as the electrolyte, and the sodium sheet as the positive electrode material to assemble a button battery. The constant current charge and discharge performance and cycle performance of the battery were tested at 25℃. The battery has a voltage window of 0.001-3V at 25℃ and a charge current of 0.05Ag. -1 The first cycle discharge capacity of the current density is 343.44 mAh g -1 , the first charge and discharge efficiency reached 91.67%. Figure 12 The battery has a voltage window of 0.001-3 V at 25°C and a charge of 0.05 Ag. -1 The battery still has a capacity retention rate of 92.3% after 10,000 cycles.

[0090] Example 4

[0091] (1) Preparation of sisal hard carbon microspheres

[0092] Wash the sisal fiber and dry it. Take 5g of the clean and dry sisal fiber and put it into 100ml 2mol L -1In a hydrochloric acid solution, hydrothermal reaction was carried out at 180 ° C for 12 hours. After the reaction was completed, it was naturally cooled to room temperature, filtered and collected, and washed until neutral. The precursor was placed in an oven for drying and dehydration for 24 hours. 1g of the precursor, ferric chloride and 0.05g of a silane coupling agent (KBM-503) were co-dispersed in 30ml of toluene dehydrated by 3A molecular sieves, where the concentration of ferric chloride in toluene was 5mmol / L. Ultrasonic dispersion was performed for 10 minutes and heated to reflux. After 5 hours of reaction, the reaction was stopped and cooled, centrifuged at 10000r / min, and the precipitate was washed several times with anhydrous ethanol. After drying, a single-molecule coated precursor was obtained;

[0093] The single-molecule-coated precursor is placed in an environment containing 3,4-ethylenedioxythiophene monomer vapor. The ambient temperature is 0°C and the concentration of 3,4-ethylenedioxythiophene monomer vapor in the environment is 20-30%. In this environment, an ordered poly (3,4-ethylenedioxythiophene) (PEDOT) layer can be grown in situ within 24 hours. The precursor with the poly (3,4-ethylenedioxythiophene) (PEDOT) layer grown on the surface is then placed in a crucible and sintered at 900°C in a nitrogen atmosphere in a tubular furnace for 1 hour to obtain a black powder, which is sisal hard carbon microspheres with a double coating layer (AHC-Si@C-2D PEDOT).

[0094] (2) Preparation of sodium ion batteries

[0095] The prepared sisal hard carbon microspheres were uniformly mixed with conductive carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and then N-methyl-2-pyrrolidone (NMP) was used as a solvent to make a slurry, which was coated on aluminum foil (250 μm) and dried in a 90°C forced air oven for 12 hours to obtain a hard carbon negative electrode, in which the sisal hard carbon microspheres were approximately 1-1.1 mg.

[0096] Assemble the battery: The hard carbon negative electrode is used as the negative electrode material. NaClO4 is dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC), where the volume ratio of ethylene carbonate to dimethyl carbonate is 1:1 and the concentration of NaClO4 is 1 mol L -1 , and added 5vol% fluoroethylene carbonate (FEC) as the electrolyte, and the sodium sheet as the positive electrode material to assemble a button battery. The constant current charge and discharge performance and cycle performance of the battery were tested at 25℃. The battery has a voltage window of 0.001-3V at 25℃ and a charge current of 0.05Ag. -1 The first cycle discharge capacity of the current density is 346.86 mAh g -1 The first charge and discharge efficiency reached 87.25%. The battery has a voltage window of 0.001-3V at 25°C and a charge and discharge efficiency of 0.05Ag. -1 The current density of charge and discharge still has a capacity retention rate of 90.87% after 10,000 cycles.

[0097] Example 5

[0098] (1) Preparation of sisal hard carbon microspheres

[0099] Wash the sisal fiber and dry it. Take 5g of the clean and dry sisal fiber and put it into 100ml 2mol L -1 In a hydrochloric acid solution, hydrothermal reaction was carried out at 180 ° C for 12 hours. After the reaction was completed, it was naturally cooled to room temperature, filtered and collected, and washed until neutral. The precursor was placed in an oven for drying and dehydration for 24 hours. 1g of the precursor, ferric chloride and 0.05g of silane coupling agent (KBM-502) were co-dispersed in 30ml of toluene dehydrated by 3A molecular sieves, where the concentration of ferric chloride in toluene was 5mmol / L. Ultrasonic dispersion was performed for 10 minutes and heated to reflux. After 5 hours of reaction, the reaction was stopped and cooled, centrifuged at 10000r / min, and the precipitate was washed several times with anhydrous ethanol. After drying, a single-molecule coated precursor was obtained;

[0100] The single-molecule-coated precursor is placed in an environment containing 3,4-ethylenedioxythiophene monomer vapor. The ambient temperature is 0°C and the concentration of 3,4-ethylenedioxythiophene monomer vapor in the environment is 20-30%. In this environment, an ordered poly (3,4-ethylenedioxythiophene) (PEDOT) layer can be grown in situ within 24 hours. The precursor with the poly (3,4-ethylenedioxythiophene) (PEDOT) layer grown on the surface is then placed in a crucible and sintered at 900°C in a nitrogen atmosphere in a tubular furnace for 1 hour to obtain a black powder, which is sisal hard carbon microspheres with a double coating layer (AHC-Si@C-2D PEDOT).

[0101] (2) Preparation of sodium ion batteries

[0102] The prepared sisal hard carbon microspheres were uniformly mixed with conductive carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and then N-methyl-2-pyrrolidone (NMP) was used as a solvent to make a slurry, which was coated on aluminum foil (250 μm) and dried in a 90°C forced air oven for 12 hours to obtain a hard carbon negative electrode, in which the sisal hard carbon microspheres were approximately 1-1.1 mg.

[0103] Assemble the battery: The hard carbon negative electrode is used as the negative electrode material. NaClO4 is dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC), where the volume ratio of ethylene carbonate to dimethyl carbonate is 1:1 and the concentration of NaClO4 is 1 mol L -1 , and added 5vol% fluoroethylene carbonate (FEC) as the electrolyte, and assembled a button battery with sodium sheet as the positive electrode material. Then the constant current charge and discharge performance and cycle performance of the battery were tested at 25℃. The battery has a voltage window of 0.001-3V at 0.05Ag -1The first cycle discharge capacity of the current density is 343.11 mAh g -1 The first charge and discharge efficiency reached 85.93%. The battery has a voltage window of 0.001-3V at 25℃ and a charge and discharge efficiency of 0.05A g -1 The current density of charge and discharge still has a capacity retention rate of 88.45% after 10,000 cycles.

[0104] Example 6

[0105] (1) Preparation of sisal hard carbon microspheres

[0106] Wash the sisal fiber and dry it. Take 5g of the clean and dry sisal fiber and put it into 100ml 2mol L -1 In a hydrochloric acid solution, hydrothermal reaction was carried out at 180 ° C for 12 hours. After the reaction was completed, it was naturally cooled to room temperature. The precursor was collected after filtration and washed until neutral. The precursor was placed in an oven for drying and dehydration for 24 hours. 1g of the precursor, ferric chloride and 0.05g of silane coupling agent (KBM-303) were co-dispersed in 30ml of toluene dehydrated by 3A molecular sieves, where the concentration of ferric chloride in toluene was 5mmol / L. Ultrasonic dispersion was performed for 10 minutes and heated to reflux. After 5 hours of reaction, the reaction was stopped and cooled, centrifuged at 10000r / min, and the precipitate was washed several times with anhydrous ethanol. After drying, a single-molecule coated precursor was obtained;

[0107] The single-molecule-coated precursor is placed in an environment containing pyrrole monomer vapor at a temperature of 0°C and a concentration of 20-30%. In this environment, an ordered polypyrrole (PPy) layer can be in situ grown within 24 hours. The precursor with the polypyrrole (PPy) layer grown on the surface is then placed in a crucible and sintered at 900°C in a nitrogen atmosphere in a tubular furnace for 1 hour to obtain a black powder, which is the sisal hard carbon microspheres with a double coating layer (AHC-Si@C-2D PPy).

[0108] (2) Preparation of sodium ion batteries

[0109] The prepared sisal hard carbon microspheres were uniformly mixed with conductive carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and then N-methyl-2-pyrrolidone (NMP) was used as a solvent to make a slurry, which was coated on aluminum foil (250 μm) and dried in a 90°C forced air oven for 12 hours to obtain a hard carbon negative electrode, in which the sisal hard carbon microspheres were approximately 1-1.1 mg.

[0110] Assemble the battery: The hard carbon negative electrode is used as the negative electrode material. NaClO4 is dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC), where the volume ratio of ethylene carbonate to dimethyl carbonate is 1:1 and the concentration of NaClO4 is 1 mol L -1, and added 5vol% fluoroethylene carbonate (FEC) as the electrolyte, and the sodium sheet as the positive electrode material to assemble a button battery. The constant current charge and discharge performance and cycle performance of the battery were tested at 25℃. The battery has a voltage window of 0.001-3V at 25℃ and a charge current of 0.05Ag. -1 The first cycle discharge capacity of the current density is 350.37 mAh g -1 The first charge and discharge efficiency reached 89.55%. The battery has a voltage window of 0.001-3V at 25°C and a charge and discharge efficiency of 0.05Ag. -1 The current density of charge and discharge still has a capacity retention rate of 89.54% after 10,000 cycles.

[0111] Example 7

[0112] (1) Preparation of sisal hard carbon microspheres

[0113] Wash the sisal fiber and dry it. Take 5g of the clean and dry sisal fiber and put it into 100ml 2mol L -1 In a hydrochloric acid solution, hydrothermal reaction was carried out at 180 ° C for 12 hours. After the reaction was completed, it was naturally cooled to room temperature, filtered and collected, and washed until neutral. The precursor was placed in an oven for drying and dehydration for 24 hours. 1g of the precursor, ferric chloride and 0.05g of silane coupling agent (KA-1003) were co-dispersed in 30ml of toluene dehydrated by 3A molecular sieves, where the concentration of ferric chloride in toluene was 5mmol / L. Ultrasonic dispersion was performed for 10 minutes and heated to reflux. After 5 hours of reaction, the reaction was stopped and cooled, centrifuged at 10000r / min, and the precipitate was washed several times with anhydrous ethanol. After drying, a single-molecule coated precursor was obtained;

[0114] The single-molecule-coated precursor is placed in an environment containing pyrrole monomer vapor at a temperature of 0°C and a concentration of 20-30%. In this environment, an ordered polypyrrole (PPy) layer can be in situ grown within 24 hours. The precursor with the polypyrrole (PPy) layer grown on the surface is then placed in a crucible and sintered at 900°C in a nitrogen atmosphere in a tubular furnace for 1 hour to obtain a black powder, which is the sisal hard carbon microspheres with a double coating layer (AHC-Si@C-2D PPy).

[0115] (2) Preparation of sodium ion batteries

[0116] The prepared sisal hard carbon microspheres were uniformly mixed with conductive carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and then N-methyl-2-pyrrolidone (NMP) was used as a solvent to make a slurry, which was coated on aluminum foil (250 μm) and dried in a 90°C forced air oven for 12 hours to obtain a hard carbon negative electrode, in which the sisal hard carbon microspheres were approximately 1-1.1 mg.

[0117] Assemble the battery: The hard carbon negative electrode is used as the negative electrode material. NaClO4 is dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC), where the volume ratio of ethylene carbonate to dimethyl carbonate is 1:1 and the concentration of NaClO4 is 1 mol L -1 , and added 5vol% fluoroethylene carbonate (FEC) as the electrolyte, and assembled a button cell with sodium sheet as the positive electrode material. Then the constant current charge and discharge performance and cycle performance of the battery were tested at 25℃. The battery has a voltage window of 0.001-3V at 0.05A g -1 The first cycle discharge capacity of the current density is 344.89 mAh g -1 The first charge and discharge efficiency reached 92.31%. The battery has a voltage window of 0.001-3V at 25°C and a charge and discharge efficiency of 0.05Ag. -1 The current density of charge and discharge still has a capacity retention rate of 85.39% after 10,000 cycles.

[0118] Comparative Example 5

[0119] The preparation process of the sodium ion battery in this comparative example is similar to that in Example 1, except that the sisal hard carbon microspheres used are preformed using chemical vapor deposition of polymer monomolecules after the monomolecule-coated precursor. The specific steps are as follows:

[0120] (1) Preparation of sisal hard carbon microspheres

[0121] Wash the sisal fiber and dry it. Take 5g of the clean and dry sisal fiber and put it into 100ml 2mol L -1 In a hydrochloric acid solution, hydrothermal reaction was carried out at 180 ° C for 12 hours. After the reaction was completed, it was naturally cooled to room temperature, filtered and collected, and washed until neutral. The precursor was placed in an oven for drying and dehydration for 24 hours. 1g of the precursor and 0.05g of silane coupling agent (KBM-503) were co-dispersed in 30ml of toluene dehydrated with 3A molecular sieves, ultrasonically dispersed for 10 minutes, and heated to reflux. After the reaction was stopped after 5 hours and cooled, centrifuged at 10000r / min, the precipitate was washed several times with anhydrous ethanol, and dried to obtain a single-molecule coated precursor;

[0122] After drying, the precursor was placed in an OCVD (Oxidized Chemical Vapor Deposition) (OCVD) chamber. Aniline monomer mixed with nitrogen and the oxidant antimony pentachloride were heated to 60°C, with a constant flow rate of 1.5 sccm (standard cubic centimeters per minute) provided by a low-flow precision metering valve. Nitrogen, an inert carrier gas, was sprayed onto the precursor surface at a flow rate of 1.5 sccm through a mass flow controller, maintaining the reactor pressure (P) at 50 mTorr. The deposition time was 1 hour, resulting in a polyaniline (PANI) layer. The precursor, with the polypyrrole (PANI) layer grown on its surface, was then placed in a crucible and sintered at 900°C in a nitrogen atmosphere in a tube furnace for 1 hour to produce a black powder, namely, oxidative chemical vapor deposition polymer carbonization layer-coated sisal hard carbon microspheres (AHC-OCVD PANI).

[0123] (2) Preparation of sodium ion batteries

[0124] The prepared sisal hard carbon microspheres were uniformly mixed with conductive carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and then N-methyl-2-pyrrolidone (NMP) was used as a solvent to make a slurry, which was coated on aluminum foil (250 μm) and dried in a 90°C forced air oven for 12 hours to obtain a hard carbon negative electrode, in which the sisal hard carbon microspheres were approximately 1-1.1 mg.

[0125] Assemble the battery: Use the above hard carbon negative electrode as the negative electrode, dissolve NaClO4 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC), where the volume ratio of ethylene carbonate to dimethyl carbonate is 1:1, and the concentration of NaClO4 is 1 mol L -1 , and added 5vol% fluoroethylene carbonate (FEC) as the electrolyte, and the sodium sheet as the positive electrode material to assemble a button battery. The battery has a voltage window of 0.001-3V at 25℃ and a capacitance of 0.05Ag -1 The first cycle discharge capacity of the current density is 346.19 mAh g -1 , the first charge and discharge efficiency reached 86.23%. The battery has a capacity retention rate of 85.8% after 5000 cycles in a voltage window of 0.001-3V at 25°C.

[0126] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not limitations on the implementation methods of the present invention. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for preparing hard carbon microspheres with double coating layers, characterized in that: The steps include: S1. Place the hard carbon source in an autoclave and perform a hydrothermal reaction at 150-220°C for 12-24 hours; S2. After the reaction is completed, the precursor is filtered and collected, and the precursor is dehydrated and dispersed in anhydrous toluene together with an oxidant and a silane coupling agent, and heated under reflux for 1-10 hours to obtain a single molecule-coated precursor; S3. The single-molecule coated precursor is placed in an environment containing conductive polymer monomer vapor to carry out in-situ polymerization reaction, and an ordered polymer layer is obtained on the outer layer of the single-molecule coated precursor, which is then sintered in an inert atmosphere to obtain hard carbon microspheres with double coating layers.

2. The preparation method according to claim 1, characterized in that The hard carbon source is selected from one or more of a biomass hard carbon source, a resin-based hard carbon source, and a pitch-based hard carbon source.

3. The preparation method according to claim 1, characterized in that The silane coupling agent is selected from one or more of an aminosilane coupling agent, an epoxysilane coupling agent, a methacryloxysilane coupling agent and a vinylsilane coupling agent.

4. The preparation method according to claim 1, characterized in that The mass ratio of the silane coupling agent to the precursor is 5-10:

100.

5. The preparation method according to claim 1, characterized in that The conductive polymer monomer is selected from one or more of pyrrole, aniline, thiophene and 3,4-ethylenedioxythiophene.

6. The preparation method according to claim 1, characterized in that In step S3, the volume concentration of the conductive polymer monomer vapor in the environment containing the conductive polymer monomer vapor is 20-30%.

7. The preparation method according to claim 1, characterized in that In step S3, the reaction temperature of the in-situ polymerization is -4 to 4°C, and the reaction time is 12 to 36 hours.

8. The preparation method according to claim 1, characterized in that In step S3, the sintering temperature is 800-1100° C., and the sintering time is 1-3 hours.

9. A hard carbon microsphere with a double coating layer, characterized in that: The method is prepared by the method according to any one of claims 1 to 8.

10. A negative electrode material, characterized in that: The negative electrode material contains the hard carbon microspheres with double coating layers as claimed in any one of claims 9.

11. A sodium ion battery, characterized in that: The invention comprises the negative electrode material according to claim 10, a positive electrode material, a separator arranged between the positive electrode material and the negative electrode material, and an electrolyte.

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