A method for purifying and pre-crosslinking coupled preparation of hard carbon by using medium and low temperature coal tar-based pitch

Abstract

This invention discloses a method for preparing hard carbon by coupling purification and pre-crosslinking of medium- and low-temperature coal tar-based pitch, comprising the following steps: (S1) mixing medium- and low-temperature coal tar pitch with a mixed extraction solvent of aromatic and aliphatic hydrocarbon solvents, and collecting the lower layer as the extract; (S2) mixing the extract with a ternary composite purification agent for purification, followed by horizontal screw discharge sedimentation and centrifugation separation, and collecting the liquid phase to obtain purified pitch liquid; (S3) removing the extraction solvent and light components from the purified pitch liquid by vacuum distillation to obtain purified pitch; (S4) reacting the purified pitch with a composite pre-crosslinking agent composed of phenolic compounds and multifunctional epoxy resin under an inert atmosphere by heating and stirring to obtain a pre-crosslinking precursor; (S5) carbonizing the pre-crosslinking precursor at high temperature under an inert atmosphere to obtain hard carbon. This invention achieves process coupling of purification and pre-crosslinking, and the resulting hard carbon has a typical disordered layered structure and excellent electrochemical sodium storage performance. The process is green and environmentally friendly and easy to industrialize.

Description

Technical Field

[0001] This invention relates to the field of hard carbon materials, specifically to a method for preparing hard carbon by purifying and pre-crosslinking coal tar-based pitch at medium and low temperatures. Background Technology

[0002] Hard carbon materials, due to their unique disordered layered structure and abundant nanopores, have shown great application potential in the field of sodium-ion battery anodes. Coal tar pitch, especially medium- and low-temperature coal tar pitch, is one of the ideal precursors for preparing hard carbon due to its rich aroma and high carbon content. However, there are significant technical bottlenecks in directly preparing hard carbon from medium- and low-temperature coal tar pitch: First, pitch contains various metallic impurities such as Fe, Na, Ca, and Mg, as well as ash. These impurities act as graphitization catalysts during carbonization, inducing the carbon material to form an ordered graphitized structure, destroying the disordered layered characteristics of hard carbon, and significantly reducing sodium storage capacity and cycle stability. Second, pitch is prone to melting and merging and molecular chain ordering transformation during heating and carbonization, easily generating soft carbon that is easily graphitized, making it difficult to form the graphite-like disordered layered structure required for hard carbon. Third, the light components and gums in pitch interfere with subsequent crosslinking reactions, resulting in insufficient crosslinking network construction and an inability to effectively suppress the tendency to graphitize.

[0003] Existing technologies for addressing the aforementioned problems have several shortcomings: using strong acids such as concentrated sulfuric acid and concentrated nitric acid for oxidation purification or cross-linking not only causes severe environmental pollution but also excessively oxidizes the aromatic ring structure of asphalt molecules, reducing carbon yield; using a single complexing agent or a single cross-linking agent results in low purification efficiency and poor cross-linking effect, and easily introduces secondary impurities; purification and pre-cross-linking processes are mostly independent operations without coupling design, making it easy for purified asphalt to re-adsorb impurities, and conventional filtration separation methods are inefficient, difficult to implement continuously, and unsuitable for industrial production; furthermore, existing technologies lack direct characterization and verification of the cross-linking agent's regulation of the hard carbon microstructure, and cannot intuitively demonstrate the role of the cross-linking agent in inhibiting graphitization and constructing hard carbon characteristic structures, resulting in insufficient technological innovation and experimental evidence.

[0004] Therefore, developing a continuous, in-depth, and green asphalt-based precursor purification process, combining a high-efficiency composite purifier with a composite pre-crosslinking agent to achieve process coupling of purification and pre-crosslinking, and visually verifying the regulatory effect of the pre-crosslinking agent on the hard carbon structure through microscopic characterization, in order to prepare a high-performance sodium-ion battery anode material with typical hard carbon characteristics, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the challenge of preparing high-performance hard carbon materials suitable for sodium battery anodes from medium- and low-temperature coal tar-based pitch using existing technologies, this invention employs an integrated pretreatment process combining solvent extraction, deep complexation, and continuous impurity removal via decanter centrifugation. This process utilizes a non-traditional ternary composite purifying agent to achieve deep purification of the pitch-based precursor, and a composite pre-crosslinking agent combining phenolic compounds and bisphenol-type epoxy resin to modify the precursor. This invention overcomes the problems of conventional purifying agents, low separation efficiency, discontinuous processes, and lack of direct characterization and verification of crosslinking effects in existing technologies. The resulting hard carbon precursor exhibits high purity and structural stability. The hard carbon material obtained after carbonization possesses typical hard carbon microstructure characteristics and excellent electrochemical performance. Furthermore, the entire process is environmentally friendly, easily scalable for industrial production, and simultaneously achieves high-value utilization of coal chemical byproducts.

[0006] Specifically, the present invention provides the following technical solutions to solve the above-mentioned technical problems.

[0007] A method for preparing hard carbon by purifying and pre-crosslinking coal tar-based pitch at medium and low temperatures includes the following steps:

[0008] (S1) Low-temperature coal tar pitch is mixed with an extraction solvent for extraction, and the lower layer is taken as the extract material. The extraction solvent includes aromatic solvents and aliphatic hydrocarbon solvents.

[0009] (S2) The extract and the ternary composite purifying agent are mixed and purified to obtain a purified slurry; the purified slurry is separated into solid and liquid phases, and the liquid phase is collected to obtain a purified asphalt slurry; the ternary composite purifying agent is a compound of polybasic acid, ethylenediaminetetraacetic acid salt and aminophosphonic acid compound.

[0010] (S3) Liquid-phase purified asphalt slurry is subjected to vacuum distillation to remove the extraction solvent and light components, thereby obtaining purified asphalt;

[0011] (S4) The purified asphalt and the composite pre-crosslinking agent are mixed and heated and stirred under an inert atmosphere to carry out a pre-crosslinking reaction. After the reaction is completed, the mixture is cooled to room temperature to obtain a pre-crosslinking precursor. The composite pre-crosslinking agent includes phenolic compounds and multifunctional epoxy resin.

[0012] (S5) Under an inert atmosphere, the pre-crosslinked precursor is carbonized at high temperature to obtain the product hard carbon.

[0013] Further, in step (S1), the aromatic solvent is selected from at least one of wash oil, anthracene oil, and naphthalene oil; the aliphatic hydrocarbon solvent is selected from at least one of petroleum ether, cyclohexane, and kerosene. Preferably, the extraction solvent is a mixture of aromatic solvent and aliphatic hydrocarbon solvent in a volume ratio of 3-5:1. The mixture of aromatic solvent and aliphatic hydrocarbon solvent achieves preliminary purification by selectively extracting medium- and low-temperature coal tar pitch. Aromatic solvents such as wash oil and anthracene oil have chemical structures very similar to the polycyclic aromatic hydrocarbons in pitch. Based on the principle of "like dissolves like," these solvents can efficiently dissolve and extract large molecular aromatic components, i.e., hard carbon precursors, from asphalt. Petroleum ether, cyclohexane, and other aliphatic hydrocarbon solvents have poor solubility for large molecular polycyclic aromatic hydrocarbons in asphalt. When aliphatic and aromatic solvents are mixed, the solubility of the mixed solvent decreases, resulting in the inability to effectively dissolve lighter components with smaller molecular weights and lower aromatization levels in the asphalt. Simultaneously, it promotes the easier sedimentation and separation of impurities such as ash and solid particles that are not tightly bound to asphalt molecules after extraction, facilitating subsequent mixing with ternary composite purifying agents for efficient removal of metallic impurities. If only a single aromatic solvent is used for dissolution, impurities and lighter components will remain in the system, increasing the difficulty of subsequent purification.

[0014] Further, in step (S1), the softening point of the medium-low temperature coal tar pitch is 60-90℃, the ash content is 2-5%, and the total content of metal impurities is 1000-2000ppm; even further, the mass-volume ratio of the medium-low temperature coal tar pitch to the extraction solvent is 1kg:2-3L; the extraction temperature is 20-60℃, the extraction time is 1-5h, and the mixing speed is 200-500rpm.

[0015] Further, in step (S2), the solid-liquid separation is centrifugal separation, preferably a horizontal screw discharge sedimentation centrifugation; the amount of the ternary composite purifying agent is 3-5 wt% of the extract mass; preferably, the polyacid is selected from at least one of oxalic acid, malic acid, and citric acid; the ethylenediaminetetraacetic acid salt is selected from at least one of sodium ethylenediaminetetraacetic acid and potassium ethylenediaminetetraacetic acid; and the aminophosphonic acid compound is selected from at least one of aminotrimethylphosphonic acid, hydroxyethylidene diphosphonic acid, and ethylenediaminetetramethylenephosphonic acid. The inventors have found that using the above-mentioned polyacid, ethylenediaminetetraacetic acid salt, and aminophosphonic acid compound as a composite purifying agent can stably control the metal impurities in hard carbon products to below 20 ppm and the ash content to below 0.2%.

[0016] Furthermore, in step (S2), the ternary composite purifying agent is a mixture of a polybasic acid, a salt of ethylenediaminetetraacetic acid, and an aminophosphonic acid compound in a mass ratio of 3-5:1-2:0.5-1.

[0017] Further, in step (S2), purification involves mixing and stirring the extractant and the ternary composite purifying agent at 200-400 rpm for 3-6 hours at a stirring temperature of 15-40℃; the process parameters for the horizontal screw discharge sedimentation centrifuge are: drum speed 3000-5000 rpm, differential speed 5-20 rpm, and feed flow rate 0.5~2 m³ / min. 3 / h.

[0018] Further, in step (S3), the vacuum degree of the vacuum distillation is -0.08 to -0.095 MPa, the distillation temperature is 120~180℃, and the distillation time is 1~3h.

[0019] Further, in step (S4), the phenolic compound is selected from at least one of methylphenol, dimethylphenol, and resorcinol; the multifunctional epoxy resin is selected from at least one of trimethylolpropane triglycidyl ether, glycerol triglycidyl ether, glycerol propoxy triglycidyl ether, pentaerythritol tetraglycidyl ether, triglycidyl isocyanurate, N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane, and tetraglycidyl diaminodiphenylmethane. Preferably, the multifunctional epoxy resin containing N is selected from at least one of triglycidyl isocyanurate, N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane, and tetraglycidyl diaminodiphenylmethane. Under heating conditions, phenolic hydroxyl groups can react with active sites (such as oxygen-containing groups or unsaturated bonds) and epoxy groups in asphalt molecules; phenols themselves can also undergo condensation reactions upon heating to form methylene bridges, connecting asphalt molecules together. The inventors also discovered that using a blend of nitrogen-containing multifunctional epoxy resin and phenolic compounds as a pre-crosslinking agent, the multifunctional epoxy resin can increase the density and complexity of the crosslinking network. The nitrogen atoms in the molecule, acting as heteroatoms, make it difficult for carbon layers to stack together to form graphite, stabilizing this disordered microstructure. The final hard carbon material exhibits the characteristic of having no sharp graphitization peaks in XRD.

[0020] Further, in step (S4), the inert atmosphere is nitrogen and / or argon; the mass ratio of purified asphalt to composite pre-crosslinking agent is 100:4-8. The pre-crosslinking reaction temperature is 190-210℃, the reaction time is 1-4h, and the stirring speed is 100-300rpm.

[0021] Furthermore, in step (S4), the softening point of the obtained pre-crosslinked precursor is ≥150℃, the total content of metal impurities is less than 20ppm, and the ash content is less than 0.2%. The metal impurities are Fe, Na, Ca, and Mg.

[0022] Furthermore, in step (S5), the inert atmosphere is nitrogen and / or argon, and the high-temperature carbonization is to heat to 1000-1300℃ and hold for 2-5 hours. There is no particular limitation on the heating rate, such as 1-20℃ / min, preferably 5-10℃ / min.

[0023] The present invention also provides a sodium-ion battery, wherein the negative electrode comprises hard carbon prepared by the above preparation method.

[0024] Compared with the prior art, the present invention achieves the following beneficial effects:

[0025] I. This invention organically combines two key processes: purification and pre-crosslinking. First, purification creates a pure reaction environment for effective crosslinking. Then, crosslinking fixes the purification results and constructs the required structure for hard carbon. The process is simple and efficient. This invention constructs an integrated pretreatment process of solvent extraction, deep complexation, and continuous impurity removal via decanter centrifugation in the preparation of hard carbon from medium- and low-temperature coal tar pitch-based precursors. This replaces traditional simple dissolution and conventional filtration. The high-speed rotating drum of the decanter centrifuge achieves efficient sedimentation of solid impurities, and continuous unloading avoids the clogging problem of traditional filtration. Separation efficiency is improved, and the feed flow rate can be flexibly adjusted to adapt to different scales of industrial production, realizing continuous and industrialized purification of pitch-based precursors.

[0026] Second, a ternary system of small molecule acid, EDTA salt and phosphonic acid complexing agent is adopted to achieve synergistic effect on metal impurities with different properties. It achieves a good effect of removing metal impurities with a low amount of purifying agent. The purification effect is far superior to the single acid washing method. It can stably control metal impurities below 20 ppm and ash content below 0.2%.

[0027] Third, a composite pre-crosslinking system of phenolic compounds and multifunctional epoxy resins is used to achieve efficient crosslinking of asphalt molecules under relatively mild conditions, significantly increasing its softening point (≥150℃) and effectively suppressing the graphitization tendency in the subsequent carbonization process. This invention directly confirms the key role of the composite pre-crosslinking agent in suppressing graphitization and constructing the characteristic hard carbon structure through three microscopic characterization methods: TEM, XRD, and Raman spectroscopy. The carbon material obtained after adding the pre-crosslinking agent exhibits a typical disordered layered graphite-like structure, with no sharp graphitization peaks in XRD and a Raman ID / IG ratio of 0.9-1.1. In contrast, the carbon material without the pre-crosslinking agent shows a clearly graphitized ordered structure. This solves the problem of no direct characterization and verification of the crosslinking effect in existing technologies, enhancing the technological innovation and the sufficiency of experimental evidence.

[0028] Fourth, this invention avoids the use of highly corrosive solvents such as concentrated nitric acid and sulfuric acid, making the production process green and environmentally friendly.

[0029] Fifth, the raw material precursor of the hard carbon material obtained by this invention is purified, has high purity and few impurities, which can reduce graphitization during the carbonization process, and has excellent sodium storage capacity and cycle performance, making it very suitable for use as the negative electrode of sodium-ion batteries. Attached Figure Description

[0030] Figure 1 This is a SEM image of the hard carbon material obtained in Example 1.

[0031] Figure 2 This is the Raman spectrum of the hard carbon material obtained in Example 1.

[0032] Figure 3 These are XRD patterns of the hard carbon material obtained in Example 1 and the hard carbon material prepared by carbonization of unpurified and pre-crosslinked pitch. Detailed Implementation

[0033] The reagents and equipment used in the embodiments of this invention can all be purchased from conventional channels.

[0034] The present invention will be further described in detail below with reference to specific embodiments. The scope of protection of the present invention is not limited to the following embodiments, and all technical solutions based on the present invention are within the scope of protection of the present invention. The reagents and equipment used in the embodiments of the present invention are all commercially available products; the performance indicators of the medium and low temperature coal tar pitch used are: softening point 75℃, ash content 2.8%, and total metal impurity content 1500ppm (of which Fe: 800ppm, Na: 450ppm, Ca: 200ppm, Mg: 50ppm); the horizontal screw centrifuge used is a WL450 type horizontal screw discharge sedimentation centrifuge; microscopic characterization is performed using transmission electron microscopy (TEM, JEM-2100), X-ray diffraction (XRD, D8 Advance), and laser Raman spectroscopy (Raman, inVia Reflex).

[0035] Example 1

[0036] (S1) Take 100g of medium-low temperature coal tar pitch, add 200mL of anthracene oil and cyclohexane in a volume ratio of 3:1, stir at 40℃ and 300rpm for 2h, let stand and separate into layers, and take the lower layer as the extract.

[0037] (S2) Add 5 wt% of a composite purifying agent (oxalic acid: disodium EDTA: aminotrimethylphosphonic acid = 3:1:0.5, mass ratio) to the extract, and stir at 20℃ and 500 rpm for 5 hours to obtain a purified slurry; dilute the purified slurry with 1 m 3 The feed flow rate is fed into the WL450 horizontal screw centrifuge at a rate of / h. The drum speed is set to 4000rpm and the differential speed is 10rpm for centrifugal separation. The liquid phase is collected to obtain purified asphalt slurry.

[0038] (S3) The liquid-phase purified asphalt slurry was distilled under reduced pressure at -0.09MPa and 150℃ for 2 hours to remove anthracene oil, cyclohexane and light components, and purified asphalt was obtained.

[0039] (S4) Add 6.2 wt% of a composite pre-crosslinking agent (a mixture of m-cresol and triglycidyl isocyanate in a mass ratio of 3:1) to the purified asphalt. Under nitrogen protection, the mixture is heated to 200°C with stirring at 200 rpm and reacted for 2 hours. After cooling, a blocky solid is obtained, which is the pre-crosslinking precursor P1.

[0040] (S5) The pre-crosslinked precursor was carbonized under a nitrogen atmosphere by heating to 1100℃ at 5℃ / min and holding for 4 hours to obtain hard carbon material.

[0041] Figure 1 This is a SEM image of the hard carbon material obtained in Example 1. From... Figure 1 It can be seen that the prepared hard carbon material has an irregular blocky and sheet-like morphology, the particle surface is relatively rough, and the surface has abundant micropores and crack structures. There is no obvious graphitized layered stacking morphology, indicating that the carbonization product has non-graphitized hard carbon structural characteristics.

[0042] Figure 2 This is the Raman spectrum of the hard carbon material obtained in Example 1. From... Figure 2 It can be seen that at approximately 1350 cm -1 and 1580cm -1 D and G peaks appeared at the specified locations. The D peak is attributed to vibrational modes caused by disordered structures and defects in the carbon material, while the G peak is attributed to the in-plane stretching vibrations of sp² carbon atoms. The calculated ID / IG ratio is approximately 1.0, indicating that the content of disordered carbon structures and graphitized carbon structures in the material is similar, exhibiting high defect and disorder levels, consistent with typical Raman spectral characteristics of hard carbon materials. The broadening of both the D and G peaks further confirms the high degree of disorder in the carbon layer arrangement.

[0043] Figure 3 The images show the XRD patterns of the hard carbon material obtained in Example 1 and the hard carbon material prepared by carbonization of untreated and pre-crosslinked pitch. The XRD patterns reveal that the untreated pitch exhibits two sharp and distinct diffraction peaks during direct carbonization: a (002) peak at approximately 26° and a (100) peak at approximately 43°. This closely matches the characteristics of graphite carbon materials, indicating that the sample has an ordered atomic arrangement and high crystallinity. In contrast, the carbon material prepared from the modified pitch in Example 1 exhibits significantly different characteristics, with a broad peak at approximately 23° and only a weak peak at approximately 43°. The peaks are broadened and have lower intensity, demonstrating that the graphite domains in this carbon material are smaller and the internal structure is highly disordered.

[0044] Example 2

[0045] Everything else is the same as in Example 1, except that in step (S1), the mixed solvent is 300 mL of wash oil and petroleum ether in a volume ratio of 4:1. Step (S4) yields the pre-crosslinked precursor P2.

[0046] Example 3

[0047] Everything else is the same as in Example 1, except that in step (S2), the composite purifying agent is a mixture of malic acid, disodium EDTA, and hydroxyethylidene diphosphonic acid in a mass ratio of 5:2:1. Step (S4) yields the pre-crosslinked precursor P3.

[0048] Example 4

[0049] Everything else is the same as in Example 1, except that in step (S4), the composite pre-crosslinking agent is a mixture of m-cresol and glycerol propoxy triglycidyl ether in a mass ratio of 3:1. Step (S4) yields the pre-crosslinking precursor P4.

[0050] Comparative Example 1

[0051] (1) Take 100g of the same medium-low temperature coal tar pitch, without any purification or pre-crosslinking treatment, and use it directly as the control sample C1.

[0052] (2) The high-temperature carbonization step is the same as in Example 1.

[0053] Comparative Example 2

[0054] Everything else is the same as in Example 1, except that in step (S1), the extraction solvent is 200 mL of anthracene oil. Step (S4) yields a pre-crosslinked precursor of C2.

[0055] Comparative Example 3

[0056] Everything else is the same as in Example 1, except that in step (S2), the composite purifying agent is a mixture of oxalic acid and disodium EDTA in a mass ratio of 3:1, meaning that no aminophosphonic acid compounds are added to the composite purifying agent. Step (S4) yields a pre-crosslinked precursor of C3.

[0057] Comparative Example 4

[0058] Everything else is the same as in Example 1, except that in step (S4), the pre-crosslinking agent is entirely m-cresol, and no multifunctional epoxy resin is added. The pre-crosslinking precursor obtained in step (S4) is C4.

[0059] Comparative Example 5

[0060] The rest is the same as in Example 1, except that step (S4) is omitted. That is, the purified asphalt is not pre-crosslinked and is directly carbonized in a nitrogen atmosphere at a temperature of 5°C / min to 1100°C and held for 4 hours to obtain hard carbon material. The comparative sample is marked as C5.

[0061] The test results of the precursor are shown in Table 1 below.

[0062] Table 1 Precursor Testing

[0063]

[0064] Application examples

[0065] The hard carbon materials obtained in the above examples and comparative examples were mixed with conductive agent Super P and binder sodium carboxymethyl cellulose at a mass ratio of 8.5:0.75:0.75, respectively. Deionized water was added to make a slurry, which was then coated on copper foil and dried to prepare a sodium-ion battery negative electrode sheet. The negative electrode sheet was then assembled with a sodium sheet counter electrode, a glass fiber separator, and a 1 mol / L NaClO4-propylene carbonate / dimethyl carbonate (volume ratio 1:1) electrolyte to form a coin cell sodium-ion battery. Electrochemical performance tests were conducted, and the results are shown in Table 2 below.

[0066] Table 2 Electrochemical Performance Tests of Hard Carbon

[0067]

[0068] Table 2 shows the battery performance test results, illustrating the synergistic relationship between the various steps in the method of this invention. Extraction is performed using a mixed solvent of aromatic and aliphatic hydrocarbon solvents, followed by purification with a ternary purifying agent consisting of a polybasic acid, a salt of ethylenediaminetetraacetic acid, and an aminophosphonic acid compound. Finally, crosslinking is achieved using a composite pre-crosslinking agent composed of phenolic compounds and multifunctional epoxy resins, ultimately yielding a hard carbon material with excellent electrochemical performance.

Claims

1. A method for preparing hard carbon by purifying and pre-crosslinking coal tar-based pitch at medium and low temperatures, characterized in that, Includes the following steps: (S1) Low-temperature coal tar pitch is mixed with an extraction solvent for extraction, and the lower layer is taken as the extract material. The extraction solvent includes aromatic solvents and aliphatic hydrocarbon solvents. (S2) The extract and the ternary composite purifying agent are mixed and purified to obtain a purified slurry; the purified slurry is separated into solid and liquid phases, and the liquid phase is collected to obtain a purified asphalt slurry; the ternary composite purifying agent is a compound of polybasic acid, ethylenediaminetetraacetic acid salt and aminophosphonic acid compound. (S3) Liquid-phase purified asphalt slurry is subjected to vacuum distillation to remove the extraction solvent and light components, thereby obtaining purified asphalt; (S4) The purified asphalt and the composite pre-crosslinking agent are mixed and heated and stirred under an inert atmosphere to carry out a pre-crosslinking reaction. After the reaction is completed, the mixture is cooled to room temperature to obtain a pre-crosslinking precursor. The composite pre-crosslinking agent includes phenolic compounds and multifunctional epoxy resin. (S5) Under an inert atmosphere, the pre-crosslinked precursor is carbonized at high temperature to obtain the product hard carbon.

2. The preparation method according to claim 1, characterized in that, In step (S1), the aromatic solvent is selected from at least one of wash oil, anthracene oil, and naphthalene oil; the aliphatic hydrocarbon solvent is selected from at least one of petroleum ether, cyclohexane, and kerosene.

3. The preparation method according to claim 1, characterized in that, In step (S1), the extraction solvent is a mixture of aromatic solvent and aliphatic hydrocarbon solvent in a volume ratio of 3-5:

1.

4. The preparation method according to claim 1, characterized in that, In step (S1), the softening point of the medium-low temperature coal tar pitch is 60-90℃, the ash content is 2-5%, and the total content of metal impurities is 1000-2000ppm; furthermore, the mass-volume ratio of the medium-low temperature coal tar pitch to the extraction solvent is 1kg:2-3L; the extraction temperature is 20-60℃, the extraction time is 1-5h, and the mixing speed is 200-500rpm.

5. The preparation method according to claim 1, characterized in that, In step (S2), the solid-liquid separation is centrifugal separation, preferably horizontal screw discharge sedimentation centrifugation; the amount of ternary composite purifying agent is 3-5 wt% of the extract mass; preferably, the polyacid is selected from at least one of oxalic acid, malic acid, and citric acid; the ethylenediaminetetraacetic acid salt is selected from at least one of sodium ethylenediaminetetraacetate and potassium ethylenediaminetetraacetate; the aminophosphonic acid compound is selected from at least one of aminotrimethylphosphonic acid, hydroxyethylidene diphosphonic acid, and ethylenediaminetetramethylenephosphonic acid.

6. The preparation method according to claim 1, characterized in that, In step (S2), the ternary composite purifying agent is a mixture of polybasic acid, ethylenediaminetetraacetic acid salt and aminophosphonic acid compound in a mass ratio of 3-5:1-2:0.5-1.

7. The preparation method according to claim 1, characterized in that, In step (S2), purification involves mixing and stirring the extractant and the ternary composite purifying agent at 200-400 rpm for 3-6 hours at a stirring temperature of 15-40℃. The process parameters for the horizontal screw discharge sedimentation centrifuge are: drum speed 3000-5000 rpm, differential speed 5-20 rpm, and feed flow rate 0.5~2 m³ / min. 3 / h.

8. The preparation method according to claim 1, characterized in that, In step (S4), the phenolic compound is selected from at least one of methylphenol, dimethylphenol, and resorcinol; the multifunctional epoxy resin is selected from at least one of trimethylolpropane triglycidyl ether, glycerol triglycidyl ether, glycerol propoxy triglycidyl ether, pentaerythritol tetraglycidyl ether, triglycidyl isocyanurate, N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane, and tetraglycidyl-diaminodiphenylmethane. Preferably, the multifunctional epoxy resin containing N is selected from at least one of triglycidyl isocyanurate, N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane, and tetraglycidyl-diaminodiphenylmethane.

9. The preparation method according to claim 1, characterized in that, In step (S4), the inert atmosphere is nitrogen and / or argon; the mass ratio of purified asphalt to composite pre-crosslinking agent is 100:4-8. The pre-crosslinking reaction temperature is 190-210℃, the reaction time is 1-4h, and the stirring speed is 100-300rpm. Furthermore, the softening point of the obtained pre-crosslinked precursor is ≥150℃, the total content of metal impurities is less than 20ppm, and the ash content is less than 0.2%; the metal impurities are Fe, Na, Ca, and Mg. Furthermore, in step (S5), the inert atmosphere is nitrogen and / or argon, and the high-temperature carbonization is to heat to 1000-1300℃ and hold for 2-5 hours.

10. A sodium-ion battery, characterized in that, Its negative electrode comprises hard carbon prepared by any one of claims 1-9.

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