A lignite-based sodium-ion battery negative electrode material with soft and hard carbon composites, and a preparation method and application thereof

Abstract

The application provides a lignite-based sodium ion battery negative electrode material with soft and hard carbon composites and a preparation method and application thereof, and belongs to the technical field of electrochemical energy storage. First, inert mineral impurities are removed and intrinsic multi-stage pores of lignite are exposed through high-concentration HCl / HF combined acid washing. Then, with anhydrous ethanol as a medium, pitch molecules with a specific softening point are allowed to penetrate into pores and interfaces of the deashed lignite. Finally, at a specific high temperature, the softened and molten pitch is in-situ converted into a long-range ordered soft carbon network, which not only repairs high-activity amorphous defects on the surface of the hard carbon and inhibits excessive growth of a solid electrolyte interface (SEI) film, but also converts part of open pores into closed pores suitable for sodium storage, thereby realizing the unity of a high-conductivity network and high-density sodium storage active sites. The application breaks through the initial efficiency and capacity limitation of the lignite-based negative electrode at a very low cost, and has a wide industrialization prospect.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage technology, and in particular to a soft and hard carbon composite lignite-based sodium-ion battery anode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries, with their advantages of abundant resources, low cost, and environmental friendliness, have shown great application potential in large-scale energy storage and low-speed electric vehicles. However, the commercialization of sodium-ion batteries is still constrained by insufficient performance of key materials, with the technological bottleneck in anode materials being particularly prominent. Therefore, developing high-performance anode materials is one of the key steps in promoting the industrialization of sodium-ion batteries.

[0003] Hard carbon is a non-graphitized carbon material composed of disordered graphite-like microcrystals and abundant nanopores. Its rich nanopores and expanded interlayer spacing effectively alleviate the stress during sodium ion insertion, while its disordered carbon layer structure provides more sodium storage sites. These structural characteristics enable hard carbon materials to exhibit excellent cycle stability and rate performance, making it one of the most promising anode materials for sodium-ion batteries for industrial application.

[0004] Currently, the preparation of hard carbon materials mainly utilizes precursors such as biomass (e.g., coconut shells, wood), synthetic polymers (e.g., phenolic resins), and fossil fuel derivatives (e.g., coal, as an ideal precursor for hard carbon materials, exhibits significant advantages in large-scale preparation. Its rich aromatic structure is more conducive to the formation of conductive networks and stable sodium storage sites. Coal-based hard carbon materials combine the advantages of wide availability of raw materials, controllable costs, and tunable microstructure, demonstrating excellent rate performance and cycle stability, making them the most industrially feasible choice for sodium-ion battery anode materials.

[0005] Lignite, as a low-rank coal, is characterized by high volatile matter, high oxygen content, and abundant oxygen-containing functional groups. Hard carbon materials prepared using lignite as a precursor readily form abundant nanopores and expanded interlayer spacing during carbonization. However, the current technology for directly preparing high-performance hard carbon materials from raw lignite still faces two major challenges: first, the high ash impurities severely interfere with the purity and microstructure of the carbon matrix; second, pure lignite-based hard carbon has too many inherent defects, which easily lead to excessive electrolyte consumption on the surface, resulting in low initial coulombic efficiency and poor overall electronic conductivity.

[0006] To address the aforementioned issues, incorporating soft carbon precursors (such as pitch) with readily graphitizable and highly conductive properties into lignite is considered an effective strategy. However, current technologies primarily utilize pitch to coat carbon materials, thereby reducing their specific surface area. For instance, CN118343755A provides a method for preparing coal-based carbon materials for sodium-ion battery anodes. This method utilizes tetrahydrofuran (THF) to extract and dissolve coal liquefaction pitch. After solvent evaporation, the pitch merely adheres to the porous carbon surface through solution evaporation.

[0007] For example, CN120423529A provides a negative electrode material and its preparation method, a negative electrode sheet, and a sodium-ion battery. The asphalt precursor plays two roles: firstly, in the anode sheet fabrication, asphalt acts as a binder, binding the composite precursor powder together for extrusion molding; secondly, during the high-temperature sintering process of the modified composite precursor, asphalt acts as a coating agent to modify the surface of the material, reducing the specific surface area and improving the initial efficiency of the hard carbon material. These technologies generally involve high-temperature carbonization first, causing the substrate pores to shrink and close, before allowing the asphalt to adhere to the outermost layer of the material. This approach offers limited improvement to the sodium storage capacity and other performance characteristics of sodium-ion batteries. Summary of the Invention

[0008] To address the aforementioned problems in existing technologies, this invention provides a lignite-based sodium-ion battery anode material with both soft and hard carbon composites, along with its preparation method and applications. This invention employs a high-concentration acid washing and deep deashing technique combined with a soft-hard carbon composite strategy. The preparation process is rigorous, cost-effective, and yields a material with excellent sodium storage performance, suitable for large-scale commercial production.

[0009] The technical solution of the present invention is as follows: This invention provides a soft and hard carbon composite lignite-based sodium-ion battery anode material, the preparation method of which includes the following steps: S1. After crushing lignite, pass it through a standard sieve to obtain lignite powder; after freezing and embrittlement of asphalt below zero degrees Celsius, grind and sieve it to obtain asphalt powder. Preferably, the lignite in step S1 is sieved through a 200-300 mesh sieve; the softening point of the asphalt is 180-250℃, and the asphalt is sieved through a 270-325 mesh sieve after grinding.

[0010] Furthermore, the softening point of the asphalt is preferably 200°C.

[0011] S2. The lignite powder obtained in step S1 is sequentially acid-washed with hydrochloric acid solution and hydrofluoric acid solution for 20-30 hours each, then filtered and washed with water until the pH value is neutral, and dried to obtain the pretreated deashed lignite precursor. Preferably, the concentration of the hydrochloric acid solution in step S2 is 2-6 mol / L. -1The mass ratio of the lignite powder to the hydrochloric acid solution is 1:4 to 1:8.

[0012] Preferably, the concentration of the hydrofluoric acid solution in step S2 is 5-10 wt%, and the mass ratio of the lignite powder to the hydrofluoric acid solution is 1:8-1:12.

[0013] Furthermore, the concentration of the HCl solution in step S2 is preferably 4 mol / L. -1 The concentration of the HF solution is preferably 10 wt%.

[0014] Preferably, in step S2, after acid washing with hydrochloric acid solution, the mixture needs to be filtered and washed with water until the pH value is neutral, and then acid washed with hydrofluoric acid solution; the drying is carried out at 50-80℃ for 10-16 h.

[0015] Furthermore, the acid washing and stirring time in step S2 is preferably 24 h, and the drying time is preferably 12 h.

[0016] In step S2 above, the thorough acid washing and stirring effectively removes metal oxides (such as SiO2, Al2O3, Fe2O3, etc.) and insoluble aluminosilicates and other deep mineral impurities from lignite, significantly improving the purity of the lignite carbon matrix. Subsequently, deionized water washing removes residual acid radicals and soluble impurities. This deep purification process not only significantly reduces ash content but also effectively preserves the organic framework structure of lignite, providing a high-purity precursor for subsequent composite preparation of high-performance materials with asphalt.

[0017] S3. Mix the pretreated deashed lignite precursor obtained in step S2 with pitch powder, add anhydrous ethanol for ball milling, then dry and grind to obtain soft and hard carbon composite precursor powder. Preferably, the grinding process described in step S3 is followed by passing the material through a 230-325 mesh sieve, and the operating temperature in step S3 is not higher than 15°C.

[0018] Preferably, in the mixture of lignite precursor and pitch powder in step S3, the mass of the pitch powder accounts for 8-12% of the mass of the mixture; and the mass of the anhydrous ethanol is 5-15 times the mass of the mixture.

[0019] Furthermore, in step S3, the mass fraction of bitumen in the mixture relative to the total carbon content is preferably 12%.

[0020] Preferably, the ball milling in step S3 is liquid-phase assisted ball milling; the process conditions for liquid-phase assisted ball milling include: a ball milling speed of 250-300 r·min. -1The ball milling time is 2-6 hours; the grinding media used in the ball milling is zirconia balls with a diameter of 1-15 mm, and the mass ratio of the zirconia balls to the mixture composed of lignite precursor and pitch powder is 5:1-30:1; during the ball milling process, the sum of the volumes of the mixture and the zirconia balls accounts for 30-65% of the total volume inside the ball mill jar.

[0021] Furthermore, the drying temperature in step S3 is 60-80℃, and the drying time is 8-10 h.

[0022] In step S3 above, anhydrous ethanol was added as a liquid medium during ball milling, promoting uniform mixing of the lignite precursor and pitch through mechanochemical action. The addition of anhydrous ethanol effectively prevented particle agglomeration and facilitated the uniform dispersion and penetration of pitch onto the surface of lignite particles. Subsequently, drying at 60-80℃ removed the ethanol solvent, avoiding structural damage during subsequent high-temperature heat treatment and providing an ideal substrate for microstructural fusion during the carbonization process.

[0023] S4. Place the soft and hard carbon composite precursor powder obtained in step S3 into a reactor, and calcine it at a carbonization temperature of 1000-1400℃ for 4-8 hours under a protective atmosphere. After cooling, the soft and hard carbon composite lignite-based anode material is obtained.

[0024] Preferably, the protective atmosphere in step S4 is an inert protective atmosphere; the heating rate is 3-7℃ min. -1 .

[0025] Furthermore, the carbonization temperature in step S4 is preferably 1400℃, and the calcination time is preferably 4 h.

[0026] In step S4 above, through systematic experimental optimization and parameter control, this invention discovered that at a carbonization temperature of 1000-1400℃ and a holding time of 4-8 hours, asphalt, as a soft carbon precursor, melts and gradually carbonizes, effectively penetrating deeply and filling in situ the pores and defects of lignite-based hard carbon, forming a continuous conductive network and sealing some closed pores. This synergistic effect of soft and hard carbon preserves the expanded interlayer spacing and abundant sodium storage active sites of hard carbon, while also compensating for the shortcomings of single pure lignite-based hard carbon, such as poor conductivity, numerous electrolyte side reactions, and low initial coulombic efficiency.

[0027] The present invention also provides the application of the soft and hard carbon composite lignite-based sodium-ion battery anode material prepared by the above method, wherein the soft and hard carbon composite lignite-based anode material is used in sodium-ion batteries to achieve efficient and stable sodium storage.

[0028] The beneficial technical effects of this invention are as follows: 1. This invention achieves deep deashing of lignite raw materials through high-concentration acid washing by systematically controlling process parameters, effectively balancing the development of hard carbon pores in lignite with the overall graphitization degree of the carbon skeleton. High-concentration acid washing removes mineral impurities that hinder ion transport, and utilizes the multi-level channels exposed by acid washing as microscopic physical containers, preserving suitable interlayer spacing and active sites for sodium storage, effectively overcoming the bottleneck of limited sodium storage capacity in unmodified lignite-based carbon materials.

[0029] 2. This invention introduces pitch with an appropriate softening point (e.g., 200℃) as a soft carbon source. Utilizing the polarity of anhydrous ethanol, the surface tension of the particles is significantly reduced. The pitch particles, refined through grinding and ball milling, are uniformly compounded with the lignite precursor. During this process, a large number of pitch particles are pressed into the macropores / mesopores of the lignite precursor by the mechanical collision and friction of the ball mill. In the subsequent high-temperature carbonization process, the softened and molten pitch not only adheres tightly to the surface of the hard carbon particles in the lignite but also effectively impregnates and fills excess defects and open pores on their surface. This synergistic soft and hard carbon structure constructs a continuous three-dimensional conductive network, reducing interfacial charge transfer impedance and thus significantly improving the electronic conductivity and structural stability of the anode material.

[0030] 3. This invention involves high-temperature co-carbonization of the coal-based framework and the bitumen penetrating into it at 1000-1400℃. During this synergistic pyrolysis process, the bitumen not only transforms into short-range ordered soft carbon with excellent conductivity and pore-closure regulation capabilities, but also undergoes deep interfacial fusion with the coal-based hard carbon framework, constructing a unique soft-hard carbon heterostructure. This soft carbon network, deeply embedded in the pores and derived in situ, effectively passivates highly active defect sites both inside and outside the lignite matrix, reducing excessive electrolyte consumption on the electrode surface. This deep repair and modification of the microscopic defects and pores of the hard carbon framework helps induce the formation of a uniform and stable solid electrolyte interphase (SEI) film during the first charge-discharge cycle, effectively reducing irreversible side reactions and thus improving the initial coulombic efficiency and long-term cycle stability of the sodium-ion battery.

[0031] 4. Unlike technologies such as CN120423529A, which rely on introducing third-party carbon dots for solid-phase crosslinking to suppress graphitization, this invention requires no additional modifiers such as carbon dots. Seamless splicing of soft and hard carbons is achieved solely through deep liquid-phase impregnation and in-situ derivatization of homologous coal tar pitch within the deashing channels. This not only completely avoids the problems of heterogeneous interface incompatibility and increased impedance caused by introducing third-party carbon dots but also significantly simplifies the material preparation process.

[0032] 5. This invention achieves excellent electrochemical performance without pre-sodiumization, primarily due to the unique mechanism of in-situ sealing through open pores via internal permeation and high-temperature co-carbonization, fundamentally eliminating internal open defects that lead to irreversible capacity loss. Under specific process parameter combinations (e.g., carbonization temperature 1400℃, holding time 4 h, asphalt softening point 200℃, and a percentage of 12%), the composite anode material prepared by this invention achieves a first-cycle charge specific capacity of 349.16 mAh·g. -1 This method demonstrates excellent electrochemical performance. The raw materials used in this preparation method are widely available and inexpensive, and the process is easily scalable, providing a practical and feasible technical path for developing low-cost, high-performance coal-based sodium-ion battery anode materials. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the process for the soft and hard carbon composite lignite-based anode material of the present invention; Figure 2 a, 2b, and 2c are scanning electron microscope (SEM) images of the negative electrode materials obtained in Comparative Example 1, Example 4, and Example 8, respectively. Figure 3 The Raman spectra of the negative electrode materials obtained in Comparative Example 1, Example 4 and Example 8 are compared. Figure 4 The negative electrode materials obtained in Comparative Examples 1, 4, and 8 were measured at 0.03 A g. -1 Comparison of first charge-discharge performance curves at current density; Figure 5 These are the rate performance test curves of the negative electrode materials obtained in Comparative Examples 1, 4, and 8. Detailed Implementation

[0034] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0035] Unless otherwise specified, all raw materials used are commercially available products in this field, and all processing techniques are conventional.

[0036] The hard carbon-based precursors used in the embodiments of this invention are derived from lignite from the Chuncheng Mine in Xiwuqi, Inner Mongolia. As a preferred substrate for preparing high-performance hard carbon anodes, this lignite intrinsically contains cross-linked oxygen-containing functional groups and a large number of volatile components. This unique composition means that during the subsequent pyrolysis and carbonization stage, the violent release of a large amount of volatiles will induce topological rearrangement of the carbon matrix in situ, thereby generating non-graphitized intrinsic micro / mesoporous networks and highly active edge defects. This provides an important physical container and attachment site for the subsequent in-pore impregnation and defect modification in this invention.

[0037] To accurately control the physicochemical evolution of precursors, this invention conducted industrial analysis tests on coal samples with different particle sizes, and the results are shown in Table 1. The test data confirms that although the particle size varied between 100 mesh and 300 mesh, the air-dried volatile matter (Vad) remained consistently in the extremely high range of 39-41%. This invention preferentially selected 300-mesh lignite powder as the reference raw coal, with the following specific indicators: volatile matter (Vad) as high as 39.81%, ash content (Aad) at 22.04%, fixed carbon (FCad) at 27.91%, and moisture (M) controlled at a low 10.24%. The core advantage of using 300-mesh powder lies in its extremely high specific surface area, which significantly enhances reaction kinetics, while the low moisture content combined with nearly 40% volatile matter ensures that local structural collapse is avoided during carbonization.

[0038] Table 1. Industrial analysis results of lignite Furthermore, the natural inorganic mineral impurities in the raw coal matrix are a key condition for constructing the microscopic open channels in this invention. Quantitative characterization by inductively coupled plasma optical emission spectroscopy (ICP-OES) yielded results shown in Table 2. It can be seen that the main associated mineral elements in untreated lignite are silicon (Si, 6.58%), aluminum (Al, 1.96%), and calcium (Ca, 1.203%), accompanied by trace amounts of iron (Fe, 0.297%) and magnesium (Mg, 0.21%). It is precisely because the primary ash impurities, accounting for as much as 22.04%, occupy a large portion of the internal volume that the subsequent HCl and HF acid washing deashing process of this invention can achieve the desired structure. Specific pretreatment is necessary to remove silicates and metal oxides from the carbon skeleton in order to expose the multi-level pore network that allows for deep penetration of liquid-phase pitch, thereby ensuring the excellent electrochemical sodium storage performance of the resulting anode material.

[0039] Table 2. Inductively Coupled Plasma Optical Spectroscopy (ICP-OES) Test of 300-mesh Raw Coal The soft carbon precursor used in this invention is modified coal-based pitch. Coal-based pitch is a mixture of polycyclic aromatic hydrocarbons, with a wide relative molecular mass distribution of its original components (200-1800). To ensure the composite effect, the modified coal-based pitch selected in this invention is a raw material that has undergone pre-treatment to remove light small molecules (pre-treated by the manufacturer at 180-250℃ before purchase), and its effective components have a relative molecular mass concentrated in the range of 400-1200. As an amorphous organic material, due to the release of light components and enhanced intermolecular forces before leaving the factory, the glass transition temperature of this modified pitch is higher than that of untreated crude pitch, resulting in better thermal stability.

[0040] The specific physicochemical parameters of the asphalt raw materials used in the examples and comparative examples are shown in Table 3. The softening point of the selected modified asphalt has a certain impact on its physical properties (the softening point depends on the degree of heat treatment of the raw material before it leaves the factory). If the softening point of the selected asphalt is too low, it indicates that a large amount of volatile components remain inside, and gas will be generated violently during the subsequent high-temperature carbonization stage; if the softening point is too high, the melt fluidity of the asphalt will decrease significantly.

[0041] Table 3. Main physicochemical properties of coal tar pitch with different softening points In this composite system, the aforementioned changes in physical properties directly affect the two-phase composite effect. For example, when the selected asphalt has a low softening point (e.g., a product with a pre-treatment temperature of only about 180°C), a large number of volatile components escape rapidly during carbonization at 1400°C. The airflow impact will damage the composite interface between the asphalt and lignite, causing defects in the carbon matrix structure. Conversely, when the selected asphalt has a high softening point (e.g., a product with a pre-treatment temperature of 250°C), its melt fluidity is insufficient, making it difficult to uniformly impregnate lignite particles during the heating process, resulting in uneven two-phase composite. Therefore, in this embodiment of the invention, modified asphalt with a moderate softening point (e.g., specifications corresponding to a pre-treatment state of about 200°C) is preferred, allowing it to achieve sufficient impregnation with lignite in the molten state. After co-carbonization at 1400°C, the resulting composite hard carbon possesses a suitable microstructure and optimal sodium storage performance.

[0042] Furthermore, it should be specifically noted that anhydrous ethanol is used as the liquid-phase auxiliary ball milling medium in the specific implementation of this invention. Under the strong mechanical shearing action of liquid-phase ball milling, anhydrous ethanol can greatly reduce the surface tension of particles, effectively prevent powder agglomeration, and allow the non-water-soluble asphalt to be fully and uniformly dispersed, penetrated, and adhered to the surface of the porous skeleton of lignite that has undergone deep acid washing and deashing, as well as the interior of some open pores.

[0043] During the subsequent heating and high-temperature carbonization process, the dispersed asphalt softens, melts, and transforms in situ into soft carbon with a locally long-range ordered microcrystalline structure. This soft carbon not only fills and deeply stitches together the excessive highly reactive amorphous defects inside and outside the hard carbon framework, playing a crucial bridging role, but also transforms some open pores into closed-pore structures conducive to sodium storage in the plateau region, thereby significantly reducing irreversible sodium loss caused by excessive SEI film growth during the first charging process. Simultaneously, the soft carbon constructs a continuous three-dimensional electron transport network between the hard carbon particles, greatly improving the electrochemical kinetics of the material interface and the overall conductivity.

[0044] It should also be noted that in the following embodiments and comparative examples, the operating temperature of ball milling in step S3 is not higher than 15°C, in order to avoid the problem of asphalt softening.

[0045] Example 1: This embodiment provides a method for preparing a lignite-based anode material with soft and hard carbon composites, the specific steps of which are as follows: Figure 1 As shown: S1. Take lignite from Chuncheng Mine in Xiwuqi, Inner Mongolia as raw material, crush it and pass it through a 300-mesh standard sieve to obtain lignite powder; then grind asphalt with a softening point of 180℃ through a 300-mesh sieve after freezing and embrittlement at a temperature below zero degrees Celsius to obtain asphalt powder. S2. The lignite powder obtained in step S1 is successively treated with 4 mol L... -1 Deep acid washing was performed using HCl solution and 10 wt% HF solution. Specifically, acid washing was carried out sequentially at solid-liquid mass ratios of 1:6 (lignite powder: hydrochloric acid solution) and 1:10 (lignite powder: hydrofluoric acid solution), with stirring on a magnetic stirrer for 24 h. After initial acid washing with hydrochloric acid solution, the mixture was filtered and washed with water until the pH was neutral, followed by acid washing with hydrofluoric acid solution. After acid washing, the mixture was filtered and washed with deionized water until the pH was neutral, then dried at 60℃ for 12 h to obtain the pretreated deashed lignite precursor. S3. The lignite precursor obtained in step S2 is mixed with pitch powder to obtain a mixture, such that the mass fraction of pitch in the mixture is 8% of the total mass of the mixture. Anhydrous ethanol is added as a liquid medium, and the mass of the anhydrous ethanol is 10 times the total mass of the mixture. Subsequently, liquid-phase assisted ball milling is performed. The specific ball milling conditions are as follows: zirconia balls with a diameter of 5 mm are used as the grinding medium, the ball-to-material mass ratio is set to 10:1, and the total volume of the mixture and the zirconia balls occupies 50% of the total volume inside the ball mill jar; the ball milling speed is 300 r·min. -1 The ball milling time was 4 hours. After ball milling, the mixture was dried at 60°C for 8 hours, and finally ground and passed through a 300-mesh sieve to obtain a soft and hard carbon composite precursor powder. S4. Place the obtained powder in a crucible and heat it at 5°C for 5 minutes under an argon atmosphere.-1 The temperature was increased to 1000℃ at a certain heating rate and held for 8 hours for calcination. After cooling, a soft and hard carbon composite lignite-based anode material was obtained.

[0046] Example 2: Similar to Example 1, the difference lies in that the asphalt powder content in step S3 is 12%, the carbonization temperature in step S4 is 1200℃, and the carbonization time is 6 hours. The process parameters for Examples 1-10 are detailed in Table 7.

[0047] Example 3: Similar to Example 1, the difference is that the softening point of the asphalt raw material is 200°C and the carbonization temperature of step S4 is 1200°C.

[0048] Example 4: This embodiment provides a method for preparing a soft and hard carbon composite lignite-based anode material, the specific steps of which are as follows: S1. Take lignite from Chuncheng Mine in Xiwuqi, Inner Mongolia as raw material, crush it and pass it through a 300-mesh standard sieve to obtain lignite powder; then grind asphalt with a softening point of 200℃ through a 300-mesh sieve after freezing and embrittlement at a temperature below zero degrees Celsius to obtain asphalt powder. S2. The lignite powder obtained in step S1 is successively treated with 4 mol L... -1 The mixture was acid-washed with HCl and 10 wt% HF, stirred, washed and dried, and the specific operation was the same as in Example 1 to obtain lignite precursor; S3. Mix the lignite precursor with pitch powder so that the pitch accounts for 10% of the total mass of the mixture. Add 10 times the mass of anhydrous ethanol to the mixture and ball mill (300 r / min). -1 The ball milling process conditions were the same as in Example 1 (4 h); then dried at 80°C for 8 h and ground, and passed through a 300-mesh sieve to obtain composite precursor powder; S4. The obtained powder is placed under an argon atmosphere at 5°C for 5 min. -1 The temperature was increased to the carbonization temperature of 1400℃ at a certain heating rate and held for 4 hours for calcination. After cooling, a soft and hard carbon composite lignite-based anode material was obtained.

[0049] Example 5: Similar to Example 1, the difference is that the softening point of the asphalt raw material is 200°C, the asphalt content ratio in step S3 is 12%, and the carbonization time in step S4 is 6 hours.

[0050] Example 6: Similar to Example 1, the difference is that the softening point of the asphalt raw material is 250°C and the carbonization temperature of step S4 is 1400°C.

[0051] Example 7: Similar to Example 1, the difference is that the softening point of the asphalt raw material is 250°C, the asphalt powder content in step S3 is 10%, the carbonization temperature in step S4 is 1200°C, and the carbonization time is 4 hours.

[0052] Example 8: This embodiment provides a method for preparing a soft and hard carbon composite lignite-based anode material, the specific steps of which are as follows: S1. Take lignite from Chuncheng Mine in Xiwuqi, Inner Mongolia as raw material, crush it and pass it through a 300-mesh standard sieve to obtain lignite powder; then grind asphalt with a softening point of 200℃ through a 300-mesh sieve after freezing and embrittlement at a temperature below zero degrees Celsius to obtain asphalt powder. S2. The same pickling process as in Example 1 (4 mol L) is used. -1 HCl and 10 wt% HF were stirred for 24 h and then dried to obtain lignite precursor; S3. Mix the lignite precursor with pitch powder so that the pitch accounts for 12% of the total mass of the mixture. Add 10 times the mass of anhydrous ethanol to the mixture and ball mill (300 r / min). -1 The ball milling process conditions were the same as in Example 1 (4 h); then the mixture was dried at 80°C for 10 h and ground, and passed through a 300-mesh sieve to obtain the composite precursor powder. S4. The obtained powder is placed under an argon atmosphere at 5°C for 5 min. -1 The temperature was increased to 1400℃ at a certain heating rate and held for 4 hours for calcination. After cooling, a soft and hard carbon composite lignite-based anode material with the best sodium storage performance was obtained.

[0053] Example 9: Similar to Example 1, the difference is that the softening point of the asphalt raw material is 250°C, the asphalt powder content in step S3 is 12%, and the carbonization time in step S4 is 4 hours.

[0054] Example 10: Similar to Example 1, the difference is that the asphalt powder content in step S3 is 10%, the carbonization temperature in step S4 is 1400℃, and the carbonization time is 6h.

[0055] Comparative Example 1: This comparative example provides a method for preparing anode materials containing only pure lignite-based hard carbon, without any soft or hard carbon composite treatment, and includes the following steps: (1) Take lignite from Chuncheng Mine in Xiwuqi, Inner Mongolia as raw material, crush it and pass it through a 300-mesh standard sieve to obtain lignite powder; (2) The lignite precursor was obtained by using the same acid washing process as in Example 8; (3) Without adding any bitumen, the lignite precursor was directly added to 10 times its own weight of anhydrous ethanol for ball milling (300 r / min). -1 The ball milling process conditions were the same as in Example 8 (4 h); then dried and ground before passing through a 300-mesh sieve. (4) Under an argon atmosphere, at 5℃ min -1 The heating rate was increased to 1400℃ and held for 4 hours for calcination. After cooling, uncomposite lignite-based hard carbon anode material was obtained.

[0056] Comparative Example 2: This comparative example provides a method for preparing a soft and hard carbon composite material with an asphalt content exceeding the optimal range, similar to Example 8, except that the asphalt powder content in step S3 is 14%. The process parameters of Comparative Examples 1-8 are shown in Table 8.

[0057] Comparative Example 3: This comparative example provides a method for preparing a soft and hard carbon composite material with an asphalt content lower than the optimal range. It is similar to Example 8, except that the asphalt powder content in step S3 is 6%.

[0058] Comparative Example 4: This comparative example provides a method for preparing soft and hard carbon composite materials without acid washing and deashing treatment, including the following steps: (1) Take lignite from Chuncheng Mine in Xiwuqi, Inner Mongolia as raw material, crush it and pass it through a 300-mesh standard sieve to obtain lignite raw coal powder; grind asphalt with a softening point of 200℃ through a 300-mesh sieve after freezing and embrittlement below zero degrees Celsius to obtain asphalt powder. (2) The lignite raw material powder is not subjected to any pickling treatment with hydrochloric acid and hydrofluoric acid, and the subsequent steps are carried out directly; (3) Mix raw coal powder with asphalt powder, with the asphalt powder accounting for 12% by mass. Add 10 times the mass of anhydrous ethanol to the mixture and ball mill it under the same process conditions as in Example 8. Then dry and grind the mixture and pass it through a 300-mesh sieve. (4) Under an argon atmosphere, at 5℃ min -1 The heating rate was increased to 1400℃, and the temperature was held for 4 hours for calcination. After cooling, a soft and hard carbon composite material without acid washing was obtained.

[0059] Comparative Example 5: This comparative example provides a method for preparing soft and hard carbon composite materials with pickling process parameters exceeding the optimal range. It is similar to Example 8, except that in the pickling process, the solid-liquid mass ratio of hydrochloric acid is increased to 1:10, and the solid-liquid mass ratio of hydrofluoric acid is increased to 1:14.

[0060] Comparative Example 6: This comparative example provides a method for preparing soft and hard carbon composite materials with pickling process parameters below the optimal range. It is similar to Example 8, except that in the pickling process, the solid-liquid mass ratio of hydrochloric acid is reduced to 1:2 and the solid-liquid mass ratio of hydrofluoric acid is reduced to 1:6.

[0061] Comparative Example 7: This comparative example provides a method for preparing soft and hard carbon composite materials without ball milling, similar to Example 8, except that in step S3, after mixing the lignite precursor and pitch powder, ball milling is not performed, but the mixture is directly ground and passed through a 300-mesh sieve.

[0062] Comparative Example 8: This comparative example provides a method for preparing a negative electrode material with an excessively low carbonization temperature, including the following steps: (1) Take lignite raw material from Chuncheng Mine in Xiwuqi, Inner Mongolia, and pass it through a 300-mesh sieve; grind asphalt with a softening point of 200℃ after freezing and embrittlement below zero degrees Celsius and pass it through a 300-mesh sieve to obtain asphalt powder; (2) The lignite precursor was obtained by using the same acid washing process as in Example 8; (3) Mix lignite with 200°C pitch powder, with the pitch powder accounting for 12% by mass. Add 10 times the mass of water-ethanol to the mixture and ball mill it. The ball milling process conditions are the same as in Example 8. Then dry and grind the mixture and pass it through a 300-mesh sieve. (4) Under an argon atmosphere, at 5℃ min -1 The heating rate was only raised to the carbonization temperature of 800℃, and the temperature was held for 4 hours for calcination. After cooling, a low-temperature carbonized composite anode material was obtained.

[0063] Test Example 1: The Effect of Acid Washing on Lignite Precursors (1) Inductively Coupled Plasma Optical Spectroscopy (ICP-OES) Analysis To accurately verify the purification and pore-forming effects of the combined HCl and HF deep acid washing process used in this invention on lignite precursors, this invention employs inductively coupled plasma optical emission spectrometry (ICP-OES) to analyze 300-mesh lignite raw coal and its 4mol / L... -1 The sample (i.e. the sample obtained in step S2 of Examples 1-10) was subjected to elemental quantitative analysis after being acid washed with HCl and 10 wt% HF for 24 h.

[0064] The results, as shown in Table 4, indicate that the unwashed raw coal matrix contains a large amount of inert mineral templates, with silicon (Si), aluminum (Al), and calcium (Ca) being the most abundant elements at 6.58%, 1.96%, and 1.203%, respectively. These elements mainly exist in the form of dense inorganic salts such as silica and aluminum silicate, severely clogging the intrinsic micropores of the coal matrix and occupying a large amount of internal space. After the deep acid washing process of this invention, all impurity elements show a significant decrease. Specifically, the core framework impurity Si decreased to 0.02%, Al to 0.06%, and Ca to 0.09%, while other metallic elements such as Fe, Mg, and K decreased to below 0.1%. This comparative data clearly demonstrates that this acid washing step efficiently removes inorganic minerals embedded in the carbon framework.

[0065] Table 4. Inductively Coupled Plasma Optical Spectroscopy (ICP-OES) Analysis of Lignite Before and After Pickling Furthermore, the control of acid washing process parameters is crucial to the pretreatment effect of lignite precursors. In Comparative Example 6 (insufficient acid washing), the low acid concentration failed to fully dissolve the dense inorganic minerals such as aluminosilicates embedded in the coal matrix, resulting in a high content of impurity elements remaining inside the sample after acid washing, and the pores were not effectively opened. Conversely, in Comparative Example 5 (excessive acid washing), although the excessively high acid concentration could remove impurities, the highly corrosive environment would cause excessive erosion of the intrinsic carbon skeleton of lignite, leading to the collapse and destruction of the original pore structure, making it unsuitable as a good carrier for subsequent composite materials.

[0066] (2) Specific surface area (BET) test analysis To further confirm the effect of deashing treatment on the expansion and unblocking of the multi-level pore network of lignite, the present invention tested the specific surface area and pore structure of pure lignite precursors before and after acid washing, and the results are shown in Table 5.

[0067] Table 5 Pore structure parameters before and after pickling Test results show that the specific surface area of ​​unwashed lignite is only 4.1 m², due to the large amount of inorganic mineral impurities (such as silicon and aluminum oxides) that severely block its intrinsic pores. 2 g -1 The micropore volume is very small, only 0.0015 cm. 3 g -1 After combined deep acid washing with HCl and HF, the specific surface area of ​​the lignite precursor significantly increased to 7.5 m². 2 g -1 The micropore volume has increased dramatically to 0.0072 cm³. 3 g-1 This represents an increase of nearly five times.

[0068] The significant improvement in pore parameters indicates that the deep pickling process successfully released the space originally occupied by inorganic impurities during removal, not only unblocking the blocked intrinsic micropores for sodium storage but also effectively expanding the open pore network from the inside out. This multi-level pore system exposed through deashing and pore-forming provides the space and interfacial contact sites for the efficient injection of liquid-phase asphalt, deep impregnation, and in-situ pore filling and defect modification at high temperatures in subsequent processes, forming the basis for the excellent sodium storage performance of this invention.

[0069] In contrast, the comparative examples that deviated from the optimal acid washing process failed to achieve the best results in improving the pore structure. For Comparative Example 6 (insufficient acid washing), because inorganic mineral impurities were not completely removed, some blockage remained in the internal micropores, limiting the increase in specific surface area and micropore volume, both of which were lower than the optimal acid washing state. For Comparative Example 5 (over-acid washing), the excessive acid erosion not only removed impurities but also caused local collapse of the intrinsic carbon skeleton of lignite, severely damaging the expanded micro-nano pore structure. This also led to a decrease in specific surface area and effective micropore volume, further demonstrating that the specific acid washing parameters used in this invention achieved the optimal balance between efficient pore formation and maintaining the stability of the carbon skeleton.

[0070] Test Example 2: Particle size of lignite and pitch before and after ball milling (1) Dry laser particle size distribution (LPSA) test and analysis Particle size analysis was performed on lignite precursor and pitch powder (pitch purchased from Jining Chenxing Carbon Co., Ltd.). The results are shown in Table 6. The median diameter (D50) of the lignite precursor after acid washing was 6.578 μm, and the median diameter (D50) of the pitch powder was 0.587 μm. The particle size of the pitch powder was much smaller than that of the lignite precursor particles.

[0071] Table 6. Laser intensity test analysis of lignite and bitumen after pickling. Generally, micron-sized solid asphalt particles cannot be directly physically filled into the micro-nano pores of lignite. However, in this invention, the asphalt infiltration process is achieved through a combination of liquid-phase media and high-temperature thermophysical changes. First, in the mixing stage, anhydrous ethanol is used as the liquid-phase media for auxiliary ball milling. Anhydrous ethanol effectively wets the pores of lignite. Under the mechanical force of ball milling, the flowing anhydrous ethanol introduces the dispersed, smaller-diameter asphalt powder into the lignite pores amplified by acid washing. After drying and removing the ethanol, the asphalt is initially distributed inside and on the surface of the lignite pores. Then, in the high-temperature carbonization stage, when the temperature reaches the softening point of the asphalt, the pre-distributed asphalt transforms into a liquid melt. Under the action of capillary force, the liquid asphalt further flows into and fills the micropores deep within the lignite. The particle size test results in Table 6 also show that the asphalt powder size used in this invention can achieve effective initial dispersion and adhesion in the anhydrous ethanol medium, providing a physical basis for subsequent high-temperature liquid infiltration.

[0072] Furthermore, Comparative Example 7 further verified the necessity of the aforementioned synergistic effect of liquid phase and mechanical force. Comparative Example 7 did not employ ball milling; it only underwent conventional simple mixing. Due to the lack of mechanical shearing and impact driving force during ball milling, the asphalt powder could not effectively penetrate the pores inside the lignite, remaining largely free or agglomerated on the outside of the lignite particles. During subsequent high-temperature carbonization, this free asphalt only formed an uneven carbon layer on the outside of the lignite particles, failing to achieve deep penetration and filling of the micro-nano pores within the framework. This comparative result confirms the irreplaceable role of ball milling in enabling asphalt to enter the pores.

[0073] Test Example 3: Analysis of Microscopic Morphology and Structural Features (1) Morphological analysis by scanning electron microscopy (SEM) Figure 2 a, 2b, and 2c are scanning electron microscope (SEM) images of the lignite-based anode materials obtained in Comparative Examples 1, 4, and 8, respectively. The comparison clearly demonstrates the reconstructive effect of the soft-hard carbon composite process on the microstructure of the materials. like Figure 2 As shown in Figure a, the particle surface of Comparative Example 1 (pure lignite-based hard carbon without composite bitumen) exhibits a highly rough and irregular shape. Numerous fine debris and obvious open structural defects are distributed around the particles, resulting in a relatively loose and porous morphology. While this unmodified surface provides a rich specific surface area, the excessive exposure of highly active defect sites and open channels significantly exacerbates side reactions in the electrolyte, consumes a large amount of active sodium ions, and the loose particle contact leads to poor overall electronic conductivity of the material.

[0074] like Figure 2As shown in b, the morphology of Example 4 (10% asphalt) changed significantly. Compared with Comparative Example 1, the surface roughness of the particles was greatly reduced, and the loose debris was significantly decreased. This indicates that during the high-temperature carbonization process, the asphalt softened and melted, and the in-situ generated soft carbon began to effectively penetrate into the deep layers of the hard carbon skeleton and form a tight interfacial bond, initially filling some of the open channels and micro-defects. The stacking between particles became more compact, initially constructing a conductive network conducive to electron transport.

[0075] like Figure 2 As shown in Figure c, Example 8 (12% bitumen) exhibited the densest and smoothest microstructure. With further optimization of the soft carbon ratio, the bitumen-derived soft carbon not only achieved comprehensive and uniform penetration and repair of the hard carbon particle surface, but also played a significant bridging role, deeply filling the deep pores and microscopic defects inside and outside the hard carbon skeleton. The originally scattered particles were seamlessly connected by the soft carbon network, forming a continuous and robust three-dimensional cross-linked conductive matrix. This extremely closed microstructure largely shields the direct contact between the electrolyte and the internal highly active sites, fundamentally inhibiting irreversible sodium consumption; at the same time, the continuous long-range soft carbon network provides a high-speed channel for the rapid transport of electrons and sodium ions. This excellent microstructure characteristic, mechanistically, explains why Example 8 achieved the highest first-cycle charging specific capacity (349.16 mAh·g). -1 The reason is...

[0076] In contrast, Comparative Examples 2 and 3, with asphalt additions deviating from the optimal range, failed to form the best microstructure. In Comparative Example 3 (low asphalt content), the amount of soft carbon generated was insufficient to completely fill the defects and deep pores of the hard carbon skeleton in situ, resulting in the exposure of some highly active sites. In Comparative Example 2 (high asphalt content), excessive soft carbon easily leads to disordered accumulation and agglomeration on the outside of the particles, increasing not only the risk of adhesion but also the diffusion resistance of sodium ions.

[0077] (2) Raman spectroscopy defect feature analysis To further reveal the effect of sodium supplementation on the degree of graphitization and microscopic defects, Raman spectroscopy analysis was performed on the samples, such as... Figure 3 As shown. The Raman spectrum mainly contains two characteristic peaks: one located at 1350 cm⁻¹. -1 The nearby D peak represents the carbon material composed of sp 3 Defects and disordered structure introduced by hybridization. Located at 1590 cm. -1 The nearby G peak represents sp in carbon materials 2 Hybridized graphite microcrystalline ordered structure. The intensity ratio of the two peaks (I0.05) D / I G It is often used to measure the disorder and defect density of carbon materials.

[0078] from Figure 3 As can be seen, the I of pure lignite-based hard carbon (Comparative Example 1) without compounding... D / I G The ratio was 1.42. However, after introducing bitumen-derived soft carbon, the disorder of the material did not decrease; instead, it showed an increasing trend. Examples 4 and 8, I... D / I G The ratios increased to 1.61 and 1.64, respectively. This significant change indicates that during the assisted ball milling and high-temperature co-carbonization of asphalt with lignite-based hard precursors, the differences in shrinkage and microcrystal development between soft and hard carbon led to the formation of numerous soft-hard carbon heterogeneous interfaces and boundary defects at the interface between the two carbon matrices. This microstructural feature synergistically constructed by soft and hard carbon plays a crucial role in improving the electrochemical performance of the material. D / I G The increased ratio signifies the formation of more abundant topological defects and edge sites within the composite material. These moderately defective structures provide a large number of additional adsorption and storage active sites for sodium ions. Example 8 exhibits the highest Ig. D / I G Value (1.64), its first-cycle charging specific capacity (349.16 mAh·g) -1 The highest concentration of asphalt fully demonstrates that, under the specific process parameter control of this invention, a 12% asphalt content achieves the best balance between the conductive network and the density of sodium storage active sites, thereby maximizing the sodium storage potential of the material.

[0079] In contrast, Comparative Examples 2 (too much bitumen) and 3 (too little bitumen), which deviated from the optimal range, had lower overall material disorder than Example 8. This is because too little bitumen cannot construct sufficient heterogeneous interfaces, while too much bitumen leads to soft carbon agglomeration, thus masking effective defects. This further confirms that only at the optimal ratio can the disorder and sodium storage active sites be maximized.

[0080] Test Example 4: Electrochemical Performance To fully verify the regulatory effect of deep deashing combined with liquid-phase assisted soft and hard carbon composite process on the microstructure of lignite-based anode materials and the enhancement of electrochemical performance, systematic tests were conducted on each example and comparative example.

[0081] (1) Half-cell assembly and conventional testing methods The negative electrode materials prepared in the examples and comparative examples were assembled into half-cells using conventional methods to determine their electrochemical performance. The specific method is as follows: The aforementioned anode material (80 wt%), acetylene black (10 wt%), and polyvinylidene fluoride (PVDF, 10 wt%) were combined to form an anode slurry. The slurry was uniformly spread on copper foil, with the loading of the anode material controlled between 1.0 and 1.3 mg, and then dried overnight at 80°C to obtain the electrode sheet.

[0082] The electrode was cut into circular pieces with a diameter of 10 mm. Argon was used as the filling gas, and a glass fiber diaphragm was used inside the glove box. Na was used as the counter electrode. 1 mol L -1 A half-cell was assembled using a diethylene glycol dimethyl ether (DEGDME) solution of NaCF3SO3 as the electrolyte and a CR2032 battery case. Its electrochemical performance was measured using a CHI660E electrochemical workstation, and the results are shown in Tables 7 and 8.

[0083] Table 7. Preparation process and initial charge / discharge performance of hard carbon anode materials in each embodiment. Table 8. Processes and initial charge-discharge performance of hard carbon anode materials for each comparative example. (2) Electrochemical sodium storage performance testing and analysis ① Initial constant current charge / discharge performance and irreversible capacity compensation Figure 4 The figures show the initial charge-discharge curves of the anode materials prepared in Comparative Examples 1, 4, and 8 at specific current densities. As shown, the discharge curves of all samples exhibit typical characteristics of hard carbon anodes in sodium-ion batteries, namely a ramp above 0.1 V and a low-voltage plateau below 0.1 V. The comparison clearly demonstrates that the soft-hard carbon composite process plays a decisive role in improving the sodium storage capacity and initial coulombic efficiency (ICE) of the materials.

[0084] The comparative example 1, which did not undergo composite charging, had a first-cycle charging capacity of 277.2 mAh·g. -1 The initial coulombic efficiency was only 87%. Its low initial efficiency and relatively short low-pressure plateau region are mainly attributed to the presence of a large number of highly active amorphous defects and open channels on the surface of pure lignite-based hard carbon. These exposed defect sites will cause severe irreversible decomposition of the electrolyte during the first discharge, forming a thick solid electrolyte interphase (SEI) film, thereby consuming a large number of active sodium ions.

[0085] The electrochemical performance of the composite material was greatly improved after the introduction of pitch-derived soft carbon. The first-charge specific capacity of Example 4 increased to 338.94 mAh·g. -1The coulombic efficiency was significantly improved to 94.8% for the first time. Example 8, with further optimization of process parameters, exhibited the most significant extension in its low-voltage plateau region, achieving a first-cycle charge specific capacity of 349.16 mAh·g. -1 The initial coulomb efficiency was as high as 96%.

[0086] The significant increase in initial efficiency from 87% to 96% and the substantial improvement in capacity strongly demonstrate the optimization of the material's microstructure. On one hand, the high-temperature molten pitch-derived soft carbon deeply impregnates and uniformly modifies the hard carbon framework, effectively passivating previously exposed highly active defect sites and drastically reducing the side reaction area at the electrode / electrolyte interface. This fundamentally inhibits the excessive growth of the irreversible SEI film, thus achieving a leap in initial efficiency. On the other hand, the bridging and filling effect of the soft carbon transforms some disordered open channels into nanopores capable of accommodating sodium ions, directly leading to a significant increase in sodium storage capacity in the low-pressure plateau region. The excellent data in Example 8 fully demonstrate that a 12% pitch content balances interfacial stability and the abundance of closed-pore structures, maximizing the sodium storage potential of the lignite-based carbon material.

[0087] In contrast, Comparative Examples 2-8, deviating from the optimal process conditions of this invention, exhibited a significant decline in both their initial charge specific capacity and initial coulombic efficiency. Specifically, deviations in the amount of asphalt added from the optimal range (Comparative Examples 2 and 3) prevented optimal pore filling, leading to increased interfacial side reactions or hindered ion diffusion. Inadequate deep pickling or improper pickling parameters (Comparative Examples 4-6) left behind a large amount of mineral impurities, severely clogging sodium storage channels and significantly reducing initial efficiency. The absence of ball milling (Comparative Example 7) prevented the asphalt from penetrating deep into the skeleton with liquid-phase assistance, weakening the in-situ repair effect of soft carbon. Furthermore, excessively low carbonization temperature (Comparative Example 8) resulted in incomplete carbon microcrystal development, failing to form a stable closed-cell structure. The absence or deviation of these individual process parameters mechanistically disrupted the construction of the optimal microstructure, ultimately leading to a decrease in the material's sodium storage capacity and accelerated irreversible consumption.

[0088] ② Rate dynamic performance under stepped current density Figure 5 The figures show the rate performance test curves of the anode materials prepared in Comparative Examples 1, 4, and 8. Rate performance is a key indicator for evaluating the kinetic characteristics of anode materials under rapid charge and discharge conditions, and is mainly affected by the electronic conductivity and ion diffusion rate of the material.

[0089] As shown in the figure, Comparative Example 1, without the soft-hard carbon composite process, maintains a certain capacity at low current densities. However, its specific capacity decreases sharply with a stepwise increase in the test current density. At the highest current density, its capacity has decreased to an extremely low level. This is mainly due to the loose contact between pure lignite-based hard carbon particles, poor overall electronic conductivity, and excessive interfacial impedance caused by surface porosity defects. During high-current charge and discharge, severe polarization occurs, preventing rapid insertion and extraction of sodium ions, resulting in a poor rate response.

[0090] The introduction of pitch-derived soft carbon significantly improved the rate performance of the composite material. Examples 4 and 8 exhibited specific capacities far exceeding those of Comparative Example 1 across all current density levels, with extremely gradual capacity decay. Example 8, in particular, under optimal processing conditions, demonstrated excellent rate response and polarization resistance. Test current densities of 0.03, 0.05, 0.1, 0.2, 0.5, and 1 A·g were also measured. -1 Under the condition of progressively increasing specific capacity, the reversible specific capacity of Example 8 remained stable at 337.48, 318.21, 305.57, 289.30, 261.68 and 208.69 mAh·g, respectively. -1 Even at 1 A·g -1 Even at high current densities, it can still maintain a capacity of over 200 mAh·g. -1 Its high capacity level demonstrates excellent rapid sodium storage kinetics.

[0091] Furthermore, after a series of high-current charge-discharge cycles, the test current density returned to its initial value of 0.03 A·g. -1 At that time, the specific capacity of Example 8 rapidly recovered to 313.5 mAh·g -1 It exhibits extremely high capacity retention. This excellent test result confirms the significant improvement in the material's microstructure. Pitch-derived soft carbon plays a deep adhesion and bridging role on the surface and between particles of lignite hard carbon, constructing a continuous and robust three-dimensional long-range conductive network. Simultaneously, the soft carbon-modified surface effectively reduces charge transfer impedance and promotes Na... + Rapid diffusion at the solid-liquid interface. The robust composite carbon framework withstood the structural stress caused by the rapid insertion and extraction of sodium ions under high current, ensuring the stability of the material under long-cycle and high-rate conditions.

[0092] Due to deviations from optimal process parameters, the overall rate performance of Comparative Examples 2-7 was inferior to that of Example 8. Whether it was the imperfect conductive network caused by the imbalance of the asphalt ratio or the obstructed ion diffusion caused by uneven impurity removal and mixing, these microscopic defects increased the charge transfer impedance at the electrode interface, fundamentally limiting the sodium storage kinetics response of the material under high current.

[0093] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, and for those of ordinary skill in the art, various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. Therefore, the present invention is not limited to the specific details without departing from the general concept defined by the claims and their equivalents.

Claims

1. A soft and hard carbon composite lignite-based sodium-ion battery anode material, characterized in that, The method for preparing the battery negative electrode material includes the following steps: S1. After crushing lignite, pass it through a standard sieve to obtain lignite powder; after freezing and embrittlement of asphalt below zero degrees Celsius, grind and sieve it to obtain asphalt powder. S2. The lignite powder obtained in step S1 is sequentially acid-washed with hydrochloric acid solution and hydrofluoric acid solution for 20-30 h each, then filtered and washed with water until the pH value is neutral, and dried to obtain the pretreated deashed lignite precursor. S3. Mix the pretreated deashed lignite precursor obtained in step S2 with pitch powder, add anhydrous ethanol for ball milling, then dry and grind to obtain soft and hard carbon composite precursor powder. S4. Place the soft and hard carbon composite precursor powder obtained in step S3 into a reactor, and calcine it at a carbonization temperature of 1000-1400℃ for 4-8 hours under a protective atmosphere. After cooling, the soft and hard carbon composite lignite-based anode material is obtained.

2. The battery negative electrode material according to claim 1, characterized in that, The lignite mentioned in step S1 is passed through a 200-300 mesh sieve; the softening point of the asphalt is 180-250℃, and the asphalt is passed through a 270-325 mesh sieve after grinding.

3. The battery negative electrode material according to claim 1, characterized in that, The concentration of the hydrochloric acid solution in step S2 is 2-6 mol / L. -1 The mass ratio of the lignite powder to the hydrochloric acid solution is 1:4 to 1:

8.

4. The battery negative electrode material according to claim 1, characterized in that, The concentration of the hydrofluoric acid solution in step S2 is 5-10 wt%, and the mass ratio of the lignite powder to the hydrofluoric acid solution is 1:8-1:

12.

5. The battery negative electrode material according to claim 1, characterized in that, In step S2, after acid washing with hydrochloric acid solution, it needs to be filtered and washed with water until the pH value is neutral, and then acid washing with hydrofluoric acid solution is performed; the drying is carried out at 50-80℃ for 10-16 hours.

6. The battery negative electrode material according to claim 1, characterized in that, In the mixture of lignite precursor and pitch powder in step S3, the mass of the pitch powder accounts for 8-12% of the mass of the mixture; the mass of the anhydrous ethanol is 5-15 times the mass of the mixture.

7. The battery negative electrode material according to claim 1, characterized in that, The ball milling in step S3 is liquid-phase assisted ball milling; the process conditions for liquid-phase assisted ball milling include: a ball milling speed of 250-300 r·min. -1 The ball milling time is 2-6 hours; the grinding media used in the ball milling is zirconia balls with a diameter of 1-15 mm, and the mass ratio of the zirconia balls to the mixture composed of lignite precursor and pitch powder is 5:1-30:1; during the ball milling process, the sum of the volumes of the mixture and the zirconia balls accounts for 30-65% of the total volume inside the ball mill jar.

8. The battery negative electrode material according to claim 1, characterized in that, After grinding in step S3, the material is passed through a 230-325 mesh sieve. The operating temperature in step S3 is not higher than 15℃.

9. The battery negative electrode material according to claim 1, characterized in that, The protective atmosphere described in step S4 is an inert protective atmosphere; the heating rate is 3-7℃ min. -1 .

10. The application of a soft-hard carbon composite lignite-based sodium-ion battery anode material prepared according to any one of claims 1-9, characterized in that, The lignite-based anode material, composed of soft and hard carbon composites, is used in sodium-ion batteries to achieve efficient and stable sodium storage.

Images