High-specific-capacity hard carbon material as well as preparation method and application thereof

Abstract

The invention discloses a high-specific-capacity hard carbon material as well as a preparation method and application thereof. The preparation method comprises the following steps: S1, preparing raw materials; s2, raw material pretreatment; s3, carrying out a puffing reaction; s4, carbonizing the puffed biomass material; and S5, impurity removal treatment. A starch-free biomass precursor is innovatively pretreated by adopting a popcorn puffing technology, a high-performance hard carbon material is successfully prepared in combination with a gradient carbonization process, and the carbon material forms unique structural characteristics by accurately regulating and controlling steam explosion parameters; due to the special structure, the material shows excellent electrochemical performance in the sodium ion battery, the first discharge capacity reaches 420 mAh / g and is improved by 35% compared with that of an unexpanded sample, the popcorn technology is applied to microstructure regulation and control of the carbon material for the first time, and through a puffing-carbonization synergistic effect mechanism, the specific surface area of the material is increased, and the specific surface area of the material is increased. And a brand new technical route is provided for low-cost preparation of a high-performance sodium ion battery negative electrode material.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy storage materials and electrochemical devices, and specifically to a high-specific-capacity hard carbon material and a preparation method and application thereof. Background Art

[0002] Among the many components of lithium-ion batteries, the performance of the anode material plays a crucial role in overall battery performance. In recent years, hard carbon materials, with their unique and exceptional structure and properties, have gradually emerged as promising anode materials for lithium-ion batteries. Hard carbon materials possess a rich microporous structure and complex surface functional groups, which make them highly adaptable to the insertion and extraction of lithium ions. Theoretically, they can accommodate more lithium ions, potentially achieving higher energy density. Furthermore, hard carbon materials possess excellent chemical stability and mechanical strength, maintaining their relative structural stability during the battery's charge and discharge cycles, thereby extending the battery's lifespan.

[0003] With the rapid development of renewable energy, sodium-ion batteries (Na-ion batteries) have become an important supplement to lithium-ion batteries due to their advantages such as abundant raw materials and low cost. Hard carbon materials are considered to be highly promising anode materials for Na-ion batteries due to their unique disordered carbon layer structure and sodium storage mechanism. However, traditional hard carbon materials generally suffer from narrow interlayer spacing (usually ≤0.38nm), slow sodium ion diffusion kinetics, and low specific capacity <300mAh / g, which severely limit their application in high-energy-density energy storage devices. Currently, the preparation methods of hard carbon materials mainly include direct carbonization of biomass, template method, and chemical vapor deposition method. Among them, biomass carbonization has attracted much attention due to its wide raw material source and simple process. However, traditional carbonization processes make it difficult to precisely control the microstructure of hard carbon, resulting in insufficient interlayer spacing and uneven pore distribution, which affects the sodium storage performance. In recent years, puffing techniques, such as popcorn puffing, have been tried for biomass pretreatment to improve the pore structure of hard carbon. For example, Langmuir, vol. 32, no. 32, pp. 8042-8049, 2016. reported the preparation of hard carbon using the conventional popcorn puffing method. However, due to the single temperature control during the puffing process, the interlayer spacing of the obtained material is still limited, and the pore distribution is uneven, which affects the electrolyte infiltration and ion transport. In summary, the existing technology still has the following problems:

[0004] 1. The interlayer spacing of hard carbon materials prepared by traditional methods is insufficient. The interlayer spacing of traditional hard carbon materials is generally ≤0.40nm, which is difficult to meet the requirements of efficient storage of sodium ions.

[0005] 2. The microstructure of hard carbon materials is uncontrollable, and conventional expansion processes cannot accurately regulate the proportion of graphite-like crystal areas and pore distribution, resulting in insufficient active sites for sodium storage. In addition, the specific capacity is relatively low: the reversible specific capacity of existing hard carbon negative electrodes is usually <300mAh / g, which is difficult to meet the needs of high-energy-density batteries. Summary of the Invention

[0006] In view of the problems that the interlayer spacing of hard carbon materials prepared by traditional methods in the existing technology is insufficient, which makes it difficult to meet the efficient storage requirements of sodium ions, and the microstructure of hard carbon materials is uncontrollable, resulting in insufficient sodium storage active sites. The purpose of the present invention is to provide a high-specific capacity expanded hard carbon material and its preparation method and application. Through the innovative "expansion-carbonization" synergistic mechanism, it provides a new technical route for the low-cost preparation of high-performance sodium ion battery negative electrode materials.

[0007] In order to solve the problems of the prior art, the technical solution adopted by the present invention is:

[0008] A method for preparing a high specific capacity expanded hard carbon material comprises the following steps:

[0009] S1. Raw material preparation: Select biomass raw materials that do not contain starch;

[0010] S2. Raw material pretreatment: After cleaning the biomass raw material, place it in a drying oven at 60-120°C and dry it for more than 24 hours, crush and grind it to less than 200 mesh to obtain a biomass precursor;

[0011] S3. Puffing reaction: After mixing the biomass precursor with the puffing agent, place it in a puffing device, control the temperature at 200°C-500°C, the pressure at 0.5MPa-1.5MPa, and the puffing time at 10min-30min to make the biomass raw material undergo puffing reaction to form puffed biomass;

[0012] S4. Carbonizing the expanded biomass material: placing the expanded biomass in a tubular furnace, heating it to 800-1200°C at a heating rate of 5-10°C / min under inert gas protection, keeping the temperature for 2-4 hours, and cooling it to obtain a black hard carbon material;

[0013] S5. Impurity removal treatment: The hard carbon material is sequentially placed in 1 mol / L hydrochloric acid and deionized water for washing until neutrality, and then filtered and dried to obtain a sodium ion battery hard carbon material.

[0014] Preferably, the biomass raw material in step S1 is any one or more of corn cobs, rice husks, bamboo fibers or palm peels, and the moisture content is controlled at 5-15 wt%.

[0015] Preferably, in step S3, the mixing mass ratio of the biomass raw material to the bulking agent is 10:1, and the bulking agent is a mixture of water and ethanol.

[0016] Preferably, in step S3, a gradient temperature increase program is adopted: first increase the temperature to 200-300°C at 5°C / min and keep warm for 0.5h, then increase the temperature to 300-500°C at 3°C / min and keep warm for 1-2h.

[0017] Preferably, in step S4, the inert gas is either nitrogen or argon.

[0018] Preferably, in step S5, after washing with hydrochloric acid, the mixture is washed with deionized water until neutral, and finally dried at 80-120° C. for 12-24 hours.

[0019] The high-specific-capacity expanded hard carbon material prepared based on the above preparation method includes wide interlayer spacing, nanoscale pores and highly distributed graphite domains. The wide interlayer spacing is 0.36–0.50 nm, and the nanoscale pores include closed pores and open pores.

[0020] Application of the above-mentioned high specific capacity expanded hard carbon material in the preparation of battery negative electrode or electrothermal composite material.

[0021] Specifically, the negative electrode material is prepared by weighing high-capacity expanded hard carbon material, acetylene black, and polyvinylidene fluoride in a mass ratio of 7:2:1, placing them in a mortar, and grinding them thoroughly to make a coating; the coating is applied to the surface of the aluminum foil, placed in a tray, and then placed in an oven and dried at 80°C for 12 hours, and the cut pieces are taken out, and the weight of all the cut negative electrode sheets is weighed and recorded to complete the production of the negative electrode sheets.

[0022] Beneficial effects:

[0023] Compared with existing technologies, this invention innovatively uses popcorn puffing technology to pretreat starch-free biomass precursors, and combines this with a gradient carbonization process to successfully prepare high-performance hard carbon materials. By precisely controlling the steam explosion parameters, the carbon material forms unique structural characteristics. This special structure enables the material to exhibit excellent electrochemical performance in sodium-ion batteries: the initial discharge capacity reaches 420mAh / g, a 35% increase over the unpuffed sample, and the low-potential plateau capacity accounts for as much as 75%.

[0024] Compared with traditional methods, the present invention not only solves the problems of low graphitization degree and insufficient specific capacity of biomass hard carbon, but also avoids the high energy consumption defect of high-temperature graphitization, reducing raw material costs by more than 40% and process energy consumption by 30%.

[0025] This invention is the first to apply popcorn technology to the microstructure regulation of carbon materials. Through the innovative "puffing-carbonization" synergistic mechanism, it provides a new technical route for the low-cost preparation of high-performance sodium-ion battery negative electrode materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a flow chart of a method for preparing a high specific capacity expanded hard carbon material according to the present invention;

[0027] Figure 2 A physical diagram of a method for preparing a high specific capacity expanded hard carbon material according to the present invention;

[0028] Figure 3 This is a diagram showing the mechanism of the present invention using biomass combined with an expansion process to produce graphite crystal regions;

[0029] Figure 4 This is the XRD pattern of the palm bark expanded carbon material of the present invention;

[0030] Figure 5 This is the FTIR test result of the palm bark expanded carbon material of the present invention;

[0031] Figure 6 This is a graph showing the rate performance of the palm bark expanded carbon material of the present invention;

[0032] Figure 7 This is a graph showing the cycling performance of the palm peel expanded carbon material EPHC at a current density of 2000 mA / g;

[0033] Figure 8 A voltage diagram showing continuous changes in the electrothermal properties of the composite film of the present invention;

[0034] Figure 9 This is a temperature stability diagram of the 10%-BHC composite film under voltage of the present invention;

[0035] Figure 10 TEM images and interlayer spacing of the palm peel expanded carbon material (EPHC) of the present invention, unexpanded palm peel hard carbon (PHC), and commercial hard carbon (CHC), among which (a), (d), and (g) are EPHC, (b), (e), and (h) are PHC, and (c), (f), and (i) are CHC. DETAILED DESCRIPTION

[0036] The technical solution of the present invention is described in detail below through specific embodiments.

[0037] A method for preparing a high specific capacity expanded hard carbon material comprises the following steps:

[0038] S1. Raw material preparation: Select biomass raw materials that do not contain starch;

[0039] S2. Raw material pretreatment: After cleaning the biomass raw material, place it in a drying oven at 60-120°C and dry it for more than 24 hours, crush and grind it to less than 200 mesh to obtain a biomass precursor;

[0040] S3. Puffing reaction: After mixing the biomass precursor with the puffing agent, place it in a puffing device, control the temperature at 200°C-500°C, the pressure at 0.5MPa-1.5MPa, and the puffing time at 10min-30min to make the biomass raw material undergo puffing reaction to form puffed biomass;

[0041] S4. Carbonizing the expanded biomass material: placing the expanded biomass in a tubular furnace, heating it to 800-1200°C at a heating rate of 5-10°C / min under inert gas protection, keeping the temperature for 2-4 hours, and cooling it to obtain a black hard carbon material;

[0042] S5. Impurity removal treatment: The hard carbon material is placed in 1 mol / L hydrochloric acid, washed, filtered and dried in sequence to obtain a sodium ion battery hard carbon material.

[0043] Preferably, the biomass raw material in step S1 is any one or more of corn cobs, rice husks, bamboo fibers or palm peels, and the moisture content is controlled at 5-15 wt%.

[0044] Preferably, in step S3, the mixing mass ratio of the biomass raw material to the bulking agent is 10:1, and the bulking agent is a mixture of water and ethanol.

[0045] Preferably, in step S3, a gradient temperature increase program is adopted: first increase the temperature to 200-300°C at 5°C / min and keep warm for 0.5h, then increase the temperature to 300-500°C at 3°C / min and keep warm for 1-2h.

[0046] Preferably, in step S4, the inert gas is either nitrogen or argon.

[0047] Preferably, in step S5, after washing with hydrochloric acid, the mixture is washed with deionized water until neutral, and finally dried at 80-120° C. for 12-24 hours.

[0048] Using starch-free biomass as a precursor, an innovative popcorn puffing pretreatment process is used to perform gradient puffing treatment in the temperature range of 200-500°C and the pressure of 0.1-2MPa, followed by carbonization treatment at 800-1200°C, so that the biomass precursor forms a hard carbon material with an ultra-wide interlayer spacing. The puffed sample is washed and dried to prepare an electrode sheet for sodium ion battery negative electrode material. It has the characteristics of simple process and low energy consumption, which is conducive to the large-scale industrial production of the product. The microstructure of the biomass carbon material is regulated by popcorn puffing technology, and the prepared hard carbon material exhibits excellent electrochemical properties such as high specific capacity (≥420mAh / g) and excellent cycle stability (capacity retention rate ≥92% after 100 cycles).

[0049] Example 1

[0050] A method for preparing a high specific capacity expanded hard carbon material comprises the following steps:

[0051] S1. Raw material preparation: Select a biomass raw material that does not contain starch; the selected biomass raw material is palm bark, and the water content of the selected biomass raw material is controlled at 5-15wt%;

[0052] S2. Raw material pretreatment: Place the biomass precursor in a drying oven, dry it at 80°C for more than 24 hours, and crush and grind it to less than 200 mesh;

[0053] S3. Puffing reaction: The pretreated biomass raw material is mixed with a puffing agent, and the biomass raw material is placed in a puffing device. The temperature is controlled at 200°C and the pressure is 1.5 MPa. The puffing time is 10 minutes to make the biomass raw material undergo a puffing reaction to form puffed biomass. The mixing ratio of the biomass raw material to the puffing agent is 10:1. The puffing agent is water or ethanol. A gradient temperature program is used: first, the temperature is increased to 200°C at 5°C / min and kept for 0.5 hours, and then the temperature is increased to 300°C at 3°C / min and kept for 1-2 hours.

[0054] S4. Carbonizing the expanded biomass material: placing the dried expanded biomass in a tubular furnace, heating it to 800°C at a heating rate of 5°C / min under inert gas protection, keeping the temperature for 2-4 hours, and cooling it to obtain a hard carbon material; the inert gas is nitrogen;

[0055] S5. Impurity removal treatment: The black hard carbon material is placed in 1 mol / L hydrochloric acid, washed, filtered and dried in sequence to obtain a sodium ion battery hard carbon material; after washing with hydrochloric acid, it is washed with deionized water until neutral, and finally dried at 80°C for 24 hours.

[0056] The above-mentioned high specific capacity hard carbon material is used to prepare a high specific capacity hard carbon negative electrode, and the specific steps are as follows:

[0057] (1) Preparing a negative electrode of a battery: using the above-mentioned method for preparing a negative electrode of a battery, preparing a negative electrode sheet of the battery;

[0058] (2) Selecting a suitable diaphragm, electrolyte and battery casing: the diaphragm material is glass fiber, and the electrolyte is 1MN ACLO4+DMC+EMC+EC;

[0059] (3) Assembling button batteries: All battery materials are placed in a glove box, and the inside of the glove box is kept in an anhydrous and oxygen-free state. A sufficient amount of negative electrode shells are placed in the battery plate in sequence, and an equal amount of negative electrode sheets are taken, with the active material of the negative electrode sheets facing upwards, and the negative electrode sheets are placed in the center of the positive electrode shell; a diaphragm paper is placed on top of the negative electrode sheet, and the electrolyte is sucked by a pipette and 200 μL is dropped on the diaphragm paper; a sodium membrane, a gasket, a spring and a negative electrode shell are placed in sequence to complete the battery assembly; the assembled battery is placed in a sealing machine for packaging, and the packaging time is about 30 seconds; the overflowed electrolyte is wiped off with dust-free paper to complete the battery preparation;

[0060] (4) Perform battery testing: Battery testing items include XRD and TEM.

[0061] from Figure 3 It can be seen that the expanded palm bark carbon material sample prepared in this embodiment has two obvious diffraction peaks near 24° and 43°, which correspond to the (002) characteristic peak and (100) characteristic peak of carbon, respectively. From the XRD spectrum, it can be obtained that the expanded palm bark carbon material has d002 = 0.51 nm;

[0062] Figure 4 TEM test of the expanded palm bark carbon material prepared in Example 1 shows an interlayer spacing of -0.51 nm. This large interlayer spacing can accelerate the diffusion rate of sodium ions and is conducive to the storage of sodium ions, thereby improving the rate performance and cycle performance of the electrode material.

[0063] Figure 5 FT-IR test of the expanded palm bark carbon material prepared in Example 1. As can be seen from the figure, the infrared spectrum shows that the carbon material has oxygen-containing functional groups such as -OH (-3433 cm-1), COC (-1030 cm-1), -CH2 (2940 and 2840 cm-1) and -C=O (-1708 cm-1);

[0064] Figure 6 This is a rate performance diagram of the expanded palm bark carbon material prepared in Example 1. Specifically, when used as a negative electrode for a sodium ion battery, after 10 cycles at different current densities of 50 mA / g, 100 mA / g, 200 mA / g, 500 mA / g, 1000 mA / g, and 2000 mA / g, the discharge specific capacities were 381 mAh / g, 295 mAh / g, 243 mAh / g, 182 mAh / g, 155 mAh / g, and 126 mAh / g, respectively. It can be seen that it has good rate performance at high current density.

[0065] In summary, the capacity retention rate was 90% after 3000 cycles at a current density of 2A / g using a Xinwei tester, which indicates that the prepared sodium ion battery hard carbon material has good structural stability.

[0066] Example 2

[0067] Application of a high specific capacity expanded hard carbon material for preparing electrothermal composite materials;

[0068] In this embodiment, the preparation process of the electrothermal composite material is as follows:

[0069] (1) Purchasing enough coconut shells as raw materials, dividing the coconut shells into small pieces and then simply drying them, with the maximum length of each piece being less than 5 cm;

[0070] (2) After cleaning the coconut shell pieces, place them in a drying oven, dry them at 100° C. for more than 24 hours, and crush and grind them to less than 200 mesh to obtain a coconut shell biomass precursor;

[0071] (3) mixing the coconut shell biomass precursor with a bulking agent at a mass ratio of 10:1 (the bulking agent is water or ethanol), placing the mixture in a bulking device, controlling the temperature at 200°C, the pressure at 1.5 MPa, and the bulking time at 10 min, so that the biomass raw material undergoes a bulking reaction to form bulked biomass;

[0072] (4) placing the expanded biomass in a tubular furnace, heating it to 800°C at a heating rate of 5°C / min under inert gas protection, keeping the temperature for 3 h, and cooling it to obtain a black hard carbon material;

[0073] (5) washing the hard carbon material with 1 mol / L hydrochloric acid and deionized water until neutral, filtering, and drying to obtain an expanded hard carbon material;

[0074] (6) Weigh 0.4 g of PEO powder and add it to deionized water solution. Stir magnetically in a constant temperature oil bath at 75°C until the PEO powder is completely dissolved to obtain a PEO solution.

[0075] (6) Add expanded hard carbon material, carbon nitride, and multi-walled carbon nanotubes to the PEO solution and stir magnetically for 30 minutes to form a uniform BHC / g-C3N4 / MWCNT / PEO dispersion with a biomass carbon content of 5-30 wt.%; (the total amount is 0.76 g, PEO is fixed at 0.4 g, if the biomass carbon accounts for 10%, it is 0.36*0.1=0.036 g, and the other two materials are equal in amount, both 0.162 g).

[0076] (7) Slowly pour the BHC / g-C3N4 / MWCNT / PEO dispersion onto a hot roller machine, adjust the temperature of the hot roller machine to 70°C to remove the deionized water solvent, and repeatedly press until the electric heating film is formed;

[0077] (8) Finally, the BHC / g-C3N4 / / MWCNT / PEO composite electric heating film is peeled off from the hot roller machine.

[0078] The composite materials prepared by the above-mentioned solution co-mixing and hot roller pressing method were named 5%-BHC, 10%-BHC, 20%-BHC, and 30%-BHC.

[0079] The electrothermal performance test found that the composite electrothermal film with a biomass carbon content of 10% BHC exhibited the best electrothermal conversion performance, with a steady-state temperature of up to 278°C and a heating rate of up to 13.9°C / s, which was significantly better than samples with other components. This excellent performance is due to the following key factors: under this component, BHC achieved the best dispersion state in the PEO / MWCNT matrix, forming a complete three-dimensional conductive network structure - MWCNT constructed a continuous electron transmission backbone, g-C3N4 sheets effectively connected the MWCNT gaps as "electronic bridges", and BHC particles (particle size of about 100nm) were precisely filled at the heterojunction interface to form an efficient electron hopping transmission node; secondly, the intrinsic polarity and surface nitrogen-containing functional groups of the g-C3N4 sheets not only promoted the interfacial polarization effect, but also formed a hydrogen bond network with the PEO molecular chain (FTIR showed that the hydrogen bond binding energy shift reached 32cm -1 ), optimizing the heat conduction path. At the same time, comparative experiments clearly revealed the necessity of component optimization: the 5% BHC sample had an incomplete conductive network and a steady-state temperature of only 242°C; while the 20% and 30% high-content samples had performance degradation due to BHC agglomeration (SEM showed that the agglomerate size was >500nm), with steady-state temperatures of 111°C and 87°C, respectively;

[0080] Figure 8 The temperature curves of composite electric heating film samples with different biomass carbon contents at 0V, 0.5V, 1V, 1.5V, 2V, 2.5V and 3V show continuous jumps. It can be seen that the composite electric heating film with a biomass carbon content of 10% reaches a higher temperature at the same voltage. Compared with the previous conductivity curve, it can be seen that the higher the conductivity of the film, the better the electrothermal performance. This is because when the conductivity is high, the movement of carriers is promoted and more heat is generated. In addition, the higher the proportion of conductive raw materials in the composite electric heating film, the higher its surface temperature. It should be that increasing the proportion of fillers increases its contact area, providing a dense, multi-channel and efficient three-dimensional network structure for phonon transport. Figure 9 The 10%-BHC film's thermal shock resistance was further analyzed by testing its cyclic heating stability in a 60s on and 60s off mode at an input voltage of 1.5V for 20,000 seconds. Aside from a slight initial increase due to the ambient temperature, the film remained stable, demonstrating the material's excellent durability and cyclic stability.

[0081] The expanded palm bark hard carbon EPHC prepared in Example 1 was compared with the unexpanded palm bark hard carbon PHC and commercial hard carbon CHC, and the microstructural characteristics of the hard carbon materials were analyzed by transmission electron microscopy (TEM). Figure 10 ) and found that EPHC has a unique layered stacking structure, and its interlayer spacing (0.5136nm) is significantly larger than that of PHC (0.278nm) and CHC (0.4133nm). This enlarged interlayer spacing is due to the "thermal expansion effect" of palm bark during the puffing process - when external energy (such as high temperature) acts on the biomass, its internal volatile components are rapidly vaporized, causing the internal pressure of the material to increase sharply; and after the pressure is released, the biomass structure expands to form a looser carbon layer structure. This process is similar to the puffing mechanism of popcorn, but because palm bark does not contain starch, its macroscopic volume change is limited, and microscopically it manifests as a significant increase in interlayer spacing.

[0082] The expansion treatment not only regulates the interlayer spacing of EPHC, but also optimizes the carbon structure through stress-induced graphitization (e.g. Figure 10 (g)). Studies have shown that pressurized carbonization can promote the graphitization process of hard carbon precursors and reduce the threshold temperature for the transformation of amorphous carbon to ordered carbon. During the preparation of EPHC, the instantaneous high pressure generated by expansion is similar to that of pressurized carbonization, which can promote the orderly arrangement of carbon layers while retaining moderate defect sites. This "expanded but ordered" structural feature enables EPHC to have both high sodium storage capacity and excellent ion transport kinetics. The enlarged interlayer spacing (0.5136nm) provides more abundant diffusion channels for sodium ions and reduces the energy barrier during the insertion / extraction process; stress-induced graphitization enhances the conductivity of the carbon skeleton, while the retained CO / C=O functional groups (XPS results) provide additional redox active sites; microscopic isotropic pressure (derived from the synergistic effect of hydrostatic pressure and shear force during the expansion process) makes the carbon layer arrangement more uniform, reduces structural distortion, and thus improves cycle stability. In contrast, PHC, due to its lack of expansion treatment, has a very small interlayer spacing (0.278nm), severely limiting the accessibility of sodium ions. While CHC has a moderate interlayer spacing (0.4133nm), its carbon structure lacks directional control, resulting in inferior sodium storage performance compared to EPHC. This invention, through a synergistic "expansion-carbonization" strategy, achieves precise control of the microstructure of hard carbon materials, providing new insights into the design of high-capacity, long-life sodium-ion battery anodes.

[0083] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing a high specific capacity expanded hard carbon material, characterized in that: The following steps are involved: S1. Raw material preparation: Select biomass raw materials that do not contain starch; S2. Raw material pretreatment: After cleaning the biomass raw material, place it in a drying oven at 60-120°C and dry it for more than 24 hours, crush and grind it to less than 200 mesh to obtain a biomass precursor; S3. Puffing reaction: After mixing the biomass precursor with the puffing agent, place it in a puffing device, control the temperature at 200°C-500°C, the pressure at 0.5MPa-1.5MPa, and the puffing time at 10min-30min to make the biomass raw material undergo puffing reaction to form puffed biomass; S4. Carbonizing the expanded biomass material: placing the expanded biomass in a tubular furnace, heating it to 800-1200°C at a heating rate of 5-10°C / min under inert gas protection, keeping the temperature for 2-4 hours, and cooling it to obtain a black hard carbon material; S5. Impurity removal treatment: The hard carbon material is sequentially placed in 1 mol / L hydrochloric acid and deionized water for washing until neutrality, and then filtered and dried to obtain a sodium ion battery hard carbon material.

2. The method for preparing a high specific capacity expanded hard carbon material according to claim 1, characterized in that: The biomass raw material in step S1 is any one or more of corn cobs, rice husks, bamboo fibers or palm peels, and the moisture content is controlled at 5-15 wt%.

3. The method for preparing a high specific capacity expanded hard carbon material according to claim 1, wherein: In step S3, the mixing mass ratio of the biomass raw material to the swelling agent is 10:1, and the swelling agent is a mixture of water and ethanol.

4. The method for preparing a high specific capacity expanded hard carbon material according to claim 1, wherein: In step S3, a gradient temperature increase program is adopted: first increase the temperature to 200-300°C at 5°C / min and keep it for 0.5h, then increase the temperature to 300-500°C at 3°C / min and keep it for 1-2h.

5. The method for preparing a high specific capacity expanded hard carbon material according to claim 1, wherein: In step S4, the inert gas is either nitrogen or argon.

6. The method for preparing a high specific capacity expanded hard carbon material according to claim 1, wherein: In step S5, after washing with hydrochloric acid, the product is washed with deionized water until neutral, and finally dried at 80-120° C. for 12-24 hours.

7. A high specific capacity expanded hard carbon material prepared by the preparation method according to any one of claims 1 to 6, characterized in that: The high-specific-capacity expanded hard carbon material comprises wide interlayer spacing, nanoscale pores and highly distributed graphite domains, wherein the wide interlayer spacing is 0.36-0.50 nm, and the nanoscale pores comprise closed pores and open pores.

8. Use of the high specific capacity expanded hard carbon material according to claim 7 in preparing battery negative electrodes or electrothermal composite materials.

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