Preparation method and application of high-capacity biomass hard carbon material

Abstract

This invention relates to the field of sodium-ion battery technology, and provides a method for preparing high-capacity biomass hard carbon materials and their applications. The method involves mixing a biomass hard carbon precursor with polyamic acid, followed by a two-step carbonization process to obtain a closed-cell hard carbon material with excellent pore size and structural strength. Polyamic acid acts as both a pore-forming and pore-closing agent, resulting in a rich closed-cell structure. The hard carbon material obtained by this invention has a closed-cell structure that matches the diameter of sodium ions, exhibiting high plateau capacity and high initial charge-discharge efficiency, and has a wide range of applications.

Description

Technical Field

[0001] This application relates to the field of sodium-ion battery technology, specifically to a method for preparing a high-capacity biomass hard carbon material and its application. Background Technology

[0002] Driven by the national dual-carbon goals, energy is transitioning towards green development, with rapid growth in green energy sources such as wind, solar, tidal, and geothermal power. However, intermittent power generation has a significant impact on the power grid, necessitating the deployment of corresponding electrochemical energy storage power stations for efficient energy storage and conversion. Currently, lithium-ion batteries have been successfully commercialized as energy storage batteries, but they suffer from uneven lithium resource distribution and poor performance at high and low temperatures. Against this backdrop, developing new energy storage systems with lower costs and superior performance is crucial. Due to abundant sodium reserves, environmental friendliness, and similar electrochemical properties to lithium-ion batteries, sodium-ion batteries are considered an ideal choice for next-generation energy storage devices. Furthermore, sodium-ion batteries offer significant advantages in low-temperature environments and high-rate charge / discharge conditions. Sodium-ion batteries demonstrate a very broad application prospect in the energy storage field and warrant further development and research.

[0003] In the research of sodium-ion batteries, carbon-based materials have become the preferred target for sodium storage anode active materials due to their advantages such as wide availability, abundant resources, diverse structures, and long lifespan. Hard carbon materials, with their large interlayer spacing, numerous nanopores, and many defect sites, can store a large number of sodium ions and exhibit high specific capacity. Therefore, hard carbon materials are currently one of the most promising anode active materials for sodium-ion batteries. However, the metallic or non-metallic impurities contained in hard carbon materials consume sodium ions, reducing the cycle performance of sodium-ion batteries. Furthermore, the excessively large specific surface area of ​​hard carbon materials leads to a mismatch between the pore structure and interlayer spacing and the diameter of sodium ions, resulting in low initial coulombic efficiency, poor rate performance, and unsatisfactory cycle performance, thus hindering the development of sodium-ion batteries. Summary of the Invention

[0004] Based on the shortcomings of existing hard carbon materials, this invention proposes a method for preparing high-capacity biomass hard carbon materials. The aim is to make the prepared hard carbon materials have a closed-cell structure with superior pore size and structural strength, so that when the hard carbon materials are used as negative electrode active materials, the batteries can have a large energy density and first coulombic efficiency.

[0005] To achieve its objectives, the present invention employs the following technical solution:

[0006] In a first aspect, the present invention provides a method for preparing a high-capacity biomass hard carbon material, comprising the following steps:

[0007] S1. Pretreatment: The biomass carbon source is successively dissolved in water and acidic solution for washing, and then dried to obtain biomass hard carbon precursor powder;

[0008] S2. Ball milling and mixing: The biomass hard carbon precursor powder and polyamic acid powder are ball milled and mixed evenly to obtain a mixed powder;

[0009] S3. Pre-carbonization: The mixed powder is subjected to a first carbonization treatment in a first protective atmosphere to obtain a composite material;

[0010] S4. Post-treatment: In a second protective atmosphere, the composite material obtained in step S3 is further subjected to a second carbonization treatment to obtain a hard carbon material.

[0011] In the technical solution of this invention: the biomass carbon source has multiple functional groups such as carbonyl, conjugated ester, phenolic hydroxyl, and aryl groups, and possesses multiple unit linkages and acylation or cross-linking structures, thus having a good basis for modification. Its three-dimensional aromatic hydrocarbon structure makes it suitable for synthesizing porous carbon materials. Adding a pore-forming agent to treat the biomass carbon source can generate a porous structure, and further treatment with a pore-closing agent and a second carbonization process can yield a hard carbon material with a closed-pore structure. Preferably, the biomass carbon source includes at least one of coconut shell, starch, bamboo powder, straw, walnut shell, potato, and wheat.

[0012] Preferably, in step S1: the acidic solution includes at least one of hydrochloric acid solution and sulfuric acid solution, the concentration of the acidic solution is 0.5–2 mol / L, and the biomass carbon source is treated in the acidic solution for 2–10 h; the drying temperature is 60–100 °C, and the drying time is 20–30 h. Washing the biomass carbon source sequentially with water and acidic solution can improve the surface functional group composition of the biomass hard carbon precursor, thereby improving the performance of the prepared hard carbon material. After washing, drying is performed to fully dry the biomass hard carbon precursor and reduce its surface free water content.

[0013] Preferably, in step S2: the polyamic acid includes at least one of aliphatic polyamic acid, furanyl polyamic acid, and aromatic polyamic acid; the biomass hard carbon precursor powder and the polyamic acid powder are mixed at a mass ratio of 1:(0.1-0.5). Within the aforementioned mass ratio range of precursor powder to polyamic acid powder, the prepared hard carbon material exhibits good electrochemical performance and structural stability.

[0014] Preferably, in step S2, the ball milling speed is 300–500 r / min, and the time is 120–240 min. Within the aforementioned ball milling speed and time range, the precursor powder and polyamic acid can be thoroughly mixed.

[0015] Preferably, in step S3, the temperature of the first carbonization treatment is 400–800℃, the time is 2–4 h, and the heating rate is 2–10℃ / min. In step S4, the temperature of the second carbonization treatment is 1100–1500℃, the time is 2–4 h, and the heating rate is 2–10℃ / min. When the conditions of the first carbonization treatment are within the aforementioned range, the biomass hard carbon precursor powder and the pore-forming agent are mixed and pyrolyzed, and the molecules of the precursor are changed; when the conditions of the second carbonization treatment are within the aforementioned range, the pore-closing agent is decomposed at high temperature to prepare a closed-cell and structurally stable hard carbon material. Polyamic acid powder acts as a pore-forming agent in the first carbonization treatment and as a pore-closing agent in the second carbonization treatment.

[0016] Preferably, the first protective atmosphere and the second protective atmosphere each independently comprise one of N2 and an inert gas. In a protective gas atmosphere, side reactions such as oxidation during the pyrolysis of biomass hard carbon precursors can be reduced.

[0017] In a second aspect, the present invention provides a negative electrode active material comprising the hard carbon material prepared by the aforementioned method. The hard carbon material prepared by the aforementioned method has a closed-pore molecular structure with high pore structure stability and is not prone to collapse, thus it can be used as a negative electrode active material for sodium-ion batteries.

[0018] In a third aspect, the present invention provides a sodium-ion battery comprising the aforementioned negative electrode active material. Therefore, the sodium-ion battery using this negative electrode active material exhibits high energy density and high initial charge-discharge efficiency.

[0019] Compared with existing technologies, the beneficial effects of this invention are reflected in:

[0020] This invention proposes a novel method for preparing hard carbon materials. It utilizes a mixture of biomass hard carbon precursor and polyamic acid, followed by a two-step carbonization process to obtain a closed-cell hard carbon material with excellent pore size and structural strength. The polyamic acid acts as both a pore-forming and pore-closing agent, resulting in a rich closed-cell structure. The hard carbon material obtained by this invention exhibits a closed-cell structure that matches the diameter of sodium ions, resulting in high plateau capacity and high initial charge-discharge efficiency, thus broadening its application range. Attached Figure Description

[0021] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0022] Figure 1 This is a scanning electron microscope image of the carbon material with an open-pore structure in Embodiment 1 of the present invention that has not undergone the second carbonization treatment;

[0023] Figure 2This is a scanning electron microscope image of the hard carbon material obtained by the second carbonization treatment in Example 1 of the present invention;

[0024] Figure 3 The image shows the XRD pattern of the hard carbon material in Example 1 of this invention.

[0025] Figure 4 This is a charge-discharge curve of the sodium-ion battery assembled with hard carbon material in Example 1 of the present invention. Detailed Implementation

[0026] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] This invention provides a method for preparing high-capacity biomass hard carbon material, comprising the following steps:

[0028] S1. Pretreatment: The biomass carbon source is successively dissolved in water and acidic solution for washing, and then dried to obtain biomass hard carbon precursor powder;

[0029] S2. Ball milling and mixing: The biomass hard carbon precursor powder and polyamic acid powder are ball milled and mixed evenly to obtain a mixed powder;

[0030] S3. Pre-carbonization: The mixed powder is subjected to a first carbonization treatment in a first protective atmosphere to obtain a composite material;

[0031] S4. Post-treatment: In a second protective atmosphere, the composite material obtained in step S3 is further subjected to a second carbonization treatment to obtain a hard carbon material. Let the temperature of the second carbonization treatment be T2, and the temperature of the first carbonization treatment be T1, then T2~T1>300℃.

[0032] Biomass carbon source molecules contain a variety of functional groups and exhibit multiple unit linkages and acylation or cross-linking structures, thus possessing a good basis for modification. Based on the number of methoxy groups attached to the benzene ring in the phenylpropane units they contain, biomass carbon sources can be classified into syringylpropane units (S-type biomass hard carbon precursors), guaiacylpropane units (G-type biomass hard carbon precursors), and p-hydroxyphenylpropane units (H-type biomass hard carbon precursors). Among these, the G-type and S-type biomass hard carbon precursors, in particular, possess a high number of methoxy groups on their phenylpropane units, which can form methyl aryl ether bonds. Intermolecular bonds can be established through ether bonds or carbon-carbon bonds, forming a three-dimensional network molecular structure. After further modification, these precursors can exhibit excellent electrochemical properties.

[0033] When biomass hard carbon precursors and polyamic acid are mixed, their properties allow for the formation of a microscopic three-dimensional structure. Leveraging the advantages of polyamic acid, anode materials with complex structures and superior performance can be designed. Specifically, biomass hard carbon precursors are mixed with polyamic acid. After a first carbonization treatment, the polyamic acid undergoes a dehydration cyclization reaction. Under heat, the amide groups within the polyamic acid molecule lose water molecules, forming an imide ring. The water molecules then escape from the reaction system in gaseous form. The dehydration cyclization reaction gradually accelerates with increasing temperature until the polyamic acid is completely converted into polyimide. During this process, the escaped water molecules activate the bio-based hard carbon material, initially carbonizing it to form a porous microstructure. After a second carbonization treatment, the polymer in the biomass hard carbon precursor... The components can be further carbonized, and the chemical bonds in the polyimide molecular chain gradually break, leading to the depolymerization and degradation of the polymer chain. This releases a variety of small molecule gases that reactivate the bio-based hard carbon material. Unformed polyamic acid also undergoes a high-temperature decomposition reaction and may produce gas. After high-temperature pyrolysis, carbonization products are formed to seal the pores formed by the two activations of the bio-based hard carbon material. This allows the final hard carbon material to maintain a rich closed-pore structure. The pore size of this structure matches the diameter of sodium ions, which helps sodium ions move relatively freely within it.

[0034] Pore-forming agents are additives that promote the formation of porous structures within materials. In the preparation of hard carbon materials, pore-forming agents, through specific chemical reactions or physical actions, create pore structures within the material, influencing properties such as specific surface area, porosity, and pore size distribution. Closing agents are chemical additives that promote the formation of closed-pore structures within materials. Through specific chemical reactions or physical actions, they create closed pore structures within hard carbon materials, helping to improve the material's sodium storage performance and cycle stability. Polyamic acid acts as both a pore-forming and closing agent in bio-based materials, resulting in abundant closed-pore structures. These closed-pore structures provide hard carbon materials with abundant sodium storage sites, giving them high reversible capacity. This helps improve the energy density and range of sodium-ion batteries. The closed-pore structure helps reduce volume changes during charge and discharge, thereby improving the material's cycle stability. The closed-pore structure facilitates electrolyte penetration and rapid ion transport, thus improving the material's rate performance.

[0035] In the technical solution of this invention, the biomass carbon source is first cleaned. The three-dimensional network structure of the biomass carbon source molecules means that some impurities may be mixed in the biomass carbon source, which may interfere with the carbonization process. The cleaning process can remove the impurities from the biomass carbon source raw material, thereby improving the purity of the reactants.

[0036] In some embodiments, the biomass carbon source is sequentially dissolved in deionized water and acidic solution using a funnel for washing, repeated three times. A glass rod is used to stir and assist in dissolving the biomass carbon source in the deionized water or acidic solution to ensure thorough washing. After washing, the sample is dried in an oven. Water washing can initially remove soluble impurities and ash from the biomass carbon source, improving its purity. Acid washing can further remove inorganic impurities from the biomass carbon source, improving the purity and stability of the obtained biomass hard carbon precursor. Furthermore, acid washing can alter the functional group composition of the biomass carbon source surface, increasing the number of oxygen-containing functional groups, improving the surface chemical properties of the hard carbon material, and enhancing its compatibility and stability with other materials.

[0037] In some embodiments, the ball milling process is performed at a rotation speed of 300–500 r / min for a time of 120–240 min. Within the aforementioned range of rotation speed and time, the precursor powder and polyamic acid can be thoroughly mixed.

[0038] In some embodiments, the polyamic acid includes at least one of aliphatic polyamic acid, furanyl polyamic acid, and aromatic polyamic acid; the biomass hard carbon precursor powder and the polyamic acid powder are mixed at a mass ratio of 1:(0.1-0.5). By controlling the amount of polyamic acid, the morphology and proportion of pores and channels in the biomass hard carbon material can be controlled, thereby controlling the size, morphology, and structure of the micropores in the material. Considering the electrochemical performance, structural stability, and preparation cost of the prepared hard carbon material, a more ideal effect can be obtained when the mass ratio of the biomass hard carbon precursor to the polyamic acid is within the aforementioned range. As an example, the mass ratio of the biomass hard carbon precursor to the furanyl polyamic acid can be 1:0.3.

[0039] In some embodiments, the temperature T1 of the first carbonization treatment is 400–800°C, and the time is 2–4 hours. At the aforementioned treatment time and temperature, the first carbonization treatment helps to release hydrogen atoms and other molecules from the biomass hard carbon precursor in a volatile form, increasing the active sites on the material surface, promoting organic cross-linking between molecules, increasing structural disorder, facilitating the migration and diffusion of Na+, and increasing the sodium storage capacity. As an example, T1 can be 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, or 800°C.

[0040] In some embodiments, the temperature T2 of the second carbonization treatment is 1100–1500°C, and the time is 2–4 hours. When the second carbonization treatment is performed at the aforementioned treatment time and temperature, the microstructure of the composite material can be further adjusted and optimized through carbonization to obtain a hard carbon material with a suitable interlayer spacing. As an example, T2 can be 1100°C, 1150°C, 1200°C, 1250°C, 1300°C, 1350°C, 1400°C, 1450°C, or 1500°C.

[0041] In some embodiments, the heating rates of the first carbonization treatment and the second carbonization treatment are independently 2 to 10 °C / min. During the carbonization treatment, when the heating rate is within the aforementioned range, the carbonized polymer, i.e., the biomass hard carbon precursor used in this invention, can reduce the degree of side reactions such as oxidation and decomposition that may occur during its carbonization process.

[0042] In some embodiments, the first protective atmosphere and the second protective atmosphere each independently comprise N2 and an inert gas; and / or, the apparatus for the carbonization treatment is a high-temperature tube furnace.

[0043] In a second aspect, the present invention provides a negative electrode active material comprising the aforementioned hard carbon material.

[0044] In a third aspect, the present invention provides a sodium-ion battery comprising the aforementioned sodium-ion battery negative electrode active material.

[0045] In some embodiments, the hard carbon material is ground and screened, then mixed with carbon black and binder at a mass ratio of 90:5:5 and ground again. The binder is carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) to obtain a slurry. The slurry is coated on copper foil and vacuum dried to obtain the negative electrode sheet of a sodium-ion battery.

[0046] The present invention will be described below through specific embodiments. It should be noted that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the present application. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with the techniques or conditions described in the literature in the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0047] The preparation method of poly(pyromellitic dianhydride-furandiamine) ammonium acid powder used in the following examples is as follows: Under nitrogen protection, 2,5-furandiamine is dissolved in pre-cooled (0-5℃) N-methylpyrrolidone (NMP) solvent and stirred until completely dissolved. Pyromellitic dianhydride (PMDA) powder is added in batches, controlling the molar ratio of diamine:dianhydride = 1:1.02-1.03, and the reaction temperature is maintained ≤5℃ to avoid violent exothermic reactions that could lead to gelation. Stirring is continued for 12-24 hours until a viscous polyamic acid (PAA) solution is formed. The reaction solution is slowly added dropwise to a deionized water / methanol mixture (volume ratio 1:1), resulting in the precipitation of a white flocculent precipitate. After filtration, the solution is washed successively with methanol and deionized water to remove residual solvent and oligomers. After vacuum drying (80℃, 12 hours), the powder is ground to obtain poly(pyromellitic dianhydride-furandiamine) ammonium acid powder.

[0048] Example 1

[0049] This embodiment prepares high-capacity biomass hard carbon material according to the following steps:

[0050] S1. Pretreatment: Dissolve 20g of biomass carbon source (bamboo powder) in water and 1mol / L hydrochloric acid solution sequentially for washing. Repeat three times while stirring with a glass rod. After washing, dry in an oven at 80℃ for 24h to obtain biomass hard carbon precursor powder.

[0051] S2. Ball milling and mixing: The biomass hard carbon precursor powder from step S1 is mixed with poly(pyromellitic dianhydride-furandiamine)amic acid powder at a mass ratio of 1:0.3, weighed, and ball milled at a speed of 300 r / min for 120 min to obtain mixed powder.

[0052] S3. Pre-carbonization: The mixed powder obtained in step S2 is placed in a tube furnace and subjected to the first carbonization treatment in a N2 atmosphere. The carbonization temperature is 600℃, the heating rate is 5℃ / min, and the time is 2h. The composite material is obtained, and its SEM image is shown below. Figure 1 As shown.

[0053] S4. Post-processing: The composite material obtained in step S3 is placed in a tube furnace and subjected to a second carbonization treatment in a N2 atmosphere. The carbonization temperature is 1400℃, the heating rate is 5℃ / min, and the time is 2h, to obtain hard carbon material. Its SEM image is shown below. Figure 2 As shown.

[0054] Example 2

[0055] This embodiment prepares high-capacity biomass hard carbon material using the same method as in Example 1, except that the temperature of the second carbonization treatment is 1300℃.

[0056] Example 3

[0057] This embodiment prepares high-capacity biomass hard carbon material using the same method as in Example 1, except that the temperature of the second carbonization treatment is 1500℃.

[0058] Example 4

[0059] This embodiment prepares high-capacity biomass hard carbon material using the same method as in Example 1, except that the acid solution used in the acid treatment process of step S1 is a 4 mol / L hydrochloric acid solution.

[0060] Example 5

[0061] This embodiment prepares high-capacity biomass hard carbon material using the same method as in Example 1, except that the ball milling time in step S2 is 4 hours.

[0062] Example 6

[0063] In this embodiment, high-capacity biomass hard carbon material is prepared using the same method as in Example 1, except that in step S2, the mass ratio of biomass hard carbon precursor powder to poly(pyromellitic dianhydride-furandiamine)amic acid powder is 1:0.6.

[0064] Comparative Example 1

[0065] This comparative example prepared biomass hard carbon material according to the following steps:

[0066] S1. Pretreatment: 20g of biomass carbon source (bamboo powder) was dissolved in water and 1mol / L hydrochloric acid solution for washing, repeated three times, and stirred with a glass rod. After washing, the powder was dried in an oven at 80℃ for 24h to obtain biomass hard carbon precursor powder.

[0067] S2. Pre-carbonization: The biomass hard carbon precursor powder obtained in step S1 is placed in a tube furnace and subjected to the first carbonization treatment in a N2 atmosphere. The carbonization temperature is 600℃, the heating rate is 5℃ / min, and the time is 2h to obtain the composite material.

[0068] S3. Post-processing: The composite material obtained in step S2 is placed in a tube furnace and subjected to a second carbonization treatment in a N2 atmosphere. The carbonization temperature is 1400℃, the heating rate is 5℃ / min, and the time is 2h to obtain hard carbon material.

[0069] Comparative Example 2

[0070] This comparative example prepared biomass hard carbon material according to the following steps:

[0071] S1. Pretreatment: 20g of biomass carbon source (bamboo powder) was dissolved in water and 1mol / L hydrochloric acid solution for washing, repeated three times, and stirred with a glass rod. After washing, the powder was dried in an oven at 80℃ for 24h to obtain biomass hard carbon precursor powder.

[0072] S2. Ball milling and mixing: The biomass hard carbon precursor powder from step S1 is mixed with poly(pyromellitic dianhydride-furandiamine)amic acid powder at a mass ratio of 1:0.3 and ball milled at 300 r / min for 120 min to obtain mixed powder.

[0073] S3. Carbonization: The mixed powder obtained in step S2 is placed in a tube furnace and carbonized in a N2 atmosphere. The carbonization temperature is 1400℃, the heating rate is 5℃ / min, and the time is 2h to obtain hard carbon material.

[0074] Comparative Example 3

[0075] This embodiment prepares high-capacity biomass hard carbon material according to the following steps:

[0076] S1. Pretreatment: Dissolve 20g of biomass carbon source (bamboo powder) in water and 1mol / L hydrochloric acid solution sequentially for washing. Repeat three times while stirring with a glass rod. After washing, dry in an oven at 80℃ for 24h to obtain biomass hard carbon precursor powder.

[0077] S2. Ball milling and mixing: The biomass hard carbon precursor powder from step S1 is mixed with poly(pyromellitic dianhydride-furandiamine)amic acid powder at a mass ratio of 1:0.3, weighed, and ball milled at a speed of 300 r / min for 120 min to obtain mixed powder.

[0078] S3. Pre-carbonization: The mixed powder obtained in step S2 is placed in a tube furnace and subjected to the first carbonization treatment in a N2 atmosphere. The carbonization temperature is 600℃, the heating rate is 5℃ / min, and the time is 2h to obtain carbon material.

[0079] Using the hard carbon materials prepared in the above embodiments and comparative examples as negative electrode active materials, coin cells were assembled. The specific steps are as follows:

[0080] Hard carbon material was mixed with carbon black, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in a mass ratio of 90:5:2.5:2.5. Using deionized water as a dispersant, the mixture was thoroughly mixed in a mortar to form a slurry. This slurry was then evenly coated onto copper foil using a coating machine and placed in a vacuum drying oven at 80°C for 12 hours. Afterward, a negative electrode sheet with a diameter of 14 mm was prepared using a die-cutting machine. A glass fiber disc was used as the separator, a sodium metal sheet as the counter electrode and reference electrode, and sodium hexafluorophosphate (1 mol / L) dissolved in diethylene glycol dimethyl ether was used as the electrolyte. The sodium-ion battery was assembled in a glove box filled with high-purity argon gas according to the construction of a CR2032 standard coin cell.

[0081] Sodium-ion batteries assembled from the materials prepared in each embodiment and comparative example were subjected to charge-discharge tests on a battery testing platform at a current density of 50 mA / g. The results are shown in Table 1.

[0082] Table 1

[0083] <![CDATA[Reversible specific capacity / mAh·g ～1 > First Coulomb Efficiency / % <![CDATA[Specific surface area m 2 / g]]> Example 1 385 93 4.8 Example 2 351 89 7.1 Example 3 332 87 5.3 Example 4 324 80 21.4 Example 5 307 83 9.3 Example 6 313 86 14.7 Comparative Example 1 212 82 102.1 Comparative Example 2 327 88 30.8 Comparative Example 3 127 30 417.0

[0084] See Figure 1 Carbon materials that have only undergone the first carbonization treatment and not the second carbonization treatment exhibit irregular pores of varying sizes, ranging from 1 to 5 μm. (See also...) Figure 2 The hard carbon material obtained after the second carbonization treatment has an irregular shape, uneven particle size ranging from 2 to 10 μm, a relatively smooth surface, and moderate porosity. Table 1, comparing Example 1 and Comparative Example 3, clearly shows that the specific surface area of ​​the activated carbon material after high-temperature carbonization significantly decreases, suggesting that the pores of the carbon material collapse, forming a closed-cell structure.

[0085] The XRD pattern of the hard carbon material in Example 1 is shown below. Figure 3 As shown in the figure, the diffraction peak half-width of the (002) crystal plane of this carbon material is relatively large and the angle is relatively small, indicating that the microcrystal disorder of this hard carbon material is relatively high and the interlayer spacing is relatively large, which is conducive to the insertion and extraction of sodium ions.

[0086] The charge-discharge curves of the sodium-ion battery assembled from the hard carbon material in Example 1 are shown in the figure below. Figure 4 As shown in the figure, the assembled sodium-ion battery has a high specific capacity of 385 mAh·g. ～1 The initial charge / discharge efficiency is 93%.

[0087] Test results show that the carbon material with the specific structure obtained by this invention is beneficial to improving the capacity of sodium-ion batteries and the first charge-discharge efficiency.

[0088] It should be noted that the present invention is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments that have the same structure and perform the same effects as the technical concept within the scope of the present invention are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of the present invention.

Claims

1. A method for preparing a high-capacity biomass hard carbon material, characterized in that, Includes the following steps: S1. Pretreatment: The biomass carbon source is successively dissolved in water and acidic solution for washing, and then dried to obtain biomass hard carbon precursor powder; S2. Ball milling and mixing: The biomass hard carbon precursor powder and polyamic acid powder are ball milled and mixed evenly at a mass ratio of 1:0.1~0.5 to obtain a mixed powder; the polyamic acid includes at least one of aliphatic polyamic acid, furanyl polyamic acid and aromatic polyamic acid; S3. Pre-carbonization: The mixed powder is subjected to a first carbonization treatment in a first protective atmosphere to obtain a composite material; the temperature of the first carbonization treatment is 400~800 ℃, the time is 2~4 h, and the heating rate is 2~10 ℃ / min. S4. Post-treatment: In the second protective atmosphere, the composite material obtained in step S3 is further subjected to a second carbonization treatment to obtain a hard carbon material; the temperature of the second carbonization treatment is 1100~1500 ℃, the time is 2~4 h, and the heating rate is 2~10℃ / min.

2. The method for preparing a high-capacity biomass hard carbon material according to claim 1, characterized in that, In step S1: the acidic solution includes at least one of hydrochloric acid solution and sulfuric acid solution, the concentration of the acidic solution is 0.5~2 mol / L, and the biomass carbon source is treated in the acidic solution for 2~10 h; the drying temperature is 60~100 ℃, and the drying time is 20~30 h.

3. The method for preparing a high-capacity biomass hard carbon material according to claim 1, characterized in that, In step S2: the ball mill rotates at a speed of 300~500 r / min for a time of 120~240 min.

4. The method for preparing a high-capacity biomass hard carbon material according to claim 1, characterized in that, The biomass carbon source includes at least one of coconut shell, starch, bamboo powder, straw, walnut shell, potato, and wheat.

5. A high-capacity biomass hard carbon material prepared by the preparation method according to any one of claims 1 to 4.

6. A negative electrode active material, characterized in that, Includes the hard carbon material as described in claim 5.

7. A sodium-ion battery, characterized in that, Includes the negative electrode active material as described in claim 6.

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