Hard carbon negative electrode material with customized pore structure and preparation method thereof

Abstract

This invention relates to the field of sodium battery anode technology, specifically disclosing a hard carbon anode material with a customized pore structure and its preparation method: The method involves pre-crushing a biomass carbon source and screening it to obtain materials of the target particle size; mixing the materials with carbonates to embed the carbonates within the materials; using the product from the previous step as a framework material at temperature T1, coating the framework material with carbon materials to obtain a core-shell structure; subjecting the core-shell structure obtained in the previous step to medium-temperature heat treatment at temperature T2; subjecting the product from the previous step to high-temperature heat treatment at temperature T3; and finally, sieving; wherein: T3 > T2 > T1. The preparation method proposed in this invention enables a gradient design of the hard carbon anode material from core to shell pore size, ensuring unobstructed sodium ion transport channels. The resulting product has abundant sodium storage sites, which helps to improve the specific capacity of sodium batteries.

Description

Technical Field

[0001] This invention relates to the field of sodium electrode anode materials technology, and in particular to a hard carbon anode material with a customized pore structure and its preparation method. Background Technology

[0002] Hard carbon materials are commonly used anode materials in sodium batteries. Compared with graphite anode materials, they have advantages such as long cycle life, high safety and specific capacity, and good conductivity, showing broad application prospects in the field of lithium-ion batteries, especially in new energy vehicles and energy storage. With the continuous improvement of battery performance requirements, the application of hard carbon anode materials will become increasingly widespread. The pore structure of hard carbon materials has a significant impact on the battery's initial efficiency, specific capacity, cycle performance, and safety. Current technologies often use biomass as a carbon source, preparing hard carbon anode materials through processes such as low-temperature pre-carbonization, high-temperature carbonization, purification, and pulverization. However, the resulting hard carbon anode materials have poor rate performance, and in practical applications, prolonged high-current charge and discharge can easily lead to sodium deposition, resulting in reduced safety and cycle stability of sodium batteries.

[0003] To this end, Chinese invention patent application No. 202410134036.2 (publication number CN 117963885A) proposes a high-rate biomass-based hard carbon anode material and its preparation method, which includes the following steps: (1) impregnating biomass raw materials in an alkaline solution and drying them to obtain alkaline-impregnated biomass; (2) subjecting the alkaline-impregnated biomass to pre-oxidation crosslinking and low-temperature etching at a low temperature and in an air atmosphere, and then subjecting it to pre-carbonization, crushing, purification and drying processes to obtain biomass-based precursor powder; (3) mixing the biomass-based precursor powder with asphalt and carbonizing it at high temperature with an inert gas to obtain biomass-based hard carbon anode material. The existing technology uses alkaline activators to obtain precursor powder with micropores through low-temperature pre-oxidation cross-linking and curing, and pre-high-temperature carbonization. Then, it obtains hard carbon anode material with high capacity, high initial efficiency and high rate performance through high-temperature asphalt coating and simultaneous high-temperature carbonization, which can solve the problem of low rate performance of hard carbon anode to a certain extent.

[0004] It should be noted that the existing technology has the following problems: (1) To activate the pores with alkaline solution, in order to avoid the alkaline solution severely etching the pores during subsequent high-temperature carbonization and thus reducing the first efficiency and cycle stability of the hard carbon anode, it is necessary to crush and purify before high-temperature carbonization, which leads to complex process, cumbersome process and reduced production efficiency; (2) Although the asphalt coating layer helps to balance the defects of hard carbon, its micropore size and the micropore size of the biomass hard carbon precursor cannot be controlled and customized. The micropore size of the asphalt coating layer is smaller than that of the biomass hard carbon precursor, which obstructs the transport channel of sodium ions and is not conducive to the insertion and extraction of sodium ions. Even if there are enough sodium insertion sites in the biomass hard carbon precursor, a high specific capacity will not be obtained.

[0005] Therefore, developing a hard carbon anode material with a customized pore structure and its preparation method to enrich its sodium storage sites and obtain a high specific capacity hard carbon anode material remains a technical challenge in this field. This application is therefore submitted for this purpose. Summary of the Invention

[0006] In view of the above-mentioned shortcomings of the prior art, the present invention provides a hard carbon anode material with a customized pore structure and a method for preparing the same.

[0007] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0008] On the one hand, this invention proposes a method for preparing a hard carbon anode material with a customized pore structure, comprising the following steps:

[0009] S1. Pre-crush the biomass carbon source and grade and screen the material to obtain the target particle size;

[0010] S2. Mix the material obtained in the previous step with carbonate, so that the carbonate is embedded inside the material.

[0011] At temperatures S3 and T1, the material obtained in the previous step is used as the skeleton material. Carbon material is used to coat the surface of the skeleton material to obtain a core-shell structure with the skeleton material as the core and the carbon material as the shell.

[0012] At temperatures S4 and T2, the core-shell structure obtained in the previous step is subjected to medium-temperature heat treatment.

[0013] The product obtained in the previous step is subjected to high-temperature heat treatment at temperatures S5 and T3.

[0014] S6. Screening;

[0015] Among them: T3 > T2 > T1.

[0016] Preparation principle: Carbonate is used as a pore-forming agent. After the material of the target particle size is mixed with carbonate, the carbonate is embedded inside. Then, carbon material is used to coat the surface at a lower temperature T1 to form a core-shell structure. Then, a medium-temperature heat treatment is carried out at a medium temperature T2. During the medium-temperature heat treatment, the volatile components of biomass in the "core" overflow. Because the biomass material framework is hard and does not expand easily, the overflowing volatile components form micropores and pores in place. The overflowing volatile components impact the coating layer, causing the soft coating layer to expand and form mesopores and macropores during the expansion process. As the medium-temperature heat treatment continues... As the processing progresses, the carbonates in the "core" decompose to produce oxides and carbon dioxide, creating specific depressions and micropores in situ. When carbon dioxide overflows, it reacts with carbon in the coating layer to form micropores. Finally, high-temperature heat treatment at a high temperature T3 causes the fine pores and micropores formed in situ when the volatile components in the "core" overflow to shrink into a large number of closed pores. During the medium-temperature heat treatment, the mesopores and macropores formed when the biomass volatile components impact the coating layer shrink into micropores. When carbon dioxide overflows, the micropores formed in the coating layer shrink into fine pores and a small number of closed pores. In this way, a gradient distribution of pore size from small to large is formed from the core to the shell.

[0017] Compared with existing technologies, this invention uses a biomass carbon source with embedded carbonates as the skeleton material and carbon materials as the coating material. Through medium-temperature heat treatment and high-temperature heat treatment, a simple, easy-to-implement, and scalable process is used to prepare hard carbon anode materials with customized pore structures. No acid or alkali activators are added during the preparation process, and there is no need for crushing or purification before high-temperature heat treatment, which simplifies the process and operation. It has the advantages of high efficiency, speed, and energy saving, which helps to improve production efficiency. The resulting hard carbon anode material has abundant sodium storage sites, which helps to significantly improve the specific capacity of sodium batteries.

[0018] Preferably, T1 is 50-350℃, T2 is 100-600℃, and T3 is 900-1500℃.

[0019] Preferably, steps S3, S4, and S5 all employ a rate-increasing heating method, and after reaching the target temperature, the reaction is kept at a constant temperature under an inert atmosphere for a period of time. The inert atmosphere in steps S3, S4, and S5 is an argon atmosphere, a nitrogen atmosphere, or a helium atmosphere, with a flow rate of 10-500 ml / min for argon, nitrogen, or helium, a heating rate of 1-10 °C / min, and a holding time of 0.5-4 h.

[0020] Preferably, in step S1, the biomass carbon source is pre-crushed to below 100μm and the material is graded and screened. The material meets the following conditions: D10≥2μm, D50=8-10μm, and maximum particle size Dmax≤50μm.

[0021] The materials after grading and screening must meet the above conditions. Biomass carbon sources within this particle size range have good uniformity, are easy to process, and help to have high specific surface area and high reactivity after processing, thereby optimizing electrochemical performance.

[0022] Preferably, in step S1, the biomass carbon source includes one or more of coconut shells, wood, straw, walnut shells, peanut shells, apricot shells, and medicinal residues. Other biomass carbon sources used in the art for preparing hard carbon materials, such as fruit peels, sawdust, and leaves, can theoretically also be used in this invention.

[0023] Preferably, in step S2, the carbonate includes one or more of sodium bicarbonate, ammonium bicarbonate, ammonium carbonate, and calcium carbonate, and the mass percentage of the material in the mixture of the material and the carbonate is 50%-99%.

[0024] Ammonium carbonate and ammonium bicarbonate decompose during heating to produce ammonia, carbon dioxide, and water vapor. Furthermore, the carbon dioxide and water vapor react with carbon, specifically: C + H₂O → H₂ + CO, CO + H₂O → H₂ + CO₂, and C + CO₂ → 2CO. The release of these gases creates numerous pores within the carbon material. Calcium carbonate and sodium carbonate decompose during heating to produce metal oxides and carbon dioxide. The carbon dioxide further reacts with carbon to form carbon monoxide, thus also creating pores. Given the varying quantities and types of gases generated by the heating and decomposition of different pore-forming agents, several experiments have confirmed that, at the same dosage, ammonium carbonate and ammonium bicarbonate exhibit the best pore-forming effect, followed by calcium carbonate, and then sodium carbonate.

[0025] The amount of carbonate added needs to be limited within a certain range; it should not be too much or too little. Too much carbonate creates large, recessed, or ineffective pores, which are not beneficial for sodium storage and may even lead to core structure collapse, negatively impacting cycle stability. Too little carbonate results in very few or no pores, insufficient active sodium storage sites, and affects sodium storage performance. This invention, through creative effort, has discovered that when the mass percentage of the material in the mixture of the material and the carbonate is 50%-99%, the carbonate content is moderate, resulting in good sodium storage performance and structural stability of the core, contributing to both high specific capacity and good cycle stability in hard carbon anode materials.

[0026] Preferably, in step S3, the carbon material includes one or more of petroleum asphalt, coal tar pitch, phenolic resin, and epoxy resin, and the mass percentage of the skeleton material in the mixture of the skeleton material and the carbon material is 50%-99%.

[0027] Carbon material should be uniformly applied to the surface of the biomass carbon source to form a continuous and dense coating layer. If there is too much carbon material, the proportion of biomass carbon source in the core will be too small, resulting in a low proportion of biomass volatile components. During medium-temperature heat treatment, the carbon material will not be able to exert a strong impact on the coating layer, resulting in fewer pores formed inside the coating layer and failing to achieve a good pore-forming effect. If there is too little carbon material, the proportion of biomass carbon source in the core will be too large, resulting in a high proportion of biomass volatile components. During medium-temperature heat treatment, the impact on the coating layer will be too strong, causing the coating layer to crack or even peel off, making it impossible to form a structurally stable core-shell structure.

[0028] Preferably, in step S6, ultrasonic sieving is performed using a 200-400 mesh sieve, and more preferably, ultrasonic sieving is performed using a 325 mesh sieve.

[0029] Generally, smaller particle size in hard carbon anode materials helps shorten the diffusion path of sodium ions in the material, reduce diffusion resistance, and achieve higher charge / discharge rates and better cycle stability. However, excessively small particle sizes increase processing difficulty and may lead to an increase in irreversible capacity and exacerbate SEI film formation, affecting the cycle stability and safety of the battery. Several experiments have verified that products sieved according to the above requirements not only have high specific capacity but also good charge / discharge rates, high cycle stability and safety, and lower processing costs. Furthermore, the narrower particle size distribution can further improve the electrode's compaction density and volumetric energy density.

[0030] Preferably, in step S1: the pre-crushing device includes one or more of ball mills, mechanical mills, and air jet mills; the grading and screening device includes one or two of mechanical classifiers and air jet classifiers; in step S2: the mixing device includes one or two of horizontal stirred tanks and vertical stirred tanks, wherein the vertical stirred tank is preferably a vertical centrifugal extrusion tank; in step S3: the device used for surface coating is a horizontal or vertical tank; the medium-temperature heat treatment in step S4 and the high-temperature heat treatment in step S5 are both carried out in a carbonization furnace, wherein the medium-temperature heat treatment is preferably carried out in a pusher kiln, and the high-temperature heat treatment is preferably carried out in a roller kiln.

[0031] It should be noted that the above-mentioned device is only the device actually used in this application. Other existing devices with similar functions in the field may also be used, and this invention does not impose any restrictions or requirements.

[0032] On the other hand, this invention proposes a hard carbon anode material with a customized pore structure prepared according to the above preparation method. The pore size exhibits a gradient distribution from small to large from the core to the shell, ensuring unobstructed sodium ion transport channels, which is conducive to the insertion and extraction of sodium ions and can significantly improve the specific capacity of sodium batteries.

[0033] Compared with existing technologies, this invention uses a biomass carbon source embedded with carbonates as the skeleton material and carbon materials as the coating material. Through medium-temperature and high-temperature heat treatment, it achieves a gradient design of the hard carbon anode material from core to shell pore size, ensuring unobstructed sodium ion transport channels. The resulting product has abundant sodium storage sites, which helps to improve the specific capacity of sodium batteries. The preparation method proposed in this invention is simple, efficient, rapid, and energy-saving, and suitable for industrial application. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 TEM image of the product prepared in Example 1;

[0036] Figure 2 SEM image of the product prepared in Example 1;

[0037] Figure 3 The adsorption-desorption curves and pore size distribution curves of the product prepared in Example 1;

[0038] Figure 4 The 0.1C constant current charge-discharge curve is obtained after the product prepared in Example 1 is assembled into a coin cell. Detailed Implementation

[0039] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0040] All raw materials, reagents, instruments, and equipment used in the following examples and comparative examples can be purchased from the market or prepared by existing methods. Unless otherwise specified, they are all commercially available products.

[0041] Example 1

[0042] The preparation method is as follows:

[0043] S1. Using medicinal residue as biomass raw material, first use a mechanical mill to grind it to D50 of 20μm, then use an air classifier to classify the ground material and screen out materials with D10 of 2μm, D50 of 9μm and maximum particle size Dmax of 30μm.

[0044] S2. Mix the material obtained in the previous step with ammonium carbonate in a vertical centrifugal extruder at a mass ratio of 85:15.

[0045] S3. The product obtained in the previous step and phenolic resin are coated in a horizontal reactor at a mass ratio of 85:15. The specific conditions for surface coating are: heating to 120°C at 2°C / min, under nitrogen atmosphere protection, nitrogen flow rate of 200ml / min, and maintaining the temperature at 120°C for 2h.

[0046] S4. Place the product from the previous step into a pusher kiln for pore formation. The specific conditions for pore formation are: heating to 400℃ at a rate of 2℃ / min, under nitrogen atmosphere protection, with a nitrogen flow rate of 250ml / min, and maintaining the temperature at 400℃ for 3 hours.

[0047] S5. Place the product from the previous step into a roller kiln for carbonization. The specific conditions for carbonization are: heat up to 1200℃ at a rate of 2℃ / min, under nitrogen atmosphere protection, with a nitrogen flow rate of 300ml / min, and maintain the temperature at 1200℃ for 2 hours.

[0048] S6. Pass the product from the previous step through a 325-mesh sieve and then ultrasonically sieve to obtain the finished product.

[0049] Example 2

[0050] The preparation method is as follows:

[0051] S1. Using walnut shells as biomass raw materials, the material is first ground by mechanical mill to a D50 of 15μm, and then the ground material is classified by air classifier to screen out materials with a D10 of 2.5μm, a D50 of 9.5μm, and a maximum particle size Dmax of 25μm.

[0052] S2. Mix the material obtained in the previous step with ammonium carbonate in a vertical centrifugal extruder at a mass ratio of 90:10.

[0053] S3. The product obtained in the previous step and phenolic resin are coated in a horizontal reactor at a mass ratio of 90:10. The specific conditions for surface coating are: heating to 250°C at 2°C / min, under nitrogen atmosphere protection, nitrogen flow rate of 200ml / min, and maintaining the temperature at 250°C for 2h.

[0054] S4. Place the product from the previous step into a pusher kiln for pore formation. The specific conditions for pore formation are: heating to 600℃ at a rate of 2℃ / min, under nitrogen atmosphere protection, with a nitrogen flow rate of 250ml / min, and maintaining the temperature at 600℃ for 3 hours.

[0055] S5. Place the product from the previous step into a roller kiln for carbonization. The specific conditions for carbonization are: heat up to 1250℃ at a rate of 2℃ / min, under nitrogen atmosphere protection, with a nitrogen flow rate of 300ml / min, and maintain the temperature at 1250℃ for 2 hours.

[0056] S6. Pass the product from the previous step through a 325-mesh sieve and then ultrasonically sieve to obtain the finished product.

[0057] Example 3

[0058] The preparation method is as follows:

[0059] S1. Using apricot shells as biomass raw materials, the material is first ground by mechanical milling until D50 is 15μm, and then the ground material is classified by air classifier to screen out the material with D10 of 2μm, D50 of 8μm and maximum particle size Dmax of 25μm.

[0060] S2. Mix the material obtained in the previous step with ammonium bicarbonate in a vertical centrifugal extruder at a mass ratio of 85:15.

[0061] S3. The product obtained in the previous step is coated with phenolic resin in a horizontal reactor at a mass ratio of 85:15. The specific conditions for surface coating are: heating to 280℃ at 2℃ / min, under nitrogen atmosphere protection, nitrogen flow rate of 200ml / min, and maintaining the temperature at 280℃ for 2h.

[0062] S4. Place the product from the previous step into a pusher kiln for pore formation. The specific conditions for pore formation are: heating to 500℃ at a rate of 2℃ / min, under nitrogen atmosphere protection, with a nitrogen flow rate of 250ml / min, and maintaining the temperature at 500℃ for 3 hours.

[0063] S5. Place the product from the previous step into a roller kiln for carbonization. The specific conditions for carbonization are: heat up to 1300℃ at a rate of 2℃ / min, under nitrogen atmosphere protection, with a nitrogen flow rate of 300ml / min, and maintain the temperature at 1300℃ for 2 hours.

[0064] S6. Pass the product from the previous step through a 325-mesh sieve and then ultrasonically sieve to obtain the finished product.

[0065] Example 4

[0066] Compared with Example 1, the pore-forming agent ammonium carbonate was replaced with an equal amount of ammonium bicarbonate, and all other aspects remained the same as in Example 1.

[0067] Example 5

[0068] Compared with Example 1, the pore-forming agent ammonium carbonate was replaced with an equal amount of sodium carbonate, and all other aspects remained the same as in Example 1.

[0069] Example 6

[0070] Compared with Example 1, the pore-forming agent ammonium carbonate was replaced with an equal amount of calcium carbonate, and all other aspects remained the same as in Example 1.

[0071] Example 7

[0072] The preparation method is as follows:

[0073] S1. Using medicinal residue as biomass raw material, first use a mechanical mill to grind it to D50 of 20μm, then use an air classifier to classify the ground material and screen out materials with D10 of 5μm, D50 of 10μm and maximum particle size Dmax of 40μm.

[0074] S2. Mix the material obtained in the previous step with ammonium carbonate in a vertical centrifugal extruder at a mass ratio of 85:15.

[0075] S3. The product obtained in the previous step and phenolic resin are coated in a horizontal reactor at a mass ratio of 85:15. The specific conditions for surface coating are: heating to 50°C at 1°C / min, under nitrogen atmosphere protection, nitrogen flow rate of 200ml / min, and maintaining the temperature at 50°C for 2 hours.

[0076] S4. Place the product from the previous step into a pusher kiln for pore formation. The specific conditions for pore formation are: heating to 100℃ at a rate of 3℃ / min, under nitrogen atmosphere protection, with a nitrogen flow rate of 250ml / min, and maintaining the temperature at 100℃ for 5 hours.

[0077] S5. Place the product from the previous step into a roller kiln for carbonization. The specific conditions for carbonization are: heat up to 900°C at a rate of 5°C / min, under nitrogen atmosphere protection, with a nitrogen flow rate of 300 ml / min, and maintain the temperature at 900°C for 10 hours.

[0078] S6. Pass the product from the previous step through a 325-mesh sieve and then ultrasonically sieve to obtain the finished product.

[0079] Example 8

[0080] The preparation method is as follows:

[0081] S1. Using medicinal residue as biomass raw material, first use a mechanical mill to grind it to D50 of 20μm, then use an air classifier to classify the ground material and screen out the material with D10 of 20μm, D50 of 8μm and maximum particle size Dmax of 40μm.

[0082] S2. Mix the material obtained in the previous step with ammonium carbonate in a vertical centrifugal extruder at a mass ratio of 85:15.

[0083] S3. The product obtained in the previous step and phenolic resin are coated in a horizontal reactor at a mass ratio of 85:15. The specific conditions for surface coating are: heating to 350℃ at 2℃ / min, under nitrogen atmosphere protection, nitrogen flow rate of 200ml / min, and maintaining the temperature at 350℃ for 2h.

[0084] S4. Place the product from the previous step into a pusher kiln for pore formation. The specific conditions for pore formation are: heating at 4℃ / min to 600℃, nitrogen atmosphere protection, nitrogen flow rate of 250ml / min, and maintaining the temperature at 600℃ for 5h.

[0085] S5. Place the product from the previous step into a roller kiln for carbonization. The specific conditions for carbonization are: heat up to 1500℃ at a rate of 6℃ / min, under nitrogen atmosphere protection, with a nitrogen flow rate of 300ml / min, and maintain the temperature at 1500℃ for 10 hours.

[0086] S6. Pass the product from the previous step through a 325-mesh sieve and then ultrasonically sieve to obtain the finished product.

[0087] Comparative Example 1

[0088] Compared with Example 1, no pore-forming agent ammonium carbonate was added, i.e. step S2 was omitted, and all other steps remained the same as in Example 1.

[0089] Comparative Example 2

[0090] Compared with Example 1, the proportion of pore-forming agent added was adjusted. The material in S2 was mixed with ammonium carbonate in a vertical centrifugal extruder at a mass ratio of 40:60. All other aspects remained the same as in Example 1.

[0091] Comparative Example 3

[0092] Compared with Example 1, the material specifications screened in step S1 have been adjusted. The material is graded and screened to be crushed and graded: first, it is ground with a mechanical mill to a D50 of 20μm, and then graded and screened to obtain materials with a D10 of 5μm, a D50 of 15μm, and a maximum diameter Dmax of 60μm. The rest are consistent with Example 1.

[0093] Comparative Example 4

[0094] Compared with Example 1, the addition ratio of the pore-forming agent was adjusted. The material in S2 was mixed with ammonium carbonate in a vertical centrifugal extruder at a mass ratio of 99.5:0.5. All other aspects remained the same as in Example 1.

[0095] Comparative Example 5

[0096] Compared with Example 1, the amount of carbon material was adjusted, and in step S3, the product and phenolic resin were mixed in a horizontal reactor at a mass ratio of 40:60. All other steps remained the same as in Example 1.

[0097] Comparative Example 6

[0098] Compared with Example 1, the amount of carbon material was adjusted, and in step S3, the product and phenolic resin were mixed in a horizontal reactor at a mass ratio of 99.5:0.5. All other steps remained the same as in Example 1.

[0099] The D50, specific surface area, micropore volume, and mesopore volume of the products obtained in the above examples and comparative examples were tested (high-purity nitrogen was used as the adsorbent gas, the test stability was liquid nitrogen temperature, and the relative pressure range was 1% to 99% of the saturated vapor pressure). Simultaneously, these were used as active materials to assemble coin cells and their reversible specific capacity and initial coulombic efficiency at 0.1C constant current charge-discharge were tested. When assembling the coin cells, a negative electrode slurry was prepared using a mass ratio of test product: conductive carbon black: CMC: SBR = 94.6:1.5:1.4:2.5. After coating and drying, a hard carbon negative electrode was obtained, and a sodium metal sheet was used as the counter electrode. 1.0M NaPF6 in diglyme = 100Vol% was used as the electrolyte. Specific test results are shown in Table 1 below, where the 0.1C constant current charge-discharge curve of the coin cell assembled in Example 1 is shown in Table 1 below. Figure 4 As shown.

[0100] Table 1

[0101]

[0102]

[0103] From Table 1, Figure 2 , Figure 3 and Figure 4 It can be known that:

[0104] (1) Under the technical parameters defined in this invention, the hard carbon anode materials obtained in each embodiment possess both high reversible specific capacity and high first-time efficiency. The morphology of the product obtained in Example 1 is as follows: Figure 1 and Figure 2 As shown, the morphology of the products in the other embodiments is similar, so no further drawings are provided. Figure 3 As shown: The adsorption-desorption curves of the product obtained in Example 1 exhibit strong microporous characteristics, with the pore size distribution curve showing a large number and high intensity of pores smaller than 2 nm. Figure 4As shown, the reversible specific capacity of the product obtained in Example 1 is as high as 390 mAh / g at 0.1C, with a first-efficiency of up to 91%. Clearly, both the reversible specific capacity and the first-efficiency are significantly superior to the prior art shown in the background section. More importantly, the preparation method proposed in this invention is simple, efficient, rapid, and energy-saving, contributing to improved production efficiency and thus leading to considerable economic benefits.

[0105] Furthermore, this invention investigated the pore-forming effects of various pore-forming agents. Data from Examples 1, 4, 5, and 6 show that the pore-forming agents specified in this invention can all achieve good pore-forming effects. Under the same dosage and other conditions, the pore-forming effects of the pore-forming agents are as follows: ammonium carbonate is the best, followed by ammonium bicarbonate, then calcium carbonate, and finally sodium carbonate.

[0106] (2) As can be seen from the test data of Example 1 and Comparative Example 1, for products with the same particle size, the specific surface area of ​​the product with added pore-forming agent is larger, the micropore volume and mesopore volume are larger, and thus it has more sodium storage sites, so the reversible specific capacity and first-time efficiency are significantly improved.

[0107] Furthermore, the amount of pore-forming agent added is subject to strict requirements. Test data from Example 1 and Comparative Examples 2 and 4 show that when the amount of pore-forming agent added exceeds the limit, although the micropore volume and mesopore volume in the product increase significantly, the reversible specific capacity and first-time efficiency deteriorate significantly compared to when no pore-forming agent is added. This indicates that the pores generated by excessive pore-forming agent do not contribute to sodium storage and therefore do not improve the relevant electrochemical performance. When the amount of pore-forming agent added is less than the limit, the micropore volume and mesopore volume in the product increase compared to when no pore-forming agent is added, and the reversible specific capacity and first-time efficiency are slightly improved, but the improvement is not significant.

[0108] Moreover, even with the addition of an appropriate amount of pore-forming agent, resulting in a rich number of micropores and mesopores, an inappropriate amount of outer carbon material coating can hinder the improvement of reversible specific capacity and first-time efficiency. As shown in Comparative Example 5: Excessive carbon material coating results in a thick and dense coating layer, which prevents the relatively small proportion of biomass volatile components during intermediate-temperature heat treatment from effectively impacting the thick and dense coating layer, thus leading to poor pore-forming effect and very small micropore and mesopore volumes in the product. As shown in Comparative Example 6: Insufficient carbon material coating results in a relatively large proportion of biomass volatile components during intermediate-temperature heat treatment, causing excessive impact on the coating layer and resulting in an unstable coating layer structure, which is not conducive to improving reversible specific capacity and first-time efficiency.

[0109] (3) As can be seen from the test data of Example 1 and Comparative Example 3, when the particle size of the biomass raw material is larger than the particle size range defined in this invention, its specific surface area is smaller, resulting in lower reactivity. Although it can improve the reversible specific capacity and first-time efficiency of sodium batteries, the improvement effect is relatively limited. Therefore, biomass raw materials need to be selected within the particle size range defined in this invention in order to obtain a higher specific surface area and higher reactivity, so as to fully improve the performance of the negative electrode material.

[0110] In summary, this invention utilizes a biomass carbon source embedded with carbonates as the skeleton material and carbon materials as the coating material. Through medium-temperature and high-temperature heat treatments, a gradient design of the hard carbon anode material's pore size from small to large from the core to the shell is achieved, ensuring unobstructed sodium ion transport channels. The resulting product has abundant sodium storage sites, which helps improve the specific capacity of sodium batteries. The preparation method proposed in this invention is simple, efficient, rapid, and energy-saving, making it suitable for industrial-scale application.

[0111] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make modifications, alterations, substitutions, and variations to the above embodiments within the scope of the present invention. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of the different embodiments or examples.

Claims

1. A method for preparing a hard carbon anode material with a customized pore structure, characterized in that, The following steps are included: S1. Pre-crush the biomass carbon source and grade and screen the material to obtain the target particle size; S2. Mix the material obtained in the previous step with carbonate, so that the carbonate is embedded inside the material. At temperatures S3 and T1, the material obtained in the previous step is used as the skeleton material. Carbon material is used to coat the surface of the skeleton material to obtain a core-shell structure with the skeleton material as the core and the carbon material as the shell. At temperatures S4 and T2, the core-shell structure obtained in the previous step is subjected to medium-temperature heat treatment. The product obtained in the previous step is subjected to high-temperature heat treatment at temperatures S5 and T3. S6. Screening; Where: T3 > T2 > T1; In step S1, the biomass carbon source is pre-crushed to below 100μm and the material is graded and screened. The material meets the following conditions: D10≥2μm, D50=8-10μm, and maximum particle size Dmax≤50μm. In step S2, the carbonate includes one or more of sodium bicarbonate, ammonium bicarbonate, ammonium carbonate, and calcium carbonate, and the mass percentage of the material in the mixture of the material and the carbonate is 50%-99%. In step S3, the carbon material includes one or more of petroleum asphalt, coal tar pitch, phenolic resin, and epoxy resin, and the mass percentage of the skeleton material in the mixture of the skeleton material and the carbon material is 50%-99%.

2. The method for preparing a hard carbon anode material with a customized pore structure according to claim 1, characterized in that, T1 is 50-350℃, T2 is 100-600℃, and T3 is 900-1500℃.

3. The method for preparing a hard carbon anode material with a customized pore structure according to claim 1, characterized in that, In step S1, the biomass carbon source includes one or more of the following: coconut shells, wood, straw, walnut shells, peanut shells, apricot shells, and medicinal residue.

4. The method for preparing a hard carbon anode material with a customized pore structure according to claim 1, characterized in that, In step S6, ultrasonic sieving is performed using a 200-400 mesh sieve.

5. The method for preparing a hard carbon anode material with a customized pore structure according to claim 1, characterized in that, In step S6, ultrasonic sieving is performed using a 325-mesh sieve.

6. The method for preparing a hard carbon anode material with a customized pore structure according to claim 2, characterized in that, Steps S3, S4, and S5 all employ a rate-increasing heating method, and after reaching the target temperature, the reaction is kept at an inert atmosphere for a period of time. The inert atmosphere in steps S3, S4, and S5 is argon, nitrogen, or helium, with a flow rate of 10-500 ml / min, a heating rate of 1-10 °C / min, and a holding time of 0.5-4 h.

7. The method for preparing a hard carbon anode material with a customized pore structure according to claim 1, characterized in that, In step S1: the pre-pulverizing device includes one or more of ball mill, mechanical mill, and air jet mill; the grading and screening device includes one or two of mechanical classifier and air jet classifier; in step S2: the mixing device includes one or two of horizontal stirred tank and vertical stirred tank; in step S3: the device used for surface coating is a horizontal or vertical tank; the medium-temperature heat treatment in step S4 and the high-temperature heat treatment in step S5 are both carried out in a carbonization furnace.

8. The method for preparing a hard carbon anode material with a customized pore structure according to claim 7, characterized in that, The vertical mixing vessel is selected from the vertical centrifugal extrusion vessel.

9. The method for preparing a hard carbon anode material with a customized pore structure according to claim 7, characterized in that, The medium-temperature heat treatment is carried out in a pusher kiln, and the high-temperature heat treatment is carried out in a roller kiln.

10. A hard carbon anode material with a customized pore structure prepared by the preparation method according to any one of claims 1-9.

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