Preparation method of a high-rate modified biomass hard carbon material

Abstract

The present invention relates to a preparation method of a high-rate modified biomass hard carbon material, which comprises the following steps: after the biomass material is pretreated, two-stage carbonization is carried out, namely, the first-stage carbonization carried out in a nitrogen atmosphere containing a small amount of oxygen, and the second-stage carbonization in a nitrogen atmosphere; after carbonization, epoxidation and doping modification are carried out. The preparation process of the present invention is simple, the raw materials are cheap and easy to obtain, the obtained hard carbon material has excellent electrochemical performance, especially the rate performance is significantly improved, and it can meet the requirements of current sodium-ion batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of hard carbon materials for sodium-ion batteries, and particularly relates to a preparation method of a high-rate modified biomass hard carbon material. Background Art

[0002] Lithium-ion batteries have been widely used at present. However, due to the limited lithium resources, the price of lithium sources has been increasing year by year. Scientists have been looking for other types of batteries that can replace lithium batteries, such as aluminum-ion batteries and sodium-ion batteries. Among them, sodium-ion batteries are a form that has been studied more and is expected to be industrialized. However, compared with lithium-ion batteries, sodium-ion batteries mainly have the following defects: low energy density, poor cycle stability, and low initial efficiency. This is because the relatively large radius and atomic mass of Na + ions result in poor sodium ion deintercalation kinetics performance, making it difficult for sodium ions to be inserted into the anode material, so that conventional silicon-based anode materials and graphite anode materials are difficult to be used in sodium-ion batteries. Currently, hard carbon and soft carbon are the most developed anode materials for sodium-ion batteries. Both belong to disordered carbon materials. The difference is that hard carbon has a low degree of graphitization, a more abundant disordered structure, and a larger carbon layer spacing, which is convenient for providing more active sites for Na + insertion. It is a promising anode material for sodium-ion batteries for industrialization. Currently, hard carbon materials are a hot research direction for sodium-ion batteries, especially biomass hard carbon materials, because of their wide sources and low prices, which are practical industrial production solutions for low-cost and large-scale solving of anode materials for sodium-ion batteries. However, based on the performance of biomass hard carbon materials, there are still many disadvantages, such as low capacity, low initial efficiency, and low energy density. Therefore, currently, biomass hard carbon materials are mostly in the research stage and have not been widely applied in practice.

[0003] When preparing biomass hard carbon materials for the anode of sodium-ion batteries, generally, biomass materials are sequentially subjected to carbonization and activation steps to form hard carbon materials with a rich disordered structure. Currently, when preparing hard carbon materials, much research has been done on the selection of biomass materials, the process conditions of carbonization and calcination, etc., and the better process conditions have been basically explored. However, the electrochemical performance of the obtained hard carbon materials is still not satisfactory, and there is still a certain gap from commercialization. In order to further improve the electrochemical performance of hard carbon materials in sodium-ion batteries for early practical application and industrialization, currently, modifying hard carbon materials is a hot research direction. The modification mainly has two methods. One is to modify the biomass materials before carbonization, and the other is to modify after carbonization. The modification methods include impregnation, pretreatment, grafting, doping, crosslinking, etc.

[0004] CN1165553518A discloses a preparation method of a hard carbon negative electrode material, which involves crushing biomass materials such as cotton seeds, walnut shells, and apricot shells, carbonizing them at 1000 - 1200 °C, pickling to remove impurities, drying, adding a cross-linking agent, a dispersing agent, and a pore-forming agent to form nano micropores on the surface of the carbon source. The modified hard carbon material improves the sodium ion transport channels, obtains more sodium storage sites, and enhances the electrochemical performance of the hard carbon material. However, the patent process is complex, and when adding the pore-forming agent, if the process is not properly controlled, the porosity of the hard carbon material will be too large, which is instead not conducive to the performance of the negative electrode electrochemistry.

[0005] CN11323440A discloses a hard carbon negative electrode material, which has a high first efficiency and excellent cycle life. However, carbon nanotubes are added to the preparation raw materials to increase the material conductivity. The price of carbon nanotubes is expensive, and even if the addition amount is small, it will still increase the production cost of the hard carbon material, deviating from the original intention of using biomass carbon sources that are cheap and easily available.

[0006] CN116854075A discloses a chemically surface-modified biomass hard carbon material and its preparation method. After the biomass raw materials are dried, crushed, and pre-carbonized, they are placed in an atmospheric plasma sintering furnace and processed by high-temperature carbonization. Through the plasma reduction atmosphere treatment, this patent can effectively reduce the surface oxygen-containing functional groups. On the one hand, it can improve the hydrophobicity of the hard carbon material surface, solve the problem of battery gas production and bulging during the charge and discharge process caused by the easy adsorption of water molecules by the surface hydrophilic groups, and improve the cycle stability. On the other hand, it can reduce the irreversible adsorption of sodium ions during the first charge and discharge process, effectively improving the low first Coulomb efficiency of the hard carbon material. However, plasma modification requires expensive equipment and complex operations, which is also not conducive to large-scale industrial production.

[0007] In addition, for hard carbon materials, the interlayer spacing is large, so they have a larger specific capacity. However, due to the complex pore structure, sodium that cannot be inserted / extracted is generated, resulting in a low first-cycle Coulomb efficiency and a reversible capacity lower than the theoretical capacity.

[0008] The inventor's previous patent CN202311081553.X discloses a modified biomass hard carbon material, in which the carbonized particles are successively impregnated and modified with epoxidation and tris[2-(3-mercaptopropionate)ethyl]isocyanurate solution, and then calcined to obtain modified carbonized particles. However, there is surface deposition of sodium metal during the high-rate cycling process. After analysis, we believe that in order to reduce the oxygen-containing functional groups of the carbon material, carbonization is carried out in a non-oxidizing atmosphere, such as a nitrogen atmosphere. However, this will reduce the efficiency of subsequent epoxidation modification, resulting in low modification efficiency, or else the surface modification is not uniform enough. Summary of the Invention

[0009] To solve the defect that the performance of biomass hard carbon anode materials in the prior art cannot meet the actual needs, especially the electrochemical performance at high rates is not satisfactory, the present invention proposes a preparation method of a high-rate modified biomass hard carbon material. The present invention is an improvement based on the previous patent CN202311081553.X. The main change is in the process conditions. Instead of carbonization in a nitrogen atmosphere, it is changed to two-stage carbonization. First, short-time carbonization is carried out in a nitrogen atmosphere containing a small amount of oxygen, and then the temperature is raised to carry out long-time carbonization in a nitrogen atmosphere. According to the above two-stage carbonization, the number of functional groups on the surface of the obtained carbon material can be increased and enriched, which is convenient for subsequent modification such as epoxidation. However, it is necessary to control the oxygen content and the carbonization process conditions. Otherwise, not only the electrochemical performance of the hard carbon material cannot be improved, but it may also affect the cycle performance of the battery and even the safety performance. The present invention solves the above technical problems through the following technical solutions:

[0010] A preparation method of a high-rate modified biomass hard carbon material, comprising the following steps:

[0011] (S1) The biomass is successively subjected to crushing, hydrothermal reaction under alkaline conditions, water washing, alcohol washing, and drying;

[0012] (S2) The biomass obtained in step (S1) is subjected to two-stage carbonization. The first-stage carbonization is carried out in a nitrogen atmosphere containing oxygen, where the volume percentage of oxygen is 5-10%, the carbonization temperature is 300-400 °C, and the carbonization time is 2-3 h; the second-stage carbonization is carried out in a nitrogen atmosphere at 500-600 °C for a carbonization time of 4-6 h;

[0013] (S3) The carbonized material is subjected to flotation, hydrochloric acid rinsing, water washing, and grinding to obtain carbonized particles;

[0014] (S4) The carbonized particles are successively subjected to epoxidation, impregnation with tris[2-(3-mercaptopropionate)ethyl] isocyanurate solution, water washing, and alcohol washing to obtain modified carbonized particles;

[0015] (S5) The modified carbonized particles are calcined in a graphite furnace, followed by secondary hydrochloric acid rinsing, water washing, drying, and grading to obtain the high-rate modified biomass hard carbon material.

[0016] Further, in step (S1), the biomass is selected from at least one of coconut shell, palm shell, bamboo block, walnut shell, and apricot shell. The present invention uses raw materials from large-scale sources to ensure the needs of large-scale industrial production.

[0017] Further, in step (S1), the crushing is to crush to 80 - 325 mesh. The crushing method is not particularly limited as long as the specified mesh number is achieved after crushing. For example, jet milling, high-speed crushing, ball milling, etc. can be used. The hydrothermal reaction under alkaline conditions is to react the crushed biomass material in an aqueous solution of 5 - 10 wt% NaOH and / or KOH, heat it to 80 - 100 °C and react for 4 - 6 h. The water washing is that the effluent of the water washing is neutral (pH = 7 - 7.5), the alcohol washing is to wash with ethanol, and the drying is not particularly limited. For example, oven drying, vacuum drying can be used.

[0018] Further, in step (S2), the carbonization is carried out in a rotary kiln with a rotation speed of 0.5 - 2 rpm. Preferably, in the first-stage carbonization, the volume content of oxygen is 6.2 - 7.5%. The present invention needs to control appropriate first-stage carbonization process conditions, including oxygen content, carbonization temperature and time. Otherwise, not only the electrochemical performance of the hard carbon material cannot be improved, but it may also be reduced.

[0019] Further, in step (S3), the flotation is by the way of flowing and foaming flotation to separate the light components and heavy components of the carbonized material. By installing an electromagnetic device in the flotation cell, iron impurities and ash components such as sediment and sand grains can be preliminarily removed, reducing the ash content and iron content of the carbonized material. The hydrochloric acid rinsing is to mix and stir evenly the dilute hydrochloric acid solution and the carbonized material after flotation in a reaction kettle, and treat it under reflux conditions for 2 - 4 h. The concentration of the dilute hydrochloric acid is 3 - 8 wt%, and the dosage of the dilute hydrochloric acid is 4 - 6 times the mass of the carbonized material after flotation. The purpose of the rinsing is to remove metal ash impurities. The water washing is to wash until the effluent is neutral (pH = 6.8 - 7), and the grinding is to grind the material to D50 of 5 - 10 μm. The grinding equipment is not particularly limited and can be conventional in the art. For example, air jet grinding equipment can be used.

[0020] Further, in step (S4), for the epoxidation, the carbonized particles are fed into a 3-5 wt% NaOH and / or KOH solution with a mass multiple of 5-10, heated to 50-60 °C, and under an inert atmosphere, epichlorohydrin with a mass multiple of 0.22-0.28 of the carbonized particles is slowly added, and the mixture is kept warm and reacted for 4-6 h. Then, solid NaOH and / or KOH with a mass multiple of 0.1-0.15 of the carbonized particles is added, and the reaction continues under warming for 2-4 h. The solvent and unreacted epichlorohydrin are removed by vacuum distillation, and after drying, epoxidized carbonized particles are obtained; for the impregnation with tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate solution, the obtained epoxidized carbonized particles are impregnated into a solution of tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate with a mass multiple of 10-15, an organic amine is added, and the reaction is carried out under warming at 50-60 °C for 4-6 h. The solvent in the solution is at least one of tetrahydrofuran and dioxane, the mass concentration of tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate is 10-15 wt%, and the addition amount of the organic amine is 10-20% of the mass of tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate. The organic amine is selected from at least one of triethylamine and trimethylamine. The reaction activity of epoxy groups and mercapto groups is slightly insufficient, and adding a small amount of organic amine as a catalyst can significantly improve the reaction efficiency.

[0021] Before modifying the carbonized particles, the present invention adopts a two-stage carbonization process. In particular, in the first stage, oxygen-containing functional groups such as hydroxyl groups and carboxyl groups are formed on the surface of the carbonized particles, which facilitates subsequent epoxidation modification, with a higher and more uniform degree of epoxidation. After epoxidation modification, the number of oxygen-containing functional groups on the surface of the carbon material decreases, which will not affect the electrochemical performance and stability of the hard carbon material.

[0022] The present invention creatively modifies the biomass carbon source. After epoxidation, it reacts with tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate under the catalysis of organic amine to introduce the doping of N and S, and form a certain cross-linked structure, reducing the defects of the hard carbon material after sintering, improving the micro-morphology, and being more conducive to enhancing the electrochemical performance of the hard carbon material. Tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate serves the purpose of both cross-linking modification and doping modification at the same time. Heteroatom doping has been proven to be an effective means to improve the electrochemical performance of hard carbon anode materials. Introducing N doping is the most studied doping method, which can improve the activity and electronic conductivity of hard carbon materials. Some studies have used nitrogen sources such as urea and melamine to calcine together with biomass carbon sources to obtain N-doped hard carbon materials. The inventor unexpectedly found that modifying the biomass carbon source with tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate, simultaneously performing the doping of N and S, and cross-linking modification, and exerting a synergistic effect, can significantly improve the various electrochemical performances of the hard carbon material, and its reversible specific capacity, initial efficiency, cycle stability, and rate performance are all improved at the same time. Sulfur atoms have a relatively large size. When introduced into the hard carbon material, the interlayer spacing can be increased, facilitating the insertion / extraction of sodium ions; moreover, sulfur atoms themselves have electrochemical activity, increasing the sodium storage capacity. The inventor has tried other substances containing nitrogen and sulfur elements, such as cysteine, poly(3,4-ethylenedioxythiophene) (Advanced Science, 2015, 2(12), 195-201), etc., but none of them have achieved obvious improvement in the electrochemical performance of the hard carbon material obtained with tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate as the doping source. Although the reason is unknown, using [2-(3-mercaptopropionyloxy)ethyl] isocyanurate as the doping source for hard carbon materials is reported for the first time, and it greatly improves the electrochemical performance of hard carbon materials.

[0023] Further, in step (S5), the calcination is carried out in a staged gradient manner. The gradient calcination treatment is carried out in a graphite furnace. First, the temperature is raised at a heating rate of 5-10 °C / min to 700-800 °C and held for 1-2 h, then the temperature is raised at a heating rate of 5-10 °C / min to 1000-1300 °C and held for 2-4 h, then cooled to 300-500 °C and held for 1-2 h, and finally cooled to room temperature. The gradient calcination treatment is beneficial to adjusting the micro-morphology of the carbonized material and obtaining a hard carbon material suitable for the interlayer spacing.

[0024] Further, in step (S5), the secondary rinsing is carried out under the same conditions as the aforementioned rinsing step. Specifically, in the reaction kettle, the dilute hydrochloric acid solution and the carbonized material after gradient calcination are mixed and stirred evenly, and treated under reflux conditions for 2 - 4 h. The concentration of the dilute hydrochloric acid is 3 - 8 wt%, and the dosage of the dilute hydrochloric acid is 4 - 6 times the mass of the carbonized material after gradient calcination. The purpose of rinsing is to further remove metal ash impurities and improve the quality of the hard carbon material. The drying is carried out until the moisture content ≤ 1%. The drying process is not particularly limited, such as paddle dryer, vacuum drying, etc.; the classification is to classify the dried semi-finished product to meet the particle size requirements: D10 > 2 μm, D50 is 5 - 10 μm, D90 ≤ 25 μm. After classification, the finished modified biomass hard carbon material is obtained.

[0025] The present invention also provides a sodium ion battery, the negative electrode active material of which comprises the modified biomass hard carbon material prepared by the above preparation method.

[0026] The present invention uses biomass materials, which are cheap and easily available, as the carbon source. After two-stage carbonization, the first-stage carbonization is carried out at a lower temperature and for a shorter time in an atmosphere containing a small amount of oxygen, and then the second-stage carbonization is carried out at a higher temperature and for a longer time in a nitrogen atmosphere. The surface of the carbon particles after two-stage carbonization is rich in active functional groups and is more evenly distributed, which facilitates the subsequent epoxidation modification. Then, the impregnation reaction of tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate is carried out, and finally the modified carbonized particles are obtained. After calcination, the biomass hard carbon material has excellent electrochemical performance, especially the electrochemical performance at high rates is significantly improved. This is attributed to the combination of the two-stage carbonization process and the modification process of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the XRD pattern of the hard carbon material prepared in Example 1.

[0028] Figure 2 is the SEM image of the hard carbon material prepared in Example 1.

[0029] Figure 3 is the SEM image of the hard carbon material prepared in Example 2. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be described in detail below. The following embodiments facilitate a better understanding of the present invention, but do not limit the present invention. The experimental methods in the following embodiments are all conventional methods unless otherwise specified.

[0031] Example 1

[0032] (S1) The coconut shell is successively crushed to 200 meshes by a high-speed crusher and then impregnated in a 5wt% NaOH aqueous solution at 50°C under stirring conditions of 100 rpm for 4 hours. After that, it is washed with water until the pH of the effluent is 7.5;

[0033] (S2) The crushed coconut shell obtained in step (S1) is fed into a rotary atmosphere furnace for two-stage carbonization. The rotation speed of the rotary furnace is set at 1 rpm. The first-stage carbonization is carried out in a mixed atmosphere of nitrogen and oxygen (where the volume percentage of oxygen is 6.2%) at 300°C for 3 hours, and then the second-stage carbonization is carried out in a nitrogen atmosphere at 600°C for 5 hours. After two-stage carbonization, coconut shell charcoal is obtained with a volatile content of ≤3wt%;

[0034] (S3) The coconut shell charcoal is subjected to flowing bubbling flotation to separate the light and heavy components of the carbonized material and remove ash components such as iron impurities, sediment, and sand grains. The flotation-treated coconut shell charcoal is fed into a reaction kettle, and 5wt% dilute hydrochloric acid 5 times the mass of the coconut shell charcoal is added, and the coconut shell charcoal is rinsed with hydrochloric acid under heating and reflux conditions. After that, it is washed with water until the effluent is neutral, and then ground to a particle size of about 7μm by a pneumatic grinding device to obtain carbonized particles;

[0035] (S4) The carbonized particles are fed into a 5wt% NaOH aqueous solution 6 times the mass, heated to 50°C, and 0.25 times the mass of the carbonized particles of epichlorohydrin is slowly added under stirring conditions in a nitrogen atmosphere, and the reaction is carried out at a constant temperature for 4 hours. Then, 0.1 times the mass of the carbonized particles of solid NaOH is added, and the reaction continues at a constant temperature for 2 hours. The solvent and unreacted epichlorohydrin are removed by vacuum distillation, and dried in a vacuum oven to obtain epoxidized carbonized particles. The obtained epoxidized carbonized particles are impregnated in a tetrahydrofuran solution of tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate 10 times the mass of the epoxidized carbonized particles, 10% of triethylamine based on the mass of tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate is added, and the reaction is carried out at 50°C for 6 hours. After cooling to room temperature, the obtained carbonized particles are first washed with water, then washed with ethanol, and dried to obtain modified carbonized particles;

[0036] (S5) The obtained modified carbonized particles are subjected to gradient calcination treatment in a graphite furnace. First, it is heated to 700°C at a heating rate of 5°C / min and held for 1 hour; then it is heated to 1000°C at a heating rate of 5°C / min and held for 4 hours, cooled to 500°C and held for 1 hour, and finally cooled to room temperature. The obtained carbon material is fed into a reaction kettle, and 5wt% dilute hydrochloric acid 4 times the mass of the carbon material is added, and the second hydrochloric acid rinse is carried out under heating and reflux conditions to further remove metal ash impurities. It is washed with water to remove excess salts and other impurities, dried in a vacuum oven until the moisture content is ≤1%, and the dried semi-finished product is classified to meet the particle size requirements: D10>2μm, D50 is 5 - 10μm, D90≤25μm. After classification, the finished modified biomass hard carbon material is obtained.

[0037] Figure 1 It is the XRD pattern of the hard carbon material obtained in Example 1.

[0038] Figure 2 It is the SEM image of the hard carbon material obtained in Example 1. The particle size of the obtained hard carbon material is around 5 - 10 μm, and the particle size distribution is uniform.

[0039] Example 2

[0040] Other conditions and operations are the same as those in Example 1, except that the biomass material coconut shell is replaced by palm shell. Figure 3 It is the SEM image of the hard carbon material obtained in Example 2, and its microstructure is similar to that of the hard carbon material prepared with coconut shell as the raw material.

[0041] Example 3

[0042] Other conditions and operations are the same as those in Example 1, except that in step (S2), in the first - stage carbonization, the volume percentage of oxygen is 7.5%.

[0043] Example 4

[0044] Other conditions and operations are the same as those in Example 1, except that in step (S2), in the first - stage carbonization, the volume percentage of oxygen is 5%.

[0045] Example 5

[0046] Other conditions and operations are the same as those in Example 1, except that in step (S2), in the first - stage carbonization, the volume percentage of oxygen is 10%.

[0047] Comparative Example 1

[0048] Other conditions and operations are the same as those in Example 1, except that step (S4) is omitted, and the carbonized particles obtained in step (S3) are directly subjected to gradient calcination and other treatments in step (S5).

[0049] Comparative Example 2

[0050] Other conditions and operations are the same as those in Example 1, except that step (S2) is changed to: carbonize at 300 °C for 8 h in a mixed nitrogen - oxygen atmosphere (where the volume percentage of oxygen is 6.2%).

[0051] Comparative Example 3

[0052] Other conditions and operations are the same as those in Example 1, except that step (S2) is changed to: carbonize at 600 °C for 6 h in a nitrogen atmosphere.

[0053] Comparative Example 4

[0054] Other conditions and operations are the same as those in Example 1, except that step (S2) is changed to: the first-stage carbonization is carried out in a nitrogen atmosphere at 600 °C for 5 h; the second-stage carbonization is carried out in a mixed nitrogen-oxygen atmosphere (where the volume percentage of oxygen is 6.2%) at 300 °C for 3 h.

[0055] Comparative Example 5

[0056] Other conditions and operations are the same as those in Example 1, except that step (S4) is changed to: the carbonized particles are put into a 5 wt% NaOH aqueous solution with a mass multiple of 6, heated to 50 °C, and epichlorohydrin with a mass multiple of 0.25 of the carbonized particles is slowly added under stirring in a nitrogen atmosphere. After holding the temperature for reaction for 4 h, solid NaOH with a mass multiple of 0.1 of the carbonized particles is added, and the reaction is continued while holding the temperature for 2 h. The solvent and unreacted epichlorohydrin are removed by vacuum distillation, and then dried in a vacuum oven to obtain epoxidized carbonized particles; the obtained epoxidized carbonized particles are impregnated in a 10 wt% ethanol solution of cysteine with a mass multiple of 10 of the epoxidized carbonized particles, triethylamine with 10% of the mass of cysteine is added, and the reaction is carried out while holding the temperature at 50 °C for 6 h. After cooling to room temperature, the obtained carbonized particles are filtered, washed with water first, then washed with ethanol, and dried to obtain modified carbonized particles; that is, tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate is replaced with an equal mass of cysteine.

[0057] Comparative Example 6

[0058] Other conditions and operations are the same as those in Example 1, except that step (S4) is changed to: the carbonized particles are put into a 5 wt% NaOH aqueous solution with a mass multiple of 6, heated to 50 °C, and epichlorohydrin with a mass multiple of 0.25 of the carbonized particles is slowly added under stirring in a nitrogen atmosphere. After holding the temperature for reaction for 4 h, solid NaOH with a mass multiple of 0.1 of the carbonized particles is added, and the reaction is continued while holding the temperature for 2 h. The solvent and unreacted epichlorohydrin are removed by vacuum distillation, and then dried in a vacuum oven to obtain epoxidized carbonized particles; the obtained epoxidized carbonized particles are impregnated in a 10 wt% ethanol solution of polyethylenedioxythiophene with a mass multiple of 10 of the epoxidized carbonized particles, triethylamine with 10% of the mass of cysteine is added, and the reaction is carried out while holding the temperature at 50 °C for 6 h. After cooling to room temperature, the obtained carbonized particles are filtered, washed with water first, then washed with ethanol, and dried to obtain modified carbonized particles; that is, tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate is replaced with an equal mass of polyethylenedioxythiophene.

[0059] Effect Example

[0060] The modified biomass hard carbon materials obtained from the above examples and comparative examples were tested. Specifically, the hard carbon materials prepared in the examples or comparative examples were used as active ingredients and mixed with 1% CMC as the binder, Super P and SBR as conductive agents according to a mass ratio of 1.88:2.4:0.05:0.12. They were mixed and dispersed in a degassing machine into a uniform negative electrode slurry. An automatic film drying machine was used, and the negative electrode slurry was evenly coated on an aluminum foil current collector with a doctor blade. The coating thickness was 140 ± 1 μm, and a circular electrode sheet with a diameter of 14 mm was made after vacuum drying. A sodium sheet was used as the counter electrode, a glass fiber was used as the separator, and the electrolyte was 1 M NaClO4 in EC:PC = 1:1 with 5% FEC. The half-cell was assembled in a glove box. After assembly, it was tested at a current density of 12 mAh·g -1 (0.1C), and the test range was 0.001 - 2.0 V. Table 1 below is the ingredient list of the negative electrode material for the sodium-ion battery of the present invention.

[0061] Table 1 Ingredient List of Negative Electrode Material for Sodium-Ion Battery

[0062]

[0063] The test results of the electrochemical performance are shown in Table 2 below.

[0064] Table 2 Performance Test of Hard Carbon Material

[0065]

[0066]

[0067] It can be seen that through the improved preparation method of the present invention, specifically by two-stage carbonization, where the first-stage carbonization is low-temperature carbonization in a nitrogen atmosphere containing a small amount of oxygen, the temperature is increased in the second-stage carbonization, and carbonization is carried out in a nitrogen atmosphere, followed by epoxidation of the carbonized particle surface and doping modification with N- and S-containing substances. The finally obtained modified biomass hard carbon material has excellent electrochemical performance, especially the rate performance is significantly improved, which further promotes the practical application of such hard carbon materials in sodium-ion batteries.

Claims

1. A preparation method of a high-rate modified biomass hard carbon material, characterized in that It includes the following steps: (S1) Biomass is successively subjected to crushing, hydrothermal reaction under alkaline conditions, water washing, alcohol washing, and drying; (S2) The biomass obtained in step (S1) is subjected to two-stage carbonization. The first-stage carbonization is carried out in a nitrogen atmosphere containing oxygen, where the volume percentage of oxygen is 5-10%, the carbonization temperature is 300-400 °C, and the carbonization time is 2-3 h; the second-stage carbonization is carried out in a nitrogen atmosphere at 500-600 °C for 4-6 h; (S3) The carbonized material is subjected to flotation, hydrochloric acid rinsing, water washing, and grinding to obtain carbonized particles; (S4) The carbonized particles are successively subjected to epoxidation, impregnation with tris[2-(3-mercaptopropionyloxy)ethyl]isocyanurate solution, water washing, and alcohol washing to obtain modified carbonized particles; (S5) The modified carbonized particles are calcined in a graphite furnace, followed by secondary hydrochloric acid rinsing, water washing, drying, and grading to obtain the high-rate modified biomass hard carbon material.

2. The preparation method according to claim 1, wherein In step (S1), the biomass is selected from at least one of coconut shell, palm shell, bamboo block, walnut shell, and apricot shell; the crushing is to crush to 80-325 mesh; the hydrothermal reaction under alkaline conditions is to heat the crushed biomass material in a 5-10 wt% NaOH and / or KOH aqueous solution to 80-100 °C and react for 4-6 h; the water washing is such that the effluent is neutral, the alcohol washing is with ethanol, and the drying is oven drying or vacuum drying.

3. The preparation method according to claim 1, characterized in that, In step (S2), the carbonization is carried out in a rotary kiln with a rotation speed of 0.5-2 rpm; in the first-stage carbonization, the volume content of oxygen is 6.2-7.5%.

4. The preparation method according to claim 1, characterized in that, In step (S3), the flotation is over-flow bubbling flotation; the hydrochloric acid rinsing is to mix and stir evenly the dilute hydrochloric acid solution and the carbonized material after flotation in a reaction kettle, and treat under reflux conditions for 2-4 h, the concentration of the dilute hydrochloric acid is 3-8 wt%, and the dosage of the dilute hydrochloric acid is 4-6 times the mass of the carbonized material after flotation; the water washing is to wash until the effluent is neutral, and the grinding is to grind the material to D50 of 5-10 μm.

5. The preparation method according to claim 1, wherein In step (S4), the epoxidation is to feed the carbonized particles into a 3-5 wt% NaOH and / or KOH solution with a mass multiple of 5-10, heat to 50-60 °C, and slowly add epichlorohydrin with a mass multiple of 0.22-0.28 of the carbonized particles under an inert atmosphere, keep the temperature for reaction for 4-6 h, then add solid NaOH and / or KOH with a mass multiple of 0.1-0.15 of the carbonized particles, keep the temperature and continue to react for 2-4 h, distill off the solvent and unreacted epichlorohydrin under reduced pressure, and dry to obtain epoxidized carbonized particles.

6. The preparation method according to claim 1, characterized in that, In step (S4), the impregnation with tris[2-(3-mercaptopropionyloxy)ethyl]isocyanurate solution is to immerse the obtained epoxidized carbonized particles in a solution of tris[2-(3-mercaptopropionyloxy)ethyl]isocyanurate with a mass multiple of 10-15, add organic amine, and keep the temperature for reaction for 4-6 h at 50-60 °C, and the solvent in the solution is at least one of tetrahydrofuran and dioxane.

7. The preparation method according to claim 6, characterized in that, The mass concentration of tris[2-(3-mercaptopropionate)ethyl] isocyanurate is 10-15 wt%, and the addition amount of the organic amine is 10-20% of the mass of tris[2-(3-mercaptopropionate)ethyl] isocyanurate. The organic amine is selected from at least one of triethylamine and trimethylamine.

8. The preparation method according to claim 1, characterized in that, In step (S5), the calcination is carried out in a staged gradient manner. The gradient calcination treatment is carried out in a graphite furnace. First, the temperature is raised at a heating rate of 5-10 °C / min to 700-800 °C and held for 1-2 h. Then, the temperature is raised at a heating rate of 5-10 °C / min to 1000-1300 °C and held for 2-4 h. Then, it is cooled to 300-500 °C and held for 1-2 h. Finally, it is cooled to room temperature.

9. The preparation method according to claim 1, characterized in that, In step (S5), the secondary hydrochloric acid rinsing is carried out in a reaction kettle. The dilute hydrochloric acid solution and the carbonized material after gradient calcination are mixed and stirred evenly, and treated under reflux conditions for 2-4 h. The concentration of the dilute hydrochloric acid is 3-8 wt%, and the dosage of the dilute hydrochloric acid is 4-6 times the mass of the carbonized material after gradient calcination. The drying is carried out until the water content ≤ 1%. The classification is carried out on the dried semi-finished product to meet the particle size requirements: D10 > 2 μm, D50 is 5-10 μm, and D90 ≤ 25 μm.

10. A sodium-ion battery, the negative electrode active material of which comprises the high-rate modified biomass hard carbon material prepared by the preparation method according to any one of claims 1-9.

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