A method for preparing modified biomass hard carbon material for sodium ion batteries

Abstract

The present invention relates to a method for preparing a modified biomass hard carbon material. The modified biomass is subjected to ultraviolet light grafting and crosslinking modification using nitrogen-containing compounds containing two or more -C=C- double bonds, followed by co-calcination with an inorganic salt. This yields a modified biomass hard carbon material for sodium-ion batteries with excellent electrochemical performance, particularly significantly improved rate performance, meeting current sodium-ion battery requirements. The method utilizes ultraviolet light grafting modification, resulting in a short reaction time, high efficiency, low energy consumption, and a simple and easy-to-operate process.

Description

Technical Field

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

[0002] Lithium-ion batteries are currently widely used, but due to the limited availability of lithium resources, the price of lithium sources has been rising year by year. Sodium-ion batteries (SIBs) are the most promising next-generation commercial batteries after lithium-ion batteries (LIBs). The electrochemical performance of sodium-ion battery anode materials currently cannot meet actual industrial needs. The large radius and atomic mass of sodium ions result in poor sodium ion intercalation and deintercalation kinetics, making it difficult for them to be embedded in the anode material, resulting in poor electrochemical performance of sodium-ion batteries in many aspects, such as first efficiency, cycle stability, and rate performance. Hard carbon materials are currently the most widely studied sodium-ion battery anode materials. Biomass-derived hard carbon materials, due to their wide availability and low price, are currently promising sodium-ion battery anode materials for industrial large-scale production. However, current biomass hard carbon materials form low surface defects and narrow interlayer spacing structures during high-temperature carbonization, resulting in low sodium storage capacity and reduced initial charge and discharge capacity.

[0003] CN1165553518A discloses a method for preparing a hard carbon negative electrode material. This involves crushing biomass materials such as cotton seeds, walnut shells, and apricot shells, carbonizing them at 1000-1200°C, acid-washing to remove impurities, and drying them. A crosslinking agent, dispersant, and pore-forming agent are then added to form nanopores on the carbon source surface. The modified hard carbon material improves sodium ion transport channels, provides more sodium storage sites, and enhances the electrochemical performance of the hard carbon material. However, the patented process is complex, and the addition of a pore-forming agent can lead to excessive porosity in the hard carbon material if the process is not properly controlled, which can negatively impact the electrochemical performance of the negative electrode.

[0004] CN11323440A discloses a hard carbon anode material with high initial efficiency and excellent cycle life. Carbon nanotubes are added to the raw materials to increase the material's conductivity. However, carbon nanotubes are expensive, and even small additions can increase the production cost of the hard carbon material, deviating from the original intention of using biomass carbon sources, which are inexpensive and readily available.

[0005] CN114388736A discloses a method for preparing a hard carbon negative electrode for lithium / sodium ion batteries from biochar. The modified biochar is acid-washed, ball-milled, and carbonized to obtain the hard carbon negative electrode material. The modification involves immersing the biochar in a solution of DMF and m-aminophenylurea hydrochloride in a volume ratio of 1-5:3. While the process is relatively simple, the immersion modification effect is limited.

[0006] The inventors previously disclosed a method for preparing modified biomass hard carbon materials in patent CN117038976A. The method used tris[2-(3-mercaptopropionyl)ethyl]isocyanurate as a modifier, impregnating carbonized biomass particles and then calcining them to produce modified carbonized particles. However, the reversible specific capacity and cycle capacity retention at high rates were poor.

[0007] Therefore, there is an urgent need to develop a method for preparing hard carbon negative electrode materials for sodium ion batteries that is low-cost, has excellent electrochemical performance, and is suitable for industrial production. Summary of the Invention

[0008] In order to solve the problems in the prior art of high manufacturing cost of hard carbon negative electrode materials for sodium ion batteries, poor electrochemical performance of the products, especially unsatisfactory electrochemical performance at high rates, the present invention proposes a method for preparing modified biomass hard carbon materials for sodium ion batteries.

[0009] The present invention solves the above technical problems through the following technical solutions:

[0010] A method for preparing a modified biomass hard carbon material comprises the following steps:

[0011] (S1) Biomass pretreatment: The biomass is sequentially crushed, alkali washed, and washed with water;

[0012] (S2) preparing a modification solution: adding a certain amount of a nitrogen-containing compound having two or more -C=C- double bonds and a polymerization inhibitor, ferrous sulfate, to an ethanol solution of a photoinitiator, benzophenone, and stirring in a stirred reactor until the mixture is completely dissolved to obtain a modification solution;

[0013] (S3) impregnation of biomass: adding a certain amount of the biomass obtained in step (S1) to the modified solution of step (S2) for impregnation;

[0014] (S4) UV grafting: introducing an inert gas into the mixture of step (S3), exhausting the air, and sealing, irradiating with UV light to initiate a graft polymerization reaction, and then purifying the reacted material after irradiation to obtain a modified biomass;

[0015] (S5) carbonization: placing the modified biomass obtained in step (S3) and an inorganic salt in a rotary atmosphere furnace and carbonizing them under an inert atmosphere to obtain a hard carbon precursor I;

[0016] (S6) Post-treatment: The hard carbon precursor I is subjected to flotation, hydrochloric acid rinsing, water washing, and grinding to obtain a modified biomass hard carbon material.

[0017] Furthermore, in step (S1), the biomass is selected from at least one of peanut shells, coconut shells, hazelnut shells, walnut shells, and apricot shells. The present invention uses cheap and readily available bulk raw materials, ensuring the needs of large-scale industrial production.

[0018] Furthermore, in step (S1), the crushing is to crush to 325-400 mesh to facilitate the subsequent grafting reaction; the crushing method can be air flow crushing, high-speed crushing, ball milling, etc.; the alkali washing is immersed in 50-60°C, 5-10wt% NaOH and / or KOH aqueous solution under stirring conditions of 60-100rpm for 2-4h; the water washing is to wash with water until the pH of the effluent is 8-9.

[0019] Furthermore, in step (S2), the nitrogen-containing compound having two or more -C=C- double bonds is triallyl isocyanurate, which contains two or more allyl double bonds, thereby increasing the degree of crosslinking during the grafting process.

[0020] Furthermore, in step (S2), the ethanol solution of the photoinitiator benzophenone has a benzophenone concentration of 0.01-0.02 mol / L. If the concentration of the photoinitiator benzophenone is too low, the reaction is incomplete. As the concentration of the photoinitiator increases, the number of free radical active sites generated on the surface of the biomass particles increases, and the grafting rate increases. However, if the concentration of the photoinitiator is too high, a shielding effect will occur, reducing the intensity of ultraviolet light at the interface between the solution and the biomass matrix. Furthermore, as the concentration of the photoinitiator increases, it will mainly induce self-polymerization of nitrogen-containing compounds with two or more -C=C- double bonds, reducing the grafting rate.

[0021] Furthermore, in step (S2), the volume ratio of the mass of the nitrogen-containing compound having two or more -C=C- double bonds to the ethanol solution of benzophenone is 10-30 g:1 L, ferrous sulfate is 0.02-0.03 wt % of allyl isocyanurate, and the function of the polymerization inhibitor ferrous sulfate is to prevent the self-polymerization of the nitrogen-containing compound having two or more -C=C- double bonds.

[0022] Furthermore, in step (S3), the biomass is immersed in the modified liquid obtained in step (S1) at 50-60°C and kept warm for 3-5 hours; the biomass obtained in step (S1) is 5-9 times the mass of allyl isocyanurate.

[0023] Furthermore, in step (S4), the inert gas is an inert gas with an oxygen content of less than 1%, such as nitrogen and / or argon; the ultraviolet light is 300-350nm, and the irradiation time is 20-30min.

[0024] Furthermore, in step (S4), the purification is to place the reacted material into a Soxhlet extractor containing acetone for extraction, washing, and drying.

[0025] Furthermore, in step (S5), the inorganic salt is K2SO4, Na2SO 4、 At least one of KCl and NaCl, with a mass ratio of the inorganic salt to the modified biomass of 1:15-20. The inorganic metal salt can act as an isolation agent, inhibiting agglomeration and stacking of the carbon source during the calcination process.

[0026] Furthermore, in step (S5), the carbonization is carried out in a rotary kiln at a rotation speed of 0.5-2 rpm and a carbonization temperature of 800-1000°C for 6-8 hours. The carbonization atmosphere is an inert atmosphere, and the inert atmosphere is an oxygen content of less than 1%, preferably less than 0.1%, such as a protective atmosphere of nitrogen and / or argon; the volatile matter of the material after carbonization is ≤5%.

[0027] The present invention creatively modifies biomass by grafting nitrogen-containing compounds with two or more -C=C- double bonds onto the biomass's macromolecular chains under the action of a photoinitiator and a polymerization inhibitor, forming a rich cross-linked network. This reduces defects in the sintered hard carbon material, improves the micromorphology, and facilitates enhanced electrochemical performance. Nitrogen-containing compounds with two or more -C=C- double bonds, in addition to the multifunctional -C=C- double bonds, also contain a high content of nitrogen, i.e., a nitrogen source. Heteroatom doping in hard carbon materials has been shown to be an effective means of improving the electrochemical performance of hard carbon negative electrode materials. Heteroatoms can increase interlayer spacing and facilitate the insertion / deinsertion of sodium ions. Nitrogen doping is the most studied doping method, which can improve the activity and electronic conductivity of hard carbon materials. Therefore, allyl isocyanurate simultaneously acts as a grafting and cross-linking modification and nitrogen doping modification. The modified biomass has a rich cross-linked network and chemically bonded nitrogen atoms. The modified biomass is then calcined at high temperature with a metal salt, which inhibits agglomeration and stacking of the carbon source during calcination. The present invention utilizes the grafting, crosslinking, and synergistic calcination to produce an N-doped hard carbon material precursor with a large interlayer spacing. The resulting hard carbon material exhibits excellent electrochemical properties.

[0028] The inventors unexpectedly discovered that UV-grafting and cross-linking modification of biomass with nitrogen-containing compounds containing two or more -C=C- double bonds, combined with nitrogen doping, can significantly improve the electrochemical properties of hard carbon materials, including cycle stability, reversible specific capacity, and first efficiency.

[0029] Furthermore, in step (S6), the flotation is carried out by flow frothing flotation to separate the light components and heavy components of the carbonized material. In conjunction with the installation of an electromagnetic device in the flotation tank, the iron impurities and ash components such as mud, sand, etc. can be preliminarily removed, thereby reducing the ash content and iron content of the carbonized material. The purpose of the hydrochloric acid rinsing is to remove metallic ash impurities. The dilute hydrochloric acid solution and the carbonized material after flotation are mixed and stirred evenly in a reactor, and refluxed for 2-4 hours. The concentration of dilute hydrochloric acid is 3-8wt%, and the amount of dilute hydrochloric acid used is 4-6 times the mass of the carbonized material after flotation. The water washing is carried out until the effluent is neutral (pH = 6.8-7). The grinding is carried out until the D50 is 5-10μm. There is no special limitation on the grinding equipment and conventional equipment in the field can be used, such as air flow grinding equipment and ball milling.

[0030] The present invention also provides a sodium ion battery, the negative electrode active material of which includes the modified biomass hard carbon material prepared by the above preparation method.

[0031] The present invention utilizes inexpensive and readily available biomass as a carbon source, undergoes UV-light grafting and crosslinking modification with nitrogen-containing compounds containing two or more -C=C- double bonds, and finally undergoes co-calcination with an inorganic salt to produce modified biomass hard carbon for sodium-ion batteries with excellent electrochemical performance. The UV-light grafting modification method utilizes a short reaction time, high efficiency, low energy consumption, and a simple and easy-to-operate process. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a SEM image of Example 1;

[0033] Figure 2 1 is a schematic diagram of the process flow of Example 1. DETAILED DESCRIPTION

[0034] To make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be described in detail below. The following examples are convenient for better understanding of the present invention, but are not intended to limit the present invention. The experimental methods in the following examples are all conventional methods unless otherwise specified.

[0035] Example 1

[0036] (S1) Biomass pretreatment: Coconut shells were ball-milled to 325 mesh, immersed in a 10 wt% NaOH aqueous solution at 50°C for 3 h under stirring at 80 rpm, and then washed with water until the effluent had a pH of 8.

[0037] (S2) Preparation of a modification solution: 20 g of triallyl isocyanurate and 0.004 g of ferrous sulfate (a polymerization inhibitor) were added to 1 L of a 0.01 mol / L benzophenone ethanol solution and the mixture was stirred in a reactor until the mixture was completely dissolved to obtain a modification solution;

[0038] (S3) Biomass impregnation: 100 g of the biomass obtained in step (S1) was added to the modified solution of step (S2) and impregnated at 50° C. for 4 hours;

[0039] (S4) UV grafting: N2 is introduced into the mixture of step (S3), the air is removed, the mixture is sealed, and then a UV lamp is turned on and irradiated under 350 nm UV light for 25 minutes to carry out a graft polymerization reaction; after irradiation, the reacted material is placed in a Soxhlet extractor containing acetone, extracted, washed, and dried to obtain a modified biomass;

[0040] (S5) Carbonization: 100 g of the modified biomass obtained in step (S3) and 6 g of K2SO4 were added to a rotary atmosphere furnace at 1 rpm for carbonization at a temperature of 1000°C for 6 h. The volatile content of the micropowder after carbonization was 3.2%.

[0041] (S6) Post-treatment: The carbonized micropowder of step (S5) is subjected to flow flotation to separate the light and heavy components of the carbonized material, and preliminarily remove iron impurities, mud, sand and other ash components; the micropowder after flotation is fed into a reactor, and 5wt% dilute hydrochloric acid, which is 4 times the amount of the micropowder, is added, and a second hydrochloric acid rinse is performed under heating reflux to further remove metallic ash impurities; excess salts and other impurities are removed by water washing, and the semi-finished product is dried in a vacuum oven to a moisture content of ≤1%. The dried semi-finished product is ball-milled and classified to meet the particle size requirements: D10>2μm, D50 is 5-10μm, and D90≤25μm. After classification, the finished modified biomass hard carbon material is obtained.

[0042] Figure 1 This is the SEM image of Example 1.

[0043] Figure 2 1 is a schematic diagram of the process flow of Example 1.

[0044] Example 2

[0045] Other aspects are the same as in Example 1, except that peanut shells are used instead of coconut shells, and the process conditions are adjusted.

[0046] (S1) Biomass pretreatment: Peanut shells were ball-milled to 325 mesh, immersed in a 10 wt % NaOH aqueous solution at 50°C for 3 h under stirring at 80 rpm, and then washed with water until the effluent had a pH of 8.

[0047] (S2) Preparation of a modification solution: 25 g of triallyl isocyanurate and 0.005 g of ferrous sulfate (a polymerization inhibitor) were added to 1 L of a 0.01 mol / L benzophenone ethanol solution and the mixture was stirred in a reactor until the mixture was completely dissolved to obtain a modification solution;

[0048] (S3) Impregnation of biomass: 125 g of the biomass obtained in step (S1) was added to the modified solution of step (S2) and impregnated at 50° C. for 4 hours;

[0049] (S4) UV grafting: N2 is introduced into the mixture of step (S3), the air is removed, the mixture is sealed, and then a UV lamp is turned on and irradiated under 350nm UV light for 30 minutes to carry out a graft polymerization reaction; after irradiation, the reacted material is placed in a Soxhlet extractor containing acetone, extracted, washed, and dried to obtain a modified biomass;

[0050] (S5) Carbonization: 100 g of the modified biomass obtained in step (S3) and 6 g of K2SO4 were added to a rotary atmosphere furnace at 1 rpm for carbonization at a temperature of 900°C for 6 h. The volatile content of the micropowder after carbonization was 3.5%.

[0051] (S6) Post-treatment: the same as step (S6) in Example 1.

[0052] Example 3

[0053] The other steps are the same as those in Example 1, except that 20 g of triallyl cyanurate is used instead of 20 g of triallyl isocyanurate in step (S2).

[0054] Example 4

[0055] The rest is the same as Example 1, except that in step (S2), 20 g of N,N'-vinylbisacrylamide is used instead of 20 g of triallyl isocyanurate (ie, -C=C- in an equimolar ratio).

[0056] Example 5

[0057] The other steps are the same as those in Example 1, except that the concentration of benzophenone in step (S2) is 0.02 mol / L.

[0058] Example 6

[0059] The rest is the same as Example 1, except that the amount of biomass obtained in step (S1) used in step (S3) is 160 g.

[0060] Comparative Example 1

[0061] The rest is the same as Example 1, except that steps (S2), (S3), and (S4) are omitted, and carbonization and post-treatment are directly performed after the biomass pre-treatment.

[0062] Comparative Example 2

[0063] The other steps were the same as those in Example 1, except that 17 g of acrylamide was used instead of 20 g of triallyl isocyanurate.

[0064] The raw material components and illumination conditions of the hard carbon materials prepared in the above examples and comparative examples are shown in Table 1 below.

[0065] Table 1 Hard carbon material raw material components and UV irradiation conditions

[0066]

[0067] Application Examples

[0068] The modified biomass hard carbon materials obtained in the above examples and comparative examples were tested. Specifically, the hard carbon materials prepared in the examples or comparative examples were used as the active ingredient. They were mixed with a binder of 1% CMC and a conductive agent of Super P in a mass ratio of 8:1:1. NMP was added as a solvent to prepare a negative electrode slurry. Using an automatic coating dryer, the negative electrode slurry was evenly coated onto an aluminum foil current collector with a doctor blade to a thickness of 150 ± 1 μm. After vacuum drying, a 14 mm diameter circular electrode sheet was formed. A sodium sheet was used as the counter electrode, a glass fiber separator was used, and the electrolyte was 1 M NaClO₄ in an EC:PC ratio of 1:1 with 5% FEC. Half-cells were assembled in a glove box and tested at a current density of 25 mAh·g⁻¹ (0.1C) over a test range of 0.001–2.0 V. The electrochemical performance test results are shown in Table 2 below.

[0069] Table 2 Hard carbon material performance test

[0070]

[0071] It can be seen that the modified biomass hard carbon material obtained by the preparation method of the present invention, when used as the negative electrode active material for a sodium ion battery, has excellent comprehensive electrochemical properties, especially a significantly improved rate performance.

Claims

1. A method for preparing a modified biomass hard carbon material, characterized in that: The following steps are involved: (S1) Biomass pretreatment: The biomass is crushed, alkali washed, and washed with water in sequence; (S2) preparing a modifying solution: adding a nitrogen-containing compound having two or more -C=C- double bonds and a polymerization inhibitor, ferrous sulfate, to an ethanol solution of a photoinitiator, benzophenone, and mixing them uniformly to obtain a modifying solution; the nitrogen-containing compound having two or more -C=C- double bonds is selected from at least one of triallyl isocyanurate, triallyl cyanurate, and N,N'-vinylbisacrylamide; (S3) impregnation of biomass: adding the biomass obtained in step (S1) to the modified solution of step (S2) for impregnation; (S4) UV grafting: introducing an inert gas into the mixture of step (S3), exhausting the air, and sealing, irradiating with UV light to initiate a graft polymerization reaction, and then purifying the reacted material after irradiation to obtain a modified biomass; (S5) carbonization: The modified biomass obtained in step (S4) and an inorganic salt are placed in a rotary atmosphere furnace and carbonized under an inert atmosphere to obtain a hard carbon precursor I; (S6) Post-treatment: The hard carbon precursor I is subjected to flotation, hydrochloric acid rinsing, water washing, and grinding to obtain a modified biomass hard carbon material.

2. The preparation method according to claim 1, characterized in that In step (S1), the biomass is selected from at least one of peanut shells, coconut shells, hazelnut shells, walnut shells, and apricot shells; the crushing is to crush to 325-400 mesh to facilitate the subsequent grafting reaction; the crushing method adopts air flow crushing, high-speed crushing, or ball milling; the alkali washing is immersed in a 50-60°C, 5-10wt% NaOH and / or KOH aqueous solution under stirring conditions of 60-100rpm for 2-4h; and the water washing is to wash with water until the pH of the effluent is 8-9.

3. The preparation method according to claim 1, characterized in that The ethanol solution of benzophenone as a photoinitiator described in step (S2) has a benzophenone concentration of 0.01-0.02 mol / L; the volume ratio of the nitrogen-containing compound having two or more -C=C- double bonds to the benzophenone ethanol solution is 10-30 g:1 L; and the ferrous sulfate is 0.02-0.03 wt% of the nitrogen-containing compound having two or more -C=C- double bonds.

4. The preparation method according to claim 1, characterized in that The biomass soaking in step (S3) is to soak the biomass obtained in step (S1) in a modification liquid at 50-60°C and keep it warm for 3-5 hours; the biomass obtained in step (S1) is 5-8 times the mass of the nitrogen-containing compound having two or more -C=C- double bonds.

5. The preparation method according to claim 1, characterized in that In step (S4), the inert gas has an oxygen content of less than 1%; the ultraviolet light is 300-350 nm, and the irradiation time is 20-30 min; and the purification is to place the reacted material into a Soxhlet extractor containing acetone, extract, wash, and dry.

6. The preparation method according to claim 1, characterized in that The inorganic salt in step (S5) is K2SO4, Na2SO 4、 At least one of KCl and NaCl, the mass ratio of the inorganic salt to the modified biomass is 1:15-20.

7. The preparation method according to claim 1, characterized in that The carbonization in step (S5) is carried out in a rotary kiln at a rotation speed of 0.5-2 rpm and a carbonization temperature of 800-1000° C. for 6-8 hours. The carbonization atmosphere is an inert atmosphere, and the inert atmosphere has an oxygen content of less than 1%. The volatile matter of the material after carbonization is ≤5%.

8. The preparation method according to claim 1, characterized in that The flotation in step (S6) is a flow-bubbling flotation method, which separates the light components and heavy components of the carbonized material. In conjunction with the installation of an electromagnetic device in the flotation tank, iron impurities and ash components such as mud, sand, etc. can be preliminarily removed, thereby reducing the ash content and iron content of the carbonized material. The purpose of the hydrochloric acid rinsing is to remove metallic ash impurities. The dilute hydrochloric acid solution and the carbonized material after flotation are mixed and stirred evenly in a reactor, and refluxed for 2-4 hours. The concentration of dilute hydrochloric acid is 3-8wt%, and the amount of dilute hydrochloric acid used is 4-6 times the mass of the carbonized material after flotation. The water washing is to wash with water until the effluent is neutral, and the grinding is to grind the material until the D50 is 5-10μm.

9. A sodium ion battery, characterized in that: The sodium ion battery negative electrode active material comprises a modified biomass hard carbon material prepared by any one of the preparation methods of claims 1-8.

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