Preparation method and application of biomass-based hard carbon negative electrode material based on microwave solvothermal method

Abstract

The invention discloses a preparation method and application of a biomass-based hard carbon negative electrode material based on a microwave solvothermal method, and the preparation method comprises the following steps: fully mixing a pretreated biomass carbon source with a sulfuric acid solution, transferring into a microwave hydrothermal reaction kettle, and carrying out microwave hydrothermal treatment to obtain a hydrothermal reaction product; carrying out solid-liquid separation on a hydrothermal reaction product, collecting filter residues, cleaning until filtrate is neutral, and drying to obtain a hard carbon precursor; performing high-temperature carbonization on the hard carbon precursor under the protection of an inert atmosphere to obtain the biomass-based hard carbon negative electrode material; according to the method, a mixed system of the biomass carbon source and sulfuric acid is pre-carbonized in a microwave solvothermal mode, uniform deliming of the biomass carbon source is achieved under the controllable acidity condition, and the uniformity of the biomass carbon source is improved; the pre-carbonized biomass material is subjected to high-temperature carbonization in an inert atmosphere, so that a hard carbon material with a moderate extension interlayer spacing can be obtained, and the synergistic improvement of high-rate charge-discharge performance and long cycle stability is realized.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery anode material technology, and particularly relates to a preparation method and application of a biomass-based hard carbon anode material based on microwave solvothermal method. Background Technology

[0002] Sodium-ion batteries, with their advantages of abundant sodium resources, low cost, and wide temperature adaptability, show promising application prospects in large-scale energy storage and portable devices. Biomass materials, due to their environmental friendliness and sustainability, are widely used as precursors for hard carbon anodes in sodium-ion batteries. Hard carbon anodes, characterized by low operating potential, wide availability of raw materials, and high safety, have become the preferred technical approach for sodium-ion battery anode materials.

[0003] In existing technologies, the preparation of biomass hard carbon typically employs a multi-step process route of "acid washing and deashing—low-temperature pre-carbonization—high-temperature carbonization in an inert atmosphere" (Chinese invention patent applications CN120664526A and CN120664524A), which suffers from problems such as cumbersome processes, long production cycles, high energy consumption, and significant environmental burden. Furthermore, while direct carbonization (Chinese invention patent application CN118213532A) can simplify the process to some extent, the presence of ash in the biomass raw materials catalyzes excessive graphitization of the carbon structure during high-temperature treatment, leading to a reduction in the interlayer spacing and sodium-storing active sites in the resulting carbon material, making it difficult to simultaneously achieve high specific capacity and excellent rate performance. Chinese invention patent CN117486192B discloses a hydrothermal process for preparing biomass-based hard carbon precursor materials. The process employs a hydrothermal method with the addition of ternary additives (methacrylate, acrylonitrile, and pyromellitic anhydride). By adjusting the additive ratio, the precursor particle size can be controlled within the nanometer to micrometer range. The resulting mother liquor is recovered via membrane separation. The precursor is then subjected to pre-carbonization and high-temperature carbonization treatments under an inert atmosphere to obtain the hard carbon material. However, this method involves a long hydrothermal holding time, uses complex ternary additives for deashing, resulting in high raw material costs, a cumbersome process, and high energy consumption. Furthermore, the mother liquor membrane separation stage is prone to contamination and scaling. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, the present invention aims to provide a method for preparing and applying biomass-based hard carbon anode materials based on microwave solvothermal methods. The method employs microwave solvothermal techniques to pre-carbonize a mixture of biomass carbon source and sulfuric acid, achieving uniform deashing of the biomass carbon source under controllable acidity conditions and improving its uniformity. The pre-carbonized biomass material is then subjected to high-temperature carbonization in an inert atmosphere to obtain a hard carbon material with appropriately extended interlayer spacing, thereby achieving a synergistic improvement in high-rate charge / discharge performance and long-cycle stability.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing biomass-based hard carbon anode material based on microwave solvothermal method includes the following steps: Step 1: Thoroughly mix the pretreated biomass carbon source with sulfuric acid solution, transfer it to a microwave hydrothermal reactor for microwave hydrothermal treatment, and obtain hydrothermal reaction products; Step 2: The hydrothermal reaction products are subjected to solid-liquid separation, the filter residue is collected and washed until the filtrate is neutral, and then dried to obtain the hard carbon precursor; Step 3: The hard carbon precursor is carbonized at high temperature under an inert atmosphere to obtain biomass-based hard carbon anode material.

[0006] In step 1, oak is used as the biomass carbon source.

[0007] In step 1, the pretreatment process is as follows: after coarse crushing and ball milling, the oak wood is sieved through a sieve of at least 400 mesh to obtain biomass oak wood powder with an average particle size of at least 38 μm; the ball milling time is 5~7 h and the ball milling speed is 300~500 r / min.

[0008] In step 1, the molar concentration of the sulfuric acid solution is 1~3 mol / L.

[0009] In step 1, the temperature of the microwave hydrothermal reaction is 160~180℃, the holding time is 20~50min, and the microwave power is at least 300W.

[0010] In step 1, during the microwave hydrothermal reaction, the reaction solution is heated by microwave while being stirred in a microwave hydrothermal reactor.

[0011] In step 1, the biomass carbon source : sulfuric acid solution ratio is (0.6~1):50 according to the mass-volume ratio (g / mL).

[0012] In step 2, the drying temperature is 60~100℃ and the drying time is 8~12h.

[0013] In step 3, the inert atmosphere is argon, the carbonization temperature is 1200~1600℃, the heating rate is set to 5~10℃ / min, and the holding time is 1~3h. The present invention also provides the application of the biomass-based hard carbon anode material prepared by the above-mentioned method in the anode active material of sodium-ion batteries.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention uses oak wood as a biomass carbon source. Relying on its dense and uniform hardwood tissue and low ash and low impurity characteristics, high-temperature carbonization can form a structurally stable hard carbon skeleton and a multi-level porous structure with micro / mesopore synergy. The high carbon and low ash properties of oak wood raw materials are conducive to reducing side reactions caused by impurity metals. At the same time, the high carbonization efficiency and carbon yield can reduce the unit energy cost and improve the economy and consistency of large-scale preparation.

[0015] 2. This invention improves the dispersibility of the carbon source precursor powder by mixing biomass carbon source with sulfuric acid, and achieves pre-oxidation of the carbon source precursor. Appropriate oxygen-containing functional groups (such as C-OH, COC, C=O) are introduced onto the surface of the carbon material, and defective carbon structures are induced during subsequent high-temperature carbonization, enhancing the Na+... + This enhances the adsorption and filling capacity, thereby increasing sodium storage capacity. Simultaneously, combining acid treatment with pre-carbonization processes achieves uniform deashing of biomass feedstock under controllable acidity conditions, reducing product composition and structural inhomogeneity and simplifying the process flow.

[0016] 3. Compared to traditional hydrothermal treatment, this invention employs a microwave solvothermal method to pre-carbonize a mixture of biomass carbon source and sulfuric acid. Utilizing the bulk heating characteristics and selective energy coupling of microwaves, rapid and uniform heating of the system is achieved, improving the uniformity of the pre-carbonized material. Furthermore, uniform pre-carbonization and appropriate interlayer spacing expansion can be completed in a shorter time, shortening the Na... + Migration paths enhance the rate performance and rapid charge / discharge capabilities of materials.

[0017] 4. The present invention sets the holding time of the microwave solvothermal stage to 20~50 min, which can significantly compress the reaction time compared with the conventional hydrothermal method, and the obtained pre-carbonization degree is basically equivalent to that of long-term traditional hydrothermal treatment, thereby reducing energy consumption and improving process efficiency.

[0018] In summary, the microwave solvothermal pre-carbonization strategy achieves low-temperature, short-duration, and homogeneous bulk heating and structural rearrangement of biomass hard carbon precursors, significantly shortening the pretreatment cycle and reducing energy consumption. Swelling, deashing, and functional group regulation are simultaneously completed in a closed liquid phase, reducing acid washing dosage and ash content, and improving precursor uniformity and batch-to-batch consistency. Through synergistic adjustment of solvent acidity, holding time, and microwave power, a carbon framework with rapid electron / ion transport is formed, while polarization is reduced and SEI is stabilized. The biomass-based hard carbon obtained after high-temperature carbonization in an inert atmosphere exhibits excellent rate performance and long-cycle stability when used as an anode in sodium-ion batteries. Furthermore, the process is simplified, has a low environmental impact, and is suitable for batch or continuous scale-up. Attached Figure Description

[0019] Figure 1The graph shows a comparison of the cycle performance of the hard carbon anode half-cells prepared in Examples 1-5 and Comparative Examples 1-4 of this invention at 1C.

[0020] Figure 2 The chart shows a comparison of the rate performance of the hard carbon anode half-cells prepared in Examples 1-5 and Comparative Examples 1-4 of this invention.

[0021] Figure 3 The chart shows a comparison of the charge-discharge curves of the hard carbon negative electrode half-cells prepared in Examples 1-5 and Comparative Examples 1-4 of this invention at 10C. Detailed Implementation

[0022] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. For those skilled in the art, the present invention is not limited to the specific details of the exemplary embodiments. Other specific forms may be adopted without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments described should be considered exemplary rather than restrictive. The scope of the present invention is defined by the appended claims and is intended to cover all variations, substitutions, and improvements falling within the meaning and scope of the claims and their equivalents.

[0023] A method for preparing biomass-based hard carbon anode material based on microwave solvothermal method includes the following steps: Step 1: Raw material pretreatment; Using biomass oak as the biomass carbon source, the material is coarsely crushed and ball-milled, then sieved through a screen of at least 400 mesh to obtain a biomass carbon source with uniform particle size; the ball milling time is 5-7 hours, and the ball milling speed is 300-500 r / min; the average particle size of the biomass carbon source is at least 38 μm. Step 2: Microwave hydrothermal treatment; The biomass carbon source and sulfuric acid solution are thoroughly mixed and transferred to a microwave hydrothermal reactor for microwave hydrothermal treatment to achieve pre-carbonization and obtain hydrothermal reaction products. The molar concentration of the sulfuric acid solution is 1~3 mol / L. The microwave hydrothermal reaction temperature is 160~180℃, the holding time is 20~50 min, and the microwave power is at least 300W. During the microwave hydrothermal reaction, the reaction solution is stirred and microwave heated in the microwave hydrothermal reactor to improve the degree of pre-carbonization. The mass-volume ratio (g / mL) of biomass carbon source: sulfuric acid solution is (0.6~1):50. Step 3: Separation and washing; The hydrothermal reaction products were separated into solid and liquid phases by vacuum filtration. The filter residue was collected and repeatedly washed with deionized water until the filtrate was neutral (pH≈7). The filter residue was dried to obtain the hard carbon precursor. The drying temperature was 60~100℃ and the drying time was 8~12h. Step 4: High-temperature carbonization; Hard carbon precursors are placed in a tube furnace under an inert atmosphere for high-temperature carbonization to obtain biomass-based hard carbon anode materials; the inert atmosphere is argon, the carbonization temperature is 1200~1600℃, the heating rate is set to 5~10℃ / min, and the holding time is 1~3h. This invention provides the application of the biomass-based hard carbon material obtained by the above preparation method as a negative electrode active material for sodium-ion batteries. Specifically, the biomass-based hard carbon material can be mixed with a conductive agent and a binder to form a slurry, which is then coated onto the surface of a current collector for use in assembling sodium-ion batteries.

[0024] Example 1 A method for preparing biomass-based hard carbon anode material based on microwave solvothermal method includes the following steps: Step 1: The oak wood is coarsely crushed and then ball-milled for 5 hours at a speed of 500 r / min. After sieving through a 400-mesh sieve, oak wood powder with an average particle size of 38 μm is obtained. Step 2: Mix 0.8g of oak wood powder with 2mol / L, 50mL sulfuric acid solution, transfer to a microwave hydrothermal reactor, perform hydrothermal treatment at 170℃, keep warm and stir for 30min, set microwave power to 300W, to achieve precursor pre-carbonization; Step 3: The hydrothermal reaction product is separated into solid and liquid by vacuum filtration. The filter residue is collected and repeatedly washed with deionized water until the filtrate is neutral. Then it is placed in an 80℃ forced-air drying oven and dried for 12 hours to obtain the hard carbon precursor. Step 4: Under argon protection, the hard carbon precursor is placed in a tube furnace for high-temperature carbonization at a temperature of 1400℃, a holding time of 2 hours, and a heating rate of 10℃ / min to obtain biomass-based hard carbon anode material.

[0025] Example 2 The process and raw materials of this embodiment are the same as those of Embodiment 1. The difference is that in step 2, 1g of oak wood powder is mixed with 1mol / L, 50mL sulfuric acid solution, transferred to a microwave hydrothermal reactor, and subjected to hydrothermal treatment at 180℃ for 20min of heat preservation and stirring to achieve precursor pre-carbonization.

[0026] Example 3 The process and raw materials of this embodiment are the same as those of Embodiment 1. The difference is that in step 2, 0.6g of oak wood powder is mixed with 3mol / L, 50mL sulfuric acid solution, transferred to a microwave hydrothermal reactor, and subjected to hydrothermal treatment at 160℃ for 50min of heat preservation and stirring to achieve precursor pre-carbonization.

[0027] Example 4 The process and raw materials of this embodiment are the same as those of Embodiment 1. The difference is that in step 4, the hard carbon precursor is placed in a tube furnace under argon protection for high-temperature carbonization. The carbonization temperature is 1200℃, the holding time is 3h, and the heating rate is 5℃ / min to obtain biomass-based hard carbon anode material.

[0028] Example 5 The process and raw materials of this embodiment are the same as those of Embodiment 1. The difference is that in step 4, the hard carbon precursor is placed in a tube furnace under argon protection for high-temperature carbonization. The carbonization temperature is 1600℃, the holding time is 1h, and the heating rate is 10℃ / min to obtain biomass-based hard carbon anode material.

[0029] Comparative Example 1 The comparative example uses the same process and raw materials as Example 1, except that the microwave hydrothermal reaction time in step 2 is 10 minutes.

[0030] Comparative Example 2 The comparative example uses the same process and raw materials as Example 1. The difference is that in step 2, the oak wood raw material is placed in a hydrothermal reactor and then placed in a conventional oven for hydrothermal pre-carbonization treatment.

[0031] Comparative Example 3 The comparative example uses the same process and raw materials as Example 1, except that in step 2, oak wood powder is mixed with deionized water for a microwave hydrothermal reaction.

[0032] Comparative Example 4 The comparative example uses the same process and raw materials as Example 1, except that in step 2, oak powder is mixed with hydrochloric acid for a microwave hydrothermal reaction.

[0033] Preparation of the negative electrode sheet for sodium-ion batteries: The hard carbon materials obtained in Examples 1-5 and Comparative Examples 1-4 were used as active materials and mixed with a conductive agent (conductive carbon black, Carbot) and a binder (polyvinylidene fluoride, PVDF) in a mass ratio of 8:1:1. NMP was added as a dispersion medium, and the mixture was stirred evenly to form an electrode slurry. The slurry was coated onto a carbon-coated aluminum foil current collector and pre-dried at 80 °C. Then, it was transferred to a vacuum drying oven and dried at 80 °C and a vacuum degree less than -0.09 MPa for 12 h. Finally, it was cut into round sheets with a diameter of 12 mm using a punching machine to serve as the negative electrode sheet.

[0034] Electrochemical performance testing: CR2032 half-cells were assembled in an inert atmosphere glove box. Electrodes prepared in Examples 1-5 and Comparative Examples 1-4 were used as working electrodes, sodium metal as the reference electrode, glass fiber as the separator, and a 1 M NaPF6 solution prepared with DGEDMG (diethylene glycol dimethyl ether) as the electrolyte. After assembly, the cells were allowed to stand for 6 h, and constant current charge-discharge tests were performed at room temperature within a voltage window of 0.01–2 V. The current density for long-cycle testing was set at 1 C (1 C = 372 mAh / g). Rate performance tests were conducted sequentially at 0.2 C, 0.5 C, 1 C, 2 C, 5 C, and 10 C. The results of the cycle performance tests at 10 C are shown in Table 1.

[0035] Table 1 Results of Cyclic Performance Tests at 10 C Ratio As shown in Table 1, the electrochemical test results of Examples 1-3 reveal that the discharge specific capacity of the samples initially increases and then decreases with the extension of the microwave solvothermal holding time. Among these, the hard carbon materials prepared with a holding time of 30 min and a carbonization time of 2 h exhibit the best performance in terms of discharge specific capacity and capacity retention. This is because moderate holding time facilitates uniform pre-carbonization and appropriate expansion of interlayer spacing, constructing a reasonable pore structure, which reduces irreversible consumption, lowers polarization, and improves the rate capability and cycle performance of the material. However, with further extension of the holding time, excessive etching is prone to occur, leading to severe damage to the material structure and the generation of more irreversible sites, resulting in a decrease in specific capacity and cycle performance.

[0036] Meanwhile, compared with the conventional hydrothermal treatment method in Comparative Example 2, under the same raw materials and proportions, the anode material prepared by the microwave solvothermal method has greater advantages under high-rate (10C) charge-discharge conditions, such as... Figure 2 As shown. Among them, although the discharge specific capacity of the hard carbon anode material prepared in Example 5 at a current density of 1C is not much different from that of Comparative Example 1 or Comparative Example 2, its rate performance is the best. Figure 3 The charge-discharge curves of the hard carbon anodes prepared in Examples 1-5 and Comparative Examples 1-4 at a 10 C rate are shown; Figure 2 and Figure 3 It is known that in the microwave hydrothermal reaction of a mixed solution of oak powder and sulfuric acid using microwave solvothermal technology, the rapid and uniform heating of the bulk phase and the selective coupling effect of the microwave field improve the uniformity of the pre-carbonized material, enabling uniform pre-carbonization and appropriate interlayer spacing expansion in a short time, and shortening the Na... +Migration pathways enhance the high-rate charge-discharge performance of the material. Compared to Comparative Examples 3 and 4, the hard carbon material prepared using sulfuric acid as a hydrothermal solvent in this invention exhibits superior electrochemical performance compared to materials using deionized water or hydrochloric acid as solvents. Sulfuric acid promotes framework crosslinking and aromatization during pre-carbonization and introduces oxygen-containing functional groups onto the material surface, making subsequent carbonization more conducive to inducing defect generation and facilitating the construction of microporous and closed-pore structures, thereby improving the material's rate and cycle performance. In contrast, deionized water and hydrochloric acid are difficult to form a dense aromatized framework, indicating that sulfuric acid has a stronger microstructure regulation effect during the pre-carbonization stage.

Claims

1. A method for preparing a biomass-based hard carbon negative electrode material based on a microwave solvothermal method, characterized in that, The method comprises the following steps: Step 1: mixing the pretreated biomass carbon source with a sulfuric acid solution, and transferring the mixture to a microwave hydrothermal reactor for microwave hydrothermal treatment to obtain a hydrothermal reaction product; Step 2: performing solid-liquid separation on the hydrothermal reaction product, collecting the filter residue and washing until the filtrate is neutral, and drying to obtain a hard carbon precursor; Step 3: performing high-temperature carbonization on the hard carbon precursor under the protection of an inert atmosphere to obtain a biomass-based hard carbon negative electrode material.

2. The method for preparing biomass-based hard carbon anode material according to claim 1, characterized in that, In step 1, Cyclobalanopsis glauca is used as the biomass carbon source.

3. The method for preparing biomass-based hard carbon anode material according to claim 2, characterized in that, In step 1, the pretreatment process is as follows: after being coarsely crushed and ball-milled, the Cyclobalanopsis glauca is sieved through a sieve with a mesh size of at least 400 to obtain biomass Cyclobalanopsis glauca powder with an average particle size of at least 38 μm; the ball-milling time is 5-7 h, and the ball-milling rotation speed is 300-500 r / min.

4. The method for preparing biomass-based hard carbon anode material according to claim 1, characterized in that, In step 1, the molar concentration of the sulfuric acid solution is 1-3 mol / L.

5. The method for preparing biomass-based hard carbon anode material according to claim 1, characterized in that, In step 1, the temperature of the microwave hydrothermal reaction is 160-180 ℃, the holding time is 20-50 min, and the microwave power is at least 300 W.

6. The method for preparing biomass-based hard carbon anode material according to claim 1, characterized in that, In step 1, during the microwave hydrothermal reaction, the reaction solution is stirred and heated by the microwave hydrothermal reactor.

7. The method for preparing biomass-based hard carbon anode material according to claim 1, characterized in that, In step 1, the mass / volume ratio (g / mL) of the biomass carbon source to the sulfuric acid solution is (0.6-1):

50.

8. The method for preparing biomass-based hard carbon anode material according to claim 1, characterized in that, In step 2, the drying temperature is 60-100 ℃, and the drying time is 8-12 h.

9. The method for preparing biomass-based hard carbon anode material according to claim 1, characterized in that, In step 3, the inert atmosphere is argon, the carbonization temperature is 1200-1600 ℃, the heating rate is set to 5-10 ℃ / min, and the holding time is 1-3 h.

10. The biomass-based hard carbon negative electrode material prepared by the method according to any one of claims 1-9 is used as a sodium-ion battery negative electrode active material.

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