Biomass-based hard carbon material based on metal catalysis and gradient ultrasonic cavitation and preparation method thereof

Abstract

The application discloses a kind of biomass-based hard carbon materials based on metal catalysis and gradient ultrasonic cavitation and preparation method thereof, method includes the following steps: biomass raw material is dipped in soluble metal salt solution, adjust pH to alkaline, obtain precursor;The precursor is carried out gradient carbonization treatment under inert gas atmosphere, obtain hard carbon material;The hard carbon material is added to acid, under high temperature and high pressure condition, carry out activation impregnation treatment, obtain mixed slurry;The mixed slurry is carried out gradient ultrasonic treatment, then after filtration, washing, drying, obtain biomass-based hard carbon material.By introducing metal elements in precursor, realize catalytic pore forming and lattice rearrangement in high temperature carbonization stage;Subsequently combined with high temperature and high pressure activation impregnation and gradient frequency ultrasonic cavitation technology, use low-frequency ultrasonic to break free-state impurities, high-frequency ultrasonic produces transient cavitation effect to accurately strip the nanometer metal particles wrapped by carbon lattice.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery anode material technology, specifically a biomass-based hard carbon material based on metal catalysis and gradient ultrasonic cavitation, and its preparation method. Background Technology

[0002] Sodium-ion batteries are considered an important alternative technology for large-scale energy storage due to their abundant resources and low cost. Hard carbon, as the most promising anode material, has electrochemical performance highly dependent on the proportion of closed pores and the content of impurities in its microstructure. Existing biomass-based hard carbon mainly suffers from two problems: First, the closed-pore structure is difficult to control. Traditional pyrolysis processes lack a directional induction mechanism, leading to disordered carbon microcrystal arrangement and a low proportion of closed-pore volume, resulting in low initial coulombic efficiency. Second, there is a serious problem with residual metal impurities. Biomass itself is rich in Fe, Ca, Mg, and other metal ions from the soil. Combined with catalysts such as Ni, Mo, and Fe introduced to control the structure, these impurities are easily encapsulated by the carbon layer, forming a lattice embedding that cannot be effectively removed by conventional acid washing. Residual metals not only reduce specific capacity but also induce battery self-discharge or even sodium deposition and short circuits.

[0003] To address the aforementioned issues, previous studies have attempted to improve the closed-pore structure through metal-catalyzed localized graphitization or by combining acid washing with ultrasonic treatment to remove surface impurities. However, existing technologies generally treat catalytic pore formation and deep impurity removal as independent processes, failing to utilize the dual function of metal elements in the carbonization process—acting as both a structure regulator and a sensitive medium for subsequent physical exfoliation. This results in fragmented processes, low efficiency, and difficulty in achieving deep removal of nanoscale metal particles encapsulated in carbon lattices. Therefore, developing a short-process preparation method that integrates catalytic pore formation and efficient impurity removal is of great significance for improving the overall electrochemical performance of hard carbon materials. Summary of the Invention

[0004] In view of this, the present invention provides a biomass-based hard carbon material based on metal catalysis and gradient ultrasonic cavitation and its preparation method. Through the synergistic effect of metal catalysis-induced closed-pore construction and gradient ultrasonic cavitation physical stripping, the first coulombic efficiency and specific capacity of the hard carbon anode are significantly improved, while the total content of metal impurities is greatly purified.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention discloses a method for preparing biomass-based hard carbon materials based on metal catalysis and gradient ultrasonic cavitation, comprising the following steps: S1. The biomass raw material is immersed in a soluble metal salt solution, and the pH value is adjusted to alkaline to obtain the precursor. S2. The precursor is subjected to gradient carbonization treatment in an inert gas atmosphere to obtain hard carbon material. S3. Add the hard carbon material to the acid solution and perform activation impregnation treatment under high temperature and high pressure conditions to obtain a mixed slurry; S4. The mixed slurry is subjected to gradient ultrasonic treatment, and after filtration, washing and drying, biomass-based hard carbon material is obtained.

[0006] As a further aspect of the present invention: the biomass raw material is at least one of bamboo, coconut shell, pomelo peel, and straw.

[0007] As a further aspect of the present invention: the metal element of the metal salt is at least one of Fe, Ni, Co, Mn, and Cu, and the concentration of the metal salt is 0.05-0.5 mol / L.

[0008] As a further aspect of the present invention: in step S1, the pH value is adjusted to 8-10, and the soaking time is 8-12 hours.

[0009] As a further aspect of the present invention: In step S2, the gradient carbonization treatment is carried out under a protective atmosphere at a heating rate of 2-5℃ / min, and includes a pre-carbonization treatment and a high-temperature carbonization treatment in sequence; wherein, the pre-carbonization temperature is 400-600℃, and the holding time is 1.5-3.5 hours; the high-temperature catalytic carbonization temperature is 1000-1600℃, and the holding time is 3-5 hours. During this process, the metal elements catalyze the localized graphitization transformation of the carbon layer, forming a closed-pore structure around the carbon microcrystals, while some metal elements are physically encapsulated by the carbon layer to form a metal-carbon composite lattice.

[0010] As a further aspect of the present invention: In step S3, the acid solution is composed of an organic acid and an inorganic acid in a volume ratio of (1-3):(1-2); the organic acid is citric acid or ascorbic acid; the inorganic acid is hydrochloric acid or nitric acid. The organic acid acts as a complexing agent for metal ions and a buffer, preventing excessive etching of the carbon structure by the strong acid. The activation impregnation is carried out at 100-200℃ and 0.5-2.0MPa for 1-5 hours, allowing the acid solution to penetrate along the microcrack channels formed by metal catalysis to the edge of the closed pores, achieving selective pre-corrosion of lattice-encapsulated metal impurities in the closed-pore region.

[0011] The mechanism of this invention lies in the fact that microcracks are stress products accompanying the formation of closed-pore structures. Firstly, metal-catalyzed localized graphitization of hard carbon facilitates the formation of closed-pore structures. Simultaneously, some metal elements are physically encapsulated by the carbon layer to form a metal-carbon composite lattice. Due to the difference in thermal expansion coefficients and contraction rates between the metal particles and the carbon matrix, microcracks, gaps, or interface separation will inevitably occur around the metal particles during high-temperature carbonization and subsequent cooling. These microcracks serve as functional channels for subsequent acid infiltration and ultrasonic cavitation. After the removal of metal impurities, the cavities retained by the graphitized carbon layer also become closed pores.

[0012] As a further aspect of the present invention: in step S3, the high temperature and high pressure conditions are: temperature 100-200℃, pressure 0.5-2.0 MPa, and heat preservation for 1-5 hours.

[0013] As a further aspect of the present invention: in step S4, the gradient ultrasound processing specifically includes: The first stage is low-frequency coarse peeling: frequency 20-40 kHz, power density 0.5-1.5 W / mL, treatment time 30-90 minutes; The second stage is high-frequency fine removal: frequency 60-120 kHz, power density 0.2-0.8 W / mL, and treatment time 60-120 minutes.

[0014] High-frequency ultrasound generates microjets and local high temperature and pressure, which act on nanoscale metal particles wrapped in carbon lattices. Shock waves generated by the implosion of cavitation bubbles are used to strip away metal impurities within the lattice.

[0015] Secondly, this invention discloses a biomass-based hard carbon material based on metal catalysis and gradient ultrasonic cavitation, which is prepared using the above-mentioned preparation method.

[0016] Thirdly, this invention discloses the application of the aforementioned biomass-based hard carbon material in the negative electrode of a sodium-ion battery.

[0017] Compared with the prior art, the beneficial effects of the present invention are: This invention introduces a metal element as a dual-functional medium during the preparation of biomass hard carbon: in the high-temperature carbonization stage, it catalyzes the directional alignment of carbon microcrystals to form a high proportion of closed-pore structures, thereby effectively suppressing electrolyte side reactions, reducing the consumption of active sodium ions, and significantly improving the initial coulombic efficiency and reversible specific capacity of the anode material; at the same time, by utilizing the difference in thermal expansion coefficients between the metal and the carbon matrix, it serves as an ultrasonic sensitive site in the subsequent gradient ultrasonic treatment, breaking free impurities with low-frequency ultrasound and precisely peeling off nano-metal particles wrapped by the carbon lattice with high-frequency ultrasound, achieving deep physical impurity removal that cannot be accomplished by traditional acid washing.

[0018] Compared with traditional methods, the technical advantages of this invention are reflected in two dimensions: firstly, in terms of structure, the regular closed-pore structure constructed by metal catalysis endows hard carbon materials with excellent sodium storage capacity and cycle stability; secondly, in terms of purity, the synergistic effect of gradient ultrasonic cavitation and high-pressure acid leaching can control the total metal impurity content to below 50 ppm, with a significant reduction in catalytic metal residue, effectively avoiding safety hazards such as battery self-discharge and sodium deposition short circuits caused by metal impurities. The simultaneous achievement of these two effects breaks through the technical limitation of existing processes where catalytic pore formation and deep impurity removal are mutually exclusive. Attached Figure Description

[0019] Figure 1The biomass-derived hard carbon anode materials prepared for Example 1 and Comparative Example 3 were used in a 0.05 A g test. -1 The first charge-discharge curve at current density. Detailed Implementation

[0020] To facilitate understanding of the present invention, a more comprehensive description will be given below with reference to specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0022] In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the prior art, and therefore will not be described in detail; unless otherwise specified, the parts in the following embodiments refer to parts by weight.

[0023] Example 1 This embodiment proposes a biomass-based hard carbon material based on metal catalysis and gradient ultrasonic cavitation, the preparation method of which includes the following steps: (1) Bamboo powder that has passed through a 200-mesh sieve was placed in a 0.2 mol / L ferric nitrate solution and impregnated for 12 hours. During the impregnation process, when the bamboo powder was completely impregnated, ammonia was added to adjust the pH of the system to 8.0 so that iron ions were uniformly deposited on the surface and in the internal pores of the bamboo powder in the form of hydroxides or oxides. After impregnation, the mixture was filtered and dried to obtain an iron-supported hard carbon precursor.

[0024] (2) The precursor obtained in step (1) was placed in a tube furnace and subjected to gradient carbonization under a nitrogen protective atmosphere. First, the temperature was increased to 500°C at a heating rate of 5°C / min and held for 2 hours for pre-carbonization; then, the temperature was increased to 1300°C at a heating rate of 2°C / min and held for 3 hours for high-temperature catalytic carbonization. After natural cooling, iron-catalyzed hard carbon material was obtained.

[0025] (3) Add the iron-catalyzed hard carbon material obtained in step (2) into a high-pressure reactor, and add a mixed acid solution (citric acid and hydrochloric acid mixed in a volume ratio of 2:1) at a solid-liquid ratio of 1:5. Seal the reactor, heat it to 150°C, control the pressure at 1.2 MPa, and keep it at that temperature for 3 hours to allow the acid solution to fully penetrate into the microcracks and closed pore edges inside the carbon material.

[0026] (4) The mixed slurry after the reaction in step (3) is subjected to two-stage gradient ultrasonic treatment: The first stage is low-frequency coarse removal, with an ultrasonic frequency of 35 kHz, a power density of 1.0 W / mL, and a 60-minute cycle treatment, used to break up free metal impurities attached to the surface and macropores of carbon particles. The second stage is high-frequency fine stripping, with an ultrasonic frequency of 80 kHz, a power density of 0.5 W / mL, and a cyclic treatment of 90 minutes. The microjets generated by the high-frequency cavitation effect and the local high temperature and pressure are used to precisely strip the nanoscale metal particles wrapped by the carbon lattice.

[0027] After ultrasonic treatment, the slurry is repeatedly washed with deionized water until neutral, and then vacuum dried at 120°C to obtain high-purity hard carbon material with a regular closed-cell structure.

[0028] Example 2 (1) Grapefruit peel that has passed through a 200-mesh sieve was placed in a 0.2 mol / L copper nitrate solution and soaked for 8 hours. During the soaking process, when the grapefruit peel was completely soaked, ammonia was added to adjust the pH of the system to 9.0 so that copper ions were uniformly deposited on the surface and in the internal pores of bamboo powder in the form of hydroxides or oxides. After the soaking was completed, the mixture was filtered and dried to obtain an iron-supported hard carbon precursor.

[0029] (2) The precursor obtained in step (1) was placed in a tube furnace and subjected to gradient carbonization under a nitrogen protective atmosphere. First, the temperature was increased to 400°C at a heating rate of 5°C / min and held for 3.5 hours for pre-carbonization; then, the temperature was increased to 1200°C at a heating rate of 2°C / min and held for 5 hours for high-temperature catalytic carbonization. After natural cooling, copper-catalyzed hard carbon material was obtained.

[0030] (3) Add the iron-catalyzed hard carbon material obtained in step (2) into a high-pressure reactor, and add a mixed acid solution (ascorbic acid and nitric acid mixed in a volume ratio of 2:1) at a solid-liquid ratio of 1:5. Seal the reactor, heat it to 120°C, control the pressure at 1.2 MPa, and keep it at that temperature for 5 hours to allow the acid solution to fully penetrate into the microcracks and closed pore edges inside the carbon material.

[0031] (4) The mixed slurry after the reaction in step (3) is subjected to two-stage gradient ultrasonic treatment: The first stage is low-frequency coarse removal, with an ultrasonic frequency of 20 kHz, a power density of 0.5 W / mL, and a 90-minute cycle treatment, used to break up free metal impurities attached to the surface and macropores of carbon particles. The second stage is high-frequency fine stripping, with an ultrasonic frequency of 60 kHz, a power density of 0.2 W / mL, and a cycle treatment of 120 minutes. The microjets generated by the high-frequency cavitation effect and the local high temperature and pressure are used to precisely strip the nanoscale metal particles wrapped by the carbon lattice.

[0032] After ultrasonic treatment, the slurry is repeatedly washed with deionized water until neutral, and then vacuum dried at 120°C to obtain high-purity hard carbon material with a regular closed-cell structure.

[0033] Example 3 (1) Straw powder that has passed through a 200-mesh sieve was placed in a mixed solution of cobalt nitrate and manganese nitrate (molar ratio 1:1) with a concentration of 0.5 mol / L and impregnated for 10 hours. During the impregnation process, when the straw powder was completely impregnated, ammonia was added to adjust the pH of the system to 10, so that cobalt and manganese ions were uniformly deposited on the surface and in the internal pores of the bamboo powder in the form of hydroxides or oxides. After impregnation, the mixture was filtered and dried to obtain an iron-supported hard carbon precursor.

[0034] (2) The precursor obtained in step (1) was placed in a tube furnace and subjected to gradient carbonization under a nitrogen protective atmosphere. First, the temperature was increased to 600°C at a heating rate of 4°C / min and held for 1.5 hours for pre-carbonization; then, the temperature was increased to 1500°C at a heating rate of 5°C / min and held for 2 hours for high-temperature catalytic carbonization. After natural cooling, cobalt-manganese catalytic hard carbon material was obtained.

[0035] (3) Add the iron-catalyzed hard carbon material obtained in step (2) into a high-pressure reactor, and add a mixed acid solution (citric acid and nitric acid mixed in a volume ratio of 3:2) at a solid-liquid ratio of 1:5. Seal the reactor, heat it to 200°C, control the pressure at 1.2 MPa, and keep it at that temperature for 3 hours to allow the acid solution to fully penetrate into the microcracks and closed pore edges inside the carbon material.

[0036] (4) The mixed slurry after the reaction in step (3) is subjected to two-stage gradient ultrasonic treatment: The first stage is low-frequency coarse removal, with an ultrasonic frequency of 40 kHz, a power density of 1.5 W / mL, and a 30-minute cycle treatment, used to break up free metal impurities attached to the surface and macropores of carbon particles. The second stage is high-frequency fine stripping, with an ultrasonic frequency of 120kHz, a power density of 0.8 W / mL, and a 60-minute cycle. The microjets generated by the high-frequency cavitation effect and the local high temperature and pressure are used to precisely strip the nanoscale metal particles wrapped by the carbon lattice.

[0037] After ultrasonic treatment, the slurry is repeatedly washed with deionized water until neutral, and then vacuum dried at 120°C to obtain high-purity hard carbon material with a regular closed-cell structure.

[0038] Comparative Example 1 (1) Bamboo powder that has passed through a 200-mesh sieve was placed in a 0.2 mol / L ferric nitrate solution and impregnated for 12 hours. During the impregnation process, when the bamboo powder was completely impregnated, ammonia was added to adjust the pH of the system to 8.0 so that iron ions were uniformly deposited on the surface and in the internal pores of the bamboo powder in the form of hydroxides or oxides. After impregnation, the mixture was filtered and dried to obtain an iron-supported hard carbon precursor.

[0039] (2) The precursor obtained in step (1) was placed in a tube furnace and subjected to gradient carbonization under a nitrogen protective atmosphere. First, the temperature was increased to 500°C at a heating rate of 5°C / min and held for 2 hours for pre-carbonization; then, the temperature was increased to 1300°C at a heating rate of 2°C / min and held for 3 hours for high-temperature catalytic carbonization. After natural cooling, iron-catalyzed hard carbon material was obtained.

[0040] (3) The hard carbon material obtained in step (2) is directly filtered, washed and dried to obtain the comparative hard carbon material.

[0041] Comparative Example 2 (1) Bamboo powder that has passed through a 200-mesh sieve was placed in a 0.2 mol / L ferric nitrate solution and impregnated for 12 hours. During the impregnation process, when the bamboo powder was completely impregnated, ammonia was added to adjust the pH of the system to 8.0 so that iron ions were uniformly deposited on the surface and in the internal pores of the bamboo powder in the form of hydroxides or oxides. After impregnation, the mixture was filtered and dried to obtain an iron-supported hard carbon precursor.

[0042] (2) The precursor obtained in step (1) was placed in a tube furnace and subjected to gradient carbonization under a nitrogen protective atmosphere. First, the temperature was increased to 500°C at a heating rate of 5°C / min and held for 2 hours for pre-carbonization; then, the temperature was increased to 1300°C at a heating rate of 2°C / min and held for 3 hours for high-temperature catalytic carbonization. After natural cooling, iron-catalyzed hard carbon material was obtained.

[0043] (3) Add the iron-catalyzed hard carbon material obtained in step (2) into a high-pressure reactor, and add a mixed acid solution (citric acid and hydrochloric acid mixed in a volume ratio of 2:1) at a solid-liquid ratio of 1:5. Seal the reactor, heat it to 150°C, control the pressure at 1.2 MPa, and keep it at that temperature for 3 hours to allow the acid solution to fully penetrate into the microcracks and closed pore edges inside the carbon material.

[0044] (4) The mixed slurry after the reaction in step (3) is directly filtered, washed, and then vacuum dried at 120°C to obtain the comparative hard carbon material.

[0045] Comparative Example 3 (1) Bamboo powder that has passed through a 200-mesh sieve was placed in deionized water and soaked for 12 hours; during the soaking process, ammonia water was added to adjust the pH of the system to 8.0. After the soaking was completed, the mixture was filtered and dried to obtain the hard carbon precursor.

[0046] (2) The precursor obtained in step (1) is placed in a tube furnace and subjected to gradient carbonization under a nitrogen protective atmosphere. First, the temperature is raised to 500°C at a heating rate of 5°C / min and held for 2 hours for pre-carbonization; then, the temperature is raised to 1300°C at a heating rate of 2°C / min and held for 3 hours for high-temperature carbonization. After natural cooling, hard carbon material is obtained.

[0047] (3) Add the hard carbon material obtained in step (2) into a high-pressure reactor, and add a mixed acid solution (citric acid and hydrochloric acid mixed in a volume ratio of 2:1) at a solid-liquid ratio of 1:5. Seal the reactor, heat it to 150°C, control the pressure at 1.2 MPa, and keep it at that temperature for 3 hours.

[0048] (4) The mixed slurry after the reaction in step (3) is subjected to two-stage gradient ultrasonic treatment: The first stage involved low-frequency coarse desorption, with an ultrasonic frequency of 35 kHz and a power density of 1.0 W / mL, and a cycle time of 60 minutes. The second stage involved high-frequency fine desorption, with an ultrasonic frequency of 80 kHz and a power density of 0.5 W / mL, and a cycle time of 90 minutes. After ultrasonic treatment, the slurry was repeatedly washed with deionized water until neutral, and then vacuum dried at 120°C to obtain the comparative hard carbon material.

[0049] Comparative Example 4 (1) Bamboo powder that has passed through a 200-mesh sieve was placed in deionized water and soaked for 12 hours; during the soaking process, ammonia water was added to adjust the pH of the system to 8.0. After the soaking was completed, the mixture was filtered and dried to obtain the hard carbon precursor.

[0050] (2) The precursor obtained in step (1) is placed in a tube furnace and subjected to gradient carbonization under a nitrogen protective atmosphere. First, the temperature is raised to 500°C at a heating rate of 5°C / min and held for 2 hours for pre-carbonization; then, the temperature is raised to 1300°C at a heating rate of 2°C / min and held for 3 hours for high-temperature carbonization. After natural cooling, hard carbon material is obtained.

[0051] (3) The hard carbon material obtained in step (2) is directly filtered, washed and dried (120°C, vacuum) to obtain the comparative hard carbon material.

[0052] Comparative Example 5 (1) Bamboo powder that has passed through a 200-mesh sieve was placed in a 0.2 mol / L ferric nitrate solution and impregnated for 12 hours. During the impregnation process, when the bamboo powder was completely impregnated, ammonia was added to adjust the pH of the system to 8.0 so that iron ions were uniformly deposited on the surface and in the internal pores of the bamboo powder in the form of hydroxides or oxides. After impregnation, the mixture was filtered and dried to obtain an iron-supported hard carbon precursor.

[0053] (2) The precursor obtained in step (1) was placed in a tube furnace and subjected to gradient carbonization under a nitrogen protective atmosphere. First, the temperature was increased to 500°C at a heating rate of 5°C / min and held for 2 hours for pre-carbonization; then, the temperature was increased to 1300°C at a heating rate of 2°C / min and held for 3 hours for high-temperature catalytic carbonization. After natural cooling, iron-catalyzed hard carbon material was obtained.

[0054] (3) Add the iron-catalyzed hard carbon material obtained in step (2) into a high-pressure reactor, and add a mixed acid solution (citric acid and hydrochloric acid mixed in a volume ratio of 2:1) at a solid-liquid ratio of 1:5. Seal the reactor, heat it to 150°C, control the pressure at 1.2 MPa, and keep it at that temperature for 3 hours to allow the acid solution to fully penetrate into the microcracks and closed pore edges inside the carbon material.

[0055] (4) The mixed slurry after the reaction in step (3) was subjected to single-frequency ultrasonic treatment: ultrasonic frequency 35 kHz, power density 1.0 W / mL, and cyclic treatment for 150 minutes. After ultrasonic treatment, the slurry was repeatedly washed with deionized water until neutral, and then vacuum dried at 120℃ to obtain the comparative hard carbon material. This comparative example only used low-frequency ultrasonic treatment and did not undergo a high-frequency fine desorption stage.

[0056] Comparative Example 6 (1) Bamboo powder that has passed through a 200-mesh sieve was placed in a 0.2 mol / L ferric nitrate solution and impregnated for 12 hours. During the impregnation process, when the bamboo powder was completely impregnated, ammonia was added to adjust the pH of the system to 8.0 so that iron ions were uniformly deposited on the surface and in the internal pores of the bamboo powder in the form of hydroxides or oxides. After impregnation, the mixture was filtered and dried to obtain an iron-supported hard carbon precursor.

[0057] (2) The precursor obtained in step (1) was placed in a tube furnace and subjected to gradient carbonization under a nitrogen protective atmosphere. First, the temperature was increased to 500°C at a heating rate of 5°C / min and held for 2 hours for pre-carbonization; then, the temperature was increased to 1300°C at a heating rate of 2°C / min and held for 3 hours for high-temperature catalytic carbonization. After natural cooling, iron-catalyzed hard carbon material was obtained.

[0058] (3) The iron-catalyzed hard carbon material obtained in step (2) was added to deionized water at a solid-liquid ratio of 1:5 (without adding acid), and subjected to a two-stage gradient ultrasonic treatment: the first stage was low-frequency coarse desorption, with an ultrasonic frequency of 35 kHz and a power density of 1.0 W / mL, and the treatment was carried out for 60 minutes; the second stage was high-frequency fine desorption, with an ultrasonic frequency of 80 kHz and a power density of 0.5 W / mL, and the treatment was carried out for 90 minutes. The ultrasonically treated slurry was obtained.

[0059] (4) Filter the slurry after ultrasonic treatment in step (3), add the resulting solid to a high-pressure reactor, and add a mixed acid solution (citric acid and hydrochloric acid mixed at a volume ratio of 2:1) at a solid-liquid ratio of 1:5. Seal the reactor, heat to 150°C, control the pressure at 1.2 MPa, and keep it at that temperature for 3 hours. After the reaction is complete, wash the slurry repeatedly with deionized water until it is neutral, and then vacuum dry it at 120°C to obtain the comparative hard carbon material. The difference between this comparative example and Example 1 is that ultrasonic treatment is performed first, followed by acid leaching, which is the reverse order.

[0060] Test case The hard carbon prepared in Examples 1-3 and Comparative Examples 1-6 was added to aqua regia (concentrated hydrochloric acid: concentrated nitric acid = 3:1), and after microwave digestion, the concentrations of residual externally added catalytic metal elements Fe / Cu / Co / Mn and the main impurity elements Mg and Ca enriched by the carbon source were determined by inductively coupled plasma atomic emission spectrometry. The results are shown in Tables 1 and 2.

[0061] Table 1

[0062] Table 2

[0063] Table 1 compares the concentration of residual catalytic metals in the biomass-derived hard carbon anode materials prepared in Example 1 and Comparative Examples 1-6 using inductively coupled plasma atomic emission spectrometry (ICP-AES). In Example 1, the total metal concentration was less than 50 ppm, with Fe at only 12 ppm. Comparative Example 1, without any impurity removal process, had a total metal concentration as high as 3500 ppm, with Fe residue at 1850 ppm, indicating that a large amount of Fe remained embedded in the carbon matrix as metal / oxide after pyrolysis. Comparative Example 2, after only acid washing, had a total metal concentration of approximately 1050 ppm and Fe at 430 ppm, indicating that acid washing can only remove free metals on the surface and some easily accessible metals, with limited effectiveness in removing metals encased in the carbon layer or at the edges of closed pores. Catalytic pyrolysis without gradient ultrasonic impurity removal results in extremely high residual metal impurities (>1000 ppm), severely impacting the initial coulombic efficiency and self-discharge performance of the battery anode. While acid washing can remove some surface metals, its ability to remove metals encased in the carbon lattice (especially around Fe catalytic sites) is limited, leaving residual metals at hundreds to thousands of ppm. In Comparative Example 3, the carbon material lacked metal catalytic pore-forming, resulting in a dense structure after pyrolysis. This made acid treatment and ultrasonic permeation ineffective, thus even after a complete impurity removal process, the intrinsic biomass metal impurities such as Mg and Ca could not be effectively removed, leaving high residual concentrations (Mg 130 ppm, Ca 165 ppm). In Comparative Examples 4-6, incomplete or incorrectly ordered impurity removal processes also affected the removal effect, indicating that the impurity removal process designed in this invention is not a simple process superposition but rather has a progressive synergistic effect. This invention, through the synergistic effect of gradient ultrasonic cavitation and high-pressure acid impregnation, can reduce total metal impurities to <50 ppm, with the residual amount of catalytic metal Fe controlled below 20 ppm. This achieves deep physical stripping of metals within the crystal lattice, providing reliable technical support for high-purity hard carbon materials.

[0064] The hard carbon prepared in Example 1 and Comparative Example 3 was used to prepare sodium-ion batteries: Hard carbon, conductive carbon black, sodium carboxymethyl cellulose, and styrene-butadiene copolymer prepared in Example 1 and Comparative Example 3 were homogenized in deionized water at a mass ratio of 9:0.5:0.3:0.2, and then coated onto copper foil to obtain a negative electrode sheet. The prepared negative electrode sheet was then combined with a sodium metal sheet, and glass fiber was used as a separator to assemble a CR2032 coin cell. The electrochemical performance of each prepared sodium-ion battery was tested, and the results are as follows: Figure 1 As shown.

[0065] It should be noted that the closed-pore structure in hard carbon cannot be directly characterized at present, and is usually inferred indirectly through electrochemical performance. The improvement in initial coulombic efficiency indicates that electrolyte side reactions are suppressed, which is closely related to the high proportion of closed pores; the increase in reversible specific capacity, especially the improvement in low-potential plateau capacity, is also attributed to the additional sodium storage sites provided by the closed pores. Based on the above electrochemical data, it can be reasonably inferred that the closed-pore structure of the hard carbon material obtained in this invention has been significantly optimized.

[0066] Figure 1 Sodium-ion batteries prepared using the hard carbon anode materials of Example 1 and Comparative Examples 1-3 were tested at 0.05 A g. -1 The first charge-discharge curve at current density, from Figure 1 It can be seen that the biomass-derived hard carbon of Example 1, after iron catalysis and high impurity removal, significantly improves the initial coulombic efficiency and reversible capacity compared to the biomass-derived hard carbon of Comparative Example 3 without metal catalysis. This is mainly due to the localized graphitization transformation of the carbon layer catalyzed by the metal element, forming a closed-pore structure around the carbon microcrystals. The increased proportion of closed-pore volume reduces electrolyte side reactions and the consumption of active sodium ions, resulting in less volume expansion and lower structural stress during charge and discharge. In contrast, although the biomass-derived hard carbon in Comparative Examples 1-2 underwent metal catalysis to regulate its pore structure, the lack of impurity removal or incomplete removal led to irreversible reduction of these impurities during the first discharge, resulting in a large consumption of sodium ions and very low initial coulombic efficiency and reversible capacity. Furthermore, the generated elemental metal blocked the pores, further deteriorating the electrochemical performance.

[0067] As can be seen from Examples 1 and Comparative Examples 2-4, a deep functional synergy is formed between metal catalysis and gradient ultrasonic cavitation, rather than a simple process superposition. First, while metal catalysis constructs a high-density closed-pore structure, it inevitably introduces stubborn metal impurities in the form of lattice encapsulation, which is an inherent problem in existing technologies where catalysis and purification processes contradict each other. Second, this invention cleverly utilizes this residual metal as a sensitive site or cavitation nucleus for subsequent gradient ultrasonic cavitation. The significant difference in thermal expansion coefficient between the metal and the carbon matrix allows for localized stress concentration in the ultrasonic field, significantly reducing the energy threshold for ultrasonic stripping of lattice-encapsulated impurities. Simultaneously, the physical stripping effect of gradient ultrasound does not destroy the already formed closed-pore structure, achieving compatibility and mutual promotion of two opposing process paths—catalytic pore formation and deep purification—within the same material system. Therefore, the technical effect of this invention (total metal <50 ppm and significantly improved closed-pore rate) exceeds the simple superposition of the effects of metal catalysis and gradient ultrasound, demonstrating a non-obvious synergistic mechanism between the two.

[0068] Although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0069] Therefore, the above description is only a preferred embodiment of this application and is not intended to limit the scope of this application; that is, all equivalent modifications made in accordance with the scope of the claims of this application shall be within the protection scope of the claims of this application.

Claims

1. A method for preparing biomass-based hard carbon materials based on metal catalysis and gradient ultrasonic cavitation, characterized in that, Includes the following steps: S1. The biomass raw material is immersed in a soluble metal salt solution, and the pH value is adjusted to alkaline to obtain the precursor. S2. The precursor is subjected to gradient carbonization treatment in an inert gas atmosphere to obtain hard carbon material. S3. Add the hard carbon material to the acid solution and perform activation impregnation treatment under high temperature and high pressure conditions to obtain a mixed slurry; S4. The mixed slurry is subjected to gradient ultrasonic treatment, and after filtration, washing and drying, biomass-based hard carbon material is obtained.

2. The preparation method according to claim 1, characterized in that, The biomass raw material is at least one of bamboo, coconut shell, pomelo peel, and straw.

3. The preparation method according to claim 1, characterized in that, The metal salt contains at least one of Fe, Ni, Co, Mn, and Cu, and the concentration of the metal salt is 0.05-0.5 mol / L.

4. The preparation method according to claim 1, characterized in that, In step S1, adjust the pH value to 8-10 and soak for 8-12 hours.

5. The preparation method according to claim 1, characterized in that, In step S2, the gradient carbonization treatment is carried out under a protective atmosphere at a heating rate of 2-5℃ / min, and includes a pre-carbonization treatment and a high-temperature carbonization treatment in sequence; wherein, the pre-carbonization temperature is 400-600℃ and the holding time is 1.5-3.5 hours; the high-temperature catalytic carbonization temperature is 1000-1600℃ and the holding time is 3-5 hours.

6. The preparation method according to claim 1, characterized in that, In step S3, the acid solution is composed of organic acid and inorganic acid in a volume ratio of (1-3):(1-2); the organic acid is citric acid or ascorbic acid; the inorganic acid is hydrochloric acid or nitric acid.

7. The preparation method according to claim 1, characterized in that, In step S3, the high temperature and high pressure conditions are: temperature 100-200℃, pressure 0.5-2.0 MPa, and heat preservation for 1-5 hours.

8. The preparation method according to claim 1, characterized in that, In step S4, the gradient ultrasound processing specifically includes: The first stage is low-frequency coarse peeling: frequency 20-40 kHz, power density 0.5-1.5 W / mL, treatment time 30-90 minutes; The second stage is high-frequency fine removal: frequency 60-120 kHz, power density 0.2-0.8 W / mL, and treatment time 60-120 minutes.

9. A biomass-based hard carbon material based on metal catalysis and gradient ultrasonic cavitation, characterized in that, It is prepared by the preparation method described in any one of claims 1-8.

10. The application of the biomass-based hard carbon material as described in claim 9 in the negative electrode of a sodium-ion battery.

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