Organic acid cross-linking regulation closed pore structure biomass hard carbon material and preparation method and application thereof

Abstract

This disclosure presents a biomass hard carbon material with a closed-cell structure regulated by organic acid crosslinking, its preparation method, and its applications. The preparation method includes washing and drying natural biomass raw materials, followed by mechanical ball milling to obtain biomass powder; dispersing the biomass powder in a polar solvent, adding a multi-component organic acid crosslinking agent, and reacting under stirring conditions to allow the organic acid to fully esterify or amidate the biomass molecular chains, obtaining a pre-crosslinked biomass slurry; drying the pre-crosslinked biomass slurry to remove the solvent and reaction product water, obtaining a solid composite precursor; subjecting the solid composite precursor to low-temperature stabilization treatment, followed by high-temperature pyrolysis carbonization treatment to obtain a carbonized product; cooling the carbonized product to room temperature, washing it with an inorganic acid solution and deionized water respectively until the filtrate is neutral, and finally vacuum drying and classification to obtain the biomass hard carbon material with a closed-cell structure regulated by organic acid crosslinking.

Description

Technical Field

[0001] This disclosure belongs to the field of new energy materials and electrochemical technology, specifically relating to a biomass hard carbon material with a closed-pore structure regulated by organic acid crosslinking, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries (SIBs) are considered one of the most competitive next-generation battery technologies in large-scale energy storage systems and low-speed electric vehicles due to the abundance, wide distribution, and low cost of sodium resources, as well as their electrochemical performance being close to that of lithium-ion batteries. In the research of anode materials for sodium-ion batteries, hard carbon materials (difficult-to-graphitize carbon) have become the preferred choice for industrial applications due to their lower operating voltage, higher specific capacity, and good cycle stability.

[0003] The sodium storage mechanism of hard carbon is typically described as an "adsorption-intercalation" or "adsorption-filling" model. Its potential curve is divided into two parts: a ramp region (potential above 0.1V) and a plateau region (potential below 0.1V). The ramp capacity mainly originates from the adsorption of sodium ions on the hard carbon surface, edge defects, heteroatom sites, and between randomly stacked carbon layers; while the plateau capacity is considered to originate from the quasi-metallic filling process of sodium ions in the closed micropores (closed pores) formed between carbon crystallites. Increasing the closed pore volume to enhance the plateau capacity is crucial for improving the energy density and reducing the operating voltage of sodium-ion batteries.

[0004] Biomass (such as starch, lignin, and fruit shells) is a renewable carbon source with advantages such as high carbon content, low cost, and wide availability. However, hard carbon materials prepared by directly carbonizing biomass face several bottlenecks: (1) Uncontrollable microstructure: Biomass is often accompanied by violent depolymerization, melting and rearrangement of molecules during pyrolysis. Due to the lack of orderly guidance of natural biomass molecular chains, the gas emissions generated during carbonization will lead to excessive open pores (open pores), which will reduce the degree of closed pore development between carbon microcrystals and result in low plateau capacity.

[0005] (2) Low initial coulombic efficiency (ICE): Biomass carbonization products usually have a high specific surface area (tens or even hundreds of square meters / gram), which leads to the generation of an excessive solid electrode interface (SEI) film in the first charging cycle, consuming a large amount of sodium ions, resulting in an ICE usually below 80%.

[0006] (3) Ash content: Inorganic salts (such as K, Ca, Mg, Si oxides, etc.) contained in natural biomass will form impurity centers after carbonization, affecting the purity and electrochemical stability of the material.

[0007] Existing technologies often employ modification methods such as asphalt coating, high-temperature pre-oxidation, or chemical activation. However, coating materials like asphalt are derived from fossil resources and have poor environmental friendliness; high-temperature pre-oxidation processes are time-consuming and energy-intensive; and chemical activation methods tend to create more open pores, which can actually reduce the platform capacity. Therefore, how to intervene in the pyrolysis pathway of biomass at the molecular level to reduce the specific surface area while inducing the formation of abundant nanopores is a pressing technical challenge in this field. Summary of the Invention

[0008] This disclosure aims to at least solve one of the technical problems existing in the prior art, and to provide a biomass hard carbon material with an organic acid crosslinking-regulated closed-pore structure, its preparation method, and its application.

[0009] One aspect of this disclosure provides a method for preparing a biomass hard carbon material with an organic acid crosslinking-regulated closed-pore structure, the preparation method comprising: Natural biomass raw materials are cleaned, dried, and then mechanically ball-milled to obtain biomass powder. The biomass powder is dispersed in a polar solvent, and a multi-component organic acid crosslinking agent is added. The mixture is reacted under stirring conditions to allow the organic acid to fully undergo esterification or amidation crosslinking with the biomass molecular chains, resulting in a viscous pre-crosslinked biomass slurry. The pre-crosslinked biomass slurry was dried to remove the solvent and water from the reaction product, resulting in a solid composite precursor with a three-dimensional network structure. The solid composite precursor was placed in a furnace with an inert gas atmosphere for low-temperature stabilization treatment, followed by high-temperature pyrolysis carbonization treatment to obtain carbonized products. After cooling the carbonization product to room temperature, it is washed with inorganic acid solution and deionized water respectively until the filtrate is neutral. Finally, it is vacuum dried and graded to obtain biomass hard carbon material with closed-pore structure regulated by organic acid crosslinking.

[0010] Optionally, the particle size D50 of the biomass powder is 5μm-15μm; The natural biomass raw materials are selected from at least one of lignin, cellulose, hemicellulose, starch, chitosan, pectin, wood flour, rice husk, coconut shell, straw, and lees.

[0011] Optionally, the multi-component organic acid crosslinking agent is selected from one or more of citric acid, tannic acid, tartaric acid, malic acid, oxalic acid, succinic acid, EDTA, and phytic acid; The mass ratio of the biomass powder to the multi-component organic acid crosslinking agent is 1:(0.01-0.5).

[0012] Optionally, the polar solvent is selected from one or more of deionized water, ethanol, methanol, acetone, and dimethyl sulfoxide; The mass ratio of the polar solvent to the biomass powder is (5-20):1.

[0013] Optionally, the biomass powder is dispersed in a polar solvent, a multi-organic acid crosslinking agent is added, and the reaction is carried out at a temperature of 60℃-150℃ for 8-24 hours under stirring conditions.

[0014] Optionally, the low-temperature stabilization treatment is performed at a temperature of 200℃-500℃ for 1-5 hours, with a heating rate of 2-5℃ / min. The high-temperature pyrolysis carbonization treatment is carried out at a temperature of 1000℃-1500℃ for 2-10 hours, with a heating rate of 5-10℃ / min.

[0015] Optionally, the inorganic acid solution is one of hydrochloric acid, nitric acid, sulfuric acid, or hydrofluoric acid; The concentration of the inorganic acid solution is 1-3 mol / L; The washing temperature using inorganic acid solution is 40℃-90℃.

[0016] In another aspect, this disclosure proposes a biomass hard carbon material with an organic acid crosslinking-regulated closed-pore structure, wherein the biomass hard carbon material is prepared using the preparation method described above; wherein, The interlayer spacing d002 of the biomass hard carbon material is 0.375 to 0.410 nm, and the closed-cell volume is 0.06 to 0.18 cm³. 3 / g, true density is 1.45 to 1.65 g / cm³ 3 The average pore size ranges from 0.8 to 2.5 nm, and the specific surface area ranges from 1.0 to 5.0 m². 2 / g.

[0017] Another aspect of this disclosure proposes an application of biomass hard carbon materials, using the aforementioned biomass hard carbon materials in electrode materials for sodium-ion batteries, potassium-ion batteries, lithium-ion batteries, or lithium-sulfur batteries; or, The biomass hard carbon materials described above are used in electrochemical sensors, supercapacitors, or catalyst supports.

[0018] Optionally, when the biomass hard carbon material is used as the negative electrode of a sodium-ion battery at a rate of 0.1C, the initial discharge capacity is greater than 330 mAh / g, the platform capacity with a voltage below 0.1V accounts for more than 65%, and the initial coulombic efficiency is between 85% and 92%.

[0019] This disclosure presents a biomass hard carbon material with a closed-cell structure regulated by organic acid crosslinking, its preparation method, and its applications. The preparation method includes: washing and drying natural biomass raw materials, followed by mechanical ball milling to obtain biomass powder; dispersing the biomass powder in a polar solvent, adding a multi-component organic acid crosslinking agent, and reacting under stirring conditions to allow the organic acid to fully esterify or amidate the biomass molecular chains, resulting in a viscous pre-crosslinked biomass slurry; drying the pre-crosslinked biomass slurry to remove the solvent and reaction product water, obtaining a solid composite precursor with a three-dimensional network structure; placing the solid composite precursor in a furnace under a protective atmosphere of inert gas for low-temperature stabilization treatment, followed by high-temperature pyrolysis carbonization treatment to obtain a carbonized product; cooling the carbonized product to room temperature, washing it with an inorganic acid solution and deionized water until the filtrate is neutral, and finally vacuum drying and classification to obtain the biomass hard carbon material with a closed-cell structure regulated by organic acid crosslinking. This method first involves molecular-level mixing of polybasic organic acids (such as citric acid and tannic acid) with biomass precursors rich in active functional groups such as hydroxyl and amino groups (such as starch, lignin, and cellulose) in a liquid phase environment. The polycarboxyl functional groups in the organic acids then undergo esterification or amidation crosslinking reactions with the biomass molecular chains to construct a composite precursor with a three-dimensional topological network structure before carbonization. Subsequently, after low-temperature stabilization and high-temperature pyrolysis carbonization at 1000℃-1500℃, the anchoring effect of the crosslinked network on the carbon layer rearrangement induces the formation of abundant nanoscale closed micropores (closed pores). Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating a method for preparing a biomass hard carbon material with an organic acid crosslinking-regulated closed-pore structure, which is a specific embodiment of this disclosure. Detailed Implementation

[0021] To enable those skilled in the art to better understand the technical solutions of this disclosure, the disclosure will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain this disclosure and represent a part of the embodiments of this disclosure, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this disclosure without creative effort are within the protection scope of this disclosure.

[0022] As shown in Figure 1, one aspect of this disclosure provides a method S100 for preparing biomass hard carbon materials with closed-pore structures regulated by organic acid crosslinking, specifically including the following steps S110~S150: S110. Precursor crushing and pretreatment: After cleaning and drying, the natural biomass raw materials are mechanically ball-milled to obtain biomass powder.

[0023] In step S110, the biomass precursor is selected from at least one of lignin, cellulose, hemicellulose, starch, chitosan, pectin, wood flour, rice husk, coconut shell, straw, and dregs. The biomass molecule (such as cellulose, lignin, starch, etc.) is rich in hydroxyl (-OH), carboxyl or amino functional groups.

[0024] In step S110, the particle size D50 of the biomass powder is 5μm-15μm.

[0025] This embodiment, by pretreating the biomass and crushing the biomass raw materials, controls the particle size to 5-15 micrometers, which is beneficial to the uniformity of subsequent liquid phase crosslinking and also to the coating of the final electrode slurry.

[0026] S120, Molecular-level liquid-phase crosslinking reaction: The biomass powder is dispersed in a polar solvent, a multi-component organic acid crosslinking agent is added, and the reaction is carried out under stirring conditions to allow the organic acid and biomass molecular chains to undergo full esterification or amidation crosslinking, resulting in a viscous pre-crosslinked biomass slurry.

[0027] In step S120, the multi-component organic acid crosslinking agent is selected from one or more of citric acid, tannic acid, tartaric acid, malic acid, oxalic acid, succinic acid, EDTA, and phytic acid; the mass ratio of the biomass powder to the multi-component organic acid crosslinking agent is 1:(0.01-0.5). This mass ratio ensures a sufficient minimum amount of crosslinking agent to effectively and multi-point chemically crosslink with the active functional groups (hydroxyl, amino) on the biomass molecular chain, forming a preliminary three-dimensional network framework. If the ratio is too low, there will be insufficient crosslinking points, resulting in a loose network structure that cannot effectively "anchor" carbon layer rearrangement during subsequent carbonization, thus failing to regulate the process. If the ratio is too high, excessive crosslinking agent may lead to an overly dense and rigid network, potentially inhibiting necessary pyrolysis and aromatization during carbonization, affecting carbon yield and conductivity.

[0028] The polybasic organic acids selected in this invention (such as citric acid, tannic acid, malic acid, etc.) contain two or more carboxyl groups. Through subsequent liquid-phase impregnation and heat treatment processes, the carboxyl groups of the organic acid molecules can undergo esterification with the hydroxyl groups of biomass molecules or amidation with the amino groups.

[0029] In step S120, the polar solvent is selected from one or more of deionized water, ethanol, methanol, acetone, and dimethyl sulfoxide; the mass ratio of the polar solvent to the biomass powder is (5-20):1. This mass ratio ensures that the biomass powder is fully wetted and dispersed by the solvent, forming a uniform slurry system. Sufficient solvent is a prerequisite for the reactants (biomass molecular chains and organic acid molecules) to fully contact, diffuse, and undergo chemical reactions. Insufficient solvent will lead to excessively high system viscosity, uneven mixing, and the reaction being limited to a localized area; excessive solvent may reduce the reaction rate, etc.

[0030] In step S120, the biomass powder is dispersed in a polar solvent, a multi-component organic acid crosslinking agent is added, and the reaction is carried out at a temperature of 60℃-150℃ for 8-24 hours under stirring conditions.

[0031] In this embodiment, the powder is placed in a polar solvent such as deionized water or ethanol, and preferably, a polybasic organic acid is added at a mass ratio of approximately 1:0.1. Stirring at 60°C to 150°C provides sufficient reaction time and conditions to ensure the formation of a cross-linked network. During this stage, the carboxyl and hydroxyl groups begin dehydration condensation. For precursors such as starch, this process involves simultaneous gelatinization and cross-linking, ultimately forming a polymeric gel. In other words, the main function of step S120 is to construct a composite precursor with a three-dimensional network topology before carbonization through the esterification / amidation reaction of the polybasic organic acid with the biomass molecular chains. S130. Drying and curing treatment: The pre-crosslinked biomass slurry is vacuum dried or spray dried to remove the solvent and water from the reaction product, thereby obtaining a solid composite precursor with a three-dimensional network structure.

[0032] In step S130, the drying temperature is preferably 105-110℃, and the time is 6-12h.

[0033] This embodiment can preferably use vacuum drying to preserve the three-dimensional network structure and avoid structural collapse caused by direct heating. In other words, the main function of the drying process in step S130 is to remove the solvent and the water generated by the reaction, and fix the cross-linked network into a solid three-dimensional structure. That is, it is mainly for the viscous pre-cross-linked slurry that has formed a network structure. Its purpose is to solidify the network, not just to dry the moisture.

[0034] S140, Programmed temperature carbonization: The solid composite precursor is placed in a furnace with an inert gas atmosphere for low-temperature stabilization treatment, followed by high-temperature pyrolysis carbonization treatment to obtain carbonized products.

[0035] In step S140, the low-temperature stabilization treatment is carried out at a temperature of 200℃-500℃ for 1-5 hours, with a heating rate of 2-5℃ / min. In the lower temperature range of 200℃ to 500℃, the cross-linked biomass-organic acid composite precursor begins to undergo further pyrolysis, dehydration, and condensation reactions. During this stage, the three-dimensional network formed by esterification / amidation reactions further undergoes chemical bond rearrangement and aromatization (initial carbonization) to form a stable, rigid carbonaceous framework precursor.

[0036] In step S140, the high-temperature pyrolysis carbonization treatment is carried out at a temperature of 1000℃-1500℃ for 2-10 hours, with a heating rate of 5-10℃ / min. At the high temperature of 1000℃ to 1500℃, the stabilized framework undergoes complete carbonization. At this point, non-carbon elements are almost completely removed, and carbon atoms rearrange to form a typical hard carbon (difficult-to-graphitize carbon) structure with short-range order and long-range disorder in a randomized stacked structure.

[0037] In step S140, the inert atmosphere is selected from nitrogen, argon or helium, and the gas flow rate is controlled at 100-500 mL / min.

[0038] This embodiment employs a stepped heating method, performing low-temperature stabilization in the 300℃-500℃ range to further solidify the cross-linked structure; and high-temperature carbonization in the 1200℃-1400℃ range. This temperature range is the golden temperature zone for the development of hard carbon interlayer spacing and the formation of closed pores. Pyrolytic carbonization is carried out within the cross-linked network framework, utilizing the "anchoring effect" of the cross-linked network to guide the orderly rearrangement of carbon layers, inhibiting the formation of large-size open pores and promoting the development of nanoscale closed pores.

[0039] S150. Post-processing and refining: After cooling the carbonized product to room temperature, it is washed with inorganic acid solution and deionized water respectively until the filtrate is neutral. Finally, it is vacuum dried and graded to obtain biomass hard carbon material with closed-pore structure regulated by organic acid crosslinking.

[0040] In step S150, the inorganic acid solution is one of hydrochloric acid, nitric acid, sulfuric acid or hydrofluoric acid, and the concentration of the inorganic acid solution is 1-3 mol / L.

[0041] In step S150, the washing temperature using an inorganic acid solution is 40℃-90℃, and the time is 3-5h.

[0042] This embodiment removes residual metal ions and oxide ash by using dilute hydrochloric acid or nitric acid.

[0043] In this embodiment, through liquid-phase impregnation and heat treatment, the carboxyl groups of organic acid molecules can undergo esterification with the hydroxyl groups of biomass molecules or amidation with the amino groups. This reaction establishes chemical bridges between molecular chains before carbonization, constructing a cross-linked network with three-dimensional topological properties. This cross-linked network plays a key regulatory role in the subsequent pyrolysis and carbonization process. 1. Molecular pinning effect: At high temperatures, the cross-linked network constrains the directional rearrangement of carbon layers through covalent bonds. At temperatures above 1000℃, carbon layers usually tend to rearrange in parallel (graphitization tendency), but under the constraint of cross-linking points, the carbon layers are twisted and randomly arranged, thus forming a large number of closed spaces (closed pores) between microcrystals.

[0044] 2. Pore Closure Effect: Small molecule fragments produced by the pyrolysis of organic acids can fill and seal some open pores in situ, reducing the specific surface area of ​​the material from tens of m² / m² in traditional methods. 2 / g reduced to 5 m 2 / g or less.

[0045] 3. Purity Optimization: The combination of liquid-phase reaction and subsequent acid washing and refining effectively removes the original mineral components of biomass and improves the purity of carbon materials.

[0046] The preparation process disclosed herein is characterized by molecular-level controllability, low cost, and high performance. This method first involves molecular-level mixing of polybasic organic acids (such as citric acid and tannic acid) with biomass precursors rich in active functional groups such as hydroxyl and amino groups (such as starch, lignin, and cellulose) in a liquid phase environment. Utilizing the polycarboxyl functional groups in the organic acids, esterification or amidation crosslinking reactions occur with the biomass molecular chains, constructing a composite precursor with a three-dimensional topological network structure before carbonization. Subsequently, after low-temperature stabilization and high-temperature pyrolysis carbonization at 1000℃-1500℃, the anchoring effect of the crosslinked network on the carbon layer rearrangement induces the formation of abundant nanoscale closed micropores (closed pores). Furthermore, the hard carbon material prepared by this invention has an extremely low specific surface area (1.0-5.0 m²). 2 / g) and significantly developed closed-pore structures (closed-pore volume reaches 0.06-0.18 cm³). 3 When used as a negative electrode in sodium-ion batteries, this material exhibits an initial discharge specific capacity greater than 330 mAh / g at 0.1C, a low-potential plateau capacity ratio exceeding 65%, an initial coulombic efficiency as high as 85%-92%, and demonstrates excellent cycle stability and rate performance.

[0047] In another aspect of this disclosure, a biomass hard carbon material with a closed-pore structure regulated by organic acid crosslinking is proposed, which is prepared by the method described above.

[0048] In this embodiment, the interlayer spacing d002 of the biomass hard carbon material is 0.375 to 0.410 nm, and the closed-cell volume is 0.06 to 0.18 cm³. 3 / g, true density is 1.45 to 1.65 g / cm³ 3 The average pore size ranges from 0.8 to 2.5 nm, and the specific surface area ranges from 1.0 to 5.0 m². 2 / g.

[0049] In another aspect of this disclosure, an application of the biomass hard carbon material is proposed. For example, in some preferred embodiments, the biomass hard carbon material can be used as an electrode material in sodium-ion batteries, potassium-ion batteries, lithium-ion batteries, or lithium-sulfur batteries. For example, in other preferred embodiments, the biomass hard carbon material can also be used in electrochemical sensors, supercapacitors, or catalyst supports.

[0050] As a further preferred option, when biomass hard carbon material is used as the negative electrode of sodium-ion battery at a rate of 0.1C, the initial discharge capacity is greater than 330 mAh / g, the plateau capacity with a voltage below 0.1V accounts for more than 65%, and the initial coulombic efficiency is between 85% and 92%.

[0051] The following will further illustrate the preparation method of biomass hard carbon materials with closed-pore structures regulated by organic acid crosslinking with specific embodiments: Example 1: Citric acid crosslinking regulates starch-based hard carbon materials (1) Take 50g of corn starch and disperse it in 300mL of deionized water to obtain a suspension.

[0052] (2) Weigh 5g of citric acid (analytical grade) and add it to the suspension. Stir in a water bath at 95℃. As the esterification reaction proceeds, the slurry gradually turns into a transparent light yellow gel.

[0053] (3) Transfer the gel to a vacuum oven and dry it at 105°C for 12 hours, then grind it into powder.

[0054] (4) Place the powder in a tube furnace and introduce high-purity argon gas. Increase the temperature to 350°C at 3°C / min and hold for 2 hours; then increase the temperature to 1300°C at 5°C / min and hold for 4 hours.

[0055] (5) After cooling, the product was washed with 2 mol / L hydrochloric acid at 70°C for 4 hours, and finally washed with water until neutral. The product was then dried under vacuum to obtain the hard carbon material.

[0056] Test results: The interlayer spacing d002 of the material is 0.385 nm. True density testing (helium displacement method) shows its true density to be 1.51 g / cm³. 3 The calculated closed-pore volume is 0.138 cm³. 3 / g. Specific surface area is 2.1 m². 2 / g.

[0057] Electrochemical performance: At a current density of 0.1C, the initial discharge capacity is 345.8 mAh / g, the initial charge capacity is 307.8 mAh / g, and the initial coulombic efficiency is 89.0%. The plateau capacity (<0.1V) is 242.0 mAh / g. After 1000 cycles at 1C, the capacity retention is 92.4%.

[0058] Example 2: Tannic acid crosslinked lignin-based hard carbon material (1) Take 30g of industrial alkali lignin and dissolve it in 150mL of ethanol solution to obtain a suspension.

[0059] (2) Add 4.5g of tannic acid (mass ratio 1:0.15) to the suspension and reflux at 80℃ for 12 hours.

[0060] (3) The ethanol solvent was recovered and the solid product was dried at 110°C for 6 hours.

[0061] (4) Under a nitrogen atmosphere, first heat up to 400℃ at 2℃ / min for 2 hours and then heat up to 1200℃ at 8℃ / min for 5 hours for pyrolysis and carbonization.

[0062] (5) The pickling process is the same as in Example 1, and the above-mentioned hard carbon material is obtained.

[0063] Test results: The interlayer spacing d002 of the material is 0.394 nm. The closed-cell volume is 0.115 cm³. 3 / g. Specific surface area is 4.3 m². 2 / g.

[0064] Electrochemical performance: Initial discharge capacity is 318.5 mAh / g, with an initial efficiency of 87.2%. Plateau capacity is 202.0 mAh / g. At a high current rate of 2C, the capacity remains above 180 mAh / g.

[0065] Example 3: Phytic acid and malic acid synergistic crosslinking of cellulose materials (1) Weigh 20g of microcrystalline cellulose and add 100mL of deionized water to obtain a suspension.

[0066] (2) Add 1g of phytic acid and 2g of malic acid to the suspension. Perform the liquid-phase reaction in a reactor at 140℃ for 10 hours.

[0067] (3) The solid product was dried at 110°C for 6 hours.

[0068] (4) Under an argon atmosphere, the temperature is first raised to 400℃ at 2℃ / min for 2 hours for stabilization, and then raised to 1400℃ at 8℃ / min for high-temperature carbonization for 3 hours to obtain the above-mentioned hard carbon material.

[0069] (5) The pickling process is the same as in Example 1, and the above-mentioned hard carbon material is obtained.

[0070] Test results: The interlayer spacing d002 of the material is 0.378 nm. The closed-cell volume is 0.162 cm³. 3 / g. Specific surface area is 1.8 m². 2 / g.

[0071] Electrochemical performance: Initial discharge capacity is 332 mAh / g, initial efficiency is 90.5%, and plateau capacity is as high as 255 mAh / g.

[0072] Comparative Example 1: Unmodified starch hard carbon material (1) Take 50g of corn starch and dry it directly at 105℃ for 12 hours.

[0073] (2) Carbonize directly under the same conditions (1300℃) for 4 hours without acid treatment and cross-linking reaction.

[0074] (3) Wash and dry to obtain the above-mentioned hard carbon material.

[0075] Test results: The interlayer spacing d002 of the material is 0.366 nm. The true density is 2.08 g / cm³. 3 The closed-cell volume is only 0.032 cm³. 3 / g (indicating that most of the internal channels are interconnected open pores). Specific surface area is 25.8 m². 2 / g.

[0076] Electrochemical performance: The initial discharge capacity is only 241.5 mAh / g, the charging capacity is 181.1 mAh / g, and the initial efficiency is only 75.0%. Due to the lack of closed pores, its low potential plateau specific capacity is only 82 mAh / g.

[0077] In summary, a comparison of the data from Example 1 and Comparative Example 1 clearly shows that, in the absence of organic acid crosslinking, biomass tends to undergo a certain degree of microcrystalline ordering during carbonization, resulting in a higher true density and extremely small closed-pore space (0.032 cm). 3 / g). Meanwhile, the numerous micro- and nano-pores generated by pyrolysis cannot close, resulting in a huge specific surface area and causing severe electrolyte side reactions. However, after adding citric acid for pre-crosslinking, the three-dimensional network constructed by the esterification reaction effectively anchors the carbon layer structure at high temperatures, maintaining the true density at a low level (1.51 g / cm³). 3 This forces the retention of a large number of closed pores (0.138 cm). 3This directly resulted in a surge in the plateau region capacity from 82 mAh / g to 242 mAh / g. Simultaneously, in-situ passivation of surface defects reduced the specific surface area by more than tenfold, and the initial efficiency increased from 75% to 89%, demonstrating the significant technical superiority of this invention.

[0078] In summary, the hard carbon anode material prepared in this disclosure exhibits a significantly extended low-potential plateau capacity, extremely high initial coulombic efficiency, and excellent cycle stability in sodium-ion battery tests. The aforementioned preparation strategy is not only applicable to common carbon sources such as starch, but can also be extended to various lignins and agricultural wastes, playing a crucial supporting role in promoting the high-value utilization of biomass and the industrialization of sodium-ion batteries.

[0079] This disclosure presents a biomass hard carbon material with a closed-pore structure regulated by organic acid crosslinking, its preparation method, and its application, which have the following advantages over existing technologies: 1. Significantly improves the specific capacity of the plateau: The closed-cell volume of hard carbon prepared by this invention is more than 3 times higher than that of traditional processes, and the proportion of plateau capacity to total capacity is increased to more than 65%.

[0080] 2. Extremely high initial coulombic efficiency: By sealing surface defects with the help of organic acids, the specific surface area is significantly reduced, and the ICE can stably reach more than 88%, with a maximum of 92%.

[0081] 3. Long cycle life: The three-dimensional cross-linked structure gives the carbon skeleton stronger mechanical strength and strong resistance to stress damage during long-term cycling.

[0082] 4. Raw materials are cheap and readily available: Citric acid, starch, etc. are all cheap bulk industrial products, and the preparation process does not require expensive equipment.

[0083] It is understood that the above embodiments are merely exemplary embodiments used to illustrate the principles of this disclosure, and this disclosure is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and substance of this disclosure, and these modifications and improvements are also considered to be within the scope of protection of this disclosure.

Claims

1. A method for preparing biomass hard carbon materials with closed-cell structures regulated by organic acid crosslinking, characterized in that, The preparation method includes: Natural biomass raw materials are cleaned, dried, and then mechanically ball-milled to obtain biomass powder. The biomass powder is dispersed in a polar solvent, and a multi-component organic acid crosslinking agent is added. The mixture is reacted under stirring conditions to allow the organic acid to fully undergo esterification or amidation crosslinking with the biomass molecular chains, resulting in a viscous pre-crosslinked biomass slurry. The pre-crosslinked biomass slurry was dried to remove the solvent and water from the reaction product, resulting in a solid composite precursor with a three-dimensional network structure. The solid composite precursor was placed in a furnace with an inert gas atmosphere for low-temperature stabilization treatment, followed by high-temperature pyrolysis carbonization treatment to obtain carbonized products. After cooling the carbonization product to room temperature, it is washed with inorganic acid solution and deionized water respectively until the filtrate is neutral. Finally, it is vacuum dried and graded to obtain biomass hard carbon material with closed-pore structure regulated by organic acid crosslinking.

2. The preparation method according to claim 1, characterized in that, The particle size D50 of the biomass powder is 5μm-15μm; The natural biomass raw materials are selected from at least one of lignin, cellulose, hemicellulose, starch, chitosan, pectin, wood flour, rice husk, coconut shell, straw, and lees.

3. The preparation method according to claim 1, characterized in that, The multi-component organic acid crosslinking agent is selected from one or more of citric acid, tannic acid, tartaric acid, malic acid, oxalic acid, succinic acid, EDTA, and phytic acid; The mass ratio of the biomass powder to the multi-component organic acid crosslinking agent is 1:(0.01-0.5).

4. The preparation method according to claim 1, characterized in that, The polar solvent is selected from one or more of deionized water, ethanol, methanol, acetone, and dimethyl sulfoxide; The mass ratio of the polar solvent to the biomass powder is (5-20):

1.

5. The preparation method according to claim 1, characterized in that, The biomass powder is dispersed in a polar solvent, and a multi-component organic acid crosslinking agent is added. The reaction is carried out under stirring conditions at a temperature of 60℃-150℃ for 8-24 hours.

6. The preparation method according to claim 1, characterized in that, The low-temperature stabilization treatment is performed at a temperature of 200℃-500℃ for 1-5 hours, with a heating rate of 2-5℃ / min. The high-temperature pyrolysis carbonization treatment is carried out at a temperature of 1000℃-1500℃ for 2-10 hours, with a heating rate of 5-10℃ / min.

7. The preparation method according to claim 1, characterized in that, The inorganic acid solution is one of hydrochloric acid, nitric acid, sulfuric acid or hydrofluoric acid; The concentration of the inorganic acid solution is 1-3 mol / L; The washing temperature using inorganic acid solution is 40℃-90℃.

8. A biomass hard carbon material with a closed-cell structure regulated by organic acid crosslinking, characterized in that, The biomass hard carbon material is prepared using the preparation method described in any one of claims 1-7; wherein... The interlayer spacing d002 of the biomass hard carbon material is 0.375 to 0.410 nm, and the closed-cell volume is 0.06 to 0.18 cm³. 3 / g, true density is 1.45 to 1.65 g / cm³ 3 The average pore size ranges from 0.8 to 2.5 nm, and the specific surface area ranges from 1.0 to 5.0 m². 2 / g.

9. An application of a biomass hard carbon material, characterized in that, The biomass hard carbon material described in claim 8 may be used in electrode materials for sodium-ion batteries, potassium-ion batteries, lithium-ion batteries, or lithium-sulfur batteries; or, The biomass hard carbon material described in claim 8 can be used in electrochemical sensors, supercapacitors, or catalyst supports.

10. The application according to claim 9, characterized in that, When the biomass hard carbon material is used as the negative electrode of a sodium-ion battery at a rate of 0.1C, the initial discharge capacity is greater than 330 mAh / g, the plateau capacity with a voltage below 0.1V accounts for more than 65%, and the initial coulombic efficiency is between 85% and 92%.

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