Hard carbon material and preparation method and application thereof

Abstract

The invention provides a hard carbon material and a preparation method and application thereof.The hard carbon material is composed of a three-dimensional carbon layer, the three-dimensional carbon layer is provided with micropores and closed holes, defect structures are distributed at the interfaces of the micropores and the closed holes and the three-dimensional carbon layer, and the ID / IG of the hard carbon material is 1.105-1.286. The hard carbon material provided by the invention is a biomass derived hard carbon material capable of cooperatively regulating and controlling interlayer spacing, a closed pore structure and defect distribution, and cooperative controllability of closed pore size, closed pore volume and microcrystal disorder degree is realized, so that high platform capacity, relatively high ICE and excellent rate capability are considered; moreover, according to the preparation method provided by the invention, biomass component decomposition and structure reconstruction are induced through a specific chemical reaction system, so that a hard carbon structure with multi-scale pore and ordered defect composite characteristics is obtained after carbonization, and unification of high platform capacity and good cycle stability is realized.

Description

Technical Field

[0001] This invention belongs to the technical field of secondary battery anode materials, and relates to a hard carbon material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries (SIBs) are considered a strong complement to lithium-ion batteries due to the abundance and low cost of sodium resources. Hard carbon, as one of the mainstream anode materials for SIBs, combines high reversible capacity, good rate performance, and low cost, making it a key material driving the commercialization of SIBs. Biomass-derived hard carbon, with its renewable nature, low cost, and naturally porous / anisotropic precursor structure, has become the main direction for hard carbon production.

[0003] Currently, the main methods for preparing hard carbon from biomass feedstocks are as follows:

[0004] I. Pretreatment and carbonization with exogenous peracetic acid (PAA) or other strong oxidants: (1) Immerse biomass raw materials (such as lignocellulose, bamboo powder, fruit shells, etc.) in an externally prepared peracetic acid solution or H2O2 / acetic acid mixture at an appropriate solid-liquid ratio; (2) React for a certain time at room temperature or under mild heating conditions to achieve oxidation / delignination; (3) Filter, wash, neutralize, and dry; (4) Carbonize under an inert atmosphere at a predetermined temperature to obtain hard carbon samples. II. Carbonization after alkali activation to generate high specific surface area and porous structure: Mix / impregnate biomass with KOH in a certain proportion, dry, and then carbonize at high temperature under an inert atmosphere. Then, remove residual alkali metals by acid washing to obtain activated carbon. III. Carbonization after selective degradation of lignin / hemicellulose by enzymatic hydrolysis or biological methods: Treat biomass with lignin-degrading enzymes (such as laccase / peroxidase) or cellulase under mild conditions to selectively remove or reconstruct lignin components; then dry and carbonize to obtain hard carbon.

[0005] However, the hard carbon obtained by the above methods, or the methods themselves, still have the following problems: 1. Uncontrollable pore structure and insufficient closed pores. Current biomass hard carbon preparation methods mostly employ direct pyrolysis or chemical activation (KOH, ZnCl2, etc.), leading to difficulties in pore structure control. In particular, the ratio of closed pores to micropores is unreasonable, which is not conducive to the filling behavior of sodium ions on the low potential plateau. 2. Small interlayer spacing and limited sodium ion diffusion. The interlayer spacing after traditional biomass carbonization is often less than 0.37 nm, which cannot effectively promote sodium diffusion. +3. Defect structure disorder and insufficient active sites: Conventional pyrolysis processes lack control over the types and distribution of defects in the carbon structure, resulting in low density of active sites (such as edge carbon and heteroatom doping sites), affecting adsorption / filling capacity for sodium storage and electrochemical stability. 4. Complex surface functional groups and low initial coulombic efficiency: Biomass contains active groups such as carboxyl, hydroxyl, and aldehyde groups. Without effective passivation or selective decomposition, the surface is prone to side reactions with the electrolyte after carbonization, leading to a low initial coulombic efficiency (ICE). 5. Acid / alkali treatment methods have side effects: Traditional acid leaching modification (such as H2SO4, HCl) can remove impurities and adjust the structure to some extent, but it can easily cause excessive damage to the biomass skeleton or introduce too many heterofunctional groups, which is not conducive to the formation of a stable structure in subsequent carbonization. Alkali activation (KOH, NaOH) may introduce metal residues or cause structural collapse. 6. High energy consumption, complex process steps, and heavy environmental burden: Existing multi-step chemical modification and high-temperature activation processes consume a lot of energy and chemical reagents, which do not meet the needs of green and sustainable development.

[0006] Furthermore, pretreatment with exogenous PAA or strong oxidants can alter lignin composition, but it presents issues of safety, peroxidation, and insufficient selectivity. Alkali activation can significantly increase pore volume and specific surface area, but it mainly generates open pores and impairs ICE and structural stability. Although enzymatic hydrolysis and DES can more gently / precisely regulate the precursor microstructure, they have practical limitations in terms of cost, scale-up, or solvent treatment.

[0007] Therefore, this paper provides a novel biomass-based hard carbon material and its preparation method, which specifically addresses the problems of difficulty in controlling pore structure, disordered defect structure, excessively small interlayer spacing, and heavy environmental burden of modification methods in the preparation of biomass hard carbon. This is of great significance for further improving the performance of hard carbon materials. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a hard carbon material, its preparation method, and its applications. The hard carbon material provided by this invention is a biomass-derived hard carbon material with synergistic control over interlayer spacing, closed-pore structure, and defect distribution. It achieves synergistic control over closed-pore size, closed-pore volume, and microcrystalline disorder, thereby balancing high plateau capacity, high ICE (intercalation efficiency), and excellent rate performance. The preparation method provided by this invention induces the decomposition and structural reconstruction of biomass components through a specific chemical reaction system, thereby obtaining a hard carbon structure with multi-scale porosity and ordered defect composite characteristics after carbonization, achieving a balance between high plateau capacity and good cycling stability.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a hard carbon material, the hard carbon material being composed of a three-dimensional carbon layer, wherein the three-dimensional carbon layer has micropores and closed pores, and defect structures are distributed at the interface between the micropores and closed pores and the three-dimensional carbon layer, wherein the hard carbon material I D / I G The range is 1.105 to 1.286. D / I G For example, 1.105, 1.15, 1.2, 1.25, or 1.286, etc.

[0011] The hard carbon material in this invention achieves synergistic effects of multiple sodium storage mechanisms through a unique structural design: the material simultaneously constructs a two-level pore structure of micropores and small-sized closed pores. The closed-pore regions provide stable, low-potential platform sites for sodium ions, while the micropores help alleviate volume strain during charging and discharging and contribute additional sodium storage capacity through surface adsorption. These pore structures are continuously interconnected, promoting rapid wetting of the electrolyte and Na+. + It achieves efficient diffusion and, together with the regular carbon layer, forms a unique "face-pore coupling" multidimensional diffusion channel. This channel can significantly reduce ion diffusion resistance and, while ensuring the continuity of electronic conductivity of the carbon skeleton, effectively suppress the excessive growth of the solid electrolyte interface film on the electrode surface.

[0012] Furthermore, the hierarchical pore structure and carbon layer structure in the material form a synergistic sodium storage reaction mechanism of "intercalation-pore filling": sodium ions can simultaneously intercalate between carbon layers and fill the closed pores, thereby significantly increasing the platform capacity ratio of the material. In addition, the carefully regulated defect regions in the material contain Na... + This provides abundant low-energy-barrier adsorption sites, enhancing the surface adsorption and sodium storage capacity. Ultimately, the synergistic effect of this porous structure, carbon layer, and defects collectively improves the material's overall sodium storage performance.

[0013] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The technical objectives and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0014] Preferably, the pore size of the micropores is <2nm, such as 0.5nm, 0.8nm, 1nm, 1.3nm, 1.5nm, 1.7nm or 1.9nm, and the pore size of the closed pores is 2nm~5nm, such as 2nm, 2.5nm, 3nm, 3.5nm, 4nm, 4.5nm or 5nm.

[0015] In this invention, the micropores have a uniform pore size distribution, high material orderliness, and a compact structure, which is more conducive to the long-term stability of the battery.

[0016] Preferably, the pore volume of the closed pore is 0.05 cm³.3 g -1 ~0.09cm 3 g -1 For example, 0.05cm 3 g -1 0.06cm 3 g -1 0.07cm 3 g -1 0.08cm 3 g -1 or 0.09cm 3 g -1 wait.

[0017] Preferably, the interlayer spacing d(002) of the carbon layer is 0.376nm~0.387nm, such as 0.376nm, 0.38nm, 0.382nm, 0.384nm, 0.385nm or 0.387nm.

[0018] The hard carbon material provided by this invention has a moderate interlayer spacing and a uniformly distributed closed-cell structure, while ensuring Na + While enabling rapid intercalation, the material's structural compactness is ensured, thus balancing the battery's plateau capacity and long-term stability. If the interlayer spacing is too small, intercalation becomes difficult; if the interlayer spacing is too large, the interaction forces between carbon layers are significantly weakened, making the hard carbon structure looser and more prone to collapse. During cycling, the carbon layers are prone to irreversible slippage and disordered stacking, leading to material structural damage and rapid capacity decay after long-term cycling.

[0019] Preferably, the defect structure includes sp 3 -sp 2 Mixed boundary defects and oxygen vacancy regulation defects.

[0020] The performance of hard carbon sodium storage is influenced by various microstructural features, mainly including: the disorder / graphitization degree of the carbon layers, interlayer spacing (d002), crystallite size (La, Lc), pore structure (micropores, mesopores, closed pores / closed pores), surface / interfacial functional groups (oxygen-containing, nitrogen-containing groups, etc.), and defect type and density. These structures collectively determine the relative contributions of "slope capacity—interlayer intercalation behavior" and "plateau capacity—closed pore / micropore filling behavior," thus affecting the initial coulombic efficiency (ICE), reversible capacity, rate performance, and cycling stability. The hard carbon material provided by this invention exhibits excellent performance in all of the above parameters.

[0021] In a second aspect, the present invention provides a method for preparing hard carbon as described in the first aspect, the method comprising:

[0022] (1) Ethyl acetate, hydrogen peroxide and catalyst are mixed to obtain a treatment solution, and then the biomass material is mixed with the treatment solution to carry out an in-situ oxidation reaction;

[0023] (2) The product after the in-situ oxidation reaction in step (1) is subjected to pre-carbonization and carbonization in sequence to obtain the hard carbon material.

[0024] In this invention, addressing the technical challenges of existing biomass hard carbon preparation processes, such as the difficulty in synergistically controlling pore structure and defects, insufficient selectivity in oxidation treatment, and the heavy environmental burden and low safety of activation methods, a reversible reaction between ethyl acetate (EA) and hydrogen peroxide (H2O2) is utilized to generate low-concentration, slow-release peracetic acid (PAA) in situ on the surface of biomass fibers. The reaction is mild and uniform, achieving mild and controllable directional oxidation modification of biomass materials. This, in turn, enables synergistic and precise control of the hard carbon microstructure (closed pores, interlayer spacing, and defects), resulting in an ideal "pore-crystal-interface" structure.

[0025] However, the traditional method of directly adding PAA exogenously has many problems in practical applications, such as: ① Safety and operation: Peracetic acid is a strong oxidizing and easily decomposed chemical. Purchasing it or using it at high concentrations poses safety and storage risks; ② Poor controllability: Direct use of strong oxidants can easily lead to over-oxidation, loss of carbonizable carbon sources and damage to the cellulose skeleton, ultimately generating too many open pores or over-dissolved components, making it difficult to stably obtain the desired closed pore distribution; ③ Environment and cost: A large amount of washing and neutralization is required, resulting in waste liquid treatment problems.

[0026] Therefore, compared with the method of directly adding PAA exogenously, the in-situ generation method provided by this invention has higher safety (no need to transport / store high concentrations of PAA), better reaction controllability (the generation rate can be adjusted by the reaction components and conditions), and lower environmental burden (reaction byproducts are easy to handle and the dosage can be reduced). Compared with alkali activation or strong acid treatment, this method does not rely on large amounts of external strong alkali or strong acid, avoiding the risks of large-scale salt solution treatment and structural collapse, and is more green and industrially friendly.

[0027] Preferably, the catalyst in step (1) comprises sulfuric acid.

[0028] Preferably, the mass concentration of hydrogen peroxide in step (1) is 13wt%~18wt%, for example, 13wt%, 14wt%, 15wt%, 16wt%, 17wt% or 18wt%.

[0029] Preferably, the volume ratio of ethyl acetate to hydrogen peroxide in step (1) is 1:(1~3), for example 1:1, 1:1.2, 1:1.5, 1:1.8, 1:2, 1:2.2, 1:2.5, 1:2.8 or 1:3, etc.

[0030] Preferably, the mass percentage of the catalyst in the treatment liquid in step (1) is 0.8wt% to 1.2wt%, for example, 0.8wt%, 0.9wt%, 1wt%, 1.1wt% or 1.2wt%.

[0031] In this invention, only a small amount of sulfuric acid is needed as a catalyst to significantly improve the reaction efficiency.

[0032] Preferably, the mass ratio of biomass material to treatment liquid in step (1) is 1g:(18~22)mL, for example 1g:18mL, 1g:19mL, 1g:20mL, 1g:21mL or 1g:22mL, etc.

[0033] For example, in laboratory operations, up to 20g of biomass material can be selected and completely immersed in a 400mL treatment solution. During this process, the mass ratio should not be too low, otherwise incomplete reaction will occur; if it is too high, the utilization rate of peracetic acid will be low, and subsequent washing and drying costs will be high.

[0034] Preferably, the biomass material in step (1) includes plant-based biomass material.

[0035] Preferably, the mixing process in step (1) is carried out in the dark.

[0036] Preferably, before the pre-carbonization in step (2), the product after the in-situ oxidation reaction is pulverized.

[0037] In this invention, by first pulverizing the oxidized product, the reaction area of ​​the reactants can be increased, thereby allowing the two carbonization reactions to proceed more fully.

[0038] Preferably, the temperature of the in-situ oxidation reaction in step (1) is 50℃~80℃, for example, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃ or 80℃.

[0039] Preferably, the in-situ oxidation reaction in step (1) takes 12h to 20h, for example, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h or 20h.

[0040] Understandably, existing plant-based biomass raw materials are mainly composed of three organic polymer components: cellulose, hemicellulose, and lignin, collectively referred to as the three biomass elements. These three components differ significantly in chemical composition, pyrolysis characteristics, and carbonization behavior, which have a significant impact on the formation of the carbon skeleton, the evolution of pore structure, and the microstructure and electrochemical properties of the final hard carbon material during the carbonization process.

[0041] Cellulose, a linear polysaccharide, exhibits high crystallinity and regularity. During carbonization, cellulose undergoes pyrolysis at approximately 300–400°C, primarily involving dehydration, dehydroxylation, and condensation reactions to generate a relatively dense carbon framework. Cellulose is the main source of ordered carbon structures in carbon materials, contributing to the formation of short-range ordered layered structures with a high degree of graphitization, thereby improving the electrical conductivity and structural stability of carbon materials.

[0042] Hemicellulose is a branched amorphous polysaccharide with poor thermal stability, and its decomposition temperature is typically between 220 and 320°C. During carbonization, hemicellulose readily releases a large amount of gas and volatile products, causing the formation of pores within the carbon skeleton, thus significantly increasing the number of micropores and mesopores in the material. Although this process leads to a decrease in carbon yield, it is beneficial for increasing specific surface area and ion transport rate, playing a positive role in improving the specific capacity and rate performance of hard carbon materials.

[0043] Lignin is a three-dimensional cross-linked aromatic polymer, its structure consisting of phenylpropane units linked by ether bonds and carbon-carbon bonds. Lignin has a wide pyrolysis temperature range (200–500°C), decomposes slowly, and has a high carbon yield. During lignin carbonization, it retains a large amount of aromatic ring skeleton, forming an amorphous carbon structure and a large interlayer spacing, which is a key factor determining the disorder and ion intercalation characteristics of hard carbon materials. However, if lignin degradation is insufficient, its polymerization products easily agglomerate to form coke, resulting in an uneven carbonization structure and affecting the material's conductivity and stability.

[0044] During the overall carbonization of biomass, significant synergistic effects exist among the three elements (hemicellulose, lignin, and cellulose). Early decomposition of hemicellulose creates gas channels, providing diffusion pathways for the subsequent carbonization reaction of cellulose and lignin. The crystalline structure of cellulose provides support at high temperatures, preventing system collapse. The aromatic structure of lignin can coat or crosslink the carbon cores of cellulose during later carbonization, enhancing overall structural stability. The pyrolysis mechanism, interactions, and residual structure of the three biomass elements during carbonization collectively determine the formation characteristics and performance of hard carbon materials. In other words, the proportions of the three elements and their degradation behavior directly determine the microstructure, interlayer spacing distribution, pore characteristics, and electrochemical properties of hard carbon materials.

[0045] Therefore, the key technological direction for improving the performance of biomass-based hard carbon is to precisely control the degradation process of the three elements, improve lignin dispersion, and achieve structural homogenization based on the proportion of the three elements in the biomass material and the structural requirements of the hard carbon product. This will avoid the problems that often occur in the existing process, such as uneven structure of carbonized products, limited specific surface area, and low ion intercalation efficiency due to insufficient lignin degradation and dispersion.

[0046] In this invention, by adaptively controlling parameters such as reaction time, dosage, and temperature according to the ratio of the three elements in the biomass material and the structural requirements of the hard carbon product, the lignin side chains and hemicellulose-lignin linkages can be selectively broken without damaging the cellulose crystalline region. This preserves the necessary carbon source and guides the formation of a target composite microstructure with "closed pores, moderate defects, and moderate interlayer spacing" during carbonization. The system can control the degree of oxidation by adjusting the molar ratio of ethyl acetate to H₂O₂, reaction temperature, and time, achieving selective lignin breakage (oxidative opening of β-O₄ bonds) and hemicellulose removal. Compared with exogenous PAA, this method has a slower generation rate, is self-stabilizing, and exhibits no explosive oxidation behavior; the byproducts are only ethanol and water, making it safe and environmentally friendly.

[0047] Preferably, the product after the in-situ oxidation reaction in step (1) is washed and dried sequentially.

[0048] Preferably, the heating rate of the pre-carbonization in step (2) is 4℃ / h to 6℃ / h, for example, 4℃ / h, 4.5℃ / h, 5℃ / h, 5.5℃ / h or 6℃ / h.

[0049] Preferably, the pre-carbonization cutoff temperature in step (2) is 350℃~450℃, such as 350℃, 360℃, 370℃, 380℃, 390℃ or 400℃, and the heat preservation time is 2h~4h, such as 2h, 2.5h, 3h, 3.5h or 4h.

[0050] In this invention, the pre-carbonization temperature affects the graphitization degree and the number of closed pores in the hard carbon product. By controlling it within the above-mentioned preferred range, it is more beneficial to keep the ratio of micropores to closed pores in the hard carbon product within an optimal range.

[0051] Preferably, after the pre-carbonization in step (2), the pre-carbonized product is acid washed.

[0052] Preferably, the acid washing reagent in step (2) includes hydrochloric acid.

[0053] In this invention, acid washing is performed after pre-carbonization, which is more convenient and efficient than acid washing directly after oxidation. Furthermore, this invention achieves superior performance without the need for doping with other heteroatoms. This is mainly achieved by controlling and expanding the interlayer spacing and creating some oxygen vacancy defects within the original structure of the biomass-based raw material, without introducing other heteroatoms. This balances the interlayer spacing and order of the material, ensuring the plateau capacity and long-term stability of the material after it is used in batteries.

[0054] Preferably, the concentration of the hydrochloric acid is 0.8 mol / L to 1.2 mol / L, such as 0.8 mol / L, 0.9 mol / L, 1 mol / L, 1.1 mol / L, or 1.2 mol / L.

[0055] Preferably, the pickling time is 3h to 5h, for example, 3h, 3.5h, 4h, 4.5h or 5h.

[0056] Preferably, after the acid washing is completed, the product is washed and dried in sequence.

[0057] Preferably, the heating rate of carbonization in step (2) is 4℃ / h to 6℃ / h, for example, 4℃ / h, 4.5℃ / h, 5℃ / h, 5.5℃ / h or 6℃ / h.

[0058] Preferably, the carbonization cutoff temperature in step (2) is 1200℃~1400℃, such as 1200℃, 1250℃, 1300℃, 1350℃ or 1400℃, and the heat preservation time is 1h~3h, such as 1h, 1.5h, 2h, 2.5h or 3h.

[0059] Thirdly, the present invention provides a negative electrode sheet, the negative electrode sheet comprising the hard carbon material as described in the first aspect or the hard carbon material prepared by the preparation method described in the second aspect.

[0060] Fourthly, the present invention also provides a sodium-ion battery, the sodium-ion battery comprising the negative electrode sheet as described in the first aspect.

[0061] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0062] Compared with the prior art, the present invention has the following beneficial effects:

[0063] 1. Solve the problem of uncontrollable pore structure and defects. Through in-situ generated peracetic acid and gentle oxidation, the directional decomposition and modification of cellulose, hemicellulose and lignin structures in the biomass skeleton are achieved, and a uniformly distributed micropore and closed pore structure is constructed.

[0064] 2. To achieve synergistic regulation of interlayer spacing and defects, the oxidation of peracetic acid introduces appropriate oxygen-containing functional groups, promotes interlayer expansion and the formation of ordered defects during carbonization, and enhances sodium ion intercalation ability and diffusion rate.

[0065] 3. It provides a green and low-energy modification route. The in-situ synthesis of peracetic acid using the ethyl acetate and hydrogen peroxide system is mild, safe, and does not require strong acids or bases, thus realizing a green chemical modification process and reducing environmental burden and process complexity. Attached Figure Description

[0066] Figure 1 These are SEM images of the hard carbon materials prepared in Examples 1 and 4-5, as well as the control group material (untreated sample).

[0067] Figure 2 These are the Raman spectra of the hard carbon materials prepared in Examples 1, 4-5, and the control group materials.

[0068] Figure 3 These are micropore size distribution diagrams of the hard carbon materials prepared in Examples 1, 4-5, and the control group materials.

[0069] Figure 4 These are closed-pore volume distribution diagrams of the hard carbon materials prepared in Examples 1, 4-5, and the control group materials.

[0070] Figure 5 The graphs show the constant current charge-discharge curves of batteries assembled using the hard carbon materials prepared in Examples 1 and 4-5, as well as the control group material.

[0071] Figure 6 Batteries assembled using the hard carbon materials prepared in Examples 1, 4-5, and the control group material, respectively, were tested at 200 mA g. -1 The following is a long-cycle performance graph.

[0072] Figure 7 Batteries assembled using the hard carbon materials prepared in Examples 1, 4-5, and the control group material, respectively, were tested at 500 mA g. -1 The following is a long-cycle performance graph. Detailed Implementation

[0073] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0074] 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 application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion.

[0075] Example 1

[0076] This embodiment provides a hard carbon material having a three-dimensional structure composed of the following components: a hierarchical pore system consisting of micropores and closed pores; a carbon layer spatially interwoven and connected with the hierarchical pore system; and a defect structure uniformly distributed at the interface between the carbon layer and the hierarchical pore system, including sp... 3 -sp 2 Mixed boundary defects and oxygen vacancy regulation defects. Among them, I D / I G The value is 1.123, the average pore size of the micropores is 0.6 nm, the average pore size of the closed pores is 3 nm, and the pore volume of the closed pores is 0.09 cm³. 3 g -1 The interlayer spacing of the carbon layers is 0.383 nm.

[0077] The specific preparation method is as follows:

[0078] (1) Prepare a treatment solution by adding ethyl acetate + 15wt% hydrogen peroxide (1:1 volume ratio) and then adding sulfuric acid (the mass percentage of sulfuric acid in the treatment solution is 1wt%). Place the solution in a brown, light-proof glass bottle, wrap it with aluminum foil to protect it from light, and add bamboo powder, the biomass material to be treated, after the solution is prepared. React in a constant temperature water bath at 60℃ for 16 hours to simultaneously generate peracetic acid in situ and oxidize the biomass material. After the reaction is complete, remove the glass bottle from the constant temperature water bath, wash it with a large amount of deionized water to neutralize it, and then dry it at 70℃ overnight.

[0079] (2) The dried sample was pulverized into a fine powder using a ball mill, and then pre-carbonized in a high-temperature tube furnace at 400℃ for 3 hours under an Ar atmosphere, with a heating rate of 5℃ / h. The pre-carbonized material was taken out and washed with 1mol / L hydrochloric acid at room temperature for 4 hours to remove organic impurities. After acid washing, it was washed again with a large amount of deionized water until neutral, and dried overnight in a forced-air drying oven at 70℃ to obtain the precursor material.

[0080] (3) The precursor carbon material obtained after pre-carbonization, acid washing and drying is placed in a crucible and placed in a high-temperature tube furnace at 1300℃ for 2 hours to carbonize again, with a heating rate of 5℃ / min, to obtain the hard carbon material.

[0081] Example 2

[0082] This embodiment provides a hard carbon material having a three-dimensional structure composed of the following components: a multi-level pore structure consisting of micropores and closed pores; a carbon layer spatially interwoven and connected with the multi-level pore structure; and a defect structure uniformly distributed at the interface between the carbon layer and the multi-level pore structure, including sp³-sp² mixed boundary defects and oxygen vacancy regulation defects.

[0083] The specific preparation method is as follows:

[0084] (1) Prepare a treatment solution by adding ethyl acetate + 18wt% hydrogen peroxide (1:2 volume ratio) and then adding sulfuric acid (the mass percentage of sulfuric acid in the treatment solution is 0.8wt%). Place the solution in a brown, light-proof glass bottle, wrap it with aluminum foil to protect it from light, and add poplar wood (crushed and sieved to a particle size of 0.25~0.42 mm) to be treated as biomass material after the solution is prepared. React the solution in a constant temperature water bath at 50℃ for 16 h to simultaneously generate peracetic acid in situ and oxidize the biomass material. After the reaction is complete, remove the glass bottle from the constant temperature water bath, wash it with a large amount of deionized water to neutralize it, and then dry it at 70℃ overnight.

[0085] (2) The dried sample was pulverized into a fine powder using a ball mill, and then pre-carbonized in a high-temperature tube furnace at 350°C for 4 hours under an Ar atmosphere, with a heating rate of 4°C / h. The pre-carbonized material was then removed and washed with 0.8 mol / L hydrochloric acid at room temperature for 5 hours to remove organic impurities. After acid washing, the material was washed again with a large amount of deionized water until neutral, and then dried overnight in a forced-air drying oven at 70°C to obtain the precursor material.

[0086] (3) The precursor carbon material obtained after pre-carbonization, acid washing and drying is placed in a crucible and placed in a high-temperature tube furnace at 1200℃ for 3 hours for carbonization, with a heating rate of 4℃ / min, to obtain the hard carbon material.

[0087] Example 3

[0088] This embodiment provides a hard carbon material having a three-dimensional structure composed of the following components: a hierarchical pore system consisting of micropores and closed pores; a carbon layer spatially interwoven and connected with the hierarchical pore system; and a defect structure uniformly distributed at the interface between the carbon layer and the hierarchical pore system, including sp... 3 -sp 2 Mixed boundary defects and oxygen vacancy regulation defects.

[0089] The specific preparation method is as follows:

[0090] (1) Prepare a treatment solution by adding ethyl acetate and 13wt% hydrogen peroxide (1:3 volume ratio) followed by sulfuric acid (the sulfuric acid content in the treatment solution is 1.2wt%). Place the solution in a brown, light-proof glass bottle, wrapped with aluminum foil to protect it from light. After the solution is prepared, add the biomass material (wood) to be treated and react in a constant temperature water bath at 80℃ for 16 hours to simultaneously generate peracetic acid in situ and oxidize the biomass material. After the reaction is complete, remove the glass bottle from the constant temperature water bath, wash it with plenty of deionized water to neutralize it, and then dry it at 70℃ overnight.

[0091] (2) The dried sample was pulverized into a fine powder using a ball mill, and then pre-carbonized in a high-temperature tube furnace at 450°C for 2 hours under an Ar atmosphere, with a heating rate of 6°C / h. The pre-carbonized material was then removed and washed with 1.2 mol / L hydrochloric acid at room temperature for 3 hours to remove organic impurities. After acid washing, the material was washed again with a large amount of deionized water until neutral, and then dried overnight in a forced-air drying oven at 70°C to obtain the precursor material.

[0092] (3) The precursor carbon material obtained after pre-carbonization, acid washing and drying is placed in a crucible and placed in a high-temperature tube furnace at 1400℃ for 1 hour, with a heating rate of 6℃ / min, to obtain the hard carbon material.

[0093] Example 4

[0094] The difference between this embodiment and embodiment 1 is that in step (1), the reaction is carried out in a constant temperature water bath at 60°C for 12 hours.

[0095] The remaining preparation methods and parameters are consistent with those in Example 1.

[0096] Example 5

[0097] The difference between this embodiment and embodiment 1 is that in step (1), the reaction is carried out in a constant temperature water bath at 60°C for 20 hours.

[0098] The remaining preparation methods and parameters are consistent with those in Example 1.

[0099] Example 6

[0100] The difference between this embodiment and embodiment 1 is that in step (1), the reaction is carried out in a constant temperature water bath at 60°C for 24 hours.

[0101] The remaining preparation methods and parameters are consistent with those in Example 1.

[0102] Example 7

[0103] The difference between this embodiment and Example 1 is that in step (1), the volume ratio of ethyl acetate to 18wt% hydrogen peroxide is 1:0.5;

[0104] The remaining preparation methods and parameters are consistent with those in Example 1.

[0105] Example 8

[0106] The difference between this embodiment and Example 1 is that in step (1), the volume ratio of ethyl acetate to 18wt% hydrogen peroxide is 1:5;

[0107] The remaining preparation methods and parameters are consistent with those in Example 1.

[0108] Example 9

[0109] The difference between this embodiment and embodiment 1 is that in step (1), the water bath temperature is 45°C;

[0110] The remaining preparation methods and parameters are consistent with those in Example 1.

[0111] Example 10

[0112] The difference between this embodiment and embodiment 1 is that in step (1), the water bath temperature is 80°C;

[0113] The remaining preparation methods and parameters are consistent with those in Example 1.

[0114] Example 11

[0115] The difference between this embodiment and embodiment 1 is that in step (2), the pre-carbonization temperature is 700°C;

[0116] The remaining preparation methods and parameters are consistent with those in Example 1.

[0117] Example 12

[0118] The difference between this embodiment and embodiment 1 is that in step (2), the dried sample is first acid washed with 1 mol / L hydrochloric acid at room temperature for 4 hours to remove organic impurities, and then placed in a high-temperature tube furnace under Ar atmosphere for pre-carbonization. The pre-carbonized material is then taken out, pulverized with a ball mill to obtain fine powder, and then carbonized again.

[0119] The remaining preparation methods and parameters are consistent with those in Example 1.

[0120] Comparative Example 1

[0121] The difference between this comparative example and Example 1 is that, in step (1), ethyl acetate and hydrogen peroxide are replaced with peracetic acid;

[0122] The remaining preparation methods and parameters are consistent with those in Example 1.

[0123] Performance testing

[0124] The hard carbon materials prepared in Examples 1-12 and Comparative Example 1 were subjected to relevant morphological and structural characterization tests to investigate the changes in morphology and structure before and after treatment. Some test results are shown below. Figures 1-4 As shown, the control group refers to untreated biomass raw materials.

[0125] In addition, sodium-ion batteries were prepared using the hard carbon materials obtained in Examples 1-12 and Comparative Example 1 according to the following method, and the electrochemical performance of the sodium-ion batteries was tested:

[0126] (1) The hard carbon materials prepared in Examples 1-12 and Comparative Example 1 were mixed with carbon black and PVDF binder (hard carbon: SP: PVDF = 8:1:1) in NMP to form a slurry, which was then uniformly coated on copper foil and dried to form a working electrode.

[0127] (2) In a glove box filled with argon, a sodium metal sheet is used as the counter electrode and a glass fiber membrane is used as the diaphragm. An electrolyte with a concentration of 1.0M (solute is NaPF6 and solvent is Diglyme) is added dropwise to assemble a sodium ion button cell and let it stand for a period of time.

[0128] Constant current charge-discharge tests were performed on the battery testing system under the following conditions: first, constant current discharge to 0.01V, then constant current charging to 3V, repeating this cycle multiple times. Some test results are shown below. Figure 4-7 As shown.

[0129] Results analysis:

[0130] Figure 1 The images show SEM images of untreated biomass raw materials and hard carbon materials obtained after different treatment times. It can be seen that, with prolonged treatment time, the precursor structure of the treated samples shows significant deconstruction, the connection between lignin and hemicellulose gradually disappears and becomes completely fragmented, and the precursor structure gradually breaks down. Furthermore, XRD analysis shows that the d(002) of the material obtained in Example 1 is approximately 0.383 nm.

[0131] Figure 2 These are Raman spectra of untreated biomass feedstock and hard carbon materials obtained after different treatment times. It can be seen that as the treatment time increases, the Ig of the material increases. D / I G The value first decreased and then increased, indicating that the material's orderliness and the number of defects / active sites are dynamically changing, and that the optimal balance was achieved at 16h.

[0132] Figure 3 and Figure 4 These are, respectively, micropore size distribution diagrams and closed-pore volume distribution diagrams of untreated biomass raw materials and hard carbon materials obtained after different treatment times. Figure 3The image in the upper right corner shows Examples 1 and 4 when they share the same vertical axis. As can be seen from the image, the micropore size of the hard carbon material obtained in Example 1 is approximately 0.6 nm, and the closed-pore volume is approximately 0.09 cm³. 3 g -1 The micropore sizes of the hard carbon materials obtained in Examples 4 and 5 and the control group were 0.58 nm, 0.62 nm, and 0.53 nm, respectively. It should be noted that the main improvement of this invention is not a significant increase in pore volume, but rather an improvement in the carbon interlayer spacing, an improvement in the hard carbon structure, and an increase in pore size, making hard carbon more suitable for sodium storage.

[0133] Figures 5-7 These are, respectively, the constant current charge-discharge curves and long-cycle performance graphs at different current densities obtained after assembling batteries using untreated biomass raw materials and hard carbon materials obtained after different treatment times. Figure 5 It can be seen that the ramp capacity and plateau capacity corresponding to Example 1 are approximately 138 mAh / g and 228.7 mAh / g, respectively; those of Example 4 are approximately 143 mAh / g and 210 mAh / g, respectively; those of Example 5 are approximately 124 mAh / g and 200 mAh / g, respectively; while the control group is only approximately 110 mAh / g and 186 mAh / g, respectively. Figure 6 It can be seen that the initial coulombic efficiency corresponding to Examples 1, 4, and 5 is 73%~75%, while that of untreated biomass feedstock is approximately 72%. Figure 7 It can be seen that the hard carbon material prepared by this invention can significantly improve high-rate, long-cycle performance. The above data indicate that the hard carbon material obtained using the preparation method provided by this invention, when used in sodium-ion batteries, can balance high plateau capacity, high ICE (interval efficiency), and excellent rate performance.

[0134] In addition to Examples 1, 4, and 5, structural performance tests were also conducted on other examples and comparative examples, but these are not shown in the accompanying drawings. Test data show that, compared to Example 1, the uniformity of micropore and closed pore distribution in the hard carbon materials obtained in Examples 6-11 and Comparative Example 1 decreased. After assembly into the battery, their corresponding electrochemical performance also decreased to varying degrees, with Comparative Example 1 showing the most significant decrease. This indicates that the present invention significantly improves the structural performance of the hard carbon product through in-situ generation of peracetic acid and gentle oxidation. Furthermore, the reaction time in the water bath process, the ratio of ethyl acetate to hydrogen peroxide, the temperature of the oxidation reaction and pre-carbonization, and the order of acidification and pulverization all affect the structure and performance of the material. Controlling these within the preferred range provided by the present invention is more conducive to improving its overall performance.

[0135] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A hard carbon material, characterized in that, The hard carbon material is composed of three-dimensional carbon layers, which have micropores and closed pores. Defect structures are distributed at the interfaces between the micropores / closed pores and the three-dimensional carbon layers. The hard carbon material's I... D / I G The range is 1.105 to 1.

286.

2. The hard carbon material according to claim 1, characterized in that, The pore size of the micropores is <2nm, and the pore size of the closed pores is 2nm~5nm; Preferably, the pore volume of the closed pore is 0.05 cm³. 3 g -1 ~0.09cm 3 g -1 ; Preferably, the interlayer spacing d(002) of the carbon layer is 0.376 nm to 0.387 nm; Preferably, the defect structure includes sp³-sp² mixed boundary defects and oxygen vacancy regulation defects.

3. A method for preparing the hard carbon material as described in claim 1 or 2, characterized in that, The preparation method includes: (1) Ethyl acetate, hydrogen peroxide and catalyst are mixed to obtain a treatment solution, and then the biomass material is mixed with the treatment solution to carry out an in-situ oxidation reaction; (2) The product after the in-situ oxidation reaction in step (1) is subjected to pre-carbonization and carbonization in sequence to obtain the hard carbon material.

4. The preparation method according to claim 3, characterized in that, The catalyst in step (1) includes sulfuric acid; Preferably, the mass concentration of hydrogen peroxide in step (1) is 13wt%~18wt%; Preferably, the volume ratio of ethyl acetate to hydrogen peroxide in step (1) is 1:(1~3); Preferably, the catalyst in step (1) has a mass percentage of 0.8 wt% to 1.2 wt% in the treatment solution; Preferably, the mass ratio of biomass material to treatment liquid in step (1) is 1g:(18~22)mL; Preferably, the biomass material in step (1) includes plant-based biomass material.

5. The preparation method according to claim 3 or 4, characterized in that, The temperature of the in-situ oxidation reaction in step (1) is 50℃~80℃; Preferably, the in-situ oxidation reaction in step (1) takes 12 to 20 hours; Preferably, the product after the in-situ oxidation reaction in step (1) is washed and dried sequentially.

6. The preparation method according to any one of claims 3-5, characterized in that, Before the pre-carbonization in step (2), the product after the in-situ oxidation reaction is pulverized; Preferably, the heating rate of the pre-carbonization in step (2) is 4℃ / h~6℃ / h; Preferably, the pre-carbonization cutoff temperature in step (2) is 350℃~450℃, and the holding time is 2h~4h.

7. The preparation method according to any one of claims 3-6, characterized in that, After the pre-carbonization described in step (2), the pre-carbonized product is acid washed. Preferably, the pickling agent includes hydrochloric acid; Preferably, the concentration of the hydrochloric acid is 0.8 mol / L to 1.2 mol / L; Preferably, the pickling time is 3 to 5 hours; Preferably, after the acid washing is completed, the product is washed and dried in sequence.

8. The preparation method according to any one of claims 3-7, characterized in that, The heating rate for carbonization in step (2) is 4℃ / h~6℃ / h; Preferably, the carbonization cutoff temperature in step (2) is 1200℃~1400℃, and the holding time is 1h~3h.

9. A negative electrode sheet, characterized in that, The negative electrode sheet comprises the hard carbon material as described in claim 1 or 2, or the hard carbon material prepared by the preparation method described in any one of claims 3-8.

10. A sodium-ion battery, characterized in that, The sodium-ion battery includes the negative electrode as described in claim 9.

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