Hard carbon material based on composite precursor and preparation method and application thereof

Abstract

The invention belongs to the technical field of batteries, and discloses a hard carbon material based on a composite precursor and a preparation method and application thereof. Biomass, a resin-based carbon source and ethylene glycol are mixed, a composite precursor is prepared through a solvothermal method, then low-temperature pre-carbonization is carried out to obtain a preliminary product, the preliminary product is subjected to acid pickling and water washing to be neutral, the product is dried and then placed in an atmosphere furnace to be subjected to high-temperature carbonization, and the hard carbon material is obtained. According to the preparation method, respective advantages of biomass and resin and pore regulation and activation effects of ethylene glycol are combined, crosslinking and uniform coupling of biomass, resin and ethylene glycol on a molecular level are realized through a solvothermal method, and a microstructure of a precursor is optimized; and finally, the hard carbon material is comprehensively optimized in terms of key parameters such as porosity, interlayer spacing, conductivity and defect density. The sodium ion battery prepared from the hard carbon negative electrode material has excellent performances that the initial coulombic efficiency is greater than or equal to 90% and the reversible specific capacity is greater than 350mAh / g.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a method for preparing hard carbon materials. Background Art

[0002] Hard carbon materials are an important category of amorphous carbon. When applied to sodium-ion batteries, they exhibit high sodium storage capacity, relatively low sodium storage potential, and excellent cycle stability, making them the most promising anode materials for sodium-ion batteries. Hard carbon is mainly composed of curved and irregularly distributed graphene nanosheets, is non-graphitizable, and has an amorphous structure. Even at temperatures above 2800 °C, it is very difficult for them to be graphitized. By carbonizing their precursors, such as resins and biomass, at relatively high temperatures (1000 - 2000 °C) and in an inert atmosphere, a turbostratic microstructure with pores and abundant defects can be formed. The hard carbon anode in a sodium-ion battery exhibits electrochemical performance similar to that of the graphite anode in a lithium-ion battery, namely a long-term and reversible low-potential charge / discharge plateau.

[0003] The successful use of hard carbon in sodium-ion batteries has also rekindled people's interest in its complex microstructure, which is closely related to sodium storage. Among them, the highly disordered structure and large interlayer distance of hard carbon exhibit high sodium storage capacity and excellent cycle stability, but at the same time, there are obvious disadvantages, such as low initial Coulomb efficiency, cycle life, and structural consistency. Moreover, its microstructure is not as fixed as that of layered graphite materials, and it is very difficult to accurately measure, which seriously restricts the improvement of the energy density of the hard carbon anode.

[0004] In the face of the above problems, appropriate strategies are needed to regulate the microstructure of hard carbon materials, such as interlayer spacing, graphitization degree, pore structure, etc. The precursor has the most direct impact on the microstructure of hard carbon materials: in terms of the chemical composition of the precursor, the content of carbon atoms directly determines the yield of the final hard carbon material. Heteroatoms (such as oxygen, hydrogen, nitrogen, etc.) will form defects and functional groups during the carbonization process. These defects and functional groups can provide more sodium ion storage sites, but excessive impurities may also reduce the stability of the material; in terms of the molecular structure of the precursor, a high proportion of aromatic structures (such as benzene rings) will lead to the formation of more ordered graphite layers during carbonization. An appropriate interlayer spacing is conducive to the insertion and extraction of sodium ions. Aliphatic structures (such as long-chain hydrocarbons) are easily decomposed into amorphous carbon during carbonization, providing more nanopores; in terms of the physical form of the precursor, the particle size of the precursor affects the specific surface area and pore structure of the final hard carbon material. Smaller particles help to form a uniform microporous structure and improve the sodium storage capacity. Fibrous or layered precursors may form hard carbon with a layered or sheet-like structure after carbonization, and this structure is conducive to the diffusion and storage of sodium ions. Therefore, how to regulate the microstructure of hard carbon materials through effective precursor selection and preparation processes to improve their sodium storage performance has become the focus of current research. Existing technologies such as CN116395668A disclose a hard carbon material, a carbon negative electrode material prepared using the hard carbon material, and a preparation method thereof. This method simply mechanically grinds and mixes a resin carbon source, a biomass carbon source, and a porous template material (CaCO3). The two precursors do not achieve cross-linking and uniform coupling at the molecular level, resulting in the formation of discontinuous carbon-carbon interfaces in the hard carbon, hindering the cross-interface transport of sodium ions, and limiting the sodium storage kinetics and structural stability. At the same time, since the CaCO3 template particles are not embedded inside the precursor but only adhere to the surface of the mixture, they decompose into CO2 and CaO at the initial stage of carbonization (300-500 °C), and cannot effectively regulate structural parameters such as the nano-scale pore size and pore size distribution of the obtained hard carbon, and thus cannot effectively improve the electrochemical performance of the hard carbon material. Summary of the Invention

[0005] Aiming at the technical problems of poor consistency, insufficient conductivity, low initial Coulomb efficiency, and short cycle life existing in traditional hard carbon materials, the present invention provides a hard carbon material based on a composite precursor, a preparation method and an application thereof.

[0006] To achieve the above object, the technical solution of the present invention is realized as follows:

[0007] A preparation method of a hard carbon material based on a composite precursor, characterized in that the steps are as follows:

[0008] (1) Mix biomass, resin, and ethylene glycol evenly and crush them to obtain a mixed product; the mixed product is used to prepare a hard carbon composite precursor through a solvothermal method. Biomass has rich carbon sources, low cost, and environmental friendliness, but there are problems such as large differences between batches, low carbon yield, and uncontrollable carbon structure; while the hard carbon materials obtained from resin precursors have a controllable structure, high yield, and high batch stability, but are restricted by high cost, complex preparation process, and poor environmental protection and are difficult to mass-produce on a large scale. Therefore, the composite precursor takes biomass as the main body, introduces a resin-based precursor with complementary characteristics, and through the hydrogen bonding, esterification, and polycondensation reactions of ethylene glycol with functional groups such as hydroxyl and carbonyl in biomass and aromatic rings or epoxy groups in resin, realizes the uniform composite of the three, improving the uniformity and stability of the precursor.

[0009] (2) Pre-carbonize the hard carbon composite precursor in a protective atmosphere to obtain low-purity derived carbon, that is, the pre-carbonization product. Through pre-carbonization, moisture, volatile components and other impurities can be initially removed, reducing the gas release amount in the subsequent high-temperature carbonization stage and avoiding the structural collapse caused by the violent escape of volatile components.

[0010] (3) Purify the pre-carbonization product in an acid solution, then wash it with deionized water until the pH = 7 and dry it to obtain an acid-purified pre-carbonization product. The purpose of purification in the acid solution is to further remove impurities such as inorganic salts and ash in the primary carbonization derivatives.

[0011] (4) In an inert atmosphere, high-temperature carbonize the acid-purified pre-carbonization product, and then perform ball milling treatment and screening to obtain it. High-temperature carbonization is to promote the rearrangement of carbon layers in the hard carbon structure and optimize the pore structure arrangement.

[0012] In the above step (1), the mass ratio of biomass to resin is (9 - 49):1; the addition amount of ethylene glycol accounts for 5 - 20 wt% of the mass of the mixed product; the temperature of the solvothermal method treatment is 160 - 200 °C, and the time is 6 - 12 h.

[0013] In the above step (1), the biomass is any one of sawdust, wood chips, straw, cork, rice husk, peanut shell, and corn cob. If the above biomass is crushed or ground into powder, it is also within the protection scope of this application. The resin-based carbon source is any one of phenol formaldehyde resin, xylenol formaldehyde resin, urea formaldehyde resin, bisphenol A epoxy resin, bisphenol F epoxy resin, polyacrylonitrile, and polyacrylate.

[0014] In the above step (2), the pre-carbonization means heating at a heating rate of 1 - 10 °C / min to 200 - 800 °C and holding for 1 - 120 min.

[0015] In the above step (3), the mass fraction of the acid solution is 10 - 30%, and the acid solution is at least one of hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid.

[0016] In the above step (3), the purification temperature is 45 - 90 °C and the time is 12 - 24 h.

[0017] In the above step (4), high-temperature carbonization means heating to 1000 - 1500 °C at a heating rate of 1 - 10 °C / min and holding for 30 - 120 min; the mesh number of the sieve is 100 - 1000 meshes.

[0018] By using biomass and resin as composite precursors and introducing ethylene glycol, the present invention further promotes the uniform composite of biomass and resin through hydrogen bonding, polycondensation and crosslinking reactions, and optimizes the microstructure of the precursors. Next, the advantages of this application are described from four aspects: interface effect, pore structure regulation, surface chemistry and doping effect, and carbon structure and graphitization degree regulation.

[0019] a. Interface effect: During the carbonization process of biomass and resin-based precursors, a carbon-carbon interface effect will be formed, generating more defect sites and amorphous regions. These interface regions can buffer the stress during sodium ion insertion / extraction, reduce the volume expansion of the material, and extend the cycle life of the battery.

[0020] b. Pore structure: The chemical compositions and decomposition temperatures of biomass precursors and resin-based precursors are different. During the carbonization process, they will undergo decomposition and carbonization reactions at different times, thus forming different pore structures and optimizing the sodium storage capacity of hard carbon materials. In biomass precursors, cellulose, hemicellulose and lignin naturally exist, and at the same time contain abundant heteroatoms (such as oxygen, nitrogen, phosphorus, etc.). During carbonization, dehydration, decarboxylation and dehydroxylation reactions will occur, generating a large amount of volatile gases. The gases released during this process contribute to the formation of micropores and mesopores in the carbon matrix. These pores provide channels for the insertion and diffusion of sodium ions, helping to improve the specific capacity and rate performance of hard carbon. Resin-based precursors, such as phenolic resin or epoxy resin, form relatively fewer micropores during carbonization, so their mechanical strength and structural stability are relatively high. After the two are combined through an ethylene glycol crosslinking medium to form a uniform molecular form, the resin-based precursor provides a stable carbon framework, and the biomass precursor regulates micropores and amorphous regions through gas evolution and heteroatoms. The two work together to obtain an appropriate pore distribution and balanced mechanical properties.

[0021] c. Surface Chemistry and Doping Effects: Biomass precursors contain natural heteroatoms (such as oxygen, nitrogen, phosphorus, etc.). During the carbonization process, these heteroatoms will be partially retained in the hard carbon material, forming oxygen- or nitrogen-containing functional groups. These heteroatoms can increase the adsorption sites of sodium ions and further improve the sodium storage performance of the material. The surface modification of resin-based precursors is less, but in the composite with biomass precursors, heteroatom doping combines with the resin-based carbon skeleton to achieve more uniform surface chemical properties. Especially nitrogen doping can improve the conductivity and electrochemical activity of the material, further enhancing the cycle performance of the battery.

[0022] d. Regulation of Carbon Structure and Graphitization Degree: After carbonization of biomass precursors, mainly amorphous carbon is generated. Due to its large interlayer spacing, sodium ions can be embedded in the randomly stacked layers of amorphous carbon, enhancing the sodium storage capacity of the material. When resin-based precursors are carbonized, carbon domains with microcrystalline graphite structures are easily formed. These microcrystalline graphite carbons can improve the conductivity and structural stability of the material. At the same time, the formation of graphite microcrystals can also reduce the volume expansion of the material during sodium ion insertion / extraction. By combining these two precursors, amorphous carbon and microcrystalline graphite structures can be obtained simultaneously, improving the capacity, conductivity, and cycle stability of hard carbon materials.

[0023] Hard carbon materials based on composite precursors prepared by the above preparation method.

[0024] Furthermore, the interlayer spacing of the above hard carbon material is 0.37 - 0.40 nm. The median particle size D50 is 5 - 15 μm, the specific surface area ≤ 15 m 2 / g, and the tapped density ≥ 0.75 g / cm 3 .

[0025] Application of the above hard carbon materials based on composite precursors in sodium ion batteries.

[0026] The beneficial effects of the present invention are:

[0027] (1) The present invention combines the respective advantages of biomass and resin-based carbon sources. Through the introduction of ethylene glycol, the uniform composite of biomass and resin is further promoted through hydrogen bonding, polycondensation, and cross-linking reactions, optimizing the microstructure of the precursor. During the carbonization process of the composite precursor, various interactions are triggered, thereby regulating the microstructure of the hard carbon material, including characteristics such as interlayer spacing, pore structure, and graphitization degree, improving the structural consistency of the hard carbon material, and finally enhancing the sodium storage performance and cycle life.

[0028] (2) The composite precursor used in the method of the present invention forms a carbon-carbon interface effect and a stable carbon skeleton during the carbonization process, buffers the volume expansion during the insertion / extraction of sodium ions, and reduces the risk of material structure collapse. The hard carbon material has a stable structure during the cycling process, significantly improving the battery cycle life and being suitable for long-term charge and discharge scenarios. The battery prepared with the hard carbon negative electrode material of the present invention has a reversible capacity ≥ 325 mAh / g, an initial Coulombic efficiency ≥ 90%, and a capacity retention rate ≥ 80% after 500 cycles. Description of the Drawings

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0030] Figure 1 It is a scanning electron microscope image of the hard carbon materials prepared in the embodiments and comparative examples of the present invention; among them, (a) is Example 1; (b) is Comparative Example 1.

[0031] Figure 2 It is a cycling performance graph of the hard carbon negative electrode materials prepared in the embodiments and comparative examples of the present invention; among them, (a) is Example 1; (b) is Comparative Example 1.

[0032] Figure 3 It is a rate performance graph of the hard carbon negative electrode materials prepared in the embodiments and comparative examples of the present invention; among them, (a) is Example 1; (b) is Comparative Example 1. Detailed Embodiments

[0033] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0034] Example 1

[0035] The preparation method of the hard carbon material based on the composite precursor in this example is as follows:

[0036] S1: Weigh peanut shell powder (20 g) and xylenol formaldehyde resin (2 g), mix them according to a mass ratio of 10:1 to obtain a mixture. Add 2.5 g of ethylene glycol to the mixture (ethylene glycol accounts for 10% of the total mass of the mixture and ethylene glycol), stir evenly to form a suspension. After treatment by solvothermal method (180 °C, 10 h) and then drying, a composite precursor is prepared.

[0037] S2: Under nitrogen protection, the composite precursor obtained in step (1) is subjected to pre-carbonization treatment, heated to 700 °C at a heating rate of 5 °C / min, and held for 30 minutes to obtain a pre-carbonized product.

[0038] S3: Add 7.5 g of the pre-carbonized product to 40 g of hydrochloric acid solution (mass fraction 12 wt%), purify at 60 °C for 12 h, then wash with deionized water until pH = 7 and dry to obtain an acid-purified pre-carbonized product.

[0039] S4: Transfer the acid-purified pre-carbonized product to an atmosphere furnace, and under an argon atmosphere, heat to 1200 °C at a heating rate of 8 °C / min and hold for 70 minutes for high-temperature carbonization.

[0040] S5: After sintering is completed, cool the hard carbon material to room temperature, process it with a high-energy ball milling device, and screen it (mesh size of the sieve is 500 mesh) to obtain a uniform hard carbon material. Figure 1 a is the SEM image of the hard carbon material prepared in this example. It can be seen from the figure that the particle size of the material is moderate, the particle distribution is uniform, and the median particle size D50 is about 8 μm.

[0041] Example 2

[0042] The preparation method of the hard carbon material based on the composite precursor in this example is as follows:

[0043] S1: Weigh corn cob powder (15 g) and bisphenol F epoxy resin (1 g), and the mass ratio of the two is 15:1. Add 2.8 g of ethylene glycol (ethylene glycol accounts for 15% of the total mass of corn cob powder, bisphenol F epoxy resin and ethylene glycol) to bisphenol A epoxy resin (1 g), stir to form a homogeneous solution, and then add corn cob powder (15 g). Finally, after solvent thermal treatment (160 °C, 12 h) and drying, a composite precursor is prepared.

[0044] S2: The composite precursor obtained in step (1) is subjected to pre-carbonization treatment in air, heated to 200 °C at a heating rate of 5 °C / min, and held for 60 minutes to obtain a pre-carbonized product.

[0045] S3: Add 5 g of the pre-carbonized product to 30 g of hydrochloric acid solution (mass fraction 12%), purify at 60 °C for 12 h, then wash with deionized water until pH = 7 and dry to obtain an acid-purified pre-carbonized product.

[0046] S4: Transfer the acid-purified pre-carbonized product to an atmosphere furnace, and under an argon atmosphere, heat to 1100 °C at a heating rate of 5 °C / min and hold for 80 minutes for high-temperature carbonization.

[0047] S5: After sintering is completed, cool the hard carbon material to room temperature, process it using a high-energy ball milling device, and sieve it (the mesh size of the sieve is 500 mesh) to obtain a uniform hard carbon material.

[0048] Example 3

[0049] The preparation method of the hard carbon material based on the composite precursor in this example is as follows:

[0050] S1: Weigh pine sawdust (19 g) and polyacrylate (1 g), mix them in a mass ratio of 19:1 to obtain a mixture. Add 2.7 g of ethylene glycol to the mixture (ethylene glycol accounts for 12% of the total mass of the mixture and ethylene glycol), stir evenly to form a suspension. After treatment by solvothermal method (160 °C, 10 h) and then drying, prepare the composite precursor.

[0051] S2: Under argon protection, perform pre-carbonization treatment on the composite precursor obtained in step (1), heat it at a heating rate of 5 °C / min to 600 °C, and hold for 60 minutes to obtain a pre-carbonized product.

[0052] S3: Add 6 g of the pre-carbonized product to 40 g of hydrochloric acid solution (mass fraction is 12%), perform purification at 60 °C for 12 h, then wash with deionized water until pH = 7 and dry to obtain an acid-purified pre-carbonized product.

[0053] S4: Transfer the acid-purified pre-carbonized product to an atmosphere furnace, under an argon atmosphere, heat it at a heating rate of 10 °C / min to 1400 °C, and hold for 60 minutes for high-temperature carbonization.

[0054] S5: After sintering is completed, cool the hard carbon material to room temperature, process it using a high-energy ball milling device, and sieve it (the mesh size of the sieve is 500 mesh) to obtain a uniform hard carbon material.

[0055] Example 4

[0056] The preparation method of the hard carbon material based on the composite precursor in this example is as follows:

[0057] S1: Weigh straw powder and bisphenol A epoxy resin in a mass ratio of 12:1. Add 2.9 g of ethylene glycol (ethylene glycol accounts for 10% of the total mass of straw powder, bisphenol A epoxy resin and ethylene glycol) to bisphenol A epoxy resin (2 g), stir to form a homogeneous phase solution, and then add straw powder (24 g). Finally, prepare the composite precursor by solvothermal treatment (160 °C, 10 h) and then drying.

[0058] S2: Under argon protection, perform pre-carbonization treatment on the composite precursor obtained in step (1), heat it at a heating rate of 5 °C / min to 600 °C, and hold for 60 minutes to obtain a pre-carbonized product.

[0059] S3: Add 8 g of the pre-carbonized product to 50 g of hydrochloric acid solution (mass fraction: 12%), carry out purification at 60 °C for 12 h, then wash with deionized water until the pH = 7 and dry to obtain the acid-purified pre-carbonized product.

[0060] S4: Transfer the acid-purified pre-carbonized product to an atmosphere furnace, and under an argon atmosphere, heat it to 1400 °C at a heating rate of 10 °C / min and hold for 60 minutes for high-temperature carbonization.

[0061] S5: After sintering is completed, cool the hard carbon material to room temperature, process it with a high-energy ball milling device, and sieve it (the mesh number of the sieve is 500 meshes) to obtain a uniform hard carbon material.

[0062] Example 5

[0063] The preparation method of the hard carbon material based on the composite precursor in this example is as follows:

[0064] S1: Weigh cork powder (30 g) and polyacrylonitrile (1 g), mix them in a mass ratio of 30:1 to obtain a mixture. Add 7.7 g of ethylene glycol to the mixture (ethylene glycol accounts for 20% of the total mass of the mixture and ethylene glycol), stir evenly to form a suspension. After treatment by solvothermal method (200 °C, 8 h) and drying, prepare the composite precursor.

[0065] S2: Under argon protection, carry out pre-carbonization treatment on the composite precursor obtained in step (1), heat it to 750 °C at a heating rate of 5 °C / min and hold for 5 minutes to obtain the pre-carbonized product.

[0066] S3: Add 10 g of the pre-carbonized product to 50 g of hydrochloric acid solution (acid mass fraction: 12%), carry out purification at 60 °C for 12 h, then wash with deionized water until the pH = 7 and dry to obtain the acid-purified pre-carbonized product;

[0067] S4: Transfer the acid-purified pre-carbonized product to an atmosphere furnace, and under an argon atmosphere, heat it to 1000 °C at a heating rate of 3 °C / min and hold for 60 minutes for high-temperature carbonization.

[0068] S5: After sintering is completed, cool the hard carbon material to room temperature, process it with a high-energy ball milling device, and sieve it (the mesh number of the sieve is 500 meshes) to obtain a uniform hard carbon material.

[0069] Example 6

[0070] The preparation method of the hard carbon material based on the composite precursor in this example is as follows:

[0071] S1: Weigh the straw powder and bisphenol A epoxy resin at a mass ratio of 9:1. Add 1.1 g of ethylene glycol (ethylene glycol accounts for 5% of the total mass of the straw powder, bisphenol A epoxy resin and ethylene glycol) to bisphenol A epoxy resin (2 g), stir to form a homogeneous solution, and then add the straw powder (18 g). Finally, a composite precursor is prepared by solvent thermal treatment (200 °C, 6 h) followed by drying.

[0072] S2: Under argon protection, pre-carbonize the composite precursor obtained in step (1) by heating at a rate of 19 °C / min to 800 °C and holding for 1 minute to obtain a pre-carbonized product.

[0073] S3: Add 4 g of the pre-carbonized product to 25 g of hydrochloric acid solution (mass fraction of 30%), purify at 40 °C for 24 h, then wash with deionized water until pH = 7 and dry to obtain an acid-purified pre-carbonized product.

[0074] S4: Transfer the acid-purified pre-carbonized product to an atmosphere furnace, and under an argon atmosphere, heat at a rate of 6 °C / min to 1500 °C and hold for 30 minutes for high-temperature carbonization.

[0075] S5: After sintering is completed, cool the hard carbon material to room temperature, process it with a high-energy ball milling device, and sieve it (the mesh size of the sieve is 1000 mesh) to obtain a uniform hard carbon material.

[0076] Example 7

[0077] The preparation method of the hard carbon material based on the composite precursor in this example is as follows:

[0078] S1: Weigh pine sawdust (24.5 g) and phenol formaldehyde resin (0.5 g), mix them at a mass ratio of 49:1 to obtain a mixture. Add 3.4 g of ethylene glycol to the mixture (ethylene glycol accounts for 12% of the total mass of the mixture and ethylene glycol), stir evenly to form a suspension. A composite precursor is prepared by solvent thermal treatment (200 °C, 6 h) followed by drying.

[0079] S2: Under argon protection, pre-carbonize the composite precursor obtained in step (1) by heating at a rate of 1 °C / min to 200 °C and holding for 120 minutes to obtain a pre-carbonized product.

[0080] S3: Add 6 g of the pre-carbonized product to 40 g of hydrochloric acid solution (mass fraction of 10%), purify at 90 °C for 18 h, then wash with deionized water until pH = 7 and dry to obtain an acid-purified pre-carbonized product.

[0081] S4: Transfer the acid-purified pre-carbonized product to an atmosphere furnace. Under an argon atmosphere, heat it at a heating rate of 1 °C / min to 1000 °C, hold for 120 minutes, and perform high-temperature carbonization.

[0082] S5: After sintering is completed, cool the hard carbon material to room temperature, process it through a high-energy ball milling device, and screen it (the mesh number of the sieve is 100 mesh) to obtain a uniform hard carbon material.

[0083] Comparative Example 1

[0084] The preparation method of the hard carbon material in this comparative example is different from that of Example 1 in that ethylene glycol is not added in step (1), and the solvothermal method is not used, and only mixing is required. The remaining steps are the same. Figure 1 b is the SEM image of the hard carbon material prepared in this comparative example. It can be seen from the figure that obvious carbon-carbon interface separation characteristics exist on the material surface, manifested as the dense blocks formed by resin carbonization and the loose and porous structure generated by biomass carbonization being isolated from each other, and there is a gap with a width of about 200 - 500 nm at the interface between the two. This discontinuous interface causes sodium ions to repeatedly cross regions with different conductive characteristics during charge and discharge, thereby leading to increased local polarization and a decrease in material stability.

[0085] Comparative Example 2

[0086] The preparation method of the hard carbon material in this comparative example is different from that of Example 2 in that ethylene glycol is not added in step (1), and the solvothermal method is not used, and only mixing is required. The remaining steps are the same.

[0087] Comparative Example 3

[0088] The preparation method of the hard carbon material in this comparative example is different from that of Example 3 in that ethylene glycol is not added in step (1), and the solvothermal method is not used, and only mixing is required. The remaining steps are the same.

[0089] Comparative Example 4

[0090] The preparation method of the hard carbon material in this comparative example is different from that of Example 4 in that ethylene glycol is not added in step (1), and the solvothermal method is not used, and only mixing is required. The remaining steps are the same.

[0091] Comparative Example 5

[0092] The preparation method of the hard carbon material in this comparative example is different from that of Example 5 in that ethylene glycol is not added in step (1), and the solvothermal method is not used, and only mixing is required. The remaining steps are the same.

[0093] Comparative Example 6

[0094] The preparation method of the hard carbon material in this comparative example is different from that in Example 1 in that only peanut shell powder is used as the precursor, and the remaining steps are the same. The steps are as follows:

[0095] S1: Weigh 20 g of peanut shell powder, add 2.2 g of ethylene glycol to the mixture (ethylene glycol accounts for 10% of the total mass of peanut shell powder and ethylene glycol), stir evenly to form a suspension. After treatment by solvothermal method (180 °C, 10 h), it is dried to prepare a composite precursor.

[0096] S2: Under nitrogen protection, the composite precursor obtained in step (1) is pre-carbonized, heated to 700 °C at a heating rate of 5 °C / min, and kept warm for 30 minutes to obtain a pre-carbonized product.

[0097] S3: Add 6 g of the pre-carbonized product to 40 g of hydrochloric acid solution (mass fraction of 12%), purify at 60 °C for 12 h, then wash with deionized water until pH = 7 and dry to obtain an acid-purified pre-carbonized product.

[0098] S4: Transfer the acid-purified pre-carbonized product to an atmosphere furnace, and under an argon atmosphere, heat it to 1200 °C at a heating rate of 8 °C / min, and keep warm for 70 minutes for high-temperature carbonization.

[0099] S5: After sintering is completed, cool the hard carbon material to room temperature, process it with a high-energy ball milling device, and sieve it (the mesh number of the sieve is 500 mesh) to obtain a uniform hard carbon material.

[0100] Comparative Example 7

[0101] The preparation method of the hard carbon material in this comparative example is different from that in Example 1 in that only xylenol formaldehyde resin is used as the precursor, and the remaining steps are the same. The steps are as follows:

[0102] S1: Weigh 20 g of xylenol formaldehyde resin, add 2.2 g of ethylene glycol to the mixture (ethylene glycol accounts for 10% of the total mass of peanut shell powder and ethylene glycol), stir evenly to form a suspension. After treatment by solvothermal method (180 °C, 10 h), it is dried to prepare a composite precursor.

[0103] S2: Under nitrogen protection, the composite precursor obtained in step (1) is pre-carbonized, heated to 700 °C at a heating rate of 5 °C / min, and kept warm for 30 minutes to obtain a pre-carbonized product.

[0104] S3: Add 6 g of the pre-carbonized product to 40 g of hydrochloric acid solution (mass fraction of 12%), purify at 60 °C for 12 h, then wash with deionized water until pH = 7 and dry to obtain an acid-purified pre-carbonized product;

[0105] S4: Transfer the acid-purified pre-carbonized product to an atmosphere furnace. Under an argon atmosphere, heat it to 1200 °C at a heating rate of 8 °C / min and hold for 70 minutes for high-temperature carbonization.

[0106] S5: After sintering, cool the hard carbon material to room temperature, process it with a high-energy ball milling device, and screen it (the mesh number of the sieve is 500 mesh) to obtain a uniform hard carbon material.

[0107] The interlayer spacing, pore structure, graphitization degree, specific surface area, and tapped density of the carbon materials prepared in Examples 1-5 and Comparative Examples 1-7 were tested and analyzed. The specific results are shown in Table 1.

[0108] Table 1. Results of interlayer spacing, pore structure, graphitization degree, specific surface area, and tapped density

[0109]

[0110]

[0111] Through the synergistic effect of the composite precursor and ethylene glycol in the examples, the optimization of the interlayer spacing (0.37 - 0.40 nm), hierarchical pore structure, and moderate graphitization degree were achieved, solving the problems of uneven pore distribution and insufficient conductivity of traditional hard carbon materials.

[0112] Application Example

[0113] Assemble the hard carbon materials prepared in Examples 1-5 and Comparative Examples 1-7 into sodium-ion half-cells. The preparation method of the battery is as follows:

[0114] (1) Preparation of the negative electrode sheet:

[0115] Add the hard carbon material, conductive agent (Super P), and binder (sodium carboxymethyl cellulose, CMC) to deionized water at a mass ratio of 18:1:1, ball mill and mix for 4 h (rotation speed 400 rpm) to obtain a uniform slurry. Coat the slurry evenly on an aluminum foil current collector (wet film thickness 200 μm), and place it in a blast drying oven at 50 °C for 6 h. Cut the dried electrode sheet into circular pieces with a diameter of 12 mm (active material loading ≈ 2.5 mg / cm 2 ), and cold press for 30 s under a pressure of 60 MPa to ensure the density of the electrode.

[0116] (2) Assembly of the battery:

[0117] a. Component preparation:

[0118] Positive electrode: Sodium metal sheet (diameter 15.6 mm, thickness 0.5 mm)

[0119] Separator: Glass fiber membrane (Whatman GF / D)

[0120] Electrolyte: 1M NaPF6 in G2

[0121] b. Assembly process (operated in an argon glove box, H2O / O2 < 0.1 ppm):

[0122] Stack the battery negative electrode case (CR2025), negative electrode sheet, separator (80 μL of electrolyte is dropped), sodium sheet, gasket, spring sheet, and battery positive electrode case (CR2025) in sequence, seal and then press to 600 kg. Let it stand and age for 12 h to ensure that the electrolyte is fully infiltrated.

[0123] Test the electrochemical performance of the prepared battery. The specific test process and results are as follows:

[0124] (1) Cycling performance

[0125] Test process (LAND CT2001A test system): Voltage range: 0.01 - 2.0V vs. Na + / Na, current density: 0.1C (30 mA / g), 0.2C (60 mA / g), 0.5C (150 mA / g), 1C (300 mA / g), temperature: 25 °C. Figure 2 This is the cycling performance graph of the hard carbon negative electrode materials prepared in Example 1 and Comparative Example 1 of the present invention. It can be seen from the graph that the hard carbon negative electrode material of Example 1 of the present invention shows a high reversible capacity of 364 mAh / g in the initial cycle at 0.1C, and the first cycle Coulombic efficiency reaches 93.6% (significantly higher than 83.4% of Comparative Example 1), indicating that its cross-linked carbon network and gradient pore structure effectively inhibit the irreversible side reactions (such as excessive growth of the SEI film) during the sodium ion insertion / extraction process. After 500 cycles, Example 1 still maintains a capacity of 329 mAh / g (retention rate 84.6%), while the capacity of Comparative Example 1 has decayed to 196 mAh / g (retention rate 65.0%).

[0126] (2) Rate performance

[0127] The charging current is kept constant at 0.1C, and the discharging current is increased sequentially to 0.1C, 0.2C, 0.5C, 1C, 2C, 5C. After each rate cycle for 5 weeks, it is restored to 0.1C to test the capacity recovery rate. Figure 3 This is the rate performance graph of the hard carbon negative electrode materials prepared in Example 1 and Comparative Example 1 of the present invention. It can be seen from the graph that the capacity of Example 1 is still as high as 200 mAh / g at 5C (1500 mA / g), and the capacity is almost unchanged (351 mAh / g) when restored to 0.1C, while the capacity of Comparative Example 1 is only 85 mAh / g at 5C.

[0128] After cycling for three weeks at a current density of 0.1C, the current density was switched to 1C for long-term cycling tests. The specific electrochemical performance data of Examples 1-5 and Comparative Examples 1-7 are shown in Table 2.

[0129] Table 2 Electrochemical Performance

[0130]

[0131]

[0132] The hard carbon materials prepared in the examples are comprehensively superior to the comparative examples in terms of the first efficiency (≥90%), reversible capacity (≥320 mAh / g), and cycle life (≥80% after 500 cycles). Moreover, the reversible capacities of the hard carbon materials prepared in Examples 1, 3, and 5 reached over 350 mAh / g, verifying the improvement effect of the composite precursor design on sodium storage performance.

[0133] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a hard carbon material based on a composite precursor, characterized in that, The steps are as follows: (1) Mix biomass, resin and ethylene glycol to obtain a mixed product; the mixed product is treated by solvothermal method to obtain a hard carbon composite precursor; (2) Pre-carbonize the hard carbon composite precursor in a protective atmosphere to obtain a pre-carbonized product; (3) Purify the pre-carbonized product in an acid solution, and then wash and dry it to obtain an acid-purified pre-carbonized product; (4) In an inert atmosphere, high-temperature carbonize the acid-purified pre-carbonized product, and then perform ball milling treatment and screening to obtain the product.

2. The preparation method of the hard carbon material based on the composite precursor according to claim 1, wherein, In the step (1), the mass ratio of biomass to resin is (9-49):1, and the addition amount of ethylene glycol accounts for 5-20 wt% of the mass of the mixed product.

3. The preparation method of the hard carbon material based on the composite precursor according to claim 2, wherein In the step (1), the temperature of the solvothermal treatment is 160-200 °C, and the time is 6-12 h.

4. The preparation method of the hard carbon material based on the composite precursor according to any one of claims 1-3, characterized in that, In the step (1), the biomass is any one of sawdust, wood chips, straw, cork, rice husk, peanut shell and corn cob.

5. The preparation method of the hard carbon material based on a composite precursor according to claim 4, characterized in that, In the step (1), the resin is any one of phenol formaldehyde resin, xylenol formaldehyde resin, urea formaldehyde resin, bisphenol A epoxy resin, bisphenol F epoxy resin, polyacrylonitrile and polyacrylate.

6. The preparation method of the hard carbon material based on the composite precursor according to claim 5, characterized in that, In the step (2), the pre-carbonization means heating to 200-800 °C at a heating rate of 1-10 °C / min and holding for 1-120 min.

7. The preparation method of the hard carbon material based on the composite precursor according to claim 6, characterized in that, In the step (3), the mass fraction of the acid solution is 10-30%, and the acid solution is at least one of hydrochloric acid, sulfuric acid, nitric acid and hydrofluoric acid.

8. The preparation method of the hard carbon material based on the composite precursor according to claim 7, wherein, In the step (4), the high-temperature carbonization means heating to 1000-1500 °C at a heating rate of 1-10 °C / min and holding for 30-120 min; the mesh number of the sieve is 100-1000 meshes.

9. A hard carbon material based on a composite precursor prepared by the preparation method according to any one of claims 1-3 or 5-8.

10. Application of the hard carbon material based on a composite precursor according to claim 9 in a sodium ion battery.

Images