A preparation method for improving capacity of hard carbon negative electrode of sodium ion battery

Abstract

The application discloses a preparation method for improving the capacity of a sodium-ion battery negative electrode hard carbon, and relates to the technical field of sodium-ion batteries, which comprises the following steps: performing chemical structure modification on a biomass raw material, and introducing a functional group to obtain a chemical modification precursor; mixing the chemical modification precursor with an organic zinc salt, and performing segmented temperature-raising catalytic carbonization treatment to obtain a catalytic carbonization intermediate; performing purification treatment on the carbonization product to obtain a purified intermediate; performing high-temperature carbonization on the purified intermediate; and performing gradient chemical vapor deposition coating on the carbonization product, that is, first adopting a small-molecule carbon source to perform first deposition coating, and then adopting a large-molecule carbon source to perform second deposition coating. The application realizes comprehensive improvement of the reversible capacity, the first coulomb efficiency, the compaction density and the structural stability of the hard carbon, simplifies the technological process, reduces the production cost, improves the batch consistency, and provides a high-performance, easy-industrialization hard carbon negative electrode material preparation method for sodium-ion batteries.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a method for preparing hard carbon for improving the capacity of the negative electrode in sodium-ion batteries. Background Technology

[0002] Sodium-ion batteries have gained widespread attention in recent years for their abundant sodium resources, low cost, and excellent low-temperature performance, particularly in large-scale energy storage and low-speed electric vehicles. As a key component of sodium-ion batteries, the performance of the anode material directly affects the overall energy density and cycle life of the battery. Hard carbon, due to its disordered carbon layer structure and abundant nanopores, is considered the most commercially promising anode material for sodium batteries. Unlike graphite and soft carbon, hard carbon provides ample sodium storage sites, with its sodium storage capacity primarily derived from intercalation between carbon layers and filling within nanopores. In the preparation of hard carbon, the biomass-based route has become one of the main directions for industrialization due to its wide availability of raw materials, low cost, and alignment with the circular economy and "dual-carbon" strategy. However, existing biomass-based hard carbon preparation technologies still face the following technical challenges: First, capacity improvement is limited; the reversible specific capacity of hard carbon is typically only 280-300 kJ / m³. First, the ash content is low, with a low platform capacity ratio, making it difficult to meet the needs of high-energy-density sodium-ion batteries. Second, the initial coulombic efficiency is low, as biomass raw materials easily form a large number of open pores and surface defects during pyrolysis, resulting in a large specific surface area. Third, the precursor consistency is poor; due to differences in origin, variety, and growth conditions, the ash content and cellulose / lignin ratio of biomass raw materials fluctuate greatly, leading to unstable microstructure and electrochemical performance of hard carbon products, making it difficult to meet the batch consistency requirements for industrialization. Fourth, the post-catalytic carbonization processing is complex. In existing technologies, transition metal catalysts such as iron, cobalt, and nickel are often introduced to control the pore structure of hard carbon, but these catalysts are difficult to process after carbonization. To remove these impurities, multiple acid or alkali washes are required, which not only increases costs and the burden of waste liquid treatment but may also introduce residual impurities into the material, affecting electrochemical performance. Fifth, the surface coating methods are limited. Common hard carbon surface treatments often use a single carbon source for chemical vapor deposition or liquid phase coating, which can only fill micropores and reduce specific surface area or form an outer shell and increase compaction density. It is difficult to achieve both effects simultaneously, and the uniformity and density of the coating layer need to be improved. Although there are some improvement schemes in the existing technology to address the above problems, such as using resin-based or pitch-based precursors, adjusting the carbonization process, and performing doping modification, there are still shortcomings such as complex processes, high costs, or limited performance improvement.

[0003] However, current common solutions have many drawbacks, including: limited capacity improvement of existing biomass-based hard carbon preparation technologies, small carbon layer spacing, low closed-pore volume ratio, and lack of precursor molecular-level structural pre-setting methods; low initial coulombic efficiency, large specific surface area, and many surface defects, making it difficult to simultaneously achieve pore filling and shell formation with a single carbon source; cumbersome post-catalytic carbonization treatment, requiring multiple acid washings to remove transition metal catalysts such as iron, cobalt, and nickel, which is costly and prone to introducing residues; unreasonable carbonization processes, such as single-stage or rapid heating, which can easily lead to premature surface densification and insufficient internal closed-pore development; in addition, poor consistency of biomass raw materials, and conventional pretreatment cannot stabilize the precursor structure, affecting batch stability of products. Summary of the Invention

[0004] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0005] In view of the problems existing in the above-mentioned methods for preparing hard carbon for improving the negative electrode capacity of sodium-ion batteries, the present invention is proposed.

[0006] Therefore, the purpose of this invention is to provide a method for preparing hard carbon for sodium-ion battery anodes, which is applicable to solving the limitations of existing biomass-based hard carbon preparation technologies in terms of capacity improvement, small carbon layer spacing, low closed-pore volume ratio, and lack of precursor molecular-level structural pre-setting methods; low initial coulombic efficiency, large specific surface area, and many surface defects, making it difficult to simultaneously achieve pore filling and shell formation with a single carbon source coating; cumbersome post-catalytic carbonization treatment, requiring multiple acid washing removals of transition metal catalysts such as iron, cobalt, and nickel, which is costly and easily introduces residues; unreasonable carbonization processes, such as single-stage or rapid heating, which easily lead to premature surface densification and insufficient internal closed-pore development; in addition, poor consistency of biomass raw materials, and conventional pretreatment cannot stabilize the precursor structure, affecting batch stability of products.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution: In a first aspect, embodiments of the present invention provide a method for preparing hard carbon for improving the capacity of a negative electrode in sodium-ion batteries. The method includes chemically modifying a biomass raw material to introduce functional groups, thereby obtaining a chemically modified precursor; mixing the chemically modified precursor with an organozinc salt and performing a staged heating catalytic carbonization treatment to obtain a catalytic carbonization intermediate; purifying the carbonization product to obtain a purified intermediate; subjecting the purified intermediate to high-temperature carbonization to obtain a carbonized product; and coating the carbonized product with gradient chemical vapor deposition: firstly, a small-molecule carbon source is used for a first deposition coating, and then a large-molecule carbon source is used for a second deposition coating.

[0008] As a preferred embodiment of the method for preparing hard carbon for improving the negative electrode capacity of sodium-ion batteries according to the present invention, the chemical structure modification is carried out in an alkaline aqueous solution, the reaction medium is a weakly alkaline environment, and the biomass molecular chains are fully contacted and covalently bonded by heating and stirring; the functional group is a volume-expanding functional group, selected from substituents containing aromatic rings or epoxy groups, which can generate local expansion stress through thermal decomposition during subsequent carbonization, thereby expanding the carbon interlayer spacing and inducing the formation of closed pores.

[0009] As a preferred embodiment of the method for preparing hard carbon for improving the negative electrode of sodium-ion batteries according to the present invention, the biomass raw material is selected from at least one of lignin, fruit shell, bamboo base, and straw, and does not include coconut shell; the biomass raw material is crushed to a suitable particle size before use and dried and dehydrated as needed.

[0010] As a preferred embodiment of the method for preparing hard carbon for improving the negative electrode capacity of sodium-ion batteries according to the present invention, the organic zinc salt is selected from zinc oxalate or zinc acetate; the organic zinc salt and the chemically modified precursor are mixed evenly in a certain proportion and then dried; during the catalytic carbonization process, the organic zinc salt is first decomposed into nano-zinc oxide particles, which are evenly distributed inside the precursor. The nano-zinc oxide catalyzes the pyrolysis of biomass to generate a highly disordered carbon layer and primary closed pores; in the subsequent high-temperature treatment stage, the zinc oxide is reduced by carbon to metallic zinc and volatilized and removed, realizing the self-removal of the catalyst without cleaning.

[0011] As a preferred embodiment of the method for preparing hard carbon for improving the negative electrode capacity of sodium-ion batteries according to the present invention, the segmented heating catalytic carbonization is carried out under an inert atmosphere and includes at least two heating stages: the first stage involves heating to the decomposition temperature of the organic zinc salt and holding it at that temperature for an appropriate time to completely decompose the organic zinc salt into zinc oxide; the second stage involves heating to the active temperature range of zinc oxide catalytic biomass pyrolysis and holding it at that temperature to promote the growth of disordered carbon layers and closed pores; the heating rate is slow to avoid structural collapse caused by violent pyrolysis.

[0012] As a preferred embodiment of the method for preparing hard carbon for improving the negative electrode capacity of sodium-ion batteries according to the present invention, the high-temperature carbonization is carried out under a protective atmosphere and adopts a multi-stage stepped heating process, that is, multiple heat preservation platforms are set from low temperature to high temperature, including a pre-carbonization platform, a middle heat preservation platform and a final high-temperature carbonization platform; the multi-stage stepped heating helps the orderly growth of closed pores and the gradual orientation and arrangement of microcrystalline layers, and avoids premature surface densification or insufficient development of internal closed pores caused by rapid heating.

[0013] As a preferred embodiment of the method for preparing hard carbon for improving the negative electrode capacity of sodium-ion batteries according to the present invention, the first deposition coating is used to fill the micropores of the hard carbon: the selected small molecule carbon source has a molecular dynamic diameter smaller than the pore size of the hard carbon micropores, and can enter the interior of the micropores at a relatively low temperature to deposit a carbon layer, thereby filling ineffective openings and reducing the specific surface area; the second deposition coating is used to form a dense shell on the surface of the hard carbon: the selected large molecule carbon source has a larger molecular size, and cannot enter the micropores but preferentially polymerizes and carbonizes on the outer surface of the hard carbon to form a continuous, smooth, dense coating layer, thereby improving the compaction density and the first coulombic efficiency.

[0014] As a preferred embodiment of the method for preparing hard carbon for improving the negative electrode capacity of sodium-ion batteries according to the present invention, wherein: the small molecule carbon source is selected from acetylene or methane; the macromolecule carbon source is selected from styrene or toluene; the temperature of the first deposition coating is lower than the temperature of the second deposition coating; the atmosphere can be switched and purged between the two deposition coating steps to avoid mixing of carbon sources; the order of gradient coating cannot be reversed, otherwise the macromolecule carbon source will preferentially block the pore openings, hindering the small molecule carbon source from entering the micropores.

[0015] As a preferred embodiment of the method for preparing hard carbon for improving the negative electrode capacity of sodium-ion batteries according to the present invention, the method further includes: a step of pre-carbonizing the biomass raw material before the chemical structure modification, wherein the pre-carbonization is carried out at a lower temperature to allow the biomass raw material to undergo preliminary pyrolysis stabilization, thereby reducing deformation and cracking in subsequent processing; and a pulverizing and shaping step before the purification treatment to obtain hard carbon particles with uniform particle size distribution and spherical shape.

[0016] As a preferred embodiment of the method for preparing hard carbon for improving the negative electrode capacity of sodium-ion batteries according to the present invention, the purification treatment is selected from at least one of acid washing, alkaline washing or high-temperature carbonization, and is used to remove ash and residual impurities; the acid used for acid washing is an inorganic acid, and the washing is carried out under heating conditions and can be combined with stirring; the high-temperature carbonization is carried out in a chlorine-containing atmosphere to volatilize and remove metal impurities; the purification treatment can be carried out once or multiple times according to the ash content of the raw material until the ash content drops to the qualified range.

[0017] The beneficial effects of this invention are as follows: This invention introduces volume-expanding functional groups into the biomass molecular chain through chemical structure modification, expands the carbon interlayer spacing and induces closed-pore formation through a pre-set method; generates highly disordered carbon layers and primary closed pores through in-situ catalytic carbonization of organic zinc salts, and the zinc salts are automatically volatilized and removed at high temperatures, eliminating the need for acid washing; promotes the orderly growth of closed pores through multi-stage high-temperature carbonization, avoiding premature surface densification; and then coats the surface with gradient chemical vapor deposition, first filling micropores to reduce specific surface area, and then forming a dense shell to improve compaction density and initial coulombic efficiency; simultaneously, combined with pre-carbonization stabilization treatment, pulverization and shaping, and purification and impurity removal, the invention ultimately achieves a comprehensive improvement in the reversible capacity, initial coulombic efficiency, compaction density, and structural stability of hard carbon, while simplifying the process, reducing production costs, and improving batch consistency, providing a high-performance and easily industrialized method for preparing hard carbon anode materials for sodium-ion batteries. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein: Figure 1 This is a flowchart illustrating the implementation of the present invention in Example 1.

[0019] Figure 2 This is a process flow diagram of the present invention in Example 1. Detailed Implementation

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0021] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0022] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0023] Example 1 Reference Figures 1-2This is the first embodiment of the present invention, which provides a method for preparing hard carbon for improving the capacity of a sodium-ion battery negative electrode, comprising the following steps: S1: Chemically modify the structure of biomass raw materials by introducing functional groups to obtain chemically modified precursors.

[0024] Preferably, the chemical structure modification is carried out in an alkaline aqueous solution, and the reaction medium is a weakly alkaline environment. Heating and stirring are used to ensure that the biomass molecular chains come into full contact with the nucleophile and undergo covalent bonding. The functional group is a volume expansion type functional group, selected from substituents containing aromatic rings or epoxy groups. This functional group can generate local expansion stress through thermal decomposition during subsequent carbonization, thereby expanding the carbon interlayer spacing and inducing closed-cell formation.

[0025] Furthermore, the biomass raw materials are selected from at least one of lignin, fruit shells, bamboo base, and straw, but do not include coconut shells; Biomass raw materials are crushed to a suitable particle size before use and dried and dehydrated as needed.

[0026] Furthermore, the nucleophile is selected from halogenated compounds containing aromatic rings (such as p-chloromethylstyrene) or olefinic compounds containing epoxy groups (such as allyl glycidyl ether); the pH of the alkaline aqueous solution is adjusted to 9-11, the reaction temperature is controlled at 60-80℃, and the reaction time is 4-8 hours, so that the functional groups are fully grafted onto the biomass molecular chain through covalent bonds.

[0027] Specifically, after the chemical structure modification is completed, the steps also include filtering, washing and drying the modified product to remove unreacted nucleophiles and byproducts. The drying temperature is controlled at 60-80℃ and the drying time is until constant weight.

[0028] Preferably, by covalently grafting volume-expanding functional groups containing aromatic rings or epoxy groups onto biomass molecular chains in an alkaline aqueous solution, these functional groups generate local expansion stress during subsequent carbonization through thermal decomposition, actively increasing the carbon interlayer spacing (d002 ≥ 0.39 nm) and inducing the formation of a large number of nanopores (closed-pore volume ratio ≥ 45%), thereby significantly increasing the storage sites for sodium ions. At the same time, the pre-designed molecular structure can weaken the impact of batch differences in raw materials on product consistency, laying the precursor foundation for achieving high-capacity hard carbon.

[0029] S2: The chemically modified precursor is mixed with an organozinc salt and subjected to staged heating catalytic carbonization to obtain a catalytic carbonization intermediate.

[0030] Preferably, the organic zinc salt is selected from zinc oxalate or zinc acetate; Organic zinc salt and chemically modified precursor are mixed evenly in a certain proportion and then dried. During the catalytic carbonization process, the organic zinc salt is first decomposed into nano zinc oxide particles, which are evenly distributed inside the precursor. The nano zinc oxide catalyzes the pyrolysis of biomass to generate a highly disordered carbon layer and primary closed pores. In the subsequent high-temperature treatment stage, zinc oxide is reduced to metallic zinc by carbon and volatilized, achieving self-removal of the catalyst without cleaning.

[0031] Specifically, the staged heating catalytic carbonization is carried out under an inert atmosphere and includes at least two heating stages: The first stage involves heating to the decomposition temperature of the organic zinc salt and holding it at that temperature for an appropriate time to allow the organic zinc salt to completely decompose into zinc oxide. The second stage involves heating to the active temperature range of zinc oxide catalyzing biomass pyrolysis and maintaining the temperature to promote the growth of disordered carbon layers and closed pores. A slow heating rate is used to avoid structural collapse caused by violent pyrolysis.

[0032] Furthermore, the mixing method of the organic zinc salt and the chemically modified precursor is selected from one of mechanical stirring, ball milling or solution blending; the drying temperature after mixing is 80-120℃, and the drying time is 4-12 hours, so as to completely remove moisture and prevent water vapor interference during the subsequent carbonization process.

[0033] Specifically, the target temperature for the first stage of heating is 280-350℃, the heating rate is 1-5℃ / min, and the holding time is 0.5-2 hours; the target temperature for the second stage of heating is 400-600℃, the heating rate is 0.5-3℃ / min, and the holding time is 1-3 hours; the inert atmosphere is at least one of nitrogen, argon, or helium, and the gas flow rate is 0.5-2 L / min.

[0034] Preferably, zinc oxalate or zinc acetate is mixed with a chemically modified precursor. During the staged heating process, the organic zinc salt first decomposes into nano-zinc oxide particles and is uniformly distributed inside the precursor, catalyzing the pyrolysis of biomass to generate a highly disordered carbon layer and primary closed pores. At a higher temperature, the zinc oxide is reduced to metallic zinc by carbon and completely volatilized and removed, achieving "self-cleaning" of the catalyst. This avoids the cumbersome acid washing steps and metal residue problems required by traditional transition metal catalysts. At the same time, the uniform distribution of nano-zinc oxide ensures the uniformity of the pore structure, providing an ideal precursor for subsequent closed-pore control.

[0035] S3: Purify the catalytic carbonization intermediate to obtain a purified intermediate. Preferably, the purification process is selected from at least one of acid washing, alkaline washing or high-temperature carbonization, used to remove ash and residual impurities; The acid used for pickling is an inorganic acid, and the washing is carried out under heating conditions and can be combined with stirring; High-temperature carbonization is carried out in a chlorine-containing atmosphere to volatilize and remove metal impurities; Purification can be performed once or multiple times depending on the ash content of the raw material, until the ash content drops to the acceptable range.

[0036] Specifically, the acid used for pickling is at least one of hydrochloric acid, sulfuric acid, or nitric acid, with a concentration of 0.5-2 mol / L; the washing temperature is 60-90℃, the washing time is 1-4 hours; the solid-liquid ratio is 1:5-1:20; after washing, the mixture is filtered and repeatedly rinsed with deionized water until neutral, and then dried at 80-120℃.

[0037] Specifically, the alkali used for alkaline washing is sodium hydroxide or potassium hydroxide solution with a concentration of 0.1-1 mol / L, and the washing conditions are similar to those for acid washing; alkaline washing can be followed by acid washing for neutralization.

[0038] Specifically, high-temperature carbonization is carried out in a nitrogen or hydrogen chloride atmosphere at a temperature of 800-1000℃ for 0.5-2 hours. After carbonization, inert gas is introduced to purge residual chlorine.

[0039] Preferably, for the inherent ash (containing K, Ca, Mg, Fe, etc.) and trace residual impurities in biomass raw materials, at least one of acid washing, alkaline washing, or high-temperature carbonization is used for deep removal. Acid washing can dissolve most metal oxides, while high-temperature carbonization can cause some insoluble metal impurities to volatilize and escape in the form of chlorides. Depending on the ash content of the raw materials, single or multiple treatments can be flexibly selected to reduce the ash content to the qualified range, avoiding the catalytic decomposition of electrolyte by impurities in electrochemical cycles, thereby improving the initial coulombic efficiency and cycle life.

[0040] S4: The purified intermediate is carbonized at high temperature to obtain the carbonized product.

[0041] Preferably, the high-temperature carbonization is carried out under a protective atmosphere and adopts a multi-stage stepped heating process, that is, multiple heat preservation platforms are set from low temperature to high temperature, including a pre-carbonization platform, an intermediate heat preservation platform and a final high-temperature carbonization platform. Multi-stage heating helps the orderly growth of closed pores and the gradual orientation and arrangement of microcrystalline layers, avoiding premature surface densification or insufficient development of internal closed pores caused by rapid heating.

[0042] Furthermore, the temperature of the pre-carbonization platform is 400-600℃, and the holding time is 0.5-2 hours; the temperature of the medium-temperature carbonization platform is 700-900℃, and the holding time is 0.5-2 hours; the temperature of the final high-temperature carbonization platform is 1100-1400℃, and the holding time is 1-4 hours.

[0043] Specifically, the heating rate between two adjacent insulation platforms is 1-5℃ / min to maintain a mild thermal process; the protective atmosphere is nitrogen, argon, or a mixture of both, with a gas flow rate of 1-3 L / min.

[0044] Furthermore, after the high-temperature carbonization is completed, the carbonized products are allowed to cool naturally to room temperature under a protective atmosphere to prevent oxidation caused by contact with air at high temperatures.

[0045] Specifically, the equipment used for high-temperature carbonization is one of the following: box furnace, tube furnace, rotary furnace, or continuous multi-stage carbonization furnace.

[0046] Preferably, by setting up multiple heat preservation platforms such as pre-carbonization, medium-temperature carbonization, and final high-temperature carbonization and connecting them with a slow heating rate, the closed pores can fully develop and become more uniform in size during a gentle thermal process. At the same time, the microcrystalline layers gradually arrange themselves in an orderly manner while retaining sufficient disordered areas. This process effectively avoids premature surface densification and insufficient development of internal closed pores caused by rapid heating, thereby reducing the specific surface area and improving structural stability while ensuring high capacity.

[0047] S5: Gradient chemical vapor deposition coating of carbonized products: First, a small molecule carbon source is used for the first deposition coating, and then a large molecule carbon source is used for the second deposition coating.

[0048] Specifically, the first deposition coating is used to fill the micropores of hard carbon: The selected small-molecule carbon source has a molecular dynamics diameter smaller than the pore size of hard carbon micropores, which can enter the interior of micropores at relatively low temperatures to deposit a carbon layer, thereby filling in ineffective openings and reducing specific surface area. The second deposition coating is used to form a dense outer shell on the hard carbon surface: The selected macromolecular carbon source has a large molecular size and cannot enter the micropores. Instead, it preferentially polymerizes and carbonizes on the outer surface of hard carbon, forming a continuous, smooth, and dense coating layer, which improves the compaction density and the first coulombic efficiency.

[0049] Furthermore, the small molecule carbon source is selected from acetylene or methane; The carbon source for the macromolecule is selected from styrene or toluene; The temperature of the first deposition layer is lower than the temperature of the second deposition layer; Atmosphere switching and purging can be performed between the two-step deposition and coating processes to avoid mixing of carbon sources; The order of gradient coating cannot be reversed; otherwise, the large molecular carbon source will preferentially block the pore openings, preventing the small molecular carbon source from entering the micropores.

[0050] Specifically, the temperature for the first deposition coating is 600-800℃, and the deposition time is 1-4 hours; the flow rate of the small molecule carbon source is 50-150 sccm, the carrier gas is argon or nitrogen, and the total gas flow rate is 200-500 sccm.

[0051] Specifically, the temperature for the second deposition coating is 800-1000℃, and the deposition time is 1-3 hours; the macromolecular carbon source is fed into the reactor through a bubbler or evaporator, the carrier gas flow rate is 20-100 sccm, and the deposition pressure is atmospheric pressure or slightly negative pressure.

[0052] Furthermore, gradient chemical vapor deposition coating is carried out in a fluidized bed reactor, rotary kiln, or fixed bed reactor; after coating is completed, the temperature is lowered to room temperature under an inert atmosphere, and the finished hard carbon is obtained.

[0053] It should be noted that before chemical structure modification, a pre-carbonization treatment of the biomass raw material is also included. The pre-carbonization is carried out at a lower temperature to allow the biomass raw material to undergo preliminary pyrolysis stabilization, thereby reducing deformation and cracking in subsequent processing. The purification process includes a crushing and shaping step to obtain hard carbon particles with a uniform particle size distribution and a near-spherical shape.

[0054] Specifically, the pre-carbonization treatment temperature is 200-400℃, the heating rate is 2-10℃ / min, the holding time is 1-5 hours, and the pre-carbonization is carried out in an inert atmosphere.

[0055] Specifically, the pulverization and shaping process uses one of the following: air jet mill, mechanical pulverizer, or ball mill; the median particle size D50 of the pulverized hard carbon particles is controlled at 6±0.5μm, and the particle size distribution width Span value is not greater than 1.5; after pulverization and shaping, the particles can be screened and graded to remove excessively large or excessively fine particles.

[0056] Preferably, a small molecule carbon source (acetylene or methane) is first deposited at a lower temperature. Its molecular dynamics diameter is smaller than the micropore size of hard carbon, allowing it to penetrate the open pores and deposit a carbon layer, effectively filling ineffective open pores and increasing the specific surface area from 5–15 m². 2 / g decreased significantly to ≤3.5 m 2 / g; then, a macromolecular carbon source (styrene or toluene) is deposited at a higher temperature. Because its molecular size is too large to enter the micropores, it preferentially polymerizes and carbonizes on the outer surface of the hard carbon, forming a continuous, smooth, and dense shell, increasing the compaction density to ≥1.05 g / cc; the two steps cannot be reversed, achieving a synergistic modification of "filling the pores first and then forming the shell", and the coulombic efficiency is increased from the industry average of 82-86% to ≥90% for the first time.

[0057] In summary, this invention introduces volume-expanding functional groups into the biomass molecular chain through chemical structural modification, thereby expanding the carbon interlayer spacing and inducing closed-pore formation. Highly disordered carbon layers and primary closed pores are generated through in-situ catalytic carbonization of organic zinc salts, with the zinc salts volatilizing and being removed at high temperatures, eliminating the need for acid washing. Multi-stage high-temperature carbonization promotes the orderly growth of closed pores, preventing premature surface densification. Gradient chemical vapor deposition coating, starting with small molecules and then moving to large molecules, first fills the micropores to reduce specific surface area, then forms a dense shell to improve compaction density and initial coulombic efficiency. Simultaneously, combined with pre-carbonization stabilization treatment, pulverization and shaping, and purification, this invention ultimately achieves a comprehensive improvement in the reversible capacity, initial coulombic efficiency, compaction density, and structural stability of hard carbon. It also simplifies the process, reduces production costs, and improves batch consistency, providing a high-performance, easily industrialized method for preparing hard carbon anode materials for sodium-ion batteries.

[0058] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing hard carbon for improving the capacity of a sodium-ion battery negative electrode, characterized in that: include: Chemically modifying biomass raw materials to introduce functional groups yields chemically modified precursors. The chemically modified precursor was mixed with an organozinc salt and subjected to staged heating catalytic carbonization to obtain a catalytic carbonization intermediate. The carbonized product was purified to obtain a purified intermediate. The purified intermediate was carbonized at high temperature to obtain the carbonized product. Gradient chemical vapor deposition coating of carbonized products: First, a small molecule carbon source is used for the first deposition coating, and then a large molecule carbon source is used for the second deposition coating.

2. The method for preparing hard carbon for improving the capacity of a sodium-ion battery negative electrode as described in claim 1, characterized in that: The chemical structure modification is carried out in an alkaline aqueous solution, and the reaction medium is a weakly alkaline environment. Heating and stirring are used to ensure that the biomass molecular chains come into full contact with the nucleophile and undergo covalent bonding. The functional group is a volume expansion type functional group, selected from substituents containing aromatic rings or epoxy groups. This functional group can generate local expansion stress through thermal decomposition during subsequent carbonization, thereby expanding the carbon interlayer spacing and inducing closed-cell formation.

3. The method for preparing hard carbon for improving the capacity of a sodium-ion battery negative electrode as described in claim 2, characterized in that: The biomass raw material is selected from at least one of lignin, fruit shell, bamboo base, and straw, but does not include coconut shell; The biomass raw materials are crushed to a suitable particle size before use and dried and dehydrated as needed.

4. The method for preparing hard carbon for improving the capacity of a sodium-ion battery negative electrode as described in claim 3, characterized in that: The organic zinc salt is selected from zinc oxalate or zinc acetate; The organic zinc salt and the chemically modified precursor are mixed evenly in a certain proportion and then dried. During the catalytic carbonization process, the organic zinc salt is first decomposed into nano zinc oxide particles, which are evenly distributed inside the precursor. The nano zinc oxide catalyzes the pyrolysis of biomass to generate a highly disordered carbon layer and primary closed pores. In the subsequent high-temperature treatment stage, zinc oxide is reduced to metallic zinc by carbon and volatilized, achieving self-removal of the catalyst without cleaning.

5. The method for preparing hard carbon for improving the capacity of a sodium-ion battery negative electrode as described in claim 4, characterized in that: The staged heating catalytic carbonization is carried out under an inert atmosphere and includes at least two heating stages: The first stage involves heating to the decomposition temperature of the organic zinc salt and holding it at that temperature for an appropriate time to allow the organic zinc salt to completely decompose into zinc oxide. The second stage involves heating to the active temperature range of zinc oxide catalyzing biomass pyrolysis and maintaining the temperature to promote the growth of disordered carbon layers and closed pores. A slow heating rate is used to avoid structural collapse caused by violent pyrolysis.

6. The method for preparing hard carbon for improving the capacity of a sodium-ion battery negative electrode as described in claim 5, characterized in that: The high-temperature carbonization is carried out under a protective atmosphere and adopts a multi-stage stepped heating process, that is, multiple heat preservation platforms are set from low temperature to high temperature, including a pre-carbonization platform, an intermediate heat preservation platform and a final high-temperature carbonization platform. Multi-stage heating helps the orderly growth of closed pores and the gradual orientation and arrangement of microcrystalline layers, avoiding premature surface densification or insufficient development of internal closed pores caused by rapid heating.

7. The method for preparing hard carbon for improving the capacity of a sodium-ion battery negative electrode as described in claim 6, characterized in that: The first deposition coating is used to fill the micropores of hard carbon: The selected small-molecule carbon source has a molecular dynamics diameter smaller than the pore size of hard carbon micropores, which can enter the interior of micropores at relatively low temperatures to deposit a carbon layer, thereby filling in ineffective openings and reducing specific surface area. The second deposition coating is used to form a dense outer shell on the hard carbon surface: The selected macromolecular carbon source has a large molecular size and cannot enter the micropores. Instead, it preferentially polymerizes and carbonizes on the outer surface of hard carbon, forming a continuous, smooth, and dense coating layer, which improves the compaction density and the first coulombic efficiency.

8. The method for preparing hard carbon for improving the capacity of a sodium-ion battery negative electrode as described in claim 7, characterized in that: The small molecule carbon source is selected from acetylene or methane; The macromolecular carbon source is selected from styrene or toluene; The temperature of the first deposition layer is lower than the temperature of the second deposition layer; Atmosphere switching and purging can be performed between the two-step deposition and coating processes to avoid mixing of carbon sources; The order of gradient coating cannot be reversed; otherwise, the large molecular carbon source will preferentially block the pore openings, preventing the small molecular carbon source from entering the micropores.

9. The method for preparing hard carbon for improving the capacity of a sodium-ion battery negative electrode as described in claim 8, characterized in that: Before the chemical structure modification, a pre-carbonization treatment of the biomass raw material is also included. The pre-carbonization is carried out at a low temperature to allow the biomass raw material to undergo preliminary pyrolysis stabilization, thereby reducing deformation and cracking in subsequent processing. Prior to the purification process, a pulverizing and shaping step is included to obtain hard carbon particles with a uniform particle size distribution and a near-spherical shape.

10. The method for preparing hard carbon for improving the capacity of a sodium-ion battery negative electrode as described in claim 9, characterized in that: The purification process is selected from at least one of acid washing, alkaline washing or high-temperature carbonization, and is used to remove ash and residual impurities. The acid used for pickling is an inorganic acid, and the washing is carried out under heating conditions and can be combined with stirring; High-temperature carbonization is carried out in a chlorine-containing atmosphere to volatilize and remove metal impurities; Purification can be performed once or multiple times depending on the ash content of the raw material, until the ash content drops to the acceptable range.

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