A hard carbon composite material, its preparation method, and its application in sodium-ion batteries

Abstract

This application relates to a hard carbon composite material, its preparation method, and its application in sodium-ion batteries. The preparation method of the hard carbon composite material obtained by this invention involves: pulverizing and sieving biomass material, followed by a first heating and calcination under an inert atmosphere or air atmosphere to obtain a carbon precursor; then, under an inert atmosphere, subjecting the carbon precursor to a second heating and calcination to obtain hard carbon material HC; finally, subjecting the obtained hard carbon material HC to CVD vapor deposition to obtain a hard carbon composite material with a carbon layer coated on its surface. This invention, through stepwise pre-carbonization and high-temperature carbonization combined with CVD surface coating, forms a stable core-shell structure, improving the material's conductivity and structural stability.

Description

Technical Field

[0001] This application relates to the field of sodium-ion battery technology, and in particular to a hard carbon composite material, its preparation method, and its application in sodium-ion batteries. Background Technology

[0002] Sodium-ion batteries (SIBs) are considered a cost-effective alternative or competitor to lithium-ion batteries for large-scale energy storage applications, but their development has been hampered by the high cost and poor electrochemical performance of hard carbon (HC) anodes. Biomass-derived hard carbon (HC), characterized by its disordered structure consisting of bent graphitic microcrystals, stacked nanopores, and defects, has attracted significant attention as a viable anode material for SIBs; however, its capacity and rate performance are poor due to a lack of sufficient sodium storage sites and conductivity.

[0003] Since the microstructure directly affects the sodium storage capacity of HC, modifying the microstructure and enhancing conductivity through simple processes should effectively improve the aforementioned problems. Introducing nitrogen atoms can typically create defects, enhance conductivity, and improve sodium storage capacity and rate performance. Furthermore, oxygen atom doping can form oxygen functional groups on the surface of HC, and these oxygen-containing functional groups interact with Na... + The reaction helps to increase Na + Platform capacity.

[0004] Nano Energy 2019, 56, 828-839. Nitrogen-doped N-HC samples, prepared by spray drying and subsequent pyrolysis in flowing NH3, exhibited increased defects and widened interlayer spacing, resulting in a higher reversible capacity compared to undoped samples. However, too many defects resulted in its first-cycle coulomb efficiency being only about 50%.

[0005] Carbon 231 (2025) 119733. By adding an air pre-carbonization step before high-temperature carbonization, the O functional group was introduced, which improved the plateau capacity, but had a weak effect on improving electrical conductivity. Summary of the Invention

[0006] To address the above technical problems, this application provides a hard carbon composite material, its preparation method, and its application in sodium-ion batteries.

[0007] The first objective of this application is to provide a method for preparing a hard carbon composite material, comprising the following steps: S1. Provide a biomass material; S2. The biomass material is pulverized and sieved, and then subjected to a first heating and calcination in air atmosphere to obtain a carbon precursor; S3. Under an inert atmosphere, the carbon precursor is subjected to a second heating and calcination to obtain hard carbon material HC; S4. Perform CVD vapor deposition on the hard carbon material HC obtained in step S3 to obtain a hard carbon composite material with a carbon layer on the surface.

[0008] In some embodiments of this application, step S3-1 is also included: heating the hard carbon material HC obtained in step S3 with a nitrogen-containing compound under an inert atmosphere to obtain N / O co-doped hard carbon.

[0009] In some embodiments of this application, the mass ratio of hard carbon material HC to nitrogen-containing compound is 1:(0.5~2). The nitrogen-containing compounds include one or more of urea, dicyandiamide, urea nitrate, guanidine nitrate, and 5-aminotetrazole; The heating temperature is 300~500℃, and the time is 2h~6h.

[0010] In some embodiments of this application, in step S1, the biomass material is selected from one or more of coffee grounds, coconut shells, and camellia shells.

[0011] In some embodiments of this application, in step S2, the sieve used for crushing and sieving is 100-300 mesh; Inert gases in an inert atmosphere include nitrogen and / or argon; The first heating and calcination is carried out at a temperature of 250~400℃, a heating rate of 1~8℃ / min, and a time of 2~6h.

[0012] In some embodiments of this application, step S3 further includes acid washing of the hard carbon material HC to remove impurities; The acid includes one or more of hydrochloric acid, nitric acid, sulfuric acid, and citric acid; The concentration of the acid is 0.5~4 mol / L; The inert atmosphere includes nitrogen and / or argon as inert gases. The second calcination is carried out at a temperature of 1000~1500℃ for 3~5 hours.

[0013] In some embodiments of this application, in step S4, the organic gas molecules in the mixed atmosphere during CVD vapor deposition are selected from one or more of C2H2, C2H6, and CH4; The mixed atmosphere also includes inactive gases, including nitrogen and / or argon. The ratio of organic gas molecules to inactive gas flow rate is 1:1~5; The heating temperature in CVD vapor deposition is 650~1100℃, the time is 2~6 hours, and the heating rate is 1~8℃ / min.

[0014] The second objective of this application is to provide a hard carbon composite material prepared by the aforementioned preparation method. The hard carbon composite material has a core-shell structure, with hard carbon material HC as the core and a carbon layer covering the surface as the shell.

[0015] In some embodiments of this application, the core and shell of the hard carbon composite material further include an N / O co-doped hard carbon layer.

[0016] A third objective of this application is to provide a negative electrode comprising the aforementioned hard carbon composite material.

[0017] The fourth objective of this application is to provide a sodium-ion battery including the aforementioned negative electrode.

[0018] The technical solution described in this application has the following advantages over the prior art: 1. By pre-carbonization and high-temperature carbonization in a stepwise manner, combined with CVD surface coating, a stable core-shell structure is formed, which improves the conductivity and structural stability of the material; 2. The introduction of N / O co-doping synergistically increases defects, active sites and interlayer spacing, significantly improving sodium storage capacity and rate performance; 3. The process conditions are mild, the raw materials are widely available, the cost is low, and it is suitable for industrial production. Attached Figure Description

[0019] To make the content of this application easier to understand, the following detailed description is provided based on specific embodiments and accompanying drawings. Figure 1 This is the N2 adsorption-desorption curve of the hard carbon anode material HC-COF@N / O obtained in Example 8 of this application.

[0020] Figure 2 These are the XRD patterns of the hard carbon composite materials obtained in Comparative Example 1 and Example 1 of this application.

[0021] Figure 3 These are the first charge-discharge curves of the hard carbon composite materials obtained in Comparative Example 1 and Example 1 of this application at a current density of 10 mA / g.

[0022] Figure 4 This describes the long-cycle performance of the hard carbon composite materials obtained in Comparative Example 1 and Example 1 of this application at a current density of 30 mA / g.

[0023] Figure 5 This is a rate performance diagram of the hard carbon composite materials obtained in Comparative Example 1 and Example 1 of this application.

[0024] Figure 6 These are the EIS spectra of the hard carbon composite materials obtained in Comparative Example 1 and Example 1 of this application. Detailed Implementation

[0025] The present application will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present application, but the embodiments are not intended to limit the present application.

[0026] 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 belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0027] In the description of this application, the terms "(1)", "(2)", "first", and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "(1)", "(2)", "first", or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified. In this application, when describing various components in the capacity layer, sodium supplement layer, and carbon layer, such as binders and conductive agents, the prefixes "first", "second", and "third" are used to define them, respectively. This is only to distinguish the components in the three layers in terms of description, and does not imply that the corresponding components in these three layers are necessarily different. The scope of protection of this application includes embodiments in which the same or different types of binders and conductive agents are used in the capacity layer, sodium supplement layer, and carbon layer.

[0028] In the description of this application, unless otherwise stated, the term "or" is inclusive. That is, the phrase "A or (or) B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0029] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the weights mentioned in the embodiments of this application can be well-known units of mass in the chemical industry, such as μg, mg, g, and kg.

[0030] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0031] Unless otherwise specified, all technical features and optional technical features of this application can be combined to form new technical solutions.

[0032] To address the technical problems identified in the background section, this application achieves its objective through the following solutions: First aspect This application provides a method for preparing a hard carbon composite material, comprising the following steps: S1. Provide a biomass material; S2. The biomass material is pulverized and sieved, and then subjected to a first heating and calcination in air atmosphere to obtain a carbon precursor; S3. Under an inert atmosphere, the carbon precursor is subjected to a second heating and calcination to obtain hard carbon material HC; S4. Perform CVD vapor deposition on the hard carbon material HC obtained in step S3 to obtain a hard carbon composite material with a carbon layer on the surface.

[0033] Furthermore, it also includes step S3-1: heating the hard carbon material HC obtained in step S3 with a nitrogen-containing compound under an inert atmosphere to obtain N / O co-doped hard carbon.

[0034] Further, in step S3-1, the mass ratio of hard carbon material HC to nitrogen-containing compound is 1:(0.5~2). Furthermore, in step S3-1, an inert atmosphere is used in the preparation of N / O co-doped hard carbon in order to prevent the "high temperature + oxygen" process of air pre-oxidation from causing premature decomposition of nitrogen-containing organic matter such as urea and loss of nitrogen source through oxidation, which would directly destroy the doping basis; and the oxygen-containing functional groups introduced by air pre-oxidation will occupy the reaction sites of nitrogen atoms, causing nitrogen species to be unable to be stably anchored.

[0035] Furthermore, many researchers, when modifying hard carbon materials, often only improve one aspect of the hard carbon's performance while neglecting other aspects. For example, some researchers have improved the reversible capacity and conductivity of hard carbon by doping it with N, but the increase in defects inevitably leads to a decrease in coulombic efficiency. This invention utilizes the simultaneous introduction of N and O to generate a synergistic effect. Through the co-doping of light elements, the surface oxygen-containing functional groups, defects, and interlayer spacing of HC can be increased, thereby improving the slope capacity and plateau capacity of HC. The synergistic modification of N / O can improve the reversible specific capacity and exhibit excellent rate performance.

[0036] The nitrogen-containing compounds include one or more of urea, dicyandiamide, urea nitrate, guanidine nitrate, and 5-aminotetrazole; The heating temperature is 300~500℃, and the time is 2-4 hours.

[0037] Further, in step S1, the biomass material is selected from one or more of coffee grounds, coconut shells, camellia shells, rice husks, sugarcane bagasse, melon seed shells, and peanut shells. The biomass materials listed in this invention are mainly composed of cellulose (30wt%~50wt%), hemicellulose (20wt%~35wt%), and lignin (15wt%~30wt%), with a carbon content of ~25wt%; they naturally possess a fibrous / porous microstructure; they are widely available and extremely low in cost.

[0038] Furthermore, coffee grounds are preferred; coffee grounds have a special composition, with oil and protein components having a "synergistic pore-forming effect"; coffee grounds have a low ash content; the interlayer spacing of coffee grounds is controllable, and the preparation process is simple and can improve electrochemical performance.

[0039] Furthermore, in step S2, the sieve used for crushing and sieving is 100-300 mesh; by controlling the size of the sieve, the consistency of the original particle size can be controlled, and experimental errors can be reduced.

[0040] Inert gases in an inert atmosphere include nitrogen and / or argon; The first heating and calcination temperature is 250~400℃, and can be, for example, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400℃, or any range between any two values; the heating rate is 1~8℃ / min, and can be, for example, 1, 2, 3, 4, 5, 6, 7, 8℃ / min, or any range between any two values; the time is 2~6 hours. Pre-carbonizing biomass materials under this temperature condition maximizes ash removal efficiency and introduces the most oxygen-containing functional groups, while ensuring minimal carbon loss. The ash from coffee grounds specifically includes alkali metal oxides such as K₂O, CaO, and MgO.

[0041] Furthermore, in step S2, when heating and calcining are performed using an air atmosphere, oxygen-containing functional groups are introduced (to inhibit graphitization) and micropores / mesopores are generated (to increase sodium storage sites) in order to gently modify the carbon precursor, while removing some impurities.

[0042] Furthermore, the precise control of the interlayer spacing in the hard carbon material of this invention is mainly related to the content of cellulose, hemicellulose, and lignin in the raw materials. Higher lignin content leads to a higher degree of carbon chain aromatization, making them easier to oriented and tightly stack at high temperatures, and thus more difficult to control the interlayer spacing. Higher cellulose / hemicellulose content provides greater space for carbon chain recombination after pyrolysis, making the interlayer spacing easier to control. Coffee grounds have a lower lignin content and a higher cellulose and hemicellulose content, therefore their interlayer spacing is relatively easy to control.

[0043] Furthermore, step S3 also includes acid washing of the hard carbon material HC to remove impurities; The acid includes one or more of hydrochloric acid, nitric acid, sulfuric acid, and citric acid; The concentration of the acid is 0.5~4 mol / L; too low an acid concentration will result in insufficient removal efficiency; too high a concentration will damage the carbon structure and may leave residues.

[0044] Furthermore, step S3 also includes washing and drying the hard carbon material HC at a temperature of 60~120℃ for 12~48h. At this drying temperature, free water can evaporate rapidly, and the residual moisture content is <0.3% within 12h; if the temperature is <60℃, the drying time needs to be extended to more than 24h (low efficiency), and the hard carbon is prone to agglomeration due to residual moisture.

[0045] The inert atmosphere includes nitrogen and / or argon as inert gases. The second calcination temperature is 1000~1500℃, and exemplarily, it can be 1000, 1100, 1200, 1300, 1400, 1500℃, or any range between any two values; the time is 3~5 hours, and the heating rate is 1~8℃ / min, and exemplarily, it can be 1, 2, 3, 4, 5, 6, 7, 8℃ / min, or any range between any two values. This invention achieves high-performance hard carbon by densifying the carbon skeleton and precisely controlling the interlayer spacing under oxidation-free conditions, thus solidifying the high-quality structure formed by pre-oxidation. The inert atmosphere isolates oxygen, preventing the oxidation and consumption of carbon materials, while also ensuring the integrity of the carbon structure and preventing the generation of impurities. Using an air atmosphere would lead to a sharp decrease in carbon yield and damage to the carbon layer structure; using a reducing atmosphere would not only reduce carbon yield and damage the carbon layer structure but also introduce impurities, thereby affecting battery performance.

[0046] Furthermore, in this invention, the temperature T2 of the second heating and calcination is greater than the temperature T1 of the first heating and calcination. The purpose of temperature T1 in this invention is primarily to introduce oxygen-containing functional groups and remove volatiles; temperature T2 is the temperature required to prepare hard carbon. The sintering temperature of hard carbon needs to be at least 1000℃; if T2 is not greater than T1, the sintered material will not be hard carbon.

[0047] Furthermore, in step S4, the organic gas molecules in the mixed atmosphere during CVD vapor deposition are selected from one or more of C2H2, C2H6, and CH4; the use of organic gas molecules can effectively control the pore structure of hard carbon materials.

[0048] The mixed atmosphere also includes inactive gases, including nitrogen and / or argon. The ratio of organic gas molecules to inactive gas flow rate is 1:1~5; When the gas used in CVD vapor deposition is acetylene, the heating temperature is 650–750 °C for 2–6 hours; when the gas is C₂H₆, the heating temperature is 700–900 °C; and when it is CH₄, the heating temperature is 900–1100 °C. The heating rate is 1–8 °C / min. Within this temperature range, the thermal decomposition of acetylene and other gases is incomplete. Above 800 °C, acetylene and other gases undergo excessive decomposition, producing a large amount of gaseous hydrocarbons. Within this temperature range, the optimal decomposition rate of acetylene and other gases can be ensured, resulting in sufficient activated carbon species.

[0049] Furthermore, the introduction of O-containing functional groups during pre-carbonization in step S2 reduces defect formation and provides a rapid transport channel for sodium ions, thereby increasing the platform capacity. The subsequent acetylene CVD treatment further optimizes the pore structure of the hard carbon through C coating, resulting in better electrochemical performance. Moreover, the acetylene CVD treatment fills the surface pores of the hard carbon material. After acetylene CVD treatment, carbon radicals generated during pyrolysis diffuse into the microporous channels, reacting with electron-rich pore walls and dangling bonds to form covalent bonds, ultimately forming sp... 3 Hybridized carbon. This process disrupts the in-plane conjugated structure, causes lattice distortion, weakens interlayer van der Waals forces, and ultimately leads to a further increase in interlayer spacing.

[0050] Second aspect This application also provides a hard carbon composite material prepared by the aforementioned preparation method. The hard carbon composite material has a core-shell structure, with hard carbon material HC as the core and a carbon layer covering the surface as the shell.

[0051] Furthermore, the core and shell of the hard carbon composite material also include an N / O co-doped hard carbon layer.

[0052] Third aspect This application also provides a negative electrode sheet comprising the aforementioned hard carbon composite material.

[0053] Fourth aspect This application also provides a sodium-ion battery, including the negative electrode.

[0054] The sodium-ion battery also includes an electrolyte, a separator, and a positive electrode.

[0055] The separator can be any material suitable for separators in electrochemical energy storage devices, for example, including but not limited to at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers.

[0056] Fifth aspect This application also provides an electrochemical device including the sodium-ion battery.

[0057] The electronic devices include, but are not limited to: computers, media players, telephones, fax machines, copiers, printers, headphones, video recorders, televisions, calculators, memory cards, radios, backup power supplies, motors, automobiles, motorcycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, large household batteries, energy storage or lithium-ion capacitors, etc.

[0058] The testing methods used in this application are: 1. Test method for charging specific capacity: At a 0.1 C rate, constant current charging was performed from 0.01 V to 3.0 V, and the charging capacity was recorded. 2. Test method for discharge specific capacity: At a 0.1 C rate, constant current discharge was performed from 3.0 V to 0.01 V, and the discharge capacity was recorded. 3. Calculation method for first-effect: First-time efficiency = (First-time charge specific capacity / First-time discharge specific capacity) × 100%; 4. Rct test method: Electrochemical impedance spectroscopy (EIS) was used, with a frequency range of 0.01 Hz to 10 Hz. 5 Hz, amplitude of 5 mV, the charge transfer impedance Rct is obtained by fitting the equivalent circuit; 5. Interlayer spacing: The interlayer spacing of hard carbon materials can be calculated using Bragg's equation. ; Where: d (002) λ is the spacing between the (002) crystal planes; n is the diffraction order, usually n=1; λ is the incident X-ray wavelength, here λ=0.154 nm; θ is the angle between the incident X-ray and the crystal plane.

[0059] Example 1

[0060] This embodiment provides a method for preparing a hard carbon anode material for sodium-ion batteries, as detailed below: S1: Place the coffee grounds in a dry oven to remove moisture, then grind and sieve using a 200-mesh sieve to obtain particles with a diameter of 150~45μm.

[0061] S2: The particles obtained in step S1 are heated to 300°C in air at a rate of 5°C / min and held at this temperature for 2 hours, and then cooled to room temperature to obtain the carbon precursor.

[0062] S3: The carbon precursor obtained in step S2 is heated to 1300°C in an inert atmosphere (N2) at a rate of 5°C / min and held at this temperature for 4 hours. It is then cooled to room temperature to obtain hard carbon material.

[0063] S4: The hard carbon material obtained in step S3 is acid-washed with 2 mol / L HCl aqueous solution for 6 hours at room temperature, filtered, thoroughly washed with deionized water, and then dried in an oven at 80℃ for 12 hours to obtain the dried hard carbon material.

[0064] S5: Mix 1g of dried hard carbon material from step S4 with urea at a mass ratio of 1:1; place it in a tube furnace and co-fire at 500℃ for 2h under N2 atmosphere to obtain N / O co-doped hard carbon.

[0065] S6: In a mixed atmosphere of N2 and C2H2 (N2: 50 sccm; C2H2: 10 sccm), the N / O co-doped hard carbon obtained in step S5 was heated to 700℃ at a rate of 5℃ / min and held at this temperature for 2 hours to obtain the hard carbon anode material HC-COF@N / O. The obtained hard carbon anode material was subjected to BET analysis, and the results are shown below. Figure 1 ,Depend on Figure 1 It can be seen that the adsorption-desorption curve of HC-COF@N / O belongs to type IV. No obvious adsorption saturation plateau appears in the high relative pressure region, and an H3-type hysteresis loop is observed, indicating the simultaneous presence of micropores and mesopores in the material. The specific surface area of ​​HC-COF@N / O is calculated to be 7.1 m² using the BET equation. 2 / g, a smaller specific surface area means less sodium ions are consumed during the SEI film formation process, resulting in less irreversible capacity.

[0066] Example 2

[0067] Unlike Example 1, the biomass material is a coconut shell.

[0068] Example 3

[0069] Unlike Example 1, the biomass material used is camellia shell.

[0070] Example 4

[0071] Unlike Example 1, the sieve has a mesh size of 300.

[0072] Example 5

[0073] Unlike Example 1, the sieve has a mesh size of 100.

[0074] Example 6

[0075] Unlike Example 1, N2: 50 sccm; CH4: 10 sccm.

[0076] Example 7

[0077] Unlike Example 1, N2: 50 sccm; C2H6: 10 sccm.

[0078] Example 8

[0079] Unlike Example 1, the nitrogen source was selected from melamine.

[0080] Comparative Example 1 Unlike Example 1, step S2 (no pre-sintering at 300°C) is missing, as detailed below: S1: Place the coffee grounds in a dry oven to remove moisture, then grind and sieve using a 200-mesh sieve to obtain particles with a diameter of 75μm. S2: Heat to 1300°C in an inert atmosphere (N2) at a rate of 5°C / min and hold at this temperature for 4 hours. After cooling to room temperature again, hard carbon material is obtained; S3: The hard carbon material obtained in step S2 is acid-washed with 2 mol / L HCl aqueous solution for several hours at room temperature, filtered, thoroughly washed with deionized water, and then dried in an oven at 80°C for 12 hours. S4: In a mixed atmosphere of N2 and C2H2 (N2: 50 sccm; C2H2: 10 sccm), heat the dried material to 700°C at a rate of 5°C / min and maintain this temperature for 2 hours.

[0081] Comparative Example 2 This comparative example provides a preparation method involving pre-sintering in an inert atmosphere at 300℃, followed by CVD vapor deposition and high-temperature carbonization at 1300℃, as detailed below: S1: Place the coffee grounds in a dry oven to remove moisture, then grind and sieve using a 200-mesh sieve to obtain particles with a diameter of 75μm. S2: The particles obtained in step S1 are heated to 300°C in an inert atmosphere (N2) at a rate of 5°C / min, held at this temperature for 2 hours, and then cooled to room temperature to obtain the carbon precursor; S3: The carbon precursor obtained in step S2 was acid-washed with 2 mol / L HCl aqueous solution for several hours at room temperature, filtered, thoroughly washed with deionized water, and then dried in an oven at 80°C for 12 hours. S4: In a mixed atmosphere of N2 and C2H2 (N2: 50 sccm; C2H2: 10 sccm), the dried carbon precursor was heated to 700°C at a rate of 5°C / min and held at this temperature for 2 hours; S5: Subsequently, the mixture was heated to 1300°C at a rate of 5°C / min in an inert atmosphere (N2) and held at that temperature for 4 hours. After cooling to room temperature again, the hard carbon anode material for sodium-ion batteries was obtained.

[0082] Comparative Example 3 (This illustrates the comparison between carbon coating followed by sintering and sintering followed by carbon coating) This comparative example provides a method for preparing a hard carbon anode material for sodium-ion batteries, similar to Example 9, except that it lacks the pre-carbonization step S2 and the carbon coating step S6, and only includes N doping, as detailed below: S1: Place the coffee grounds in a dry oven to remove moisture, then grind and sieve using a 200-mesh sieve to obtain particles with a diameter of 75μm. S2: Subsequently, the material was heated to 1300°C at a rate of 5°C / min in an inert atmosphere (N2) and held at that temperature for 4 hours. After cooling to room temperature again, hard carbon material was obtained. S3: The obtained hard carbon material was acid-washed with 2 mol / L HCl aqueous solution for several hours at room temperature, filtered, thoroughly washed with deionized water, and then dried in an oven at 80℃ for 12 hours. S4: Mix the dried and cleaned hard carbon with urea at a mass ratio of 1:1; heat to 700°C at a rate of 5°C / min in an inert atmosphere (N2), and maintain this temperature for 2 hours. Cool to room temperature again.

[0083] Comparative Example 4 This comparative example provides the preparation method of hard carbon composite material given in the literature JOURNAL OF GUANGXI UNIVERSITY OF SCIENCE AND TECHNOLOGY 2024, 35(3), 83-90: mixing before high-temperature sintering, which may affect the structure of hard carbon during high-temperature co-firing. However, in this invention, mixing is performed after high temperature, and the subsequent mild doping will not affect the structure of hard carbon. At the same time, using coffee grounds as a precursor requires fewer cumbersome pretreatment steps than using potatoes. The specific preparation method of the hard carbon composite material in this comparative example is as follows: (1) Pretreatment and precarbonization of potato starch: Weigh 5 g of potato starch and add it to 500 mL of prepared acetate-sodium acetate buffer solution (pH=5.5). After preheating in a 55 ℃ water bath for 15 min, add a mixture of α-amylase and saccharifying enzyme at a mass ratio of 1:2. Stir for 10 h, then wash the starch with deionized water until pH=7 to obtain enzymatically hydrolyzed potato starch. Soak the enzymatically hydrolyzed potato starch in a 20% (NH4)2HPO4 solution for 2 h, then filter and freeze-dry. Place the freeze-dried potato starch in a tube furnace and heat at 210 ℃ for 5 h under N2 atmosphere protection, then raise the temperature to 500 ℃ for pre-carbonization pyrolysis for 2 h. After cooling, wash with deionized water until neutral and dry in a forced-air drying oven for 12 h. The product obtained after complete drying is potato starch carbon. (2) Preparation of nitrogen-doped hard carbon anode materials: After thoroughly mixing potato starch carbon and dicyandiamine at a mass ratio of 1:1, the mixture is placed in a nickel crucible, which is then placed in a muffle furnace. The temperature is increased to 1100 °C at a heating rate of 2 °C / min under a N2 atmosphere and held for 2 hours. After natural cooling, nitrogen-doped hard carbon material is obtained.

[0084] Comparative Example 5 This comparative example provides a method for preparing a hard carbon anode material for sodium-ion batteries, as detailed below: S1: Place the coffee grounds in an oven at 50℃ for 24 hours to remove moisture, then grind and sieve using a 200-mesh sieve to obtain particles with a diameter of 150~45μm. S2: The particles obtained in step S1 are heated to 300°C at a rate of 5°C / min in an inert atmosphere (N2), and held at this temperature for 2 hours, and then cooled to room temperature to obtain the carbon precursor; S3: The carbon precursor obtained in step S2 is heated to 1300°C in an inert atmosphere (N2) at a rate of 5°C / min and held at that temperature for 4 hours. Then it is cooled to room temperature to obtain hard carbon material. S4: The hard carbon material obtained in step S3 is acid-washed with 2 mol / L HCl aqueous solution for 6 hours at room temperature, filtered, thoroughly washed with deionized water, and then dried in an oven at 80℃ for 12 hours to obtain the dried hard carbon material. S5: In a mixed atmosphere of N2 and C2H2 (N2: 50 sccm; C2H2: 10 sccm), the dried hard carbon material is heated to 700°C at a rate of 5°C / min and held at this temperature for 2 hours to finally obtain a hard carbon material with a carbon layer on its surface, which is the hard carbon anode material for sodium-ion batteries.

[0085] application The hard carbon composite materials obtained in Examples 1-9 and Comparative Examples 1-5 were used to prepare corresponding negative electrode sheets and batteries. The specific preparation methods are as follows: Using the hard carbon material obtained in the examples and comparative examples as the active material, Super P as the conductive agent, and PVDF as the binder, an electrode slurry was prepared with a mass ratio of active material: conductive agent: binder = 8:1:1. The slurry was coated on copper foil, vacuum dried at 70°C for 12 hours, and cut into battery negative electrode sheets with a diameter of 13 mm.

[0086] The positive electrode is a sodium sheet, the separator is glass fiber, and the electrolyte is 1M NaClO4 (solvent is EC:PC=1:1, v / v, containing 5wt% FEC).

[0087] Battery assembly: Place the positive electrode shell, place the negative electrode plate, add electrolyte, place the separator, add electrolyte to wet it, place the sodium plate and gasket, place the spring sheet, place the negative electrode shell, press at 50 MPa for 30 seconds to obtain the assembled CR2032 type half cell.

[0088] Electrochemical performance testing The electrochemical performance of the CR2032 type half-cells obtained above was tested, and the results are shown in Table 1.

[0089] Table 1 Example 1 demonstrates that pre-carbonization stabilizes the carbon framework, facilitating the formation of a more disordered, larger interlayer spacing (larger d(002)), and more defective ideal hard carbon structure. The post-coating strategy in Example 1 avoids the carbon layer destruction at high temperatures caused by the pre-coating followed by high-temperature sintering in Comparative Example 2. It perfectly encapsulates the hard carbon particles, providing high-speed electron channels and stabilizing the electrode / electrolyte interface.

[0090] As can be seen from Examples 1 and 3 and 4, compared with coconut shells / camellia shells, coffee grounds have a larger and controllable interlayer spacing, which can form more defects and pores, resulting in a higher sodium storage capacity.

[0091] As can be seen from Examples 2 and 1, air pre-carbonization can suppress graphitization, increase interlayer spacing, introduce defects and micropores, thereby improving the capacity and first-time efficiency of sodium-ion battery anode materials.

[0092] As can be seen from Examples 1 and 7, the carbon layer deposited by methane may be more disordered and defective, providing additional sodium ion adsorption sites, thus slightly increasing the capacity; the more disordered deposited carbon layer may have a larger specific surface area, leading to an increase in irreversible side reactions that form the SEI film during the first charge and discharge, and a decrease in ICE; the carbon layer deposited by methane has a lower degree of graphitization and poorer conductivity, which is not conducive to rapid charge transfer, thus increasing Rct.

[0093] As can be seen from Examples 1 and 6, the deposited carbon layer may provide a small number of additional adsorption sites, but the bulk capacity remains unchanged; the density and graphitization degree of the ethane-deposited carbon layer are lower than those of acetylene, resulting in slightly poorer surface stability, which may lead to a slight increase in side reactions, thus reducing ICE; the conductivity of the deposited carbon layer is lower than that of acetylene, leading to an increase in charge transfer resistance. Therefore, Rct increases.

[0094] As shown in Example 8, the addition of N doping before carbon coating provides additional adsorption and pseudocapacitive sodium storage capacity through the defects, active sites, and expanded interlayer spacing introduced by N doping, significantly improving the capacity. C coating stabilizes the interface and reduces side reactions, but N doping sites may trigger irreversible reactions. The combined effect of both results in little change in ICE. N doping greatly improves the electronic conductivity of the carbon matrix. The synergistic effect of C coating optimizes interfacial ion transport, significantly improving kinetics, thus greatly reducing Rct. The incorporation of N atoms into the carbon layer causes lattice distortion and expansion, thereby slightly expanding the interlayer spacing, which is more conducive to the rapid insertion / extraction of sodium ions.

[0095] As shown in Examples 9 and 8, by selecting melamine with a higher nitrogen content, a higher nitrogen doping level is expected to introduce more surface defects and active sites, thereby providing a stronger contribution to pseudocapacitance. More uniform doping may form a more stable carbon-nitrogen structure. Under the protection of subsequent acetylene carbon coating, it helps to maintain or slightly reduce the first irreversible side reaction, thus improving ICE. Melamine doping may be more conducive to forming a highly conductive graphitic nitrogen configuration, and in synergy with the dense carbon coating layer, constructing a better electronic conductivity network, further reducing Rct.

[0096] As shown in Comparative Example 1, removing the pre-calcination step leads to a decrease in both intercalation and adsorption capacity; at the same time, the specific surface area increases significantly, side reactions become more severe, irreversible capacity loss is huge, and ICE decreases; and the high specific surface area results in extremely high interfacial impedance; the carbon layers are excessively ordered at high temperatures, developing into a graphitized structure, and the interlayer spacing is significantly reduced.

[0097] As shown in Comparative Example 2, the method of first performing CVD vapor deposition and then high-temperature sintering may cause the CVD carbon layer to become unstable or generate new defects at the final high temperature, which will eventually lead to a significant decrease in capacity and ICE. At the same time, after reforming at 1300℃, the conductivity continuity of the CVD carbon layer may be disrupted, resulting in a higher Rct. The high graphitization and tight arrangement of the carbon layer also significantly reduce the interlayer spacing.

[0098] As shown in Comparative Example 3, the lack of pre-oxidation leads to a tightly ordered arrangement of carbon layers at high temperatures, resulting in a significant reduction in intercalation capacity. The gases produced by urea decomposition may create some pores, providing limited adsorption capacity. The lack of pre-oxidation and the nitrogen-containing gases and residues produced by urea decomposition exacerbate irreversible side reactions, thus significantly reducing ICE. The high specific surface area results in a huge interfacial impedance, which significantly increases Rct. Due to the lack of cross-linking structures, the carbon layers rearrange freely at 1300℃, forming graphite-like microcrystals, and the interlayer spacing shrinks significantly.

[0099] Depend on Figure 2 It can be seen that the (002) peak of Example 1 is shifted to a lower angle than that of Comparative Example 1, thus it has a larger interlayer spacing, which is beneficial to the improvement of intercalation capacity.

[0100] Depend on Figure 3 It can be seen that the initial efficiency of Example 1 is 70%, and that of Comparative Example 1 is 50%, which proves the effect of N / O co-doping on improving the initial efficiency of hard carbon.

[0101] Depend on Figure 4 It can be seen that the reversible capacity of Example 1 is as high as 295mAh / g, which is about 75mAh / g higher than that of Comparative Example 1.

[0102] Depend on Figure 5 It can be seen that the rate performance of Example 1 is improved compared with Comparative Example 1, and the reversible capacity of Example 1 is higher than that of Comparative Example 1 at all current densities.

[0103] Depend on Figure 6 It can be seen that: the Rct of Example 1 is 79Ω, which is much smaller than that of Comparative Example 1 (238Ω), indicating that its interfacial charge transfer impedance is smaller; at the same time, the slope of the straight line in the low-frequency region of Example 1 is higher than that of Comparative Example 1, indicating that their sodium ion diffusion rate is faster.

[0104] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A method for preparing a hard carbon composite material, characterized in that, Includes the following steps: S1. Provide a biomass material; S2. The biomass material is pulverized and sieved, and then subjected to a first heating and calcination in air atmosphere to obtain a carbon precursor; S3. Under an inert atmosphere, the carbon precursor is subjected to a second heating and calcination to obtain hard carbon material HC; S4. Perform CVD vapor deposition on the hard carbon material HC obtained in step S3 to obtain a hard carbon composite material with a carbon layer on the surface.

2. The preparation method according to claim 1, characterized in that, It also includes step S3-1: heating the hard carbon material HC obtained in step S3 with a nitrogen-containing compound under an inert atmosphere to obtain N / O co-doped hard carbon.

3. The preparation method according to claim 2, characterized in that, The mass ratio of hard carbon material HC to nitrogen-containing compounds is 1:(0.5~2); The nitrogen-containing compounds include one or more of urea, dicyandiamide, urea nitrate, guanidine nitrate, and 5-aminotetrazole; The heating temperature is 300~500℃, and the time is 2-6 hours.

4. The preparation method according to claim 1, characterized in that, In step S1, the biomass material is selected from one or more of coffee grounds, coconut shells, and camellia shells; In step S2, the sieve used for crushing and sieving is 100-300 mesh; Inert gases in an inert atmosphere include nitrogen and / or argon; The first heating and calcination is carried out at a temperature of 250-400℃, with a heating rate of 1-8℃ / min and a duration of 2-6h.

5. The preparation method according to claim 1, characterized in that, In step S3, the inert gas in the inert atmosphere includes nitrogen and / or argon. The second calcination is carried out at a temperature of 1000~1500℃ for 2-6 hours, with a heating rate of 1~8℃ / min.

6. The preparation method according to claim 1, characterized in that, In step S4, the organic gas molecules in the mixed atmosphere during CVD vapor deposition are selected from one or more of C2H2, C2H6, and CH4; The mixed atmosphere also includes inactive gases, including nitrogen and / or argon. The ratio of the organic gas molecules to the inert gas flow rate is 1:1~5; The heating temperature in CVD vapor deposition is 650~750℃, the time is 2-6 hours, and the heating rate is 1-8℃ / min.

7. A hard carbon composite material, characterized in that, The hard carbon composite material prepared by any one of claims 1 to 6 has a core-shell structure, with hard carbon material HC as the core and a carbon layer covering the surface as the shell.

8. The hard carbon composite material according to claim 7, characterized in that, The hard carbon composite material also includes an N / O co-doped hard carbon layer between the core and shell.

9. A negative electrode sheet, characterized in that, Includes the hard carbon composite material described in claim 8.

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

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