Hard carbon material, and preparation method therefor and use thereof

Abstract

Provided in the present application are a hard carbon material, and a preparation method therefor and the use thereof. In the Raman spectrum of the hard carbon material, the ratio of the peak intensity at the wavelength of 1345 cm-1 to the peak intensity at the wavelength of 1600 cm-1 is 0.9-1.3; and the powder impedance of the hard carbon material under the pressure of 16-20 MPa is 0.02-0.07 Ω·cm. The present application contributes to improving the reversible specific capacity, initial Coulombic efficiency and rate capability of a battery.

Description

A hard carbon material, its preparation method and application

[0001] This application claims priority to Chinese Patent Application No. 202510138334.3, filed on February 8, 2025, entitled "A Hard Carbon Material and Its Preparation Method and Application", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to anode materials, specifically to a hard carbon material, its preparation method, and its application. Background Technology

[0003] Sodium-ion batteries have wide applications in electric two-wheelers, low-speed electric vehicles, and energy storage systems. Currently, the main bottleneck in the industrialization of sodium-ion batteries lies in hard carbon anode materials, including their high price, insufficient production capacity, and low industry maturity. Therefore, there is an urgent need to develop high-capacity, low-cost hard carbon anode materials for sodium-ion batteries. Biomass-based hard carbon, due to its abundant raw material supply, low cost, and high sodium storage capacity, is the preferred choice for large-scale production of hard carbon anodes.

[0004] However, biomass-based hard carbon currently suffers from low sodium storage capacity and low initial coulombic efficiency. Summary of the Invention

[0005] This application provides a hard carbon material, its preparation method, and its application. The hard carbon material has a high sodium storage capacity and a high initial coulombic efficiency.

[0006] This application provides a hard carbon material, wherein the Raman spectrum of the hard carbon material has a wavelength of 1345 cm⁻¹. - 1 The peak intensity and wavelength at 1600 cm⁻¹ -1 The ratio of peak intensity at the point is 0.9 to 1.3, and the powder impedance of the hard carbon material under a pressure of 16 to 20 MPa is 0.02 to 0.07 Ω·cm.

[0007] Optionally, the interlayer spacing d of the hard carbon material (002) crystal planes 002 The thickness is 0.37–0.40 nm; and / or, the hard carbon material comprises graphite microcrystals and amorphous carbon, wherein the width La of the graphite microcrystals along the a-axis is 1.4–2.0 nm, the thickness Lc along the c-axis is 0.7–1.0 nm, and the number of stacked layers is 2.9–3.5.

[0008] Optionally, the hard carbon material includes a core, a first coating layer on the surface of the core, and a second coating layer on the surface of the first coating layer, wherein the first coating layer includes carbon and the second coating layer includes nitrogen and carbon.

[0009] Optionally, the number of nitrogen atoms accounts for 0.1% to 2.0% of the total number of atoms in the hard carbon material; and / or, the sum of the thicknesses of the first coating layer and the second coating layer is 5 to 100 nm.

[0010] Optionally, the porosity of the hard carbon material is 1% to 10%; and / or, the specific surface area of ​​the hard carbon material is 0.5 to 20 m². 2 / g.

[0011] This application provides a method for preparing the hard carbon material as described above, comprising: carbonizing biomass raw materials in a first inert atmosphere to obtain a hard carbon precursor; and subjecting the hard carbon precursor to pore-forming treatment under the action of a pore-forming agent to obtain a specific surface area of ​​300-900 m². 2 / g porous hard carbon precursor; the porous hard carbon precursor and a first coating agent are mixed and then subjected to a first sintering treatment in a second inert atmosphere to obtain a coated porous hard carbon precursor, wherein the first coating agent includes a carbon source; the coated porous hard carbon precursor and a second coating agent are mixed and then subjected to a second sintering treatment in a third inert atmosphere to obtain the hard carbon material, wherein the second coating agent includes a carbon-nitrogen source compound, wherein the carbon-nitrogen source compound includes amine and alkyl groups.

[0012] Optionally, the biomass raw material includes one or more of glucose, sucrose, starch, coconut shell, walnut shell, hazelnut shell, and wood; and / or, in the process of carbonizing the biomass raw material to obtain a hard carbon precursor, the carbonization temperature is 300–800℃, the carbonization time is 1–10 h, and the heating rate is 0.5–3℃ / min; and / or, the process of carbonizing the biomass raw material to obtain a hard carbon precursor further includes: after carbonizing the biomass raw material, pulverizing and sieving it to obtain the hard carbon precursor with a mesh size of 300–500 mesh; and / or, the pore-forming agent includes a physical pore-forming agent, which includes water vapor and / or carbon dioxide, and the pore-forming treatment temperature is 650–950℃; and / or, the pore-forming agent includes a chemical pore-forming agent, which includes one or more of potassium hydroxide, sodium hydroxide, zinc chloride, and phosphoric acid, and the pore-forming treatment temperature is 500–900℃.

[0013] Optionally, the carbon source includes pitch and / or resin; and / or, the temperature of the first sintering treatment is 1100–1500°C, the holding time is 1–8 h, and the heating rate is 0.5–3°C / min; and / or, the temperature of the second sintering treatment is 700–950°C; and / or, the carbon-nitrogen source compound further includes a carboxyl group, and the carbon-nitrogen source compound includes one or more of isoleucine, leucine, alanine, glycine, proline, valine, tyrosine, phenylalanine, tryptophan, serine, threonine, cysteine, asparagine, glutamine, aspartic acid, glutamine, lysine, and arginine; and / or, during the second sintering treatment, the heating rate is 0.5–3°C / min, and the holding time is 2–6 h.

[0014] This application provides a negative electrode sheet, the negative electrode sheet including a negative electrode active material layer, the negative electrode active material layer including the hard carbon material as described above or the hard carbon material prepared according to the preparation method of the hard carbon material as described above.

[0015] This application provides a battery, which includes a negative electrode as described above.

[0016] This application provides a hard carbon material, its preparation method, and its application. The hard carbon material has a large number of active sites for storing active ions (e.g., sodium storage active sites) and also has low powder impedance, which helps to improve reversible capacity, first coulombic efficiency, rate performance, and cycle performance. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 is a scanning electron microscope (SEM) image of the hard carbon material prepared in Example 1;

[0019] Figure 2 shows a scanning electron microscope (SEM) image of the hard carbon material prepared in Comparative Example 1;

[0020] Figure 3 shows the X-ray diffraction (XRD) pattern of the hard carbon material prepared in Example 1;

[0021] Figure 4 shows the Raman spectrum of the hard carbon material prepared in Example 1;

[0022] Figure 5 shows the charge-discharge curves of the hard carbon material prepared in Example 1. Detailed Implementation

[0023] To enable those skilled in the art to better understand the solutions of this application, a further detailed description of this application is provided below. The specific embodiments listed below are merely descriptions of the principles and features of this application; the examples are only for explaining this application and are not intended to limit its scope. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.

[0024] In existing technologies, biomass-based hard carbon generally suffers from low sodium storage capacity and low initial coulombic efficiency.

[0025] To overcome the shortcomings of the prior art, this application provides a hard carbon material (hard carbon anode material) with a Raman spectrum of 1345 cm⁻¹. -1 The peak intensity and wavelength at 1600 cm⁻¹ -1 The ratio of peak intensity at the point is 0.9 to 1.3, and the powder impedance of hard carbon material under pressure of 16 to 20 MPa is 0.02 to 0.07 Ω·cm.

[0026] According to research and analysis: In the Raman spectrum of the aforementioned hard carbon materials, at a wavelength of 1345 cm⁻¹... -1 and 1600cm -1 Characteristic peaks (D peak and G peak, respectively) are present at all locations, with a wavelength of 1345 cm⁻¹. -1 Peak intensity I at D and wavelength of 1600cm -1 Peak intensity I at G The ratio (I) D / I G The concentration of 0.9 to 1.3 improves the structure of hard carbon materials, resulting in more active sites for storing active ions (e.g., sodium storage sites). Furthermore, the hard carbon material also has low powder impedance, which helps to improve reversible capacity, first coulombic efficiency, rate performance, and cycle performance.

[0027] Understandably, in the Raman spectrum of the aforementioned hard carbon materials, at a wavelength of 1345 cm⁻¹... -1 and 1600cm -1 Characteristic peaks (D peak and G peak, respectively) are present at each location, corresponding to amorphous carbon structure and graphite-like microcrystalline structure, that is, the above-mentioned hard carbon material includes amorphous carbon and graphite-like microcrystalline structure. The embodiments of this application, by controlling the structure of amorphous carbon and graphite-like microcrystalline structure in the hard carbon material, enable the hard carbon material to possess excellent reversible specific capacity and high coulombic efficiency.

[0028] For example, the wavelength mentioned above is 1345cm. -1 Peak intensity I at D and wavelength of 1600cm -1Peak intensity I at G The ratio (I) D / I G The range can be 0.9, 1.0, 1.1, 1.2, 1.3 or any two of them.

[0029] For example, the powder impedance of hard carbon material under a pressure of 16 to 20 MPa can be a range of 0.02, 0.03, 0.04, 0.05, 0.06, 0.07 Ω·cm or any combination thereof.

[0030] In some embodiments, the interlayer spacing d of the crystal planes of the aforementioned hard carbon material (002) 002 The wavelength ranges from 0.37 to 0.40 nm.

[0031] In some embodiments, the hard carbon material comprises graphite microcrystals and amorphous carbon, wherein the width La(002) of the graphite microcrystals along the a-axis is 1.4–2.0 nm, and the thickness Lc along the c-axis is 0.7–1.0 nm; the number of stacked layers (N = Lc / d) 002 +1) is 2.9 to 3.5. The above-mentioned hard carbon material is a hard carbon structure composed of short-range ordered graphite-like microcrystals and amorphous carbon with appropriate average width, stacking thickness and number of stacked layers. It has a large number of sodium (or lithium) storage active sites, and the hard carbon material also has low powder impedance, thus exhibiting good electrochemical performance, which helps to improve reversible capacity, first coulombic efficiency, rate performance and cycle performance.

[0032] In some embodiments, the hard carbon material includes a core, a first coating layer on the surface of the core, and a second coating layer on the surface of the first coating layer, wherein the first coating layer includes carbon and the second coating layer includes nitrogen and carbon.

[0033] Specifically, the proportion of nitrogen atoms in the hard carbon material can be 0.1% to 2.0% (i.e., the nitrogen doping ratio in the hard carbon material is 0.1 at% to 2.0 at%). A small amount of nitrogen doping can enhance the defect active sites and conductivity of the hard carbon material, thereby increasing the reversible capacity, increasing the ramp capacity, and improving the rate performance.

[0034] For example, the ratio of nitrogen atoms to the total number of atoms in the hard carbon material can be a range of 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, or any combination thereof.

[0035] The first coating layer may include carbon elements, and the second coating layer may also include carbon elements. That is, the hard carbon material is coated with nitrogen elements and also includes a soft carbon coating layer. This can avoid the side effect of irreversible capacity increase caused by nitrogen doping, improve the interface of the hard carbon material, reduce the side reactions between the defective surface caused by doping and the electrolyte, and improve the first coulombic efficiency and rate performance of the hard carbon material.

[0036] Furthermore, the sum of the thicknesses of the first coating layer and the second coating layer can be 5 to 100 nm, for example, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 nm or any combination thereof.

[0037] In some embodiments, the porosity of the hard carbon material is 1% to 10%, for example, a range of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination thereof.

[0038] In some embodiments, the specific surface area of ​​the hard carbon material is 0.5–20 m². 2 / g, for example 0.5, 1, 5, 10, 20m 2 / g or a range consisting of any two of them.

[0039] This application also provides a method for preparing the above-mentioned hard carbon material, comprising: carbonizing biomass raw materials in a first inert atmosphere to obtain a hard carbon precursor; and subjecting the hard carbon precursor to pore-forming treatment (pore size control) under the action of a pore-forming agent to obtain a specific surface area of ​​300-900 m². 2 / g porous hard carbon precursor; the porous hard carbon precursor and the first coating agent are mixed and then subjected to a first sintering treatment in a second inert atmosphere to obtain a coated porous hard carbon precursor, wherein the first coating agent includes a carbon source; the coated porous hard carbon precursor and the second coating agent are mixed and then subjected to a second sintering treatment in a third inert atmosphere to obtain a hard carbon material, wherein the second coating agent includes a carbon-nitrogen source compound, wherein the carbon-nitrogen source compound includes amine and alkyl groups.

[0040] Research analysis shows that pore-forming treatment helps suppress graphite microcrystallization during the carbonization process of biomass raw materials, facilitating the preparation of hard carbon materials including graphite-like microcrystallization and amorphous carbon. Furthermore, the hard carbon material undergoes a two-layer coating, with the second layer employing a carbon-nitrogen synergistic coating. On one hand, nitrogen doping enhances the defective active sites and conductivity of the hard carbon material, thereby increasing reversible capacity, ramp capacity, and rate performance. On the other hand, the carbon-nitrogen synergistic coating avoids the side effect of increased irreversible capacity caused by nitrogen doping, improves the interface of the hard carbon material, reduces side reactions between doped defect surfaces and the electrolyte, and enhances the first coulombic efficiency and rate performance of the hard carbon material.

[0041] Specifically, biomass raw materials can include one or more of glucose, sucrose, starch, coconut shells, walnut shells, hazelnut shells, and wood, and have advantages such as abundant raw material yield, low cost, and high sodium (or lithium) storage capacity.

[0042] In the process of carbonizing biomass raw materials (low-temperature pyrolysis carbonization) to obtain hard carbon precursor, the carbonization temperature can be 300-800℃ and the carbonization time (holding time) can be 1-10h.

[0043] In addition, the heating rate during the carbonization process can be 0.5 to 3 °C / min.

[0044] In practice, the process of carbonizing biomass raw materials to obtain hard carbon precursors further includes: carbonizing the biomass raw materials to obtain carbonized material, and then pulverizing and sieving the carbonized material (e.g., pulverizing, classifying, and sieving). The equipment used in the pulverizing process may include an air classifier, and the sieve mesh size can be 300–500 mesh. The specific surface area of ​​the obtained hard carbon precursor is generally less than 300 m². 2 / g, for example, the specific gravity is approximately 10–250m 2 / g, hard carbon precursors with a particle size of approximately 4–15 μm are beneficial to the uniformity and sufficiency of subsequent sintering and doping / coating reactions.

[0045] More specifically, for example, a carbonized material is obtained by using an air jet mill and classifier at frequencies of 40Hz (air jet mill and classifier), and then sieved through a 325-mesh sieve to obtain a hard carbon precursor with a particle size of approximately 4-6 μm. This particle size is beneficial to the uniformity and sufficiency of subsequent reactions such as sintering and doping / coating, eliminating the need for secondary crushing and classification, requiring only sieving. The specific surface area of ​​the hard carbon precursor is generally less than 300 m². 2 / g, for example 10-250m 2 / g.

[0046] During the high-temperature carbonization process, biomass feedstock (hard carbon feedstock) releases volatile gases such as water vapor, carbon dioxide, and carbon monoxide, forming pores within the hard carbon material. This results in high porosity and a large specific surface area, leading to problems such as low compaction density and low initial coulombic efficiency (first-cycle coulombic efficiency). The pore structure of most hard carbon materials is naturally formed during the high-temperature carbonization process, making pore size control difficult. This often results in hard carbon having numerous open pores, which can lead to electrolyte consumption and sodium deposition. While creating pores can increase the reversible sodium (or lithium) storage capacity to some extent, the significant increase in specific surface area of ​​hard carbon greatly reduces the initial coulombic efficiency. Currently, there is still a lack of effective methods to control the pore structure of hard carbon. Therefore, the sodium (or lithium) storage capacity of hard carbon materials needs further improvement, and they also suffer from poor cycle performance and rate performance. Furthermore, in general, long-range ordered structures (such as graphite microcrystals) in sodium storage carbon materials have difficulty accommodating the insertion of sodium ions. Therefore, long-range ordered carbon structures reduce sodium storage active sites, leading to a loss of reversible capacity.

[0047] The abundance of closed-pore structures is key to improving the reversible storage performance of active ions (such as sodium ions or lithium ions) in hard carbon. In this application, the specific surface area of ​​the porous hard carbon precursor after pore-forming treatment is controlled at 300-900 m². 2 / g, by controlling the graphitization structure of hard carbon through pore engineering, the hard carbon precursor is subjected to pore-forming treatment to increase its porosity. The large number of pores hinders the formation of hard carbon graphite crystallites, inhibits the tendency of excessive graphitization of hard carbon materials, increases the interplanar spacing of carbon layer (002), and increases the active sites for sodium (or lithium) storage. This porous hard carbon precursor can increase its reversible capacity after high-temperature carbonization. By controlling the structure of graphite crystallites and amorphous carbon in hard carbon materials within a suitable range, it is helpful to obtain I D / I G Hard carbon materials with a strength of 0.9–1.3 can balance high reversible capacity with initial coulombic efficiency and cycle performance.

[0048] For example, the specific surface area of ​​the above-mentioned porous hard carbon precursor can be 300, 350, 400, 450, 500, 600, 700, 800, or 900 m². 2 / g or a range consisting of any two of them.

[0049] The aforementioned pore-forming treatments can include physical pore-forming methods or chemical pore-forming methods. Specific explanations are as follows.

[0050] In the physical pore-forming treatment method, the aforementioned pore-forming agent includes a physical pore-forming agent, which may include water vapor and / or carbon dioxide. Accordingly, the temperature of the pore-forming treatment (i.e., the pyrolysis process of the physical pore-forming agent) can be 650–950°C, for example, a range of 650, 700, 800, 900, 950°C or any combination thereof.

[0051] More specifically, the aforementioned physical pore-forming agent may also include nitrogen, i.e., a mixture of water vapor and / or carbon dioxide with nitrogen as a physical pore-forming agent.

[0052] Furthermore, in specific implementations, the aforementioned physical pore-forming agent, such as water vapor, carbon dioxide, a mixture of water vapor and / or carbon dioxide and nitrogen, can be introduced into the reaction system at a flow rate of 0.15–0.25 L / min, for example, 0.15, 0.2, 0.25 L / min, or any combination thereof. In the chemical pore-forming treatment method, the aforementioned pore-forming agent includes a chemical pore-forming agent, which may include one or more of potassium hydroxide, sodium hydroxide, zinc chloride (ZnCl2), and phosphoric acid (H3PO4). Accordingly, the temperature of the pore-forming treatment (i.e., the pyrolysis process of the chemical pore-forming agent) can be 500–900 °C, for example, 500, 600, 700, 800, 900 °C, or any combination thereof.

[0053] In practice, a solution containing a chemical pore-forming agent can be prepared first, such as a solution containing one or more of potassium hydroxide, sodium hydroxide, zinc chloride (ZnCl2), and phosphoric acid (H3PO4). The hard carbon precursor is then impregnated in this solution containing the chemical pore-forming agent, dried, and then pyrolyzed at 500–900°C to obtain a specific surface area of ​​300–900 m². 2 / g porous hard carbon precursor.

[0054] The dosage of the aforementioned pore-forming agent and the pore-forming time can be adjusted appropriately according to the actual situation and the expected specific surface area of ​​the porous hard carbon precursor.

[0055] Conventional coating processes typically involve coating and mixing hard carbon samples after high-temperature sintering, followed by a second high-temperature carbonization. This conventional coating process generally leads to increased defects and specific surface area, resulting in a decrease in initial coulombic efficiency. Furthermore, hard carbon prepared from biomass raw materials generally has a low compaction density, while hard carbon prepared using pore-forming techniques exhibits even lower compaction density due to the creation of numerous pores. However, in the preparation method of this application, coating the porous hard carbon precursor with a first coating agent and a second coating agent effectively reduces the specific surface area, increases the compaction density of the hard carbon material, reduces powder impedance, and improves initial coulombic efficiency and rate performance.

[0056] The process of mixing a porous hard carbon precursor and a first coating agent (e.g., using a mixing and modifying machine for VC mixing treatment) and then performing a first sintering treatment (high-temperature carbonization) in a second inert atmosphere to obtain a coated porous hard carbon precursor may include: mixing the porous hard carbon precursor and a first coating agent (e.g., VC mixing treatment), and then performing a first sintering treatment in a second inert atmosphere to obtain a coated porous hard carbon precursor.

[0057] Specifically, in the mixture obtained by mixing the porous hard carbon precursor and the first coating agent, the mass percentage (mixing ratio) of the first coating agent is 1wt% to 10wt%, for example, 1wt%, 3wt%, 5wt%, 7wt%, 9wt%, 10wt%, or any combination thereof.

[0058] In addition, the mixing time of the porous hard carbon precursor and the first coating agent can be 15 to 60 minutes, and the stirring speed can be 1000 to 2000 rpm.

[0059] The aforementioned first coating agent (or its carbon source) may include asphalt and / or resin, including but not limited to one or more of petroleum asphalt, coal tar pitch, β resin, phenolic resin, and epoxy resin.

[0060] In practice, the temperature of the first sintering treatment (high-temperature carbonization) can be 1100-1500℃, and the holding time can be 1-8h.

[0061] In addition, during the first sintering treatment (high-temperature carbonization), the heating rate can be 0.5 to 3 °C / min.

[0062] Traditional heteroatom doping processes generally lead to an increase in defects and specific surface area in carbon materials, resulting in a significant decrease in the first coulombic efficiency of hard carbon. At the same time, coating and doping processes are difficult to coordinate, making the process complex.

[0063] In this embodiment, the second coating agent includes a carbon-nitrogen source compound comprising an amino group (-NH2) and an alkyl group (-R). This compound performs a one-step modification of the hard carbon material by nitrogen doping and synergistic carbon coating. The resulting hard carbon material (hard carbon anode material) exhibits nitrogen doping and contains a soft carbon coating layer. The small amount of nitrogen doping enhances the defective active sites of the hard carbon, improves conductivity, thereby increasing reversible capacity, increasing ramp capacity, and improving rate performance. Simultaneously, the concurrently constructed soft carbon coating layer avoids the side effect of increased irreversible capacity caused by nitrogen doping, improves the hard carbon interface, reduces side reactions between the defective surface generated by doping and the electrolyte, and enhances the first coulombic efficiency and cycle performance of the hard carbon material (hard carbon anode material).

[0064] Specifically, the carbon-nitrogen source compound may include a carboxyl group, and the carbon-nitrogen source compound includes one or more of isoleucine, leucine, alanine, glycine, proline, valine, tyrosine, phenylalanine, tryptophan, serine, threonine, cysteine, asparagine, glutamine, asparagine, glutamine, lysine, and arginine.

[0065] Furthermore, the aforementioned carbon and nitrogen source compounds may also include carboxyl groups (-COOH), which facilitate cross-linking reactions with oxygen-containing functional groups on the surface of hard carbon materials to form cross-linked structures such as ester groups and acid anhydrides. This hinders the rearrangement of the microcrystalline structure of hard carbon graphite, helps increase sodium storage capacity, and helps improve reversible capacity, first coulombic efficiency, rate performance, and cycle performance.

[0066] Understandably, the temperature of the second sintering treatment should be determined in conjunction with the specific type of carbon and nitrogen source compound in order to achieve a better coating effect.

[0067] In some embodiments, the temperature of the second sintering process described above is 700 to 950°C, for example, it can be a range of 700, 800, 900, 950°C or any combination thereof.

[0068] In specific implementation, during the second sintering process described above, the heating rate can be 0.5 to 3℃ / min, for example, 0.5, 1, 2, 3℃ / min or any combination thereof, and the holding time can be 2 to 6h, for example, 2, 3, 4, 5, 6h or any combination thereof.

[0069] It is understood that the inert atmosphere (first inert atmosphere, second inert atmosphere, third inert atmosphere) in the embodiments of this application may include one or more of nitrogen, argon, and helium.

[0070] Understandably, in practical applications, the aforementioned hard carbon material can be pulverized and sieved to facilitate its use in preparing negative electrode sheets. During this sieving process, the mesh size of the sieve can be 300–500 mesh.

[0071] The aforementioned hard carbon material can be used to prepare lithium-ion battery anode sheets or sodium-ion battery anode sheets, preferably sodium-ion battery anode sheets, and then used to prepare lithium-ion batteries or sodium-ion batteries, preferably sodium-ion batteries.

[0072] This application also provides a negative electrode sheet, which includes a negative electrode active material layer. The negative electrode active material layer includes the aforementioned hard carbon material or a hard carbon material prepared according to the aforementioned method for preparing hard carbon material. Based on this hard carbon material, the negative electrode sheet has corresponding effects, which will not be elaborated here.

[0073] This application also provides a battery, which includes the aforementioned negative electrode. Based on the negative electrode or hard carbon material, the battery has corresponding effects, which will not be elaborated here.

[0074] In some embodiments, the battery described above includes a sodium-ion battery.

[0075] Understandably, the aforementioned battery also includes a positive electrode, a separator, and an electrolyte.

[0076] The aforementioned positive electrode sheet specifically includes a positive current collector and a positive active layer disposed on at least one functional surface of the positive current collector, the positive active layer comprising a positive active material.

[0077] In the specific preparation of the positive electrode sheet, for example, the positive electrode active material, conductive agent and binder can be dispersed in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and thoroughly stirred and mixed to form a uniform positive electrode slurry; the positive electrode slurry is uniformly coated on the positive electrode current collector, and after drying, rolling and cutting, the positive electrode sheet is obtained.

[0078] The positive electrode current collector can be made of at least one of aluminum foil or nickel foil; the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, carbon fiber, carbon nanotubes, and conductive graphite; the binder can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0079] The aforementioned positive electrode active material may include one or more of layered oxides, polyanionic compounds, Prussian blue compounds, sodium peroxides, and sodium superoxides.

[0080] The embodiments of this application do not strictly limit the selection of electrolyte, and may include one or more of the solvents commonly used in sodium-ion battery electrolytes, as well as the electrolyte sodium salts commonly used in sodium-ion electrolytes. For example, the solvent may be ethylene carbonate, propylene carbonate, butene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, γ-butyrolactone, etc.; the electrolyte may be one or more of sodium hexafluorophosphate, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide, and sodium fluorotrifluoromethanesulfonylimide.

[0081] The embodiments of this application do not strictly limit the choice of membrane material. It can be one of the membrane materials commonly used in sodium-ion batteries, such as polypropylene membrane (PP), polyethylene membrane (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun membrane (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven membrane, and membrane with ceramic coating.

[0082] In the preparation of sodium-ion batteries, the positive electrode, separator, and negative electrode are wound or stacked to obtain a bare cell, which is then packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery is dried at 85°C, the electrolyte is injected into the dried battery. The battery undergoes resting, formation, and secondary sealing to complete the preparation of the sodium-ion battery.

[0083] The present application will be further described below through specific embodiments and comparative examples. Unless otherwise specified, the reagents, materials and instruments used below are all conventional reagents, materials and instruments, all of which are commercially available, and the reagents and materials involved can also be synthesized by conventional synthetic methods.

[0084] Example 1

[0085] This embodiment provides a method for preparing a hard carbon material (hard carbon anode material) for sodium-ion batteries, including:

[0086] 1) Coconut shells were carbonized in nitrogen at a temperature of 300℃, a heating rate of 2℃ / min, and a carbonization time of 4 hours. The carbonized shells were then pulverized and sieved using a 325-mesh sieve to obtain a hard carbon precursor with a particle size of 6 μm. The specific surface area of ​​this hard carbon precursor was measured to be approximately 250 m². 2 / g;

[0087] 2) Carbon dioxide gas was introduced into the hard carbon precursor to perform pore-forming treatment. The pore-forming treatment temperature was 800℃, the time was 1 hour, the heating rate was 3℃ / min, and the flow rate of carbon dioxide gas was 0.2 L / min, resulting in a specific surface area of ​​650 m². 2 / g porous hard carbon precursor;

[0088] 3) At a stirring speed of 1500 rpm, a specific surface area of ​​650 m² was added. 2 / g of porous hard carbon precursor and asphalt are mixed. The mass percentage of asphalt in the mixture obtained after mixing the porous hard carbon precursor and asphalt is 5wt%, and the mixing time is 30min. Then, a first sintering treatment is carried out in nitrogen to obtain the coated porous hard carbon precursor. The temperature of the first sintering treatment is 1300℃, the holding time is 4h, and the heating rate can be 3℃ / min.

[0089] 4) After mixing the coated porous hard carbon precursor and glutamic acid, a second sintering treatment is carried out in nitrogen. The temperature of the second sintering treatment is 900℃, the heating rate is 2℃ / min, and the holding time is 4h. Then, the material is crushed and sieved. The mesh size of the sieve is 325 mesh to obtain the hard carbon material (hard carbon anode material) with a carbon coating layer (coating layer) thickness of 10nm.

[0090] Example 2

[0091] This embodiment is basically the same as Embodiment 1, except that:

[0092] 1) Replace coconut shells with walnut shells, and adjust the carbonization temperature to 400℃;

[0093] 2) Replace asphalt with β resin;

[0094] 3) Replace glutamic acid with glycine, with a nitrogen doping ratio (at%) of 2.0 at%.

[0095] 4) Adjust the amount of coating agent to make the thickness of the carbon coating layer (coating layer) 5nm;

[0096] All other conditions remain unchanged.

[0097] Example 3

[0098] This embodiment is basically the same as Embodiment 1, except that:

[0099] 1) The carbonization temperature was adjusted to 800℃;

[0100] 2) Step 2) is adjusted to mix the hard carbon precursor with potassium hydroxide at a mass ratio of 1:1, and then perform pore-forming treatment (pyrolysis) at 800℃ with a heating rate of 3℃ / min and a reaction time of 2h to obtain a porous hard carbon precursor.

[0101] 3) Adjust the amount of coating agent to make the thickness of the carbon coating layer (coating layer) 100nm;

[0102] All other conditions remain unchanged.

[0103] Example 4

[0104] This embodiment is basically the same as Embodiment 1, except that:

[0105] In step 1), the coconut shell was replaced with glucose, the carbonization time was adjusted to 10 hours, the heating rate was adjusted to 0.5℃ / min, and the mesh size of the sieve used for sieving was 300 mesh; other conditions remained unchanged.

[0106] Example 5

[0107] This embodiment is basically the same as Embodiment 1, except that:

[0108] In step 1), the coconut shell was replaced with starch, the carbonization time was adjusted to 1 hour, the heating rate was adjusted to 3℃ / min, and the mesh size of the sieve used for sieving was 500 mesh; other conditions remained unchanged.

[0109] Example 6

[0110] This embodiment is basically the same as Embodiment 1, except that:

[0111] In step 2), water vapor is used to replace carbon dioxide, and the temperature for the pore-forming treatment is adjusted to 650℃. After the pore-forming treatment, a specific surface area of ​​300m² is obtained. 2 / g porous hard carbon precursor; other conditions remain unchanged.

[0112] Example 7

[0113] This embodiment is basically the same as Embodiment 1, except that:

[0114] In step 2), carbon dioxide is replaced with a mixture of carbon dioxide and nitrogen (volume ratio 1:3), and the pore-forming temperature is adjusted to 950℃. After the pore-forming process, a specific surface area of ​​900 m² is obtained. 2 / g porous hard carbon precursor; other conditions remain unchanged.

[0115] Example 8

[0116] This embodiment is basically the same as embodiment 3, except that:

[0117] In step 2), zinc chloride is used instead of potassium hydroxide, and the temperature for the hole-forming treatment is 500℃; other conditions remain unchanged.

[0118] Example 9

[0119] This embodiment is basically the same as embodiment 3, except that:

[0120] In step 2), potassium hydroxide is replaced with phosphoric acid, and the temperature for pore formation is 900℃; other conditions remain unchanged.

[0121] Example 10

[0122] This embodiment is basically the same as Embodiment 1, except that:

[0123] In step 3), the temperature of the first sintering treatment is 1100℃, the holding time is 8h, and the heating rate is 0.5℃ / min;

[0124] In step 4), glutamic acid is replaced with phenylalanine. The temperature of the second sintering treatment is 950℃, the heating rate is 0.5℃ / min, and the holding time is 6h.

[0125] Example 11

[0126] This embodiment is basically the same as Embodiment 1, except that:

[0127] In step 3), the temperature of the first sintering treatment is 1500℃, the holding time is 1h, and the heating rate is 3℃ / min;

[0128] In step 4), proline is used to replace glutamic acid, the temperature of the second sintering treatment is 700℃, the heating rate is 3℃ / min, and the holding time is 2h.

[0129] Comparative Example 1

[0130] This comparative example is basically the same as Example 1, except that:

[0131] No pore-forming or coating treatment is performed; all other conditions remain unchanged.

[0132] This comparative example provides a method for preparing a hard carbon material (hard carbon anode material) for sodium-ion batteries, including:

[0133] 1) Coconut shells were carbonized in nitrogen at a temperature of 300℃, a heating rate of 2℃ / min, and a carbonization time of 4 hours. The carbonized shells were then pulverized and sieved using a 325-mesh sieve to obtain a hard carbon precursor with a particle size of 6 μm. The specific surface area of ​​this hard carbon precursor was measured to be approximately 250 m². 2 / g;

[0134] 2) Then, the first sintering treatment is carried out in nitrogen, followed by crushing and sieving. The sieve mesh size is 325 mesh to obtain hard carbon material (hard carbon anode material). The temperature of the first sintering treatment is 1300℃, the holding time is 4h, and the heating rate can be 3℃ / min.

[0135] Comparative Example 2

[0136] This comparative example is basically the same as Example 1, except that:

[0137] No coating treatment is performed; all other conditions remain unchanged.

[0138] This comparative example provides a method for preparing a hard carbon material (hard carbon anode material) for sodium-ion batteries, including:

[0139] 1) Coconut shells were carbonized in nitrogen at a temperature of 300℃, a heating rate of 2℃ / min, and a carbonization time of 4 hours. The carbonized shells were then pulverized and sieved using a 325-mesh sieve to obtain a hard carbon precursor with a particle size of 6 μm. The specific surface area of ​​this hard carbon precursor was measured to be approximately 250 m². 2 / g;

[0140] 2) Carbon dioxide gas was introduced into the hard carbon precursor to perform pore-forming treatment. The pore-forming treatment temperature was 800℃, the time was 1 hour, the heating rate was 3℃ / min, and the flow rate of carbon dioxide gas was 0.2 L / min, resulting in a specific surface area of ​​650 m². 2 / g porous hard carbon precursor;

[0141] 3) Then, the first sintering treatment is carried out in nitrogen, followed by crushing and sieving. The sieve mesh size is 325 mesh to obtain hard carbon material (hard carbon anode material). The temperature of the first sintering treatment is 1300℃, the holding time is 4h, and the heating rate can be 3℃ / min.

[0142] Comparative Example 3

[0143] This comparative example is basically the same as Example 1, except that:

[0144] No hole-making process is performed; other conditions remain unchanged.

[0145] This comparative example provides a method for preparing a hard carbon material (hard carbon anode material) for sodium-ion batteries, including:

[0146] 1) Coconut shells were carbonized in nitrogen at a temperature of 300℃, a heating rate of 2℃ / min, and a carbonization time of 4 hours. The carbonized shells were then pulverized and sieved using a 325-mesh sieve to obtain a hard carbon precursor with a particle size of 6 μm. The specific surface area of ​​this hard carbon precursor was measured to be approximately 250 m². 2 / g;

[0147] 2) The hard carbon precursor and asphalt are mixed at a stirring speed of 1500 rpm. The mass percentage of asphalt in the mixture is 5 wt% and the mixing time is 30 min. Then, a first sintering treatment is carried out in nitrogen to obtain a coated porous hard carbon precursor. The temperature of the first sintering treatment is 1300℃, the holding time is 4 h, and the heating rate can be 3℃ / min.

[0148] 3) After mixing the coated porous hard carbon precursor and glutamic acid, a second sintering treatment is carried out in nitrogen. The temperature of the second sintering treatment is 900℃, the heating rate is 2℃ / min, the holding time is 4h, and the nitrogen doping ratio (at%) is 0.5%. Then, the material is crushed and sieved. The sieve mesh size is 325 mesh to obtain the hard carbon material (hard carbon anode material).

[0149] Comparative Example 4

[0150] This comparative example is basically the same as Example 1, except that:

[0151] No nitrogen-carbon synergistic coating treatment was performed; other conditions remained unchanged.

[0152] This comparative example provides a method for preparing a hard carbon material (hard carbon anode material) for sodium-ion batteries, including:

[0153] 1) Coconut shells were carbonized in nitrogen at a temperature of 300℃, a heating rate of 2℃ / min, and a carbonization time of 4 hours. The carbonized shells were then pulverized and sieved using a 325-mesh sieve to obtain a hard carbon precursor with a particle size of 6 μm. The specific surface area of ​​this hard carbon precursor was measured to be approximately 250 m². 2 / g;

[0154] 2) Carbon dioxide gas was introduced into the hard carbon precursor to perform pore-forming treatment. The pore-forming treatment temperature was 800℃, the time was 1 hour, the heating rate was 3℃ / min, and the flow rate of carbon dioxide gas was 0.2 L / min, resulting in a specific surface area of ​​650 m². 2 / g porous hard carbon precursor;

[0155] 3) At a stirring speed of 1500 rpm, a specific surface area of ​​650 m² was added. 2 / g of porous hard carbon precursor and asphalt are mixed. The mass percentage of asphalt in the mixture obtained after mixing the porous hard carbon precursor and asphalt is 5wt%, and the mixing time is 30min. Then, a first sintering treatment is carried out in nitrogen to obtain the coated porous hard carbon precursor. The temperature of the first sintering treatment is 1300℃, the holding time is 4h, and the heating rate can be 3℃ / min.

[0156] 4) Then crush and sieve the material. The sieve mesh size is 325 mesh to obtain hard carbon material (hard carbon anode material).

[0157] Comparative Example 5

[0158] This comparative example is basically the same as Example 1, except that:

[0159] During the pore-forming process in step 2), parameters such as the pore-forming time and the amount of pore-forming agent are adjusted to obtain a specific surface area of ​​250 m². 2 / g porous hard carbon precursor; other conditions remain unchanged.

[0160] Comparative Example 6

[0161] This comparative example is basically the same as Example 1, except that:

[0162] During the pore-forming process in step 2), parameters such as the pore-forming time and the amount of pore-forming agent are adjusted to obtain a specific surface area of ​​950 m².2 / g porous hard carbon precursor; other conditions remain unchanged.

[0163] Comparative Example 7

[0164] This comparative example is basically the same as Example 1, except that:

[0165] In step 4), the temperature of the second sintering treatment is adjusted to 650℃; other conditions remain unchanged.

[0166] Comparative Example 8

[0167] This comparative example is basically the same as Example 1, except that:

[0168] In step 4), the temperature of the second sintering treatment is adjusted to 1000℃; other conditions remain unchanged.

[0169] Test case

[0170] 1. The following parameters of the hard carbon materials in each embodiment and comparative example were tested:

[0171] 1) Scanning electron microscopy (SEM) test: SEM images were obtained using a Hitachi Regulus 8100 / SU 8010 scanning electron microscope;

[0172] 2) X-ray diffraction (XRD): XRD was obtained using an X-ray powder diffractometer, and the interplanar spacing d of (002) was calculated. 002 The average width La of the graphite crystallites along the a-axis, the thickness Lc of the stacked layers along the c-axis, and the number of stacked layers N, where d 002 =λ / (2sinθ); La=1.84λ / (βcosθ); Lc=0.9λ / (βcosθ); N=Lc / d 002 +1, where λ is the X-ray wavelength, β is the full width at half maximum (FWHM) of the (002) crystal plane, and θ is the diffraction angle of the (002) crystal plane;

[0173] 3) Raman testing: Raman spectra were obtained using a laser confocal Raman spectrometer. I was calculated based on the intensity ratio of the D peak and the G peak in the Raman spectrum. D / I G The value;

[0174] 4) X-ray photoelectron spectroscopy (XPS): The nitrogen doping ratio is obtained by measuring with an X-ray photoelectron spectrometer;

[0175] 5) The thickness of the carbon coating layer was measured by transmission electron microscopy (TEM), and the thickness of the carbon coating layer was obtained by measuring the high-resolution TEM image.

[0176] 6) Cross-sectional porosity: Cross-sectional porosity is obtained by taking SEM images of the material cross-section and then analyzing them using OPTO software.

[0177] 7) Specific surface area (BET): BET was obtained by using Bestech 3H-2000BET-A according to the national standard GB / T 19587-2017;

[0178] 8) Impedance (powder impedance): The impedance was obtained by using the four-probe method according to the national standard GB / T 30835-2014.

[0179] 2. The above-mentioned hard carbon materials were used to fabricate negative electrode sheets and assembled into CR2032 coin cells. The method for assembling the CR2032 cell is as follows: Hard carbon material (hard carbon negative electrode material) was placed in a slurry mixer with conductive carbon black and sodium carboxymethyl cellulose at a mass ratio of 80:10:10. Then, N-methylpyrrolidone solvent was added and mixed evenly to obtain a slurry. The slurry was evenly coated on the surface of carbon-coated aluminum foil and dried in a vacuum drying oven at 100°C for 12 hours. At the same time, sodium metal was used as the counter electrode, glass fiber was used as the separator, and a 1 mol / L sodium hexafluorophosphate (NaPF6) solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 1:1) was used as the electrolyte to assemble the CR2032 cell.

[0180] The above-mentioned button cells were subjected to constant current charge-discharge tests at 25°C on the Xinwei multi-channel test system. The voltage ranged from 0 to 2.0V, the rate was 0.1C, and 1C = 300mA / g.

[0181] Reversible capacity: The discharge capacity obtained by constant current discharge to 0V at a rate of 0.1C is the initial discharge capacity. Then, the charging capacity obtained by constant current charging to 2V at a rate of 0.1C is the initial charging capacity. The initial charging capacity is the reversible capacity.

[0182] First-charge efficiency: the percentage obtained by dividing the first-charge capacity by the first-discharge capacity;

[0183] 1C 50-cycle capacity retention rate: The percentage obtained by dividing the charging capacity of the 50th cycle at 1C rate by the charging capacity of the 1st cycle;

[0184] 2C capacity retention rate: The percentage obtained by dividing the charging capacity obtained at a 2C rate by the reversible capacity.

[0185] Test results:

[0186] Table 1

[0187] Data Analysis: As can be seen from the table above, the pore-forming treatment (pore size control) and the synergistic nitrogen-carbon coating in each embodiment help to obtain a wavelength of 1345 cm⁻¹ in the Raman spectrum. -1 Peak intensity I at D and wavelength of 1600cm -1 Peak intensity I at G The ratio (I) D / I G Hard carbon materials with a resistivity of 0.9 to 1.3 and low powder impedance.

[0188] Table 2

[0189] Data Analysis: As can be seen from Tables 1 and 2, compared with the comparative example, the hard carbon in the embodiments of this application contains a composite carbon structure of amorphous carbon and graphite-like microcrystals and has a suitable Raman spectral peak intensity ratio I. D / I G The high specific capacity (0.9–1.3) of the precursor results in a large number of sodium-storing active sites, giving the hard carbon material excellent reversible specific capacity and high coulombic efficiency. Furthermore, by controlling the pore size of the precursor and then applying nitrogen doping to synergistically coat the carbon layer, a hard carbon material with a suitable Raman peak intensity ratio (0.9–1.3), a large interlayer spacing (0.37–0.40 nm), and low powder impedance (0.02–0.07 Ω·cm) was obtained in the examples. This contributes to improving the reversible capacity, initial coulombic efficiency, rate performance, and cycling performance of the hard carbon material.

[0190] Figure 1 and Figure 2 are scanning electron microscope images of the hard carbon materials prepared in Example 1 and Comparative Example 1, respectively. By comparison, it can be seen that both have a blocky morphology, while the particle size in Figure 1 is slightly larger and the surface has a wrinkled coating layer, which proves that the hard carbon material prepared in Example 1 has a surface coating modification effect compared with the hard carbon material prepared in Comparative Example 1.

[0191] From the X-ray diffraction pattern of the hard carbon material prepared in Example 1 in Figure 3, it can be seen that it exhibits two characteristic bulge peaks typical of hard carbon materials, corresponding to the (002) and (100) crystal planes, respectively. The interplanar spacing d of the (002) crystal plane was calculated. 002 The average width La of the graphite crystals along the a-axis and the thickness Lc of the stacked graphite crystals along the c-axis are 1.48 nm and 0.72 nm, respectively, and the number of stacked layers N is 2.9.

[0192] Based on the Raman spectrum of the hard carbon material prepared in Example 1 in Figure 4, its wavelength at 1345 cm⁻¹ can be calculated. -1 Peak intensity I at point D D and wavelength of 1600cm -1 peak intensity I at G G The ratio (I)D / I G The value is 1.1;

[0193] As can be seen from the charge-discharge curve of the hard carbon material prepared in Example 1 in Figure 5, the material exhibits excellent sodium storage performance, with a reversible specific capacity of 337 mAh / g and an initial coulombic efficiency of 92%.

[0194] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A hard carbon material, characterized in that, The Raman spectrum of the hard carbon material has a wavelength of 1345 cm⁻¹. -1 The peak intensity and wavelength at 1600 cm⁻¹ -1 The ratio of peak intensity at the point is 0.9 to 1.3, and the powder impedance of the hard carbon material under a pressure of 16 to 20 MPa is 0.02 to 0.07 Ω·cm.

2. The hard carbon material according to claim 1, characterized in that, The interlayer spacing d of the hard carbon material (002) crystal plane 002 The wavelength ranges from 0.37 to 0.40 nm. And / or, the hard carbon material includes graphite microcrystals and amorphous carbon, wherein the width La of the graphite microcrystals along the a-axis is 1.4 to 2.0 nm, the thickness Lc along the c-axis is 0.7 to 1.0 nm, and the number of stacked layers is 2.9 to 3.

5.

3. The hard carbon material according to claim 1 or 2, characterized in that, The hard carbon material includes a core, a first coating layer on the surface of the core, and a second coating layer on the surface of the first coating layer. The first coating layer includes carbon, and the second coating layer includes nitrogen and carbon.

4. The hard carbon material according to claim 3, characterized in that, The nitrogen element comprises 0.1% to 2.0% of the total number of atoms in the hard carbon material. And / or, the sum of the thicknesses of the first coating layer and the second coating layer is 5 to 100 nm.

5. The hard carbon material according to claim 1 or 2, characterized in that, The porosity of the hard carbon material is 1% to 10%; And / or, the specific surface area of ​​the hard carbon material is 0.5 to 20 m². 2 / g.

6. A method for preparing the hard carbon material according to any one of claims 1-5, characterized in that, include: The biomass raw material is carbonized in a first inert atmosphere to obtain a hard carbon precursor; The hard carbon precursor was subjected to pore-forming treatment with a pore-forming agent to obtain a specific surface area of ​​300–900 m². 2 / g porous hard carbon precursor; After mixing the porous hard carbon precursor and the first coating agent, a first sintering treatment is performed in a second inert atmosphere to obtain a coated porous hard carbon precursor. The first coating agent includes a carbon source. The coated porous hard carbon precursor and the second coating agent are mixed and then subjected to a second sintering treatment in a third inert atmosphere to obtain the hard carbon material. The second coating agent includes a carbon-nitrogen source compound, which includes amine and alkyl groups.

7. The method for preparing hard carbon material according to claim 6, characterized in that, The biomass raw materials include one or more of glucose, sucrose, starch, coconut shell, walnut shell, hazelnut shell, and wood; And / or, in the process of carbonizing biomass raw materials to obtain hard carbon precursor, the carbonization temperature is 300-800℃, the carbonization time is 1-10h, and the heating rate is 0.5-3℃ / min. And / or, the process of carbonizing biomass raw materials to obtain hard carbon precursor further includes: after carbonizing the biomass raw materials, crushing and sieving them to obtain the hard carbon precursor with a mesh size of 300 to 500 mesh; And / or, the pore-forming agent includes a physical pore-forming agent, which includes water vapor and / or carbon dioxide, and the temperature of the pore-forming treatment is 650–950°C; And / or, the pore-forming agent includes a chemical pore-forming agent, which includes one or more of potassium hydroxide, sodium hydroxide, zinc chloride, and phosphoric acid, and the temperature of the pore-forming treatment is 500–900°C.

8. The method for preparing the hard carbon material according to claim 6 or 7, characterized in that, The carbon source includes pitch and / or resin; And / or, the temperature of the first sintering treatment is 1100-1500℃, the holding time is 1-8h, and the heating rate is 0.5-3℃ / min; And / or, the temperature of the second sintering treatment is 700–950°C; And / or, the carbon-nitrogen source compound further includes a carboxyl group, and the carbon-nitrogen source compound includes one or more of isoleucine, leucine, alanine, glycine, proline, valine, tyrosine, phenylalanine, tryptophan, serine, threonine, cysteine, asparagine, glutamine, asparagine, glutamine, lysine, and arginine; And / or, during the second sintering process, the heating rate is 0.5 to 3 °C / min, and the holding time is 2 to 6 h.

9. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode active material layer, which includes the hard carbon material according to any one of claims 1-5 or the hard carbon material prepared according to the preparation method of the hard carbon material according to any one of claims 6-8.

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

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