Nitrogen-doped porous hard carbon material, preparation method and application thereof

Abstract

The application discloses a nitrogen-doped porous hard carbon material and a preparation method and application thereof, and relates to the technical field of carbon materials. The nitrogen-doped porous hard carbon material is prepared by the following steps: pretreating a biomass raw material to obtain biomass powder; activating the biomass powder to obtain activated biomass; adding a nitrogen source and a pore-forming agent into the activated biomass to perform medium-temperature pre-doping to obtain nitrogen-doped biomass; and performing high-temperature sintering on the nitrogen-doped biomass to obtain the nitrogen-doped porous hard carbon material. The raw material is cheap and easy to obtain, is environment-friendly, low-toxic or non-toxic, and has a simple preparation process, so that the preparation cost can be reduced, industrial production is easy, and the feasibility of commercial application is improved.

Description

Technical Field

[0001] This application relates to the field of sodium-ion battery technology, specifically to a nitrogen-doped porous hard carbon material and its preparation method and application. Background Technology

[0002] In recent years, with the clarification of national goals for carbon neutrality and carbon peaking, and the continued consumption of non-renewable energy, more and more companies have begun to explore sustainable energy development. Against this backdrop, secondary batteries, represented by lithium-ion batteries, have gradually taken over the new energy and energy storage markets.

[0003] However, due to the low abundance and limited storage of lithium resources on Earth, the price of lithium-ion batteries remains high and fluctuates significantly. Therefore, attention has shifted to resources with similar energy levels to lithium. Sodium-ion batteries operate on a similar principle to lithium-ion batteries and offer excellent rate performance, high and low temperature performance, and reliable safety, thus attracting unprecedented enthusiasm from researchers. Research focuses on improving the coulombic efficiency, energy density, and lifespan of sodium-ion batteries to enhance their overall electrochemical performance and facilitate their practical application. However, due to thermodynamic reasons and atomic size limitations, graphite-based materials cannot exhibit good performance in sodium-ion batteries, making the development of high-energy-density anode materials an urgent priority.

[0004] Because the atomic radius of sodium ions is much larger than that of lithium ions, and the pore size of graphite layers is relatively small, sodium ions cannot freely intercalate and deintercalate. Therefore, graphite anodes, currently suitable for lithium-ion batteries, are not suitable as anode materials for sodium-ion batteries. Researchers have turned their attention to biomass materials. The carbon skeleton structure in biomass can provide stable conductive channels, offering good electronic conductivity for the sodium ion intercalation / deintercalation process. Simultaneously, the oxygen-containing functional groups and other surface functional groups present in biomass also help improve the diffusion rate and charge transfer efficiency of sodium ions. Therefore, appropriate design and protection of the hard carbon anode structure can mitigate these problems. Summary of the Invention

[0005] To address the aforementioned deficiencies in this field, this application aims to provide a nitrogen-doped porous hard carbon material, its preparation method, and its application.

[0006] According to one aspect of this application, a method for preparing a nitrogen-doped porous hard carbon material is provided, comprising:

[0007] Biomass raw materials are pretreated to obtain biomass powder;

[0008] The biomass powder is activated to obtain activated biomass;

[0009] A nitrogen source and a pore-forming agent are added to the activated biomass, and medium-temperature pre-doping is performed to obtain nitrogen-doped biomass;

[0010] The nitrogen-doped biomass is sintered at high temperature to obtain the nitrogen-doped porous hard carbon material.

[0011] According to some embodiments of this application, the activation treatment includes: activation at 300-600°C under a carbon dioxide atmosphere for 1-4 hours.

[0012] 4. According to some embodiments of this application, the nitrogen source is selected from one or more of ethylenediamine, dopamine, aniline, melamine, and urea, preferably urea;

[0013] According to some embodiments of this application, the predoping temperature is 500-800℃ and the predoping time is 1-4h.

[0014] According to some embodiments of this application, the mass ratio of the activated biomass, nitrogen source, and pore-forming agent is 1:(1-3):(1-3).

[0015] According to some embodiments of this application, the pore-forming agent is a water-soluble salt;

[0016] According to some embodiments of this application, the water-soluble salts include: NaCl and KCl.

[0017] According to some embodiments of this application, the sintering temperature is 1000-1300℃, the sintering time is 1-4h, and the sintering atmosphere is one or more of nitrogen, argon, helium, and xenon.

[0018] According to some embodiments of this application, the biomass raw material is selected from one or more of the following: coconut shell, rice, orange peel, sugarcane, rapeseed, cotton, barley, wheat, corn, reed, and sisal; preferably, it is coconut shell.

[0019] According to some embodiments of this application, the pretreatment includes: washing, crushing, and acid washing to purify the biomass raw material;

[0020] The pickling agent used in the pickling process is selected from hydrochloric acid and hydrofluoric acid.

[0021] According to some embodiments of this application, the process before sintering further includes: washing the nitrogen-doped biomass with water to remove the pore-forming agent.

[0022] According to another aspect of this application, a nitrogen-doped porous hard carbon material is also provided, with a pore size ≤2nm and a specific surface area of ​​10-16m². 2 / g; nitrogen doping content is 3.0-8.5wt%; the ratio of amorphous carbon peaks to graphitized carbon peaks (I D / I GThe value is 1.20-1.30.

[0023] According to one aspect of this application, a negative electrode is provided, comprising a nitrogen-doped porous hard carbon material prepared by the above preparation method, and / or a nitrogen-doped porous hard carbon material.

[0024] According to one aspect of this application, a sodium-ion battery is provided, comprising the aforementioned negative electrode.

[0025] According to another aspect of this application, an electrochemical device is provided, including the sodium-ion battery described above.

[0026] Compared with the prior art, this application has at least the following beneficial effects:

[0027] This application provides a method for preparing a nitrogen-doped porous hard carbon material, which uses biomass such as coconut shells as raw materials, water-soluble salts as templates, and nitrogen sources such as urea as nitrogen dopant to prepare a porous hard carbon material with high microporosity, large specific surface area, and high nitrogen doping rate.

[0028] The nitrogen-doped porous hard carbon material of this application exhibits an increased specific surface area of ​​the hard carbon anode due to its porous structure, thereby improving its electrochemical reaction rate and energy storage density. This results in higher performance and a higher charge-discharge rate in energy storage devices. The porous structure also enhances the mechanical strength of the hard carbon anode, thus improving its fatigue resistance and exhibiting better stability during long-term cycling.

[0029] The method for preparing nitrogen-doped porous hard carbon materials in this application uses inexpensive and readily available raw materials that are environmentally friendly, low-toxic or non-toxic, and has a simple preparation process that can reduce preparation costs, facilitate industrial production, and improve the feasibility of its commercial application. Attached Figure Description

[0030] Figure 1 This is a SEM image of the nitrogen-doped porous hard carbon material of Example 1 of this application.

[0031] Figure 2 SEM image of the hard carbon material used in Example 1 of the application.

[0032] Figure 3 The image shows the Raman diagrams of the hard carbon materials prepared in Example 1 and Comparative Example 1.

[0033] Figure 4 The images show the XRD patterns of the hard carbon materials prepared in Example 1 and Comparative Example 1.

[0034] Figure 5 The image shows the pore size distribution of the hard carbon materials prepared in Example 1 and Comparative Example 1. Detailed Implementation

[0035] The technical solution of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0036] It should be particularly noted that similar substitutions and modifications made to this application are obvious to those skilled in the art, and they are all considered to be included in this application. Those skilled in the art can obviously make modifications or appropriate alterations and combinations to the methods and applications described herein without departing from the content, spirit, and scope of this application to implement and apply the technology of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0037] Unless otherwise specified, this application is conducted under standard conditions or conditions recommended by the manufacturer. The raw materials or excipients used, as well as the reagents or instruments used, whose manufacturers are not specified, are all conventional products that can be obtained commercially.

[0038] The following is a detailed description of this application.

[0039] Due to the extremely low content and uneven distribution of lithium resources in the Earth's crust, sodium-ion batteries have received increasing attention due to the abundance of natural sodium resources and are considered an effective supplement to lithium-ion batteries. Hard carbon (HC) materials possess advantages such as low redox potential, abundant sodium storage sites, high specific capacity determined by the sodium storage method, high electronic and ionic conductivity, low cost, wide availability, and no environmental pollution, making them considered the most ideal anode material for sodium-ion batteries (SIBs) at present.

[0040] Existing research indicates that nitrogen doping can increase the lattice spacing of hard carbon materials, forming a localized graphite-like structure, which is beneficial for the storage and transport of larger Na ions in batteries. In other words, doping carbon-based materials with heteroatoms (such as N, B, O, S, and P) results in high electron mobility, good sodium mobility, and higher capacity, showing great potential for sodium storage.

[0041] Current research uses melamine as a nitrogen dopant source. However, due to its high volatility, melamine easily clogs experimental equipment during reheating, and most of it cannot be completely incorporated into the bulk, leading to waste of raw materials. Existing technologies also use aqueous solutions containing nitrogen sources for nitrogen doping, followed by freeze-drying to recover the nitrogen, resulting in a porous structure in the nitrogen-containing hard carbon anode. However, freeze-drying is not suitable for large-scale industrial production. Alternatively, aniline can be used as the amine source, and formaldehyde, generated from the high-temperature hydrolysis of hexamethylenetetramine, can be used as the aldehyde source to provide alkaline conditions for the reaction. Under the action of the surfactant polyoxyethylene-polyoxypropylene ether block copolymer, the system undergoes an aldehyde-amine condensation reaction, forming micron-sized resin spheres. This method eliminates the need for additional heteroatom introduction steps, directly utilizing nitrogen atoms provided by the reactants to complete the heteroatom doping. While this method is innovative, it suffers from a long experimental process and the use of potentially hazardous raw materials, making it unsuitable for large-scale industrial production.

[0042] Based on this, this application provides a method for preparing nitrogen-doped porous hard carbon material, comprising the following steps:

[0043] (1) Pre-treat the biomass raw materials;

[0044] (2) Activate the pretreated biomass powder;

[0045] (3) Add a pore-forming agent and a nitrogen source to the activated biomass powder, and simultaneously perform pore-forming and medium-temperature pre-doping at a temperature of 500-800℃ and a time of 1-4h to obtain nitrogen-doped biomass.

[0046] (4) The nitrogen-doped biomass is washed with water to remove the pore-forming agent, dried, and then sintered at high temperature under a protective gas atmosphere to obtain the nitrogen-doped porous hard carbon material of this application.

[0047] In step (1): the pretreatment includes: washing and drying the biomass raw material, crushing (preliminary crushing and final crushing) into powder with a particle size of about 400 mesh, and purifying the powder by acid washing to remove ash and impurities; the acid washing agent is selected from hydrochloric acid and HF acid.

[0048] The biomass is selected from one or more of the following: rice, orange peel, sugarcane, rapeseed, cotton, barley, wheat, corn stalks, corn cobs, reeds, sisal, bamboo, peanuts, seaweed, loofah, pumpkin, jujube wood, oak, peach wood, and coconut shell. Among these, coconut shell is characterized by its wide availability and low price. Due to its high density, low ash content, and low H / C and O / C molar ratio, coconut shell hard carbon exhibits high energy density and excellent electrochemical performance in sodium-ion batteries (SIBs).

[0049] This application selects biomass as the raw material because the carbon skeleton structure in biomass can provide a stable conductive channel, providing good electronic conduction performance for the insertion / extraction process of sodium ions.

[0050] In step (2): the activation treatment includes activation at 300-600℃ in a CO2 atmosphere for 1-4 hours. This application uses a carbon dioxide activation step to modulate the material's pore structure, specific surface area, and surface active sites, thereby improving the material's adsorption performance and facilitating subsequent nitrogen doping.

[0051] In step (3): the mass ratio of the activated biomass, nitrogen source, and pore-forming agent is 1:(1-3):(1-3). The nitrogen source includes one or more organic nitrogen sources selected from ethylenediamine, dopamine, aniline, melamine, and urea. Melamine has a high nitrogen content, which is irreplaceable by other materials. However, it has strong volatility, therefore urea is preferred in this application. The pore-forming agent is a water-soluble salt; the water-soluble salt includes NaCl and KCl.

[0052] The incorporation of nitrogen sources imparts high electron mobility, good sodium mobility, and improved capacity retention and rate performance to hard carbon materials. This application employs a medium-temperature nitrogen doping step. Activated biomass powders possess a good microporous structure, and raw materials that have not undergone high-temperature carbonization have an even richer microporous structure. High-temperature carbonization reduces the microporous structure of the material, significantly impacting the amount of nitrogen incorporated. Therefore, this application utilizes medium-temperature nitrogen doping, allowing nitrogen to fully penetrate along the pore-forming agent and its own micropores, forming a stable structure that does not volatilize significantly during subsequent high-temperature carbonization.

[0053] This application adds water-soluble salt as a pore-forming agent while performing nitrogen doping at medium temperature. The pore-forming agent is used to create pores in the material, which shortens the diffusion distance of sodium ions in the material. The appearance of pores can also store more sodium ions. The main reason for using water-soluble salt templates is that they have the advantages of being environmentally friendly, less corrosive to equipment, and safer. Commonly used alkaline pore-forming agents in this field are more corrosive to equipment and have certain safety issues.

[0054] In step (4): the sintering temperature is 1000-1300℃, the sintering time is 1-4h, and the sintering atmosphere is one or more of nitrogen, argon, helium and xenon.

[0055] Using the above preparation method, this application yields a nitrogen-doped porous hard carbon material with a pore size ≤2nm and a specific surface area of ​​10-16m². 2 / g; optionally, the specific surface area is 10-11m². 2 / g、11-12m 2 / g、12-13m 2 / g、13-14m 2 / g, 14-15m 2 / g, 15-16m 2 / g; nitrogen doping content is 3.0-8.5wt%, optionally, the nitrogen doping content is 3.0-3.5wt%, 3.5-4.0wt%, 4.0-4.5wt%, 4.5-5.0wt%, 5.0-5.5wt%, 5.5-6.0wt%, 6.0-6.5wt%, 6.5-7.0wt%, 7.0-7.5wt%, 7.5-8.0wt%, 8.0-8.5wt%; the ratio of amorphous carbon peak to graphitized carbon peak (I D / I G The value is 1.20-1.30.

[0056] The nitrogen-doped porous hard carbon material of this application, with its porous structure and heteroatom doping, provides an effective diffusion channel and a short sodium ion diffusion distance for sodium ions; the defects generated by the porous structure vacancies and nitrogen atom doping can serve as sodium storage sites; the mechanical stress of volume expansion / contraction during sodium insertion / extraction is small, resulting in good cycle stability and excellent rate performance.

[0057] The technical solution of this application will be further described below with reference to specific embodiments.

[0058] Preparation Example

[0059] The coconut shell raw material is cleaned and dried. The dried coconut shell is then crushed. After preliminary crushing and final crushing, the coconut shell becomes powder with a particle size of 400 mesh. The powder is then purified by acid washing with hydrochloric acid and HF to obtain purified coconut shell powder.

[0060] The purified coconut shell powder was activated at 500°C for 2 hours under a CO2 atmosphere to obtain activated coconut shell powder.

[0061] Example 1

[0062] Take 10g of activated coconut shell powder and mix it with urea and NaCl in a mass ratio of 1:2:1. Under nitrogen atmosphere, heat treat at 600℃ for 2h to dope nitrogen. Before heat treatment, first purge the air in the tubular furnace by passing nitrogen gas for half an hour.

[0063] Nitrogen-doped coconut shell powder was washed with water and then dried at 120°C. The dried material was then subjected to high-temperature carbonization at 1300°C for 2 hours to obtain porous hard carbon material.

[0064] Example 2

[0065] Take 10g of activated coconut shell powder and mix it with urea and NaCl in a mass ratio of 1:1:1. Under nitrogen atmosphere, heat treat at 600℃ for 2h to dope nitrogen; before heat treatment, first purge the air in the tubular furnace for half an hour.

[0066] Nitrogen-doped coconut shell powder was washed with water and then dried at 120°C. The dried material was then subjected to high-temperature carbonization at 1000°C for 4 hours to obtain porous hard carbon material.

[0067] Example 3

[0068] Take 10g of activated coconut shell powder and mix it with urea and NaCl in a mass ratio of 1:3:1. Under nitrogen atmosphere, heat treat at 500℃ for 4h to dope nitrogen. Before heat treatment, first purge the air in the tubular furnace with nitrogen for half an hour.

[0069] Nitrogen-doped coconut shell powder was washed with water and then dried at 120°C. The dried material was then subjected to high-temperature carbonization at 1200°C for 3 hours to obtain porous hard carbon material.

[0070] Example 4

[0071] Take 10g of activated coconut shell powder and mix it with urea and NaCl in a mass ratio of 1:2:1. Under nitrogen atmosphere, heat treat at 500℃ for 2h to dope nitrogen. Before heat treatment, first purge the air in the tubular furnace by passing nitrogen gas for half an hour.

[0072] Nitrogen-doped coconut shell powder was washed with water and then dried at 120°C. The dried material was then subjected to high-temperature carbonization at 1300°C for 1 hour to obtain porous hard carbon material.

[0073] Example 5

[0074] Take 10g of activated coconut shell powder and mix it with urea and NaCl in a mass ratio of 1:2:1. Under a nitrogen atmosphere, heat treat at 800℃ for 2h to dope nitrogen. Before heat treatment, first purge the air in the tubular furnace by passing nitrogen gas for half an hour.

[0075] Nitrogen-doped coconut shell powder was washed with water and then dried at 120°C. The dried material was then subjected to high-temperature carbonization at 1300°C for 2 hours to obtain porous hard carbon material.

[0076] Example 6

[0077] Take 10g of activated coconut shell powder and mix it with melamine and KCl in a mass ratio of 1:2:1. Under nitrogen atmosphere, heat treat at 800℃ for 1h to dope nitrogen; before heat treatment, first purge the air in the tube furnace for half an hour.

[0078] Nitrogen-doped coconut shell powder was washed with water and then dried at 120°C. The dried material was then subjected to high-temperature carbonization at 1300°C for 2 hours to obtain porous hard carbon material.

[0079] Comparative Example 1

[0080] The preparation steps are basically the same as in Example 1, except that: 10g of activated coconut shell powder is taken, ground evenly and placed in a graphite crucible for carbonization at 1300℃, without adding any exogenous additives.

[0081] Comparative Example 2

[0082] The preparation steps are basically the same as in Example 1, except that: the activated coconut shell powder and nitrogen source are mixed evenly in a mass ratio of 1:2, and no pore-forming agent is added.

[0083] Comparative Example 3

[0084] The preparation steps are basically the same as in Example 1, except that: the activated coconut shell powder, nitrogen source and pore-forming agent are mixed evenly in a mass ratio of 1:2:1, and no medium-temperature nitrogen doping is performed. The carbonization is carried out directly at 1300℃.

[0085] Comparative Example 4

[0086] The preparation steps are basically the same as in Example 1, except that: the activated coconut shell powder is carbonized at 1300℃ to obtain carbonized material, and then nitrogen source and pore-forming agent are mixed in a mass ratio of 1:2:1 and calcined at 600℃ for 2 hours.

[0087] Comparative Example 5

[0088] The preparation steps are basically the same as in Example 1, except that: purified coconut shell powder is mixed with nitrogen source and pore-forming agent, and CO2 activation is not performed.

[0089] Experimental Example

[0090] 1. The hard carbon materials of the examples and comparative examples were tested.

[0091] Figure 1 This is a SEM image of the nitrogen-doped porous hard carbon material prepared in Example 1. Figure 2 Here is a SEM image of the hard carbon material prepared in Comparative Example 1, combined with... Figures 1-2 As shown, the nitrogen-doped porous hard carbon material of this application has a better morphology and a smoother surface.

[0092] Figure 3 The Raman spectra of Example 1 and Comparative Example 1 are shown. The ratio I of the amorphous carbon peak to the graphitized carbon peak in the nitrogen-doped porous hard carbon material was calculated. D / I G The value of I in the hard carbon material of Comparative Example 1 is 1.30. D / I G The specific capacity is only 1.21. In Example 1, the hard carbon material has a higher degree of graphitization, which is beneficial to improving the specific capacity and the first coulombic efficiency.

[0093] Figure 4 The XRD patterns of Example 1 and Comparative Example 1 are shown. As can be seen from the XRD, the characteristic diffraction peaks of amorphous carbon are displayed near 25.6°, 44° and 80°, which correspond to the (002), (100) and (110) crystal planes of the graphite region in hard carbon, respectively. Compared with Comparative Example 1, the peak angle of the characteristic peaks in Example 1 has shifted to a lower angle, which proves that the interlayer spacing of Example 1 has been expanded.

[0094] The nitrogen content in the material was determined by organic element analyzer EA, and the nitrogen content in the hard carbon material in Example 1 was 6.5 wt%.

[0095] Figure 5 The pore size distribution diagrams for Example 1 and Comparative Example 1 are shown below. Figure 5 It can be observed that the porous nitrogen-doped hard carbon material prepared in Example 1 has abundant pores, mainly consisting of micropores ≤2nm, with a specific surface area of ​​15.88m². 2 / g.

[0096] 2. The hard carbon materials obtained in the examples and comparative examples were used to prepare sodium-ion batteries according to the following method.

[0097] A method for preparing a button-type sodium-ion battery includes the following steps:

[0098] According to the ratio of active material:SP:CMC:SBR=92:2:2:4, weigh out the negative electrode material, SP, CMC and SBR respectively and mix them evenly in deionized water to prepare a slurry; coat the evenly mixed slurry onto the aluminum foil current collector, bake it in an oven at 80℃ for 1 hour, and then take it out and cool it to room temperature.

[0099] Adjust the roller spacing and roll the electrode sheets. Cut the rolled electrode sheets into small round pieces with a diameter of 14mm and weigh them as m1. Similarly, cut the aluminum foil current collector into aluminum foil round pieces with a diameter of 14mm and weigh them as m2. (m1-m2)*0.94 represents the mass of the active material, denoted as m3. Place the weighed round pieces in an 80℃ oven and vacuum dry for 12 hours.

[0100] The vacuum-dried small discs were transferred to a glove box, and sodium discs were used as the counter electrode and auxiliary electrode. The electrolyte was 1M NaPF6 / EC:DMC:DEC = 2:2:1, and a glass fiber diaphragm was used as the separator. Sodium-ion button cells were assembled in a glove box where the oxygen and water content were both less than 0.01ppm.

[0101] The assembled button-type sodium-ion batteries were left to stand for 12 hours. The electrochemical performance of the stood button-type sodium-ion batteries was then tested under constant current using the Wuhan Landian Battery Testing System.

[0102] The test data for the examples and comparative examples are shown in the table below:

[0103]

[0104]

[0105] As can be seen from the data in the table above, the nitrogen-doped porous hard carbon materials prepared in the embodiments of this application have a pore size ≤2nm and a specific surface area greater than 10.8m². 2 / g, up to 15.88m 2 / g; and the nitrogen doping content is high, reaching up to 8.3wt%, with a ratio of I between the amorphous carbon peak and the graphitized carbon peak. D / I G With a specific capacity greater than 1.2 and a maximum of 1.30, based on the above performance, the charge / discharge specific capacity and first-time efficiency of the embodiments of this application are higher than those of the comparative hard carbon materials. The nitrogen-doped porous hard carbon material prepared in the embodiments of this application has high reversible capacity and excellent first-time efficiency. The porous nitrogen-doped hard carbon material has a charge specific capacity greater than 320 mAh / g, a discharge specific capacity greater than 280 mAh / g, and a first-time coulombic efficiency greater than 87%. Moreover, the highest charge specific capacity can reach 347.83 mAh / g; the highest discharge specific capacity can reach 320 mAh / g; and the highest first-time efficiency can reach 92.1%.

[0106] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of this application. It should be noted that, for those skilled in the art, several improvements and modifications can be made to this application without departing from the principles of this application, and these improvements and modifications also fall within the protection scope of the claims of this application.

Claims

1. A method for preparing a nitrogen-doped porous hard carbon material, characterized in that, include: Biomass raw materials are pretreated to obtain biomass powder; The biomass powder is activated to obtain activated biomass; A nitrogen source and a pore-forming agent are added to the activated biomass, and medium-temperature pre-doping is performed to obtain nitrogen-doped biomass; The nitrogen-doped biomass is sintered at high temperature to obtain the nitrogen-doped porous hard carbon material. The pre-doping temperature is 500-800℃; The activation treatment includes: activation at 300-600℃ in a carbon dioxide atmosphere for 1-4 hours.

2. The preparation method according to claim 1, characterized in that, The nitrogen source is selected from one or more of ethylenediamine, dopamine, aniline, melamine, and urea.

3. The preparation method according to claim 2, characterized in that, The nitrogen source is urea.

4. The preparation method according to claim 1, characterized in that, The predoping time is 1-4 hours.

5. The preparation method according to claim 1, characterized in that, The mass ratio of the activated biomass, nitrogen source, and pore-forming agent is 1:(1-3):(1-3).

6. The preparation method according to claim 1, characterized in that, The pore-forming agent is a water-soluble salt.

7. The preparation method according to claim 6, characterized in that, The water-soluble salts include: NaCl and KCl.

8. The preparation method according to claim 1, characterized in that, The sintering temperature is 1000-1300℃, the sintering time is 1-4h, and the sintering atmosphere is one or more of nitrogen, argon, helium, and xenon.

9. The preparation method according to claim 1, characterized in that, The biomass raw materials are selected from one or more of the following: coconut shell, rice, orange peel, sugarcane, rapeseed, cotton, barley, wheat, corn, reed, and sisal.

10. The preparation method according to claim 1, characterized in that, The biomass raw material is coconut shell.

11. The preparation method according to claim 1, characterized in that, The pretreatment includes: washing, crushing, and purifying the biomass raw materials with acid; The pickling agents used in the pickling process are hydrochloric acid and hydrofluoric acid.

12. The preparation method according to claim 8, characterized in that, The process before sintering also includes: washing the nitrogen-doped biomass with water to remove the pore-forming agent.

13. A nitrogen-doped porous hard carbon material, characterized in that, Pore ​​size ≤ 2nm; specific surface area 10-16m² 2 / g; nitrogen doping content is 3.0-8.5wt%; the ratio of amorphous carbon peaks to graphitized carbon peaks (I D / I G The value is 1.20-1.

30.

14. A negative electrode sheet, characterized in that, This includes nitrogen-doped porous hard carbon materials prepared by any of the preparation methods described in claims 1-12, and / or nitrogen-doped porous hard carbon materials as described in claim 13.

15. A sodium-ion battery, characterized in that, Includes the negative electrode sheet as described in claim 14.

16. An electrochemical device, characterized in that, Including the sodium-ion battery as described in claim 15.

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