An alkali-activated synergistic nitrogen and phosphorus co-doped porous biomass hard carbon material, a preparation method therefor and applications thereof

Abstract

This invention provides an alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material, its preparation method, and its application. This invention only requires mixing pre-carbonized biomass material, an activator, and a dopant, followed by calcination under an inert atmosphere to obtain a porous hard carbon material. Then, the porous hard carbon material is calcined again under an inert atmosphere to obtain the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material. This simple and effective method achieves the stepped pore design and nitrogen-phosphorus doping of the hard carbon material, and the obtained nitrogen-phosphorus co-doped porous hard carbon material exhibits high specific capacity and initial coulombic efficiency.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of hard carbon materials, in particular to an alkali-activated and nitrogen-phosphorus co-doped porous biomass hard carbon material and a preparation method and application thereof. BACKGROUND

[0002] Sodium-ion (Na + ) batteries with cost advantages are considered to be a beneficial supplement to lithium-ion batteries, and the performance of electrode materials is one of the key factors determining whether sodium-ion batteries can be applied on a large scale. In terms of negative electrode materials, hard carbon has the advantages of easy availability of carbon source, flexible preparation method, and controllable structure, and has great commercial application potential. Among the many precursors of hard carbon materials, biomass is widely available, green and environmentally friendly, and has a unique microstructure itself, which can be used as a precursor for preparing hard carbon.

[0003] The biomass precursor is pyrolyzed at high temperature, in which the organic components decompose and form short-range crystalline regions with local order but long-range disorder. These carbon micro-regions are composed of curved graphene sheet layers stacked in disorder. In addition, the pyrolysis temperature will also directly affect the structure of the biomass hard carbon: low temperature (600-800℃) generates high porosity and low graphitization structure, and high temperature (>1000℃) promotes the order of the crystallites and the formation of closed pores.

[0004] In the sodium storage mechanism of the biomass hard carbon material, the excellent sodium intercalation performance of the prepared sample is closely related to the unique pore structure of the material. The existence of micropores not only helps the material to fully contact with the electrolyte, but also makes the material have a large number of defect sites, providing active sites for Na + storage and improving the sodium storage capacity; the existence of mesopores and macropores shortens the transport distance of sodium ions, improves the migration rate of Na + , and provides channels and more sodium intercalation sites for Na + transport in the material. In addition, high-temperature pyrolysis can further optimize the pore distribution, form more regular pore structures through the contraction and recombination of the carbon skeleton, reduce the pore blockage problem, and thus significantly improve the initial coulombic efficiency (>80%) and capacity of the sodium-ion battery.

[0005] In research reports on hard carbon materials, alkali activation plays a crucial role as an important material modification method in the preparation and performance optimization of hard carbon materials. By precisely controlling the microstructure (pore size, interlayer spacing) and chemical composition of hard carbon, its sodium storage capacity can be significantly enhanced. Furthermore, heteroatom doping, as another effective strategy to improve the electrochemical performance of biomass hard carbon materials, can further optimize ion transport pathways, enhance pore wall chemical stability, and extend cycle life. The synergistic effect of alkali activation and heteroatom doping not only utilizes activation to create efficient ion channels but also further expands the pore size distribution and optimizes electrochemical activity through chemical doping. The coupling effect of these two methods can simultaneously improve the capacity, rate performance, and cycle stability of biomass hard carbon.

[0006] Chinese patent CN117985692A discloses nitrogen-phosphorus co-doped biomass hard carbon materials, their preparation methods, and applications. This patent incorporates nitrogen and phosphorus sources into the carbon interlayer through liquid-phase mixing and high-temperature calcination, expanding the interlayer spacing and thus increasing sodium storage sites. However, the melamine used in this patent only reacts with carbonyl groups of a specific structure, resulting in uneven distribution of doping sites. The morphology of the phosphate ester crosslinks (such as chain length and crosslinking density) directly affects the pore structure, and the stirring reaction makes it difficult to precisely control the molecular weight and distribution of the crosslinks, potentially leading to pore collapse or blockage during subsequent carbonization. Furthermore, the stirring reaction easily causes local concentrations to be too high or too low, affecting the doping effect. Moreover, this patent requires balancing liquid-phase reaction time and energy consumption, resulting in a cumbersome production process, high production costs, and significant bottlenecks in industrial production.

[0007] Therefore, there is an urgent need to develop a simple and controllable preparation method to effectively solve the problems of cumbersome processes and unsatisfactory sodium storage performance. Summary of the Invention

[0008] The purpose of this invention is, on the one hand, to significantly improve the capacity, rate performance, and cycle stability of biomass hard carbon, and on the other hand, to simplify the process, reduce production costs, and improve production efficiency. This invention provides an alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material, its preparation method, and its application. This method achieves the generation of stepped pores and uniform and stable heteroatom doping coupling in the material, further improving the specific capacity and first coulombic efficiency of the hard carbon material.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A method for preparing an alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material, the method comprising the following steps:

[0011] (1) The biomass raw material is pyrolyzed and pre-carbonized under a certain atmosphere to obtain a pre-carbonized precursor;

[0012] (2) The pre-carbonized precursor obtained in step (1) is crushed and sieved (it can pass through a 300-mesh sieve) and mixed with activator and dopant. It is then heat-treated (activated and perforated) and co-doped with nitrogen and phosphorus under an inert atmosphere (nitrogen or argon). It is then stirred and soaked in hydrochloric acid solution, filtered and dried.

[0013] (3) The material after step (2) is heat-treated in an inert atmosphere (nitrogen or argon) to obtain nitrogen-phosphorus co-doped porous biomass hard carbon material.

[0014] Further, in step (1), the biomass raw material is one or more of the following: pine fuel pellets, straw, water-soluble starch, lignin, corn cobs, coconut shells, walnut shells, and hazelnut shells.

[0015] Further, in step (1), the pre-carbonization heating rate is 2~5℃ / min, the pyrolysis temperature is 300~500℃, and the holding time is 3~6h. The atmosphere during the heating process is a mixture of air and argon, with an air-to-argon flow rate ratio of 1:3. The holding process uses pure argon. The main function of air is to pre-oxidize the biomass. During pre-oxidation, biomass (such as cellulose and lignin) undergoes dehydration and cross-linking reactions to form a stable thermosetting structure, preserving the carbon skeleton for subsequent carbonization.

[0016] Further, in step (2), the activator is one or more of potassium hydroxide, sodium hydroxide, sodium carbonate and potassium carbonate, and the dopant is one or more of ammonium polyphosphate, ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0017] Further, in step (2), the mass ratio of the pre-carbonized precursor, activator and dopant is 1:0.1~1:0.01~0.1, and the blending is carried out by dry mixing with a small mixer for 1~60 min.

[0018] Further, in step (2), the heating rate of the heat treatment is 1~4℃ / min, the temperature is 600~900℃, and the holding time is 0.5~4h; the drying temperature is 70~90℃, and the time is 8~12h.

[0019] Further, in step (2), the concentration of the hydrochloric acid solution is 0.1~2M, the stirring and soaking time is 3~5h, and then it is filtered and washed until neutral.

[0020] Furthermore, in step (3), the heating rate of the heat treatment is 1~3℃ / min, the temperature is 1300~1500℃, and the time is 1~4h.

[0021] An alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material is prepared by the above-mentioned method.

[0022] The above-mentioned alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material is used as a negative electrode material for sodium-ion batteries.

[0023] The advantages of this invention over the prior art are as follows:

[0024] 1. This invention achieves the coupling of stepped porosity generation and heteroatom doping through the simultaneous action of an alkaline activator and nitrogen-phosphorus dopant. Compared to traditional methods where activation and doping are performed in separate steps, this not only reduces energy consumption but also avoids material loss in multiple steps, making it suitable for large-scale production.

[0025] 2. The present invention provides more active sites for heteroatom insertion through the porous structure (micropore-mesopore-macropore hierarchical network) formed during the alkali activation process, promotes the anchoring of nitrogen and phosphorus atoms, and further improves the doping efficiency and site uniformity.

[0026] 3. This invention partially repairs the defects (edge ​​sites, graphene layer defects) introduced by activation at high temperatures, while retaining some active sites, thus balancing structural stability and electrochemical activity and improving the electrochemical performance of the material. Attached Figure Description

[0027] Figure 1 This is a SEM image of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 1;

[0028] Figure 2 EDS image of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 1;

[0029] Figure 3 The XRD patterns are those of the intermediate material and the finished product after activation and doping in Example 1, and the XRD diffraction patterns of Comparative Example 1.

[0030] Figure 4 The images show the isotherm curves and pore size distribution diagrams of the intermediate material and the finished product after activation and doping in Example 1.

[0031] Figure 5 This is the first charge-discharge curve of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 1;

[0032] Figure 6 This is the first charge-discharge curve of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material in Example 2;

[0033] Figure 7 This is the first charge-discharge curve of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material in Example 3;

[0034] Figure 8This is the first charge-discharge curve of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material in Example 4;

[0035] Figure 9 The first charge-discharge curve of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material in Example 5 is shown.

[0036] Figure 10 This is the first charge-discharge curve of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 6;

[0037] Figure 11 The first charge-discharge curve of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material in Example 7 is shown.

[0038] Figure 12 The first charge-discharge curve of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material in Example 8 is shown.

[0039] Figure 13 SEM image of the biomass hard carbon material in Comparative Example 1;

[0040] Figure 14 The first charge-discharge curve of the biomass hard carbon material in Comparative Example 1 is shown.

[0041] Figure 15 The first charge-discharge curve of the alkali-activated biomass hard carbon material in Comparative Example 2 is shown.

[0042] Figure 16 The charging cycle curve of the nitrogen-phosphorus doped biomass hard carbon material in Comparative Example 3 is shown.

[0043] Figure 17 The charging cycle curve of the nitrogen-phosphorus doped biomass hard carbon material in Comparative Example 4 is shown.

[0044] Figure 18 This is a process flow diagram of the present invention;

[0045] Figure 19 Example 1 shows its charge-discharge chemical cycle curves at a current density of 0.5C.

[0046] Figure 20 The charge-discharge chemical cycle curves of Comparative Example 1 were tested at a current density of 0.5C. Detailed Implementation

[0047] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0048] Example 1

[0049] (1) Take 40g of pine fuel pellets and put them into a high-temperature tube furnace. The temperature is raised from room temperature to 500℃ at a rate of 5℃ / min and kept at that temperature for 5h. The pine fuel pellets are pyrolyzed and carbonized to obtain 12g of pre-carbonized precursor. The atmosphere during the heating process is a mixture of air and argon, with an air to argon flow rate ratio of 1:3. The heat preservation process is pure argon.

[0050] (2) Crush 12g of the pre-carbonized precursor obtained in step (1) and sieve it. Then add 6g of KOH and 0.6g of ammonium polyphosphate powder to a mixer containing the pre-carbonized precursor and mix for 30min.

[0051] (3) The material processed in step (2) is placed in a high-temperature tube furnace and heated from room temperature to 700°C at a rate of 4°C / min under an argon protective atmosphere. The temperature is held for 1 hour, cooled, and then taken out and sieved to obtain nitrogen-sulfur doped porous carbon material.

[0052] (4) Put the material treated in step (3) into 200ml of 1.4M hydrochloric acid solution and stir for 3h. Filter, wash until neutral, and put into a blower dryer to dry at 80℃.

[0053] (5) The material processed in step (4) is placed in a high-temperature box furnace and heated from room temperature to 1300℃ at a rate of 3℃ / min under argon protection atmosphere. The temperature is maintained for 3 hours. After cooling, the material is taken out and sieved to obtain nitrogen-sulfur doped biomass porous hard carbon powder.

[0054] Figure 1 Figure a shows the SEM images of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 1. Figure a shows that the particle size of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material is between 1 and 30 μm. Figure b shows that after alkali activation synergistic nitrogen-phosphorus co-doping, the material exhibits obvious macropores, providing a wider channel and more active sites for ion conduction.

[0055] Figure 2 The image shows the EDS diagram of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 1. The figure shows that N and P elements are uniformly distributed on the surface of the material, indicating that the synergistic effect of activation and doping promotes the anchoring of nitrogen and phosphorus atoms, further improving the doping efficiency and site uniformity.

[0056] Figure 3The XRD diffraction patterns of the activated doped intermediate material, the finished product, and Comparative Example 1 are shown. As can be seen from the figures, compared to Comparative Example 1, the diffraction angle of the (002) crystal plane peak of the activated doped intermediate material in Example 1 shifts to a smaller angle, and the diffraction peak is significantly broadened. This is partly due to the activation process increasing the defect density and lattice distortion of the material, thus reducing the degree of graphitization; and partly due to the introduction of heteroatoms (nitrogen, phosphorus) which expands the interlayer spacing of the carbon layers. While there is no significant angle shift between the (002) crystal plane peak of the activated doped intermediate material in Example 1 and the finished product in Example 1, the broadening of the (002) and (100) crystal plane peaks is significantly reduced, indicating that high-temperature pyrolysis reduces defects and partially restores structural order.

[0057] Figure 4 The isotherm curves and pore size distribution of the activated doped intermediate and finished materials in Example 1 are shown in Table 1. As can be seen from Table 1, on the one hand, the alkali-activated synergistic doping strategy achieves a stepped pore structure design for the material, not only constructing efficient ion channels for sodium ion insertion / extraction but also providing abundant anchoring sites for nitrogen and phosphorus heteroatoms, enhancing the chemisorption of sodium ions and further increasing the material's capacity. On the other hand, through high-temperature calcination, some defects on the material surface are repaired, and the micropore volume is reduced. Simultaneously, carbon atoms diffuse and migrate, forming a more regular mesoporous structure. This process not only retains some active sites but also balances structural stability and electrochemical activity.

[0058] The alkaline-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 1 was used to fabricate an electrode. Using metallic sodium as the counter electrode and 1 mol / L NaPF6-EC / DMC / DEC (1:1:1) as the electrolyte, a button cell was assembled. Its charge-discharge curve was tested at a current density of 0.1C. Figure 5 As shown, the test results indicate that the initial sodium insertion specific capacity is 397.28 mAh / g, the initial sodium removal specific capacity is 360.33 mAh / g, and the initial coulombic efficiency is 90.70%. Its charging chemical cycle curve was tested at a current density of 0.5C, as shown below. Figure 19 As shown, the test results indicate that its capacity retention rate after 45 cycles is 87.04%.

[0059] Table 1. Pore size analysis of intermediate materials and finished products after activation and doping in Example 1.

[0060]

[0061] Example 2

[0062] (1) Take 40g of pine fuel particles and put them into a high-temperature tube furnace. The temperature is raised from room temperature to 500℃ at a rate of 5℃ / min and kept at that temperature for 5h. The pine fuel particles are pyrolyzed and carbonized to obtain 12g of pre-carbonized precursor. The atmosphere during the heating process is a mixture of air and argon, with an air to argon flow rate ratio of 1:3. The heat preservation process is pure argon.

[0063] (2) Crush 12g of the pre-carbonized precursor obtained in step (1) and sieve it. Then add 2.4g of KOH and 0.6g of ammonium polyphosphate powder to a mixer containing the pre-carbonized precursor and mix for 30min.

[0064] (3) The material processed in step (2) is placed in a high-temperature tube furnace and heated from room temperature to 700°C at a rate of 4°C / min under an argon protective atmosphere. The temperature is held for 1 hour, cooled, and then taken out and sieved to obtain nitrogen-sulfur doped porous carbon material.

[0065] (4) Put the material treated in step (3) into 200ml of 1.4M hydrochloric acid solution and stir for 3h. Filter, wash until neutral, and put into a blower dryer to dry at 80℃.

[0066] (5) The material processed in step (4) is placed in a high-temperature box furnace and heated from room temperature to 1300℃ at a rate of 3℃ / min under argon protection atmosphere. The temperature is maintained for 3 hours. After cooling, the material is taken out and sieved to obtain nitrogen-sulfur doped biomass porous hard carbon powder.

[0067] The alkaline-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 2 was used to fabricate an electrode. Using metallic sodium as the counter electrode and 1 mol / L NaPF6-EC / DMC / DEC (1:1:1) as the electrolyte, a button cell was assembled. Its charge-discharge curve was tested at a current density of 0.1C. Figure 6 As shown, the test results indicate that the initial sodium insertion specific capacity is 353.80 mAh / g, the initial sodium removal specific capacity is 331.21 mAh / g, and the initial coulombic efficiency is 93.61%.

[0068] Example 3

[0069] (1) Take 40g of pine fuel pellets and put them into a high-temperature tube furnace. The temperature is raised from room temperature to 500℃ at a rate of 5℃ / min and kept at that temperature for 5h. The pine fuel pellets are pyrolyzed and carbonized to obtain 12g of pre-carbonized precursor. The atmosphere during the heating process is a mixture of air and argon, with an air to argon flow rate ratio of 1:3. The heat preservation process is pure argon.

[0070] (2) Crush 12g of the pre-carbonized precursor obtained in step (1) and sieve it. Then add 6g of KOH and 0.12g of ammonium polyphosphate powder to a mixer containing the pre-carbonized precursor and mix for 30min.

[0071] (3) The material processed in step (2) is placed in a high-temperature tube furnace and heated from room temperature to 700°C at a rate of 4°C / min under an argon protective atmosphere. The temperature is held for 1 hour, cooled, and then taken out and sieved to obtain nitrogen-sulfur doped porous carbon material.

[0072] (4) Put the material treated in step (3) into 200ml of 1.4M hydrochloric acid solution and stir for 3h. Filter, wash until neutral, and put into a blower dryer to dry at 80℃.

[0073] (5) The material processed in step (4) is placed in a high-temperature box furnace and heated from room temperature to 1300℃ at a rate of 3℃ / min under argon protection atmosphere. The temperature is maintained for 3 hours. After cooling, the material is taken out and sieved to obtain nitrogen-sulfur doped biomass porous hard carbon powder.

[0074] The alkaline-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 3 was used to fabricate an electrode. Using metallic sodium as the counter electrode and 1 mol / L NaPF6-EC / DMC / DEC (1:1:1) as the electrolyte, a button cell was assembled. Its charge-discharge curve was tested at a current density of 0.1C. Figure 7 As shown, the test results indicate that the initial sodium insertion capacity is 372.47 mAh / g, the initial sodium removal capacity is 342.12 mAh / g, and the initial coulombic efficiency is 91.85%.

[0075] Example 4

[0076] (1) Take 40g of pine fuel particles and put them into a high-temperature tube furnace. The temperature is raised from room temperature to 500℃ at a rate of 5℃ / min and kept at that temperature for 5h. The pine fuel particles are pyrolyzed and carbonized to obtain 12g of pre-carbonized precursor. The atmosphere during the heating process is a mixture of air and argon, with an air to argon flow rate ratio of 1:3. The heat preservation process is pure argon.

[0077] (2) Crush 12g of the pre-carbonized precursor obtained in step (1) and sieve it. Then add 6g of KOH and 1.2g of ammonium polyphosphate powder to a mixer containing the pre-carbonized precursor and mix for 30min.

[0078] (3) The material processed in step (2) is placed in a high-temperature tube furnace and heated from room temperature to 700°C at a rate of 4°C / min under an argon protective atmosphere. The temperature is maintained for 1 hour, cooled, and then taken out and sieved to obtain nitrogen-sulfur doped biomass porous material.

[0079] (4) Put the material treated in step (3) into 200ml of 1.4M hydrochloric acid solution and stir for 3h. Filter, wash until neutral, and put into a blower dryer to dry at 80℃.

[0080] (5) The material processed in step (4) is placed in a high-temperature tube furnace and heated from room temperature to 1300°C at a rate of 3°C / min under an argon protective atmosphere. The temperature is maintained for 3 hours. After cooling, the material is taken out and sieved to obtain nitrogen-sulfur doped biomass porous hard carbon powder.

[0081] The alkaline-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 4 was used to fabricate an electrode. Using metallic sodium as the counter electrode and 1 mol / L NaPF6-EC / DMC / DEC (1:1:1) as the electrolyte, a button cell was assembled. Its charge-discharge curve was tested at a current density of 0.1C. Figure 8 As shown, the test results indicate that the initial sodium insertion specific capacity is 386.52 mAh / g, the initial sodium removal specific capacity is 352.25 mAh / g, and the initial coulombic efficiency is 91.13%.

[0082] Example 5

[0083] (1) Take 40g of pine fuel pellets and put them into a high-temperature tube furnace. The temperature is raised from room temperature to 500℃ at a rate of 5℃ / min and kept at that temperature for 5h. The pine fuel pellets are pyrolyzed and carbonized to obtain 12g of pre-carbonized precursor. The atmosphere during the heating process is a mixture of air and argon, with an air to argon flow rate ratio of 1:3. The heat preservation process is pure argon.

[0084] (2) Crush and sieve 12g of the pre-carbonized precursor obtained in step (1), and then add 2g of KOH and 1.2g of ammonium dihydrogen phosphate powder to a mixer containing the pre-carbonized precursor and mix for 30min.

[0085] (3) The material processed in step (2) is placed in a high-temperature tube furnace and heated from room temperature to 700°C at a rate of 4°C / min under an argon protective atmosphere. The temperature is held for 1 hour, cooled, and then taken out and sieved to obtain nitrogen-sulfur doped porous carbon material.

[0086] (4) Put the material treated in step (3) into 200ml of 1.4M hydrochloric acid solution and stir for 3h. Filter, wash until neutral, and put into a blower dryer to dry at 80℃.

[0087] (5) The material processed in step (4) is placed in a high-temperature tube furnace and heated from room temperature to 1300°C at a rate of 3°C / min under an argon protective atmosphere. The temperature is maintained for 3 hours. After cooling, the material is taken out and sieved to obtain nitrogen-sulfur doped biomass porous hard carbon powder.

[0088] The alkaline-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 5 was used to fabricate an electrode. Using metallic sodium as the counter electrode and 1 mol / L NaPF6-EC / DMC / DEC (1:1:1) as the electrolyte, a button cell was assembled. Its charge-discharge curve was tested at a current density of 0.1C. Figure 9 As shown, the test results indicate that the initial sodium insertion capacity is 338.34 mAh / g, the initial sodium removal capacity is 318.25 mAh / g, and the initial coulombic efficiency is 94.06%.

[0089] Example 6

[0090] (1) Take 40g of straw and put it into a high-temperature tube furnace. The temperature is raised from room temperature to 500℃ at a rate of 5℃ / min and kept for 5h. The straw is pyrolyzed and carbonized to obtain 12g of pre-carbonized precursor. The atmosphere during the heating process is a mixture of air and argon, with an air to argon flow rate ratio of 1:3. The heat preservation process is pure argon.

[0091] (2) Crush and sieve 12g of the pre-carbonized precursor obtained in step (1), and then add 4g of K2CO3 and 1.2g of diammonium hydrogen phosphate powder to a mixer containing the pre-carbonized precursor and mix for 30min.

[0092] (3) The material processed in step (2) is placed in a high-temperature tube furnace and heated from room temperature to 900°C at a rate of 4°C / min under an argon protective atmosphere. The temperature is maintained for 1 hour, cooled, and then taken out and sieved to obtain nitrogen-sulfur doped porous carbon material.

[0093] (4) Put the material treated in step (3) into 200ml of 1.4M hydrochloric acid solution and stir for 3h. Filter, wash until neutral, and put into a blower dryer to dry at 80℃.

[0094] (5) The material processed in step (4) is placed in a high-temperature tube furnace and heated from room temperature to 1300°C at a rate of 3°C / min under an argon protective atmosphere. The temperature is maintained for 3 hours. After cooling, the material is taken out and sieved to obtain nitrogen-sulfur doped biomass porous hard carbon powder.

[0095] The alkaline-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 6 was used to fabricate an electrode. Using metallic sodium as the counter electrode and 1 mol / L NaPF6-EC / DMC / DEC (1:1:1) as the electrolyte, a button cell was assembled. Its charge-discharge curve was tested at a current density of 0.1C. Figure 10 As shown, the test results indicate that the initial sodium insertion capacity is 347.47 mAh / g, the initial sodium removal capacity is 317.43 mAh / g, and the initial coulombic efficiency is 91.36%.

[0096] Example 7

[0097] (1) Take 40g of walnut shells and put them into a high-temperature tube furnace. The temperature is raised from room temperature to 500℃ at a rate of 5℃ / min and kept at that temperature for 5h. The walnut shells are pyrolyzed and carbonized to obtain 12g of pre-carbonized precursor. The atmosphere during the heating process is a mixture of air and argon, with an air to argon flow rate ratio of 1:3. The heat preservation process is pure argon.

[0098] (2) Crush and sieve 12g of the pre-carbonized precursor obtained in step (1), and then add 4g of Na2CO3 and 1.2g of ammonium dihydrogen phosphate powder to a mixer containing the pre-carbonized precursor and mix for 30min.

[0099] (3) The material processed in step (2) is placed in a high-temperature tube furnace and heated from room temperature to 900°C at a rate of 4°C / min under an argon protective atmosphere. The temperature is maintained for 1 hour, cooled, and then taken out and sieved to obtain nitrogen-sulfur doped porous carbon material.

[0100] (4) Put the material treated in step (3) into 200ml of 1.4M hydrochloric acid solution and stir for 3h. Filter, wash until neutral, and put into a blower dryer to dry at 80℃.

[0101] (5) The material processed in step (4) is placed in a high-temperature tube furnace and heated from room temperature to 1300°C at a rate of 3°C / min under an argon protective atmosphere. The temperature is maintained for 3 hours. After cooling, the material is taken out and sieved to obtain nitrogen-sulfur doped biomass porous hard carbon powder.

[0102] The alkaline-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 7 was used to fabricate an electrode. Using metallic sodium as the counter electrode and 1 mol / L NaPF6-EC / DMC / DEC (1:1:1) as the electrolyte, a button cell was assembled. Its charge-discharge curve was tested at a current density of 0.1C. Figure 11 As shown, the test results indicate that the initial sodium insertion capacity is 373.17 mAh / g, the initial sodium removal capacity is 339.71 mAh / g, and the initial coulombic efficiency is 91.03%.

[0103] Example 8

[0104] (1) Take 40g of hazelnut shells and put them into a high-temperature tube furnace. The temperature is raised from room temperature to 500℃ at a rate of 5℃ / min and kept at that temperature for 5h. The hazelnut shells are pyrolyzed and carbonized to obtain 12g of pre-carbonized precursor. The atmosphere during the heating process is a mixture of air and argon, with an air to argon flow rate ratio of 1:3. The heat preservation process is pure argon.

[0105] (2) Crush and sieve 12g of the pre-carbonized precursor obtained in step (1), and then add 6g of NaOH and 0.6g of ammonium dihydrogen phosphate powder to a mixer containing the pre-carbonized precursor and mix for 30min.

[0106] (3) The material processed in step (2) is placed in a high-temperature tube furnace and heated from room temperature to 700°C at a rate of 4°C / min under an argon protective atmosphere. The temperature is held for 1 hour, cooled, and then taken out and sieved to obtain nitrogen-sulfur doped porous carbon material.

[0107] (4) Put the material treated in step (3) into 200ml of 1.4M hydrochloric acid solution and stir for 3h. Filter, wash until neutral, and put into a blower dryer to dry at 80℃.

[0108] (5) The material processed in step (4) is placed in a high-temperature tube furnace and heated from room temperature to 1300°C at a rate of 3°C / min under an argon protective atmosphere. The temperature is maintained for 3 hours. After cooling, the material is taken out and sieved to obtain nitrogen-sulfur doped biomass porous hard carbon powder.

[0109] The alkaline-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material of Example 8 was used to fabricate an electrode. Using metallic sodium as the counter electrode and 1 mol / L NaPF6-EC / DMC / DEC (1:1:1) as the electrolyte, a button cell was assembled. Its charge-discharge curve was tested at a current density of 0.1C. Figure 12 As shown, the test results indicate that the initial sodium insertion capacity is 368.41 mAh / g, the initial sodium removal capacity is 348.72 mAh / g, and the initial coulombic efficiency is 94.65%.

[0110] Comparative Example 1

[0111] (1) Take 30g of pine wood particles and put them into a high-temperature tube furnace. Under the protection of argon, the temperature is raised from room temperature to 1300℃ at a rate of 3℃ / min and kept at the temperature for 3h. The pine wood particles are pyrolyzed and carbonized to obtain hard carbon powder.

[0112] (2) The hard carbon powder obtained in step (1) was placed in a 1.4M hydrochloric acid solution and stirred for 3 hours. Then it was washed with deionized water and ethanol until neutral. The product was placed in a forced-air drying oven and dried at 80°C. After passing through a sieve, the biomass hard carbon material was obtained.

[0113] Figure 13 The image shows a SEM image of the biomass hard carbon material in Comparative Example 1, which shows that the particle size of the biomass hard carbon material is between 1 and 30 μm.

[0114] The biomass hard carbon material from Comparative Example 1 was used to fabricate electrode sheets. Using metallic sodium as the counter electrode and 1 mol / L NaPF6-EC / DMC / DEC (1:1:1) as the electrolyte, a button cell was assembled. Its charge-discharge curve was tested at a current density of 0.1C. Figure 14 As shown, the test results indicate that the initial sodium insertion specific capacity is 296.54 mAh / g, the initial sodium removal specific capacity is 277.21 mAh / g, and the initial coulombic efficiency is 93.48%. Its charging chemical cycle curve was tested at a current density of 0.5C, as shown below. Figure 20 As shown, the test results indicate that its capacity retention rate after 45 cycles is 80.21%.

[0115] Comparative Example 2

[0116] (1) Take 30g of pine fuel pellets and put them into a high-temperature tube furnace. Under the protection of argon, the temperature is raised from room temperature to 500℃ at a rate of 5℃ / min and kept at that temperature for 5h. The pine fuel pellets are pyrolyzed and carbonized to obtain 9g of pre-carbonized precursor.

[0117] (2) Crush 9g of the pre-carbonized precursor obtained in step (1) and sieve it. Then add 4.5g of KOH to the mixer containing the pre-carbonized precursor and mix for 30min.

[0118] (3) The material processed in step (2) is placed in a high-temperature tube furnace and heated from room temperature to 700°C at a rate of 4°C / min under an argon protective atmosphere. The temperature is maintained for 1 hour, cooled, and then taken out and sieved to obtain nitrogen-sulfur doped biomass porous material.

[0119] (4) Put the material treated in step (3) into 200ml of 1.4M hydrochloric acid solution and stir for 3h. Filter until neutral and put into a blower dryer to dry at 80℃.

[0120] (5) The material processed in step (4) is placed in a high-temperature box furnace and heated from room temperature to 1300℃ at a rate of 3℃ / min under argon protection atmosphere. The temperature is maintained for 3 hours. After cooling, the material is taken out and sieved to obtain nitrogen-sulfur doped biomass porous hard carbon powder.

[0121] The sulfur-doped biomass hard carbon material from Comparative Example 2 was used to fabricate electrodes. Using metallic sodium as the counter electrode and 1 mol / L NaPF6-EC / DMC / DEC (1:1:1) as the electrolyte, a button cell was assembled. Its charge-discharge curve was tested at a current density of 0.1C. (The results are shown in the original text.) Figure 15 As shown, the test results indicate that the initial sodium insertion capacity is 353.76 mAh / g, the initial sodium removal capacity is 329.59 mAh / g, and the initial coulombic efficiency is 93.17%.

[0122] Comparative Example 3

[0123] (1) Take 30g of pine fuel pellets and put them into a high-temperature tube furnace. Under the protection of argon, the temperature is raised from room temperature to 500℃ at a rate of 5℃ / min and kept at that temperature for 5h. The pine fuel pellets are pyrolyzed and carbonized to obtain 9g of pre-carbonized precursor.

[0124] (2) Crush 9g of the pre-carbonized precursor obtained in step (1) and sieve it. Then add 0.45g of ammonium polyphosphate to a mixer containing the pre-carbonized precursor and mix for 30min.

[0125] (3) The material processed in step (2) is placed in a high-temperature tube furnace and heated from room temperature to 700°C at a rate of 4°C / min under an argon protective atmosphere. The temperature is maintained for 1 hour, cooled, and then taken out and sieved to obtain nitrogen-sulfur doped biomass porous material.

[0126] (4) Put the material treated in step (3) into 200ml of 1.4M hydrochloric acid solution and stir for 3h. Filter until neutral and put into a blower dryer to dry at 80℃.

[0127] (5) The material processed in step (4) is placed in a high-temperature box furnace and heated from room temperature to 1300℃ at a rate of 3℃ / min under argon protection atmosphere. The temperature is maintained for 3 hours. After cooling, the material is taken out and sieved to obtain nitrogen-sulfur doped biomass porous hard carbon powder.

[0128] The sulfur-doped biomass hard carbon material from Comparative Example 3 was used to fabricate electrodes. Using metallic sodium as the counter electrode and 1 mol / L NaPF6-EC / DMC / DEC (1:1:1) as the electrolyte, a button cell was assembled. Its charge-discharge curve was tested at a current density of 0.1C. Figure 16 As shown, the test results indicate that the initial sodium insertion capacity is 339.47 mAh / g, the initial sodium removal capacity is 311.08 mAh / g, and the initial coulombic efficiency is 91.64%.

[0129] Comparative Example 4

[0130] (1) Take 30g of pine fuel pellets and put them into a high-temperature tube furnace. Under the protection of argon, the temperature is raised from room temperature to 500℃ at a rate of 5℃ / min and kept at that temperature for 5h. The pine fuel pellets are pyrolyzed and carbonized to obtain 9g of pre-carbonized precursor.

[0131] (2) Crush 9g of the pre-carbonized precursor obtained in step (1) and sieve it. Then add 0.9g of ammonium polyphosphate to a mixer containing the pre-carbonized precursor and mix for 30min.

[0132] (3) The material processed in step (2) is placed in a high-temperature tube furnace and heated from room temperature to 700°C at a rate of 4°C / min under an argon protective atmosphere. The temperature is maintained for 1 hour, cooled, and then taken out and sieved to obtain nitrogen-sulfur doped biomass porous material.

[0133] (4) Put the material treated in step (3) into 200ml of 1.4M hydrochloric acid solution and stir for 3h. Filter until neutral and put into a blower dryer to dry at 80℃.

[0134] (5) The material processed in step (4) is placed in a high-temperature tube furnace and heated from room temperature to 1300°C at a rate of 3°C / min under an argon protective atmosphere. The temperature is maintained for 3 hours. After cooling, the material is taken out and sieved to obtain nitrogen-sulfur doped biomass porous hard carbon powder.

[0135] The sulfur-doped biomass hard carbon material from Comparative Example 4 was used to fabricate electrodes. Using metallic sodium as the counter electrode and 1 mol / L NaPF6-EC / DMC / DEC (1:1:1) as the electrolyte, a button cell was assembled. Its charge-discharge curve was tested at a current density of 0.1C. Figure 17 As shown, the test results indicate that the initial sodium insertion capacity is 333.74 mAh / g, the initial sodium removal capacity is 299.87 mAh / g, and the initial coulombic efficiency is 89.85%.

[0136] In summary, this invention employs the simultaneous action of alkali activation and nitrogen-phosphorus doping to achieve the coupling of stepped pore formation and heteroatom doping, enabling nitrogen-phosphorus co-doped hard carbon materials to possess high specific capacity and initial coulombic efficiency. The optimal mass ratio of pre-carbonized precursor, KOH, and ammonium polyphosphate is 1:0.5:0.05. In contrast, the microporous structure formed solely by nitrogen-phosphorus co-doping of hard carbon materials is limited and cannot provide a richer variety of active sites and ion migration channels.

[0137] This invention achieves simultaneous alkaline activation and nitrogen-phosphorus doping, which, compared to traditional methods where activation and doping are performed in separate steps, not only reduces energy consumption but also avoids material loss in multiple steps, making it suitable for large-scale production.

[0138] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing an alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material, characterized in that: The method includes the following steps: (1) The biomass raw material is pyrolyzed and pre-carbonized under a certain atmosphere to obtain a pre-carbonized precursor; (2) The pre-carbonized precursor obtained in step (1) is crushed and sieved and mixed with activator and dopant. It is then heat-treated (activated and pore-drilled) and co-doped with nitrogen and phosphorus under an inert atmosphere. It is then stirred and soaked in hydrochloric acid solution, filtered and dried. (3) The material after step (2) is heat-treated in an inert atmosphere to obtain nitrogen and phosphorus co-doped porous biomass hard carbon material.

2. The method for preparing an alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material according to claim 1, characterized in that: In step (1), the biomass raw material is one or more of the following: pine fuel pellets, straw, water-soluble starch, lignin, corn cobs, coconut shells, walnut shells, and hazelnut shells.

3. The method for preparing an alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material according to claim 1, characterized in that: In step (1), the heating rate of the pre-carbonization is 2~5℃ / min, the pyrolysis temperature is 300~500℃, and the holding time is 3~6h. The atmosphere during the heating process is a mixture of air and argon, with an air to argon flow rate ratio of 1:

3. The holding process is pure argon.

4. The method for preparing an alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material according to claim 1, characterized in that: In step (2), the activator is one or more of potassium hydroxide, sodium hydroxide, sodium carbonate and potassium carbonate, and the dopant is one or more of ammonium polyphosphate, ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

5. The method for preparing an alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material according to claim 1 or 4, characterized in that: In step (2), the mass ratio of the pre-carbonized precursor, activator and dopant is 1:0.1~1:0.01~0.1, and the blending is carried out by dry mixing with a small mixer for 1~60 min.

6. The method for preparing an alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material according to claim 1, characterized in that: In step (2), the heating rate of the heat treatment is 1~4℃ / min, the temperature is 600~900℃, and the holding time is 0.5~4h; the drying temperature is 70~90℃, and the time is 8~12h.

7. The method for preparing an alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material according to claim 1, characterized in that: In step (2), the concentration of the hydrochloric acid solution is 0.1~2M, the stirring and soaking time is 3~5h, and then the solution is filtered and washed until neutral.

8. The method for preparing an alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material according to claim 1, characterized in that: In step (3), the heating rate of the heat treatment is 1~3℃ / min, the temperature is 1300~1500℃, and the time is 1~4h.

9. A porous biomass hard carbon material with alkali activation and synergistic nitrogen-phosphorus co-doping, characterized in that: It is prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the alkali-activated synergistic nitrogen-phosphorus co-doped porous biomass hard carbon material as a sodium-ion battery anode material according to claim 9.

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