A nitrogen-doped hard carbon material and its preparation method

Abstract

This invention relates to a nitrogen-doped hard carbon material and its preparation method, belonging to the field of sodium-ion battery technology. The preparation method of this invention includes the following steps: S1, urea and a glycosyl carbon source are thoroughly mixed and heated to melt, then cooled to obtain a eutectic salt; S2, under a protective atmosphere, the eutectic salt is prepolymerized, pyrolyzed, and carbonized to obtain the nitrogen-doped hard carbon material. First, urea and a glycosyl carbon source form a eutectic salt. During this process, the strongly polar functional groups in the glycosyl carbon source form a hydrogen bond network with the amino and carbonyl groups in the urea. This hydrogen bond network can restrict the free movement of urea molecules and constrain their thermal decomposition path, thereby delaying and inhibiting premature decomposition of urea, while simultaneously improving the nitrogen retention rate. A three-stage heating program is used to achieve layered nitrogen doping. This segmented temperature control strategy constructs a layered doping mechanism of "first building a carbon framework, then introducing nitrogen atoms, and finally stabilizing the carbon framework structure."

Description

Technical Field

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

[0002] With the rapid development of new energy technologies, sodium-ion batteries have attracted much attention due to the abundant reserves of sodium salt raw materials and the wide availability of anode materials. Among them, hard carbon materials, with their disordered amorphous structure, abundant nanopores, and large carbon interlayer spacing, are considered the most suitable anode materials for sodium-ion batteries. However, traditional hard carbon materials still face problems such as poor rate performance and cycle stability, making it difficult to meet the requirements for anode materials in sodium-ion batteries.

[0003] To improve the electrochemical performance of hard carbon materials, engineers have explored various modification methods, with nitrogen doping emerging as an effective strategy. Nitrogen incorporation not only alters the electronic structure of hard carbon and improves conductivity but also provides more active sites for sodium ion storage. However, existing nitrogen doping methods typically suffer from limitations such as complex synthesis processes, environmentally unfriendly procedures, and difficulty in controlling pore size, hindering efficient nitrogen doping and resulting in low nitrogen retention. Furthermore, the microstructure of hard carbon materials, such as interlayer spacing and specific surface area, is difficult to precisely control, further impacting the material's electrochemical performance.

[0004] Currently, to address the issues of low nitrogen retention and unstable carbon material performance during the preparation of nitrogen-doped hard carbon materials, relevant invention patents have been published. For example, patent CN 117638063 A discloses a method for preparing heteroatom-doped hard carbon anode materials for sodium-ion batteries. This method rapidly fixes the microstructure of carbon materials within minutes by burning carbon precursors in air. Based on the rich pore structure and heteroatom doping, the resulting hard carbon material exhibits excellent sodium storage performance. However, there is still room for optimization in the selection of the type and ratio of heteroatom dopants and structure modifiers, which in turn affects the electrochemical performance of the hard carbon anode material. Patent CN 116675211... A discloses a method for preparing nitrogen-doped porous carbon materials. The method uses glucose, sucrose, or N-acetyl-D-glucosamine as a carbon source, urea or N-acetyl-D-glucosamine as a nitrogen source, and potassium oxalate and nano-calcium carbonate as pore-forming agents. After high-temperature carbonization and acid treatment, nitrogen-doped porous carbon materials are obtained. However, the method still needs further optimization in terms of the ratio of carbon source to nitrogen source, the type and amount of pore-forming agent, etc., to improve the performance of nitrogen-doped porous carbon materials.

[0005] In summary, developing a simple, efficient, and environmentally friendly method to prepare nitrogen-doped hard carbon materials is of great significance. Furthermore, by controlling the microstructure of the materials, such as interlayer spacing and specific surface area, the electrochemical performance of the materials can be further optimized. However, how to achieve efficient nitrogen doping and precise control of the microstructure while maintaining the excellent physicochemical properties of hard carbon materials remains a technical challenge that urgently needs to be solved. Summary of the Invention

[0006] Therefore, the technical problem to be solved by the present invention is to overcome the problem that the nitrogen retention rate of hard carbon materials is generally low and the microstructure is difficult to control precisely during the nitrogen doping process, which leads to unsatisfactory electrochemical performance.

[0007] To address the aforementioned technical problems, this invention provides a nitrogen-doped hard carbon material and its preparation method.

[0008] The first objective of this invention is to provide a method for preparing nitrogen-doped hard carbon materials, comprising the following steps: S1. Urea is thoroughly mixed with a sugar-based carbon source and then heated to melt. After cooling, a eutectic salt is obtained. S2. Under a protective atmosphere, the eutectic salt described in S1 is prepolymerized, pyrolyzed, and carbonized to obtain the nitrogen-doped hard carbon material.

[0009] In one embodiment of the present invention, in S1, the glycosyl carbon source is selected from one or more of glucose, sucrose, fructose, sorbitol, and mannitol. The molecules of this type of glycosyl carbon source are rich in strongly polar functional groups that can act as hydrogen bond donors (-OH, -NH2, -COOH, etc.) or hydrogen bond acceptors (C=O, -O-, etc.). These functional groups can generate dense intermolecular interactions with -NH2 (which has both hydrogen bond donor and acceptor properties) and C=O (strong hydrogen bond acceptor) in urea molecules, thereby forming an entangled network structure during the heating and melting process. This restricts the movement of urea molecules, thereby inhibiting the decomposition of urea and increasing the nitrogen retention rate.

[0010] In one embodiment of the present invention, in S1, the mass ratio of urea to glycosyl carbon source is (31-39):(61-69). By controlling the content of glycosyl carbon source in the system, the carbon skeleton arrangement, nitrogen doping site distribution and pore structure formation can be precisely controlled, thereby achieving effective control of the interlayer spacing and specific surface area of ​​hard carbon materials. When the ratio of the two is appropriate, the carbon layer can form a moderately expanded interlayer spacing under the "support" of nitrogen atoms, which is conducive to the insertion and extraction of sodium ions, and ultimately optimizes the sodium storage performance and electrochemical stability of hard carbon materials. The mass ratio of glycosyl carbon source to urea has an unreliable linear relationship with the final material's microstructure and electrochemical performance. Instead, there is a critical threshold range: when the glycosyl carbon source content is below 61%, although the material has a high nitrogen content, excessive urea decomposition leads to a loose and porous carbon skeleton with too many defects. While it can provide a certain sodium storage capacity, it significantly reduces the initial coulombic efficiency and cycle stability. When the glycosyl carbon source content is above 69%, the carbon skeleton tends to be overly dense. At the same time, insufficient nitrogen doping leads to a significant reduction in sodium storage active sites, and the carbon interlayer spacing shrinks to a range unfavorable for rapid sodium ion transport, resulting in a precipitous drop in the material's specific capacity. Only by precisely controlling the glycosyl carbon source content within the range of 61-69% can the system simultaneously obtain a moderately expanded interlayer spacing, a moderate and stable specific surface area, and a sufficient number and reasonable distribution of pyridine nitrogen active sites, achieving a synergistic improvement in the material's comprehensive electrochemical performance. Moreover, this effect of simultaneous optimization of multiple indicators cannot be obtained through conventional ratio adjustment or linear derivation by orthogonal experiments.

[0011] In one embodiment of the present invention, in S1, the heating and melting temperature is 100℃-150℃, and the time is 2h-4h.

[0012] In one embodiment of the present invention, in S1, the cooling is cooling to 5°C-35°C.

[0013] In one embodiment of the present invention, in S2, the prepolymerization temperature is 150℃-300℃ and the time is 1h-2h. During this low-temperature prepolymerization stage, the glycosyl carbon source undergoes dehydration and condensation reactions to form a carbon-rich polymer skeleton, and urea begins to partially decompose simultaneously. Under the encapsulation of the hydrogen bond network and the polymer, some nitrogen elements remain in the system in the form of nitrogen-containing intermediates, laying the foundation for the subsequent nitrogen doping process.

[0014] In one embodiment of the present invention, in S2, the pyrolysis temperature is 400℃-600℃, and the time is 0.5h-1h; during this medium-temperature pyrolysis process, the carbon-rich polymer is further carbonized to form a carbon skeleton, and the decomposition products of urea (such as NH3, CN) are decomposed. - (etc.) react synchronously with the carbon framework, and nitrogen atoms are embedded in the carbon layer edge and defect sites in the form of pyridine nitrogen and pyrrole nitrogen, thereby achieving uniform doping of nitrogen element.

[0015] In one embodiment of the present invention, in S2, the carbonization temperature is 700℃-900℃ and the time is 1h-3h; this high-temperature carbonization process can promote the further graphitization of the carbon skeleton, making the overall structure of the material tend to be stable, while some unstable nitrogen in the system will be removed, and the nitrogen that is finally retained mainly exists in the form of graphitic nitrogen and stable pyridine nitrogen, thereby effectively enhancing the conductivity and structural stability of nitrogen-doped hard carbon materials.

[0016] In one embodiment of the present invention, in S2, the protective atmosphere is a nitrogen atmosphere.

[0017] A second objective of this invention is to provide a nitrogen-doped hard carbon material prepared by the method described above.

[0018] The technical solution of the present invention has the following advantages compared with the prior art: (1) The preparation method of the present invention first forms a eutectic salt with urea and a glycosyl carbon source. During this process, the strong polar functional group in the glycosyl carbon source forms a hydrogen bond network with the amino (-NH2) and carbonyl (C=O) groups in urea. This hydrogen bond network can form a structure similar to a "molecular cage" or "solvation shell", which can restrict the free movement of urea molecules and constrain their thermal decomposition path, thereby delaying and inhibiting the premature decomposition of urea. At the same time, the eutectic salt system will form a stable intermediate at low temperature, laying the structural foundation for the subsequent nitrogen doping process. In the subsequent carbonization process, nitrogen can be embedded in the carbon skeleton in a more stable form such as pyridine nitrogen and graphitic nitrogen, rather than escaping in a gaseous form, thereby significantly improving the nitrogen retention rate.

[0019] (2) The preparation method described in this invention adopts a three-stage heating program to achieve nitrogen layered doping. This segmented temperature control strategy constructs a layered doping mechanism of "first constructing a carbon skeleton, then introducing nitrogen atoms, and finally stabilizing the carbon skeleton structure", which effectively avoids the problem of nitrogen element loss and carbon material structure inhomogeneity caused by high-temperature one-step processing. At the same time, it overcomes the defects of traditional nitrogen doping methods, such as complex synthesis process and environmentally unfriendly preparation process. Detailed Implementation

[0020] As described in the background section, traditional hard carbon materials suffer from poor rate performance and cycle stability, making it difficult to meet the requirements for sodium-ion battery anode materials. Existing nitrogen doping modification methods generally suffer from complex synthesis processes and environmentally unfriendly preparation processes, and it is difficult to achieve efficient nitrogen doping, resulting in low nitrogen retention. At the same time, the microstructure parameters of hard carbon materials, such as interlayer spacing and specific surface area, are difficult to control precisely, further affecting the electrochemical performance of the materials.

[0021] To address the aforementioned technical problems, this invention provides a nitrogen-doped hard carbon material and its preparation method.

[0022] The first objective of this invention is to provide a method for preparing nitrogen-doped hard carbon materials, comprising the following steps: S1. Urea is thoroughly mixed with a sugar-based carbon source and then heated to melt. After cooling, a eutectic salt is obtained. S2. Under a protective atmosphere, the eutectic salt described in S1 is prepolymerized, pyrolyzed, and carbonized to obtain the nitrogen-doped hard carbon material.

[0023] In one embodiment of the present invention, in S1, the glycosyl carbon source is selected from one or more of glucose, sucrose, fructose, sorbitol and mannitol.

[0024] In one embodiment of the present invention, in S1, the mass ratio of urea to glycosyl carbon source is (31-39):(61-69); for example, it can be 31:69, 32:68, 33:67, 34:66, 35:65, 36:64, 37:63, 38:62, 39:61, etc.

[0025] In one embodiment of the present invention, in S1, the heating and melting temperature is 100℃-150℃, for example, it can be 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, 140℃, 145℃, 150℃, etc.; the time is 2h-4h, for example, it can be 2h, 2.5h, 3h, 3.5h, 4h, etc.

[0026] In one embodiment of the present invention, in S1, the cooling is cooling to 5°C-35°C.

[0027] In one embodiment of the present invention, in S2, the prepolymerization temperature is 150℃-300℃, for example, it can be 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃, 300℃, etc.; the time is 1h-2h, for example, it can be 1h, 1.5h, 2h, etc.

[0028] In one embodiment of the present invention, in S2, the pyrolysis temperature is 400℃-600℃, for example, it can be 400℃, 410℃, 420℃, 430℃, 440℃, 450℃, 460℃, 470℃, 480℃, 490℃, 500℃, 510℃, 520℃, 530℃, 540℃, 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, etc.; the time is 0.5h-1h, for example, it can be 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, 1h, etc.

[0029] In one embodiment of the present invention, in S2, the carbonization temperature is 700℃-900℃, for example, it can be 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, 790℃, 800℃, 810℃, 820℃, 830℃, 840℃, 850℃, 860℃, 870℃, 880℃, 890℃, 900℃, etc.; the time is 1h-3h, for example, it can be 1h, 1.5h, 2h, 2.5h, 3h, etc.

[0030] In one embodiment of the present invention, in S2, the protective atmosphere is a nitrogen atmosphere.

[0031] A second objective of this invention is to provide a nitrogen-doped hard carbon material prepared by the method described above.

[0032] The present invention will be further described below with reference to specific embodiments, so that those skilled in the art can better understand and implement the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. It should be understood that the specific embodiments are only used to explain the present invention, but the embodiments are not intended to limit the present invention.

[0033] In this invention, unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0034] In this invention, unless otherwise stated, the term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0035] In this invention, unless otherwise specified, the experimental methods used in the embodiments of this invention are conventional methods, and the materials and reagents used are commercially available unless otherwise specified. Example 1

[0036] The nitrogen-doped hard carbon material and its preparation method in this embodiment specifically include the following steps: S1. Preparation of eutectic salt: Weigh 60g of urea and 112g of glucose (glucose accounts for about 65% of the mass), mix them thoroughly, and melt them at 120℃ for 3 hours; after cooling to room temperature, pulverize them to 200 mesh to obtain eutectic salt; S2. Preparation of nitrogen-doped hard carbon materials: Under a nitrogen atmosphere, the eutectic salt is heated from room temperature to 200℃ and held for 2 hours to complete prepolymerization. The temperature is then increased to 450℃ and held for 1 hour to complete pyrolysis and achieve nitrogen doping. Finally, the temperature is increased to 800℃ and held for 2 hours to complete carbonization and stabilize the carbon skeleton structure, thus obtaining nitrogen-doped hard carbon materials. Comparative Example 1

[0037] It is basically the same as Example 1, except that the glucose content is about 45%. Comparative Example 2

[0038] It is basically the same as Example 1, except that the glucose content is about 50%. Comparative Example 3

[0039] It is basically the same as Example 1, except that the glucose content is about 55% by mass. Comparative Example 4

[0040] It is basically the same as Example 1, except that the glucose content is about 60%. Comparative Example 5

[0041] It is basically the same as Example 1, except that the glucose content is about 70%. Comparative Example 6

[0042] It is basically the same as Example 1, except that the glucose content is about 75% by mass. Comparative Example 7

[0043] The process is basically the same as in Example 1, except that the urea and glucose are not melted and are used directly as precursor powder. Comparative Example 8

[0044] It is basically the same as Example 1, except that prepolymerization is not performed. Comparative Example 9

[0045] It is basically the same as Example 1, except that pyrolysis is not performed. Comparative Example 10

[0046] It is basically the same as Example 1, except that prepolymerization and pyrolysis are not performed. Test Example 1

[0047] The nitrogen content, specific surface area, and interlayer spacing of the nitrogen-doped hard carbon materials prepared in the examples and comparative examples were tested: (1) Nitrogen content test: X-ray photoelectron spectroscopy (XPS) was used to detect the material, and the mass fraction of nitrogen was quantitatively calculated by peak fitting of the N 1s spectrum peak; (2) Specific surface area test: The specific surface area of ​​the material was determined by the BET specific surface adsorption method; (3) Interlayer spacing test: The material was tested by X-ray diffraction (XRD) and the carbon interlayer spacing of the material was obtained by refining the XRD pattern. Table 1 shows the relevant test results: Table 1

[0048] As shown in Table 1, with the gradual increase in the glucose content, the nitrogen content, specific surface area, and interlayer spacing of the material all showed a decreasing trend. During the process of increasing the glucose content from 45% to 75%, the nitrogen content decreased from 11.5 wt% to 7.8 wt%, and the specific surface area increased from 78 m² / s² to 78 m² / s². 2 / g decreased to 38m 2 / g, the interlayer spacing decreased from 0.385nm to 0.355nm.

[0049] Comparing Example 1 and Comparative Examples 1-4, it can be seen that as the glucose content gradually increases from 45% to 65%, the glucose content in the system increases, the hydrogen bond network formed with urea becomes more compact, the inhibitory effect on the thermal decomposition of urea molecules gradually strengthens, and the loss of nitrogen tends to stabilize, so the decrease in nitrogen content gradually slows down; at the same time, the carbon skeleton becomes more complete, the pore structure is optimized, the number of micropores and defects decreases, and the specific surface area continues to decrease and tends to be moderate; while the "supporting" effect of nitrogen atoms on the carbon layer gradually becomes more reasonable as the nitrogen content decreases, the orderliness of the carbon layer arrangement increases, and the decrease in interlayer spacing gradually narrows, finally reaching the ideal interlayer spacing value at the glucose content of Example 1.

[0050] Comparing Example 1 and Comparative Examples 5-6, it can be seen that when the glucose content exceeds 65% and continues to increase to 70% and 75%, the relative proportion of urea in the system decreases significantly. Although the constraint effect of the hydrogen bond network on urea still exists, the reduction of nitrogen source directly leads to a further decrease in nitrogen doping, and the "supporting" effect of nitrogen atoms on the carbon layer continues to weaken. At the same time, after excessive glucose carbonizes, it forms a highly dense carbon skeleton, which fills some of the pores inside the material, further reducing the specific surface area. The order of the carbon skeleton also continues to increase due to insufficient nitrogen doping, and the carbon layers are arranged more tightly, ultimately causing a further reduction in the interlayer spacing, and the rate of decrease is more obvious than that in the comparison stage of Example 1 and Comparative Examples 1-4. Test Example 2

[0051] (1) Battery assembly: Negative electrode: The negative electrode active material is hard carbon material prepared in the examples and comparative examples, the conductive agent is acetylene black, the binder is polyvinylidene fluoride, and the negative electrode current collector is 6μm aluminum foil. The negative electrode active material, conductive agent, and binder are mixed at a mass ratio of 8:1:1, and N-methylpyrrolidone is added and stirred to form a uniform and stable negative electrode slurry. The negative electrode slurry is uniformly coated onto the surface of the negative electrode current collector using a 200μm doctor blade coating. After drying and cold pressing, a mass loading of approximately 2.5 mg / cm³ is obtained. 2 The negative electrode.

[0052] Counter electrode: Sodium metal sheet.

[0053] Separating membrane: Polyethylene film, 9μm thick.

[0054] Electrolyte: Sodium hexafluorophosphate is dissolved in polycarbonate to prepare an electrolyte with a concentration of 1 mol / L.

[0055] Sodium-ion battery assembly: Arrange the negative electrode, separator, counter electrode, and separator in sequence to assemble a CR2032 coin cell sodium-ion battery.

[0056] (2) Performance testing: First coulombic efficiency test: The prepared CR2032 coin cell sodium-ion battery to be formed was placed at 60℃ for 40 min, then discharged at a rate of 0.1C to the lower limit of activation voltage (0V), and then charged to the upper limit of activation voltage (2.2V). The first coulombic efficiency was measured. The first coulombic efficiency = the ratio of the first discharge capacity to the first charge capacity. Specific capacity test: The sodium-ion battery was charged / discharged at a rate of 0.1C within a voltage range of 0V-2.2V, according to the formula C=Q. D / M is used to calculate the specific capacity of the active material, where Q D M represents the discharge capacity, and M represents the mass of the active material. Cyclic stability testing was performed by first calibrating the discharge capacity at 1C and recording it as C0. Then, 1C charge-discharge (0-2.2V) was used, and the discharge capacity C0 was recorded after the 500th cycle. 500 Capacity retention ratio R=C 500 / C0*100% is used to evaluate the cycle stability of sodium-ion batteries after 500 cycles; Table 2 shows the relevant test results: Table 2

[0057] As can be seen from Table 2, with the gradual increase of the glucose mass ratio, the specific capacity of the material shows a trend of first increasing and then decreasing. The initial coulombic efficiency and the capacity retention rate after 500 cycles both show an increasing trend. The changes in various electrochemical performance indicators are highly correlated with the glucose ratio and the microstructure parameters of the material (nitrogen content, specific surface area, interlayer spacing).

[0058] Comparing Example 1 and Comparative Examples 1-4, it can be seen that as the glucose content gradually increases from 45% to 65%, the microstructure of the material is continuously optimized, the carbon skeleton becomes more complete, the pore structure is reasonably controlled, the interlayer spacing and specific surface area tend to be moderate, and the doping sites of nitrogen atoms are more uniformly distributed, providing better structural conditions for the insertion and extraction of sodium ions. Therefore, the specific capacity continues to increase and reaches a peak of 362 mAh / g with the glucose content in Example 1. At the same time, as the specific surface area gradually decreases, the side reactions between the electrode and the electrolyte decrease, and the initial coulombic efficiency and cycle stability also gradually improve. The overall electrochemical performance of the system develops towards a better direction.

[0059] Comparing Example 1 and Comparative Examples 5-6, it can be seen that when the glucose content exceeds 65% and continues to increase to 75%, the nitrogen source content in the system decreases significantly, the nitrogen doping amount continues to decrease, the sodium storage active sites provided by nitrogen atoms decrease, and the carbon skeleton becomes highly dense due to excessive glucose, further reducing the interlayer spacing, which is not conducive to the insertion and transport of sodium ions. Therefore, the specific capacity of the material decreases significantly. However, the dense carbon skeleton formed after the carbonization of excessive glucose further improves the structural stability of the material, and the continuous reduction in specific surface area further suppresses the electrode side reactions, so that the initial coulombic efficiency and the capacity retention rate after 500 cycles still show an upward trend. Although the cycle stability is excellent, the sodium storage specific capacity of the material is sacrificed, and the overall electrochemical performance decreases.

[0060] As can be seen from Tables 1 and 2, when the glucose content increases from 60% to 65%, although the nitrogen content of the material decreases slightly from 9.4 wt% to 9.2 wt%, the specific capacity does not decrease accordingly. Instead, it increases slightly from 355 mAh / g to 362 mAh / g. At the same time, the coulombic efficiency and cycle stability continue to increase for the first time. This indicates that within this range, the synergistic effect of nitrogen doping efficiency and carbon framework order reaches its optimal level. The "supporting" effect of nitrogen atoms on the carbon layer and the regular arrangement of the carbon layer form the most favorable microenvironment for sodium ion insertion, extraction, and transport. At this time, the interlayer spacing, specific surface area, and nitrogen doping site distribution of the material are all in an ideal state suitable for sodium storage. Even if the nitrogen content decreases slightly, the optimized microstructure can compensate for the slight changes in active sites and achieve the improvement of electrochemical performance. However, when the glucose content increased only from 65% to 70%, the material's specific capacity showed a significant non-linear decrease, plummeting from 362 mAh / g to 340 mAh / g, a drop of 22 mAh / g. This decrease far exceeded the cumulative increase of 14 mAh / g in specific capacity during the process of increasing the glucose content from 55% to 65%. This abrupt change proves that there is a critical point for material structure evolution near the 65% glucose content. Once this critical point is crossed, the urea content in the system decreases significantly, leading to a further decrease in nitrogen content to 8.5 wt%. The sodium storage active sites provided by nitrogen atoms are greatly reduced, and the "supporting" effect of nitrogen atoms on the carbon layer continues to weaken. The carbon interlayer spacing shrinks to 0.365 nm, and the excessively dense carbon skeleton formed after the carbonization of excess glucose fills some of the internal pores of the material, reducing the specific surface area to 45 m². 2 / g, at this point, the reduction of active sites and the shrinkage effect of interlayer spacing caused by insufficient nitrogen content begin to dominate. The excessive densification of the carbon skeleton not only fails to provide favorable conditions for sodium ion storage, but also hinders the rapid transport of sodium ions. Even if the initial coulombic efficiency and cycle stability are slightly improved due to structural densification, the core sodium storage performance of the material still deteriorates rapidly, and the overall electrochemical performance drops significantly. Test Example 3

[0061] Based on Test Example 2, the electrical performance of CR2032 coin-type sodium-ion batteries prepared from hard carbon materials of Comparative Examples 7-10 was tested, and the results are shown in Table 3: Table 3

[0062] As can be seen from Table 3, the specific capacity, initial coulombic efficiency and capacity retention rate after 500 cycles of Example 1 are the best among all samples, which fully verifies the technical advantages of the eutectic salt pretreatment combined with the three-stage heating process.

[0063] Comparing Example 1 and Comparative Example 7, it can be seen that Comparative Example 7 did not involve melting urea and glucose to form a eutectic salt; it only performed simple physical mixing. This failed to allow the glycosyl carbon source to form a hydrogen bond network with urea, thus failing to restrict the thermal decomposition pathway of urea molecules. During the heating process, urea decomposed extensively at low temperatures, with nitrogen escaping in gaseous form, resulting in a significant decrease in nitrogen retention and a sharp reduction in sodium storage active sites. Simultaneously, the uneven mixing of the carbon and nitrogen sources led to a loose, porous carbon skeleton with excessive defects after carbonization, resulting in an abnormally large specific surface area and exacerbating side reactions between the electrode and the electrolyte. Ultimately, its performance was far inferior to that of Example 1. This demonstrates that the hydrogen bond network formed by the eutectic salt pretreatment is crucial for achieving nitrogen locking and optimizing the carbon skeleton structure, and is also a fundamental prerequisite for improving the overall electrochemical performance of the material.

[0064] Comparing Example 1 and Comparative Example 8, it can be seen that Comparative Example 8 omits the prepolymerization stage and directly enters the pyrolysis stage. Glucose fails to fully complete the dehydration and condensation reactions at low temperatures, thus failing to form a structurally complete carbon-rich polymer framework. During subsequent pyrolysis and carbonization, the carbon framework remains loose and has numerous internal defects, resulting in uneven distribution of nitrogen doping sites and causing some nitrogen atoms to be intercalated in an unstable form, making it difficult to form effective sodium storage active sites. Compared to Example 1, its specific capacity, initial coulombic efficiency, and cycle stability are all significantly reduced, indicating that the prepolymerization stage is a necessary and indispensable step in constructing a stable carbon framework and providing structural support for subsequent uniform nitrogen doping.

[0065] Comparing Example 1 and Comparative Example 9, it can be seen that Comparative Example 9 omits the pyrolysis stage, allowing the prepolymerized carbon skeleton to directly enter the high-temperature carbonization process. The reaction between urea decomposition products and the carbon skeleton at intermediate temperatures is omitted. Nitrogen atoms lack sufficient conditions to embed into the carbon layer edges and defect sites, causing urea decomposition products to rapidly escape in gaseous form. This significantly reduces nitrogen doping efficiency and results in a substantial shortage of effective sodium storage active sites. Although the basic carbon skeleton formed in the prepolymerization stage slightly outperforms Comparative Example 8, the specific capacity is still lower, and the initial coulombic efficiency and cycle stability are also lower than in Example 1. This demonstrates that intermediate-temperature pyrolysis is a key step in achieving efficient and uniform nitrogen doping and improving the core performance of sodium storage.

[0066] Comparing Example 1 and Comparative Example 10, it can be seen that Comparative Example 10 eliminated both the prepolymerization and pyrolysis stages, directly carbonizing the eutectic salt at high temperature. On the one hand, the absence of a prepolymerization process resulted in the carbon skeleton lacking any structural guidance, leading to a chaotic arrangement and dense defects after molding. On the other hand, the absence of a pyrolysis stage caused uncontrolled urea decomposition, resulting in a significant loss of nitrogen and near-ineffective nitrogen doping. Simultaneously, the one-step high-temperature carbonization made the interlayer spacing and specific surface area of ​​the carbon material completely uncontrollable, failing to provide favorable conditions for sodium ion transport and storage. Ultimately, its performance was the worst among all samples. This result highlights the overall synergistic effect of the three-stage heating process in Example 1, where the prepolymerization, pyrolysis, and carbonization stages are progressive, mutually supportive, and indispensable.

[0067] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a nitrogen-doped hard carbon material, characterized in that, Includes the following steps: S1. Urea is thoroughly mixed with a sugar-based carbon source and then heated to melt. After cooling, a eutectic salt is obtained. S2. Under a protective atmosphere, the eutectic salt described in S1 is prepolymerized, pyrolyzed, and carbonized to obtain the nitrogen-doped hard carbon material.

2. The method for preparing nitrogen-doped hard carbon material according to claim 1, characterized in that, In S1, the glycosyl carbon source is selected from one or more of glucose, sucrose, fructose, sorbitol and mannitol.

3. The method for preparing nitrogen-doped hard carbon material according to claim 1, characterized in that, In S1, the mass ratio of urea to glycosyl carbon source is (31-39):(61-69).

4. The method for preparing nitrogen-doped hard carbon material according to claim 1, characterized in that, In S1, the heating and melting temperature is 100℃-150℃, and the time is 2h-4h.

5. The method for preparing nitrogen-doped hard carbon material according to claim 1, characterized in that, In S1, the cooling is cooling to 5°C-35°C.

6. The method for preparing nitrogen-doped hard carbon material according to claim 1, characterized in that, In S2, the prepolymerization temperature is 150℃-300℃ and the time is 1h-2h.

7. The method for preparing nitrogen-doped hard carbon material according to claim 1, characterized in that, In S2, the pyrolysis temperature is 400℃-600℃ and the time is 0.5h-1h.

8. The method for preparing nitrogen-doped hard carbon material according to claim 1, characterized in that, In S2, the carbonization temperature is 700℃-900℃ and the time is 1h-3h.

9. The method for preparing nitrogen-doped hard carbon material according to claim 1, characterized in that, In S2, the protective atmosphere is a nitrogen atmosphere.

10. A nitrogen-doped hard carbon material prepared by the method according to any one of claims 1-9.

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