Preparation method of efficient sodium storage material based on high pseudo-graphite domain nitrogen-doped porous network structure hard carbon

Abstract

The invention provides a preparation method of a high-efficiency sodium storage material based on high-pseudo graphite domain nitrogen-doped porous network structure hard carbon, and belongs to the technical field of sodium ion battery negative electrode material preparation. The invention aims at forming micro-area structures with different order degrees in a nitrogen atom doping induction carbon network and optimizing the content of a pseudo-graphite area, so that the storage capacity and the cycling stability of sodium ions are improved. The nitrogen-doped hard carbon material is prepared by taking polyvinylpyrrolidone and melamine as precursors through a two-step high-temperature carbonization method. The introduction of nitrogen atoms can effectively induce formation of a pseudo-graphite region, a highly disordered region and a graphite-like region, and the pseudo-graphite region has a suitable interlayer spacing (0.36-0.40 nm) and a rich closed-pore structure, which is beneficial to rapid embedding and storage of sodium ions. The prepared hard carbon material still keeps the reversible capacity of 309mAh g <-1 > after 140 times of circulation under the current density of 0.03 A g <-1 >, the capacity retention rate exceeds 90% after 2000 times of circulation under the current density of 1A g <-1 >, and the hard carbon material shows excellent electrochemical performance and structural stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium ion battery electrode material preparation, and particularly relates to a preparation method of high-efficiency sodium storage material based on high pseudo-graphitic domain nitrogen-doped porous network structure hard carbon. BACKGROUND

[0002] With the increasing demand for renewable energy worldwide and the highlighting of the scarcity and cost of lithium resources, sodium ion batteries have become one of the important alternative energy storage technologies for lithium ion batteries due to the advantages of abundant sodium resources and low cost. However, sodium ions have a larger ionic radius and slower diffusion kinetics, which puts higher requirements on the design of negative electrode materials. Commercial graphite negative electrodes have poor sodium storage performance and are difficult to meet the practical application requirements of sodium ion batteries, so the development of negative electrode materials with high reversible capacity and excellent cycle stability has become the key to promoting the commercial application of sodium ion batteries.

[0003] Among the many negative electrode materials, hard carbon materials have a larger interlayer spacing than traditional graphite and an open pore structure, which is beneficial to the rapid insertion / extraction of sodium ions and cycle stability, and shows good commercial application prospects. Hard carbon materials can be prepared from various precursors, such as biomass (e.g., lignin, glucose), polymers (e.g., phenol formaldehyde resin, polyacrylonitrile), and pitch or tar products (e.g., coal tar, petroleum pitch). Such materials usually have an expanded interlayer spacing (>0.37 nm), which can achieve efficient insertion and extraction of sodium ions, while also having the advantages of low cost and wide source of raw materials, so it has become a research hotspot.

[0004] The dispersed regions in hard carbon mainly include three parts: (i) highly disordered regions (interlayer spacing >0.40 nm), which store sodium ions through adsorption mechanism and are related to the slope region in the charge-discharge curve; (ii) pseudo-graphitic regions (interlayer spacing between 0.36-0.40 nm), which store sodium ions through intercalation and closed pore filling, corresponding to the platform region; (iii) graphite-like regions (interlayer spacing <0.36 nm), which are not conducive to sodium ion insertion. Therefore, the suitable interlayer spacing and large closed pore volume of the pseudo-graphitic region are crucial to improve the platform capacity. Currently, the preparation of hard carbon negative electrodes with rich pseudo-graphitic microdomains, nanopores, and adjustable interlayer spacing has become a research focus.

[0005] However, the electrochemical performance of hard carbon materials, such as reversible specific capacity and the first cycle coulombic efficiency, still cannot meet the requirements of practical applications. To overcome these limitations, foreign atom doping (such as nitrogen, oxygen, sulfur) is widely studied as an effective modification strategy. In recent years, hard carbon materials with rich defects and expanded interlayer spacing have been designed through the "doping-escape" strategy, solving the trade-off problem between capacity and first cycle efficiency in traditional modification methods. However, most existing literature focuses on the role of nitrogen atoms in improving electronic conductivity or creating additional active sites, and the mechanism of regulating the microstructure of carbon materials is not clear, which may have a decisive impact on sodium storage performance. SUMMARY

[0006] The purpose of the present application is to improve the electrochemical performance by designing the structure of sodium ion battery anode materials, precisely controlling the proportion of three types of regions during preparation, increasing the content of pseudo-graphite domains, and coordinating the relationship between defects and pore structure. Thus, the technical problems of low reversible capacity and poor cycle stability caused by insufficient content of pseudo-graphite regions, improper regulation of interlayer spacing, and imbalance of the proportion of three types of microregions (highly disordered region, pseudo-graphite region, and graphite-like region) in the prior art of nitrogen-doped hard carbon anode materials are solved.

[0007] To achieve the above-mentioned purpose, the technical solutions of the present application are as follows:

[0008] A preparation method of a high-efficiency sodium storage material based on a high-pseudo-graphite-domain nitrogen-doped porous network structure hard carbon, characterized in that it comprises the following steps:

[0009] Step 1: 0.9-1g of polyvinylpyrrolidone is added to a mortar and ground;

[0010] Step 2: 0.5-3g of melamine is added to the material obtained in step (1), and then ground to obtain a uniform mixed precursor;

[0011] Step 3: The precursor is calcined in a tube furnace under an argon atmosphere at a heating rate of 3℃ / min to 500-700 degrees for 2 hours, and then continues to be calcined at a heating rate of 3℃ / min to 1100-1300℃ for 2 hours, and after cooling, a nitrogen-doped hard carbon with a porous network structure is obtained.

[0012] In the above technical solution, in step 3, the temperature is raised to 600 degrees at a rate of 3℃ / min for 2 hours, and then the temperature is raised to 1200℃ at a rate of 3℃ / min for 2 hours.

[0013] In the above technical solution, the polyvinylpyrrolidone in step 1 is 1g.

[0014] In the above technical solution, the molecular weight of the polyvinylpyrrolidone in step 1 is K30.

[0015] In the above technical solution, the melamine in step 2 is 1.5 g.

[0016] In the above technical solution, the volume ratio of the pseudo-graphite region in the nitrogen-doped hard carbon material is higher than that of the graphite-like region and the highly disordered region, and the (002) interlayer spacing d002 thereof is 0.36-0.40 nm.

[0017] The application also provides a sodium ion battery, wherein the high-efficiency sodium storage material serves as a negative electrode.

[0018] As described above, due to the adoption of the above technical solution, the application has the following beneficial effects:

[0019] 1. After adding melamine, the g-C3N4 is formed in the carbonization process at 600 DEG C through a two-step pyrolysis method, which promotes the growth of the layered structure, and the subsequent high-temperature annealing decomposes the g-C3N4 into gas, thereby forming more porous structures in the preparation process, increasing the graphene sheet structure and the bending degree of the carbon layer structure, and being more conducive to the growth of the pseudo-graphite region, and the pseudo-graphite region has a suitable interlayer spacing and rich closed pore structure, which provides an ideal channel for the rapid embedding and extraction of sodium ions, and significantly improves the reversible specific capacity and rate performance of the material.

[0020] 2. In the application, the appropriate amount of melamine is doped to induce the formation of more short-range disordered structures, increase the interlayer spacing of the hard carbon material, and promote the formation of the pseudo-graphitic region, thereby controlling the overall disorder degree of the material, limiting the volume expansion of the hard carbon material, reducing the stress in the charge and discharge cycle, and effectively improving the cycle stability of the material.

[0021] 3. There are also disclosures of nitrogen-containing hard carbon in the prior art, which mostly focus on using the active sites provided by nitrogen atoms, improving the conductivity, or generally expanding the interlayer spacing. The application quantitatively affects the formation and proportion of different ordered microzones in the hard carbon by different nitrogen doping, thereby synergistically optimizing the sodium ion storage performance. By controlling the amount of melamine, the optimized sample (HC-1.5) has lower charge transfer resistance and higher sodium ion diffusion coefficient, which significantly improves the electrochemical performance at high rates. The introduction of pyridine nitrogen and pyrrole nitrogen can destroy the regularity of the graphene plane, introduce local stress and distortion, cause the carbon layer to bend, prevent it from transforming into a highly graphitized structure, and thereby promote the formation of the "pseudo-graphite region" and the "highly disordered region". BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 The electron microscope image of the sodium ion battery electrode material of the application;

[0023] Figure 2X-ray diffraction pattern of the sodium ion battery electrode material of the present application;

[0024] Figure 3 Physical characterization chart of the sodium ion battery electrode material of the present application;

[0025] Figure 4 Electrochemical performance test chart of the sodium ion battery electrode material of the present application;

[0026] Figure 5 Cyclic voltammetry test chart, constant current intermittent titration technique and corresponding fitting analysis of the sodium ion battery electrode material of the present application;

[0027] Figure 6 Electrochemical performance chart of the sodium ion battery electrode material of the present application combined with NVPF (hard carbon) to assemble a full battery. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is described in further detail below in combination with embodiments and drawings.

[0029] A preparation method of a high-safety sodium ion battery material, comprising the following steps:

[0030] Example 1

[0031] Step 1: 1g of polyvinylpyrrolidone was added to a mortar and ground for 5 minutes;

[0032] Step 2: 1.5g of melamine was added to the above container and ground for 20 minutes, and mixed uniformly;

[0033] Step 3: calcination at 600 degrees under argon atmosphere in a tube furnace for 2 hours, with a heating rate of 3℃ / min, and then calcination at 1200℃ with a heating rate of 3℃ / min, and finally nitrogen-doped hard carbon HC-1.5 with a porous network structure was obtained.

[0034] Example 2

[0035] Step 1: 1g of polyvinylpyrrolidone was added to a mortar and ground for 5 minutes;

[0036] Step 2: calcination at 600 degrees under argon atmosphere in a tube furnace for 2 hours, with a heating rate of 3℃ / min, and then calcination at 1200℃ with a heating rate of 3℃ / min, and finally hard carbon HC-0 was obtained.

[0037] Example 3

[0038] In example 1, the melamine in step 2 was adjusted to 0.5g, and the others remained unchanged, and finally nitrogen-doped hard carbon (HC-0.5) was obtained.

[0039] Example 4

[0040] The melamine in step 2 in Example 1 was adjusted to 1 g, and the rest was the same, and finally a porous structure nitrogen-doped hard carbon (HC-1) was obtained.

[0041] Example 5

[0042] The melamine in step 2 in Example 1 was adjusted to 3 g, and the rest was the same, and finally a porous structure nitrogen-doped hard carbon (HC-3) was obtained.

[0043] The appropriate amount of melamine can induce the formation of hard carbon with high pseudo-graphitic microdomains, and excessive melamine can destroy the three-dimensional network structure and reduce the interlayer spacing, and too little will form less pore structure, and less pseudo-graphitic area will lead to poor performance, and 1.5 g of content has the best performance.

[0044] Figure 1 For the electron microscope images of Example 1, etc., a stable nitrogen-containing hard carbon (HC-1.5) with a three-dimensional network structure was prepared by a two-step pyrolysis method using polyvinylpyrrolidone (PVP) and melamine as precursors. First, it was pretreated at 600°C for 2 hours under an argon atmosphere, and then heated to 1200°C to obtain hard carbon with different microstructure content (such as Figure 1 The scanning electron microscope (SEM) images (b and c in Figure 1 ) show that HC-1.5 has an interconnected pore network, and the transmission electron microscope (TEM) image (d in Figure 1 ) confirms the uniform distribution of the porous structure. High-resolution TEM (e and f in Figure 1 ) further reveals the different ordered carbon layer arrangements in HC-1.5, showing obvious graphitic, highly disordered, and a small amount of graphite-like regions. The graphitic region is characterized by short and curved carbon layers, forming additional nanopores (d-spacing ≈ 0.387 nm). The highly disordered region presents more randomly oriented carbon layers, while the graphite-like region has stacked carbon layers with minimal curvature and a d-spacing of ≈ 0.357 nm. Figure 1 The inset of (e) shows the SAED image of the region, which presents a diffuse diffraction ring, indicating a high degree of disorder and a low degree of graphitization in the structure. The elemental distribution (g in Figure 1 ) of HC-1.5 shows that nitrogen (N) and oxygen (O) are uniformly distributed in the carbon skeleton, clearly verifying the successful incorporation of nitrogen elements.

[0045] Figure 2 For the physical characterization data of the material of the present application, Figure 2In the figure, 'a' represents the X-ray electron diffraction (XRD) patterns of five implementation cases. The (002) and (100) crystal planes produce characteristic reflections at 24° and 43°, respectively. With increasing melamine loading, the diffraction intensity of (002) systematically decreases, indicating damage to the graphite crystal structure. This is attributed to stacking defects induced by nitrogen dopant. We analyzed empirical parameters such as the R-factor, average interlayer spacing (d002), longitudinal dimension (La), average stacking distance (Lc), and number of parallel stacked layers (Np). Figure 2 As observed in b, the HC-1.5 sample exhibits the largest d002 and La values, but the smallest Lc, R factor, and Np values, indicating the presence of more pseudo-graphite domains, thereby enhancing Na+ transport and intercalation. Among these parameters, interlayer spacing determines the applicability of sodium ion transport channels; therefore, d002 is an important parameter for characterizing the HC microstructure. Consequently, we divided the interlayer spacing into three regions based on the following size ranges: highly disordered region (interlayer spacing > 0.40 nm), pseudo-graphite region (interlayer spacing 0.36 nm–0.40 nm), and graphite-like region (interlayer spacing < 0.36 nm). This is consistent with the structure observed in the transmission electron microscopy images, and the proportions of each region are as follows: Figure 2 As shown in c in Figure 2 and d in Figure 3, HC-0 is mainly dominated by graphite-like structures; in HC-0.5 and HC-1, the graphite-like structures gradually decrease, while highly disordered and graphite-like structures gradually increase; while HC-1.5 has the fewest graphite-like regions but the most graphite-like regions. In the HC-1.5 sample, the incorporation of an appropriate amount of melamine leads to nitrogen atom doping, inducing the formation of more short-range disordered structures, promoting the formation of pseudo-graphitized regions, and thus regulating the overall disorder of the material.

[0046] To further characterize the microstructure of the implementation cases, Raman spectroscopy tests were performed on all implementation cases. Figure 3 Two characteristic peaks were observed in band a: band D (1348 cm⁻¹). -1 ) and G-band (1578cm) -1 These correspond to defect-induced disordered and regular graphite structures in carbon materials, respectively. With increasing melamine content, the intensity ratio (ID / IG) gradually increases, reaching 0.969, 1.030, 1.092, 1.123, and 1.167 for HC-0, HC-0.5, HC-1, HC-1.5, and HC-3 samples, respectively. This indicates that nitrogen atoms were successfully introduced into the material and formed pseudo-graphite regions, creating a more suitable microstructural environment for sodium ion storage. Furthermore, Figure 3In Example 1 (b), the high-resolution N 1s peak was decomposed into peaks at 398.8, 401.3, 402.9, and 405.1 eV. The unique structures of pyridine nitrogen and pyrrole nitrogen disrupt the regular plane of graphene, introducing local stress and distortion, leading to increased bending or structural disorder of the carbon layer, which may result in the formation of more pseudo-graphite regions. The C 1s region of HC-1.5 showed five distinct carbon states: OC=O (291.1 eV), C=O (288.7 eV), C=N (286.3 eV), CN (285.2 eV), and C=C (284.8 eV). These bonds are attributed to the formed porous microstructure.

[0047] To demonstrate the improved electrochemical performance of HC-1.5 synthesized in Example 1 compared to Examples 2 (HC-0), 3 (HC-0.5), 4 (HC-1), and 5 (HC-3), half-cells were assembled using sodium metal as the counter electrode, and the electrochemical performance of the five examples was tested. Figure 4 'a' in the figure shows different implementation cases at 0.1mV s -1 The initial cyclic voltammetry curves under the given conditions show a significant redox peak in the 0-0.16 V range, indicating a reversible sodium ion intercalation / deintercalation process in the non-porous graphitic carbon matrix. The redox peak of the HC-1.5 sample exhibits the highest current density, indicating a more pronounced electrochemical reaction and thus a higher reversible capacity. An irreversible cathode peak appears in the 0.3-0.6 V range during the initial anodic scan, indicating electrolyte decomposition and the formation of a solid electrolyte interphase (SEI) layer. Figure 4 Figure b shows the discharge / charge curve of the initial cycle. The HC-1.5 sample exhibits a capacity of 299.7 mAh g / L. -1 The reversible capacity is significantly higher than that of HC-0 (167.9 mAh g). -1 ), HC-0.5 (218.0mAh g) -1 ), HC-1 (258.4mAh g) -1 ) and HC-3 (258.9mAh g) -1 The superior performance of HC-1.5 stems from its high content of pseudo-graphite microregions and rich microstructure. These microstructures inhibit the long-range ordered stacking of carbon layers and promote the formation of curved closed pores, thus facilitating the sodium ion storage process. Figure 4 The 'c' quantifies the capacity contribution of each region during the second discharge cycle. It is evident that HC-1.5 exhibits excellent plateau region capacity (155.9 mAh g⁻¹). -1 ) and slope capacity (150.1 mAh g) -1The results showed that the sodium ion concentration was significantly higher than that of other samples. This may be attributed to the increased closed-pore volume and interlayer spacing, which created more pseudo-graphite regions that provided suitable pathways for sodium ion transport, thereby promoting the diffusion and storage of sodium ions. Figure 4 The d in the figure compares different implementation cases at 0.03A g. -1 Cyclic stability under certain conditions. It is evident that HC-1.5 exhibits excellent cycling performance: at 0.03A g... -1 It still maintains 309 mAh g after 140 cycles under the conditions. -1 The capacity (compared to the initial value of 306 mAh g) -1 The capacity retention rate reached 101.1%, which was significantly better than HC-0 (83.4%), HC-0.5 (88.3%), HC-1 (92.1%) and HC-3 (88.5%). This clearly indicates that the more disordered microstructure and more closed pores in the pseudo-graphite region of HC-1.5 are conducive to sodium ion storage. Figure 4 The comparison of rate performance revealed significant differences between the samples, with HC-1.5 exhibiting the best rate performance. Specifically, HC-1.5 showed the best rate performance from 0.03 to 2.0 A g. -1 At current densities, the values ​​were 310, 269, 261, 252, 242, 230, and 212 mAh g. -1 The reversible capacity of these materials is higher than that of other carbon-based anode materials reported in the literature. Figure 4 f in the middle. Figure 4 As shown in g, HC-1.5 exhibits excellent long-term cycling stability. At 1A g... -1 After 2000 cycles under the specified conditions, its capacity retention reached 90% (initial capacity 215.3 mAh g). -1 The coulombic efficiency is close to 100%, significantly better than other control samples: HC-3 (193.9 mAh g). -1 ), HC-1 (186.5mAh g) -1 ), HC-0.5 (162.5mAh g) -1 ) and HC-0 (103.3mAh g) -1 ).

[0048] Subsequently, cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic titration techniques, along with their fitting analysis, were performed on the five example samples (HC-0, HC-0.5, HC-1, HC-1.5, and HC-3). Figure 5 ). Figure 5 Figure 'a' shows different scan rates (0.1, 0.2, 0.4, 0.6, and 0.8 mV s). -1The cyclic voltammetry curves of HC-1.5 under varying scan rates show that the shape of the curves remains unchanged while the peak current gradually increases, indicating that this electrode material possesses excellent rate performance. The contribution of the capacitance effect can be quantitatively described by the following equation:

[0049] i(v)=k1v+k2v 1 / 2

[0050] Where k1v represents the capacitor-dominated behavior, k2v 1 / 2 This corresponds to the diffusion-limited process. By fitting data at different voltages, the capacitance contribution rate at each scan rate can be calculated (e.g., ...). Figure 5 (As shown in b). The capacitance contribution of HC-1.5 is 62.8% (0.1mV s). -1 The percentage increased to 82.8% (0.8 mV s). -1 ()( Figure 5 c) indicates that the capacitive process is dominated by a scan rate dependence over the diffusion-controlled process. In contrast, the capacitive contribution of HC-0 only increased from 20.9% to 38.1%, indicating that it is still dominated by a diffusion-controlled process. To further investigate the electrochemical reaction kinetics of HC-1.5, Nyquist plots were obtained by electrochemical impedance spectroscopy (EIS) (see c). Figure 5 (d) The semicircle in the high-frequency region reflects the charge transfer resistance (R). ct ) and internal resistance (R) s The slope in the low-frequency region characterizes Na. + Diffusion kinetics. R of HC-1.5 ct The value was only 1.68 Ω, significantly lower than HC-0 (9.73 Ω), HC-0.5 (7.07 Ω), HC-1 (2.34 Ω), and HC-3 (2.78 Ω). HC-1.5 showed a significantly lower R value compared to HC-0. ct The values ​​indicate that the interfacial charge transfer process on the former electrode surface is significantly enhanced compared to the latter. Furthermore, the R of HC-1.5... s The value (4.285Ω) is also lower than that of HC-0 (24.81Ω), indicating that it has higher conductivity, which has a positive impact on the aforementioned rate performance.

[0051] Figure 5 The superior kinetics of HC-1.5 were further verified using girders-intermittent titration (GITT). HC-1.5 exhibited a high discharge slope of 5.01 × 10⁻⁶ ppm. -10 cm 2 s -1 The Na+ diffusion coefficient decreases slightly to approximately 0.891 × 10⁻⁶ in the plateau region. -10 cm 2 s -1However, it is still significantly higher than other electrodes. Overall, HC-1.5 consistently exhibits a higher diffusion coefficient during cycling, which is mainly attributed to its optimized pseudo-graphitized region content, which provides an appropriate degree of disorder for sodium ion storage.

[0052] Finally, to verify the commercial potential of Example 1 (HC-1.5), the present invention used NVPF as the cathode and paired it with Example 1 to assemble a full cell (HC-1.5 / / NVPF). Figure 6 Figure 'a' shows the overall charge-discharge curves of the HC-1.5 anode and NVP F cathode in the half-cell. The HC-1.5 / NVPF full cell exhibits excellent rate performance. Figure 6 In b), at 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1213, 202, 242, 182 and 150 mAh g -1 The corresponding charge / discharge curves are summarized in [the table / source]. Figure 6 c in the text. Furthermore, the device has an initial reversible capacity of 223 mAh g. -1 (0.05A g -1 It remained stable after three cycles of testing. Figure 6 d) in 0.05A g -1 Long-term cycling tests under these conditions demonstrated excellent stability, maintaining a capacity of 181.7 mAh g⁻¹ even after continuous operation. -1 capacity ( Figure 6 The results (e) in the figure fully demonstrate that the HC-1.5 / NVPF full cell has excellent cycle stability. Figure 6 The image in f shows an assembled full cell that can drive a light-emitting diode (LED), highlighting the feasibility of this material in the field of energy storage.

[0053] This invention synthesizes a high-efficiency sodium storage material with a nitrogen-doped porous network structure of hard carbon based on a high pseudo-graphite domain. The introduction of nitrogen atoms provides a powerful and simple way to finely adjust the content and characteristics of these key microstructural regions (pseudo-graphite, graphite-like, and highly disordered regions). By optimizing the nitrogen doping level, we designed a hard carbon anode (HC-1.5), achieving an ideal balance between highly active and inactive graphite-like regions. The hard carbon in the high pseudo-graphite region has more closed pores and a larger interlayer spacing, providing more active sites for sodium storage at 0.03 A g. -1 Under these conditions, after 140 cycles, the sample with optimized active pseudo-graphite region and inactive graphite region content exhibited 309 mAh g⁻¹. -1 Stable and reversible capacity. Furthermore, the optimized sample exhibited excellent long-term cycling performance at 1 A g. -1Under these conditions, it retains >90% of its capacity after 2000 cycles.

[0054] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar features unless otherwise specified. All disclosed features, or steps in all methods or processes, may be combined in any way except for mutually exclusive features and / or steps.

Claims

1. A method for preparing a high-efficiency sodium storage material based on a nitrogen-doped porous network structure of hard carbon with high pseudo-graphite domains, characterized in that, Includes the following steps: Step 1: Add 0.9-1g of polyvinylpyrrolidone to a mortar and grind it; Step 2: Add 0.5-3g of melamine to the material obtained in step (1), and then grind it to obtain a uniformly mixed precursor; Step 3: The precursor is heated to 500-700 degrees Celsius for 2 hours in a tube furnace under an argon atmosphere at a heating rate of 3°C / min, and then heated to 1100-1300 degrees Celsius for 2 hours at a heating rate of 3°C / min. After cooling, nitrogen-doped hard carbon with a porous network structure is obtained.

2. The preparation method according to claim 1, characterized in that, The polyvinylpyrrolidone content is 1g.

3. The preparation method according to claim 1, characterized in that, The polyvinylpyrrolidone has a molecular weight of K30.

4. The preparation method according to claim 1, characterized in that, The amount of melamine is 1.5g.

5. The preparation method according to any one of claims 1-4, characterized in that, The volume ratio of the pseudo-graphite region in the nitrogen-doped hard carbon material is higher than that of the graphite-like region and the highly disordered region, and its (002) crystal plane interlayer spacing d002 is 0.36-0.40 nm.

6. A sodium-ion battery, characterized in that, The high-efficiency sodium storage material obtained by any of the preparation methods described in claims 1-4 is used as the negative electrode.

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