A crystalline carbon nitride / nitrogen-doped carbon composite nanosheet and its preparation method and application

Abstract

The present invention relates to a crystalline carbon nitride / nitrogen-doped carbon composite nanosheet, a preparation method and application thereof, and belongs to the field of negative electrode materials for alkali metal ion batteries. A crystalline carbon nitride / nitrogen-doped carbon composite nanosheet, wherein the composite nanosheet is composed of a mixture of crystalline carbon nitride nanosheets having a PTI structure and nitrogen-doped carbon nanosheets. The crystalline carbon nitride presents a porous nanosheet with a thickness of about 10 to 15 nm, having abundant open channels and microporous structures. The carbon material in the carbon nitride / carbon composite material exists in the form of nitrogen-doped carbon, and among the three nitrogen configurations doped, the total proportion of pyridinic nitrogen and pyrrolic nitrogen with high sodium storage activity is greater than 80%. A new composite material consisting of C3N4 allotrope PTI and nitrogen-doped carbonaceous material with a two-dimensional layered structure and a high theoretical capacity was successfully synthesized through a molten salt strategy.

Description

Technical Field

[0001] The invention relates to a crystalline carbon nitride / nitrogen-doped carbon composite nanosheet and a preparation method and application thereof, and belongs to the field of negative electrode materials for alkali metal ion batteries. Background Art

[0002] The development of advanced energy storage technology is of great significance for achieving large-scale utilization of sustainable energy, especially with the increasing concern about the supply of fossil fuels and their environmental problems. Considering the low cost and environmental friendliness of carbonaceous materials, carbonaceous materials are still the first choice for the commercialization of sodium ion batteries (SIBs). Commonly studied carbonaceous anodes, including hard carbon, graphene, carbon fiber or expanded graphite, still have problems such as low actual capacity, large volume change and poor cycle stability. Therefore, the development of fast Na + High-performance all-carbon anode materials with both storage capacity and long cycle life remain challenging.

[0003] As a novel two-dimensional layered nitrogen-containing carbon-based material, carbon nitride (CN) has shown increasing potential in recent years for energy storage applications due to its advantages, including low cost, abundant reaction sites, high theoretical capacity, and convenient synthesis. However, its poor conductivity and severe structural collapse during electrochemical reactions limit its use as anodes for SIBs. Furthermore, the reduced crystallinity caused by partial reaction with alkali metal ions also leads to poor stability and low capacity. Coupling with conductive carbon is considered an attractive strategy to offset its shortcomings by improving the structural durability and electronic conductivity of CN. However, a fundamental understanding of the interactions between CN and carbon additives is still lacking, and many unresolved issues remain. CN in most cases displays an amorphous "melon" polymer composed of heptazine rings, which is considered an amorphous or semicrystalline phase with low in-plane electronic conduction and poor electrochemical activity. Therefore, precise control of the CN / carbon interaction and detailed investigation of effective crystallinity enhancement are crucial for the rational design of advanced CN anodes. Summary of the Invention

[0004] To address the aforementioned issues of the prior art, the present invention aims to provide a molten salt method for preparing crystalline carbon nitride / nitrogen-doped carbon composite nanosheets for use as negative electrode materials in sodium-ion batteries. Using a molten salt strategy, the present invention successfully synthesized a novel composite material consisting of polytriazineimide (PTI) and nitrogen-doped carbonaceous material with a two-dimensional layered structure and high theoretical capacity. High pyridinic and pyrrolic nitrogen doping was achieved in the carbon material, and the polymerization process of C3N4 in different alkali metal molten salts was explored.

[0005] At the same time, in order to verify the effect of N configuration on the interaction between C3N4 and carbon materials, and emphasize the role of pyrrole-N, which is not common in previous studies. In the present invention, nitrogen-doped carbon and highly crystalline C3N4 composites were prepared by a one-step molten salt strategy. Unlike the amorphous C3N4 with a melon structure synthesized by traditional thermal polymerization, a crystalline C3N4 allotrope-polytriazine imide (PTI) was introduced by molten salt treatment, which improved its crystallinity and electrochemical properties. The simple molten salt treatment also achieved a close interaction between the components and introduced a hierarchical porous structure into the composite material, all of which are prerequisites for achieving high sodium storage performance. More importantly, the N configuration can be finely controlled by the molten salt component. Compared with the ternary salt LiCl / KCl / NaCl (melting point, mp = 450-500 ° C), it was found that the eutectic binary salt LiCl / KCl (mp = 352 ° C) with a lower melting point can better regulate the in-plane polymerization of C3N4 and promote the conversion of graphite-N to pyrrole-N. Density functional theory (DFT) calculations revealed the superior effect of PTI coupling with pyrrole-N carbon groups, which provides us with clues for the rational design of C3N4 anodes with high sodium adsorption and storage capacity. At the same time, the button full battery assembled with the prepared anode and the commercial-grade cathode Na3V2(PO4)3 also showed very excellent performance.

[0006] The molten salt method is highly operable, environmentally friendly, and non-toxic, and can prepare efficient and stable sodium-ion battery negative electrode materials with a safer and faster synthesis process. In different molten salt systems, the present invention emphasizes that the LiCl / KCl binary molten salt better optimizes the corresponding chemical structure and properties of the product, forming a 2D layered structure stacked by nanosheets, finely controlling the polymerization of C3N4 and the nitrogen configuration of the composite material, and achieving a material with less graphite-N content and high pyrrole-N content. More importantly, for the first time, both experimental data and theoretical calculations have emphasized the role of pyrrole-N in enhancing the charge transfer of C3N4-based composite materials and promoting Na + DFT calculations show that the introduction of pyrrole-N can promote the + The adsorption of PTI and pyrrole-N significantly improves the conductivity of the composite material. PTI and pyrrole-N gather more delocalized charges at the interface, which is beneficial to improve the interfacial charge distribution of the entire two-dimensional structure and accelerate the charge transfer and transmission between interfaces. The excellent electrochemical performance of the electrode material benefits from its strong structural stability, fast interfacial reaction kinetics, the unique structural arrangement of the nano SEI layer on the electrode surface and Na +This invention provides insights into the design and manufacture of high-performance carbon anodes and broadens the application of C3N4 in energy storage. The overall preparation process is simple, the reaction conditions are mild, and the material structure is reproducible and stable, offering significant advantages.

[0007] In order to achieve the above objectives, the main technical solutions adopted by the present invention include:

[0008] A crystalline carbon nitride / nitrogen-doped carbon composite nanosheet is formed by mixing a crystalline carbon nitride nanosheet with a PTI structure and a nitrogen-doped carbon nanosheet.

[0009] The crystalline carbon nitride / nitrogen-doped carbon composite nanosheets of the present invention are a mixture of crystalline carbon nitride nanosheets having a PTI structure and nitrogen-doped carbon nanosheets. The specific ratio between the two can be controlled by the ratio of the raw materials.

[0010] The carbon nitride nanosheets in the crystalline carbon nitride / carbon composite nanosheets of the present invention have high crystallinity and exist in the form of PTI crystals.

[0011] Furthermore, the crystalline carbon nitride nanosheet with the PTI structure is a porous nanosheet with a thickness of 10 to 15 nm, and has open channels and abundant microporous structures.

[0012] Furthermore, the nitrogen-doped carbon nanosheet is a porous nanosheet with a thickness of 10 to 15 nm, having open channels and abundant microporous structures.

[0013] The BET characterization results of the present invention show that the micropores on the crystalline carbon nitride nanosheets and the nitrogen-doped carbon nanosheets are the main components of the porous structure of the obtained material, and the pore diameter is about 1 nm.

[0014] The crystalline carbon nitride / carbon composite nanosheets of the present invention, wherein the nitrogen-doped carbon nanosheets exist in the form of nitrogen-doped carbon, and among the three doped nitrogen configurations, the total proportion of pyridinic nitrogen and pyrrolic nitrogen is higher than 80%.

[0015] The carbon material in the crystalline carbon nitride / carbon composite nanosheets of the present invention exists in the form of nitrogen-doped carbon, and among the three doped nitrogen configurations (pyridinic nitrogen, pyrrolic nitrogen and graphitic nitrogen), the total proportion of pyridinic nitrogen and pyrrolic nitrogen with high sodium storage activity is greater than 80%.

[0016] Furthermore, in the nitrogen-doped carbon nanosheets, the contents of pyridinic nitrogen, pyrrolic nitrogen and graphitic nitrogen are 52.23%, 33.35% and 14.42% respectively.

[0017] Another object of the present invention is to provide a method for preparing the above-mentioned crystalline carbon nitride / carbon composite nanosheets.

[0018] A method for preparing crystalline carbon nitride-nitrogen-doped carbon composite nanosheets comprises grinding and uniformly mixing a carbon nitride precursor, a carbon precursor, and ammonium chloride; adding a solid salt to the mixture and carefully grinding it; then calcining the mixture in an inert gas atmosphere; and after cooling to room temperature, thoroughly washing the sample with deionized water to remove metal salts, and drying the sample.

[0019] Preferably, the carbon nitride precursor is melamine, dicyandiamide, urea or thiourea.

[0020] Preferably, the carbon precursor is glucose or citric acid.

[0021] Preferably, the mass ratio of the carbon nitride precursor, the carbon precursor and the ammonium chloride is 1:1:1; the mass ratio of all precursors to the salt is 1:10-40, preferably 1:1-20.

[0022] In the present invention, ammonium chloride is added into the reaction system as a gas template and a nitrogen source for nitrogen doping.

[0023] Preferably, the solid salt in the reaction system is a ternary molten salt of NaCl / LiCl / KCl (mass ratio 15:11:15) or a binary molten salt of LiCl / KCl (molar ratio KCl / LiCl+KCl=0.6).

[0024] The above-mentioned solid salts are selected, wherein different salts are selected to have a ratio with the lowest eutectic point.

[0025] Preferably, the composite material precursor is placed in a nitrogen or argon atmosphere, prepolymerized at 300° C. for 1 hour, and then calcined at 500-600° C. for 2-4 hours to obtain the composite nanosheets.

[0026] Another object of the present invention is to provide the use of the above-mentioned crystalline carbon nitride / carbon composite nanosheets as a negative electrode material for sodium ion batteries.

[0027] The beneficial effects of the present invention are:

[0028] (1) A novel all-carbon composite composed of C3N4 allotrope PTI and nitrogen-doped carbonaceous materials with a two-dimensional layered structure and high theoretical capacity was successfully synthesized through a molten salt strategy.

[0029] (2) The polymerization process of C3N4 in different alkali metal molten salts was explored. By selecting the binary salt LiCl / KCl with a lower eutectic temperature (mp = 352°C) and higher ion diffusion rate compared to the ternary salt LiCl / KCl / NaCl (mp = 450-500°C), it was found that the polymerization pathway was indeed modified, which showed a unique guiding polymerization process.

[0030] (3) Traditionally, doping temperature has a significant impact on the nitrogen doping content and configuration. In our work, we found that at the same heat treatment temperature, the binary LiCl-KCl molten salt environment is more conducive to promoting the formation of pyrrolic-N and reducing the content of graphitic-N. In the binary LiCl-KCl molten salt with a lower eutectic point, the solid-solid reaction is transformed into a solid-liquid reaction at 352°C, allowing the reactants to achieve atomic-scale mixing in the liquid phase more quickly, greatly accelerating the ion diffusion rate and mass transfer rate. The eutectic molten salt method has opened up a method for N doping configuration transformation that no longer relies solely on adjusting the doping temperature.

[0031] (4) As the negative electrode material of SIBs, the prepared carbon nitride / carbon composite material has a high conductivity at 2A g -1 After 5000 cycles at the same current density, the battery exhibited excellent sodium storage capacity with reverse capacity growth. This ultra-long cycle performance is one of the highest values ​​reported for SIBs carbonaceous anodes. In addition, the assembled full battery exhibited excellent sodium storage capacity with reverse capacity growth after 5000 cycles at the same current density. -1 It still shows good reversible capacity in the next 300 cycles.

[0032] (5) The present invention does not contain expensive raw materials, which is in line with the concept of low cost of sodium ion batteries. The present invention provides insights into the design and manufacture of high-performance carbon negative electrodes and widely expands the application of C3N4 in the field of energy storage. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 (a) to (f) are morphological characterization images of the BMSCNC composite material produced in Example 1 at different magnifications.

[0034] Figure 2 This is the FT-IR graph of the BMSCNC composite material produced in Example 1.

[0035] Figure 3 (a) to (d) are XPS images of the BMSCNC composite material produced in Example 1.

[0036] Figure 4 (a) to (c) are the sodium storage properties of the BMSCNC composite material produced in Example 1.

[0037] Figure 5 (a) to (e) are the electrochemical performance curves of the sodium ion battery negative electrode made of the BMSCNC composite material product of Example 1 and the sodium ion full battery tested after assembly. DETAILED DESCRIPTION

[0038] The following non-limiting examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0039] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are all commercially available unless otherwise specified.

[0040] The basic concept of the present invention is as follows: carbon nitride and a carbon precursor are ground and mixed uniformly. Solid salts (NaCl / LiCl / KCl) are added to the mixture and carefully ground. The mixture is then calcined in an inert gas atmosphere. After cooling to room temperature, the sample is thoroughly washed with deionized water to remove the metal salts and then dried overnight before collection. This product is then used to prepare a negative electrode material for sodium-ion batteries, enabling them to achieve excellent charge-discharge specific capacity and ideal cycle stability.

[0041] Example 1

[0042] A method for preparing crystalline carbon nitride / carbon composite nanosheets using a molten salt method, comprising:

[0043] The carbon nitride and carbon precursors were ground and mixed thoroughly. Solid salts (NaCl / LiCl / KCl) were added to the mixture and carefully ground. The mixture was then calcined in an inert gas atmosphere. After cooling to room temperature, the sample was washed thoroughly with deionized water to remove the metal salts and then dried overnight before collection.

[0044] Specifically, the precursors are selected as melamine, glucose and ammonium chloride in a mass ratio of 1:1:1. The total mass of melamine, glucose and ammonium chloride is in a mass ratio of 1:13 to the binary salt LiCl+KCl. The binary salt LiCl / KCl (molar ratio KCl / LiCl+KCl=0.6) is added to the mixture and carefully ground. The mixture is then calcined in a N2 atmosphere at 300°C for 1h, 550°C for 2h and 600°C for 2h. After cooling to room temperature, the sample is thoroughly washed with deionized water to remove the metal salt, and then dried at 60°C overnight to obtain a crystalline carbon nitride / carbon composite material (denoted as BMSCNC).

[0045] 1. Morphology characterization of BMSCNC composites

[0046] The molten salt strategy successfully synthesized a novel composite material consisting of PTI and nitrogen-doped carbon materials with a two-dimensional layered structure and high theoretical capacity.

[0047] See also Figure 1, which are the SEM and TEM images of the BMSCNC composite material, the product of this example, showing a rich open channel assembled by porous nanosheets with a thickness of 10 to 15 nm, with a tubular structure with directional arrangement and abundant micropores. The ultra-thin tube wall can effectively reduce the density of the electrode material and improve its reversible capacity. These nanosheets are loosely and directionally stacked together, leaving a large number of gaps in them. These gaps will facilitate the penetration of electrolyte into the interior and rapid ion diffusion, and provide sufficient space to accommodate volume changes during cycling. The EDS image of the BMSCNC composite material confirms the uniform distribution of C, N and O elements (carbon materials contain trace oxygen-containing functional groups) in the composite material. At the same time, the TEM image of the BMSCNC composite material, further structural research revealed that BMSCNC contains ultra-thin multilayer nanosheets with many micropores. HRTEM detected clear lattice fringes with a spacing of 0.32 nm, which originated from the (002) crystal plane of the triazine-based C3N4 stacked between its layers. TEM and HRTEM results confirmed the formation of a crystalline C3N4-based carbonaceous composite.

[0048] 2. Structural Characterization of BMSCNC Composites

[0049] In order to study the influence of the reaction environment on the polymerization process of melamine, we also selected NaCl / LiCl / KCl ternary molten salt (the mass ratio of the total mass of melamine, glucose and ammonium chloride to the ternary salt NaCl / LiCl / KCl was 1:13, and the mass ratio of NaCl / LiCl / KCl was 15:11:15) and a molten salt-free reaction environment. Other experimental conditions were exactly the same, and the obtained samples were recorded as TMSCNC and CNC, respectively.

[0050] See also Figure 2 The chemical structures and functional groups of BMSCNC, TMSCNC, and CNC were further analyzed using Fourier transform infrared spectroscopy (FT-IR). The three samples showed the chemical structures of carbon nitride and carbon-based materials, indicating the successful construction of the composite materials. –1 The multiple peaks in the range represent the stretching mode of the CN ring, and the multi-peak signals of the CN ring in BMSCNC and TMSCNC are significantly stronger than that in CNC, which indicates that the molten salt reaction system promotes the core chemical structure of carbon nitride. –1 The peak near 640 cm can be attributed to the breathing mode of the triazine unit. It is worth noting that the intensity of the breathing mode of the triazine unit in BMSCNC is much higher than that in TMSCNC and CNC. In addition, a peak at 640 cm –1 The typical characteristic peaks of PTI are at 810 and 640 cm –1The characteristic peaks of BMSCNC confirm the high structural order of BMSCNC. Furthermore, the -C≡N vibrational band intensity in BMSCNC is much lower than that in TMSCNC and CNC, likely due to the decomposition or incomplete polymerization of the structural units, indicating that BMSCNC possesses superior structural order. Comparison of the chemical structures of the three samples illustrates the key role of the binary salts LiCl / KCl in improving the structural order of PTI.

[0051] See also Figure 3 The high-resolution XPS spectrum of C 1s deconvoluted into four peaks, corresponding to CC, CO, N=CN, and C-NH2 in the materials. Quantitative analysis of C-NH2 based on peak area integration revealed that the percentage of C-NH2 generated from uncondensed amino groups of C3N4 was lowest in BMSCNC, further confirming that the LiCl-KCl binary molten salt system favors the condensation of C3N4 and the high polymerization degree of C3N4 in the BMSCNC composite. High-resolution N1s spectra revealed that the pyridinic-N, pyrrolic-N, and graphitic-N contents in CNC, TMSCNC, and BMSCNC materials were 47.50% / 53.16% / 52.23%, 19.31% / 22.18% / 33.35%, and 33.19% / 24.66% / 14.42%, respectively. Data comparison revealed that the percentages of pyridinic-N were similar among the three samples, with significant variations occurring between pyrrolic-N and graphitic-N. The percentage of pyrrolic-N gradually increased, while the percentage of graphitic-N gradually decreased, in the CNC, TMSCNC, and BMSCNC composites. The highest percentage of pyrrolic-N and the lowest percentage of graphitic-N were detected in the BMSCNC material. Pyrrolic-N has been reported to have more suitable adsorption energy and provide more electrochemically active sites for efficient Na adsorption, while graphitic-N species can disrupt the crystallinity of C3N4, leading to poor electrochemical performance. C3N4 materials with minimal graphitic-N and high pyrrolic-N content are considered a key solution to improve the storage performance of sodium-ion batteries. Therefore, the BMSCNCs shown in this chapter, with their unique N configuration, lay the foundation for achieving high electrochemical performance.

[0052] 3. Sodium storage performance of BMSCNC composite materials

[0053] The BMSCNC composite material prepared in this example was used as the active material for the negative electrode of a sodium ion battery. It was mixed with acetylene black and a binder, BMSCNC, in a mass ratio of 70:20:10 to form a slurry, which was then coated on copper foil to form a film. After vacuum drying at 80°C for 12 hours, the sheets were cut and pressed. A sodium metal sheet was used as the counter electrode, sodium perchlorate electrolyte (NC-008), and a Whatman glass microfiber membrane, model GF / D, were assembled into a CR2032 button half-cell in an argon-filled glove box. In the experiment, a LAND2001CT battery performance tester was used to perform charge, discharge, and cycle performance tests in the voltage range of 0.01 to 3.0 V. The rate performance and coulombic efficiency curve of the battery were tested.

[0054] See also Figure 4 , CNC, TMSCNC and BMSCNC electrode materials at 0.2Ag -1 Cycling performance curves at different current densities. After 500 cycles, the CNC and TMSCNC electrodes only provided 146.7 mAh g -1 and 209.1mAh g -1 The reversible capacity and capacity retention rates of the BMSCNC electrode were 76.2% and 90.1%, respectively, while the BMSCNC electrode showed a reverse capacity growth with a reversible capacity of 317.2 mAh g -1 , the corresponding capacity retention rate is 116.3%. The rate performance of CNC, TMSCNC and BMSCNC electrodes was studied, among which BMSCNC electrode showed the best electrochemical performance. -1 At a current density of 1.5 Å, the reversible capacities of CNC, TMSCNC, and BMSCNC electrodes are 202.8 / 262 / 338, 183.5 / 231.4 / 298.2, 144.6 / 202.2 / 274.9, 116.9 / 167.7 / 243.9, and 87.9 / 126.6 / 193.3 mAh g, respectively. -1 Even at 5Ag -1 At a high current density of 161.4 mAh g -1 The high reversible capacity of CNC and TMSCNC is only 51.9 / 85 mAh g -1 When the current density returns to 0.1A -1 The reversible capacity of the BMSCNC electrode is 316.5 mAh g -1 After 20 cycles, the reversible capacity increased to 325 mAh g -1, showing excellent rate performance. After the high current test, when the current is restored to a low level, the reverse capacity can still maintain a continuous growth, indicating that the high current does not damage the structure of the electrode material.

[0055] 4. BMSCNC composite material assembly full battery

[0056] See also Figure 5 Given the outstanding performance of BMSCNC electrodes in half-cells, NVP was used as the positive electrode to construct a sodium-ion full-cell. Before assembling the whole cell, the BMSCNC negative electrode was heated to 0.1Ag -1 Three cycles of pre-sodiumization were performed at a current density of 1.5 Å to prevent side reactions between the electrode and the electrolyte. The electrochemical performance of the full battery was measured based on the active mass of the negative electrode material. For all charge and discharge processes, the voltage cutoff window of BMSCNC / / NVP was designed to be 0.01-4 V. The rate performance of BMSCNC / / NVP was 0.1, 0.2, 0.5, 1.0, and 2.0 A g -1 The current densities were 406.7, 309.4, 250.7, 205.2, and 154 mAh g -1 reversible capacity. Importantly, when the current density is restored to 0.1 A g -1 After 20 cycles, BMSCNC / / NVP can still provide 273.6mAh g -1 , with excellent rate performance. At the same time, the full battery is 0.1-2Ag -1 The charge and discharge curves at different current densities show good cycle stability. -1 After 300 cycles, it can show 180.6mAh g -1 The reversible capacity of the BMSCNC material was 1.337W, with a capacity decay rate of only 0.079% per cycle. Compared with traditional SIBs full cells, our configured full cell has superior cycling stability. These results confirm that the BMSCNC material has great application prospects and can be used as a promising anode for SIBs portable consumer electronics.

Claims

1. A method for preparing crystalline carbon nitride / nitrogen-doped carbon composite nanosheets, characterized by: The carbon nitride precursor, the carbon precursor and the ammonium chloride are ground and mixed uniformly; the solid salt is added to the mixture and carefully ground; the mixture is then calcined in an inert gas atmosphere; after cooling to room temperature, the sample is thoroughly washed with deionized water to remove the metal salt, and dried to obtain a carbon nitride precursor, wherein the mass ratio of the carbon nitride precursor, the carbon precursor and the ammonium chloride is 1:1:1, and the mass ratio of all the precursors to the solid salt is 1:10~40.

2. The method according to claim 1, wherein: The carbon nitride precursor is melamine, dicyandiamide, urea, and thiourea; the carbon precursor is glucose and citric acid.

3. The method according to claim 1, wherein: The solid salt in the reaction system is a ternary molten salt of NaCl / LiCl / KCl with a mass ratio of 15:11:15 or a binary molten salt of LiCl / KCl with a molar ratio of KCl / LiCl+KCl of 0.

6.

4. The method according to claim 1, wherein: The composite material precursor is placed in a nitrogen or argon atmosphere, prepolymerized at 300°C for 1 h, and then calcined at 500-600°C for 2-4 h to obtain composite nanosheets.

5. The crystalline carbon nitride / nitrogen-doped carbon composite nanosheet prepared by the preparation method according to any one of claims 1 to 4, characterized in that: The composite nanosheets are composed of a mixture of crystalline carbon nitride nanosheets with a PTI structure and nitrogen-doped carbon nanosheets, wherein the nitrogen-doped carbon nanosheets exist in the form of nitrogen-doped carbon, and among the three doped nitrogen configurations, the total proportion of pyridinic nitrogen and pyrrolic nitrogen is greater than 80%.

6. The composite nanosheet according to claim 5, wherein: The crystalline carbon nitride nanosheet with a PTI structure is a porous nanosheet with a thickness of 10 to 15 nm and has open channels and abundant microporous structures.

7. The composite nanosheet according to claim 5, wherein: The nitrogen-doped carbon nanosheet is a porous nanosheet with a thickness of 10 to 15 nm and has open channels and abundant microporous structures.

8. Use of the crystalline carbon nitride / nitrogen-doped carbon composite nanosheet according to any one of claims 5 to 7 as a negative electrode material for sodium ion batteries.

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