Two-dimensional nanoplatelet-like porous carbon active negative electrode material and preparation method and application thereof

Abstract

The application belongs to the technical field of electrode materials, and provides a two-dimensional nanosheet-shaped porous carbon active negative electrode material, a preparation method and application thereof. The preparation method comprises the following steps: mixing 2-methyl imidazole, a molten salt, potassium nitrate and water, and then sequentially performing freeze-drying, carbonization and post-treatment on the mixture to obtain the two-dimensional nanosheet-shaped porous carbon active negative electrode material. The two-dimensional nanosheet-shaped porous carbon active negative electrode material with two-dimensional nanosheet morphology and hierarchical porous structure is prepared by using a molten salt synthesis method of liquid bath and combustion activation, and has the advantages of simple preparation process and easily obtained raw materials. The two-dimensional nanosheet-shaped porous carbon active negative electrode material has the morphology of two-dimensional nanosheets and hierarchical porous structure, improves the diffusion rate, and increases the contact area of the electrode and the electrolyte. The synergistic effect of nitrogen and sulfur doping increases the specific surface area, defect number and Na + adsorption sites of the porous carbon, and the two-dimensional nanosheet-shaped porous carbon active negative electrode material has good sodium storage capacity and rate performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrode materials, and particularly relates to a two-dimensional nanosheet-shaped porous carbon active negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] Among various negative electrode materials of sodium ion batteries, porous carbon has high conductivity and cycle stability due to its tough skeleton, rich pores and good conductivity. By adjusting the morphological structure and chemical composition of the porous carbon, the sodium storage performance of the porous carbon can be further improved. Among various methods for preparing the porous carbon, the template method and the chemical activation method are most common. Although the template method can control the morphological structure and pore size distribution of the porous carbon, it has problems such as single product shape and size, high cost, complex process and easy environmental pollution. The chemical activation method can construct a layered porous structure, but it can corrode equipment.

[0003] The salt template method is widely used in the field of material synthesis due to its easy availability, environmental protection and renewability. Salt has a specific melting point. When the temperature is lower than the melting point, the salt is a crystal; when the temperature is higher than the melting point, the salt is a liquid; when the temperature is further increased to the critical point, the salt will form a gaseous state, and even a decomposition reaction will occur. Mixing two or more kinds of salt can increase the entropy and strengthen the disorder of the crystal lattice, so that the melting point of the salt is reduced. The molten salt not only can provide a liquid reaction environment, but also can participate in the reaction as a precursor. Because the diffusion of the precursor is enhanced in the molten salt state, the reaction temperature required by the molten salt system is lower than that of the solid state reaction system. In addition, the salt template can not only introduce heteroatom doping in the preparation process, but also effectively control the morphological structure of the porous carbon. Therefore, it is of great significance to use the salt template method in the preparation process of the battery negative electrode material to make up for the shortcomings of the traditional template method and the chemical activation method. SUMMARY

[0004] The present application aims at providing a two-dimensional nanosheet-shaped porous carbon active negative electrode material and a preparation method and application thereof to solve the problems in the prior art.

[0005] In order to achieve the above-mentioned application purpose, the present application provides the following technical scheme:

[0006] The present application provides a preparation method of a two-dimensional nanosheet-shaped porous carbon active negative electrode material. 2-methylimidazole, molten salt, potassium nitrate and water are mixed, and the mixture is sequentially subjected to freeze-drying, carbonization and post-treatment to obtain the two-dimensional nanosheet-shaped porous carbon active negative electrode material.

[0007] Preferably, the molten salt comprises potassium chloride and potassium sulfate, and the molar ratio of the potassium chloride to the potassium sulfate is 2-3:1.

[0008] As preferred, the mass-volume ratio of the 2-methylimidazole, molten salt, potassium nitrate and water is 2-8 g: 4-15 g: 1-2 g: 70-200 mL.

[0009] As preferred, the temperature of the freeze-drying is -60 to -35℃, and the time of the freeze-drying is 18-30 h.

[0010] As preferred, the carbonization is carried out in a protective gas, the protective gas is argon and / or nitrogen, the flow rate of the protective gas is 65-90 mL / min; the temperature of the carbonization is 650-850℃, the time of the carbonization is 0.5-2 h, and the heating rate of the carbonization is 3-6℃ / min.

[0011] As preferred, the post-treatment comprises the steps of impurity removal, washing and drying, which are sequentially carried out.

[0012] As preferred, the temperature of the impurity removal is 15-35℃, and the time of the impurity removal is 20-30 min; the reagent used for the impurity removal is hydrochloric acid, and the concentration of the hydrochloric acid is 0.01-0.5 mol / L.

[0013] As preferred, the temperature of the drying is 80-120℃, and the time of the drying is 15-27 h.

[0014] The application further provides a two-dimensional nanosheet-shaped porous carbon active negative electrode material prepared by the preparation method.

[0015] The application further provides an application of the two-dimensional nanosheet-shaped porous carbon active negative electrode material in a sodium ion battery.

[0016] The application has the following advantages:

[0017] 1) The application adopts a molten salt synthesis method of liquid bath and combustion activation to prepare a nitrogen and sulfur double-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material with a two-dimensional nanosheet morphology and a hierarchical porous structure, and the preparation process is simple, the raw materials are easy to obtain and non-toxic, and the application overcomes many shortcomings of traditional template methods, such as high cost, complex process, single product shape and size, weak activation effect of physical activation method, easy corrosion of production equipment and environmental pollution of chemical activation method, and low yield of solid-state synthesis method.

[0018] 2) The nitrogen and sulfur double-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material prepared by the application has a two-dimensional nanosheet morphology and a hierarchical porous structure, which is beneficial to shorten the diffusion distance of ions and electrons, improve the diffusion rate, increase the contact area of the electrode and the electrolyte, and buffer the volume expansion caused by storage; the synergistic effect of nitrogen and sulfur doping is beneficial to increase the specific surface area, defect number and Na + adsorption site of the porous carbon, widen the carbon layer spacing, and promote the Na + storage capacity and rate performance of the sodium ion battery. +The transmission and diffusion of nitrogen and sulfur double doping cooperate with the morphology and hierarchical porous structure of the two-dimensional nanosheet, so that the nitrogen and sulfur double-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material prepared by the application has good sodium ion storage capacity and rate performance. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The scanning electron microscope image of the nitrogen and sulfur double-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material ECSK-15 prepared for Example 1.

[0020] Figure 2 The scanning electron microscope image of the porous carbon active negative electrode material EK-15 prepared for Comparative Example 1.

[0021] Figure 3 The scanning electron microscope image of the porous carbon active negative electrode material ECK-15 prepared for Comparative Example 2.

[0022] Figure 4 The scanning electron microscope image of the porous carbon active negative electrode material ESK-15 prepared for Comparative Example 3.

[0023] Figure 5 The scanning electron microscope image of the porous carbon active negative electrode material ECS prepared for Comparative Example 4.

[0024] Figure 6 The scanning electron microscope image of the porous carbon active negative electrode material ECSK-9 prepared for Comparative Example 5.

[0025] Figure 7 The scanning electron microscope image of the porous carbon active negative electrode material ECSK-21 prepared for Comparative Example 6.

[0026] Figure 8 The X-ray diffraction pattern of the porous carbon active negative electrode material prepared for Example 1 and Comparative Examples 1-6.

[0027] Figure 9 The first cycle charge-discharge curve of the porous carbon active negative electrode material prepared for Example 1 and Comparative Examples 1-4 as a negative electrode of a sodium ion battery at 0.05 A / g.

[0028] Figure 10 The first cycle charge-discharge curve of the porous carbon active negative electrode material prepared for Example1, Comparative Example 5, and Comparative Example 6 as a negative electrode of a sodium ion battery at 0.05 A / g.

[0029] Figure 11 The rate performance of the porous carbon active negative electrode material prepared for Example 1 and Comparative Examples 1- 6 as a negative electrode of a sodium ion battery.

[0030] Figure 12Rate capability graph of the porous carbon active negative electrode material prepared for example 1, comparative example 5, comparative example 6 as a negative electrode of a sodium ion battery.

[0031] Figure 13 Electrochemical impedance spectrogram of the porous carbon active negative electrode material prepared for example 1, comparative examples 1-6 as a negative electrode of a sodium ion battery.

[0032] Figure 14 Electrochemical impedance spectrogram of the porous carbon active negative electrode material prepared for example 1, comparative examples 5, comparative example 6 as a negative electrode of a sodium ion battery.

[0033] Figure 15 Cyclic voltammetry curve graph of the nitrogen and sulfur double-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material ECSK-15 prepared for example 1 as a negative electrode of a sodium ion battery at 0.2 mV / s.

[0034] Figure 16 Long cycle performance graph of the nitrogen and sulfur double-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material ECSK-15 prepared for example 1 as a negative electrode of a sodium ion battery at 5 A / g.

[0035] Figure 17 Long cycle performance graph of the nitrogen and sulfur double-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material ECSK-15 prepared for example 1 as a negative electrode of a sodium ion battery at 2 A / g. DETAILED DESCRIPTION

[0036] The application provides a preparation method of a two-dimensional nanosheet-shaped porous carbon active negative electrode material, which comprises the following steps: mixing 2-methylimidazole, a molten salt, potassium nitrate and water, and then sequentially performing freeze-drying, carbonization and post-treatment on the mixture to obtain the two-dimensional nanosheet-shaped porous carbon active negative electrode material.

[0037] In the application, the molten salt can provide a wide temperature range and form a molten phase during carbonization, thereby providing a liquid-sealed reaction environment; the potassium nitrate and the 2-methylimidazole undergo a combustion activation reaction to release CO, CO2 and NO and a large amount of heat, which further promotes the melting of the molten salt.

[0038] In the application, the molten salt preferably comprises potassium chloride and potassium sulfate, and the molar ratio of the potassium chloride to the potassium sulfate is preferably 2-3:1, further preferably 2.25-2.85:1, and more preferably 2.5:1.

[0039] In the present application, the mass-volume ratio of the 2-methylimidazole, molten salt, potassium nitrate and water is preferably 2-8 g: 4-15 g: 1-2 g: 70-200 mL, further preferably 4-7 g: 6-12 g: 1.2-1.8 g: 100-150 mL, and more preferably 5-6 g: 8.5-10.048 g: 1.4-1.515 g: 120-130 mL.

[0040] In the present application, the mixing is preferably carried out under stirring, and the stirring rate is preferably 250-600 r / min, further preferably 350-450 r / min, and more preferably 400 r / min; the stirring time is preferably 20-28 h, further preferably 22-26 h, and more preferably 24 h.

[0041] In the present application, the temperature for the freeze-drying is preferably -60 to -35℃, further preferably -50 to -40℃, and more preferably -45℃; the freeze-drying time is preferably 18-30 h, further preferably 20-26 h, and more preferably 22-24 h.

[0042] In the present application, the carbonization is preferably carried out in a protective gas, and the protective gas is preferably argon and / or nitrogen; the flow rate of the protective gas is preferably 65-90 mL / min, further preferably 70-85 mL / min, and more preferably 75-80 mL / min; the carbonization temperature is preferably 650-850℃, further preferably 700-800℃, and more preferably 750℃; the carbonization time is preferably 0.5-2 h, further preferably 1-1.5 h; and the temperature rising rate for the carbonization is preferably 3-6℃ / min, further preferably 4-5℃ / min.

[0043] In the present application, the post-treatment preferably comprises the steps of impurity removal, washing and drying, which are carried out in sequence.

[0044] In the present application, the temperature for the impurity removal is preferably 15-35℃, further preferably 20-30℃, and more preferably 25℃; the impurity removal time is preferably 20-30 min, further preferably 23-27 min, and more preferably 25 min; and the reagent used for the impurity removal is preferably hydrochloric acid, and the concentration of the hydrochloric acid is preferably 0.01-0.5 mol / L, further preferably 0.1-0.4 mol / L, and more preferably 0.2-0.3 mol / L.

[0045] In the present application, the washing is preferably carried out at room temperature, and the reagent used for the washing is preferably water; and the washing is preferably carried out 2-3 times.

[0046] In the present application, the temperature for drying is preferably 80-120℃, further preferably 90-110℃, and more preferably 100℃; the time for drying is preferably 15-27h, further preferably 20-25h, and more preferably 22h.

[0047] The present application also provides a two-dimensional nanosheet-shaped porous carbon active negative electrode material prepared by the preparation method.

[0048] The present application also provides application of the two-dimensional nanosheet-shaped porous carbon active negative electrode material in sodium ion batteries.

[0049] The technical solutions provided by the present application will be described in detail below in conjunction with examples, but they should not be understood as limiting the scope of protection of the present application.

[0050] Example 1

[0051] First, 4g of 2-methylimidazole, 5.517g of potassium chloride and 4.531g of potassium sulfate were added to 100mL of deionized water, and then 1.515g of potassium nitrate was added, and stirring was carried out at a rotation speed of 400r / min for 24h to obtain a mixed solution; the mixed solution was frozen at-45℃ for 24h, and then carbonization was carried out in nitrogen gas with a flow rate of 75mL / min (ramp to 750℃ at a rate of 5℃ / min, and carbonization at 750℃ for 1h). The carbonization product was placed in hydrochloric acid with a concentration of 0.2mol / L at a temperature of 25℃ for 25min for impurity removal, and then washed with deionized water for 3 times, and finally dried at 100℃ for 22h to obtain a nitrogen-sulfur dual-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material.

[0052] The nitrogen-sulfur dual-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material prepared in this example is denoted as ECSK-15.

[0053] Example 2

[0054] First, 6g of 2-methylimidazole, 6.2g of potassium chloride and 5.8g of potassium sulfate were added to 150mL of deionized water, and then 1.8g of potassium nitrate was added, and stirring was carried out at a rotation speed of 600r / min for 20h to obtain a mixed solution; the mixed solution was frozen at-50℃ for 20h, and then carbonization was carried out in argon gas with a flow rate of 70mL / min (ramp to 700℃ at a rate of 4℃ / min, and carbonization at 700℃ for 1.5h). The carbonization product was placed in hydrochloric acid with a concentration of 0.1mol / L at a temperature of 20℃ for 27min for impurity removal, and then washed with deionized water for 2 times, and finally dried at 90℃ for 25h to obtain a nitrogen-sulfur dual-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material.

[0055] The nitrogen-sulfur dual-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material prepared in this example is denoted as ECSK-18.

[0056] Example 3

[0057] First, 7 g of 2-methylimidazole, 2.94 g of potassium chloride, and 3.06 g of potassium sulfate were added to 200 mL of deionized water, and 1.2 g of potassium nitrate was added, and stirring was performed at a rotation speed of 250 r / min for 28 h to obtain a mixed solution; the mixed solution was frozen at -40 °C for 26 h, and then carbonization was performed in nitrogen gas at a flow rate of 85 mL / min (ramp to 800 °C at a rate of 3 °C / min, and carbonization at 800 °C for 0.5 h). The carbonization product was placed in hydrochloric acid at a temperature of 30 °C and a concentration of 0.4 mol / L for 23 min to remove impurities, then washed with deionized water for 3 times, and finally dried at 110 °C for 20 h to obtain a nitrogen and sulfur double-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material.

[0058] The nitrogen and sulfur double-doped two-dimensional nanosheet-shaped porous carbon active negative electrode material prepared in this example is denoted as ECSK-12.

[0059] Comparative Example 1

[0060] The difference from Example 1 is that potassium chloride and potassium sulfate are not added, and the others are the same as Example 1.

[0061] The nitrogen-doped porous carbon active negative electrode material prepared in this comparative example is denoted as EK-15.

[0062] Comparative Example 2

[0063] The difference from Example 1 is that potassium sulfate is not added, and the others are the same as Example 1.

[0064] The nitrogen-doped porous carbon active negative electrode material prepared in this comparative example is denoted ECK-15.

[0065] Comparative Example 3

[0066] The difference from Example 1 is that potassium chloride is not added, and the others are the same as Example 1.

[0067] The nitrogen and sulfur double-doped porous carbon active negative electrode material prepared in this comparative example is denoted as ESK-15.

[0068] Comparative Example 4

[0069] The difference from Example 1 is that potassium nitrate is not added, and the others are the same as Example 1.

[0070] The nitrogen and sulfur double-doped porous carbon active negative electrode material prepared in this comparative example was denoted as ECS.

[0071] Comparative Example 5

[0072] The difference from Example 1 is that the amount of potassium nitrate added is 0.909 g, and the others are the same as Example 1.

[0073] The nitrogen-sulfur dual-doped two-dimensional nanosheet porous carbon active anode material prepared in this comparative example is designated as ECSK-9.

[0074] Comparative Example 6

[0075] The difference from Example 1 is that the amount of potassium nitrate added is 2.121g, otherwise it is the same as Example 1.

[0076] The nitrogen-sulfur dual-doped two-dimensional nanosheet porous carbon active anode material prepared in this comparative example is designated as ECSK-21.

[0077] The microstructure of the porous carbon active anode materials prepared in Example 1 and Comparative Examples 1-6 was observed by scanning electron microscopy. Figures 1-7 .

[0078] Depend on Figures 1-7 It can be seen that the nitrogen-sulfur dual-doped two-dimensional nanosheet porous carbon active anode material prepared in Example 1 has a two-dimensional nanosheet porous structure, the porous carbon active anode materials prepared in Comparative Examples 1 to 4 only have a sheet porous structure, the porous carbon active anode material prepared in Comparative Example 5 has both a sheet porous structure and a bulk structure, and the porous carbon active anode material prepared in Comparative Example 6 has a disordered powder stacking structure.

[0079] The phase composition of the porous carbon active anode materials prepared in Example 1 and Comparative Examples 1-6 was observed by X-ray diffraction. Figure 8 .

[0080] The specific surface area and pore volume of the porous carbon active anode materials prepared in Examples 1-3 and Comparative Examples 1-6 were determined by nitrogen adsorption-desorption at 77K, as shown in Table 1.

[0081] The nitrogen and sulfur content of the porous carbon active anode materials prepared in Examples 1-3 and Comparative Examples 1-6 was determined by X-ray photoelectron spectroscopy, as shown in Table 1.

[0082] Table 1 Performance of porous carbon active anode materials

[0083]

[0084] As shown in Table 1, the porous carbon active anode materials prepared in Examples 1-3 and Comparative Examples 3-6 are doped with nitrogen and sulfur diatomic dopants. Among them, the porous carbon active anode materials prepared in Comparative Examples 3 and 4 have a lower sulfur doping content, while the porous carbon active anode materials prepared in Comparative Examples 1 and 2 are doped with single atoms.

[0085] The porous carbon active anode materials prepared in Examples 1-3 and Comparative Examples 1-6 were assembled into sodium-ion batteries. The assembly method was as follows: A mixture of porous carbon active anode material, conductive agent Li-90, and polyvinylidene fluoride (PVDF) in a mass ratio of 7.5:1.5:1 was added to N-methylpyrrolidone (N-methylpyrrolidone to PVDF in a volume mass ratio of 2L:1g) to obtain a mixed solution. The mixed solution was uniformly dropped onto a stainless steel sheet to form a 200μm thick sheet, which was then dried at 80°C for 10 hours to obtain the anode sheet. In an argon-filled glove box, the anode sheet, a glass fiber separator (Whatman GF / D glass fiber separator, 1823-047 glass fiber filter paper), and a sodium metal disc were assembled into a sodium-ion battery. The electrolyte used sodium difluorosulfonamide as the solute and diethylene glycol dimethyl ether as the solvent, with a solute concentration of 1 mol / L.

[0086] Using a Chenhua CHI660E electrochemical workstation and a Xinwei CT-4008T battery testing cabinet, the assembled sodium-ion battery was subjected to cyclic voltammetry, charge-discharge curves, rate performance, and electrochemical impedance spectroscopy tests at room temperature. The test results are as follows: Figures 9-17 As shown in Table 2.

[0087] Table 2 Performance of sodium-ion batteries assembled with porous carbon active anode materials

[0088] Example 1 ECSK-15 310.4148 6.355 Example 2 ECSK-18 305.2751 8.33 Example 3 ECSK-12 298.4645 10.24 Comparative Example 1 EK-15 255.3897 27.9 Comparative Example 2 ECK-15 277.5498 22.53 Comparative Example 3 ESK-15 274.2885 24.29 Comparative Example 4 ECS 290.266 20.52 Comparative Example 5 ECSK-9 248.9442 14.64 Comparative Example 6 ECSK-21 286.7861 33.72

[0089] As shown in Table 2, the impedance of the sodium-ion batteries assembled with the nitrogen-sulfur dual-doped two-dimensional nanosheet porous carbon active anode materials of Examples 1-3 of the present invention is significantly lower than that of the comparative examples, indicating that the two-dimensional nanosheet porous structure in the two-dimensional nanosheet porous carbon active anode material of the present invention can promote charge transfer.

[0090] Depend on Figure 15 It can be seen that in the first volt-ampere curve of the sodium-ion battery assembled with ECSK-15 anode material, the cathode peak close to 0.01V indicates the underpotential deposition of Na atoms or Na… + Embedded, other cathode peaks indicate electrolyte decomposition, SEI layer formation, and Na+. + Adsorption at the defect site; the second and third voltammetric curves almost overlap, indicating the high stability of the ECSK-15 anode; the rectangular voltammetric curves above 1.5V indicate that the ECSK-15 anode exhibits capacitive sodium storage behavior, including the double layer's ability to store Na. + The physical adsorption of Na and the effects of defects and edge positions on Na + Reversible chemisorption.

[0091] Depend on Figure 9 , 10It can be seen that the reversible capacity of the sodium-ion battery assembled with ECSK-15 anode material is 310.41 mAh / g at 0.05 A / g, and the coulombic efficiency is 57.9% at this time, indicating that more active sodium can participate in the sodium storage process of ECSK-15 anode.

[0092] Depend on Figure 11 , 12 It can be seen that the specific capacities of the sodium-ion battery assembled with ECSK-15 anode material at 0.05A / g, 0.1A / g, 0.2A / g, 0.5A / g, 1A / g, 2A / g, 3A / g, 4A / g, 5A / g and 10A / g are 310.41mAh / g, 266.08mAh / g, 241.76mAh / g, 217.94mAh / g, 195.87mAh / g, 176.17mAh / g, 158.39mAh / g, 146.71mAh / g, 137.52mAh / g and 116.68mAh / g, respectively. When returning to 0.05A / g, it has a specific capacity of 319.11mAh / g, indicating that the ECSK-15 anode has high reversibility and stability.

[0093] Depend on Figure 16 , 17 It can be seen that the specific capacity of the sodium-ion battery assembled with ECSK-15 anode material is 90.31 mAh / g after 5000 cycles at 5A / g and 135.60 mAh / g after 1000 cycles at 2A / g.

[0094] In summary, the two-dimensional nanosheet porous carbon active anode material of the present invention exhibits excellent Na+ properties. + The high storage performance, reversible specific capacity, and coulombic efficiency are mainly attributed to the fact that the two-dimensional nanosheet structure can promote the storage of Na+. + Hierarchical porous structures can improve the transport and storage of electrons in Na+. + Stability during storage also accelerates Na + The transport rate, nitrogen and sulfur dual doping and high doping levels can increase the wettability of the electrode material and Na + The adsorption of Na. The simple layered porous structure of EK-15 porous carbon active anode material increases the adsorption capacity of Na. + The diffusion barrier of Na was lowered. + The migration rate of Na decreases, reducing the contact area between the electrode material and the electrolyte; simple nitrogen doping, compared to nitrogen-sulfur dual doping, reduces the number of defects and pores, making Na... +The adsorption sites decreased sharply, ultimately resulting in sodium-ion batteries assembled from EK-15 porous carbon active anode materials and ECK-15 porous carbon active anode materials exhibiting lower performance than the nitrogen-sulfur dual-doped two-dimensional nanosheet porous carbon active anode material of this invention. The simple layered porous structure of ESK-15 porous carbon active anode materials and ECS porous carbon active anode materials increased the adsorption sites. + The diffusion barrier of Na was lowered. + The migration rate of Na decreases, reducing the contact area between the electrode material and the electrolyte; nitrogen and a small amount of sulfur doping, compared to high-content nitrogen-sulfur dual doping, reduces the number of defects and pores, making Na... + The adsorption sites decreased sharply, ultimately resulting in sodium-ion batteries assembled from ESK-15 porous carbon active anode materials and ECS porous carbon active anode materials exhibiting lower performance than the nitrogen-sulfur dual-doped two-dimensional nanosheet porous carbon active anode material of this invention. The layered porous structure and bulk structure of ECSK-9 porous carbon active anode material increased the adsorption sites. + The diffusion barrier of Na was lowered. + The migration rate of the electrolyte reduces the contact area between the electrode material and the electrolyte, ultimately resulting in the sodium-ion battery assembled with the ECSK-9 porous carbon active anode material having lower performance than the nitrogen-sulfur dual-doped two-dimensional nanosheet porous carbon active anode material of this invention. The disordered powder packing structure of the ECSK-21 porous carbon active anode material increases the Na+ migration rate. + The diffusion barrier of Na was lowered. + The migration rate of the electrode material is reduced, which decreases the contact area between the electrode material and the electrolyte. Ultimately, this results in the performance of sodium-ion batteries assembled with ECSK-21 porous carbon active anode material being lower than that of the nitrogen-sulfur dual-doped two-dimensional nanosheet porous carbon active anode material of this invention.

[0095] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a two-dimensional nanosheet porous carbon active anode material, characterized in that, 2-methylimidazole, molten salt, potassium nitrate and water are mixed, and the mixture is sequentially freeze-dried, carbonized and post-treated to obtain a two-dimensional nanosheet porous carbon active anode material.

2. The preparation method according to claim 1, characterized in that, The molten salt contains potassium chloride and potassium sulfate, with a molar ratio of 2 to 3:

1.

3. The preparation method according to claim 1 or 2, characterized in that, The mass-to-volume ratio of the 2-methylimidazole, molten salt, potassium nitrate, and water is 2–8 g: 4–15 g: 1–2 g: 70–200 mL.

4. The preparation method according to claim 3, characterized in that, The freeze-drying temperature is -60 to -35°C, and the freeze-drying time is 18 to 30 hours.

5. The preparation method according to claim 4, characterized in that, The carbonization is carried out in a protective gas, which is argon and / or nitrogen, at a flow rate of 65–90 mL / min; the carbonization temperature is 650–850 °C, the carbonization time is 0.5–2 h, and the carbonization heating rate is 3–6 °C / min.

6. The preparation method according to claim 4 or 5, characterized in that, The post-processing includes sequential impurity removal, washing, and drying.

7. The preparation method according to claim 6, characterized in that, The purification temperature is 15–35°C, and the purification time is 20–30 min; the reagent used for purification is hydrochloric acid, and the concentration of hydrochloric acid is 0.01–0.5 mol / L.

8. The preparation method according to claim 7, characterized in that, The drying temperature is 80–120°C, and the drying time is 15–27 hours.

9. The two-dimensional nanosheet porous carbon active anode material prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the two-dimensional nanosheet porous carbon active anode material according to claim 9 in sodium-ion batteries.

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