Nitrogen and fluorine doped carbon dots, preparation method thereof and application of nitrogen and fluorine doped carbon dots in aqueous zinc ion battery

Abstract

The invention belongs to the technical field of battery materials, and particularly relates to a nitrogen-fluorine-doped carbon dot, a preparation method thereof and application of the nitrogen-fluorine-doped carbon dot in an aqueous zinc ion battery. The nitrogen and fluorine doped carbon dots provided by the invention are prepared by taking a nitrogen source, sodium fluoride and a carbon source as raw materials and combining a hydrothermal reaction. The nitrogen and fluorine doped carbon dots prepared by the method have the particle size of less than 10nm and are uniformly distributed, and abundant nucleation sites can be provided for zinc ions. After the nitrogen-fluorine-doped carbon dot is used as an electrolyte additive, the problem of dendritic crystal growth caused by non-uniform nucleation sites in the zinc ion battery can be effectively solved, the specific capacity and cycling stability of the water-based zinc ion battery are effectively improved, and the nitrogen-fluorine-doped carbon dot has wide application prospects in preparation of the high-performance water-based zinc ion battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and particularly relates to a nitrogen and fluorine co-doped carbon dot, a preparation method thereof, and an application thereof in an aqueous zinc-ion battery. Background Art

[0002] With the growth of the economy and the development of technology, the conversion and storage of renewable energy and clean energy have become increasingly important. Aqueous zinc-ion batteries (AZIBs) are a type of battery that uses zinc as the negative electrode material and a water-based or water-containing electrolyte as the electrolyte. Among various energy storage systems, AZIBs have received extensive attention from the academic and industrial communities in recent years due to their advantages such as low cost, environmental friendliness, high theoretical specific capacity, and low redox potential.

[0003] However, although the zinc negative electrode has many inherent advantages, it still faces a series of technical challenges in practical applications, including uncontrollable growth of zinc dendrites, hydrogen evolution reaction (HER), and corrosion passivation. These challenges not only lead to capacity decay of the zinc negative electrode and reduction of Coulomb efficiency (CE), but may even cause safety hazards such as diaphragm piercing and battery short circuit in extreme cases. More severely, due to the uneven charge distribution on the electrode surface and fewer nucleation sites, the disordered growth of zinc dendrites will significantly increase the contact exposure area between the zinc negative electrode and the electrolyte, and the interaction effect between these problems will further exacerbate the deterioration of the negative electrode performance, thus restricting the practical application prospects of AZIBs.

[0004] Therefore, how to effectively alleviate the dendrite growth caused by uneven nucleation sites in zinc-ion batteries and at the same time effectively improve the specific capacity and cycle stability of zinc-ion batteries is of great significance for further promoting the application of AZIBs in depth and breadth. Summary of the Invention

[0005] To overcome the deficiencies in the prior art, the purpose of the present invention is to provide a nitrogen and fluorine co-doped carbon dot, a preparation method thereof, and an application thereof in an aqueous zinc-ion battery. The nitrogen and fluorine co-doped carbon dot provided by the present invention is a carbon dot material containing nitrogen and fluorine functional groups, and the particle size is less than 10 nm. During the charge and discharge process of the zinc-ion battery, the uneven surface of the zinc negative electrode will cause uneven zinc deposition. Due to the tip effect, dendrites will form on the negative electrode surface as the charge and discharge proceed, and then pierce the diaphragm. However, the nitrogen and fluorine co-doped carbon dot of the present invention can effectively adsorb on the zinc surface to provide uniform sites. After being used as an electrolyte additive, it can effectively alleviate the dendrite growth caused by uneven nucleation sites in zinc-ion batteries, and thus significantly improve the specific capacity and cycle stability of zinc-ion batteries.

[0006] To achieve the above purpose, in the first aspect of the present invention, a preparation method of a nitrogen and fluorine co-doped carbon dot is provided, which includes the following steps:

[0007] A nitrogen source, sodium fluoride, a carbon source and water are mixed to perform a hydrothermal reaction, and after the reaction, the resulting solution is separated, dialyzed and dried to obtain the nitrogen-fluorine-doped carbon dots;

[0008] The nitrogen source is one or more of urea, melamine, dicyandiamide, and ethylenediamine; and the carbon source is one or more of citric acid monohydrate, glucose, acetic acid, and ethylenediaminetetraacetic acid.

[0009] As a preferred embodiment, the mass ratio of the nitrogen source, sodium fluoride and carbon source is (1.5-2): (0.1-0.4): (3-5).

[0010] As a preferred embodiment, the nitrogen source is urea, the carbon source is citric acid monohydrate, and the mass ratio of the nitrogen source, sodium fluoride, and carbon source is 1.6:0.24:3.6. With this raw material type and ratio, the zinc-ion battery can achieve excellent improvements in specific capacity and cycle stability.

[0011] As a preferred embodiment, the temperature of the hydrothermal reaction is 160-200°C, and the hydrothermal reaction time is 4-8 hours; the separation is centrifugal separation; the dialysis bag used for dialysis has a molecular weight cutoff of 500-1000Da, and the dialysis time is 1-5 days; the drying temperature is 50-90°C, and the drying time is 6-20 hours.

[0012] The second aspect of the present invention provides nitrogen-fluorine-doped carbon dots prepared by the above preparation method.

[0013] A third aspect of the present invention provides an application of the nitrogen-fluorine-doped carbon dots as described above, wherein the application is the application of the nitrogen-fluorine-doped carbon dots as electrolyte additives in aqueous zinc-ion batteries.

[0014] In a fourth aspect of the present invention, an electrolyte for an aqueous zinc ion battery is provided, comprising a zinc salt, nitrogen-fluorine-doped carbon dots and water; the nitrogen-fluorine-doped carbon dots are nitrogen-fluorine-doped carbon dots prepared by the above-mentioned preparation method.

[0015] The present invention does not impose any particular restrictions on the type of zinc salt; it only needs to be able to release zinc ions to serve as a transmission medium during the charge and discharge process. As a preferred embodiment, the zinc salt is one or more of zinc sulfate (ZnSO4), zinc chloride (ZnCl2), zinc trifluoromethanesulfonate (Zn(CF3SO3)2), and zinc bis(trifluoromethanesulfonyl)imide (Zn(TFSI)2).

[0016] As a preferred embodiment, the mass concentration of the nitrogen-fluorine-doped carbon dots is 0.2-0.8 mg / mL, more preferably 0.6 mg / mL. The mass concentration of the zinc salt is 0.5-3 mol / L, more preferably 2 mol / L.

[0017] In the fifth aspect of the present invention, a water-based zinc-ion battery is provided, which includes a positive electrode, a negative electrode, a separator, and the electrolyte for the water-based zinc-ion battery as described above.

[0018] As a preferred solution, the positive electrode is one or more of vanadium-based compounds, manganese-based oxides, Prussian blue and its analogues; the negative electrode is zinc or a zinc alloy; and the separator is a nanocellulose separator, a polypropylene separator or a glass fiber separator.

[0019] As a further preferred solution, the positive electrode is sodium vanadate; the negative electrode is a zinc foil; and the separator is a glass fiber separator.

[0020] The technical solution of the present invention has the following advantages and beneficial effects:

[0021] The nitrogen- and fluorine-doped carbon dots provided by the present invention are prepared by using a nitrogen source, sodium fluoride and a carbon source and combining a hydrothermal reaction. The whole process is simple to operate, low in cost, environmentally friendly, pollution-free, with a high utilization rate of raw materials and great commercial potential. At the same time, the nitrogen- and fluorine-doped carbon dots prepared by the present invention have a particle size of less than 10 nm and are evenly distributed, can provide abundant nucleation sites for zinc ions, promote the nucleation and deposition of zinc, improve the stability of the zinc negative electrode, and are suitable for use as an electrolyte additive for water-based zinc-ion batteries.

[0022] The electrolyte for the water-based zinc-ion battery provided by the present invention contains nitrogen- and fluorine-doped carbon dots and has the following advantages in application: First, the functionalized carbon material has good conductivity, can promote the migration of electrons during charge and discharge, accelerate the electron transfer, and improve the kinetics of the zinc-ion battery reaction; second, the nitrogen- and fluorine-doped amorphous carbon layer constructed on the surface of the zinc negative electrode by the carbon material can inhibit zinc dendrites and guide zinc deposition; third, the preparation process of the carbon material is simple and the raw material price is low, which is conducive to commercial application.

[0023] The water-based zinc-ion battery provided by the present invention is constructed by adding carbon dots with nitrogen and fluorine functional groups to the electrolyte. On the one hand, it can provide more active sites on the surface of the zinc negative electrode to achieve uniform deposition of zinc ions, effectively improving the cycle stability of the zinc-ion battery. On the other hand, the fluorine- and nitrogen-doped carbon material has excellent conductivity and can accelerate the electron transfer during charge and discharge, further improving the cycle stability of the zinc-ion battery, which is conducive to commercial application. Description of the Drawings

[0024] Figure 1 It is the EDS diagram of the carbon dots provided in Example 1 and Comparative Examples 1-2 of the present invention;

[0025] Figure 2 It is the ultraviolet fluorescence diagram of the nitrogen- and fluorine-doped carbon dots provided in Example 1 of the present invention;

[0026] Figure 3 TEM image of the nitrogen and fluorine co-doped carbon dots provided in Example 1 of the present invention;

[0027] Figure 4 XPS image of the nitrogen and fluorine co-doped carbon dots provided in Example 1 of the present invention;

[0028] Figure 5 Ion conductivity test results of the electrolytes for aqueous zinc-ion batteries provided in Example 1, Comparative Examples 1-2 of the present invention;

[0029] Figure 6 Ion conductivity test results of the electrolytes for aqueous zinc-ion batteries provided in Examples 1-4, Comparative Example 3 of the present invention;

[0030] Figure 7 Charge-discharge curves and rate performance test results of the aqueous zinc-ion batteries assembled with the electrolytes for aqueous zinc-ion batteries based on Example 1, Comparative Example 3 of the present invention at different current densities;

[0031] Figure 8 Cyclic stability curve diagram of the aqueous zinc-ion batteries assembled with the electrolytes for aqueous zinc-ion batteries based on Example 1, Comparative Examples 1-3 of the present invention at a current density of 5 A / g. Detailed implementation manners

[0032] To make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be clearly and completely described below in conjunction with specific embodiments. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention. The raw materials and reagents not described in the present invention are all materials that can be obtained through commercial channels.

[0033] Example 1

[0034] This example provides a kind of nitrogen and fluorine co-doped carbon dots, and its preparation method includes the following steps:

[0035] Mix 1.6 g of urea, 0.24 g of sodium fluoride and 3.6 g of citric acid monohydrate in 34.5 mL of deionized water, then transfer it to a high-pressure autoclave (capacity 100 mL) with a polytetrafluoroethylene inner liner, place the high-pressure autoclave in an oven at 180 °C for hydrothermal reaction for 6 h. After the reaction, centrifuge the obtained reactant solution (8000 rpm, 5 min), then take the precipitate and dialyze it in double-distilled water for 48 h. The cut-off molecular weight of the dialysis bag used during dialysis is 1000 Da. Finally, dry the dialyzed solution at 80 °C for 12 h to obtain a dark powder, which is the nitrogen and fluorine co-doped carbon dots of this example.

[0036] This embodiment also provides an electrolyte for an aqueous zinc-ion battery, which is composed of a zinc salt, the above-mentioned nitrogen-fluorine doped carbon dots, and water; wherein, the zinc salt is zinc sulfate; the mass concentration of the nitrogen-fluorine doped carbon dots in the electrolyte is 0.6 mg / mL; the mass concentration of the zinc salt in the electrolyte is 2 mol / L.

[0037] This embodiment also provides an aqueous zinc-ion battery, which includes a positive electrode, a negative electrode, a separator, and the above-mentioned electrolyte for an aqueous zinc-ion battery. Among them, the positive electrode is sodium vanadate; the negative electrode is a zinc foil; the separator is a glass fiber separator.

[0038] Example 2

[0039] This embodiment provides a kind of nitrogen-fluorine doped carbon dots, and its preparation method is the same as that of Example 1.

[0040] This embodiment also provides an electrolyte for an aqueous zinc-ion battery. The difference from Example 1 is that the mass concentration of the nitrogen-fluorine doped carbon dots in the electrolyte is 0.2 mg / mL, and other parameters, conditions are the same as those in Example 1.

[0041] Example 3

[0042] This embodiment provides a kind of nitrogen-fluorine doped carbon dots, and its preparation method is the same as that of Example 1.

[0043] This embodiment also provides an electrolyte for an aqueous zinc-ion battery. The difference from Example 1 is that the mass concentration of the nitrogen-fluorine doped carbon dots in the electrolyte is 0.4 mg / mL, and other parameters, conditions are the same as those in Example 1.

[0044] Example 4

[0045] This embodiment provides a kind of nitrogen-fluorine doped carbon dots, and its preparation method is the same as that of Example 1.

[0046] This embodiment also provides an electrolyte for an aqueous zinc-ion battery. The difference from Example 1 is that the mass concentration of the nitrogen-fluorine doped carbon dots in the electrolyte is 0.8 mg / mL, and other parameters, conditions are the same as those in Example 1.

[0047] Comparative Example 1

[0048] This comparative example provides a kind of nitrogen-doped carbon dots, and its preparation method includes the following steps:

[0049] 1.6 g of urea and 3.6 g of citric acid monohydrate were placed in 34.5 mL of deionized water and mixed, and then transferred to a autoclave with a polytetrafluoroethylene liner (capacity 100 mL). The autoclave was placed in an oven at 180 °C for hydrothermal reaction for 6 h. After the reaction, the resulting reactant solution was centrifuged (8000 rpm, 5 min), and then the precipitate was taken and dialyzed in double-distilled water for 48 h. The cut-off molecular weight of the dialysis bag used during dialysis was 1000 Da. Finally, the dialyzed solution was dried at 80 °C for 12 h to obtain a dark powder, which was the nitrogen-doped carbon dots of this comparative example.

[0050] This comparative example also provides an electrolyte for an aqueous zinc-ion battery, which is composed of a zinc salt, the above-mentioned nitrogen-doped carbon dots and water; wherein, the zinc salt is zinc sulfate; the mass concentration of the nitrogen-doped carbon dots in the electrolyte is 0.6 mg / mL; the mass concentration of the zinc salt in the electrolyte is 2 mol / L.

[0051] This comparative example also provides an aqueous zinc-ion battery, which includes a positive electrode, a negative electrode, a separator and the electrolyte for the aqueous zinc-ion battery of this comparative example. Among them, the positive electrode is sodium vanadate; the negative electrode is a zinc foil; the separator is a glass fiber separator.

[0052] Comparative Example 2

[0053] This comparative example provides a fluorine-doped carbon dot, and its preparation method includes the following steps:

[0054] 0.24 g of sodium fluoride and 3.6 g of citric acid monohydrate were placed in 34.5 mL of deionized water and mixed, and then transferred to a autoclave with a polytetrafluoroethylene liner (capacity 100 mL). The autoclave was placed in an oven at 180 °C for hydrothermal reaction for 6 h. After the reaction, the resulting reactant solution was centrifuged (8000 rpm, 5 min), and then the precipitate was taken and dialyzed in double-distilled water for 48 h. The cut-off molecular weight of the dialysis bag used during dialysis was 1000 Da. Finally, the dialyzed solution was dried at 80 °C for 12 h to obtain a light yellow powder, which was the fluorine-doped carbon dots.

[0055] This comparative example also provides an electrolyte for an aqueous zinc-ion battery, which is composed of a zinc salt, the above-mentioned fluorine-doped carbon dots and water; wherein, the zinc salt is zinc sulfate; the mass concentration of the fluorine-doped carbon dots in the electrolyte is 0.6 mg / mL; the mass concentration of the zinc salt in the electrolyte is 2 mol / L.

[0056] This comparative example also provides an aqueous zinc-ion battery, which includes a positive electrode, a negative electrode, a separator and the electrolyte for the aqueous zinc-ion battery of this comparative example. Among them, the positive electrode is sodium vanadate; the negative electrode is a zinc foil; the separator is a glass fiber separator.

[0057] Comparative Example 3

[0058] This comparative example provides an electrolyte for an aqueous zinc-ion battery, which consists of a zinc salt and water; among them, the zinc salt is zinc sulfate; the mass concentration of the zinc salt in the electrolyte is 2 mol / L.

[0059] This comparative example also provides an aqueous zinc-ion battery, which includes a positive electrode, a negative electrode, a separator, and the electrolyte for the aqueous zinc-ion battery of this comparative example. Among them, the positive electrode is sodium vanadate; the negative electrode is a zinc foil; the separator is a glass fiber separator.

[0060] Experimental Example 1. Characterization of the Carbon Dot Structure

[0061] In this experiment, energy-dispersive X-ray spectroscopy (EDS) was first used to analyze the element distribution of the nitrogen-fluorine co-doped carbon dots prepared in Example 1, the nitrogen-doped carbon dots prepared in Comparative Example 1, and the fluorine-doped carbon dots prepared in Comparative Example 2. Then, combined with ultraviolet fluorescence analysis technology, transmission electron microscopy analysis technology (TEM), and X-ray photoelectron spectroscopy analysis technology (XPS), the structural composition of the nitrogen-fluorine co-doped carbon dots prepared in Example 1 was analyzed.

[0062] Figure 1 Figure 13 shows the EDS characterization results of the nitrogen-fluorine co-doped carbon dots of Example 1, the nitrogen-doped carbon dots of Comparative Example 1, and the fluorine-doped carbon dots of Comparative Example 2. Figure 1 In it, Figures a - c are the EDS diagrams of the carbon dots of Example 1, Comparative Example 1, and Comparative Example 2, respectively.

[0063] As Figure 1 can be seen, in the nitrogen-fluorine co-doped carbon dots of Example 1, C, N, O, and F elements are evenly distributed (Figure a). In the nitrogen-doped carbon dots of Comparative Example 1, C, N, and O elements are evenly dispersed (Figure b). In the fluorine-doped carbon dots of Comparative Example 2, C, F, and O elements are evenly dispersed (Figure c).

[0064] Furthermore, after the carbon dots of Example 1 of the present invention were formulated into a solution, the successful synthesis of the carbon dots was verified through fluorescence characteristics. Figure 2 Figure 14 shows the ultraviolet fluorescence diagram of the nitrogen-fluorine co-doped carbon dots of Example 1. As Figure 2 shown, the solution prepared from the carbon dots of Example 1 of the present invention is dark brown under natural light conditions, while it shows bright blue fluorescence emission under 365 nm ultraviolet excitation. This phenomenon can be used as a direct optical basis for the successful synthesis of the carbon dots of the present invention.

[0065] Furthermore, the nitrogen-fluorine co-doped carbon dots prepared in Example 1 of the present invention were analyzed by TEM, and the results are as Figure 3 shown. As Figure 3 can be seen, the nitrogen-fluorine co-doped carbon dots prepared in Example 1 are shown as uniformly dispersed nanoparticles, and the size is less than 10 nm, further confirming the successful preparation of the nano-carbon dots of the present invention.

[0066] Further, the nitrogen- and fluorine-doped carbon dots prepared in Example 1 were subjected to X-ray photoelectron spectroscopy analysis, and the XPS spectral results are as Figure 4 shown. As can be seen from Figure 4 this, the XPS spectrum shows four distinct peaks, corresponding to the C1s, O 1s, N 1s, and F1s peaks, respectively. In addition, the high-resolution spectrum of C 1s can be divided into four peaks of C-C, C-O / C-N, C=O, and C-F, located at 284.8, 286.4, 288.3, and 289.6 eV, respectively (Figure a). The high-resolution spectrum of O 1s presents four peaks at 529.8, 531.2, 532.0, and 533.2 eV, corresponding to C-O, C=O, O-C-O, and O-C=O, respectively (Figure b). The high-resolution spectrum of N 1s can be fitted into three peaks, which are attributed to the characteristic peaks of pyridine N (398.9 eV), pyridine N (399.9 eV), and graphitic N (401.4 eV), respectively (Figure c). The F 1s peak is located at two peaks of 684.4 and 685.5 eV, corresponding to the covalent bond and semi-ionic bond of C-F (Figure d).

[0067] The elemental quantitative results of the XPS spectral analysis are shown in Table 1.

[0068] Table 1. Elemental quantitative results of XPS spectral analysis

[0069] Name Peak position Atomic ratio / % O 1s 531.5 29.82 C 1s 284.8 58.04 F 1s 683.93 1.16 N 1s 399.86 10.98

[0070] As can be seen from Table 1, the atomic ratios of the four elements in the carbon dot material are: C (58.04%), N (10.98%), O (29.82%), and F (1.16%). Thus, it can be seen that the XPS analysis further proves that the carbon dots (CDs) synthesized in the present invention are doped with both N and F.

[0071] Test Example 2. Application effect test

[0072] (1) Conductivity test

[0073] The conductivities of the aqueous zinc-ion battery electrolytes prepared from the nitrogen- and fluorine-codoped carbon dots of Example 1, the nitrogen-doped carbon dots of Comparative Example 1, and the fluorine-doped carbon dots of Comparative Example 2 were tested and compared with the aqueous zinc-ion battery electrolyte without carbon dots of Comparative Example 3. The test results of the conductivity are as Figure 5 shown.

[0074] As can be seen from Figure 5It can be seen that when the carbon dots added to the 2 mol / L ZnSO4 electrolyte are nitrogen and fluorine co-doped carbon dots (N,F-COD), nitrogen-doped carbon dots (N-COD), and fluorine-doped carbon dots (F-COD), the conductivity values are 35.1 mS / cm, 18.2 mS / cm, and 20.8 mS / cm, respectively. The conductivity of the electrolyte (ZnSO4) without carbon dots in Comparative Example 3 is 15.8 mS / cm. This shows that after adding the electrolyte additive, the electrolyte obtained by adding both nitrogen and fluorine elements at the same concentration has the highest conductivity.

[0075] Further, the conductivities of the aqueous zinc-ion battery electrolytes prepared in Examples 1 to 4 and the electrolyte in Comparative Example 3 (carbon dot concentration 0 mg / mL) were tested respectively, and the results Figure 6 are shown as follows.

[0076] It can be seen from Figure 6 the figure that when the carbon dot concentrations added to the ZnSO4 electrolyte are 0.0, 0.2, 0.4, 0.6, and 0.8 mg / mL, the conductivity values are 15.8 mS / cm, 17.6 mS / cm, 27.9 mS / cm, 35.1 mS / cm, and 18.7 mS / cm, respectively. It can be known therefrom that after adding the electrolyte additive, the conductivity is the highest when the carbon dot concentration is 0.6 mg / mL, and the highest conductivity, uniform electric field distribution, and fast electro-chemical reaction kinetics can be obtained.

[0077] (2) Specific capacity and rate performance tests

[0078] The aqueous zinc-ion battery electrolytes provided in Example 1 and Comparative Example 3 were used to assemble aqueous zinc-ion batteries. Specifically, the assembled aqueous zinc-ion battery is a Zn||NVO full battery, which uses sodium vanadate as the positive electrode, a glass fiber membrane (GF / D, Whatman) as the separator, a zinc foil with a thickness of 100 μm as the negative electrode, and is encapsulated in a CR2032 button battery, and the electrolyte dosage is 125 μL. The Zn||NVO battery was subjected to charge and discharge tests at current densities of 1 A / g and 5 A / g, and the change in specific capacity with the number of cycles was recorded. The rate performance was tested at current densities of 0.2, 0.5, 1, 2, and 5 A / g, and the change in specific capacity with the number of cycles was recorded.

[0079] Figure 7 In the figure, Figure a is the charge-discharge cycle diagram of the zinc-ion battery assembled with the electrolytes of Example 1 and Comparative Example 3; Figure b is the charge-discharge curve of the rate performance of the zinc-ion battery assembled with the electrolyte of Example 1 at room temperature; Figure c is the charge-discharge curve of the rate performance of the zinc-ion battery assembled with the electrolyte of Comparative Example 3 at room temperature.

[0080] It can be seen from Figure 7It can be seen that at a current density of 1 A / g, the discharge specific capacity of the zinc-ion battery (N,F-COD) using the electrolyte of Example 1 is 267.36 mAh / g, while the discharge specific capacity of the zinc-ion battery (ZnSO4) assembled with the zinc sulfate electrolyte without carbon dots in Comparative Example 3 is 213.29 mAh / g. This indicates that after adding the carbon dots of Example 1 to the electrolyte, the electrochemically reactive capacity is significantly improved (Figure a).

[0081] Furthermore, to evaluate the rate performance and cycle stability of the battery, the specific capacity of the full battery was tested at different current densities (0.2 A / g to 5 A / g), and the results are shown in Figure 7 Figures b and c in. At current densities of 0.2, 0.5, 1, 2, and 5 A / g, the specific capacities of the Zn||NVO battery assembled with the electrolyte of Example 1 are 364.1, 281.2, 242.5, 206.8, and 160.5 mAh / g respectively, while the specific capacities using the electrolyte of Comparative Example 3 are 296.3, 244.8, 215.7, 187.4, and 147.2 mAh / g respectively. This shows that using nitrogen-fluorine doped carbon dots as an electrolyte additive in the present invention significantly improves the electrochemical performance of the battery.

[0082] (3) Cycle performance test

[0083] The aqueous zinc-ion battery electrolytes provided in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 were used to assemble aqueous zinc-ion batteries. Except for the different electrolytes, the remaining parts of the battery assembly method were the same as those in the above specific capacity and rate performance test parts. The assembled aqueous zinc-ion batteries were subjected to cycle performance tests. Figure 8 Figure for the cycle performance of zinc-ion batteries assembled based on the electrolytes of Example 1 and Comparative Examples 1 to 3 at room temperature.

[0084] It can be seen from Figure 8 that at a current density of 5 A / g, the initial specific capacity of the battery using the electrolyte of Comparative Example 3 is 210.29 mAh / g, and after 300 cycles, the specific capacity is 120.82 mAh / g. The initial specific capacities of the batteries assembled with the electrolytes of Comparative Example 1 and Comparative Example 2 are 200.85 mAh / g and 200.93 mAh / g respectively, and after 300 cycles, the specific capacities are 142.75 mAh / g and 154.58 mAh / g respectively. While the initial specific capacity of the battery using the electrolyte of Example 1 is 244.36 mAh / g, and after 300 cycles, the specific capacity is 178.90 mAh / g. It can be seen that using nitrogen-fluorine doped carbon dots as an electrolyte additive in the present invention can significantly improve the cycle stability of aqueous zinc-ion batteries compared with the electrolyte additives of Comparative Examples 1 to 3.

[0085] Based on the above results, it can be seen that the aqueous zinc-ion battery assembled with the electrolyte of the present invention still has excellent cycle stability at a relatively high current density, indicating that the nitrogen and fluorine co-doped carbon dots can effectively improve the problems of lithium dendrites and side reactions in zinc-ion batteries, thereby effectively improving the specific capacity and cycle stability of aqueous zinc-ion batteries.

[0086] In summary, the nitrogen and fluorine doped carbon dots provided by the present invention have a particle size of less than 10 nm and are evenly distributed, which can provide abundant nucleation sites for zinc ions. After being used as an electrolyte additive, it can effectively alleviate the problem of dendrite growth caused by uneven nucleation sites in zinc-ion batteries, effectively improve the specific capacity and cycle stability of aqueous zinc-ion batteries, and have broad application prospects in the preparation of high-performance aqueous zinc-ion batteries.

[0087] The above description is only a preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of nitrogen and fluorine co-doped carbon dots, characterized in that, It includes the following steps: Mix a nitrogen source, sodium fluoride, a carbon source and water for hydrothermal reaction. After the reaction, separate, dialyze and dry the obtained solution to obtain the nitrogen- and fluorine-doped carbon dots. Among them, the nitrogen source is one or more of urea, melamine, dicyandiamide, and ethylenediamine; the carbon source is one or more of citric acid monohydrate, glucose, acetic acid, and ethylenediaminetetraacetic acid.

2. The preparation method of the nitrogen- and fluorine-doped carbon dots according to claim 1, wherein The mass ratio of the nitrogen source, sodium fluoride, and carbon source is (1.5 - 2)∶(0.1 - 0.4)∶(3 - 5).

3. The preparation method of the nitrogen- and fluorine-doped carbon dots according to claim 2, characterized in that, The nitrogen source is urea; the carbon source is citric acid monohydrate; the mass ratio of the nitrogen source, sodium fluoride, and carbon source is 1.6∶0.24∶3.

6.

4. The preparation method of the nitrogen and fluorine doped carbon dots according to claim 1, wherein, The temperature of the hydrothermal reaction is 160 - 200 °C, and the time of the hydrothermal reaction is 4 - 8 h; the separation is centrifugal separation; the cut-off molecular weight of the dialysis bag used for dialysis is 500 - 1000 Da, and the dialysis time is 1 - 5 days; the drying temperature is 50 - 90 °C, and the drying time is 6 - 20 h.

5. A nitrogen- and fluorine-doped carbon dot prepared by the preparation method according to any one of claims 1 to 4.

6. Use of the nitrogen- and fluorine-doped carbon dots as described in claim 5, characterized in that, The application is the application of nitrogen- and fluorine-doped carbon dots as an electrolyte additive in an aqueous zinc-ion battery.

7. An electrolyte for an aqueous zinc-ion battery, characterized in that, It includes a zinc salt, nitrogen- and fluorine-doped carbon dots and water; the nitrogen- and fluorine-doped carbon dots are the nitrogen- and fluorine-doped carbon dots prepared by the preparation method according to any one of claims 1 to 4.

8. The electrolyte for an aqueous zinc-ion battery according to claim 7, wherein The zinc salt is one or more of zinc sulfate, zinc chloride, zinc trifluoromethanesulfonate, and zinc bis(trifluoromethanesulfonyl)imide; the mass concentration of the nitrogen- and fluorine-doped carbon dots is 0.2 - 0.8 mg / mL; the mass concentration of the zinc salt is 0.5 - 3 mol / L.

9. An aqueous zinc ion battery, characterized in that: It includes a positive electrode, a negative electrode, a separator, and an electrolyte for an aqueous zinc-ion battery according to claim 7 or 8.

10. The aqueous zinc ion battery according to claim 9, wherein, The positive electrode is one or more of vanadium-based compounds, manganese-based oxides, Prussian blue and its analogues; the negative electrode is zinc or a zinc alloy; the separator is a nanocellulose separator, a polypropylene separator or a glass fiber separator.

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