A nitrogen-doped carbon dot based on pomelo inner pericarp and a preparation method and application thereof

Abstract

The application discloses a kind of nitrogen-doped carbon dots based on pomelo inner pericarp, with pomelo inner pericarp and 2-methylimidazole as raw material, by hydrothermal reaction, filtration, dialysis and freeze-drying to obtain nitrogen-doped carbon dots NPP-CPDs, size is 10-20nm, with fluorescent emission peak;Carbon dots surface load imidazole group, carboxyl and hydroxyl, nitrogen site can be coordinated with Zn 2+ Stable. Its preparation method comprises the following steps: 1, pretreatment of pomelo inner pericarp PP;2, preparation of R-NPP-CPDs;3, purification of R-NPP-CPDs. A kind of NPP-CPDs zinc ion battery electrolyte additive, the concentration of NPP-CPDs is 100-800mg L ‑1 , by coordination with the Zn 2+ On the surface of zinc anode Form stable Zn-N bond;Zinc-zinc symmetrical battery cycle performance is 1400-1600h.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbon dots and the technical field of zinc ion batteries, and in particular to nitrogen-doped carbon dots based on pomelo inner pericarp and a preparation method and application thereof. BACKGROUND

[0002] Zinc ion batteries (ZIBs) have excellent theoretical capacity (820 mAh g -1 ), low redox potential (-0.763 V vs SHE), and high safety due to the use of aqueous electrolytes. However, the hydrogen evolution reaction (HER) seriously affects the cycle stability of ZIBs. According to the existing literature 1 (Cui, Yunwei, et al. "Challenges, strategies, and perspectives of anode protection in aqueous zinc-ion batteries." ACS Materials Letters 6.2 (2024): 611-626.), the current methods for improving the performance of aqueous zinc batteries mainly include optimizing the zinc electrode structure, constructing a functional protective layer, and adding electrolyte additives. Among them, improving the electrode structure and constructing a protective layer can effectively alleviate the corrosion problem, but such methods have the problems of high production cost and complex preparation process. The technical solution of adding corrosion inhibitors, on the other hand, has the advantages of low cost and simple operation.

[0003] Currently, the principle of improving the performance of zinc ion batteries through electrolyte additives is that the additive replaces H2O in the solvation structure through coordination, i.e., coordination effect. For example, existing literature 2 (Lin, Yunhui, et al. "Highly reversible aqueous zinc-ion battery using the chelating agent triethanolamine as an electrolyte additive." CrystEngComm 24.45 (2022): 7950-7961.) uses triethanolamine (TEA) as an electrolyte additive. TEA has the characteristic of preferentially coordinating with Zn 2+ , thereby replacing H2O in the solvation structure, effectively preventing direct contact between H2O and Zn 2+ on the zinc electrode surface, and significantly inhibiting the HER. However, since TEA only contains one -NH2 coordination site, the coordination stability with Zn 2+ is weak, resulting in a high coordination dissociation rate, which allows H2O to re-coordinate with Zn 2+ . 2+ ​

[0004] A direct method to solve the above problems is to provide an additive molecule with multiple coordination sites to enhance the coordination stability with Zn 2+ . Among them, fluorescent carbon dots as multi-coordination site particles can be used as additives for zinc ion batteries. For example, existing document 3 (Yang, Shuhua, et al. "Hydrophilic and nanocrystalline carbon quantum dots enable highly reversible zinc-ion batteries." Green Chemistry 26.12 (2024): 7293-7301.) uses chitosan powder and acetic acid solution as raw materials to obtain CQDs by hydrothermal reaction. The CQDs are used as electrolyte additives in zinc-zinc symmetric batteries, which improves the cycle life from the original 106h to 1000h. The problem of this technical solution is that since the obtained CQDs are not doped, the surface functional group activity of the CQDs is weak, and cannot form stable coordination with Zn 2+ .

[0005] In order to further improve the force between carbon dots and the surface of zinc negative electrode, nitrogen doping can be performed on the carbon dots to introduce nitrogen coordination sites with stronger force. Since biomass materials naturally contain nitrogen, the problem of lacking strong coordination sites in the above traditional preparation method can be solved. For example, existing document 4 (Preparation method and application of a porous carbon material based on pomelo peel: CN108584947A[P]. 2018-09-28.) uses biomass material pomelo peel as raw material, and obtains a nitrogen-containing pomelo peel-based carbon material through low-temperature carbonization and activation treatment of porous carbon material. The carbon material obtained by the technical solution has good energy storage performance when applied to supercapacitor electrode materials, reaching 117-315F / g at a current density of 0.5A / g. However, since the technical field of this technical solution is supercapacitors, the principle of supercapacitors determines that the required carbon material only needs to be doped with a low amount of nitrogen, and too high a nitrogen doping amount will actually lead to a decrease in performance, which is a substantial difference from the requirements of carbon materials in the field of zinc batteries. The preparation method of the pomelo peel-based carbon material in existing document 4 is not suitable for the field of zinc batteries. The specific reason is that the number of nitrogen coordination sites of the carbon material of this technical solution is small, and the activity of the nitrogen coordination sites is weak.

[0006] Therefore, according to the existing documents, the current technology has the following technical problems:

[0007] 1. Single coordination site coordination stability is weak, and the coordination dissociation rate is high, which will make H2O recombine with Zn 2+Coordination, therefore, needs to provide multiple coordination sites to enhance the coordination stability with Zn 2+ ;

[0008] 2, the carbon dots without nitrogen doping treatment, due to the weak activity of surface functional groups, leading to weak coordination stability with Zn 2+ ;

[0009] 3, when the carbon material is applied to zinc ion battery, the nitrogen content and nitrogen source activity in the conventional biomass material cannot meet the application requirements of nitrogen doping. SUMMARY

[0010] The purpose of the present application is to provide a kind of based on nitrogen doped carbon dots of shaddock inner pericarp and its preparation method and application. For the technical problems existing in the prior art, the present application uses the following principles and methods to solve the above problems:

[0011] 1, select the carbon dots with multiple coordination sites on the surface as additive molecules to solve the problem of weak coordination stability of single coordination site and high coordination dissociation rate;

[0012] 2, by introducing additional high-activity nitrogen source, efficient nitrogen doping is realized, and the problem of insufficient nitrogen content in traditional biomass material leading to fewer coordination sites of carbon dots and weak activity is solved.

[0013] In order to achieve the above-mentioned purpose of the present application, the technical scheme adopted by the present application is:

[0014] A kind of nitrogen doped carbon dots based on shaddock inner pericarp, characterized by: with shaddock inner pericarp and 2-methyl imidazole as raw material, after hydrothermal reaction, filtration, dialysis and freeze-drying, nitrogen doped carbon dots NPP-CPDs are obtained;

[0015] The size of the NPP-CPDs is 10-20nm, and has a fluorescence emission peak at 455nm under the condition of excitation wavelength 380nm;The NPP-CPDs surface is loaded with imidazole group-Im, carboxyl-COOH and hydroxyl-OH, wherein, nitrogen site can be coordinated with Zn 2+ Stable coordination.

[0016] A preparation method of nitrogen doped carbon dots based on shaddock inner pericarp, comprising the following steps:

[0017] Step 1, pretreatment of shaddock inner pericarp, first, with shaddock inner pericarp, i.e. white sponge-like fruit skin as raw material, dry under certain conditions, then, dry white shaddock fruit skin under certain conditions, ball milling is carried out, and shaddock skin powder, simply referred to as PP, is obtained;

[0018] In the step 1, the drying conditions are that the drying temperature is 85 DEG C, and the drying time is 8-12h;

[0019] In the step 1, the ball milling speed is 300-500 rpm, and the ball milling time is 4-6 h;

[0020] In the step 2, the R-NPP-CPDs are prepared as follows: first, 2-methyl imidazole is dissolved in ultrapure water to obtain an imidazole solution, then the PP obtained in the step 1 is placed in the imidazole solution, and a hydrothermal reaction is carried out under certain conditions to obtain raw NPP-CPDs, which are referred to as R-NPP-CPDs.

[0021] In the step 2, the mass ratio of the PP to 2-methyl imidazole is 1:(1-1.5).

[0022] In the step 2, the hydrothermal reaction is carried out at a temperature of 160-200 ℃ for 8-12 h.

[0023] In the step 3, the R-NPP-CPDs are purified as follows: first, the R-NPP-CPDs obtained in the step 2 are filtered at room temperature to obtain a filtrate, then the filtrate is dialyzed under certain conditions, after the dialysis is completed, the solution in the dialysis bag is the purified NPP-CPDs, and finally, the purified NPP-CPDs are freeze-dried to obtain the NPP-CPDs based on the inner pericarp of grapefruit.

[0024] In the step 3, the filtration is carried out by using a microporous membrane with a pore size of 0.22 μm, and the dialysis is carried out by using a dialysis bag with a MW of 1000.

[0025] In the step 3, the dialysis method is to replace the ultrapure water every 8 h, and the total dialysis time is 24 h.

[0026] In the step 3, the freeze-drying method is to freeze at a temperature of -20 to -25 ℃ for 24 h, and then freeze-dry at a temperature of -45 ℃ for 24-72 h.

[0027] A zinc ion battery electrolyte additive based on the NPP-CPDs of the inner pericarp of grapefruit, the concentration of the NPP-CPDs in the ZnSO4 electrolyte is 100-800 mg / L -1 The NPP-CPDs can form stable Zn-N bonds with Zn 2+ on the surface of the zinc negative electrode through coordination, and when used as a zinc ion battery electrolyte additive, the cycle performance of the zinc-zinc symmetrical battery is 1400-1600 h.

[0028] The technical effects of the present application are as follows:

[0029] The basic characteristics of the NPP-CPDs are proved by transmission electron microscopy (TEM) test and fluorescence spectrum test.

[0030] The imidazole group-Im, carboxyl group-COOH and hydroxyl group-OH on the surface of the NPP-CPDs are proved by Fourier transform infrared spectroscopy (FTIR) test.

[0031] The NPP-CPDs are proved to be capable of stable coordination with Zn 2+ and forming Zn-N bond by X-ray photoelectron spectroscopy (XPS) test.

[0032] In the application of zinc-zinc symmetric battery, the zinc-zinc symmetric battery can be stably cycled for 1400 hours after the NPP-CPDs are used as electrolyte additives.

[0033] In the application of supercapacitor, it is found that the NPP-CPDs are only suitable for zinc ion battery but not suitable for supercapacitor after the NPP-CPDs are used as electrolyte additives, which proves the specificity of the NPP-CPDs in application.

[0034] Therefore, the present application has the following advantages over the prior art:

[0035] 1. Since the carbon dots with multiple coordination sites on the surface are selected as zinc ion battery additives,

[0036] 1.1. The carbon dot surface contains multiple coordination sites, and this multi-site coordination mode significantly improves the coordination stability with Zn 2+ , and avoids the coordination instability problem caused by the high dissociation rate of single coordination site;

[0037] 1.2. The carbon dot preparation process is simple, controllable, and the raw materials are widely available and low in cost;

[0038] 1.3. The carbon dot has good chemical stability and can maintain the structure intact in the electrolyte solution and is not prone to side reactions;

[0039] 2. Since the efficient nitrogen doping is realized by introducing an additional high-activity nitrogen source,

[0040] 2.1. The nitrogen-doped carbon dot surface functional group has high activity and high coordination stability with Zn 2+ , and can form a stable Zn-N bond;

[0041] 2.2. Nitrogen doping can improve the conductivity of the carbon dot, which is helpful to the electron transmission efficiency of the zinc ion battery and improves the overall cycle life of the battery;

[0042] 2.3 The type and ratio of nitrogen doping can be precisely controlled through the preparation process to meet the performance requirements of different application scenarios. Attached Figure Description

[0043] Figure 1 TEM image of NPP-CPDs prepared in Example 1;

[0044] Figure 2 The fluorescence emission spectrum of the NPP-CPDs prepared in Example 1 is shown below.

[0045] Figure 3 FTIR chromatogram of NPP-CPDs prepared in Example 1;

[0046] Figure 4 XPS images of zinc sheets immersed in two electrolytes, ZnSO and NPP-CPDs-ZnSO, in Example 1.

[0047] Figure 5 The time-voltage curves of zinc-zinc symmetric cells with four electrolytes in Example 1 are shown.

[0048] Figure 6 The CV curves are for the supercapacitors using NaSO and NPP-CPDs-NaSO electrolytes in Example 1. Detailed Implementation

[0049] The present invention will be further described in detail through embodiments and with reference to the accompanying drawings, but this is not intended to limit the scope of the invention.

[0050] Example 1

[0051] A method for preparing nitrogen-doped carbon dots based on grapefruit endocarp includes the following steps:

[0052] Step 1, Pre-treatment of grapefruit inner peel: First, grapefruit inner peel, i.e., spongy white peel, is used as raw material. It is dried at a drying temperature of 85℃ for 12 hours. Then, the dried white grapefruit peel is ball-milled at a ball mill speed of 400 rpm for 5 hours to obtain grapefruit peel powder, abbreviated as PP.

[0053] Step 2, preparation of R-NPP-CPDs: First, 0.6g of 2-methylimidazole was dissolved in 60mL of ultrapure water to obtain an imidazole solution. Then, 0.6g of PP obtained in Step 1 was placed in the imidazole solution, and a hydrothermal reaction was carried out at a hydrothermal temperature of 180℃ for 8h to obtain unpurified Raw-NPP-CPDs, abbreviated as R-NPP-CPDs.

[0054] Step 3, purification of R-NPP-CPDs, first, the R-NPP-CPDs obtained in step 2 were filtered with a microporous membrane with a pore size of 0.22 μm at room temperature to obtain a filtrate, then the filtrate was dialyzed with a dialysis bag with MW 1000, and the ultra-pure water was replaced every 8 h, and the total dialysis time was 24 h, after dialysis, the solution in the dialysis bag was the purified NPP-CPDs, finally, the purified NPP-CPDs were freeze-dried to obtain the nitrogen-doped carbon dots based on pomelo inner pericarp, referred to as NPP-CPDs.

[0055] The method of freeze-drying was first frozen at a freezing temperature of -20℃ for 24 h, and then freeze-dried at a freeze-drying temperature of -45℃ for 48 h.

[0056] In order to prove that NPP-CPDs are carbon dots, TEM test and fluorescence emission spectrum test were performed.

[0057] The TEM test results of NPP-CPDs are shown in Figure 1 , the micro-morphology of NPP-CPDs is ellipsoidal, and the size is 15 nm;

[0058] The fluorescence emission spectrum test results of NPP-CPDs are shown in Figure 2 , under the condition of excitation wavelength of 380 nm, NPP-CPDs has a fluorescence emission peak at 455 nm;

[0059] The results of TEM test and fluorescence emission spectrum test show that NPP-CPDs are carbon dots.

[0060] In order to prove the active sites on the surface of NPP-CPDs, FTIR test was performed. The test results are shown in Figure 3 , NPP-CPDs contain characteristic peaks of C=N, N-H bond, ring structure and stretching vibration of -OH, C=O, wherein C=N, N-H bond, ring structure are derived from imidazole group-Im. The test results show that NPP-CPDs surface is loaded with imidazole group-Im, carboxyl group-COOH and hydroxyl group-OH.

[0061] In order to prove the role of NPP-CPDs as zinc ion battery electrolyte additive, XPS test was performed. The specific test method was first, with the concentration of 2 mol L -1 ZnSO4 electrolyte as the basic electrolyte, referred to as ZnSO, then the concentration of NPP-CPDs was 200 mg L -1NPP-CPDs electrolyte, referred to as NPP-CPDs-ZnSO, was obtained by adding NPP-CPDs into the base electrolyte. Finally, the zinc sheet was soaked in the NPP-CPDs-ZnSO for 3 days, and then the XPS test was performed. At the same time, in order to prove the effect of NPP-CPDs on the zinc sheet, the zinc sheet was soaked in the base electrolyte, and the XPS test was performed as a reference. The test results are shown in Figure 4

[0062] The test results of ZnSO are that no characteristic peaks are contained;

[0063] The test results of NPP-CPDs-ZnSO are that, in addition to the characteristic peaks of C-N and C=N of NPP-CPDs, the characteristic peaks of Zn-N bond are also contained;

[0064] The test results show that the addition of NPP-CPDs can form a stable coordination with Zn 2+ to form a Zn-N bond.

[0065] In order to prove the effect of NPP-CPDs as zinc ion battery electrolyte additive on the performance of zinc ion battery, the cycle performance test of zinc-zinc symmetric battery was performed, and the cycle performance test of zinc-zinc symmetric battery was performed with ZnSO as electrolyte as a reference. The test results are shown in Figure 5 Under the conditions of current density 1 mA cm -2 and capacity 0.5 mAh cm -2 , ZnSO can be stably cycled for 150 h;

[0066] NPP-CPDs-ZnSO can be stably cycled for 1400 h;

[0067] The test results show that the addition of NPP-CPDs can significantly improve the cycle performance of zinc-zinc symmetric battery, and the improvement amplitude reaches 833.3%.

[0068] In order to prove the specificity of NPP-CPDs, i.e. suitable for zinc ion battery electrolyte additive, CV test was performed with NPP-CPDs as supercapacitor electrolyte additive. The specific preparation method of supercapacitor is that, first, Na2SO4 electrolyte with a concentration of 2 mol L -1 is used as base electrolyte, referred to as NaSO, and then the concentration of NPP-CPDs is 200 mg L -1 ​NPP-CPDs-NaSO, and finally, with CMK-3 as the positive and negative electrode material, NPP-CPDs-NaSO as the electrolyte, supercapacitors were assembled for CV test, and NaSO was used as the electrolyte to assemble supercapacitors as a reference. The test results are shown in Figure 6 As shown in the figure, at a scan rate of 10 mV / s, the potential is in the range of -2.5 V to 2.5 V,

[0069] The curve of NaSO is a typical rectangular shape.

[0070] The curve shape of NPP-CPDs-NaSO is similar to that of NaSO, which is a typical rectangular shape, but the difference is that the current density of NPP-CPDs-NaSO increases, because NPP-CPDs-NaSO not only does not have a corrosion effect in supercapacitors, but also triggers hydrogen or oxygen evolution side reactions, which makes the performance of supercapacitors decline.

[0071] The test results show that although the battery and supercapacitor have similar requirements for additives under normal circumstances, NPP-CPDs are only suitable for use as zinc ion battery electrolyte additives, but not as supercapacitor additives, which proves that NPP-CPDs have specificity.

[0072] Similarly, compared with the prior art 4 described in the background, although the technical solution also uses pomelo peel as a biomass raw material, because the technical field involved in the technical solution is supercapacitors, a calcination process is used to prepare porous carbon materials, and the corresponding technical effect is improved; but as the background analysis shows, the technical solution of prior art 4 is also not suitable for the zinc battery technical field of the present application, which also proves that the NPP-CPDs of the present application have specificity.

[0073] In order to prove the influence of pomelo inner peel on the performance of nitrogen-doped carbon dots, i.e. the role of pomelo inner peel in the technical solution, a comparative example 1, a kind of nitrogen-doped carbon dots based on orange peel, is provided.

[0074] Comparative Example 1

[0075] A preparation method of a kind of nitrogen-doped carbon dots based on orange peel, the steps not specially explained are the same as example 1, the difference is that: in step 1, orange peel is used instead of pomelo inner peel as a biomass carbon source, and the obtained carbon dots are referred to as NOP-CPDs.

[0076] The cycle performance test results of zinc-zinc symmetric battery of NOP-CPDs are shown in Figure 5 As shown in the figure, at a current density of 1 mAcm -20.5mAh capacity -2 Under these conditions, NOP-CPDs-ZnSO can be stably cycled for 180 hours;

[0077] Compared with ZnSO, the addition of NOP-CPDs can slightly improve the cycle performance of zinc-zinc symmetric cells, with an improvement of only 20%.

[0078] Compared with the NPP-CPDs-ZnSO of Example 1, it can be seen that there are substantial and significant differences in the cycle performance of the zinc-zinc symmetric battery between nitrogen-doped carbon dots prepared based on grapefruit endocarp and those prepared based on orange peel.

[0079] The reason is that, according to reference 1 (Feng Kun. Study on the components and activities of the peel of major grapefruits in China [D], 2019), the cellulose content in grapefruit peel reaches 30%-40%, and the structure of its fiber tissue determines that it can form a dense and stable carbon skeleton structure after carbonization, providing better surface area and pore structure, which is conducive to loading active substances.

[0080] Meanwhile, according to reference 2 (Liu Xuefeng, Yang Mei, Xiang Pingwei, et al. Research progress on important components and their applications in citrus peel [J], Food and Fermentation Industries, 2024, 50(2):379-388), the cellulose content of orange peel is only 20%-30%, and the structure of its fiber tissue determines that the carbon skeleton structure formed after carbonization is a loose structure, which makes it impossible to maintain stability after loading active substances.

[0081] Therefore, according to existing literature, although grapefruit and orange belong to the same genus Citrus in the Rutaceae family, meaning they are highly similar as biomass raw materials, the differences in their components, represented by cellulose content and fibrous tissue structure, directly lead to substantial differences in the structure of the carbon dots obtained after carbonization.

[0082] To demonstrate the effect of 2-methylimidazole on the performance of nitrogen-doped carbon dots, i.e. the role of 2-methylimidazole in the technical solution, Comparative Example 2 is provided, a grapefruit inner peel carbon dot without the addition of 2-methylimidazole.

[0083] Comparative Example 2

[0084] A method for preparing carbon dots from grapefruit inner peel without adding 2-methylimidazole. Unless otherwise specified, the steps are the same as in Example 1, except that 2-methylimidazole is not added in step 2, and the resulting carbon dots are referred to as PP-CPDs.

[0085] The cycle performance test results of zinc-zinc symmetric cells of PP-CPDs are as follows: Figure 5 As shown, at a current density of 1 mA / cm² -2, capacity 0.5 mAh cm -2 PP-CPDs-ZnSO can be stable for 240 h under the condition of 0.5 mA cm

[0086] Compared with ZnSO, the addition of PP-CPDs can slightly improve the cycle performance of zinc-zinc symmetric battery, and the improvement is only 60%;

[0087] Compared with NPP-CPDs-ZnSO of Example 1, nitrogenation by adding 2-methyl imidazole can significantly improve the cycle stability.

[0088] The reason is that 2-methyl imidazole is used as a nitrogen source, and the loading of 2-methyl imidazole on the pomelo inner pericarp can be doped with nitrogen, which can improve the active sites of the obtained PP-CPDs, and then make the PP-CPDs and the Zn 2+ form Zn-N bond, improve the adsorption effect of PP-CPDs carbon dots on the surface of zinc electrode, reduce the contact of H2O and zinc electrode, thereby inhibit the occurrence of hydrogen evolution reaction, and effectively improve the cycle stability of zinc electrode.

Claims

1. A method for preparing nitrogen-doped carbon dots based on pomelo inner pericarp, characterized by It comprises the following steps: Step 1, pretreatment of the inner pericarp of pomelo peel, first, taking the inner pericarp of pomelo, i.e. the white sponge-like pericarp, as the raw material, drying it under certain conditions, then, under certain conditions, ball-milling the dried white pomelo pericarp to obtain pomelo peel powder, abbreviated as PP; In step 1, the drying conditions are that the drying temperature is 85 ℃, and the drying time is 8-12 h; In step 1, the ball-milling conditions are that the ball-milling speed is 300-500 rpm, and the ball-milling time is 4-6 h; Step 2, preparation of R-NPP-CPDs, first, dissolving 2-methylimidazole in ultrapure water to obtain an imidazole solution, then, placing the PP obtained in step 1 in the imidazole solution to perform a hydrothermal reaction under certain conditions to obtain raw NPP-CPDs, abbreviated as R-NPP-CPDs; In step 2, the mass ratio of PP to 2-methylimidazole is 1: (1-1.5); In step 2, the hydrothermal reaction conditions are that the hydrothermal temperature is 160-200 ℃, and the hydrothermal time is 8-12 h; Step 3, purification of R-NPP-CPDs, first, filtering the R-NPP-CPDs obtained in step 2 at room temperature to obtain a filtrate, then, dialyzing the filtrate under certain conditions, after dialysis is completed, the solution in the dialysis bag is the purified NPP-CPDs, finally, freeze-drying the purified NPP-CPDs to obtain the NPP-CPDs based on the inner pericarp of pomelo, abbreviated as NPP-CPDs; In step 3, the filtering conditions are that the filtration is performed with a microporous membrane with a pore size of 0.22 μm; the dialysis conditions are that the dialysis is performed with a dialysis bag with MW 1000; In step 3, the dialysis method is that the ultrapure water is replaced every 8 h, and the total dialysis time is 24 h; In step 3, the freeze-drying method is that first, freezing at a freezing temperature of -20 to -25 ℃ for 24 h, then, freeze-drying at a freeze-drying temperature of -45 ℃ for 24 to 72 h.

2. The method of claim 1, wherein: The obtained nitrogen-doped carbon dots based on pomelo inner pericarp are prepared by taking pomelo inner pericarp and 2-methylimidazole as raw materials, after hydrothermal reaction, filtration, dialysis and freeze-drying, to obtain nitrogen-doped carbon dots NPP-CPDs; the size of the NPP-CPDs is 10-20 nm, and under the condition of an excitation wavelength of 380 nm, the NPP-CPDs have a fluorescence emission peak at 455 nm; the NPP-CPDs are loaded with imidazole group-Im, carboxyl-COOH and hydroxyl-OH on the surface, wherein the nitrogen sites can be coordinated with Zn 2+ stable coordination.

3. The method of claim 1, wherein: When the nitrogen-doped carbon dots obtained from the inner peel of grapefruit are used as additives in zinc-ion battery electrolytes, the concentration of NPP-CPDs in the ZnSO4 electrolyte is 100-800 mg / L. -1 NPP-CPDs can coordinate with Zn on the zinc anode surface. 2+ Stable Zn-N bonds are formed; the cycle performance of the zinc-zinc symmetric cell is 1400-1600 h.

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