Preparation method and application of sodium-ion battery positive electrode material vanadium sodium fluorophosphate modified by N-doped carbon based on ZIF-8

Abstract

This invention discloses a method for preparing and applying sodium vanadium fluorophosphate, a nitrogen-doped carbon-modified cathode material for sodium-ion batteries based on ZIF-8. The method includes the following steps: (1) using Zn(NO3)2·6H2O and 2-methylimidazole as raw materials and methanol as solvent, a zeolite-type imidazole framework-8 ZIF-8 is prepared; (2) using ZIF-8 as raw material, the material is calcined at high temperature, soaked in hydrochloric acid, filtered, and washed to obtain a black powder nitrogen-doped porous carbon material N-PC; (3) using ammonium metavanadate as the vanadium source, ammonium dihydrogen phosphate as the phosphorus source, sodium fluoride as the fluorine source, citric acid as the reducing agent, nitrogen-doped porous carbon material N-PC as the carbon source, and ultrapure water as the solvent, a Na3V2(PO4)2F3 / N-PC composite material is prepared in situ by spray drying combined with high-temperature sintering. In the composite material, Na3V2(PO4)2F3 has a hollow sphere structure, and nitrogen-doped porous carbon material N-PC is tightly attached to the surface of Na3V2(PO4)2F3 and inside the hollow sphere. This is beneficial for sodium ion insertion and extraction in the material and helps to suppress the capacity decay caused by the collapse of the microstructure after multiple charge-discharge cycles.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery cathode material preparation technology, specifically relating to a preparation method and application of sodium vanadium fluorophosphate, an N-doped carbon-modified sodium-ion battery cathode material based on ZIF-8. Background Technology

[0002] Na3V2(PO4)2F3 (NVPF), with its NASICON structure, has attracted widespread attention due to its high ionic conductivity, high operating voltage, and stable three-dimensional framework structure. However, Na3V2(PO4)2F3 (NVPF) has been reported to have poor conductivity and poor ion diffusion rate, making it difficult to exhibit excellent electrochemical performance such as high rate capability and long cycle life. This problem is considered a major factor hindering the rate improvement and capacity retention of NVPF in battery cathode materials. Researchers have proposed several strategies to overcome these shortcomings of Na3V2(PO4)2F3, the most popular of which is the integration of Na3V2(PO4)2F3 with conductive porous nanomaterials (such as graphene, carbon nanotubes, and carbon black). Heteroatom doping (nitrogen, sulfur, phosphorus, and iodine) of carbon materials has always been an effective strategy for regulating the electrochemical activity and improving the chemical reactivity of electrode materials. In particular, nitrogen doping is an effective method to improve the reactivity and electronic conductivity of carbon materials by generating external defects.

[0003] Literature reports indicate that doping carbon materials with nitrogen atoms improves both conductivity and specific capacity. Huang et al. reported that N-doped carbon nanofibers using polypyrrole as a precursor could provide a reversible capacity of 73 mAh / g at a current density of 20 A / g. Yang's group found that nitrogen-doped carbon sheets using polydopamine as a nitrogen source exhibited a capacity of 382 mA h / g at a current density of 50 mA / g. These studies clearly demonstrate that nitrogen-doped porous carbon (N-PC) can exhibit better electrochemical performance in sodium-ion batteries (SIBs).

[0004] In recent years, direct pyrolysis of metal-organic frameworks (MOFs) has proven to be a simple method for synthesizing porous carbon materials. Unlike ordinary carbon materials, MOF-derived carbon materials exhibit advantages due to their simple synthesis route. Furthermore, nitrogen-doped porous carbon materials can be obtained through a simple one-step high-temperature carbonization process without the need for additional nitrogen or carbon sources. These MOF-derived porous carbon materials can then be combined with existing electrode materials as carbon sources, leveraging their structural advantages to enhance the electrochemical performance of the original electrode materials, thereby achieving modification.

[0005] Zeolite-type imidazole framework-8 (ZIF-8), a metal-organic framework (MOF), undergoes pyrolysis to evaporate all Zn in its structure, yielding nitrogen-doped porous carbon material (N-PC). The porosity and nitrogen doping of N-PC make it an ideal carbon material for modification. This patent utilizes the nitrogen-doped porous carbon material (N-PC) obtained from ZIF-8 as the conductive support for the positive electrode of sodium-ion batteries (SIBs), aiming to solve the problems of poor conductivity and poor ion diffusion rate in Na3V2(PO4)2F3 electrode materials. Summary of the Invention

[0006] Purpose of the invention: The purpose of this invention is to overcome the shortcomings of poor conductivity and poor ion diffusion rate of Na3V2(PO4)2F3, and to provide a method for modifying Na3V2(PO4)2F3 to improve its electrochemical performance by utilizing the structural advantages of preparing nitrogen-doped porous carbon materials with ZIF-8.

[0007] To achieve the above-mentioned objectives, the following technical solution is proposed:

[0008] A nitrogen-doped carbon-modified sodium-ion battery cathode material, Na3V2(PO4)2F3, based on ZIF-8 template, is disclosed. This cathode material consists of Na3V2(PO4)2F3 and nitrogen-doped porous carbon material (N-PC) obtained through ZIF-8 modification. Its microstructure features a hollow spherical structure in Na3V2(PO4)2F3, with the N-PC tightly adhered to the surface of Na3V2(PO4)2F3 and the interior of the hollow spheres. The specific surface area is 5.100 m². 2 / g, with an average pore size of 14.684 nm.

[0009] A method for preparing Na3V2(PO4)2F, an N-doped carbon-modified sodium-ion battery cathode material based on ZIF-8 template, includes the following steps:

[0010] 1. Preparation of ZIF-8

[0011] 1.468 g of Zn(NO3)2·6H2O was weighed and dissolved in 100 mL of methanol solution. After stirring at room temperature (25℃) for 5 min, mixture A was obtained. 1.622 g of 2-methylimidazole (2-MeIm) was then weighed and dissolved in 100 mL of methanol solution. The mixture was also stirred at room temperature for 5 min to obtain mixture B. Mixture B was rapidly combined with mixture A and stirred vigorously at room temperature for 1 h to obtain a white emulsion. The white emulsion was sealed with plastic wrap and allowed to stand for 24 h. The white emulsion, after standing for 24 h, was centrifuged at 10000 r / min. The precipitate was washed three times with methanol and finally dried in a vacuum drying oven at 60℃ for 12 h to obtain the ZIF-8 sample material.

[0012] 2. Preparation of nitrogen-doped porous carbon materials

[0013] A certain amount of ZIF-8 white powder sample was weighed and poured into a crucible. Under an argon (Ar) atmosphere, the temperature was increased to 900℃ at a heating rate of 5℃ / min and held for 3 h. The temperature was then decreased at a cooling rate of 3℃ / min until room temperature was reached, yielding a black powder sample. 0.2 g of the black powder was weighed into a beaker and added to 4 mol / L hydrochloric acid. The mixture was stirred overnight on a magnetic stirrer to remove elemental Zn from the ZIF-8 sample. Finally, the sample was filtered, and the precipitate was washed with deionized water until neutral. The precipitate was then placed in a forced-air drying oven and dried at 60℃. The resulting black powder nitrogen-doped porous carbon material was denoted as N-PC.

[0014] 3. Preparation of Na3V2(PO4)2F3 / N-PC composite material

[0015] Na3V2(PO4)2F3 / C composite material was prepared by spray drying combined with high-temperature sintering. 2.3398 g of ammonium metavanadate (NH4VO3), 2.3004 g of ammonium dihydrogen phosphate (NH4H2PO4), 1.2597 g of sodium fluoride (NaF), and citric acid were weighed as reducing agents, and N-PC was added last. All the weighed reagents were added to round-bottom flasks, and 30 mL of ultrapure water was added as a solvent. The mixture was stirred in an oil bath at 80℃ for 6-8 h until completely dissolved in the solvent. The mixed solution was then spray-dried at 200℃, and the Na3V2(PO4)2F3 / N-PC precursor powder was collected. Subsequently, the Na3V2(PO4)2F3 / N-PC precursor powder was transferred to an alumina crucible and sintered at high temperature in a tube furnace under Ar protection. First, the temperature was raised to 300℃ for 3 h (heating rate of 5℃ / min), and then the temperature was programmed to 650℃ for 6 h (heating rate of 10℃ / min). After the apparatus was allowed to cool naturally to room temperature, the sample was taken out, and the Na3V2(PO4)2F3 / N-PC composite material was obtained.

[0016] Compared with existing Na3V2(PO4)2F3 carbon-modified composite materials, the present invention has the following advantages: (1) By utilizing the structural advantages of nitrogen-doped porous carbon materials prepared by ZIF-8, Na3V2(PO4)2F3 is modified, which greatly improves the electrochemical performance of Na3V2(PO4)2F3. (2) The Na3V2(PO4)2F3 / N-PC composite material has a hollow sphere structure, and the nitrogen-doped porous carbon material (N-PC) is tightly attached to the surface of Na3V2(PO4)2F3 and the interior of the hollow sphere. This structure is beneficial to the insertion and extraction of sodium ions in the material. On the other hand, the nitrogen-doped porous carbon material (N-PC) can increase the conductivity of the material and support the material structure. This is beneficial to suppress the significant capacity decay caused by the collapse of the microstructure after multiple charge-discharge cycles. Attached Figure Description

[0017] Figure 1 (a) and (b) are SEM images of the Na3V2(PO4)2F3 / N-PC composite material at different magnifications in the embodiments of the present invention.

[0018] Figure 2 The adsorption-desorption curves are for the Na3V2(PO4)2F3 / N-PC composite material in this embodiment.

[0019] Figure 3 (a) and (b) are the charge-discharge curves and rate performance diagrams of the Na3V2(PO4)2F3 / N-PC composite material in this embodiment at different rates. Detailed Implementation

[0020] The present invention will now be described in detail with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited thereto.

[0021] A method for preparing Na3V2(PO4)2F, an N-doped carbon-modified sodium-ion battery cathode material based on ZIF-8 template, includes the following steps:

[0022] 1. Preparation of ZIF-8

[0023] 1.468 g of Zn(NO3)2·6H2O was weighed and dissolved in 100 mL of methanol solution. After stirring at room temperature (25℃) for 5 min, mixture A was obtained. 1.622 g of 2-methylimidazole (2-MeIm) was then weighed and dissolved in 100 mL of methanol solution. The mixture was also stirred at room temperature for 5 min to obtain mixture B. Mixture B was rapidly combined with mixture A and stirred vigorously at room temperature for 1 h to obtain a white emulsion. The white emulsion was sealed with plastic wrap and allowed to stand for 24 h. The white emulsion, after standing for 24 h, was centrifuged at 10000 r / min. The precipitate was washed three times with methanol and finally dried in a vacuum drying oven at 60℃ for 12 h to obtain the ZIF-8 sample material.

[0024] 2. Preparation of nitrogen-doped porous carbon materials

[0025] Five grams of ZIF-8 white powder sample were weighed and poured into a crucible. Under an argon (Ar) atmosphere, the temperature was increased to 900℃ at a heating rate of 5℃ / min and held for 3 h. The temperature was then decreased at a cooling rate of 3℃ / min until room temperature was reached, yielding a black powder sample. 0.2 g of this black powder was weighed into a beaker and added to 100 mL of 4 mol / L hydrochloric acid. The mixture was stirred overnight on a magnetic stirrer to remove elemental Zn from the ZIF-8 sample. The sample was then filtered, and the precipitate was washed with deionized water until neutral. The precipitate was then placed in a forced-air drying oven and dried at 60℃. The resulting black powder nitrogen-doped porous carbon material was designated N-PC.

[0026] 3. Preparation of Na3V2(PO4)2F3 / N-PC composite material

[0027] Na3V2(PO4)2F3 / C composite material was prepared by spray drying combined with high-temperature sintering. 2.3398 g of ammonium metavanadate (NH4VO3), 2.3004 g of ammonium dihydrogen phosphate (NH4H2PO4), 1.2597 g of sodium fluoride (NaF), and 0.479 g of citric acid were weighed as reducing agents, and finally 0.59 g of N-PC was added. All the weighed reagents were added to round-bottom flasks, and 30 mL of ultrapure water was added as a solvent. The mixture was stirred for 7 h in an oil bath at 80℃ until completely dissolved in the solvent. The mixed solution was then spray-dried at 200℃, and the Na3V2(PO4)2F3 / N-PC precursor powder was collected. Subsequently, the Na3V2(PO4)2F3 / N-PC precursor powder was transferred to an alumina crucible and sintered at high temperature in a tube furnace under Ar protection. First, the temperature was raised to 300℃ for 3 h (heating rate of 5℃ / min), and then the temperature was programmed to 650℃ for 6 h (heating rate of 10℃ / min). After the apparatus was allowed to cool naturally to room temperature, the sample was taken out, and the Na3V2(PO4)2F3 / N-PC composite material was obtained.

[0028] The morphology characterization results of the Na3V2(PO4)2F3 / N-PC composite material are shown in Figure 1(a) and Figure 1(b). Figure 1 It is easy to see from (a) that a large number of uniform hollow spherical Na3V2(PO4)2F3 particles can be observed at low magnification. The size of the spheres is uniformly distributed in the range of 5-10 μm. Figure 1 As can be seen in (b), the nitrogen-doped porous carbon material is tightly attached to the surface of Na3V2(PO4)2F3 and the interior of the hollow spheres, which increases the conductivity of the material and supports the material structure. This helps to suppress the significant capacity decay caused by the collapse of the microstructure after multiple charge-discharge cycles.

[0029] The specific surface area test results of the Na3V2(PO4)2F3 / N-PC composite material are shown in Figure 2. Figure 2 It can be seen that the BET surface area of ​​the Na3V2(PO4)2F3 / N-PC composite material is 5.100 m². 2 / g, the average pore size measured by the BJH method is 14.684 nm, compared with 1.365 nm for pure phase Na3V2(PO4)2F3 material. 2 The specific surface area of ​​the Na3V2(PO4)2F3 / N-PC composite material is increased, and there are more active sites for the reaction.

[0030] The electrochemical performance test results of the Na3V2(PO4)2F3 / N-PC composite material are shown in the figure. Figure 3 (a) and Figure 3(b). From above Figure 3 As can be seen in (a), the Na3V2(PO4)2F3 / N-PC composite material can maintain a high discharge specific capacity at different rates. At a rate of 0.1 C, the capacity reaches 109.56 mAh / g, which is 26.56% higher than the 86.57 mAh / g of the pure phase material. Moreover, a complete plateau appears during the high-rate cyclic charge and discharge process. Figure 3 (b) shows that when the discharge rate is changed from 0.1 C to 2 C and then back to 0.1 C, the discharge specific capacity of the Na3V2(PO4)2F3 / N-PC composite material can be approximately returned to the discharge specific capacity at the initial 0.1 C, which also shows that the material has excellent rate performance.

Claims

1. A method for preparing sodium vanadium fluorophosphate, an N-doped carbon-modified sodium-ion battery cathode material based on ZIF-8, characterized in that, Includes the following steps: (1) Preparation of ZIF-8: Weigh 1.468 g of Zn(NO3)2·6H2O and dissolve it in 100 mL of methanol solution. Stir at room temperature (25℃) for 5 min to obtain mixture A. Weigh 1.622 g of 2-methylimidazole and dissolve it in 100 mL of methanol solution. Stir at room temperature for 5 min to obtain mixture B. Mix B with A quickly and stir vigorously at room temperature for 1 h to obtain a white emulsion. Cover the white emulsion with plastic wrap and let it stand for 24 h. Centrifuge the white emulsion that has stood for 24 h at 10000 r / min. Take the precipitate and wash it three times with methanol. Finally, place it in a vacuum drying oven and dry it at 60℃ for 12 h to obtain ZIF-8 sample material. (2) Preparation of nitrogen-doped porous carbon material: Weigh 5 g of the ZIF-8 white powder sample obtained in step (1) above, transfer it into a crucible, heat it to 900℃ at a heating rate of 5℃ / min under an argon atmosphere, and keep it at that temperature for 3 h. The cooling rate is 3℃ / min. After cooling to room temperature, a black powder sample is obtained. Weigh 0.2 g of the black powder in a beaker, add 100 ml of hydrochloric acid with a concentration of 4 mol / L, stir on a magnetic stirrer overnight, filter, wash the precipitate with deionized water until neutral, put it in a forced-air drying oven, and dry it at 60℃ to obtain the black powder nitrogen-doped porous carbon material N-PC. (3) Preparation of Na3V2(PO4)2F3 / N-PC composite material: Weigh 2.3398 g of ammonium metavanadate NH4VO3, 2.3004 g of ammonium dihydrogen phosphate NH4H2PO4, 1.2597 g of sodium fluoride NaF and 0.479 g of citric acid, and finally add 0.59 g of nitrogen-doped porous carbon material N-PC obtained in step (2) above. Add all the above-weighed reagents to a round-bottom flask and add 30 mL of ultrapure water as solvent. Stir for 6-8 minutes in an oil bath at 80°C. The solution was completely dissolved in the solvent. The mixed solution was then dried in a spray dryer at 200°C to collect the Na3V2(PO4)2F3 / N-PC precursor powder. The Na3V2(PO4)2F3 / N-PC precursor powder was then transferred to an alumina crucible and sintered at high temperature in a tube furnace under Ar protection. The heating rate was first controlled at 5°C / min, and the temperature was raised to 300°C for 3 h for preheating. Then the heating rate was controlled at 10°C / min, and the temperature was raised to 650°C for calcination for 6 h. After the apparatus was allowed to cool naturally to room temperature, the sample was taken out to obtain the Na3V2(PO4)2F3 / N-PC composite material.

2. A sodium vanadium fluorophosphate cathode material for N-doped carbon modified sodium-ion batteries based on ZIF-8 template, prepared by the method described in claim 1, characterized in that... The cathode material consists of Na3V2(PO4)2F3 and nitrogen-doped porous carbon material N-PC obtained through ZIF-8. The structural characteristics of this sodium-ion battery cathode material are that Na3V2(PO4)2F3 has a hollow sphere structure, and the nitrogen-doped porous carbon material N-PC is tightly attached to the surface of Na3V2(PO4)2F3 and inside the hollow spheres, with a specific surface area of ​​5.100 m². 2 / g, with an average pore size of 14.684 nm.

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