A cobalt / nitrogen-doped carbon material-modified Na3V2(PO4)2F3 composite material with bimetallic MOF as template and its preparation method.

Abstract

This invention discloses a nitrogen-doped modified Na3V2(PO4)2F3 composite material using a bimetallic MOF as a template, which can be used as a cathode material for sodium-ion batteries. The preparation of this composite material includes the following steps: (1) preparing a bimetallic MOF, denoted as Co / Zn-ZIF, by a solvothermal method; (2) obtaining a nitrogen-doped derived carbon material Co / N-PC by high-temperature calcination of Co / Zn-ZIF; (3) preparing the Na3V2(PO4)2F3 / Co / N-PC composite material by spray drying-high-temperature sintering. The purpose of this invention is to solve the problems of poor conductivity and poor ion diffusion rate of Na3V2(PO4)2F3 electrode material. A high-temperature pyrolysis bimetallic MOF is proposed to synthesize porous nitrogen-doped carbon nanocube material containing Co nanoparticles. Then, it is combined with the polyanionic compound Na3V2(PO4)2F3 to obtain Na3V2(PO4)2F3 / Co / N-PC composite material, thereby overcoming the problems of poor conductivity and poor ion diffusion rate, and thus improving its electrochemical performance as a cathode material for sodium-ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery cathode material preparation technology, specifically relating to a cobalt / nitrogen-doped carbon material modified Na3V2(PO4)2F3 composite material with a bimetallic MOF as a template, which can be used as a sodium-ion battery cathode material, and its preparation method. Background Technology

[0002] Lithium-ion batteries have flourished as a green and renewable energy source over the past three decades. They have not only served the emerging markets of electric vehicles and portable electronic devices but have also been used as an alternative power source for grid storage facilities. However, lithium-ion batteries also have significant drawbacks. The relative abundance of lithium in the Earth's crust is only 20 ppm, and lithium resources are mainly distributed in South America, making lithium supply dependent on imports. Sodium, as a clear substitute for lithium, exhibits a relatively low standard redox potential (relative to the standard hydrogen electrode of -2.71 V), has similar chemical properties to lithium, and has abundant resource reserves. It holds promise for producing sodium-ion batteries that are low-cost, efficient, and less prone to resource shortages.

[0003] Sodium-ion battery cathode materials are currently classified into three main categories: transition metal oxides, polyanionic compounds, and Prussian blue compounds. Na3V2(PO4)2F3, as a polyanionic compound, has been found to possess characteristics such as long cycle life, strong structural stability, and high safety. However, the poor conductivity and ion diffusion rate of Na3V2(PO4)2F3 are considered important factors hindering its application as a battery cathode material in terms of both rate improvement and capacity retention. Therefore, research on the modification of Na3V2(PO4)2F3 is of great significance for improving its performance in sodium-ion battery cathode materials.

[0004] Doping has always been an effective strategy for regulating the electrochemical activity and improving the reactivity of electrode materials. Nitrogen doping is an effective method to enhance the reactivity and electronic conductivity of carbon materials by generating external defects. The doping modification of transition metals, such as Fe, Co, Ni, and Mo, has attracted widespread attention due to its ability to improve conductivity and provide more active sites. However, some problems, such as the aggregation of metal nanoparticles and low density of active sites, still hinder the development of transition metal doping. In recent years, MOF-derived nitrogen-doped carbon materials have gained widespread attention due to their simple synthesis route and the ability to obtain nitrogen-doped porous carbon materials through a simple one-step high-temperature carbonization without the need for additional nitrogen and carbon sources. When these MOF-derived porous carbon materials are combined with existing electrode materials, they not only retain the high specific surface area of ​​MOFs but also retain transition metal elements, thus exhibiting excellent conductivity and catalytic performance, achieving the purpose of modification.

[0005] This patent addresses the synergistic effect of cobalt nanoparticles and porous nitrogen-doped carbon materials to improve the electrochemical performance of materials. A bimetallic Co / Zn-ZIF was synthesized and subjected to high-temperature pyrolysis to obtain nitrogen-doped porous carbon nanocubes containing Co nanoparticles (Co / N-PC). During the high-temperature carbonization process, Zn is released as vapor. This Zn release creates numerous pores on the surface of the Co / N-PC, resulting in a rougher surface that significantly increases the specific surface area and provides more reactive sites. 2+ The reducing gases (NH3 and H2) generated during high-temperature pyrolysis reduce the carbon material to Co nanoparticles. The nano-Co formed in nitrogen-doped porous carbon materials enhances the conductivity of the carbon materials and promotes charge transfer in the electrode materials. The results also show that using Na3V2(PO4)2F3 / Co / N-PC as the cathode material for sodium-ion batteries exhibits excellent electrochemical performance, including stable charge-discharge curves and long cycle life. Summary of the Invention

[0006] Purpose of the invention: The purpose of this invention is to solve the problems of poor conductivity and poor ion diffusion rate of Na3V2(PO4)2F3 electrode material. It proposes a high-temperature pyrolysis bimetallic MOF to synthesize a porous nitrogen-doped carbon nanocube material containing Co nanoparticles, and then composite it with the polyanionic compound Na3V2(PO4)2F3 to obtain Na3V2(PO4)2F3 / Co / N-PC composite material, so as to overcome the problems of poor conductivity and poor ion diffusion rate, thereby improving its electrochemical performance as a cathode material for sodium-ion batteries.

[0007] If the above-mentioned objectives are not achieved, the following technical solution is proposed:

[0008] A method for preparing a cobalt / nitrogen-doped modified Na3V2(PO4)2F3 composite material using a bimetallic MOF as a template, which can be used as a cathode material for sodium-ion batteries, includes the following steps:

[0009] 1. Preparation of bimetallic MOFs

[0010] Weigh 223.12 mg of Zn(NO3)2·6H2O, 72.76 mg of Co(NO3)2·6H2O, and 5 mg of methyltrimethylammonium bromide (CTAB) and dissolve them in 10 mL of deionized water. Stir for 5 min to form solution A. Then weigh 4540 mg of 2-methylimidazole and dissolve it in 70 mL of deionized water. Stir for 5 min to form solution B. Quickly add solution A to solution B and stir vigorously at 25°C for 20 min to obtain a pale purple emulsion. Seal the emulsion with plastic wrap at room temperature and age for 24 h. Then centrifuge at 4000 r / min. The precipitate is collected using a mixture of ethanol and deionized water (V... 乙醇 V 去离子水 =1:1) Repeat washing 5-6 times, and finally place in a vacuum drying oven and dry at 60℃ for 12 h. The resulting purple powder sample is denoted as Co / Zn-ZIF and weighed.

[0011] 2. Preparation of cobalt / nitrogen-doped carbon materials

[0012] All the Co / Zn-ZIF samples obtained above were transferred into a crucible and heated to 900℃ at a heating rate of 5℃ / min under an argon atmosphere. After holding at this temperature for 3 h, the temperature was lowered at a cooling rate of 3℃ / min until the temperature reached room temperature, thus obtaining a black powder sample. 0.2 g of this black powder was weighed and added to hydrochloric acid with a concentration of 4 mol / L. The mixture was stirred overnight on a magnetic stirrer to remove elemental Zn from the sample. Finally, the sample was filtered and the precipitate was washed with deionized water until the filtrate was neutral. The precipitate was then placed in a forced-air drying oven and dried at 60℃. The resulting black powder was designated as Co / N-PC and weighed.

[0013] 3. Preparation of Na3V2(PO4)2F3 / Co / N-PC composite material

[0014] Weigh 2.3398 g of ammonium metavanadate NH4VO3, 2.3004 g of ammonium dihydrogen phosphate NH4H2PO4, 1.2597 g of sodium fluoride NaF, and 0.467 g of citric acid as a reducing agent, and add them to a round-bottom flask. Finally, add all the Co / N-PC obtained in step (2) above and 30 mL of deionized water as a solvent. Stir for 6-8 h in an oil bath at 80°C to completely dissolve the solution in the solvent. After passing the dissolved solution through a spray dryer at 200°C, collect the Na3V2(PO4)2F3 / Co / N-PC precursor powder. Then, transfer the Na3V2(PO4)2F3 / Co / N-PC precursor powder to an alumina crucible and sinter it at high temperature in a tube furnace under argon protection. First, preheat the temperature to 300°C at a heating rate of 5°C / min. After h, the temperature was raised to 650℃ and calcined for 6 h at a heating rate of 10℃ / min. After the device was allowed to cool naturally to room temperature, the sample was taken out, and the Na3V2(PO4)2F3 / Co / N-PC cathode material was obtained. Compared with the existing Na3V2(PO4)2F3, the present invention has the following advantages: (1) The metal / carbon material prepared by high-temperature pyrolysis of MOFs can retain the high specific surface area of ​​MOFs, and at the same time avoid the disadvantages of metal nanoparticle aggregation and low active site density, thus exhibiting excellent conductivity and catalytic performance. (2) Co nanoparticles can enhance the conductivity of carbon materials and promote charge transfer of electrode materials. The doping of N atoms can generate external defects, thereby improving the electronic conductivity of carbon materials. At the same time, the presence of N shortens the time of Na + The insertion and extraction channels play a positive role in improving electrochemical performance. Attached Figure Description

[0015] Figure 1 (ab) and Figure 1 (cd) are SEM images of the cobalt / nitrogen-doped carbon material Co / N-PC and the Na3V2(PO4)2F3 / Co / N-PC composite material in the examples of this invention at different magnifications.

[0016] Figure 2 The adsorption-desorption curves are shown for the Na3V2(PO4)2F3 / Co / N-PC composite material in the embodiments of the present invention.

[0017] Figure 3 The image shows the CV curve of the Na3V2(PO4)2F3 / Co / N-PC composite material in the embodiments of the present invention.

[0018] Figure 4 (a) and Figure 4 (b) are the charge-discharge curves and rate performance diagrams of the Na3V2(PO4)2F3 / Co / N-PC composite material in the embodiments of the present invention, respectively. Detailed Implementation

[0019] 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.

[0020] A method for preparing a cobalt / nitrogen-doped modified Na3V2(PO4)2F3 composite material using a bimetallic MOF as a template, which can be used as a cathode material for sodium-ion batteries, includes the following steps:

[0021] 1. Preparation of bimetallic MOFs

[0022] Weigh 223.12 mg of Zn(NO3)2·6H2O, 72.76 mg of Co(NO3)2·6H2O, and 5 mg of methyltrimethylammonium bromide (CTAB) and dissolve them in 10 mL of deionized water. Stir for 5 min to form solution A. Then weigh 4540 mg of 2-methylimidazole and dissolve it in 70 mL of deionized water. Stir for 5 min to form solution B. Quickly add solution A to solution B and stir vigorously at 25°C for 20 min to obtain a pale purple emulsion. Seal the emulsion with plastic wrap at room temperature and age for 24 h. Then centrifuge at 4000 r / min. The precipitate is collected using a mixture of ethanol and deionized water (V... 乙醇 :V 去离子水 The mixture was washed 6 times with a ratio of 1:1, and then placed in a vacuum drying oven and dried at 60°C for 12 h. The resulting purple powder sample was denoted as Co / Zn-ZIF and weighed.

[0023] 2. Preparation of Co / N-PC, a Co / Nitrogen-Doped Derivative Carbon Material

[0024] All the Co / Zn-ZIF samples obtained in step 1 above were transferred into a crucible and heated to 900℃ at a heating rate of 5℃ / min under an argon atmosphere. After holding at this temperature for 3 h, the temperature was lowered at a cooling rate of 3℃ / min until the temperature reached room temperature, thus obtaining a black powder sample. 0.2 g of the black powder was weighed and added to hydrochloric acid with a concentration of 4 mol / L. The mixture was stirred overnight on a magnetic stirrer to remove elemental Zn from the sample. Finally, the sample was filtered and the precipitate was washed with deionized water until the filtrate was neutral. The precipitate was then placed in a forced-air drying oven and dried at 60℃. The resulting black powder was designated as Co / N-PC and weighed.

[0025] 3. Preparation of Na3V2(PO4)2F3 / Co / N-PC composite material

[0026] Weigh 2.3398 g ammonium metavanadate NH4VO3, 2.3004 g ammonium dihydrogen phosphate NH4H2PO4, 1.2597 g sodium fluoride NaF, and 0.467 g citric acid as a reducing agent, and add them to a round-bottom flask. Finally, add all the Co / N-PC obtained in step (2) above and 30 mL of deionized water as a solvent. Stir for 8 h in an oil bath at 80°C to completely dissolve it in the solvent. After passing the dissolved solution through a spray dryer at 200°C, collect the Na3V2(PO4)2F3 / Co / N-PC precursor powder. Then transfer the Na3V2(PO4)2F3 / Co / N-PC precursor powder to an alumina crucible and sinter it at high temperature in a tube furnace under argon protection. First, preheat the temperature to 300°C at a heating rate of 5°C / min. After h, the temperature was raised to 650℃ and calcined for 6 h at a heating rate of 10℃ / min. After the device was allowed to cool naturally to room temperature, the sample was taken out, and the Na3V2(PO4)2F3 / Co / N-PC cathode material was obtained.

[0027] The morphology characterization of Co / N-PC materials is shown in the appendix. Figure 1 (ab) shows that the Co / N-PC prepared in this patent has a nanocubic structure, and its rough surface can increase the specific surface area. (Appendix) Figure 1 (cd) shows the morphological characterization of the Na3V2(PO4)2F3 / Co / N-PC composite material. The material exhibits a hollow sphere structure in its microstructure, with many small nanocube structures, namely Co / N-PC, attached to its surface and interior, proving the successful preparation of the Na3V2(PO4)2F3 / Co / N-PC composite material.

[0028] The adsorption-desorption curves of the Na3V2(PO4)2F3 / Co / N-PC composite material are shown in the appendix. Figure 2 From the appendix Figure 2 It can be seen that the BET surface area of ​​the Na3V2(PO4)2F3 / Co / N-PC composite material is 21.107 m². 2 / g, the average pore size measured by the BJH method is 7.170 nm, compared with 1.365 nm for pure phase Na3V2(PO4)2F3 material. 2 / g, the composite material has a larger specific surface area and more active sites for the reaction, a conclusion consistent with the attached... Figure 1 The results of the increased specific surface area were consistent with the analysis, further proving the successful preparation of the Na3V2(PO4)2F3 / Co / N-PC composite material.

[0029] The CV curve of the Na3V2(PO4)2F3 / Co / N-PC composite material is attached. Figure 3As shown, within a voltage range of 2.3–4.7 V, the electrode material exhibits three pairs of redox peaks during charge and discharge, indicating that the material undergoes three Na+ oxidation-reduction peaks during this process. + The three oxidation peaks are located at 3.34 V, 3.57 V, and 4.05 V, respectively, while the three reduction peaks are located at 3.44 V, 3.65 V, and 4.12 V. The peak voltage difference ΔEp for each pair of redox peaks is relatively small, which proves that the electrode reaction of the material has good reversibility. The electrochemical performance of the Na3V2(PO4)2F3 / Co / N-PC composite material modified with Co / N-PC is better than that of pure phase Na3V2(PO4)2F3 material. Since the active sites of Co nanoparticles inserted into the nanocube can promote charge transfer in carbon materials, and N doping has the ability to shorten the ion insertion / extraction channel, the synergistic effect of the two makes the electrochemical performance of Na3V2(PO4)2F3 / Co / N-PC composite material superior to that of Na3V2(PO4)2F3 / N-PC composite material.

[0030] Appendix Figure 4 (a) is the charge-discharge curve of the Na3V2(PO4)2F3 / Co / N-PC composite material. It can be seen that this composite material maintains a high discharge specific capacity at different rates. At a rate of 0.1 C, the specific capacity reaches 126.66 mAh / g, and a complete plateau appears during high-rate cyclic charge-discharge. The three plateau voltages correspond to the three pairs of redox peak voltages in the CV curve. (See attached image.) Figure 4 (b) is a rate performance diagram of the Na3V2(PO4)2F3 / Co / N-PC composite material. As can be seen from the figure, when the 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 / Co / N-PC composite material can also be approximately returned to the discharge specific capacity at the initial 0.1 C. This also shows that the material has excellent rate performance.

Claims

1. A sodium-ion battery cathode material, characterized in that: The cathode material is a cobalt / nitrogen-doped carbon-modified Na3V2(PO4)2F3 composite material prepared using a bimetallic MOF as a template, namely Na3V2(PO4)2F3 / Co / N-PC composite material, with a specific surface area of ​​21.107 m². 2 The Na3V2(PO4)2F3 / Co / N-PC composite material, with an average pore size of 7.170 nm and a g / g, was prepared using the following steps: (1) Preparation of bimetallic MOF: 223.12 mg of Zn(NO3)2·6H2O, 72.76 mg of Co(NO3)2·6H2O and 5 mg of methyltrimethylammonium bromide (CTAB) were weighed and dissolved in 10 mL of deionized water. After stirring for 5 min, solution A was formed. Then, 4540 mg of 2-methylimidazole was weighed and dissolved in 70 mL of deionized water. After stirring for 5 min, solution B was formed. Solution A was quickly added to solution B and stirred vigorously at 25 °C for 20 min to obtain a light purple emulsion. The emulsion was sealed with plastic wrap at room temperature and aged for 24 h. Then, it was centrifuged at 4000 r / min. The precipitate was precipitated with a mixture of ethanol and deionized water. 乙醇 :V 去离子水 The ratio of co-zinc to zinc was 1:

1. The washing was repeated 5-6 times. Finally, the sample was placed in a vacuum drying oven and dried at 60°C for 12 hours. The resulting purple powder sample was denoted as Co / Zn-ZIF and weighed. (2) Preparation of Co / N-PC composite material: All Co / Zn-ZIF samples obtained in step (1) above were transferred into a crucible and heated to 900℃ at a heating rate of 5℃ / min under an argon atmosphere. After holding at the temperature for 3 h, the temperature was lowered at a cooling rate of 3℃ / min. After the temperature dropped to room temperature, a black powder sample was obtained. 0.2 g of the black powder was weighed and hydrochloric acid with a concentration of 4 mol / L was added. The mixture was stirred overnight on a magnetic stirrer to remove the elemental Zn in the sample. Finally, the sample was filtered and the precipitate was washed with deionized water until the filtrate was neutral. The precipitate was then placed in a forced-air drying oven and dried at 60℃. The obtained black powder was named Co / N-PC and weighed. (3) Preparation of Na3V2(PO4)2F3 / Co / N-PC composite material: Weigh 2.3398 g ammonium metavanadate NH4VO3, 2.3004 g ammonium dihydrogen phosphate NH4H2PO4, 1.2597 g sodium fluoride NaF and 0.467 g citric acid as reducing agent, and add them to a round-bottom flask. Finally, add all the Co / N-PC obtained in step (2) above and 30 mL of deionized water as solvent, and stir for 6-8 minutes in an oil bath at 80°C. The Na3V2(PO4)2F3 / Co / N-PC precursor powder was completely dissolved in the solvent and then dried in a spray dryer at 200°C. The powder was then transferred to an alumina crucible and sintered at high temperature in a tube furnace under argon protection. The temperature was first raised to 300°C for 3 hours at a rate of 5°C / min, and then calcined at 650°C for 6 hours at a rate of 10°C / min. After the apparatus was allowed to cool naturally to room temperature, the sample was removed to obtain the Na3V2(PO4)2F3 / Co / N-PC composite material.

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