A polyimide in-situ composite porous sodium vanadium phosphate positive electrode material, a preparation method and application thereof

Abstract

The application belongs to the technical field of sodium ion batteries, and aims to solve the problems of low active material utilization rate and poor electrochemical performance under large current of sodium vanadium phosphate, and provides a polyimide in-situ composite porous sodium vanadium phosphate positive electrode material, a preparation method and application thereof. Ammonium metavanadate, sodium dihydrogen phosphate and citric acid are used as raw materials, and polyamide acid modified by triethylamine is used as an additional carbon source. In the pre-sintering process of sodium vanadium phosphate, the polyamide acid is in-situ thermally polymerized into porous polyimide, and after carbonization, a porous carbon skeleton is formed and uniformly distributed with sodium vanadium phosphate. The polyimide composite sodium vanadium phosphate is prepared by a liquid phase method assisted high-temperature carbon thermal reduction method. The polyimide composite sodium vanadium phosphate has a dual effect. The carbonization into a porous skeleton is beneficial to increasing the contact area between sodium vanadium phosphate and electrolyte, improving the active material utilization efficiency, and buffering the stress caused by large current. After carbonization, the polyimide is partially decomposed to form a nitrogen-doped carbon coating layer, which has a large number of defects, is beneficial to the rapid transmission of electrons and sodium ions, and improves the electrochemical kinetic characteristics of the material.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium-ion battery cathode materials, specifically relating to a polyimide in-situ composite porous sodium vanadium phosphate cathode material, its preparation method, and its application. Background Technology

[0002] Clean energy technologies addressing the traditional fossil fuel crisis face cyclical and intermittent drawbacks, urgently requiring the support of large-scale energy storage facilities. Lithium-ion batteries, as the most widely used electrochemical energy storage battery, are constrained by the scarcity and uneven distribution of lithium resources, making them highly susceptible to oligopolistic monopolies and geopolitical influences. Their high cost also hinders their application in large-scale energy storage systems. Sodium-ion batteries, due to their similar working principle and industrialization process to lithium-ion batteries, are considered the most promising alternative. However, the performance of sodium-ion batteries still needs further improvement, with the development of cathode materials currently being a bottleneck.

[0003] Sodium vanadium phosphate, as a polyanionic material, possesses high stability and an ultrafast sodium-ion transport channel, making it an ideal cathode material for sodium-ion batteries. Furthermore, it allows for simultaneous high-voltage sodium insertion and low-voltage sodium removal, promising applications in the electronics industry and other fields. However, the strong bonding in the stable PO4 tetrahedra and VO6 octahedra restricts electron movement within the crystal, resulting in low intrinsic conductivity. Moreover, during the sintering process, sodium vanadium phosphate inevitably agglomerates during crystal growth, leading to low utilization of the active material.

[0004] Improving the intrinsic conductivity of electrodes is typically achieved through carbon coating. However, the ordered nature of the carbon layer hinders sodium ion diffusion, negatively impacting charge-discharge behavior. Furthermore, controlling the morphology of the material helps increase the contact area between the active material and the electrolyte, thereby increasing the utilization efficiency of active sites. However, conventional methods for controlling morphology are cumbersome and not easily industrialized. Summary of the Invention

[0005] This invention addresses the problems of low intrinsic conductivity and low utilization efficiency of active materials in sodium vanadium phosphate by providing a polyimide-derived porous sodium vanadium phosphate cathode material, its preparation method, and its applications. The nitrogen-doped carbon layer derived from polyimide possesses a multi-defect structure, which facilitates the rapid diffusion of sodium ions. Simultaneously, the polyimide forms a porous carbon framework during sintering, increasing the contact area between the electrode and electrolyte, improving the utilization efficiency of the active material, and resulting in excellent electrochemical performance. Finally, the porous carbon framework helps alleviate the stress and strain experienced by sodium vanadium phosphate particles under high current impact, enhancing the material's electrochemical performance under high current. When the prepared material is used as a cathode in a 2025 coin cell, it exhibits excellent electrochemical performance. Therefore, the prepared electrode material has great application potential in sodium-ion battery systems.

[0006] This invention is achieved by the following technical solution: a polyimide in-situ composite porous sodium vanadium phosphate cathode material, wherein the cathode material is made from ammonium metavanadate, sodium dihydrogen phosphate and citric acid as raw materials, and polyamic acid modified with triethylamine as an additional carbon source. During the pre-sintering process of sodium vanadium phosphate, the polyamic acid is in-situ polymerized into porous polyimide and carbonized to form a porous carbon skeleton, which is uniformly distributed with sodium vanadium phosphate. The polyimide in-situ composite porous sodium vanadium phosphate cathode material is prepared by a liquid-phase assisted high-temperature carbothermal reduction method.

[0007] The specific steps for preparing the polyimide in-situ composite porous sodium vanadium phosphate cathode material are as follows: (1) Under ice-water bath conditions, 4,4'-diaminodiphenyl ether is dissolved in a good solvent and the concentration of 4,4'-diaminodiphenyl ether in the prepared solution is 15 wt%; wherein the good solvent is any one of N,N-diaminocarboxamide, N,N-diaminoacetamide, dimethyl sulfoxide, N-methylpyrrolidone or m-methylphenol; (2) Maintain ice-water bath conditions, add pyromellitic dianhydride to the solution prepared in step (1), and stir for 12 hours; wherein the molar ratio of pyromellitic dianhydride to 4,4'-diaminodiphenyl ether is 1:1; (3) Add triethylamine and stir at room temperature for 8 hours; the molar ratio of triethylamine to 4,4'-diaminodiphenyl ether is 2:1, and then pour in the unsuitable solvent, wherein the mass ratio of the unsuitable solvent to the good solvent is 5:1; the unsuitable solvent is any one of ethyl acetate, acetone, methanol or dichloromethane; filter and wash the solution with the unsuitable solvent through a circulating water vacuum pump to obtain polyamic acid; (4) Place the polyamic acid obtained in step (3) in a vacuum oven and dry it under vacuum at 60°C for 12 hours; (5) Take sodium dihydrogen phosphate, ammonium metavanadate, and citric acid in a molar ratio of 82.21:5.46:1; add sodium dihydrogen phosphate and ammonium metavanadate to 100 mL of deionized water, heat to 70 °C and stir continuously to form a yellow transparent solution; slowly add citric acid to the yellow transparent solution, and the solution color eventually stabilizes in blue. (6) Add the polyamic acid prepared in step (4) to the blue solution obtained in step (5), wherein the mass ratio of polyamic acid to citric acid is 0.2~1:1; stir at constant temperature until the precursor solution is concentrated into 20ml of viscous colloid; (7) The viscous colloid obtained in step (6) is placed in a forced-air drying oven and dried at 80°C for 12 hours to obtain a precursor. The precursor is sintered in a nitrogen atmosphere. The heating process is to raise the temperature to 200°C at a rate of 2°C per minute and hold for 1 hour, raise the temperature to 300°C at the same rate and hold for 1 hour, and finally raise the temperature to 400°C at the same rate and hold for 4 hours. Then, the sintering continues in a nitrogen atmosphere, and the temperature is raised to 700°C at a rate of 10°C per minute and held for 6 hours to obtain the final product, polyimide in-situ composite porous sodium vanadium phosphate cathode material.

[0008] The present invention also provides the application of the polyimide in-situ composite porous sodium vanadium phosphate cathode material as a cathode material in sodium-ion batteries.

[0009] The specific method is as follows: 1.6 ml of N-methylpyrrolidone is used as an organic solvent. 0.03 g of polyvinylidene fluoride binder, 0.06 g of acetylene black conductive filler, and 0.21 g of polyimide in-situ composite porous sodium vanadium phosphate positive electrode material are added sequentially to the organic solvent. The mixture is then placed in a planetary ball mill jar and ball-milled at 40 Hz for 4 hours to obtain a slurry, which is then coated onto a single-sided carbon-coated aluminum foil. The aluminum foil coated with the slurry is then dried at 40°C for 4 hours and vacuum-dried at 120°C for 6 hours. The resulting circular electrode is the positive electrode. The positive electrode is then assembled into a 2025 type button battery in a vacuum glove box. The negative electrode is metallic sodium, the separator is a ceramic Celgard separator, and the electrolyte is prepared by dissolving 1 M sodium perchlorate in a 1:1 volume ratio ethylene carbonate / diethyl carbonate system, with sodium perchlorate as the calculation basis, and 5 wt% fluoroethylene carbonate is added.

[0010] This invention utilizes polyimide as an additional carbon source to synthesize sodium vanadium phosphate with a porous framework and a nitrogen-doped carbon coating in a one-step liquid-phase method, significantly improving the intrinsic conductivity and ion diffusion performance of the material. Specifically, during sintering, the in-situ polymerization of polyamic acid into porous polyimide and subsequent carbonization to form a porous carbon framework are achieved. The nitrogen-doped, highly conductive carbon coating has numerous defects, which facilitates the rapid transport of electrons and sodium ions. Furthermore, the porous carbon framework not only improves the utilization efficiency of the active material but also effectively mitigates the stress caused by current surges.

[0011] Polyimide-modified sodium vanadium phosphate (CNP) composites serve a dual purpose. First, the triethylamine-modified polyamic acid polymerizes into fibrous polyimide during the pre-calcination stage, ensuring a uniform distribution of the CNP precursor. During the final calcination stage, it carbonizes into a porous framework. This increases the contact area between CNP and the electrolyte, improving the utilization efficiency of the active material and buffering the stress caused by high currents. Second, the partial decomposition of the carbonized polyimide forms a nitrogen-doped carbon coating layer. This coating layer contains numerous defects, facilitating the rapid transport of electrons and sodium ions and enhancing the electrochemical kinetics of the material.

[0012] This invention utilizes triethylamine-modified polyamic acid as an additional carbon source to synthesize a sodium vanadium phosphate cathode material with a porous carbon framework and a nitrogen-doped carbon coating. The preparation steps are simple, the raw materials are inexpensive, and it is easy to industrialize. During the sintering of sodium vanadium phosphate, the triethylamine-modified polyamic acid polymerizes in situ to form polyimide, creating a porous carbon framework that significantly alleviates the stress and strain caused by current surges. After carbonization, the polyimide partially decomposes, forming a nitrogen-doped carbon coating. This multi-defect coating facilitates the rapid transport of electrons and sodium ions, enhancing the intrinsic conductivity and sodium ion diffusion performance of the electrode material. The material obtained by this invention has a large electrode-electrolyte contact area, fully utilizing the active material and exhibiting excellent electrochemical performance. Attached Figure Description

[0013] Figure 1 The 1H NMR spectrum of polyamic acid modified with triethylamine is shown in Figure 1. Figure 2 The image is a 10,000x magnified SEM image of the pre-calcined polyimide composite sodium vanadium phosphate precursor prepared in Example 1. It can be seen that the sodium vanadium phosphate precursor is uniformly distributed on the fibrous polyimide. Figure 3 The image shows the XRD pattern of the polyimide in-situ composite porous sodium vanadium phosphate cathode material prepared in Example 1. As can be seen from the image, the added polyimide did not affect the growth of sodium vanadium phosphate crystals, and the characteristic peaks are consistent with the standard spectrum of sodium vanadium phosphate. Figure 4 The image shows the Raman spectrum of the polyimide in-situ composite porous sodium vanadium phosphate cathode material prepared in Example 1. The nitrogen-doped carbon coating layer formed after polyimide carbonization has a high degree of disorder. Figure 5 The image is a 50,000x SEM image of the polyimide in-situ composite porous sodium vanadium phosphate cathode material prepared in Example 1 after etching sodium vanadium phosphate crystals. The porous framework characteristics of polyimide after carbonization can be seen. Figure 6The image is a TEM image with a magnification of 70,000x showing the porous framework characteristics of polyimide in situ composite porous sodium vanadium phosphate cathode material prepared in Example 1 after etching sodium vanadium phosphate crystals. Figure 7 The constant current charge-discharge curve of the polyimide in-situ composite porous sodium vanadium phosphate cathode material prepared in Example 1 was measured when it was assembled into a 2025 button battery, with a current density of 0.1 C. Figure 8 The cycling curve of the polyimide in-situ composite porous sodium vanadium phosphate cathode material prepared in Example 1 when assembled into a 2025 button cell at a current density of 15 C. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0015] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains, and all materials publicly cited herein and cited by them are incorporated herein by reference.

[0016] Equivalent technologies of the specific embodiments described herein that are readily apparent to those skilled in the art through routine experimentation are included in this application.

[0017] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the instruments and equipment used in the following examples are all standard laboratory instruments and equipment; unless otherwise specified, the experimental materials used in the following examples were all purchased from regular biochemical reagent stores.

[0018] Example 1: Preparation of polyimide in-situ composite porous sodium vanadium phosphate cathode material: Under ice-water bath conditions, 2.0433 g of 4,4'-diaminodiphenyl ether was dissolved in 26.0599 g of N,N-diaminoformamide. Maintaining the ice-water bath conditions, 2.2257 g of pyromellitic dianhydride was added to the solution in small, repeated additions, with continuous stirring for 12 hours. Then, 2.0340 g of triethylamine was added at room temperature, and the mixture was stirred for 8 hours. The resulting precursor was poured into 130.2995 g of ethyl acetate and washed three times using a circulating water vacuum pump. The resulting solid was then placed in a vacuum drying oven and dried under vacuum at 60°C for 12 hours to obtain polyamic acid.

[0019] Dissolve 1.0263 g of ammonium metavanadate in 100 ml of deionized water and stir continuously at 70 °C for 1 hour to form a slightly yellow and transparent solution. Add 0.3385 g of citric acid and maintain stirring at 70 °C for 2 hours. Finally, add 1.5789 g of sodium dihydrogen phosphate and 0.2 g of polyimide, and maintain stirring at 70 °C to concentrate the liquid to 20 ml. Place the prepared precursor solution in a forced-air drying oven and dry at 80 °C for 12 hours. The obtained solid precursor is heated under a nitrogen atmosphere at a heating rate of 2 °C per minute to 200 °C and 300 °C for 1 hour each, and then at 400 °C for 4 hours. After natural cooling, heat at 700 °C for 6 hours at a heating rate of 10 °C per minute to obtain the final product.

[0020] The structural characteristics of the triethylamine-modified polyamic acid in all examples are as follows: Figure 1 As shown. The characteristic peak of -NH- at 11.5 ppm indicates that -NH2 reacts with the acid anhydride. Meanwhile, the characteristic peaks at 8.1 ppm, 8.0 ppm, and 7.85 ppm are characteristic peaks reflecting the change in the chemical environment of the hydrogen on the benzene ring after the anhydride of 1,2,4,5-pyromellitic dianhydride undergoes ring opening. In conclusion, ODA-PMDA type polyamic acid has been successfully prepared. The characteristic peaks of the methyl and methylene groups in triethylamine at 11 ppm and 3.0 ppm indicate that triethylamine has been successfully introduced into the polyamic acid, reacting with the carboxylic acid and binding the carboxylic acid.

[0021] In this embodiment, the SEM image of the pre-calcined polyimide-composite sodium vanadium phosphate precursor is as follows: Figure 2 As shown, sodium vanadium phosphate precursor is uniformly distributed on the fibrous polyimide.

[0022] The X-ray diffraction pattern obtained through testing in this embodiment is as follows: Figure 3 As shown in the figure, the added polyimide did not affect the growth of sodium vanadium phosphate crystals, and the characteristic peaks were consistent with the standard spectrum of sodium vanadium phosphate.

[0023] Raman spectroscopy analysis was performed using this embodiment, and the Raman spectrum is as follows: Figure 4 As shown in the figure, the nitrogen-doped carbon coating layer formed after polyimide carbonization has a high degree of disorder.

[0024] SEM image after etching sodium vanadium phosphate crystal, as shown below Figure 5 As shown, the TEM image is as follows Figure 6 As shown, the porous framework characteristics of polyimide after carbonization can be observed.

[0025] The active material prepared in this embodiment was used as a positive electrode material in a 2025-type button cell after slurry preparation and coating. First, 1.6 ml of N-methylpyrrolidone was added as an organic solvent. Then, 0.03 g of polyvinylidene fluoride binder, 0.06 g of acetylene black conductive filler, and 0.21 g of active material were added sequentially to the organic solvent. The mixture was placed in a planetary ball mill jar and ball-milled at 40 Hz for 4 hours to obtain a slurry, which was then coated onto a single-sided carbon-coated aluminum foil. The aluminum foil coated with the slurry was then dried at 40 °C for four hours, followed by vacuum drying at 120 °C for six hours, and cut into 16 mm circular electrode sheets to obtain a positive electrode sheet suitable for assembly in a 2025-type button cell. The 2025 type button cell was assembled in a vacuum glove box. The negative electrode was metallic sodium, the separator was a ceramic Celgard separator, and the electrolyte was prepared by dissolving 1M sodium perchlorate in a 1:1 volume ratio of ethylene carbonate / diethyl carbonate system, with 5 wt% fluoroethylene carbonate added.

[0026] The assembled coin cells were subjected to constant current charge-discharge tests at room temperature within a voltage range of 2.3-4.1 V. Specifically, the first charge-discharge curves are shown below. Figure 7 The charge-discharge curves at different rates are as follows: Figure 8 .

[0027] Electrochemical tests show that the material achieves a discharge specific capacity of 115.4 mAh g⁻¹ at 0.1 C. -1 Furthermore, it exhibits excellent cycling performance at high current densities. After 350 cycles at a current density of 15C, the capacity retention is 95.50%.

[0028] The above embodiments illustrate that this invention utilizes a rapid sol-gel method to synthesize a polyimide-based in-situ composite porous sodium vanadium phosphate cathode material in one step. Polyamic acid polymerizes during a unique pre-sintering process and rapidly carbonizes into a porous carbon framework during final sintering, providing support for the in-situ growth of sodium vanadium phosphate. This porous carbon framework helps increase the contact area between sodium vanadium phosphate and the electrolyte, enhancing the utilization efficiency of active sites. Furthermore, the carbonized polyimide portion forms a nitrogen-doped carbon coating layer. This defect-rich, highly conductive carbon layer facilitates the rapid transport of electrons and sodium ions, improving the electrochemical kinetics of the electrode. The material preparation process is simple, low-cost, and easily industrialized, offering broad application advantages.

[0029] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A polyimide in-situ composite porous sodium vanadium phosphate cathode material, characterized in that: The cathode material is prepared by using ammonium metavanadate, sodium dihydrogen phosphate, and citric acid as raw materials, and polyamic acid modified with triethylamine as an additional carbon source. During the pre-sintering process of sodium vanadium phosphate, the polyamic acid is polymerized in situ to form porous polyimide and carbonized to form a porous carbon framework, which is uniformly distributed with sodium vanadium phosphate. The in-situ composite porous sodium vanadium phosphate cathode material of polyimide is prepared by liquid-phase assisted high-temperature carbothermal reduction method. The specific steps for preparing the polyimide in-situ composite porous sodium vanadium phosphate cathode material are as follows: (1) Under ice-water bath conditions, 4,4'-diaminodiphenyl ether is dissolved in a good solvent to prepare a solution with a concentration of 15 wt%; wherein the good solvent is any one of dimethyl sulfoxide, N-methylpyrrolidone or m-methylphenol; (2) Maintain ice-water bath conditions, add pyromellitic dianhydride to the solution prepared in step (1), and stir for 12 hours; wherein the molar ratio of pyromellitic dianhydride to 4,4'-diaminodiphenyl ether is 1:1; (3) Add triethylamine and stir at room temperature for 8 hours; the molar ratio of triethylamine to 4,4'-diaminodiphenyl ether is 2:1, and then pour in the unsuitable solvent, wherein the mass ratio of the unsuitable solvent to the good solvent is 5:1; the unsuitable solvent is any one of ethyl acetate, acetone, methanol or dichloromethane; filter and wash the solution with the unsuitable solvent through a circulating water vacuum pump to obtain polyamic acid; (4) Place the polyamic acid obtained in step (3) in a vacuum oven and dry it under vacuum at 60°C for 12 hours; (5) Take sodium dihydrogen phosphate, ammonium metavanadate, and citric acid in a molar ratio of 82.21:5.46:1; add sodium dihydrogen phosphate and ammonium metavanadate to 100 mL of deionized water, heat to 70 °C and stir continuously to form a yellow transparent solution; slowly add citric acid to the yellow transparent solution, and the solution color eventually stabilizes in blue. (6) Add the polyamic acid prepared in step (4) to the blue solution obtained in step (5), wherein the mass ratio of polyamic acid to citric acid is 0.2~1:1; stir at constant temperature until the precursor solution is concentrated into 20ml of viscous colloid; (7) The viscous colloid obtained in step (6) is placed in a forced-air drying oven and dried at 80°C for 12 hours to obtain a precursor. The precursor is sintered in a nitrogen atmosphere. The heating process is to raise the temperature to 200°C at 2°C / min and hold for 1 hour, raise the temperature to 300°C at the same rate and hold for 1 hour, and finally raise the temperature to 400°C at the same rate and hold for 4 hours. Then, the sintering continues in a nitrogen atmosphere, and the temperature is raised to 700°C at a rate of 10°C / min and held for 6 hours to obtain the final product, polyimide in-situ composite porous sodium vanadium phosphate cathode material.

2. The application of the polyimide in-situ composite porous sodium vanadium phosphate cathode material according to claim 1, characterized in that: The polyimide in-situ composite porous sodium vanadium phosphate cathode material is used as a cathode material in sodium-ion batteries.

3. The application according to claim 2, characterized in that: The specific method is as follows: 1.6 ml of N-methylpyrrolidone is used as an organic solvent. 0.03 g of polyvinylidene fluoride binder, 0.06 g of acetylene black conductive filler, and 0.21 g of polyimide in-situ composite porous sodium vanadium phosphate positive electrode material are added sequentially to the organic solvent. The mixture is then placed in a planetary ball mill jar and ball-milled at 40 Hz for 4 hours to obtain a slurry, which is then coated onto a single-sided carbon-coated aluminum foil. The aluminum foil coated with the slurry is then dried at 40°C for 4 hours and vacuum-dried at 120°C for 6 hours. The resulting circular electrode is the positive electrode. The positive electrode is then assembled into a 2025 type button battery in a vacuum glove box. The negative electrode is metallic sodium, the separator is a ceramic Celgard separator, and the electrolyte is prepared by dissolving 1 M sodium perchlorate in a 1:1 volume ratio ethylene carbonate / diethyl carbonate system, with sodium perchlorate as the calculation basis, and 5 wt% fluoroethylene carbonate is added.

Images