A high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode, a preparation method and application thereof

Abstract

The application relates to the technical field of flexible electrodes, and discloses a high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode, a preparation method and application. The raw materials of the high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode include Na3V2(PO4)2F3 powder, polyacrylonitrile and N,N-dimethylformamide solvent; a mixture of the Na3V2(PO4)2F3 powder, the polyacrylonitrile and the N,N-dimethylformamide solvent is prepared into a flexible fiber membrane containing Na3V2(PO4)2F3 in an electrostatic spinning mode; and the flexible fiber membrane is subjected to drying and calcination treatment to obtain the Na3V2(PO4)2F3@C flexible self-supporting positive electrode. The flexible self-supporting positive electrode has a high-voltage platform and meets the requirements of flexible sodium ion batteries on electrochemical performance and mechanical performance.

Description

Technical Field

[0001] This invention relates to the field of flexible electrode technology, and in particular to a high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode, its preparation method, and its application. Background Technology

[0002] With the development and progress of science and technology, humans have placed higher demands on the portability and wearability of intelligent electronic devices, giving rise to flexible electronic devices with enormous development potential. Developing corresponding flexible energy storage systems is crucial, yet challenging, for the widespread adoption of flexible electronic devices. Flexible energy storage devices must meet the portability and flexibility standards of flexible electronic devices and continue to provide uninterrupted power even after repeated folding. Significant progress has been made in the application of flexible lithium-ion batteries in flexible devices and wearable devices. Compared to lithium-ion batteries, sodium-ion batteries have significant safety and cost advantages, and their energy storage mechanism is similar to that of lithium-ion batteries. Due to the abundance and low cost of sodium resources and a similar production process to lithium-ion batteries, sodium-ion batteries are considered the most likely next-generation energy storage battery to complement lithium-ion batteries.

[0003] In recent years, research on flexible sodium-ion batteries and related materials has been gradually established. Graphene oxide / reduced graphene oxide, carbon nanotubes, and carbon cloth are used as carbon substrates. Electrode materials are attached to the flexible substrates through methods such as dip coating, electrodeposition, and coating printing. However, the production methods of graphene oxide / reduced graphene oxide and carbon nanotubes are complex, and the cost of preparing high-quality graphene oxide and carbon nanotubes is high. At the same time, the contact between the electrode material and the flexible substrate is a rigid contact, and the active material is prone to detachment during repeated charge and discharge processes, thus affecting its electrochemical performance.

[0004] Electrospinning is considered one of the most effective methods for preparing carbon nanofibers with controllable diameter and orientation. Electrospinning offers advantages such as fewer and simpler operation steps, one-step forming, and controllable morphology. In electrospinning, a polymer solution is stretched into filaments using a high-voltage power supply to form nanofibers, which in turn yield nanofiber films. At the spinning needle nozzle, polymer droplets generate an electric field opposite to the surface tension under a high-voltage electric field. As the electric field increases, Taylor cones gradually form. When the electric field overcomes the surface tension, the Taylor cones form charged jets, are stretched into filaments, and solvent evaporation causes the fibers to solidify, ultimately distributing them randomly on the receiving device. Furthermore, electrospinning effectively encapsulates nanomaterials within carbon nanofibers through in-situ chemical methods, not only meeting the mechanical requirement of flexible self-support but also shortening the electron and ion transport paths. However, carbon nanofibers have size requirements for electrode materials; excessively large electrode materials can damage the integrity of the carbon nanofibers, thus compromising the excellent mechanical properties of the flexible self-supporting electrode.

[0005] As a polyanionic compound, Na3V2(PO4)2F3 exhibits a reversible intercalation / deintercalation capacity of 2 mol sodium ions per mol of Na3V2(PO4)2F3, resulting in a discharge specific capacity of 128 mAh g. -1 It also exhibits good structural and thermal stability. Due to F... - Its extremely high electronegativity results in a very high voltage plateau (close to 3.9V). However, due to the separation of VO6 octahedra by PO4 tetrahedra in its structure, Na3V2(PO4)2F3 suffers from low conductivity and a small sodium ion diffusion coefficient, which greatly limits its further development and application. The limitations on electrode material size imposed by electrospinning, coupled with the inherent drawbacks of Na3V2(PO4)2F3, have led to the absence of reports on the preparation of high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathodes with both good electrochemical and mechanical properties using electrospinning technology. Summary of the Invention

[0006] The purpose of this invention is to propose a high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode, which has a high voltage platform and meets the requirements of flexible sodium-ion batteries for electrochemical and mechanical performance.

[0007] Another objective of this invention is to propose a method for preparing a high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode. By combining electrospinning technology and Na3V2(PO4)2F3 powder preparation technology, a flexible self-supporting cathode with a high voltage platform, excellent mechanical properties and good electrochemical performance is obtained.

[0008] Another objective of this invention is to provide a sodium-ion battery that can be applied to flexible electronic components and smart wearable products.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] A high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode, the raw materials of which include Na3V2(PO4)2F3 powder, polyacrylonitrile and N,N-dimethylformamide solvent;

[0011] A flexible fiber membrane containing Na3V2(PO4)2F3 was prepared by electrospinning a mixture of Na3V2(PO4)2F3 powder, polyacrylonitrile, and N,N-dimethylformamide solvent. The flexible fiber membrane was then dried and calcined to obtain the Na3V2(PO4)2F3@C flexible self-supporting positive electrode.

[0012] Furthermore, the mass ratio of the polyacrylonitrile to Na3V2(PO4)2F3 powder is 1:(0.5-2).

[0013] A method for preparing a high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode, the method being used to prepare the high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode as described in claim 1 or 2, the method comprising the following steps:

[0014] (1) VPO4 powder was prepared by the sol-gel method;

[0015] (2) Mix VPO4 powder and NaF to prepare Na3V2(PO4)2F3 powder by solid-state method;

[0016] (3) Add polyacrylonitrile to N,N-dimethylformamide solvent to form a polymer solution, and then disperse Na3V2(PO4)2F3 powder in the polymer solution to obtain spinning solution;

[0017] (4) Electrospinning the spinning solution to obtain a flexible fiber membrane containing Na3V2(PO4)2F3;

[0018] (5) The flexible fiber membrane is subjected to vacuum drying and calcination treatment in sequence to obtain the high voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode.

[0019] Furthermore, in step (1), the method for preparing VPO4 powder using the sol-gel method is as follows:

[0020] C6H8O7·H2O, NH4VO3 and NH4H2PO4 in a molar ratio of 1:1:1 were dissolved in deionized water and magnetically stirred in an oil pan at 75-85℃ for 3-6 hours. After the water evaporated, a dark blue gel was obtained.

[0021] After freeze-drying the gel for 30-36 hours, a loose and porous bulk material is formed. The bulk material is then calcined twice to obtain VPO4 powder.

[0022] Furthermore, the bulk material is subjected to two calcinations as follows:

[0023] First calcination: carried out under an inert atmosphere at 320–400℃ for 3–5 hours;

[0024] Second calcination: carried out under an inert atmosphere at 650–800℃ for 6–7 hours.

[0025] Furthermore, in step (2), the method for preparing Na3V2(PO4)2F3 powder using the solid-state method is as follows:

[0026] VPO4 and NaF in a molar ratio of 2:(3-3.15) are mixed and ground for 5-20 minutes, pressed into tablets, and calcined at 600-700℃ for 1-6 hours under an inert atmosphere to obtain Na3V2(PO4)2F3 powder.

[0027] Furthermore, in step (3), the mass ratio of polyacrylonitrile to Na3V2(PO4)2F3 powder is 1:(0.5~2).

[0028] Furthermore, in step (4), the electrospinning conditions are as follows: voltage is 21kV, feed flow rate is 0.2~0.6ml / h, roller speed is 200rpm, needle back-and-forth distance is 30mm, moving speed is 10mm / s, and distance between needle and roller is 8~12cm.

[0029] Furthermore, in step (5), after the flexible fiber membrane is vacuum dried at 90-120°C for 3-11 hours, it is pre-calcined in air at 300-350°C for 2.8-3.8 hours with a heating rate of 2°C / min;

[0030] The pre-calcined flexible fiber membrane was calcined at 500–700°C for 1–7 hours under an inert atmosphere.

[0031] A sodium-ion battery comprising the aforementioned high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode.

[0032] The technical solution provided by this invention may include the following beneficial effects:

[0033] This technical solution selects Na3V2(PO4)2F3 as the electrochemical active material and obtains a high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode with good electrochemical and mechanical properties through electrospinning technology, thus meeting the electrochemical and mechanical performance requirements of flexible sodium-ion batteries. Moreover, the flexible self-supporting cathode prepared by this technical solution does not require coating and can be directly used as the cathode for sodium-ion batteries. Therefore, the high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode prepared by this technical solution is expected to promote the practical application of flexible sodium-ion batteries, alleviate the current supply and demand tension of flexible lithium-ion batteries, and promote the rapid development of flexible electronic devices and smart wearable products. Attached Figure Description

[0034] Figure 1 This is a SEM analysis image of the high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode of Example 1;

[0035] Figure 2 This is a SEM analysis image of the high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode of Example 2;

[0036] Figure 3 This is the XRD analysis diagram of the high-voltage Na3V2(PO4)2F3@C powder material in Example 3;

[0037] Figure 4 This is the XRD analysis diagram of the high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode of Example 3;

[0038] Figure 5 The charge-discharge curves of the coin cell assembled with the high-voltage Na3V2(PO4)2F3@C flexible self-supporting electrode as the positive electrode and metallic sodium as the negative electrode in Example 4 are shown.

[0039] Figure 6 The CV diagram is shown for a coin cell assembled with a high-voltage Na3V2(PO4)2F3@C flexible self-supporting electrode as the positive electrode and sodium vanadium phosphate as the negative electrode, as described in Example 5.

[0040] Figure 7 The charge-discharge curves of the coin cell assembled using the high-voltage Na3V2(PO4)2F3@C flexible self-supporting electrode of Example 5 as the positive electrode and sodium vanadium phosphate as the negative electrode are shown.

[0041] Figure 8 This is a mechanical performance state diagram of the high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode of Example 6. Detailed Implementation

[0042] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0043] This invention provides a high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode, the raw materials of which include Na3V2(PO4)2F3 powder, polyacrylonitrile and N,N-dimethylformamide solvent;

[0044] A flexible fiber membrane containing Na3V2(PO4)2F3 was prepared by electrospinning a mixture of Na3V2(PO4)2F3 powder, polyacrylonitrile, and N,N-dimethylformamide solvent. The flexible fiber membrane was then dried and calcined to obtain the Na3V2(PO4)2F3@C flexible self-supporting positive electrode.

[0045] This technical solution selects Na3V2(PO4)2F3 as the electrochemical active material and obtains a high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode with good electrochemical and mechanical properties through electrospinning technology, thus meeting the electrochemical and mechanical performance requirements of flexible sodium-ion batteries. Moreover, the flexible self-supporting cathode prepared by this technical solution does not require coating and can be directly used as the cathode for sodium-ion batteries. Therefore, the high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode prepared by this technical solution is expected to promote the practical application of flexible sodium-ion batteries, alleviate the current supply and demand tension of flexible lithium-ion batteries, and promote the rapid development of flexible electronic devices and smart wearable products.

[0046] Based on electrospinning technology, Na3V2(PO4)2F3 powder can be uniformly distributed within and on the surface of carbon nanofibers without compromising their integrity. This maintains the good mechanical properties of the flexible self-supporting electrode. The three-dimensional network structure formed by the carbon nanofibers significantly improves the ion and electron transport rates, optimizing the electrochemical performance of Na3V2(PO4)2F3. This approach addresses the problem of electrode material size affecting the integrity of carbon nanofibers and thus the mechanical properties of the electrode, while also optimizing the low intrinsic electronic conductivity of Na3V2(PO4)2F3.

[0047] Specifically, this technical solution uses Na3V2(PO4)2F3 as the electrochemically active material. Na3V2(PO4)2F3 has the ability to release and absorb sodium ions. Within the voltage range of 2.0–4.3V (relative to Na+ / Na), 1 mol of Na3V2(PO4)2F3 can reversibly intercalate and deintercalate 2 mol of sodium ions, achieving a discharge specific capacity of 128 mAh g. -1 The voltage plateau is close to 3.9V, exhibiting a high voltage platform. The Na3V2(PO4)2F3 flexible self-supporting positive electrode prepared by this technical solution can directly serve as the positive electrode for sodium-ion batteries without coating. The flexible electrode consists only of the active material and a flexible substrate, without containing binders, conductive carbon, or current collectors. The active material Na3V2(PO4)2F3 of the flexible self-supporting positive electrode prepared by this technical solution is loaded on carbon nanofibers, therefore it can be directly used as a positive electrode.

[0048] It is worth noting that during the electrospinning process, polyacrylonitrile forms a polymer skeleton with good mechanical properties under high voltage, which will form carbon nanofibers after high-temperature calcination in an inert atmosphere.

[0049] Furthermore, the mass ratio of polyacrylonitrile to Na3V2(PO4)2F3 powder is 1:(0.5-2). The amount of Na3V2(PO4)2F3 powder added has a significant impact on the electrochemical performance, quality, and toughness of the flexible fiber membrane prepared later. When the mass ratio of added polyacrylonitrile to Na3V2(PO4)2F3 powder is within a certain range, the flexible electrode obtained after carbonization exhibits excellent electrochemical performance and good mechanical properties. When the mass ratio is too high, i.e., the amount of Na3V2(PO4)2F3 powder added is too low, the electrochemical performance of the flexible electrode will decrease; if the mass ratio is too low, i.e., the amount of Na3V2(PO4)2F3 powder added is too high, the flexibility of the Na3V2(PO4)2F3 flexible self-supporting cathode will be reduced, affecting the electrochemical and mechanical properties of the flexible electrode.

[0050] Accordingly, the present invention also provides a method for preparing a high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode. This method is used to prepare the high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode as described in claim 1 or 2. The method includes the following steps:

[0051] (1) VPO4 powder was prepared by the sol-gel method;

[0052] (2) Mix VPO4 powder and NaF to prepare Na3V2(PO4)2F3 powder by solid-state method;

[0053] (3) Add polyacrylonitrile to N,N-dimethylformamide solvent to form a polymer solution, and then disperse Na3V2(PO4)2F3 powder in the polymer solution to obtain spinning solution;

[0054] (4) Electrospinning the spinning solution to obtain a flexible fiber membrane containing Na3V2(PO4)2F3;

[0055] (5) The flexible fiber membrane is subjected to vacuum drying and calcination treatment in sequence to obtain the high voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode.

[0056] The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode obtained by the above preparation method has a high-voltage platform and can be directly used as a cathode in sodium-ion batteries. Furthermore, compared to the solid-phase method, this invention uses a sol-gel method to prepare VPO4 powder, resulting in VPO4 powder with higher purity and smaller particle size.

[0057] Polyacrylonitrile (PAN), as a polymer, can be used as a viscosity modifier in electrospinning solutions. Simultaneously, PAN can form nanofibers with large specific surface area and good uniformity under high voltage, constituting a three-dimensional network structure. After high-temperature calcination in an inert atmosphere, it forms carbon nanofibers. Specifically, in preparing the spinning solution, PAN is first added to an N,N-dimethylformamide solvent. After complete dissolution, Na3V2(PO4)2F3 powder is added to the polymer solution and mixed thoroughly to ensure uniform dispersion of the Na3V2(PO4)2F3 powder. More specifically, PAN powder is added to a glass bottle containing N,N-dimethylformamide (DMF) solvent and magnetically stirred at room temperature for 1–2 hours. Then, Na3V2(PO4)2F3 powder is added to the mixture, and the mixture is magnetically stirred at room temperature for 20–40 hours, followed by ultrasonic treatment for 0.2–0.8 hours to obtain the spinning solution.

[0058] Furthermore, in step (1), the method for preparing VPO4 powder using the sol-gel method is as follows:

[0059] C6H8O7·H2O, NH4VO3 and NH4H2PO4 in a molar ratio of 1:1:1 were dissolved in deionized water and magnetically stirred in an oil pan at 75-85℃ for 3-6 hours. After the water evaporated, a dark blue gel was obtained.

[0060] After freeze-drying the gel for 30-36 hours, a loose and porous bulk material is formed. The bulk material is then calcined twice to obtain VPO4 powder.

[0061] Vacuum freeze-drying freezes moisture into a solid state below 0°C, then sublimates it directly into a gaseous state under vacuum, achieving the drying effect. Compared to traditional thermal drying, freeze-drying takes longer, but the dried material is porous and less prone to agglomeration, which is beneficial for obtaining VPO4 powder with smaller particle size. The sol-gel method involves uniformly mixing raw materials in the liquid phase and allowing hydrolysis or condensation reactions to occur. The components react within the nanometer range, resulting in a faster reaction rate and higher product purity. It should be noted that when the molar ratio of C6H8O7·H2O, NH4VO3, and NH4H2PO4 is 1:1:1, VPO4 can be effectively generated under high-temperature calcination at 650–800°C, resulting in a high VPO4 content in the obtained VPO4 powder.

[0062] Specifically, C6H8O7·H2O and NH4VO3 are dissolved in deionized water. After the solution changes from yellow to blue-green, NH4H2PO4 is added. Once the NH4H2PO4 is completely dissolved, a deep blue solution is obtained. This order of adding raw materials ensures that each raw material reacts fully, reduces the occurrence of impurities, and conserves raw materials.

[0063] Furthermore, the bulk material is subjected to two calcinations as follows:

[0064] First calcination: carried out under an inert atmosphere at 320–400℃ for 3–5 hours;

[0065] Second calcination: carried out under an inert atmosphere at 650–800℃ for 6–7 hours.

[0066] When preparing VPO4 powder using the sol-gel method, citric acid is required as a reducing agent to reduce VPO4. 5+ Restored to the low-valence state V 3+ To ensure that vanadium is in the trivalent state, the gel needs to be ground and then calcined in a tube furnace for the first time; then calcined a second time in a tube furnace at a high temperature of 650-800°C to crystallize into VPO4. If the temperature of the second calcination is too low, the content of VPO4 generated will be less, resulting in low purity of Na3V2(PO4)2F3 generated in step (2). For example, the inert atmosphere mentioned above is argon or nitrogen.

[0067] Furthermore, in step (2), the method for preparing Na3V2(PO4)2F3 powder using the solid-state method is as follows:

[0068] VPO4 and NaF in a molar ratio of 2:(3-3.15) are mixed and ground for 5-20 minutes, pressed into tablets, and calcined at 600-700℃ for 1-6 hours under an inert atmosphere to obtain Na3V2(PO4)2F3 powder.

[0069] In the solid-state preparation of Na3V2(PO4)2F3 powder, this invention first mixes and grinds VPO4 and NaF in a predetermined mixing ratio to ensure thorough and uniform mixing. Then, the mixture is pressed into sheets to ensure closer and more complete contact between VPO4 and NaF, allowing for a complete reaction during calcination. This improves the purity of the Na3V2(PO4)2F3 powder and avoids the formation of impurities. Furthermore, the calcination temperature is limited to 600–700°C. If the calcination temperature is too low, the reaction between VPO4 and NaF will be incomplete, resulting in a lower content of Na3V2(PO4)2F3 and thus a decrease in the electrochemical performance of the flexible self-supporting cathode. If the calcination temperature is too high, fluorine will be lost, leading to the formation of Na3V2(PO4)3, which will also affect the electrochemical performance of the flexible self-supporting cathode.

[0070] Furthermore, in step (4), the electrospinning conditions are as follows: voltage of 21kV, feed flow rate of 0.2-0.6ml / h, roller speed of 200rpm, needle reciprocating distance of 30mm, moving speed of 10mm / s, and distance between needle and roller of 8-12cm. Under these spinning conditions, the flexible fiber membrane obtained has good uniformity of nanofibers and a large specific surface area, and Na3V2(PO4)2F3 powder is uniformly dispersed in the interior and surface of the nanofibers.

[0071] Furthermore, in step (5), after the flexible fiber membrane is vacuum dried at 90–120°C for 3–11 hours, it is pre-calcined in air at 300–350°C for 2.8–3.8 hours at a heating rate of 2°C / min; the pre-calcined flexible fiber membrane is then calcined in an inert atmosphere at 500–700°C for 1–7 hours at a heating rate of 3°C / min. It is worth noting that pre-calcination is a pre-oxidation process of the flexible fiber membrane. During this calcination process, a complex structural transformation process occurs inside the flexible fiber membrane in air, mainly involving cyclization, oxidation, and dehydrogenation reactions. Calcination is a carbonization process; polyacrylonitrile can only carbonize to form carbon nanofibers under an inert atmosphere. These carbon nanofibers serve as the flexible substrate for the flexible self-supporting positive electrode. If the calcination is performed in an oxidizing atmosphere, the carbon nanofibers will disappear, and Na3V2(PO4)2F3 will also undergo an oxidation reaction, generating other substances.

[0072] Accordingly, the present invention also provides a sodium-ion battery, comprising the aforementioned high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode. This sodium-ion battery is a flexible battery with excellent mechanical and electrochemical properties, and can be applied to flexible electronic components and smart wearable products.

[0073] The present invention is further illustrated below through examples.

[0074] Example 1

[0075] The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode of this embodiment is prepared from Na3V2(PO4)2F3 powder, polyacrylonitrile, and N,N-dimethylformamide solvent by the following method:

[0076] (1) Weigh C6H8O7·H2O, NH4VO3 and NH4H2PO4 in a molar ratio of 1:1:1. Dissolve C6H8O7·H2O, NH4VO3 and NH4H2PO4 in 70 ml of deionized water to obtain a dark blue solution. Place the solution in an oil bath at 80 °C and stir magnetically for 4 h. After the water evaporates, a dark blue gel is obtained. After freeze-drying the gel for 30 h, a loose and porous bulk material is formed. Grind the gel and place it in a tube furnace for the first calcination under an inert gas atmosphere. The specific conditions are calcination at 320 °C for 5 h. After cooling with the furnace, a VPO4 precursor is obtained. After grinding the VPO4 precursor, place it in a tube furnace for the second calcination under an inert gas atmosphere. The specific conditions are calcination at 750 °C for 7 h. After cooling with the furnace, grind the precursor to obtain VPO4 powder.

[0077] (2) Weigh VPO4 and NaF in a molar ratio of 2:3, mix and grind for 20 min, press the mixed powder into sheets, calcine at 630℃ for 5 h, cool with the furnace, grind, and obtain Na3V2(PO4)2F3 powder;

[0078] (3) Take 7 ml of DMF solvent into a glass bottle, then add 0.74 g of polyacrylonitrile powder to the above DMF solution, place it in a magnetic stirrer and stir for 1 h at a stirring speed of 1120 rpm; then weigh 0.7 g of Na3V2(PO4)2F3 powder and add it to the above mixed solution, place it in a magnetic stirrer and stir magnetically for 24 h at a stirring speed of 720 rpm, and then sonicate for 0.5 h to obtain the spinning solution;

[0079] (4) The spinning conditions are as follows: 5 ml spinning solution, voltage of 21 kV, feed speed of 0.6 ml / h, drum rotation speed of 200 rpm, needle reciprocating distance of 30 mm, moving speed of 10 mm / s, distance between needle and drum of 8 cm, and needle size of 19 G. After spinning, a flexible fiber membrane containing Na3V2(PO4)2F3 particles can be obtained on the drum.

[0080] (5) The flexible fiber membrane is first placed in a vacuum drying oven and dried at a constant temperature of 105℃ for 9 hours. After cooling, it is placed in a muffle furnace and calcined at 300℃ for 2.8 hours. After cooling, it is placed in a tube furnace and calcined at 500℃ for 7 hours. Argon gas is introduced and after cooling, a high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode can be obtained. The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode prepared in this embodiment does not need to be coated and can be directly used as a cathode for battery assembly.

[0081] SEM analysis was performed on the high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode of this embodiment, and the results are as follows: Figure 1 As shown, from Figure 1It can be seen that the carbon nanofibers in the high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode are relatively uniformly distributed, with a diameter of about 500 nm. The Na3V2(PO4)2F3 particles are encapsulated or embedded in the carbon nanofibers, proving that the active material Na3V2(PO4)2F3 does indeed exist on the carbon nanofibers. The one-dimensional carbon nanofibers act as carrier channels for sodium ions during the charge and discharge process, which is conducive to the extraction and insertion of sodium ions and improves the electrochemical performance.

[0082] Example 2

[0083] The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode of this embodiment is prepared from Na3V2(PO4)2F3 powder, polyacrylonitrile, and N,N-dimethylformamide solvent by the following method:

[0084] (1) Weigh out C6H8O7·H2O, NH4VO3 and NH4H2PO4 in a molar ratio of 1:1:1. Dissolve C6H8O7·H2O, NH4VO3 and NH4H2PO4 in 80 ml of deionized water to obtain a dark blue solution. Place the solution in an oil bath at 85°C and stir magnetically for 3 h. After the water evaporates, a dark blue gel is obtained. After freeze-drying the gel for 32 h, a loose and porous block material is formed. Grind the gel and place it in a tube furnace for the first calcination at 350°C for 4 h. After cooling in the furnace, a VPO4 precursor is obtained. Grind the VPO4 precursor and place it in a tube furnace for the second calcination at 650°C for 6 h. After cooling in the furnace, grind the precursor to obtain VPO4 powder.

[0085] (2) Weigh VPO4 and NaF in a molar ratio of 2:3, mix and grind for 15 min, press the mixed powder into sheets, calcine at 650℃ for 2 h, cool with the furnace, grind, and obtain Na3V2(PO4)2F3 powder.

[0086] (3) Take 8.5 ml of DMF solvent into a glass bottle, then add 0.95 g of polyacrylonitrile powder to the above DMF solution, place it in a magnetic stirrer and stir for 1.5 h at a stirring speed of 1120 rpm; then weigh 1.9 g of Na3V2(PO4)2F3 powder and add it to the above mixed solution, place it in a magnetic stirrer and stir magnetically for 36 h at a stirring speed of 720 rpm, and then sonicate for 0.8 h to obtain the spinning solution;

[0087] (4) The spinning conditions were as follows: 5 ml spinning solution, voltage of 21 kV, feed rate of 0.4 ml / h, drum rotation speed of 200 rpm, needle reciprocating distance of 30 mm, moving speed of 10 mm / s, distance between needle and drum of 9 cm, and needle size of 19 G. After spinning, a flexible fiber membrane containing Na3V2(PO4)2F3 particles was obtained on the drum.

[0088] (5) The flexible fiber membrane is first placed in a vacuum drying oven at a constant temperature of 120℃ for 3 hours. After cooling, it is placed in a muffle furnace at 350℃ for 3.8 hours. After cooling, it is placed in a tube furnace at 580℃ for 3 hours. Argon gas is introduced and after cooling, a high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode can be obtained. No coating is required, and it can be directly used as a positive electrode for battery assembly.

[0089] Figure 2 The SEM image obtained from the SEM analysis of the high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode in Example 2 shows that the carbon nanofibers are relatively uniformly distributed in the high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode, with a diameter of about 300 nm. Na3V2(PO4)2F3 particles are present between the carbon nanofibers, indicating that the active material Na3V2(PO4)2F3 is not well encapsulated or embedded in the carbon nanofibers; the fibers are clearly discontinuous and have undergone a certain degree of breakage. The increased amount of Na3V2(PO4)2F3 powder leads to uneven embedding and distribution of the powder on the carbon nanofibers, and agglomeration occurs, forming larger secondary particles. These larger secondary particles disrupt the continuity of the fibers, reduce the mechanical properties of the flexible electrode, and impair its flexibility. The mechanical properties of the high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode in this embodiment are inferior to those in Example 1, but it has comparable electrochemical properties. Therefore, in this invention, the mass ratio of polyacrylonitrile to Na3V2(PO4)2F3 powder is not less than 1:2.

[0090] Example 3

[0091] The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode of this embodiment is prepared from Na3V2(PO4)2F3 powder, polyacrylonitrile, and N,N-dimethylformamide solvent by the following method:

[0092] (1) Weigh out C6H8O7·H2O, NH4VO3 and NH4H2PO4 in a molar ratio of 1:1:1. Dissolve C6H8O7·H2O, NH4VO3 and NH4H2PO4 in 110 ml of deionized water to obtain a dark blue solution. Place the solution in an oil bath at 82°C and stir magnetically for 6 h. After the water evaporates, a dark blue gel is obtained. After freeze-drying the gel for 36 h, a loose and porous bulk material is formed. Grind the gel and place it in a tube furnace for the first calcination at 400°C for 4.5 h. After cooling in the furnace, a VPO4 precursor is obtained. Grind the VPO4 precursor and place it in a tube furnace for the second calcination at 700°C for 6.5 h. After cooling in the furnace, grind the precursor to obtain VPO4 powder.

[0093] (2) Weigh VPO4 and NaF in a molar ratio of 2:3.15, mix and grind for 5 min, press the mixed powder into sheets, calcine at 690℃ for 1 h, cool with the furnace, grind, and obtain Na3V2(PO4)2F3 powder.

[0094] (3) Take 9 ml of DMF solvent into a glass bottle, then add 0.87 g of polyacrylonitrile powder to the above DMF solution, place it in a magnetic stirrer and stir for 1.25 h at a stirring speed of 1120 rpm; then weigh 1.2 g of Na3V2(PO4)2F3 powder and add it to the above mixed solution, place it in a magnetic stirrer and stir magnetically for 24 h at a stirring speed of 720 rpm, and then sonicate for 0.2 h to obtain spinning solution;

[0095] (4) The spinning conditions are as follows: 5 ml spinning solution, voltage of 21 kV, feed speed of 0.2 ml / h, roller speed of 200 rpm, needle reciprocating distance of 10 mm, moving speed of 30 mm / s, distance between needle and roller of 10 cm, and needle size of 19 G. After spinning, a flexible fiber membrane containing Na3V2(PO4)2F3 particles can be obtained on the roller;

[0096] (5) The flexible fiber membrane is first placed in a vacuum drying oven and dried at a constant temperature of 95°C for 10 hours. After cooling, it is placed in a muffle furnace and calcined at 330°C for 3.1 hours. After cooling, it is placed in a tube furnace and calcined at 630°C for 5 hours. Argon gas is introduced and after cooling, a high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode can be obtained. The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode prepared in this embodiment does not need to be coated and can be directly used as a cathode for battery assembly.

[0097] XRD analysis was performed on the Na3V2(PO4)2F3 powder material prepared in this embodiment, and the results are as follows: Figure 3 As shown, by Figure 3As can be seen from the comparison with the PDF card of the standard substance, the intensity and position of the peaks in the XRD pattern of the Na3V2(PO4)2F3 powder material in this embodiment are consistent with those of the standard Na3V2(PO4)2F3 substance.

[0098] XRD analysis was performed on the high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode prepared in this embodiment, and the results are as follows: Figure 4 As shown, by Figure 4 As can be seen from the comparison with the PDF card of the standard material, the intensity and position of the peaks in the XRD pattern of the high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode in this embodiment are consistent with those of the standard Na3V2(PO4)2F3 active material, which proves that the high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode prepared in this embodiment contains Na3V2(PO4)2F3 electrochemical active material.

[0099] Example 4

[0100] The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode of this embodiment is prepared from Na3V2(PO4)2F3 powder, polyacrylonitrile, and N,N-dimethylformamide solvent by the following method:

[0101] (1) Weigh C6H8O7·H2O, NH4VO3 and NH4H2PO4 in a molar ratio of 1:1:1. Dissolve C6H8O7·H2O, NH4VO3 and NH4H2PO4 in 65 ml of deionized water to obtain a dark blue solution. Place the solution in an oil bath at 75°C and stir magnetically for 3.5 h. After the water evaporates, a dark blue gel is obtained. After freeze-drying the gel for 34 h, a loose and porous bulk material is formed. Grind the gel and place it in a tube furnace for the first calcination at 380°C for 5 h. After cooling in the furnace, a VPO4 precursor is obtained. Grind the VPO4 precursor and place it in a tube furnace for the second calcination at 720°C for 6.8 h. After cooling in the furnace, grind the precursor to obtain VPO4 powder.

[0102] (2) Weigh VPO4 and NaF in a molar ratio of 2:3.15, mix and grind for 12 min, press the mixed powder into sheets, calcine at 700℃ for 3 h, cool with the furnace, grind, and obtain Na3V2(PO4)2F3 powder.

[0103] (3) Take 10 ml of DMF solvent into a glass bottle, then add 1.2 g of polyacrylonitrile powder to the above DMF solution, place it in a magnetic stirrer and stir for 2 h at a stirring speed of 1120 rpm; then weigh 1.7 g of Na3V2(PO4)2F3 powder and add it to the above mixed solution, place it in a magnetic stirrer and stir magnetically for 30 h at a stirring speed of 720 rpm, and then sonicate for 0.4 h to obtain the spinning solution;

[0104] (4) The spinning conditions are: 5 ml of spinning solution, voltage of 21 kV, feed speed of 0.3 ml / h, drum speed of 200 rpm, needle back-and-forth distance of 30 mm, moving speed of 10 mm / s, distance between needle and drum of 12 cm, needle size of 19 G. After spinning, a flexible fiber membrane containing Na3V2(PO4)2F3 particles can be obtained on the drum.

[0105] (5) The flexible fiber membrane is first placed in a vacuum drying oven and dried at a constant temperature of 90°C for 11 hours. After cooling, it is placed in a muffle furnace and calcined at 310°C for 3.2 hours. After cooling, it is placed in a tube furnace and calcined at 700°C for 1 hour. Argon gas is introduced and after cooling, a high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode can be obtained. The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode prepared in this embodiment does not need to be coated and can be directly used as a cathode for battery assembly.

[0106] The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode prepared in this embodiment was assembled into an experimental coin cell, and its charge-discharge electrochemical performance was tested. The charge-discharge curve at 0.2C is shown below. Figure 5 As shown, the electrochemical characteristics are consistent with those of Na3V2(PO4)2F3 powder, exhibiting charge-discharge capability between 2-4.3V and two high-voltage plateaus around 3.6V and 4V. These high-voltage plateaus persist even after 200 cycles. This demonstrates that the high-voltage Na3V2(PO4)2F3 flexible self-supporting cathode possesses excellent electrochemical performance.

[0107] Example 5

[0108] The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode of this embodiment is prepared from Na3V2(PO4)2F3 powder, polyacrylonitrile, and N,N-dimethylformamide solvent by the following method:

[0109] (1) Weigh out C6H8O7·H2O, NH4VO3 and NH4H2PO4 in a molar ratio of 1:1:1. Dissolve C6H8O7·H2O, NH4VO3 and NH4H2PO4 in 85 ml of deionized water to obtain a dark blue solution. Place the solution in an oil bath at 77°C and stir magnetically for 4 h. After the water evaporates, a dark blue gel is obtained. After freeze-drying the gel for 33 h, a loose and porous bulk material is formed. Grind the gel and place it in a tube furnace for the first calcination at 340°C for 4.2 h. After cooling in the furnace, a VPO4 precursor is obtained. Grind the VPO4 precursor and place it in a tube furnace for the second calcination at 780°C for 6.2 h. After cooling in the furnace, grind the precursor to obtain VPO4 powder.

[0110] (2) Weigh VPO4 and NaF in a molar ratio of 2:3.1, mix and grind for 10 min, press the mixed powder into sheets, calcine at 600℃ for 4 h, cool with the furnace, grind, and obtain Na3V2(PO4)2F3 powder;

[0111] (3) Take 8 ml of DMF solvent into a glass bottle, then add 0.64 g of polyacrylonitrile powder to the above DMF solution, place it in a magnetic stirrer and stir for 1 h at a stirring speed of 1120 rpm; then weigh 0.4 g of Na3V2(PO4)2F3 powder and add it to the above mixed solution, place it in a magnetic stirrer and stir magnetically for 24 h at a stirring speed of 720 rpm, and then sonicate for 0.2 h to obtain the spinning solution;

[0112] (4) The spinning conditions are: 5 ml of spinning solution, voltage of 21 kV, feed speed of 0.5 ml / h, drum speed of 400 rpm, needle back-and-forth distance of 30 mm, moving speed of 10 mm / s, distance between needle and drum of 11 cm, needle size of 19 G. After spinning, a flexible fiber membrane containing Na3V2(PO4)2F3 particles can be obtained on the drum.

[0113] (5) The flexible fiber membrane is first placed in a vacuum drying oven and dried at a constant temperature of 100°C for 6 hours. After cooling, it is taken out and placed in a muffle furnace and calcined at 320°C for 3.4 hours. After cooling, it is placed in a tube furnace and calcined at 600°C for 2 hours. Argon gas is introduced and after cooling, a high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode can be obtained. The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode prepared in this embodiment does not need to be coated and can be directly used as a cathode for battery assembly.

[0114] The high-voltage Na3V2(PO4)2F3@C flexible self-supporting electrode prepared in this embodiment was used as the positive electrode, and Na3V2(PO4)3 was used as the negative electrode to assemble a coin cell, and its electrochemical performance was tested. Figure 6 The cyclic voltammetry curve of the full cell at 0.3 mV / s is shown. Figure 7 This is the charge-discharge curve of the full cell at 0.2C. Cyclic voltammetry and charge-discharge tests show that the high-voltage Na3V2(PO4)2F3@C flexible self-supporting electrode can be used to assemble a full cell and successfully undergo charge-discharge cycles.

[0115] Example 6

[0116] The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode of this embodiment is prepared from Na3V2(PO4)2F3 powder, polyacrylonitrile, and N,N-dimethylformamide solvent by the following method:

[0117] (1) Weigh C6H8O7·H2O, NH4VO3 and NH4H2PO4 in a molar ratio of 1:1:1. Dissolve C6H8O7·H2O, NH4VO3 and NH4H2PO4 in 90 ml of deionized water to obtain a dark blue solution. Place the solution in an oil bath at 83°C and stir magnetically for 3.8 h. After the water evaporates, a dark blue gel is obtained. After freeze-drying the gel for 31 h, a loose and porous bulk material is formed. Grind the gel and place it in a tube furnace for the first calcination at 390°C for 3 h. After cooling in the furnace, a VPO4 precursor is obtained. Grind the VPO4 precursor and place it in a tube furnace for the second calcination at 800°C for 6.3 h. After cooling in the furnace, grind the precursor to obtain VPO4 powder.

[0118] (2) Weigh VPO4 and NaF in a molar ratio of 2:3.05, mix and grind for 18 min, press the mixed powder into sheets, calcine at 680℃ for 6 h, cool with the furnace, grind, and obtain Na3V2(PO4)2F3 powder.

[0119] (3) Take 9.5 ml of DMF solvent into a glass bottle, then take 1.1 g of polyacrylonitrile powder and add it to the above DMF solution. Place it in a magnetic stirrer and stir for 1 h at a stirring speed of 1120 rpm. Then weigh 0.55 g of Na3V2(PO4)2F3 powder and add it to the above mixed solution. Place it in a magnetic stirrer and stir magnetically for 24 h at a stirring speed of 720 rpm. Then sonicate for 0.4 h to obtain the spinning solution.

[0120] (4) The spinning conditions are: 5 ml of spinning solution, voltage of 21 kV, feed speed of 0.45 ml / h, drum speed of 200 rpm, needle back-and-forth distance of 30 mm, moving speed of 10 mm / s, distance between needle and drum of 10.5 cm, needle size of 19 G. After spinning, a flexible fiber membrane containing Na3V2(PO4)2F3 particles can be obtained on the drum.

[0121] (5) The flexible fiber membrane is first placed in a vacuum drying oven and dried at a constant temperature of 110°C for 8 hours. After natural cooling, it is taken out and placed in a muffle furnace and calcined at 340°C for 3 hours. After cooling, it is placed in a tube furnace and calcined at 650°C for 6 hours. Argon gas is introduced and after cooling, a high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode can be obtained. The high-voltage Na3V2(PO4)2F3@C flexible self-supporting cathode prepared in this embodiment does not need to be coated and can be directly used as a cathode for battery assembly.

[0122] The high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrodes of Examples 1-6 were bent without cracking, indicating that they meet the mechanical performance requirements for assembling flexible batteries. A bending photograph of Example 6 is shown below. Figure 8 As shown.

[0123] Other components and operations of a high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode, its preparation method, and its application according to embodiments of the present invention are known to those skilled in the art and will not be described in detail here.

[0124] In the description of this specification, references to terms such as "embodiment," "example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0125] Although embodiments of the invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. A method for preparing a high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode, characterized in that, Includes the following steps: (1) VPO4 powder was prepared by sol-gel method; (2) Mix VPO4 powder and NaF to prepare Na3V2(PO4)2F3 powder by solid-state method; (3) Add polyacrylonitrile to N,N-dimethylformamide solvent to form a polymer solution, and then disperse Na3V2(PO4)2F3 powder in the polymer solution to obtain spinning solution; (4) Electrospinning the spinning solution to obtain a flexible fiber membrane containing Na3V2(PO4)2F3; (5) The flexible fiber membrane is subjected to vacuum drying and calcination treatment in sequence to obtain the high voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode; In step (2), the method for preparing Na3V2(PO4)2F3 powder using the solid-state method is as follows: Mix and grind VPO4 and NaF in a molar ratio of 2:(3~3.15) for 5~20 min, press into tablets, and calcine at 600~700 ℃ for 1~6 h under an inert atmosphere to obtain Na3V2(PO4)2F3 powder; In step (5), the flexible fiber membrane is vacuum dried at 90~120 ℃ for 3~11 h, and then pre-calcined in air at 300~350 ℃ for 2.8~3.8 h; the pre-calcined flexible fiber membrane is calcined at 500~700 ℃ for 1~7 h in an inert atmosphere, with a heating rate of 3℃ / min.

2. The method for preparing a high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode according to claim 1, characterized in that, In step (1), the method for preparing VPO4 powder using the sol-gel method is as follows: C6H8O7·H2O, NH4VO3 and NH4H2PO4 in a molar ratio of 1:1:1 were dissolved in deionized water and magnetically stirred in an oil pan at 75-85 ℃ for 3-6 h. After the water evaporated, a dark blue gel was obtained. After freeze-drying the gel for 30-36 h, a loose and porous bulk material is formed. The bulk material is then calcined twice to obtain VPO4 powder.

3. The method for preparing a high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode according to claim 2, characterized in that, The bulk material is subjected to two calcinations as follows: First calcination: carried out under an inert atmosphere at 320~400 ℃ for 3-5 h; Second calcination: carried out under an inert atmosphere at 650~800 ℃ for 6-7 h.

4. The method for preparing a high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode according to claim 1, characterized in that, In step (3), the mass ratio of polyacrylonitrile to Na3V2(PO4)2F3 powder is 1:(0.5~2).

5. The method for preparing a high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode according to claim 1, characterized in that, In step (4), the electrospinning conditions are as follows: voltage is 21 kV, feed flow rate is 0.2~0.6 ml / h, roller speed is 200 rpm, needle back-and-forth distance is 30 mm, moving speed is 10 mm / s, and distance between needle and roller is 8~12 cm.

6. A sodium-ion battery, characterized in that, The high-voltage Na3V2(PO4)2F3@C flexible self-supporting positive electrode prepared by the method described in any one of claims 1-5.

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