Solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium iron pyrophosphate composite cathode material and preparation method and application thereof

Abstract

The application belongs to the technical field of sodium ion battery positive electrode materials, and relates to a solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium iron pyrophosphate phosphate composite positive electrode material and a preparation method and application thereof. The preparation method is that sodium iron pyrophosphate phosphate is taken as a matrix, ammonium fluoride is taken as a solid source, and sodium iron pyrophosphate phosphate is modified by fluorine and nitrogen element co-doping by using plasma technology to obtain a sodium iron pyrophosphate phosphate composite positive electrode material. The composite positive electrode material has high capacity and excellent cycle stability, and has a wide market application prospect. Meanwhile, the preparation method is simple, rapid, efficient, convenient and easy to control, and is helpful to promote the development and application of high-performance sodium ion battery positive electrode materials.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery cathode material technology, specifically to a high-capacity fluorine and nitrogen co-doped sodium iron pyrophosphate composite cathode material constructed by solid-source plasma, its preparation method, and its application. Background Technology

[0002] With the increasing global demand for large-scale energy storage systems (such as smart grids and renewable energy grid integration) and low-cost electric vehicles, the development of resource-rich, low-cost, safe, and reliable electrochemical energy storage technologies has become a focus of research. While lithium-ion batteries (LIBs) dominate the market, the uneven geographical distribution of lithium resources and continuously rising costs limit their widespread application in large-scale energy storage. Against this backdrop, sodium-ion batteries (SIBs), due to their abundant and widely distributed sodium resources and low cost, are considered one of the most promising alternative technologies. One of the core challenges of sodium-ion batteries lies in developing high-performance cathode materials. Among numerous candidate materials, polyanionic compounds are favored due to their stable crystal framework, tunable operating voltage, and excellent thermal stability. Among these, pyrophosphate cathode materials, especially sodium iron phosphate pyrophosphate (Na4Fe3(PO4)2(P2O7), NFPP), have attracted widespread attention from academia and industry in recent years.

[0003] NFPP belongs to the orthorhombic crystal system with space group Pn21a. Its structure can be understood as a robust three-dimensional framework formed by the interconnection of FeO6 octahedra, PO4 tetrahedra, and P2O7 bitetrahedra. Sodium ions occupy various vacancies within this framework, providing multidimensional ion migration channels. (P2O7) 4- Group ratio (PO4) 3- The group has stronger electronegativity, which can further reduce Fe. 3+ / Fe 2+ The energy level of the NFPP increases the average operating voltage. The average voltage of the NFPP is approximately 3.2 V (vs. Na). + The sodium ion intercalation / deintercalation ratio ( / Na) is higher than that of pure pyrophosphate (~3.0 V). Furthermore, the three-dimensional framework constructed by the two strongly covalently bonded polyanionic groups is extremely stable, exhibiting minimal volume change (<1%) during sodium ion intercalation / deintercalation, which provides a solid foundation for its ultra-long cycle life. The extremely small volumetric strain allows the material to maintain a very high capacity retention rate after thousands of cycles, with a lifespan far exceeding that of many layered oxide cathodes.

[0004] Despite the promising prospect of NFPP, it also faces similar inherent challenges as other polyanionic materials: low intrinsic electronic conductivity: this is the main factor limiting its rate performance; low tap density: resulting in relatively low volumetric energy density of the electrode. Therefore, it is urgent to develop interface optimization strategies to strengthen its ion transport, obtain high safety, long life, low cost sodium ion battery cathode materials. And the current mainstream modification strategies include: carbon coating, nanocrystallization, element doping, etc. For example, the existing patent CN116281922A Sodium-rich fluorine-doped sodium iron pyrophosphate composite material and preparation method and application realizes fluorine doping by adding fluoride such as ammonium fluoride during the preparation of sodium iron pyrophosphate material. However, this technology needs to be carried out during the preparation of sodium iron pyrophosphate material, and it is difficult to realize the modification treatment of the material surface, and it requires sodium-rich treatment to improve the modification effect, and only realizes single fluorine doping modification, which still has further improvement space.

[0005] Based on this, the present application proposes a kind of solid source plasma constructs high capacity fluorine, nitrogen synergic doped sodium iron pyrophosphate composite cathode material and its preparation method and application, effectively combine the above strategies, double modification is carried out to sodium iron pyrophosphate, synergic strengthening of cathode / electrolyte interface sodium ion transport dynamics, fluorine ion has stronger electronegativity, can strengthen the strength of metal-oxygen / flourine bond, sometimes slightly expand sodium ion migration channel, reduce diffusion energy barrier, nitrogen element is introduced into carbon layer, and C-N bond is formed by rearrangement with carbon. The two realize the synergistic optimization of the bulk structure and the surface interface of the material. SUMMARY

[0006] The present application mainly aims at the problems of low theoretical and actual mass energy density, extremely low intrinsic electronic conductivity and complex process of sodium iron pyrophosphate cathode material, and provides a kind of solid source plasma constructs high capacity fluorine, nitrogen synergic doped sodium iron pyrophosphate composite cathode material and its preparation method. By using plasma technology, with solid ammonium fluoride as the activation source in the plasma environment, high-energy electrons and ions will bombard and decompose the solid ammonium fluoride, generating various high-activity fluorine-containing and nitrogen-containing radicals and ions. Fluoride anions partially replace oxygen ions and enter the bulk phase for doping. Nitrogen is doped into sp 2 The carbon network, nitrogen and carbon rearrangement form a close-packed network that can greatly enhance the electronic conductivity of the carbon layer and the wettability of the electrolyte. Thus, the interface electronic conductivity and ion mobility of sodium iron pyrophosphate are improved, and the first cycle coulombic efficiency and long cycle stability are greatly improved.

[0007] The technical scheme adopted by the present application to solve its technical problems is:

[0008] The application discloses a preparation method of a solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium iron pyrophosphate composite positive electrode material.

[0009] Preferably, the method comprises the following steps:

[0010] (1) sodium iron pyrophosphate Na4Fe3(PO4)2(P2O7) and a solid source ammonium fluoride are ground and mixed to obtain a mixture, the mixture is placed in a plasma reaction device, and the plasma reaction device is connected with a plasma generator, and a reaction area of the plasma reaction device is subjected to vacuumizing treatment;

[0011] (2) under a heating condition, plasma is ignited by adjusting a radio frequency power and a vacuum degree, and a plasma reaction is carried out between the sodium iron pyrophosphate and the plasma, after a certain reaction time, the radio frequency power is turned off, and the sodium iron pyrophosphate co-doped with fluorine and nitrogen is obtained.

[0012] The following are preferred technical schemes of the application:

[0013] Preferably, in step (1), the selected sample is at least one of a sheet and a powder, more preferably, the sample can be more uniformly treated by rotating the plasma device, and the solid mixing and grinding can make the sample more fully mixed and react. The particle size range is preferably 1-5 μm.

[0014] Preferably, in step (1), the vacuum degree is 1-50 Pa, and the vacuum state is in a vacuum state.

[0015] Preferably, in step (1), the mass percentage content of the solid source in the mixture is 10-50%.

[0016] Preferably, in step (2), the system temperature is 100-900 ℃ during heating, and more preferably, the temperature is 200-800 ℃.

[0017] Preferably, in step (2), the reaction conditions are as follows: the radio frequency power is 100-800 W, the system vacuum degree is preferably 5-100 Pa, the reaction temperature is 100-900 ℃, and the reaction time is 3-30 min, and more preferably, the vacuum degree is 30 Pa.

[0018] Preferably, in step (2), the sodium iron pyrophosphate composite positive electrode material is co-doped and modified by fluorine and nitrogen elements; more preferably, the doping amount of fluorine and nitrogen in the sodium iron pyrophosphate composite positive electrode material is 1-3%.

[0019] The application also provides the solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium iron pyrophosphate composite positive electrode material prepared by any one of the preparation methods.

[0020] The application also provides application of the solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium iron pyrophosphate composite positive electrode material in the field of sodium ion batteries.

[0021] The application is prepared by one-step reaction of solid source plasma excitation and sodium iron pyrophosphate material, high-energy electrons and ions bombard and decompose ammonium fluoride solid, generate various high-activity fluorine and nitrogen-containing radicals and ions, fluorine anions partially replace oxygen ions, and enter the bulk phase for doping.

[0022] Compared with the prior art, the application has the beneficial effects as follows:

[0023] (1) One-step efficient realization of double functional doping: fluorine and nitrogen are introduced at the same time in one process, which simplifies the process flow, improves the efficiency and reduces the cost.

[0024] (2) Safety and environmental protection: solid NH4F is used as the source, which avoids the use of highly toxic and corrosive F2 gas, greatly improves the safety of experiments and future production.

[0025] (3) Surface and bulk phase collaborative modification: fluorine doping mainly acts on the bulk lattice, stabilizes the structure, improves the voltage and expands the ion channel. Nitrogen doping is more inclined to the surface and the carbon coating layer, which greatly improves the interface electron conductivity and ion mobility. The combination of the two improves the performance of NFPP from the bulk phase to the interface.

[0026] The application is prepared by one-step reaction of solid source plasma excitation and sodium iron pyrophosphate material, high-energy electrons and ions bombard and decompose ammonium fluoride solid, generate various high-activity fluorine and nitrogen-containing radicals and ions, fluorine anions partially replace oxygen ions, and enter the bulk phase for doping. 2In the carbon network, nitrogen and carbon rearrangement forms a close-packed network, which can greatly enhance the electronic conductivity of the carbon layer and the wettability of the electrolyte. At the same time, the ion transmission can be effectively strengthened, and a sodium ion battery positive electrode material with high capacity and long life is obtained. The synergistic optimization of the bulk structure and the surface interface of the material helps to promote the development of the interface optimization strategy of the sodium ion battery positive electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The schematic diagram of the reaction device for the plasma technology in Example 1 is shown in the figure.

[0028] Figure 2 The TEM image and element distribution diagram of the fluorine and nitrogen co-doped sodium iron pyrophosphate composite material prepared in Example 1 are shown in the figure.

[0029] Figure 3 The rate performance comparison diagram of the coin-type half battery of the fluorine and nitrogen co-doped sodium iron pyrophosphate composite material prepared in Example 1 and the comparative example 1 is shown in the figure.

[0030] Figure 4 The cycle stability performance comparison diagram of the coin-type half battery of the fluorine and nitrogen co-doped sodium iron pyrophosphate composite material prepared in Example 1 and the comparative example 1 is shown in the figure. DETAILED DESCRIPTION

[0031] For the convenience of understanding, the technical solutions and embodiments of the present application are further clearly, completely and specifically described below by means of specific embodiments and in conjunction with the drawings. It should be understood that the embodiments described in the present application are implemented on the premise of the technical solutions of the present application, and detailed implementation manners and specific operation processes are given, but only a part of the embodiments of the present application, not all the embodiments. The specific implementation manners described are only limited to the description and explanation of the present application, and do not limit the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0032] Unless otherwise specified, the experimental methods and conditions used in the following examples are conventional methods and conventional conditions. The materials, reagents or instrument devices used in the examples, unless otherwise specified, are conventional substances or devices known to those skilled in the art and can be obtained commercially. The reaction conditions embodied in the inventive content of the present application can achieve the described reactions and obtain the expected effect of the product. Due to the limitation of the length, only some examples are listed below to further illustrate the advantages of the technical solutions of the present application.

[0033] Example 1

[0034] The pyrophosphoric iron sodium phosphate (can refer to the method of comparative example 1) powder sample is mixed with solid powder ammonium fluoride by grinding and placed in the reaction zone of a rotating plasma reaction device, wherein the mass percentage content of the solid source ammonium fluoride in the mixture is 10%; copper rings are connected at both ends of the plasma reaction device, and the copper rings are connected with the generator (plasma generator) of the radio frequency power source by wires, then the plasma device is vacuumed to 10 Pa and kept in a vacuum state, and the temperature is heated to 500 ℃. Turn on the radio frequency power switch, adjust the radio frequency power, and adjust the vacuum pump to maintain the vacuum degree at 30 Pa. After ignition, high-energy electrons and ions will bombard and decompose the ammonium fluoride solid to produce various high-activity fluorine-containing and nitrogen-containing free radicals, which react with the surface of the pyrophosphoric iron sodium phosphate. After 10 minutes of plasma reaction, the radio frequency power is turned off, and a fluorine and nitrogen co-doped pyrophosphoric iron sodium phosphate composite material is obtained.

[0035] Examples 2-16

[0036] On the basis of example 1, the reaction conditions including the percentage content of the solid source, the reaction temperature, the reaction power (radio frequency power), the reaction time, etc. are changed, and the specific conditions are shown in Table 1 as follows:

[0037]

[0038] Comparative example 1

[0039] CH3COONa·3H2O, MgSO4·7H2O, FeSO4·7H2O, NH4H2PO4 and C6H8O7·H2O are added in deionized water in proportion. After stirring for 10 min, the precursor is obtained by drying at 160℃. Then, the precursor powder is ground and precalcined at 300℃ for 3 h under vacuum, with a heating rate of 5℃·min -1 . The calcined powder is pressed at a pressure of 15 MPa, and then calcined at 550℃ for 10 h under vacuum, with a heating rate of 5℃·min -1 . Finally, the block is crushed to obtain pyrophosphoric iron sodium phosphate powder.

[0040] Comparative example 2

[0041] The pyrophosphoric iron sodium phosphate material in comparative example 1 is mixed with solid powder ammonium fluoride by grinding and placed in a sealed supercritical high-pressure reaction kettle, and vacuumed to a state of 8.0 MPa. Then the supercritical device after argon is transferred to a 40℃ oven and kept for 8 h, and then cooled naturally, and the gas valve is quickly opened to obtain a modified pyrophosphoric iron sodium phosphate material, but the fluorine and nitrogen doping is not successful.

[0042] Comparative example 3

[0043] The pyrophosphate sodium iron phosphate material in Comparative Example 1 was mixed with solid powder ammonium fluoride by grinding and placed in a sealed supercritical high-pressure reaction kettle, and vacuumized. Argon gas was introduced into the supercritical device at a gas rate of 40 seem to make the gas pressure reach 7.0 MPa. Then the supercritical device after argon introduction was transferred to a 40 °C oven and left to stand, and after 8 h of heat preservation, it was naturally cooled, the gas valve was quickly opened, and the modified pyrophosphate sodium iron phosphate material was obtained, but fluorine and nitrogen doping was not successfully achieved.

[0044] Comparative Example 4

[0045] The pyrophosphate sodium iron phosphate material in Comparative Example 1 was mixed with solid powder ammonium fluoride by grinding and placed in a general tube furnace, argon protective gas was introduced, and then the temperature was raised to 500 °C at a temperature rising rate of 5 °C·min -1 -1. The temperature was raised to 500 °C at a temperature rising rate of 5 °C·min

[0046] Comparative Example 5

[0047] The pyrophosphate sodium iron phosphate material in Comparative Example 1 was mixed with solid powder ammonium fluoride by grinding and placed in a general tube furnace, argon protective gas was introduced, and then nitrogen gas was introduced at a gas rate of 30 seem. Then the temperature was raised to 500 °C at a temperature rising rate of 5 °C·min -1 -1. The temperature was raised to 500 °C at a temperature rising rate of 5 °C·min

[0048] Comparative Example 6

[0049] The pyrophosphate sodium iron phosphate material in Comparative Example 1 was mixed with solid powder ammonium fluoride by grinding and placed in a general tube furnace, argon protective gas was introduced, and then nitrogen gas was introduced at a gas rate of 50 seem. Then the temperature was raised to 500 °C at a temperature rising rate of 5 °C·min -1 -1. The temperature was raised to 500 °C at a temperature rising rate of 5 °C·min

[0050] Comparative Example 7

[0051] The pyrophosphate sodium iron phosphate material in Comparative Example 1 was mixed with solid powder ammonium fluoride by grinding and placed in a general tube furnace, argon protective gas was introduced, and then nitrogen gas was introduced at a gas rate of 70 seem. Then the temperature was raised to 500 °C at a temperature rising rate of 5 °C·min -1 -1. The temperature was raised to 500 °C at a temperature rising rate of 5 °C·min

[0052] Comparative Example 8

[0053] The pyrophosphate sodium iron phosphate material in Comparative Example 1 was ground and mixed with solid powder ammonium hexafluorophosphate and placed in the reaction zone of a rotating plasma reaction device, copper rings were connected to both ends of the plasma reaction device, and a wire was used to connect the copper rings to the generator of the radio frequency power source (plasma generator), then the plasma device was vacuumed to 10 Pa and kept in a vacuum state, the temperature was heated to 500 ℃. Turn on the radio frequency power switch, adjust the radio frequency power, adjust the radio frequency power to 500 W, adjust the vacuum pump to maintain the vacuum degree at 30 Pa. After 10 min of treatment, the modified pyrophosphate sodium iron phosphate material was obtained by natural cooling, but fluorine and nitrogen doping was not successfully achieved.

[0054] Comparative Example 9

[0055] The pyrophosphate sodium iron phosphate material in Comparative Example 1 was ground and mixed with solid powder ammonium hexafluorophosphate and placed in the reaction zone of a rotating plasma reaction device, copper rings were connected to both ends of the plasma reaction device, and a wire was used to connect the copper rings to the generator of the radio frequency power source (plasma generator), then the plasma device was vacuumed to 10 Pa and kept in a vacuum state, the temperature was heated to 500 ℃. Turn on the radio frequency power switch, adjust the radio frequency power, adjust the radio frequency power to 500 W, adjust the vacuum pump to maintain the vacuum degree at 30 Pa. After 10 min of treatment, the modified pyrophosphate sodium iron phosphate material was obtained by natural cooling, but fluorine and nitrogen doping was not successfully achieved.

[0056] Comparative Example 10

[0057] The pyrophosphate sodium iron phosphate material in Comparative Example 1 was ground and mixed with solid powder ammonium hexafluorophosphate and placed in the reaction zone of a rotating plasma reaction device, copper rings were connected to both ends of the plasma reaction device, and a wire was used to connect the copper rings to the generator of the radio frequency power source (plasma generator), then the plasma device was vacuumed to 10 Pa and kept in a vacuum state, the temperature was heated to 500 ℃. Turn on the radio frequency power switch, adjust the radio frequency power, adjust the radio frequency power to 500 W, adjust the vacuum pump to maintain the vacuum degree at 30 Pa. After 10 min of treatment, the modified pyrophosphate sodium iron phosphate material was obtained by natural cooling, but fluorine and nitrogen doping was not successfully achieved.

[0058] Comparative Example 11

[0059] The pyrophosphate sodium iron phosphate material in Comparative Example 1 was ground and mixed with solid powder ammonium dihydrogen phosphate and placed in the reaction zone of a rotating plasma reaction device. Copper rings were connected to both ends of the plasma reaction device, and a wire was used to connect the copper rings to the generator of a radio frequency power source (plasma generator). The plasma device was then vacuumed to 10 Pa and maintained in a vacuum state. The temperature was heated to 500°C. Carbon dioxide gas was introduced, and the radio frequency power switch was turned on. The radio frequency power was adjusted to 500 W, and the vacuum pump was adjusted to maintain a vacuum degree of 30 Pa. After 10 min of treatment, the modified pyrophosphate sodium iron phosphate material was obtained by natural cooling, but fluorine and nitrogen doping was not successfully achieved.

[0060] Comparative Example 12

[0061] The pyrophosphate sodium iron phosphate material in Comparative Example 1 was ground and mixed with 70% solid powder ammonium fluoride and placed in a general tube furnace. Argon gas was introduced as a protective gas, and nitrogen gas was introduced at a gas rate of 30 sccm. The temperature was then increased to 500°C at a rate of 5°C·min -1 After holding for 2 h, the modified pyrophosphate sodium iron phosphate material was obtained by natural cooling, and fluorine and nitrogen doping was achieved.

[0062] Comparative Example 13

[0063] The pyrophosphate sodium iron phosphate material in Comparative Example 1 was placed in the reaction zone of a rotating plasma reaction device. Copper rings were connected to both ends of the plasma reaction device, and a wire was used to connect the copper rings to the generator of a radio frequency power source (plasma generator). The plasma device was then vacuumed to 10 Pa and maintained in a vacuum state. The temperature was heated to 500°C. Trifluoromethane gas was introduced into the plasma reaction device. The radio frequency power switch was turned on, and the radio frequency power was adjusted to 500 W. The vacuum pump was adjusted to maintain a vacuum degree of 30 Pa. After 10 min of treatment, the plasma-modified pyrophosphate sodium iron phosphate material was obtained by natural cooling, and fluorine doping was achieved.

[0064] Performance Test

[0065] The sodium iron pyrophosphate samples prepared in Examples 1-16 and Comparative Examples 1-11 were homogenously mixed with Super P and PVDF at a mass ratio of 7:2:1 to form the electrode. The coated electrode was dried in a vacuum drying oven at 120°C for 12 h to ensure complete removal of the solvent and was cut into a circular electrode with a diameter of 10 mm. A type 2032 button cell was assembled using a sodium metal sheet as the negative electrode and the aforementioned surface double-modified composite sodium iron pyrophosphate circular electrode as the positive electrode. The electrolyte was 1M NaPF6 in DME = 100 vol%, and the separator was a glass fiber separator. The half-cell was assembled in the order of positive electrode shell, positive electrode, separator, electrolyte, sodium sheet, gasket, spring, and negative electrode shell, and was tightly sealed with a fully automatic packaging machine. After the battery was left to stand for 24 h, electrochemical tests were performed using a Neware electrochemical workstation. The electrochemical tests were performed at 25°C and mainly included constant current charge-discharge tests and electrochemical impedance analysis. The long cycle performance of the battery was tested at a rate of 1 C within the potential window of 1.7-4.3 V for sodium iron pyrophosphate, and the rate performance of the battery was tested at rates of 1 C, 2 C, 5 C, 10 C, 20 C, 30 C, 40 C, and 50 C.

[0066] The results of the rate performance tests are shown in Table 2 below.

[0067]

[0068] As can be seen from the results of the rate performance tests of the examples and comparative examples in Table 2, the sodium iron pyrophosphate materials in Examples 1-16, which were optimized by the plasma technology described in the present application, all exhibited specific capacities of more than 90 mAh / g and first coulombic efficiencies of more than 90 % at a current density of 1 C, and Example 1 exhibited a high specific capacity of 122 mAh / g and a first coulombic efficiency of more than 99 % at a current density of 1 C. In addition, from the data in the table and in combination with the results of the long cycle performance tests, it can be seen that the cycle performance and rate performance of the materials were excellent. Figure 2 and Figure 3 It can be seen that the rate performance of the materials was greatly improved at 1 C, 30 C, 40 C, and 50 C, and the cycle performance and rate performance were excellent.

[0069] In Comparative Examples 1-11, in comparison with Comparative Example 1, the modification effect of Comparative Examples 2-11 was not obvious, the initial capacity of the sodium iron pyrophosphate positive electrode material was less than 80 mAh / g at a rate of 1 C, and the first coulombic efficiency was not more than 83 %, and the reversible specific capacity at rates of 30 C, 40 C, and 50 C was much lower than that of Examples 1-16, which indicates that the plasma technology of the present application is very effective and unpredictable for improving the rate performance and cycle performance of the modified sodium iron pyrophosphate positive electrode material.

[0070] Figure 1 The schematic diagram of the actual device for treating the sodium iron pyrophosphate powder sample by the plasma technology in Example 1; Figure 2 The TEM image and element distribution diagram of the fluorine and nitrogen co-doped sodium iron pyrophosphate composite material prepared in Example 1; Figure 3 、 Figure 4 The rate performance comparison diagram and the cycle stability comparison diagram of the button half-cell of the fluorine and nitrogen co-doped sodium iron pyrophosphate composite material prepared in Example 1 and the unmodified sodium iron pyrophosphate material of Comparative Example 1, respectively. Figure 2 It can be seen from the TEM and Mapping element distribution that the high-activity plasma fluorine radicals are successfully doped to form sodium fluoride in the phosphate polyanion phase; the fluorine and nitrogen element co-doping is to optimize the electrochemical behavior of the positive electrode / electrolyte interface, improve the interface transmission kinetics of the sodium ion battery and release the volume change stress. 2 The nitrogen is doped into the carbon network, and the nitrogen and carbon rearrangement form a close-packed network, which can greatly enhance the electronic conductivity of the carbon layer and the wettability of the electrolyte. At the same time, the ion transmission can be effectively strengthened, and a sodium ion battery positive electrode material with high capacity and long life is obtained.

[0071] The solid ammonium fluoride is used as an activation source in the plasma environment, and high-energy electrons and ions will bombard and decompose the solid ammonium fluoride to generate various high-activity fluorine-containing and nitrogen-containing radicals and ions. The active substances generated by the plasma first slightly etch the carbon layer or the body of the sodium iron pyrophosphate particles on the surface, creating defects and active sites. The high-activity fluorine radicals react with the carbon, oxygen and iron elements on the surface of the material to introduce fluorine elements in the form of lattice oxygen substitution. The nitrogen-containing radicals react with the carbon or defect sites on the surface of the material to introduce nitrogen elements into the carbon layer, rearrange the carbon to form a close-packed network and introduce C-N bonds. The two form a synergistic effect: the introduction of fluorine and nitrogen is not completely independent. The introduction of nitrogen produces vacancies or lattice distortion, which may provide a channel for further doping of fluorine, and vice versa, forming a synergistic doping effect. Thus, the first-cycle coulombic efficiency and long cycle stability are greatly improved. The synergistic optimization of the material bulk structure and the surface interface helps to promote the development of the interface optimization strategy of the sodium ion battery positive electrode material.

[0072] The above-described embodiments are only the preferred schemes of the present application, and do not limit the present application in any form, and other variants and modifications are also available without exceeding the technical solutions recited in the claims.

Claims

1. A method for preparing a solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium iron pyrophosphate composite cathode material, characterized in that, The method is based on sodium pyrophosphate iron phosphate as the matrix, ammonium fluoride as the solid source, and sodium pyrophosphate iron phosphate is modified by one-step plasma method to obtain sodium pyrophosphate iron phosphate composite positive electrode material; comprising the following steps: (1) sodium pyrophosphate iron phosphate and ammonium fluoride are ground and mixed to obtain a mixture, the mixture is placed in a plasma reaction device and connected with a plasma generator, and vacuum treatment is performed; the percentage content of ammonium fluoride in the mixture is 10-50%; (2) under heating conditions, adjust the radio frequency power and vacuum degree, and after ignition, reaction is carried out to obtain sodium pyrophosphate iron phosphate composite positive electrode material.

2. The preparation method of the solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium iron pyrophosphate composite cathode material according to claim 1, characterized in that, In step (1), the vacuum is drawn to a vacuum degree of 1-50 Pa.

3. The method for preparing a solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium iron pyrophosphate composite cathode material according to claim 1, characterized in that, In step (2), the heating temperature is 100-900℃.

4. The method for preparing a solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium iron pyrophosphate composite cathode material according to claim 1, characterized in that, In step (2), the reaction conditions are: radio frequency power is 100-800W, vacuum degree is 5-100Pa, reaction temperature is 100-900℃, and reaction time is 3-30min.

5. The method for preparing a solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium iron pyrophosphate composite cathode material according to claim 1, characterized in that, In step (2), the reaction conditions are: radio frequency power is 500W, vacuum degree is 30Pa, reaction temperature is 500℃, and reaction time is 10min.

6. The method for preparing a solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium iron pyrophosphate composite cathode material according to claim 1, characterized in that, The doping amount of fluorine and nitrogen elements in sodium pyrophosphate iron phosphate composite positive electrode material is 1-3%.

7. A solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium pyrophosphate iron phosphate composite positive electrode material prepared by the preparation method of any one of claims 1-6.

8. The application of the solid source plasma constructed high-capacity fluorine and nitrogen co-doped sodium pyrophosphate iron phosphate composite positive electrode material in the field of sodium ion batteries.

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