Gas-liquid mixture source plasma constructed high-capacity interface dual-modified sodium ferric pyrophosphate composite positive electrode material as well as preparation method and application of high-capacity interface dual-modified sodium ferric pyrophosphate composite positive electrode material

Abstract

The invention belongs to the technical field of sodium ion battery positive electrode materials, and relates to a gas-liquid mixture source plasma constructed high-capacity interface dual-modified sodium ferric pyrophosphate composite positive electrode material and a preparation method and application thereof. The preparation method comprises the following steps: taking sodium ferric phosphate pyrophosphate as a matrix, taking sulfur hexafluoride as a gas source, introducing a liquid source, and carrying out fluorine and sulfur element synergistic doping modification and surface carbon layer coating on sodium ferric phosphate pyrophosphate by utilizing a plasma technology to obtain the sodium ferric phosphate pyrophosphate composite positive electrode material. The composite positive electrode material shows excellent sodium storage cycle stability and rate capability, and has a wide market application prospect. Meanwhile, the preparation method provided by the invention is simple, rapid, efficient and convenient, is easy to control, and contributes to promoting the development and application of the high-performance sodium-ion battery positive electrode material.

Description

Technical Field

[0001] The present invention relates to the technical field of cathode materials for sodium-ion batteries, and particularly to a gas-liquid hybrid plasma constructed high-capacity interface dual-modified sodium iron pyrophosphate composite cathode material, its preparation method and application. Background Art

[0002] High specific energy and long-life secondary batteries are the core power sources and key equipment for modern electric transportation and distributed energy storage. Compared with the high cost and low reserves of lithium sources, considering factors such as resources and the environment, high specific energy and long-life sodium-ion batteries with resource cost advantages have become the new favorites in the international research on power and energy storage batteries. Among them, the cathode material is the key to achieving high performance of sodium-ion batteries, including layered structure oxides, polyanion compounds, and Prussian blue coordination polymers, etc. Compared with other cathode materials, polyanion-type compounds, especially sodium iron pyrophosphate (Na4Fe3(PO4)2(P2O7), NFPP), are considered to be potential next-generation high-performance sodium-ion battery cathode materials due to their excellent low cost, environmental friendliness, excellent cycle stability, stable crystal structure, etc. Sodium iron pyrophosphate has a typical NASICON-type (sodium superionic conductor) structure, in which the three-dimensional crystal framework and the strong inductive effect of the pyrophosphate (P2O7) group provide strong support for the fast Na + transport pathway and effectively reduce the sodium ion transport barrier. However, the low electronic conductivity and interfacial diffusion kinetics of sodium iron pyrophosphate seriously limit its sodium ion transport kinetics, resulting in insufficient long-term cycle stability and unsatisfactory rate performance. Therefore, it is urgent to develop an interface optimization strategy to strengthen its ion transport and obtain a sodium-ion battery cathode material with both high capacity and long life.

[0003] To address the above problems, existing research mainly uses interface modification strategies to optimize it, including interface coating modification and gradient interface modification. The interface coating strategy mainly uses conductive materials such as graphite and carbon nanotubes and inorganic materials such as oxides, phosphates, or organic carbon sources such as citric acid, glucose, sucrose, maltose, etc. as carbon sources, which can not only effectively inhibit the structural changes of sodium iron pyrophosphate during the electrochemical reaction process, but also improve the electrode structure stability and electronic conductivity, thereby effectively improving its cycle stability and rate performance. For example, Wuhan University synthesized a graphene (rGO) coated NFPP / rGO composite structure by a simple spray drying method, effectively improving the electronic conductivity of sodium iron pyrophosphate and the Na + diffusion kinetics. The above composite structure has a high reversible capacity of 128 mAh·g at 0.1 C -1And long cycle life. University of Shanghai for Science and Technology synthesized the NFPP / C composite structure through the sol-gel method. This composite material has excellent rate performance and excellent cycle stability, with the number of cycles exceeding 4,400 times. Chinese patents CN117810414A Coating modification preparation method of single-crystalline sodium iron pyrophosphate phosphate cathode material, CN116344823A A carbon-coated composite material and its preparation method and application, etc. mostly use sodium iron pyrophosphate phosphate precursor and carbon source to achieve carbon coating through sintering to improve the electrochemical performance of the cathode material. However, the synthesis temperature resistance of sodium iron pyrophosphate phosphate is insufficient, usually lower than 550 °C. Therefore, the traditional carbon coating method has a complex process and the quality of conventional low-temperature carbon coating is low and the performance is insufficient. In addition, although single-element doping (such as single fluorine doping or sulfur doping) can partially improve the performance, it is difficult to balance the requirements of high capacity and long cycle life at the same time.

[0004] Based on this, the present invention proposes a gas-liquid hybrid plasma to construct a high-capacity interface double-modified sodium iron pyrophosphate phosphate composite cathode material, its preparation method and application. The above strategies are effectively combined to double-modify the interface of sodium iron pyrophosphate phosphate, thereby synergistically strengthening the sodium ion transport kinetics at the cathode / electrolyte interface, which helps to promote the development and application of sodium ion battery cathode materials. Summary of the Invention

[0005] The present invention mainly aims at the problems of low electronic conductivity, low theoretical capacity, and insufficient cycle stability of sodium ion battery cathode materials, and provides a gas-liquid hybrid plasma to construct a high-capacity interface double-modified sodium iron pyrophosphate phosphate composite cathode material, its preparation method and application. By using plasma technology, sulfur hexafluoride gas with extremely strong reducibility is used as the gas source, and a liquid carbon source such as ethanol carbon source is used to plasma-excite a large number of highly active carbon radicals, so as to achieve double doping of fluorine and sulfur elements and surface carbon layer coating on sodium iron pyrophosphate phosphate at low temperature, thereby optimizing the surface and interface sodium ion transport kinetics of sodium iron pyrophosphate phosphate and improving the electrochemical performance of the material.

[0006] The technical solution adopted by the present invention to solve its technical problems is: A preparation method of a gas-liquid hybrid plasma to construct a high-capacity interface double-modified sodium iron pyrophosphate phosphate composite cathode material, the method uses sodium iron pyrophosphate phosphate as the matrix, a liquid carbon source as the liquid source, and sulfur hexafluoride as the gas source, and performs double doping of fluorine and sulfur elements and surface carbon layer coating on sodium iron pyrophosphate phosphate through a one-step plasma method to obtain a sodium iron pyrophosphate phosphate composite cathode material.

[0007] Preferably, the method includes the following steps: (1)Place sodium iron pyrophosphate phosphate Na4Fe3(PO4)2(P2O7) inside the plasma reaction device and connect it to the plasma generator. Evacuate the reaction area of the plasma reaction device. (2)Introduce sulfur hexafluoride gas as the gas source and bring the liquid source into the plasma reaction device in the form of vapor to a certain vacuum degree. Under heating conditions, control the initiation of the plasma of the gas source and the liquid source by adjusting the radio frequency power, and carry out a plasma reaction with the sodium iron pyrophosphate phosphate material. After reacting for a certain time, turn off the radio frequency power supply to obtain a sodium iron pyrophosphate phosphate material with dual doping of fluorine and sulfur elements and surface carbon layer coating.

[0008] The following are the preferred technical solutions of the present invention: Preferably, in step (1), the selected sample is at least one of sheet and powder. More preferably, the sample can be processed more uniformly by a rotating plasma device, and can react more fully with the introduced gas and liquid carbon source.

[0009] Preferably, in step (1), the vacuum is pumped to a vacuum degree of 1-50 Pa and is in a vacuum state.

[0010] Preferably, in step (2), the liquid carbon source includes at least one of ethanol, toluene, methanol, benzyl alcohol, acetone, and ethylene glycol, and more preferably is an ethanol carbon source.

[0011] Preferably, in step (2), the vacuum degree of the system after introducing the gas source and the liquid source is preferably 5-100 Pa.

[0012] Preferably, in step (2), the temperature of the system when introducing the gas source is 100-900 °C. More preferably, the flow rate of the gas source is 10-50 sccm.

[0013] Preferably, in step (2), the reaction conditions are: the power of the radio frequency power supply is 100-800 W, the reaction temperature is 100-900 °C, the reaction time is 3-30 min, and more preferably, the vacuum degree is 30 Pa.

[0014] Preferably, in step (2), the thickness of the surface carbon layer coating is preferably 1-5 nm, and the fluorine and sulfur elements are co-doped and modified; more preferably, the doping amounts of fluorine and sulfur are both 1-3%.

[0015] The present invention also provides a gas-liquid hybrid source plasma constructed high-capacity interface dual-modified sodium iron pyrophosphate phosphate composite cathode material prepared by any of the above preparation methods.

[0016] Preferably, the carbon layer thickness of the sodium iron pyrophosphate phosphate composite cathode material is 1-5 nm, and the fluorine and sulfur elements are co-doped and modified.

[0017] The present invention also provides an application of the gas-liquid hybrid source plasma-constructed high-capacity interface dual-modified sodium pyrophosphate iron phosphate composite cathode material prepared by any of the above preparation methods in the field of sodium-ion battery cathode materials.

[0018] In the present invention, the gas source SF6 and liquid sources such as ethanol carbon source are subjected to plasma excitation by the above plasma technology to form a large number of highly active free radicals, and sulfur, fluorine doping and surface carbon layer coating of sodium pyrophosphate iron phosphate are realized under low-temperature heating conditions, thereby optimizing the surface and interface sodium-ion transport kinetics of sodium pyrophosphate iron phosphate. Plasma-derived carbon free radicals have strong reducibility, which will change the local coordination environment and atomic valence state of Fe in the original structure of sodium pyrophosphate iron phosphate, improve the stability of the Fe-O octahedron, and promote Na + diffusion kinetics. And the synergistic doping of fluorine and sulfur will have a great influence on the redox process of Fe 2+ / Fe 3+ and the sodium-ion deintercalation behavior, thereby endowing the modified sodium pyrophosphate iron phosphate with excellent cycle stability and rate performance. The thickness of the surface carbon layer prepared by the present invention can be controlled by parameters such as action time, radio frequency power, and heating temperature. In addition, this method is generally applicable to powder samples and other metal battery fields, such as Li, K, Zn, etc. can be widely used. The present invention analyzes the influence of the carbon radical-derived carbon coating layer and the synergistic doping of fluorine and sulfur on the sodium-ion deintercalation behavior, and clarifies its in-situ sodium storage mechanism. Establish the structure-activity relationship from the local electronic structure to the microscopic crystal structure and then to the macroscopic electrochemical performance and mechanism, establish a reasonable carbon coating and doping process model, and prepare a sodium pyrophosphate iron phosphate composite cathode material with both high capacity and long life.

[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) Through the "gas-liquid-plasma" ternary synergistic effect, a sodium pyrophosphate iron phosphate material with synergistic doping of fluorine and sulfur elements and carbon layer coating is prepared by a one-step method. The preparation method is simple, fast and efficient, and the conditions are mild, without the high-temperature conditions of conventional carbon coating; (2) The synergistic doping of fluorine and sulfur can synchronously optimize the bulk structure and surface interface characteristics of the material, and solve the limitations of single doping. It can stabilize the redox process of Fe 2+ / Fe 3+ , accelerate the interface transport kinetics of sodium-ion batteries and slow down its volume change stress, and effectively improve the material performance; (3) Through the synergistic effect of temperature and plasma, the chemical composition, crystal structure and interface characteristics of the carbon layer are accurately regulated, the controllability of the material performance is realized, and the application of the material is further expanded; The present invention is prepared by a one-step reaction of a gas source bringing a liquid carbon source into plasma excitation and sodium iron pyrophosphate phosphate material, realizing the co-doping of highly active plasma carbon free radicals and fluorine and sulfur elements to modify the surface and interface of polyanionic phosphates. The co-doping of fluorine and sulfur elements and the carbon layer coating are relatively effective ways to optimize the cathode / electrolyte interface, which can stabilize the 2+ Fe / Fe 3+ redox process, accelerate the interfacial transport kinetics of sodium-ion batteries and relieve their volume change stress. At the same time, it can effectively strengthen ion transport, obtain a sodium-ion battery cathode material with both high capacity and long life, and the dual surface and interface modification helps to promote the development of the interface optimization strategy of sodium-ion battery cathode materials. Description of the Drawings

[0020] Figure 1 It is a reaction device diagram of the plasma technology of bringing ethanol liquid source by sulfur hexafluoride gas source in Example 1; Figure 2 It is a TEM image and element distribution diagram of sodium iron pyrophosphate phosphate with co-doping of fluorine and sulfur elements and surface carbon coating in Example 1; Figure 3 It is a rate performance diagram of the button half-cell of sodium iron pyrophosphate phosphate with co-doping of fluorine and sulfur elements and surface carbon coating prepared in Example 1; Figure 4 It is a cyclic stability performance diagram of the button half-cell of sodium iron pyrophosphate phosphate with co-doping of fluorine and sulfur elements and surface carbon coating prepared in Example 1. Detailed Embodiments

[0021] For the convenience of understanding, the technical solutions and implementation manners of the present invention will be further clearly, completely and detailedly described below through specific examples and in combination with the drawings. It should be noted that the described embodiments are implemented on the premise of the technical solutions of the present invention, and the detailed implementation manners and specific operation processes are given, but they are only a part of the embodiments of the present invention, rather than all the embodiments. The described specific implementation manners are only for explaining and interpreting the present invention, and do not limit the present invention. Based on the embodiments of the present invention, all other implementation manners obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0022] 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 are, unless otherwise specified, conventional substances or devices known to those skilled in the art and can be obtained commercially. The reaction conditions reflected in the content of the present invention can all achieve the said reaction and obtain the products with the expected effects. Due to space limitations, only some examples are listed below to further illustrate the advantages of the technical solutions of the present invention.

[0023] Example 1

[0024] Place the powder sample of sodium iron pyrophosphate phosphate (which can refer to the method of Comparative Example 1) in the reaction zone of the rotating plasma reaction device. Connect copper rings at both ends of the plasma reaction device, and connect the copper rings to the generator of the radio frequency power supply (plasma generator) with wires. Subsequently, evacuate the plasma device to 10 Pa and maintain the evacuated state, heat the temperature to 500 °C, introduce sulfur hexafluoride gas, and then the gas will bring ethanol vapor into the plasma reaction device, and the vacuum degree rises to 50 Pa. Turn on the radio frequency power supply switch, adjust the radio frequency power, and adjust the radio frequency power to 500 W. Adjust the vacuum pump to maintain the vacuum degree at 30 Pa. After ignition, the sulfur hexafluoride gas mixed with ethanol vapor excites a large number of highly active carbon free radicals in the plasma reaction zone, and the synergistic doping of fluorine and sulfur elements reacts with the surface of sodium iron pyrophosphate phosphate simultaneously. After the plasma reaction for 10 min, turn off the radio frequency power supply to obtain a composite cathode material of sodium iron pyrophosphate phosphate with synergistic doping of fluorine and sulfur elements and surface carbon coating.

[0025] Examples 2 - 26

[0026] On the basis of Example 1, change the reaction conditions including liquid source, reaction temperature, reaction power, reaction time, etc. The specific conditions are shown in Table 1 below: Table 1 Summary of reaction conditions for each example

[0027] Comparative Example 1 Add CH3COONa·3H2O, MgSO4·7H2O, FeSO4·7H2O, NH4H2PO4, and C6H8O7·H2O to deionized water in proportion. After stirring for 10 min, dry at 160 °C to obtain a precursor. Then, grind the precursor powder and pre-calcine it in vacuum at 300 °C for 3 h, with a heating rate of 5 °C / min -1 . Press the calcined powder into a mold under a pressure of 15 MPa and calcine it in vacuum at 550 °C for 10 h, with a heating rate of 5 °C / min -1 . Finally, crush the block to obtain sodium iron pyrophosphate phosphate powder.

[0028] Comparative Example 2

[0029] Place the sodium iron pyrophosphate material in Comparative Example 1 into a sealed supercritical high-pressure reactor, evacuate it to a vacuum state, and introduce carbon dioxide gas into the supercritical device at a gas rate of 30 sccm until the air pressure reaches 8.0 MPa. Subsequently, transfer the supercritical device after introducing carbon dioxide to an oven at 40 °C and let it stand still. After heat preservation for 8 h, naturally cool it, and quickly open the gas valve to obtain the modified sodium iron pyrophosphate material, but carbon coating was not successfully achieved.

[0030] Comparative Example 3

[0031] Place the sodium iron pyrophosphate material in Comparative Example 1 into a sealed supercritical high-pressure reactor, evacuate it to a vacuum state, and introduce carbon dioxide gas into the supercritical device at a gas rate of 40 sccm until the air pressure reaches 7.0 MPa. Subsequently, transfer the supercritical device after introducing carbon dioxide to an oven at 40 °C and let it stand still. After heat preservation for 8 h, naturally cool it, and quickly open the gas valve to obtain the modified sodium iron pyrophosphate material, but carbon coating was not successfully achieved.

[0032] Comparative Example 4

[0033] Place the sodium iron pyrophosphate material in Comparative Example 1 into a sealed supercritical high-pressure reactor, evacuate it to a vacuum state, and introduce sulfur hexafluoride gas into the supercritical device at a gas rate of 30 sccm until the air pressure reaches 8.0 MPa. Subsequently, transfer the supercritical device after introducing carbon dioxide to an oven at 40 °C and let it stand still. After heat preservation for 8 h, naturally cool it, and quickly open the gas valve to obtain the modified sodium iron pyrophosphate material, but carbon coating was not successfully achieved.

[0034] Comparative Example 5

[0035] Place the sodium iron pyrophosphate material in Comparative Example 1 into a sealed supercritical high-pressure reactor, evacuate it to a vacuum state, and introduce sulfur hexafluoride gas into the supercritical device at a gas rate of 40 sccm until the air pressure reaches 7.0 MPa. Subsequently, transfer the supercritical device after introducing carbon dioxide to an oven at 40 °C and let it stand still. After heat preservation for 8 h, naturally cool it, and quickly open the gas valve to obtain the modified sodium iron pyrophosphate material, but doping was not successfully achieved.

[0036] Comparative Example 6

[0037] Place the sodium iron pyrophosphate material in Comparative Example 1 into an ordinary tubular furnace, introduce argon protective gas, and introduce methane gas at a gas rate of 30 sccm. Subsequently, -1 heat it at a heating rate of 5 °C·min to 500 °C, keep it warm for 2 h and then naturally cool it to obtain the modified sodium iron pyrophosphate material, but carbon coating was not successfully achieved.

[0038] Comparative Example 7

[0039] Place the sodium iron pyrophosphate material in Comparative Example 1 into an ordinary tubular furnace, introduce argon as the protective gas, and introduce sulfur hexafluoride gas at a gas rate of 30 sccm. Subsequently, heat it at a heating rate of 5 °C·min -1 to 500 °C, keep it at this temperature for 2 h, and then cool it naturally to obtain the modified sodium iron pyrophosphate material, but doping was not successfully achieved.

[0040] Comparative Example 8

[0041] Place the sodium iron pyrophosphate material in Comparative Example 1 into a hydrothermal autoclave, add ethanol liquid to two-thirds of the autoclave, place it in a hydrothermal oven at 180 °C and keep it for 10 h. After natural cooling, transfer it to an ordinary tubular furnace, introduce argon as the protective gas, and then heat it at a heating rate of 5 °C·min -1 to 500 °C, keep it at this temperature for 2 h, and then cool it naturally to obtain the modified sodium iron pyrophosphate material, but carbon coating was not successfully achieved.

[0042] Comparative Example 9

[0043] Place the sodium iron pyrophosphate material in Comparative Example 1 into a hydrothermal autoclave, add methanol liquid to two-thirds of the autoclave, place it in a hydrothermal oven at 180 °C and keep it for 10 h. After natural cooling, transfer it to an ordinary tubular furnace, introduce argon as the protective gas, and then heat it at a heating rate of 5 °C·min -1 to 500 °C, keep it at this temperature for 2 h, and then cool it naturally to obtain the modified sodium iron pyrophosphate material, but carbon coating was not successfully achieved.

[0044] Comparative Example 10

[0045] Place the sodium iron pyrophosphate material in Comparative Example 1 into a hydrothermal autoclave, add toluene liquid to two-thirds of the autoclave, place it in a hydrothermal oven at 180 °C and keep it for 10 h. After natural cooling, transfer it to an ordinary tubular furnace, introduce argon as the protective gas, and then heat it at a heating rate of 5 °C·min -1 to 500 °C, keep it at this temperature for 2 h, and then cool it naturally to obtain the modified sodium iron pyrophosphate material, but carbon coating was not successfully achieved.

[0046] Comparative Example 11

[0047] Place the sodium iron pyrophosphate material in Comparative Example 1 into a hydrothermal autoclave, add ethylene glycol liquid to two-thirds of the autoclave, place it in a hydrothermal oven at 180 °C and keep it for 10 h. After natural cooling, transfer it to an ordinary tubular furnace, introduce argon as the protective gas, and then heat it at a heating rate of 5 °C·min -1The heating rate was increased to 500 °C, and after holding for 2 h, it was naturally cooled to obtain the sodium iron pyrophosphate phosphate modified material, but carbon coating was not successfully achieved.

[0048] Comparative Example 12

[0049] The sodium iron pyrophosphate 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 the copper rings were connected to the generator of the radio frequency power supply (plasma generator) with wires. Subsequently, the plasma device was evacuated to 10 Pa and maintained in the evacuated state, and the temperature was heated to 500 °C. Sulfur hexafluoride gas was introduced, the radio frequency power supply switch was turned on, the radio frequency power was adjusted to 500 W, and the vacuum pump was adjusted to maintain the vacuum degree at 30 Pa. After treatment for 10 min, it was naturally cooled to obtain the sodium iron pyrophosphate phosphate modified material, but carbon coating was not successfully achieved.

[0050] Comparative Example 13

[0051] The sodium iron pyrophosphate 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 the copper rings were connected to the generator of the radio frequency power supply (plasma generator) with wires. Subsequently, the plasma device was evacuated to 10 Pa and maintained in the evacuated state, and the temperature was heated to 500 °C. Carbon dioxide gas was introduced, the radio frequency power supply switch was turned on, the radio frequency power was adjusted to 500 W, and the vacuum pump was adjusted to maintain the vacuum degree at 30 Pa. After treatment for 10 min, it was naturally cooled to obtain the sodium iron pyrophosphate phosphate modified material, but carbon coating was not successfully achieved.

[0052] Comparative Example 14

[0053] The sodium iron pyrophosphate 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 the copper rings were connected to the generator of the radio frequency power supply (plasma generator) with wires. Subsequently, the plasma device was evacuated to 10 Pa and maintained in the evacuated state, and the temperature was heated to 500 °C. Methanol was introduced as a liquid source, the radio frequency power supply switch was turned on, the radio frequency power was adjusted to 500 W, and the vacuum pump was adjusted to maintain the vacuum degree at 30 Pa. After treatment for 10 min, it was naturally cooled to obtain the sodium iron pyrophosphate phosphate modified material, but carbon coating was not successfully achieved.

[0054] Comparative Example 15

[0055] Place the sodium iron pyrophosphate phosphate material in Comparative Example 1 in the reaction zone of a rotating plasma reaction device. Connect copper rings at both ends of the plasma reaction device, and use wires to connect the copper rings to the generator of the radio frequency power supply (plasma generator). Subsequently, evacuate the plasma device to 10 Pa and maintain the evacuated state, and heat the temperature to 500 °C. Introduce sulfur hexafluoride gas, and then the gas will bring isopropanol vapor into the plasma reaction device. Turn on the radio frequency power supply switch, adjust the radio frequency power, and adjust the radio frequency power to 500 W. Adjust the vacuum pump to maintain the vacuum degree at 30 Pa. After treatment for 10 min, cool naturally to obtain the plasma-modified sodium iron pyrophosphate phosphate material.

[0056] Comparative Example 16

[0057] Place the sodium iron pyrophosphate phosphate material in Comparative Example 1 in the reaction zone of a rotating plasma reaction device. Connect copper rings at both ends of the plasma reaction device, and use wires to connect the copper rings to the generator of the radio frequency power supply (plasma generator). Subsequently, evacuate the plasma device to 10 Pa and maintain the evacuated state, and heat the temperature to 500 °C. Introduce carbon dioxide gas, and then the gas will bring ethanol vapor into the plasma reaction device. Turn on the radio frequency power supply switch, adjust the radio frequency power, and adjust the radio frequency power to 500 W. Adjust the vacuum pump to maintain the vacuum degree at 30 Pa. After treatment for 10 min, cool naturally to obtain the plasma-modified sodium iron pyrophosphate phosphate material.

[0058] Comparative Example 17

[0059] Place the sodium iron pyrophosphate phosphate material in Comparative Example 1 in the reaction zone of a rotating plasma reaction device. Connect copper rings at both ends of the plasma reaction device, and use wires to connect the copper rings to the generator of the radio frequency power supply (plasma generator). Subsequently, evacuate the plasma device to 10 Pa and maintain the evacuated state. Introduce sulfur hexafluoride gas at room temperature, and then the gas will bring ethanol vapor into the plasma reaction device. Turn on the radio frequency power supply switch, adjust the radio frequency power, and adjust the radio frequency power to 500 W. Adjust the vacuum pump to maintain the vacuum degree at 30 Pa. After treatment for 10 min, cool naturally to obtain the plasma-modified sodium iron pyrophosphate phosphate material, but carbon coating was not successfully achieved.

[0060] Performance test

[0061] The sodium iron pyrophosphate samples prepared in Examples 1-26 and Comparative Examples 1-17 were homogenized and coated with Super P and PVDF in a mass ratio of 7:2:1 to form a positive electrode sheet. The coated positive electrode sheet was dried in a vacuum drying oven at 120 °C for 12 h to ensure complete removal of the solvent in the positive electrode sheet, and then cut into circular positive electrode sheets with a diameter of 10 mm. Using a sodium metal sheet as the negative electrode and the above-prepared circular positive electrode sheet of the surface-dually modified composite sodium iron pyrophosphate as the positive electrode, a button cell of model 2032 was assembled. The electrolyte was 1M NaPF6 in DME = 100 vol%, and the separator was a glass fiber separator. A half-cell was assembled in the order of the positive electrode case, the positive electrode sheet, the separator, the electrolyte, the sodium sheet, the gasket, the spring piece, and the negative electrode case, and tightly sealed with a fully automatic encapsulation machine. After the battery was left standing for 24 hours, electrochemical tests were carried out using a Neware and an electrochemical workstation. The electrochemical tests were all carried out under the condition of 25 o °C, mainly including constant current charge and discharge tests and electrochemical impedance analysis. The potential window of the sodium iron pyrophosphate in phosphoric acid was 1.7-4.3 V, and the long-term cycling performance of the battery was tested at a rate of 1 C; the rate performance of the battery was tested at rates of 0.5 C, 1 C, 2 C, 5 C, 10 C, 15 C, and 0.5 C.

[0062] The test results of the rate performance and cycling performance are shown in Table 2 below: Table 2 Summary of the rate performance test results of each example and comparative example

[0063] It can be found from the rate performance test results of each example and comparative example in Table 2 that the sodium iron pyrophosphate materials optimized by the plasma technology described in the present invention in Examples 1-26 all showed a high specific capacity of more than 90 mAh / g and a first Coulombic efficiency of more than 90% at a current density of 1 C; and when the liquid carbon source was ethanol, it showed more excellent performance compared with other carbon sources of the present invention. In addition, the rate performance of the material was greatly improved at 0.5 C, 1 C, 2 C, 5 C, 10 C, 15 C, and 0.5 C, and the capacity retention rate was 98% after 100 cycles at 1 C, indicating excellent cycling performance and rate performance. On the contrary, in Comparative Examples 1-17, compared with Comparative Example 1, the modification effects of Comparative Examples 2-17 were not obvious. For the sodium iron pyrophosphate positive electrode material at a rate current density of 1 C, the initial capacity of the material was low, all lower than 80 mAh / g, and the first Coulombic efficiency did not exceed 80%. The reversible specific capacity at rates of 1 C and 10 C was much lower than that of Examples 1-26, indicating that the plasma technology of the present invention is very effective and unexpected for improving the rate performance and cycling performance of the modified sodium iron pyrophosphate positive electrode material.

[0064] Figure 1 Schematic diagram of the treatment of sodium iron pyrophosphate phosphate powder sample by plasma technology in Example 1; Figure 2 TEM test diagram and Mapping element distribution diagram of sodium iron pyrophosphate phosphate co-doped with fluorine and sulfur elements and surface carbon-coated in Example 1; Figure 3 , Figure 4 Rate performance diagram and cycle stability diagram of the button half-cell of sodium iron pyrophosphate phosphate co-doped with fluorine and sulfur elements and surface carbon-coated prepared in Example 1. From Figure 2 The TEM and Mapping element distribution of

[0065] It can be seen that highly active plasma carbon radicals construct a 4-nm-thick carbon coating layer on the surface and interface of phosphate polyanions; the co-doping of fluorine and sulfur elements optimizes the electrochemical behavior of the cathode / electrolyte interface, improves the interfacial transport kinetics of sodium-ion batteries and releases its volume change stress. Plasma-derived carbon radicals have strong reducibility and will change the local coordination environment and atomic valence state of Fe in the original structure of sodium iron pyrophosphate phosphate, thereby improving its cycle performance and rate performance. 2+ / Fe 3+ The redox process of + is stabilized, the interfacial transport kinetics of sodium-ion batteries is accelerated and its volume change stress is released. Plasma-derived radicals have strong reducibility and will change the local coordination environment and atomic valence state of Fe in the original structure of sodium iron pyrophosphate phosphate, enhance the stability of the Fe-O hexahedron, and promote the

[0066] The above-described embodiments are only preferred solutions of the present invention and do not impose any form of limitation on the present invention. There are other variations and modifications without exceeding the technical solutions described in the claims.

Claims

1. A preparation method of a sodium iron pyrophosphate composite cathode material with a high-capacity interface double modification constructed by a gas-liquid mixture source plasma, characterized in that, The method described above uses sodium iron pyrophosphate as the matrix, a liquid carbon source as the liquid source, and sulfur hexafluoride as the gas source. The sodium iron pyrophosphate is modified by a one-step plasma method to obtain a sodium iron pyrophosphate composite cathode material.

2. The preparation method of a sodium iron pyrophosphate composite cathode material with a high-capacity interface dual modification constructed by a gas-liquid mixture source plasma according to claim 1, characterized in that, It includes the following steps: (1) Place sodium iron pyrophosphate inside a plasma reaction device and connect it to a plasma generator, then perform a vacuum pumping process. (2) Introduce the gas source and bring the liquid source into the plasma reaction device to a certain vacuum degree. Under heating conditions, adjust the radio frequency power. After ignition, react for a certain period of time to obtain a sodium iron pyrophosphate composite cathode material.

3. The preparation method of a sodium iron pyrophosphate composite cathode material with a high-capacity interface dual modification constructed by a gas-liquid mixture source plasma according to claim 2, characterized in that, In step (1), pump the vacuum to a vacuum degree of 1 - 50 Pa.

4. The preparation method of a sodium iron pyrophosphate composite cathode material with a high-capacity interface double modification constructed by a gas-liquid hybrid source plasma according to claim 2, characterized in that, In step (2), the liquid source includes at least one of ethanol, toluene, methanol, benzyl alcohol, acetone, and ethylene glycol.

5. The preparation method of a sodium iron pyrophosphate composite cathode material with a high-capacity interface double modification constructed by a gas-liquid mixture source plasma according to claim 4, characterized in that, In step (2), the liquid source is ethanol.

6. The preparation method of a sodium iron pyrophosphate composite cathode material with a high-capacity interface dual modification constructed by a gas-liquid hybrid source plasma according to claim 2, characterized in that, In step (2), after introducing the gas source and the liquid source, the vacuum degree of the system is 5 - 100 Pa; and / or, the temperature when introducing the gas source is 100 - 900 °C; and / or, the flow rate of the gas source is 10 - 50 sccm.

7. The preparation method of a sodium iron pyrophosphate composite cathode material with a high-capacity interface dual modification constructed by a gas-liquid mixture source plasma according to claim 2, characterized in that, In step (2), the power of the radio frequency power supply is 100 - 800 W, the reaction temperature is 100 - 900 °C, and the reaction time is 3 - 30 min.

8. The preparation method of a sodium iron pyrophosphate phosphate composite cathode material with a high-capacity interface double modification constructed by a gas-liquid mixed body source plasma according to claim 2, wherein, The thickness of the surface carbon coating layer of the sodium iron pyrophosphate composite cathode material is 1 - 5 nm, and / or, the doping amounts of fluorine and sulfur elements are 1 - 3% respectively.

9. A gas-liquid hybrid source plasma constructed high-capacity interface dual-modified sodium iron pyrophosphate composite cathode material prepared by the preparation method according to any one of claims 1 - 8.

10. An application of the gas-liquid hybrid source plasma constructed high-capacity interface dual-modified sodium iron pyrophosphate composite cathode material according to claim 9 in the field of cathode materials for sodium ion batteries.

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