Gas-liquid mixture source plasma construction of high-capacity interface dual-modified sodium iron pyrophosphate composite cathode material and its preparation method and application

Abstract

The present 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 its preparation method and application. The preparation method is to use sodium ferric pyrophosphate as a matrix, sulfur hexafluoride as a gas source, introduce a liquid source, and use plasma technology to perform fluorine and sulfur element coordinated doping modification on sodium ferric pyrophosphate and coat the surface with a carbon layer to obtain a sodium ferric pyrophosphate composite positive electrode material. The composite positive electrode material of the present invention exhibits excellent sodium storage cycle stability and rate performance, and has broad market application prospects. At the same time, the preparation method provided by the present invention is simple, fast, efficient, convenient, and easy to control, which helps to promote the development and application of high-performance sodium ion battery positive electrode materials.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion battery positive electrode materials, and in particular to a gas-liquid mixture source plasma-constructed high-capacity interface dual-modified sodium iron pyrophosphate composite positive electrode material, as well as a preparation method and application thereof. Background Art

[0002] High-energy-density, long-life secondary batteries are the core power source 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 environment, high-energy-density, long-life sodium-ion batteries with resource cost advantages have become the new favorite in international power and energy storage battery research. Among them, the positive electrode material is the key to achieving high performance of sodium-ion batteries, including layered oxides, polyanion compounds, and Prussian blue coordination polymers. Compared with other positive electrode materials, polyanion compounds, especially sodium iron pyrophosphate (Na4Fe3(PO4)2(P2O7), NFPP), are considered to be the next generation of high-performance sodium-ion battery positive electrode materials with great potential due to their outstanding low cost, environmental friendliness, excellent cycle stability, and stable crystal structure. Sodium iron pyrophosphate has a typical NASICON-type (sodium superion conductor) structure, in which the strong inductive effect of the three-dimensional crystal framework and pyrophosphate (P2O7) groups is the rapid Na + The transport pathway provides strong support, effectively reducing the sodium ion transport barrier. However, the low electronic conductivity and interfacial diffusion kinetics of sodium iron pyrophosphate severely limit its sodium ion transport kinetics, resulting in insufficient long-term cycling stability and unsatisfactory rate performance. Therefore, it is urgent to develop interface optimization strategies to enhance its ion transport and obtain sodium ion battery cathode materials with both high capacity and long life.

[0003] In response to the above problems, existing research mainly uses interface modification strategies to optimize them, including interface coating modification and gradient interface modification. The interface coating strategy mainly uses conductive materials graphite and carbon nanotubes and inorganic materials 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, but also improve the electrode structure stability and electronic conductivity, thereby effectively improving its cycle stability and rate performance. For example, Wuhan University used a simple spray drying method to synthesize a graphene (rGO)-coated NFPP / rGO composite structure, which effectively improved the electronic conductivity and Na + Diffusion kinetics. The composite structure has a high reversible capacity of 128 mAh g at 0.1 C. -1and long cycle life. The University of Shanghai for Science and Technology synthesized the NFPP / C composite structure through the sol-gel method. The composite material has excellent rate performance and excellent cycle stability, with more than 4,400 cycles. Chinese patents CN117810414A "Method for coating and modifying single-crystal sodium iron pyrophosphate positive electrode material" and CN116344823A "A carbon-coated composite material and its preparation method and application" often use a sodium iron pyrophosphate precursor and a carbon source to achieve carbon coating through sintering to improve the electrochemical performance of the positive electrode material. However, the temperature resistance of sodium iron pyrophosphate synthesis is insufficient, usually below 550°C. Therefore, the traditional carbon coating process is relatively complicated and the conventional low-temperature carbon coating is of low quality and insufficient performance. In addition, although single element doping (such as single fluorine doping or sulfur doping) can partially improve 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 mixture source plasma to construct a high-capacity interface dual-modified sodium iron pyrophosphate composite positive electrode material and its preparation method and application. The above strategies are effectively combined to perform dual modification of the sodium iron pyrophosphate interface, thereby synergistically enhancing the sodium ion transfer kinetics at the positive electrode / electrolyte interface, which helps to promote the development and application of sodium ion battery positive electrode materials. Summary of the Invention

[0005] The present invention mainly addresses the problems of low electronic conductivity, low theoretical capacity, and insufficient cycle stability of sodium ion battery positive electrode materials, and provides a high-capacity interface dual-modified sodium ferric pyrophosphate composite positive electrode material constructed by gas-liquid mixture source plasma, as well as its preparation method and application. By adopting plasma technology, using highly reducing sulfur hexafluoride gas as the gas source, and using a liquid carbon source such as ethanol carbon source plasma to excite a large number of highly active carbon free radicals, the sodium ferric pyrophosphate is dual-doped with fluorine and sulfur elements and coated with a surface carbon layer at low temperature, thereby optimizing the surface interface sodium ion transmission kinetics of the sodium ferric pyrophosphate and improving the electrochemical performance of the material.

[0006] The technical solution adopted by the present invention to solve its technical problem is:

[0007] A preparation method for a high-capacity interface dual-modified sodium ferric pyrophosphate composite cathode material constructed by gas-liquid mixture source plasma. The method uses sodium ferric pyrophosphate as a matrix, a liquid carbon source as a liquid source, and sulfur hexafluoride as a gas source. The sodium ferric pyrophosphate is dual-doped with fluorine and sulfur elements and coated with a carbon layer on the surface through a one-step plasma method to obtain the sodium ferric pyrophosphate composite cathode material.

[0008] Preferably, the method comprises the following steps:

[0009] (1) placing sodium iron pyrophosphate Na4Fe3(PO4)2(P2O7) inside a plasma reaction device, connecting it to a plasma generator, and vacuuming the reaction area of the plasma reaction device;

[0010] (2) Sulfur hexafluoride gas as a gas source is introduced, and the liquid source is brought into the plasma reaction device in the form of vapor to a certain vacuum degree. Under heating conditions, the ignition of the gas source and liquid source plasma is controlled by adjusting the radio frequency power, and the plasma is reacted with the sodium iron pyrophosphate material. After a certain reaction time, the radio frequency power is turned off to obtain a sodium iron pyrophosphate material doped with fluorine and sulfur elements and coated with a surface carbon layer.

[0011] The following are the preferred technical solutions of the present invention:

[0012] Preferably, in step (1), the selected sample is in at least one of a flake and a powder form. 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.

[0013] Preferably, in step (1), the vacuum is evacuated to a vacuum degree of 1-50 Pa, which is a vacuum state.

[0014] Preferably, in step (2), the liquid carbon source includes at least one of ethanol, toluene, methanol, benzyl alcohol, acetone, and ethylene glycol, and is more preferably an ethanol carbon source.

[0015] Preferably, in step (2), the vacuum degree of the system after the gas source and the liquid source are introduced is preferably 5-100 Pa.

[0016] Preferably, in step (2), the system temperature is 100-900° C. when the gas source is introduced, and more preferably, the flow rate of the gas source is 10-50 sccm.

[0017] Preferably, in step (2), the reaction conditions are: RF power of 100-800 W, reaction temperature of 100-900 °C, reaction time of 3-30 min, and more preferably, vacuum degree of 30 Pa.

[0018] Preferably, in step (2), the thickness of the surface carbon coating is preferably 1-5 nm, and fluorine and sulfur elements are synergistically doped and modified; more preferably, the doping amounts of fluorine and sulfur are both 1-3%.

[0019] The present invention also provides a gas-liquid mixture source plasma-constructed high-capacity interface dual-modified sodium iron pyrophosphate composite positive electrode material prepared by any of the above preparation methods.

[0020] Preferably, the carbon layer thickness of the sodium iron pyrophosphate composite positive electrode material is 1-5 nm, and the fluorine and sulfur elements are synergistically doped and modified.

[0021] The present invention also provides an application of a gas-liquid mixture source plasma-constructed high-capacity interface dual-modified sodium iron pyrophosphate composite positive electrode material prepared by any of the above preparation methods in the field of sodium ion battery positive electrode materials.

[0022] The present invention uses the above-mentioned plasma technology to plasma excite the gas source SF6 and the liquid source such as ethanol carbon source to form a large number of highly active free radicals, and realizes the sulfur and fluorine doping and surface carbon layer coating of sodium ferric pyrophosphate under low temperature heating conditions, thereby optimizing the surface interface sodium ion transport kinetics of sodium ferric pyrophosphate. Plasma-derived carbon free radicals have strong reducing properties, which will change the local coordination environment and atomic valence state of Fe in the original structure of sodium ferric pyrophosphate, improve the stability of Fe-O octahedron, and promote Na + Diffusion dynamics. And the synergistic doping of fluorine and sulfur will affect Fe 2+ / Fe 3+ The redox process and the deintercalation behavior of sodium ions have a great influence, thereby giving the modified sodium iron pyrophosphate 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, the 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 effects of carbon radical-derived carbon coating layers and the synergistic doping of fluorine and sulfur on the deintercalation behavior of sodium ions, and clarifies its in situ sodium storage mechanism. Establish a structure-activity relationship from local electronic structure to microscopic crystal structure to macroscopic electrochemical performance and mechanism, establish a reasonable carbon coating and doping process model, and prepare a sodium iron pyrophosphate composite positive electrode material with both high capacity and long life.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) Through the ternary synergistic effect of "gas-liquid-plasma", a one-step preparation of sodium iron pyrophosphate material doped with fluorine and sulfur elements and coated with a carbon layer is achieved. The preparation method is simple, fast, efficient, and mild, without the need for the high temperature conditions of conventional carbon coating;

[0025] (2) The coordinated doping of fluorine and sulfur can simultaneously optimize the bulk structure and surface interface properties of the material, solving the limitations of single doping. It can stabilize Fe 2+ / Fe 3+ The redox process accelerates the interface transport dynamics of sodium-ion batteries and relieves their volume change stress, effectively improving material performance;

[0026] (3) Through the synergistic effect of temperature and plasma, the chemical composition, crystal structure and interface characteristics of the carbon layer can be precisely controlled to achieve the controllability of material properties and further expand the application of materials;

[0027] The present invention is prepared by a one-step reaction of a gas source into a liquid carbon source plasma excitation and sodium iron pyrophosphate material, realizing the surface modification of phosphate polyanion by the synergistic doping of high-activity plasma carbon radicals and fluorine and sulfur elements. The synergistic doping of fluorine and sulfur elements and the carbon layer coating are more effective ways to optimize the positive electrode / electrolyte interface, which can stabilize Fe 2+ / Fe 3+ The redox process accelerates the interfacial transport dynamics of sodium-ion batteries and relieves their volume change stress. At the same time, it can effectively enhance ion transport, obtaining sodium-ion battery cathode materials with both high capacity and long life. The dual surface and interface modification helps to advance the development of interface optimization strategies for sodium-ion battery cathode materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a diagram of the reaction device for the plasma technology of introducing ethanol liquid source into the gas source sulfur hexafluoride in Example 1;

[0029] Figure 2 TEM image and element distribution diagram of sodium iron pyrophosphate co-doped with fluorine and sulfur elements and coated with carbon on the surface in Example 1;

[0030] Figure 3 This is a rate performance diagram of a button half-cell of sodium iron pyrophosphate co-doped with fluorine and sulfur elements and coated with carbon on the surface, prepared in Example 1;

[0031] Figure 4 This is a cycle stability performance diagram of the button half-cell of sodium iron pyrophosphate co-doped with fluorine and sulfur elements and surface-coated with carbon prepared in Example 1. DETAILED DESCRIPTION

[0032] For ease of understanding, the technical solutions and implementation methods of the present invention are further described clearly, completely and in detail through specific examples and in combination with the accompanying drawings. It should be noted that the embodiments described in the present invention are implemented on the premise of the technical solution of the present invention, and detailed implementation methods and specific operating procedures are given, but they are only part of the embodiments of the present invention, not all of the embodiments. The specific implementation methods described are limited to illustrating and explaining the present invention and do not limit the present invention. Based on the embodiments of the present invention, all other implementation methods obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0033] The experimental methods and conditions used in the following examples are conventional methods and conventional conditions unless otherwise specified. The materials, reagents, instruments, devices, etc. used in the examples are conventional substances or equipment known to those skilled in the art and can be obtained from commercial channels unless otherwise specified. The reaction conditions embodied in the summary of the invention are all capable of achieving the described reactions and obtaining products with the desired effects. Due to space limitations, only some examples are listed below to further illustrate the advantages of the technical solutions of the present invention.

[0034] Example 1

[0035] A powdered sample of sodium ferric pyrophosphate (SFP) (see Comparative Example 1) was placed in the reaction zone of a rotating plasma reactor. Copper rings were attached to both ends of the plasma reactor and connected to the RF power generator (plasma generator) via wires. The plasma reactor was then evacuated to 10 Pa and maintained in this state. The temperature was then heated to 500°C, and sulfur hexafluoride gas was introduced. The gas then carried ethanol vapor into the plasma reactor, raising the vacuum to 50 Pa. The RF power was turned on, and the RF power was adjusted to 500 W. The vacuum pump was then adjusted to maintain a vacuum of 30 Pa. After ignition, the sulfur hexafluoride gas mixed with the ethanol vapor in the plasma reaction zone excited a large number of highly active carbon radicals, which co-doped with fluorine and sulfur elements and reacted with the surface of the SFP. After a 10-minute plasma reaction, the RF power was turned off, yielding a SFP composite cathode material co-doped with fluorine and sulfur elements and coated with carbon on the surface.

[0036] Example 2-26

[0037] On the basis of Example 1, the reaction conditions including liquid source, reaction temperature, reaction power, reaction time, etc. were changed. The specific conditions are shown in Table 1 below:

[0038] Table 1 Summary of reaction conditions for each example

[0039]

[0040] Comparative Example 1

[0041] CH3COONa·3H2O, MgSO4·7H2O, FeSO4·7H2O, NH4H2PO4, and C6H8O7·H2O were added to deionized water in proportion. After stirring for 10 minutes, the precursor was dried at 160°C to obtain the precursor. The precursor powder was then ground and pre-calcined at 300°C in vacuum for 3 hours at a heating rate of 5°C min -1 The calcined powder was pressed under a pressure of 15 MPa and calcined in vacuum at 550 °C for 10 h at a heating rate of 5 °C min -1Finally, the block is crushed to obtain sodium ferric pyrophosphate powder.

[0042] Comparative Example 2

[0043] The sodium ferric pyrophosphate material in Comparative Example 1 was placed in a sealed supercritical autoclave and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical apparatus at a gas rate of 30 sccm to a pressure of 8.0 MPa. The supercritical apparatus, after the introduction of carbon dioxide, was then transferred to a 40°C oven and allowed to stand for 8 hours. After natural cooling, the gas valve was quickly opened to obtain a modified sodium ferric pyrophosphate material, but carbon coating was not successfully achieved.

[0044] Comparative Example 3

[0045] The sodium ferric pyrophosphate material in Comparative Example 1 was placed in a sealed supercritical autoclave and evacuated to a vacuum state. Carbon dioxide gas was introduced into the supercritical apparatus at a gas rate of 40 sccm to a pressure of 7.0 MPa. The supercritical apparatus after the introduction of carbon dioxide was then transferred to a 40°C oven for 8 hours, then naturally cooled. The gas valve was quickly opened to obtain a modified sodium ferric pyrophosphate material, but carbon coating was not successfully achieved.

[0046] Comparative Example 4

[0047] The sodium ferric pyrophosphate material in Comparative Example 1 was placed in a sealed supercritical autoclave and evacuated to a vacuum state. Sulfur hexafluoride gas was introduced into the supercritical apparatus at a gas rate of 30 sccm to a pressure of 8.0 MPa. The supercritical apparatus, after the introduction of carbon dioxide, was then transferred to a 40°C oven and allowed to stand for 8 hours. After natural cooling, the gas valve was quickly opened to obtain a modified sodium ferric pyrophosphate material, but carbon coating was not successfully achieved.

[0048] Comparative Example 5

[0049] The sodium ferric pyrophosphate material from Comparative Example 1 was placed in a sealed supercritical autoclave and evacuated to a vacuum state. Sulfur hexafluoride gas was then introduced into the supercritical apparatus at a rate of 40 sccm to a pressure of 7.0 MPa. The supercritical apparatus, after the carbon dioxide was introduced, was then transferred to a 40°C oven and allowed to stand for 8 hours before naturally cooling. The gas valve was then quickly opened to obtain a modified sodium ferric pyrophosphate material, but doping was unsuccessful.

[0050] Comparative Example 6

[0051] The sodium iron pyrophosphate material in Comparative Example 1 was placed in a common tube furnace, argon protective gas was introduced, and methane gas was introduced at a gas rate of 30 sccm. -1The heating rate was increased to 500 °C, kept at this temperature for 2 h and then cooled naturally to obtain the sodium iron pyrophosphate modified material, but carbon coating was not successfully achieved.

[0052] Comparative Example 7

[0053] The sodium iron pyrophosphate material in Comparative Example 1 was placed in a common tube furnace, and argon protective gas was introduced, and sulfur hexafluoride gas was introduced at a gas rate of 30 sccm. -1 The heating rate was increased to 500 °C, kept at this temperature for 2 h and then cooled naturally to obtain the sodium iron pyrophosphate modified material, but doping was not successfully achieved.

[0054] Comparative Example 8

[0055] The sodium iron pyrophosphate material in Comparative Example 1 was placed in a hydrothermal high-pressure reactor, ethanol liquid was added to two-thirds of the reactor, and the reactor was placed in a hydrothermal oven at 180°C for 10 h. After natural cooling, the reactor was transferred to an ordinary tube furnace, argon protective gas was introduced, and the reaction temperature was adjusted to 5°C·min. -1 The heating rate was increased to 500 °C, kept at this temperature for 2 h and then naturally cooled to obtain the sodium iron pyrophosphate modified material, but carbon coating was not successfully achieved.

[0056] Comparative Example 9

[0057] The sodium iron pyrophosphate material in Comparative Example 1 was placed in a hydrothermal high-pressure reactor, methanol liquid was added to two-thirds of the reactor, and the reactor was placed in a hydrothermal oven at 180°C for 10 h. After natural cooling, the reactor was transferred to an ordinary tube furnace, argon protective gas was introduced, and the reaction temperature was kept at 5°C / min. -1 The heating rate was increased to 500 °C, kept at this temperature for 2 h and then naturally cooled to obtain the sodium iron pyrophosphate modified material, but carbon coating was not successfully achieved.

[0058] Comparative Example 10

[0059] The sodium iron pyrophosphate material in Comparative Example 1 was placed in a hydrothermal high-pressure reactor, toluene liquid was added to two-thirds of the reactor, and the reactor was placed in a hydrothermal oven at 180°C for 10 h. After natural cooling, the reactor was transferred to an ordinary tube furnace, argon protective gas was introduced, and the reaction temperature was adjusted to 5°C / min. -1 The heating rate was increased to 500 °C, kept at this temperature for 2 h and then naturally cooled to obtain the sodium iron pyrophosphate modified material, but carbon coating was not successfully achieved.

[0060] Comparative Example 11

[0061] The sodium iron pyrophosphate material in Comparative Example 1 was placed in a hydrothermal high-pressure reactor, ethylene glycol liquid was added to two-thirds of the reactor, and the reactor was placed in a hydrothermal oven at 180°C for 10 h. After natural cooling, the reactor was transferred to an ordinary tube furnace, argon protective gas was introduced, and the reaction temperature was adjusted to 5°C·min. -1 The heating rate was increased to 500 °C, kept at this temperature for 2 h and then naturally cooled to obtain the sodium iron pyrophosphate modified material, but carbon coating was not successfully achieved.

[0062] Comparative Example 12

[0063] The sodium iron pyrophosphate material in Comparative Example 1 was placed in the reaction zone of a rotating plasma reactor. Copper rings were connected to both ends of the plasma reactor and connected to the generator (plasma generator) of the radio frequency power supply with wires. The plasma reactor was then evacuated to 10 Pa and maintained in this evacuated state, and the temperature was heated to 500°C. Sulfur hexafluoride gas was introduced, 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 treating for 10 minutes, the material was naturally cooled to obtain a sodium iron pyrophosphate modified material, but carbon coating was not successfully achieved.

[0064] Comparative Example 13

[0065] The sodium ferric pyrophosphate material from Comparative Example 1 was placed in the reaction zone of a rotating plasma reactor. Copper rings were attached to both ends of the plasma reactor and connected to the RF power generator (plasma generator) with wires. The plasma reactor was then evacuated to 10 Pa and maintained in this state, and the temperature was heated to 500°C. Carbon dioxide gas was introduced, the RF power switch was turned on, the RF power was adjusted to 500 W, and the vacuum pump was adjusted to maintain a vacuum of 30 Pa. After a 10-minute treatment, the material was naturally cooled to obtain a modified sodium ferric pyrophosphate material, but carbon coating was not successfully achieved.

[0066] Comparative Example 14

[0067] The sodium ferric pyrophosphate material from Comparative Example 1 was placed in the reaction zone of a rotating plasma reactor. Copper rings were attached to both ends of the plasma reactor and connected to the RF power generator (plasma generator) with wires. The plasma reactor was then evacuated to 10 Pa and maintained in this evacuated state, and the temperature was heated to 500°C. Methanol was introduced as a liquid source, the RF power switch was turned on, the RF power was adjusted to 500 W, and the vacuum pump was adjusted to maintain a vacuum of 30 Pa. After treating for 10 minutes, the material was naturally cooled to obtain a modified sodium ferric pyrophosphate material, but carbon coating was not successfully achieved.

[0068] Comparative Example 15

[0069] The sodium iron pyrophosphate material in Comparative Example 1 was placed in the reaction zone of a rotating plasma reactor. Copper rings were connected to both ends of the plasma reactor and connected to the generator of the radio frequency power supply (plasma generator) with wires. The plasma reactor was then evacuated to 10 Pa and maintained in the evacuated state, and the temperature was heated to 500°C. Sulfur hexafluoride gas was introduced, and the gas then brought the isopropyl alcohol vapor into the plasma reactor. 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 the vacuum degree at 30 Pa. After treating for 10 minutes, the plasma-modified sodium iron pyrophosphate material was obtained by natural cooling.

[0070] Comparative Example 16

[0071] The sodium iron pyrophosphate material in Comparative Example 1 was placed in the reaction zone of a rotating plasma reactor. Copper rings were connected to both ends of the plasma reactor and connected to the generator (plasma generator) of the radio frequency power supply with wires. The plasma reactor was then evacuated to 10 Pa and maintained in the evacuated state, and the temperature was heated to 500°C. Carbon dioxide gas was introduced, and the gas then brought ethanol vapor into the plasma reactor. 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 the vacuum degree at 30 Pa. After treating for 10 minutes, the plasma-modified sodium iron pyrophosphate material was obtained by natural cooling.

[0072] Comparative Example 17

[0073] The sodium iron pyrophosphate material in Comparative Example 1 was placed in the reaction zone of a rotating plasma reactor. Copper rings were connected to both ends of the plasma reactor and connected to the generator (plasma generator) of the radio frequency power supply with wires. The plasma reactor was then evacuated to 10 Pa and maintained in a vacuum state. Sulfur hexafluoride gas was introduced at room temperature, and the gas then brought ethanol vapor into the plasma reactor. 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 treating for 10 minutes, the plasma-modified sodium iron pyrophosphate material was obtained by natural cooling, but carbon coating was not successfully achieved.

[0074] Performance Testing

[0075] The sodium iron pyrophosphate samples prepared in Examples 1-26 and Comparative Examples 1-17 were slurried with Super P and PVDF in a mass ratio of 7:2:1 to form electrodes. The coated electrodes were dried in a vacuum drying oven at 120°C for 12 h to ensure complete removal of the solvent in the electrodes and cut into circular electrodes with a diameter of 10 mm. Sodium metal sheets were used as negative electrodes, and the composite sodium iron pyrophosphate circular electrodes with double surface modifications prepared above were used as positive electrodes to assemble into button batteries of model 2032. The electrolyte was 1M NaPF6 in DME = 100 vol%, and the diaphragm was a glass fiber diaphragm. The half-cell was assembled in the order of positive electrode shell, positive electrode sheet, diaphragm, electrolyte, sodium sheet, gasket, shrapnel, and negative electrode shell, and sealed with a fully automatic packaging machine. After the battery was allowed to stand for 24 hours, electrochemical tests were performed using a Xinwei and electrochemical workstation. The electrochemical tests were all conducted at 25 o The battery was tested at a rate of 1 C in a potential window of 1.7-4.3 V for sodium phosphate-sodium iron pyrophosphate, mainly including constant current charge-discharge tests and electrochemical impedance spectroscopy. The long-cycle 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.

[0076] The test results of rate performance and cycle performance are shown in Table 2 below:

[0077] Table 2 Summary of rate performance test results of various embodiments and comparative examples

[0078]

[0079] From the rate performance test results of the various examples and comparative examples in Table 2, it can be found that the sodium iron pyrophosphate materials optimized by the plasma technology described in Examples 1-26 all exhibited high specific capacities exceeding 90 mAh / g and initial coulombic efficiencies exceeding 90% at a current density of 1 C. Furthermore, when the liquid carbon source was ethanol, they exhibited superior performance compared to other carbon sources of the present invention. Furthermore, the rate performance of the materials was significantly 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, demonstrating excellent cycling and rate performance. On the contrary, in Comparative Examples 1-17, compared with Comparative Example 1, the modification effect of Comparative Examples 2-17 is not obvious. At a rate current density of 1 C, the initial capacity of the sodium iron pyrophosphate positive electrode material is low, all lower than 80 mAh / g, and the first coulombic efficiency does not exceed 80%. The reversible specific capacity at rates of 1 and 10 C is much lower than that of Example 1-26. This shows that the plasma technology of the present invention is very effective and unexpected in improving the rate performance and cycle performance of the modified sodium iron pyrophosphate positive electrode material.

[0080] Figure 1 This is a schematic diagram of the treatment of a sodium iron pyrophosphate powder sample by plasma technology in Example 1; Figure 2 TEM test image and mapping element distribution map of sodium iron pyrophosphate co-doped with fluorine and sulfur elements and coated with carbon on the surface in Example 1; Figure 3 、 Figure 4 The figure shows the rate performance and cycle stability of the button half-cell of the sodium iron pyrophosphate doped with fluorine and sulfur and coated with carbon on the surface prepared in Example 1. Figure 2 TEM and mapping elemental distributions reveal that highly active plasma carbon radicals form a 4 nm carbon coating on the surface of the phosphate polyanion. Synergistic doping with fluorine and sulfur optimizes the electrochemical behavior of the cathode / electrolyte interface, enhancing interfacial transport kinetics and mitigating volumetric stress in sodium-ion batteries. Plasma-derived carbon radicals possess strong reducing properties, altering the local coordination environment and atomic valence state of Fe in the original structure of sodium iron pyrophosphate, thereby improving its cycling and rate performance.

[0081] The present invention uses highly active plasma free radicals to form sulfur-fluorine doping and surface carbon coating on the surface of phosphate polyanion, which is a more effective way to optimize the positive electrode / electrolyte interface and stabilize Fe 2+ / Fe 3+ The redox process accelerates the interface transport kinetics of sodium ion batteries and relieves their volume change stress. The plasma-derived free radicals have strong reducing properties, which will change the local coordination environment and atomic valence state of Fe in the original structure of sodium iron pyrophosphate, enhance the stability of Fe-O hexahedron, and promote Na + Diffusion dynamics to enhance its ion transport. The composite positive electrode exhibits good cycle stability and rate performance, and the dual surface and interface modification helps to promote the development of interface optimization strategies for sodium ion battery positive electrode materials.

[0082] The above-described embodiments are only preferred solutions of the present invention and are not intended to limit the present invention in any form. Other variations and modifications are possible without exceeding the technical solutions described in the claims.

Claims

1. A method for preparing a high-capacity interface double-modified sodium iron pyrophosphate composite cathode material constructed by gas-liquid mixture source plasma, characterized in that: The method uses sodium ferric pyrophosphate as a matrix, a liquid carbon source as a liquid source, and sulfur hexafluoride as a gas source to modify the sodium ferric pyrophosphate through a one-step plasma method, comprising the following steps: (1) placing sodium iron pyrophosphate inside a plasma reaction device, connecting it to a plasma generator, and performing vacuum treatment to a vacuum degree of 1-50 Pa; (2) introducing a gas source and bringing a liquid source into a plasma reaction device to a certain vacuum degree, adjusting the radio frequency power under heating conditions, and reacting for a certain time after ignition to obtain a sodium iron pyrophosphate composite positive electrode material; wherein the liquid source includes at least one of ethanol, toluene, methanol, benzyl alcohol, acetone, and ethylene glycol; After the gas source and liquid source are introduced, the system vacuum degree is 5-100 Pa; The temperature when the gas source is introduced is 100-900°C; The flow rate of the gas source is 10-50 sccm; The RF power is 100-800 W, the reaction temperature is 100-900 ℃, and the reaction time is 3-30 min.

2. The method for preparing a high-capacity interface double-modified sodium iron pyrophosphate composite cathode material constructed by gas-liquid mixture source plasma according to claim 1, characterized in that: In step (2), the liquid source is ethanol.

3. The method for preparing a high-capacity interface double-modified sodium iron pyrophosphate composite cathode material constructed by gas-liquid mixture source plasma according to claim 1, characterized in that: The thickness of the carbon coating layer on the surface of the sodium iron pyrophosphate composite positive electrode material is 1-5 nm, and / or the doping amounts of fluorine and sulfur elements are 1-3%, respectively.

4. A gas-liquid mixture 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 to 3.

5. An application of the gas-liquid mixture source plasma-constructed high-capacity interface dual-modified sodium iron pyrophosphate composite positive electrode material according to claim 4 in the field of sodium ion battery positive electrode materials.

Images