A positive electrode material, a preparation method and application thereof

Abstract

The application relates to the technical field of batteries, in particular to a positive electrode material and a preparation method and application thereof. The positive electrode material comprises a substrate and a coating layer coated on at least part of the surface of the substrate; the substrate comprises sodium pyrophosphate; the coating layer comprises carbon material and sodium sulfide; the carbon material comprises a doping element; and the doping element comprises S and F. Compared with the prior art, the application improves the electronic conductivity of the sodium pyrophosphate positive electrode material by doping the double coating layer with carbon and sodium sulfide in combination with sulfur and fluorine, and further applies the sodium pyrophosphate positive electrode material in a sodium ion battery to improve the electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a cathode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries work on the same principle as lithium-ion batteries and are known as "rocking chair batteries." During charging and discharging, sodium ions shuttle back and forth between the cathode and anode, accompanied by the migration of electrons. Compared to lithium resources, sodium resources are widely available in nature, giving sodium-ion batteries the advantage of low cost. They also possess advantages such as a wide temperature range and high thermal stability, making them more suitable for use in extreme environments.

[0003] Currently, sodium-ion batteries are being widely used in two-wheeled vehicles, large-scale energy storage, and A00 electric vehicles. In the components of a sodium-ion battery, the cathode material determines the battery's energy density and low-temperature performance. Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, abbreviated as NFPP) cathode material has advantages such as structural stability, small volume change, environmental friendliness, and low cost, making it a commonly used active cathode material for energy storage and start-stop power supplies. However, existing NFPP cathode materials have poor electronic conductivity, which increases the battery's internal resistance. Furthermore, during high-rate cycling, the inefficient charge transport within the material further reduces the battery's power density, leading to increased energy loss over long cycles. Summary of the Invention

[0004] In view of this, the present invention aims to at least partially solve one of the technical problems in the related art. To this end, the present invention provides a cathode material, its preparation method, and its application. By combining carbon and sodium sulfide double coating with sulfur and fluorine doping of the double coating layer, the NFPP cathode material is modified, improving its electronic conductivity without altering its morphology. Further application in sodium-ion batteries can enhance electrochemical performance.

[0005] To solve the above-mentioned technical problems, the present invention is implemented as follows: According to one aspect of the present invention, a positive electrode material is provided, comprising: The substrate and the covering layer covering at least a portion of the surface of the substrate; The matrix comprises sodium iron pyrophosphate; The coating layer comprises carbon material and sodium sulfide; the carbon material comprises doping elements; the doping elements comprise S and F.

[0006] In some of these embodiments, the general chemical formula of the sodium iron pyrophosphate is Na4Fe3(PO4)2P2O7.

[0007] In some embodiments, the sodium ferric pyrophosphate has a particle size of 3.0 μm to 7.5 μm.

[0008] In some of these embodiments, the thickness of the coating layer is 2 nm to 7 nm.

[0009] In some embodiments, the sodium sulfide in the coating layer accounts for 10% to 45% by mass.

[0010] In some embodiments, the carbon material in the coating layer accounts for 55% to 90% of the total mass.

[0011] In some embodiments, the carbon material comprises amorphous carbon; the S element reconstructs the charge distribution near the defects of the amorphous carbon to form SO and CSC functional groups, and the F element reconstructs the charge distribution near the defects of the amorphous carbon to form CF and FCF functional groups.

[0012] According to another aspect of the present invention, the present invention provides a method for preparing the cathode material described in the above technical solution, comprising the following steps: Sodium iron pyrophosphate and a coating agent were subjected to a first plasma treatment in an inert atmosphere; then, the inert atmosphere was replaced with sulfur hexafluoride gas for a second plasma treatment to obtain the cathode material.

[0013] In some of these embodiments, the coating agent includes at least one of carbon disulfide, tricarbon disulfide, dodecyl mercaptan, and carbon dichlorosulfide.

[0014] In some embodiments, the inert atmosphere includes at least one of argon, nitrogen, and helium.

[0015] In some of these embodiments, the mass ratio of sodium ferric pyrophosphate to the coating agent is (2~10):(15~35).

[0016] In some embodiments, the first plasma treatment process includes: Sodium iron pyrophosphate was sealed in a quartz tube filled with an inert atmosphere; Seal the coating agent in the beaker; then connect the sealed beaker to the plasma argon gas line; The sealed quartz tube is placed into the plasma generating chamber. The flow rate of argon gas in the plasma argon gas path is adjusted so that the coating agent enters the quartz tube with the argon gas. After a certain period of time, the quartz tube is filled with the coating agent. The first plasma treatment is performed under the first vacuum condition through a glow emitter.

[0017] In some of these embodiments, the beaker is sealed to a flange using a flange shut-off valve.

[0018] In some embodiments, the argon flow rate in the plasma argon gas path is adjusted to 20 sccm to 80 sccm.

[0019] In some of these embodiments, the inlet time is 1 min to 10 min.

[0020] In some of these embodiments, the vacuum pressure of the first vacuum condition is -0.1 × 10⁻⁶. -3 Pa ~ 50 Pa.

[0021] In some of these embodiments, the glow discharge power of the first plasma treatment is 100W to 500W.

[0022] In some of these embodiments, the duration of the first plasma treatment is 30 seconds to 5 minutes.

[0023] In some of these embodiments, the flow rate of the sulfur hexafluoride gas is 15 sccm to 50 sccm.

[0024] In some of these embodiments, the sulfur hexafluoride gas is introduced for 5 to 15 minutes.

[0025] In some embodiments, the second plasma treatment process includes: Sulfur hexafluoride gas is introduced to completely remove the coating agent, and a second plasma treatment is performed under a second vacuum condition using a glow emitter.

[0026] In some embodiments, the vacuum pressure of the second vacuum condition is -0.1 × 10⁻⁶. -3 Pa ~ 20 Pa.

[0027] In some of these embodiments, the glow discharge power of the second plasma treatment is 400W to 800W.

[0028] In some of these embodiments, the temperature of the second plasma treatment is 260°C to 530°C, and the heating rate is 5°C / min to 20°C / min.

[0029] In some of these embodiments, the second plasma treatment time is 5 min to 15 min.

[0030] According to another aspect of the present invention, a battery is provided, including a positive electrode sheet; the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector along the thickness direction, the positive active material layer including the positive electrode material described in the above-described technical solution or the positive electrode material prepared by the preparation method described in the above-described technical solution.

[0031] Implementing the technical solution of the present invention has at least the following beneficial effects: 1. This invention uses low-temperature plasma technology to transform an organic liquid containing carbon and sulfur sources into an amorphous conductive carbon and sodium sulfide coating layer. Based on this, sulfur hexafluoride gas is used as a gas source dopant to dope the coating layer with S and F, which solves the problem of poor intrinsic conductivity of the composite sodium iron pyrophosphate material, enhances the electron transport capability of the material, and improves the transport capability of sodium ions between the electrolyte and the cathode material interface.

[0032] 2. The presence of the S,F-doped carbon-sodium sulfide coating layer in this invention avoids the reaction between the cathode material and the electrolyte, significantly improves the long-cycle performance of the battery, and at the same time improves the discharge specific capacity and rate performance of the cathode material under high and low current conditions.

[0033] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0034] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0035] Figure 1 The images show SEM images of the cathode materials prepared in Example 1 and Comparative Example 1 of this application.

[0036] Figure 2 The charge-discharge curves of the coin cells prepared in Example 1 and Comparative Example 1 of this application are shown for the first cycle at 0.1C.

[0037] Figure 3 This is long-cycle data of 200 cycles at 2C for the coin cells prepared in Example 1 and Comparative Example 1 of this application.

[0038] Figure 4 The discharge data of the coin cells prepared in Example 1 and Comparative Example 1 of this application at different rates are shown.

[0039] Figure 5 The impedance diagrams of the coin cells prepared in Example 1 and the comparative example of this application before cycling are shown.

[0040] The accompanying drawings have illustrated specific embodiments of the invention, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0041] The present application will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0042] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges or individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0043] In the description of this application, "same chemical composition" should be interpreted broadly, that is, the main components of the two have the same chemical composition, or the two have substantially the same chemical composition, but may have errors or impurities within the acceptable range that can be understood by those skilled in the art.

[0044] In the description of this application, "A and / or B" can include any of the cases of A alone, B alone, or A and B, where A and B are merely examples and can be any technical feature connected by "and / or" in this application.

[0045] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0046] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0047] Unless otherwise specified, all technical features and optional technical features of this invention can be combined to form new technical solutions.

[0048] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order; for example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0049] Currently, sodium-ion batteries are being widely used in two-wheeled vehicles, large-scale energy storage, and A00 electric vehicles. In the components of a sodium-ion battery, the cathode material determines the battery's energy density and low-temperature performance. Sodium iron pyrophosphate (SOP) cathode material has advantages such as structural stability, small volume change, environmental friendliness, and low cost, making it a commonly used active cathode material for energy storage and start-stop power supplies. Because SOP crystals contain a large number of phosphate groups, which are covalently bonded to oxygen, the high bond energy makes them difficult to break, resulting in structural stability. However, this structure also restricts the electron transport path within the material, leading to poor electronic conductivity and increased internal resistance. According to Joule's law, when the charging and discharging current is the same, increased internal resistance inevitably generates more heat during charging and discharging, exacerbating battery temperature rise and causing safety issues. Simultaneously, during high-rate cycling, the inefficient charge transport within the material further reduces the battery's power density, leading to increased energy loss over long cycles.

[0050] To improve the electronic conductivity of sodium iron pyrophosphate, the inventors of this invention considered using a coating modification method, such as coating an ultrathin conductive carbon layer onto the material surface using a single carbon coating method. This layer not only improves the material's conductivity but also reduces side reactions between the electrolyte and the cathode material, improving the battery's long-cycle performance. However, a single carbon coating layer is not a sodium ion conductor and does not improve the transport capacity of sodium ions at the interface between the electrolyte and the cathode material. Therefore, using coating modification technology alone cannot achieve the desired improvement effect. Further doping treatment of the coating layer is considered to enhance its adsorption and desorption capacity for sodium ions. In addition, commonly used coating modification methods in the prior art mainly include hydrothermal coating, spray drying coating, and high-temperature sintering coating. These coating modification methods typically use expensive coating agents such as reduced graphene oxide and require long processing times and high energy consumption.

[0051] Based on this, the present invention uses a liquid coating agent containing sulfur and carbon sources at room temperature to perform a one-step double coating treatment of sodium iron pyrophosphate cathode material with carbon and sodium sulfide. Subsequently, sulfur hexafluoride gas is used to dope the double coating layer with sulfur and fluorine. The morphology of the resulting cathode material does not change significantly, and the sodium-ion battery further prepared using this cathode material exhibits excellent electrochemical performance, with better rate performance, first discharge specific capacity, and capacity retention than the unmodified sodium iron pyrophosphate cathode material. Specifically, the present invention adopts the following technical solution: According to one aspect of the present invention, a positive electrode material is provided, comprising: The substrate and the covering layer covering at least a portion of the surface of the substrate; The matrix comprises sodium iron pyrophosphate; The coating layer comprises carbon material and sodium sulfide; the carbon material comprises doping elements; the doping elements comprise S and F.

[0052] In a specific embodiment of the present invention, the general chemical formula of the sodium iron pyrophosphate is Na4Fe3(PO4)2P2O7; the particle size of the sodium iron pyrophosphate is preferably 3.0μm~7.5μm, specifically 3.0μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, etc.; by selecting the above-mentioned suitable particle size, the present invention can improve the capacity and density of the cathode material, while reducing the damage to the structural stability during sodium ion insertion and extraction, and improving the cycle stability of the cathode material.

[0053] In a specific embodiment of the present invention, the thickness of the coating layer is preferably 2nm to 7nm, specifically 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, etc. If the coating layer is too thick, the transport of sodium ions between the electrolyte and the positive electrode material is hindered, resulting in a high battery impedance. If the coating layer is too thin, it cannot effectively protect the material during cycling, leading to poor cycle performance. The present invention selects the aforementioned suitable coating layer thickness, which can improve the conductivity and ion transport capacity of the positive electrode material, and reduce side reactions between the electrolyte and the positive electrode material, thereby improving the long-cycle performance of the battery.

[0054] In a specific embodiment of the present invention, the mass percentage of sodium sulfide in the coating layer is preferably 10% to 45%, specifically 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, etc.; the mass percentage of carbon material in the coating layer is preferably 55% to 90%, specifically 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, etc. Excessive carbon material content leads to a decrease in the specific capacity of the cathode material, thereby reducing the energy density of the battery; insufficient carbon material content results in poor conductivity of the cathode material and increased internal resistance of the battery. The present invention selects the appropriate proportion of carbon material described above to improve the conductivity of the cathode material, thereby increasing the battery capacity.

[0055] In a specific embodiment of the present invention, the carbon material preferably comprises amorphous carbon; the carbon material includes doping elements, namely S and F. The S element reconstructs the charge distribution near the defects of the amorphous carbon, forming stable SO and CSC functional groups, while simultaneously widening the carbon interlayer spacing and enhancing the reaction kinetics at high rates. The F element, possessing the strongest electronegativity, reconstructs the charge distribution near the defects of the amorphous carbon, forming stable CF and FCF functional groups, enhancing the wettability of the coating layer with the electrolyte, and increasing the conductivity of the amorphous carbon.

[0056] According to another aspect of the present invention, the present invention provides a method for preparing the cathode material described in the above technical solution, comprising the following steps: Sodium iron pyrophosphate and a coating agent were subjected to a first plasma treatment in an inert atmosphere; then, the inert atmosphere was replaced with sulfur hexafluoride gas for a second plasma treatment to obtain the cathode material.

[0057] In a specific embodiment of the present invention, the coating agent preferably includes at least one selected from carbon disulfide, tricarbon disulfide, dodecyl mercaptan, and carbon dichlorosulfide. The coating agent has a simple elemental composition, with C and S as the main elements, and can be decomposed into highly reactive plasma states of carbon and sulfur in a low-temperature plasma generator. The use of the above-mentioned coating agent in this invention ensures that the resulting coating layer has good performance, thereby further obtaining a cathode material that meets the performance requirements of the present invention.

[0058] In a specific embodiment of the present invention, the inert atmosphere preferably includes at least one of argon atmosphere, nitrogen atmosphere, and helium atmosphere, and more preferably argon atmosphere; the present invention uses the above-mentioned inert atmosphere to isolate oxygen, moisture and active impurities, and avoid the sodium iron pyrophosphate matrix from being oxidized or reacting with impurities during plasma treatment.

[0059] In a specific embodiment of the present invention, the preferred mass ratio of sodium iron pyrophosphate to the coating agent is (2~10):(15~35), specifically it can be 1:1.5, 1:3.5, 1:4, 1:5, 2:15, 2:35, etc.; the present invention can ensure that the coating layer obtained subsequently has good performance by using the above-mentioned suitable mass ratio, thereby further obtaining a cathode material that meets the performance requirements of the present invention.

[0060] In a specific embodiment of the present invention, the first plasma treatment process preferably includes: Sodium iron pyrophosphate was sealed in a quartz tube filled with an inert atmosphere; Seal the coating agent in the beaker; then connect the sealed beaker to the plasma argon gas line; The sealed quartz tube is placed into the plasma generating chamber. The flow rate of argon gas in the plasma argon gas path is adjusted so that the coating agent enters the quartz tube with the argon gas. After a certain period of time, the quartz tube is filled with the coating agent. The first plasma treatment is performed under the first vacuum condition through a glow emitter.

[0061] In this invention, the coating agent is decomposed into highly reactive plasma states of carbon and sulfur in a low-temperature plasma generator. The plasma sulfur reacts with residual sodium on the surface of sodium ferric pyrophosphate, such as sodium carbonate or hydroxide, to generate a sodium sulfide coating layer. The plasma carbon is deposited on the surface of sodium ferric pyrophosphate to form an amorphous carbon coating layer.

[0062] In a specific embodiment of the present invention, the beaker is preferably sealed with a flange stop valve; the argon flow rate in the plasma argon gas path is preferably 20 sccm to 80 sccm, specifically 20 sccm, 30 sccm, 40 sccm, 50 sccm, 60 sccm, 70 sccm, 80 sccm, etc.; the introduction time is preferably 1 min to 10 min, specifically 1 min, 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, etc.; the vacuum pressure of the first vacuum condition is preferably -0.1 × 10⁻⁶. -3 Pa~50 Pa, specifically -0.1×10 Pa. -3Pa, 1 Pa, 50 Pa, etc.; the glow discharge power of the first plasma treatment is preferably 100W~500W, specifically 100W, 200W, 300W, 400W, 500W, etc.; the time of the first plasma treatment is preferably 30s~5min, specifically 30s, 1min, 2min, 3min, 4min, 5min, etc. By selecting the reaction conditions of the above-mentioned first plasma treatment process, this invention can ensure that the subsequently obtained coating layer has good performance, thereby further obtaining a cathode material that meets the performance requirements of this invention.

[0063] In a specific embodiment of the present invention, the second plasma treatment process preferably includes: introducing sulfur hexafluoride gas to completely remove the coating agent, and performing the second plasma treatment under a second vacuum condition through a glow emitter.

[0064] In a specific embodiment of the present invention, the flow rate of the sulfur hexafluoride gas is preferably 15 sccm to 50 sccm, specifically 15 sccm, 20 sccm, 25 sccm, 30 sccm, 35 sccm, 40 sccm, 45 sccm, 50 sccm, etc.; the introduction time of the sulfur hexafluoride gas is preferably 5 min to 15 min, specifically 5 min, 10 min, 15 min, etc. The present invention selects the above-mentioned suitable sulfur hexafluoride gas flow rate and introduction time to introduce sulfur and fluorine elements, ensuring saturated adsorption of sulfur and fluorine elements at the defects of carbon materials, forming stable functional groups.

[0065] In a specific embodiment of the present invention, the vacuum pressure of the second vacuum condition is preferably -0.1 × 10⁻⁶. -3 Pa ~ 20 Pa, specifically -0.1 × 10 Pa. -3 Pa, 1 Pa, 20 Pa, etc.; the glow discharge power of the second plasma treatment is preferably 400W~800W, specifically 400W, 500W, 600W, 700W, 800W, etc.; the temperature of the second plasma treatment is preferably 260℃~530℃, specifically 260℃, 300℃, 360℃, 400℃, 460℃, 500℃, 530℃, etc.; the heating rate is preferably 5℃ / min~20℃ / min, specifically 5℃ / min, 10℃ / min, 15℃ / min, 20℃ / min, etc.; the time of the second plasma treatment is 5min~15min, specifically 5min, 10min, 15min, etc. By selecting the above-mentioned reaction conditions for the second plasma treatment process, this invention can ensure that S and F elements are doped into the coating layer, thereby further obtaining a cathode material that meets the performance requirements of this invention.

[0066] According to another aspect of the present invention, a battery is provided, including a positive electrode sheet; the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector along the thickness direction, the positive active material layer including the positive electrode material described in the above-described technical solution or the positive electrode material prepared by the preparation method described in the above-described technical solution.

[0067] In a specific embodiment of the present invention, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector along the thickness direction. The positive current collector has two surfaces opposite to each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0068] In specific embodiments of the present invention, the positive electrode current collector can be a metal foil or a composite current collector. For example, aluminum foil can be used as the metal foil. The composite current collector can include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.). The present invention does not have any special restrictions on the source of the positive electrode current collector; commercially available products or self-made products well known to those skilled in the art can be used.

[0069] In a specific embodiment of the present invention, the positive electrode active material layer includes, in addition to the positive electrode material, a binder and a conductive agent; wherein, the binder includes, but is not limited to, one or more of polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS); the conductive agent includes, but is not limited to, one or more of conductive carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The present invention does not impose any special restrictions on the source of the above-mentioned binder and conductive agent; commercially available products well known to those skilled in the art can be used.

[0070] In a specific embodiment of the present invention, the positive electrode sheet is prepared by a method well known to those skilled in the art, which involves thoroughly mixing the positive electrode material, binder, conductive agent and solvent to prepare a slurry, coating the slurry onto the positive electrode current collector, and then drying, cold pressing and slitting to obtain the positive electrode sheet. The present invention does not have any special limitations on this.

[0071] In this invention, the battery further includes a negative electrode, a separator, and an electrolyte. Conventional negative electrode, separator, and electrolyte known to those skilled in the art for use in batteries can be used, and this invention does not impose any special limitations on them. In a preferred embodiment of this invention, the negative electrode is a conventional negative electrode used in sodium-ion batteries.

[0072] In a specific embodiment of the present invention, the separator can be a commercially available separator for batteries that is well known to those skilled in the art; the present invention does not impose any particular limitation on the type of separator, and any porous structure separator with good chemical and mechanical stability can be selected.

[0073] In a specific embodiment of the present invention, the material of the diaphragm preferably includes one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride; the diaphragm can be a single-layer film or a multi-layer composite film, and the present invention does not have any special restrictions on this; when the diaphragm is a multi-layer composite film, the materials of each layer can be the same or different, and the present invention does not have any special restrictions on this.

[0074] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This invention does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0075] The present invention does not impose any special restrictions on the assembly method of the battery. Any technical solution known to those skilled in the art for assembling the above-mentioned positive electrode, separator, negative electrode and electrolyte to prepare the battery can be used.

[0076] In a specific embodiment of the present invention, the outer packaging of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell, or it can be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic, and examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate. The present invention does not impose any particular limitation on the shape of the lithium-ion battery; it can be cylindrical, square, or any other arbitrary shape, and those skilled in the art can choose according to specific practical needs.

[0077] The present application will be described in detail below with reference to the accompanying drawings and embodiments. However, the implementation and protection of the present invention are not limited thereto. The following embodiments are only some embodiments of the present application and are not intended to limit the present application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0078] Example 1 (1) Weigh 5g of sodium ferric pyrophosphate in the glove box and seal it in a quartz tube; weigh 20g of carbon disulfide and pour it into a wide-mouth flask, then seal the beaker with a flange stop valve and a flange, and connect the sealed beaker to the plasma argon gas path; place the sealed quartz tube into the plasma generating chamber, adjust the argon gas flow meter in the plasma argon gas path so that the carbon disulfide enters the quartz tube with the argon gas, set the flow rate to 50 sccm, and turn off the argon gas flow meter after 5 minutes; turn on the vacuum pump and adjust the vacuum pressure to 6×10 -2 Pa, set the glow discharge power to 200W, turn on the glow discharge emitter for plasma treatment for 2 minutes, then turn off the glow discharge emitter and shut off the argon gas path.

[0079] (2) Open the sulfur hexafluoride gas path and control the flow rate of sulfur hexafluoride at 20 sccm. Introduce the gas for 10 min to completely remove carbon disulfide. Turn on the vacuum pump and adjust the vacuum pressure to 0.6 Pa. Set the glow discharge power to 500 W. Turn on the heating setting of the plasma generation chamber and set the temperature to 380 °C with a heating rate of 10 °C / min. After reaching the set temperature, turn on the glow discharge emitter to perform plasma treatment for 10 min to obtain the sodium iron pyrophosphate NFPP cathode material with S,F doped carbon-sodium sulfide coating. The material obtained above is sieved in a glove box to obtain the final modified NFPP cathode material, denoted as PLA-NFPP (see SEM). Figure 1 As shown in the figure, the NFPP particle size is 5.0 μm to 5.5 μm and the coating thickness is 3 nm; Na2S accounts for 30% of the mass of the coating and SF-doped carbon material accounts for 70% of the mass of the coating.

[0080] (3) 0.2g of binder polyvinylidene fluoride (PVDF) was poured into 18g of solvent N-methylpyrrolidone (NMP). After complete dissolution, 1.6g of the above-mentioned NFPP positive electrode material and 0.2g of conductive agent carbon black (Super P) were added for homogenization to obtain a positive electrode slurry with a solid content of 10%. The positive electrode slurry was coated onto aluminum foil using a 100μm scraper and dried in an 80℃ forced-air oven for 3h. Then, it was transferred to a 120℃ vacuum oven for 24h and punched into a positive electrode sheet with a diameter of 9mm in a drying room with a dew point of -50℃. Finally, the above-mentioned positive electrode sheet was used as the positive electrode, the composite sodium sheet as the negative electrode, and the glass fiber as the separator in an Ar atmosphere in a glove box to assemble a CR2032 button cell.

[0081] Example 2 The modified NFPP cathode material was obtained by the preparation method provided in Example 1, with the only difference being that: dodecyl mercaptan was used to replace carbon disulfide as the coating agent; the thickness of the coating layer of the modified NFPP cathode material was 4 nm, the mass ratio of Na2S in the coating layer was 10%, and the mass ratio of SF-doped carbon material in the coating layer was 90%.

[0082] Example 3 The modified NFPP cathode material was obtained by the preparation method provided in Example 1, with the only difference being that carbon trichlorosulfide was used instead of carbon disulfide as the coating agent; the thickness of the coating layer of the modified NFPP cathode material was 3.5 nm, the mass ratio of Na2S in the coating layer was 15%, and the mass ratio of SF-doped carbon material in the coating layer was 85%.

[0083] Example 4 (1) Weigh 2g of sodium ferric pyrophosphate in the glove box and seal it in a quartz tube; weigh 15g of carbon disulfide and pour it into a wide-mouth flask, then seal the beaker with a flange stop valve and a flange, and connect the sealed beaker to the plasma argon gas path; place the sealed quartz tube into the plasma generating chamber, adjust the argon gas flow meter in the plasma argon gas path so that the carbon disulfide enters the quartz tube with the argon gas, set the flow rate to 20 sccm, and turn off the argon gas flow meter after 10 minutes; turn on the vacuum pump and adjust the vacuum pressure to 0.1×10 -3 Pa, set the glow discharge power to 500W, turn on the glow discharge emitter for plasma treatment for 4 minutes, then turn off the glow discharge emitter and shut off the argon gas path.

[0084] (2) Open the sulfur hexafluoride gas circuit and control the flow rate of sulfur hexafluoride at 15 sccm. Introduce the gas for 15 minutes to completely remove carbon disulfide; turn on the vacuum pump and adjust the vacuum pressure to 0.1 × 10⁻⁶. -3 Pa was used to set the glow discharge power to 800W, and the plasma generation chamber heating settings were turned on. The temperature was set to 530℃, and the heating rate was 20℃ / min. After reaching the set temperature, the glow discharge emitter was turned on for plasma treatment for 5 minutes to obtain an S,F-doped carbon-sodium sulfide coated sodium iron pyrophosphate NFPP cathode material. The obtained material was then sieved in a glove box to obtain the final modified NFPP cathode material. Testing showed that the thickness of the coating layer of the modified NFPP cathode material was 7nm; the mass percentage of Na2S in the coating layer was 20%, and the mass percentage of SF-doped carbon material in the coating layer was 80%.

[0085] (3) 0.2g of binder polyvinylidene fluoride (PVDF) was poured into 18g of solvent N-methylpyrrolidone (NMP). After complete dissolution, 1.6g of the above-mentioned NFPP positive electrode material and 0.2g of conductive agent carbon black (Super P) were added for homogenization to obtain a positive electrode slurry with a solid content of 10%. The positive electrode slurry was coated onto aluminum foil using a 100μm scraper and dried in an 80℃ forced-air oven for 3h. Then, it was transferred to a 120℃ vacuum oven for 24h and punched into a positive electrode sheet with a diameter of 9mm in a drying room with a dew point of -50℃. Finally, the above-mentioned positive electrode sheet was used as the positive electrode, the composite sodium sheet as the negative electrode, and the glass fiber as the separator in an Ar atmosphere in a glove box to assemble a CR2032 button cell.

[0086] Example 5 (1) Weigh 10g of sodium iron pyrophosphate in the glove box and seal it in a quartz tube; weigh 35g of carbon disulfide and pour it into a wide-mouth flask, then seal the beaker with a flange stop valve and a flange, and then connect the sealed beaker to the plasma argon gas path; put the sealed quartz tube into the plasma generating chamber, adjust the argon gas flow meter in the plasma argon gas path so that carbon disulfide enters the quartz tube with argon gas, set the flow rate to 80sccm, and turn off the argon gas flow meter after 1min; turn on the vacuum pump, adjust the vacuum pressure to 50Pa, set the glow discharge power to 100W, turn on the glow discharge emitter to perform plasma treatment for 5min and then turn off the glow discharge emitter and close the argon gas path.

[0087] (2) Open the sulfur hexafluoride gas circuit and control the flow rate of sulfur hexafluoride at 15 sccm. Introduce the gas for 15 minutes to completely remove carbon disulfide; turn on the vacuum pump and adjust the vacuum pressure to 0.1 × 10⁻⁶. -3 Pa was used to set the glow discharge power to 800W, and the plasma generation chamber heating settings were turned on. The temperature was set to 530℃, and the heating rate was 20℃ / min. After reaching the set temperature, the glow discharge emitter was turned on for plasma treatment for 5 minutes to obtain an S,F-doped carbon-sodium sulfide coated sodium iron pyrophosphate NFPP cathode material. The obtained material was then sieved in a glove box to obtain the final modified NFPP cathode material. Testing showed that the thickness of the coating layer of the modified NFPP cathode material was 6nm; the mass percentage of Na2S in the coating layer was 10%, and the mass percentage of SF-doped carbon material in the coating layer was 90%.

[0088] (3) 0.2g of binder polyvinylidene fluoride (PVDF) was poured into 18g of solvent N-methylpyrrolidone (NMP). After complete dissolution, 1.6g of the above-mentioned NFPP positive electrode material and 0.2g of conductive agent carbon black (Super P) were added for homogenization to obtain a positive electrode slurry with a solid content of 10%. The positive electrode slurry was coated onto aluminum foil using a 100μm scraper and dried in an 80℃ forced-air oven for 3h. Then, it was transferred to a 120℃ vacuum oven for 24h and punched into a positive electrode sheet with a diameter of 9mm in a drying room with a dew point of -50℃. Finally, the above-mentioned positive electrode sheet was used as the positive electrode, the composite sodium sheet as the negative electrode, and the glass fiber as the separator in an Ar atmosphere in a glove box to assemble a CR2032 button cell.

[0089] Example 6 (1) Weigh 2g of sodium ferric pyrophosphate in the glove box and seal it in a quartz tube; weigh 15g of carbon disulfide and pour it into a wide-mouth flask, then seal the beaker with a flange stop valve and a flange, and connect the sealed beaker to the plasma argon gas path; place the sealed quartz tube into the plasma generating chamber, adjust the argon gas flow meter in the plasma argon gas path so that the carbon disulfide enters the quartz tube with the argon gas, set the flow rate to 20 sccm, and turn off the argon gas flow meter after 10 minutes; turn on the vacuum pump and adjust the vacuum pressure to 0.1×10 -3 Pa, set the glow discharge power to 500W, turn on the glow discharge emitter for plasma treatment for 30 seconds, then turn off the glow discharge emitter and shut off the argon gas path.

[0090] (2) Open the sulfur hexafluoride gas path and control the flow rate of sulfur hexafluoride at 50 sccm. Introduce the gas for 5 minutes to completely remove carbon disulfide. Turn on the vacuum pump, adjust the vacuum pressure to 20 Pa, set the glow discharge power to 400 W, turn on the plasma generation chamber heating setting, set the temperature to 260 °C, and the heating rate to 5 °C / min. After reaching the set temperature, turn on the glow discharge emitter to perform plasma treatment for 5 minutes to obtain S,F-doped carbon-sodium sulfide coated sodium iron pyrophosphate NFPP cathode material. The material obtained above is sieved in a glove box to obtain the final modified NFPP cathode material. The thickness of the coating layer of the modified NFPP cathode material is 2 nm; the mass ratio of Na2S in the coating layer is 30%, and the mass ratio of SF-doped carbon material in the coating layer is 70%.

[0091] (3) 0.2g of binder polyvinylidene fluoride (PVDF) was poured into 18g of solvent N-methylpyrrolidone (NMP). After complete dissolution, 1.6g of the above-mentioned NFPP positive electrode material and 0.2g of conductive agent carbon black (Super P) were added for homogenization to obtain a positive electrode slurry with a solid content of 10%. The positive electrode slurry was coated onto aluminum foil using a 100μm scraper and dried in an 80℃ forced-air oven for 3h. Then, it was transferred to a 120℃ vacuum oven for 24h and punched into a positive electrode sheet with a diameter of 9mm in a drying room with a dew point of -50℃. Finally, the above-mentioned positive electrode sheet was used as the positive electrode, the composite sodium sheet as the negative electrode, and the glass fiber as the separator in an Ar atmosphere in a glove box to assemble a CR2032 button cell.

[0092] Example 7 (1) Weigh 10g of sodium iron pyrophosphate in the glove box and seal it in a quartz tube; weigh 35g of carbon disulfide and pour it into a wide-mouth flask, then seal the beaker with a flange stop valve and a flange, and then connect the sealed beaker to the plasma argon gas path; put the sealed quartz tube into the plasma generating chamber, adjust the argon gas flow meter in the plasma argon gas path so that carbon disulfide enters the quartz tube with argon gas, set the flow rate to 80sccm, and turn off the argon gas flow meter after 1min; turn on the vacuum pump, adjust the vacuum pressure to 50Pa, set the glow discharge power to 100W, turn on the glow discharge emitter to perform plasma treatment for 5min and then turn off the glow discharge emitter and close the argon gas path.

[0093] (2) Open the sulfur hexafluoride gas path and control the flow rate of sulfur hexafluoride at 50 sccm. Introduce the gas for 5 minutes to completely remove carbon disulfide. Turn on the vacuum pump and adjust the vacuum pressure to 20 Pa. Set the glow discharge power to 400 W. Turn on the heating setting of the plasma generation chamber and set the temperature to 260 °C with a heating rate of 5 °C / min. After reaching the set temperature, turn on the glow discharge emitter for plasma treatment for 5 minutes to obtain sodium iron pyrophosphate NFPP cathode material with S,F doped carbon-sodium sulfide coating. The material obtained above is sieved in a glove box to obtain the final modified NFPP cathode material. The thickness of the coating layer of the modified NFPP cathode material is 6 nm. The mass percentage of Na2S in the coating layer is 35%, and the mass percentage of SF doped carbon material in the coating layer is 65%.

[0094] (3) 0.2g of binder polyvinylidene fluoride (PVDF) was poured into 18g of solvent N-methylpyrrolidone (NMP). After complete dissolution, 1.6g of the above-mentioned NFPP positive electrode material and 0.2g of conductive agent carbon black (Super P) were added for homogenization to obtain a positive electrode slurry with a solid content of 10%. The positive electrode slurry was coated onto aluminum foil using a 100μm scraper and dried in an 80℃ forced-air oven for 3h. Then, it was transferred to a 120℃ vacuum oven for 24h and punched into a positive electrode sheet with a diameter of 9mm in a drying room with a dew point of -50℃. Finally, the above-mentioned positive electrode sheet was used as the positive electrode, the composite sodium sheet as the negative electrode, and the glass fiber as the separator in an Ar atmosphere in a glove box to assemble a CR2032 button cell.

[0095] Example 8 (1) Weigh 4g of sodium iron pyrophosphate in the glove box and seal it in a quartz tube; weigh 20g of carbon disulfide and pour it into a wide-mouth flask, then seal the beaker with a flange stop valve and a flange, and then connect the sealed beaker to the plasma argon gas path; put the sealed quartz tube into the plasma generating chamber, adjust the argon gas flow meter in the plasma argon gas path so that carbon disulfide enters the quartz tube with argon gas, set the flow rate to 40sccm, and turn off the argon gas flow meter after 7min; turn on the vacuum pump, adjust the vacuum pressure to 0.15Pa, set the glow discharge power to 300W, turn on the glow discharge emitter to perform plasma treatment for 3min and then turn off the glow discharge emitter and close the argon gas path.

[0096] (2) Open the sulfur hexafluoride gas circuit and control the flow rate of sulfur hexafluoride at 30 sccm. Introduce the gas for 10 minutes to completely remove carbon disulfide. Turn on the vacuum pump and adjust the vacuum pressure to 3.5 × 10⁻⁶. -2 Pa was used to set the glow discharge power to 600W, and the plasma generation chamber heating settings were turned on. The temperature was set to 400℃, and the heating rate was 9℃ / min. After reaching the set temperature, the glow discharge emitter was turned on for plasma treatment for 7 minutes to obtain an S,F-doped carbon-sodium sulfide coated sodium iron pyrophosphate NFPP cathode material. The obtained material was then sieved in a glove box to obtain the final modified NFPP cathode material. Testing showed that the thickness of the coating layer of the modified NFPP cathode material was 3nm; the mass percentage of Na2S in the coating layer was 25%, and the mass percentage of SF-doped carbon material in the coating layer was 75%.

[0097] (3) 0.2g of binder polyvinylidene fluoride (PVDF) was poured into 18g of solvent N-methylpyrrolidone (NMP). After complete dissolution, 1.6g of the above-mentioned NFPP positive electrode material and 0.2g of conductive agent carbon black (Super P) were added for homogenization to obtain a positive electrode slurry with a solid content of 10%. The positive electrode slurry was coated onto aluminum foil using a 100μm scraper and dried in an 80℃ forced-air oven for 3h. Then, it was transferred to a 120℃ vacuum oven for 24h and punched into a positive electrode sheet with a diameter of 9mm in a drying room with a dew point of -50℃. Finally, the above-mentioned positive electrode sheet was used as the positive electrode, the composite sodium sheet as the negative electrode, and the glass fiber as the separator in an Ar atmosphere in a glove box to assemble a CR2032 button cell.

[0098] Example 9 (1) Weigh 5g of sodium iron pyrophosphate in the glove box and seal it in a quartz tube; weigh 20g of carbon disulfide and pour it into a wide-mouth flask, then seal the beaker with a flange stop valve and a flange, and then connect the sealed beaker to the plasma argon gas path; put the sealed quartz tube into the plasma generating chamber, adjust the argon gas flow meter in the plasma argon gas path so that carbon disulfide enters the quartz tube with argon gas, set the flow rate to 60sccm, and turn off the argon gas flow meter after 4min; turn on the vacuum pump, adjust the vacuum pressure to 0.8Pa, set the glow discharge power to 200W, turn on the glow discharge emitter to perform plasma treatment for 4min and then turn off the glow discharge emitter and close the argon gas path.

[0099] (2) Open the sulfur hexafluoride gas path and control the flow rate of sulfur hexafluoride at 40 sccm. Introduce the gas for 10 min to completely remove carbon disulfide. Turn on the vacuum pump, adjust the vacuum pressure to 0.7 Pa, set the glow discharge power to 450 W, turn on the plasma generation chamber heating setting, set the temperature to 340 °C, and the heating rate to 12 °C / min. After reaching the set temperature, turn on the glow discharge emitter to perform plasma treatment for 9 min to obtain S,F-doped carbon-sodium sulfide coated sodium iron pyrophosphate NFPP cathode material. The material obtained above is sieved in a glove box to obtain the final modified NFPP cathode material. The thickness of the coating layer of the modified NFPP cathode material is 4 nm; the mass ratio of Na2S in the coating layer is 35%, and the mass ratio of SF-doped carbon material in the coating layer is 65%.

[0100] (3) 0.2g of binder polyvinylidene fluoride (PVDF) was poured into 18g of solvent N-methylpyrrolidone (NMP). After complete dissolution, 1.6g of the above-mentioned NFPP positive electrode material and 0.2g of conductive agent carbon black (Super P) were added for homogenization to obtain a positive electrode slurry with a solid content of 10%. The positive electrode slurry was coated onto aluminum foil using a 100μm scraper and dried in an 80℃ forced-air oven for 3h. Then, it was transferred to a 120℃ vacuum oven for 24h and punched into a positive electrode sheet with a diameter of 9mm in a drying room with a dew point of -50℃. Finally, the above-mentioned positive electrode sheet was used as the positive electrode, the composite sodium sheet as the negative electrode, and the glass fiber as the separator in an Ar atmosphere in a glove box to assemble a CR2032 button cell.

[0101] Comparative Example 1 Sodium iron pyrophosphate was used as the positive electrode material. 0.2g of polyvinylidene fluoride (PVDF) binder was poured into 18g of N-methylpyrrolidone (NMP) solvent. After complete dissolution, 1.6g of sodium iron pyrophosphate positive electrode material and 0.2g of conductive carbon black (Super P) were added and homogenized to obtain a positive electrode slurry with a solid content of 10%. The positive electrode slurry was coated onto aluminum foil using a 100μm scraper and dried in an 80℃ forced-air oven for 3 hours, then transferred to a 120℃ vacuum oven for 24 hours. The resulting positive electrode was punched into 9mm diameter sheets using a punching machine in a drying room with a dew point of -50℃. Finally, CR2032 coin cells were assembled using the above-mentioned positive electrode sheets as the positive electrode, a composite sodium sheet as the negative electrode, and glass fiber as the separator under an Ar atmosphere in a glove box.

[0102] Performance testing: The performance of the cathode materials and sodium-ion batteries prepared by Examples 1-9 and Comparative Example 1 was tested, as follows: Morphology testing: Scanning electron microscopy (SEM) projects high-energy electrons onto the sample and then uses secondary electron imaging to obtain surface image information, thereby analyzing the morphological characteristics of the sample. Hitachi High Technologies SU8020 high-resolution field emission scanning electron microscope was used.

[0103] The morphology test results of the cathode materials prepared in Example 1 and Comparative Example 1 are shown in the figure. Figure 1 As shown. The S,F-doped carbon-sodium sulfide coated sodium iron pyrophosphate NFPP cathode material (PLA-NFPP) obtained in Example 1 has good surface morphology and shows no significant morphological change compared to the untreated material (NFPP).

[0104] Electrical performance testing: (1) Charge / discharge specific capacity test: After the assembled button cell battery was left to stand for 24 hours, the first charge and discharge test was carried out on the Xinwei battery system. The test steps are as follows: ① Stand for 2 hours; ② Constant current charging, cutoff voltage is 4.3V, current density is 12mAh / g; ③ Stand for 2 hours; ④ Constant current discharging, current density is 12mAh / g, cutoff voltage is 1.7V.

[0105] (2) Cyclic Test: After the assembled button cells were left to stand for 24 hours, a cyclic test was performed on the Xinwei Battery System. The test steps were as follows: ① Stand for 30 minutes; ② Constant current charging, cutoff voltage 4.3V, current density 240mAh / g; ③ Stand for 30 minutes; ④ Constant current discharging, current density 240mAh / g, cutoff voltage 1.7V; ⑤ Repeat steps ① to ④ 200 times. The discharge specific capacity of the first cycle was recorded as C0, the discharge specific capacity of 200 cycles was recorded as C1, and the capacity retention rate was calculated as (C1 / C0)×100%.

[0106] (3) Rate test: After the assembled button cells were left to stand for 24 hours, a cycle test was performed on the Xinwei battery system. The test steps were as follows: ① Stand for 30 minutes; ② Constant current charging, cutoff voltage of 4.3V, current density of 12mAh / g; ③ Stand for 30 minutes; ④ Constant current discharging, current density of 12mAh / g, cutoff voltage of 1.7V; ⑤ Repeat steps ① to ④ 5 times. Repeat the above test steps ① to ⑤, and set the test current density to 24mAh / g, 48mAh / g, 120mAh / g, 240mAh / g, 480mAh / g, 960mAh / g, 1920mAh / g, and 3840mAh / g respectively. The rate retention rate can be obtained by calculating the ratio of the average discharge specific capacity at a current density of 1920mAh / g for 5 times to the average discharge specific capacity at an initial current density of 12mAh / g for 5 times.

[0107] (4) Impedance test: After the assembled button cell was left to stand for 24 hours, an AC impedance test was performed on the Chenhua Electrochemical Workstation. The test frequency was 0.1~1MHz and the amplitude of the sine wave was 5mV.

[0108] The test results are shown in Tables 1 and 2 below. Figures 2-5 As shown.

[0109] Table 1. Charge-discharge test and cycle test data for each battery group. Through Table 1, Figures 2-3As can be seen, the modified cathode material of the present invention exhibits improved discharge and charge specific capacities, indicating that the coating layer with doped elements solves the problem of poor intrinsic conductivity of the cathode material, thereby improving its capacity. Thanks to the presence of the coating layer, side reactions between the cathode material and the electrolyte are suppressed during cycling, thus improving the cycling performance of the cathode material.

[0110] By changing the type of coating agent, coating layers of different thicknesses and compositions were obtained. As shown in Examples 1-3, when a coating agent with excessive carbon content is used, the resulting coating layer contains too much SF-doped carbon material, resulting in a decrease in the specific capacity of the first cycle; when a coating agent with less sulfur content is used, the proportion of Na2S in the resulting coating layer decreases, leading to a deterioration in cycling performance.

[0111] By changing the mass ratio of coating agent to sodium iron pyrophosphate, the coating reaction time, and the intensity of the reaction, coating layers of different thicknesses were obtained. Examples 5 and 1 show that excessive coating time and excessive coating agent result in a thicker coating layer, hindering sodium ion transport and reducing the initial specific capacity. Examples 6 and 1 show that excessively short coating time results in a thinner coating layer, failing to adequately protect the material during cycling and leading to poor cycling performance. Examples 5 and 7 show that, with the same coating layer thickness and composition, appropriately increasing the heating temperature and glow discharge power during doping can promote the entry of impurities into amorphous carbon, enhancing the stability of the coating layer and further improving cycling stability. Examples 4 and 9 show that, by simultaneously changing the coating layer thickness and composition, and adjusting the glow discharge power, plasma treatment time, and temperature during doping, the material's initial discharge performance and 200-cycle performance exhibit significant differences.

[0112] Table 2. Rate test and impedance test data for each group of batteries. Through Table 2, Figures 4-5 It can be seen that the modified cathode material of the present invention has improved rate retention and reduced impedance value, indicating that the coating layer with doped elements has successfully solved the problem of residual alkali on the surface of the cathode material. The coating layer formed by the reaction reduces the residual alkali content on the surface, thereby reducing the impedance value. At the same time, due to the presence of the coating layer of sodium ion fast conductor, sodium ions can still be transported quickly under high current density, resulting in high discharge specific capacity.

[0113] By varying the mass ratio of the coating agent to sodium iron pyrophosphate, the coating reaction time, and the intensity of the reaction, coating layers of different thicknesses were obtained. Examples 1-3 show that using a coating agent with excessive carbon content results in an excessive mass proportion of SF-doped carbon material in the coating layer; using a coating agent with less sulfur content reduces the proportion of Na₂S in the resulting coating layer. At high current densities, the rate performance deteriorates due to the reduction in the thin layer of fast sodium ion conductor Na₂S. Examples 5 and 1 show that excessive coating time and excessive coating agent content result in a thicker coating layer, hindering electron and sodium ion transport and leading to higher battery impedance. Examples 6 and 1 show that excessively short coating time results in a thinner coating layer; at high current densities, sodium ion transport is suppressed, leading to poorer rate performance. As shown in Examples 5 and 7, when the coating layer thickness and composition are the same, appropriately increasing the heating temperature and glow discharge power during doping can promote the entry of impurity elements into amorphous carbon, enhance the electronic conductivity of the coating layer, and further reduce the battery impedance. As shown in Examples 4 and 9, simultaneously changing the coating layer thickness and composition, doping power, time, and temperature results in significant differences in the material rate performance and impedance.

[0114] The parts of this invention not described in detail are techniques known to those skilled in the art.

[0115] The basic principles of the present invention have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in the present invention are merely examples and not limitations, and should not be considered as essential features of each embodiment of the present invention. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the present invention to the necessity of employing the aforementioned specific details.

[0116] In the foregoing description of this specification, references to terms such as "one embodiment," "another embodiment," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment is included in at least one embodiment of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples, without contradiction. Additionally, it should be noted that in this specification, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features.

[0117] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode material, characterized in that, include: The substrate and the covering layer covering at least a portion of the surface of the substrate; The matrix comprises sodium iron pyrophosphate; The coating layer comprises carbon material and sodium sulfide; the carbon material comprises doping elements; the doping elements comprise S and F.

2. The cathode material according to claim 1, characterized in that, The general chemical formula of the sodium iron pyrophosphate is Na4Fe3(PO4)2P2O7; And / or, the particle size of the sodium ferric pyrophosphate is 3.0 μm to 7.5 μm.

3. The cathode material according to claim 1, characterized in that, The thickness of the coating layer is 2nm~7nm; And / or, the sodium sulfide in the coating layer accounts for 10% to 45% by mass; And / or, the carbon material in the coating layer accounts for 55% to 90% of the total mass.

4. The cathode material according to claim 1, characterized in that, The carbon material includes amorphous carbon; the S element reconstructs the charge distribution near the defects of the amorphous carbon to form SO and CSC functional groups, and the F element reconstructs the charge distribution near the defects of the amorphous carbon to form CF and FCF functional groups.

5. A method for preparing the cathode material according to any one of claims 1 to 4, characterized in that, Includes the following steps: Sodium iron pyrophosphate and a coating agent were subjected to a first plasma treatment in an inert atmosphere; then, the inert atmosphere was replaced with sulfur hexafluoride gas for a second plasma treatment to obtain the cathode material.

6. The preparation method according to claim 5, characterized in that, The preparation method satisfies at least one of the following features (1) to (4): (1) The coating agent includes at least one of carbon disulfide, carbon trisulfide, dodecyl mercaptan, and carbon dichlorosulfide; (2) The inert atmosphere includes at least one of argon atmosphere, nitrogen atmosphere, and helium atmosphere; (3) The mass ratio of sodium iron pyrophosphate to the coating agent is (2~10):(15~35); (4) The first plasma treatment process includes: Sodium iron pyrophosphate was sealed in a quartz tube filled with an inert atmosphere; Seal the coating agent in the beaker; then connect the sealed beaker to the plasma argon gas line; The sealed quartz tube is placed into the plasma generating chamber. The flow rate of argon gas in the plasma argon gas path is adjusted so that the coating agent enters the quartz tube with the argon gas. After a certain period of time, the quartz tube is filled with the coating agent. The first plasma treatment is performed under the first vacuum condition through a glow emitter.

7. The preparation method according to claim 6, characterized in that, The preparation method satisfies at least one of the following features (1) to (6): (1) The beaker is sealed with a flange stop valve and a flange; (2) The argon flow rate in the plasma argon gas path is adjusted to 20 sccm~80 sccm; (3) The time for the inlet is 1 min to 10 min; (4) The vacuum pressure of the first vacuum condition is -0.1×10 -3 Pa~50Pa; (5) The glow discharge power of the first plasma treatment is 100W~500W; (6) The time for the first plasma treatment is 30s to 5min.

8. The preparation method according to claim 5, characterized in that, The preparation method satisfies at least one of the following features (1) to (3): (1) The flow rate of the sulfur hexafluoride gas is 15 sccm to 50 sccm; (2) The sulfur hexafluoride gas is introduced for 5 min to 15 min; (3) The second plasma treatment process includes: Sulfur hexafluoride gas is introduced to completely remove the coating agent, and a second plasma treatment is performed under a second vacuum condition using a glow emitter.

9. The preparation method according to claim 8, characterized in that, The preparation method satisfies at least one of the following features (1) to (4): (1) The vacuum pressure of the second vacuum condition is -0.1×10 -3 Pa ~ 20 Pa; (2) The glow discharge power of the second plasma treatment is 400W~800W; (3) The temperature of the second plasma treatment is 260℃~530℃, and the heating rate is 5℃ / min~20℃ / min; (4) The second plasma treatment time is 5 min to 15 min.

10. A battery, characterized in that, The positive electrode includes a positive electrode sheet; the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector along the thickness direction, the positive active material layer including the positive electrode material according to any one of claims 1 to 4 or the positive electrode material prepared by the preparation method according to any one of claims 5 to 9.

Images