Sodium-ion battery layered cathode material and application

Abstract

The application discloses a sodium ion battery layered positive electrode material and application, the layered positive electrode material, including layered positive electrode active material and the basic coating layer of coating in the outer surface of layered positive electrode active material, the basic coating layer is at least one of NaOH, NaO, Na2CO3, NaHCO3. The basic coating layer can effectively enhance the air stability of the positive electrode material, prevent the performance deterioration caused by the loss of sodium in the bulk phase, and can improve the storage time of the positive electrode material in the air environment. In addition, the application is sprayed by oxalic acid solution in the atmosphere rotary furnace and reacts with the basic coating layer, which can convert the basic coating layer into sodium oxalate, reduce the overall alkalinity of the positive electrode material, ensure that the material has good processing performance, and sodium oxalate can decompose in the battery formation stage and provide sodium ions to participate in the generation of the negative electrode SEI film, reduce the consumption of sodium ions in the bulk phase, improve the specific capacity of the positive electrode material, and improve the energy density of the battery.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a layered cathode material for sodium-ion batteries and its application. Background Technology

[0002] The large-scale application of lithium-ion batteries in electric vehicles is rapidly depleting already scarce lithium resources, severely hindering the long-term development of lithium-ion batteries. Sodium, on the other hand, is abundant in the Earth's crust, inexpensive, and has high energy density. Furthermore, sodium and lithium belong to the same group of elements, sharing similar physicochemical properties and working principles. In the long run, sodium-ion batteries hold promise as a beneficial supplement to lithium-ion batteries. Among these, layered cathode materials for sodium-ion batteries exhibit higher specific capacity and energy density compared to other cathode materials, making them one of the most promising choices for the industrialization of sodium-ion batteries.

[0003] During the high-temperature solid-state sintering process of sodium-ion battery layered cathode materials, after sodium salts and metal oxides form a layered structure through the breaking and recombination of chemical bonds, some sodium salts do not enter the bulk structure of the material but remain on the material surface, forming alkaline substances, resulting in excessive alkalinity of the material. In addition, patent CN 111370664 A and literature NanoLett.2019,19(1),182-188 both indicate that sodium-ion battery layered cathode materials have poor air stability. When the material is stored in an air environment containing water and carbon dioxide, sodium in the bulk phase is easily released to form alkaline substances such as sodium carbonate and sodium hydroxide on the particle surface, which reduces the capacity of the cathode material itself. This makes it necessary to store such cathode materials in a vacuum or protective atmosphere environment, resulting in high storage costs. In addition, during the preparation of the positive electrode slurry, alkaline substances on the surface of the main material will attack the fluorine-containing PVDF binder in the positive electrode adhesive, remove HF and generate unstable double bonds, which will further crosslink with the active sites on the surface of the positive electrode active material particles, eventually forming a gel. This will affect the coating consistency and even cause the slurry to be scrapped and unable to be coated. Furthermore, the damage to PVDF will lead to a significant decrease in the bonding strength of the positive electrode sheet and a deterioration in the cell performance.

[0004] Currently, the main approach to addressing the gelation problem in high-alkalinity sodium-ion battery cathode material slurries is to reduce the residual alkali content of the main material. For example, methods such as water washing, alcohol washing, or acid washing are used to reduce the alkalinity of the main material (e.g., CN 108807069A; CN108878863A). These methods can effectively reduce the residual alkali content. However, such solutions often involve prolonged immersion and stirring of the cathode material in solution, followed by multiple washes. This can lead to the dissolution of certain ions from the surface of the cathode active material, damaging its surface structure. Furthermore, a small amount of residual water molecules can diffuse into the particles, making them difficult to remove. In addition, while water washing, alcohol washing, or acid washing can remove residual alkali from the material surface, it inevitably results in the loss of sodium from the bulk phase, causing a loss of cathode material capacity and affecting battery capacity consistency.

[0005] Therefore, for layered oxide cathode materials in sodium-ion batteries, it is of great significance to develop a method that can both improve the air stability of layered cathode materials and increase battery energy density by supplementing sodium. Summary of the Invention

[0006] In view of the problems existing in the related technologies, the purpose of this invention is to provide a layered cathode material for sodium-ion batteries and its application, which can improve the air stability of the layered cathode material and increase the battery energy density by adding sodium.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] According to a first aspect of the present invention, a layered cathode material for sodium-ion batteries is provided that can be stably stored in air for a long period of time and prevent the loss of bulk sodium.

[0009] The present invention discloses a layered cathode material for sodium-ion batteries, comprising a layered cathode active material and an alkaline coating layer covering the outer surface of the layered cathode active material, wherein the alkaline coating layer is at least one selected from NaOH, NaO, Na2CO3, and NaHCO3.

[0010] Preferably, the layered positive electrode active material is an O3, P2, or P3 phase layered positive electrode active material with the chemical formula Na. x MO y Where M is one or more elements selected from Ni, Cu, Mn, Fe, Zn, Co, Al, Cr, Zr, and Mo; x and y satisfy charge balance; 0.5 < x < 1.5; y ≥ 2; and the elements in the chemical formula satisfy charge balance.

[0011] Preferably, the thickness of the alkaline coating layer is 10-100 nm.

[0012] Preferably, the pH of the layered cathode material is 12.5.

[0013] According to a second aspect of the invention, a method is provided for removing the alkaline coating layer of a layered cathode material for a sodium-ion battery and converting it into a sodium-replenishing additive.

[0014] This invention discloses a method for removing the alkaline coating layer of a layered cathode material for a sodium-ion battery and converting it into a sodium-replenishing additive, comprising the following steps:

[0015] S1. Place the layered cathode material with alkaline coating to be treated into an atmosphere rotary furnace, heat it to 60°C at a heating rate of 2°C / min and hold it at that temperature.

[0016] S2. Then, spray a 20mM-100mM oxalic acid solution into the furnace at a constant flow rate for 2-6 hours. After the reaction is complete, remove the oxalic acid spray device and introduce nitrogen gas at a constant flow rate.

[0017] S3. Heat the atmosphere rotary furnace to 150°C at a heating rate of 2°C / min and hold for 1-2 hours to remove moisture and excess oxalic acid from the furnace. The reaction is complete when the outflowing gas is no longer acidic. Cool to room temperature to obtain the processed layered cathode material.

[0018] Preferably, in step S2, the constant flow rate is 2 mL / min to 10 mL / min.

[0019] Preferably, in step S2, the mass ratio of oxalic acid in the oxalic acid solution to the mass of the layered cathode material is 0.005-0.05.

[0020] Preferably, in step S3, the pH of the treated layered cathode material is 12.

[0021] According to a third aspect of the present invention, a sodium-ion battery cathode material containing a sodium-supplementing additive is provided.

[0022] The present invention discloses a sodium-ion battery cathode material, which is obtained by removing the alkaline coating layer of the aforementioned sodium-ion battery layered cathode material and converting it into a sodium-replenishing additive.

[0023] According to a fourth aspect of the present invention, a high-energy-density sodium-ion battery containing a sodium-supplemented additive cathode material is provided.

[0024] The present invention provides a sodium-ion battery, wherein the sodium-ion battery includes the aforementioned sodium-ion battery positive electrode material.

[0025] The technical solution provided by this invention can achieve the following beneficial effects:

[0026] 1) The present invention coats the surface of the layered cathode material of sodium-ion battery with an amorphous and dense alkaline coating layer of NaOH, NaO, Na2CO3 or NaHCO3. The alkaline coating layer can act as a physical barrier to isolate air, effectively enhance the air stability of the cathode material, prevent sodium loss in the bulk phase from causing performance deterioration, and increase the storage time of the cathode material in ordinary air environment without the need for storage under special conditions.

[0027] 2) This invention involves spraying oxalic acid solution into a rotary furnace to react with the alkaline coating layer on the surface of the layered oxide cathode material of a sodium-ion battery. This reaction transforms the alkaline coating layer into sodium oxalate (Na2C2O4), reducing the overall alkalinity of the cathode material, ensuring good processing performance, and simplifying the process. Furthermore, the generated sodium oxalate remains in the cathode material. When the formation voltage is higher than the decomposition potential of sodium oxalate, the sodium oxalate can decompose during the battery formation stage and provide sodium ions to participate in the formation of the SEI film on the negative electrode, reducing the consumption of sodium ions in the bulk phase, improving the specific capacity of the cathode material, and increasing the energy density of the battery.

[0028] 3) The oxalic acid spraying method provided by this invention is simple, has low energy consumption, and high processing efficiency. The cathode material does not need to be soaked in water / acid solution for a long time, and it has no effect on the bulk structure of the cathode material. It can effectively convert all the alkaline coating layer into sodium supplementation additive. Moreover, the production process of this method is clean and environmentally friendly. Excess oxalic acid solution sprayed in can be sublimated and recovered, and no waste liquid is generated. At the same time, it can avoid the adverse effects of oxalic acid residue in the cathode material on subsequent processing and battery performance. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of the method of removing the alkaline coating layer of the layered cathode material and converting it into a sodium-supplementing additive according to the present invention.

[0031] Figure 2 The image shows the XRD pattern of the layered cathode material containing an alkaline coating, as presented in this invention.

[0032] Figure 3 The XRD pattern of the layered cathode material with the alkaline coating removed according to the present invention.

[0033] Figure 4 This is one of the SEM images of the layered cathode material containing an alkaline coating layer according to the present invention.

[0034] Figure 5This is the second SEM image of the layered cathode material containing an alkaline coating layer of the present invention.

[0035] Figure 6 This is one of the SEM images of the layered cathode material with the alkaline coating removed according to the present invention.

[0036] Figure 7 This is the second SEM image of the layered cathode material with the alkaline coating removed according to the present invention. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0038] According to a first aspect of the present invention, the present invention provides a layered cathode material for sodium-ion batteries, comprising a layered cathode active material and an alkaline coating layer covering the outer surface of the layered cathode active material, wherein the alkaline coating layer is at least one of amorphous and dense NaOH, NaO, Na2CO3, and NaHCO3.

[0039] The alkaline coating does not change the surface morphology and crystal structure of the layered cathode material. Moreover, the alkaline coating can act as a physical barrier to effectively prevent H2O and CO2 in the air from corroding the surface of the layered cathode material, thereby improving the air stability of the layered cathode material and enhancing its storage capacity in the air environment.

[0040] Among them, the alkaline coating method can be solid phase coating method, liquid phase coating method or gas phase coating method; solid phase coating method includes mechanochemical method, solid phase reaction method, high energy method, polymer encapsulation method, microcapsule modification method; liquid phase coating method includes hydrothermal method, sol-gel method, precipitation method, heterogeneous nucleation method, chemical plating method, microemulsion method, heterogeneous flocculation method; gas phase coating method includes physical vapor deposition and chemical vapor deposition.

[0041] The alkaline coating layer on the surface of the layered positive electrode active material is preferably formed by a solid-phase reaction method, which specifically includes the following steps:

[0042] 1) Mix the salt solutions of sodium salt and metal salt to obtain a precursor mixture, wherein the sodium salt and metal salt are added according to the atomic molar ratio of the layered positive electrode active material, and then an excess of sodium salt is added and mixed evenly.

[0043] 2) The above precursor mixture was reacted at a pressure of 5-50 MPa and a temperature of 120-200℃ for 5-20 h to obtain the product;

[0044] 3) The sodium-ion battery layered cathode material is obtained by calcining the product.

[0045] The metal salt is at least one of the sulfate, chloride, nitrate, and carbonate salts of the metal elements Ni, Cu, Mn, Fe, Zn, Co, Al, Cr, Zr, and Mo.

[0046] The excess sodium salt added cannot enter the bulk phase of the positive electrode active material and forms a coating layer on the outside of the positive electrode active material particles.

[0047] In one embodiment of the present invention, the layered positive electrode active material is an O3, P2, or P3 phase layered positive electrode active material with the chemical formula Na. x MO y Where M is one or more elements selected from Ni, Cu, Mn, Fe, Zn, Co, Al, Cr, Zr, and Mo; x and y satisfy charge balance; 0.5 < x < 1.5; y ≥ 2; and the elements in the chemical formula satisfy charge balance.

[0048] In one embodiment of the present invention, the thickness of the alkaline coating layer is 10-100 nm, preferably 40-80 nm, and more preferably 60 nm.

[0049] In one embodiment of the invention, the pH of the layered cathode material is 12.5.

[0050] According to a second aspect of the invention, a method is provided for removing the alkaline coating layer of a layered cathode material for a sodium-ion battery and converting it into a sodium-replenishing additive.

[0051] This invention discloses a method for removing the alkaline coating layer of a layered cathode material for a sodium-ion battery and converting it into a sodium-replenishing additive, comprising the following steps:

[0052] S1. Place the layered cathode material with alkaline coating to be treated into an atmosphere rotary furnace, heat it to 60°C at a heating rate of 2°C / min and hold it at that temperature.

[0053] S2. Then, spray a 20mM-100mM oxalic acid solution into the furnace at a constant flow rate for 2-6 hours. After the reaction is completed, remove the oxalic acid spray device and introduce nitrogen gas at a constant flow rate. The reaction time can be determined according to the content of the alkaline coating material in the layered cathode material and the corresponding oxalic acid solution spray concentration.

[0054] S3. Heat the atmosphere rotary furnace to 150°C at a heating rate of 2°C / min and hold for 1-2 hours to remove moisture and excess oxalic acid from the furnace. The reaction is complete when the outflowing gas is no longer acidic. Cool to room temperature to obtain the processed layered cathode material.

[0055] In this process, a weakly acidic oxalic acid solution spray reacts with the alkaline coating layer on the surface of the layered cathode material to neutralize the alkaline substances on the surface of the layered cathode material particles. The reaction product is sodium oxalate. After the reaction is completed, the residual oxalic acid in the furnace sublimates and volatilizes (oxalic acid sublimates rapidly at 125℃ and in large quantities at 150℃), and enters the oxalic acid aqueous solution collection bottle with the nitrogen gas flow.

[0056] This method converts the alkaline substances coated on the surface of the layered cathode material into sodium oxalate through the reaction of oxalic acid. The sodium oxalate remains in the cathode material and can decompose during the battery formation stage, providing sodium ions to participate in the formation of the SEI film of the anode, reducing the consumption of sodium ions in the bulk phase, improving the specific capacity of the cathode material, and increasing the energy density of the battery.

[0057] In one embodiment of the present invention, in step S2, the constant flow rate is 2 mL / min-10 mL / min, preferably 5-8 mL / min.

[0058] In one embodiment of the present invention, in step S2, the mass ratio of oxalic acid in the oxalic acid solution to the mass of the layered cathode material is 0.005-0.05, preferably 0.01-0.02.

[0059] In one embodiment of the present invention, in step S3, the pH of the treated layered cathode material is 12.

[0060] According to a third aspect of the present invention, a sodium-ion battery cathode material containing a sodium-supplementing additive is provided.

[0061] The present invention discloses a sodium-ion battery cathode material, which is obtained by removing the alkaline coating layer of the aforementioned sodium-ion battery layered cathode material and converting it into a sodium-replenishing additive.

[0062] According to a fourth aspect of the present invention, a high-energy-density sodium-ion battery containing a sodium-supplemented additive cathode material is provided.

[0063] This invention discloses a sodium-ion battery, comprising the aforementioned sodium-ion battery positive electrode material. The invention converts the alkaline coating layer into sodium oxalate, a sodium-supplementing additive. Sodium oxalate decomposes during the battery formation stage and provides sodium ions to participate in the formation of the SEI film on the negative electrode, reducing sodium ion consumption in the bulk phase, improving the specific capacity of the positive electrode material, and increasing the battery's energy density.

[0064] As an example, a sodium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes, to obtain a battery cell. Alternatively, the battery cell can be obtained by winding the electrodes. The battery cell is then placed in a packaging shell, filled with electrolyte, and sealed to obtain a sodium-ion battery.

[0065] The positive electrode includes a positive current collector and a positive active material layer stacked on the positive current collector, the positive active material layer including the aforementioned positive electrode material. For example, the positive current collector includes two opposing surfaces, and the positive active material layer is stacked on either or both of the two surfaces of the positive current collector.

[0066] The positive electrode active material layer may also include a binder and / or a conductive agent. There are no specific limitations on the types of binders and conductive agents; those skilled in the art can select them according to actual needs. For example, the binder used for the positive electrode sheet may be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), and polyvinyl alcohol (PVA); the conductive agent may be one or more of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon quantum dots, carbon nanotubes, graphene, and carbon nanofibers.

[0067] The positive electrode current collector can be made of metal foil, carbon-coated metal foil, or porous metal plate, with aluminum foil being the preferred material.

[0068] The above-mentioned positive electrode sheet can be prepared according to conventional methods in the art. Typically, the positive electrode material and optional conductive agent and binder are dispersed in a solvent (e.g., N-methylpyrrolidone, abbreviated as NMP) to form a uniform positive electrode slurry. The positive electrode slurry is coated on the positive electrode current collector, and after drying, rolling and other processes, the positive electrode sheet is obtained.

[0069] The negative electrode can be a sodium metal sheet, or it can include a negative current collector and a layer of negative active material stacked on the negative current collector. For example, the negative current collector has two opposing surfaces, and the negative active material layer is stacked on either or both of the two surfaces of the negative current collector.

[0070] The negative electrode active material layer typically includes the negative electrode active material and optional conductive agents, binders, and thickeners.

[0071] As an example, the negative electrode active material can be one or more of natural graphite, artificial graphite, mesophase micro carbon spheres (MCMB), hard carbon and soft carbon, preferably hard carbon.

[0072] The conductive agent can be one or more of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon quantum dots, carbon nanotubes, graphene, and carbon nanofibers.

[0073] The binder may be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, and carboxymethyl cellulose (CMC), and the thickener may be carboxymethyl cellulose (CMC).

[0074] However, the present invention is not limited to the materials mentioned above. Other materials that can be used as active materials, conductive agents, binders, or thickeners for sodium-ion batteries can also be used.

[0075] The negative electrode current collector can be made of materials such as metal foil, carbon-coated metal foil, or porous metal plate, with aluminum foil being the preferred material.

[0076] The above-mentioned negative electrode sheet can be prepared according to conventional methods in this field. Typically, the negative electrode active material and optional conductive agent, binder and thickener are dispersed in a solvent, which can be deionized water, to form a uniform negative electrode slurry. The negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet is obtained.

[0077] The sodium-ion battery provided by this invention does not have any particular restrictions on the separator. Any well-known porous structure separator with electrochemical and chemical stability can be selected, such as one or more single-layer or multi-layer films selected from glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride.

[0078] The sodium-ion battery provided by this invention uses an electrolyte that comprises an organic solvent and a sodium electrolyte salt. As an example, the organic solvent may be one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC); the sodium electrolyte salt may be one or more of NaPF6, NaClO4, NaBCl4, NaSO3CF3, and Na(CH3)C6H4SO3.

[0079] The following embodiments describe the disclosure of this invention in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of this disclosure. Furthermore, all reagents used in the embodiments are commercially available or synthesized using conventional methods and can be used directly without further processing, as are the instruments used in the embodiments.

[0080] Example

[0081] Preparation of layered cathode materials with alkaline coating :

[0082] 1) Sodium carbonate (Na2CO3), copper sulfate (CuSO4), ferric chloride (FeCl3), and manganese carbonate (MnCO3) are mixed in a molar ratio of Na2CO3:CuSO4:FeCl3:MnCO3 = 0.45:0.22:0.30:0.48. Then, excess Na2CO3 is added and the mixture is stirred until homogeneous to obtain the precursor mixture.

[0083] 2) The above precursor mixture was reacted at a pressure of 20 MPa and a temperature of 180 °C for 10 h to obtain the product;

[0084] 3) The product is calcined at 800°C for 8 hours to obtain a layered cathode material with an alkaline coating.

[0085] The XRD pattern of the prepared layered cathode material is as follows: Figure 2 As shown; the SEM image of the prepared layered cathode material is as follows. Figures 4-5 As shown in the figure, a distinct coating layer can be observed on the surface of the layered cathode material.

[0086] Furthermore, the prepared layered cathode material has a pH of 12.5. After being exposed to air for 15 days, the pH of the layered cathode material remained at 12.5, indicating that the coating layer effectively isolates the air and prevents the layered cathode material from reacting with water and carbon dioxide in the air, thus effectively increasing the storage time of the layered cathode material in the air without the need for special protection measures.

[0087] Removal of alkaline coating and preparation of cathode materials containing sodium-supplementing additives :

[0088] S1, see also Figure 1 500g of sodium-ion battery layered cathode material with sodium carbonate coating to be processed... 0.9 Cu 0.22 Fe 0.30 Mn 0.48 O2 (pH=12.5) is placed in the atmosphere rotary furnace, and the atmosphere rotary furnace rotates at a constant speed to heat up to 60°C at a heating rate of 2°C / min.

[0089] S2. At 60℃, a 50mM oxalic acid solution is sprayed into the furnace at a constant speed of 8mL / min for 2 hours. After the reaction is completed, the oxalic acid spraying device is removed and nitrogen gas is introduced at a constant speed.

[0090] S3. Heat the atmosphere rotary furnace to 150°C at a heating rate of 2°C / min and hold it at that temperature to remove moisture and excess oxalic acid from the furnace until the outflowing gas is no longer acidic, at which point the reaction is complete. Cool the furnace to room temperature to obtain the processed layered cathode material.

[0091] The XRD pattern of the processed layered cathode material is shown below. Figure 3 As shown, the crystal structure of the cathode material remained unchanged after oxalic acid spray treatment, indicating that by controlling the concentration, flow rate, and reaction time of the oxalic acid spray, the reaction of removing the surface alkaline coating layer by oxalic acid spray in a rotary kiln can be carried out without damaging the bulk phase of the cathode material. Furthermore, the XRD diffraction peaks of sodium oxalate were newly added in the spectrum, indicating that the surface-coated sodium carbonate reacted and transformed into sodium oxalate. The SEM spectrum of the obtained cathode material is shown below. Figures 6-7 As shown, observations show that after oxalic acid spray treatment, the surface of the cathode material becomes smooth, the surface coating is removed, and there are no signs of damage to the bulk phase of the cathode material; moreover, the pH of the cathode material decreases to 12.0.

[0092] Preparation of sodium-ion batteries

[0093] The prepared layered cathode material was used as the cathode material. The conductive agents were acetylene black (SP), graphene, carbon quantum dots, and carbon nanotubes (CNTs), and the binder was polyvinylidene fluoride (PVDF). The weight ratio of each component was cathode material:SP:CNT:graphene:carbon quantum dots:PVDF = 95:1.5:0.5:0.5:0.5:2. The solvent was N-methylpyrrolidone (NMP), and the solid content of the cathode slurry was 55%. The specific operation was as follows: PVDF was added; the conductive agent was added, and the mixture was stirred at high speed of 1500 rpm for 30-60 minutes to allow it to separate as much as possible. The slurry is evenly dispersed; the positive electrode material is added in 2-3 batches and stirred at high speed of 1500 rpm for 60-120 minutes until the slurry is evenly dispersed; the speed is reduced to 500 rpm and vacuum is applied for about 30-60 minutes to remove air bubbles from the slurry; the viscosity of the slurry is adjusted to the range of 6000-10000 mPa·s; the slurry is filtered through a 150-mesh sieve to obtain the final slurry; then the slurry is evenly coated on carbon-coated aluminum foil using a 200 μm scraper, dried, and sliced ​​to obtain the positive electrode sheet; the positive electrode sheet is combined with a hard carbon negative electrode, a separator, and an electrolyte to make a full battery, and its charge and discharge capacity is tested.

[0094] Comparative Example

[0095] The cathode material used is a conventional uncoated cathode material. The prepared cathode material has a pH of 12.2. After being exposed to air for 15 days, the pH of the cathode material increased to 12.8, indicating that alkaline substances stored in the cathode material were released in the air. The cathode material was also made into a slurry and cathode sheet, and then combined with a hard carbon anode, separator and electrolyte to make a full battery. Its charge and discharge capacity was then tested.

[0096] Performance tests were conducted on the embodiments and comparative examples, and the test results are as follows:

[0097] The slurry and electrode conditions of the examples and comparative examples are shown in the table below (viscosity unit: mPa·s):

[0098] Example 7865 8025 9066 good Comparative Example 13656 38689 gel Scratches appeared

[0099] As shown in the table above, the comparative slurry has a high residual alkali content, resulting in high viscosity and gelation upon discharge, making it prone to scratches during coating. The gelation worsens after standing for a period of time. In contrast, the slurry in the example has suitable flowability, normal viscosity, and remains stable after 24 hours of standing, with good coating condition on the electrode sheets.

[0100] The full-cell charge-discharge test data for the examples and comparative examples are shown in the table below (capacity unit: mAh):

[0101]

[0102] As can be seen from the table above, the battery discharge capacity of the embodiment of the present invention is effectively improved, and the first efficiency and stability are also improved. This is because sodium oxalate can decompose during the battery formation stage and provide sodium ions to participate in the formation of the negative electrode SEI film, reduce the consumption of sodium ions in the bulk phase, improve the specific capacity of the positive electrode material, and improve the battery energy density.

[0103] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments described above, and any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. A sodium-ion battery cathode material, characterized in that: The sodium-ion battery layered cathode material is obtained through processing. The layered cathode material comprises a layered cathode active material and an amorphous, dense alkaline coating layer covering the outer surface of the layered cathode active material. The alkaline coating layer is at least one of NaOH, NaO, Na2CO3, and NaHCO3, and is used to improve the air stability of the sodium-ion battery layered cathode material. The alkaline coating layer is applied to the surface of the layered cathode active material using a solid-phase reaction method, a liquid-phase coating method, or a gas-phase coating method. The processing of the sodium-ion battery layered cathode material involves removing the alkaline coating layer and converting it into a sodium-replenishing additive, including the following steps: S1. Place the sodium-ion battery layered cathode material with alkaline coating to be processed into an atmosphere rotary furnace, heat it to 60°C at a heating rate of 2°C / min and keep it at that temperature. S2. Then, spray a 20mM-100mM oxalic acid solution into the furnace at a constant flow rate for 2-6 hours. After the reaction is complete, remove the oxalic acid spray device and introduce nitrogen gas at a constant flow rate. S3. Heat the atmosphere rotary furnace to 150°C at a heating rate of 2°C / min and hold for 1-2 hours to remove moisture and excess oxalic acid from the furnace. The reaction is complete when the outflowing gas is no longer acidic. Cool to room temperature to obtain the processed layered cathode material.

2. The sodium-ion battery cathode material according to claim 1, characterized in that: The layered positive electrode active material is an O3, P2, or P3 phase layered positive electrode active material with the chemical formula NaxMOy; wherein, M is one or more elements selected from Ni, Cu, Mn, Fe, Zn, Co, Al, Cr, Zr, and Mo; x and y satisfy charge balance; 0.5 < x < 1.5; y ≥ 2; and the elements in the chemical formula satisfy charge balance.

3. The sodium-ion battery cathode material according to claim 1, characterized in that: The thickness of the alkaline coating layer is 10-100 nm.

4. The sodium-ion battery cathode material according to claim 1, characterized in that: The pH of the layered cathode material for the sodium-ion battery is 12.

5.

5. The sodium-ion battery cathode material according to claim 1, characterized in that: In step S2, the constant flow rate is 2 mL / min-10 mL / min.

6. The sodium-ion battery cathode material according to claim 1, characterized in that: In step S2, the mass ratio of oxalic acid in the oxalic acid solution to the mass of the layered cathode material of the sodium-ion battery is 0.005-0.

05.

7. The sodium-ion battery cathode material according to claim 1, characterized in that: In step S3, the pH of the treated layered cathode material is 12.

8. A sodium-ion battery, characterized in that, The sodium-ion battery includes the sodium-ion battery cathode material as described in claim 1, 5, 6, or 7.

Images