Sodium-ion positive electrode material and preparation method thereof

Abstract

This invention provides a method for preparing a sodium-ion cathode material. The method includes: annealing and cooling the sintered material under an active gas and a carrier gas atmosphere; wherein the annealing and cooling time is 0.5–5 h, preferably 2–4 h; the active gas is selected from one or more of ammonia, carbon dioxide, and formaldehyde; the carrier gas is an inert gas, preferably selected from one or more of helium, nitrogen, and argon; and the volume content of the active gas is 1–30%, preferably 1%–20%, based on the total gas volume of the active gas and the carrier gas. This invention also provides the cathode material prepared by the method and its applications. This invention can accelerate the annealing process, reduce the moisture content of the material, and thus reduce the formation of residual alkali.

Description

Technical Field

[0001] This invention belongs to the field of cathode material technology for new energy sodium-ion batteries, specifically relating to a sodium-ion cathode material and its preparation method. Background Technology

[0002] With the rapid development of the new energy industry, lithium-ion batteries play a crucial role in energy storage and power batteries. However, due to limited lithium resources and the small amount of lithium salt reserves in China, their prices have risen rapidly. Sodium-ion batteries, on the other hand, benefit from abundant sodium resources, low manufacturing costs, and compatibility with existing production lines, thus offering a partial alternative. The main cathode materials for sodium-ion batteries include layered oxides, polyanionic compounds, and Prussian blue analogues. Among these, layered oxide materials exhibit relatively high energy density, and their manufacturing process is similar to that of existing ternary materials, making them a promising candidate for future development.

[0003] The high residual alkali content on the surface of sodium ion layered cathode materials may be due to the reaction of the material with oxygen, carbon dioxide, and water in the air when it comes into contact with air, producing stable sodium carbonate and sodium hydroxide. This will have a significant impact on the performance of the battery cell: (1) The generated sodium carbonate and sodium hydroxide will hinder the movement of sodium ions, thereby increasing the impedance of the battery cell; (2) Excessive residual alkali may cause gelation during the slurry mixing stage; (3) The generation of sodium carbonate and sodium hydroxide will inevitably consume some of the sodium in the bulk phase of the material, thereby causing changes in the valence state of the metal elements on the surface of the material and reducing the specific capacity; (4) The high sodium carbonate content will increase during the cycling and storage of the battery cell. Among the existing methods, CN114171737A uses water washing to reduce the residual alkali content on the surface of the material. This method is relatively complex in terms of material preparation process, has a low yield, and will have a certain impact on the internal crystal structure of the material. CN111370664A discloses a method for reacting volatile acidic gas with sodium ion cathode material in an atmosphere rotary kiln. However, strong acidic gas is very likely to cause over-reaction, which will have a significant impact on the performance of the material. Weak acidic gas may only react under high pressure conditions, but it is difficult to achieve the high pressure atmosphere of a rotary kiln.

[0004] Therefore, it is urgent to find a preparation method to reduce the residual alkali content of the cathode material and generate a layered material with good stability in air. Summary of the Invention

[0005] The purpose of this invention is to accelerate the annealing process and reduce the moisture content of the material by simultaneously introducing active gas and carrier gas during high-temperature annealing, thereby reducing the production of residual alkali by reacting with air during the cooling stage and reducing the loss of sodium content in the bulk phase.

[0006] The first aspect of this invention provides a method for preparing a sodium-ion cathode material, the method comprising: annealing and cooling the sintered material under an active gas and a carrier gas atmosphere; wherein the annealing and cooling time is 0.5-5 h, preferably 2-4 h, the active gas is selected from one or more of ammonia, carbon dioxide and formaldehyde, the carrier gas is an inert gas, preferably selected from one or more of helium, nitrogen and argon, and the volume content of the active gas is 1-30%, preferably 1%-20%, based on the total gas volume of the active gas and the carrier gas.

[0007] In one or more embodiments, the method includes the following steps:

[0008] (1) Mix the precursor, sodium salt and dopant evenly, and sinter to obtain a sintered material;

[0009] (2) Anneal the calcined material to room temperature under the atmosphere of the active gas and the carrier gas;

[0010] (3) The sintering material obtained in step (2) is crushed, mixed evenly with the coating material, and subjected to a second sintering treatment to obtain the sodium ion cathode material.

[0011] in,

[0012] The precursor is Ni x Fe y Mn 1-x-y OH2, where x is 0.2-0.4, y is 0.2-0.4, and x+y is 0.4-0.8;

[0013] The sodium salt is selected from one or more of sodium carbonate, sodium hydroxide, sodium oxalate, and sodium citrate;

[0014] The dopant is selected from one or more compounds of Li, Ni, Mg, Cu, Mn, Zn, Co, Ca, Ba, Sr, Al, B, Cr, Zr, Ti, Sn, V, Mo, Ru, Nb, Sb, or Nb;

[0015] The molar ratio of the precursor, sodium salt, and dopant is (0.9-1.2):1:(0.001-0.05), preferably (0.9-1.1):1:(0.003-0.0005).

[0016] The coating material is selected from one or more of phosphoric acid, aluminum oxide, and titanium dioxide;

[0017] The mass of the coating agent is 500-1500 pmm of the total mass of the sintering material.

[0018] In one or more embodiments, the process of obtaining a burning material further includes a heat preservation step.

[0019] In one or more embodiments, in step (1), the heating rate of the sintering process is 2~10℃ / min, preferably 5~10℃ / min.

[0020] In one or more embodiments, in step (1), the sintering temperature is 500~1500℃, preferably 700~1400℃.

[0021] In one or more embodiments, in step (1), the holding time of the sintering treatment is 8 to 24 hours, preferably 10 to 20 hours.

[0022] In one or more embodiments, the temperature of the second sintering treatment is 100~300℃, preferably 150~300℃.

[0023] The present invention also provides a positive electrode sheet, wherein the positive electrode material layer of the positive electrode sheet includes a positive electrode material, a conductive agent and a binder, and the positive electrode material is prepared by the method described in any embodiment of the present invention.

[0024] In one or more embodiments, the conductive agent is selected from one or more of conductive carbon black (SP), carbon fiber (CF), acetylene black, conductive graphite, graphene, carbon nanotubes, and carbon microspheres.

[0025] In one or more embodiments, the binder is selected from one or more of vinylidene fluoride (PVDF), polytetrafluoroethylene, polyvinyl alcohol, polyolefins, styrene-butadiene rubber, fluorinated rubber, polyurethane, and sodium alginate. In one or more embodiments, the mass fraction of the positive electrode material is 90-98 wt%, preferably 90-96 wt%, based on the total mass of the positive electrode material layer.

[0026] In one or more embodiments, the mass fraction of the conductive agent is 0.5 to 5 wt%, preferably 1 to 5 wt%, based on the total mass of the positive electrode material layer.

[0027] In one or more embodiments, the binder has a mass fraction of 0.5 to 5 wt%, preferably 1 to 5 wt%, based on the total mass of the positive electrode material layer.

[0028] The present invention also provides a method for preparing a sodium-ion battery, the method comprising the steps described in any embodiment herein.

[0029] The present invention also provides a sodium-ion battery, wherein the sodium-ion battery contains a positive electrode sheet as described in any embodiment herein or is prepared by the method described in any embodiment herein.

[0030] In one or more embodiments, the sodium-ion battery exhibits a discharge specific capacity ≥121 mAh·g under a charge-discharge test at room temperature and a rate of 4.0~2.0V 0.2C. -1 Preferred concentration: ≥124 mAh·g -1 More preferably ≥127 mAh·g -1 .

[0031] In one or more embodiments, the sodium-ion battery retains a capacity of ≥95%, preferably ≥96%, and more preferably ≥98% after 50 cycles.

[0032] In one or more embodiments, the residual alkali growth of the sodium-ion battery after 7 days is ≤10%, preferably ≤9%, and more preferably ≤8%.

[0033] In one or more embodiments, the total alkaline content of the sodium-ion battery is ≤3.5 at.%, preferably ≤3.1 at.%, and more preferably ≤2.5 at.%.

[0034] The present invention also provides the application of the method as described in any embodiment herein in sodium-ion batteries, or its application in reducing residual alkali content in sodium-ion batteries, reducing residual alkali rebound, increasing discharge capacity and cycle performance. Attached Figure Description

[0035] Figure 1 This is the XRD pattern of the sodium ion cathode material. Detailed Implementation

[0036] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0037] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0038] In this document, the terms “contains,” “includes,” “containing,” and similar terms encompass the meanings of “basically composed of” and “composed of.” For example, when this document discloses “A contains B and C,” “A is basically composed of B and C” and “A is composed of B and C” should be considered as having been disclosed in this document.

[0039] In this document, all features defined in the form of numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0040] Unless otherwise specified, "percentage" in this article refers to mass percentage, and "ratio" refers to mass ratio.

[0041] In this article, "atmosphere" refers to the gaseous environment in which the raw materials or materials are located, which is usually circulating.

[0042] In this document, when describing embodiments or examples, it should be understood that it is not intended to limit the invention to those embodiments or examples. Rather, all alternatives, modifications, and equivalents of the methods and materials described herein are covered within the scope defined by the claims.

[0043] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0044] Through extensive and in-depth research, the inventors discovered that introducing active gas and carrier gas during high-temperature annealing can accelerate the annealing process, reduce the moisture content of the material, reduce the formation of residual alkali by reacting with air during the cooling stage, and reduce the loss of sodium content in the bulk phase.

[0045] Cathode materials and their preparation methods

[0046] The preparation method of positive electrode (active) materials includes sintering, annealing and cooling, and a second sintering. This invention first provides a method for preparing a positive electrode material, the method comprising annealing and cooling the sintered material under an active gas and a carrier gas atmosphere.

[0047] The preparation steps of the cathode material include optional pre-sintering treatment, a first sintering treatment, annealing and cooling, and a second sintering treatment. Typically, the pre-sintering treatment involves uniformly mixing the precursor and dopant and then pre-sintering. This pre-sintering treatment can be performed by heating at a rate of 1–10 °C / min to 400–800 °C and holding at that temperature for 2–10 hours. Alternatively, those skilled in the art may choose not to perform pre-sintering treatment and proceed directly to sintering, depending on the specific needs.

[0048] Sintering is a common process in this field, involving the homogenization of the precursor, sodium salt, and dopant, followed by high-temperature sintering to obtain a high-temperature material, i.e., a sintered material. The high temperature of the sintering process can be achieved through gradual heating, for example, a heating rate of 2–10 °C / min, preferably 5–10 °C / min, and a sintering temperature of 500–1500 °C, preferably 700–1400 °C. In this paper, the precursor can be Ni. x Fe y Mn 1-x-y OH₂, where x is 0.2~0.4, y is 0.2~0.4, and x+y is 0.4~0.8. In some embodiments, the precursor is Ni. 0.33 Fe 0.33 Mn 0.34 (OH)2. The sodium salt can be a sodium-containing inorganic or organic salt commonly used in the art, preferably selected from one or more of sodium carbonate, sodium hydroxide, sodium oxalate, and sodium citrate, with sodium carbonate being the most preferred. The dopant can be a metal oxide or metal complex, preferably selected from one or more compounds of Li, Ni, Mg, Cu, Mn, Zn, Co, Ca, Ba, Sr, Al, B, Cr, Zr, Ti, Sn, V, Mo, Ru, Nb, Sb, or Nb. In an exemplary embodiment, the dopant is alumina. During the mixing process, the molar ratio of the precursor, sodium salt, and dopant is (0.9-1.2):1:(0.001-0.05). In some embodiments, the molar ratio of the precursor, sodium salt, and dopant is (0.9-1.1):1:(0.003-0.0005). Furthermore, after the first sintering treatment, a heat preservation process can be performed for 8-24 hours, preferably 10-20 hours.

[0049] The annealing cooling step refers to cooling down after the holding time in the sintering process. The annealing cooling time can be 0.5~5h, preferably 2~4h. The inventors' experiments show that if the annealing cooling time is longer than the cooling time of this invention, it will increase the residual alkali content on the material surface. It should be understood that cooling refers to lowering the temperature to a temperature suitable for production or the next process step, usually referring to lowering the temperature to room temperature. In this article, the annealing cooling method includes transferring the material to an atmosphere, preferably transferring the high-temperature material to an atmosphere of active gas and carrier gas. It should be understood that the annealing cooling step involves first introducing active gas and carrier gas, and then transferring the high-temperature material to the atmosphere of active gas and carrier gas. It should be understood that the temperature of the introduced active gas and carrier gas is lower than the material temperature; during the transfer process, the high-temperature material is kept sealed. In addition, the annealing cooling method may also include circulating water cooling, such as increasing the cooling water flow rate, increasing the cooling water flow rate, increasing the atmosphere intake, and increasing the atmosphere exhaust.

[0050] In the annealing and cooling step, the active gas refers to a small molecule that can combine with water molecules through chemical bonds (e.g., hydrogen bonds) to form a low-boiling-point substance. Preferred active gases are selected from one or more of dry ammonia, carbon dioxide, and formaldehyde. In some embodiments, the active gas is ammonia or formaldehyde. The carrier gas can be a carrier gas commonly used in the art, preferably selected from one or more of dry nitrogen, argon, and helium. The proportion of active gas can be 1-30%, preferably 1-20%, based on the total volume of the introduced gas. It should be understood that the temperature of the active gas and the carrier gas is lower than the temperature of a burning material. Furthermore, the introduction of the active gas and the carrier gas is preferably continuous, and the flow rate of the introduced gas can be as commonly used in the art.

[0051] Before the second sintering treatment, the first sintering material is pulverized, mixed with a coating agent, and sintered to obtain the second sintering material, which is the layered sodium ion cathode material. Pulverization can be performed using methods such as ball milling or mortar and pestle. The coating agent can be a metal oxide such as boric acid, alumina, or titanium dioxide, preferably boric acid. The mass of the coating agent is 500-1500 μm of the total mass of the first sintering material. The temperature of the second sintering treatment can be 100-300℃, preferably 150-300℃. Those skilled in the art will know that a third or fourth sintering treatment can be optionally performed as needed.

[0052] Therefore, in some embodiments, the present invention also provides a layered sodium ion cathode material prepared by the method described in any embodiment herein.

[0053] Positive electrode sheet

[0054] The positive electrode sheet includes a positive current collector and a positive electrode material layer formed on the surface of the positive current collector. The positive electrode material layer includes a positive electrode (active) material, a conductive agent, and a binder. Therefore, the present invention also provides a positive electrode sheet comprising a layered sodium ion positive electrode material as described in any embodiment herein.

[0055] The positive electrode material layer is obtained by coating a positive electrode slurry containing positive electrode (active) material, conductive agent, binder, and solvent onto a positive electrode current collector, followed by rolling, die-cutting, and baking. The solvent for the positive electrode slurry can be N-methylpyrrolidone (NMP). The positive electrode current collector can be copper foil, aluminum foil, titanium foil, nickel foil, iron foil, zinc foil, etc. The conductive agent for the positive electrode can be one or more selected from conductive carbon black (SP), carbon fiber (CF), acetylene black, conductive graphite, graphene, carbon nanotubes, and carbon microspheres. The binder for the positive electrode can be one or more selected from vinylidene fluoride (PVDF), polytetrafluoroethylene, polyvinyl alcohol, polyolefins, styrene-butadiene rubber, fluorinated rubber, polyurethane, and sodium alginate. In some embodiments, the conductive agent in the positive electrode material layer is SP, and the binder is PVDF. The content ratio of each component in the positive electrode material layer can be conventional. For example, the mass fraction of the positive electrode material can be 90~98wt%, preferably 90~96wt%, the mass fraction of the conductive agent can be 0.5~5wt%, preferably 1~5wt%, and the mass fraction of the binder can be 0.5~5wt%, preferably 1~5wt%.

[0056] Negative electrode sheet

[0057] The negative electrode sheet includes a negative current collector and a negative electrode material layer formed on the surface of the negative current collector. The negative current collector can be copper foil. The negative electrode material layer includes a negative electrode active material, a conductive agent, and a binder. The negative electrode material layer is obtained by coating a negative electrode slurry containing a negative electrode active material, a conductive agent, a binder, and a solvent onto the positive electrode current collector, followed by rolling and baking. The solvent for the negative electrode slurry can be water. The negative electrode active material can be one or more selected from carbon materials (e.g., graphite), silicon, silicon compounds, lithium titanate, tin, and tin compounds. The negative electrode conductive agent can be one or more selected from conductive carbon black (SP), acetylene black, carbon nanotubes, carbon nanowires, carbon microspheres, carbon fibers, and graphene. The negative electrode binder can be one or more selected from polyvinylidene fluoride, polytetrafluoroethylene, acrylonitrile copolymers, polybutylene acrylate, polyacrylonitrile, and styrene-butadiene rubber (SBR). The negative electrode material layer and the negative electrode slurry may also contain a thickener, such as sodium carboxymethyl cellulose (CMC). In some embodiments, the negative electrode material in the negative electrode material layer is graphite, the conductive agent is conductive carbon black, the binder is styrene-butadiene rubber, and the thickener is sodium carboxymethyl cellulose. The mass ratio of the components in the negative electrode material layer can be conventional; for example, the mass fraction of the negative electrode active material can be 90-98 wt%, preferably 90-96 wt%, the mass fraction of the conductive agent can be 0.5-5 wt%, preferably 1-5 wt%, the mass fraction of the binder can be 0.5-5 wt%, preferably 1-5 wt%, and the mass fraction of the thickener can be 0-5 wt%.

[0058] Sodium-ion batteries

[0059] The sodium-ion cathode material prepared by this invention can reduce residual alkali and accelerate the annealing and cooling process, thereby reducing the consumption of sodium element content and increasing the specific capacity and rate performance of sodium-ion batteries. Therefore, this invention also provides a sodium-ion battery containing the layered sodium-ion cathode material and / or cathode sheet as described in any embodiment herein. In some embodiments, this invention also provides a method for preparing the sodium-ion battery, the method comprising the step of preparing the layered sodium-ion cathode material described above.

[0060] Sodium-ion batteries consist of a cell and an electrolyte. A sodium-ion battery cell comprises a positive electrode, a negative electrode, and a separator. The cell is manufactured by stacking or winding the positive electrode, negative electrode, and separator according to design requirements (e.g., Z-shaped stacking or wound stacking).

[0061] The membrane can be a polymer porous membrane, an inorganic porous membrane, or a polymer-inorganic composite porous membrane. Polymer porous membranes include single-layer polymer porous membranes and multi-layer polymer porous membranes. The material of the polymer membrane can be PE, PP, etc.

[0062] The electrolyte typically comprises a solvent and a sodium salt. The electrolyte suitable for this invention can be conventional; for example, the solvent can be one or more selected from dimethyl carbonate (DMC), propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), or 1,4-butylpropyl carbonate, more preferably one or more selected from PC, EC, and EMC. The sodium salt can be one or more selected from sodium tetrafluoroborate, sodium hexafluorophosphate, sodium trifluoromethanesulfonate, or sodium perchlorate. In some embodiments, the sodium salt in the electrolyte is NaPF6, and its concentration in the electrolyte can be 0.8~1.0 mol / L. The solvent in the electrolyte is PC, EC, and EMC, and the mass ratio of the three can be 1:1:1.

[0063] After obtaining the battery cell, it is encapsulated in an outer casing, and then dried, injected with electrolyte, sealed, left to stand, formed, and shaped to produce a sodium-ion battery. The form of the sodium-ion battery of this invention is not particularly limited; it can be a cylindrical sodium-ion battery, a button-type sodium-ion battery, a pouch sodium-ion battery, or an aluminum-cased sodium-ion battery. The sodium-ion battery of this invention exhibits a discharge capacity ≥121 mAh·g under charge-discharge testing at 25±2℃ and 0.2C 4.0~2.0V. -1 Preferred concentration: ≥124 mAh·g -1 More preferably ≥127 mAh·g -1 After 50 cycles, the capacity retention rate is ≥95%, preferably ≥96%, more preferably ≥98%, and the total alkali content is ≤3.5 at.%, preferably ≤3.1 at.%, more preferably ≤2.5 at.%.

[0064] In some embodiments, the present invention also provides the application of the sodium-ion cathode material as described above in reducing residual alkali on the surface of sodium-ion batteries and improving the specific capacity and rate performance of sodium-ion batteries.

[0065] The present invention has the following beneficial effects:

[0066] This invention employs a high-temperature annealing method to reduce the formation of residual alkali by reacting with air during the cooling stage, thereby reducing the loss of sodium content in the bulk phase. Simultaneously, the introduction of active gas and carrier gas accelerates the annealing process and reduces the moisture content of the material, further reducing the formation of residual alkali.

[0067] The present invention will be further described below by way of specific embodiments. It should be understood that these embodiments are merely illustrative and are not intended to limit the scope of the invention. Unless otherwise stated, the methods and reagents used in the embodiments are conventional methods and reagents in the art.

[0068] Detection methods

[0069] Residual alkali content test: The residual alkali content was tested using a potentiometric titrator; 7-day outdoor residual alkali test: The percentage of residual alkali after 7 days of outdoor exposure compared to the residual alkali after the first day of outdoor exposure, with the outdoor environment being normal temperature and dry.

[0070] Specific capacity and cycle test conditions: The battery was charged and discharged at 25±2℃, with a charge and discharge voltage of 4.0~2.0V and a rate of 0.2C. The discharge specific capacity and 50-cycle performance were tested respectively.

[0071] Example 1

[0072] ① Ni 0.33 Fe 0.33 Mn 0.34 OH2 precursor, sodium carbonate, and alumina were mixed at a molar ratio of sodium salt / precursor / alumina of 1.05 / 1 / 0.003. After uniform mixing, the mixture was heated to 1200℃ at a rate of 5℃ / min and sintered for 12 hours to prepare a sintered material. The material was then rapidly annealed and cooled to room temperature for 2 hours by introducing active gas NH3 and carrier gas N2 (ratio of 1%:99%). The material was then transferred from the sintering furnace to an atmosphere of NH3 and N2 (ratio of 1%:99%).

[0073] ② After crushing the calcined material prepared in ① above, mix it with coated boric acid accounting for 1000 ppm of the calcined material. After mixing evenly, sinter at 200℃ to prepare the required layered sodium-ion battery cathode material.

[0074] Example 2

[0075] ① Ni0.33 Fe 0.33 Mn 0.34 OH2 precursor, sodium carbonate, and alumina were mixed at a molar ratio of sodium salt / precursor / alumina of 1.05 / 1 / 0.003. After uniform mixing, the mixture was heated to 1200℃ at a rate of 5℃ / min and sintered for 12 hours to prepare a sintered material. The material was then rapidly annealed and cooled to room temperature by introducing active gas NH3 and carrier gas N2 (in a ratio of 20%:80%). The material was then transferred from the sintering furnace to an atmosphere of NH3 and N2 (in a ratio of 20%:80%).

[0076] ② The calcined material prepared in ① above is crushed and mixed with 1000 ppm of coated boric acid. After being mixed evenly, it is sintered at 200°C to prepare the required layered sodium-ion battery cathode material.

[0077] Example 3

[0078] ① Ni 0.33 Fe 0.33 Mn 0.34 OH2 precursor, sodium carbonate, and alumina were mixed at a molar ratio of sodium salt / precursor / alumina of 1.05 / 1 / 0.003. After uniform mixing, the mixture was heated to 1200℃ at a rate of 5℃ / min and sintered for 12 hours to prepare a sintered material. The material was then rapidly annealed and cooled to room temperature for 4 hours under the influence of active gas formaldehyde HCHO and carrier gas N2 (ratio 1%:99%). The material was then transferred out of the sintering furnace and placed under the influence of active gas formaldehyde HCHO and carrier gas N2 (ratio 1%:99%).

[0079] ② The calcined material prepared in ① above is crushed and mixed with 1000 ppm of coated boric acid. After being mixed evenly, it is sintered at 200°C to prepare the required layered sodium-ion battery cathode material.

[0080] Comparative Example 1

[0081] ① Ni 0.33 Fe 0.33 Mn 0.34 OH2 precursor, sodium carbonate, and alumina are mixed in a certain ratio (the molar ratio of Na / precursor / alumina is 1.05 / 1 / 0.003). After the mixture is homogeneous, the temperature is raised to 1200℃ at a rate of 5℃ / min and sintered for 12 hours to prepare a sintered material. The material is then cooled to room temperature in the sintering furnace for 12 hours.

[0082] ② The calcined material prepared in ① above is crushed and mixed with 1000 ppm of coated boric acid. After being mixed evenly, it is sintered at 200°C to prepare the required layered sodium-ion battery cathode material.

[0083] Comparative Example 2

[0084] ① Ni 0.33 Fe 0.33 Mn 0.34 OH2 precursor, sodium carbonate, and alumina are mixed in a certain ratio (the molar ratio of Na / precursor / alumina is 1.05 / 1 / 0.003). After uniform mixing, the mixture is heated to 1200℃ at a rate of 5℃ / min and sintered for 12 hours to prepare a sintered material. No active gas or carrier gas is introduced. The material is then directly transferred out of the sintering furnace and cooled to room temperature for 2 hours.

[0085] ② The calcined material prepared in ① above is crushed and mixed with 1000 ppm of coated boric acid. After being mixed evenly, it is sintered at 200°C to prepare the required layered sodium-ion battery cathode material.

[0086] Comparative Example 3

[0087] ① Ni 0.33 Fe 0.33 Mn 0.34 OH2 precursor, sodium carbonate, and alumina are mixed at a molar ratio of sodium salt / precursor / alumina of 1.05 / 1 / 0.003. After uniform mixing, the mixture is heated to 1200℃ at a rate of 5℃ / min and sintered for 12 hours to prepare a sintered material. Pure active gas NH3 is introduced for rapid annealing and cooling to room temperature for 2 hours. The material is then transferred from the sintering furnace to a pure active gas NH3 atmosphere.

[0088] ② The calcined material prepared in ① above is crushed and mixed with 1000 ppm of coated boric acid. After being mixed evenly, it is sintered at 200°C to prepare the required layered sodium-ion battery cathode material.

[0089] Comparative Example 4

[0090] ① Ni 0.33 Fe 0.33 Mn 0.34 OH2 precursor, sodium carbonate, and alumina are mixed at a molar ratio of sodium salt / precursor / alumina of 1.05 / 1 / 0.003. After uniform mixing, the mixture is heated to 1200℃ at a rate of 5℃ / min and sintered for 12 hours to prepare a sintered material. Pure active gas formaldehyde is then introduced for rapid annealing and cooling to room temperature for 2 hours. The material is then transferred from the sintering furnace to pure active gas formaldehyde.

[0091] ② The calcined material prepared in ① above is crushed and mixed with 1000 ppm of coated boric acid. After being mixed evenly, it is sintered at 200°C to prepare the required layered sodium-ion battery cathode material.

[0092] Comparative Example 5

[0093] ① Ni 0.33 Fe 0.33 Mn 0.34 OH2 precursor, sodium carbonate, and alumina are mixed at a molar ratio of sodium salt / precursor / alumina of 1.05 / 1 / 0.003. After uniform mixing, the mixture is heated to 1200℃ at a rate of 5℃ / min and sintered for 12 hours to prepare a sintered material. Inert nitrogen gas is introduced for rapid annealing and cooling to room temperature for 2 hours. The material is then transferred from the sintering furnace to an inert nitrogen atmosphere.

[0094] ② The calcined material prepared in ① above is crushed and mixed with 1000 ppm of coated boric acid. After being mixed evenly, it is sintered at 200°C to prepare the required layered sodium-ion battery cathode material.

[0095] The positive electrode materials obtained in the examples and comparative examples were mixed at a mass ratio of positive electrode active material: conductive carbon black: binder PVDF = 90:5:5. The mixture was then coated onto aluminum foil using NMP as a solvent and vacuum dried at 90°C to obtain the positive electrode sheet. The negative electrode sheet (sodium sheet), positive electrode sheet, electrolyte (1 mol / L NaPF6, PC:EC:EMC mass ratio = 1:1:1), and separator were then assembled into a coin cell. The discharge capacity and cycle performance of the coin cell were tested, and the results are shown in the table below.

[0096] Table 1: Battery Physicochemical Parameters and Performance

[0097]

[0098] As shown in Table 1, compared to Example 1, the residual alkali value of Examples 2-3 increased and the specific capacity decreased, possibly due to their slightly longer annealing time, resulting in the formation of some residual alkali on the surface. Compared to the Examples, the residual alkali value of Comparative Example 1 increased significantly, possibly because rapid cooling reduced the material's reaction with the external environment at medium and low temperatures, and the decrease in specific capacity was likely due to excessive sodium content consumed in the formation of residual alkali.

[0099] Comparing Comparative Examples 5 and 2 with Example 1, the increase in residual alkali and the decrease in specific capacity are likely due to the absence of an active atmosphere, resulting in excessive water absorption and ultimately affecting the discharge specific capacity. Comparative Examples 3 and 4, which only introduced active gas without a carrier gas, effectively reduced residual alkali, but the discharge specific capacity decreased. The inventors speculate that this may be due to the deterioration of the material by the active gas.

Claims

1. A method for preparing a sodium-ion cathode material, characterized in that, The method includes the following steps: (1) Mix the precursor, sodium salt and dopant evenly, and sinter to obtain a sintered material; (2) Anneal the sintered material to room temperature under an atmosphere of active gas and carrier gas; (3) The sintering material obtained in step (2) is crushed, mixed evenly with the coating material, and subjected to a second sintering treatment to obtain the sodium ion cathode material; in, The precursor is Ni x Fe y Mn 1-x-y OH2, where x is 0.2-0.4, y is 0.2-0.4, and x+y is 0.4-0.8; The annealing and cooling time is 0.5~5 hours; The active gas is selected from one or more of ammonia and formaldehyde; The carrier gas is an inert gas; The volume content of the active gas is 1-30% based on the total gas volume of the active gas and the carrier gas.

2. The method as described in claim 1, characterized in that, The sodium salt is selected from one or more of sodium carbonate, sodium hydroxide, sodium oxalate, and sodium citrate; The dopant is selected from one or more compounds of Li, Mg, Cu, Zn, Co, Ca, Ba, Sr, Al, B, Cr, Zr, Ti, Sn, V, Mo, Ru, Sb or Nb; The molar ratio of the precursor, sodium salt, and dopant is (0.9-1.2):1:(0.001-0.05); The coating material is selected from one or more of boric acid, aluminum oxide and titanium dioxide; The mass of the coating material is 500 to 1500 ppm of the total mass of the sintered material.

3. The method as described in claim 1, characterized in that, The annealing and cooling time is 2-4 hours.

4. The method as described in claim 1, characterized in that, The carrier gas is selected from one or more of helium, nitrogen, and argon.

5. The method as described in claim 1, characterized in that, The volume content of the active gas is 1% to 20% based on the total gas volume of the active gas and the carrier gas.

6. The method as described in claim 1, characterized in that, The process of obtaining a burning material also includes a heat preservation step.

7. The method as described in claim 2, characterized in that, In step (1), the heating rate of the sintering process is 2~10℃ / min.

8. The method as described in claim 2, characterized in that, In step (1), the sintering temperature is 500~1500℃.

9. The method as described in claim 2, characterized in that, In step (1), the holding time for the sintering treatment is 8~24h.

10. The method as described in claim 7, characterized in that, In step (1), the heating rate of the sintering process is 5~10℃ / min.

11. The method as described in claim 8, characterized in that, In step (1), the sintering temperature is 700~1400℃.

12. The method as described in claim 9, characterized in that, In step (1), the holding time for the sintering treatment is 10~20h.

13. The method as described in claim 2, characterized in that, The temperature of the second sintering process is 100~300℃.

14. The method as described in claim 13, characterized in that, The temperature of the second sintering process is 150~300℃.

15. A positive electrode plate, characterized in that, The positive electrode material layer of the positive electrode sheet includes a positive electrode material, a conductive agent, and a binder, wherein the positive electrode material is prepared by the method of any one of claims 1-14.

16. The positive electrode sheet as described in claim 15, characterized in that, The conductive agent is selected from one or more of conductive carbon black, carbon fiber, conductive graphite, graphene, carbon nanotubes, and carbon microspheres; and / or The adhesive is selected from one or more of PVDF, polytetrafluoroethylene, polyvinyl alcohol, polyolefin, styrene-butadiene rubber, fluorinated rubber, polyurethane, and sodium alginate.

17. The positive electrode sheet as described in claim 16, characterized in that, The conductive agent is acetylene black.

18. The positive electrode sheet as described in claim 16, characterized in that, The mass fraction of the positive electrode material is 90-98 wt% based on the total mass of the positive electrode material layer.

19. The positive electrode sheet as described in claim 16, characterized in that, The mass fraction of the conductive agent is 0.5~5wt% based on the total mass of the positive electrode material layer.

20. The positive electrode sheet as described in claim 16, characterized in that, The mass fraction of the binder is 0.5~5wt% based on the total mass of the positive electrode material layer.

21. The positive electrode sheet as described in claim 18, characterized in that, The mass fraction of the positive electrode material is 90-96 wt% based on the total mass of the positive electrode material layer.

22. The positive electrode sheet as described in claim 19, characterized in that, The mass fraction of the conductive agent is 1~5wt% based on the total mass of the positive electrode material layer.

23. The positive electrode sheet as described in claim 20, characterized in that, The mass fraction of the binder is 1-5 wt% based on the total mass of the positive electrode material layer.

24. A method for preparing a sodium-ion battery, characterized in that, The method includes the steps of the method as described in any one of claims 1-14.

25. A sodium-ion battery, characterized in that, The sodium-ion battery contains a positive electrode sheet as described in any one of claims 15-23 or is prepared by the method described in claim 24.

26. The sodium-ion battery as described in claim 25, characterized in that, The sodium-ion battery was tested at room temperature, 4.0~2.0V, and a 0.2C rate charge / discharge: Discharge capacity ≥121 mAh·g -1 , and / or The sodium-ion battery retains ≥95% capacity after 50 cycles, and / or The sodium-ion battery is said to have a residual alkali increase of ≤10% after 7 days of storage, and / or The total alkalinity of the sodium-ion battery is ≤3.5 at.

27. The sodium-ion battery as described in claim 26, characterized in that, The sodium-ion battery was tested at room temperature, 4.0~2.0V, and a 0.2C rate charge / discharge: The discharge capacity is ≥124 mAh·g -1 , and / or The sodium-ion battery retains ≥96% capacity after 50 cycles, and / or The sodium-ion battery exhibits a residual alkali increase of ≤9% after 7 days of storage, and / or The total alkalinity of the sodium-ion battery is ≤3.1 at.

28. The sodium-ion battery as described in claim 26, characterized in that, The sodium-ion battery was tested at room temperature, 4.0~2.0V, and a 0.2C rate charge / discharge: The discharge capacity is ≥127 mAh·g -1 , and / or The sodium-ion battery retains ≥98% capacity after 50 cycles, and / or The sodium-ion battery exhibits a residual alkali increase of ≤8% after 7 days of storage, and / or The total alkalinity of the sodium-ion battery is ≤2.5 at.%.

29. The application of the method according to any one of claims 1-14 in the preparation of sodium-ion batteries.

30. The application of the method according to any one of claims 1-14 in reducing the residual alkali content of sodium-ion batteries.

31. The application of the method according to any one of claims 1-14 in reducing residual alkali rebound in sodium-ion batteries.

32. The application of the method according to any one of claims 1-14 in increasing the discharge capacity and cycle performance of sodium-ion batteries.

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