Single-crystal sodium-electric positive electrode material, and preparation method and application thereof

Abstract

The application provides a single-crystal sodium electric positive electrode material and a preparation method and application thereof, wherein the preparation method comprises the following steps: mixing a metal source, a sodium source, a surfactant and a solvent to obtain a dispersion mixed solution, and then performing spray pyrolysis and secondary sintering on the dispersion mixed solution in sequence, so that the single-crystal sodium electric positive electrode material is prepared. The preparation method can prevent particle agglomeration, is beneficial to uniform distribution of metal elements, optimizes grain growth and crystal structure, and obtains the single-crystal sodium electric positive electrode material with good crystallinity and low cation mixing, so that the energy density and cycle performance of a sodium ion battery are improved.

Description

Technical Field

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

[0002] As the material that accounts for the largest proportion of the cost of sodium-ion batteries, sodium-ion battery cathode materials have become a core focus of the new energy industry. Currently, research on sodium-ion battery cathode materials mainly focuses on crystalline materials, including layered oxides, polyanionic compounds, and Prussian blue compounds. Layered oxides for sodium-ion batteries, due to their advantages such as wide availability of raw materials, good processing performance, and high specific capacity, show great application potential in low-cost, large-scale energy storage. Based on microstructure, layered oxides for sodium-ion batteries can be divided into monocrystalline and polycrystalline types. Monocrystalline materials have many advantages over polycrystalline materials in terms of battery performance, including higher load voltage, better safety and cycle performance, and smaller interfacial side reactions. This means that during battery use, monocrystalline cathode materials are more likely to maintain higher battery efficiency, reduce energy loss, and extend battery life. Therefore, monocrystalline sodium-ion battery cathode materials may become an important research and application direction in the development of battery technology.

[0003] Solid-state sintering is a commonly used method for preparing single-crystal sodium-ion battery cathode materials. A typical process involves first preparing a hydroxide precursor via co-precipitation, then mixing these washed and post-treated hydroxide precursors with a sodium source and subjecting them to multiple high-temperature calcinations. Firstly, this process is cumbersome, has a long production cycle, and consumes a lot of energy. It can also introduce impurities or oxide residues, negatively impacting the performance of the single-crystal cathode material. Secondly, during sintering, uneven mixing of the two raw materials can lead to grain agglomeration and uneven elemental distribution, resulting in surface cracks and decreased crystallinity, thus affecting the quality of the single-crystal structure. Therefore, although solid-state sintering has certain advantages in the preparation of single-crystal cathode materials, its process complexity and energy consumption issues need to be recognized, and improvements should be sought to enhance material quality and preparation efficiency.

[0004] Molten salt method is also a major method for preparing single-crystal sodium-ion battery cathode materials. For example, CN117577814A discloses a single-crystal sodium-ion battery cathode material and its preparation method, which uses a sacrificial molten salt combined with a surfactant to prepare a single-crystal cathode. The molten salt is doped into the crystal lattice and transformed into a coating layer in a one-step process, eliminating the need for washing to remove excess molten salt and secondary heat treatment. However, the sintering temperature of the molten salt method is relatively high, resulting in energy consumption issues.

[0005] In summary, there is a need to develop a novel method for preparing single-crystal sodium cathode materials. Summary of the Invention

[0006] In view of the problems existing in the prior art, the present invention provides a single-crystal sodium-ion battery cathode material, its preparation method, and its application. The preparation method involves uniformly mixing a metal source, a sodium source, a surfactant, and a solvent. The resulting dispersed mixture is then subjected to spray pyrolysis and secondary sintering to obtain the single-crystal sodium-ion battery cathode material. The preparation method of the present invention can prevent particle agglomeration, promote uniform distribution of metal elements, optimize grain growth and crystal structure, and obtain a single-crystal sodium-ion battery cathode material with good crystallinity and low cation mixing, thereby improving the energy density and cycle performance of sodium-ion batteries.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] One objective of this invention is to provide a method for preparing a single-crystal sodium cathode material, the method comprising the following steps:

[0009] The metal source, sodium source, surfactant and solvent are mixed evenly, and the resulting dispersed mixed solution is subjected to spray pyrolysis and secondary sintering in sequence to obtain a single crystal sodium electrode material.

[0010] Wherein, the molar ratio of Ni, Mn and metal M in the metal source is a:b:(1-ab); wherein, 0.1≤a≤0.3, 0.3≤b≤0.5, and the metal M includes at least one of Co, Fe, Cu, Mg, Al, Ca, Zn, Cr, Ti, Zr, W, Mo, Ru, V or Nb; the molar ratio of sodium content in the sodium source to total metal content in the metal source is x:1; wherein, 0.7≤x≤1.

[0011] The preparation method of this invention involves uniformly mixing a metal source, a sodium source, a surfactant, and a solvent. The resulting dispersed mixed solution is then subjected to spray pyrolysis and secondary sintering to obtain a single-crystal sodium-ion battery cathode material. This method first uses spray pyrolysis to prevent particle agglomeration, effectively controlling the particle size and morphology of the sodium-ion battery cathode material. Then, secondary sintering achieves single-crystal growth and optimizes the crystal structure. Furthermore, this method allows for precise control of the spray pyrolysis reaction conditions, such as the type of metal salt, the concentration of the metal salt solution, the sodium source ratio, the type and content of the surfactant, the pyrolysis temperature, and the air pressure. This enables atomic-level mixing of Ni, Mn, metallic M, and Na in the crystal lattice, resulting in a single-crystal sodium-ion battery cathode material with uniform metal element distribution and low cation mixing, which helps improve the energy density and cycle life of sodium-ion batteries.

[0012] As a preferred technical solution of the present invention, the metal salt in the metal source exists in at least one of chloride, sulfate, acetate or nitrate.

[0013] As a preferred embodiment of the present invention, the sodium source includes at least one of sodium hydroxide, sodium carbonate, sodium oxalate, sodium chloride, or sodium nitrate.

[0014] As a preferred embodiment of the present invention, the surfactant includes at least one of polyvinylpyrrolidone, ascorbic acid, oxalic acid, polypropylene glycol, or citric acid.

[0015] As a preferred embodiment of the present invention, the solvent includes at least one of water, ethanol, methanol or acetone.

[0016] As a preferred technical solution of the present invention, the total concentration of metal salt in the dispersed mixed solution is 100-300 g / L, such as 100 g / L, 150 g / L, 200 g / L, 250 g / L or 300 g / L, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0017] It should be noted that the total concentration of metal salts in the dispersed mixed solution described in this invention refers to the total concentration of metal salts corresponding to the metal source, that is, the total concentration of metal salts excluding the sodium source.

[0018] Preferably, the surfactant content in the dispersion mixture is 0.1-5 wt%, such as 0.1 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, or 5 wt%, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0019] As a preferred technical solution of the present invention, the spray pyrolysis includes: atomizing the dispersed mixed solution, wherein the resulting atomized droplets pass through four temperature control zones from top to bottom; the temperature of the first temperature control zone is 300-350℃, the temperature of the second temperature control zone is 600-800℃, the temperature of the third temperature control zone is 850-1050℃, and the temperature of the fourth temperature control zone is 300-350℃.

[0020] In this invention, the temperature of the first temperature control range is 300-350℃, such as 300℃, 310℃, 320℃, 330℃, 340℃ or 350℃, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0021] In this invention, the temperature of the second temperature control range is 600-800℃, such as 600℃, 630℃, 650℃, 680℃, 700℃, 730℃, 750℃, 770℃ or 800℃, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0022] In this invention, the temperature of the third temperature control range is 850-1050℃, such as 850℃, 880℃, 900℃, 930℃, 950℃, 980℃, 1000℃, 1020℃ or 1030℃, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0023] In this invention, the temperature of the fourth temperature control range is 300-350℃, such as 300℃, 310℃, 320℃, 330℃, 340℃ or 350℃, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0024] It should be noted that the spray pyrolysis described in this invention is carried out in a spray pyrolysis device. First, the power supply of the spray pyrolysis device is turned on, the temperature of the calcination furnace is adjusted to the required temperature, and the peristaltic pump is turned on to pump the dispersed mixed solution into the atomizer. After atomization, it enters the calcination furnace for full pyrolysis under the blowing of compressed air. The nozzle of the atomizer is selected from at least one of air atomizing nozzle, fine atomizing nozzle, hollow cone nozzle, fan nozzle or solid cone nozzle. The pressure at the nozzle of the atomizer is controlled at 7-70 kPa, the frequency of the peristaltic pump is controlled at 40-220 rpm / min, and the flow rate of the compressed air is controlled at 0-50 L / min.

[0025] As a preferred technical solution of the present invention, the sintering atmosphere of the secondary sintering includes at least one of air, oxygen, nitrogen or hydrogen.

[0026] Preferably, the sintering temperature of the secondary sintering is 600-900℃, such as 600℃, 650℃, 700℃, 750℃, 800℃, 850℃ or 900℃, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0027] Preferably, the sintering time for the secondary sintering is 2-24h, such as 2h, 4h, 6h, 8h, 10h, 12h, 14h, 16h, 18h, 20h, 22h or 24h, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0028] Preferably, the heating rate of the secondary sintering is 3-10℃ / min, such as 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min or 10℃ / min, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0029] The second objective of this invention is to provide a single-crystal sodium cathode material, which is prepared according to the preparation method described in the first objective, and has the chemical formula Na. x Ni a Mn b M 1-a-b O2; wherein, 0.1≤a≤0.3, 0.3≤b≤0.5, 0.7≤x≤1; the metal M includes at least one of Co, Fe, Cu, Mg, Al, Ca, Zn, Cr, Ti, Zr, W, Mo, Ru, V or Nb.

[0030] As a preferred technical solution of the present invention, the particle size D50 of the single crystal sodium electrode material is 5-20μm, such as 5μm, 7μm, 8μm, 10μm, 12μm, 13μm, 15μm, 16μm, 18μm or 20μm, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0031] The third objective of this invention is to provide an application of a single-crystal sodium-ion battery cathode material, which is used in the preparation of the cathode sheet of a sodium-ion battery.

[0032] The single-crystal sodium-ion cathode material is prepared according to the preparation method described in one of the objectives, or the single-crystal sodium-ion cathode material is the single-crystal sodium-ion cathode material described in the second objective.

[0033] Compared with existing technical solutions, the present invention has at least the following beneficial effects:

[0034] (1) The preparation method of the present invention first uses spray pyrolysis to prevent particle agglomeration, which can effectively control the particle size and morphology of sodium battery cathode material, and then achieves single crystal growth and optimizes crystal structure through secondary sintering;

[0035] (2) The preparation method described in this invention can precisely control the spray pyrolysis reaction conditions, such as the type of metal salt, the concentration of the metal salt solution, the sodium source ratio, the type and content of surfactant, the pyrolysis temperature, and the air pressure. This can achieve atomic-level mixing of Ni, Mn, metallic M, and Na in the crystal lattice, resulting in a single-crystal sodium electrode material with uniform distribution of metal elements and low cation mixing, which helps to improve the energy density and cycle life of sodium-ion batteries. Attached Figure Description

[0036] Figure 1 This is a SEM image of the single-crystal sodium cathode material prepared in Example 1 of this invention. Detailed Implementation

[0037] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0038] To better illustrate the present invention and facilitate understanding of its technical solutions, typical but non-limiting embodiments of the present invention are as follows:

[0039] Example 1

[0040] This embodiment provides a method for preparing a single-crystal sodium-ion cathode material, the method comprising the following steps:

[0041] According to the molar ratio n(Ni) 2+ ):n(Mn 2+ ):n(Fe 2+ ):n(Cu 2+ The ratio of sodium nitrate to total metal content in the sodium source is 0.9:1. Accurately weigh nickel nitrate, manganese nitrate, ferrous nitrate, and copper nitrate as metal sources. Accurately weigh sodium carbonate as sodium source, based on the molar ratio of sodium content in the sodium source to total metal content in the metal source being 0.9:1.

[0042] A metal source, a sodium source, polyvinylpyrrolidone (PVP) and ultrapure water are mixed evenly to obtain a dispersion solution. The total concentration of metal salts in the dispersion solution is 200 g / L, and the content of polyvinylpyrrolidone (PVP) in the dispersion solution is 3 wt%.

[0043] Start the spray pyrolysis equipment and set four temperature control zones in the calcining furnace from top to bottom. The temperature of the first temperature control zone is 350℃, the temperature of the second temperature control zone is 700℃, the temperature of the third temperature control zone is 850℃, and the temperature of the fourth temperature control zone is 300℃. Turn on the peristaltic pump to pump the dispersed mixture into the atomizer. The peristaltic pump frequency is 150 rpm / min. The nozzle of the atomizer is a fine atomizing nozzle, and the pressure at the nozzle is 40 kPa. Turn on the air compressor and adjust the gas flow meter to control the flow rate of compressed air to 25 L / min. The dispersed mixture enters the calcining furnace under the blowing of compressed air for complete pyrolysis.

[0044] The fully pyrolyzed material was collected and transferred to a tube furnace for secondary sintering. The sintering atmosphere in the tube furnace was air. The temperature was increased to 700℃ at a heating rate of 5℃ / min and held for 12 hours. After natural cooling, washing, and drying, the material with the chemical formula Na was obtained. 0.9 Ni 0.2 Mn 0.35 Fe 0.35 Cu 0.1 O2 and a single-crystal sodium cathode material with a particle size D50 of 14μm.

[0045] Figure 1The SEM image of the single-crystal sodium cathode material described in this embodiment is shown. It can be seen that the single-crystal sodium cathode material described in this embodiment has a single-crystal morphology under an electron microscope, and the particle shape is a hexagonal plate-like structure.

[0046] Example 2

[0047] This embodiment provides a method for preparing a single-crystal sodium electrode material. The only difference from Example 1 is that the total concentration of metal salt in the dispersed mixed solution is 80 g / L.

[0048] Example 3

[0049] This embodiment provides a method for preparing a single-crystal sodium electrode material. The only difference from Example 1 is that the total concentration of metal salt in the dispersed mixed solution is 380 g / L.

[0050] Example 4

[0051] This embodiment provides a method for preparing a single-crystal sodium cathode material. Compared with Example 1, the only difference is that in the dispersed mixed solution, the molar ratio of sodium content in the sodium source to total metal content in the metal source is 0.7:1.

[0052] Example 5

[0053] This embodiment provides a method for preparing a single-crystal sodium cathode material. The only difference from Embodiment 1 is that the molar ratio of sodium content in the sodium source to total metal content in the metal source in the dispersed mixed solution is 1:1.

[0054] Example 6

[0055] This embodiment provides a method for preparing a single-crystal sodium-ion cathode material, the method comprising the following steps:

[0056] According to the molar ratio n(Ni) 2+ ):n(Mn 2+ ):n(Fe 2+ ):n(Cu 2+ The ratio of sodium nitrate to total metal content in the sodium source is 0.9:1. Accurately weigh nickel nitrate, manganese nitrate, ferrous nitrate, and copper nitrate as metal sources. Accurately weigh sodium carbonate as sodium source, based on the molar ratio of sodium content in the sodium source to total metal content in the metal source being 0.9:1.

[0057] A metal source, a sodium source, polyvinylpyrrolidone (PVP) and ultrapure water are mixed evenly to obtain a dispersed mixed solution. The total concentration of metal salts in the dispersed mixed solution is 100 g / L, and the content of polyvinylpyrrolidone (PVP) in the dispersed mixed solution is 0.1 wt%.

[0058] Start the spray pyrolysis equipment and set four temperature control zones in the calcining furnace from top to bottom. The temperature of the first temperature control zone is 300℃, the temperature of the second temperature control zone is 600℃, the temperature of the third temperature control zone is 850℃, and the temperature of the fourth temperature control zone is 300℃. Turn on the peristaltic pump to pump the dispersed mixture into the atomizer. The peristaltic pump frequency is 150 rpm / min. The nozzle of the atomizer is a fine atomizing nozzle, and the pressure at the nozzle is 40 kPa. Turn on the air compressor and adjust the gas flow meter to control the flow rate of compressed air to 25 L / min. The dispersed mixture enters the calcining furnace under the blowing of compressed air for complete pyrolysis.

[0059] The fully pyrolyzed material was collected and transferred to a tube furnace for secondary sintering. The sintering atmosphere in the tube furnace was air. The temperature was increased to 600℃ at a heating rate of 3℃ / min and held for 12 hours. After natural cooling, washing, and drying, the material with the chemical formula Na was obtained. 0.9 Ni 0.2 Mn 0.35 Fe 0.35 Cu 0.1 O2 and a single-crystal sodium cathode material with a particle size D50 of 14μm.

[0060] Example 7

[0061] This embodiment provides a method for preparing a single-crystal sodium-ion cathode material, the method comprising the following steps:

[0062] According to the molar ratio n(Ni) 2+ ):n(Mn 2+ ):n(Fe 2+ ):n(Cu 2+ The ratio of sodium nitrate to total metal content in the sodium source is 0.9:1. Accurately weigh nickel nitrate, manganese nitrate, ferrous nitrate, and copper nitrate as metal sources. Accurately weigh sodium carbonate as sodium source, based on the molar ratio of sodium content in the sodium source to total metal content in the metal source being 0.9:1.

[0063] A metal source, a sodium source, polyvinylpyrrolidone (PVP) and ultrapure water are mixed evenly to obtain a dispersed mixed solution. The total concentration of metal salts in the dispersed mixed solution is 300 g / L, and the content of polyvinylpyrrolidone (PVP) in the dispersed mixed solution is 5 wt%.

[0064] Start the spray pyrolysis equipment and set four temperature control zones in the calcining furnace from top to bottom. The temperature of the first temperature control zone is 350℃, the temperature of the second temperature control zone is 800℃, the temperature of the third temperature control zone is 1050℃, and the temperature of the fourth temperature control zone is 350℃. Turn on the peristaltic pump to pump the dispersed mixture into the atomizer. The peristaltic pump frequency is 150 rpm / min. The nozzle of the atomizer is a fine atomizing nozzle, and the pressure at the nozzle is 40 kPa. Turn on the air compressor and adjust the gas flow meter to control the flow rate of compressed air to 25 L / min. The dispersed mixture enters the calcining furnace under the blowing of compressed air for complete pyrolysis.

[0065] The fully pyrolyzed material was collected and transferred to a tube furnace for secondary sintering. The sintering atmosphere in the tube furnace was air. The temperature was increased to 900℃ at a heating rate of 5℃ / min and held for 24 hours. After natural cooling, washing, and drying, the material with the chemical formula Na was obtained. 0.9 Ni 0.2 Mn 0.35 Fe 0.35 Cu 0.1 O2 and a single-crystal sodium cathode material with a particle size D50 of 14μm.

[0066] Comparative Example 1

[0067] This comparative example provides a method for preparing a sodium-ion cathode material, which differs from Example 1 only in that polyvinylpyrrolidone (PVP) is completely omitted.

[0068] The sodium-ion battery positive electrode materials prepared in the above examples and comparative examples were used to make positive electrode sheets for sodium-ion batteries. Then, sodium sheets were used as negative electrode sheets, Celgard 2400 was used as separators, and 1 mol / L LiPF6 was used as electrolytes to assemble CR2032 coin cells. Electrochemical performance tests were then conducted under the following conditions: voltage range 2.5-4.0V, current density 0.2C, and 50 cycles. The test results are shown in Table 1.

[0069] Table 1

[0070] Serial Number <![CDATA[0.2C capacity (mAh·g -1 )]]> Volume retention rate after 50 laps (%) Example 1 215.6 95.4 Example 2 165.4 76.3 Example 3 163.0 72.8 Example 4 173.6 81.9 Example 5 181.5 86.2 Example 6 201.9 92.6 Example 7 203.1 92.9 Comparative Example 1 160.7 72.5

[0071] In summary, the preparation method of this invention first uses spray pyrolysis to prevent particle agglomeration, effectively controlling the particle size and morphology of the sodium-ion battery cathode material. Then, single crystal growth and crystal structure optimization are achieved through secondary sintering. The preparation method of this invention can precisely control the spray pyrolysis reaction conditions, such as the type of metal salt, the concentration of the metal salt solution, the sodium source ratio, the type and content of surfactant, the pyrolysis temperature, and the air pressure. This allows for atomic-level mixing of Ni, Mn, metallic M, and Na in the crystal lattice, resulting in a single-crystal sodium-ion battery cathode material with uniform metal element distribution and low cation mixing, which helps to improve the energy density and cycle life of sodium-ion batteries.

[0072] The present invention has been illustrated with the above embodiments to illustrate its detailed structural features. However, the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the components used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

[0073] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0074] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0075] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing a single-crystal sodium cathode material, characterized in that, The preparation method includes the following steps: The metal source, sodium source, surfactant and solvent are mixed evenly, and the resulting dispersed mixed solution is subjected to spray pyrolysis and secondary sintering in sequence to obtain a single crystal sodium electrode material. Wherein, the molar ratio of Ni, Mn and metal M in the metal source is a:b:(1-ab); wherein, 0.1≤a≤0.3, 0.3≤b≤0.5, and the metal M includes at least one of Co, Fe, Cu, Mg, Al, Ca, Zn, Cr, Ti, Zr, W, Mo, Ru, V or Nb; the molar ratio of sodium content in the sodium source to total metal content in the metal source is x:1; wherein, 0.7≤x≤1; The surfactant content in the dispersion mixture is 0.1-5 wt%; the surfactant includes at least one of polyvinylpyrrolidone, ascorbic acid, oxalic acid, polypropylene glycol, or citric acid. The spray pyrolysis includes: atomizing the dispersed mixed solution, with the resulting atomized droplets passing through four temperature control zones from top to bottom; the temperature of the first temperature control zone is 300-350℃, the temperature of the second temperature control zone is 600-800℃, the temperature of the third temperature control zone is 850-1050℃, and the temperature of the fourth temperature control zone is 300-350℃. The sintering temperature for the secondary sintering is 600-900℃; The single-crystal sodium cathode material has a particle shape that is roughly hexagonal and sheet-like.

2. The preparation method according to claim 1, characterized in that, The metal salt in the metal source exists in at least one of the following forms: chloride, sulfate, acetate, or nitrate.

3. The preparation method according to claim 1, characterized in that, The sodium source includes at least one of sodium hydroxide, sodium carbonate, sodium oxalate, sodium chloride, or sodium nitrate.

4. The preparation method according to claim 1, characterized in that, The solvent includes at least one of water, ethanol, methanol, or acetone.

5. The preparation method according to claim 1, characterized in that, The total concentration of metal salts in the dispersed mixed solution is 100-300 g / L.

6. The preparation method according to claim 1, characterized in that, The sintering atmosphere for the secondary sintering includes at least one of air, oxygen, nitrogen, or hydrogen.

7. The preparation method according to claim 1, characterized in that, The sintering time for the secondary sintering is 2-24 hours.

8. The preparation method according to claim 1, characterized in that, The heating rate for the secondary sintering is 3-10℃ / min.

9. A single-crystal sodium cathode material, characterized in that, The single-crystal sodium-ion cathode material is prepared according to any one of claims 1-8, and has the chemical formula Na. x Ni a Mn b M 1-a-b O2; wherein, 0.1≤a≤0.3, 0.3≤b≤0.5, 0.7≤x≤1; the metal M includes at least one of Co, Fe, Cu, Mg, Al, Ca, Zn, Cr, Ti, Zr, W, Mo, Ru, V or Nb.

10. The single-crystal sodium cathode material according to claim 9, characterized in that, The particle size D50 of the monocrystalline sodium cathode material is 5-20 μm.

11. An application of a single-crystal sodium cathode material, characterized in that, Applying single-crystal sodium cathode materials to the preparation of cathode sheets for sodium-ion batteries; The single-crystal sodium electrode material is prepared by the preparation method according to any one of claims 1-8, or the single-crystal sodium electrode material is the single-crystal sodium electrode material according to claim 9 or 10.

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