A high-magnification long-cycle sodium-ion positive electrode material and a preparation method thereof

Abstract

The application provides a high-rate long-cycle sodium-ion positive electrode material and a preparation method thereof. x M1 y )Ni a Fe b Mn c M2 d O2@M, M represents a coating layer coated on the surface of the core material, and M is a mixture of NaCoO2 and Co3O4. The high-rate long-cycle sodium-ion positive electrode material of the application combines the doping of sodium sites and metal sites of the internal core material and the composite coating of NaCoO2 and Co3O4 on the surface of the core material, and synergistically improves the rate performance and cycle performance of the material.

Description

Technical Field

[0001] This invention relates to the field of battery cathode materials, specifically to a high-rate, long-cycle sodium-ion cathode material and its preparation method. Background Technology

[0002] Sodium-ion batteries have a high positive electrode redox potential, which can improve the overall energy density of the battery; they have high stability in the electrolyte and a stable structure during charge and discharge; they have high electronic conductivity and ionic conductivity, and are simple to prepare, environmentally friendly, safe and non-toxic.

[0003] Na x Ni a Fe b Mn c Although O2-type battery materials have advantages in terms of capacity, cycle performance, and energy density, they are prone to phase transitions during charging and discharging, which severely negatively impacts their specific capacity and rate performance. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a high-rate, long-cycle sodium-ion cathode material and its preparation method.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a high-rate, long-cycle sodium-ion cathode material, wherein the high-rate, long-cycle sodium-ion cathode material has a core-shell structure, comprising a core material located inside and a coating layer covering the surface of the core material, and the general chemical formula of the high-rate, long-cycle sodium-ion cathode material is (Na... x M1 y Ni a Fe b Mn c M2 d O2@M, where @M represents a coating layer on the surface of the core material, M is a mixture of NaCoO2 and Co3O4, and the coating layer accounts for 0.3% to 1.2% of the mass of the sodium-ion cathode material in high-rate long-cycle operation, wherein 0.8≤x+y≤1.05, 0.1≤a≤0.6, 0.1≤b≤0.5, 0.1≤c≤0.5, and a+b+c+d=1, 0.01≤d≤0.1, M1 is at least one of Ca or K, and M2 is at least one of Zr, Al, Sn, Sb, Mg, Ti, W, Y, and La.

[0006] The aforementioned high-rate, long-cycle sodium-ion cathode material has a core-shell structure. In the core material, sodium ions are occupied by doping with Ca or K at sodium sites, while nickel, iron, and manganese ions are occupied by doping with Zr, Al, Sn, Sb, Mg, Ti, W, Y, and La at metal sites. This stabilizes the crystal structure, especially during phase transitions in charge and discharge processes. The synergistic doping of sodium and metal sites effectively supports the crystal, reduces structural changes during cycling, and provides good vacancy channels for sodium ion extraction and insertion. This maintains the rate performance while improving cycle performance. (NaCoO2 and Co3O4 groups) The mixture forms a coating layer around the core material. During charge and discharge, the O3 phase NaCoO2 undergoes an O3-O′3-P′3-P3-P'3 phase transition. The P3-P'3 phase has a faster Na ion diffusion pathway, which is beneficial for further improving the material's rate performance. Co3O4 is a stable inorganic compound that can prevent further erosion of the material surface by the electrolyte, reducing side reactions during charge and discharge and improving cycle performance. The composite coating layer of NaCoO2 and Co3O4, with their synergistic effect, can improve the material's rate performance and cycle performance, thus enhancing its electrical properties. The aforementioned high-rate, long-cycle sodium-ion cathode material, combining the doping of sodium and metal sites in the core material with the composite coating of NaCoO2 and Co3O4 on the core material surface, synergistically improves the material's rate performance and cycle performance.

[0007] Preferably, the mass ratio of NaCoO2 to Co3O4 in the coating layer is 1:(0.3 to 1.7).

[0008] Preferably, the mass ratio of NaCoO2 to Co3O4 in the coating layer is 1:(0.6-1.0).

[0009] In the aforementioned high-rate, long-cycle sodium-ion cathode materials, the composite coating of NaCoO2 and Co3O4, through adjustments to the sintering temperature and time of the coating layer, allows for the preparation of sodium-ion cathode materials with varying mass ratios of NaCoO2 to Co3O4 in the coating layer. x M1 y Ni a Fe b Mn c M2 dIn O2@M, the inventors discovered that the mass ratio of NaCoO2 to Co3O4 affects the rate performance and cycle performance of the material, and that a mass ratio of NaCoO2 to Co3O4 of 1:(0.6~1.0) in the coating layer can better improve the rate performance and cycle performance of the sodium-ion cathode material. This invention controls the proportion of the surface coating by controlling the atmosphere and sintering temperature during the sintering stage, forming a composite coating layer that conforms to the above ratio.

[0010] Preferably, the coating layer accounts for 0.5% to 1.0% of the mass of the sodium-ion cathode material used in high-rate, long-cycle applications.

[0011] In the above-mentioned high-rate, long-cycle sodium-ion cathode materials, when the coating layer accounts for 0.5% to 1.0% of the mass of the high-rate, long-cycle sodium-ion cathode material, the sodium-ion cathode material has better rate performance and cycle performance.

[0012] Preferably, M1 is at least one of Ca or K, and M2 is at least one of Zr, Sn, Mg, Ti, W, Y, and La.

[0013] More preferably, M1 is Ca and M2 is a combination of Zr and Mg, or M2 is a combination of Mg and Ti.

[0014] During the screening and optimization of doping elements, the inventors discovered that when NaCoO2 and Co3O4 are used as a composite coating layer, the sodium ion sites are occupied by Ca sodium doping in the core material, and M2 is a combination of Zr and Mg, or M2 is a combination of Mg and Ti doping in the metal sites to occupy the nickel, iron and manganese sites. This can further stabilize the crystal structure of the material and improve the battery performance of the material.

[0015] Preferably, the high-rate, long-cycle sodium-ion cathode material is prepared by a two-step sintering method, wherein the first step sintering prepares (Na) as the core material. x M1 y Ni a Fe b Mn c M2 d O2, the second step of sintering will (Na) x M1 y Ni a Fe b Mn c M2 d The high-rate, long-cycle sodium-ion cathode material is prepared by sintering O2 with the raw materials of the coating layer.

[0016] This invention also provides a method for preparing any of the above-described high-rate, long-cycle sodium-ion cathode materials, the method comprising the following steps:

[0017] (1) Nickel-iron-manganese raw materials, M1 compound, M2 compound and sodium source are mixed evenly and sintered in an oxygen-containing atmosphere at 900℃~1100℃ to obtain (Na) as the core material. x M1 y Ni a Fe b Mn c M2 d O2;

[0018] (2) The Na obtained in step (1) is used as the core material x M1 y Ni a Fe b Mn c M2 d O2 and cobalt source are mixed in deionized water and the pH of the mixture is adjusted to 7.0-8.5. The solid particles are collected, dried, and sintered in an oxygen-containing atmosphere at 500-800℃ for 5-8 hours.

[0019] The above-mentioned method for preparing high-rate, long-cycle sodium-ion cathode materials involves a two-step sintering process, (Na... x M1 y Ni a Fe b Mn c M2 d O2 and a cobalt source are mixed in deionized water, and the pH of the mixture is adjusted to 7.0–8.5 for wet treatment, followed by calcination of the matrix (Na). x M1 y Ni a Fe b Mn c M2 d Some residual sodium carbonate and sodium hydroxide on the O2 surface will dissolve into the water. At the same time, after heat treatment, the residual sodium content on the surface is very low, resulting in good air stability and excellent processing performance of the product.

[0020] Preferably, the sintering time in step (1) is 10 to 20 hours.

[0021] Preferably, the sodium source is one or two of sodium hydroxide, sodium carbonate, or sodium bicarbonate; the cobalt source is at least one of cobalt nitrate, cobalt chloride, or cobalt sulfate; the M1 compound is the oxide, hydroxide, or oxyacid salt corresponding to its element, and the M2 compound is the oxide, hydroxide, or oxyacid salt corresponding to its element.

[0022] Preferably, in step (2), (Na) is used as the core material. x M1 y Ni a Fe b Mnc M2 d The weight ratio of O2 to deionized water is (1-3):1.

[0023] The beneficial effects of this invention are as follows: This invention provides a high-rate, long-cycle sodium-ion cathode material and its preparation method. The high-rate, long-cycle sodium-ion cathode material of this invention has a core-shell structure. In the internal core material, sodium ions are occupied by doping with Ca or K at sodium sites, and nickel, iron, and manganese ions are occupied by doping with Zr, Al, Sn, Sb, Mg, Ti, W, Y, and La at metal sites. This stabilizes the crystal structure of the material, especially during the phase transition in the charge-discharge process. The synergistic doping of sodium and metal sites effectively supports the crystal, reduces structural changes during cycling, and provides good vacancy channels for sodium ion extraction and insertion. This maintains the rate performance of the material while improving its efficiency. The cycling performance of the material is improved by a mixture of NaCoO2 and Co3O4 that coats the core material. During charge and discharge, the O3 phase of NaCoO2 undergoes an O3-O′3-P′3-P3-P'3 phase transition. The P3-P'3 phase has a faster Na ion diffusion pathway, which is beneficial to further improving the rate performance of the material. Co3O4 is a stable inorganic compound that can prevent further erosion of the material surface by the electrolyte, reducing side reactions during charge and discharge, and thus improving cycle performance. The composite coating of NaCoO2 and Co3O4, with their synergistic effect, can improve the rate performance and cycle performance of the material, thereby enhancing its electrical properties. The aforementioned high-rate, long-cycle sodium-ion cathode material, combining the doping of sodium and metal sites in the core material with the composite coating of NaCoO2 and Co3O4 on the surface of the core material, synergistically improves the rate performance and cycle performance of the material. Attached Figure Description

[0024] Figure 1 The images show the XRD patterns of the sodium-ion cathode material of Example 1 and the sodium-ion cathode material of Comparative Example 1.

[0025] Figure 2 This is a SEM image of the sodium ion cathode material of Example 1 of the present invention.

[0026] Figure 3 The diagram shows the cycle retention rate of the sodium-ion cathode materials of Examples 1, 5, and 12 of this invention, and the sodium-ion cathode materials of Comparative Examples 1, 2, 3, 4, and 5.

[0027] Figure 4 This is the X-ray photoelectron spectrum (XPS) of the sodium ion cathode material obtained in Example 1 of the present invention.

[0028] Figure 5This is a transmission electron microscope (TEM) image of the sodium ion cathode material obtained in Example 1 of the present invention. Detailed Implementation

[0029] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0030] Example 1

[0031] As an embodiment of the present invention, a high-rate, long-cycle sodium-ion cathode material is a core-shell structure, comprising an internal core material and a coating layer covering the surface of the core material. The general chemical formula of the high-rate, long-cycle sodium-ion cathode material is (Na... 0.95 Ca 0.05 Ni 0.3465 Fe 0.297 Mn 0.3465 Zr 0.01 O2@M, where @M represents the coating layer on the surface of the core material, and M is a mixture of NaCoO2 and Co3O4.

[0032] The preparation method of the high-rate, long-cycle sodium-ion cathode material in this embodiment includes the following steps:

[0033] (1) Sodium carbonate, calcium carbonate, and nickel-iron-manganese precursor Ni 0.35 Fe 0.3 Mn 0.35 (OH)2 and zirconium oxide were added according to the molar ratio Na:Ca:Ni:Fe:Mn:Zr = 0.95:0.05:0.3465:0.297:0.3465:0.01. After initial mixing in a self-sealing bag, the mixture was added to a high-speed mixer. The mixture was first stirred at a low speed of 15Hz for 5 minutes, and then stirred at a high speed of 50Hz for 30 minutes. After mixing, the material was placed in a mortar and sintered in a box furnace at 1000℃ in air atmosphere for 12 hours. After natural cooling, the mixture was coarsely crushed, finely crushed, and sieved to obtain (Na)2 as the core material. 0.95 Ca 0.05 Ni 0.3465 Fe 0.297 Mn 0.3465 Zr 0.01 O2;

[0034] (2) The obtained (Na) 0.95 Ca 0.05 Ni 0.3465 Fe 0.297 Mn 0.3465 Zr 0.01O2 core material and deionized water were added to a mixing tank at a solid-liquid ratio of 2:1 and stirred. The temperature of the solid-liquid mixture in the mixing tank was controlled at 35°C. Then, cobalt nitrate was added to the mixing tank, wherein the cobalt nitrate reacted with (Na) 0.95 Ca 0.05 Ni 0.3465 Fe 0.297 Mn 0.3465 Zr 0.01 The O2 core material weight ratio is 4:100. The pH value of the reaction solution is adjusted to 7.8 using ammonia water. After the ammonia water is added, the reaction is stirred for 30 minutes and then filtered. The filtered material is vacuum dried at 180℃. After drying, the material is placed in a box furnace and sintered at 700℃ in air atmosphere for 6 hours. After crushing, sieving, and demagnetizing, a high-rate, long-cycle sodium ion cathode material is obtained.

[0035] According to X-ray photoelectron spectroscopy (XPS), in the high-rate, long-cycle sodium-ion cathode material prepared in this embodiment, the coating layer accounts for 0.81% of the mass of the high-rate, long-cycle sodium-ion cathode material, and the mass ratio of NaCoO2 to Co3O4 in the coating layer is 1:1.01.

[0036] Example 2

[0037] As a high-rate, long-cycle sodium-ion cathode material according to an embodiment of the present invention, the only difference between this embodiment and Example 1 is: the ratio of cobalt nitrate to (Na+) is adjusted. 0.95 Ca 0.05 Ni 0.3465 Fe 0.297 Mn 0.3465 Zr 0.01 The O2 core material has a weight ratio of 1:100.

[0038] According to X-ray photoelectron spectroscopy (XPS), in the high-rate, long-cycle sodium-ion cathode material prepared in this embodiment, the coating layer accounts for 0.36% of the mass of the high-rate, long-cycle sodium-ion cathode material, and the mass ratio of NaCoO2 to Co3O4 in the coating layer is 1:1.03.

[0039] Example 3

[0040] As a high-rate, long-cycle sodium-ion cathode material according to an embodiment of the present invention, the only difference between this embodiment and Example 1 is: the ratio of cobalt nitrate to (Na+) is adjusted. 0.95 Ca 0.05 Ni 0.3465 Fe 0.297 Mn 0.3465 Zr 0.01 The weight ratio of O2 core materials is 2:100.

[0041] According to X-ray photoelectron spectroscopy (XPS), in the high-rate, long-cycle sodium-ion cathode material prepared in this embodiment, the coating layer accounts for 0.45% of the mass of the high-rate, long-cycle sodium-ion cathode material, and the mass ratio of NaCoO2 to Co3O4 in the coating layer is 1:1.02.

[0042] Example 4

[0043] As a high-rate, long-cycle sodium-ion cathode material according to an embodiment of the present invention, the only difference between this embodiment and Example 1 is: the ratio of cobalt nitrate to (Na+) is adjusted. 0.95 Ca 0.05 Ni 0.3465 Fe 0.297 Mn 0.3465 Zr 0.01 The weight ratio of O2 core materials is 3:100.

[0044] According to X-ray photoelectron spectroscopy (XPS), in the high-rate, long-cycle sodium-ion cathode material prepared in this embodiment, the coating layer accounts for 0.62% of the mass of the high-rate, long-cycle sodium-ion cathode material, and the mass ratio of NaCoO2 to Co3O4 in the coating layer is 1:0.99.

[0045] Example 5

[0046] As a high-rate, long-cycle sodium-ion cathode material according to an embodiment of the present invention, the only difference between this embodiment and Example 1 is: the ratio of cobalt nitrate to (Na+) is adjusted. 0.95 Ca 0.05 Ni 0.3465 Fe 0.297 Mn 0.3465 Zr 0.01 The weight ratio of O2 core materials is 5:100.

[0047] According to X-ray photoelectron spectroscopy (XPS), in the high-rate, long-cycle sodium-ion cathode material prepared in this embodiment, the coating layer accounts for 0.98% of the mass of the high-rate, long-cycle sodium-ion cathode material, and the mass ratio of NaCoO2 to Co3O4 in the coating layer is 1:1.02.

[0048] Example 6

[0049] As a high-rate, long-cycle sodium-ion cathode material of this invention, the only difference between this embodiment and Embodiment 1 is that the material is placed in a box furnace and sintered at 500°C in air atmosphere for 7 hours, and then crushed, sieved, and demagnetized to obtain the high-rate, long-cycle sodium-ion cathode material.

[0050] According to X-ray photoelectron spectroscopy (XPS), the mass ratio of NaCoO2 to Co3O4 in the coating layer of the high-rate, long-cycle sodium-ion cathode material prepared in this embodiment is 1:0.38.

[0051] Example 7

[0052] As a high-rate, long-cycle sodium-ion cathode material of this invention, the only difference between this embodiment and Embodiment 1 is that the material is placed in a box furnace and sintered at 800°C in air atmosphere for 5 hours, and then crushed, sieved, and demagnetized to obtain the high-rate, long-cycle sodium-ion cathode material.

[0053] According to X-ray photoelectron spectroscopy (XPS), the mass ratio of NaCoO2 to Co3O4 in the coating layer of the high-rate, long-cycle sodium-ion cathode material prepared in this embodiment is 1:0.67.

[0054] Example 8

[0055] As a high-rate, long-cycle sodium-ion cathode material of this invention, the only difference between this embodiment and Embodiment 1 is that the material is placed in a box furnace and sintered at 800°C in air atmosphere for 6 hours, and then crushed, sieved, and demagnetized to obtain the high-rate, long-cycle sodium-ion cathode material.

[0056] According to X-ray photoelectron spectroscopy (XPS), the mass ratio of NaCoO2 to Co3O4 in the coating layer of the high-rate, long-cycle sodium-ion cathode material prepared in this embodiment is 1:0.91.

[0057] Example 9

[0058] As a high-rate, long-cycle sodium-ion cathode material of this invention, the only difference between this embodiment and Embodiment 1 is that the material is placed in a box furnace and sintered at 650°C in air atmosphere for 8 hours, and then crushed, sieved, and demagnetized to obtain the high-rate, long-cycle sodium-ion cathode material.

[0059] According to X-ray photoelectron spectroscopy (XPS), the mass ratio of NaCoO2 to Co3O4 in the coating layer of the high-rate, long-cycle sodium-ion cathode material prepared in this embodiment is 1:1.69.

[0060] Example 10

[0061] As a high-rate, long-cycle sodium-ion cathode material according to an embodiment of the present invention, the only difference between this embodiment and Embodiment 1 is that the chemical formula of the high-rate, long-cycle sodium-ion cathode material is (Na... 0.95 Ca 0.05 Ni 0.3325 Fe0.285 Mn 0.3325 Mg 0.02 Ti 0.03 O2@M.

[0062] Example 11

[0063] As a high-rate, long-cycle sodium-ion cathode material according to an embodiment of the present invention, the only difference between this embodiment and Embodiment 10 is that the chemical formula of the high-rate, long-cycle sodium-ion cathode material is (Na... 0.95 K 0.05 Ni 0.3325 Fe 0.285 Mn 0.3325 Mg 0.02 Ti 0.03 O2@M.

[0064] Example 12

[0065] As a high-rate, long-cycle sodium-ion cathode material according to an embodiment of the present invention, the only difference between this embodiment and Embodiment 1 is that the chemical formula of the high-rate, long-cycle sodium-ion cathode material is (Na... 0.95 Ca 0.05 Ni 0.3325 Fe 0.285 Mn 0.3325 Mg 0.02 Zr 0.03 O2@M.

[0066] Comparative Example 1

[0067] As a sodium-ion cathode material for comparative examples of the present invention, the only difference between this comparative example and Example 1 is that it does not contain a coating layer.

[0068] The general chemical formula of the sodium ion cathode material is (Na) 0.95 Ca 0.05 Ni 0.3465 Fe 0.297 Mn 0.3465 Zr 0.01 O2.

[0069] The preparation method, as described in Example 1, includes the following steps:

[0070] (1) Sodium carbonate, calcium carbonate, Ni 0.35 Fe 0.3 Mn 0.35(OH)2 and zirconium oxide were added according to the molar ratio Na:Ca:Ni:Fe:Mn:Zr = 0.95:0.05:0.3465:0.297:0.3465:0.01. After initial mixing in a self-sealing bag, the mixture was added to a high-speed mixer. The mixture was first stirred at a low speed of 15Hz for 5 minutes, and then stirred at a high speed of 50Hz for 30 minutes. After mixing, the material was placed in a mortar and sintered in a box furnace at 1000℃ in air atmosphere for 12 hours. After natural cooling, the mixture was coarsely crushed, finely crushed, and sieved to obtain (Na)2 as the core material. 0.95 Ca 0.05 Ni 0.3465 Fe 0.297 Mn 0.3465 Zr 0.01 O2;

[0071] Comparative Example 2

[0072] As a comparative example of the present invention, the only difference between this comparative example and Example 1 is that the core material is different. The chemical formula of the high-rate, long-cycle sodium-ion cathode material is NaNi. 0.3465 Fe 0.297 Mn 0.3465 Zr 0.01 O2@M.

[0073] That is, this comparative example is the same as Example 1 except that the sodium site is not doped with Ca.

[0074] Comparative Example 3

[0075] As a comparative example of the present invention, the only difference between this comparative example and Example 1 is that the core material is different, and the nickel-iron-manganese metal sites are not doped. The chemical formula of the high-rate, long-cycle sodium-ion cathode material is (Na... 0.95 Ca 0.05 Ni 0.35 Fe 0.3 Mn 0.35 O2@M, where @M represents the coating layer on the surface of the core material, and M is a mixture of NaCoO2 and Co3O4.

[0076] Comparative Example 4

[0077] As a comparative example of the present invention, the only difference between this comparative example and Example 1 is that the material is placed in a box furnace and sintered at 500°C for 4 hours, and then crushed, sieved, and demagnetized to obtain a high-rate, long-cycle sodium ion cathode material.

[0078] According to X-ray photoelectron spectroscopy (XPS), the mass ratio of NaCoO2 to Co3O4 in the coating layer of the high-rate, long-cycle sodium-ion cathode material prepared in this embodiment is 1:0.09.

[0079] Comparative Example 5

[0080] As a comparative example of the present invention, the only difference between this comparative example and Example 1 is that the material is placed in a box furnace and sintered at 800°C for 4 hours, and then crushed, sieved, and demagnetized to obtain a high-rate, long-cycle sodium ion cathode material.

[0081] According to X-ray photoelectron spectroscopy (XPS), the mass ratio of NaCoO2 to Co3O4 in the coating layer of the high-rate, long-cycle sodium-ion cathode material prepared in this embodiment is 1:2.1.

[0082] Experimental methods

[0083] I. Material Characterization

[0084] Figure 1 The XRD patterns of the sodium ion cathode material obtained in Example 1 and the sodium ion cathode material in Comparative Example 1 are shown; both have an O3-type structure. Figure 2 This is a SEM image of the sodium ion cathode material obtained in Example 1. Figure 3 The graph shows the cycle capacity retention rate of the sodium-ion cathode materials obtained in Examples 1, 5, and 12, compared with that of the sodium-ion cathode materials in Comparative Examples 1, 2, 3, 4, and 5. Figure 4 The image shows the X-ray photoelectron spectrum (XPS) of the sodium ion cathode material obtained in Example 1. Figure 5 This is a transmission electron microscope (TEM) image of the sodium ion cathode material obtained in Example 1.

[0085] II. Battery Performance

[0086] The prepared sodium-ion positive electrode material, conductive carbon black (SP), and polyvinylidene fluoride (PVDF) were weighed in a mass ratio of 90:5:5, with a solid content of 45% added. The concentration of the PVDF solution was 8%. The sodium-ion positive electrode material coating slurry was evenly dispersed by centrifugal high-speed stirring and uniformly coated onto aluminum foil using a small coating machine. After baking at 120℃, a dry electrode sheet was obtained. After rolling, the electrode sheet was cut into 14mm diameter sheets. A coin cell half-cell was assembled using sodium sheet as negative electrode, ceramic fiber as separator, and 1mol / L NaPF6 (EMC+PC+FEC(2%)) solution as electrolyte. The test voltage was 2.0–4.0V. The experimental results are shown in Table 1.

[0087] Table 1 Battery performance of sodium-ion cathode materials

[0088]

[0089]

[0090] As shown in Table 1, in the core material, sodium ions are occupied by Ca or K doping at sodium sites, and nickel, iron, and manganese ions are occupied by Zr, Al, Sn, Sb, Mg, Ti, W, Y, and La doping at metal sites. Combined with a mixture of NaCoO2 and Co3O4, a coating layer is formed on the core material. During charge and discharge, the O3 phase of NaCoO2 undergoes an O3-O′3-P′3-P3-P'3 phase transition. P3-P'3 has a faster Na ion diffusion pathway, which is beneficial to further improving the rate performance of the material. Co3O4 is a stable inorganic compound that can prevent further erosion of the material surface by the electrolyte, reduce side reactions during charge and discharge, and improve cycle performance. The composite coating layer of NaCoO2 and Co3O4, with their synergistic effect, can improve the rate performance and cycle performance of the material, and better enhance the electrical performance of the material. The aforementioned high-rate, long-cycle sodium-ion cathode material combines the doping of sodium and metal sites in the core material with the composite coating of NaCoO2 and Co3O4 on the surface of the core material, synergistically improving the rate performance and cycle performance of the material. When the mass ratio of NaCoO2 to Co3O4 in the coating layer is 1:(0.6–1.0), the rate performance and cycle performance of the sodium-ion cathode material can be further enhanced.

[0091] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A high-rate long-cycle sodium-ion cathode material, characterized in that, The high-rate long-cycle sodium-ion positive electrode material is a core-shell structure, comprising a core material in the inside and a coating layer coated on the surface of the core material, and a chemical general formula of the high-rate long-cycle sodium-ion positive electrode material is ((Na x M1 y )Ni a Fe b Mn c M2 d O2@M, @M represents the coating layer coated on the surface of the core material, M is a mixture of NaCoO2 and Co3O4, the coating layer accounts for 0.5% to 1.0% of the mass of the high-rate long-cycle sodium-ion positive electrode material, wherein 0.8≤x+y≤1.05, 0.1≤a≤0.6, 0.1≤b≤0.5, 0.1≤c≤0.5, and a+b+c+d=1, 0.01≤d≤0.1; M1 is Ca, and M2 is Zr. The mass ratio of NaCoO2 to Co3O4 in the coating layer is 1:(0.6-1.0). The high-rate long-cycle sodium-ion positive electrode material is prepared by a two-step sintering method, a core material (Na x M1 y )Ni a Fe b Mn c M2 d O2 is prepared by the first step of sintering, and the high-rate long-cycle sodium-ion positive electrode material is prepared by sintering (Na x M1 y )Ni a Fe b Mn c M2 d O2 and raw materials of a coating layer in the second step of sintering. In the second sintering step, the (Na x M1 y )Ni a Fe b Mn c M2 d O2 obtained in step (1) as the core material is mixed with a cobalt source in deionized water and the pH of the mixture is adjusted to 7.0-8.5, and the solid particles are collected, vacuum dried, and sintered at 800°C in an oxygen-containing atmosphere for 5-8h.

2. The method for preparing the high-rate, long-cycle sodium-ion cathode material as described in claim 1, characterized in that, The method comprises the following steps: (1) Nickel-iron-manganese raw materials, M1 compound, M2 compound and sodium source are mixed evenly and sintered in an oxygen-containing atmosphere at 900℃~1100℃ to obtain (Na) as the core material. x M1 y Ni a Fe b Mn c M2 d O2; (2) The (Na x M1 y )Ni a Fe b Mn c M2 d O2 obtained in step (1) as a core material is mixed with a cobalt source in deionized water and the pH of the mixture is adjusted to 7.0-8.5, and the solid particles are collected, vacuum dried, and sintered at 800°C in an oxygen-containing atmosphere for 5-8h.

3. The method of claim 2, wherein the sodium-ion cathode material is prepared by the following steps: (1) preparing a precursor of the sodium-ion cathode material; (2) mixing the precursor with a solvent to form a mixture; (3) heating the mixture to obtain the sodium-ion cathode material. The sintering time in step (1) is 10-20 hours.

4. The method for preparing the high-rate, long-cycle sodium-ion cathode material according to claim 2, characterized in that, The sodium source is one or two of sodium hydroxide, sodium carbonate or sodium bicarbonate; the cobalt source is at least one of cobalt nitrate, cobalt chloride and cobalt sulfate; the M1 compound is an oxide, a hydroxide or an oxyacid salt corresponding to the element thereof, and the M2 compound is an oxide, a hydroxide or an oxyacid salt corresponding to the element thereof.

5. The method for preparing the high-rate, long-cycle sodium-ion cathode material according to claim 2, characterized in that, In step (2), the core material (Na x M1 y )Ni a Fe b Mn c M2 d O2 and deionized water is (1~3): 1.

Images