In-situ formed sodium ferric phosphate coated sodium ion positive electrode layered oxide material, sodium ion battery positive electrode and sodium ion battery

Abstract

The invention discloses an in-situ formed sodium ferric phosphate coated sodium ion positive electrode layered oxide material, a sodium ion battery positive electrode and a sodium ion battery, the method comprises the following steps: preparing a sodium ion positive electrode layered oxide material to be coated, and measuring the residual alkali content of the sodium ion positive electrode layered oxide material; the method comprises the following steps: mixing ferric pyrophosphate with the content equal to that of residual alkali with a raw material for preparing a sodium ion positive electrode layered oxide to form a premix, sintering and slowly cooling, and forming a sodium ferric phosphate coating layer on the sodium ion positive electrode layered oxide material in situ by a residual sodium substance separated out from the surface of the sodium ion positive electrode layered oxide material and ferric pyrophosphate to prepare the sodium ion positive electrode layered oxide material. The sodium iron phosphate in-situ coated sodium ion positive electrode layered oxide material is prepared. According to the sodium ion positive electrode layered oxide material, a dense, uniform and thickness-controllable sodium ferric phosphate coating layer is formed in situ, so that residual alkali on the surface of the material can be effectively reduced, the pH value is reduced, meanwhile, the cell processing performance of the material is improved, the electrolyte corrosion resistance of the surface of the material is effectively enhanced, and the service life of the material is prolonged. The subsequent complicated process flow for reducing the residual alkali is reduced, the capacity of the sodium-ion battery positive electrode material is improved, and the production cost of the sodium-ion battery positive electrode material is reduced.

Description

Technical Field

[0001] The invention belongs to the field of sodium ion positive electrode layered oxide materials, and in particular relates to a sodium ion positive electrode layered oxide material coated with sodium iron phosphate formed in situ, a sodium ion battery positive electrode and a sodium ion battery. Background Art

[0002] Lithium-ion batteries have been widely used in various energy storage devices, especially in electric vehicles. However, due to the uneven distribution of lithium in the earth's crust, the small reserves, and the development of large-scale smart grids, the current lithium resources are difficult to meet the current energy storage needs. As a naturally abundant and low-cost sodium-containing raw material, sodium-ion batteries have been considered as an alternative solution to the shortage of lithium sources in my country. Therefore, sodium-ion batteries have developed rapidly in recent years, and some material systems have already been used in products.

[0003] There are still some difficulties in the positive electrode materials of sodium ion batteries. One of the thorny difficulties is to form a uniform coating layer with adjustable thickness on the surface of the substrate. At present, the common methods for surface coating of electrode materials in industrial production mainly include spray drying, vapor deposition, liquid phase method and mechanical mixing method. Although the first three methods can form a good coating layer, they require a liquid medium. Most of the sodium ion layered positive electrode materials are sensitive to water and easily lead to the loss of sodium ions in the substrate, reduced structural stability and reduced cycle stability. They are limited in the industrial production of positive electrode materials for sodium ion batteries. Although the mechanical mixing method can achieve good mixing, the coating agent is generally added in a small amount. After calcination after solid-solid mixing, it is difficult to prepare a uniform coating layer with adjustable thickness, resulting in unsatisfactory coating effect. For the newly developed atomic thin layer deposition technology in recent years, it can form a uniform coating layer with adjustable thickness on the surface of the material, but the expensive equipment will significantly increase the production cost. At the same time, it is still necessary to carry out a secondary coating process, increase the process route, and increase the production cost of the material.

[0004] For example, the prior art CN107293729A discloses a method for preparing an alumina-coated sodium ion battery positive electrode material, wherein the method dopes rare earth metals in the positive electrode material, and the band gap of the formed rare earth metal element -O is smaller than that of VO, making it easier for electrons to be excited from the valence band to the conduction band, which is beneficial to promoting electron conduction. The coating material of the prior art forms a solid solution with the contact agent, which is easy to coat evenly, and is tightly bonded after firing, and the coating layer is not easy to fall off. This method not only shortens the production cycle, but also ensures the uniformity of the coating, so that the coating material can also be subjected to high temperature to form an ideal glassy coating, thereby improving the performance of the material.

[0005] For another example, the prior art CN114824222A discloses that the present application discloses a method for coating positive electrode materials with an organosilicon coupling agent, which comprises the following steps: a reaction solution preparation step, in which deionized water and an organosilicon coupling agent are added to an organic solvent to prepare a reaction solution; a mixing reaction step, in which the positive electrode material is placed in the reaction solution and mixed and stirred evenly, and then the temperature of the reaction solution is maintained at 80 to 130°C for 1 to 5 hours; a filtering and drying step, in which the positive electrode material is filtered out of the reaction solution, and the positive electrode material is placed in an environment of 60 to 120°C and dried for 6 to 10 hours. After the positive electrode material prepared by the method adopted in the prior art is assembled into a battery, the high temperature and high pressure performance of the battery is significantly improved, and the cycle performance is also greatly improved.

[0006] In addition, most of the positive electrode materials for sodium ion batteries are coated with oxides, such as aluminum oxide, titanium dioxide, silicon dioxide, magnesium oxide and other coating agents. There are also coating layers formed by using organic coupling agents. However, this type of coating layer material itself has the problem of low ionic and electronic conductivity, which to a certain extent affects the electrochemical properties of the main material.

[0007] Based on this, a sodium ion positive electrode layered oxide material is studied that uses ferric pyrophosphate combined with residual alkali to achieve the in-situ formation of sodium iron phosphate-coated sodium ion positive electrode layered oxide material, which can form a dense, uniform and thickness-controllable coating layer. Summary of the invention

[0008] Purpose of the invention: The technical problem to be solved by the present invention is to provide a sodium ion positive electrode layered oxide material, a sodium ion battery positive electrode and a sodium ion battery, which use ferric pyrophosphate combined with residual alkali to form a dense, uniform and thickness-controlled sodium iron phosphate coating layer in situ.

[0009] Technical solution: The present invention forms in situ sodium iron phosphate-coated sodium ion positive electrode layered oxide material, which is prepared by the following steps:

[0010] (1) preparing a sodium ion positive electrode layered oxide material to be coated and determining its residual alkali content;

[0011] (2) mixing ferric pyrophosphate having a content equivalent to that of residual alkali with a raw material for preparing a sodium ion positive electrode layered oxide to form a premix, and then sintering and slowly cooling the premix. The residual sodium substance precipitated on the surface of the sodium ion positive electrode layered oxide material and the ferric pyrophosphate in situ form a sodium iron phosphate coating layer on the sodium ion positive electrode layered oxide material, thereby obtaining a sodium ion positive electrode layered oxide material in situ coated with sodium iron phosphate.

[0012] The present invention starts with the process of high-temperature solid-phase sintering of the positive electrode material of the sodium ion battery, and according to the residual sodium content of the coated sodium ion positive electrode layered oxide material tested, an equal percentage of pyrophosphate is added to mix and form a premix. Based on the good thermal stability and structural stability of the pyrophosphate ion, after the calcination of the premix with pyrophosphate ion added, the pyrophosphate ion will not enter the structure of the layered oxide main material, but the residual sodium substances precipitated on the surface of the material during the slow cooling process, such as sodium carbonate, sodium bicarbonate, sodium hydroxide, etc., will react with pyrophosphate ion to generate sodium iron phosphate in situ and coat the surface of the material, forming a sodium iron phosphate coating layer in situ, and the reaction mechanism involved is as follows:

[0013] Fe4(P2O7)3+3Na2CO3→2Na3Fe2(PO4)3+3CO2;

[0014] Fe4(P2O7)3+2Na2CO3+2NaOH→2Na3Fe2(PO4)3+2CO2+H2O.

[0015] That is, the preparation method can controllably generate a dense and uniform coating layer on the surface of the positive electrode material of the sodium ion battery in situ, and the a ratio in the molecular formula corresponding to the same product is different, resulting in different residual sodium. In this way, the residual sodium is different for the same product, and the thickness of the coating layer can be controlled. In addition, ferric pyrophosphate is converted into a sodium iron phosphate coating layer according to the residual alkali content of the substrate. The in-situ sodium iron phosphate coating of the sodium ion positive electrode layered oxide material can effectively reduce the residual alkali on the surface of the material, reduce the pH, and improve the material's battery processing performance, effectively enhance the material surface's resistance to electrolyte corrosion, reduce the subsequent complex process of reducing residual alkali, increase the production capacity of sodium ion battery positive electrode materials, and reduce the production cost of sodium ion battery positive electrode materials.

[0016] Furthermore, in the preparation step (1) of the material of the present invention, the chemical formula of the sodium ion positive electrode layered oxide material is Na a M b O2, 0.6<a≤1.1, 0.9<b≤1.0; a and b satisfy charge balance; the elements in the chemical formula satisfy charge balance; M is one or more elements selected from Ni, Fe, Mn, Cu, Co, Zn, Ca, Ba, Al, B, Ti, and Mo.

[0017] Furthermore, in the preparation step (2) of the material of the present invention, the particle size D50 of the ferric pyrophosphate is 10 to 500 nm.

[0018] In step (2), the slow cooling rate is 0.1 to 5°C / min.

[0019] Furthermore, in the preparation step (2) of the material of the present invention, the temperature of the sintering insulation section is 700-980° C., the insulation time is 4-15 hours, and the sintering atmosphere is air, oxygen or nitrogen.

[0020] Furthermore, in the preparation step (2) of the material of the present invention, the chemical formula of the sodium iron phosphate formed is Na3Fe2(P04)3.

[0021] The sodium ion battery positive electrode of the present invention comprises the above-mentioned sodium ion positive electrode layered oxide material based on in-situ coating of sodium iron phosphate.

[0022] The sodium ion battery of the present invention comprises the above-mentioned sodium ion battery positive electrode, negative electrode, separator and electrolyte.

[0023] Beneficial effects: Compared with the prior art, the significant advantages of the present invention are: the sodium ion positive electrode layered oxide material is based on the in-situ formation of a dense, uniform and thickness-controlled sodium iron phosphate coating layer, which can effectively reduce the residual alkali on the surface of the material, reduce the pH, and at the same time improve the material's battery processing performance, effectively enhance the material surface's resistance to electrolyte corrosion, reduce the subsequent complex process of reducing residual alkali, increase the production capacity of sodium ion battery positive electrode materials, and reduce the production cost of sodium ion battery positive electrode materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Na is not coated with sodium iron phosphate 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 SEM image of O2 sodium ion positive electrode layered oxide material;

[0025] Figure 2 Na coated with sodium iron phosphate 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 SEM image of O2 sodium ion positive electrode layered oxide material;

[0026] Figure 3 Na of uncoated and coated sodium iron phosphate 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 XRD pattern of O2 sodium ion positive electrode layered oxide material;

[0027] Figure 4 Na of uncoated and coated sodium iron phosphate 0.92 Ni 0.20 Mn0.40 Fe 0.25 Cu 0.15 Comparison of the first cycle charge and discharge performance of O2 sodium ion positive electrode layered oxide materials;

[0028] Figure 5 Na is not coated with sodium iron phosphate 0.95 Ni 0.33 Mn 0.33 Fe 0.33 SEM image of O2 sodium ion positive electrode layered oxide material;

[0029] Figure 6 Na coated with sodium iron phosphate 0.95 Ni 0.33 Mn 0.33 Fe 0.33 SEM image of O2 sodium ion positive electrode layered oxide material;

[0030] Figure 7 Na of uncoated and coated sodium iron phosphate 0.95 Ni 0.33 Mn 0.33 Fe 0.33 Comparison of the first cycle charge and discharge performance of O2 sodium ion positive electrode layered oxide materials;

[0031] Figure 8 Na is not coated with sodium iron phosphate 0.97 Ni 0.4 Mn 0.2 Fe 0.4 SEM image of O2 sodium ion positive electrode layered oxide material;

[0032] Fig. 9 Na coated with sodium iron phosphate 0.98 Ni 0.4 Mn 0.2 Fe 0.4 SEM image of O2 sodium ion positive electrode layered oxide material;

[0033] Fig.10 Na of uncoated and coated sodium iron phosphate 0.98 Ni 0.4 Mn 0.2 Fe 0.4 Comparison of the first cycle charge and discharge performance of O2 sodium ion positive electrode layered oxide materials. DETAILED DESCRIPTION

[0034] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0035] It should be noted that the residual alkali in the sodium ion positive electrode layered oxide material of the present invention refers to the sodium salt that fails to enter the bulk structure of the material during the material synthesis process, but is an alkaline substance remaining on the surface of the material, such as NaOH, NaHCO3 and Na2CO3. And the content of the residual alkali is determined after the corresponding sodium ion layered positive electrode material is prepared first. The chemical formula of the sodium iron phosphate formed in the present invention is Na3Fe2(P04)3. The particle size D50 of the pyrophosphate iron used in the following embodiments of the present invention is 10 to 500nm.

[0036] Based on the sodium ion positive electrode layered oxide material coated with sodium iron phosphate formed in situ of the present invention, the corresponding sodium ion battery positive electrode and sodium ion battery can be further prepared. Among them, the sodium ion battery also includes a negative electrode, a separator and an electrolyte.

[0037] Example 1

[0038] The sodium ion layered positive electrode material of Example 1 has the molecular formula Na 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 O2.

[0039] In this embodiment 1, a sodium ion positive electrode layered oxide material coated with sodium iron phosphate is formed in situ, which is prepared by the following steps:

[0040] (1) According to the sodium ion layered cathode material Na 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 Nickel oxide, manganese trioxide, iron trioxide, copper oxide and sodium carbonate were mixed in a molar ratio of 0.20:0.40:0.25:0.15:0.92 among Ni, Mn, Fe, Cu and Na elements, as mixture A, and 0.02 mol of ferric pyrophosphate with a percentage equal to the residual sodium was added at the same time, as mixture B. Then the two mixtures A and B were run at a speed of 1200 rpm for 15 min respectively;

[0041] (2) The two mixed materials A and B were placed in a sagger, heated to 890°C at a heating rate of 5°C per minute in an air atmosphere, sintered and kept at the temperature for 10 hours, and finally cooled naturally to room temperature. The mixed materials A and B were then ground using a jet mill to obtain the final material, the sodium ion positive electrode layered oxide material Na that was not coated with sodium iron phosphate. 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 O2 and sodium ion positive electrode layered oxide material Na coated with sodium iron phosphate0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 O2.

[0042] The ferric pyrophosphate in the mixture B is obtained by measuring the residual alkali of the positive electrode material prepared from the mixture A, and the same is true for the following embodiments.

[0043] The sodium ion positive electrode layered oxide material Na prepared in Example 1 without sodium iron phosphate coating 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 O2 and sodium ion positive electrode layered oxide material Na coated with sodium iron phosphate 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 O2 was used for structural characterization, and the results obtained were as follows Figure 1 and Figure 2 As shown. Na coated with sodium iron phosphate 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 There is no obvious change in the morphology of O2, but the surface is smoother and the particle distribution material is less. It can be seen that coating with sodium iron phosphate effectively improves the microscopic state of the material surface and significantly improves the residual sodium on the surface.

[0044] The sodium ion positive electrode layered oxide material Na which is not coated with sodium iron phosphate synthesized in Example 1 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 O2 and sodium ion positive electrode layered oxide material Na coated with sodium iron phosphate 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 O2 was characterized by X-ray diffraction (XRD), and the results obtained were as follows Figure 3 As shown in the figure, it can be seen that the Na coated with sodium iron phosphate 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 The residual sodium peak near 43° of O2 is significantly reduced, indicating that iron phosphate converts the residual sodium on the surface of the material into the target coating layer of sodium iron phosphate, revealing that coating with sodium iron phosphate helps to enhance the structural stability of the material and inhibit the loss of sodium in the crystal structure.

[0045] The sodium ion positive electrode layered oxide material Na which is not coated with sodium iron phosphate synthesized in Example 1 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 O2 and sodium ion positive electrode layered oxide material Na coated with sodium iron phosphate 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 The first cycle charge and discharge performance comparison of O2 is as follows: Figure 4 As shown. Figure 4 The cycling performance results show that Na 0.92 Ni 0.20 Mn 0.40 Fe 0.25 Cu 0.15 The discharge specific capacity of O2 is significantly higher than that of uncoated sodium iron phosphate, with a lower median charge voltage and a higher median discharge voltage. This is because a good coating helps maintain the structural stability of the material and helps to reduce the side reactions of the electrolyte.

[0046] Example 2

[0047] The sodium ion layered positive electrode material of Example 2 has the molecular formula Na 0.95 Ni 0.33 Mn 0.33 Fe 0.33 O2.

[0048] In this embodiment 2, a sodium ion positive electrode layered oxide material coated with sodium iron phosphate is formed in situ, which is prepared by the following steps:

[0049] (1) According to the sodium ion layered cathode material Na 0.95 Ni 0.33 Mn 0.33 Fe 0.33 The molar ratio of O2 was prepared by conventional co-precipitation of the precursor Ni 0.33 Mn 0.33 Fe 0.33 (OH)2, then weighed sodium carbonate in a stoichiometric ratio, and no ferric pyrophosphate was added as mixed material A and 0.03 mol ferric pyrophosphate was added as mixed material B, and finally a high-speed mixer was used to mix at a speed of 1200 rpm for 15 min;

[0050] (2) The two mixed materials A and B were placed in a sagger, heated to 890°C at a rate of 5°C per minute in an air atmosphere, sintered and kept at a temperature for 10 h, and then naturally cooled to room temperature. The final material was then crushed using a jet mill, and the uncoated sodium iron phosphate Na 0.95 Ni 0.33 Mn 0.33 Fe 0.33 O2 sodium ion positive electrode layered oxide material and Na2O2 coated with sodium iron phosphate 0.95 Ni 0.33 Mn 0.33 Fe 0.33 O2 sodium ion positive electrode layered oxide material.

[0051] The uncoated sodium iron phosphate Na synthesized in Example 2 0.95 Ni 0.33 Mn 0.33 Fe 0.33 O2 sodium ion positive electrode layered oxide material and Na2O2 coated with sodium iron phosphate 0.95 Ni 0.33 Mn 0.33 Fe 0.33 The O2 sodium ion positive electrode layered oxide material was characterized by SEM, and the results obtained are as follows Figure 5 and Figure 6 As shown in Figure 5 and Figure 6 The SEM characterization results show that the Na 0.95 Ni 0.33 Mn 0.33 Fe 0.33 The morphology of O2 is more rounded and the surface is smoother, indicating that the material coated with sodium iron phosphate has less residual sodium, which is beneficial to battery cell processing, cycle stability and rate performance.

[0052] The uncoated sodium iron phosphate Na synthesized in Example 2 0.95 Ni 0.33 Mn 0.33 Fe 0.33 O2 sodium ion positive electrode layered oxide material and Na2O2 coated with sodium iron phosphate 0.95 Ni 0.33 Mn 0.33 Fe 0.33 The first cycle charge and discharge performance of O2 sodium ion positive electrode layered oxide materials was compared, and the results obtained are as follows Figure 7 As shown in . Figure 7 The cycling performance results show that Na 0.95 Ni 0.33 Mn 0.33 Fe 0.33The discharge specific capacity of O2 is slightly higher than that of uncoated sodium iron phosphate, the charging median voltage is lower, and the discharging median voltage is higher. This is because a good coating helps maintain the structural stability of the material. At the same time, the coating helps to reduce the side reactions of the electrolyte. The material coated with sodium iron phosphate has less residual sodium, which is beneficial to battery cell processing, cycle stability and rate performance.

[0053] Example 3

[0054] The sodium ion layered positive electrode material of Example 3 has the molecular formula Na 0.98 Ni 0.40 Fe 0.20 Mn 0.40 O2.

[0055] In this embodiment 3, a sodium ion positive electrode layered oxide material coated with sodium iron phosphate is formed in situ, which is prepared by the following steps:

[0056] (1) According to the sodium ion layered positive electrode material composition, Na 0.98 Ni 0.40 Fe 0.20 Mn 0.40 The molar ratio of O2 was prepared by conventional co-precipitation of the precursor Ni 0.40 Fe 0.20 Mn 0.40 (OH)2, then weighed sodium carbonate in a stoichiometric ratio, and no ferric pyrophosphate was added as mixed material A and 0.035 mol ferric pyrophosphate was added as mixed material B, and finally a high-speed mixer was used to mix at a speed of 1200 rpm for 15 min;

[0057] (2) The two mixed materials A and B were placed in a sagger, heated to 890°C at a rate of 5°C per minute in an air atmosphere, sintered and kept at a temperature for 10 h, and then naturally cooled to room temperature. The final material was then crushed using a jet mill, and the uncoated sodium iron phosphate Na 0.98 Ni 0.40 Fe 0.20 Mn 0.40 O2 sodium ion positive electrode layered oxide material and Na2O2 coated with sodium iron phosphate 0.98 Ni 0.40 Fe 0.20 Mn 0.40 O2 sodium ion positive electrode layered oxide material.

[0058] The uncoated sodium iron phosphate Na synthesized in Example 3 0.98 Ni 0.40 Fe 0.20 Mn 0.40 O2 sodium ion positive electrode layered oxide material and Na2O2 coated with sodium iron phosphate 0.98 Ni 0.40 Fe0.20 Mn 0.40 The SEM characterization of the O2 sodium ion positive electrode layered oxide material was performed, and the results obtained were as follows Figure 8 and Fig. 9 As shown. Figure 8 and Fig. 9 The SEM characterization results show that the Na 0.98 Ni 0.40 Fe 0.20 Mn 0.4 The morphology of O2 is more rounded and the surface is smoother, indicating that the material coated with sodium iron phosphate has less residual sodium, which is beneficial to battery cell processing, cycle stability and rate performance.

[0059] The uncoated sodium iron phosphate Na synthesized in Example 3 0.98 Ni 0.40 Fe 0.20 Mn 0.40 O2 sodium ion positive electrode layered oxide material and Na2O2 coated with sodium iron phosphate 0.98 Ni 0.40 Fe 0.20 Mn 0.40 The first cycle charge and discharge performance of O2 sodium ion positive electrode layered oxide materials was compared, and the results obtained are as follows Fig.10 As shown. Fig.10 The cycling performance results show that Na 0.98 Ni 0.40 Fe 0.20 Mn 0.4 The discharge specific capacity of O2 is significantly higher than that of uncoated sodium iron phosphate, with a lower median charge voltage and a higher median discharge voltage. This is because a good coating helps maintain the structural stability of the material, and the coating helps to reduce the side reactions of the electrolyte.

[0060] In addition to the above embodiments, the preparation method of the present invention is used for the chemical formula Na a M b The sodium ion positive electrode layered oxide materials with O2, 0.6<a≤1.0, 0.9<b≤1.0 can achieve the above-mentioned technical effects under the preparation process conditions of the present invention. Therefore, they are not listed one by one.

Claims

1. A sodium ion positive electrode layered oxide material coated with sodium iron phosphate formed in situ, characterized in that: Prepared by the following steps: (1) preparing a sodium ion positive electrode layered oxide material to be coated and determining its residual alkali content; (2) The pyrophosphate having the same residual alkali content as the raw material for preparing the sodium ion positive electrode layered oxide is mixed to form a premix, and then the premix is ​​sintered and cooled to room temperature. The residual sodium substance precipitated on the surface of the sodium ion positive electrode layered oxide material and the pyrophosphate in situ form a sodium iron phosphate coating layer on the sodium ion positive electrode layered oxide material, thereby obtaining a sodium ion positive electrode layered oxide material in situ coated with sodium iron phosphate.

2. The sodium ion positive electrode layered oxide material coated with sodium iron phosphate formed in situ according to claim 1, characterized in that: In step (1), the chemical formula of the sodium ion positive electrode layered oxide material is Na a M b O2, 0.6<a≤1.0, 0.9<b≤1.0; a and b satisfy charge balance; the elements in the chemical formula satisfy charge balance; M is one or more elements selected from Ni, Fe, Mn, Cu, Co, Zn, Ca, Ba, Al, B, Ti, and Mo.

3. The sodium ion positive electrode layered oxide material coated with sodium iron phosphate formed in situ according to claim 1, characterized in that: In step (2), the particle size D50 of the ferric pyrophosphate is 10 to 500 nm.

4. The sodium ion positive electrode layered oxide material coated with sodium iron phosphate formed in situ according to claim 1, characterized in that: In step (2), the temperature of the sintering insulation section is 700-980° C., the insulation time is 4-15 hours, and the sintering atmosphere is air, oxygen or nitrogen.

5. The sodium ion positive electrode layered oxide material coated with sodium iron phosphate formed in situ according to claim 1, characterized in that: In step (2), the chemical formula of the sodium iron phosphate formed is Na3Fe2(P04)3.

6. A sodium ion battery positive electrode, characterized in that: The sodium ion battery positive electrode comprises the sodium ion positive electrode layered oxide material coated with in-situ formed sodium iron phosphate as claimed in claim 1.

7. A sodium ion battery, characterized in that: The sodium ion battery comprises the sodium ion battery positive electrode, negative electrode, separator and electrolyte as described in claim 6.

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