Na a Fe b X c Y d (BO3) e Positive electrode active materials, their preparation and application in sodium ion batteries

Abstract

The present invention belongs to battery materials, specifically to Na a Fe b X c Y d (BO3) e A positive electrode active material, wherein X is at least one of F, Cl, and Br; Y is at least one of a phosphorus oxide anion and a silicon oxide anion; a is in the range of 1 to 4, b is in the range of 1 to 4, c is in the range of 0.3 to 2, d is in the range of 0.3 to 2; and e is in the range of 0.01 to 0.1. The present invention also includes methods for preparing and using the material. The material of the present invention has excellent performance, can adapt to the requirements of high electrode thickness loading, and can exhibit excellent electrochemical performance at high loading thicknesses.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and particularly relates to the technical field of positive electrode materials for sodium ion batteries. Background Art

[0002] Lithium-ion batteries, with their advantages of high energy density, high stability and long life, have rapidly captured the portable electronic product market and gradually extended to the field of electric vehicles. However, the reserves of lithium in the earth's crust are small and unevenly distributed, resulting in rising costs of lithium-ion batteries and limiting their wider application. Sodium and lithium belong to the same family and have some similar physical and chemical properties, but the atomic radius of sodium is larger than that of lithium, making many materials and construction theories suitable for lithium-ion batteries inapplicable in the field of sodium-ion batteries, making it difficult to achieve similar technical effects as lithium-ion batteries. This is also one of the main reasons why sodium-ion batteries have not been widely used like lithium-ion batteries.

[0003] In contrast, iron-based composite phosphates have significant cost advantages among sodium-ion electrode materials due to the abundant sources of Fe elements in the earth's crust and the low price of raw materials, making them one of the most commercially promising sodium-ion battery positive electrode materials.

[0004] Iron-based polyanionic materials mainly include sodium ferric fluorophosphate, sodium ferric pyrophosphate, etc., but such components have more crystal forms, are prone to mixed phases during the preparation process, and are generally not ideal for electrical conductivity. For this reason, the main improvement ideas of the prior art are to construct carbon composite structures and doping of Fe positions. For example, the Chinese patent publication number CN114242968A discloses a carbon-coated sodium ferric fluorophosphate material and its preparation method and application. The Chinese patent publication number CN116262608A discloses a preparation method and its application of a polyanionic material in which a crystalline phase and an amorphous phase coexist. For another example, CN103915628A reports a scheme for iron-doping by K, Mg, Al, Cu, Mn, Ti, Zn, etc.

[0005] As mentioned above, although the existing technology can improve the problems faced by iron-based polyanion materials to a certain extent, it is still unable to adapt to the needs of electrochemical applications under high loads and is difficult to achieve scale-up applications. Summary of the Invention

[0006] In view of the problem of unsatisfactory high-load activity faced by existing iron-based sodium electrolytic materials, the first purpose of the present invention is to provide a Na a Fe b X c Y d (BO3) e Cathode active materials are designed to improve their electrochemical performance under high loads.

[0007] The second object of the present invention is to provide the Na a Fe b X c Y d (BO3) e Preparation method of positive electrode active material and its application.

[0008] The third object of the present invention is to provide a method comprising a Fe b X c Y d (BO3) e Sodium ion battery with positive electrode active material and positive electrode and positive electrode material thereof.

[0009] A Na a Fe b X c Y d (BO3) e Positive electrode active material, wherein X is at least one of F, Cl, and Br; and Y is at least one of phosphorus oxide anion and silicon oxide anion;

[0010] The range of a is 1 to 4, the range of b is 1 to 4, the range of c is 0.3 to 2, the range of d is 0.3 to 2; the range of e is 0.01 to 0.1.

[0011] The present invention provides an iron-based sodium electroactive material with a synergistic composite of three anions, X, Y, and BO3. Studies have shown that through the synergistic effect of the three anions, X, Y, and BO3, and further combined control of parameters, unexpected synergy can be achieved, which can significantly improve the structural stability of the material, regulate the induction effect, optimize the voltage platform during the material's charge and discharge process, and slow down the capacity loss during the charge and discharge process, thereby unexpectedly and significantly improving its electrochemical performance under high load.

[0012] In the present invention, the combination of BO3 and X and Y is key to synergistically improving its structural stability and electrochemical performance under high loads. Research in the present invention also shows that optimizing the ratio of X, Y, and other elements can further synergistically improve the electrochemical performance of the material under high loads.

[0013] Preferably, the X comprises F. The present invention surprisingly shows that the use of F in combination with the Y and BO3 can unexpectedly further synergistically improve the electrochemical performance of the material under high load.

[0014] Preferably, in Y, the phosphorus oxide anion is at least one of phosphate and pyrophosphate, preferably phosphate; and the silicon oxide anion is silicate.

[0015] In the present invention, the Na a Fe b X c Y d (BO3) e Satisfies electrical neutrality.

[0016] Preferably, the Na a Fe b X c Y d (BO3) e In the positive electrode active material, X is F, Y is phosphate or silicate; a is 2 to 3, b is 1 to 2, c is 1 to 2, d is 0.5 to 1, and e is 0.01 to 0.1;

[0017] More preferably, a is 2, b is 1, X is F, c is 1, Y is phosphate, e is 0.02 to 0.07, preferably 0.04 to 0.06, more preferably 0.045 to 0.055, and d + e = 1. This preferred material has higher high-load stability.

[0018] Preferably, the Na a Fe b X c Y d (BO3) e Carbon materials are also allowed to exist in the positive electrode active material. In the present invention, the conductive carbon can be coated on the Na a Fe b X c Y d (BO3) e There is no particular requirement for the carbon content, and for example, it may be below 5 wt %, and further below 2 wt %.

[0019] The present invention also provides a Na a Fe b X c Y d (BO3) e The preparation method of the positive electrode active material comprises mixing the liquid phases of the raw materials with stoichiometric amounts (the molar ratio of Na:Fe:X:Y:B is a:b:c:d:e), controlling the pH of the mixed system to be 7.1-9.0, then desolventizing and drying to obtain a precursor, and then calcining the precursor to obtain the positive electrode active material.

[0020] The present invention has found that the Na a Fe b X c Y d (BO3) eDuring the preparation of positive electrode active materials, problems such as difficulty in synchronous recombination of X, Y and BO3 three anions and difficulty in exerting their target effects are easily encountered. The present invention innovatively recombines the raw materials of each element according to a stoichiometric ratio and jointly controls the parameters such as pH and sintering temperature. This unexpectedly achieves synergy, enables the synchronous recombination of the three anions, and controls the impurity phase in the synthesis process, thereby improving the electrochemical properties of the prepared material, especially improving the electrochemical properties of the prepared material under high load.

[0021] In the present invention, the raw materials include raw materials that can provide Na, Fe, X, Y and B;

[0022] In one embodiment of the present invention, the raw materials include a sodium source, an iron source, an X source, a Y source, and a boric acid source;

[0023] In the present invention, the sodium source includes at least one of sodium carboxylate, sodium carbonate, sodium nitrate, sodium chloride, sodium fluoride, and sodium bromide. The sodium carboxylate may be, for example, sodium acetate, sodium formate, sodium citrate, sodium oxalate, or the like.

[0024] In the present invention, the iron source includes at least one of ferric nitrate, organic acid iron, ferric oxalate, ferrous oxalate, and ferric phosphate. The organic acid iron may be, for example, ferrous gluconate, ferrous acetate, or the like.

[0025] In the present invention, the X source is a salt containing the X anion and the cation is H, Na and / or iron, and can further include at least one of sodium fluoride, ammonium fluoride, ammonium chloride, sodium chloride, ammonium bromide, and sodium bromide.

[0026] In the present invention, the Y source is a salt containing the Y anion and the cation is H, Na and / or iron, and can also be an ester containing Y, and can further include at least one of phosphoric acid, sodium phosphate, sodium hydrogen phosphate, sodium dihydrogen phosphate, ammonium phosphate, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, iron phosphate, sodium silicate, and silicate.

[0027] In the present invention, the boric acid source is at least one of boric acid, sodium borate, and ammonium borate;

[0028] Preferably, in the raw materials, Fe, X, Y and B are measured in a stoichiometric molar ratio.

[0029] In the present invention, the raw materials may also contain a carbon source, such as a polymer, a sugar, or an organic acid. The organic acid may be a compound that can provide an organic acid anion (such as oxalate, malate, citrate) and at least one cation selected from H, Na, Fe, and ammonium ions.

[0030] Research in the present invention shows that the combination of materials with the elemental components, further coordinated with the joint control of the ratio of the elements, can further synergize and improve the stability of the prepared materials at high load thickness. For example, research in the present invention shows that when the raw materials are controlled to be 2-3, b is 1-2, c is 1-2, d is 0.5-1, and e is 0.01-0.1; preferably, a is controlled to be 2, b is 1, X is F, and c is 1, Y is phosphate, e is 0.02-0.07, preferably 0.04-0.06, and d+e=1. This can further improve the electrochemical stability of the prepared materials at high electrode load thickness.

[0031] In the present invention, the solvent in the liquid phase mixing stage is at least one of water and C1 to C4 alcohols;

[0032] In the present invention, the liquid phase mixing process is carried out with the assistance of at least one of ball milling, uniform milling and sand milling;

[0033] In the present invention, ammonia water is used to regulate the pH;

[0034] In the present invention, under the above-mentioned innovation of joint control of raw materials and proportions, further coordinated with the joint control of pH during the treatment process, the electrochemical performance of the prepared material under high positive electrode material loading thickness can be further synergistically improved.

[0035] Preferably, the pH is 7.5 to 8.7, more preferably 8.2 to 8.4. Studies have surprisingly shown that, under the above preparation process, further combined with the preferred control of the pH, the performance of the prepared material can be further synergistically improved, further adapting to the application requirements of high positive electrode material load electrodes, and achieving even better performance.

[0036] In the present invention, the desolventizing and drying method is spray drying;

[0037] In the present invention, the feed rate in the spray drying stage is 100-500 ml / h, the inlet temperature is 160-300°C, the outlet temperature is 90-150°C, the air inlet rate is 10-50 L / h, and the carrier gas is at least one of dry air, nitrogen, and argon.

[0038] In the present invention, the atmosphere in the calcination stage is a protective atmosphere, and may further be at least one of nitrogen and an inert gas.

[0039] In the present invention, under the above process, further combined with the control of the calcination temperature helps to solve the problems faced by the prepared material under high positive electrode load thickness, and can still show excellent stability under higher electrode load thickness.

[0040] In the present invention, the temperature in the calcination stage is 575-630°C, preferably 595-605°C;

[0041] Preferably, the heat preservation and roasting time is 4 to 12 hours, and further can be 9 to 11 hours;

[0042] The present invention also provides a Na a Fe b X c Y d (BO3) e Application of positive electrode active materials: as positive electrode active materials, used in the preparation of sodium ion batteries.

[0043] In the present invention, the Na a Fe b X c Y d (BO3) e The positive electrode active material is used to prepare the required sodium ion battery and its positive electrode and positive electrode material.

[0044] The present invention also provides a positive electrode for a sodium ion battery, comprising a current collector and a positive electrode material composited on the surface thereof, wherein the positive electrode material comprises a positive electrode active material, and the positive electrode active material comprises the Na a Fe b X c Y d (BO3) e Positive electrode active material.

[0045] In the present invention, in the positive electrode active material, the Na a Fe b X c Y d (BO3) e The content of the positive electrode active material may be more than 10 wt %, further more than 50 wt %, further more than 90 wt %, further more than the Na a Fe b X c Y d (BO3) e Positive electrode active material.

[0046] Preferably, the content of the positive electrode active material in the positive electrode material is above 60 wt.%, preferably 70 to 90 wt.%;

[0047] Preferably, the positive electrode material may also contain a conductive agent and a binder;

[0048] Preferably, the content of the conductive agent and the binder is below 15 wt.%, preferably 1-10 wt.%, and further can be 4-7 wt.%. The technical solution of the present invention can still achieve excellent electrochemical performance under low conductive agent compounding conditions and can still adapt to application requirements of high load thickness.

[0049] In the present invention, there is no special requirement for the thickness of the positive electrode material, for example, it can be above 100 μm, but considering the special advantages of the material described in the present invention, it can also be applied to pole pieces with a thickness of 150 to 200 μm (further 170 to 190 μm), so that better technical advantages can be obtained.

[0050] The present invention also provides a sodium ion battery, comprising a battery core and an electrolyte for soaking the battery core, wherein the battery core comprises a positive electrode, a separator and a negative electrode that are composited in sequence, and the positive electrode is a positive electrode comprising the positive electrode active material of the present invention.

[0051] In the present invention, in addition to the positive electrode active material of the present invention, the other components and parts of the sodium ion battery can be well-known.

[0052] Beneficial effects

[0053] The present invention provides an X, Y, BO3 three-anion synergistic composite iron-based sodium electroactive material, which has excellent electrochemical performance under high load.

[0054] The present invention innovatively compounds the raw materials and jointly controls the pH, thereby unexpectedly achieving synergy, realizing the synchronous recombination of the three anions, and effectively controlling the impurity phase during the synthesis process, thereby improving the electrochemical properties of the prepared material, especially improving the electrochemical properties of the prepared material under high load. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 This is the SEM image of the material prepared in Example 1.

[0056] Figure 2 This is the first cycle charge and discharge curve of the material prepared in Example 1. DETAILED DESCRIPTION

[0057] Example 1

[0058] (1) Sodium fluoride (3 mol) and ferric phosphate (2.85 mol) were added to 1 L of pure water and mechanically stirred at 500 rpm for 0.5 h to obtain a fluorophosphate anion mixture. Boric acid (0.15 mol) was added to the fluorophosphate anion mixture and mechanically stirred at 500 rpm for 0.5 h to obtain a polyanion coordination slurry.

[0059] (2) Sodium citrate (1 mol) and ferrous oxalate (0.15 mol) were added to the above-mentioned polyanion coordination slurry, and mechanical stirring was performed at a stirring rate of 500 rpm for 0.5 h. The pH value of the mixed system was controlled to 8.3 to obtain a slurry.

[0060] (3) The slurry was coarsely ground in a ball mill. The ball-to-material ratio was controlled at 20:1. The coarse grinding speed was set to 500 rpm and the grinding time was set to 60 min to obtain a coarsely ground slurry. The coarsely ground slurry was further finely ground in a sand mill at a speed of 2000 rpm and a grinding time of 1 h to obtain a finely ground slurry.

[0061] (4) The finely ground slurry was spray granulated to obtain a precursor. The feed rate of the spray granulation was 300 ml / h, the inlet temperature was 220°C, the outlet temperature was 100°C, the air inlet rate was 10-50 L / h, and the carrier gas was dry air.

[0062] (5) The precursor is placed in a box furnace for heating and sintering to obtain an iron-based composite anion material. The heating time is set to 300 min, the sintering temperature is 600 ° C, the sintering time is 10 h, and the sintering atmosphere is nitrogen atmosphere to obtain Na2FeF(PO4) 0.95 (BO3) 0.05 / C material.

[0063] Example 2

[0064] Compared with Example 1, the only difference is that the amount of ferric phosphate in step (1) is 2.79 mol, the amount of boric acid is 0.21 mol; the amount of ferrous oxalate in step (2) is 0.21 mol, and the other operations and parameters are the same as those in Example 1, and Na2FeF(PO4) is finally obtained. 0.93 (BO3) 0.07 / C material.

[0065] Example 3

[0066] Compared with Example 1, the only difference is that the amount of ferric phosphate in step (1) is 2.94 mol, the amount of boric acid is 0.06 mol; the amount of ferrous oxalate in step (2) is 0.06 mol, and the other operations and parameters are the same as those in Example 1, and Na2FeF(PO4) is finally obtained. 0.98 (BO3) 0.02 / C material.

[0067] Example 4

[0068] Compared with Example 1, the only difference is that an equimolar amount of NaBr is used to replace the NaF, and the other operations and parameters are the same as those in Example 1. The other operations and parameters are the same as those in Example 1, and finally Na2FeBr(PO4) is prepared. 0.95 (BO3) 0.05 / C material.

[0069] Example 5

[0070] Compared with Example 1, the only difference is that an equimolar amount of NaCl is used to replace the NaF, and the other operations and parameters are the same as those in Example 1. The other operations and parameters are the same as those in Example 1, and Na2FeCl(PO4) is finally prepared. 0.95 (BO3) 0.05 / C material.

[0071] Example 6

[0072] Compared with Example 1, the only difference is that in step (1), ferrous phosphate is replaced by ferrous oxalate with an equal molar amount of iron, and tetraethyl silicate is added (the molar amount of Si is the same as P in Example 1). Other operations and parameters are the same as those in Example 1. The obtained Na2FeF(SiO4) 0.95 (BO3) 0.05 / C material.

[0073] Example 7

[0074] Compared with Example 1, the only difference is that in step (2), the pH value of the mixed system is controlled to be 8.7, and the other operations and parameters are the same as in Example 1.

[0075] Example 8

[0076] Compared with Example 1, the only difference is that in step (2), the pH value of the mixed system is controlled to be 7.6, and other operations and parameters are the same as in Example 1.

[0077] Example 9

[0078] Compared with Example 1, the only difference is that in step (5), the sintering temperature is set to 580° C. and the sintering time is 11 h. Other operations and parameters are the same as in Example 1.

[0079] Example 10

[0080] Compared with Example 1, the only difference is that in step (5), the sintering temperature is set to 620° C. and the sintering time is 9 h. Other operations and parameters are the same as in Example 1.

[0081] Comparative Example 1

[0082] Compared with Example 1, the only difference is that in step (1), ammonium molybdate is used to replace the boric acid (wherein the added molar amounts of Mo and B are the same), and other operations and parameters are the same as in Example 1.

[0083] Comparative Example 2

[0084] Compared with Example 1, the only difference is that in step (1), ammonium tungstate is used to replace the boric acid (wherein the added molar amounts of W and B are the same), and other operations and parameters are the same as in Example 1.

[0085] Comparative Example 3

[0086] Compared with Example 1, the only difference is that in step (1), ammonium phosphate is used to replace the boric acid (wherein the added molar amounts of P and B are the same), and other operations and parameters are the same as in Example 1.

[0087] Comparative Example 4

[0088] Compared with Example 1, the only difference is that in step (1), the amount of ferric phosphate is 3 mol, the amount of sodium fluoride is 3 mol, and boric acid is absent; the amount of ferrous oxalate in step (2) is 0 mol, and other operations and parameters are the same as in Example 1.

[0089] Comparative Example 5

[0090] Compared with Example 1, the only difference is that in step (2), the pH value of the mixed system is controlled to be 9.5, and the other operations and parameters are the same as in Example 1.

[0091] Comparative Example 6

[0092] Compared with Example 1, the only difference is that in step (1), the amount of ferric phosphate is 2.4 mol, the amount of boric acid is 0.6 mol; in step (2), the amount of ferrous oxalate is 0.6 mol, and other operations and parameters are the same as in Example 1.

[0093] Comparative Example 7

[0094] Compared with Example 1, the only difference is that in step (5), the sintering temperature is set to 500° C., and other operations and parameters are the same as Example 1.

[0095] Comparative Example 8

[0096] Compared with Example 1, the only difference is that in step (5), the sintering temperature is set to 750° C., and other operations and parameters are the same as Example 1.

[0097] The main steps of the test are:

[0098] (1) Using a 2032 model battery case, the positive electrode is a pole piece of the prepared iron-based composite phosphate material (the current collector is aluminum foil, the active material: conductive carbon: PVDF = 90:6:4), with a thickness of 180 μm, the negative electrode is sodium metal, and a glass fiber separator (model Whatman Grade GF / D) is used. The electrolyte is 1M NaClO4 (pure PC + 5% FEC) to assemble the battery;

[0099] (2) The rest time is 12 h, and a program for testing cyclic stability is set up, cycling 5 times at 0.1C, followed by 200 cycles at 1C.

[0100] (3) The theoretical specific capacity of the positive electrode active material is set to 124 mAh / g;

[0101] The test results are shown in Table 1:

[0102] Discharge capacity at 0.1C (mAh / g) Median voltage (V) Capacity retention rate at 1C Example 1 110.38 2.8874 90％ Example 2 109.6 2.8804 88％ Example 3 108.9 2.8857 85％ Example 4 105.2 2.8793 83％ Example 5 106.1 2.8789 85％ Example 6 107.1 2.8856 89％ Example 7 104.4 2.8759 82.4％ Example 8 103.8 2.8824 81.9％ Example 9 102.7 2.8811 81％ Example 10 103.2 2.8824 81.4％ Comparative Example 1 97.1 2.8746 79％ Comparative Example 2 96.6 2.8730 72％ Comparative Example 3 95.4 2.8695 73％ Comparative Example 4 88.8 2.8613 67.3％ Comparative Example 5 84.6 2.8697 68.9％ Comparative Example 6 81.3 2.8661 64.1％ Comparative Example 7 84.5 2.8679 66.7％ Comparative Example 8 76.2 2.8559 58.1％

[0103] In summary, by compounding the raw materials and jointly controlling the pH, the synchronous compounding of the three anions can be achieved, and the impurities in the synthesis process can be effectively controlled, thereby improving the electrochemical properties of the prepared materials, especially improving the electrochemical properties of the prepared materials under high load.

Claims

1. Na a Fe b X c Y d (BO3) e A positive electrode active material, characterized in that The X is at least one of F, Cl, and Br; the Y is at least one of phosphorus oxide anion and silicon oxide anion; The range of a is 1-4, the range of b is 1-4, the range of c is 0.3-2, the range of d is 0.3-2; the range of e is 0.01-0.

1.

2. Na as claimed in claim 1 a Fe b X c Y d (BO3) e A positive electrode active material, characterized in that The X includes Cl and F.

3. Na as claimed in claim 1 a Fe b X c Y d (BO3) e A positive electrode active material, characterized in that In the Y, the phosphorus oxyanion is at least one of phosphate and pyrophosphate; The silicon oxide anion is silicate.

4. Na as claimed in claim 1 a Fe b X c Y d (BO3) e A positive electrode active material, characterized in that The Na a Fe b X c Y d (BO3) e In the positive electrode active material, a is 2 to 3, b is 1 to 2, c is 1 to 2, d is 0.5 to 1, and e is 0.01 to 0.

1.

5. Na as claimed in claim 1 a Fe b X c Y d (BO3) e A positive electrode active material, characterized in that The a is 2, the b is 1, the X is F, and c is 1, the Y is a phosphate group, the e is 0.02-0.07, and d+e=1.

6. Na as claimed in claim 1 a Fe b X c Y d (BO3) e A positive electrode active material, characterized in that The Na a Fe b X c Y d (BO3) e Carbon materials are also present in the positive electrode active materials.

7. A Na according to any one of claims 1 to 6 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: The stoichiometric liquid raw materials are mixed, and the pH of the mixed system is controlled to be 7.1-9, followed by desolventization and drying to obtain a precursor, and then the precursor is calcined at a temperature of 550-700°C to obtain the product.

8. Na as claimed in claim 7 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: The raw materials include raw materials capable of providing Na, Fe, X, Y and B.

9. Na as claimed in claim 7 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: The raw materials include a sodium source, an iron source, an X source, a Y source and a boric acid source.

10. Na as claimed in claim 9 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: The sodium source includes at least one of sodium carboxylate, sodium carbonate, sodium nitrate, sodium chloride, sodium fluoride, and sodium bromide; The iron source includes at least one of ferric nitrate, organic acid iron, ferric oxalate, ferrous oxalate, and ferric phosphate; The X source is a salt containing the X anion and the cation is H, Na and / or iron; The Y source is a salt containing the Y anion and the cation is H, Na and / or iron, or an ester containing Y; The boric acid source is at least one of boric acid, sodium borate and ammonium borate.

11. Na as claimed in claim 7 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: The solvent in the liquid phase mixing stage is at least one of water and C1 to C4 alcohol.

12. Na as claimed in claim 11 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: The liquid phase mixing process is carried out with the assistance of at least one of ball milling, homogenizing milling and sand milling.

13. Na as claimed in claim 7 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: The desolventizing and drying method is spray drying.

14. Na as claimed in claim 7 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: Ammonia water was used to adjust the pH.

15. Na as claimed in claim 7 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: The pH is 7.5-8.

7.

16. Na as claimed in claim 13 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: The feed rate of the spray drying stage is 100-500 ml / h, the inlet temperature is 160-300°C, the outlet temperature is 90-150°C, the air inlet rate is 10-50 L / h, and the carrier gas is at least one of dry air, nitrogen, and argon.

17. Na as claimed in claim 7 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: The temperature of the calcination stage is 575-630 °C.

18. Na as claimed in claim 17 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: The holding time for roasting is 4~12 h.

19. Na as claimed in claim 17 a Fe b X c Y d (BO3) e A method for preparing a positive electrode active material, characterized in that: The atmosphere during the calcination stage is a protective atmosphere.

20. A Na according to any one of claims 1 to 6 a Fe b X c Y d (BO3) e Positive electrode active material or Na prepared by any one of claims 7 to 19 a Fe b X c Y d (BO3) e The application of positive electrode active material is characterized in that As positive electrode active material, it is used to prepare sodium ion batteries.

21. A positive electrode for a sodium ion battery, comprising a current collector and a positive electrode material composited on the surface of the current collector, wherein the positive electrode material comprises a positive electrode active material, characterized in that: The positive electrode active material comprises the Na according to any one of claims 1 to 6 a Fe b X c Y d (BO3) e Positive electrode active material or Na prepared by any one of claims 7 to 19 a Fe b X c Y d (BO3) e Positive electrode active material.

22. The positive electrode of the sodium ion battery according to claim 21, characterized in that The content of the positive electrode active material in the positive electrode material is above 60 wt.%.

23. The positive electrode of the sodium ion battery according to claim 22, characterized in that The content of the positive electrode active material in the positive electrode material is 70-90 wt.%.

24. The positive electrode of the sodium ion battery according to claim 21, characterized in that The positive electrode material also includes a conductive agent and a binder.

25. The positive electrode of the sodium ion battery according to claim 24, characterized in that The content of the conductive agent and the binder is below 15 wt.%.

26. The positive electrode of the sodium ion battery according to claim 25, characterized in that The content of the conductive agent and the binder is 1-10 wt.%.

27. The positive electrode of a sodium ion battery according to any one of claims 21 to 26, wherein The thickness of the positive electrode material is above 100 μm.

28. The positive electrode of the sodium ion battery according to claim 27, characterized in that The thickness of the positive electrode material is 150-200 μm.

29. A sodium ion battery comprising a battery cell and an electrolyte for soaking the battery cell, wherein the battery cell comprises a positive electrode, a separator and a negative electrode which are composited in sequence, characterized in that: The positive electrode is the positive electrode according to any one of claims 21 to 28.

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