Positive electrode active material, and preparation method therefor and use thereof

Abstract

Provided in the present application are a positive electrode active material, and a preparation method therefor and the use thereof. The positive electrode active material comprises a core and a carbon coating layer arranged on at least part of the surface of the core, wherein the core has the chemical composition Na4Fex(PO4)2(P2O7), where 2.8≤x≤3.0. Moreover, in an X-ray diffraction pattern of the positive electrode active material, a first diffraction peak is present at the position where 2θ is 33.6°, a second diffraction peak exists is present at the position where 2θ is 32.9°, and the ratio of the peak intensity of the first diffraction peak to the peak intensity of the second diffraction peak is larger than 22.5. The positive electrode active material has relatively high specific capacity, relatively high sodium ion diffusion capability and electronic conductivity, and can effectively improve the energy density and low-temperature cycle performance of a battery.

Description

A positive electrode active material and a preparation method and application thereof

[0001] The present application claims priority to the Chinese patent application No. 202411390154.6, filed on September 30, 2024, and entitled "A positive electrode active material and a preparation method and application thereof", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application belongs to the field of sodium ion batteries, and relates to a positive electrode active material, in particular to a positive electrode active material and a preparation method and application thereof. BACKGROUND

[0003] With the deepening of the research on sodium battery materials, sodium iron phosphate pyrophosphate (NFPP) positive electrode material has attracted attention in the energy storage market due to its high structural stability and wide selection of raw materials for synthesis, and is expected to become the next generation of energy storage positive electrode material. However, the low energy density of the NFPP positive electrode material has greatly limited its application. At present, the energy density of the battery is generally improved by increasing the working voltage of the sodium ion battery or the compaction density of the positive electrode sheet, but the improvement is limited. Moreover, the low-temperature cycle performance of the positive electrode material is also poor, which seriously hinders the development of sodium iron phosphate pyrophosphate in the battery field.

[0004] Therefore, how to further improve the sodium iron phosphate pyrophosphate (NFPP) positive electrode material to improve the energy density and low-temperature cycle performance of the sodium ion battery is a problem that needs to be solved in the field. SUMMARY

[0005] In view of the above defects, the present application provides a positive electrode active material, which has a high gram capacity and high sodium ion diffusion capacity and electronic conductivity, and can effectively improve the energy density and low-temperature cycle performance of the battery.

[0006] The present application also provides a preparation method of the above positive electrode active material, and the positive electrode active material prepared by the preparation method not only has a high gram capacity, but also has a high sodium ion diffusion capacity and electronic conductivity, so that the sodium ion battery has a high energy density and low-temperature cycle performance.

[0007] The present application also provides a positive electrode sheet comprising the above positive electrode active material or the positive electrode active material prepared by the above preparation method, and the positive electrode sheet can effectively improve the energy density and low-temperature cycle performance of the battery when applied to the sodium ion battery.

[0008] This application provides a sodium-ion battery, including the above-mentioned positive electrode active material, or the positive electrode active material prepared by the above-mentioned preparation method, or the above-mentioned positive electrode sheet. Therefore, the sodium-ion battery has high energy density and low-temperature cycle performance.

[0009] This application provides a positive electrode active material, the positive electrode active material comprising a core and a carbon coating layer disposed on at least a portion of the surface of the core, the core comprising a chemical composition shown in Formula 1, Na4Fe x (PO4)2(P2O7) Equation 1

[0010] In Equation 1, 2.8 ≤ x ≤ 3.0;

[0011] In the X-ray diffraction pattern of the positive electrode active material, there is a first diffraction peak at 2θ = 33.6° and a second diffraction peak at 2θ = 32.9°. The ratio of the peak intensity of the first diffraction peak to the peak intensity of the second diffraction peak is >22.5.

[0012] Furthermore, the carbon coating layer in the positive electrode active material has a mass percentage C of 1.50wt%-2.15wt%.

[0013] Furthermore, the median particle size D50 of the positive electrode active material is 8μm-10μm, and the specific surface area BET of the positive electrode active material is not higher than 13.5m². 2 / g.

[0014] Furthermore, the positive electrode active material satisfies any one of Equations 2, 3, and 4.

[0015] 1.5% < C < 1.7% and 8.5 < BET < 10.5 (Equation 2);

[0016] 1.7% ≤ C < 1.9% and 10.5 ≤ BET < 11.5 (Equation 3);

[0017] 1.9% ≤ C < 2.15% and 11.5 ≤ BET < 13.5 Equation 4.

[0018] Furthermore, the compacted density of the positive electrode active material powder is 1.91-2.01 g / cm³. 3 .

[0019] Furthermore, the positive electrode active material also includes doping elements;

[0020] The doping element includes at least one of B, F, Al, and N.

[0021] Furthermore, the mass percentage of doped elements in the positive electrode active material is 500ppm-2000ppm.

[0022] The application also provides a preparation method of the positive electrode active material according to any one of the above, comprising the following steps:

[0023] 1) mixing raw materials including a sodium source, an iron source, a phosphorus source and a carbon source with deionized water to obtain a mixture; grinding the mixture to obtain a ground material; the D10 of the ground material is 0.10-2 μm, the D50 is 0.25-0.3 μm, and the D90 is 0.7-1.0 μm;

[0024] 2) spray drying the ground material to obtain a spray material; the water content of the spray material is ≤3%, and the loose bulk density is ≤0.8 g / cm 3 ;

[0025] 3) fluidized drying the spray material, and during the process, the fluidization temperature is 600-1000℃, the gas flow rate is 5-20 m / s, and the fluidization time is 3-5 s to obtain a fluidized material;

[0026] 4) under a protective atmosphere, using a rotary kiln, sequentially sintering the fluidized material to obtain the positive electrode active material;

[0027] The temperature of the first sintering is 200-250℃, and the holding time is 1-2 h; the temperature of the second sintering is 400-550℃, and the holding time is 10-20 h.

[0028] Further, in step 1), the grinding is performed by a ball milling method;

[0029] The grinding comprises: sequentially performing first grinding and second grinding on the mixture to obtain the ground material; the rotation speed of the first grinding is 1000-1300 r / min, the grinding time is 30-50 min, and the particle size of the grinding beads is 0.6-0.8 μm; the rotation speed of the second grinding is 1400-1600 r / min, the grinding time is 30-60 min, and the particle size of the grinding beads is 0.3-0.4 μm.

[0030] Further, in step 2), the inlet air temperature during the spray drying process is 175-205℃, and the outlet air temperature is 80-110℃, and the difference between the inlet air temperature and the outlet air temperature is 95-105℃.

[0031] The application also provides a positive electrode sheet comprising the positive electrode active material according to any one of the above, or the positive electrode active material prepared by the above preparation method.

[0032] The application also provides a sodium ion battery comprising the positive electrode active material according to any one of the above, or the positive electrode active material prepared by the above preparation method, or the positive electrode sheet.

[0033] The present application effectively improves the specific capacity of the positive electrode active material, and improves the electronic conductivity and sodium ion migration rate of the positive electrode active material, thereby effectively improving the energy density and low-temperature cycle performance of the sodium ion battery, by making the positive electrode active material include a core having a chemical composition of Na4Fe x (PO4)2(P2O7) and a carbon coating layer located on at least part of the surface of the core, while making the ratio of the peak intensity of the first diffraction peak at 33.6° to the second diffraction peak at 32.9° in the X-ray diffraction pattern of the positive electrode active material > 22.5. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is an SEM image of the fluidized material in Example 1 of the present application;

[0035] Figure 2 is an SEM image of the positive electrode active material in Example 1 of the present application at 1.00k;

[0036] Figure 3 is an SEM image of the positive electrode active material in Example 3 of the present application at 1.00k;

[0037] Figure 4 is an SEM image of the positive electrode active material in Example 3 of the present application at 30.0k;

[0038] Figure 5 is an SEM image of the positive electrode active material in Comparative Example 8 of the present application at 30.0k;

[0039] Figure 6 is an XRD spectrum of the positive electrode active material in Example 1 of the present application. DETAILED DESCRIPTION

[0040] To make the purpose, technical scheme and advantages of the present application clearer, the technical scheme in the embodiments of the present application will be described clearly and completely below in combination with the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present application.

[0041] The first aspect of the present application provides a positive electrode active material, which includes a core and a carbon coating layer arranged on at least part of the surface of the core, and the core includes a chemical composition shown in Formula 1, Na4Fe x (PO4)2(P2O7) Formula 1

[0042] In Formula 1, 2.8≤x≤3.0;

[0043] The X-ray diffraction pattern of the positive electrode active material has a first diffraction peak at 2θ of 33.6° and a second diffraction peak at 2θ of 32.9°, and the ratio of the peak intensity of the first diffraction peak to the peak intensity of the second diffraction peak is greater than 22.5.

[0044] Further, the positive electrode active material in the application is a polycrystalline positive electrode material.

[0045] The X-ray diffraction pattern of the positive electrode active material in the application is obtained by performing XRD testing on the positive electrode active material and refining the obtained X-ray diffraction spectrum, and the refining coefficient Rwp is less than 9. The testing parameters are as follows: the radiation line is Cu target, Kα ray, the wavelength is 0.154056 nm, the scanning angle range is 10°-80°, the scanning rate is 5° / min, and the step is 0.0065°.

[0046] In the application, the positive electrode active material comprises a core and a carbon coating layer located at least part of the outer surface of the core, and the core comprises a chemical composition Na4Fe x (PO4)2(P2O7), wherein 2.8≤x≤3, which can effectively improve the electronic conductivity of the positive electrode active material; at the same time, the positive electrode active material is a iron-deficient phase, which can significantly improve the diffusion rate of sodium ions, thereby effectively improving the charge-discharge performance of the sodium ion battery, and making it have higher low-temperature cycle performance; at the same time, in the X-ray diffraction pattern of the positive electrode active material, there is a first diffraction peak at 2θ of 33.6° and a second diffraction peak at 2θ of 32.9°, and the ratio of the peak intensity of the first diffraction peak to the peak intensity of the second diffraction peak is greater than 22.5, wherein the first diffraction peak is a characteristic main peak of sodium iron pyrophosphate (NFPP), and the second diffraction peak is a characteristic impurity peak of sodium iron phosphate with a sodium iron phosphate structure, and sodium iron phosphate has no electrochemical activity. When the ratio of the peak intensity of the first diffraction peak to the peak intensity of the second diffraction peak is greater than 22.5, the content of the sodium iron phosphate impurity phase in the positive electrode active material is low, and the purity is high, which can effectively improve the specific capacity of the positive electrode active material, thereby effectively improving the energy density of the sodium ion battery.

[0047] In addition, since the content of the sodium iron phosphate impurity phase in the positive electrode active material in the application is low, the occurrence of side reactions between the positive electrode active material and the electrolyte can also be effectively inhibited, which helps to improve the cycle performance and safety of the battery.

[0048] In a specific embodiment, the mass percentage content C of the carbon coating layer in the positive electrode active material is 1.50wt%-2.15wt%. Within this range, the positive electrode active material not only has high electronic conductivity and powder compaction density, but also can reduce the energy density loss caused by non-active substance carbon, further improving the energy density and low-temperature cycle performance of the battery.

[0049] Exemplarily, the mass percentage content C of the carbon coating layer in the positive electrode active material is 1.50 wt%, 1.55 wt%, 1.60 wt%, 1.65 wt%, 1.70 wt%, 1.75 wt%, 1.80 wt%, 1.85 wt%, 1.90 wt%, 1.95 wt%, 2.00 wt%, 2.05 wt%, 2.10 wt%, or 2.15 wt%.

[0050] The mass percentage content of the carbon coating layer in the positive electrode active material in the present application can be obtained by an elemental analyzer.

[0051] In a specific embodiment, the median particle size D50 of the positive electrode active material is 8-10 μm, and the specific surface area BET of the positive electrode active material is not higher than 13.5 m 2 / g. Under this range, the positive electrode active material has a higher powder compaction density, which is beneficial to improve the energy density of the battery; and the carbon coating layer is coated more uniformly, which can further improve the electrical conductivity of the positive electrode active material, so that the battery has higher low-temperature cycle performance; in addition, the specific surface area of the positive electrode active material is lower, which can effectively reduce the side reaction between the electrolyte and the positive electrode, thereby effectively improving the cycle performance of the battery.

[0052] D50 in the present application refers to the particle size corresponding to the cumulative particle size distribution percentage of 50% of the sample. It can be obtained by a laser particle size analyzer.

[0053] In a specific embodiment, the positive electrode active material satisfies any one of formula 2, formula 3, formula 4,

[0054] 1.5% < C < 1.7% and 8.5 < BET < 10.5 Formula 2;

[0055] 1.7% < C < 1.9% and 10.5 < BET < 11.5 Formula 3;

[0056] 1.9% < C < 2.15% and 11.5 < BET < 13.5 Formula 4.

[0057] When the positive electrode active material satisfies any one of the above formula 2, formula 3, formula 3, the coating effect of the carbon coating layer is better and more uniform, which is beneficial to improve the electronic conductivity of the positive electrode active material; at the same time, it can further reduce the loss of energy density caused by non-active substance carbon, so that the sodium ion battery has higher low-temperature cycle performance and energy density.

[0058] In a specific embodiment, the powder compaction density of the positive electrode active material is 1.91-2.01 g / cm 3 . For example, the compaction density is 1.91 g / cm 3 , 1.92 g / cm3 1.93 g / cm3 3 1.94 g / cm3 3 1.95 g / cm3 3 1.96 g / cm3 3 1.97 g / cm3 3 1.98 g / cm3 3 1.99 g / cm3 3 2.00 g / cm3 3 or 2.01 g / cm3 3 Within this range, the powder compaction density of the positive electrode active material is higher, which can achieve a higher compaction density, thereby further improving the energy density of the sodium ion battery.

[0059] The powder compaction density in the present application is tested under a pressure of 1T (equivalent to 74.0Mpa).

[0060] In a specific embodiment, the positive electrode active material further comprises a doping element; the doping element comprises at least one of B, F, Al, and N.

[0061] It should be noted that the aforementioned doping element is coated on the surface of the carbon coating layer.

[0062] When the positive electrode active material further comprises the aforementioned doping element, on the one hand, the structural stability of the positive electrode active material can be improved, the voltage of the positive electrode active material can be improved, and the capacity can be fully utilized during the charging and discharging process; on the other hand, the problems of low balling rate and easy breakage of particles during the spray drying process can be effectively solved, the regularity of the particle morphology of the positive electrode active material is improved, the side reaction between the electrolyte and the positive electrode is reduced, thereby further improving the energy density and the cycle stability of the sodium ion battery.

[0063] In a specific embodiment, the mass percentage content of the doping element in the positive electrode active material is 500ppm-2000ppm. Within this range, not only can the structural stability of the positive electrode active material and the regularity of the morphology be further improved, but also the problem of reduced charging and discharging capacity caused by excessive doping amount can be avoided.

[0064] Exemplarily, the mass percentage content of the doping element in the positive electrode active material is 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm, 1500ppm, 1600ppm, 1700ppm, 1800ppm, 1900ppm, or 2000ppm.

[0065] The mass percentage of the doping element in the present application can be obtained by inductively coupled plasma optical emission spectrometer (ICP-OES Avio200 / Avio500).

[0066] The second aspect of the present application provides a preparation method of the positive electrode active material of the first aspect, comprising the following steps:

[0067] 1) mixing raw materials including a sodium source, an iron source, a phosphorus source, and a carbon source with deionized water to obtain a mixed material; grinding the mixed material to obtain a ground material; the D10 of the ground material is 0.10 μm-2 μm, the D50 is 0.25 μm-0.3 μm, and the D90 is 0.7 μm-1.0 μm;

[0068] 2) spray drying the ground material to obtain a sprayed material; the water content of the sprayed material is ≤3%, and the loose bulk density is ≤0.8 g / cm 3 ;

[0069] 3) fluidized drying the sprayed material, and in the process: the fluidization temperature is 600℃-1000℃, the gas flow rate is 5-20 m / s, and the fluidization time is 3-5 s, to obtain a fluidized material;

[0070] 4) under a protective atmosphere, using a rotary kiln, sequentially sintering the fluidized material to obtain a positive electrode active material;

[0071] The first sintering temperature is 200-250℃, and the holding time is 1-2 h; the second sintering temperature is 400-550℃, and the holding time is 10-20 h.

[0072] Specifically, in step 1), the raw materials including a sodium source, an iron source, a phosphorus source, and a carbon source are added to deionized water and uniformly mixed to obtain a mixed material; then the mixed material is ground, so that the D10 of the obtained ground material is 0.10 μm-2 μm, the D50 is 0.20 μm-0.3 μm, the D90 is 0.7 μm-1.0 μm, and further the D10 is 0.10 μm-2 μm, the D50 is 0.25 μm-0.3 μm, and the D90 is 0.7 μm-1.0 μm. At this time, the particle size distribution of the ground material is more uniform, which is conducive to improving the balling rate in the subsequent spray drying process, improving the uniformity of carbon coating, and also conducive to the capacity of the positive electrode active material.

[0073] The sodium source in the present application refers to a raw material providing sodium element, the iron source refers to a raw material providing iron element, the phosphorus source refers to a raw material providing phosphorus element, and the carbon source refers to a raw material providing carbon element. As long as the raw material contains the target element (Na, Fe, P, C), it belongs to the definition of the present application, and one target element can be introduced into the reaction system by one or more raw materials. For example, iron phosphate can be used as both an iron source and a phosphorus source, and sodium pyrophosphate can be used as both a sodium source and a phosphorus source.

[0074] The present application does not specifically limit the specific types of sodium source, iron source, phosphorus source and carbon source. For example, the sodium source includes at least one of sodium carbonate, sodium pyrophosphate, sodium dihydrogen phosphate, sodium acetate, disodium dihydrogen pyrophosphate, ammonium dihydrogen phosphate, and sodium phosphate; the iron source includes at least one of iron oxide, iron phosphate, ferrous oxalate, iron powder, iron acetate, and iron nitrate; the phosphorus source includes at least one of iron phosphate, sodium pyrophosphate, sodium dihydrogen phosphate, phosphoric acid, and disodium hydrogen phosphate; and the carbon source includes at least one of glucose, vitamin C, citric acid, Tween 60, sucrose, polyethylene glycol 2000, and oxalic acid.

[0075] The present application does not specifically limit the molar ratio of the sodium source, the iron source, and the phosphorus source, as long as the chemical composition of the core in the prepared positive electrode active material satisfies formula 1.

[0076] The present application does not specifically limit the mass percentage of the carbon source in the raw material, and further, the mass percentage of the carbon coating layer in the positive electrode active material can be further controlled by controlling the addition amount of the carbon source.

[0077] The present application does not specifically limit the specific source of the sodium source, the iron source, the phosphorus source, and the carbon source, and any product available on the market or prepared by a conventional method known to those skilled in the art can be used.

[0078] The present application does not specifically limit the mixing method, as long as the mixture is uniformly mixed, for example, the mixture can be mixed by mechanical stirring.

[0079] The present application does not specifically limit the solid content of the mixture, for example, the solid content is 15wt%-35wt%.

[0080] The present application does not specifically limit the grinding method, as long as the D10, D50, and D90 of the ground material are within the aforementioned ranges.

[0081] D10 in the present application refers to the particle size corresponding to 10% of the cumulative particle size distribution percentage of the sample, D50 refers to the particle size corresponding to 50% of the cumulative particle size distribution percentage of the sample, and D90 refers to the particle size corresponding to 90% of the cumulative particle size distribution percentage of the sample.

[0082] In step 2), the above-mentioned grinding material is subjected to a spray drying treatment to obtain a spray material with a water content ≤ 3%, a loose bulk density ≤ 0.8 g / cm 3 At this time, the evaporation of water in the fluidized drying process is facilitated, and the carbon corrosion problem caused by water in the subsequent sintering process is avoided, and the uniformity of the carbon coating layer is improved; at the same time, the preliminary formation of crystal nuclei and the preliminary coating of carbon sources in the fluidization process are facilitated, which is beneficial to the further growth of crystals and the effective coating of carbon sources in the subsequent sintering process.

[0083] Further, the median particle size D50 of the spray material can be further controlled so that the median particle size D50 of the positive electrode active material is between 8 μm and 10 μm; preferably, the median particle size D50 of the spray material is 8 μm to 10 μm.

[0084] Further, the balling rate of the spray material is not less than 95%.

[0085] The balling rate in the present application can be obtained by importing the SEM image under 30K into the Nano Measurer software.

[0086] The present application does not make specific limitations on the inlet air temperature and outlet air temperature in the spray drying process, only the water content and loose bulk density of the prepared spray material need to meet the above-mentioned ranges.

[0087] In step 3), the above-mentioned spray material is subjected to a fluidized drying treatment, the fluidized temperature is maintained at 600-1000℃, the gas flow rate is 5-20 m / s, and the fluidized time is 3-5 s, to obtain a fluidized material. In this process, on the one hand, the water in the spray material can be fully evaporated, avoiding the water corrosion of the carbon coating layer in the subsequent sintering process, and affecting the carbon coating effect; on the other hand, the spray material can undergo a preliminary reaction to generate crystal nuclei, improve the reaction activity, and facilitate the growth of the positive electrode active material in the subsequent sintering process, so that the element distribution is more uniform, and the uniformity of the carbon coating layer is improved.

[0088] Illustratively, the fluidized temperature is 600℃, 650℃, 700℃, 750℃, 800℃, 850℃, 900℃, 950℃ or 1000℃; the gas flow rate is 5 m / s, 8 m / s, 11 m / s, 14 m / s, 17 m / s or 20 m / s; and the fluidized time is 3 s, 4 s or 5 s.

[0089] The present application does not make specific limitations on the gas in the fluidized drying process, for example, it can be argon and / or nitrogen.

[0090] In step 4), the fluidized material is first sintered at 200-250°C for 1-2h and then sintered at 400-550°C for 10-20h in a rotary kiln under a protective atmosphere to prepare the positive electrode active material. In this process, the metal elements grow rapidly along the crystal plane of the crystal nucleus, and a uniform carbon coating layer is formed to obtain the positive electrode active material.

[0091] For example, the first sintering temperature is 200°C, 210°C, 220°C, 230°C, 240°C or 250°C; and the first sintering time is 1h, 1.2h, 1.4h, 1.6h, 1.8h or 2h.

[0092] For example, the second sintering temperature is 400°C, 430°C, 460°C, 490°C, 520°C or 550°C; and the second sintering time is 10h, 12h, 14h, 16h, 18h or 20h.

[0093] The protective atmosphere is not specifically limited in the present application, for example, nitrogen and / or argon can be used as the protective atmosphere.

[0094] Further, the furnace pressure M (MPa) and the sintering capacity T (tons) in the sintering process satisfy any one of formula 5, formula 6, formula 7,

[0095] 0 < T < 1 ton, then M > 80 MPa Formula 5

[0096] 1 < T < 3 tons, then 60 MPa < M < 80 MPa Formula 6

[0097] T > 3 tons, then 50 MPa < M < 60 MPa Formula 7.

[0098] The furnace pressure M in the present application refers to the pressure in the rotary kiln, and the sintering capacity T refers to the amount of feed per unit rotary kiln.

[0099] Since the reaction occurring in the sintering process produces gas, which affects the furnace pressure, the corresponding furnace pressure needs to be set according to the amount of feed per unit kiln to stabilize the production of uniform positive electrode active material. When the furnace pressure in the rotary kiln and the sintering capacity satisfy the above relationship, the uniformity of the performance of the positive electrode active material can be improved.

[0100] The preparation method of the positive electrode active material in the present application first obtains a grinding material with uniform particle size distribution by grinding to improve the balling rate in the spray drying process; secondly, the grinding material is subjected to spray drying treatment to obtain a water content of ≤3%, a loose bulk density of ≤0.8g / cm 3The spray material is subjected to fluidized drying treatment, which helps to evaporate the moisture in the fluidized drying process and preliminarily form the crystal nucleus and the carbon coating layer; then, the metal elements in the spray material are preliminarily reacted to form the crystal nucleus by controlling the fluidized temperature, the gas flow rate and the fluidized time during the fluidized drying treatment, and part of the carbon source is coked to coat at least part of the outer surface of the spray material particles to obtain the fluidized material; subsequently, the fluidized material is heat treated and sintered at two temperatures for a period of time, so that the metal elements grow rapidly along the crystal nucleus, and the carbon source is completely coked to coat the surface of the particles to obtain the positive electrode active material with a core-shell structure, the core has a chemical composition of Na4Fe x (PO4)2(P2O7)(2.8≤x≤3.0), the shell layer is a carbon coating layer, and in the X-ray diffraction pattern of the positive electrode active material, there is a first diffraction peak at 2θ of 33.6° and a second diffraction peak at 2θ of 32.9°, and the ratio of the peak intensity of the first diffraction peak to the peak intensity of the second diffraction peak is > 22.5; therefore, the positive electrode active material not only has high electronic conductivity and high sodium ion diffusion rate, but also has low content and high purity of sodium iron phosphate impurities, which can effectively improve the gram capacity of the positive electrode active material, so that the sodium ion battery including the positive electrode active material has high energy density and low-temperature cycle performance.

[0101] In addition, the preparation method in the present application carries out fluidized drying treatment before sintering, so compared with the traditional sintering process, the time for forming the crystal nucleus in the sintering process is saved, the sintering period is shortened, and the sintering temperature is reduced, thereby achieving the purpose of reducing cost and increasing efficiency; and compared with the roller kiln sintering, the use of the rotary kiln for sintering in the present application can make the fluidized material be heated more uniformly, which can effectively improve the uniformity of the reaction of the metal elements and improve the electrochemical performance of the positive electrode active material, and the production consumables such as the sagger used in the roller kiln sintering are saved, thereby reducing the production energy consumption and cost; at the same time, the heating environment in the rotary kiln in the present application can be provided by the heat of the airflow in the fluidized drying treatment, further reducing the cost.

[0102] In a specific embodiment, the grinding includes: in step 1), the grinding is carried out by a ball milling method; the grinding includes: the mixed material is subjected to first grinding and second grinding in sequence to obtain the ground material; the rotation speed of the first grinding is 1000-1300 r / min, the grinding time is 30-50 min, and the particle size of the grinding beads is 0.6-0.8 μm; the rotation speed of the second grinding is 1400-1600 r / min, the grinding time is 30-60 min, and the particle size of the grinding beads is 0.3-0.4 μm. When the foregoing grinding method is used, the particle size distribution of the ground material can be further made more uniform, thereby further improving the balling rate in the spray drying process.

[0103] Exemplarily, the rotation speed of the first grinding is 1000 r / min, 1050 r / min, 1100 r / min, 1150 r / min, 1200 r / min, 1250 r / min or 1300 r / min; the time of the first grinding is 30 min, 35 min, 40 min, 45 min or 50 min; and the particle size of the grinding beads in the first grinding is 0.6 μm, 0.7 μm or 0.8 μm.

[0104] Exemplarily, the rotation speed of the second grinding is 1400 r / min, 1450 r / min, 1500 r / min, 1550 r / min or 1600 r / min; the time of the second grinding is 30 min, 35 min, 40 min, 45 min, 50 min, 55 min or 60 min; and the particle size of the grinding beads in the second grinding is 0.3 μm or 0.4 μm.

[0105] In an embodiment, the inlet air temperature in the spray drying process is 175-205 ℃, the outlet air temperature is 80-110 ℃, and the difference between the inlet air temperature and the outlet air temperature is 95-105 ℃. At this time, the balling rate of the spray material can be further improved, and the D50, the water content and the bulk density of the spray material are reduced, which is beneficial to improve the powder compaction density of the positive active material and the uniformity of the carbon coating layer, thereby improving the energy density and the low-temperature cycle performance of the battery.

[0106] Exemplarily, the inlet air temperature is 175 ℃, 180 ℃, 185 ℃, 190 ℃, 195 ℃, 200 ℃ or 205 ℃; and the outlet air temperature is 80 ℃, 85 ℃, 90 ℃, 95 ℃, 100 ℃, 105 ℃ or 110 ℃.

[0107] In an embodiment, the raw material further comprises a dopant, and the dopant comprises at least one of Al2O3, NaBF4 and oleylamine. The structural stability of the prepared positive active material can be effectively improved, the morphology regularity of the particles is improved, which is beneficial to the capacity performance in the charging and discharging process and reduces the side reaction between the electrolyte and the positive electrode, thereby improving the cycle performance of the sodium ion battery.

[0108] The mass percentage content of the dopant in the raw material is not specifically limited in the present application, and further, the mass percentage content of the dopant in the raw material can be controlled to further control the mass percentage content of the dopant element in the positive active material to be 500-2000 ppm.

[0109] The third aspect of the present application provides a positive electrode sheet comprising the positive electrode active material of the first aspect or the positive electrode active material prepared by the preparation method of the second aspect. Since the positive electrode active material contained therein has high electronic conductivity and sodium ion migration rate, and also has high gram capacity, when the positive electrode sheet is used in a sodium ion battery, the energy density and low-temperature cycle performance of the battery can be effectively improved.

[0110] The fourth aspect of the present application provides a sodium ion battery comprising the positive electrode active material of the first aspect or the positive electrode active material prepared by the preparation method of the second aspect or the positive electrode sheet of the third aspect. Therefore, the sodium ion battery has high energy density and low-temperature cycle performance.

[0111] Hereinafter, the positive electrode active material of the present application will be described in detail through specific examples.

[0112] Example 1

[0113] 1) Mix raw materials including sodium carbonate, iron phosphate, glucose, aluminum oxide, and sodium tetrafluoroborate, wherein the molar ratio of the sodium source, the iron source, and the phosphorus source is 1:0.7:1, and the mass percentage of the carbon source in the raw materials is 10wt%, add deionized water, mix uniformly, and obtain a mixed material with a solid content of 30wt%; carry out first grinding on the mixed material at 1200r / min, the grinding time is 50min, and the particle size of the grinding beads is 0.7μm, then carry out second grinding at 1400r / min, the grinding time is 60min, and the particle size of the grinding beads is 0.3μm, to obtain a ground material with D10 of 0.25μm, D50 of 0.29μm, and D90 of 0.85μm;

[0114] 2) Spray dry the above ground material, set the inlet temperature to 200℃ and the outlet temperature to 105℃, to obtain a spray material with a median particle size D50 of 9μm, a water content of 2%, and a loose bulk density of 0.7g / cm 3 ;

[0115] 3) Carry out fluidized drying on the above spray material, set the fluidization temperature to 800℃, the gas flow rate to 17m / s (volume flow rate 17Nm 3 / s), and the fluidization time to 3s, to obtain a fluidized material; as shown in FIG. 1, it is the SEM graph of the fluidized material, and from the graph, it can be seen that the surface of the fluidized material is smoother and more regular, which is conducive to improving the uniformity of the carbon coating layer;

[0116] 4) under nitrogen atmosphere, the above fluidized material is transferred into a rotary kiln to sequentially perform first sintering and second sintering, the temperature of the first sintering is set to 200℃, the holding time is 2h, the temperature of the second sintering is set to 500℃, the holding time is 11h, to obtain the positive electrode active material of the present example, through ICP test, the chemical composition of the core is Na4Fe 2.8 (PO4)2(P2O7), the doping amount of Al is 800ppm, the doping amount of B is 1000ppm; through the element analyzer, the mass content of the carbon coating layer in the positive electrode active material is 1.95wt%.

[0117] Example 2

[0118] 1) the raw materials including sodium carbonate, iron phosphate, glucose, aluminum oxide, sodium tetrafluoroborate are mixed, the molar ratio of the sodium source, the iron source and the phosphorus source is 1:0.725:1, the mass percentage content of the carbon source in the raw materials is 7wt%, deionized water is added, after uniform mixing, a mixed material with a solid content of 30wt% is obtained; the mixed material is subjected to first grinding at 1100r / min, the grinding time is 40min, the particle size of the grinding beads is 0.8μm, then it is subjected to second grinding at 1500r / min, the grinding time is 40min, the particle size of the grinding beads is 0.3μm, to obtain a ground material with D10 of 1.2μm, D50 of 0.22μm and D90 of 0.91μm;

[0119] 2) the above ground material is subjected to spray drying treatment, the inlet air temperature is set to 205℃, the outlet air temperature is set to 100℃, to obtain a spray material with a median particle size D50 of 8μm, a water content of 1% and a loose bulk density of 0.6g / cm 3 ;

[0120] 3) the above spray material is subjected to fluidized drying treatment, the fluidized temperature is set to 600℃, the gas flow rate is set to 5m / s (5Nm 3 / s), the fluidized time is 5s, to obtain a fluidized material;

[0121] 4) under nitrogen atmosphere, the above fluidized material is transferred into a rotary kiln to sequentially perform first sintering and second sintering, the temperature of the first sintering is set to 250℃, the holding time is 1h, the temperature of the second sintering is set to 400℃, the holding time is 20h, to obtain the positive electrode active material of the present example, through ICP test, the chemical composition of the core is Na4Fe 2.9 (PO4)2(P2O7), the doping amount of Al is 800ppm, the doping amount of B is 1000ppm; through the element analyzer, the mass content of the carbon coating layer in the positive electrode active material is 1.52wt%.

[0122] Example 3

[0123] 1) Mix raw materials including sodium carbonate, iron phosphate, glucose, aluminum oxide, wherein the molar ratio of sodium source, iron source, phosphorus source is 1:0.7:1, the mass percentage of carbon source in the raw materials is 11wt%, add deionized water, mix uniformly, and obtain a mixture with a solid content of 30wt%; the mixture is first ground at 1000r / min, the grinding time is 30min, the particle size of the grinding beads is 0.8μm, and then secondly ground at 1600r / min, the grinding time is 30min, the particle size of the grinding beads is 0.4μm, and the ground material with D10 of 1.9μm, D50 of 0.28μm, and D90 of 0.96μm is obtained;

[0124] 2) Spray dry the above ground material, set the inlet temperature to 205℃ and the outlet temperature to 100℃, and obtain a spray material with a median particle size D50 of 8μm, a water content of 1%, and a loose bulk density of 0.6g / cm 3 ;

[0125] 3) Fluidized dry the above spray material, set the fluidization temperature to 1000℃, the gas flow rate to 20m / s (20Nm 3 / s), and the fluidization time to 4s, and obtain a fluidized material;

[0126] 4) Under a nitrogen atmosphere, transfer the above fluidized material into a rotary kiln to sequentially perform first sintering and second sintering, set the temperature of the first sintering to 240℃, the holding time to 1.5h, the temperature of the second sintering to 450℃, and the holding time to 15h, and obtain the positive electrode active material of the present embodiment, wherein the core chemical composition thereof is Na4Fe 2.9 (PO4)2(P2O7) by ICP test, and the doping amount of Al is 800ppm; the mass content of the carbon coating layer in the positive electrode active material is 2.15wt% measured by an elemental analyzer.

[0127] Example 4

[0128] The preparation method of the positive electrode active material in the present embodiment is basically the same as that in Example 1, except that in step 1), the aluminum oxide is not included in the raw materials, and the mass content of the carbon coating layer in the positive electrode active material is 1.89wt% measured by an elemental analyzer.

[0129] Example 5

[0130] The preparation method of the positive electrode active material in the present embodiment is basically the same as that in Example 1, except that in step 1), the aluminum oxide in the raw materials is replaced by oleylamine, and the doping amount of the doping element N in the prepared positive electrode active material is 1000ppm, and the doping amount of B is 1000ppm; the mass content of the carbon coating layer in the positive electrode active material is 2.03wt% measured by an elemental analyzer.

[0131] Example 6

[0132] The preparation method of the positive electrode active material in this example is basically the same as that in Example 3, except that the mass percentage of the carbon source in the raw material is adjusted to 12wt% in step 1), and the others remain unchanged. The mass content of the carbon coating layer in the positive electrode active material is 2.27wt% measured by an elemental analyzer.

[0133] Example 7

[0134] The preparation method of the positive electrode active material in this example is basically the same as that in Example 3, except that the content of aluminum oxide in the raw material is adjusted so that the mass percentage of the doping element in the positive electrode active material is 400ppm.

[0135] Example 8

[0136] The preparation method of the positive electrode active material in this example is basically the same as that in Example 3, except that the content of aluminum oxide in the raw material is adjusted so that the mass percentage of the doping element in the positive electrode active material is 2100ppm.

[0137] Comparative Example 1

[0138] The preparation method of the positive electrode active material in this example is basically the same as that in Example 3, except that the molar ratio of the sodium source, the iron source and the phosphorus source is adjusted to 1:0.675:1 in step 1), and then the chemical composition of the core of the positive electrode active material prepared is Na4Fe 2.7 (PO4)2(P2O7), and the mass content of the carbon coating layer in the positive electrode active material is 1.93wt% measured by an elemental analyzer.

[0139] Comparative Example 2

[0140] The preparation method of the positive electrode active material in this example is basically the same as that in Example 3, except that the temperature of the second sintering is adjusted to 600℃ in step 4).

[0141] Comparative Example 3

[0142] The preparation method of the positive electrode active material in this example is basically the same as that in Example 3, except that the D10 of the grinding material is adjusted to 0.27μm, the D50 is adjusted to 0.38μm, and the D90 is adjusted to 1.10μm in step 1).

[0143] Comparative Example 4

[0144] The preparation method of the super active material in the present comparative example is basically the same as that in Example 3, except that in step 2), the inlet air temperature is adjusted to 225℃, and the outlet air temperature is adjusted to 115℃, at this time, the spray material with a median particle size D50 of 11 μm, a water content of 3%, and a loose bulk density of 0.9 g / cm 3 .

[0145] Comparative Example 5

[0146] The preparation method of the super active material in the present comparative example is basically the same as that in Example 3, except that in step 3), the fluidization temperature is adjusted to 1100℃, the gas flow rate is adjusted to 25 Nm 3 / s, and the fluidization time is 2 s.

[0147] Comparative Example 6

[0148] The preparation method of the super active material in the present comparative example is basically the same as that in Example 3, except that the spray material is directly subjected to the first sintering and the second sintering without fluidization drying treatment.

[0149] Comparative Example 7

[0150] The preparation method of the super active material in the present comparative example is basically the same as that in Example 3, except that in step 4), the temperature of the first sintering is adjusted to 300℃, and the holding time is adjusted to 3 h; the temperature of the second sintering is adjusted to 560℃, and the holding time is adjusted to 9 h.

[0151] Comparative Example 8

[0152] The preparation method of the super active material in the present comparative example is basically the same as that in Example 3, except that in step 4), the rotary kiln is adjusted to a roller kiln.

[0153] The basic parameters are shown in Table 1.

[0154] Table 1

[0155] Test Example

[0156] 1. The positive electrode active materials prepared in Example 1 and Example 3 are subjected to SEM testing, and the test results are shown in Figures 2 and 3.

[0157] Figure 2 is an SEM image of the positive electrode active material in Example 1 at 1.00k, and Figure 3 is an SEM image of the positive electrode active material in Example 3 at 1.00k. As can be seen from Figures 2 and 3, the positive electrode active material without doping elements has partial breakage and a rough surface morphology; while the positive electrode active material in Example 3 contains doping elements, the particle surface is relatively smooth, and the sphericity is relatively high. Therefore, by doping elements into the positive electrode active material, the structural stability of the positive electrode active material can be effectively improved, the particle breakage problem can be improved, and the regularity of the particle morphology of the positive electrode active material can be improved.

[0158] 2. The positive electrode active materials prepared in Example 3 and Comparative Example 8 were subjected to SEM testing, and the test results are shown in Figures 4 and 5.

[0159] Figure 4 is an SEM image of the positive electrode active material in Example 3 at 30.0k, and Figure 5 is an SEM image of the positive electrode active material in Comparative Example 8 at 30.0k. As can be seen from Figures 4 and 5, compared with the roller kiln, the positive electrode active material sintered by the rotary kiln has a more regular surface and a better carbon coating effect.

[0160] 3. The positive electrode active materials prepared in the above examples and comparative examples were subjected to XRD testing, and the test results are shown in Table 2 and Figure 6.

[0161] Figure 6 is an XRD pattern of the positive electrode active material in Example 1. As can be seen from Figure 6, there is a first diffraction peak with a peak intensity of 1082 at 33.6°, a second diffraction peak with a peak intensity of 33 at 32.9°, and the ratio of the peak intensity of the first diffraction peak to the second diffraction peak is 32.78, which is greater than 22.5.

[0162] Table 2

[0163] As can be seen from Table 2:

[0164] The positive electrode active materials in Examples 1-8 have a first diffraction peak at 2θ of 33.6° and a second diffraction peak at 2θ of 32.9°, and the ratio of the peak intensity of the first diffraction peak to the second diffraction peak is >22.5, while the ratio of the peak intensity of the positive electrode active materials in Comparative Examples 1-8 is <22.5.

[0165] 4. The specific capacity, electronic conductivity and sodium ion mobility of the positive electrode active materials prepared in the above examples and comparative examples were tested, including the following steps:

[0166] The positive electrode active material, conductive carbon black Super P, and polyvinylidene fluoride (PVDF) prepared in the above examples and comparative examples were mixed in a mass ratio of 9:0.5:0.5, and uniformly ground in a maroon mortar. Then, an appropriate amount of N-methyl pyrrolidone (NMP) solvent was added to prepare a uniform slurry. The prepared slurry was uniformly coated on a clean aluminum foil, baked at 110°C for 20 min, and then transferred to a vacuum oven at 60°C for drying for 12 h. The dried electrode sheet was cut into a circular electrode sheet with a diameter of 13 mm using a button cell slicer. The surface density of the electrode sheet was 1.25 g / 100 cm 2 , and the compacted density was 2.00 g / cm 3 . The mass of each electrode sheet was measured and recorded. Finally, the weighed electrode sheet was placed in a vacuum oven at 100°C for drying for 12 h, and used to assemble a button half-cell.

[0167] 1) Gravimetric capacity

[0168] The button cell prepared above was subjected to charge-discharge test. The test voltage was 2-3.4 V, and the test condition was 0.2C. The charge-discharge curve was obtained. The discharge gravimetric capacity of the cell was calculated according to the discharge curve, and the discharge gravimetric capacity (mAh / g) = discharge current (mA) x discharge time (h) / cell mass (g).

[0169] 2) Electronic conductivity

[0170] The button cell prepared above was subjected to cyclic voltammetry test using an Autolab electrochemical workstation of Swiss Metrohm Company. The test voltage range was 2-3.4 V, the initial voltage and the terminal voltage were both the open circuit voltage of the cell, the scanning direction was low voltage to high voltage, and the scanning rate was 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 mV·s -1 . The electronic conductivity was obtained.

[0171] 3) Sodium ion transference number

[0172] The button cell prepared above was subjected to alternating current impedance test using an Autolab electrochemical workstation of Swiss Metrohm Company. The test frequency range was 105-10-2 Hz, and the amplitude was 10 mV. The data were fitted by Autolab software Nova 2.1 to obtain the corresponding circuit elements and the converted impedance value to obtain the sodium ion transference number.

[0173] The test results are shown in Table 3.

[0174] 5. The positive electrode active materials prepared in the above examples and comparative examples are made into sodium ion batteries, the specific steps including:

[0175] The positive electrode active materials prepared in the above examples and comparative examples, conductive carbon black Super P, carbon nanotubes, polyvinylidene fluoride (PVDF, 7% 5130 binder), and dispersant polyvinylpyrrolidone (PVP) are mixed in a mass ratio of 93:3.2:0.1:3.5:0.2, then an appropriate amount of N-methylpyrrolidone (NMP) solvent is added to prepare a uniform slurry. Then the prepared slurry is uniformly coated on a clean aluminum foil, and after baking at 110°C for 20 min, it is transferred to a 60°C vacuum oven for drying for 12 h. After rolling and slitting, a positive electrode sheet with a surface density of 12.5 mg / cm 2 and a compacted density of 1.8 g / cm 3 is obtained. Hard carbon (Kureha Type 2), conductive carbon black Super P, carbon nanotubes, and polyvinylidene fluoride (PVDF, 5% 5130 binder) are uniformly mixed in a mass ratio of 93:1.9:0.1:5, and then deionized water is added. After mixing uniformly, a negative electrode slurry is obtained. Then the negative electrode slurry is uniformly coated on a clean copper foil, and after baking at 110°C for 20 min, it is transferred to a 60°C vacuum oven for drying for 12 h. After rolling and slitting, a negative electrode sheet with a surface density of 5 mg / cm 2 and a compacted density of 0.9 g / cm 3 is obtained. A 16 μm aluminum foil is used as a separator, and an electrolyte including a sodium salt sodium hexafluorophosphate and an organic solvent is used. The organic solvent includes methyl ethyl carbonate, diethyl carbonate, vinylene carbonate, propylene carbonate, 1,3 propane sultone, and ethyl acetate in a volume ratio of 52.51:0.60:1.78:22.18:1.55:21.38, and the molar concentration of the sodium salt is 1M. The positive electrode sheet, the separator, and the negative electrode sheet are assembled into a cell, and after baking, liquid injection, formation, and capacity distribution, a sodium ion battery is obtained, and the NP ratio (the ratio of the capacity of the negative electrode to the capacity of the positive electrode) is 1.2. The formation is performed by the following steps:

[0176] First cycle: sequentially charging at 0.05C for 1 h, 0.1C for 1 h, 0.2C to 3.4V, then constant voltage charging at 0.05C, and then discharging at 0.1C to 2.0V;

[0177] Second cycle: charging at 0.2C to 3.4V, then constant voltage charging at 0.05C, and then discharging at 0.2C to 2.0V;

[0178] Third cycle: charging at 1C to 3.4V, then constant voltage at 0.05C, and then discharging at 1C to 2.0V, and the formation is completed.

[0179] The energy density and low-temperature cycle performance of the sodium ion battery prepared above were tested.

[0180] 1) Energy density

[0181] The positive active material in the sodium ion battery above was weighed in g before testing. Then the sodium ion battery was charged at 0.33 C with a cut-off voltage of 3.4 V, and discharged at 0.33 C with a cut-off voltage of 2 V, the actual discharge capacity of the cell was measured in Ah, and the discharge voltage of the battery was recorded in V. The product of the discharge voltage and the capacity and the ratio of the active material mass is the mass energy density of the sodium ion battery, which is Wh / kg.

[0182] 2) Low-temperature cycle performance

[0183] At 25℃, the sodium ion battery was charged to 3.4V at 0.33C, and then the battery was placed at -20℃, and after the battery body temperature dropped to -20℃, it was discharged to 2.0V at 0.33C, and the discharge capacity was recorded as C0; according to the foregoing charge and discharge mechanism, the cycle was carried out, and after 100 cycles, the discharge capacity was recorded as C1, and the low-temperature cycle capacity retention rate (%) = C1 / C0 x 100%.

[0184] The test results are shown in Table 3.

[0185] Table 3

[0186] From Table 3, it can be seen that:

[0187] Compared with Comparative Examples 1-8, the batteries in Examples 1-8 have higher gram capacity, electronic conductivity and sodium ion transference rate, and also have higher energy density and low-temperature cycle capacity retention rate, wherein the low-temperature cycle capacity retention rate of Example 5 is as high as 90.1%, which is significantly higher than that of Comparative Examples 1-8. It can be seen that the positive active material in the present application can significantly improve the energy density and low-temperature cycle performance of the battery.

[0188] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A positive electrode active material, wherein, The positive electrode active material includes a core and a carbon coating layer disposed on at least a part of a surface of the core, the core including a chemical composition represented by Formula 1, Na4Fe x (PO4)2(P2O7) Formula 1 In formula 1, 2.8≤x≤3.0; The X-ray diffraction pattern of the positive electrode active material has a first diffraction peak at 2θ of 33.6° and a second diffraction peak at 2θ of 32.9°, and the ratio of the peak intensity of the first diffraction peak to the peak intensity of the second diffraction peak is > 22.

5.

2. The positive electrode active material according to claim 1, wherein The mass percentage content C of the carbon coating layer in the positive electrode active material is 1.50wt%-2.15wt%.

3. The positive electrode active material according to claim 1 or 2, wherein The median particle size D50 of the positive electrode active material is 8 μm - 10 μm, the specific surface area BET of the positive electrode active material is not higher than 13.5 m 2 / g.

4. The positive electrode active material according to claim 2 or 3, wherein The positive electrode active material satisfies any one of formula 2, formula 3, formula 4, 1.5%<C<1.7% and 8.5<BET<10.5 Formula 2; 1.7%≤C<1.9% and 10.5≤BET<11.5 Formula 3; 1.9%≤C<2.15% and 11.5≤BET<13.5 Formula 4.

5. The positive electrode active material according to any one of claims 1 to 4, wherein The powder tap density of the positive electrode active material is 1.91 to 2.01 g / cm3 3 .

6. The positive electrode active material according to any one of claims 1 to 5, wherein The positive electrode active material further comprises a doping element; The doping element comprises at least one of B, F, Al and N.

7. The positive electrode active material according to claim 6, wherein The mass percentage content of the doping element in the positive electrode active material is 500ppm-2000ppm.

8. A method for producing the positive electrode active material as claimed in any one of claims 1 to 7, wherein Comprising the following steps: 1) mixing raw materials comprising a sodium source, an iron source, a phosphorus source and a carbon source with deionized water to obtain a mixture; grinding the mixture to obtain a ground material; the D10 of the ground material is 0.10μm-2μm, the D50 is 0.25μm-0.3μm, and the D90 is 0.7μm-1.0μm; 2) spray-drying the abrasive material to obtain a spray material; the spray material has a water content of ≤ 3% and a loose bulk density of ≤ 0.8 g / cm3 3 ; 3) performing fluidized drying treatment on the spray material, and during the treatment, the fluidization temperature is 600℃-1000℃, the gas flow rate is 5-20m / s, and the fluidization time is 3-5s to obtain a fluidized material; 4) under a protective atmosphere, using a rotary kiln, sequentially performing first sintering and second sintering on the fluidized material to obtain the positive electrode active material; The temperature of the first sintering is 200-250℃, and the holding time is 1-2h; the temperature of the second sintering is 400-550℃, and the holding time is 10-20h.

9. The method of producing a positive electrode active material according to claim 8, wherein In step 1), the grinding is performed by a ball milling method; The grinding comprises sequentially performing first grinding and second grinding on the mixture to obtain the ground material; the rotation speed of the first grinding is 1000-1300r / min, the grinding time is 30-50min, and the particle size of the grinding beads is 0.6-0.8μm; the rotation speed of the second grinding is 1400-1600r / min, the grinding time is 30-60min, and the particle size of the grinding beads is 0.3-0.4μm.

10. The method of producing a positive electrode active material according to claim 8 or 9, wherein In step 2), the inlet air temperature during the spray drying is 175-205℃, the outlet air temperature is 80-110℃, and the difference between the inlet air temperature and the outlet air temperature is 95-105℃.

11. A positive electrode sheet, wherein The positive electrode active material according to any one of claims 1-7, or prepared by the preparation method according to any one of claims 8-10.

12. A sodium-ion battery, wherein, The positive electrode active material according to any one of claims 1-7, or prepared by the preparation method according to any one of claims 8-10, or the positive electrode sheet according to claim 11.

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