A positive electrode active material, its preparation method and application

Abstract

This invention provides a positive electrode active material, its preparation method, and its application. The positive electrode active material includes a core and a carbon coating layer disposed on at least a portion of the surface of the core. The core comprises Na4Fe. x The cathode active material is composed of (PO4)2(P2O7), where 2.8 ≤ x ≤ 3.0. Simultaneously, in the X-ray diffraction pattern of this material, a first diffraction peak exists at 2θ = 33.6°, and a second diffraction peak exists at 2θ = 32.9°, with the ratio of the peak intensity of the first to the second peak being greater than 22.5. This cathode active material exhibits high specific capacity, high sodium ion diffusion capability, and high electronic conductivity, effectively improving the battery's energy density and low-temperature cycling performance.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion batteries, and relates to a positive electrode active material, and more particularly to a positive electrode active material and its preparation method and application. Background Technology

[0002] With the deepening research on sodium-ion battery materials, sodium iron pyrophosphate (NFPP) cathode material has attracted much attention in the energy storage market due to its high structural stability and wide selection of raw materials, and is expected to become the next generation of energy storage cathode material. However, the low energy density of NFPP cathode material greatly limits its application. Currently, the energy density of batteries is generally improved by increasing the operating voltage of sodium-ion batteries or the compaction density of the cathode sheet, but the improvement is limited; moreover, the low-temperature cycling performance of this cathode material is also poor, which seriously hinders the development of sodium iron pyrophosphate in the battery field.

[0003] Therefore, how to further improve sodium iron pyrophosphate (NFPP) cathode materials to enhance the energy density and low-temperature cycling performance of sodium-ion batteries is an urgent problem to be solved in this field. Summary of the Invention

[0004] To address the aforementioned deficiencies, this invention provides a positive electrode active material that has a high specific capacity, as well as high sodium ion diffusion capability and electronic conductivity, which can effectively improve the energy density and low-temperature cycle performance of the battery.

[0005] The present invention also provides a method for preparing the above-mentioned positive electrode active material. The positive electrode active material prepared by this method not only has a high specific capacity, but also has a high sodium ion diffusion capacity and electronic conductivity, which can enable sodium ion batteries to have high energy density and low temperature cycle performance.

[0006] The present invention also provides a positive electrode sheet, comprising the above-mentioned positive electrode active material or the positive electrode active material prepared by the above preparation method. Applying this positive electrode sheet to a sodium-ion battery can effectively improve the energy density and low-temperature cycle performance of the battery.

[0007] The present invention provides a sodium-ion battery, comprising 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.

[0008] This invention 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) Formula 1

[0009] In Equation 1, 2.8 ≤ x ≤ 3.0;

[0010] 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.

[0011] Furthermore, the carbon coating layer in the positive electrode active material has a mass percentage C of 1.50wt%-2.15wt%.

[0012] Furthermore, the median particle size D50 of the positive electrode active material is 8μm-10μm, and the specific surface area of ​​the positive electrode active material is no higher than 13.5m². 2 / g.

[0013] Furthermore, the positive electrode active material satisfies any one of Equations 2, 3, and 4.

[0014] 1.5% < C < 1.7% and 8.5 < BET < 10.5 (Equation 2);

[0015] 1.7% ≤ C < 1.9% and 10.5 ≤ BET < 11.5 (Equation 3);

[0016] 1.9% ≤ C < 2.15% and 11.5 ≤ BET < 13.5 Equation 4.

[0017] Furthermore, the compacted density of the positive electrode active material powder is 1.91-2.01 g / cm³. 3 .

[0018] Furthermore, the positive electrode active material also includes doping elements;

[0019] The doping element includes at least one of B, F, Al, and N.

[0020] Furthermore, the mass percentage of doped elements in the positive electrode active material is 500ppm-2000ppm.

[0021] The present invention also provides a method for preparing the positive electrode active material according to any one of the above claims, comprising the following steps:

[0022] 1) Raw materials including sodium source, iron source, phosphorus source and carbon source are mixed with deionized water to obtain a mixture; the mixture is ground to obtain a grinding material; the grinding material has a D10 of 0.10μm-2μm, a D50 of 0.25μm-0.3μm and a D90 of 0.7μm-1.0μm;

[0023] 2) The abrasive is spray-dried to obtain a spray-dried material; the moisture content of the spray-dried material is ≤3%, and the loose bulk density is ≤0.8g / cm³. 3 ;

[0024] 3) The spray material is subjected to fluidized drying treatment. 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 fluidized material.

[0025] 4) Under a protective atmosphere, the fluidized material is subjected to a first sintering and a second sintering in a rotary kiln to obtain the positive electrode active material;

[0026] The first sintering temperature is 200-250℃, and the holding time is 1-2h; the second sintering temperature is 400-550℃, and the holding time is 10-20h.

[0027] Furthermore, in step 1), grinding is performed by ball milling;

[0028] The grinding process includes: sequentially subjecting the mixture to a first grinding and a second grinding to obtain the grinding material; the first grinding is performed at a rotation speed of 1000-1300 r / min for 30-50 min, and the grinding beads have a particle size of 0.6-0.8 μm; the second grinding is performed at a rotation speed of 1400-1600 r / min for 30-60 min, and the grinding beads have a particle size of 0.3-0.4 μm.

[0029] And / or, in step 2), the inlet air temperature during 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℃.

[0030] The present invention also provides a positive electrode sheet, comprising the positive electrode active material described in any one of the above claims, or the positive electrode active material prepared by the above preparation method.

[0031] The present invention also provides a sodium-ion battery, comprising the positive electrode active material described in any one of the above methods, or the positive electrode active material prepared by the above preparation method, or the above positive electrode sheet.

[0032] This invention involves making the positive electrode active material include Na4Fe, which has the chemical composition Na4Fe xThe (PO4)2(P2O7) core and the carbon coating layer located on at least part of the surface of the core, while ensuring that the peak intensity ratio of the first diffraction peak at 33.6° and the second diffraction peak at 32.9° in the X-ray diffraction pattern of the positive electrode active material is >22.5, can effectively improve the specific capacity of the positive electrode active material and increase its electronic conductivity and sodium ion mobility, thereby effectively improving the energy density and low-temperature cycle performance of sodium-ion batteries. Attached Figure Description

[0033] Figure 1 This is a SEM image of the fluidized material in Embodiment 1 of the present invention;

[0034] Figure 2 This is a SEM image of the positive electrode active material in Example 1 of the present invention at 1.00 kJ.

[0035] Figure 3 This is a SEM image of the positive electrode active material in Example 3 of the present invention at 1.00 kJ.

[0036] Figure 4 This is a SEM image of the positive electrode active material in Example 3 of the present invention at 30.0 K;

[0037] Figure 5 This is a SEM image of the positive electrode active material in Comparative Example 8 of the present invention at 30.0 K;

[0038] Figure 6 The image shows the XRD pattern of the positive electrode active material in Example 1 of this invention. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0040] A first aspect of the present invention provides a 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.

[0041] Na4Fe x (PO4)2(P2O7) Formula 1

[0042] In Equation 1, 2.8 ≤ x ≤ 3.0;

[0043] 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.

[0044] Furthermore, the positive electrode active material in this invention is a polycrystalline positive electrode material.

[0045] The X-ray diffraction pattern of the positive electrode active material in this invention is obtained by performing XRD tests on the positive electrode active material and refining the obtained X-ray diffraction pattern. The refinement coefficient Rwp < 9. The test parameters are: radiation is Cu target, Kα rays, wavelength 0.154056 nm, scanning angle range 10°-80°, scanning rate 5° / min, and step size 0.0065°.

[0046] This invention comprises a positive electrode active material including a core and a carbon coating layer located on at least a portion of the outer surface of the core, and the core comprises a chemical composition of Na4Fe. x The (PO4)2(P2O7) composition, where 2.8 ≤ x ≤ 3, effectively improves the electronic conductivity of the positive electrode active material. Simultaneously, this positive electrode active material is an iron-deficient phase, which significantly enhances the diffusion rate of sodium ions, thereby effectively improving the charge-discharge performance of sodium-ion batteries and giving them higher low-temperature cycling performance. Furthermore, in the X-ray diffraction pattern of this positive electrode active material, a first diffraction peak exists at 2θ = 33.6°, and a second diffraction peak exists 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. The first diffraction peak is the characteristic main peak of sodium iron phosphate pyrophosphate (NFPP), and the second diffraction peak is the characteristic impurity peak of sodium iron phosphate with phosphonite structure. 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 >22.5, the content of 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 sodium-ion battery.

[0047] Furthermore, since the content of sodium iron phosphate impurity phase in the positive electrode active material of the present invention is low, it can effectively suppress the occurrence of side reactions between the positive electrode active material and the electrolyte, which helps to improve the cycle performance and safety of the battery.

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

[0049] For example, the mass percentage 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 of the carbon coating layer in the positive electrode active material in this invention can be obtained by elemental analysis.

[0051] In one specific embodiment, the median particle size D50 of the positive electrode active material is 8 μm-10 μm, and the specific surface area of ​​the positive electrode active material is no higher than 13.5 m². 2 / g. Within this range, the positive electrode active material has a higher powder compaction density, which is beneficial to improving the energy density of the battery; and the carbon coating layer is more uniformly coated, which can further improve the conductivity of the positive electrode active material, enabling the battery to have higher low-temperature cycle performance; in addition, the positive electrode active material has a lower specific surface area, which can effectively reduce the side reactions between the electrolyte and the positive electrode, thereby effectively improving the cycle performance of the battery.

[0052] In this invention, D50 refers to the particle size corresponding to a cumulative particle size distribution percentage of 50% for the sample. It can be obtained by testing with a laser particle size analyzer.

[0053] In one specific embodiment, the positive electrode active material satisfies any one of formulas 2, 3, and 4.

[0054] 1.5% < C < 1.7% and 8.5 < BET < 10.5 (Equation 2);

[0055] 1.7% ≤ C < 1.9% and 10.5 ≤ BET < 11.5 (Equation 3);

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

[0057] When the positive electrode active material satisfies any one of the above formulas 2, 3, and 3, the coating effect of the carbon coating layer is better and more uniform, which is conducive to improving the electronic conductivity of the positive electrode active material. At the same time, it can further reduce the energy density loss caused by inactive carbon, thereby enabling sodium-ion batteries to have higher low-temperature cycle performance and energy density.

[0058] In one specific embodiment, the compacted density of the positive electrode active material powder is 1.91-2.01 g / cm³. 3 For example, the compacted density is 1.91 g / cm³. 3 1.92g / cm 31.93g / cm 3 1.94 g / cm 3 1.95g / cm 3 1.96g / cm 3 1.97g / cm 3 1.98g / cm 3 1.99g / cm 3 2.00g / cm 3 Or 2.01 g / cm 3 Within this range, the powder compaction density of the positive electrode active material is relatively high, which can achieve a higher compaction density and thus further improve the energy density of sodium-ion batteries.

[0059] The powder compaction density in this invention was obtained by testing under a pressure of 1T (equivalent to 74.0 MPa).

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

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

[0062] When the positive electrode active material also includes the aforementioned doping elements, on the one hand, it can improve the structural stability of the positive electrode active material and increase its voltage, allowing the capacity to be fully utilized during charging and discharging; on the other hand, it can effectively solve the problems of low pelletizing rate and easy breakage of particles during spray drying, improve the regularity of the particle morphology of the positive electrode active material, reduce the side reactions between the electrolyte and the positive electrode, thereby further improving the energy density and cycle stability of sodium-ion batteries.

[0063] In one specific embodiment, the mass percentage of dopant elements in the positive electrode active material is 500ppm-2000ppm. Within this range, not only can the structural stability and morphological regularity of the positive electrode active material be further improved, but the problem of reduced charge and discharge capacity due to excessive doping can also be avoided.

[0064] For example, the mass percentage of doped elements 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 doping elements in this invention can be obtained by measuring an inductively coupled plasma optical emission spectrometer (ICP-OES Avio200 / Avio500).

[0066] A second aspect of the present invention provides a method for preparing a positive electrode active material according to the first aspect, comprising the following steps:

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

[0068] 2) The abrasive is spray-dried to obtain spray-dried material; the moisture content of the spray-dried material is ≤3%, and the loose bulk density is ≤0.8g / cm³. 3 ;

[0069] 3) The sprayed material is subjected to fluidized drying treatment. During the treatment process, the fluidization temperature is 600℃-1000℃, the gas flow rate is 5-20m / s, and the fluidization time is 3-5s to obtain fluidized material.

[0070] 4) Under a protective atmosphere, a rotary kiln is used to sequentially perform the first sintering and the second sintering of the fluidized material to obtain the positive electrode active material;

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

[0072] Specifically, in step 1), raw materials including sodium, iron, phosphorus, and carbon sources are added to deionized water and mixed uniformly to obtain a mixture. This mixture is then ground to obtain a finely ground material with a D10 of 0.10 μm-2 μm, a D50 of 0.25 μm-0.3 μm, and a D90 of 0.7 μm-1.0 μm. At this point, the particle size distribution of the finely ground material is more uniform, which is beneficial for improving the pelletizing rate in subsequent spray drying, enhancing the uniformity of carbon coating, and simultaneously improving the capacity utilization of the positive electrode active material.

[0073] In this invention, sodium source refers to a raw material that provides sodium, iron source refers to a raw material that provides iron, phosphorus source refers to a raw material that provides phosphorus, and carbon source refers to a raw material that provides carbon. As long as the target element (Na, Fe, P, C) is present, it is within the scope of this invention. Furthermore, a target element can be introduced into the reaction system through 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] This invention does not specifically limit the types of sodium, iron, phosphorus, and carbon sources. 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, ferric phosphate, ferrous oxalate, iron powder, ferric acetate, and ferric nitrate; the phosphorus source includes at least one of ferric 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] This invention does not impose specific limitations on the molar ratio between sodium, iron, and phosphorus sources; it only requires that the chemical composition of the core in the prepared positive electrode active material satisfies Formula 1.

[0076] This invention does not specifically limit the mass percentage of carbon source in the raw materials. Furthermore, the mass percentage of carbon coating in the positive electrode active material can be further controlled by controlling the amount of carbon source added.

[0077] This invention does not impose any special limitations on the specific sources of sodium, iron, phosphorus, and carbon; products prepared using commercially available products or conventional preparation methods known to those skilled in the art are acceptable.

[0078] The present invention does not specify the mixing method; it is sufficient to mix the materials evenly. For example, the mixing can be carried out by mechanical stirring.

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

[0080] The present invention does not specify the grinding method, as long as the D10, D50 and D90 of the grinding material are within the aforementioned range.

[0081] In this invention, D10 refers to the particle size corresponding to a cumulative particle size distribution percentage of 10%, D50 refers to the particle size corresponding to a cumulative particle size distribution percentage of 50%, and D90 refers to the particle size corresponding to a cumulative particle size distribution percentage of 90%.

[0082] In step 2), the above-mentioned abrasive is spray-dried to obtain a moisture content ≤3% and a bulk density ≤0.8g / cm³. 3 The sprayed material helps to evaporate moisture during the fluidized bed drying process, avoiding carbon corrosion caused by moisture during subsequent sintering and improving the uniformity of the carbon coating layer. At the same time, it helps to initially form crystal nuclei and initially coat the carbon source during fluidization, which is beneficial to the further growth of crystals and effective coating of the carbon source during subsequent sintering.

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

[0084] Furthermore, the pelleting rate of the sprayed material is not less than 95%.

[0085] The pellet formation rate in this invention can be obtained by importing SEM images at 30K using Nano Measurer software.

[0086] This invention does not specifically limit the inlet and outlet air temperatures during the spray drying process; it only requires that the moisture content and loose density of the prepared spray material meet the aforementioned ranges.

[0087] In step 3), the above-mentioned spray material is subjected to fluidized bed drying treatment, maintaining a fluidization temperature of 600℃-1000℃, a gas flow rate of 5-20 m / s, and a fluidization time of 3-5 s to obtain a fluidized material. This process ensures that the moisture in the spray material is fully evaporated, preventing moisture corrosion of the carbon coating layer during subsequent sintering and affecting the carbon coating effect. Furthermore, the spray material undergoes a preliminary reaction, generating crystal nuclei, increasing its reactivity, which is beneficial for the growth of the positive electrode active material during subsequent sintering, resulting in a more uniform element distribution and improved uniformity of the carbon coating layer.

[0088] For example, the fluidization temperature is 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C, 950°C, or 1000°C; 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 fluidization time is 3 s, 4 s, or 5 s.

[0089] The present invention does not specifically limit the gas used in the fluidized drying process; for example, it can be argon and / or nitrogen.

[0090] In step 4), a rotary kiln is used to first sinter the fluidized material at 200-250℃ under a protective atmosphere for 1-2 hours, followed by a second sintering at 400-550℃ for 10-20 hours, thus preparing the positive electrode active material. During this process, metal elements grow rapidly along the crystal nucleus and crystal plane, simultaneously forming a uniform carbon coating layer, resulting in the positive electrode active material.

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

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

[0093] The present invention does not specifically limit the protective atmosphere; for example, nitrogen and / or argon may be used as the protective atmosphere.

[0094] Furthermore, the furnace pressure M (MPa) and sintering capacity T (tons) during the sintering process satisfy any one of equations 5, 6, and 7.

[0095] If 0 < T < 1 ton, then M > 80 MPa. (Equation 5)

[0096] If 1 ≤ T < 3 tons, then 60 MPa < M ≤ 80 MPa. (Equation 6)

[0097] If T≥3 tons, then 50MPa<M≤60MPa (Equation 7).

[0098] In this invention, furnace pressure M refers to the pressure in the rotary kiln, and sintering capacity T refers to the feed rate per unit of rotary kiln.

[0099] Since the reaction that occurs during sintering produces gas, which affects the furnace pressure, it is necessary to set the corresponding furnace pressure according to the feed rate in the unit kiln in order to stably produce uniform positive electrode active materials. When the furnace pressure in the rotary kiln and the sintering capacity meet the above relationship, the uniformity of the positive electrode active material performance can be improved.

[0100] The preparation method of the positive electrode active material in this invention involves, firstly, obtaining a uniformly sized abrasive by grinding to improve the pelletizing rate during spray drying; secondly, spray drying the abrasive to obtain a material with a moisture content ≤3% and a bulk density ≤0.8 g / cm³. 3 The sprayed material facilitates moisture evaporation and the initial formation of crystal nuclei and carbon coatings during fluidized bed drying. Then, by fluidizing and drying the sprayed material, controlling the fluidization temperature, gas flow rate, and fluidization time, the metal elements in the sprayed material undergo initial reactions to form crystal nuclei, while some carbon sources coke and coat at least part of the outer surface of the sprayed material particles, resulting in a fluidized material. Subsequently, the fluidized material is sintered at two temperature stages for a period of time, allowing the metal elements to grow rapidly along the crystal nuclei, and the carbon source to be completely coked and coated on the particle surface, yielding a core-shell structured positive electrode active material with a core containing Na4Fe. xThe positive electrode active material has a chemical composition of (PO4)2(P2O7) (2.8≤x≤3.0) and a carbon coating layer. In the X-ray diffraction pattern of this 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. Therefore, this positive electrode active material not only has high electronic conductivity and sodium ion diffusion rate, but also has a low content of sodium iron phosphate impurity phase and high purity, which can effectively improve the specific capacity of the positive electrode active material. This results in sodium-ion batteries containing this positive electrode active material having high energy density and low-temperature cycling performance.

[0101] Furthermore, the preparation method of this invention, due to the fluidized drying treatment before sintering, eliminates the time for crystal nucleation during sintering, shortens the sintering cycle, and lowers the sintering temperature compared to traditional sintering processes, thereby achieving the goal of cost reduction and efficiency improvement. Moreover, compared to roller kiln sintering, the rotary kiln sintering method of this invention allows for more uniform heating of the fluidized material, effectively improving the uniformity of metal element reactions and enhancing the electrochemical performance of the positive electrode active material. It also eliminates the need for production consumables such as saggers used in roller kiln sintering, reducing production energy consumption and costs. At the same time, the heating environment in the rotary kiln of this invention can be provided by the heat from the airflow during the fluidized drying treatment, further reducing costs.

[0102] In one specific embodiment, the grinding includes: in step 1), grinding is performed by ball milling; the grinding includes: sequentially subjecting the mixture to a first grinding and a second grinding to obtain abrasive 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 aforementioned grinding method is used, the particle size distribution of the abrasive material can be further made more uniform, thereby further improving the balling rate during the spray drying process.

[0103] For example, the rotational 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 grinding time 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] For example, the rotational speed of the second grinding is 1400 r / min, 1450 r / min, 1500 r / min, 1550 r / min or 1600 r / min; the grinding time 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 one specific embodiment, the inlet air temperature during the spray drying process is 175-205℃, the outlet air temperature is 80-110℃, and the temperature difference between the inlet and outlet air is 95-105℃. This can further improve the pelletizing rate of the sprayed material and reduce its D50, moisture content, and bulk density, which is beneficial for improving the powder compaction density of the positive electrode active material and the uniformity of the carbon coating layer, thereby enhancing the battery's energy density and low-temperature cycle performance.

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

[0107] In one specific embodiment, the raw materials further include a dopant, which includes at least one of Al2O3, NaBF4, and oleylamine. This can effectively improve the structural stability of the prepared positive electrode active material, enhance the morphological regularity of the particles, facilitate capacity utilization during charge and discharge, reduce side reactions between the electrolyte and the positive electrode, and improve the cycle performance of sodium-ion batteries.

[0108] This invention does not specifically limit the mass percentage of dopants in the raw materials. Furthermore, by controlling the mass percentage of dopants in the raw materials, the mass percentage of dopants in the positive electrode active material can be further controlled to be 500ppm-2000ppm.

[0109] A third aspect of this invention 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 included positive electrode active material has high electronic conductivity and sodium ion mobility, as well as high specific capacity, using this positive electrode sheet in sodium-ion batteries can effectively improve the battery's energy density and low-temperature cycle performance.

[0110] A fourth aspect of this invention 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, this sodium-ion battery exhibits high energy density and low-temperature cycling performance.

[0111] The positive electrode active material of the present invention will be described in detail below through specific embodiments.

[0112] Example 1

[0113] 1) Raw materials including sodium carbonate, ferric phosphate, glucose, alumina, and sodium tetrafluoroborate are mixed, wherein the molar ratio of sodium source, iron source, and phosphorus source is 1:0.7:1, and the mass percentage of carbon source in the raw materials is 10wt%. The mixture is added to deionized water and mixed evenly to obtain a mixture with a solid content of 30wt%. The mixture is first ground at 1200 r / min for 50 min, and the particle size of the grinding beads is 0.7 μm. Then it is second ground at 1400 r / min for 60 min, and the particle size of the grinding beads is 0.3 μm, to obtain abrasive with D10 of 0.25 μm, D50 of 0.29 μm, and D90 of 0.85 μm.

[0114] 2) The above-mentioned abrasive material was spray-dried with an inlet air temperature of 200℃ and an outlet air temperature of 105℃ to obtain a median particle size D50 of 9μm, a moisture content of 2%, and a bulk density of 0.7g / cm³. 3 aerosol;

[0115] 3) The above-mentioned spray material is subjected to fluidized bed drying treatment, with the fluidization temperature set at 800℃ and the gas flow rate at 17 Nm. 3 / s, fluidization time is 3s, resulting in fluidized material; such as Figure 1 The image shown is an SEM image of the fluidized material. As can be seen from the image, the surface of the fluidized material is smoother and more regular, which is beneficial to improving the uniformity of the carbon coating layer.

[0116] 4) Under a nitrogen atmosphere, the above fluidized material was transferred into a rotary kiln for a first sintering and a second sintering. The temperature of the first sintering was set at 200°C and the holding time was 2 hours. The temperature of the second sintering was set at 500°C and the holding time was 11 hours. The positive electrode active material of this embodiment was obtained. ICP testing showed that the core chemical composition was Na₄Fe. 2.8 (PO4)2(P2O7), with Al doping at 800 ppm and B doping at 1000 ppm; the carbon coating content in the positive electrode active material was measured to be 1.95 wt% by elemental analysis.

[0117] Example 2

[0118] 1) Raw materials including sodium carbonate, ferric phosphate, glucose, alumina, and sodium tetrafluoroborate are mixed, wherein the molar ratio of sodium source, iron source, and phosphorus source is 1:0.725:1, and the mass percentage of carbon source in the raw materials is 7wt%. The mixture is added to deionized water and mixed evenly to obtain a mixture with a solid content of 30wt%. The mixture is first ground at 1100 r / min for 40 min, and the particle size of the grinding beads is 0.8 μm. Then it is second ground at 1500 r / min for 40 min, and the particle size of the grinding beads is 0.3 μm, resulting in a grinding material with D10 of 1.2 μm, D50 of 0.22 μm, and D90 of 0.91 μm.

[0119] 2) The above-mentioned abrasive material was spray-dried with an inlet air temperature of 205℃ and an outlet air temperature of 100℃ to obtain a median particle size D50 of 8μm, a moisture content of 1%, and a bulk density of 0.6g / cm³. 3 aerosol;

[0120] 3) The above-mentioned spray material is subjected to fluidized bed drying treatment, with the fluidization temperature set at 600℃ and the gas flow rate at 5 Nm. 3 / s, fluidization time is 5s, and fluidized material is obtained;

[0121] 4) Under a nitrogen atmosphere, the above fluidized material was transferred into a rotary kiln for a first sintering and a second sintering. The temperature of the first sintering was set at 250°C and the holding time was 1 hour. The temperature of the second sintering was set at 400°C and the holding time was 20 hours. The positive electrode active material of this embodiment was obtained. ICP testing showed that the core chemical composition was Na4Fe. 2.9 (PO4)2(P2O7), with Al doping at 800 ppm and B doping at 1000 ppm; the carbon coating content in the positive electrode active material was measured to be 1.52 wt% by elemental analysis.

[0122] Example 3

[0123] 1) Raw materials including sodium carbonate, ferric phosphate, glucose, and alumina are mixed, wherein the molar ratio of sodium source, iron source, and phosphorus source is 1:0.7:1, and the mass percentage of carbon source in the raw materials is 11wt%. After adding deionized water and mixing evenly, a mixture with a solid content of 30wt% is obtained. The mixture is first ground at 1000 r / min for 30 min, and the particle size of the grinding beads is 0.8 μm. Then, it is second ground at 1600 r / min for 30 min, and the particle size of the grinding beads is 0.4 μm, resulting in a grinding material with D10 of 1.9 μm, D50 of 0.28 μm, and D90 of 0.96 μm.

[0124] 2) The above-mentioned abrasive material was spray-dried with an inlet air temperature of 205℃ and an outlet air temperature of 100℃ to obtain a median particle size D50 of 8μm, a moisture content of 1%, and a bulk density of 0.6g / cm³. 3 aerosol;

[0125] 3) The above-mentioned spray material is subjected to fluidized bed drying treatment, with the fluidization temperature set at 1000℃ and the gas flow rate at 20 Nm. 3 / s, fluidization time is 4s, and fluidized material is obtained;

[0126] 4) Under a nitrogen atmosphere, the above fluidized material was transferred into a rotary kiln for first and second sintering sequentially. The temperature for the first sintering was set at 240°C and the holding time was 1.5 h. The temperature for the second sintering was set at 450°C and the holding time was 15 h. The positive electrode active material of this embodiment was obtained. ICP testing showed that the core chemical composition was Na4Fe. 2.9 (PO4)2(P2O7), with an Al doping amount of 800 ppm; the mass content of the carbon coating layer in the positive electrode active material was measured to be 2.15 wt% by elemental analysis.

[0127] Example 4

[0128] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step 1), the raw materials do not include alumina, and the mass content of the carbon coating layer in the positive electrode active material measured by the elemental analyzer is 1.89 wt%.

[0129] Example 5

[0130] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step 1), the alumina in the raw material is replaced with oleylamine. The doping amount of N in the prepared positive electrode active material is 1000ppm and the doping amount of B is 1000ppm. The mass content of carbon coating in the positive electrode active material measured by the elemental analyzer is 2.03wt%.

[0131] Example 6

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

[0133] Example 7

[0134] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 3. The difference is that the content of alumina in the raw materials is adjusted so that the mass percentage of doped elements in the positive electrode active material is 400 ppm.

[0135] Example 8

[0136] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 3. The difference is that the content of alumina in the raw materials is adjusted so that the mass percentage of doped elements in the positive electrode active material is 2100 ppm.

[0137] Comparative Example 1

[0138] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 3. The difference is that in step 1), the molar ratio of sodium source, iron source and phosphorus source is adjusted to 1:0.675:1. Then the chemical composition of the core in the prepared positive electrode active material is Na4Fe 2.7 The carbon coating content in the positive electrode active material was found to be 1.93 wt% (PO4)2(P2O7) as determined by elemental analysis.

[0139] Comparative Example 2

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

[0141] Comparative Example 3

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

[0143] Comparative Example 4

[0144] The preparation method of the highly active material in this 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, a median particle size D50 of 11μm, a moisture content of 3%, and a loose packing density of 0.9g / cm³ are obtained. 3 Spray material.

[0145] Comparative Example 5

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

[0147] Comparative Example 6

[0148] The preparation method of the highly active material in this comparative example is basically the same as that in Example 3, except that the sprayed material is directly subjected to the first and second sintering without fluidized drying.

[0149] Comparative Example 7

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

[0151] Comparative Example 8

[0152] The preparation method of the highly active material in this 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]

[0156]

[0157] Test case

[0158] 1. The positive electrode active materials prepared in Examples 1 and 3 were subjected to SEM testing. The test results are shown in [Figure 1]. Figure 2 and Figure 3 .

[0159] Figure 2 This is a SEM image of the positive electrode active material in Example 1 at 1.00 K. Figure 3 This is a SEM image of the positive electrode active material in Example 3 at 1.00 K. Figure 2 and Figure 3 It can be seen that the cathode active material without doped elements exhibits partial breakage and a relatively rough surface morphology; while the cathode active material in Example 3 contains doped elements, resulting in a smoother particle surface and higher sphericity. Therefore, it is evident that doping cathode active materials with elements can effectively improve their structural stability, alleviate particle breakage, and enhance the regularity of particle morphology.

[0160] 2. The positive electrode active materials prepared in Example 3 and Comparative Example 8 were subjected to SEM testing. The test results are shown in [Figure 1]. Figure 4 and Figure 5 .

[0161] Figure 4This is a SEM image of the positive electrode active material in Example 3 at 30.0 K. Figure 5 The image shows the SEM image of the positive electrode active material in Comparative Example 8 at 30.0 K. Figure 4 and Figure 5 It can be seen that, compared with roller kiln, the surface of the positive electrode active material obtained by rotary kiln sintering is more regular and the carbon coating effect is better.

[0162] 3. XRD tests were performed on the positive electrode active materials prepared in the above examples and comparative examples. The test results are shown in Table 2 and... Figure 6 .

[0163] Figure 6 The image shown is the XRD pattern of the positive electrode active material in Example 1. Figure 6 It can be seen that there is a first diffraction peak with an intensity of 1082 at 33.6° and a second diffraction peak with an intensity of 33 at 32.9°, and the ratio of the peak intensities of the first diffraction peak to the second diffraction peak is 32.78, which is greater than 22.5.

[0164] Table 2

[0165]

[0166] As shown in Table 2:

[0167] The positive electrode active materials in Examples 1-8 have a first diffraction peak at 2θ = 33.6° and a second diffraction peak at 2θ = 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, while the ratio of the peak intensity of the positive electrode active materials in Comparative Examples 1-8 is less than 22.5.

[0168] 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:

[0169] The positive electrode active material, conductive carbon black Super P, and polyvinylidene fluoride (PVDF) prepared in the above examples and comparative examples were mixed at a mass ratio of 9:0.5:0.5 and ground evenly in an agate mortar. Then, an appropriate amount of N-methylpyrrolidone (NMP) solvent was added to prepare a uniform slurry. The prepared slurry was then evenly coated onto a clean aluminum foil, baked at 110°C for 20 min, and then transferred to a vacuum oven at 60°C for 12 h to dry. The dried electrode sheet was then shaped into circular electrode sheets with a diameter of 13 mm using a button cell slicing machine, resulting in an areal density of 1.25 g / 100 cm³. 2 The compacted density is 2.00 g / cm³. 3The positive electrode sheets were weighed and their mass recorded. The weighed electrodes were then dried in a 100℃ vacuum oven for 12 hours for use in assembling button half-cells. Battery assembly was carried out in a sealed glove box filled with argon gas. The assembly sequence was: negative electrode shell—positive electrode sheet—separator—sodium sheet—gasket—spring sheet—electrolyte—positive electrode shell. The assembled battery was then pressed and sealed on a battery packaging machine. Afterward, the battery was removed from the glove box and allowed to stand for 24 hours before electrochemical testing.

[0170] 1) Capacity

[0171] The button batteries prepared above were subjected to charge-discharge tests at a voltage of 2-3.4V and a test condition of 0.2C to obtain charge-discharge curves. The discharge capacity of the battery was calculated based on the discharge curves. The discharge capacity (mAh / g) = discharge current (mA) × discharge time (h) / battery mass (g).

[0172] 2) Electronic conductivity

[0173] Cyclic voltammetry tests were performed on the button cells prepared above using an AutoLab electrochemical workstation from Metrohm, Switzerland. The test voltage range was 2V-3.4V, with the initial and termination voltages being the open-circuit voltages of the cells. The scan direction was from low voltage to high voltage, and the scan rates were 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1 mV·s. -1 The electronic conductivity was obtained through testing.

[0174] 3) Sodium ion mobility

[0175] The coin cells prepared above were subjected to AC impedance testing using an Autolab electrochemical workstation from Metrohm, Switzerland. The test frequency range was 10⁵–10⁻² Hz, and the amplitude was 10 mV. The test data were fitted using Autolab software Nova 2.1 to obtain the corresponding circuit components and their corresponding impedance values. The sodium ion mobility was then calculated.

[0176] The test results are shown in Table 3.

[0177] 2. The positive electrode active materials prepared in the above examples and comparative examples are used to fabricate sodium-ion batteries. The specific steps include:

[0178] The positive electrode active material, conductive carbon black Super P, carbon nanotubes, polyvinylidene fluoride (PVDF, 7% 5130 binder), and dispersant polyvinylpyrrolidone (PVP) prepared in the above examples and comparative examples were mixed in a mass ratio of 93:3.2:0.1:3.5:0.2. Then, an appropriate amount of N-methylpyrrolidone (NMP) solvent was added to prepare a uniform slurry. The prepared slurry was then uniformly coated onto clean aluminum foil, baked at 110°C for 20 minutes, and then transferred to a vacuum oven at 60°C for 12 hours of drying. After rolling and slitting, a surface density of 12.5 mg / cm³ was obtained. 2 The compacted density is 1.8 g / cm³. 3 The positive electrode sheet was prepared by uniformly mixing hard carbon (Kuraray Type 2), conductive carbon black Super P, carbon nanotubes, and polyvinylidene fluoride (PVDF, 5% 5130 binder) at a mass ratio of 93:1.9:0.1:5. Deionized water was added, and the mixture was thoroughly mixed to obtain the negative electrode slurry. This slurry was then uniformly coated onto a clean copper foil, baked at 110℃ for 20 minutes, and then transferred to a 60℃ vacuum oven for drying for 12 hours. After rolling and slitting, a surface density of 5 mg / cm³ was obtained. 2 The compacted density is 0.9 g / cm³. 3 The negative electrode is used. A 16μm aluminum foil is used as the separator. The electrolyte includes sodium hexafluorophosphate and an organic solvent. The organic solvent consists of ethyl methyl carbonate, diethyl carbonate, vinylene carbonate, propylene carbonate, 1,3-propane sulpholactone, and ethyl acetate in a volume ratio of 52.51:0.60:1.78:22.18:1.55:21.38. The molar concentration of the sodium salt is 1M. The positive electrode, separator, and negative electrode are assembled into a cell. After baking, electrolyte injection, formation, and capacity testing, a sodium-ion battery is obtained with an NP ratio (the ratio of negative electrode capacity to positive electrode capacity) of 1.2. The formation process includes the following steps:

[0179] First cycle: charge at 0.05C for 1 hour, charge at 0.1C for 1 hour, charge at 0.2C to 3.4V, then charge at a constant voltage of 0.05C, and then discharge at 0.1C to 2.0V.

[0180] Second cycle: Charge at 0.2C to 3.4V, then charge at a constant voltage of 0.05C, and then discharge at 0.2C to 2.0V;

[0181] Third cycle: Charge at 1C to 3.4V, then maintain constant voltage at 0.05C, and then discharge at 1C to 2.0V, thus ending the transformation.

[0182] The energy density and low-temperature cycling performance of the sodium-ion batteries prepared above were tested:

[0183] 1) Energy density

[0184] Before testing, the positive electrode active material in the sodium-ion battery was weighed in grams. The sodium-ion battery was then charged at a constant current of 0.33C with a cutoff voltage of 3.4V, followed by a constant current discharge at 0.33C with a cutoff voltage of 2V. The actual discharge capacity of the cell was measured in Ah, and the discharge voltage was recorded in V. The ratio of the product of the discharge voltage and capacity to the mass of the active material is the gravimetric energy density of the sodium-ion battery, expressed in Wh / kg.

[0185] 2) Low-temperature cycling performance

[0186] At 25°C, the sodium-ion battery is charged to 3.4V at 0.33C. The battery is then placed at -20°C. After the battery body temperature drops to -20°C, it is discharged to 2.0V at 0.33C, and the discharge capacity is recorded as C0. The battery is cycled according to the aforementioned charge-discharge mechanism. After 100 cycles, the discharge capacity is recorded as C1. The low-temperature cycle capacity retention rate (%) = C1 / C0 × 100%.

[0187] The test results are shown in Table 3.

[0188] Table 3

[0189]

[0190]

[0191] As shown in Table 3:

[0192] Compared to Comparative Examples 1-8, the batteries in Examples 1-8 exhibit higher specific capacity, electronic conductivity, and sodium ion mobility, while also possessing higher energy density and low-temperature cycling capacity retention. Specifically, Example 5 achieves the highest low-temperature cycling capacity retention of 90.1%, significantly higher than Comparative Examples 1-8. Therefore, the positive electrode active material in this invention can significantly improve the energy density and low-temperature cycling performance of the battery.

[0193] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode active material, characterized in that, The positive electrode active material includes a core and a carbon coating layer disposed on at least a portion of the surface of the core, the core having a chemical composition shown in Formula 1. Na4Fe x (PO4)2(P2O7) Formula 1 In Equation 1, 2.8 ≤ x ≤ 3.0; 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. The positive electrode active material also includes doping elements; the doping elements include at least one of B, F, Al, and N.

2. The positive electrode active material according to claim 1, characterized in that, The carbon coating layer in the positive electrode active material has a mass percentage (C) of 1.50 wt% to 2.15 wt%. And / or, 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.

3. The positive electrode active material according to claim 2, characterized in that, The positive electrode active material satisfies any one of formulas 2, 3, and 4. 1.5% < C < 1.7% and 8.5 < BET < 10.5 (Equation 2); 1.7%≤C<1.9% and 10.5≤BET<11.5 (Equation 3); 1.9%≤C<2.15% and 11.5≤BET<13.5 Equation 4.

4. The positive electrode active material according to any one of claims 1-3, characterized in that, The compacted density of the positive electrode active material is 1.91-2.01 g / cm³. 3 .

5. The positive electrode active material according to claim 1, characterized in that, The mass percentage of doped elements in the positive electrode active material is 500ppm-2000ppm.

6. A method for preparing the positive electrode active material according to any one of claims 1-5, characterized in that, Includes the following steps: 1) Raw materials including sodium source, iron source, phosphorus source and carbon source are mixed with deionized water to obtain a mixture; the mixture is ground to obtain a grinding material; the grinding material has a D10 of 0.10μm-2μm, a D50 of 0.25μm-0.3μm and a D90 of 0.7μm-1.0μm; 2) The abrasive is spray-dried to obtain a sprayable material; the moisture content of the sprayable material is ≤3%, and the loose bulk density is ≤0.8g / cm³. 3 ; 3) The spray material is subjected to fluidized drying treatment. 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 fluidized material. 4) Under a protective atmosphere, the fluidized material is subjected to a first sintering and a second sintering in a rotary kiln to obtain the positive electrode active material; The first sintering temperature is 200-250℃, and the holding time is 1-2h; the second sintering temperature is 400-550℃, and the holding time is 10-20h.

7. The method for preparing the positive electrode active material according to claim 6, characterized in that, In step 1), grinding is performed by ball milling; The grinding process includes: sequentially subjecting the mixture to a first grinding and a second grinding to obtain the grinding material; the first grinding is performed at a rotation speed of 1000-1300 r / min for 30-50 min, and the grinding beads have a particle size of 0.6-0.8 μm; the second grinding is performed at a rotation speed of 1400-1600 r / min for 30-60 min, and the grinding beads have a particle size of 0.3-0.4 μm. And / or, in step 2), the inlet air temperature during 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℃.

8. A positive electrode plate, characterized in that, It includes the positive electrode active material according to any one of claims 1-5, or the positive electrode active material prepared by the preparation method according to claim 6 or 7.

9. A sodium-ion battery, characterized in that, It includes the positive electrode active material according to any one of claims 1-5, or the positive electrode active material prepared by the preparation method according to claim 6 or 7, or the positive electrode sheet according to claim 8.

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